Photothermal spectroscopic method and photothermal detection element

Abstract

A photothermal spectroscopy method using a photothermal detection element (1), wherein the photothermal detection element (1) comprises a frame (2), a membrane (3) supported by the frame (2), and an electrical circuit (5), wherein the electrical circuit (5) comprises a first measurement resistor (6) arranged on or in the membrane (3), wherein the membrane (3) comprises a substantially planar receiving surface (4) configured for receiving an analyte, the method comprising the following steps.

Description

[0001] This invention relates to a photothermal spectroscopic method utilizing a photothermal detection element and to a photothermal detection element.

[0002] The present invention also relates to photothermal detection elements, such as disposable sampling and detector chips based on thermal resistance readings that are suitable for chemical analysis of samples by photothermal spectroscopy and, in particular, photothermal infrared (IR) spectroscopy.

[0003] Infrared (IR) absorption spectroscopy is a key technique in analytical science, primarily focusing on the interaction between infrared radiation and matter. This widely used method is crucial for identifying and quantifying substances through the absorption of IR light, which is intrinsically related to the vibrational and rotational motion of molecules. The core of this technique lies in its reliance on transmission measurements, where the absorption of IR light is measured directly as it passes through the sample. While efficient, this method primarily focuses on the transmission characteristics of the sample.

[0004] However, the advent of photothermal infrared spectroscopy marks a significant evolution in absorption spectroscopy techniques. Unlike traditional transmission-based methods, photothermal IR spectroscopy utilizes the subtle but measurable thermal effect caused by the absorption of infrared radiation. When a sample absorbs IR light, it experiences a small temperature rise. While this temperature change is typically negligible in traditional transmission spectroscopy, it forms the cornerstone of photothermal detection.

[0005] The advantages of photothermal infrared spectroscopy are multifaceted. One of its most compelling advantages is its ability to analyze samples that are difficult or impossible to study using transmission methods. This includes optically opaque or highly scattering materials, where transmission measurements would yield limited or no information. Furthermore, photothermal IR spectroscopy exhibits higher sensitivity (see Bialkowski, Stephen E., Nelson GC Astrath, and Mikhail A. Proskurnin. Photothermal spectroscopy methods. John Wiley & Sons, 2019.), making it particularly adept at detecting low concentrations of analytes.

[0006] One particular photothermal method utilizes the excellent temperature responsiveness of nanomechanical resonators (see, for example, West, Robert G., Kostas Kanellopulos and Silvan Schmid. “Photothermal Microscopy and Spectroscopy with Nanomechanical Resonators.” The Journal of Physical Chemistry C (2023).). Nanomechanical photothermal infrared spectroscopy has enabled the analysis of samples in the picometer range (see Kurek, Maksymilian et al., “Nanomechanical infrared spectroscopy with vibrating filters for pharmaceutical analysis.” Angewandte Chemie 129.14 (2017): 3959-3963; or Kurek, Maksymilian et al., “Nanomechanical infrared spectroscopy with vibrating filters for pharmaceutical analysis.” Angewandte Chemie 129.14 (2017): 3959-3963; or Luhmann, Niklas et al., “Nanoelectromechanical Infrared Spectroscopy with In Situ Separation by Thermal Desorption: NEMS-IR-TD.” ACSsensors 8.4 (2023): 1462-1470). Here, the sample to be analyzed is directly collected on a disposable nanomechanical resonator. When the sample is irradiated with probe light (e.g., infrared light), it photothermally heats the nanomechanical resonator, causing frequency detuning of the resonant mode.

[0007] The nanomechanical detectors involved require a high vacuum to eliminate gas damping in the resonator. The necessity of maintaining a high vacuum environment in some infrared spectroscopy methods presents significant limitations and challenges, making these methods less optimized for a range of applications. First, the requirement for a high vacuum imposes substantial limitations on the instrument's practicality and versatility. Creating and maintaining a high vacuum requires complex, often bulky and expensive equipment, which can be prohibitive for many laboratories, especially those with limited resources or space. Furthermore, the process of purging the system to achieve a high vacuum is time-consuming, reducing the throughput of analytical procedures and limiting the efficiency of the method in fast-paced or high-throughput environments.

[0008] Furthermore, the high vacuum environment limits the types of samples that can be analyzed. For example, volatile compounds, biological specimens, or materials sensitive to low-pressure conditions may degrade or change under vacuum, leading to potential inaccuracies in spectroscopic analysis. This limitation significantly narrows the scope of applications, especially in fields such as biological research, environmental science, and materials science where samples are typically sensitive to extreme conditions.

[0009] Furthermore, the operational complexity of maintaining a high vacuum and the need for specialized handling can increase the risk of operational errors and instrument downtime, requiring operators to have a higher level of expertise and more stringent maintenance procedures. These factors collectively lead to higher operating costs and reduced ease of use, making high-vacuum infrared spectroscopy less ideal compared to other spectroscopic techniques that operate under environmental or less demanding conditions.

[0010] Furthermore, the operation of nanomechanical resonators requires specialized feedback electronics (such as phase-locked loops or self-sustaining oscillators) to monitor the resonant frequency shift during photothermal sensing. To analyze samples using frequency-shift-based photothermal nanomechanical resonators, users must configure the feedback electronics, which requires specialized training and is time-consuming.

[0011] WO 2015 109 410 A1 discloses a nanowire exposed to chemical molecules adsorbed on it. The nanowire is exposed to infrared light, and the change in its impedance parameters is measured using an LC resonance detection scheme to perform chemical analysis of the molecules via IR spectroscopy.

[0012] US 2013 170 517 A1 discloses a radiative heat meter with a microsystem comprising a support and a movable mass block suspended from a beam above the support.

[0013] WO 2023 183 958 A1 illustrates an infrared detector and an infrared spectroscopy method based on the variation of the resonant frequency of a membrane supported by a frame. The detector is used in a vacuum.

[0014] US 9,121,761 B2 discloses an infrared detector. An optical resonator element, used as a light absorber, is suspended above a substrate. Photothermal heating is measured using a thermal resistance element.

[0015] WO 2023 / 183958 A1 shows another infrared detector.

[0016] JP 2013 003014 A illustrates an infrared detector with increased responsiveness.

[0017] JPH0634448A illustrates a thermometer with an infrared detection unit.

[0018] WO 2022 / 268892 A2 illustrates a thermal fluid sensor for sensing the concentration or composition of a fluid.

[0019] Another infrared detector is known from WO 2020 / 047572 A2.

[0020] The purpose of this invention is to mitigate or eliminate at least some of the disadvantages of the prior art. In particular, the purpose of this invention is to provide a photothermal spectroscopic method and a photothermal detection element that are particularly easy to use and provide high sensitivity.

[0021] This objective is achieved through a photothermal spectroscopy method using a photothermal detection element comprising a frame, a membrane supported by the frame, and circuitry, wherein the circuitry includes a first measuring resistor disposed on or within the membrane, and wherein the membrane includes a substantially flat receiving surface configured to receive an analyte, the method comprising the following steps:

[0022] - The analyte is deposited on the receiving surface;

[0023] - Irradiate the receiving surface with electromagnetic radiation, especially IR radiation;

[0024] - Detect the change in resistance of the circuit; and

[0025] - Determine the properties of the analyte based on the detected changes.

[0026] For example, the membrane may be a silicon nitride membrane.

[0027] Other materials used for fabricating the film are—unlimited—any 2D material (e.g., graphene, molybdenum disulfide, etc.), polymer (e.g., SU-8), pyrolytic carbon, diamond, silicon carbide, aluminum nitride, silicon dioxide, silicon, gallium arsenide, or titanium nitride. The film's outline can be, for example, generally rectangular or circular, particularly circular. The film can be, for example, circular and include a diameter. The film can be, for example, square and include a side length. The film can include a thickness in the range of 10 nm to 1 µm, particularly in the range of 30 nm to 500 nm, preferably in the range of 50 nm to 200 nm. The film can include silicon, silicon carbide, aluminum oxide, diamond, silicon oxide, and / or similar dielectric materials.

[0028] The membrane is supported by a frame. The membrane can be attached to the frame such that the membrane is under tension. The intrinsic tensile stress of the membrane can be at or below 1 gigapascal (GPa), optionally below 200 megapascals (MPa), optionally below 50 MPa, and optionally below 10 MPa. For silicon nitride membranes, the tensile stress can be controlled stoichiometrically during deposition. Alternatively, low tensile stress values ​​can be achieved, for example, by appropriate oxygen plasma exposure, as disclosed by N. Luhmann, A. Jachimowicz, J. Schalko, P. Sadeghi, M. Sauer, A. Foelske-Schmitz, and S. Schmid, “Effect of oxygen plasma on nanomechanical siliconnitride resonators”, Appl. Phys. Lett., Vol. 111, p. 63103, 2017.

[0029] The membrane can be attached to the frame such that there are no holes and / or gaps and / or grooves between the membrane and the frame. The membrane may include a main extension plane and an outer edge within the main extension plane. The main extension plane is defined such that the membrane has maximum extension in the main extension plane, while the membrane has less extension in a plane transverse to the main extension plane. The outer edge of the membrane may be fully attached to the frame. Alternatively, the membrane may include one or more holes and / or gaps and / or grooves. The outer edge of the membrane may be partially attached to the frame and may be partially separated from the frame (i.e., isolated). In particular, the membrane and frame may include a trampoline configuration, i.e., the membrane may be suspended from the frame by two or more (optionally three or more, particularly four) tethers. The tethers may be part of the membrane. In this case, the membrane may also be referred to as a nanomechanical trampoline. The four narrow tethers of the nanomechanical trampoline ensure that high stress is localized to the tethers, while the central membrane (i.e., the rest of the membrane excluding the tethers) exhibits much lower stress. In addition to stress engineering, the narrow tethers also improve thermal insulation to the frame. The thermal insulation of the membrane relative to the frame results in a decrease in thermal conductivity from the membrane to the frame, and thus leads to higher detector sensitivity. Therefore, the trampoline configuration is particularly sensitive to absorbed radiation and heat dissipated into the membrane. The tethers can have a width, for example, from 300 nm to 300 µm. The tethers can be placed at the corners of the membrane and can extend substantially along the diagonal of the membrane. We understand the tethers as branches (limbs) at, for example, the boundaries of the membrane. The tethers can typically be narrow, strip-shaped portions of the membrane (“tether sections”), but are not limited to a rectangular outline of such portions. The tethers can be made of the same material as the rest of the membrane; in particular, the membrane ground material in the absorption region can be the same material as any tether section. The use of tethers also facilitates the connection between the membrane and the frame.

[0030] The membrane may include a lateral dimension (i.e., within the main extension plane) ranging from 100 µm to 1 cm, particularly in the range of 250 µm to 5 mm, and preferably in the range of 500 µm to 3 mm. The lateral dimension may, for example, be related to the side length or diameter of the membrane.

[0031] The photothermal detection element includes a circuit having a first measuring resistor at least partially disposed on or within the film. The first measuring resistor is in thermal contact with the film and may substantially be at the same temperature as the film. The first measuring resistor may be an electrical conductor. For example, the first measuring resistor may include metals such as, for example, platinum, aluminum, titanium, nickel, chromium, molybdenum, tungsten, copper, gold, and / or metal alloys and / or metal oxides (such as titanium oxide or nickel oxide). The film may optionally comprise multiple layers. The film may include a metallization layer. The metallization layer may be the top layer of the film. The metallization layer may, for example, include gold. The thickness of the metallization layer (e.g., a gold layer) may range from 5 nm to 1 µm, particularly from 10 nm to 500 nm, preferably from 30 nm to 200 nm. Due to the low thickness of the metallization layer, the metallization may be substantially permeable to infrared radiation. The first measuring resistor may be formed by conductors within the metallization layer. The first measuring resistor has resistance. The resistance of the first measuring resistor is temperature-dependent. The first measuring resistor may include a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). For example, gold has a positive temperature coefficient. Due to the thermal contact between the first measuring resistor and the film, the resistance of the first measuring resistor depends on the temperature of the film. Therefore, the resistance of the first measuring resistor is a function of the film temperature. The temperature of the film can be estimated or determined based on the resistance of the first measuring resistor. The first measuring resistor (and optionally any other resistor) is a thermal resistance element. An analyte is excited or probed with electromagnetic radiation (especially infrared (IR) radiation). The analyte's response to radiation is determined by utilizing the temperature rise of the film caused by photons absorbed by the analyte and the thermal conduction between the analyte and the film (and ultimately the first measuring resistor) through the thermal resistance element. The reading of this response can be called the thermal resistance reading. In contrast, existing techniques rely primarily on the frequency shift of the film, which is independent of the thermal resistance reading.

[0032] The membrane includes a substantially flat receiving surface configured to receive analytes. The analytes can be organic or inorganic substances. They can be solids or liquids. The analytes can include particles, particularly aerosol particles. Due to the substantially flat receiving surface, various analytes can be deposited on the membrane in a simple and reproducible manner. For example, analytes can be deposited by drop casting. The flat receiving surface is particularly advantageous for drop casting because droplets of analyte (or a solution containing the analyte) can be deposited in a controlled manner. Alternatively, the analytes can include aerosol particles and can be deposited by impaction. The flat receiving surface can be used as an impactor plate. Alternatively, aerosol particles can be deposited, for example, by diffusion, electrostatic precipitation, or thermophoresis. Furthermore, due to the substantially flat receiving surface, the analytes can be irradiated in a controlled and reproducible manner.

[0033] Photothermal spectrometry methods include the following steps:

[0034] - The analyte is deposited on the receiving surface;

[0035] - Irradiate the receiving surface with electromagnetic radiation (especially IR radiation);

[0036] - Detect the change in resistance of the circuit; and

[0037] - Determine the properties of the analyte based on the detected changes.

[0038] When the deposited analyte is irradiated, it absorbs photons when the wavelength of the analyte resonates with that of the photons. The energy of the absorbed photons is then converted into heat and transferred from the analyte to the membrane, causing the membrane temperature to rise. When the wavelength (i.e., energy) of the photons does not correspond to the resonance of the analyte, the photons may be reflected or transmitted through the membrane, which essentially does not affect the membrane temperature. Therefore, the temperature of the membrane (especially the temperature rise) indicates the absorbed radiation and thus the resonance of the wavelength of the analyte with that radiation. The circuit (especially the first measuring resistor) is also affected by the rise in membrane temperature, which will cause a change in the circuit resistance. The resistance of the circuit can be, for example, the resistance of the first measuring resistor. Alternatively, the resistance of the circuit can be a function of the resistance of the first measuring resistor and optionally other resistors.

[0039] Changes in circuit resistance can be detected using resistance measuring units, such as multimeters, oscilloscopes, or integrated circuits for measuring circuit resistance. This resistance measurement can be fully automated and requires no user adjustments. Therefore, it is easier to operate than, for example, photothermal sensors based on nanomechanical resonators.

[0040] Based on the detected changes, the properties of the analyte are determined. For example, the temperature of the membrane can be detected as a function of the wavelength of radiation. Therefore, the absorption spectrum of the analyte can be determined. The absorption spectrum can be a characteristic of the analyte. Based on the absorption spectrum, other properties of the analyte, such as chemical properties, can be determined. In short, the analyte can be deposited in a particularly simple, universal, and reproducible manner. Compared to prior art nanomechanical detectors and detection schemes, the method according to the invention does not require a high vacuum to eliminate gas damping. Prior art relies primarily on the shift in the resonant frequency of the membrane, which is caused by the temperature shift of the membrane. Compared to the resonant frequency of the membrane, the resistance of the first measuring resistor is almost unaffected by the background gas. Therefore, the measurement scheme according to the invention does not rely on a high vacuum, is therefore particularly easy to use, and allows for the analysis of a wider variety of analytes.

[0041] Furthermore, the objective of this invention is achieved through a photothermal detection element, which has:

[0042] - Framework;

[0043] - A membrane supported by the frame, wherein the membrane includes a substantially flat receiving surface configured to receive the analyte; and

[0044] - A circuit, the circuit having:

[0045] - A first measuring resistor, at least partially disposed on or in the membrane;

[0046] - First reference resistor; and

[0047] - At least a first electrical contact pad, a second electrical contact pad, and a third electrical contact pad for contacting the circuit, wherein the first electrical contact pad, the second electrical contact pad, and the third electrical contact pad are at least partially disposed on the frame, wherein the resistance of the first electrical contact pad, the second electrical contact pad, and the third electrical contact pad is less than 20% (particularly less than 10% or less than 5%) of the resistance of the first measuring resistor and / or the first reference resistor, wherein the first measuring resistor is connected to the first electrical contact pad and the second electrical contact pad, and wherein the first reference resistor is at least connected to the third electrical contact pad;

[0048] The average distance between the first reference resistor and the center of the membrane is greater than the average distance between the first measuring resistor and the center of the membrane.

[0049] In addition to the first measuring resistor, the circuit also includes a first reference resistor. The reference resistor includes a reference resistor. The reference resistor may include a positive temperature coefficient or a negative temperature coefficient. The first reference resistor may include the same temperature coefficient as the first measuring resistor. The first reference resistor may be formed at least partially by wires within the metallization layer of the film and / or the metallization layer of the frame. The thermal coupling between the film and the first measuring resistor may be less than the thermal coupling between the film and the reference resistor. The reference resistor may be substantially thermally decoupled from the film. Furthermore, the circuit includes at least a first electrical contact pad, a second electrical contact pad, and a third electrical contact pad for contacting the circuit. The contact pads (i.e., the first contact pad, the second contact pad, the third contact pad, and optionally a fourth electrical contact pad, and optionally additional contact pads) may be configured to connect the circuit to (external) resistance measurement (particularly the resistance of the first measuring resistor and / or the first reference resistor) for measuring the resistance of the circuit. The first electrical contact pad, the second electrical contact pad, and the third electrical contact pad are at least partially disposed on the frame. The metallization layer of the frame may include these pads. The respective resistances of the first, second, and third electrical contact pads are less than 20% of the resistance of the first measuring resistor and / or the first reference resistor, particularly less than 10% or less than 5%. The resistance of the electrical contact pads can be the maximum resistance between any two points on the respective contact pads. The resistance of the contact pads is less than the resistance of the first and measuring resistors, such that the contact pads, and particularly the precise contact positions on these contact pads, have no significant effect on the resistance measurement of the first reference resistor. The contact pads can include rectangular or circular (particularly circular) shapes. The contact pads can include a diameter or minimum side length of at least 0.5 mm (preferably at least 1 mm or at least 2 mm). The (lateral) area of ​​the contact pads can be at least 5 times larger than the (lateral) area of ​​the wires forming the resistor, preferably at least 10 times, particularly at least 20 times. The electrical contact pads and the resistor can be formed from different regions of the (same) metallization layer. The resistor can have an elongated shape.

[0050] A first measuring resistor is connected to a first electrical contact pad and a second electrical contact pad. Therefore, the resistance of the first measuring resistor can be measured between the first and second electrical contact pads. For example, a first measuring circuit can contact the first and second electrical contact pads to measure the resistance between them. A first reference resistor is connected to at least a third electrical contact pad. Optionally, the first reference resistor is connected to either the first or second electrical contact pad. Therefore, the resistance of the first reference resistor can be measured between the third electrical contact pad and either the first or second electrical contact pad. For example, a second measuring circuit can contact the third electrical contact pad and either the first or second electrical contact pad to measure the resistance between them. In other words, one of the contact pads can be used to measure the resistance of both the first measuring resistor and the first reference resistor. Alternatively, the circuit may include a fourth electrical contact pad, wherein the first reference resistor can be connected to both the third and fourth electrical contact pads.

[0051] The average distance between the first reference resistor and the center of the film is greater than the average distance between the first measuring resistor and the center of the film. The center of the film is related to the centroid of the film. A flat receiving surface of the film is preferably arranged at and around the center of the film. Furthermore, the film can preferably be irradiated with infrared radiation at the center. Therefore, the film can be heated mainly at the center. Thus, the film can include a transversely extending temperature distribution due to absorbed radiation, wherein this temperature distribution may include a peak or plateau at the center. Because the average distance between the first reference resistor and the center of the film is greater than the average distance between the first measuring resistor and the center of the film, the first measuring resistor heats up faster and more significantly than the first reference resistor when the analyte absorbs IR radiation. Therefore, the first measuring resistor indicates the temperature rise due to heating, wherein the first reference resistor provides a reference.

[0052] Optionally, the first measuring resistor comprises a conductive material, wherein the film comprises a non-conductive material and / or a semiconductor material.

[0053] Optionally, the circuit includes a first reference resistor, preferably arranged on or in the frame, wherein the average distance between the first reference resistor and the center of the film is greater than the average distance between the first measuring resistor and the center of the film.

[0054] Optionally, the membrane includes a permeable deposition section, wherein the receiving surface is the surface of the permeable deposition section, and the deposited analytes optionally include:

[0055] - Guides aerosols containing analytes through a permeable deposition zone.

[0056] Aerosol particles can be deposited particularly simply and repeatedly through permeable deposition sections. These permeable deposition sections can be configured as membrane filters that capture aerosol particles (i.e., analytes) guided through the filter. The membrane can include openings (such as pores) extending through the membrane perpendicular to the main extension plane. These openings can be etched. The openings can include diameters ranging from 1 µm to 50 µm, particularly from 2 µm to 25 µm, preferably from 3 µm to 10 µm, for example. The diameter can be the maximum diameter of these openings and can be evaluated using an optical microscope. The receiving surface can be coated with a thin metal film, such as a gold film. The gold film can be grounded to avoid charging effects that could negatively affect analyte deposition. Charging effects can occur, particularly in the case of depositing aerosol particles. Therefore, the combination of a gold film and a permeable deposition section is particularly useful. Alternatively, the analyte can be deposited on the permeable deposition section using a drop casting method.

[0057] The circuit may include a second measuring resistor disposed on or within the membrane and a second reference resistor disposed on or within a frame, wherein the first measuring resistor, the second measuring resistor, the first reference resistor, and the second reference resistor are electrically connected in a Wheatstone bridge configuration. The Wheatstone bridge (configuration) is a highly sensitive and accurate tool for measuring small resistance changes, making it an ideal arrangement for detecting photothermal signals. It offers advantages such as zero-adjustment for precise calibration when integrated on a chip, and is therefore cost-effective. Furthermore, it compensates for external variables such as temperature variations, ensuring reliable measurements.

[0058] Optionally, the circuit includes:

[0059] - A second measuring resistor, wherein the second measuring resistor is at least partially disposed on or in the film; and

[0060] - A second reference resistor, wherein the second reference resistor is at least particularly at least partially disposed on or in the frame;

[0061] - Four electrical contact pads;

[0062] The first measuring resistor, the second measuring resistor, the first reference resistor, and the second reference resistor are electrically connected in a Wheatstone bridge configuration, wherein each of the first measuring resistor, the second measuring resistor, the first reference resistor, and the second reference resistor is electrically connected to two electrical contact pads, such that the four electrical contact pads, the first measuring resistor, the second measuring resistor, the first reference resistor, and the second reference resistor are connected in a loop.

[0063] The first measuring resistor may include a first measuring resistor R. m1 The second measuring resistor may include a second measuring resistor R. m2 The first reference resistor may include a first reference resistor R. ref1 The second reference resistor may include a second reference resistor R. ref2 At a reference temperature (e.g., 23°C), R m1 It can be basically equal to R m2 And R ref1 It can be basically equal to R ref2 Optionally, R m1 R m2 R ref1 and R ref2 They can be substantially equal at a reference temperature. The Wheatstone bridge configuration is well known in the context of measuring resistance. In this configuration, the membrane may include a fourth electrical contact pad, wherein each of the four resistors (i.e., the first measuring resistor and the second measuring resistor, and the first reference resistor and the second reference resistor) is electrically connected to two electrical contact pads, such that the resistors and pads are arranged in alternating closed loops of pads and resistors. Preferably, the Wheatstone bridge configuration includes the following sequential connection: first measuring resistor—first electrical contact pad—first reference resistor—second electrical contact pad—second measuring resistor—third electrical contact pad—second reference resistor—fourth electrical contact pad; wherein the first measuring resistor is connected to the fourth electrical contact pad, such that the loop is closed.

[0064] A bias voltage is applied between two diagonally opposite pads in a ring arrangement of resistors and pads, with the bridge voltage measured between the remaining two pads. For example, a bias voltage can be applied between the first and third electrical contact pads, with the bridge voltage measured between the second and fourth electrical contact pads. When the bridge voltage is zero, the bridge is balanced, i.e., R... ref1 / R m1 = R m2 / R ref2 The first and second measuring resistors indicate temperature changes in the film (due to absorbed radiation), while the first and second reference resistors provide a reference. A Wheatstone bridge configuration can be balanced at the reference temperature. The first and second measuring resistors can be arranged symmetrically with respect to the center of the film. Therefore, temperature changes in the film can affect R equally. m1 and R m2 R m1 and / or R m2 The change (relative to the reference temperature) causes an imbalance in the Wheatstone bridge, i.e., the bridge voltage V g Non-zero. Bridge voltage V gThe bridge voltage V is proportional to the temperature difference between (one or more) measuring resistors and (one or more) reference resistors. g It is proportional to the temperature difference between the membrane and the frame. Therefore, the bridge voltage ultimately depends on whether the analyte is included in the resonance at the wavelength of the incident radiation.

[0065] Optionally, the membrane includes a permeable deposition section, wherein the receiving surface is the surface of the permeable deposition section.

[0066] Optionally, the first reference resistor is disposed on or within the frame. Optionally, the second reference resistor is disposed on or within the frame. The frame may include a thermal mass significantly higher than that of the film. The thermal coupling from the film to the frame can be lower than the thermal coupling from the film to the first (and second) measuring resistors. Therefore, when the film is heated, the frame, and thus the first (and second) reference resistors, may not experience a significant temperature rise. With this configuration, the first (and second) reference resistors are particularly insensitive to temperature fluctuations and provide a particularly stable reference.

[0067] Optionally, the first measuring resistor may be at least partially disposed within the deposition section of the film. The film is heated within the deposition section due to photons absorbed by the deposited analyte. Therefore, the temperature rise of the film is most pronounced in the deposition section. Thus, it is advantageous to arrange the first measuring resistor at least partially, and preferably entirely, within or on the deposition section. This modification also applies to the second measuring resistor.

[0068] Optionally, the first measuring resistor and / or the first reference resistor includes a resistor in the range of 10 ohms to 100 k ohms, particularly between 50 ohms and 10 k ohms, and optionally between 100 ohms and 1 k ohms.

[0069] Optionally, the second measuring resistor and / or the second reference resistor includes a resistor in the range of 10 ohms to 100 k ohms, particularly between 50 ohms and 10 k ohms, and optionally between 100 ohms and 1 k ohms.

[0070] Optionally, the first measuring resistor and / or the first reference resistor includes a wire. The wire may be referred to as a conductive trace. The wire is easy to manufacture and can be monolithically integrated on or within the film. The wire may be a conductive connection between electrical contact pads. The wire may be strip-shaped.

[0071] Optionally, the first measuring resistor may include a conductive material, wherein the film includes a non-conductive material and / or a semiconductor material. The (bulk) thermal conductivity of the film may be lower than that of the first measuring resistor. Conductive materials (such as gold) typically include high thermal conductivity. Non-conductive and semiconductor materials include lower thermal conductivity compared to conductive materials.

[0072] For example, the first measuring resistor and the first reference resistor may include substantially the same resistance value, at least at a reference temperature. The reference temperature could be, for example, 23°C. The term "substantially equal" can be understood as such that the difference between the resistance values ​​of the first measuring resistor and the first reference resistor is less than 3% of the maximum resistance value at the reference temperature, particularly less than 1%, preferably less than 0.1%. Optionally, the first measuring resistor and the first reference resistor include substantially the same temperature dependence, such that their resistance values ​​are substantially equal when the first measuring resistor and the first reference resistor are at the same temperature. The first measuring resistor and the first reference resistor may include the same material. For example, both the first measuring resistor and the first reference resistor may include gold. Alternatively or additionally, the first measuring resistor and the first reference resistor may include substantially the same geometry. For example, the first measuring resistor and the first reference resistor may include substantially the same thickness, width, and length. When the first measuring resistor and the first reference resistor include the same material and substantially the same geometry, the temperature dependence of their resistances is substantially equal. This, with the necessary modifications, also applies to the second measuring resistor and the second reference resistor. The first and second measuring resistors can include substantially the same resistance value, at least at a reference temperature. The first and second reference resistors can also include substantially the same resistance value, at least at a reference temperature. The resistances of the resistors can be selected such that, in a Wheatstone bridge configuration, the bridge is balanced at the reference temperature, i.e., the bridge voltage is zero. The chip (i.e., the photothermal sensing element) is preferably inherently balanced by manufacturing equal resistors in parallel during photolithography. This ensures that the resistances are matched as perfectly as possible within tolerances. The bridge voltage can be zero as long as the film is not heated by absorbed radiation. A non-zero bridge voltage can indicate absorbed radiation. Therefore, the bridge voltage can be used, for example, to evaluate the absorbance of an analyte.

[0073] A photothermal spectrometer may include:

[0074] - A photothermal detection element according to the present invention includes an analyte deposited on or in the membrane;

[0075] - A radiation source, wherein the radiation source is configured to emit electromagnetic radiation, particularly IR radiation, wherein the electromagnetic radiation is incident on the deposited analyte;

[0076] - An electronic reading device connected to an electronic circuit, wherein the electronic reading device is configured to detect changes in the resistance of the circuit due to absorbed electromagnetic radiation.

[0077] The photothermal detection element can be removable, allowing it to be replaced. The radiation source can include a thermal infrared emitter or a laser, such as, for example, a quantum cascade laser. The photothermal spectrometer can be configured to i) perform dispersive spectroscopy by varying the wavelength of the electromagnetic radiation, or ii) perform Fourier transform spectroscopy using time-modulated broadband electromagnetic radiation. The photothermal spectrometer can be configured to determine the absorption spectrum of the deposited analyte. The photothermal spectrometer can include a preamplifier configured to amplify the electronic signal from an electronic readout device. Alternatively, the electronic readout device can include a preamplifier. The preamplifier can include high gain.

[0078] Compared to nanomechanical detection schemes based on resonant frequency shift, the photothermal detection method according to the present invention can benefit from high vacuum but does not require it.

[0079] Preferably, the membrane does not include a (separate) infrared absorbing layer, for example, disposed on the membrane and / or frame and / or receiving surface. At least the membrane and / or frame may be substantially permeable or substantially reflective to electromagnetic radiation (especially infrared radiation). At least the frame and / or membrane may substantially not absorb incident radiation, such that in the absence of (absorbed) analyte, incident electromagnetic radiation does not cause a significant increase in the temperature of the photothermal detection element. In particular, the frame, membrane and / or electrical contact pad and / or the first and second measuring resistors and / or the first and second reference resistors may be substantially permeable or substantially reflective to incident electromagnetic radiation (e.g., infrared radiation). In particular, the frame, membrane and / or electrical contact pad and / or the first and second measuring resistors and / or the first and second reference resistors may substantially not absorb incident electromagnetic radiation (e.g., infrared radiation).

[0080] For example, the absorptivity of the membrane (i.e., the ratio of absorbed intensity to incident intensity) can be less than 0.01, for example, at wavelengths between 2 µm and 8 µm.

[0081] For example, the overall (average) absorbance of the photothermal detection element can be from 0.00001 to 0.0005, for example, at wavelengths from 2 µm to 10 µm.

[0082] In other words, the temperature rise of the membrane can be primarily attributed to the absorption of incident electromagnetic radiation by deposited analytes; in the absence of deposited analytes, the temperature rise can be small. The membrane can exhibit significant absorptivity (e.g., at least 0.01) at specific wavelengths. For example, the membrane can be a silicon nitride membrane, and at 835 cm⁻¹... -1The absorbance can be as low as 0.1 (i.e., 10%) at the wavenumber. This difference in absorbance at a defined (narrow) wavelength can be used for calibration, for example, to normalize the signal and quantitatively compare different measurements (of different photothermal detection elements).

[0083] The receiving surface can be unobstructed (before the analyte is deposited), allowing the analyte to be deposited on the receiving surface.

[0084] The electrical contact pad can be unobstructed, allowing it to be (releasably) attached to, for example, an electrical probe.

[0085] As examples, some selected embodiments shown in the accompanying drawings further illustrate this disclosure for illustrative purposes. However, these embodiments should not be considered as limiting the scope of this disclosure.

[0086] Figure 1A The photothermal detection element of the first embodiment is schematically shown, which has a first measuring resistor and a second measuring resistor, as well as a first reference resistor and a second reference resistor, in a Wheatstone bridge configuration.

[0087] Figure 1B yes Figure 1A A cross-sectional view of the photothermal detection element;

[0088] Figure 2 schematically shown Figure 1A The equivalent circuit diagram of the Wheatstone bridge configuration;

[0089] Figure 3 A second embodiment of the photothermal detection element is schematically shown;

[0090] Figure 4 A third embodiment of the photothermal detection element is schematically shown, wherein the receiving surface is coated with a thin metal film;

[0091] Figure 5 A fourth embodiment of the photothermal detection element is schematically shown, which is related to... Figure 4 Compared to the previous embodiment, it has a smaller electrical contact pad;

[0092] Figure 6 It schematically shows the relationship with Figure 5 The fifth embodiment is similar to the previous one and receives a photothermal detection element with a surface coated with a thin metal film;

[0093] Figure 7 A sixth embodiment of the photothermal detection element is schematically shown, wherein the first and second measuring resistors are partially arranged on a transparent deposition section.

[0094] Figure 8 It schematically shows the relationship with Figure 7A seventh embodiment of a photothermal detection element that is similar to the previous embodiment but has a smaller electrical contact pad;

[0095] Figure 9 An eighth embodiment of the photothermal detection element is schematically shown, which has a first measuring resistor and a first reference resistor without a Wheatstone bridge configuration;

[0096] Figure 10 schematically shown Figure 9 The equivalent circuit diagram of the circuit;

[0097] Figure 11 A photothermal spectrometer with a photothermal detection element is schematically shown; and

[0098] Figure 12A and 12B A ninth embodiment of a photothermal detection element with a trampoline membrane is schematically shown.

[0099] Figure 1A The photothermal detection element 1 is schematically shown, which has:

[0100] - Frame 2;

[0101] - A membrane 3 supported by a frame 2, wherein the membrane 3 includes a substantially flat receiving surface 4 configured to receive the analyte. Furthermore, the photothermal detection element 1 includes a circuit 5 (see also...). Figure 2 ), which has:

[0102] - A first measuring resistor 6, which is (at least partially) arranged on or in the membrane 3 (in this embodiment, it is arranged entirely on the membrane).

[0103] - First reference resistor 7; and

[0104] At least a first electrical contact pad 8, a second electrical contact pad 9, and a third electrical contact pad 10 are used to contact the circuit 5. The first electrical contact pad 8, the second electrical contact pad 9, and the third electrical contact pad 10 are (at least) partially arranged on the frame 2. The resistance of the first electrical contact pad 8, the second electrical contact pad 9, and the third electrical contact pad 10 is less than 5% of the resistance of the first measuring resistor 6 and the first reference resistor 7. The first measuring resistor 6 is connected to the first electrical contact pad 8 and the second electrical contact pad 9, wherein the first reference resistor 7 is at least connected to the third electrical contact pad 10. The average distance between the first reference resistor 6 and the center 11 of the membrane 3 is greater than the average distance between the first measuring resistor 7 and the center 11 of the membrane 3. The membrane 3 has a square shape.

[0105] The membrane 3 includes permeable deposition sections 12 (indicated as spots), wherein the receiving surface 4 is the surface of the permeable deposition sections 12. The deposition sections 12 include pores, allowing aerosols to be guided through the permeable deposition sections 12. The permeable deposition sections 12 serve as particulate filters.

[0106] The first reference resistor 7 is arranged on the frame 2. Due to the thermal mass of the frame 2, the reference resistor 7 is heated less than the first measuring resistor 6 arranged on the membrane 3.

[0107] In addition, circuit 5 includes:

[0108] - A second measuring resistor 13, wherein the second measuring resistor 13 is disposed on the membrane 3; and

[0109] - Second reference resistor 14, wherein the second reference resistor 14 is arranged on frame 2;

[0110] - Four electrical contact pads 8, 9, 10 and 15 (i.e., first electrical contact pad 8, second electrical contact pad 9, third electrical contact pad 10 and fourth electrical contact pad 15).

[0111] The first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 are configured in a Wheatstone bridge configuration 16 (see also...). Figure 1B Electrical connections are made, wherein each of the first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 is electrically connected to two electrical contact pads 8, 9, 10, and 15, such that the four electrical contact pads 8, 9, 10, 15, the first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 are connected to form a loop 17 (see...). Figure 1B ).

[0112] The first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 are indicated only schematically.

[0113] The first measuring resistor 6 and the second measuring resistor 13 are arranged on the membrane 3, while the first reference resistor 7 and the second reference resistor 14 are arranged on the frame 2.

[0114] A photothermal spectroscopic method using a photothermal detection element 1 includes the following steps:

[0115] - The analyte is deposited on the receiving surface 4;

[0116] - Irradiate the receiving surface 4 with electromagnetic radiation, especially IR radiation;

[0117] - Detect the change in resistance of circuit 5; and

[0118] - Determine the properties of the analyte based on the detected changes.

[0119] Circuit 5 includes a first reference resistor 7 arranged on frame 2, wherein the average distance between the first reference resistor 7 and the center 11 of membrane 3 is greater than the average distance between the first measuring resistor 6 and the center 11 of membrane 3.

[0120] Membrane 3 includes a permeable deposition section 12, wherein the receiving surface 4 is the surface of the permeable deposition section 12. Deposited analytes include:

[0121] - Guide aerosols containing analytes through permeable deposition sections 12.

[0122] Circuit 5 includes a second measuring resistor 13 disposed on membrane 3 and a second reference resistor 14 disposed on frame 2, wherein the first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7 and the second reference resistor 14 are configured in a Wheatstone bridge configuration 16 (see details). Figure 2 Electrical connection.

[0123] Figure 1B It shows Figure 1A A cross-sectional view of the photothermal detection element 1. The membrane 3 is supported by the frame 2. The membrane 3 is suspended from the frame 2. From Figure 1A As can be seen from the top view, the frame 2 surrounds the membrane 3. In this exemplary embodiment, the membrane 3 has a square shape. The membrane 3 is formed of one or more thin layers (two layers in this case) that partially extend over the frame 2. In this exemplary embodiment, the membrane 3 includes a dielectric layer 3A and a metallization layer 20 disposed on top of the dielectric layer 3A. However, the membrane 3 is defined only as the portion of the thin layer(s) that is not deposited (i.e. disposed) on top of the frame 2 but is surrounded by the frame 2. The metallization layer 20 is laterally smaller than the dielectric layer 3A and the frame 2, such that there is an edge between the outer ends of the metallization layer 20 and the frame 2.

[0124] Figure 2 schematically shown Figure 1A The equivalent circuit diagram 18 of the Wheatstone bridge configuration 16 of the photothermal detection element 1 is shown. A first measuring resistor 5, a second measuring resistor 13, a first reference resistor 7, a second reference resistor 14, and first electrical contact pads 8, 9, 10, and 15 are connected in a loop. Due to radiation absorbed by the analyte deposited on the receiving surface 4 of the film 3, the film 3 becomes hot. This also affects the first measuring resistor 6 and the second reference resistor 13. The first measuring resistor 6 includes a first measuring resistor R. m1 The second measuring resistor 13 includes a second measuring resistor R. m2 The first reference resistor 7 includes a first reference resistor R. ref1The second reference resistor 14 includes a second reference resistor R. ref2 .

[0125] A bias voltage is applied to circuit 5 between two diagonally opposite pads (e.g., between the first electrical contact pad 8 and the third electrical contact pad 10). The bridge voltage is measured between the remaining pads (e.g., between the second electrical contact pad 9 and the fourth electrical contact pad 15). The bridge voltage is zero when the Wheatstone bridge is balanced. The bridge voltage is non-zero when the bridge is unbalanced (and the bias voltage is non-zero). The nominal resistance values ​​of the resistors at the reference temperature are known. These nominal resistances can be selected such that the Wheatstone bridge is balanced at the reference temperature and unbalanced if the membrane is heated due to radiation absorbed by the deposited analyte. The measured bridge voltage (and bias voltage) can be used to estimate the temperature rise of membrane 3—the temperature dependence of the specified resistors is known.

[0126] Figure 3 A second embodiment of the photothermal detection element 1 is schematically shown. The first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 each include a conductor 19. Due to the shape of the conductor 19 (particularly the width of the conductor), the resistances of the first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 are significantly higher than the resistances of the electrical contact pads 8, 9, 10, and 15. The first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14, as well as the electrical contact pads 8, 9, 10, and 15, are formed by a metallization layer 20, which in this embodiment is a gold metallization layer, disposed (i.e., deposited) on top of the dielectric layer 3A of the film 3.

[0127] The first measuring resistor 6 comprises a conductive material, in this case gold, while the film 3 comprises a non-conductive material.

[0128] The first measuring resistor 6 and the first reference resistor 7 have substantially the same resistance value, at least at a reference temperature. In this example, the reference temperature is 23°C.

[0129] Figure 4 A third embodiment of the photothermal detection element 1 is schematically shown. (Compared to...) Figure 3Compared to the previous embodiment, the receiving surface 4 (particularly the permeable deposition section 12) is coated with a thin metal film, in this case by a metallization layer 20. The coated receiving surface 4 is electrically connected to a second electrical contact pad 9. During aerosol particle deposition, the second electrical contact pad 9 can be grounded to avoid the charging effect of the permeable deposition section 12. Alternatively, the second contact pad 9 can be biased to enhance the deposition of charged aerosol particles. The coated permeable deposition section 12 is coated such that the pores are not blocked by the metallization layer 20. In other words, aerosols can also be guided through the (coated) permeable deposition section 12 of the membrane 3.

[0130] Figure 5 A fourth embodiment of the photothermal detection element 1 is schematically shown. (Compared to...) Figure 3 Compared to the previous embodiment, the first electrical contact pad 8, the second electrical contact pad 9, the third electrical contact pad 10, and the fourth electrical contact pad 15 extend less on the membrane 3. Therefore, the thermal coupling from the membrane 3 to the frame 2 is... Figure 3 The embodiments are relatively low.

[0131] Figure 6 A fifth embodiment of the photothermal detection element 1 is schematically shown. (Compared to...) Figure 5 Compared to the previous embodiment, the receiving surface 4 (particularly the permeable deposition section 12) is coated with a thin metal film, in this case by a metallization layer 20 (similar to...). Figure 4 (Example). The coated receiving surface 4 is electrically connected to the first electrical contact pad 8.

[0132] Figure 7 A sixth embodiment of the photothermal detection element 1 is schematically shown. (Compared to...) Figure 5 Compared to the previous embodiment, the first reference resistor 7 and the second reference resistor 13 are farther from the center 11 of the film. Due to the greater distance, the first reference resistor 7 and the second reference resistor 14 are less affected by the temperature rise of the film, and thus provide a more stable reference. Furthermore, the first measuring resistor 6 is partially disposed in the permeable deposition section 12 of the film 3. The second measuring resistor 13 is also partially disposed in the deposition section 12. According to claim 7 or 8, the photothermal detection element, wherein the first measuring resistor is at least partially disposed in the deposition section of the film. The analyte is primarily deposited in the permeable deposition section 12, therefore, radiation is primarily absorbed in the permeable deposition section 12. Therefore, the film 3 experiences the fastest and most significant temperature rise in the permeable deposition section. As a result, the arrangement of the first measuring resistor 6 and (optionally) the second measuring resistor 13 on or in the permeable deposition section 12 makes the photothermal detection element 1 respond particularly quickly to the absorbed radiation.

[0133] Figure 8 A seventh embodiment of the photothermal detection element 1 is schematically shown. (Compared to...) Figure 7Compared to the previous embodiment, the first electrical contact pad 8, the second electrical contact pad 9, the third electrical contact pad 10, and the fourth electrical contact pad 15 extend less on the membrane 3 (similar to...). Figure 6 (Example). Therefore, the thermal coupling from membrane 3 to frame 2 and Figure 7 The embodiments are relatively low.

[0134] Figure 9 An eighth embodiment of the photothermal detection element 1 is schematically shown, which is similar to... Figure 1A The first embodiment includes a first measuring resistor 6 and a first reference resistor 7. Compared to the aforementioned embodiments, particularly with... Figure 1A Compared to the previous embodiment, Figure 9 The eighth embodiment does not include the second measuring resistor 13 (see, for example) Figure 1A It also does not include a second reference resistor (see example). Figure 1A Therefore, this embodiment does not include Wheatstone bridge configuration 16 (see example...). Figure 1A and Figure 2 The photothermal detection element 1 includes a fourth electrical contact pad 15, which is not connected to any resistor in this example. However, the photothermal detection element 1 includes the fourth electrical contact pad to have a symmetrical arrangement of the pads.

[0135] Figure 10 schematically shown Figure 9 The equivalent circuit diagram of circuit 5 of photothermal detection element 1. The first measuring resistor 6, the first reference resistor 7, and the first electrical contact pad 8, the second electrical contact pad 9, and the third electrical contact pad 10 are connected in series. The first measuring resistor 6 includes a first measuring resistor R. m1 The first reference resistor 7 includes a first reference resistor R. ref1 .

[0136] The resistance of the first reference resistor 7 can be measured between the first electrical contact pad 8 and the third electrical contact pad 10. The resistance of the first measuring resistor 6 can be measured between the second electrical contact pad 9 and the first electrical contact pad 8. The first measuring resistor 6 and the first reference resistor 7 have the same resistance at the reference temperature (23°C in this example) (i.e., their resistances have the same value). During the measurement, the frame 2 is maintained at the reference temperature (e.g., in the measurement chamber). The membrane 3 initially has the same temperature as the frame 2. However, once the membrane 3 is heated due to radiation absorbed by the analyte, the membrane 3 and the first measuring resistor 6 become heated, which causes a change in the resistance of (at least) the first measuring resistor 6. The change in resistance of the first measuring resistor is proportional to the absorbed radiation. However, the frame 2 and the first reference resistor 7 do not experience the same temperature rise. Therefore, the first reference resistor 7 provides a reference for the measurement.

[0137] Figure 11A photothermal spectrometer 21 is schematically shown. The photothermal spectrometer 21 includes a photothermal detection element 1 having an analyte deposited on a film 3 (see example...). Figure 1A Furthermore, the photothermal spectrometer 21 includes a radiation source 22 configured to emit electromagnetic radiation (IR radiation in this exemplary embodiment), which is incident on the deposited analyte. Additionally, the photothermal spectrometer 21 includes a circuit connected to electronic circuitry 5 (see, for example...). Figure 1A or Figure 2 The electronic reading device 23 is configured to detect changes in the resistance of the circuit 5 due to absorbed electromagnetic radiation.

[0138] Figure 12A and 12B A ninth embodiment of the photothermal detection element 1 is schematically shown, which is similar to... Figure 3 The second embodiment shown. (Compared to...) Figure 3 Compared to the second embodiment, the membrane 3 includes four grooves 24. For better understanding, the grooves 24 are schematically depicted with square shading lines, making them visually distinct from the rest of the membrane 3. The membrane 3 is suspended from the frame 2 by four tethers 25. The tethers 25 are arranged at the corners of the membrane 3 and extend substantially along the diagonal of the membrane 3. Alternatively, two or three tethers 25 may be used to suspend the membrane 3 from the frame 2. This configuration of the membrane 3 may be referred to as a trampoline, a trampoline configuration, a trampoline membrane, or a trampoline resonator.

[0139] Figure 12B It shows Figure 12A The cross-section (AA) of the photothermal detection element 1 is shown, which illustrates the groove 25. In this illustration, the groove 25 is not shaded.

[0140] Compared to the first through eighth embodiments, the ninth embodiment exhibits lower thermal conductivity from the membrane 3 to the frame 2 due to the groove 24. Furthermore, the membrane 3 has a smaller thermal mass compared to the aforementioned embodiments. Therefore, this embodiment is particularly sensitive to absorbed radiation and heat dissipated into the membrane 3. Optionally, the photothermal detection element 1 may comprise only a single wire 19 extending on the membrane 3 (i.e., the second measuring resistor 13 may be optionally omitted) to further reduce the thermal conductivity from the membrane 3 to the frame 2 and further improve sensitivity.

[0141] If the first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 cover the same temperature range, then they will have substantially the same resistance value. The resistance value will vary with each individual temperature. The wires 19 forming the first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 will be made of substantially the same material (gold in this example) and have the same thickness. The widths of the individual wires 19 may differ such that the resistances of the first measuring resistor 6, the second measuring resistor 13, the first reference resistor 7, and the second reference resistor 14 are matched (if the resistors cover the same temperature range). Alternatively, the widths and lengths may be substantially equal respectively, such that the resistances are matched. Due to the geometry of the first measuring resistor 6 and the second measuring resistor 13 (which is due to the groove 24 and the tether 25), the first measuring resistor 7 and the second measuring resistor 14 will each include a bent section 26 such that the total resistance of the resistors is matched accordingly. The bent section 26 is, in particular, only schematically depicted.

Claims

1. A photothermal spectroscopy method utilizing a photothermal detection element (1), wherein the photothermal detection element (1) includes a frame (2), a membrane (3) supported by the frame (2), and a circuit (5), wherein the circuit (5) includes a first measuring resistor (6) disposed on or in the membrane (3), wherein the membrane (3) includes a substantially flat receiving surface (4) configured to receive an analyte, the method comprising the following steps: - The analyte is deposited on the receiving surface (4); - Irradiate the receiving surface (4) with electromagnetic radiation; - Detect the change in resistance of the circuit (5); and - Determine the properties of the analyte based on the detected changes.

2. The method according to claim 1, wherein the first measuring resistor (6) comprises a conductive material, and wherein the film (3) comprises a non-conductive material and / or a semiconductor material.

3. The method according to claim 1 or 2, wherein the circuit (5) includes a first reference resistor (7), preferably arranged on or in the frame (2), wherein the average distance between the first reference resistor (6) and the center (11) of the membrane (3) is greater than the average distance between the first measuring resistor (7) and the center (11) of the membrane (3).

4. The method according to any one of claims 1 to 3, wherein the membrane (3) comprises a permeable deposition section (12), wherein the receiving surface (4) is the surface of the permeable deposition section (12), wherein depositing the analyte optionally comprises: - Guide the aerosol containing the analyte through the permeable deposition section (12).

5. The method according to any one of claims 1 to 4, wherein the circuit (5) comprises a second measuring resistor (13) disposed on or in the membrane (3) and a second reference resistor (14) disposed on or in the frame (3), wherein the first measuring resistor (6), the second measuring resistor (13), the first reference resistor (7) and the second reference resistor (14) are electrically connected in a Wheatstone bridge configuration (16).

6. The method according to any one of claims 1 to 5, wherein the electromagnetic radiation is IR radiation.

7. A photothermal detection element (1), comprising: - Frame (2); - A membrane (3) supported by the frame (2), wherein the membrane (3) includes a substantially flat receiving surface (4) configured to receive the analyte; and - Circuit (5), said circuit (5) having: - A first measuring resistor (6) that is at least partially disposed on or in the membrane (3); - First reference resistor (7); and - At least a first electrical contact pad (8), a second electrical contact pad (9), and a third electrical contact pad (10) for contacting the circuit (5), wherein the first electrical contact pad (8), the second electrical contact pad (9), and the third electrical contact pad (10) are at least partially disposed on the frame (2), wherein the resistance of the first electrical contact pad (8), the second electrical contact pad (9), and the third electrical contact pad (10) is less than 20% of the resistance of the first measuring resistor (6) and / or the first reference resistor (7), wherein the first measuring resistor (6) is connected to the first electrical contact pad (8) and the second electrical contact pad (9), wherein the first reference resistor (7) is at least connected to the third electrical contact pad (10); The average distance between the first reference resistor (7) and the center (11) of the membrane (3) is greater than the average distance between the first measuring resistor (6) and the center (11) of the membrane (3).

8. The photothermal detection element (1) according to claim 7, wherein the resistance of the first (8), second (9) and third electrical contact pads (10) is less than 10% or less than 5% of the resistance of the first measuring resistor (6) and / or the first reference resistor (7).

9. The photothermal detection element (1) according to claim 7 or 8, wherein the film (3) includes a permeable deposition section (12), wherein the receiving surface (4) is the surface of the permeable deposition section (12).

10. The photothermal detection element (1) according to any one of claims 7 to 9, wherein the first reference resistor (7) is arranged on or in the frame (3).

11. The photothermal detection element according to claim 9 or 10, wherein the first measuring resistor (6) is at least partially disposed in the deposition section (12) of the film (3).

12. The photothermal detection element (1) according to any one of claims 7 to 11, wherein the first measuring resistor (6) and / or the first reference resistor (7) comprises a resistance in the range of 10 ohms to 100 k ohms.

13. The photothermal detection element (1) according to claim 12, wherein the first measuring resistor (6) and / or the first reference resistor (7) comprises a resistance between 50 ohms and 10 k ohms.

14. The photothermal detection element (1) according to claim 13, wherein the first measuring resistor (6) and / or the first reference resistor (7) comprises a resistance between 100 ohms and 1 k ohm.

15. The photothermal detection element (1) according to any one of claims 7 to 14, wherein the first measuring resistor (6) and / or the first reference resistor (7) comprises a wire.

16. The photothermal detection element (1) according to any one of claims 7 to 15, wherein the first measuring resistor (6) comprises a conductive material, and wherein the film (3) comprises a non-conductive material and / or a semiconductor material.

17. The photothermal detection element (1) according to any one of claims 7 to 16, wherein the first measuring resistor (6) and the first reference resistor (7) have substantially the same resistance value at least at the reference temperature.

18. The photothermal detection element according to any one of claims 7 to 17, wherein the circuit (5) comprises: - A second measuring resistor (13), wherein the second measuring resistor (13) is at least partially disposed on or in the membrane (3); as well as - A second reference resistor (14), wherein the second reference resistor (14) is at least partially disposed on or in the frame (3); - Four electrical contact pads (8, 9, 10, 15); The first measuring resistor (6), the second measuring resistor (13), the first reference resistor (7), and the second reference resistor (14) are electrically connected in a Wheatstone bridge configuration (16), wherein each of the first measuring resistor (6), the second measuring resistor (13), the first reference resistor (7), and the second reference resistor (14) is electrically connected to two electrical contact pads (8, 9, 10, 15), such that the four electrical contact pads (8, 9, 10, 15), the first measuring resistor (6), the second measuring resistor (13), the first reference resistor (7), and the second reference resistor (14) are connected in a loop (17).

19. A photothermal spectrometer (21), comprising: - The photothermal detection element (1) according to claims 7 to 18, wherein the photothermal detection element (1) comprises an analyte deposited on or in the membrane (3); - A radiation source (22), wherein the radiation source (22) is configured to emit electromagnetic radiation, wherein the electromagnetic radiation is incident on the deposited analyte; - An electronic reading device (23) connected to the electronic circuit (5), wherein the electronic reading device (23) is configured to detect changes in the resistance of the circuit (5) due to absorbed electromagnetic radiation.

20. The photothermal spectrometer (21) according to claim 19, wherein the radiation source (22) is configured to emit IR radiation.

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