N‐Doped Carbon‐Quantum‐Dot‐Integrated Colorimetric System for Visible Photoprinting and Oxygen Sensing

Abstract The design of light‐responsive color switching systems (LCSs) is essential to the implementation of rewritable paper and oxygen detection. Previous LCSs rely on metal‐based semiconductor photocatalysts which are expensive and pose metal‐leaking risk. To solve the issue, for the first time, nitrogen‐doped carbon quantum dots (N‐CQDs) are reported as metal‐free photocatalyst for visible photoprinting and oxygen sensing. On N‐CQD surface, the decorated amino and hydroxyl groups act as hole scavengers and thus realize visible‐light‐responsive single and multiple color switching, after mixing with redox‐sensitive dyes. Combined with polymeric substrates, LCS rewritable papers can be fabricated and exhibit rapid decoloration performances under 420 nm light irradiation, due to the reduction of dye by photogenerated electrons from N‐CQDs. After the decoloration, efficient color reversion can be realized by the oxidization of dye in ambient air (O₂). By taking advantage of the controlled recoloration kinetics, an oxygen sensor can be attained for visual quantitative analysis of atmospheric conditions with color variations. These results assist the future development of nonmetal color switching systems for superior functional design of photoprintable papers and colorimetric oxygen sensors.