Antiterrorism and Homeland Defense - American Chemical Society

Chapter 2

Downloaded by NORTH CAROLINA STATE UNIV on August 4, 2012 | http://pubs.acs.org Publication Date: December 31, 2007 | doi: 10.1021/bk-2007-0980.ch002

Synthesis and Spectroscopic Characterization of Molecularly Imprinted Polymer Phosphonate Sensors G. E. Southard, K. A. Van Houten, Edward W. Ott Jr., and G. M. Murray* Applied Physics Lab, The Johns Hopkins University, Laurel, MD 20723

Molecularly imprinted polymers (MIPs) that are capable of sensing specific organophosphorous compounds, such as pinacolyl methylphosphonate (PMP), by luminescence have been synthesized and characterized. The polymers have been synthesized using conventional free radical polymerization and using Reversible Addition Fragmentation Transfer (RAFT) polymerization. The R A F T polymers exhibited many advantages over conventional free radical processes but are more difficult to make porous.

The preparation of molecularly imprinted polymers (MIPs) normally employs conventional free radical polymerization. While this is a relatively simple process, it is prone to problems that can affect the imprinting site. R A F T polymerization can overcome some of the problems associated with conventional free radical polymerizations and offers an opportunity to make block copolymers, thus localizing the crosslinking of the polymer around the imprinting site.

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In Antiterrorism and Homeland Defense; Reynolds, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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The Synthesis of Imprinted Polymers for Luminescence Detection Molecular imprinting is a process for making selective binding sites in synthetic polymers (7). The process may be approached by designing the recognition site or by simply choosing monomers that may have favorable interactions with the imprinting molecule. To successfully apply the methodology to chemical sensing requires the designed approach. The process involves building a complex of an imprint molecule and complementary polymerizable ligands. At least one of the molecular complements must exhibit a discernable physical change associated with binding. This change in property can be any measurable quantity, but a change in luminescence is the most sensitive and selective analytical technique. By copolymerizing the complexes with a matrix monomer and a suitable level of crosslinking monomer, the imprint complex becomes bound in a polymeric network. The network must be mechanically and chemically processed to liberate the imprinting species and create the binding site (2-4). The design of the binding site requires chemical insight. These insights are derived from studies of molecular recognition and self-assembly and include considerations of molecular geometry, size and shape, as well as molecule-to-ligand thermodynamic affinity. For the purpose of sensing, a reporting molecule must be a part of the cavity to indicate when the target molecule (e.g., pinacolyl methylphosphonate) is bound. The best reporters generate fluorescence, since photoluminescence is highly sensitive and adds greater selectivity through the two wavelengths of light that must be employed. Compounds like pinacolyl methylphosphonate are complex organic molecules that exhibit a variety of chemical functional groups as substituents. Many of these groups possess an affinity for metal ions in solution and can form complexes. By using these natural affinities, metal incorporating organic copolymers can be made that have a high thermodynamic affinity for binding molecules. The organic matrix of the polymer improves vapor detection since it can selectively condense organic vapor from air, as is the case in solid phase micro-extraction (SPME) methods. The judicious choice of a metal ion with useful spectroscopic properties, in addition to a high thermodynamic binding affinity, results in the formation of highly selective and sensitive sensors. The mechanism to detect pinacolyl methylphosphonate is to discern its effect of the luminescence of a lanthanide ion, E u (5). The optical absorption and emission spectra of the triply charged free lanthanide ions, which are assignable to f-»f transitions, generally consist of very narrow lines (0.1-0.01 nm). Organic compounds with affinity to form metal ion complexes are called 3 +



21 ligands. When ligands are placed around the lanthanide ions, the spectral lines are shifted in position (wavelength), they may be split (usually -1.0 nm), and they are often altered in relative intensity (some of them may even be eliminated) due to electrostatic interactions called the Stark Effect. The exact character and degree of these changes are specific to each different ligand environment, the major factors being the coordination number, the coordination symmetry (geometry), and the type(s) of ligands. Even though the ligand field is sufficient to produce these shifts, splits, and intensity alterations, the ligand to f electron coupling is generally not adequate to result in broadening of the lines. Another consequence of the weak coupling and the inter-configurational nature of f->f transitions, radiative lifetimes of lanthanide ions in compounds are quite long, on the order of milliseconds, simplifying time resolved measurements that enhance sensitivity. By a judicious choice of coordinating ligand, the absorptivity or quantum efficiency can be enhanced and a broad band source, such as a light emitting diode (LED) can be used. O f the lanthanide ions, the E u ion has the simplest set of splitting possibilities and most favorable level structure for sensitization. These spectroscopic properties make E u ions attractive for chemical sensing (6). Thus, the exchange of one ligand for another results in spectral changes that are useful for sensing. In the case of a vapor sensor, the analyte need only displace a solvent molecule (typically water) to bind to E u ion. This process is facilitated by the imprinted cavity geometry and the affinity of organic polymers to absorb organic molecules. In order to develop a binding site with appropriate characteristics for sensing, several candidate complexes must be screened. The selection process is based on finding a complex that has a spectral shift with complexation and that will absorb light from an inexpensive and low power source. The complexing species also has to bind E u tightly enough to keep it from being removed from the polymer when the imprinting species is removed. In order to satisfy these criteria, we selected β-diketones. Once the pair of complexes with and without imprinting ion were shown to exhibit the correct spectroscopic properties, it was necessary to functionalize the complexing ligands with vinyl groups. However, it is vital that the ligands be polymerizable, so that the imprinted site will be bound to the polymer. Incorporation of the functionalized complex into the polymer matrix required some experimentation (7). The metal ion complexes may have limited solubility in the monomer solution. This can be addressed by adding an appropriate solvent to the mixture. The solvent has the further purpose of acting to help increase the porosity of the finished polymer. This simplifies the removal of the imprint molecule and increases the speed of the sensor response. Typically, a range of complex loading, crosslinking, and solvent addition are examined for optimal optical response. 3 +

3 +

3 +

3+


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Experimental Details


Reagents 4-Bromobenzoylacetone was synthesized by a modified Shea method (8). The new bromine substituted β-diketones were synthesized by modified literature procedures (9-11). The l-acetyl-4-bromonaphthalene was synthesized by standard Friedel-Crafts acylation of 1-bromonaphthalene with acetyl chloride and aluminum chloride. The synthesis of the vinyl substituted β-diketones, see Figure 1, has been reported elsewhere (19). A l l chemicals were provided by Sigma-Aldrich unless otherwise stated, and were used without further purification.

Instrumentation A n in-house detection system was employed for screening lanthanide complexes that includes: an A r Ion Laser, Model 543 Head and Model 170 Power Supply, (Omnichrome, Chino, C A ) and an f/4, 0.5 meter monochromator (Chromex, Albuquerque, N M ) equipped with a Model ST-6 C C D detector (Santa Barbara Instrument Group, Inc., Santa Barbara, C A ) . KestrelSpec software (Rhea Corp. Wilmington, DE) was used to operate the C C D and record the compound luminescence. A Cary 50 UV-vis spectrophotometer (Varian, Walnut Creek, C A ) was used to obtain absorbance spectra. The luminescence titrations were obtained using a Model QM-2 Fluorimeter/Phosphorimeter, (Photon Technologies International, Monmouth, NJ). Thermogravimetry was performed using a Model SDT 2960 Simultaneous D S C - T G A (TA Instruments, New Castle, DE). A Hewlett-Packard Model 5400 ICP-MS (Yokogawa Analytical Systems, Tokyo, Japan) was used to verify metal concentrations in all sample solutions. N M R was performed using either a Bruker AC-200 M H z spectrometer or a Model EFT 90 M H z spectrometer (Anasazi Instruments, Indianapolis, IN). The purity of synthesized organics was established using a Model QP 5050A G C / M S (Shimadzu, Columbia, M D ) . The Heck coupling reaction was performed with either a Model H C 677 100-mL reactor (PanInstruments, Moline, IL) or an L C series 300-mL reactor (Pressure Products Industries, Warminster, PA).

Synthesis of HDBNTFA (8): Dithiobenzoic acid l-(4-(4,4,4-trffluorobutane-l,3-dione)-naphthalen-l-yl)-ethyl ester (HDBNTFA) Compound 6a (2.92 g, 10 mmol), dithiobenzoic acid (1.54 g, 10 mmol), and carbon tetrachloride (6 mL) were placed together into a 15-mL, round-bottomed


23 flask equipped with a reflux condenser under an argon atmosphere. The reaction was heated to 70 °C for 16 h when a 2 aliquot of dithiobenzoic acid (0.77 g, 5 mmol) and the reaction was continued for another 4 h. The solvent was removed by vacuum and the final product was isolated by column chromatography through silica gel with 60/40 hexanes/chloroform as eluent to give a viscous red oil (2.2 g, 50% yield). *H-NMR (90 M H z , 25 °C, CDC1 ): δ 8.52-8.45 (dd, 2 H), 8.09-7.36 (bm, 9 H), 6.80 (s, 1 Η), 6.11--6.04 (q, 1 H), 2.00-1.92 ( d , 3 H ) . n d


3

Pd /DMF/TTP 2+

2 3

Η X-|, X3, X j Br — X2 Η X2, X3, X i B r X

Antiterrorism and Homeland Defense - American Chemical Society

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