**4. Conclusions**

The aBC was successfully generated at various treatment temperatures without transforming the original physical features. The treatment temperature is known to significantly impact the surface chemical structure of aBC; hence, we were able to simplify the investigation into the effect of surface chemical properties of aBC on human epithelial cells; aBC with a nano size effectively penetrated the plasma membrane, leading to cell damage. It was found that an increase in the presence of oxygen functional groups on the surface of aBC directly affected cell viability and oxidative stress in A549 cells. The remaining limitation of this research is that aBC is still far from real-world BC, which has a complex mixture of chemical components, however, the results of the relationship between cytotoxicity and surface oxidation of aBC can be the foundation to conduct further research. Furthermore, aggregation and surface morphological changes of aBC in cell medium culture need further investigation, and aBC-induced bio-toxicity with proof of cell uptake is also needed to improve the quality of the cytotoxicity test. The semi-quantification of cell uptake should be investigated to support the mechanism of cell death. An in vivo test and in vitro test with an air–liquid interface approach would be needed for further toxicological models for our synthesized aBC, rather than a submerged cell culture to minimize the current limitations. Future studies will focus on aBC synthesized with sulfates, nitrates, metals, and organic compounds that are commonly emitted with BC to the atmosphere to better simulate real-world experimentation.

**Author Contributions:** Y.T.-H.L.: methodology, investigation, writing—origin draft; J.-S.Y.; supervision, writing—review & editing; X.-Y.C. and D.-I.K.; experimentation, formal analysis; H.-W.C.; experimentation, investigation; K.-H.L. and H.-G.M.; supervision, validation; K.-J.J.; conceptualization, supervision, validation, writing—review & editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Korea Institute of Toxicology (KIT, KK −1905-02) and supported by the Korea Ministry of Environment (MOE) as part of the Environmental Health Action Program (2016001360005).

**Data Availability Statement:** Data are contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
