Metal organic frameworks for gas sensing

Abstract

The invention relates to methods of detecting a volatile acid in a gas, the method comprising the steps of exposing a sensor cartridge to a gas. The sensor cartridge comprises an acid collapsable metal-organic framework (MOF) with a fluorescent molecule encapsulated within the MOF framework. The sensor cartridge is illuminated during and / or after the exposure of the sensor to the gas to excite the fluorescent molecule. The fluorescence emitted by the fluorescent molecule after exposure to the gas is detected. An increase of fluorescence emitted by the sensor cartridge after exposure to the gas is indicative of the presence of an acid in the gas.

Description

[0001] METAL ORGANIC FRAMEWORKS FOR GAS SENSING

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the use of MOFS and fluorescuent dyes in the detection of volatile acids.

[0004] BACKGROUND OF THE INVENTION

[0005] Monitoring volatile acid levels is important in occupational hygiene to comply with established exposure limits (long-term exposure acetic acid: 10 ppmv). A preferred detector combines robustness, affordability, and sensitivity in the relevant range. Fluorescence is the phenomenon in which a molecule absorbs light and subsequently emits a photon of lowered energy due to vibrational relaxation. Fluorescence-based chemical sensors are increasingly favored due to their high sensitivity and low-tech device integration.

[0006] Dye@MOF structures have been used for gas sensing in the past Lin et al. (2016) Adv. Sci., 3, 1500434. Towards acid vapor detection, Sanchez etal. (2022) ACS Appl. Mater. Interfaces, 14, 42656 encapsulated proton transfer dyes into MOFs. Their findings confirmed that the frameworks remain stable under saturated HCI vapor exposure, suggesting that the MOFs act as carriers and the dyes are the sensitive elements. In contrast, Wu et al. (2022) Microchim. Acta, 189, 87 reported a method for pH sensing through a degradation-mediated mechanism. Both a fluorophore and a quencher were encapsulated into a ZIF-8 framework. When the framework degrades in acid solutions, the ZIF-8 degrades, and the encapsulated molecules are released, alleviating the quenching.

[0007] Previous studies indicated that the integration of fluorescent dyes into ZIF-8 can result in aggregation-induced quenching [Glembockyte et al. (2018) J. Am. Chem. Soc 140, 16882]. The degradation of the ZIF-8 framework, and the release of dye molecules, leads to fluorescence enhancement [Li et al. (2019) Sens. Actuators B Chem. 301, 127110],

[0008] Lin et al. cited above, discuss the fluorescence of MOFs and factors influencing the fluoresce of the MOF itself.

[0009] Wu et al. cited above, discloses MOFs comprising upconversion nanoparticles (UCNP) wich are quenched by doxorubin. Collapse of the MOF releases doxorubicin results in fluorescent recovery of the UCNP. Sanchez et al. cited above, combine two proton transfer dyes, the presence of acids results in a spectral shift.

[0010] Glembockyte et al. cited above, disclose MOFs with two types of dyes to enhance the fotostabikity of BODIPY dyes.

[0011] Li et al. cited above disclose a MOF with uranine for the detection of inorganic phosphate.

[0012] Summary of the invention

[0013] The triggered collapse of metal-organic frameworks (MOFs) allows to fabricate low-cost acid vapor detectors. Fluorescence-based detection allows low-cost and simple device integration. The present invention disloses the encapsulation of fluorescent dyes such as fluorescein, coumarin 343, and rhodamine B in a zeolitic imidazolate framework such as ZIF-8. When the dye is incorporated into a ZIF-8 framework, the fluorescence of the dye is quenched. Upon degradation an collapse of the MOF by acid vapor exposure, the dye molecules are released from the framework by co, enhancing the fluorescence and signaling framework collapse.

[0014] The present invention exploits the triggered collapse of metal-organic frameworks (MOFs) for volatile acid detection. Zeolitic imidazolate framework-8 (ZIF-8, zinc nodes connected by 2-methyl imidazolate linker molecules) emerged as a promising material showing single-digit ppmv-level detection.

[0015] Fluorescent dyes that fit into the pores of the ZIF-8 framework , though larger than the pore inlet, have been integrated during synthesis. The dye molecules were selected based on their water solubility and size.

[0016] In embodiments of the present invention, three fluorescent dye@ZIF-8 structures were screened for their acetic acid vapor detection capabilities: fluorescein@ZIF-8, coumarin 343@ZIF-8, and rhodamine B@ZIF-8. Rhodamine B@ZIF-8 exhibited the most desirable behavior due to the lower pKa of the dye, therefore showing no dye-intrinsic acetic acid sensitivity. ZIF-8 degradation was detected between 10 and 41 ppmv of acetic acid. Initially, the structures were investigated by fluorescence spectroscopy, but a proof-of-concept LED-based read-out was implemented to showcase the viability of low-tech detection. The invention is further summarized in the following statements:

[0017] 1. A method of detecting a volatile acid in a gas, the method comprising the steps of:

[0018] - exposing a sensor cartridge to a gas, wherein the sensor cartridges comprises an acid collapsable metal-organic framework (MOF), with a fluorescent molecule encapsulated within the MOF framework,

[0019] - illuminating the sensor cartridge during and / or after the exposure of the sensor to the gas to excite the fluorescent molecule,

[0020] - detecting the fluorescence emitted by the fluorescent molecule during and / or after exposure to the gas,

[0021] - wherein an increase of fluorescence emitted by the sensor cartridge after exposure to the gas is indicative of the presence of an acid in the gas.

[0022] 2. The method according to statement 1, wherein the sensor comprises, apart from the MOF, no further compounds quenching the fluorescence of the fluorescent molecule, and wherein one single type of quenching compound is present within the MOF.

[0023] 3. The method according to statement 1 or 2, wherein the MOF is a Zeolitic Imidazolate Framework (ZIF), such as a ZIF-7 or a ZIF-8.

[0024] 4. The method according to any one of statements 1 to 3, wherein the fluorescent molecule is fluorescein, coumarin 343, or rhodamine B.

[0025] 5. The method according to any one of statements 1 to 4, wherein the gas comprises water vapour.

[0026] 6. The method according to any one of statements 1 to 5, wherein the sensor cartridge is exposed to a gas outside a sensor device, and the cartridge is subsequently analysed in a sensor device wherein the fluorescent dye is exited and fluorescence is detected.

[0027] 7. The method according to any one of statements 1 to 5, wherein the sensor cartridge is present within a sensor device wherein the fluorescent dye is exited and fluorescence is detected, and wherein a gas to be analysed comes in contact with the sensor cartridge.

[0028] 8. The method according to statement any one of statement 1 to 7, wherein the method detects organic acids released from paper or from movies.

[0029] 9. A sensor cartridge for detecting volatile acids in a gas, the sensor comprising an acid collapsable metal-organic framework (MOF), characterized in the presence of a fluorescent molecule encapsulated within the MOF framework, the sensor comprises, apart from the MOF, no further compounds quenching the fluorescence of the fluorescent molecule, and wherein one single type of fluorescent molecule is present within the MOF.

[0030] 10. The sensor cartridge according to statement 9, wherein the MOF is a ZIF-7 or a ZIF-8.

[0031] 11. The sensor cartridge according to statement 9 or 10, wherein the fluorescent molecule is fluorescein, coumarin 343, or rhodamine B.

[0032] 12. Use of a sensor cartridge according to statement 9 or 10 , for the detection of volatile acids in a gas.

[0033] 13. The use according to statement 12, for the detection of volatile acids in paper archives or film archives.

[0034] Detailed description

[0035] Figure 1: Fluorescence spectroscopy of dye@ZIF-8 powders exposed to static acetic acid dosing overnight. Emission spectra with acetic acid concentration screening, excitation wavelength 485nm, 430 nm, and 550 nm for a) fluorescein@ZIF-8, b) coumarin 343@ZIF-8, and c) rhodamine B@ZIF-8 respectively (lines in the figures represent, from top to bottom 104, 62, 41, 10, 3, 0 ppmv acetic acid, and air). Figure 2: 380 nm LED-based fluorescence read-out. a) Images of dye@ZIF-8 samples after static acetic acid exposure , b) and exported intensities normalized to the intensity measured at 104 ppmv.

[0036] Figure 3: Acetic acid sensitivities of dyes by comparing fluorescence spectra of a 0.5 mM aqueous solution and a 0.5 mM aqueous solution with 1% v / v of acetic acid, a) Fluorescein excitation at 485 nm, b) coumarin 343 excitation at 430 nm, and c) rhodamine B excitation at 550 nm.

[0037] Figure 4: Long (67 h) static acetic acid dosing, a) Fluorescein@ZIF-8 excitation at 485 nm, b) coumarin 343@ZIF-8 excitation at 430 nm, and c) rhodamine B@ZIF-8 excitation at 550 nm. (lines in the fiures represent, from top to bottom 10, 1, 0 ppmv acetic acid, and air)

[0038] Figure 5: Non-normalized intensities from LED-based setup after static acetic acid dosing.

[0039] Figure 6: Long (67 h) static acetic acid dosing monitored with 380 nm LED-based read-out. a) Images of the dye@ZIF-8 samples after static acetic acid exposure and b) exported non-normalized intensities from panel (a). A first aspect of the invention relates to a sensor cartridge for detecting volatile acids in a gas, the sensor comprising an acid collapsable metal-organic framework (MOF), characterized in the presence of a fluorescent molecule encapsulated within the MOF framework. Other acid lable porouos materials are equally suitable.

[0040] In preferred embodients, the sensor comprises, apart from the MOF, no further compounds quenching the fluorescence of the fluorescent molecule.

[0041] In preferred embodients, the sensor comprises, within the pores of the MOF a single type of fluorescent molecule.

[0042] "Acid labile" or "acid collapsible" MOF in the context of the present invention refers to porous coordination polymers that degrade upon acid exposure, and can be assayed by enclosing the MOF-material in a gas-impermeable closed volume with a separate vial containing a diluted acid solution, then the acid concentration in the vapor phase is controlled by the concentration in solution through Henry's Law or under flow by utilizing specialized dosing systems capable of creating the desired acid vapor compositions.

[0043] Typically the MOF is a Zeolitic imidazolate Framework (ZIF), such as a ZIF-7 or ZIF-8.

[0044] Typically the fluorescent molecule fits in the pores of the MOF.

[0045] Fluorescent molecules suitable in the context of the present invention at the one end fit within the pores and at the other hand are larger than the pore inlet. Computer based docketing can identify compounds which fullfill these size distriction Monitoring fluorescence can be done by e.g. fluorescence spectroscopy, photodiodes, or a camera.

[0046] Preferably the fluorescent molecule is water soluble. Water soluble fluorescent molecules facilitate the synthesis of the cartridges. Furthermore, after collapse of the MOF, water in a water containing gas will dissolve the dye leading to less quenching. Preferably, the fluorescence of a fluorescent molecule is pH indepent between pH 1 and pH7.

[0047] Preferably, the difference between waverlength of excitation and emission of the fluorescent molecule is at least 20 nm.

[0048] A second aspect of the invention method of detecting a volatile acid in a gas, the method comprising the steps of:

[0049] - exposing a sensor cartridge as described above in the first aspect, - illuminating the sensor cartridge during and / or after the exposure of the sensor to the gas to excite the fluorescent molecule,

[0050] - detection the fluorescence emitted by the fluorescent molecule after exposure to the gas,

[0051] - wherein an increase of fluorescence emitted by the sensor cartridge after exposure to the gas is indicative of the presence of an acid in the gas, the acid degrading the MOF structure and decreasing the quenching of the fluorescent molecule by the MOF. The gas that is analysed may contain water vapour.

[0052] In embodiments of the method the sensor cartridge is exposed to a gas outside a sensor device, and the cartridge is subsequently analysed in a sensor device wherein the fluorescent dye is exited and fluorescence is detected.

[0053] This allows to place different cartridges a various positions for a longer period whereafter the cartridges are collected and analysed. To provision of pores or other openings allows the access of the gas to the MOF, without the need of ventilators or other means of active gas transport.

[0054] In other setting the sensor cartridge is present within a sensor device wherein the fluorescent dye is exited and fluorescence is detected, and wherein a gas to be analysed comes in contact with the sensor cartridge. This allows a real time measurent and monitoring of volatile acids in gasses.

[0055] A third aspect of the present invention is use of the sensor cartridge of the first aspect in detection of volatile organic acids in a gas, for organic acids released by paper, or organic acids released by movies made of cellulose acetate film.

[0056] The invention is suitable for the detection of gasses of organic acids, such as formic acid, acetic acid, propionic acid, or butyric acid.

[0057] Detection levels of gasses are below 1500 ppmv , below 100 ppmv, 50 ppmv or even below 10 ppmv.

[0058] Example 1. Methods and materials

[0059] Dve(a)ZIF-8 synthesis: The procedure of Li et al. was followed for fluorescein@ZIF-8 synthesis [Li et al. (2019) Sensors and Actuators B: Chemical 301, 127110]. First, 0.2 g of Zn(NO3)2-6H2O (99%, Thermo Fisher) was dissolved in 0.8 mL deionized water with an adjusted pH of 8 (using a NaOH solution, 99.5%, Chemsolute). Then, a solution of 12 mg fluorescein (95% Sigma-Aldrich) in 4 mL deionized water was added and the mixture was stirred for 1 min at room temperature. Next, 2 g of 2- methyl imidazole (99%, Sigma-Aldrich) was dissolved in 8 mL deionized water and added dropwise to the stirred mixture. This mixture was further stirred at room temperature for 15 minutes. Afterward, the fluorescein@ZIF-8 was collected by centrifugation and washed three times with water / ethanol (1 / 1, v / v, >99.8%, Fisher Scientific). Lastly, the powder was dried overnight under vacuum at 60 °C. Coumarin 343@ZIF-8 and rhodamine B@ZIF-8 powder were synthesized using a similar procedure. The fluorescein was replaced by coumarin 343 (97%, Sigma-Aldrich) and rhodamine B (>99.8%, Sigma-Aldrich), though the pH of the Zn(NOs)2-6H2O solution was not adjusted since the pKa of both coumarin 343 and rhodamine B are lower, between 4.65 and 6.0 for coumarin 343 and 4.2 for rhodamine B, while the pKa of fluorescein is 6.43.

[0060] Static acetic acid dosing-. A sample is enclosed overnight (19h, unless otherwise specified) with a vial containing an aqueous acetic acid solution. By changing the concentration of the acetic acid, the concentration in the vapor phase can be controlled through Henry's Law. Acid exposures according to static acetic acid dosing occur at 100% relative humidity. In all measurements, a control with pure water as the solution is included, which is referred to as "0 ppmv" in the figures.

[0061] Fluorescence spectroscopy: The sample was transferred to a custom 3D printed sample holder. First, excitation-emission maps were recorded utilizing an Edinburgh FLS980 fluorimeter, to determine the optimal excitation wavelength of each sample. Afterward, the optimal excitation wavelength was used for generating emission spectra after static acetic acid dosing.

[0062] LED-based read-out: Fluorescence images using an in-house built setup consisting of a camera and 380 nm LED. The images were taken with identical camera settings, and were also processed using Image! processing software.

[0063] Fourier transform infrared spectroscopy fFTIR): A Varian 620 FTIR imaging microscope with a slide-on Ge ATR tip attached to a Varian 670 Fourier Transform-IR spectrometer with an actively cooled mercury cadmium telluride detector was used for all powder measurements.

[0064] Powder X-ray diffraction: Powder X-ray diffraction (PXRD) patterns of the powders were recorded on a Malvern PANalytical Empyrean diffractometer using a Cu source (Cu Kai: 1.5406 X and Cu KQ2: 1.5444 X) and PIXcel 3D solid-state detector. The measurements were taken in continuous mode in a 20 range from 1.3° to 45° with a step size of 0.0131° and scan speed of 79 seconds per step. Thermogravimetric analysis Thermogravimetric analyses (TGA) were performed on a STA 449F3 Jupiter (Netsch, Germany). The measurements were performed in a continuous flow of dry synthetic air (50 mL / min) and the samples were heated from ambient temperature to 700 °C with a heating rate of 5 °C / min. Before analysis, the samples were reactivated overnight in vacuum at 60°C.

[0065] High-performance liquid chromatography CHPLC) HPLC measurements were performed on an Agilent 1200 Series SL quaternary system equipped with a G1322A degasser, a G1311A quaternary pump, a G1367A automated sample injector, a G1316A thermostatted column compartment, and a G1314A variable wavelength detector. Separation of the different dyes was achieved on an Ascentis RP-Amide column (150 mm x 4.0 mm i.d ., 5.0 pm particles), maintained at 40 °C. The mobile phase consists of a 70:30 mixture of 1 wt% H3PO4 solution in milli-Q water and acetonitrile at a flow rate of 0.7 mL / min. The injection volume was 2 pL and chromatograms were monitored at 445 nm for coumarin 343, 485 nm for fluorescein and 541 nm for rhodamine B.. Afterward, 5 mg of each dye@ZIF-8 was dissolved in 25 mL of 10 v% acetic acid in water. For the analysis of the mixtures containing Coumarin 343, a 50x dilution was performed prior to the analysis.

[0066] Dye sensitivity testing in solution : 0.5 mM solutions were prepared for all dyes. The sensitivity to acetic acid was investigated by making 0.5 mM aqueous dye solutions with 1% v / v acetic acid. The zinc ion sensitivity was studied by making 0.5 mM aqueous dye solutions and adding 0.5 mM of Zn(NO3)2-6H2O. Afterward, the fluorescence spectra were measured in cuvettes using an Edinburgh FLS980 fluorimeter, and the optimal excitation wavelength for each dye was employed (485, 430, and 550 nm for fluorescein, coumarin 343, and rhodamine B respectively).

[0067] EXAMPLE 2. Dye@ZIF-8 characterization

[0068] Subsequent to the synthesis of dye@ZIF-8 powders, the samples underwent a series of material characterization. High-performance liquid chromatography reveals a dye content of 18 wt%, 22 wt%, and 11 wt% for, respectively, fluorescein@ZIF-8, coumarin 343@ZIF-8, and rhodamine B@ZIF-8, which is confirmed by infrared spectroscopy. Only the characteristic coumarin 343 peaks are detected in the dye@ZIF-8 samples. X-ray diffraction confirmed the presence of the characteristic ZIF-8 peaks for all samples, indicating successful framework formation . Thermogravimetric analysis shows simultaneous decreases in mass at 430°C for all samples, but the dye@ZIF-8 samples show a less sudden decrease than the pristine ZIF-8, indicating increased chemical heterogeneity. All dyes were insensitive to the addition of a Zn salt in an aqueous solution, showing that the quenching is not related to the proximity to the zinc nodes . This illustrated that fluorescence quenching is caused by an aggregation-based mechanism. These powders will be exposed to acid vapor via static acetic acid dosing , in which the dye@ZIF-8 powders are enclosed overnight together with an aqueous acetic acid solution. By controlling the concentration of acetic acid in the solution, the concentration in the vapor phase is controlled through Henry's Law.

[0069] To facilitate fluorescence screening, excitation-emission maps are constructed of acetic acid-degraded dye@ZIF-8 samples (41 ppmv). These maps enable the determination of an optimal excitation wavelength: 485nm, 430 nm, and 550 nm for fluorescein@ZIF-8, courmarin343@ZIF-8, and rhodamine B@ZIF-8, respectively. Next, the acetic acid concentration is screened using the previously established optimal excitation wavelengths. Fluorescein@ZIF-8 exhibits a progressive increase in emission with increasing acetic acid concentration. This subdued response can be attributed to fluorescein's pH sensitivity, as its emission intensity diminishes upon protonation. Exposure of the fluorescein@ZIF-8 to acetic acid vapor promotes degradation, alleviating the aggregation-induced quenching. On the other hand, acetic acid (pKa = 4.75) can protonate fluorescein (pKa = 6.43, Figure 3a), counteracting the degradation-based sensing mechanism.

[0070] Reported values for the pKa of coumarin 343 vary, ranging from 4.65 to 6.0. When coumarin 343 is dissolved in a 1% v / v acetic acid solution, a reduction in fluorescence is observed (Figure 3b). Since the pKa of acetic acid and coumarin 343 are in the same range, a high concentration of acetic acid is required to promote significant protonation. At low acetic acid concentrations, limited dye-intrinsic behavior will likely be observed. Figure 1 demonstrates a pronounced step in the fluorescence enhancement for coumarin 343@ZIF-8 when the acetic acid concentration rises from 10 ppmv to 41 ppmv. A distinct redshift is observed from a sample just exposed to air relative to a sample exposed to water (Figure 1). As a solvatochromic dye, Coumarin 343's fluorescent behavior is sensitive to its chemical environment, and its fluorescence redshifts when it is in more polar surroundings. Rhodamine B has a pKa of 4.2. Therefore, dye protonation by acetic acid is not viable. Thus, no dye-specific behavior is expected for acetic acid-sensing (Figure 3c). Rhodamine B@ZIF-8 shows the most desirable behavior in Figure 2 of all tested dye@ZIF-8 samples, showing the most distinct step in fluorescence when the acetic acid concentration is increased from 10 ppmv to 41 ppmv. The detection threshold for acetic acid vapor detection lies between 10 and 41 ppmv for coumarin 343@ZIF-8 and rhodamine B@ZIF-8, while thhe acetic acid collapse of ZIF-8 thin films at approximately 2.5 ppmv. At prolonged static acetic acid dosing times, a fluorescence enhancement can be observed between 1 ppmv and 10 ppmv (Figure 4).

[0071] Example 3. LED-based read-out

[0072] One of the main advantages of fluorescence-based detection is its compatibility with simple readout electronics. Herein a 380 nm LED is used for excitation coupled to a camera module for detection (Figure 2a). This method shows the degradation step between 10 and 41 ppmv acetic acid for coumarin 343@ZIF-8 and rhodamine B@ZIF-8 (Figure 2b-c), as observed above. However, enhanced sensitivity is observed for extended static acetic acid dosing times (between 1 and 10 ppmv, Figure 6). Figure 2b shows the most intense signal for coumarin 343@ZIF-8, while significantly less for rhodamine B@ZIF-8. To explain this observation, the ideal excitation wavelengths, need to be considered. The wavelength of the LED (380 nm) is closest to the ideal excitation wavelength of coumarin 343@ZIF-8 (430 nm) and furthest away from rhodamine B@ZIF-8 (550 nm). When the intensities are normalized relative to the highest measured intensity (at 104 ppmv), a similar concentration profile is observed for all dyes (Figure 2c).

Claims

CLAIMS1. A method of detecting a volatile acid in a gas, the method comprising the steps of:- exposing a sensor cartridge to a gas, wherein the sensor cartridges comprises an acid collapsable metal-organic framework (MOF), with a fluorescent molecule encapsulated within the MOF framework,- illuminating the sensor cartridge during and / or after the exposure of the sensor to the gas to excite the fluorescent molecule,- detecting the fluorescence emitted by the fluorescent molecule during and / or after exposure to the gas,- wherein an increase of fluorescence emitted by the sensor cartridge after exposure to the gas is indicative of the presence of an acid in the gas.

2. The method according to claim 1, wherein the sensor comprises, apart from the MOF, no further compounds quenching the fluorescence of the fluorescent molecule, and wherein one single type of quenching compound is present within the MOF.

3. The method according to claim 1 or 2, wherein the MOF is a Zeolitic Imidazolate Framework (ZIF), such as a ZIF-7 or a ZIF-8.

4. The method according to any one of claims 1 to 3, wherein the fluorescent molecule is fluorescein, coumarin 343, or rhodamine B.

5. The method according to any one of claims 1 to 4, wherein the gas comprises water vapour.

6. The method according to any one of claims 1 to 5, wherein the sensor cartridge is exposed to a gas outside a sensor device, and the cartridge is subsequently analysed in a sensor device wherein the fluorescent dye is exited and fluorescence is detected.

7. The method according to any one of claims 1 to 5, wherein the sensor cartridge is present within a sensor device wherein the fluorescent dye is exited andfluorescence is detected, and wherein a gas to be analysed comes in contact with the sensor cartridge.

8. The method according to claim any one of claim 1 to 7, wherein the method detects organic acids released from paper or from movies.

9. A sensor cartridge for detecting volatile acids in a gas, the sensor comprising an acid collapsable metal-organic framework (MOF), characterized in the presence of a fluorescent molecule encapsulated within the MOF framework, the sensor comprises, apart from the MOF, no further compounds quenching the fluorescence of the fluorescent molecule, and wherein one single type of fluorescent molecule is present within the MOF.

10. The sensor cartridge according to claim 9, wherein the MOF is a ZIF-7 or a ZIF- 8.

11. The sensor cartridge according to claim 9 or 10, wherein the fluorescent molecule is fluorescein, coumarin 343, or rhodamine B.

12. Use of a sensor cartridge according to claim 9 or 10 , for the detection of volatile acids in a gas.

13. The use according to claim 12, for the detection of volatile acids in paper archives or film archives.

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