**2.2 Metal-based nanobiosensors**

Metal-based structures at the nanoscale are used in many fields due to their functionability, catalytic and electrical properties, surface plasmon resonance behavior, charge conduction, and sensing properties, and these properties provide unique advantages, especially for biosensor applications. Core@shell nanomaterials have been successfully synthesized by different methods, such as the hydrothermal method [31], sonochemical method [32], microwave irradiation [33], laser ablation [34] etc. In 2022, Mallick et al. highlighted the requirement of metal@metal oxide (M@MO) core@shell nanoparticles (NPs) with chemical, physical properties, and unique morphologies at the nanoscale in high-performance biosensor applications [35]. Various characterization methods such as photoluminescence (PL), UV-visible spectrophotometer, dynamic light scattering (DLS), scanning electron microscopy (SEM), wet-scanning electron microscopy (wet-SEM), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectrometer, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) are used to characterize nanostructures. However, thanks to new studies demonstrating advancements in nanotechnology, it is proven that the sensing performance of chemosensors and biosensors improves depending on the size and shape of nanostructures, including TiO2, SnO2, CuO, ZnO, WO3, and Fe2O3 nanoparticles with different morphologies.

Metals (Au, Ag, Pt, etc.) and metal oxides (ZnO, ZrO2, CuO, Fe2O3, TiO2, etc.) were investigated in metal-based biosensor studies. Among metal nanoparticles, noble metal nanoparticles (such as silver (Ag) and gold (Au) NPs) have been widely studied for use in biosensor applications [21, 36]. In particular, Ag NPs and Au NPs have been used for the detection of heavy metals such as cadmium, arsenic, mercury, etc. [21]. Santhosh et al. [37] synthesized novel plant-derived Ag NPs and tested them for the detection of heavy metals. In this study, waste *Allium cepa L* peels were prepared as a plant extract and then Ag NPs were synthesized in a greener way by chemical precipitation via using the plant extract.

#### *Nanostructures in Biosensors: Development and Applications DOI: http://dx.doi.org/10.5772/intechopen.108508*

The synthesized Ag NPs were tested in the solution of various heavy metal salts and a significant color change was observed in the HgCl2 solution. This result paves the way for the detection of mercury in the liquid phase by observation, which is easy and does not require any tools. The authors stated that green-produced Ag NPs can be used as a simple and inexpensive sensing partner in biosensor applications. In light of the metal-based nanosystems with sensing performances, Hasan et al. [38] worked on developing a gold nanoparticle-based biosensor that provides observation in a colorimetric way for the early diagnosis of ovarian cancer, which is one of the most common cancer types in women. In their study, a novel aptamer-based fluorescence sensor was develpoed for the detection of platelet-derived growth factor (PDGF). It aimed to develop a biosensor based on the fact that Au NPs show significant color change after aggregation. The authors stated that after aptamers and platelets were added to the Au NPs solution, aggregations occurred and changed the particle size of the NPs. In this case, the color of the solution changed from a pinkish color to a purplish color. Also, they said that the aptamers and Au NPs-based structure have potential for such applications by providing the observability of such an important disease in a simple, fast, inexpensive, and effective way for early diagnosis. Another metal-based nanostructure widely used in biosensor applications is metal-oxide nanoparticles (MO NPs). Many different forms of synthesized nano-sized metal oxides have many uses as transducers in biosensors, and many studies are being conducted on them [17]. Karakuş et al. [39] synthesized novel green polyphenol matrix CuO nanoparticles with the help of matcha tea powder extract by a low-cost, easy, green, and fast sonochemical method and then studied the development of the biosensor for the smartphone-based digital colorimetric detection of ammonia. In the study, the prepared CuO NPs solution was coated onto the gold electrochemical transducer by drop-casting and then dried at 40°C, and CuO NPs-based electrochemical biosensors were prepared. It was clear that the prepared biosensor carried out with the detection of color changes by performing red-green-blue (RGB) analysis in the examinations with the help of a smartphone provides an easy and highly selective detection of ammonia at nano levels.
