Detecting and characterizing various biomolecules have been extensively researched in recent years^[1–5]. Biochemical detection technology with high specificity, sensitivity, and precision is critical in a global disease epidemic, which not only involves the medical and public health security of disease diagnosis, treatment, and prevention but also has vital applications in pharmaceutics, biological monitoring, national security, and even military fields. In the identification of biomolecules, one of the most important is the detection of chirality. A substance has chirality if its mirror image cannot overlap with itself through basic rotations and translations, as shown in Fig. 1(a). The qualitative and quantitative analysis of chiral biochemical substances with complex stereochemical structures plays an essential role in detecting biochemical samples^[6–10]. More than 60% of drugs and 25% of pesticides are chiral compounds. Chiral characteristics significantly affect the function of biochemical substances. Chiral isomers have the same chemical composition but may have completely different physical and chemical properties and biological activities, and even a chiral molecule has a therapeutic effect on diseases, while its isomers are toxic. Therefore, accurate and efficient detection of biochemical substances, especially the identification and quantitative analysis of chiral biochemical substances, has practical application needs and is of crucial scientific significance.

Currently, the optical characterization and detection methods for chiral substances primarily utilize infrared vibrational circular dichroism (VCD), visible electronic circular dichroism (ECD), and Raman optical activity (ROA) spectra, shown in Fig. 1(d)^[11]. These techniques are also integrated with complex analytical methods such as X-ray diffraction, nuclear magnetic resonance spectroscopy, and mass spectrometry to achieve a comprehensive understanding of these substances. However, the collective vibration mode of biomacromolecules is located in the terahertz (THz, 1THz=1012Hz) band^[12–16], and there is often no discernible fingerprint specificity in the chiral spectrum within the visible light band. Consequently, many biochemical substances cannot be specifically identified using existing vision-infrared spectroscopy technology, and it is necessary for the development of new chiral spectroscopy technology operating in the THz band to address these issues^[17–22].

The THz wave is located between microwave and infrared waves, which has the characteristics of low photon energy, non-ionization, wide band, and strong coherence. Traditional THz biochemical sensing is mostly combined with the THz absorption spectrum^[23]. However, the progress of this technology is primarily impeded by two factors. First, conventional THz absorption spectroscopy enables the measurement of single-polarization amplitude and phase changes in the THz wave band following interaction with a sample. It is difficult to capture the complete polarizations and chirality that reflect crucial structural information of biochemical substances. Second, when dealing with bioactive substances dissolved in aqueous solutions, the strong absorption of THz waves by water molecules poses a challenge for obtaining characteristic spectra using traditional THz absorption spectroscopy^[24–31].

To acquire structural information in biochemical samples, THz polarization/chiral spectrum technology has emerged as an effective approach. It is well known that distinct polarized light exhibits diverse spatial electromagnetic chirality, as illustrated in Fig. 1(b). Chiral molecules exhibit different spectral responses when subjected to different chiral light fields, thereby serving as a crucial means for characterizing the structure of biochemical molecules^[32]. For example, THz chiral spectra have been successfully employed for the detection of chiral enantiomers of biochemical substances, as depicted in Fig. 1(c)^[23]. However, the observed difference in chirality remains very weak. To enhance the chiral response and the interaction between chiral matters and chiral electromagnetic fields^[33–39], it is imperative to generate a robust chiral light field in the THz band, which can be effectively achieved through metamaterials. Chiral metamaterials and active chiral regulation techniques have emerged as prominent research areas in recent years^[40–42], employing various structural designs such as multiple helices and multilayer metamaterials to produce intense chiral light fields^[43,44]. Additionally, by incorporating phase change materials and electric drives^[45,46], active chirality control of microstructures has been accomplished. Despite significant advancements in chiral metamaterials and regulatory methods, their application for detecting chiral substances remains limited. The interaction of the metamaterial chiral light field with chiral matter is expected to be a new technique for detecting the chirality of matter.

Currently, the existing THz biochemical detection methods, whether based on traditional THz absorption spectroscopy or utilizing metamaterial sensors, necessitate a certain sample thickness for measurement. However, the detection sensitivity remains weak, posing novel challenges in trace sample detection. In this context, employing specific sensing technology based on metamaterials emerges as an effective approach to address this issue^[47,48]. Metamaterials play a pivotal role in this aspect, offering two significant advantages. First, the utilization of a strong local field in metamaterials can enhance the interaction of THz waves and samples^[49–60] and facilitate high-sensitivity and high-precision sensing of trace substances. Second, by integrating metamaterials with biochemical modifications and functional nanomaterials like graphene and gold nanoparticles, it becomes feasible for achieving targeted detection of minute samples.

In summary, in recent years, significant breakthroughs have been achieved in the two crucial aspects: the identification of chiral enantiomers of biochemical substances and the specific enhancement of trace sample detection. These advancements are pivotal for revealing the interaction mechanism between THz waves and biochemical molecules, as well as for developing THz biochemical sensing technology. In this review, we summarize the advances achieved in THz chiral and special detection of biochemical substances, aiming to develop THz chiral spectroscopy and specific sensing theory. Specifically, the review is structured into three sections, beginning with an introduction to THz sensing systems, polarization sensing methods, and metamaterial sensors. Subsequently, the following two parts delve into a detailed discussion of research progress in chiral enantiomer recognition and specific sensing of biochemical molecules, summarizing the methods employed in these works as well as their significant conclusions. A final section is presented at the end to summarize the main conclusions and prospects of THz biosensing.

In general, THz time-domain spectroscopy (THz-TDS) techniques remain the primary methods for THz spectroscopy and sensing characterization, illustrated in Fig. 2(a)^[61]. The femtosecond laser is divided into two beams after passing through the beam-splitting lens. One laser beam excites the GaAs photoconductive antenna to generate THz waves. After being reflected by four off-axis parabolic mirrors, the THz wave meets with another femtosecond laser in the optical delay path on the ZnTe crystal. After passing through the quarter-wave plate and Wollaston prism, the femtosecond laser is sampled and detected by the differential photodetector. The optical delay path is controlled by a precise translation stage, through which the time-coherent position of the THz wave and laser pulse is changed to detect the THz signal in the time domain. The standard THz-TDS system is modified to the THz time domain polarization spectroscopy (THz-TDPS) system to obtain polarization characteristics. In the THz-TDPS system, two THz polarizers are added in the front of the incident wave and the back of the output wave, which can control the polarization direction of the incident wave and detect the complete polarization state of the emitted wave. By rotating the polarizers, a pair of orthogonal linear polarization (LP) signals can be detected, and the THz spectral information of any polarization state can be reconstructed by calculation.

In addition, the sensing system can be further divided into transmitted and reflected modes. In the transmitting system, the sensing sample usually has low absorption. It is generally placed in the focus position of the THz optical path for detection, which can enhance the interaction of the THz wave and the sample^[62]. Reflective sensing can be achieved by adding reflective modules to the THz parallel optical path of a standard THz-TDS system shown in Fig. 2(b)^[62]. In a reflected system, metal mirrors are used to reflect THz waves onto the surface of the sample pool. The THz wave reflected by the sample will convey the sample information. This method is usually used to measure samples with high absorption, which can effectively avoid the strong absorption of THz via liquid samples, especially aqueous solution, and has significant advantages in biosensing.

Traditional spectral detection based on the THz-TDS system uses LP light excitation, only THz signals corresponding to the polarization direction can be detected, and linear polarization transmission or reflection spectra of the samples are obtained by T(ω)orR(ω)=As(ω)/Ar(ω),(1)Δφ(ω)=φs(ω)−φr(ω),(2)where T(ω) and R(ω) are amplitude transmission and reflection spectra, respectively; As(ω) is the detection amplitude signal when the sample is added; and Ar(ω) is the reference transmission amplitude signal in the case without the sample, while in the case of the reflection, it is also the reference amplitude signal when using a metal mirror. Δφ(ω) is the transmission or reflection phase difference spectrum, and φs(ω) and φr(ω) are the phase signals of the sample and the blank reference signal, respectively.

According to wave optics theory, an LP wave can be decomposed into the superposition of a left-handed circular polarization wave and a right-handed circular polarization wave (LCP and RCP) with equal amplitude and phase. In addition, two LP waves with orthogonal polarization directions and a phase difference of π/2 can be synthesized into circular polarization (CP) waves. An orthogonal set of LP signals or CP signals can be used as basis vectors to describe any polarization state of the electromagnetic wave. Therefore, by excitation and detection of LP signals in two orthogonal directions, the detection of multiple polarization states can be realized through calculation. For example, take transmission sensing in the 0° and 90° orthogonal directions (i.e., the x- and the y-axis directions). Through experimental measurement, the amplitude and phase transmission spectra of the THz signal in two directions can be obtained by Fourier transform and, through calculation, the complete polarization state of the detected signal can be reconstructed and can be expressed as^[40](ExAx)2+(EyAy)2−2ExEyAxAycosΔφ=sin2Δφ,(3)where Ex and Ey are the x and y components of the electric field vector E of the detection signal, respectively, and Δφ=φx−φy. The components of LCP and RCP can be calculated by the following equation^[40]: TLCP(ω)=12(Ax(ω)eiφx(ω)−iAy(ω)eiφy(ω)),(4)TRCP(ω)=12(Ax(ω)eiφx(ω)+iAy(ω)eiφy(ω)).(5)

The polarization spectrum refers to the polarization elliptical angle (PEA) and the polarization rotation angle (PRA) spectra. For any polarization state, PEA and PRA can be used to characterize it completely, covering the whole Poincare sphere. They reflect the polarization state of the THz signal. The two parameters PEA ‘ε’ and PRA ‘ψ’ can be calculated as follows^[40]: sin2ε(ω)=sin2β(ω)sinΔφ(ω),(6)tan2ψ(ω)=tan2β(ω)cosΔφ(ω),(7)where tanβ=Ax/Ay. PEA is used to describe the ellipticity of the polarization state of the output wave. Positive and negative values reflect the chirality of the output wave. Positive values represent RCP, and negative values represent LCP, ranging from −45° to +45°; 0° corresponds to linear polarization, and ±45° correspond to circular polarization. PRA is used to describe the rotation angle of the output wave’s polarization direction, ranging from −90° to +90°. Positive values denote the clockwise direction, and negative values denote the counterclockwise direction.

In addition, if the polarization states of the incident wave and the detected wave are considered comprehensively, taking the transmission spectra in which excitation and detection are both x- and y-axis polarization directions as an example, the polarization conversion equation is written as follows^[42]: (T++T+−T−+T−−)=12[(Txx+Tyy)+i(Txy−Tyx)(Txx−Tyy)−i(Txy+Tyx)(Txx−Tyy)+i(Txy+Tyx)(Txx+Tyy)−i(Txy−Tyx)],(8)where “+” and “−” represent the RCP and LCP components, and “x” and “y” represent the linear polarization component in the x- and y-directions. The first quantity of the lower angular notation represents the direction of the output polarization state, and the second represents the direction of the incident polarization state. Among these transmission spectra, Txx, Tyy, Txy, and Tyx can be measured experimentally. Therefore, the polarization retention (co-polarization, T++ and T−−) and polarization transformation (cross-polarization, T+− and T−+) spectra of the CP transmission can be calculated. Furthermore, to adapt to the experimental system, the x- and y- axis directions can be changed into +45° and −45° orthogonal directions with similar calculation methods. Chirality in optics is often used to describe the difference in the response of a substance to LCP and RCP, mainly characterized by two aspects: circular dichroism (CD) and optical activity (OA). The chiral spectrum can be reflected by the CD spectrum and the OA spectrum, which are calculated as follows^[42]: CD(ω)=arctan(|T++(ω)|2−|T−−(ω)|2|T++(ω)|2+|T−−(ω)|2),(9)OA(ω)=12arg(T−−(ω)T++(ω)),(10)where ‘arg’ takes the complex angle of the complex function, CD represents the difference in transmission amplitude between the LCP and RCP waves, and OA represents the optical rotation characteristics of the sample and is the difference in phase angle between the LCP and RCP waves.

According to the different materials and structures, THz metamaterial sensors can be divided into different types. Figure 3 shows several different types of sensor structures with different sensing performances. Figure 3(a) shows the single-layer metal metasurface structure^[63,64], which was usually made by plating an Au film on the substrate and then patterning using conventional mask lithography. As the intrinsic losses of the constituent metals usually impose limitations, an emerging direction to circumvent such restraints is using all-dielectric metamaterials that have low optical loss and high refractive index, as shown in Fig. 3(b)^[65,66]. This type of metamaterials is often made of Si, SiO2, GaAs, and Ge using the reactive ion beam etching process. With the development of manufacturing technology, some flexible materials are also used to make metasurfaces to achieve wearable biosensors, as shown in Fig. 3(c)^[67]. This type of metamaterial usually uses flexible polyimide (PI) film as substrate. 3D printing technology can also be used to make metamaterials, and with the improvement of accuracy, more complex patterns and even 3D metamaterials can be manufactured, as shown in Fig. 3(d)^[68].

The THz polarization sensing method and metamaterial sensors are effective tools for THz biochemical sensing, providing high sensitivity, precision, and more information on the anisotropy and chirality of samples. The strong local field generated by metamaterials enhances the interaction between the THz waves and matter, offering irreplaceable advantages in trace detection. In this section, we summarize recent advances in the successful application of these two technologies: biochemical substances with polarization sensing and the enhancement of the chiral response of biochemical substances with the metasurface.

The circular polarization spectra of chiral isomers have fingerprint characteristics. Chiral molecules have different special responses to different chiral optical field distributions, which is one of the important methods to characterize molecular stereochemical structure. The circular polarization spectrum can be utilized for identifying chiral isomers. For instance, the CD spectrum of chiral isomers shows fingerprint properties, and it has been successfully used to identify the isomers in both the optical band and the THz band^[23,69]. In addition to the identification of the chiral isomer, the THz polarization sensing methods have been successfully applied in various fields such as protein physicochemical processes, cell proliferation, and virus recognition. These methods exhibit significant advantages in biochemical sensing. For example, our group achieved some progress in the investigation of the protein physicochemical process, as illustrated in Fig. 4. Specifically, it displays the thermal denaturation, hydrolysis, and crystallization of proteins. The physicochemical activity of proteins is typically manifested in solution to mitigate water absorption, and pertinent investigations are conducted employing the reflective THz-TDPS system. Zhang et al. conducted a flexible twisted dual-layer metasurface sensor for protein concentration sensing^[70]. The curves in Fig. 4(a) show the peak value and difference of the CP spectrum when the denaturation degree of the bovine serum albumin (BSA), the whey protein (WP), and the ovalbumin (OVA) protein solutions increases from 0% to 100%. Moreover, the CP spectra of the three proteins are different during thermal denaturation due to their different molecular structures.

In another work, as shown in Fig. 4(b), the resonance enhancement effect of a flexible chiral metasurface sensor was applied in the sensing of the BSA under the proteolysis of papain in a reflective polarization system^[71]. The sensor adopts a double-layer structure with the same pattern, but the structural direction is rotated by 60°. Because the mirror symmetry is broken, the sensor has strong chirality and brings a strong polarization response. It can be seen from the CP spectrum that the higher the concentration of papain, the deeper the degree of BSA hydrolysis. From the polarization ellipse, increasing the concentration will change the polarization state. This method can also detect the molecular structure changes of other proteins during hydrolysis. In addition, the crystallization process of the WP was characterized by five different polarization parameters^[72] performed by Zhang et al., shown in Fig. 4(c). The above results all show that the THz polarization sensing method has higher detection sensitivity and accuracy compared with traditional LP sensing.

THz polarization detection is not only suitable for the detection of proteins but also for the quantitative detection and qualitative identification of cell and virus species. For instance, Liu et al. demonstrated that aspirin’s inhibitory effect on cell proliferation was demonstrated by using a metal metasurface as a polarization sensor, as shown in Fig. 5(a)^[73]. The experimental results showed that aspirin with a certain concentration could inhibit cell proliferation and cause changes in polarization parameters of the THz wave output. This polarization state is susceptible to changes in the sensor surface, so we can detect changes in the number of cells on the sensor by detecting changes in the polarization state. The minimum cell concentration obtained by polarization sensing was 3.0×103cells/mL. In addition, the quality factor (Q factor) and figure of merit (FoM) are 4–5 times higher than that in traditional resonant sensing. Zhang et al. reached a similar conclusion in their study, as shown in Fig. 5(b). Different from the above, a reflected system combined with polarization sensing was used for the quantitative detection of cell concentration and was realized in a liquid environment^[74]. The utilization of a silicon subwavelength grating as a microstructure sensor enhances the optical response of the sample, resulting in a minimum detectable cell concentration of up to 104cells/mL.

Amin et al. presented a method for differentiating three viruses with similar optical properties using a graphene-modified metasurface combined with polarization sensing^[75]. As shown in Fig. 5(c), the electric field distribution presents opposite circular directions at two different frequencies, which indicates that the surface plasmon current has chirality. The graphene surface plasma provides high near-field enhancement, and the reflected THz wave polarization state changes rapidly with the sample refractive index (RI). For three very similar influenza viruses, H1N1, H5N2, and H9N2, due to the difference in the RI and extinction coefficient, the polarization state of the THz wave reflected by them will be changed to different degrees to determine the virus type.

The above studies both used the polarization sensing method to analyze the non-chiral biochemical substances. The results show that polarization sensing has the advantages of high sensitivity, high measurement accuracy, and wide application, and has rich detection dimensions compared with traditional resonant sensing.

As mentioned above, metamaterials can generate a strong local field that enhances the interaction between the THz waves and samples and amplifies the chiral response of matter, which provides a novel approach for recognizing chiral enantiomers of molecules. At present, this method has been successfully applied to identifying chiral enantiomers such as sugars and amino acids.

For example, Zhong et al. distinguished sugar isomers in trace doses via the THz polarization spectra and the chiral metasurface, as shown in Fig. 6(a)^[61]. The curves in the figure show that the peak frequency of galactose in the PEA and PRA spectrum is farthest from that of the reference curves. In the PRA spectrum, the polarization angle of lactose is always greater than that of glucose to qualitatively distinguish the three sugars. In addition, the polarization ellipses of the THz wave affected by three kinds of sugars also have significant differences at specific frequencies. The differences in these polarization parameters were used to selectively identify different sugars, including a pair of isomers.

In another work, Zhang et al. demonstrated the advantage of using a metasurface to identify chiral amino acid isomers, as shown in Fig. 6(b)^[76]. The polarization sensing method is combined with the all-dielectric metasurface to enhance the chiral response of the substance. The comparison of the CD spectra with and without the metasurface shows that the presence of the metasurface significantly enhances the chiral response of the substance. In the range of 0.6–1.46 THz, the CD spectra of tyrosine, cysteine, arginine, and their chiral enantiomers were amplified by the same metasurface, and the maximum enhancement was about 97 times. The CD difference of chiral isomers was as high as 34.22°. This indicates that metasurface sensors using chiral sensing methods have great potential in improving the chiral response of amino acids.

Liu et al. proposed a novel Pancharatnam–Berry (PB) metasurface that has been used to enhance the chiral response of substances^[77]. As shown in Fig. 6(c), the spin beam deflection and separation of the PB metasurface can effectively amplify the chirality response of the substance, and significantly improve the chirality characteristics. The experimental results show that the transmission of D-tyrosine at a negative angle deflection is always greater than that at a positive angle deflection, and the results of L-tyrosine are opposite. The CD values of D-tyrosine and L-tyrosine reach 16.4° and −11.6°, respectively, which are 9.3 and 11.9 times higher than that without the PB metasurface. This method has been successfully applied to distinguish chiral isomers of amino acids, which provides a new way to determine chiral substances. The above studies on the sensing of chiral substances are all based on metasurfaces without chiral properties. Although the chiral response of substances is enhanced, it is still limited. Next, the role of chiral metasurface in chiral substance sensing is analyzed.

For chiral substances, their chirality response is weak, and the difference between chiral enantiomers is not apparent. Chiral metamaterials usually refer to multilayer microstructures in space that satisfy spatial mirroring or central-reversal symmetry breaking. The chirality of the metasurfaces will enhance the chirality response of the substance, making this difference significantly perceived. Hwang has shown a chiral THz multilayer metamaterial fabricated using lithography-free techniques^[78]. This advanced microstructure can significantly enhance the difference of chiral enantiomers, shown in Fig. 7(a). The experimental results show that using chiral metamaterials can significantly enhance the chirality difference of enantiomeric CYT (Cys–Cys linked via S–S bonds) biocrystals by 7 times. The effect of the chiral microstructure on enhancing the chirality response has been fully demonstrated. This achievement paves the way for the applicability of the detection of chiral enantiomers in the THz band. In another study, Zhang et al. also demonstrated that chiral multilayer microstructures play an important role in chiral biological enantiomer recognition, shown in Fig. 7(b)^[79]. The chiral response of the microstructure can be adjusted flexibly by controlling the number of layers and the torsion angle between the adjacent layers. The different electric field distributions of RCP and LCP in the figure show that the proposed microstructure is chiral. This chiral property successfully allows the chiral enantiomers of the proline to be recognized without labels. In the frequency range of 0.3–0.8 THz, there are significant chiral differences between proline enantiomers. In addition, the sample concentration can also affect the chiral response. These novel chiral microstructures provide a new strategy for detecting chiral substances.

In addition to the three-dimensional microstructure, Zhang et al. have demonstrated that the chirality of a two-dimensional metasurface can also be excited by oblique incidence. As shown in Fig. 7(c), for a single-layer spiral metasurface, the strong chiral response is excited by oblique incidence, which enforces the strong interaction between the metasurface and chiral samples^[62]. The measured CD spectrum of the metasurface confirms that the structure has strong polarization conversion and optical chirality. The results show significant chiral differences between the D- and L-proline in the frequency range of 0.4–0.7 THz. The chirality difference of the measured proline spectrum decreased with an increase in concentration. This difference in the chiral isomers of alanine and tyrosine was also detected. It shows that this method has a certain universality for chiral solution samples. It can be seen that the optical chirality response of the sample can be enhanced by chiral metamaterials. This novel THz sensing mechanism is expected to be an efficient, non-contact, and label-free sensing method for chiral isomer recognition.

As we mentioned above, THz waves have attracted a lot of attention in biosensing with their unique advantages and have already shown exciting performance in chiral sensing. In addition to the distinction of chiral molecules, how to realize the specific detection of target biomolecules is also a key issue that cannot be ignored in biosensing because the identification of some biomolecules is of great importance as they might be related to specific diseases and early identification can be crucial to treat them in time, such as viruses, DNA, RNA, and other disease markers^[80–82].

Although metasurfaces have emerged as excellent label-free sensing devices in biosensing applications, it works essentially in response to changes in the refractive index of their surrounding environment. Therefore, the specific detection of certain biomolecules cannot be realized only by metasurface-based sensors. To address this inherent limitation in THz metasensors, in recent years, a sensing strategy for metasurface functionalization has been introduced and experimentally validated for the specific detection of target biomolecules at ultra-low concentrations^[83]. Such sensing strategy usually uses biological modification to make the metasurface have the function of selective binding^[84] or to introduce functional nanomaterials, such as gold nanoparticles (GNPs), gold nanorods (GNRs), graphene^[85,86], to the metasurface via biomolecular interactions (including immune binding and molecular self-assembly) and both specific detection and signal amplification can be realized.

Due to the importance of specific sensing in chemical analysis and medical diagnosis, THz-specific sensing technology has attracted great interest from the scientific community although it is a newly developing research. Depending on the modification method, this section is structured into two sub-sections: specific detection based on modified metamaterials and GNPs-assisted THz specific sensing.

The sensing strategy of modifying metamaterials with biomolecules (including antibodies and proteins) or nanomaterials, such as graphene and hydrogels, was applied to enable metamaterials to specifically recognize target molecules. Large-scale epidemics caused by virus infections have always been a major crisis for human society, especially the SARS-CoV-2 pandemic in recent years. Its rapid spread has devastated many countries’ healthcare systems and seriously threatened social and economic stability^[87]. Most of these diagnosis processes have the limitations of being time-consuming and labor-intensive. Also, the sensitivity is poor and may give rise to false negative results^[88,89]. Therefore, metamaterials-based THz-specific sensing technology is expected to address these shortcomings with its ultra-high sensitivity and real-time label-free detection^[90–92].

In the work of Ahmadivand et al.^[93], the research team pioneered a method to achieve specific detection of the target virus (ZIKV envelope protein) by using the immune binding effect between antibodies and viruses. Going beyond conventional planar metasurfaces, a unit cell consisting of planar plasmonic resonators, which can support magnetic toroidal modes, was proposed [as shown in Fig. 8(a)] by launching a THz beam in the z-direction, which can excite magnetic and toroidal resonances. To make the metasurface obtain the ability of specific detection, the metasurface was modified with Zika antibodies. Thus, the ZIKV envelope protein can be captured by the metasurface. Then, the transmission spectra of the plasmonic metasurface for different concentrations of the ZIKV envelope protein were obtained, corresponding frequency shifts were recorded, and the experimental results demonstrated the high sensitivity of the metasurface to the ZIKV envelope protein. The limit of detection (LoD) of the metasurface was 24 pg/mL, and the sensitivity of the structure reached 6.47 GHz/log(pg/mL).

In addition to antibodies, graphene can also be combined with metasurfaces to improve sensing performance. As a special two-dimensional nanomaterial, graphene can not only efficiently identify trace chemical elements but also achieve specific recognition through surface modification^[94]. Zhou et al. incorporated graphene with a THz metasurface into a microfluidic cell for sensitive detection of DNA sequences^[95] [as shown in Fig. 8(b)]. This metasurface consists of a series of complementary asymmetry split ring (CASR), and the graphene was transferred onto the surface of the CASR metasurface and then assembled into a THz microfluidic cell (TMFC) to detect tiny amounts of liquid. DCH aqueous solutions with different concentrations were tested based on the pure TMFC, CASR-based TMFC, and graphene-based TMFC, respectively. The results show the enhanced sensing ability of the hybrid structure of CASR-graphene. Then, CASR-graphene was modified with a probe DNA through π−π stacking to detect target DNA molecules, and the comparative tests for other kinds of DNA were also conducted. The authors verified that this method can be used to specifically recognize target DNA sequences of E. coli O157:H7 with high selectivity. This work provides an important reference for the application of the photoelectric behavior of nanomaterials in THz biosensing.

Moreover, as demonstrated by Shi et al., a protein-modified metasurface can be used to distinguish and even recognize amino acid enantiomers, as shown in Fig. 8(c)^[96]. This metasurface was modified with BSA, which will carry a negative charge in the PBS buffer with a pH=7.4. According to the isoelectric point theory^[97,98], when the isoelectric point of an amino acid is greater than the pH of the solvent, it will carry a positive charge. Therefore, the isoelectric point of Arg (arginine) is 10.7, which is positively charged and is different from other amino acids such as Pro, Cys, and Ala. Thus, only Arg can be attached to the functionalized metasurface through electrostatic adsorption, and the distinction between Arg and the other three amino acids can be realized. In addition to the electrostatic adsorption, L-Arg can also bind to BSA through the intermolecular interaction of polypeptide bonds, which is a property that D-Arg does not have, thus enabling the identification of enantiomers of Arg. The detection precision for L-Arg in this work is 2.5×10−5g/mL, and the detection sensitivity and precision are higher than the other amino acids. Furthermore, the D- and L-chiral enantiomers of Arg were distinguished due to their different binding abilities to the functionalized metasurface.

For THz biosensing, the most meaningful molecular signals are obscured by the strong THz absorption of solvent water. To solve this problem, the same authors demonstrated a chiral selective bio-molecular film constructed on an anapole metasurface sensor with a reflective THz sensing system^[48] and realized the chiral recognition of the amino acid enantiomers [as shown in Fig. 9(a)]. This metasurface can provide an ultrahigh localized field with a strong THz reflected signal even in a water environment. In this work, different amino acid enantiomers are detected, and the highest detection sensitivity is 0.516GHz·mL/μmol. The LoD of this sensor can reach the nanomole level. More importantly, since the sensor surface is modified by the BSA through amine coupling, which has different binding abilities to D- and L-amino acids. Therefore, the modified metasurface can exhibit different responses to amino acid enantiomers.

In addition to using reflective sensing systems, some special materials can also be used to modify metasurfaces to avoid the strong absorption of THz waves by water. Zhou et al.^[99] prepared a molecular-specific THz metasurface biosensor functionalized by aptamer hydrogels [shown in Fig. 9(b)], benefitting from the strong interaction with the localized electric field of the metamaterial. Trace thrombin-induced variations in the hydration state of the hydrogel can be sensitively probed. The sensing performance of the optimized THz biosensor was investigated for h-TB (human α-thrombin), and the LoD of this biosensor can reach 0.21 pmol/L. Then, the specificity of the proposed THz biosensor was investigated by comparing the signal variation of h-TB with other samples. The method proposed in this work could achieve selective detection in aqueous environments without the need for any further tedious isolation and purification processes. From the above, in chiral recognition, reflective chiral sensing strategies have been developed to avoid the absorption of water. In addition, some chiral selective biofilms modified on the metasurface can not only improve the chiral recognition ability of the metasurface but also reduce the absorption of THz waves by water. In specific sensing, it is also proposed that the sensing method based on microfluidic devices can greatly reduce the absorption of water. Furthermore, some functional materials such as hydrogels can not only maintain the water environment of the sample but also produce extremely high specificity through modification.

Then, for DNA detection, Lee et al.^[85] reported an advanced marker-free detection method for identifying single-stranded DNA (ssDNA) [as shown in Fig. 9(c)], which transfers a thin layer of graphene onto a nanogap metasurface so that DNA molecules can be fully absorbed by the graphene. THz near-field enhancement, induced by a nanogap resonance structure, can increase the absorption cross section of biomolecules and the graphene thin layer so that changes in the biomolecules can be observed at low concentration levels, which can significantly improve the sensing performance of the graphene nanogap metamaterials. From the above, the modified THz metamaterial sensing platform will provide a broad field of application for future research, especially for biomedical applications, such as ultra-sensitive specific sensing, and has great potential for avoiding water absorption.

Among the recent works for this biosensing mechanism, the integration of Au nanoparticles (AuNPs) with the THz metamaterial has been introduced as a promising mechanism to overcome and address the weak binding between biomolecules (like DNA, RNA, or antigen^[100]) and metamaterial, as well as to obtain more obvious frequency shift responses. It is worth mentioning that the frequency shifts in metamaterials have been proven to increase with the substrate’s refractive index^[101]. Therefore, AuNPs are often used to bind target biomolecules in experiments via their extremely high refractive index in the THz band and then produce a more robust THz response.

Shi et al.^[102] considered a dual-band all-dielectric metasurface consisting of a split half-cylinder array [as shown in Fig. 10(a)]. They modified AuNPs with a specific antibody (Anti-HA) to prepare functional AuNPs and then they are dispersed onto the metasurface. After the introduction of functional AuNPs, the metasurface can form an immunobinding with the HA antigen of human influenza. Different concentrations of the HA antigen (from 20 to 50 µg/mL) were detected, and results for different polarization incident THz radiation were also discussed. The highest resonance frequency undergoes an overall shift of 87.5 GHz, which is around two times that if AuNPs were not introduced to the metasurface. Then, the sensitivity of this biosensor was calculated for both polarizations in the case of with or without AuNPs. When the AuNPs were used, the highest value of sensitivity was 2.96 GHz·mL/nmol for the y-polarization incident THz wave.

Additionally, in the work proposed by Ahmadivand et al., the authors designed a new toroidal metamaterial biosensor [as shown in Fig. 10(b)] to detect SARS-CoV-2 spike proteins^[103]. The toroidal dipole extreme is a dominant contributor to the metasurface response, while the other dipolar and multipolar electromagnetic modes are strongly suppressed and thus can generate a distinct toroidal peak at around ∼0.4THz. Moreover, to further improve the sensitivity and specificity of the sensor, the functionalized AuNPs have been introduced in combination with a toroidal metasurface to optimize the binding strength between the sample and sensor. Via the AuNPs with specific antibodies (SARS-CoV-2 Spike S1-His recombinant protein), the functionalized AuNPs can bind with the spike protein of the SARS-CoV-2 virus. Therefore, an ultra-high sensitivity specific detection can be achieved. Sensor performance was monitored as a frequency shift with the change of spike protein concentrations, and the experiment results show the LoD for the sensor around ∼4.2fmol. These studies have confirmed that the highly specific and sensitive detection of viruses can be achieved through the modification of AuNPs.

In addition to the detection of viruses, AuNPs can also be used for specific detection of other disease markers. Liu et al. designed a bow-tie array THz metamaterial biosensor [shown in Fig. 10(c)] and modified it with AuNPs for the specific epidermal growth factor receptor (EGFR) detection^[104]. EGFR is a transmembrane protein that is important in the development of many types of cancer, such as gastrointestinal cancer, lung cancer, and oral squamous cell carcinoma^[82,105]. Different concentrations of EGPR were detected by bare metamaterial first. The LoD of the sensor is 100 fM (1 M = 1 mol/L). Then, the same samples were detected by introducing antibody-modified AuNPs. The LoD can be promoted to 10 fM. Moreover, the authors found that compared to the antibody-modified metamaterial, the GNP-Ab functionalized metamaterial has a larger frequency shift and a higher sensitivity for EGFR detection. They also found that the metamaterial functionalized by GNP-Ab with a larger GNP diameter achieves a larger resonance frequency shift.

Yang et al.^[106] presented a metamaterial biosensor based on AuNPs and rolling circle amplification (RCA). Different concentrations of S. aureus DNA were detected. The RCA process is used to amplify the S. aureus DNA fragments. Then, the generated long single-strand DNA molecules will conjugate with the AuNPs to form complexes [as shown in Fig. 11(a)]. These DNA-GNPs complexes enable the metasurface to recognize samples at very low concentrations. In the experiment of detecting synthetic S. aureus DNA, the frequency shifts of the metamaterial displayed a linear relationship with the change of concentrations, and the LoD of the biosensor can reach 2.77 fmol. Then the authors detected genomic DNA in clinical bacterial strains, and the biosensor also showed a linear response with an LoD of 0.08 pg/µL. In addition, the authors also compared the sensor’s response to S. aureus genomic DNA with that of three other bacteria, E. coli, A. baumannii, and P. aeruginosa, and proved that the proposed biosensor exhibited a remarkable capacity to resist interference in the detection of S. aureus genomic DNA.

For RNA detection, Yang et al. developed a novel THz biosensor [as shown in Fig. 11(b)] comprising a planar array of SRRs on high-resistivity silicon substrates^[107] to detect microRNA (miRNA) samples based on AuNPs and strand displacement amplification (SDA). The SDA reaction amplifies the target miRNA and generates copious yields of secondary DNA molecules (Trigger DNA), which are subsequently conjugated to AuNPs that form nanoparticle-Trigger DNA complexes. It is worth noting that the authors reveal the effect of the nanoparticle diameter on frequency shift. Their experimental results show that the AuNPs with larger diameter can lead to more significant sensing effects. The sensitivity of the THz metamaterial biosensor was evaluated by detecting miRNA-21 under the optimal experimental conditions, and a good linear relationship could be observed between the frequency shift Δf and the logarithm of the concentration of miRNA-21 in the range of 1 fM to 10 pM with an LoD of 14.54 aM. Then, the specificity of this biosensor for miRNA-21 detection is also discussed, and the signals of the other four different DNA samples and mixed samples were detected and compared. The frequency shifts of the target miRNA-21 sample were significantly greater than other samples.

Based on these studies, the development of AuNPs-assisted THz biosensors that can be used for clinical sample detection has gradually become the focus of researchers, and more practical wearable detection devices are also the development trend of metamaterials-based biosensors. Li et al. produced a flexible THz metamaterial biosensor for ultrasensitive detection of HBV DNA in clinical serum samples^[108]. In this work, high fidelity and specificity long single-stranded DNA was obtained by the RCA reaction. The synthesized ssDNA was then coupled with detection probes fixed onto AuNPs to form a GMNPs-RCA-AuNPs sandwich complex [as shown in Fig. 11(c)]. The quantitative analysis of HBV DNA was realized, and the resonant frequency shift exhibited a good linear relationship with the logarithm of the concentration. The LoD of this sensor was estimated to be 1.27×102IU/mL. Then, the authors continued to assess the specificity of their sensing strategy by comparing the frequency shift caused by the HBV target sequence with the single-base and six-base mismatched oligonucleotides. The results show that the frequency shift of the HBV target was significantly greater than other samples.

In summary, this review gives a comprehensive summary of the research methods and the latest progress of THz metamaterial sensors in chiral recognition and specific sensing, providing readers with the latest references for this emerging research topic. Since the qualitative and quantitative analysis of the enantiomers play an important role in the detection of the biochemical samples, we first focused on the THz chiral sensing, revealing that the THz chiral spectrum and chiral metamaterials are irreplaceable in the identification, structural characterization, and functional analysis of enantiomers. This showed the importance and frontier of the research of THz chiral spectroscopy and sensing technology. In addition, research on THz-specific sensing is also detailed, as high-specificity and high-precision specific biochemical detection techniques are particularly important in today’s global epidemics. In response to this problem, this paper reviews several common biological modification methods that can fully extend the practical application range of metamaterial sensors and achieve specificity and sensitivity that are far higher than traditional solutions.

In short, THz biochemical detection technology has unparalleled advantages in other bands, but the research of related technologies is still in its infancy. Many key scientific problems and technical bottlenecks have not been fully studied and solved, limiting the development and application of THz biochemical detection technology. To realize the detection of biochemical substances with high specificity, high sensitivity, and high precision, it is necessary to make full use of the vector characteristics of the THz light field, such as polarization and chirality, in the broadband spectrum range, and to develop the unique chiral spectrum and sensing technology with full vector characterization in the THz band. Its significance is as follows: on the one hand, more abundant sensing information can be obtained in multi-parameters and multi-dimensions to improve the sensitivity and accuracy of detection. On the other hand, the intrinsic electromagnetic interaction between the THz chiral light field and substance structure is used to increase the specificity of detection.

Therefore, the development of chiral spectroscopy and its sensing technology in the THz band, which has rich specific responses to the chirality and spatial structure of biochemical substances, is not only a major demand for the current global epidemic prevention and control but also has important scientific research value. It is expected to break through the bottleneck and limitation of traditional THz absorption spectroscopy and optical band chiral spectroscopy technologies and to develop into an irreplaceable new means of characterization and sensing detection of biochemical substances. Seizing the strategic opportunity that THz chiral spectroscopy technology offers and using basic and cutting-edge research on the THz chiral spectrum response law of biochemical substances, the direction of future exploration includes: (1)investigate the far/near-field combined full vector THz time-domain spectral detection technology to improve the accuracy of chiral recognition with more abundant characterization parameters, such as the distribution of vector light field in space;(2)develop the chirality enhancement principle of THz light field, especially the regulation law of chirality light field under the joint action of biochemical molecule chirality and artificial electromagnetic chirality;(3)based on current metamaterial modification methods, we can further develop new materials and new methods of sensor surface modification-assisted sensing enhancement and specific biochemical binding under a physiological active environment;(4)improve the mechanism of the interaction between artificial electromagnetic metamaterials and biomolecules, establish a more reliable biosensing simulation and prediction model, and design a metamaterial sensor with a stronger local electric field and higher sensitivity.

To promote the rapid development of THz biochemical detection technology, we suggest improving and developing the theoretical system of THz chiral spectrum characterization and sensing detection technology around the new mechanism and effect of THz chiral light field excitation, manipulation, and enhancement; developing key devices and detection systems with independent intellectual property rights; and realizing the detection of biochemical substances with high specificity, high sensitivity, and high precision.