# Nanomaterials for Healthcare Biosensing Applications
## 3 Applications of Inorganic Nanomaterials in Healthcare Biosensing
### Gold Nanoparticles  
Gold nanoparticles (AuNPs) are conductive materials, which possess a large surface area and exhibit unique optical properties. In AuNPs, a surface plasmon is confined, giving rise to LSPR. Therefore, their colour changes from red to yellow as their size increases from 100 nm [2]. AuNPs undergo oscillations, which are analogous to their metallic lattice. Depending on the shape of AuNPs, the heat and light scattering arising from the surface plasmonic decay may be affected. Hence, colour changes in nanospheres are less pronounced than GNRs. Another factor that can tune the optical properties of AuNPs is their degree of aggregation. This parameter can be harnessed for developing biosensors and optical immunoassays [133]. Multifunctional AuNPs are now being widely used for detecting various biomarkers for cancer [56; 68; 75; 139; 140; 188], neurological disorders [31], diabetes mellitus [135; 138], nucleic acids [17], amino-acids [70; 99], hemoglobin [189], and a variety of pathogens [79; 136; 190; 191]. Shan and co-workers developed a piezoelectric QCM aptasensor using silver enhanced AuNPs as labels for the determination of CCRF-CEM cells, which are T-cell biomarkers of acute lymphoblastic leukemia (ALL) [140]. The use of aminophenylboronic acid-functionalised AuNPs (APBA-AuNPs) resulted in signal amplification, providing linear responses in the concentration range of \(2\times 10^{3}\)-\(1\times 10^{5}\) cells mL\({}^{-1}\) (LOD: 1160 cells mL\({}^{-1}\)). Yan et al. detected CD-10, another biomarker for ALL, using a label-free QCM-based sandwich immunosensor, relying on antibody immobilised glutathione functionalised AuNPs (Ab\({}_{2}\)-Glut-AuNPs) as signal amplification agents. The large surface area and superior conductivity of AuNPs allowed detection in the range of \(1.0\times 10^{-11}\)-\(1.0\times 10^{-10}\) M with a \(2.4\times 10^{-12}\) M limit. Chaichi and Ehsani immobilised GOx on chitosan shells with Fe\({}_{3}\)O\({}_{4}\) cores and integrated it with a lumininol chemiluminescence system to develop an optical sensor for glucose [137]. They used gold nanoparticles to catalyse the lumininol CL reaction as well as the reaction between GOx and glucose that lead to the generation of H\({}_{2}\)O\({}_{2}\). The linear range of the neoteric sensor was \(1\times 10^{-4}\)-\(8.5\times 10^{-7}\) M and the detection limit was \(4.3\times 10^{-7}\) M. Guo and co-workers reported a similar enzymatic sensor with an electrochemical detection mechanism [135]. Following the initial electrophoretic deposition of rhodium nanoparticles on a platinum electrode, AuNPs, Nafion and GOx were deposited. The sensor was reported to be selective against electroactive non-specific biomolecules, such as acetaminophen, UA, and AA, and had a lifespan of up to 90 days. The sensor provided a linear amperometric response from 0.05 to 15 mM with an LOD of 30 \(\upmu\)M.  
AuNPs are also materials of choice for pathogen quantification. A FRET-based immunoassay used AuNPs for the competitive fluorescence quenching of CdTe quantum dots to detect outer membrane protein W (OmpW) of _Vibrio cholerae_[192]. The AuNPs were functionalised with 11-mercaptoethanolanoic acid (11-MUA) and then conjugated with OmpW, as shown in Figure 9a. Figure 9b depicts the integration of carboxyl functionalised CdTeQDs with pAb of OmpW. When OmpW reacts with its antibody, the distance among the two nanoparticles reduces below 10 nm, causing energy transfer from CdTeQDs to AuNPs, thereby lowering the emission intensity. The fluorescence quenching increases with an elevation in the AuNP-OmpQ concentration by FRET. Competitive binding, as illustrated in Figure 9c, takes place when free OmpW is added to the system, ultimately leading to fluorescence recovery. The sensor allowed the quantification of OmpW in a linear range between 2 and 10 nM [192]. In another study, an enzyme-free electrochemical sensor was developed using a nanocomposite of Cu-Zr MOFs with aptamer as the signalling probe for the detection of _Pseudomonas aeruginosa_ (_P.