Krishna M. R. Kallury

Direct synthesis of aryldipyrromethanes

Abstract The synthesis of aryldipyrromethanes from the direct condensation of pyrroles with aromatic aldehydes is described. Yields are improved with strong electronegative substituents on the aromatic aldehyde.

Communication. Network analysis: acoustic energy transmission detection of polynucleotide hybridization at the sensor–liquid interface

Palladium has been sputtered onto the surface of gold-plated thickness-shear mode acoustic wave sensors. Adsorption of polynucleotides onto the PdO surface is confirmed by X-ray photoelectron spectroscopy and by gas-phase sensor measurements. Hybridization of complementary strands of polynucleotide at the device–liquid interface results in series resonant frequency signals (network analysis) that are significantly higher than expected from mass-based responses. This observation is interpreted in terms of the perturbation of acoustic energy transmission by changes in interfacial properties on hybridization. Preliminary measurements of changes in motional resistance from network analysis are also presented.

Characterization of the polypyrrole film-piezoelectric sensor combination.

Polymerization of pyrrole onto the electrode surfaces of thickness-shear-mode acoustic wave sensors at various levels of oxidation has been performed with electrochemical methods. The resulting films of polypyrrole have been characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. Frequency decreases for the polypyrrole-coated sensors exposed to methanol, toluene and ammonia have been evaluated in terms of the various interactions occurring at the polymer surface.

Silanization of oxidized silicon and aluminum surfaces with functionalized silanes with characterization by wettability, ellipsometry, XPS and quartz crystal microbalance studies

Abstract The synthesis of N -octadecanoyl, N-(12-acetoxydodecanoyl), N -(8-carbomethoxyoctanoyl) and N -(12-nitrododecanoyl) derivatives of 3-triethoxysilylpropanamine ( N -acyl APTES derivatives) along with that of 11-triethoxysilylundecanoic acid t -butyldimethylsilyl ester has been carried out. The covalent binding of all these silanes to oxidized silicon and aluminum surfaces has been effected by both adsorption and Langmuir—Blodgett monolayer transfer techniques. Contact angle and ellipsometric measurements indicate a polarity gradient for the functionalized silane-treated surfaces and variations in orientation with respect to the surface normal compared to similar surfaces treated with octadecyltrichlorosilane and functionalized thiols. Surface coverages of 80–90% are estimated from the Si(2p) silane : Si(2p) substrate binding energy peak ratios determined by XPS. Using the bulk acoustic wave propagation measurements, the reactivity of the silanized surfaces towards 2-propanol and octadecanoic acid is followed. The observed differences in frequency shifts correlate well with the polarities and proposed orientation of the terminal functionalities.

Immobilization of phospholipids on silicon, platinum, indium/tin oxide and gold surfaces with characterization by x-ray photoelectron spectroscopy and time-of-flight secondary-ion mass spectrometry

Abstract Simple procedures are described for the covalent binding of phospholipids to silicon, platinum, indium/tin oxide and gold surfaces. The protocol for the first three surfaces consists of silanization with γ-amino propyltriethoxysilane, followed by reaction of the amino function with a derivative of a long-chain aliphatic dicarboxylic acid used as a cross-linker and treatment with a lyso-lipid. The seposition of a lipid on gold emplys the reactivity of a sulphur substituent at the terminus on one of the acyl chains of the lipid. Surface coverages ranging from 60 to 75% are obtained as deduced from elemtnal determinations and N(1 s )-to-P(2 p binding energy peak ratios obtained in x-ray photoelectron spectroscopic study of the immobilized species. Functional group determinations were done through a high-resolutions of the C(1 s peaks together with comparison of spectra from intermediate surface structures. Depth profiling of the lipid-bound silicon surface through argon ion etching indicated a model involving the orientation of the sn -2 chain perpendicular to the surface plane. The molecular structure of the immobilized lipid on silicon is established by positive and negative time-of-flight secondary-ion mass spectrometry.

Covalent binding of amino, carboxy, and nitro-substituted aminopropyltriethoxysilanes to oxidized silicon surfaces and their interaction with octadecanamine and octadecanoic acid studied by X-ray photoelectron spectroscopy and ellipsometry

Three silanized silicon substrates were prepared by treating cleaned oxidized silicon wafers with N-(3-triethoxysilylpropyl)octadecanamide (1), 9-[N-(triethoxysilylpropyl)amino]-9-oxononanoic acid methyl ester (2), and N-(3-triethoxysilylpropyl)-12-nitrododecanamide (4). The carbomethoxyl of 2 immobilized on silicon was hydrolysed to yield the corresponding ω-carboxylic surface (3) and the nitro group of the surface 4 reduced to afford the ω-amino surface (5). All five silanized surfaces were treated with octadecanoic acid and octadecanamine used as models for acidic and basic polymeric adhesives and the interactions were followed by ellipsometry and X-ray photoelectron spectroscopy. While the surfaces 1 and 2 reacted only by physisorption, the carboxylic surface 3 and the amino surface 5 showed a strong reaction with octadecanamine and octadecanoic acid, respectively. The nitro surface 4 exhibited a strong interaction with both probes, but by different pathways.

Covalent immobilization of amphiphilic monolayers containing urease onto optical fibers for fluorimetric detection of urea

Abstract Bifunctional 11 or 12 carbon chain length amphiphiles with a triethoxy chlorosilane group at one terminus and an amine functionality at the other terminus are covalently immobilized onto planar quartz wafers and optical fibers. A small amount of the fluorescent probe nitrobenzaoxadiazole dipalmitoylphosphatidylethanolamine (NBD-PE) is partitioned into the membranes from an aqueous suspension. For coated wafers or fibers placed into aqueous solutions, alterations of pH change the physical and electrostatic structure of the membranes, which in turn alters the emission intensity of the NBD-PE owing to changes in self-quenching. The fluorescence intensity decreases as the degree of ionization of headgroups within the membrane decreases, consistent with an increase in self-quenching. Samples which are immobilized onto optical fibers have better sensitivity to changes of pH than those immobilized onto planar wafers. The enzyme urease is covalently linked onto the functional groups at the surface of some membranes to investigate immobilized membranes that are chemically selective. A small amount of NBD-PE is partitioned from water into the membrane either before or after immobilization of urease. Addition of urea to this system produces ammonia and carbonic acid, and results in changes in fluorescence intensity from the immobilized layer owing to alteration of surface charge at the membrane. The detection system is sensitive to changes in the bulk concentration of urea as small as 20 μM, with a limit of detection of 40 μM of urea.

Transduction of the reaction between urea and covalently immobilized urease by fluorescent amphiphilic membranes

Abstract Amphiphiles with chain lengths of 12 and 16 carbons having a triethoxy chlorosilane group at one terminus were covalently immobilized by reaction with hydroxyl groups on the surfaces of planar quartz wafers and optical fibres. The enzyme urease was covalently immobilized onto either carboxylic acid or amine functionalities at the other terminus of the immobilized amphiphiles, resulting in a surface coverage of about 60% of a close-packed monolayer of protein. A small amount (2–3 mol.%) of the fluorescent probe nitrobenzoxadiazole dipalmitoylphosphatidylethanolamine (NBD-PE) was partitioned into the membranes from an aqueous suspension following immobilization of the urease. Addition of urea to coated wafers or optical fibers placed into aqueous solutions resulted in substantial changes of fluorescence intensity from both the carboxylic acid and amine functionalized membranes. A 20μM change in the concentration of urea could be detected, with a limit of detection of 40μM of urea. The sensitivity degraded ten-fold over a period of 7 days when the samples were stored in buffer at a temperature of 4°C. An investigation into the mechanism of the fluorescence response revealed that local alterations of pH at the surface of membranes due to enzymatic hydrolysis of urea resulted in changes in the extent of ionization of both the membrane and the urease. The resulting changes in the electrostatic interactions between the membrane and the enzyme produced alterations in the rotational mobility of the amphiphiles and fluorophores which affected the self-quenching of NBD-PE.