## 1 Introduction  
Carbon \"quantum\" dots or carbon dots (CDots) may be defined generally as small carbon nanoparticles with various surface passivation schemes [1, 2, 3], which may be considered loosely as core-shell nanostructures, each with a carbon nanoparticle core and a thin shell of soft organic materials (Figure 1) [3]. There has been widespread interest in CDots and related carbon/organic nanostructures, with the emergence of a rapidly advancing and expanding research field [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. Among actively pursued technological applications of CDots are those exploiting their visible/natural light-activated antimicrobial properties [15], with many experimental demonstrations on the effective inactivation of various microorganisms, including multidrug-resistant bacterial pathogens, and viruses [17, 18, 19, 20, 21].  
Low-dimensional carbon materials have historically been employed in various uses that combine the carbon with metals and/or metal oxides, for which a good example is the application in some popular catalysts. In the more recent development of the different nanoscale carbon allotropes, from carbon nanotubes and graphene nanosheets to carbon nanoparticles, there has been significant experimental evidence suggesting structural compatibility of the nanoscale carbon with other popular nanomaterials, including especially conventional nanoscale semiconductors, for the preparation of hybrid nanostructures [22, 23, 24, 25]. For small carbon nanoparticles, which represent zero-dimensional carbon allotrope, their ready coating by selected metals in simple photoreductive deposition was demonstrated [26, 27], with the corresponding hybrid CDots serving as highly potent photocatalysts [26]. Additionally demonstrated was the similarly successful coating of small carbon nanoparticles with conventional semiconductors such as ZnS and ZnO for their derived hybrid CDots to exhibit much brighter fluorescence emissions [28].  
Colloidal TiO\({}_{2}\) has been used extensively for a variety of purposes, including applications exploiting the light-activated antimicrobial function [29]. However, the required excitation of TiO\({}_{2}\) nanoparticles by hazardous UV light represents a serious limitation. Hybrid nanostructures containing both TiO\({}_{2}\) and carbon have attracted recent attention, taking advantage of the ability of the carbon domains in the nanostructures to harvest photons effectively over the visible spectrum. For example, hybrid nanostructures of small or larger TiO\({}_{2}\) nanoparticles combined with carbon and organic functionalization were prepared [18; 25; 30], and such nanostructures, denoted as C/TiO\({}_{2}\) hybrid dots, were evaluated for their antimicrobial properties [18; 30]. Interestingly, the C/TiO\({}_{2}\) hybrid dots derived from the commercially available larger TiO\({}_{2}\) nanoparticles (25-30 nm in diameter, Figure 1) were found to be much more effective than those from the specifically synthesized small TiO\({}_{2}\) nanoparticles (8 nm in diameter, for example) in the visible light-driven antibacterial activities [18]. The valuable implication of the finding is such that the larger TiO\({}_{2}\) nanoparticles are not only available commercially in extremely large quantities but also very inexpensive, making their derived hybrid dots particularly promising for potentially widespread antimicrobial applications.  
In this study, we further investigated our previously reported C/TiO\({}_{2}\) hybrid dots (Figure 1) for their visible light-activated antibacterial activities and related mechanistic implications, with particular emphasis on the examination and understanding of the biological consequences to the targeted bacterial cells in terms of major damage to the cell structures and functions. Additionally presented and discussed are opportunities and challenges in the development of carbon-based/derived hybrid dots for antimicrobial applications.