Flexible plasmonic microneedle array-based SERS sensor for pH monitoring of skin interstitial fluid

Interstitial fluid (ISF) is a prevalent accessible and information-rich biofluid, and monitoring pH level of interstitial fluid plays a crucial role in the physiological health assessment. However, facile extraction of ISF and sensitive detection of pH is still a great challenge. Herein, a flexible plasmonic microneedle array (PMNA)-based SERS sensor was developed for facile and sensitive pH monitoring of skin ISF. The flexible microneedle arrays were fabricated by norland optical adhesive 65 (NOA65), and coated with pH-sensitive 4-Mercaptobenzoic acid (4-MBA)-labeled silver nanoparticles (Ag NPs) by electrostatic assembly. The proposed flexible PMNA-based SERS sensor has a good linear response in the pH range of 5 to 9 which covers the normal pH level in skin interstitial fluid. The ISF extracted from porcine skin was chosen as the test model, and the pH levels detected by the PMNA-based SERS sensors were well consistent with the theoretical results. Besides, the proposed flexible SERS sensors show good mechanical robustness, sensing stability, and biocompatibility before and after skin insertion, which can be a potential analytical tool for practical detection of pH levels in ISF and provide technical support for SERS-based wearable biosensing.