# Nanomaterials for Healthcare Biosensing Applications
## 3 Applications of Inorganic Nanomaterials in Healthcare Biosensing
### Magnetic Nanoparticles  
Fe\({}_{2}\)O\({}_{3}\), Fe\({}_{3}\)O\({}_{4}\), FePt, and many other similar nanoparticles are superparamagnetic in nature. Depending on their synthesis mechanism, these particles may have varying size distributions and their sizes range from 10 nm to 1000 nm. MNPs are generally employed as either transducers, which may be electrochemical, piezoelectric, optical, or colorimetric in nature, or as labels in conjugation with biomolecules [146]. MNP biosensors are useful in various disciplines, including the food industry, medical diagnosis, and environmental investigations [177]. There are three prerequisites that need to be considered for utilising MNPs in healthcare biosensing: (i) MNPs must retain a high saturation magnetisation to enable the manipulation of their movement in blood without the requirement of very strong magnetic fields. allowing the movement of MNPs in close proximity to the targeted tissue; (ii) MNPs must be biocompatible and non-toxic; (iii) the size of MNPs should range between 10 and 50 nm to avoid aggregation or precipitation owing to gravitational forces and to ensure the colloidal stability, especially in water at pH 7.0, thereby yielding a large surface area for a specific volume of the material [146; 178; 179]. The application of MNPs in early stage cancer sensing has exhibited tremendous potential. For example, Pal et al. multiplexed MNPs with monoclonal antibodies (mAbs) for sensing various ovarian cancer biomarkers (cancer antigen 125 (CA-125), Apo-lipoprotein A1 (ApoA1), and \(\beta\)2-microglobulin (\(\beta\)2-M)) [180]. A sandwich assay was developed with the help of polyclonal antibodies (pAbs). The sandwiched particles were subsequently extracted from the sensing medium with the help of magnetic force. This was simultaneously accompanied by a real-time monitoring of the fluorescence change against a standard concentration. The study was validated using a comparative analysis with SPR to ensure reproducibility. The assays resulted in LODs of 0.26 U mL\({}^{-1}\), 7.7 ng mL\({}^{-1}\), and 0.55 ng mL\({}^{-1}\) for CA-125, ApoA1, and \(\beta\)2-M, respectively. The sensor succeeded in distinguishing ovarian cancer patients from healthy individuals with 94% sensitivity and 98% specificity. Suaifan and Zourob developed electrochemical and optical-based biosensors for the analysis of PSA. They used carboxyl functionalised MNPs on a gold platform. The functionalised MNPs bonded with the N terminus of the PSA specific peptide and Au bonded with the thiol functionalities. Following proteolysis, an external magnetic field was applied to cleave the MNP-PSA peptide moieties from the Au platform. EIS revealed an LOD of 1 pg mL\({}^{-1}\) with a detection range up to 1 ug mL\({}^{-1}\). The optical analyses revealed relatively higher LODs of 100 pg mL\({}^{-1}\) for SPR and 1 ng mL\({}^{-1}\) for visual evaluation [166]. Lee et al. prepared the functionalised Fe\({}_{3}\)O\({}_{4}\) core-Au shell structures on GSPE for the detection of eosinophil cationic protein (ECP), a biomarker for asthma [181]. The CYA labelled heparin (Hep) modified core-shell magnetic nanostructures amplified the electrochemical signal difference, thus improving the sensitivity of the biosensor. CV and SWV measurements revealed that the sensor can be used for a wide investigation range (1-1000 nM) with quite high sensitivity (LOD: 0.30 nM). Due to rising cases of myocardial infarction, various MNP-based SPR biosensors for cTnI have been reported in the last five years. In one such study, PDA wrapped Fe\({}_{3}\)O\({}_{4}\) doped MWCNT (MMWCNT) was implemented to enrich cTnI dAb and used for magnetic extraction [182]. The sensing medium in this case was made up of a film of PDA and hollow gold nanoparticles (HGNP), which underwent self-assembly on a mercapto-functionalised gold platform prior to cAb immobilisation. The high surface area and the magnetic nature of MMWCNT improved target enrichment and allowed magnetic extraction.