Red-emissive carbon dots from spinach: Characterization and application in visual detection of time

As a new type of carbon nanomaterial, carbon dots (CDs) have attracted more and more attention due to their excellent properties. Here, we developed a simple, green and environmentally friendly method for preparing red-emissive CDs from spinach. The emission color of the CDs under UV light changed from red luminescence to green luminescence after one to four weeks. This was due to the change of the aggregated molecules on the surface of CDs. It was demonstrated that the nitrogen content of the CDs gradually decreased with the increase of time, since CDs combined with oxygen to form aggregates to eliminate excess nitrogen. It was further found that CDs showed the increase intensity of the emission in 500 nm wavelength and decrease intensity of the emission in 665 nm wavelength, which could be attributed to their respective emission from different molecular structures. Compared with the CDs stored for one week, the agglomeration of the CDs particles after four weeks was obvious, indicating that its light stability depended on the agglomeration structure of CDs. Due to their special properties, the CDs could be used for visual detection of time.