Dual-Mode Photochromic Luminescence of Carbon Dots Induced by Photoinduced Electron Transfer

• • Na-CDs exhibit a 180 nm red-shift in emission (450 → 630 nm) under 365 nm excitation, enabling broad-spectrum color modulation that exceeds typical organic photochromic systems; this spectral range is critical for high-contrast anti-counterfeiting and multiplexed optical encoding. • • Na,B-CDs achieve blue-shifted multicolor emission (orange → yellow → green) within 30 s of UV irradiation, a response time that is an order of magnitude faster than many spiropyran-based materials, directly addressing the slow response bottleneck in dynamic security inks. • • Photochromic states spontaneously revert to initial configurations without external stimuli, and the process remains reversible over multiple cycles; this self-recovery eliminates the need for chemical or thermal resetting, reducing operational complexity in UV detection and encryption platforms. • • Na-CDs-PVA films darken rapidly under sunlight (UV index = 6) and recover after sunset, demonstrating daylight-responsive UV detection with reversible operation; this provides a low-cost, passive dosimeter for occupational UV exposure monitoring, though quantitative dose-response linearity and humidity sensitivity require further validation.