A Biomimetic Nose by Microcrystals and Oriented Films of

ARTICLE pubs.acs.org/crystal

A Biomimetic Nose by Microcrystals and Oriented Films of Luminescent Porous Metal
Organic Frameworks Tu Lee,* Zheng Xin Liu, and Hung Lin Lee Department of Chemical & Materials Engineering, National Central University, 300 Jhong-Da Road, Jhong-Li City 320, Taiwan, Republic of China

bS Supporting Information ABSTRACT: Guest-free and porous [In(OH)(bdc)]n (bdc =1,4-benzenedicarboxylate) microcrystals were produced by heating indium(III) nitrate hydrate (In(NO3)3 3 xH2O) and terephthalic acid (H2bdc) in N,N-dimethylformamide (DMF) with the addition of ethyl acetate as a crystal habit modifier at 100 °C for 10 h and then vacuum drying them in an oven at 250 °C for 48 h to remove lattice DMF. These microcrystals were monodispersed and hexagonal rod-like in shape with a length of 30 μm, an aspect ratio of 7.5, a BET surface area of 1148.74 m2/g, and the total pore volume of 0.57 cm3/g. The oriented films of those hexagonal rods were fabricated first through the self-assembly of as-synthesized [In(OH)(bdc) 3 2DMF]n microcrystals at the water/n-heptane interface and then followed by the removal of DMF upon vacuum drying in an oven at 250 °C for 48 h. The stereochemical sensing capability of both guest-free and porous [In(OH)(bdc)]n microcrystals and oriented films toward odorants emanated from the analytes such as cumin, cinnamon, vanillin, p-xylene, m-xylene, o-xylene, water, and ethanol, was successfully transduced to the first set of photoluminescence (PL) emission responses. The original emission of the guest-free, porous [In(OH)(bdc)]n framework might be attributed to the ligand-to-metal charge transfer (LMCT). The inclusions of guest odorant molecules in the guess-free, porous [In(OH)(bdc)]n microcrystals could have quenched the excitons through: (1) delocalization over the conjugated polymer backbone, (2) interchain energy migration in the solid state, and (3) a highly organized molecular stacking structure, attributed to the pore confinement of the analyte inside the molecular-sized cavities of [In(OH)(bdc)]n framework which facilitated strong interactions between the analyte (or the odorant) and the host framework. Therefore, specific guest
host stereochemical interactions would dictate the characteristic quenching response of a given analyte. Red shifts were observed for cumin-, cinnamon-, vanillin-, p-xylene-, m-xylene-, o-xylene-, water-, and ethanol-adsorbed samples of [In(OH)(bdc)]n microcrystals, with emission λmax at 390, 425, 343, 428, 422, 400, 390, and 389 nm respectively, upon excitation at 270 nm in solid state. A second set of PL emission responses of the same analytes produced from the analyte-adsorbed Zn4O(bdc)3 (MOF-5) microcrystals for a demonstration purpose was also determined and coplotted with the first set of PL emission values to construct a 2D map of PL emission responses for our MOF-based “biomimetic nose”. The biomimetic nose could distinguish the odors of the analytes based on a pattern recognition method (i.e., principal component analysis) because on the 2D map of PL emission responses, ethanol, mxylene, o-xylene, vanillin, cumin, p-xylene, cinnamon, and water, were represented by a line, a point, a point, a point, a line, a point, a rectangle and a point respectively.

’ INTRODUCTION The difference between a sensor and a human nose is that the sensor responds in a highly selective fashion to a specific target analyte or class of target analytes through the use of a “lock-andkey” design, whereas the human nose is designed to discriminate between various kinds of analytes (i.e., odorants).1 The odorant is first introduced in the nasal cavity and exposed to a small area of about 2.5 cm2 containing around 50 million primary sensor cells. Each type of cell is replicated. Identical cells are connected to a glomerulus in the olfactory bulb. The information sent to the cortex upon sensing the odorant is not the signal from a specific receptor but rather it is a 2D map of the responses of all glomeruli projected on the surface of the olfactory bulb. The brain then recognizes a distributed pattern formed by an ensemble of signals r 2011 American Chemical Society

coming from all the glomeruli and not a single response from a specific cell.2 To mimic the working principle of a human nose, artificial electro-optical noses3
9 operate on exposing odorants to an array of sensors and analyze the data using principal component analysis.10 Artificial electro-optical noses have a broad spectrum of interesting and extraordinary applications, such as diabetes diagnosis11 and lung cancer identification12 by the analysis of breath, cigarette brand identification,13 online monitoring of composting procedures,14 evaluation of coffee ripening,15 monitoring of plant health in greenhouse,16 identification of paper Received: June 7, 2011 Revised: July 17, 2011 Published: July 20, 2011 4146

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Crystal Growth & Design quality,17 determination of the ripening state of cheese,18 detection of land mines,19 discrimination of perfumes,20 characterization of wastewater,21 aroma analysis of dairy products,22 and recognition of olive oils.23 In general, metal-oxide-based and silicon-based catalytic arrays in the artificial electro-optical noses provide pores for size discrimination of the analytes. But they allow for neither deliberate chemical control of the response of elements in the arrays nor reproducibility of response from array to array due to the lack of understanding of catalytic function.1 As for the electrically conductive polymer-based arrays, they do allow for chemical modification of sensing elements to respond to a broad range of analytes but suffer from providing a large surface area for stereochemical recognition.7 To embrace both the advantages of metal oxide catalysts and electrically conductive polymers, we propose the use of a new class of zeolite-like crystalline materials —metal
organic frameworks (MOFs) (or porous coordination polymer particles (CPPs)).24
26 MOFs contain well-defined environments for lumophores, accessible large pore volumes and cavities constructed by the extension of luminescent multidentate linkers27 and the metal centers in space. Under these conditions, the olfactory-like, host
guest chemistry can be transduced to detectable changes in luminescence. MOFs have already been shown as ideal candidates for chemical sensing applications in many different areas, such as detection of high explosives,28 aromatic molecules in water29 and ethanol in air,30 sensing of organic solvent molecules,31 cations32 and anions,33 inclusion of ferrocene molecules,34 selective binding of organic guests35
37 and identification of subatomic particles.38 If additional functions such as enantioselectivity and chiral recognition were desired, then unsymmetric linkers could even be substituted in the supramolecular structures of MOFs.39
44 Interestingly, all of the concepts mentioned above have only been realized in the form of microcrystals and rarely in the form of thin films.28,45 Even though there were several means to grow films directly on functionalized substrates, films grown by those methods usually resulted in films of intergrown crystals.46
55 But, sometimes, there exists a need for MOF thin films with performance properties comparable to those of single crystals which can be synthesized economically in large size and quantity.56 This goal may be achieved if the film microstructures are first optimized through the control of size, shape, and orientation of MOF microcrystals which are then self-assembled into ordered two-dimensional arrays at the air
liquid interface by the self-assembly of the MOF microcrystals.57,58 Therefore, the aim of this work is 4-fold: (1) to select guest-free, porous [In(OH)(bdc)]n (bdc =1,4-benzenedicarboxylate)59,60 microscrystals as a model system because its production and crystal habit optimization by solvent (i.e., crystal habit modifier) screening was relatively simple. Also, indium ion was chosen as a functional trivalent metal center61 for MOF because it possessed fascinating coordination properties originating from their variable coordination geometry and coordination numbers,60,62,63 (2) to understand the miscibility relationship between solvent and fabrication of the [In(OH)(bdc)]n oriented film, (3) to use [In(OH)(bdc)]n microcrystals and a [In(OH)(bdc)]n oriented film as the stereochemical sensing component for the odorants such as cumin, cinnamon, vanillin, p-xylene, m-xylene, o-xylene, water, and ethanol to obtain the corresponding photoluminescent (PL) responses, and (4) to add a different type of MOF microcrystals: [Zn4O(bdc)3] (MOF-5)64,65 as the second stereochemical sensing component for the same odorants to achieve

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another set of photoluminescence (PL) responses for demonstrating a pattern recognition concept (i.e., principal component analysis (PCA)).10

’ RESULTS AND DISCUSSION Reaction crystallization of [In(OH)(bdc) 3 2DMF]n was carried out individually in DMF with the addition of 19 different common solvents (i.e., crystal habit modifiers) via solvent screening. The 19 crystal habit modifiers were: p-xylene, ethyl acetate, toluene, MTBE, benzene, MEK, chloroform, THF, acetone, 1,4-dioxane, nitrobenzene, n-butyl alcohol, IPA, benzyl alcohol, acetonitrile, ethanol, DMSO, methanol, and water, which were arranged in the ascending order according to the values of a Hildebrand parameter.66,67 The POM images of [In(OH)(bdc) 3 2DMF]n microcrystals produced under the influence of each crystal habit modifier were illustrated in Figure S1 of the SI correspondingly. Figures S1p and S1b of the SI revealed that the original crystal habit of short thin needles of [In(OH)(bdc) 3 2DMF]n microcrystals in pure DMF could be easily turned to monodispersed long thick hexagonal rods with a length of 30 μm and a width of 4 μm (i.e., aspect ratio of 7.5) by the addition of ethyl acetate. Since the hexagonal, long and thick [In(OH)(bdc) 3 2DMF]n microcrystals were ideal for the fabrication of oriented films, all [In(OH)(bdc)]n microcrystals used in our experiments were mass produced in DMF-ethyl acetate cosolvent and followed by vacuum drying at 250 °C for 48 h. In the FTIR spectrum of [In(OH)(bdc)]n microcrystals (Figure S2 of the SI), a broad peak centered at 3400 was mainly attributed to the ν(O
H). The absence of strong adsorption bands in the area of 1684 to 1720 cm
1 verified the fully deprotonation of the carboxylate groups of bdc. The bands centered at 1558(s) and 1397(s) cm
1 corresponded to a bound carboxylate group νasym and νsym (COOIn), respectively, indicating that the carboxylate groups of the bdc ligands were coordinated to indium metal atoms in the chelating bidentate mode. The peaks of 1506(m), 1176(w), 1154(w), 1107(w), 1019(m), 881(m), 823(w), 813 (w), and 551(s) cm
1 were associated with the para-disubstituted phenyl ring.59,61 In the TGA scan (Figure S3a of the SI), the first weight loss of 19.69% from 50° to 250 °C and the second weight loss of 40.02% from 450° to 550 °C of the as-synthesized [In(OH)(bdc) 3 2DMF]n microcrystals oven-dried at 40 °C overnight indicated that (1) a complete desolvation of DMF happened around 250 °C, and (2) the molar ratio of DMF: In(OH)(bdc) was indeed 2:1. At 550 °C, the residue weighted 40.29% (i.e., 100%
19.69%
40.02%) was In2O3.61 Therefore, if guest-free, porous [In(OH)(bdc)]n microcrystals were desired, the as-synthesized [In(OH)(bdc) 3 2DMF]n microcrystals must be vacuumed dried at 250 °C for 48 h to remove all lattice DMF. According to the TGA scan (Figure S3b of the SI), the resulting guest-free, porous [In(OH)(bdc)]n microcrystals were found to be thermally stable up to 460 °C. The PXRD diffraction pattern (Figure S4a of the SI) of [In(OH)(bdc)]n microcrystals with characteristic peaks at 2θ = 9.4°, 14.16°, 16.32°, 17.04°, and 18.82° matched very well with the ones in the published work of Cho et al.59 But unfortunately, both Cho et al.59 and our research group had difficulties in growing a single crystal of [In(OH)(bdc)]n. Therefore, we had adopted a modification of the single-crystal X-ray structure60 of In(OH)(bdc) 3 0.75H2bdc (monoclinic, space group P21/c, a = 18.228(3)Å, b = 11.9701(19)Å, c = 34.062(6)Å, β = 122.355(2)°, V = 3652.03(10)Å3, F = 1.77955 g/cm3, Z = 4) with the 4147

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Figure 2. Top view POM image of the oriented film of guest-free, porous [In(OH)(bdc)]n microcrystals on a microscope cover slide.

Figure 1. (a) Schematic representation of the oriented film fabrication for MOF microcrystallites at the water/n-heptane interface and the transfer procedure. The glass substrate was gradually pulled upward (red arrow), and (b) schematic illustration of the thin meniscus formed at the contact line, as indicated in a red dotted box in (a). [In(OH)(bdc)]n microcrystals were represented as green rods. The red arrow on the top represented the direction of the frictional force, f, generated by the deposition of microcrystals at the contact line. The green arrow at the bottom represented the moving direction of [In(OH)(bdc)]n microcrystals caused by the convective force. Two blue arrows represented γf and γL. θ was the contact angle between n-heptane and the glass substrate.

removal of 0.75H2bdc to approximate the lattice structure of our [In(OH)(bdc)]n microcrystals as suggested by Cho et al.59 The elemental analysis calculated for [In(OH)(bdc)]n microcrystals was C, 32.44%; H, 1.68%; and O, 27.03% which was in a good agreement with experimentally found values of C, 32.13%; H, 1.75%; and O, 26.85%. N2 gas adsorption isotherm of guest-free, porous [In(OH)(bdc)]n microcrystals (Figure S5 of the SI) was measured at 77.35 K after pretreatment under a dynamic vacuum at 250 °C for 24 h. The N2 adsorption isotherm showed type I behavior typical for microporous material.

The BET and Langmuir surface areas of the isotherm were 1148.74 and 1521.88 m2/g respectively. The total pore volume was 0.57 cm3/g. The self-assembly of the as-synthesized [In(OH)(bdc) 3 2DMF]n long thick rods at the water/n-heptane interface was illustrated in Figure 1a. When the slurry of the as-synthesized [In(OH)(bdc) 3 2DMF]n microcrystals suspended in n-heptane and IPA was introduced dropwise on the water/n-heptane interface, presumably the hydrophobic surface of the microcrystals enabled the microcrystals to float on the water surface by the surface tension of water and a high contact angle between water and the hydrophobic surface. A thin meniscus was formed at the glass substrate-n-heptane-air interface. As n-heptane evaporated, convective transport drove [In(OH)(bdc) 3 2DMF]n microcrystals to the contact line and deposited [In(OH)(bdc) 3 2DMF]n microcrystals onto the glass substrate. The surface roughness was generated by the deposition of [In(OH)(bdc) 3 2DMF]n microcrystals. A frictional force, f, was produced which, together with nheptane surface tension, γf, pinned the position of the contact line. As the evaporation proceeded, n-heptane would have more contact with [In(OH)(bdc) 3 2DMF]n microcrystals and less contact with glass substrate. The capillary force, γL, pulled the n-heptane inward, and the contact line became depinned, slipped, and reached another equilibrium position. n-heptane was then again in contact with the glass substrate (Figure 1b).68 Capillary force, γL, densified the rod matrix into bundles by rearranging domains of rod-like [In(OH)(bdc) 3 2DMF]n microcrystals and creating a maximum of rod-like crystal face contacts (Figure 1b).57 The guest-free, porous [In(OH)(bdc)]n oriented films were prepared by vacuum drying the self-assemble film of as-synthesized [In(OH)(bdc) 3 2DMF]n microcrystals at 250 °C for 48 h. The top view of the film by POM (Figure 2) showed the total film coverage of >80%. The anisotropic birefringence illustrated domains of oriented rods produced by the arrangement process. The side view by SEM (Figure 3) showed that the oriented film was not a monolayer and had a thickness of 3 to 5 μm. Since no significant random aggregation of [In(OH)(bdc) 3 2DMF]n microcrystals were observed, we speculated that the microcrystals were electrically stabilized by high zeta potential during the self-assembly process in water. In addition, the PXRD pattern of the [In(OH)(bdc)]n oriented film (Figure S4b of the SI) exhibited 4148

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Figure 5. Diagram of the relative positions of indium atoms for the interconnection between polyhedral chains of the guest-free, porous [In(OH)(bdc)]n framework.

Figure 3. SEM side view image of the oriented film of guest-free, porous [In(OH)(bdc)]n microcrystals on a microscope cover slide.

Figure 4. Diagram of the coordination of In(1) in the guest-free, porous [In(OH)(bdc)]n framework.

not only the same characteristic peaks as [In(OH)(bdc)]n microcrystals (Figure S4a of the SI) but also pronounced intensity for peaks at 2θ of 9.4° and 18.82°. This evidenced the film’s anisotropy. In principle, the long-time stability of the MOF coat between MOF microcrystals and the substrate could be further improved by a covalent bonding through the anchoring of MOF microcrystals to a carboxylic acid-terminated self-assemble monolayer (SAM)49 and a functionalized polymer surface.52 On the basis of the crystal data of In(OH)(bdc) 3 0.75H2bdc given by Anokhina et al.,60 each In(III) atom: In1, in the guestfree, porous [In(OH)(bdc)]n framework was bound to four carboxylate oxygen atoms: O6, O12, O20, and O13, from four bdc linkers, and two other bridging oxygen atoms to form an asymmetric octahedron coordination geometry for the In(III) atom: In1 (Figure 4).61 In the In1 center, the tetragonal plane was formed by four carboxylate oxygen atoms: O6
O12
O20
O13, in which the four oxygen atoms and In1 were almost coplanar with a mean deviation 2.1 Å, while two axial positions were occupied by O2 and O1 from two μ-OH groups with the O2
In1
O1 bond angle of 179.4° (Figure 4).61 The octahedra were linked into infinite chains, the axial oxygen atom corners were shared by neighboring octahedral to form a zigzag 3 3 3 OH
In
OH-In- 3 3 3 backbone with In
OH
In angles of 119.2° (Figure 5). Furthermore, two adjacent octahedra in one chain were bridged by two carboxylate arms. And yet, the octahedra were severely tilted as reflected by In5
O2
In1 angles of 119.2° which was very far from the value of 180° if the chains of octahedral were perfectly linear (Figure 5). The guest-free, porous [In(OH)(bdc)]n framework constructed from chains of corner-linked octahedral connected by

Figure 6. Views of the interaction between polyhedral chains and bdc groups in the In(OH)bdc 3 0.75H2bdc framework upon the removal of 0.75H2bdc in order to simulate our guest-free, porous [In(OH)(bdc)]n framework. Schematic representation of conceptual diagrams of [In(OH)(bdc)]n framework: (a) front view, (b) first side view, (c) second side view, and (d) top view.

carboxylate linkers possessed 23.2 Å  11.9 Å channels along the c-axis (Figure 6a). Figure 6b showed the side view of the framework where carboxylate arms were coplanar with the phenyl ring. Each bdc carboxylate arm was chelated to an In(III) atom. All phenyl rings were perpendicular to the directions of the In(III) atom chains.69 Figure 6c showed the side view in the bcplane of the corrugated layer of phenyl rings with an acute angle of 38.7°. Similarly, Figure 6d showed the top view in the ac-plane of the corrugated layer of phenyl rings with an obtuse angle of 113.5°. To examine the potential capabilities of guest-free, porous [In(OH)(bdc)]n framework for sensing of small gaseous guest molecules, its PL properties after exposing to the odorants emanated from cumin, cinnamon, and vanillin, and vapor phase analytes originated from p-xylene, m-xylene, o-xylene, water, and 4149

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Figure 7. Solid-state PL emission spectra of: (a) guest free, porous [In(OH)(bdc)]n microcrystals (λmax = 326 nm), (b) H2bdc (λmax = 388 nm), (c) cumin-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 390 nm), (d) p-xylene-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 428 nm), (e) m-xylene-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 422 nm), (f) water-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 390 nm), (g) o-xylene-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 400 nm), and (h) cinnamon-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 425 nm), obtained at an excitation wavelength of 270 nm at room temperature.

Figure 8. Solid-state PL emission spectra of: (a) guest free, porous [In(OH)(bdc)]n microcrystals (λmax = 326 nm), (b) H2bdc (λmax = 388 nm), (c) vanillin-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 343 nm), and (d) ethanol-adsorbed [In(OH)(bdc)]n microcrystals (λmax = 389 nm), obtained at an excitation wavelength of 270 nm at room temperature.

ethanol were investigated (Figures 7 and 8). Upon excitation at 270 nm at room temperature, guest-free, porous [In(OH)(bdc)]n microcrystals and H2bdc solids exhibited ultraviolet and violet luminescence centered at λmax emission = 326 nm (Figures 7a and 8a) and λmax emission = 388 nm (Figures 7b and 8b), respectively. Because H2bdc emitted luminescence at 388 nm attributed to the π* f n transitions,70 the emission of the guest-free, porous [In(OH)(bdc)]n framework might be attributed to the ligand-tometal charge transfer (LMCT).27 The conjugated π-systems of bdc linkers in the guess-free, porous [In(OH)(bdc)]n framework

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Figure 9. Solid-state PL emission spectra of (a) guest free, porous [In(OH)(bdc)]n oriented film (λmax = 384 nm), (b) o-xylene-adsorbed [In(OH)(bdc)]n oriented film (λmax = 400 nm), (c) water-adsorbed [In(OH)(bdc)]n oriented film (λmax = 397 nm), (d) m-xylene-adsorbed [In(OH)(bdc)]n-oriented film (λmax = 390 nm), (e) cumin-adsorbed [In(OH)(bdc)]n oriented film (λmax = 490 nm), (f) p-xylene-adsorbed [In(OH)(bdc)]n oriented film (λmax = 395 nm), and (g) cinnamonadsorbed [In(OH)(bdc)]n oriented film (λmax = 419 nm), obtained at an excitation wavelength of 270 nm at room temperature.

Figure 10. Solid-state PL emission spectra of (a) guest free, porous [In(OH)(bdc)]n oriented film (λmax = 384 nm), (b) ethanol-adsorbed [In(OH)(bdc)]n oriented film (λmax = ill-defined), and (c) vanillinadsorbed [In(OH)(bdc)]n oriented film (λmax = ill-defined).

acted as both the sources of luminescence and binding sites for chemical recognition. The inclusions of guest odorant molecules of cumin, cinnamon and vanillin, and gaseous analytes of p-xylene, m-xylene, o-xylene, water, and ethanol in the guess-free, porous [In(OH)(bdc)]n microcrystals could have quenched the excitons through (1) delocalization over the conjugated polymer backbone, (2) interchain energy migration in the solid state, and (3) a highly organized molecular stacking structure, attributed to the pore confinement of the analyte inside the molecular-sized cavities of [In(OH)(bdc)]n framework which facilitated strong interactions between the analyte (or the odorant) and the host framework.28,37 Therefore, specific guest
host stereochemical 4150

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interactions would dictate the characteristic quenching response of a given analyte. Red shifts were observed for cumin-, cinnamon-, vanillin-, p-xylene-, m-xylene-, o-xylene-, water-, and ethanol-adsorbed samples of [In(OH)(bdc)]n microcrystals, with emission λmax at 390, 425, 343, 428, 422, 400, 390, and 389 nm respectively, upon excitation at 270 nm in solid state (Figures 7c
h and 8c,d). Although PL responses of all analyte-adsorbed [In(OH)(bdc)]n microscrystals were quite strong, TGA scans of the analyte-adsorbed [In(OH)(bdc)]n microcrystals could actually be classified into two types. They were as follows: (1) [In(OH)(bdc)]n microcrystals with high loading of analytes: 19.3 wt % for p-xylene, 22.9 wt % for o-xylene, 20.5 wt % for m-xylene, 9.7 wt % for water, 21.8 wt % for cinnamon and 10.1 wt % for cumin (Figure S6 of the SI), and (2) [In(OH)(bdc)]n microcrystals with low loading of analytes: 2.5 wt % for vanillin and 3.5 wt % for ethanol (Figure S7 of the SI). We speculated that the sorption amount depended on molecular sizes and geometry of the analyte.37

Figure 11. Solid-state PL emission spectra of (a) guest free, porous MOF-5 microcrystals (λmax = 366 nm), (b) ethanol-adsorbed MOF-5 microcrystals (λmax = 339 nm), (c) m-xylene-adsorbed MOF-5 microcrystals (λmax = 342 nm), (d) o-xylene-adsorbed MOF-5 microcrystals (λmax = 347 nm), (e) vanillin-adsorbed MOF-5 microcrystals (λmax = 351 nm), (f) cumin-adsorbed MOF-5 microcrystals (λmax = 344 nm), (g) p-xylene-adsorbed MOF-5 microcrystals (λmax = 346 nm), (h) cinnamon-adsorbed MOF-5 microcrystals (λmax = 399 nm), and (i) water-adsorbed MOF-5 microcrystals (λmax = 339 nm), obtained at an excitation wavelength of 270 nm at room temperature.

The intrinsic loading capacity of a given analyte in the guestfree, porous [In(OH)(bdc)]n framework played a significant role on the PL response of an analyte-adsorbed [In(OH)(bdc)]n oriented film because the amount of [In(OH)(bdc)]n framework available for sorption was drastically reduced in the form of a thin film. Therefore, we would expect guest-free, porous [In(OH)(bdc)]n oriented films were capable of sensing p-xylene, o-xylene, m-xylene, water, cinnamon, and cumin well, but not vanillin and ethanol, based on the previous TGA results in Figures S6 and S7 of the SI, respectively. This speculation was substantiated by the pronounced PL responses observed for o-xylene-, water-, mxylene-, cumin-, p-xylene-, and cinnamon-adsorbed [In(OH)(bdc)]n oriented films, with emission λmax at 400, 397, 390, 490, 395, and 419 nm, respectively, upon excitation at 270 nm in solid state (Figures 9b
g). As for ethanol- and vanillin-adsorbed [In(OH)(bdc)]n-oriented films, their emission wavelengths were ill-defined (Figures 10b,c). Moreover, the difference between the PL response of guest-free, porous [In(OH)(bdc)]n microcrystals (Figure 7a) and the one of a guest-free, porous [In(OH)(bdc)]n-oriented film (Figure 9a), and the discrepancy between the emission feature of analyte-absorbed [In(OH)(bdc)]n microcrystals (Figures 7c
h) and the one of analyte-absorbed [In(OH)(bdc)]n-oriented film (Figures 9b
g) for a particular analyte, were thought to be caused by the orientations of the microcrystals relative to the detector and the incident beam.30 Finally, blue-shifts were observed for ethanol-, m-xylene-, oxylene, vanillin-, cumin-, p-xylene, cinnamon-, and water-adsorbed MOF-5 microcrystals with PL emission wavelengths of 339, 342, 347, 351, 344, 346, 345 and 399; and 339 nm respectively (Figure 11). This phenomenon might be attributed to the structural reinforcement caused by the adsorption of relatively rigid guest odorant molecules, which weakened the skeleton vibration and stabilized the framework. Consequently, the intraligand πfπ* energy was enhanced.37 If those values were assigned as the first principal component (i.e y-coordinate), and the emission wavelengths of ethanol-, m-xylene-, o-xylene, vanillin-, cumin-, p-xylene, cinnamon-, and water-adsorbed [In(OH)(bdc)]n microcrystals of 339 and 389 and 430; 422, 400, 343, 339 and 390; 428, 343 and 425; and 390 nm, respectively (Figures 7 and 8) were treated as the second principal component (i.e., x-coordinate) (Table 1), a 2D map could be constructed1 when two sets of data were combined by PCA (Figure 12). Therefore, the “odor or fragrance” of each analyte: ethanol, m-xylene, o-xylene, vanillin, cumin, p-xylene, cinnamon, and water, could be represented by pattern recognition such as a

Table 1. Photoluminescence Emission Wavelengths of Analyte-Adsorbed MOF-5 and [In(OH)(bdc)]n Microcrystals Were Assigned As 1st and 2nd Principal Components, Respectivelya 1st principal component (y-coordinate):

2nd principal component (x-coordinate): emission

emission wavelength of analyte-adsorbed MOF-5 microcrystals (nm)

wavelength of analyte-adsorbed [In(OH)(bdc)]n microcrystals (nm)

ethanol

339

339 and 389 and 430

m-xylene

342

422

odorants or analytes

line (3 black squares) point (1 blue star)

o-xylene

347

400

point (1 blue diamond)

vanillin

351

343

point (1 orange triangle)

cumin

344

339 and 390

line (2 pink triangles)

346 345 and 399

428 343 and 425

point (1 green triangle) rectangle (4 gray hexagons)

339

390

p-xylene cinnamon water a

2D map by PCA

point (1 red open circle)

Each analyte was represented as a point, a line or a rectangle on a 2D map by a principle component analysis (PCA) of the data. 4151

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wavelengths in a visible range,71 (3) using the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates for principal component analysis,71 and (4) investigating film fabrication techniques46
55 directly on light sources of LED or OLED.72 In addition to the quantification of olfactory senses by “biomimetic nose”, taste applications in defining the degrees of saltiness, sourness, bitterness, sweetness, and umami73 by “biomimetic tongue” will also be studied.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experiments, Materials, Instrumentations, and Figures S1
S7. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Figure 12. Plot of the PCA of the emission wavelengths of analyteadsorbed [In(OH)(bdc)]n microcrystals from Figures 8 and 9 as the xcoordinates and the emission wavelengths of analyte-adsorbed MOF-5 microcrystals from Figure 12 as the y-coordinates (Table 1). Each analyte (or odorant): ethanol, m-xylene, o-xylene, vanillin, cumin, pxylene, cinnamon, and water, was represented by a line (3 black squares), a point (1 blue star), a point (1 blue diamond), a point (1 orange triangle), a line (2 pink triangles), a point (1 green triangle), a rectangle (4 gray hexagons) and a point (1 red open circle), respectively, on a 2D map.

line (3 black squares), a point (1 blue star), a point (1 blue diamond), a point (1 orange triangle), a line (2 pink triangles), a point (1 green triangle), a rectangle (4 gray hexagons), and a point (1 red open circle) respectively (Figure 12). Intriguingly, isomers, such as o-xylene, m-xylene, and p-xylene could even be easily distinguished on a 2D map of the PL responses from our MOF-based “biomimetic nose”!

’ CONCLUSIONS Current studies had clearly demonstrated that the concept of applying pattern recognition method (i.e., principal component analysis) on an ensemble of photoluminescence emission responses collecting from the analyte-adsorbed [In(OH)(bdc)]n and MOF-5 microcrystals of our “biomimetic nose”. The inclusions of guest analyte (or the odorant) molecules in the guessfree, porous MOF microcrystals could have quenched the excitons through: (1) delocalization over the conjugated polymer backbone, (2) interchain energy migration in the solid state, and (3) a highly organized molecular stacking structure, attributed to the pore confinement of the analyte inside the molecularsized cavities of MOF framework, which facilitated strong interactions between the analyte (or the odorant) and the host framework. Therefore, specific guest
host stereochemical interactions would dictate the characteristic quenching response of a given analyte. Consequently, the biomimetic nose was able to distinguish the “odor or fragrance” emanated not only from ethanol, vanillin, cumin, cinnamon, and water, but also from isomers such as m-xylene, o-xylene and p-xylene! The powder form of biomimetic nose could further be fabricated into a thin film device simply by allowing the self-assembly of MOF microcrystals at the water/n-heptane interface. Our future studies will be as follows: (1) focusing on increasing the total degree of microcrystalline coverage, (2) exploring some other types of MOF materials which can be excited and emitted with

Corresponding Author

*Phone: +886-3-422-7151 ext. 34204. Fax: +886-3-425-2296. Email: [email protected].

’ ACKNOWLEDGMENT This research was supported by the grants from the National Science Council of Taiwan, ROC (NSC 99-2119-M-008-016). We are greatly indebted to Ms. Ching-Wei Lu with her assistance in EA at the Department of Chemistry in National Taiwan University, Ms. Ching-Tien Lin in SEM, Ms. Shew-Jen Weng in PXRD, and Ms. Jui-Mei Huang in BET and TGA at the Precision Instrument Center in National Central University. The authors would like to thank Prof. A. S. T. Chiang at the Department of Chemical and Materials Engineering in National Central University for his advice of integrating biomimetic nose with visible light sources. ’ REFERENCES (1) L
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