Developing functional carbon dots for enhanced antimicrobial activity and development of ion-imprinted carbon dots for lead ion detection

Abstract

The increasing need for efficient and renewable antimicrobial nanomaterials for biomedical and environmental applications has stimulated extensive research into novel carbon-based nanomaterials. In parallel, heavy metal contamination continues to pose a significant global environmental and public health concern. Lead (Pb2+) is a persistent environmental contaminant that poses a serious risk to human health even at trace amounts. Carbon dots (CDs), a new class of carbon-based nanomaterials, have attracted significant attention owing to their biocompatibility, fluorescence sensing, tunable surface chemistry, green synthesis, and antibacterial properties. However, the relationship between synthesis conditions, physicochemical characteristics, and antibacterial performance remains incompletely understood. This thesis aimed to optimize the hydrothermal synthesis of Zn/N-doped carbon dots derived from carboxymethyl cellulose (CMC) by investigating the effects of reaction temperature (250 °C, 300 °C and 350 °C) and time (1 h, 2 h, 3 h,) on their structural, optical, and antibacterial properties against a Gram-negative and a Gram-positive species (Escherichia coli and Staphylococcus aureus) using response surface methodology (RSM). The results showed that temperature is the dominant factor governing CDs properties, increasing crystallinity and fluorescence performance, with a maximum quantum yield (QY) of 71% and crystallinity of 48% achieved at 350 °C and 3 h. In contrast, optimal antimicrobial activity was observed at intermediate synthesis conditions, around 300 °C and 2 h, reaching a maximum zone of inhibition of 15.2 mm. Antioxidant activity was found to be more dependent on synthesis time, with the highest antioxidant activity observed in carbon dots synthesized for 1 h. Furthermore, this thesis explored the fabrication of ionic-imprinted carbon dots to enhance the selectivity and sensitivity of Pb2+ detection for environmental and biomedical monitoring. Overall, carbon dots synthesized at 300 °C for 2 hrs showed the best antimicrobial performance in both species, with MICs (minimum inhibitory concentrations) of 1 mg/mL for E.coli and 2mg/mL for S.aureus, with near-neutral surface charge and medium crystallinity compared to other samples and medium functional groups, indicating the synergistic effect of all properties instead of one enhanced property. Ion-imprinted carbon dots successfully demonstrated enhanced sensitivity and selectivity toward Pb²⁺ ions in blood and water samples, achieving a limit of detection (LOD) of 1.3 × 10⁻¹ μM.