No abstract

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 cm 2 ) 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 surfacecleaved 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 × 10 7 ) 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/cm 2 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.

Noble metal complex nanomaterials were widely investigated in bioscientific systems for surface-enhanced Raman scattering (SERS) imaging. Silver (Ag) has higher enhancement factors, arising from better optical properties, in comparison to gold (Au), which has better chemical stability and a broader localized surface plasmon resonance (LSPR) region extending to red-NIR, and both should be taken into account to enhance SERS performance. Among them, nanostars (NSs) have emerged as one promising geometry because of their strong enhanced electromagnetic fields induced by the lightning-rod effect from their branches. Therefore, core–shell NSs have been employed as efficient SERS substrates for trace detection because bimetallic nanoparticles provide richer plasmonic modes from their integrated material-dependent and size/shape-correlated plasmonics. Herein, we prepared two uniform and tunable core–shell NSs with different cores (Ag or Au) and the same Au shell, tailoring size and branch morphology to investigate their induced LSPR properties. As a result, their extinction spectra nearly coincide when the size and shape of two NSs become similar. A parallel electric field distribution and response wavelength were obtained as the shells are alike. Thus, it strongly suggests that the shell dominates its LSPR region in the core–shell NS. Meanwhile, an improved SERS activity with an enhancement factor of 1.5 × 107 was achieved in Ag@Au compared to Au@Au NSs because of the addition of Ag. This work offered certain theoretical and experimental references for the plasmonic metal NS with extensively enhanced electromagnetic fields, providing an effective SERS substrate for imaging and detection in life sciences.