Iridium Anodic Oxidation to Ir(III) and Ir(IV) Hydrous Oxides

Iridium oxide films (IROFs) are known to have an enhanced or the so‐called super‐Nernstian (<59 mV/pH) pH‐sensitivity. The intention in the present study was to find out the reasons of such behavior and also to elucidate the nature of iridium anodic oxidation processes. The methods employed were combined cyclic voltammetry and chronopotentiometry. Iridium layers of 0.1 to 0.2 μm thickness, deposited thermally on titanium or gold‐plated titanium substrates, were used for investigations. IROFs on the surface of working electrodes were formed anodically by applying a constant potential in deaerated and oxygen‐containing solutions of 0.5 M H₂SO₄, 0.1 M KOH and 0.5 M H₃PO₄+KOH. Linear pH‐dependences of the stationary open‐circuit potential with the slopes close to 59 mV/pH were found for iridium electrode oxidized at 0.4 V–0.8 V (RHE) in deaerated and at 0.8 V–1.2 V (RHE) in O₂‐containing solutions. They were attributed to reversible Ir/Ir(OH)₃ and Ir/ IrO₂?nH₂O metal‐oxide electrodes, respectively. It has been suggested that the main current peaks seen in the voltammograms of iridium electrode in acid and alkaline solutions are of different nature. The difference between iridium electrode surface states in acid and alkaline solutions has been presumed to be the main reason of super‐Nernstian pH‐sensitivity of the IROFs. On the basis of the results obtained standard potential of Ir/Ir(OH)₃ electrode and the solubility product of Ir(OH)₃ have been evaluated: =0.78±0.02 V and K_sp=3.3×10^?64.     

Electrodeposition of Three‐Dimensional Porous Platinum Film on Removable Polyaniline Template for High‐Performance Electroanalysis

Three‐dimensional porous platinum (Pt_por) films are prepared based on Pt electrodeposition on polyaniline (PANI) modified electrodes followed by selective dissolution of PANI with HNO₃. Electrochemical quartz crystal microbalance data suggest that the PANI‐H₂PtCl₆ interaction involves redox and coordination reactions, depending on the working potential. The Pt_por shows better electrocatalytic performance than the Pt/PANI and conventionally electrodeposited Pt. The Pt_por modified glassy carbon electrode (GCE) can electrocatalyze the oxidation of H₂O₂ with a sensitivity of 414 µA cm^?2 mM^?1 and a detection limit of 9 nM, and the chitosan‐glucose oxidase/Pt_por/GCE can sense glucose with a sensitivity of 93.4 µA cm^?2 mM^?1.     

A Biofuel Cell Based on Biocatalytic Reactions of Glucose on Both Anode and Cathode Electrodes

Biofuel cells based on electrocatalytic oxidation of NADH and reduction of H₂O₂ have been prepared using carbon fiber electrodes functionalized with graphene nano‐flakes. The electrochemical oxidation of NADH was catalyzed by Meldola's blue (MB), while the reduction of H₂O₂ was catalyzed by hemin, both catalysts were adsorbed on the graphene flakes due to their π‐π staking. In the next set of experiments, the MB‐ and hemin‐electrodes were additionally modified with glucose dehydrogenase (GDH) and glucose oxidase (GOx), respectively. The enzyme catalyzed reactions in the presence of glucose, NAD⁺ and O₂ resulted in the production of NADH and H₂O₂ in situ. The produced NADH and H₂O₂ were oxidized and reduced, respectively, at the bioelectrocatalytic electrodes, thus producing voltage and current generated by the biofuel cell. The enzyme‐based biofuel cells operated in a human serum solution modelling an implantable device powered from the natural biofluid. Finally, two enzyme‐based biofuel cell connected in series and operating in the serum solution produced electrical power sufficient for activation of an electronic watch used as an example device.     

纳米Ir/IrOx电极的制备及在过氧化氢检测中的应用

在150μm的铱电极上,采用高电位氧化和低电位还原的方法制备纳米多孔Ir/IrOx电极,并用扫描电镜、循环伏安方法对其结构和电极的电化学进行表征。纳米多孔Ir/IrOx电极用于过氧化氢的检测,灵敏度高,线性范围宽,分别为10～590μmol/L和2～60 mmol/L,检测限为2.77μmol/L。实验结果令人满意。     

Simple Electrodeposition of Dendritic Pd Without Supporting Electrolyte and Its Electrocatalytic Activity Toward Oxygen Reduction and H2O2 Sensing

Metallic palladium (Pd) electrocatalysts for oxygen reduction and hydrogen peroxide (H₂O₂) oxidation/reduction are prepared via electroplating on a gold metal substrate from dilute (5 to 50 mM) aqueous K₂PdCl₄ solution. The best Pd catalyst layer possessing dendritic nanostructures is formed on the Au substrate surface from 50 mM Pd precursor solution (denoted as Pd‐50) without any additional salt, acid or Pd templating chemical species. The Pd‐50 consisted of nanostructured dendrites of polycrystalline Pd metal and micropores within the dendrites which provide high catalyst surface area and further facilitate reactant mass transport to the catalyst surface. The electrocatalytic activity of Pd‐50 proved to be better than that of a commercial Pt (Pt/C) in terms of lower overpotential for the onset and half‐wave potentials and a greater number of electrons (n) transferred. Furthermore, amperometric i–t curves of Pd‐50 for H₂O₂ electrochemical reaction show high sensitivities (822.2 and ?851.9 µA mM^?1 cm^?2) and low detection limits (1.1 and 7.91 µM) based on H₂O₂ oxidation H₂O₂ reduction, respectively, along with a fast response (<1 s).     

A Complete Electroenzymatic Choline Microprobe Based on Nanostructured Platinum Microelectrodes and an IrOx On‐probe Reference Electrode

A microelectrode array microprobe with a choline sensing site and an on‐probe reference electrode was constructed by depositing permselective polymer films and choline oxidase (ChOx) on one microelectrode, and iridium oxide (IrOx) on another, both of which were coated previously with a nanostructured Pt deposit. Scanning electron microscopy (SEM) of the nanostructured Pt layer revealed a unique pillar‐like, “nanograss” structure. Polyphenylenediamine (PPD) and Nafion were coated sequentially on the working (i. e. sensing) electrode surface to serve as the permselective polymer films. The microsensor exhibited high sensitivity to choline (123±13 μA mM^?1 cm^?2), low detection limit (3.2±0.8 μM), and fast response time (3–5 s). The choline sensor also was tested at physiological concentrations of electroactive interfering species common to brain extracellular fluid (i. e. ascorbic acid, dopamine, DOPA, and DOPAC) and showed excellent selectivity. Selectivity likely was aided by the relatively low potential of 0.35 V vs. IrOx that was made possible by the enhanced H₂O₂ electrooxidation activity of the underlying nanostructured Pt‐coated working electrode. Thus, Pt “nanograss” appears to be an excellent electrode surface modification for creation of high performance electroenzymatic biosensors.     

Hemin Modified SnO2 Films on ITO‐PET with Enhanced Activity for Electrochemical Sensing

We present a simple and efficient method for the preparation of hemin‐modified SnO₂ films on low cost, flexible, conducting ITO‐PET substrates to enable the development of a sensitive electrochemical sensor for the determination of H₂O₂. Using a hydrothermal processing method meant that the SnO₂ films can be prepared at low temperatures, compatible with the PET substrate. The properties of the electrodes enable a high hemin loading to be achieved in a stable and functional way, allowing the direct reduction and oxidation of the immobilized hemin and maintaining its high electrocatalytic activity in the reduction of H₂O₂ on the surface. The results showed a sensitive response linearly proportional to the concentration of H₂O₂ in the range 1.5 to 90 μM.     

Synthesis of nanoporous activated iridium oxide films by anodized aluminum oxide templated atomic layer deposition

Iridium oxide (IrOx) has been widely studied due to its applications in electrochromic devices, pH sensing, and neural stimulation. Previous work has demonstrated that both Ir and IrOx films with porous morphologies prepared by sputtering exhibit significantly enhanced charge storage capacities. However, sputtering provides only limited control over film porosity. In this work, we demonstrate an alternative scheme for synthesizing nanoporous Ir and activated IrOx films (AIROFs). This scheme utilizes atomic layer deposition to deposit a thin conformal Ir film within a nanoporous anodized aluminum oxide template. The Ir film is then activated by potential cycling in 0.1 M H₂SO₄ to form a nanoporous AIROF. The morphologies and electrochemical properties of the films are characterized by scanning electron microscopy and cyclic voltammetry, respectively. The resulting nanoporous AIROFs exhibit a nanoporous morphology and enhanced cathodal charge storage capacities as large as 311 mC/cm².     

pH and H2O2 dual-responsive carbon dots for biocatalytic transformation monitoring

Development of sensitive biosensors for biocatalytic transformations monitoring is in high demand but remains a great challenge. It is ascribed to the current strategies that focused on the single metabolite detection, which may bring about the relatively low sensitivity and false diagnosis result. Herein, we report the design and fabrication of novel carbon dots (CDs) with strong orange light emission, pH and H₂O₂ dual-responsive characteristics. The fluorescence quenching of CDs by H⁺ and H₂O₂ enables the highly sensitive detection of H⁺/H₂O₂-generating biocatalytic transformations. This is exemplified by the glucose oxidase-mediated catalytic oxidation reaction on glucose, in which H⁺ and H₂O₂ would be formed. As compared to the case in which glucose is present, significant fluorescence reduction is detected, and the fluorescence intensity is negatively proportional to glucose concentration. Thus, highly sensitive detection of glucose was readily achieved with a detection limit down to 10.18 nmol/L. The prepared CDs not only realize the highly sensitive detection of glucose, but also allows the probing other substances by changing the enzymes, thus providing a versatile platform, and demonstrating good potential to be used for biocatalytic transformations effective monitoring.     

Minimization of Interferences in Flow Injection Amperometric Glucose Biosensor Based on Oxidation of Enzymatically‐produced H2O2

One of the major problems in amperometric biosensors based on detection of H₂O₂ produced by enzymatic reaction between oxidase enzymes and substrate is the interference of redox active compounds such as ascorbic acid (AA), dopamine (DA) and uric acid (UA). To minimize these interferences, sodium bismuthate was used for the first time as an insoluble pre‐oxidant in the flow injection (FI) amperometric glucose biosensor at a Glucose oxidase (GOx) immobilized Pt/Pd bimetallic modified pre‐anodized pencil graphite electrode (p.PGE). In this context, these interfering compounds were injected into a flow injection analysis (FIA) system using an injector which was filled with NaBiO₃. Thus, these interferents were converted into their redox inactive oxidized forms before reaching the electrode in the flow cell. While glucose was not influenced by the pre‐oxidant in the injector, the huge oxidation peak currents of the interferents decreased significantly in the biosensor. FI amperometric current time curves showed that the AA, DA and UA were minimized by 96 %, 86 %, and 98 % respectively, in the presence of an equivalent concentration of interferences in a 1.0 mM glucose solution. The proposed FI amperometric glucose biosensor exhibits a wide linear range (0.01–10 mM, R²=0.9994) with a detection limit of 2.4×10^?3 mM. Glucose levels in the artificial serum and two real samples were successfully determined using the fabricated FI amperometric biosensor.     

Oxidase-functionalized Fe(3)O(4) nanoparticles for fluorescence sensing of specific substrate

This study reports the development of a reusable, single-step system for the detection of specific substrates using oxidase-functionalized Fe₃O₄ nanoparticles (NPs) as a bienzyme system and using amplex ultrared (AU) as a fluorogenic substrate. In the presence of H₂O₂, the reaction pH between Fe₃O₄ NPs and AU was similar to the reaction of oxidase and the substrate. The catalytic activity of Fe₃O₄ NPs with AU was nearly unchanged following modification with poly(diallyldimethylammonium chloride) (PDDA). Based on these features, we prepared a composite of PDDA-modified Fe₃O₄ NPs and oxidase for the quantification of specific substrates through the H₂O₂-mediated oxidation of AU. By monitoring fluorescence intensity at 587 nm of oxidized AU, the minimum detectable concentrations of glucose, galactose, and choline were found to be 3, 2, and 20 μM using glucose oxidase–Fe₃O₄, galactose oxidase–Fe₃O₄, and choline oxidase–Fe₃O₄ composites, respectively. The identification of glucose in blood was selected as the model to validate the applicability of this proposed method.     

A Membraneless Glucose/O2 Biofuel Cell Based on Pd Aerogels

In this study, we introduce the first membraneless glucose/O₂ biofuel cell using Pd‐based aerogels as electrode materials. The bioanode was fabricated with a coimmobilized mediator and glucose oxidase for the oxidation of glucose, in which ferrocenecarboxylic acid was integrated into a three‐dimensional porous beta‐cyclodextrin‐modified Pd aerogel to mediate the bioelectrocatalytic reaction. Bilirubin oxidase and Pd–Pt alloy aerogel were confined to an electrode surface, which realized the direct bioelectrocatalytic function for the reduction of O₂ to H₂O with a synergetic effect at the biocathode. By employing these two bioelectrodes, the assembled glucose/O₂ biofuel cell showed a maximum power output of 20 μW cm^?2 at 0.25 V.     

Effect of substrate on phase formation and surface morphology of sol-gel lead-free KNbO3, NaNbO3, and K0.5Na0.5NbO3 thin films

Environmentally acceptable lead-free ferroelectric KNbO₃ (KN) or NaNbO₃ (NN) and K_0.5Na_0.5NbO₃ (KNN) thin films were prepared using a modified sol-gel method by mixing potassium acetate or sodium acetate or both with the Nb-tartrate complex, deposited on the Pt/Al₂O₃ and Pt/SiO₂/Si substrates by a spin-coating method and sintered at 650°C. X-ray diffraction (XRD) analysis indicated that the NN and KNN films on the Pt/SiO₂/Si substrate possessed a single perovskite phase, while NN and KNN films on the Pt/Al₂O₃ substrate contained a small amount of secondary pyrochlore phase, as did KN films on both substrates. Scanning electron microscopic (SEM) and atomic force microscopic (AFM) analyses confirmed that roughness R _q of the thin KNN/Pt/SiO₂/Si film (?? 7.4 nm) was significantly lower than that of the KNN/Pt/Al₂O₃ film (?? 15 nm). The heterogeneous microstructure composed of small spherical and larger needle-like or cuboidal particles were observed in the KN and NN films on both substrates. The homogeneous microstructure of the KNN thin film on the Pt/SiO₂/Si substrate was smoother and contained finer spherical particles (?? 50 nm) than on Pt/Al₂O₃ substrates (?? 100 nm). The effect of different substrates on the surface morphology of thin films was confirmed.     

A New Technique for Chemical Deposition of Pt Nanoparticles and its Applications on Biosensor Design

A new glucose biosensor design based on glucose oxidase (GOD) immobilized by polypyrrole has been described in this paper. The polymerization of pyrrole was initiated by a hexachloroplatinate which itself was reduced into Pt nanoparticles and thus served as a catalyst for the H₂O₂ oxidation. Properties of the produced GOD modified electrode were examined and the activity of the entrapped enzyme was determined by basic application on the amperometric detection of glucose. Much better results were found comparatively with the enzyme electrode for which the enzyme was entrapped by the electrochemically polymerized polypyrrole. This kind of technique for Pt nanoparticles deposition can be generalized to many cases where polypyrrole is used.     

High‐Performance Amperometric Sensors Using Catalytic Platinum Nanoparticles‐Thionine‐Multiwalled Carbon Nanotubes Nanocomposite

Thionine (TH) adsorbed on multiwalled carbon nanotubes (MWCNTs) increases the load and dispersion of platinum nanoparticles (PtNPs) generated by chemical reduction of H₂PtCl₆ with NaBH₄. Under the optimum conditions, the PtNPs‐TH‐MWCNTs/Au electrode electrocatalyzed the reduction and oxidation of H₂O₂ with high sensitivity, and after glucose oxidase (GOx) adsorption it responded to glucose concentration with a sensitivity of 0.14 A M^?1 cm^?2. The cyclic voltammetric cathodic peak current for NO₂^? reduction on PtNPs‐TH‐MWCNTs/Au responded linearly to NO₂^? concentration from 0.5 to 150 µM, with a sensitivity of 5.52 A M^?1 cm^?2 and a detection limit of 0.2 µM.     

A Nonenzymatic Glucose Sensor Using Nanoporous Platinum Electrodes Prepared by Electrochemical Alloying/Dealloying in a Water‐Insensitive Zinc Chloride‐1‐Ethyl‐3‐Methylimidazolium Chloride Ionic Liquid

We report here a nonenzymatic sensor by using a nanoporous platinum electrode to detect glucose directly. The electrode was fabricated by electrochemical deposition and dissolution of PtZn alloy in zinc chloride‐1‐ethyl‐3‐methylimidazolium chloride (ZnCl₂‐EMIC) ionic liquid. Both SEM and electrochemical studies showed the evidences for the nanoporous characteristics of the as‐prepared Pt electrodes. Amperometric measurements allow observation of the electrochemical oxidation of glucose at 0.4 V (vs. Ag/AgCl) in pH 7.4 phosphate buffer solution. The sensor also demonstrates significant reproducibility in glucose detection; the higher the roughness factor of the Pt electrode, the lower the detection limit of glucose. The interfering species such as ascorbic acid and p‐acetamidophenol can be avoided by using a Pt electrode with a high roughness factor of 151. Overall, the nanoporous Pt electrode is promising for enzymeless detection of glucose at physiological condition.     

EQCM Study of Iridium Anodic Oxidation in H2SO4 and KOH Solutions

In the present study anodic oxidation of iridium layer formed thermally on a gold‐sputtered quartz crystal electrode has been investigated by electrochemical quartz crystal microgravimetry (EQCM) in the solutions of 0.5 M H₂SO₄ and 0.1 M KOH. The emphasis here has been put on the microgravimetric behavior of iridium as a metal, because a few previous EQCM studies reported in literature have been devoted to iridium oxide films (IROFs). The objective pursued here has been to elucidate the nature of the main voltammetric peaks, which occur at different ranges of potential in the solutions investigated. It has been found that anodic oxidation of iridium electrode in 0.5 M H₂SO₄ and 0.1 M KOH solutions is accompanied by irregular fluctuations of the electrode mass at 0.4 V<E<0.8 V followed by regular increase in mass at 0.8 V<E<1.2 V. The cathodic process initially, at 1.2 V>E>0.9 V, proceeds without any or with slight increase in electrode mass, whereas at E<0.8 V a regular decrease in mass is observed. It has been found that mass to charge ratio characterizing the processes of interest is 2 to 3 g F^?1in acidic medium, whereas in the case of alkaline one it is 4 to 6 g F^?1. The main pair of peaks seen in the voltammograms of Ir electrode in alkaline medium at E<0.8 V is attributable to redox transition Ir(0)→Ir(III), whereas those observed in the case of acidic medium at E>0.8 V should be related to the redox process Ir(0)→Ir(IV) going via intermediate stage of Ir(III) formation. As a consequence of these redox transitions, the gel‐like surface layer consisting of Ir(III) or Ir(IV) hydrous oxides forms on the electrode surface.     

Copper Nanoflowers on Carbon Cloth as a Flexible Electrode Toward Both Enzymeless Electrocatalytic Glucose and H2O2

Herein, a novel electrochemical sensor for glucose and hydrogen peroxide (H₂O₂) detection was successfully developed through the use of Cu nanoflowers (CuNFs) combined with flexible carbon cloth (CC) substrate. The 3D flower-like CuNFs in size uniformly and firmly grow on CC substrate by a facile, scalable, one-step hydrothermal strategy. Morphology, size and surface property of the prepared CuNFs/CC were examined by SEM, EDS and TEM, respectively. The electrochemical mechanism of CuNFs/CC for glucose and H₂O₂ detection was investigated by cyclic voltammetry. High electrochemical performances were displayed with amperometric i-t curves including a wide linear range from 1.0 nM to 6.0 mM for glucose and from 1.0 μM to 36.66 mM for H₂O₂, respectively. Good sensitivity, repeatability, and stability, as well as anti-interference ability, the carbon cloth-supported CuNFs will be the promising materials for fabricating practical non-enzymatic glucose and H₂O₂ sensors.     

Response Behavior of Amperometric Glucose Biosensors Based on Different Carbon Substrate Transducers Coated with Enzyme‐Active Layer: A Comparative Study

Carbon electrodes (glassy carbon, GC, screen‐printed carbon, SPC, and carbon fiber, CF) were used as substrate transducers to prepare glucose biosensors of different sizes and geometries, based on iron‐ruthenium hexacyanoferrate as H₂O₂ reduction mediator and glucose oxidase immobilized in a poly(1,2‐phenylenediamine) membrane. Their response behavior under hydrodynamic amperometric conditions at an operating potential of ?0.02 V vs. Ag/AgCl was studied and compared. While the GC and SPC based conventional size biosensors showed enzymatic catalysis controlled current response with nonlinear concentration dependence, the CF based micro‐biosensor exhibited, due to diffusion‐controlled current response, extended linear range calibration curves with relatively lower sensitivity and longer response times. Several preparation parameters responsible for the improvement of biosensor performance were also investigated.     

β‐AgVO3 Nanorods as Peroxidase Mimetic for Colorimetric Determination of Glucose

β‐AgVO₃ nanorods have been demonstrated to exhibit intrinsic peroxidase‐like activity. The oxidation of glucose can be catalyzed by glucose oxidase (GOx ) to generate H₂O₂ in the presence of O₂ . The β‐AgVO₃ nanorods can catalytically oxidize peroxidase substrates including o‐phenylenediamine (OPD ), 3,3′,5,5′‐tetramethylbenzidine (TMB ), and diammonium 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonate) (ABTS ) by H₂O₂ to produce typical color reactions: OPD from colorless to orange, TMB from colorless to blue, and ABTS from colorless to green. The catalyzed reaction by the β‐AgVO₃ nanorods was found to follow the characteristic Michaelis–Menten kinetics. Compared with horseradish peroxidase and AgVO₃ nanobelts, β‐AgVO₃ nanorods showed a higher affinity for TMB with a lower Michaelis–Menten constant (K _m) value (0.04118 mM ) at the optimal condition. Taking advantage of their high catalytic activity, the as‐synthesized β‐AgVO₃ nanorods were utilized to develop a colorimetric sensor for the determination of glucose. The linear range for glucose was 1.25–60 μM with the lower detection limit of 0.5 μM . The simple and sensitive GOx ‐β–AgVO₃ nanorods–TMB sensing system shows great promise for applications in the pharmaceutical, clinical, and biosensor detection of glucose.