Fullerene modification of gold electrodes and gold nanoparticles based on application of aromatic thioacetate-functionalized C₆₀

Abstract

A new procedure for the synthesis of S-acetyl-derivatized fullerene is described. The deprotected aromatic S-acetyl-derivatized fullerene was employed for efficient modification of a gold electrode and gold nanoparticles. The proposed deposition procedure of fullerene derivative on Au surfaces allows the control of the thickness of the self-assembled fullerene layer. The modified surfaces were characterized by electrochemical methods and X-ray photoelectron spectroscopy (XPS). Functionalized fullerenes largely retain favorable redox electronic properties, behaving as an electron sink and revealing 4 reversible sequential 1e electrode processes at slightly more negative potentials than those observed for unsubstituted C₆₀. For the first time, fullerene-capped gold nanoparticles were obtained by a two-step ligand exchange procedure involving substitution of alkanethiol with thiolated fullerene derivative following the synthesis of alkanethiol capped nanoparticles. The ligand exchange procedure was very efficient – the number of fullerene moieties per single gold nanoparticle was found to be as high as 30.

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Introduction

Fullerenes and their derivatives attract reasonable attention due to their unique electronic properties including reversible reduction with up to six electrons per single C₆₀ molecule.¹ This gives them huge potential as building blocks for the construction of nanodevices² including well-ordered monolayers or thin films for practical applications in photovoltaic cells, field effect transistors, sensors, molecular wires or optoelectronics.^3,4

The thiol moiety is the most widely used anchoring group for the modification of metals surfaces. Although the literature on thiol self-assembly on gold surface is very extensive and these structures have been thoroughly investigated, thiol functionalized fullerenes applications were limited to alkyl derivatives.⁵ Due to the conductive and electronic properties, the aromatic thiols are more interesting for future implementations but reports on the subject are scarce⁶ even though linker's structure and length may improve the electrical contact between the fullerene moiety and the gold surface. In this paper we focused on short aromatic linker originating from thioacetate protected thiol functionalized fullerene. In contrary to free thiols which are readily oxidized, the thioesters are much more stable⁷ and can be easily converted into thiols when needed.

Functionalization of gold nanoparticles (AuNPs) with C₆₀ fullerene has been recently subject of a number of reports but usually alkyl thiols were employed^8–12 while aromatic moieties on the gold surface were attached to only indirectly.^13,14

In the present paper, we describe the synthesis of a new C₆₀ fullerene functionalized with a short aromatic thioacetate linker. Following deprotection, the fullerene derivative is used to self-assembly into densely packed fullerene layer on Au (111) surface of gold electrode or gold nanoparticles. Two variants, including the in situ procedure for the deprotection of the thioacetyl-derivatized fullerene by pyrrolidine were elaborated (Fig. 1). Both approaches can be of general interest because of their simplicity and lack of side reactions with the fullerene cage.

Fig. 1 Scheme for preparation of thioacetyl-functionalized C₆₀ fullerene films on the gold surface.

Electronic properties of the novel C₆₀ derivative bound to both gold electrode and AuNPs were investigated using the electroanalytical methods. The gold nanoparticles showed narrow size distribution, well defined composition and high fullerene content, calculated to be ca. 30 fullerene substituents per single AuNP.

Experimental section

Reagents

Acetyl chloride, dimethylformamide (DMF) and pyrrolidine were obtained from Alfa Aesar. Hydrogen tetrachloroaurate(III), 4-(methylthio)benzaldehyde, sodium tert-butyl sulfide (tBuSNa), N-methylglycine, tetrabutylammoniumhexafluorophosphate (TBAHFP), tetraoctylammoniumbromide (TOAB) and toluene were bought from Sigma-Aldrich. Dichloromethane, ethanol, ethyl acetate, methanol, n-hexane, acetonitrile (AN) and sodium sulfate were purchased from POCh (Poland). Fullerene C₆₀, 99.9%, was obtained from MER Corp. DMF and toluene were dried and purified before use, according to standard procedures, other solvents were analytical grade reagents and were used as received.

Synthesis of fullerene thioacetate

4-(S-Acetylthio)benzaldehyde was synthesized using the combination and modification of two methods described before.^15,16 To a suspension of tBuSNa (2.82 g; 25 mmol) in dry DMF (35 mL), a solution of 4-(methylthio)benzaldehyde (1.9 g; 12.5 mmol) in DMF (5 mL) was injected via a syringe while stirring vigorously. Reaction was kept under argon atmosphere at 153 °C for 4 h and suspension became dark brown. It was then cooled to 0 °C and acetyl chloride (4.3 mL; 60 mmol) was added in strong Ar stream. After 45 min, the resulting suspension was poured into 400 mL of water, followed by addition 80 mL of CH₂Cl₂, resulting two phases were stirred for 30 min. and the organic layer was separated, extracted with water (2 × 10 mL) and dried over anhydrous sodium sulfate. After evaporation of the solvent under reduced pressure the purification was accomplished by means of flash column chromatography (3 : 1 n-hexane–ethyl acetate) to give desired product as white solid.

Yield: 78%, mp 46 °C, the mass spectrum (ESI-MS) showed a [M + H]⁺ peak at 181.0 (Fig. S1, ESI†); IR (KBr disk) ν_max (cm⁻¹) 2972.7, 2547.0, 1698.0, 1593.4, 1561.1, 1424.2, 1400.1, 1365.7, 1309.4, 1291.6, 1278.1, 1174.9, 1126.2, 1016.3, 941.1, 859.6, 810.4, 712.1, 625.3, 537.1 (Fig. S5, ESI†); δ ¹H NMR (200 MHz; CDCl₃; TMS) 2.47 (3H, s), 7.58–7.61 (2H, d, J = 8.4 Hz), 7.89–7.92 (2H, d, J = 8.3 Hz), 10.04 (1H, s) ppm (Fig. S7, ESI†); δ ¹³C NMR (125 MHz; CDCl₃) 30.53, 130.02, 134.52, 135.44, 136.47, 191.46, 192.31 ppm (Fig. S8, ESI†).

Preparation of aromatic thioacetate functionalized C₆₀ fullerene (ATF-C₆₀) was accomplished using the modified Prato synthesis.¹⁷ A stirred solution of C₆₀ fullerene (144 mg, 0.2 mmol), 4-(S-acetylthio)benzaldehyde (36 mg, 0.2 mmol) and N-methylglycine (89 mg, 1 mmol) in dry toluene (130 mL) was heated at reflux under argon atmosphere for 8 h. The reaction mixture was then cooled, filtered to remove insoluble precipitate and the solvent was removed under reduced pressure. The residue was chromatographed (2 : 1 n-hexane–toluene) to give pure ATF-C₆₀ as dark brown powder.

Yield: 8%, the mass spectrum (ESI-MS) showed a [M + Na]⁺ peak at 950.7 (Fig. S2, ESI†); IR (KBr disk) ν_max (cm⁻¹) 2946.1, 2841.4, 2777.9, 1708.0, 1488.3, 1461.2, 1423.3, 1332.0, 1177.8, 1114.3, 1089.3, 1016.4, 944.6, 766.2, 728.0, 608.6, 552.4, 526.2, see (Fig. S6, ESI†); δ ¹H NMR (200 MHz; CDCl₃; TMS) 2.41 (3H, s), 2.83 (3H, s), 4.26–4.31 (1H, d, J = 9.4 Hz), 4.97 (1H, s), 4.97–5.02 (1H, d, J = 10.1 Hz), 7.46–7.51 (2H, d, J = 8.5 Hz), 7.88–7.93 (2H, d, J = 7.4 Hz) ppm (Fig. S9, ESI†); δ ¹³C NMR (125 MHz; CDCl₃) 30.30, 40.29, 69.19, 70.14, 83.02, 128.17, 135.70, 135.96, 138.58, 139.93, 141.90, 140.02, 140.05, 142.07, 142.12, 142.16, 142.26, 142.60, 144.43, 145.21, 145.30, 145.34, 145.37, 145.56, 145.57, 145.97, 145.99, 146.17, 146.31, 147.34, 152.92, 153.10, 153.94, 156.11, 193.93 ppm (Fig. S10, ESI†).

Deprotection of fullerene thioacetate

At the very beginning we discarded reported sodium hydroxide¹⁸ and sodium thiomethoxide¹⁹ methods that required use of ethanol and methanol respectively, solvents which synthesized fullerene compound was insoluble in. However, aromatic thioacetate group present in some non-related compounds was previously deprotected also in solvents suitable for most fullerene derivatives using tetrabutylammonium hydroxide (TBAH)²⁰ or aliphatic amines, i.e. triethylamine.^20,21 Seemingly, synthesized fullerene thioacetate should follow the same route. Unexpectedly, we observed difficulties that turned out to be inherent to aromatic thioacetate fullerene derivatives while trying to apply these common deprotection procedures. Despite good solubility of fullerene thioacetate in tetrahydrofuran the use of TBAH solution was unable to deprotect ATF-C₆₀ because brown precipitate formed soon after addition of tetrabutylammonium hydroxide solution. The probable explanation for this fact can be that TBAH is well known catalyst of fullerene hydroxylation²² and some insoluble fullerenol was formed instead of desired free thiol. The second studied deprotection agent was triethylamine;^20,21 which can also react with C₆₀ fullerene as reported previously by Kajii et al.²³ and in our case even large excess of Et₃N in toluene did not deprotect ATF-C₆₀ to form free thiol. After that, we tried to use a modified procedure given by Yelm²⁴ who applied pyrrolidine to deprotect non-fullerene aromatic thioacetates in solvents suitable for fulleropyrrolidines. Despite the fact that fulleropyrrolidines are less reactive than pure C₆₀ fullerene²⁵ we were still considering possible side-reactions of pyrrolidine with C₆₀ fullerene cage under aerobic conditions^26,27 and the chance that heat also may promote reaction of secondary amines with fullerene,²⁸ so the deprotection was finally done under inert gas atmosphere and without heating. This time results were fully satisfying, thiolate anion of deprotected ATF-C₆₀ has been detected by means of ESI-MS (Fig. S3, ESI†). Thus, pyrrolidine at room temperature and in inert gas atmosphere was found to be the best deprotection agent for synthesized fullerene thioacetate. However, in situ deprotection with gold plate immersed into a solution of ATF-C₆₀ with 20 eq. of pyrrolidine led to formation of multilayer with remaining traces of amine whereas removal of the pyrrolidine excess before self-assembly allowed producing stable monolayers on both gold electrode and nanoparticles. In typical procedure to a 0.5 mM solution of ATF-C₆₀ in toluene (10 mL) 7.1 mg (8.3 μL, 0.1 mmol) of pyrrolidine was added. Resulting mixture was stirred for 3 h at room temperature under argon atmosphere. Then, mixture was concentrated under reduced pressure, removing the pyrrolidine at the same time, and 10 mL of methanol was added. Precipitate was centrifuged (10 000 rpm, 5 min), washed with methanol and monitored by TLC and ESI-MS for complete removal of formed N-acetylpyrrolidine (detected in methanol solution by ESI-MS, see Fig. S4, ESI†) and any remaining traces of pyrrolidine (usually not present in supernatant). Then it was dissolved in 10 mL of toluene to form stock solution S and proceeded to self-assembly.

Self-assembly of thioacetate derivative on surface of gold electrode

Attempts to obtain self-assembled monolayer (SAM) of fullerene thioacetate at the gold electrode surface without previous deprotection procedure failed, so we can assume that from the mixture of deprotected fullerene derivative and fullerene thioacetate only the first one should deposit on Au surface. Self-assembly of deprotected ATF-C₆₀ on gold electrode was done by dipping the gold substrates into the solution S diluted 100× with toluene or 1,2,4-trichlorobenzene for 24 h at RT. Resulting films were washed with the corresponding solvent and dried under stream of nitrogen. After successful approach with chemisorption of C₆₀ derivative on the surface of gold electrode we decided to synthesize gold nanoparticles covered with C₆₀ using deprotected fullerene thioacetate as a ligand.

Synthesis of gold nanoparticles

Gold NPs were prepared using a modified Brust–Schiffrin method reaction.²⁹ An aqueous solution of hydrogen tetrachloroaurate (III) (30 mL, 30 mM) was extracted with 80 mL of tetraoctylammonium bromide (2.19 g, 4 mmol) solution in toluene. Most of the tetrachloroaurate ions were transferred to the organic layer as the yellow aqueous layer faded until colorless. 1.44 μL (8.3 × 10⁻⁶ M) of 1-octanethiol was added to the organic phase, and the mixture was reduced with freshly prepared aqueous solution of sodium borohydride (378 mg, 200 mmol in 25 mL of deionized water). A solution of borohydride was very slowly added under vigorous stirring. After further stirring for 3 h, the organic phase was separated, evaporated to 10 mL in a rotary evaporator, and mixed with 400 mL of 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, the precipitate was dissolved in a small amount of toluene (10 mL), precipitated with ethanol (400 mL), and centrifuged. Finally, all samples were dissolved in 10 mL of toluene. Obtained nanoparticles were characterized by TEM, UV-Vis and DLS measurements.

AuNPs ligand-exchange reaction

An organic ligand deprotected ATF-C₆₀ toluene solution S (10 mL) was added to the toluene solution of octanethiol stabilized gold NPs (200 μL), and the reaction proceeded for ca. 72 h upon mixing at room temperature. 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 small volume of toluene (max. 2 mL). In the next step, the NPs were precipitated using ethanol and centrifuged 2 h later (13 000 rpm, 5 min). The precipitate was dissolved in toluene, sonicated and the suspension was centrifuged again at 13 000 rpm for 20 minutes The toluene washing process was repeated several times until the supernatant fluid became colorless. Resulting fullerene functionalized nanoparticles were characterized by UV-Vis, DLS and XPS measurements.

Characterization methods and instrumentation

ESI-MS spectra were acquired on a Micromass LCT ESI-TOF mass spectrometer equipped with an orthogonal electrospray ionization source. ¹H NMR and ¹³C NMR spectra were recorded on Varian Unity Plus 200 and 500 MHz spectrometers, respectively, using CDCl₃ as a solvent. The infrared experiments were carried out using the Nicolet 8700.

XPS measurements were carried out using a VG ESCALAB 210 electron spectrometer equipped with an Al Kα source (1486.6 eV). XPS data were calibrated using the binding energy of C 1s 285.0 eV as the internal standard.

The morphology and size of obtained gold nanoparticles were obtained using a Libra TEM (Carl Zeiss) operating at 120 kV.

UV-Vis spectra were collected from 300 to 700 nm using Evolution 60 UV-Vis Spectrophotometer from Thermo Scientific and a quartz glass cuvette.

The dynamic light scattering (DLS) measurements were performed using Zetasizer Nano ZS from Malvern Instruments.

Thermogravimetric analysis (TG) was performed under high purity nitrogen atmosphere using TA Instruments Q50 Thermal Gravimetric Analyzer with heating rate of 5 K min⁻¹.

CV (cyclic voltammetry) and DPV (differential pulse voltammetry) experiments were done using the Autolab potentiostat (ECO Chemie, Netherlands) in a three electrode arrangement, with a silver/silver chloride (Ag/AgCl) electrode as the reference, platinum foil as the counter and the glassy carbon electrode (GCE, BAS, 3 mm diameter) or the modified gold electrodes (gold (200 nm)) evaporated on glass with a pre-layer of 2–4 nm of chromium (Gold Arrandee, Gmbh) as the working electrodes. The reference electrode was separated from the working solution by an electrolytic bridge filled with 0.1 M TBAHFP–toluene–AN (4 : 1) solution. The reference potential electrode was calibrated using the ferrocene (Fc) redox process in the same TBAHFP/toluene/AN solution, which was also used as the supporting electrolyte. Argon was used to deaerate solution and an argon blanket was maintained over the solution during the experiments. All experiments were carried out at 25 °C.

The atomic force microscopy (AFM) images were acquired using Agilent Technologies 5500AFM (Agilent Technologies, Santa Clara, CA, USA). The non-contact mode was used for imaging. Ultra sharp probes were purchased from NANOSENSORS (SSS-NCLR with 2 nm tip radius). Single gold monocrystal was applied as substrate for AFM measurements. The atomic flat Au (111) surface at monocrystal surface was used for image acquisition. Substrate before fullerene adsorption and AFM imaging was flame annealed and cleaned in piranha solution (concentrated H₂SO₄–30% H₂O₂ 3 : 1 v/v) for 30 minutes and rinsed with Milli-Q ultrapure water. AFM images of fullerene derivative-modified Au substrate were recorded after its immersion in fullerene solutions for 24 hours.

Results and discussion

TEM measurements

One or two droplets of as-prepared nanoparticle solutions diluted with toluene to a very small concentration were placed on a standard copper grid (400 mesh) coated with carbon. The grid was left overnight to dry. The TEM image of the synthesized AuNPs is shown in Fig. 2. The dispersed nanoparticles have an average diameter of 4.72 nm.

Fig. 2 TEM image of AuNPs with average size of 4.72 nm obtained at 120 kV.

UV-Vis spectroscopy

UV-Vis spectroscopy was used to control the formation of ATF-C₆₀-AuNPs by observation of a peak referring to the maximum of absorption at 522 nm. Data were collected from 300 to 700 nm for 3 different species: the starting solution of AuNPs (20 μL of NPs diluted 40× with toluene), the solution of gold nanoparticles modified with fullerene derivative (250 μL of modified NPs diluted 3.2× with toluene) and ATF-C₆₀ solution in toluene (C = 0.0176 mg mL⁻¹). As we can observe in Fig. 3 fullerene functionalized nanoparticles retain peak coming from AuNPs–octanethiol (λ_max = 522 nm) and show additional one referring to the maximum of absorption at 336 nm for C₆₀ derivative ATF-C₆₀ indicating the successful exchange of the ligand.¹²

Fig. 3 UV-Vis spectra of bare octanethiol AuNPs, ATF-C₆₀ and ATF-C₆₀@AuNPs collected from 300 to 700 nm.

Dynamic light scattering

Size of single fullerene thioacetate molecule was theoretically estimated from fully optimized geometry structure using the DFT/B3LYP/6-31g*,^30–33 approach to be 1.37 nm. Dynamic Light Scattering measurements (Fig. S11, ESI†) sized the bare octanethiol–AuNPs as 4.71 nm which is in excellent agreement with TEM results (4.72 nm). AuNPs coated with fullerene derivative indicated the average diameter of 7.27 nm (Fig. S12, ESI†). Subtracting the size of fullerene coating which increases the whole size of nanoparticle by 2.74 nm, the average size of coated nanoparticle can be assessed to be 4.53 nm, thus slightly less than that from DLS measurements. This induces that we clearly obtain a monolayer and the orientation of fullerene substituents is probably not perpendicular to the AuNP surface but slightly tilted from the surface normal.

FTIR spectroscopy of SAMs

Fig. 4 shows FTIR spectra of both ATF-C₆₀ and its deprotected SAMs on the surface of gold electrode. Deprotection of ATF-C₆₀ and its deposition on Au surface causes several changes in the infrared spectrum. The intense signal at 1708 cm⁻¹ attached to the C=O stretching mode almost completely disappeared confirming the efficient deprotection of thioacetyl group and removal of N-acetylpyrrolidine. Four characteristic fulleropyrrolidine bands are visible in both spectra and their positions are similar. Bands at 1113–1114 cm⁻¹ and 1423–1424 cm⁻¹ are associated with C₆₀ fullerene cage and contribution of pyrrolidine ring is visible in the range of 2750–3000 cm⁻¹.³⁴

Fig. 4 IR spectra of ATF-C₆₀ in KBr pellet (A) and the reflectance spectrum of the thin film obtained after its deprotection and self-assembly from solution S diluted 100× with 1,2,4-trichlorobenzene (B).

Atomic force microscopy

The surface morphology of deprotected ATF-C₆₀ layer deposited, on the Au (111) surface, from pyrrolidine free solution S diluted 100× with two different solvents: 1,2,4-trichlorobenzene (Fig. 5A) and toluene (Fig. 5B) was studied by AFM. Both images provide a precise insight into the surface morphology and allow for assessing the influence of the solvents used for the self-assembly on the topography of the films of deprotected thiolated fullerene derivatives deposited on gold. In both cases, the gold surface is covered by small granular aggregates. Analysis of the AFM images enables us to evaluate the average size of these grains. Diameter of the smallest fullerene aggregates was estimated to be in the range of 65–80 nm in the case of toluene and 10–15 nm for 1,2,4-trichlorobenzene. In addition, there are more morphological differences between the films obtained in toluene and trichlorobenzene solutions. Namely, the aggregated grains in the Fig. 5B have no clearly marked spherical shapes and exhibit a tendency to form large domains with diameters up to ca. 200 nm which are visible as bright spots. Comparing images in Fig. 5A and B it is obvious that the morphology of fullerene aggregates depends strongly on the solvent properties. Smaller fullerene grains obtained in case of 1,2,4-trichlorobenzene solution are not surprising, considering dielectric constant, ε of each solvent and the higher solubility of fullerene derivatives in 1,2,4-trichlorobenzene,³⁵ both factors favor the formation of smaller aggregates³⁶ as clearly demonstrated in the presented figures. Obtained AFM results confirm earlier reports that fullerene derivatives can not self-assemble into ordered monomolecular layers.^37,38

Fig. 5 Non-contact AFM images of self-assembled films of deprotected ATF-C₆₀ on Au (111) from solution S diluted 100× with (A) 1,2,4-trichlorobenzene, (B) toluene.

XPS spectroscopy results

X-ray photoelectron spectroscopy was used to assess the composition of nanomaterials obtained from self-assembly of in situ deprotected thioacetate, chemisorption after removal of pyrrolidine and N-acetyl pyrrolidine as well as of nanoparticles functionalized with C₆₀ fullerenes. XPS spectra of all investigated samples show the presence of gold, sulfur, nitrogen, carbon while the nanoparticles sample also contained bromine atoms. Binding energies obtained from XPS spectrum confirmed the presence of functionalized fullerenes on the surface of gold electrode and gold nanoparticles attached to Au by the sulfur atoms.

The S 2p region of spectrum obtained for the in situ deprotection sample (Fig. 6A) shows two doublets, each with area ratios of 2 : 1 and splitting of 1.2 eV. First one is observed at 161.9 eV and 163.1 eV assigned to S 2p_3/2 and S 2p_1/2 respectively, this binding energy value is typical for the S–Au bond³⁹ and it was found to be 53 at% of all sulfur atoms in the sample. Second doublet at 163.8 eV for S 2p_3/2 and 165.0 eV for S 2p_1/2 corresponds to surface unbound sulfur atoms⁷ and equals remaining 47 at%. These results are in good agreement with AFM data suggesting that the proposed in situ method results in thick multilayer formation.

Fig. 6 Partial XPS spectra of the S 2p and N 1s regions for films obtained using in situ ATF-C₆₀ deprotection method.

Results for the N 1s region were deconvoluted into two peaks (Fig. 6B). According to literature data the bigger one observed at 399.7 eV is assigned to nitrogen atoms from fulleropyrrolidine⁴⁰ with atomic percent value of 81 at%. Second peak with the lower binding energy of 398.4 eV is assigned to pyrrolidine,⁴¹ indicating that amine remains in obtained film (19 at%). However, taking into account the small molecular mass of pyrrolidine in comparison to the functionalized fullerene molecule the overall weight percent of both components are 98 wt% for fulleropyrrolidine and only 2 wt% for pyrrolidine. These results show that in situ deprotection of aromatic thioacetate can be used whenever the thickness of the film and remaining small amount of amine still meet application requirements.

In the S 2p region of the XPS spectra of the thin film obtained using pyrrolidine free solution S of deprotected ATF-C₆₀ diluted 100× with 1,2,4-trichlorobenzene (Fig. 7A) one doublet at 161.9 eV and 163.1 eV is observed, with expected 2 : 1 ratio. This results show that all sulfur atoms in the investigated sample are bound to gold surface.

Fig. 7 Partial XPS spectra of the S 2p and N 1s regions for films obtained using pyrrolidine free solution of deprotected ATF-C₆₀. S 2p_3/2 : S 2p_1/2 intensity ratio 2 : 1 and splitting of 1.2 eV.

The N 1s photoelectron peak appears at 399.8 eV (Fig. 7B) and we can assign it to the fulleropyrrolidine nitrogen atoms. As a result, the pyrrolidine free mixture lead to deposition of functionalized fullerenes without any impurities and formation of thin film on the gold electrode surface.

Results obtained for AuNPs covered with functionalized fullerenes reveal a doublet in the S 2p region (Fig. 8A) (2.0 at% overall atoms). It was deconvoluted into two peaks centered at 162.0 and 163.2 eV corresponding to S 2p_3/2 and S 2p_1/2 atoms, confirming that all ATF-C₆₀ molecules are bound to the surface of gold nanoparticles by S–Au bonds and, no fullerene–gold interaction is observed. A double-peak observed in the N 1s scanning range (Fig. 8B) from 396 to 404 eV was deconvoluted with very good correlation in two peaks with binding energies 399.3 eV and 402.1 eV associated with fulleropyrrolidine and quaternary ammonium nitrogen⁴² respectively (38 at% and 62 at% of all nitrogen atoms, 0.8 at% and 1.3 at% overall atoms). XPS spectrum of the Br 3d region confirms the presence of bromine from tetraoctylammonium salt with intensity 1.1 at% overall atoms which is in good correlation with results obtained for their counter tetrabutylammonium ion nitrogen atoms (1.3%). Results presented above allow to calculate ATF-C₆₀–TOAB–octanethiol ratio on AuNPs surface from corresponding signals (0.8 at% : 1.3 at% : (2% of total S amount − 0.8 at% S from ATF-C₆₀)), that is exactly 1 : 1.625 : 1.5.

Fig. 8 Partial XPS spectra of the S 2p and N 1s regions for gold nanoparticles functionalized with deprotected ATF-C₆₀.

In summary, thioacetate group is not observed according to XPS results for all samples, which is confirmed by lack of S 2p_3/2 signal at around 163 eV, typical for S–C bond.^43,44 Thus confirming the efficiency of deprotection and selective deposition of free thiol instead of S-acetyl functionalized fullerene from any remaining traces of protected compound. Moreover, the absence of peaks above 166 eV for S 2p region indicates that the obtained fullerene–gold nanocomposites are free from oxidized sulfur species usually found at higher binding energies.⁴⁵

Thermogravimetric analysis

TG analysis results (Fig. 9.) showed that 8.5% of total mass of nanoparticles is organic coating and remaining 91.5% was clearly visible pure Au. Assuming the spherical shape of nanoparticles, the average number of gold atoms per AuNP (N_Au) was calculated according to eqn (1) proposed by Huo et al.⁴⁶ whereas the number of fullerene ligands per nanoparticle (N_L) was determined using XPS results by eqn (2) (where wt_org% and wt_Au% are thermogravimetrically determined percentage mass values for organic coating and Au core respectively, D is the average diameter of AuNPs in nm, M_L and M_oct are molecular masses of corresponding ATF-C₆₀ and octanethiol thiolate anions, M_Au is atomic weight of gold and M_TOAB stands for the molecular mass of tetraoctylammonium bromide) and was found to be ca. 30. The remaining organic mass at the AuNP surface consists of tetraoctylammonium ions and octanethiol molecules. Note, that the left factor in the eqn (2) corresponds to the total mass of organic coating and the right one is connected to the fullerene content.

N_Au = 30.89602D³ (1)

Fig. 9 Results of TG analysis obtained for the AuNPs covered with deprotected ATF-C₆₀.

Electrochemical behavior of thioacetate derivative in the solution

Electrochemical properties of C₆₀ and thioacetate derivative ATF-C₆₀ in 0.1 M TBAHFP toluene–acetonitrile solution were investigated using voltammetry (CV and DPV). The fullerene moiety preserves sequential one-electron reversible redox processes characteristic of the extended π-electron system (Fig. 10A).^1,47 The one-to-one ratio of CV peak currents and the close to 60 mV peak-to-peak separation confirm the reversibility. The thioacetate derivative exhibits the voltammetric behavior similar to that of C₆₀, however, the peak potentials are shifted to more negative values due to the presence of substituent, and the peak currents are much smaller indicating smaller diffusion coefficients of the derivative compared to that of unmodified fullerene.

Fig. 10 Voltammograms of (black line) C₆₀ and (red line) compound ATF-C₆₀ in 0.1 M TBAHFP–toluene–acetonitrile (4 : 1), (A) CV, v = 100 mV s⁻¹; (B) DPV, tp = 3 ms, ΔE = 50 mV (characteristic parameters of the voltammetric behavior of compound ATF-C₆₀ are given in ESI, Table S1†).

Four reversible peaks were also recorded in the DPV experiment. The fullerene centered processes appear again at more negative potentials, compared to of C₆₀ (Fig. 10B). Such shifts are due to the presence of a substituent containing an electron donating moiety, in this case the aromatic ring. As the result, the fullerene derivative becomes a weaker electron acceptor than the unsubstituted molecule, however the reversibility of each of the first 4 steps of the electrode processes is retained. All peak currents of the thioacetate derivative are smaller than those of the unsubstituted fullerene reflecting the smaller diffusion coefficients – the molecule is larger due to the presence of the substituent.

Application of deprotected thioacetate derivative ATF-C₆₀ for Au electrode and AuNPs modification

The C₆₀ thioacetate derivative following its deprotection was self-assembled on the gold electrode surface. The cyclic voltammogram of the gold electrode modified with self-assembled monolayer of C₆₀ derivative recorded in acetonitrile containing 0.1 M TBAHFP is shown on Fig. 11A.

Fig. 11 Cyclic voltammogram of deprotected C₆₀ derivative ATF-C₆₀ (A) self-assembled monolayer recorded in 0.1 M TBAHFP solution in AN, v = 1 V s⁻¹ and (B) desorption of adsorbed monolayer in 0.5 M KOH in water, v = 50 mV s⁻¹.

The two redox peaks observed in Fig. 11A, correspond to the first and second redox step of the immobilized fullerene derivatives. The redox potentials are close to the values reported for fullerene lipoic acid derivative or fullerene with long alkane thiol chain self-assembled on the gold substrates.^48,49 The peak currents increase linearly with scan rate as predicted for the surface immobilized species. The surface concentration of the deprotected ATF-C₆₀ derivative was calculated based on eqn (3):

Γ = Q/nFA (3)

where n is the number of exchanged electrons, A is the electrode surface area in cm², Γ is the surface concentration in mol cm⁻², Q is the charge under the peak in C. The surface concentration of the fullerene is 7.1 × 10⁻¹¹ mol cm⁻².

Γ_CV is calculated from the charge of the fullerene reduction peak while Γ_des is obtained from the charge of the reductive desorption peak of the compound from the Au surface. Assuming that fullerene occupies on the surface ca. 95 Ų, the surface concentration should be 1.75 × 10⁻¹⁰ mol cm⁻². The surface concentration Γ_des is slightly smaller (8.3 × 10⁻¹¹ mol cm⁻²) than the value corresponding to a monolayer. The values of Γ_CV are similar to those of Γ_des.

Fig. 12 compares the cyclic voltammograms of ATF-C₆₀ and of the tethered gold nanoparticles, ATF-C₆₀@AuNPs. It can be seen that there are four pairs of voltammetric waves for the ATF-C₆₀ solution (red curve), which are all close to reversible. For the ATF-C₆₀@AuNPs, a series of two well-defined voltammetric peaks at −0.56 and −0.95 V can be found within the potential range of −1.4 to 0 V. These signals can be ascribed to the successive 1ē reduction steps of the C₆₀ molecules, i.e., C₆₀^0/1− and C₆₀^1−/2− couples, respectively.^47,50,51 The capacitance is much larger for the ATF-C₆₀@AuNPs compared to the faradaic process. Interestingly the shift of peak potentials indicates stronger electron acceptor properties of the fullerenes when bound to gold nanoparticles compared to those of ATF-C₆₀ dissolved in the solution. The dependence of peak current versus v^1/2 in both cases ATF-C₆₀@AuNPs and pure fullerene derivative is linear with R² = 0.982 and 0.991, respectively (Fig. 13). It indicates that the process is controlled by diffusion.

Fig. 12 Voltammograms of (black line) ATF-C₆₀ and (red line) the C₆₀ derivative tethered gold nanoparticles (ATF-C₆₀@AuNPs) in solution 0.1 M TBAHFP–toluene–acetonitrile (4 : 1), CV, v = 100 mV s⁻¹.

Fig. 13 Dependence of peak current versus v^1/2 for ATF-C₆₀@AuNPs (A) and the fullerene derivative ATF-C₆₀ in a solution 0.1 M TBAHFP–toluene–acetonitrile (4 : 1) (B).

Conclusions

A new procedure for efficient modification of gold surfaces with fullerenes has been elaborated. Firstly, a new aromatic thioacetate functionalized fullerene derivative ATF-C₆₀ was synthesized in a two step procedure and was thoroughly characterized using a wide range of analytical techniques. The electrochemical behavior of the thioacetate functionalized fullerene derivative was evaluated in toluene–acetonitrile solution and 4 reversible sequential 1e electrode processes were resolved both in the cyclic and differential pulse voltammograms.

In the next step, the newly synthesized molecules following suitable deprotection, were used for the preparation of the C₆₀ fullerene-modified gold electrode, and C₆₀ fullerene-modified gold nanoparticles. The proposed deprotection procedure for S-acetyl-derivatized aromatic fullerene is very efficient and allows the control of the thickness of the self-assembled fullerene layer providing complete removal of the excessive pyrrolidine and resulting N-acetylpyrrolidine. Otherwise, traces of the deprotection reagents can promote multilayer formation as found from the XPS data.

The described two-step approach proved to be an efficient way for the preparation of fullerene modified gold nanoparticles. Interestingly, the gold nanoparticles coverage is higher than achieved using one-step procedures. DLS and TEM measurements show that octanethiol–AuNPs of 4.71 nm diameter following ligand exchange procedure gave fullerene modified AuNPs of 7.27 nm diameter with high population of 30 bound fullerene moieties per one nanoparticle. Anchoring the fullerene on gold nanoparticle lead also to improved acceptor properties of the fullerene moieties that was confirmed by easier uptake of electrons from the electrode in the electrochemical experiments.

Acknowledgements

We gratefully acknowledge the financial support from the National Science Centre (Projects: Piotr Piotrowski 2012/07/N/ST5/01966 and Joanna Pawłowska 2011/03/N/ST5/04825). Theoretical calculations were performed using the computational resources of the Interdisciplinary Center for Mathematical and Computational Modeling at Warsaw University (Grant G15-11).

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Footnote

† Electronic supplementary information (ESI) available: Additional characterization data including ESI-MS, IR and ¹H NMR spectra for synthesized compounds and dynamic light scattering (DLS) data are presented. See DOI: 10.1039/c4ra12139b

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