**Abstract**

Nanocomposites based on carbon nanomaterial particularly in graphene oxide, graphene quantum dots, and doped graphene quantum dots with improved biocompatibility have been increasing interests in the field of drug delivery, biosensor, energy, imaging and electronic. These nanomaterials as new kinds of fluorescent probes and electrochemical sensors all display ultrasmall size, good photostability, and excellent biocompatibility. In this chapter, we summarize an updated advance in the development of graphene and its related derivatives of synthesis methods and biomedical applications as nanosensors for detection of metal ions, inorganic ions, amino acids, proteins, saccharides and small molecules, drug molecules, and so on.

**Keywords:** nanocomposite, graphene oxide, graphene quantum dots, sensor, detection, application

### **1. Introduction**

Graphene, as an atom-thick sp2 -hybridized carbon nanosheet, has been extensively studied since it was first separated and characterized by Andre Geim and Konstantin Novoselov in 2004 [1]. The graphene presents a unique property including large specific surface area, easy functionalization, unique optical properties, chemical stability, high electronic conductivity, and photonic and mechanical properties and provides a promising platform for the design and construction of useful nanomaterials. Recently, the field involving graphene nanomaterials is a rapidly developing area due to their potential applications in biomedical and clinical medicine field. One of the most valuable virtues of graphene is their applications in sensors, particularly in fluorescence sensors and electrochemical sensors [2].

Mainly graphene-based nanomaterials are graphene oxide (GO, 2-D), graphene quantum dots (GQDs, 0-D), and heteroatom (N, P, S atom)-doped graphene quantum dots (doped GQDs, 0-D). The structures of different materials are shown in **Figure 1**. GO is a layered stack of nanosheets, while GQDs are regarded as GO nanosheets cutting into nanodots in oxidation process showing excellent performance of graphene. Recently, the GQDs have greatly attracted attention of scientific workers due to the good biocompatibility, excellent water solubility, and stable photoluminescence (PL) and chemical inertness [3, 4]. The GQDs contain carboxyl group, hydroxyl group, and epoxy groups at the edge and show similar structure to graphene and can be easily functionalized by various biological and non-biological species. Meantime, heteroatom-doped GQDs showed enhanced chemical activity, higher fluorescence quantum yields, and effectively modulated performance of bandgap.

**Figure 1.** *Structure of graphene oxide (GO) and nitrogen-doped graphene quantum dots (N-GQDs).*

In this chapter, efforts have been made on summarizing the design, synthesis, and applications of nanocomposite-based graphene. We mainly focused on the recent development of graphene-based nanocomposites as fluorescence sensors and electrochemical sensors for the detection of biological species and non-biological species in human serum, respectively.
