The electrochemical (EC) behavior of mechanically exfoliated graphene and highly oriented pyrolytic graphite (HOPG) is studied at high spatial resolution in aqueous solutions using Ru(NH 3 ) 6 3+/2+ as a redox probe whose standard potential sits close to the intrinsic Fermi level of graphene and graphite. By coupling scanning electrochemical cell microscopy (SECCM)

ABSTRACT:There is considerable interest in understanding the interaction and activity of single entities, such as (electro)catalytic nanoparticles (NPs), with (electrode) surfaces. Through the use of a high bandwidth, high signal/noise measurement system, NP impacts on an electrode surface are detected and analyzed in unprecedented detail, revealing considerable new mechanistic information on the process. Taking the electrocatalytic oxidation of H2O2 at ruthenium oxide (RuOx) NPs as an example, the rise time of currenttime transients for NP impacts is consistent with a hydrodynamic trapping model for the arrival of a NP with a distancedependent NP diffusion-coefficient. NP release from the electrode appears to be aided by propulsion from the electrocatalytic reaction at the NP. High frequency NP impacts, orders of magnitude larger than can be accounted for by a single pass diffusive flux of NPs, are observed that indicate the repetitive trapping and release of an individual NP that has not previously recognized. The experiments and models described could readily be applied to other systems and serve as a powerful platform for detailed analysis of NP impacts.An important frontier in electrochemistry is measuring the behavior of individual nano-entities such as nanoparticles (NPs), nanowires and nanorods and relating this to other properties such as size, structure and electronic characteristics, so as to develop fundamental understanding and rational applications. 1-3 An interesting approach for observing the electrochemical properties of catalytic NPs is to monitor their impact (or landing) from solution onto a collector electrode, as introduced by Bard et al.,4,5 and developed by several groups. [6][7][8][9][10][11][12] In order to resolve such impacts, the use of a small-sized ultramicroelectrode (UME) is mandatory to reduce both background currents and the impact frequency. To enhance the impact signal to background current, electrode surfaces have been modified with Hg or Bi 7 and borondoped diamond 12 has also been used as an UME material. Alternatively, scanning electrochemical cell microscopy (SECCM) functioning as an ultramicro-electrochemical cell system offers particularly low background currents by reducing the area of the collector electrode, as well as offering the widest range of support electrodes. This is because the electrochemical cell is formed by meniscus confinement, rather than electrode encapsulation (Figure 1). 13 Despite these innovations, detailed analysis of the form of the current-time profile which is the primary signal for the landing (and detachment) of a single NP on an electrode has not yet been forthcoming, but would represent a huge advance towards understanding the impact process. Herein, we are able to analyze this process as never before and deduce key information on the NP arrival and release process from individual impact transients. Moreover, we show that impact frequencies can be orders of magnitude higher than expected based on single pass diffusion due to the repetitiv...

A simple method that uses graphene to fabricate nanotopographic substrata was reported for stem cell engineering. Graphene-incorporated chitosan substrata promoted adhesion and differentiation of human mesenchymal stem cells (hMSCs). In addition, we proposed that nanotopographic cues of the substrata could enhance cell-cell and cell-material interactions for promoting functions of hMSCs.

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