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.

Green Switching and Light-Harvesting Abilities of Red-Emissive Carbon Nanodot

“…The serendipitous observation of excitation-energy-dependent photoluminescence from nanoscale carbon particles in 2004 has rejuvenated the search for solution-processed, biocompatible, and photoluminescent carbon quantum dots. , There are a vast number of reports on the strategies for the synthesis of such water-soluble, photoluminescent dots, usually termed carbon nanodots (CNDs). − Their potential applications cover the most demanding areas of research in recent times, such as intracellular colocalization, light-emitting diodes, hydrogen gas production, and enhanced photosynthesis. − The photoluminescence of the reported CNDs is highly sensitive to precursors and solvents used in the synthesis method, and the emissions of these CNDs can easily be tuned by the doping of nitrogen, sulfur, and others, yielding a color gamut. − …”

“…The impetus generated by the carbon nanodot for violating the photochemical principleKasha-Vavilov’s rulefailed to gather steam due to the unavailability and uncertainty in the structural features of the carbon nanodot and the mechanism of elemental doping. − The emergence of excitation-energy-independent photoluminescence in this context challenges the foundation of the best-known carbon nanodot. ,, Recently, we have shown that aggregates of citrazinic acid alone at room temperature can exhibit excitation-energy-dependent emission and graphitic nanocarbon features . Hence, the term nano in CNDs can be restricted to only solid-state structural aspects, rather than emission characteristics in solution.…”

“…It is worthy to mention here that the science-based concerns on the origin of their photoluminescence have not been settled yet. A prominent hypothesis on the involvement of graphitic carbon in dictating its emission properties is one-dimensional and devoid of the basic principles of organic transformations. − Although the emerging hypothesis of molecular crystals in solid state is more insightful, it needs to be substantiated with experimental evidence of direct molecular transformability. ,− This has highly intrigued us to identify the routes of molecular transformability for carbon nanodots. To the best of our understanding, this is the first report where such a challenging yet compelling attempt of switching the red-emissive carbon dot to a green-emissive one has been made (Scheme .…”

Preparing Colour-Tunable Tannic Acid-Based Carbon Dots by Changing the pH Value of the Reaction System

“…In this study, we report that polychromatic CDs are obtained by the reaction of TA with o-phthalaldehyde under different reaction conditions ( Figure 1 ). The creation of a larger conjugated system during CD synthesis is ensured by the high reactivity between many phenolic hydroxyl groups and benzaldehyde, facilitating the further reaction of CDs [ 24 , 25 ]. Interestingly, the luminous colour of CDs is controlled by the pH value of the isomer structure reaction environment in the o-phthalaldehyde region.…”

Red, green, and blue light-emitting carbon dots prepared from gallic acid for white light-emitting diode applications

“…1), and is widely found in fruits and plants. Although its structure is very favorable for the preparation of CDs, multicolor luminescent CDs based on GA have not been reported because it is difficult to obtain an extended conjugating structure with only the phenolic hydroxyl group of GA [18][19][20] . Herein, we report the preparation of multicolor luminescent CDs based on GA, which can react with o-phthalaldehyde via the aldol condensation reaction 22 .…”