A simple strategy to improve the interfacial activity of true Janus gold nanoparticles: a shorter hydrophilic capping ligand

A simple s a egy o imp o e he in e acial ac i i y o ue Janus gold nanopa icles: a sho e hyd ophilic capping ligand† Miguel Angel Fe nandez-Rod iguez,aLimei Chen,bCh is ophe P. Deming,bMiguel An- gel Rod iguez-Val e de,aShaowei Chen,bMiguel Angel Cab e izo-Vilchez,aand Roque Hidalgo-Al a ez∗a Janus gold nanopa icles (JPs) o ∼4 nm-diame e hal unc ionalized wi h 1-hexane hiol as hy- d ophobic capping ligand exhibi signi ican ly highe in e acial ac i i y, ep oducibili y and heo- logical esponse when he o he hal is unc ionalized wi h 1,2-me cap op opanediol (JPs-MPD) han wi h 2-(2-me cap oe hoxy)e hanol (JPs-MEE), bo h ac ing as he hyd ophilic capping ligand. The in e acial p essu e measu ed by pendan d op ensiome y eaches 50 mN/m and 35 mN/m o he JPs-MPD a he wa e /ai and wa e /decane in e ace, espec i ely. A he same a ea pe pa icle, he JPs-MEE e eal signi ican ly lowe in e acial p essu e: 15 mN/m and 5 mN/m a he wa e /ai and wa e /decane in e ace, espec i ely. In e acial dila a ional heology measu emen s also show an elas ic shell beha iou a highe comp ession s a es o JPs-MPD while he JPs- MEE p esen nea -ze o elas ici y. The enhanced in e acial ac i i y o JPs-MPD is explained in e ms o chemical and hyd a ion di e ences be ween he MPD and MEE ligands, whe e MPD has a sho e hyd oca bon chain and wice mo e hyd oxyl e minal g oups han MEE. Picke ing emulsions can be he modynamically s abilized by amphiphilic Janus nanopa icles (JPs) wi h a we abili y aniso opy1–3.I is known ha JPs show h ee imes mo e adso p- ion ene gy han homogeneous nanopa icles2,4. S ong e o s ha e been made o syn hesize and simula e JPs wi h di e en mo phologies and su ace chemis y o con ol he way in which hese pa icles sel -assemble a luid in e aces5–9. Gold nanopa icles andmonly unc ionalized wi h 1- undecane hiol and N,N,N- ime hyl (11-me cap oundecyl) ammonium chlo ide a e epo ed o become Janus-like when he capping ligands ea ange a he wa e /ai in e ace10. Ne e heless, ecen ly Regue a e al.11 demons a ed by neu on e lec i i y ha such ea angemen does no happen wi h gold nanopa icles unc ionalized wi h 1-oc ane hiol and 6-me cap o- 1-hexanol a he wa e /ai in e ace. A way o ob ain gold JPs wi h ue sepa a e domains is o selec i ely unc ionalize each hemisphe e o he co e wi h he desi ed capping ligands immobilizing he nanopa icles in a Langmui balance12,13. I is undamen al o selec he app opia e capping ligands ha aBiocolloid and Fluid Physics G oup, Applied Physics Depa men , Facul y o Sciences, Uni e si y o G anada, G anada, Spain; E-mail: hidalgo@ug .es bDepa men o Chemis y and Biochemis y, Uni e si y o Cali o nia, 1156 High S ee , San a C uz, CA 95064, USA. †Elec onic Supplemen a y In o ma ion (ESI) a ailable: HRTEM mic og aphies and pendan d ops pic u es. See DOI: 10.1039/b000000x/ will con e he Janus cha ac e o he nanopa icles because o he signi ican dependence be ween hese capping ligands and he in- e acial ac i i y o he inal JPs. We p opose a simple s a egy o enhance he in e acial ac i i y, heological esponse and ep o- ducibili y h ough colloidal s abili y o ue gold Janus nanopa - icles. We syn hesized Janus gold nanopa icles hal capped by 1- hexane hiol and he o he hal by 2-(2-me cap o-e hoxy)e hanol (JPs-MEE) o 1,2-me cap op opanediol (JPs-MPD) in su ac an - ee condi ions as desc ibed in p e ious wo ks (see Fig. 1)12–14. The sizes ob ained by high esolu ion TEM measu emen s (see Fig. S1†) a e 3.5±0.9nm and 3.7±1.9nm o he JPs-MEE and JPs-MPD, espec i ely. The elec opho e ic mobili y o bo h JPs was measu ed wi h a Ze aSize Nano (Mal e n) in a 10−2M sodium ci a e MilliQ wa e solu ion o s abilize he elec ical double laye ob aining µe,JPs−MEE = (−2.2±1.5)·10−8m2/(V·s) and µe,JPs−MPD = (−2.9±0.4)·10−8m2/(V·s). F om p e ious wo ks, he JPs-MEE showed an a e age mac oscopic con ac an- gle o (56.1±1.8)◦and (49.0±1.3)◦in each hemisphe e, whe eas he JPs-MPD showed (63.3±2.7)◦and (53.4±2.9)◦in hei espec- i e hyd ophobic and hyd ophilic hemisphe es12,13. The con ac angles a e sligh ly lowe o he JPs-MEE likely due o he ab i- ca ion p ocess: o MEE adso p ion, he close-packed monolaye o 1-hexane hiol co e ed gold nanopa icles was imme sed in a wa e solu ion con aining MEE13 and his migh p oduce g ea e 1–5 | 1 SH OOH CH3 SH OH OH Fig. 1 MEE (le ) and MPD ( igh ) capping ligands. The SH g oup is he ancho g oup a he gold nanopa icle su ace. 56 60 64 68 72 0 200 400 600 800 1000 1200 Su ace Tension γ (mN/m) Time (s) 1.6⋅1012 JPs−MPD 3.2⋅1012 JPs−MPD 4.8⋅1012 JPs−MPD 8.0⋅1012 JPs−MPD Fig. 2 Su ace ension e olu ion o e he ime a e he deposi ion o di e en numbe o JPs-MPD in THF on he su ace o an ini ial 5µL MilliQ wa e pendan d op and g ow h up o 45 µL. Same colo cu es co espond o di e en expe imen s wi h he same numbe o deposi ed JPs-MPD. A e he sol en e apo a ion, he su ace ension emained s able. hyd ophilic capping ligand exchange and hus lowe con ac an- gle han o he JPs-MPD in which he unc ionaliza ion wi h MPD was pe o med di ec ly in he Langmui balance, exchanging he MPD ligands om he wa e subphase12. The con ac angle alues in he hyd ophilic hemisphe es a e simila wi hin e o s, which e lec s ha he di e ences be ween MPD and MEA a e no e lec ed mac oscopically in he con ac angle and he we abili y con as (i.e. con ac angle di e ences be ween hemisphe es) is simila in bo h JPs-MEE and JPs-MPD wi hin e o s. The pendan d op ensiome y was conduc ed as ollows: di e en amoun s o Janus nanopa icles (JPs-MEE and JPs-MPD) dispe sed in e ahy- d o u an (THF, HPLC g ade) we e deposi ed on a wa e pendan d op wi h a handheld mic osy inge and a mic oposi ione . The su ace ension was ob ained by axisymme ic d op shape anal- ysis upon THF e apo a ion, while he pendan d op olume was kep cons an . The esul s in Fig. 2 show a dec ease in he inal su ace ension a e e apo a ion o THF which is highe as he concen a ion o JPS-MPD is inc eased ( e e o p e ious wo k o he JPs-MEE simila cha ac e iza ion15). The expe imen s a e highly ep oducible as can be seen in he di e en uns o a ixed concen a ion o JPs (di e en cu es wi h same colo in Fig. 2). A e he THF e apo a ion, g owing and sh inking expe imen s we e pe o med a 0.08 µL/s o each JP concen a ion. Nex , he pendan d op was imme sed in decane and he g owing and sh inking expe imen s we e pe o med again. We plo he in e a- cial p essu e (Π=γ0−γ, whe e γ0is 72.5mN/m o he wa e /ai (W/A) and 52.3mN/m o he wa e /decane (W/O) in e aces and γis he measu ed in e acial ension) agains he d op a ea pe pa icle (Ap, he a ea o he pendan d op di ided by he num- be o deposi ed JPs). A piecewise comp ession iso he m can be seen in Fig. 3a and Fig. 3b o W/A and W/O in e aces, espec- i ely. The i s ema kable ac is he lowe in e acial ac i i y o JPs-MEE compa ed o JPs-MPD a he same Ap alues. A he lowes Ap eached o JPs-MPD, Πis 50mN/mand 35mN/mand 15mN/mand 5mN/m o he JPs-MEE a he W/A and W/O in e - aces, espec i ely. The highes Π alues ob ained o JPs-MEE, a e u he comp ession, a e 30mN/mand 20mN/ma he W/A and W/O in e aces, espec i ely. Con a y o JPs-MEE, he com- p ession iso he ms o JPs-MPD a he W/A in e ace exhibi open cycles a he beginning o he expe imen s poin ing ou ha he colloidal monolaye ea anges in o a inal s a e ha is p ese ed in u he comp ession cycles. This beha iou is a enua ed a he W/O in e ace as he hys e esis cycles a e much smalle (i.e. he uppe comp ession and lowe expansion cu es a e close ) which migh be due o he ac ha enough ene gy is p o ided o each a mo e elaxed s a e when i is imme sed in decane. Mo eo e , he low hys e esis o he JPs-MEE comp ession cycles compa ed o he JPs-MPD migh be due o he lowe in e acial ac i i y o hese pa icles. Since he ab ica ion p ocess, hyd ophobic cap- ping ligand, we abili y con as , size and cha ge we e simila o bo h JPs-MEE and JPs-MPD, he di e ences be ween in e acial ac i i y mus come om he in e acial ac i i y o he MEE and MPD hyd ophilic capping ligands. Whe eas MEE has a longe hy- d oca bon chain ( ou CH2and one oxygen) and one hyd oxyl e minal g oup, he MPD has a sho e hyd oca bon chain ( h ee CH2) and wo hyd oxyl e minal g oups (see Fig. 1). The lowe numbe o hyd oca bon g oups and highe numbe o hyd oxyl g oups o MPD migh esul in highe hyd a ion o he hyd ophilic hemisphe e o he JPs (i.e. es ablishing hyd ogen bonds be ween wa e and he hyd ophilic capping ligands). These chemical di - e ences migh play a decisi e ole in he inal in e acial ac i i y o hese JPs. The in e acial dila a ional heology o he JPs was e alua ed by en pe iodic olume a ia ions o 1µL o di e en pe iods. When a pe iodic injec ion/ex ac ion o olume is pe o med o he pendan d op, he in e ace ies o e-es ablish he equilib- ium. This coun e ac ion is ep esen ed by a complex quan i y composed by a s o age pa and a loss pa : E=Ed+iωηd(1) whe e Eis he su ace dila a ional modulus ha accoun s o he change in su ace ension p oduced by a small change in a su ace a ea, Edis he in e acial dila a ional elas ici y, ωis he oscilla ion equency and ηdis he in e acial dila a ional iscosi y16. I he iscosi y is negligible du ing he elaxa ion p ocess a e pe u - ba ion o he in e ace, he in e ace p esen an essen ially elas ic beha io . The ex ao dina y in e acial ac i i y o JPs-MPD is e- lec ed in he heology esul s in Fig. 4a and 4b (see Fig. S2 and S3†). Fo bo h W/A and W/O in e aces, E dec eases sligh ly and ηinc eases o inc easing pe iods. Fo he W/A in e ace, Edand ηdinc eases clea ly wi h he comp ession s a e o he col- loidal monolaye . This end is also obse ed o he W/O in e - 2 | 1–5 (a) Wa e /ai in e ace 0 10 20 30 40 50 0 5 10 15 20 25 30 35 Π (mN/m) A eapa icle (nm2/pa icle) 1.6⋅1012 JPs−MEE 3.3⋅1012 JPs−MEE 5.0⋅1012 JPs−MEE 8.3⋅1012 JPs−MEE 16.6⋅1012 JPs−MEE 33.1⋅1012 JPs−MEE 49.7⋅1012 JPs−MEE 1.6⋅1012 JPs−MPD 3.2⋅1012 JPs−MPD 4.8⋅1012 JPs−MPD 8.0⋅1012 JPs−MPD (b) Wa e /decane in e ace 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Π (mN/m) A eapa icle (nm2/pa icle) 1.7⋅1012 JPs−MEE 5.0⋅1012 JPs−MEE 16.6⋅1012 JPs−MEE 1.6⋅1012 JPs−MPD 3.2⋅1012 JPs−MPD 4.8⋅1012 JPs−MPD 8.0⋅1012 JPs−MPD Fig. 3 Su ace p essu e agains he a ea pe pa icle o di e en numbe o JPs-MEE and JPs-MPD deposi ed a he (a) W/A and (b) W/O in e aces. Fo a mo e de ailed cha ac e iza ion o he JPs-MEE, please e e o Fe nandez e al.15 ace bu wi h lowe alues o bo h Edand ηd. The high Ed alue is a signal ha he colloidal monolaye c ea es an elas ic shell on he pendan d op a highe comp ession s a es. This elas ic beha iou again sugges s he abili y o he JPs-MPD as emulsi- ie s. Fu he heology expe imen s we e pe o med o a ixed pe iod o 10scompa ing he esponse o JPs-MEE and JPs-MPD. The esul s in Fig. 4c and 4d poin ou ha he JPs-MPD each sig- ni ican ly highe Edand ηd alues upon comp ession (i.e. lowe Ap) han he JPs-MEE (∼10 imes highe Edand ηd o JPs-MPD han JPs-MEE a he W/A in e ace and 2 imes a he W/O in- e ace), sugges ing ha he elas ic shell beha iou is no p esen o he JPs-MEE. A inal conside a ion mus be aken in o accoun o gold nanopa icles in he ange o a ew nanome e s (i.e. less han 10nm), he adso p ion ene gy a he in e ace is o he o de o KBT17 and hey a e expec ed o easily lea e he in e ace. Ne - e heless, he s able in e acial ension o e ime a e he THF e apo a ion, he closed g owing/sh inking cycles and he dila a- ional heology seem o poin ou ha he JPs-MEE and JPs-MPD a e i e e sibly ancho ed a he W/A and W/O in e aces, p oba- bly due o i s Janus cha ac e . In conclusion, he JPs-MEE and JPs-MPD a e simila in ab i- ca ion p ocess, hyd ophobic capping ligand, we abili y con as , size and cha ge, bu a e unc ionalized wi h di e en hyd ophilic capping ligand. The JPs-MPD exhibi a signi ican ly highe in e - acial ac i i y a W/A and W/O in e aces. Mo eo e , he dila a- ional heology sugges s an elas ic shell-like beha iou o he pen- dan d op when he JPs-MPD a e deposi ed a W/A and W/O in- e aces. This elas ic shell beha iou seems o be absen wi h he JPs-MEE. This poin s ou he impo ance o he chemical s uc u e o he capping ligands in JPs o p edic he in e acial ac i i y and he e o e hei abili y as emulsi ie s. Sho e hyd oca bon chain and mo e hyd oxyl e minal g oups in he hyd ophilic capping ligands seems o be a ou e o ob ain enhanced in e acial ac i - i y o his kind o JPs ia enhanced hyd a ion o he hyd ophilic hemisphe e o he JPs. To he bes o ou knowledge, i is he i s ime ha such high in e acial ac i i y is ob ained wi h ∼4nm- diame e gold nanopa icles in su ac an - ee condi ions. Acknowledgemen s This wo k was suppo ed by he Spanish MINECO (p ojec s MAT2013-44429-R and MAT2014-60615R), by “Jun a de An- dalucía” and FEDER (p ojec s P10-FQM-5977 and P12-FQM- 1443) and by US Na ional Science Founda ion (DMR-1409396). Au ho s hank o D . J.A. Holgado-Te iza o he so wa e Con ac o c used o su ace ension measu emen s. Re e ences 1 J. Hu, S. Zhou, Y. Sun, X. Fang and L. Wu, Chem. Soc. Re ., 2012, 41, 4356–4378. 2 A. Kuma , B. J. Pa k, F. Tu and D. Lee, So Ma e , 2013, 9, 6604–6617. 3 B. T. T. Pham, C. H. Such and B. S. Hawke , Polym. Chem., 2015, 6, 426–435. 4 R. 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