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Electrochemical Nucleation of Gold Nanoparticles in a Polymer Film at a Liquid-Liquid Interface Rene´ Knake,† Amir W. Fahmi,‡ Syed A. M. Tofail,† Jason Clohessy,† Miroslav Mihov,† and Vincent J. Cunnane*,† Materials and Surface Science Institute, University of Limerick, Limerick, Ireland, and School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. Received July 9, 2004. In Final Form: October 28, 2004 Simultaneous nucleation of gold nanoparticles and polymerization of tyramine has been carried out at an immiscible electrolyte interface. By transferring the gold ion of tetraoctylammoniumtetracloroaurate (TOAAuCl4) from the organic to the aqueous phase, a fast homogeneous electron transfer from the tyramine monomer reduces the gold ion. Electropolymerization then proceeds, and gold nanoparticles form. The newly formed nanoparticles act as nucleation sites for the deposition of the oligomers/polymer (and possibly vice versa). This results in gold nanoparticles stabilized in a polytyramine matrix. The size of the nanoparticles is controlled by the concentration of oligomers/polymer in solution. The polymer nanoparticle composite film was analyzed with TEM, XPS, and AFM.

Introduction In recent years, nanoparticle research has witnessed tremendous growth. The vast number of publications on nanoparticles demonstrates the versatility of synthesis procedures and the variety of potential applications for nanoparticles. To prepare nanoparticles, approaches such as controlled chemical reduction,1 electrochemical reduction,2 metal vaporization techniques,3 sonochemical process,4 and γ-irradiation technique5 have been successfully developed. In addition, during nanoparticle synthesis, surfactants have been used as surface-capping agents in order to stabilize the nanoparticle and to prevent coagulation. Such covered particles have been produced mainly by using citrate,6-8 thiol,9-12 or dendrimer13-15 capping compounds. * Author to whom correspondence should be addressed. Tel.: +35361202640. Fax: +353 61 202912. E-mail: vincent.cunnane@ ul.ie. † University of Limerick. ‡ University of Nottingham. (1) Pileni, M. P. Langmuir 1997, 13, 3266. (2) Bandyopadhyay, S.; Chakravorty, D. J. Mater. Res. 1997, 12, 2719. (3) Devenish, R. W.; Goulding, T.; Heaton, B. T.; Whyman, R. J. Chem. Soc., Dalton Trans. 1996, 673. (4) Kumar, R. V.; Diamant, Y.; Gedanken, A. Chem. Mater. 2000, 12, 2301. (5) Yin, Y.; Xu, X.; Ge, X.; Xia, C.; Zhang, Z. Chem. Commun. 1998, 1641. (6) Frens, G. Nature-Phys. Sci. 1973, 241, 20. (7) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55. (8) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (9) Xu, W.; Liu, W.; Zhang, D. Q.; Xu, Y.; Wang, T. X.; Zhu, D. B. Colloids Surf., A 2002, 204, 201. (10) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Supramol. Sci. 1997, 4, 329. (11) Zhang, P.; Kim, P. S.; Sham, T. K. Appl. Phys. Lett. 2003, 82, 1470. (12) Manna, A.; Chen, P. L.; Akiyama, H.; Wei, T. X.; Tamada, K.; Knoll, W. Chem. Mater. 2003, 15, 20. (13) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256. (14) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157. (15) Taubert, A.; Wiesler, U. M.; Mullen, K. J. Mater. Chem. 2003, 13, 1090.

Comparable to thiol capping, polymer capping has the added advantage that it allows the preparation of highly ordered colloidal monolayers and nanoparticle-coated surfaces, which are attractive candidates for the construction of nanoscaled devices for various applications. Polymer-protected nanosized noble metal colloids are usually prepared from suitable metal precursors (mainly HAuCl4) by various reactions that occur in the presence of the protective polymer16 (poly(methyl methacrylate), PMMA) or monomer (aniline species17and pyrrole18-21) or with the help of γ-radiation22 (acrylamide as the monomer). Other methods are the synthesis of coated nanoparticles and their further polymerization23 (pyrrole species) and the incorporation of derivatized gold nanoparticles into a polymer.24 Synthesis of nanoparticles, in particular electrochemical synthesis at liquid-liquid interfaces, is a relatively new field of research. While electron- and ion-transfer reactions at the interface between two immiscible electrolyte solutions (ITIES) have been studied extensively on simple reversible systems,25-29 electrochemical synthesis is only described in a few limited cases. The first metal deposition (16) Liu, F. K.; Hsieh, S. Y.; Fu-Hsiang, K.; Chu, T. C.; Dai, B. T. Jpn. J. Appl. Phys., Part 1 2003, 42, 4147. (17) Tan, Y. W.; Li, Y. F.; Zhu, D. B. Synth. Met. 2003, 135, 847. (18) Selvan, S. T.; Nogami, M. J. Mater. Sci. Lett. 1998, 17, 1385. (19) Selvan, S. T.; Hayakawa, T.; Nogami, M.; Moller, M. J. Phys. Chem. B 1999, 103, 7441. (20) Henry, M. C.; Hsueh, C. C.; Timko, B. P.; Freund, M. S. J. Electrochem. Soc. 2001, 148, D155. (21) Bhattacharjee, R. R.; Chakraborty, M.; Mandal, T. K. J. Nanosci. Nanotechnol. 2003, 3, 487. (22) Ni, Y. H.; Ge, X. W.; Zhang, Z. C.; Ye, Q. Mater. Lett. 2002, 55, 171. (23) Hata, K.; Fujihara, H. Chem. Commun. 2002, 2714. (24) Peng, Z. Q.; Wang, E. K.; Dong, S. J. Electrochem. Commun. 2002, 4, 210. (25) Geblewicz, G.; Schiffrin, D. J. J. Electroanal. Chem. 1988, 244, 27. (26) Cunnane, V. J.; Geblewicz, G.; Schiffrin, D. J. Electrochim. Acta 1995, 40, 3005. (27) Cunnane, V. J.; Schiffrin, D. J.; Beltran, C.; Geblewicz, G. J. Electroanal. Chem. 1988, 247, 203. (28) Zamec, Z.; Maracek, V.; Koryta, J.; Khalil, M. W. J. Electroanal. Chem. 1977, 83, 393. (29) Cheng, Y.; Schiffrin, D. J. J. Electroanal. Chem. 1991, 314, 153.

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(Cu) study was carried out by Guainazzi et al.30 In recent years, Cheng and Schiffrin31 have shown that it is possible to deposit metallic gold nanoparticles at the water/1,2dichloroethane (DCE) interface (Schiffrin initiated work on the chemical production of gold nanoparticles32). Johans et al. have reported in several publications the nucleation of palladium nanoparticles at an electrified liquid/liquid interface and have developed theoretical models for such electrodeless nucleation.33-35 Dryfe’s group has characterized Pd and Pt nanoparticles electrodeposited at bare and templated liquid/liquid interfaces.36-38 Furthermore, silver particles have been formed at micro and nano liquid/liquid interfaces.39 Electropolymerization at the ITIES was shown for the first time by Cunnane and Evans,40 where pyrrole oligomers were formed in the organic phase by electron transfer between an aqueous-based redox system and an organic-based monomer units. Polymer layers with derivatives of pyrrole have been formed at a liquid/liquid interface.41 Recently poly(2,2′:5′2′′-terthiophene)42,43 has been prepared electrochemically in a similar way. From these, separate metal nucleation and polymerization studies have investigated the concept to combine both innovations to synthesize polymer-covered nanoparticles (or nanoparticles in a polymer matrix) by using suitable precursors in the aqueous (w) and organic phase (o), respectively. The electrosynthesis of polyphenylpyrrolecoated silver nanoparticles 44 starting from Ag2SO4 (w) and phenylpyrrole (o) was the first outcome arising from this approach. Gold is the most commonly studied metallic species for nanoparticle research, and therefore, attempts have been made to extend this new method to incorporate gold into a polymer matrix. Initially, it was shown that the simple replacement of silver ions by gold ions (in the form of AuCl4-) in the aqueous phase does not lead to the desired result. A spontaneous precipitation takes place on contacting the aqueous gold solution with an organic solution containing a wide range of suitable monomers. This is due to the ease of transfer of the AuCl4- ion from the aqueous phase and its high standard electrode potential of approximately 1 V vs SHE.45 These properties have been used to form gold nanoparticle composites from a simple AuCl4- and monomer system17-21 in a single-phase system. The reverse approach of utilizing an organicsoluble gold species and an aqueous-soluble monomer (30) Guainazzi, M.; Silvestri, G.; Serravalle, G. Chem. Commun. 1975, 200. (31) Cheng, Y. F.; Schiffrin, D. J. J. Chem. Soc., Faraday Trans. 1996, 92, 3865. (32) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801. (33) Johans, C.; Lahtinen, R.; Kontturi, K.; Schiffrin, D. J. J. Electroanal. Chem. 2000, 488, 99. (34) Johans, C.; Kontturi, K.; Schiffrin, D. J. J. Electroanal. Chem. 2002, 526, 29. (35) Johans, C.; Liljeroth, P.; Kontturi, K. S. PCCP 2002, 4, 1067. (36) Platt, M.; Dryfe, R. A. W.; Roberts, E. P. L. Chem. Commun. 2002, 2324. (37) Platt, M.; Dryfe, R. A. W.; Roberts, E. P. L. Electrochim. Acta 2003, 48, 3037. (38) Platt, M.; Dryfe, R. A. W.; Roberts, E. P. L. Electrochim. Acta 2004, 49, 3937. (39) Guo, J.; Tokimoto, T.; Othman, R.; Unwin, P. R. Electrochem. Commun. 2003, 5, 1005. (40) Cunnane, V. J.; Evans, U. Chem. Commun. 1998, 19, 2163. (41) Maeda, K.; Janchenova, H.; Lhotsky, A.; Stibor, I.; Budka, J.; Marecek, V. J. Electroanal. Chem. 2001, 516, 103. (42) Gorgy, K.; Fusalba, F.; Evans, U.; Kontturi, K.; Cunnane, V. J. Synth. Met. 2002, 125, 365. (43) Evans-Kennedy, U.; Clohessy, J.; Cunnane, V. J. Macromolecules 2004, 37, 3630. (44) Johans, C.; Clohessy, J.; Fantini, S.; Kontturi, K.; Cunnane, V. J. Electrochem. Commun. 2002, 4, 227. (45) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill: New York, 1992.

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forms the basis of the present work. As such, tetraoctylammoniumtetrachloroaurate (TOAAuCl4)san organic and ionic-soluble Au species which was previously used to deposit Au nanoparticles31shas been favored. Tyramine (4-hydroxyphenylethylamine) a biogenous, water-soluble monomer, which has been electropolymerized at low oxidation potentials in acidic and basic solutions, is the redox species of choice in the aqueous phase. The chemical and electrochemical properties of tyramine and polytyramine have been previously described.46-48 The polymer film shows a low conductivity if polymerized at a solid electrode. As a result of its structure, the polymer film presents one reactive amine group per moiety and has been used both as an immobilization matrix for enzymes and oligonucleotides48 and also for the construction of biosensors.49-52 The object of the present work was to demonstrate that liquid-liquid electrochemistry opens a new and controlled way to produce metal-polymer nanocomposites and a basis to form thin layers of metallic nanoparticle-embedded polymer layers. Experimental Section Li2SO4 (99%, Aldrich) and tyramine (99%, Aldrich) were used in the aqueous phase. Millipore water (18 MΩ cm) used throughout was prepared using the Maxima ultrapure water system (Elga). Tetraoctylammoniumteracloroaurate was synthesized as previously described31 from HAuCl4.3 H2O (Aldrich) and tetraoctylammonium bromide (99%, Fluka). The organic base electrolyte used, tetraphenylarsonium tetrakis (pentafluorophenyl) borate (TBAsTPBF20) was prepared from tetraphenylarsoniumchloride (TPACl) (97%, Aldrich) and lithium tetrakis(pentafluorophenyl) borate etherate (LiTPBF20) (Boulder Scientific Co.) by a standard procedure.42 1,2-Dichlorethane (DCE) (99,5%, Fluka) was used as received in all experiments. TetrapropylammoniumTPBF20 was synthesized by metathesis of tetrapropylammoniumchloride (TPrACl) (in water) and LiTPBF20 (in DCE). A typical four-electrode cell configuration for liquid-liquid studies was used.53 The interfacial area was approximately 0.2 cm2. The electrochemical cells are chemically described as follows

Ag|AgCl| 1 mM TPAsCl (w) | 1 mM TPAsTPBF20 (o) + 0.2 mM TOAAuCl4 (o) || 10 mM Li2SO4(w) | Pt (Cell 1) Ag|AgCl| 1 mM TPAsCl (w) | 1 mM TPAsTPBF20 (o) + 0.2 mM TOAAuCl4 (o) || 10 mM Li2SO4(w) + x mM tyramine (w) | Pt (Cell 1a) Ag|AgCl| 1 mM TPAsCl (w) | 1 mM TPAsTPBF20 (o) + 0.1 mM TPrATPBF20 (o) + 0.2 mM TOAAuCl4 (o) || 10 mM Li2SO4 (w) + x mM tyramine (w) | Pt (Cell 1b) where || indicates the polarizable liquid/liquid interface and x indicates the initial concentrations of tyramine in each experiment. (46) Dubios, J. E.; Lacaze, P. C.; Pham, M. C. J. Electroanal. Chem. 1981, 117, 233. (47) Pham, M. C.; Lacaze, P. C.; Dubois, J. E. Electrochim. Acta 1984, 131, 77. (48) Tran, L. D.; Piro, B.; Pham, M. C.; Ledoan, T.; Angiari, C.; Dao, L. H.; Teston, F. Synth. Met. 2003, 139, 251. (49) Situmorang, M.; Gooding, J. J.; Hibbert, D. B.; Barnett, D. Electroanalysis 2002, 14, 17. (50) Situmorang, M.; Gooding, J. J.; Hibbert, D. B.; Barnett, D. Electroanalysis 2001, 13, 1469. (51) Situmorang, M.; Gooding, J. J.; Hibbert, D. B. Anal. Chim. Acta 1999, 394, 211. (52) Palmisano, F.; DeBenedetto, G. E.; Zambonin, C. G. Analyst 1997, 122, 365. (53) Samec, Z.; Marecek, V.; Koryta, J.; Khalil, M. W. J. Electroanal. Chem. 1977, 83, 393.

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Cyclic voltammetry studies were carried out using an Autolab PGSTAT 100 controlled by GPES 4.9 for Windows software. All potentials reported have been corrected to the Galvani scale by using a standard ion transfer potential of ∆w°φTPrA+ ) -93 mV upon addition of TPrACl to the aqueous phase in selected experiments. A platinum microelectrode with 25 µm diameter (BAS) has been used for the determination of the electrochemical properties of tyramine in a standard three-electrode cell combined with platinum wire as counter electrode and SCE as reference, as described in cell 2.

SCE | 10 mM Li2SO4(w) + 5 mM tyramine (w) | Pt (Cell 2) To produce sufficient amounts of nanocomposites for TEM, XPS, and AFM, analysis partitioning ion experiments27 (electrodeless electrochemistry) have been utilized to control the electrochemical potential established at the interface (due to the small interfacial area in Cell 1 and with it the small amount of transferring material insufficient material could be produced). The cells used to establish electrochemically suitable potentials, are

Figure 1. Potential window of Cell 1 in the absence (dashed line) and presence of 0.2 mM TOAAuCl4 (o) (solid line).

1 mM TPAsTPBF20 (o) + 0.2 mM TOAAuCl4 (o) + 1 mM LiTPBF20 (o) || 10 mM Li2SO4 (w) + 1 mM tyramine (w) (Cell 3)

H2O/H2O2/NH4OH 30% solution for 20 min at 65 °C. Then, the Si substrate was rinsed with water several times and dried in a nitrogen gas stream.

1 mM TPAsTPBF20 (o) + 0.2 mM TOAAuCl4 (o) + 1 mM TPrATPBF20 (o) || 10 mM Li2SO4 (w) + 1 mM tyramine (w) + 2 mM TPrACl (w) (Cell 4) respectively utilizing Li+ and TPrA+ as partitioning ions, which allows a wide range of Galvani potentials to be established. For this purpose, 5 mL of organic and aqueous solutions were filled in a test tube, the mixture was shaken intensely, and the reaction was allowed to proceed for some days. Duplicate experiments were also performed at an undisturbed interface, i.e., without shaking. The resulting Au-nanoparticle-polymer films were transferred onto 3 mm carbon covered copper grids (400 mesh) (Agar Scientific Ltd.) lying on absorbent paper by carefully pipetting a small drop of the solution from the interfacial region. The deposit was washed with water and DCE to ensure that no electrolyte salt was left on the surface and was dried at room temperature. TEM images were recorded with a transmission electron microscope (JEOL 2010). Samples for XPS were prepared in a similar way on aluminum foil. XPS analysis was performed by using a Kratos Axis 165 spectrometer. A monochromatic Al KR (excitation energy ) 1486.6 eV) radiation source at a voltage of 15 kV and a current of 5 mA was used. The energy scale calibration of the instrument was performed by taking the Cu 2p 3/2 and Au 4f 7/2 peak position at 20 eV pass energy and checking the linearity of the energy scale with the Ag 3d 5/2 peak. The machine work function was then set to ensure the correct binding energy position of Ag 3d 5/2 peak. The intensities have been corrected for photoionization cross sections to work out the elemental concentration. Individual narrow-range spectra for the elements of interest were acquired at high resolution with 20 eV pass energy, 0.05 eV step size, and 100 ms dwell time. Due to the insulating nature of the sample under investigation, neutralization of surface charge was necessary. This was accomplished by the use of a charge neutralizer, which is coaxial to the electron optic system and creates a cloud of low-energy electrons on the surface of the sample to be analyzed. The settings of the charge neutralizer were optimized so as to achieve an overcompensation of charge by 2-3 eV. The spectra acquired in this way were then corrected using the C 1s position at 285 eV as reference. AFM analysis was performed in air using a Digital Instruments Multimode instrument using Si cantilevers (spring constant 40 N m-1, resonance frequency of 150-190 kHz) working in tapping mode (TMAFM). The Au-polymer film was either transferred directly onto the substrate or into toluene and dispersed in an ultrasonic bath. A spin-coating method was also used to spread the colloidal nanoparticles onto wide areas of SiOx wafers. The SiOx surface was prepared by soaking the silicon disk in 5:1:1

Results and Discussion The potential window and the transfer of the AuCl4ion in the absence of tyramine are shown in Figure 1. The dotted line shows a region of approximately 600 mV of low current density, where no electron or significant ion transfer occurs. The potential window is limited by the transfer TPAs+ from the organic to the aqueous phase at negative potentials and the transfer of Li+ from the aqueous to the organic phase at positive potentials. Upon addition of TOAAuCl4 (full line) to the organic phase, a reversible ion transfer of AuCl4- at approximately 92 mV can be obtained. By convention, the current direction for a negatively charged ion (AuCl4-) transferring from the organic to the aqueous phase is a positive current. This charge-transfer reaction is characterized by the typical 60 mV peak-to-peak separation attributed to a single charge transfer. It should be mentioned at this point, that sometimes a depressed prewave (at about -107 mV) is observable.31,54 The presence of this prewave has been discussed in the literature, and the currents observed are dependent on the AuCl4- concentration.31 The addition of tyramine to the aqueous phase of the liquid-liquid system (in the absence of TOAAuCl4) does not effect any changes to the base electrolyte cyclic voltammograms. In Figure 2, the electrochemical behavior of tyramine in a simple three-electrode cell with a platinum microelectrode as the working electrode is shown. The irreversible electropolymerization sets in at a very low potential of about 130 mV vs SCE. Values between -50 and 700 mV vs Ag/AgCl have been measured at different electrode/ solvent combinations.48,50,51,55-57 The generated polymer film effectively covers the electrode after the first sweep; the second sweep (dashed line) shows greatly decreased activity. Using appropriately small concentrations of monomer in the aqueous phase in combination with TOAAuCl4 in (54) Cheng, Y.; Cunnane, V. J.; Schiffrin, D. J.; Mutoma¨ki, L.; Kontturi, K. J. Chem. Soc., Faraday Trans. 1991, 87, 107. (55) Losic, D.; Shapter, J. G.; Gooding, J. J. Electrochem. Commun. 2002, 4, 953. (56) Inoue, T.; Kirchhoff, J. R.; Hudson, R. A. Anal. Chem. 2002, 74, 5321. (57) Situmorang, M.; Gooding, J. J.; Hibbert, D. B.; Barnett, D. Biosens. Bioelectron. 1998, 13, 953.

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Figure 2. Oxidation (and polymerization) of 5 mM tyramine in 10 mM Li2SO4; scan rate 50 mV sec-1, 25 µm Pt microelectrode (Cell 2). First (solid line) and second scan (dashed line) are shown.

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Figure 4. Effect of tyramine concentration. Voltammograms (second sweep) without tyramine (solid line), 0.5 mM tyramine (dashed line), 1 mM tyramine (dotted line) and 10 mM tyramine (dashed-dotted line); scan rate 50 mV sec-1 (Cell 1a).

The homogeneous reaction between tyramine and AuCl4- may be described by an initial electron transfer from the tyramine to the gold ion, resulting in reduction of the gold ion to metallic gold and simultaneous initiation of the radical cation polymerization of tyramine. The driving force of the overall reaction, approximately -80 kJ mol-1, calculated from the redox potentials, is high due to the low standard electrode potential of tyramine in comparison to the high potential of AuCl4- ion. The overall reaction scheme may be expressed by the following equations:

AuCl4- (o) a AuCl4- (w) Ion transfer AuCl4- (w) + 3H-Mon (w) f Au (s) + 3Mon• (w) + 3H+ + 4ClHomogeneous electron transfer

Figure 3. Effect of the addition of 1 mM tyramine into the aqueous phase in the presence of 0.2 mM TOAAuCl4 and 0.1 mM TPrATPBF20 in organic phase, 5 consecutive scans; A indicates forward and B indicates return peak for AuCl4-; scan rate 25 mV sec-1 (Cell 1b).

Mon• (w) + xMon (w) f Dimer f Oligomer f Polymer (s) Polymerization

the organic phase, the liquid-liquid reaction may be followed by cyclic voltammetry. Figure 3 shows five consecutive sweeps of Cell 1a, where the tyramine concentration is equal to 1 mM. With every sweep as the potential is swept positive of 90 mV, AuCl4- ion is transferred to the aqueous phase (see position A in Figure 3). Not only can a reduction (or the ultimate disappearance) of the return peak (see position B in Figure 3) be seen but the forward peak also gets smaller with each further sweep (see position A in Figure 3). This effect on the AuCl4- ion transfer is, on one hand, due to a fast reaction of the transferred AuCl4- with the excess tyramine in the aqueous phase and, on the other hand, is due to the formation of a layer at the interface which is blocking the interface for further transfer. The blocking of the interface by a formed layer is demonstrated by the addition of TPrA+ to the electrochemical cell (Cell 1b). The ion transfer of TPrA+ is not influenced by any chemical reactions with the tyramine monomer, but with increasing number of sweeps (and the gradual formation of a blocking polymer layer), the increase in peak-to-peak separation and the formation of a current plateau (Figure 3) show typical behavior of a blocked interface.

Other possible reaction schemes would include a heterogeneous electron transfer (HET) reaction pathway and a pathway involving the possible partition of tyramine into the organic phase. However, the charge transfer potential is clearly that of the AuCl4- ion, and the subsequent decrease in current with each sweep is therefore associated with the ion transfer reaction (a slow HET is evident in the partitioning ion experiments [see below]; however, this aspect is not evident in the time scale of the present voltammetric experiments). A potential homogeneous reaction in the organic phase initiated by tyramine partition would lead to the generation of H+ and Cl- ions in the organic phase. These ions would transfer to the aqueous phase. However, these ions have markedly different transfer potentials to those seen in Figure 3, and as such, there is no evidence for such a pathway. A contribution of alternative pathways to that described above, however, cannot be completely ruled out. The use of different concentrations of tyramine in the aqueous phase influences the electrochemical system dramatically. In Figure 4, the second sweep of voltammograms using different tyramine concentrations are shown for Cell 1a (during the first scan, all forward peaks

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Figure 5. Sweep rate dependence of Cell 1a from 2.5 to 200 mV sec-1, tyramine concentration ) 0.4 mM.

Figure 7. (a) TEM picture of the Au-polymer film electrodeposited at the water/DCE interface, Cell 3 shaken. (b) Size distribution.

Figure 6. Plot of peak current ratio vs log(kfτ); experimental data for 0.2 (0) and 0.4 mM (*) tyramine and theoretical fit (solid line).

are similar to the forward peak of the charge transfer due to AuCl4- in the absence of tyramine and are dependent only on the initial AuCl4- concentration). The decreasing peak height with increasing monomer concentration shows that the AuCl4- ion is reacting within the time frame of the sweep, once a sufficient monomer concentration is available. When a concentration of 0.5 mM tyramine is used with 0.2 mM TOAAuCl4 (used throughout), a scaleddown return peak can be obtained, while at a 1mM monomer concentration, the return and forward peaks are reduced to a minimum. In the case of an excess of 10 mM tyramine, no ion transfer can be seen (on the second sweep) and the sweep indicates a highly capacitive (blocked) interface. The extent of the irreversible reaction is also dependent on the switching time. In Figure 5, the voltammetric curves of scan rates between 2.5 and 200 mV sec-1 are shown for Cell 1a, where the tyramine concentration equals 0.4 mM. By choosing a low tyramine concentration, it is possible to obtain stable voltammetric scans which were then analyzed as shown in Figure 6. The peak current ratio of forward and return scan (A and B in Figure 3) is dependent on the switching time (τ, the time that elapses between the forward and return peaks), indicating a homogeneous reaction mechanism. Attempts to fit the measured values to the theory suggested by Nicholson et al.58 results in a partial fit. From the partial fit, a heterogeneous forward rate constant, kf, of 0.8 × (58) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706.

10-1 s-1 was obtained. This value indicates a fast reaction in comparison to a kf of 0.8 × 10-3 s-1 for the generation of silver particles in a similar system.44 Only a partial fit is obtained due to the following reasons: the measurement at shorter switching times (to get values smaller than -0.5 for log(kfT)) by using a faster sweep rate (>200 mV sec-1) is impractical, as no peaks can be obtained. Additionally, the current ratios at longer switching times do not fit the theory. This is due to the low monomer concentration in comparison to the AuCl4- concentration. At longer switching times, the reaction becomes more dependent on the monomer concentration. At this point, there is simply insufficient monomer to react with the AuCl4- ions. As shown in Figure 6, the use of higher monomer concentration improves the fit when compared with 0.2 mM monomer, otherwise higher concentrations lead to a total disappearance of the return peak, as was also observed at slower scan rates (see Figures 3 and 4). To obtain sufficient nanocomposite material for further analysis, partitioning ion experiments were carried out. By this method, ions, which partition between two phases at a liquid-liquid interface, set up an interfacial Galvani potential difference. In Cell 3, a common ion, Li+, in the form of Li2SO4 (w) and LiTPBF20 (o) sets up a Galvani potential of approximately 450 mV44 (as anticipated from the voltammogram in Figure 1) which is more positive than the AuCl4- transfer. In the shaken test tube experiments, multiple swirled, dark violet polymer films are to be seen after 1 day at the interface. It takes several days to complete the formation of a (closed, rough) layer at the interface of approximately 1 mm thickness. The reaction is considered to be complete when the yellow organic phase (TOAAuCl4) is completely colorless. A wide difference in the formed film exists by the two methods. The thin films obtained in shaken test tubes look (optically) homogeneous (unvarying colors), whereas a thicker layer, obtained from the unshaken test tubes, is inhomogeneous such that a striation is evident in the sequence (bottomup): DCE phaseswhite polymer excesssviolet gold excesssaqueous phase. This unpredictable striation is in particular remarkable because this enrichment occurs in the opposite phase to that where the educts originate and

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Figure 8. AFM of the Au-polytyramine film produced at the liquid-liquid interface on a SiOx slide.

may be explained by density differences or possibly by hydrophilic-hydrophobic properties of the nanocomposites. The use of TPrA+ as partitioning ion (interfacial potential -93 mV) utilized in Cell 4 should inhibit the transfer of AuCl4- from the organic to the aqueous phase (as the potential established is negative to that of the ion transfer potential) and with it the polymerization and nucleation reaction. However, the establishment of such a low potential does not completely block the ion-transfer process, and a slow reaction occurs, resulting in the formation of the first polymer-particle composites after 1 week in the shaken test tubes, possibly due to a very slow heterogeneous electron-transfer reaction. The results obtained with Cell 4 were in fact similar to these obtained, with a control sample where no partitioning ion is present and the galvanic potential is undefined. The thin film prepared in the manner of Cell 3 was analyzed by TEM. Figure 7a shows an almost equal distribution of the small gold nanoparticles in the polymer matrix. The particle size is in the range from 2 to 13 nm, with a majority of particles of about 8 nm. The usual occurrence of aggregation of the particles is not obtained and is inhibited by the fixation of the Au particles in the polymer film. Figure 7b shows the size distribution for circular particles only. It shows a broad distribution from 2 to 12 nm, with maxima at 2 and 8 nm. A possible reason for this bimodal distribution may be due to two different reaction mechanisms: one forms Au seeds that act as nucleation sites for further Au and polymer deposition, and/or formed polytyramine deposits can act as nucleation sites for the Au atoms, leading to differently sized particles. Beside the circular particles, some elliptical particles can be seen in Figure 7. Their smaller diameter is in the range of 6-12 nm, and the size contribution is in agreement with the circular ones. The formation of elliptical particles may be reasoned by the aggregation of two circular particles or an overlay of two particles, which cannot be distinguished by simple TEM techniques.

Figure 9. AFM of Au-nanoparticle dispersed (spin coated) on SiOx: (a) 2D amplitude. (b) 3D high; peak height is approximately 7 nm.

The AFM image in Figure 8 of the polymer-nanoparticle film shows a closed ‘rough’ layer (ra ) 255 nm) on a SiOx substrate. The roughness may be caused by overlapping of several smaller layers during drying on the substrate. Also, the sample was shaken several times and subjected to turbulences during the formation of the film at the liquid-liquid interface. However, by employing spincoating techniques, single metallic particles can be obtained on the wafer surface. The particles in Figure 9 have a lateral size of approximately 70 nm diameter, while their height is approximately 7 nm. While the measured width is affected by the tip geometry, the height correlates with the results of the TEM analysis. An XPS survey scan is shown in Figure 10, and it can be noted that the main elements that are present in the Au-polytyramine film can be ascertained. Various photoelectron peaks of Zn, F, O, N, C, B, Al, and Au can be

Electrochemical Nucleation of Gold Nanoparticles

Langmuir, Vol. 21, No. 3, 2005 1007 Table 1. Elements Present in the XPS Sample element

C

N O F

Figure 10. XPS survey scan of the polytyramine-Au film. Beside the expected Au, C, N, and O peaks, several signals from the matrix (Al and Zn) and F are obtained.

identified. The presence of Al and Zn arise from the surface of the Al plate. The electrolyte used for electropolymerization contained both F and B, and this accounts for their presence in the scan. The proportions of C, O, and N were found to be approximately 64%, 20%, and 5%, respectively. Of considerable interest is the higher amount of F (∼11%) and the very small amount of Au (0.33%). Figure 11a shows the high-resolution spectra in the C 1s region. A main peak at around 285 eV and a broad shoulder, typical of polytyramine,48 at the higher binding energy side dominate the spectra. The C 1s spectrum has been decomposed into a number of components, the binding energy and relative amount of which are listed in Table 1. The main C1s peak at 284.9 eV is due to C bonded to C. The aromatic nature of the C-C bonding can be seen from the presence of a

bonding

binding energy (eV)

relative amount (at.%)

C*-C C*-N C*-O-C C*-F RCOOH Sat1 N*-C N+ C-O*-C RCOOH F*-C

284.9 285.6 286.5 287.7 289.2 291.6 400.257 402.102 533.311 531.661 687.8

25.89 7.25 18.22 11.17 1.06 0.37 4.46 0.63 18.93749 0.993511 10.69

satellite peak (291.6 eV) due to a π f π* transition in the aromatic ring. A small contribution (7.25 at.%) from C bonded to N can be seen at around 285.6 eV. The peak appearing at 286.5 eV has been ascribed to C singly bonded to O, in the form of C-O-C. This inter-ring O in polytyramine is similar to that found for the ester functional group, and the binding energy corresponds to that published in the literature for polytyramine48 or ester.59 Please note that the combined contribution of C bonded to N and C bonded to O as C-O-C to that of C-C is approximately 1, indicating two couplings per monomer. The theoretical ratio for a single coupling per monomer is 0.6, while that for two couplings per monomer is 1.0.48 There is also a contribution (about 11 at.%) from F single bonded to C. Apart from this and a small contribution from carboxylic acid (1.06 at.%), the film was found to be reasonably free from other C contaminations. The correspondence of the results of such decomposition of the C 1s peak can be found from the O 1s and N 1s spectra (Figure 11b and 11c) and the relative amounts of the synthetic components of these spectra in Table 1. For example, the O 1s spectra have been decomposed into two components: the major one being that of the O inter-ring,

Figure 11. High-resolution XPS spectra of the informative elements for the structure determination of the Au-polymer composite.

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in the form of C-O-C between two tyramine monomers (19 at.%). There is also a small contribution (∼1 at.%) from the O in the carboxylic acid. The amount of N bonded to C (∼4.5 at.%) is lower than that calculated for the C bonded to N and indicates that the amount of C bonded to N is overestimated. Most N, however, is in the form of neutral amine, as the amount of protonated amine (N+ in Figure 11c) is very low, only approximately 12% of the total N, indicating a weak protonation of the amine group. The Au 4f spectra show a possible indication of oxidation and/or a small size effect. The main Au 4f 7/2 and Au 4f 5/2 doublet appears at 84.4 (Au1 in Figure 11d) and 88.1 eV, respectively, and thus experience a shift of approximately 0.4 eV to the higher binding energy, in all probability due to oxidation. The shoulder, approximately 1 eV higher in binding energy than the main peaks, might arise from Au in two different chemical states (core and shell of the nanoparticle) or due to electrically disconnected Au particles charging during the measurement.60 The rela(59) Gardner, S. D.; Singamsetty, C. S. K.; Booth, G. L.; He, G.-R.; Pittman, C. U., Jr. Carbon 1995, 33, 587. (60) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H. G.; Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D. S.; Schlogl, R.; Yasuda, A.; Vossmeyer, T. J. Phys. Chem. B 2003, 107, 7406.

Knake et al.

tively small amount of Au (0.33 at.%) had negligible influence on the O 1s spectra. Conclusion An electrified liquid-liquid interface has been used to produce gold-polytyramine films. The controlled transfer of AuCl4- ions induces a homogeneous electron transfer reaction between the gold ion and the monomer. The gold nanoparticles, in the 10 nm size scale, are stabilized in the polytyramine film. A significant amount of the nanocomposite can be prepared electrochemically in an electrodeless system by the use of partitioning ions. The formed nanocomposite is reasonably homogeneous. The polymer appears to have two couplings per monomer, and the N content is predominantly neutral amine. There is some evidence of the oxidation of the Au colloids; however, this aspect needs further investigation. Acknowledgment. The author thanks the European Commission for funding the SUSANA Research and Training Network (Supramolecular Self-Assembly of Interfacial Nanostructures) Project No. HPRN-CT-200200185. LA048277Q