The electrochemical responses of the MHF-GNC multilayer film-modified ITO electrode were also investigated in acidic (pH = 1) and neutral (pH = 7) solutions containing ClO—, NO—, or SO4 —. The peak potentials of the wave due to the redox of the ferrocene/ferrocenium cation (Fc/Fc+) couple were almost independent of the pH but strongly dependent on the nature of the electrolyte as shown in Table 1. The results are in good agreement with those reported for ferrocene thiol SAMs-modified gold electrodes,[42-44] indicating that the ferrocene molecules adsorbed on the GNCs behaved essentially in the same manner as those of the SAMs on planar electrodes.
The amounts of redox-active ferrocene groups of the multilayer-modified gold and ITO electrodes estimated from the anodic charge of the redox peak are plotted as a function of the number of deposition cycles in Fig. 3. Although the amount of the electroactive Fc moiety increased with increase in the number of deposition cycles in both cases, the manner and the magnitude of increase in the amount of the electroactive ferrocene moiety with increase in the number of deposition cycles depended on the substrate. While the amount of the redox-active Fc group on the gold substrate increased almost linearly with increase in deposition cycles, the increment of that on the ITO substrate at each deposition cycle became smaller at higher deposition cycle. The increment of the adsorbed Fc groups at each deposition cycle on gold was 1.8 x 10"10 mol/cm2 in average and that on ITO was 12.5 x 10"10 mol/cm2 initially and decreased to 5.5 x 10"10 mol/cm2 as the number of deposition cycles increased. Because one GNC contained 10 FcC6SH molecules on the surface as mentioned above, the numbers of adsorbed GNCs per deposition cycle on ITO and Au substrates were estimated to be 7.6-3.4 x 1013/cm2 and 1.1 x 1013/cm2, respectively. Both values are greater than that calculated for a hexagonally packed monolayer of thiol-covered GNCs, (3.8-8.8) x 1012/cm2,[40] as well as that of a previously reported GNC assembly with a comparable core size.[14] One possible reason for this discrepancy is that not the monolayers but the relatively thick GNC/PAH layers were adsorbed on both surfaces at each deposition cycle under the present preparation conditions. We used a rather high concentration of salt in the PAH adsorbing solution (1 M NaCl), and it is known that the use of an adsorbing solution of high ionic strength usually results in the thicker adsorbed layers.[33] The reason for the larger amount of deposited GNC and the decrease in the deposited amount as the number of deposition cycles increased on the ITO substrate will be discussed later.
Table 1 Redox potential and peak separation of Fc/Fc+ couple of an MHF-GNC/PAH multilayer-modified ITO electrode in aqueous solutions containing 0.1 M of various anions at two pH values.
Fig. 3 Amount of electroactive Fc groups as a function of number of MHF-GNC deposition cycles on ITO (O) and Au (□) electrodes.
Fig. 4 Apparent differential capacitances before (4 and □) and after (O and O) oxidation of ferrocene moiety as a function of MHF-GNC deposition cycles on the ITO (4 and O) and Au (□ and O) electrodes.
Differential Capacitance of MHF-Gold Nanocluster/Poly(Allylamine Hydrochloride) Multilayers
Fig. 4 shows the deposition cycle dependencies of the apparent differential capacitances of GNC multilayers on ITO and gold electrodes calculated by dividing the charging currents observed before and after the redox peak by the scan rate.a For all cases, the apparent differential capacitances after the ferrocene oxidation were greater than those before the oxidation and increased with increase in the number of deposition cycles. Increase in capacitance upon oxidation of the ferrocene moiety has been ob served at a ferrocene SAM-modified electrode and is known to be a result of an increase in hydrophilicity.[45]
The capacitance increased with increase in the number of deposition cycles in both cases, suggesting that the immobilized GNCs act as an ensemble of numerous nanosized electrodes. It is notable that the capacitance increased linearly with increase in the number of deposition cycles at the GNC-modified gold electrode, but the capacitance increment per deposition cycle became smaller as the number of deposition cycles increased at the GNC-modified ITO electrode. This observation is in accordance with the observation for the deposition cycle dependencies of the amount of redox-active Fc groups. The surface area of one GNC with a core size of 1.8 nm is ca. 1.02 x 10—13 cm2. Using the number of GNCs adsorbed on the ITO surface estimated from Fig. 3, the total electrode surface area of the GNCs assembled on an ITO surface of 1 cm2 can be calculated to be ca. 7.5 (1 cycle)-54 (10 cycles) cm2. Because the capacitances of the GNC-modified ITO electrode before and after the redox peak were ca. 50 (1 cycle)-140 (10 cycles) pF/cm2 and 73 (1 cycle)-280 (10 cycles) pF/cm2 based on the apparent surface area of the ITO electrode, the capacitances of the GNC assembly on the ITO surface in reduced and oxidized states of the ferrocene moiety based on the real electrode surface area were ca. 6.7-2.6 and 9.7-5.2 pF/cm2, respectively. Similarly, the capacitances of the GNC assembly on the Au electrode surface based on the real electrode surface area were estimated to be ca. 7.0 and 13.0 pF/cm2 in reduced and oxidized states of the ferrocene moiety, respectively. Thus although the apparent capacitances of the GNC/PAH-modified ITO electrode were much greater than those of the GNC/PAH-modified gold electrode, the capacitances of the GNC/ PAH-modified ITO electrode based on the real surface area were smaller than those of the GNC/PAH-modified gold electrode. The difference between the capacitances of the GNCs on the ITO electrode and those on the gold electrode may reflect the higher conformational order of the SAMs on the GNCs assembled on the ITO electrode than those on the gold electrode. Actually, the redox peaks at the GNC-modified ITO electrode were sharper than those at the gold electrode as mentioned before, indicating a stronger interaction between the ferrocene groups on the GNCs assembled on the ITO electrode than on the gold electrode. Thus the difference is essentially because of the difference in the amount of deposited GNCs on these surfaces, and the GNC/PAH layer on the ITO electrode is more compact than that on the gold electrode.
These capacitance values are comparable to those of ferrocene SAMs on a planar Au surface,[42] confirming that the order of ferrocene SAMs formed on a GNC surface is essentially the same to that of ferrocene SAMs formed on a planar Au surface.[7]
Effect of the Underlying Substrate on the Formation of Gold Nanocluster/ Poly(Allylamine Hydrochloride) Multilayers
The amounts of adsorbed GNC multilayers on the ITO electrode were always greater than those on the Au electrode of the same deposition cycle, suggesting that the underlying substrate plays an important role in GNC multilayer formation. Atomic force microscopy measurements of the two substrates showed that while the Au surface consisted of very large and almost atomically flat terraces, a number of hemispherical islands of ca. 100 nm in diameter and several tens of nanometers in height were observed on the ITO surface. An ordered and closely packed MUA SAM is expected to form on a gold surface as it has been well documented for the formation of various alkanethiol SAMs, and both the adsorbed PAH and the GNC layers are expected to be well organized and closely packed when the gold surface with uniformly distributed charged sites of the carboxylate group was alternately immersed in PAH and GNC solutions.[33]
The amount of adsorbed GNC of the multilayer-modified ITO electrode was 3-7 times greater than that of the Au electrode, although the surface roughness of the ITO electrode was only ca. 2.5 times greater than that of the Au electrode, as determined from the AFM images. These findings suggest that the multilayer formation of GNC on an ITO surface and therefore the electrochemical behavior were related to but not solely determined by surface roughness. The nature of the underlying metal oxide layers on the ITO electrode as well as the adsorbed MUA molecules should play important roles in the GNC packing arrangement and therefore its electrochemical behavior. We have already demonstrated that alkanethiol molecules tend to form an island structure on the ITO surface with surface coverage of only 1/3-1/4 of that on the gold surface, although the formed SAM was well organized.[46] A nonuniform MUA SAM layer on the ITO surface and/or intrinsically dangling Sn-O— bonds resulted in disordered polymer layer adsorption. The GNCs can both adsorb on top of the polymer surface and diffuse into the layers, binding to internal sites of PAH through ion exchange of uncompensated anions. This is one possible reason for the large amount of adsorbed GNC on the ITO. The deviations from linear relations of both the adsorption amount and the capacitance of the ITO-modified electrode with the number of deposition cycles shown in Figs. 3 and 4, respectively, may be the result of the less-ordered multilayer structure on ITO than that on Au.
The decreases in the deposited amount of GNC per deposition cycle and the capacitance per unit real area with increase in the number of deposition cycles can also be explained by considering that the surface became smoother as more GNC/PHC layers were deposited.
Electrochemical Characteristics of the Multilayers Consisting of Only One MHF-Gold Nanocluster Layer and MH-Gold Nanocluster as the Other Layers with Two Different Sequences
The electrochemical characteristics of multilayers of MHF-GNC and MH-GNC with two different sequences containing only one layer of MHF-GNC, as described in Scheme 1a and 1b, were investigated. In the first system, MHF-GNC existed in the layer closest to the electrode surface, i.e., the first layer, and MH-GNC existed in the other layers [Scheme 1a: Au or ITO/ MUA/PAH/MHF-GNC/(PAH/MH-GNC)n, n = 1-9]. In the other system, MHF-GNC existed only in the outermost layer and MH-GNC existed in the other layers [Scheme 1b: (Au or ITO/MUA/(PAH/MH-GNC)n/PAH/ MHF-GNC, n = 1-9].
The X-ray photoelectron spectra of the multilayer assemblies (a) Au(111)/MUA/PAH/MHF-GNC/(PAH/ MH-GNC)n for n = 1, 3, and 5 and (b) Au(111)/MUA/ (PAH/MH-GNC)n/PAH/MHF-GNC for n =1,3, 5, and 9 in the Fe2p region showed that while the Fe peak was observed only for n = 1 and no peak was observed even at n = 3 in the former, the peak intensity did not change with n in the latter, confirming that the ferrocene group definitely existed at the innermost and outermost layers in the former and the latter systems, respectively, as suggested in Scheme 1a and 1b.
The redox peaks of the ferrocene group were observed in the CVs of the former and the latter multilayer-modified gold and ITO electrodes measured in a 0.1 M HClO4 aqueous solution. Although the double-layer charging currents increased with increase in the number of MH-GNC deposition cycles, suggesting sequential LBL deposition of MH-GNC on the ITO surface, the charges in the Fc redox peaks did not depend on the number of MH-GNC deposition cycles and are in good agreement with that of the electrodes modified with only one MHF-GNC layer, implying that all of the Fc groups in the innermost or outermost layer were electrochemically active regardless of the number of MH-GNC layers, showing that the transport of both electrons and anions through these two types of multilayers was rather facile.
Charge Transfer Through the Gold Nanocluster Multilayers
Because the redox process of the surface-attached ferrocene group is known to be represented by:
where — Fc and — Fc+ are the neutral and oxidized states of the ferrocene group, respectively, Xs" is the solvated anion in solution, — Fc+X" is an ion pair between the ferricenium cation and the anion, and solv. is a solvent molecule; the electron and anion should be transferred between the ferrocene moiety and the electrode, and between the electrolyte solution and the ferrocene moiety, respectively, upon the redox reaction of the ferrocene group. Thus the results shown in Figs. 2 and 4 strongly suggest that perchlorate anion, which forms an ion pair with Fc+, is transferred between the electrolyte solution and the ferrocene group at the innermost layer through the 1-9 PAH/MH-GNC layers at the Au/MUA/PAH/ MHF-GNC/(PAH/MH-GNC)n (n = 1, 3, 5, and 9) electrodes, and an electron is transferred between the ferrocene group at the outermost layer and the gold electrode through the 1-9 PAH/MH-GNC layers at the Au/MUA/ (PAH/MH-GNC)n/PAH/MHF-GNC (n = 1, 3, 7, and 9) electrodes.
Fig. 5 shows the potential dependencies of current and EQCM frequency shift at the Au/MUA/PAH/MHF-GNC/ (PAH/MH-GNC)9 electrode obtained simultaneously in 0.1 M HClO4 aqueous solution at a scan rate of 10 mV sec"1 A significant frequency decrease and increase, i.e., weight increase and decrease, were observed when the current due to the oxidation and reduction of the ferrocene moiety, respectively, flowed. A linear relation was observed between the mass change calculated from the frequency change by using Saubrey’s equation and the charge in the region of the ferrocene oxidation (250550 mV) with a slope of 216 g/mol electron. A similar EQCM response was observed for the Au/MUA/PAH/ MHF-GNC/(PAH/MH-GNC)3 and Au/MUA/PAH/MHF-GNC/(PAH/MH-GNC)6 electrodes. The slopes for the mass change-charge relations, i.e., mass per electron (mpe), for the former and the latter were 233 and 177, respectively. Furthermore, essentially the same EQCM response was obtained at the Au/MUA/(PAH/MH-GNC)9/ PAH/MHF-GNC. These results clearly show that the EQCM response was not affected by the number of the PAH/MH-GNC overlayers, confirming that perchlorate ions are transferred between the electrolyte solution and the ferrocene group at the innermost layer through the PAH/ MH-GNC. The mpe values obtained for Au/MUA/PAH/ MHF-GNC/(PAH/MH-GNC)n (n = 3, 6, and 9) are in good agreement with the value observed at the gold electrode modified with PAH/MHF-GNC multilayers (245 g/mol electron) and are much larger than the molar mass of perchlorate ion (99.5 g/mol), suggesting that each perchlo-rate ion moves into and is released from the modified layer accompanied by 5-7 water molecules upon the oxidation and reduction of the ferrocene moiety, respectively.
Fig. 5 EQCM response of the Au(111)/MUA/PAH/MHF-GNC/(PAH/MH-GNC)9 electrode measured in 0.1 M HClO4 aqueous solution at a scan rate of 10 mV sec"1.
Because the distance between the ferrocene group in the outermost layer and the gold electrode in the GNC multilayers with 10 deposition cycles is far too long for an electron to tunnel, i.e., more than 50 nm as evidenced from both cross-sectional transmission electron microscopy and ellipsometry, electron should be transferred between the ferrocene group and the gold electrode not by direct tunneling, but through the GNCs by hopping.
Scheme 2 Schematically idealized model for the charge transfer during the redox of the ferrocene moiety.
CONCLUSION
Two types of GNCs, one covered by SAMs of MUA, C6SH, and FcC6SH, MHF-GNC, and the other with MUA and C6SH, MH-GNC, were used for the construction of GNC multilayers on the MUA-modified Au(111) and ITO surfaces based on the carboxylate/polycation (PAH)/ carboxylate electrostatic interaction. Electrochemical measurements at the GNC/PAH multilayer-modified gold and ITO electrodes showed that the amount of GNC adsorbed on the ITO surface was larger than that of the Au electrode, indicating the different packing arrangements of GNC on the two surfaces. Both the underlying substrate and the adsorbed MUA molecules seem to play important roles in the GNC/polymer multilayer organization. Results of electrochemical studies on the two series of multilayers of MHF-GNC and MH-GNC layers of different sequences on Au and ITO surfaces by placing MHF-GNC either as the layer closest to the gold electrode, i.e., the first layer, or as the outermost layer with MH-GHC in the other showed that both electrons and anions transferred facilely through these multilayers. The results of the present study imply that 1) stable GNC multilayers can be constructed on gold and ITO surfaces through LBL assembling processes, 2) the amount of GNCs and their packing arrangement in the multilayers can be tuned by the number of deposition cycles and selection of underlying substrate, and 3) the charges, i.e., electron and anion, are transported through these multilayers as shown in Scheme 2 and the charge transport is facile.
