Hybrid Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Ammonium Nitrate, Thiram, and Nile Blue

Abstract

We report the fabrication and performance evaluation of hybrid surface-enhanced Raman scattering (SERS) substrates involving laser ablation and chemical routes for the trace-level detection of various analyte molecules. Initially, picosecond laser ablation experiments under ambient conditions were performed on pure silver (Ag) and gold (Au) substrates to achieve distinct nanosized features on the surface. The properties of the generated surface features on laser-processed portions of Ag/Au targets were systematically analyzed using UV-visible reflection and field emission scanning electron microscopy studies. Later, hybrid-SERS substrates were achieved by grafting the chemically synthesized Au nanostars on the plain and laser-processed plasmonic targets. Subsequently, we employed these as SERS platforms for the detection of a pesticide (thiram), a molecule used in explosive compositions [ammonium nitrate (AN)], and a dye molecule [Nile blue (NB)]. A comparative SERS study between the Au nanostar-decorated bare glass, silicon, Ag, Au, and laser-processed Ag and Au targets has been established. Our studies and the obtained data have unambiguously determined that laser-processed Ag structures have demonstrated reasonably good enhancements in the Raman signal intensities for distinct analytes among other substrates. Importantly, the fabricated hybrid SERS substrate of "Au nanostar-decorated laser-processed Ag" exhibited up to eight times enhancement in the SERS intensity compared to laser-processed Ag (without nanostars), as well as up to three times enhancement than the Au nanostar-loaded plain Ag substrates. Additionally, the achieved detection limits from the Au nanostar-decorated laser-processed Ag SERS substrate were ∼50 pM, ∼5 nM, and ∼5 μM for NB, thiram, and AN, respectively. The estimated enhancement factors accomplished from the Au nanostar-decorated laser-processed Ag substrate were ∼10⁶, ∼10⁶, and ∼10⁴ for NB, thiram, and AN, respectively.

Figure 1

UV–visible absorption spectra of the synthesized Au nanostars.

Figure 2

(a) Reflectivity of (i) plain Ag, (ii) plain Au (dotted line), (iii) laser-processed Au, and (iv) laser-processed Ag substrates, and the (b) inset shows the plain Ag reflectivity spectrum.

Figure 3

(a) TEM images of Au nanostars and (b) histogram plot for the core size of Au nanostars.

Figure 4

FESEM micrographs and the corresponding EDX spectra of Au nanostars decorated on (a,b) plain Ag and (c,d) plain Au substrates, respectively.

Figure 5

(a–c) FESEM micrographs of laser-processed regions of Ag obtained at different magnifications (5 KX, 20 KX, and 50 KX, respectively). Yellow square highlighted with arrow symbols in (a) is where the higher-magnification image (b) was collected. (d) Gray value (arb. units) vs horizontal position corresponding to (c) was achieved using Gwyddion S/W. Few nanoholes and nanoprotrusions are highlighted with yellow arrows in (c).

Figure 6

(a–c) FESEM micrographs of laser-processed regions of Au obtained at different magnifications (5 KX, 20 KX, and 50 KX, respectively). Yellow square highlighted with arrow symbols in (a) is where the higher-magnification image (b) was collected. (d) Gray value (arb. units) vs horizontal position corresponding to (c) achieved using Gwyddion S/W. Few nanoholes and nanoprotrusions are highlighted with yellow arrows in (c).

Figure 7

FESEM micrographs of Au nanostar-decorated laser-processed portions of (a) Ag and (b) Au substrates. Portions with yellow circles show a few of the highlighted nanostars.

Figure 8

(a) SERS spectra of thiram obtained with the concentrations varying from 5 mM to 5 nM and collected from the Au nanostar-decorated plain Au substrate. (b) Log plot of the 1375 cm^–1 peak SERS intensity vs thiram concentrations. (c) 3D waterfall Raman spectra plotted by collecting the data of thiram (5 mM) at 20 randomly selected sites from the Au nanostar-decorated plain Au substrate. (d) Histogram plot for the Raman intensities of 560, 1138, and 1375 cm^–1 peaks measured at 20 sites, and the inset shows the calculated RSD values for the respective peak intensities.

Figure 9

(a) SERS spectra of thiram obtained with concentrations varying from 500 μM to 5 nM recorded from the Au nanostar-decorated plain Ag substrate, (b) log plot of the 1375 cm^–1 peak SERS intensity vs thiram concentration, (c) 3D waterfall Raman spectra plotted by collecting the data of thiram (5 mM) randomly at 20 sites from the Au nanostar-decorated plain Ag substrate, and (d) histogram plot for the Raman intensities of 560, 1138, and 1375 cm^–1 peaks measured at 20 random sites, and the inset shows the calculated RSD values for the respective peak intensities.

Figure 10

(a) SERS spectra of thiram (5 μM) acquired from (i) laser-processed Au (without nanostars), (ii) laser-processed Ag (without nanostars), and Au nanostars decorated on (iii) plain Au, (iv) plain Ag, (v) laser-processed Au, and (vi) laser-processed Ag substrates. (b) SERS intensity for a major mode of thiram (1375 cm^–1) obtained from the respective substrates. The error bars are estimated based on the standard deviation of the average SERS intensity at the 1375 cm^–1 peak achieved by repetitive SERS measurements for four times.

Figure 11

(a) SERS spectra of NB with the concentrations ranging from 5 μM to 50 pM recorded from the Au nanostar-decorated laser-processed Ag substrate, (b) log plot of the 590 cm^–1 peak SERS intensity vs NB concentration, (c) SERS spectra of AN with the concentrations varying from 5 mM to 5 μM recorded from the Au nanostar-decorated laser-processed Ag substrate, and (d) log plot of the 1041 cm^–1 peak SERS intensity vs AN concentration.