Impedance-derived Capacitive Spectroscopy Research Articles

Electrochemistry at 2D and 3D nanoelectrodes: The interplay between interface kinetics and surface density of states

Heterogenous Electron Transfer (HET) at electrode-electrolyte interfaces depends strongly on the morphological features or geometry of the nanostructure used for modifying the electrode surface. A swift HET results in faster interface kinetics, which has significant impact on the development/calibration of electrochemical devices like biomolecular sensors, supercapacitors, batteries and electrochromic platforms. The interface electrochemistry depends strongly on the electronic Density of States (DOS) of electrode materials. Over the past years, the 2D electron gas nanomaterials - primarily graphene, Graphene Oxide (GO) and Reduced Graphene Oxide (RGO), have garnered significant interest in electrochemical applications due to promising DOS features. However, the electroanalytical dependency on DOS of 3D nanostructures such as Zinc Oxide Tetrapods (ZnOT) is yet unexplored. The current work focusses on a comparative electrochemical analysis of interface kinetics at RGO (2D) and ZnOT (3D) coated screen printed electrodes, with the intention of selecting the suitable material geometry. The analyses were performed using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). While the dependence of HET on DOS of RGO and ZnOT nanomaterials were studied using both DFT analysis and impedance-derived capacitance spectroscopy – the latter giving insights on quantum capacitance. It was observed that, the 2D RGO nanostructures exhibit higher surface DOS near the Fermi level, along with a high quantum capacitance (∼345 nF) as compared to 3D ZnOT (∼276 nF). This results in enhanced HET at the former, thereby indicating its suitability in developing futuristic electrochemical devices for various applications as desired.

Porous 3D-graphene functionalized with MnO2 nanospheres and NiO nanoparticles as highly efficient electrodes for asymmetric capacitive deionization: Evaluation by impedance-derived capacitance spectroscopy

Capacitive deionization (CDI) is a novel desalination technology for brackish water purification. Here, we report the rational design of an efficient asymmetric CDI system considering several steps. First, three-dimensional graphene nanostructures (3DG) were synthesized using the hydrothermal method, followed by the freeze-drying process, and then drop casting onto the carbon fiber paper (CFP) substrate. Second, a facile and controllable electrodeposition technique was used for the in-situ synthesis of MnO2 nanospheres and NiO nanoparticles onto the CFP/3DG surface to prepare CFP/3DG/MnO2 cathode and CFP/3DG/NiO anode. Third, the prepared nanocomposite electrodes were used to fabricate an asymmetric CDI cell. Here, MnO2 (pI = 4.5) and NiO (pI = 10) nanostructures provide opposite surface charges due to their dissimilar isoelectric points, enhancing cations and anions electrosorption. The surface morphology, chemical and electrochemical characteristics, as well as the capacitive performance of the fabricated CFP/3DG/MnO2 and CFP/3DG/NiO electrodes, were investigated using SEM, EDX, XRD, FTIR, TGA, BET/BJH, and electrochemical techniques. Besides, impedance-derived capacitive analysis was used to evaluate the ions retention time on the fabricated electrodes. Finally, the fabricated CFP/3DG/NiO||CFP/3DG/MnO2 asymmetric CDI cell was used in water desalination with high reversibility, good regeneration rate, and high electrosorption capacity (21.01 mg g−1) for the initial salt concentration of 1000 mg L−1 in 1.2 V. These results suggest that the proposed asymmetric CDI system is a promising candidate for the efficient capacitive desalination process.

Measuring quantum conductance and capacitance of graphene using impedance-derived capacitance spectroscopy

Here, we demonstrate that impedance-derived capacitance spectroscopy is a useful method for measuring and analyzing the quantum of conductance and capacitance of a single-layer graphene sheet. Both quantum conductance and capacitance exhibit the expected traditional V-shape when measured as a function of bias potential (with the minima of both parameters being at the Dirac point) in a 1-Butyl-3-methylimidazolium tetrafluoroborate ionic liquid environment. Measurements were conducted at a specific low-frequency limit in which the conductance and capacitance were scanned as a function of bias potential to attain the quantum conductive and capacitive V-shapes. The V-shapes agree in terms of magnitude and profile with those reported in the literature using different methodologies and/or techniques. This validates impedance-derived capacitance spectroscopy as a suitable method to acquire key information of graphene and its derivatives. This method represents an alternative for rapid construction of both conductive and capacitive V-shapes of graphene in a single, simple experimental run.

Nanostructured functional peptide films and their application in C-reactive protein immunosensors

Peptides with an active redox molecule are incorporated into nanostructured films for electrochemical biosensors with stable and controllable physicochemical properties. In this study, we synthesized three ferrocene (Fc)-containing peptides with the sequence Fc-Glu-(Ala)n-Cys-NH2, which could form self-assembled monolayers on gold and be attached to antibodies. The peptide with two alanines (n = 2) yielded the immunosensor with the highest performance in detecting C-reactive protein (CRP), a biomarker of inflammation. Using electrochemical impedance-derived capacitive spectroscopy, the limit of detection was 240 pM with a dynamic range that included clinically relevant CRP concentrations. With a combination of electrochemical methods and polarization-modulated infrared reflection–absorption spectroscopy, we identified the chemical groups involved in the antibody-CRP interaction, and were able to relate the highest performance for the peptide with n = 2 to chain length and efficient packing in the organized films. These strategies to design peptides and methods to fabricate the immunosensors are generic, and can be applied to other types of biosensors, including in low cost platforms for point-of-care diagnostics.

Label-free Capacitive Diagnostics: Exploiting Local Redox Probe State Occupancy

An electrode surface confined redox group contributes to a substantial potential-dependent interfacial charging that can be sensitively probed and frequency-resolved by impedance-derived capacitance spectroscopy. In utilizing the sensitivity of this charging fingerprint to redox group environment, one can seek to generate derived sensory configurations. Exemplified here through the generation of mixed molecular films comprising ferrocene and antibody receptors to two clinically important targets, the label-free methodology is able to report on human prostatic acid phosphatase (PAP), a tumor marker, with a limit of detection of 11 pM and C-reactive protein with a limit of detection of 28 pM. Both assays exhibit linear ranges encompassing those of clinical value.

Fundamentals and Applications of Impedimetric and Redox Capacitive Biosensors

Electroanalyses have brought a huge amount to our understanding of interfaces generally. When applied to surfaces which have been specifically engineered so as to recruit targets from an analytical solution, potent sensors can be derived. These may be based on a multitude of different analytical methods all typified by specific requirements and surface configurations. This short review examines the application of amperometric and impedimetric methods to the detection of biomarkers of clinical relevance. Basic principles are introduced with examples at both planar and “nanofunctionalised” interfaces comprising immobilized antibodies/antigens and oligonucleotide receptors. A particular focus is made of new developments in impedance and impedance derived capacitance spectroscopy.

Label free redox capacitive biosensing

A surface confined redox group contributes to an interfacial charging (quantifiable by redox capacitance) that can be sensitively probed by impedance derived capacitance spectroscopy. In generating mixed molecular films comprising such redox groups, together with specific recognition elements (here antibodies), this charging signal is able to sensitively transduce the recognition and binding of specific analytes. This novel transduction method, exemplified here with C-reactive protein, an important biomarker of cardiac status and general trauma, is equally applicable to any suitably prepared interfacial combination of redox reporter and receptor. The assays are label free, ultrasensitive, highly specific and accompanied by a good linear range.

Elucidating Capacitance and Resistance Terms in Confined Electroactive Molecular Layers

Electrochemical analyses on confined electroactive molecular layers, herein exemplified with electroactive self-assembled monolayers, sample current contributions that are significantly influenced by additional nonfaradaic and uncompensated resistance effects that, though unresolved, can strongly distort redox analysis. Prior work has shown that impedance-derived capacitance spectroscopy approaches can cleanly resolve all contributions generated at such films, including those which are related to the layer dipolar/electrostatic relaxation characteristics. We show herein that, in isolating the faradaic and nonfaradaic contributions present within an improved equivalent circuit description of such interfaces, it is possible to accurately simulate subsequently observed cyclic voltammograms (that is, generated current versus potential patterns map accurately onto frequency domain measurements). Not only does this enable a frequency-resolved quantification of all components present, and in so doing, a full validation of the equivalent circuit model utilized, but also facilitates the generation of background subtracted cyclic voltammograms remarkably free from all but faradaic contributions.