On-column silver substrate synthesis and surface-enhanced Raman detection in capillary electrophoresis

A new, simple, and efficient approach for on-column surface-enhanced Raman scattering (SERS) detection in capillary electrophoresis (CE) is reported. A ∼50-µm SERS substrate spot was prepared by laser-induced growth of silver particles in the 100-µm inner diameter CE capillary window or in a flow cell consisting of a 250-µm inner diameter fused silica capillary connector. For this purpose, the Raman laser was focused by a 20× objective into the detection window filled with a 0.5 mM silver nitrate and 10 mM citrate buffer solution. During the CE runs, the silver substrate spot was formed in a few seconds after the analyte injection, hence the analytes adsorbed sequentially to the silver surface when the detection window was reached, followed by desorption from the silver surface and continuing the electrophoretic migration to the capillary end. Thus, beyond migration time, valuable molecular specific information was delivered by the SERS spectra. Accurate separations and high-intensity SERS spectra are shown by CE-SERS time-dependent 3D electropherograms for the analytes rhodamine 6G, 4-(2-pyridylazo)resorcinol (PAR), PAR complex with Cu(II) and methylene blue at 0.25-25 ppm concentrations, by using 1.4-3.6 mW HeNe laser power and an acquisition time of 5 s for each spectrum. Before and after each analyte passes the detection window, clean background spectra were recorded and no memory effects perturbed the SERS detection. The silver substrate is characterized by a fast preparation rate, good reproducibility, a preparation success rate of over 95% and no mentionable influence on the electrophoretic migration time, the CE-SERS and CE-UV electropherograms being in good agreement. The successful coupling of CE and on-column SERS detection opens new perspectives for monitoring CE separations.