## 3 Results and Discussion
Both PEG-C/TiO\({}_{2}\)-Dots and PEI&PEG-C/TiO\({}_{2}\)-Dots with exposure to visible light were found to kill bacterial cells, as reported previously [18]. With each hybrid dot consid  
Figure 3: Absorption and fluorescence spectra of PEI&PEG-C/TiO\({}_{2}\)-Dots (solid, black) and PEG-C/TiO\({}_{2}\)-Dots (dash-dot-dot, blue), with those of PEI-CDots (dash, purple) and aqueous suspended small carbon nanoparticles (dash-dot, green, absorption only) for comparison. ered as structurally analogous to a TiO\({}_{2}\) nanoparticle bound with many CDot-like entities, the observed antibacterial activities may be contributed by both TiO\({}_{2}\) and CDot-like components in the dot structure. When exposed to visible light, the CDot-like domain is responsible for the photon harvesting, with the harvested excitation energy driving its own antibacterial action and also activating the attached TiO\({}_{2}\) nanoparticle in a scheme rather similar to the known photosensitization in dye-modified colloidal TiO\({}_{2}\)[31; 32; 33]. As related, the observed meaningfully lower fluorescence quantum yields of the hybrid dots than those of their neat counterparts could be attributed to the intra-dot quenching effects due to electron transfers [18], again similar to those known in dye-sensitized TiO\({}_{2}\) systems [31; 32; 33]. Mechanistically on neat (no TiO\({}_{2}\)) CDots, the photoexcitation drives rapid charge transfers and separation for electrons and holes. The separated redox pairs are trapped at the likely abundant surface defect sites of the carbon nanoparticles, which in the CDots are stabilized by the organic functionalization [3]. The available experimental results from the ultrafast transient absorption probing [34; 35] and the photoinduced reductive noble metal deposition on the dot surface [26; 27] are consistent with the generation of the redox pairs following photoexcitation. The radiative recombination of the separated redox pairs results in longer-lived emissive excited states (in nanoseconds as determined experimentally) responsible for the observed bright and colorful fluorescence emissions [3; 36]. On the light-activated antimicrobial function of CDots, the mechanistic rationale based on available experimental results called for combined actions of the two sets of highly reactive species, the initially formed electron-hole pairs and the classical reactive oxygen species (ROS) generated in the emissive excited states [37]. In the same mechanistic framework, the TiO\({}_{2}\) nanoparticles in the hybrid dots act as a quencher of the photoexcited CDot-like entities and by extension their antimicrobial actions, but at the same time the sensitization due to the quenching activates the antimicrobial function of the TiO\({}_{2}\) nanoparticles. An interesting question is whether the sensitization of the TiO\({}_{2}\) nanoparticles is associated with the quenching of the initially formed redox pairs or the longer-lived emissive excited states or both, though it is beyond the scope of this work and best left for a dedicated investigation. Nevertheless, the results of ROS scavenging experiments discussed as follows support the notion that the observed antimicrobial properties of the C/TiO\({}_{2}\) hybrid dots, similar to those of their neat counterparts, are beyond just the classical ROS. Experimentally for the detection and quantification of the intracellular ROS levels in the treatment of _Bacillus subtilis_ cells with the hybrid dots and visible light, dihydrohadamine (DHR 123) was used as a probe [38; 39]. DHR 123 can be oxidized by ROS to form rhodamine 123, which with 500 nm excitation is brightly fluorescent around 536 nm [38; 39]. Shown in Figure 4A is the significant increase in intracellular ROS generated in _B. subtilis_ cells after their being treated with the hybrid dots and visible light illumination. For the treatment with 0.1 mg/mL PEG&PEI-C/TiO\({}_{2}\)-Dots and visible light for 2 h, the intracellular ROS level in the cells is about 6 times of that in the untreated cells (Figure 4A).