# Nanomaterials for Healthcare Biosensing Applications
## 3 Applications of Inorganic Nanomaterials in Healthcare Biosensing
### Silver Nanoparticles  
Analogous to AuNPs, silver nanoparticles ( AgNPs) are commonly used in medical diagnostics. The colour of AgNP solutions is also dependent on the particle size, owing to LSPR absorption. Similarly, their optical properties are a function of their degree of aggregation, shape, and size. However, the antimicrobial nature of AgNPs set them apart from AuNPs. In addition, they possess attractive electrical properties [2], with a more affordable cost than AuNPs. AgNPs are also utilised for enhancing the performance in a biosensing system. AgNPs have been widely used for SERS-based biosensors [133; 140]. They demonstrate a higher extinction coefficient than AuNPs of the same size and undergo electrochemical oxidation more easily. Though the instability and functionalisation of AgNPs was considered challenging in the past, the synthesis methods and modification techniques have substantially improved them in recent years [195]. AgNPs have been used for the detection of various pharmaceutical and narcotic drugs, as well as for monitoring their effects on human. Raj and Goyal modified pyrolytic graphite (PyG) with a nanocomposite of ERGO and AgNPs to detect caffeine (CAF) and determine its effect on the concentration of estradiol, 1,3,5 (10)-estratrien-3,17 \(\upbeta\)-diol (EST) in women of child-bearing age [196]. EST is primarily a female growth hormone. AgNPs in conjugation with ERGO have a synergistic electrocatalytic effect arising from the storage capacity of electrons and the ability to supply electrons on demand. The voltammetric sensor provided wide detection ranges of 0.001-200 \(\upmu\)M and 0.001-175 \(\upmu\)M and LODs of 0.54 nM and 0.046 nM for EST and CAF, respectively. The sensor was highly selective against AA,UA, xanthine, and hypoxanthine [196]. Mao et al. developed a SERS sensor for the determination of methylamphetamine (MAMP), a popular illicit drug, using Au\({}_{\text{shell}}\)-Ag\({}_{\text{core}}\) nanoparticles [197]. The SERS performance of the shell-core structures was much more pronounced in comparison to AuNPs. The sensor quantified MAMP with the help of MAMP aptamer in the range of 0.5-40 ppb, and the detection limit was found to be 0.16 ppb. Bagheri and co-workers developed a patulin sensor with the help of AgNP-ZnMOF capped with a molecularly imprinted copolymer of 3-Aminopropyl}triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) [198]. AgNPs displayed a mimetic activity in the ZnMOF, thereby increasing the number of active sites for the H\({}_{2}\)O\({}_{2}\)-terephthalatic acid reaction, resulting in high fluorescence. The sensor provided a linear response between 0.1 and 10 \(\upmu\)M, and the detection limit was 0.06 \(\upmu\)M.  
Zheng and co-workers reported a highly sensitive and simultaneous sensing of multiple breast cancer biomarkers by developing a SERS microfluidic chip sensor [199]. The AgNPs were functionalised with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and subsequently modified with 4 mercaptobenzoic acid (4 MBA) and antibodies as Raman reporters. The unreacted sites were capped with bovine serum albumin (BSA). Antibodies were immobilised on a SERS substrate and a sandwich immunoassay provided a linear detection response for cancer antigens CA 153, CA 125, and CEA in the respective ranges of 0.001 U mL\({}^{-1}\)-1 kW mL\({}^{-1}\), 0.001 U mL\({}^{-1}\)-1 kW mL\({}^{-1}\), and 0.1 pg mL\({}^{-1}\)-100 ng mL\({}^{-1}\) in serum. The LODs were 0.01 U mL\({}^{-1}\), 0.01 U mL\({}^{-1}\), and 1 pg mL\({}^{-1}\), respectively. In another study for PSA detection, a nanomaterial-conductive polymer nanocomposite, graphene-poly(3-aminobenzoic acid) (GP-P3 ABA), was used for electrode modification and porous-hollowed-silver-gold core-shell nanoparticles (PHAu\({}_{\text{shell}}\)-Ag\({}_{\text{core}}\)NPs) amplified the signals by acting as labels in the system [200]. A 3-fold current response was obtained for PHAg\({}_{\text{core}}\)-Au\({}_{\text{shell}}\)NPs against pure AuNPs.