Flow Analysis-Based Surface-Enhanced Raman Spectroscopy Employing Exchangeable Microbeads as SERS-Active Surfaces

Abstract

This paper reports on a new fully automated approach for highly sensitive and robust surface-enhanced Raman spectroscopic (SERS) detection using flow systems. An aqueous solution of silver nitrate was reduced by the addition of sodium borohydride and the thus formed silver sols adsorbed on Sephadex anion exchanger microbeads (dry bead size: 50–120 μm) present in the reaction mixture. In this way microbeads carrying a SERS-active layer were generated that could be easily pumped in the flow system without facing problems due to Ag-sol adsorption on the Teflon® tubing. After online generation of the SERS-active microbeads, the analyte (test analyte: nicotinic acid) was added, and the homogeneous SERS–microbead–analyte suspension pumped into a specially designed flow cell where the beads were retained and the SERS spectra measured with a Fourier transform (FT)-Raman instrument with a laser wavelength of 1064 nm. Afterwards the SERS microbeads were discarded by flow reversal and new, freshly prepared SERS microbeads together with the analyte could be introduced automatically into the flow system. The developed method is characterized by complete computer control of all operation steps, high sensitivity of the SERS measurement, high precision, and long-term stability. The concentration dependence of the SERS signals was investigated over a range from 5·10−6 M to 0.1 M nicotinic acid. The signal intensities increased until 1 mM, after which no further rise in the SERS intensities was found. Typical precision achieved was 8% throughout the whole calibration range.