Gold Nanoparticles Tethered to Gold Surfaces Using Nitroxyl Radicals

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Gold Nanoparticles Tethered to Gold Surfaces Using Nitroxyl Radicals Olga Swiech, Natalia Hrynkiewicz-Sudnik, Barbara Palys, Andrzej Kaim, and Renata Bilewicz* Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland ABSTRACT:

We describe the interaction of Au with >NO radicals—the basis of a new method of binding gold nanoparticles to gold substrates. In the sandwich-like system, the gold electrode is separated from the gold nanoparticle layer by thiolated TEMPO radicals. The formation of the NO
Au bond was confirmed by FAR IR spectroscopy. The properties of the Au substrate
Au nanoparticle assembly were studied by cyclic voltammetry and scanning tunneling microscopy. Binding of nanoparticles by means of nitroxyl radicals instead of linking them using an alkanedithiol leads to a higher population of nanoparticles at the electrode surface, as shown by scanning tunneling microscopy.

’ INTRODUCTION TEMPO (2,2,6,6-tetramethypiperidine-1-oxyl radical) and its derivatives have been widely used as spin labels,1 spin traps,2 and antioxidants3 in biomedical applications; mediators in “living/ controlled” free radical polymerization;4 catalysts in aerobic oxidation processes;5 and key components for electrode-active organic coatings.6 They were used for the studies of the kinetics of surface partitioning and lateral diffusion of redox active amphiphiles in the aqueous/vapor interfacial region.7 Thiolated TEMPO molecules have been anchored on gold electrodes as self-assembled monolayers and studied as versatile electroactive centers in aqueous and organic media.8
10 Zhang and co-workers used stable paramagnetic probes to study interactions of gold nanoparticles and nitroxyl radicals.11
13 Interaction between nitroxyl radicals, such as TEMPO and TEMPAMINE, and gold nanoparticles (NPs) resulted in the loss of the electron paramagnetic resonance (EPR) signal. Following Freed and coworkers,14 these authors suggested that the exchange interaction of unpaired electrons with conduction-band electrons of the metallic particle is responsible for eliminating the EPR signal. More recently, Krukowski and co-workers used scanning tunneling microscopy (STM), cyclic voltammetry (CV), and EPR spectroscopy to investigate the behavior of 2,2,6,6-tetramethylpiperidine, known as TMP, on Au(111). The authors suggest that TMP could be oxidized to the nitroxyl TEMPO radical, r 2011 American Chemical Society

which adsorbs on Au in the form of an oxoammonium cation. Such a oxoammonium cation has been postulated to form a complex with gold at 0.5 V that is easily desorbed during STM imaging.15 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 devices consisting of two conductive planes separated by an organic linker. Such nanostructured electrodes with a TEMPO organocatalyst can be employed for the chemoselective oxidations, for example, of primary and secondary alcohols.15 In the present study, bis[2-(4-oxy-2,2,6,6-tetramethylpiperidine-1-oxyl) ethyl] disulfide (TEMPO-DiSS) (Figure 1a) and alkanethiols with 3 (C4SH) (Figure 1c) or 11 (C12SH) (Figure 1d) methylene units in the chain were used for the formation of monolayerprotected nanoparticles building one of these planes. Bare gold, 1,9-nonanedithiol, or TEMPO thiol-modified gold electrodes formed the other side of the sandwich. CV, STM, and infrared spectroscopy (FAR IR) revealed the interaction of the gold nanoparticles with the gold electrode by means of the TEMPO radical. This new way of binding AuNPs to gold Received: January 26, 2011 Revised: March 13, 2011 Published: March 25, 2011 7347

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Figure 1. Structures of (a) bis[2-(4-oxy-2,2,6,6-tetramethylpiperidine-1-oxyl)ethyl] disulfide (TEMPO-DiSS) and Au nanoparticles protected with (b) C4SH, (c) C4SH/TEMPO thiol, and (d) C12SH/TEMPO thiol monolayers.

surfaces was compared with AuNP attachment by alkanedithiol linkers. Both the TEMPO-modified nanoparticles and the surfaces of electrodes covered by TEMPO-modified nanoparticles can find several applications. The use of the nanostructures in controlled “living” radical polymerization will lead to well-defined polymer chains on the gold surface. These hybrid structures with planned architectures containing polymeric materials and metallic cores should have different properties, for example, higher strength and modified conductivity, compared with the conventional polymers. Blends of polymers, for example, polystyrene, with gold nanoparticles capped with the appropriate thiol can serve as electrode-sensitive bipolar resistive switchings.16 Thus, instead of blends, TEMPO-coated gold nanoparticles grafted with polystyrene chains synthesized according to nitroxide mediated radical polymerization (NMRP) will probably enhance the processing and chemical stability of the devices. TEMPO thiol attached with the thiol terminal group to the gold NP and with the other terminal moiety, NO, to another NP or the gold electrode can provide new types of networked asymmetric NP junctions.17
19 Gold nanoparticles coated with TEMPO radicals can be employed as a new class of spin markers, for the studies of dynamics of biological systems, for example, membranes and proteins, and new materials for nanoparticlebased spintronics.20

’ MATERIALS AND METHODS Synthesis of Bis[2-(4-oxy-2,2,6,6-tetramethylpiperidine1-oxyl)ethyl] Disulfide (TEMPO-DiSS). TEMPO-DiSS was

synthesized according to the procedure given by Matyjaszewski et al.21 A round-bottom flask was filled with 3,3-dithiodipropionic acid (6.3084 g), 4-hydroxy-TEMPO (15.5032 g),

dicyclohexylcarbodiimide (18.5826 g), and 4-(dimethylamino) pyridine (2.209 g) and purged for 15 min with dry oxygen-free argon. Anhydrous tetrahydrofuran (50 mL) was then added. The mixture became turbid and orange-red. Subsequently, the content of the flask was stirred at room temperature. After stirring for 12 h, the reaction mixture was filtered and the solvent vaporized, which created a red oil. On the TLC plate (cyclohexane/ethyl acetate, 8:2), apart from obtaining the required product (TEMPO-DiSS), a trace of an unreacted substrate of 4-hydroxy-TEMPO was observed. Flash chromatography was used to purify this biradical: the column was filled with Al2O3 (activity I), and cyclohexane/ethyl acetate 8:2 was used as a developing system. The product was recrystallized from n-hexane. Orange crystals (4.0066 g, yield = 26%) with the melting point of 94.4 °C were then obtained. MS (Mass Spectrometer Quattro LC), 1H NMR, and 13C NMR (Varian Unity Plus 200 MHz, CDCl3, in the presence of phenyl hydrazine) proved the TEMPO-DiSS identity. Preparation of Nanoparticles Covered with C4SH, C12SH, C4SH/TEMPO Thiol, and C12SH/TEMPO Thiol. Gold NPs were prepared as previously described22 using a modified Brust
Schiffrin method.23 An aqueous solution of hydrogen tetrachloroaurate (90 mL, 30 mmol dm
3) was extracted (three times with 200 mL) with a toluene solution of methyltrioctylammonium bromide (5.38 g, 1.38 mmol). Most of the tetrachloroaurate ions were transferred to the organic layer as the yellow aqueous layer faded until colorless. Alkanethiol was added to the toluene solution (2:1 mol ratio of alkanethiol to AuCl4
), and the mixture was reduced at 15 °C with a freshly prepared aqueous solution of sodium borohydride (1.40 g, 30 mmol in 10 mL of deionized H2O). A solution of borohydride was quickly added under vigorous stirring. After further stirring for 3 h, the organic phase was separated, washed with pure water (2  50 mL), evaporated to 5 mL in a rotary evaporator, and mixed with 200 mL of 7348

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The Journal of Physical Chemistry C absolute ethanol to precipitate NPs. The mixture was kept for 12 h at
4 °C. The dark brown precipitate was sonicated for 60 s and centrifuged (5 min, 13 000 rpm). Again, precipitate was dissolved in a small amount of toluene (5 mL), precipitated with ethanol (100 mL), and centrifuged. Finally, all samples were dissolved in 20 mL of hexane and centrifuged for 5 min. The procedure was repeated until no trace of excess thiol was found, as determined by 1H NMR spectra and TLC. Obtained nanoparticles were characterized by X-ray measurements. The gold core size was ca. 2 nm. Ligand-Exchange Reaction. An organic ligand (20 mg, bis[2-(4-oxy-2,2,6,6-tetramethylpiperidine-1-oxyl)ethyl] disulfide, DiSS) was added to the toluene solution (20 mL) of starting gold NPs (20 mg), and the reaction proceeded upon mixing at room temperature for ca. 72 h. As no precipitation and/or color change occurred, we inferred that this ligand-exchange reaction was not accompanied by side effects, such as irreversible aggregation or significant gold core size modifications. The clear burgundy solution was rotary evaporated, and the blackish solid was dissolved in a minimum volume of toluene (max. 2 mL). In the next step, the NPs were precipitated using acetone, cooled to 4 °C, and centrifuged 2 h later (13 000 rpm, 5 min). This washing process was repeated four times until no trace of excess DiSS was found, as determined by TLC. Preparing Electrode Surfaces Coated by NPs Modified with C4SH, C4SH/TEMPO Thiol, C12SH/TEMPO Thiol, and 1,9-Nonanedithiol. For electrochemical measurements, a gold electrode (BAS) was polished mechanically with 1.0, 0.3, and 0.05 μm alumina powder on a Buehler polishing cloth to obtain a gold mirror. After several washings with water, the electrodes were placed in an ultrasound bath for 10 min. Next, electrochemical cleaning was carried out in 1 M NaOH by holding the electrode at
1.5
1V for 60 s and scanning five times in the potential range of 1.5
1V. The last step of the electrode pretreatment was electrochemical cycling in 0.5 M H2SO4 from
0.2 to 1.6 V until the characteristic shape of the voltammogram for a gold electrode was observed.24 For the SPM experiments, gold evaporated on glass slides with a chromium underlayer was used (Arrandee) and annealed several times over a flame and then in an oven at 700 °C for 30 min. For the IR experiments, the gold electrodes were cleaned in a mixture of 25% ammonia, 30% hydrogen peroxide, and water in a 1:1:5 ratio at 80 °C for 30
60 min. Pretreated electrodes were immediately used for monolayer deposition. Self-assembly of the thiolated TEMPO derivative (TEMPO thiol) on the gold electrode was carried out in a 5 mM solution of TEMPO-DiSS in acetonitrile for 18 h at room temperature. During self-assembly, the solution was deoxygenated using argon. For further comparison experiments, the monolayer of nonanedithiol was also self-assembled from 1 mM 1,9-nonanedithiol solution in ethanol for 24 h. The electrodes were immersed into the solution of nanoparticles in toluene for 18 h; after vigorous washing with toluene and ethanol, they were used in the experiments. Electrochemistry. Electrochemical measurements were performed using the PGSTAT Autolab (Eco Chemie BV, Utrecht, The Netherlands). All electrochemical experiments were done in a three-electrode arrangement with a silver/silver chloride (Ag/ AgCl) electrode as the reference, the platinum foil as the counter, and the Au electrode (BAS, 2 mm diameter) as the working electrode. The supporting electrolyte solution was 0.1 M TBAHFP in acetonitrile.

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Scanning Tunneling Microscopy. STM measurements were performed using a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) and commercially available Pt
Ir tips. The images were taken under ambient conditions. Infrared Reflection Adsorption Spectroscopy. The infrared (IR) experiments were carried out using the Nicolet 8700 spectrometer (Thermo Scientific). The reflectance spectra of layers adsorbed on plates were recorded using the variable-angle accessory by Thermo Electron. For the far IR range (200
600 cm
1), the DTGS detector with a polyethylene window was used. The spectra in the mid-IR range (400
4000 cm
1) were recorded using the DTGS detector with a KBr window.

’ RESULTS AND DISCUSSION Interaction of Gold Nanoparticles Covered with C4SH, C4SH/TEMPO Thiol, and C12SH/TEMPO Thiol with a Bare Gold Electrode Surface. Figure 2A shows the voltammetric

curve recorded for a bare gold electrode immersed for 18 h in a solution of NP C4SH/TEMPO thiol. A pair of peaks, A1/C1 at 0.950 V, characteristic of the TEMPO/TEMPOþ redox couple, proves the adsorption of TEMPO-decorated NPs on the surface of the bare gold electrode. When C12SH was used as the diluent in the monolayer protecting the NPs, the A1/C1 peaks could not be resolved from the high background current (not shown). However, the desorption peak at
1 V was observed for both C4SH/TEMPO thiol- and C12SH/TEMPO-protected NPs selfassembled on gold electrodes. (Figure 2B, curve b) It confirmed the presence of some adsorbed nanoparticles on the bare gold surface also for C12SH/TEMPO-protected NPs, hence covered with a long-chain diluent thiol. No desorption peak was observed for gold electrodes after 18 h of contact with a solution of C4SH NP devoid of TEMPO radicals (Figure 2B, curve c). This result excludes any physical adsorption of monolayer-protected nanoparticles 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. Therefore, a nitroxyl radical
gold electrode interaction was considered (Figure 3). This suggested that C4SH/TEMPO thiol-protected NPs, with the short alkanethiol as the diluent in the monolayer, could be useful for the construction of a sandwich-like assembly consisting of a gold NP layer and a gold electrode. The gold electrodes covered with these NPs were, therefore, imaged by STM. After 18 h of contact with a solution of C4SH/TEMPO thiol NPs, the nanoparticles were tightly packed on the bare gold electrode surface (Figure 4A) and could not be removed by thorough rinsing of the gold surface with toluene and ethanol. However, in the case of C12SH/TEMPO thiol-modified NPs, the STM image showed the presence of only a few NPs (Figure 4B), which was attributed to the too long alkyl chains of the diluent C12SH. The TEMPO groups were buried in the C12SH diluent, which made their access to the gold electrode surface much more difficult (Figure 3). Interaction of Gold NPs Covered with C4SH, C4SH/TEMPO Thiol, and C12SH/TEMPO Thiol with a TEMPO Thiol-Modified Electrode. To confirm binding through nitroxyl radicals, we conducted a series of experiments with electrodes covered by TEMPO thiol monolayers. When thiol and TEMPO terminal groups are competing in binding to the gold substrate, the TEMPO groups of the monolayer are directed toward the solution while thiol binds to the gold electrode. The 7349

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Figure 2. (A) Voltammetric curves recorded using a gold electrode exposed for 18 h to a solution of NPs protected with a C4SH/TEMPO thiol mixed monolayer. (B) Desorption voltammograms recorded in 1 M NaOH for gold electrodes after 18 h of immersion in the toluene solution of NPs protected with monolayers of (a) C4SH/TEMPO thiol, (b) C12SH/TEMPO thiol, and (c) C4SH. Supporting electrolyte: 0.1 M TBAHFP in AN. Scan rate: 1 V/s.

Figure 3. Scheme of adsorption of gold nanoparticles covered with thiolated TEMPO radicals on (bare) gold.

voltammogram of the TEMPO thiol monolayer-covered electrode shows a reversible pair of peaks, A1/C1, at 0.907 V (Figure 5). The average surface concentration and area per one molecule of TEMPO thiol are 4.39 ( 0.35  10
10 mol/cm2 and 42.0 ( 4.3 Å2, respectively. The peak current increases after contact with a solution of NPs covered with the C4SH/TEMPO thiol monolayer for 18 h (Figure 5). For all monolayers, values of surface concentrations, calculated from the area under the thiol desorption peaks at ca.
1 V and under the peak of the nitroxyl radical oxidation at ca. þ0.9 V, are compared (Table 1). Increased surface concentration, calculated from the total thiol desorption charges, was also noted for the TEMPO thiol monolayer-modified electrodes following contact with NPs

protected with a one-component C4SH monolayer (Table 1), proving adsorption of these NPs on the monolayer-coated gold surface. STM images taken for electrodes modified with a TEMPO derivative and covered with C4SH and C4SH/TEMPO thiolprotected nanoparticles showed high surface coverage by NPs and confirmed their binding by the TEMPO terminal groups of the monolayer covering the electrode. Interaction of Gold NPs Protected by C4SH, C4SH/TEMPO Thiol, and C12SH/TEMPO Thiol with the Electrode Covered with a 1,9-Nonanedithiol Monolayer. The common binding procedure via alkanedithiol was employed for all three types of NPs in order to compare the efficiency of binding with nitroxyl 7350

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Figure 4. 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: 18 h.

Figure 5. Voltammograms of TEMPO/TEMPOþ electrode processes in a TEMPO thiol monolayer-covered Au electrode. Self-assembly solution: 5 mM TEMPO-DiSS in AN. Voltammograms recorded before (a) and after 18 h of contact with a toluene solution of C4SH/TEMPO thiol (b) or C4SH (c) protected NPs. Supporting electrolyte: 0.1 M TBAHFP in AN. Scan rate: 1 V/s.

radical bonding. Figure 6 shows the voltammetric curves recorded for a 1,9-nonanedithiol-modified gold electrode immersed for 18 h in a solution of NP: C4SH/TEMPO thiol (Figure 6, curve a) and C12SH/TEMPO thiol (Figure 6, curve b). Gold NPs covered with a C4SH/TEMPO thiol monolayer gave a pair of peaks at a formal potential equal to 0.950 V, identical with the formal potential for the same NPs immobilized on bare gold surfaces. Gold NPs coated with mixed monolayers with a longer alkanethiol—C12SH/TEMPO thiol—gave a pair of peaks with a formal potential equal to 0.993 V. The more positive value potential of the A1/C1 couple reflects the more hydrophobic environment of the nitroxyl radical groups. The values of surface concentrations calculated from the voltammetric curves of the TEMPO moiety oxidation (A1 peak) and desorption curves (C2 peak) are compared in Table 1. The alkanedithiol approach allows the binding of NPs protected with both long and short alkanethiols

as diluents, but the population of NPs is always larger in the case of nitroxyl radicals as the binding unit. STM pictures recorded for the 1,9-nonanedithiol-modified electrode after 18 h of contact with NPs protected with a C4SH/ TEMPO thiol mixed monolayer confirm that the electrode surface is less densely covered with the NPs than in cases of the bare gold substrate or modified with TEMPO thiol. In addition, NPs are shown to agglomerate and occupy mainly the edges of the terraces and cracks of the electrode (Figure 7). IR Studies of NP-Covered Electrodes. To prove that binding of NO
Au is responsible for the immobilization of the NPs at the gold electrode surface, the FAR IR spectra were recorded. The curve a in Figure 8 shows the spectrum of the gold electrode after 18 h of contact with a toluene solution of TEMPO molecules not containing thiol groups. For these molecules, the bond of NO
Au (electrode) is the only means to immobilize 7351

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Table 1. Values of TEMPO Thiol Surface Concentrations Calculated from the Area under the TEMPO Oxidation Peak (ΓTEMPO) and the Thiol Desorption Peak (Γthiol) covered with NPs C4SH/TEMPO thiola

without NPs

C12SH/TEMPO thiolb

C4SHc

Γthiol [mol/cm2] ΓTEMPO [mol/ Γthiol [mol/cm2] ΓTEMPO [mol/ Γthiol [mol/cm2] ΓTEMPO [mol/ Γthiol [mol/cm2] ΓTEMPO [mol/ from desorption electrode Au alkanedithiol-


10

(10

)

cm2] TEMPO
10

groups (10

8.23 ( 0.31

)

from desorption
10

(10

)

cm2] TEMPO
10

groups (10

57.8 ( 5.7 17.8 ( 1.5

1.48 ( 0.11 0.78 ( 0.07

18.1 ( 4.0

4.66 ( 0.3

)

from desorption
10

(10

)

11.2 ( 2.5 10.9 ( 1.1

cm2] TEMPO
10

groups (10

)

from desorption cm2] TEMPO (10
10)

0.34 ( 0.05

11.8 ( 1.2

4.12 ( 0.36

12.9 ( 3.6

groups (10
10)

modified Au TEMPO thiol-

4.84 ( 0.86

4.39 ( 0.35

4.18 ( 0.6

3.45 ( 0.2

modified Au a C4SH/TEMPO thiol: NPs modified with a mixed monolayer of butanethiol and TEMPO thiol. b C12SH/TEMPO thiol: NPs modified with a mixed monolayer of dodecanethiol and TEMPO thiol. c C4SH: nanoparticles modified with a monolayer of butanethiol.

them on the surface of gold. The spectrum is characterized by a group of three peaks at frequencies of 530, 470, and 435 cm
1 and a single peak at the frequency of 280 cm
1, which corresponds to the binding of NO
Au (electrode). The observed vibration at frequencies higher than 430 cm
1 can be assigned to ONC and NCC deformation modes. In the spectrum of TEMPO registered in the KBr pellet (not shown), wide overlapping bands between 450 and 600 cm
1 were also observed. The appearance of that group of bands in the TEMPO spectrum in the absence of a gold surface proves that the peaks at frequencies of 530, 470, and 435 cm
1 cannot correspond to the molecule
gold surface interaction. The ONC and NCC deformation modes shift toward lower frequencies in the spectra of the TEMPO monolayer-covered electrode immersed for 18 h in the C4SH/TEMPO thiolprotected NP solution (Figure 8, spectrum b). The frequency shift is probably caused by the interaction with gold NPs and the C4SH neighboring the TEMPO molecules. Indeed, this type of interaction may lead to the high population of NPs observed in the STM images (Figure 4). In addition, the spectra of a pure gold electrode after placing on it C4SH NPs in toluene solution and evaporating the solvent (not shown) do not show the peaks at frequencies about 400 and 280 cm
1, confirming the assignment of these vibration frequency peaks to binding by means of the NO
Au interaction of the NPs modified with TEMPO thiol. Spectrum c in Figure 8 was recorded for the gold electrode coated with a monolayer of TEMPO thiol. In this case, we observed the intense peak at a frequency of 360 cm
1, which corresponds to the S
Au bond. This confirms that thiols have a larger affinity to the gold substrates than the nitroxyl moiety. After 18 h of contact with a toluene solution of NP C4SH/ TEMPO, we observed the appearance of peaks at a frequency of 420 cm
1, hence corresponding to that of the ONC and NCC deformation modes, and peaks at a frequency of 280 cm
1, corresponding to the NO
Au (NP) bond (Figure 8, spectrum d). The low-intensity peaks compared to the peak of the S
Au bond can be explained by the small population of NO in the mixed monolayer covering the NPs, resulting in a small number of NO
Au electrode bonds and by the good organization of the TEMPO thiol monolayer, providing vertical orientation of the S
Au bound molecules. Selection rules for the reflectance spectra enhance vibration perpendicular to the metal surface.25

Figure 6. Voltammograms recorded in 0.1 M TBAHFP/AN solution using a 1,9-nonanedithiol modified-gold electrode after 18 h of immersion in a toluene solution of NPs protected with (a) C12SH/TEMPO thiol and (b) C4SH/TEMPO thiol. Scan rate: 1 V/s.

Because the fraction of TEMPO molecules bonded to gold NPs oriented parallel to the gold electrode is small, their contribution is not visible in the spectrum.

’ CONCLUSION In summary, we propose building sandwich assemblies consisting of a gold electrode and a layer of gold nanoparticles separated by a TEMPO derivative with the nitroxyl radical playing the role of a binding unit. It is a new binding scheme compared to the common approach based on alkanedithiols. The interactions of Au with the >NO• radicals were noted earlier.14,15 Decreased EPR signals were observed in the gold
TEMPO system and ascribed to the exchange interaction of unpaired electrons with conduction-band electrons of the metallic particle. In the present study, gold
gold NP assemblies separated by thiolated TEMPO radicals have been successfully developed using the to and from approach. In the to approach, Au
NP assemblies are formed due to NO
Au bond via the NO• radical of the TEMPO thiol-modified gold NP pointed toward the bare 7352

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Figure 7. STM images of a gold electrode coated with a monolayer of 1,9-nonanedithiol and Au NPs protected with a C4SH/TEMPO thiol monolayer. NP deposition time: 18 h.

gold (either on NPs or on the Au electrode), then bonding by means of the NO radical can take place, and it is easily recognized in the spectra. The monolayer of TEMPO thiol self-assembled on the gold electrode is able to bind all kinds of Au NPs by means of the same NO
Au binding. We also show that linking through nitroxyl radicals leads to gold surfaces more densely covered by NPs than in the case of binding by an alkanedithiol. The smaller population of NPs in the case of alkanedithiol binding is due to the overly tight packing of 1,9-nonanedithiol molecules on the gold surface and restricted access of the terminal thiol group to the surface of the NPs. Nitroxyl groups present in a small population in the butanethiol monolayer protecting the NPs (ca. 10%) protrude above it and, therefore, provide easily accessible sites for tethering the NPs to the electrode surface.

’ AUTHOR INFORMATION Figure 8. IR spectra of a 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.

gold surface. The complementary from approach generates Au
Au NPs sandwich assemblies in which the NO• radicalforming NO
Au bond resides on the gold electrode surfaces. Thiolated TEMPO adsorbs on the NP via the thiol because of its stronger affinity toward gold. Indeed, the S
Au bond is stronger than NO
Au, and the latter cannot compete in binding to gold. In addition, the TEMPO group is much larger than the thiol group and has bulky substituents: four methyl groups in the ring. Therefore, in the FAR IR spectrum of the TEMPO monolayer, the band corresponding to the NO
Au bond is absent. When all thiol moieties formed from the disulfide groups are anchored on

Corresponding Author

*Tel: þ48 22 8220211. Fax: þ48 22 8225996. E-mail: bilewicz@ chem.uw.edu.pl.

’ ACKNOWLEDGMENT The authors would like to thank Michal Wojcik and Wiktor Lewandowski from the Laboratory of Natural Products Chemistry for their assistance in preparing C4SH and C12SH monolayer-protected nanoparticles. ’ REFERENCES (1) Borbat, P. P.; Costa-Filho, J.; Earle, K. A.; Moscicki, J. K.; Freed, J. H. Science 2001, 291, 266–269. (2) Mason, R. P. Free Radical Biol. Med. 2004, 36, 1214–1223. (3) Mitchell, J. B.; Krishna, M. C.; Kuppusamy, P.; Cook, J. A.; Russo, A. Exp. Biol. Med. (Maywood, NJ, U.S.) 2001, 226, 620–621. 7353

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