Mass Transport Investigated with the Electrochemical and Electrogravimetric Impedance Techniques. 3. Complex Charge Transport in PPy/PSS Films

For the first time, the complex charge transport mechanism for polypyrrole/poly(polystyrenesulfonate) (PPy/PSS) films in aqueous electrolyte solutions has been revealed by employing the cyclic/step electrochemical quartz crystal microbalance (EQCM) technique and the electrochemical/electrogravimetric impedance technique. Especially from the impedance experiments, the relative diffusion coefficient of charge transport has been obtained, and also the ion transport behavior in the slow charge transport process has been revealed. In PPy/PSS films, anion transport is considerable in the slow charge transport process, while cation transport is dominant in the fast charge transport process. It is shown that the presence of the slow charge transport processes is due to a slow electron transport process, not a slow ion transport process

Enhancement of Electrocatalytic Activity of DNA-Conjugated Gold Nanoparticles and Its Application to DNA Detection

Hydrazine electrooxidation readily occurs on bare Au nanoparticles (AuNPs), whereas it does not on DNA-conjugated AuNPs. Thus, when DNA-conjugated AuNPs are used as electrocatalytic labels in electrochemical DNA detection, anodic current of hydrazine is not easily observed within the potential window because of the high overpotential caused by the slow electron-transfer kinetics on DNA-conjugated AuNPs as well as the slow electron tunneling between the AuNP and the indium−tin oxide (ITO) electrode. NaBH4 treatment significantly enhances the electrocatalytic activity of DNA-conjugated AuNPs. This enhancement substantially decreases the overpotential caused by the slow electron-transfer kinetics, and the anodic current of hydrazine can be measured within the potential window if the distance between the AuNP and the ITO electrode is not too large. The enhancement with NaBH4 treatment allows high signal current, and the low intrinsic electrocatalytic activity of ITO electrodes allows low background current. The high signal-to-background ratio enables us to detect 1 fM target DNA without target amplification or enzymatic signal amplification. The ultrasensitive detection using versatile AuNP and simple chemical treatment is practically appealing

Mass Transport Investigated with the Electrochemical and Electrogravimetric Impedance Techniques. 1. Water Transport in PPy/CuPTS Films

Water transport in poly(pyrrole/copper phthalocyaninetetrasulfonate) (PPy/CuPTS) films, where cation transport prevails, has been investigated by employing the electrochemical quartz crystal microbalance (EQCM) technique, the electrochemical impedance technique, and the electrogravimetric impedance technique. The approximate numbers of accompanying waters per cation for PPy/CuPTS films have been obtained from both cyclic EQCM experiment and impedance experiment. It is shown that the number of accompanying waters depends on the nature and concentration of an electrolyte solution as well as the redox state of the film. It increases with the hydration number of cation in an aqueous electrolyte solution and exhibits hysteresis behavior during redox cycle. It is found that ionic conductivity of cation in a film depends on the number of accompanying waters and the ion−ion interactions inside the film. It is also found that a substantial amount of water moves with the cation during the break-in process at the first cathodic scan. Moreover, the electromechanical impedance technique has been employed to verify the relation between mass and resonant frequency of an oscillating quartz crystal and to monitor morphology changes (viscoelastic change and volume change) during the redox reaction of PPy/CuPTS films. It is found that mass change can be obtained from resonant frequency without consideration of morphology changes of PPy films. It is also found that morphology changes of PPy/CuPTS films relate to the amount of water moving into or out of the film

Dependence of the Electrochemical Behavior of Poly(<i>N</i>-Phenylpyrrole) Films on the Type of Anion and Solvent Used in the Electropolymerization

The mass transport behavior during the redox reaction of poly(N-phenylpyrrole) (PPhPy) films in propylene carbonate (PC) and acetonitrile (AN) solutions has been studied using the cyclic electrochemical quartz crystal microbalance (EQCM) technique and the electrochemical impedance technique. It was found that the type of dopant anion and solvent used in the electropolymerization affects the amount and the direction of solvent transport during the redox reaction of a PPhPy film. It was also shown that the charge capacity and ionic resistance of a PPhPy film depends highly on the type of dopant anion and solvent used in the electropolymerization. The PPhPy film grown in a PF6- -containing solution shows larger charge capacity and lower ionic resistance than the film grown in a ClO4--containing solution. Moreover, the PPhPy film grown in a PC solution shows similar to or larger charge capacity and lower ionic resistance than the film grown in an AN solution. These dependences on the electropolymerization condition were discussed in connection with the morphology of a film

Glucose-Oxidase Label-Based Redox Cycling for an Incubation Period-Free Electrochemical Immunosensor

Catalytic reactions of enzyme labels in enzyme-linked immunosorbent assays require a long incubation period to obtain high signal amplification. We present herein a simple immunosensing scheme in which the incubation period is minimized without a large increase in the detection limit. This scheme is based on electrochemical-enzymatic (EN) redox cycling using glucose oxidase (GOx) as an enzyme label, Ru­(NH₃)₆³⁺ as a redox mediator, and glucose as an enzyme substrate. Fast electron mediation of Ru­(NH₃)₆³⁺ between the electrode and the GOx label attached to the electrode allows high signal amplification. The acquisition of chronocoulometric charges at a potential in the mass transfer-controlled region excludes the influence of the kinetics of Ru­(NH₃)₆²⁺ electrooxidation and also facilitates high signal-to-background ratios. The reaction between reduced GOx and Ru­(NH₃)₆³⁺ is rapid even in air-saturated Tris buffer, where the faster competitive reaction between reduced GOx and dissolved oxygen also occurs. The direct electrooxidation of glucose at the electrode and the direct electron transfer between glucose and Ru­(NH₃)₆³⁺ that undesirably increase background levels occur relatively slowly. The detection limit for the EN redox cycling-based detection of cancer antigen 125 (CA-125) in human serum is slightly higher than 0.1 U/mL for the incubation period of 0 min, and the detection limits for the incubation periods of 5 and 10 min are slightly lower than 0.1 U/mL, indicating that the detection limits are almost similar irrespective of the incubation period and that the immunosensor is highly sensitive

Electrochemical Signal Enhancement via Redox Cycling Involving Iron Oxide Magnetic Particles (Adaptable, Reversible Redox Reservoirs) and Its Application in Sensitive Cu²⁺ Detection

Electrochemical signals may be affected by the presence of iron oxide magnetic particles (MPs) on an electrode owing to their distinct magnetic and redox properties. Recognizing their significance in electrochemical detection, we investigated the changes in electrochemical signals in the presence of MPs and their underlying causes. In the presence of MPs, the cyclic voltammograms of (quasi)reversible redox species (e.g., Fe(CN)64– and Ru(NH3)63+) exhibit different current-enhancing behaviors, depending on their formal potentials. Several redox species, such as Fe(CN)63– and Os(2,2′-bipyridyl)2Cl2, display non-zero initial currents at non-oxidizing or -reducing applied potentials in the presence of MPs. These findings are primarily attributed to the rapid redox reaction between the redox species and MP rather than the enhancement of mass transfer via magnetoconvection. The reaction between a redox species and an MP leads to a positive or negative shift in the equilibrium potential of the MP, which depends on the formal potential of the redox species. This enables MPs to act as adaptable, reversible redox reservoirs, facilitating current enhancement via redox cycling involving the MPs at specific potentials or during anodic and cathodic scanning. We applied signal enhancement via redox cycling to electrochemical Cu2+ detection. Cu2+ is rapidly reduced to Cu+ by the MPs during incubation, and Cu+ is then measured by using redox cycling. The calculated detection limit is approximately 15 nM, which is ∼100-fold lower than that observed without using MPs

Ultrasensitive Detection of DNA in Diluted Serum Using NaBH₄ Electrooxidation Mediated by [Ru(NH₃)₆]³⁺ at Indium−Tin Oxide Electrodes

There is a crucial need for simple and highly sensitive techniques to detect DNA in complicated biological samples such as serum. Here we present an ultrasensitive electrochemical DNA sensor using (i) single DNA hybridization with peptide nucleic acid (PNA), (ii) selective binding of [Ru(NH3)6]3+ to hybridized DNA, (iii) fast NaBH4 electrooxidation mediated by [Ru(NH3)6]3+, and (iv) low background currents of NaBH4 at indium−tin oxide (ITO) electrodes. The [RuIII(NH3)5NH2]2+ formed from [RuIII(NH3)6]3+ in borate buffer (pH 11.0) is readily electrooxidized to both [RuIV(NH3)5NH2]3+ and Ru complex with a higher oxidation state. In the absence of [Ru(NH3)6]3+ bound to the DNA-sensing ITO electrodes, the oxidation currents of NaBH4 are very low. However, in the presence of [Ru(NH3)6]3+, the oxidation currents of NaBH4 are highly enhanced due to electron mediation of the oxidized Ru complexes. The significant enhancement in the electrocatalytic activity of sensing electrodes after [Ru(NH3)6]3+ binding facilitates to obtain high signal-to-background ratios. PNA and ethylenediamine on DNA-sensing electrodes significantly decrease [Ru(NH3)6]3+ binding, also allowing for high signal-to-background ratios. The oxidation charges of NaBH4 obtained from chronocoulometry are highly reproducible. All combined effects enable the detection of DNA with a detection limit of 1 fM in ten-fold diluted human serum. The simple and fast detection procedure and the ultrasensitivity make this approach highly promising for practical DNA detection

A Nanocatalyst-Based Assay for Proteins: DNA-Free Ultrasensitive Electrochemical Detection Using Catalytic Reduction of <i>p</i>-Nitrophenol by Gold-Nanoparticle Labels

This communication reports a nanocatalyst-based electrochemical assay for proteins. Ultrasensitive detection has been achieved by signal amplification combined with noise reduction:  the signal is amplified both by the catalytic reduction of p-nitrophenol to p-aminophenol by gold-nanocatalyst labels and by the chemical reduction of p-quinone imine to p-aminophenol by NaBH4; the noise is reduced by employing an indium tin oxide electrode modified with a ferrocenyl-tethered dendrimer and a hydrophilic immunosensing layer

Electroreduction-Based Electrochemical-Enzymatic Redox Cycling for the Detection of Cancer Antigen 15‑3 Using Graphene Oxide-Modified Indium–Tin Oxide Electrodes

We compare herein biosensing performance of two electroreduction-based electrochemical-enzymatic (EN) redox-cycling schemes [the redox cycling combined with simultaneous enzymatic amplification (one-enzyme scheme) and the redox cycling combined with preceding enzymatic amplification (two-enzyme scheme)]. To minimize unwanted side reactions in the two-enzyme scheme, β-galactosidase (Gal) and tyrosinase (Tyr) are selected as an enzyme label and a redox enzyme, respectively, and Tyr is selected as a redox enzyme label in the one-enzyme scheme. The signal amplification in the one-enzyme scheme consists of (i) enzymatic oxidation of catechol into <i>o</i>-benzoquinone by Tyr and (ii) electroreduction-based EN redox cycling of <i>o</i>-benzoquinone. The signal amplification in the two-enzyme scheme consists of (i) enzymatic conversion of phenyl β-d-galactopyranoside into phenol by Gal, (ii) enzymatic oxidation of phenol into catechol by Tyr, and (iii) electroreduction-based EN redox cycling of <i>o</i>-benzoquinone including further enzymatic oxidation of catechol to <i>o</i>-benzoquinone by Tyr. Graphene oxide-modified indium–tin oxide (GO/ITO) electrodes, simply prepared by immersing ITO electrodes in a GO-dispersed aqueous solution, are used to obtain better electrocatalytic activities toward <i>o</i>-benzoquinone reduction than bare ITO electrodes. The detection limits for mouse IgG, measured with GO/ITO electrodes, are lower than when measured with bare ITO electrodes. Importantly, the detection of mouse IgG using the two-enzyme scheme allows lower detection limits than that using the one-enzyme scheme, because the former gives higher signal levels at low target concentrations although the former gives lower signal levels at high concentrations. The detection limit for cancer antigen (CA) 15-3, a biomarker of breast cancer, measured using the two-enzyme scheme and GO/ITO electrodes is ca. 0.1 U/mL, indicating that the immunosensor is highly sensitive

“Outer-Sphere to Inner-Sphere” Redox Cycling for Ultrasensitive Immunosensors

This paper reports chemical–chemical (CC) and electrochemical–chemical–chemical (ECC) redox cycling, for use in ultrasensitive biosensor applications. A triple chemical amplification approach using an enzymatic reaction, CC redox cycling, and ECC redox cycling is applied toward electrochemical immunosensors of cardiac troponin I. An enzymatic reaction, in which alkaline phosphatase converts 4-aminophenyl phosphate to 4-aminophenol (AP), triggers CC redox cycling in the presence of an oxidant and a reductant, and electrochemical signals are measured with ECC redox cycling after an incubation period of time in an air-saturated solution. To obtain high, selective, and reproducible redox cycling without using redox enzymes, two redox reactions [the reaction between AP and the oxidant and the reaction between the oxidized form of AP (4-quinone imine, QI) and the reductant] should be fast, but an unwanted reaction between the oxidant and reductant should be very slow. Because species that undergo outer-sphere reactions (OSR-philic species) react slowly with species that undergo inner-sphere reactions (ISR-philic species), highly OSR-philic Ru­(NH₃)₆³⁺ and highly ISR-philic tris­(2-carboxyethyl)­phosphine (TCEP) are chosen as the oxidant and reductant, respectively. The OSR- and ISR-philic QI/AP couple allows fast redox reactions with both the OSR-philic Ru­(NH₃)₆³⁺ and the ISR-philic TCEP. Highly OSR-philic indium–tin oxide (ITO) electrodes minimize unwanted electrochemical reactions with highly ISR-philic species. Although the formal potential of the Ru­(NH₃)₆³⁺/Ru­(NH₃)₆²⁺ couple is lower than that of the QI/AP couple, the endergonic reaction between Ru­(NH₃)₆³⁺ and AP is driven by the highly exergonic reaction between TCEP and QI (via a coupled reaction mechanism). Overall, the “outer-sphere to inner-sphere” redox cycling in the order of highly OSR-philic ITO, highly OSR-philic Ru­(NH₃)₆³⁺/Ru­(NH₃)₆²⁺ couple, OSR- and ISR-philic QI/AP couple, and highly ISR-philic TCEP allows high, selective, and reproducible signal amplification. The electrochemical data obtained by chronocoulometry permit a lower detection limits than those obtained by cyclic voltammetry. The detection limit of an immunosensor for troponin I in serum, calculated from the anodic charges in chronocoulometry, is ca. 10 fg/mL