Robust and cost-effective silver dendritic nanostructures for SERS-based trace detection of RDX and ammonium nitrate

Abstract

We report the fabrication and performance evaluation of cost-effective, reproducible silver nanodendrite (AgND) substrates, possessing high-density trunks and branches, achieved by a simple electroless etching process and subsequently utilized them for the trace detection of 1,3,5-trinitroperhydro-1,3,5-triazine (Research Development Explosive, RDX) and Ammonium Nitrate (AN). The intricate structural features in AgNDs offer high-density hotspots for effective molecular detection based on the surface enhanced Raman scattering (SERS) technique. The active SERS-substrate was initially tested with standard Rhodamine 6G (R6G) molecules at 1 nM concentration, which established an effective enhancement factor (EF) of ∼10⁸. The AgNDs were subsequently utilized in the detection of the explosives RDX and AN, down to concentrations of 1 μM. The typical EF achieved in the case of RDX and AN was ∼10⁴. The sensitivity of 1 μM R6G was further enhanced by two-fold through the deposition of Au nanoparticles on the AgNDs. The reproducibility of the low-cost substrate was also demonstrated, with a ∼9% RSD value in the measurements.

Fig. 1. Schematic representation of the experimental process to achieve AgNDs and Au@AgNDs.

Fig. 2. Morphological evolution of AgNDs at various AgNO₃ concentrations: (a) 5 mM (0.8 μg μL⁻¹); (b) 10 mM (1.6 μg μL⁻¹); (c) 15 mM (2.5 μg μL⁻¹); (d) 20 mM (3.3 μg μL⁻¹); (e) 25 mM (4.2 μg μL⁻¹); (f) 30 mM (5.09 μg μL⁻¹); (g) 35 mM (5.94 μg μL⁻¹); (h) 40 mM (6.79 μg μL⁻¹). (i) Corresponding EDS spectrum.

Fig. 3. The modulation of AgNDs at various AgNO₃ deposition temperatures: (a) 25 °C; (b) 30 °C; (c) 40 °C; (d) 50 °C; (e) 60 °C. (f) Corresponding elemental confirmation through EDX data.

Fig. 4. R6G molecular detection on various SERS substrates grown at different deposition temperatures (left) and the intensity of the 613 cm⁻¹ peak with increasing deposition temperature (right).

Fig. 5. (a) The SERS spectra of R6G molecules tested at: (i) 50 μM; (ii) 10 μM; (iii) 1 μM; (iv) 0.1 μM; (v) 10 nM; (vi) 1 nM. (b) Linear dependence of the Raman intensity versus concentration for the prominent modes of R6G molecules. Spectra in (a) are displaced on the Y-axis for clarity.

Fig. 6. (a) The SERS spectra of RDX explosive at: (i) 100 μM; (ii) 80 μM; (iii) 30 μM; (iv) 10 μM; (v) 5 μM. (b) Corresponding linear relationship of intensity vs. concentration.

Fig. 7. (a) The SERS spectra of AN explosive at: (i) 70 μM; (ii) 50 μM; (iii) 10 μM; (iv) 5 μM; (v) 1 μM. (b) Linear relationship of intensity vs. concentration.

Fig. 8. (a) FESEM images of the gold-deposited AgNDs at room temperature; (b) and (c) are higher magnification images of (a) for clarity. (d) EDX data of the AgNDs.

Fig. 9. Black spectrum (bottom curve) represents the SERS spectrum of R6G molecules (1 μM concentration) recorded for pure AgNDs, while the red spectrum (top curve) represents the SERS spectrum obtained for Au@AgNDs.

Fig. 10. (a) Reproducibility of the SERS spectra of 1 μM R6G molecules detected at 10 different spots on AgNDs and (b) corresponding histogram with RSD values.

Fig. 11. Raman intensity versus concentration for (a) R6G, 613 cm⁻¹; (b) RDX, 884 cm⁻¹; (c) AN, 1048 cm⁻¹. (d–f) Linear dependence of the SERS intensities of corresponding molecules at lower analyte concentrations.