Electroactive Complex Research Articles

π-π+ Interaction Promoting the Absorption of Electroactive Chiral Selectors into the Cavity of Conductive Covalent Organic Framework for Enantioselective Sensing of Electrochemically Silent Molecules.

To date, achieving enantioselective electroanalysis for electrochemically silent chiral molecules is still highly desired. Here, an ionic covalent organic framework (COF) consisting of the pyridinium cation was derived from the tripyridinium Zincke salt and 1,4-phenylenediamine in a one-pot reaction. The electrochemical measurements revealed that the ionic backbone contributed to the electron transfer with a low charge transfer resistance. Besides, the π-π+ interaction between the pyridinium cation and ferrocenyl unit can promote the absorption of electroactive chiral ferrocenyl reagents into the hole of COF, so as to afford the electrochemical signals by themselves, replacing the testing enantiomers. As a result, the electroactive complex used as an electrochemical platform was highly effective at enantiomerically recognizing amino alcohols (prolinol, valinol, leucinol, and alaninol) and amino acids (methionine, serine, and penicillamine), giving the ratios of current intensity between l- and d-enantiomers in the range of 1.46-1.72. Moreover, the density functional theory calculations determined the possible intermolecular interactions between the testing enantiomers and chiral selector: namely, hydrogen bonds and electrostatic attractions. Overall, the present work offers an effective strategy to enlarge the electrochemical scope for chiral recognition based on electroactive chiral COFs.

Ultrasensitive functionalized CeO2/ZnO nanocomposite sensor for determination of a prohibited narcotic in sports pethidine hydrochloride

The extraordinary features of cerium oxide (CeO2) and zinc oxide (ZnO) nanostructures have encouraged substantial attention to those nanocomposites as probable electroactive complexes for sensing and biosensing purposes. In this study, an advanced novel factionalized CeO2/ZnO nanocomposite-aluminum wire membrane sensor was designed to assess pethidine hydrochloride (PTD) in commercial injection samples. Pethidine-reineckate (PTD-RK) was formed by mixing pethidine hydrochloride and ammonium reineckate (ARK) in the presence of polymeric matrix (polyvinyl chloride) and o-nitrophenyl octyl ether as a fluidizing agent. The functionalized nanocomposite sensor displayed a fast dynamic response and wide linearity for the detection of PTD. It also revealed excellent selectivity and sensitivity, high accuracy, and precision for the determination and quantification of PTD when compared with the unmodified sensor PTD-RK. The guidelines of analytical methodology requirements were obeyed to improve the suitability and validity of the suggested potentiometric system according to several criteria. The developed potentiometric system was suitable for the determination of PTD in bulk powder and commercial products.

A closed-shell phenalenyl-based dinuclear iron(III) complex as a robust cathode for a one-compartment H2O2 fuel cell.

Closed-shell phenalenyl (PLY) systems are increasingly becoming more attractive as building blocks for developing promising catalysts and electroactive cathode materials, as they have tremendous potential to accept electrons and participate in redox reactions. Herein, we report a PLY-based dinuclear [FeIII2(hmbh-PLY)3] complex, 1, and its utility as a cathode material in a H2O2 fuel cell. Complex 1 was synthesized from a new Schiff base ligand, (E)-9-(2-(2-hydroxy-3-methoxybenzylidene)hydrazineyl)-1H-phenalen-1-one, hmbh-PLYH2, designed using a PLY precursor, Hz-PLY. The newly derived ligand and complex 1 were characterized by various analytical techniques, including single-crystal X-ray diffraction (SCXRD). The cyclic voltammetry (CV) study revealed that complex 1 undergoes five electron reductions under an applied electric potential. When the electroactive complex 1 was employed as a cathode in a membrane-less one-compartment H2O2 fuel cell, with Ni foam as the corresponding anode, the designed fuel cell exhibited an exceptionally high peak power density (PPD) of 2.41 mW cm-2, in comparison with those of all the previously reported Fe-based molecular complexes. DFT studies were performed to gain reasonable insights into the two-electron catalytic reduction (pathway I) of H2O2 by the Fe-center of complex 1 and to explore the geometries, energetics of the electrocatalyst, reactive intermediates and transition states.

Cyclic voltammetric studies of electron transfer reactions associated with bioactive iron–KA complexation at different biological pH

This study explores electrochemical interactions of a pharmaceutical drug, kojic acid (5-hydroxy-2-hydroxymethyl-4 H-pyran-4-one, KA), with bioactive iron Fe(III) at different pH systems by using cyclic voltammetric technique. For this study, buffers relevant to physiological pH of blood and digestive system have been selected. Bioactive iron was found to form an electroactive complex [Fe(III)L3] in the presence of KA at all studied pH, whereas electrochemical behavior of KA was also changed by complex formation. This electrochemical investigation reveals that under the experimental conditions, [Fe(III)L3] complex undergoes a diffusion controlled, one electron redox process that is quasi-reversible in nature. It was also found that [Fe(III)L3] complex undergoes coupled chemical reaction according to EC (an electron transfer reaction followed by a chemical reaction) mechanism, and this interaction is pH dependent. Modified Nicholson and Shain, Gileadi, and Kochi methods were used to determine standard heterogeneous electron transfer rate constant ( k0) of [Fe(III)L3] complex. It was found to be in the range of 10−3 to 10−5 cm/s.

The Use of a Solid Bismuth Microelectrode for Vanadium Quantification by Adsorptive Stripping Voltammetry in Environmental Water Samples.

This paper presents for the first time the use of an environmentally friendly solid bismuth microelectrode for the voltammetric quantification of V(V) in natural water samples. These studies were designed to replace the film bismuth electrode that had been introduced to eliminate the conventional sensors based on highly toxic mercury. In the proposed procedure, V(V) is preconcentrated at the solid bismuth microelectrode surface via the formation of electroactive complexes with cupferron from a solution of 0.1-mol L−1 acetate buffer, pH = 4.6 at a potential of −0.4 V. The linearity of the calibration graph is in the V(V) concentration range from 8 × 10−10 to 1 × 10−7 mol L−1 with a preconcentration time of 1 min. The limit of detection (calculated as 3 σ) is 2.5 × 10−10 mol L−1 for a preconcentration time of 1 min. It was also demonstrated that significant improvement in analytical parameters was achieved as a result of the activation of the solid electrode surface at a potential of −2.5 V for 2 s. The developed procedure is highly selective for the presence of foreign ions and organic compounds in tested samples. The accuracy of the recommended procedure was checked using SPS-WW1 waste water-certified reference materials of a complex composition, in which the concentration of V(V) determined by the proposed method was 95.1 ± 1.6 ng mL−1. Moreover, in keeping with the outlined procedure, river, tap and rain water samples were analyzed without any pretreatment, and recovery values from 96% to 106% were obtained.

Highly Functionalized Modified Metal Oxides Polymeric Sensors for Potentiometric Determination of Letrozole in Commercial Oral Tablets and Biosamples.

The advanced and high-functional activities of magnesium oxide and copper oxide nanoparticles encourage the extensive use of these metal oxides as remarkable electroactive materials in electrochemical and sensing detections. The current study described a comparative sensing activity and selectivity of modified coated wire membrane sensors enriched with magnesium oxide and copper oxide nanoparticles for quantifying the breast cancer medication letrozole (LTZ) in its pharmaceutical form and human plasma. The fabricated sensors were based on the incorporation of LTZ with phosphomolybdic acid (PMA) to form the electroactive complex letrozole-phosphomolybate (LTZ-PM) in the presence of o-nitrophenyloctyl ether (o-NPOE) as a solvent mediator. Under optimum conditions, the modified sensors LTZ-PM-MgONPs and LTZ-PM-CuONPs demonstrated linear relationships of 1.0 × 10−8–1.0 × 10−2 and 1.0 × 10−10–1.0 × 10−2 mol L−1, respectively. Least square equations were calculated as EmV = (56.4 ± 0.7) log [LTZ] + 569.6 and EmV = (58.7 ± 0.3) log [LTZ] + 692.6 for LTZ-PM-MgONPs and LTZ-PM-CuONPs, respectively. The conventional type LTZ-PM showed a potential response EmV = (53.3 ± 0.5) log [LTZ] + 451.4 over concentration range of 1.0 × 10−6–1.0 × 10−2 mol L−1. The suggested sensors were successfully used to determine LTZ in pharmaceutical formulations and biosamples. Method validation ensured the suitability of the suggested potentiometric sensors.

Quantum-Chemical Study of the Electron Transfer in CaTiF6+12CaCl2 Model System

Today, quantum-chemical studies of the mechanism of electron transfer in molten salts based on a direct calculation of the transition state are faced with practically insurmountable computational difficulties. As our experience shows, model systems assigned to investigate the mechanism of charge transfer should include, in addition to the electroactive complex, two more of its coordination spheres. For electrolytes based on alkaline earth metal halides, this can be, for example, MTiF6+12MX2 type systems (M – Mg, Ca, Sr, Ba; X – F, Cl). The search for a transition state by standard methods will require enormous computer time and is almost unrealistic. In this paper, we proposed another approach, which is based on the analysis of frontier molecular orbitals (FMO) under various deformations of the initial structure.The aim of this work was the using of the method of frontier molecular orbitals to study the mechanism of the electron transfer in the CaTiF6+12CaCl2 model system.The geometry optimization of structures was performed with the Firefly program package, partially based on the source code of the GAMESS(US) program, by the density functional theory DFT/UHF method with the use of the B3LYP hybrid functional. For the F and Cl atoms quasi-relativistic basis set Stuttgart RLC ECP was used; for Ti and Ca – Stuttgart RSC 1997 ECP basis set.In this work, we were interested in the state of the complex TiF6 2- near the cathode surface. For this reason, on one side of the model system, a flat boundary layer consisting of 12-15 chlorine and calcium ions was formed. The titanium complex is in contact with one of the calcium cations belonging to the boundary layer, because our previous studies indicate the bridge nature of electron transfer to the TiF6 2- complex. Note that adding of carbon cluster to the main system CaTiF6+12CaCl2 would require a drastic increase of the computation time. However, for qualitative conclusions, it is enough to analyze the CaTiF6+12CaCl2 system.Based on the experimental data and the quantum-chemical analysis of FMO in the CaTiF6 + 12CaCl2 system, a mechanism for the charge transfer was proposed. The geometric structure of the transition state was shown to be not intermediate between the initial and final states of the system. As a result, it was found that the structure of this state is much less ordered than the initial structures of the boundary layer. The high efficiency of FMO method allows recommending this method as the instrument for testing hypotheses on the mechanism of electron transfer in molten salts.

Electrodeposition of tin(II) from citrate complexes

This paper provides the calculation of the distribution of the concentrations of complex particles of the Sn(II) - citrate - water system in the solution volume, on the surface of the electrode, and in the diffusion layer for the tin citrate electrolyte containing an excess of the ligand. Based on the calculations for the electrodeposition of tin, an electrolyte containing [SnCit]$^{2-}$ complex at pH 8.0 was chosen. The kinetic parameters of the discharge stage, the diffusion coefficient of the electrochemical active ion, and the current efficiency of tin were determined by the methods of stationary voltammetry and chronovoltamperometry. It is shown that the electroreduction of tin is governed by the laws of mixed kinetics and the transfer stage of the second electron is the limiting one. The discharge mechanism and the composition of the electroactive complex, which is [SnCit]$^{2-}$, are proposed. The current density of the deposition of tin coatings with different functional properties is determined as protective coatings and coatings of anodes of Li-ion accumulators.

Dual molecular recognition strategy for highly sensitive electrochemical detection of dopamine based on amplification of DNA–Au bio–bar codes

We had reported an electrochemical assay for dopamine with high sensitivity and selectivity based on dual molecular recognition strategy and amplification of DNA–Au bio–bar codes. In this assay, carbon nanotubes were first immobilized on the glass carbon electrode surface. Subsequently, a boronic acid-functionalized pyrene probe was self-assembled on the sidewalls of carbon nanotubes via π–π stacking interactions as a capture probe. 3,3′-Dithiodipropionic acid di(N-hydroxysuccinimide ester) (DSP) and double strands DNA functionalized AuNPs (DSP/AuNPs/dsDNA) were designed as signal probes and characterized by transmission electron microscopy and spectroscopic techniques. Upon the binding of dopamine with boronic acid followed by DSP/AuNPs/dsDNA, the AuNPs were captured on the electrode surface to enhance the signal. RuHex, an electroactive complex, which could bind to the anionic phosphate of DNA strands through electrostatic interaction, serves as signaling transducer. Signal amplification and dual recognition were used to quantify dopamine concentration from 30 pM to 30 nM, with a detection limit of 18 pM. Other small molecules could be measured by this assay after modification with differential recognition molecules.

Electroactive polyion complex polypeptide hydrogel locally supplies methylprednisolone and microsphere-entrapped neurotrophin-3 for spinal cord injury repair

Electroactive polyion complex polypeptide hydrogel locally supplies methylprednisolone and microsphere-entrapped neurotrophin-3 for spinal cord injury repair

A superstructure-based electrochemical assay for signal-amplified detection of DNA methyltransferase activity

DNA methyltransferase (MTase) activity is highly correlated with the occurrence and development of cancer. This work reports a superstructure-based electrochemical assay for signal-amplified detection of DNA MTase activity using M.SssI as an example. First, low-density coverage of DNA duplexes on the surface of the gold electrode was achieved by immobilized mercaptohexanol, followed by immobilization of DNA duplexes. The duplex can be cleaved by BstUI endonuclease in the absence of DNA superstructures. However, the cleavage is blocked after the DNA is methylated by M.SssI. The DNA superstructures are formed with the addition of helper DNA. By using an electroactive complex, RuHex, which can bind to DNA double strands, the activity of M.SssI can be quantitatively detected by differential pulse voltammetry. Due to the high site-specific cleavage by BstUI and signal amplification by the DNA superstructure, the biosensor can achieve ultrasensitive detection of DNA MTase activity down to 0.025U/mL. The method can be used for evaluation and screening of the inhibitors of MTase, and thus has potential in the discovery of methylation-related anticancer drugs.

Investigation of Photophysical and Electrofluorochromic Properties of Gold Nanoparticles Functionalized by a Luminescent Electroactive Complex

Gold nanoparticles (6 nm diameter) functionalized by a luminescent electroactive iridium complex have been synthesized, leading to stable colloidal suspensions in o-dichlorobenzene, a high boiling point solvent. The photophysical properties of these functionalized nanoparticles have been investigated showing a partial quenching of iridium emission by the gold core. Fluorescence correlation spectroscopy has been used to demonstrate that all the iridium complex is actually located on the nanoparticle surface, allowing the investigation of the electrofluorochromic behavior to be investigated for the first time in colloidal plasmonic systems. Compared with the free complex in solution, which shows a well-defined reversible electrochemically monitored luminescence (electrofluorochromism), the iridium-functionalized nanoparticles display a much less pronounced and less potential-dependent electrofluorochomic behavior, unless the potential is pushed toward values where gold stripping occurs.

Supramolecular Approach to Decorate Multi-Walled Carbon Nanotubes with Negatively Charged Iron(II) Complexes

Poly(4-vinylpyridine) (P4VP) and P4VP/pentacyanoferrate(II) metallopolymer were used to suspend up to 1 mg mL-1 of multi-walled carbon nanotubes (MWCNT) in ethanol/water mixtures, providing noncolavent decoration of MWCNT with electroactive iron complex. The major interaction between polymer side chains and nanotubes is via π-π stacking, rather than charge-transfer interaction reported for many nitrogenated interacting molecules. Preservation of structural integrity of the nanotubes after functionalization was attested by Raman, which was reflected in the semiconducting character of the dried nanocomposites. Proximity of pentacyanoferrate(II) to MWCNT walls was suggested by its solvatochromic shift of metal-to-ligand charge transfer band attributed to [Fe(CN)5]3− bound to P4VP pyridyl moieties. Electronic microscopy revealed smooth films and good adhesion between MWCNT bundles and the matrix. The procedure described herein can be used to combine unique properties of transition metal compounds able to coordinate to pyridyl groups, with the film-forming ability of P4VP and conductivity provided by MWCNT.

Phosphate Detection through a Cost-Effective Carbon Black Nanoparticle-Modified Screen-Printed Electrode Embedded in a Continuous Flow System.

An automatable flow system for the continuous and long-term monitoring of the phosphate level has been developed using an amperometric detection method based on the use of a miniaturized sensor. This method is based on the monitoring of an electroactive complex obtained by the reaction between phosphate and molybdate that is consequently reduced at the electrode surface. The use of a screen-printed electrode modified with carbon black nanoparticles (CBNPs) leads to the quantification of the complex at low potential, because CBNPs are capable of electrocatalitically enhancing the phosphomolybdate complex reduction at +125 mV versus Ag/AgCl without fouling problems. The developed system also incorporates reagents and waste storage and is connected to a portable potentiostat for rapid detection and quantification of phosphate. Main analytical parameters, such as working potential, reagent concentration, type of cell, and flow rate, were evaluated and optimized. This system was characterized by a low detection limit (6 μM). Interference studies were carried out. Good recovery percentages comprised between 89 and 131.5% were achieved in different water sources, highlighting its suitability for field measurements.

Electrodeposition of CoPt Alloys from the Stable Hexachloroplatinate Solution: Electrochemical Studies

A novel plating solution for CoPt alloys electrodeposition containing CoSO4 as a source of Co and H2PtCl6 as a source of Pt, has been developed. The solution was stable for more than one year. It was proposed that the most abundant six-coordinate PtIVL62− electroactive complex in new plating solution is PtCl5(OH)2−. The electrochemical conversion of PtIVL62− → PtIIL42−, occurring through two electron transfer and two ligand loss, was studied by using several electroanalytical techniques. It was concluded that the mechanism of the electron- transfer-PtL-bond-breaking process in the electrochemical reduction of PtIVL62- complex is a function of electrode materials (Pt, Ru). On a Pt electrode the reduction through stepwise pathway occurs, while on Ru the concerted mechanism prevails. The composition of CoPt alloys is strongly dependent on potential and agitation of the solution. Electrodeposition of CoPt alloys is demonstrated at a potential positive to that required to deposit elemental Co, which is attributed to the negative enthalpy of CoPt formation.

Fabrication of a novel naltrexone biosensor based on a computationally engineered nanobiocomposite

A computationally engineered impedimetric naltrexone (NLT) biosensor based on immobilization of bovine serum albumin (BSA) onto fullerene-C60/glassy carbon electrode (FLR/GCE) has been developed using initial characterization by computational methods and complementing them by experimental ones. Computational results showed that BSA hydrophobically binds to FLR which is energetically favorable and leads to the spontaneous formation of the stable nanobiocomposite and also showed that interaction of NLT with BSA is mainly driven by hydrogen bonding and hydrophobic interactions. Besides complementing the computational studies, experimental results showed that addition of FLR to the surface of the electrode facilitated electron transfer reactions, and also showed that the presence of BSA inhibits the interfacial electron transfer in some extent due to the non-conductive properties of BSA. The presence of NLT may form a negatively charged electroactive complex with BSA which repels the negatively charged redox probe and decelerates interfacial electron transfer leading to obvious faradaic impedance change. The faradaic impedance responses were linearly related to naltrexone concentration between 0.1nM and 80nM and limit of detection (LOD) was calculated to be 0.01nM 3Sb/b. Finally, the proposed biosensor was successfully applied to determination of NLT in urine samples of both healthy and addict volunteers.

Voltammetric and spectroscopic analysis of interactions of pyridine mono-carboxylic acid isomers with cysteine at physiological pH

The interactions of pyridine mono-carboxylic acid isomers (PCAIs) (nicotinic acid (NA), isonicotinic acid (INA) and picolinic acid (PA)) with cysteine (RSH) at physiological pH (7.40) have been investigated by square-wave and cyclic voltammetry (SWV and CV), and UV-Vis and infrared spectroscopy. By the addition of isomeric pyridine mono-carboxylic acids, the reduction peak current of mercurous cysteine thiolate could be decreased and also its peak potential E p was shifted to more positive values. Also, the significant changes in formal potential E 0′, electron transfer coefficient α and electrode reaction standard rate constant k s of mercurous cysteine thiolate (Hg2(SR)2) in the presence and absence of PCAIs were observed. The results of voltammetric measurements indicated that binding reactions were occurred between PCAIs and RSH and new electroactive molecular complexes were formed, which resulted in the decrease of free cysteine concentration and the decrease of the reduction peak current of mercurous cysteine thiolate. The logarithmic values of binding constants of NA, INA and PA were calculated as 13.4, 17.7 and 18.9, respectively. The binding ratios for NA-RSH, INA-RSH and PA-RSH complexes were determined as 1: 3, 1: 4 and 1: 4, respectively. Both UV-Vis and FTIR studies also confirmed these interaction reactions.

Fabrication of an ultrasensitive impedimetric buprenorphine hydrochloride biosensor from computational and experimental angles.

For the first time, an ultrasensitive impedimetric buprenorphine hydrochloride (BN) biosensor based on immobilization of bovine serum albumin (BSA) onto multi-walled carbon nanotubes (MWCNTs)/glassy carbon electrode (BSA/MWCNTs/GCE) has been developed using initial characterization by computational methods and complementing them by experimental observations. Computational results showed that the BSA hydrophobically binds to MWCNTs which is energetically favorable and leads to spontaneous formation of the stable BSA/MWCNTs nanobiocomposite (bioconjugate). Computational results also showed that the interaction of BN with BSA is mainly driven by hydrophobic interactions. The interactions of BSA with MWCNTs and BN with BSA were also monitored by fluorescence and UV-vis spectroscopic techniques, and their results were consistent with the computational results. Morphology and electrochemical properties of the fabricated composite electrodes were examined by scanning electron microscopy (SEM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Besides complementing the computational studies, experimental results showed that the addition of MWCNTs to the surface of the GCE greatly facilitated the electron transfer reactions, and also showed that the presence of BSA inhibits the interfacial electron transfer in some extent due to the non-conductive properties of BSA. On the other hand, the presence of BN may form an electroactive complex with BSA which accelerates the interfacial electron transfer and leads to obvious Faradaic impedance changes. The Faradaic impedance responses were linearly related to BN concentration between 5.0 nM and 72.0 nM and a limit of detection (LOD, 3S(b)/b) of 1.5 nM was achieved. Finally, the proposed biosensor was successfully applied to determination of BN in urine samples of both healthy and addict volunteers. The results were satisfactory and comparable to those obtained by applying the reference method based on high performance liquid chromatography-ultraviolet detection (HPLC-UV). It is expected that the distinctive features of BSA/MWCNTs nanobiocomposite would make it potentially advantageous for a broad range of biosensing, and clinical applications.

Magnetic bead-based hybridization assay for electrochemical detection of microRNA

Aberrant expression of microRNAs (miRNAs), short non-coding RNA molecules regulating gene expression, is often found in tumor cells, making the miRNAs suitable candidates as cancer biomarkers. Electrochemistry is an interesting alternative to current standard methods of miRNA detection by offering cheaper instrumentation and faster assays times. In this paper, we labeled miRNA in a quick, simple, two-step procedure with electroactive complex of osmium(VI) and 2,2′-bipyridine, Os(VI)bipy, which specifically binds to the ribose at the 3′-end of the miRNA, and hybridized such labeled miRNA with biotinylated capture probe attached to the streptavidin magnetic beads. Labeled miRNA was then detected at hanging mercury drop electrode at femtomole level due to an electrocatalytic nature of the peak from the Os(VI)bipy label. We obtained good selectivity of the assay using elevated hybridization temperatures for better discrimination of perfect duplex from single and double mismatches. After optimization of the protocol, we demonstrated feasibility of our assay by detecting target miRNA in real total RNA samples isolated from human cancer cells.

Os(VI)bipy-based electrochemical assay for detection of specific microRNAs as potential cancer biomarkers

Recently, altered expression levels of microRNAs (miRNAs) – short noncoding RNA molecules which bind to mRNAs and thus regulate gene expression – were observed in many cancer cells. miRNA expression profiling is therefore of great interest, but current standard methods are still considered relatively laborious and expensive. Electrochemistry has a potential to become quick and inexpensive alternative. Here, we describe modification of miRNA with an electroactive complex composed of six-valent osmium and 2,2′-bipyridine, Os(VI)bipy, specifically binding to the 3′-end of the ribose, which is detectable at hanging mercury drop electrode at femtomole level due to an electrocatalytic nature of a resulting signal. By combining miRNA labeling step with magnetic beads-based hybridization assay, detection of specific miRNA sequence from a mixture of other noncomplementary miRNAs was possible.