Bionic Thermosensation Inspired Temperature Gradient Sensor Based on Covalent Organic Framework Nanofluidic Membrane with Ultrahigh Sensitivity

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021Bionic Thermosensation Inspired Temperature Gradient Sensor Based on Covalent Organic Framework Nanofluidic Membrane with Ultrahigh Sensitivity Weipeng Xian†, Pengcheng Zhang†, Changjia Zhu, Xiuhui Zuo, Shengqian Ma and Qi Sun Weipeng Xian† Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 †W. Xian and P. Zhang contributed equally to this work.Google Scholar More articles by this author , Pengcheng Zhang† Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 †W. Xian and P. Zhang contributed equally to this work.Google Scholar More articles by this author , Changjia Zhu Department of Chemistry, University of North Texas, Denton, TX 76201 Google Scholar More articles by this author , Xiuhui Zuo Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 Google Scholar More articles by this author , Shengqian Ma *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, University of North Texas, Denton, TX 76201 Google Scholar More articles by this author and Qi Sun *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101125 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The ability to precisely monitor temperature at a high resolution is an important task, particularly in terms of safety. Inspired by natural thermosensitive transient receptor potential cation channels, we developed a temperature sensor based on thermal-driven ionic charge separation. To mimic the function of nature, an ionic covalent organic framework-based nanofluidic membrane was fabricated. By engineering the membrane to separate two electrolyte solutions, the temperature difference across the membrane can synchronously induce a potential. The high charge density and narrow channel size render extraordinary permselectivity to the membrane, thus offering a thermosensation selectivity of up to 1.25 mV K−1, superior to that of any known natural system. Additionally, the generated potential is linearly related to the introduced temperature gradient, thus allowing for precise detection. With these attributes, an alarm device with high thermosensation sensitivity was constructed, demonstrating great promise for environmental temperature monitoring. Download figure Download PowerPoint Introduction Temperature is one of the fundamental parameters in science and engineering.1–3 Respective sensors are ubiquitous in daily life and in almost all industrial processes. It is estimated that temperature sensors occupy ∼80% of the worldwide sensor market.4–6 The physicochemical underpinnings of the current thermal sensation systems include (1) thermometers based on the thermal expansion of encased liquids, (2) optical sensors based on thermochromic color switching, and (3) thermocouples based on the Seebeck effect. Given the synchronization and precision of thermoelectric conversion, thermocouples have become the most widely used industrial sensors.7 However, the tremendous impact of the electromagnetic field on the movement of electrons and the directionality of temperature measurement (from hot to cold ends) limit their wide applications, thus calling for advanced techniques and materials for monitoring temperature. Nature is the source of principle and functional innovation, inspiring scientists to develop sophisticated functional systems. Adaptive evolution has resulted in the development of specialized temperature-sensing mechanisms, enabling organisms to perceive subtle temperature stimuli rapidly. In mammals, this role is performed by a specialized family of membrane proteins containing transient receptor potential (TRP) channels, whose gating is sensitive to temperature variation.8,9 The molecular basis of sensory perception is the thermal-driven ionic charge separation that ultimately results in an electrochemical potential difference. However, the fragility of the lipid bilayer diminishes the prospects for developing practical sensors using this technology. Recent significant theoretical and experimental advances in the nanofluidic field have enabled the replication of the functions of biological channels,10–20 providing a potential opportunity to generate a new type of thermosensation device.21–31 However, effectively regulating the thermophoretic mobilities of cations and anions over a wide concentration range of salt solutions remains a challenge. In addition, thermoelectric voltage under confinement is positively proportional to the surface charge density of the channel when the pore diameter is smaller or comparable with the thickness of the electric double layer (EDL).32 This set of prerequisites necessitates the high-precision nanoscale design of ion channels from materials exhibiting high intrinsic charge density. Covalent organic frameworks (COFs) are regular open matrices linked by organic struts.33–45 In two-dimensional (2D) COFs, the layered sheets are stacked face-to-face in the third dimension, forming continuous transport channels. With such stunning structures, these materials have received considerable attention in the material community. The practicality of COFs in comparison with other materials for the design of nanofluidic membranes comes from their purely organic composition, which provides stability. Additionally, the monomer design can be fine-tuned due to the endless number of organic reactions at a chemist’s disposal, resulting in a material that retains its robustness while providing a large number of available functional groups in nanospaces.46–54 In this contribution, we demonstrate that COFs possess all the necessary traits for the design of high-performance thermosensation devices based on thermal-driven ionic charge separation. To address this challenging topic, a carboxylic acid-functionalized COF membrane (COF-COOH) was fabricated by pairing 1,3,5-triformylphloroglucinol (Tp) and 2,5-diaminobenzoic acid (Dba). Owing to its ultrasmall one-dimensional channel size (1.2 nm) and highly negatively charged surface, the resulting membrane was highly permselectivity, screening the transport of anions while favoring that of cations, thereby leading to significant transmembrane potentials in response to temperature gradients with a sensitivity of up to 1.25 mV K−1 (Figure 1). In addition, the thermoelectric response was rapid, stable, reproducible, and reversible, thus providing an alternative option for constructing a temperature monitoring system. Figure 1 | Schematic illustration of the thermoelectric conversion principle based on temperature gradient triggered transmembrane ionic charge separation. Download figure Download PowerPoint Experimental Methods Fabrication of the free-standing COF-COOH membrane The free-standing COF-COOH membrane was synthesized via interface condensation of Tp and Dba. The Dba (36.0 mg, 0.236 mmol) dispersed in a p-toluene sulfonic acid (TsOH, 89.8 mg, 0.472 mmol) aqueous phase (20 mL) was gently placed on top of the Tp (33.1 mg, 0.157 mmol) dichloromethane (20 mL) solution. The system was kept at 35 °C for 3 days. The free-standing COF-COOH was obtained as a brown powder after being washed thoroughly with water and ethanol in sequence and then dried under vacuum for further characterization. Fabrication of COF-COOH/PAN COF active layers were formed via interface polymerization on the surface of an asymmetric polyacrylonitrile (PAN) ultrafiltration membrane. The PAN support was vertically placed in the middle of a homemade diffusion cell, resulting in a volume of 7 cm3 in each cell ( Supporting Information Figure S1). An aqueous solution of Dba (12.6 mg, 0.083 mmol) and TsOH (31.5 mg, 0.165 mmol) (7 mL) and the CH2Cl2 solution (7 mL) of Tp (11.6 mg, 0.055 mmol) were separately introduced into the two sides of the diffusion cell. The reaction mixture was kept at 35 °C for 3 days. The resulting membrane was rinsed with methanol to remove any residual monomers and catalyst. Finally, each membrane was rinsed with water for 24 h and then used for tests or air-dried for physicochemical characterization. Results and Discussion Nanofluidic membrane fabrication and characterization The free-standing COF-COOH membrane was synthesized by interfacial polymerization, as depicted in Figure 2a, whereby the Dba dispersed in the TsOH aqueous phase was gently placed on top of the Tp dichloromethane solution (Figure 2b). The structure and crystallinity of the resulting COF membrane were scrutinized by powder X-ray diffraction (PXRD). The diffraction peak of 4.54° at 2θ is ascribed to the (100) plane of COF-COOH. The appearance of a characteristic peak of 26.6°at 2θ, attributable to the (001) plane, suggests the preferential planar orientation of the 2D sheets in the membrane ( Supporting Information Figure S2). The experimental PXRD profile displays only limited deviations from the eclipsed stacking mode (Figure 2c and Supporting Information Table S1). The intensity of the diffraction peaks remained unchanged after treatment with 3M KCl, indicating the excellent salinity tolerance of COF-COOH, which is critical for using in a wide concentration range of salt solutions ( Supporting Information Figure S2). The permanent porosity and pore size of the COF-COOH membrane were evaluated by N2 sorption isotherms, which exhibited typical type-I reversible isotherms, characteristic of a microporous structure ( Supporting Information Figure S3). The Brunauer–Emmett–Teller (BET) surface area was estimated to be 399 m2 g−1. Fitting of the sorption isotherms using the nonlocal density functional theory (NLDFT) model resulted in a pore size distribution centered at 1.2 nm, in conformity with the predicted value of the eclipsed stacking model. The zeta potential of COF-COOH is −11.6 ± 2.4, validating its highly negatively charged surface. Figure 2 | Fabrication of membrane. (a) Schematic illustration of the free-standing COF-COOH membrane by the interfacial polymerization of Tp and Dba in the presence of p-toluene sulfonic acid. (b) Synthetic scheme of COF-COOH. (c) Graphic views of the eclipsed stacking structure of COF-COOH (blue: N; gray: C; red: O; white: H). Download figure Download PowerPoint To increase the operability and stability of the COF membrane, we grew the COF layer on PAN ultrafiltration membranes. PAN was selected because of its flexibility and hydrophilicity, which is not only easy to handle but also highly permeable to ions. To control the COF active layers exclusively on the PAN support, the Tp-dichloromethane solution and the Dba-TsOH aqueous solution were separately introduced into a homemade diffusion cell segregated by PAN into two chambers ( Supporting Information Figure S1). Over 3 days, a vermilion film was formed on PAN facing the organic phase (COF-COOH/PAN, Supporting Information Figure S4). Primary characterization of the resulting membrane was thoroughly performed prior to the thermoelectric response study. The Fourier-transform infrared (FT-IR) spectrum of COF-COOH/PAN revealed that the adsorption features of –NH2 (∼3300 cm−1) and –CHO (∼2880 and ∼1650 cm−1) for the monomers disappeared with concomitant emergence of a peak for C=C at 1562 cm−1, indicating a high condensation degree of the membrane and no monomer being trapped in the pore channel ( Supporting Information Figure S5).55 Scanning electron microscopy (SEM) images revealed continuous film surfaces that contoured the underlying PAN support with a thickness of approximately 220 nm ( Supporting Information Figures S6 and S7). Contact angle measurements revealed that COF-COOH/PAN was hydrophilic with a water contact angle of 52 ± 4° ( Supporting Information Figure S8). Dye molecule exclusion tests indicated that the COF active layer is porous but without large defects because the negatively charged thymol blue, with size (1.0 × 1.1 nm), penetrated the membrane, whereas methyl blue, with a greater size (1.5 × 2.0 nm), was fully withheld (see details in the Experimental Section in the Supporting Information Figure S9). Ion permselectivity evaluation Encouraged by these characterization results, we next investigated the ion transport properties of COF-COOH/PAN. Given the similar bulk diffusivities of K+ and Cl−, KCl was the electrolyte of choice (unbuffered KCl was used unless otherwise specified). COF-COOH/PAN was located between the two chambers of a conductivity cell. The transmembrane current was recorded using symmetric KCl solutions with concentrations ranging from 0.01 mM to 3 M ( Supporting Information Figure S10). The variation of the normalized transmembrane conductance derived from the slope of the current–voltage (I–V) curve versus KCl concentrations confirmed the nanoscale channels and the absence of KCl leakage through large cracks (Figure 3a). Ions were transported across COF-COOH/PAN in a fashion parallel to that in the bulk electrolyte at high electrolyte concentrations (>100 mM). At lower ionic strengths (<100 mM), the ion conductivity of COF-COOH/PAN deviated from the bulk behavior and demonstrated saturation, which is typical of surface-charge-governed ionic transport. Under such circumstances, the electrostatic force of the overlapped EDL could screen ions, repel co-ions, and attract counter-ions. Moreover, considering that the surface-charge-governed regime of COF-COOH/PAN extended above 100 mM, under which the Debye screening lengths was around 1 nm ( Supporting Information Table S2), we can therefore conclude that the pore size of the membrane is smaller than 2 nm. These results further validate that there are no big cracks in the membrane.56 Figure 3 | Investigation of transmembrane ion transport. (a) Measured steady-state ionic conductance of COF-COOH/PAN, as well as a bulk prediction for KCl solutions (dashed line). The bulk conductivity represents the conditions where the effect of the surface charge can be neglected. (b) I–V curves recorded under various KCl concentration gradients separated by COF-COOH/PAN. (c) Numerical simulation of the distributions of K+ (solid line) and Cl− (dashed line) with various concentration gradients (the left side was fixed at 0.1 mM, while the right side increased from 0.1 mM to 1 M) inside the nanochannels of COF-COOH. Download figure Download PowerPoint To quantitatively evaluate the permselectivity of COF-COOH/PAN, its reversal potentials (Vr) under various KCl concentration gradients were evaluated. To do so, COF-COOH/PAN was mounted between asymmetric KCl solutions, with one side facing the COF active layer, denoted as the cis side, and the other side named as the trans side. The x-intercepts (Vr) of the I–V plots exhibited average reversal potentials (three times) of –58.4, –112.4, and –109.6 mV for cis/trans = 1.0/0.1, 10/0.1, and 100/0.1 mM KCl, respectively (Figure 3b). The permeability ratios of K+/Cl− calculated based on the Goldman equation (eq 1) were 449, 416, and 78, respectively, confirming the preferential passage of cations over a wide range of concentration gradients.57 P K + P Cl − = a Cl − , cis · exp ( − V r F / R T ) − a Cl − , trans a K + , cis − a K + , trans · exp ( − V r F / R T ) (1) To gain additional insight into the ion transport profiles inside the nanopores, theoretical calculations using a continuum approach based on the Nernst–Planck–Poisson equations were performed using COMSOL. The calculation model is illustrated in the Supporting Information Figure S11, whereby a negatively charged nanochannel separates KCl solutions with different concentration gradients. The dimension of the nanochannel was set according to the characterization results (200 nm length and 1.2 nm diameter). To gain affordable computation scale, a 2D model was employed and the fluidic pathway was simplified to be a 200 nm long single channel with a width of 1.2 nm. The radial ion distribution from the channel axis to the channel wall was calculated, which revealed the presence of high concentration of K+ ions throughout the channel, even at low bath concentrations. Moreover, the concentration of Cl− ions was always lower than that of K+ ions, although the discrepancy decreased with an increase in the concentration gradient, consistent with the experimental results (Figure 3c). These phenomena can be rationalized by the high negative charge density and ultrasmall channel size of COF-COOH that produce co-ion exclusion.58,59 Theoretical principle of biomimetic thermosensation Developments in the principles of nanofluidic transport allow us to mimic the function of biological pores. It is established that the transmembrane diffusion potential (φdiff) is a function of temperature and salt activity of the electrolytes (eq 2 and see detailed derivation process in the Supporting Information), where t+ is the transference number of cations, T is temperature, and a is the electrolyte activity. ϕ diff = ϕ cis − ϕ trans = ( 2 t + − 1 ) R F ( T trans ln a trans − T cis ln a cis ) (2) Experimentally, we can measure the open-circuit potential (Voc) directly. According to the equivalent circuit of our experimental setup, Voc is the sum of φdiff, the redox potential of the Ag/AgCl electrodes (Eredox), and the voltage drops across the membrane (iRmembrane) and solution (iRsolution), where i is the ionic current, and Rmembrane and Rsolution are the internal resistance of the membrane and the solution resistance, respectively (eq 3). V oc = V cis − V trans = − ( ϕ diff + E redox + i R membrane + i R solution )(3) Eredox can be calculated using eq 4. E redox = E cis − E trans = R F ( T trans ln a trans − T cis ln a cis ) (4) Under open-circuit conditions, i is close to 0 and the drops in iR can be ignored. Therefore, the equation Voc relating to the solution temperature and concentration can be established as eq 5. V oc = − ( ϕ diff + E redox ) = − 2 t + R F ( T trans ln a trans − T cis ln a cis ) (5) To study the potential changes in response to the temperature gradients between the two electrolytes, we used symmetric solutions (acis = atrans = a). Both φdiff and Voc are equal to zero at the initial state (Tcis = Ttrans). Upon introducing a temperature gradient, ΔT (ΔT = |Ttrans – Tcis|), the magnitude of the potential changes can be derived from eqs 6 and 7. Δ V oc ( T ) = − 2 t + R F Δ T ln a(6) Δ ϕ diff ( T ) = ( 2 t + − 1 ) R F Δ T ln a(7) Thermoelectric response evaluation Once we confirmed the high permselectivity of COF-COOH/PAN, we proceeded to study its thermoelectric response properties. To investigate the influence of the temperature gradient between the two chambers on the transmembrane potential, COF-COOH/PAN was placed in contact with symmetric KCl solutions (1 mM). In the steady state, negligible zero-voltage currents were detected. When a slight temperature gradient was induced by briefly heating or cooling one end of the solution, an apparent voltage was observed, the sensitivity of ion transport ( Supporting Information Figure To quantitatively study the thermoelectric a was whereby ΔT was recorded by a of and the changes in voltage were by Ag/AgCl which were by an electrochemical ( Supporting Information Figure Figure the evolution curves of ΔT and recorded with COF-COOH/PAN, which the temperature of the trans side increased from °C to approximately 35 °C and then to thermal whereby synchronously in response to temperature In the cooling and ΔT decreased with and with a value greater than was obtained by the variation of Voc temperature in with the theoretical derivation (eq 6 and Figure over different the thermosensation sensitivity derived from the slope of the plots was mV K−1 for COF-COOH/PAN, the thermosensation sensitivity of organisms as well as the systems ( Supporting Information Table Moreover, this potential was reversible and and no of was for at indicating a between the temperature gradient and ion transport potential (Figure and Supporting Information Table S4). the device response to the temperature with an average sensitivity of (see details in the Supporting Information Figure Figure | Thermosensation evaluation of COF-COOH/PAN. (a) The evolution of in response to the solution temperature with the initial of and 35 respectively, The of ΔT according to eq (b) of thermosensation sensitivity for COF-COOH/PAN with of other nanochannels (c) and evolution curves of and ΔT recorded with COF-COOH/PAN membrane in 1 mM KCl for Download figure Download PowerPoint Given that is of the initial temperature but not we further investigated the thermosensation of COF-COOH/PAN. we evaluated the thermoelectric of the developed thermosensation system other temperature The permselectivity of COF-COOH/PAN was in response to hot and cold the t+ for the systems the temperature of °C and °C were calculated to be and Moreover, between and ΔT were observed, to the thermosensation of and mV K−1, respectively, and its wide temperature ( Supporting Information Figures and To evaluate the of COF-COOH/PAN to the temperature gradients of various we investigated its thermoelectric response electrolytes with a wide range of concentrations. profiles of in response to ΔT were by ΔT the proportional potential difference to the temperature gradient under various ionic strengths ( Supporting Information Figure The thermoelectric derived from the were to be in the range of mV K−1 over KCl concentrations of mM ( Supporting Information Table The decreased with electrolyte which can be by eq the slope decreased as to evaluate the thermosensation properties of COF-COOH/PAN in the presence of salt concentration M and 0.01 M KCl were used as electrolytes, which as of and a heating was on the side, a Voc value of mV was to the t+ value of which the high permselectivity of COF-COOH/PAN. Fitting the experimental results, a of and ΔT was also observed, a thermosensation sensitivity of mV K−1 ( Supporting Information Figure these results the potential of COF-COOH/PAN as a temperature The thermosensation sensitivity of COF-COOH/PAN with other electrolyte were also that the thermosensation sensitivity in the of KCl mV mV mV This can be rationalized by their difference in diffusion of cations, in the K+ ( Supporting Information Figures and The the cation the the charge separation which is a typical behavior of membranes. The high thermosensation sensitivity of COF-COOH/PAN potential in temperature gradient into To do so, COF-COOH/PAN was placed between the asymmetric aqueous solutions M) to mimic the salinity gradient at a Given that the concentration gradient and hot the ion transport in the temperature gradient was on the concentration side. The initial Voc and were to be mV and according to = Voc × a density of was introducing a temperature gradient of approximately the Voc and increased to mV and respectively, and the density was calculated to be ( Supporting Information Figure Temperature alarm device The high tolerance various conditions in terms of salt concentrations and temperature as well as the thermosensation sensitivity of COF-COOH/PAN, us to the of a temperature alarm system ( Supporting Information Figure the magnitude of the voltage linearly with an increase in the temperature gradient, we could precisely set an alarm To the alarm we an system that can temperature gradient into voltage that are then up and by a (Figure The when the in temperature the alarm Figure a of the experimental To as a sensor for monitoring the changes in the temperature, a conductivity cell from different materials and volume was Owing to the different thermal of and the temperature of electrolyte mM in the cell with large volume can be as unchanged in a when the temperature but that in the cell with volume In a steady state, the potential between the two remains the the voltage is zero at the When the temperature a temperature gradient between the two chambers is and the sensor a which the alarm circuit that the after the set Given that the magnitude of the temperature gradient the value of the the gating temperature can be precisely set by the Additionally, the thermosensation selectivity of the developed system a temperature variation to be detected. To demonstrate a we set the temperature gradient around 1 K as a or the sensor and the temperature gradients to the of the It was that when the temperature gradient of the electrolytes the