Citation: Peponi, L.; Barrera-Rivera, K.A.; Kenny, J.M.; Marcos-Fernandez, Á.; Martinez-Richa, A.; López, D. Bio-Catalysis for the Functionalization of Cellulose Nanocrystals. Nanomaterials 2022, 12, 4064. https://doi.org/10.3390/ nano12224064 Academic Editor: Alexey Pestryakov Received: 13 October 2022 Accepted: 15 November 2022 Published: 18 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Bio-Catalysis for the Functionalization of Cellulose Nanocrystals Laura Peponi 1, * , Karla A. Barrera-Rivera 2 , Jos é M. Kenny 1 , Ángel Marcos-Fernandez 1 , Antonio Martinez-Richa 2 and Daniel López 1, * 1 Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC., Calle Juan de la Cierva 3, 28006 Madrid, Spain 2 Departamento de Química, Universidad de Guanajuato, Noria Alta s/n, Guanajuato 36050, Guanajuato, Mexico * Correspondence: lpeponi@ictp.csic.es (L.P.); daniel.l.g@csic.es (D.L.) Abstract: In this work, the chemical modification of cellulose nanocrystals (NCs) using an enzyme as a catalyst has been performed by a “grafting from” reaction, in order to covalently functionalize the external surface of NCs with both poly(L-lactic acid) (PLLA) and poly(ε-caprolactone) (PCL) by ring-opening polymerization. Firstly, cellulose nanocrystals were prepared from commercial cellulose microcrystals by acid hydrolysis and then functionalized by using Yarrowia lipolytica lipase immobilized on Lewatit resin as a catalyst. To confirm the success of the grafting reactions, 1 H-NMR has been performed as well as FT-IR and Raman spectroscopy. Moreover, thermogravimetric analysis has been used to determine the amount of polymeric chains grafted onto the surface of cellulose nanocrystals. Furthermore, the crystalline nature of the polymeric chains grafted onto the cellulose surface has been studied by DSC, X-ray scattering, as well as SAXS analysis. To our knowledge, it is the first time that a biocatalyst approach has been used to obtain biopolymeric functionalized cellulose nanocrystals. Keywords: bio-catalysis; enzyme; covalent functionalization; cellulose nanocrystals; green chemistry 1. Introduction Bio-catalysis is an emerging research area with both substantial scientific and tech- nological interests, as well as a remarkable impact on environmental issues considering the emerged principles for the circular economy [1–4]. The capability to graft synthetic polymers onto carbohydrates is interesting for obtaining new materials’ applications, such as detergents, packaging, and pharmaceuticals, among others [5–7]. From an environ- mental point of view, this approach can lead to an increment in the biodegradability of polyesters [7]. However, selective functionalization of carbohydrates is a challenging task to accomplish considering that carbohydrates contain a variety of hydroxyl groups with different chemical reactivities [8–10]. In general, enzymes are highly selective cata- lysts, being used to region-selectively acylate carbohydrates [11,12]. In this regard, the synthesis of polyester based on the lipase-catalyzed reaction can be considered an attrac- tive alternative to poorly selective chemical catalysts [13–16]. Among them, immobilized Yarrowia lipolytica lipase (YLL) has proved to be useful for bulk ring-opening polymerization of PCL in the presence of isosorbide [13,17–19]. In this work, a bio-catalytic approach for the modification of covalently functionalized cellulose nanocrystals is reported. To our knowledge, it is the first time that the grafting reaction onto the surface of the cellulose nanocrystals has been reported by using both biocatalysts as well as biopolymers to functionalize them. In fact, in the scientific literature, the functionalization of the cellulose nanocrystals with biopolymers has been reported, but a common catalyst has always been used [8,19–21]. Here, the covalently functionalized cellulose nanocrystals have been modified by grafting biopolymeric chains onto the external surface of the cellulose nanocrystals by using an enzyme as a biocatalyst. Therefore, an environmental-friendly approach has been used to increase the compatibility of hydrophilic Nanomaterials 2022, 12, 4064. https://doi.org/10.3390/nano12224064 https://www.mdpi.com/journal/nanomaterials