Photonic-Crystal-Integrated Plasmonic Hydrogel Microcylinder Arrays for Enhanced and Fouling-Resistant SERS

Surface-enhanced Raman scattering (SERS) enables highly sensitive molecular identification but can be compromised by plasmonic-surface fouling and inefficient utilization of excitation light. Here, we report a strategy that combines molecular-size-selective transport with spectrally selective excitation recycling to enhance SERS sensitivity while resisting surface fouling. A colloidal photonic crystal (PC) reflector was fabricated by shear-assisted casting of monodisperse silica particles in a photocurable medium, with its stopband tuned to the 633 nm excitation wavelength by controlling particle size and processing conditions. Poly(ethylene glycol) diacrylate (PEGDA) microcylinders were transferred onto the reflector and rendered SERS-active by AgNO3 infiltration followed by ultraviolet-induced photoreduction, producing Ag nanoparticles within the hydrogel network. Stopband matching enhanced Raman intensity relative to substrates without a reflector and off-resonant PC films, consistent with selective back-reflection and recycling of excitation light through the microcylinders. The narrow reflection band predominantly overlapped the excitation wavelength, potentially limiting broadband optical-background reflection. The PEGDA network also exhibited size-dependent molecular permeation and restricted access of larger proteins, preserving Raman responses of small analytes in their presence. This multifunctional architecture integrates photonic excitation management and molecular sieving to improve SERS sensitivity and resistance to surface fouling while retaining a compact and readily fabricated sensing format.