Conventional sensing platforms for plant health monitoring are often limited by high operating temperatures, rigid substrates, and poor compatibility with ambient, power-constrained, or biologically sensitive environments. These limitations hinder their integration into emerging platforms such as smart agriculture and plant-interfaced electronics, where mechanical flexibility, energy efficiency, and low thermal budgets are essential. This paper reports a scalable, thermally passive NO₂ sensor based on light-activated 3D TiO₂ nanoarchitectures. Fabricated via sequential glancing angle deposition, the highly ordered porous nanoarchitectures exhibit tunable broadband light scattering and defect-mediated sub-bandgap activation under ambient light. Integrated with a wireless microcontroller and mobile application, the sensor enables autonomous NO₂ monitoring in real-world conditions. Field deployment on Mentha suaveolens plants demonstrates real-time tracking of gas-induced physiological stress, establishing practical ecological relevance. This platform overcomes the key limitations of conventional sensors, offering a structurally tunable, spectrally adaptive, and fabrication-scalable solution for light-powered, bio-integrated environmental monitoring.