**4. Metamaterials in Microwave Sensors**

Nowadays, the characterization and quantification of liquids have become crucial in different fields such as biomedical engineering, agriculture, pharmaceutical, etc. [192–195]. In general, the characteristics of various liquids can be analyzed with the help of their polar nature and specific electrical properties. Moreover, the performance of the microwave devices is greatly influenced by the electrical properties of the liquids. Further, the interaction of the electromagnetic radiation with the polar liquid materials led to variation of the direction of the polarization for different molecules. Especially, microwave sensors employ that kind of interaction to manipulate the dielectric properties of the liquids for characterization. Due to simple procedure, non-invasiveness and quick response, the electromagnetic approach-based sensors offer several advantages than the normal ones. Further, owing to their unique properties the electromagnetic metamaterials gained much attention in the advancement of the electromagnetic devices over last two decades. Recently, the designs based on electromagnetic metamaterials have been employed for microfluidic sensing applications [196,197]. These sensors exhibit high sensitivity and strong interaction between the analytes and electric field. However, the requirement of large sample volume is one of the shortcomings of the microfluidic sensors. This can be mitigated by using unit cell resonation structures in microfluidic sensors [198,199]. Still, achieving the significant sensitivity in microlevel is a major problem. To overcome this, numerous researchers are putting their continuous efforts in designing the miniaturized sensors with significant sensitivity and selectivity for consistent characterization of a liquid. In continuing this, Xu et al. [200] demonstrated a lightweight, low cost, portable, biocompatible, and flexible metamaterial (metaflex) based photonic device for the biological and chemical sensing and high sensitivity strain applications. The device can be operated invisible as well as IR regions. The proposed device consists U-shaped SRRs of 30 nm thick gold (Au) or silver (Ag) which are deposited on poly(ethylene-naphthalate) substrate with the help of electron beam lithography [201]. In addition, the U-shaped SRRs metamaterials display an electric resonance of 542 nm and magnetic resonance of 756 nm. Both magnetic and electric resonant modes give highly sensitive response to surrounding dielectric media, bending strain, and surface chemical environment. Owing to coupling of electric and magnetic fields, the proposed metamaterial-based photonic device shows greater response to nonspecific bovine serum albumin protein binding with a shift of magnetic resonance of 4.5 nm. It also shows superior response for self-assembled monolayer of 2-naphthalenethiol with a shift of magnetic resonance of 65 nm. These outcomes suggest that the proposed device is a prominent candidate for chemical and biological sensing. Microwave metamaterials are electromagnetic devices which are synthesized to control microwave fields. These materials can be used as future healthcare systems that can overcome technical restrictions

after they are interfaced with human body. John S. Ho et al. [202] studied the working principles and applications of microwave metamaterials for biomedical sensing. Further, Kayal et al. [203] demonstrated a compact microwave sensor with the help of mu negative (MNG) metamaterial for liquid characterization. They reported that the prepared sensor exhibits high sensitivity along with noticeable compactness. The square spiral metamaterial (MNG) plays a significant role in accomplishing this sensitivity in small cross-sectional area as well as notable compactness. In addition, the sensing behaviour of the prepared device is confirmed through least square technique followed by development of two nonlinear equations for calibration purpose. These nonlinear equations (Equations (9) and (10) of Ref. [203]) are useful for finding the permittivity of the unknown samples. Hence, the compactness and high sensitivity of the prepared sensor make it a prominent candidate for liquid sensing applications.

In addition, the electromagnetic waves in tera Hertz (THz) region shows sensitive responses to intra and intermolecular vibration modes and low photon energy (few meV) [204,205]. Due to this the THz electromagnetic waves display potential applications in bio-sensing, microscopy, and spectroscopy. In THz region, with the help of tens of metamaterials and graphene, Xu et al. [206] introduced a platform for bio molecular sensing. Further, by using graphene assisted nano metamaterials, Lee et al. [207] demonstrated a label free sensing technique for discrimination of single-stranded deoxyribonucleic acids (ssDNAs) in THz region. The combination of unusual properties of metamaterial and electrooptical properties of graphene provides biomolecule sensing property even using THz photons with very low energy. Additionally, they reported that the enhancement of THz field at resonance frequency causes the rise in absorption cross section of the graphene sheet which in turn provides ultrahigh sensitivity. The sensing mechanism includes the direct transfer of graphene onto a nano-slot metamaterial and tightly binding the targeted DNA molecules without modifying the structure. Here, the nano-slot metamaterials enhance the THz transmittance which is proportional to the absorption cross-section of DNA adsorbed graphene layer. So, greater number of DNA molecules can be observed through the strongly focused THz electromagnetic waves. Further, adsorbed molecules change the intrinsic electrical properties of the graphene which can easily be detected. This mechanism is allowed to sense of different biomolecules. Specifically, considering suitable receptor to capture DNA molecules followed by rapid primary screening, finally applied for sequencing the DNA. Hence, the graphene assisted THz metamaterial sensing platform is suitable for biological sensing applications also understanding the electro-optic behaviour of 2D materials.

The sensitivity of the microwave sensor can be enhanced by coupling the transmission lines with metamaterial based open loop resonators [208]. However, during coupling the resonator with transmission line there exist shift in resonant frequency which strongly affects attain high sensitivity. To overcome this, Abdolrazzaghi et al. [209] proposed a novel metamaterial based planar microwave sensor which can be operated at 2.5 GHz. They prepared the above proposed sensor by coupling negative refractive indexed metamaterials with transmission lines which exhibits greatly improved resonant properties [210]. Afterwards, they developed the signal flow analysis to estimate the transmission response of the prepared sensor. In comparison with the microstrip or conventional sensor the proposed sensor shows very high sensitivity along with large complex permittivity. They reported that the proposed sensor displays superior properties, particularly in water host medium and high permittivity materials. The concentration measurements of the methanol or ethanol in water medium reveals the outstanding performance of the proposed sensor over conventional sensor. Hence, the proposed metamaterial based planar microwave sensor is useful for characterization of high permittivity materials, highly sensitive concentration measurement of methanol or ethanol in water and also biomolecule detection.
