## 7 Conclusions and Future Perspectives  
The current review reports various methods for the extraction of nanocellulose from waste lignocellulosic biomass, and different possibilities for its application in a range of commercial industries. Green bio-based renewable raw materials and more ecologically friendly and sustainable resources are currently one of the major sustainable development goals set forth by the United Nations. Since cellulose is the most prevalent source on earth, it has undergone extensive research by scientists from all over the globe. Nanocellulose has attracted a lot of interest in the recent years because of its biocompatibility, renewability, bioavailability and other noteworthy characteristics. The removal of all non-cellulosic materials requires the biomass to be pretreated because lignin, hemicelluloses and other materials firmly aggregate in the cell walls of waste biomass. Pretreatment of lignocellulosic biomass is quite necessary for the extraction of cellulose, removal of lignin and other impurities, which enhance the quality of crystalline nanocellulose and nanofibers. Their structural and physicochemical characteristics are significantly influenced by the method used to separate nanocellulose.  
Compared to traditional extraction techniques, green cellulose extraction technologies from agro-industrial food waste have a number of benefits. The advancements in cellulose recovery employing ultrasonic and microwaves can lower the treatment time and the quantity of the energy used, in line with ecological and sustainable goals. The use of mild acid could cut down on the water needed to get the material pH level to neutral. DESs have been suggested as safe compounds for cellulose extraction from agro-food waste. The ongoing studies in this area demonstrate how reliable and repeatable procedures are necessary to guarantee the adoption of nanocellulose within the industry. Cellulose nanomaterials have received an extensive interest as biomaterials, with a variety of applications in different food industries. However, it is essential to evaluate the material safety and any negative impacts on human beings and the environment before developing nanoscale materials and bringing them into the market.  
The development of nanocellulose from waste biomass has opened up new avenues for scientific study, as well as engineers working in the area of nanoscience and nanotechnology. One of the main problems that restrict nanocellulose applications is the low thermal stability of cellulose materials. However, different methods are being adopted to improve the thermal stability of nanocellulose, including the removal of toxic acid groups and neutralization by alkaline solutions. The structural and functional characteristics of cellulosic derivatives are being investigated in numerous other research studies, such as alternatives to conventional packaging materials and metal ion detection, besides their major applications in drug delivery, tissue engineering and wastewater treatment. Undoubtedly, there are obstacles to be surmounted in order to produce innovative materials for sustainable applications. Therefore, more effective research outputs are required for the wider application range of nanocellulose-derived materials for sustainable development.  
Conceptualization, P.N. and C.-H.K.; Validation, V.S. and P.N.; Formal Analysis, V.S. and P.N.; Investigation, V.S.; Writing--Original Draft Preparation, P.N.; Writing--Review and Editing, H.-M.D.W. and C.-H.K.; Resources, M.-L.T. and S.-L.H.; Supervision, C.-D.D. and C.-H.K.; Funding Acquisition, C.-H.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by research funding grants provided by the National Science and Technology Council of Taiwan (MOST 111-2221-E-992-005-MY3).  
Not applicable.  
Author [PERSON] thankfully acknowledges the National Kaohsiung University of Science and Technology, Kaohsiung City, Taiwan, for providing the post-doctoral fellowship.