Anchoring Gold Nanoparticles to Gold Surfaces

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and TEMPO thiol were produced with in a ligand exchange reaction similar to that described by .... F. Minisci, F. Recupero, G. F. Pedulli andM. Lucarini, J. Mol.
ECS Transactions, 35 (25) 39-45 (2011) 10.1149/1.3655509 © The Electrochemical Society

Anchoring Gold Nanoparticles to Gold Surfaces through Nitroxyl Radicals O. Święch, A. Kaim,N. Hrynkiewicz-Sudnik, B. Pałys, R. Bilewicz Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

Abstract This report describes the interaction of the gold surface with nitroxyl radicals that can be the basis of a new method for binding gold nanoparticles to gold substrates The gold surface is separated from the gold nanoparticle layer by thiolated TEMPO radicals in the sandwich-like system. TEMPO derivatives that separate gold electrode from the layer of gold nanoparticles were oriented either to or from electrode. In both cases however nitroxyl radicals play the role of binding units.The sandwich assemblies of both types were studied by cyclic voltammetry, scanning tunneling microscopy and far-infrared reflectance spectroscopy proving high efficiency of the method sincebinding of nanoparticles by means of nitroxylmoiety resulted in higher population of nanoparticles at the electrode surface when compared to the conventional dithiol approach. Introduction There are many reports on constituents of the investigated assemblies containing free or mobile electronsconstructed from gold nanoparticles(NPs) and the gold surface.The stable radical used in this work, TEMPO (2,2,6,6-tetramethypiperidine-1-oxyl radical), and its derivatives have been applied as spin labels (1), spin traps (2), antioxidants (3), mediators in “living/controlled” free radical polymerization (4), catalysts in oxidation processes of primary and secondary alcohols to corresponding aldehydes and ketons (5), and key components for electrode-active organic coatings (6).Systems of gold NPs or plates grafted with organic molecules bearing mobile electrons were proposed as components for various type of switchings and spintronic devises (7,8,9,10,11). So, these interesting technological prospects prompted us to studycomposite systemsthat could combine the functionalities mentioned above. Thus, our aim was to prepare gold nanoparticles (NPs) modified with a TEMPO derivative - alkanethiol mixed monolayer and anchor them to gold electrodes in order to produce sandwich-like composites consisting of two conductive planes separated by an organic linker.

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ECS Transactions, 35 (25) 39-45 (2011)

Experimental Gold electrode was prepared for electrochemical measurements according to standard procedures. Gold NPs protected with monolayer of butanethiol(C4SHthiol)or dodecanethiol(C12SHthiol) were prepared using a modified Brust- Schiffrin method (12,13).Gold nanoparticles densely covered (Figure 1) with mixed layer of alkanethiol and TEMPO thiol were produced with in a ligand exchange reaction similar to that described by Chechik et al. (14).

Figure1. The thermogravimetrical curve for NPs covered with the C12SH/TEMPO thiollayer indicating high 28,33% organic contents in the system. Decomposition onset at 212.20C. Source of free radicals in our systems,bisnitroxide disulfide(Figure 2),in short TEMPO DiSS, was synthesized according to the procedure given by Matyjaszewski et. al. (15) O

O

N

S

O

S

O

N

O

O

Figure 2. Bis[2-(4-oxy-2,2,6,6-tetramethylpiperidine-1-oxyl)ethyl] disulfide (TEMPODiSS) used in a ligand exchange reaction to decorate NPs with TEMPO thiol ligands. Results and Discussion Interaction of Protected Gold Nanoparticles with Gold Electrode The voltammetric measurements present evidences for interaction of gold nanoparticles with gold electrode by way of nitroxyl radicals (Figure 3). A pair of peaks, A1/C1 at 0.950 V, characteristic of the TEMPO/TEMPO+ redox couple, proves the adsorption of TEMPO-decorated NPs on the surface of the bare gold electrode.

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ECS Transactions, 35 (25) 39-45 (2011)

Figure3. The voltammetric curve for a bare gold electrode modified with NPs covered with mixed butanethiol/TEMPO thiol monolayer (Figure 4) (A); desorption voltammograms for the gold electrode after 18h contact with a solution of nanoparticles covered with: a) C4SH/TEMPO thiol, b) C12SH/TEMPO thiol and c) C4SH/toluene. We verified the conclusion by comparing it with desorption voltamogramms(Figure 3, Part B).The desorption peak at -1 V was observed for both C4SH/TEMPOthiol (curve a)and C12SH/TEMPO thiol-protected NPs self-assembled on gold electrodes (curve b).

Figure4.Gold electrode covered with C4SH/TEMPOthiol or C12SH/TEMPO thiol monolayer. However, no desorption peak was registered for gold electrodes after contact with a solution of C4SH NP devoid of TEMPO radicals. This result excludes any physical adsorption of nanoparticles without nitroxide radicals on the bare gold electrode surface. It also confirms that the presence of TEMPO groups in the coating area of the NP is indispensable to adsorb NPs on the gold surface. The STM images showed that the bare gold electrode surface is tightly packed gold nanoparticles after contact with a solution of C4SH/TEMPO thiol NPs (Figure 5A). The

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ECS Transactions, 35 (25) 39-45 (2011)

bond between the gold electrode surface and gold NPs is strong enough for the nanoparticle layer could not be removed even by thorough rinsing of the gold surface with toluene and ethanol. However, in the case of C12SH/TEMPOthiol-modifiedNPs, the STM image showed the presence of only a few NPs (Figure 5B). The reason is that the TEMPO groups are buried in the C12SH diluent, which makes their access to the gold electrode surface much more difficult.

Figure5.STM images of gold electrodes coated with gold nanoparticles protected with C4SH/TEMPO thiol (A) and C12SH/TEMPO thiol (B) mixed monolayers. NP deposition time: 18h. The voltammogram of the electrode covered with TEMPO thiol monolayer only shows a reversible pair of peaks, A1/C1, at 0.907 (Figure 5a) . The peak current increases visible after contact with a solution of NPs covered with the C4SH/TEMPO thiol monolayer (Figure 6b). The increase is not very big as there is only about 10% terminal TEMPO groups in the coating area of NPs. Additionally, the background is 3 times higher because of nanostructuringof the electrode surface by nanoparticles. However, in total, the results prove that the number of free nitroxide groups in the system increased.

Figure 6. Voltammograms of TEMPO/TEMPO+ electrode processes in a TEMPO thiol monolayer-covered Au electrode. Self-assembly solution: 5mM TEMPO thiol in

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ECS Transactions, 35 (25) 39-45 (2011)

AN. Voltammograms recorded before (A) and after 18 h of contact with a toluene solution of C4SH/TEMPO thiol (B). Experiments were performed to compare the binding ability of the gold electrode modified with 1,9-nonanedithiol and TEMPO as the dithiol approach is one of the most common method to attach functional groups to the gold electrode surface. Here, we compare the values of surface concentration of NPs TEMPO nitroxyls on the gold electrode modified with 1,9-nonanedithiol and TEMPO thiol immersed in a toluene solution of NPs protected with C4SH/TEMPO thiol and C12SH/TEMPO thiol (Figure 7).From the values of TEMPO thiol surface concentrations calculated from the area under the TEMPO oxidation peak (not shown) arises that linking through nitroxyl radicals leads to gold surfaces 5-8 times more densely covered with NPs than in the case of binding by an alkanedithiol (Figure 8).In addition, the STM pictures recorded for the 1,9nonanedithiol-modified electrode after 18 h of contact with NPs protected with a C4SH/ TEMPO thiol mixed monolayer revealed that NPs agglomerate and occupy mainly the edges of the terraces and cracks of the electrode. The results showed however that the binding of NPs is possible with both long and short alkanethiols as diluents.

Figure7.NPs TEMPO nitroxyls on the gold electrode modified with TEMPO thiol(A) and 1,9-nonanedithiol (B).

Figure 8. STM images of a gold electrode coated with a monolayer of 1,9nonanedithiol and Au NPs protected with a C4SH/TEMPO thiol monolayer.NP deposition time: 18 h.

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ECS Transactions, 35 (25) 39-45 (2011)

According to the literature data the NO-Au bond can be interpreted as a result of two phenomena: the exchange interaction between the unpaired electron of TEMPO and conduction-band electrons of the metal (including Au) postulated by Freed et al. (16) and Zhang et al. (17), and complex formation of gold with oxoammoniumcation formed by nitroxyl radical in the adsorbed state proposed lately by Krukowskiet al. (18).The NO-Au bond can be seen in the FAR IR reflectance spectra (Figure 9).

Figure 9. IR spectra of an Au electrode after 18 h of contact with (a) TEMPO- and (b) C4SH/TEMPO thiol-protected NP solution and spectra of TEMPO thiol SAMs on a Au electrode before (c) and after (d) 18 h of contact with C4SH/TEMPO thiol-protected NP solution in toluene. The spectrum a shows the spectrum of the gold electrode after 18h contact with a toluene solution of TEMPO molecules, thus, not containing thiol groups. Apart from a group of three peaks at frequencies of 530 cm-1, 470 cm-1and 445 cm-1 that can be assigned to ONC and NCC deformation modes (they were not recorded in absence of gold), a single peak at the frequency 280 cm-1 corresponds to the binding of NO-Au (electrode). The ONC and NCC deformation modes are shifted toward lower frequencies (in the range 380 – 435 cm-1) in the spectra of gold electrode immersed for 18h in the C4SH/TEMPO thiol-protected NP solution (spectrum b). The frequency shift is probably caused by the interaction with gold NPs and the C4SH neighboring the TEMPO molecules. Spectrum c was recorded for the gold electrode coated with monolayer of TEMPO thiols only. In this case an intense peak at 360 cm-1 corresponding to S (thiol)-Au (electrode) bond. When the electrode was then contacted with a toluene solution of NP C4SH/TEMPO (spectrum d), peaks at frequency 420 cm-1 and 280 cm-1 were again observed, hence corresponding to the TEMPO molecule deformation modes and the NOAu (electrode) bond, respectively. The low intensity of both peaks can be explained by small population of NO group in the mixed monolayer covering the NPs and, consequently, in a small number of NO-Au electrode bonds. In addition, good organization of the TEMPO thiol monolayer at the gold electrode can also weaken the absorption band, providing vertical orientation of the bound molecules.

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ECS Transactions, 35 (25) 39-45 (2011)

Conclusions Following main conclusion can be drawn from the presented study: - gold NPs modified with derivative of TEMPO can attach to gold surface by NO-Au bond, - monolayer of TEMPO thiol derivative at gold surface is able to bind gold NPs, - linking through nitroxyl radicals leads to gold surfaces more densely covered by NPs than it is the case of binding by an alkanedithiol, - C4SH/TEMPO thiol-protected NPs can be useful for the construction of sandwichlike assemblies consisting of a gold NPs layer and a bare or protected gold surface, in both the to and from approach. Acknowledgments This work was supported by the Polish Ministry of Sciences and Higher Education and The National Center for Research and Development (NCBiR), grant NR05-0017-10/2010 (PBR-11) References 1. P. P.Borbat, J.Costa-Filho,K. A.Earle, J. K.Moscicki andJ. H.Freed, Science,291, 266 (2001). 2. R. P. Mason, Free Radical Biol. Med.,36, 1214 (2004). 3. J. B.Mitchell, M. C.Krishna, P.Kuppusamy, J. A.Cook and A. Russo,Exp. Biol. Med. (Maywood, NJ, U.S.), 226, 620 (2001). 4. C. J. Hawker, A. W.Bosman,E. Harth, Chem. Rev.,101, 3661 (2001). 5. F. Minisci, F. Recupero, G. F. Pedulli andM. Lucarini, J. Mol. Catal. A: Chem.,204_205, 63 (2003). 6. I.Takeshi, B.Rainer, B.Frings, A.Lachowicz, K.Soichi and N. Hiroyuki,Chem. Commun. 46, 3475 (2010). 7. J. Y. Ouyang and Y. Yang, Appl. Phys. Lett., 96, 063506 (2010). 8. S. Sek, R. Bilewicz and K. Slowinski, Chem. Commun., 404 (2004). 9. K Stolarczyk and R. Bilewicz, Electrochim. Acta, 51, 2358 (2004). 10. T. Sugawara and M. M. Matsushita,J. Mater. Chem., 19, 1738 (2009). 11. S. Sek, A. Tolak, A. Misicka and R. Bilewicz,J. Phys. Chem. B, 109, 18433 (2005). 12. M.Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman,. J. Chem. Soc., Chem. Commun., 801 (1994). 13. M. Wojcik, W. Lewandowski, J. Matraszek,J. Mieczkowski, J. Borysiuk, D. Pociecha and E.Gorecka, Angew. Chem., Int. Ed., 48, 5167 (2009). 14. P. Ionita, A. Caragheorgheopol, B. C. Gilbert andV. Chechik,J. Am. Chem. Soc., 124, 9048 (2002) 15. R. Nicolay, L. Marx, P. Hémery and K.Matyjaszewski, Macromolecules, 40, 9217(2007). 16. P. G.Barkley, J. P. Hornak and J. H. Freed, J. Chem. Phys., 84, 1886 (1986). 17. Z. Zhang, A. Berg, H. Levanon, R.W. Fessenden, andD. Meisel, J. Am. Chem. Soc., 125, 7959 (2003). 18. P.Krukowski, P. J. Kowalczyk, P. Krzyczmonik, W. Olejniczak, Z. Klusek, M. Puchalski and K. Gwozdzinski, Appl. Surf. Sci.,255, 3946(2009).

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