**4. Conclusions**

In conclusion, a series of novel cationic CNCs were successfully synthesized via SI-SET-LRP at room temperature. The cationic CNCs displayed rod-like structures with different sizes and positive surface charges. The morphological features and structural integrity of the cationic CNCs were retained as evidenced by both TEM and AFM analyses. In general, the cationic CNCs showed low toxicity and the slight decreases in cell viability were cell-type dependent and did not indicate any correlation with physico-chemical characteristics. This initial analysis is crucial for further biological applications, suggesting that these cellulose-based nanomaterials would be good candidates to be investigated as immunomodulators and further developed as potential vaccine nanoadjuvants.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/8/1603/s1: Preparation of CNC-initiators and cationic CNCs, Figure S1: FTIR spectra of pristine CNCs, CNC-BriB-1 and CNC-BriB-2, Table S1: Amounts of reactants and reagents used for the preparation of cationic CNCs, Figure S2: FTIR spectra of cationic CNCs, Figure S3: Intensity-averaged size distribution profiles for pristine CNCs and CNC-METAC-1B in water, Figure S4: TEM images of cationic CNCs, Figure S5: AFM height and phase images of cationic CNCs, Figure S6: controls and gating for flow cytometry, Figure S7: Typical representative flow cytometry SSC vs. FSC plots.

**Author Contributions:** Conceptualization, R.S. and K.C.; investigation, Y.I., B.T., C.W.S., and A.R., (synthesis and characterization), Y.L. and S.H. (AFM and TEM analysis), and L.N. and K.C. (cytotoxicity assays); data curation, Y.I., B.T., Y.L., and K.C.; writing—review and editing, R.S. and K.C.; supervision, R.S. and K.C.; project administration, R.S. and K.C.; funding acquisition, R.S. and K.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This material is based upon work supported by the National Science Foundation under Grant No 1703890.

**Acknowledgments:** The authors would like to thank InnoTech Alberta Inc., Edmonton, AB, Canada for generously providing spray-dried sulfated CNC for this study. Dante Quinones and Angela Pacherille are acknowledged for scaling up the preparation of CNC-BriB samples.

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