Clovia I. Holdsworth

Molecularly imprinted polymers (MIPs): sensing, an explosive new opportunity?

Our group is currently developing in-field detection systems alongside the Australian Federal Police Forensic Services utilising molecularly imprinted polymers as the recognition elements. This review looks at MIP synthesis and our perceptions of future directions from an Australian and forensic perspective.

Synthesis and Evaluation of a Molecularly Imprinted Polymer Selective to 2,4,6-Trichlorophenol

Molecularly imprinted polymers (MIPs) selective for the phenolic contaminant 2,4,6-trichlorophenol (2,4,6-TCP) were prepared and evaluated in three porogens of differing character (hexane, acetonitrile, dichloromethane). Rebinding of 2,4,6-TCP was found to be most effective in dichloromethane (imprinting factor: 13.2). Competitive binding studies performed against a range of close structural analogues showed a high preference for the target molecule, although partial recognition towards 2,4-dichlorophenol was also observed. Specificity was found to be dependent upon the presence of ring chlorine on the target, which suggested that these atoms participate in secondary binding interactions that are essential for successful recognition in the polymer cavity.

Formulation of Cocaine-Imprinted Polymers Utilizing Molecular Modelling and NMR Analysis

Molecular imprinted polymers (MIPs) have distinctive features that make them attractive as an inexpensive, reusable, and robust field-based detection system for illicit substances. Optimizing MIP performance is traditionally attained by the synthesis and evaluation of a plethora of individual formulations. A non-covalently imprinted polymer for cocaine has been prepared using a commercially available molecular modelling package (Spartan 02) to predict energetically favourable monomer–template interactions between the target (T) and two different functional monomers (FM)—methacrylic acid (MAA) and 4-vinylpyridine (4VP). NMR studies undertaken to assess target–monomer behaviour in solution were in good agreement with the computational data. MIPs involving three target-to-functional monomer ratios (1 : 2, 1 : 6, and 1 : 14) were prepared and evaluated. Target rebinding was found to be most favourable in the 1 : 2 formulation with a target-selective binding of 0.48 ppm and an imprinting factor (I) of 2.8 obtained for 10 mg of test polymer.

Microwave induced MIP synthesis: comparative analysis of thermal and microwave induced polymerisation of caffeine imprinted polymers

Caffeine templated molecularly imprinted polymers (MIPs) have been prepared using identical polymer formulations by thermal and microwave induced initiation, and the binding performance of the systems compared using solid phase extraction (SPE). While the binding capacity of MIP(Microwave) was found to be lower than MIP(Thermal), MIP(Microwave) recorded a higher imprinting factor (IF) due to comparatively low levels of non-specific binding. Selectivity against theophylline in cross-reactivity studies was also found to be greater for MIP(Microwave). While large time savings are achievable through the use of microwave irradiation conditions, physical analysis of the polymers (surface area analysis, thermal gravimetric analysis and differential scanning calorimetry) reveals that the different polymerisation methods lead to differences in both polymer structure and performance.

Effect of template on the formation of phase-inversed molecularly imprinted polymer thin films: an assessment

A range of molecularly imprinted polymer films (MIPFs) consisting of acrylonitrile (AN) as a matrix monomer, methacrylamide (MAM) as a functional monomer (96 : 4) for 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT) were prepared using a phase inversion method with post polymerisation introduction of the template. The films exhibited no rebinding activity following template removal, prompting speculation of a collapse in the pore structure of the materials. Addition of extremely low concentrations of cross-linker (0.01% ethylene glycol dimethacrylate (EDGMA)) successfully stabilised the macroscopic structure of the films. The same approach was then applied to the preparation of conventionally imprinted films. The average molecular weight of the polymer chains in the MIPFs was found to decrease with increasing template concentration, confirming results from previous studies identifying TNT as a chain transfer agent. Failure to observe rebinding in the films in either the liquid or gas phases suggested that template removal precipitates irreversible changes in microscopic pore structure of the MIPFs. Thermogravimetric analysis (TGA) showed a proportionate loss in mass of the template from the non-extracted MIPFs, confirming the successful encapsulation of the template. Differential scanning calorimetry (DSC) measurements revealed distinct differences between the non-extracted films and the non-imprinted control polymer that become more exaggerated with the presence of increasing amounts of template in the MIPF formulation. Following template removal, the MIPF thermograms closely resembled the non-imprinted control, suggesting that template removal promotes changes in the MIPF microstructure consistent with the loss of template cavities and the formation of a more homogeneous structure.

Investigation of the doping efficiency of poly(styrene sulfonic acid) in poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) dispersions by capillary electrophoresis

CE can efficiently separate poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) complexes and free PSS in dispersions and can be used to estimate the degree of PSS doping. We investigated the doping efficiency of PSS on PEDOT in dispersions using CE and its effect on the conductivity of the resulting PEDOT/PSS films. Results of this study indicate that dispersions containing 1:2.5–3 EDOT:PSS feed ratio (by weight) exhibiting 72–73% PSS doping generate highly processable and highly conductive films. Conductivity can be optimized by limiting the time of reaction to 12 h. At this point of the reaction, the PEDOT/PSS segments, appearing as broad band in the electropherogram, could still exist in an extended coil conformation favoring charge transport resulting in high conductivity. Above a threshold PEDOT length formed at reaction times longer than 12 h, the PEDOT/PSS complex, appearing as spikes in the electropherogram, most likely have undergone a conformational change to coiled core‐shell structure restricting charge transport resulting in low conductivity. The optimal conductivity (5.2 S/cm) of films from dispersions synthesized for 12 h is significantly higher than those from its commercial equivalent Clevios P and other reported values obtained under similar conditions without the addition of codopants.

N-2-Propenyl-(5-dimethylamino)-1-naphthalene Sulfonamide, a Novel Fluorescent Monomer for the Molecularly Imprinted Polymer-Based Detection of 2,4-Dinitrotoluene in the Gas Phase

Fluorescent molecularly imprinted polymers (MIP) specific for 2,4-dinitrotoluene (DNT) have been synthesised using a novel monomer N-2-propenyl-(5-dimethylamino)-1-naphthalene sulfonamide. Three formats of the polymer were produced: a traditional bulk monolith ground into particles, a flexible, but highly cross-linked plasticiser-modified free standing membrane, and a hybrid material consisting of particles embedded in a poly(acrylonitrile) phase inversed film. Within all materials, a clearly defined imprinting effect was observed upon exposure to DNT vapour at room temperature. In all cases, preferential rebinding of DNT to the molecularly imprinted materials (3–5 times) over their non-imprinted (NIP) equivalents was evident within <10 min of contact with the DNT vapour stream. Fluorographic images of the fluorescent polymers showed the DNT binding-induced quenching to be significantly higher in the MIP material than in the non-imprinted control polymer.

Nonequilibrium Plasma Polymerization of HFC-134a in a Dielectric Barrier Discharge Reactor: Polymer Characterization and a Proposed Mechanism for Polymer Formation

Nonequilibrium plasma polymerization of hydrofluorocarbon HFC-134a (CF3CH2F) in argon bath gas has been studied in a dielectric barrier discharge reactor at atmospheric pressure and in the absence of oxygen and nitrogen. The reaction resulted in the formation of a polymeric solid fraction and the noncrosslinked properties of this material assisted in its characterization by solution state 13C and 19F nuclear magnetic resonance spectroscopy. Gel permeation chromatography revealed that the polymers include low (number average molecular weight, Mn values between 900 and 3000 g mol-1) and high (Mn approximately 60000 g mol-1) molecular weight fractions. A detailed polymerization mechanism is proposed, based on the published literature and the findings of the current investigation.

Molecularly imprinted films of acrylonitrile/methyl methacrylate/acrylic acid terpolymers: influence of methyl methacrylate in the binding performance of L-ephedrine imprinted films

Molecularly imprinted polymeric films (MIPFs) highly selective to 1R,2S(-)ephedrine (L-ephedrine, EPD) were produced by phase inversion post-polymerization imprinting on poly(acrylonitrile-co-methyl methacrylate-co-acrylic acid) (PAMA) terpolymers. The inclusion of methyl methacrylate (MMA) to the polymer formulation resulted in enhanced EPD selectivity which appears to be dictated by polymer composition to achieve the necessary balance between polymer rigidity and porosity. Substitution of MMA with methyl acrylate, ethyl acrylate and n-butyl acrylate resulted in a loss of EPD selectivity and EPD entrapment within the polymer matrix not observed in PAMA MIPFs. MMA, by virtue of its methyl group, is able to provide the scaffolding and rigidity necessary for stability and preservation of imprinted cavities within the PAMA MIPF leading to high EPD selectivity.

Capillary electrophoresis with photodiode array detection of processable poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate aqueous dispersions

The application of capillary electrophoresis (CE) with on-line photodiode array detection to the analysis of processable poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) aqueous dispersions is presented. The dispersions were prepared by oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) in the presence of PSS. The separation of PEDOT/PSS complexes and PSS was achieved using sodium borate (pH 9.2) in a fused silica capillary and positive polarity. The electrophoretic behavior and UV spectra of the negatively charged complexes were correlated to the amount of PEDOT attached to PSS. The ability of CE to predict the conductivity of films produced from spin coating of PEDOT/PSS dispersions was also proposed.