# Nanomaterials for Healthcare Biosensing Applications
## 2 Carbon Allotrope-Based Nanomaterial Applications in Healthcare Biosensing
### Fullerene Nanomaterials  
C\({}_{60}\), the smallest and most common fullerene, is made up of five and six membered sp\({}^{2}\) hybridised carbon rings forming a truncated icosahedron. In addition to good electrical conductivity and high specific surface area, fullerene shows an excellent biocompatibility, inert behaviour, and structural stability, good affinity towards various organic molecules. It is easy to use and is free from metallic impurities [24; 50; 51; 52]. Since it avoids \(\pi\) bonds in pentagonal rings, leading to inadequate electron delocalisation, C\({}_{60}\) is not \"superaromatic\". Therefore, it possesses superior electron acceptance capacity, enabling it to react with amines and to be decorated with various nanoparticles [25]. Moreover, due to its inner redox activity [53], fullerene can act as an electrophile as well as a nucleophile, allowing its use as a mediator [54] in electrochemical biosensing devices, as illustrated in Figure 4.  
Due to poor solubility in aqueous medium, C\({}_{60}\) tends to aggregate, thus inhibiting its redox activity. This makes the electrochemical behaviour irreversible and unstable in water. This issue can be tackled with covalent functionalisation of fullerene with hydroxyl, carboxyl, or amine groups [25; 54]. We can thus obtain fullerenes with tailored properties, such as hydroxyl fullerene, which is water soluble and protects the biological activity of proteins by forming noncovalent complexes [55]. The properties of C\({}_{60}\) can also be manipulated by forming nanocomposites with metallic nanoparticles to enhance its loading capacity as well as the electrical conductivity. For example, Yuan and co-workers fabricated a state-of-the-art sandwich-type biosensor for the detection of \(\alpha\)2,3-sialylated glycans (\(\alpha\)2,3-sial-Gs), a biomarker for early stage cancer diagnosis, using a composite of C\({}_{60}\) with metallic nanoparticles [56]. The surface of the glassy carbon electrode (GCE) was modified with bimetallic palladium-platinum alloy nanocrystals coupled with amino-functionalised fullerene (n-C\({}_{60}\)-PdPt). 4-mercaptoethanol/bironylboronic acid (4-MPBA) was immobilised on the n-C\({}_{60}\)-PdPt, as the mercapto functional group can strongly adsorb on PdPt alloy. The coordination of the boron group of the 4-MPBA with the amide group of \(N\)-acetylheuraminic acid (Neu5 Ac) in the structure of \(\alpha\)2,3-sial-Gs is responsible for recognition. The redox species consist of Au-poly(methylene blue) (Au-PMB) with covalent immobilisation of maackia amurensis lectin (MAL). A schematic representation of the experimental protocol is illustrated in Figure 5. The current response of the sensor was obtained with the help of differential pulse voltammetry (DPV). The sensor harnessed the excellent electron transfer capacity afforded by fullerene and its large surface area for the in-situ reduction of PdPt alloy nanocrystals. The sensor was effective over a broad concentration range of 10 fg mL\({}^{-1}\)-100 ng mL\({}^{-1}\), with a very low detection limit (LOD) of 3 fg mL\({}^{-1}\)[56]. Figure 4: Fullerene as a mediator at the interface of the electrode and the recognition site. Another approach involved a molecularly imprinted polymer (MIP, synthetic antibodies)-based sensor using functionalised C\({}_{60}\) as a monomer [57]. Vinyllic-C\({}_{60}\)-monoadduct was allowed to self-assemble in a sodium dodecylsulfate (SDS) micellar system to obtain neoteric water-compatible C\({}_{60}\)-monoadduct in the presence of chloramphenicol template (anti-cancer drug). The polymerisation of the fullerene monoadducts was performed with ammonium persulfate as an initiator and \(N\),\(N\)-methylenebisacrylamide as a crosslinker. Following template removal by dynamic incubation in acetonitrile, the immobilisation of imprinted micellar nanoparticles on the surface of ionic liquid decorated ceramic carbon electrode (IL-CCE) was performed.