# Nanomaterials for Healthcare Biosensing Applications
## 2 Carbon Allotrope-Based Nanomaterial Applications in Healthcare Biosensing
### Carbon Nanotubes  
CNTs are regarded as auspicious building blocks of biosensors as a result of their high aspect ratio, large surface area, excellent thermal and chemical stability, superior mechanical strength, and exceptional optical and electronic properties [18]. CNTs offer an edge to biosensors due to their high sensitivity, excellent signal-to-noise ratio, low background, broad absorption spectrum, label-free detection, and real-time monitoring [22]. They serve as scaffolds for biomolecule immobilisation, thereby ameliorating signal transduction and subsequently recognition [74]. The semiconducting nature of carbon nanotubes allows their use as nanoscale field effect transistors (FET) [32]. They can be used to manufacture top-of-the-line nanoscale electrodes due to the superior excellent conductivity along their length. CNTs have exceptional wavelength conversion function, particularly the adjustable near-infrared emission, which represents fluctuations in local dielectric function but is resistant to permanent photobleaching. In addition, they show high luminous intensity and excellent luminous properties, which are ideal for optical biosensing [75; 76; 77; 78; 79; 80]. CNTs have only a sixth of the density of steel but are 100 times stronger, allowing their use in fabricating piezoresistive sensors [81; 82; 83; 84]. It is also possible to synthesize CNT-based calorimetric sensors which rely on changes in the size of nanotubes caused by thermal variations [18; 85]. Numerous CNT-based biosensors for glycaemic biomarkers of diabetes mellitus have been recently reported. Hatada et al. reported a label-free chemiresistor-type FET affinity sensor for haemoglobin A1c (HbA1c) using single-walled carbon nanotubes (SWCNT) as a transducing element and a bacterial periplasmic protein (SocA) as a receptor [32] HbA1c, on proteolytic hydrolysis, produces fructosyl valine (FV) that could be quantified by the sensor in a concentration range of 1.2-1909 nM. Comba and co-workers developed a durable enzymatic biosensor for glucose using a mucin-CNT (CNT-muc) nanocomposite immobilised on a platinum surface [86]. The large surface area of CNT promoted the immobilisation of GOx enzyme. The Pt-CNT-muc-GOx sensor could sense glucose in a range of 0.002-3.2 mM by employing chronoamperometry and the LOD was found to be 3 \(\upmu\)M. Another study eliminated the requirement of the GOx by fabricating MWCNT scaffolds with cobalt functionalised MoS\({}_{2}\)[87]. This scaffold was suitable for glucose sensing over a linear concentration range of 0.2-16.2 mM with an extremely low detection limit (80 nM). Aryal and Jeong reported a thermally reduced graphene oxide-MWCNT (TRGO-MWCNT) nanocomposite modified with ambient plasma and \(\beta\)-cyclodextrinrin (\(\beta\)CD) for uric acid (UA) detection [88]. The LOD of this sensor was 0.06 \(\upmu\)M and the sensor could provide linear responses from 10 \(\upmu\)M to 300 \(\upmu\)M. Bollella et al. exploited the exceptional electron exchange properties between MWCNT and poly(methylene blue) (pMB) to develop the first second-generation Au microneedle sensor for continuous lactate detection in dermal interstitial fluid [89]. Lactate detection between 10 and 200 \(\upmu\)M was possible following lactate oxidase (LOx) immobilisation on the sensor and a very low detection limit (2.4 \(\upmu\)M) was obtained. Shen et al. reported a chemiresistive paper-based label-free immunosensor for cost-effective POC detection using SWCNT [33]. They harnessed the non-covalent \(\uppi\)-\(\uppi\) stacking interactions between SWCNT and pyrene carboxylic acid (PCA) to synthesise a water-based ink prior to human serum albumin (HSA) antibody immobilisation. The SWCNT-PCA-antiHSA ink was able to sense 0.015-9.43 nM HSA and demonstrated an LOD of 1 pM. Huang and co-workers developed an immunomod  
5000 ng mL\({}^{-1}\) with LOD of 50 ng mL\({}^{-1}\)[76]. Optical sensors based on surface plasmon resonance (SPR) using CNT have also been reported.