Redox-tagged peptide for capacitive diagnostic assays

Capacitive spectroscopy as transduction mechanism for wearable biosensors: opportunities and challenges

Wearable sensors would revolutionize healthcare and personalized medicine by providing individuals with continuous and real-time data about their bodies and environments. Their integration into everyday life has the potential to enhance well-being, improve healthcare outcomes, and offer new opportunities for research. Capacitive sensors technology has great potential to enrich wearable devices, extending their use to more accurate physiological indicators. On the basis of capacitive sensors developed so far to monitor physical parameters, and taking into account the advances in capacitive biosensors, this work discusses the benefits of this type of transduction to design wearables for the monitoring of biomolecules. Moreover, it provides insights into the challenges that must be overcome to take advantage of capacitive transduction in wearable sensors for health.

Designing Quantum Capacitive Peptide Interfaces for Electroanalytical Applications

Redox-active moieties assembled on metallic interfaces have been shown to follow quantum mechanical rules, where the quantum capacitance of the interface (directly associated with the electronic structure of the redox-active moieties) plays a key role in the electron transfer dynamics of the interface. Modifying these interfaces with biological receptors has significant advantages (simplifying molecular diagnostics methods, reducing size, time, and cost while maintaining high sensitivity), enabling the fabrication of miniaturized electroanalytical devices that can compete with traditional ELISA and RT-PCR benchtop assay methods. Owing to their intrinsic characteristics, the use of peptide-based redox-active moieties is a promising chemical route for modifying metallic surfaces, resulting in a high quantum capacitive signal sensitivity. In the present work, different ferrocene-tagged peptides with a structure of Fc-Glu-XX-XX-Cys-NH2 (XX = serine, phenylalanine, glycine) were used to form self-assembled monolayers on gold. The feasibility of using these interfaces in an electroanalytical assay was verified by detecting the NS1 DENV (Dengue Virus) biomarker to compare the efficiency of peptide structures for biosensing purposes. Parameters such as the formal potential of the interface, normalized electronic density of states (DOS), quantum capacitance, and electron transfer rate constants were obtained for Ser-, Phe-, and Gly-peptides. The Gly-peptide structure presented the highest analytical performance for sensing NS1 with a sensitivity of 5.6% per decade and the lowest LOD (1.4 ng mL-1) and LOQ (2.6 ng mL-1), followed by Phe-peptide, whereas Ser-peptide had the lowest performance. This work demonstrates that the use of peptides to fabricate a self-assembled monolayer as a biosensor component has advantages for low-cost point-of-care diagnostics. It also shows that the performance of the sensing interface depends strongly on how the chemistry of the surface is designed as a whole, not only on the redox-active group.

Aptamer-functionalized capacitive biosensors

The growing use of aptamers as target recognition elements in label-free biosensing necessitates corresponding transducers that can be used in relevant environments. While popular in many fields, capacitive sensors have seen relatively little, but growing use in conjunction with aptamers for sensing diverse targets. Few reports have shown physiologically relevant sensitivity in laboratory conditions and a cohesive picture on how target capture modifies the measured capacitance has been lacking. In this review, we assess the current state of the field in three areas: small molecule, protein, and cell sensing. We critically analyze the proposed hypotheses on how aptamer-target capture modifies the capacitance, as many mechanistic postulations appear to conflict between published works. As the field matures, we encourage future works to investigate individual aptamer-target interactions and to interrogate the physical mechanisms leading to measured changes in capacitance. To this point, we provide recommendations on best practices for developing aptasensors with a particular focus on considerations for biosensing in clinical settings.

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.

Utilising Commercially Fabricated Printed Circuit Boards as an Electrochemical Biosensing Platform

Printed circuit boards (PCBs) offer a promising platform for the development of electronics-assisted biomedical diagnostic sensors and microsystems. The long-standing industrial basis offers distinctive advantages for cost-effective, reproducible, and easily integrated sample-in-answer-out diagnostic microsystems. Nonetheless, the commercial techniques used in the fabrication of PCBs produce various contaminants potentially degrading severely their stability and repeatability in electrochemical sensing applications. Herein, we analyse for the first time such critical technological considerations, allowing the exploitation of commercial PCB platforms as reliable electrochemical sensing platforms. The presented electrochemical and physical characterisation data reveal clear evidence of both organic and inorganic sensing electrode surface contaminants, which can be removed using various pre-cleaning techniques. We demonstrate that, following such pre-treatment rules, PCB-based electrodes can be reliably fabricated for sensitive electrochemical biosensors. Herein, we demonstrate the applicability of the methodology both for labelled protein (procalcitonin) and label-free nucleic acid (E. coli-specific DNA) biomarker quantification, with observed limits of detection (LoD) of 2 pM and 110 pM, respectively. The proposed optimisation of surface pre-treatment is critical in the development of robust and sensitive PCB-based electrochemical sensors for both clinical and environmental diagnostics and monitoring applications.

Introducing polymer conductance in diagnostically relevant transduction

In this work we demonstrate that an impedance derived capacitance method is able to cleanly resolve the resonant conductance characteristics of an electrode-confined polymer film. In decorating the film with receptors, this conductance is thereafter modulated by the capturing of specific targets, demonstrated herein with C-reactive protein. This entirely reagentless and single step marker quantification is relevant to the drive of moving assays to a scaleable format requiring minimal user intervention.

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Relaxation conductance was used to create a new transducer signal concept.

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This relaxation conductance transducer signal concept was proved to be valid.

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Conductive redox polymer was used as sensitive function for the new signal concept.

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C-reactive protein was successfully detected in suitable low concentrations.

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This transducer principle is equally applicable to any other target.

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 assaying of biomarkers for molecular diagnostics

The label-free analysis of biomarkers offers important advantages in developing point-of-care (PoC) biosensors. In contrast to label-based methodologies, such as ELISA, label-free analysis enables direct detection of targets without additional steps and labeled reagents. Nonetheless, label-free approaches require high sensitivity to detect the intrinsic features of a biomarker and low levels of nonspecific signals. Electrochemical capacitance, [Formula: see text], is a feature of electroactive nanoscale films that can be measured using electrochemical impedance spectroscopy. [Formula: see text] is promising as an electrochemical transducing signal for the development of high-sensitivity, reagentless and label-free molecular diagnostic assays. We used a proprietary ferrocene (Fc)-tagged peptide that is able to self-assemble onto gold electrodes (thicknesses <2 nm) to which any biological receptor can be coupled. When coupled with biological receptors (e.g., a monoclonal antibody), [Formula: see text] exhibited by the redox-tagged peptide changes as a function of the target concentration. We provide herein the steps for the qualitative and quantitative detection of dengue non-structural protein 1 (NS1) biomarker. Detection of NS1 can be used to diagnose dengue virus infection, which causes epidemics each year in tropical and subtropical regions of the world. Including the pre-treatment of the electrode surface, the analysis takes ~25 h. This time can be reduced to minutes if the electrode surface is fabricated separately, demonstrating that [Formula: see text] is promising for PoC applications. We hope this protocol will serve as a reference point for researchers and companies that intend to further develop capacitive devices for molecular diagnostic assays.

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.

Perspectives on and Precautions for the Uses of Electric Spectroscopic Methods in Label-free Biosensing Applications

Label-free approaches for molecular diagnostic applications are appealing because of their inherent point-of-care advantages. Nonetheless, technical challenges impose a limit on the use of these methods as will be discussed in this paper. Electrochemical spectroscopic methods, such as impedance and impedance-derived methods, are highly effective in the development of label-free diagnostic assays, but they require careful control of the dynamics of the sensing interface. We herein report the strength and challenges of the current methodologies associated with the applications of impedance and impedance-derived methods by focusing on their principles of operation. We demonstrate that the uses of their potentialities are not based on the know-how of these methods, but on how to combine the spectroscopic features with the required chemical design for the associated sensing interfaces. Predominantly, we illustrate how to use the resistive and capacitive terms of the interface to improve its sensitivity to the target. For instance, with the proper signal amplification strategy, limitations related to target-to-receptor size ratio can be overcome. The target-to-receptor ratio is one of the difficulties that we use as an example to illustrate how the sensing of an electric signal can be improved by controlling the properties of the interface on the nanometer scale.

Recent progress on the sensitive detection of cardiovascular disease markers by electrochemical-based biosensors

Cardiovascular disease is the most reason for deaths in all over the world. Hence, biomarkers of cardiovascular diseases are very crucial for diagnosis and management process. Biomarker detection demand is opened the important way in biosensor development field. Rapid, cheap, portable, precise, selective and sensitive biomarker sensing devices are needed at this point to detect and predict disease. A cardiac biomarker can be orderable as C-reactive protein, troponin I or T, myoglobin, tumor necrosis factor alpha, interleukin-6, interleukin-1, lipoprotein-associated phospholipase, low-density lipoprotein and myeloperoxidase. They are used for prediction of cardiovascular diseases. There are many methods for early diagnosis of cardiovascular diseases, but these have long time process and expensive devices. In recent studies, different biosensors have been developed to remove the problems in this field. Electrochemical devices and developed biosensors have many superiorities than others such as low cost, mobile, reliable, repeatable, need a little amount of solution. In this review, recent studies were presented as details for cardiovascular disease biomarkers detection using electrochemical methods.

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.

Controlled self-assembling structures of ferrocene-dipeptide conjugates composed of Ala-Pro-NHCH2CH2SH chain

Bioorganometallic ferrocene-dipeptide conjugates with the Ala-Pro-cysteamine chain, Fc-L-Ala-L-Pro-NHCH₂CH₂SH (2) and Fc-L-Ala-D-Pro-NHCH₂CH₂SH (4) (Fc=ferrocenoyl), were prepared by the reduction of the ferrocene-dipeptide conjugates, Fc-L-Ala-L-Pro-cystamine-L-Pro-L-Ala-Fc (1) or Fc-L-Ala-D-Pro-cystamine-D-Pro-L-Ala-Fc (3), respectively. Control of the self-assembling structures of the ferrocene-dipeptide conjugates was demonstrated by changing the chirality of the amino acid. The molecular structure of 2 composed of the L-Ala-L-Pro-NHCH₂CH₂SH chain confirmed the formation of intramolecular hydrogen bond of N-H⋯N pattern between the NH of cysteamine moiety and the nitrogen of Pro moiety. Furthermore, intermolecular hydrogen bonds between NH (Ala) and CO (Pro of another molecule) and between NH (cysteamine) and CO (the ferrocenoyl moiety of another molecule) were formed, wherein each molecule is connected to four neighboring molecules by continuous intermolecular hydrogen bonds to form the hydrogen-bonded molecular assembling structure. On the contrary, the left-handed helical assembly through an intermolecular hydrogen-bonding network of 15-membered intermolecularly hydrogen-bonded ring between NH (Ala) and CO (the ferrocenoyl moiety of another molecule) and between NH (the cysteamine moiety of another molecule) and CO (Ala) was observed in the crystal packing of 4 composed of the L-Ala-D-Pro-NHCH₂CH₂SH chain.

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.