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Abstract
Biosensors have various potential applications in biomedical research and clinical diagnostic, especially in detection of biomolecules in highly diluted solutions. In this study, a high-performance Bloch surface wave biosensor was constructed for the detection of hemoglobin. The procedure consisted of designing a porous silicon-based Kretschmann configuration to ensure excitation of the Bloch surface wave. The performance of the resulting sensor was then optimized by adjusting the buffer layer parameters based on the impedance matching method. The results showed an increase in the quality factor and figure of merit of the biosensor as a function of the decrease in thickness and refractive index of the buffer layer. The combination of the two optimization methods resulted in the quality factor and figure of merit of the optimized biosensor reaching as high as Q = 6967.4 and FOM = 11050RIU−1, respectively. In sum, the designed biosensor with high performance looks promising for future detection of hemoglobin.
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1. Introduction
Sensors have gradually become the backbone of modern information technology in terms of technology development and applications [1]. So far, sensors based on optical signals have widely been used in medical diagnosis, environmental monitoring, food safety, defense industry, and other fields [2]. In particular, optical sensors are considered important devices for bio-sensing applications owing to their inherent advantages, such as strong anti-interference ability, remote sensing, and rapid response time [3–6]. In this category, optical biosensors collect changes in the information of test substances by monitoring the variations in certain characteristics of light in interaction with the test object.
Recently, rapid progress has been made in optical sensing and detection technology based on the surface wave. Surface plasmon wave (SPW), surface-enhanced Raman, surface-enhanced fluorescence, and other technologies have received increasing attention as typical representative technologies [7–10]. The most widely used in commercial applications are SPW-based biosensors, which are very sensitive to the refractive index changes at the sensor surface [9,10]. However, the loss and absorption of the metal film broaden the SPW resonance mode, thereby limiting the detection sensitivity and preventing the detection of small molecules. Thus, one key to achieving higher sensitivity is by narrowing the SPW resonance, which can be done by micro-ring resonator using the whispering-gallery-mode to yield ultrahigh quality factor (Q) values and extremely narrow resonances, thereby high resolution [11]. The disadvantages of this kind of sensor lie in probing light closely limited by the resonant, where only a very small part of the optical mode interacts with the analysis solution to yield relatively low sensitivities. The other has to do with the ultrahigh Q resonator with thermal instability and the difficulty in achieving experimental data [11–13]. In such processes, Bloch surface wave (BSW) excited by a moderate-Q one-dimensional photonic crystal (1DPC) structure is a new bio-sensing mechanism [14], offering multiple design freedoms to modulate the Q value and electromagnetic field distributions. The BSW mode originates from the violation of a uniform waveguide system caused by edge effects, leading to a structure with a buffer layer on top of a 1DPC. In other words, an effective Fabry-Perot resonator forms in the buffer layer by the photonic band gap and the total internal reflection boundary [15,16]. The excited BSW mode strongly localizes at the interface between the biosensor and bio-solution. The strong field enhancement renders BSW greatly sensitive to changes in bio-solution. In reflected spectra of biosensors, BSW manifests itself as resonant depressions. Thus, the behaviors of biomolecules in bio-solution can be clarified by looking at shifts in the resonance spectra either of angular, wavelength, phase, or intensity domain [17–22]. Variations in bio-solution concentration can induce slight changes in its refractive index, leading to shifts in BSW mode. The sensitivity of biosensors can be defined by the ratio between the BSW mode shift and the refractive index change. In addition, the sensing performances of biosensors can be quantified by their Q values and figure of merit (FOM) [23–25].
Porous silicon (P-Si) is a new type of nanomaterial with a unique geometric structure and material properties, making it useful in biomedical sensing, optoelectronics, drug delivery, and energy conversion [26–29]. Besides, P-Si is inexpensive and readily fabricated through the electrochemical etching of bulk silicon [30]. Moreover, the application of moderate current densities would tune the porosity of P-Si films during electrochemical etching, thereby modifying the refractive index. Also, films with different refractive indices can create 1DPC structures [31–36]. Hence, P-Si-based 1DPC coupled with prism has been used to excite BSW and yield excellent sensing signals [37–44].
As part of the standard complete blood count, the determination of total hemoglobin concentration along with the hematocrit and the mean erythrocyte hemoglobin are important factors. Independent optical methods can be used to evaluate these blood parameters and help develop quality control for blood analysis. This is called blood optics, which are principal for bio-photonic and clinical applications, both in therapy and diagnostics [45]. Blood cells include red blood cells, white blood cells, and platelets. Hemoglobin (Hb) is an important constituent of blood and its accurate measurement in the blood can help detect various diseases like anemia, diabetes, and leukemia [46,47]. Meanwhile, the blood refractive index depends on Hb concentration, with volume fraction routinely used to determine and monitor Hb [48]. For the detection of Hb and other biological molecules, low concentration markers are key requirements for early detection of diseases. This requires continuous efforts to develop highly sensitive techniques. For example, Pader et al. [49] demonstrated the application potential of a BSW-based sensor for the detection of protein aggregation $(40\deg /RIU)$. Goyal et al. [48] proposed a BSW based method for the detection of Hb with sensitivity $(69\deg /RIU)$. Rodriguez et al. [50] reported a size-selective grating coupled P-Si BSW biosensor with high sensitivity $(72\deg /RIU)$.
The performances of biosensors can be optimized by raising the Q values and the sensitivity of the probing light. However, such studies rely on numerical parameter sweeping, which provides little physical insights and sometimes cannot fully reflect the influence of system parameters [49,50]. As a result, the design of high-performance biosensors becomes a tedious task. The impedance matching method has been adopted to study this kind of problem theoretically, but the influences of the buffer layer have not yet been fully examined.
In this work, a high-performance Bloch surface wave biosensor was constructed for the detection of Hb. The performances of the resulting sensor were then optimized by adjusting the buffer layer parameters based on the impedance matching method. The results showed an increase in the Q and FOM of the biosensor as a function of the decrease in thickness and refractive index of the buffer layer. The combination of the two optimization methods resulted in the Q and FOM of the optimized biosensor reaching as high as $Q = 6967.4$ and $FOM = 11050RI{U^{ - 1}}$, respectively. In sum, the designed biosensor with high performance looks promising for future detection of Hb.
2. Theory of the optical biosensor
The design scheme of the BSW biosensor based on the Kretschmann configuration is shown in Fig. 1. Here, the prism was made of BK7 $({n_{pri}} = 1.457)$ and the considered 1DPC structure consisted of $|{(BA)^N}|C|$. The mediums A and $B$ corresponded to 50% P-Si and 80% P-Si, respectively. At a working wavelength much larger than the diameter of P-Si pores, the refractive index of the entire P-Si layer can be calculated by the Bruggeman dielectric constant approximation theory [34]:
The thicknesses of mediums A and $B$ were set to ${d_A} = 151nm$ and ${d_B} = 246nm$, respectively. A buffer layer C made of $ZnS$ was inserted between the bio-solution and 1DPC. The $ZnS$ thickness was ${d_c} = 230nm$, and the refractive index was ${n_c} = 2.28$. The flat and hard surfaces of the biosensor were very convenient for manufacturing and cleaning, thereby reducing the cost of sensors. To balance the requirements of a high-Q reflected beam and compact configuration, the period number of 1DPC was set at $N = 6$. The probing light was assumed as TE wave, and the incident angle $\theta$ consisted of the angle between the probing light and the normal of the biosensor. Although TE wave was considered as probing light, the TM wave also can be used as the probing light. For the 1DPC structure, both TM wave and TE wave can generate the photonic bandgap necessary to excite BSW. Note that BSW excited by TM wave and TE wave corresponded to different system parameters.
The theoretical analysis revealed forward- and backward-decaying propagation patterns of the excited BSW near the surface [51,52]. The surface impedance at each side of the interface was defined as the ratio of the tangential electric and magnetic fields. The forward-decaying mode can be defined by Eq. (2):
The surface Bloch impedance of the backward-decaying mode can be determined according to Eq. (3):
The positive and negative directions of Bloch impedances of the 1DPC in Eq. (3) can be expressed by Eq. (4):
According to the dispersion relationship, the Bloch wave vector of the 1DPC can be written by Eq. (5):
The surface impedance mismatch function of forwarding- and backward-decaying modes can be defined by Eq. (6):
The BSW would be excited when the impedance matching ${Z_S} = 0$. However, the changes in Hb molecules would destroy the balance, leading to impedance mismatch as a function of external variables ${n_{Hb}}$, $\theta$, and $\lambda$ for a given configuration.
Here, the Hb concentration and the refractive index of the bio-solution to be measured would change slightly. Namely, the first term on the right side of Eq. (7) would induce an impedance mismatch. To ensure excited BSW, the incident angle or probing wavelength was modified. The shift in resonant spectra of the incident angle domain required fixing the wavelength $\Delta \lambda = 0$. Thus, the incident angle sensitivity can be defined according to Eq. (8):
The Q can be used to describe the sharpness of the resonant peak, employed to express the relationship between the total resonant electromagnetic wave energy and the energy consumed by attenuation. This can be expressed by Eq. (9) in the spectrum calculation:
where FWHM represents the full width at half maximum of the resonance dip.FOM can be used as an important parameter to measure the ability of the biosensor to respond to any changes in the resonance angle. FOM may be given as a ratio between the sensitivity S (deg /RIU) and $FWHM$:
For further evaluation of the performance of the biosensor, the limit of detection (LOD) of the biosensor can be calculated as follows [53]:
where $\sigma$ is the resolution of the instrument set as 0.005 deg.3. Results and discussion
The performance of the resulting BSW biosensor in the detection of the Hb concentration was analyzed. A biosensor optimization scheme was built from the perspective of impedance matching. This was achieved by combining the optimization methods of the buffer layer thickness and refractive index to study the Q and FOM of the biosensor. To this end, the impedance mismatch function of the biosensor based on Eq. (6) was first plotted (red dashed line in Fig. 2(a)). The reflection spectrum of the biosensor was then calculated with the transfer matrix method (black solid line in Fig. 2(a)) [54]. As can be seen, the impedance matching and the reflection dip both occurred at $\theta \textrm{ = }72.311\deg$. Thus, the impedance matching condition can be used as the criterion to determine the BSW excitation, helping to build the BSW excitation system. In Fig. 2(b), the electric field intensity distribution inside the biosensor was given at the reflection dip $(\theta \textrm{ = }72.311\deg$ and $\lambda = 1060nm)$ [55]. The electric field of BSW looked strongly confined at the surface between the biosensor and the bio-solution, and an evanescent wave appeared in the bio-solution. These characteristics revealed BSW as excellent sensing signals for the detection of small changes in the bio-solution.
Fig. 2. (a) The variation in impedance mismatch function with the incident angle (red dashed line) and the reflection spectrum (black solid line) at incident wavelength $\lambda = 1060nm$. (b) The electric field distribution inside the biosensor at $\theta \textrm{ = }72.311\deg$ and $\lambda = 1060nm$.
A displacement of the BSW reflectance spectrum as a function of the concentration of the Hb solution is provided in Fig. 3. By considering the laser sources and detectors, the incident wavelength was fixed as $\lambda = 1060nm$. The refractive index of Hb solution (${n_{Hb}}$) varied with Hb concentration (${c_{Hb}}$) and the incident wavelength. This can be calculated by the model function shown in Eq. (11) [56]:
Fig. 3. The shifts in BSW reflectance spectra as a function of Hb concentration. The corresponding resonance angles were determined as ${\theta _1} = 72.320\deg ({c_{Hb}} = 10g/dl)$, ${\theta _2} = 72.414\deg ({c_{Hb}} = 11g/dl)$, ${\theta _3} = 72.578\deg ({c_{Hb}} = 12g/dl)$ and ${\theta _4} = 72.736\deg ({c_{Hb}} = 13g/dl)$, respectively.
In Eqs. (8)–(11), the sensitivity, FWHM, Q, FOM, and LOD were set to ${S_\theta } = 52.49\deg /RIU$, $FWHM = 0.04\deg$, $Q = 1812$, $FOM = 1312.25RI{U^{ - 1}}$, and $LOD = 9.5\ast {10^{ - 5}}RIU$, respectively. These values were the average of four resonance dips, with errors within the allowable measurement range.
Since the buffer layer parameters significantly impact the impedance matching, the biosensor performances were optimized in terms of buffer layer parameters. To this end, the effect of the buffer layer thickness on the biosensor performance was first studied and the results are given in Fig. 4. Here, the variations in the sensitivity, FWHM, Q, and FOM of the biosensor with buffer layer thickness were all calculated. As shown in Fig. 4(a), the sensitivity of the biosensor increased as a function of the decrease in buffer layer thickness, while FWHM showed a declining trend. In Fig. 4(b), the Q and FOM of the biosensor also increased with the decrease in thickness of the buffer layer, attributed to the definition of Q and FOM. Fortunately, the performance of the biosensor can be doubled by reducing the thickness of the buffer layer from 240 nm to 215 nm. This process was very effective in improving the performance of the biosensor.
The effect of the buffer layer refractive index on the biosensor performance was also considered. The variations in sensitivity, FWHM, Q, and FOM of the biosensor with buffer layer refractive index were all calculated and the results are gathered in Fig. 5. The sensitivity of the biosensor increased almost linearly as the buffer layer refractive index decreased (Fig. 5(a)). Also, the changes in FWHM were still not significant. In Fig. 5(b), the Q and FOM of the biosensor incremented with the decline in the refractive index of the buffer layer. At refractive indices, less than 2.27, the Q and FOM of the biosensor increased significantly, mainly due to the smaller FWHM in Fig. 5(a). Therefore, changes in the refractive index of the buffer layer can also improve the performance of biosensors.
Combining these two optimization methods in Figs. 4 and 5 resulted in buffer layer thickness of the optimized biosensor of ${d_c} = 215nm$ and buffer layer refract index of ${n_c} = 2.26$. The reflection spectrum of the optimized sensor is shown in Fig. 6 (the incident wavelength is 1060 nm). As mentioned earlier, the Hb concentration was supposed to evenly shift from $10g/dl$ to $13g/dl$ by a step of $1g/dl$. Hence, the shifts in the BSW reflectance spectra were calculated as a function of Hb concentration. The corresponding resonance angles were estimated to ${\theta _5} = 69.368\deg$ $({c_{Hb}} = 10g/dl)$, ${\theta _6} = 69.525\deg$ $({c_{Hb}} = 11g/dl)$, ${\theta _7} = 69.771\deg$ $({c_{Hb}} = 12g/dl)$, and ${\theta _8} = 70.031\deg ({c_{Hb}} = 13g/dl)$, respectively. The FWHM of the reflection dips and angle shifts between the neighboring reflection dips were recorded as $FWHM = 0.01\deg$ and $\Delta \theta = 0.221\deg$, respectively. By considering the slight variations in bio-solution refractive index $\Delta {n_{Hb}} = 0.002$, the biosensor sensitivity was calculated as ${S_\theta } = 110.5\deg /RIU$ and LOD was determined as $4.52\ast {10^{ - 5}}RIU$. Importantly, the Q and FOM of the optimized biosensor reached as high as $Q = 6967.4$ and $FOM = 11050RI{U^{ - 1}}$, respectively. Such Q and FOM were much larger than those of similarly reported biosensors [53,57].
Fig. 6. The shifts in BSW reflectance spectra as a function of the Hb concentrations. The corresponding resonance angles are ${\theta _5} = 69.368\deg ({c_{Hb}} = 10g/dl)$, ${\theta _6} = 69.525\deg$ $({c_{Hb}} = 11g/dl)$, ${\theta _7} = 69.771\deg ({c_{Hb}} = 12g/dl)$, and ${\theta _8} = 70.031\deg ({c_{Hb}} = 13g/dl)$, respectively.
4. Conclusions
In this paper, a high-performance Bloch surface wave biosensor was successfully constructed for the detection of Hemoglobin concentration. To this end, a porous silicon-based Kretschmann configuration was first designed to ensure excitation of the Bloch surface wave. The performances of the sensor were then optimized by adjusting the buffer layer parameters based on the impedance matching method. The results revealed an increase in the quality factor and figure of merit of the biosensor with the decrease in thickness and refractive index of the buffer layer. By combining the two optimization methods, the quality factor and figure of merit of the optimized biosensor reached as high as $Q = 6967.4$ and $FOM = 11050RI{U^{ - 1}}$, respectively. Overall, the proposed method looks very promising for the fabrication of Bloch surface wave biosensors for the detection of Hemoglobin concentrations.
Funding
Department of Science and Technology of Liaoning Province (22-BS-294); Scientific Research Fund of Liaoning Provincial Education Department (L2020049); National Natural Science Foundation of China (62105132).
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.
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