Ag@BiOCl super-hydrophobic nanostructure for enhancing SERS detection sensitivity

Abstract

Surface-enhanced Raman scattering (SERS) has received widespread attention in the rapid detection of trace substances. The super-hydrophobic surface of structures has a significant impact on improving SERS performance. Usually a low concentration of objective molecules is randomly distributed in a large area on a non-hydrophobic SERS substrate, resulting in the Raman signals of the molecules not being easily detected. As a solution, a super-hydrophobic surface can gather a large number of probe molecules around the plasmon hot spots to effectively improve Raman SERS detection sensitivity. In this work, a chloride super-hydrophobic surface is fabricated, for the first time, by a simple and low-cost method of combining surface hydrophobic structures with surface modification. The dispersed and uniform hierarchical Ag@BiOCl nanosheet (Ag@BiOCl NSs) substrate has a higher surface-to-volume ratio and rich nano-gap. Such a chip with a high static contact angle of 157.4° exhibits a Raman signal detection limit of R6G dyes up to 10^-9 M and an enhancement factor up to 10⁷. This SERS chip with a super-hydrophobic surface offers great potential in practical applications owing to its simple fabricating process, low cost, large area, and high sensitivity.

Fig. 1. Preparation process of Ag@BiOCl NSs structure.

Fig. 2. (a and b) SEM image and CA of BiO_x thin film. (c and d) SEM image and CA of BiOCl NSs. (e and f) SEM image and CA of BiOCl NSs of surface modification. (g and h) SEM image and CA of Ag@BiOCl NSs. Above inset in (g and h): SEM image after surface modification and CA of Ag nanoparticles on Si substrate after surface modification. Bottom insets in (a), (c), (e), and (g): the corresponding cross-sectional SEM images of different surfaces, respectively. The scale bar is 300 nm.

Fig. 3. (a) SEM image of BiOCl NSs with super-hydrophobic surface. (b) SEM image of Ag@BiOCl NSs with super-hydrophobic surface. Statistics of (c) length and (d) thickness distribution of BiOCl NSs. Statistics of (e) size and (f) distance between particles distribution of Ag nanoparticles on BiOCl NSs.

Fig. 4. (a) SERS spectra of R6G solution with different concentration (10⁻⁶ to 10⁻⁹ M) of Ag@BiOCl NSs substrate. (b) The detailed images of Raman spectra at concentrations of 10⁻⁸ M and 10⁻⁹ M.

Fig. 5. (a) Schematic diagram of periodic unit structure of Ag@BiOCl NSs for calculation. The electric field distribution and normalized electric field intensity (|E|/|E₀|) of (b) between adjacent Ag particles on the single side of BiOCl NSs and (c) of Ag particles on adjacent BiOCl NSs.