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Separation and fractionation of nanoparticles in solutions by membranes Asmaa Khaled Selim Thesis submitted to the Faculty of Science at the University of Zaragoza, Spain in partial fulfilment of the requirements for the degree of Master Erasmus Mundus In Membrane Engineering Supervisors: Dr. Reyes Mallada Dr. Pilar Lobera June 24, 2014 Zaragoza, Spain 2 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Disclaim BG Този проект е финансиран с подкрепата на Европейската комисия. Тази публикация (съобщение) отразява само личните виждания на нейния автор и от Комисията не може да бъде търсена отговорност за използването на съдържащата се в нея информация. CS Tento projekt byl realizován za finanční podpory Evropské unie. Za obsah publikací (sdělení ) odpovídá výlučně autor. Publikace (sdělení) nereprezentují názory Evropské komise a Evropská komise neodpovídá za použití informací, jež jsou jejich obsahem. DA Dette projekt er finansieret med støtte fra Europa-Kommissionen. Denne publikation (meddelelse) forpligter kun forfatteren, og Kommissionen kan ikke drages til ansvar for brug af oplysningerne heri. DE Dieses Projekt wurde mit Unterstützung der Europäischen Kommission finanziert. Die Verantwortung für den Inhalt dieser Veröffentlichung (Mitteilung) trägt allein der Verfasser; die Kommission haftet nicht für die weitere Verwendung der darin enthaltenen Angaben. ΕΛ Το σχέδιο αυτό χρηματοδοτήθηκε με την υποστήριξη της Ευρωπαϊκής Επιτροπής.Η παρούσα δημοσίευση (ανακοίνωση) δεσμεύει μόνο τον συντάκη της και η Επιτροπή δενευθύνεται για τυχόν χρήση των πληροφοριών που περιέχονται σε αυτήν. EN This project has been funded with support from the European Commission. This publication (communication) reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein. ES El presente proyecto ha sido financiado con el apoyo de la Comisión Europea. Esta publicación (comunicación) es responsabilidad exclusiva de su autor. La Comisión no es responsable del uso que pueda hacerse de la información aquí difundida. ET Projekti on rahaliselt toetanud Euroopa Komisjon. Publikatsiooni sisu peegeldab autori seisukohti ja 3 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Euroopa Komisjon ei ole vastutav selles sisalduva informatsiooni kasutamise eest. FI Hanke on rahoitettu Euroopan komission tuella.Tästä julkaisusta (tiedotteesta) vastaa ainoastaan sen laatija, eikä komissio ole vastuussa siihen sisältyvien tietojen mahdollisesta käytöstä. FR Ce projet a été financé avec le soutien de la Commission européenne. Cette publication (communication) n’engage que son auteur et la Commission n’est pas responsable de l’usage qui pourrait être fait des informations qui y sont contenues. GA Maoiníodh an tionscadal seo le tacaíocht ón gCoimisiún Eorpach. Tuairimí an údair amháin atá san fhoilseachán (scéala) seo, agus ní bheidh an Coimisiún freagrach as aon úsáid a d’fhéadfaí a bhaint as an eolas atá ann. HU Az Európai Bizottság támogatást nyújtott ennek a projektnek a költségeihez. Ez a kiadvány (közlemény) a szerzõ nézeteit tükrözi, és az Európai Bizottság nem tehetõ felelõssé az abban foglaltak bárminemû felhasználásért. IT Il presente progetto è finanziato con il sostegno della Commissione europea. L’autore è il solo responsabile di questa pubblicazione (comunicazione) e la Commissione declina ogni responsabilità sull’uso che potrà essere fatto delle informazioni in essa contenute. NL Dit project werd gefinancierd met de steun van de Europese Commissie. De verantwoordelijkheid voor deze publicatie (mededeling) ligt uitsluitend bij de auteur; de Commissie kan niet aansprakelijk worden gesteld voor het gebruik van de informatie die erin is vervat. LT Šis projektas finansuojamas remiant Europos Komisijai. Šis leidinys (pranešimas) atspindi tik autoriaus požiūrį, todėl Komisija negali būti laikoma atsakinga už bet kokį jame pateikiamos informacijos naudojimą. LV Šis projekts tika finansēts ar Eiropas Komisijas atbalstu. Šī publikācija (paziņojums) atspoguļo vienīgi autora uzskatus, un Komisijai nevar uzlikt atbildību par tajā ietvertās informācijas jebkuru iespējamo 4 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) izlietojumu. MT Dan il-proġett ġie finanzjat bl-għajnuna tal-Kummissjoni Ewropea. Din il-publikazzjoni tirrifletti (Dan il-komunikat jirrifletti) l-opinjonijiet ta’ l-awtur biss, u l-Kummissjoni ma tistax tinżamm responsabbli għal kull tip ta’ uzu li jista’ jsir mill-informazzjoni li tinsab fiha ( fih). PL Ten projekt został zrealizowany przy wsparciu finansowym Komisji Europejskiej. Projekt lub publikacja odzwierciedlają jedynie stanowisko ich autora i Komisja Europejska nie ponosi odpowiedzialności za umieszczoną w nich zawartość merytoryczną. PT Projecto financiado com o apoio da Comissão Europeia. A informação contida nesta publicação (comunicação) vincula exclusivamente o autor, não sendo a Comissão responsável pela utilização que dela possa ser feita. RO Acest proiect a fost finanţat cu sprijinul Comisiei Europene.<0}Această publicaţie (comunicare) reflectă numai punctul de vedere al autorului şi Comisia nu este responsabilă pentru eventuala utilizare a informaţiilor pe care le conţine. SK Tento projekt bol financovaný s podporou Európskej Komisie. Táto publikácia (dokument) reprezentuje výlučne názor autora a Komisia nezodpovedá za akékoľvek použitie informácií obsiahnutých v tejto publikácii (dokumente). SL Izvedba tega projekta je financirana s strani Evropske komisije. Vsebina publikacije (komunikacije) je izključno odgovornost avtorja in v nobenem primeru ne predstavlja stališč Evropske komisije. SV Projektet genomförs med ekonomiskt stöd från Europeiska kommissionen. Föruppgifterna i denna publikation (som är ett meddelande) ansvarar endast upphovsmannen. Europeiska kommissionen tar inget ansvar för hur dessa uppgifter kan komma att användas 5 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Acknowledgment First of all, I express my utmost thanks to Almighty God the omnipresent and the creator of the worlds, who has endowed me brain and instable instinct, construction of knowledge and body to accomplishs work. After this, I would like express my deepest gratitude to my advisor, Professor Reyes Mallada for her support and guidance through this experience. Whatever kind of problem was discussed, her advice and encouragement always provided new perspectives. The expertise that she shared with me remains a source for my professional carrier. I am very thankful for everything she has done, for me. I warmly thank Dr. Maria Pilar Lobera and Ana for their advice and participation as my committee members. Also, I would like to voice my appreciation to Maciej Zieba and Dr. Nuria Navascues Garcia for giving technical suggestions, and providing me the use of many lab facilities, their time, encouragement, suggestions, and the sharing of their expertise and special thanks for Nuria for her help with transmission electron microscopy. I am also grateful to Carlos Ayllόn for his help as well for scanning electron microscopy. I would also like to thank my mother, my sister and the rest of my family and my fiancé, for their continued support over the two years of this master. I would also like to appreciate all my friends namely Usman, Vinit, Ghada, Meron and Cheryl for their supporting and motivation. 6 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Table of contents Disclaim .......................................................................................................................................... 2 Acknowledgment ............................................................................................................................ 5 Table of contents ............................................................................................................................. 6 List of figures ................................................................................................................................ 10 List of tables .................................................................................................................................. 14 Resumen ........................................................................................................................................ 15 Abstract ......................................................................................................................................... 16 Chapter one: Introduction ............................................................................................................. 17 1.1 Background ......................................................................................................................... 17 1.2 Separation and fractionation methods ................................................................................. 19 1.3 Separation and fractionation of nanoparticles in solutions by membranes ......................... 21 Chapter two: Objective ................................................................................................................. 31 Chapter three: Experimental work ................................................................................................ 32 3.1 Chemicals ............................................................................................................................ 32 3.2 Membrane ........................................................................................................................... 32 A) Titania membrane ........................................................................................................ 33 B) Alumina membrane ...................................................................................................... 34 3.3 Nanoparticles synthesis procedure ...................................................................................... 36 3.3.1 Synthesis of silica nanoparticles .................................................................................. 36 A) Ultrasonic method ........................................................................................................ 36 7 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) B) Conventional Stirring methods .................................................................................... 37 3.3.2 Synthesis of silver nanaoparticles ............................................................................... 37 3.4 Experimental set-up ............................................................................................................ 38 3.4.1 Filtration/Separation methods ...................................................................................... 38 a) Dead-end process .......................................................................................................... 39 b) Cross-flow process ....................................................................................................... 40 3.4.2 Set-up ........................................................................................................................... 40 3.5 Characterization and analysis Methods .............................................................................. 42 3.5.1 Dynamic Light Scattering - DLS ................................................................................. 42 3.5.2 Scanning Electron Microscope - SEM ......................................................................... 43 3.5.3 Transmission electron microscopy - TEM ................................................................... 43 3.5.4 Ultraviolet–visible spectroscopy - UV-vis................................................................... 44 3.5.5 Microbalance - Concentration and yield measurements .............................................. 45 5.3.6 Microwave coupled plasma atomic emission spectroscopy (MCP-AES) ................... 48 Chapter four: Results and discussion ............................................................................................ 49 4.1 Nanoparticles synthesis and their characterization ............................................................. 49 4.1.1 Silica nanoparticles characterization ............................................................................ 49 4.1.1.1 Ultrasonic Procedure ............................................................................................. 49 A) Dynamic Light Scattering ........................................................................................ 49 B) Scanning Electron Microscop Characterization....................................................... 51 4.1.1.2 Conventional stirring method ............................................................................... 53 A) Dynamic Light Scattering ........................................................................................ 53 8 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) B) Scanning Electron Microscop Characterization....................................................... 55 4.1.2 Silver nanoparticles characterization ........................................................................... 56 4.1.2.1 DLS characterization ............................................................................................ 56 4.1.2.2 Transmission Electron Microscope characterization ............................................ 58 4.1.2.3 Ultraviolet visible spectroscopy (UV-vis) ............................................................ 59 4.2 Fractionation and separation process .................................................................................. 61 4.2.1 Silica nanoparticles solutions ....................................................................................... 61 4.2.1.1 Filtration of polydisperse solutions ....................................................................... 61 4.2.1.2 Monodisperse silica solution................................................................................. 65 4.2.2 Separation and filtration of silver nanoparticles .......................................................... 69 4.2.2.1 Dead-end procedure .............................................................................................. 69 A) Feed with 0.01 mg/ml .............................................................................................. 70 B) Feed with 0.1 mg/ml ................................................................................................ 72 C) Feed with 0.25 mg/ml .............................................................................................. 74 4.2.2.2 Cross - flow procedure .......................................................................................... 78 A) Feed with 0.5 mg/ml ................................................................................................ 79 B) Feed with 1 mg/ml ................................................................................................... 81 Chapter five: Conclusion .............................................................................................................. 85 Bibliography ................................................................................................................................. 86 Appendix ....................................................................................................................................... 89 Appendix A: Methods of separation and fractionation of nanoparticles .................................. 89 1. Magnetic field ................................................................................................................... 89 9 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 2. Chromatography ............................................................................................................... 90 3. Centrifugal and Density Gradient Centrifugal .................................................................. 91 4. Electrophoresis .................................................................................................................. 93 5. Size selective precipitation ............................................................................................... 94 6. L-L extraction ................................................................................................................... 95 7. Field-flow fraction (FFF) .................................................................................................. 96 Appendix B: Pictures of some equipment used for characterization ........................................ 98 1. Dynamic light scattering ................................................................................................... 98 2. Transmission Electron Microscope................................................................................... 98 3. Ultraviolet–visible spectroscopy -UV-vis......................................................................... 99 4. CEM discover single-mode microwave ............................................................................ 99 16 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Abstract The intriguing size and shape dependent properties of nanoparticles have garnered recent attention in many science and engineering areas. Nowadays, nanoparticles have wide range of applications in the water purification field. When applying nanoparticles for removing contaminants / water purification, nanoparticles and the contaminant should be removed from the purified stream. For these reasons it is important to develop a method for size selective separation and purification of nanoparticles. Membrane filtration technique has been chosen for the separation and purification of water from those nanoparticles, because of their advantages of being easy, fast and green technique. This thesis presents novel approaches of using ultrafiltration membranes for nanoparticle separation and purification, using dead-end and cross-flow filtration techniques. A commercial ceramic membrane was evaluated for the fractionation and separation of silver and silica nanoparticles. In order to study the filtration process aspects, silver nanoparticles and silica nanoparticles were synthesized. Characterization of both nanoparticles and the solution before and after filtration was adopted using by Scanning Electron Microscopy (SEM), TEM to disclose their formation and corresponding morphologies, dynamic light scattering (DLS) particle size analyser for particle size distribution, RadWag for concentration and finally Ultraviolet visible (UV-vis) scanning spectrophotoscopy to detect the distinct spectrum of the silver nanoparticles produced. 17 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Chapter one: Introduction 1.1 Background Nanotechnology is the science and engineering of examining, monitoring, and modifying the behavior and performance of materials at nanoscale that is, at the atomic or molecular level. The techniques and principles have gained recognition in the field of medicine, water filtration and, in several industries and underdevelopment into transportation applications. Nanotechnology is multidisciplinary and has motivated collaboration among engineers, scientists, innovators, and researchers in sustainable development.[1] Nanotechnology is an emerging field that covers a wide range of technologies, which are presently under development in nanoscale. It plays a major role in the development of innovative methods to produce new products, to substitute existing production equipment and to reformulate new materials and chemicals with improved performance resulting in less consumption of energy and materials and reduced harm to the environment as well as environmental remediation. Similar to nanotechnology’s success in consumer products and other sectors, nanoscale materials have the potential to improve the environment, both through direct applications of those materials to detect, prevent, and remove pollutants, as well as indirectly by using nanotechnology to design cleaner industrial processes and create environmentally responsible products [2]. In the area of water purification, nanotechnology offers the possibility of an efficient removal of pollutants and germs [3]. Nanoparticles have been used for remediation of the ground water and for many other applications. It must be remembered that nanoparticles– unlike atoms– are never monodisperse and no two particles are identical. If the nanoparticles are monodisperse, they are a single-size and their properties are easily defined; however, if the nanoparticles are produced by attrition, they are polydisperse and their properties over a broad range when compared to single-size nanoparticles 18 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) [1]. This inherent polydispersity complexities of self-assembly, affects the overall characteristics deriving from the size-dependent properties of individual nanoparticles [4]. The impact of manufactured nanoparticles on living organisms is still controversial, albeit most researchers consider them as toxic. Engineered nanoparticles have the ability to travel from the lung to systematic sites in case of inhaling them. In addition, they have high rate of pulmonary deposition, penetrate dermal barriers and general high inflammatory potency. [5] Therefore, the sustainable growth of nanotechnology requires an environmentally-friendly technology to remove nanoparticles from potential drinking water sources [6]. These nanoparticles are synthesised in liquid media and usually the concentration of nanoparticles in the final solution is very low. For these reasons is important to develop a method for size selective separation and purification of nanoparticles. Nanoparticles size is one of the most important characterization parameter. In addition, size, morphology and structure are the effective factors on the nanoparticles performance. Producing controlled size and size distribution of nanoparticles was and still one of the confrontations, which stand up to nanoparticles production. Long time ago, producing a monodisperesd nanoparticles had been achieved by either controlling the production process or through the fractionation and separations techniques of nanoparticles. [7] The purification and separation of nanoparticles is also challenging because nanoparticles do not typically dissolve in solution, but instead disperse in the solvent: each individual nanoparticle exists as a single particle or crystal in the liquid system. 19 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 1.2 Separation and fractionation methods Approaches to separation and purification of nanoparticles are based on methods routinely applied to inorganic and organic materials and macromolecules, and may be selective for material property or size. Nanoparticles often consist of an inorganic (metal) core decorated with organic ligands. Purification and separation strategies for these molecules are capitalize on differences in their chemical and physical properties, such as polarity, solubility, and molecular weight, and due to the size differences of nanoparticles and their byproducts , it is possible to isolate them. Techniques such as magnetic fields, chromatography, density gradient centrifugal, electrophoresis, selective precipitation, liquidliquid extraction, and field-flow fractionation were usually used to separate and fractionate the nanoparticles. Table 1, is showing a summary of those different technique in terms of the driving force, main advantages and disadvantages of each. In addition, table one is showing the most famous techniques which are used in nanoparticles separation specifically. 20 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Table 1: Summary of separation and fractionation methods Method Driving force& Properties Advantages Disadvantages Most famous technique Ref. Magnetic field -Magnetic force - Based on the NPs magnetic susceptibility - Doesn't need addition of surfactant - Fast technique Limited by 50 nm - Magnetic field flow fractionation - High gradient magnetic separation [4] Chromatography - Based on differences in partition coefficient between mobile and stationary phases -Working in a wide range of NPs (few tens to few hundred) - Clogging the pores of the packing material - High performance liquid chromatography - Size-exclusion chromatography [1] Centrifugal - Based on density differences -Simple and easy - cheap - Time consuming - Limited amount of sample depending on the tube size - Required high speed for high efficiency - Density gradient centrifugal and ultracentrifugal [4], [1] Electrophoresis - Electric field - Based on the charge of the NPs which force them to migrate towards the opposite polarity electrode - Low consumption of sample and reagent - Fewer surface effect -More useful compared to Chromatography - Time consuming - Capillary electrophoresis (Ag, Au , inorganic oxides and CNTs) - Gel electrophoresis ( DNA and RNA) [8], [9], [10] Size selective precipitation - Based on difference in size , shape and surface coverage - High efficiency ≥ 90% - Most of the original sample will be discarded - DNA induced size selective separation for Au NPs [11], [12] L - L Extraction - Based on relative solubilities in two different immiscible liquids - Ability to extract two or more different NPs in same time -------- - Solvent extraction - Cloud point extraction [13] Field flow fractionation - The dual effect of the flow behavior and field distribution in a thin open channel - Small amount of samples in the ng-µg range - Parallel processing with multiple separation channels - Sample preparation - Magnetic field flow fractionation - Electrical field flow fractionation [14], [15] 21 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 1.3 Separation and fractionation of nanoparticles in solutions by membranes Filtration through a membrane is another alternative for the purification and size-fractionation of NPs. Membrane filtration has been used to separate or concentrate desired products via careful selection of the membrane pore size. Ultrafiltration and nanofiltration membranes mostly cover the nanometer range. In 1996 IUPACnomenclature defines nanofiltration as the process of rejecting particles and dissolved molecules smaller than 2 nm and defines ultrafiltration as lying between 2 nm and 0.1 μm.[16] Membrane-based filtration using media of a stated pore size has the following benefits; estimation of size-exclusion performance, easy scale-up by expanding the membrane area greener process that saves solvent during the purification and separation steps, convenient connections between processing tools such as reactors and analysis devices.[1] Nowadays, using membranes for the fractionation and separtaion of nanoparticles is most widely used, because of the high efficiency and easy operation. By the same way, membrane separation and fractionation received high attention due to its environmental impact as it is considered as a green process. [7] The work by Liang et al. [17], highlight the use of the carbonaceous nanofiber membrane for the filtration and separation of the nanoparticles. They demonstrated high efficient separation for three different types of nanoparticles Au, Ag, SiO2 using three membranes consisting of different size CNFs, namely, CNF-50, CNF-71, and CNF-98. Additionally, different particle were chosen for the testing i.e., 5, 25, and 60 nm Au, 25 nm Ag, and 150 nm SiO2. The separations technique had been done by using batch dead-end mechanism. The solution of binary mixture of Au nanoparticles 5 nm and 25 nm has volume of 14 ml in the ratio (2:12 ml) had been filtered with CNF-50. The retentate from this filtration process had been 22 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) dispered again in 20 ml of water for further filtration. Figure 1 is showing the results from the filtration process of the 25 nm Au nanoparticles solution. Figure 1: UV-vis spectra of the 25 nm Au nanoparticles feed solution, the concentrated solution and the filtrate permeated through the CNF-50 membrane. b) Optical images of the corresponding Au solution.[17] Another mixture solution of Au had been separated with the CNF-71 and CNF-50 membranes successively. This mixture of volume 24 ml of mixed nanoparticle containing 5, 25, and 60 nm Au nanoparticles (4:8:12 mL). Three fractions; upper retentate on the CNF-71 membrane, middle retentate on the CNF-50 membrane, and the bottom permeate were collected and analyzed by TEM. Figure 2 is showing the TEM analysis with a scale bar 200 nm for the mixed feed, the permeate and the retantae solutions from the separation process. 23 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Figure 2: a) The original mixture consisted of 5 nm, 25 nm, and 60 nm Au nanoparticles. b) The bottom filtrate fraction permeated through the CNF-71 and CNF-50 membranes successively. c) The middle fraction retained on the feed side of the CNF-50 membrane. d) The upper fraction retained on the feed side of the CNF-71 membrane.[17] On the other hand, another success has been obtained when Ag nanoparticles were tested with the CNF-50. The rejection of 25 nm Ag nanoparticles solution was almost 100%. Figure 3 is showing the UV-vis spectra for the feed and the filtrate solution. 24 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Figure 3: UV-vis spectra of the 25 nm Ag nanoparticles feed solution and the filtrate permeated.[17] In the same work, silica nanoparticles solution with 150 nm diameter was filtrated using CNF-98 membrane. The filtrate had been characterized with SEM. Figure 4 is showing the SEM images for the silica nanoparticles solution before and after the filtration. Figure 4: Filtration of 150 nm SiO 2 spheres using the CNF-98 membrane.[17] 25 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Mekawy et al. [18] have studied using membrane in nanoparticles separation. In this work, Ag nanopaticles have a diameter in the range of 1-16 nm had been separated using trimethylsilyl (TMS) -silica hybrid (anodic alumina membranes) AAM membrane. Anodic Alumina Membrane (AAM) with pore size of 200 nm, diameter 2.5 cm, and thickness 60 µm was used. The separation system was based on size - exclusive of Ag nanoparticles. Figure 5A is showing the separated amount of Ag nanoparticles from this system, which was monitored by measuring UV–Vis absorbance after saturation AAM pores. Additionally, it shows that 20% of the original amount of Ag nanoparticles could be separated in monodispersed and uniform Ag nanoparticles. However, the nanoparticles, with particle diameter larger than 4.5 nm, were blocked into the pore size or onto the surface of hybrid AAM as figure 5B and 5C is showing. Figure 5: Absorbance spectra (A) of Ag NPs, and the color change of Ag NPs solution (B) and mesoporous TMS-silica hybrid AAM membranes (C) before (a) and after (b) filtration process.[18] 32 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Chapter three: Experimental work 3.1 Chemicals Ethanol (C2H6O) with molecular weight 46.07 g/mol and purity 99.8% and ammonia withmolecular weight 17.03 g/mol and concentration 25% - were purchased from PanReac . Inc. (Barcelona, Spain). Tetraethyl orthosilicate (C8H20O4Si) with molecular weight 208.33 g/mol and purity 98% - , ethylene glycol (C2H6O2) with molecular weight 62.07 g/mol and purity ≥99 % - were purchased from Sigma Aldrich (Laborchemikalien GmbH, Seelze, Germany). Ammonium hydroxide withmolecular weight 35.05 g/mol and concentration 28% was purchased from Sigma-Aldrich Quimica SL (Madrid, Spain). Polyvinyl pyrrolidone (C6H9NO)n with molecular weight 10000 g/mol - and Silver nitrate (AgNO3) , molecular weight 169.87 g/mol and purity 99.999 % - were purchased from Sigma Aldrich (St. Louis, Missouri, USA). 3.2 Membrane Ceramic membranes were used in this study were Inopor®nano (inocermic GmbH, Hermsdorf, Germany) membranes. Two different pore sizes membranes had used for separation and fractionation process. Both ceramic membranes were used in this study membranes had an outer diameter 5 mm and an inner diameter of 3 mm and thickness of 1 mm. These membranes are asymmetric with a mean pore size of 5 nm and 60 nm. Both membranes had been cut to small membrane with 4.5 cm length. The filtration length was controlled to be 2.5 cm by enameling both ends to totally block the pores of the ends of the sectioned membranes. Thereafter, calcinations step had been done, in the calcinations step the polished membrane had been introduced to a high temperature oven at 850 oC for 1 hr. The membrane after calcinations step would have a shiny ended. 33 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) A) Titania membrane With reference to the smaller pores size membrane, the active membrane layer was composed of Titania (TiO2), which was deposited on a coarse-porous support Alumina (Al2O3). The whole thickness of the membrane is 1 mm. While the thickness of the small particles and pores layers was 60 µm and the rest was the big pores and coarse alumina support layer. The alumina support layer was divided to four layers with different thickness Coarseness and particles size. Figure 10 shows the scanning electron microscope image of the whole membrane thickness and the thickness and the structure for each layer separately. As shown in the images, the titania layer has a thickness around 600 nm and the four different alumina layers have thickness around 940 µm, 14.4 µm , 25 µm and 20 µm for the bigger pores and coarser particles to the smaller pores and finer particles respectively. 34 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Figure 10 : SEM image for TiO2 membrane B) Alumina membrane On the other hand, the 60 nm pores size membrane was an alumina membrane. The 1 mm thickness of this membrane is divided to four layers as same as the smaller pores size membrane but without the titania layer. The thicknesses of the four layers are 940 µm, 15 µm, 25 µm and 20 µm from the coarser to the finer layer respectively. Figure 11 is showing the scanning electron microscope image for the whole 60 nm pores membrane thickness. 35 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Figure 11 : SEM images for alumina 60 nm pores membrane characterization. 36 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 3.3 Nanoparticles synthesis procedure 3.3.1 Synthesis of silica nanoparticles The Stöber process [20] is used for the preparation of monodispersed silica colloids by means of hydrolysis of alkyl silicates and subsequent condensation of silicic acid in alcoholic solutions using ammonia as catalyst. The diameter of silica particles from this process is controlled by the relative contribution from nucleation and growth processes. The hydrolysis and condensation reactions provide precursor species and the necessary supersaturation for the formation of particles. During the hydrolysis reaction, the ethoxy group of tetraethyl orthosilicate "TEOS" reacts with the water molecule to form the intermediate (Si (OC2H5)4-X (OH)X) with hydroxyl group substituting ethoxy groups. Ammonia/ammonium hydroxide works as a basic catalyst to this reaction; the hydrolysis reaction is initiated by the attacks of hydroxyl anions on tetraethyl orthosilicate "TEOS" molecules [21]. Two procedures of the silica nanoparticles were performed based on Stöber method to produce polydisperse and monodisperse silica nanoparticles. The difference between both procedures is the amount of the precursor and the amount of catalyst. The molar composition between tetraethyl orthosilicate: ammonium hydroxide is as following 1:1 in the ultrasonic method and 1: 2 for the conventional stirring method. A) Ultrasonic method The synthesis procedure was as follows: 4 ml of tetraethyl orthosilicate "TEOS", 50 ml of ethanol and 4 ml of ammonium hydroxide 28% were taken. The mixture of ethanol and tetraethyl orthosilicate "TEOS" was stirred for 2 minutes, and then the ammonium hydroxide was added to the mixture during the stirring process to make sure of completely mixed solution. The mixture after the stirring process had been inserted to ultrasonic bath for 2 hr at room temperature. In the last mentioned step, the silica nanoparticles must have already been synthesized. 37 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) B) Conventional Stirring methods The synthesis procedure was as follows: 50 ml of ethanol, 3 ml of ammonium hydroxide 28-30% and 1.5 ml of tetraethyl orthosilicate "TEOS" were taken. The mixture of ethanol and ammonium hydroxide was stirred for 2 min and during stirring the tetraethyl orthosilicate "TEOS" was added drop by drop very slowly. The mixture then had been stirred for 1 hr or until the solution color start to be converted from clear transparent solution to milky turbid solution at temperature 30oC .[22] Thereafter for both methods, the mixture was centrifuged at 10000 rpm for 10 min to separate the synthesized particle from the colloidal solution. For finishing the synthesis, the silica nanoparticles were washed by absolute ethanol for three times to remove non-reacted tetraethyl orthosilicate "TEOS". The solution after washing by using ethanol has been washed by deionized water to prepare the solutions for filtration. 3.3.2 Synthesis of silver nanaoparticles Silver nanoparticles had been synthesized by microwave assisted polyol synthetic approach. The microwave-polyol method is a promising method for the rapid synthesis of rods, wires, polygonal plates, sheets, and dendrite types of silver nanostructures. Yamamoto et al. [23] prepared triangular Ag nanoplates by using microwave promoted reduction of silver nitrate "AgNO3" in aqueous solutions involving polyvinyl pyrrolidone "PVP". In the other hand, by using the microwave polypl method only spherical Ag nanoparticles were prepared by Pal et al.[24] . Ehylene glycol "EG" acts as a reducing agent as well as a solvent with a high boiling point of 198 oC, and high reduction ability. Since ethylene glycol "EG" has a large dielectric loss constant of 41.0, it is an ideal solvent in microwave heating synthesis of metallic nanostructures. Polyvinyl pyrrolidone (PVP, Mw: 10,000), silver nitrate (99,9999%) , ethylene glycol (anhydrous, 99 %) and deionized water were used to prepare aqueous solutions. First, 20 ml of 38 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) ethylene glycol "EG" was transferred to a glass beaker and was stirred with a magnetic stirrer. While stirring 2.4 g of polyvinyl pyrrolidone "PVP" was added slowly to the solution. The solution was covered and stirred for 2 hr. An amount of 0.158 g of silver nitrate "AgNO3" powder was added to the solution. The beaker was then covered with aluminium foil to protect it from the light, as the silver is a photosensitive material. The mixture of silver nitrate, polyvinyl pyrrolidone and ethylene glycol was then agitated continuously for 20 minutes. The mixture was then subjected to microwaves in order to synthesize the nano-spheres of specific diameters. In the CEM discover single-mode microwave which was enabled with Synergy program SintNPsAG23s which exposing the mixture to 23 seconds at 2 00 Watts of power and maximum temperature of 200⁰C, which intern synthesized spheres of diameters 5-30 nm in diameter. The sample temperature was monitored by an optical fibber immersed in the reaction mixture. Thereafter, the resulting solution was cooled rapidly in an ice bath. The mixture then was transfered to centrifugal tubes to introduce it to the centrifuge. The tubes were subjected to centrifugation at 21000 rpms for 1 hour; this process is repeated 6 times. In the first 5 times 10 ml of the supernatant was removed and replaced with a fresh deionized water. While in the sixth wash almost all the supernatant was removed and replaced with fresh deionized water. 3.4 Experimental set-up 3.4.1 Filtration/Separation methods Filtration processes are defined as dead-end or cross-flow by the solution stream direction. In this study, both filtration processes had been used with the silver and silica nanoparticles separation. Figure 12 is showing a schematic for both techniques. 39 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Figure 12: Separation mechanism ; (a) dead-end flow filtration, (b) cross-flow filtration.[1] a) Dead-end process The most basic form of filtration is dead-end filtration. The feed suspension flows perpendicularly to the membrane. Any particles in the feed solution that are larger than the pore size of the membrane are accumulated on the membrane surface forming a cake of solids and the rest of particles which are lower size than the pore size is passing through the membrane . The solution, which passed through the membrane, is called filtrate. The accumulated particle" cake" on the surface of the membrane decreases the filtration rate due to clogging. In spite of the flushing step had to be done after each batch to remove the filtrate cake and clean the membrane surface, in case of low concentrated solutions, dead end process is the most useful technique for concentrating compounds.[25], [26] 40 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) b) Cross-flow process In cross-flow filtration, the flow direction is parallel to the membrane surface and the feed flow and filtration flow direction have a 90 degrees angle. The feed flow through the membrane has an elevated pressure as driving force for the filtration process and a high flow speed to create turbulent conditions. This set-up has proved effective in reducing the clogging of membrane pores because of the turbulent flow along the membrane surface, which prevents the accumulation of matter on the membrane surface. In addition, this technique is scalable method that allows continuous-flow operation. The outlets from this technique are two streams: a permeate stream (or filtrate stream) which is perpendicular on the feed and a retentate stream, which flows tangentially to the feed stream.[26], [27] 3.4.2 Set-up A simple experimental set-up have been designed in the laboratory. Using SHIMADZU LC10ATVP High Performance Liquid Chromatography (HPLC) pump connected to stainless steel connections to introduce the solution to the membrane. Pressure was measured using pressure meter. Figure 13.a, 13.b, 13.c showing the experimental set-up for both separation/filtration technique cross-flow, dead-end and the backwash set-up respectively. The needle valve in the cross-flow set-up was used to control the retentate amount and the plug in the backwash and the dead-end was for blocking the other side from the membrane and force the solution to go through the opened side of the membrane. 41 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) (a) (b) (c) Figure 13 : Experimental set-up simple design. 48 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) With similar measurements of the concentration and calculations of the yield, the concentrations and yields had been measured and calculated for all the samples of both nanoparticles silica and silver nanoparticles before and after the filtration process. In addition to the yield calculations, the % of rejection for each membrane had been calculated based on the concentration as the % of rejection value is given by the following equation [26] : Rejection value = 5.3.6 Microwave coupled plasma atomic emission spectroscopy (MCP-AES) Microwave coupled plasma atomic emission spectroscopy (MCP-AES) also referred to as microwave coupled plasma optical emission spectrometry (MCP-OES). MCP is an analytical technique used for the detection of trace metals. Basically,it had been used to distinguish quantitatively the amount of silver in the final product after the filtration / separation process. The filtrate and the backwash concentration from the dead-end process for the low concentrated silver nanoparticles solution had been observed using Agilent Technologies, 4100 MP-AES. The measurement of the samples should be preceded by calibration. Using a standard samples of known concentration 1, 3, 5, 7, 9 ppm of a standard silver solution with the addition of aqua regia (HNO3: 3HCl) and fresh deionized water in the ratio 1: 3 . In addition, a blank sample had been prepared without addition of the silver slandered as a reference for the following measurements. The samples had been prepared in the same conditions of the patrons and the ratio between sample: water: aqua regia was 1:3:1. The measurement had been done at two wavelengths 328 and 338 nm. The measurement had been repeated twice to ensure the results for each sample and in the end the value of the concentration will be estimated from the average of these two measurement at the different wavelength. 49 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Chapter four: Results and discussion 4.1 Nanoparticles synthesis and their characterization 4.1.1 Silica nanoparticles characterization Four different batches had been prepared using the ultrasonic procedure and three using the conventional stirring method. All the batches had been characterized using the Dynamic Light Scattering (DLS) and the Scanning electron microscope (SEM). 4.1.1.1 Ultrasonic Procedure A) Dynamic Light Scattering As discussed in the previous chapter, the DLS results had been plotted using Origin software .In addition some samples' results had been fitted with the Gauss non-linear fitting to observe the mean diameter of the nanoparticles. Results in table 3 show that the ultrasonic procedure for synthesis silica nanoparticles is nonreproducible in terms of nanoparticles size, concentration, and the synthesis yield. The temperature of the ultrasonic bath and the exact position of the synthesis mixture could be the main reasons for the non-reproducibility of this procedure. As the temperature changing from one position to another inside the ultrasonic bath, this could affect on the conversion of the reaction and consequently the concentration and the yield could be changed conversely. On the other hand, from the polydipersity point of view the ultrasonic procedure is giving a monodisperse distribution of nanoparticles as the result of the polydispersity index show for the all batches. 50 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Table 3: DLS characterization for 4 batches of silica Synthesis using Ultrasonic Sample/ Data 1 2 3 4 Mean Diameter (nm) 188 226.5 381.4 145.2 Mean Diameter fitting (nm) 140.53 No fitting No fitting 140.25 FWHM (nm) 19.2 No fitting No fitting 54.7 PDI 0.084 0.059 0.005 0.063 Concentration (mg/ml) 20.56 5.7 15.64 8.34 Yield (%) 91.8 79.54 90.94 31.8 PH 9.48 9.07 9.15 8.77 Figure 15 (a-d), is representative for the 4 batches plotted and the fitting curves for some of them respectively. As shown in figure 15 for batch 1 and 4, the Gauss non-linear fitting curve is in red color while the main distribution from DLS is in the black color. 0100 200 300 400 0 20 40 60 80 100 120 140 160 180 1000 1010 1020 1030 1040 1050 1060 Number Diameter (nm) Batch 1 R=0.9 220 230 240 250 0 20 40 60 80 100 Number Diameter( nm) Batch 2 (a) (b) 51 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 370 375 380 385 390 0 20 40 60 80 100 Number Diameter(nm) Batch 3 0100 200 300 400 500 0 20 40 60 80 100 Number Diameter(nm) Batch 4 R = 0.9 (c) (d) Figure 15: DLS characterization for the four samples; (a), (b), (c), (d) represent samples from 1 to 4 respectively. B) Scanning Electron Microscop Characterization In addition to the DLS analysis, the SEM images are illustrating the polydispersity of samples 1 and 4. The bigger particles for sample 3 with the high degree of monodispersity are also shown clearly in the image. In addition, the image for sample 4 shows high degree of agglomerations. (a) (b) 52 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) (c) (d) Figure 16: SEM characterization for the four samples; (a), (b), (c), (d) represent samples from 1 to 4 respectively. The size distribution of the nanoparticles was also measured using he SEM images and the National Instruments IMAQ vision builder software. Figure 17 shows the results corresponding to measuring 100 nanoparticles of batch number 3. The mean nanoparticle diameter obtained from the IMAQ was 300±20 nm and it is smaller in accordance to the DLS measurements. Because DLS is measuring the hydrodynamic diameter of the nanoparticles, the results from DLS are not the same from the IMAQ. 200 250 300 350 400 450 500 0.00 0.05 0.10 0.15 0.20 0.25 Relative Frequency Diameter(nm) Figure 17 : Particle size distribution obtained by IMAQ for batch 3. 53 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 4.1.1.2 Conventional stirring method A) Dynamic Light Scattering Table 4 shows the results from synthesis of three solutions and their characterization using DLS. Evidently, the value for the polydisperse index is a clear indication of the high degree of monodispersity for the three samples. By the same way as dedicated in the table, the nanoparticles have a mean particles size in the range of 140±15 nm. In addition, the reproducibility of the conventional stirring method in terms of the synthesis yield and the concentration of the produced silica nanoparticles had been observed. The reproducibility of this method lies in three reasons: (1) controlling the addition of the tetraethyl orthosilicate as slow as possible, (2) Fixing the stirring rate from the beginning of the process and keep it constant, (3) Controlling the temperature of the stirring plat while the whole process at 30oC. Table 4: DLS characterization for 3 batches of silica synthezed using conventional stirring method Sample/ Data 1 2 3 Mean Diameter (nm) 155.3 143.9 139.3 Mean Diameter fitting (nm) No fitting No fitting 135.4 FWHM (nm) No fitting No fitting 53 PDI 0.031 0.022 0.03 Concentration (mg/ml) 3.48 3.22 3.25 Yield (%) 25.04 23.96 24.2 PH 8.6 8.9 8.7 54 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) As same as in the ultrasonic batches, the third batch had been fitted to observe the mean particles size diameter. The black curve and the red curve represent the DLS distribution and the fitting curve according to Gauss non-linear fitting. Figure 18 shows the DLS characterization for the three samples. 150 152 154 156 158 160 0 20 40 60 80 100 Number Diameter(nm) Batch 1 140 142 144 146 148 0 20 40 60 80 100 Number Diameter(nm) Batch 2 (a) (b) 0100 200 300 400 500 600 0 20 40 60 80 100 Number Diameter(nm) Batch 3 R= 0.923 (c) Figure 18: DLS characterization for the four samples; (a), (b), (c) represent samples from 1 to 3 respectively. 55 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) B) Scanning Electron Microscop Characterization The three samples were characterized by SEM and three of them show the same particles arrangement and morphology. Figure 19.a shows the SEM images for one sample of them. A second method was also used to confirm the size of the nanoparticles produced, the size was determined statistically using National Instruments IMAQ vision builder software as shown in figure 19.b. The mean nanoparticle diameter determined by IMAQfor measuring 100 nanoparticles was of 110±10 nm. 050 100 150 200 250 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Relative Frequency mean diameter(nm) (a) (b) Figure 19: Particle size distribution obtained by: (a) SEM , (b) IMAQ . Figure 19 (b) shows that there were small nanoparticles in the solution, which, the DLS could not recognize them because it measures the hydrodynamic diameter. 56 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 4.1.2 Silver nanoparticles characterization Six different batches had been prepared using the Microwave assisted polyol synthetic approach. All the batches had been characterized using the Dynamic Light Scattering and the Transmission electron microscope and Ultraviolet–visible spectroscopy. 4.1.2.1 DLS characterization In order to estimate the nanoparticl diameter, DLS characterization had been done for the 6 different samples (figure 20). 15 20 25 30 35 40 45 0 20 40 60 80 100 Number Diameter(nm) Batch 1 15 20 25 30 35 40 45 50 0 20 40 60 80 100 Number Diameter(nm) Batch 2 (a) (b) 15 20 25 30 35 40 45 50 55 60 0 20 40 60 80 100 120 Number Diameter(nm) Batch 3 R=0.925 0 5 10 15 20 25 30 35 40 0 20 40 60 80 100 Number Diameter(nm) Batch 4 (c) (d) 57 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 0 5 10 15 20 25 30 35 40 0 20 40 60 80 100 Number Dameter(nm) Batch 5 0 5 10 15 20 25 0 20 40 60 80 100 Number Diameter(nm) Batch 6 (e) (f) Figure 20: DLS characterization for the six samples from (a) to (f) represent samples from 1 to 6 respectively. For batches 1, 2, 3 and 5 the nanoparticles have a particles size in the range of 10 - 30 nm .While in the batches 4 and 6 the particle size distribution is different as the particles have a diameter in the range of 2.5 - 5 nm. The third batch distribution had been fitted and according to Gauss nonlinear fitting the mean diameter is around 32.7 nm and FWHM 8.77 nm. In addition, table 5 is summarizing the particle diameter, concentration, yield and the polydispearsity index for each batch. In all the batches bigger particles have been appeared in the volume distribution Table 5: Comparison between six samples of siver nanoparticles characterization Sample/ Data 1 2 3 4 5 6 Mean Diameter (nm) 21.2 25.6 33.1 6.8 13.7 2.7 PDI 0.063 0.094 0.218 0.298 0.305 0.321 Concentration (mg/ml) 4.26 3.28 1.7 2.05 3.1 12.54 Yield (%) ----- ------ 33.35 41.51 15.3 31 64 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) The results from DLS show that the solution has a polydispersity index of 0.124 and mean particle diameter 238.5±0.1 nm. 220 230 240 250 260 270 280 0 20 40 60 80 100 Number Diameter(nm) Backwash Figure 26: DLS characterization for the backwash solution for the polydispersity silica solution. According to the previous filtration process results, the fractionation process was successful for the polydisperse silica nanoparticles solution by having a smaller particles in the range of 73±5 nm in the permeate side while the bigger particles in the range of 239±0.4 nm were collected in the retentate side. Additionally, the concentration of permeate solution has measured and it was 0.34 mg/ml. Thereafter the % of rejection[26] had calculated resulting in 83 % of the polysiperse silica solution had been rejected using the 60 nm membrane. 65 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 4.2.1.2 Monodisperse silica solution A mixture solution had been prepared from the initial samples. The mixture particles size distributions were measured by using DLS. Figure 27 shows the number and volume distribution for the mixture solution. 160 170 180 190 200 210 0 20 40 60 80 100 Number Diameter(nm) Feed 160 170 180 190 200 210 0 20 40 60 80 100 Volume Diameter(nm) Feed volume (a) (b) Figure 27: DLS characterization for the conventional stirring batches mixture. The results from DLS distinct the mean particles diameter for the mixture is 180±0.2 nm. The polydispersity index for this mixture was 0.083, which depict the similarity between the number and the volume distribution. The feed mixture was prepared with a concentration of 2 mg/ml then the mixture had been fractionated using the 60 nm pores membrane. The fractionation had been done by operating cross flow, constant flow process. The cross flow process had been done at 2 ml /m flow for feed solution. 66 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) The pressure was fluctuating during the process between 0.025 - 1.005 MPa. The process was perpetuated until the pressure started to be constant and reach a plateau. The maximum pressure was 1.005 MPa during the 10 minutes of filtration process but it started to be constant after 8 minutes at value of 0.885 MPa. Figure 28 is showing the fluctuation of the pressure during the filtration process versus time. Figure 28: Pressure vs. time during the cross-flow separation process for the conventional stirring silica mixture solution. Although, the permeate solution was almost clear colorless solution while the retentate was dark milky solution. The particles in the permeate solution were having diameters in the range of 76.3±0.09 nm. The permeate solution concentration was measures in order to estimate the % of rejection[26]. The measurement shows that the permeate has a concentration of 0.86 mg/ml. Thereafter the % rejection was 57%. This result means that either there were smaller particles in the feed solution were successful to path through the 60 nm pore size membrane or these smaller particle could be due to fragmentation of the nanoparticles according to the pressure which was applied during of the process. Figure 29 is showing the size distribution of the particles in both permeate and retentate solutions. 0 0.2 0.4 0.6 0.8 1 1.2 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 Pressure MPa Time (m) Conventional stirring silica solution Silica solution 2 ml/m Backwash 5 ml/m 67 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 50 60 70 80 90 100 0 20 40 60 80 100 Number Diameter(nm) Permeate 200 210 220 230 240 250 0 20 40 60 80 100 Number Diameter(nm) Retentate (a) (b) Figure 29: DLS characterization for (a) permeate, (b) retentate solutions. In order to estimate the particle size for the nanoparticles, which were in the filtration cake on the membrane surface, a backwash process was done. The backwash process has been done at constant higher flow than the cross flow process in order to achieve the strong and high pushing for the nanoparticles. The pressure during the backwash process was plotted versus the time. Figure 28 shows that the pressure in the backwash process was fluctuating between 0.05 - 0.825 MPa and the process was lasted for 8 minutes but the process engrossed around 5 minutes to reach a fixed pressure at 0.825 MPa. Similarly to the previous results , the backwash pressure profile indicate that after a sharp increase the membrane is clean. The particle size distribution was measured by using DLS in order to estimate the mean Particles size of the backwash solution, figure 30 show the DLS graph. The mean diameter for the particles was of 194±0.38 nm. 68 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 0100 200 300 400 500 600 700 0 20 40 60 80 100 Number Diameter(nm) Backwash Figure 30: DLS characterization for the backwash solution. Table 6 shows a summary for filtration process for both the ultrasonic and the conventional stirring processes in terms of polydispersity index, concentration and rejection. Table 6: Summary of silica nanoparticles fractionation. Method /Data Polydisperse solution Monodisperse solution Feed Permeate Retentate Backwash Feed Permeate Retentate Backwash Particle mean diameter (nm) 40±10 &250±20 73±5 239±0.4 238±0.1 180±0.2 76.3±0.09 218±0.6 194±0.038 PDI 0.085 0.134 0.085 0.124 0.085 0.139 0.073 0.08 Rejection % 83% 53% In Wang et al [17] work, the % rejection for the monodisperse silica nanoparticles solution filtrated by dead end was given of ≥ 99%, but they only eliminate the silica nanoparticles having diameters ≤ the pores of their membrane CNF-98. While based on the stated results in this thesis a fractionation process was successful to reject 83 % of polydispere and 53% of monodisperse silica nanoparticles, producing monodiperse solution of the smaller particles in the permeate. 69 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 4.2.2 Separation and filtration of silver nanoparticles According to the different particles diameter, concentrations, polydiepersity and the yield of the produced silver nanoparticles batches. Two different mixtures of the silver nanoparticles had been used. The filtration of the silver nanoparticles had been done by using the 5 nm pores membrane due to the smaller nanoparticles that the silver has compared to the silica nanoparticles. Additionally, the silver solution had been filtrated with two different filtration procedures. Dead-end procedure was used for the low concentrations and cross-flow for the higher concentrations. 4.2.2.1 Dead-end procedure Three different concentration solutions (0.01, 0.1 and 0.25 mg/ml) had been prepared to be tested with dead-end filtration using three different 5 nm membrane. The filtration processes were done at constant flow 2 ml/m for the feed solution. The mixture feed solution (S1) particles size had been characterized by using DLS. Figure 31 shows the particle size distribution for both number andvolume. The volume distribution is in order to obtain the bigger particle. 010 20 30 40 50 0 20 40 60 80 100 Number Diameter(nm) S1 010 20 30 40 50 0 20 40 60 80 100 S1 Volume Diameter(nm) (a) (b) Figure 31: DLS characterization for mixed silver nanoparticles solution (S1) ;(a) Number and (b) Volume distribution. 70 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) A) Feed with 0.01 mg/ml A feed solution of concentration 0.01 mg/ml was filtrated using 5 nm membrane. The filtrate solution was totally clear and colorless. Figure 32 (a) shows the solutions before and after filtration. The filtrate concentration was very low, so it was not possible to measure the size distribution for the nanoparticles using the DLS. Alternatively, the filtrate nanoparticles were characterized by TEM and UV-vis respectively. Although, the TEM images did not show the nanoparticles size also due to the diluted solution as it shown in figure 32 (b) and the UV-vis spectra shows almost 100 % rejection figure 33(b). The solution had been concentrated by evaporating the water from the solution in order to estimate the particle size distribution and measure the exact concentration and rejection % of the filtrate solution. The evaporation process was at 60oC and lasted for 4 hr in which, initial volume for the filtrate was 22.8 ml and became 2.5 ml after evaporation. The solution after evaporation had a concentration of 0.0021 mg/ml (2.1 ppm) measured by MCP-AES. A rejection percentage of 88% was calculated based on the concentration value. Although, it was not possible again to measure the nanparticles size distribution by using the DLS analysis but the TEM for the filtrate after evaporation could show the nanoparticle of size 6±2.5 nm. Figure 32 (c) shows the TEM image for the filtration solution after evaporation. Figure 32: Filtration process for 0.01 mg/ml feed ; (a) solution before and after filtration , (b) TEM image for filtrate , and (c) TEM image for filtrate after evaporating the water . 71 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Therefore, the UV-vis spectroscopy alone or the TEM images for high diluted solution are not good techniques for characterize the silver nanoparticles after a filtration process. Consequently, the rejection percentage of 100 % which, estimated in Wang et al. work[17] or the 20 % estimated by Mekawy et al. [18] are not accourt as it had been calculated based on the Uv-vis spectra only. Despite this, comparing the rejection % obtained in this work with published work, the 88 % rejection obtained for the silver nanoparticles solution in the range of 5-20 nm at variance to the monodisperse solution of a particle diameter of 25 nm in Wang et. al[17]. In order to estimate the nanoparticles size IMAQ software was used to count the nanoparticles, figure 33 (a) is showing the particle size distribution of 100 nanoparticles obtained by IMAQ. Thereafter, in order to recover the nanoparticles, which were blocked in the membranes pore a back flush process had been done. The flow (2 ml/m) for the back flush was not enough to push the nanoparticles from the pores and the resulted solution was clear. Additionally, the absorption spectra of the feed, the filtrate before and after the evaporation and the backwash solution have been measured using UV-vis and are shown in figure 33 (b). 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0.00 0.05 0.10 0.15 0.20 Relative Frequency Diameter(nm) 200 300 400 500 600 700 800 900 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020 Absorbance Wavelength(nm) 0.01 feed 0.01 filtrate before evaporation Backwash 0.01 filtrate after evaporation (a) (b) Figure 33: (a) Particle size distribution obtained by IMAQ, and (b) UV-vis spectra for feed, filtrate before and after evaporation and backwash solutions. 72 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) From the figure, the nanoparticles have diameters in the range of 4 - 13 nm. The higher portion of the nanoparticles was in the range of 5 - 8 nm these nanoparticles, which are bigger than the 5 nm pores of the membrane could be formed during the evaporation process. On the other hand, the curve for the filtrate after evaporation has higher absorbance than before as it shown in figure 33 (b) the red and purple curves. B) Feed with 0.1 mg/ml A feed solution of concentration 0.1 mg/ml was filtrated using 5 nm membrane. The filtrate solution was light yellow. Thereafter, a backwash process had been done in order to recover the nanoparticles, which were crammed inside the 5 nm pores of the membrane. The flow for the backwash process was 5 ml/m in order to be higher than the feed flow. Figure 34 shows the pressure versus time graph for the backwash process and for the filtration process. Figure 34: Pressure vs. time during the dead - end process for the 0.1 silver solutions. The figure shows that the pressure for the backwash process is higher than the filtrate process and this is due to the nanoparticles, which were stuck in the membrane pores. The filtrate process lasted for 11 minutes until reaching a fixed pressure while it took only 6 minutes for the backwsh. The pressure for the filtrate process reached a plateau at 0.13 MPa while for the backwash process it was 0.26 MPa. 0 0.05 0.1 0.15 0.2 0.25 0.3 0 2 4 6 8 10 12 Pressure Mpa Time (m) Silver 0.1 mg/ml Silver 2 ml/m B.W 5 ml/m 73 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) The filtrate and backwash size distribution for the nanoparticles was measured using the DLS. Figure 35 (a) shows the filtrate and the backwash solutions, (b) and (c) shows the size distribution and TEM image for the permeate solution, (d) shows the size distribution for backwash and (e) shows the UV-vis spectra for feed, filtrate and backwash solutions respectively. 010 20 30 40 50 60 0 20 40 60 80 100 Number Diameter(nm) Filtrate (a) (b) 10 12 14 16 18 20 22 0 20 40 60 80 100 Number Diameter(nm) Backwash (c) (d) 80 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 2 4 6 8 10 12 14 16 0 20 40 60 80 100 Number Diameter(nm) Permeate 0 5 10 15 20 25 30 35 40 45 0 20 40 60 80 100 Number Diameter(nm) Retentate (a) (b) 810 12 14 16 18 20 22 0 20 40 60 80 100 Number Diameter(nm) Backwash 200 300 400 500 600 700 800 900 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance Wavelength(nm) Feed 0.5 mg/ml Permeate Retentate Backwash (c) (d) Figure 39: 0.5 mg/ml Filtration process; (a) (b) and (c) DLS characterization for filtrate retentate and backwash and (d) UV-vis spectra for feed, filtrate and backwash solutions respectively. The results from DLS show that, the permeate solution has nanoparticles in the range of 58 nm and the mean diameter is 5.5±0.5 nm. While for the retentate the nanoparticles were in the range of 19-32 nm and the mean diameter is 21.4±0.05 nm. Figure 40 shows TEM image for the permeate solution. 81 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Figure 40: TEM characterization for the permeate solution. B) Feed with 1 mg/ml A filtration process for a 1 mg/ml solution had been done at constant flow rate of 5 ml/m, the process had lasted for 9 min. The permeate and the retentate solutions were collected in order to estimate the size distribution of both of them. Additionally, a backwash process had been done in order to recover the nanoparticles which were stuck in the membrane pores. The backwash process had been done at same conditions of the filtrate process. The size particles distribution of the solutions had been measured using DLS. Additionally, the absorbance spectra of the feed, permeate, retentate and the backwash solution was analyzed using UV-vis spectroscopy. Figure 41, is showing the DLS characterization for all the solutions. 82 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 0 2 4 6 8 10 12 14 0 20 40 60 80 100 Number Diameter(nm) Permeate 15 20 25 30 35 0 20 40 60 80 100 Number A Retentate (a) (b) 10 15 20 25 30 35 40 45 50 0 20 40 60 80 100 Number Diameter(nm) Backwash 200 300 400 500 600 700 800 900 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Absorbance Wavelength(nm) Feed Permeate Retentate Backwash (c) (d) Figure 41: 1 mg/ml Filtration process; (a) (b) and (c) DLS characterization for filtrate retentate and backwash and (d) UV-vis spectra for feed, filtrate and backwash solutions respectively. The results from DLS are showing that, the permeate solution has mean diameter of 8.7±0.123 nm while the retentate nanoparticles having mean particle diameter of 21.4±0.05 nm. On the other hand, the backwash nanoparticles were has mean diameter of of 16±0.06 nm. The 83 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) permeate solution was additionally characterized using TEM, Figure 42, is showing TEM image for the permeate solution. Figure 42: TEM characterization for the permeate solution. Table 8 is the summary of the cross flow filtration in terms of the particle size and the polydispersity index in addition to the amount of silver in each stream and the mass balance percentage. Table 8: Summary of the cross flow filtration process. Data/ sample 0.5 mg/ml 1 mg/ml Feed Permeate Retentate Backwash Feed Permeate Retentate Backwash Particles diameter rage(nm) 5-51 5-8 19-32 15.8 5-51 7.9 14-32 16-38 PDI 0.306 0.029 0.246 0.217 0.306 0.271 0.255 0.272 Amount of silver (mg) 27.5 1.35 16.24 0.702 45 23.32 15.75 5.76 Mass balance (%) 66.5% 99.6% Rejection(%) 95% 48.2% 84 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) According to the result from the filtration of the 1 mg/ml silver solution, almost all the silver nanoparticles had been collected. Therefore, the membrane became a place of interest in order to explore whether there are silver nanoparticles inside the pores of the membrane or not. In order to achieve that, the membrane had been characterized by SEM. In addition, Energy-dispersive Xray spectroscopy (EDX) had been used to analyze the nanoparticles, which, were found in the membrane, pores. Figure 43(a) and (b), is showing a thin layer of nanoparticles, the EDX analysis for this layer indicated to these were silver nanoparticles. (a) (b) Figure 43: SEM and EDX analysis for the 5 nm membrane after filtration. Table 9 shows the results from EDX analysis for the nanoparticles which existing in the pores of the membrane and approving the existence of silver nanoparticles in all the places in addition to the titanium and aluminum. Table 9: EDX analysis for the 5 nm membrane. Spectrum 1 2 3 4 5 6 mean Ag 1.70 1.92 2.64 1.83 1.56 0.88 1.75 85 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Chapter five: Conclusion The ultrasonic procedure of synthesis silica nanoparticles is non-reproducible method in terms of the particle mean diameter and the yield of the reaction. The conventional stirring method is a reproducible method taking into account : reasons:(1) controlling the addition of the tetraethyl orthosilicate as slow as possible, (2) Fixing the stirring rate from the beginning of the process and keep it constant, (3) Controlling the temperature of the stirring plat while the whole process at 30oC. The fractionation process of the polydisperse and monodisperse silica solution using a cross flow filtration was successful using 60 nm membrane. Independently on the concentration of small nanoparticles is the feed, the membrane was able to fractionate the solution. Additionally, the dead end filtration process was successful for purification and removal of the silver nanoparticles at low concentration 10 ppm (0.01 mg/ml) in 88 % rejection. The concentration of the silver in the filtrate solution was 2.1 ppm (0.0021mg/ml). The Uv-vis technique is not enough to evaluate the residue of the silver nanoparticles. The cross flow filtration of polydisperse solutions of silver nanoparticles using 5 nm TiO2/Al2O3 membrane was effective for fractionation the silver nanoparticles. The rejection of the membrane is inversely proportional to the concentration of the feed solution. As the silver nanoparticles of 0.5 mg/ml concentration lead to 95% rejection, while the 1 mg/ml concentration resulted to 48.2%, which is almost the half. 86 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Bibliography 1. 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Mines, Editor. 89 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Appendix Appendix A: Methods of separation and fractionation of nanoparticles 1. Magnetic field According to the nanoparticles size and/or their magnetic susceptibility the magnetic field can separate them with a magnetic force given by FM=μ0χVpH∇H ; H is the external magnetic field, Vp is the particle volume and χ is the magnetic susceptibility . For iron oxide nanoparticle, theoretically the limit size for separation is ≈50 nm in the low magnetic field (< 100 T/m). Thermal diffusion and Brownian motion overcome the magnetic force affecting on the nanoparticle and it is not possible to get full fractionation in case of nanoparticle lower than 50 nm. In the absence of the external magnetic field, the dipole-dipole interactions between particles' magnetic moments might be present. The formation of large nanoparticles aggregate could be happen because of these forces, these aggregation characterized by strong magnetic response. During high gradient magnetic separation (HGMS) of magnetic nanoparticles those aggregation were observed by Moeser et al. [31]. The formation of these aggregations also explains why it is possible to separate nanoparticles smaller than predicted by theory. Capillary magnetic field flow fractionation (MFFF) is another system based on the magnetic field separation. According to this system, the nanoparticles can be separated not only by the different in the size but also by the different in the material composition. This technique depends on (i) the perpendicular magnetic forces to capillary flow and of magnitude proportional to the particles' magnetic susceptibility; and (ii) hydrodynamic forces and particle diffusion.[4] 96 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) The silver nanoparticles has selectively extracted through a strong ligandnanoparticle interaction into the organic phase resulting in a solution of Ag monolayer-protected clusters. In addition, the remaining gold dendrimer encapsulated nanoparticles could be extracted after by the addition of an organic phase containing an n-alkanethiol. It is also possible to extract both metals simultaneously or selectively extract Au nanoparticles from a mixture of Ag and Au dendrimer encapsulated nanoparticles. Figure f is showing the separations schemes for both silver and gold dendrimer encapsulated nanoparticles. Figure f : Extraction separation schemes for both silver and gold dendrimer encapsulated nanoparticles [13]. 7. Field-flow fraction (FFF) Field-flow fractionation is a family of analytical and separation techniques that depend on the dual effect of the flow behavior and field distribution in a thin open channel. In addition, fieldflow fractionation separations are carried out in a single phase compared to other techniques such as chromatography. The flow profile in field-flow fractionation channel can be described by a parabola with the highest flow velocity at the center of the channel and decreasing flow velocity 97 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) with increasing proximity to the channel walls. Field-flow fractionation channels typically consist of a thin spacer enclosed by two parallel plates, modified to impart the external field. The sample mixture migrates toward one side of the channel by virtue of the channel flow, is lifted by diffusion and hydrodynamic forces, and sinks under the influence of the downward flow. Thus, the particle mixture distributes in the vertical direction; and particles are separated into zones of differing transport velocity[34] [1, 14]. Figure g: Field flow fraction technique for particle size analysis[1]. 98 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) Appendix B: Pictures of some equipment used for characterization 1. Dynamic light scattering - Brookhaven 90 plus photo correlation spectroscope used for particle size analysis (DLS) 2. Transmission Electron Microscope - FEI Technai T20 (LTEM) transmission electron microscope used for TEM characterization 99 "The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities". (www.em3e.eu) 3. Ultraviolet–visible spectroscopy -UV-vis Hellmanex JASCO V-670 spectrophotometer used for UV-vis characterization. 4. CEM discover single-mode microwave CEM microwave system used to synthesize silver nanospheres.