Photoelectrochemical　(PEC)　sensing　has　been　rapidly　evolving　in　recent　years,　while　the　introduction　of　small　molecules　with　specific　recognition　functions　into　the　sensing　interface　remains　a　nascent　area　of　study.　In　this　work,　we　reported　a　PEC　biosensor　for　formaldehyde　(FA)　detection　based　on　photoinduced　electron　transfer　(PET)-gated　electron　injection　between　organic　small　molecules　and　inorganic　semiconducting　substrates.　Specifically,　an　FA-responsive　probe　(NA-FA-COOH)　and　TiO2　nanoarrays　were　integrated　to　construct　a　PEC　platform　(NFC/TiO2)　via　a　coordination　bond.　NFC　served　simultaneously　as　a　target-specific　recognition　element　and　a　modulator　of　photoinduced　electron　injection.　Treatment　of　NFC/TiO2　by　FA　would　suppress　the　intramolecular　PET　process,　with　the　quenched　photocurrent　signal　due　to　the　changed　carrier　transfer　pathway,　thus　establishing　the　PEC　platform　for　FA　based　on　effective　PET　modulation.　The　proposed　PEC　system　exhibited　high　selectivity　and　sensitivity,　with　a　low　detection　limit　of　0.071　μM.　This　study　presents　a　novel　perspective　on　the　use　of　organic　small　molecules　with　a　PET　effect　for　advanced　PEC　bioanalysis.　©　2023　American　Chemical　Society.