Self-Assembly of Strain-Adaptable Surface-Enhanced Raman Scattering Substrate on Polydimethylsiloxane Nanowrinkles

Abstract

Flexible surface-enhanced Raman scattering (SERS) substrates adaptable to strains enable effective sampling from irregular surfaces, but the preparation of highly stable and sensitive flexible SERS substrates is still challenging. This paper reports a method to fabricate a high-performance strain-adaptable SERS substrate by self-assembly of Au nanoparticles (AuNPs) on polydimethylsiloxane (PDMS) nanowrinkles. Nanowrinkles are created on prestrained PDMS slabs by plasma-induced oxidation followed by the release of the prestrain, and self-assembled AuNPs are transferred onto the nanowrinkles to construct the high-performance SERS substrate. The results show that the nanowrinkled structure can improve the surface roughness and enhance the SERS signals by ∼4 times compared to that of the SERS substrate prepared on flat PDMS substrates. The proposed SERS substrate also shows good adaptability to dynamic bending up to ∼|0.4| 1/cm with excellent testing reproducibility. Phenolic pollutants, including aniline and catechol, were quantitatively tested by the SERS substrate. The self-assembled flexible SERS substrate proposed here provides a powerful tool for chemical analysis in the fields of environmental monitoring and food safety inspection.

Figure 1

Design and fabrication of the self-assembly SERS substrate on PDMS nanowrinkles. (a) Working protocol of fabricating the self-assembly SERS substrate, (b, c) pictures of the 3D-printed bending mold (c) before and (c) after sample loading, (d) 3D AFM image of nanowrinkles on the PDMS surface and (e) cross-sectional profile of the nanowrinkles, (f) experimental images of self-assembly of AuNPs, (g) AFM image of the morphology of the SERS substrate and (h) zoomed-in view of the SERS substrate, (i) SEM image of the SERS substrate (scale bar, 1 μm) and (j) zoomed-in view of the AuNPs (scale bar, 100 nm), and (k) size distribution of the AuNPs characterized by SEM.

Figure 2

Performance of the self-assembled SERS substrates. (a) SERS signals of RhB of the same concentration detected by SERS substrates prepared on bare PDMS, nanowrinkled PDMS, and flat PDMS, (b) numerical simulation example of the electric field distribution around the nanogap, (c) gap size effect on the enhancement of the electromagnetic field, (d) AFM image of the SERS substrate surface prepared on flat PDMS, (e) SERS signals of RhB solutions of various concentrations tested on a flat PDMS SERS substrate and (f) summary of the Raman intensity peaking at 1361 cm^–1, (g) AFM image of the SERS substrate surface prepared on nanowrinkled PDMS, (h) SERS signals of RhB solutions of various concentrations tested on a nanowrinkled PDMS SERS substrate, and (i) summary of the Raman intensity peaking at 1360 cm^–1.

Figure 3

Repeatability and stability of the self-assembled SERS substrate. (a) SERS spectra of 5 × 10^–3 M aniline solutions at different positions, (b) RSD of 5 × 10^–3 M aniline solution at the 1001 cm^–1 characteristic peak, (c) SERS spectra of 5 × 10^–3 M aniline detected on different batches of SERS substrates, and (d) peak intensity of different batches of SERS substrates detected at 1001 cm^–1.

Figure 4

Bending test of the self-assembled nanowrinkled SERS substrates by using 10^–3 M 4-ATP as the probe. (a) Schematics of the bending tests, (b) SERS spectra obtained under the condition of different bending curvatures, (c) summary of the spectra peaking @ 1082 cm^–1, and (d) SERS spectra of 4-ATP recorded under the condition of 100 cycle bending tests.

Figure 5

Detection of aniline and catechol. (a) Comparison of SERS spectra of aniline detected by nanowrinkled SERS substrates and flat SERS substrate, (b) SERS spectra of aniline solutions in the range of 5 × 10^–1 to 5 × 10^–8 M by the nanowrinkled SERS substrate, (c) linear fitting of the intensity of the spectra peak at 1001 cm^–1 versus the logarithm of the concentration of aniline, (d) SERS of catechol by SERS substrates prepared by flat and nanowrinkled PDMS, (e) SERS spectra of catechol solutions in the range of 10^–1 to 10^–7 M measured by the nanowrinkled SERS substrate, and (f) linear fitting of the intensity of the spectra peaking at 1245 cm^–1 versus the logarithm of the concentration of catechol solution.