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Biomedical subjects

Richard M Crooks

Publications and source records attributed to Richard M Crooks.

At least 37 records · Page 2Linked to original sources
PubMed2004

Comparison of nanoparticle size and electrophoretic mobility measurements using a carbon-nanotube-based coulter counter, dynamic light scattering, transmission electron microscopy, and phase analysis light scattering.

The precision and accuracy of measurements of the diameter and electrophoretic mobility (mu) of polymeric nanoparticles is compared using four different analytical approaches: carbon-nanotube-based Coulter counting, dynamic light scattering (DLS), transmission electron microscopy (TEM), and phase analysis light scattering (PALS). Carbon-nanotube-based Coulter counters (CNCCs) use a 132 nm diameter channel to simultaneously determine the diameter (28-90 nm) and mu value for individual nanoparticles. These measurements are made without calibration of the CNCC and without labeling the sample. Moreover, because CNCCs measure the properties of individual particles, they provide true averages and polydispersities that are not convoluted into the intrinsic instrumental response function of the CNCC. CNCCs can be used to measure the size of individual nanoparticles dispersed in aqueous solutions, which contrasts with the TEM-measured size of individual dehydrated particles and the ensemble size averages of dispersed particles provided by DLS. CNCCs provide more precise values of mu than PALS.

Journal Article↗
PubMed2004

Pore-bridging poly(dimethylsiloxane) membranes as selective interfaces for vapor-phase chemical sensing.

A new kind of polymer-based sensor is described in which the experimental parameters controlling selectivity and sensitivity are decoupled. The sensor is based on a surface acoustic wave (SAW) gravimetric transducer modified with a high-surface-area, nanoporous alumina coating. A very thin ( approximately 40 nm) poly(dimethylsiloxane) (PDMS) coating resides atop the porous alumina structure. In this configuration, the total surface area of the nanoporous alumina coating controls the sensitivity of the device, while the chemical properties of the PDMS membrane control selectivity. In conventional polymer-based sensors, the polymer plays the dual role of controlling both selectivity (via the chemical composition of the coating) and sensitivity (via the volume of the film). In this paper, we show that PDMS acts as a chemically selective gate that absorbs polar and nonpolar VOCs but does not transport these analytes to the underlying pore volume. In contrast, water vapor is absorbed by the PDMS to a very minor extent but is easily transported through it to the underlying walls of the porous substructure. Specifically, there was little difference in the mass-loading response arising from polar and nonpolar VOCs dosed onto planar and nanoporous SAW devices modified with PDMS. In contrast, SAW devices having nanoporous coatings responded up to 24 times more selectively to water than planar sensors modified identically.

Journal Article↗
PubMed2004

Immobilization of DNA onto poly(dimethylsiloxane) surfaces and application to a microelectrochemical enzyme-amplified DNA hybridization assay.

This paper describes immobilization of DNA onto the interior walls of poly(dimethylsiloxane) (PDMS) microsystems and its application to an enzyme-amplified electrochemical DNA assay. DNA immobilization was carried out by silanization of the PDMS surface with 3-mercaptopropyltrimethoxysilane to yield a thiol-terminated surface. 5'-acrylamide-modified DNA reacts with the pendant thiol groups to yield DNA-modified PDMS. Surface-immobilized DNA oligos serve as capture probes for target DNA. Biotin-labeled target DNA hybridizes to the PDMS-immobilized capture DNA, and subsequent introduction of alkaline phosphatase (AP) conjugated to streptavidin results in attachment of the enzyme to hybridized DNA. Electrochemical detection of DNA hybridization benefits from enzyme amplification. Specifically, AP converts electroinactive p-aminophenyl phosphate to electroactive p-aminophenol, which is detected using an indium tin oxide interdigitated array (IDA) electrode. The IDA electrode eliminates the need for a reference electrode and provides a steady-state current that is related to the concentration of hybridized DNA. At present, the limit of detection of the DNA target is 1 nM in a volume of 20 nL, which corresponds to 20 attomoles of DNA.

Alkaline Phosphatase↗
PubMed2004

Synthesis, characterization, and surface immobilization of platinum and palladium nanoparticles encapsulated within amine-terminated poly(amidoamine) dendrimers.

Platinum and palladium dendrimer-encapsulated nanoparticles (DENs) were prepared within commercially available, fourth-generation, amine-terminated, poly(amidoamine) dendrimers (G4-NH2). The synthesis is carried out by selectively encapsulating metal complexes within the dendrimer and then reducing the resulting composite. Intradendrimer complexation requires control over the solution pH to prevent attachment of the metal complexes to primary amine groups on the dendrimer periphery. That is, the surface primary amines of the dendrimer must be selectively protonated in the presence of the interior tertiary amines. The metal-ion encapsulation and reduction processes were characterized by UV-vis spectroscopy. Forty-atom Pt and Pd DENs were examined by high-resolution transmission electron microscopy, which showed that the mean particle sizes were 1.4 and 1.5 nm, respectively, and that both were nearly monodisperse (standard deviation = 0.3 nm). The free amine groups on the dendrimer surface were used to link Pd DENs to monolithic Au surfaces via an intermediate self-assembled monolayer adhesion layer.

Amines↗
PubMed2004

A carbon nanotube-based coulter nanoparticle counter.

This Account reports on the properties and applications of carbon nanotube-based Coulter counters (CNCCs). CNCCs provide a means for determining the diameter and electrophoretic mobility (or electrokinetic surface charge) of individual nanoparticles dispersed in aqueous solutions, as well as the nanoparticle concentration. Such measurements do not require CNCC calibration or sample labeling. Because CNCCs measure the characteristics of individual particles, they provide the true average and polydispersity distribution of nanoparticle properties. CNCCs can differentiate between individual nanoparticles based on their surface charge and size, and CNCCs can be used to determine the apparent surface pK(a) of polymeric nanoparticles. Nanoparticle characterization by CNCC, electron microscopy, conductometric titration, and light scattering are compared.

Journal Article↗
PubMed2004

The resurgence of Coulter counting for analyzing nanoscale objects.

This review discusses recent advances in the science and technology of Coulter counting. The Coulter counting principle has been used to determine the size, concentration, and in favorable cases the surface charge, of nanometer-scale colloidal particles, viruses, DNA and other polymers, and metal ions. A resurgence of interest in the field of COulter counting is occurring because of the advent of new technologies that permit fabrication of membranes containing single, robust, and chemically well-defined channels having smaller and more uniform sizes than could be prepared in the past. These channels are prepared from biological materials, such as self-assembling membrane proteins, and from synthetic materials such as polymers, carbon nanotubes, and silicon-based inorganic materials. In addition to particle characterization, there have been a few recent examples of using Coulter counters to study chemical processes, such as the dehybridization of DNA.

Animals↗
PubMed2003

Electrokinetic trapping and concentration enrichment of DNA in a microfluidic channel.

We report a simple and efficient method for enriching the concentration of charged analytes within microfluidic channels. The method relies on exerting spatial control over the electrokinetic velocity of an analyte. Specifically, the electroosmotic (eo) velocity of the buffer solution in one region of the microfluidic system opposes the electrophoretic (ep) velocity of the analyte in the other region. This results in ep transport of DNA to the location where the ep and eo velocities are equal and opposite. Accumulation of the analyte occurs at this location. This enrichment method is conceptually distinct from field-amplification stacking, isotachophoresis, micelle sweeping, size exclusion, and other methods that have been previously reported. The method requires no complex microfabricated structures, no special manipulation of the solvent, and the enriched analyte remains in solution rather than being captured on a solid support. A concentration enrichment factor of 800 can be achieved for 20mer DNA in a fluidic channel having dimensions of 100 mum x 25 mum x 5 mm. The time required to achieve this level of enrichment is 300 s, and the enriched zone has a minimum width of 100 mum.

DNA↗
PubMed2003

Extraction of monodisperse palladium nanoparticles from dendrimer templates.

We describe a strategy for extracting nearly size-monodisperse, 1.7-nm-diameter Pd nanoparticles from within dendrimer hosts. The approach involves two steps. First, an aqueous solution of dendrimer-encapsulated Pd nanoparticles is prepared, followed by the addition of a toluene solution containing an n-alkanethiol. When the two-phase system is stirred, n-alkanethiols presumably self-assemble onto the surface of the Pd nanoparticles, extract them from the dendrimer, and transport them to toluene phase. UV-vis spectroscopy indicates that the Pd nanoparticles, in the form of monolayer-protected clusters, are transported into the toluene phase. FT-IR spectroscopy shows that the dendrimers remain in the aqueous phase. High-resolution transmission electron microscopy shows that the average size and size distribution of the particles do not change during the extraction process.

Journal Article↗
PubMed2003

Microelectrochemical logic circuits.

We report microelectrochemical devices that mimic some functions of solid-state circuit components such as diodes and transistors. The operation of this new family of devices is enabled by a network of microfluidic electrochemical cells that communicate with one-another via conductive solutions and, in some cases, bipolar electrodes. The output of the devices can be either electrochemical or optical. The approach reported here is a first step toward the development of large-scale integrated electrochemical systems having parallel processing capabilities.

Journal Article↗
PubMed2003

Gas adsorption gates based on ultrathin composite polymer films.

High surface area alumina coatings were prepared on surface acoustic wave (SAW) mass balances. These coatings were fabricated by anodic etching of evaporated aluminum films. The coatings consisted of roughly collinear pores penetrating through the monolithic alumina film. The nanoporous (NP) coatings were characterized by scanning electron microscopy, and the pore number density and diameter were found to be (3.8 +/- 0.5) x 10(3) pores/microm2 and 6.8 +/- 4.8 nm, respectively. The mass of volatile organic compounds that adsorbed onto naked and chemically modified NP alumina coatings was measured using SAW mass balances and compared to the mass absorbed onto SAW devices having planar aluminum coatings. Thirty-four times more heptane adsorbed to the naked NP coating than to the planar coating. The mass loading response was also measured after modification with organic thin films (3-12 nm thick) that spanned the pores of the NP coating. These organic thin films were composed of sixth-generation, amine-terminated poly(amido amine) dendrimers and poly(maleic anhydride)-c-poly(methyl vinyl ether) (Gantrez). The key result of this study is that these organic thin films modulate adsorption of VOCs onto the pore walls of the NP alumina. Specifically, a single 3-nm-thick monolayer of the dendrimer reduces permeability of the VOCs by approximately 17%, whereas a 12-nm-thick G6-NH2/Gantrez composite reduces permeability by 100%. Thus, the polymer composite acts as a nonselective gate that controls access of VOCs to the underlying surface area of the pores.

Journal Article↗
PubMed2003

Measurement of enzyme kinetics using a continuous-flow microfluidic system.

This paper describes a microanalytical method for determining enzyme kinetics using a continuous-flow microfluidic system. The analysis is carried out by immobilizing the enzyme on microbeads, packing the microbeads into a chip-based microreactor (volume approximately 1.0 nL), and flowing the substrate over the packed bed. Data were analyzed using the Lilly-Hornby equation and compared to values obtained from conventional measurements based on the Michaelis-Menten equation. The two different enzyme-catalyzed reactions studied were chosen so that the substrate would be nonfluorescent and the product fluorescent. The first reaction involved the horseradish peroxidase-catalyzed reaction between hydrogen peroxide and N-acetyl-3,7-dihydroxyphenoxazine (amplex red) to yield fluorescent resorufin, and the second the beta-galactosidase-catalyzed reaction of nonfluorescent resorufin-beta-D-galactopyranoside to yield D-galactose and fluorescent resorufin. In both cases, the microfluidics-based method yielded the same result obtained from the standard Michaelis-Menten treatment. The continuous-flow method required approximately 10 microL of substrate solution and 10(9) enzyme molecules. This approach provides a new means for rapid determination of enzyme kinetics in microfluidic systems, which may be useful for clinical diagnostics, and drug discovery and screening.

Catalysis↗
PubMed2003

Simultaneous determination of the size and surface charge of individual nanoparticles using a carbon nanotube-based Coulter counter.

A resistive-pulse Coulter counter based on a membrane containing a single multiwall carbon nanotube (MWNT) channel was used to simultaneously determine the size and surface charge of carboxy-terminated polystyrene nanoparticles. The membrane was prepared from an epoxy section containing a MWNT channel mounted on a poly(dimethylsiloxane) (PDMS) support structure. The PDMS support reduced the background noise level by a factor of > 20 compared to the Si/Si3N4 support structure used in our previous study. The lower noise level makes it possible to accurately measure the height and width of resistive-pulse signals resulting from transport of individual particles through the MWNT channel. Particle sizes, calculated from current pulse heights, were comparable to those determined by transmission electron microscope (TEM). The width of the current pulses is a measure of the nanoparticle transport time, and it permits calculation of the electrokinetic surface charge. Different types of polystyrene nanoparticles having nearly the same size, but different electrokinetic surface charge, could be resolved on the basis of the difference in their transport time.

Electric Impedance↗
PubMed2003

Bimetallic palladium-platinum dendrimer-encapsulated catalysts.

We report the synthesis, characterization, and catalytic activity of bimetallic palladium-platinum dendrimer-encapsulated catalysts (DECs). These materials are prepared by co-complexation of different ratios of palladium and platinum salts to the interior tertiary amines of fourth-generation, hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers. Chemical reduction of these composites yields stable, fairly monodisperse, water-soluble bimetallic DECs having sizes on the order of 1.9 +/- 0.4 nm. Evidence that these nanoparticles are bimetallic comes from single-particle X-ray energy dispersive spectroscopy (EDS) and catalysis experiments. The latter indicate that the hydrogenation rate of allyl alcohol is enhanced in the presence of the bimetallic nanoparticles compared to DECs containing only platinum or only palladium nanoparticles. EDS results indicate that the percentage composition of the bimetallics is reflected by the percentage of metal salts initially complexed with the dendrimer.

Journal Article↗
PubMed2003

A multichannel microfluidic sensor that detects anodic redox reactions indirectly using anodic electrogenerated chemiluminescence.

Here, we describe a new approach for detecting redoxactive targets by electrochemical oxidation and reporting their presence by electrogenerated chemiluminescence (ECL) based on electrochemical oxidation of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and tripropylamine (TPA). This new strategy, which complements our previous reports of using ECL to signal the presence of targets undergoing electrochemical reduction, takes advantage of many of the attractive attributes of microfluidic-based electrochemical cells. These attributes include close proximity of multiple flow channels and electrodes, ability to move reagents through channels under laminar flow conditions, and the capacity to precisely place device components relative to one another using photolithography. Specifically, the microfluidic electrochemical sensor described here consists of three channels. The analyte and ECL reporting cocktail flow through separate channels, but they share a common anode. The cathode resides in a channel containing a sacrificial reductant. In this configuration, the target analyte competes with Ru(bpy)3(2+) and TPA to provide electrons for the reductant. Accordingly, in this competitive assay approach, the presence of the analyte is signaled as a lowering of the ECL intensity. In this report, the device performance characteristics are reported, and the detection of both ferrocyanide and dopamine is demonstrated at micromolar concentrations.

Journal Article↗
PubMed2003

A two-channel microfluidic sensor that uses anodic electrogenerated chemiluminescence as a photonic reporter of cathodic redox reactions.

This paper describes a new approach for sensing electrochemically active substrates in microfluidic systems. This two-electrode sensor relies on electrochemical detection at one electrode and electrogenerated chemiluminescent (ECL) reporting at the other. Each microfabricated indium tin oxide electrode is located in a separate microfluidic channel, but the channels are connected downstream of the electrodes to maintain a complete electrical circuit. Because of laminar flow, there is no bulk mixing of the fluids in the detecting and reporting channels. This approach allows the ECL reaction to be physically and chemically decoupled from the sensing channel of the device, which greatly expands the number of analytes that can be detected. However, because the cathode and anode are connected, electron-transfer processes occurring at the sensing electrode are electrically coupled to the ECL reaction. Charge balance permits the ECL light output to be quantitatively correlated to electrochemical reductions at the cathode. The system is used to detect Fe(CN)6(3-), Ru(NH3)6(3+), and benzyl viologen and report their presence via Ru(bpy)3(2+) (bpy = bipyridine) luminescence. Each different redox target initiates ECL at a unique potential bias related to its standard redox potential. The influence of the concentrations of Ru(bpy)3(2+) and the target analytes is discussed.

Journal Article↗
PubMed2003

A microfluidic bioreactor based on hydrogel-entrapped E. coli: cell viability, lysis, and intracellular enzyme reactions.

Viable E. coli cells were entrapped in hydrogel micropatches photopolymerized within microfluidic systems. The microfluidic channels and the micropatches have sizes on the order of 100-500 microm. Small molecules, such as dyes and surfactants, present in the solution surrounding the hydrogel, are able to diffuse into the gel and encounter the cells, but the cells are sufficiently large to be retained. For example, sodium dodecyl sulfate is a lysis agent that is able to penetrate the hydrogel and disrupt the cellular membrane. Entrapment of viable cells within hydrogels, followed by lysis, could provide a convenient means for preparing biocatalysts without the need for enzyme extraction and purification. Hydrogel-immobilized cells are able to carry out chemical reactions within microfluidic channels. Specifically, a nonfluorescent dye, BCECF-AM, is able to penetrate both the hydrogel and the bacterial membrane and be converted into a fluorescent form (BCECF) by the interior cellular machinery. These results suggest that cells immobilized within microfluidic channels can act as sensors for small molecules and as bioreactors for carrying out reactions.

Bioreactors↗
PubMed2002

Efficient mixing and reactions within microfluidic channels using microbead-supported catalysts.

A strategy for efficiently mixing solutions and carrying out multistep catalytic reactions in microfluidic systems is described. The approach involves immobilizing catalysts on microbeads, placing the beads into well-defined microreactor zones, and then passing reactants through one or more of the reactor zones to yield products. The catalyst-modified beads effectively mix reactants and increase the effective surface area of the channel interior, both of which improve reaction velocities compared to open channels. This approach is demonstrated using two sequential reactions catalyzed by glucose oxidase and horseradish peroxidase. In addition to providing a general route to chemical synthesis within microfluidic systems, this design strategy may also be applicable to modeling reaction pathways within cells and to bio/chemical sensing applications.

Biosensing Techniques↗