Compared with traditional biosensors, the electron transfer efficiency, surface area, and biocompatibility of sensors made of 2D materials are much higher as the materials enhance the performance of optical fiber biosensors. Optical signals produce EWs that enter the optically thinner medium when it completely reflects off the optical fiber and metal film surfaces. This optically thinner medium contains a specific plasma wave. The collision of the two waves causes resonance. The energy of the reflected light significantly diminishes when the EWs resonate with the surface plasma wave because the surface plasma wave absorbs the majority of the incoming light’s energy. Therefore, metal films are usually used to excite SPR for the detection of biological substances. Compared with the SPR of traditional metal films, the SPR of 2D nanomaterials is very sensitive to the change in the optical phase. Based on this principle, some biological probes are modified on the surface of 2D materials. When they capture the target molecules, the phase change of SPR at the interface of the 2D materials is very drastic, which can realize the highly sensitive sensing of a series of biological molecules. Optical biosensors have more stable signals and can avoid the interference of temperature and other factors. Therefore, some typical 2D nanomaterials such as GO, MXene, and MoS2 are favored by researchers. All-fiber photonics, photo-electronics, and optical biosensors have been successfully combined in recent years, thanks to the widespread usage of graphene and other 2D materials. The use of 2D materials, which can be used to control the polarization, phase, intensity, and frequency of light beams as well as to realize active photoelectric conversion and electro-optic modulation, improves the interaction between light and matter in silica fiber devices. This opens new possibilities for the study and development of optical fiber biosensors. This section primarily examines the fiber optic biosensor applications of a few well-known 2D materials.
4.1. Metal Films
In recent years, increasingly sensitive and unlabeled optical device-based fiber SPR using metal thin films and nanostructures have been developed [
141]. SPR is an optical phenomenon generated by the propagation and collection of free electron oscillations in metal films and nanostructures surrounded by dielectric media [
27]. EWs form and enter into the photophobic medium, and there is a certain plasma wave in the medium when the total reflection of light occurs on the surface of the optical fiber and metal film. Resonance may occur when the two waves meet. The detected reflected light intensity is greatly weakened when EWs resonate with surface plasmon waves. The energy is transferred from the photon to the surface plasma, and most of the energy of the incident light is absorbed by the surface plasma wave, which drastically reduces the energy of the reflected light. With a variety of fiber architectures, biosensor probes for depositing metal films and NPs have been developed (e.g., side-polished, tapered, U-shaped, hyper-fiber, nanofiber, etc.) to generate strong plasma-matter interactions. SPR sensors are sensor probes that generate plasma using metal films and nanostructures and are applied to light fields to detect chemical, physical [
142], biomolecular [
143], DNA [
41], and microbial agents. In order to improve the sensitivity of the sensor, several optical geometries have been studied and developed. Recent sensor fabrication technologies indicate that the combination of nano-deposition, thin film coating, and optical fiber is a hot topic for measuring RI.
Wang et al. proposed and demonstrated an unlabeled fiber optic SPR biosensor for the specific detection of C-reactive protein (CRP) [
144]. The uncoated MMF was used as the sensing area for the fiber optic sensor, which was then fabricated by depositing an Au film. The sensitivity of the developed sensor was then assessed by detecting various concentrations of NaCl solutions.
Figure 8A depicts the SPR probe and measurement equipment in detail. The anti-CPR monoclonal antibody was fixed on the surface of the sensor to provide CPR-specific tests with the use of the biological crosslinked membrane (polydopamine). Further, the anti-CRP monoclonal antibody was subsequently linked to the sensor surface for CPR-specific detection. The experimental results, as shown in
Figure 8B, were obtained by optimizing the fixation time of the anti-CRP monoclonal antibody and antigen-antibody reaction time. The results showed that the sensor displayed a reasonable linear (R
2 = 0.97) response in the CRP concentration range of 0.01–20 μg/mL. The maximum CRP sensitivity under the circumstances was 1.17 nm/lg(μg/mL). While the resonance wavelength shifted in BSA detection, it increased with the increase in CRP concentration in the concentration range of 10–100 mg/mL. The anti-CRP monoclonal antibody immobilized sensors did not show non-specific detection, as evidenced by BSA detection. According to the experimental findings, when CRP concentration increased, the resonance wavelength shift also increased.
Gahlaut et al. coated the fiber with a silver film and dengue biomarker NS1 protein antigen and developed a fiber optic SPR biosensor for dengue virus detection by studying various self-assembled monolayers (SAM) with different chain lengths and surface parts to optimize the surface motifs attached to anti-NS1 antibodies.
Figure 8C depicts the structure of the fiber, the steps of probe fabrication, and the mechanism of interaction with the antigen. This type of sensing probe was used for the quantification of the NS1 antigen. NS1 antigen detection depends on the interaction of the NS1 antigen present in the infected serum sample with the anti-NS1 antibody attached to the fiber probe with the help of the SAM layer on the silver film. When the NS1 antigen in the sample solution binds to the antibody on the probe, the RI around the silver layer changes, which is manifested as a change in the resonance wavelength of the SPR spectrum. During probe fabrication, the antibody concentrations were first changed to 1:100, 1:50, 1:10, and 1:5. The results showed that the probe with an antibody concentration of 1:10 showed the best performance in terms of resonance wavelength shift in the above NS1 antigen concentration range. The reasons for this trend can be understood as follows: lower antibody concentrations may not be sufficient to have sufficient sensing sites on the probe; however, high concentrations may cause steric hindrance between adjacent antibodies, resulting in reduced sensing performance.
Figure 8D shows the detection response at two antibody concentrations of 1:100 and 1:10. For both probes, a redshift in resonance wavelength was observed as the antigen concentration increased from 0–2 μg/mL, as shown in
Figure 8D(a,b). The redshift of the resonance wavelength means that the RI around the Ag layer increased when the NS1 antigen was bound to its antibody on the probe surface. Calibration curves describing the resonance wavelength as a function of NS1 antigen concentration are shown in
Figure 8D(c) for the 1:100 and 1:10 probes. The experiment was repeated five times for each probe to find the average resonance wavelength, which is shown as the standard deviation of the error line in the calibration plot. It can be noticed from
Figure 8D(d) that when the antibody concentration increases, the resonance wavelength will be blueshifted. In SPR-based sensors, the resonance wavelength shift is related to the change in RI around the metal layer. The probe gives a very satisfactory response to real samples and can work in a physiological range with high sensitivity and selectivity, demonstrating the feasibility of its clinical use [
145]. In the field of food safety detection, optical fiber SPR sensors coated with Ag films are also used for the detection of glucose and fructose. A simple, rapid, unlabeled technique can be used to detect glucose/fructose in pure honey.
Hossain et al. used the finite element method for the numerical and theoretical analysis of a hollow PCF-SPR biosensor. The sensor based on plasma metal and an Ag layer has high sensitivity and low constraint loss and can achieve a long-time detection of 1.33–1.42 RIU. Therefore, SPR sensors can be used for the detection and sensing applications of unknown analytes, such as biomolecular components, lipids, proteins, and carbohydrates [
24]. In another study, Wang et al. used an Au film-modified fiber U-shaped structure cascaded multi-channel fiber SPR sensor to detect glucose and sucrose [
15]. The sensor is useful for the simultaneous measurement of numerous parameters and may be immediately put into a tiny space for measurement. Further, Zhang et al. also proposed a glucose detection platform based on the MMF-SMF-MMF structure of SPR and enzymatic reaction, providing a new way for the detection of food additives and medical immunoassays [
5]. The dual-parameter SPR biosensor was developed by Zheng et al. based on the HPCF modified with an Au film and AuNPs. The developed sensor offers a fresh approach to dual-parameter measurement and broadens the scope of applications for optical fiber biosensors in the biomedical industry [
146]. However, the low detection range reduces the practical value of dual-parameter sensors to a certain extent. A double-layer Cu fiber optic biosensor based on SPR has been proposed and verified experimentally [
147]. On a polished optical fiber surface, a uniform copper layer was deposited using an electron beam-assisted sputtering technique. The thickness of the deposited Cu film was found to be 50.9 nm and 51.3 nm, respectively. Due to the high RI and unique photoelectric characteristics of Cu, the biosensor based on Cu film SPR has been applied to detect various concentrations of bovine serum albumin (BSA) solution and has achieved satisfactory sensitivity (1.907 nm/(mg/mL)) and LoD (5.70 × 10
−7 mg/mL). Notably, this biosensor shows sensitivity to sub-microliter doses, promising biochemical applications in DNA hybridization, cancer screening, medical examination, and environmental engineering.
Metal thin films of Au, Ag, and Cu are widely used in the development of biosensor probes based on fiber optic SPR. Some recent research results of SPR biosensors based on metal film modification are shown in
Table 3. The application of metal composite film in the field of fiber optic biosensors can be considered in the future development of sensor probes.
4.2. Graphene Oxide
Graphene-based nanomaterials have significant and independent optical properties, including tunability and broadband adsorption, as well as influence dependent on material polarization [
149,
150]. The development of optical system-based biosensors has been facilitated by the distinctive optical characteristics of materials based on graphene.
In 2018, Wang et al. verified the effect of GO on improving sensitivity in the development of an SPR immunosensor. The results show that the sensitivity of the GO-modified fiber probe was 68% higher than that of the unmodified fiber probe [
151]. In 2019, Yang et al. proposed a glucose biosensor probe based on LSPR for glucose detection at a concentration of 0–10 mM and achieved a sensitivity of 0.93 nm/mM [
57]. Later, Yang et al. used GO to modify the previous LSPR-based sensor probe in order to improve the sensitivity of the biosensor [
152]. The fiber probe was tested by the device shown in
Figure 9A. The comparison of the two results is shown in
Figure 9B.
Figure 9B(a,b) are the results of the LSPR sensor probe without GO modification, and
Figure 9B(c,d) are the results of the LSPR sensor probe with GO modification. As can be seen from the
Figure 9B, the detection linear range of the sensor probe modified by GO changed from 0–11mm, and the sensitivity increased from 0.93 nm/mM to 1.06 nm/mM. The slight increase in sensitivity demonstrates the feasibility of GO in the development of LSPR-based biosensors.
The layers of GO also have an impact on the sensitivity and detection range of biosensor probes. Sun et al. investigated a hemoglobin (Hb) biosensor using an excessively tilted fiber grating (Ex-TFG) coated with GO [
28]. The sensing system included a biosensor based on Ex-TFG coated with GO monolayers for the detection of Hb biomolecules, as shown in
Figure 9C. The GO layer on the Ex-TFG had a high surface-to-volume area and abundant functional groups, which was conducive to the adsorption of biomolecules. Fiber optic sensing relies on changes in the evanescent field to measure RI changes. The Hb biomolecules adsorbed by the GO layer through π-π interaction and hydrogen bond interaction can greatly induce the perturbation of the grating evanescent field, leading to the detection of the wavelength shift of the cladding mode resonance of Ex-TFG. GO was adsorbed onto the surface of the Ex-TFG sensor probe through π-π and H-bond interaction. Then the sensitivity of the Hb biosensor with different layers of GO coating was tested. The results indicated that GO coating might increase the Ex-TFG’s bioactivity and render Ex-TFG responsiveness to the Hb biomolecular solution.
Figure 9D shows that three biosensor probes had sensitivity values of 3.83 nm/(mg/mL), 4.33 nm/(mg/mL), and 8.21 nm/(mg/mL), respectively. Unfortunately, the detection range was drastically decreased and changed to 0.8 mg/mL, 0.6 mg/mL, and 0.4 mg/mL, respectively.
Wang et al. demonstrated a label-free biosensor platform using GO nanosheet functionalized micro-taper long period grating (MTLPG) for hemoglobin detection in various solvents [
153].
Figure 9E shows the schematic diagram of the GO nanosheets decorated on the surface of the fiber sensing probe. As the hemoglobin molecule is absorbed onto the surface of the fiber, the resonance wavelength shifts, allowing detection of the hemoglobin concentration. As shown in
Figure 9F(a–c), the sensitivities reached −2 nm/(mg/mL), −1.03 nm/(mg/mL) and −0.73 nm/(mg/mL) in the range of 0–2.0 mg/mL for hemoglobin concentration in DI water, urea solution, and glucose solution, respectively.
Figure 9F(d) shows a control group without GO-modified probes. It can be concluded that the inherent excellent optical and biochemical properties of GO can provide high stability, strong light wave interference, and excellent biocompatibility for the biosphere surface layer.
Currently, GO and other fiber optic biosensor probes have been developed for glucose and hemoglobin sensing. In the future, GO will have a broad prospect in the field of fiber optic biosensors.
Table 4 summarizes some research results of fiber optic biosensor probes modified by GO.
4.3. Molybdenum Disulfide
Molybdenum is one of the trace dietary elements necessary for human survival. Aldehyde oxidase, sulfite oxidase, and xanthine oxidase are MO-containing enzymes involved in key metabolic activities of the human body. Therefore, some molybdenum-based compounds have been widely used in biomedical research because of their good biocompatibility [
159,
160,
161]. MoS
2 consists of S-Mo-S bonded by weak van der Waals forces and is known as a 2D nanocrystalline material “beyond graphene” [
162]. MoS
2 has several advantages compared to graphene, including greater efficiency at optical absorption, a wider band gap, greater electron mobility, higher surface-to-volume ratio, less poisonous, and biocompatible. As a result, it has been successfully used for sensing purposes. MoS
2 nanomaterials have attracted more and more attention in biomolecular detection [
7,
163], bacterial analysis [
19,
103,
164,
165], virus detection [
166,
167], and cancer diagnosis [
160,
168,
169,
170,
171] due to their unique physical and chemical properties. Due to easy modification and large specific surface area, MoS
2 can adsorb a variety of biomolecules and drug molecules through covalent or non-covalent interactions, increasing the stability of its protein targets as well as the accuracy and sensitivity of detecting particular biomarkers [
161,
172,
173].
Kaushik et al. developed an antibody-functionalized fiber optic SPR sensing probe for the unlabeled detection of BSA [
174]. Two kinds of BSA sensor probes were prepared with MoS
2 and without MoS
2 modification. The experimental results showed that compared with the fiber SPR sensor without MoS
2, the LoD of the MoS
2-modified sensor showed a significant decrease from 0.45 μg/mL to 0.29 μg/mL. Thus, MoS
2 can be used to reduce LOD and improve sensitivity for the development of fiber sensing probes.
Since then, some researchers have used MoS
2 for bacteria or virus detection. MoS
2 nanosheets were fixed to the Au membrane interface of an SPR immunosensor via Au-S bonding and then bio-conjugated effectively with
E. coli monoclonal antibody through hydrophobic interaction [
165].
E. coli with a population density of 1000–8000 CFU/mL was detected without label. The results showed that the proposed SPR immunosensor platform could sensitively detect
E. coli as low as 94 CFU/mL. The target analyte (
E. coli) has been developed for the immunosensor to achieve accurate and selective detection in the presence of other interfering bacteria. However, the developed fiber optic SPR system has some limitations, including high cost, short effective time, and other shortcomings. In order to study the spectral characteristics of the fiber optic SPR sensor, the resonance wavelength obtained after the antibody was fixed onto the surface of the SPR immunosensor was considered as the reference resonance wavelength. Different concentrations of
E. coli were detected using the SPR sensing device shown in
Figure 10A. The SPR spectrum is shown in
Figure 10B(a). It can be observed that with the increase of
E. coli concentration, the resonance intensity in the SPR spectrum decreased and showed a redshift, thus determining the resonance wavelength of each concentration.
Figure 10B(c) shows the corresponding change of Ab/MoS
2 at the resonance wavelength and
E. coli concentration. The resonance wavelength rose linearly with
E. coli concentration in the range of 1000–8000 CFU/mL for the Ab/Au/fiber and Ab/MoS
2 immunosensors, as can be observed from
Figure 10B(b,d). When
E. coli and monoclonal antibodies interact on the Au layer of the MoS
2 functional SPR immunosensor, the RI of the environment changes due to the formation of an antibody-antigen complex, which also changes the EWs characteristics of the interaction. The amount of
E. coli in the buffer solution was directly related to the change in RI. The Ab/MoS
2/Au fiber exhibited a greater wavelength shift, improving sensitivity of the sensor. The functionalized nanosheet’s huge surface area boosted the antibody’s binding density, which resulted in more target analytes being captured.
MoS
2 not only plays a role in improving sensitivity and reducing detection limits in the bacterial sensor probe but also plays an important role in other fiber optic biosensors. Zhu et al. proposed a biosensor probe based on LSPR for the detection of acetylcholine [
163]. A controlled experiment was set up to control a single variable to verify that MoS
2 improved the performance of biosensor probes. As shown in
Figure 10C, AuNPs/acetylcholinesterase and AuNPs/MoS
2/acetylcholinesterase were used to modify fiber sensing probes with different structures, respectively. An AuNPs and acetylcholinesterase functionalized MMF-tapered MCF-MMF was used as sensing probe 1, and an AuNPs, MoS
2, and acetylcholinesterase functionalized MMF-tapered MCF-MMF was used as sensing probe 2. The LSPR sensor spectrum and resonance wavelength fitting of probe 1 are shown in
Figure 10D(a,b), and the LSPR sensor spectrum and resonance wavelength fitting of probe 2 are shown in
Figure 10D(c,d). As shown in
Figure 10D(b,d), in comparison to probe 1, probe 2 had a higher linear fit, higher sensitivity (increased by 33.8%), and lower LoD (14.28 μM).
MoS
2 possesses a unique resilience, electron mobility, and huge surface-to-volume ratio in its lattice structure. It is important to emphasize that adding a spatial structure with a sizable specific area would raise the target biomolecule’s number of interaction sites and boost the probe’s sensitivity. So far, it has been proved that MoS
2 modification of sensor probe surfaces can effectively improve sensor performance. MoS
2, as a 2D nanomaterial of transition metal dichalcogenides (TMDCs), has set off a new trend in the field of biosensor research. However, in the field of fiber optic biosensors, there have been no spectacular achievements. In the future, fiber optic biosensor probes will gradually enter the field of clinical research.
Table 5 lists some of the achievements of MoS
2 applied in the field of fiber optic biosensors.
4.4. MXene
MXene, as a novel 2D transition metal carbide and nitride nanomaterial, is characterized by its unique morphology and properties, such as good electrical conductivity, high surface area, excellent hydrophilicity, and abundant surface functional groups [
175,
176,
177,
178,
179]. The general formula for MXene can be expressed as M
n+1X
nTx (n = 1–4), where “M” is a transition metal, “X” is C or N, and “T” is a surface terminal, such as -O, -F, or -OH. Compared to graphene, MXene exhibits a large number of hydrophilic functional groups on its surface, which can effectively trap molecules in an aqueous solution. Synthesis of MXenes and exploitation of their inherent properties have been explored in the field of biosensors to achieve high sensitivity and high selectivity. The unique characteristics of MXenes, especially their optical properties and good biocompatibility, make them a good candidate for biosensor construction. In the last five years, MXenes have made remarkable progress in the field of sensing and biosensor analysis devices based on different technologies proposed by researchers. In addition to its applications in photoluminescence and colorimetric sensing, MXenes-based materials have now been explored in SPR, SERS, and LSPR fiber sensing. Recently, a wide range of biomedical applications has been found in biosensors. Previous studies have shown that Ti
3C
2MXenes have inherent peroxidase-like activity [
180,
181]. However, the catalytic performance of MXenes alone still lags behind other nano-enzymes such as metals and metal oxides. Therefore, developing easy-to-use methods to manufacture MXenes nanomaterials with high catalytic efficiency remains a challenge.
It is worth mentioning that 2D niobium carbide (Nb
2C) MXene nanomaterials are highly biocompatible, biodegradable, and have an inherent light response. Therefore, Nb
2CTx MXene is also widely used in the development of biosensor probes [
16,
178,
182]. Li et al. decorated the convex fiber-tapered MCF-convex fiber (CTC) structure with AuNPs and Nb
2CTx MXene and developed a biosensor based on the LSPR effect for the detection of creatinine substances [
16]. The creatinine biosensing probe model diagram and its detection schematic diagram are shown in
Figure 11A. The strong EWs were obtained by tapering the total internal reflection transmission mode of the fiber probe to induce the LSPR effect of AuNPs, so as to improve the probe sensitivity. Nb
2CTx MXene adsorbed more creatinase enzyme to enhance the specific recognition ability of the probe. Creatinine was adsorbed on the surface of the sensing probe to undergo biochemical decomposition, resulting in RI changes in the surrounding medium, and the resonance wavelength gradually redshifted. The results showed that the wavelength shift increased with increasing creatinine concentration. The LSPR sensor spectrum is shown in
Figure 11B(a) by testing sample solutions with different concentrations of creatinine. After analyzing the above detection data, the relationship between sample solution concentration and resonance wavelength is shown in
Figure 11B(b). The sensitivity of the biosensors based on LSPR was 3.1 pm/μM, and the LoD was found to be 86.12 μM, respectively.
MXene not only plays a positive role in the field of human health detection but also attracts researchers’ attention in the field of water quality monitoring. Kumar et al. proposed a highly sensitive SPR sensor based on D-shaped PCF (DPCF) of Ti
3C
2Tx MXene with different thicknesses for biomolecular detection [
12]. To promote the SPR effect, an MXene/Au mixed metal layer was applied to the DPCFs flat top surface. Simulated biomolecules have RIs between 1.33 and 1.39. According to the findings, in the absence of MXene, the wavelength sensitivity was 2000 nm/RIU. However, the wavelength sensitivity was 7000 nm/RIU and 13,000 nm/RIU, respectively, when the MXene layer was 14 nm and 27 nm thick. According to numerical findings, the MXene films’ wavelength sensitivity rose by 3.5 and 6.5 times, respectively. As a result, the biosensor based on Ti
3C
2Tx/Au layered DPCF had a high sensing capability and may be utilized to detect biological and chemical samples with RI values between 1.33 and 1.39. Liu et al. successfully developed an MXene-based biosensor for the detection of biological substances such as GDF11. The proposed fiber SPR sensor was decorated with 2D material Ti
3C
2 MXene [
176]. The structure of the fiber sensing probe is shown in
Figure 11C. Ti
3C
2Tx MXene/Au film/AuNPs were used to decorate the plastic-coated multimode fiber to obtain the sensing probe, as shown in
Figure 11D. The MXene-modified fiber SPR sensor’s sensitivity was raised to 4804.64 nm/RIU. The LoD of the fiber optic SPR sensor was 0.577 pg/L after functionalization of the GDF11 antibody, which was 100 times lower than the single-molecule enzyme-linked immunosorbent assay (ELISA), enabling the sensor to uniquely identify GDF11.
Figure 11E depicts the sensing probe test curve for GDF11 based on SPR.
Figure 11E(a) shows that the resonance wavelength and the light intensity decreased when the concentration of GDF11 antigen solution increased. As shown in
Figure 11E(b), as detection time rose, the resonance wavelength excursion first increased quickly before gradually becoming steady. The resonant wavelength shift increased with decreased GDF11 antigen solution concentration. Ti
3C
2-MXene and AuNPs can effectively improve the sensitivity of the sensor probe. After processing the data of the above detection results, the relationship between GDF11 concentration and resonance wavelength was obtained, as shown in
Figure 11E(c). It could accurately detect GDF11 with an LoD of 0.577 pg/L when the fiber SPR biosensor was functionalized with the GDF11 antibody.
Synthesis of MXenes and exploitation of their inherent properties have been explored in the field of biosensors to achieve high sensitivity and high selectivity. The unique characteristics of MXenes, especially its optical properties and good biocompatibility, make it a good candidate for biosensor construction. In the last five years, MXenes has made remarkable progress in the field of sensing, and biosensor analysis devices based on different technologies have been proposed by researchers. In addition to its applications in photoluminescence and colorimetric sensing, MXenes-based materials have now been explored in SPR, SERS and LSPR fiber sensing. The applications of MXene in the field of optical fiber biosensing are listed in
Table 6.
4.5. Other Novel 2D Nanomaterials
Rahman designed the SPR biosensor using a novel 2D material, tin selenide (SnSe) allotrope fiber [
185]. Due to the improved optical features of its 2D graphene-like structure, single-layer SnSe has gained a lot of attention lately as a biomolecular recognition element (BRE) in sensor design. For sensing applications, the sensitivity of BRE fiber sensors with three different single allotrope types was examined. The findings demonstrate that the sensitivity of α-SnSe, δ-SnSe, and ε-SnSe sensors, respectively, were 3225 nm/RIU, 3300 nm/RIU, and 3475 nm/RIU. The findings demonstrate that the suggested sensor’s increased sensitivity outperforms the conventional sensor built on a metal film. In order to create exceptionally sensitive fiber SPR biosensors for DNA hybridization, the suggested SnSe allotrope may be a viable substitute for conventional BRE.
Similar to graphene, TMDC has also attracted a lot of attention. TMDC MoSe
2 was applied to SPR fiber optic biosensor by Liu et al. for the detection of Goat-Anti-Rabbit IgG [
186]. Cysteamine hydrochloride with enrichment groups can generate self-assembled membranes to adhere tightly to the surface of MoSe
2. In this study, the researchers combined the fast response of the fiber optic SPR sensor with the enhanced sensitivity of the MoSe
2 nanofilm. A suggested and constructed fiber optic SPR biosensor with a MoSe
2-Au nanostructure offers impressive sensitivity and LoD. Additionally, scientists tested the bio-affinity of the biosensor using BSA as the target molecule with MoSe
2 deposition cycles ranging from zero to eight. Finally, the immunoassay was performed with goat anti-rabbit IgG and a LoD of 0.33 μg/mL was reached. The fast response and high bio-affinity indicate that the proposed MoSe
2-Au SPR immunosensor has strong applicability in specific interactions and immunotherapy.
Tungsten disulfide (WS
2), a TMDC, also exhibits many unique photoelectric properties, such as a high compound RI ratio, direct band gap, and large surface-to-volume ratio [
187]. However, there are relatively few experimental studies on WS
2 in SPR sensors, which mainly focus on prism-based SPR sensors [
188]. WS
2 modified sensors can improve the sensitivity of fiber SPR sensors. Cai et al. proposed a theoretical model for glucose detection based on an SPR fiber optic biosensor coated with Au/ZnO/WS
2 multilayer film. Compared with traditional SPR sensors [
189], the sensitivity of biosensors may be increased when using WS
2 materials. In order to load glucose oxidase, WS
2’s absorptive capability is exploited to create glucose-sensitive areas. Once the solution’s RI has been determined, the glucose concentration may be computed using the connection between the RI and the glucose concentration. The suggested WS
2-based SPR fiber biosensor has a special function in the measurement of glucose levels. It is clear that WS
2 modification-based sensors may provide a platform for biochemical detection that is sensitive, inexpensive, straightforward, and environmentally protective.