GB2568311A

GB2568311A - A Spectroscopy cell

Info

Publication number: GB2568311A
Application number: GB1718794.9A
Authority: GB
Inventors: Balocco Claudio; Shirshova Natasha
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-11-14
Filing date: 2017-11-14
Publication date: 2019-05-15
Also published as: GB201718794D0

Abstract

A porous matrix having plurality of cavities as a spectroscopy cell. A spectroscopy cell 120 suitable for probing an analyte using electromagnetic (EM) radiation, the spectroscopy cell having a porous matrix with a plurality of cavities for receiving the analyte. The porous matrix can be a polymer or inorganic porous bulk material. The polymer may also be a macroporous material comprising a high internal phase emulsion. A method of manufacturing the spectroscopy cell may comprise preparing and shaping said emulsion, and polymerising said emulsion to obtain a porous matrix. The spectroscopy cell may also be included as part of an optical device. The optical device may comprise a source of EM radiation 105, a dispersive element 130, and an EM radiation detector 140, one of which may be a micro-electric-mechanical systems component.

Description

A SPECTROSCOPY CELL

Technical Field

The present disclosure relates to a spectroscopy cell for probing an analyte. In particular, the present disclosure relates to a spectroscopy cell for probing an analyte using infrared or far-infrared electromagnetic radiations.

Background

Complex molecules display many vibrational and rotational modes which lie within the infrared and the far-infrared spectrum. As a result, infrared (IR)-spectroscopy is a powerful technique for the identification of many substances. Different techniques, based for example on absorption or transmission measurements, have been designed to probe specific spectral ranges. At the low energy side, terahertz (THz) spectroscopy, such as THz time-domain spectroscopy, can be used to investigate the very far 1R region. Fourier Transform Infrared Spectroscopy (FT1R) may be used to investigate samples between the mid 1R up to the terahertz region. Mid and near 1R spectroscopy can be used to measure samples within the high energy side of the 1R spectrum.

The analysis of complex chemicals in liquid solutions requires careful handling and preparation, as the sample solution needs to be physically constrained in a chemically inert and 1R transparent holder. Moreover, samples in aqueous solutions suffer from high losses in the far-lR/THz region due to water absorption. This constrains the geometry and reduces the optical thickness of the sample medium. Conventionally two main approaches are used for overcoming these problems.

The first approach relies on the use of microfluidic cells. Microfluidic cells are frequently used for liquid sample analysis in the THz region. They can be relatively bulky as they require special syringe pumps and large sample holders. Microfluidic cells are also relatively expensive to manufacture as they often require to be designed for individual pieces of equipment. Furthermore, microfluidic cells are commonly built using polydimethylsiloxane (PDMSj, which has an undesirable high absorption in the infrared and far-infrared region.

The second approach relies on the use of cuvettes-type containers. Such containers are cheap and disposable; however, their shape is not always compatible with the instrumentation and technique chosen to analyse the sample. They also require a lid to maintain the liquid sample within the container.

It is an object of the disclosure to address one or more of the above-mentioned limitations.

Summary

According to a first aspect of the disclosure, there is provided a spectroscopy cell for probing an analyte using an electromagnetic radiation, the spectroscopy cell comprising a porous matrix having a plurality of cavities for receiving the analyte.

Optionally, the cavities have an average diameter that is sufficient to prevent scattering effects at the electromagnetic radiation wavelength. For example, the cavities may have an average size that is large enough to prevent scattering effects, while at the same time being sufficiently small to retain the analyte.

Optionally, the wavelength may be greater than about 1 micrometre, and the cavities have an average diameter of less than about 100 micrometres.

For example, the wavelength may range from a few microns to one or more millimetres.

Optionally, the cavities have an average diameter ranging from about 1 micrometre to about 100 micrometres.

Optionally, the cell is a terahertz spectroscopy cell, wherein the cavities have an average diameter ranging from about 10 micrometres to about 50 micrometres.

Optionally, the cell is a medium-infrared spectroscopy cell, wherein the cavities have an average diameter ranging from about 1 micrometre to about 10 micrometres.

Optionally, the porous matrix comprises a polymer or an inorganic porous bulk material. For example, the polymer may be an organic polymer or an inorganic polymer. The inorganic porous bulk material may be a porous silica.

Optionally, the polymer may be a macroporous polymer.

Optionally, the macroporous polymer may comprise a polymerised high internal phase emulsion.

Optionally, the plurality of cavities may be interconnected.

Optionally, the cell may comprise the analyte. For instance, the analyte may be provided within the cavities.

According to a second aspect of the disclosure there is provided an optical device comprising a spectroscopy cell according to the first aspect. For example, the optical device may be a spectroscopic device.

Optionally, the optical device comprises a source of electromagnetic radiations; a dispersive element; and an electromagnetic radiation detector.

Optionally, at least one of the source of electromagnetic radiation, the dispersive element and the electromagnetic radiation detector is a microelectro-mechanical systems component.

Optionally, the source of electromagnetic radiation is adapted to provide infrared radiations or terahertz radiations.

According to a third aspect of the disclosure, there is provided a method of manufacturing a spectroscopy cell according to the first aspect. The method comprises preparing an emulsion; shaping the emulsion; and polymerising the emulsion to obtain a porous matrix.

For example, shaping the emulsion may be performed by casting process. For example, the emulsion may be shaped into a three-dimensional shape such as a thin film or a cylindrical shape.

Optionally, the emulsion may be a high internal phase emulsion. For example, the emulsion may contain at least 74 volume percent of internal phase.

Optionally, the emulsion comprises an internal phase comprising a salt solution or an ionic liquid; and an external phase comprising at least one monomer and at least one of a surfactant and solid particles. For example, the solid particles may be Pickering particles.

For example, the salt solution may be an aqueous solution or an organic solvent based solution. The monomer may be a multifunctional monomer, also referred to as crosslinker. If only one monomer is present in the external phase, then the monomer should be a multifunctional monomer.

Optionally, the external phase comprises an initiator for initiating polymerization. The initiator may be a thermal initiator or a photo initiator.

Optionally, the porous matrix comprises a plurality of cavities having an average diameter ranging from about 1 microns to about 100 microns.

Optionally, the emulsion comprises the analyte. For example, the internal phase may comprise the analyte.

According to a fourth aspect of the disclosure, there is provided the use of a porous matrix having a plurality of cavities as a spectroscopy cell. For example, the porous matrix may be a matrix according to the first aspect of the disclosure.

The options described with respect to the first aspect of the disclosure are also common to the second, third and fourth aspects of the disclosure.

Description

The disclosure is described in further detail below by way of example and with reference to the accompanying drawings, in which:

figure 1 is a diagram of a spectroscopy experimental setup;

figure 2 is an image of a polyHlPE microstructure obtained by scanning electron microscopy;

figure 3 is a flow chart of a method for manufacturing a spectroscopy cell using a macroporous polymer;

figure 4 is the terahertz spectrum of a polyHlPE cell filled with a solution of paracetamol and caffeine.

Figure 1 is a schematic representation of an experimental set up 100 for performing a spectroscopic measurement of a sample. The step up includes an electromagnetic source 105 such as a light source to provide a radiation beam 110, a cell 120, a dispersive element 130 and a detector 140.

The dispersive element may be a prism, a grating or a photonic crystal, and can be provided as part of a spectrometer. The detector 140 is optically aligned with the dispersive element 130 and the spectroscopy cell 120. Such an optical alignment may be achieved using various geometries via additional reflective components such as mirrors.

The spectroscopy cell 120 is provided by a porous matrix, also referred to as porous substrate, having a plurality of cavities 122. These cavities may contain an analyte to be probed using electromagnetic radiations. The porous matrix may be based on different materials. For instance, the porous matrix may be an inorganic porous bulk material, such as a porous silica bulk. Alternatively, the porous matrix may be based on a polymer such as a macroporous polymer. The polymer may be an organic polymer or an inorganic polymer.

The spectroscopy cell may be suitable for use with electromagnetic radiations in the mid-lR and far-lR region. For instance, the average diameter of the cavities may be selected to prevent scattering effects at a specific wavelength of the electromagnetic radiation, while at the same time being capable of retaining the analyte. As the wavelength of the electromagnetic radiation chosen to investigate the sample is increased, the average diameter of the cavities present in the matrix can be increased. For instance, the average diameter of the cavities located in the porous matrix of a Mid-lR spectroscopy cell may range between about lpm to about 10pm. The average diameter of the cavities present in the porous matrix of a far-lR spectroscopy cell may range between about 10pm to about 50pm or more.

Macroporous polymers have a plurality of cavities also referred to as pores which may have different shape, including cylindrical, slit-shaped and spherical shapes. The size of the pores may be defined with respect to the smallest dimension, referred to as pore-width, of the pore geometry. For example, if the cavity has a cylindrical shape, the pore-width corresponds to the diameter of the cylinder. If the cavity has a spherical shape, the pore-width also corresponds to the diameter of the sphere. Macropores have widths typically larger than 50nm. The macroporous polymer may be provided by a polymerised high internal phase emulsions polymer, polyHlPE.

The spectroscopy cell may be used in different optical configurations. For instance, the spectroscopy cell may be coupled to a waveguide or a transmission line. In this case the spectroscopy cell may be in direct contact with the waveguide or the transmission line.

Figure 2 shows a scanning electron microscopy image of a polyHlPE microstructure. Polymerised high internal phase emulsions polymers are macroporous polymers, also referred to as emulsion-templated polymers, which possess controllable pore structure and functionalities as well as high chemostability. PolyHlPEs can be synthesised from highly viscous high internal phase emulsions, HIPEs, using various mechanisms, ranging from free radical and controlled polymerisation to so called click-polymerisation. The pores of polyHlPEs can be interconnected and their size can be tailored depending on the application. Figure 2 shows a polymer structure having a plurality of cavities or pores 210a, 210b. Each pore is interconnected with other pores via a plurality of holes 220, also called pore windows, present in the polymer walls 230. In this example, the pores 210 are approximately spherical in shape with a diameter of about 4 microns.

Figure 3 is a flow chart of a method of manufacturing a spectroscopy cell suitable for use with electromagnetic radiations having a wavelength ranging from about a few micrometres to a one or more millimetres.

At step 310, an emulsion is prepared. The emulsion can be prepared by mixing the continuous phase of the emulsion, also referred to as external phase with the dispersed phase of the emulsion, also referred to as the internal phase. For example, the emulsion may be a high internal phase emulsion H1PE. As their name suggest, HIPEs possess a large volume of internal phase compared to the external phase. For example, the internal phase may constitute at least 74% of the volume of the emulsion. This results in the deformation of the internal phase droplets into for example polyhedral shapes, which are separated by thin layers of external phase. The external phase may comprise at least one monomer, a polymeric surfactant and a catalyst or initiator for initiating polymerization of the monomer. The monomer or monomers may have a single functional group, such as double bound, or a plurality of functional groups. Monomers with a plurality of functional groups can be referred to as multifunctional monomers or crosslinkers. Such crosslinkers are used to maintain the integrity of the porous structure. If the monomer has a single functional group, then a crosslinker is added to the external phase. The emulsion may be stabilised using solid particles such as modified colloidal silica particles. Such particles may be used as an alternative to the polymeric surfactant or in combination with the polymeric surfactant. The initiator may be a thermal initiator or a photoinitiator for initializing a polymerization reaction of the monomer(s). The internal phase may be provided by a salt solution, which may be an aqueous solution or an organic solvent-based solution, or an ionic liquid, hence a molten salt. An advantage of using an ionic liquid instead of a salt solution, lies on the possibility of recovering the ionic liquid after synthesis. Optionally, the internal phase may also include the analyte to be studied.

At step 320, the emulsion is shaped into a desired geometry. This may be achieved by casting process. For example, a film shape may be achieved by casting the emulsion on a flat surface, such as a glass slide.

At step 330, the shaped emulsion is polymerised. For example, polymerization may be photo inducted, also referred to as photopolymerization. The presence of internal phase droplets during photopolymerization shapes the cavities or pores shown in figure 2. The internal phase solution may be evacuated from the cavities during a drying process. A solvent can be used to remove salts which may be present in the internal solution, prior to drying the film. Drying may be performed under vacuum.

The form and thickness of the macroporous polymer may be varied depending on the application, and whether the cell is designed to be used with 1R radiations of relatively short wavelengths or relatively long wavelengths. A film of macroporous polymer could be as thin as tens of micrometres, and could be made as thick as required. For example, the film thickness may range between about 100 pm to hundreds of micrometres or more. A H1PE film can also be moulded in a complex shape prior to polymerisation. This could be achieved using different techniques, including 3D printing. In this case the H1PE may be used as printing material to print a spectroscopic cell of any required shape.

The emulsion may be varied to obtain a particular pore structure and pore size, to suit a particular range of wavelength. For example, if the spectroscopy cell is intended for performing THz spectroscopy, with a wavelength of approximately 300qm, the pore size should be in the order of tens of micrometres in order to avoid scattering effects. A pore size smaller than necessary to avoid scattering effects is undesirable as it would hinder the inclusion of liquid sample in the pores.

The pore structure and the pore size can be controlled by varying the composition of the emulsion, the conditions for preparing the emulsion, or both. For instance, different amount or types of monomers may be selected, as well as crosslinkers. Different types, amount and/or mixtures of surfactant may be chosen. A surfactant may also be combined with solid particles. The condition of preparation may include parameters such as the speed at which the internal phase is added, the mixing rate, and the type of polymerization used. This allows controlling the properties of the H1PE emulsion including its viscosity, wettability, droplet size, and stability. The interconnection between pores may also be adjusted. For instance, using Pickering particles instead of a surfactant it is possible to obtain closed pore structures.

The average pore-width of a Mid-IR spectroscopy cell may range between about lpm to about 10pm. The average pore-width of a THz spectroscopy cell may range between about 10pm to about 50pm. A Mid-IR spectroscopy cell may be suitable for use with radiations having a wavelength ranging from about 3pm to about 50pm. A THz spectroscopy cell may be suitable for use with radiations having a wavelength ranging from about 50pm to about 1mm.

The analyte to be optically probed can be embedded in the porous structure of the macroporous polymer either during emulsification or after polymerisation. For example, the analyte can be mixed in the internal phase of the emulsion such that the analyte is present in the emulsion used to form the film or other shape. The analyte itself may form the internal phase. Alternatively, the analyte may be introduced into the macroporous polymer film after polymerisation. This may be achieved by capillary motion or wicking.

For instance, the film may be simply submerged into a solution of analyte. If the analyte is aqueous and the film is made of hydrophobic polymer, the procedure may be performed using solvent exchange. In this case, the film would be submerged first into a solvent that is less hydrophilic than water, for example acetone, then the solvent would be slowly substituted by water, followed by the analyte. The analyte may also be introduced by suction. This can be achieved by providing a first and a second tube on opposite sides of the cell. The first tube is connected to a syringe and the second tube is connected to the analyte solution. The syringe can then be used to provide suction and fill the cell with the analyte solution.

The manufacturing method described above provides a simple and reliable method for the synthesis of macroporous polymers suitable for cost effective high-throughput processing technique like printing.

In a specific example, the spectroscopy cell may be manufactured as follows. The external phase of the emulsion is prepared using a polymeric surfactant, for example Hypermer B246, a monomer, for example butyl acrylate, a crosslinker, for example 1.6-hexanediol diacrylate (HDDA), and a photoinitiator, for example Darocure 1173. An amount of 0.603g of Hypermer B246 (corresponding to 25 weight percent wt.% of the monomer mixture) and 1 mol.% (calculated with respect to the total number of double bounds present in the monomer and crosslinker) of Darocure 1173 are dissolved in a mixture of 1.44 ml of butyl acrylate (BA) and 1.12 ml of 1.6-hexanediol diacrylate (HDDA) in a glass sample tube. The internal phase of the emulsion is obtained by preparing a solution of calcium chloride CaCL. A 0.5 wt.% CaCL aqueous solution (80 vol.%) is then added drop by drop into the external phase over a period of approximately 12 minutes and stirred at a stirring rate of about 400 rpm. Once the internal phase has been added, the stirring rate is then increased to 2000 rpm for 5 minutes to further homogenize the emulsion. Stirring may be performed using a homogeniser.

The HIPE emulsion is then cast on a glass slide or any other suitable substrate, and optionally covered, depending on the thickness of the film to minimize the contact of the HIPE film with air. For instance, the emulsion may be covered with a glass film or a polyethylene terephtalate (PET) film. The HIPE film is then polymerized under a UV lamp for 60 minutes or until desirable conversion is achieved. For example, the lamp may be an LED lamp such as a 24W, 1600 lumens lamp.

Alternatively, the macroporous polymer matrix of the spectroscopy cell may be synthetized using different approaches which may include precipitation polymerisation or reaction induced phase separation. As mentioned above, the porous matrix of the spectroscopy cell may also be made out of porous silica. For instance, a porous silica bulk may be obtained using a sol-gel process.

The spectroscopy cell obtained can be mounted on a relatively small optical device, such as a portable spectrometer. For example, a compact spectroscopic device may include a source of electromagnetic radiation, a dispersive element, a detector and the spectroscopy cell within a housing. A spectroscopic device, such as a spectrometer, may be built at least in part in micro-electro-mechanical systems MEMS technology. Such a spectroscopic device may then be used in different types of applications.

THz spectroscopy can be used in numerous applications including quality control and detection of counterfeit products, as well as for the detection of illicit drugs and explosives. The presence of complex molecules in perfumes and whiskies can be used to verify the origin of the product, control its quality, and identify fake products. This can be achieved using a reference sample. For instance, a sample of the genuine product can be sealed permanently into a polyHIPE film and used as a reference.

Water quality may also be controlled by monitoring the presence of organic compounds. THz spectroscopy can be used to detect the presence of pesticides in runoff water. Of particular interest is imidacloprid, used in rice cultivation, which while banned in the European Union, remains widely used in Asian countries. Infrared spectroscopy may also be used in diagnostic applications for instance in blood analysis for identification of infections such as blood fungal infections.

Figure 4 shows the terahertz spectrum of a spectroscopy cell filled with a sample containing 1% paracetamol and 0.05% caffeine. The spectroscopy cell was provided by a polyHlPE polymer film obtained from an emulsion having a continuous phase including butyl-acrylate and 1,6-hexanediol diacrylate. The spectrum 410 illustrates the result obtained for a cell filled with a solution sample. The spectrum 420 illustrates the result obtained once the sample solution has dried out. In this case the analyte (paracetamol and caffeine) is trapped in the pores of the cell in powder form.

A skilled person will appreciate that variations of the disclosed spectroscopy cell are possible without departing from the disclosure. Accordingly, the above description of the specific embodiments is made by way of example only and not for the purposes of limitation.

Claims

1. A spectroscopy cell for probing an analyte using an electromagnetic radiation, the spectroscopy cell comprising a porous matrix having a plurality of cavities for receiving the analyte.

2. The spectroscopy cell as claimed in claim 1, wherein the electromagnetic radiation has a wavelength, and wherein the cavities have an average diameter that is sufficient to prevent scattering effects at the wavelength.

3. The spectroscopy cell as claimed in claim 2, wherein the wavelength is greater than about 1 micrometre and, wherein the cavities have an average diameter of less than about 100 micrometres.

4. The spectroscopy cell as claimed in any of the preceding claims, wherein the cavities have an average diameter ranging from about 1 micrometre to about 100 micrometres.

5. The spectroscopy cell as claimed in any of the preceding claims wherein the cell is a terahertz spectroscopy cell, wherein the cavities have an average diameter ranging from about 10 micrometres to about 50 micrometres.

6. The spectroscopy cell as claimed in any of the claims 1 to 4, wherein the cell is a medium-infrared spectroscopy cell, wherein the cavities have an average diameter ranging from about 1 micrometre to about 10 micrometres.

7. The spectroscopy cell as claimed in any of the preceding claims, wherein the porous matrix comprises a polymer or an inorganic porous bulk material.

8. The spectroscopy cell as claimed in claim 7, wherein the polymer is a macroporous polymer.

9. The spectroscopy cell as claimed in claim 8, wherein the macroporous polymer comprises a polymerised high internal phase emulsion.

10. The spectroscopy cell as claimed in any of the preceding claims, wherein the cavities are interconnected.

11. The spectroscopy cell as claimed in any of the preceding claims comprising the analyte.

12. An optical device comprising a spectroscopy cell as claimed in any of the claims 1 to 11.

13. The optical device as claimed in claim 12, further comprising a source of electromagnetic radiation; a dispersive element; and an electromagnetic radiation detector.

14. The optical device as claimed in claim 13, wherein at least one of the source of electromagnetic radiation, the dispersive element and the electromagnetic radiation detector is a micro-electromechanical systems component.

15. The optical device as claimed in any of the claims 12 to 14 wherein the source of electromagnetic radiation is adapted to provide infrared radiations or terahertz radiations.

16. A method of manufacturing a spectroscopy cell according to any of the claims 1 to 11, the method comprising:

preparing an emulsion;

shaping the emulsion; and polymerising the emulsion to obtain a porous matrix.

17. The method as claimed in claim 16, wherein the emulsion is a high internal phase emulsion.

18. The method as claimed in claim 16 or 17, wherein the emulsion comprises an internal phase comprising a salt solution or an ionic liquid; and an external phase comprising at least one monomer and at least one of a surfactant and solid particles.

19. The method as claimed in any of the claims 16 to 18, wherein the external phase comprises an initiator for initiating polymerization.

20. The method as claimed in any of the claims 16 to 19, wherein the porous matrix comprises a plurality of cavities having an average diameter ranging from about 1 microns to about 100 microns.

21. The method as claimed in any of the claims 16 to 20, wherein the emulsion comprises the analyte.

22. Use of a porous matrix having a plurality of cavities as a spectroscopy cell.