Redox-Active Ionic-Liquid-Assisted One-Step General Method for

Oct 13, 2010 - This IL ([TBP][Ci]) is prepared by the simple neutralization of tetrabutylphosphonium ... Sacrificial template synthesis of (Co x Ni 1â...
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Redox-Active Ionic-Liquid-Assisted One-Step General Method for Preparing Gold Nanoparticle Thin Films: Applications in Refractive Index Sensing and Catalysis Enakshi Dinda, Md. Harunar Rashid, Mrinmoy Biswas, and Tarun K. Mandal* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received August 3, 2010. Revised Manuscript Received September 24, 2010 We describe a general one-step facile method for depositing gold nanoparticle (GNP) thin films onto any type of substrates by the in situ reduction of AuCl3 using a newly designed redox-active ionic liquid (IL), tetrabutylphosphonium citrate ([TBP][Ci]). Various substrates such as positively charged glass, negatively charged glass/quartz, neutral hydrophobic glass, polypropylene, polystyrene, plain paper, and cellophane paper are successfully coated with a thin film of GNPs. This IL ([TBP][Ci]) is prepared by the simple neutralization of tetrabutylphosphonium hydroxide with citric acid. We also demonstrate that the [TBP][Ci] ionic liquid can be successfully used to generate GNPs in an aqueous colloidal suspension in situ. The deposited GNP thin films on various surfaces are made up of mostly discrete spherical GNPs that are well distributed throughout the film, as confirmed by field-emission scanning electron microscopy. However, it seems that some GNPs are arranged to form arrays depending on the nature of surface. We also characterize these GNP thin films via UV-vis spectroscopy and X-ray diffractometry. The as-formed GNP thin films show excellent stability toward solvent washing. We demonstrate that the thin film of GNPs on a glass/quartz surface can be successfully used as a refractive index (RI) sensor for different polar and nonpolar organic solvents. The as-formed GNP thin films on different surfaces show excellent catalytic activity in the borohydride reduction of p-nitrophenol.

Introduction Today, one of the major motivations for research in nanotechnology is the engineering of nanoscale materials of novel optical, electrical, and nonlinear properties.1 There are several techniques for generating a wide range of metal nanoparticles (MNPs), especially gold nanoparticles (GNPs) of various sizes and properties and the assembly of these nanoparticles (NPs) into tailormade arrangements.1 The assembly of MNPs or semiconductor NPs to obtain thin films is one such method that has interesting and unique optical and electronic properties with many promising applications.1-7 In fact, the size, shape, and distribution of such MNPs generally govern the properties of the thin film.1 The potential applications of these types of MNP thin films include building blocks of next-generation nanoelectronic and *Corresponding author. E-mail: [email protected]. Fax: 91-33-2473 2805. (1) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (2) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (3) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (4) Musick, M. D.; Keating, C. D.; Lyon, L. A.; Botsko, S. L.; Pena, D. J.; Holliway, W. D.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Chem. Mater. 2000, 12, 2869. (5) Fishelson, N.; Shkrob, I.; Lev, O.; Gun, J.; Modestov, A. D. Langmuir 2001, 17, 403. (6) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (7) Izgorodin, A.; Winther-Jensen, O.; Winther-Jensen, B.; MacFarlane, D. R. Phys. Chem. Chem. Phys. 2009, 11, 8532. (8) Kruis, F. E.; Fissan, H.; Peled, A. J. Aerosol Sci. 1998, 29, 511. (9) Markovich, G.; Leff, D. V.; Chung, S.-W.; Soyez, H.; Dunn, B.; Heath, J. R. Appl. Phys. Lett. 1997, 70, 3107. (10) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Mater. 1999, 11, 13. (11) Lahav, M.; Shipway, A. N.; Willner, I.; Nielsen, M. B.; Stoddart, J. F. J. Electroanal. Chem. 2000, 482, 217. (12) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mullen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551.

17568 DOI: 10.1021/la103084t

optoelectronic devices,8,9 electrochemical detection of redoxactive analytes in the liquid phase,10,11 and chemical vapor sensors.12,13 Because we know that the surface plasmon resonance (SPR) property of thin films of MNPs, especially gold (Au) and silver (Ag) nanoparticles (NPs), is dependent on the refractive index of the local environment.14,15 Thus, these MNP thin films can also be utilized as a good sensory material for measuring the refractive index of a medium. Our group has been demonstrated the use of GNP thin films as a sensor for measuring the refractive index of various organic solvents.16 Also, the SPR property of GNPs is sensitive to changes in the dielectric constant of the surrounding medium (εm) in which the particles are embedded.14,17 Hence, the changes in the dielectric environment of GNPs resulted in a measurable shift of the SPR peak position or magnitude that can be used for the detection of label-free chemicals.16 Thus, GNP thin films with such a property can be effectively used as a tool for making varieties of other sensor devices.1 To date, numerous techniques have been demonstrated by many researchers for obtaining such MNP thin films. The effort has been made to deposit MNPs as thin films using the LangmuirBlodgett technique.18 Electrostatic layer-by-layer (LBL) deposition techniques of oppositely charged polyelectrolytes19,20 (13) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; M€ullen, K.; Yasuda, A. Adv. Mater. 2002, 14, 238. (14) Mulvaney, P. Langmuir 1996, 12, 788. (15) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (16) Rashid, M. H.; Bhattacharjee, R. R.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 9684. (17) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (18) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (19) Lvov, Y.; Essler, F.; Decher, G. J. Phys. Chem. 1993, 97, 13773. (20) Lvov, Y.; Haas, H.; Decher, G.; Mohwald, H.; Kalachev, A. J. Phys. Chem. 1993, 97, 12835.

Published on Web 10/13/2010

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Dinda et al.

and dendrimer13 and metal or semiconductor NPs has also been applied as a facile means of creating ordered functionalized thin films.21-23 An efforts has also been made to create nanostructured films of MNPs by direct electrodeposition.24-26 Beside these techniques, the electroless deposition of metal on solid substrates is also very important.27 However, the preparation of such thin metal films on solid substrates via an in situ wet chemical approach is very rare.16,28 Following this approach, we have prepared GNP thin films on glass substrates from a gold precursor solution using 1,4,8,11-tetraazacyclotetradecane (cyclam) as a reducing-cumdepositing agent.16 Ionic liquids (ILs) are an important class of compounds having different properties such as a negligibly low vapor pressure, high thermal stability, and high chemical and electrochemical stability, making these compounds effective “green” solvents.29 Because of the high electrochemical stability of ILs, they have been efficiently used in the electrodeposition process of metal nanoparticles.26,30 Ionic liquids containing different biologically relevant molecules such as amino acids,29,31 sugars and sugar derivatives,32,33 lactic acid,34,35 and ascorbic acid36 as the counteranions have also been prepared with the expectation that the different ILs would exhibit different physicochemical properties. Several research groups including our group have also synthesized inorganic nanomaterials of different sizes and shapes with novel properties using varieties of ILs.36-49 In this article, we demonstrate that a newly designed and synthesized redox-active IL, tetrabutylphosphonium citrate ([TBP][Ci]), is successfully used for the in situ deposition of GNP thin films onto glass/quartz substrates from an aqueous solution of AuCl3. In our earlier work, we reported a series of new (21) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (22) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. (23) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (24) Emery, V. J.; Kivelson, S. A. Nature 1995, 374, 434. (25) Nakato, Y.; Murakoshi, K.; Imanishi, A.; Morisawa, K. Electrochemistry 2000, 68, 556. (26) El Abedin, S. Z.; Polleth, M.; Meiss, S. A.; Janek, J.; Endres, F. Green Chem. 2007, 9, 549. (27) Yu, Y.; Addai-Mensah, J.; Losic, D. Langmuir 2010, 26, 14068. (28) Pernstich, K. P.; Schenker, M.; Weibel, F.; Rossi, A.; Caseri, W. R. ACS Appl. Mater. Interfaces 2010, 2, 639. (29) Fukumoto, K.; Yoshizawa, M.; Ohno, H. J. Am. Chem. Soc. 2005, 127, 2398. (30) El Abedin, S. Z.; Moustafa, E. M.; Hempelmann, R.; Natter, H.; Endres, F. ChemPhysChem 2006, 7, 1535. (31) Kagimoto, J.; Fukumoto, K.; Ohno, H. Chem. Commun. 2006, 2254. (32) Handy, S. T.; Okello, M.; Dickenson, G. Org. Lett. 2003, 5, 2513. (33) Carter, E. B.; Culver, S. L.; Fox, P. A.; Goode, R. D.; Ntai, I.; Tickell, M. D.; Traylor, R. K.; Hoffman, N. W.; Davis, J. H., Jr. Chem. Commun. 2004, 630. (34) Pernak, J.; Goc, I.; Mirska, I. Green Chem. 2004, 6, 323. (35) Earle, M. J.; McCormac, P. B.; Seddon, K. R. Green Chem. 1999, 1, 23. (36) Dinda, E.; Si, S.; Kotal, A.; Mandal, T. K. Chem.;Eur. J. 2008, 14, 5528. (37) Dash, P.; Scott, R. W. J. Chem. Commun. 2009, 812. (38) Khare, V.; Li, Z. H.; Mantion, A.; Ayi, A. A.; Sonkaria, S.; Voelkl, A.; Thunemann, A. F.; Taubert, A. J. Mater. Chem. 2010, 20, 1332. (39) Kim, K. S.; Choi, S.; Cha, J. H.; Yeon, S. H.; Lee, H. J. Mater. Chem. 2006, 16, 1315. (40) Li, Z. G.; Friedrich, A.; Taubert, A. J. Mater. Chem. 2008, 18, 1008. (41) Ma, Z.; Yu, J.; Dai, S. Adv. Mater. 2010, 22, 261. (42) Redel, E.; Walter, M.; Thomann, R.; Hussein, L.; Kruger, M.; Janiak, C. Chem. Commun. 2010, 46, 1159. (43) Ryu, H. J.; Sanchez, L.; Keul, H. A.; Raj, A.; Bockstaller, M. R. Angew. Chem. Int. Ed. 2008, 47, 7639. (44) Tatumi, R.; Fujihara, H. Chem. Commun. 2005, 83. (45) Taubert, A. Angew. Chem. Int. Ed. 2004, 43, 5380. (46) Tsuda, T.; Seino, S.; Kuwabata, S. Chem. Commun. 2009, 6792. (47) Zhu, J.; Shen, Y.; Xie, A.; Qiu, L.; Zhang, Q.; Zhang, S. J. Phys. Chem. C 2007, 111, 7629. (48) Taubert, A.; Li, Z. Dalton Trans. 2007, 723. (49) Li, Z.; Jia, Z.; Luan, Y.; Mu, T. Curr. Opin. Solid State Mater. Sci. 2008, 12, 1.

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Article Scheme 1. Reaction Scheme for the Synthesis of the [TBP][Ci] Ionic Liquid

alkyl imidazolium-based ILs with ascorbate as a counteranion in order to utilize them as reducing-cum-capping agent to generate anisotropic Au nanostructures in the colloidal state.36 It is worthwhile to mention that our ascorbate-based ILs did not produce any GNP thin films on solid substrates during their reaction with gold precursors. Also, to our knowledge, until today, no group has utilized ILs to produce GNP thin films on solid surfaces via an in situ deposition technique. It is also demonstrated that the as-fabricated GNP thin films could be used as a refractive index (RI) sensor for the different organic solvents as well as supported catalysts. For catalysis, we first coat the inner wall of a quartz cuvette with GNPs by this technique and perform the borohydride reduction of p-nitrophenol in the same GNPs-coated cuvette. The progress of this reaction is then monitored using UV-vis spectroscopy, as has been done by many research groups including ours.50-54

Experimental Section Materials. Auric(III) chloride trihydrate (AuCl3 3 3H2O), tetrabutylphosphonium hydroxide (40 wt % aqueous solution), triethoxy(octyl)silane (TEOS), and hexadecyltrimethoxysilane (HDTMS) were purchased from Sigma-Aldrich. Poly(diallyldimethylammonium chloride) (PDDAC, Mw = 100-200 kD) as a 20 wt % aqueous solution was obtained from Aldrich. Citric acid monohydrate, sodium citrate monohydrate, and sodium borohydride (NaBH4) were purchased from Merck, India. p-Nitrophenol (4-C6H5NO3, 4-NP) was purchased from Loba Chemicals. All of these compounds were used as received. Acrylic acid (Aldrich 99%) monomer was purified according to the procedure provided in our earlier paper and was distilled under reduced pressure.55 Glass slides were purchased from Blue Star, India Ltd. and were cleaned prior to use in the process of thin film deposition. All of the aqueous solutions were made with triple-distilled water. Freshly distilled reagent-grade organic solvents were used for all other synthesis purposes as well as for the measurement of refractive indices. Synthesis of Tetrabutylphosphonium Citrate ([TBP][Ci]) Ionic Liquid. In a typical synthesis, 0.61 g (2.89 mM) of citric acid was added to 2 mL of a 40 wt % aqueous solution of tetrabutylphosphonium hydroxide (2.89 mM) and the mixture was stirred magnetically for 12 h at room temperature. The synthesis is schematically shown in Scheme 1. The resultant [TBP][Ci] IL was isolated by freeze drying. Finally, the isolated IL was dissolved in dry dichloromethane (DCM), and the DCM solution was filtered to remove the residual solid impurities. The filtrate containing the IL was then dried in a rotary evaporation unit, and a waxy white solid product was obtained. Finally, the purified IL was dried in a vacuum oven for 3 days at 70 °C. The IL was fully characterized via 1H NMR spectroscopy using D2O as a solvent. The 1H NMR, mass spectra, and C, H, and N analysis data of the (50) Rashid, M. H.; Bhattacharjee, R. R.; Kotal, A.; Mandal, T. K. Langmuir 2006, 22, 7141. (51) Rashid, M. H.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 16750. (52) Tang, R.; Liao, X. P.; Shi, B. Chem. Lett. 2008, 37, 834. (53) Xia, Y.; Shi, Z.; Lu, Y. Polymer 2010, 51, 1328. (54) Zhang, M.; Liu, L.; Wu, C.; Fu, G.; Zhao, H.; He, B. Polymer 2007, 48, 1989. (55) Si, S.; Kotal, A.; Mandal, T. K.; Giri, S.; Nakamura, H.; Kohara, T. Chem. Mater. 2004, 16, 3489.

DOI: 10.1021/la103084t

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Article IL are given below and will be discussed later in the Results and Discussion section. [TBP][Ci]. 1H NMR (300 MHz, D2O, TMS): δ 2.83 (d, J = 9.1 Hz, 2H, -CH2-COO-), 2.71 (d, J = 9.3 Hz, 2H, -CH2-COOH), 2.01 (t, J = 8.4 Hz, 8H, P-CH2-CH2-CH2-CH3), 1.49-1.47 (m, 8H, P-CH2-CH2-CH2-CH3), 1.43-1.38 (m, 8H, P-CH2CH2-CH2-CH3), 0.87 (t, J = 4.2 Hz, 12H, P-CH2-CH2CH2-CH3). MS (35 eV) m/z (%): 259 (100) [Mþ þ Naþ]. Elemental analysis calcd (%) for C22H43PO7: C 58.66, H 9.55. Found: C 58.40, H 9.38

Cleaning of Glass Vials, Glass Slides, and Quartz Cuvettes. At first, all of the glass vials/slides and quartz cuvettes were washed with soap solution, followed by washing with distilled water. Following these steps, all substrates were treated with freshly prepared piranha solution (concentrated H2SO4/30% H2O2 = 7:3) for 1 h. (Caution! Piranha is a strong oxidizing agent that causes severe burns when placed in contact with skin and reacts violently with organic materials.) These substrates were then further exhaustively washed with double-distilled water. The next step was to wash the substrates with a mixture of acetone and methanol (1:1) in a bath sonicator for 1 h. Finally, these substrates were dried in an air oven and were immediately used for coating with GNPs.

Dinda et al. Table 1. Reaction Recipes for the Preparation of Colloidal GNPs and GNP Thin Films on Various Surfaces sample ida

sample idb

[AuCl3] (mM)

[ [TBP][Ci]] (mM)

[[TBP][Ci]]/ [AuCl3]

[TBP][Ci]-Au-1 GNP-film-1 0.5 2.0 4.0 [TBP][Ci]-Au-2 GNP-film-2 0.5 1.5 3.0 [TBP][Ci]-Au-3 GNP-film-3 0.5 1.0 2.0 [TBP][Ci]-Au-4 GNP-film-4 0.5 0.5 1.0 a Sample id for the colloidal GNP suspension. b Sample id for GNP thin films on any type of functionalized (positively charged/negatively charged/neutral hydrophobic) surface or unfunctionalized commercial substrate after 6 h of reaction using the recipe as used for the preparation of the respective colloidal suspension.

Scheme 2. Schematic Representation of the Preparation of GNP Colloids and GNP Thin Films on Various Solid Substrates and Their Applications in Refractive Index (RI) Sensing and Catalysis

Coating of Glass Slides/Vials with Cationic Polyelectrolyte. To coat the glass slides/vials with a cationic polyelectrolyte, typically they were first cleaned according to the above-mentioned procedure. Typically, a piranha-treated glass slide (small piece) was then immersed in an aqueous solution containing PDDAC (0.2 wt %) and NaCl (0.5M) in a piranha-treated vial and allowed to stand overnight as described elsewhere.16 PDDAC coats both the vial and the slide. The excess PDDAC solution was then decanted, and the coated slide/vial was rinsed several times with triple-distilled water and dried under vacuum. GNP thin films were then deposited on these slides/vials with positively charged surfaces.

Coating of Glass Slides/Vials with Anionic Polyelectrolyte. A positively charged glass slide (substrate) obtained from the above-mentioned procedure was immersed in an aqueous solution of 1 wt % poly(acrylic acid) (PAA) taken in a positively charged glass vial (substrate) for 15 min, as followed elsewhere, in order to coat the slide with anionic polyelectrolyte.56 It should be noted that PAA was synthesized in our laboratory using the procedure mentioned elsewhere.55 After the PAA solution was removed from the vial, both the slide and the vial were washed repeatedly with triple-distilled water with the expectation that the entire physically adsorbed polyelectrolyte would be rinsed off. The substrates were then air dried and were used for coating with GNPs. Note that the surfaces of these substrates are negatively charged.

Coating of Glass Slides/Vials with Hydrophobic Alkyl Silane. The silanization of glass vials/slides was performed by

following the procedure reported elsewhere.57,58 Typically, a piranha-treated glass vial was filled with a 5% (v/v) toluene solution of an alkyl silane, and then a piranha-treated glass slide was dipped into this solution and left undisturbed overnight. The toluene solution was then removed, and the vial was filled with fresh, hot toluene (70 °C) for an hour. Both the slide and the vial were washed with a mixture of methanol and toluene (1:1) and then with plenty of triple-distilled water. Finally, these substrates were dried in an air oven. Two different alkyl silanes of varying alkyl chain length such as triethoxy(octyl)silane (TEOS) and hexadecyltrimethoxysilane (HDTMS) were used to make the surfaces hydrophobic/neutral. The functionalized surface was characterized by attenueted total reflection infrared (ATR-IR) spectroscopy. The TEOS monolayer on the glass slide shows (56) Das, B. C.; Pal, A. J. ACS Nano 2008, 2, 1930. (57) Plueddemann, E. P. Silane Coupling Agents, 2nd ed.; Plenum: New York, 1991. (58) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268.

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bands at 2834 and 2928 cm-1 corresponding to the alkyl group present in the silane moiety. (For spectra, Figure S1 in the SI).

Preparation of a Thin Film of Gold Nanoparticles (GNPs) on Various Substrates. In a typical film-preparation procedure,

an aqueous solution of [TBP][Ci] (400 μL, 10 mM) was taken in a 4 mL glass vial that was either piranha treated or functionalized with any one of these compounds such as positively charged PDDAC, negatively charged PAA, or neutral alkyl silane (TEOS or HDTMS) according the above-mentioned procedure. This solution was then diluted by adding 1.5 mL of distilled water. An aqueous solution of AuCl3 (100 μL, 10 mM) was then added to the above solution. A functionalized (positive/negative/neutral) glass slide was then immediately dipped into this reaction mixture in the functionalized glass vial. After 1 h of reaction, a portion of the reaction mixture was taken out of the vial to characterize the formed [TBP][Ci]-Au nanoconjugates in the colloid state. The reaction set and the formed colloidal nanoconjugates are designated as [TBP][Ci]-Au-1. This suspension taken after 1 h was directly used for analysis of the nanoconjugate via UV-vis spectroscopy, XRD, and TEM. The rest of the mixture in the vial containing the glass slide was left undisturbed for 6 h at ambient temperature to make sure that the deposition of GNP as a thin film was complete. Note that the positively charged slide was dipped into the positively charged vial containing the reaction mixture. A similar order was also followed for negative and neutral substrates. Finally, the GNP-coated glass slide was taken out of the mixture, and the reaction mixture was poured out of the vial. By this process, the inner wall of the vial and both sides of the glass slide were coated with a thin film of GNPs. Finally, both the GNP-coated slide and the vial were washed thoroughly with triple-distilled water and were air dried. The GNP thin films on Langmuir 2010, 26(22), 17568–17580

Dinda et al. both of these substrates are designated as GNP-film-1 (Table 1). The whole procedure is schematically shown in Scheme 2. To optimize the reaction conditions to obtain good-quality thin films on various substrates, three similar sets of reactions were also carried out using different concentrations of [TBP][Ci] such as 1.5, 1.0, and 0.5 mM as depicted in Table 1, and the corresponding reaction sets are designated as [TBP][Ci]-Au-2, [TBP][Ci]-Au-3 and [TBP][Ci]-Au-4, respectively. In all of these cases, the concentration of AuCl3 was kept constant at 0.5 mM. The GNP thin films that formed on substrates by these reactions sets are designated as GNP-film-2, GNP-film-3, and GNP-film-4, respectively (Table 1). Note that among all of these reaction sets the [TBP][Ci]-Au-1 set was used to deposit thin films onto various substrates. The reason that we chose this set for making GNP thin films on various surfaces will be discussed later in the Results and Discussion section. Thus, the [TBP][Ci]-Au-1 set was also used to coat the surfaces of various commercially available materials such as polystyrene (PS), polypropylene (PP), cellophane paper (cellulose acetate), and plain paper (cellulose) with GNP-film-1 (entry 1 in Table 1). These different surfaces coated with GNPfilm-1 were then analyzed using various instrumental techniques such as UV-vis spectroscopy, scanning electron microscopy, and XRD.

Borohydride Reduction of p-Nitrophenol Catalyzed by GNP Thin Films. For the catalytic activity study of the assynthesized GNP thin films, the inner wall of a quartz cuvette (a negatively charged surface) was first coated with GNP-film-1 (entry 1 of Table 1). The procedure is schematically shown in Scheme 2. As a model reaction, the borohydride reduction of p-nitrophenol was then performed in this GNP-film-1 coated quartz cuvette. Earlier, we studied the catalytic activities of assynthesized gold and silver NPs using the same model reaction.50,51,59 Typically, a reaction mixture of water (2.8 mL) and an aqueous solution of p-nitrophenol (0.10 mL; 3  10-3 M) was first taken in the GNP-film-1-coated quartz cuvette. The total amount of GNPs present in this film was 1.46  10-7 mol. (For the calculation, see S11 of the SI.) Aqueous NaBH4 (0.10 mL; 3  10-1 M) was then added to the above mixture, and the cuvette was quickly placed in a spectrophotometer. The time-dependent UV-vis absorption spectra of the reaction mixture were then recorded. The progress of the reaction was monitored by the disappearance of the peak at λmax= 400 nm corresponding to the p-nitrophenolate ion as a result of its conversion to the p-aminophenolate ion with time. Similarly, we performed the same catalytic reaction using GNP-film-1 deposited on both the positively charged and neutral hydrophobic glass slides. In this case, instead of a GNP-coated quartz cuvette, we used positively charged and neutral hydrophobic glass slides that are coated with GNPs as prepared by the above-mentioned procedure. The GNP-thin-filmcoated glass slides were separately dipped vertically in the uncoated quartz cuvette containing the catalytic reaction mixture.

Measurement of Refractive Indices of Different Organic Solvents Using a GNP-Thin-Film-Coated Quartz Cuvette. The measurement of refractive indices (RIs) of different solvents was carried out by following our earlier reported procedure.16 The procedure is schematically shown in Scheme 2 and is briefly as follows: typically, the inner walls of a quartz cuvette were first coated with GNP-film-1 (entry 1 of Table 1). Three milliliters of an organic solvent was then poured into the coated quartz cuvette, and the spectra of GNPs in the thin film were recorded. Note that after every use the cuvette was emptied, washed properly, and then dried in vacuum for every time after each measurement of the spectrum in the presence of solvent to ensure that the GNP film was free from any residual solvent prior to the next measurement. From these spectra, the transverse SPR band maxima (λmax,t) of GNP thin films in the presence of different organic solvents were measured. The obtained λmax,t values were then plotted against (59) Rashid, M. H.; Mandal, T. K. Adv. Funct. Mater. 2008, 18, 2261.

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Article their respective standard refractive index (n) values to obtain a calibration curve. To measure the refractive index of an unknown solvent, first the λmax, t value of GNPs present in GNP-film-1 in the presence of that solvent was determined using a spectrophotometer. Using the obtained λmax,t value, we obtained the corresponding refractive index from the calibration curve. This technique was used to determine the n value of three solvents (methanol, dimethylformamide, and tetrahydrofuran), considered to be unknowns, as well as a dichloromethane/methanol (1:1) mixture. As mentioned above, our method is so general that we can coat any surface with a GNP thin film. To determine whether the GNP thin films on positively charged (PDDAC coated, as mentioned above) and/or neutral surface (TEOS coated) can also be used for the determination of the RI of organic solvents, we carried out the following experiments. Typically, a small piece of functionalized glass (modified with either TEOS or HDTMS) coated with GNPfilm-1 was first immersed in a quartz cuvette (without a coating) containing 3 mL of each organic solvent. The GNP-thin-filmcoated glass slide was immersed in the cuvette in such a manner that UV light can pass through the thin film on the slide. The λmax,t values of GNPs in different solvents were then recorded. The n of these solvents was then determined according to a similar procedure that used the GNP-thin-film-coated quartz cuvette mentioned above. Characterization. NMR Study. The 1H NMR spectrum of [TBP][Ci] in D2O (1-10 mM) was acquired using a Bruker DPX 300 MHz spectrometer. ESI Mass Spectrometry Study. The ESI mass spectrum of assynthesized ionic liquid [TBP][Ci] was recorded from a methanol solution in a quadrupole time-of-flight (Q-TOF) Micro YA263 mass spectrometer. C, H, N Analysis. The elemental analyses of [TBP][Ci] were carried out using a Perkin-Elmer 2400 series II CHN analyzer. Thermogravimetric Analysis. The purified IL, [TBP][Ci], and neat citric acid were subjected to thermal treatment using a TA SDT Q600 instrument at a heating rate of 20 °C min-1 under an N2 atmosphere. UV-Vis Spectroscopy Study. UV-vis spectra of all of the [TBP][Ci]-assisted gold nanoparticle (GNP) thin films on various substrates were acquired in transmission mode by placing the substrate vertically in a Hewlett-Packard 8453 UV-vis spectrophotometer using the uncoated glass slide as a blank over the region of 200-1000 nm. The spectra of the colloidal GNPs prepared using [TBP][Ci] were recorded after 1 h of reaction using the same spectrophotometer. The appropriate amount of colloid was transferred to a cuvette and quickly placed in the spectrophotometer (to avoid the deposition of the GNPs in the inner wall of the cuvette), and the spectra were recorded. The evolution of the absorption spectra of GNPs was recorded in the kinetic mode of operation.

Fourier-Transform Infrared (FTIR) Spectroscopy Study. For FTIR study, GNPs were removed from the GNP-film-1 on a glass substrate by sonication in water and then freeze dried to remove water. The spectrum of the dried [TBP][Ci]-gold nanoconjugates was recorded with KBr in a 1:100 (w/w) ratio using a Shimadzu FTIR-8400S spectrometer. The spectrum of the IL, [TBP][Ci], was similarly acquired. For the detection of the triethoxy(octyl)silane (TEOS) layer on the glass slide, the functionalized glass slide was examined with the same FT-IR instrument operating in attenuated total reflection infrared (ATR-IR) mode. X-ray Diffraction (XRD) Study. For XRD analysis, a clean glass slide was coated with GNP thin films using the abovementioned procedure and air dried. For XRD analysis of the colloidal [TBP][Ci]-Au nanoconjugate, one drop of its colloidal suspension obtained after 1 h of reaction was deposited on a microscope glass slide as a thin film and air dried. The diffractogram of the GNPs was then recorded by using a Bruker AXS D8 DOI: 10.1021/la103084t

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Advance diffractometer at an accelerating voltage of 40 kV using Cu KR (λ = 1.5405 A˚) as the X-ray radiation source. Transmission Electron Microscopy (TEM) Study. To ascertain the morphology of GNPs present in an aqueous colloidal suspension, one drop of such a suspension was placed on a carbon-coated Cu grid and air dried. To examine the morphology of GNPs present in the thin film, the coated glass substrate was first dipped in water in a small beaker and then was sonicated for 15 min to move the particles from the substrate to the solution. One drop of such an aqueous suspension of the [TBP][Ci]functionalized GNPs was placed on a carbon-coated copper grid and was allowed to air dry. The image was taken by placing the sample-containing grid in a JEOL JEM2010 high-resolution transmission electron microscope operating at an accelerating voltage of 200 kV.

Field Emission Scanning Electron Microscopy (FESEM) Study. For the FESEM study, the GNP thin films on various substrates were first cut into small pieces and were then mounted on a metal stub. These thin film samples (without sputtering with Au or Pt) on the metal tab were then placed under a JEOL JSM6700F electron microscope, operating at an electron voltage of 5 kV, for imaging. Atomic Force Microscopy (AFM) Study. AFM studies of the GNP thin films on negatively charged glass surfaces were conducted on a Veeco digital instruments CPII microscope operating in tapping mode to observe the morphology of the formed GNPs and the extent of their coverage/distribution in the thin film.

Results and Discussion Synthesis and Characterization of the [TBP][Ci] Ionic Liquid. In recent studies, we have shown that redox-active tyrosine/tryptophan-based peptides and redox-active ascorbic acid-based ILs can assist in the formation of spherical as well as anisotropic Au and Ag NPs.36,55,60-63 In this article, we designed and synthesized a different redox-active ionic liquid (IL), tetrabutylphosphonium citrate ([TBP][Ci]), for the in situ deposition of GNPs as thin films on various substrates. This IL was prepared simply by the neutralization of tetrabutylphosphonium hydroxide with citric acid, the details of which are mentioned in the Experimental Section (Scheme 1). Because we used equimolar amounts of both citric acid (acid) and tetrabutylphosphonium hydroxide (base), it is expected that the two components should react in an equimolar ratio to produce a neutalized product. Furthermore, after the neutralization reaction, when the mixture was washed with DCM a solid residue was obtained. This residue is nothing but excess citric acid because it is insoluble in dry DCM. Therefore, it is expected that all the tetrabutylphosphonium hydroxide was consumed in this reaction. Again, the aqueous solution of the obtained IL has a neutral pH, which indicates the absence of any acid or base in the prepared IL. The 1H NMR spectrum (Figure S2 in the SI) shows a doublet peak at δ = 2.83 corresponding to two H atoms of the -CH2-COO- group of the citrate moiety of the IL. Whereas another doublet peak at δ = 2.71 also appeared that was due to 2H of -CH2-COOH of same citrate moiety of the IL. A triplet peak at δ = 2.01 also appeared that was due to eight H atoms of four R-CH2 groups of four butyl groups (-CH2-CH2-CH2-CH3) of [TBP] cation of the IL. Again, another two multiplet signals appeared at δ = 1.49-1.47 and δ =1.43-1.38 corresponding to eight H atoms of four β-CH2 and four γ-CH2- groups of four butyl moieties of [TBP] cation, respectively. The spectrum also exhibit a triplet (60) (61) (62) (63)

Dinda, E.; Rashid, M. H.; Mandal, T. K. Cryst. Growth Des. 2010, 10, 2421. Si, S.; Dinda, E.; Mandal, T. K. Chem.;Eur. J. 2007, 13, 9850. Si, S.; Dinda, E.; Mandal, T. K. J. Nanosci. Nanotechnol. 2008, 8, 5934. Si, S.; Mandal, T. K. Chem.;Eur. J. 2007, 13, 3160.

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Figure 1. UV-vis absorption spectra of as-prepared suspensions of different GNP samples prepared under different conditions as mentioned in Table 1: (a) [TBP][Ci]-Au-1, (b) [TBP][Ci]-Au-2, (c) [TBP][Ci]-Au-3, and (d) [TBP][Ci]-Au-4. The spectra were recorded after 1 h of reaction.

signal for 12 H atoms at δ = 0.87 corresponds to the -CH3 of the -CH2-CH2-CH2-CH3 group of the butyl moiety of [TBP][Ci]. The ESI mass spectrum of the IL shows the presence of only a strong signal due to its cationic part. No other signal was found in this spectrum, indicating that the as-synthesized IL is free from any impurity such as excess citric acid or TBP base. The elemental analysis data well matches the composition of the element present in the IL. The 1H NMR spectrum reveals that the integral value of the peak at δ = 2.83 corresponding to two H atoms of -C-CH2-COO- of the citrate moiety [Ci]- anion is 1.0, whereas the integral value of the peak at δ = 2.01 for eight H atoms of four -CH2- groups of the four-butyl group of the TBP cation ([P-(CH2-CH2-CH2CH3)4]þ) is 4.04. Thus, the integral value for one H atom of the citrate moiety is almost the same as that of one H atom of the butyl group of the [TBP] cation, which indicates that the IL is composed of a single [TBP] cation and a single citrate anion. The TGA thermograms of the purified [TBP][Ci] IL reveal that it is stable up to a temperature of 160 °C, after which its decomposition starts and continues up to 400 °C (Figure S3a of the SI). The decomposition of neat citric acid starts from 100 °C and continues up to 200 °C (Figure S3b of the SI). This result suggests that the ILs have better thermal stability than citric acid. Preparation of Colloidal Gold Nanoparticles (GNPs) Using the [TBP][Ci] Ionic Liquid. The colloidal GNP suspension was obtained by simply mixing an aqueous solution of ILs with an aqueous solution of AuCl3 in a piranha-treated glass vial as schematically shown in Scheme 2. We observed a pinkish-blue hue in the solution mixture after 30 min of reaction. After 1 h, the color of the solution changed to light pink and darkened with time, indicating the formation of GNPs. However, we noticed that some of the formed GNPs were deposited on the inner walls of the vial after 1 h. The mixed solution was undisturbed for another 5 h. Surprisingly, we found that the reaction mixture solution was colorless after a total of 6 h of reaction but that the vial was dark pink in color. This is because of the fact that all of the formed GNPs are deposited as a pink thin film onto the inner walls of the vial. To confirm this, we acquired the absorption spectra of the reaction mixture with time during the formation of GNPs in the set [TBP][Ci]-Au-1 (entry 1 of Table 1; Figure S4 in the SI). It should be noted that the spectrum of the suspension taken after 6 h of reaction did not exhibit any SPR band due to GNPs (Figure S4f in the SI). However, all of the spectra acquired before 6 h of reaction exhibit a prominent transverse SPR band Langmuir 2010, 26(22), 17568–17580

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Figure 2. (A) TEM image of the as-prepared colloidal GNPs (sample [TBP][Ci]-Au-1). (B) Electron diffraction pattern of flakelike GNPs for the image in panel A. (C) Electron diffraction pattern of spherical GNPs for the same image in panel A. Image and ED patterns were taken after 1 h of reaction.

centered at 553 nm along with a shoulder peak at 800 nm due to the logitudinal SPR band of GNPs. This result indicates that the formed GNPs are completely deposited as a thin film onto the inner wall of the vial after 6 h of reaction. This is the reason that we have used a time span of 6 h for GNP thin film deposition at the surface of any substrate. The details of the fabrication of GNP thin films on various substrates will be discussed later in this section. As mentioned above that most of the formed GNPs are deposited as a thin film after 6 h of reaction, for the characterization of GNPs in the colloidal state, the sample suspensions were withdrawn after 1 h of reaction. The UV-vis spectra (taken after 1 h of reaction) of all of the GNP samples prepared according to the recipes in Table 1 are shown in Figure 1. The spectra of samples [TBP][Ci]-Au-1, [TBP][Ci]-Au-2, and [TBP][Ci]-Au-3 exhibit both a transverse (shorter wavelength) SPR band at 553, 555, and 559 nm and a longitudinal (longer wavelength) SPR band at 766, 850, and 782 nm (Figure 1a-c). These types of SPR bands may arise from the formation of either nonspherical or assembled GNPs.64 However, the [TBP][Ci]-Au-4 sample exhibits a single broad SPR absorption band at λmax = 560 nm (Figure 1d). Such a broad SPR absorption band may also be attributed to the formation of GNPs with nonspherical morphology, spherical particles with high polydispesity, or aggregated spherical particles.15,64 Among all of these reaction sets, the [TBP][Ci]-Au-1 set is used to prepare GNP thin films on various substrates (Table 1). Therefore, we examined only the morphology of this sample as a representative case. The reason for choosing this reaction set to depositing a thin film will be discussed later in this section. The TEM image of sample [TBP][Ci]-Au-1 obtained after 1 h of reaction shows the presence of aggregated spherical GNPs with a diameter ranging from 15 to 20 nm (Figure 2A). Some flakelike, flat-lying particles are also seen in the image. It seems that the aggregated spherical GNPs are on the top of these flakes. The electron diffraction (ED) patterns, obtained by focusing the electron beam separately on flakelike, flat-lying GNPs and single isolated spherical GNPs, are shown in Figure 2B,C, respectively. These two ED patterns with the hexagonal symmetry of diffracted spots suggests that both the spherical and flakelike GNPs are face-centered cubic (fcc) single crystal bounded by {111} facets (JCPDS file no. 4-0784).65 The presence of aggregated and flakelike GNPs may be responsible for showing two SPR bands for the [TBP][Ci]-Au-1 sample as mentioned above (Figure 1a).15 (64) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (65) Chu, H. C.; Kuo, C. H.; Huang, M. H. Inorg. Chem. 2006, 45, 808.

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Figure 3. XRD patterns of as-prepared GNP samples prepared under different conditions according to the recipe provided in Table 1: (a) [TBP][Ci]-Au-1, (b) [TBP][Ci]-Au-2, (c) [TBP][Ci]Au-3, and (d) [TBP][Ci]-Au-4. The samples were taken after 1 h of reaction.

The crystalline nature of all of the GNP samples obtained from their suspensions after 1 h of reaction was confirmed through X-ray diffraction (XRD) analysis. The XRD patterns of samples [TBP][Ci]-Au-1, [TBP][Ci]-Au-2, [TBP][Ci]-Au-3, and [TBP][Ci]Au-4 (Table 1) exhibit an intensive diffraction peak centered at 2θ = 38.18° corresponding to the (111) lattice plane of the fcc gold (Figure 3).65,66 In contrast, the intensities of the peaks at 2θ = 44.4 and 64.6° corresponding to (200) and (220) lattice planes are quite weak for all of these GNP samples. The lattice constant (a) calculated from the XRD patterns (considering the peak at 2θ = 38.18°) is 4.08A°, which is in good agreement with the literature-reported value of 4.07 (JCPDS file no. 4-0784). Formation of GNP Thin Films on the Surfaces of Different Substrates. As mentioned above, when the reaction of aqueous AuCl3 with aqueous [TBP][Ci] was performed in a clean glass vial, in the initial stage the formed GNPs stay partially in suspension and partially deposited as a thin film with a pink hue on the inner walls of the reaction vials. However, if the reaction is continued for more than 6 h, then almost all of the formed GNPs are deposited as a thin film on the inner walls of the reaction vial. The film deposition procedure is schematically shown in Scheme 2. It should be noted that the thin film of GNPs on the glass surface is so stable that it cannot be removed from the surface by repeated washing with any polar/nonpolar solvents. (66) Shao, Y.; Jin, Y. D.; Dong, S. J. Chem. Commun. 2004, 1104.

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Figure 5. FESEM images of the GNP thin films deposited on different negatively charged glass substrates after 6 h of reaction: (A) GNP-film-1 on a piranha-treated glass substrate and (B) GNPfilm-1 on a PAA-modified glass substrate. The insets in these two images are enlarged views of a representative marked portion of the respective images. Figure 4. UV-vis spectra of GNP-film-1 on negatively charged surfaces prepared with the [TBP][Ci]-Au-1 sample: (a) a piranhatreated glass substrate and (b) a PAA-modified glass substrate. The spectra were recorded after 6 h of deposition.

This observation prompted us to coat piranha-treated glass slides or glass slides functionalized with positively charged polyelectrolyte/negatively charged polyelectrolyte/neutral hydrophobic silane or piranha-treated quartz cuvette as well as various unfunctionalized commercially available substrates such as polystyrene, polypropelene, plain paper, and cellophane paper with GNP thin films using the recipe for GNP-film-1 (entry 1 in Table 1). Interestingly, we found that the formed GNPs can be deposited as a thin film on any of these surfaces no matter what the nature of the surface. It should also be noted that among all of these sets of reactions (Table 1) the concentration of IL used is the maximum in set [TBP][Ci]-Au-1, which produces GNP-film-1. The IL is actually reduced from Au3þ to metallic Au, which is eventually deposited as a thin film on the functionalized surfaces. Therefore, one could expect that the quantity of GNPs deposited as a thin film would be maximized in this case. As further proof, we acquired the UV-vis spectra of four films (GNP-film-1, GNPfilm-2, GNP-film-3, and GNP-film-4) on piranha-treated glass, which are shown in Figure S5A of the SI. All of the films exhibited a transverse SPR band along with the longitudinal band. The absorbance values at λmax,t of the transverse SPR band of all of these films were plotted against the concentration of IL used (at fixed concentration of AuCl3, Table 1) for the preparation of these films (Figure S5B in the SI). Figure S5B clearly reveals that the absorbance at λmax,t for GNP-film-1 is maximized compared to that of all other films. This is why we have chosen to use the recipe of set [TBP][Ci]-Au-1 to make GNP-film-1 on various functionalized/unfunctionalized surfaces. Furthermore, we have added NaBH4 to the supernatant, which is obtained after GNPfilm-1 formation. This confirms whether any unreacted Au3þ ions are present in the supernatant after film formation. We found that the color of the supernatant is unchanged after the addition of NaBH4, indicating no unreacted Au3þ ions in the supernatant. Thus, this result proves that all of the Au3þ ions present in the solution are converted to GNPs and are deposited as a thin film on a surface in contact with the solution. GNP Thin Films on a Negatively Charged Glass Surface. The negative charge on the glass surface was created by treating it with piranha solution as mentioned in the Experimental Section and elesewhere.67 To make sure that there is enough negative charge on the surface, the piranha-treated glass was first positively charged by modification with PDDAC. This positively charged (67) Liu, B. F.; Ma, J.; Xu, Q. Y.; Cui, F. Z. Colloids Surf., B 2006, 53, 175.

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glass was further treated with a negatively charged polyelectrolyte, PAA.68 Both the PAA-treated and piranha-treated negatively charged glass surfaces were used for depositing GNP thin films onto them using set [TBP][Ci]-Au-1 (entry 1 of Table 1). The obtained GNP-film-1 on a piranha-treated glass slide showed both transverse and longitudinal SPR bands centered at 531 and 689 nm, respectively (Figure 4a). The presence of two bands indicates that the GNPs present in this film have either anisotropic morphology or spherical particles with some shot of assembly.64 Note that a blue shift (553 to 531 nm) in the transverse SPR band was observed as a result of the deposition of the GNPs as a thin film from a colloidal suspension (cf. Figures 4a and 1a). The change in the environment after the deposition of GNPs as a thin film might be responsible for such a blue shift. Liz-Marzan et al. have also observed a similar kind of change in optical absorption during GNP thin film formation by the layer-by-layer assembly technique.68 However, the GNP-film-1 on the PAAmodified glass surface shows a maximum transverse SPR band centered at 534 nm along with a clear, weak soldier peak at 674 nm (Figure 4b). This result primarily indicates the deposition of GNPs onto the PAA-modified glass surface. A similar blue shift in the transverse SPR band of GNPs present in thin film compared to that of GNPs in suspension was also observed in this case (cf. Figures 4b and 1a). FESEM was used to study the morphology of GNP-film-1 on a negatively charged glass surface. The GNP-film-1 on a piranhatreated glass surface shows the presence of spherical NPs, which are well distributed throughout the film along with very few quasispherical NPs (Figure 5A). Besides this, it seems that some of the spherical particles are arranged in regular arrays, some of which are marked with a white circle as shown in Figure 5A. The enlarged view of one such marked portion is shown in the inset of Figure 5A. The GNPs are polydisperse in nature with sizes ranging from 15 to 50 nm. The AFM image (Figure S6 in the SI) of GNP-film-1 on the negatively charged glass surface also shows some array of spherical GNPs (diameter of ∼30 nm). The height profile image indicates that the particles are not touching each other. It also shows some larger spherical GNPs in the film with a diameter of 50 nm or even larger as observed through FESEM. However, the height profile of the larger (50 nm) spherical particle indicates that it is composed of two smaller GNPs of diameter ∼18 nm (Figure S6). It seems that some of the initially formed smaller GNPs are aggregated during their deposition on the solid surface. This is probably the reason that such polydisperse GNPs (15-50 nm) are seen in the FESEM image as (68) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 2002, 18, 3694.

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Figure 6. TEM image of GNPs of GNP-film-1 on a piranhatreated glass surface. The GNPs were obtained from the GNPcoated glass slide by sonicating them with water.

mentioned above. Again, the GNP-film-1 deposited on the PAAmodified surface (Figure 5B) also shows a similar distribution of GNPs to that observed in the case of GNP-film-1 on a piranhatreated glass surface. In this case, we also observed some kind of array of spherical GNPs, some of which are also marked by a circle in Figure 5B (inset). The size of the GNPs present in this film is also in a similar range (15 to 50 nm). Such an array of GNPs in the thin film on a negatively charged glass surface might be responsible for generating two SPR bands as mentioned above.15,64 For a better understanding of the morphology of GNPs present in GNP-film-1, we first removed the GNPs from the piranhatreated glass surface by ultrasonication in water (details in the Experimental Section). The obtained GNP suspension was then examined via TEM. The image shows spherical GNPs with diameters ranging between 15 and 50 nm (Figure 6). The size range of these GNPs matches well with that of the GNPs present in the thin film as measured by FESEM images mentioned above. The X-ray diffraction pattern of the thin film of GNPs on a piranha-treated negatively charged glass surface exhibits all four peaks at 2θ = 38.2, 44.4, 64.6, and 77.4° corresponding to the (111), (200), (220), and (311) lattice planes, respectively, for the fcc metallic Au (Figure S7 in the SI). All of these peaks are identical to those of colloidal GNPs for the same sample (compare Figure 3a with Figure S7 of the SI), indicating that the crystal structures of the GNPs present in thin film and in the colloidal state are similar (i.e., both are grown along the (111) direction).65,66 GNP Thin Films on a Positively Charged Glass Surface. The glass surface was positively charged by modifying the piranha-treated surface with a cationic polyelectrolyte, PDDAC, following the modified procedure described elsewhere (details in the Experimental Section).68 Figure 7A shows the UV-vis spectrum of GNP-film-1 on the positively charged PDDACcoated glass surface. The spectrum exhibits a sharp transverse SPR band centered at 535 nm along with a broad peak at 810 nm. In this case, the transverse SPR band of GNPs was blue-shifted from 553 (colloid state) to 535 nm (film state) because of the transformation of GNPs from the colloidal state to the film state. This blue shift might be due to the change in the surrounding environment of the thin film as mentioned above in this section and elsewhere.68 The FESEM image analysis of GNP-film-1 on a positively charged surface shows that most of the deposited GNPs are spherical in nature and are well distributed throughout the matrix as shown in Figure 7B. We also observed some kind of array of spherical GNPs in this thin film (some of them are marked by a circle), as also noticed in the GNP thin film on a negatively charged surface mentioned above. The enlarged portion of one Langmuir 2010, 26(22), 17568–17580

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Figure 7. (A) UV-vis spectrum and (B) FESEM image of GNPfilm-1 on a positively charged (modified with PDDAC) glass substrate. The spectrum and image were recorded after 6 h of deposition. The inset in panel B is an enlarged view of a representative marked portion of the same image.

such marked area is shown in the inset of Figure 7B, indicating that the GNPs are well-separated. It was also confirmed from the image that the spherical GNPs were polydisperse in nature and that their sizes were in the range of 15 to 50 nm. The X-ray diffraction pattern of GNP-film-1 on a positively charged glass surface shows the most intense peak to be centered at 2θ = 38.2°, corresponding to the (111) lattice plane of the fcc metallic Au (Figure S8 of the SI).65,66 Along with this peak, a very weak peak at 2θ = 44.4° due to the (200) lattice plane (enlarged view of Figure S8 in the SI) was also observed. However, we did not find any peak corresponding to the (220) and (311) lattice planes as observed in the case of GNP-film-1 on a negatively charged glass surface as mentioned above. At this point, we do not know the exact reason behind this. To determine whether any impurity such as gold oxide is formed during this deposition process, we acquired the XRD pattern of this GNP thin film at a lower angle (Figure S8 in the SI). The absence of any peak at a low angle (around 2θ = 25.6, JCPDS file no. 4-0784) indicates that the GNPs present in the film are in the pure state and no gold oxide is produced. Thin Films of GNPs on a Hydrophobic Neutral Glass Surface. The piranha-treated glass vials/slides were made hydrophobic by treating them with two alkyl silanes, such as TEOS and HDTMS. The GNP thin films was then deposited on these hydrophobically neutral surfaces using the recipe for GNP-film1 (entry 1 in Table 1). The UV-vis spectra of GNP-film-1 on a TEOS-modified glass surface exhibit both transverse and longitudinal SPR bands centered at 532 and 649 nm, respectively (Figure 8a). The spectrum of GNP-film-1 on an HDTMSmodified glass surface also exhibits both the transverse SPR band (534 nm) and a long-range longitudinal band (850 nm) (Figure 8b). The presences of two SPR bands in the above two cases may indicate that the spherical GNPs undergo some kind of assembly from the colloidal suspension during their deposition as a thin film, as also observed for two other cases as mentioned above (GNP-film-1 deposited on negatively and positively charged surfaces as shown in Figures 4 and 7A, respectively). To understand how the GNPs are arranged in this thin film on a hydrophobic neutral substrate, we examined this film via FESEM. Interestingly, we found that the spherical GNPs were assembled to a large extent into some kind of structure in this thin film (Figure 9). To be precise, in the GNP-film-1 on a TEOSmodified glass surface, the spherical GNPs were organized into self-assembled chainlike structures (Figure 9A). The size of the spherical GNPs that formed the assembly ranged from 15 to 50 nm. An enlarged view of a portion of this chainlike assembled structure DOI: 10.1021/la103084t

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Figure 8. UV-vis spectra of GNP-film-1 deposited on different neutral hydrophobic glass substrates: (a) a TEOS-modified glass substrate and (b) a HDTMS-modified glass substrate. Spectra were recorded after 6 h of deposition.

Figure 9. FESEM images of GNP-film-1 on various neutral hydrophobic substrates: (A) a TEOS-modified glass substrate and (B) an HDTMS-modified glass substrate. Images were taken after 6 h of deposition. The insets in these two images are enlarged views of a representative marked portion of the respective images.

reveals that two spherical GNPs are not touching each other (inset of Figure 9A). The presence of this type of self-assembled structure in GNP-film-1 on a TEOS-modified glass surface may be responsible for exhibiting two SPR bands as mentioned above (Figure 8a).15 However, GNP-film-1 on a HDTMS-modified glass surface mostly contained well-dispersed spherical GNPs throughout the matrix (Figure 9b). Here, the sizes of spherical GNPs are also in the range of 15 to 50 nm. We did not observe such a chainlike structure in this case, but some array of spherical GNPs can be seen in the image (Figure 9b) as observed in the cases of a thin film on positively/negatively charged glass surfaces. An enlarged view of one such short array is given in the inset of Figure 9b. Furthermore, we also observed a small number of platelike GNPs, which are also well distributed throughout the HDTMS-modified glass surface. The presence of GNPs with some kinds of arrays as well as the presence of other shaped particles might be responsible for the exhibition of two SPR bands by the GNP-film-1 on an HDTMS-modified glass surface as mentioned above (Figure 8b), as also noticed by other groups.15 Overall, the arrangement of GNPs in the thin film on a TEOS-modified glass surface is different from that of GNPs present in the thin film on an HDTMS-modified glass surface. The X-ray diffraction pattern of GNP-film-1 on a TEOSmodified surface showed only the most intense peak centered at 2θ = 38.2 corresponding to the (111) lattice plane for the fcc metallic Au (Figure S9 in the SI).65,66 No other peaks at 2θ = 44.4, 64.6, and 77.4 corresponding to the (200), (220), and (311) planes were observed, even in the enlarged view of this XRD pattern. Other group have also shown only a single peak corresponding to (69) Dhara, K.; Sarkar, K.; Roy, P.; Nandi, M.; Bhaumik, A.; Banerjee, P. Tetrahedron 2008, 64, 3153.

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Figure 10. FESEM images of GNP-film-1 deposited on various commercially available substrates: (A) polystyrene (PS), (B) polypropylene (PP), (C) plain paper, and (D) cellophane paper (cellulose acetate membrane). The images were taken after 6 h of reaction. The insets in panels A-D show enlarged views of a representative marked portion of the respective panel.

the (111) plane of metallic Au in the GNPs prepared using a complex organic molecule.69 Thin Films of GNPs on the Surfaces of Commercially Available Substrates. As mentioned above, our technique is so general that it can also be successfully used to deposit GNP thin films on various commercially available surfaces. Thus, to check this, we chose some easily available commercial substrates such as polystyrene (PS), polypropylene (PP), plain paper, and cellophane paper (cellulose acetate) to coat with GNP thin films using this method (details in the Experimental Section). We observed that the thin film on different surfaces is very stable and cannot be leached out after washing with both the polar and nonpolar solvents. Photographs of the GNP thin films on different substrates were taken with a digital camera; these images clearly show the formation of a pink film on the respective substrates (Figure S10 in the SI). The UV-vis spectrum of the thin film of GNPs deposited on the PS surface showed only one sharp SPR band centered at 549 nm, indicating the deposition of GNPs on this surface (Figure S11 in the SI). However, we are unable to acquire the spectra of GNP thin films deposited on other substrates because these materials are opaque (such as paper and PP) and we do not have the ability to acquire the spectra in reflectance mode. Figure 10A-D shows the FESEM images of GNP-film-1 deposited on PS, PP, paper, and cellophane paper surfaces, respectively. The GNPs present in GNP-film-1 on the PS surface shows spherical and quasi-spherical morphology with diameters in the range of 15 to 50 nm. The GNPs are also well dispersed throughout the matrix. In this case, however, there are fewer assembled structure compared to the number observed for GNP-film-1 on functionalized and piranha-treated glass surfaces (cf. Figures 5, 7B, and 9 with Figure 10A). This may be the reason for obtaining the single SPR band in the case GNP thin films on the PS surface (Figure S11 in the SI). The distribution of GNPs in GNP-film-1 on the PP surface is also similar to that of GNPs of GNP-film-1 on the PS surface (Figure 10B). GNP-film-1 on plain paper and cellophane paper (Figure 10C,D respectively) shows well-dispersed GNPs (D ≈ 15-50 nm) as observed in the films on PS (Figure 10A) and PP (Figure 10B) surfaces. Control Experiments. To understand the role of [TBP][Ci] IL in the deposition process of GNPs as a thin film on various surfaces, we have performed two different sets of control reactions. In the first set, an aqueous solution of sodium citrate (NaCi) Langmuir 2010, 26(22), 17568–17580

Dinda et al. Scheme 3. Schematic Representation of the Possible Adsorption of [TBP][Ci]-Coated Au NPs onto Negatively Charged, Positively Charged, and Neutral Hydrophobic Substrates

Article Table 2. Standard Refractive Index Values of Different Pure Solvents and λmax,t Values of GNPs in GNP-Film-1 in the Presence of These Solvents solvents

na

λmax,t (nm)

ethanol 1.361 dichloromethane 1.424 chloroform 1.445 toluene 1.496 xylene 1.505 acetonitrile 1.344 ethyl acetate 1.372 dioxane 1.422 a Standard values of the refractive index (RI) (n).

(10 mM, 400 μL), instead of [TBP][Ci] IL, was added to an aqueous AuCl3 solution (10 mM, 100 μL) taken in a piranhatreated glass vial. Similarly, in another set, an aqueous citric acid (10 mM, 400 μL) solution was added to an aqueous AuCl3 solution (10 mM, 100 μL) taken in a piranha-treated glass vial. Note that the concentration of NaCi in the first set and the concentration of citric acid in the second set are the same and are equal to that of the IL used to make any GNP-film-1 on any substrate (entry 1 in Table 1). We did not observe any GNP film deposition on the inner wall of the glass vial in these two cases, even after several days of storage. Rather, in the first set, a blue colloid containing the as-formed GNPs and in the second set, a pink colloid of as-formed GNPs were obtained. Note that the rate of reduction of AuCl3 to Au NPs by citric acid is relative slower than that of NaCi, but there was no thin film formation on the wall of the vial in either case. Furthermore, the use of a mixture of tetrabutylphosphonium hydroxide and sodium borohydride, instead of [TBP][Ci] IL, resulted in the formation of aggregated GNPs that precipitated at the bottom of the vial but not as a thin film on the wall. These results substantiated that the [TBP][Ci] IL, which is prepared from citric acid and tetrabutylphosphonium hydroxide, has the capability to reduce Au3þ to metallic Au, which is completely deposited as a GNP thin films on the surfaces of these used substrates. Furthermore, to determine whether our previously reported imidazolium ascorbate-based ILs36 are capable of producing this type of thin film on a solid surface, we performed the reaction of AuCl3 (10 mM, 100 μL) with one such IL (10 mM, 400 μL) containing the ascorbate anion. Interestingly, the formation of colloidal GNPs took place very quickly, but no GNP film was deposited on the inner walls of the reaction vial. These results further confirmed the fact that only the IL, [TBP][Ci], has the capability to deposit an in situ film of GNPs on the solid surface. FTIR Study. FTIR characterization was performed on GNPfilm-1 on a negatively charged glass surface as well as on neat [TBP][Ci] IL. The FTIR spectrum of a neat IL exhibited clear symmetric and asymmetric stretching band at 2958 and 2876 cm-1 as a result of the -CH group of the IL (Figure S12a). Again, a band at 1602 cm-1 appeared because of the carboxylate group of the citrate moiety of the IL (Figure S12a). However, the FTIR spectrum of GNPs, which was removed from the thin film, exhibited three bands at 2923, 2890, and 1614 cm-1 (Figure S12b). The first two bands correspond to the alkane group of the cationic [TBP] moiety of the IL, but the third band is due to the carboxylate Langmuir 2010, 26(22), 17568–17580

540 547 549 553 553 539 541 544

group of the anionic [Ci] moiety of the IL. These results indicate that the IL molecules are also present on the surfaces of GNPs obtained from a thin film, but at this point, we do not know its extent of adsorption. Possible Mechanism of Formation of GNP Thin Films on Solid Substrates. As mentioned above, the thin films of GNPs were deposited on all kinds of surfaces as a result of the reaction of AuCl3 and IL, no matter what the charge of the surface. It is also mentioned that each molecule in the IL, [TBP][Ci], contains a cationic and an anionic part. Again, the FTIR results confirmed that the IL molecules were adsorbed on the GNP surface. On the basis of these results, we may propose the following mechanism. In the case of a negatively charged glass substrate, the IL containing GNPs is probably deposited through the electrostatic interaction between cationic [TBP] parts of the IL with the negative charge on the glass surface (Scheme 3). However, in the case of the positively charged glass surface, the anionic [Ci] part of the IL on the GNPs probably interacts with the positive charge on the glass surface (Scheme 3). Again, this IL contains four butyl groups that are attached to the phosphorus atom of the [TBP] moiety, which helps to deposit the GNPs on the hydrophobic surface through the van der Waals interaction (Scheme 3). This is the reason that IL-containing GNPs are deposited on three different types of surfaces. Measurement of Refractive Indices (RIs) of Organic Solvents Using GNP Thin Films. We utilized the as-fabricated GNP thin films on a glass substrate for the measurement of the refractive index (RI) of different organic solvents that were measured earlier using cyclam-based GNP thin films.16 For measurement of RI, the spectra of GNPs of GNP-film-1 on the inner walls of the quartz cuvette were acquired by filling the cuvette with some of the common organic solvents (details in the Experimental Section). The measured absorption maxima of the transverse SPR band (λmax,t) of GNPs in GNP-film-1 for different solvents are provided in Table 2. It is clear that the λmax,t values are dependent on the nature of the solvents. It has been discussed by Mie in his theory that the change in the SPR property of GNPs is influenced by the surrounding environment.70 According to this theory, the SPR band maximum (λmax,t) of GNPs is susceptible to changes in the dielectric constant of the surrounding medium (εm) in which the particles are embedded/dispersed, and the relation between λmax,t and εm is as follows ðλmax, t Þ2 ¼ λp 2 ðεd þ 2εm Þ where εd is the high-frequency value of the dielectric function (13.2 for gold),71 εm is the dielectric constant of the medium, (70) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (71) Kobayashi, Y.; Correa-Duarte, M. A.; Liz-Marzan, L. M. Langmuir 2001, 17, 6375.

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Figure 11. Plot showing (A) the variation of (λmax,t)2 values against εm (dielectric constant) of different organic solvents (values given in Table 2) and (B) the variation of the measured λmax,t of GNP-film-1 deposited on the inner walls of the quartz cuvette in the presence of different solvents with their respective refractive indices.

which is equal to the square of its refractive index (n) value, and λp is the plasmon wavelength of the bulk metal. According to the above equation, the measured values of (λmax,t)2 of GNPs of the thin film in the presence of different solvents were plotted against 2εm, which gives a linear curve as shown in Figure 11A. Note that the values of εm are obtained from the standard values of n for the same solvents. From the linear plot (Figure 11A), the calculated value of λp for GNPs is 131.45, which well matches the reported theoretical value of 131.70 This proved that the fabricated GNP thin film on the inner walls of the quartz cuvette responded well to changes in the dielectric constant of the local environment. For comparison, a calibration curve was prepared (Figure 11B) by plotting the measured λmax,t of GNP thin films present on the inner walls of the cuvette in the presence of different solvents versus their respective standard n values as given in Table 2. Typically, to measure the RI of a solvent by this method, first the λmax,t value of GNP thin films in the presence of this solvent was recorded. The corresponding n value for this solvent was then determined from the calibration curve given in Figure 11B. By this technique, we measured the RI of three solvents such as methanol, THF, and DMF (considered to be unknown) as well as that of the (1:1) mixture of methanol and dichloromethane using the same GNP-thin-film-coated cuvette. The obtained n values of these solvents and the mixed solvent are also provided in Table 3. It is very clear from the data in Table 3 that the n values of these solvents are very close to the standard values obtained from the literature. In our previous report, we measured the n values of these solvents by using cyclam-based GNP thin films coated on quartz cuvettes, which are given in column 5 of Table 3 for comparison. We also measured the n values of these solvents using the Chatelaine wedge method, and these are also presented in column 6 of Table 3 for comparison.16 Thus, the data in Table 3 clearly shows that the measured values of n for these three pure solvents as well as for the mixture obtained by the present method matched very well with that measured using other methods, as has been described in the previous report. Note that the λmax,t values of GNPs in solvents (such as THF, DMF, and dioxane) remain unchanged, although their n values are different, as we also noticed in our earlier work where an explanation of this discrepancy has been provided.16 Additionally, we also tried to determine the RI of different organic solvents using GNP-film-1 on positively charged (PDDAC-coated) and neutral hydrophobic (TEOS-coated) glass slides using a technique similar to that discussed above. For this, the λmax,t values of GNP-film-1 on these two surfaces were 17578 DOI: 10.1021/la103084t

Table 3. RI (n) Values of Various Solvents Using Different Methods solvents

λmax,t (nm)

na

nb

nc

nd

methanol 538 1.328 1.325 1.329 1.315 DMF 544 1.43 1.44 1.438 1.420 THF 543 1.407 1.41 1.390 1.420 methanol/DCM (1:1) mixture 540 1.383 1.388 1.375 a Standard values of n obtained from the literature. b Values of n obtained using GNP thin films by the present method. c Values of n obtained from our previous paper as measured by the Chatelaine wedge method.16 d Values of n obtained from our previous paper as measured using cyclam-based GNP thin films.16

measured in the presence of different solvents, and the values are reported in Tables S1 and S2 in the SI. The (λmax,t)2 values for GNPs on these two surfaces were then plotted against 2εm. Again, linear curves were obtained for both cases (Figures S13A and S14A in the SI). Linear calibration curves were also obtained when the values of λmax,t of GNP-film-1 on these surfaces were plotted against the refractive index values of different solvents (Figures S13B and S14B in the SI). From these two calibration curves for the GNPs in these two cases, we can easily determine the RI of different organic solvents according to the same procedure as described above. The obtained RI values of these three solvents (methanol, THF, and DMF) using GNP-film-1 on these two surfaces are well matched to that obtained using GNPfilm-1 on a quartz surface. Study of the Catalytic Activity of GNP Thin Films. To study the catalytic activity, we first deposited GNP-film-1 on the inner walls of a piranha-treated quartz cuvette. The borohydride reduction of nitrophenol is chosen as a model reaction for studying the catalytic activity of this GNP thin film. In our earlier study of the catalytic activities of differently shaped Au and Ag nanoparticles, we have chosen the same reaction.50,51,59 It is known that the reaction mixture of 4-NP and NaBH4 is inactive in the absence of a catalyst. When this reaction was carried out in a GNP-film-1-coated cuvette, a gradual fading of the yellow color of the reaction mixture was observed. To monitor this reaction systematically, we recorded the time-dependent UV-vis spectra of the reaction mixture. It is also known that 4-NP shows absorption at 317 nm that red shifted to 400 nm because of the formation of 4-nitrophenolate in the presence of NaBH4. Therefore, we monitored the peak due to nitrophenolate (400 nm) to study the activity of this thin film toward this reaction. Figure 12A shows typical UV-vis spectra of the reaction mixture catalyzed by a thin film of GNPs present on the inner walls of the quartz cuvette. It can be seen from the spectra that the absorbance of the Langmuir 2010, 26(22), 17568–17580

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Figure 12. (A) Successive UV-vis spectra of p-nitrophenol during its reduction by NaBH4 in the presence of GNP-film-1 deposited on the inner walls of a quartz cuvette. (B) Variation of ln A (obtained from part A) vs time for this film. Table 4. Comparison of Apparent Catalytic Rate Constants (kapp) for the Borohydride Reduction of 4-NP Using Different Supported and Bare GNP Catalysts catalyst cyclodextrin-capped GNPs78 GNPs on a polymer support53 GNPs on a polymer support54 GNP diatom replica27 GNP thin films on negatively charged quartz glass (present case) GNP thin films on positively charged glass (present case) GNP thin films on neutral hydrophobic glass (present case)

catalyst dosages (mol % wrt 4NP)

kapp (s-1)

190 48.6

4.6  10-3 6.1  10-4 3.2  10-3 3.9  10-3 1  10-2

21

3.7  10-2

29

2.6  10-2

17.6

peak at 400 nm rapidly decreases with time; subsequently, a new peak is generated at 300 nm that corresponds to the formation of p-aminophenolate. This result indicates that the GNPs present in the thin film successfully catalyze the borohydride reduction of 4-NP. Thus, to quantify the activity of the GNP-film-1, we plotted ln A (A = absorbance of 4-nitrophenolate at 400 nm) versus time (t) and a linear curve was obtained (Figure 12B). It is known that in this reduction process the concentration of NaBH4 far exceeds the concentration of 4-NP and hence the rate is assumed to follow pseudo-first-order kinetics as also reported elsewhere.72 Therefore, according to this report and our earlier reports,50,51,59 the apparent rate constant (kapp) of this reaction in the presence of GNP-film-1 on negatively charged quartz glass is calculated from Figure 12B. We obtained a value of kapp = 1  10-2 s-1 (Table 4). Similarly, the obtained values of kapp of this reaction using GNPsfilm-1 on the positively charged glass slide and GNP-film-1 on the neutral hydrophobic surface were 3.7  10-2 and 2.6  10-2 s-1, respectively (Table 4). The latter two kapp values were calculated from Figures S14 and S15 in the SI. Several other research groups have also studied the catalytic activities of GNPs that bind to various biopolymers such as collagen/chitosen/calcium alginate using this reaction as a model reaction.54,73,74 Furthermore, this model reaction has also been used to examine the (72) Hayakawa, K.; Yoshimura, T.; Esumi, K. Langmuir 2003, 19, 5517. (73) Wei, D.; Ye, Y.; Jia, X.; Yuan, C.; Qian, W. Carbohydr. Res. 2010, 345, 74. (74) Saha, S.; Pal, A.; Kundu, S.; Basu, S.; Pal, T. Langmuir 2010, 26, 2885. (75) Kumar, S. S.; Kumar, C. S.; Mathiyarasu, J.; Phani, K. L. Langmuir 2007, 23, 3401. (76) Liu, W.; Yang, X.; Huang, W. J. Colloid Interface Sci. 2006, 304, 160. (77) Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. C 2010, 114, 8814.

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catalytic activities of GNPs supported on different polymeric substrates.52-54,75-77 The kapp values for GNP-film-1 on different surfaces are higher than those reported for polymer-supported GNPs (Table 4).53,54,76 Table 4 also clearly indicates that the obtained kapp values for our GNP thin films are much higher than those reported for differently sized/shaped gold nanoparticles prepared by different methods (Table 4).27,59,78 Note that in the present case the GNP-film-1 on a quartz cuvette exhibited poor catalytic activity in the second cycle for the same catalytic reaction. However, at this point, we do not know why our catalyst is not active in the second cycle. In this contest, it should be noted that Wei et al. have also observed a similar deactivation of the chitosen gold nanocomposite catalyst when tested in the second cycle of the same catalytic reaction.73 They proposed that the product obtained from the reaction may absorb onto the surface of the catalyst, poisoning it and resulting in a loss of activity.

Conclusions A newly designed redox-active tetrabutylphosphonium citrate ([TBP][Ci]) ionic liquid with thermal stability up to 160 °C has been successfully used in situ to deposit GNP thin films on glass substrates whose surfaces are positively charged, negatively charged, or neutral hydrophobic in nature. It has also been demonstrated that this technique can be efficiently used to coat different commercially available unfunctionalized substrates (polystyrene, polypropylene, plain paper, and cellophane paper) with a thin film of GNPs. This IL also has the capability to produce GNPs in an aqueous colloidal suspension. An FESEM study showed that the deposited GNP thin films on various surfaces were made up of mostly spherical GNPs that were well distributed throughout the film. It has also been observed that some of the GNPs are organized to form arrays. The as-formed GNP thin films on a glass/quartz surface have been successfully and efficiently used for the determination of the refractive indices of various organic solvents. Again, we have demonstrated that the GNP thin films on a quartz surface can be utilized as an excellent catalyst for the borohydride reduction of 4-NP. The advantages of this technique over the other chemical and physical methods for film formation are as follows: (i) This is a simple method for producing GNP thin films via a greener approach and requires only one step. (ii) The technique is so general and easy that it can applied to coat almost any surface with a thin film of GNPs. (iii) No complex chemistry is involved in functionalizing the glass/quartz surface for GNP film (78) Huang, T.; Meng, F.; Qi, L. J. Phys. Chem. C 2009, 113, 13636.

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formation. (iv) The as-formed GNP thin films can be directly used to determine the refractive indices of liquids/solvents. Therefore, this technique does not require any sophisticated instrumentation or laser-light setup. (v) The as-formed GNP thin films can be directly utilized in catalyzing the organic reaction, which means that no further isolation/purification of the catalyst is required. Acknowledgment. E.D., Md.H.R., and M.B. thank the CSIR, Government of India for providing fellowships. This research was partially supported by grants from CSIR, India. We are also grateful for financial support from the DST, New Delhi, India under the Nanoscience and Nanotechnology Initiative.

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Supporting Information Available: ATR-IR spectrum of a TEOS-modified glass surface, NMR spectrum of IL, TGA thermograms of IL and citric acid, time-dependent kinetic plot of GNP formation, UV-vis spectra of GNP thin films, AFM images of GNP-film-1, XRD patterns of GNP-film-1 on different glass surfaces, photographs of GNP-film-1 on various substrates, UV-vis absorption spectra, calibration curve for RI measurements, variation of λmax of GNP-film-1 on hydrophobic and neutral surfaces with different organic solvents, and catalytic data of GNP-film-1 on positive and neutral glass surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(22), 17568–17580