Max-phase-based optical device and method for determining residual max-phase in mxenes

Abstract

The group of inventions relates to the field of optics of nanostructured materials and in particular to MAX-phase-based optical devices and source material quality verification methods, and namely to the methods for determining residual MAX-phase in MXenes after etching out the element A. The optical device comprises a transparent base 1 and photoactive particles 2 formed from the MAX-phase having the chemical formula Mn+1AXn. The content of residual aluminum therein in relation to the initial content in the MAX-phase comprises not less than 1%. The method for determining the residual MAX-phase in MXenes comprises exposing them to an exciting radiation and obtaining the output spectral characteristics. The amount of the residual MAX-phase in MXenes is determined from the intensity of the emission band at 694 nm. The inventions make it possible to create simple and efficient method for measuring the MAX-phase derivatives and to expand the range of MAX-phase technology.

Description

MAX-PHASE-BASED OPTICAL DEVICE AND METHOD FOR DETERMINING RESIDUAL MAX-PHASE IN MXENES

[0001] The group of inventions relates to the field of optics of nanostructured materials and in particular to MAX-phase-based optical devices and source material quality verification methods, and namely to the methods for determining residual MAX-phase in MXenes after etching out the element A.

[0002] MAX-phase is a family of layered compounds that are carbides or nitrides formed from transition metals (early transition metals of the periodic table groups III to VII), elements of group IIIA or IVA (for example, aluminum, gallium, silicon, germanium) and carbon or nitrogen. The MAX-phase has the following general formula: Mn+1AXn, where М is transition metals, А is an element of group IIIA or IVA, and Х is carbon or nitrogen. The MAX-phase-based materials have unique properties, such as high thermal and electric conductivity, low density, reduced elasticity modulus, high strength, corrosion stability, resistance to high-temperature oxidation and thermal shocks, and also they can be easily machined and have high melting temperature. Due to their unique properties, the MAX-phases are promising for using in various applications.

[0003] Aluminum-based materials are particularly unique as they provide good electric conductivity of the MAX-phase and, moreover, aluminum is much easier to etch out of the MAX-phase as against silicon to obtain MXenes that, in their turn, can be exfoliated into flakes. Furthermore, the flakes of MXenes are used in various applications, for example, in production of filters, membranes, solar cell panels, in biomedicine, and for various functional coatings.

[0004] MXenes are a new family of 2D-materials that have been obtained by selective etching of a MAX-phase. Using relatively weaker М-А bonds in comparison with М-Х bonds, the layer А can be selectively etched out by chemical or electrochemical methods, while keeping the М and X layers, which are the layers that form two-dimensional MXenes. After the etching, the multilayer (ML)-MXenes are submerged into dimethyl sulfoxide, tetrabutyl ammonium hydroxide, alcohols, choline hydroxide or n-butylamine for further exfoliation by ultrasound treatment or centrifugation. By now, approximately 100 MAX-phases have been predicted and about 30 MXenes have been synthesized and described, which makes them the largest group in the 2D family. Also, MXenes have non-trivial properties, which makes them suitable for many various applications.

[0005] For example, the prior art discloses a hybrid thin-film photoconverter, containing a transparent substrate, which is successively coated with a transparent electrode and a photoactive layer, based on hybrid perovskites АРbХ3modified with MXenes (see patent RU2694086, cl. H01L51 / 42, published 09.07.2019). The prior art solution makes it possible to enhance the photoconverter energy conversion efficiency due to increasing no-load voltage of the devices by more than 10%, as well as to enhance the current-voltage characteristic filling factor of the devices by more than 5% due to reduced current shunting leaks and increased contact resistance.

[0006] Also, the prior art discloses a humidity sensor and a gas analyzing multisensor chip based on a two-dimensional titanium-vanadium carbide MXene structure, which are produced by preparing a suspension of a two-dimensional titanium carbide based material, by applying this suspension in the form of a layer onto a dielectric substrate in such a way that the particles of the coating material form percolation paths between the electrodes, and by soldering the substrate with the applied coating into a housing (see patent RU2804013, cl. B82Y 40 / 00, published 26.09.2023). The disclosed solution makes it possible to create a new generation of sensors functioning at a room temperature as well.

[0007] The closest, in terms of technical substance, to the claimed invention as related to the device is the contact lens for the eye photothermal therapy and laser protection, comprising a transparent base and photoactive particles in the form of MXenes sprayed as a solution onto the surface of a dehydrated transparent polymer-based contact lens (see patent CN113568188, cl. G02C7 / 04, published 02.12.2022). The obtained lenses have excellent characteristics of photothermal conversion and antimicrobial effect, as well as high water retentiveness and light transmittance properties.

[0008] The main disadvantage of the prior art devices is their limited functionality, which makes it impossible to use the potential of the MAX-phase-based materials in full.

[0009] It has been found that some MXenes also have extraordinary optical properties, for example, they demonstrate an adjustable photo-induced fluorescence in ambient environment, with a narrowband peak at 694 nm having the half-width smaller than 1 nm (see A.V. Syuy, S.M. Novikov, G.I. Tselikov, V.V. Krishtop, D.S. Shtarev, A.V. Arsenin, V.S. Volkov, Photoluminescent properties of Mo2TiC2Txand Mo2Ti2C3TxMXenes; Physics: fundamental and applied research, education: XXI Regional Scientific Conference Proceedings; Blagoveshchensk: Amur State University, 2023; DOI:10.22250 / 9785934934195_97). This phenomenon can be used as a basis to form unique optical devices.

[0010] Since the properties of nanostructured materials critically depend even on small variations in their composition, the vital problem when using the MAX-phase-based particles is to determine the MAX-phase residual content in MXenes. It is known that etching out aluminum of a МАХ-phase results in the X-ray diagram intensity peak displacement corresponding to the direction (002) from 10 degrees to 6 degrees (see Babak Anasori, Yu Xie, Majid Beidaghi, Jun Lu, Brian C. Hosler, Lars Hultman, Paul R. C. Kent, Yury Gogotsi, and Michel W. Barsoum. Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes) / ACS Nano. 2015. Vol. 9. # 10. P. 9507–9516). The displacement of this peak can be used to estimate the MXene purity from residual aluminum. This conventional approach is the closest to the disclosed invention as related to the method, but has significant disadvantages. The X-ray phase analysis measure of sensitivity is a minimum substance quantity in the mixture, giving a set of its characteristic reflexes sufficient to determine the presence in the sample, i.e. sensitivity is different for different phases and different phase mixtures — it increases with the increase in the reflecting power of the atomic planes of the phase whose presence needs to be detected, and with the weakening of the diffractogram background. The absorption factor ratio between the whole mixture and the phase being determined is essential as well. Substances that highly scatter X-ray radiation can be easily detected in a low scattering mixture. Whereas the compounds of light elements in a mixture with heavy elements can only be detected in case of their large concentrations. In the case of MXenes and MAX-phases based on Al, sensitivity of such a method is within 1-2% of the Al content in the mixture. Besides, the equipment for X-ray analysis is rather bulky and expensive, the measurements are rather long-lasting and involve certain radiation hazard for personnel.

[0011] Thus, the technical problem is to eliminate said disadvantages and to create relatively simple and efficient method for measuring the MAX-phase derivatives and also to expand the range of technology that uses the MAX-phase-based materials.

[0012] The technical effect, as related to the device, consists in expanding functionality of the MAX-phase-based devices. Said problem has been solved and the technical effect has been achieved by that in the optical device comprising a transparent base and photoactive particles said particles are formed from the MAX-phase having the chemical formula Mn+1AXn, where M is transition metals, A is aluminum, X is carbon or nitrogen, n = 1, 2 or 3, wherein the photoactive particles are formed with the size from 5 nm to 200 nm, and the content of residual aluminum in these particles in relation to the initial content in the MAX-phase comprises not less than 1%. In an embodiment, the optical device can be implemented as a contact lens whose body contains a built-in power source and at least one light emitting diode with the emission wavelength ranging from 350 nm to 650 nm disposed on the contact lens inner surface side and directed towards its external surface. Each said light emitting diode can be disposed at the periphery of the contact lens whose body can contain a built-in at least one waveguide going from said light emitting diode to the lens center where at least one diffractive optical element is disposed that turns the emission from the waveguide inside the eye, whereas said photoactive particles are distributed inside the waveguide. In another embodiment, said photoactive particles can be applied onto the peripheral part of the contact lens external surface as a layer with the thickness from 1 µm to 30 µm, onto which a protective layer of chitosan having the thickness from 1 µm to 30 µm is applied. In another embodiment, the optical device can be implemented as a laser module comprising a source of emission with the wavelength ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2, a ring photonic resonator and an output waveguide, with the core formed from a material with the refractive index not lower than 2.4, wherein a layer of said particles having the thickness ranging from 1 µm to 30 µm is disposed between the source of emission and the resonator. In another embodiment, the optical device can be implemented as a temperature sensor comprising a source of emission ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2, and a photodiode equipped with a 694 nm band-pass filter, wherein a base with a layer of said particles having the thickness ranging from 1 µm to 30 µm is disposed between the light emitting diode and the photodiode.

[0013] The technical effect, as related to the method, consists in simplifying determination of the residual MAX-phase, having the chemical formula Mn+1AXn, where M is transition metals, A is aluminum, X is carbon or nitrogen, n = 1, 2 or 3, in MXenes by enabling the use of a wider range of measurement equipment. Said problem has been solved and the technical effect has been achieved by that the residual MAX-phase in MXenes is determined by exposing them to exciting radiation and obtaining the output spectral characteristics, wherein the exciting radiation that is used has the wavelength ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2, the output spectral characteristics are measured as the intensity of the emission band at 694 nm, and the amount of the residual MAX-phase in MXenes is determined from the intensity of said band.Fig.1

[0014] shows a schematic view of a MAX-phase based on Ti3AlC2(M – Ti; A – Al; X – C; n=2);Fig.2

[0015] shows the MAX-phase ofafter etching, with residual inclusions of Al2O3:Cr;Fig.3

[0016] shows the absorption and emission spectra for a photoluminescence band at the 694 nm wavelength;Fig.4

[0017] shows a most simple embodiment of the disclosed optical device;Fig.5

[0018] shows an embodiment of the disclosed optical device in the form of a contact lens with light emitting diodes, with MAX-phase-based photoactive particles distributed within the contact lens body;Fig.6

[0019] shows an embodiment of the disclosed optical device in the form of a contact lens with light emitting diodes, waveguides with MAX-phase-based photoactive particles, and diffractive optical elements;Fig.7

[0020] shows a drug delivery diagram using MAX-phase-based photoactive particles;Fig.8

[0021] shows a schematic operation diagram of an embodiment of the disclosed optical device in the form of a contact lens with a GRS-sensor;Fig.9

[0022] shows an embodiment of the disclosed optical device in the form of a laser module with a ring resonator;Fig.10

[0023] shows a diagram of intensity variation in case of thermal quenching;Fig.11

[0024] shows an embodiment of the disclosed optical device in the form of a temperature sensor operating on a pass-through principle;Fig.12

[0025] shows an embodiment of the disclosed optical device in the form of a temperature sensor operating on a waveguide principle;Fig.13

[0026] shows the characteristic 694 nm photoluminescence band spectra for different residual Al concentrations;Fig.14

[0027] shows a general MXene quality verification diagram according to the disclosed method.

[0028] The group of the disclosed inventions relates to the practical application of the discovered phenomenon of the MAX-phase-based particles photoluminescence at the wavelength of 694 nm. The characteristic size of such particles ranges from 5 nm to 200 nm, which results from the particles synthesis method (a lower size results in losing the structure effect in the nanocomposite material, while a larger size is limited by the particles synthesis method). The source MAX-phase has a layered structure with alternating layers М, А, and Х. The number of layers depends on the MAX-phase formula. As an example,shows the structural diagram of М3АХ2(that is: Ti3AlC2, where three titanium layers alternate with two carbon layers, and this group is separated from another identical group by one layer of aluminum).

[0029] Once aluminum is etched in a mixture of acids, for example, HCl+LiF, aluminum should be absent in the MAX-phase, and the broken bonds with aluminum are covered by so called tail endings O, H, OH, F, Li, Cl. However, since the acids cannot penetrate the MAX-phase structure completely, there is always a certain amount of aluminum that remains. Aluminum, in its turn, is easily oxidized in water or air. Also, a small amount of chromium lingers in the air in almost any laboratory. Therefore, the process of oxidization and bonding with chromium in the etched MAX-phase (MXene) results in that some concentration of Al2O3:Cr is present. The resulting MAX-phase structure after etching is shown in. In the case of the source MAX-phase (100% Al concentration), the existing aluminum naturally oxidizes, and also the Al2O3:Cr compound is formed.

[0030] The photoactive centers Al2O3:Cr are the basis for the ruby laser and have a narrow intensive band of photoluminescence at the 694 nm wavelength with the width of approximately 1 nm, which is excited by the emission with the wavelength ranging from 350 nm to 650 nm (the ruby laser absorption spectrum is represented by the curve in the left part of). To observe said band, the residual aluminum concentration in relation to the source concentration in the MAX-phase must be from 1 to 100%. As an example,shows the emission spectra of the analyzed MAX-phase-based photoactive particles when excited at the wavelength of 532 nm and with the residual aluminum concentration of 1, 2, and 5 %.

[0031] It has been experimentally found that said phenomenon of reemission at 694 nm is only observed for the particles having the residual aluminum concentration of not less than 1% in relation to the source concentration in the MAX-phase. When the Al concentration is lower, the formation of photoactive centers in the form of residual Al2O3:Cr inclusions practically stops and the 694 nm band intensity remains at the noise level (as it will be described below, this phenomenon can be effectively used to verify the MXene quality).

[0032] The 694 nm emission band was observed in all of the analyzed particles based on an aluminum-containing MAX-phase, such as, in particular: Ti3AlC2, Ti2AlN, Ti2AlC, Cr2AlC, V2AlC, Nb2AlC, Ta2AlC, Ta4AlC3, Nb4AlC3, V4AlC3, Ti4AlN3, (Ti1−xNbx)2AlC, (Ti1−xVx)2AlC, (V1−xNbx)2AlC, (V1−xCrx)2AlC, (V1−xCrx)3AlC2, (Ti1−xVx)3AlC2, (Ta1−xTix)3AlC2, (Nb0.8Ti0.2)4AlC3, (Nb0.8Zr0.2)4AlC3, (Mo1−xVx)4AlC3, (Nb0.975W0.025)4AlC3, (TixTa1−x)4AlC3, (Mo1−xVx)5AlC4, (Nb0.67Sc0.33)2AlC, Mo4 / 3Sc2 / 3AlC, Mo4 / 3Y2 / 3AlC, W4 / 3Sc2 / 3AlC, W4 / 3Y2 / 3AlC, Mo4 / 3Y2 / 3AlC, Mo2TiAlC2, Cr2TiAlC2, Mo2Ti2AlC3, Mo2Nb2AlC3, and others, and, based on the physical nature of the phenomenon, it can be concluded that said band may be expected to appear in any structure of the type Mn+1AXn, where M is transition metals, A is aluminum, X is carbon or nitrogen, n = 1, 2 or 3. As well as in MXenes produced based on aluminum-containing MAX-phases: Ti3C2, Ti2N, Ti2C, Cr2C, V2C, Nb2C, Ta2C, Zr3C5, Ta4C3, Nb4C3, V4C3, Ti4N3, (Ti1−xNbx)2C, (Ti1−xVx)2C, (V1−xNbx)2C, (V1−xCrx)2C, (V1−xCrx)3C2, (Ti1−xVx)3C2, (Ta1−xTix)3C2, (Nb0.8Ti0.2)4C3, (Nb0.8Zr0.2)4C3, (Mo1−xVx)4C3, (Nb0.975W0.025)4C3, (TixTa1−x)4C3, (Mo1−xVx)5C4, (Nb0.67Sc0.33)2C, Mo4 / 3Sc2 / 3C, Mo4 / 3Y2 / 3C, W4 / 3Sc2 / 3C, W4 / 3Y2 / 3C, Mo4 / 3Y2 / 3C, Mo2TiC2, Cr2TiC2, Mo2Ti2C3, Mo2Nb2C3, and others.

[0033] In general, the optical device (see) that enables the practical use of the discovered photoluminescence phenomenon at 694 nm must comprise a transparent base 1 and the photoactive particles 2 formed from a MAX-phase based on aluminum (Al). The base 1 can be represented by any solid substance (amorphous, crystalline), for example, glass, Si, SiO2, foil, etc. with a smooth surface. Using a spraying (for example, with an airbrush) or spin-coating (centrifugation) method, a thin layer of photoactive particles 2 is applied onto the base. Then, the base 1 with the particles 2 is dried at the room temperature for 1 hour. To ensure the required stability, the obtained structure is covered with the protective layer 3, for example, based on chitosan or a transparent polymer, to protect against mechanical impacts.

[0034] In a more complex embodiment of the optical device, its base 1 can be implemented as a contact lens containing the photoactive particles 2 integrated inside the lens volume (for example, by means of the lens hydratation in an aqueous solution of such particles) and formed from an aluminum (Al) based MAX-phase having a spherical form with the diameter of 20-60 nm. The particles can be produced in two ways: using the method of mechanical grinding in a ball grinder or using the method of laser ablation.

[0035] Such particles can be used, for example, as a contact lens shelf-life sensor. When the lens is removed overnight, it is put into a dedicated storage box, which is equipped with an excitation source with the wavelength ranging from 350 nm to 650 nm (for example, 532), and with a 694 nm emission sensor. When the lens body is exposed to the exciting radiation, the sensor detects a signal at 694 nm: if there is no signal, then the contact lens is good; if there is a signal, then the contact lens shelf-life has expired and it needs replacing. As a matter of fact, the sensor operates as a lens air permeability indicator, as oxidation of the MAX-phase with residual aluminum (hence, Al2O3:Cr) results in degrading the lens permeability.

[0036] The contact lens form-factor is also suitable for solving the problem of curing diabetic retinopathy — it is well known that the 694 nm emission has a positive therapeutic effect on the eye retina 4 (see N. Fijalkowski, D. M. Moshfeghi. New Laser Technologies for Diabetic Retinopathy. Curr Ophthalmol Rep (2013) 1:134–143. DOI: 10.1007s40135-013-0017-01), and the placement of the photoactive particles 2 reemitting in this range enables such an effect during the daily use of the lenses. In this embodiment, the excitation can be provided by the scattered ambient visible light (sunlight or an artificial source of white light) or one or more wide-aperture light emitting diodes 5 (see) encapsulated into the lens / base 1.

[0037] The light emitting diodes 5 with the emission wavelength ranging from 350 nm to 650 nm (for example, 532 nm or 633 nm) are arranged evenly along the periphery on the contact lens inner surface side and directed towards its outer surface. For operation, the light emitting diodes 5 are equipped with a power source 6 in the form of a battery or a wireless antenna (for convenience of use, the following control mechanism can be implemented: a single tap on the contact lens turns the light emitting diodes on, the second tap turns them off). When exposed to the exciting radiation, the particles 2, due to the presence of residual aluminum and the Al2O3:Cr compound, photoluminesce at the wavelength of 694 nm and reemit the light emitting diode energy in all directions, including backward direction — i.e. towards the user’s eye retina 4, subjecting it to the required therapeutic action. As the refractive index of a polymeric contact lens is noticeably lower than that of the air, the edge emission of the light emitting diodes will be additionally led into the body of the base 1 due to the complete inner reflection (waveguide effect), which will to a certain extent enhance the efficiency of reemission.

[0038] To increase the energy conversion efficiency even more, the contact lens is equipped with the waveguides 7 having the refractive index higher than that of the contact lens main body (see). The waveguides can be formed directly with the particles 2 — then, reemission at the required wavelength will occur concurrently as the excitation emission spreads from the light emitting diodes 5 through the waveguides 7. In the lens central area, the generated emission with the wavelength of 694 nm from the waveguides 7 turns towards the eye retina 4 by means of the diffractive elements 8 (for example, holographic arrays). Such a design makes it possible to increase the reemission efficiency by a few orders in comparison with simple scattering.

[0039] Alternatively, the therapeutic action by means of the contact lens containing the particles 2 that reemit at 694 nm is provided by targeted drug delivery into the eye, using biomaterials that are sensitive to such an emission (see Abdelmohsen HAM, Copeland NA, Hardy JG. Light-responsive biomaterials for ocular drug delivery. Drug Deliv Transl Res. 2023 Aug;13(8):2159-2182. doi: 10.1007 / s13346-022-01196-5. Epub 2022 Jun 24. PMID: 35751001; PMCID: PMC10315357). Among different ways of drug delivery in medicine and veterinary, the delivery of ophthalmological preparations is the most complex despite easy access to the eyes, and the eye protection by various barriers and protective mechanisms requires that the efficient drug delivery systems be developed. The drug loss is mainly caused by a high tear flow rate (0.5–32.2 μL / min), drainage of the lacrimal pathway (primary way of lachrymation), and reflectory tear secretion. Light is a promising method for controlling the drug release remotely, especially in ophthalmology. Exposing photosensitive materials to radiation results in various photophysical effects, including photochemical (for example, isomerization, decomposition, dimerization / polymerization, photosensibilization), and photothermal effects. Such systems are particularly interesting due to the possibility of precise space-time control, limited generation of by-products, convenience and ease of use. Light can be easily turned on and off, and the drug release rate can be optimized by controlling the emission intensity and duration, which enables precise control of the drug delivery for specific applications.

[0040] For the targeted drug delivery to the patient's eye, it is suggested to implement the following sequence of procedures (see):

[0041] - the photoactive particles 2 are incorporated from an aluminum (Al) based MAX-phase having a spherical form with the diameter of 20-60 nm, by using a contact lens as the base 1, wherein the lens is hydrated in an aqueous solution of such particles 2;

[0042] - the obtained contact lens is put onto the user's eye;

[0043] - photoactivated polymeric capsules with a biomaterial are introduced into the back part of the eye via systemic, intravitreal or periocular routes;

[0044] - the contact lens is illuminated tangentially by an external laser emission with the wavelength of 532 nm, the particles 2 reemit at the wavelength of 694 nm, the emission at the wavelength of 694 nm, in its turn, activates the biomaterial, which performs its therapeutic action on the targeted part of the eye.

[0045] The advantage of this method is that the uniform exposure to the 694 nm emission from the whole contact lens area results in the uniform activation of the drug over the whole required volume inside the eye. Furthermore, it should be noted that the ruby luminescence wavelength resides within the biological tissue spectral transparency window, i.e. so called NIR-I window, and will not be absorbed by biological tissues, while providing efficient activation of the biomaterial. Unlike the potential option of activating the biomaterial directly by ruby laser emission, the disclosed scheme has a number of advantages: the compact size of the 532 nm source as compared with a ruby laser, and uniform exposure of the back part of the eye to nanoparticles emitting at 694 nm. The remotely activated drug delivery system enables repeated on-demand dozing of the drugs, which will be adapted to the patient’s routine and enable delivering several doses at a time.

[0046] Another application of the disclosed contact lens is to use it as a sensor based on the Giant Raman Scattering (GRS provides 105–106times amplification of the peak intensity of a dye applied onto metal surfaces), which enables detection of a trace quantity of analyte in the lacrimal fluid. Experiments have shown (see Shevchuk, K., Sarycheva, A. & Gogotsi, Y. Evaluation of two-dimensional transition-metal carbides and carbonitrides (MXenes) for SERS substrates. MRS Bulletin 47, 545–554, 2022) that the Mo2TiC2MXenes demonstrate possibility to receive a signal from the Rodamine 6G molecules having the concentration approximately 10−8mol / L, which suggests a high potential efficiency of their use as part of GRS-sensors.

[0047] To form a GRS-sensor, the outer surface of the periphery of the base 1 in the form of a contact lens is covered with a suspension of the above described MAX-phase-based photoactive particles 2 (see) that have chemically reacted with a dye (Rodamin, Crystal violet, Metilen blue) by using a drop method, in the form of an area of 1–2 mm2. On top, the protective layer 3 is applied, for example, made from chitosan, which provides resistance to external influences, but allows the lacrimal fluid to pass through. The obtained contact lens it put onto the user's eye. Nearby, the device 9 is disposed, which operates confocally and is a source of emission with the wavelength ranging from 350 nm to 650 nm and a Raman signal receiver in the respective range (the spectra can be obtained and visualized using a portable Raman microspectrometer, which can be coupled via a fiber-optic input with an external camera of a personal computer, smartphone, etc.). The device 9 is disposed in such a way that, when the segment with the particles 2 is illuminated, the focused laser emission does not hit the user's pupil aperture. The receiver detects the Raman scattering intensity, which is used to determine the content (concentration) of analyte in the lacrimal fluid.

[0048] Sugar (glucose), cholesterol or other typical organic complexes can be selected as the analyte. For example, the sugar level in the lacrimal fluid is about 10−4mol / L, and can increase reaching approximately 10−3mol / L, which can be easily detected using the disclosed GRS-sensor. Such blood sugar level monitoring is noninvasive, painless, safe, causes no microtrauma, and can be used in domestic conditions.

[0049] Apart from using in a contact lens form-factor, the schematic diagram of the disclosed optical device can be used to form an integrated laser module. Ruby lasers are stable and efficient sources of emission at the wavelength of 694 nm, however their use in integrated photonics is complicated due to the necessity to grow the Al2O3crystal on a base. Such problems can be overcome by forming an optical device based on the photoactive particles 2 produced from a MAX-phase based on aluminum (Al). Such particles can be transferred onto any base 1 and demonstrate a narrow (up to 0.5 nm wide) peak of photoluminescence at the wavelength of 694 nm, which indicates the presence of not only the residual aluminum, but also Al2O3:Cr nanocrystals in a concentration sufficient for consistent observation of this peak at a relatively low pumping power (about 1834.4 W / cm2) at the wavelength of 532 nm.

[0050] The disclosed laser module can be produced as described below. The base 1 in the form of a flat base with the photon waveguides 7 based on a high refractive transparent material is coated, using a spin-coating (centrifugation) method, with the above described photoactive particles 2 (for example, Mo2TiC2) to form a layer covering the waveguide 7. Linking the outgoing emission and the waveguide mode is provided by direct adherence of the particles 2 material to the waveguide 7 core, as well as by sufficiently high Parcell factor due to high localization of the mode field in the high refractive core. To counteract destruction and degradation of the particles 2 layer, once the transfer has been completed, it is necessary to isolate it from the environment by means of the polymeric protective layer 3.

[0051] In the preferred embodiment, the design comprises a layer of the particles 2 that has the thickness from 1 µm to 30 µm and is disposed on the surface of the ring resonator 10 having the refractive index not lower than 2.4 (for example, GaP or TiO2), which is bonded with the output waveguide 7 whose one end is equipped with the Bragg grating based integrated reflector 11, which enables passing the emission through the output waveguide in one direction. At the same time, provided the resonator 10 has sufficiently high quality factor, it becomes possible to achieve the laser generation threshold at the wavelength of 694 nm, which opens a prospect to enhance energy efficiency of the disclosed laser module since the target emission into the output waveguide is amplified by the MXene layer, while the emission to the free space is not. Unlike conventional laser epitaxial heterostructures consisting of III–V type semiconductors having identical grating constant, the layered structure of the MAX-phase-based particles 2 is compatible with any bases and encapsulating layers.

[0052] The disclosed optical device in the form of a laser module is an integrated source of emission at the wavelength of 694 nm with optical pumping. The pumping is carried out by exposing to an external source of emission at the wavelength ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2(for example, using the light emitting diode 5), which focuses on the layer of particles 2. An alternative (non-laser based) method of generating the emission in the waveguide can be implemented using a grating coupler (see Cheng, L.; Mao, S.; Li, Z.; Han, Y.; Fu, H.Y. Grating Couplers on Silicon Photonics: Design Principles, Emerging Trends and Practical Issues. Micromachines 2020, 11, 666. https: / doi.org / 10.3390 / mi11070666), however, this will require precise positioning of the external source in relation to the base with the waveguide. The disclosed device has no such a disadvantage: the external pumping source can be disposed arbitrarily in relation to the device, while achieving that the emission spot is focused on the layer of particles 2.

[0053] Another option to use the disclosed optical device in practice is to use it as a temperature sensor. Experiments have proved that the photoluminescence peak intensity of the particles 2 at 694 nm when excited in the range from 350 nm to 650 nm (for example, using a laser with the wavelength of 532 nm) and with the intensity ranging from 0.5 mW / µm2to 100 mW / µm2demonstrates sensitivity to the change in temperature (see): this is reflected in the fact that the intensity of said peak reduces and a blue shift occurs as the temperature increases from 10 °С and up to 60 °С and returns to the previous values as the temperature decreases, i.e. reversibility is observed. Such a behavior of the particles 2 can be used to implement a contactless luminescent thermometry for moving and extremely small objects, as well as in the cases when physical contact is impossible. One of the key advantages of the disclosed temperature sensor is that the temperature can be measured within the physiological range (from 35 to 44 °C), which is essential for developing the methods of diagnostics and therapy, for example, of oncological diseases.

[0054] To produce a MAX-phase-based temperature sensor, a layer of photoactive particles 2 with the thickness from 1 µm to 30 µm should be uniformly applied (for example, using centrifugation) onto the surface of the object under observation or onto some other base 1, which will be acting as a sensor. To enhance the sensor durability, a passivating protective layer 3 is also formed, for example, as a transparent film. Photoluminescence is induced by the light emitting diodes and detected using the receiver 12 (for example, a photodiode) equipped with the narrow-band optical filter 13 (692–695 nm transmittance band) to separate the emission characteristic wavelength of 694 nm.

[0055] The design of such sensors can vary, but fundamentally, devices operating on a pass-through principle (see) and on a waveguide principle (see) can be distinguished.

[0056] In the scheme based on a pass-through principle (see), the exciting radiation from one or several light emitting diodes is delivered from below, the particles 2 reemit at the wavelength of 694 nm, this emission passes through the optical filter 13 and hits the receiver 12 installed above (on the opposite side from the base 1). When the MAX-phase-based particles 2 are heated up by 50 °С (from 10 °С to 60 °С), the intensity peak is shifted from 693.6 nm to 694.2 nm in a linear manner, and a change in its intensity occurs, which can be used to estimate the change in temperature of the base 1 and, consequently, of the test specimen.

[0057] Referring to the scheme based on a waveguide principle (see), the receiver 12 and the optical filter 13 are installed at the base 1 end face, which makes the device more compact.

[0058] Apart from implementing all kinds of optical devices, the discovered phenomenon of photoluminescence at 694 nm can be used to measure residual MAX-phase in MXenes, i.e., as a matter of fact, to verify their quality and to carry out incoming inspection of the materials in a laboratory or in production.

[0059] MXenes have become widely used in various areas, such as biomedicine, optoelectronics, environment protection, ecology, sensorics, catalysis, electromagnetic protection, solar cell panels, high-capacity batteries, energy storage units. The production of MXenes involves etching out aluminum or silicon of the MAX-phase. The MAX-phase synthesis most often utilizes aluminum (Al), as it is relatively easy to etch out using various mixtures of acids. However, in practice, it is impossible to completely etch out aluminum of the MAX-phase (having the chemical formula Mn+1AXn, where M is transition metals, A is aluminum, X is carbon or nitrogen, n = 1, 2 or 3), and the presence of residual aluminum results in technological complications when exfoliating MXenes into separate flakes, which are then used in various technical devices, for example, sensors, membrane filters, thin films, etc. Residual aluminum has strong bonds with titanium and carbon, and prevents exfoliation of MXene. Thus, determination of residual aluminum in MXenes is an important practical task.

[0060] As already stated above, when MXenes contain residual aluminum, the process of natural oxidation results in formation of Al2O3oxide, and residual chromium, which commonly occurs in the air in any laboratory, almost always intrudes into the composition during the laboratory synthesis. Even a small amount of chromium results in formation of Al2O3:Cr, which is the basis for ruby laser and has a narrow reemission line at 694 nm. When scanning with a laser beam with the wavelength ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2(for example, 532 nm or 633 nm), a narrow intensive band (half-width smaller than 1 nm) at 694 nm is excited in some segments over the MXene surface, whose presence can be used to estimate the presence of residual aluminum, i.e. the residual Al-based MAX-phase in MXenes (see: the band at 694 becomes clearly visible in the presence of oxidized residual aluminum).

[0061] Implementing the disclosed method includes the following successive steps:

[0062] - (step I) using a drop method, the MXene powder being examined is applied onto a base;

[0063] - (step II) the base with MXenes is exposed to the exciting radiation at the wavelength ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2;

[0064] (step III) output spectral characteristics are obtained by measuring the emission band intensity at 694 nm (using a Raman scattering spectrometer – in the range up to 5000 cm-1);

[0065] (step IV – computer-aided processing, not shown in the drawings) quantitative content of the residual MAX-phase in MXenes is determined using the calibration curve of the emission intensity versus Al concentration (the absence of the characteristic band in the spectrum indicates the high quality of MXenes).

[0066] Sensitivity of the disclosed method is comparable with sensitivity of X-ray phase analysis, however, the equipment that is used is far much faster, more compact, and more cost effective. Today, portable compact smartphone-size spectrometers are available, and the spectrum measurement takes less than a minute.

[0067] Thus, the disclosed group of inventions makes it possible to create relatively simple and efficient method for determining the MAX-phase derivatives and also to expand the range of technology that uses the MAX-phase-based materials.

Claims

An optical device comprising a transparent base 1 and photoactive particles 2, wherein said particles 2 are formed from the MAX-phase having the chemical formula Mn+1AXn, where M is transition metals, A is aluminum, X is carbon or nitrogen, n = 1, 2 or 3, wherein the photoactive particles 2 are formed with the size from 5 nm to 200 nm, and the content of residual aluminum in these particles 2 in relation to the initial content in the MAX-phase comprises not less than 1%.The optical device according to claim 1, wherein it is implemented as a contact lens.The optical device according to claim 2, wherein the contact lens body contains a built-in power source 6 and at least one light emitting diode 5 with the emission wavelength ranging from 350 nm to 650 nm disposed on the contact lens inner surface side and directed towards its external surface.The optical device according to claim 3, wherein each said light emitting diode 5 is disposed at the periphery of the contact lens whose body contains a built-in at least one waveguide 7 going from said light emitting diode 5 to the lens center where at least one diffractive optical element 8 is disposed that turns the emission from the waveguide 7 inside the eye, wherein said photoactive particles 2 are distributed inside the waveguide 7.The optical device according to claim 2, wherein said photoactive particles 2 are applied onto the peripheral part of the contact lens external surface as a layer with the thickness from 1 µm to 30 µm, onto which a protective layer 3 of chitosan having the thickness from 1 µm to 30 µm is applied.The optical device according to claim 1, wherein it is implemented as a laser module comprising a source of emission with the wavelength ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2, a ring photonic resonator 10 and an output waveguide 7, with the core formed from a material with the refractive index not lower than 2.4, wherein a layer of said particles having the thickness ranging from 1 µm to 30 µm is disposed between the source of emission and the ring resonator 10.The optical device according to claim 1, wherein it is implemented as a temperature sensor comprising a source of emission ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2, and a photodiode 12 equipped with a 694 nm band-pass filter 13, wherein a base 1 with a layer of said particles 2 having the thickness ranging from 1 µm to 30 µm is disposed between the light emitting diode 5 and the photodiode 12.A method for determining the residual MAX-phase having the chemical formula Mn+1AXn, where M is a transition metal, A is aluminum, X is carbon or nitrogen, n = 1, 2 or 3, in MXenes by exposing them to exciting radiation and obtaining the output spectral characteristics, wherein the exciting radiation that is used has the wavelength ranging from 350 nm to 650 nm and the intensity ranging from 0.5 mW / µm2to 100 mW / µm2, the output spectral characteristics are measured as the intensity of the emission band at 694 nm, and the amount of the residual MAX-phase in MXenes is determined from the intensity of said band.

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