Intensity modulation sensing method with selectable excitation wavelength and aluminum as surface plasma resonance material

Abstract

The invention relates to the technical field of optical sensing detection, in particular to an intensity modulation sensing method with an optional excitation wavelength and aluminum as a surface plasma resonance material. According to the linear relation between the excitation wavelength and the optimal thickness of the aluminum film, the optimal thickness of the aluminum film with the selected excitation wavelength is obtained; preparing an aluminum film with the corresponding optimal thickness according to a thermal evaporation technology; determining a reflection angle spectrum of the aluminum film through a surface plasma resonance detection system, and calibrating a fixed incident angle of intensity modulation; the medium environment contacted with the aluminum film is changed, the reflection intensity is detected in real time, and the refractive index of the medium environment is obtained according to the linear relation between the refractive index and the reflection light intensity. According to the invention, aluminum is used as a surface plasma resonance material, surface plasma resonance from deep ultraviolet to near infrared is realized, and random selection of excitation light wavelength in the wavelength range is realized by utilizing the approximate linear characteristics of a real part and an imaginary part of the refractive index in the range of 400-700nm.

Description

Technical Field

[0001] The invention relates to the technical field of optical sensing detection, and in particular to an intensity modulation sensing method with optional excitation wavelength and aluminum as a surface plasma resonance material. Background Art

[0002] The principle of surface plasmon resonance (SPR) is that light waves excite free electron density waves at the interface of metal / environmental dielectric. Using prism coupling, resonance occurs when the wave vector and frequency of the excitation light and the free electron density wave match, and the energy of the incident light is completely converted into the energy of the free electron density wave, and an intensity valley appears at the reflection end. During resonance, the surface free electron density wave propagates along the interface between the metal and the medium, and generates an evanescent wave in the direction perpendicular to the interface inside the metal film and the medium, in the form of exponential decay. The effective depth in the medium is about 100 to 200 nm, and a nanometer-sized optically enhanced near field is formed on the surface of the metal film. When the wave vector cannot match, SPR cannot be excited, and the reflectivity is close to total reflection.

[0003] At present, most of the SPR sensing materials used in the market are based on precious metals such as gold and silver, which are very expensive and have certain disadvantages. Gold SPR sensor chips mostly use other metals such as chromium to improve the adhesion of gold to the glass substrate, which damages the sensitivity of the SPR sensor chip. The chemical stability of silver materials is extremely poor, and it is easy to produce a black oxide layer, which makes the SPR sensor completely unusable. In addition, commonly used surface plasmon materials such as gold and silver will undergo interband electron transitions in the blue light band, so they cannot support surface plasmon resonance in the blue light band. Most of the current SPR sensors are limited to using red excitation light for refractive index sensing. Because the excitation wavelength is single, the selective detection of target molecules can only be achieved by surface treatment on the SPR sensor chip. Due to the above reasons, the widespread promotion of SPR sensing technology has been greatly restricted for SPR sensor chips based on precious metal materials. Therefore, it is necessary to broaden the working frequency band of SPR sensing. The electronic transition frequency of many biological molecules is in the blue light band, which is conducive to combining different light-matter interactions, such as fluorescence spectroscopy, surface-enhanced Raman spectroscopy, etc., and can perform multi-parameter sensing detection of biological molecules based on the nano-optical near-field provided by SPR.

[0004] In addition, the commonly used surface plasmon resonance modulation methods are mainly angle modulation and wavelength modulation, and both modulation methods have certain disadvantages. The angle modulation method requires changing the incident angle to find the resonance angle, which makes the mechanical structure of the system more complicated. Due to the accuracy of the rotating stage, detection errors may be introduced; the wavelength modulation method requires a spectrometer to detect the resonance wavelength, and the mechanical structure is relatively stable, but it has high requirements for the collimation of the incident white light, the collection of spectral data, and the post-data processing, which greatly increases the complexity of the detection. Summary of the invention

[0005] In order to overcome the problems of limited excitation wavelength, complex detection system and high cost in surface plasmon resonance detection technology, the technical problem solved by the present invention is to provide an intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material.

[0006] The technical solution of the present invention is as follows: an intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material, comprising:

[0007] Step a), after the excitation wavelength is selected, the optimal thickness of the aluminum film at the selected excitation wavelength is obtained according to the linear relationship between the excitation wavelength and the optimal thickness of the aluminum film;

[0008] Step b), preparing an aluminum film with an optimal thickness according to a thermal evaporation technique;

[0009] Step c), measuring the reflection angle spectrum of the aluminum film by a surface plasmon resonance detection system, and calibrating the fixed incident angle of intensity modulation;

[0010] Step d), changing the medium environment that the aluminum film contacts, detecting the reflection intensity in real time, and obtaining the refractive index of the medium environment according to the linear relationship between the refractive index and the reflected light intensity.

[0011] The linear relationship between the excitation wavelength and the optimal aluminum film thickness is y=-0.02662x+30.86496, where y is the aluminum film thickness in nm, and x is the excitation wavelength in nm.

[0012] The excitation wavelength is selected within the range of 400 to 700 nm.

[0013] The fixed incident angle of the intensity modulation is according to the formula θ 1 / 4 =θ R -(θ R -θ tot ) / 4, the θ 1 / 4 is the fixed incident angle for intensity modulation, θ tot is the total reflection point of the reflection angle spectrum, θ R is the resonance point of the reflection angle spectrum.

[0014] The surface plasmon resonance detection system comprises an LED light source, an aperture, a half-wave plate, a reflector, a polarization beam splitter prism, a rotating stage and a photodetector;

[0015] The LED light source provides excitation light, and the excitation light controls the excitation light spot size through an aperture; the half-wave plate, the reflector and the polarization beam splitter prism are arranged in sequence; the half-wave plate is used to control the excitation light power; the reflector is used to collimate the excitation light; the output light of the polarization beam splitter prism is P polarized light, which is used to excite the SPR effect; the photodetector is used to detect the reflection intensity; the control system controls the angle of the rotating table and controls the incident angle of the excitation light.

[0016] The reflection intensity is measured in real time by an optical power meter.

[0017] Beneficial effects of the present invention:

[0018] (1) The present invention uses aluminum as the surface plasmon resonance material. Since aluminum has a higher electron density and a higher plasma frequency, the excitation wavelength can be extended to the deep ultraviolet band and the near infrared band. Therefore, a suitable excitation wavelength can be selected according to the characteristics of the detected substance.

[0019] (2) The present invention uses an intensity modulation detection method. Not only is the detection system relatively simple, easy to build, and conducive to integrated miniaturization, but the detection process is also relatively simple. It can be fixed at a suitable angle and a photoelectric detector can be used to detect the reflected light intensity.

[0020] (3) The intensity modulation sensitivity of the present invention under 450nm, 520nm and 633nm excitation light is greater than the intensity modulation sensitivity of gold under 633nm excitation wavelength, while gold does not have a surface plasmon resonance effect when the excitation wavelength is less than 520nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The excitation light wavelength range is 400-700nm, the aluminum film thickness range is 5-40nm, the incident angle is 72°, and the COMSOL Multiphysics simulation diagram.

[0022] Figure 2 It is a linear relationship diagram between the excitation light wavelength and the optimal thickness of the aluminum film.

[0023] Figure 3 It is a schematic diagram of intensity modulation with fixed angle.

[0024] Figure 4 This is a sensor chip with 450nm excitation light and 19nm thick aluminum film. When different concentrations of NaCl solutions are passed through it, the reflection intensity changes with time.

[0025] Figure 5 This is the intensity modulation curve of 450nm excitation light and 19nm thick aluminum film sensor chip.

[0026] Figure 6 This is a graph showing the change in reflection intensity over time when 520nm excitation light and 17nm thick aluminum film sensor chip are introduced into NaCl solutions of different concentrations.

[0027] Figure 7 This is the intensity modulation curve of 520nm excitation light and 17nm thick aluminum film sensor chip.

[0028] Figure 8 This is a graph showing the change in reflection intensity over time when 633nm excitation light and 14nm thick aluminum film sensor chip are introduced into NaCl solutions of different concentrations.

[0029] Fig. 9 This is the intensity modulation curve of 633nm excitation light and 14nm thick aluminum film sensor chip.

[0030] Fig.10 This is the optical path diagram of the surface plasmon resonance detection system.

[0031] In the figure: 1-LED light source; 2-aperture; 3-half-wave plate; 4-reflector a; 5-reflector b; 6-polarization beam splitter prism; 7-photodetector; 8-SPR chip; 9-rotating table; 10-computer. DETAILED DESCRIPTION

[0032] First, this method uses aluminum as a surface plasmon resonance material. Aluminum has a higher plasma frequency than precious metals such as gold and silver due to its higher electron density. It can support surface plasmon resonance in the blue light band and even the ultraviolet band, and has a higher quality factor. Therefore, the SPR sensor chip of aluminum film will have higher sensitivity and resolution. In addition, aluminum material has an oxide layer under natural conditions, has extremely high optical transmittance, and can provide damage protection for aluminum film. In particular, aluminum is cheap and abundant, and its processing method is compatible with traditional CMOS, which can greatly reduce the cost of SPR sensor chips.

[0033] Secondly, this method selects the intensity modulation detection mode, and uses the SPR phenomenon to fix the incident angle of the excitation light in the linear region between the highest point of reflection intensity and the lowest point of reflection intensity. When the dielectric environment of the aluminum film interface changes, the SPR reflection angle spectrum will shift left and right, causing the reflected light intensity at a fixed angle position to change in magnitude. The intensity modulation system has a simple structure, and since there is no mechanical movement, it is extremely stable, and the detection process is relatively simple, which reduces the probability of introducing detection errors and facilitates data processing.

[0034] Based on the above two changes, the wavelength range of the excitation light source is broadened, the overall complexity of the SPR detection system is simplified, and the cost of detection is reduced.

[0035] The following is a specific embodiment, an intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material, comprising:

[0036] Step a) According to the actual application or limiting conditions, a suitable excitation wavelength is selected, taking the common 450nm, 520nm and 633nm LED as examples; according to the linear relationship between the excitation wavelength and the optimal aluminum film thickness, the optimal thickness of the aluminum film at different excitation wavelengths is calculated;

[0037] Step b) preparing an aluminum film having an optimal thickness by using a thermal evaporation technique;

[0038] Step c) measuring the reflection angle spectrum of the aluminum film by a surface plasmon resonance detection system to calibrate the fixed incident angle of intensity modulation;

[0039] Step d) changing the medium environment that the aluminum film contacts, detecting the reflection intensity in real time, and obtaining the refractive index of the medium environment according to the linear relationship between the refractive index and the reflected light intensity.

[0040] In this sensing method, the excitation wavelength can be selected between 400 and 700 nm. Through COMSOL Multiphysics software simulation, it is determined that aluminum has a surface plasma resonance effect in the wavelength range of 400 to 700 nm, and the real and imaginary parts of the refractive index of the aluminum film in this wavelength range are both linear. According to the simulation results, the optimal thickness of the aluminum film and the excitation wavelength satisfy a linear relationship, which is y = -0.02662x + 30.86496, which can be used to calculate the optimal thickness of the aluminum film required in this wavelength range.

[0041] Furthermore, the intensity modulation system of surface plasmon resonance includes: an LED light source provides excitation light; an aperture controls the size of the excitation light spot; the output light of the polarization beam splitter prism is P polarized light; a half-wave plate is placed before the polarization beam splitter prism to control the power of the excitation light; two reflectors are used to collimate the excitation light; a photodetector detects the reflection intensity; and a control system controls the angle of the rotating stage, that is, the incident angle of the excitation light.

[0042] Furthermore, LED is a small semiconductor laser having the characteristics of small size, stable output power, good monochromaticity and low price.

[0043] Furthermore, based on the linear relationship between the excitation wavelength and the optimal thickness of the aluminum film, the optimal aluminum film thicknesses corresponding to the excitation wavelengths of 450nm, 520nm and 633nm were obtained to be 19nm, 17nm and 14nm, respectively, excluding the aluminum oxide layer of about 3nm.

[0044] Furthermore, the aluminum film is prepared using a vacuum evaporator, which requires a high vacuum degree and a high deposition rate to ensure the quality of the prepared aluminum film.

[0045] Furthermore, the total reflection point θ of the angle spectrum tot and the resonance point θ R , according to the formula θ 1 / 4 =θ R -(θ R -θ tot ) / 4, and we can calculate θ 1 / 4 is a fixed incident angle for intensity modulation.

[0046] Furthermore, the reflected light intensity is measured in real time using an optical power meter.

[0047] Numerical simulation: COMSOL Multiphysics simulates the surface plasmon resonance effect of aluminum under the conditions of excitation light wavelength: 400~700nm, aluminum film thickness: 5~40nm, and fixed angle of 72°, such as Figure 1 As shown. From the simulation results, it can be seen that aluminum has a surface plasma resonance effect in the excitation wavelength range of 400 to 700nm, that is, aluminum can be used for intensity modulation in this wavelength range; COMSOL Multiphysics simulates the angle modulation curve and intensity modulation curve of 405nm, 450nm, 520nm and 633nm excitation wavelengths in a certain aluminum film thickness range, and selects the best intensity modulation aluminum film thickness for the four excitation wavelengths. The selection criteria are the length of the linear interval of the modulation curve and the size of the detection sensitivity. Through the simulation data, it can be concluded that the best film thickness corresponding to 405nm is 20nm, the best film thickness corresponding to 450nm is 19nm, the best aluminum film thickness corresponding to 520nm is 17nm, and the best aluminum film thickness corresponding to 633nm is 14nm. The best aluminum film thicknesses of the four excitation wavelengths are linearly fitted, as shown in Figure 3 When a certain wavelength of excitation light is selected in an actual experiment, the optimal thickness of the aluminum film can be determined based on the linear relationship between the excitation wavelength and the thickness of the aluminum film. Here, a wavelength is selected in each of the three bands of 400-500, 500-600 and 600-700, and three wavelengths of 450nm, 520nm and 633nm are selected as the excitation light source.

[0048] Preparation and characterization of aluminum film: Use a vacuum evaporator to prepare aluminum film of corresponding thickness, use an ellipsometer to calibrate the n and k coefficients of the aluminum film, use a step meter to calibrate the thickness of the aluminum film, and use an atomic force microscope to calibrate the surface roughness of the aluminum film. Finally, it is determined that the prepared aluminum film meets the index requirements.

[0049] Angle spectrum measurement: The aluminum film is made into a surface plasmon resonance sensor chip, and the surface plasmon resonance detection system is used to measure the angle spectrum. The optical path of the surface plasmon resonance detection system is as follows: Fig.10 As shown. It includes: LED light source 1 provides excitation light; aperture 2 controls the size of the excitation light spot; the output light of polarization beam splitter prism 6 is P polarized light; half-wave plate 3 is placed in front of polarization beam splitter prism 6 to control the power of excitation light; two reflectors are used to collimate the excitation light; photodetector 7 detects the reflection intensity of SPR chip 8; computer 10 is used as a control system to control the angle of the rotating stage, that is, the incident angle of the excitation light. Ultrapure water is passed into the sample pool, and the sample stage starts to rotate. The angle interval of the rotating stage is 0.05°, the starting angle is 68°, and the ending angle is 83°. The rotation angle range includes the total reflection point θ tot and SPR resonance point θ R Each time the rotating stage rotates, the reflected intensity of the laser is measured using an optical power meter to obtain the angle-modulated reflected angle spectrum, such as Figure 2 shown.

[0050] Select a fixed angle of intensity modulation: determine the total reflection point θ of the reflection angle spectrum tot and the resonance point θ R , calculate θ 1 / 4 =θ R -(θ R -θ tot ) / 4, and θ 1 / 4 As a fixed angle of intensity modulation, such as Figure 2 shown.

[0051] Intensity modulation detection: using a fixed angle θ 1 / 4 , fix the optical power meter at the corresponding reflection angle direction, use 450nm, 520nm and 633nm excitation light sources for intensity modulation detection, and introduce different concentrations of sodium chloride solution 0%, 2%, 5%, 7%, 10%, 12%, 15% into the sample pool. The corresponding refractive indices are 1.33112, 1.33321, 1.339, 1.343, 1.34793, 1.35071, 1.35548, respectively. Each time a sodium chloride solution of different concentration is introduced, use the optical power meter to record the change of reflected light intensity during and after the introduction in real time, such as Figure 4 , 6 , 8; analyze the experimental data of intensity modulation and determine the sensitivity and linear range of intensity modulation, such as Figure 5 , 7, 9. From the experimental results, it can be seen that when the sodium chloride concentration is between 0%, 2%, 5%, and 7%, that is, the refractive index ranges from 1.33112 to 1.343, the intensity modulation curve remains linear, and the linear relationship between the refractive index and the reflectivity is y=43.61x-57.91, y=52.68x-69.91, and y=43.89x-58.26, respectively, where x is the refractive index and y is the reflectivity.

[0052] It should be noted that the user can select an excitation wavelength in the range of 400 to 700 nm, determine the required aluminum film thickness based on the linear relationship between the excitation wavelength and the optimal thickness of the aluminum film, and plate the aluminum film. Calibrate according to steps c) and d) of the present invention, measure the reflection angle spectrum of the aluminum film, and calibrate the fixed incident angle of intensity modulation; measure the reflected light intensity of NaCl solutions of different concentrations with known refractive index; perform linear fitting on the measured data to obtain a linear relationship between the refractive index and the reflectivity, which can be used for other refractive index detection.

Claims

1. An intensity modulation sensing method with selectable excitation wavelength and aluminum as surface plasmon resonance material, characterized in that: include: Step a), after the excitation wavelength is selected, the optimal thickness of the aluminum film at the selected excitation wavelength is obtained according to the linear relationship between the excitation wavelength and the optimal thickness of the aluminum film; Step b), preparing an aluminum film with an optimal thickness according to a thermal evaporation technique; Step c), measuring the reflection angle spectrum of the aluminum film by a surface plasmon resonance detection system, and calibrating the fixed incident angle of intensity modulation; Step d), changing the medium environment that the aluminum film contacts, detecting the reflection intensity in real time, and obtaining the refractive index of the medium environment according to the linear relationship between the refractive index and the reflected light intensity.

2. The intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material according to claim 1, characterized in that: The linear relationship between the excitation wavelength and the optimal aluminum film thickness is y=-0.02662x+30.86496, where y is the aluminum film thickness in nm, and x is the excitation wavelength in nm.

3. The intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material according to claim 1, characterized in that: The excitation wavelength is arbitrarily selected within the range of 400 to 700 nm.

4. The intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material according to claim 1, characterized in that: The fixed incident angle of the intensity modulation is according to the formula θ 1 / 4 =θ R -(θ R -θ tot ) / 4, the θ 1 / 4 is the fixed incident angle for intensity modulation, θ tot is the total reflection point of the reflection angle spectrum, θ R is the resonance point of the reflection angle spectrum.

5. The intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material according to claim 1, characterized in that: The surface plasmon resonance detection system comprises an LED light source (1), an aperture (2), a half-wave plate (3), a reflector, a polarization beam splitter prism (6), a rotating stage (9) and a photodetector (7); An LED light source (1) provides excitation light, and the excitation light controls the size of the excitation light spot through an aperture (2); a half-wave plate (3), two reflectors and a polarization beam splitter prism (6) are arranged in sequence; the half-wave plate (3) is used to control the power of the excitation light; reflectors a (4) and b (5) are used to collimate the excitation light; the output light of the polarization beam splitter prism (6) is P polarized light, which is used to stimulate the SPR effect; a photodetector (7) is used to detect the reflection intensity; and a control system controls the angle of a rotating table (9) to control the incident angle of the excitation light.

6. The intensity modulation sensing method with optional excitation wavelength and aluminum as surface plasmon resonance material according to claim 1, characterized in that: The reflection intensity is measured in real time by an optical power meter.

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