A novel dark-field microscopy technique coupled with capillary electrophoresis for visual analysis of single nanoparticles

Abstract

A system for interfacing dark field microscopy (DFM) with capillary electrophoresis (CE) was demonstrated and applied to investigate the movement of single gold nanoparticles (AuNPs) visually and in real time for the first time. When using ~60 nm cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) capped AuNPs as target analytes, preliminary data indicated that differently charged AuNPs had different movement velocities (v). From the result of the effect of applied voltage and buffer pH value on the movement of single AuNPs, we could "image" that positively charged CTAB-capped AuNPs (red) moved faster than negatively charged SDS-capped AuNPs (green), and the former could catch up with and pass the latter. The results visually confirmed the separation rules and some fundamental theories of CE including the effect of buffer pH value on the electroosmotic flow (EOF) and the plug-like profile of EOF. This novel integration was successfully utilized to evaluate the size distribution and homogeneity of AuNPs, from which the results obtained were in accordance with transmission electron microscopy (TEM) results. The proposed system offers a new insight into CE analysis and has potential applications in many fields, such as the study of interaction between NPs and biomolecules in bioanalysis.