Evaluation of Antioxidant Activity of Zinc-Doped Carbon Quantum Dots

Carbon quantum dots (CQDs) are a class of fluorescent carbon-based nanomaterials that have attracted substantial attention due to their small size (<10 nm), tunable photoluminescence, good aqueous solubility, and low cytotoxicity [1]. Since their discovery in 2004 during the purification of single-walled carbon nanotubes, CQDs have been widely studied for applications in bioimaging, drug delivery, photocatalysis, and biosensing . Unlike conventional semiconductor quantum dots, carbon-based QDs offer the advantages of high biocompatibility and environmental friendliness [2]. CQDs can be synthesized from a variety of carbon-rich precursors using top-down or bottom-up methods. Among these, hydrothermal synthesis is considered a green and cost-effective approach that allows easy doping with heteroatoms or metal ions. [3].The incorporation of metal dopants into CQDs significantly alters their physicochemical and biological properties. Zinc (Zn), a biologically essential trace element, has been shown to improve antioxidant activity when introduced into nanomaterials . Zn-doped CQDs (Zn-CQDs) have thus emerged as a promising candidate for therapeutic applications, particularly in the management of oxidative stress. Carboxylic acid combination with zinc nitrate in a hydrothermal environment facilitates the formation of Zn-CQDs with surface functionalities such as hydroxyl, amine, and thiol groups, which contribute to the bioactivity and stability of the nanomaterials . These surface groups play key roles in modulating enzyme inhibition, metal chelation, and radical scavenging activities [4]. Antioxidant therapies have been extensively explored to mitigate these effects [5]. In this context, we report the synthesis of Zn-CQDs using a green hydrothermal route and evaluate their physicochemical properties and biological activities, focusing on their antioxidant potential.

2.1 SYNTHESIS OF Zn-CQDS

CQDs were synthesized hydrothermally by dissolving the precursors  in distilled water. The solution was transferred to a Teflon-lined autoclave and heated at 180°C for 6 hours. The product was cooled, filtered, and lyophilized to obtain dry Zn-CQDs.[6]

2.2 CHARACTERIZATION

FT-IR spectra of the sample was recorded on a FT-IR spectrometer (ACIC, Thiruchirapalli). The morphology of nanoparticles was analysed at HRTEM facility, PSG, Coimbatore. The UV-VIS spectra of the sample to measure the absorbance were done by using the double beam UV- 2400 PC series spectrometer (ACIC, Tiruchirappalli).

2.3 BIOLOGICAL ASSAYS

Antioxidant capacity was measured by the phosphomolybdenum method, with results expressed as ascorbic acid equivalents.[7]

OPTICAL PROPERTIES

 The UV-Vis peak at 260 nm indicates π–π* transitions, a hallmark of graphitic carbon domains, confirming successful carbonization of the precursor. This confirms the presence of conjugated domains in the carbon structure, commonly associated with CQDs.[8]

Figure.1 UV Spectrum of Carbon Quantum Dots

3.2 STRUCTURAL AND ELEMENTAL ANALYSIS

FTIR peaks showed typical bands for -OH, -NH2, -COOH, and C=S, indicative of thiourea and ascorbic acid residues. FTIR spectra revealed characteristic peaks at ~3400 cm⁻¹ (O–H and N–H stretching), 1630 cm⁻¹ (C=O stretching), and 1050–1250 cm⁻¹ (C–N and C–S stretching), confirming functionalization with oxygen, nitrogen, and sulfur-containing groups. .[8]

Figure.2.   IR Spectrum of Carbon Quantum Dots

3.3MORPHOLOGY

TEM analysis showed nearly spherical Zn-CQDs with uniform dispersion, important for consistent biological activity. The high dispersion and small size are favorable for bio-interactions and stability. .[9]

Figure.5- TEM Image Of Carbon Quantum Dots

3.5 ANTIOXIDANT ACTIVITY

The total antioxidant capacity of the extract was evaluated by the phospho-molybdenum method, according to the procedure described by Prieto et al.The absorbance of the solution was measured at 695 nm using a UV-VIS spectrophotometer against blank after cooling to room temperature. The total antioxidant activity is expressed as the number of gram equivalent of ascorbic acid. [10].The results obtained are listed in the table below

Table.2 Results of Antioxidant activity of synthesized CQDs

Zn-CQDs displayed significant antioxidant activity (148.27 µg/mg AAE), confirming their potential to scavenge free radicals.

 Zn-CQDs synthesized from the mentioned precursors demonstrate promising antioxidant properties. Comprehensive characterization confirmed the quantum dots' amorphous structure, functional surface groups, and elemental composition. Zn-CQDs demonstrated promising antioxidant capacity , suggesting their potential as therapeutic agents in oxidative stress. Future work may focus on improving bioactivity via surface functionalization or targeting delivery mechanisms.

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