Biogenic carbon quantum dots with tunable surface charge for biocompatible antibacterial applications

Aim: Antimicrobial resistance poses a major global health crisis, with some bacterial strains now resistant to nearly all available antibiotics. Carbon quantum dots (CQDs) have emerged as promising nanomaterials for broad-spectrum infection prevention owing to their multiple antibacterial mechanisms, biocompatibility, and cost-effectiveness. Many reported CQD fabrication methods rely on synthetic chemicals, which increase production costs and potentially compromise the biocompatibility of the resulting CQDs. Although green-synthesized CQDs have attracted considerable attention for antibacterial applications, limited studies have investigated the use of natural, food-derived components to tune CQD surface charge and its influence on antibacterial activity and mammalian cell compatibility. This study aims to develop CQDs with tunable surface charges from natural food-derived carbon sources for broad-spectrum antibacterial applications. Methods: Whole-meal bread and soybean flour were used as biogenic precursors to synthesize negatively charged CQDs via a simple hydrothermal method. Surface charge was adjusted to neutral and positive by incorporating lemon juice and chitosan during synthesis. Antibacterial activity and mammalian cell viability were evaluated. Results: Bread- and soybean-derived CQDs exhibited negative surface charges (–15 mV) due to abundant carboxyl and hydroxyl groups formed during precursor decomposition. Addition of lemon juice altered the surface chemistry by introducing balanced protonated and deprotonated species, producing zwitterionic CQDs with near-neutral charge (–0.1 mV). Further incorporation of chitosan introduced protonated amine groups (–NH3+), yielding positively charged CQDs (+10 mV). At an optimal concentration of 10 µg/mL, both neutral and positively charged CQDs demonstrated moderate broad-spectrum antibacterial activity (30–40% inhibition) against Gram-negative and Gram-positive bacteria. Their antibacterial effect was attributed to favorable electrostatic interactions with negatively charged bacterial cell envelopes, causing membrane disruption, reactive oxygen species (ROS)-induced damage, and intracellular interference. In contrast, mammalian cells maintained 100% viability, likely due to their flexible cholesterol-rich membranes, stronger antioxidant defense systems, and intracellular compartmentalization. Conclusions: This study demonstrates a reagent-free and sustainable approach for producing CQDs with controlled surface charges from natural precursors. The resulting CQDs show strong potential as safe and effective antibacterial nanomaterials for biomedical applications.