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Biomedical subjects

Charles R Martin

Publications and source records attributed to Charles R Martin.

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

Electrophoretic capture and detection of nanoparticles at the opening of a membrane pore using scanning electrochemical microscopy.

Stochastic electrophoretic capture of individual nanometer-scale particles at the small opening of a conically shaped nanopore in a synthetic membrane is described. Particle capture is sensed using a scanning electrochemical microscope (SECM) to measure the decrease in the transport rate of a redox-active molecule through the pore. The SECM tip is positioned at the larger backside opening of pore and used to amperometrically monitor the transport rate prior, during, and after particle capture. Following capture, the particle is released by electrophoretically driving it out of the pore opening and back into the solution. The capture and release method is demonstrated by detection of charged polystyrene spheres (43-150-nm diameter) using a polycarbonate membrane with conically shaped pores, the small opening of the pore having a diameter of 60 nm. The inverse of the time to capture polystyrene spheres increases with particle concentration over the range 10(8)-10(10) particles/mL. Selective detection based on nanoparticle charge and size is also demonstrated. A quantitative theoretical description of the rate of particle capture is presented, and the physical mechanism of particle capture, based on the balance of electrostatic and entropic forces, is considered.

Journal Article↗
PubMed2004

Conical-nanotube ion-current rectifiers: the role of surface charge.

Ion channels are protein pores that span cell membranes and open and close in response to stimuli like changes in the transmembrane potential, binding of a ligand, or mechanical stress. When open, ions pass through the pore, and hence across the cell membrane, and when closed, ion-transport is precluded. Hence, these channels are nanodevices that have a current-rectification function. There is intense research effort aimed at understanding the molecular-level mechanism for this function. One approach for elucidating the mechanism is to construct a simple abiotic system that mimics this function and to use the mechanistic details of this mimic as a guide to understand the more complex biological channel. We describe here such an abiotic mimic: a synthetic membrane that contains a single conical gold nanotube. The advantage of this mimic is that the surface charge and chemistry of the nanotube wall can be varied, at will, by judicious choice of electrolyte or by thiol chemisorption. This has allowed us to make conical Au nanotubes that rectify the ion current and, just as importantly, to definitively elucidate the mechanism of this function.

Gold↗
PubMed2004

DNA-functionalized nanotube membranes with single-base mismatch selectivity.

We describe synthetic membranes in which the molecular recognition chemistry used to accomplish selective permeation is DNA hybridization. These membranes contain template-synthesized gold nanotubes with inside diameters of 12 nanometers, and a "transporter" DNA-hairpin molecule is attached to the inside walls of these nanotubes. These DNA-functionalized nanotube membranes selectively recognize and transport the DNA strand that is complementary to the transporter strand, relative to DNA strands that are not complementary to the transporter. Under optimal conditions, single-base mismatch transport selectivity can be obtained.

Base Pair Mismatch↗
PubMed2004

Redox modulation of electroosmotic flow in a carbon nanotube membrane.

Electroosmotic flow (EOF) is widely used to manipulate solutions in capillaries and microfluidic devices, and more recently in the nanotubes of a carbon nanotube membrane. In all of these applications it is important to control both the rate and direction of EOF through the system, independently of the electric field that drives EOF. For this reason, there has been considerable recent effort devoted to developing ways of modulating the rate and direction of EOF. We describe here a new method, and we use the carbon nanotube membrane (CNM) system to demonstrate this method. This new method entails coating the inside walls of the carbon nanotubes within the CNM with redox-active polymer films. The redox polymer, poly(vinylferrocene), can be reversibly electrochemically switched between an electrical neutral and a polycationic form. This provides a way for controlling both the magnitude and the sign of the surface charge on the nanotube walls, which in turn allows for control of both the rate and direction of EOF through the CNM.

Journal Article↗
PubMed2004

Layer-by-layer nanotube template synthesis.

Electroless deposition of gold on the pore walls of polycarbonate templates is currently the best known method for controlling inside diameters of template-synthesized nanotubes. It would be very useful to have alternative template-based synthetic chemistries that yield nanotubes composed of other materials, but which still allow for precise control over the nanotube wall thickness and i.d. A film-formation process that is based on layer-by-layer deposition of the film-forming material along the pore walls of the template membrane provides this desired alternative synthetic chemistry. We describe here the use of Mallouk's alpha,omega-diorganophosphonate/Zr layer-by-layer film-forming method for preparing nanotubes within the pores of alumina template membranes. We have found that this method allows accurate, quantitative, and predictable control over the wall thickness, and thus i.d., of the layered nanotubes obtained.

Journal Article↗
PubMed2004

Conical nanopore membranes. Preparation and transport properties.

We have been investigating applications of nanopore membranes in analytical chemistry-specifically in membrane-based bioseparations, in electroanalytical chemistry, and in the development of new approaches to biosensor design. Membranes that have conically shaped pores (as opposed to the more conventional cylindrical shape) may offer some advantages for these applications. We describe here a simple plasma-etch method that converts cylindrical nanopores in track-etched polymeric membranes into conically shaped pores. This method allows for control of the shape of the resulting conical nanopores. For example, the plasma-etched pores may be cylindrical through most of the membrane thickness blossoming into cones at one face of the membrane (trumpet-shaped), or they may be nearly perfect cones. The key advantage of the conical pore shape is a dramatic enhancement in the rate of transport through the membrane, relative to an analogous cylindrical pore membrane. We demonstrate this here by measuring the ionic resistances of the plasma-etched conical pore membranes.

Journal Article↗
PubMed2004

Template synthesis of gold nanotubes in an anodic alumina membrane.

Nanotube-containing membranes prepared by the template method show promise for use as highly selective filters for membrane-based chemical and biological separations. Most of the work to date has been done on gold nanotubes prepared by electroless deposition of Au within the pores of polymeric filtration membranes. These polymeric filters have very low porosities (< 1%), and, as a result, the flux through Au nanotube membranes based on these templates is very low. In contrast, the other popular template membranes-anodic aluminas-have high porosities-30% to 50%. In spite of this potential advantage of anodic alumina templates, there have been no reports of electrolessly plated Au nanotubes within the pores of these templates. This is because the electroless plating method used to deposit Au nanotubes in polymeric templates does not work in aluminas. We have developed a modified electroless plating strategy that can be used to deposit high-quality Au nanotubes within the pores of the alumina templates. We describe this new plating method here.

Aluminum↗
PubMed2004

A ligand-gated ion-channel mimetic nanopore membrane with an on-board transmembrane microbattery.

We have been investigating synthetic nanopore membranes that mimic the function of ligand-gated ion channels. We showed previously that the transmembrane ion current in a hydrophobic alumina nanopore membrane can be switched from an "off" state to an "on" state by exposure of the membrane to hydrophobic ionic surfactants. In these prior experiments, external electrodes and an external power supply were used to drive the ion current when the membrane was in its "on" state. In biological channels there are no electrodes, and the ion current is driven by an electrochemical potential difference across the cell membrane. In this article we mimic this function of the ligand-gated ion channel by applying a porous battery cathode film to one face of the hydrophobic alumina membrane and a porous battery anode film to the other face. Hence, in analogy to the naturally occurring channel case, we have a membrane with a built in electrochemical potential difference across the membrane. We show here that in the absence of the ligand (again, a hydrophobic ionic surfactant), the membrane is in its "off" state, and the electrochemical potential difference cannot be utilized to drive a transmembrane ion current. In contrast, when the ligand is detected, the membrane switches to its "on" state and the transmembrane battery discharges, producing a corresponding transmembrane ion current.

Biomimetic Materials↗
PubMed2003

Synthetic single-nanopore and nanotube membranes.

There is increasing interest in investigating transport and electrochemical phenomena in synthetic membrane samples that contain a single pore of nanoscopic diameter. Approaches used to date for preparing such single-nanopore membranes include microfabrication-based methods, the track-etch method, and a method based on the incorporation of a single fullerene nanotube within a synthetic membrane. We describe here an alternative approach that we believe is easier and more accessible than the previously described methods. This method is based on a very low pore density track-etch membrane obtained from commercial sources. Fluorescence microscopy is used to identify and isolate a single nanopore in this membrane. Membrane samples containing single nanopores with diameters as small as 30 nm have been prepared. Furthermore, we show here that an electroless plating method can be used to deposit a gold nanotube within the single nanopore, and this provides a route for further decreasing the inside diameter of the pore. A single-nanotube membrane with an electrochemically determined inside diameter of approximately 2 nm was prepared and evaluated.

Journal Article↗
PubMed2003

Electrophoretic protein transport in gold nanotube membranes.

Gold nanotube membranes are ideal model systems for exploring how pore size affects the rate and selectivity of protein transport in synthetic membranes. This is because these membranes have cylindrical, monodisperse pores (the nanotubes) with diameters that can be varied at will from tens of nanometers down to less than 1 nm. We report here on the effects of nanotube inside diameter, solution pH, and applied transmembrane potential on the rate and selectivity of protein transport in PEG-thiol-treated gold nanotube membranes. The transport properties of four proteins of differing sizes and pI values--lysozyme, bovine serum albumin, carbonic anhydrase, and bovine hemoglobulin--were investigated. In general, membranes containing larger diameter nanotubes showed higher fluxes and lower selectivities than membranes with smaller diameter nanotubes. Transmembrane electrophoresis can be used to augment the diffusive transport selectivity. For example, for proteins that are oppositely charged, a combination of a large transmembrane potential and a large nanotube diameter can be used to optimize both selectivity and flux. In addition to transmembrane potential and nanotube diameter, solution pH value plays an important role in determining the transport selectivity. This is because pH determines the net charge on the protein molecule and this, in turn, determines the importance of the electrophoretic transport term.

Animals↗
PubMed2003

The emerging field of nanotube biotechnology.

Nanoparticles are being developed for a host of biomedical and biotechnological applications, including drug delivery, enzyme immobilization and DNA transfection. Spherical nanoparticles are typically used for such applications, which reflects the fact that spheres are easier to make than other shapes. Micro- and nanotubes--structures that resemble tiny drinking straws--are alternatives that might offer advantages over spherical nanoparticles for some applications. This article discusses four approaches for making micro- and nanotubes, and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.

Biotechnology↗
PubMed2003

Smart nanotubes for biomedical and biotechnological applications.

Nanoparticles are being developed for a host of biomedical and biotechnological applications including drug delivery, enzyme immobilization and DNA transfection. Spherical nanoparticles are typically used for such applications, but this only reflects the fact that spheres are easier to make than other shapes. Micro- and nanotubes--structures that resemble tiny drinking straws--are alternatives and may offer advantages over spherical nanoparticles for some applications. This article discusses four different approaches to making micro- and nanotubes and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.

Biotechnology↗
PubMed2002

Smart nanotubes for bioseparations and biocatalysis.

Tube-shaped nanostructures (nanotubes) have a number of attributes that make them potentially useful for biomedical applications such as drug delivery/detoxification and enzyme immobilization. Template synthesis provides a route for preparing monodisperse nanotubes of nearly any size and composed of nearly any material. We show here that template-synthesized silica nanotubes can be biochemically functionalized such that they act as biocatalysts and highly selective nano-phase extraction agents for bioseparations. For example, nanotubes containing an enantioselective antibody selectively extract the enantiomer of a drug molecule that binds to the antibody, relative to the enantiomer that has no specific interaction with the antibody. Nanotubes containing the enzyme glucose oxidase function as nanophase bioreactors to catalyze the oxidation of glucose.

Antibodies↗
PubMed2002

Electromodulated molecular transport in gold-nanotube membranes.

We have developed a new class of synthetic membranes that contain monodisperse Au nanotubes with inside diameters of molecular dimensions (<1 nm). The Au nanotubes span the complete thickness of the membrane and can act as conduits for molecule and ion transport between solutions placed on either side of the membrane. We have recently become interested in the concept of electromodulating neutral molecule transport across these membranes. This communication describes a novel approach for accomplishing this objective. This approach makes use of an anionic surfactant which, when a positive potential is applied to the Au nanotube membrane, partitions into the nanotubes to charge the solution side of the electrical double layer at the tube walls. Because of the hydrophobic tail of the surfactant, this renders the nanotube interior hydrophobic, and the membrane now preferentially extracts and transports neutral hydrophobic molecules. Because the anionic surfactant can be expelled from the nanotubes by applying a negative potential, this provides a route for reversibly electromodulating neutral molecule transport in these membranes.

Journal Article↗
PubMed2002

Template-synthesized nanotubes for chemical separations and analysis.

We have developed a new class of synthetic membranes that consist of a porous polymeric support that contains an ensemble of gold nanotubes that span the thickness of the support membrane. The support is a commercially-available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubes are prepared by electroless deposition of Au onto the pore walls, that is, the pores acts as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These nanotube membranes can be used to cleanly separate small molecules on the basis of molecular size. Furthermore, use of these membranes as a novel electrochemical sensor is also discussed. This new sensing scheme involves applying a constant potential across the Au nanotube membrane and measuring the drop in the transmembrane current upon the addition of the analyte. This paper reviews our recent progress on size-based based transport selectivity and sensor applications in this new class of membranes.

Journal Article↗
PubMed2002

Antibody-based bio-nanotube membranes for enantiomeric drug separations.

Synthetic bio-nanotube membranes were developed and used to separate two enantiomers of a chiral drug. These membranes are based on alumina films that have cylindrical pores with monodisperse nanoscopic diameters (for example, 20 nanometers). Silica nanotubes were chemically synthesized within the pores of these films, and an antibody that selectively binds one of the enantiomers of the drug was attached to the inner walls of the silica nanotubes. These membranes selectively transport the enantiomer that specifically binds to the antibody, relative to the enantiomer that has lower affinity for the antibody. The solvent dimethyl sulfoxide was used to tune the antibody binding affinity. The enantiomeric selectivity coefficient increases as the inside diameter of the silica nanotubes decreases.

Aluminum Oxide↗
PubMed2002

Ion channel mimetic micropore and nanotube membrane sensors.

This paper describes synthetic micropore and nanotube membranes that mimic the function of a ligand-gated ion channel; that is, these membranes can be switched from an "off" state (no or low ion current through the membrane) to an "on" state (higher ion current) in response to the presence of a chemical stimulus. Ion channel mimics based on both microporous alumina and Au nanotube membranes were investigated. The off state was obtained by making the membranes hydrophobic by chemisorbing either a C18 silane (alumina membrane) or a C18 thiol (Au nanotube membrane). Water and electrolyte are forbidden from entering these very hydrophobic pores/nanotubes. The transition to the on state was induced by the partitioning of a hydrophobic ionic species (e.g., a drug or a surfactant) into the membrane. The membrane switches to the on state because at a sufficiently high concentration of this ionic analyte species, the pores/nanotubes flood with water and electrolyte. A pH-responsive membrane was also prepared by attaching a hydrophobic alkyl carboxylic acid silane to the alumina membrane.

Biosensing Techniques↗