sensors Article
Molecularly Imprinted Filtering Adsorbents for Odor Sensing Sho Shinohara 1 , You Chiyomaru 2 , Fumihiro Sassa 2 , Chuanjun Liu 2,3 and Kenshi Hayashi 2, * 1 2
3
*
Graduate School of Systems Life Sciences, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan;
[email protected] Graduate School of Information Science and Electrical Engineering, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan;
[email protected] (Y.C.);
[email protected] (F.S.);
[email protected] (C.L.) Research Laboratory, U.S.E Co. Ltd., 1-10-4 Hiroo, Shibuya-ku, Tokyo 150-0012, Japan Correspondence:
[email protected]; Tel.: +81-92-802-3747
Academic Editors: Takeshi Onodera and Kiyoshi Toko Received: 9 September 2016; Accepted: 16 November 2016; Published: 23 November 2016
Abstract: Versatile odor sensors that can discriminate among huge numbers of environmental odorants are desired in many fields, including robotics, environmental monitoring, and food production. However, odor sensors comparable to an animal’s nose have not yet been developed. An animal’s olfactory system recognizes odor clusters with specific molecular properties and uses this combinatorial information in odor discrimination. This suggests that measurement and clustering of odor molecular properties (e.g., polarity, size) using an artificial sensor is a promising approach to odor sensing. Here, adsorbents composed of composite materials with molecular recognition properties were developed for odor sensing. The selectivity of the sensor depends on the adsorbent materials, so specific polymeric materials with particular solubility parameters were chosen to adsorb odorants with various properties. The adsorption properties of the adsorbents could be modified by mixing adsorbent materials. Moreover, a novel molecularly imprinted filtering adsorbent (MIFA), composed of an adsorbent substrate covered with a molecularly imprinted polymer (MIP) layer, was developed to improve the odor molecular recognition ability. The combination of the adsorbent and MIP layer provided a higher specificity toward target molecules. The MIFA thus provides a useful technique for the design and control of adsorbents with adsorption properties specific to particular odor molecules. Keywords: odor sensor; adsorbents; molecularly imprinted filtering adsorbent
1. Introduction With progress in organic materials, electronics, and micro-fabrication technologies, a number of highly advanced odor sensors have been proposed. However, compared with physical or chemical sensors such as inertial [1] or blood glucose sensors [2], commercial application of these odor sensors is not widespread. A major reason for this is the complexity of the analytes that make up an odor. An odor usually consists of a mixture of several gases, and the possible number of combinations of these gases is enormous. One of the simplest approaches for the detection and discrimination of odors is to use a large number of odor-specific sensors in an array; this is known as the electronic nose (e-nose) approach. However, it is often impossible to prepare such large numbers of odor-specific sensors as an array. In contrast, it has recently been reported that the human nose can discriminate a single odor from over one trillion odors [3]. When animals smell odors, the olfactory bulb is activated by stimulation of the olfactory receptors. This information from the activated olfactory bulb is called an odor map [4,5], and it is composed of several hundreds of glomeruli on the olfactory Sensors 2016, 16, 1974; doi:10.3390/s16111974
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odor cluster [6–9]. With the resulting cluster map, a huge number of odors can easily be bulb. Glomeruli activated by odorants with similar molecular characteristics are located close together, discriminated. and form an odor cluster [6–9].cluster With the resulting cluster the map,measurement a huge number of odors The generation of odor maps thus enables of odors in can the easily same be discriminated. manner as that used in animal olfaction. Each cluster corresponds to a particular type of molecular The generation of odor cluster maps thus enables measurement in the manner structure information, for example functional groups,the molecular sizes,oforodors shapes, andsame recognizing as that used in animal olfaction. Each cluster corresponds to a particular type of molecular structure the molecular structure of the odorant enables qualitative and quantitative analysis of the odor information, example functional groups, molecular or shapes, recognizing molecular based on its for visible pattern information [10–12]. Thus,sizes, the design andand flexible control the of the sensor structure the odorant enablesbased qualitative and cluster quantitative of the odor based its visible specificityoftoward parameters on odor maps analysis are important factors for on odor sensor pattern information [10–12]. Thus, the design and flexible control of the sensor specificity toward development. parameters on odor cluster are important factorssystem for odoraimed sensoratdevelopment. In ourbased previous study, we maps developed a gas sensing recognizing complex In our structures previous study, wea developed a gas sensing system sensing aimed atdevice recognizing molecular molecular using gas concentrating-separating [13]. complex It was possible to structures a gas from concentrating-separating It was possible to regenerate regenerateusing odor maps sensor responses to sensing odorantsdevice using[13]. this system. The system had eight odor maps from responses to odorants using this system.unit Theand system eight unit; sensing sensing cells, andsensor each cell consisted of an adsorbent separation a gashad sensing the cells, and each cell consisted of an adsorbent separation unit and a gas sensing unit; the adsorbent adsorbent separation unit contained an adsorbent film on a micro-ceramic heater, and the separated separation unit contained adsorbent film on unit a micro-ceramic and the separated odorants odorants were quantified an with a gas sensing made up ofheater, a metal oxide semiconductor gas were quantified a gas unit made upofofthis a metal semiconductor sensor. Figure 1 sensor. Figure 1with shows thesensing conceptual design odoroxide sensing system. Thegas selectivity of this shows conceptual design of thisrecognition odor sensing system. of Thethe selectivity of this device depends on device the depends on the molecular properties adsorbents; it requires adsorbents with high selectivity in molecular The sensor system can selectively detect odors ifin it the molecular recognition propertiesrecognition. of the adsorbents; it requires adsorbents with high selectivity incorporates adsorbents appropriate various However, the ability of molecular recognition. Thewith sensor system canselectivities selectively for detect odorsodorants. if it incorporates adsorbents with the previously used adsorbents insufficient for the ability recognition a wide used variety of odor appropriate selectivities for variouswas odorants. However, of the of previously adsorbents molecules. was insufficient for the recognition of a wide variety of odor molecules.
Figure 1.1.Conceptual Conceptualapproach approachforfor a bio-inspired odor sensing system. (a) Adsorption-separation Figure a bio-inspired odor sensing system. (a) Adsorption-separation odor odor sensing system; (b) Resulting odor cluster map forevaluation odor evaluation sensing system; (b) resulting odor cluster map for odor [13]. [13].
In In this this study, study, aa new new molecular molecular selective selective adsorbent adsorbent for for gas gas preconcentrator preconcentrator was was developed developed by by combinations imprinted polymer (MIP). First, we we focused on combinationsof ofadsorbent adsorbentmaterials materialsand anda amolecularly molecularly imprinted polymer (MIP). First, focused polydimethylsiloxane (PDMS), whichwhich has superior fabrication processability and a highand adsorption on polydimethylsiloxane (PDMS), has superior fabrication processability a high capacity andcapacity specificity forspecificity odorants, for as confirmed previousinwork. In addition, other adsorption and odorants, in asour confirmed our previous work. Inadsorbent addition, polymers, such aspolymers, divinylbenzene (DVB) and polyvinyl alcohol werealcohol compounded other adsorbent such ascopolymer divinylbenzene copolymer (DVB) and(PVA), polyvinyl (PVA), into a matrix with a PDMS substrate, with the aim of controlling the solubility parameters of the were compounded into a matrix with a PDMS substrate, with the aim of controlling the solubility adsorbents. chloride (PVC)—containing plasticizer was also examined. parameters Aofpolyvinyl the adsorbents. A polyvinyl chloride (PVC)—containing plasticizer was also To improve the selectivity of the adsorbents toward the odor molecules, the surface of the examined. adsorbent substrates was modified with a molecularly imprinted filter (MIF), which is anofMIP To improve the selectivity of the adsorbents toward the odor molecules, the surface the layer formed by a sol-gel method, in which the MIP retains molecular void which space in its MIP structure. adsorbent substrates was modified with a molecularly imprinted filter (MIF), is an layer
formed by a sol-gel method, in which the MIP retains molecular void space in its structure. The
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modified adsorbent is called a molecularly imprinted filtering adsorbent (MIFA). Figure 2 shows a The modified adsorbent is called a molecularly imprinted filtering adsorbent (MIFA). Figure 2 shows structural the molecules moleculespassing passingthrough throughthis thisthin thin MIF film will a structuralschematic schematicdrawing drawingof ofaaMIFA. MIFA. Only Only the MIF film will be be concentrated into the adsorbent substrate layer; this gives the composite molecular sieving concentrated into the adsorbent substrate layer; this gives the composite molecular sieving properties. properties. Consequently, adsorbents can be developed using composite materials Consequently, gas-selectivegas-selective adsorbents can be developed using composite materials prepared from prepared adsorbent polymers and the adsorption properties can becontrolled designed adsorbentfrom polymers and MIP layers,and andMIP the layers, adsorption properties can be designed and and using adsorbents and MIF the target analytes. usingcontrolled combinations of combinations adsorbents andofMIF layers specific to thelayers target specific analytes.toMIFAs are expected to MIFAs are expected to have good adaptability for an odor-clustering sensing system. have good adaptability for an odor-clustering sensing system.
Figure Figure 2. 2. Structure Structure of of aa MIFA. MIFA.
2. 2. Materials Materialsand andMethods Methods 2.1. 2.1. Preparation Preparation of of Composite Composite Adsorbents Adsorbents PDMS-based PDMS-based composite composite adsorbents adsorbentscontaining containingDVB DVB(PDMS-DVB) (PDMS-DVB)and andPVA PVA(PDMS-PVA) (PDMS-PVA) were were prepared. PDMS is known to be a good adsorbent for a broad range of odorant molecules, and can prepared. PDMS is known to be a good adsorbent for a broad range of odorant molecules, and can in in particular particular adsorb adsorb large large amounts amounts of of ketones. ketones. However, However, fatty fatty acids acids and and alcohols alcohols are are not not well well adsorbed adsorbed by by PDMS PDMS because because of of its its low low hydrophilicity hydrophilicity [13]. [13]. We We therefore therefore made made a PDMS composite composite adsorbent adsorbent containing DVB (2%), which was expected to adsorb fatty acids, and a composite with PVA containing DVB (2%), which was expected to adsorb fatty acids, and a composite with PVA (30%), (30%), which which can adsorb alcohols, to modify the adsorption properties of PDMS. We We also prepared PVC-dioctyl PVC-dioctyl phthalate phthalate (PVC-DOP) (PVC-DOP)and andpolyethylene polyethyleneglycol-PVA glycol-PVA(PEG-PVA) (PEG-PVA) (1:1) composite adsorbents to enhance the adsorption of alcohols. PVC is a relatively hydrophilic (1:1) composite adsorbents to enhance the adsorption of alcohols. PVC is a relatively hydrophilic material properties can be be controlled by by mixing thisthis polymer withwith the material and andits itschemical chemicaland andphysical physical properties can controlled mixing polymer hydrophobic plasticizing agent DOP. To control the gas adsorption properties by varying the hydrophobic plasticizing agent DOP. To control the gas adsorption properties by varying the the hydrophobicity, hydrophobicity, DOP DOP was was mixed mixed with with PVC PVC in in ratios ratios of of 1:1, 1:1, 1:2, 1:2, 1:5, 1:5, and and 1:10. 1:10. A A mass mass of of 7.5 7.5 gg of of each each composite composite was was dissolved dissolved in in 55 mL mL tetrahydrofuran tetrahydrofuran (THF) (THF) and and stirred stirred for for 22 h. h. Films Films of of the the adsorbent adsorbent substrates were then formed by evaporation of THF at room temperature. Finally, cured adsorbents substrates were then formed by evaporation of THF at room temperature. Finally, cured adsorbents were were obtained. obtained. The The adsorbent adsorbent materials materials used used were were chosen chosen based based on on their their solubility solubility parameters, parameters, which which are are as as 33)1/2 1/2 follows: PDMS, 14.9; DVB, 9.3; PVA, 12.6; PVC, 9.5; and DOP, 12.1 (cal/cm [14]. follows: PDMS, 14.9; DVB, 9.3; PVA, 12.6; PVC, 9.5; and DOP, 12.1 (cal/cm ) [14]. 2.2. Deposition Deposition of of MIFs MIFs on on the the Adsorbent Adsorbent Substrates Substrates 2.2. To obtain control over the properties of the adsorbents, permselective To obtainsophisticated sophisticated control overadsorption the adsorption properties of the aadsorbents, a filter was prepared on the adsorbents. this research, PVC-DOP, PDMS, permselective filter by wasdeposition prepared ofbyMIPs deposition of MIPs on In the adsorbents. In this research, PDMS-DVB, and PDMS-PVA were as adsorbent for the MIFAs. We for developed two PVC-DOP, PDMS, PDMS-DVB, andused PDMS-PVA were substrates used as adsorbent substrates the MIFAs. types of MIFs composed (PAA) and methacrylic acid (MAA). We developed two typesofoffrom MIFspolyacrylic composedacid of from polyacrylic acid (PAA) and methacrylic acid The deposition method for the MIFA prepared using PAA (MIFPAA ) is shown in Figure 3. (MAA). The deposition method for the MIFA prepared using PAA (MIFPAA) is shown in Figure 3.
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1.1.
The was prepared by mixing PAA (10 mM), (50HCl mM),(50 a template The polymer polymersolution solution was prepared by mixing PAA (10 HCl mM), mM), a molecule template (5 mM), and ethanol to obtain a certain concentration, and then stirring the mixture for h. molecule (5 mM), and ethanol to obtain a certain concentration, and then stirring the4mixture 2. The alkoxide solution was prepared by mixing ethanol (1.5 mL), toluene (1.5 mL), and for 4titanium h. 2. titanium(VI) The titaniumbutoxide alkoxide (100 solution mM).was prepared by mixing ethanol (1.5 mL), toluene (1.5 mL), and titanium(VI) mM). 3. A gel layer ofbutoxide titanium(100 oxide cannot be attached directly onto the PDMS surface because of its 3. highly A gel layer of titaniumnature. oxide cannot be attached directly onto theactivated PDMS surface because of its water-repellent The surface of PDMS was therefore by hydrophilization highly water-repellent nature. The surface of PDMS was therefore activated by by treatment with an O2 plasma (Harrick plasma, PDC-002). hydrophilization treatment O2 plasma plasma,by PDC-002). 4. A single titanium by alkoxide layerwith wasan formed on the(Harrick PDMS surface dipping hydrophilization4. treated A single titanium alkoxide layer was formed on the PDMS surface PDMS into a titanium alkoxide solution for 20 min. Thereafter, the surfaceby wasdipping rinsed hydrophilization-treated intopolymer a titanium alkoxide forPAA 20 min. the with ethanol and dippedPDMS into the solution to solution give a MIF film Thereafter, including the surface was rinsed with ethanol and dipped into the polymer solution to give a MIFPAA film template molecule. including the molecule template was molecule. 5. The template removed from the MIF by rinsing with ethanol and heating at 80 ◦ C 5. for The1template molecule was removed from the MIF by rinsing with ethanol and heating at 80 °C h. for 1 h. In this way, an MIFPAA -coated adsorbent was obtained. In this way, an MIFPAA-coated adsorbent was obtained. The MIF prepared using MAA (MIFMAA ) was synthesized by copolymerization of polymerized The MIF prepared using MAA (MIFMAA) was synthesized by copolymerization of polymerized MAA and a crosslinker in the presence of a template molecule. Here, ethylene glycol dimethacrylate MAA and a crosslinker in the presence of a template molecule. Here, ethylene glycol dimethacrylate (EGDMA) was used as the crosslinker. The deposition method for MIFMAA is described below. (EGDMA) was used as the crosslinker. The deposition method for MIFMAA is described below.
1.1.
2.2.
3.3.
The was prepared by mixing MAAMAA (2 mmol), EGDMAEGDMA (1 mmol),(1a mmol), templatea The prepolymer prepolymersolution solution was prepared by mixing (2 mmol), molecule (2 mmol),(2 acetonitrile (40 mmol)(40 as the diluent, and azobis-isobutyronitrile (2 mg) as template molecule mmol), acetonitrile mmol) as the diluent, and azobis-isobutyronitrile the initiator and then stirring this mixture for 4 h. (2 mg) as the initiator and then stirring this mixture for 4 h. A gel layer of titanium titanium oxide oxide was was formed formed on on the the hydrophilization-treated hydrophilization-treatedPDMS PDMSusing usingthe thesame same method as as for for the the preparation preparation of ofMIF MIFPAA PAA [15]. [15]. The PDMS was then then rinsed rinsed with with ethanol ethanol and and ◦ C. dipped into the prepolymer solution for 1 h at 80 into the prepolymer solution for 1 h at 80 °C. The template template molecule molecule was was removed removed as as described described above above for for MIF MIFPAA PAA..
Inthis thisway, way,an anMIF MIFMAA-coated adsorbent was obtained. In MAA -coated adsorbent was obtained.
Figure3. 3. Procedure Procedurefor forMIFA MIFAfabrication. fabrication. Figure
2.3. Characterization Characterizationofofthe theAdsorption AdsorptionProperties Propertiesofofthe theAdsorbent Adsorbent 2.3. Toevaluate evaluate specific adsorption of the adsorbent, wechromatography–mass used gas chromatography–mass To thethe specific adsorption of the adsorbent, we used gas spectrometry spectrometry (GC-MS) with a solid-phase microextraction (SPME) fiber auto-sampler (GC-MS) with a solid-phase microextraction (SPME) fiber auto-sampler [16], as described below.[16], as described below. 1. The adsorbents were placed in a sealed chamber and exposed to odorant gases (Figure 4). 1. The adsorbents were placed in a sealed chamber and exposed to odorant gases (Figure 4). The The odorants were volatilized at 50 ◦ C in a permeator (PD-1B, GASTEC, Ayase, Japan) or a glass odorants were volatilized at 50 °C in a permeator (PD-1B, GASTEC, Ayase, Japan) or a glass desiccator, and then left in the chamber for 1 h under gas flow at 0.5 L/min. desiccator, and then left in the chamber for 1 h under gas flow at 0.5 L/min. 2. The adsorbents were transferred to screw tube vials and were introduced into the SPME 2. The adsorbents were transferred to screw tube vials and were introduced into the SPME auto-sampler (AOC5000 plus, Shimadzu, Japan) with an SPME fiber (23-gauge, 50/30 µm, auto-sampler (AOC5000 plus, Shimadzu, Japan) with an SPME fiber (23-gauge, 50/30 µm, DVB/CAR/PDMS, SPELCO, Bellefonte, PA, USA). The samples were then heated from 40 to DVB/CAR/PDMS, SPELCO, Bellefonte, PA, USA). The samples were then heated from 40 to 240 240 ◦ C to desorb the odor molecules from the adsorbents. Finally, the amounts of the detached °C to desorb the odor molecules from the adsorbents. Finally, the amounts of the detached gases were determined using GC-MS (GCMS-QP2010, Shimadzu, Kyoto, Japan). This method is referred to below as the “GC-MS/SPME method”.
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gases were determined using GC-MS (GCMS-QP2010, Shimadzu, Kyoto, Japan). This method is referred Sensors 2016,to 16,below 1974 as the “GC-MS/SPME method”. 5 of 10
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Figure 4. Adsorption experiment system. CMS:carbon carbonmolecular molecularsieve sievecolumn columnfor forobtaining obtainingclean cleanair. Figure 4. Adsorption experiment system. CMS: air. mass MFC:flow masscontroller. flow controller. Sample adsorbents placed in the sample chamber. MFC: Sample adsorbents werewere placed in the sample chamber.
Resultsand andDiscussion Discussion 3. 3. Results Figure 4. Adsorption experiment system. CMS: carbon molecular sieve column for obtaining clean air. MFC: mass flow controller. Sample adsorbents were placed in the sample chamber.
InfluenceofofDOP DOPConcentration Concentrationon on the the Adsorption Adsorption Properties Adsorbents 3.1.3.1 Influence PropertiesofofPVC-DOP PVC-DOPComposite Composite Adsorbents 3. Results and Discussion
The specific adsorption properties of PVC-DOP composite adsorbents different mixing The specific adsorption properties of PVC-DOP composite adsorbents withwith different mixing ratios ratios were evaluated using the GC-MS/SPME method. In this experiment, 10 odorant gases were evaluated using theConcentration GC-MS/SPME method.Properties In this ofexperiment, 10 odorant gases (benzene, 3.1 Influence of DOP on the Adsorption PVC-DOP Composite Adsorbents (benzene, heptanal, 2-hexanone, heptanal, anisole,propanoic 2-nonanone, propanoic acid, salicylaldehyde, methyl 2-hexanone, 2-nonanone, salicylaldehyde, methyl salicylate, The specificanisole, adsorption properties of PVC-DOPacid, composite adsorbents with different mixing o-cresol, salicylate, o-cresol, hexanoic acid) were used as odor samples. These gases are typical bio-volatile hexanoicratios acid)were wereevaluated used as using odor the samples. These gases areIntypical bio-volatile compounds GC-MS/SPME method. this experiment, 10 organic odorant gases organic compounds (BVOCs) from various chemical groups whichacid, are important analytes for food (benzene, 2-hexanone, heptanal, anisole, 2-nonanone, propanoic methyl (BVOCs) from various chemical groups which are important analytes forsalicylaldehyde, food production and living production and living environmental monitoring. Figure 5 illustrates the dependence of the amount salicylate, o-cresol, hexanoic acid) were used as odor samples. These gases are typical bio-volatile environmental monitoring. Figure 5 illustrates the dependence of the amount of adsorbed gases on the of adsorbed on the DOP mixing ratio, chemical and the groups amounts of adsorbed gasanalytes normalized to those organic gases compounds (BVOCs) from various which are important for food DOP mixing ratio, and the amounts of adsorbed gas normalized to those adsorbed by a pure PDMS production and PDMS living environmental monitoring. illustrates the dependence the amount adsorbed by a pure adsorbent are shown in Figure Figure5 6. The normalized valuesofindicate whether adsorbent shown in Figure The normalized values indicate whether the adsorbs greater of are adsorbed gases ongreater the 6. DOP mixing ratio, and the amounts adsorbed gas composite to those the composite adsorbs amounts of gas than PDMS ofdoes. Figure 6normalized shows the individual amountsadsorbed of gas than PDMS does. Figure 6areshows the individual values forvalues 2-hexanone, propanoic acid, by a pure PDMS adsorbent shown in Figure 6. The normalized indicate whether values for 2-hexanone, propanoic acid, benzene, and o-cresol. The adsorbed gas amounts increased the composite adsorbs greater amounts of gas increased than PDMSwith does. Figure 6 shows the ratio individual benzene, and o-cresol. The adsorbed gas amounts the increasing DOP for benzene, with the increasing DOP ratio for benzene, 2-hexanone, and propanoic acid; the change in the valuesand for propanoic 2-hexanone, acid; propanoic change acid, benzene, o-cresol. amounts The adsorbed propanoic gas amountsacid increased 2-hexanone, in theand adsorbed is adsorbed amounts of propanoicthe acid is especially large. In contrast, theof adsorbed amount of especially o-cresol with the increasing DOP ratio for benzene, 2-hexanone, and propanoic acid; the change in the large. In contrast, the adsorbed amount of o-cresol decreased with the increasing DOP content. decreased with the increasing DOP content. These specific absorption properties can be attributed to adsorbed amounts of propanoic acid is especially large. In contrast, the adsorbed amount of o-cresol These specific absorption properties can be attributed to the change in solubility parameters on mixing the change in solubility parameters oncontent. mixingThese DOPspecific and PVC; hydrophobic adsorbents have atohigher decreased with the increasing DOP absorption properties can be attributed DOP and hydrophobic adsorbents have a higher concentrating ability toward hydrophobic concentrating toward hydrophobic odorants. This result shows that the specific of thePVC; changeability in solubility parameters on mixing DOP and PVC; hydrophobic adsorbents have adsorption a higher toward result shows that the specific adsorption of This odorants. resultability shows that hydrophobic the specificodorants. adsorption of gases PVC-DOP adsorbents gases concentrating byThis PVC-DOP composite adsorbents can be This controlled bybyadjusting thecomposite mixing ratio. gases is byespecially PVC-DOP composite adsorbents befor controlled by adjusting the mixing ratio. This cancomposite be controlled by adjusting the mixing ratio.can This composite is especially promising as an adsorbent promising as an adsorbent fatty acids. composite is especially promising as an adsorbent for fatty acids. for fatty acids.
5. Adsorbed odorant amount (TIC:total totalion ion current from gas chromatography–mass Figure 5.Figure Adsorbed odorant amount (TIC: currentobtained obtained from gas chromatography–mass spectrometry/solid-phase microextraction) of polyvinyl chloride–dioctyl phthalate (PVC-DOP) Figure 5. Adsorbed odorant amount (TIC: total current obtained from gas chromatography–mass spectrometry/solid-phase microextraction) ofion polyvinyl chloride–dioctyl phthalate (PVC-DOP) adsorbents as a function of DOP content. spectrometry/solid-phase microextraction) of polyvinyl chloride–dioctyl phthalate (PVC-DOP) adsorbents as a function of DOP content. adsorbents as a function of DOP content.
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Figure 6. Adsorption properties of polyvinyl chloride–dioctylphthalate phthalate (PVC-DOP) as as a Figure 6. Adsorption properties of polyvinyl chloride–dioctyl (PVC-DOP)adsorbents adsorbents a function of DOP content, relative to those of polydimethylsiloxane (PDMS) adsorbents. Adsorption function of DOP content, relative to those of polydimethylsiloxane (PDMS) adsorbents. Adsorption of (a) 2-hexanone; (b) propanoic acid; (c) benzene; (d) o-cresol. of (a) 2-hexanone; (b) propanoic acid; (c) benzene; (d) o-cresol.
3.2. Modification of the Adsorption Properties of PDMS by Addition of Other Adsorbents
3.2. Modification of the Adsorption Properties of PDMS by Addition of Other Adsorbents Next, the specific adsorption of PDMS mixed with other adsorbents (PVA and DVB) was evaluated
Next, theGC-MS/SPME specific adsorption PDMS mixed with other adsorbents DVB) was using the method.of The PEG-PVA adsorbent was also evaluated.(PVA In thisand experiment, evaluated using the GC-MS/SPME method. The adsorbent was evaluated. In this three odorant gases (2-heptanone, 1-heptanol and PEG-PVA heptanoic acid) were used as also the gas samples, and experiment, three odorant gases (2-heptanone, 1-heptanol and heptanoic acid) were used as the their structures are shown in Figure 7. The molecular structures of these three gases are similar and the gas lengths their structures carbon chains identical. Table 17.shows the normalized amounts of adsorbed gases are samples, andoftheir areare shown in Figure The molecular structures of these three gases relative to the amounts adsorbed by the pure PDMS adsorbent. Table 2 shows the adsorption ratios of of similar and the lengths of their carbon chains are identical. Table 1 shows the normalized amounts 1-heptanol heptanoic acidamounts to 2-heptanone. The by results that PDMS-DVB PEG-PVA are the adsorbed gasesand relative to the adsorbed the indicate pure PDMS adsorbent.and Table 2 shows effective adsorbents for heptanoic acid, which is a fatty acid, and that PEG-PVA can be used for the adsorption ratios of 1-heptanol and heptanoic acid to 2-heptanone. The results indicate that alcohol 1-heptanol. PDMS-DVB and PEG-PVA are effective adsorbents for heptanoic acid, which is a fatty acid, and that The results described above show that the amount of gas adsorbed by PVC and PDMS adsorbents PEG-PVA can be used for the alcohol 1-heptanol. can be varied by mixing these adsorbents with materials that have different solubility parameters. In this way, composite adsorbents can be used to achieve rough selectivity toward different gases. Table 1. Adsorption specificity of composite adsorbents. Adsorption amounts (total ion current (TIC) values) were normalizedspecificity to the TIC values of polydimethylsiloxane (PDMS).(total ion current (TIC) Table 1. Adsorption of composite adsorbents. Adsorption amounts values) were normalized to the TIC values of polydimethylsiloxane (PDMS).
PDMS-DVB/pure PDMS PEG-PVA/pure PDMS PDMS-DVB/pure PDMS PDMS-PVA/pure PDMS PEG-PVA/pure PDMS PDMS-PVA/pure PDMS
2-Heptanone 0.82 2-Heptanone 0.3 0.82 1.0 0.3 1.0
1-Heptanol Heptanoic Acid 0.92 Heptanoic Acid 8.71 1-Heptanol 1.1 6.33 0.92 8.71 3.02 1.1 1.08 6.33 1.08
3.02
Table 2. Adsorption specificity of composite adsorbents. Adsorption amounts determined from the total ion current (TIC values) were normalized to the TIC values for adsorbed 2-heptanone on each adsorbent.
PDMS PDMS-DVB PEG-PVA
1-Heptanol/2-Heptanone 0.479 0.541 1.779
Heptanoic Acid/2-Heptanone 0.00215 0.0229 0.046
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Table 2. Adsorption specificity of composite adsorbents. Adsorption amounts determined from the total ion current (TIC values) were normalized to the TIC values for adsorbed 2-heptanone on each adsorbent.
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PDMS PDMS-DVB PEG-PVA PDMS-PVA
1-Heptanol/2-Heptanone
Heptanoic Acid/2-Heptanone
0.479 0.541 1.779 0.515
0.00215 0.0229 0.046 0.00647
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Figure 7. Amounts of of gas adsorbed imprintedfiltering filtering adsorbent fabricated Figure 7. Amounts gas adsorbedby by aa molecularly molecularly imprinted adsorbent fabricated usingusing hexanoic template.The The amounts amounts are relative to the of gas of adsorbed by hexanoic acidacid as as thethe template. arenormalized normalized relative toamounts the amounts gas adsorbed polyvinyl chloride–dioctyl phthalate (PVC-DOP) adsorbent withoutwithout a molecularly imprinted imprinted filter. by aapolyvinyl chloride–dioctyl phthalate (PVC-DOP) adsorbent a molecularly filter. 3.3. Controlling the Adsorption Properties of the PVC-DOP Adsorbent with a MIF
TheToresults above that the aamount of gas of adsorbed byadsorbent PVC and PDMS controldescribed the specificity of theshow gas adsorption, MIFA composed a PVC-DOP with adsorbents can be was varied by mixing these adsorbents with materials that have different solubility a MIFPAA layer fabricated and its adsorption properties were evaluated. For this experiment, a PVC-DOP containing 67% DOP used. Two types of rough MIFAs were fabricated parameters. In composite this way,adsorbent composite adsorbents can was be used to achieve selectivity toward usinggases. different template materials (2-decanone and 2-nonanone), and an adsorbent without a MIF different layer was used as a negative control. The amount of gas adsorbed by each adsorbent was measured using the GC-MS/SPME method. The of sample gas consisted of a mixture 2-decanone, 2-nonanone, 3.3. Controlling the Adsorption Properties the PVC-DOP Adsorbent with aofMIF hexanoic acid, nonanol, and 5-nonanone. For both MIFAs, the adsorption of the gas used as the To control specificity of the gas a MIFA composed of a PVC-DOP adsorbent template was the increased compared with theadsorption, amount adsorbed by the negative control (data not shown). withInacontrast, MIFPAAthe layer wasof fabricated and its adsorption properties amount adsorbed non-template gases decreased, which were shows evaluated. that the MIFFor PAA this layers behave as MIFs. composite These resultsadsorbent show that containing the specificity of the gaswas adsorption was enhanced experiment, a PVC-DOP 67% DOP used. Two types of by MIFAs addition of MIFs to the adsorbents. were fabricated using different template materials (2-decanone and 2-nonanone), and an adsorbent alsolayer tested theused gas specificity of the MIFA The in detail usingofnine (heptanal, without aWe MIF was as a negative control. amount gas different adsorbedgases by each adsorbent hexanoic acid ethyl, anisole, propanoic acid, benzaldehyde, nonanol, methyl salicylate, hexanoic acid, of was measured using the GC-MS/SPME method. The sample gas consisted of a mixture and guaiacol). The MIFA fabricated with a hexanoic acid template was used in this experiment. 2-decanone, 2-nonanone, hexanoic acid, nonanol, and 5-nonanone. For both MIFAs, the adsorption Figure 7 shows the normalized amounts of adsorbed gas relative to those for the adsorbent without of the gas used as the template was increased compared with the amount adsorbed by the negative a MIF. The amount of gas adsorbed increased only for hexanoic acid, and decreased for all of the other control (data not shown). In contrast, the amount of adsorbed non-template gases decreased, which gases. The normalized amount of adsorbed target gas, hexanoic acid, was seven times greater than the shows that the MIFPAA layers adsorbed amounts of the otherbehave gases. as MIFs. These results show that the specificity of the gas adsorption enhanced by addition of MIFs toadsorbents the adsorbents. Thewas adsorption properties of PDMS-DVB with a MIFMAA filtering layer were also We also tested the gas specificity of the MIFA in detail using acid, nine were different gases (heptanal, evaluated. The sample gases, 2-heptanone, 1-heptanol, and heptanoic adsorbed onto the hexanoic ethyl, anisole, using propanoic acid, nonanol, The methyl salicylate, hexanoic MIFA,acid which was prepared heptanoic acidbenzaldehyde, as the template molecule. adsorption properties
acid, and guaiacol). The MIFA fabricated with a hexanoic acid template was used in this experiment. Figure 7 shows the normalized amounts of adsorbed gas relative to those for the adsorbent without a MIF. The amount of gas adsorbed increased only for hexanoic acid, and decreased for all of the other gases. The normalized amount of adsorbed target gas, hexanoic acid, was seven times greater than the adsorbed amounts of the other gases.
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were measured using the same method as used for the other experiments in this paper. A non-imprinted filtering adsorbent (NIFA), which is an adsorbent made following the same procedure as that used to prepare MIFAs but without a template molecule, was prepared as a negative control. The normalized amounts of adsorbed gas, relative to the amounts adsorbed with the NIFA, are shown in Figure 8. The normalized amount of adsorbed gas increased for heptanoic acid but decreased for 2-heptanone Sensors 2016, 16, 1974 8 of 10 and 2-heptanol. The ratios between heptanoic acid and 2-heptanone, and heptanoic acid and 2-heptanol, were 9.5 could and 18,filter respectively. These results thatThus, the MIF layer couldcontrol filter out a wide that the MIF layer out a wide range of gasshow species. considerable over the range of gas species. Thus, considerable control over the gas adsorption properties of adsorbents could gas adsorption properties of adsorbents could be achieved. In our previous study, we confirmed the beregeneration achieved. Inand our degradation previous study, we confirmed theThe regeneration and degradation of MIF [17]. of MIF layers [17]. sensing system using MIFAs has alayers heating The sensing system using MIFAs has a heating unit, and then the adsorbed gas can be desorbed unit, and then the adsorbed gas can be desorbed in a higher temperature condition (80 °C). Such indesorption a higher temperature condition (80 ◦ C). Such evaluation, desorption i.e., wastemplate confirmed GC-MS/SPME was confirmed before GC-MS/SPME gasbefore molecules cannot be ◦ C vacuum-heating process. evaluation, i.e., template gas molecules cannot be detected after the 80 detected after the 80 °C vacuum-heating process. The lifetime of the MIFPAA is 7 h under a high The lifetime of the 7 h under a high concentration of PAA solvent concentration gasMIF of PAA (ethanol). However, thegas MIFA can be used (ethanol). for severalHowever, weeks inthe a PAA issolvent MIFA cangaseous be usedconcentration for several weeks in a normal gaseousthe concentration In addition, normal environment. In addition, MIF layer canenvironment. be refreshed and rebuilt the layer can be process refreshed and rebuilt in a reconfigurable process [17]. in MIF a reconfigurable [17].
Figure8. Adsorption 8. Adsorption specificity of a molecularly imprinted filtering adsorbent on a Figure specificity of a molecularly imprinted filtering adsorbent on a polydimethylsiloxane– polydimethylsiloxane–divinylbenzene (PDMS-DVB) adsorbent layer. template divinylbenzene (PDMS-DVB) adsorbent layer. The template odorant was The heptanoic acid.odorant was heptanoic acid.
3.4. Comparison of the Filtration Efficiency of MIFMAA and MIFPAA 3.4. Comparison of the Filtration Efficiency of MIFMAA and MIFPAA To examine the filtration efficiencies of different MIF materials, the amounts of gases adsorbed To examine the filtration efficiencies of different MIF materials, the amounts of gases adsorbed by MIFMAA and MIFPAA were compared. In this experiment, both types of MIFs were deposited by MIFMAA and MIFPAA were compared. In this experiment, both types of MIFs were deposited on a on a pure PDMS substrate. Two fatty acids (heptanoic acid and nonanoic acid) and two alcohols pure PDMS substrate. Two fatty acids (heptanoic acid and nonanoic acid) and two alcohols (heptanol and nonanol) were used as templates for the MIF layer. The normalized amounts of adsorbed (heptanol and nonanol) were used as templates for the MIF layer. The normalized amounts of gases, relative to the amount adsorbed by the NIFA, are shown in Figure 9. For both fatty acids and adsorbed gases, relative to the amount adsorbed by the NIFA, are shown in Figure 9. For both fatty alcohols, MIF a better filtering performance than MIFPAAthan did.MIF MIF MAA showed MAA is believed to acids and alcohols, MIFMAA showed a better filtering performance PAA did. MIFMAA is have specific adsorption sites in its fixed crosslinked structure, which make it a stable MIF. it Inacontrast, believed to have specific adsorption sites in its fixed crosslinked structure, which make stable MIF has flexibility and the fabrication of its loose MIF structure is easy [13]. Thus, the selection PAA MIF. In contrast, MIFPAA has flexibility and the fabrication of its loose MIF structure is easy [13].of MIF materials is another important factor in the important design of afactor MIFAincomposite selectivity for Thus, the selection of MIF materials is another the designwith of ahigh MIFA composite specific odorants. with high selectivity for specific odorants.
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Figure 9. (a) Adsorption specificityof ofaa molecularly molecularly imprinted adsorbent (MIFA) for fatty Figure 9. (a) Adsorption specificity imprintedfiltering filtering adsorbent (MIFA) for fatty acids; (b) Adsorption specificity of a MIFA for alcohols. Adsorbent samples are named as in acids; (b) Adsorption specificity of a MIFA for alcohols. Adsorbent samples are named asthein the following example: MIFA (heptanol)==MIFA MIFA with with aamethacrylic acid (MAA)-MIF layer prepared MAA following example: MIFA MAA (heptanol) methacrylic acid (MAA)-MIF layer prepared using heptanol as the template. using heptanol as the template.
4. Conclusions
4. Conclusions
Highly selective gas adsorbents based on a MIFA were developed. The combination of a filter layer Highly selective gas adsorbents based on a MIFA were developed. The combination of a filter that is selective to specific molecules and a gas adsorbent with a high adsorption capacity and a rough layergas that is selective to specific molecules andofa the gasadsorption adsorbentproperties. with a high adsorption selectivity enabled highly tunable control The rough gas capacity selectivityand a rough gasbase selectivity enabled highly tunable control adsorption properties. The rough of the adsorbent materials, PDMS and PVC, could of be the modified by mixing these materials with gas selectivity of the base adsorbent materials, PDMS and PVC, could be modified by mixing other polymer adsorbents, and these composite adsorbents functioned as broad gas-filtering materials.these The combination these composite adsorbents gave greater control over their specific materials with otherof polymer adsorbents, and with theseMIFs composite adsorbents functioned as broad adsorption toward various odor molecules. Because of this high control over the adsorption properties, gas-filtering materials. The combination of these composite adsorbents with MIFs gave greater MIFAs aretheir promising materials for the production of a setodor of odor sensors forBecause the detection of clustered control over specific adsorption toward various molecules. of this high control odorants. The set of sensors is a promising information source of a pattern recognition machine which over the adsorption properties, MIFAs are promising materials for the production of a set of odor usually requires a great deal of independent information for discrimination. The cluster maps were sensors for the detection of clustered odorants. The set of sensors is a promising information source synthesized using the preliminary system [18]. This novel concept involving the use of a MIF provides of a pattern recognition machine which usually requires a great deal of independent information for a simple means by which to add molecular selectivity to other chemical sensors.
discrimination. The cluster maps were synthesized using the preliminary system [18]. This novel Acknowledgments: byaJSPS KAKENHI, Numbers: JP15H01713, JP26620206. concept involving theThis usework of awas MIFsupported provides simple meansGrant by which to add molecular selectivity to other chemical sensors.Kenshi Hayashi and Chuanjun Liu conceived and designed the experiments; You Chiyomaru Author Contributions: and Sho Shinohara performed the experiments; Kenshi Hayashi and You Chiyomaru analyzed the data;
Acknowledgments: This work wasand supported by JSPS KAKENHI, Fumihiro Sassa, Kenshi Hayashi Sho Shinohara wrote the paper. Grant Numbers: JP15H01713, JP26620206. Conflicts of Interest: The authorsHayashi declare noand conflict of interest. Author Contributions: Kenshi Chuanjun Liu conceived and designed the experiments; You Chiyomaru and Sho Shinohara performed the experiments; Kenshi Hayashi and You Chiyomaru analyzed the data; Fumihiro Sassa, Kenshi Hayashi and Sho Shinohara wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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