Direct current optical excitation-based light addressable potentiometric sensor frequency detection method

Abstract

The invention belongs to the technical field of light addressable potentiometric sensor frequency detection, and discloses a light addressable potentiometric sensor frequency detection method based on direct current light excitation, and the specific technical scheme is as follows: a direct current bias circuit provides bias voltage, so that an LAPS is in a working state; a direct-current modulation light source is adopted to scan and irradiate all positions of an LAPS substrate, so that the parts of the LAPS substrate sequentially enter an inversion state and do not respond to changes of surface potentials, other parts which are not illuminated keep responsiveness to the surface potentials, and the changes of the surface potentials of all the illuminated parts are indirectly measured; the LAPS is embedded into the relaxation oscillation circuit to form an oscillation circuit, the measurement result of the LAPS is obtained by measuring the output pulse frequency, the method is different from a traditional LAPS amplitude or phase measurement method adopting alternating current light excitation, a measurement method adopting a direct current light excitation and frequency measurement mode is adopted, the anti-noise capability is higher, and the measurement accuracy is higher. The LAPS detection precision and accuracy can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of frequency detection of optically addressed potentiometric sensors, and in particular relates to a frequency detection method of an optically addressed potentiometric sensor based on direct current light excitation. Background Art

[0002] The Light Addressable Potentiometric Sensor (LAPS) is an electrochemical sensor based on semiconductor field-effect technology, widely used in fields such as biomedical testing and environmental monitoring. It detects target substances by converting the capacitance change between the sensitive membrane and the electrolyte solution into an easily measurable current or voltage signal. External light source excitation gives LAPS flexible light addressing capabilities, enabling it to freely define detection locations on the sensor surface and achieve simultaneous multi-point and multi-parameter measurement. However, LAPS currently only supports two detection modes: amplitude and phase. Due to the limitations of LAPS's detection mechanism, frequency measurement technology cannot be directly applied to LAPS systems. This, to a certain extent, limits LAPS's performance and faces problems such as weak anti-interference ability and low detection sensitivity. Summary of the Invention

[0003] In order to solve the technical problems existing in the prior art, the present invention provides a frequency detection method for an optically addressable potential sensor based on DC light excitation, which converts the capacitor charging and discharging process of the optically addressable potential sensor into a pulse signal of a specific frequency for transmission, thereby improving the anti-interference ability, improving the detection performance of the optically addressable potential sensor by single-point detection, and realizing two-dimensional imaging of the surface of the optically addressable potential sensor through multi-point detection.

[0004] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a frequency detection method of a light-addressable potential sensor based on DC light excitation, wherein the bottom of the solution pool is arranged with a sensitive layer, an insulating layer, a depletion layer and an N-type silicon substrate from top to bottom, the bottom end of the N-type silicon substrate is connected to a contact electrode, and a movable DC light source is arranged below the N-type silicon substrate. The solution pool is filled with an electrolyte solution, the lower section of the reference electrode is inserted into the electrolyte solution, and the upper section of the reference electrode is electrically connected to a relaxation oscillation module and a frequency-voltage conversion module in sequence.

[0005] The DC bias signal circuit provides bias voltage for the light-addressable potential sensor, and the capacitance multiplication circuit multiplies the equivalent capacitance of the light-addressable potential sensor through impedance transformation technology, thereby enhancing the output signal strength of the light-addressable potential sensor; the relaxation oscillation circuit converts the voltage output by the charge and discharge of the light-addressable potential sensor into a certain pulse frequency transmission, thereby improving the anti-interference ability of the light-addressable potential sensor; the frequency-voltage conversion circuit converts the pulse signal output by the relaxation oscillation circuit into a voltage signal, making the voltage output more convenient and flexible to detect.

[0006] A DC light source is used to scan the N-type silicon substrate. When the sensor is not in contact with the solution, the depletion layer capacitance C d The equivalent calculation formula is:

[0007]

[0008] Among them, C nm is the depletion layer capacitance directly above the light source scanning point, m is the number of DC light source scanning points moved along the Y direction, and n is the number of DC light source scanning points moved along the X direction;

[0009] When the surface of the photo-addressable potentiometric sensor contacts the solution, the depletion layer capacitance is equal to C d +ΔC d , ΔC d is the change in total depletion layer capacitance after the surface of the light-addressable potentiometric sensor contacts the solution, and the calculation formula is:

[0010]

[0011] When the DC light source scans the N-type silicon substrate, the holes in the photogenerated electron-hole pairs generated below the N-type silicon substrate will move vertically upward, pass through the N-type silicon substrate and reach the depletion layer to recombine with the electrons there. This process will reduce the thickness of the depletion layer, causing the sensitive film above the light source irradiation area to be sensitive to H in the electrolyte solution. + The ions lose sensitivity, causing the depletion layer capacitance of the region to change by ΔC mn Close to zero.

[0012] The frequency detection method of the light-addressable potentiometric sensor is to convert the capacitance charging and discharging process of the light-addressable potentiometric sensor into a pulse signal of a specific frequency for transmission. Finally, the capacitance change of the light-addressable potentiometric sensor is converted into a voltage signal through a frequency-voltage conversion circuit for measurement.

[0013] When a DC light source scans the N-type silicon substrate, the depletion layer capacitance of the region changes by ΔC. mn is close to zero, thus the capacitance measurement model of the light-addressable potentiometric sensor can be obtained:

[0014]

[0015] In the capacitance measurement model of the light-addressable potentiometric sensor, the total depletion layer capacitance is expressed as C dmn (n, m are 1, 2, ...), represents the capacitance value at different measurement points, which is obtained by actual measurement; C mn (n, m are 1, 2, ...) represents the basic capacitance value under no light conditions. These are known constants. The unknown quantity to be solved is the change in capacitance under light conditions, ΔC.mn (n, m are 1, 2, ...). Each equation in the capacitance measurement model of the light-addressable potentiometric sensor corresponds to the capacitance change relationship of a measurement point. The known total capacitance value and basic capacitance value can be used to solve these unknown capacitance changes.

[0016] This paper proposes an innovative frequency detection method for light-addressable potentiometers (LAPSs), which can improve the problems of insufficient detection accuracy and weak anti-interference ability of light-addressable potentiometers. A DC modulated light source is used to scan and illuminate various locations on the LAPS substrate, causing local areas to enter an inversion state and not respond to changes in surface potential, while other areas not illuminated remain responsive to their surface potential. A capacitance measurement model for the light-addressable potentiometer is established, and the change in capacitance ΔC under illumination conditions is analyzed and solved. mn , using impedance transformation technology to multiply the equivalent capacitance of the sensor, improve the sensor output signal strength and detection sensitivity, and then use the relaxation oscillation circuit to convert the voltage output changes of the light-addressable potential sensor capacitor charge and discharge into a certain pulse frequency transmission, thereby improving the LAPS anti-interference ability. The pulse signal output by the relaxation oscillation circuit is converted into a voltage signal through the frequency-voltage conversion circuit, making the voltage output more convenient and flexible to detect. This measurement method of DC light excitation + frequency measurement mode has stronger anti-noise ability, can more accurately capture small changes in substance concentration, and effectively improve the detection precision and accuracy of LAPS. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the detection device of the present invention.

[0018] Figure 2 Schematic diagram of a traditional detection device based on AC light excitation.

[0019] Figure 3 This is the depletion layer equivalent capacitance diagram.

[0020] Figure 4 This is the LAPS frequency measurement circuit diagram based on impedance transformation.

[0021] Figure 5 This is the relaxation oscillation circuit diagram based on LAPS impedance transformation.

[0022] Figure 6 This is the LAPS frequency-voltage conversion circuit diagram.

[0023] Figure 7 This is the output characteristic curve of the traditional amplitude measurement circuit when random noise (10mV) is applied.

[0024] Figure 8 This is a graph showing the circuit output characteristics when random noise (10mV) is applied and measured based on frequency.

[0025] Figure 9 The normalized characteristic curves of the frequency measurement method based on impedance transformation and without impedance transformation (the measured object is a standard pH buffer solution, pH = 7.0).

[0026] Figure 10 This is a normalized characteristic curve diagram of pH = 7.0 using the frequency measurement method without impedance transformation for 5 times.

[0027] Figure 11 This is a normalized characteristic curve diagram of the frequency measurement method five times (pH = 7.0) with an impedance transformation ratio of R1:R2 = 1:1.

[0028] Figure 12 This is the density distribution diagram of the “L” pattern on the sensor surface based on amplitude detection.

[0029] Figure 13 This is the concentration distribution diagram of the “O” pattern on the sensor surface based on amplitude detection.

[0030] Figure 14 This is the density distribution diagram of the “L” pattern on the sensor surface based on frequency detection.

[0031] Figure 15 This is the density distribution diagram of the “O” pattern on the sensor surface based on frequency detection. DETAILED DESCRIPTION

[0032] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0033] A frequency detection method for a light-addressable potentiometric sensor based on DC light excitation is proposed. During the LAPS (light-addressable potentiometric sensor) detection process, pulses are transmitted at a certain frequency to improve noise interference during the LAPS measurement process, thereby improving the detection accuracy and anti-interference ability of the LAPS.

[0034] The structure of LAPS and the traditional detection method based on AC light excitation are as follows Figure 2As shown, a LAPS primarily consists of metal contact electrodes, a silicon substrate, an insulating layer, and a sensitive layer. Taking an N-type silicon substrate as an example, a negative DC bias voltage applied to the LAPS surface via a reference electrode forms a depletion layer at the silicon-insulator interface. The capacitance of this depletion layer is directly related to the surface potential of the sensitive layer (which is proportional to the concentration of the substance being measured in the electrolyte solution). When an AC-modulated light source is used to illuminate a specific location on the LAPS substrate, the excited photogenerated carriers drift toward the depletion layer under the influence of the internal electric field, generating a photocurrent. Because the depletion layer capacitance varies at different locations, the photocurrent generated by each location illuminated by the light source also varies. Measuring this current can be used to determine the depletion layer capacitance. Because changes in the depletion layer capacitance directly reflect changes in the LAPS surface potential, the distribution of the LAPS surface potential at each location can be determined by moving the light source to illuminate different locations on the substrate.

[0035] The frequency detection principle diagram of the optically addressable potentiometric sensor based on DC light excitation is shown in the figure below. Figure 1 As shown, C i is the insulation layer capacitance, C d is the depletion layer capacitance, and the depletion layer capacitance is equivalent to Figure 3 As shown, the depletion layer capacitance C d It is equivalent to multiple capacitors in parallel. A DC light source is used to scan the LAPS substrate. When the sensor is not in contact with the solution, the LAPS depletion layer capacitance C d The equivalent calculation formula is:

[0036]

[0037] Among them, C nm is the depletion layer capacitance directly above the DC light source scanning point, m is the number of DC light source scanning points moved along the Y direction, and n is the number of DC light source scanning points moved along the X direction.

[0038] When the sensor surface contacts the solution, the depletion layer capacitance is equal to C d +ΔC d , ΔC d is the change in total depletion layer capacitance after the sensor surface contacts the solution, and the calculation formula is:

[0039]

[0040] Among them, C nm is the depletion layer capacitance just above the DC light source scanning point, ΔC mn is the change in depletion layer capacitance in this region.

[0041] In formula (2), the depletion layer capacitance C d The calculation formula is:

[0042]

[0043] Where ε0 is the dielectric constant of vacuum, ε rs is the relative dielectric constant of the silicon substrate, d s is the depletion layer thickness.

[0044] When the DC light source scans the N-type silicon substrate, the holes in the photogenerated electron-hole pairs generated below the N-type silicon substrate will move vertically upward, pass through the N-type silicon substrate and reach the depletion layer and recombine with the electrons. This process will reduce the thickness of the depletion layer, causing the sensitive film above the light source irradiation area to be sensitive to H in the electrolyte solution. + The ions lose sensitivity, causing the depletion layer capacitance of the region to change by ΔC mn Close to zero, the LAPS detection method based on frequency measurement is to convert the LAPS capacitance charging and discharging process into a pulse signal of a specific frequency for transmission.

[0045] Thus, the capacitance measurement model of LAPS can be obtained:

[0046]

[0047] In the LAPS capacitance measurement model, the total depletion layer capacitance is expressed as C dmn (n, m are 1, 2, ...), represents the capacitance value at different measurement points, which is obtained by actual measurement; C mn (n, m are 1, 2, ...) represents the basic capacitance value under no light conditions. These are known constants; the unknown quantity to be solved is the change in capacitance under light conditions, ΔC mn (n, m are 1, 2, ...). Each equation in the LAPS capacitance measurement model corresponds to the capacitance change relationship of a measurement point. The known total capacitance value and base capacitance value can be used to solve these unknown capacitance changes.

[0048] The specific implementation steps of the frequency detection method of the light-addressable potentiometric sensor are as follows:

[0049] Step 1: Use impedance transformation technology to achieve capacitance multiplication, reduce the impedance between the reference electrode and the solution, and thus improve the detection performance. U1 uses the OPA2227 operational amplifier, and forms a capacitance multiplication circuit with resistors R1 and R2 as shown below: Figure 4 As shown, the equivalent impedance of LAPS is the output impedance Z without impedance transformation. o1 for:

[0050]

[0051] Among them, R f is the impedance of the reference electrode, R e is the impedance of the electrolyte being measured, C i Represents the capacitance of the insulating layer, C drepresents the capacitance of the depletion layer, ω is the angular frequency, and j is the imaginary unit;

[0052] The output impedance Z of the impedance conversion circuit is used o2 for:

[0053]

[0054] After the impedance transformation, an additional resistance term is added to the output impedance of the measurement circuit, namely R1R2 / (R1+R2), and the insulation layer capacitance C i and the depletion layer capacitance C d The value is expanded by 1+R1 / R2 times, and the impedance R e The attenuation rate is 1+R1 / R2 times. The impedance conversion circuit can not only reduce the influence of nonlinear factors in the measurement, but also increase the equivalent capacitance value of the same LAPS.

[0055] Step 2: The voltage output by the LAPS capacitor charge and discharge is converted into a certain pulse frequency through the relaxation oscillation circuit, thereby improving the LAPS anti-interference ability. Figure 5 The relaxation oscillation circuit diagram based on LAPS is shown in Figure 2. U2 is an LM311 open-drain voltage comparator, and the supply voltage of the entire circuit is ±5V. The three-element analysis method is used to analyze the dynamic process of capacitor charging and discharging in the circuit. The formula for the three-element analysis method is:

[0056]

[0057] In formula (7), u c (t) is the instantaneous value of the capacitor voltage changing with time, u c (∞) is the steady-state value of the capacitor voltage, u c (0 + ) is the initial value of the capacitor voltage, τ is the time constant, and t is the time variable.

[0058] The charging time is determined as the process of the capacitor voltage rising to a certain set threshold value. Under the combined effect of the resistance and capacitance in the circuit, equations (8) and (9) are the calculation process of the relaxation oscillation circuit charging time.

[0059]

[0060] Where t1 is the charging time of the relaxation oscillation circuit, capacitor C is the equivalent capacitance of the LAPS insulation layer capacitance and the depletion layer capacitance in series, and resistor R is the equivalent resistance of the reference electrode resistance, electrolyte solution resistance, and LAPS internal resistance in series.

[0061] Similar to the charging time, the discharge time is calculated based on the time required for the capacitor voltage to drop to a set threshold. The discharge time is calculated as formulas (10) and (11).

[0062]

[0063] Where t2 is the discharge time of the relaxation oscillation circuit, capacitor C is the equivalent capacitance of the LAPS insulation layer capacitance and the depletion layer capacitance in series, and resistor R is the equivalent resistance of the reference electrode resistance, electrolyte solution resistance, and LAPS internal resistance in series.

[0064] Through the above formula, the charging time and discharging time of the relaxation oscillator circuit can be accurately calculated, and then the frequency and period of the output signal can be obtained. The final output frequency of the relaxation oscillator is represented by f, and formula (12) is its calculation formula.

[0065]

[0066] Step 3: Use the frequency-voltage conversion circuit to convert the pulse signal into a voltage signal output, which is convenient for flexible detection and analysis. Figure 6 The LAPS frequency-to-voltage conversion circuit, composed of the AD8602 (U3), capacitor C1, and resistors R7 and R8, forms an active differential circuit that differentially converts the rectangular pulse signal output by the relaxation oscillation circuit into a periodic, sharp, and narrow impulse signal, achieving a preliminary change in the signal's shape. The 74LS04 inverter (U4) reverses the impulse signal, converting its rising edge into a falling edge. Whenever a falling edge occurs, the monostable circuit based on the NE555 timer (U5) is triggered, generating a pulse of fixed width. Resistor R9 and capacitor C2 together form a timing network that determines the width of the monostable output pulse. The output pulse width t W It can be calculated by formula (13).

[0067]

[0068] Finally, the pulse signal output by the monostable circuit is filtered by a second-order active low-pass filter with OPA2227 (U6) as the core. The DC component is extracted from the pulse signal and converted into a stable voltage signal. The conversion from frequency to voltage is completed by converting the frequency change of the input pulse signal into the amplitude change of the voltage signal.

[0069] Step 4: Test the anti-interference capability of the constructed DC light-excited optically addressable potentiometric sensor frequency detection system. A random noise signal with a peak voltage of 10mV was introduced. The electrolytic solution under test was a phosphate pH buffer (PBS buffer), composed of two phosphate solutions, NaH2PO4 and Na2HPO4. The ratio of the two phosphates was adjusted to create a buffer solution with a pH of 6.86. Under the same conditions, a comparative test was conducted between traditional amplitude detection and frequency detection methods to observe their performance in a noisy environment. Figure 7 This is the output characteristic curve of the traditional voltage amplitude measurement method with random noise (10mV) applied. Figure 8 The output characteristic curve of the frequency measurement method with random noise (10mV) applied is shown. The measurements were performed 20 times. Through comparative analysis, we can clearly see the advantage of the frequency detection method in suppressing noise interference.

[0070] Step 5. Design five different measurement conditions: no impedance transformation, impedance transformation coefficient ratio R1:R2 = 1:1, impedance transformation coefficient ratio R1:R2 = 2:1, impedance transformation coefficient ratio R1:R2 = 3:1, and impedance transformation coefficient ratio R1:R2 = 5:1. Use a standard buffer solution with a pH value of 7.0 as the object to explore the effect of impedance transformation on the output signal strength of the LAPS sensor. Figure 9 The normalized characteristic curves of the frequency detection method without impedance transformation and with different impedance transformation coefficients are presented. As the impedance transformation ratio increases, the slope of the output characteristic curve increases significantly, and the capacitance change of LAPS has a more significant impact on the output signal, making the output signal more sensitive to the capacitance change.

[0071] Step 6: Using two measurement circuits without and with impedance transformation, perform five repeated measurements on a buffer solution with a pH value of 7.00 to obtain normalized current-voltage (IV) curves and the stability and consistency under different measurement conditions. Figure 10 is the characteristic curve without impedance transformation. Figure 11 This is the characteristic curve after impedance transformation. It can be seen that Figure 11 The curve displacement in is significantly smaller than Figure 10 This shows that different measurement circuits have different degrees of influence on the curve during repeated measurements. By adjusting the impedance characteristics of the measurement circuit, the random fluctuation of the measurement results can be effectively reduced, thereby enhancing the stability and reliability of the measurement.

[0072] Step 7: 2D imaging of the LAPS surface was achieved through multi-point detection. Transparent silicone rubber was spin-coated to create "L" and "O"-shaped patterns on the LAPS sensor's sensitive layer. A standard buffer solution with a pH of 7.00 was selected as the sample solution for detection. The bias voltage was set to -0.9V. This value was chosen based on experimental requirements and sensor characteristics to ensure accuracy and repeatability. The LAPS sensor's sensitive layer was scanned using a lateral scanning strategy. In this experiment, 25 data points were collected.

[0073] Figure 12 、 Figure 13 These are the concentration distribution diagrams of “L” and “O” patterns on the sensor surface based on amplitude detection. Figure 14 、 Figure 15These are the density distribution maps of "L" and "O" patterns on the sensor surface, respectively, based on frequency detection. Compared to amplitude detection, the frequency detection method displays "L" and "O" patterns more clearly. Frequency detection technology can more sensitively capture subtle changes in density, enabling more accurate identification of pattern edges and details. Furthermore, this method helps reduce the impact of ambient noise on detection results, thereby improving their accuracy and reliability.

[0074] The present invention can solve the problems of insufficient LAPS detection accuracy and weak anti-interference ability. Through an innovative LAPS frequency detection method, the capacitor charging and discharging process of LAPS is converted into a pulse signal of a specific frequency for transmission, thereby improving the anti-interference ability and detection sensitivity of LAPS.

[0075] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of the present invention.

Claims

1. A frequency detection method for an optically addressable potentiometric sensor based on DC light excitation, characterized in that: The invention comprises a solution pool, wherein the bottom of the solution pool is sequentially arranged with a sensitive layer, an insulating layer, a depletion layer and an N-type silicon substrate, the bottom end of the N-type silicon substrate is connected to a contact electrode, a movable DC light source is arranged below the N-type silicon substrate, the solution pool is filled with an electrolyte solution, the lower section of the reference electrode is inserted into the electrolyte solution, and the upper section of the reference electrode is sequentially electrically connected to a relaxation oscillation module and a frequency-voltage conversion module; The DC bias signal circuit provides a DC bias voltage to the N-type silicon substrate through the contact electrode; The relaxation oscillation circuit converts the voltage outputted by the charge and discharge of the light-addressable potential sensor capacitor into a pulse frequency transmission; The frequency-voltage conversion module converts the pulse signal output by the relaxation oscillation circuit into a voltage signal.

2. The method for detecting frequency of an optically addressable potentiometric sensor based on direct current light excitation according to claim 1, wherein: A DC light source is used to scan the N-type silicon substrate. When the light-addressable potential sensor is not in contact with the solution, the depletion layer capacitance C d The equivalent calculation formula is: Among them, C nm is the depletion layer capacitance directly above the DC light source scanning point, m is the number of DC light source scanning points moved along the Y direction, and n is the number of DC light source scanning points moved along the X direction; When the surface of the photo-addressable potentiometric sensor contacts the solution, the depletion layer capacitance is equal to C d +ΔC d , ΔC d is the change in the total depletion layer capacitance after the sensor surface contacts the solution. The specific calculation formula is: When the DC light source scans the N-type silicon substrate, the holes in the photogenerated electron-hole pairs generated below the N-type silicon substrate move vertically upward until they pass through the substrate and reach the depletion layer and recombine with the electrons. The sensitive film above the DC light source irradiation area is sensitive to H in the electrolyte solution. + The ions lose sensitivity, causing the depletion layer capacitance in this area to change by ΔC mn Close to zero.

3. The frequency detection method of an optically addressable potentiometric sensor based on DC light excitation according to claim 2, characterized in that: When the DC light source scans the substrate, the depletion layer capacitance of the region changes by ΔC. mn Close to zero, the capacitance measurement model of the light-addressable potentiometric sensor is obtained: In the capacitance measurement model of the light-addressable potentiometric sensor, the total depletion layer capacitance is expressed as C dmn (n, m are 1, 2, ...), indicating the capacitance value at different measurement points; C mn (n, m are 1, 2, ...) represents the basic capacitance value under no light conditions; the unknown quantity to be solved is the change in capacitance under light conditions ΔC mn (n, m are 1, 2, ...), each equation in the capacitance measurement model of the light-addressable potentiometric sensor corresponds to the capacitance change relationship of a measurement point, and the capacitance change is solved using the known total capacitance value and base capacitance value.

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