Observation of Discrete Au Nanoparticle Collisions by Electrocatalytic Amplification Using Pt Ultramicroelectrode Surface Modification

After growing a thin layer of oxide (PtOx) by anodization of a Pt electrode, it changed from catalytically active for electrochemical NaBH4 oxidation into an inactive electrode. When held at a potential where the oxide film was maintained, collisions of individual 14 nm diameter Au nanoparticles (NPs) that catalyzed NaBH4 oxidation were successfully observed as discrete current pulses (spikes or blips) for each NP interaction with the modified Pt electrode via amplification fromNaBH4 oxidation. The current response is affected byNPconcentration and the applied potential. SECTION Nanoparticles and Nanostructures P revious reports fromthis laboratorydescribeda scheme to observe the currents caused by collisions of individual NPs at an electrode by electrocatalytic amplification. The protocol mainly involves three components, (1) choice of an inner sphere heterogeneous electron-transfer reaction, whose reaction kinetics strongly depend on the electrode material (e.g., proton reduction or hydrazine oxidation), (2) a colloidal solution of a low concentration (∼pM) of NPs that are electrocatalytic for this inner sphere reaction, (3) an inert conductive ultramicroelectrode (UME), which, at the applied potential, shows essentially no electrocatalytic effect for this reaction. The selection of electrode material is critical in observing NPcollisions. To obtain a negligible current from the electrode at the applied potential, its electrocatalysis for the chosen reaction should be small; typically C or Au electrodes have been used. The detecting electrodemust be anUME (radius≈ nm to μm) to minimize collisions of more than one NP at a time and to obtain a good signal/noise ratio since the current that results from the collision of a single NP is very small. The NPmust be a good electrocatalyst for the reaction, and Pt NPs have usually been used. Moreover, as we have found previously, theNPmustbe incontactwith theelectrode surface so that the residence times of NPs at the electrode surface are sufficiently long to allow the current from a single NPcollision to be detected. Indeed in the previous studies, the particles stuck to the electrode; therefore, each showed a characteristic current step and an overall staircase response. As shown in Scheme 1, we changed an active Pt electrode into one with an inactive surface by simply growing a thin layer of oxide via anodization. The oxide layer inhibited the inner sphere reaction on the Pt surface but still allowed electron tunneling, for example, to a NP from the solution, to take place. NaBH4 was used as a heterogeneous inner sphere reductant. Borohydride oxidation (E = -1.24 vs NHE), asmanyothermultielectron-transfer reactions,depends strongly on the electrode material. To suppress the hydrolysis of NaBH4, 10mMNaBH4 was dissolved in 0.1 M NaOH. 12 As shown in Figure 1a, the oxidation of NaBH4 starts at negative potentials at both Au and Pt UMEs. The oxidation at the Pt UME occurs more easily (i.e., shows a negative shift in the onset potential by about 0.3 V) compared to the Au UME (region I). However, injecting Pt NPs and recording the current at a Au UME, following previous studies on hydrazine oxidation, only resulted in steady current increases with no distinguishable current steps from single NP collisions (Figure S1, Supporting Information). This inability to observe individual NP collisions has also been seen previously with other electrocatalytic systems (e.g., with Pt NPs stabilized with polyvinylpyrrolidone (PVP)on a Au electrode for hydrazine oxidation). However, when the potential was scanned tomore positive values, the current due to NaBH4 oxidation at both Au and Pt UMEsdecreasedanddropped tonear zero at potentials>0.5V (vs Ag/AgCl). This is caused by the gradual formation of adsorbed oxygen or oxide films on the metal electrodes. Oxidized Pt and Au are not electrocatalytic toward NaBH4 oxidation, as withmany other reactions that occur at themetal surface, for example, hydrogen oxidation. When the potential is scanned back in the negative direction, the oxidation current at a Au UME increases, starting at ∼0.2 V (vs Ag/AgCl), but the oxidation current at a Pt UME remains near zero until ∼-0.25 V (vs Ag/AgCl) (region II in Figure 1a) because Au Received Date: July 15, 2010 Accepted Date: August 24, 2010