Preliminary investigation of a bioelectrochemical sensor for the detection of phenol vapours

This work investigates the feasibility of constructing a bioelectrochemical sensor that can operate directly in gases. A series of experiments are described, resulting in a sensor that is responsive to phenol vapours. The sensor was based on ionically conducting films that incorporate a biological redox catalyst at the surface of an array of interdigitated microband electrodes. Exposure to phenol vapour drives the bioelectrochemical reaction, providing a basis for a current signal under constant potential conditions. Ionic materials included Nafion and films based on tetrabutylammonium toluene-4-sulphonate (TBATS). The quasi-reversible electrode reaction of potassium hexacyanoferrate (II) within TBATS was investigated as a function of drying time. E^o′ and K⁰ were determined at a TBATS modified microdisc electrode under steady-state conditions. Drying time (water loss) from the TBATS film had the effect of increasing the film ionic strength. It was shown that as the film ionic strength increased, E⁰′ for potassium hexacyanoferrate (II) shifts toward positive potentials (because of ion pairing) and there was a corresponding increase in the heterogeneous rate constant K⁰. The latter effect was attributed to increasing ion-ion (cation-ferrocyanide ion) interactions as the film dried and the enhancing effect this had on the prevention of surface poisoning reactions at the electrode. These factors are discussed in relation to sensor design.     

Label-free electrochemical DNA sensor based on functionalised conducting copolymer

A simple and label-free electrochemical sensor for recognition of the DNA hybridization event was prepared based on a new functionalised conducting copolymer, poly[pyrrole-co-4-(3-pyrrolyl) butanoic acid]. This precursor copolymer can be easily electrodeposited on the electrode surface and shows high electroactivity in an aqueous medium. An amino-substituted oligonucleotide (ODN) probe was covalently grafted onto the surface of the copolymer in a one step procedure and tested on hybridization with complementary ODN segments. The cyclic voltammogram of ODN probe-modified copolymer showed very little change when incubated in presence of non-complementary ODN, while a significant, and reproducible, modification of the voltammogram was observed after addition of complementary ODN. The AC impedance spectrum showed an increased charge transfer resistance (Rct) and double layer capacitance of the sensor film after hybridisation. Sensors with thinner films showed higher sensitivity than thicker films, suggesting that hybridisation at or near the surface of the film produces a larger change in electrical properties than that within the body of the film.     

Langmuir and Langmuir-Blodgett films of organophosphorus acid anhydrolase

In this paper, we describe the preparation and characterization of Langmuir and Langmuir-Blodgett (LB) monolayers of the enzyme organophosphorus acid anhydrolase (OPAA). Langmuir films of OPAA were characterized on different subphases, such as phosphate, ammonium carbonate, and bis-tris-propane buffers. Monolayers at the air-water interface were characterized by measuring the surface pressure and surface potential-area isotherms. In situ UV-vis absorption spectra were also recorded from the Langmuir monolayers. The enzyme activity at the air-water interface was tested by the addition of diisopropylfluorophosphate (DFP) to the subphase. LB films of OPAA were transferred to mica substrates to be studied by atomic force microscopy. Finally, a one-layer LB film of OPAA labeled with a fluorescent probe, fluorescein isothiocyanate (FITC), was deposited onto a quartz slide to be tested as sensor for DFP. The clear, pronounced response and the stability of the LB film as a DFP sensor show the potential of this system as a biosensor.     

Triboelectrification‐Enabled Self‐Charging Lithium‐Ion Batteries

Li‐ion batteries as energy storage devices need to be periodically charged for sustainably powering electronic devices owing to their limited capacities. Here, the feasibility of utilizing Li‐ion batteries as both the energy storage and scavenging units is demonstrated. Flexible Li‐ion batteries fabricated from electrospun LiMn₂O₄ nanowires as cathode and carbon nanowires as anode enable a capacity retention of 90% coulombic efficiency after 50 cycles. Through the coupling between triboelectrification and electrostatic induction, the adjacent electrodes of two Li‐ion batteries can deliver an output peak voltage of about 200 V and an output peak current of about 25 µA under ambient wind‐induced vibrations of a hexafluoropropene–tetrafluoroethylene copolymer film between the two Li‐ion batteries. The self‐charging Li‐ion batteries have been demonstrated to charge themselves up to 3.5 V in about 3 min under wind‐induced mechanical excitations. The advantages of the self‐charging Li‐ion batteries can provide important applications for sustainably powering electronics and self‐powered sensor systems.     

Surface structure of chitosan and hybrid chitosan-amylose films-restoration of the antibacterial properties of chitosan in the amylose film

The surface structure of films prepared by casting aqueous solutions of mixtures of water soluble chitosan (WSC) and amylose as well as a fully deacetylated chitosan was studied. Zeta potential measurements indicated that the surface of WSC and fully deacetylated chitosan films is positively charged but very weakly, whereas, a film of amylose blended with WSC exhibited an obvious positive charge. X-ray photoelectron spectra of these films suggest that less amino groups are exposed on the surface of WSC and fully deacetylated chitosan films, whereas, more amino groups are exposed on the surface of a WSC film blended with amylose. A sheet structure in which free amino groups are less exposed on the surface of the film of WSC or fully deacetylated chitosan is proposed. This accounts for the loss of antibacterial activity of chitosan on the WSC film surface. When blended with amylose, the morphology of the film may be disrupted, resulting in strong antibacterial properties.     

Non-isothermal potential of phospholipid bilayer films Influence of cholesterol and macrocyclic carrier effects

The effect of cholesterol on the ion selective behavior of phospholipid (phosphatidylcholine or phosphatidylethanolamine) bilayer films is studied through the measurement of the membrane non-isothermal potential.It is shown how the mixed phosphatidylcholine-cholesterol membrane can be either cation of anion permselective according to the film composition (cationic behavior is met in the 0–10% cholesterol composition range while anionic selectivity appears in the 20–50% range).On the contrary, mixed phosphatidylethanolamine-cholesterol membranes show the absence of ionic selectivity already met with pure phosphatidylethanolamine films.The presence of a cationic carrier as Dibenzo-18-crown-6 in the film transforms all the studied films (cationic, anionic and no selective bilayers) into ideally cationic selective membranes.These results are discussed on the basis of the current ideas on the charge distribution through the bilayer membranes. Moreover, the role of the permeating ions as potential determining species is stressed.     

Probing electrostatic interactions in cytochrome c using site-directed chemical modification.

This communication reports the generation of an electrostatic probe using chemical modification of methionine side chains. The alkylation of methionine by iodoacetamide was achieved in a set of Saccharomyces cerevisiae iso-1-cytochrome c mutants, introducing the nontitratable, nondelocalized positive charge of a carboxyamidomethylmethionine sulfonium (CAMMS) ion at five surface and one buried site in the protein. Changes in redox potential and its variation with temperature were used to calculate microscopic effective dielectric constants operating between the probe and the heme iron. Dielectric constants (epsilon) derived from deltadeltaG values were not useful due to entropic effects, but epsilon(deltadeltaH) gave results that supported the theory. The effect on biological activity of surface derivatization was interpreted in terms of protein-protein interactions. The introduction of an electrostatic probe in cytochrome c often resulted in marked effects on activity with one of two physiological partners: cytochrome c reductase, especially if introduced at position 65, and cytochrome c oxidase, if at position 28.     

A physical model of potassium channel activation: from energy landscape to gating kinetics

We have developed a method for rapidly computing gating currents from a multiparticle ion channel model. Our approach is appropriate for energy landscapes that can be characterized by a network of well-defined activation pathways with barriers. To illustrate, we represented the gating apparatus of a channel subunit by an interacting pair of charged gating particles. Each particle underwent spatial diffusion along a bistable potential of mean force, with electrostatic forces coupling the two trajectories. After a step in membrane potential, relaxation of the smaller barrier charge led to a time-dependent reduction in the activation barrier of the principal gate charge. The resulting gating current exhibited a rising phase similar to that measured in voltage-dependent ion channels. Reduction of the two-dimensional diffusion landscape to a circular Markov model with four states accurately preserved the time course of gating currents on the slow timescale. A composite system containing four subunits leading to a concerted opening transition was used to fit a series of gating currents from the Shaker potassium channel. We end with a critique of the model with regard to current views on potassium channel structure.     

Surface Plasmon Resonance Based Fiber Optic Ammonia Sensor Utilizing Bromocresol Purple

In this paper, a surface plasmon resonance (SPR) based fiber optic ammonia gas sensor has been designed and fabricated using bromocresol purple (BCP) as sensing element. The sensor works under wavelength modulation scheme. The detection of ammonia gas has been carried out at room temperature. Three different kinds of film coating configurations, namely silver + BCP, gold + BCP, and silver + silicon + BCP on the unclad portion of the fiber have been used for studying the role of each layer. Further, to optimize the performance of the sensor, the films of varying thicknesses were coated using thermal evaporation technique. Experiments have been performed for the ammonia concentrations ranging from 0 to 150 ppm around the probe. To record the SPR spectrum, light from a polychromatic source is launched in the fiber and the spectrum is recorded at the other end of the fiber. The spectrum has a peak at lower wavelength while a dip at the higher wavelength. The dip corresponds to SPR while the peak appears to be due to fluorescence properties of the dye. It has been observed that as the ammonia gas comes in contact of the BCP layer, it changes the refractive index of the BCP dye which, in turn, changes the resonance wavelength. Further, the change in refractive index increases as the concentration of ammonia gas increases up to certain concentration of ammonia after that it saturates. Silicon layer has been shown as a protection layer for silver and gold from oxidation and acts as a tuner of wavelength. The proposed ammonia sensor has small response as well as recovery time.     

Effect of pectin charge density on formation of multilayer films with chitosan

The effect of pectin charge density on the formation of multilayer films with chitosan (PEC/CHI) is studied by means of electro-optics. Pectins of low (21%) and high (71%) degrees of esterification, which are inversely proportional to the pectin charge density, are used to form films on colloidal beta-FeOOH particles at pH 4.0 when the CHI is fully ionized. We find that, after deposition of the first 3-4 layers, the film thickness increases linearly with the number of adsorbed layers. However, the increase in the film thickness is larger when the film is terminated with CHI. Irregular increase of the film thickness is more marked for the PEC with higher density of charge. Oscillation in the electrical polarizability of the film-coated particles with the number of deposited layers is also registered in the PEC/CHI films. The charge balance of the multilayers, calculated from electrical polarizability of the film-coated particles, is positive, with larger excess of positive charge within the film constructed from CHI and less charged PEC. This is attributed to the ability of CHI to diffuse into the film at each deposition step. Despite the CHI diffusion, the film thickness increases linearly due to the dissolution of unstable PEC/CHI complexes from the film surface.     

Modeling of spherical fluorescent glucose microsensor systems: design of enzymatic smart tattoos

A two-substrate mathematical model of microspherical optical enzymatic glucose sensors is presented. The sensors are based on the well-known oxidation of glucose by glucose oxidase, and are constructed by the encapsulation of glucose oxidase within hydrogel microspheres coated with ultrathin polyelectrolyte multilayer films. In order to measure glucose via changes in oxygen concentration, a fluorescent oxygen indicator is co-encapsulated with the enzyme. The model was used to predict the temporal and spatial distributions of glucose and oxygen within the sphere for step increases in bulk glucose concentration. In addition, the model was used to observe the effect of varying sensor parameters, namely sphere size, film thickness, enzyme concentration, and mass transport of substrate and co-substrate within the sphere and film coatings, on the response of the sensors. A major finding was that the application of {PSS/PAH} films as thin as 12 nm can drastically improve the sensor performance over uncoated sensors based on calcium alginate microspheres. The model is proposed as an important tool for a priori design of these complex sensor structures.     

Molecular mobility in the monolayers of foam films stabilized by porcine lung surfactant.

Certain physical properties of a range of foam film types that are believed to exist in vivo in the lung have been investigated. The contribution of different lung surfactant components found in porcine lung surfactant to molecular surface diffusion in the plane of foam films has been investigated for the first time. The influence of the type and thickness of black foam films, temperature, electrolyte concentration, and extract composition on surface diffusion has been studied using the fluorescence recovery after photobleaching technique. Fluorescent phospholipid probe molecules in foam films stabilized by porcine lung surfactant samples or their hydrophobic extracts consisting of surfactant lipids and hydrophobic lung surfactant proteins, SP-B and SP-C, exhibited more rapid diffusion than observed in films of its principal lipid component alone, L-alpha-phosphatidylcholine dipalmitoyl. This effect appears to be due to contributions from minor lipid components present in the total surfactant lipid extracts. The minor lipid components influence the surface diffusion in foam films both by their negative charge and by lowering the phase transition temperature of lung surfactant samples. In contrast, the presence of high concentrations of the hydrophillic surfactant protein A (SP-A) and non-lung-surfactant proteins in the sample reduced the diffusion coefficient (D) of the lipid analog in the adsorbed layer of the films. Hysteresis behavior of D was observed during temperature cycling, with the cooling curve lying above the heating curve. However, in cases where some surface molecular aggregation and surface heterogeneity were observed during cooling, the films became more rigid and molecules at the interfaces became immobilized. The thickness, size, capillary pressure, configuration, and composition of foam films of lung surfactant prepared in vitro support their investigation as realistic structural analogs of the surface films that exist in vivo in the lung. Compared to other models currently in use, foam films provide new opportunities for studying the properties and function of physiologically important alveolar surface films.     

Steady-state availability of sodium channels. Interactions between activation and slow inactivation.

Changes in holding potential (Vh), affect both gating charge (the Q(Vh) curve) and peak ionic current (the F(Vh) curve) seen at positive test potentials. Careful comparison of the Q(Vh) and F(Vh) distributions indicates that these curves are similar, having two slopes (approximately 2.5e for Vh from -115 to -90 mV and approximately 4e for Vh from -90 to -65 mV) and very negative midpoints (approximately -86 mV). Thus, gating charge movement and channel availability appear closely coupled under fully-equilibrated conditions. The time course by which channels approach equilibration was explored using depolarizing prepulses of increasing duration. The high slope component seen in the F(Vh) and Q(Vh) curves is not evident following short depolarizing prepulses in which the prepulse duration approximately corresponds to the settling time for fast inactivation. Increasing the prepulse duration to 10 ms or longer reveals the high slope, and left-shifts the midpoint to more negative voltages, towards the F(Vh) and Q(Vh) distributions. These results indicate that a separate slow-moving voltage sensor affects the channels at prepulse durations greater than 10 ms. Charge movement and channel availability remain closely coupled as equilibrium is approached using depolarizing pulses of increasing durations. Both measures are 50% complete by 50 ms at a prepulse potential of -70 mV, with proportionately faster onset rates when the prepulse potential is more depolarized. By contrast, charge movement and channel availability dissociate during recovery from prolonged depolarizations. Recovery of gating charge is considerably faster than recovery of sodium ionic current after equilibration at depolarized potentials. Recovery of gating charge at -140 mV, is 65% complete within approximately 100 ms, whereas less than 30% of ionic current has recovered by this time. Thus, charge movement and channel availability appear to be uncoupled during recovery, although both rates remain voltage sensitive. These data suggest that channels remain inactivated due to a separate process operating in parallel with the fast gating charge. We demonstrate that this behavior can be simulated by a model in which the fast charge movement associated with channel activation is electrostatically-coupled to a separate slow voltage sensor responsible for the slow inactivation of channel conductance.     

Theoretical Considerations on the Responses of Gold-Deposited Surface Plasmon Resonance-Based Glass Rod Sensors Using a Three-Layer Fresnel Equation

The response curves of gold (Au)-deposited surface plasmon resonance-based glass rod sensors were calculated using a three-layer Fresnel equation while considering various parameters for the sensor system calculations. Au films with thicknesses of 30, 45, and 70 nm were deposited on half of the surfaces of the glass rods, which were 2 mm in diameter, with a deposition length of 100 mm. Sensor elements with Au film thicknesses of 45 nm on glass rods with diameters of 1 and 4 mm and with deposition lengths of 10, 20, and 50 mm were also prepared. The sensor system consists of a light-emitting diode (LED) with a wavelength of 654 nm as the light source with a mini-spectrometer as the detector. The LED intensity distribution, the range of the angle of incidence of light into the sensor element, and the thickness distributions of the Au films deposited on the glass rods were considered to be the important parameters for the calculations. The minimum positions of all the theoretical response curves agreed well with those of the experimental response curves within the limits of the experimental and theoretical uncertainties. Most of the overall response characteristics of the theoretical curves agreed well with those of the experimental curves within the limits of both types of uncertainty. It was found that the thickness distribution of the deposited Au film in the cross-sectional direction dominates the sensor response and thus is the most important parameter for calculation of the sensor properties. The agreements between the experimental and theoretical response curves indicate both the potential and the usefulness of the sensor performance estimation process based on the three-layer Fresnel equation.     

DNA detection and cell adhesion on plasma‐polymerized pyrrole

This study investigates the application of Plasma‐polymerized pyrrole (ppPY) as bioactive platform for DNA immobilization and cell adhesion based on the fundamental properties of ppPY, such as chemical structure, electrochemical property, and protein adsorption. Variations in electrochemical properties of the ppPY film deposited under different plasma conditions before and after DNA immobilization were measured using electrochemical impedance spectroscopy (EIS). The equilibrium concentration of the probe DNA immobilized on the ppPY surface was deduced by detecting the variations in the surface charge transfer resistance (R_ct) of the ppPY films after DNA immobilization with different concentrations. In addition, the detection limit of the target DNA hybridization with probe DNA, the association constant, K_a, and the dissociation constant were deduced from Langmuir isotherm equations simulated using the experimental data collected by EIS. Moreover, inverted microscope was used to observe the cell adhesions onto the surface of the ppPY films prepared under different plasma conditions. Different adhesive behaviors of cells were observed, demonstrating that ppPY films could be an alternative biomaterial used as the sensitive layer for DNA sensor or cell adhesion. © 2013 Wiley Periodicals, Inc. Biopolymers 101: 496–503, 2014.     

Energy barrier presented to ions by the vestibule of the biological membrane channel.

The role of the vestibule in influencing the permeation of ions through biological ion channels is investigated. We derive analytical expressions for the electric potential satisfying Poisson's equation with prolate spheroidal boundary conditions. To allow more realistic geometries we devise an iterative method to calculate the electric potential arising from a fixed charge and an arbitrary dielectric boundary, and confirm that the analytical expressions and iterative method give similar potential values. We then investigate the size of the potential barrier presented to an ion by model vestibules of conical and catenary shapes. The height of the potential barrier increases steeply as an ion enters the vestibule and moves toward the constricted region of the channel. We show that the barrier presented by, for example, a 15 degrees conical vestibule can be canceled by placing dipoles with a total moment of about 50 Debyes near the constricted region of the pore. The selectivity of cations and anions can result from the polarity of charge groups or the orientation of dipoles located near the constricted region of the channel.     

Electrical Performance of Organic Solar Cells with Additive‐Assisted Vertical Phase Separation in the Photoactive Layer

Understanding the vertical phase separation of donor and acceptor compounds in organic photovoltaics is requisite for the control of charge transport behavior and the achievement of efficient charge collection. Here, the vertically phase‐separated morphologies of poly(3‐hexylthiophene):[6,6]phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) blend films are examined with transmission electron microtomography, dynamic secondary ion mass spectroscopy, and X‐ray photoelectron spectroscopy. The 3D morphologies of the processed films are analyzed and how the solvent additive causes vertical segregation is determined. The photocurrent–voltage characteristics of the vertically segregated blend films are strongly dependent on the 3D morphological organization of the donor and acceptor compounds in the photoactive layer. This dependence is correlated with asymmetric carrier transport at the buried interface and the air surface in the vertically segregated blend films.     

Colorimetric/fluorescent bacterial sensing by agarose-embedded lipid/polydiacetylene films

Aim: Development of a new chromatic (colorimetric/fluorescence) bacterial sensor, for rapid, sensitive and versatile detection of bacterial proliferation. Methods and Results: We constructed agarose‐embedded chromatic films which produce dramatic colour changes and fluorescence transformations in response to bacterial growth. The sensing constructs comprise glass‐supported Langmuir–Schaeffer phospholipid/polydiacetylene films that undergo both blue‐red transformations and induction of intense fluorescence following interactions with bacterially secreted amphiphilic compounds that diffuse through the agarose. The agarose matrix coating the sensor film further contains growth nutrients, facilitating signal amplification through promotion of bacterial culture proliferation. The agarose layer also constitutes an effective barrier for reducing background signals not associated with the bacteria. We demonstrate the applications of the new sensor for the detection of Gram‐negative and Gram‐positive bacteria, and for screening specimens of physiological fluids (blood and urine) and foods (meat) for bacterial contaminations. Conclusions: The experiments demonstrate that the new agarose‐embedded film constructs are capable of bacterial detection through visible colour transitions and fluorescence emission recorded in conventional apparatuses. Significance and Impact of the Study: This work demonstrated a new simple chromatic platform for bacterial detection, based on the generation of easily recorded colour and fluorescence changes. The new bacterial detection scheme is highly generic and could be employed for varied practical uses, in which, rapid reporting on bacterial presence is required.     

The inner membrane barrier of lipid membranes experienced by the valinomycin/Rb+ complex: charge pulse experiments at high membrane voltages.

The kinetic analysis of charge pulse experiments at planar lipid membranes in the presence of macrocyclic ion carriers has been limited so far to the low voltage range, where, under certain simplifying conditions, an analytical solution is available. In the present study, initial voltages of up to 300 mV were applied to the membrane, and the voltage decay through the conductive pathways of the membrane was followed as a function of time. The system of differential equations derived from the transport model was solved numerically and was compared with the experimental data. The generalized kinetic analysis of charge pulse experiments and of steady-state current-voltage curves was used to study the voltage dependence of the individual transport steps and to obtain information on the shape of the inner membrane barrier. The data were found to be consistent with a comparatively broad inner barrier such as a trapezoidal barrier or an image force barrier. The inner barrier was found to sense 70-76% of the voltage applied to the membrane. As a consequence, 24-30% of the voltage acts on the two interfacial barriers between membrane and water. The data refer to membranes formed from monoolein, monoeicosenoin, or monoerucin in n-decane.     

Development of a sensor prepared by entrapment of MIP particles in electrosynthesised polymer films for electrochemical detection of ephedrine

A voltammetric sensor for (-)-ephedrine has been prepared by a novel approach based on immobilisation of an imprinted polymer for ephedrine (MIPE) in an electrosynthesised polypyrrole (PPY) film. Composite films were grown potentiostatically at 1.0 V vs. Pt (QRE) on a glassy carbon electrode using an unconventional "upside-down" (UD) geometry for the three-electrode cell. As a consequence, a high MIP loading was obtained, as revealed by SEM. The sensor response was evaluated, after overoxidation of PPY matrix, by cyclic voltammetry after pre-concentration in a buffered solution of analyte in 0.5-3 mM concentration range. An ephedrine peak at approximately 0.9 V increasing with concentration and saturating at high concentrations was evident. PPY-modified electrode showed a response, which was distinctly lower than the MIP response for the same concentration of the template. The effect of potential interferences including compounds usually found in human fluids (ascorbic acid, uric acid, urea, glucose, sorbitol, glycine, dopamine) was examined.