Michael A. Markowitz

Nanoporous Organosilicas as Preconcentration Materials for the Electrochemical Detection of Trinitrotoluene

We describe the use of nanoporous organosilicas for rapid preconcentration and extraction of trinitrotoluene (TNT) for electrochemical analysis and demonstrate the effect of template-directed molecular imprinting on TNT adsorption. The relative effects of the benzene (BENZ)- and diethylbenzene (DEB)-bridged organic-inorganic polymers, having narrow or broad pore size distributions, respectively, on electrochemical response and desorption behavior were examined. Sample volumes of 0.5-10 mL containing 5-1000 ppb TNT in a phosphate-buffered saline buffer were preconcentrated in-line before the detector using a microcolumn containing 10 mg of imprinted BENZ or DEB. Square-wave voltammetry was used to detect the first reduction peak of TNT in an electrochemical flow cell using a carbon working electrode and a Ag/AgCl reference electrode. Imprinted BENZ released TNT faster than imprinted DEB with considerably less peak tailing and displayed enhanced sensitivity and an improvement in the limit of detection (LOD) owing to more rapid elution of TNT from that material with increasing signal amplitude. For imprinted BENZ, the slope of signal versus concentration scaled linearly with increasing preconcentration volume, and for preconcentrating 10 mL of sample, the LOD for TNT was estimated to be 5 ppb. Template-directed molecularly imprinted DEB (TDMI-DEB) was 7-fold more efficient in adsorption of TNT from aqueous contaminated soil extract than nonimprinted DEB.

Control of surface expression of functional groups on silica particles

The effect of the time of addition of organosilanes to silica particle formation reaction mixtures on the resulting surface availability of the added functional groups was investigated. Base catalyzed particle formation was initiated by the addition of tetraethyl orthosilicate (TEOS) to a water-in-oil microemulsion. Subsequently, amine, mono-carboxylate, ethylenediaminetriacetic acid, or dihydroimidazole-terminated organosilanes were added to the microemulsion. Continuous growth in size of monodispersed spherical particles over time was monitored by transmission electron microscopy and light-scattering measurements. Surface primary amine and carboxylate groups on the resulting particles were labeled with fluorescamine and 1-pyrenyldiazomethane (PDAM), respectively, and the effect of varying the time of organosilane addition to the microemulsion was determined by fluorescence spectroscopy. The effect of the time of organosilane addition on surface expression of dihydroimidazole groups was determined using a direct titration method. The results demonstrate that the degree of surface functionalization of the silica particles varied with the time of organosilane addition. For each type of amine terminated organosilane used, the highest surface availability of organo-functional groups was obtained when the organosilane was added 30 min after particle growth was initialized. A progressive decrease in the surface availability of primary amine groups was observed when 3-aminopropyltrimethoxysilane (APTMS) was added later than 30 min after particle formation was initiated. For each type of carboxylate-terminated organosilane used, the highest surface availability of organo-functional groups was obtained when the organosilane was added 5.5 h after particle growth was initialized.

Palladium ion assisted formation and metallization of lipid tubules

Abstract Tubules have been formed at pH 5.6 from the negatively charged diacetylenic phospholipid 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphohydroxyethanol in the presence of tetraammine palladium (II) chloride. Transmission electron microscopy revealed the formation of predominantly 0.07 μm–0.16 μm diameter tubules. Unbound palladium salt was removed by dialyzing tubule dispersion against water. The palladium bound to the lipid headgroups acted as a catalyst for the electroless metal plating of the tubules with nickel and cobalt metals. Sustained dialysis prior to metallization resulted in an increase in the diameter of the tubules. Palladium salt did not bind to the neutral phosphocholine headgroup of the analogous diacetylenic lipid and therefore no metallization was observed. Thin slices of metal coated tubules embedded in epoxy resin were characterized by atomic force and scanning tunneling microscopy.

The influence of the polar headgroups of acidic diacetylenic phospholipids on tubule formation, microstructure morphology and Langmuir film behavior

Abstract The effect of modification of the polar headgroup of diacetylenic phospholipids on bilayer morphology and monolayer properties has been investigated. The lipids were prepared by substituting the choline portion of 1,2-bis(10, 12-tricosadiynoyl)-sn-glycero-3-phosphocholine with the glycols HO(CH 2 ) n OH ( n = 2, 3, 4), by means of a phospholipase D catalyzed transphosphatidylation. In the presence of CuCl 2 (pH 5.6 and ionic strength 0.18), the lipid with n = 2, produced a bimodal population of small diameter (0.1 μm) and large diameter (0.9 μm) tubules, the lipid for n = 3 produced a similar bimodal distribution of tubules except that the diameter of the smaller tubules increased to 0.3 μm. No small diameter tubules were formed from the lipid where n = 4. An increase or decrease either in the pH or ionic strength decreased the yield of tubules and affected the formation of the small diameter tubules, respectively. The counteranion of the metal salt present in the aqueous medium was also found to affect tubule diameter. The molecular area of the lipids was 70 A 2 /molecule ( n = 2), 105 A 2 /molecule ( n = 3) and 160 A 2 /molecule ( n = 4). Monolayer studies suggested that the packing behavior of lipids at the air-water interface in the low surface pressure region was influenced by the subphase pH, ionic strength and counteranions.

Materials fabrication via polymerizable selforganized membranes: An overview

Abstract Molecular self-assembly of amphiphilic phospholipid molecules (containing a hydrophobic acyl chain and a hydrophilic phosphate group attached to glycerol backbone) and other amphiphiles offers a versatile approach to form ordered structures such as vesicles and lipid microcylinders. Stabilization of lipid microstructures by polymerization renders them useful for practical applications in the areas ranging from controlled release technology to template mediated synthesis of metals. Our efforts are focussed on the fabrication and use of derived microstructures from polymerizable diacetylenic phospholipids as templates for making advanced materials. The surfaces of the phospholipid membrane templates consist of the hydrophilic headgroup region of phospholipids that are made active by chemical modification. Phospholipids with chemically reactive sites were incorporated into lipid membranes formed from negatively charged and charge neutral phospholipids and used for binding metal ions and growing fine metal particles.

Phosphatidylhydroxyalkanols as Versatile Intermediates in the Synthesis of Headgroup Modified Diacetylenic Phospholipids

Abstract Phospholipase D extract (from savoy cabbage) mediates exchange of omega hydroxyalkanols (n= 2−4) with the choline moiety of phosphatidylcholines to produce corresponding phosphatidylhydroxyalkanols in > 70% yield. The phosphatidyl hydroxyalkanol and its chloro analogue were further reacted with molecules containing carboxylic and amine functionality respectively, providing an easy and efficient method to produce headgroup modified phospholipids.

Synthesizing submicrometer and nanoscale particles via self-assembled molecular membranes

The unique properties of submicrometer and nanoscale materials have stimulated a great deal of research into the synthesis of such structures. One avenue being explored is the use of organic precursors. This article describes some recent work in the use of self-assembled molecular membranes as templates for submicrometer and nanoscale structures.

Diffusion and transfer of antibody proteins from a sugar-based hydrogel

Diffusion of antibody protein from hydrogel films and hydrogel encapsulated in a microcapillary was studied. Thin hydrogel films were formed by crosslinking 6-acryloyl-B-O-methylgalactoside withN,N’-methylene-bis-acrylamide and the diffusive transport of monoclonal antimouse IgG-FITC into and out of the hydrated films was measured. Diffusion coefficients in 2 and 4% crosslinked hydrogel films were measured. The measured diffusion constants determined for IgG in both the 2 and 4% hydrogel films were comparable to the free diffusion of IgG in bulk water (Dmean ∼ 10-7cm2/s). In addition, 2% crosslinked hydrogels were prepared in a capillary tube and the transport of antimouse IgG-FITC into and out of the hydrated hydrogel was measured. Kinetic analysis indicated that the protein transport through the capillary hydrogel was faster than would be expected for a simple diffusion process. Finally, by utilizing the diffusion of antibody from the capillary hydrogel, transfer of antibody to a silica surface was demonstrated. A capillary hydrogel loaded with antimouse IgG-FITC was used to transfer the protein to a silica surface forming a 30-μm spot of antibody, which was imaged using fluorescence microscopy. These results may lead to the development of a nonlithographic method of patterning antibodies on surfaces for use in integrated microimmunosensors.

Modulation of bilayer structures derived from diacetylenic phosphocholines containing oxygen linker β to diacetylene

Phospholipids with diacetylenes present in the acyl chains form tubules and helices in aqueous dispersions. In order to modulate the morphology of bilayer structures and to understand the role of diacetylene in lipid-bilayer assembly, two diacetylenic phosphocholines, 1,2-bis(9,16-dioxa-hexacosa-11,13-diynoyl)-sn-3-phosph ocholine and 1,2-bis(15-oxa-pentacosa-10,12-diynoyl)-sn-3-phosphocholine, in which the diacetylene is linked to the acyl chain by an oxygen spacer have been synthesized. Lipid dispersions were characterized by calorimetric, film balance and microscopic techniques. Placement of oxygen spacer influences the morphology of the bilayer assemblies formed in aqueous solution. When both ends of the diacetylene were linked to the acyl chain by oxygen atoms, liposomes (diameters ranging from 0.3-3.4 microns) were observed by optical microscopy. Linking only the terminal portion of the acyl chain to the diacetylene with an oxygen atom resulted in a lipid which formed tubular microstructures as well as vesicles. Diameter of the tubular structures ranged from 0.4-4.7 microns. Transmission electron microscopic (TEM) analysis of replicas of a freeze fractured sample of the dispersion revealed that the tubular structures were hollow cylinders consisting of an aqueous core surrounded by a wall of lipid.

Effect of surfactant and oil type on the solution synthesis of nanosized silica

Abstract We have investigated the effects of using two different oils, cyclohexane and chloroform, and two different surfactants, nonylphenyl pentaethylene glycol (NP-5) and dimethyldodecylamine oxide (DDAO) in a modified Stober process for synthesizing silica colloids. Mean particle size was affected by both the oil and the surfactant used, and a significant interaction was also observed between these two factors: DDAO produced larger particles in cyclohexane, and NP-5 produced larger particles in chloroform. Water-to-surfactant and water-to-silicate ratios, as well as volume of oil added, were also observed to affect mean particle size. Only the volume of added oil was determined to affect the size dispersity of particles, with the use of a smaller volume of oil leading to a lower dispersity in particle size.