Explosive Detection by Fluorescent Electrospun Polymer Membrane

Chapter 30

Explosive Detection by Fluorescent Electrospun Polymer Membrane Sensor

Downloaded by PENNSYLVANIA STATE UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch030

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Xianyan Wang , Christopher Drew , Soo-Hyoung Lee , Kris J. Senecal , Jayant Kumar *, and Lynne A. Samuelson * 1,

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Center for Advanced Materials, Departments of Chemistry and Physics, University of Massachusetts at Lowell, Lowell, M A 01854 Natick Soldier Center, U.S. Army Soldier and Biological Chemical Command, Natick, M A 01760 2

Introduction Chemical sensors for explosive detection have attracted increasing attention recently due to heightened awareness of terrorist and criminal activities. Numerous methods of direct explosive detection, including ion mobility spectrometry, neutron analysis, X-ray backscattering, and electron capture detection have been developed. However, there remains urgent need for new approaches that not only complement existing methods, but improve on them in terms of lower cost and greater instrumental simplicity (1). Fluorescent optical chemical sensors are of particular interest due to their inherent sensitivity and simplicity (2). These types of sensors have many other advantages that optical sensors, in general, offer. One of the most attractive features is that they do not require a separate reference sensor, as a potentiometric chemical sensor does. In addition, they are not affected by electrical interference, sample flow rate, and stir speed which can be serious problems with electrochemical sensors. Fluorescent optical chemical sensors have been widely used for quantitative measurements of various analytes in environmental, industrial, clinical, medical, and biological applications (2).

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© 2005 American Chemical Society In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.


389 Over the last decade, polymeric materials have attracted tremendous interest in optical sensor applications due to their unique attributes and advantages for sensor technologies (3). They are relatively low-cost materials. Their processing and fabrication techniques are quite simple, i.e. there is no need for special clean-room or high-temperature processes. They can be deposited on various types of substrates. Polymers posses a wide variety of molecular structures expanded by the possibility to build in various side chains to endow the films with desirable physical and chemical properties. However, conventional polymer materials alone are generally not active sensing materials in that their optical parameters cannot be significantly affected by the environment. Therefore, suitablefluorescenceindicators are used as molecular recognition materials in polymeric sensors (4). These indicators must exhibit changes in fluorescence intensity in the presence of the desired analyte to be detected. The procedures for immobilization, the materials used, and the morphology of the sensingfilmshave strong effects on the performance of the sensor in tarns of stability and sensitivity. In many cases,fluorescentdyes are immobilized by physical or chemical procedures onto the polymeric materials for fabrication. The physical procedures used for immobilization include adsorption (5), dissolution (6), entrapment in a porous network (7) and ion exchange (8). These methods are simple but suffer from the problem of insolubility of dyes in the polymeric support, which results in the dyes leaching-out. The chemical procedure to immobilize the dye entails the formation of covalent bonds between the dyes and support materials. Sensors with covalently immobilized dyes have the advantage of not sufferingfromdye loss over time. However, attaching thefluorescentgroup to the polymer is not always a trivial task in that the reaction can be quite complicated, involving multiple steps and difficultreactantpurification (9). It is well established that the sensitivity of the sensing film in a sensor is proportional to the surface area of thefilmto volumeratio.Thinfilmswith very large surface areas can be easily fabricated by electrospinning, wherein a polymer solution is exposed to a high static voltage creating sub-micron or nanometer scalefiberscollected as a non-woven membrane (10). Electrospun nanofibrous membranes can have a surface area approximately one to two orders of the magnitude higher than those found in continuous thinfilms.It has been demonstrated that this high surface area has the potential to provide unusually high sensitivity and fast response time for sensing applications (11). In this chapter, the on-going research into the effects of the polymeric system on the performance of the fluorescent electrospun polymer nanofibrous membrane sensors for the detection of 2,4-dinitrotoluene are discussed.

In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Techniques


Fluorescence quenching Thefluorescence-quenchingphenomenon has been used in the detection of explosive molecules such as 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) (1). Since TNT and DNT contain electron-withdrawing nitro groups on the aromatic benzene ring, they have relatively strong interaction with electronrich species, such as many fluorescent dyes. These interactions result in quenching of thefluorescenceof the dye. The degree of quenching is dependent on the amount of the quencher, TNT or DNT, present In a homogeneous medium, such as a solution, the quantitative measure offluorescencequenching is described by the Stem-Volmer constant, Ks in the equation I

η COOCH

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Enzyne, AIBN PMMA-PM

Figure 2 Synthesis of PMMA-PM.

Poly (acrylic acid-pyrene methanol) (PAA-PM) Poly (acrylic acid-pyrene methanol) was synthesized as shown in Figure 3 (11). Catalysts 1,1 'K^rbonyldiimidazole (GDI) and 1,8diazabicyclo[5A0]undee-7-ene (DBU) dissolved in N, N'-dimethylfonnamide was added to a solution of polyacrylic acid. After stirring the solution at elevated temperature until the evolution of carbon dioxide subsided, a solution of pyrene methanol solution was added and the solution was stirred for one day. The solution was then poured into ethyl ether to precipitate the polymer. After filtration, the obtained polymer was extensively washed with ether and acetone and dried in a vacuum oven. This method resulted in a degree of fimctionalization of about seven percent by NMR measurement. That is, about seven percent of the acid groups on the polymer had reacted with the pyrene methanol. Earlier investigations showed that there is a strongfluorescenceselfquenching when a higher degree (>10%) offimctionalizationwas used (28). Therefore, the polyacrylic acid can befiinctionalizedwith appropriately low pyrene methanol levels to minimize the self-quenching effect The synthesized poly (acrylic acid-pyrene methanol) polymer shows the characteristic UV-Vis absorption and emission spectra of the pyrene methanol indicator with maximum UV-Vis absorbance andfluorescenceemission at 348 nm and 453 nm respectively.



Sensor Fabrication and Performance For poly (methylmethacrylate-pyrenemethanol), the spin-dope solution, which consisted of a 26%, by weight, solution of PMMA-PM, was dissolved in propylene glycol methyl ether acetate. Veryfinefiberswere collected on a glass slide coated with tin oxide. The applied electrospinning voltages rangedfrom15 - 20 kv. The working distance between thetipof the pipette and the glass slide ranged from 15 to 20 cm. The collection time was between 30 to 45 seconds. For poly (acrylic acid-pyrene methanol), the spin-dope solution, consisted of an 18.6% by weight solution of the copolymer and 36.5% of crosslinkable polyurethane, dissolved in DMF. Fibers were again collected on a glass slide coated with tin oxide. The applied electrospinning voltages, working distances, and collection times were the same as previously described. The electrospun membranes were dried in a vacuum oven at 80°C for one day. The polyurethane contained a melamine cross-linker that required a cure time of one to two minutes at about 250°C. The fluorescent polymer was immobilized with the cross-linked polyurethane to form an interpenetrating network structure and a water insoluble sensing membrane. The scanning electron microscope (SEM) images of electrospun membranes of the PMMA-PM and cross-linked PAA-PM are shown in Figure 4. It was observed that the membranes have three-dimensional structures with a random fiber orientation that is uniformly distributed on the substrate. The diameters of thefiberswere approximately 400 to 1000 nm for PMMA-PM and 100 to 400 nm for PAA-PM. The porous structure of electrospun membrane provides 1 to 2 orders of magnitude higher surface area to volume ratio than that known for continuous thin films.



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Figure 4. SEM images ofelectrospun membranes: PMMA-PA (left) and PAA-PM (right). Fluorescence spectra as a function of different concentrations of 2,4-dinitro toluene (DNT) were measured from the electrospun membranes of ΡΜΜΑ-ΡΜ. The fluorescence intensity of an electrospun membrane decreased with DNT concentration and the degree of quenching depended on the amount of DNT, as shown in Figure 5.

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Wavelength (nm) Figure 5. Fluorescence emission spectra ofa PMMA -PM electrospun membrane with varying DNT concentration.



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DNT Concentrations (χ 10' M) Figure 6. Stern-Volmer plots ofelectrospun membranes as a function of quencher concentration.

The ratio offluorophoremonomer to structural monomer was varied at 0.5%, 2 % and 4 % denoted as RI, R2 and R3. Similar behavior was observed in all the polymers. Linear relationships between concentration of quencher (DNT) and Io/T were obtained. Stern-Volmer constants (Κ, ) were calculated from the slopes of each plot and are 1.96 χ 10 , 1.30 χ Mr and 8.70 xl0 for RI, R2 and R3 respectively. These values are one order of magnitude higher than those obtained from continuous films of the similar polymer system by electrostatic layer-by-layer self-assembly technique, which were previously reported from our group (28). It is interesting to note that a higher value of Κ* was observed in the polymer RI which contained a lower concentration offluorophore.This may be explained in that the change of thefluorescenceintensity due to small amount of quencher is more significant if the baselinefluorescenceintensity (I ) is lower. Therefore, a lower concentration offluorophoreshould be advantageous in increasing the sensitivity of the device. Similar studies with an electrospun membrane of poly acrylic acid-pyrene methanol showing the change influorescencespectra as a function of different concentrations of DNT were measured and are shown in Figure 7. It was found that the fluorescence intensities decreased with increasing DNT concentration. This fluorescence intensity decrease is expected and believed to be due to the quenching of the pyrene-based indicator by electron poor species, DNT. ν

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Figure 7. Fluorescence emission spectra of a PAA-PM electrospun membrane with varying DNT concentration. 7

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For quencher concentrations in the range of 10" to 10" mol/L, a linear relationship between quencher concentration and yi is obtained showing a Stem-Volmer relationship, where the Stern-Volmer constant is (Κ* ) 9.8 χ ΙΟ M" . This value is roughly two to three orders of magnitude greater than that obtained previously from the thin film sensors (28). The sensitivities of these devices are in the range of tens parts per billion. The fact that PAA-PM system is more sensitive than PMMA-PM is presumably due to the smaller size of electrospun fibers for PAA-PM and the solvent swelling effect of the crosslinked network structure of the polymer in analyte solution. Thus the crosslinked, PAA-PM system has a greater effective surface area than the PMMAP M system. 5

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Conclusions We have successfully demonstrated nanofibrous optical chemical sensors for DNT detection using electrospun fluorescent polymers. The device sensitivity is 1 to 2 orders of magnitude higher than that observed for continuous


398 thin films. Further efforts will focus on exploring new sensing materials including fluorescent conjugated polymers, controlling the structural properties of the electrospun films and optimizing the sensitivities for the detection of explosives both from liquid phase and vapor phase.


Acknowledgements Financial support from the U.S. Army is acknowledged. The authors are grateful to Dr. J. He, Dr. S. Balasubramanian, and Dr. L. L i for discussions of optical sensing. The authors also recognize Professor C. Sung and Ms. B. Kang for the SEM images, Mr. Bon-Cheol K u for assistance with polymer synthesis and Dr. H . Schreuder-Gibson for advice and assistance with electrospinning. Thanks are also expressed to Soluol Chemical Co., INC for providing the crosslinkable polyurethane and helpful discussions with Dr. John Reisch. This work is dedicated to Professor S. K. Tripathy.

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Explosive Detection by Fluorescent Electrospun Polymer Membrane

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