Large-Scale Flexible Surface-Enhanced Raman Scattering (SERS) Sensors with High Stability and Signal Homogeneity

Flexible surface-enhanced Raman scattering (SERS) sensors have attracted great attention as a portable and low-cost device for chemical and bio-detection. However, flexible SERS sensors tend to suffer low signal spatial homogeneity due to the uneven distribution of active plasmonic nanostructures (hot spots) and quick degradation of their sensitivity due to low adhesion of hot spots and flexible substrates during fast sampling. Herein, a large-area (20 × 20 cm²) polyimide (PI)-based SERS sensor is exploited for trace detection with high signal homogeneity and stability. The SERS sensor is constructed from PI through in situ growth of silver and gold core-shell nanoparticles (Ag@Au NPs) based on chemical reduction and galvanic replacement processes. Benefiting from the abundant carboxyl groups on the surface-cleaved PI, densely and uniformly distributed Ag@Au NPs are successfully prepared on the film under ambient conditions. The high Raman enhancement factor (EF) (up to 1.07 × 10⁷) and detection capability with low nanomolar (10^-9 M) detection limits are obtained for this flexible SERS sensor. The uniform Raman signals in the random region show good signal homogeneity with a low variation of 8.7%. Moreover, the flexible SERS sensor exhibited superior efficiency and durability after storage for 30 days even after 500 cycles of mechanical stimuli (bending or torsion). The residue of pesticide thiram (tetramethylthiuram disulfide, TMTD) has been rapidly traced by direct sampling from the apple surface, and a sensitivity of 10 ng/cm² for TMTD was achieved. These findings show that the PI-based SERS sensor is a very strong candidate for broad and simple utilization of flexible SERS for both laboratory and commercial applications in chemical and biomolecule detections.