Squarewave voltammetry (SWV) is a form of linear potential sweep voltammetry that uses a combined square wave and staircase potential applied to a stationary electrode. [1] It has found numerous applications in various fields, including within medicinal and various sensing communities.
When first reported by Barker in 1957, [2] the working electrode utilized was primarily a dropping mercury electrode (DME). When using a DME, the surface area of the mercury drop is constantly changing throughout the course of the experiment; for this reason, complex mathematical modeling was at times required in order to analyze collected electrochemical data. The squarewave voltammetric technique allowed for the collection of the desired electrochemical data within one mercury drop, meaning that the need for mathematical modeling to account for the changing working electrode surface area was no longer needed. In short, the introduction and development of this technique allowed for the rapid collection of reliable and easily reproducible electrochemical data using DME or SDME working electrodes. With continued improvements from many electrochemists (particularly the Osteryoungs), SWV is now one of the primary voltammetric techniques available on modern potentiostats. [3]
In a squarewave voltammetric experiment, the current at a (usually stationary) working electrode is measured while the potential between the working electrode and a reference electrode is pulsed forward and backward at a constant frequency. The potential waveform can be viewed as a superposition of a regular squarewave onto an underlying staircase (see figure above); in this sense, SWV can be considered a modification of staircase voltammetry.
The current is sampled at two times - once at the end of the forward potential pulse and again at the end of the reverse potential pulse (in both cases immediately before the potential direction is reversed). As a result of this current sampling technique, the contribution to the current signal resulting from capacitive (sometimes referred to as non-faradaic or charging) current is minimal. As a result of having current sampling at two different instances per squarewave cycle, two current waveforms are collected - both have diagnostic value, and are therefore preserved. When viewed in isolation, the forward and reverse current waveforms mimic the appearance of a cyclic voltammogram (which corresponds to the anodic or cathodic halves, however, is dependent upon experimental conditions).
Despite both the forward and reverse current waveforms having diagnostic worth, it is almost always the case in SWV for the potentiostat software to plot a differential current waveform derived by subtracting the reverse current waveform from the forward current waveform. This differential curve is then plotted against the applied potential. Peaks in the differential current vs. applied potential plot are indicative of redox processes, and the magnitudes of the peaks in this plot are proportional to the concentrations of the various redox active species according to:
where Δip is the differential current peak value, A is the surface area of the electrode, C0* is the concentration of the species, D0 is the diffusivity of the species, tp is the pulse width, and ΔΨp is a dimensionless parameter which gauges the peak height in SWV relative to the limiting response in normal pulse voltammetry. [4]
It is important to note that in squarewave voltammetric analyses, the diffusion layer is not renewed between potential cycles. Thus, it is not possible/accurate to view each cycle in isolation; the conditions present for each cycle is a complex diffusion layer which has evolved through all prior potential cycles. The conditions for a particular cycle are also a function of electrode kinetics, along with other electrochemical considerations.
Because of the minimal contributions from non-faradaic currents, the use of a differential current plot instead of separate forward and reverse current plots, and significant time evolution between potential reversal and current sampling, high sensitivity screening can be obtained utilizing SWV. For this reason, squarewave voltammetry has been utilized in numerous electrochemical measurements and can be viewed as an improvement to other electroanalytical techniques. For instance, SWV suppresses background currents much more effectively than cyclic voltammetry - for this reason, analyte concentrations on the nanomolar scale can be registered utilizing SWV over CV.
SWV analysis has been used recently[ when? ] in the development of a voltammetric catechol sensor, [5] in the analysis of a large number of pharmaceuticals, [6] and in the development and construction of a 2,4,6-TNT and 2,4-DNT sensor [7]
In addition to being utilized in independent analyses, SWV has also been coupled with other analytical techniques, including but not limited to thin-layer chromatography (TLC) [8] and high-pressure liquid chromatography. [9]
Analytical technique is a method used to determine a chemical or physical property of a chemical substance, chemical element, or mixture. There is a wide variety of techniques used for analysis, from simple weighing to advanced techniques using highly specialized instrumentation.
In electrochemistry, cyclic voltammetry (CV) is a type of potentiodynamic measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is reached in a CV experiment, the working electrode's potential is ramped in the opposite direction to return to the initial potential. These cycles of ramps in potential may be repeated as many times as needed. The current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram trace. Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution or of a molecule that is adsorbed onto the electrode.
Differential pulse voltammetry (DPV) is a voltammetry method used to make electrochemical measurements and a derivative of linear sweep voltammetry or staircase voltammetry, with a series of regular voltage pulses superimposed on the potential linear sweep or stairsteps. The current is measured immediately before each potential change, and the current difference is plotted as a function of potential. By sampling the current just before the potential is changed, the effect of the charging current can be decreased.
A potentiostat is the electronic hardware required to control a three electrode cell and run most electroanalytical experiments. A Bipotentiostat and polypotentiostat are potentiostats capable of controlling two working electrodes and more than two working electrodes, respectively.
Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied. The analytical data for a voltammetric experiment comes in the form of a voltammogram which plots the current produced by the analyte versus the potential of the working electrode.
In electrochemistry, chronoamperometry is an analytical technique in which the electric potential of the working electrode is stepped and the resulting current from faradaic processes occurring at the electrode is monitored as a function of time. The functional relationship between current response and time is measured after applying single or double potential step to the working electrode of the electrochemical system. Limited information about the identity of the electrolyzed species can be obtained from the ratio of the peak oxidation current versus the peak reduction current. However, as with all pulsed techniques, chronoamperometry generates high charging currents, which decay exponentially with time as any RC circuit. The Faradaic current - which is due to electron transfer events and is most often the current component of interest - decays as described in the Cottrell equation. In most electrochemical cells, this decay is much slower than the charging decay-cells with no supporting electrolyte are notable exceptions. Most commonly a three-electrode system is used. Since the current is integrated over relatively longer time intervals, chronoamperometry gives a better signal-to-noise ratio in comparison to other amperometric techniques.
Polarography is a type of voltammetry where the working electrode is a dropping mercury electrode (DME) or a static mercury drop electrode (SMDE), which are useful for their wide cathodic ranges and renewable surfaces. It was invented in 1922 by Czech chemist Jaroslav Heyrovský, for which he won the Nobel prize in 1959. The main advantages of mercury as electrode material are as follows: 1) a large voltage window: ca. from +0.2 V to -1.8 V vs reversible hydrogen electrode (RHE). Hg electrode is particularly well-suited for studying electroreduction reactions. 2) very reproducible electrode surface, since mercury is liquid. 3) very easy cleaning of the electrode surface by making a new drop of mercury from a large Hg pool connected by a glass capillary.
Electroanalytical methods are a class of techniques in analytical chemistry which study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte. These methods can be broken down into several categories depending on which aspects of the cell are controlled and which are measured. The four main categories are potentiometry, amperometry, coulometry, and voltammetry.
In analytical chemistry, linear sweep voltammetry is a method of voltammetry where the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly in time. Oxidation or reduction of species is registered as a peak or trough in the current signal at the potential at which the species begins to be oxidized or reduced.
Staircase voltammetry is a derivative of linear sweep voltammetry. In linear sweep voltammetry the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly in time. Oxidation or reduction of species is registered as a peak or trough in the current signal at the potential at which the species begins to be oxidized or reduced.
In electrochemistry, the working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three-electrode system. Depending on whether the reaction on the electrode is a reduction or an oxidation, the working electrode is called cathodic or anodic, respectively. Common working electrodes can consist of materials ranging from inert metals such as gold or platinum, to inert carbon such as glassy carbon, boron-doped diamond or pyrolytic carbon, and mercury drop and film electrodes. Chemically modified electrodes are employed for the analysis of both organic and inorganic samples.
In electrochemistry, the auxiliary electrode, often also called the counter electrode, is an electrode used in a three-electrode electrochemical cell for voltammetric analysis or other reactions in which an electric current is expected to flow. The auxiliary electrode is distinct from the reference electrode, which establishes the electrical potential against which other potentials may be measured, and the working electrode, at which the cell reaction takes place.
In analytical chemistry, a rotating ring-disk electrode (RRDE) is a double working electrode used in hydrodynamic voltammetry, very similar to a rotating disk electrode (RDE). The electrode rotates during experiments inducing a flux of analyte to the electrode. This system used in electrochemical studies when investigating reaction mechanisms related to redox chemistry and other chemical phenomena.
In analytical chemistry, hydrodynamic voltammetry is a form of voltammetry in which the analyte solution flows relative to a working electrode. In many voltammetry techniques, the solution is intentionally left still to allow diffusion-controlled mass transfer. When a solution is made to flow, through stirring or some other physical mechanism, it is very important to the technique to achieve a very controlled flux or mass transfer in order to obtain predictable results. These methods are types of electrochemical studies which use potentiostats to investigate reaction mechanisms related to redox chemistry among other chemical phenomenon.
Bulk electrolysis is also known as potentiostatic coulometry or controlled potential coulometry. The experiment is a form of coulometry which generally employs a three electrode system controlled by a potentiostat. In the experiment the working electrode is held at a constant potential (volts) and current (amps) is monitored over time (seconds). In a properly run experiment an analyte is quantitatively converted from its original oxidation state to a new oxidation state, either reduced or oxidized. As the substrate is consumed, the current also decreases, approaching zero when the conversion nears completion.
A liquid metal electrode is an electrode that uses a liquid metal, such as mercury, Galinstan, and NaK. They can be used in electrocapillarity, voltammetry, and impedance measurements.
Fast-scan cyclic voltammetry (FSCV) is cyclic voltammetry with a very high scan rate (up to 1×106 V·s−1). Application of high scan rate allows rapid acquisition of a voltammogram within several milliseconds and ensures high temporal resolution of this electroanalytical technique. An acquisition rate of 10 Hz is routinely employed.
In electrochemistry, protein film voltammetry is a technique for examining the behavior of proteins immobilized on an electrode. The technique is applicable to proteins and enzymes that engage in electron transfer reactions and it is part of the methods available to study enzyme kinetics.
Electrochemical stripping analysis is a set of analytical chemistry methods based on voltammetry or potentiometry that are used for quantitative determination of ions in solution. Stripping voltammetry have been employed for analysis of organic molecules as well as metal ions. Carbon paste, glassy carbon paste, and glassy carbon electrodes when modified are termed as chemically modified electrodes and have been employed for the analysis of organic and inorganic compounds.
An electrochemical aptamer-based (E-AB) biosensor has the ability to generate an electrochemical signal in response to specific target binding in vivo The signal is measured by a change in Faradaic current passed through an electrode. E-AB sensors are advantageous over previously reported aptamer-based sensors, such as fluorescence generating aptamers, due to their ability to detect target binding in vivo with real-time measurements. An E-AB sensor is composed of a three-electrode cell: an interrogating electrode, a reference electrode, and a counter electrode. A signal is generated within the electrochemical cell then measured and analyzed by a potentiostat. There are several biochemical and electrochemical parameters to optimize signal gain for E-AB biosensors. The density packing of DNA or RNA aptamers, the ACV frequency administered by the potentiostat, and the chemistry of the SAM are all factors that determine signal gain as well as the signal to noise ratio of target binding. E-AB biosensors provide a promising mechanism for in-situ sensing and feedback-controlled drug administration.
