Applications of Nanomaterials in RFID Wireless Sensor Components

Abstract

Radio Frequency Identification (RFID) technology, capable of wirelessly processing large amounts of information, is gaining attention with the advancement of IoT technology. RFID systems can be utilized as Wireless Sensor Network (WSN) technology by introducing sensing materials responsive to external environmental stimuli. To achieve effective information communication and sensing capabilities, various types of nanomaterials are being used as various components of RFID sensors. This paper provides an overview of the RFID sensor system and the nanomaterials used in their composition. Polymers that can achieve flexibility are attracting more attention as user-friendly substrates as demand for them increases in the wearable market. Additionally, advancements in inkjet printing technology, allowing cost-effective and simple production of components, introduce inks and manufacturing methods utilizing various nanomaterials such as metal nanoparticles, carbon materials, and composites. Furthermore, nanomaterials utilized as sensing materials enable the detection of various external environments with high sensitivity and a wide detection range. Consequently, RFID sensor systems that achieve wireless detection in a variety of environments are actively utilized in many applications. By analyzing the current research progress and problems faced in RFID sensor technology, this paper suggests future research directions for its development as a next-generation wireless sensor system.

1. Introduction

Wireless communication technology facilitates the exchange of information between distributed points without the need for physical lines or direct plug-ins. With the rapid expansion of the Internet of Things (IoT), the integration of sensors and communication functions into various objects for internet connectivity has gained increased significance [1,2,3,4,5]. Wireless Sensor Network (WSN) is a specific type of wireless communication technology that collaboratively exchanges data by centrally collecting spatially distributed monitoring records [6,7,8]. WSN establishes a network structure by gathering data from sensor nodes and transmitting them externally to sink nodes [9,10].

Radio Frequency Identification (RFID), a subset of WSN technologies, enables the real-time collection and processing of information using radio frequency waves as a communication medium [11,12,13,14,15]. RFID is extensively employed in everyday life and industry due to its notable attributes such as high data throughput, contactless recognition, and data readability from multiple directions [16,17,18,19,20,21,22,23]. Notably, ongoing research is exploring the establishment of a wireless sensor network based on RFID communication methods capable of Wireless Information and Power Transfer (WIPT), with applications to various sensor devices [24,25,26].

Nanomaterials, characterized by a high surface area for chemical reactions and a unique surface structure that facilitates the implementation of flexible elements, enhance the superior performance and efficiency of RFID sensors [27,28,29,30]. Thoughtful material selection and the exploration of diverse structures contribute to the sensor’s heightened reliability, selectivity, and rapid response and recovery performance to external stimuli [31,32,33,34,35]. Utilizing various materials in the fabrication of RFID sensor components allows for the effective integration of RFID wireless communication technology and sensing capabilities [36,37,38,39,40].

This review assesses the impact of applying various nanomaterials to RFID systems on wireless sensor performance, summarizing their contributions. Additionally, it introduces the fundamental mechanisms based on the configuration of RFID sensor systems and changes in electrical resistance. The review explores research trends related to various materials such as polymers, metal nanoparticles, and carbon-based materials applied to each component. Finally, an analysis of RFID sensor systems applied in diverse fields aims to predict the applicability and prospects of next-generation wireless sensors.

2. RFID System

Radio Frequency Identification (RFID) is a type of contactless wireless communication technology that utilizes radio waves as a transmission medium [41,42,43]. It consists of three components: tags, RFID readers, and IT systems (Figure 1). Tags, which contain unique information, record data through an integrated circuit chip (IC chip) and transmit and receive wireless signals via an antenna [44,45]. The wireless interaction between tags and RFID readers is based on digital or backscattered modulation, which involves altering the amplitude, phase, and frequency of radio waves [46]. Specifically, when a tag exists within the permissible range of a RFID reader, the tag receives the transmitted signal from the reader, modifies it, and sends it back as a modulated reflected signal through the antenna. The RFID reader converts the returned signal into digital format, decodes the embedded information, and transmits it to IT systems. Ultimately, IT systems collect data from distributed readers, manage information, and control the overall system.

Compared to traditional barcode recognition systems, RFID systems can process relatively large amounts of data over long distances due to their reliance on radio waves [21]. Additionally, RFID does not require direct or physical patterns, enabling recognition regardless of vertical or horizontal arrangements. Consequently, RFID can capture signals over a wide range and in all directions without direct contact. Furthermore, using electronic circuits (IC chips) instead of visual pattern arrays allows RFID to store more information than barcodes [47].

The operating frequency range of the tag is broadly categorized into three ranges, determining the communication method and antenna type with the RFID reader (Figure 2a). For low operating frequencies, specifically in the range of 120–140 kHz (LF), the tag and RFID reader operate through inductive coupling, with the antenna used adopting a coil shape. In the case of an operating frequency of 13.56 MHz (HF), the tag and RFID reader operate through inductive or capacitive coupling, and the antenna used primarily employs a planar inductor [48,49]. In a high operating frequency range of 868–962 MHz (UHF), the tag and RFID reader are radiatively coupled according to the backscattering principle, and the antenna used in this case is either a dipole or microstrip [50].

3. RFID Sensor Technology

The principles of systems operating through inductive coupling, such as LF and HF, are as follows (Figure 2b) [51]. The process initiates by creating a magnetic field in the RFID reader antenna through a driving current. When the tag is positioned in the corresponding magnetic field region, current is induced, resulting in the generation of voltage, ultimately activating the RFID system. The signal transmitted from the reader is modified by the unique pattern of the tag, and the reader decodes this information through demodulation. In this case, since the current flowing through the tag is very weak, only short-range transmission is possible. Nevertheless, it exhibits less sensitivity to interference, allowing signals to be transmitted and received through objects such as metals or liquids [52]. LF RFID is effectively used in underground object detection such as mines, military munitions management, and urban construction tasks like subterranean manholes [53,54,55,56]. HF RFID, with its relatively higher frequency, achieves improved reading range and processing speed, making it suitable for applications that require a slightly longer distance, such as library management, animal identification, and cold chain tracking [57,58,59].

On the other hand, in scenarios operating through backscattering, such as UHF, the RFID reader transmits an electric field signal to the tag to initiate the process (Figure 2c) [60]. The tag, activated by the signal from the RFID reader, conveys its information to the reflected signal by altering the RFID reader’s radio signal. Subsequently, the RFID reader processes the information by recognizing the difference between the transmitted and received information. RFID, utilizing radio signals and receiving energy from its own battery, can continuously transmit and receive signals over much longer distances. In modern society, where there is a growing demand for automation and remote monitoring, the interest in UHF RFID applications, which offer a wide reading range and enable rapid data processing, continues to rise. UHF RFID, well suited for large-scale inventory management, is used in various fields such as supply chain management of large supermarkets, farm machinery, and automatic machine identification [61,62,63]. However, this outstanding performance not only increases the price but also increases the size and shortens the lifespan. Therefore, for effective RFID utilization, understanding the usage environment and necessary conditions is crucial, necessitating optimized design considerations.

Research on utilizing active substances with specific sensing capabilities for materials in RFID system tags to construct sensing tags and employ them as wireless sensors is being actively conducted. As shown in Figure 3a, a wireless sensor using a RFID system consists of a substrate, IC chip, sensing area, and antenna [64].

The operational principle of RFID-based wireless sensors involves a sensing tag, which carries the detection material, modulating and reflecting the signal transmitted by the RFID reader to interpret the reflected signal and acquire detection information (Figure 3b). Specifically, the RFID reader initiates the operation by transmitting the initial signal, P1, to the sensing tag. The sensing tag receives P1, and the detection area, occupying a small portion within the sensing tag, detects external stimuli. The detection area reacts to chemical environmental changes, causing a resistance change in the detection material. This change leads to alterations in reception signal parameters such as amplitude, frequency, and phase within the sensing tag. The modified reflected signal, P2, resulting from the material detection, is backscattered from the sensing tag to the RFID reader. Particularly, by analyzing the degree of change in the modulated P2 signal received by the RFID reader, one can calculate the quantity of the detected material. The reflected signal information read by the RFID reader is transmitted to the server via wireless communication methods, enabling users to utilize the collected information [65].

The substrate serves as the foundational material supporting various elements in constructing a sensing tag. It determines aspects such as the tag’s size, shape, and strength, providing the mechanical structure for the sensing tag. The selection of the substrate must accommodate all components of the sensing tag without affecting the operating principles and antenna design frequencies. Substrates can be chosen from a variety of materials, including metal, ceramic, textile, plastic, and more [66,67,68]. Recently, attempts have been made to develop RFID yarns that can be immediately integrated into tag manufacturing, considering the widespread use of RFID in the clothing market. Additionally, there is growing attention to substrates that can provide flexibility and stretchability, especially with increasing demand for wearables (Figure 3c) [69]. The flexibility of the substrate allows it to withstand external mechanical impacts and makes it more suitable for attachment to the human biome. Stretchability, based on high elasticity and reversibility, demands properties that do not deform even with repeated strain and release. Research on flexible and stretchable RFID substrates is ongoing, with expectations that it will lead to various applications.

The Integrated Circuit (IC chip) is responsible for storing tag information and processing data. Sensing tags are distinguished based on the presence of the IC chip [70]. A sensing tag with an attached IC chip follows a conventional structure, where the IC chip not only handles the ID, data, and encryption key storage and processing but also includes digital logic required for data processing (Figure 3d) [71]. Such sensing tags allow for long-distance data reading and complex data processing. However, due to the inclusion of semiconductor-based IC chips, the manufacturing cost is higher. To address the need for more affordable and simpler sensing tags, a structure without the necessity of an IC chip has emerged. In cases where there is no IC chip, integrated patterns made of materials such as metal nanoparticles, conductive polymer inks, carbon materials, etc., serve as substitutes for IC chips [72,73,74]. Patterns replacing IC chips can be designed using various methods such as ultra-wide band (UWB), polarization diversity, phase quantization encoding, etc. [75,76,77]. This structure is cost-effective, has a long lifespan, and is suitable for use in harsh environments [78]. However, compared to cases with IC chips, sensitivity and selectivity are limited, requiring further research [79].

A certain portion of the sensing tag is configured as a sensing area using active substances to induce changes in the properties of the active substances in response to external physical or chemical stimuli (Figure 3e) [80]. Specifically, materials such as PEDOT, which changes in electrical conductivity based on pH, phenanthrene with a discontinuous dielectric constant above a certain temperature, and CNT, which alters resistance upon gas molecule adsorption, fall into this category [81,82,83]. The changes in the properties of the active substances result in impedance variations in the sensing tag, and the detection process is completed as the RFID reader detects these changes. This sensing area can detect various elements such as light, moisture, and temperature, making it applicable to diverse fields such as environmental monitoring, healthcare, agriculture, and food [84,85,86]. The integration of the sensing area into RFID sensor systems is achieved through various methods, including coating the antenna with active substances, attaching it across the antenna and IC chip, and inserting the sensing area by perforating holes in the antenna [87,88,89,90]. When designing the sensing area, it is crucial to select materials with high sensitivity to the target substance and to exercise caution to avoid influencing other components simultaneously.

In a RFID sensor system, the antenna is responsible for the data transmission and reception between the tag and the reader (Figure 3f) [91]. As detection occurs, the impedance matching between the antenna and chip of the tag changes, consequently altering the communication characteristics between the sensing tag and the RFID reader. The antenna serves as a mediator by transmitting and receiving the corresponding radio waves, enabling the identification of external stimuli. When designing the antenna, it is crucial to consider factors such as operating frequency, coupling mechanisms, bandwidth, efficiency, quality factor, etc., to achieve optimal antenna conditions. For RFID applications, microstrip patch antennas, typically composed by attaching a conductor to a ground plane, are widely preferred. These antennas are predominantly rectangular in shape, although circular shapes for uniform phase excitation of all antenna elements, and meander line shapes for maximizing surface area with a combination of horizontal and vertical lines, have also been reported. Recently, efforts have been ongoing to introduce antenna patterns by printing them directly onto the RFID substrate surface [92,93,94]. In comparison to traditional methods, antenna printing allows for more sophisticated and complex structures, along with lower manufacturing costs, rapid production, and customization of components [95]. This is expected to significantly contribute to the future design and manufacturing of RFID antennas.

In this way, the RFID sensor tag consists of a substrate that provides the tag’s framework, an IC chip that processes information, a sensing area that detects external stimuli, and an antenna that transmits and receives data. With the recent advancements in IoT, the escalating volume of information necessitates the enhancement of RFID performance through the integration of nanomaterials. The development of inkjet printing technology, which can efficiently produce parts with minimal materials under atmospheric pressure or low vacuum compared to existing deposition methods and is compatible with flexible substrates, is also increasing the demand for nanomaterials. Nanomaterials, which have superior surface and interfacial properties compared to bulk materials, can increase the reactivity and achieve excellent sensitivity as sensors by maximizing their surface area. Nanomaterials also have various advantages such as miniaturization, light weight, cost-effectiveness, low power consumption, and efficient integration. Accordingly, many studies have been conducted to implement RFID sensor systems with nanomaterials, which are summarized in

Table 1. Nanomaterials in RFID sensor system.

Classification	Materials	Component	Property	Analytes	Sensing Range	Sensitivity	LOD	Application	Ref.
Polymer	PET	Substrate	Flexible	-	-	-	-	Gas sensing	[96]
	Nafion 117	Sensing material	Humidity-sensitive	Humidity	3–33%	0.012–0.05%	-	Humidity sensing	[97]
	PDMS	Substrate	Stimuli-responsive	-	-	-	-	-	[98]
	Wheat gluten	Sensing material	CO2-sensitive	CO2	0~40%	-	40% CO2	Food	[99]
	pHEMA	Sensing material	Humidity-sensitive	Humidity	30–80%RH	172% (ΔC/C0)	-	Agriculture	[100]
Conducting polymer	Carboxyl group-functionalized PPy nanoparticles	Sensing material	NH3-sensitive	NH3	0.1–25 ppm	>3%	0.1 ppm	Gas sensing	[64]
	PEDOT:PSS	Sensing material	-	-	-	-	-	Printing technology	[101]
	PANI	Sensing material	NH3-sensitive	NH3	0–18 ppm	-	3 ppm	Food	[102]
	PTS-PANI	Sensing material	NH3-sensitive	NH3	5–200 ppm	225%(R/R0)	5 ppm	Food	[103]
Metal nanoparticle	Patterned Ag	Antenna, sensing material	Humidity-sensitive	Humidity	-	-	-	Humidity sensing	[104]
	AgNP/HRP-assisted AuNP	Antenna, sensing material	H2O2- and glucose-sensitive	H2O2, glucose	10−6—10−3 M, 1–20 mM	71% (Δf)	10−6 M, 1 mM	Healthcare	[105]
	Dextrin-capped AuNP	Antenna, sensing material	Biosensitive	E. coli	-	-	--	Food	[106]
	Core-shell CuNP	Antenna	High-performance and anti-oxidant	-	-	-	-	Printing technology	[107]
Carbon-based	rGO conductive pattern	Antenna,	Mechanical durability; high electrical conductivity	-	-	-	-	Printing technology	[108]
	SWCNT film (buckypaper)	Antenna, sensing material	Gas-sensitive; isotropic conductivity; good mechanical strength; flexibility	NH3	-	-	-	Gas sensing	[109]
	MWCNT	Antenna, sensing material	Gas- and temperature-sensitive; lightweight	CO2, temperature	-	-	-	Temperature sensing, Gas sensing	[110]
	GO film	Antenna	Humidity-sensitive	Humidity	30–99%RH	6.25 MHz/%RH	30%RH	Humidity sensing	[111]
	Graphene	Sensing material	Biochemical-sensitive	Ethanol, KCl, Staphylococcus aures	1–5 ppm, 0.008–1 M, 0–1.0 × 106 CFU/mL	-	1 ppm, 0.0009 M, 105 CFU/mL	Healthcare	[112]
	Graphene oxide	Sensing material	Humidity-sensitive	Humidity	-	-	-	Agriculture	[113]
Composite	Pt-decorated rGO	Sensing material	H2 gas-sensitive	H2	1–100 ppm-	-	1 ppm	Gas sensing	[65]
	Ag-rGO	Sensing material	NH3 gas-sensitive	NH3	5–100 ppm	1.25%	5 ppm	Gas sensing	[96]
	PANI-incorporated P-GO	Sensing material	NH3 gas-sensitive	NH3	2–200 ppm	15.9	1 ppm	Gas sensing	[114]
	rGO/Ag-ink	Antenna, sensing material	NH3 gas-sensitive	NH3	0–200 ppm	-	-	Gas sensing	[115]
	ZnO/CuO/rGO	Sensing material	CO2 gas-sensitive	CO2	500–1600 ppm	0.009 ppm/dB	500 ppm	Gas sensing	[116]
	Fe2O3 hollow nanoparticle/PANI:PSS	Sensing material	NO2 gas-sensitive	NO2	1–100 ppm	-	0.5 ppm	Gas sensing	[117]
	ZnO/MoS2/rGO	Antenna, sensing material	Temperature sensitive	Temperature	0–200 °C	1.46 °C/dB	-	Temperature sensing	[118]
	Graphite-polyurethane	Sensing material	pH-sensitive; stretchable	pH	0–14 pH	11.13 ± 5.8 mV/pH	-	Bio sensing	[119]
	Ag ink/CNT	Sensing material	Strain-sensitive	Strain	0–50%	-	-	Healthcare	[120]
	SnO2/MoS2/RGO, ZnO/MoS2/RGO, TiO2/Co3O4/RGO, In2O3/CuO/RGO	Sensing material	Humidity-sensitive; temperature-sensitive; light-sensitive; gas-sensitive	Humidity, temperature, light, CO2	10–50% RH, 5–25 °C, 700–1900 lux, 800–1600 ppm	2 MHz/%RH, 0.63 dB/°C, 0.02 dB/Lux, 0.012 dB/ppm	-	Agriculture	[121]
Others	Aluminum nitride thin film	Sensing material	Temperature-sensitive	Temperature	20–120 °C	64 ppm/°C	-	Temperature sensing	[120
	Cu-doped ionic liquid	Sensing material	Temperature-sensitive	Temperature	8 °C (threshold)	-	-	Temperature sensing	[122]
	Xerogel-based Cu electrode	Antenna	High pattern resolution; mechanical stability	-	-	-	-	Printing technology	[123]
	MXene/PdNPs	Antenna, sensing material	Decent printing resolution; conductivity; mechanical robustness; gas-sensitive	Ethylene	0.5–20 ppm	-	0.084 ppm	Gas sensing	[124]
	Aerosolized Ag/MWCNT nanocomposite	Antenna	Mechanical durability; strain-sensitive	Strain	0–75°	-	>20%	Strain sensor	[125]
	MXene ink	Antenna, sensing material	Large single-layer ratio; narrow flake size distribution; metallic conductivity	Temperature, humidity	20–55 °C, 20–80%RH	-	-	Temperature sensing, humidity sensing	[126]
	PAM hydrogel	Sensing material	Wide range of elastic moduli; bio-analytes-sensitive	Pressure, sweat, temperature	0–120 kPa, 0–70 mg/dL, 3–6 pH, 25–45 °C	-	-	Healthcare	[127]
	TiO2	Sensing material	Humidity-sensitive	Humidity	0–100%RH	-	-	Agriculture	[128]

. The introduction of nanomaterials improves the performance of IC chips and antennas, evolving RFID systems into effective communication systems with high throughput and efficient data transmission. Moreover, the use of flexible substrates and various sensing areas configured for wider applications enables the development of next-generation sensor systems that are user-friendly and achieve high sensitivity.

Figure 3. (a) The structure of RFID-based wireless sensing tag and (b) its operating principles. Each component of the sensing tag: (c) the substrate [69], (d) the IC chip [71], (e) the sensing area [80], and (f) the antenna pattern [91].

Figure 3. (a) The structure of RFID-based wireless sensing tag and (b) its operating principles. Each component of the sensing tag: (c) the substrate [69], (d) the IC chip [71], (e) the sensing area [80], and (f) the antenna pattern [91].

4. Nanomaterials for RFID-Based Sensing System

RFID-based sensing devices achieve wireless detection systems capable of effectively sensing various external stimuli, relying on a variety of nanomaterials. Polymers are predominantly used as substrates due to their cost-effectiveness and ease of processing. Conducting polymers, in particular, garner attention as sensing materials because they can indicate activity for specific substances. Metal nanoparticles and carbon-based materials are used as conductive inks, applied in patterns to RFID components such as antennas and sensing materials. Furthermore, various materials with different compositions and structures are being researched for the advancement of RFID sensors.

4.1. Polymer

Polymers are favored materials for the main substrate of sensor tags. Commonly used polymers include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), polyimide (PI), polycarbonate (PC), polydimethylsiloxane (PDMS), and more [129]. For example, Zhang et al. fabricated a RFID sensor based on the cost-effective and mechanically and thermally superior PET substrate [96]. The PET surface was activated by oxygen plasma treatment, and the formed hydroxyl and carboxyl groups facilitated easy drop casting. This resulted in a RFID sensor based on PET that maintained mechanical stability even when bent and preserved integrity in environmental conditions (Figure 4a).

Polymers not only serve as substrates but also provide an environment where other elements such as sensing materials, antennas, and IC chips can operate optimally. For instance, Marchi et al. investigated the impact of the substrate’s dielectric constant on the sensitivity of sensing materials for sensor performance optimization (Figure 4b) [97]. A polymer substrate with a high dielectric constant is ineffective in low humidity conditions, which reduce sensor sensitivity. Conversely, in high humidity environments, a polymer substrate with a high dielectric constant proves to be more effective.

Table 2. Frequency shifts and signal amplitude variation of humidity sensor based on dielectric substrates [97].

Substrate	Dielectric Constant	RH Low [%]	RH High [%]	Frequency Shift [MHz/%]	Δ|S21|min [dB/%]
DiClad	2.33	15	70	3.909	0.012
RO4003C	3.88	15	75	2.683	0.018
FR4	4.6	10	65	3.454	0.017
RO3010	11.2	10	90	1.562	0.05

shows the frequency shift and signal amplitude variation values of humidity sensors composed of substrates with different dielectric constants. The dielectric constant of the substrate influences the sensor’s sensitivity by varying its mechanism under different conditions. Furthermore, the substrate’s dielectric constant affects factors such as impedance matching systems, antenna bandwidth, and overall readout efficiency of a RFID system [130]. Therefore, careful consideration is required in the selection of polymers due to their properties, such as dielectric constant, that influence the sensitivity and communication performance of the RFID sensor.

When designing tags, the influence of the sensing process on the polymer substrate must be taken into account. For example, Belsey et al. observed the state of PDMS based on exposure to solvent vapors. PDMS serves as the substrate for vapor sensors, and by printing an ink solution of silver, a conductive loop is formed (Figure 4c) [98]. During gas detection, PDMS expands mechanically and forms cracks in the printed silver loop in the presence of solvent vapors. However, upon solvent removal, PDMS contracts, and the silver loop recovers, demonstrating reversible characteristics. Thus, PDMS proves to be a suitable substrate for gas detection environments. Therefore, while numerous polymers exist for sensing-tag substrates, a careful selection of polymers considering their interaction with other elements is essential for withstanding harsh sensing environments.

Conducting polymers can exhibit various properties due to the conjugation structure of their chains. In particular, the increased density of charge carriers in the polymer chain structure not only enhances electrical conductivity but also allows for high sensitivity to specific compounds by utilizing diverse monomer chemical structures and additional dopants. Commonly used conductive polymers in RFID sensor systems include polyaniline (PANI), polypyrrole (PPy), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), polythiophene, and poly(p-phenylene vinylene) [131].

The high electrical conductivity of conductive polymers enables the fabrication of antennas for sensing tags. Guerchouche et al. designed a compact bandwidth antenna for UHF RFID using PEDOT:PSS (Figure 5a) [101]. Despite having lower conductivity than copper, organic polymer-based antennas demonstrated readable distances of up to 75 cm, indicating sufficient performance as antennas. Additionally, these antennas were printed as films on transparent glass substrates, providing optical transparency useful for next-generation displays.

Conducting polymers can selectively respond to specific analytes and are widely used as detection materials due to the ease of adjusting selectivity by controlling various parameters. Nanostructures of conductive polymers, in particular, enhance effective detection of target substances by increasing the interaction sites with the analyte due to a high surface-area-to-volume ratio. For instance, Jun et al. utilized carboxyl-functionalized polypyrrole (C-PPy) nanoparticles as the active material for a sensing tag for detecting ammonia gas (Figure 5b) [64].

While conducting polymers can respond to external environmental stimuli, their resistance changes are relatively small compared to other materials. To overcome this limitation, conducting polymers can form heterojunctions with their opposite semiconductor materials to enhance sensitivity. Tanguy et al. improved the sensitivity of a RFID sensor by introducing n-type phosphonium-functionalized reduced graphene oxide (P-rGO) into p-type semiconductor PANI, forming localized heterojunctions (Figure 5c) [114]. The p-n heterojunction induced a rapid resistance change in the material by altering the depth of the depletion layer, depending on sensing conditions, enhancing the sensor’s performance.

4.2. 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]. Luo 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 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 °C, the conductivity was 0.206 × 10⁷ S/m, which was higher than 0.192 × 10⁷ S/m at 170 °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, Sahu 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, Larpant 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, Karuppuswami et al. implemented RFID sensing by designing a smart vial for dextrin detection and controlling the surface charge of gold nanoparticles (Figure 6c) [106]. Specifically, in samples with a high concentration of E. coli, a decrease in the number of gold nanoparticles attached to the smart vial was observed, resulting in a reduction in resonance frequency shift.

Furthermore, Wang et al. manufactured a stable copper printing paste by controlling the structure of copper nanoparticles (CuNP) through nanostructure control as a replacement for silver paste (Figure 6d) [107]. Surface treatment and heat treatment using formate ligands (FA) were performed to uniformly produce the shell layer of copper particles. The controlled sintering time and temperature in this process improved conductivity. The RFID pattern antenna made from copper paste with added micron-sized copper flakes demonstrated an excellent identification distance of 6.0 cm and superior performance. Additionally, the antenna made through printing showed excellent durability, remaining undamaged against bending and twisting.

4.3. 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. Song 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. Lv 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. Occhiuzzi 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. Habib 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. This passive RFID tag demonstrated the ability to transmit 5 bits of data and effectively function as a sensor without electronic components. Although its performance, such as electrical conductivity, stable ink formation, and relatively low sensitivity, still needs to be improved, carbon materials are expected to enable the low-cost and easy implementation of next-generation RFID sensor tags with flexibility and excellent durability.

4.4. 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. Lamanna 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. Vivaldi 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 °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. Wang 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.

MXene’s high conductivity, hydrophilicity, and dispersibility make it suitable as ink in the field of printed electronics. Li 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.

Min 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]. Aerosolization, achieved by using compressed air’s excitation and purging, allows the rapid deposition of nanomaterials without chemical bonding or heat treatment, enabling direct and simple printing. The printed composite nanomaterial demonstrated transparency, flexibility, and effective functionality as a RFID-based sensor.

5. Applications

RFID-based sensing systems formed using various materials can be used in various fields as follows in real life.

5.1. Gas Sensing

Invisible harmful gases, which affect not only human respiratory systems but also the overall metabolism, need to be detected swiftly, even in trace amounts. RFID sensor systems enabling low-cost wireless detection of hazardous substances utilize various nanomaterials to effectively detect gas molecules [142]. Lee et al. implemented a wireless hydrogen gas detection system by introducing Pt-decorated reduced graphene oxide (Pt-rGO) nanocomposites into the antenna of a UHF-RFID tag (Figure 9a) [65]. When the sensor is exposed to gas, Pt particles adsorb hydrogen gas molecules, causing electrons to move to rGO, leading to a change in material resistance. The change in antenna resistance causes impedance mismatch, allowing the RFID system to identify the detection. Pt_rGO firmly fixed on a flexible plastic substrate exhibits significant mechanical durability and demonstrates high detection capabilities for hydrogen gas without an external power source.

Miao et al. used a nanocomposite composed of three components, ZnO, CuO and rGO, to effectively detect carbon dioxide (Figure 9b) [116]. The formation of rod-shaped ZnO and petal-shaped CuO composites increases adsorption sites for gas molecules, and the introduction of rGO with excellent electrical conductivity enhances the sensitivity of the detection material. The three-layered structured composite material shows a sensitivity of 0.009 ppm/dB, a response time of 29.1 s, and exhibits repeatability and stability, effectively detecting carbon dioxide at room temperature.

Kim et al. created a detection pattern for nitrogen dioxide using multi-dimensional and porous Fe₂O₃ nanoparticles produced through dual nozzle electrospinning (Figure 9c) [117]. A composite paste, produced by introducing PANI:PSS as a conductive matrix, is incorporated into the antenna pattern, demonstrating detection performance over a wide range of NO₂ gas concentrations from 0.5 ppm to 50 ppm. The selection of various nanomaterials, structural control, and composite formation enhance the sensitivity of the RFID system’s detection material, enabling effective detection.

5.2. Environmental Monitoring

Monitoring the quality of air, water, and other environmental factors is crucial to protect human health and life from external harmful elements [143]. RFID systems are widely utilized in applications related to this field because they can wirelessly and in real time collect and report continuous information about the environment.

High humidity can generate harmful substances like mold, leading to the corrosion of structures and adversely affecting human health [144]. Additionally, rapid temperature changes can cause expansion or contraction of structures, resulting in damage and causing conditions such as dehydration or hypothermia that hinder normal human body function [145]. From an inventory management perspective, uncontrolled temperatures can degrade the quality of goods transported and stored in large quantities [146]. RFID tags can analyze abnormal behavior in temperature and humidity by collecting real-time information from a distance.

Xue et al. proposed a chipless RFID humidity sensor based on paper with high sensitivity and quality factors (Figure 10a) [111]. To optimize resonance characteristics, a toroidal-shaped inductor-interdigitated electrode capacitor and double copper-coated paper with a significantly shifted resonance frequency were employed in the fabrication of the RFID humidity sensor. The moisture-sensitive GO-coated electrode achieved a detection range of 60%RH to 90%RH and an improved sensitivity of 6.25 MHz/%RH. This is attributed to the synergy between the moisture absorption capacity of the paper substrate itself and water molecule adsorption in GO.

Miao et al. fabricated an ultra-high speed and high-sensitivity RFID temperature sensor using ZnO/MoS₂/RGO nanocomposite material (Figure 10b) [118]. Introducing MoS₂ with a large surface area and rGO with high electrical conductivity to ZnO, a well-known temperature-sensitive material, resulted in a nanocomposite with excellent temperature detection characteristics. The temperature-sensitive microstrip antenna, fabricated using thermal transfer technology, demonstrated outstanding temperature detection performance with a linear relationship in the range of 0 to 200 °C, a recovery time of less than 10 s, and temperature hysteresis of less than 0.34 °C.

Shao et al. produced a functional flexible electronic device capable of detecting both temperature and humidity by printing it with additive-free MXene aqueous ink at room temperature (Figure 10c) [126]. The fabricated RFID sensor undergoes changes in electrical conductivity as the PDMS substrate expands with increasing temperature, stretching the MXene conductive network. Moreover, exposure of MXene to humidity causes swelling in the interlayer structure of the nanosheets, altering electrical conductivity and enabling humidity detection. The flexible sensor, printed using printing technology, effectively detects temperature and humidity when attached to plants or the human body.

5.3. Healthcare

Measuring physiological signals and automatically diagnosing them, mobile healthcare that provides medical services freely without being constrained by time, space, or location is currently attracting significant attention. Wireless sensors collect the patient’s bio and environmental information, allowing real-time and organic feedback to the overall healthcare system [147].

Xu et al. proposed a wireless sensor tag capable of analyzing gases, ions, bacteria, and more [112]. As shown in Figure 11a, when the concentration of the analyzed substance exceeds the threshold marked by the red line, the smartphone recognizes it and can read the corresponding value. By varying the resistance of the rheostat within a single tag, various threshold values can be set without the need for multiple tags. Graphene-coated carbonic interdigitated electrodes grown by chemical vapor deposition (CVD) were used for ethanol detection. The graphene exposed to the target gas undergoes a change in impedance by physically adsorbing gas molecules. Potassium chloride ions and Staphylococcus aureus bacteria were detected through gold and platinum electrodes, respectively. The sensor demonstrated low limits of detection (LOD) of 1 ppm for ethanol, 0.009 M for KCl, and 105 CFU/mL for bacteria, showing excellent linearity and sensitivity, effectively detecting various biochemical substances.

Glucose monitoring is recognized as a powerful tool for early diagnosis of diabetes. Gao et al. proposed a biosensor for non-contact detection of glucose concentration based on a RFID system [148]. The sensor tag is fabricated by integrating a capacitor produced by Cu plating and a PDMS microchannel providing excellent biological characteristics and compatibility onto a glass substrate. The tag exhibits variations in permittivity based on the presence and concentration of glucose, altering the effective capacitance of the Cu-based squared spiral capacitor. Glucose, with its relatively higher molecular weight compared to water molecules, induces a viscous effect that interferes with the rotation of the AC field, resulting in changes in permittivity. Based on this mechanism, the reader induces reflection parameters, generating deviations in resonance frequency, thereby achieving glucose detection. The glucose sensor demonstrates larger shifts in resonance frequency with increasing glucose concentrations within the range of 25 mg/dL to 600 mg/dL, proving excellent linearity. The RFID-based bio-sensor tag, composed of a PDMS channel and a copper-based spiral capacitor, has successfully achieved non-contact glucose detection. Ongoing research is actively exploring further advancements in glucose detection.

Dang et al. developed a wearable sweat pH monitoring sensor that can maintain contact even with human movements (Figure 11b) [119]. Since sweat contains valuable information such as glucose, lactate, and cortisol, reflecting the intensity of physical activity and dehydration levels, its analysis is highly beneficial for health assessment. The researchers manufactured pH-sensitive electrodes at low temperatures, selecting a graphite–polyurethane composite for flexibility and rapid responsiveness to analyze sweat. This led to effective pH monitoring with a high sensitivity of 11.13 ± 5.8 mV/pH on a wearable substrate.

Dutta et al. implemented a sensor with LEDs attached to the resonator of an NFC sensor circuit, enabling intuitive understanding of the user’s state (Figure 11c) [127]. They structured hydrogel as an interlayer, advancing RF resonators to wearable applications without rigid materials. This device effectively reported the body’s response to pressure, sweat, and temperature changes as a soft sensor skin.

Niu et al.’s sensor system, bodyNET, comprehensively monitors human physiological signals through networking, utilizing RFID technology in the healthcare field (Figure 11d) [120]. They used a flexible polymer substrate made of poly(styrene-b-ethylene-butadiene) (SEBS), inductor patterns, and a flexible capacitor bottom electrode with silver conductive ink and CNT as the strain-detecting substance. By attaching the sensor to various positions on the body, they could analyze pulse, respiration, and body movement simultaneously and continuously. The bodyNET system, capable of securely adhering to the body and interpreting multiple signals comprehensively, is expected to contribute effectively to the next generation of personal health monitoring. The flexibility of components friendly to the human body and the analysis capability for various substances make RFID sensor systems even more promising in the healthcare field.

5.4. Food Fresh Management

RFID, which has strengths in inventory management, allows for convenient and effective monitoring of food quality [149]. Detecting food spoilage is crucial due to its potential consequences for human health and environmental issues. Monitoring of food involves the analysis of gases or bacteria occurring within the packaging [150].

Bibi et al. conducted research on food quality through carbon dioxide monitoring using the intriguing properties of wheat gluten, a polar substance, at high relative humidity (Figure 12a) [99]. Wheat gluten, when in contact with carbon dioxide at 90% relative humidity, serves as a detecting material by changing its electrical and dielectric characteristics, effectively integrated into a UHF RFID system operating at 868 MHz.

Karuppuswami et al. proposed a biosensor using Dextrin-capped gold nanoparticles (d-AuNP) as markers for quality management of milk (Figure 12b) [106]. The resonant frequency shifts in capacitive loading, depending on the bacteria count in contaminated milk, were measured to analyze the concentration of E. coli C3000. Detection through the attachment of conductive nanoparticles and cells enabled the development of a RFID sensor, demonstrating a low threshold of 5 log CFU/mL bacteria.

Ammonia is a harmful gas generated by bacteria during the protein-containing food spoilage process. Karuppuswami et al. applied highly conductive polymer PANI with an affinity for ammonia to a wireless sensor system (Figure 12c) [102]. The system, combining PANI thin films with high ammonia affinity with a RFID system, enabled wireless and long-distance monitoring of food quality, exhibiting the capability to detect small amounts of ammonia (3 ppm) at room temperature and atmospheric pressure. Ma et al. enhanced detection performance by adding p-toluene sulfonate hexahydrate (PTS) to PANI (PTS-PANi). As shown in Figure 12d, PTS-PANI formed a porous structure with a larger surface area and smaller diameter, demonstrating sensitivity to analytes [103]. This detection material successfully achieved detection not only for ammonia but also for actual biological amines such as putrescine and cadaverine, showing ΔR/R₀ values of 46% and 17%, respectively.

5.5. Agriculture

As the scale of farms expands, the need for effective management and supervision in agriculture has become increasingly essential. RFID sensor tags can detect various environmental parameters such as temperature, humidity, and light. Through their large-scale information processing capabilities, they enhance the efficiency of crop and environmental management.

Cappelli et al. developed a passive RFID sensor tag for soil moisture monitoring (Figure 13a) [128]. They first formed a thin-film circuit on a silicon substrate through lithography, sputtering, and lift-off processes to establish the foundation for measuring soil impedance and fringe capacitance. Subsequently, they coated the sensor with TiO₂ nanoparticles for humidity measurement. The functionalization using TiO₂ nanoparticles achieved significant sensitivity, especially at humidity levels exceeding 50%RH. This sensor operates independently of soil composition and effectively measures various soil moisture levels without the need for frequent calibration.

Reddy et al. designed a humidity sensor using Ag nanoparticles and poly (2-hydroxyethyl methacrylate) (pHEMA) (Figure 13b) [100]. The sensor, printed on a flexible PET substrate, introduced Ag nanoparticles as interdigitated capacitors. Following this, they deposited humidity-sensitive pHEMA on the interdigitated capacitors with a fixed thickness. This sensor exhibited excellent linearity in humidity measurements, ranging from 30%RH to 80%RH, with a maximum error range of 0.6 to 0.8%, demonstrating its stability. This sensor is expected to be actively used in the smart agriculture field.

Miao et al. implemented a multifunctional sensor capable of detecting humidity, temperature, light, and CO₂ gas (HTLC) by combining various nanomaterials (Figure 13c) [121]. Composites composed of pure SnO₂, MoS₂, ZnO, TiO₂, Co₃O₄, In2O₃, CuO, and RGO solutions were manufactured using a thermal method. By combining these materials, the researchers created sensors with specific characteristics, such as SnO₂/MoS₂/RGO for humidity detection, ZnO/MoS₂/RGO for temperature sensitivity, TiO₂/Co₃O₄/RGO for light intensity sensitivity, and In₂O₃/CuO/RGO for CO₂ gas sensitivity. Each environmental parameter was measured at separate frequencies or amplitudes, ensuring no interference. The HTLC multidimensional sensor based on RFID systems is expected to effectively monitor complex agricultural environments.

Li et al. utilized graphene-based films to construct an effective system for agriculture information monitoring (Figure 13d) [113]. They proposed a system capable of real-time collection and transmission of diverse and large amounts of data through wireless networks of various sensors. Among the numerous sensors, those utilizing graphene active films demonstrated non-destructive and accurate monitoring of leaf moisture. The high surface area of graphene effectively increased the monitoring sensitivity of the sensor. Consequently, the real-time and mass information processing capabilities of RFID systems contribute to the systematic management of the agricultural field, where monitoring of various environmental variables is crucial.

5.6. Others

In addition, RFID sensor systems are widely utilized in various fields such as stock management, railway, shipping, electronic toll systems, automotive, and aerospace. With the expanding applications of RFID technology, there is an increasing demand for efficient measurement of physical and chemical parameters, as well as diverse information collection and processing in different environments. Consequently, there is a growing movement to enhance the performance of RFID components using nanomaterials. As an example, Machiels et al. suggested an efficient inventory management through the implementation of smart packaging using a paper substrate and highly compatible silver ink [151]. They facilitated the potential of smart boxes capable of effectively managing large inventories by connecting high-frequency RFID chips to the screen-printed conductive adhesive on paper. Mejias-Morillo et al. attempted to improve the performance of passive wireless sensors used in the space industry through Kapton wrapping of the antenna conductive surface [152]. As a result, the sensor achieved a detection range of up to 3.2 m and a mean measurement error of less than 1%. These efforts would lead to broader applications of RFID technology. Additionally, with the increasing demand for user-friendly electronic devices, research is actively progressing to advance RFID sensor components, ensuring their superior performance and stable operation even in wearable environments. Tekcin et al. reported on the impact of the sintering process on RF antenna performance after printing conductive ink using the pad printing method [153]. The sintered printed sensor enabled wireless data transmission at shorter range and demonstrated stable operation in wearable and moisture-sensitive environments. In this way, research is actively underway to implement diverse nanomaterials and manufacturing methods for RFID systems to be applied in various fields.

6. Summary and Perspectives

The RFID sensor system is a type of wireless communication technology that detects specific stimuli from the external environment remotely and responds to them. By selecting appropriate materials for each component and processing them in various ways, improved communication functionality and detection performance can be achieved (Figure 14). Polymers can successfully implement flexible and elastic substrates, and nanomaterials such as conductive polymers, metal nanoparticles, and carbon-based substances can be produced with conductive inks for effective integration into the system through printing. These materials can be used as functional patterns that replace various components of RFID systems, including sensing materials, antennas, and IC chips. The introduction of each component through printing is expected to make future RFID sensor manufacturing more convenient and efficient.

The necessity for research on nanomaterials to advance RFID as an efficient communication system and sensor continues to grow. In the case of the substrate, which is the basis for other components, it is required to design it considering the effects of polymer properties, such as dielectric constant and mechanical properties, on other components. With the expansion of the wearable market, achieving not only excellent flexibility but also transparency is expected to broaden the applications of RFID sensor systems. In addition, it is necessary to improve the properties and performance of metal nanoparticles, which provide excellent electrical properties, while developing carbon-based materials to replace them at a relatively low price. As there is an increasing demand to produce RFID components using relatively simple and cost-effective inkjet printing, research on improving the viscosity, conductivity, resolution, and performance of the inks is essential.

The introduction of nanomaterials has transformed RFID into a more effective sensor system. Currently, RFID sensor tags are being applied across various fields, effectively monitoring gas, temperature, humidity, human sensing, food quality, and agriculture. Ongoing research will guide RFID towards achieving enhanced performance, including a broader operating range, superior data transmission capabilities, and excellent sensitivity to external environments, enabling its diverse applications. In the future, the RFID system is anticipated to be more widely utilized as a smart and user-friendly next-generation sensor in the Internet of Things (IoT) environment.