1 INTRODUCTION
The recent pandemic has once more underscored the substantial potency, swift and extensive dissemination, and the threat to human health posed by microorganisms. In particular, the infections of bacterial pathogen continue to be a problem of public health concern.[1] Recent estimates further suggest that up to 80% of bacterial and archaeal cells reside in biofilms, in which bacteria display characteristics and behavior distinct from their planktonic counterparts.[2] It has been reported that bacteria in biofilms are 10–1000 times more tolerant of bactericidal reagents than the planktonic counterparts.[3] Therefore, addressing the resulting recalcitrant infection of pathogenic bacteria and biofilm formation has been the central to health care, veterinary, and disease prevention and control.[4, 5] Antibiotics are widely used weapons against bacterial infection, but they have been losing their punch due to the emergence and propagation of drug-resistance, caused by their overuse and abuse.[1, 6] In 2019, approximately 4.95 million people worldwide deaths were linked to bacterial antibiotic resistance.[7] Furthermore, by 2050, antibiotic-resistant bacterial infections were estimated to annually cause up to 10 million deaths worldwide, and a loss of over 3.8% of the global gross domestic product.[8, 9]
Nanostructured materials have gained popularity in the post-antibiotic era, due to their associated unique properties.[9, 10] It has been recognized that nanomaterials hardly causes antibiotic resistance while exhibiting long-lasting antibacterial activity.[11] Recently, people have evolved to exploit nanomaterials as an alternative to combat bacterial infection, including antibacterial nanomaterials.[9] Up to date, noble metal (e.g., Ag[12] and Au[13]) nanoparticles and metal oxide (e.g., ZnO,[14] CuO,[15] and TiO2,[16]) nanoparticles have been employed as antimicrobial reagents, attributable to the associated reactive ion release, photocatalytic activity, reactive oxygen species (ROS) generation, and/or physical contact with bacterial cells.[17] However, most of these nanomaterials are associated with high production expenses and/or the potential emission of toxic substances that can pose a health threat to human beings and the nature. In addition, researchers from microbial and biomedical fields may encounter with synthetic difficulty of nanomaterials and, therefore, the antibacterial applications of nanomaterials has remained limited.[18] The welfare of humans, animals, and the ecosystem are still pointing to the pressing need for the development of effective yet biocompatible nanomaterials to combat resistant strains and biofilms across various contexts.
As an emerging class of nanomaterials, carbon dots (CDs) can be converted from various carbon sources of wide availability. Interestingly, they have demonstrated their antibacterial property.[19] Nevertheless, the currently adopted procedures for synthesis of CDs are time-consuming and complicated, making them less accessible to people without synthesis background.[20] Further, their antibacterial mechanisms are not as well studied as some other materials, such as Ag and metal oxide nanoparticles. Here we showcase the synthesis of tiny yet uniform CDs with a quasi-spherical shape and a size of 3.3 nm through a one-pot, one-step hydrothermal process. The use of citric acid and polyethyleneimine as the precursor are responsible to the resultant positively charged surface, facilitating the interaction with bacterial cells and implement of antibacterial effect. The insights into the bactericidal activity of the CDs elucidate the damaged bacterial membrane structure and elevated ROS level inside bacterial cells in the presence of the CDs. In particular, a great enhancement in their antibacterial activity is observed, which can be ascribed to the generation of 1O2 by the CDs under light irradiation. We further demonstrated the negligible cytotoxicity of the CDs even at high concentration, and their capability of efficient elimination of biofilms, making them superior candidates for antibacterial applications in fields ranging from food packing, woundplast production, antibacterial medical device coating, and home sterilizing spray as well.