Thin silica shell coated Ag assembled nanostructures for expanding generality of SERS analytes

Abstract

Surface-enhanced Raman scattering (SERS) provides a unique non-destructive spectroscopic fingerprint for chemical detection. However, intrinsic differences in affinity of analyte molecules to metal surface hinder SERS as a universal quantitative detection tool for various analyte molecules simultaneously. This must be overcome while keeping close proximity of analyte molecules to the metal surface. Moreover, assembled metal nanoparticles (NPs) structures might be beneficial for sensitive and reliable detection of chemicals than single NP structures. For this purpose, here we introduce thin silica-coated and assembled Ag NPs (SiO2@Ag@SiO2 NPs) for simultaneous and quantitative detection of chemicals that have different intrinsic affinities to silver metal. These SiO2@Ag@SiO2 NPs could detect each SERS peak of aniline or 4-aminothiophenol (4-ATP) from the mixture with limits of detection (LOD) of 93 ppm and 54 ppb, respectively. E-field distribution based on interparticle distance was simulated using discrete dipole approximation (DDA) calculation to gain insight into enhanced scattering of these thin silica coated Ag NP assemblies. These NPs were successfully applied to detect aniline in river water and tap water. Results suggest that SiO2@Ag@SiO2 NP-based SERS detection systems can be used as a simple and universal detection tool for environment pollutants and food safety.

Fig 1. Schematic illustration of detection concept and fabrication process.

(a) Simultaneous quantitative detection of aniline and 4-aminothiophenol with thin silica shell coated Ag NP assembled structure (SiO₂@Ag@SiO₂ NP), (b) Overall fabrication scheme of desired structure.

Fig 2. Transmission electron microscopic images of fabricated nanostructure.

(a) Silica nanoparticle (NP), (b) SiO₂@Ag NPs, and (c) SiO₂@Ag@SiO₂ NPs.

Fig 3. Comparison of surface-enhanced Raman scattering (SERS) spectra of a non-thiol analyte (i; aniline), a thiol analyte (ii; 4-ATP) and their mixture (iii; same quantities of aniline and 4-ATP).

(a) Raman spectra of SiO₂@Ag NPs and (b) SiO₂@Ag@SiO₂ NPs. Raman spectra were obtained by 532 nm photoexcitation and 10s acquisition. Intensities were normalized to Raman intensity of the ethanol peak at 882 cm⁻¹. The characteristic aniline peaks were not detected in the spectrum of the mixture when the SiO₂@Ag NP SERS substrate was used. However, both peaks of aniline and 4-ATP were detected at similar intensities when the SiO₂@Ag@SiO₂ NPs was used as SERS substrate. Baselines were adjusted for the clarity of comparison.

Fig 4. Limit of detection analysis with two different molecule.

Limit of detection of (a) aniline, (b) 4-ATP at various concentrations based on their corresponding surface-enhanced Raman scattering (SERS) signals using SiO₂@Ag@SiO₂ NPs. All Raman spectra were measured at laser power of 10 mW with acquisition time of 10 s. Intensities were normalized to Raman intensity of ethanol peak at 882 cm⁻¹.

Fig 5. Theoretical simulation of optical fields for SiO₂@Ag NPs with different inter-particle distance.

The wavelength of incident light was at 532 nm. (a) An illustrated model for the nanostructure used for calculation. (b) An E-field distribution map around the nanostructure when the center-to-center distance of two spheres is smaller than the outer diameter of a sphere. The area depicted in red circle is the brightest area on silica shell surface. (c) The same as in (b) when the center-to-center distance of two spheres is larger than the outer diameter of a sphere. The area depicted in red circle is the brightest area on silica shell surface. The area depicted in red circle of (b) and (c) is the maximum (E/E₀)² of silica shell surface and same region of interest (d). A plot of (E/E₀)² at the brightest spot on silica shell surface versus the center-to-center distance of Ag NPs.

Fig 6. Comparison of surface-enhanced Raman scattering (SERS) spectra.

(a) Raman spectra of river water. (i) River water only, (ii) aniline in river water with SiO₂@Ag NPs, and (iii) aniline in river water with SiO₂@Ag@SiO₂ NPs. (b) Raman spectra of tap water. (i) Tap water only, (ii) aniline in tap water with SiO₂@Ag NPs, (iii) aniline in tap water with SiO₂@Ag@SiO₂ NPs. All Raman spectra were measured at laser power of 10 mW with acquisition time of 10 s. Intensities were normalized to Raman intensity of ethanol peak at 882 cm⁻¹.