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.