Facile chemical routes to mesoporous silver substrates for SERS analysis

Abstract

Mesoporous silver nanoparticles were easily synthesized through the bulk reduction of crystalline silver(I) oxide and used for the preparation of highly porous surface-enhanced Raman scattering (SERS)-active substrates. An analogous procedure was successfully performed for the production of mesoporous silver films by chemical reduction of oxidized silver films. The sponge-like silver blocks with high surface area and the in-situ-prepared mesoporous silver films are efficient as both analyte adsorbents and Raman signal enhancement mediators. The efficiency of silver reduction was characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The developed substrates were applied for SERS detection of rhodamine 6G (enhancement factor of about 1-5 × 10⁵) and an anti-ischemic mildronate drug (meldonium; enhancement factor of ≈10²) that is known for its ability to increase the endurance performance of athletes.

Keywords: meldonium; mesoporous silver substrates; silver oxide; surface-enhanced Raman spectroscopy.

Figure 1

(a,b) SEM micrographs of Ag₂O crystallites produced by alkaline sedimentation in excess of PVP, namely, (a) AgNO₃/PVP 1:10 and (b) AgNO₃/PVP 1:5. (c,d) SEM micrographs of the samples obtained by Ag₂O reduction using NaBH₄ with the following molar ratios: (c) Ag₂O/NaBH₄ 1:2 and (d) Ag₂O/NaBH₄ 1:10. The reducing treatment time was 40 min for both the samples. (e) XRD data for pristine Ag₂O (1) and reduction products at double excess of NaBH₄ (2) and ten-fold excess of NaBH₄ (3). (f) XPS spectra of silver mesoporous particles (mp-Ag) obtained in a Ag₂O/NaBH₄ 1:10 ratio.

Figure 2

SEM images of mesoporous silver films after immersion for 40 min in NaBH₄ (a) without PVP and (b) with 5 mM of PVP, respectively. In (c) and (d) the micrographs of initial magnetron silver film and Ag₂O films are presented.

Figure 3

Optical absorbance spectra replotted from total reflectance data collected by an integrating sphere (d = 150 mm): (a) (1) reduced by NaBH₄ silver and (2) pristine Ag₂O polycrystals, and (b) (1) mp-Ag/Ag (PVP) film, (2) pristine Ag₂O film.

Figure 4

(a) The principal scheme of SERS experiment and (b) corresponding SERS spectra of rhodamine 6G in 10⁻⁸ M, 10⁻⁷ M and 10⁻⁶ M concentrations obtained using the substrates of mesoporous Ag microcubes synthesized at Ag₂O/NaBH₄ 1:10 ratio. (c) SERS spectra of rhodamine 6G in 10⁻⁷ M and 10⁻⁸ M 1–2 μL aliquots collected using mp-Ag/Ag slides prepared with or without PVP.

Figure 5

SERS spectra of mildronate in (1) 10⁻³ M and (2) 10⁻² M concentrations collected using mp-Ag/Ag slides prepared in the presence of PVP. The volume of the aliquot was 2 μL. As a reference sample, 2 µL of 100 mg/mL standard solution was dried on the glass substrate (3). The excitation laser was 514.4 nm, at 5% power of 20 mW, and 10 s acquisition time.

Figure 6

Micrographs of mp-Ag/Ag slides prepared in the presence of PVP after sintering for 1 h in air at (a) 110 °C and (b) 120 °C. (c) Raman/SERS spectra of meldonium (probe: dried 2 μL probe of 1 × 10⁻² M concentration) after sintering at (1) 110 °C and (2) 120 °C for 1 h in air. The excitation laser wavelength was 514.4 nm, at 5% power of 20 mW, and 10 s acquisition time.