Multifunctional Carbon Dots with Solid–Liquid State Orange Light Emission for Vitamin B12 Sensing, Cellular Imaging, and Red/White Light-Emitting Diodes

In the current research work, safranine T (ST) was chosen to be the guest, and the preparation of carbon dots (CDs) was done by a one-step hydrothermal methodology with citric acid (CA) to be the precursor, besides the surface passivation species in the consequent host–guest CDs. In addition, the CDs that had high-luminescence properties were prepared in the solid and liquid states, respectable fluorescence quantum yield of 15.2% (liquid) and 39.9% (solid), which is higher as compared with the majority of the reported orange fluorescent CDs. Meanwhile, red light-emitting diodes (LEDs) were fabricated with the use of a combination of CDs and UV LED chips. The constructed white LEDs possess excellent white light properties with the Commission Internationale de L’Eclairage chromaticity coordinate of (0.33, 0.34), coupled with the normal correlated color temperature of 5347 K as well as elevated color rendering index of 81 (>80), which seems appropriate for indoor lighting. Moreover, vitamine B12 (VB12) has the potential of productively quenching the fluorescence of CDs on the basis of the inner filter effect. Besides that, the system possessed the elevated selectivity with regard to VB12 that had the detection limit of 60.78 nM, which could further have been illustrated in not just VB12 tablets but in vitamin drink and human serum samples as well. Moreover, the CDs with the positive biocompatibility as well as ignorable cytotoxicity were extended for sensing VB12 in the living cell. These properties suggest that CDs have immense potential in the field of sensors, cell imaging, and optical devices.