Chronic disease management can benefit from continuous real time tracking of disease relevant biochemical levels using minimally invasive implants. Current methods however rely on electrochemical sensing methods integrated into rigid, invasive transdermal probes that are expensive, short lived, and require frequent calibration. The development of new optically interrogatable sensing methods that can be housed in soft biocompatible hydrogel matrixes and implanted subcutaneously can provide a means to extend biosensor life, reliability, and comfort. This work demonstrates the development of pH responsive surface enhanced Raman spectroscopy (SERS) assays using 4-mercaptobenzoic acid (MBA) capped gold nanoparticles (AuNP). By encapsulating MBA-AuNPs into polyelectrolyte multilayer (PEM) microcapsules (MCs) and then casting MCs into a bulk hydrogel matrix, a soft biocompatible sensor that exhibited pH sensitivity and reproducibility in SERS scattering signal could be developed. To extend this sensing approach to more relevant analytes, enzymatic glucose sensors were prepared with MBA-AuNPs co-encapsulated with glucose oxidase (GOx). SERS spectra recorded from the hydrogels exhibited apparent trends of decreasing pH from 6.50 to 4.50 due to glucose oxidization within the microdomains. To demonstrate multi-modal sensing capabilities with these SERS approaches, pH and oxygen sensitive microdomains were integrated into a single hydrogel that could be interrogated via SERS and phosphorescence lifetime optical modalities respectively. The PEM nanofilms were effective at minimizing optical interferences by spatially separating optical probes. To highlight the benefits of using a PEM MC embedded hydrogel approach, an aggregation based SERS assay with metal nanoparticle encapsulation was studied. The PEM MCs provide vacant pockets where the aggregation-based SERS assay specifically interacts with the analyte, while excluding large proteins which result in non-specific aggregation. As a proof of concept, hybridization-based microRNA-17 sensing assay was embedded in the PEM MCs. In the presence of large protein molecules, microRNA17 sensing PEM MCs exhibited target level dependent SERS signal changes. The above results demonstrate significant advances in enabling SERS sensing assays to be integrated into soft biocompatible hydrogels while still retaining selectivity and repeatability via the use of PEM MCs. This lays the foundation for developing implantable SERS biosensors that can be integrated with other sensing strategies to develop multimodal multi-analyte sensors.
