Biomedical subjects

Michael J Tarlov

Publications and source records attributed to Michael J Tarlov.

13 recordsLinked to original sources

PubMed2007

Synthesis and structural characterization of glucopyranosylamide films on gold.

Self-assembled monolayers (SAMs) of glucose derivatives on gold have been prepared from alpha- and beta-glucopyranosylamide derivatives. The glucosyl conjugates were synthesized stereoselectively via the in situ generation of glucosyl isoxazolines followed by treatment with thiopyridyl esters. The resulting film structures were characterized by atomic force microscopy, reflection Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The experimental data indicated that alpha- or beta-linked glucopyranosylamide derivatives with free hydroxyl groups attach to gold via the thiol linker. Both derivatives form monolayer films with high packing densities--comparable to those typically observed for alkanethiol monolayers on gold. Acetate analogues of these conjugates do not form SAMs on gold; they form multilayered films under identical deposition conditions.

Adsorption↗

PubMed2006

Independent control of grafting density and conformation of single-stranded DNA brushes.

We describe self-assembly of ssDNA brushes that exploits the intrinsic affinity of adenine nucleotides (dA) for gold surfaces. The grafting density and conformation of these brushes is deterministically controlled by the length of the anchoring dA sequences, even in the presence of thymine nucleotides (dT). We produce and characterize brushes of model block-oligonucleotides, d(T(m)-A(n)), with systematically varied lengths m and n of the thymine and adenine blocks [denoted d(T(m)) and d(A(n)), respectively]. The hairpin conformation, dominant for self-complementary d(T(m)-A(n)) oligos in solution, is disrupted by the high preferential affinity of dA for gold surfaces. As a result, the d(T(m)-A(n)) oligos adsorb as a brush of d(T) strands immobilized via the d(A) blocks. Quantitative analysis by FTIR spectroscopy and x-ray photoelectron spectroscopy (XPS) reveals a unique feature of DNA immobilization via d(A) blocks: The surface density of dA nucleotides is close to saturation and is nearly independent of d(A) block length. Accordingly, the lateral spacing (grafting density) of the d(T) blocks is determined by the length of the d(A) blocks. The d(T) blocks extend away from the surface in a brush-like conformation at a lateral spacing 2-3 times larger (a grafting density 5-10 times lower) than in analogous films immobilized via standard thiol linkers. This combination of brush-like conformation and low saturation grafting density is expected to increase the efficiency of DNA hybridization at surfaces. Therefore, immobilization via d(A) blocks offers a method of producing DNA brushes with controlled properties for applications in biotechnology and nanotechnology.

Adsorption↗

PubMed2006

Alkanethiols on platinum: multicomponent self-assembled monolayers.

We have studied the formation of self-assembled monolayers (SAMs) of n-alkanethiols on platinum thin films using X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and contact angle (CA) measurements. Specifically, SAMs of 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were grown on polycrystalline Pt films, and the effects of Pt surface preparation, deposition conditions, and solvent treatments on the initial quality and stability of the monolayer in air were investigated. The SAMs prepared under ambient conditions on piranha-cleaned and UV/ozone-cleaned substrates were compared to monolayers formed on template-stripped Pt in an inert atmosphere. We found that alkanethiols deposited from 1 mM ethanolic solutions on piranha-cleaned Pt formed densely packed monolayers in which alkyl chains were oriented close to the surface normal. Stored in the laboratory ambient, these monolayers were unchanged over about 1 week but were largely oxidized in about 1 month. No evidence was found of molecules being weakly bound within the monolayer or having undergone C-S bond scission; however, three distinct sulfur states were observed for all samples in the XPS of the S 2p region. The lowest- and highest-binding-energy components are assigned to alkylthiolate and partially oxidized alkylthiolate species, respectively. The remaining S 2p component (approximately one-third of the sulfur layer), intermediate in binding energy between the other two components, is attributed to a chemisorbed species with a S binding configuration distinct from the majority alkylthiolate: for example, S bound to Pt bound to O, S with a different Pt coordination number, or S in an adsorbed disulfide.

Journal Article↗

PubMed2006

Nucleobase orientation and ordering in films of single-stranded DNA on gold.

We demonstrate how the orientation and ordering of DNA bases in ultrahigh vacuum (UHV) and ambient environments can be determined using complementary spectroscopic methods. Near-edge X-ray absorption fine structure (NEXAFS) with fluorescence detection, X-ray photoelectron (XPS), and Fourier transform infrared (FTIR) spectroscopies are used to quantify the coverage, chemical composition, orientation, and ordering of thymine bases in model self-assembled monolayers of thymine homo-oligonucleotides [oligo(dT)] on gold. We find that, in monolayers of thiol-modified oligo(dT), thymine bases tend to orient parallel to the Au substrate, and this preferential orientation is significantly more pronounced in monolayers of thiolated 5-mers compared to 25-mers. We interpret this preferential orientation as a signature of significant correlations (local ordering) between individual nuleobases, which offers a way to quantify and compare nucleobase interactions in films under both ambient and UHV conditions.

DNA, Single-Stranded↗

PubMed2006

Hydrogel-immobilized antibodies for microfluidic immunoassays: hydrogel immunoassay.

The integration of immunoassays in microfluidic devices is a rapidly developing research area combining the power of immunoassays with the inherent benefits of microfluidics. Here, a general overview of microfluidic-based immunoassays is presented along with a method for immobilizing antibodies in polyacrylamide gel plugs set in microfluidic channels. These antigen-specific hydrogels can be rapidly formed by photopolymerizing monomer solutions mixed with antibodies or other large proteins. The resulting antigen-specific hydrogels contain pore sizes appropriate for physical entrapment of large antibodies while remaining permeable to smaller proteins. The open structure of these hydrogels enables the capture and concentration of target antigens present at low concentrations. Such physical entrapment provides a conceptually simple method of immobilization compared with immobilization of proteins on surfaces and offers advantages such as resistance to chemical and thermal denaturation.

Acrylic Resins↗

PubMed2005

Nanosecond imaging of microboiling behavior on pulsed-heated au films modified with hydrophilic and hydrophobic self-assembled monolayers.

Fast transient microboiling has been characterized on modified gold microheaters using a novel laser strobe microscopy technique. Microheater surfaces of different hydrophobicity were prepared using self-assembled monolayers of hexadecane thiol (hydrophobic) and 16-mercaptohexadecanol (hydrophilic) as well as the naturally hydrophilic bare gold surface. The microheater was immersed in a pool of water, and a 5-micros voltage pulse to the heater was applied, causing superheating of the water and nucleation of a vapor bubble on the heater surface. Light from a pulsed Nd:Yag laser was configured to illuminate and image the sample through a microscope assembly. The timing of the short duration (7.5 ns) laser flash was varied with respect to the voltage pulse applied to the heater to create a series of images illuminated by the flash of the laser. These images were correlated with the transient resistance change of the heater both during and after the voltage pulse. It was found that hydrophobic surfaces produced a bubble that nucleated at an earlier time, grew more slowly to a smaller maximum size, and collapsed more rapidly than bubbles formed on hydrophilic surfaces.

Journal Article↗

PubMed2004

DNA hybridization assays using temperature gradient focusing and peptide nucleic acids.

Two types of DNA hybridization assays are demonstrated with temperature gradient focusing (TGF) and peptide nucleic acids (PNAs). In TGF, the application of a controlled temperature gradient along the length of a microchannel filled with an appropriate temperature-dependent buffer results in the formation of a gradient in both the electric field and electrophoretic velocity. Ionic species move in this gradient and concentrate at a unique point where the total velocity sums to zero. The first assay is a mixing assay in which PNA is allowed to flow through spatially focused DNA targets within a capillary. The second assay detects single base pair mutations (SBPM) by monitoring the fluorescence intensity of PNA/DNA duplexes as a function of temperature within the capillary. The SBPM analysis can be performed in less than 5 min with 100-fold more dilute analyte compared to conventional UV melting measurements.

Buffers↗

PubMed2004

DNA displacement assay integrated into microfluidic channels.

This paper describes the development of a unique fluorescence-based DNA diagnostic microfluidic assay that does not require labeling of the target sequence prior to analysis. The assay is based on the displacement of a short sacrificial fluorescent-tagged indicator oligomer by a longer untagged target sequence as it is electrophoresed through a DNA-containing hydrogel plug immobilized in a microfluidic channel. The distinct advantages of this assay are the short sensing times, as a result of directed electrophoretic transport of target DNA to the sensing element, combined with the ability to detect nonlabeled target DNA.

DNA↗

PubMed2004

Quantitative characterization of DNA films by X-ray photoelectron spectroscopy.

We describe the use of self-assembled films of thiolated (dT)25 single-stranded DNA (ssDNA) on gold as a model system for quantitative characterization of DNA films by X-ray photoelectron spectroscopy (XPS). We evaluate the applicability of a uniform and homogeneous overlayer-substrate model for data analysis, examine model parameters used to describe DNA films (e.g., density and electron attenuation length), and validate the results. The model is used to obtain quantitative composition and coverage information as a function of immobilization time. We find that when the electron attenuation effects are properly included in the XPS data analysis, excellent agreement is obtained with Fourier transform infrared (FTIR) measurements for relative values of the DNA coverage, and the calculated absolute coverage is consistent with a previous radiolabeling study. Based on the effectiveness of the analysis procedure for model (dT)25 ssDNA films, it should be generally valid for direct quantitative comparison of DNA films prepared under widely varying conditions.

DNA, Single-Stranded↗

PubMed2003

Thermolithographic patterning of sol-gel metal oxides on micro hot plate sensing arrays using organosilanes.

Sol-gel-derived SnO2 and Fe2O3 were selectively deposited on elements of micro hot plate (microHP) arrays. The silicon micromachined microHP arrays contain heating elements (100 microm x 100 microm) that are electronically addressable and thermally isolated from each other. Thin films of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (TFS) or hexyltrichlorosilane (HFS) assembled on surfaces of the arrays served as thermally sensitive resists whereby heating of specific microHPs resulted in removal of organosilane films only in heated areas. TFS-masked surfaces were characterized with condensation figures and secondary ion mass spectrometry (SIMS) imaging. TFS was removed from regions heated above 400 degrees C to expose hydrophilic surfaces, while TFS films in unheated areas were unaffected and remained hydrophobic. Sol-gel tin oxide spin-coated on the thermally patterned arrays adhered only to the hydrophilic regions and was repelled from the hydrophobic areas masked by the TFS films. By using HFS films, it was possible to selectively deposit two sol-gel materials, SnO2 and Fe2O3, on different microHPs in the same array as confirmed by SIMS imaging. Both materials showed varying degrees of electrical response to hydrogen and methanol in gas-sensing measurements.

Journal Article↗

PubMed2003

Base-dependent competitive adsorption of single-stranded DNA on gold.

We characterize the room-temperature adsorption of single-stranded DNA homo-oligonucleotides from solution onto polycrystalline Au films, including competitive adsorption between all possible pairs of unmodified oligomers. Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy analysis of the resulting films shows that oligonucleotides adsorb with a strongly base-dependent affinity, adenine (A) > cytosine (C) >/= guanine (G) > thymine (T). In competitive adsorption experiments on Au, oligo(dA) strongly dominates over the other oligonucleotides. The relative adsorption affinity of oligo(dA) is so great that it competes effectively against adsorption of thiolated oligomers and even causes hybridized oligo(dA).oligo(dT) duplexes to denature in the presence of Au.

Adsorption↗

PubMed2003

Quantitative analysis and characterization of DNA immobilized on gold.

We describe the complementary use of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy to quantitatively characterize the immobilization of thiolated (dT)(25) single-stranded DNA (ssDNA) on gold. When electron attenuation effects are accurately accounted for in the XPS analysis, the relative coverage values obtained by the two methods are in excellent agreement, and the absolute coverage can be calculated on the basis of the XPS data. The evolution of chemically specific spectral signatures during immobilization indicates that at lower coverages much of the DNA lies flat on the surface, with a substantial fraction of the thymine bases chemisorbed. At higher immobilization densities, the (dT)(25) film consists of randomly coiled ssDNA molecules each anchored via the thiol group and at possibly one or two other bases. We use two examples to demonstrate how the quantitative analysis can be applied to practical problems: the effects of different buffer salts on the immobilization efficiency; the immobilization kinetics. Buffers with divalent salts dramatically increase the efficiency of immobilization and result in very high surface densities (>5 x 10(13)/ cm(2)), densities that may only be possible if the divalent counterions induce strong attractive intermolecular interactions. In contrast with previous reports of alkanethiol adsorption kinetics on gold, ssDNA immobilization in 1 M phosphate buffer does not occur with Langmuir kinetics, a result attributable to rearrangement within the film that follows the initial adsorption.

DNA, Single-Stranded↗

PubMed2002

Immobilization of DNA hydrogel plugs in microfluidic channels.

Acrylamide-modified DNA probes are immobilized in polycarbonate microfluidic channels via photopolymerization in a polyacrylamide matrix. The resulting polymeric, hydrogel plugs are porous under electrophoretic conditions and hybridize with fluorescently tagged complementary DNA. The double-stranded DNA can be chemically denatured, and the chip may be reused with a new analytical sample. Conditions for photopolymerization, hybridization, and denaturation are discussed. We also demonstrate the photopolymerization of plugs containing different DNA probe sequences in one microfluidic channel, thereby enabling the selective detection of multiple DNA targets in one electrophoretic pathway.

Biopolymers↗