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Carbon dots (CDots) are small carbon nanoparticles with effective surface passivation by organic functionalization. In the reported work, the surface functionalization of preexisting small carbon nanoparticles with N-ethylcarbazole (NEC) was achieved by the NEC radical addition. Due to the major difference in microwave absorption between the carbon nanoparticles and organic species such as NEC, the nanoparticles could be selectively heated via microwave irradiation to enable the hydrogen abstraction in NEC to generate NEC radicals, followed by in situ additions of the radicals to the nanoparticles. The resulting NEC-CDots were characterized by microscopy and spectroscopy techniques including quantitative proton and 13C NMR methods. The optical spectroscopic properties of the dot sample were found to be largely the same as those of CDots from other organic functionalization schemes. The high structural stability of NEC-CDots benefiting from the radical addition functionalization is highlighted and discussed.

Carbon dots (CDots) are generally defined as small-carbon nanoparticles with surface organic functionalization and their classical synthesis is literally the functionalization of preexisting carbon nanoparticles. Other than these “classically defined CDots”, however, the majority of the dot samples reported in the literature were prepared by thermal carbonization of organic precursors in mostly “one-pot” processing. In this work, thermal processing of the selected precursors intended for carbonization was performed with conditions of 200 °C for 3 h, 330 °C for 6 h, and heating by microwave irradiation, yielding samples denoted as CS200, CS330, and CSMT, respectively. These samples are structurally different from the classical CDots and should be considered as “nano-carbon/organic hybrids”. Their optical spectroscopic properties were found comparable to those of the classical CDots, but very different in the related photoinduced antibacterial activities. Mechanistic origins of the divergence were explored, with the results suggesting major factors associated with the structural and morphological characteristics of the hybrids.

The carbon/TiO2 hybrid dots (C/TiO2-Dots) are structurally TiO2 nanoparticles (in the order of 25 nm in diameter from commercially available colloidal TiO2 samples) surface-attached by nanoscale carbon domains with organic moieties, thus equivalent to hybrids of individual TiO2 nanoparticles each decorated with many carbon dots. These hybrid dots with exposure to visible light exhibit potent antibacterial properties, similar to those found in neat carbon dots with the same light activation. The results from the use of established scavengers for reactive oxygen species (ROS) to “quench” the antibacterial activities, an indication for shared mechanistic origins, are also similar. The findings in experiments on probing biological consequences of the antibacterial action suggest that the visible light-activated C/TiO2-Dots cause significant damage to the bacterial cell membrane, resulting in higher permeability, with the associated oxidative stress leading to lipid peroxidation, inhibiting bacterial growth. The induced bacterial cell damage could be observed more directly in the transmission electron microscopy (TEM) imaging. Opportunities for the further development of the hybrid dots platform for a variety of antibacterial applications are discussed.

Carbon dots (CDots) of small carbon nanoparticles with oligomeric polyethylenimine for surface functionalization, coupled with visible light exposure, were found highly effective in the inactivation of bacterial pathogens. In this study, using a representative strain of a major foodborne pathogen – Listeria monocytogenes , as a target, the effects of the CDots treatment at sublethal concentrations on bacterial functions/behaviors related to the biofilm formation ability/potential, including cell attachment and swimming motility, were assessed. On the consequence at molecular level, the expression levels of the genes that are related to cell attachment/adhesion, motility, flagellar synthesis, quorum sensing, and environmental stress response and virulence were found all being up-regulated.