Comparison of Glyphosate Detection by Surface-Enhanced Raman Spectroscopy Using Gold and Silver Nanoparticles at Different Laser Excitations

Abstract

Glyphosate is one of the most widely used pesticides in the world, but it has been shown to persist in the environment and therefore needs to be detected in food. In this work, the detection of glyphosate by surface-enhanced Raman scattering (SERS) using gold and silver nanoparticles and three different commonly used laser excitations (532, 632, and 785 nm wavelengths) of a Raman microscope complemented with a portable Raman spectrometer with 785 nm excitation is compared. The silver and gold nanosphere SERS substrates were prepared by chemical synthesis. In addition, colorimetric detection of glyphosate using cysteamine-modified gold and silver nanoparticles was also tested. The best results were obtained with Ag NPs at 532 nm excitation with a detection limit of 1 mM and with Au nanoparticles at 785 nm excitation with a detection limit of 100 µM. The SERS spectra of glyphosate with cysteamine-modified silver NPs improved the detection limits by two orders of magnitude for 532 nm excitation, i.e., up to 10 µM, and by one order of magnitude for 632 and 785 nm excitation wavelengths.

Keywords: SERS; colloid; glyphosate; gold; pesticides; silver; substrate.

Figure 1

(A) UV–Vis spectra of synthesized nanoparticles and glyphosate solution (2.4 mM); TEM images of (B) Ag NPs and (C) Au NPs.

Figure 2

Raman spectra of glyphosate powder recorded with different laser excitations: (A) 532 nm (B) 785 nm, (C) 632 nm, (D) 785 nm (portable Raman; baseline corrected), and (E) Raman spectrum of 10 mM glyphosate solution excited at 632 nm on Si wafer.

Figure 3

SERS spectra of different glyphosate concentrations: (A) with Ag NPs at 532 nm excitation; (B) with Au NPs at 632 nm excitation; (C) with Au NPs at 785 nm excitation; (D) with Au NPs at 785 nm excitation (baseline corrected; portable Raman).

Figure 4

UV–Vis absorption spectra after the addition of different concentration of glyphosate for: (A) Au NPs and (B) Ag NPs.

Figure 5

SERS spectra of different glyphosate concentrations with cysteamine-modified NPs: (A) Ag with 532 nm excitation, (B) Au with 632 excitation and (C) Au with 785 nm excitation. (cys: cysteamine powder).