Calcium ion selective optical sensor and manufacturing method thereof

Abstract

An ion-selective optical sensor for detecting calcium ions and a method of manufacturing an ion-selective sensor, the method comprising the steps of: treating a surface of the sensor with (3-aminopropyl) triethoxysilane (APTES) and then treating the surface with glutaraldehyde (GA); and coating the APTES-GA surface with a crown ether ionophore to produce a functionalized layer; wherein when the functionalized layer is in contact with a target ion in a sample, a complex is formed between the crown ether and the target ion, thus changing the material index of the functionalized layer, the change corresponding to the concentration of the target ion in the sample.

Description

Technical Field

[0001] The present invention generally relates to the detection of metal cations, and more particularly to sensors for detecting and measuring the calcium (Ca 2+ ) ion content in a sample and methods for manufacturing the sensors. Background Art

[0002] Calcium is one of the essential electrolytes in the human body and also plays an important role in various applications in the fields of chemistry, biology, and the environment. Therefore, active research has been conducted on the determination of calcium content. For example, in clinical chemistry, the measurement of calcium levels can be used for diagnostic purposes as an indicator of certain pathological conditions. Calcium also plays a crucial role as one of the soil health indicators for plant growth. Due to the recognition of the importance of calcium, the development of materials and devices capable of measuring or quantifying calcium ion content has attracted considerable interest in recent years.

[0003] There are several modes for detecting the presence of calcium in a sample. One of them is the binding of calcium ions to a compound, which usually results in the formation of a strong binding complex indicating the presence of the target ions in the sample and can be used for quantification. However, there are various conditions and variables that can affect ion recognition, stability, and selectivity in such sensors, including macrocyclic cavity size, shape, substituent effects, conformational flexibility, donor atom type, and the solvent used.

[0004] One of the key challenges in measuring calcium content is sustainability and cost, as the vast majority of devices and equipment require complex operating steps and complex sample pretreatment processes. Considering that most of these devices and equipment are laboratory-limited, this not only makes them unsuitable for on-site or in-situ measurements but also increases the manufacturing cost. Summary of the Invention

[0004] In one aspect, the present invention provides a method for forming an ion-selective optical sensor, the method comprising the steps of: treating the surface of the sensor with (3-aminopropyl)triethoxysilane (APTES), and then treating the surface with glutaraldehyde (GA); and coating the APTES-GA surface with a crown ether-ionophore to produce a functionalized layer; wherein when the ionophore-functionalized layer contacts the target ions in the sample, a complex is formed between the crown ether and the target ions, thus changing the material index of the functionalized layer, and the change corresponds to the concentration of the target ions in the sample.

[0006] Generally, the surface of the sensor includes a resonant structure.

[0007] Generally, changing the material index shifts the resonant wavelength of the layer, thus enabling the measurement of ion concentration.

[0008] In one embodiment, the crown ether compound comprises five ether moieties capable of binding to a target ion. Typically, the target ion is Ca 2+ .

[0009] In another embodiment, the method further includes a pretreatment step of modifying the surface of the sensor to achieve hydroxylation before treating the surface with APTES and GA.

[0010] In yet another embodiment, the pretreatment step includes plasma irradiation treatment with oxygen.

[0011] In one embodiment, the crown ether is benzo-15-crown-5. Typically, the crown ether is 4-aminobenzo-15-crown-5.

[0012] In one embodiment, the step of treating the surface with APTES and GA further includes diluting APTES in ethanol and diluting GA in deionized water, typically 2% (v / v) APTES and 0.1% (v / v) GA.

[0013] In one embodiment, the step of coating the APTES-GA surface with the crown ether ionophore includes diluting the crown ether compound in methanol.

[0014] In one embodiment, the method further includes diluting the crown ether compound in methanol.

[0015] In another aspect, the present invention provides an optical sensor for detecting ions in a sample, the optical sensor comprising a resonant structure having a functionalized ionophore layer prepared with a crown ether compound, wherein when the target ion contacts the functionalized ionophore layer, the material index of the functionalized ionophore layer changes due to the absorption of the target ion, and thus the concentration of the target ion in the sample can be determined.

[0016] In one embodiment, the resonant structure includes a pair of waveguides in the form of a sensing arm and a reference arm, the sensing arm being coated with the functionalized ionophore layer, and the reference arm including a barrier coating (or not coated with the functionalized layer) on the functionalized ionophore layer. Typically, the resonant structure is formed in silicon or silica.

[0017] Typically, the channel width of the waveguide is 15 nm to 1000 nm, and the length is 200 μm to 1 cm. Typically, the thickness of the waveguide is about 220 nm.

[0018] In another embodiment, the light source is configured to emit light of at least one wavelength, which is guided through the pair of waveguides after a predetermined exposure time, thereby generating corresponding patterns that can be compared to determine the concentration of the target ion. Typically, the light source emits light waves in the wavelength range of 1500 nm to 1600 nm.

[0019] In one embodiment, the exposure time for detecting ions is 20 seconds or more.

[0020] In another embodiment, the sensor includes a spectral interrogator for detecting and measuring ion concentration.

[0021] Typically, the target ion is Ca 2+ , and the crown ether compound is benzo-15-crown-5.

[0022] In one embodiment, the concentration of ions detected in a sample is based on the measurement of the resonance wavelength shift versus time. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be better understood by reference to the following description in conjunction with the accompanying drawings herein:

[0024] Figure 1 A schematic diagram of a detection mechanism according to an embodiment of the present invention is provided;

[0025] Figure 2A A top view of a sensor chip according to an embodiment of the present invention is shown;

[0026] Figure 2B An example of a waveguide according to an embodiment of the present invention is provided;

[0027] Figure 3 A flowchart of a method according to an embodiment of the present invention is shown;

[0028] Figure 4 An example of the wavelength shift relative to the change in material index according to an embodiment of the present invention is shown;

[0029] Figures 5A to 5B An example of the wavelength shift caused by an increase in ion concentration is shown DETAILED DESCRIPTION

[0030] Based on the above overview, the following description of several specific alternative embodiments is provided to understand the inventive features of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. To avoid obscuring the present invention, some details may not be described in detail. For ease of reference, when referring to the same or similar features shared with the drawings, common reference numerals will be used throughout the drawings.

[0031] Embodiments of the present invention are described by way of illustration. As will be recognized, the present invention is capable of having other and different embodiments, and several details thereof can be modified in various aspects, all without departing from the scope of the present invention. It should be noted that the drawings include flowcharts of how to perform the method according to the preferred embodiments. Standard devices or components may not be shown as they are known in the art.

[0032] The present invention provides a calcium ion-selective optical sensor that includes a functionalized ion carrier layer prepared with a crown ether compound. As Figure 1 shown, when the target ion contacts the functionalized layer, the material index of the crown ether layer changes due to the absorption kinetics of the target ion on the functionalized layer, and thus calcium ions in the sample can be detected. Therefore, the concentration of the ions in the sample can be obtained by measuring the resonance wavelength shift versus time. In a preferred embodiment, the sensor is an optical sensor that includes a resonant surface and waveguide sections of different widths.

[0033] Now refer to Figure 2A , a light source (13) projects a light beam that passes through an input waveguide before being equally split at a Y-junction and then guided to propagate along a sensing exposure arm (10) and a reference arm (12) in the form of straight waveguides. In a preferred embodiment, the reference arm (12) is coated with a material that prevents calcium ion sensing, thus providing a reference wavelength pattern corresponding to the condition of no calcium ions, while the sensing arm (10) is coated with a functionalized layer to detect calcium ions and provide a wavelength pattern corresponding to the calcium ion concentration. The chemical reaction in the sensing arm (10) can be measured by the interference intensity at the output waveguide, which is then measured by an interrogator (15). Therefore, the sensor can be calibrated and optimized based on each pair of different channels with different characteristics to provide a maximum shift at a specific or selected wavelength. Advantageously, this comparison allows for accurate measurement of a shift of <500 pm, corresponding to a sensitivity of 10 ppb calcium ions.

[0034] In one embodiment, the functionalized layer can be provided on the surface of a waveguide-based sensor that includes a chip having a Mach-Zehnder interferometer (MZI) configuration. An example of an MZI optical waveguide sensor with a silicon-on-insulator (SOI) platform including the functionalized layer of the present invention is shown in Figure 2B . In this embodiment, the sensing section (20) includes silicon oxide SiO 2A waveguide of the cladding, having a thickness of about 220 nm, a width of 15 nm - 1000 nm, and a length of about 200 μm - 1 cm. The coupler is a vertical grating coupler (11), preferably in a transverse mode. The wavelength range is 1500 nm - 1600 nm, and the resolution measurement is as low as 1 picometer (pm). The integrator (15) is connected to the output, where the interrogator (15) may include a pair of MMI-based reflectors (15A, 15B) and two pairs of microring resonators (16, 17, 18, 19). Appropriately, the readout parameter is in-phase or λ.

[0035] In use, when performing a test with an MZI system, the following sequence can be observed. First, a drop of distilled water is added as a blank sample on the surface of the MZI system. Second, the distilled water droplet is removed from the surface. Third, a drop of a solution with a known Ca 2+ concentration is added on the surface. After 30 seconds to 2 minutes, the solution is removed from the surface. Then, a drop of distilled water is added to the surface again. Last but not least, the laser scans at a wavelength of 1500 nm to 1600 nm, and the output of the sensor is measured.

[0036] Therefore, the change in the material index (i.e., the change or alteration of properties) due to the ion absorption of the functionalized layer fixed on the waveguide surface enables the detection and measurement of the concentration of calcium ions.

[0037] The present invention also provides a method for manufacturing or forming the ion-selective sensor for determining the presence of ionic substances in a sample. More specifically, the present invention provides a method for manufacturing a sensor for detecting calcium (Ca 2+ ) ions, wherein a functionalized ionophore prepared based on a crown ether compound is coated on the surface of the sensor such that it forms a layer on the surface of the sensor. The crown ether contains five ether moieties that are generally selective for cations (i.e., calcium), and thus can detect such substances in a sample.

[0038] In a preferred embodiment, the method for manufacturing an ion-selective sensor includes the following steps: treating the surface of the sensor with (3-aminopropyl)triethoxysilane (APTES), and then treating the surface with glutaraldehyde (GA); coating the APTES-GA surface with a crown ether ionophore to produce a functionalized layer; wherein when the ionophore functionalized layer contacts the target ion, the material index of the functionalized crown ether ionophore layer changes, and thus the target ion in the sample can be detected.

[0039] Generally, the crown ether compound is benzo-15-crown-5, more specifically 4-aminobenzo-15-crown-5. However, other benzo-crown compounds and derivatives having moieties selective or capable of selectively binding calcium ions are expected to be used in this method.

[0040] The method includes using non-toxic solvents such as water, ethanol, and methanol in the step of preparing the functionalized ionophore layer.

[0041] In a preferred embodiment, the surface is treated with (3-aminopropyl)triethoxysilane (APTES) and glutaraldehyde (GA) to initiate a chemical cross-linking reaction for covalent binding, and then 4-aminobenzo-15-crown-5 ionophore is immobilized on the surface of the sensor.

[0042] Figure 3 A flowchart depicting the steps of a method for fabricating an ion-selective optical sensor is provided. The method includes: pre-modifying the target surface of the sensor using oxygen via plasma treatment to change the surface and achieve hydroxylation reaction (S101). After plasma treatment, the pre-modified sensor surface is treated with a 2% (v / v) APTES solution diluted in absolute ethanol at room temperature for 1 hour (S102), and then the APTES-treated surface is dried at a high temperature (e.g., 80 °C) for 1 hour (S103). Then, the dried APTES-treated surface is treated with a solution of 0.1% (v / v) GA in deionized water at room temperature for 20 minutes (S104). The APTES-GA modified surface is further treated with a 50 mL methanol solution of 100 mM self-synthesized 4-aminobenzo-15-crown-5 and left to stand to allow the solvent to evaporate (S105).

[0043] The functionalized ionophore layer is heterogeneously integrated with the resonant surface of the sensor such that the resonant condition of the sensor is correspondingly shifted according to the change in the material index caused by the absorption of ions. Therefore, the concentration of ions in a sample can be measured and quantified by detecting the shift of the resonant wavelength curve relative to the time curve. The same technique can also be used to train an artificial neural network for deriving ion concentration.

[0044] An example of the wavelength shift of an optical sensor integrated with a functionalized layer due to the change in the material index is shown in Figure 4 . Thus, when the layer absorbs ions, the change in the material index causes a shift in the wavelength, which helps to measure or quantify the ion concentration in a sample. This is clearly shown in Figure 5A and Figure 5B where the wavelength shifts as the functionalized layer absorbs more ions from the sample. The presence of calcium ions can be detected within a predetermined exposure time, e.g., within 20 seconds to 120 seconds. It has been found that after 120 seconds, there is essentially no further change in the change or wavelength shift.

[0045] In one embodiment, the target surface of the sensor includes a resonant surface, where the functionalized ionophore prepared according to the method can be applied on the resonant structure or resonant surface of the sensor (such as Mach-Zehnder interferometer, mirror resonator) or on an integrated photonics platform (i.e., III-V, silicon photonics).

[0046] The concentration of calcium ions can be adjusted to optimize the detection range of calcium ions, where a higher ion detection range can be achieved by increasing the concentration of ions and the sensitivity of the sensor. In one embodiment, the sensor can detect and measure ion concentrations from 10 ppb to 200 ppm.

[0047] Different from conventional methods or devices such as ICP-MS / OES that require complex sample pretreatment and highly skilled personnel operation, the integration of the functionalization of the sensor with the resonant surface enables the reduction of the complexity of in-situ measurements because the sensor can be implemented in a compact and portable form.

[0048] The sensor can be mass-produced by varying the concentration of the crown ether and its linker on a single sensor area, thus providing sensors that can be deployed in any system or environment.

[0049] Although the present invention has been described in accordance with the preferred embodiments and the requirements of specific operating ranges and conditions, those skilled in the art will understand that the invention described herein can be varied and modified in addition to the specifically described content.

Claims

1. A method for forming an optical ion-selective sensor, the method comprising the steps of: - treating the surface of the sensor with (3-aminopropyl)triethoxysilane (APTES), and then treating the surface with glutaraldehyde (GA); and - coating the APTES-GA surface with a crown ether ionophore to produce a functionalized layer; wherein when the functionalized layer contacts a target ion in a sample, a complex is formed between the crown ether and the target ion, thereby changing the material index of the functionalized layer, and the change corresponds to the concentration of the target ion in the sample.

2. The method according to claim 1, wherein the surface of the sensor comprises a resonant structure.

3. The method according to claim 1 or 2, wherein changing the material index shifts the resonant wavelength of the layer, thereby enabling the measurement of ion concentration.

4. The method according to any one of the preceding claims, wherein the crown ether compound comprises five ether moieties capable of binding to the target ion.

5. The method according to any one of the preceding claims, wherein the target ion is Ca 2+ .

6. The method according to any one of the preceding claims, wherein the method further comprises a pretreatment step to modify the surface to achieve hydroxylation before treating the surface of the sensor with APTES and GA.

7. The method according to claim 6, wherein the pretreatment step comprises plasma irradiation treatment with oxygen.

8. The method according to any one of the preceding claims, wherein the crown ether is benzo-15-crown-5.

9. The method according to any one of the preceding claims, wherein the crown ether is 4-aminobenzo-15-crown-5.

10. The method according to any one of the preceding claims, wherein the step of treating the surface with APTES and GA further comprises diluting APTES in ethanol and diluting GA in deionized water.

11. The method according to any one of the preceding claims, wherein the step of coating the APTES-GA surface with the crown ether ionophore comprises diluting the crown ether compound in methanol.

12. The method according to any one of the preceding claims, wherein the surface is treated with a 2% (v / v) APTES solution diluted in ethanol and a 0.1% (v / v) GA solution in deionized water.

13. An optical sensor for detecting ions in a sample, the optical sensor comprising a resonant structure having a functionalized ionophore layer prepared with a crown ether compound, wherein when a target ion contacts the functionalized ionophore layer, the material index of the functionalized ionophore layer changes due to the absorption of the target ion, thereby enabling the determination of the concentration of the target ion in the sample.

14. The optical sensor according to claim 13, wherein the resonant structure comprises a pair of waveguides in the form of a sensing arm and a reference arm, the sensing arm is coated with the functionalized ionophore layer, and the reference arm comprises a barrier coating over the functionalized ionophore layer.

15. The optical sensor according to claim 14, wherein the channel width of the waveguide is in the range of 15 nm to 1000 nm, and the length is in the range of 200 μm to 1 cm.

16. The optical sensor according to claim 14, wherein the light source is configured to emit light of at least one wavelength, and the light is guided through the pair of waveguides after a predetermined exposure time, thereby generating corresponding patterns that can be compared to determine the concentration of the target ion.

17. The optical sensor according to claim 16, wherein the light source emits light having a wavelength range of 1500 nm to 1600 nm.

18. The optical sensor according to any one of claim 16, wherein the concentration of the ions detected in the sample is obtained based on the measurement of the resonance wavelength shift versus time.

19. The optical sensor according to claim 16, wherein the exposure time for detecting ions is 20 seconds or more.

20. The optical sensor according to claim 13, wherein the sensor further comprises a spectral interrogator for detecting and measuring the ion concentration.

21. The optical sensor according to any one of claims 13 to 20, wherein the target ion is Ca 2+ , and the crown ether compound is benzo-15-crown-5.

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