Cr(VI)-ResponsiveInk with Four-Dimensional Printingof an Ultracompact Hydrogel Optical Fiber Microsensor

Hydrogel is emerging as a promising material for smart sensors due to its remarkable stimuli-responsiveness and biocompatibility. However, traditional methods like ultraviolet curing or imprinting could not yield ultracompact hydrogel microstructures with sophisticated design and controllable morphology, posing challenges in developing highly integrated microfluidic sensors. With the advanced femtosecond laser (Fs) direct writing technology, an intelligent hydrogel optical microsensor is prepared for real-time monitoring of trace hexavalent chromium ions [Cr(VI)] in water. First, a Cr(VI)-responsive hydrogel ink containing 3-acrylamidopropyl-trimethyammonium chloride (ACTC) is developed, boasting a printing resolution of ∼250 nm. Subsequently, a fiber-tip Fabry–Perot cavity (FPC) Cr(VI) microsensor is printed using a multimaterial TPP strategy. The sensor shows an ultracompact size (∼100 μm) and high specificity for detecting trace liquid samples. The detection limit of 1.48 × 10^–9 M makes it suitable for rapidly detecting trace Cr(VI). The on-chip direct writing of smart hydrogel MEMS sensors provides an ultracompact detection platform for environmental protection and analytical science fields.