# Nanomaterials for Healthcare Biosensing Applications
## 2 Carbon Allotrope-Based Nanomaterial Applications in Healthcare Biosensing
### Graphene and Graphene Derivatives  
Graphene is a relatively recent type of carbon allotrope made up of sp\({}^{2}\) hybridised carbon atoms assembled in a hexagonal configuration. The electrons in graphene impart unusual properties, such as ambipolar electric field effects, excellent thermal conductivity, and quantum hall effects at room temperature. It has a 2D structure, giving rise to extremely high surface area and high porosity. This makes graphene suitable for the adsorption of various gases, such as methane, hydrogen, and carbon dioxide [2]. Properties of graphene can be tuned by manipulating the number of layers and the stacking order. It is highly transparent with a strong resistance to fracture and a high modulus of elasticity. In addition, graphene is capable of interacting with various biomolecules via physisorption, making it an ideal candidate for biosensors [18]. Graphene derivatives can display interesting properties, including graphene oxide (GO), which exhibits fluorescence. GO, RGO, and graphene quantum dots (GQDs) are the most essential graphene derivatives employed in the biosensing field [95; 96]. Graphene-based biosensors are highly versatile and can be used for the detection of cysteine [70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80], glycaemic biomarkers [58], cholesterol [2], neurotransmitters [60; 64], H\({}_{2}\)O\({}_{2}\)[101], cancer cells [102], nucleic acids [2], pharmaceutical drugs [39], and infectious bacteria [96]. Graphene has been used in the fabrication of many optical and electrochemical biosensors for the detection of various amino acids. Kumar et al. exploited the large surface area of electrochemically-reduced graphene oxide (ERGO) by chelating it with a complex of 1,10-phenanthroline-5,6-dione(phen-dione) and Cu\({}^{+2}\) to obtain a composite GCE for cysteine (Cys) detection [97]. This sensor presented a linear response between 10.0 and 32,344.0 uM with a detection limit of 2.0 uM. In another study, a bifunctional optical sensor based on the photophysical properties of AuNP decorated GQD was used for the determination of lysine and Cys [98]. The sensor could determine Cys and Lys in the concentration ranges of 0.05-0.5 mM and 0.047-0.8 mM, respectively. The corresponding LODs were found to be 16.14 uM and 5.88 uM. Graphene-based biosensors play a pivotal role in the sensing of incurable and fatal diseases, such as diabetes and various kinds of cancer. Jaberi et al. developed an RGO-Au nanostructure paper-based electrochemical nano-genosensor for HbA1c determination on a flexible and cost-effective graphite sheet (GS) electrode [103]. A wide concentration of 1-13.83 uM could be successfully determined by the thiolated aptamer-based genosensor. The amount of HbA1c can be affected by various diseases, such as sickle cell anaemia, haemolytic anaemia, and haemoglobinpathy, making it an unreliable biomarker for conclusive diagnosis of diabetes mellitus. Apiwat and co-workers overcame this issue by substituting HbA1c with glycated HAS (GHSA) as a biomarker [104]. They immobilised biotinylated aptamer on fluorescent quenching GO and obtained a linear response between 0.05 and 0.3 mg mL\({}^{-1}\) and a detection limit of 50 ug mL\({}^{-1}\). A study on LZM detection reported a state-of-the-art ultrasensitive aptamer-based chemiluminescent sandwich biosensor using a GQD-GO-carbon fibre (CF) nanocomposite with an LOD of 12.5 pg L\({}^{-1}\) and a wide linear range of \(2.64\times 10^{-10}\)-\(6.6\times 10^{-8}\) g L\({}^{-1}\)[105]. Shahrokhian and Salimian developed a highly sensitive genosensor for detecting BRCA1, a genetic biomarker of breast cancer, using a GCE-ERGO electrode with poly (pyrro-3-carboxylic acid) (PP3A) and E-DNA functionalisation [102]. The GCE-ERGO-PP3A-E-DNA electrochemical sensor could successfully detect the biomarker in the concentration range of 10 fM-0.1 uM BRCA1 with a detection limit of 3 fM. Dong et al.