(Invited) Ultrasensitive Electroanalytical Detection and Study of Single Nanoparticle Catalysts

The continued interest in nanoparticles (NPs) is due to their numerous properties such as active components in sensors, optics,  energy conversion and energy storage applications.   The development of new analytical tools for characterizing NP properties  in a fast and reproducible manner is sorely needed.  Especially for those to correlate structure-function relationships of NPs and optimize NPs activity for in homogeneous and heterogeneous catalysis. The precise characterization of individual NPs in terms of their size, shape and composition would faciliate better fundamental understanding.  The development of "electrocatalytic amplification" is one such method where single NPs are detected  by measuring electrocatalytic current (or potential) due to the electrochemical processes (oxidation/reduction of the species present in solution) occurring on the surface of the NP whenever a NP collides with an inert UME (gold, platinum, platinum oxide and carbon) which otherwise cannot catalyze the reaction.

           Here we report the application of electrocatalytic amplification method employing inert (non-catalytic) UMEs  (Hg, Bi, Ga) as electrodes for evaluating the catalytic activity of Pt NPs of different shapes (hexagones and cubes) for hydrazine oxidation.  To precisely correlate structure function relationship of Pt NPs, Pt NPs without ligands referred as "naked Pt NPs" were used in this study.  "Naked Pt NPs" were obtained by following the procedure reported previously where polyacrylate capped Pt NPs were synthesized first followed by a strong base treatment (potassium hydroxide) to remove the ligand (polyacrylate) off the NP's surface.  From TEM the average size of polyacrlylate capped hexagonal and cubic Pt NPs was found to be 11.2 ± 2.8 nm and 11.8 ± 1.7 nm respectively.  Hexagonal Pt NPs possess (100) and (111) facets where cubic Pt NPs possess (111) planes as preferential surface facets.  Complete removal of polyacrylate ligand from the Pt NP's surface was confirmed by obtaining CVs, TGA and IR spectra of polyacrylate capped Pt NPs before and after KOH treatment.  Also, Pt NPs retained their size and structure after base treatment.  The integrated charge passed per spike during single NP collision for polyacrylate capped hexagonal and cubic Pt NPs concentrated over a range of (120-210) and (40-100) pC respectively.  Whereas, for naked hexagonal and cubic Pt NPs the integrated charge passed per spike during single NP collision concentrated over a range of (180-400) and (60- 180) respectively.  Naked Pt NPs displayed higher catalytic activity for hydrazine oxidation compared to polyacrylate capped Pt NPs of similar size and size.