Photoacoustic detection device and method with a membrane forming a contact surface - Patent Application 20070122997

Abstract

The invention relates to an optical acoustic detection device (1) intended to be applied via a contact surface (3) to a medium to be analyzed (2). The device comprises a hollow cavity (20), a pulsed or amplitude modulated light source (10) and an acoustic detector (28) for detecting sound waves (12) extending through the cavity. The device comprises an interface membrane (23) forming the contact surface (3). The interface membrane (23) forms an interface between the gas filling the cavity (20) and the medium to be analyzed (2), is configured to prevent the passage of the medium to be analyzed (2) and the cavity (20) or of the gel, and to generate acoustic pressure waves in the cavity (20) under the effect of a temperature change of the interface membrane (23). The temperature change of the interface membrane (23) is caused by heating of the medium due to its irradiation.

Description

[Technical field]

[0001] The technical field of the invention is the detection of analytes by photoacoustic detection. [Background technology]

[0002] Photoacoustic detection is based on the detection of sound waves generated by the effect of absorption by the analyzed medium of an incident pulsed or amplitude modulated electromagnetic wave. The sound waves are formed after heating of the target molecules present in the analyzed medium under the effect of absorption of the incident wave. The heating leads to a modulated thermal expansion of the medium, which expansion is the origin of the sound waves.

[0003] Photoacoustic detection can be specialized for one particular analyte by tuning the wavelength of the incident electromagnetic wave to the absorption wavelength of the analyte. Photoacoustic detection has therefore been applied to the detection of gas species in gases and the presence of specific molecules in biological tissues. The wavelength of the incident wave is often in the infrared region.

[0004] The photoacoustic detection device comprises an amplitude modulated light source, e.g. a laser light source, operating at a frequency of tens of Hz to tens of kHz. The modulation frequency defines the frequency of the photoacoustic waves caused by the periodic heating of the target molecules present in the analyzed medium. The photoacoustic detection device comprises an acoustic detector configured to detect the periodic photoacoustic waves. The response function of the photoacoustic detection device may be calibrated to establish a correlation between the amplitude of the measured pressure oscillations and the amount of analyte in the analyzed medium.

[0005] Devices are described that allow the application of photoacoustic detection in biological media, for example for the quantification of specific biomolecules such as glucose. Examples of devices are described in US Patent Publication No. 2014 / 0073899 and US Patent Publication No. 20210302387. In these devices, the medium to be analyzed is periodically heated under the effect of periodic illumination. The periodic heating of the medium is propagated to the interface between the medium and an air-filled cavity. The temperature change at the interface generates periodic pressure waves in the cavity. Summary of the Invention [Problem to be solved by the invention]

[0006] The inventors have designed an optical acoustic detection device based on the same principles as the device described in the previous paragraph, but suitable for application in liquid media. [Means for solving the problem]

[0007] A first subject of the invention is a photoacoustic detection device intended to be applied via a contact surface to a medium to be analyzed, a hollow cavity filled with gas and opening toward the contact surface; a light source configured, upon operation, to emit an incident light beam in a pulsed or amplitude modulated emission spectral band through the cavity toward the interface; an acoustic detector connected to the cavity configured to detect acoustic waves propagating through the cavity to detect acoustic waves generated by heating of the medium under the influence of illumination of the medium by the incident light beam; Equipped with Further comprising an interface film forming a contact surface; The interface film is forming an interface between the gas filling the cavity and the medium to be analyzed; configured to prevent passage of the medium to be analyzed into the cavity; A photoacoustic detection device.

[0008] According to one embodiment, the interface membrane is not apertured.

[0009] According to one embodiment, the interface membrane comprises a through opening with a radius of less than 50 μm or less than 30 μm. The interface membrane comprises a hydrophobic coating within the through opening.

[0010] According to one embodiment, the light source is positioned such that, in operation, an incident light beam passes through the interface membrane before reaching the medium to be analyzed; the interface film includes cross segments corresponding to portions of the interface film through which the incident light beam has passed; At least in the crossing segments, the interface film is made of a transparent material having a transmission greater than 0.4 in the emission spectral band.

[0011] The interface film then allows optimizing the amount of light propagating through the medium. The transparent material may be at least one material selected from Si, Ge, AlN, ZnSe, BaF2, CaF2, KBr, ZnS, and sapphire. The interface film may be monolithic and made of the transparent material.

[0012] According to one embodiment, the interface membrane is removable.

[0013] Advantageously, the interface membrane extends between an inner surface in contact with the gas filling the cavity and an outer surface intended to be applied to a medium to be analyzed; the inner surface of the interface film is provided with an anti-reflective coating or microstructure configured to minimize reflection of the incident light beam; and / or The outer surface of the interface film includes an anti-reflective coating or microstructure configured to minimize reflection of the incident light beam.

[0014] The thickness of the interface film may be in the range of 20 μm to 1 mm.

[0015] According to one embodiment, the cavity is bounded by a distal membrane, the distal membrane being located opposite the interfacial membrane such that the cavity extends between the distal membrane and the interfacial membrane; The light source is positioned such that, in operation, an incident light beam passes through the distal membrane before reaching the interface membrane.

[0016] The distal membrane may comprise or be made of a material having a transmission higher than 0.4 in the emission spectral band, in particular it is a matter of a material chosen from Si, Ge, AlN, ZnSe, BaF2, CaF2, KBr, ZnS, sapphire.

[0017] The distal membrane and the permeable membrane may be made of the same material.

[0018] According to one embodiment, the cavity is bounded by a distal wall and a side wall, the side wall extending between the distal wall and the interface membrane; The interface membrane extends between opposing edges of the sidewalls.

[0019] The volume of the cavity may be less than 50 μL.

[0020] A second subject of the invention is a method for detecting an analyte in a medium, the analyte absorbing light at an absorption wavelength, the method comprising the steps of: - applying a device according to the first subject of the invention to a medium such that the interface film is in contact with the medium; operating a light source in an emission spectral band that includes an absorption wavelength of the analyte; detecting photoacoustic pressure waves by an acoustic detector and estimating the amount of analyte depending on the detected photoacoustic pressure waves, more precisely depending on the amplitude of the photoacoustic pressure waves; Includes.

[0021] The medium may be or may include a liquid or a gel.

[0022] The light source may be pulsed or amplitude modulated with a pulse or modulation frequency of less than 500 Hz.

[0023] The present invention will be better understood by reading the description of example embodiments presented in the remainder of the detailed description, with reference to the drawings listed below. [Brief description of the drawings]

[0024] [Figure 1] 1 shows an example of a photoacoustic detection device. [Diagram 2] 1 shows an example of an interface film. [Figure 3A] 1 shows an example of application of a photoacoustic detection device to a duct through which a liquid flows. [Figure 3B]1 shows an example of application of a photoacoustic detection device to a duct through which a liquid flows. [Figure 4] Another example of the interface film is shown. [Diagram 5] Along the horizontal axis perpendicular to the interfacial film, modeling of the amplitude of thermal vibrations occurring within the medium and cavity during cyclic irradiation of the medium (y-axis) is shown. The x-axis corresponds to the position along the horizontal axis. [Figure 6] 1 shows a modelling of the amplitude of thermal vibrations (y-axis) occurring in the analyzed medium and in the cavity under the effect of periodic illumination of the medium as a function of the modulation frequency of the illumination beam (x-axis). [Figure 7A] 1 shows schematic diagrams of process steps for fabricating the cavity and interfacial membrane of the device. [Figure 7B] 1 shows schematic diagrams of process steps for fabricating the cavity and interfacial membrane of the device. [Figure 7C] 1 shows schematic diagrams of process steps for fabricating the cavity and interfacial membrane of the device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Figure 1 shows diagrammatically a device 1 making it possible to implement the invention. The device 1 is configured to be applied to a medium 2 to be analyzed. It comprises a contact surface 3 intended to be applied to the medium to be analyzed. In the example shown, the medium 2 is a liquid flowing through a duct 5. The liquid does not have to be flowing. The invention is applicable to the analysis of static liquids placed in a container. The invention is also applicable to the analysis of solids or gels.

[0026] The device comprises a light source 10 configured to emit a light beam 11 that propagates into a medium 2 to be analyzed. The light source 10 is pulsed or amplitude modulated. The light beam 11 is focused at an absorption wavelength λ of an analyte 4 present in the medium. a One purpose of the device 1 is to detect the presence of an analyte 4 and, if possible, to estimate its concentration.

[0027] The analyte 4 may be a molecule present in a fluid, possibly a body fluid or a fluid used in an industrial process. For example, it may be a matter of glucose, alcohol, ethanol.

[0028] Preferably, the emission spectral band Δλ is in the visible or infrared, for example extending between wavelengths of 2 μm and 15 μm. Preferably, the emission spectral band Δλ is narrow enough for the device 1 to be specific for a single analyte. If the analyte is glucose, the emission spectral band is narrow enough to extend beyond the absorption wavelength of glucose, for example 1034 cm -1 The light source 10 may in particular be a pulsed laser light source, for example a tunable quantum cascade laser (QCL). According to other embodiments, the light source may be a filament-type light source or a light-emitting diode. According to these embodiments, the light source is preferably associated with a band-pass filter in order to define a sufficiently narrow emission spectrum band centered on the absorption wavelength of interest.

[0029] The device 1 is intended to be applied to a medium 2 to be analyzed. It comprises a body 21 in which a cavity is formed. The cavity 20 is filled with a gas, for example air. The cavity 20 opens onto an interface membrane 23, which forms an interface between the cavity 20 and the medium 2 to be analyzed. The membrane is configured to prevent the passage of a liquid or gel between the medium 2 to be analyzed and the cavity 20. The membrane is therefore non-porous to liquids or gels. The membrane may also be porous to gases, as will be explained below. In the example shown, the membrane is flush with the inner surface of a duct through which the liquid flows. The membrane may be flexible or rigid.

[0030] The light source 10 is configured to emit a light beam 11 so that it arrives at a normal or substantially normal angle of incidence on the medium 2 to be analyzed. By substantially normal, it is meant that it is normal within an angular tolerance of ±30°. Preferably, the light beam 11 passes through an interface membrane 23 before reaching the medium 2 to be analyzed.

[0031] Under the influence of the presence of the analyte 4 in the medium 2, an acoustic wave called photoacoustic wave 12 is formed. The photoacoustic wave 12 is an acoustic wave formed as a result of the periodic heating of the medium by the light beam 11, the latter being pulsed or amplitude modulated. The periodic heating is transferred by thermal conduction to the interface between the medium 2 and the cavity 20. At the interface, a periodic photoacoustic wave is formed and propagates through the cavity 20. The photoacoustic wave, in particular its amplitude, is detected by an acoustic detector 28. The acoustic detector 28 is connected to the cavity 20 by an acoustic channel 27. The acoustic detector may be a microphone with a detection spectral range that includes the frequency of the photoacoustic wave. The photoacoustic wave is amplitude modulated at the pulsed or amplitude modulated frequency of the light beam 11. Thus, in the acoustic detector, the pressure is amplitude modulated. The measured amplitude of the acoustic wave correlates with the concentration of the analyte in the medium.

[0032] The cavity 20 is bounded by an interface membrane 23 and extends in the body 21 between a side wall 22 and a distal wall 24. The side wall 22 extends between the interface membrane 23 and the distal wall 24 around an axis parallel to the transverse axis Z. The transverse axis Z is perpendicular to the interface membrane 23. The distal wall 24 faces the interface membrane 23. In the illustrated example, the distal wall 24 is formed by a membrane called distal membrane 24 that is parallel to the interface membrane 23. The light source 10 is arranged outside the internal space bounded by the cavity. An incident light beam propagates through the distal membrane 24, forming the distal wall of the cavity, and then through the interface membrane 23 into the medium to be analyzed. Preferably, the light beam 11 propagates at a distance from the side wall 22 bounding the cavity 20 so as to avoid heating the latter. Otherwise, parasitic acoustic signals that are not specific to the analyte being sought would be formed. This is also the reason why the distal wall 24 of the cavity is formed by a thin distal membrane, preferably made of a material that is considered transparent in the emission spectrum band. The distal membrane has a high transmission in the emission spectrum band so as to maximize the amount of light that propagates to the medium to be analyzed. The more light that reaches the medium, the higher the sensitivity of the device.

[0033] The volume of the cavity may be a few μL or tens of μL (microliters). It is estimated that the volume of the cavity is preferably less than 50 μL in order to increase the amplitude of the pressure wave detected by the acoustic detector.

[0034] The light beam 11 reaching the medium 2 is gradually absorbed by the medium 2, in particular by the analytes 4, according to the Beer-Lambert absorption law. The absorption rate increases with increasing concentration of the analytes absorbing the incident light beam. Thus, the higher the concentration of the analytes, the smaller the thickness of the medium penetrated by the incident light beam. The optical energy absorbed by the analytes is transferred to the medium in the form of heat. The heat diffuses through the medium to the interface film. The interface film 23 then undergoes a periodic temperature change caused by the periodic heating of the medium. The periodic heating of the medium is caused by the periodic irradiation by the light beam 11.

[0035] The periodic temperature change of the interface membrane 23 generates periodic pressure waves 12 in the cavity, the latter being detected by the acoustic detector 25. Under the effect of periodic heating of the membrane 23, pressure waves are formed. The photoacoustic effect is indirect, since it is generated in the interface membrane under the effect of heat conduction through the medium. The period of the pressure waves formed in the cavity corresponds to the period of the incident light beam 11.

[0036] For optimal conversion of temperature changes of the medium into changes in the amplitude of the pressure wave, the material forming the interface film preferably has a high thermal conductivity, for example higher than 0.5 W / mK, 1 W / mK, 5 W / mK or 10 W / mK. It is also preferable that the material forming the film has a low density. Materials such as plastics and fibers are preferably avoided. It is preferable that the material forming the interface film has a low specific heat capacity. Finally, the interface film is preferably thin, as will be explained with reference to FIG. 2.

[0037] Another function of the interface membrane is to prevent the penetration of part of the medium to be analyzed into the cavity. The interface membrane therefore forms a fluid barrier, especially with respect to liquids or gels that may be present in the medium to be analyzed. This prevents contamination inside the cavity. This also makes it possible to control the volume of gas in the cavity. In fact, variations in the internal volume of the cavity can introduce a bias into the interpretation of the measurements made by the acoustic detector. This bias can affect the relationship between the amplitude of the photoacoustic wave and the concentration of the analyte. Thus, by blocking the passage of liquids or gels inside the cavity, the interface membrane is able to stabilize the response function of the device.

[0038] FIG. 2 shows a schematic representation of an example of an interface membrane 23. The interface membrane 23 has an outer surface 23 intended to come into contact with the medium to be analyzed. e and an inner surface 23 in contact with the gas present in the cavity. i Between the outer and inner faces, the membrane has a small thickness ε, preferably less than 1 mm. Advantageously, the thickness ε of the interface membrane 23 is comprised between 25 μm and 100 μm. The thicker the membrane, the less heat it will conduct to the cavity.

[0039] The interface film has a cross segment 23 corresponding to the portion of the interface film through which the light beam 11 passes. int At least 23 cross segments int In the embodiment, the interface membrane is made of a material with high transmission in the spectral band Δλ of the radiation beam 11. High transmission means a transmission preferably higher than 0.4, more preferably higher than 0.8, for example of the order of 0.9 or higher. Transmission means the part of the light intensity that is transmitted by the interface membrane 23. The interface membrane may be made partially or entirely of Si or other materials transparent to infrared light, for example porous Si, Ge, AlN, ZnSe, BaF2, CaF2, KBr, ZnS or sapphire. The same applies to the distal membrane 24 described above. High transmission allows to maximize the amount of light reaching the medium.

[0040] The transmittance is especially high on the inner surface. i , preferably the inner surface 23i and outer surface 23 e The reflective coefficient may be increased to reach a value close to 1 by applying an anti-reflective coating to at least the intersecting segment 23. int Alternatively, the anti-reflection treatment may take the form of an unapertured "quarter wave" layer deposited in the form of a thin layer at at least the intersecting segment 23 through which the incident light beam propagates. int In the inner surface 23 i and optionally an outer surface 23 e The microstructure may comprise a microstructure of, for example, a diffraction grating, which can optimize the transmission of light in the emission spectrum band. An example of a microstructure is described in Douglas S. Hobbs, Bruce D. MacLeod, and Juanita R. Riccobono, "An update on the development of high performance antireflective surface relief microstructures," Proc. SPIE 6545.

[0041] The interface membrane may be monolithic, formed from a single material, or may be a composite material. If the interface membrane is a composite material, it may be a composite material. int The cross segments may comprise a material that is deemed sufficiently transparent in the emission spectral band within the cross segments and another material outside the cross segments that is a very good thermal conductor, such as copper or aluminum.

[0042] Figure 3A shows diagrammatically one possible way in which a device 1 as described above can be applied to a duct 5 through which a liquid 2 flows. An interface membrane 23 is placed at an opening in the duct, at the interface between the liquid and the cavity. Figure 3B is a cutaway view of Figure 3A showing the central plane.

[0043] In the example shown in Fig. 2, the interface membrane 23 is not apertured. According to one possibility shown in Fig. 4, the interface membrane 23 has a plurality of through apertures 23 extending through the membrane. oPreferably, the radius of the through openings is between 5 μm and 50 μm, preferably between 20 μm and 30 μm. Advantageously, the openings are functionalized, for example by a hydrophobic treatment, to prevent the passage of liquids through the membrane. Such a configuration makes it possible to utilize the photoacoustic waves generated at each opening at the interface between the cavity medium and the air. According to this possibility, the through openings 23 o The intersecting segments 23 of the film 23 are arranged such that they do not impede the propagation of the incident light beam through the interface film. int is preferably not opened.

[0044] In the configuration of Figure 4, it is advantageous, but not essential, for the membrane to be a good thermal conductor, and thus in this embodiment the membrane may be a polymer containing a number of openings.

[0045] The amplitude of thermal oscillations as a function of position along the transverse axis Z was simulated in the medium 2 and in the cavity 20 under the effect of periodic illumination. Figure 5 shows the amplitude of thermal oscillations (y-axis - in Km) as a function of z-position along an axis parallel to the axis Z. 2 ) whose axis is centred on the cavity and on the membrane. This axis is represented by a dashed-dotted line in FIG. 1. The position z<0 corresponds to the medium 2 to be analysed. The position z>0 corresponds to the device. The coordinate z=0 corresponds to the medium / device interface. Firstly, the device without the membrane was considered and the corresponding results plotted in curve a). Then, a membrane 23 with a thickness of 50 μm was considered. The modelling parameters are: -Sample: Water at 25℃, thickness greater than 1mm, -Irradiation beam: Frequency 100Hz, wave number 1035cm -1 A 10mW output laser source modulated by Film composition: Si (curve b) or Ge (curve c) without anti-reflection treatment.

[0046] In FIG. 5, the curves corresponding to the Si or Ge films are nearly identical.

[0047] FIG. 6 shows the change in amplitude of the signal measured at the acoustic detector (y-axis-units V) representative of the photoacoustic wave as a function of the modulation frequency (x-axis-Hz).

[0048] Figure 5 shows that the photothermal effect, i.e. the heating of the analyzed medium by the laser beam, occurs at a depth of less than 100 μm from the interface film. Furthermore, a temperature stability was observed in the film thickness. This is due to the large thermal diffusion distances of Si and Ge. This confirms that these materials are suitable to form interface films. Moreover, the very similar values ​​obtained for Si and Ge are due to the good optical transmission properties in the infrared of these two materials.

[0049] 6 shows that at a modulation frequency of 100 Hz, the amplitude of the photoacoustic wave is attenuated by about 10 times in the presence of the membrane (curves b and c in FIG. 6) compared to the configuration without the membrane (curve a in FIG. 6). The attenuation induced by the interface membrane is observed when the light beam 11 propagates through the interface membrane, in particular at the inner surface 23. i and outer surface 23 e These unwanted reflections are caused by the effect of reflections at the inner and outer surfaces. These unwanted reflections can be avoided by applying an anti-reflection treatment to these two surfaces. The inventors estimate that applying an anti-reflection treatment to the inner and outer surfaces can increase the amplitude of the photoacoustic pressure waves by a factor of about four.

[0050] It should be noted that in all three modeled configurations, no membrane, Si membrane, and Ge membrane, the amplitude of the photoacoustic wave decreases as the modulation frequency increases. This is due to the fact that as the frequency increases, the thermal diffusion distance in the membrane and in the air decreases. In FIG. 6, it can be seen that the difference between curve a (no membrane) and curves b) and c) (Si membrane or Ge membrane) increases as the modulation frequency increases. Therefore, it is preferable that the modulation frequency is relatively low, for example, below 100 Hz or below several hundred Hz. The membrane acts as a low-pass filter for the photoacoustic wave.

[0051] The interface membrane 23 may be manufactured separately from the cavity and attached to the cavity. It may also be removable.

[0052] According to one possibility, the cavity is obtained by assembling two substrates. A first substrate 101 is etched to form a part of the cavity and an interface membrane 23, see FIG. 7A. At the level of the interface membrane 23, the substrate is thinned to obtain a membrane of the desired thickness. A second substrate 102 is etched to form a complementary part of the cavity and optionally a distal membrane 24, see FIG. 7B. The substrates 101 and 102 are sealed to each other to form a cavity bounded on the one hand by the interface membrane 23 and on the other hand by the distal membrane 24, see FIG. 7C.

[0053] The device may comprise a biocompatible material configured to extend beyond the interface between the medium to be analyzed and the membrane. The biocompatible material is selected to ensure long-term biocompatibility and to allow optimal transfer of heat transferred by the medium to be analyzed to the membrane. The biocompatible material may be a metal, for example aluminum or copper, and may have a thickness of less than or of the order of 1 μm. The biocompatible material may also be a plastic, in which case its thickness is tens or hundreds of microns.

[0054] The invention may be used to measure the concentration of a molecule of interest in a medium, in particular in a liquid medium or a gel. Applications of the invention are relevant not only in the health sector but also in industrial sectors such as the food, pharmaceutical or chemical industry.

Claims

1. A photoacoustic detection device (1) intended to be applied via a contact surface (3) to a medium (2) to be analyzed, comprising: a hollow cavity (20) filled with gas and opening towards the contact surface; a light source (10) configured, upon operation, to emit an incident light beam (11) in a pulsed or amplitude modulated emission spectral band (Δλ) through the cavity (20) towards the contact surface; an acoustic detector (28) connected to the cavity to detect acoustic waves generated by heating of the medium (2) when the medium is illuminated by the incident light beam; Equipped with Further provided is an interface film (23) that forms the contact surface, The interface film is forming an interface between the gas filling the cavity and the medium; configured to prevent passage of the medium into the cavity; the acoustic detector is configured to detect acoustic pressure waves generated in the cavity under the influence of temperature changes of the interface film, the temperature changes of the interface film being caused by heating of the medium due to irradiation of the medium; Photoacoustic detection device.

2. The interface film (23) is not apertured. The photoacoustic detection device according to claim 1 .

3. The interface membrane has through-openings (23) with a radius of less than 50 μm or less than 30 μm. ｏ ), including The photoacoustic detection device according to claim 1 .

4. the interface membrane includes a hydrophobic coating within the through opening; The photoacoustic detection device according to claim 3 .

5. the light source is positioned such that, in operation, the incident light beam passes through the interface film before reaching the medium; The interface film has a cross segment (23) corresponding to a portion of the interface film through which the incident light beam has passed. ｉｎｔ ), At least in the crossing segments, the interface film is made of a transparent material having an optical transmittance higher than 0.4 in the emission spectral band. The photoacoustic detection device according to claim 1 .

6. The transparent material is Si, Ge, AlN, ZnSe, BaF ２ , CaF ２ , KBr, ZnS, and sapphire; The photoacoustic detection device according to claim 5 .

7. The interface membrane is removable. The photoacoustic detection device according to claim 1 .

8. The interface film is formed on the inner surface (23) in contact with the gas filling the cavity. ｉ ) and an outer surface (23) intended to be applied to said medium. ｅ ) and the inner surface of the interface film comprises an anti-reflective coating or microstructure configured to minimize reflection of the incident light beam; and / or the outer surface of the interface film comprises an anti-reflective coating or microstructure configured to minimize reflection of the incident light beam. The photoacoustic detection device according to claim 1 .

9. The thickness of the interfacial film is in the range of 20 μm to 1 mm. The photoacoustic detection device according to claim 1 .

10. the cavity (20) is bounded by a distal membrane (24), the distal membrane (24) being located opposite the interface membrane such that the cavity extends between the distal membrane and the interface membrane; the light source (10) is positioned such that, in operation, the incident light beam passes through the distal membrane before reaching the interface membrane; The photoacoustic detection device according to claim 1 .

11. the cavity is bounded by a distal wall (24) and a side wall (22), the side wall extending between the distal wall (24) and the interface membrane; the interface film extends between opposing edges of the sidewalls; The photoacoustic detection device according to claim 1 .

12. The volume of the cavity is less than 50 μL. The photoacoustic detection device according to claim 1 .

13. the acoustic detector is connected to the cavity by an acoustic channel; The photoacoustic detection device according to claim 1 .

14. The interface film is formed from a material having a thermal conductivity higher than 0.5 W / mK. The photoacoustic detection device according to claim 1 .

15. 1. A method for detecting an analyte in a medium, wherein the analyte absorbs light at an absorption wavelength, the method comprising: applying a device (1) according to any one of claims 1 to 4 to the medium so that the interface film is in contact with the medium; operating the light source (10) in the emission spectral band that includes the absorption wavelength of the analyte; detecting photoacoustic pressure waves with the acoustic detector and estimating the amount of the analyte in response to the detected photoacoustic pressure waves; A method comprising:

16. The medium (2) is a liquid or a gel.

16. The method of claim 15.

17. the light source is pulsed or amplitude modulated at a pulse or modulation frequency of less than 500 Hz; 16. The method of claim 15.

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