Curvature-Insensitive Transparent Surface-Enhanced Raman Scattering Substrate Based on Large-Area Ag Nanoparticle-Coated Wrinkled Polystyrene/Polydimethylsiloxane Film for Reliable In Situ Detection

Abstract

Flexible and transparent surface-enhanced Raman scattering (SERS) substrates have attracted considerable attention for their ability to enable the direct in situ detection of analytes on curved surfaces. However, the curvature of an object can impact the signal enhancement of SERS during the measurement process. Herein, we propose a simple approach for fabricating a curvature-insensitive transparent SERS substrate by depositing silver nanoparticles (Ag NPs) onto a large-area wrinkled polystyrene/polydimethylsiloxane (Ag NP@W-PS/PDMS) bilayer film. Using rhodamine 6G (R6G) as a probe molecule, the optimized Ag NP@W-PS/PDMS film demonstrates a high analytical enhancement factor (AEF) of 4.83 × 10⁵, excellent uniformity (RSD = 7.85%) and reproducibility (RSD = 3.09%), as well as superior mechanical flexibility. Additionally, in situ measurements of malachite green (MG) on objects with diverse curvatures, including fish, apple, and blueberry, are conducted using a portable Raman system, revealing a consistent SERS enhancement. Furthermore, a robust linear relationship (R² ≥ 0.990) between Raman intensity and the logarithmic concentration of MG detected from these objects is achieved. These results demonstrate the tremendous potential of the developed curvature-insensitive SERS substrate as a point-of-care testing (POCT) platform for identifying analytes on irregular objects.

Keywords: Ag NP-coated wrinkled PS/PDMS film; SERS; curvature insensitivity; point-of-care testing; quantitative detection.

Figure 1

Schematic illustration of the fabrication procedure of the Ag NP@W-PS/PDMS film and its POCT analysis on objects with diverse curvatures.

Figure 2

(a) Photographs of a flat PDMS film (left) and a PS/PDMS bilayer film after thermal treatment (right). (b) Optical microscope image and (c) AFM image of surface wrinkles on the PS/PDMS bilayer film. (d) The height profile of the wrinkled structure is marked with a red line in (c).

Figure 3

SEM images of the Ag NP@W-PS/PDMS films with (a) 30 s, (b) 45 s, (c) 60 s, and (d) 75 s Ag sputtering times. (e) SERS spectra of 10⁻⁵ M R6G collected from different Ag NP@W-PS/PDMS films. The inset shows the comparison of SERS intensity between different substrates. (f) Photograph of a large-area transparent Ag NP@W-PS/PDMS film on a flat paper. Inset shows a photograph of the deformed Ag NP@W-PS/PDMS film, demonstrating the substrate flexibility.

Figure 4

(a) SERS spectra of R6G with different concentrations obtained from the Ag NP@W-PS/PDMS-60 film. (b) Linear correlation of peak intensity at 1510 cm⁻¹ with the logarithmic concentration of R6G. (c) SERS spectra of R6G (10⁻⁵ M) collected from 100 random sites. (d) The histogram of SERS intensity of R6G at 1510 cm⁻¹ at the 100 sites. The red line represents the average intensity. (e) SERS spectra of R6G (10⁻⁵ M) collected from five different Ag NP@W-PS/PDMS-60 films. (f) The histogram of SERS intensity of R6G at 1510 cm⁻¹ from 5 different substrates. The red line represents the average intensity.

Figure 5

SERS spectra of R6G collected from Ag NP@W-PS/PDMS-60 film in (a) bending, (b) twisting, and (c) adhesion test. SERS intensities of R6G at 1510 cm⁻¹ recorded from Ag NP@W-PS/PDMS-60 film in (d) bending, (e) twisting, and (f) adhesion test.

Figure 6

Schematic illustration of SERS measurements under (a) front-side and (b) back-side excitation mode. Comparison of SERS spectra of (c) MG and (d) CV with concentrations ranging from 10⁻³ to 10⁻⁶ M obtained from both sides of the Ag NP@W-PS/PDMS-60 film.

Figure 7

SERS spectra of MG with different concentrations on the surfaces of (a) fish and (c) apple and (e) blueberry acquired by in situ detection using Ag NP@W-PS/PDMS-60 film. The plot of peak intensity at 1616 cm⁻¹ versus the logarithm of MG concentration collected from the surfaces of (b) fish and (d) apple and (f) blueberry. The insets in (b,d,f) are photographs showing portable in situ detection measurements.