###### Abstract
In the presence of a base, such as NaOH, the hydroxyl groups on the CNCs surface can become more nucleophilic and reactive, which can facilitate the covalent linkage formation with PEG [16]. In the context of this paper, surface modification of CNCs with PEG and NaOH was performed to improve their biocompatibility and hydrophilicity, as well as to potentially enhance their performance as coating agents to develop a dual-contrast composite material [17]. In order to design effective magnetic nanoparticle (MNPs)-based contrast agents for biological applications, it is crucial to produce high-quality nanoparticles with desirable physical properties, size and shape control, good colloidal stability, and low cytotoxicity. Meeting these requirements can enable the synthesized magnetic nanoparticles (MNPs) to exhibit their full potential and optimal performance in MRI applications. Magnetic resonance imaging (MRI) is a widely used imaging technique in clinical diagnosis due to its nonradioactive and non-invasive nature and high spatial resolution [18]. However, obtaining high sensitivity remains a challenge. Contrast agents (CAs) have been developed to improve image contrast by affecting the relaxation time of water protons during MRI relaxivity measurement, thereby increasing the sensitivity of MRI [19; 20]. There are two types of MRI CAs based on the contrast enhancement mechanism: positive (T\({}_{1}\)) CAs, which reduce longitudinal relaxation time and result in brighter T\({}_{1}\)-weighted images, and negative (T\({}_{2}\)) CAs, which shorten transverse relaxation time and lead to darker T\({}_{2}\)-weighted images [21]. T\({}_{1}\) CAs are composed of materials that increase the relaxation of water protons in the spin-lattice direction, resulting in an increased r\({}_{1}\) relaxivity value. These agents are commonly made from gadolinium (Gd) and manganese (Mn) ions. In addition, Gd-based complexes have undesirable characteristics such as toxicity, short circulation time, and moderate r\({}_{1}\) value, which restrict their use as T\({}_{1}\) CAs [22; 23]. Conversely, superparamagnetic materials are primarily used as T\({}_{2}\) CAs, which accelerate the spin-spin relaxation of water protons and increase the relaxivity (r\({}_{2}\)) value [24]. However, their use is limited due to drawbacks such as signals from calcification or bleeding and magnetic susceptibility artefacts [25; 26]. As previously mentioned, T\({}_{1}\) and T\({}_{2}\) contrast agents each have their respective limitations, which is why developing a new type of contrast agent that can act as both T\({}_{1}\) and T\({}_{2}\) agents would be advantageous. Dual-contrast agents provide more comprehensive information for accurate diagnosis. Creating dual agents is challenging, but it can be achieved through factors such as controlling surface coating and nanoparticle size. The modification of nanoparticle surfaces is crucial in determining their colloidal stability, biocompatibility, hydrodynamic volume, and magnetic fluid viscosity. In this study, the acid hydrolysis of MCC was used to synthesize CNCs as MCC is a widely used cellulose source in the food and pharmaceutical industries and for CNCs production. The aim of this study was to characterize CNCs and to explore their potential as a coating agent to develop a dual-contrast composite material. CNCs are prepared using acid hydrolysis and combined with polyethylene glycol and sodium hydroxide (PEG/NaOH) to form a biocompatible, stable, and hydrophilic substrate.