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Michihiro Nishikawa, Tomohiro Murata, [Shinsuke Ishihara](https://orcid.org/0000-0001-6854-6032), [Kota Shiba](https://orcid.org/0000-0001-7775-0318), [Lok Kumar Shrestha](https://orcid.org/0000-0003-2680-6291), [Genki Yoshikawa](https://orcid.org/0000-0002-9136-8964), [Kosuke Minami](https://orcid.org/0000-0003-4145-1118), [Katsuhiko Ariga](https://orcid.org/0000-0002-2445-2955)

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[Discrimination of Methanol from Ethanol in Gasoline Using a Membrane-type Surface Stress Sensor Coated with Copper(I) Complex](https://mdr.nims.go.jp/datasets/b8f376f0-c392-4be0-abb3-0236af9c0066)

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Selected PaperArticle for Frontiers of Molecular ScienceDiscrimination of Methanol from Ethanol in GasolineUsing a Membrane-type Surface Stress Sensor Coatedwith Copper(I) Complex#Michihiro Nishikawa,*1,2 Tomohiro Murata,3 Shinsuke Ishihara,1 Kota Shiba,1,4,5Lok Kumar Shrestha,1 Genki Yoshikawa,*1,4,6 Kosuke Minami,*1,4,7 and Katsuhiko Ariga*1,31World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA),National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan2Research Center for Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278-8510, Japan3Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo,5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan4Center for Functional Sensor & Actuator (CFSN), Research Center for Functional Materials,National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan5John A. Paulson School of Engineering and Applied Sciences, Harvard University,9 Oxford Street, Cambridge, Massachusetts 02138, USA6Materials Science and Engineering, Graduate School of Pure and Applied Science, University of Tsukuba,1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan7International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS),1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: nishikawa5@rs.tus.ac.jp (M. Nishikawa), YOSHIKAWA.Genki@nims.go.jp (G. Yoshikawa),MINAMI.Kosuke@nims.go.jp (K. Minami), ARIGA.Katsuhiko@nims.go.jp (K. Ariga)Received: October 30, 2020; Accepted: November 12, 2020; Web Released: January 13, 2021Michihiro NishikawaMichihiro Nishikawa received his Ph.D. from The University of Tokyo in 2013. He was an assistant professorat Seikei University from 2013 to 2018. He joined NIMS as a NIMS postdoctoral researcher from 2018to 2019. He joined the Department of Chemistry, School of Science, The University of Tokyo as projectassistant professor in 2019 and moved to Tokyo University of Science in 2020. His research interests arecoordination chemistry, photochemistry, and electrochemistry of metal complexes especially for copper(I)complex.Genki YoshikawaGenki Yoshikawa is a Group Leader at the Center for Functional Sensor & Actuator (CFSN), NationalInstitute for Materials Science (NIMS). He received his Ph.D. in science from The University of Tokyo in2004. His research interests are focused on the development of olfactory sensors.Document type: Article648 | Bull. Chem. Soc. Jpn. 2021, 94, 648–654 | doi:10.1246/bcsj.20200347 © 2021 The Chemical Society of Japanhttps://doi.org/10.1246/bcsj.20200347Kosuke MinamiKosuke Minami received his B.Sc. from The University of Tokyo in 2008, majoring in insect biology. Heobtained his Ph.D. in organic chemistry under the guidance of Prof. Eiichi Nakamura from The University ofTokyo in collaboration with Prof. Eisei Noiri at Faculty of Medicine, University Hospital, The University ofTokyo in 2013. He started his work as a postdoctoral fellow at NIMS from 2013 and became an ICYSresearcher as an independent research position in 2019.Katsuhiko ArigaKatsuhiko Ariga received his Ph.D. from Tokyo Institute of Technology in 1990. He is currently the Leader ofthe Supermolecules Group and Principal Investigator at the World Premier International Research Centre forMaterials Nanoarchitectonics (MANA), NIMS. He has also been appointed as Professor at the University ofTokyo.AbstractWe successfully fabricated a novel sensing platform, aMembrane-type Surface stress Sensor (MSS) coated with cop-per(I) complex bearing phen and BINAP ligands, [Cu(phen)-((«)-BINAP)]PF6 (1, phen = 1,10-phenanthroline, BINAP =2,2¤-bis(diphenylphosphino)-1,1¤-binaphthyl), for specificmolecular sensing. Based on the transduction of mechanicalstresses derived from sorption-induced deformation of Cu(I)complex, the detection performance of various volatile organiccompounds (VOCs) has been investigated. The fabricatedsensor devices showed selective responses to methanol over awide range of VOCs. In addition, distinct MSS signals uponexposure to methanol were observed for mixing samples ofmethanol in n-hexane and gasoline with clear discrimination ofethanol mixtures. In fact, gasoline vapor with 1% methanolexhibited much higher MSS responses than 20% ethanol con-taining gasoline samples. Methanol contamination in gasolineand related petroleum samples is a world-wide common prob-lem in the automobile and fuel sectors where detection ofmethanol contaminants with portable devices by easy proce-dures is required. The current research results will contribute tofulfilling these social demands.Keywords: Membrane-type Surface stress Sensor (MSS) jCopper(I) complex j Methanol fuel1. IntroductionOne of the crucial demands in current society is a develop-ment of sensing systems for the detection and removal ofenvironmental risks and biological dangers with simple andinexpensive procedures.1­3 For example, illegal usages ofmethanol in alcoholic beverages and gasoline as fake-ethanolhave to be instantly detected to avoid serious health problemsand automobile damage, respectively. However, the rather fea-tureless nature of methanol and its characteristic resemblanceto ethanol make methanol sensing unexpectedly difficult.Although the discrimination between methanol and ethanolcan be analyzed by proper analytical instrumentation (e.g. gaschromatography-mass spectrometry, NMR), facile and selectivedetection of methanol is still a challenging subject amongvarious sensing demands.Various research efforts on methanol sensing and relatedmatters have been actively made and been reported.4­16 Forexample, Ishihara et al. reported the colorimetric discrimina-tion of methanol from ethanol using a composite film based onporphyrin-type dye (oxoporphyrinogen) and layered doublehydroxide.4 Because of the visible color change upon metha-nol exposure, the composite films can be applied as low-cost and portable colorimetric sensing systems for methanol ingasoline.4,5 The other promising approach to methanol sensingis Raman-based sensors.7­9 The characteristic Raman signal ofmethanol can be discriminated from gasoline,8 leading to thedetection of methanol in gasoline. However, the sensingdevices of methanol from mixtures such as gasoline usingrepeatable and mobile systems are still limited, preventing apractical implementation of the methanol sensors. Facile detec-tion systems with high sensitivity for methanol in gasoline orhigh ethanol content samples are awaited to be explored.As a novel sensing system for versatile usages, we haverecently developed a new type of static mode nanomechanicalsensor®Membrane-type Surface stress Sensor (MSS). TheMSS is composed of a silicon-based membrane suspended byfour piezoresistive sensing beams, composing a fullWheatstonebridge, achieving high sensitivity with compactness.17,18 TheMSS exhibits higher repeatability than a conventional canti-lever-type nanomechanical sensor, due to the high robust-ness.19,20 The MSS transduces surface stresses derived from theabsorption/desorption-induced deformation (i.e. swelling orshrinking) of a receptor layer coated on the membrane. There-fore, any kind of interaction between target molecules and thereceptor materials accompanied by mechanical deformation ispotentially detected by the MSS. Among various candidates ofsensing materials, the selectivity can be tuned by appropriatefabrication of sensing materials,21­30 while it is still limited tofabricate the sensing materials, which have high sensitivity tomethanol, distinguishing methanol and ethanol.22,30 In the pres-ent paper, we have found that Cu(I) complex bearing diimineand diphosphine ligands have a high capability for selectivesensing of methanol.Cu(I) complexes bearing diimine and diphosphine ligandsfrom a cheap abundant metal source are promising metal com-plexes for many applications such as photoredox reactions,31­33light emitting devices,34 and luminescence sensors for oxy-Bull. Chem. Soc. Jpn. 2021, 94, 648–654 | doi:10.1246/bcsj.20200347 © 2021 The Chemical Society of Japan | 649https://doi.org/10.1246/bcsj.20200347gen.35 However, the fabrication of electric sensors based onsolid of this type of Cu(I) complex has not been fully explored.Here, we report a gas sensor, which is capable of detectingmethanol contamination in gasoline, based on MSS coated withsolid Cu(I) complex (Figure 1). Aiming to fabricate solid ofCu(I) complex as a receptor for MSS, we selected [Cu(phen)-((«)-BINAP)]PF6 (1, phen = 1,10-phenanthroline, BINAP =2,2¤-bis(diphenylphosphino)-1,1¤-binaphthyl). Since the com-plex metal center is surrounded by hydrogen atoms in aromaticC-H bonds or fluorine atoms in PF6 anion, weak interactionbetween these moieties would work for the effective absorp-tion of methanol and the rigid and sterically hindered ligandseffectively form a densely packed bulk receptor layer, whichgives limited intermolecular space, leading to the discrimina-tion of small molecules sensitively. This type of Cu(I) complexenables us to expect methanol molecule to interact with thesemoieties for facile and selective detection of methanol by MSSdevices.2. Experimental2.1 Materials. Tetrakis(acetonitrile)copper(I) hexafluoro-phosphate ([Cu(CH3CN)4]PF4) and 1,10-phenanthroline (phen)were purchased from Aldrich. («)-2,2¤-bis(diphenylphosphi-no)-1,1¤-binaphthyl ((«)-BINAP) was purchased from TokyoChemical Industry Co. Ltd. Dichloromethane, diethyl ether,N,N-dimethylacetamide (DMA), and all deuterated solventswere purchased from Aldrich, Tokyo Chemical Industry Co.Ltd., and Wako Pure Chemical, and used as purchased. As forthe sensing measurement, methanol, ethanol, 2-propanol, n-hexane, n-heptane, benzene, toluene, acetone, and ethyl acetatein analytical grades were used. Regular gasoline was purchasedfrom ENEOS corporation. To obtain water vapor, MilliQ waterwas used (Merck MilliPore).2.2 Synthesis of [Cu(phen)((«)-BINAP)]PF6 1.[Cu(phen)((«)-BINAP)]PF6 1 was synthesized according to amodified procedure described in the literature.36 [Cu(CH3CN)4]-PF4 (186mg, 0.500mmol) was added to a solution of («)-BINAP (313mg, 0.503mmol; 1.0 equiv.) and phen (90.2mg,0.501mmol; 1.0 equiv.) in CH2Cl2 (7.5mL). After stirring for1.5 h at room temperature under air, diethyl ether (0.5mL) wasadded to the reaction mixture to obtain a yellow suspension.The suspension was collected by filtration, washed with diethylether (2.5mL), and dried under reduced pressure to yield[Cu(phen)((«)-BINAP)]PF6 1 (297mg, 59%). 1HNMR (ace-tone-d6, 300.40MHz) ¤ = 9.27 (d, J = 4.8Hz, 2H), 8.91 (d,J = 8.1Hz, 2H), 8.34 (s, 2H), 8.13 (dd, J = 4.8, 8.1Hz, 2H),7.88 (d, J = 8.1Hz, 2H), 7.76 (d, J = 8.1Hz, 2H), 7.48­7.38(m, 8H), 7.36­7.10 (m, 12H) 6.96 (d, J = 8.0Hz, 2H), 6.85(dd, J = 7.8, 7.8Hz, 2H), 6.70 (dd, J = 7.8, 7.8Hz, 4H)(Figure S1).1HNMR spectra were recorded with a JEOL AL300 FT-NMR spectrometer. We note that the related parent complexes,[Cu(phen)((S)-BINAP)]PF636 are known compounds with suffi-cient identifications including NMR data.2.3 Fabrication of Cu(I) Complex-Coated MSS.[Cu(phen)((«)-BINAP)]PF6 1 was directly coated onto thesurface of the MSS by inkjet spotting using an inkjet spotter(LaboJet-500SP, Microjet Co. Ltd.) equipped with a nozzle(IJHBS-300, Microjet Co. Ltd.). [Cu(phen)((«)-BINAP)]PF6 1(0.72mg) was dissolved in 2.5mL of DMA and then sonicatedfor ca. 5min. The resulting yellow solution of 1 was loadedinto the inkjet nozzle, and 300 sequential droplet depositionswere performed. The inkjet stage was heated to 80 °C to controlevaporation of DMA.2.4 Sensing System and Procedure. The Cu(I) complex-coated MSS chip was loaded in a Teflon chamber placed in anincubator (HCRCSIV-A series) at a controlled temperature of25.00 « 0.02 °C. The chamber was connected to a gas flow sys-tem consisting of two mass flow controllers (MFCs), a mixingchamber, a purging gas line, and a vial for target sample liquid,including a single component guest and guest mixtures. Thecarrier nitrogen gas, which was saturated vapor with targetsolvent vapors, was then diluted at 20% with pure nitrogen gasby using two MFCs. The diluted vapor was introduced to thesensor for 30 sec. Four injection-purging (ON-OFF) cycleswere performed and data were recorded at a sampling rate of 20Hz by applying a bridge voltage of ¹0.5V to the Wheatstonebridge. The selectivity tests were performed using 8 differentvapors (i.e. water, methanol, ethanol, 2-propanol, acetone, ethylacetate, n-hexane, benzene, and toluene were used in thispresent study). As a reference, the signal response of bare MSSto methanol was also measured (see supporting information).As the model of methanol fuel, detections of methanol wereperformed using the mixtures of methanol in n-hexane. Metha-nol was diluted with n-hexane in concentrations ranging from 0to 100mol%. The resulting solvent vapors were introduced tothe sensor. In the actual gas sensing, 1 vol% of methanol inregular gasoline was used, and 20 vol% of ethanol in gasoline(commonly known as E20 fuel) was used as a reference.Figure 1. Membrane-type Surface stress Sensor (MSS)coated with copper(I) complex 1. (a) The schematic illus-trations of the configuration of MSS and the structureof Cu(I) complex bearing phen and BINAP ligands,[Cu(phen)((«)-BINAP)]PF6 1 (phen = 1,10-phenanthro-line, BINAP = 2,2¤-bis(diphenylphosphino)-1,1¤-binaphth-yl). (b) Working principle of the detection of gases using theMSS. (c) Typical signal outputs of Cu(I) complex-coatedMSS for various VOCs.650 | Bull. Chem. Soc. Jpn. 2021, 94, 648–654 | doi:10.1246/bcsj.20200347 © 2021 The Chemical Society of Japanhttps://doi.org/10.1246/bcsj.20200347To identify the chemical selectivity of the Cu(I) complex-coated MSS for the varieties of VOCs, we evaluated the signalintensity for each VOC extracted from the last three signalresponses from 90 to 330 sec out of four signal responses ineach measurement, because the latter signal responses canprovide more reproducible signal responses than the formerones, which exhibit initial fluctuations associated with mixingof sample gases and pre-adsorbed gases.3. Results and Discussion[Cu(phen)((«)-BINAP)]PF6 1 was synthesized according tothe modified procedure described in literature.36 As-synthesizedCu(I) complex 1 exhibited high solubility in several organicsolvents, especially polar solvents, such as chloroform, acetone,dimethylsulfoxide, and so on (Figure S1­S3).31 The Ramanspectrum of the as-synthesized solid of 1 shows obvious signalsof 1209, 1299, 1342, 1375, 1422, 1450, 1511, and 1589 cm¹1(Figure S4). These signals in a range from 1400 to 1600 cm¹1can be interpreted as bands derived from the aromatic groupsdue to C=C or C=N bending of aromatic rings such as phenyl,binaphthyl, and diimine moieties. In the case of this type ofheteroleptic Cu(I) complexes, the complex metal center is sur-rounded by sterically bulky aromatic ligands, preventing thereactivity of the complexes with solvent molecules.36 We alsoevaluated the reactivity of 1 to methanol. The chemical shift ofCu(I) complex 1 in acetone-d6 in the presence of methanol-d6was measured. As shown in Figure S5, the chemical shift of 1in acetone-d6­methanol-d6 (v/v 100:1) is similar to that inacetone-d6, indicating that the reaction of Cu(I) complex 1 tomethanol is negligible at least in the solution state even in thepresence of an excess amount of methanol, due to the metalcenter surrounded by the aromatic ligands. As the complexmetal center is surrounded by hydrogen atoms in aromatic C-Hbonds or fluorine atoms in PF6 anion, weak interaction betweenthese moieties is expected to work for discrimination of smallmolecules sensitively.Chemical stability is one of the most important factors ofsensing materials for practical applications. We evaluated thechemical stability of the Cu(I) complex by using 1HNMR mea-surement. The chemical shifts of all signals of the 1HNMRspectrum after dissolving the Cu(I) complex 1 in acetone-d6at room temperature within 1 hour are the same as those of1HNMR after 1 day and 6 days under the same conditions(Figure S1). In addition, the chemical shifts of Cu(I) complex 1in acetone-d6­methanol-d6 (v/v 100:1) showed no significantchanges for 8 days (Figure S5). These results indicate that theacetone solution of Cu(I) complex 1 is stable enough under airat room temperature for more than 8 days. Importantly, thepowder 1, which was kept at room temperature under air for 6months, was dissolved in acetone-d6 and then 1HNMR mea-surement was performed. As shown in Figure S1, the signalsare similar to as-synthesized complex 1 in acetone-d6, indicat-ing that the Cu(I) complex 1 in the solid phase is highly stableunder ambient conditions. These results suggest that the Cu(I)complex 1 is suitable for the sensing materials of nano-mechanical sensing.We fabricated the Cu(I) complex-based MSS by inkjetspotting directly onto the membrane of the MSS. The sensingresponses to the various solvent vapors are shown in Figure 2(detailed responses are shown in Figure S6). The Cu(I) com-plex film effectively absorbs various solvent vapors and gen-erates the absorption-induced deformation (see also Figure S7for the signal response of bare MSS). The Cu(I) complex-basedMSS yields significantly high intensity to methanol vapor upto 8mV, and approximately 8­10 fold higher selectivity tomethanol can be observed compared to a wide range of theother organic solvent vapors including polar (i.e. ethanol, 2-propanol, acetone, and ethyl acetate) and non-polar solvents(i.e. n-hexane, benzene, and toluene). It should be noted thatthe sensitivity of the nanomechanical sensing based on gas­solid equilibrium, including the MSS, generally depends onpartial vapor pressure, which is a vapor pressure divided by theFigure 2. Responses of [Cu(phen)((«)-BINAP)]PF6 1 to various vapors. (a­f ) Signal responses of 1 to water (a); alcohols (i.e.methanol (solid line), ethanol (dashed line), and 2-propanol (dotted line)) (b); acetone (c); ethyl acetate (d); alkanes (i.e. n-hexane(solid line) and n-heptane (dashed line)) (e); and aromatics (i.e. benzene (solid line) and toluene (dashed line)) (f ). Details areshown in Figure S6. (g) Intensities of 1 to various vapors. Colors correspond to those used in (a)­(f ).Bull. Chem. Soc. Jpn. 2021, 94, 648–654 | doi:10.1246/bcsj.20200347 © 2021 The Chemical Society of Japan | 651https://doi.org/10.1246/bcsj.20200347saturated vapor pressure, rather than the absolute concentrationof vapors.24,37 Although 20% saturated methanol vapor (33,000ppm) is slightly smaller than that of n-hexane (39,000 ppm) andis slightly higher than that of acetone (44,000 ppm) calculatedby the saturated vapor pressure (Table S1),38 the partial vaporpressure of the samples was fixed at 20% by precisely control-ling the two MFCs. Thus, the specific interaction of methanolwith hydrogen atoms in aromatic C­H bonds and/or fluorineatoms in PF6¹ anion of the Cu(I) complex 1 may enhance theabsorption of methanol molecules in the receptor layer, leadingto the superior selectivity to methanol.Importantly, the Cu(I) complex yields high selectivity tomethanol even among the series of alcohols (i.e. methanol,ethanol, and 2-propanol). Figure S8 depicts the signal inten-sities of VOCs including water as a function of molecularweight, which is related to the volume of each molecule. Thesignal intensities to the samples, whose molecular weights arehigher than methanol, dramatically decreased to less than 1mV,while the intensity to water yields relatively high intensity(3mV) compared to the other organic vapors. This result sug-gests that methanol molecule is small enough to interact withthe Cu(I) complex in the solid phase of the receptor layer, dueto the rigid structures of aromatic ligands without any flexiblefunctional groups (e.g. alkyl chains). Although water moleculeis smaller than methanol, the sensitivity to water is lower thanmethanol, possibly due to the hydrophobic nature of aromaticligands on the Cu(I) complex 1.With the superior selectivity of the Cu(I) complex-basedMSS to methanol compared to n-hexane, we also examined thespecific detection of methanol from the mixture of methanoland other organic solvents (i.e. n-hexane). The gas sensingmeasurements were performed using a wide range of mixturesof methanol/n-hexane (Figure S9). The signal intensities forthe mixtures are plotted as a function of molar ratios of alco-holic contents in n-hexane (Figure 3). It has to be noted that themixture of methanol and n-hexane with 76mol% is obviouslyseparated into two phases, due to the low miscibility of metha-nol in n-hexane (up to 21mol%).39 Since the molar ratio ofmethanol in the vapor phase as well as the partial pressure ofmethanol are almost constant in a range from 14.4mol% (16.01kPa) to 94.0mol% (16.70 kPa) at 25 °C,39 the signal intensitiesreached constant values over 14mol% (ca. 6mV). In contrast tothe case of the high content of methanol in n-hexane, the signalintensities are drastically enhanced with linear correlation tothe increase of methanol contents at the range from 1.6mol% to6.1mol% (Figure 3b). This linear correlation is attributed to theincrease of the concentration of methanol in the vapor phase.Indeed, the reported molar ratio of methanol and total pres-sure are drastically changed in a range from 0mol% in liquid(0mol% in vapor, 16.16 kPa), 0.8mol% in liquid (29mol% invapor, 22.37 kPa), and 8.5mol% in liquid (42mol% in vapor,27.50 kPa).39 Importantly, ethanol in n-hexane exhibited insen-sitive nature over a wide range of mixing ratios. In contrastto methanol, ethanol can be freely miscible with n-hexane,whereas the signal intensity did not show an increase evenin the high concentration of ethanol vapor (Figure 3a andFigure S10). These results clearly suggested the utility of theCu(I) complex-based MSS for the detection of methanol ingasoline as well as the mixture of high ethanol content samples.Finally, we have demonstrated the availability of the Cu(I)complex-coated MSS for the methanol sensing of methanol-containing gasoline. The Cu(I) complex-coated MSS wasexposed to the vapor of gasoline with 1% methanol (Figure 4).The Cu(I) complex-coated MSS yielded significant sensitivityto methanol-containing gasoline as compared to the ethanol-containing gasoline, even when the ethanol content in gasolineis 20 times higher than methanol in gasoline. This result clearlyindicates the efficient detection and discrimination betweenmethanol and ethanol in gasoline are successfully achieved.Figure 3. Selective response of Cu(I) complex 1 to metha-nol in n-hexane. (a) Intensities of 1 to methanol (redcircles) and ethanol (blue circles) in n-hexane ranging from0 to 100mol%. (b) Magnified plots of methanol in n-hexane ranging from 0 to 15mol%. A black line indicatesthe line fit with R2 value.Figure 4. Detection of methanol in gasoline by Cu(I)complex-coated MSS. Red and blue lines are 1 vol% ofmethanol in gasoline and 20 vol% of ethanol in gasoline(the model of E20 fuel).652 | Bull. Chem. Soc. Jpn. 2021, 94, 648–654 | doi:10.1246/bcsj.20200347 © 2021 The Chemical Society of Japanhttps://doi.org/10.1246/bcsj.20200347Detailed mechanisms on the superior sensitivity of the MSScoated with Cu(I) complex 1 are not fully clear. The highaffinity of methanol to this Cu(I) complex would be one of theeffective factors of such selective sensitivity to methanol. Wespeculate from the experimental facts that crystals of this type ofCu(I) complexes often include methanol molecules as a co-existing component in the crystals.40 The results would suggestthe existence of intermolecular space among the Cu(I) complex1 in the solid phase, which may fit the size of methanol mole-cule. In contrast to methanol, other relatively large analytes maynot penetrate into the receptor layer, resulting in the low signalresponses. We note that powder XRD patterns of Cu(I) complex1 film on glass plate are not much influenced by methanol vapor(Figure S11), suggesting that the aforementioned intermolecu-lar space may be in amorphous and/or short-range order states.Inappropriate contamination of expensive safe ethanol byinexpensive toxic methanol in gasoline as well as in beveragesis a serious problem. Contaminated methanol in gasolinecauses heavy damage to automobile engines, while that indrinks induces serious health problems. The sensor systemspresented in this study would make significant contributionsto safe and reliable social activities. In addition, the Cu(I)complex-based MSS yielded low sensitivity to water vapor,suggesting a highly advantageous feature for sensor usagesunder practical conditions, such as in the presence of moistureor humidified conditions.4. ConclusionWe successfully evaluated the utility of heteroleptic cop-per(I) complex bearing phen and BINAP ligands for methanolsensing. The fabricated sensor exhibited high selectivity tomethanol vapor over a wide range of VOCs. In addition, dis-tinct signals of the Cu(I) complex MSS upon exposure tomethanol were observed for mixing samples of methanol in n-hexane and in gasoline, which can be clearly discriminatedfrom those for ethanol mixtures. Because of the varieties ofligands with various functional groups,31­35 the present studyprovides an effective platform for nanomechanical sensormaterials using structure-designed sensitive complexes such asa metal-coordinated complex with structured ligands.Since the MSS can be portable, the developed sensing sys-tem whose detection range of methanol contaminants underambient conditions covers 1­10% could be practically useful.Not limited by the necessity of ultrahigh sensitive detection forhighly toxic substances, 1­10% range selective detection forcommon chemicals such as methanol is in high demand fordaily life activities. For example, methanol-contaminated gaso-line and related petroleum products is a world-wide commonproblem in the automobile and fuel sectors where detection ofmethanol contaminants with portable devices by easy proce-dures is required. The current research results will make signifi-cant contribution to these social demands.This study was partially supported by JSPS KAKENHIGrant Number JP16H06518 (Coordination Asymmetry),JP20H00392, JP20H00316, JP18H04168, JP20K20554,CREST JST Grant Number JPMJCR1665, and the Public/Private R&D Investment Strategic Expansion Program(PRISM), Cabinet Office, Japan.Supporting InformationDetailed characterization of Cu(I) complex, detailed sensingperformances and supporting tables are summarized in Sup-porting Information. This material is available on https://doi.org/10.1246/bcsj.20200347.References# Dedicated to Professor Eiichi Nakamura on the occasion ofhis 70th birthday.1 G. Sai-Anand, A. Sivanesan, M. R. Benzigar, G. Singh,A.-I. Gopalan, A. V. 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