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Lane A Baker

Publications and source records attributed to Lane A Baker.

9 recordsLinked to original sources
PubMed2006

Resistive-pulse DNA detection with a conical nanopore sensor.

In this paper, we describe resistive-pulse sensing of two large DNAs, a single-stranded phage DNA (7250 bases) and a double-stranded plasmid DNA (6600 base pairs), using a conically shaped nanopore in a track-etched polycarbonate membrane as the sensing element. The conically shaped nanopore had a small-diameter (tip) opening of 40 nm and a large-diameter (base) opening of 1.5 microm. The DNAs were detected using the resistive-pulse, sometimes called stochastic sensing, method. This entails applying a transmembrane potential difference and monitoring the resulting ion current flowing through the nanopore. The phage DNA was driven electrophoretically through the nanopore (from tip to base), and these translocation events were observed as transient blocks in the ion current. We found that the frequency of these current-block events scales linearly with the concentration of the DNA and with the magnitude of the applied transmembrane potential. Increasing the applied transmembrane potential also led to a decrease in the duration of the current-block events. We also analyzed current-block events for the double-stranded plasmid DNA. However, because this DNA is too large to enter the tip opening of the nanopore, it could not translocate the pore. As a result, much shorter duration current-block events were observed, which we postulate are associated with bumping of the double-stranded DNA against the tip opening.

DNA↗
PubMed2006

Alternating current impedance imaging of high-resistance membrane pores using a scanning electrochemical microscope. Application of membrane electrical shunts to increase measurement sensitivity and image contrast.

Whether an individual pore in a porous membrane can be imaged using scanning electrochemical microscopy (SECM), operated in ac impedance mode, is determined by the magnitude of the change in the total impedance of the imaging system as the SECM tip is scanned over the pore. In instances when the SECM tip resistance is small relative to the internal pore resistance, the total impedance changes by a negligible amount, rendering the pore invisible during impedance imaging. A simple solution to this problem is to introduce a low-impedance electrical shunt (i.e., a salt bridge) across the membrane. This principle is demonstrated by imaging polycarbonate membranes (6-12-microm thickness) containing between 1 and 2000 conical-shaped pores (60-nm- and 2.5-microm-diameter openings) using an approximately 1-microm-radius Pt tip. Theory and experiments show that image contrast (the change in ac current measured as the probe is scanned over the pore) is inversely proportional to the total resistance of the membrane and can be increased by a factor of approximately 50x by introducing a low-resistance electrical shunt across the membrane. Remarkably, SECM images of membranes containing a single high-resistance (approximately 1 G Omega) pore can only be imaged by short-circuiting the membrane. Image contrast also becomes independent of membrane resistance when an electrical shunt is used, allowing for more quantitative comparisons of the features in ac impedance images of different membranes.

Journal Article↗
PubMed2006

Biosensing with conically shaped nanopores and nanotubes.

In this review we consider recent results from our group that are directed towards developing "smart" synthetic nanopores that can mimic the functions of biological nanopores (transmembrane proteins). We first discuss the preparation and characterization of conical nanopores synthesized using the track-etch process. We then consider the design and function of conical nanopores that can rectify the ionic current that flows through these pores under an applied transmembrane potential. Finally, two types of sensors that we have developed with these conical nanopores are described. The first sensor makes use of molecular recognition elements that are bound to the nanopore mouth to selectively block the nanopore tip, thus detecting the presence of the analyte. The second sensor makes use of conical nanopores in a resistive-pulse type experiment, detecting the analyte via transient blockages in ionic current.

Biosensing Techniques↗
PubMed2005

Solvent-extraction and Langmuir-adsorption-based transport in chemically functionalized nanopore membranes.

We have investigated the transport properties of nanopore alumina membranes that were rendered hydrophobic by functionalization with octadecyltrimethoxysilane (ODS). The pores in these ODS-modified membranes are so hydrophobic that they are not wetted by water. Nevertheless, nonionic molecules can be transported from an aqueous feed solution on one side of the membrane, through the dry nanopores, and into an aqueous receiver solution on the other side. The transport mechanism involves Langmuir-type adsorption of the permeating molecule onto the ODS layers lining the pore walls, followed by solid-state diffusion along these ODS layers; we have measured the diffusion coefficients associated with this transport process. We have also investigated the transport properties of membranes prepared by filling the ODS-modified pores with the water-immiscible (hydrophobic) liquid mineral oil. In this case the transport mechanism involves solvent extraction of the permeating molecule into the mineral oil subphase confined with the pores, followed by solution-based diffusion through this liquid subphase. Because of this different transport mechanism, the supported-liquid membranes show substantially better transport selectivity than the ODS-modified membranes that contain no liquid subphase.

Adsorption↗
PubMed2005

Effect of crown ether on ion currents through synthetic membranes containing a single conically shaped nanopore.

Ion-current measurements were made on synthetic polymer membranes that contained a single conically shaped nanopore. This entailed placing an electrolyte solution on either side of the membrane, using an electrode placed in each solution to control the transmembrane potential, and measuring the resulting transmembrane ion current. The effect of the crown ether commonly called 18-crown-6 (18C6) on the measured ion current was investigated. Adding 18C6 to the electrolyte solution on one side of a conical nanopore membrane provides a way to rectify the ion current flowing through the nanopore. This chemical rectification is observed only when the cation of the electrolyte is complexed by 18C6 (e.g., K+), and when the mouth diameter of the conical nanopore is of molecular dimensions, in this case approximately 1.5 nm. This chemical rectification can either augment or diminish the inherent electrostatic rectification observed with these small mouth-diameter nanopores. We have interpreted these results using a model based on the formation of a junction potential at the membrane-solution interface. This junction potential arises because the transference number for the K+-18C6 complex in bulk solution is larger than its transference number in the mouth of the conical nanopore.

Crown Ethers↗
PubMed2005

Alternating current impedance imaging of membrane pores using scanning electrochemical microscopy.

Alternating current impedance imaging of a 6-microm thick membrane containing conical-shaped pores (60-nm and 2.5-microm diameter openings) using scanning electrochemical microscopy (SECM) is described. Impedance images of the pore openings were obtained by rastering a glass-sealed conically shaped Pt tip (approximately 1-microm radius) above the membrane surface, while measuring the total impedance between the tip and a large area Pt electrode located on the opposite side of the membrane. Individual pore openings in the high pore density membrane (approximately 8 x 10(4) pores/cm2) are observed in the SECM impedance image. The image contrast is due to the decrease in tip and membrane resistance, in the vicinity of the pore opening. An equivalent circuit for the SECM cell and membrane is proposed and evaluated against the measured SECM imaging impedance. Criteria for employing SECM in impedance mode to image membranes are discussed.

Electric Impedance↗
PubMed2005

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↗
PubMed2005

Detecting single porphyrin molecules in a conically shaped synthetic nanopore.

We report here the first example of abiotic resistive-pulse sensing of a molecular (as opposed to a particle or macromolecular) analyte. This was accomplished by using a conically shaped nanopore prepared by the track-etch method as the sensing element. It is possible to sense the molecular analyte because the small diameter opening of the conical nanopore (approximately 4.5 nm) is comparable to the diameter of the analyte molecule (approximately 2 nm).

Journal Article↗