Dmitry Pestov

High surface area polymer coatings for SAW-based chemical sensor applications

Abstract High surface-to-volume ratio coatings consisting of nano-scale polymer particles were applied to surface acoustic wave (SAW) transducers. The resulting sensors were tested upon exposure to analyte vapors and were compared with sensors developed from bulk films of the same polymers. The relative importance of surface adsorption and bulk absorption was investigated and it is shown that high surface area coatings can be used to improve the sensitivity and response time particularly in the case of polymers exhibiting low vapor permeability. Furthermore, because the mean particle diameter is small compared to the spacing of the SAW interdigital electrodes, nano-scale particulate coatings appear acoustically uniform and are therefore, compatible with SAW technology. The high surface-to-volume ratio coatings were deposited using a spray-on technique known as rapid expansion of supercritical solutions (RESS) and both glassy and viscoelastic polymers were studied.

An electrospray-based, ozone-free air purification technology

A zero-pressure-drop, ozone-free air purification technology is reported. Contaminated air was directed into a chamber containing an array of electrospray wick sources. The electrospray sources produce an aerosol of tiny, electrically charged aqueous droplets. Charge was transferred from the droplets onto polar and polarizable species in the contaminated air stream and the charged contaminants were extracted using an electric field and deposited onto a metal surface. Purified air emerged from the other end of the chamber. The very small aqueous electrospray droplets completely evaporate so that the process is essentially dry and no liquid solvent is collected or recirculated. The air purification efficiency was measured as a function of particle size, air flow rate, and specific system design parameters. The results indicate that the electrospray-based air purification system provides high air purification efficiency over a wide range of particle size and, due to the very low power and liquid consumption ...

Steady-state and frequency-domain lifetime measurements of an activated molecular imprinted polymer imprinted to dipicolinic acid.

We recently demonstrated the synthesis and fluorescence activity associated with an optical detector incorporating a molecular imprinted polymer (MIP). Steady-state and time-resolved (lifetime) fluorescence measurements were used to characterize the binding activity associated with MIP microparticles imprinted to dipicolinic acid (DPA). DPA is a unique biomarker associated with the sporulation phase of endospore-forming bacteria. Vinylic monomers were polymerized in a dimethylformamide solution containing DPA as a template. The resulting MIP was then pulverized and sorted into small microscale particles. Tests were conducted on replicate samples of biologically active cultures representing both vegetative stationary phase and sporulation phase of Bacillus subtilis in standard media. Samplers were adapted incorporating the MIP particles within a dialyzer cartridge (500 MW). The permeability of the dialyzer membrane permitted diffusion of lighter molecular weight constituents from microbial media effluents to enter the dialyzer chamber and come in contact with the MIP. Results showed dramatic (10-fold over background) steady-state fluorescence changes (as a function of excitation, emission and intensity) for samples associated with high endospore biomass (DPA), and a frequency-domain lifetime of 5.3 ns for the MIP–DPA complex.

Molecular imprinting using monomers with solid-state polymerization

Molecular imprinting of two diolefinic compounds with solid-state photopolymerization, 2,5-distyrylpyrazine (DSP) and diethyl p-phenylenediacrylate (EPA), was demonstrated. Solid nanoscale particles of the monomer were produced and deposited onto the surface of a surface acoustic wave (SAW) transducer using the technique known as rapid expansion of supercritical solutions (RESS). The particles were polymerized by UV light in the presence of an alkane template vapor. Both imprinted and non-imprinted devices were tested upon exposure to a variety of alkane vapors in the gas phase. The results demonstrate an enhanced sensitivity to vapors at or below the size of the template. A size exclusion mechanism of recognition is proposed.

Photosensitive 2,5-distyrylpyrazine particles produced from rapid expansion of supercritical solutions

Solvent-free, photoreactive particles of 2,5-distyrylpyrazine (DSP) monomer were developed by rapid precipitation from an expanding supercritical chlorodifluoromethane solution. DSP polymer particles were produced by solid-state photopolymerization. DSP particles below a critical diameter of about 0.5 μm were found to be mechanically stable and did not fragment upon photopolymerization. The rate of DSP photopolymerization was shown to be size-sensitive. Nano-scale particles demonstrated superior photoreactivity in the solid state in comparison to micro-scale crystals. UV spectra of DSP at different degrees of conversion were investigated and the extinction coefficients were calculated for the DSP monomer and polymer in sulfuric acid.

ELECTROSPUN POLYMER-FIBER SOLAR CELL

A novel electrospun polymer-fiber solar cell was synthesized by electrospinning a 1 : 2.5 weight% ratio mixture of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) resulting in bulk heterojunctions. Electrospinning is introduced as a technique that may increase polymer solar cell efficiency, and a list of advantages of the technique applied to solar cells is discussed. The device achieved a power conversion efficiency of %. The absorption and photoluminescence of MEH-PPV nanofibers are compared to thin films of the same material. Electrospun nanofibers are discussed as a favorable structure for application in polymer solar cells.

Novel low-temperature fabrication process for integrated high-aspect ratio zinc oxide nanowire sensors

The authors present a new low-temperature nanowire fabrication process that allows high-aspect ratio nanowires to be readily integrated with microelectronic devices for sensor applications. This process relies on a new method of forming a close-packed array of self-assembled high-aspect-ratio nanopores in an anodized aluminum oxide (AAO) template in a thin (2.5 μm) aluminum film deposited on a silicon substrate. This technique is in sharp contrast to the traditional free-standing thick film methods, and the use of an integrated thin aluminum film greatly enhances the utility of such methods. The authors have demonstrated the method by integrating ZnO nanowires onto the metal gate of a metal-oxide-semiconductor (MOS) transistor to form an integrated chemical field-effect transistor (ChemFET) sensor structure. The novel thin film AAO process uses a novel multistage aluminum anodization, alumina barrier layer removal, ZnO atomic layer deposition (ALD), and pH controlled wet release etching. This new process ...

Room Temperature Synthesis of Highly Magnetic Cobalt Nanoparticles by Continuous Flow in a Microfluidic Reactor

Cobalt nanoparticles were synthesized using continuous-flow (CF) chemistry in a stainless steel microreactor for the first time at high output based on the ethanol hydrazine alkaline system (EHAS) producing a yield as high as 1 g per hour [1, 2]. Continuous-flow (CF) synthetic chemistry provides uninterrupted product formation allowing for advantages including decreased preparation time, improved product quality, and greater efficiency. This successful synthetic framework in continuous-flow of magnetic Co nanoparticles indicates feasibility for scaled-up production. The average particle size by transmission electron microscopy (TEM) of the as-synthesized cobalt was 30±10 nm, average crystallite size by Scherrer analysis (fcc phase) was 15±2 nm, and the estimated magnetic core size was 6±1 nm. Elemental surface analysis (X-ray photoelectron spectroscopy [XPS]) indicates a thin CoO surface layer. Assynthesized cobalt nanoparticles possessed a saturation magnetization (Ms) of 125±1 emu/g and coercivity (Hc) of 120±5 Oe. The actual Ms is expected to be greater since the as-synthesized cobalt mass was not weight-corrected (nonmagnetic mass: reaction by-products, solvent, etc.). Our novel high-output, continuous-flow production (>1 g/hr) of highly magnetic cobalt nanoparticles opens an avenue toward industrial-scale production of several other single element magnetic nanomaterials.

Chemical point detection using differential fluorescence from molecularly imprinted polymers

Fluorescence represents one of the most attractive approaches for chemical sensing due to the abundant light produced by most fluorophores, resulting in excellent detection sensitivity. However, the broad and overlapping emission spectra of target and background species have made it difficult to perform species identification in a field instrument because of the need to perform spectral decomposition and analysis. This paper describes a new chemical sensing strategy based on differential fluorescence measurements from molecularly imprinted polymers, which eliminates the need to perform any spectral analysis. Species identification is accomplished by measuring the differential light output from a pair of polymers-one imprinted to a target species and the other identical, but not imprinted. The imprinted polymer selectively concentrates the target molecule and controls the energy (wavelength) of the emitted fluorescence signal and the differential output eliminates common mode signals associated with non-specific background interference. Because no spectral analysis is required, the sensors can be made extremely small and require very little power. Preliminary performance parameters from a prototype sensor are presented and discussed.

Fiber Bragg grating multichemical sensor

Fiber optic-based chemical sensors are created by coating fiber Bragg gratings (FBG) with the glassy polymer cellulose acetate (CA). CA is a polymeric matrix capable of localizing or concentrating chemical constituents within its structure. Some typical properties of CA include good rigidity (high modulus) and high transparency. With CA acting as a sensor element, immersion of the gratings in various chemical solutions causes the polymer to expand and mechanically strain the glass fiber. This elongation of the fiber sections containing the grating causes a corresponding change in the periodicity of the grating that subsequently results in a change in the Bragg-reflected wavelengths. A high-resolution tunable fiber ring laser interrogator is used to obtain room-temperature reflectance spectrograms from two fiber gratings at two different wavelengths - 1540nm and 1550nm. The graphical representation from this device enables the display of spectral shape, and not merely shifts in FBG central wavelength, thereby allowing for more comprehensive analysis of how different physical conditions cause the reflectance profile to move and alter overall form. Wavelength shifts on the order of 1 to 80 pm in the FBG transition edges and changes in spectral shape are observed in both sensors upon immersion in a diverse selection of chemical analytes.