New approach to electrode kinetic measurements in square-wave voltammetry: amplitude-based quasireversible maximum

Genuine differential voltammetry

A novel form of differential voltammetry is proposed, developed through the implicit anodic and cathodic current components of the experimentally accessible conventional net current measured in a voltammetric experiment. By employing basic mathematical modelling of an electrode reaction of a dissolved redox couple at a conventional, macroscopic electrode within the framework of the Butler-Volmer electrode kinetic model, the implicit anodic and cathodic current components of the net conventional current are clearly defined and can be estimated. Consequently, a novel form of differential current, calculated as the difference between anodic and cathodic implicit current components associated with a single potential of the voltammetric experiment, can be established. This differential current demonstrates remarkable characteristics in terms of electrode kinetics and analytical performance, particularly in cases involving fast, seemingly electrochemically reversible electrode processes. It holds promise to be analytically superior to the best-known differential voltammetric techniques so far (e.g., square-wave voltammetry), as well as provides a means for estimating the rate constants of very fast, apparently reversible electrode processes at macroscopic electrodes under mild experimental conditions (i.e., studied at slow potential scan rates). The practical implication of the novel methodology is significant: a simple linear sweep voltammogram of a quasi-reversible electrode reaction with unknown electrode kinetic parameters can be readily transformed into the novel type of differential voltammogram through a convolution procedure of the conventional net current, paving a new way for studying electrode processes by voltammetry.

Biotin-Pt(IV)-Ru(II)-Boron-Dipyrromethene Prodrug as "Platin Bullet" for Targeted Chemo- and Photodynamic Therapy

Using the principle of "Magic Bullet", a cisplatin-derived platinum(IV) prodrug heterobimetallic Pt(IV)-Ru(II) complex, cis,cis,trans-[Pt(NH3)2Cl2{Ru(tpy-BODIPY)(tpy-COO)}(biotin)]Cl2 (Pt-Ru-B, 2), having two axial ligands, namely, biotin as water-soluble B-vitamin for enhanced cellular uptake and a BODIPY-ruthenium(II) (Ru-B, 1) photosensitizer having N,N,N-donor tpy (4'-phenyl-2,2':6',2″-terpyridine) bonded to boron-dipyrromethene (BODIPY), is developed as a "Platin Bullet" for targeted photodynamic therapy (PDT). Pt-Ru-B exhibited intense absorption near 500 nm and emission near 513 nm (λex = 488 nm) in a 10% dimethyl sulfoxide-Dulbecco's phosphate-buffered saline medium (pH 7.2). The BODIPY complex on light activation generates singlet oxygen as the reactive oxygen species (ROS) giving a quantum yield (ΦΔ) of ∼0.64 from 1,3-diphenylisobenzofuran experiments. Pt-Ru-B exhibited preferential cellular uptake in cancer cells over noncancerous cells. The dichlorodihydrofluorescein diacetate assay confirmed the generation of cellular ROS. Confocal images revealed its mitochondrial internalization. Pt-Ru-B showed submicromolar photocytotoxicity in visible light (400-700 nm) in A549 and multidrug-resistant MDA-MB-231 cancer cells. It remained nontoxic in the dark and less toxic in nontumorigenic cells. Cellular apoptosis and alteration of the mitochondrial membrane potential were evidenced from the respective Annexin V-FITC/propidium iodide assay and JC-1 dye assay. A wound healing assay using A549 cells and Pt-Ru-B revealed inhibition of cancer cell migration, highlighting its potential as an antimetastatic agent.

Genuine anodic and cathodic current components in cyclic voltammetry

Implicit anodic and cathodic current components associated with the real net current at a given potential of a simple quasireversible electrode reaction can be accurately estimated using basic mathematical modeling within the framework of Butler-Volmer electrode kinetics. This methodology requires only prior knowledge of the formal potential of the dissolved redox couple, offering direct insight into the electrode kinetics. The proposed approach facilitates a unique transformation of a conventional cyclic voltammogram, allowing the replacement of the common, net current with authentic anodic and cathodic current components. This simple methodology introduces a novel perspective in analyzing voltammetric data, particularly enabling the kinetic characterization of fast, seemingly electrochemically reversible electrode processes on macroscopic electrodes at slow scan rates. Theoretical predictions are experimentally demonstrated using the electrode reaction for the reduction of the hexaammineruthenium(III) complex, serving as an example of one of the fastest electrode processes involving a dissolved redox species.

A Current Averaging Strategy for Maximizing Analyte and Minimizing Redox Interference Signals with Square Wave Voltammetry

Square wave voltammetry (SWV) is commonly used in electroanalytical applications to enhance analyte faradaic signals and minimize nonfaradaic processes. However, little attention is given as to how best use SWV to minimize faradaic interference signals that arise from redox species present in solution that have redox potentials that convolute with that of the analyte. In conventional SWV, a series of current-time (i-t) transients are collected, and i is averaged over a specified window of each transient (potentiostat dependent). This average i is reported against the electrode potential, E. As the i-t response is governed by the type of electron transfer reaction under investigation, we show how by collecting all i-t data and through judicious choice of the current averaging window, it is possible to enhance the analyte response while at the same time reducing the interferent signal. We look at three different electron transfer reactions, fast electron transfer outer sphere, metal electrodeposition/stripping, and surface-confined proton-coupled electron transfer (PCET) and demonstrate different i-t behaviors in SWV, visually aided by the use of 3D i-t-E plots. In the case of PCET quinone-based voltammetric sensing of pH in the presence of a heavy metal (here Cu2+), we show that the use of a much earlier current averaging window (2-10% of the i-t response) results in the pH signal being clearly distinguished from that of the overlapping heavy metal.

Strategy, Design, and Fabrication of Electrochemical Biosensors: A Tutorial

Advanced healthcare requires novel technologies capable of real-time sensing to monitor acute and long-term health. The challenge relies on converting a real-time quantitative biological and chemical signal into a desired measurable output. Given the success in detecting glucose and the commercialization of glucometers, electrochemical biosensors continue to be a mainstay of academic and industrial research activities. Despite the wealth of literature on electrochemical biosensors, reports are often specific to a particular application (e.g., pathogens, cancer markers, glucose, etc.), and most fail to convey the underlying strategy and design, and if it is transferable to detection of a different analyte. Here we present a tutorial review for those entering this research area that summarizes the basic electrochemical techniques utilized as well as discusses the designs and optimization strategies employed to improve sensitivity and maximize signal output.

Biotin and boron-dipyrromethene-tagged platinum(IV) prodrug for cellular imaging and mito-targeted photocytotoxicity in red light

A platinum(IV) prodrug, cis,cis,trans-[Pt(NH3)2Cl2(biotin)(L)] (1), derived from cisplatin, where HL is the PEGylated red-light active boron-dipyrromethene (BODIPY) ligand, was synthesized, characterized and its photocytotoxicity evaluated. The complex showed a near-IR absorption band at 653 nm (ε ∼9.19 × 104 M-1 cm-1) in dimethyl sulfoxide and Dulbecco's phosphate-buffered saline (1 : 1 v/v) at pH 7.2. When excited at 630 nm, it showed an emission band at 677 nm in DMSO with a fluorescence quantum yield of 0.13. The 1,3-diphenylisobenzofuran titration experiment gave a singlet oxygen quantum yield (ΦΔ) of ∼0.32. A mechanistic DNA photocleavage study revealed singlet oxygen as the reactive oxygen species (ROS). The complex with biotin and PEGylated-distyryl-BODIPY showed significantly higher cellular uptake in A549 cancer cells as compared to non-cancerous Beas-2B cells from flow cytometry, indicating selectivity towards cancer cells. A dichlorodihydrofluorescein diacetate assay showed cellular ROS generation. Confocal images revealed predominant internalization in the mitochondria. The prodrug showed remarkable photodynamic therapy (PDT) activity in cancerous A549 and multidrug-resistant MDA-MB-231 cells with a high photocytotoxicity index value (half-maximal inhibitory concentration (IC50): 0.61-1.54 μM in red light), while being non-toxic in the dark. The chemo-PDT activity was significantly less in non-tumorigenic lung epithelial cells (Beas-2B). The prodrug effectively triggered cellular apoptosis, which was confirmed by the Annexin V-FITC/propidium iodide assay, and the alteration of the mitochondrial membrane potential was substantiated by the JC-1 dye assay. The β-tubulin immunofluorescence assay confirmed that incubating the cells with a light-treated complex resulted in the rapture of the cytoskeletal structure and the formation of apoptotic bodies. The results demonstrate that the prodrug triggered apoptosis via DNA damage, a reduction in mitochondrial function and disruption of the cytoskeletal framework.

Cyclic voltammetry, square wave voltammetry or electrochemical impedance spectroscopy? Interrogating electrochemical approaches for the determination of electron transfer rates of immobilized redox proteins

In this article, we cross-examine three well-established electrochemical approaches, namely cyclic voltammetry (CV), cyclic square-wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) to dissect the electron transfer (ET) rate of electrostatically immobilized cytochrome c on Ag electrodes. A detailed analysis supported by simulations of redox transition provided three distinct values for the heterogeneous electron transfer (HET) rate constant of cyt c interfaced on COOH-terminated C₁₀-long alkanethiol, i.e., k_HET= 47.8 (±2,91) s^-1 in CV, k_HET= 64.8 (±1,27) s^-1 in SWV, and k_HET= 26.5 s^-1 in EIS. We discuss the obtained discrepancies obtained from electrochemical methods and compare them with the data from spectro-electrochemical experiments. A comprehensive selection list is created from which the most applicable approach can be chosen for studying proteins of interest. CV is most applicable to study the interfaced proteins exhibiting k_HET of ca. 0.5 - 70 s^-1, SWV is suitable for a broader range of k_HET of 5 - 120 s^-1 and EIS for k_HET of 0.5 to 5 s^-1 if alkanethiols are used as immobilization strategy.

Electrochemical and Spectroscopic (FTIR) Evidence of Conducting Polymer-Cu Ions Interaction

In this work, we provide electrochemical and spectroscopic evidence of the conducting polymer-heavy metal ion interaction by comparing the electrochemical and spectroscopic behavior (FTIR) of two different conducting polymer-modified electrodes based on 3,4-alkoxythiophenes: 3,4-ethylenedioxythiophene (EDOT) and ortho-xylen-3,4-dioxythiophene (XDOT) during the potentiodynamic stripping of copper. By analyzing the electrochemical and spectroscopic results, it is possible to propose two different copper dissolution processes during the electrochemical stripping process, which depend on the conducting polymer used. With PEDOT matrix, stripping occurs in a two-step pathway, observed as two anodic peaks, involving the formation of the Cu⁺-PEDOT complex and the subsequent oxidation step of the Cu⁺ complex to release Cu²⁺ ions. On the other side, the experiments carried out let us propose the formation of a poorly stable Cu²⁺-PXDOT complex or a superficial mechanism for the Cu²⁺ release, characterized by a single stripping signal for this process. Thus, the incorporation of Cu ions into the matrix and the stripping release are intimately related to the chemical structure of the polymer used.

Electrode kinetics from a single experiment: multi-amplitude analysis in square-wave chronoamperometry

The recently introduced technique of square-wave chronoamperometry (SWCA) is studied under conditions of progressively increasing height of potential pulses (square-wave amplitude) within a single experiment. In multi-amplitude square-wave chronoamperometry (MA-SWCA) a potential modulation consisting of square-wave forward and reverse potential pulses is imposed on a constant mid-potential; the amplitude of pulses increases progressively during the experiment. This allows the fast and reliable estimation of kinetic parameters at a constant pulse frequency in a single experiment, based on the resulting feature known as the amplitude-based quasireversible maximum. The proposed methodology is tested by simulating the responses of a simple quasireversible electrode reaction of a dissolved redox couple and a surface confined electrode reaction. Compared with conventional square-wave voltammetry (SWV) and SWCA, MA-SWCA shows advantages in estimation of the standard rate constant in terms of simplicity, speed and efficiency for both studied electrode mechanisms.

Cobalt(III) Complexes for Light-Activated Delivery of Acetylacetonate-BODIPY, Cellular Imaging, and Photodynamic Therapy

Cobalt(III) complexes [Co(TPA)(L¹)](ClO₄)₂ (1), [Co(4-COOH-TPA)(L¹)](ClO₄)₂ (2), [Co(TPA)(L²)]Cl₂ (3), and [Co(4-COOH-TPA)(L²)]Cl₂ (4) having acetylacetonate-linked boron-dipyrromethene ligands (L¹, acac-BODIPY; L², acac-diiodo-BODIPY) were prepared and characterized, and their utility as bioimaging and phototherapeutic agents was evaluated (TPA, tris-(2-pyridylmethyl)amine; 4-COOH-TPA, 2-((bis-(2-pyridylmethyl)amino)methyl)isonicotinic acid). HL¹, HL², and complex 1 were structurally characterized by X-ray crystallography. Complexes 1 and 2 on photoactivation or in a reducing environment (excess GSH, ascorbic acid, and 3-mercaptopropionic acid) released the acac-BODIPY ligand. They exhibited strong absorbance near 501 nm (ε ∼ (5.2-5.8) × 10⁴ M^-1 cm^-1) and emission bands near 513 nm (Φ_F ∼ 0.13, λ_ex = 490 nm) in dimethyl sulfoxide (DMSO). Complexes 3 and 4 with absorption maxima at ∼536 and ∼538 nm (ε ∼ (1.2-1.8) × 10⁴ M^-1 cm^-1), respectively, afforded high singlet oxygen quantum yield (Φ_Δ ∼ 0.79) in DMSO. Complexes 1-4 showed Co(III)-Co(II) redox responses near -0.2 V versus saturated calomel electrode (SCE) in dimethylformamide (DMF)-0.1 M tetrabutylammonium perchlorate (TBAP). The photocleavage of pUC19 DNA by complex 4 revealed the formation of both singlet oxygen and superoxide anion radicals as the reactive oxygen species (ROS). Confocal fluorescence microscopy showed the selective accumulation of complex 1 in the endoplasmic reticulum (ER) in A-549 cells. Complex 4 exhibited a high phototherapeutic index value (PI > 7000) in HeLa cancer cells (IC₅₀ ∼ 0.007 μM in visible light of 400-700 nm, total dose ∼5 J cm^-2). The ancillary ligands in the complexes demonstrated a structure-activity relationship and modulated the Co(III)-Co(II) redox potential, the complex solubility, acac-BODIPY ligand release kinetics, and phototherapeutic efficacy.

Modeling and Experimental Study of the Electron Transfer Kinetics for Non-ideal Electrodes Using Variable-Frequency Square Wave Voltammetry

The knowledge of nonequilibrium electron transfer rates is paramount for the design of modern hybrid electrocatalysts. Herein, we propose a general simulation-based approach to interpret variable-frequency square wave voltammetry (VF-SWV) for heterogeneous materials featuring reversible redox behavior. The resistive and capacitive corrections, inclusion of the frequency domain, and statistical treatment of the surface redox kinetics are used to account for the non-ideal nature of electrodes. This approach has been validated in our study of Co^II/Co^I redox transformation for Co tetraphenylporphyrin (CoTPP) immobilized on carbon cloth and multiwalled carbon nanotubes (CNTs) - one of the most active heterogeneous molecular catalysts in carbon dioxide (CO₂) electroreduction. It is demonstrated that the modeling of experimental data furnishes the capacitance of the surface double layer C, uncompensated resistance R_u, symmetry coefficients α, kinetic constants k₀, and equilibrium redox potentials E⁰ in one experiment. Moreover, the proposed method yields a stochastic map of the redox kinetics rather than a single value, thus exposing the inhomogeneous nature of the electrochemically active layer. The computed parameters are in excellent agreement with the results of the classic methods such as cyclic voltammetry and fall in line with the reported CoTPP catalytic activity. Thus, VF-SWV is suitable for the study of high-level composites such as covalent organic frameworks and organometallic-CNT mixtures. The resulting insights into the electron transfer mechanisms are especially useful for the rational development of the catalyst-support interfaces and immobilization methods.

Real-Time Electrochemical Detection of Pseudomonas aeruginosa Phenazine Metabolites Using Transparent Carbon Ultramicroelectrode Arrays

Here, we use a recently developed electrochemical sensing platform of transparent carbon ultramicroelectrode arrays (T-CUAs) for the in vitro detection of phenazine metabolites from the opportunistic human pathogen Pseudomonas aeruginosa. Specifically, redox-active metabolites pyocyanin (PYO), 5-methylphenazine-1-carboxylic acid (5-MCA), and 1-hydroxyphenazine (OHPHZ) are produced by P. aeruginosa, which is commonly found in chronic wound infections and in the lungs of cystic fibrosis patients. As highly diffusible chemicals, PYO and other metabolites are extremely toxic to surrounding host cells and other competing microorganisms, thus their detection is of great importance as it could provide insights regarding P. aeruginosa virulence mechanisms. Phenazine metabolites are known to play important roles in cellular functions; however, very little is known about how their concentrations fluctuate and influence cellular behaviors over the course of infection and growth. Herein we report the use of easily assembled, low-cost electrochemical sensors that provide rapid response times, enhanced sensitivity, and high reproducibility. As such, these T-CUAs enable real-time electrochemical monitoring of PYO and another extremely reactive and distinct redox-active phenazine metabolite, 5-methylphenazine-1-carboxylic acid (5-MCA), from a highly virulent laboratory P. aeruginosa strain, PA14. In addition to quantifying phenazine metabolite concentrations, changes in phenazine dynamics are observed in the biosynthetic route for the production of PYO. Our quantitative results, over a 48-h period, show increasing PYO concentrations during the first 21 h of bacterial growth, after which PYO levels plateau and then slightly decrease. Additionally, we explore environmental effects on phenazine dynamics and PYO concentrations in two growth media, tryptic soy broth (TSB) and lysogeny broth (LB). The maximum concentrations of cellular PYO were determined to be 190 ± 5 μM and 150 ± 1 μM in TSB and LB, respectively. Finally, using desorption electrospray ionization (DESI) and nanoelectrospray ionization (nano-ESI) mass spectrometry we confirm the detection and identification of reactive phenazine metabolites.

Highly Ordered Macroporous Poly-3,4- ortho-xylendioxythiophene Electrodes as a Sensitive Analytical Tool for Heavy Metal Quantification

Highly ordered macroporous electrodes of the conducting polymer poly-3,4- ortho-xylendioxythiophene (PXDOT) are presented as a sensitive analytical tool for heavy metal ion quantification due to a controlled gain in electroactive area. They were designed by using colloidal crystal templates. A direct correlation between the final number of porous layers and the deposition charge ( Q_d) employed for electropolymerization is observed. All the electrodes exhibit a surface-templated structure due to an interaction between the radical cation, formed during the electropolymerization, and the surface groups of the silica beads. The voltamperometric response of the macroporous PXDOT electrodes shows a rather fast electron transfer with Δ E_p values between 70 mV and 110 mV. Square wave anodic stripping voltammetric (SWASV) analysis of Cu²⁺ as a representative heavy metal ion shows a linear response in the subppm range. As a model application, the efficient quantification of Cu²⁺ in a commercial mezcal sample is validated by the standard addition method and the results correlate adequately with the values obtained by atomic absorption spectroscopy.

Properties of Site-Specifically Incorporated 3-Aminotyrosine in Proteins To Study Redox-Active Tyrosines: Escherichia coli Ribonucleotide Reductase as a Paradigm

3-Aminotyrosine (NH₂Y) has been a useful probe to study the role of redox active tyrosines in enzymes. This report describes properties of NH₂Y of key importance for its application in mechanistic studies. By combining the tRNA/NH₂Y-RS suppression technology with a model protein tailored for amino acid redox studies (α₃X, X = NH₂Y), the formal reduction potential of NH₂Y₃₂(O^•/OH) ( E°' = 395 ± 7 mV at pH 7.08 ± 0.05) could be determined using protein film voltammetry. We find that the Δ E°' between NH₂Y₃₂(O^•/OH) and Y₃₂(O^•/OH) when measured under reversible conditions is ∼300-400 mV larger than earlier estimates based on irreversible voltammograms obtained on aqueous NH₂Y and Y. We have also generated D₆-NH₂Y₇₃₁-α2 of ribonucleotide reductase (RNR), which when incubated with β2/CDP/ATP generates the D₆-NH₂Y₇₃₁^•-α2/β2 complex. By multifrequency electron paramagnetic resonance (35, 94, and 263 GHz) and 34 GHz ¹H ENDOR spectroscopies, we determined the hyperfine coupling (hfc) constants of the amino protons that establish RNH₂^• planarity and thus minimal perturbation of the reduction potential by the protein environment. The amount of Y in the isolated NH₂Y-RNR incorporated by infidelity of the tRNA/NH₂Y-RS pair was determined by a generally useful LC-MS method. This information is essential to the utility of this NH₂Y probe to study any protein of interest and is employed to address our previously reported activity associated with NH₂Y-substituted RNRs.

Simulation-Based Approach to Determining Electron Transfer Rates Using Square-Wave Voltammetry

The efficiency with which square-wave voltammetry differentiates faradic and charging currents makes it a particularly sensitive electroanalytical approach, as evidenced by its ability to measure nanomolar or even picomolar concentrations of electroactive analytes. Because of the relative complexity of the potential sweep it uses, however, the extraction of detailed kinetic and mechanistic information from square-wave data remains challenging. In response, we demonstrate here a numerical approach by which square-wave data can be used to determine electron transfer rates. Specifically, we have developed a numerical approach in which we model the height and the shape of voltammograms collected over a range of square-wave frequencies and amplitudes to simulated voltammograms as functions of the heterogeneous rate constant and the electron transfer coefficient. As validation of the approach, we have used it to determine electron transfer kinetics in both freely diffusing and diffusionless surface-tethered species, obtaining electron transfer kinetics in all cases in good agreement with values derived using non-square-wave methods.

Maximizing the Signal Gain of Electrochemical-DNA Sensors

Electrochemical DNA (E-DNA) sensors have emerged as a promising class of biosensors capable of detecting a wide range of molecular analytes (nucleic acids, proteins, small molecules, inorganic ions) without the need for exogenous reagents or wash steps. In these sensors, a binding-induced conformational change in an electrode-bound "probe" (a target-binding nucleic acid or nucleic-acid-peptide chimera) alters the location of an attached redox reporter, leading to a change in electron transfer that is typically monitored using square-wave voltammetry. Because signaling in this class of sensors relies on binding-induced changes in electron transfer rate, the signal gain of such sensors (change in signal upon the addition of saturating target) is dependent on the frequency of the square-wave potential pulse used to interrogate them, with the optimal square-wave frequency depending on the structure of the probe, the nature of the redox reporter, and other features of the sensor. Here, we show that, because it alters the driving force of the redox reaction and thus electron transfer kinetics, signal gain in this class of sensors is also strongly dependent on the amplitude of the square-wave potential pulse. Specifically, we show here that the simultaneous optimization of square-wave frequency and amplitude produces large (often more than 2-fold) increases in the signal gain of a wide range of E-DNA-type sensors.

Electrochemical aptasensor for lung cancer-related protein detection in crude blood plasma samples

The development of an aptamer-based electrochemical sensor for lung cancer detection is presented in this work. A highly specific DNA-aptamer, LC-18, selected to postoperative lung cancer tissues was immobilized onto a gold microelectrode and electrochemical measurements were performed in a solution containing the redox marker ferrocyanide/ferricyanide. The aptamer protein targets were harvested from blood plasma of lung cancer patients by using streptavidin paramagnetic beads and square wave voltammetry of the samples was performed at various concentrations. In order to enhance the sensitivity of the aptasensor, silica-coated iron oxide magnetic beads grafted with hydrophobic C8 and C4 alkyl groups were used in a sandwich detection approach. Addition of hydrophobic beads increased the detection limit by 100 times. The detection limit of the LC-18 aptasensor was enhanced by the beads to 0.023 ng/mL. The formation of the aptamer – protein – bead sandwich on the electrode surface was visualized by electron microcopy. As a result, the electrochemical aptasensor was able to detect cancer-related targets in crude blood plasma of lung cancer patients.

Cyclic Square Wave Voltammetry of Surface-Confined Quasireversible Electron Transfer Reactions

The theory for cyclic square wave voltammetry of surface-confined quasireversible electrode reactions is presented and experimentally verified. Theoretical voltammograms were calculated following systematic variation of empirical parameters to assess their impact on the shape of the voltammogram. From the trends obtained, diagnostic criteria for this mechanism were deduced. These criteria were experimentally confirmed using two well-established surface-confined analytes. When properly applied, these criteria will enable non-experts in voltammetry to assign the electrode reaction mechanism and accurately measure electrode reaction kinetics.