Search

We describe experimentally and theoretically the fluoride-induced negative differential resistance (NDR) phenomena observed in conical nanopores operating in aqueous electrolyte solutions. The threshold voltage switching occurs around 1 V and leads to sharp current drops in the nA range with a peak-to-valley ratio close to 10. The experimental characterization of the NDR effect with single pore and multipore samples concern different pore radii, charge concentrations, scan rates, salt concentrations, solvents, and cations. The experimental fact that the effective radius of the pore tip zone is of the same order of magnitude as the Debye length for the low salt concentrations used here is suggestive of a mixed pore surface and bulk conduction regime. Thus, we propose a two-region conductance model where the mobile cations in the vicinity of the negative pore charges are responsible for the surface conductance, while the bulk solution conductance is assumed for the pore center region.

We describe the fabrication of a chemical-sensitive nanofluidic device based on asymmetric nanopores whose transport characteristics can be modulated upon exposure to hydrogen peroxide (H2O2). We show experimentally and theoretically that the current–voltage curves provide a suitable method to monitor the H2O2–mediated change in pore surface characteristics from the electronic readouts. We demonstrate also that the single pore characteristics can be scaled to the case of a multipore membrane whose electric outputs can be readily controlled. Because H2O2is an agent significant for medical diagnostics, the results should be useful for sensing nanofluidic devices.

Electrical transduction from fluctuating external signals is central to energy conversion based on nanoscale electrochemical devices and bioelectronics interfaces. We demonstrate theoretically and experimentally a significant energy transduction from white noise signals using the electrical rectification of asymmetric nanopores in polymeric membranes immersed in aqueous electrolyte solutions. Load capacitor voltages of the order of 1V are obtained within times of the order of 1min by means of nanofluidic diodes which convert zero time-average potentials of amplitudes of the order of 1V into average net currents. We consider single-nanopore and multipore membranes to show that the conversion processes can be significantly increased by scaling. The results concern a wide range of operating electrolyte concentrations. Because these concentrations dictate the pore resistance, the tuning of the load capacitance to optimize the system response at each concentration is also addressed. Finally, the experimental results are described theoretically by using simple equivalent circuits with a voltage-dependent resistance in series with a load capacitance.

We designed and characterized a cylindrical nanopore that exhibits high electrochemical current rectification ratios at low and intermediate electrolyte concentrations. For this purpose, the track-etched single cylindrical nanopore in polymer membrane was coated with a gold (Au) layer via electroless plating technique. Then, a non-homogeneous fixed charge distribution inside the Au-coated nanopore was obtained by incorporating thiol-terminated uncharged poly(N-isopropylacrylamide) chains in series to poly(4-vinylpyridine) chains, which were positively charged at acidic pH values. The functionalization reaction was checked by measuring the current–voltage curves prior to and after the chemisorption of polymer chains. The experimental nanopore characterization included the effects of temperature, adsorption of chloride ions, electrolyte concentration, and pH of the external solutions. The results obtained are further explained in terms of a theoretical continuous model. The combination of well-established chemical procedures (thiol and self-assembled monolayer formation chemistry, electroless plating, ion track etching) and physical models (two-region pore and Nernst–Planck equations) permits the obtainment of a new nanopore with high current rectification ratios. The single pore could be scaled up to multipore membranes of potential interest for pH sensing and chemical actuators.

Nanochannel-based biosensing devices have been proposed for selective detection of protein analyte molecules. However, the design and miniaturization of reusable channel-based biosensors is still a challenge in nanoscience and biotechnology. We present here a reusable nanofluidic biosensor based on reversible lectin-carbohydrate interactions. The nanochannels are fabricated in heavy ion tracked polymer membranes. The channel walls are functionalized with p-aminophenyl α-d-mannopyranoside (APMP) monolayers through carbodiimide coupling chemistry. The chemical (mannopyranoside) groups on the inner channel walls serve as binding sites and interact with specific protein molecules. The binding (bioconjugation)/unbinding of proteins inside the confined geometry gives measurable changes in the electrical conductance for the case of single channel and in the permeation rate for a multichannel membrane. The modified-channel selectively recognizes concanavalin A (ConA) protein, but not the control proteins (lysozyme and bovine serum albumin), because ConA specifically binds with the mannopyranoside moieties. The method permits ConA detection in the range 10 nM to 1000 nM. Moreover, the ConA binding/unbinding is reversible, allowing several measuring cycles by washing the bioconjugated-channels with mannose solution. The experimental results are explained qualitatively by introducing a phenomenological model that incorporates the basic experimental trends observed in the current–voltage curves.

Calcium binding to fixed charge groups confined over nanoscale regions is relevant to ion equilibrium and transport in the ionic channels of the cell membranes and artificial nanopores. We present an experimental and theoretical description of the dissociation equilibrium and transport in a single conical nanopore functionalized with pH-sensitive carboxylic acid groups and phosphonic acid chains. Different phenomena are simultaneously present in this basic problem of physical and biophysical chemistry: (i) the divalent nature of the phosphonic acid groups fixed to the pore walls and the influence of the pH and calcium on the reversible dissociation equilibrium of these groups; (ii) the asymmetry of the fixed charge density; and (iii) the effects of the applied potential difference and calcium concentration on the observed ionic currents. The significant difference between the carboxylate and phosphonate groups with respect to the calcium binding is clearly observed in the corresponding current–voltage (I–V) curves and can be rationalized by using a simple molecular model based on the grand partition function formalism of statistical thermodynamics. The I–Vcurves of the asymmetric nanopore can be described by the Poisson and Nernst–Planck equations. The results should be of interest for the basic understanding of divalent ion binding and transport in biological ion channels, desalination membranes, and controlled drug release devices.

We present an experimental and theoretical characterization of single cigar-shaped nanopores with pH-responsive carboxylic acid and lysine chains functionalized on the pore surface. The nanopore characterization includes (i) optical images of the nanostructure obtained by FESEM; (ii) different chemical procedures for the nanopore preparation (etching time and functionalizations; pH and electrolyte concentration of the external solution) allowing externally tunable nanopore responses monitored by the current–voltage (I–V) curves; and (iii) transport simulations obtained with a multilayer nanopore model. We show that a single, approximately symmetric nanopore can be operated as a reconfigurable diode showing different rectifying behaviors by applying chemical and electrical signals. The remarkable characteristics of the new nanopore are the sharp response observed in the I–Vcurves, the improved tunability (with respect to previous designs of symmetric nanopores) which is achieved because of the direct external access to the nanostructure mouths, and the broad range of rectifying properties. The results concern both fundamental concepts useful for the understanding of transport processes in biological systems (ion channels) and applications relevant for tunable nanopore technology (information processing and drug controlled release).

4‐oxo‐4‐(pyren‐4‐ylmethoxy) butanoic acid is used as a photolabile protecting group to show the optical gating of nanofluidic devices based on synthetic ion channels. The inner surface of the channels is decorated with monolayers of photolabile hydrophobic molecules that can be removed by irradiation, which leads to the generation of hydrophilic groups. This process can be exploited in the UV‐light‐triggered permselective transport of ionic species in aqueous solution through the channels. The optical gating of a single conical nanochannel and multichannel polymeric membranes is characterised experimentally and theoretically by means of current–voltage and selective permeation measurements, respectively. It is anticipated that the integration of nanostructures into multifunctional devices is feasible and can readily find applications in light‐induced controlled release, sensing, and information processing.

Nanoporous and nanofluidic structures can be coated with metal and insulating layers deposited on the pore surface: when an electrolyte solution is in contact with the internal insulating layer, well-defined ionic conductance levels could be tuned by applying a gate potential to the external metallic layer. We study theoretically the dependence of the effective gate potential at the insulating layer/solution interface with the applied gate potential at the metallic surface as well as the change of the nanopore conductance with the gate potential for different electrolyte solution concentrations and nanopore radii. We solve the Poisson−Boltzmann equation to obtain the electrical potential distribution in the two regions of the pore cross-section, the insulating layer, and the inner pore solution. The model provides estimations of the effective nanopore surface charge density that could be achieved by gating the nanopore (this charge determines the nanopore selectivity in practical cases). As an application, we have shown that NORand NANDlogic gate schemes based on input and output electrical signals could be implemented by exploiting the gating of the nanopore conductance.

The use of fixed charge nanopores in practical applications requires tuning externally the electrostatic interaction between the charged groups and the ionic permeants in order to allow integrating a variety of functions on the same nanostructure. We design, produce, and characterize, theoretically and experimentally, a single-track amphoteric nanopore functionalized with lysine and histidine chains whose positive and negative charges are very sensitive to the external pH. This nanofluidic diode with amphoteric chains attached to the pore surface allows for a broad set of rectification properties supported by a single nanodevice. A definite plus is to functionalize these groups on a conical nanopore with well-defined, controlled structural asymmetry which gives virtually every rectification characteristic that may be required in practical applications. Nanometer-scaled amphoteric pores are of general interest because of the potential applications in drug delivery systems, ion-exchange membranes for separation of biomacromolecules, antifouling materials with reduced molecular adsorption, and biochemical sensors.

We present the study of osmotic energy harvesting with soft-etched nanoporous polyimide membranes for a wide range of salt concentration differences, ion irradiation fluences, and multipore orientations (parallel or network). The effect of solution pH and divalent cations on the membrane performance is also investigated. The membrane behavior is evaluated in terms of the reversal potential (Vrev), the maximum power generation (Pmax), and the energy conversion efficiency (ηmax). For an exposed membrane area of ∼1 cm2, the maximum values obtained for these parameters are Vrev = 123 mV, Pmax = 0.45 μW, and ηmax = 40%, respectively. A significant decrease in the membrane performance is observed for membrane samples obtained with ion fluences higher than 4 × 109ions cm−2because of pore overlapping and the diffusional resistance of the external solution boundary layers at very low ionic concentrations. The soft-etched membranes are able to hold the reversal potential for several days.

A two-region model for describing the conductivity of porous fixed charge membranes is proposed. In the surface region, the conductivity is due to the mobile positive ions (counterions) around the negative fixed charges. In the pore center region, the conductive properties resemble those of the external electrolyte solution because the fixed charges are assumed to be effectively neutralized by the counterions in the surface region. Activation energies and surface diffusion coefficients are estimated by assuming that the counterion jump from a fixed charge group is the rate limiting process for surface transport. The barrier energy for this jump is calculated using a simple electrostatic model with two microscopic parameters, the sum of the counterion and fixed charge hydration radii and the local dielectric constant. The bulk conductivity is obtained from experimental data. The total membrane conductivity and the counterion transport number are then calculated as functions of the external solution concentration for several pore radii and membrane fixed charge concentrations. The results are compared with those given by the Donnan model for homogeneous membranes and by the numerical solution of a continuous model based on the Poisson–Boltzmann equation extended to finite size ions. The study of the membrane conductivity for a series of electrolytes allows to distinguish clearly between the mechanism characteristic of the bulk ionic conductivity and that of surface conductivity. The surface conductivity is found to be significant for narrow pores at low external solution concentrations.