Ag(NP)

Oct 19, 2012 - GC/OligoThiophene/Ag(NP) hybrid interfaces are synthesized and characterized: GC is the glassy carbon surface; OligoThiophene stands fo...
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On the Hybrid Glassy Carbon Electrode/OligoThiophene/Ag(NP) Interface Francesco Tassinari,† Erik Tancini,† Massimo Innocenti,‡ Luisa Schenetti,† and Claudio Fontanesi*,† †

University of Modena, Reggio Emilia, Department of Chemistry, Via Campi 183, 41125 Modena, Italy University of Firenze, Department of Chemistry, via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy

‡

S Supporting Information *

ABSTRACT: GC/OligoThiophene/Ag(NP) hybrid interfaces are synthesized and characterized: GC is the glassy carbon surface; OligoThiophene stands for both an ultrathin bithiophene grafted film and a 4-Br-Bithiophene grafted polymer; Ag(NP) stands for silver nanoparticles. The hybrid interface preparation involves different steps: first, the electrode surface is functionalized through a combination of electrochemically assisted grafting (under reduction regime) and polymerization (under oxidation regime); then, silver nanoparticles are chemisorbed by dipping. In particular, an ultrathin film of grafted bithiophene can be obtained by applying one cyclic voltammetry reduction cycle (GC/BT surface), while subsequent cyclic voltammetry cycling under oxidation regime yields an immobilized 4Br-Bithiophene polymer (GC/4BrBT surface). AFM and TEM images were recorded to investigate the morphology and chemical composition of the Ag(NP). FeII/ FeIII cyclic voltammetry, Zn underpotential deposition (UPD), XPS, LA-ICP-MS, and Raman techniques were exploited to characterize both the GC/OligoThiophene and GC/OligoThiophene/Ag(NP) interfaces. Theoretical calculation, at the B3LYP/ 6-311G** level of the theory, enabled rationalization of the electroreduction mechanism and the Raman results.

1. INTRODUCTION In recent decades, the research field regarding the functionalization of conducting surfaces has attracted increasing interest from both scientists and engineers. Surface modification from the chemical adsorption of suitable functional groups on various surfaces has been widely studied and exploited in a great variety of applications, ranging through biosensors, nanobioelectronics, catalysis, and solar cells.1−4 Among a great variety of conducting metallic/nonmetallic substrates, specific carbon surfaces have been particularly studied: carbon nanotubes5,6 and graphene7 have become two areas of great interest due to their unique physical and chemical properties and the possibility of improving their performance through surface modifications. Within this context, glassy carbon surfaces have also been extensively studied: several examples of different surface functionalization with a large number of different functional groups have been reported.8 The first example of a covalently modified carbon surface is found in the work of Pinson, who formed a stable and compact ultrathin organic layer by electrochemical oxidation of amine-containing compounds, the great reactivity of their corresponding cation radicals with the carbon surface allowed functionalization of the GC: “electrochemical assisted grafting”.9 The reactivity of these radicals leads to subsequent chemisorption on the surface, through the formation of a carbon−nitrogen covalent bond. Following the groundbreaking work of Pinson, a large number of organic compounds proved to be effective in the grafting of carbon surfaces: not only aliphatic and aromatic amines, but also organic cations such as diazonium, iodonium, and © 2012 American Chemical Society

sulfonium salts, and iodide and bromide substituted organic compounds.8,10−13 Of further interest is the surface binding of molecules carrying another suitable (electrochemically inert) chemical functionality, in addition to the one reacting in the grafting process. Such a strategy would allow for a further, onestep modification of the surface layer, leading to the immobilization of functional groups prone to successive surface reactivity (postfunctionalization). Amino acids, metal complexes, and monomers are some examples of molecules that have been covalently grafted to the surface of a GC electrode and subsequently submitted to other processes.10 A monomerfunctionalized surface can be subjected to a polymerization reaction in suitable conditions, and this topic has been widely explored: several examples of polymerization on carbon modified surfaces are reported in the literature.14,15 Conducting polymers such as polythiophenes, polyanilines, and polypyrroles can than be used as linking media between an electrode and a third species, helping to immobilize the latter (both chemi- and physisorption are possible) and guarantee an effective electrical contact. The result is a polymeric film functionalized with different elements such as enzymes, metal complexes, and metallic nanoparticles.16−18 Silver nanoparticles, in particular, exhibit peculiar optical (surface plasmon resonance19) and catalytic (oxygen reduction reaction,20 under potential deposition,21 electrocatalytic dehalogenation22) propReceived: June 28, 2012 Revised: October 12, 2012 Published: October 19, 2012 15505

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were conducted in ambient air and room temperature. Images were recorded at an optimized linear scan rate of 0.5−1.0 Hz, with image resolution of 256 × 256 pixel and variable scan size. Scratching experiments were performed using a Park CP instrument. NSG11 probes were purchased from NT-MDT (Head office in MoscowRussia) with resonant frequency in the range 190−300 kHz. The force used for scratching was 1 μN. Scratches were made in contact mode, while height measurements were made in noncontact mode configuration. The microscope Z-axis was calibrated using a standard grid provided by NT-MDT. 2.5. LA-ICP-MS. The New Wave laser ablation UP 213 unit was equipped with a solid state laser, exciting wavelength of 213 nm, energy density >27 J/cm2, tunable impulse frequency in the 1 to 20 Hz frequency, and followed by a plasma spectrometer HR-MC-ICPMS Neptune, by Thermo Fisher Scientific. 2.6. Raman. The Raman microscope features an He−Ne laser exiting at 632.81 nm, with a max 20 mW power. It is equipped with a 1024 × 256 × 16 CCD detector, Peltier cooled. 2.7. Computational Details. Ab initio molecular orbital calculations were performed using the Gaussian and GAMESS programs.24,25 Screening full optimization geometry calculations were performed at the B3LYP/3-21G* level of the theory, while final geometries are obtained at the B3LYP/6-311G** level of the theory. Geometries optimized in the gas phase were used to calculate the solvation energy of the various species involved in the determination of the redox potential, using Barone and Cossi’s polarizable conductor model (CPCM) method, which is based on the polarized continuum model (PCM) of Tomasi. Ionization potentials and solvation energies, required to assess the reduction and oxidation standard potentials following the Cramer and Truhlar modelistic approach,26 were obtained at the B3LYP/6-311G** level of the theory.

erties. Within this context, the main purpose of the present study is to obtain a nanostructured, precious metal, Ag surface immobilized on a less expensive and easy-to-use substrate. The final and general research aim is Co decoration of a Ag surface. The overall process can be obtained through electroless Co2+ reduction, coupled with the oxidation of previously deposited Zn layers, exploiting the UPD process, on Ag, as illustrated in detail in ref 20. Thus, in this study an investigation is conducted on the electrochemically assisted grafting process of the 4-Brbithiophene monomer onto the GC surface (yielding an ultrathin GC/BT surface, i.e., a glassy carbon surface grafted with bithiophene moieties). The subsequent electro-oxidative polymerization of the monomer on the GC/BT surface, using this as the working electrode, is discussed. The GC/ OligoThiophene interface is used to immobilize silver nanoparticles, and a nanostructured Ag surface is obtained. The latter surface is used as a standalone working electrode on which Zn electrodeposition UPD is observed.

2. EXPERIMENTAL SECTION 2.1. Reagents. All the reagents were purchased from Sigma Aldrich and used without further purification. The solvents (diethyl ether 99.7% anhydrous; acetonitrile 99.8% anhydrous) were purchased from Carlo Erba Reagenti and used without further purification. The GC electrodes (rods) with an area of 0.072 cm2 were purchased from Metrohm, and the GC plates (2.5 cm ×2.5 cm × 1 mm) were purchased from HTW (SIGRADUR G). The alumina used for the cleaning of the surface had a mesh of 1 and 0.05 μm and was purchased from Buehler. The GC surface was cleaned using consecutively a slurry of 1 μm and 0.05 μm of alumina on polishing cloth, subsequently washed with bidistilled water, and sonicated first in water then ACN for 10 min each. 2.2. Synthesis of 4-Bromo-2,2′-bithiophene (4-BrBT).23 The debromination of 2,3,5-tribromothiophene with butyl lithium at −40 °C in Et2O under Ar atmosphere gave product 1 in good yield (81%) and purity after distillation under reduced pressure. A Kumada coupling reaction between the Grignard reagent of the 2bromothiophene and product 1 was then used to obtain product 2. The solution of the Grignard reagent was added to an Et2O solution of product 1 and catalyst Pd(dppf)Cl2 at −20 °C under Ar atmosphere and then allowed to warm up to room temperature. After the usual workup, the crude product was purified by flash chromatography (silica gel, AcOEt/EtPet 1:5), obtaining a green liquid (75% yield).

3. RESULTS AND DISCUSSION 3.1. Grafting and Electropolymerization. The preparation of the hybrid GC/4-BrBT/Ag(NP) interface was carried out in three successive steps: (1) electrochemically assisted grafting, under reduction regime conditions, one single CV cycle; (2) electrochemical polymerization of the 4-BrBT, under oxidation regime conditions; (3) finally, Ag(NPs) are chemisorbed by dipping the GC/4-BrBT surface in the Ag(NP) suspension, Ag(NPs) are obtained by silver electro-oxidation (vide infra). 3.1.1. Electrochemical Assisted Grafting. Grafting of the 4BrBT is achieved with an electroreductive procedure in 0.1 M TBATBF in ACN solution with a single cyclic voltammetric cycle. Figure 1 shows repetitive cyclic voltammograms recorded for 4-BrBT, at a scan rate (ν) of 0.1 V s−1. A current peak, at about −1.9 V, is present, the relevant underlying redox process appears irreversible: as is suggested by the absence of any peak in the backward scan. On subsequent cycling, the peak current remains almost constant up to the fourth/fifth cycle, then it decreases rapidly, eventually becoming almost negligible after the tenth cycle. Such behavior is consistent with slow progressive electrode passivation strongly indicating electrode surface derivatization. The analysis of CVs recorded as a function of the scan rate shows that the peak current, relevant to the reduction process, ip, is found to be a linear function of ν1/2 (the inset in Figure 1 shows CVs recorded at various scan rates: 0.05, 0.25, 0.50, 0.75 V s−1). The whole of the electrochemical results, under reduction regime, suggests that the electrochemical process, underlying the reduction peak in the CV, is an irreversible and diffusion-limited process. A quantitative analysis of the ip vs ν1/2 results was not attempted, because the relevant outcome may be affected, to an unknown extent, by the occurrence of the

H NMR (200 MHz, CDCl3): δ 7.02 (1H, dd, J = 3.67−5.14 Hz, H4′), 7.08 (1H, d, J = 1.47 Hz, H3), 7.10 (1H, d, J = 1.47 Hz, H5), 7.18 (1H, dd, J = 1.05- 3.56, H3′), 7.25 (1H, dd, J = 1.26−5.03 Hz, H5′). 2.3. Electrochemical. Cyclic voltammetry (CV) measurements were performed using both Autolab PGSTAT 20 and CHI660A potentiostats and employing a typical three-electrode electrochemical cell arrangement. Tetrabutylammonium tetrafluoroborate (TBATBF) 0.1 M was used as the supporting electrolyte in ACN solution; KOH 0.1 M was used as the supporting electrolyte in aqueous solution. GC surfaces, a Pt wire, and a Ag/AgCl/KClsat electrode were used as the working, counter, and reference electrodes, respectively. 2.4. AFM. AFM measurements were performed with an APE Research AFM A-10 Microscope (Italy) operated in contact mode with a constant applied force of 1.3 nN. Images were obtained with an NSC19/NoAl uncoated silicon-etched probe (MikroMasch), with a cantilever length of 125 μm, spring constant 0.6 N/m, and 10 nm nominal probe tip radius (manufacturer’s values). All measurements 1

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Figure 2. PES as a function of the C−Br bond distance. 4Br-BT anion open-shell species. B3LYP/6-311G** level of theory. Triangle up, relaxed scan. Triangle down, unrelaxed scan.

(both aromatic and aliphatic) exhibit markedly different behavior from the quantitative point of view, in that their PES vs dC−Cl curves are characterized by a single dissociation activation energy, which is typically less than 10 kJ mol−1.29−31 3.1.2. Electrochemical Polymerization of the 2-BrBithiophene. Oxidation Regime. Figure 3 sets out the CV of a 1 mM solution of 4-BrBT in ACN, 0.1 M TBATBF as base electrolyte, recorded on a GC electrode.

Figure 1. Repetitive cyclic voltammograms of a 1 mM 4-BrBT in ACN solution, ten successive cycles are shown, TBATBF 0.1 M base electrolyte. GC working electrode. 0.1 V s−1 scan rate. The inset shows CVs recorded as a function of the scan rate: 0.05, 0.25, 0.50, 0.75 V s−1.

concurrent grafting process and possible parasitic side reactions. Following the electrochemically induced dissociation, the thiophene radical (compare reaction 2, Scheme 1) can attack Scheme 1. Relevant Reduction Process Steps

the GC surface, thus yielding a chemiadsorbed bithiophene moiety covalently bound to the carbon surface, likely via a C−C covalent bond. The relevant film thickness estimate is established from AFM measurements, indicating that the formation of an ultrathin film is achieved during the first CV cycle (between one and three monolayers): the latter surface (i.e., obtained by applying a single CV scan in reduction regime) is indicated as GC/BT. This result is in general agreement with the electrochemically assisted grafting mechanism proposed by Pinson.9 The stability of the 4-BrBT radical anion was studied theoretically. Figure 2 shows the potential energy surface (PES) vs the C−Br bond distance relevant to the 4-BrBT open-shell radical anion species, calculated at the B3LYP/6-311g** level of the theory (relaxed and unrelaxed PES scans are shown). Both the PES vs dC−Br patterns show a minimum at about 0.195 nm. This minimum is followed by an energy barrier of about 10 and 20 kJ mol−1 for the unrelaxed and relaxed scans, respectively. A second minimum is found at 0.27 nm, featuring an energy dissociation barrier of about 30 kJ mol−1. Therefore, the radical anion dissociation is assumed to be a spontaneous process at room temperature (note that the C−Br bond distance of the second minimum, 0.27 nm, corresponds to a very large carbon bromine distance, indicative more of an exit complex rather than a stable compound) as is found in the case of similar bromine substituted organic compounds.27,28 However, organic chloride radical anions

Figure 3. CV of a 1 mM 4-BrBT in ACN solution, TBATBF 0.1 M base electrolyte. GC working electrode. 0.05 V s−1 scan rate.

A current peak, in the oxidation regime, is found around 1.6 V in the forward curve. The backward curve shows a nonnegligible current peak suggesting the occurrence of a quasireversible oxidation process. Potential cycling in the oxidation regime, in the present case using a GC/BT electrode, yields the growth of a “thick” bithiophene-based polymer film, eventually obtaining a GC/4-BrBT surface. The total polymeric film thickness was estimated by AFM microscopy: after 10 oxidative cycles the film on the electrode was 1 μm (each CV cycle corresponds to an approximate increase in polymer film thickness of 100 nm). Note that the electropolymerization process, under oxidation regime, can proceed through a large number of different mechanisms (parasitic reactions occurring in the bulk and involving radical/radical or radical/neutral reactions cannot be excluded: leading, for instance, to bulk dimerization). Moreover, both the chemisorbed (grafted) bithiophene moiety and the bulk 4-BrBT species can undergo an electrochemical oxidation process. This would yield two 15507

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structurally different interfaces: (i) if polymerization proceeds through oxidation of the grafted bithiophene, the polymer film is expected to be covalently bound to the GC surface; (ii) if electrochemical oxidation concerns the bulk 4-BrBT attaching a bulk 4-BrBT molecule, a “simple” physically adsorbed polymer would be formed. Figure 4 sets out the effect of the potential

Figure 5. CVs recorded as a function of the scan rate: 0.05 (black), 0.10 (red), 0.25 (blue), 0.50 (green), 0.75 (red), 1.00 (black) V s−1. GC/4BrBT working electrode. Solution of base electrolyte only: 0.1 M TBATBF in ACN.

CV cycling suggests that the film is covalently immobilized on the GC surface. CV measurements recorded in the case of simple drop cast thiophene-based polymer films do not permit recording of reproducible successive CVs after the second/third repetitive cycle, thus preventing the recording of CVs as a function of the scan rate.32,33 3.1.3. Ag(NP) Characterization. The electrochemical preparation of Ag(NP) was achieved following the procedure proposed by Starowicz et al.34 Although the present results appear in close agreement to those reported by Starowicz, in this section the morphology of the Ag(NP) used is briefly illustrated. Figure 6 shows an AFM topography image of dried

Figure 4. CVs, four successive cycles, of a 1 mM 4-BrBT in ACN solution, 0.1 M TBATBF base electrolyte. GC/BT working electrode. 0.1 V s−1 scan rate. Black curve, first cycle; blue curve, fourth cycle.

cycling under oxidation regime: four successive CV cycles are shown using a GC/BT (glassy carbon functionalized with a single CV cycle in reduction regime: first CV in Figure 1) working electrode, 1 mM 4-BrBT in ACN solution, 0.1 M TBATBF base electrolyte. Examination of the CV forward curves, in Figure 4, shows the presence of two peaks: the first, Iox, is found, roughly, at 1.40 V and becomes more evident as a function of repetitive cycling (a shallow shoulder which grows into a relatively sharp peak as a function of CV cycling). The relevant current increases as a function of the number of cycles. Moreover, in the backward curve the Iox peak is matched by a reduction peak Ired, the latter featuring the same potential value of Iox and characterized by the same dependence between the current peak height and the number of cycles. At more positive potentials, a second oxidation peak is found, IIox, whose current value is independent of the number of cycles and without any corresponding peak in the backward curve, which can be assigned to 4BrBT(bulk) oxidation (compare Figure 3). Thus, the process underlying the Iox and Ired peaks appears to be as a quasi-reversible oxidation process (even if a contribution from the electrochemical process underlying the IIox peak cannot be completely excluded; compare Figure 3 CV) and a function of the film thickness, strongly indicating that oxidation involves the grafted “thiophene-based” film. Conversely, the IIox peak may be due to the irreversible oxidation of the bulk 4Br-BT species. Figure 5 shows CVs recorded as a function of the scan rate, GC/4BrBT working electrode in contact with a 0.1 M TBATBF in ACN solution (i.e., a solution containing only the base electrolyte). An oxidation peak is found in the forward scan, mirrored by a reduction peak in the backward scan, both compare well to peaks Iox and Ired in Figure 4. The analysis of the CVs recorded as a function of the scan rate shows that the peak current is a linear function of the scan rate, indicating that the redox couple is in the adsorbed state (a result consistent with the electrochemical oxidation/reduction of a polymer film). Moreover, the temporal stability exhibited after repetitive

Figure 6. (Left) AFM topography image of silver nanoparticles dried on flat silicon wafer. (Right) Higher resolution scan of the area highlighted in the left figure.

Ag(NP) deposited from the suspension: image line profiles allow estimation of average particle radius varying between 10 and 20 nm. The Ag(NPs) are assumed to be strongly adsorbed through the chemical interaction that occurs between the sulfur atoms contained in the polymeric chain and the silver atoms of the nanoparticles. AFM microscopy was used to characterize the surface of the final electrode, showing that a relatively homogeneous distribution of AgNP on the surface is obtained. In some cases, there is Ag(NPs) coalescence, eventually forming aggregates of dimensions ranging from 0.5 to 2 μm. Note that no surfactants were used to avoid this phenomenon. Figure 7 shows TEM images of the Ag(NPs). TEM results show that the larger aggregates are formed by smaller particles of dimensions in the 5 to 50 nm range and quasi-spherical in shape. Simultaneous microanalysis measurements show that the 15508

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(Note: The images were analyzed using the image processing software package SPIP 5.1.5 (Image Metrology A/S, Hørsholm, Denmark), further supporting the idea of similar geometrical and morphology characteristics for the GC and GC/4-BrBT surfaces, while the greater roughness value of the GC/4-BrBT/ Ag(NP) surface indicates a completely different structure. On the whole, AFM analysis further supports the effective Ag(NP) adsorption on the GC/4BrBT surface, even if featuring a weaker mechanical stability with respect to that of the electrochemically grown polymer substrate. 3.2.2. Electrochemical Characterization. The electrochemical behavior of the hybrid interface is characterized electrochemically using the GC/4-BrBT/Ag(NP) surface as a working electrode: (i) CVs recorded exploiting the K3Fe(CN)6/ K4Fe(CN)6 redox couple as a surface probe; (ii) characterization of Zn electrodeposition, which is known to yield UPD on the bulk Ag. Figure 9 shows CV curves recorded on the

Figure 7. TEM images of the silver nanoparticles.

NPs are composed of pure Ag (spectr1, spectr2, spectr3 spots in Figure 7). These results are in general agreement with the results reported in the Starowicz et al. paper.34 3.2. Hybrid Interface Characterization. 3.2.1. AFM Imaging. Ag(NP) adsorption is obtained by dipping a GC/ 4BrBT surface in a stirred suspension of Ag(NP). The silver nanoparticles are obtained by electrochemical oxidation, 20 s, of silver wire. Figure 8a,b,c shows three topographic images of the freshly polished GC, GC/4-BrBT, and GC/4-BrBT/ Ag(NP) surfaces. The GC surface features a relatively flatgrained structure: the 2D average grain size is found to range between100 and 200 nm with a height of 4 to 10 nm. The GC/ 4-BrBT surface resembles the bare GC surface. and the polymer film appears to quite closely copy the topography (both for simple appearance and grain dimensions) of the bare GC surface, save the appearance of roughly circular aggregates of about 1 μm diameter and 6 to 12 nm higher than the “average” surface level. Two of these aggregates can be seen in Figure 8b: top left corner and almost center positions. Figure 8c shows a topographic image of the GC/4-BrBT/Ag(NP) surface. The image was recorded in noncontact mode, because the contact mode used for the GC and GC/4-BrBT analysis was not able to yield sufficiently clear pictures. The surface is different (with respect to the GC/4BrBT surface, Figure 8b), comprising larger grains, roughly of 400 nm diameter. A substructure is clearly present, with the larger aggregates due to the coalescence of native Ag nanoparticles, with an average radius of 20 nm. Compare with section 3.1.3 for the dimensions of freshly synthesized Ag(NP). Surfaces shown in Figure 8a,b,c feature 5.80, 4.96, and 36.06 nm roughness average values, respectively

Figure 9. CVs of a 5 mM K3Fe(CN)6/K4Fe(CN)6 aqueous solution, 0.3 M K2SO4 base electrolyte. 0.05 V s−1 scan rate. Black line: GC/4BrBT working electrode. Red curve: GC/4-BrBT/Ag(NP) working electrode.

GC/4-BrBT surface and on the hybrid GC/4-BrBT/Ag(NP) surface, where an acceptable grade of electrochemical reversibility is maintained. Note that a significant increase, about 20%, in the peak current is observed in the case of the hybrid system; tentatively, this result can be related to an increase in the electrode effective surface area, even if the comparison of the average roughness values (compare Figure 8 AFM results) were consistent with a larger increase in the

Figure 8. AFM topography images of (a) freshly polished GC surface 3 μm × 3 μm, (b) GC/4-BrBT surface 3 μm × 3 μm, (c) GC/4-BrBT/ Ag(NP) surface 0.8 μm × 0.8 μm. 15509

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comparison, a spectrum monitoring the S32 signal was recorded on the bare GC surface (green line). Note that a systematic increase in intensity is observed GC/ 4Br-BT (red line) > GC/BT (blue line) > GC background (green line). This indicates effective “grafting” of the GC surface by the thiophene moiety obtained with the electrochemical assisted grafting under reduction regime (confirming the formation of an ultrathin film of thiophene grafted on the GC). Figure 11b shows the statistical weight associated with the various measurements, indicating that the three different LAICP-MS mass signals in Figure 11 (with particular reference to the comparison of the blue and green line signals) are statistically significant and actually reflect different S 32 quantities present on the GC surface. Figure 12 shows the LA-ICP-MS Ag107 spectra from a GC/4BrBT interface before and after dipping in the Ag(NPs)

current when changing from the GC/4-BrBT to the GC/4BrBT/Ag(NP) working electrode surface. Figure 10 shows the CV38 for Zn electrodeposition, GC/4BrBT/Ag(NP) working electrode: three different curves

Figure 10. CVs of a 0.08 M ZnO aqueous solution, 1 M NaOH base electrolyte. GC/4-BrBT/Ag(NP) working electrode. 0.1 V s−1 scan rate. The potential is held constant at −1.55 V: long dashed line (5 s), dotted line (10 s), solid line (15 s). Inset: −0.1 to −0.5 V potential range, −0.5 to −1 μA the current range.

(recorded successively on the same physical surface) are shown, the potential was held constant for 5, 10, 15 s at −1.55 V. A stripping UPD peak is found at −0.24 V, with current increasing proportionally to the rest time at −1.55 V (as shown in the Figure 10 inset). These results are in close agreement with the findings of Adzic, regarding the UPD of Zn on bulk polycrystalline Ag.35 Comparison of Zn electrodeposition on the GC, GC/BT, and GC/4Br-BT surfaces can be found in the Supporting Information. LA-ICP-MS spectra were recorded to probe the chemical composition of the different surfaces, i.e., to monitor the effective presence of thiophene moiety (sulfur) and Ag(NPs), on the GC surface. Figure 11 shows LA-ICP-MS S32 mass signal for a 4-BrBT “grafted” GC surface, that is, the spectrum of sulfur for a GC surface after a single CV cycle under reduction regime, i.e., a GC/BT surface (blue line spectrum) and the spectrum for a similar surface after 5 CV cycles under oxidation regime, i.e., a GC/4Br-BT surface (red line). For the sake of

Figure 12. Cps/time Ag107 mass spectra are reported: dotted line, bare GC surface; dashed line, GC dipped in Ag(NP) suspension; solid line, GC/4-BrBT/Ag(NP) surface.

suspension. The Ag 107 signal of the hybrid interface (continuous line after five consecutive careful washing and sonication cycles) is about 100 times greater than the Ag107 MS signal of the background (dotted line), indicating chemisorption of the Ag(NPs) induced by the GC/4-BrBT interface (sulfur atoms). 3.2.1. Raman, SERS Observations. Figure 13 shows Raman spectra (uncorrected raw data) recorded on three different surfaces: (a) black line, the bare GC surface; (b) blue line, the GC/BT/Ag(NP) hybrid surface, ultrathin (in the monolayer range) chemisorbed organic film; (c) red line, the GC/4Br-BT/ Ag(NP) hybrid interface. Note that the scattering intensity in arbitrary units is scaled to a different extent for the three

Figure 11. (a) Cps/time S32 mass signal is reported: (i) green line is the GC background, (ii) blue line the GC/BT surface, (iii) red line the GC/4Br-BT surface; (b) the median value of each spectra is reported with the error (gray box), as well as the 5th and 95th percentiles (black dots).

Figure 13. Raman spectra: black curve, freshly polished GC surface; blue curve, GC/BT/Ag(NP) surface (ultrathin BT polymer film thickness); red curve, GC/4-BrBT/Ag(NP) surface (1 μm 4-BrBT polymer film thickness). 15510

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different surfaces. In particular, the red and black spectra are multiplied by a 10−4 and 10−2 factor, respectively, due to the SERS effect. The black curve shows the Raman scattering spectrum of the bare GC surface: two major peaks are found at 1330 and 1600 cm−1, in close agreement with the results in the literature.36,37 The blue curve is the Raman spectrum recorded on a hybrid interface, i.e., GC/BT/Ag(NP), where the organic film is estimated to be 1 to 3 ML thick (it is obtained with a single CV scan, under reduction regime), two small amplitude peaks are evident at 1062 and 1448 cm−1. The red spectrum is relevant to the hybrid interface where the organic film is the result of five CV cycles, i.e., the GC/4Br-BT/Ag(NP) interface (produced by a single CV scan in reduction regime, followed by four CV scans in oxidation regime). The theoretical Raman spectrum calculated for the isolated 4-BrBT species, at the B3LYP/6311G** level of the theory, shows a strong peak at 1485 cm−1, which is in close agreement with the experimental peak at 1448 cm−1, such a peak is absent in the spectrum of the bare GC surface, and it is of extremely low intensity in the hybrid interface featuring the ultrathin film. Finally, the 1448 cm−1 peak is the dominant peak in the spectrum recorded on the hybrid interface characterized by a 0.5 μm film thickness of the electropolymerized 4-BrBT compound. The same line of reasoning can be roughly applied to the peak found at 1062 cm−1; the shift to lower wavenumber (about 30 cm−1) is probably due to the more extended polymeric structure. The vertical dotted lines in Figure 13 are centered on the peak positions of the ultrathin organic film interface, for the sake of comparison of exact peak position in the three spectra.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Italian Ministry of University and Research (MIUR) through PRIN 2008 (Project 2008N7CYL5) directed by Prof. M. L. Foresti (national coordinator). The “Centro Interdipartimentale Grandi Strumenti” of the University of Modena and Reggio Emilia is kindly acknowledged for LA-ICP-MS and TEM measurements.

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REFERENCES

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4. CONCLUSIONS 1. A novel procedure, top-down, to prepare a hybrid interface is proposed aiming to exploit Ag properties in both bulk/massive form and nanoparticle states. The preparation strategy exploits the electrochemically assisted grafting of the glassy carbon surface by means of a suitable organic compound: the 4Brbithiophene. The latter compound features a suitable leaving group upon reduction, good electronic conductivity, and sulfur atoms (allowing the chemical adsorption of the Ag(NP). 2. The structure of both the grafted thiophene and the Ag(NP) was probed by typical surface sensitive techniques together with electrochemical characterization of the final hybrid interface used as a working electrode. 3. Finally, the overall picture concurs to establish that the obtained GC/4Br-BT/Ag(NP) interface incorporates both bulk silver (i.e., CV for the Fe(II)/Fe(III) redox couple and Zn UPD) and silver nanoparticle (Raman SERS) behavior, with the advantage of an extremely low quantity of silver required to prepare such a surface, compared to the surface of bulk Ag. Although some interesting results were achieved, some drawbacks are still present. These deserve further study and are currently under investigation in this lab. In particular, this includes the selection/design of suitable organics, still thiophene based compounds, aiming to achieve better control of the polymerized film structure and the overall conductivity of the GC/thiophenepolymer/Ag(NP) hybrid interface.

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(34) Starowicz, M.; Stypuła, B.; Banas, J. Electrochemical synthesis of silver nanoparticles. Electrochem. Commun. 2006, 8, 227−230. (35) Adzic, G.; McBreen, J.; Chu, M. G. Adsorption and Alloy Formation of Zinc Layers on Silver. J. Electrochem. Soc. 1981, 128, 1691−1694. (36) Yoshikawa, M.; Nagai, N.; Matsuki, M.; Fukuda, H.; Katagiri, G.; Ishida, H.; Ishitani, J. Raman scattering from sp2 carbon clusters. Phys. Rev. B 1992, 46, 7169−7174. (37) Marshall, I.; Nathan, I.; Smith, J. E., Jr.; Tu, K. N. Raman spectra of glassy carbon. J. Appl. Phys. 1974, 45, 2370−2373. (38) The experimental measure is actually a chronoamperometric measurement (steps and sweeps). In the present case, the current is recorded applying a first potential linear sweep (from 0 V to −1.55 V, 0.1 V s−1 scan rate), then the potential is maintained constant for a fixed amount of time (three experiments are reported: 5, 10, and 15 s per step), a final potential linear sweep (from −1.55 to 0 V, 0.1 V s−1 scan rate) is applied. Thus, the plot in Figure 10 is a parametric plot, as an implicit function of time (it is different from a classical CV, in that the potential is held constant for a desired amount of time at the vertex inversion potential.

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