Melatonin is a key endocrine output of the circadian system, but its low endogenous abundance and dynamic fluctuation make accurate detection in biological samples challenging. Here, we report a highly sensitive needle-type organic electrochemical transistor (OECT) biosensor for melatonin detection based on an Au nanocoral-gated acupuncture needle. A hierarchically nanoporous coral-like Au structure was fabricated on the needle surface via a dynamic hydrogen bubble templating strategy, generating a high-surface-area and highly electroactive gate for immobilization of a thiolated melatonin aptamer. By integrating this recognition-enhancing nanostructured gate with the intrinsic signal amplification capability of the OECT, the platform efficiently converts interfacial molecular recognition into amplified electrical outputs. The optimized biosensor achieved a detection limit of 160 fM, showed good selectivity against structurally related interferents, and maintained reliable performance in serum-containing media. In rat plasma, the biosensor showed satisfactory agreement with ELISA and accurately tracked the circadian secretion profile of melatonin. Moreover, the needle-type configuration enabled skin-interfaced measurements and interstitial-fluid detection without causing obvious extensive tissue damage under short-term insertion conditions. The platform further resolved circadian phase shifts induced by light-dark reversal, demonstrating its ability to study physiologically meaningful endocrine dynamics. These results highlight this OECT platform as a promising strategy for low-abundance melatonin analysis, minimally invasive biosensing, and circadian rhythm analysis.
