## 4 Summary and Conclusions  
Nanoscale carbon is compatible with colloidal TiO\({}_{2}\) for C/TiO\({}_{2}\) hybrid structures, which with the surface organic moieties are equivalent to individual TiO\({}_{2}\) nanoparticles each surface-attached by many CDot-like nanoscale entities, hence C/TiO\({}_{2}\) hybrid dots. These hybrid dots are optically absorptive in the visible spectrum, due exclusively to the absorptions of the nanoscale carbon domains. The photon energies thus harvested by the hybrid dots could drive their antibacterial activities, similar to those found in neat CDots (no TiO\({}_{2}\)) with the same visible light exposure. The mechanistic origins of the antibacterial function shared by the C/TiO\({}_{2}\) hybrid dots and their neat counterparts, as supported by the similar outcomes of the experiments on using established scavengers of reactive oxygen species (ROS) to \"quench\" the antibacterial activities, are also likely to be similar. In experiments for probing biological consequences of the antibacterial action, the results suggest that the visible light-activated C/TiO\({}_{2}\) hybrid dots cause significant damage to the bacterial cell membrane to result in higher permeability, and the associated oxidative stress leads to lipid peroxidation, inhibiting bacterial growth. For the bacterial cell damage, more direct evidence is provided by the TEM imaging results. Since the colloidal TiO\({}_{2}\) is widely available commercially in extremely large quantities and very low costs, from which C/TiO\({}_{2}\)-Dots can be produced in a facile fashion, the hybrid dots represent a highly promising platform for a variety of antibacterial applications. Beyond antimicrobial uses, the hybrid dots are more advantageous than the widely pursued dye-sensitized colloidal TiO\({}_{2}\) for broad applications, such as in photocatalysis for CO\({}_{2}\) conversion and waste management.  
Figure 6: Representative TEM images of _B. subtilis_ cells: (**A**) untreated, and (**B**) after treated with 0.1 mg/mL. PEI&PEG-C/TiO\({}_{2}\)-Dots with visible light for 2 h.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12199633/s1](https://www.mdpi.com/article/10.3390/app12199633/s1)]([https://www.mdpi.com/article/10.3390/app12199633/s1](https://www.mdpi.com/article/10.3390/app12199633/s1)), Information about Materials, Measurement, Synthesis of C/TiO\({}_{2}\) Hybrid Dots, Microscopy results, and Antibacterial Experiments; Figure S1: Representative AFM (upper) and TEM (lower) images of PEI&PEG-C/TiO\({}_{2}\)-Dots. Inset: Statistical analysis of dot sizes based on multiple AFM images.  
**Author Contributions:** A.F.A.: investigation, methodology. W.L.: investigation, methodology. P.A.O.: investigation, methodology. X.D.: data curation, investigation, methodology. K.S.: investigation, methodology. P.W.: data curation, investigation, methodology. I.S.F.: investigation. S.-J.H.: investigation, methodology. Y.-PS.: conceptualization, funding acquisition, resources, supervision, writing--original draft, writing--review & editing, L.Y.: conceptualization, funding acquisition, investigation, resources, supervision, writing--original draft, writing--original draft, writing--review & editing. All authors have read and agreed to the published version of the manuscript.  
**Funding:** Financial support from NSF (2102021 and 2102056, and 1855905) and USDA (2019-67018-29689) is gratefully acknowledged.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** All data is contained within the article or Supplementary Materials.  
**Conflicts of Interest:** The authors declare no conflict of interest.