Zuzanna Siwy

Diode-like single-ion track membrane prepared by electro-stopping

The prepn. of an asym. membrane in poly(ethylene terephthalate) (PET) is described, using a combination of chem. and electro-stopping. For this purpose, a single-ion-irradiated PET film is inserted ...

Voltage-Gated Sodium Channel Expression and Potentiation of Human Breast Cancer Metastasis

Purpose: Ion channel activity is involved in several basic cellular behaviors that are integral to metastasis (e.g., proliferation, motility, secretion, and invasion), although their contribution to cancer progression has largely been ignored. The purpose of this study was to investigate voltage-gated Na+ channel (VGSC) expression and its possible role in human breast cancer. Experimental Design: Functional VGSC expression was investigated in human breast cancer cell lines by patch clamp recording. The contribution of VGSC activity to directional motility, endocytosis, and invasion was evaluated by in vitro assays. Subsequent identification of the VGSC α-subunit(s) expressed in vitro was achieved using reverse transcription-PCR, immunocytochemistry, and Western blot techniques and used to investigate VGSCα expression and its association with metastasis in vivo. Results: VGSC expression was significantly up-regulated in metastatic human breast cancer cells and tissues, and VGSC activity potentiated cellular directional motility, endocytosis, and invasion. Reverse transcription-PCR revealed that Nav1.5, in its newly identified “neonatal” splice form, was specifically associated with strong metastatic potential in vitro and breast cancer progression in vivo. An antibody specific for this form confirmed up-regulation of neonatal Nav1.5 protein in breast cancer cells and tissues. Furthermore, a strong correlation was found between neonatal Nav1.5 expression and clinically assessed lymph node metastasis. Conclusions: Up-regulation of neonatal Nav1.5 occurs as an integral part of the metastatic process in human breast cancer and could serve both as a novel marker of the metastatic phenotype and a therapeutic target.

Ionic Selectivity of Single Nanochannels

There has been an increasing interest in single nanochannel ionic devices, such as ionic filters that control the type of transported ions and ionic diodes that rectify the ionic flow. In this article, we theoretically investigate the importance of the dimensions, surface charge, electrolyte concentration, and applied bias on nanopore performance. We compare numerical solutions of the Poisson, Nernst-Planck (PNP), and Navier-Stokes (NS) equations with their one-dimensional, analytical approximations. We show that by decreasing the length of the nanopore, the ionic current and ionic selectivity become affected by processes outside the nanochannel. The contribution of electroosmosis is noticeable, especially for highly charged nanochannels, but is insignificant, justifying the use of the simple one-dimensional approximation in many cases. Estimates for the critical electric field at which the nanopore selectivity decreases and the ion current starts to saturate are provided.

Biosensing with Nanofluidic Diodes

Recently reported nanofluidic diodes with highly nonlinear current-voltage characteristics offer a unique possibility to construct different biosensors. These sensors are based on local changes of the surface charge on walls of single conical nanopores induced by binding of an analyte. The analyte binding can be detected as a change of the ion-current rectification of single nanopores defined as a ratio of currents for voltages of one polarity, and currents for voltages of the opposite polarity. In this article, we provided both modeling and experimental studies of various biosensing routes based on monitoring changes of the rectification degree in nanofluidic diodes used as a biosensing platform. A prototype of a sensor for the capsular poly gamma-D-glutamic acid (gammaDPGA) from Bacillus anthracis is presented. The nanopore used for the sensing was locally modified with the monoclonal antibody for gammaDPGA. The proof of principle of the rectification degree-based sensing was further shown by preparation of sensors for avidin and streptavidin. Our devices also allowed for determination of the isoelectric point of the minute amounts of proteins immobilized on the surface.

Tuning transport properties of nanofluidic devices with local charge inversion.

Nanotubes can selectively conduct ions across membranes to make ionic devices with transport characteristics similar to biological ion channels and semiconductor electron devices. Depending on the surface charge profile of the nanopore, ohmic resistors, rectifiers, and diodes can be made. Here we show that a uniformly charged conical nanopore can have all these transport properties by changing the ion species and their concentrations on each side of the membrane. Moreover, the cation versus anion selectivity of the pores can be changed. We find that polyvalent cations like Ca(2+) and the trivalent cobalt sepulchrate produce localized charge inversion to change the effective pore surface charge profile from negative to positive. These effects are reversible so that the transport and selectivity characteristics of ionic devices can be tuned, much as the gate voltage tunes the properties of a semiconductor.

Electric-field-induced wetting and dewetting in single hydrophobic nanopores

The behaviour of water in nanopores is very different from that of bulk water. Close to hydrophobic surfaces, the water density has been found to be lower than in the bulk, and if confined in a sufficiently narrow hydrophobic nanopore, water can spontaneously evaporate. Molecular dynamics simulations have suggested that a nanopore can be switched between dry and wet states by applying an electric potential across the nanopore membrane. Nanopores with hydrophobic walls could therefore create a gate system for water, and also for ionic and neutral species. Here, we show that single hydrophobic nanopores can undergo reversible wetting and dewetting due to condensation and evaporation of water inside the pores. The reversible process is observed as fluctuations between conducting and non-conducting ionic states and can be regulated by a transmembrane electric potential.

Preparation of synthetic nanopores with transport properties analogous to biological channels

Abstract Conically shaped pores have been prepared in polyethylene terephthalate (PET) and polyimide foils by applying the track-etching technique. For this purpose, a thin polymer foil was penetrated by a single heavy ion (e.g. Au, Bi, U) of total kinetic energy of several hundred MeV to some GeV, followed by preferential chemical etching of the ion track. Asymmetric etching conditions allowed the preparation of charged pores of conical shape, similar to biological voltage-sensitive channels. The nanopores in PET and polyimide behave as ion current rectifiers where the preferential direction of the cation flow is from the narrow entrance towards the wide aperture of the pore. The PET pore shows voltage-dependent ion current fluctuations with opening and closing kinetics similar to voltage-gated biological ion channels. In contrast to PET, the polyimide nanopore exhibits a stable ion current signal. We discuss the possibility of using the synthetic nanopores as model for voltage-gated biochannels.

Nanofluidic Ionic Diodes. Comparison of Analytical and Numerical Solutions

Recently reported experimental and theoretical studies of nanofluidic nonlinear devices, such as bipolar and unipolar ionic diodes, have yet to answer the question about the possibility of their further miniaturization. In this Article, we theoretically investigate the effects of size reduction, applied bias, and solution ionic strength in such devices. We compare the numerical solutions of the Poisson, Nernst-Planck (PNP), and Navier-Stokes (NS) equations with their one-dimensional, analytical approximations. We demonstrate that the contribution of electroosmosis is insignificant and find analytical approximations to PNP for bipolar and unipolar diodes that are in good agreement with numerical 3D solutions. We identify the minimal dimensions for such diodes that demonstrate ion current rectification behavior and demonstrate the importance of the edge effect in very short diodes.

Poisson-Nernst-Planck model of ion current rectification through a nanofluidic diode.

We have investigated ion current rectification properties of a recently prepared bipolar nanofluidic diode. This device is based on a single conically shaped nanopore in a polymer film whose pore walls contain a sharp boundary between positively and negatively charged regions. A semiquantitative model that employs Poisson and Nernst-Planck equations predicts current-voltage curves as well as ionic concentrations and electric potential distributions in this system. We show that under certain conditions the rectification degree, defined as a ratio of currents recorded at the same voltage but opposite polarities, can reach values of over 1000 at a voltage range <-2 V,+2 V>. The role of thickness and position of the transition zone on the ion current rectification is discussed as well. We also show that the rectification degree scales with the applied voltage.

Ion transport through asymmetric nanopores prepared by ion track etching

Transport properties of single asymmetric nanopores in polyethylene terephthalate (PET) and polyimide (Kapton) membranes are investigated. The pores are produced by the track-etching technique based on irradiation of the polymer with heavy ions and subsequent chemical etching. Electrolytic conductivity measurements show that asymmetric pores in both polymeric materials rectify the ionic current. The PET and Kapton pores differ however significantly in their transient transport properties. The ion current through the PET nanopore fluctuates with the amplitudes reaching even 100% of the mean current, whereas nanopores in Kapton exhibit a stable current signal. We show that the transient properties of the pores depend on the chemical structure of the polymer as well as on the irradiation and etching procedures used in this work.