Stable and Reusable Lace-like Black Silicon Nanostructures Coated with Nanometer-Thick Gold Films for SERS-Based Sensing

Abstract

We propose a simple, fast, and low-cost method for producing Au-coated black Si-based SERS-active substrates with a proven enhancement factor of 10⁶. Room temperature reactive ion etching of silicon wafer followed by nanometer-thin gold sputtering allows the formation of a highly developed lace-type Si surface covered with homogeneously distributed gold islands. The mosaic structure of deposited gold allows the use of Au-uncovered Si domains for Raman peak intensity normalization. The fabricated SERS substrates have prominent uniformity (with less than 6% SERS signal variations over large areas, 100 × 100 μm²). It has been found that the storage of SERS-active substrates in an ambient environment reduces the SERS signal by less than 3% in 1 month and not more than 40% in 20 months. We showed that Au-coated black Si-based SERS-active substrates can be reused after oxygen plasma cleaning and developed relevant protocols for removing covalently bonded and electrostatically attached molecules. Experiments revealed that the Raman signal of 4-MBA molecules covalently bonded to the Au coating measured after the 10th cycle was just 4 times lower than that observed for the virgin substrate. A case study of the reusability of the black Si-based substrate was conducted for the subsequent detection of 10^-5 M doxorubicin, a widely used anticancer drug, after the reuse cycle. The obtained SERS spectra of doxorubicin were highly reproducible. We demonstrated that the fabricated substrate permits not only qualitative but also quantitative monitoring of analytes and is suitable for the determination of concentrations of doxorubicin in the range of 10^-9-10^-4 M. Reusable, stable, reliable, durable, low-cost Au-coated black Si-based SERS-active substrates are promising tools for routine laboratory research in different areas of science and healthcare.

Figure 1

Steps of bSi/Au SERS substrate synthesis.

Figure 2

SEM micrographs of bSi (A, B) and bSi sputtered with gold (C, D): angle-side (A, C) and top–bottom (B, D) views.

Figure 3

Raman spectrum of 4-MBA powder (bottom) and SERS spectra of 4-MBA on SiO₂/Au, bSi/Au* (conical), bSi/Au (conical), and bSi/Au substrates (from bottom to top, respectively). *30% thicker gold layer sputtered on the surface of bSi than in other cases (bSi/Au (conical) and bSi/Au).

Figure 4

Evaluation of the photothermal effect on characteristic bands in SERS spectra of 4-MBA SAM on the bSi/Au substrate. SERS peak intensities of 1076 (A) and 1588 cm^–1 (D); dynamic curves of intensities (in cps) obtained for 1076 cm^–1 (B) and 1588 cm^–1 (E) bands; the dependencies of the peak positions of 1076 cm^–1 (C) and 1588 cm^–1 (F) bands on the laser power and time of exposure of the probed 4-MBA molecules to excitation irradiation. The excitation wavelength is 785 nm. Ranges on the false color maps that correspond to photothermally affected and unaffected spectral characteristics are separated by red dashed lines.

Figure 5

SERS spectra of a 4-MBA monolayer on bSi/Au in the range of 990–1030 cm^–1. The exposure time varies from 1 to 60 s. The excitation powers are (A) 20.3, (B) 41.8, and (C) 89.3 mW.

Figure 6

Two-dimensional (2D) distribution of the peak intensities at 1076 (A) and 1586 cm^–1 (B) of SERS spectra of the 4-MBA monolayer deposited on top of the bSi/Au substrate. (C) 4-MBA SERS spectra from the whole scanned area of 100 × 100 μm² (shown in gray color); an orange spectrum corresponds to the mean SERS spectrum of 4-MBA. (D) Spatial distribution of accidental impurities. Measurement parameters: λ_ex = 785 nm, exposure time is 1s, laser power is 0.05 mW, scanning step is 1 μm, 50× objective.

Figure 7

Storage stability of the bSi/Au substrate evaluated with 4-MBA as a probe analyte. Each SERS spectrum corresponds to a 4-MBA monolayer on the surface of freshly manufactured bSi/Au samples and bSi/Au samples stored for 1, 2, 3, 4, and 20 months.

Figure 8

Versatility of the bSi/Au substrate reuse demonstrated for (A, B) covalently bonded molecules of 4-MBA and (C, D) noncovalently bonded molecules of DOX. (A) SERS spectra of 4-MBA on the bSi/Au substrate over 10 reusability cycles; (B) dependence of the 1076 and 1588 to 521 cm^–1 intensity ratios on the number of reuse cycles; (C) Raman spectrum of DOX (powder) and SERS spectrum of 10^–5 M of DOX on SiO₂/Au; (D) SERS spectrum of 10^–5 M of DOX on bSi/Au substrate before oxygen plasma cleaning (mustard line), spectra of bSi/Au substrate after 15 min of oxygen plasma treatment (red line), and SERS spectrum of 10^–5 M DOX on the reused bSi/Au substrate (blue line).

Figure 9

Calibration plot for the determination of DOX concentration by the bSi/Au substrate. Inset: SERS spectra of DOX at different concentrations (1.0 nM to 1.0 mM).