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[Subrata Maji](https://orcid.org/0000-0003-3985-7522), [Jan Hynek](https://orcid.org/0000-0003-1883-9464), Kazushi Nakada, Akiko Hori, [Kosuke Minami](https://orcid.org/0000-0003-4145-1118), [Jan Labuta](https://orcid.org/0000-0002-8329-0634), [Mandeep K. Chahal](https://orcid.org/0000-0002-8810-2196), [Daniel T. Payne](https://orcid.org/0000-0001-7707-8381), Gary J. Richards, [Lok Kumar Shrestha](https://orcid.org/0000-0003-2680-6291), [Katsuhiko Ariga](https://orcid.org/0000-0002-2445-2955), Yusuke Yamauchi, [Genki Yoshikawa](https://orcid.org/0000-0002-9136-8964), [Jonathan P. Hill](https://orcid.org/0000-0002-4229-5842)

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[Molecular Nanoarchitectonic Sensing Layer for Analysis of Volatile Fatty Acids in Bioreactor Headspaces Using a Nanomechanical Sensor](https://mdr.nims.go.jp/datasets/0a2c9571-c0f4-44b9-ae3d-2ba65cc6e16e)

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Molecular Nanoarchitectonic Sensing Layer for Analysis of Volatile Fatty Acids in Bioreactor Headspaces Using a Nanomechanical SensorRESEARCH ARTICLEwww.advmattechnol.deMolecular Nanoarchitectonic Sensing Layer for Analysis ofVolatile Fatty Acids in Bioreactor Headspaces Using aNanomechanical SensorSubrata Maji,* Jan Hynek, Kazushi Nakada, Akiko Hori, Kosuke Minami, Jan Labuta,Mandeep K. Chahal, Daniel T. Payne, Gary J. Richards, Lok Kumar Shrestha,Katsuhiko Ariga, Yusuke Yamauchi, Genki Yoshikawa, and Jonathan P. Hill*Anaerobic digestors (ADs) have the potential to become a major green energysource in a future sustainable society. To achieve this, effective operationalparameters of the reactors must be established and maintained preferably byautomation. In situ monitoring and control of AD systems is currently achallenging matter based on inconsistent feedstuff composition and thecomplexity of the biological processes involved, so that reactor shutdownsassociated with costly downtime are a common problem. Here, a sensingdevice is developed for the monitoring of volatile fatty acids (VFAs), keyintermediates during bioreactor operation, in the headspaces of anaerobicreactors. The device is compact, contains a sensing element based on aself-assembled molecular layer in conjunction with a nanomechanical sensor,and can be used to monitor VFA contents in bioreactors by estimating theirrelative contents in the reactor headspace in real time. This device allows theclose monitoring of VFA concentrations in admixture toward cost-effectiveoperation of bioreactors by reducing or eliminating reactor downtime. Itrepresents a simple solution to the problem of real-time reactor monitoring,making anaerobic digesters a realistic alternative energy source.S. Maji, J. Hynek, J. Labuta, M. K. Chahal, D. T. Payne, L. K. Shrestha,K. Ariga, J. P. HillResearch Center for Materials Nanoarchitectonics (MANA)National Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: Maji.Subrata@nims.go.jp; Jonathan.Hill@nims.go.jpK.Nakada, A.Hori,G. J. RichardsDepartment of AppliedChemistryGraduate School of Engineering andScienceShibaura Institute of TechnologyFukasaku307,Minuma-ku, Saitama-shi, Saitama337–8570, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/admt.202500294© 2025 The Author(s). Advanced Materials Technologies published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License,which permits use and distribution in any medium, provided the originalwork is properly cited, the use is non-commercial and no modificationsor adaptations are made.DOI: 10.1002/admt.2025002941. IntroductionAnaerobic digestion (AD) has the poten-tial to reduce greenhouse gas emissionsby 15% and could produce 6–10% ofglobal primary energy demand throughbiogas production. Worldwide, thereare ≈132000 medium-to-large-scale di-gesters representing only 2% of the totalestimated capacity of the AD industry.[1,2]AD is a complicated biological processinvolving various interacting microor-ganisms that degrade organic matter,including food waste, crop residues,animal manure, and sewage sludge, toproduce biogas, bioplastics or organicfertilizers.[3–6] Effective and efficientproduction of biogas is only possiblewhen all the biological processes (hy-drolysis, acidogenesis, acetogenesis,and methanogenesis) at the digesterK. Minami, G. YoshikawaResearch Center for Macromolecules and BiomaterialsNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanL. K. ShresthaDepartment of Materials ScienceInstitute of Pure and Applied SciencesUniversity of Tsukuba1-1-1, Tennodai, Tsukuba, Ibaraki 305–8573, JapanK. ArigaDepartment of Advanced Materials ScienceGraduate School of Frontier SciencesThe University of Tokyo5-1-5 Kashiwanoha, Kashiwa, Chiba 277–8561, JapanY. YamauchiDepartment of Materials Process EngineeringGraduate School of EngineeringNagoya UniversityNagoya 464−8603, JapanY. YamauchiAustralian Institute for Bioengineering and Nanotechnology (AIBN) andSchool of Chemical EngineeringThe University of QueenslandBrisbane, QLD 4072, AustraliaAdv. Mater. Technol. 2025, 10, e00294 e00294 (1 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbHhttp://www.advmattechnol.demailto:Maji.Subrata@nims.go.jpmailto:Jonathan.Hill@nims.go.jphttps://doi.org/10.1002/admt.202500294http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadmt.202500294&domain=pdf&date_stamp=2025-05-28www.advancedsciencenews.com www.advmattechnol.deinterior operate smoothly. If one of these processes is in-hibited or arrested, then there is an immediate impact on theother processes, which leads to losses in production or neces-sitates shutdown of the plant.[7–10] Organic overload, where thevolume of organic matter fed to the digester exceeds the degra-dation capacity of the microorganism, is a common problem inAD.[11–13] In that case, organic matter is only partially degradedto volatile fatty acids (VFAs; acetic, propionic, butyric acids),which then accumulate inside the reactor, causing digesteracidification. Therefore, the concentrations of individual VFAsat digester interiors must be monitored to establish processstability by controlling feedstock composition and to understandthe interaction and inhibition of different groups of microorgan-isms in the reactor.[14–17] A moderate accumulation of acetic acidin the digester is normal, as acetic acid is the final precursorto methane. However, the accumulation of the longer chainfatty acids (propionic acid, butyric acid) indicates severe processinstability, which may lead to reduced biogas production orplant failure. Therefore, the monitoring of acetic acid, propionicacid, and butyric acid levels in the digester is very importantto follow the biogas production. It should also be noted herethat other types of AD based on different microorganisms (e.g.,propionic acid bacteria) operate under high levels of propionicacid[18] or butyric acid,[19] and monitoring of their levels remainscritical in both cases. High-performance liquid chromatography(HPLC), gas chromatography (GC), or GC-mass spectrometry(MS) are used to monitor individual VFAs.[20–22] These offlinelaboratory-based techniques are expensive, time-consuming,and require expert operation to obtain reliable results. Therefore,it would be highly beneficial to develop high-performance VFAsensors for the simultaneous detection and discrimination ofVFAs in the digester for remote real-time sensing and interfac-ing with Internet of Things (IoT) based technologies, towardthe automation of the biogas plant. To date, there are relativelyfew reports of vapor sensors for VFAs, including those foracetic acid (for recent examples see A. Akhtar et al.[23] and N. J.Pineau et al.[24]), propionic acid,[25,26] and butyric acid.[27] Theseare largely based on inorganic nanomaterials[28–31] in differentsensing devices, including quartz crystal microbalance[32,33] orelectronic nose configurations.[34,35] Other examples have reliedon a metal-organic-framework (MOF) component[36] or otherreceptor films[37,38] for operation, and bioelectrochemical meth-ods are known.[39] A comparison of the available performancemetrics of selected sensors is shown in Table S1 (SupportingInformation). Reports of simultaneous VFA monitoring arealso rare[40–42] mostly involving mixtures of acetic and propionicacids, although monitoring of more complex mixtures has alsobeen investigated.[43,44] Finally, while interference from otherY. YamauchiDepartment of Chemical and Biomolecular EngineeringYonsei University50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South KoreaG. YoshikawaMaterials Science and EngineeringGraduate School of Pure and Applied ScienceUniversity of Tsukuba1-1-1 Tennodai, Tsukuba, Ibaraki 305–8571, Japananalytes can be a serious drawback in sensing operations,[45] inthe case of VFAs, oligomerization of the carboxylic acid analytesis a specific interaction[46] that is difficult to account for sincelittle is known about the dynamics of that interaction in vapormixtures.Nanomechanical sensors present a promising gas or vapormonitoring technology based on their high sensitivity, miniaturesize, and low power consumption.[47–49] These sensors can beused to detect mechanical changes caused by the interaction ofa selective receptor layer (adhered at the sensor active surface)with targeted analytes. For example, the Membrane-Type Surfacestress Sensor (MSS) is an advanced, optimized nanomechani-cal sensor exhibiting excellent sensitivity due to its unique struc-ture, which consists of a silicon membrane embedded with fourpiezoresistors sensing beams in a Wheatstone bridge-type con-figuration to maximize the output signal.[50–53] Receptor-layer-coated MSS generates adsorption-induced surface stress at thesilicon membrane in the presence of targeted analytes, especiallythose in the vapor phase, and the resulting surface stress is trans-duced through the four piezoresistors sensing beams, which gen-erate sensing responses. In principle, any kind of material canbe used as a receptor layer for MSS, including polymers, nano-materials or small organic molecules, but they must exhibit me-chanical deformation caused by interactionswith the analyte. Thereceptor layer material, regardless of its identity, must also be at-tached firmly to the MSS silicon membrane in order that adsorp-tion of the analyte induces sufficient mechanical deformation ofthe receptor layer in turn generating maximum stress at the sili-con membrane. In the case of AD process monitoring, other pa-rameters, including selectivity for VFAs and low sensitivity to wa-ter, are both critical for the implementation of MSS to monitorreal-time acid concentrations. We have demonstrated the mea-surements of VFAs in silage samples for their quality evaluationusing common polymers as receptormaterials coated onMSS,[54]while advanced functionalmaterials with higher performance areanticipated for practical on-site monitoring in AD. Recently, wehave developed porphyrin-based coordination complexes whoseunique structures allow for solution-based casting of porous ar-chitectures onto the MSS membrane by using inkjet printing.[55]These unique materials have large specific surface areas avail-able for gas or vapor adsorption and are capable of generatingvery high strain at the MSS surface, leading to large sensing re-sponses. These materials also show robust performance underconditions of high humidity (>90% RH), a critical property inbiological process monitoring applications, and they can also bemodified using organic synthesis techniques.In this work, we demonstrate the dynamic analysis of differ-ent volatile fatty acids in the vapor phase at sub-ppm sensitivityunder high relative humidity conditions. For this purpose, we in-troduce an advanced 2nd generation porphyrin receptor layer re-ferred to as 𝜷-NiOx1, which forms a nanoporous polycrystallinefilm when deposited on the MSS sensor chip and shows ex-cellent selectivity toward volatile fatty acid vapors. Importantly,the 𝜷-NiOx1@MSS hybrid device can be used to analyze mix-tures of low concentration VFA vapors in the high humidityregime (>90% RH) due its general low response to humidity asan interfering agent.[56] This device 𝜷-NiOx1@MSS allows theclose monitoring of volatile fatty acid concentrations in admix-ture toward cost-effective operation of bioreactors by reducing orAdv. Mater. Technol. 2025, 10, e00294 e00294 (2 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.deeliminating reactor downtime. Overall, this 𝜷-NiOx1 receptormaterial, used in conjunction with MSS, is the most impor-tant development so far to address the critical challenge of fullautomation of biogas plants. Such solution processable porousreceptor materials are highly suitable for applications involv-ing nanomechanical sensors for monitoring different targetedVOCs.2. ResultsThe porphyrin[57] molecules used in this work are known by thegeneral term NiOx, and are available in 1st generation and now amore advanced 2nd generation of compound (“𝜶” and “𝜷”, resp.).Each NiOx compound can be obtained in two different oxida-tion states (“Ox1” and “Ox3”). For example, the most effective1st generation material is 𝜶-NiOx3, based on its high specific sur-face area and the high sensitivity and selectivity of the resulting𝜶-NiOx3@MSS hybrid sensors to acetone under high humidity(see M. K. Chahal et al.[55] for the structure of 𝜶-NiOx3). Here,we have developed the 2nd generation 𝜷-NiOx1 (see Figure 1a forthe structure of 𝜷-NiOx1 and its direct precursor; see SupportingInformation for details of their synthesis).In contrast to 1st generation 𝜶-NiOx1, which has a small spe-cific surface area (for 1st gen. materials, 𝜶-NiOx3 has a large sur-face area), 𝜷-NiOx1 exhibits microporosity in its as-isolated pow-der state, it is highly soluble in different organic solvents (𝜷-NiOx3 has a low specific surface area), and provides microporoussolids on evaporation of its solutions. Thus, 𝜷-NiOx1 can be con-sidered a “smart” building block for sensor nanoarchitectonics[58]by in situ synthesis of porous monoliths. For example, inkjet de-position of this material onto an MSS sensing chip (Figure 1b,c)yields a receptor layer film composed of a porous polycrystallinesheet network (Figure 1d,e), with an average thickness of 60 nm.Such a porous polycrystalline sheet attached with a sensor plat-form can be used to detect volatile organic compounds (VOCs),especially targeted VFAs, with strong specific interaction.In order to assess the possibility of using the 𝜷-NiOxmaterialsas MSS receptor layers, their specific surface areas (SSA) wereestimated by determining their nitrogen adsorption-desorptionisotherms (surface area is a critical parameter, often a determin-ing factor in the sensing efficacy ofMSS receptor layermaterials).N2 adsorption isotherms of 𝜷-NiOx1 (and 𝜷-NiOx3) are shown inFigure 1f revealing that, while it appears to possess a microp-orous structure, 𝜷-NiOx3 has a much smaller surface area (123m2 g−1) than 𝜷-NiOx1 (670 m2 g−1), which also exhibits microp-orosity with a Type III hysteresis loop indicating the copresenceof meso- and micropores. Additionally, the presence of the hys-teresis loop indicates the formation of plate-like structures, whichcan be seen in SEM images (Figure 1d,e). The pore size data for𝜷-NiOx1 (Figure 1g,h) indicate a narrow distribution dominatedby micropores with nanometric pore diameters. These data (andSEM images; see Figure 1d,e) establish that 𝜷-NiOx1 is a strongcandidate for application as an MSS receptor layer based on itshighly porous interleaved polycrystalline structure. Porous recep-tor materials with uniform pore size and high surface area canproduce a high level of sensitivity when incorporated in multi-component gas/vapor sensor devices.In order to determine the origin of its microporous struc-ture, single crystals of 𝜷-NiOx1 were grown by slow diffusion ofmethanol into a chloroform solution of the compound. The sin-gle crystal X-ray structure is shown in Figure 2. Figure 2a showsthemolecular structure with an extended 𝜋 electronic surface dueto the fused benzimidazole component (see also Figures S1 andS2, Supporting Information).Peripheral meso-substituents are not coplanar with the conju-gated core except where fused (Figure 2b; see caption for details),and the conjugated surface of the molecule is slightly curved,probably to maintain the coordination to Ni(II). Molecules forma dimer unit in the crystals (Figure 2c; Figure S3, Supporting In-formation) involving several supramolecular interactions espe-cially C═O…H─C hydrogen bonding, C─H…Br─C H─bonding,and C─H…N H─bonding. Dimer units pack to form a porousstructure (Figure 2d; Figure S4, Supporting Information) wherevoids in the crystal are initially occupied by chloroformmoleculesof solvation, none of which contribute to the structure. Notethat methyl and 4-bromophenyl substituents protruding into thechannels are mutually remote so that voids running through thecrystal are continuous parallel to the a-axis (Figure 2e). Interest-ingly, themultiple supramolecular interactions operating in crys-tals of 𝜷-NiOx1 are sufficiently strong to maintain its structure inthe absence of solvent so that it exhibit a large SSA (Figure 1g)even after annealing at 120 °C. This is similar to the case for𝜶-NiOx3, whose crystal structure could be remeasured even af-ter solvent evacuation[55] and other porous structures such ashydrogen-bonded organic frameworks (HOFs).[59,60] Chloroformcontained in the channels is present as chains of solvent, whichare non-structural in nature. Chains are maintained by C─H…Clhydrogen bonding and Cl…Cl interactions (Figure S5a,b; Sup-porting Information). An additional smaller pore is present or-thogonal to the main pore (Figure 2f).𝜷-NiOx1 can be inkjet printed onMSSmembrane surfaces (forSEMmorphology see Figure S6, Supporting Information), giving𝜷-NiOx1@MSS, which was tested using the experimental set-upshown in Figure 3a. Figure 3b shows a survey of the sensing re-sponses of 𝜷-NiOx1@MSS in the presence of water and threemajor volatile acids: acetic acid, propionic acid, and butyric acidvapors. Importantly, 𝜷-NiOx1 shows excellent sensitivity towardvapors of each of the acids, with the highest sensitivity to aceticacid. Additionally, 𝜷-NiOx1 shows the lowest sensitivity and se-lectivity to water vapor, which is a significant advantage for real-time applications of this receptor material. In contrast, 𝜷-NiOx3with a lower surface area (123 m2 g−1) shows much weaker sens-ing responses (up to several orders of magnitude weaker), mostlikely due to its lower adsorption capacity for the different VOCs(Figure 3c). This contrasts with the 1st generation materials 𝜶-NiOx1/𝜶-NiOx3 (previously referred to as 1 and 1-ox, resp.)[55]where the higher oxidation state 𝜶-NiOx3 material possesses asignificantly larger specific surface area (475m2 g−1 versus 15m2g−1 for 𝜶-NiOx1) with excellent selectivity to acetone and alcoholvapors.Based on the speculation that selectivity might be tuned byvarying molecular structure in the 𝜶-NiOx1 system, we soughta receptor sensitive to vapor phase VFAs, because there is a gen-eral dearth of materials suitable for this purpose.[61] By makingsimple modifications of the functional group in 𝜶-NiOx1[55] toprepare 𝜷-NiOx1, we have successfully varied the sensitivity ofthe materials from acetone to volatile fatty acids. It was grati-fying to note that 𝜷-NiOx1@MSS hybrid sensor shows intense,Adv. Mater. Technol. 2025, 10, e00294 e00294 (3 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.deFigure 1. Synthesis and textural properties of polycrystalline nanoporous receptor material and interfacial layer for nanomechanical sensing de-vices (MSS). a) Synthesis of 𝜷-NiOx1 (see also Scheme S1, Supporting Information): treatment of the precursor with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in dichloromethane. b) corresponding solution of 𝜷-NiOx1 in toluene. Film formed by inkjet deposition on MSS sensor chipusing 𝜷-NiOx1 dissolved in DMF. c) Optical micrograph of receptor material 𝜷-NiOx1 inkjet-printed onto MSS sensor device. d) SEM morphology of𝜷-NiOx1 film shows interleaved polycrystalline sheets. e) Edge-view SEM image of 𝜷-NiOx1 crystalline sheet indicates uniformly thin (≈60 nm) crystal-lites. f) Nitrogen adsorption isotherms for 𝜷-NiOx1 (and 𝜷-NiOx3) obtained following sample annealing at 120 °C for 24 h. g) Pore size distributionprofile obtained by using the density functional theory (DFT) method. h) Pore size distribution profile obtained by the Barrett—Joyner–Halenda (BJH)method.rapid responses in the presence of acetic, propionic, and butyricacid vapors, and the responses are 4–6-fold greater than thatto water vapor. The relative intensity of the different acid re-sponses (Figure 3b) decreases with increasing molecular weightas expected due to their relative saturated vapor pressures (aceticacid: 20.9 mmHg; propionic acid: 3.3 mmHg; butyric acid:1.65 mmHg at 25 °C), which control their concentrations inthe gas phase. Time-dependent adsorption-desorption kineticsplay an important role to understand the interaction of differ-ent vapors with the porous receptor material. The adsorption-desorption-induced MSS signals can be fitted with the double-exponential (DE) model, which assumes that there are two ki-netic barriers for the adsorption of VFAs into the porous recep-tor materials.[62,63] The first kinetic barrier is due to the diffusionthrough the window of the porous framework or outer layer ofthe receptor layer, which is the faster process. The second bar-rier is due to the diffusion through the pore cavities, which isthe slower process and depends solely on the receptor materialidentity. The DE curve fitting model is given by the followingEquation (1):Adv. Mater. Technol. 2025, 10, e00294 e00294 (4 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.deFigure 2. X-ray crystal structure of 𝜷-NiOx1. a) Molecular structure showing the extension of the 𝜋-electronic system by fusion at a bay area (Bondlengths and angles: Ni-Nav: 1.94 Å; C-Br: 1.89 Å; spiro─C═O: 1.22 Å; phenol C─O: ≈1.37 Å). b) Edge view of the molecule showing the slight “S”-shape(Dihedral angles with average porphyrin plane: 4-BrPh: 48.2°, 43.5°; phenols (clockwise from fused groups): 44.1°, 48.8°. c) Dimer structure of 𝜷-NiOx1with important interactions highlighted (Interaction lengths and angles: spiro-C═O…H─C: 2.74 Å; C─Har…N: 2.66 Å; C─H…𝜋: 2.76, 2.77 Å. For moreinformation, see Figure S3, Supporting Information). d) Packing structure of 𝜷-NiOx1 viewed along the a-axis showing solvent-filled voids highlightedin green. The red ellipse identifies a single dimer unit. (See also Figures S1–S5, Supporting Information). e,f) Pore structures containing chloroformmolecules in 𝜷-NiOx1. e) Main pore viewed parallel to the a-axis. f) Minor pore (yellow circle) viewed perpendicular to the main pore channel. The greenarrow denotes the direction of the a-axis. Green shading denotes the main pore; pink ellipses in (e) indicate CHCl3 bound close to the pore wall.MtMe= A1(1 − e−k1t)+ A2(1 − e−k2t)(1)whereMt is the mass uptake at time t andMe is the equilibriummass uptake, k1 and k2 represent the kinetic rate constants (k1 >k2). A1 and A2 are the relative contributions of the two energeticbarriers controlling the adsorption process, with A1 + A2 = 1. Inthe case of MSS, the mass uptake is directly proportional to thestrain and the corresponding output voltage.[64] Therefore, theabove equation can be further correlated with the MSS signals asEquation (2):VtVe= A1(1 − e−k1t)+ A2(1 − e−k2t)(2)Adv. Mater. Technol. 2025, 10, e00294 e00294 (5 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.deFigure 3. MSS sensing using 𝜷-NiOx1. a) Schematic showing MSS vapor measurement setup including standard and portable MSS unit. b) MSSresponse of 𝜷-NiOx1@MSS in the presence of water vapor and VFAs: acetic acid (AcH), propionic acid (PrH), and butyric acid (BuH). c) Relativesensitivities of highly porous 𝜷-NiOx1@MSS and low porosity 𝜷-NiOx3@MSS (see also Figure S7, Supporting Information). d) Adsorption kinetics ofdifferent VFAs (AcH, PrH, and BuH) by the receptor material 𝜷-NiOx1. e) The relative contribution of the slower and faster components for the differentacids.The desorption process can also be modeled as a similar two-stage process using the following Equation (3):VeVt= A1(e−k1t)+ A2(e−k2t)(3)where Vt is the output voltage at time t and Ve is the equi-librium output voltage. Figure 3d shows the well-fitted ad-sorption curves based on Equation 2 with a regression coeffi-cient R2> 99%. Figure 3e shows the relative contribution offast and slow diffusion processes at the receptor material in-terior for the three acids. Here we can see that the contribu-tion from the slower process (diffusion inside the pore cav-ity) increases from acetic to butyric acid. This increase in theslower component can be attributed to the slower diffusion ofbutyric acid and propionic acid inside the pore cavity of the𝜷-NiOx1 receptor material relative to acetic acid. As a result, ata fixed time, the quantities of acid adsorbed inside the receptormaterial follow the order: acetic acid > propionic acid > butyricacid, which is exactly similar to the MSS sensitivity (Figure 3b).The differences in the diffusion process for different VFAs arecritically important for their discrimination in complex mixturesandmight allow for their analysis by theMSS hybrid sensor with-out the requirement for sophisticated machine learning tech-niques or other expensive devices. Fitting of desorption processesof MSS signals also shows a similar trend for different VFAsas shown in Figure S8 (Supporting Information). Desorptionprocesses are increasingly dominated by the slower component,which can be attributed to the slower desorption process fromthe pore cavity.Previous studies indicate that different VFAs form during theacidogenesis process occurring in AD plants, with acetic acid be-ing the dominant intermediate, accounting for more than 75%of AD methane production.[65,66] Propionic and butyric acids areof secondary importance, being eventually converted to aceticacid and hydrogen.[67,68] Analysis suggests that the total VFAconcentration should be in the range of 1000–4300 mg L−1(≈15–100 mM) to maintain process stability of biogas plants.While the individual fatty acids concentration may vary, the to-tal concentration should lie in this range for stable reactor op-eration, and slightly acidic to neutral pH 5.5–7.0 is optimum forVFA production/conversion inside the biogas plant. VFAs are po-lar molecules forming strong hydrogen bonds with water, mak-ing their analysis in vapor or gas phase at reactor interiors achallenging task. To simulate biogas plant conditions, we pre-pared different solutions of acetic acid, propionic acid, and bu-tyric acid in water in the operating concentration range. Proton-transfer reactionmass spectrometry (PTR-MS) was used to deter-mine the headspace concentrations over the acid solutions, lead-ing to vapor phase acid concentrations in the sub-ppm (0.45 ppm)to tens of ppm (70 ppm) range for all the acids, depending ontheir concentrations in solution (Table S3, Supporting Informa-tion). The headspaces are also saturated with water vapor (RH= 100%), simulating the expected condition at the interiors ofAD reactors of biogas plants so that the receptor material is re-quired also to be relatively insensitive toward water vapor. The𝜷-NiOx1@MSS shows excellent performance for the detectionAdv. Mater. Technol. 2025, 10, e00294 e00294 (6 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.deFigure 4. Sensing response of 𝜷-NiOx1@MSS in headspace analysis over different concentrated (0-500mM) VFA solutions in water. a) Sensing responseof acetic acid (AcH) solution with magnified desorption curve (b). c) Sensing response of propionic acid (PrH) solution with magnified desorption curve(d). e) Sensing response of butyric acid (BuH) solution with magnified desorption curve (f). The red dashed line in all the magnified desorption curvesindicates the selected point to construct the calibration plot. g) Relative contribution of the slower components in the desorption curves for the differentacid solutions. h) Plot of MSS desorption intensity (393 s) versus solution concentration for calibration of headspace acid concentration of aqueoussolution.of different VFAs in the headspaces of the low-concentration so-lutions in water, as shown in Figure 4. Typical sensing responsesof 𝜷-NiOx1@MSS to acetic acid (AcH) (Figure 4a,b), propionicacid (PrH) (Figure 4c,d), and butyric acid (BuH) (Figure 4e,f) va-pors reveal the clear discrimination of low-concentration acid so-lutions even at sub-ppm headspace acid concentration. All thesignal profiles differ from that of pristine water vapor and fol-low adsorption-desorption profiles similar to pure acid, even un-der these high-humidity conditions. Sensing responses (adsorp-tion and desorption curves) of individual VFAs (AcH, PrH, andBuH) differ and also follow similar trends of desorption of thepure acids (Figure S8, Supporting Information) and can be usedto differentiate the responses at different concentrations. Signalprofiles obtained over acetic acid solution (Figure 4a,b) show fastadsorption and desorption due to its low molecular weight andhigh vapor pressure, which induce fast diffusion inside the porecavity of the receptor layer. In contrast, response over butyric acidsolutions (Figure 4e,f) shows the slowest adsorption/desorptioncharacteristics due to its relatively low vapor pressure and largesize, which leads to slow diffusion in the receptor material.Adv. Mater. Technol. 2025, 10, e00294 e00294 (7 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.deFigure 5. Calibration of 𝜷-NiOx1@MSS response over aqueous solutions of binary acidmixtures (i.e., a ternarymixture, two acids plus water). a) Sensingresponse to headspace vapors over different mixtures with fixed acetic acid (AcH) and variable propionic acid (PrH) concentration (10–200 mM) atroom temperature (25 °C). b) Sensing response to headspace vapors over different mixtures of fixed acetic acid (AcH) and variable butyric acid (BuH)concentration (10-200 mM) at room temperature (25 °C). c) Plot near saturation MSS intensity (390 s) versus propionic or butyric acid concentrationto monitor any abrupt change in propionic or butyric acid level.Fitting of the desorption curves of individual acids (Figure S9,Supporting Information) clearly indicates that the desorption isdominated by slow desorption from the pore cavity of the 𝜷-NiOx1 receptor material. Figure 4g shows the relative contribu-tion of slow desorption of the different VFAs from the porousreceptor material. On average, 40–70% of the total decay curve isdominated by slow desorption of the acids, and the rate of decaydepends on the individual acids. Therefore, differences in des-orption intensity at fixed time have been used to calibrate the re-lationship between desorption intensity and acid concentration.Here, desorption intensity at 393 s has been selected arbitrarilyto quantify the acid concentration in the headspace vapor over so-lutions of the individual acids. Desorption at this point is domi-nated by slow desorption from the pore cavity, which is directly re-lated to the receptor layer property. Figure 4h shows the variationof desorption intensity at 393 s (i.e., 3 s after N2 injection) at differ-ent acid concentrations. There is a nonlinear correlation betweenacid concentration and MSS desorption intensity for individualacids. At low concentration (0–100 mM), the variation follows al-most linear correlation with evidence of saturation at higher con-centration (>100 mM). Saturation at higher concentrations canbe attributed toMSS signal saturation due to excessive acid vaporconcentration. Intensity calibration curves shown in Figure 4hcan be used to clearly differentiate the acids at low concentra-tions even under high humidity conditions, which is a signifi-cant development toward continuous VFA monitoring in biogasplants. For purposes of comparison, the response of the 1st gen-eration receptor layer 𝜶-NiOx3@MSSwas alsomeasured (FigureS10, Supporting Information), revealing that 𝜶-NiOx3 can alsobe used to analyze the concentration of butyric acid by headspaceanalysis. However, the performance of 𝜶-NiOx3@MSS is poorcompared to 𝜷-NiOx1@MSS (see Figure S10c, Supporting Infor-mation), illustrating the importance of molecular structure andthus crystal morphology in the excellent selectivity of 𝜷-NiOx1toward VFAs.The possibility of using the 𝜷-NiOx1 receptormaterial to differ-entiate propionic acid or butyric acid from acetic acid in aqueousmixtures similar to those encountered in AD reactors was also in-vestigated. Basically, the concentrations of propionic acid and bu-tyric acid are important determinants of AD process imbalanceand indicators for AD plant shutdown. Therefore, the monitor-ing of these acids in the presence of acetic acid while challengingis also a critical matter. Solutions containing a fixed concentra-tion of acetic acid (40 mM) with a range of propionic and butyricacid concentrations (10–200 mM) were prepared in water, andthe headspace vapor of the acid mixture was monitored using𝜷-NiOx1@MSS.Figure 5a shows the sensing responses of the mixed acid sam-ples of 40 mM acetic acid with varied concentrations of propionicacid, illustrating the capability to differentiate propionic acidfrom acetic acid in aqueous mixtures by using receptor material𝜷-NiOx1. Small variations in propionic acid concentration inthe aqueous binary mixtures (i.e., a ternary mixture with watereffectively being eliminated by its rapid desorption) are reflectedin the 𝜷-NiOx1@MSS sensing responses. Saturated intensity ofthe MSS signal increases with propionic acid concentration ascan be seen from Figure 5a. Similarly, variation of butyric acidconcentration can be easily differentiated from acetic acid at fixedconcentration in aqueous mixture by using the 𝜷-NiOx1@MSSresponses as shown in Figure 5b. Nearly saturated intensity of𝜷-NiOx1@MSS signals has been used to correlate the variationin propionic and butyric acid concentrations in headspace withfixed acetic acid concentration in the mixture. Figure 5c showsthe correlation between MSS-saturated intensity with propionicand butyric acid concentrations (10–200 mM). It is indispens-able to construct such calibration curves for monitoring of thereal-time variation of propionic or butyric acid concentrationsand maintaining stable AD plant operation. By calibrating theheadspace analysis, any sudden increases in propionic or butyricacid concentrations in the reactor can be detected in real-timesimply by using the 𝜷-NiOx1@MSS sensor signal. These char-acteristics of the 𝜷-NiOx1@MSS device make it highly suitablefor continuous monitoring of VFAs in biogas plants to maintainreactor stability and improve the efficiency of AD processes toestablish carbon-sustainable processes.3. Discussion and ConclusionAdsorption/desorption processes lead to high sensitivities forthese porous receptor materials and depend strongly on theirAdv. Mater. Technol. 2025, 10, e00294 e00294 (8 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.desolution processability, where in situ crystallization and adhesionon the sensor platform result in an effective porous material-sensor interface. Most of the known porous receptor materialsform poor quality films, which are incompatible with sensingapplications and exhibit low sensitivity and selectivity. In con-trast, compound 𝜷-NiOx1 is a microcrystalline powder with ahighly porous structure and is readily soluble in common or-ganic solvents to form a clear solution as shown in Figure 1b.Upon inkjet deposition on MSS (Figure 1c), 𝜷-NiOx1 forms auniform interleaved polycrystalline network film, as shown inFigure 1d,e. SEM analysis of this inkjet film shows the forma-tion of supramolecular sheets on the MSS membrane, wherecross-sectional analysis of the supramolecular sheet using SEMshows a uniform thickness of the crystal components of ≈60 nm(Figure 3f). 𝜷-NiOx1 also exhibits a similar sheet-likemorphologyby solution drop-casting (Figure S6a,b, Supporting Information).Additionally, the sheet formation of 𝜷-NiOx1 onMSSmembraneis not affected by inkjet droplet number (Figure S6c–h, Sup-porting Information), which indicates the advantages associatedwith the novel solution-processable 𝜷-NiOx1 receptor material.Finally, sensor stability is an important factor, especially in appli-cations involving potentially corrosive analytes or where continu-ous monitoring is expected. This issue has been addressed by ex-posing the sensor to a fixed concentration of acetic acid (40 mMin water) in humid air, involving a repeated exposure-readout-desorption mode for 2000 cycles. Figure S11a,b (Supporting In-formation) shows the change in intensity with cycle number. The𝜷-NiOx1@MSS sensor shows extended cycle life with 85% reten-tion of initial intensity even after 2000 cycles, which confirms that𝜷-NiOx1 is an excellent receptor material for VFA sensing. Also,we have confirmed that sensor devices are stable under storagein the dark for periods beyond 2 years indicating excellent possi-ble shelf-lives of the 𝜷-NiOx1@MSS sensor devices (see FigureS11c,d, Supporting Information).In summary, the receptormaterial 𝜷-NiOx1 can be used in con-junction with MSS nanomechanical sensor technology (as the𝜷-NiOx1@MSS sensor device) for the estimation of low molec-ular weight volatile fatty acids (VFAs) in admixture at high rel-ative humidity at the low concentrations suitable for monitor-ing of AD reactor biogas plants. This work thus obviates the ex-isting requirement of expensive equipment or time-consuminglaboratory testing for monitoring the acid concentration makingit possible to replace those with an inexpensive, disposable (orreusable) chip sensor used in conjunction with a mobile device.The substantial differentiation of analytes at low concentrationin aqueous media (here in the vapor phase) represents a mile-stone for sensing technologies aimed at monitoring and diagno-sis since it is possible not only to adapt the technology from themolecular level up to customize the sensor response to partic-ular analytes but also to employ machine learning techniques toparse data for the estimation of the compositions of complexmix-tures. In the case described here, monitoring of AD reactors canbe used to optimize reaction conditions and also to prevent costlyreactor downtime toward an efficient carbon sustainable society.Additionally, we note that the presence and composition of VFAscontained in the human gut biome are an important factor in de-termining general digestive health[69] but also affect several dis-ease states,[70] including cancer,[71] and has even been reportedto affect cognitive function[72] further improving the prospects oftechnologies aimed at monitoring VFAs in situ or batch analyses.We are currently investigating how the present system might beadopted for this purpose.4. Experimental SectionGeneral Experimental: Reagents and dehydrated solvents (in septum-sealed bottles) used for syntheses and spectroscopic measurementswere obtained from Tokyo Kasei Chemical Co., Wako Chemical Co.,or Aldrich Chemical Co. and were used without further purification.Electronic absorption spectra were measured using a JASCO V-570UV/Vis/NIR spectrophotometer or a Princeton Applied Research (PAR)diode array rapid scanning spectrometer. ATR-FTIR spectra were obtainedusing a Thermo-Nicolet 760X FTIR spectrophotometer equipped with aSMART-iTX ATR accessory. Electron spin resonance (ESR) spectra weremeasured from solid samples using a JEOL JES-FA200 spectrometerequipped with an ES-CT470 liquid He variable temperature system,with data recorded and processed using A-System version 1.6.5 PCIJ/X-Band and FA-Manager version 1.2.9 V2 series software. PowderX-ray diffraction (pXRD) patterns were recorded at room temperatureusing with Cu-K𝛼 radiation (𝜆 = 1.54184 Å) on a Rigaku MiniFlex 600diffractometer. 1H-NMR spectra were obtained using JEOL AL300BXor JEOL AL400SSS spectrometers operating respectively at 300 and400 MHz and using tetramethylsilane as an internal standard. Protondecoupled 13C-NMR spectra were obtained using JEOL AL300BX or JEOLAL400SSS spectrometers operating respectively at 76 and 101 MHz andusing tetramethylsilane as an internal standard. Data was processedon Delta version 5.0.5.1, Always JNM-AL version 6.2, and MestReNova6.0.2. 1H NMR chemical shifts (𝛿) were reported in ppm relative toTMS for CDCl3 (𝛿 0.00) or the residual solvent peak for other solvents.13C NMR chemical shifts (𝛿) were reported in ppm relative to thesolvent reported. Coupling constants (J) were expressed in Hertz (Hz),shift multiplicities were reported as singlet (s), doublet (d), triplet (t),quartet (q), double doublet (dd), multiplet (m), and broad singlet (bs).High-resolution electrospray ionization time-of-flight mass spectra(ESI-TOF-MS) were measured using a Thermo Scientific Q-Exactive Plusinstrument. Tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphinatonickel(ii)([T(DtBHP)P]Ni), 2-formyl-5,10,15,20-tetrakis-(3,5-di-t-butyl-4-hydroxyphenyl)porphinatonickel(ii)[55] ([T(DtBHP)P]Ni-CHO) and5,6-bis(4-bromophenyl)pyrazine-2,3-diamine[73] were prepared accordingto literature methods.Synthesis: Comprehensive details of the synthesis of receptormolecules are given in the Supporting Information. Briefly for 𝜷-NiOx1and 𝜷-NiOx3, a round-bottom flask was charged with 4BrPh-Ni(Benzimd)(400mg, 62 μmol) dissolved in CH2Cl2 (10 mL). 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (56mg, 0.25mmol) was added. Themixture was stirredfor 1 h at room temperature with monitoring by thin layer chromatog-raphy, then the solvents were removed under reduced pressure, and thesolid residue was purified by column chromatography (SiO2) eluting withCH2Cl2, yielding 𝜷-NiOx1 (186 mg, 32% yield) as a dark olive solid. In-creasing the polarity of the eluent to 199:1 CH2Cl2/methanol mixture pro-vided 𝜷-NiOx3 (210 mg, 28% yield) as a brown solid. 𝜷-NiOx1: UV/Vis(CH2Cl2): 𝜆max (𝜖, M−1 cm−1)= 379 (33 200), 451 (176 000), 552 (16 500),593 (29 000) nm. 1H NMR (400 MHz, CDCl3): 𝛿 9.64 (s, 1H), 9.42 (d, J =5.2 Hz, 1H), 8.95 (d, J = 4.8 Hz, 1H), 8.85 (d, J = 5.2 Hz, 1H), 8.81 – 8.76(m, 3H), 7.84 (s, 2H), 7.77 (d, J = 3.2 Hz, 4H), 7.48 (s, 1H), 7.45 (d, J =2.8 Hz, 5H), 7.27 (s, 1H), 7.25 (s, 1H), 7.14 (s, 2H), 5.57 (s, 1H), 5.51 (s,1H), 5.50 (s, 1H), 1.60 (s, 18H), 1.57 (s, 18H), 1.56 (s, 18H), 1.20 (s, 18H).13C NMR (100.5 MHz, CDCl3): 𝛿 186.2, 154.2, 153.9, 151.4, 149.6, 147.7,146.2, 145.6, 144.5, 144.3, 144.1, 144.0, 143.3, 142.5, 140.8, 140.1, 139.9,138.5, 138.0, 135.2, 134.7, 134.5, 134.4, 134.3, 133.6, 133.4, 133.2, 133.1,132.1, 132.1, 131.5, 131.2, 131.1, 130.9, 125.6, 122.8, 122.7, 122.4, 122.0,121.3, 108.8, 63.9, 35.5, 34.6, 30.6, 29.3 ppm. FTIR (ATR): 𝜈 = 3638 (m),2956 (s), 2912 (w), 2871 (m), 1724 (w), 1703 (w), 1665 (m), 1644 (m),1589 (w), 1515 (w), 1485 (m), 1458 (m), 1433 (s), 1389 (m), 1363 (w),1350 (s), 1312 (m), 1232 (m), 1223 (m), 1196 (s), 1144 (m), 1121 (m),Adv. Mater. Technol. 2025, 10, e00294 e00294 (9 of 11) © 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2025, 17, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/admt.202500294 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmattechnol.dewww.advancedsciencenews.com www.advmattechnol.de1072 (m), 1010 (s), 959 (w), 940 (m), 915 (w), 881 (m), 856 (w), 830 (s),800 (m), 782 (w), 773 (w), 754 (w), 723 (m), 710 (s), 694 (w), 685 (m), 668(w), 654 (w), 633 (w), 618 (w) cm−1. HRMS: calc’d for C93H99Br2N8NiO4:1608.5538; found: 1608.5464 ([M + H]+).Specific Surface Area Determinations: Nitrogen adsorption/desorptionisotherms were recorded on a BELSORB Max instrument for the estima-tion of Brunauer–Emmett–Teller (BET) surface area. Samples were loadedinto a cell for measurement and heated at 120 °C under reduced pressurefor 24 h prior to the collection of the adsorption/desorption isotherms.X-Ray Crystallography: Crystals of 𝜷-NiOx1 were grown by diffusion ofmethanol into a solution in chloroform. The single crystal X-ray structurewas determined using a Bruker D8 Venture diffractometer with a multi-layered confocal X-ray mirror and MoK𝛼 radiation (𝜆 = 0.71073 Å) gen-erated at 50 kV and 1.6 mA. Details of data collection and refinementare given in the Supporting Information. Crystallographic data (exclud-ing structure factors) have been deposited with the Cambridge Crystal-lographic Data Centre with CCDC reference number 2 350 299 (𝜷-NiOx1).Copies of the data can be obtained, free of charge, on application to CCDC,12Union Road, Cambridge CB2 1EZ, UK http://www.ccdc.cam.ac.uk/perl/catreq/catreq.cgi, e-mail: data_request@ ccdc.cam.ac.uk, or fax:+44 1223336 033.Sensor Preparation: Sensing receptor materials were deposited di-rectly onto the four-channel MSS membrane by inkjet deposition usingan inkjet spotter (LaboJet-500SP) with a selected nozzle (IJHBS-300) sup-plied by MICROJET Corporation. Sensor receptor material 𝜷-NiOx1 wasdissolved in dimethylsulfoxide (DMSO) at c = 1 mg mL−1 for the inkjetdeposition. Different numbers of sequential droplet (50-300) depositionswere performed. The inkjet stage temperature was set at 70 °C to controlthe evaporation rate during the deposition process.Selectivity Tests: Selectivity tests of the inkjet-spotted receptor materi-als on the MSS chip were performed at ambient temperature using thedifferent VOCs at 20% vapor saturation. Analytes included acids, alco-hols, and aldehydes. Ketones and hydrocarbons. Vapors were introducedto the sensor chamber for 30 s using a custom-built setup (Figure 5a)then purged with nitrogen gas for 60 s. Four sampling-purging cycles wereperformed for each vapor and data were recorded at a sampling rate of20 Hz by applying a bridge voltage of −0.5 V to the Wheatstone bridge.Here only the 4th cycle was shown and discussed the results accord-ingly. Sensitivity of the receptor materials to different low-concentrationVFAs was performed under an atmosphere saturated with water vapor.To perform different low-concentration (10–500 mM) solutions of aceticacid, propionic acid, and butyric acid in water medium, and measure theirheadspace concentration to identify the amount of acids in the headspacewas prepared. Naturally, this headspace vapor was saturated with watervapor which exactly replicates the ideal condition of the biogas reactor.The headspace acid concentration of the acid solution was also checkedby highly sensitive proton-transfer reaction mass spectrometry (PTR-MS)methods as a reference. Several different mixtures of propionic acid andbutyric acid with a fixed amount of acetic acid were prepared and measuretheir headspace concentration to see the variation in MSS signals. Solu-tions containing a fixed concentration of acetic acid (40 mM) with a rangeof propionic and butyric acid concentrations (10–200 mM) were preparedin water, and the headspace vapor of the acidmixture wasmonitored using𝜷-NiOx1@MSS. To check the real-time detection limit the MSS saturationintensity was calibrated with the solution concentration of propionic andbutyric acid.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThe authors are grateful for support by the JST-ERATO Yamauchi Materi-als Space-Tectonics Project (JPMJER2003), and the Queensland Node ofthe Australian National Fabrication Facility (ANFF-Q). The authors alsothank Japan Society for the Promotion of Science (JSPS) through KAKENHIGrant Nos. JP20H00392 and JP23H05459.Conflict of InterestS.M., M.K.C., G.Y., and J.P.H. are inventors on WIPO patent applicationnumber WO2023282200A1, submitted by the National Institute for Mate-rials Science (NIMS). J.H., K.N., A.H., K.M., J.L., D.T.P., G.J.R., K.A., Y.Y.,and L.K.S. declare no conflict of interest.Author ContributionsS.M. and J.P.H. designed and initiated the project. J.H. and M.K.C. per-formed preliminary synthesis and characterization. K.N., A.H, and G.J.H.performed X-ray crystallographic analyses and refinements. D.T.P. under-took spectroscopic characterization of the materials. S.M., K.M., and G.Y.performed and analyzed the sensing properties of themolecules. J.L. mod-eled the sensing signals elicited from the sensing devices. K.A., Y.Y., andL.K.S. measured and analyzed the textural parameters of the materials.J.P.H. supervised the research, proposed and established the moleculardesign, and performed the large-scale synthesis of the compounds. S.M.and J.P.H. wrote the manuscript with contributions from all the authors.All authors analyzed the results and contributed to the writing and editingof the manuscript.Data Availability StatementThe data that support the findings of this study are available in the sup-plementary material of this article.Keywordsmicroporous coordination complex, molecular sensing layer, nanoarchi-tectonics, nanomechanical device, volatile fatty acid sensorReceived: March 12, 2025Revised: May 2, 2025Published online: May 28, 2025[1] Global Potential of Biogas, http://www.worldbiogasassociation.org/global-potential-of-biogas/ (accessed: July 2019).[2] A. Costa, C. Ely, M. Pennington, S. Rock, C. Staniec, J. Turgeon,Anaerobic Digestion and its Applications, Environmental ProtectionAgency Bulletin, 2015 (EPA/600/R-15/304), United States.[3] M. Zamanzadeh, L. H. Hagen, K. Svensson, R. Linjordet, S. J. Horn,Sci. Rep. 2017, 7, 17664.[4] E. D. Erickson, P. A. Tominac, V. M. Zavala,Nat. 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Introduction 2. Results 3. Discussion and Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Author Contributions Data Availability Statement Keywords