**5. Conclusions**

In this study, BC-NC was conveniently synthesized by sequential ultrasonic irradiation and microwave treatment. A simple and an eco-friendly approach was developed to control the degradation of bacterial cellulose at very low hydrochloric acid concentration. The results demonstrated that ultrasonic irradiation of BC in water/ethanol mixture led to cellulose depolymerization. The CrI increased from 60.7% to 69.1% during the first 30 min of irradiation. Yet, amorphous region distortion remained constant or increased slightly after 2 h, i.e., by 71.4%. It was found that microwave treatment using MnCl2 as Lewis acid exhibited excellent catalytic activity and promoted the hydrolysis in diluted HCl. The reaction rate and the selectivity of BC-NC formation are shown to be optimal at 0.1 M of HCl with 5% *w*/*v* of MnCl2 for 30 min. These conditions can enhance the removal of amorphous regions, yielding BC-NC that possesses an initiating decomposition temperature of 318.6 ◦C, and led to improve the CrI of up to 79.4%. The BC-NC is typically a mixture of small needles of (164.51 ± 7.56) nm in length and (25.05 ± 2.80) nm diameter that can form fibrillated structures. This procedure yields nanocrystalline bacterial cellulose having similar features to commercially-available nanocrystalline cellulose.

**Author Contributions:** E.Y.W. took care about Sections 1–5, proposed the subject of the review to all the other authors and took care about the general planning of the work; H.W. and S.A. prepared the figures, tables and organized references; M.P.P. organized Sections 2 and 2.3.1; D.C. prepared Sections 2.3.3, 3.3, 4.1 and 4.2; F.O. took care about Sections 2.3.1 and 3.2; E.G. prepared Sections 2.3.4, 3.1 and 4 wrote the abstract, Section 1, Section 4 and the conclusions, K.S. supervised all the work and Finally, all the authors contributed equally to the general organization of the manuscript and its revision, with helpful suggestions about the content and the style of the text.

**Funding:** This research was founded by Ministry of Research, Technology and Higher Education of the Republic of Indonesia (Kemenristekdikti).

**Acknowledgments:** Technology and Higher Education of the Republic of Indonesia (Kemenristekdikti), Universitas Sultan Agung Tirtayasa (UNTIRTA). Within the framework of the chair: "Green Chemistry and Processes" this research was also sponsored by the European Union through FEDER funding, by the <sup>R</sup>égion Haut de France, the Ecole Supérieur de Chimie Organique et Minérale (ESCOM) and Université de Technologie de Compiègne (UTC).

**Conflicts of Interest:** The authors declare no conflict of interest. Moreover, the funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. The design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
