**1. Introduction**

In recent years, there has been an interest in the production of nanocrystalline cellulose (NCC) from cellulosic material because of its biodegradability, renewability, abundance and excellent mechanical properties [1]. In this world, cellulose is one of the most numerous natural renewable and biodegradable polysaccharides. NCC is the nano-scaled of needle or rod-shaped crystalline which has hundreds of nanometers in length and 1–10 nm in width [2,3]. NCC is obtained when cellulose undergoes acid hydrolysis with conditions where the amorphous regions are selectively hydrolyzed [4]. Mineral acids including hydrochloric acid and sulfuric acid are used in the mixture of hydrolysis of cellulose to prepare NCC [5]. Thus, NCC is constitutively acidic and exhibits a lyotropic phase behavior depending on the concentration. NCC has the potential in various applications as a rheology modifier such as drilling fluids, consumer products, drug delivery, artificial tissue formation and injectable hydrogels [6–8].

To enhance the NCC properties, the hydroxyl functional group in NCC can be modified by using several methods [9–12]. In this present work, NCC has been cationically modified using hexadecyltrimethylammonium bromide (CTA). CTA can enhance the absorption by improving hyperchromicity and sensitization of NCC [13–17]. To further increase the performance, this modified NCC was chosen as a matrix for carboxyl functionalized graphene quantum dots (CGQD) as CGQD has beneficial and unique properties including hydrophilicity, strong photoluminescence and photo-stability [18–20]. Due to its outstanding properties, CGQD provides unprecedented opportunities for different fields of application such as optical sensing, catalysis and bioimaging [21–25].

As far as we know, the optical properties of the CTA cationically modified NCC/CGQD (CTA-NCC/CGQD) nanocomposite thin film and its potential application for detection of glucose using surface plasmon resonance technique (SPR) have yet to be reported. SPR is known as a simple optical method for surface studies of thin films and can act as a very sensitive spectroscopy for detection of a variety of targets [26–42]. Hence in this study, the fabrication of the CTA-NCC/CGQD nanocomposite thin film, its characterization and potential sensing application were explored.
