Photonics Research, Volume. 10, Issue 1, 126(2022)
Performance improvement approaches for optical fiber SPR sensors and their sensing applications
Optical fiber surface plasmon resonance (SPR) sensors point toward promising application potential in the fields of biomarker detection, food allergen screening, and environmental monitoring due to their unique advantages. This review outlines approaches in improving the fiber SPR sensing performance, e.g., sensitivity, detection accuracy, reliability, cross-sensitivity, selectivity, convenience and efficiency, and corresponding sensing applications. The sensing principles of SPR sensors, especially the performance indicators and their influencing factors, have been introduced. Current technologies for improving the fiber SPR performance and their application scenarios are then reviewed from the aspects of fiber substrate, intrinsic layer (metal layer), and surface nanomaterial modification. Reasonable design of the substrate can strengthen the evanescent electromagnetic field and realize the multi-parameter sensing, and can introduce the in situ sensing self-compensation, which allows corrections for errors induced by temperature fluctuation, non-specific binding, and external disturbances. The change of the intrinsic layer can adjust the column number, the penetration depth, and the propagation distance of surface plasmon polaritons. This can thereby promote the capability of sensors to detect the large-size analytes and can reduce the full width at half-maximum of SPR curves. The modification of various-dimensionality nanomaterials on the sensor surfaces can heighten the overlap integral of the electromagnetic field intensity in the analyte region and can strengthen interactions between plasmons and excitons as well as interactions between analyte molecules and metal surfaces. Moreover, future directions of fiber SPR sensors are prospected based on the important and challenging problems in the development of fiber SPR sensors.
1. INTRODUCTION
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Surface plasmon resonance (SPR) sensing methods can realize analyses of biomolecular interactions with high sensitivity, rapid response, and no labeling [1–3]. Optical fiber-based SPR effect can be regarded as the nonlinear multiple integral of the prism-based SPR effect at the incident angle and the incident plane. Optical fiber SPR sensors possess the advantages of anti-electromagnetic interference, corrosion resistance, small volume, and great flexibility, and they can easily form a sensor network through cascading or other manners. This allow us to achieve the real-time, online, remote, and multi-parameter detection of target analytes [4,5]. Therefore, the research on fiber SPR sensors has made significant progress in the field of biochemical sensing. However, on the one hand, the size of viruses, cells, proteins, nucleic acids, and some other biological macromolecules is in the order of micrometers or even millimeters and is much larger than the penetration depth of surface plasmon polaritons (SPPs), which is on a scale of hundreds of nanometers [6,7]. This indicates that the above substances cannot be detected sensitively via conventional SPR sensors. On the other hand, the full width at half-maximum (FWHM) of conventional SPR sensors is relatively large, which exceeds 100 nm, and will result in a low detection accuracy (DA). Furthermore, the detection repeatability and the anti-interference capability of conventional SPR sensors need to be improved. Therefore, performance improvement approaches for fiber SPR sensors have been widely studied and are mainly categorized into the following three branches.
Reasonable and flexible design of fiber substrates by means of cascading, polishing, and other methods can increase the evanescent field leakage and can introduce the in situ interference self-compensation function. This will promote the capability of sensors in detecting the variety and the intensity of external factors.
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Adjusting intrinsic layers (metal layers) of fiber SPR sensors can modulate the original light field, which makes sensors suitable for different application scenarios. Attenuated total reflection (ATR)-based long range SPR (LRSPR) [8,9], coupled plasmon waveguide resonance (CPWR) [10,11], nearly guided wave SPR (NGWSPR) [11], and waveguide coupled SPR (WCSPR) [12] can be considered as optimization parameters to improve the performance of conventional SPR sensors from different aspects.
The modification of various-dimensionality nanomaterials with high refractive index (RI) on the sensor surfaces by physical sputtering or chemical bond coupling can cause large disturbance of the evanescent field [9,13], can strengthen the confined electric field intensity [11], and can promote the capability of sensors for detecting small changes in the ambient environment.
According to above discussions, an overview of performance improvement approaches for fiber SPR sensors and their sensing applications is presented. There have been some reviews on SPR, but they are different from our report in terms of research scope and focus. In 2011, Shalabney
Optical fiber SPR sensors based on commercial fibers, customized fiber structures, and metastructure platform have been outlined in Refs. [14–19], but they are more focused on the classification of fiber SPR sensors. Meanwhile, applications of fiber SPR sensors in various scenarios (e.g., environmental monitoring, food safety detection, and biological immunoassay) have also been involved in Refs. [15,16,20–23]. Both classification and application scenarios are obviously different from the performance improvement approaches discussed in our paper. In addition, the sensing mechanism and applications of specific types of fiber SPR sensors (e.g., D-type fiber SPR sensors, hybrid plasmonic fiber sensors, molecular imprinting improved fiber SPR sensors, and surface plasmon cavities) have been systematically reviewed in Refs. [24–30]. The focuses of these reviews are particular, which are different from the broader focuses of our paper. We hope that this review can provide an insightful overview of fiber SPR sensors and inspirations for the development of SPR sensors.
2. THEORETICAL FOUNDATION OF FIBER SPR SENSORS
A. Sensing Mechanism of Fiber-Based SPR
The disassembly of cylindrical surfaces in the optical fiber allows the SPR phenomenon at each point to be explained by a three-layer structure. As shown in Fig. 1(a), the layer configuration of a traditional SPR sensor consists of a substrate layer, an intrinsic layer, and a dielectric layer. The substrate can employ commercial fibers (e.g., multi-mode fiber (MMF) [32], photonic crystal fiber (PCF) [33], hollow-core fiber (HCF) [34]), customized fiber such as suspended-core fiber [35], and the fiber grating (e.g., tilted fiber Bragg grating (TFBG) [5], long period fiber grating (LPFG) [36]). The intrinsic layer utilizes materials, with negative real-part and positive imaginary-part in dielectric constants, such as mono/bimetallic film with high reflectivities and low absorption rates (e.g., Au [37], Ag [38], Cu [39], Al [40]). The dielectric layer refers to the analyte layer.
Figure 1.(a) The layer configuration and (b) resonance spectrum of SPR sensor. (c) Field/current distributions of the insulator-metal-insulator model corresponding to SPR sensing structure [31]. Note: I, II, and III are three modes with the lowest loss; red line, black line, and orange arrow represent electric field distribution, magnetic field distribution, and current conduction, respectively.
The total reflection will occur when the transverse magnetic (TM) polarized light is projected on the substrate-metal layer interface at an angle larger than the critical value. The resulted evanescent wave (EW) enters the metal layer, and its energy decreases exponentially. This excites SPPs to propagate along the upper surface of the metal layer [7,11]. When wave vector components of the EW and SPPs on the interface are equal, SPPs will be strongly coupled with the EW. At this point, most of the energy of the incident light is coupled into SPPs [14] while a resonance dip appears in the transmission spectrum, as shown in Fig. 1(b). This phenomenon is called SPR. The SPP mode, which is strongly coupled to the EW, is called the resonance SPP mode, i.e., SPR mode. The SPP mode that is weakly coupled to the evanescent wave is called the non-resonance SPP mode. The wavelength of the incident light, which corresponds to the occurrence of the strong coupling, is defined as the resonance wavelength (). When the RI of the dielectric on the metal layer surface changes, a new resonance will occur for the incident light at another angular frequency. This can be macroscopically manifested in the change of the resonance wavelength. According to the change of the resonance wavelength, we can measure the change in the average RI of the dielectric on the metal layer surface, which is caused by the change of a merging of parameters belonging to bulk/volume sensing and others belonging to surface sensing, to realize the detection of the analyte.
B. Performance Indicators of Fiber SPR Sensors and Their Influencing Factors
The performance improvement of fiber SPR sensors mainly is targeted at some indicators, and this section introduces the typical performance indicators of the sensor and their influencing factors. Since it is not convenient to adjust the incident angle at the fiber core-cladding interface, most fiber SPR sensors are designed based on the wavelength scanning. The depth of the resonance dip is determined by the energy coupling between the resonance SPP modes and the evanescent wave. The stronger the coupling is, the deeper the resonance dip will be. The FWHM of resonance dip is determined by the difference between the resonance SPP modes-evanescent wave coupling and the non-resonance SPP modes-evanescent wave coupling. The greater the difference is, the narrower the FWHM will be. Meanwhile, some researchers claim that the observed SPR curve band is the result of the convolution of the resonance dips excited by separate optical modes [41], and the Ohmic loss of metal and the radiative loss of SPPs on the parallel axis [in Fig. 1(c)] lead to the broadening of the resonance dip [11].
The sensitivity () of the sensor is defined as the resonance wavelength shift () caused by the unit average RI change () surrounding the sensing region, as expressed in Eq. (1) [42]. Its magnitude mainly depends on the overlap integral of the SPP electric field intensity on the vertical axis, as shown in Fig. 1(c), in the analyte region [11]. The DA is defined as the reciprocal of the FWHM value in the resonance dip, as expressed in Eq. (2) [42]. Some research works also employ the depth-width ratio in the SPR curve to describe the DA [43]. Both definitions have similar properties. The narrower the FWHM is, the easier it is to find the resonance wavelength and the higher the DA is. The signal to noise ratio (SNR) is inversely proportional to the FWHM, as expressed in Eq. (3) [44]. The figure of merit (FOM) and the quality factor (QF) are comprehensively quantitative indicators for characterizing the performance of the sensor, as expressed in Eqs. (4) and (5) [45], respectively.
The limit of detection (LOD) of the sensor represents the minimum concentration or the quantity of the target components that can be detected from the test sample within a given degree of reliability. Equations (6)–(8) [46–48] display the different ways to evaluate LOD. Slightly different from LOD, the limit of quantification (LOQ) represents the lowest detectable analyte concentration near blank considering the standard deviation of calibration curve, as expressed in Eq. (9) [46]. In general, the kinetic adsorption curve, acquired from biomass detection by using the sensor, is obtained by fitting the Langmuir adsorption isotherm equation [49] or logistic regression equation [50]. The sensitivity () utilized to calculate the LOD and the LOQ can be obtained via the first-order derivative or the linear fitting of the low-concentration data (near zero) in the adsorption curve [46,47,51]. The aforementioned performance indicators of SPR sensors are listed in Table 1. Parameter Indices to Evaluate the Performance of SPR SensorsIndex Equation Parametric Meaning Ref. [ DA – SNR [ FOM – [ QF – [ LOD LOQ [
3. PERFORMANCE IMPROVEMENT APPROACHES FOR FIBER SPR SENSORS AND THEIR SENSING APPLICATIONS
A. Reasonable Design of Fiber Substrate to Improve Sensor Performance
1. Increasing the Evanescent Field Leakage to Amplify SPR Signal
The use of different commercial optical fibers as the substrate to construct SPR sensors will obtain different sensing characteristics. PCF-based SPR sensors possess high sensitivity due to the high confinement loss of SPPs excited by a Gaussian-like leaky mode of the PCF element [15]. HCF-based SPR sensors can realize all-fiber distributed sensing, since the entire inner wall of the fiber can be coated with chemicals to form the sensing area [34]. Fiber grating-based SPR sensors can obtain a smaller FWHM and a higher SNR [36].
SPR is the electromagnetic resonance stimulated by the evanescent field leakage when the total reflection occurs at the substrate interface. Therefore, research works are carried out to increase the evanescent field leakage for enhancing the sensitivity by customizing the fiber structure through micro/nano processing (e.g., fused tapering [52], side polishing [53], bending into U-shape [54]). In 2014, Cennamo
Figure 2.(a) Schematic of the sensing structure and (b) concentration response as well as (c) time response spectra for the nicotine detection using the tapered plastic fiber SPR sensor [55]. (d) Schematic of the sensing structure and (e) glucose concentration response as well as (f) temperature response spectra of the SPF-SPR sensor [56]. (g) Schematic of the sensing structure and (h) human IgG concentration response spectra of the U-shaped fiber SPR sensor [57].
When the cladding and even the core of the fiber on one side are polished away, the evanescent field energy will leak from the polished area. This allows the side-polished fiber (SPF)-based SPR sensor to have a higher sensitivity [53]. In 2009, Tsai
2. Multi-Parameter Sensing
The signal spectral response of the fiber SPR sensor has a wide wavelength range; and thus, multi-parameter sensing can be realized by capturing different central wavelengths in different bands [59,60]. The implementations are mainly categorized into the following three branches. First, multiple detection lanes are constructed based on different fiber substrates [61–65]. In 2021, Xiao
Figure 3.(a) Schematic of the PCF-SPR sensor for the simultaneous measurement of magnetic field, RI, and temperature [61]. (b) Loss spectrum of the PCF-SPR sensor for the measurement of magnetic field [61]. (c) Schematic of the experimental setup for the characterization process of the SPR fiber tip sensor [66]. (d) Transmission spectrum of the SPR fiber tip sensor for the measurement of the liquid level [66]. (e) Schematic of the LRSPR sensor and experimental setup for the simultaneous measurement of RI and temperature [67]. (f) Transmission spectrum of the LRSPR sensor for the measurement of temperature [67].
Second, double detection lanes are constructed in the same sensing area [66,70]. In 2019, Alonso-Murias
Thirdly, multi-parameter measurement is carried out based on the sensitive characteristics of dielectric materials [67,71]. In 2020, Wang
The optical fiber sensors used for multi-parameter measurement can meet the needs of different complex scenarios, which is of great significance to application.
3. Self-Compensation for Temperature Fluctuation
Multi-parameter measurement is generally used to solve the cross-influence among parameters. In 2019, Zhang
The relationship between the wavelength shift in the optical grating resonance peak () and the change in the concentration () and the temperature () of the glucose is given as follows:
According to Eq. (12), the resonance wavelength shift caused by the change in the concentration and the temperature of the glucose can be calculated separately. Meanwhile, the SPR sensing unit and the LPFG temperature compensation unit were set close to each other, and thus the sensor could realize the in situ temperature self-compensation in the detection of the glucose concentration.
Similarly, Weng
4. Self-Compensation for Non-Specific Binding
When the fiber SPR sensor is employed to detect the biomass based on the antigen-antibody specific binding, the resonance wavelength shift caused by the interaction between the antigen and the non-specific binding site on the sensor surface will affect the detection reliability. In 2021, Wang
Similarly, Wang
B. Attenuated Total Reflection-Based SPR Modes
1. Long Range SPR
LRSPR refers to an electromagnetic resonance mode stimulated by adding a lossless dielectric buffer layer (e.g., Cytop [9], Teflon [77], [78], nano- film [79]), with an RI similar to that of the intended buffer layer but lower than that of the substrate, between the substrate and the metal layer. The layer configuration is shown in Fig. 4(a). Compared with the traditional SPR sensor, in which there only exists a single row of SPPs, the SPPs in the LRSPR sensor exist on both sides of the metal layer. The constructive interference between the two rows of SPPs forms a symmetrical mode, i.e., long range SPPs (LRSPPs) [43]. Its energy is mainly focused in the dielectric on both sides of the metal layer, which shows a property of low damping [82]. The penetration depth of the LRSPP’s electromagnetic energy in the dielectric layer and the propagation distance at the interface between the dielectric layer and the metal layer are both an order of magnitude higher than that of traditional SPPs [6]. Therefore, the LRSPR sensor exhibits more advantages in detecting biological macromolecules. In 2019, Zhang
Figure 4.(a) Layer configuration of LRSPR sensor. (b) Schematic of the sensing structure and (c) resonance spectrum for the detection of different BSA concentrations in the
2. Coupled Plasmon Waveguide Resonance and Nearly Guided Wave SPR
The CPWR sensor incorporates a thick (typically 500 nm [11]) dielectric layer with high-complex permittivity (e.g., ITO [83], ZnO [80], [80]) as the waveguide layer over the traditional SPR sensor surface, as shown in Fig. 4(d). When the plane wave is incident on the waveguide layer in the TM or the transverse electric (TE) mode, the light will be both reflected and refracted at the waveguide layer interface. The resulted multiple rays will interfere due to the existence of the optical path difference, to form the waveguide resonance mode and to generate the waveguide mode resonance [43]. In 2018, Liu
Similar to CPWR, NGWSPR can be stimulated by coating a thin (typically 10 nm [11]) dielectric layer with high real part of the dielectric constant (e.g., [84,85], [11,85], PbS [86], [85]) on the metal layer surface. Since the thickness of the dielectric layer is insufficient to support a guided mode, this leads to the spread of SPPs along the dielectric layer, where part of the evanescent field energy is coupled to the covered materials (to be measured) [11]. In 2020, Prakashan
3. Waveguide Coupled SPR
WCSPR refers to an electromagnetic resonance mode generated by coating an additional metal layer on the surface of the waveguide layer, based on CPWR. The layer configuration is shown in Fig. 4(g). When the light ray is incident at a small angle, the SPPs propagating along the interface between the lower metal layer and the waveguide layer will be excited by the TM polarized light. With the increase of the incident angle, the SPPs on the interface between the upper metal layer and the waveguide layer will also be excited [43]. Due to the coupling between the SPR and the waveguide resonance modes generated by the interference between multiple rays, narrow resonance dips and interference valleys that do not shift with the change of the external RI appear in the transmission spectrum [81]. In 2019, Ma
4. Comparison of ATR-Based SPR Modes
Due to unique sensing characteristics of ATR-based SPR modes, corresponding sensors are suitable for different applications, e.g., stimulating stronger surface enhanced Raman scattering (SERS) signal for pH sensing [87] and obtaining biological information regarding the cellular micromotion [88] and the DNA hybridization [89]. Different types of ATR-based SPR modes are summarized in Table 2 for a comparison. Comparison of Different ATR-Based SPR ModesATR-Based Layer Configuration Polarization State of Excitation Light Advantage Disadvantage SPR Substrate/metal layer/analyte TM The layer configuration is simple, and the sensitivity is high. The FWHM is wide, and the DA is low. LRSPR Substrate/dielectric layer with permittivity is pure real-number and lower than that of substrate/metal layer/analyte TM The FWHM is narrow, the DA is high, and the sensor is suitable for biomacromolecules detection. The sensitivity depends heavily on symmetric configuration. CPWR Substrate/metal layer/thick waveguide layer with high-complex permittivity/analyte TM or TE The FWHM is narrow, and the DA is high. The sensitivity is low. NGWSPR Substrate/metal layer/thin waveguide layer/analyte TM The sensitivity is high, and the resonance dip is deep. The DA is low. WCSPR Substrate/metal layer/waveguide layer/metal layer/analyte TM or TE The sensitivity is slightly higher, and the sensor possesses self-reference function. The layer configuration is complex.
C. Applications of Various-Dimensionality Nanomaterials in Fiber SPR Sensing
1. Zero-Dimensional Nanomaterials
Zero-dimensional nanomaterials are confined in the nanoscale range (1–100 nm) in all three directions, and their electrons cannot move freely, e.g., nanospheres [90] and nanoparticles [91]. In 2020, Wang
Figure 5.(a) Fabrication process in the SPP coupling-based fiber biosensor [75]. (b) Variation of resonance wavelength for human IgG detection [75]. (c) Schematic of the fiber SPR sensor fabricated by PDA accelerated ELP for immunoassay. Inset, scanning electron microscopy (SEM) image of the Au seeds formed Au layer [92]. (d) Sensitivity fitting curve of the sensor for detecting different concentrations of human IgG [92]. (e) Fabrication process in the HGNPs modified fiber LRSPR biosensor [51]. (f) Resonance spectrum for human IgG detection [51].
Due to the assistance of the coupling between the inner and the outer walls, the hollow gold nanoparticles (HGNPs) have a stronger plasmonic field compared to solid GNPs [93]. In 2021, Cheng
Based on the above discussion, typical application examples of zero-dimensional nanomaterials applied in improving the SPR sensing performance are listed in Table 3. Application Examples of Zero-Dimensional Nanomaterials in SPR SensingSensing Structure Target Analyte Simulated/Experimental Results Ref. Sensitivity LOD Notes Fiber/Au layer/AuNPs/analyte RI 1.3332–1.3710 3074.34 nm/RIU – – [ Fiber/Au seeds formed Au layer/PDA/anti-IgG/IgG RI 1.333–1.359/1.359–1.386 2054/3980 nm/RIU – The FOM of the sensor was [ Human IgG 2–100 μg/mL 0.41 nm/(μg/mL) 0.90 μg/mL Fiber/PDA/Au seeds formed Au layer/PDA/anti-IgG/IgG RI 1.328–1.386 1391–5346 nm/RIU – – [ Human IgG 0.5–40 μg/mL 0.65 nm/(μg/mL) 0.22 μg/mL Fiber/DML/Au layer/PDA-HGNPs/anti-IgG/IgG Human IgG 1–40 μg/mL 1.84 nm/(μg/mL) 0.20 μg/mL DML refers to dielectric matching layer. Combination of LRSPR and electric field coupling effects. The spike-and-recovery for serum samples detection was 107.62%. [ Fiber/Au layer-PMBA/glucose/AuNPs-AET-PMBA Glucose – 80 nM PMBA and AET refer to p-mercaptophenylboronic acid and 2-aminoethanethiol, respectively. [ PCF/Au layer/GO/anti-IgG/AuNPs-IgG RI 1.3323–1.3359 13,592.36 nm/RIU Synergistic sensitization of zero-dimensional AuNPs and two-dimensional graphene oxide (GO). [ Human IgG 1–35 μg/mL 1.36 nm/(μg/mL) 0.015 μg/mL Fiber core/Au Goat-anti-IgG 5–25 μg/mL 0.054 μg/mL Synergistic sensitization of zero-dimensional AuNPs and two-dimensional [ Fiber core/Ag layer/ERY imprinted nanoparticles ERY 0.205 nm/nM 1.62 nM ERY refers to erythromycin. The spike-and-recovery for real samples detection was 98.2%–102.0%. [ Fiber/Ag core- 2 nm/lg[M] The LOD for serum samples was [ Fiber/triangular AgNPs/GO/analyte RI 1.3318–1.3495 1114.80 nm/RIU – The apices generated greater electric field amplification. [ Fiber/Au nanostars arrays SERS – – The apices generated greater electric field amplification, and the proposed sensor was demonstrated with 45 times electric field intensity enhancement compared with Au nanorods design. [
2. One-Dimensional Nanomaterials
One-dimensional nanomaterials are confined in the nanoscale range in two directions and are extended to the macroscale range in the last direction. Their electrons can only move freely in one direction, e.g., nanotubes [38], nanorods [104], nanowires [105], and nanofibers [106]. Carbon nanotubes (CNTs) have been widely applied in the fields of biochemical sensing, nano-optics, and photothermal conversion owing to their extraordinary chemical, electrical, and optical characteristics. In 2021, Pathak
Figure 6.(a) Schematic of the PdNPs embedded/PPy shell coated MWCNT-based fiber SPR probe and the laboratorial setup. Inset, schematic and SEM image of the PdNP embedded/PPy shell coated MWCNTs and SEM image of the fiber probe surface [38]. (b) Resonance spectrum obtained by detecting hydrazine with different concentrations [38]. (c) The fabrication process of the double-layer Au nanorods and GO sensitized PCF-SPR sensor [104]. (d) Resonance spectrum obtained by detecting human IgG with different concentrations [104]. (e) Schematic of the
The modification of Au nanorods on the surface of SPR sensor can effectively enhance the sensitivity of the sensor because of the transverse and the longitudinal plasmon electric field coupling between nanorods and the Au layer [91]. In 2020, Xia
Based on the above discussion, typical application examples of one-dimensional nanomaterials applied in improving the SPR sensing performance are listed in Table 4. Application Examples of One-Dimensional Nanomaterials in SPR SensingSensing Structure Target Analyte Simulated/Experimental Results Ref. Sensitivity FOM LOD Notes Fiber core/ITO/Si/SWCNTs/analyte RI 1.330–1.335 9780 nm/RIU – ITO and SWCNTs refer to indium tin oxide and singlewalled CNTs, respectively. [ Fiber core/Au layer/MWCNTs-PtNPs/analyte RI 1.3385–1.3585 5923.14 nm/RIU – Synergistic sensitization of zero-dimensional platinum nanoparticles (PtNPs) and one-dimensional MWCNTs. [ Fiber/Au layer/MWCNTs-CuNPs/analyte Nitrate 3.25 nm/lg[M] – – Simultaneous measurement of two parameters. Synergistic sensitization of zero-dimensional cuprum nanoparticles (CuNPs) and one-dimensional MWCNTs. [ Temperature – – Fiber core/Ag layer/MWCNTs-CuNPs/analyte Nitrate 0.08062 nm/nM – 4 nM Synergistic sensitization of zero-dimensional CuNPs and one-dimensional MWCNTs. [ Fiber core/Ag layer/PdNPs-PPy-MWCNTs/analyte Hydrazine 0.09 nm/nM – 20 nM Synergistic sensitization of zero-dimensional PdNPs and one-dimensional MWCNTs. The spike-and-recovery rate of real sample detection was 97.5%–102.8%. [ Fiber core/Ag layer/MWCNTs/analyte Sulfamethaxazole – 891.80 nM – [ Fiber core/Ag layer/PPy-MWCNTs/analyte Dopamine 68.58 nm/lg[M] – 0.0189 nM – [ Fiber core/Ag layer/graphene-MWCNTs-poly-(methyl methacrylate)/analyte Methane gas 10–100 ppm (parts per million) – – – Synergistic sensitization of one-dimensional MWCNTs and two-dimensional graphene. The maximum shift in the resonance wavelength was 30 nm for methane gas detection. [ Fiber core/Ag Xanthine 0.0262 nm/nM – 12.70 nM The sensor worked well for the detection of xanthine in green tea samples. [ Fiber core/Ag layer/ZnO: graphene nanofibers/analyte Nicotine – 74 nM The sensor worked well for the detection of nicotine in cigarette samples. [ PCF/Au layer/Au nanorods/GO/Au nanorods/analyte RI 1.3323–1.3361 22,248.22 nm/RIU – Synergistic sensitization of one-dimensional Au nanorods and two-dimensional GO. [ Human IgG 1–15 μg/mL 3.28 nm/(μg/mL) – 6.10 ng/mL PCF/Au layer/double-layer Au nanorods/GO/analyte RI 1.3320–1.3366 25,642.65 nm/RIU – Synergistic sensitization of one-dimensional Au nanorods and two-dimensional GO. [ Human IgG 1–15 μg/mL 4.35 nm/(μg/mL) – 4.60 ng/mL U-bent fiber/Au layer/ITO nanorods/graphene/analyte RI 1.3330–1.3634 690.70 nm/RIU – – Synergistic sensitization of one-dimensional ITO nanorods and two-dimensional graphene. [ DNA 0.1–100 nM – – 0.10 nM PCF/Ag nanowires/analyte RI 1.33–1.38 9314.28 nm/RIU – [ PCF/Au nanowires/analyte RI 1.32–1.38 10,286 nm/RIU – [ Fiber/bimetallic nanowire gratings/analyte RI 1.33–1.49 643.75 nm/RIU – – – [
3. Two-Dimensional Nanomaterials
Two-dimensional nanomaterials are confined in the nanoscale range in one direction and are expanded to the macroscale range in other two directions. Their electrons can only move freely in such two directions. Two-dimensional nanomaterials popularly used for improving the performance of SPR sensors are listed in Table 5. Graphene possesses lower energy losses (e.g., Ohmic loss and radiative loss), high electron transport mobility, and high surface-to-volume ratio, and it can accommodate more organic and biological molecules because of the stacking interaction between the carbon-based ring structure of molecules and hexagonal cells of graphene [122]. The position of each graphene atom can be involved in the interaction between biomolecules, which means that graphene may eventually realize the single-molecule detection. GO has a hexagonal carbon structure similar to graphene and also contains the oxygen functional group such as hydroxyl, alkoxy, and carboxyl. This results in its higher solubility and easier surface functionalization for various types of biological receptors [122]. Phosphorene has also been applied in photonic sensor research works due to its high carrier mobility, strong carrier confinement, tunable direct bandgap, and anisotropic electron and photon characteristics [123,124]. The high carrier mobility and light absorption of TMDCs enable the effective use of light energy in the electron transmission. This leads to a strong coupling on the interface, thereby will enhance the surface energy density of the sensing layer and will amplify the response of the sensor to the RI disturbance in the sensitive layer [125]. Popular Two-Dimensional Materials for SPR Sensor Performance ImprovementCategory Basic Chemical Formula Ensample Graphene and derivatives – GO, reduced GO (rGO) Phosphorene – Black phosphorus (BP), BlueP Transition metal dichalcogenides (TMDCs) MXene Perovskite Notes M, X, Y, Z, and
Different two-dimensional layered materials can be vertically stacked together and integrated to form van der Waals heterostructures and superlattices. This can manipulate optical and electronic characteristics of their interlayer interactions, to provide technical support for the development of next-generation devices [126]. In 2019, Rahman
Figure 7.(a) Schematic of the phosphorene-graphene/TMDC heterostructure-based fiber SPR biosensor [127]. (b) Resonance spectrum of the biosensor for DNA hybridization detection [127]. (c) Schematic of the
MXene can realize the attractive biosensing due to its chemical stability, fully functionalized surface termination, high metal conductivity, tunable direct bandgap, small work function, large adsorption energy, and hydrophilic biocompatible surface [123,126,131]. In 2020, Chen
Based on the above discussion, typical application examples of two-dimensional nanomaterials applied in improving the SPR sensing performance are listed in Table 6. Application Examples of Two-Dimensional Nanomaterials in SPR SensingSensing Structure Target Analyte Simulated/Experimental Results Sensitivity FOM LOD Notes Ref. Fiber core/Ag layer/Pt layer/ITO/graphene/analyte RI 1.33–1.36 4150 nm/RIU – – [ Fiber core/Ag layer/GO/analyte Glucose adulterant 21,140 nm/RIU – – [ Fructose adulterant 18,890 nm/RIU – Fiber core/Ag layer/Au RI 1.3318–1.3701 3061.84 nm/RIU – – [ Fiber core/Au BSA 10–50 μg/mL 0.9234 nm/(μg/mL) – 0.29 μg/mL – [ RI 1.3314–1.3623 6184.40 nm/RIU – – [ Human IgG 5–70 μg/mL 1.014 nm/(μg/mL) – Fiber core/Cr layer/Au RI 1.333–1.358 2793.36 nm/RIU – – [ Goat-anti-rabbit IgG – – 0.33 μg/mL Fiber core/Al layer/ RI 1.330–1.332 6200 nm/RIU – – – [ SPF/Cr layer/Au Glucose 6708.87 nm/RIU – – PBA refers to pyrene-1-boronic acid. [ Graphene 4050 nm/RIU – 2-D materials refer to two-dimensional materials. The sensor was also utilized successfully to detect DNA hybridization. [ Fiber core/Au layer/BP/2-D materials/analyte RI 1.33–1.39 3950 nm/RIU – 3975 nm/RIU – 3975 nm/RIU – 4000 nm/RIU – Fiber core/NaF layer/Ag layer/BlueP/2-D materials/analyte – – – [ – – Fiber core/Ag layer/SnSe/analyte RI 1.33–1.37 3475 nm/RIU – – – [ Fiber core/Ag layer/GNP- Dopamine 0–100 μM 10.66 nm/μM – 0.031 μM GNP refers to graphene nanoplatelet. [ Fiber core/Ag Acetylcholine 0–10 μM 8.709 nm/μM – 0.038 μM – [ Heterocore fiber/Au RI 1.3343–1.3658 2180.20 nm/RIU – – – [ Fiber core/Au RI 1.333–1.335 3725 nm/RIU – – [ PTOF/Au RI 1.333–1.343 6710 nm/RIU – – – [ SPF/Au RI 1.3332–1.3710 2543.33 nm/RIU – – – [ RI 1.3710–1.4140 6040.42 nm/RIU – –
4. Three-Dimensional Nanomaterials
Three-dimensional nanomaterials represent composites composed of one or more basic structural units from zero-dimensional, one-dimensional, and two-dimensional materials, e.g., three-dimensional superlattice structures [146,147] and nanostructured metal architectures [148,149].
In 2020, Li
Figure 8.(a) Preparation procedure of the three-dimensional composite-based fiber LSPR biosensor [150]. (b) Schematic of the three-dimensional composite on the fiber surface, transmission electron microscopy image of Au nanoparticles covered by multilayer graphene, and schematic of the DNA detection process [150]. (c) Real-time wavelength redshift for DNA detection [150]. (d) Schematic of the bioreceptor patterning onto the Au coated fiber surface using DNA nanotechnology: three-dimensional DNA lateral surface (LS) origami, distal ends (DE) origami, and tetrahedron. (Dark green, bioreceptors; dark gray spheres, thiol groups; light green and red, ssDNA [151].) (e) Calibration curves for thrombin bioassay on the fiber SPR biosensing platform [151].
Based on the quantum size effect, the surface effect, and the dielectric confinement effect, nanomaterials with new functions are expected if there are further breakthroughs in the structure, the composition, the arrangement, and the size of three-dimensional nanomaterials.
4. FUTURE DIRECTIONS OF DEVELOPMENT
A. End-Face Reflective Sensing Structure
Compared with the transmission-type optical fiber SPR sensor [152] shown in Fig. 9(a), the end-face reflective optical fiber SPR sensor can be inserted into a small piece of sample or cramped space to directly acquire the signal in the dip-and-read way. Combined with mature fiber communication technology, this can achieve the high-efficiency, low-damage, and invasive measurement. At present, there are mainly three types of end-face reflective fiber SPR sensing probes [14], as shown in Fig. 9(a). On the cylindrical surface at the end of fiber, the cladding is removed, and a metal layer is coated to stimulate the SPR. Meanwhile, the end-face of fiber is also coated with the metal to serve as a mirror. Moreover, when the light is reflected from the end-face and enters the fiber again, the optical path is doubled, which enhances the plasmon resonance effect. In 2019, Kong
Figure 9.(a) Transmission-type fiber SPR sensor [152] based on core mismatch I and reflective fiber SPR sensor [14] based on flat tip II, tapered tip III, and angle polished tip IV. (b) Schematic of the sensing structure of the protruding-shaped fiber plasmonic microtip probe and the testbed [153]. (c) SEM image of the microtip probe and schematic of the bio-probe [153]. (d) Langmuir adsorption curve and (e) sensitivity fitting line for human IgG detection with different concentrations [153].
B. Combination of Nanoarray Structure and Fiber SPR Sensing
A highly ordered plasmonic nanoarrays, produced by self-assembly technology [154], focused ion/electron beam lithography [155], and two-photon laser direct writing [37], can stimulate various coupling optical effects such as Fano resonance [156], guided mode resonance [157], surface lattice resonance (SLR) [158], and SERS [154]. The combination of above effects and the SPR sensing can further strengthen the light-matter interaction, can reduce the radiative loss, and can thus improve the sensor performance. The periodic nanoarray also provides tunable structure parameters, wider spectral range, and higher degree of freedom. The SPR sensor based on the nanoarray has higher sensitivity and FOM, which expands the application scenarios of SPR. Furthermore, it is compatible with existing imaging equipment, microfluidic chips, and photonic circuits so that the fabricated devices can be highly integrated and miniaturized. Based on the above advantages [157], nanoarray structures have attracted more and more attention.
In 2016, He
Figure 10.(a) Optical micrograph of the plasmonic crystal cavity on the SMF end-face [159]. (b) Resonance spectrum of the SPR device for the detection of different solutions [159]. (c) Process in the fabrication of nanotriangular arrays on the reflective fiber SPR sensor end-face based on colloidal lithography technology and the SEM image of the nanotriangular arrays [160]. (d) Sensitivity fitting lines for the RI detection of the Au triangularly patterned and non-patterned sensors [160]. (e) Block diagram of the nanotrimer arrays on the bent fiber end-face and the SEM image of the nanotrimer arrays [161]. Resonance spectra of the (f) SLR-based and (g) LSPR-based sensors [158].
In 2018, Wang
C. Plasmonic Antenna Integrated on Tapered Fiber Tip
Lab on tip based probe devices provide high-precision dynamic tuning of plasmonic antenna and high-resolution detection through the strong interaction between plasmonic hotspots and samples. The test objects include the molecular adsorption or deformation [162], molecular vibration spectrum [163], quantum tunneling [164,165], etc.
As shown in Figs. 11(a) and 11(b), the coupling between the tapered fiber (or dielectric waveguide) and metal nanowires (or SPP waveguide) can greatly improve the light wave coupling and sub-wavelength focusing efficiency of the near-field optical microscope probe. This realized the scanning Raman spectroscopy imaging with a spatial resolution of 1 nm [166,169]. As shown in Fig. 11(c), it was observed by Danckwerts
Figure 11.(a) Near-field optical microscope probe based on the high-efficiency coupling of Ag nanowires and tapered optical fiber (AgNW-OF) [166]. (b) Polarization-resolved k-space imaging of the light emitted from the nanofocused SPP mode at the AgNW-OF probe tip [166]. (c) The four-wave-mixing produced signal increased sharply when the Au nanoparticle-fiber probe approached another Au nanoparticle [167]. (d) The molecular fluorescence changed from enhancement to quenching when the Au nanoparticle-fiber probe approached a single molecule [168].
D. Initial Conceptions
Although the fiber SPR sensors can realize the detection of physical and biochemical parameters, there is still some space to improve fiber SPR sensors in terms of accuracy, cross-sensitivities, selectivity, costs, and reliability. In the future research, it will be promising to involve microelectromechanical systems [170,171] for improving the stability of the device and to employ liquid metals [172] for developing flexible optical devices. The integrated and miniaturized design of fiber SPR sensors can utilize the two-photon three-dimensional micro/nano film printing to achieve the accurate, real-time, and on-site detection [171,173]. Terahertz biosensors based on metamaterials can also be vigorously developed to achieve the non-invasive detection for biomarkers or biochemical indicators in living organisms [156,173]. According to the sensing characteristics of fiber SPR sensors modified by different nanomaterials, nanomaterials can be reversely designed at the atomic level to modify sensors to obtain desired sensing characteristics, e.g., to locate the resonance valley in the optical band. In order to improve the performance of fiber SPR sensors from different aspects, various approaches can be jointly applied in the implementation of sensors. For example, the modification of nanomaterials on the surface of the LRSPR sensor with improved metal layer quality can enhance the sensitivity of the sensor while ensuring the DA [174,175].
5. CONCLUSION
This review work summarized current major approaches for improving the performance of fiber SPR sensing and the applications of the manufactured sensors mainly in the field of biochemical detection. The sensing principle and the performance indicators including determinants of SPR sensors were demonstrated to clarify the research direction of the performance improvement. The application examples and the internal mechanisms in the SPR sensing performance improvement were reviewed from three aspects, corresponding to the basic layer configuration of fiber SPR sensors, namely substrate, intrinsic layer, and surface nanomaterial modification. Relevant contents were summarized and compared in detail. In general, the performance improvement of fiber SPR sensors mainly lies in heightening the overlap integral of the SPP electric field intensity on the sensor surface to enhance the sensitivity, reducing the energy losses of SPP to narrow down the FWHM of SPR curve and to improve the DA and the SNR, and introducing the sensing self-compensation to enhance the detection reliability. Furthermore, the development direction of fiber SPR sensors was discussed based on the state of the art in plasmonic detection technologies.
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Jianying Jing, Kun Liu, Junfeng Jiang, Tianhua Xu, Shuang Wang, Jinying Ma, Zhao Zhang, Wenlin Zhang, Tiegen Liu. Performance improvement approaches for optical fiber SPR sensors and their sensing applications[J]. Photonics Research, 2022, 10(1): 126
Category: Surface Optics and Plasmonics
Received: Sep. 22, 2021
Accepted: Nov. 1, 2021
Published Online: Dec. 14, 2021
The Author Email: Kun Liu (beiyangkl@tju.edu.cn)