Colorimetric detection, quantification and extraction of Fe(III) in water by acrylic polymers with pendant Kojic acid motifs

Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 1 Colorimetric detection, quantification and extraction of Fe(III) in water by acrylic polymers with pendant Kojic acid motifs Saúl Vallejos,1 Asunción Muñoz,1 Félix Clemente García1, Roberta Colleoni,2 Raffaela Biesuz,2 Giancarla Alberti,2,* José Miguel García1,* 1 Departamento de Química, Facultad de Ciencias, Universidad de Burgos, Plaza de Misael Bañuelos s/n, 09001 Burgos, Spain. Fax: (+) 34 947 258 831, Tel: (+) 34 947 258 085. E-mail: [email protected] 2 Dipartimento di Chimica, Università di Pavia, via Taramelli 12, 27100 Pavia, Italy. Tel: (+) 39 0382 987347. E-mail: [email protected] Graphical abstract Research highlights • A colorimetric sensory polymer for the detection of Fe(III) was synthesized. • This polymer was prepared from a monomer derived from Kojic acid, which is a natural product. • The film-shaped polymer was cut to obtain manageable solid sensory kits. • Fe(III) was efficiently extracted, detected, discriminated and quantified from water. • UV/vis and computer vision-based techniques were used for Fe(III) analysis. ABSTRACT We synthesized a solid sensory material for the extraction, detection and quantification of iron(III) in aqueous media. The material is a film-shaped colorless polymer membrane that exhibits gel behavior. The Fe(III) extraction and sensing characteristics 1.65x10 -6 M 1.65x10 -1 M Fe(III) Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 2 are imparted by a new monomer derived from a natural product (i.e., Kojic acid), which exhibits chelating properties toward Fe(III). The sorption of Fe(III) on the membrane in water has been thoroughly characterized, including the sorption kinetics, sorption isotherms and profiles as a function of the pH. Fe(III) sorption followed pseudo firstorder kinetics and required approximately 30 min to reach equilibrium. The maximum sorption capacity was approximately 0.04 mmol/g, and the sorption isotherms are well modeled by the Langmuir equation. The complexes that were found in the solid phase are in good agreement with those previously identified in the aqueous phase. Moreover, the sorption is highly specific (i.e., a recognition process) and results from the formation of a colored complex (iron(III)-Kojic acid derivative moieties). Therefore, the colorless sensory membrane turns red upon immersion in aqueous solutions containing Fe(III). The color output allows for both the qualitative visual determination of the Fe(III) concentration as well as also titration of Fe(III) using a) a UV/vis technique (limit of detection of 3.6x10-5 M; dynamic range of five decades, lower concentration = 1.65x106 M) and b) a computer vision-based analytical chemistry approach via color definition of the sensory membrane (RGB parameters) obtained from an image recorded with a handy device (e.g., a smartphone) (limit of detection of 2.0x10-5 M). Keywords Sensory polymers, Kojic acid, visual detection, iron detection, iron extraction 1. INTRODUCTION The wide distribution of iron as cations in different oxidation states in the environment has both natural and anthropogenic origins. The latter is caused by its multiple applications in agriculture, industry, construction, medicine, households, and advanced Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 3 technological fields, which has led to increasing concerns over the potential effects of iron on the environment as well as in living things [1]. Therefore, the detection and quantification of iron salts is a fundamental task in chemistry due to its crucial role in living organisms and being extremely dangerous when its concentration exceeds a critical level. Its quantification is still a fundamental task in clinical chemistry but it is also of interest in environmental and agri-food fields. Currently, routine analyses are performed using methods that require trained staff and expensive equipment and techniques, such as atomic absorption spectroscopy (AAS) or inductively coupled plasma mass spectrometry (ICP-MS). However, chemical sensors allow for conversion of the iron salt concentration into signals that can be read by widely available instruments or even by an untrained observer, especially if the signal is a color change (chromogenic sensor). A sensor is a self-contained analytical device that is able to convert a physical quantity, which is related to a chemical species concentration, into a signal that can be read by an observer or an instrument [2-8]. Sensors are used in everyday objects and have innumerable applications that are often overlooked [9]. In recent years, we have focused on investigating solid-phase sensors for iron(III) analysis to develop a quick, easy and cheap method for the quantitative determination of metal ions [10-13]. In this study, we report a novel polymeric membrane for visual iron(III) sensing that features a derivative of Kojic acid as the receptor moiety. Kojic acid, which is a γ-pyrone derivative (5-hydroxy-2- (hydroxymethyl)-4-pyrone), is a natural heterocyclic chelating ligand [14], and anions of Kojic acid and its derivatives act as bidentate ligands to strongly coordinate to iron(III) via carbonyl and phenolic hydroxyl groups [15]. Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 4 The film-shaped membrane is a manageable material that can be cut into solid sensory kits (e.g., small sensory discs). To develop a solid-phase sensor, the sorption of the selected target species as well as the kinetic and thermodynamic properties must be characterized. Therefore, a full characterization of the material was performed to determine the kinetics, isotherms and sorption profiles of iron(III) as a function of pH. For the colorimetric iron(III) sensing behavior of the membrane, the membrane is initially colorless and transparent but after immersion in iron(III) solutions, it turns red within minutes. Its performance was evaluated using ultraviolet-visible spectroscopy (UV/vis). In addition, a titration curve was constructed using the UV/vis data from the spectra recorded from sensory discs placed in contact with water solutions containing different iron(III) concentrations. Moreover, the titration was also performed using an image of the sensory discs (i.e., color digital definition (RGB parameters) of the previously mentioned sensory discs (computer vision-based analytical chemistry). Therefore, the solid sensory discs are manageable materials that can be handle without care, stored under ambient conditions for long periods of time, and used to measure the concentration of Fe(III) even by untrained personnel using images recorded using widely available devices, such as mobile phones and tablets. 2. EXPERIMENTAL SECTION The materials, synthetic procedures and measurements methods and methodologies are described in the Supporting Information (ESI) in Section S1. 2.1. Preparation of sensory materials Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 5 The sensory monomer derived from Kojic acid (3) was prepared according with the experimental procedure described in the ESI (Section S1) and shown schematically in Scheme 1. The film-shaped sensory membranes were prepared via bulk radical polymerization of hydrophilic monomers including 1-vinyl-2-pyrrolidone (VP), 2hydroxyethyl acrylate (2HEA), and a monomer derived from Kojic acid (3). Ethylene glycol dimethacrylate (EGDMA) was used as a cross-linking agent (Scheme 2). The VP/2HEA/(3)/EGDMA comonomer molar ratio was 75/24/1/10. AIBN (1 wt%) was employed as a thermal radical initiator. The bulk radical polymerization reaction was carried out in a 200 µm thick silanized glass mold in an oxygen-free atmosphere at 60ºC overnight. After demolding, the film was conditioned at 60ºC for an additional night. The solid sensory substrates were manufactured from the film-shaped membrane by using a puncher to cut out sensory discs (8 mm diameter). Scheme 1. Scheme 2 3. RESULTS AND DISCUSSION Our objective was to prepare an easily handled solid material for the extraction, detection and quantification of Fe(III) in pure water. The material consists a film-shaped polymer network with derivative of Kojic acid in its structure, which acts as a receptor and chemosensory core for Fe(III). The polymer has a cross-linked structure that is hydrophilic and acts as a membrane with gel behavior, allowing Fe(III) ions in water to enter into the material as solvated species where they interact with the receptors to give Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 6 rise to the extraction and sensing phenomena. Kojic acid was chosen as the receptor because it is a natural product and a well-known chelating ligand that strongly coordinates to iron(III), which results in red-colored complexes [14-16]. Moreover, Kojic acid has been used for many years for the spectrophotometric determination of iron in different contexts (e.g., in ores) [17,18]. Therefore, we designed and prepared an acrylic monomer (3) containing a Kojic acid sensory motif to synthesize a membrane for use as a colorimetric sensor and solid-phase extractant (Scheme 1). The mechanism of detection and extraction of Fe(III) involves the formation of red Fe(III)n:(Kojate motifs)m chelates with primary stoichiometries (n:m) of 1:1, 1:2 and 1:3 (Scheme S1, ESI). The stability constants for the interaction between iron(III) and Kojic acid have been reported by Muraki and are log K1 = 10.20, log K2 = 8.78, and log K3 = 7.53 [16]. The stoichiometry of the Fe(III)n:(Kojate motifs)m species in the solid state (i.e., inside the membrane) and the relative exchange coefficients that are described below for our system are in agreement with those for the species in solution [16]. 3.1. Material preparation and characterization The methacrylate sensory monomer (3) could not be conventionally prepared in a single step from Kojic acid and methacryloyl chloride but was synthesized using a two-step procedure. First, the treatment of Kojic acid with thionyl chloride led to the reaction of the primary alcohol with the formation of a Kojic acid primary chloro-derivative [19] that led to (3) by reaction with potassium methacrylate at 100ºC (no thermal initiated polymerization was observed) [20]. The reaction steps are schematically shown in Scheme 1. The 1H and 13C NMR as well as FTIR spectra of the intermediates and monomers are provided in the experimental section and in the ESI, Section S1. The potential applicability of the sensory membrane was confirmed by the fact that less than Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 7 1.6% by weight of the sensory synthetic monomer (3) was used in the preparation of the sensory material and >98.4% by weight of commercial and inexpensive comonomers. The membrane or film exhibits a good physical appearance and was creasable and easily handled. The thermal resistance was evaluated by thermogravimetric analysis (TGA). The degradation temperatures that resulted in a 5% and 10% weight loss under inert and oxidizing atmospheres (T5) were ~280ºC and ~280ºC, respectively, which is in agreement with the TGA data for the VP and 2HEA copolymers [10,21]. The thermal degradation patterns were affected by the sorption of Fe(III) by the membrane. The immersion of the membrane in water containing a relatively low concentration of Fe(III) increased T5 to 355ºC due to the additional crosslinking caused by the formation of Fe(III)1:(Kojic acid moieties)2 and Fe(III)1:(Kojic acid moieties)3 (Figure S5, ESI). In contrast, immersion in water with a higher concentration of Fe(III) decreased T5 to 300ºC due to partial displacement or complexation to Fe(III)1:(Kojic acid moieties)1 species with a concomitant decrease in the crosslinking density (please see the sorption and sensing results below). Gel behavior is relevant for a membrane to sense in pure water because the target species enter into the material as solvated species by diffusion. However, the water uptake has to be modulated to maintained good mechanical properties in the swelled state. Therefore, a moderate water swelling percentage ranging from 40% to 100% is desirable. The membrane composition was designed to meet this criterion, and its water swelling percentage was 65%. 3.2. Sorption studies 3.2.1. Sorption kinetics and isotherms Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 8 For example, the kinetic profiles of iron(III) sorption on the membrane are shown in Figure 1. Kinetic experiments are useful to determine the time required to reach equilibrium between the two phases. The experiments were performed under acid conditions (pH 2.5) to avoid iron(III) hydrolysis reactions. Iron(III) sorption on the membrane required approximately 30 min to achieve equilibrium. The experimental data are fitted using a HPDM model (homogeneous particle diffusion model) where the diffusion of ions from the solution to the sorbent is the slowest step. The relationship between the amount of sorbed metal ion in the solid phase (q, mmol g-1) and time (t, min) is formally equal to the pseudo first order kinetic equation [22], and the rate constant (k) was determined to be 0.20(2) min-1 (R2=0.979; SE(y) = 0.015). Figure 1. The sorption isotherm of iron(III) on the membrane in a 0.1 M KNO3 solution at 25°C and pH 2.5 are shown in Figure 2. Figure 2. The Langmuir and Freundlich models are typically employed to describe the relationship between q (sorbed quantity in the solid phase, mmol g-1) and Ceq (solute concentration at equilibrium). Their varied performances have been previously reviewed [23]. The Langmuir model provided the best description of the sorption of Fe(III) on the membrane. Based on non-linear fitting of the data in Figure 2, the maximum sorption capacity qmax was 0.041(2) mmol g-1, and KL= 6.5(9) × 103 M-1 (R2 = 0.984; Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 9 M H L MH L H r pn nq++  SE(y) = 0.002). The theoretical total number of active sites calculated from the weight percentages of (3) in the membrane was 0.07 mmol g-1, which is in agreement with the obtained qmax. 3.2.2. Sorption profiles The thermodynamic characterization of ion-exchange and chelating materials is fundamental for describing the separation process and predicting the behavior of analyte sorption on solid phases in different systems. As previously mentioned, especially for ion-exchange and complexing resins [23-25], a metal ion (M) can be sorbed onto a chelating solid phase via complexation reactions, and the general equilibrium can be expressed as follows: (1) where M is the metal ion, HrL is the r-protonated form of the active site of the solid phase, H is the proton, and the overbar represents species in the solid phase. Charges are omitted for simplicity. This reaction is studied based on the sorption profiles as a function of pH. This method has been previously described in several papers and reviews [23-28]. This method enables us to describe the sorption curve with an equation obtained using a combination of exchange coefficients ( β 1npex) that are associated with each possible reaction between the metal ion and the active site and selected to minimize the difference between the calculated and experimental sorption profiles. The exchange coefficient (β1npex) can be expressed as follows: n r q np np ] LH[[M] [H]]LMH[ = ex1 ⋅ ⋅ β (2) Sensors and Actuators B: Chemical, 2016, 233, 120-126 doi: 10.1016/j.snb.2016.04.040 Link to the published article: http://www.sciencedirect.com/science/article/pii/S092540051630510X 16 tablets and smartphones, allow for out-of-lab quantification of target species in an in situ, rapid and inexpensive fashion. Acknowledgments We gratefully acknowledge the financial support provided by FAR (Fondi Ateneo per la Ricerca) of the University of Pavia, by the Spanish Ministerio de Economía y Competitividad-Feder (MAT2014-54137-R) and by the Consejería de Educación – Junta de Castilla y León (BU232U13). Supporting information Experimental (intermediates and membrane characterization) and principal component analysis (PCA) data. References and notes [1] H. G. Seiler, A. Sigel, H. Sigel, Handbook on Metals in Clinical and Analytical Chemistry, Marcel Dekker, Inc., NY, 1994, p.13. [2] V. 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García, Sub-ppm quantification of Hg(II) in aqueous media using both the naked eye and digital information from pictures of a colorimetric sensory polymer membrane taken with the digital camera of a conventional mobile phone, Anal. Methods 5 (2013) 54-58. 1 FIGURES AND SCHEMES Captions Scheme 1. Synthesis of acrylic monomer (2). Scheme 2. Monomers and chemical structure of the sensory membrane. The picture shows the physical aspect of the membrane on a notebook. Figure 3. Sorption profiles of Fe(III) on 26.5 mg of dry membrane in 1 M KNO3, V = 10 mL, [Fe(III)] = 1.8 µM. Red circles = profile in absence of competitive ligand, blue diamonds = profile in presence of pyridine dicarboxylic acid (PDCA) 0.5 mM, green circles = profile in presence of sodium oxalate 0.05 M. Figure 2. Sorption isotherm of Fe(III) on the membrane (sorbed metal ion in the solid phase (q) as a function of the solute concentration when the equilibrium is reached (Ceq)). Conditions: volume = 10 mL 0.1 M KNO3, pH 2.5, temperature = 25ºC, 36.5 mg of dry membrane. Gray circles: experimental points; continuous line: best fitting by Langmuir model. Figure 1. Kinetic profile of Fe(III) uptake on the membrane (sorbed metal ion in the solid phase (q) as a function of time (t)). Conditions: volume = 10 mL 0.1 M KNO3, pH = 2.5, temperature = 25ºC, [Fe(III)] = 1.34·10-4 M, 13.7 mg of dry membrane. The lines represent the fit obtained using a pseudo first order equation. Figure 4. Colorimetric determination of the Fe(III) concentration in water using sensory discs cut from the membrane: a) UV/vis spectra (inset: picture of the discs, left disc is control disc); b) UV/vis titration curve; c) titration using the RGB parameters from the digital image taken from the sensory materials (see inset, Figure 5a). The three parameters (R, G and B) defining the color of each disc were reduced to one principal component (PC1) by principal component analysis. Conditions: prior to each measurement, each disc was immersed overnight in Milli-Q water (temperature = 25ºC, pH = 2 -buffer HCl/KCl-) containing a Fe(III) concentration ranging from 1.65x10-6 to 1.65 x10-1 M and then removed from the medium and dried at rt. Figure 5. Response time. Selected UV/vis spectra as a function of the time that a piece of membrane (discs, 8 mm diameter) was immersed in water (pH = 2, buffer: KCl-HCl, 2 mL) in a UV/vis quartz cuvette upon addition of Fe(III) (concentration = 5x10-3 M). Inset = Absorbance (490 nm) as a function of time. Figure 6. Interference study. a) UV/vis absorbance variation at 483 nm for (3) in solution (DMAc/H2O, 50/50; pH = 2, buffer: KCl-HCl, 2 mL; [(3)] = 1.6x10-3 M) after addition of various cations. Each cation was added individually in a (3)/cation molar ratio of 1/10 (the concentration of each cation was 1.6x10-2 M). Inset: sample UV/vis spectra of solution (3) after addition of various cations (i.e., Fe(III), Co(II) and Hg(II)); and b) picture of two sensory discs after immersion overnight in water (pH = 2, buffer: KCl-HCl, 2 mL) containing Fe(III) ([Fe(III)] = 1 x 10-2 M) and two mixtures of cations (the concentration of each cation was 1 x 10-2 M). 2 Scheme 1. Synthesis of acrylic monomer (3). O OH O HOO OH O ClO OH O O O SOCl 2 0ºC, 4h OK O, 2-pentanone 100ºC, 2h (1) (2) (3) 3 Scheme 2. Monomers and chemical structure of the sensory membrane . The picture shows the physical aspect of the membrane on a notebook. Monomers: OH O O 2-hydroxyethyl acrylate (2HEA) O O O O Crosslinker: ethylene glycol dimethacrylate (EGDMA) XZ Y Membrane chemical structure: X:Y:Z:V = 75:24:1:10 O O O HO O (5-hydroxy-4-oxo-4H-pyran-2yl)-methyl methacrylate (3) N 1-vinyl-2pyrrolidinone (VP) O O O O O OH O O HO O OO N OV O 4 Figure 1. Kinetic profile of Fe(III) uptake on the membrane ( sorbed metal ion in the solid phase (q) as a function of time (t)). Conditions: volume = 10 mL 0.1 M KNO3, pH = 2.5, temperature = 25ºC, [Fe(III)] = 1.34·10-4 M, 13.7 mg of dry membrane. The lines represent the fit obtained using a pseudo first order equation. 0,000 0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040 0,045 0,050 020 40 60 80 100 q, mmol g-1 t, min 5 Figure 2. Sorption isotherm of Fe(III) on the membrane ( sorbed metal ion in the solid phase (q) as a function of the solute concentration when equilibrium is reached (Ceq)). Conditions: volume = 10 mL 0.1 M KNO3, pH 2.5, temperature = 25ºC, 36.5 mg of dry membrane. Gray circles: experimental points; continuous line: best fitting by Langmuir model. 0,000 0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040 0,045 0,050 0,0E+00 5,0E-04 1,0E-03 1,5E-03 2,0E-03 q, mmol g-1 Ceq, M 6 Figure 3. Sorption profiles of Fe(III) on 26.5 mg of dry membrane in 1 M KNO3, V = 10 mL, [Fe(III)] = 1.8 µM. Red circles = profile in absence of competitive ligand, blue diamonds = profile in presence of pyridine dicarboxylic acid (PDCA) 0.5 mM, green circles = profile in presence of sodium oxalate 0.05 M. 0,00 0,20 0,40 0,60 0,80 1,00 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 f pH 7 Figure 4. Colorimetric determination of the Fe(III) concentration in water using sensory discs cut from the membrane: a) UV/vis spectra (inset: picture of the discs, left disc is control disc); b) UV/vis titration curve; c) titration using the RGB parameters from the digital image taken from the sensory materials (see inset, Figure 5a). The three parameters (R, G and B) defining the color of each disc were reduced to one principal component (PC1) by principal component analysis. Conditions: prior to each measurement, each disc was immersed overnight in Milli-Q water (temperature = 25ºC, pH = 2 -buffer HCl/KCl-) containing a Fe(III) concentration ranging from 1.65x10-6 to 1.65 x10-1 M and then removed from the medium and dried at rt.