Bilirubin is an endogenous substance produced by the destruction of blood cells in the body. Abnormal bilirubin metabolic processes often lead to serious consequences such as mental retardation, cerebral palsy, nuclear jaundice, and even death [1]. Consequently, the removal of excess bilirubin from the blood is of great importance in clinical practice. Currently, the generally adopted strategies for bilirubin removal mainly involve hemoperfusion, hemodialysis, and affinity-membrane chromatography [2]. Hemoperfusion is one of the earliest and most widely used clinical techniques for blood purification. As the core of the hemoperfusion technique, absorbents in the forms of various carbon-based materials, bilirubin-imprinted materials, silica particles, and polymer resins have been developed [3].

Reduced graphene oxide (rGO) has outstanding mechanical strength, a large and easily accessible specific surface area, and sterilizes viruses and bacteria [4]. Moreover, the widely delocalized π-electron region on rGO endows it with a high adsorption capacity for bilirubin. Owing to these advantages, rGO-based materials are considered promising bilirubin adsorbents [5]. For example, Li et al. [6] fabricated regular macromesoporous reduced graphene aerogel beads that exhibited a high bilirubin adsorption capacity of 791.49 mg/g. Ma et al. [7] fabricated a type of three-dimensional porous graphene monolith with a maximum adsorption capacity of 126.1 mg/g. Unfortunately, while rGO-based adsorbents have been greatly developed, their excellent properties cannot be fully exploited because they disperse poorly in the matrix [8]. This deficiency compromises the adsorption capacity and repeated use of rGO, seriously hindering its further hemoperfusion applications. Therefore, excellent dispersion is necessary to completely decompose rGO stacks into an individually dispersed state. Current approaches for dispersing rGO in water mainly involve physical dispersion, chemical modification, the addition of surfactants, and polymer packaging [9,10]. Although physical dispersion is simple and inexpensive, its effects are often poor [11]. While chemical modification is the most effective, it is unexpectedly environmentally unfriendly and inevitably destroys the conjugated rGO system and its own physicochemical properties as a consequence [12]. Coating rGO with a polymer is another promising new method [13], in which mutual-contact interference prevents hydrophobic interactions, improves compatibility with the matrix, and avoids chemical-structure destruction.

Chitosan (CS) is a common bilirubin-adsorbing material that is nontoxic, nonhemolytic, and rich in active groups that are easily chemically modified [14]. However, CS-based adsorbents exhibit poor mechanical strengths and tend to rupture easily, which seriously compromises their practical value. Therefore, combining the respective characteristics of CS and rGO appears to be a better strategy. CS possesses good blood compatibility, while rGO is highly mechanically strong and a exhibits bilirubin-adsorbing capacity. Forming a CS/rGO hybrid adsorbent by dispersing rGO in a CS solution is expected to endow the adsorbent with mechanical strength, blood compatibility, and a high bilirubin-adsorbing capacity.

In this context, herein, we report a simple strategy based on a CS-assisted rGO dispersion for preparing high-performance bilirubin adsorbents. In this strategy, we employed a CS solution dissolved in a LiOH/KOH/urea/H2O mixture as the rGO dispersant. A homogeneous and stable CS/rGO mixed dispersion was subsequently achieved by gentle stirring followed by freeze-thaw cycling, which successfully exfoliated the rGO to form a stable dispersion. The prepared CS/rGO dispersion was used to fabricate CS/rGO microspheres using a simple oil/water emulsion method. The hybridized adsorbent exhibited a good adsorption capacity and fast adsorption during bilirubin-adsorption testing owing to the hierarchically porous structure of each CS/rGO microsphere.