# Applications of Nanomaterials in RFID Wireless Sensor Components
## 4 Nanomaterials for RFID-Based Sensing System
### Others  
To enhance RFID performance, the utilization of new forms and composite materials beyond conventional ones is required. Among the improved RFID technologies, Surface Acoustic Wave (SAW) RFID operating in the GHz range is considered one that can operate over long distances and penetrate obstacles like metal and liquids. Aluminum nitrate (AlN) possesses characteristics such as high acoustic velocity, low dielectric and propagation loss, and good chemical resistance, making it a widely researched material in SAW applications. [PERSON] et al. have fabricated a disposable SAW RFID sensor capable of high-frequency operation based on AlN and a flexible substrate (Figure 8a) [139]. The AlN pattern deposited on a PEN film allows Lamb SAW to propagate with a rapid wave phase velocity of 10,600 m/s. There is another type of sensor based on acoustic waves, utilizing Bulk Acoustic Wave (BAW). Unlike SAW, which propagates along the surface of the material, BAW propagates through the bulk of the medium [140]. BAW-based RFID sensors which operate in higher frequency ranges provide a wider reading range compared to SAW-based sensors. Additionally, they are less sensitive to external environmental factors such as temperature and bending deformation and can achieve relatively high sensitivity [141]. However, due to the complex manufacturing processes and costs, BAW sensors are still in the early stages of development, requiring further research. Research is also underway for RFID sensors utilizing ionic liquids (IL) that exist in a liquid state even below a certain temperature. [PERSON] et al. have developed a temperature-detecting RFID sensor tag for cold chain monitoring using copper ionic liquid (Cu-IL) (Figure 8b) [122]. Cu-IL integrates into the RFID sensor by being placed in small drops between two electrodes. When the temperature exceeds the melting point of Cu-IL (8 \({}^{\circ}\)C), it melts, achieving detection by transitioning the electrodes from an open to a closed state. The sintering process of metal nanoparticles requires high temperatures, causing deformation in typical plastic substrates. To address this issue, researchers are exploring the enhancement of the adhesive, thermal, and structural properties of metal nanoparticles through the assistance of other substances. [PERSON] et al. have produced a copper electrode usable as an antenna for RFID tags by employing soft and mesoporous Xerogel (Figure 8c) [123]. After creating the Xerogel framework by printing copolymer-based ink, they loaded a catalyst through ion exchange, providing active sites for copper particle growth. The resulting Xerogel-based Cu electrode antenna demonstrated stable signal transmission and reception even under bending conditions. Figure 7: (**a**) rGO-based ammonia-sensitive hybrid materials and their antenna applications. (**b**) In situ patterning and reduction process of rGO patterns called reactive inkjet printing [108]. (**c**) Passive RFID sensor tag based on CNT film integrated in antenna. (**d**) Chipless RFID sensor manufactured with full inkjet printing using MWCNT solution [110]. MXene's high conductivity, hydrophilicity, and dispersibility make it suitable as ink in the field of printed electronics. [PERSON] et al. have created an ethylene-modulated resonator for RFID-based sensors by integrating palladium nanoparticles (PdNP) into additive-free MXene ink (Figure 8d) [124]. The approach of growing PdNP in MXene ink has led to the flexibility of devices, enabling effective detection and in situ sensing. The RF resonator based on MXene-reduced Pd NP ink exhibited low LOD (0.084 ppm), appropriate selectivity at room temperature, and effective wireless sensing performance. [PERSON] et al. proposed a straightforward process by aerodynamically focused printing Ag/MWCNT composites, maintaining electrical properties without the need for post-processing (Figure 8e) [125].