Background: Carbon dots (CDots) have recently been demonstrated their effective visible light-activated antimicrobial activities toward bacteria. This study was to evaluate and understand the roles of the surface functionalities in governing the antimicrobial activity of CDots. Methods: Using the laboratory model bacteria Bacillus subtilis , the photo-activated antimicrobial activities of three groups of CDots with specifically selected different surface functionalization moieties were evaluated and compared. The first group consisting of CDots with surface functionalization by 2,2-(ethylenedioxy)bis(ethylamine) (EDA) vs. 3-ethoxypropylamine (EPA), was evaluated to determine the effect of different terminal groups/charges on their photo-activated antibacterial activities. The second group consisting of CDots functionalized with oligomeric polyethylenimine (PEI) and those prepared by the carbonization of PEI – citric acid mixture, was to evaluate the effects of dot surface charges vs. fluorescent quantum yields on their antimicrobial activities. The third group consisting of CDots functionalized with PEI of 1,200 vs. 600 in average molecular weight was evaluated for the effect of molecular weight of surface passivation molecular on their antimicrobial activities. Results: The results indicated the EDA-CDots in the first group was more effective and was attributed to the positive charges from the protonation of the amino groups (–NH 2 ) being more favorable to interactions with bacterial cells. The evaluation of the second group CDots suggested the same surface charge effect dominating the antibacterial performance over the fluorescent quantum yields. The evaluation of the third group CDots functionalized with PEI of 1,200 vs. 600 in average molecular weight, indicated the latter was significantly more effective. Conclusions: The results from this study highlighted the dominant role of surface functionalities in governing CDots’ light activated antimicrobial activity and should have significant implications to the further design and development of CDots as a new class of visible light-activated antibacterial agents.

BackgroundCarbon quantum dots (CDots) have recently been reported as a new class of visible light activated antimicrobial nanomaterials. This study reports the synergistic photoactivated antimicrobial interactions of CDots with photosensitizers on bacterial cells.MethodsThe antimicrobial effects of the CDots with surface passivation molecules 2,2′-(ethylenedioxy)bis(ethylamine) in combination with photosensitizer methylene blue (MB) or toluidine blue (TB) at various concentrations were evaluated against Escherichia coli cells with and without 1-hour visible light illumination. The broth microdilution checkerboard method and isobologram analysis were used for determining if synergistic effect existed between CDots and MB or TB.ResultsThe results showed that CDots alone at a concentration of 5 μg/mL did not display antimicrobial effects, 1 μg/mL MB alone only decreased 1.86 log of viable cell numbers, but the combination treatment with 5 μg/mL CDots combined with 1 μg/mL MB completely inhibited bacteria growth, resulted in 6.2 log viable cell number reduction, suggesting synergistic interaction between the two. The antimicrobial effects of CDots/TB combination exhibited similarly synergistic effects on E. coli cells. These synergistic effects between CDots and MB or TB were further confirmed using the checkerboard microdilution methods, where the fractional inhibitory concentration index value (0.5) and the isobologram analyses. The synergistic interactions were also correlated to the increased generation of intracellular reactive oxygen species in E. coli cells upon the combination treatments of CDots/MB or CDots/TB.ConclusionThe study demonstrated the synergistic photoactivated antimicrobial effects of CDots in combination with other photosensitizers. Such synergistic effect may open new strategies for developing highly effective antimicrobial methods.

Carbon dots (CDots) are characterized by their optical properties including strong absorptions and bright and colorful fluorescence emissions in the visible spectrum and by their photoinduced redox characteristics as both potent electron acceptors and donors. The reported study was for a systematic comparison of CDots from different synthetic approaches based on the use of pre-existing small carbon nanoparticles sourced from pure carbon soot versus the formation of similar nanoparticles in situ via a one-pot thermal carbonization of organic molecular precursors, emphasizing spectroscopic characterizations over the visible spectrum. The results show that the CDots prepared by the latter under sufficiently robust processing conditions are generally no different from those from the former in terms of the observed optical properties and associated photoinduced redox characteristics in the application-wise more meaningful visible spectral region, suggesting a high stability or general applicability of the definition on CDots as surface-passivated small-carbon nanoparticles. Implications of the reported findings to the further understanding and mechanistic explorations of CDots, including the necessity to focus on the core carbon nanoparticles in CDots in such explorations, are highlighted and discussed.

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