Single molecule/particle detection/observation

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

Thus, the decrease of particle activity was mainly due to the blockage of the catalytic surface sites for IS reactions such as hydrazine electrooxidation. The examples of applications of NP collision measurements listed earlier suggest the possibility of a wider range of applications. In terms of analytical applications, stochastic electrochemical detection based on observation of NP label individual collision events provides the single-molecule sensitivity, that is, individual analyte species are detected one at a time. As with other similar analytical schemes, the lowest concentrations that can actually be detected are limited by factors such as strength of interactions, selectivity, and the time required to detect the particle. In the field of fundamental studies, the study of individual NP collisions should allow extraction of information that is buried or averaged out in ensemble measurements. An example is an examination of factors that contribute to adsorption or sticking... 

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