# Applications of Nanomaterials in RFID Wireless Sensor Components
## 4 Nanomaterials for RFID-Based Sensing System
### Metal Nanoparticles  
Metal nanostructures can be utilized as components of sensing tags due to their large surface area, excellent electrical characteristics, easy processability, and mechanical durability [132]. Metal nanoparticles with high electrical conductivity are desirable materials for conductive inks used in inkjet printing. One significant advantage of metal nanoparticle-based inkjet printing is the ability to easily implement electrical circuits with minimal material usage [133; 134]. In the context of RFID sensor systems, it is commonly utilized for the fabrication of patterns or antennas that can replace IC chips. This can reduce the production cost of RFID sensor tags while achieving high throughput. Representative metal nanoparticles used include gold, silver, and copper, and the ink is manufactured by dissolving or suspending these nanoparticles [135]. Particularly, silver nanoparticles, known for their high electrical and thermal conductivity, chemical stability, and low melting point, are widely employed [136]. [PERSON] et al. fabricated a sensing tag without an IC chip by easily replacing it with an antenna through the in-situ metallization of silver nanoparticles (Figure 6a) [104]. This sensing tag demonstrated a capacity of 2 bits without an IC chip, maintaining durability under repetitive tensile and compressive stress while proving excellent adhesion and flexibility. The conductivity of metal nanoparticle ink directly influences the performance of patterns and antennas in constructing RFID systems. High conductivity indicates active electron mobility within the material, resulting in reduced resistance in electrical circuits and rapid transmission of electrical signals. In other words, improving the conductivity of patterns composed of metal nanoparticles leads to the effective advancement of  
Figure 5: Application of conducting polymers as sensing tag components: (**a**) antenna pattern [101], (**b**) sensing area [64]. (**c**) Application of composite materials formed by bonding conducting polymer with other materials (P-rGO) as sensing area components [114]. communication in RFID systems. Various methods have been proposed to enhance the conductivity of metal nanoparticle patterns. One of the representative methods is sintering, which improves conductivity by removing binders and organic stabilizers and forming connections between nanoparticles, and controlling its temperature. For instance, when the sintering temperature of a pattern based on silver nanoparticles was 220 \({}^{\circ}\)C, the conductivity was 0.206 \(\times\) 10\({}^{7}\) S/m, which was higher than 0.192 \(\times\) 10\({}^{7}\) S/m at 170 \({}^{\circ}\)C [137]. High sintering temperatures increase the bond strength between particles and grow the crystalline structure, providing an efficient path for electrons to move freely. However, high sintering temperatures damage substrates such as polymers and paper. Therefore, recent efforts focus on maintaining low sintering temperatures while achieving high conductivity. Accordingly, [PERSON] et al. reported that the limited conductivity of silver nanoparticles can be improved through Cu electroplating [137]. Given the critical role of ink conductivity in effectively advancing RFID systems, numerous studies are required to enhance it. To overcome the limitations of pure metal nanoparticles, research is underway to enhance properties and characteristics through the combination with other substances. For instance, [PERSON] et al. designed a biosensor tag capable of wireless monitoring without a battery by controlling the redox conversion of silver nanoparticles (Figure 6b) [105]. The sensing area of the sensing tag was created by pipetting a mixture of silver nanoparticles and horseradish peroxidase (HRP). The interdigitated electrode made from the enzyme-nano material combination achieved high sensitivity through a significant resistance difference of up to 15 times of Ag/AgCl and an accelerated oxidation-reduction process. In another example, [PERSON] et al.