Recent advances in surface modification of nanocellulose by chemical routes

Nanocellulose (NC) is one of the great inventions of the late twentieth century that may address some of the recent challenges regarding environmental sustainability. NC has great potential in a multitude of applications, such as medicine, sensors, automotive, electronics and many others due to its origin from renewable sources, excellent mechanical resistance, biocompatibility, and biodegradability. For most of these applications, NC's surface must be chemically modified to meet the specific requirements of each intended use. The presence of hydroxyl groups at its surface opens plenty of possibilities for tuning its surface properties by chemical reactions. However, choosing the most suitable method in terms of efficiency, simplicity, health and environmental risks, costs, etc. requires thorough research of the existing literature which is extremely voluminous. The review proposed herein aims to provide a comprehensive overview of the recent efforts, progress, and challenges in the chemical surface modification of NC as cellulose nanocrystals, nanofibrillated cellulose and bacterial cellulose for imparting new properties to nanocelluloses for cutting-edge applications. The goals of this review reside in selecting relevant papers on this topic especially from the last decade, in order to provide a critical description of the methods used for the chemical modification of NC and an objective comparison between the results reported by different authors when applying similar methods. Particular attention was paid to the presentation of the novelties that different methods brought in terms of increasing efficiency, reducing toxicity, simplifying the procedures, or enlarging the spectrum of NC's applications.