N-Doped Yellow-Emissive Carbon Nanodots from Gallic Acid: Reaction Engineering, Stimuli-Responsive Red Emission, and Intracellular Localization

Abstract

An easy one-pot method is utilized for the synthesis of carbon nanodots, which has been investigated primarily from material perspectives. However, this preferred one-pot synthesis method suffers from the untraceability of steps leading to the infusion of the photoluminescence property in the nanodot. To resolve the steps involved, a single-precursor approach has been adopted here for the synthesis of the yellow-fluorescent probe by using a bioresource, gallic acid. The synthesized probe exhibits excitation-energy-independent emission, a typical molecular behavior, but shows an emission-wavelength-dependent lifetime that is often attributed to the red-edge effect most commonly observed for nanodots. We propose here that the observation of the red-edge effect for a synthesized probe in neat solvents is due to proton-transfer controlled solvation, a unique photoprocess. Characterization and computational results of the synthesized product clearly suggest the formation of a molecular fluorophore. Unprecedentedly, the synthesized probe shows proton-triggered red emission. The addition of a proton resulted in molecular aggregation due to a strong H-bonded interaction among product molecules. The presence of multiple phenolic−OH groups in the probe allows the easy formation of molecular chains of crystals on transmission electron microscopy grids, causing the observation of lattice spacing that remains unchanged in an aggregated state as well. We have also explored its possible applications in bioimaging and trivalent metal ion sensing. It has better cell-membrane permeability but remains predominantly in the cytoplasm. Both green and red fluorescence are observed inside the cell. The product molecule possesses a submicromolar detection limit for aluminum ions as well, further confirming the unaltered position of phenolic–OH groups present in the product and substrate.