SENSOR AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/023,463, filed on May 12, 2020, entitled “PHOTOTHERMAL SENSORY APPLICATION OF MXENE CONFINED ION CHANNELS,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to a photothermal sensory application of MXene confined ion channels, and more particularly, to a sensor that uses MXene-based sub-nanometer ion channels to convert external temperature changes into electrical signals via preferential diffusion of cations under thermal gradient within the ion channels.

DISCUSSION OF THE BACKGROUND

[0003] With recent advances in the nanofabrication technologies and the growing interest in bioinspired smart nanochannels, significant progresses have been made in the design and fabrication of artificial nanofluidic channels, mimicking tailored functions of biological ion channels [1, 2] For instance, human skins possessing various sensory receptors have inspired extensive research to develop nanofluidic sensory systems having fast and selective responses to different chemical and physical stimuli. Among various types of biological ion channels, thermosensitive transient receptor potential (thermo TRP) cation channels have been of interest in such novel nanofluidic applications as ionic sieving, energy storage and harvesting. The thermo TRP channels in the thermoreceptor cells of the skin convert external thermal stimuli into electrical signals associated with action potentials.

[0004] In earlier studies, temperature-sensitive gatekeepers such as elastin- like polypeptide loops or polyNIPAM brushes, which were grafted onto synthetic or protein pores, described various roles in controlling the ionic flow in response to temperature changes. Most recently, intrinsic nanochannels with ultra-confined pores and plenty of surface charges revealed their potential applications to a variety of external stimuli-response ionic sensory systems [3-6] When the confined space of the nanochannels is as small as the Debye screening length near charged walls, an overlapped electrical double layer (EDL) mostly composed of surface charge- compensating counterions can be created inside the nanoconfined channels, which enables charge-selective ion transportation in response to external stimuli [6] Especially under an electrochemical potential driven by temperature or salinity, such charge-selective ion transportation can serve as a driving force to generate an electrical potential. In light of this feature, stimuli-responsive artificial nanochannels with surface charges have been explored with a wide range of fluidic conduits such as perforated nanopores, nanoslits, or nacre-inspired lamellae nanochannels [6] However, the existing systems are not very sensitive to the thermal gradient that is usually encountered by the human or animal skin.

[0005] Thus, there is a need for a new sensor that is very sensitive to small thermal gradients and also is highly cationic selective for mimicking the human skin, and overcome some of the limitations of the existing systems.

BRIEF SUMMARY OF THE INVENTION

[0006] According to an embodiment, there is a photothermoelectric sensor for transforming photothermal energy into electrical energy. The sensor includes a housing having an internal chamber; an MXene membrane located inside the housing and positioned to divide the internal chamber into a first chamber and a second chamber; an electrolyte located in the internal chamber so that the electrolyte has a first salt concentration in the first chamber and a second salt concentration in the second chamber; a first electrode located in the first chamber; and a second electrode located in the second chamber. The MXene membrane has confined ion channels extending along a longitudinal axis X, and ions from the electrolyte in the second chamber move from a first largest face of the MXene membrane to a second largest face of the MXene membrane, along the confined ion channels, into the first chamber, to transform light energy into the electrical energy. [0007] According to another embodiment, there is a photothermoelectric sensor for transforming photothermal energy into electrical energy. The sensor includes a housing having an internal chamber; an MXene membrane located inside the housing and positioned to divide the internal chamber into a first chamber and a second chamber; an electrolyte located in the internal chamber so that the electrolyte has a first salt concentration in the first chamber and a second salt concentration in the second chamber; a first electrode located in the first chamber; and a second electrode located in the second chamber. The MXene membrane has confined ion channels extending along a longitudinal axis X, and ions from the electrolyte in the second chamber move from one end of the MXene membrane to an opposite end of the MXene membrane, along the confined ion channels, into the first chamber, to transform the photothermal energy into the electrical energy.

[0008] According to yet another embodiment, there is a method for sensing a thermal gradient due to light. The method includes heating with light a first side of an MXene membrane, wherein the MXene membrane has plural confined ionic channels; generating a diffusion of cations from an electrolyte located at a second side of the MXene membrane, to the first side of the MXene membrane, along the plural confined ionic channels, wherein the second side is opposite to the first side; and generating a voltage difference between the first and second sides of the MXene membrane. The MXene membrane is located inside a housing and positioned to divide the internal chamber into a first chamber and a second chamber, and the electrolyte is located in the internal chamber so that the electrolyte has a first salt concentration in the first chamber and a second salt concentration in the second chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0010] Figures 1A and 1B are schematic diagrams of a photothermoelectric application of an MXene membrane for transforming a temperature gradient into an electrical signal;

[0011] Figure 2 illustrates the X-ray diffraction of the lamellar T13C2T _X membrane with highly aligned interplanar spacing;

[0012] Figure 3 illustrates the photothermal conversion of the MXene membrane under sunlight illumination;

[0013] Figure 4 illustrates a photothermoelectric sensor that uses an MXene membrane for transforming a temperature gradient into an electrical signal;

[0014] Figure 5 shows the surface-charge-governed ion transport along the MXene membrane;

[0015] Figure 6 shows the current-voltage transport under salinity concentration gradient at neutral pH through the MXene membrane;

[0016] Figures 7A and 7B show the cation transference number and maximum osmotic power density, respectively, through the MXene channels;

[0017] Figure 8 shows PDMS-encapsulated MXene cation channels used as a photothermoelectric sensor; [0018] Figure 9 shows the temperature gradient and consequential photothermal voltages under elevated light intensity;

[0019] Figure 10 shows the proportional increment of temperature gradient at elevated light intensity;

[0020] Figure 11 shows the directionality of photothermal voltage depending on the irradiation position for the MXene membrane;

[0021] Figure 12 shows the static thermoelectric carrier transport for MXene membrane;

[0022] Figure 13 shows the opposite directionality in thermoelectric transport under thermal gradient for two different MXene membranes;

[0023] Figure 14 shows the Seebeck coefficient measurement of the Nb2CT lamellar membrane;

[0024] Figure 15 shows KCI concentration-dependent photothermal voltages for the MXene membrane;

[0025] Figure 16A shows photothermal voltages under KCI concentration gradient for the MXene membrane;

[0026] Figure 16B shows the temporal thermoelectric response under salt concentration gradient at incremental light intensity for a irradiation duration of 60 sec for the MXene membrane;

[0027] Figures 17A and 17B show a sensor having MXene lamellar membranes in contact with an electrolyte; [0028] Figures 18A and 18B show the thermal voltage generation under light with elevated intensity and the stable photothermal voltage in response to a constant temperature gradient;

[0029] Figure 19 shows a photothermoelectric sensor that uses an MXene membrane; and

[0030] Figure 20 is a method for generating an electrical signal from a thermal gradient using an MXene membrane.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a photothermoelectric sensor that uses an MXene membrane for generating an electrical voltage. However, the embodiments to be discussed next are not limited to such sensor, but may be applied to other sensors or systems.

[0032] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0033] According to an embodiment, a novel photothermal sensory application of a MXene nanoporous lamellae structure is used for mimicking the thermoelectric feature of the skin by taking advantage of the thermosensitive transient receptor potential (thermo TRP) cationic channels formed in the MXene structure. The thermo TRP channels in the thermoreceptor cells of the skin convert external thermal stimuli into electrical signals associated with action potentials. The cation channels of metal carbide and nitride (MXene) lamellar membranes are implemented in a sensor in pursuit of mimicking the thermoelectric feature of the skin. As the lamellar membranes provide densely interconnected and interplanar nanocapillaries with subnanometer dimensions, it is beneficial to detect collective motion of ions in response to electrochemical potential. Additionally, the nanoconfined channels (also called ionic confined channels herein) are configured as EDL, enabling permselective transport. The MXene-based 2D nanochannels have been increasingly utilized as an ion-selective conductor, due to their negatively-charged surface terminal groups (-0, -OH or -F) and well-defined structural confinement [7, 8]

[0034] In one embodiment, the ionic thermoelectric transport through MXene- based fluidic channels is investigated, especially under a temperature gradient created by light-driven heating. Coupled with the inherent photothermal conversion performance of the MXene materials, the sub-nanometer ionic channels in the MXene membranes exhibit thermo-osmotic cation flow and consequential thermoelectric response of up to 1.0 mV K ^-1 under local sunlight exposure. Even under a very small temperature difference below 1 K, the MXene channels generated a very sensitive thermoelectric response. Such non-isothermal voltage is found to be enhanced with increasing cationic permselectivity of channels, which is associated with ionic concentration or pH of permeant fluids. Furthermore, the MXene ion channels revealed stable, reproducible and reversible responses to temperature change under light illumination. Thus, in one embodiment, the MXene cation channels can be used to mimick the biological thermosensation process, which would open many potential applications, including temperature sensing, photodetection or photothermoelectric energy harvesting.

[0035] In this regard, Figures 1A and 1B schematically illustrate an MXene membrane 100 having plural lamellae 102-1 (with I being a whole number larger than 2, usually in the hundreds to millions) that extend in a XY plane (see Figure 1B), and are stacked along a Z direction. The lamellae are substantially parallel to the plane XY and they define plural interplanar nano-sized channels 104-J, where J is a whole number larger than 2. As shown in Figure 1A, two adjacent lamellae 102-1 and 102- 2 define a nano-sized ionic channel 104-J. As discussed later, a distance d (height of the channel) between two adjacent lamellae is smaller than 100 nm, even smaller than 1 nm. Various ions 110 (K ions in the figures) are moving through the nanosized ionic channels 104-J, demonstrating the ionic thermoelectric voltage generation through the confined ion channels under local light 120 irradiation-driven temperature gradient.

[0036] The transverse fluidic ionic channels 104-J in the lamellar MXene membrane 100 offer the cation-exchange features enabled by the EDL inside the nanoconfined channels. In this regard, note that negative charges 112, usually electrons, pave the external surfaces of the lamellae 102-1, as shown in Figure 1A, thus, enabling the double EDL for each nano-sized ionic channel 104-J. When the channel 104-J’s height d is comparable to the electrostatic screening length given by the Debye length (e.g., between twice the Debye length and half the Debye length), the EDL formed on each interplane can overlap, enabling the strong cation-selective passage under chemical potentials that are generated by the reservoirs 130 and 132, which are located at the ends of the membrane 100. Note that reservoirs 130 and 132 may include the same or different salt based fluids. In one application, the salt based fluid is KCI. The reservoirs and the membrane may be placed inside a housing 106. In this or another embodiment, the reservoir 130 is heated with the light 120, so that its temperature is higher than the reservoir 132. For this reason, the reservoir 130 is called herein the hot reservoir and the reservoir 132 is called the cold reservoir. In one application, the hot reservoir has a different K (salt) concentration than the cold reservoir, for example, the hot reservoir has a higher salt concentration than the cold reservoir. The opposite is also possible.

[0037] When the nano-sized channel 104-J is stimulated by localized thermal excitation 120, a thermochemical potential of permeating ions 110 is created along the temperature gradient, and consequentially, a thermo-osmotic ion transport directed from the cold reservoir 132 to the hot reservoir 130 can develop across the stimulated ionic channels 104-J [3] In principle, an electric charge transport takes place in a direction producing more entropy, and more specifically, heat diffuses from the high temperature to the low temperature, whereas the confined permeant 110 flows against the thermal gradient. Aiming to create such thermodynamic conditions under light illumination, the MXene lamellar membrane 100 is exploited herein, offering outstanding photothermal conversion performance as well as plenty of surface charge groups accommodating hydrated cations.

[0038] As a prerequisite for the ion-selective passage, the nanoconfined interplanar ionic channel 104-J present in the lamellar MXene membrane 100 was verified by the X-ray diffraction analysis of fully hydrated T C2T _x lamellar membranes. Figure 2 shows the incremental increase of the fully hydrated interlayer spacing d by 4 A, from 1.27 nm, for ambient-state membranes. This volumetric expansion in hydration is strongly correlated with accumulated water molecules between lamellae ionic channels 104-J. Taking into account its theoretical monolayer thickness of ~ 0.98 nm, the effective interplanar height d for the fluidic motions is estimated to be ~ 0.69 nm, which is large enough for hydrated small ions such as K ⁺ or Na ⁺ to pass through. The effective transport area of a single nano-sized channel 104-J (i.e., cross-section area in the XZ plane in Figure 1B) is estimated to be 3.27 E ⁰⁹ m ² with the porosity of around 41.7 % for a 2 pm-thick membrane 100. The lamellar structure was confirmed from the cross-section areal scanning electron microscopy image as shown in the inset of Figure 2.

[0039] The high optical absorptivity of the MXene membrane 100, which is essential for photothermal heating, was verified from its rapid temperature changes under light, which is further supported by the lower diffuse reflectance of T13C2T _X films compared to that of cellulose acetate, as shown in Figure 3. Figure 3 plots on the Y axis the temperature of the membrane while it plots on the X axis the time in seconds during which the thermal irradiation is on.

[0040] To investigate the charge-selective ion transport through the MXene nanoconfined ionic channels 104-J, the transverse ion flux was measured under an electric potential as well as under a salt concentration gradient, based on the system 400 illustrated in Figure 4. Figure 4 shows that the MXene membrane 100 was encapsulated by polydimethylsiloxane (PDMS) layers 402 and 404, and the ends of the membrane 100 were placed in the hot and cold reservoirs 130 and 132, respectively. The entire system was placed in the housing 106. The housing 106 has an internal chamber 108, which is divided by the membrane 100 into a first chamber 108A and a second chamber 108B. The first chamber 108A corresponds to the first reservoir 130 while the second chamber 108B corresponds to the second reservoir 132. The longitudinal axis X of the membrane extends from the first chamber 108A to the second chamber 108B. Thus, the ions 110 move along the confined ionic channels 104- J , from one reservoir to the other reservoir, when heat is provided to one of the first and second chambers 108A and 108B. Note that the ionic channels 104-J also extend along the longitudinal axis X. The reservoirs include a same electrolyte, e.g., salt 109, including ions 110 having different concentrations, for example, a higher concentration in the first reservoir 130 than in the second reservoir 132. Note that both phases (liquid and solid) of the electrolyte may be used. The salt 109 can be KCI. In this embodiment, the ions 110 travel along the confined ionic channels 104-J, along the longitudinal axis X of the membrane 100. The width, height, and length of a singular two-dimensional channel 104-J are approximately 3 mm, 1 nm or smaller (e.g., 0.695 nm), and 10 mm, respectively. Unless otherwise mentioned, all the ion transport measurements were conducted using a membrane thickness of 2 pm. A source electrode 410 was placed in the hot reservoir 130 and a ground electrode 412 was placed in the cold reservoir 132. The two electrodes 410 and 412 were connected to an electrometer for measuring the current produced through the membrane 100. The electrodes may be Ag/AgCI electrodes. Figure 4 also shows the electrical circuit corresponding to the system 400, with the membrane 100 acting as a power source having a resistance Rch. [0041] As shown in Figure 5, the ionic conductance (G _channel ) 500 of the membrane 100 deviates from the bulk transport curve 510 below KCI 1 mol-L ¹ , confirming the dominant surface charge effect at lower salt concentration. Furthermore, a scaling behaviour is observed at low salt concentrations, which is caused by salt concentration-dependent variation of surface charges as previously reported. The current-voltage ( IV) transport was also investigated under a salt concentration gradient (C _higt jQ _ow ) of 10 ³ to 1 at a fixed <¾ _w of 10 ^-4 mol-L· ¹ , as shown in Figure 6. All the IV measurements were carried out at room temperature and neutral pH. To avoid redox potentials ( E _redox ) arising from the electrodes in the different salt concentrations, salt bridge Ag/AgCI electrodes were used to the measurement. A direction of the short circuit current {I _dirr ) in absence of bias is consistent with a net flow of positive charges, and this charge-selective osmotic flow produces an open- circuit voltage {E _diff ) across the membrane 100. With increasing the salt concentration gradient between the hot and cold reservoirs 130 and 132, the IV curves show obvious increment in both l _diff and E _diff . A cation transference number (t ₊ ), calculated by the equation: t ₊ = 0.5 (1 + E _diff /E _redox ), approaches 0.97 under a 10-fold difference in the salt concentration, nearly close to unity cation selectivity, as shown in Figure 7A. Note that the transference number is defined as the fraction of the current carried either by the anion or the cation to the total electric current. The higher transference number under the 10-fold difference is attributed to the stronger cationic selectivity observed in the lower concentration regime. The salinity gradient- induced osmotic power, as illustrated in Figure 7B, further supports their charge- selective ion transport through the confined channels 104-J with negative surface charges 112.

[0042] To introduce a thermochemical potential across the fluidic channels 104-J, a localized photothermal heating was applied by the source 120, creating an axial temperature gradient along the longitudinal axis X, as illustrated in Figure 8.

The array of transverse ionic channels 104-J generates a parallel flux of electrical double layers under thermal potential, and their directionality is strongly dependent on the light exposure position. The ionic thermoelectric response under light 120 is characterized with the ionic Seebeck coefficient calculated from an open-circuit voltage through ionic channels by a given temperature difference. Due to the encapsulated structure by PDMS layers 402 and 404, interferential factors influencing the thermoelectric response could be ruled out, such as thermal evaporative cooling and derived fluidic interferences. As the applied PDMS layers 402 and 404 are optically transparent and nearly thermally insulating, the optical scattering under light irradiation or thermal dissipation is minimized.

[0043] For a light intensity spanning a range of 74 to 127 mW-cnr ² , the temperature gradient and derived photothermal voltages were investigated across the MXene channels 104-J, as shown in Figures 9 and 10. Figure 9 shows that with increasing the light intensity (on the X axis), a proportional photothermal response is observed, yielding positive ionic Seebeck coefficient. The thermal voltage shows a proportional increase with the light intensity while sustaining its polarity. The magnitude of the temperature gradient is dependent on the light intensity (see Figure 10), but irrespective of the light illumination position (see Figure 9). Note that the applied light intensity in Figure 9 is turned off before applying a larger light intensity, which explains the fall of the temperature difference and change in voltage in Figure 9 between the application of increasing light intensities. Moreover, the spontaneous heating effect under the light stimulation could be observed. When the MXene ionic channels 104-J are locally irradiated by simulated sunlight, the temporal temperature changes at designated positions exhibit a stable thermal flux from the hot to the cold reservoirs (see Figure 10), in accordance with Fourier’s law of heat conduction. This conductive thermal feature drives a stable temperature difference across the interlayer water in the transverse cation channels. Furthermore, the prolonged light illumination over 60 sec avoids provoking an osmotic ionic diffusion against the photothermal flux.

[0044] With the MXene cation channels 104-J of the membrane 100, even temperature differences under 1 K could be thermoelectrical ly converted to the electrical signals, as shown in Figure 11. The polarity and magnitude of the responsive voltages depends on the irradiation position and light intensity. As a result, the ionic Seebeck coefficient of the MXene T13C2T _X cation channels 104-J is in the range of 0.6 to 0.9 mV-K ^-1 and it depends on the averaged temperature of the irradiated MXene channels. The observed dependence possibly involves thermal enhancement of ionic mobilities. It is noted that, according to recent reports, a photoelectric effect giving rise to asymmetric charge distribution can drive a directional and charge-selective ionic flow for charge compensation. In such systems associated with wide band-gap semiconducting materials, the ionic flux directly correlates with concomitant electric potential built by a photoinduced charge separation.

[0045] To understand the effects of optoelectronic responses of conduits on the permeating ionic flux, a MXene Nb2CT _x membrane 1200 showing different thermoelectric feature is comparatively investigated, as illustrated in Figure 12. The Nb2CT _x membrane 1200 exhibits an identical lamellar configuration as the T13C2T _X membrane 100, and the hydrated interlayer spacing is verified to be below 1.3 nm, for example, around 1.278 nm, with highly oriented structures. Furthermore, the Nb2CT _x membrane 1200 demonstrate comparable optical absorptivity to that of the T13C2T _X membrane 100. Static thermoelectric measurements of both T13C2T _X and Nb2CT _x , reveal opposite polarity under a temperature gradient, as illustrated in Figure 13. Given that the MXene T13C2T _X membrane behaves as a metal-like conductor, such contrasting response from the Nb2CT _x membrane suggests the dominant presence of the hole charge carriers. The majority carriers in the Nb2CTx membrane 1200 could be reconfirmed by Hall effect measurement as well as Seebeck coefficient measurement. However, under light illumination, the Nb2CT _x ionic channels 104-J show no discrepancy in magnitude and polarity of the ionic Seebeck coefficient when compared to those of the T13C2T _X membrane 100. Furthermore, both species of T13C2T _X and Nb2CT _x membranes exhibit the same directional flux of cations even under direct joule heating-induced temperature gradient, as illustrated in Figure 14. Hence, it is believed that the photothermal potential along the MXene ionic channels 104-J is dominantly governed by temperature gradient-derived chemical potential difference. [0046] The charge selectivity of the ionic channels 104-J play a large role in the regulation of trans-nanochannel photothermal potential ($ _m ) under temperature difference, according to previously suggested non-isothermal ion transport models given by: where R is the gas constant, F is the Faraday constant, and and c _t are, respectively, the temperature and the salt concentration at the hot and cold reservoirs. Due to a small temperature change, a contribution of the thermophoretic ion transport to the potential is assumed to be negligibly limited. To experimentally discern the relationship between cation permselectivity and ionic Seebeck coefficient, the inventors explored the photothermal voltages at various salt concentrations and under regulated pH.

[0047] The ionic Seebeck coefficient at different salt concentrations shows a strong dependence on the salt concentration, as illustrated in Figure 15. As the salinity decreases at equimolar state, the enhanced cation flux over the anion resulted in a proportional increment of the ionic Seebeck coefficient, reaching up to 1.0 mV-K T Such molarity-dependency is comparable to those from commercially available cation exchange membranes. Furthermore, a suppressed photothermal voltage is observed at acidic condition of pH 3.34. When the pH is decreased, the protonation of the terminal groups leads to reduced negative surface charges, accumulating less counterions on individual MXene nanosheets. As a result, the fewer cations 110 inside the ionic channels 104-J and the associated decrease of ionic selectivity constrain the preferential flux of cations, yielding a lower ionic Seebeck coefficient. Such pH-dependent thermoelectric response is consistent with the retarded ionic conductance.

[0048] The ionic thermoelectric response is further investigated in the presence of an electrolyte concentration gradient as shown in Figures 16A and 16B. When the light is irradiated on the membrane 100’s end located at the diluted side (ciow), as shown by the inset of Figure 16A, the thermo-osmotic ionic flux 1610 from cold to hot is aligned to the direction of self-diffusive ion flux 1620, as also shown in the inset. In this embodiment, the Chigh is 0.5 molL ¹ and ci _ow is 10 ^-2 molL ¹ . The estimated ionic Seebeck coefficient from the interpolated output voltages is averaged to be 0.81 mV K ¹ while showing a slight temperature-dependent increase. The observed higher ionic Seebeck coefficient under the salt gradient, taking the applied higher concentration into account, is most likely due to self-diffusion-driven, lowered energy barrier cation passage. The thermoelectric response under the salt concentration gradient was stable and reproducible for at least a 1000 sec time scale. The temporal thermoelectric response under the salt concentrations noted above at incremental light intensity for an irradiation of 60 sec. are shown in Figure 16B.

[0049] In one embodiment, as illustrated in Figures 17A and 17B, a photothermo-sensation system 1700 with a large-areal transmembrane in contact with an ionic gel electrolyte 1710 is introduced. The system 1700 may include the MXene membrane 100 or 1200, which is placed into an enclosure 1702. The membrane 100/1200 is placed inside the enclosure 1702 so that it divides an internal chamber 1710 into two chambers, a first chamber 1710A and a second chamber 171 OB. The two chambers 1710A and 171 OB correspond to the two reservoirs 130 and 132 in Figure 4. Each chamber is filled with the ionic gel electrolyte 1720. The enclosure 1702 may have a top portion open. This open part may be closed with a photothermal material layer 1730, so that the electrolyte 1720 cannot escape from the internal chamber 1710. When light 120 is impinging on the photothermal layer 1730, this material is configured to transform the light into heat. Thus, under light illumination, a stable heat transfer through the electrolyte 1720, which in this embodiment is a quasi-solid state gel, creates a stable temperature gradient across the membrane 100/1200. Note that as illustrated in Figure 17B, the longitudinal axis X of the membrane does not coincide with the thermal gradient direction as in the embodiment of Figures 4 or 8. In this embodiment, the thermal gradient direction is along axis Y, which is perpendicular to the axis X. This means that the ions 110 have an overall move perpendicular to the largest surface of the membrane, although the ions 110 move through the plural parallel channels 104-J that are oriented along the axis X. In other words, after the ions 110 move along a given channel 104-J, the ions move to a next channel 104-(J+1), which is parallel to the channel 104-J, then they move along this new channel, and then again move to another parallel channel, until arriving at the largest surface of the membrane 100/1200. Thus, in this embodiment, the entire largest surface of the membrane is heated, not only one end as in Figure 4. However, the opposite largest surface of the membrane is not heated, and this corresponds to the cold reservoir. The device 1700, despite a limited temperature change across the membrane 100/1200, displays a light intensity-sensitive response as well as reproducible thermal voltages, as illustrated by the measurements of Figures 18A and 18B.

[0050] Figure 19 shows more details about the device 1700. In this embodiment, the electrolyte 1720 is Agarose gel 3w/v% KCL 0.1 mol/L, the enclosure 1702 is made of PDMS, the photothermal material layer 1730 is a carbon nanotube-polyvinyl alcohol composite, the source and ground electrodes are made of Ag/AgCI, the membrane 100 is T13C2T _X , the entire enclosure 1702 is formed on a PET film 174, a distance D between the bottom part 1702A of the enclosure 1702 and the membrane 100 is about 3-4 mm, and a width Wof the chamber 1710 is about 10 mm. Other sizes may be used for the device 1700. In this embodiment, the housing 1702 is made of plural parts 1702A and 1702B, and the membrane 100/1200 is placed over a lower part 1702A and the upper part 1702B is placed over the lower part 1702A and over the membrane 100/1200 so that the membrane is physically sandwiched between the lower and upper parts. The lower and upper parts 1702A and 1702B may be glued together with an adhesive 1740. Other means may be used for holding together the various parts of the housing 1702. In one embodiment, the membrane may be glued to the walls of the housing and the housing may be made of a single part. Various other methods for placing the membrane inside the housing may be used.

[0051] The T13C2T x membrane 100 and the Nb2CT _x membrane 1200 can be synthesized as now discussed. Note that both types of MXene materials can be employed for realizing the thermosensory channels. However, all types of MXene species, not only being photothermally active, but also forming easily lamellae structure, can be applied for the proposed applications. The MAX phase T13AIC2 etchant was prepared by a method in which 1 g lithium fluoride (LiF 98 %) was slowly added into 20 ml. 9 mol L ^-1 hydrochloric (HCI 35-38 %) and stirred at 500 rpm for 5 min. The 1g T13AIC2 powder (98 wt.%, 400 mesh) was immersed in as-prepared etching solution at 40 °C for 24 h. The slurry was centrifuged and rinsed several times using deionized water until the pH of the supernatant is about 6. Then, the sediment was intercalated by 10 ml 1 mol-L ¹ tetramethylammonium hydroxide (TMAOH) at room temperature for 24 h and centrifuged three times at 11 ,000 rpm for 10 min. After intercalation, the dark green T13C2T _X MXene suspension can be collected after 5 min centrifugation at 3500 rpm using deionized water. The experiment procedure for the synthesis of Nb2CT _x MXene is the same as above, except that the raw material Nb2AIC was etched by 20 ml 49 % hydrofluoric acid. The lamellar MXene membranes were fabricated by filtering specific amounts of MXene dispersion through a polyvinylidene fluoride (PVDF) membrane (0.22 pm pore size and a diameter of 43 mm). All filtrated membranes were ambient-dried overnight and could be easily detached from the support.

[0052] The above discussed embodiments used sub-nanometer confined ionic channels in the lamellar MXene membranes as a bionic thermo-sensation platform. The nanoconfined channels covered by electrical double layer lead to highly charge- selective ion diffusion under the electrochemical potential. Coupled with the inherent photothermal properties of MXenes, the cation-selective transport by localized photothermal stimuli is converted to a trans-nanochannel diffusion potentials, corresponding to open-circuit voltage over temperature gradient. A thermoelectric response of up to 1.0 mV-K ^-1 was obtained under one sunlight illumination, comparable to that of biological thermosensory channels. Such thermoelectric responses show not only a dependency on the cationic permselectivity of the ionic nanochannels, as anticipated from the theoretical consideration, but also its high sensitivity even under very small thermal gradients, e.g., < 1 K. Moreover, the thermoelectric response is stable and reproducible in the absence and presence of the ionic concentration gradient. Thus, the devices in these embodiments can be used as photo-thermo-osmotic ion transport devices, i.e., the application of MXene membranes in the biomimetic sensory systems.

[0053] A method for sensing a thermal gradient due to light is now discussed with regard to Figure 20. The method includes a step 2000 of heating a first side of an MXene membrane, wherein the MXene membrane has plural confined ionic channels, a step 2002 of generating a diffusion of cations from an electrolyte located at the first side of the MXene membrane, to a second side of the MXene membrane, which is opposite to the first side, along the plural confined ionic channels, and a step 2004 of generating a voltage difference between the first and second sides of the MXene membrane. The MXene membrane is located inside a housing and positioned to divide the internal chamber into a first chamber and a second chamber. The electrolyte is located in the internal chamber so that the electrolyte has a first salt concentration in the first chamber and a second salt concentration in the second chamber. In one application, the first salt concentration is higher than the second salt concentration. The confined ion channel has a height smaller than 1 nm. [0054] The disclosed embodiments provide a photothermoelectric MXene based sensor that is capable of detecting small external temperature changes and generate a corresponding electrical current. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0055] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. [0056] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims. References

The entire content of all the publications listed herein is incorporated by reference in this patent application.

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