SERS Detection of Hydrophobic Molecules: Thio-β-Cyclodextrin-Driven Rapid Self-Assembly of Uniform Silver Nanoparticle Monolayers and Analyte Trapping

Abstract

High-sensitivity and repeatable detection of hydrophobic molecules through the surface-enhanced Raman scattering (SERS) technique is a tough challenge because of their weak adsorption and non-uniform distribution on SERS substrates. In this research, we present a simple self-assembly protocol for monolayer SERS mediated by 6-deoxy-6-thio-β-cyclodextrin (β-CD-SH). This protocol allows for the rapid assembly of a compact silver nanoparticle (Ag NP) monolayer at the oil/water interface within 40 s, while entrapping analyte molecules within hotspots. The proposed method shows general applicability for detecting hydrophobic molecules, exemplified as Nile blue, Nile red, fluconazole, carbendazim, benz[a]anthracene, and bisphenol A. The detection limits range from 10^-6to 10^-9 M, and the relative standard deviations (RSDs) of signal intensity are less than 10%. Moreover, this method was used to investigate the release behaviors of a hydrophobic pollutant (Nile blue) adsorbed on the nanoplastic surface in the water environment. The results suggest that elevated temperatures, increased salinities, and the coexistence of fulvic acid promote the release of Nile blue. This simple and fast protocol overcomes the difficulties related to hotspot accessibility and detection repeatability for hydrophobic analytes, holding out extensive application prospects in environmental monitoring and chemical analysis.

Keywords: SERS; hydrophobic analyte; self-assembly film; silver nanoparticle; β-CD-SH.

Figure 1

Schematic diagram of SERS detection utilizing the synergistic interaction between substrate self-assembly and target analyte capture.

Figure 2

(a) Images of the self-assembly process of Ag NPs assisted by β-CD-SH, (b) SEM image of the film formed by Ag NPs with β-CD-SH modification, (c) SEM image of the film formed by Ag NPs without β-CD-SH modification, (d) SERS spectra of NB at different concentrations collected on a monolayer film (10⁻⁵ M β-CD-SH), (e) SERS spectra of 10⁻⁷ M NB collected from 10 random points on a monolayer film, (f) Intensity of the 593 cm⁻¹ peak at 10 random points on the substrate, (g) SERS spectra of NB (10⁻⁷ M) detected on 10 batches of monolayer films prepared under the same conditions, and (h) Statistical distribution of SERS intensity at 593 cm⁻¹ (NB, 10⁻⁷ M) corresponding to 10 batches of co-assembled films.

Figure 3

(a) SERS spectra of 10⁻⁷ M NB collected from 10 random points on a monolayer film (without β-CD-SH modification), (b) Intensity of the 593 cm⁻¹ peak at 10 random points on the substrate (without β-CD-SH modification), (c) SERS spectra of NB (10⁻⁷ M) at 10 random points on a β-CD-SH-Ag film detected by droplet application, (d) Intensity of the 593 cm⁻¹ peak at 10 random points on a β-CD-SH-Ag film detected by droplet application, (e) SERS spectra of NB (10⁻⁷ M) at 10 random points on a β-CD-SH-Ag film detected by soaking, (f) Intensity of the 593 cm⁻¹ peak at 10 random points on a β-CD-SH-Ag film detected by soaking, (g) SERS spectra of NB (10⁻⁷ M) at 10 random points on a PVP-Ag film detected by droplet application, and (h) Intensity of the 593 cm⁻¹ peak at 10 random points on a PVP-Ag film detected by droplet application.

Figure 4

SERS spectra of (a) Nile red, (b) fluconazole, (c) carbendazim, (d) benz[a]anthracene, and (e) bisphenol A at different concentrations.

Figure 5

SERS spectra of NB released from PMMA under the influence of different (a) temperatures, (b) salinities, (c) pH values, and (d) concentrations of fulvic acid.