# Nanomaterials for Healthcare Biosensing Applications
## 2 Carbon Allotrope-Based Nanomaterial Applications in Healthcare Biosensing  
Carbon allotrope-based nanomaterials have drawn plenty of research interest in the field of medical biosensing in the last few years. Because of the presence of a variety of carbon allotropes, such as graphite, fullerenes, diamonds and lonsdaleite, as well as more novel forms such as graphene, nanohorns, and nanotubes, as shown in Figure 3, carbon allotrope-based nanomaterials are highly valued [18]. Each of these allotropes possesses inimitable and unique features, which lead to their extensive exploitation for diverse biological applications, ranging from cancer therapy, tissue engineering, and drug delivery to medical diagnostics, bioimaging, and biosensing [19; 20]. Carbon allotrope-based nanomaterials possess an unparalleled combination of optical, electrical, and mechanical properties, generating miniaturised sensors with superior performance and low power requirements. These nanomaterials are flexible and thermally stable in nature, with superior strength-to-weight ratio as well as high electron mobilities [21]. A broad spectrum of compounds that find applications in healthcare diagnosis and POC analysis of diseases can be detected with such biosensor materials [22]. Carbon allotrope-based nanomaterials consisting of fullerenes, [23; 24; 25; 26; 27] nanotubes (CNT) [28; 29; 30; 31; 32; 33], films of graphene and its derivatives [34; 35; 36; 37], quantum dots [38; 39; 40; 41], and nanodiamonds [42; 43; 44; 45; 46; 47] play a substantial role in recent advancements in the biosensor domain. In addition to greater sensitivity and novel mechanisms, such sensors offer a higher spatial resolution in case of localised detection along with real-time and label-free non-destructive sensing.  
The use of carbon-based nanomaterials for the detection of various biological analytes has been rising over the past decade. In spite of their exceptional material properties, carbon allotrope-based nanomaterials are marred by a deficiency of surface heterogenic reactivity, which is essential for the surface immobilisation of clinically relevant biomarkers. To enhance the biomolecule attachment to the functionalised surface as well as the subsequent protein-protein recognition, precise engineering of nanomaterial interface is required. To this end, a majority of these nanomaterials need to undergo covalent or non-covalent modifications [30; 48; 49]. A wide array of biomolecules, such as aptamers, enzymes, peptide nucleic acid (PNA), deoxyribonucleic acid (DNA), viruses, antigens, antibodies, ribonucleacic acid (RNA), and microRNA can be immobilised on these materials via functionalisation. In all such cases, carbon allotrope-based nanomaterials act as transducers by offering suitable interfaces for the translation of biorecognition inputs to highly sensitive and quantifiable outputs [48]. Although  
Figure 3: Various crystalline allotropes of carbon. (**a**) Fullerene–C\({}^{60}\), (**b**) Graphene monolayer, (**c**) carbon nanotube (CNT), (**d**) carbon nanoonion, (**e**) nanodiamond, (**f**) carbon nanohorn.  
almost all crystalline and amorphous allotropes of carbon have been exploited for healthcare biosensing, this review primarily focuses on sensors incorporating fullerenes, nanotubes, graphene films, quantum dots, and nanodiamonds.