Amita Pathak

The present study aims to explore the origin of fluorescence in nitrogen‐doped carbon dots (NCDs) and the impact of their distinct molecular domain characteristics on sensing applications where bilirubin (BR) and Cu ²⁺ have been chosen as model analytes. The transformation of H‐bonded molecular clusters of fluorophores [citrazinic acid and 4‐hydroxy‐1H‐pyrrolo[3,4‐c] pyridine‐1,3,6(2H, 5H)‐trione (HPPT)] in dual emissive (blue/green) NCD1 into amorphous single emissive (green, HPPT dominated) carbon nanodots, NCD2, synthesized by thermal pyrolysis of citric acid and urea at the two optimized temperatures of 140°C and 240°C, respectively, was observed. Depending on the synthesis temperature, the nature and population of molecular constituents of NCDs can be altered, revealed through time‐resolved and steady‐state optical spectroscopy, HRTEM, XPS and Raman analysis. Furthermore, these NCDs were used as fluorescent probes for the detection of BR and Cu ²⁺ ions in aqueous solution. Cu ²⁺ detection, monitored through fluorescence quenching of the NCDs, occurred via electron transfer between the NCDs (donor) and Cu ²⁺ (acceptor), with limit of detection differences attributed to the distinct molecular compositions of NCD1 and NCD2. On the contrary, BR sensing was found to occur via ground‐state complex formation for NCD1 (evidenced by isosbestic points) and through the inner filter effect for NCD2.

We report tunable electrocatalytic activity for CO₂ reduction (CO₂ER) and the hydrogen evolution reaction (HER), exhibited by nitrogen-doped carbon dots (NCDs) synthesized via thermolysis of citric acid and urea (1:3 molar ratio) at 140, 240, 300, and 400 °C. Microscopic and spectroscopic analyses revealed a structural transition from hydrogen-bonded molecular clusters to disordered graphitic domains with an increase in the synthesis temperature. Linear Sweep Voltammetry (LSV) and Chronoamperometry studies revealed the highest current density for NCDs synthesized at 300 °C in a CO₂-saturated medium (4.3 mA cm^-2) and a maximum Faradaic efficiency (FE) of 69.96% for the formation of CO₂-reduced products (methanol, acetate, and n-propanol in 26.13:27.55:21.87% ratio) with minimum HER activity (FE of 25.03%). In contrast, NCDs synthesized at 400 °C exhibited a strong selectivity toward HER with an FE of 64.82% while demonstrating poor catalytic activity toward CO₂ER (FE of 30.18%). The electrocatalytic activity of the synthesized NCDs and their selectivity toward either CO₂ER or HER has been quantitatively correlated with the evolution of their domain behavior as well as the bonding environment of nitrogen.

In this study, the photophysical mechanism of ratiometric detection of the uranyl ion (UO ₂ ²⁺ ), a critical aspect of environmental monitoring and nuclear waste management, has been elucidated.

Nitrogen‐doped carbon dots (NCDs) synthesized via thermal pyrolysis of citric acid and urea have been explored as basic heterogeneous catalysts for Knoevenagel‐Doebner and Aldol condensation reactions. Two distinct NCDs were prepared at different carbonization temperatures: 140 °C (NCD1) and 300 °C (NCD2). Structural characterization revealed that NCD1 comprises hydrogen‐bonded clusters of small organic molecules with abundant surface functional groups (─OH, ─NH ₂ , and ─CONH ₂ ), while NCD2 features a crystalline graphitic carbon core with minimal surface‐passivated functional groups, similar to carbon quantum dots (CQDs). Comparative catalytic studies demonstrated superior performance of NCD1 over NCD2, achieving 75% conversion of benzaldehyde to cinnamic acid in Knoevenagel‐Doebner condensation and 71% conversion to benzalacetone in Aldol condensation. Based on XPS and CO ₂ ‐TPD (Temperature programmed Desorption) studies, the enhanced catalytic activity of NCD1 could be attributed to its elevated surface Lewis basicity, arising from the presence of molecular domain rich in ─NH ₂ functionalities.

Blue-emissive nitrogen-doped chiral carbon dots (d-NCD230 and l-NCD230) exhibiting antipodal chiroptical activity, synthesized from the thermal pyrolysis of citric acid and d/l-aspartic acid in 1:2 molar ratios, have been explored as chirality-based fluorescent turn-off/on probes for the detection of Hg²⁺ and l-cysteine (l-Cys). Circular dichroism (CD) spectroscopy revealed that the chiroptical activity originates from a synergy among intrinsic chirality, chiral precursors on the NCD surface, and hybridization of lower energy levels within the embedded chiral chromophore. Quantitative analysis of optical asymmetry using the Kuhn asymmetry factor (g) at the CD signal of 312 nm showed a higher value for d-NCD230 (1.03 × 10^-4) compared to l-NCD230 (1.13 × 10^-5). Moreover, we have demonstrated chirality transfer and chiral inversion phenomena in d/l-NCDs by preparing carbon dots with different precursor ratios at different temperatures and probing them through CD spectroscopy. The NCDs exhibited selective fluorescence quenching in the presence of Hg²⁺, demonstrating linearity in the Stern-Volmer plot. Limits of detection (LODs) for Hg²⁺ were calculated to be 129 and 192 nM for d-NCD230 and l-NCD230, respectively, in the 0-150 μM concentration range. The quenching mechanism involves nonradiative electron transfer due to Hg²⁺ binding to oxygen-rich functional groups on the d/l-NCD230 surface. The slight variation in LOD values between d-NCD230 and l-NCD230 indicates the negligible effect of the chirality on Hg²⁺ sensing. Notably, the fluorescence intensity of d/l-NCD230 could be restored upon adding l-cysteine, with d-NCD230 showing a more pronounced enhancement than l-NCD230. This differential response is attributed to a preferential stereoselective interaction arising from the homochirality of d-NCD230/Hg²⁺ and l-cysteine. These findings demonstrate the potential of chiral nitrogen-doped carbon dots as sensitive and selective probes for Hg²⁺ and l-cysteine, with implications for environmental monitoring and biological sensing applications.

The hydrogenation of 4-nitrophenol using carbon dot-stabilized gold (Au) nanoparticles is well-studied, with Au-H species known to catalyze the reaction. However, the impact of specific nitrogen moieties in nitrogen-doped carbon dots on Au-H formation and catalytic activity remains unexplored. These nitrogen species, acting as surface ligands, may influence the catalytic properties through the generation of Au-H species via H^• radicals. In this regard, modulation of the catalytic properties of Au nanoparticles has been explored by conjugating their surface with nitrogen-doped carbon dots (NCDs). Three distinct nanohybrid formulations comprising NCDs and Au nanoparticles (i.e., NCDs-Au) have been prepared, where the NCDs were derived from different carbon sources (e.g., citric acid and l-malic acid) and varying mole ratios of the nitrogen source (i.e., urea). The impact of NCDs on Au nanoparticle-mediated catalysis has been investigated using the model reaction of hydrogenation of 4-nitrophenol (4-NP) in the presence of NaBH₄. The fractions of different nitrogen species (such as pyrrolic, pyridinic, and amidic) in the different NCDs-Au nanohybrids were quantified through XPS analysis, and their roles in catalytic performance have been studied. Further, the size, shape, crystallinity, defects, and exposed facets of the NCDs-Au nanohybrids have also been assessed (through XRD, HRTEM, and Raman studies), and their structure-activity relationships have been corroborated. The hydrogenation of 4-NP is proposed to happen through the formation of gold-hydride (Au-H) species facilitated by H^• radicals, as confirmed by EPR analysis. The NCDs-Au nanohybrid, synthesized from NCDs derived from a 1:3 molar ratio of l-malic acid and urea (MU13-Au), exhibits superior catalytic efficiency with a rate constant of 1.013 min^-1, attributed to its abundant defects and a notably high relative content of catalytically favorable pyridinic nitrogen species compared to other tested nanohybrids.

Human serum albumin (HSA) is a crucial protein in the human body, serving as a carrier for various small molecules and playing a vital role in maintaining osmotic pressure in the bloodstream. However, misfolding and fibrillation of HSA can lead to a range of pathological conditions, including amyloidosis. In recent years, researchers have explored several innovative strategies to inhibit HSA fibrillation, and one of the promising avenues is the utilization of chiral compounds that can offer an effective means of modulating the 'structural integrity of the protein. Among the most prevalent chiral compounds, chiral carbon dots (CCDs) are renowned for their distinctive physicochemical properties, offering unique stereochemical properties, assuming a pivotal role in attaining circularly polarized luminescence that can be harnessed for inhibiting HSA fibrillation. Recent advancement of carbon-based chiral nanostructures has attracted widespread attention in the field of nanobiotechnology owing to their overwhelming chemical, optical and electronic properties. However, being an emerging type of chiral nanomaterial, chiral CDs (CCDs) are still at their primary research stage. Herein, we have developed a simple methodology to quickly synthesize chiral carbon dots (L/D-TA-C-dots) of size ranging from 3 to 7 nm by microwave-assisted pyrolysis of L/D-tartaric acid, respectively. The circular dichroism spectrum of L/D-TA-C-dots has demonstrated characteristic peaks with opposite nature at around 230–232 nm corresponding to the n → π* transition of CO groups. The synthesized L/D-C-dots have exhibited stable and intense luminescence at 460 nm. A variety of biophysical techniques such as thioflavin-T fluorescence, circular dichroism, fluorescence microscopy, transmission electron microscopy and atomic force microscopy have suggested that the L-TA-C-dots are more effective in hindering the fibrillation process of HSA than D-TA-C-dots. Further, the potential of the synthesized L-TA-C-dots to inhibit the formation of human serum albumin (HSA) fibrils has been investigated in vitro. The biocompatibility of water-soluble L-TA-C-dots has been confirmed by determining the cellular uptake efficiency through a localization study after exposing them to MDA-MB-231 cells. Besides, the thermodynamic parameters for the interaction of HSA monomers with L/D-TA-C-dots have also been calculated, which suggests that the preferential hydrophobic interaction between HSA monomers and L-C-dots restricts the self-assembly of HSA at an early stage, leading to inhibition of fibrillation. The promising results of chiral L-TA-C-dots provide an opportunity to exploit them in the field of in vivo bioimaging and can be used as promising therapeutic agents for various amyloidogenic disorders.