A ZIF-8 Derived Nano-Porous Carbon as Solid Transducing Layer in High-Performance All-Solid- State Sodium Ion Selective Electrodes

A ZIF-8 Derived Nano-Porous Carbon as Solid Transducing Layer in High-Performance All-Solid- State Sodium Ion Selective Electrodes | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A ZIF-8 Derived Nano-Porous Carbon as Solid Transducing Layer in High-Performance All-Solid- State Sodium Ion Selective Electrodes Peike Wang, Shuheng Fan, Haipeng Liu, Shiqiang Zhou, Jingjing Luo, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9279543/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract Graphene and carbon nanotubes, common solid transducing materials for solid-contact ion-selective electrodes (SC-ISEs), face limitations due to their intrinsic microstructures, which motivates the development of alternative carbon-based materials to address these performance bottlenecks. Herein, we report the synthesis of hydrophobic nanoporous-carbon (NPC) using a zeolitic imidazolate framework-8 (ZIF-8) template under a mixed nitrogen-hydrogen atmosphere. This integrated design strategy simultaneously achieves high specific surface area, nitrogen-doped double-layer capacitance, and superior hydrophobicity by minimizing polar oxygen-containing functional groups. These combined attributes effectively enhance sensitivity and long-term stability by suppressing aqueous layer formation at the interface. For Na⁺ sensing, the resulting SC-ISEs exhibited a Super-Nernstian sensitivity of 64.11 mV·dec –1 and a detection limit of 10 –4.7 M, demonstrating that the innovative application of ZIF-8-derived hydrophobic NPC provides a superior pathway for developing high-performance SC-ISEs that outperform most reported solid transducing materials. solid transducing layer nanoporous-carbon double-layer capacitance hydrophobicity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Ion-selective electrodes (ISEs) are widely applied in various fields, including environmental monitoring and disease diagnosis[ 1 ]. The development of miniaturized, calibration-free, low-cost, and high-precision ISEs for real-time in situ monitoring has become an important research direction[ 2 ]. Compared with liquid-contact ISEs (LC-ISEs), solid-contact ISEs (SC-ISEs) exhibit higher sensitivity, lower detection limits, and better biocompatibility[ 3 ]. A typical SC-ISE consists of an ion-selective membrane (ISM), a solid transducing layer (STL), and a supporting electrode[ 4 ]. The STL serves as the ion-to-electron transducer between the ISM and SE, converting the ionic signal into an electronic one. Rational design of the composition and structure of the STL is therefore critical to further improving the performance of SC-ISEs. An ideal STL should possess high capacitance and excellent hydrophobicity[ 5 ]. Solid transducing materials can be classified into two categories: double-layer capacitive materials and pseudocapacitive materials[ 6 ]. Double-layer capacitive materials generally exhibit better stability than pseudocapacitive materials[ 7 ]. Carbon-based materials, as typical double-layer capacitive materials, have attracted extensive attention owing to their tunable composition and structure, which enable adjustable solid transducing performance[ 8 ]. Carbon nanotubes (CNTs)[ 9 – 11 ], graphene[ 12 ] and carbon black (CB)[ 13 , 14 ] are the three most commonly used carbon-based materials in SC-ISEs. Although these materials have enabled SC-ISEs to achieve near-Nernstian responses and improved potential stability[ 15 – 17 ], their performance remains constrained by inherent microstructural limitations, which hinder further enhancement of their capacitive properties. To overcome these microstructural constraints, nanoporous-carbon (NPC) have emerged as a promising alternative, offering greater flexibility in the regulation of composition and morphological characteristics[ 18 , 19 ]. Organic framework-derived NPC generally exhibit ultrahigh double-layer capacitance, which is favorable for improving the sensitivity of SC-ISEs[ 20 , 21 ]. However, many such materials suffer from poor hydrophobicity, leading to the formation of an undesirable water layer and insufficient long-term stability. While some progress has been made—for instance, the Bühlmann group utilized three-dimensionally ordered microporous carbon to achieve an ultralow potential drift of 11.7 mV·h –1 [ 22 ]—developing NPC that simultaneously possess high capacitance and robust hydrophobicity remains a significant challenge. Zeolitic imidazolate framework-8 (ZIF-8), a typical metal-organic framework (MOF), is an ideal precursor for synthesizing such materials due to its uniform pore size, high nitrogen content, and ease of carbonization, resulting in carbon structures with abundant nanopores[ 23 , 24 ]. Despite these advantages, the application of ZIF-8-derived NPC as the STL in SC-ISEs remains largely unexplored. In this work, we employ ZIF-8 templates to synthesize NPC via pyrolysis under a mixed nitrogen-hydrogen atmosphere. The as-prepared NPC inherits the high specific surface area of the ZIF-8 template, providing high double-layer capacitance. Furthermore, the introduction of hydrogen during synthesis inhibits the formation of polar oxygen-containing functional groups on the surface, endowing the material with excellent hydrophobicity. Sodium-ion SC-ISEs using the as-prepared NPC as the STL exhibit Nernstian sensitivity, low detection limits, and excellent long-term stability, outperforming most previously reported counterparts. Experimental segment Chemicals The synthetic raw materials zinc nitrate, ammonium chloride, potassium ferricyanide, isopropyl alcohol, tetrahydrofuran, 2-methylimidazole, sodium chloride, potassium chloride, sodium tetrakis [3,5-bis(trifluoromethyl) phenyl] borate, bis(2-ethylhexyl)sebacate, poly(vinyl chloride) and sodium ionophore x were obtained from Beijing Innochem Science & Technology Co., Ltd. Commercial materials graphene and multi-walled carbon nanotubes (MWCNTs) were obtained from Jiangsu Xianfeng Nano Material Technology Co., Ltd. All materials were obtained from commercial sources and used without further treatment. Synthesis of ZIF-8 Methanol solutions of zinc nitrate (0.094 mol·L –1 ) and 2-methylimidazole (0.375 mol·L –1 ) were prepared separately. Subsequently, the zinc nitrate solution was added to the 2-methylimidazole solution under stirring at a volume ratio of 1:1. The mixed solution was left to stand at room temperature for 24 h. After thorough washing with deionized water and ethanol, ZIF-8 was collected via centrifugation. The product was dried under vacuum at 60°C for 24 h to remove residual solvents. Synthesis of ZIF-8 derived nano-porous carbon ZIF-8 was pyrolyzed under a N 2 /H 2 flow (90%/10% volume ratio) at 950°C for 2 h, with a heating rate of 5°C·min –1 . The resulting product was denoted as NPC. Preparation of SC-ISEs Solid transducing layer Solid transducing layer ink was prepared by dispersing solid transducing materials (2 mg) in the mixed solution of deionized water (800 µL), isopropanol (200 µL) and 5 wt.% Nafion (40 µL). After sonicated for 30 min, the ink (60 µL) was cast onto the glassy carbon electrode with a diameter of 6 mm to form the solid transducing layer. Na + selective membrane The Na + selective membrane cocktail solution was prepared by dispersing 65.45 wt.% sebacic acid (261.8 mg), 0.55 wt.% sodium borate (2.2 mg), 33 wt.% polyvinyl chloride (132 mg) and 1 wt.% tetraethyl tetraacetate (4 mg) in tetrahydrofuran (2640 µL). The cocktail solution (20 µL) was cast onto the solid transducing layers to form the ion selective membrane. Characterization Scanning electron microscopy (SEM) images were obtained by a Hitachi S-4800 with electron acceleration energy of 5 kV. Transmission electron microscopy (TEM) images were collected on a JEOL JEM-2100F with electron acceleration energy of 200 kV. The crystal structure of the catalysts was examined by a DX-2700BH with Cu K α radiation (λ = 1.5406 Å) with a step size of 0.2°. Specific surface area was measured based on Brunauer-Emmett-Teller method using AutosorbiQ instrument (Quantachrome) at liquid nitrogen temperature. Detailed chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS) on an Axis Supra instrument from Kratos Analytical Ltd. using a monochromatic Al K α X-ray beam (1,486.6 eV). A Shirley background was applied to all spectra. All the spectra were charge referenced to 284.8 eV. Raman spectra were performed in backscattering mode on a Thermo Fisher Raman microscope using a 532 nm laser. Fourier transform infrared spectroscopy (FTIR) were measured on a Thermo Fisher IS10 infrared spectrometer. Capacitive and SC-ISEs performance analysis All the electrochemical tests including cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were conducted on CHI 760E workstations in a typical three-electrode system. A double salt bridge Ag/AgCl was used as the reference and a Pt foil was used as the counter electrode. The electrochemical active surface areas (ECSA) were calculated using the Randles–Ševčík equation: $$\:{i}_{p}=(2.69\times\:{10}^{5}){n}^{3/2}AC{D}^{1/2}{v}^{1/2}$$ 1 Where i p is the peak current obtained by cyclic voltammetry in ferrocyanide solution, n is the number of electrons transferred during redox, A is the ECSA, C is the concentration, D is the diffusion coefficient and v is the scan rate. Results and discussion Materials characterization To establish a foundation for subsequent performance evaluations, a comprehensive suite of characterization techniques was employed to verify the morphology and microstructure of the synthesized NPC. SEM and TEM were utilized to observe the material's morphology. As depicted in Fig.s 1 b and 1 c, the NPC retains a rhombic dodecahedron shape with an average particle size of 113.7 nm, aligning with previously reported ZIF-8 templates[ 25 ]. High-resolution TEM (HRTEM) and X-ray diffraction (XRD) provided insights into the crystal structure. The HRTEM image (Fig. 1 d.) reveals the amorphous nature of the NPC, a finding further corroborated by the XRD pattern in Fig. 1 e. For structural comparison with traditional carbon materials, the XRD patterns of control graphene and MWCNT samples are provided in Fig. S2. Raman spectroscopy was further employed to analyze the carbon framework. As shown in Fig. 1 f, the Raman spectrum of the NPC is characterized by three prominent bands: the D band (disordered carbon, ~ 1300–1400 cm –1 ), the G band (graphitic carbon, ~ 1580 cm –1 ), and the 2D band (~ 2680 cm –1 )[ 26 ]. The calculated ID/IG ratio of 0.92 indicates a high degree of structural disorder and defects within the carbon skeleton, which is consistent with the amorphous nature observed via HRTEM and XRD[ 27 , 28 ]. Raman spectra of graphene and MWCNT controls (Fig. S3) serve as structural benchmarks and align with literature values. To investigate the porosity, N₂ adsorption-desorption measurements were conducted. The isotherm (Fig. 1 g) displays characteristic features of nanoporous materials[ 29 , 30 ], including a sharp nitrogen uptake at low relative pressures (P/P₀ < 0.05) and a gradual increase between P/P₀ = 0.2 and 0.9, signifying the presence of abundant micropores. This is further confirmed by the pore size distribution curve (Fig. S4), which shows that the pores are primarily concentrated within the 0–3 nm range. In summary, the synthesized NPC possesses a high specific surface area and a rich microporous architecture. These structural attributes are highly advantageous for its application as a solid transducer layer, providing a robust platform for the subsequent electrochemical performance discussions. To elucidate the chemical origins of the material's hydrophobicity, XPS and FTIR were performed. The high-resolution C 1s XPS spectrum (Fig. 2 a) reveals peaks corresponding to C–C (284.8 eV)[ 31 , 32 ], C–N (286.25 eV)[ 18 ], C–O (288.14 eV)[ 33 ], and C = O (290.22 eV)[ 33 ]. Nitrogen species are further identified in the N 1s spectrum (Fig. 2 b) as pyridinic N (398.50 eV), pyrrolic N (400.66 eV), and graphitic N (401.96 eV), while the O 1s spectrum confirms the presence of C–O (532.02 eV) and C = O (533.79 eV) bonds. These results, compared alongside the control graphene and MWCNT samples (Fig. S5)[ 34 , 35 ], highlight the specific surface chemistry of the NPC. FTIR characterization provides further evidence: the spectrum of NPC exhibits C–H out-of-plane bending (672 cm –1 )[ 36 ], C–O stretching (1150–1300 cm –1 )[ 37 , 38 ], and C = O stretching (1588 cm⁻¹)[ 36 ]. Notably, the hydroxyl (–OH) stretching vibration peak at 1734 cm –1 [ 39 ] is completely absent, which contrasts sharply with the control graphene and MWCNT samples where –OH groups are clearly observed (Fig. S6). This lack of hydrophilic functional groups (Table S1 ) directly contributes to the superior hydrophobicity of NPC[ 40 ]. In water contact angle measurements, NPC achieves a value of 157.7°, outperforming both graphene (154.7°) and MWCNTs (151.4°). This chemical state and the resulting exceptional hydrophobicity are critical for suppressing the formation of an interfacial water layer at the ISM/STL interface, thereby ensuring the long-term potential stability of SC-ISEs. Capacitive performance analysis To establish the potential of NPC as a high-performance solid transduction layer, its capacitive performance was systematically evaluated and compared with graphene and MWCNTs in a 0.1 M NaCl solution. CV measurements revealed quasi-rectangular shapes for all samples (Fig. 3 a, Fig. S7a–S7c), characteristic of typical double-layer capacitance. Notably, NPC exhibited the largest integrated area, signifying a superior double-layer capacitance compared to the other materials[ 41 ]. GCD profiles (Fig. 3 b, Fig. S7d–S7f) further confirmed this behavior through their linear and symmetric shapes, indicating excellent electrochemical reversibility and high coulombic efficiency[ 42 ]. Calculations derived from these curves yielded a specific capacitance of 163 F·g –1 for NPC, remarkably exceeding that of graphene (15.63 F·g –1 ) and MWCNT (7.52 F·g –1 ). ECSA was assessed using a ferricyanide redox probe. Consistent with the Randles-Sevcik model (Fig. 3 c)[ 43 ], all samples exhibited reversible redox characteristics. According to the Randles-Sevcik equation (Eq. 1 , Supporting Information), ECSA is proportional to the peak current. NPC displayed the highest peak current (353.9 µA) relative to graphene (300.9 µA) and MWCNT (162.6 µA), confirming it possesses the most extensive ECSA. To further elucidate charge transfer kinetics, electrochemical impedance spectroscopy (EIS) was employed. Nyquist plots (Fig. 3 d) demonstrate that NPC exhibits the smallest semicircle diameter and the steepest low-frequency slope, representing the lowest charge transfer resistance and ion diffusion resistance among the tested materials[ 42 ]. These results demonstrate that NPC provides a large ECSA and abundant adsorption sites for electrolyte ions, facilitating significantly enhanced double-layer capacitance. This superior electrochemical performance is highly consistent with the unique nanoporous architecture and favorable chemical properties established in the preceding characterization section. Importantly, the combination of high capacitance, large ECSA, and minimized resistance fulfills the critical requirements for an ideal STL, providing a robust theoretical foundation for the subsequent evaluation of its performance in SC-ISEs[ 44 ]. SC-ISEs performance analysis The assembly structure of the sodium-ion SC-ISEs is illustrated in Fig. 4 a. A typical sodium-ion SC-ISE consists of a glassy carbon electrode (GCE), an STL, and a Na + -selective ISM. The open-circuit potential (OCP) response and corresponding calibration curves for SC-ISEs using bare GCE, NPC, MWCNT, and graphene as the STL were measured in a series of NaCl solutions to evaluate electrode sensitivity. The NPC and graphene-based SC-ISEs demonstrated an extended continuous response range from 10 –6 to 10 –1 M, whereas the MWCNT-based electrode and the bare GCE were restricted to ranges of 10 –5 –10 –1 M and 10 –3 –10 –1 M, respectively (Fig. 4 b). This broader response range for NPC and graphene directly demonstrates their advantages as STLs, which is attributed to their higher capacitance and larger electrochemical active surface area. As summarized in Fig. 4 c and Table S2, the NPC-based SC-ISE demonstrated a superior sensitivity of 64.11 mV·dec –1 and a detection limit of 10 –4.7 M, outperforming the graphene-based (61.21 mV·dec –1 ; 10 –4.8 M), MWCNT-based (59.85 mV·dec –1 ; 10 –4.5 M), and bare GCE (42.9 mV·dec –1 ; 10 –2.3 M) electrodes. This super-Nernstian response of the NPC electrode indicates its optimal performance, which is intrinsically linked to its high capacitance. Aqueous layer formation tests were performed by alternately exposing the SC-ISEs to primary ion (Na + ) and interfering ion (NH₄ + ) solutions for 1 h each. Although all SC-ISEs exhibited relatively stable potential responses in both NaCl and NH₄Cl solutions (Fig. 4 d), the NPC-based electrode demonstrated the most robust stability. While the hydrophobicity of the STLs generally helps prevent water layer formation, the superior stability of the NPC electrode is specifically driven by the synergy between its excellent hydrophobicity and high capacitance. Chronopotentiometry at ± 1 nA cm –2 was further performed to evaluate the short-term potential stability of the SC-ISEs. As shown in Fig. 4 e, the NPC-based system exhibited the smallest potential fluctuations, confirming that its unique structural properties effectively stabilize the phase boundary and minimize potential drift. Figure 5 schematically illustrates the Na + transduction mechanism of the NPC-based SC-ISEs. Initially, Na + in the solution complexes with the ionophore L within the ISM to form Na + -L complexes. Specifically, the high specific surface area and abundant porous structure of the NPC provide a multitude of active sites for the formation of an electric double layer between the electrons in the STL and the Na + -L complexes. This high-capacitance characteristic of the NPC ensures a stable and efficient charge transfer at the electrode interface, thereby establishing an interfacial potential. This interfacial potential is measured to determine the Na + activity in the solution. Conclusions The integration of high-surface-area ZIF-8 derivatives with controlled atmospheric pyrolysis yields a superior nanoporous-carbon (NPC)-based STL for SC-ISEs. The high double-layer capacitance of this NPC is derived from its substantial specific surface area and nitrogen-doped structural motifs. Crucially, pyrolysis under a mixed nitrogen-hydrogen atmosphere significantly enhances material hydrophobicity, thereby mitigating the formation of detrimental aqueous layers and bolstering electrode stability. Synergizing high double-layer capacitance with excellent hydrophobicity allows the Na + -selective SC-ISEs to achieve a superior sensitivity of 64.11 mV·dec –1 , a low detection limit of 10 –4.7 M, and exceptional potential stability. These performance metrics surpass most reported STL materials, marking a significant advancement in electrochemical ion sensing. By demonstrating a rational design strategy for optimizing carbon materials, this work provides a robust methodology for developing advanced carbon-based architectures for diverse electrochemical applications beyond SC-ISEs. Future research could extend this strategy to the fabrication of multi-ion sensing platforms or further refine the pore structure of ZIF-derived carbons to achieve even lower detection limits in complex biological matrices. Declarations Supplementary Information The online version contains supplementary material available at Acknowledgments This work was financially supported by Shenzhen Science and Technology Innovation Program (Grant No. KQTD20200820113045083, ZDSYS20190902093220279, JCYJ20220818102403007), Shenzhen Research Fund for Returned Scholars (DD4500100125, DD2440800624) Author contributions Peike Wang: Conceptualization, Methodology, Investigation, Writing – original draft. Shuheng Fan: Formal analysis, Investigation, Writing – original draft. Haipeng Liu: Formal analysis, Investigation. Shiqiang Zhou: Investigation. Jingjing Luo: Investigation. Yuan Meng: Investigation. Jin Ning: Investigation. Suzhu Yu: Conceptualization, Formal analysis, Writing – review & editing, Supervision; Jun Wei: Conceptualization, Writing – review & editing, Supervision, Project Administration, Funding Acquisition. Data availability Data will be made available on request. Conflict of interest The authors declare that they have no known com­peting financial interests or personal relationships that could have ap­peared to influence the work reported in this paper. References Wardak C, Morawska K, Pietrzak K (2023) New Materials Used for the Development of Anion-Selective Electrodes-A Review. https://doi.org/10.3390/ma16175779 . MATERIALS 16 Shao YZ, Ying YB, Ping JF (2020) Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends. Chem Soc Rev 49:4405–4465. https://doi.org/10.1039/c9cs00587k Ding JW, Qin W (2020) Recent advances in potentiometric biosensors. 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RSC Adv 7:44896–44903. https://doi.org/` 10.1039/c7ra08535d Deng X, Li J, Zhu S et al (2019) Boosting the capacitive storage performance of MOF-derived carbon frameworks via structural modulation for supercapacitors. Energy Storage Mater 23:491–498. https://doi.org/10.1016/j.ensm.2019.04.015 Zhang M, Zhao N, Sha J et al (2014) Synthesis of novel carbon nano-chains and their application as supercapacitors. J Mater Chem A 2:16268–16275. https://doi.org/10.1039/c4ta02623c Yeung KK, Li JW, Huang T et al (2022) Utilizing Gradient Porous Graphene Substrate as the Solid-Contact Layer To Enhance Wearable Electrochemical Sweat Sensor Sensitivity. https://doi.org/10.1021/acs.nanolett.2c01969 . NANO LETTERS Wang L, Wang C, Wang H et al (2018) ZIF-8 nanocrystals derived N-doped carbon decorated graphene sheets for symmetric supercapacitors. Electrochim Acta 289:494–502 https://doi.org/10.1016/j.electacta.2018.09.091 Additional Declarations No competing interests reported. Supplementary Files 260331Supplementary.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 18 May, 2026 Reviewers agreed at journal 05 May, 2026 Reviewers invited by journal 20 Apr, 2026 Editor assigned by journal 09 Apr, 2026 Submission checks completed at journal 09 Apr, 2026 First submitted to journal 31 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9279543","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":626484288,"identity":"11ebdde5-e477-432e-a900-5e47bb36b0bb","order_by":0,"name":"Peike Wang","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Peike","middleName":"","lastName":"Wang","suffix":""},{"id":626484289,"identity":"079201ba-5eb1-4996-9efe-c29795bae8a3","order_by":1,"name":"Shuheng Fan","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABAElEQVRIie3PMUsDMRTA8XcE7PJC1h4R/QopgTsKxX6Vk0KnQ3RxNVC46XC++xBCJ+ccWWtvLdRNcCrScotoh6ZT6WB6boL5D+E9eL8hAD7fH6yjAAiAtiPu9wEyptwE9TEZn4eF/h0xA6GSE6TzYpq77PUiZk/V2+1XjQJ0sN6kDoI3Y15m77JffIxk+bjEmCgSls8/kyGkEaGZuZ4uZhGn+RL7Sp8R6iDIVrKx5MGS+JvmcxQ6OUG6qeCWJKLOI4Kfug1ZRRznpjddoORUjTAsqonzL8hS2eC9uRT1rNfg9mrI2KRabxzkUDcBCLL9FKg29zam7bNteezz+Xz/qh3jCFRtMbjF0gAAAABJRU5ErkJggg==","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":true,"prefix":"","firstName":"Shuheng","middleName":"","lastName":"Fan","suffix":""},{"id":626484290,"identity":"59b7f124-a194-4696-a822-5b083739234f","order_by":2,"name":"Haipeng Liu","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Haipeng","middleName":"","lastName":"Liu","suffix":""},{"id":626484291,"identity":"08e90f3e-5b9d-4e27-af43-39afb201b125","order_by":3,"name":"Shiqiang Zhou","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Shiqiang","middleName":"","lastName":"Zhou","suffix":""},{"id":626484292,"identity":"9bb979aa-b262-4c6f-a586-666b61d770b2","order_by":4,"name":"Jingjing Luo","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Jingjing","middleName":"","lastName":"Luo","suffix":""},{"id":626484293,"identity":"710802e3-a095-4ca3-b7b3-d7d6cf95aa73","order_by":5,"name":"Yuan Meng","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Yuan","middleName":"","lastName":"Meng","suffix":""},{"id":626484294,"identity":"eba90e9d-4e8b-4240-8230-db9a8618c697","order_by":6,"name":"Jin Ning","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Jin","middleName":"","lastName":"Ning","suffix":""},{"id":626484295,"identity":"212a5b98-3f85-405e-8f3c-6e38cd269d68","order_by":7,"name":"Suzhu Yu","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Suzhu","middleName":"","lastName":"Yu","suffix":""},{"id":626484296,"identity":"11887b54-3fad-465f-9288-32b742ca93e8","order_by":8,"name":"Jun Wei","email":"","orcid":"","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Jun","middleName":"","lastName":"Wei","suffix":""}],"badges":[],"createdAt":"2026-03-31 12:08:36","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9279543/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9279543/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108186948,"identity":"932bf259-fd17-475c-a0c7-0be87b3f3f6a","added_by":"auto","created_at":"2026-04-30 09:20:07","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":202984,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a)\u003c/strong\u003e Synthetic scheme of NPC. \u003cstrong\u003e(b)\u003c/strong\u003e SEM image, \u003cstrong\u003e(c)\u003c/strong\u003e TEM image, \u003cstrong\u003e(d)\u003c/strong\u003e HRTEM image, \u003cstrong\u003e(e)\u003c/strong\u003e XRD pattern, \u003cstrong\u003e(f)\u003c/strong\u003e Raman spectrum, and \u003cstrong\u003e(g)\u003c/strong\u003e N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm of NPC\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9279543/v1/4b1b3bbc051e398f8f3b239c.jpg"},{"id":108490936,"identity":"a83ab6c3-09dc-492c-9d1d-e05bfdfd6161","added_by":"auto","created_at":"2026-05-05 09:50:13","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":119821,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a)\u003c/strong\u003e C 1s, \u003cstrong\u003e(b)\u003c/strong\u003e N 1s, and \u003cstrong\u003e(c)\u003c/strong\u003e O 1s XPS spectra of the NPC. \u003cstrong\u003e(d)\u003c/strong\u003e FTIR spectrum of NPC. Water contact angle images of \u003cstrong\u003e(e)\u003c/strong\u003e MWCNT, \u003cstrong\u003e(f)\u003c/strong\u003e graphene, and \u003cstrong\u003e(g)\u003c/strong\u003e NPC\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9279543/v1/706648826b7c1f15c0d35c05.jpg"},{"id":108490978,"identity":"c6aabaf1-bbb9-483f-816b-586006b8f07f","added_by":"auto","created_at":"2026-05-05 09:50:41","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":133608,"visible":true,"origin":"","legend":"\u003cp\u003eCapacitive performance evaluation of NPC, graphene and MWCNT. \u003cstrong\u003e(a)\u003c/strong\u003e CV curves tested in a 0.1 M NaCl solution at 50 mV·s\u003csup\u003e–1\u003c/sup\u003e. \u003cstrong\u003e(b)\u003c/strong\u003e GCD curves at 0.5 A·g\u003csup\u003e–1\u003c/sup\u003e. \u003cstrong\u003e(c)\u003c/strong\u003e CV curves in a 10 mM ferricyanide and 1 M NaCl solution at 30 mV·s\u003csup\u003e–1\u003c/sup\u003e. \u003cstrong\u003e(d)\u003c/strong\u003e Nyquist plots at open-circuit potential in a 0.1 M NaCl solution; inset is the enlargement at the high-frequency region\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9279543/v1/c9c27b37a418d74f4304cb28.jpg"},{"id":108186951,"identity":"36b0cb44-fc49-4ce4-8eac-4ee279f5564a","added_by":"auto","created_at":"2026-04-30 09:20:07","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":117201,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical measurements of SC-ISEs using bare glass carbon electrode, NPC, MWCNT, and graphene as solid transducing layers. \u003cstrong\u003e(a)\u003c/strong\u003e Schematic of the SC-ISE fabrication process, \u003cstrong\u003e(b)\u003c/strong\u003e OCP response plots in a series of NaCl solutions, \u003cstrong\u003e(c) \u003c/strong\u003eresponse calibration curves, \u003cstrong\u003e(d)\u003c/strong\u003e water layer test results, \u003cstrong\u003e(e)\u003c/strong\u003e chronopotentiograms under a constant current of ±1 nA in 1 mM NaCl solution\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9279543/v1/a56be1a7d1733ed99604c463.jpg"},{"id":108491056,"identity":"15af11de-f596-4a54-9c44-ce7737f6aa32","added_by":"auto","created_at":"2026-05-05 09:51:40","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":133226,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the solid transduction mechanism of NPC in SC-ISEs\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9279543/v1/dc62a5b75a527beb42861212.jpg"},{"id":109067550,"identity":"399f21c9-8001-4406-8a1e-b7835993c02f","added_by":"auto","created_at":"2026-05-12 09:55:47","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":977921,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9279543/v1/9b46bddf-c4e5-4e0d-891d-a5b122f3adb3.pdf"},{"id":108186947,"identity":"bab9d9d8-c20c-45ca-9569-4a174ed9b5b3","added_by":"auto","created_at":"2026-04-30 09:20:07","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1208690,"visible":true,"origin":"","legend":"","description":"","filename":"260331Supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-9279543/v1/d30e9a40fc654af1b41f1118.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"A ZIF-8 Derived Nano-Porous Carbon as Solid Transducing Layer in High-Performance All-Solid- State Sodium Ion Selective Electrodes","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIon-selective electrodes (ISEs) are widely applied in various fields, including environmental monitoring and disease diagnosis[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The development of miniaturized, calibration-free, low-cost, and high-precision ISEs for real-time in situ monitoring has become an important research direction[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Compared with liquid-contact ISEs (LC-ISEs), solid-contact ISEs (SC-ISEs) exhibit higher sensitivity, lower detection limits, and better biocompatibility[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. A typical SC-ISE consists of an ion-selective membrane (ISM), a solid transducing layer (STL), and a supporting electrode[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The STL serves as the ion-to-electron transducer between the ISM and SE, converting the ionic signal into an electronic one. Rational design of the composition and structure of the STL is therefore critical to further improving the performance of SC-ISEs. An ideal STL should possess high capacitance and excellent hydrophobicity[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Solid transducing materials can be classified into two categories: double-layer capacitive materials and pseudocapacitive materials[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Double-layer capacitive materials generally exhibit better stability than pseudocapacitive materials[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCarbon-based materials, as typical double-layer capacitive materials, have attracted extensive attention owing to their tunable composition and structure, which enable adjustable solid transducing performance[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Carbon nanotubes (CNTs)[\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], graphene[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] and carbon black (CB)[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] are the three most commonly used carbon-based materials in SC-ISEs. Although these materials have enabled SC-ISEs to achieve near-Nernstian responses and improved potential stability[\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], their performance remains constrained by inherent microstructural limitations, which hinder further enhancement of their capacitive properties. To overcome these microstructural constraints, nanoporous-carbon (NPC) have emerged as a promising alternative, offering greater flexibility in the regulation of composition and morphological characteristics[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOrganic framework-derived NPC generally exhibit ultrahigh double-layer capacitance, which is favorable for improving the sensitivity of SC-ISEs[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. However, many such materials suffer from poor hydrophobicity, leading to the formation of an undesirable water layer and insufficient long-term stability. While some progress has been made\u0026mdash;for instance, the B\u0026uuml;hlmann group utilized three-dimensionally ordered microporous carbon to achieve an ultralow potential drift of 11.7 mV\u0026middot;h\u003csup\u003e\u0026ndash;1\u003c/sup\u003e [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u0026mdash;developing NPC that simultaneously possess high capacitance and robust hydrophobicity remains a significant challenge. Zeolitic imidazolate framework-8 (ZIF-8), a typical metal-organic framework (MOF), is an ideal precursor for synthesizing such materials due to its uniform pore size, high nitrogen content, and ease of carbonization, resulting in carbon structures with abundant nanopores[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Despite these advantages, the application of ZIF-8-derived NPC as the STL in SC-ISEs remains largely unexplored.\u003c/p\u003e \u003cp\u003eIn this work, we employ ZIF-8 templates to synthesize NPC via pyrolysis under a mixed nitrogen-hydrogen atmosphere. The as-prepared NPC inherits the high specific surface area of the ZIF-8 template, providing high double-layer capacitance. Furthermore, the introduction of hydrogen during synthesis inhibits the formation of polar oxygen-containing functional groups on the surface, endowing the material with excellent hydrophobicity. Sodium-ion SC-ISEs using the as-prepared NPC as the STL exhibit Nernstian sensitivity, low detection limits, and excellent long-term stability, outperforming most previously reported counterparts.\u003c/p\u003e"},{"header":"Experimental segment","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eChemicals\u003c/h2\u003e \u003cp\u003eThe synthetic raw materials zinc nitrate, ammonium chloride, potassium ferricyanide, isopropyl alcohol, tetrahydrofuran, 2-methylimidazole, sodium chloride, potassium chloride, sodium tetrakis [3,5-bis(trifluoromethyl) phenyl] borate, bis(2-ethylhexyl)sebacate, poly(vinyl chloride) and sodium ionophore x were obtained from Beijing Innochem Science \u0026amp; Technology Co., Ltd. Commercial materials graphene and multi-walled carbon nanotubes (MWCNTs) were obtained from Jiangsu Xianfeng Nano Material Technology Co., Ltd. All materials were obtained from commercial sources and used without further treatment.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSynthesis of ZIF-8\u003c/h3\u003e\n\u003cp\u003eMethanol solutions of zinc nitrate (0.094 mol\u0026middot;L\u003csup\u003e\u0026ndash;1\u003c/sup\u003e) and 2-methylimidazole (0.375 mol\u0026middot;L\u003csup\u003e\u0026ndash;1\u003c/sup\u003e) were prepared separately. Subsequently, the zinc nitrate solution was added to the 2-methylimidazole solution under stirring at a volume ratio of 1:1. The mixed solution was left to stand at room temperature for 24 h. After thorough washing with deionized water and ethanol, ZIF-8 was collected via centrifugation. The product was dried under vacuum at 60\u0026deg;C for 24 h to remove residual solvents.\u003c/p\u003e\n\u003ch3\u003eSynthesis of ZIF-8 derived nano-porous carbon\u003c/h3\u003e\n\u003cp\u003eZIF-8 was pyrolyzed under a N\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e flow (90%/10% volume ratio) at 950\u0026deg;C for 2 h, with a heating rate of 5\u0026deg;C\u0026middot;min\u003csup\u003e\u0026ndash;1\u003c/sup\u003e. The resulting product was denoted as NPC.\u003c/p\u003e\n\u003ch3\u003ePreparation of SC-ISEs\u003c/h3\u003e\n\u003cp\u003e \u003cstrong\u003eSolid transducing layer\u003c/strong\u003e \u003cp\u003eSolid transducing layer ink was prepared by dispersing solid transducing materials (2 mg) in the mixed solution of deionized water (800 \u0026micro;L), isopropanol (200 \u0026micro;L) and 5 wt.% Nafion (40 \u0026micro;L). After sonicated for 30 min, the ink (60 \u0026micro;L) was cast onto the glassy carbon electrode with a diameter of 6 mm to form the solid transducing layer.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eNa\u003csup\u003e+\u003c/sup\u003e selective membrane\u003c/strong\u003e \u003cp\u003eThe Na\u003csup\u003e+\u003c/sup\u003e selective membrane cocktail solution was prepared by dispersing 65.45 wt.% sebacic acid (261.8 mg), 0.55 wt.% sodium borate (2.2 mg), 33 wt.% polyvinyl chloride (132 mg) and 1 wt.% tetraethyl tetraacetate (4 mg) in tetrahydrofuran (2640 \u0026micro;L). The cocktail solution (20 \u0026micro;L) was cast onto the solid transducing layers to form the ion selective membrane.\u003c/p\u003e \u003c/p\u003e\n\u003ch3\u003eCharacterization\u003c/h3\u003e\n\u003cp\u003eScanning electron microscopy (SEM) images were obtained by a Hitachi S-4800 with electron acceleration energy of 5 kV. Transmission electron microscopy (TEM) images were collected on a JEOL JEM-2100F with electron acceleration energy of 200 kV. The crystal structure of the catalysts was examined by a DX-2700BH with Cu K\u003csub\u003eα\u003c/sub\u003e radiation (λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;) with a step size of 0.2\u0026deg;. Specific surface area was measured based on Brunauer-Emmett-Teller method using AutosorbiQ instrument (Quantachrome) at liquid nitrogen temperature. Detailed chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS) on an Axis Supra instrument from Kratos Analytical Ltd. using a monochromatic Al K\u003csub\u003eα\u003c/sub\u003e X-ray beam (1,486.6 eV). A Shirley background was applied to all spectra. All the spectra were charge referenced to 284.8 eV. Raman spectra were performed in backscattering mode on a Thermo Fisher Raman microscope using a 532 nm laser. Fourier transform infrared spectroscopy (FTIR) were measured on a Thermo Fisher IS10 infrared spectrometer.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eCapacitive and SC-ISEs performance analysis\u003c/h2\u003e \u003cp\u003eAll the electrochemical tests including cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were conducted on CHI 760E workstations in a typical three-electrode system. A double salt bridge Ag/AgCl was used as the reference and a Pt foil was used as the counter electrode.\u003c/p\u003e \u003cp\u003eThe electrochemical active surface areas (ECSA) were calculated using the Randles\u0026ndash;Ševč\u0026iacute;k equation:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{i}_{p}=(2.69\\times\\:{10}^{5}){n}^{3/2}AC{D}^{1/2}{v}^{1/2}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003ei\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e is the peak current obtained by cyclic voltammetry in ferrocyanide solution, \u003cem\u003en\u003c/em\u003e is the number of electrons transferred during redox, \u003cem\u003eA\u003c/em\u003e is the ECSA, \u003cem\u003eC\u003c/em\u003e is the concentration, \u003cem\u003eD\u003c/em\u003e is the diffusion coefficient and \u003cem\u003ev\u003c/em\u003e is the scan rate.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eMaterials characterization\u003c/h2\u003e \u003cp\u003eTo establish a foundation for subsequent performance evaluations, a comprehensive suite of characterization techniques was employed to verify the morphology and microstructure of the synthesized NPC. SEM and TEM were utilized to observe the material's morphology. As depicted in Fig.s\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, the NPC retains a rhombic dodecahedron shape with an average particle size of 113.7 nm, aligning with previously reported ZIF-8 templates[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHigh-resolution TEM (HRTEM) and X-ray diffraction (XRD) provided insights into the crystal structure. The HRTEM image (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed.) reveals the amorphous nature of the NPC, a finding further corroborated by the XRD pattern in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee. For structural comparison with traditional carbon materials, the XRD patterns of control graphene and MWCNT samples are provided in Fig. S2. Raman spectroscopy was further employed to analyze the carbon framework. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef, the Raman spectrum of the NPC is characterized by three prominent bands: the D band (disordered carbon, ~\u0026thinsp;1300\u0026ndash;1400 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e), the G band (graphitic carbon, ~\u0026thinsp;1580 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e), and the 2D band (~\u0026thinsp;2680 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e)[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. The calculated ID/IG ratio of 0.92 indicates a high degree of structural disorder and defects within the carbon skeleton, which is consistent with the amorphous nature observed via HRTEM and XRD[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Raman spectra of graphene and MWCNT controls (Fig. S3) serve as structural benchmarks and align with literature values.\u003c/p\u003e \u003cp\u003eTo investigate the porosity, N₂ adsorption-desorption measurements were conducted. The isotherm (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg) displays characteristic features of nanoporous materials[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], including a sharp nitrogen uptake at low relative pressures (P/P₀ \u0026lt; 0.05) and a gradual increase between P/P₀ = 0.2 and 0.9, signifying the presence of abundant micropores. This is further confirmed by the pore size distribution curve (Fig. S4), which shows that the pores are primarily concentrated within the 0\u0026ndash;3 nm range. In summary, the synthesized NPC possesses a high specific surface area and a rich microporous architecture. These structural attributes are highly advantageous for its application as a solid transducer layer, providing a robust platform for the subsequent electrochemical performance discussions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the chemical origins of the material's hydrophobicity, XPS and FTIR were performed. The high-resolution C 1s XPS spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) reveals peaks corresponding to C\u0026ndash;C (284.8 eV)[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], C\u0026ndash;N (286.25 eV)[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], C\u0026ndash;O (288.14 eV)[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], and C\u0026thinsp;=\u0026thinsp;O (290.22 eV)[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Nitrogen species are further identified in the N 1s spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) as pyridinic N (398.50 eV), pyrrolic N (400.66 eV), and graphitic N (401.96 eV), while the O 1s spectrum confirms the presence of C\u0026ndash;O (532.02 eV) and C\u0026thinsp;=\u0026thinsp;O (533.79 eV) bonds. These results, compared alongside the control graphene and MWCNT samples (Fig. S5)[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], highlight the specific surface chemistry of the NPC.\u003c/p\u003e \u003cp\u003eFTIR characterization provides further evidence: the spectrum of NPC exhibits C\u0026ndash;H out-of-plane bending (672 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e)[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], C\u0026ndash;O stretching (1150\u0026ndash;1300 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e)[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], and C\u0026thinsp;=\u0026thinsp;O stretching (1588 cm⁻\u0026sup1;)[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Notably, the hydroxyl (\u0026ndash;OH) stretching vibration peak at 1734 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e] is completely absent, which contrasts sharply with the control graphene and MWCNT samples where \u0026ndash;OH groups are clearly observed (Fig. S6). This lack of hydrophilic functional groups (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) directly contributes to the superior hydrophobicity of NPC[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn water contact angle measurements, NPC achieves a value of 157.7\u0026deg;, outperforming both graphene (154.7\u0026deg;) and MWCNTs (151.4\u0026deg;). This chemical state and the resulting exceptional hydrophobicity are critical for suppressing the formation of an interfacial water layer at the ISM/STL interface, thereby ensuring the long-term potential stability of SC-ISEs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eCapacitive performance analysis\u003c/h2\u003e \u003cp\u003eTo establish the potential of NPC as a high-performance solid transduction layer, its capacitive performance was systematically evaluated and compared with graphene and MWCNTs in a 0.1 M NaCl solution. CV measurements revealed quasi-rectangular shapes for all samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, Fig. S7a\u0026ndash;S7c), characteristic of typical double-layer capacitance. Notably, NPC exhibited the largest integrated area, signifying a superior double-layer capacitance compared to the other materials[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGCD profiles (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, Fig. S7d\u0026ndash;S7f) further confirmed this behavior through their linear and symmetric shapes, indicating excellent electrochemical reversibility and high coulombic efficiency[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Calculations derived from these curves yielded a specific capacitance of 163 F\u0026middot;g\u003csup\u003e\u0026ndash;1\u003c/sup\u003e for NPC, remarkably exceeding that of graphene (15.63 F\u0026middot;g\u003csup\u003e\u0026ndash;1\u003c/sup\u003e) and MWCNT (7.52 F\u0026middot;g\u003csup\u003e\u0026ndash;1\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003eECSA was assessed using a ferricyanide redox probe. Consistent with the Randles-Sevcik model (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec)[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e], all samples exhibited reversible redox characteristics. According to the Randles-Sevcik equation (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Supporting Information), ECSA is proportional to the peak current. NPC displayed the highest peak current (353.9 \u0026micro;A) relative to graphene (300.9 \u0026micro;A) and MWCNT (162.6 \u0026micro;A), confirming it possesses the most extensive ECSA.\u003c/p\u003e \u003cp\u003eTo further elucidate charge transfer kinetics, electrochemical impedance spectroscopy (EIS) was employed. Nyquist plots (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed) demonstrate that NPC exhibits the smallest semicircle diameter and the steepest low-frequency slope, representing the lowest charge transfer resistance and ion diffusion resistance among the tested materials[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. These results demonstrate that NPC provides a large ECSA and abundant adsorption sites for electrolyte ions, facilitating significantly enhanced double-layer capacitance.\u003c/p\u003e \u003cp\u003eThis superior electrochemical performance is highly consistent with the unique nanoporous architecture and favorable chemical properties established in the preceding characterization section. Importantly, the combination of high capacitance, large ECSA, and minimized resistance fulfills the critical requirements for an ideal STL, providing a robust theoretical foundation for the subsequent evaluation of its performance in SC-ISEs[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eSC-ISEs performance analysis\u003c/h2\u003e \u003cp\u003eThe assembly structure of the sodium-ion SC-ISEs is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. A typical sodium-ion SC-ISE consists of a glassy carbon electrode (GCE), an STL, and a Na\u003csup\u003e+\u003c/sup\u003e-selective ISM. The open-circuit potential (OCP) response and corresponding calibration curves for SC-ISEs using bare GCE, NPC, MWCNT, and graphene as the STL were measured in a series of NaCl solutions to evaluate electrode sensitivity. The NPC and graphene-based SC-ISEs demonstrated an extended continuous response range from 10\u003csup\u003e\u0026ndash;6\u003c/sup\u003e to 10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e M, whereas the MWCNT-based electrode and the bare GCE were restricted to ranges of 10\u003csup\u003e\u0026ndash;5\u003c/sup\u003e\u0026ndash;10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e M and 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e\u0026ndash;10\u003csup\u003e\u0026ndash;1\u003c/sup\u003e M, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). This broader response range for NPC and graphene directly demonstrates their advantages as STLs, which is attributed to their higher capacitance and larger electrochemical active surface area. As summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and Table S2, the NPC-based SC-ISE demonstrated a superior sensitivity of 64.11 mV\u0026middot;dec\u003csup\u003e\u0026ndash;1\u003c/sup\u003e and a detection limit of 10\u003csup\u003e\u0026ndash;4.7\u003c/sup\u003e M, outperforming the graphene-based (61.21 mV\u0026middot;dec\u003csup\u003e\u0026ndash;1\u003c/sup\u003e; 10\u003csup\u003e\u0026ndash;4.8\u003c/sup\u003e M), MWCNT-based (59.85 mV\u0026middot;dec\u003csup\u003e\u0026ndash;1\u003c/sup\u003e; 10\u003csup\u003e\u0026ndash;4.5\u003c/sup\u003e M), and bare GCE (42.9 mV\u0026middot;dec\u003csup\u003e\u0026ndash;1\u003c/sup\u003e; 10\u003csup\u003e\u0026ndash;2.3\u003c/sup\u003e M) electrodes. This super-Nernstian response of the NPC electrode indicates its optimal performance, which is intrinsically linked to its high capacitance.\u003c/p\u003e \u003cp\u003eAqueous layer formation tests were performed by alternately exposing the SC-ISEs to primary ion (Na\u003csup\u003e+\u003c/sup\u003e) and interfering ion (NH₄\u003csup\u003e+\u003c/sup\u003e) solutions for 1 h each. Although all SC-ISEs exhibited relatively stable potential responses in both NaCl and NH₄Cl solutions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), the NPC-based electrode demonstrated the most robust stability. While the hydrophobicity of the STLs generally helps prevent water layer formation, the superior stability of the NPC electrode is specifically driven by the synergy between its excellent hydrophobicity and high capacitance. Chronopotentiometry at \u0026plusmn;\u0026thinsp;1 nA cm\u003csup\u003e\u0026ndash;2\u003c/sup\u003e was further performed to evaluate the short-term potential stability of the SC-ISEs. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, the NPC-based system exhibited the smallest potential fluctuations, confirming that its unique structural properties effectively stabilize the phase boundary and minimize potential drift.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e schematically illustrates the Na\u003csup\u003e+\u003c/sup\u003e transduction mechanism of the NPC-based SC-ISEs. Initially, Na\u003csup\u003e+\u003c/sup\u003e in the solution complexes with the ionophore L within the ISM to form Na\u003csup\u003e+\u003c/sup\u003e-L complexes. Specifically, the high specific surface area and abundant porous structure of the NPC provide a multitude of active sites for the formation of an electric double layer between the electrons in the STL and the Na\u003csup\u003e+\u003c/sup\u003e-L complexes. This high-capacitance characteristic of the NPC ensures a stable and efficient charge transfer at the electrode interface, thereby establishing an interfacial potential. This interfacial potential is measured to determine the Na\u003csup\u003e+\u003c/sup\u003e activity in the solution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe integration of high-surface-area ZIF-8 derivatives with controlled atmospheric pyrolysis yields a superior nanoporous-carbon (NPC)-based STL for SC-ISEs. The high double-layer capacitance of this NPC is derived from its substantial specific surface area and nitrogen-doped structural motifs. Crucially, pyrolysis under a mixed nitrogen-hydrogen atmosphere significantly enhances material hydrophobicity, thereby mitigating the formation of detrimental aqueous layers and bolstering electrode stability. Synergizing high double-layer capacitance with excellent hydrophobicity allows the Na\u003csup\u003e+\u003c/sup\u003e-selective SC-ISEs to achieve a superior sensitivity of 64.11 mV\u0026middot;dec\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, a low detection limit of 10\u003csup\u003e\u0026ndash;4.7\u003c/sup\u003e M, and exceptional potential stability. These performance metrics surpass most reported STL materials, marking a significant advancement in electrochemical ion sensing. By demonstrating a rational design strategy for optimizing carbon materials, this work provides a robust methodology for developing advanced carbon-based architectures for diverse electrochemical applications beyond SC-ISEs. Future research could extend this strategy to the fabrication of multi-ion sensing platforms or further refine the pore structure of ZIF-derived carbons to achieve even lower detection limits in complex biological matrices.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupplementary Information\u003c/strong\u003eThe online version contains supplementary material available at\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003eThis work was financially supported by Shenzhen Science and Technology Innovation Program (Grant No. KQTD20200820113045083, ZDSYS20190902093220279, JCYJ20220818102403007), Shenzhen Research Fund for Returned Scholars (DD4500100125, DD2440800624)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003ePeike Wang: Conceptualization, Methodology, Investigation, Writing \u0026ndash; original draft. \u0026nbsp; Shuheng Fan: Formal analysis, Investigation, Writing \u0026ndash; original draft. Haipeng Liu: Formal analysis, Investigation. Shiqiang Zhou: Investigation. Jingjing Luo: Investigation. Yuan Meng: Investigation. Jin Ning: Investigation. Suzhu Yu: Conceptualization, Formal analysis, Writing \u0026ndash; review \u0026amp; editing, Supervision; Jun Wei: Conceptualization, Writing \u0026ndash; review \u0026amp; editing, Supervision, Project Administration, Funding Acquisition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003eData will be made available on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u0026nbsp;\u003c/strong\u003eThe authors declare that they have no known com\u0026shy;peting financial interests or personal relationships that could have ap\u0026shy;peared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWardak C, Morawska K, Pietrzak K (2023) New Materials Used for the Development of Anion-Selective Electrodes-A Review. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/ma16175779\u003c/span\u003e\u003cspan address=\"10.3390/ma16175779\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. 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Electrochim Acta 289:494\u0026ndash;502\u003c/span\u003e \u003cspan\u003e\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.electacta.2018.09.091\u003c/span\u003e\u003cspan address=\"10.1016/j.electacta.2018.09.091\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"solid transducing layer, nanoporous-carbon, double-layer capacitance, hydrophobicity","lastPublishedDoi":"10.21203/rs.3.rs-9279543/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9279543/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGraphene and carbon nanotubes, common solid transducing materials for solid-contact ion-selective electrodes (SC-ISEs), face limitations due to their intrinsic microstructures, which motivates the development of alternative carbon-based materials to address these performance bottlenecks. Herein, we report the synthesis of hydrophobic nanoporous-carbon (NPC) using a zeolitic imidazolate framework-8 (ZIF-8) template under a mixed nitrogen-hydrogen atmosphere. This integrated design strategy simultaneously achieves high specific surface area, nitrogen-doped double-layer capacitance, and superior hydrophobicity by minimizing polar oxygen-containing functional groups. These combined attributes effectively enhance sensitivity and long-term stability by suppressing aqueous layer formation at the interface. For Na⁺ sensing, the resulting SC-ISEs exhibited a Super-Nernstian sensitivity of 64.11 mV\u0026middot;dec\u003csup\u003e\u0026ndash;1\u003c/sup\u003e and a detection limit of 10\u003csup\u003e\u0026ndash;4.7\u003c/sup\u003e M, demonstrating that the innovative application of ZIF-8-derived hydrophobic NPC provides a superior pathway for developing high-performance SC-ISEs that outperform most reported solid transducing materials.\u003c/p\u003e","manuscriptTitle":"A ZIF-8 Derived Nano-Porous Carbon as Solid Transducing Layer in High-Performance All-Solid- State Sodium Ion Selective Electrodes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-30 09:20:03","doi":"10.21203/rs.3.rs-9279543/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-05-18T09:58:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"74621951934509877808561095587721652213","date":"2026-05-06T00:34:44+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-20T16:25:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-09T09:37:33+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-09T09:37:30+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2026-03-31T11:59:16+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"9ca800a7-2b78-4736-a624-73357c9c86b3","owner":[],"postedDate":"April 30th, 2026","published":true,"recentEditorialEvents":[{"type":"editorInvitedReview","content":"","date":"2026-05-18T09:58:37+00:00","index":33,"fulltext":""},{"type":"reviewerAgreed","content":"74621951934509877808561095587721652213","date":"2026-05-06T00:34:44+00:00","index":32,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-30T09:20:03+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-30 09:20:03","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9279543","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9279543","identity":"rs-9279543","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}