Electroactive nanotube membranes and redox-gating.
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Crystallization↗
Biomedical subjects
Punit Kohli
Publications and source records attributed to Punit Kohli.
Modulating fluorescence resonance energy transfer in conjugated liposomes.
We report here a novel system where the rate of energy transfer is based on changes in the spectral overlap between the emission of the donor and the absorption of the acceptor (J) as well as changes in the quantum yield of the acceptor. We use the fluorophore dansyl as the donor and polydiacetylene (PDA) as the acceptor to demonstrate the modulation of FRET through conformationally induced changes in the PDA absorption spectrum following thermal treatment that converts the PDA backbone of the liposome from the blue form to the red form. Energy transfer was found to be significantly more efficient from dansyl to the red-form PDA. These findings support the basis of a new sensing platform that utilizes J-modulated FRET as an actuating mechanism.
Journal Article↗
Protein biosensors based on biofunctionalized conical gold nanotubes.
There is increasing interest in the concept of using nanopores as the sensing elements in biosensors. The nanopore most often used is the alpha-hemolysin protein channel, and the sensor consists of a single channel embedded within a lipid bilayer membrane. An ionic current is passed through the channel, and analyte species are detected as transient blocks in this current associated with translocation of the analyte through the channel-stochastic sensing. While this is an extremely promising sensing paradigm, it would be advantageous to eliminate the very fragile lipid bilayer membrane and perhaps to replace the biological nanopore with an abiotic equivalent. We describe here a new family of protein biosensors that are based on conically shaped gold nanotubes embedded within a mechanical and chemically robust polymeric membrane. While these sensors also function by passing an ion current through the nanotube, the sensing paradigm is different from the previous devices in that a transient change in the current is not observed. Instead, the protein analyte binds to a biochemical molecular-recognition agent at the mouth of the conical nanotube, resulting in complete blockage of the ion current. Three different molecular-recognition agents, and correspondingly three different protein analytes, were investigated: (i) biotin/streptavidin, (ii) protein-G/immunoglobulin, and (iii) an antibody to the protein ricin with ricin as the analyte.
Animals↗
Smart nanotubes for biotechnology.
Nanotechnology concerns the science of very small particles and deals with both the fundamental aspects of understanding the properties of such nanoparticles and with developing technological applications of nanoparticles. Biomedical and biotechnological applications of nanoparticles have been of special recent research and development interest, with potential applications that include use of nanoparticles as drug (or DNA) delivery vehicles, and as components in medical diagnostic kits, biosensors and membranes for bioseparations. Spherical nanoparticles are typically used for such applications, but this only reflects the fact that spheres are easier to make than nanoparticles having 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 different approaches for making micro and nanotubes and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.
Biotechnology↗
DNA-nanotube artificial ion channels.
There is considerable interest in developing chemical devices that mimic the function of biological ion channels. We recently described such a device, which consisted of a single conically shaped gold nanotube embedded within a polymeric membrane. This device mimicked one of the key functions of voltage-gated ion channels: the ability to strongly rectify the ionic current flowing through it. The data obtained were interpreted using a simple electrostatic model. While the details are still being debated, it is clear that ion-current-rectification in biological ion channels is more complicated and involves physical movement of an ionically charged portion of the channel in response to a change in the transmembrane potential. We report here artificial ion channels that rectify the ion current flowing through them via this "electromechanical" mechanism. These artificial channels are also based on conical gold nanotubes, but with the critical electromechanical response provided by single-stranded DNA molecules attached to the nanotube walls.
Biomimetic Materials↗
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↗
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↗
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↗
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↗
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↗
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↗