Measuring Quantum Capacitance in Energetically Addressable Molecular Layers

On the fundamentals of quantum rate theory and the long-range electron transport in respiratory chains

It has been shown that both the electron-transfer rate constant of an electrochemical reaction and the conductance quantum are correlated with the concept of quantum capacitance. This simple association between the two separate concepts has an entirely quantum rate basis that encompasses the electron-transfer rate theory as originally proposed by Rudolph A. Marcus whether statistical mechanics is appropriately taken into account. I have prepared a concise review of the quantum mechanical rate theory principles focused on its quantum electrodynamics character to demonstrate that it can reconcile the conflicting views established on attempting to use the super-exchange (supported on electron transfer) or 'metallic-like' (supported on conductance quantum) mechanisms separately to explain the highly efficient long-range electron transport observed in the respiratory processes of living cells. The unresolved issues related to long-range electron transport are clarified in light of the quantum rate theory with a discussion focused on Geobacter sulfurreducens films as a reference standard of the respiration chain. Theoretical analyses supported by experimental data suggest that the efficiency of respiration within a long-range electron transport path is intrinsically a quantum mechanical event that follows relativistic quantum electrodynamics principles as addressed by quantum rate theory.

Capacitive immunosensing at gold nanoparticle-decorated reduced graphene oxide electrodes fabricated by one-step laser nanostructuration

Nanostructured electrochemical biosensors have ushered in a new era of diagnostic precision, offering enhanced sensitivity and specificity for clinical biomarker detection. Among them, capacitive biosensing enables ultrasensitive label-free detection of multiple molecular targets. However, the complexity and cost associated with conventional fabrication methods of nanostructured platforms hinder the widespread adoption of these devices. This study introduces a capacitive biosensor that leverages laser-engraved reduced graphene oxide (rGO) electrodes decorated with gold nanoparticles (AuNPs). The fabrication involves laser-scribed GO-Au3+ films, yielding rGO-AuNP electrodes, seamlessly transferred onto a PET substrate via a press-stamping methodology. These electrodes have a remarkable affinity for biomolecular recognition after being functionalized with specific bioreceptors. For example, initial studies with human IgG antibodies confirm the detection capabilities of the biosensor using electrochemical capacitance spectroscopy. Furthermore, the biosensor can quantify CA-19-9 glycoprotein, a clinical cancer biomarker. The biosensor exhibits a dynamic range from 0 to 300 U mL-1, with a limit of detection of 8.9 U mL-1. Rigorous testing with known concentrations of a pretreated CA-19-9 antigen from human fluids confirmed their accuracy and reliability in detecting the glycoprotein. This study signifies notable progress in capacitive biosensing for clinical biomarkers, potentially leading to more accessible and cost-effective point-of-care solutions.

Detection of a Chirality-Induced Spin Selective Quantum Capacitance in α-Helical Peptides

Advanced Kelvin probe force microscopy simultaneously detects the quantum capacitance and surface potential of an α-helical peptide monolayer. These indicators shift when either the magnetic polarization or the enantiomer is toggled. A model based on a triangular quantum well in thermal and chemical equilibrium and electron-electron interactions allows for calculating the electrical potential profile from the measured data. The combination of the model and the measurements shows that no global charge transport is required to produce effects attributed to the chirality-induced spin selectivity effect. These experimental findings support the theoretical model of Fransson et al. Nano Letters 2021, 21 (7), 3026-3032. Measurements of the quantum capacitance represent a new way to test and refine theoretical models used to explain strong spin polarization due to chirality-induced spin selectivity.

Halogen bonding and chalcogen bonding mediated sensing

Sigma-hole interactions, in particular halogen bonding (XB) and chalcogen bonding (ChB), have become indispensable tools in supramolecular chemistry, with wide-ranging applications in crystal engineering, catalysis and materials chemistry as well as anion recognition, transport and sensing. The latter has very rapidly developed in recent years and is becoming a mature research area in its own right. This can be attributed to the numerous advantages sigma-hole interactions imbue in sensor design, in particular high degrees of selectivity, sensitivity and the capability for sensing in aqueous media. Herein, we provide the first detailed overview of all developments in the field of XB and ChB mediated sensing, in particular the detection of anions but also neutral (gaseous) Lewis bases. This includes a wide range of optical colorimetric and luminescent sensors as well as an array of electrochemical sensors, most notably redox-active host systems. In addition, we discuss a range of other sensor designs, including capacitive sensors and chemiresistors, and provide a detailed overview and outlook for future fundamental developments in the field. Importantly the sensing concepts and methodologies described herein for the XB and ChB mediated sensing of anions, are generically applicable for the development of supramolecular receptors and sensors in general, including those for cations and neutral molecules employing a wide array of non-covalent interactions. As such we believe this review to be a useful guide to both the supramolecular and general chemistry community with interests in the fields of host-guest recognition and small molecule sensing. Moreover, we also highlight the need for a broader integration of supramolecular chemistry, analytical chemistry, synthetic chemistry and materials science in the development of the next generation of potent sensors.

Untapped potential of 2D charge density wave chalcogenides as negative supercapacitor electrode materials

Two-dimensional (2D) materials have opened new avenues for the fabrication of ultrathin, transparent, and flexible functional devices. However, the conventional inorganic graphene analogues are either semiconductors or insulators with low electronic conductivity, hindering their use as supercapacitor electrode materials, which require high conductivity and large surface area. Recently, 2D charge density wave (CDW) materials, such as 2D chalcogenides, have attracted extensive attention as high performance functional nanomaterials in sensors, energy conversion, and spintronic devices. Herein, TaS2 is investigated as a potential CDW material for supercapacitors. The quantum capacitance (C Q) of the different TaS2 polymorphs (1T, 2H, and 3R) was estimated using density functional theory calculations for different numbers of TaS2 layers and alkali-metal ion (Li, Na and K) intercalants. The results demonstrate the potential of 2H- and 3R-polymorphs as efficient negative electrode materials for supercapacitor devices. The intercalation of K and Na ions in 1T-TaS2 led to an increase in the CQ with the intercalation of Li ions resulting in a decrease in the C Q. In contrast, Li ions were found to be the best intercalant for the 2H-TaS2 phase (highest C Q), while K ion intercalation was the best for the 3R-TaS2 phase. Moreover, increasing the number of layers of the1T-TaS2 resulted in the highest CQ. In contrast, C Q increases upon decreasing the number of layers of 2H-TaS2. Both 1T-MoS2 and 2H-TaS2 can be combined to construct a highly performing supercapacitor device as the positive and negative electrodes, respectively.

Continuous and Polarization-Tuned Redox Capacitive Anion Sensing at Electroactive Interfaces

Continuous, real-time ion sensing is of great value across various environmental and medical scenarios but remains underdeveloped. Herein, we demonstrate the potential of redox capacitance spectroscopy as a sensitive and highly adaptable ion sensing methodology, exemplified by the continuous flow sensing of anions at redox-active halogen bonding ferrocenylisophthalamide self-assembled monolayers. Upon anion binding, the redox distribution of the electroactive interface, and its associated redox capacitance, are reversibly modulated, providing a simple and direct sensory readout. Importantly, the redox capacitance can be monitored at a freely chosen, constant electrode polarization, providing a facile means of tuning both the sensor analytical performance and the anion binding affinity, by up to 1 order of magnitude. In surpassing standard voltammetric methods in terms of analytical performance and adaptability, these findings pave the way for the development of highly sensitive and uniquely tunable ion sensors. More generally, this methodology also serves as a powerful and unprecedented means of simultaneously modulating and monitoring the thermodynamics and kinetics of host-guest interactions at redox-active interfaces.

Sensing the quantized reactivity of graphene

We demonstrated that the variations measured in the quantum capacitance of single-layer graphene, envisioned here as a conceptual molecular model, depend on the chemical reactivity of the molecule and can be used as an analytical and sensing tool for environmental conditions. The variations are quantized as a function of the environmental changes and can be correlated with chemical reactivity indexes such as chemical hardness and softness. This not only constitutes a proof-of-principle that the chemical reactivity of graphene, as a single molecule, can be determined in situ by measuring the quantum capacitance, but also that these measurements can be used as an analytical tool.

Electron transfer and conductance quantum

The electron transfer rate constant of an electrochemical reaction and the conductance quantum are fundamental concepts that drive processes ranging from nanoscale electronic circuits to photosynthesis. In this paper, it is demonstrated that they are correlated with the concept of electrochemical capacitance. The relationship between electron transfer rate, quantum transport and electrochemical capacitance encompasses the theory of electron transfer rate proposed by Rudolph A. Marcus, and potentially unites electronics and electrochemistry.

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.

Reagentless Redox Capacitive Assaying of C-Reactive Protein at a Polyaniline Interface

Methods that enable the sensitive and label-free detection of protein biomarkers are well-positioned to make potentially significant contributions to diagnostics and derived personalized healthcare. In support of this goal, a myriad of (electrochemical) methodologies have been developed; recently, electrochemical capacitance spectroscopy emerged as an impedance-derived approach which, in employing surface-confined redox-transducers, circumvents problems associated with the use of solution-phase redox-probes. Herein, we expand this scope by utilizing phytic acid-doped polyaniline as a novel redox-charging polymer support enabling the reagentless assaying of C-reactive protein in serum with good sensitivity. The construction of the sensory interface via electropolymerization allows facile tuning of the surface coverage and redox (capacitive) properties of the polymers, which, in turn, modulate both assay selectivity, fouling, and sensitivity. Significantly, this methodology is readily extendable to a wide range of electrode materials and analytes.

Charge transport and energy storage at the molecular scale: from nanoelectronics to electrochemical sensing

This tutorial review considers how the fundamental quantized properties associated with charge transport and storage, particularly in molecular films, are linked in a manner that spans nanoscale electronics, electrochemistry, redox switching, and derived nanoscale sensing.

The nanoscopic principles of capacitive ion sensing interfaces

Herein we discuss the operational principles of molecular interfaces that specifically recruit ions from an electrolyte solution and report this in a reagentless capacitive manner.

The intrinsic volumetric capacitance of conducting polymers: pseudo-capacitors or double-layer supercapacitors?

The capacitance of conducting polymers represents one of the most important material parameters that in many cases determines the device and material performances. Despite a vast number of experimental studies, the theoretical understanding of the origin of the capacitance in conducting polymers remains unsatisfactory and appears even controversial. Here, we present a theoretical method, based on first principle capacitance calculations using density functional theory (DFT), and apply it to calculate the volumetric capacitance of two archetypical conducting polymers: poly(3,4-ethylene dioxythiophene) (PEDOT) and polypyrrole (PPy). Our aim is to achieve a quantitate description of the volumetric capacitance and to provide a qualitative understanding of its nature at the atomistic level. We find that the volumetric capacitance of PEDOT and PPy is ≈100 F cm-3 and ≈300 F cm-3, respectively, which is within the range of the corresponding reported experimental results. We demonstrate that the capacitance of conducting polymers originates from charges stored in atomistic Stern layers formed by counterions and doped polymeric chains. The Stern layers have a purely electrostatic origin, since the counterions do not form any bonds with the atoms of the polymeric chains, and no charge transfer between the counterions and conducting polymer takes place. This classifies the conducting polymers as double-layer supercapacitors rather than pseudo-capacitors. Further, we analyze contributions to the total capacitance originating from the classical capacitance C C and the quantum capacitance C Q, respectively, and find that the latter provides a dominant contribution. The method of calculations of the capacitance developed in the present paper is rather general and opens up the way for engineering and optimizing the capacitive response of the conducting polymers.

Serological point-of-care and label-free capacitive diagnosis of dengue virus infection

Dengue non-structural protein 1 (NS1 DENV) is considered a biomarker for dengue fever in an early stage. A sensitive and rapid assay for distinguishing positive from negative dengue infection samples is imperative for epidemic control and public health in tropical regions because it enables the development of instantaneous updatable databases and effective surveillance systems. Presently, we successfully report, for the first time, the use of the electrochemical capacitive method for the detection of NS1 DENV biomarker in human serum samples. By using a ferrocene-tagged peptide modified surface containing anti-NS1 as the receptor, it was possible to differentiate positive from negative samples with a p < 0.01 in a reagentless and label-free capacitive format. This capacitive assay had a cut-off of 1.36% (confidence interval of 99.99%); it therefore opens new avenues for developing miniature label-free electrochemical devices for infectious diseases.

Redox Capacitive Assaying of C-Reactive Protein at a Peptide Supported Aptamer Interface

Electrochemical immunosensors offer much in the potential translation of a lab based sensing capability to a useful "real world" platform. In previous work we have introduced an impedance-derived electrochemical capacitance spectroscopic analysis as supportive of a reagentless means of reporting on analyte target capture at suitably prepared mixed-component redox-active, antibody-modified interfaces. Herein we directly integrate receptive aptamers into a redox charging peptide support in enabling a label-free low picomolar analytical assay for C-reactive protein with a sensitivity that significantly exceeds that attainable with an analogous antibody interface.

Quantum capacitance as a reagentless molecular sensing element

The application of nanoscale capacitance as a transduction of molecular recognition relevant to molecular diagnostics is demonstrated. The energy-related signal relates directly to the electron occupation of quantized states present in readily fabricated molecular junctions such as those presented by redox switchable self-assembled molecular monolayers, reduced graphene oxide or redox-active graphene composite films, assembled on standard metallic or micro-fabricated electrodes. Sensor design is thus based on the response of a confined and resolved electronic density of states to target binding and the associated change in interfacial chemical potential. Demonstrated herein with a number of clinically important markers, this represents a new potent and ultrasensitive molecular detection enabling energy transducer principle capable of quantifying, in a single step and reagentless manner, markers within biological fluid.

Mapping the ionic fingerprints of molecular monolayers

Molecular dynamics simulations support a self-assembled monolayer specific energy barrier to solution-phase ions that once surmounted, the entrapped ions support a film embedded ionic capacitance and non-faradaic relaxation (mapping through electrochemical capacitance measurements). The associated capacitance can be assigned as a particular case of general electrochemical capacitance.

Mesoscopic behaviour of multi-layered graphene: the meaning of supercapacitance revisited

The double layer capacitive phenomena is just a particular case of a more general quantum mechanical approach, wherein the electrochemical capacitance is central hence governing the super-capacitance phenomenology in general.

Molecular conductance of double-stranded DNA evaluated by electrochemical capacitance spectroscopy

Conductance was measured in two different double stranded DNA (both with 20 bases), the more conducting poly(dG)-poly(dC) (ds-DNAc) and the less conducting poly(dA)-poly(dT) (ds-DNAi), by means of Electrochemical Capacitance Spectroscopy (ECS). The use of the ECS approach, exemplified herein with DNA nanowires, is equally a suitable and time-dependent advantageous alternative for conductance measurement of molecular systems, additionally allowing better understanding of the alignment existing between molecular scale conductance and electron transfer rate.

Density functional theory and an experimentally-designed energy functional of electron density

We demonstrate that capacitance spectroscopy experimentally allows access to the energy associated with the quantum mechanical ground state of many-electron systems.

Capacitance spectroscopy and density functional theory

We relate capacitance spectroscopy with density functional theory, providing a theoretical description of redox capacitance and its electrostatic and quantum terms.