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Abstract
The presence of hyperbolic metamaterial (HMM) enables the coupling of surface plasmon polaritons (SPPs) and thus enhances the sensitivity of surface plasmon resonance (SPR) sensors. Biosensors based on the combination of HMM and two-dimensional materials possess the ability to further promote the detection sensitivity. In this paper, a monolayer graphene/HMM/D-shaped plastic optic fiber (G/HMM/D-POF) SPR sensor is proposed in the combination of the virtues of monolayer graphene, HMM, and D-POF. The sensing performance of the sensor has been proved by the numerical simulation and the experimental results. The optimal sensor structure is G/3*(Au/Al2O3)/D-POF, with a high sensitivity of 5166.7 nm/RIU for the detection of aqueous ethanol. Besides, glucose monitoring is essential for the detection and treatment of diabetes. Therefore, the G/3*(Au/Al2O3)/D-POF sensor is also used to detect aqueous glucose and achieve the sensitivity of 2767.3 nm/RIU. It is believed that the G/HMM/D-POF SPR sensor has the potential to provide a new detection path for biomolecule detection.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Sensors based on SPR gradually develop into one of the most feasible and effective techniques for detecting the tiny change of the refractive index (RI) surrounding the medium, with the advantages of label-free, point-of-care, and high-throughput [1–3]. SPR is generated by the excitation of surface plasmon polaritons, which is produced by the combination of free electron density oscillations and electromagnetic waves on the surface between dielectric medium and metal film [4]. Another form of SPR is surface plasmon wave (SPW) or SPPs, belonging to transverse magnetic or p-polarized light wave that is a vector wave perpendicular to the incident plane and propagates along the upper surface of the metal film [5]. SPR sensors are mainly divided into the traditional prism and novel optic fiber sensors; the former strongly depends on large optical components with high operational costs and difficulty in miniaturization [6,7]. Compared with the former in this regard, the latter possesses numerous virtues (such as the facility of integration, easy manipulation, excellent flexibility, long-distance sensing and low cost) [8–10]. Therefore, various optical fiber structures have been explored, such as D-shaped [11], tapered [12], U-bent fiber [13,14], and straight strip cladding [15], to enhance the behaviors of optic fiber sensors. Among them, the D-POF has lots of advantages: easier access to a large evanescent field, assistance in molecular fixation, super machinability, and low cost [16,17]. In addition to choosing different shapes of fiber structures, optimizing the materials used as sensing layers is also a method to achieve stronger SPR signal. Metal (such as Au and Ag) and composite metal construction as the sensing layers have been presented in the past few years [14,18,19]. Gong et al. proposed a G/Au/D-POF SPR sensor with a sensitivity of 1227 nm/RIU [17]. However, the SPP excited by the pure metal structure can’t attend the requirement of detection accuracy with the rapid development of biological detection. Therefore, the new sensing layer structure needs to be further designed and explored.
Hyperbolic metamaterials (HMMs), defined as a class of artificial materials that present hyperbolic dispersion due to one of their principal permittivity components, have the opposite sign to the other two [20]. Recently, numerous investigations have confirmed that the plasmonic sensors with HMMs possess the remarkable sensitivity [21–25]. The type ΙΙ HMM consisting of a multi-stack of metal/dielectric bilayers was explored in our work, and the bilayers support both the transmission coupling of waves between different layers and the propagation of surface wave along the metal surface [26]. This wave was defined as a bulk plasmon polariton (BPP), which can propagate inside the bulk of the material while maintaining the properties of a propagating surface wave. The BPPs have been experimentally illustrated to be extremely sensitive to any change in the dielectric constant within the evanescent field [23]. Furthermore, the performance of the sensors can be further improved by combining them with graphene. Graphene possesses the following attractive virtues. 1) The honeycomb arrangement of carbon atoms forms π-stacking interaction with aromatic rings contributes to the fixation of biomolecules; [27]; 2) the thickness of graphene is the atomic scale and hardly compromises the sensing performance; 3) the graphene layer can strengthen the electric field of SPP and promote the interaction between biomolecules and evanescent field [19]. Li et al. designed the sensor by combining HMM and multiple layers of graphene to achieve the sensitivity value of 4461 nm/RIU [24]. Besides, Zhang et al. have reported that the monolayer graphene can enhance the intensity of SPP by about 30.2% while the multiple graphene layers would depress the SPP intensity owing to the energy loss of electrons [19]. However, the preparation of graphene on D-type optical fiber still has some problems. For example, 1) graphene grown by chemical synthesis cannot be guaranteed to be monolayer; 2) the conventional wet-transfer graphene cannot remove the polymethyl methacrylate (PMMA) molecules completely, resulting in loss of transmission and accuracy reduction. Therefore, the efficient method of transferring monolayer graphene onto optical fiber needs to be proposed.
In this paper, monolayer graphene, n*(Au/Al2O3) (n is the number of period) composite HMM, and D-POF are combined to form a G/HMM/D-POF SPR sensor with high sensitivity. The method of directly salvaging monolayer graphene through a film of water on a glass sheet and transferring it to the optical fiber is proposed, which can avoid the transmission loss caused by PMMA residual and further strengthen the electric field of SPP. The G/HMM/D-POF sensor has been proved to have several virtues, such as low cost, simple fabrication, high sensitivity (aqueous ethanol with 5166.7 nm/RIU; aqueous glucose with 2767.3 nm/RIU), and satisfactory linear response. These high sensitivity D-POF SPR sensors would promote the detection of chemical and biological molecule.
2. Simulation results and discussion
2.1 Theoretical calculation
Considering that the thickness of both the metal and dielectric are below the excitation wavelength [28], the hyperbolic properties of the multi-stack of metal/dielectric bilayers can be explained with the effective medium theory (EMT). The components of the equivalent dielectric tensor of the HMM can be evaluated using the criteria of the EMT, expressed as:
Fig. 1. (a) The schematic of the composite structure of monolayer graphene and a multilayer metamaterial at p = 0.76. (b) The real and imaginary part of the permittivity component of εm and εd for the HMM with p = 0.76. (c) The real part relation curve between kx/k0 and kz/k0, with colored lines representing different wavelengths. (d) Dispersion curves of SPP, GPP, and BPP.
The dispersions of both modes are well beyond the light in the air. Therefore, the wavevectors in metamaterials need to be further enhanced through the polarization coupling of the multiperiod metal and dielectric. However, the specific calculation and derivation need further study. According to the wave vector condition of different mode excitation, the mode dispersion analysis diagram is divided into four areas. In area I, the wavevector is too low to excite any mode. The wavevector in area II can excite the SPP. The BPP° mode begins to appear when the wavevector further increase (in area III). Notably, the BPP is derived from the repulsion of the GPP mode. Hence, the BPPn mode appears only when the GPP satisfies a higher wavevector (in area IV).
3.2 Optimization of the HMM in simulation
Based on the LP01 mode (see Supplement 1), we further vary the number of the period and add monolayer graphene to reveal the best design sensor. The finite element method was employed to simulate the sensing performance of the HMM/D-POF and Au/D-POF sensor (the thickness of Au is 50 nm). The model diagram is provided in Fig. 2(a). The angle of incident light was fixed at 80 degrees, and the RI of the analyte was changed to obtain different transmittance spectra. In Fig. 2(b), the performance of the normalized transmission spectra of the n*(Au/Al2O3) and Au/D-POF structures are described. The spectra of the G/n*(Au/Al2O3) and G/Au/D-POF structures are presented in Fig. 2(c). The DRD represents the intensity of resonance. The DRD of the above structures is illustrated in Fig. 2(d), in which the G/3*(Au/Al2O3)/D-POF displays the best resonance characteristics. Moreover, the DRD value of 2*(Au/Al2O3) is small mainly due to the thickness of the sensitive layer is too thin to achieve the optimal resonance intensity. When n=4 or 5, the DRD values are weak, indicating that the depth of plasmon evanescent field propagation was limited. Besides, the presence of monolayer graphene can increase the resonance depth. The 3*(Au/Al2O3)/D-POF, Au/D-POF, and G/Au/D-POF also display good resonance characteristics. Therefore, the four structures with the best resonance intensity were selected to further compare their redshift and R2, and the results are shown in Fig. 2(e). It is verified that all the four structures possess a good linear correlation. The inset of Fig. 2(e) exhibits the redshift contrast of the four structures. The G/3*(Au/Al2O3)/D-POF possess the optimal parameters, with the red shift and sensitivity of 44 nm and 3666.7 nm/RIU, respectively. Moreover, the comparative analysis of sensor redshift reveals that the values of 3*(Au/Al2O3)/D-POF can be improved 10% with the adding of monolayer graphene. The reason why the redshift value is improved by 10% is that the monolayer graphene can strengthen the electric field of SPP. The biggest advantage of the SPR sensor applied in sensing and detection is that the electromagnetic field is concentrated near the metal surface. The interaction between high-intensity electromagnetic field and analyte contributes to obtaining higher sensing response sensitivity. Therefore, the electric field distributions of the different structures need to be analyzed in detail. In Fig. 2(f), the electric field distributions values of the different structures are described, with the values of Au/D-POF and G/Au/D-POF as reference line. Specifically, only the 3*(Au/Al2O3)/D-POF and G/3*(Au/Al2O3)/D-POF exceed the reference line, and the optimal structure is G/3*(Au/Al2O3)/D-POF. The detailed electric field distributions curves of the different structures are shown in Fig. 2(g) and (h). Different from the pure metal structures, the metamaterial structures exhibit step-like changes in the electric field. The electric field intensification occurs in the Al2O3 layer, with an extreme value at the interface, followed by an exponential decay. The plasmon polariton that occurred in two metal layers is defined as gap plasmon polariton (GPP) [29], which can produce a stronger electric field confinement effect. Since the BPP originates from the repulsion of GPPs, the BPPs can support extremely high momentum propagate inside the bulk of the material and produce a stronger electric field confinement effect on the surface. Additionally, the interaction between light and material on the surface can be improved through high-intensity electric field amplitude and strong electric field confinement. Notably, the electric field has an extreme value as the depth of plasmon evanescent field propagation was limited. After reaching the maximum penetration depth, the continued stacking of layers made the evanescent field attenuated rapidly [30]. The analysis results of the polarized wave propagation are illustrated in Fig. S2 (see Supplement 1). Therefore, the optimal structure is G/3*(Au/Al2O3)/D-POF after the sensor structure parameters are comprehensively optimized.
Fig. 2. (a) Simulation model of the structure n*(Au/Al2O3) (n=2-5). (b) Normalized transmission spectra of Au/D-POF and n*(Au/Al2O3)/D-POF for RI increasing from 1.34 to 1.352, respectively. (c) Normalized transmission spectra of G/Au/D-POF and G/n*(Au/Al2O3)/D-POF for the same RIs. (d) Summary of the DRD for various structures; the inset is the shape diagram of DRD. (e) Resonance wavelength of the four structures sensor as a function of ethanol solution RIs from 1.34 to 1.352; the inset is the red shift contrast diagram. (f) Summary of the NormE values for Au/D-POF and n*(Au/Al2O3)/D-POF without and with monolayer graphene. The electric field distributions of (g) the Au/D-POF and n*(Au/Al2O3)/D-POF, (h) G/Au/D-POF and G/n*(Au/Al2O3)/D-POF at their resonance wavelength for SRI=1.34.
3. Experimental setup and results
3.1 Preparation and detection of G/HMM/D-POF sensor
The preparation process of G/HMM/D-POF is exhibited in Fig. 3(a). Firstly, the POF (inner diameter:1 mm) was cut into 15 cm long strips. Regarding the 1.5 cm length fiber located in the middle of the optical fiber section, the cladding and part of the fiber core should be removed as the sensing area. Then, the HMM film is transferring to D-POF. The HMM/D-POF with the best sensing performance was adopted to salvage the monolayer graphene. The experimental setup for measuring the sensing performance of the G/HMM/D-POF sensor is provided in Fig. 3(b). A white light source (Ocean Optics, HL-2000) was introduced into the G/HMM/D-POF. The microfluidic chips made of polydimethylsiloxane (PDMS) were utilized to act as the testing reaction device. The actual size of the test chamber is 20×1×1 mm. A magnified image of the test chamber was exhibited in Fig. 3(a). The spectrometer (Idea optics Instruments, PG 2000) was applied to record the SPR spectrum, and the recorded image was presented on a computer.
3.2 Characterization of the samples
After the simulation analysis, the experiment was conducted according to the simulation parameters. The optical graph of Au/D-POF and n*(Au/Al2O3) (n=2–5) are shown in Fig. 4(a). The length of the sensing area is controlled at about 1.5 cm, and the color gradually deepens with the increase in the number of periods. The cross-sectional image of 50 nm Au film and 3*(Au/Al2O3) were characterized with the scanning electron microscope (SEM), as illustrated in Fig. 4(b) and (c). They were deposited on the silicon substrates to ensure their accurate measurement. The thickness and position of Au film (∼16 nm, bright layers) and Al2O3 film (∼5 nm, dark layers) can be easily distinguished from these SEM images. The Al2O3 layers provide an effective interval layer for Au films, acting as the crucial role for the HMM design. Figure 4(d) exhibits the atomic force microscopy (AFM) image of the 2*(Au/Al2O3). In Fig. 4(e), the corresponding height profile indicates a thickness of 45 nm, consistent with the theory thickness of 42 nm. Besides, the elemental composition of the n*(Au/Al2O3) substrates was analyzed by X-ray photoelectron spectroscopy (XPS). Figure 4(f) provides the XPS survey spectrum of the substrate, including the elements of Al 2p (56.49 eV), Au 4f, C 1s (284.88 eV), Au 4d5 (335.15 eV), Au 4d3 (354.62 eV), and O 1s (531.78 eV). The high-resolution spectra of the Au 4f, Al 2p, and O 1s were presented in Fig. 4(g)-(i), respectively. Besides, the high-resolution XPS scan of the Au 4f demonstrated two characteristic peaks of Au 4f5/2 (87.84 eV) and Au 4f7/2 (84.13 eV), supporting the insertion of crystallized Au [31,32]. Notably, the peak of Al 2p orbital of Al3+ ions are derived from Al2O3, suggesting that the Al metal has been oxidized as expected.
Fig. 4. (a) Optical graph of Au/D-POF and n*(Au/Al2O3) (n=2–5). (b) SEM cross-sectional image of a 50-nm-thick Au structure. (c) SEM cross-sectional image of 3*(Au/Al2O3). (d) AFM image of 2*(Au/Al2O3). (e) The AFM height profile. XPS spectra in (f) survey, (g) Au 4f, (h) Al 2p, and (i) O 1s regions.
3.3 Sensing performance of the designed sensor
The experiments for testing the sensing performance of Au/D-POF and n*(Au/Al2O3)/D-POF were performed based on theoretical analysis. The normalized transmission spectra of Au/D-POF and n*(Au/Al2O3)/D-POF were displayed in Fig. 5(a)-(e), which tested the ethanol solutions with RIs from 1.34 to 1.352. The insets in these figures are the model diagram. With the increase in HMM periods, the spectral width of the HMM/D-POF sensor gradually increases, and the SPR peaks from 555 to 730 nm with RI of 1.34. According to Fig. 1(b), the resonance wavelength of HMM should be greater than 580 nm. Thus, the structure of 2*(Au/Al2O3) in experiment cannot be defined as HMM. Besides, the summary of red shift and resonance wavelength for n*(Au/Al2O3)/D-POF was shown in Fig. 5(g). The optimal value of the red shift is equal to 36 nm, corresponding to 3*(Au/Al2O3) and 4*(Au/Al2O3). However, Fig. 5(d) and (e) indicate that the normalized transmission spectra become irregular because the structure thickness exceeds the depth of plasmon evanescent field propagation. Especially, the SPR peak excessive broadening results in incomplete spectral curves when n=5. It has been proved experimentally verified that the optimal structure is 3*(Au/Al2O3)/D-POF, which is consistent with the theoretical simulation. The sensitivity of the 3*(Au/Al2O3)/D-POF sensor is 3000 nm/RIU. Furthermore, the monolayer graphene was transferred to the surface of 3*(Au/Al2O3)/D-POF to form G/HMM/D-POF sensor, so as to further improve the sensing performance of the optimal structure. The normalized transmission spectra of G/3*(Au/Al2O3)/D-POF sensor are exhibited in Fig. 5(f). As observed in Fig. 5(g), the red shift of the sensor is 62 nm, corresponding to the sensitivity of 5166.7 nm/RIU. Therefore, the sensitivity of G/3*(Au/Al2O3)/D-POF sensor has been significantly improved about 1.7 times compared with 3*(Au/Al2O3)/D-POF (3000 nm/RIU). The main reasons for this phenomenon are detailed as follows. 1) The graphene can exhibit much tighter plasmons confinement and enhance the electric field of SPP [33]; 2) the graphene possesses advantages of a large surface-to-volume ratio and can efficiently adsorb various polar molecules and biomolecules. In Fig. 5(h), the characteristic peaks D (1350 cm-1), G (1582 cm-1), and 2G (2700 cm-1) in Raman spectra, and the ratio of G to 2G (1:2) confirm the existence of a monolayer graphene. The Raman mapping of monolayer graphene at 1582 cm-1 was measured with an area of 20×20 μm2. Figure 5(i) displays the typical response-recovery characteristic of a sensor with RI=1.3486 at a resonance wavelength of 692 nm. The response and recovery time for detecting ethanol solution are 0.58 and 0.73 s, respectively. The above analysis validated that the G/3*(Au/Al2O3)/D-POF sensor possesses the excellent sensing performance. In order to intuitively demonstrate the excellent performance of the sensor in this paper, the sensing performances of various D-POF sensors are compared and shown in Table 1.
Fig. 5. (a)-(e) Normalized transmission spectra of Au/D-POF and n*(Au/Al2O3)/D-POF (n=2-5) with the ethanol solution RIs from 1.34 to 1.352, respectively. (f) Normalized transmission spectra of G/3*(Au/Al2O3)/D-POF with the same RIs. (g) Red shift and resonance wavelength for various structures with the same RIs. (h) Raman spectrum and Raman mapping of monolayer graphene at 1582 cm-1 with an area of 20×20 μm2. (i) Response-recovery curve of G/3*(Au/Al2O3)/D-POF sensor for the aqueous ethanol RI at 1.3486.
Table 1. Sensing performance comparison of various D-POF SPR sensorsa
3.4 Application of the sensor
Moreover, glucose monitoring is crucial for the detection and treatment of diabetes. The G/3*(Au/Al2O3)/D-POF sensor was used for the aqueous glucose to further verify the detection performance. The schematic diagram of the sensor detecting glucose molecule is exhibited in Fig. 6(a). Figure 6(b) illustrates the normalized transmission spectra of the sensor with the aqueous glucose RIs from 1.3398 to 1.3557. In Fig. 6(c), the red shift of the sensor is 44 nm, corresponding to the sensitivity of 2767.3 nm/RIU. Besides, the R2 of the calibration curve for the sensor is 0.9974, reflecting the excellent linear detection capability. The normalized transmission spectra of 10 cycles modes were recorded (RI of 1.3557) and exhibited in Fig. 6(d), revealing the excellent reproducibility of the sensor. Therefore, the G/3*(Au/Al2O3)/D-POF sensor can be qualified for detecting the aqueous glucose.
Fig. 6. (a) Schematic diagram of a sensor detecting a glucose molecule. (b) Normalized transmission spectra of G/3*(Au/Al2O3)/D-POF sensor with the aqueous glucose RIs from 1.3398 to 1.3557. (c) Resonance wavelength redshift of the sensor as a function of aqueous glucose RIs from 1.3398 to 1.3557. (d) Normalized transmission spectra of the sensor at the aqueous glucose RI of 1.3557, recorded in a cycling mode (up to 10 cycles).
4. Conclusion
In summary, a G/HMM/D-POF SPR sensor with high sensitivity was successfully prepared. The monolayer graphene as a sensitive layer can effectively enhance sensing performance of the sensor. Besides, the optimal structure is G/3*(Au/Al2O3)/D-POF with a sensitivity of 5166.7 nm/RIU for detecting aqueous ethanol. The sensitivity of the G/3*(Au/Al2O3)/D-POF sensor has been significantly improved by 1.7 times compared to the 3*(Au/Al2O3)/D-POF sensor. Furthermore, the sensor has the relatively excellent ability to detect aqueous glucose with good sensitivity (2767.3 nm/RIU), excellent linearity (R2=0.9974), good stability, and repeatability. These results demonstrate the sensors have great potential for sensing applications related to food safety and bio-monitoring.
5. Experimental methods
5.1 Preparation of the HMMs
A series of composite HMMs [n*(Au/Al2O3)] was produced by alternating deposition of gold (thickness of 16 nm) and Al2O3 (thickness of 5 nm) layers on copper foil using the thermal evaporation method. The number of the period is varied from 2 to 5. Firstly, the Au layer was deposited on the copper foil at a rate of 0.3 Å·s-1. Subsequently, an Al2O3 layer was deposited on the Au film [35]. The 5 nm thick Al2O3 film was obtained by repeating the process of evaporating the 2.5 nm Al film (0.5 Å/s, 7 × 10−5 Pa) and oxidizing it in pure oxygen for two times. The 2-5 period structures were prepared by alternating deposition Au film and Al2O3 film. Afterward, Au film (50 nm thickness) was prepared for comparison.
5.2 Method of transferring graphene
Monolayer graphene grown on copper foil through chemical vapor deposition (CVD) and was obtained by etching copper foil with ferric chloride solution. Then, the monolayer graphene needs to be transferred to deionized water using the water film on the hydrophobic glass sheet. This cleaning process is repeated three times. Besides, the fiber is dried on a heating table at 40 ℃ for 3 mins, making the graphene and HMM fit together more closely. This method ensures the transfer of monolayer graphene and avoids the loss caused by PMMA residue during the wet transfer process.
Funding
National Natural Science Foundation of China (11674199, 12074226); Natural Science Foundation of Shandong Province (ZR2019MF025); Shandong Provincial Key Laboratory of Biophysics (FWL2021066).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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