Amol Chandekar

Anomalous vapor sensor response of a fluorinated alkylthiol-protected gold nanoparticle film.

Monolayer-protected gold nanoparticle films generally swell and increase their electrical resistance when exposed to organic vapors. Films of gold nanoparticles protected by 1H,1H,2H,2H-perfluorodecanethiol (PFDT) exhibit an anomalous response in which the resistance decreases for all vapors investigated. Electron microscopy illustrates that the PFDT-functionalized gold nanoparticles are hexagonally ordered with an interparticle separation of 3 nm. Quartz crystal microbalance measurements confirm substantial mass uptake, but the relatively large interparticle separation and insulating properties of the gold particles lead to a porous film whose electrical resistance is strongly influenced by changes in the relative permittivity and reversible, vapor-induced changes in film morphology.

Peptide surface modification of P(HEMA-co-MMA)-b-PIB-b-P(HEMA-co-MMA) block copolymers.

Peptide surface modification of poly[(methyl methacrylate-co-hydroxyethyl methacrylate)-b-isobutylene-b-(methyl methacrylate-co-hydroxyethyl methacrylate)] P(MMA-co-HEMA)-b-PIB-b-P(MMA-co-HEMA) triblock copolymers with different HEMA/MMA ratios has been accomplished using an efficient synthetic procedure. The triblock copolymers were reacted with 4-fluorobenzenesulfonyl chloride (fosyl chloride) in pyridine to obtain the activated polymers [poly{(methyl methacrylate-co-fosyloxyethyl methacrylate)-b-isobutylene-b-(methyl methacrylate-co-fosyloxyethyl methacrylate)}] P(MMA-co-FEMA)-b-PIB-b-P(MMA-co-FEMA), with an activating efficiency of 80-90%. The resulting polymers were soluble in chloroform, and their solutions were used to coat thin uniform films with a predetermined thickness on smooth steel surfaces. The presence of reactive activating groups on the film surface was confirmed by X-ray photoelectron spectroscopy (XPS), dye labeling, and confocal laser scanning microscopic studies. Activation of the triblock copolymer films was also achieved under heterogeneous conditions in polar (acetonitrile) and nonpolar (hexanes) media. The extent of activation was controlled by varying the dipping time and polarity of the medium. Peptide attachment was accomplished by immersing the coated steel strips into aqueous buffer solution of Gly-Gly or GYIGSR. XPS and solubility studies revealed successful attachment of peptides to the polymer surface. Virtually all remaining activating groups were successfully replaced in the subsequent step by a treatment with Tris(hydroxymethyl)amino methane in a buffered methanol/water mixture.

Nanoscale patterning of a conjugated oligomer

Vapor-deposited α-sexithiophene is finding important organic electronic device applications. A method of patterning dibutylphosphonate-substituted sexithiophene (DBP-α6T), a soluble version of this conjugated oligomer, with lateral dimensions as small as 50nm is described. It consists of using either microcontact printing or dip-pen nanolithography to form hydrophilic alkanethiol patterns on a gold surface. The remainder of the surface is backfilled with a hydrophobic alkanethiol. When DBP-α6T is spin-coated and annealed, it preferentially adsorbs on the hydrophilic patterns. Photoemission demonstrates the similarity in valence electronic structure of DBP-α6T films to α-sexithiophene. This patterning method may be useful for future nanoscale device fabrication.

Synthesis and Characterization of Poly(methyl methacrylate‐co‐hydroxyethyl methacrylate)‐b‐polyisobutylene‐b‐poly(methyl methacrylate‐co‐hydroxyethyl Methacrylate) Triblock Copolymers

The synthesis of poly[(methyl methacrylate‐co‐hydroxyethyl methacrylate)‐b‐isobutylene‐b‐(methyl methacrylate‐co‐hydroxyethyl methacrylate)] P(MMA‐co‐HEMA)‐b‐PIB‐b‐P(MMA‐co‐HEMA) triblock copolymers with different HEMA/MMA ratios has been accomplished by the combination of living cationic and anionic polymerizations. P(MMA‐co‐HEMA)‐b‐PIB‐b‐P(MMA‐co‐HEMA) triblock copolymers with different compositions were prepared by a synthetic methodology involving the transformation from living cationic to anionic polymerization. First, 1,1‐diphenylethylene end‐functionalized PIB (DPE‐PIB‐DPE) was prepared by the reaction of living difunctional PIB and 1,4‐bis(1‐phenylethenyl)benzene (PDDPE), followed by the methylation of the resulting diphenyl carbenium ion with dimethylzinc (Zn(CH3)2). The DPE ends were quantitatively metalated with n‐butyllithium in tetrahydrofuran, and the resulting macroanion initiated the polymerization of methacrylates yielding triblock copolymers with high blocking efficiency. Microphase separation of the thus prepared triblock copolymers was evidenced by the two glass transitions at −64 and +120°C observed by differential scanning calorimetry. These new block copolymers exhibit typical stress‐strain behavior of thermoplastic elastomers. Surface characterization of the samples was accomplished by angle‐resolved X‐ray photoelectron spectroscopy (XPS), which revealed that the surface is richer in PIB compared to the bulk. However, a substantial amount of P(MMA‐co‐HEMA) remains at the surface. The presence of hydroxyl functionality at the surface provides an opportunity for further modification.

Design and Lipase Catalyzed Synthesis of 4-Methylcoumarin-siloxane Hybrid Copolymers

Poly dimethylsiloxanes with amino end groups were copolymerized with diesters of 4-methylcoumarins enzymatically using a lipase (Candida antarctica lipase) as a biocatalyst. In a separate synthesis, 4-methylcoumarin was also incorporated into the poly siloxanes-isophthalate copolymers by functionalization of hydroxyl groups in the isophthalate moiety. The synthesis and characterization of two sets of novel copolymers are presented. The thermal and flammability properties of these polymers have also been studied using TGA and microcalorimetry, respectively.

Comparison of Solubility and Vapor Sensing Properties of Methyl‐ and Thiophene‐Terminated Alkanethiol‐Protected Gold Nanoparticle Films

Gold nanoparticles, protected by monolayers of two different lengths of methyl‐ and ω‐thienyl‐terminated alkanethiols, have been synthesized, and their solubility properties in various solvents have been evaluated by measuring the energy of the plasmon band using UV‐Vis absorbance spectroscopy and by transmission electron microscopy. Chemical sensors have been fabricated by spin‐coating films of the monolayer‐protected gold particles onto interdigitated microelectrode arrays. Exposure to chloroform, toluene, and hexane vapors causes the electrical resistance of the films to increase. A sensor based on 12‐(3‐thienyl)dodecanethiol is shown to be dramatically more sensitive to both chloroform and toluene than one based on tridecanethiol. However, in the case of hexane, these two alkanethiol‐protected gold nanoparticle sensors perform similarly. These results suggest that for very nonpolar solvents such as hexane, the functional group at the periphery of the particle is much less significant than the effect of alkanethiol chain length.