Picosecond Laser-Ablated Nanoparticles Loaded Filter Paper for SERS-Based Trace Detection of Thiram, 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX), and Nile Blue

Abstract

Recently, filter paper (FP)-based surface-enhanced Raman scattering (SERS) substrates have stimulated significant attention owing to their promising advantages such as being low-cost, easy to handle, and practically suitable for real-field applications in comparison to the solid-based substrates. Herein, a simple and versatile approach of laser-ablation in liquid for the fabrication of silver (Ag)-gold (Au) alloy nanoparticles (NPs). Next, the optimization of flexible base substrate (sandpaper, printing paper, and FP) and the FP the soaking time (5−60 min) was studied. Further, the optimized FP with 30 min-soaked SERS sensors were exploited to detect minuscule concentrations of pesticide (thiram-50 nM), dye (Nile blue-5 nM), and an explosive (RDX-1,3,5-Trinitroperhydro-1,3,5-triazine-100 nM) molecule. Interestingly, a prominent SERS effect was observed from the Au NPs exhibiting satisfactory reproducibility in the SERS signals over ~1 cm2 area for all of the molecules inspected with enhancement factors of ~105 and relative standard deviation values of <15%. Furthermore, traces of pesticide residues on the surface of a banana and RDX on the glass slide were swabbed with the optimized FP substrate and successfully recorded the SERS spectra using a portable Raman spectrometer. This signifies the great potential application of such low-cost, flexible substrates in the future real-life fields.

Keywords: SERS; explosive molecules; filter paper; flexible substrate; laser ablation; nanomaterials; pesticide; thiram.

Figure 1

(a) Photographic images of as-fabricated Ag-Au alloy NPs including pure Ag and Au NPs (top) and FP soaked with NPs (bottom) (b) Normalized UV-Visible absorption spectra of Au-Ag NPs (i) Pure Ag (black) (ii) Ag₇₀Au₃₀ (red) (iii) Ag₅₀Au₅₀ (blue) (iv) Ag₃₀Au₇₀ (pink) and (v) pure Au (green one) (c) variation of SPR peak position with increasing Au percentage in Ag-Au alloy NPs.

Figure 2

TEM images of as-fabricated NPs by laser ablation (a) Ag NPs (b) Ag₇₀ Au₃₀ NPs (c) Ag₅₀Au₅₀ NPs (d) Ag₃₀Au₇₀ NPs (e) Au NPs, respectively.

Figure 3

(a) FESEM-EDX map image of a Ag₅₀ Au₅₀ particle. The highlighted portion was utilized to collect the EDX data while the inset shows the two nanoparticles (b) Ag EDX map (c) Au EDX map (b) EDX spectra of Ag₅₀ Au₅₀ NP. The scale bar for (a–c) is 700 nm. The inset of (d) shows the respective compositions of Ag and Au in a single Ag₅₀Au₅₀ particle.

Figure 4

FESEM images of filter paper-loaded pure and alloy NPs and their corresponding EDX spectra of (a1,a2) Pure Ag, (b1,b2) Ag₇₀ Au₃₀, (c1,c2) Au₅₀Ag₅₀, (d1,d2) Ag₃₀Au₇₀, and (e1,e2) Pure Au, respectively.

Figure 5

(a) SERS spectra of NB (5 µM) recorded from (i) FPAg (ii) FPAg₇₀Au₃₀ (iii) FPAg₅₀Au₅₀ (iv) FPAg₃₀Au₇₀ (v) FPAu substrates. (b) Histogram plot of SERS signal intensity at 590 cm⁻¹ modes recorded on each substrate. Integration time is 15 s. Errors bars were estimated by measuring the standard deviation of 590 cm⁻¹ peak SERS average intensity obtained from five repetitive SERS measurements on each substrate. (c) SERS signals of NB at varying concentrations [from (i) 500 µM to (vi) 5 nM] (d) 3D SERS spectra of NB (500 µM) collected from randomly selected 15 sites on FPAu substrate. Integration time was 15 s.

Figure 6

(a) SERS spectra of thiram (5 mM) collected from (i) FPAg (ii) FPAg₇₀Au₃₀ (iii) FPAg₅₀Au₅₀ (iv) FPAg₃₀Au₇₀ (v) FPAu substrates. (b) Histogram plot of SERS signal intensity at 1368 cm⁻¹ peak recorded on each substrate. Integration time is 15 s. Errors bars were achieved by estimating the standard deviation of 1368 cm⁻¹ peak SERS average intensity obtained from five repetitive SERS measurements on each substrate. (c) SERS signals of thiram at different concentrations (from 5 mM to 50 nM). (d) 3D SERS spectra of thiram (5 mM) collected from randomly selected 10 sites on FPAu substrate. Integration time was 15 s.

Figure 7

(a) Soaking time of filter paper in Au NPs (b) relative intensity of thiram (50 µM) at 1368 cm⁻¹ with respect to FP soaking time. Error bars are measured based on the standard deviation of the average SERS intensity at 1368 cm⁻¹ peak accomplished by repetitive SERS measurements five times. Integration time is 30 s.

Figure 8

RDX SERS spectra recorded from optimized FP Au SERS substrate (a) concentration RDX SERS spectra (b) variation of SERS intensity versus concentration of the analyte (c) 3D waterfall SERS spectra (d) Histogram plot of RDX SERS signal intensity at 863 cm⁻¹ peaks collected at 10 random sites.

Figure 9

(a,b) Photograph of thiram and RDX collection by simple swabbing on banana and glass surfaces using flexible FP substrate. (c) Thiram SERS spectra were recorded by swabbing on a banana (d) RDX SERS spectra recorded by swabbing on glass and collected the data from randomly selected 10 sites on optimized FPAu SERS substrate (after swabbing).

Figure 10

SERS spectra of thiram-5 nM molecule using FP-loaded with ps laser ablated of Au NPs in the presence of distilled water (FPAu-red color) and NaCl-20 mM (FPAu NaCl-black).