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Abstract
Bovine serum albumin (BSA) label-free concentration sensor based on silica corrosion quantitative monitoring system (SCQMS) has been proposed. Anti-resonance of hollow cylindrical waveguide (HCW) in SCQMS is simulated and investigated for monitoring corrosion rate quantitatively. Hydrofluoric acid (HF) samples with different concentrations are studied respectively, and the corrosion rate is obtained by demodulating the corresponding anti-resonance dips shift and free spectral range (FSR). Therefore, a high-precision SQCMS was prepared successfully. On this basis, a highly sensitive concentration sensor based on hole-assisted dual-core fiber (HADF) is prepared. The BSA samples with concentration from 0.2 mg/mL to 0.7 mg/mL are detected. The sensor has a high sensitivity of 30.04 nm/(mg/mL) and ultra-low limit of detection (LOD) of 0.05 mg/mL for the assisted core exposed to the target solution directly. We have demonstrated the SCQMS that can be a feasible tool for precise and quantitative corrosion of silicon structure safely. In addition, the concentration sensor structure has a wide application for ultra-low LOD, simple preparation process and high integration.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Bovine serum albumin (BSA) mainly plays an important role of vascular osmotic pressure, PH buffer, carrier and nutrition in blood, which also known as the fifth component. BSA contains 583 amino acid residues with a molecular weight of 66.430 kDa and has wide applicate prospects in biochemical experiments and medical research [1–3]. The concentration detection of BSA has been investigated extensively. At present, bromocresol green spectrophotometry and ultraviolet spectrophotometer are usually used to measure the concentration of BSA, various fluorescently labeled probes and engineered molecular beacons have also been demonstrated used for concentration detection [4–6]. However, the molecular-probe is limited due to the uneven intracellular distribution on detecting probes, photobleaching, and intracellular protein binding. The accuracy of the sensors and label-free detection also need to be improved in some microcosmic aspects. Therefore, it is urgent to develop a highly sensitive and label-free biological sample solution concentration detection method.
Optic fiber sensor is a powerful detection and analysis tool for protein concentration detection [7–14]. At present, the concentration detection is mainly based on the different refractive index of target solution, molecular specific binding and fluorescence quenching, which cause parameters of optical field propagation changed in the fiber. In order to improve the detection sensitivity and resolution, fiber grating [7,8], Fabry-Perot interference [9–12], Mach Zander interference [2,13–15], surface plasmon resonance [16–18], etc. are introduced into the fiber sensor. Li et al. proposed an innovative label-free optical fiber biosensor based on a Mach-Zehnder interferometer for BSA concentration detection. The sensor device was fabricated by femtosecond laser micromachining and chemical etching [2]. Ermatov et al. utilized multispectral optical sensing technique to determine the concentration and refractive index dispersion for BSA and the accuracy can meet clinical needs [17]. Zhang et al. presented a long-range surface plasmon resonance (LRSPR) sensor based on a side-polished multimode fiber for biosensing applications. The BSA concentration detection was performed to further confirm the sensitivity enhancement of the optimized sensor in biosensing [16]. Huong et al. reported a fiber optical biosensor based on localized surface plasmon resonance (LSPR) of Au nanoparticles for the detection of BSA concentration [18].
To simplify the preparation technology and reduce cost of this kind of fiber sensor, a new preparation method is needed urgently. As an important inorganic fluorine substance, hydrogen fluoride (HF) is the basic raw material for the fluorine chemical industry. Compared with strong corrosive media such as sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), HF has strong corrosiveness and toxicity due to its small radius and strong electronegativity [19,20]. Metal, glass and silicon can be excessively etched. So, HF is used for etching glass and carving patterns. As for micromachining, quartz materials with different doping can be etched into various microcavity and molds by HF with high accuracy [2,21–23]. However, the corrosion rate of HF on silica fiber has not been well studied because of its strong corrosivity, irritation and toxicity. In order to further improve the corrosion accuracy quantitatively, it is necessary to design an effective and safe monitoring method. In this paper, we propose a BSA label-free concentration sensor based on silica corrosion quantitative monitoring system (SCQMS), which is based on three-beam interference of hole-assisted dual-core fiber (HADF) and anti-resonance sensor structure of hollow cylindrical waveguide (HCW). HF is isolated from external environment in the SQCMS and silica materials can be etched safely, accurately, and quantitatively. A corrosion monitoring channel and BSA concentration detection channel are both prepared in a polydimethylsiloxane (PDMS) microfluidics chip, which can’t be etched by HF due to its chemical inertness [24,25]. The HCW structure for monitoring corrosion rate and HADF structure for detecting concentration are embedded in channels respectively. Starting from the Fresnel formula, the anti-resonance principle of HCW is investigated and simulated when the surrounding environment of the HCW changed. The corrosion rate of silica fiber is directly monitored by HF with concentration of 25%, 50%, 66%, and 75%. By demodulating the shift of the anti-resonance dips and the free spectral range (FSR) of the transmission curve, the corrosion rate is monitored quantitatively with high sensitivity and accuracy. On this basis, the HADF is etched precisely. BSA solutions with different concentrations of with an infinitesimally small refractive index difference can be clearly differentiated by the deigned HADF sensor structure for the assisted core exposed to the target solution directly. An excellent feature of HADF sensor structure is that target solution can directly interact with the fiber core in micro channels, which leads to a higher detection sensitivity and limit of detection (LOD). Experiments demonstrated the sensor structure exhibited a BSA solution concentration sensitivity of 30.04 nm/(mg/mL) and a LOD of 0.05 mg/mL, respectively. The SCQMS has great application potentials in silicon corrosion, material production, solution concentration detection, medical research and other fields.
2. Principle and fabrication
2.1 Principle
As presented in Fig. 1(a), the proposed corrosion monitoring sensor structure consists of two single-mode fibers (Corning SMF-28e) and a silicon HCW (air core diameter ∼20 µm, outer silica cladding diameter ∼126 µm with RI of 1.452). When the beam from core of SMF propagates into the HCW, part of the light satisfied to Eq. (1) will leak into the sidewall. Reflection and transmission occur at the cladding/air interface M1, resulting in the interference of multi-beam light in HCW as shown in Fig. 1(b). FSR of the anti-resonance transmission spectrum mainly depends on the thickness of the sidewall of HCW. Assuming δ is the phase difference between two adjacent reflected beams. It can be obtained that the phase difference δ of two adjacent beams is:
where, h is the thickness of the HCF sidewall and π is the phase caused by the half-wave loss of reflected light, n1 and n0 are RI of the sidewall and air core of HCF respectively. The light satisfied to Eq. (1), will have the anti-resonance reflection, which is limited in the air core, corresponding to loss dips in the interference spectrum.
Fig. 1. (a) Schematic structure of SMF-HCW-SMF (SHS) corrosion rate monitoring sensor. (b) Principle of anti-resonance in HCW.
According to the theory of anti-resonance principle [26], the central wavelength of the mth resonant peak λm can be expressed as:
Therefore, the corrosion rate of silicon dioxide with different concentrations of HF are obtained by demodulating the shift of the anti-resonance dips, which is caused by the change of h during the corrosion process. However, as the [26,27] reported, when the surrounding environment of HCW is liquid, the anti-resonance peak intensity will disappear, which brings great difficulty in monitoring the shift of anti-resonance dip. We have analyzed the reasons for this phenomenon in principle. The total optical transmission intensity of anti-resonance light field in SMF can be calculated by
Finite Difference Beam Propagation Method (FD-BPM) is used to simulate the transmission light distribution of the air core and sidewall of HCW as shown in Fig. 2. Parts of light leak into the side wall and reflect constantly between the inner and outer surface of the HCW as a FP cavity. The air core and cladding RI of HCW are set to be 1.0 and 1.452. The core and cladding RI of SMF are set to be 1.452 and 1.442; the wavelength is set as the corresponding resonance wavelength respectively. The presence of liquids around the HCW can inhibit energy leakage and the anti-resonance dips intensity decrease. If the sensor is immersed into liquids completely, the HCW will experience such an inhibiting effect on the intensity of the anti-resonance dips and contribute no more to the energy loss. So once the sensor structure is immersed in HF completely, there will be no anti-resonance as shown in Fig. 2. When the HF in the channel are drained, the HCW sensor structure is exposed to air. According to the Fresnel formula, r’ increased caused by the decrease of refractive index of the surroundings. The increase of the anti-resonance intensity can be obtained from Eq. (4), and the anti-resonance dips appear. The anti-resonance dips can be monitored when the HCW sensor structure exposed to the air. A certain anti-resonance dip is monitored as the HCW is etched and the transmission spectrum can be demodulated. To solve this problem, we designed a device that can pump HF and air into the micro channel cyclically, and real-time monitoring of corrosion can be realized by adjusting the cycle of pumping.
Fig. 2. Simulated cross-section transmission light distribution of the SHS structure. (a) In air. (b) In liquid.
In the BSA solution concentration detection channel, the etched HADF is used for BSA concentration detecting. The side wall of HADF has been etched accurately and at the same time the suspended core was not etched, which can directly interact with the target solution. The proposed schematic structure is shown in Fig. 4(b), piece of HADF is fusion spliced between two sections of graded-index fiber (GIF, Yangtze Optical Fiber, 105/125), and the GIF acts as a collimator to expand the beam into the HADF [28,29]. Light reaches the HADF that splits into three sections. One beam travel along the cladding and the others propagate along the remained core and assisted core in the hole. The beam propagating in the hole is dissipated due to the high loss of the air cavity. The interference occurred due to the phase difference of three beams, and the phase difference affects the transmission intensity of the recombined beam. This intensity can be expressed as:
When the effective RI of the assisted core is varied, and the dip wavelength shifted by an amount Δλ, assuming the length of HADF is L, the sensitivity is estimated as:
Fig. 4. (a) Light field distribution in SMF-GIF structure. (b)The schematic configuration of the proposed sensor based on HADF. (c) Three-dimensional schematization of etched HADF. (d). Cross section of etched HADF. (d). CCD photo of cross section.
2.2 Fabrication
The corrosion rate sensor structure and concentration detection sensor are integrated in PDMS (SYLGARD 184) microfluidics chip, as shown in Fig. 3. To get a circular cross section channel, microwires with diameter of 800 µm (enameled copper and steel) is selected as molds for preparing microfluidic channels due to the smooth surface and high flexibility. In order to prepare a thinner channel for placing the optical fiber, we selected the wire with diameter of 500 µm for the enameled copper. PDMS and its curing agent are mixed evenly in a ratio of 10:1 and then poured into the square mold with a size of 5 cm*5 cm*0.5 cm. The upper layer of PDMS is produced by pouring the same solution into the mold. Then the two molds were placed in a heating furnace and heated at 80°C for five hours. The PDMS mold was placed at room temperature and cooled slowly. Microfluidic chip is taken out of the molds by the tweezer carefully, and cleaned in ultrasonic cleaner at 27°C for 30 mins. Two holes are produced by the punch in the upper layer, and two plastic injection heads are installed in the small holes for liquid and air transferring. The SHS structure for corrosion rate monitoring and the HADF structure for concentration detecting are embedded in the designed thinner microfluidic channels respectively. UV curing glue is used to fix the two sensor structures at the grooves firmly through the high-precision displacement platform. The upper cover and the lower layer with channels are heat sealed to ensure that the liquid in the channels cannot flow into the layer gap.
The proposed concentration detection sensor structure is fabricated by splicing a short section of HADF (Centre core diameter ∼13 µm, air core diameter ∼13 µm, air hole diameter ∼45 µm, outer silica cladding diameter ∼125 µm) of 1800 µm spliced between SMFs and GIFs as shown in Fig. 4(a).
FD-BPM is used to simulate the light field of the SMF-GIF structure. The intensity profile of light field reaches the maximum twice in one period due to the gradient of the refractive index as Fig. 4(a) shows. It can be seen that the divergence of the light field is greatest at the position of 1/4 pitch due to the self-focusing effect of GIF. The related content is also mentioned in [30]. Therefore, GIF with a length of 500 µm is precisely split up by the high-precision cutting platform. The beam is well diverged and collected through GIF. The side wall of the hole cavity is precisely etched to ensure that the assisted core can directly contact the target solution, as shown in Figs. 4(c)-(e).
The light emitted from broadband source (BBS, 1400–1600 nm) goes through the sensor structure in the microchips and measured by optical spectrum analyzer (OSA, AQ6370B). The pipes at both ends of the SCQMS are connected to single channel syringe pump (LSP01-1AY) and the peristaltic pump (RongBai pump, BT301LC) respectively to ensure the air-liquid circulation inside the channel as shown in Fig. 5.
3. Experimental results
In this experiment, HF (purchased from Macklin) are prepared into four concentrations of solution samples, and the volume ratios of deionized water were 1:1, 1:2, 1:3, and 3:1. The HF samples are injected into the liquid inlets and divided into two parts. The sensor structures corrosion rate monitoring channel and the BSA solution concentration detecting channel are etched and monitored simultaneously. The air and HF circulation device are set to circulate once every ten seconds. After the HF sample reacted with each structure in the channel for one minute, the HF sample was discharged out using a microfluidic pump and the transmission spectrum is collected.
Due to the presence of the hydrophobic material on the wall of the channels, there is almost no liquid remaining after the above steps. The corrosion rate is modulated by measuring the FSR of the anti-resonance dip according to Eqs. (2), (3). The transmission spectrums of the two-channel optical fiber sensing structure are obtained respectively. The above process is repeated ten times, and we obtain the result of the corrosion degree of HF on HCW within ten minutes. As shown in Fig. 6(a). The corrosion process of HCW with other concentrations of HF samples are also detected in the same way and the results are shown in Figs. 6(b)-(d). It can be seen that the anti-resonance dips shift to the short wavelength, as sidewall of HCW is etched and thinned by HF, which is consistent with in Eq. (2). The quality of the transmission spectrum deteriorates as HCW is etched further, which is believed due to the flatness of the outer surface of HCW becomes rough in tiny areas to varying degrees. The reflectivity r’ is uneven at that time, which causes the interference intensity changed as Eq. (4). It can be seen that clearly as the HF concentration increases, the anti-resonant peak moves to the short wavelength faster, that is, the corrosion rate increased. In order to further study the relationship between concentration and corrosion rate quantitatively, we monitored the movement of the anti-resonance dip at 1568.7 nm in HF with different concentration as shown in Fig. 7(a). We performed a linear fitting and found that the corrosion rate increases linearly with the increase of the HF concentration in Fig. 7(b), and the linearity is 99.8%. The insert in Fig. 7(b) is a CCD photo of HCW etched by four HF samples with different concentrations after 10 mins.
Fig. 6. The measured transmission spectra of the sensor structures with different concentration of HF (a) 1:3, (b) 1:2, (c) 1:1, (d) 3:1.
Fig. 7. The relationships between anti resonance dip and corrosion time. (b) The relationships between concentration of HF and corrosion rate. The insert is the cross CCD photo of etched HCW.
On this basis, the thinner side wall of the HADF is accurately etched to ensure that the assisted core in the side hole is not etched as shown in Fig. 4(e) and the etched transmission spectrum is shown in the insert of Fig. 8(a). Therefore, the assisted core can be directly contacted with the targeted solution to obtain higher sensitivity. A series of BSA solutions with different concentration ranging from 0.2 to 0.7 mg/mL with a step of 0.1 were prepared as target analytes. Different weights of BSA powder were dissolved in deionized water at room temperature. Magnetic stirrers are used to disperse the solution evenly and then diluted to a lower concentration. BSA dispersion at lower concentrations is injected into the concentration monitoring channel at a lower flow rate through the single-channel uniform velocity injection device. The assisted core of the HADF sensing structure in the hole is contact with the BSA dispersion directly and the transmission spectrum is monitored in real time. After the channel is cleaned, a higher concentration of BSA solution is injected, and the above steps are performed. BSA solutions with different concentration are detected.
Fig. 8. (a) Transmission spectra at different BSA concentration samples, and the insert transmission spectrum of unetched HADF sensor structure (Black line) and the etched HADF sensor structure (Red line). (b) Anti-Resonance dips shift for different BSA concentration samples.
The interference dip located at 1560.43 nm is monitored until the transmission spectrum is stable. The dip shifts to long wavelength as concentration of the BSA solution increases shown in Fig. 8(a). The linear fitting curve is implemented to intuitively illustrate the change trend of the resonance wavelength with BSA concentration in Fig. 8(b).
The above experiment is conducted for three times and the error bars are added, which shows good stability. The BSA ultra-high wavelength sensitivity of 30.04 nm/(mg/mL) was obtained with high LOD of 0.05 mg/mL. The result has greatly satisfied the actual application requirements and the sensor structure with high sensitivity and LOD has great potentials in the label-free concentration detection applications.
The response time is also worth to be focused indeed. The experimental system for response time testing is shown in Fig. 9(a). Two springe pumps (gas-liquid circulation device), a tunable laser (C-Band), photoelectric detector (Conquer, PR-200K-A) and high-speed acquisition card (USB_HRF_Dev) were used to record the sensor response continually. The output power of the tunable laser was set as 0.6 mW. The tunable laser was tuned at 1558.01 nm which located at interference dip of spectrum corresponding to the position that the highest slope of the spectrum as Fig. 9(b) shows. The air and BSA solutions with concentration of 0.2 mg/mL was pushed in the channel circularly. When the BSA solutions was pushed in the channel rapidly, the voltage variation was about 0.75 V, and the voltage could return to the initial value (around the average voltage value) after each cycle as shown in Fig. 9(c). We can see that clearly, the response time and recover time of the sensor are 230 ms and 3.1 s in the insert of Fig. 9(c). The recovery time is longer than the response time because the liquid in the channel cannot be drained quickly. In addition, the stability of the HADF sensor structure is tested. The HADF structure was sealed into a glass tube by UV-curable glue that is filled with BSA solution with concentration of 1.0 mg/mL. The transmission spectrum of this structure was monitored every 10 minutes for 6 times in thermotank. The interference dips of the sensor have little jitter or drift, and it is proved that the sensor has a good stability.
Fig. 9. (a) The experimental system for measuring the response time of the sensor. (b) The spectrum of the tunable laser and the transmission spectrum of sensor structure. (c) The response of the sensor, the insert: the response time and recovery time of the sensor.
4. Conclusions
In summary, a SCQMS has been proposed. A corrosion channel and a concentration monitor channel are prepared in a PDMS microfluidics chip. The anti-resonance sensor structure of HCW for corrosion rate monitoring quantitatively is designed and prepared. Accurate corrosion can be achieved safely, accurately and quantitatively in real time. The corrosion limitation can reach 1 micron. On this basis, we have prepared a highly integrated BSA concentration sensor with a core that can directly contact the target solvent. The higher sensitivity can reach 30.04 nm/(mg/mL) and the LOD is 0.05 mg/mL, demonstrating its ability for ultrasensitive quantitation. Compared with the conventional BSA concentration detection method at present, the optical fiber sensor device has the advantages of high sensitivity, high integration, anti-electromagnetic interference and lower detection limit, which has great application potential in the label-free detection of biological liquid concentration. The sensing system has high preparation stability, simple process, low cost and outstanding advantages in terms of productization.
Funding
National Natural Science Foundation of China (11874010); Natural Science Foundation of Shandong Province (ZR2021MF111); Natural Science Foundation of Guangxi Province (2021GXNSFAA075012, 2021GXNSFAA220057).
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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