Photochromic film composed of methylene blue and carbon nitride semiconductor: reversible color switching in a moist “quasi-solid” microenvironment

This study reports a biodegradable, reversible oxygen indicator film fabricated by coating a cellulose paper substrate with an aqueous ink containing methylene blue (MB), polymeric graphitic carbon nitride (g-C3N4), sodium hydroxide and sodium carboxymethyl cellulose (CMC). When stored in a transparent polymer vacuum bag, the film bleaches under natural solar irradiation and recovers its violet color in the dark upon oxygen ingress through the packaging barrier. UV–Vis spectroscopy, attenuated total reflectance Fourier-transform infrared spectroscopy, scanning electron microscopy/energy-dispersive x-ray spectroscopy, X-ray photoelectron spectroscopy and simultaneous thermogravimetric analysis/differential scanning calorimetry indicate that the film operates in a “quasi-solid” state in which sorbed water retained by the CMC microstructure provides a local environment that enables ionic mobility and reversible MB/leuco-MB redox cycling. Thermal analysis shows an initial low-temperature mass loss of approximately 5%, consistent with moisture retention, supporting the hypothesis that water mediates the solid-state analogue of the “blue bottle” reaction. The system is proposed as a reusable indicator for light exposure and oxygen ingress in vacuum or modified-atmosphere packaging.