# Nanomaterials for Healthcare Biosensing Applications
## 2 Carbon Allotrope-Based Nanomaterial Applications in Healthcare Biosensing
### Carbon Nanotubes
Pathak and Gupta developed a polypyrrole (PPy) MIP on carboxylated multi-walled carbon nanotubes (CMWCNT) with a permselective nafion membrane for the SPR detection of dopamine (DA) [77]. This sensor demonstrated a wide dynamic range of 10\({}^{-9}\)-10\({}^{-5}\) M with an LOD of 18.9 pM in synthetic cerebrospinal fluid. Another SPR sensor employed a radically different strategy to detect human cardiac troponin I (cTnI) using Fe\({}_{3}\)O\({}_{4}\)@PDA-dAb as immune probes on a capture antibody (cAb) immobilised Au platform, which was coated with polydopamine (PDA) and modified with AuNPs [78]. In this study, MWCNTs were used in conjunction with silver nanoparticles (AgNPs) and PDA with secondary antibody decoration (Ab\({}_{2}\)) to enhance the SPR response. Lee et al. developed a novel plasmon-assisted fluoro-immunoassay (PAFI) for quantification of influenza virus H3N2, relying on AuNP decorated MWCNT nanohybrids with immobilised virus antibodies (Abs) [79]. A low detection limit of 50 plaque forming units per mL (pfu mL\({}^{-1}\)) was obtained and the sensor provided a linear response between 50 and 10,000 pfu mL\({}^{-1}\). Interdigital electrodes (IDE) of series piezoelectric quartz crystal (SPQC) have been frequently coupled with CNTs to develop piezoelectric sensors for clinical diagnosis [81; 82; 83; 84]. Zhang et al. developed an aptasensor in this way for the detection of _Mycobacterium tuberculosis_ (H37 Rv) with the help of a single-strand deoxyribonucleic acid (ssDNA) aptamer [81]. The sensor provided a linear signal in the concentration range of \(1\times 10^{3}\)-\(1\times 10^{7}\) cfu mL\({}^{-1}\) with an LOD of 100 cfu mL\({}^{-1}\). In another study, a lysozyme (LZM) aptasensor exploiting the electron transfer between SWCNT and an IDE of series piezoelectric quartz crystal (SPQC) was reported to generate linear frequency shifts from 1 to 80 nM of LZM with a detection limit of 0.5 nM [83]. A proof-of-concept mass sensitive detection of rifampicin, a tuberculosis drug, using a nanocomposite of MWCNT and Bi\({}_{2}\)WO\({}_{6}\) as an affinity material for quartz crystal microbalance (QCM) is illustrated in Figure 6. The sensor demonstrated a linear response in the range of 1-700 uM and achieved an LOD of 0.16 uM [90]. CNT biosensors are commonly used for the detection of various cancer types and neurological disorders. For an instance, a chemiresistive paper-based CMWCNT biosensor was constructed for prostate specific antigen (PSA), a common prostate cancer biomarker. This antibody sensor allowed the quantification of the biomarker down to 1.18 ng mL\({}^{-1}\)[91]. Another antibody-based CNT sensor for the recognition of P-glycoprotein (P-gp), a leukemia biomarker, employed anti-P-gp-SWCNT film developed on a SiO\({}_{2}\)-Si substrate. The sensor could assay \(1.5\times 10^{3}\)-\(1.5\times 10^{7}\) cells mL\({}^{-1}\) with an  
Figure 6: Scheme for fabricating a quartz crystal microbalance (QCM)-based rifampicin sensor using multiwalled carbon nanotubes (MWCNT) and Bi\({}_{2}\)WO\({}_{6}\)[90]. LOD of 19 cells mL\({}^{-1}\)[92]. Keihan et al. proposed a bamboo like MWCNT (BCNT) nanocomposite with ionic liquid (IL) and Prussian Blue (PB) for the enzymatic determination of choline. With this sensor, a linear calibration was possible in between \(4.5\times 10^{-7}\) and \(1.0\times 10^{-4}\) M [93]. In a similar study for the enzyme-based quantification of serotonin, a monoamine neurotransmitter associated with memory and happiness, a GCE was functionalised with MWCNT with monoamine oxidase A (MAO-A) immobilisation. The potentiometric detection of serotonin in a concentration range of \(5.67\times 10^{-7}\)-\(2.26\times 10^{-6}\) M was achieved in simulated body fluid with a high sensitivity (LOD: \(2\times 10^{-7}\) M) [94].