# Nanomaterials for Healthcare Biosensing Applications
## 3 Applications of Inorganic Nanomaterials in Healthcare Biosensing
### Quantum Dots  
Inorganic QDs generally consist of a bimetallic alloy core with a shell layer, such as metal chalcogenide. The dominance of quantum confinement effects takes place when the diameter of QDs is less than the electron-hole Bohr radius, giving rise to unique optical properties. Stokes shifts, arising from the NIR or UV electromagnetic radiation during excited electron relaxation to holes, cause superior photoluminescence in comparison to organic dyes. QDs have been used for the detection of a variety of molecules, such as proteins [157; 158], pathogens [159], lung cancer biomarkers [160], and nucleic acids [161; 162]. Nevertheless, widespread in vivo use of quantum dots is still inhibited by the toxicity of cadmium, a common constituent in QDs, as well as by the tendency of protective coatings to undergo in vivo degradation [163]. A wide array of target molecules can be effectively detected by implementing QDs in a micro-fluidic platform. Recently, such a sensor was used for the determination and subtyping of three influenza viruses (H1N1, H3N2, and H9N2) [164]. Streptavidin-coated quantum dots (Str-QDs) with immobilised biotinylated DNA were used as labels for fluorescent imaging and DNA immobilised superparamagnetic beads (SMB) acted as capture probes (Figure 8a). The sensor operation was based on the hybridisation of nucleic acid on the microfluidic chip in the presence of a regulated micro-magnetic field (Figure 8c,d). The simultaneous detection of H1N1, H3N2, and H9N2 was performed in 80 min in the ranges of 1-150 nM, 5-150 nM, and 1-150 nM with LODs of 0.21 nM, 0.16 nM, and 0.12 nM, respectively [164]. A similar study for the detection of common peanut allergen (Ara h1) was reported using QD-aptamer-GO hybrids [165]. This system acted as a probe that underwent conformational changes due to the adsorption and desorption of GO on biotinylated aptamer functionalised Str-QDs. Within 10 min, the microfluidic sensor provided a single step homogeneous assay. The miniaturised optical sensing system was functional in a concentration range of 200-2000 ng mL\({}^{-1}\) with an LOD of 56 ng mL\({}^{-1}\)[165]. The sensing method thus exhibited a potential for on-site allergen detection. Another study on protein recognition reported the fluorescence quenching of CdSe quantum dots using metal oxide nanoparticles of Eu\({}_{2}\)O\({}_{3}\) and CuO, as well as noble metal nanoparticles of silver and gold [157]. Following the addition of analyte, the fluorescence activity of QDs was restored, as the analyte-QD interactions liberated QDs from the nanoparticle-QD conjugate. The interactions between the proteins and the QDs also improved the fluorescent intensity. The LODs in all cases were below 2 uM. CuO-CdSeQDs and Eu\({}_{2}\)O\({}_{3}\)-CdSeQDs recognised casein in the ranges of 0.5-5 uM and 2.0-50 uM, respectively [157]. AgNP-CdSeQDs were successful in papain detection from 3.0 to 20 uM and the AuNP-CdSeQDs provided a linear response between 2.0 and 50 uM. DA functionalised CdSe-ZnS QDs have been investigated for the detection of \(\alpha\)-fetoprotein (AFP), an important biomarker for various kinds of tumour and prenatal aneuploidy, with an immunoassay based on redox-mediated indirect fluorescence [158]. Tyrosine(TSA)-detection antibody conjugate acted as bridges between biomarker concentrations and QD signals by catalysing the DA oxidation on the QD surface. AFP detection was reported between 10 pM and 100 nM and the detection limit was 10 pM. Specificity of 100% and 97.5% sensitivity, with a clear distinction between positive and negative samples, was observed when the sensor system was employed to validate AFP detection in 10 different AFP-negative control samples and 40 AFP-positive samples obtained from hepatocellular carcinoma patients. A combination of QD fluorescence and immunomagnetic separation (IMS) has been reported to hold promising results in pathogen detection [159]. Magnetic nanoparticle cores with gold shells, functionalised with biotinylated _E.