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Highly Selective Sensing of Nitroaromatics Using Nanomaterials of Ellagic Acid Hong Wang,† Xiaohe Xu,† Choongkeun Lee,† Craig Johnson,‡ Karl Sohlberg,† and Hai-Feng Ji*,† †

Department of Chemistry, ‡Centralized Research Facilities, Drexel University, Philadelphia, Pennsylvania 19104, United States ABSTRACT: We report the synthesis, characterization, and application of nanobelts and nanowires of ellagic acid for sensing nitroaromatics. The nanostructures are composed of aggregated ellagic acids that are held together by hydrogen bonds and π−π interactions. The molecules are oriented with their long axis perpendicular to the nanowire or nanobelt and the π−π stacking direction parallel to the longitudinal axis of the wire or belt. Both the conductivity and fluorescence of the nanobelts change selectively in the presence of the vapor of nitrobenzene, a representative nitroaromatic compound, suggesting the nanobelt of ellagic acid may be used for selective detection of explosives.

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INTRODUCTION Substantial research in recent years has focused on detection of trace explosive vapors with the ultimate goal of thwarting possible terrorist attacks.1 Methods available for the analysis of air samples, such as ion mobility spectrometry and mass spectrometry,2 are expensive, are not easily portable, are timeconsuming, and often require laborious sample preparation. There is a need to develop methods for detecting explosives with real-time response, high sensitivity and selectivity, miniaturized size, low power requirements, and low cost. Methods for detection of explosives that rely on the interaction of sensing materials with explosives hold promise to meet the above-mentioned requirements.3 In these methods, detection of explosives depends on the binding of explosives leading to observable outputs, such as a change in color, fluorescence, or conductivity. Studies in this field have resulted in a variety of materials that change color or fluorescence characters on their interaction with explosives.4,5 In contrast, there have been few reports of detection based on conductivity changes,6,7 mainly due to a lack of selectivity. To address the problem of low selectivity, libraries of nonselective, conductive composites (such as polymer−carbon composites) were used, and pattern recognition was employed to recognize specific chemical vapors, including explosives. While selectivity has been a challenge, sensors based on conductivity changes are potentially advantageous in cost and convenience because they could eliminate the complexity and power consumption inherent to optical instruments. In the above-mentioned color and florescence methods,4 one strategy to enhance the selectivity in detecting explosives is to develop appropriate materials that can form a strong complex with explosives selectively. We expect that the same strategy can be applied for sensing devices that are based on conductance change. Adsorption of the analyte species can be expected to alter the electronic structure of the sensing material and thereby © 2012 American Chemical Society

lead to a change in conductance. To achieve selectivity, however, the sensing material needs to be selected for optimum binding to the species of interest. Since many explosives are nitroaromatics, here we seek strong binding to nitrobenzene, a representative nitroaromatic. To develop conductivity-change-based sensors, one-dimensional (1D) nanostructures are expected to achieve higher sensitivity8 than bulky structures because of their greater surface areas. In the development of organic supramolecular electronics,9−12 one-dimensional (1D) organic nanomaterials are considered among the most promising candidates in the fabrication of molecular electronic devices.13,14 In addition to sensors, potential applications of these nanostructures include nanoelectronics, photoelectronics, photoluminescent lightemitting devices, etc.15 Most organic electronic materials are based on aromatic derivatives.15 Of a big family of aromatic derivatives, ellagic acid (Scheme 1) is of our particular interest Scheme 1. Structure of Ellagic Acid

because it has several characteristics that suggest it may be especially suitable for developing a conductivity-based sensor for explosives. First, ellagic acid is reported as an electron-rich material,16 which can form a strong complex with explosives, Received: November 6, 2011 Revised: December 27, 2011 Published: January 10, 2012 4442

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morphology of the nanostructures obtained. The densities of nanobelts and nanowires obtained on glass substrates were considerably higher than those on Si- or Au-coated substrates. This phenomenon is similar to that observed for the growth of perylenediimide (PTCDI) nanobelts,22 suggesting that the deposition was facilitated by hydrogen bonding between the ellagic acid molecules and the Si−OH on the glass surface. TEM images indicate that the nanobelts (Figure 2) have a crystalline structure. However, the crystalline lattice of the

due to the fact that most explosives are a group of electrondeficient nitroaromatics. This is because ellagic acid contains four OH groups and two acyloxy groups that extend from a core of fused aromatic rings. These electron-donating groups enhance the electron density of ellagic acid. These features endow ellagic acid with the capability to participate in both hydrogen bonding and π−π interactions. Since nitrobenzene also possesses the capability to interact both by hydrogen bonding and π−π interactions, this dual functionality suggests that ellagic acid may exhibit selective binding for nitrobenzene over general aromatics (which may interact by π−π interactions but not hydrogen bonding), or general hydrogen bonding species that lack aromatic rings (and therefore cannot participate in π−π interactions). Second, the fused rings keep the ellagic acid a near planar structure, which is needed for assembling 1D nanostructures held via π−π interaction between molecules. Third, hydrogen bonding can develop between a carbonyl of one ellagic acid and a hydroxyl of an adjacent ellagic acid, which assists the formation and the stability of the 1D nanostructures. In this work, we report the synthesis and characterization of 1D nanostructures of ellagic acid and demonstrate their potential application in detecting electron deficient chemicals, with nitrobenzene as a representative compound. We show that both the fluorescence and conductivity of the 1D nanostructures of ellagic acid can be used for detection. Methods to synthesize 1D organic nanomaterials include precipitation from solutions,17,18 template synthesis,19 solid phase reaction,20 vapor deposition,21 etc. Among these methods, vapor deposition represents a facile approach to develop 1D nanostructures. In our experiments, we developed 1D nanostructures of the ellagic acid by employing a vapor deposition method reported previously.22

Figure 2. TEM image of a nanobelt (left) and the corresponding electron diffraction pattern (right) of the nanobelt of ellagic acid.

nanobelts cannot be obtained due to the small size of the nanobelt. The corresponding electron diffraction pattern shows 8.9 Å characteristic d-spacing in the perpendicular direction and 3.6 Å dspacing in the longitudinal nanobelt direction. This diffraction pattern could not perfectly match with any of two crystal structures of ellagic acid that have been previously reported,23,24 in which the crystals of ellagic acid were prepared from solutions. From the diffraction pattern, we hypothesized that the molecules are oriented with their long axis perpendicular to the belt and the π−π stacking direction parallel to the belt. The 3.6 Å π−π stacking space between two adjacent molecular planes is similar to that in crystals of PTCDI (3.6 Å),25 pentacene derivatives (3.35−3.44 Å),26 and rubrene (3.75 Å).27 The geometry of a dimer of ellagic acid has been modeled with the density functional theory, and the intermonomer separation distance between two ellagic acid molecules side-to-side bonded with hydrogen bonding is 8.7 Å, which is close to the value of 8.9 Å determined by TEM. Curving of the contours visible in the TEM image (Figure 2) indicates that the nanobelts are bent. The bending also results in the arcing of the reflections in the diffraction pattern. Figure 3 shows the ATR-IR spectra of nanostructures of ellagic acid. The IR peaks of the powder of ellagic acid at

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RESULTS AND DISCUSSION Figure 1 shows SEM images of the two 1D nanostructures. When the powder was heated to 400 °C, belt-like nanostructures

Figure 1. SEM images of nanostructures of ellagic acid at (A) 400 °C and (B) 450 °C from heating 1 mg of ellagic acid powder.

were obtained (Figure 1A). The dimensions of these belts are 400−1200 nm in width, 80−100 nm in thickness, and up to 20 μm in length. When the furnace temperature was increased to 450 °C, wire-like nanostructures were obtained. These nanowires have a diameter of 40−180 nm and a length of 20−30 μm (Figure 1B). When the amount of the powder was increased in the tube, bundles of nanobelts and nanowires were obtained at 400 and 450 °C, respectively, but the morphology of the nanostructures was not affected by the concentration of ellagic acid in the tube. When the sample was heated to 500 °C or above, nanoparticles with irregular morphologies were observed. The effects of other factors, such as heating time and substrate, have also been investigated, and the results show that the heating time does not significantly affect the dimension or

Figure 3. ATR-IR spectra of the powder (black), the nanobelts (red), and the nanowires (green) of ellagic acid.

3000−3400, 1690, and 1322 cm−1 are corresponding to OH stretching, CO group stretching, and COH stretching 4443

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Figure 4. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the nanobelts, the nanowires, and the powder of ellagic acid.

motions, respectively.28−30 The absorption at 3000−3400 cm−1 in powers of ellagic acid hydrate are partially attributed to H2O as well. Compared to the IR peaks of the powder of ellagic acid, the IR peaks of the two nanostructures of ellagic acid are shifted to higher wavenumbers at 3380, 1702, and 1340 cm−1, respectively, indicating the formation of hydrogen bonds between ellagic acid molecules in the nanobelts and nanowires, which restrict the stretching vibration and thus increase the vibrational frequency. It is also noted that the peak at 3380 cm−1 is relatively sharp. This may due to (1) the loss of the H2O molecules in these nanostructures and (2) the fact that these OH groups are in a similar hydrogen bonding environment, i.e., most of the EA molecules may be hydrogen bonded to a similar extent in an ordered structure.31 The IR spectra of the nanowires and the nanobelts are basically the same, indicating a similar molecular arrangement of ellagic acid molecules in the nanowires and the nanobelts. The IR spectra also show that, when the temperature is above 500 °C, the IR spectra of the irregular particles deposited on the substrates were totally different from that of ellagic acid, suggesting that ellagic acid decomposes or reacts to form other unknown chemicals, a finding we chose not to pursue in this work (IR spectrum not shown). Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) (Figure 4) show that the thermal properties and the stability of the powder, the nanowires, and the nanobelts of ellagic acid are basically the same, except for an 11.5% decrease in the mass of the powder at 100 °C, which corresponds to the removal of water molecules. The result suggests that the crystallinity or molecular arrangement of ellagic acid does not affect the thermal properties of the ellagic acid molecules. Evaporation of the ellagic acid molecules occurred when the temperature was above 400 °C, which is consistent with the observed formation temperature of the nanostructures. Figure 5 shows the fluorescence spectra of the powder, the nanowires, and the nanobelts of ellagic acid. The fluorescence peak of the powder is at 430 nm. The fluorescence peaks of the nanowires and nanobelts red-shifted to 455 nm. The red-shift may be attributed to the formation of aggregates in the nanostructures. Our observations show that the morphology of nanostructures of ellagic acid can be controlled by varying the heating temperature, but the electron diffraction, as well as the spectroscopic and thermal properties and the two nanostructures, indicates that the arrangement of ellagic acid molecules in nanowires and nanobelts is essentially the same. Figure 6 shows the SEM image of the device and I−V curves for a network of the nanobelts. In this experiment, a network of the nanobelts was directly deposited on two gold-covered

Figure 5. Fluorescence spectra of powder, nanowires, and nanobelts of ellagic acid.

electrodes on the surface of a glass substrate for an I−V curve measurement. The current was not affected by the intensity of incident light or the presence of oxygen. The conductivity of the nanobelt network, however, decreased significantly upon exposure to 200 ppm nitrobenzene vapor, as shown in Figure 6. The conductivity of the nanobelt network was not sensitive to a variety of other chemical vapors, including benzene, methylformate, acetone, ethanol, or nitromethane, at the same 200 ppm concentration. These chemicals were selected for comparison to investigate the effects of polarity and aromaticity of the adsorbate on the conductivity of the nanobelts. Among these, benzene and nitromethane should receive the most attention for comparison, since one is aromatic but without an electronwithdrawing group and the other has an electron-withdrawing group but is not aromatic. The results demonstrate that the change in conductivity of the nanobelts is quite selective for nitrobenzene, a representative electron deficient aromatic. This selective response may hold promise for use as the basis of a sensor for electron deficient explosive chemicals. We investigated the time dependence of conductivity for the nanobelt network upon exposure to nitrobenzene. The kinetics of adsorption provides information on the mechanism of the sorption processes. The time-dependent adsorption data was analyzed with two kinetic models: the pseudo-first-order model (eq 1) and the pseudo-second-order models (eq 2).32

θ(t ) = θe(1 − exp( − kR t ))

(1)

θ(t ) = θe2kR t /(1 + θekR t )

(2)

where θ(t) is the amount of nitrobenzene adsorbed at any time t, θe is the amount of nitrobenzene adsorbed at equilibrium, and 4444

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Figure 6. Left: The SEM image of the device and the I−V curves with and without the presence of nitrobenzene. Right: Effect of various solvent vapors on the conductivity of the nanobelt network. I0 is the current measured in air; I is the current measured in the presence of 200 ppm of chemical vapors at 20 °C. The film resistivity is calculated to be 8.79 × 107 Ω·cm in air before exposure to nitrobenzene.

Figure 7. Current response as a function of time for nanobelt networks of ellagic acid upon injection of 200 ppm of nitrobenzene: (a) the fitting data according to the pseudo-first-order model; (b) the fitting data according to the pseudo-second-order model.

kR is the binding constant. We assume that θ(t) is directly proportional to the current. Our results show poor fitting of the pseudo-first-order model (Figure 7a) but a good fit of the pseudo-second-order model (Figure 7b), indicating the adsorption of nitrobenzene on the surface of the nanobelt is not a first-order interaction but a second-order interaction. This suggests that the rate-limiting step may be chemical sorption involving strong interaction through sharing or the exchange of electrons between sorbent (ellagic acid in this case) and sorbate (nitrobenzene in this case).33 This result also supports our hypothesis that the electionrich ellagic acid can form a strong complex with the electrondeficient nitrobenzene and explains the selectivity of the nanobelt toward nitroaromatics. In addition to the conductivity change, the fluorescence of the nanowires also shows selective change to vapor of nitrobenzene, a representative nitroaromatic compound, as shown in Figure 8. The intrinsic material property governing conductivity is charge mobility. Charge conductance in organic semiconductors is believed to be predominantly due to the transport of holes,34,35 so hole mobility (μ) is the metric of interest. To understand the influence of a surface adsorbate on hole mobility in ellagic acid nanostructures, we combined a cluster model

of the ellagic acid surface with a widely used three-parameter expression for hole mobility.35 The crystal structure of ellagic acid was taken from X-ray diffraction data.23 A cluster of nine molecular monomers was selected from the crystal structure to represent the material surface (see Figure 9). On the basis of this cluster model, hole mobility was estimated using the hopping model in the presence of defects. The procedure has been described in detail elsewhere35 and will only be reviewed here. In the hopping model,35 hole mobility (μ) is written in terms of the hole diffusion coefficient (D) as

μ=

eD kBT

(3)

where e is the electronic charge, kB is the Boltzmann constant, and T is the temperature. The diffusion constant is in turn expressed as a weighted average hopping rate between a representative molecular monomer in the cluster and each of its nearest neighbors

D=

1 2n

∑ rWP i i i i

(4)

Here, n is the dimensionality, r is the com−com distance between adjacent monomers, W is the hop rate, P is the 4445

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Figure 8. Left: Fluorescence of nanobelts of ellagic acid before (black) and after (red) exposure to nitrobenzene. Right: Effect of various chemical vapors on the conductivity of the ellagic acid nanobelt network. Fl0 is the fluorscrence peak at 450 nm measured in air; Fl is the fluorescence peak at 450 nm measured in the presence of 200 ppm of chemical vapors at 20 °C. The excitation wavelength is 380 nm.

Here, V is the coupling matrix element, λ is the reorganization energy, and ℏ is the Planck constant. The coupling matrix element is related to the overlap of molecular orbitals on adjacent monomers, which is strongly dependent on the relative position and orientation of the neighboring molecules.35−40 Reorganization energy is a consequence of the geometry relaxation in a molecule when the electronic state changes. In the case of hole transport in a heterogeneous system, the reorganization energy is expressed as

λ = (E+* − E+)A + (E* − E)D (6) where E+ and E are the energies at the optimized geometries for ionized and neutral species, respectively, and E+* and E* are the energies of the ionic and neutral species at the optimized neutral and ionized geometries, respectively.35−40 ( )A and ( )D mean hole acceptor and donor, respectively. In the present case, the coupling matrix element was found by direct computation of the Fock transfer integral. This avoids complications involving the site energy difference that plague the popular dimer-splitting approach. The coupling matrix element is therefore expressed as

V = ⟨φ′A |F |φ′B⟩

(7)

where F is the Fock matrix and φ′ is an orthogonalized monomer HOMO orbital. The orthgonalized orbitals are

|φ′A ⟩ = α|φA ⟩ + β|φB⟩ |φ′B⟩ = α|φB⟩ + β|φA ⟩

(8)

where α = (1 + 3S /8) and β = (−S/2) and S is the overlap integral. Finally, the coupling matrix element is expressed as 2

V = αβ⟨φA |F |φA ⟩ + α 2⟨φA |F |φB⟩ + β2⟨φA |F |φB⟩

Figure 9. (a) Top-down view of the cluster used to represent the surface of ellagic acid, (b) side-view of the nitrobenzene adsorbed ellagic acid, (c) side-view of the benzene adsorbed ellagic acid, and (d) side-view of the nitromethane adsorbed ellagic acid.

+ αβ⟨φB|F |φB⟩ (9) The reorganization energies and coupling matrix elements were computed with the dispersion-corrected AM1-FS141 method. The cluster model was extracted from the crystal structure23 using Material Studio.42 This procedure was developed to create a fast and efficient method for estimating charge mobility, and it has shown good predicting power.43 To model different levels of surface coverage of the adsorbate, 0−9 adsorbate molecules were placed on the surface of the cluster and the hole mobility estimated with the hopping

hopping probability, and i is an index that runs over all possible dimers in the cluster that contain the representative central molecule. The hopping rate is taken to be given by the expression 1/2 ⎛ V2 ⎛ π ⎞ λ ⎞ W= ⎟ exp⎜ − ⎜ ⎟ ℏ ⎝ λkBT ⎠ ⎝ 4kBT ⎠

(5) 4446

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obtained with a Hitachi F-7000 fluorescence spectrometer. Attenuated total reflection infrared (ATR-IR) spectra were recorded on a Smith IR microscope. The thermal analyses were obtained with a thermogravimetry/differential thermal analyzer (TG/DTA 6300, SII Nanotechnology Inc.). Zeiss Supra 50VP was used to obtain scanning electron microscopy (SEM) images. A JEOL JEM2100 operated at 120 keV was used for obtaining transmission electron microscopy (TEM) images. In a typical procedure to prepare the nanostructure of ellagic acid, a quantity of ellagic acid powder was placed in a 50 mL quartz tube. The tube was heated in a vacuum for 1 h, and then, the tube was allowed to cool to room temperature. Onedimensional nanostructures formed on substrates were collected for analysis. Gas sensing experiments were performed with the nanobelt resistor device placed in a glass tube that is 5 cm in length and 1 cm in diameter. A flow of air was maintained during each experiment. 200 ppm of a chemical vapor was added to the gas flow via a low-pressure injection valve. I−V curves were measured before and after the device was exposed to the chemical vapor using a Keithyley 2636A system source meter at room temperature. To measure the fluorescence change of the nanobelt network to chemical vapors, the nanobelts were deposited onto a quartz slide. Then, the quartz slide was left in the quartz cell for fluorescence measurement. 200 ppm of chemical vapor was injected into the quartz cell by using a gas syringe.

model, as outlined above and detailed in ref 36. In the calculation of λ and V, when a surface adsorbate molecule is present on one of the molecular monomers in the ellagic acid cluster, both the ellagic acid monomer and its associated surface adsorbate molecule are taken to represent one monomer. In this way, the influence of adsorbate coverage on μ can be estimated.41 The predicted hole mobility of ellagic acid as a function of adsorbate coverage is plotted in Figure 10. In the presence

Figure 10. Predicted hole mobility in ellagic acid as a function of adsorbate surface coverage.

of nitrobenzene, hole mobility is substantially decreased with increasing adsorbate coverage. By contrast, when benzene or nitromethane is taken as the surface adsorbate, the hole mobility changes little with increasing surface coverage. Our calculations show that the origin of this selective response is more subtle than simply selective binding. The difference is due to the change in reorganization energy that occurs when an adsorbate molecule is present. Reorganization energy is the energy change associated with the geometric distortion that accompanies oxidation/ reduction and is directly related to the energy barrier to charge hopping. When an ellagic acid molecule has an adsorbed nitrobenzene, the reorganization energy is increased by 0.48 eV. In contrast, it changes very little (0.7 meV) when a benzene is adsorbed. Neither substitution influences the coupling matrix element appreciably. The computed 9-fold decrease in mobility in the presence of nitrobenzene is consistent with our experimental results, as shown in Figure 3. Nitromethane adsorption on ellagic acid was found to form a bridging structure between two nitromethane molecules. In this case, the charge transfer has two fundamental steps; the charge first transfers from ellagic acid → nitromethane and then from nitromethane → (the adjacent) ellagic acid. Both steps were treated with hopping theory and the rate controlling step identified. For this step, the reorganization energy is increased relative to the benzene-adsorbed case, which favors decreased charge mobility, but this is counterbalanced by stronger electronic coupling between ellagic acid and nitromethane than between ellagic acid monomers, leading to no dramatic change in charge mobility. It is noteworthy that the selectivity based on the calculation of two known crystal structures and our hypothesized structure is similar, although our structure is slightly different from the reported crystal structures.

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CONCLUSIONS In summary, this work demonstrates that the conductivity and fluorescence of the nanostructures of ellagic acid change selectively in the presence of nitrobenzene, indicating its potential for detection of explosive chemicals. The sensing performance of these nanostructures to explosives will be investigated and reported in due course.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 01-215-895-2562. Fax: 01-215-895-1265.

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ACKNOWLEDGMENTS This work is partially supported by the National Science Foundation (EEC-1138240) and the National Natural Science Foundation of China (20728506/B05). K.S. acknowledges support from the National Science Foundation Grant CHE0449595.

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MATERIALS AND EXPERIMENTAL Ellagic acid hydrate was purchased from Alfa Aesar and was used as received. UV−vis spectra were recorded on a Shimadzu UV-2401PC spectrophotometer. Fluorescence spectra were 4447

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