# Applications of Nanomaterials in RFID Wireless Sensor Components
## 4 Nanomaterials for RFID-Based Sensing System
### Carbon Nanostructures  
Carbon-based materials are gaining attention as low-density substances with high strength per unit weight, characterized by excellent properties such as high strength, light weight, resistance to corrosion, high heat resistance, and corrosion resistance. These properties make them ideal for the fabrication of components in sensor systems, such as RFID sensor antennas and detection materials [138]. Among them, graphene is widely utilized as a sensing tag component due to its outstanding conductivity, lightweight characteristics, and the potential for flexible component implementation. [PERSON] et al. applied reduced graphene oxide (rGO) for the fabrication of patch antenna patterns and sensing detection units (Figure 7a) [115]. As a typical p-type semiconductor, graphene can be utilized as a gas sensing material by inducing electron movement upon contact with oxidizing and reducing gas molecules. Additionally, the high surface area of graphene enhances sensitivity by increasing the surface loading of reactants. Specifically, the researchers applied an ink containing a mixture of rGO and silver nanoparticles to the patch antenna pattern and sensing material. The graphene-based patch antenna achieved a RFID wireless sensing system through changing its radiation characteristics. In this way, the high surface-to-volume ratio and excellent electrical properties of graphene promote its effective utilization as a sensing material. As mentioned above, metal nanoparticles are mostly used as ink in inkjet printing to form conductive loops. However, its expensive cost and high sintering temperature limit its usage. Consequently, there is a growing interest in graphene inks that are relatively inexpensive and can provide suitable conductivity. Additionally, graphene inks exhibit less aggregation compared to metal nanoparticles, allowing for the formation of uniform printed loops, and their durability is particularly advantageous for implementing flexible components. Notably, Graphene Oxide (GO) and Reduced Graphene Oxide (rGO) are gaining attention as printing inks due to their water dispersibility and self-assembly capabilities. [PERSON] et al. manufactured high-resolution rGO conductive patterns using a responsive inkjet printing technique. The alternate deposition of GO dispersion and reducing agents led to a simple electronic pattern print without requiring additional post-processing (Figure 7b) [108]. The produced rGO patterns exhibited high resolution and conductivity, and they were applied to RFID tags and sensors. The inkjet-printed rGO patterns were well integrated into plasma-treated PET substrates, forming antenna patterns. In this way, graphene-based inkjet printing, which can overcome the limitations of metal nanoparticle-based inks, continues to evolve in the direction of stable ink development and effective process establishment. Carbon nanotubes (CNTs) are suitable as sensing materials due to their hollow structure, high surface-to-volume ratio, high sensitivity, and rapid response. Additionally, their dispersion in water enables easy integration with flexible substrates through inkjet printing. [PERSON] et al. created a UHF RFID sensor tag by forming a CNT film that could alter its electrical conductivity in response to analytes (Figure 7c) [109]. The produced CNT film, integrated into the tag antenna in a rectangular shape, affected the radiation performance of the antenna. By modifying the resistance load and adjusting the impedance of the tag, detection was achieved in the UHF band of RFID. [PERSON] et al. implemented a chipless RFID sensor tag using an MWCNT-based sensing material through complete inkjet printing (Figure 7d) [110]. The MWCNT solution mixed with water was injected into the meandered structured detection slot of the tag at room temperature, forming the sensing area. MWCNTs responded sensitively to the concentration of analytes, altering their resistance, which, in turn, affected the electromagnetic energy returning to the reader.