Optical sensor for selectively detecting metal ions

Abstract

An optical sensor (100) for detecting at least one metal ion in a sample includes a resonant structure (102) having a pair of arms (104a, 104b) coated with a chelating agent to bind to the metal ion in the sample. A pair of arms (104a, 104b) is provided with optical waveguides (106) of unequal width to form different optical path lengths in the arms (104a, 104b) and produce an optical interference pattern with wavelength shift. The shift of the pattern wavelength is in direct proportion to the concentration of the metal ions in the sample, and can be used for quantitative and in-situ detection of the metal ions.

Description

Technical Field

[0001] The present invention generally relates to the detection of metal ions, and more particularly to an optical sensor for detecting the concentration of heavy metal ions in a sample. Background Art

[0002] Heavy metal ions in drinking water can cause temporary to permanent health problems, such as respiratory problems, DNA damage, etc. Therefore, it is necessary to detect these metal ions in water before drinking. There are several methods and devices on the market that can be used to detect various heavy metal ions in water. One such method for detecting the concentration of heavy metal ions uses inductively coupled plasma mass spectrometry / optical emission (ICP-MS / OES). However, such a method requires expensive, bulky equipment, and is time-consuming and laborious. In addition, this bulky equipment cannot be easily carried by people to detect heavy metal ions in water on the spot. On the other hand, there are several portable heavy metal ion sensors on the market, such as gold nanoparticle sensors, for the presence of heavy metal ions in water on the spot. In addition, most of these sensors are colorimetric, so they are qualitative.

[0003] Another method for detecting heavy metal ions uses a biosensor fabricated by coating a microphotonic silicon-on-insulator (SOI) Mach-Zehnder interferometer with a functionalized mesoporous silicate to detect lead heavy metals, such as lead (Pb), in water samples with a detection limit of <100 ppb. The detection method includes the step of directing a water sample to the coated MZ interferometer using a capillary sealed relative to the SOI device. By comparing the previously measured change in the coating refractive index with the phase shift caused by the presence of lead in the water sample, a quantitative measurement of lead in the water sample can be obtained. However, the biosensor uses a long spiral waveguide, which greatly increases the size of the sensor, and the optical resonance during the sensing operation is caused by the formation of the microfluidic channel, which further reduces the scalability of the sensor.

[0004] Another method for detecting heavy metals such as nickel ions uses a fiber optic sensor based on Mach-Zehnder interferometer (MZI), which is formed by cascading two tapered optical fibers functionalized with coatings. The tapered fiber sensor is compact, with a sensing length of about 1 mm, and includes multiple double layers formed by stacking carbon nanotubes / chitosan and polyacrylic acid, which increases the chelating sites for nickel ion attachment. When heavy metal ions are absorbed by the coating, the transmission spectrum of the MZI changes due to the change in the refractive index of the coating, and the concentration of heavy metal ions is measured by modulating the intensity of the spectrum.

[0005] However, the above sensors and test methods cannot be used for selective and quantitative detection of heavy metal ions in a wide range of water. In addition, the above sensors are not compact and require complicated and labor-intensive manufacturing. Therefore, a device and related methods are needed for on-site, quantitative determination of heavy metal ion concentrations in liquids such as water. Required sensors will be compact, easy to use, allowing large-scale implementation of monitoring systems that can determine the concentration of heavy metal ions over a wide range. Summary of the invention

[0006] In one aspect, the present invention provides an optical sensor for detecting at least one metal ion in a sample. The optical sensor comprises a resonant structure having a pair of arms, the arms being coated with a chelating agent that binds to the metal ions in the sample, wherein the pair of arms are provided with optical waveguides of unequal widths to generate an optical interference pattern having a wavelength shift that is proportional to the concentration of the metal ions in the sample.

[0007] In one embodiment, the resonant structure is formed on a silicon photonics based Mach-Zehnder interferometer.Typically, the resonant structure is formed by coating the surface of the Mach-Zehnder interferometer with a chelating agent.

[0008] In one embodiment, each of a pair of arms of the Mach-Zehnder interferometer has an equal length of between 900-2000 μm.

[0009] In another embodiment one arm of a pair of arms of the Mach-Zehnder interferometer has a width between 370-470 nm and the other arm has a width between 450-650 nm.

[0010] In one example, each of a pair of arms of the Mach-Zehnder interferometer has a length of 1100 μm, and widths of 420 nm and 550 nm, respectively.

[0011] In one embodiment, the percentage of propagating light that interacts with the chelator on a pair of arms with widths between 370-470nm and between 450-650nm is 19-25% and 18-20%, respectively. Typically, a pair of arms with unequal widths form a waveguide with different optical path lengths, resulting in different phase changes in the light propagating therethrough. This results in a wavelength shift in the optical interference pattern that is proportional to the concentration of the metal ions. Advantageously, this aids in the quantitative determination of heavy metal ion concentrations in liquids such as water.

[0012] In one embodiment, the chelating agent is an L-glutathione based compound.

[0013] In another embodiment, the chelating agent coated on the pair of arms is selected from the group of L-glutathione-based compounds, each of which can only bind to a specific type of metal ion in the sample when in contact. This binding changes the material refractive index of the resulting layer and produces an optical interference pattern with a wavelength shift proportional to the concentration of the specific metal ion in the sample.

[0014] In one example, the percentage of propagating light that interacted with the L-glutathione-based compound coated on the 420 and 550 nm width Mach-Zehnder arms was 24% and 20%, respectively. Advantageously, this induces different phase changes in a pair of arms, which then results in a shift in the wavelength spectrum that is proportional to the concentration of metal ions in the sample.

[0015] In one embodiment, suitable L-glutathione-based chelators are selected from the group of L-glutathione-based compounds, each of which binds only to specific metal ions selected from lead (Pb), zinc (Zn), mercury (Hg), nickel (Ni), cadmium (Cd), copper (Cu), chromium (Cr) and arsenic (As). BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be better understood by referring to the following description in conjunction with the accompanying drawings:

[0017] Figure 1 A schematic diagram is provided showing a top view of an optical sensor according to an embodiment of the present invention;

[0018] Figure 2 The embodiment according to the present invention is shown Figure 1 A cross-sectional view of the optical sensor shown; and

[0019] Figure 3 The use of the method according to the embodiment of the present invention is shown Figure 1 Example of a resonant shift in the wavelength of an optical interference pattern obtained by an optical sensor. DETAILED DESCRIPTION

[0020] In light of the foregoing invention, the following description of a number of specific and optional embodiments is provided to understand the inventive features of the present invention. However, it is apparent to those skilled in the art that the present invention may be implemented without these specific details. Some details may not be described in detail to avoid obscuring the present invention. For ease of reference, when referring to the same or similar common features in the accompanying drawings, the same reference numerals will be used in all drawings.

[0021] Embodiments of the present invention are described by way of example. As will be appreciated, the present invention can have other and different embodiments, and its several details can be modified in various aspects, all without departing from the scope of the present invention. It should be noted that standard equipment or components may not be shown because they are known in the art.

[0022] The present invention provides an optical sensor for detecting the concentration of selected metal ions in an aqueous sample (such as water). Figure 1 The optical sensor 100 shown includes a resonant structure 102 having a pair of arms 104a, 104b, and the pair of arms 104a, 104b are coated with a suitable chelating agent to bind to one or more metal ions when the arms 104a, 104b are in contact with the sample. In one example, the chelating agent is configured to bind only to selected metal ions for determining the concentration of metal ions in the sample. Typically, the chelating agent forms a chelating layer or functionalized layer when in contact with specific metal ions and changes the material refractive index of the layer. Therefore, the change in the material refractive index of the chelating layer causes a change in the effective refractive index of light propagating through the pair of arms 104a, 104b of the resonant structure 102.

[0023] In one embodiment, each of a pair of arms 104a, 104b of the resonant structure 102 has equal length and unequal width. The width difference of a pair of arms 104a, 104b causes waveguides with different widths along the arms 104a, 104b. This difference in waveguide width produces an offset of its optical path length. In an example, compared with the corresponding waveguide in the prior art, the offset produced by the width difference of the waveguide in a pair of arms 104a, 104b is compact. When light propagates through the waveguide, the optical waveguide with different optical path lengths in a pair of arms 104a, 104b produces a phase shift. According to a preferred embodiment, the phase shift of a pair of arms 104a, 104b further causes a wavelength shift of the optical interference pattern produced. The wavelength shift of the optical interference pattern is directly proportional to the concentration of the metal ion selected in the sample.

[0024] In one embodiment, the resonant structure 102 is formed on a silicon photonics based Mach-Zehnder interferometer. Figure 1 and Figure 2As shown, the resonant structure 102 is typically formed by coating or functionalizing the surface of the interferometer chip with a chelating agent. In one embodiment, the chelating agent is selected from a group of L-glutathione-based compounds, each of which is adjusted to bind to a specific metal ion in the sample when in contact. The chelating agent changes its material refractive index when in contact with the metal ion, which, when illuminated by light, results in an optical interference pattern with a wavelength shift that is proportional to the concentration of the selected metal ion. Therefore, the spectral shift can be associated with the concentration of the selected heavy metal ion in the sample in contact with the optical sensor 100.

[0025] In one example, each of the pair of Mach-Zehnder arms 104a, 104b is equal in length, which is selected between 900-2000 μm. In one embodiment, the width of each of the arms 104a, 104b is different to produce an offset in the optical path length. Typically, one 104a of a pair of arms of a Mach-Zehnder interferometer has a width between 370-470nm and the other arm 104b has a width between 450-650nm. Therefore, this results in an offset in the optical path length in a compact manner within the silicon photonics-based Mach-Zehnder interferometer 102.

[0026] In an exemplary embodiment, the physical length of each pair of Mach-Zehnder arms 104a, 104b is 1100 μm, and the width of one arm is 420 nm and the width of the other arm is 550 nm. Thus, this results in an offset of the optical path length in a compact manner within the silicon photonics based Mach-Zehnder interferometer 102.

[0027] In one embodiment, a chelating agent selected from the group of L-glutathione-based chelating agents is specifically tuned to bind to the heavy metal ion of interest. When in contact with a specific heavy metal ion, the chelating agent binds to it, forming a chelate, changing its material refractive index, which in turn results in a change in the effective refractive index of the optical mode propagating in a pair of arms 104a, 104b of the Mach-Zehnder interferometer 102.

[0028] In general, the degree of effective refractive index variation propagating in a pair of arms 104a, 104b of the Mach-Zehnder interferometer 102 is proportional to the degree of power of the light propagating through the waveguide and its percentage of interaction with the chelating layer of glutathione. In one example, the percentage of propagating light interacting with a pair of arms 104a and 104b of the Mach-Zehnder interferometer 102 with a width between 370-470nm and 450-650nm is 19-25% and 18-20%, respectively. In another example, the percentage of propagating light interacting with a pair of arms 104a, 104b of the Mach-Zehnder interferometer 102 with a width of 420 and 550nm is 24% and 20%, respectively. As a result, when exposed to heavy ions, the optical phase induced in the pair of arms 104a, 104b is different, and the wavelength spectrum produced by the unbalanced Mach-Zehnder interferometer 102 is shifted in proportion to the concentration of the selected metal ions in the sample. Therefore, the spectral shift can be correlated to the concentration of heavy metal ions contacting the optical sensor 100 .

[0029] Figure 3 An example of the wavelength shift of the optical interference pattern caused by the functionalized layer in a pair of arms 104a, 104b due to the different optical path lengths is shown. Therefore, since the pair of arms 104a, 104b coated with the chelating agent absorbs the metal ions, the change in the refractive index of the material causes a shift in the wavelength, which is beneficial for measuring the concentration of the metal ions in the sample. Figure 3 The wavelength shift is shown when the functionalized layer absorbs more metal ions from the sample. By analyzing the change in wavelength, the concentration of various types of metal ions in the sample can be detected.

[0030] In one embodiment, the L-glutathione based chelator is selected from the group of L-glutathione based compounds, wherein each chelator binds only to a specific type of metal ion selected from lead (Pb), zinc (Zn), mercury (Hg), nickel (Ni), cadmium (Cd), copper (Cu), chromium (Cr), arsenic (As), silver (Ag), iron (Fe), manganese (Mn), molybdenum (Mo), boron (B), calcium (Ca), antimony (Sb), cobalt (Co), etc.

[0031] In one embodiment, the optical sensor 100 may be obtained by functionalizing the surface of a resonant structure or a resonant surface of an optical sensor such as a Mach-Zehnder interferometer, a mirror resonator or on an integrated photonic platform, ie III-V, silicon photonics.

[0032] Compared to conventional methods or devices such as ICP-MS / OES that require complex sample pretreatment and highly skilled personnel to operate, the integration of the functionalized layer with the resonant surface of the optical sensor 100 enables less complex in-situ measurements because the sensor can be implemented in a compact and portable form. In addition, the present optical sensor 100 can be mass-produced by functionalizing the chip surface of a silicon photonics-based Mach-Zehnder interferometer.

[0033] While the present invention has been described in terms of preferred embodiments and specific operating ranges and conditions, those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described.

Claims

1. An optical sensor (100) for selectively detecting at least one metal ion in a sample, comprising: A resonant structure (102) having a pair of arms (104a, 104b) having equal lengths and unequal widths to form an optical waveguide (106); Each of the pair of arms (104a, 104b) is coated with a chelating agent, which, when in contact with the at least one metal ion in the sample, combines with the at least one metal ion to form a functionalized layer on at least a portion of the pair of arms (104a, 104b), thereby changing the refractive index of the optical waveguide (106) and generating an optical interference pattern having a wavelength shift proportional to the concentration of the at least one metal ion in the sample.

2. The optical sensor (100) according to claim 1, wherein: The resonant structure (102) is formed on a silicon photonics based Mach-Zehnder interferometer.

3. The optical sensor (100) according to claim 2, wherein: Each of a pair of arms (104a, 104b) of the Mach-Zehnder interferometer has an equal length of between 900-2000 μm.

4. The optical sensor (100) according to claim 2, wherein: One arm (104a) of a pair of arms of the Mach-Zehnder interferometer has a width between 370-470 nm, and the other arm (104b) has a width between 450-650 nm.

5. The optical sensor (100) according to claim 4, wherein: The percentage of propagating light interacting with the functionalized layer on a pair of arms (104a, 104b) having a width between 370-470 nm and between 450-650 nm is 19-25% and 18-20%, respectively.

Images