###### Abstract  
This study focused on surface modification of cellulose nanocrystals (CNCs) to create a biocompatible, stable, and hydrophilic substrate suitable for use as a coating agent to develop a dual-contrast composite material. The CNCs were prepared using acid hydrolysis. Hydrolysis was completed using 64% sulfuric acid at 45 \({}^{\circ}\)C for 1 h, which was combined with polyethylene glycol and sodium hydroxide (PEG/NaOH). The yield of samples exhibited prominent physicochemical properties. Zeta (\(\zeta\)) potential analysis showed that the CNCs sample had excellent colloidal stability with a highly negative surface charge. Transmission electron microscopy (TEM) analysis confirmed that the CNCs sample had a rod-like morphology. On the other hand, field-emission scanning electron microscopy (FESEM) analysis showed that the acid hydrolysis process caused a significant reduction in particle size and changed surface morphology. In addition, cellulose nanocrystals with polyethylene glycol and sodium hydroxide (CNCs-PEG/NaOH) have many noteworthy properties such as colloidal stability, small hydrodynamic size, and water dispersibility. Furthermore, the MTT assay test on Hep G2 cells demonstrated good biocompatibility of the CNCs-PEG/NaOH and did not exhibit any cytotoxic effects. Hence, CNCs-PEG/NaOH holds the potential to serve as a dual-contrast agent for MRI techniques and other biomedical applications. microcrystalline cellulose; cellulose nanocrystals; acid hydrolysis; particle size; magnetic resonance imaging (MRI); MTT assay +
Footnote †: journal: Applied Physics Letters B  
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Footnote 1: email: {mohd. high crystallinity, high specific surface area, modulated size/morphology, good optical properties, biodegradability, and biocompatibility. In comparison to cellulose, CNCs offer more attractive properties, such as exceptional mechanical properties (high specific strength and modulus), high aspect ratio, environmental benefits, greater malleability, and lower cost. These make CNCs a potentially better alternative to cellulose [5; 6; 7; 8; 9]. CNCs have found a wide range of applications in various fields due to their exceptional properties. They are commonly used as reinforcement fillers in polymer matrices, in the production of separation membranes and transparent barrier films, as well as in the fabrication of supercapacitors, biosensors, and coating agents. In addition, CNCs have been explored for their potential applications in biomedicine, such as in drug delivery systems and tissue engineering [10]. Nano-cellulose is typically derived from natural plant resources using methods such as acid hydrolysis, ultrasonic techniques, and enzymatic hydrolysis. Among these, acid hydrolysis is the most commonly used method due to its ease, speed, and desirable properties, such as biocompatibility, biodegradability, sustainability, nontoxicity, hydrophilicity, low cost, large surface area, high mechanical strength, and high stiffness. Previous studies have shown that nano-cellulose produced through acid hydrolysis has a relatively high crystallinity index of around 22.29% compared to other methods. Additionally, nano-cellulose obtained through acid hydrolysis has a smaller diameter/width of 5-10 nm [5; 6; 11; 12; 13; 14]. Surface modification of cellulose nanocrystals (CNCs) with polyethylene glycol (PEG) is a commonly used technique to improve their biocompatibility and stability in biological environments. PEG is a water-soluble polymer that is widely used in biomedical applications due to its non-toxicity, low immunogenicity, and its ability to reduce protein adsorption and opsonization [15]. The surface modification of CNCs with PEG typically involves the formation of covalent linkages between the hydroxyl groups on the CNCs' surfaces and the functional groups on the PEG molecules. This can be achieved using various chemical methods, such as carbodimimide-mediated coupling or thiol-ene click chemistry.