Synthesis of nitrogen-doped graphene quantum dots and its antibacterial property

Abstract

Bacterial infection presents a serious global challenge to human health nowadays, especially with increasing bacterial resistance. Graphene quantum dots (GQDs) are nano-scale graphitic fragments with sp2 hybridized carbon atoms edged with heteroatomic functional groups. As a new-age nanoparticle, it is a potential candidate for new antibacterial agent due to its low cytotoxicity and high chemical stability, compared to other nanoparticles. Most importantly, its passivation can be crafted to make it suitable in antibacterial and optoelectronic applications. This work aims to fabricate nitrogen-doped GQDs (N-GQDs) and investigate its antibacterial effect. N-GQDs were synthesized by microwave-assisted hydrothermal synthesis using p-phenylenediamine as a precursor. The as-prepared N-GQDs solution underwent column chromatography and was separated into four fractions. Each fraction of the N-GQD solution showed distinguishable photoluminescence (PL) properties. The fractions were named according to their PL peak emission color, i.e. green (g-GQDs), yellow (y-GQDs), orange (o-GQDs) and red (r-GQDs). The GQDs samples showed an absorption peak at 245 nm, indicating π-π* transitions of C=C bonds. Transmission electron microscopy (TEM) studies showed that the GQDs exhibited graphitic structure with an average size of ~ 2.5 nm. X-ray photoelectron spectroscopy (XPS) showed an increase in nitrogen content (from 9.1% to 16.2%) with a decrease in oxygen content (from 13.5% to 7.2%) according to the following order: g-GQDs, y-GQDs, o-GQDs and r-GQDs. Gram-positive S. aureus and gram-negative E. coli were treated with the four GQD fractions in order to investigate their antibacterial abilities. Minimal inhibitory concentration (MIC) was measured to evaluate the antibacterial power of the GQDs. The antibacterial effect of the GQDs was found to be more effective in S. aureus than in E. coli. The MIC value towards S. aureus decreased from g-GQDs (171 μg/mL) to r-GQDs (43 μg/mL), indicating the increasing trend in antibacterial power as the nitrogen content increase.