Red Light-Gated Charge Transfer Enables Trace Oxygen Response in Dye-Sensitized TiO₂-Carbon Nanotube Hybrids

Light offers a powerful means of activating materials with spatial and temporal precision. However, translating excited-state processes into chemically responsive electronic function in conductive solids remains challenging, particularly under low-energy red illumination. Herein, we report red light-gated charge transfer in dye-sensitized single-walled carbon nanotube-TiO₂ (SWCNT-TiO₂) hybrids that enables trace oxygen detection. Immobilization of red-absorbing squaraine sensitizers on TiO₂ domains embedded within a conductive SWCNT network reversibly generates a photostationary charge-transfer state under 628 nm excitation. Despite the low photon energy of red light, the hybrid architecture supports interfacial electron injection into TiO₂ and hole accumulation in the nanotube scaffold, while remaining electronically dormant in the dark. Interception of photogenerated electrons by O₂ perturbs this light-induced state and converts excited-state redox chemistry into a chemiresistive signal with ppb-level sensitivity. This capability enables continuous monitoring of oxygen depletion in a sealed headspace during food spoilage. Electron paramagnetic resonance spectroscopy, energy-level alignment analysis, and charge-carrier inversion experiments show that O₂ triggers light-activated changes in SWCNT carrier density. These findings demonstrate that low-energy optical excitation can gate chemical response in dye-sensitized SWCNT-TiO₂ hybrids, establishing a design strategy for temporally controlled sensing and photoresponsive electronic function.