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Trace analysis of liquid samples has a wide range of applications in the life sciences and environmental monitoring. In this work, we have developed a compact and inexpensive photometer based on metal waveguide capillaries (MCCs) for ultrasensitive determination of absorption. The optical path can be greatly increased, and much longer than the physical length of the MWC, because light scattered by the corrugated smooth metal sidewalls can be contained within the capillary regardless of the angle of incidence. Concentrations as low as 5.12 nM can be achieved using common chromogenic reagents due to new non-linear optical amplification and fast sample switching and glucose detection.
Photometry is widely used for trace analysis of liquid samples due to the abundance of available chromogenic reagents and semiconductor optoelectronic devices1,2,3,4,5. Compared to traditional cuvette-based absorbance determination, liquid waveguide (LWC) capillaries reflect (TIR) by keeping the probe light inside the capillary1,2,3,4,5. However, without further improvement, the optical path is only close to the physical length of LWC3.6, and increasing the LWC length beyond 1.0 m will suffer from strong light attenuation and a high risk of bubbles, etc.3, 7. With regard to the proposed multi-reflection cell for optical path improvements, the detection limit is only improved by a factor of 2.5-8.9.
There are currently two main types of LWC, namely Teflon AF capillaries (having a refractive index of only ~1.3, which is lower than that of water) and silica capillaries coated with Teflon AF or metal films1,3,4. To achieve TIR at the interface between dielectric materials, materials with a low refractive index and high light incidence angles are required3,6,10. With respect to Teflon AF capillaries, Teflon AF is breathable due to its porous structure3,11 and can absorb small amounts of substances in water samples. For quartz capillaries coated on the outside with Teflon AF or metal, the refractive index of quartz (1.45) is higher than most liquid samples (eg 1.33 for water)3,6,12,13. For capillaries coated with a metal film inside, transport properties have been studied14,15,16,17,18, but the coating process is complicated, the surface of the metal film has a rough and porous structure4,19.
In addition, commercial LWCs (AF Teflon Coated Capillaries and AF Teflon Coated Silica Capillaries, World Precision Instruments, Inc.) have some other disadvantages, such as: for faults. . The large dead volume of the TIR3,10, (2) T-connector (to connect capillaries, fibers, and inlet/outlet tubes) can trap air bubbles10.
At the same time, the determination of glucose levels is of great importance for the diagnosis of diabetes, cirrhosis of the liver and mental illness20. and many detection methods such as photometry (including spectrophotometry 21, 22, 23, 24, 25 and colorimetry on paper 26, 27, 28), galvanometry 29, 30, 31, fluorometry 32, 33, 34, 35, optical polarimetry 36 , surface plasmon resonance. 37, Fabry-Perot cavity 38, electrochemistry 39 and capillary electrophoresis 40,41 and so on. However, most of these methods require expensive equipment, and detection of glucose at several nanomolar concentrations remains a challenge (for example, for photometric measurements21, 22, 23, 24, 25, 26, 27, 28, the lowest concentration of glucose). the limitation was only 30 nM when Prussian blue nanoparticles were used as peroxidase mimics). Nanomolar glucose analyzes are often required for molecular-level cellular studies such as inhibition of human prostate cancer growth42 and the CO2 fixation behavior of Prochlorococcus in the ocean.
In this article, a compact, inexpensive photometer based on a metal waveguide capillary (MWC), a SUS316L stainless steel capillary with an electropolished inner surface, was developed for ultrasensitive absorption determination. Since light can be trapped inside metal capillaries regardless of the angle of incidence, the optical path can be greatly increased by light scattering on corrugated and smooth metal surfaces, and is much longer than the physical length of the MWC. In addition, a simple T-connector was designed for the optical connection and fluid inlet/outlet to minimize dead volume and avoid bubble entrapment. For the 7 cm MWC photometer, the detection limit is improved by about 3000 times compared to the commercial spectrophotometer with 1 cm cuvette due to the new enhancement of the non-linear optical path and fast sample switching, and the glucose detection concentration can also be achieved. only 5.12 nM using common chromogenic reagents.
As shown in Figure 1, the MWC-based photometer consists of a 7 cm long MWC with an EP grade electropolished inner surface, a 505 nm LED with a lens, an adjustable gain photodetector, and two for optical coupling and liquid input. Exit. A three-way valve connected to the Pike inlet tube is used to switch the incoming sample. The Peek tube fits snugly against the quartz plate and MWC, so the dead volume in the T-connector is kept to a minimum, effectively preventing air bubbles from being trapped. In addition, the collimated beam can be easily and efficiently introduced into the MWC through the T-piece quartz plate.
The beam and liquid sample are introduced into the MCC through a T-piece, and the beam passing through the MCC is received by a photodetector. Incoming solutions of stained or blank samples were alternately introduced into the ICC through a three-way valve. According to Beer’s law, the optical density of a colored sample can be calculated from the equation. 1.10
where Vcolor and Vblank are the output signals of the photodetector when color and blank samples are introduced into the MCC, respectively, and Vdark is the background signal of the photodetector when the LED is turned off. The change in the output signal ΔV = Vcolor–Vblank can be measured by switching samples. According to the equation. As shown in Figure 1, if ΔV is much smaller than Vblank–Vdark, when using a sampling switching scheme, small changes in Vblank (eg drift) can have little effect on the AMWC value.
To compare the performance of the MWC-based photometer with the cuvette-based spectrophotometer, a red ink solution was used as the color sample because of its excellent color stability and good concentration-absorbance linearity, DI H2O as a blank sample. . As shown in Table 1, a series of red ink solutions were prepared by the serial dilution method using DI H2O as solvent. The relative concentration of sample 1 (S1), undiluted original red paint, was determined as 1.0. On fig. Figure 2 shows optical photographs of 11 red ink samples (S4 to S14) with relative concentrations (listed in Table 1) ranging from 8.0 × 10–3 (left) to 8.2 × 10–10 (right).
The measurement results for sample 6 are shown in Figs. 3(a). The points of switching between stained and blank samples are marked in the figure by double arrows “↔”. It can be seen that the output voltage increases rapidly when switching from color samples to blank samples and vice versa. Vcolor, Vblank and the corresponding ΔV can be obtained as shown in the figure.
(a) Measurement results for sample 6, (b) sample 9, (c) sample 13, and (d) sample 14 using an MWC-based photometer.
The measurement results for samples 9, 13, and 14 are shown in Figs. 3(b)-(d), respectively. As shown in Figure 3(d), the measured ΔV is only 5 nV, which is almost 3 times the noise value (2 nV). A small ΔV is difficult to distinguish from noise. Thus, the limit of detection reached a relative concentration of 8.2×10-10 (sample 14). With the help of equations. 1. AMWC absorbance can be calculated from measured Vcolor, Vblank and Vdark values. For a photodetector with a gain of 104 Vdark is -0.68 μV. The measurement results for all samples are summarized in Table 1 and can be found in the supplementary material. As shown in Table 1, absorbance found at high concentrations saturates, so absorbance above 3.7 cannot be measured with MWC-based spectrometers.
For comparison, a red ink sample was also measured with a spectrophotometer and the measured Acuvette absorbance is shown in Figure 4. The Acuvette values at 505 nm (as shown in Table 1) were obtained by referring to the curves of samples 10, 11, or 12 (as shown in the inset). to Fig. 4) as a baseline. As shown, the detection limit reached a relative concentration of 2.56 x 10-6 (sample 9) because the absorption curves of samples 10, 11 and 12 were indistinguishable from each other. Thus, when using the MWC-based photometer, the detection limit was improved by a factor of 3125 compared to the cuvette-based spectrophotometer.
Dependence absorption-concentration is presented in Fig.5. For cuvette measurements, the absorbance is proportional to the ink concentration at a path length of 1 cm. Whereas, for MWC-based measurements, a non-linear increase in absorbance was observed at low concentrations. According to Beer’s law, absorbance is proportional to the optical path length, so the absorption gain AEF (defined as AEF = AMWC/Acuvette at the same ink concentration) is the ratio of MWC to the optical path length of the cuvette. As shown in Figure 5, at high concentrations, the constant AEF is around 7.0, which is reasonable since the length of the MWC is exactly 7 times the length of a 1 cm cuvette. However, at low concentrations (related concentration <1.28 × 10-5 ), AEF increases with decreasing concentration and would reach a value of 803 at related concentration of 8.2 × 10-10 by extrapolating the curve of cuvette-based measurement. However, at low concentrations (related concentration <1.28 × 10-5 ), AEF increases with decreasing concentration and would reach a value of 803 at related concentration of 8.2 × 10-10 by extrapolating the curve of cuvette-based measurement. Однако при низких концентрациях (относительная концентрация <1,28 × 10–5) AEF увеличивается с уменьшением концентрации и может достигать значения 803 при относительной концентрации 8,2 × 10–10 при экстраполяции кривой измерения на основе кюветы. However, at low concentrations (relative concentration <1.28 × 10–5), the AEF increases with decreasing concentration and can reach a value of 803 at a relative concentration of 8.2 × 10–10 when extrapolated from a cuvette-based measurement curve.然而，在低浓度（相关浓度<1.28 × 10-5 ）下，AEF 随着浓度的降低而增加，并且通过外推基于比色皿的测量曲线，在相关浓度为8.2 × 10-10 时将达到803 的值。然而 ， 在 低 浓度 （相关 浓度 <1.28 × 10-5） ， ， AEF 随着 的 降低 而 ， 并且 通过 外推 基于 比色皿 测量 曲线 ， 在 浓度 为 8.2 × 10-10 时 达到 达到 达到 达到 达到803 值。 Однако при низких концентрациях (релевантные концентрации < 1,28 × 10-5) АЭП увеличивается с уменьшением концентрации, и при экстраполяции кривой измерения на основе кюветы она достигает значения относительной концентрации 8,2 × 10–10 803 . However, at low concentrations (relevant concentrations < 1.28 × 10-5) the AED increases with decreasing concentration, and when extrapolated from a cuvette-based measurement curve, it reaches a relative concentration value of 8.2 × 10–10 803 . This results in a corresponding optical path of 803 cm (AEF × 1 cm), which is much longer than the physical length of the MWC, and even longer than the longest commercially available LWC (500 cm from World Precision Instruments, Inc.). Doko Engineering LLC has a length of 200 cm). This non-linear increase in absorption in the LWC has not been previously reported.
On fig. 6(a)-(c) show an optical image, a microscope image, and an optical profiler image of the inner surface of the MWC section, respectively. As shown in fig. 6(a), the inner surface is smooth and shiny, can reflect visible light, and is highly reflective. As shown in fig. 6(b), due to the deformability and crystalline nature of the metal, small mesas and irregularities appear on the smooth surface. In view of small area (<5 μm×5 μm), the roughness of most surface is less than 1.2 nm (Fig. 6(c)). In view of a small area (<5 μm×5 μm), the roughness of most surface is less than 1.2 nm (Fig. 6(c)). Ввиду малой площади (<5 мкм×5 мкм) шероховатость большей части поверхности составляет менее 1,2 нм (рис. 6(в)). Due to the small area (<5 µm×5 µm), the roughness of most of the surface is less than 1.2 nm (Fig. 6(c)).考虑到小面积（<5 μm×5 μm），大多数表面的粗糙度小于1.2 nm（图6（c））。考虑到小面积（<5 μm×5 μm），大多数表面的粗糙度小于1.2 nm（图6（c））。 Учитывая небольшую площадь (<5 мкм × 5 мкм), шероховатость большинства поверхностей составляет менее 1,2 нм (рис. 6(в)). Considering the small area (<5 µm × 5 µm), the roughness of most surfaces is less than 1.2 nm (Fig. 6(c)).
(a) Optical image, (b) microscope image, and (c) optical image of the internal surface of the MWC cut.
As shown in fig. 7(a), the optical path LOP in the capillary is determined by the angle of incidence θ (LOP = LC/sinθ, where LC is the physical length of the capillary). For Teflon AF capillaries filled with DI H2O, the angle of incidence must be greater than the critical angle of 77.8°, so the LOP is less than 1.02 × LC without further improvement3.6. Whereas, with MWC, the confinement of light inside the capillary is independent of refractive index or angle of incidence, so as the angle of incidence decreases, the light path can be much longer than the length of the capillary (LOP » LC). As shown in fig. 7(b), the corrugated metal surface can induce light scattering, which can greatly increase the optical path.
Therefore, there are two light paths for MWC: direct light without reflection (LOP = LC) and sawtooth light with multiple reflections between the side walls (LOP » LC). According to Beer’s law, the intensity of the transmitted direct and zigzag light can be expressed as PS×exp(-α×LC) and PZ×exp(-α×LOP) respectively, where the constant α is the absorption coefficient, which depends entirely on the ink concentration.
For high concentration ink (eg, related concentration >1.28 × 10-5), the zigzag-light is highly attenuated and its intensity is much lower than that of straight-light, due to the large absorption-coefficient and its much longer optical-path. For high concentration ink (eg, related concentration >1.28 × 10-5), the zigzag-light is highly attenuated and its intensity is much lower than that of straight-light, due to the large absorption-coefficient and its much longer optical- path. Для чернил с высокой концентрацией (например, относительная концентрация >1,28 × 10-5) зигзагообразный свет сильно затухает, а его интенсивность намного ниже, чем у прямого света, из-за большого коэффициента поглощения и гораздо более длинного оптического излучения. For high concentration ink (e.g. relative concentration >1.28×10-5), the zigzag light is strongly attenuated and its intensity is much lower than that of direct light due to the large absorption coefficient and much longer optical emission. track.对于高浓度墨水（例如，相关浓度>1.28×10-5），Z字形光衰减很大，其强度远低于直光，这是由于吸收系数大，光学时间更长。对于 高浓度 墨水 （例如 ， 浓度 浓度> 1.28 × 10-5） ， z 字形 衰减 很 大 ， 强度 远 低于 直光 ， 这 是 吸收 系数 大 光学 时间 更。。。 长 长 长 长 长 长 长 长 长 长 长 长 长Для чернил с высокой концентрацией (например, релевантные концентрации >1,28×10-5) зигзагообразный свет значительно ослабляется, и его интенсивность намного ниже, чем у прямого света из-за большого коэффициента поглощения и более длительного оптического времени. For high concentration inks (eg, relevant concentrations >1.28×10-5), the zigzag light is significantly attenuated and its intensity is much lower than that of direct light due to the large absorption coefficient and longer optical time. little road. Thus, direct light dominated the absorbance determination (LOP=LC) and the AEF was kept constant at ~7.0. In contrast, when the absorption-coefficient is decreased with decreasing ink concentration (eg, related concentration <1.28 × 10-5), the intensity of zigzag-light increases more rapidly than that of straight-light and then zigzag-light begins to play a more important role. In contrast, when the absorption-coefficient is decreased with decreasing ink concentration (eg, related concentration <1.28 × 10-5), the intensity of zigzag-light increases more rapidly than that of straight-light and then zigzag-light begins to play a more important role. Напротив, когда коэффициент поглощения уменьшается с уменьшением концентрации чернил (например, относительная концентрация <1,28 × 10-5), интенсивность зигзагообразного света увеличивается быстрее, чем у прямого света, и затем начинает играть зигзагообразный свет. On the contrary, when the absorption coefficient decreases with decreasing ink concentration (for example, the relative concentration <1.28×10-5), the intensity of the zigzag light increases faster than that of the direct light, and then zigzag light begins to play. more important role.相反，当吸收系数随着墨水浓度的降低而降低时（例如，相关浓度<1.28×10-5），Z字形光的强度比直光增加得更快，然后Z字形光开始发挥作用一个更重要的角色。相反 ， 当 吸收 系数 随着 墨水 的 降低 而 降低 时 例如 例如 ， 相关 浓度 浓度 <1.28 × 10-5） ， 字形光 的 强度 比 增加 得 更 ， 然后 z 字形光 发挥 作用 一 个 重要 重要 重要 更 更 更 更 更 更 更 更 HI的角色。 И наоборот, когда коэффициент поглощения уменьшается с уменьшением концентрации чернил (например, соответствующая концентрация < 1,28×10-5), интенсивность зигзагообразного света увеличивается быстрее, чем прямого, и тогда зигзагообразный свет начинает играть более важную роль. Conversely, when the absorption coefficient decreases with decreasing ink concentration (for example, the corresponding concentration < 1.28×10-5), the intensity of the zigzag light increases faster than the direct light, and then the zigzag light begins to play a more important role. role character. Therefore, due to the sawtooth optical path (LOP » LC), the AEF can be increased much more than 7.0. Precise light transmission characteristics of MWC can be obtained using waveguide mode theory.
In addition to improving the optical path, fast sample switching also contributes to ultra-low detection limits. Due to the small volume of MCC (0.16 ml), the time needed to switch and change solutions in MCC can be less than 20 seconds. As shown in Figure 5, the minimum detectable value of AMWC (2.5 × 10–4) is 4 times lower than that of Acuvette (1.0 × 10–3). The fast switching of the flowing solution in the capillary reduces the effect of system noise (eg drift) on the accuracy of the absorbance difference compared to the retention solution in the cuvette. For example, as shown in fig. 3(b)-(d), ΔV can be easily distinguished from a drift signal due to fast sample switching in the small volume capillary.
As shown in Table 2, a range of glucose solutions at various concentrations were prepared using DI H2O as solvent. Stained or blank samples were prepared by mixing glucose solution or deionized water with chromogenic solutions of glucose oxidase (GOD) and peroxidase (POD) 37 in a fixed volume ratio of 3:1, respectively. On fig. 8 shows optical photographs of nine stained samples (S2-S10) with glucose concentrations ranging from 2.0 mM (left) to 5.12 nM (right). Redness decreases with decreasing glucose concentration.
The results of measurements of samples 4, 9, and 10 with an MWC-based photometer are shown in Figs. 9(a)-(c), respectively. As shown in fig. 9(c), the measured ΔV becomes less stable and slowly increases during the measurement as the color of the GOD-POD reagent itself (even without adding glucose) slowly changes in the light. Thus, successive ΔV measurements cannot be repeated for samples with a glucose concentration of less than 5.12 nM (sample 10), because when ΔV is small enough, the instability of the GOD-POD reagent can no longer be neglected. Therefore, the limit of detection for glucose solution is 5.12 nM, although the corresponding ΔV value (0.52 µV) is much larger than the noise value (0.03 µV), indicating that a small ΔV can still be detected. This detection limit can be further improved by using more stable chromogenic reagents.
(a) Measurement results for sample 4, (b) sample 9, and (c) sample 10 using an MWC-based photometer.
The AMWC absorbance can be calculated using the measured Vcolor, Vblank and Vdark values. For a photodetector with a gain of 105 Vdark is -0.068 μV. Measurements for all samples can be set in the supplementary material. For comparison, glucose samples were also measured with a spectrophotometer and the measured absorbance of Acuvette reached a detection limit of 0.64 µM (sample 7) as shown in Figure 10.
The relationship between absorbance and concentration is presented in Figure 11. With the MWC-based photometer, a 125-fold improvement in detection limit was achieved compared to the cuvette-based spectrophotometer. This improvement is lower than the red ink assay due to the poor stability of the GOD-POD reagent. A non-linear increase in absorbance at low concentrations was also observed.
The MWC-based photometer has been developed for the ultra-sensitive detection of liquid samples. The optical path can be greatly increased, and much longer than the physical length of the MWC, because light scattered by the corrugated smooth metal sidewalls can be contained within the capillary regardless of the angle of incidence. Concentrations as low as 5.12 nM can be achieved using conventional GOD-POD reagents thanks to new non-linear optical amplification and fast sample switching and glucose detection. This compact and inexpensive photometer will be widely used in life sciences and environmental monitoring for trace analysis.
As shown in Figure 1, the MWC-based photometer consists of a 7 cm long MWC (inner diameter 1.7 mm, outer diameter 3.18 mm, EP class electropolished inner surface, SUS316L stainless steel capillary), a 505 nm wavelength LED (Thorlabs M505F1), and lenses (beam spread about 6.6 degrees), variable gain photodetector (Thorlabs PDB450C) and two T-connectors for optical communication and liquid in/out. The T-connector is made by bonding a transparent quartz plate to a PMMA tube into which MWC and Peek tubes (0.72 mm ID, 1.6 mm OD, Vici Valco Corp.) are tightly inserted and glued. A three-way valve connected to the Pike inlet tube is used to switch the incoming sample. The photodetector can convert the received optical power P into an amplified voltage signal N×V (where V/P = 1.0 V/W at 1550 nm, gain N can be manually adjusted in the range of 103-107). For brevity, V is used instead of N×V as the output signal.
In comparison, a commercial spectrophotometer (Agilent Technologies Cary 300 series with R928 High Efficiency Photomultiplier) with a 1.0 cm cuvette cell was also used to measure the absorbance of liquid samples.
The inner surface of the MWC cut was examined using an optical surface profiler (ZYGO New View 5022) with a vertical and lateral resolution of 0.1 nm and 0.11 µm, respectively.
All chemicals (analytical grade, no further purification) were purchased from Sichuan Chuangke Biotechnology Co., Ltd. Glucose test kits include glucose oxidase (GOD), peroxidase (POD), 4-aminoantipyrine and phenol, etc. The chromogenic solution was prepared by the usual GOD-POD 37 method.
As shown in Table 2, a range of glucose solutions at various concentrations were prepared using DI H2O as a diluent using a serial dilution method (see Supplementary Materials for details). Prepare stained or blank samples by mixing glucose solution or deionized water with chromogenic solution in a fixed volume ratio of 3:1, respectively. All samples were stored at 37°C protected from light for 10 minutes prior to measurement. In the GOD-POD method, stained samples turn red with an absorption maximum at 505 nm, and the absorption is almost proportional to the glucose concentration.
As shown in Table 1, a series of red ink solutions (Ostrich Ink Co., Ltd., Tianjin, China) were prepared by the serial dilution method using DI H2O as solvent.
How to cite this article: Bai, M. et al. Compact photometer based on metal waveguide capillaries: for determination of nanomolar concentrations of glucose. the science. 5, 10476. doi: 10.1038/srep10476 (2015).
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