Fabrication and characterization of large-scale ordered silver-coated polystyrene microspheres films for surface-enhanced Raman spectroscopy

In this work, we investigate the optical properties of large-scale, ordered silver-coated polystyrene microspheres films fabricated on glass substrate using the convective self-assembly (CSA) method, with the goal of their application as active plasmonic substrate for Surface-Enhanced Raman Spectroscopy (SERS). We fabricated the film using an optimized CSA protocol with a custom-built equipment. Scanning electron microscopy (SEM) reveals a close-packed large area of silver-coated monolayer of intact microspheres. The optical properties of the fabricated plasmonic substrate were characterized experimentally by recording the transmittance spectra and complemented by finite-difference time-domain (FDTD) numerical simulations. Additionally, the theoretically predicted electromagnetic field distribution and hot-spots localization within the two-dimensional (2D) ordered array film were validated experimentally through SERS measurements conducted at three distinct excitation laser lines. SERS mapping of the intensity at 1074 cm−1 band of 4-mercaptobenzoic acid (MBA) reveals an intense SERS activity of the plasmonic platform over large scanned areas. In particular, the signal is significantly higher and richer in regions correlated with the presence of hot-spots. Furthermore, consistent with the FDTD predicted field distribution, the fabricated substrate exhibits the highest SERS performance at 633 nm, where the map features very high SERS signals in some areas connected to the presence of hot-spots. Specifically, the average SERS enhancement factor at 633 nm laser excitation is 1.58 ± 0.33 × 106 which is in the range of typical enhancement factors on solid SERS substrates. The result emphasizes also the importance of choosing an appropriate laser wavelength to stimulate the enhancement of hot-spots localized in the gaps between the nanospheres. Consequently, we propose the as-fabricated 2D ordered silver-coated polystyrene microspheres film as promising and reliable candidate for SERS detection applications.