An optical sensor array based on photonic Fabry–Pérot films of surface-mounted metal–organic-frameworks (SURMOFs) with different homochiral structures is presented. It is used to detect and enantioselectively discriminant 3 pairs of chiral odor molecules, either pure or in binary mixtures

Heinke, Lars

Jiang, Yunzhe

Zhan, Kuo

English

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8704 |  Chem. Commun., 2023, 59, 8704–8707 This journal is © The Royal Society of Chemistry 2023Cite this: Chem. Commun., 2023,59, 8704Optical sensor array of chiral MOF-basedFabry–Pérot films for enantioselective odorsensing†Kuo Zhan,ab Yunzhe Jianga and Lars Heinke *aAn optical sensor array based on photonic Fabry–Pérot films ofsurface-mounted metal–organic-frameworks (SURMOFs) with dif-ferent homochiral structures is presented. It is used to detect andenantioselectively discriminant 3 pairs of chiral odor molecules,either pure or in binary mixtures.Chirality and enantioselectivity are fundamental in nature andchiral messenger molecules are often the key to biologicalinformation transmission.1–3 Most chiral odor molecules havean enantioselective smell, which can be typically discriminatedby the human olfactory system.4 One of the most popularexamples is limonene: While the R-isomer of limonene (R-Lim)has an odor of an orange, the S-isomer (S-Lim) smells likelemons.4 Another example is 1-phenylethanol: the R-isomer(R-PhOH) has a floral, earthy-green odor, while the odor ofthe S-isomer (S-PhOH) is described like a mild hyacinth withstrawberry nuances.5 For accurate discrimination, the chiralodors are mostly detected through enantioselective chromato-graphy with homochiral columns.6 However, due to its com-plexity, time-consumption and costs, it is not suited forpractical sensor applications. Several efforts have been madeto fabricate portable enantioselective sensors based on differ-ent chiral materials such as chiral polymers,7 supramolecularchiral systems,8 cyclodextrin,9–11 carbon-nanotubes,12 or gra-phene functionalized with chiral molecules.13 For example,DNA-functionalized carbon nanotubes have been used for thedistinction of R- and S-enantiomers of a-pinene as well as ofb-pinene and limonene (separately).14Portable enantioselective optical sensing has been demon-strated with designed fluorescence15 and colorimetric sensorsusing labels, such as chiral functionalized silver nanoparticlesor Au–Ag alloy nanoparticles.16,17 However, due to the strongintermolecular interactions between the sensor labels and theanalytes, these label-based sensors operate typically not in areversible and repeatable manner. Optical, label-free sensorsthat detect and discriminate the enantiomers of various mole-cules (even in their pure form) have not yet been presented.A Fabry–Pérot (FP) cavity is an interferometric optical struc-ture, constructed by a thin film in between two reflectors. Thephotonic properties of the FP-film are controlled by the thick-ness and reflective index (RI) of the inner film, see ESI.†18Based on this, FP-films have been widely used as opticalchemical sensors.18,19 Using the changes of the reflectionspectrum caused by the uptake of guest molecules as sensorsignal, FP-sensors can detect the targeted analytes accuratelyand conveniently. To make optical sensors based on FP-films,transparent and porous materials are required. Thin films ofmetal–organic frameworks (MOFs) seem ideally suited for suchtasks.20,21 MOFs consist of metal nodes connected by organicligand molecules, forming a regular porous scaffold. MOFspossess many unique properties, such as high specific surfaceareas, very diversified structures and tailorable functiona-lities.22 All these properties are favorable for applications insensors. For FP-based sensing, the MOF materials need to beprepared in the form of thin films. There, the layer-by-layersynthesis seems ideally suited, resulting in surface-mountedMOF (SURMOF) films which have typically a homogeneousmorphology with a high degree of structural order and a lowdefect density.23 In a previous study, a gravimetric sensor arraybased on homochiral and achiral SURMOFs was used to dis-criminate the enantiomers of 5 pairs of chiral odorssimultaneously.24 Optical sensors based on chiral MOF filmshave not yet been presented.Here, we present a label-free optical sensor array based onhomochiral and achiral FP-SURMOF films. The sensor arrayis made of SURMOFs with different structures, which areCu-based homochiral MOFs (Cu2(DCam)2(dabco)25 and Cu2-(DCam)2(Bipy)25) and achiral HKUST-1.26,27 (DCam stands forchiral (1R,3S)-(+)-camphoric acid as layer linker, dabco fora Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG),Eggenstein Leopoldshafen 76344, Germany. E-mail: Lars.Heinke@kit.edub Beijing Jiaotong University (BJTU), School of Physical Science and Engineering,Beijing 100044, China† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cc01298kReceived 15th March 2023,Accepted 5th June 2023DOI: 10.1039/d3cc01298krsc.li/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 05 June 2023. Downloaded on 7/13/2023 9:22:30 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueThis journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 8704–8707 |  87051,4-diazabicyclo[2.2.2]octane and BiPy for 4,40-bipyridine as pillarlinkers, HKUST-1 stands for Hong Kong University of Scienceand Technology 1.) Based on the three SURMOFs, we prepareda FP-chiral-SURMOF sensor array. The sensor array in combi-nation with UV-vis spectroscopy is able to reversibly detect3 pairs of different chiral odors at low concentrations withlimits of detection (LODs) in the order of 10–40 ppm. Usingsimple machine learning algorithms to analyze the data, thesensor array can enantioselectively distinguish all six isomers at100 ppm with a very high classification accuracy of 98.6%.Furthermore, the FP-chiral-SURMOF sensor array is also suc-cessfully applied to precisely discriminate the R-/S-Lim binarymixture odors in different ratios. We also demonstrate that thesensor read-out can be performed with the digital camera of acommon smart phone, avoiding expensive lab equipment.The SURMOF films with different structures were preparedin a layer-by-layer fashion,23 as outlined in Fig. 1a. The sub-strates, which are 11-mercapto-1-undecanol-functionalizedgold-coated silicon wafer, were alternatively immersed in theethanolic solutions of the metal nodes and of the linkermolecules, see SI. The metal nodes are copper acetate. Thelinker molecules are DCam and dabco for Cu2(DCam)2(dabco),DCam and Bipy for Cu2(DCam)2(Bipy) and trimesic acid (BTC)for HKUST-1 (see Fig. 1b). The syntheses were performed with adipping robot with 180 cycles each.25 To enhance the SURMOFquality and to decrease the surface roughness, ultrasonicationwas used to improve the cleaning process.28 On top of the chiralSURMOF films, platinum films of about 5 nm thickness weresputtered. The Pt film acts as a mirror, resulting in theFP-SURMOF films. The crystallinity of the SURMOF filmswas explored by X-ray diffraction (XRD), showing that the filmshave a crystalline structure which correspond to the targetedMOF structures, Fig. 1c. The diffractograms after all sensingexperiments are essentially identical to the diffractograms ofthe pristine samples, Fig. S14 (ESI†), indicating the stability ofthe SURMOFs. The samples were further characterized byinfrared reflection absorption spectroscopy (IRRAS, Fig. S1,ESI†), verifying the composition of the SURMOFs. The cross-section SEM images, Fig. S2 (ESI†), show that the SURMOFfilms have a homogeneous morphology. The film thicknessesof all SURMOFs are approximately 500 nm.The sensing performance of the FP-chiral-SURMOFs sensorarray was tested with 3 pairs of chiral odors as analytes. Theanalytes are R- and S-1-phenylethylamine (R-PEA, S-PEA), R- andS-1-phenylethanol (R-PhOH, S-PhOH) and R- and S-Limonene(R-Lim, S-Lim), see Table S2, ESI.† The UV-vis spectra of theFP-chiral-SURMOF films in a controlled vapor atmospherewere recorded in reflection. The reflectance spectra of theFP-Cu2(DCam)2(dabco), FP-Cu2(DCam)2(Bipy) and FP-HKUST-1films, either empty (pristine) or exposed to the vapors of thechiral molecules are shown in Fig. 2a, Fig. S5 and S7 (ESI†),respectively. The spectra show a characteristic photonic reflectanceFig. 1 (a) The synthesis of the FP-chiral-SURMOF films. (b) Sketches ofthe SURMOF structures, which are Cu2(DCam)2(dabco), Cu2(DCam)2(Bipy),HKUST-1, left to right, see labels. (c) X-ray diffractograms of the SURMOFs,see labels. The X-ray wavelength is 0.154 nm.Fig. 2 (a) The reflectance spectra of the FP-Cu2(DCam)2(dabco) film inthe atmosphere of the saturated analytes, see labels in (c). The same colorcode is used in all panels. (b) The wavelength shift of the reflectancepeak of the FP-Cu2(DCam)2(dabco) film in different analyte vaporsversus concentration. The UV-vis reflectance spectra for the FP-Cu2(DCam)2(Bipy) and FP-HKUST-1 films are shown in Fig. S4–S9 (ESI†). Theinset shows the wavelength shift of the reflectance peak of theFP-Cu2(DCam)2(dabco) film for 300 ppm R-Lim, testing the repeatabilityand reproducibility. (c) Radar plot of the wavelength-sensitivity (see slopein b) in the range of 0–300 ppm) in the sensors. The units of the axes are102 nm ppm1.Communication ChemCommOpen Access Article. Published on 05 June 2023. Downloaded on 7/13/2023 9:22:30 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online8706 |  Chem. Commun., 2023, 59, 8704–8707 This journal is © The Royal Society of Chemistry 2023peak (at 466 nm for Cu2(DCam)2(dabco), 462 nm for Cu2(DCam)2-(Bipy) and 578 nm for HKUST-1). These peaks shift to largerwavelengths and their intensities decrease upon analyte exposure,see also SI. This is caused by the increase of the reflective index ofthe SURMOF when the analyte molecules are adsorbed in thepores. The reflectance-peak wavelength shifts linearly with theanalyte concentration for all analytes in the considered concen-tration range (0–300ppm), Fig. 2b. The slopes in this plot areregarded as the sensitivity. This allows the calculation of the limitsof detection (LODs), Table S2, ESI.† Since the sensitivity and LODare based on the wavelength shift of the photonic peak, we refer tothem as wavelength-sensitivity and wavelength-LOD.In addition to the wavelength shift, the intensity of thephotonic reflectance peak changes. The sensitivity (see Fig. S5c,ESI†) and the so calculated LOD are referred to as intensity-sensitivity and intensity-LOD, see Table S2, ESI.†In both methods, the LODs of the sensors are in the rangebetween 8.8 ppm and 42.9 ppm, significantly smaller than100 ppm. (This is in line with the very high classificationaccuracy of the different chiral odors at 100 ppm, see below.)A closer look shows that the method based on the wavelengthresults in somewhat smaller LODs than the intensity method.The repeatability is explored by repeating the 300ppm R-Limvapor exposure, Fig. 2b inset. The data show that the sensorexposure is fully reversible and repeatable, as a result of thereversible R-Lim uptake by and release from the FP-SURMOF film.Each FP-chiral-SURMOF sensor possesses a different sensi-tivity for the same analyte. Moreover, different analytes exhibitdifferent sensitivities in the same sensor. (See Table S2 (ESI†) orthe slopes in Fig. 2b, Fig. S5b and S7b (ESI†), summarized alsoin the radar plot Fig. 2c.) This means each analyte has acharacteristic signal pattern in the sensor array, which is alsoshown in the characteristic fingerprint in the radar plot. Thispattern allows the qualitative discrimination of the (chiral)analytes.For determining the accuracy of the sensor array to discri-minate the chiral odors, we analyzed the data with simplemachine learning algorithms. In this work, we apply 3 classi-fication machine learning algorithms which are k-NearestNeighbor (k-NN), Support Vector Machine (SVM) and ArtificialNeural Network (ANN) to identify the analytes, see ESI† andref. 29, 30. The accuracies of the data classification are shownin the confusion matrix, Fig. 3. Fig. 3a shows the quantitativediscriminations of the 3 pairs of the chiral odors at 100 ppm byANN, indicating a correct identification with an average classi-fication accuracy of 98.6%. Confusion matrices by the kNN andSVM analyses as well as from other concentrations (200 ppm,300 ppm) are shown in Fig. S9–S11 (ESI†). All results show veryhigh classification accuracies, demonstrating the sensor arrayof FP-chiral-SURMOF films allows the enantioselective classifi-cation of the 6 chiral odors.The discrimination property of the sensor array towardmixtures of chiral odors was investigated by exposing thesensor array to pure R- and S-Lim as well as their binarymixtures (3 : 1, 1 : 1 and 1 : 3), Fig. 3b. The confusion matrixindicates a correct identification of the mixtures with anaverage classification accuracy of 99%. The radar plot of thereflectance intensity change and its corresponding reflectancespectra are shown in Fig. S12 (ESI†). All the above results showthat the FP-chiral-SURMOF sensor array can identify the chiralodors and also their mixtures precisely.Based on the strong performance of the sensor array todetect and classify the chiral analytes by using its UV-visspectra, we explore whether the sensor read-out can be per-formed with simple equipment. To this end, we placed thethree FP-SURMOFs next to each other in a cell with controlledanalyte concentration and took pictures with the digital cameraof a common smart phone, Fig. 4a. From the pictures, thecolors were quantified by their RGB values (Table S1, ESI†) andtheir positions on the CIE-color-space-map are shown inFig. 4b. By applying simple machine-learning algorithms, thesensor response can be clearly distinguished with an averageclassification accuracy of 97%. This demonstrates that a simpledevice (here the smart-phone-camera) can be used for thesensor read out as well, avoiding lab equipment.Summarized, we developed an optical sensor array based on FPfilms made of nanoporous chiral surface-mounted MOF films. ThisFP-chiral-SURMOF sensor array is a label-free, inexpensive andreversible sensor system. Here, the sensors are based on homo-chiral and achiral SURMOFs, which are Cu2(DCam)2(dabco), Cu2(DCam)2(Bipy) and HKUST-1. The sensor responses are obtainedfrom their UV-vis spectra. The sensors show high sensitivities andlow LODs in the range of B10–40 ppm for 3 pairs of chiral odors,which are R-/S-limonene, R-/S-1-phenylethanol and R-/S-1-phenylethylamine. In addition, the sensing results are fully repea-table and reversible. The analysis of the sensor-array data by simplemachine learning algorithms like ANN results in a classificationaccuracy of 98.6% for the enantioselective discrimination of the 3pairs of chiral odors at a concentration of 100 ppm. Moreover,different binary mixtures of R- and S-limonene could also bediscriminated very precisely. We also demonstrate that the sensorread out can be performed with the camera of a common smartphone, avoiding expensive lab equipment.Fig. 3 (a) The ANN confusion matrix of the 3 pairs of chiral odors at aconcentration of 100 ppm and the pristine (empty) sensors. (b) The ANNconfusion matrix of R- and S-Lim pure (100%) and their binary mixtures(3 : 1, 1 : 1, 1 : 3) and the pristine (empty) sensors. The total concentration(R + S) is always 300 ppm. The true class are the rows and the predictedclass are the columns. Correct classifications are shown on the maindiagonal of the matrix; misclassifications are the other values.ChemComm CommunicationOpen Access Article. Published on 05 June 2023. Downloaded on 7/13/2023 9:22:30 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThis journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 8704–8707 |  8707This work shows an inexpensive platform of very small,portable and label-free optical sensors that allows the enantio-selective discrimination of chiral odors. We foresee that variouschiral SURMOF structures can be used in such FP-based sensorarrays for advanced chiral odor and vapor sensing.The authors acknowledge financial support by the DeutscheForschungsgemeinschaft (DFG, via SPP 1928 COORNETs andvia HE 7036/5), the Chinese Fundamental Research Funds forthe Central Universities 20211YJS175 and the China Scholar-ship Council (CSC).Conflicts of interestThe authors declare no conflict of interest.Notes and references1 L. D. Barron, Space Sci. Rev., 2008, 135, 187–201.2 A. Salam, J. Mol. Evol., 1991, 33, 105–113.3 L. A. 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(c) The confusion matrix to classify the chiralodors, calculated by ANN.Communication ChemCommOpen Access Article. Published on 05 June 2023. Downloaded on 7/13/2023 9:22:30 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online

Optical sensor array of chiral MOF-based Fabry–Pérot films for enantioselective odor sensing

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Optical sensor array of chiral MOF-based Fabry–Pérot films for enantioselective odor sensing

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