Microfluidic setup for on-line SERS monitoring using laser induced nanoparticle spots as SERS active substrate

Abstract

A microfluidic setup which enables on-line monitoring of residues of malachite green (MG) using surface-enhanced Raman scattering (SERS) is reported. The SERS active substrate was prepared via laser induced synthesis of silver or gold nanoparticles spot on the bottom of a 200 μm inner dimension glass capillary, by focusing the laser beam during a continuous flow of a mixture of silver nitrate or gold chloride and sodium citrate. The described microfluidic setup enables within a few minutes the monitoring of several processes: the synthesis of the SERS active spot, MG adsorption to the metal surface, detection of the analyte when saturation of the SERS signal is reached, and finally, the desorption of MG from the spot. Moreover, after MG complete desorption, the regeneration of the SERS active spot was achieved. The detection of MG was possible down to 10^-7 M concentration with a good reproducibility when using silver or gold spots as SERS substrate.

Keywords: SERS; gold nanoparticles; malachite green; microfluidic setup; silver nanoparticles.

Figure 1

(a) Schematic diagram of the SERS active spot synthesis, followed by MG adsorption to the nanoparticles and SERS detection; (b) the on-line SERS monitoring showing selected spectra recorded during silver spot synthesis, MG adsorption, and desorption using a 10⁻⁵ M MG solution. The inset from the figure shows a SEM image of the silver spot; (c) the intensity variation of the 1617 cm⁻¹ SERS band of MG during the entire process. For all spectra in the figure, the acquisition parameters were 2 accumulations at 1 s exposure.

Figure 2

(a) Selected SERS spectra in the time range from 100 to 510 seconds, considered from the start of the experiment (silver/citrate mixture injection), during continuous flow of 10⁻⁷ M MG in the capillary. The exposure time for recording each spectrum was 4 seconds; (b) time dependence of the SERS intensity of the 1617 cm⁻¹ MG marker band.

Figure 3

Maximum intensity of the 1178 and 1617 cm⁻¹ MG marker bands recorded on nine different silver spots.

Figure 4

On-line SERS monitoring showing selected spectra recorded during the in situ synthesis of the gold spot, the adsorption, and desorption of 10⁻⁵ M MG solution on the substrate. The inset from the figure shows an optical image of the gold spot on the glass capillary surface.

Figure 5

Schematic representation of the microfluidic SERS setup. The inset shows a picture of the glass capillary.