Fabrication of Glass-Insulated Ultramicrometer to Submicrometer Carbon Fiber Electrodes to Support a Single Nanoparticle and Nanoparticle Ensembles in Electrocatalytic Investigations. 
We report a procedure to fabricate glass-insulated carbon fiber ultramicroelectrodes (CF UMEs) with sizes of 10 and 1 μm in diameter.  CF UMEs of theses sizes are commonly insulated with polymer or epoxy resins, which typically have pinholes and are less stable and difficult to polish.  Through judicious choice of polishing materials, the fabrication procedure reported here leads to CF UMEs with a surface geometry that, within experimental error, is close to that of an inlaid disk.  We demonstrate how the hardness factor of commonly used polishing materials may alter the geometry of CF UMEs, in which carbon has a larger hardness factor compared to other metals (i.e., Pt or Au).  Atomic force microscopy (AFM) and steady-state voltammetry (SSV) were used to characterize the CF UME surface and record the electrochemical response, respectively.  These results were compared to theoretical values for an inlaid disk UME.  Pt nanoparticle (NP) collision experiments using electrocatalytic amplification were used to deposit an exact number of Pt NPs on a CF UME surface.  Surface roughness was found to significantly decrease the sticking of Pt NPs on the CF surface, compared to the theoretical collision frequency.  However, decreasing surface roughness through judicious polishing led to good agreement between experimental and theoretical collision frequencies of NPs on a CF UME surface, leading to the fabrication of NP ensembles of UME dimensions (UME-NPE) and a single-nanoparticle UME (SNP-UME).  These electrodes were used to record and analyze SSVs for the hydrogen evolution reaction (HER) in acidic media.