Bimetallic Pd@CeO₂-decorated 2D-g-C₃N₄/calcium alginate conductive hydrogel: A multifunctional hybrid material for wearable electrochemical sensing of serotonin

Developing multifunctional hybrid materials with enhanced electrochemical and mechanical properties is critical for next-generation wearable biosensing technologies. Here, we report the fabrication of a hierarchically engineered hybrid nanocomposite composed of palladium-doped cerium oxide (Pd@CeO₂) nanoparticles uniformly anchored onto two-dimensional graphitic carbon nitride (g-C₃N₄) nanosheets. This hybrid nanostructure was embedded within a calcium alginate (CA)-based conductive hydrogel matrix to construct a flexible, skin-compatible composite electrode platform. The synergistic interactions between Pd@CeO₂ and the π-conjugated g-C₃N₄ framework afford superior electrocatalytic activity, improved charge transport, and enhanced surface area. Integration of CA hydrogels not only imparts mechanical flexibility but also introduces a hydration layer that mitigates biofouling, making the composite highly suitable for bioelectronic interfaces. As a proof of concept, the hybrid platform demonstrated high-performance electrochemical detection of serotonin (5-HT), achieving a detection limit (LOD) of 0.64 nM in phosphate buffer and 1.7 nM in human serum, along with broad linear ranges and high sensitivity. The sensing mechanism is driven by electrostatic and π–π interactions between the indole moiety of 5-HT and the tri-s-triazine (C₆N₇) units of g-C₃N₄, enabling selective adsorption and detection. Furthermore, the platform demonstrated reliable real-time monitoring in complex biological matrices, including artificial sweat and differentiated neuronal cell cultures. Importantly, the integrated wearable sensors were enabled in both ex situ skin-mimicking models and in situ on-body human volunteers. This work presents a versatile hybrid composite framework for wearable bioelectronic applications, bridging advanced material design with practical biomedical utility.