Wearable surface-enhanced raman spectroscopy sensor using inverted bimetallic nanopyramids for biosensing and sweat monitoring

Surface-enhanced Raman spectroscopy (SERS) has gained prominence as a pivotal tool in optical sensing technology with tremendous potential. Extensive efforts have been dedicated to improving the practical aspects of SERS, focusing on cost effectiveness, stability, reproducibility, flexibility, and robustness. Herein, we employ a multistep molding process that incorporates electron beam lithography, nanoimprinting lithography, and subsequent e-beam thin film deposition. We first fabricated a Ag/Au bimetallic inverted nanopyramid (i-NPyr) and upright nanopyramid (u- NPyr) on flexible plastic substrates. A meticulous comparison of their SERS detection capabilities revealed impressive enhancement factors of 3.88 × 106 and 7.86 × 105, respectively. Subsequently, the i-NPyr structures were successfully exploited to achieve ultrasensitive detection of the hemoglobin biomolecule at concentrations as low as 1 nM. Furthermore, the stability and resilience of i-NPyr for SERS performance were thoroughly investigated through angled bending and delicate crumpling tests. In addition, computational simulations were employed to gain a comprehensive understanding of the electromagnetic confinement within the substrate. Finally, a proof-of-concept demonstration of the i-NPyr SERS sensor as a wearable device was achieved by attaching it to a volunteer’s neck in running and walking activities, successfully detecting constituents such as urea and lactic acid excreted in sweat. Thus, by combining the lithographic technique with imprinting, we produced SERS substrates with a precise morphology and sharp asperities, offering scalability and cost effectiveness on a large-scale production level. This innovative approach holds the potential to solve the challenges in wearable sensing technology, facilitating in situ measurement of important analytes of biological processes.