Fluorescent Polypyrrole Nanospheres – Synthesis and Properties of “Wireless” Redox Probes

In this work a novel concept of monitoring of occurrence of redox reactions between conducting polymer nanospheres and redox species in a solution is proposed. The redox process is monitored in the emission mode (without wiring of the probe to an electrochemical measuring set‐up) as a change in emission spectrum of a dye (not participating in the redox process itself) but reporting the alteration of properties of highly sensitive conducting polymer nanoparticles. This approach is possible due to applied unique method of synthesis of conducting polymers nanospheres of highly active, unblocked surface. Thus the nanospheres redox state is affected by the solution redox potential, leading to change of their properties. If solvatochromic probe of sufficiently high brightness (pyrene) is present in nanospheres, a redox reaction between the conducting polymer and solution can be observed as change of emission spectrum of the probe. Thus a localized redox potential optical probe can be obtained. The emission properties of the dye incorporated were preserved in the nanospheres, moreover, the emission spectrum of the probe was affected by the change in redox potential of the solution, thus influencing the redox state and ultimately the properties of the conducting polymer. The emission changes observed were dependent on ion‐exchange properties of polypyrrole, i.e. depending on the dopant ions present in the polymer, the sensitivity of the optical probe can be tuned.     

A novel label-free electrochemical aptasensor for thrombin based on the {nano-Au/thionine}n multilayer films as redox probes

Herein, a novel label-free electrochemical aptasensor based on direct immobilization of the redox probes on an electrode surface was reported. Gold electrode coated Nafion was firstly modified with redox probe-thionine (Thi) through ion exchange adsorption. Then, with the help of chemisorption and electrostatic adsorption, negatively charged nano-Au and positively charged Thi were layer-by-layer (LBL) self-assembled onto the modified electrode surface, which formed {nano-Au/Thi⁺}_n multilayer films for improving the amount of redox probes and immobilizing thiolated thrombin aptamers (TBA). In the presence of target thrombin (TB), the TBA on the multilayer film could catch the TB onto the electrode surface, which resulted in a barrier for electro-transfer, leading to decrease of the current. The proposed method avoided the cubsome redox probe labeling process, increased the amount of redox probe and reduced the distance between the redox probe and electrode surface. Thus, the approach showed a high sensitivity and a wider linearity to TB in the range from 0.12 nM to 46 nM with a detection limit of 40 pM.     

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.     

Nanostructuring carbon fibre probes for use in central venous catheters

A carbon fibre probe is described which utilises the oxidation of an endogenous biomarker to provide diagnostic information on the condition of intravascular access lines. The probe surface was modified through anodic oxidation to provide a high selectivity towards urate which was used as a redox probe through which the pH could be determined. A Nernstian response (−60 mV/pH) was obtained which was free from the interference of other redox species common to biofluids. The electroanalytical performance of the probe has been optimised and the applicability of the approach demonstrated through testing the responses in whole blood.     

Reversible reorganization of alkyl ester groups grafted on glassy carbon electrode: Induction by a redox probe

The oxidation of the tetrabutylammonium salt of adipic acid monomethyl ester was performed to extend the concept that the oxidation of carboxylates with terminal functional groups with electrons in π-orbitals induces the covalent grafting of carbon surfaces. When 1,4-benzoquinone was used as a redox probe to identify the compaction level of the grafted film, the voltammetric behaviour of this redox probe changed by cycling. Once a reproducible and quasi-reversible wave was obtained, the repose of the electrode in the electrolyte solution during a few minutes allowed the recovery of the original voltammogram of the redox probe on the modified electrode. This behaviour is cyclic and can be understood as the result of a reversible reorganization effect of the organic film. This reorganization phenomenon is affected by the water content in the electrochemical cell, which can be explained by hydrogen bonds forming between the ester groups present in the structure film and water.     

Redox cycling amplified electrochemical detection of DNA hybridization: application to pathogen E. coli bacterial RNA

An electrochemical genosensor in which signal amplification is achieved using p-aminophenol (p-AP) redox cycling by nicotinamide adenine dinucleotide (NADH) is presented. An immobilized thiolated capture probe is combined with a sandwich-type hybridization assay, using biotin as a tracer in the detection probe, and streptavidin-alkaline phosphatase as reporter enzyme. The phosphatase liberates the electrochemical mediator p-AP from its electrically inactive phosphate derivative. This generated p-AP is electrooxidized at an Au electrode modified self-assembled monolayer to p-quinone imine (p-QI). In the presence of NADH, p-QI is reduced back to p-AP, which can be re-oxidized on the electrode and produce amplified signal. A detection limit of 1 pM DNA target is offered by this simple one-electrode, one-enzyme format redox cycling strategy. The redox cycling design is applied successfully to the monitoring of the 16S rRNA of E. coli pathogenic bacteria, and provides a detection limit of 250 CFU μL⁻¹.     

Functionalization of single-walled carbon nanotubes for direct and selective electrochemical detection of DNA

We report here a new strategy to graft both redox and DNA probes on carbon nanotubes to make a label-free DNA sensor. Oxidized single-walled carbon nanotubes are first immobilized on a self-assembled monolayer of cysteamine; then the redox probe, a quinone derivative 3-[(2-aminoethyl)sulfanyl-5-hydroxy-1,4-naphthoquinone], is grafted on the free carboxylic groups of the nanotubes. After that, for DNA probe grafting, new carboxylic sites are generated via an aryl diazonium route. After hybridization with a complementary sequence, the conformational changes of DNA could influence the redox kinetics of quinone, leading to a current increase of the redox signal, detected by square wave voltammetry. The system is selective, as it can discriminate a single mismatched sequence from the complementary one.     

Measurement of the concentration of Mn2+ and Mn3+ in the manganese-catalyzed 1,4-cyclohexanedione-acid-bromate reaction using redox-triggered magnetic resonance spectroscopy

A new nuclear magnetic resonance (NMR) experiment is reported, where the spectrometer is triggered using the output from a combination redox electrode. This technique was used to probe redox oscillations in the 1,4-cyclohexanedione-acid-bromate reaction. Manganese(III) acetate or manganese(II) sulfate was used as the catalyst, and the periodic change in concentration of Mn2+/Mn3+ ions was determined as a function of redox potential. The concentration of Mn3+ ions was at a maximum at high redox potential and at a minimum at low redox potential. Also, redox potentials were found to not be dominated by the Mn2+/Mn3+ couple.     

Sorption and Abiotic Redox Transformation of Nitrobenzene at the Smectite-Water Interface

The effect of the redox state of structural Fe on the surface reactivity of iron-bearing phyllosilicates in aqueous suspension was investigated using a molecular probe. For this purpose the structural Fe in montmorillonite and ferruginous smectite was chemically reduced by sodium dithionite in citrate-bicarbonate (CB) buffer solution under N(2) at 70 degrees C, with the excess reactants removed by washing and centrifugation. Nitrobenzene was chosen as an electron acceptor probe to react with unaltered and chemically reduced smectites. Nitrobenzene was transformed into aniline only in the presence of the reduced smectites. This abiotic reductive transformation depended on the concentration of the electron acceptor in solution and the total accessible structural Fe(II) in smectites. As much as 40% of the crystal layer structural Fe(II) of the reduced smectites was oxidized to Fe(III); these electrons migrated to the surfaces/edges and then were transported into the adjacent aqueous layer. No significant effect of the oxidation state of structural Fe on the sorption of nitrobenzene on smectites was observed, but the reduced smectites sorbed less aniline than the unaltered smectites. The electronic structure and molecular geometry of the probe were modified within the smectite-water interface. Reducing structural Fe in smectites perturbed the surface reactivity. Copyright 2001 Academic Press.     

Measuring the ionic flux of an electrochemically actuated conducting polymer using modified scanning ion conductance microscopy

We propose a modification of a scanning ion conductance microscope suitable for probing an electrode in an operating electrochemical cell. We demonstrate its use by measuring salt concentration variations near a conducting polymer electrode as the polymer is electrochemically oxidized and reduced. The electrochemical control circuit is opened to isolate the working electrode, at a frequency sufficiently high that the electrode capacitance maintains the electrode potential. The local solution conductivity variations are detected through the probe current during the open-circuit time. We demonstrate two-stage ion exchange during oxidation and reduction of poly(3,4-ethylenedioxythiophene) films that develops strongly with repeated cycling and is correlated with actuation changes. Spatial composition variations of the film, caused by redox current distribution over the surface, and electromigration to the probe tip, causing local solution composition changes, have clear and characteristic effects on the measured transients.     

Electron conduction across electrode-immobilized neutravidin bound with biotin-labeled ruthenium pentaamine

Voltammetry is reported here of a self-assembled redox-protein conjugate consisting of neutravidin conjugated with a biotin derivative redox probe, Ru(NH3)5(N-[(N-[(4-pyridyl)methyl]biotinamide], immobilized on gold electrodes modified by self-assembled monolayers of mercaptoundecanoic acid. This voltammetry indicates that self-assembly of the conjugate/electrode electronic interface, driven by electrostatic binding between the monolayer and a single redox probe, favors orientation of the conjugate, resulting in electronic accessibility of the remaining three redox probes.     

De novo designed peptidic redox potential probe: linking sensitized emission to disulfide bond formation

The design and utility of a peptidic probe capable of accurately measuring environmental redox potential via sensitized emission has been prepared. This probe is characterized by long-lived luminescence (millisecond), nanomolar detection limits, and a probe reduction potential of -0.243 V.     

Electrochemistry-assisted microstructuring of reduced graphene oxide-based microarrays with adjustable electrical behavior

An electrochemistry-assisted microstructuring process is developed for fabricating well-aligned reduced graphene oxide (rGO)-based micropatterns on arbitrary substrates using a combined method of photolithography, electrochemical reduction and wet etching techniques. The dimension of special-shaped rGO microarrays localized in an insulating GO matrix is effectively adjusted by changing GO reduction time without multi-mask patterning. The increased conductivity of rGO micropatterns by several orders of magnitude is achieved by controlling GO thickness and reduction time. The electrochemical activity of rGO micropatterns as microarray electrodes is confirmed by using ferricyanide in aqueous solution as the redox probe. The present method could be a scalable technology to conventional photolithography for fabricating arbitrary rGO micropatterns in an insulating GO matrix for their potential applications in next generation electronic and electrochemical devices.     

A reversible near-infrared fluorescence probe for reactive oxygen species based on Te-rhodamine

We have designed and synthesized a reversible near-infrared (NIR) fluorescence probe, 2-Me TeR, for reactive oxygen species (ROS), utilizing the redox properties of the tellurium (Te) atom. 2-Me TeR is oxidized to fluorescent 2-Me TeOR by various ROS, while the generated 2-Me TeOR is quickly reduced in the presence of glutathione to regenerate 2-Me TeR. This redox-induced reversible NIR-fluorescence response of 2-Me TeR allowed us to detect the endogenous production of ROS and subsequent homeostatic recovery of the intracellular reductive environment in hydrogen peroxide-stimulated HL-60 cells. This probe is expected to be useful for monitoring the dynamics of ROS production continuously in vivo.     

Effect of redox label tether length and flexibility on sensor performance of displacement-based electrochemical DNA sensors

This article summarizes the sensor performance of four electrochemical DNA sensors that exploit the recently developed displacement-replacement sensing motif. In the absence of the target, the capture probe is partially hybridized to the signaling probe at the distal end, positioning the redox label, methylene blue (MB), away from the electrode. In the presence of the target, the MB-modified signaling probe is released; one type of probe is capable of assuming a stem-loop probe (SLP) conformation, whereas the other type adopts a linear probe (LP) conformation. Independent of the sensor architecture, all four sensors showed “signal-on” sensor behavior. Unlike the previous report, here we focused on elucidating the effect of the redox label tether length and flexibility on sensor sensitivity, specificity, selectivity, and reusability. For both SLP and LP sensors, the limit of detection was 10 pM for sensors fabricated using a signaling probe with three extra thymine (T3) bases linked to the MB label. A limit of detection of 100 pM was determined for sensors fabricated using a signaling probe with five extra thymine (T5) bases. The linear dynamic range was between 10 pM and 100 nM for the T3 sensors, and between 100 pM and 100 nM for the T5 sensors. When compared to the LP sensors, the SLP sensors showed higher signal enhancement in the presence of the full-complement target. More importantly, the SLP-T5 sensor was found to be highly specific; it is capable of discriminating between the full complement and single-base mismatch targets even when employed in undiluted blood serum. Overall, these results highlight the advantages of using oligo-T(s) as a tunable linker to control flexibility of the tethered redox label, so as to achieve the desired sensor response.     

Electrochemical studies of derivatized thiol self-assembled monolayers on gold electrode in the presence of surfactants.

Electrochemical impendence spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed to investigate the barrier properties and electron transfer of derivatized thiol self-assembled monolayers (SAMs) on gold in the presence of surfactants. The thiol derivatives used included 2-mercaptoethanesulfonic acid (MES), 2-mercaptoacetic acid (MAA), and N-acetyl-L-cysteine (NAC). A simple equivalent circuit was derived to fit the impedance spectra very well. The negative redox probe [Fe(CN)6](3-/4-) was selected to indicate the electron-transfer efficiency on the interface of the studied electrodes. It was found that by changing the surface structure of SAMs, different surfactants could regulate the barrier properties and electron-transfer efficiency in different ways. A positively charged surfactant lowered the electrostatic repulsion between the negative redox probe and negatively charged surface groups of a monolayer, while enhancing the reversibility of electron transfer by virtue of increasing the redox probe concentration within the electric double-layer region. A neutral surfactant showed no significant effect, while a negative surfactant hindered the access and reaction of redox probe by electrostatic repulsion of same-sign charges.     

Gadolinium‐Decorated Silica Microspheres as Redox‐Responsive MRI Probes for Applications in Cell Therapy Follow‐Up

The redox microenvironment within a cell graft can be considered as an indicator to assess whether the graft is metabolically active or hypoxic. We present a redox‐responsive MRI probe based on porous silica microparticles whose surface has been decorated with a Gd‐chelate through a disulphide bridge. Such microparticles are designed to be interspersed with therapeutic cells within a biocompatible hydrogel. The onset of reducing conditions within the hydrogel is paralleled by an increased clearance of Gd, that can be detected by MRI.     

Electroactive Ceramic Carbon Electrode Modified with Hydrophobic Polar Solvent

Ceramic carbon electrode modified with redox probe solution in hydrophobic polar solvent was prepared and studied. The electrode consisting of graphite powder, homogeneously dispersed in hydrophobic silicate matrix, was prepared from the mixture of methyltrimethoxysilane based sol and graphite powder by sol‐gel method. It was immersed in t‐butyloferrocene solution in nitrobenzene. The electrode properties were investigated by cyclic voltammetry and chronoamperometry in KNO₃ solution of different concentration. In most cases linear polarization of the electrode towards positive potentials results in peak shaped voltammogram originating from electrooxidation of t‐butyloferrocene. Its shape changes with time, but after 5–7 scans stable curve is obtained. In all conditions the anodic to cathodic charge ratio is larger than unity. The peak current is proportional to the concentration of the redox probe in organic phase and salt in aqueous phase, whereas the midpeak potential is almost not affected by these factors. It has been concluded, that the electrooxidation of redox probe within hydrophobic silicate matrix is followed by two simultaneous processes: t‐butyloferrocenium cation transfer to the aqueous phase and anion transfer from aqueous phase. Their relative contribution depends on the ratio of concentration of the redox probe in organic phase to concentration of salt in aqueous phase.     

Signal enhancement for gene detection based on a redox reaction of [Fe(CN)(6)](4-) mediated by ferrocene at the terminal of a peptide nucleic acid as a probe with hybridization-amenable conformational flexibility

Electrochemically enhanced DNA detection was demonstrated by utilizing the couple of a synthesized ferrocene-terminated peptide nucleic acid (PNA) with a cysteine anchor and a sacrificial electron donor [Fe(CN)(6)](4-). DNA detection sensors were prepared by modifying a gold electrode surface with a mixed monolayer of the probe PNA and 11-hydroxy-1-undecanethiol (11-HUT), protecting [Fe(CN)(6)](4-) from any unexpected redox reaction. Before hybridization, the terminal ferrocene moiety of the probe was subject to a redox reaction due to the flexible probe structure and, in the presence of [Fe(CN)(6)](4-), the observed current was amplified based on regeneration of the ferrocene moiety. Hybridization decreased the redox current of the ferrocene. This occurred because hybridization rigidified the probe structure: the ferrocene moiety was then removed from the electrode surface, and the redox reaction of [Fe(CN)(6)](4-) was again prevented. The change in the anodic current before and after hybridization was enhanced 1.75-fold by using the electron donor [Fe(CN)(6)](4-). Sequence-specific detection of the complementary target DNA was also demonstrated.     

Study of the Electrochemical Behavior of Biodiesel Microemulsion

Microemulsions have become a widely employed technique for the control of biodiesel quality but are still poorly understood as regard to their electrochemical behavior. In this work, we report the fundamental importance associated with the knowledge of electrochemical behavior of microemulsions composed of water in the presence of a supporting electrolyte, soybean biodiesel and propan‐1‐ol as consolute, along with the ferrocyanide–ferricyanide redox system applied as probe. The voltammetric results showed that for different compositions of microemulsions, variation in peak currents and change in system reversibility as well as in the electron transfer process were clearly noted. Furthermore, through the study of the diffusion coefficient, three different types of microemulsions including Oil/Water, Bicontinuous, and Water/Oil were successfully identified. Electrochemical impedance spectroscopy studies were also carried out aiming at obtaining more information regarding the electrode/solution interface. All the studies performed demonstrated that different types of microemulsions were formed upon exerting a direct influence on the electrochemical behavior of the redox probe. These results, in essence, point to the possibility of choosing a more suitable and advantageous microemulsion type for the development of an analytical method, as in the case, for example, of the microemulsions ME‐2 and ME‐3 which presented high voltammetric response in redox probe oxidation.