4D patterned chitosan-based micro-nano pH sensors

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Advances in nanotechnology have enabled the development of microsensors with high sensitivity, selectivity, and real-time detection in localized microenvironments. pH is one of the most important physiochemical parameters at the micro/nanoscale, as strong temporal and spatial variations in localized environments regulate reaction kinetics and are of utmost importance in investigating cell biology, surface studies, environmental monitoring, etc. Existing pH microsensors such as fiber-optic sensors, microelectrodes, SERS devices, and fluorescent dyes have advanced the field but are limited by toxicity, invasive nature, poor scalability, and integration with solid-state compact devices. Fluorescence-based microsensors provide high spatial-temporal resolution and minimal invasiveness but are constrained by isotropic emission, toxicity, and challenges in solid-state integration. To address these issues, we report a one-step femtosecond-laser-assisted maskless technique for fabrication of micro-nanosensors (MNs) using chitosan for localized pH sensing. This technique simultaneously patterns chitosan and enables in situ formation of nitrogen-doped carbon quantum dots (NCQDs) along the laser-irradiated region. The fabricated 4D MNs exhibit a fluorescence-responsive pH owing to the surface functionalities of in situ -formed NCQDs. This maskless approach for biopolymer patterning establishes a solid-state platform for high-resolution pH sensing and offers a versatile route toward development of next-generation lab-on-chip-based nanophotonic devices.