## 1 Introduction  
In recent years, the unrestricted use of non-renewable fuels and their endless exploitation have had a significant influence not only on the environment's ability to sustain itself, but also on public health [1, 2]. As a result, research is being done on the viability of natural biomaterials, particularly the use of biopolymers made from a variety of naturally occurring biomass feedstocks [3]. Cellulose, which is the most prevalent natural polymeric substance on the planet, can serve to generate a variety of biomaterials and compounds [4]. This cellulose can be majorly derived from the agro-industrial lignocellulosic biomass (LB), which is otherwise considered to be waste creating environment issues. The utilization of LB, especially derived from the agro-food industries for the extraction of cellulose, will not only solve the issue of food waste management, but also be a part of circular bioeconomy for the production of cellulose- and non-cellulose-based value-added biomaterials [5; 6]. Out of a variety of biomaterials derived from cellulose, its most significant and promising role has emerged in the development of novel and advanced materials such as nanocellulose. Nanocellulose (NC) is a naturally occurring nanomaterial with various merits, including its nanoscale dimensions, a high surface area, renewability, high aspect ratio, distinctive shape, high strength and strong optical properties [7]. The nanometric-sized NC is broadly classified into three types, i.e., cellulose nanocrystals (CNC), cellulose nanofibrils (CNFs) and bacterial nanocellulose (BNC). Although these three forms of nanocellulose have identical chemical compositions, they have diverse physical properties, such as different particle sizes, morphologies and crystallinities [8]. Whereas CNC possess a perfect crystalline structure, the CNFs contain both crystalline and amorphous patches that can be fabricated into an entangled matrix. Due to its exceptional physical properties, mechanical strength, biocompatibility and renewable source of production, NC finds its applications in a variety of fields, including biomedical science, paper and pulp, mechanically reinforced polymer composites, antibacterial coating materials and environmental remediation. Pretreatment is a mandatory step in the fractionation of LB and the extraction of cellulose polymer for the NC production. In addition to significantly increasing the digestibility of cellulose, an effective pretreatment should also make it easier to recover other polymers such as lignin and hemicelluloses for value addition. A number of different pretreatment techniques have been applied for increasing the efficiency of cellulose separation from lignin and other components, including chemical, physical, biological and physicochemical methods [9; 10; 11; 12]. However, in the recent times certain low-cost pretreatment strategies such as deep eutectic solvents, hydrothermal pretreatment, ball milling, microwaves, ultrasonication, etc., have been given importance, owing to their remarkable biomass deconstruction properties and contribution towards increasing the overall economy of the bioprocess [8; 13; 14]. An important factor in the production of nanocellulose is the source from where cellulose is derived. Wood pulp or pure cellulose, such as microcrystalline cellulose, are the principal sources of NC. However, non-woody plant wastes such as crop waste and agro-food waste, are rich in cellulosic portions, which therefore act as starting materials for NC production. According to recent studies, nanocellulose can be derived from LB sources such as rice straw, wheat straw, coconut huks, sugarcane bagasse, pineapple leaves, corn cobs, organic kitchen waste, citrus trash, etc. [15; 16]. These waste biomasses are rich in high fiber content, which makes them a particularly intriguing feedstock for the production of nanocellulose. In the previous years, many review articles have been published in the literature on different nanocellulose production strategies [17; 18].