Expanding the Scope of Protein-Detecting Electrochemical DNA "Scaffold" Sensors

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.

Rationally Designed DNA-Based Scaffolds and Switching Probes for Protein Sensing

The detection of a protein analyte and use of this type of information for disease diagnosis and physiological monitoring requires methods with high sensitivity and specificity that have to be also easy to use, rapid and, ideally, single step. In the last 10 years, a number of DNA-based sensing methods and sensors have been developed in order to achieve quantitative readout of protein biomarkers. Inspired by the speed, specificity, and versatility of naturally occurring chemosensors based on structure-switching biomolecules, significant efforts have been done to reproduce these mechanisms into the fabrication of artificial biosensors for protein detection. As an alternative, in scaffold DNA biosensors, different recognition elements (e.g., peptides, proteins, small molecules, and antibodies) can be conjugated to the DNA scaffold with high accuracy and precision in order to specifically interact with the target protein with high affinity and specificity. They have several advantages and potential, especially because the transduction signal can be drastically enhanced. Our aim here is to provide an overview of the best examples of structure switching-based and scaffold DNA sensors, as well as to introduce the reader to the rational design of innovative sensing mechanisms and strategies based on programmable functional DNA systems for protein detection.

Building a Nucleic Acid Nanostructure with DNA-Epitope Conjugates for a Versatile Approach to Electrochemical Protein Detection

The recent surge of effort in nucleic-acid-based electrochemical (EC) sensors has been fruitful, yet there remains a need for more generalizable EC platforms for sensing multiple classes of clinically relevant targets. We recently reported a nucleic acid nanostructure for simple, economical, and more generalizable EC readout of a range of analytes, including small molecules, peptides, proteins, and antibodies. The nanostructure is built through on-electrode enzymatic ligation of three oligonucleotides for attachment, binding, and signaling. However, the generalizable detection of larger proteins remains a challenge. Here, we adapted the sensor to quantify larger proteins in a more generic manner through conjugating the protein's minimized antibody-binding epitope to the central DNA strand. This concept was verified using creatine kinase (CK-MM), a biomarker of muscle damage and several disorders for which rapid clinical sensing is important. DNA-epitope conjugates permitted a competitive immunoassay for the CK protein at the electrode via square-wave voltammetry (SWV). Sensing through a signal-off mechanism, the anti-CK antibody limit of detection (LOD) was 5 nM with a response time as low as 3 min. Antibody displacement by native protein analytes gave a signal-on response with the CK sensing range from the LOD of 14 nM up to 100 nM, overlapping with the normal (nonelevated) human clinical range (3-37 nM), and the sensor was validated in 98% human serum. While a need for improved DNA-epitope conjugate purification was identified, overall, this approach allows the quantification of a generic protein- or peptide-binding antibody and should facilitate future quantitative EC readouts of clinically relevant proteins that were previously inaccessible to EC techniques.

Electrochemical Biosensors for Whole Blood Analysis: Recent Progress, Challenges, and Future Perspectives

Whole blood, as one of the most significant biological fluids, provides critical information for health management and disease monitoring. Over the past 10 years, advances in nanotechnology, microfluidics, and biomarker research have spurred the development of powerful miniaturized diagnostic systems for whole blood testing toward the goal of disease monitoring and treatment. Among the techniques employed for whole-blood diagnostics, electrochemical biosensors, as known to be rapid, sensitive, capable of miniaturization, reagentless and washing free, become a class of emerging technology to achieve the target detection specifically and directly in complex media, e.g., whole blood or even in the living body. Here we are aiming to provide a comprehensive review to summarize advances over the past decade in the development of electrochemical sensors for whole blood analysis. Further, we address the remaining challenges and opportunities to integrate electrochemical sensing platforms.

Conformational-switch biosensors as novel tools to support continuous, real-time molecular monitoring in lab-on-a-chip devices

Recent years have seen continued expansion of the functionality of lab on a chip (LOC) devices. Indeed LOCs now provide scientists and developers with useful and versatile platforms across a myriad of chemical and biological applications. The field still fails, however, to integrate an often important element of bench-top analytics: real-time molecular measurements that can be used to "guide" a chemical response. Here we describe the analytical techniques that could provide LOCs with such real-time molecular monitoring capabilities. It appears to us that, among the approaches that are general (i.e., that are independent of the reactive or optical properties of their targets), sensing strategies relying on binding-induced conformational change of bioreceptors are most likely to succeed in such applications.

Rapid Point-of-Care Electrochemical Sensor for the Detection of Cancer Tn Antigen Carbohydrate in Whole Unprocessed Blood

Improving outcomes for cancer patients during treatment and monitoring for cancer recurrence requires personalized care which can only be achieved through regular surveillance for biomarkers. Unfortunately, routine detection for blood-based biomarkers is cost-prohibitive using currently specialized laboratories. Using a rapid self-assembly sensing interface amenable to methods of mass production, we demonstrate the ability to detect and quantify a small carbohydrate-based cancer biomarker, Tn antigen (αGalNAc-Ser/Thr) in a small volume of blood, using a test format strip reminiscent of a blood glucose test. The detection of Tn antigen at picomolar levels is achieved through a new transduction mechanism based on the impact of Tn antigen interactions on the molecular dynamic motion of a lectin cross-linked lubricin antifouling brush. In tests performed on retrospective blood plasma samples from patients presenting three different tumor types, differentiation between healthy and diseased patients was achieved, highlighting the clinical potential for cancer monitoring.

Biosensor for heavy metals detection in wastewater: A review

Pollution due to heavy metals is a global issue in recent years. Initially, there were fewer contaminants, which has increased exponentially owing to rapid industrialization and various anthropogenic activities. Toxicity due to heavy metals causes a lot of health problems and organ system failure in human beings. It also affects other forms of living beings such as plants, animals and even the microbiota. This has been reported by various press reports and research findings. In this review, the production of heavy metals, associated effects on the environment and the technologies employed for detecting these heavy metals are comprehensively discussed. The analytical instruments, including biosensors, have been found to be more beneficial than other techniques. Biosensor exhibits numerous special features, such as reproducibility, reusability, linearity, sensitivity, selectivity, and stability. Over the last three years, biosensors have also had a detection limit of 65.36 ng/mL for heavy metals. The design of biosensors, features and types were also explained in detail. The limit of detection for the heavy metals in wastewater using biosensors was also included with recent references up to the last five years.

Controlling Dynamic DNA Reactions at the Surface of Single-Walled Carbon Nanotube Electrodes to Design Hybridization Platforms with a Specific Amperometric Readout

Carbon nanotube (CNT)-based electrodes are cheap, highly performing, and robust platforms for the fabrication of electrochemical sensors. Engineering programmable DNA nanotechnologies on the CNT surface can support the construction of new electrochemical DNA sensors providing an amperometric output in response to biomolecular recognition. This is a significant challenge, since it requires gaining control of specific hybridization processes and functional DNA systems at the interface, while limiting DNA physisorption on the electrode surface, which contributes to nonspecific signal. In this study, we provide design rules to program dynamic DNA structures at the surface of single-walled carbon nanotubes electrodes, showing that specific DNA interactions can be monitored through measurement of the current signal provided by redox-tagged DNA strands. We propose the use of pyrene as a backfilling agent to reduce nonspecific adsorption of reporter DNA strands and demonstrate the controlled formation of DNA duplexes on the electrode surface, which we then apply in the design and conduction of programmable DNA strand displacement reactions. Expanding on this aspect, we report the development of novel amperometric hybridization platforms based on artificial DNA structures templated by the small molecule melamine. These platforms enable dynamic strand exchange reactions orthogonal to conventional toehold-mediated strand displacement and may support new strategies in electrochemical sensing of biomolecular targets, combining the physicochemical properties of nanostructured carbon-based materials with programmable nucleic acid hybridization.

Extracellular Biomarkers of Inner Ear Disease and Their Potential for Point-of-Care Diagnostics

Rapid diagnostic testing has become a mainstay of patient care, using easily obtained samples such as blood or urine to facilitate sample analysis at the point-of-care. These tests rely on the detection of disease or organ-specific biomarkers that have been well characterized for a particular disorder. Currently, there is no rapid diagnostic test for hearing loss, which is one of the most prevalent sensory disorders in the world. In this review, potential biomarkers for inner ear-related disorders, their detection, and quantification in bodily fluids are described. The authors discuss lesion-specific changes in cell-free deoxyribonucleic acids (DNAs), micro-ribonucleic acids (microRNAs), proteins, and metabolites, in addition to recent biosensor advances that may facilitate rapid and precise detection of these molecules. Ultimately, these biomarkers may be used to provide accurate diagnostics regarding the site of damage in the inner ear, providing practical information for individualized therapy and assessment of treatment efficacy in the future.

Continuous monitoring of molecular biomarkers in microfluidic devices

The ability to monitor molecular targets is crucial in fields ranging from healthcare to industrial processing to environmental protection. Devices employing biomolecules to achieve this goal are called biosensors. Over the last half century researchers have developed dozens of different biosensor approaches. In this chapter we analyze recent advances in the biosensing field aiming at adapting these to the problem of continuous molecular monitoring in complex sample streams, and how the merging of these sensors with lab-on-a-chip technologies would be beneficial to both. To do so we discuss (1) the components that comprise a biosensor, (2) the challenges associated with continuous molecular monitoring in complex sample streams, (3) how different sensing strategies deal with (or fail to deal with) these challenges, and (4) the implementation of these technologies into lab-on-a-chip architectures.

MOF@COF Heterostructure Hybrid for Dual-Mode Photoelectrochemical-Electrochemical HIV-1 DNA Sensing

We developed a novel metal-organic framework (MOF)@covalent-organic framework (COF) hybrid with a hierarchical nanostructure and excellent photoactivity, which further acted as the bifunctional platform of a dual-mode photoelectrochemical (PEC) and electrochemical (EC) biosensor for detecting HIV-1 DNA via immobilizing the HIV-1 DNA probe. First, the presynthesized Cu-MOF nanoellipsoids were used as the template for the in situ growth of the COF network, which was synthesized using copper-phthalocyanine tetra-amine (CoPc-TA) and 2,9-bis[p-(formyl)phenyl]-1,10-phenanthroline as building blocks through the Schiff base condensation. In view of the large specific surface area, abundant reserved amino group, excellent electrochemical activity, and high photoactivity, the obtained Cu-MOF@CuPc-TA-COF heterostructure not only can serve as the sensitive platform for anchoring the HIV-1 DNA probe strands but also can be utilized as the signal transducers for PEC and EC biosensors. Thereby, the constructed biosensor shows the sensitive and selective analysis ability toward the HIV-1 target DNA via the complementary hybridization between probe and target DNA strands. The dual-mode PEC and EC measurements revealed that the Cu-MOF@CuPc-TA-COF-based biosensor displayed a wide linear detection range from 1 fM to 1 nM and an extremely low limit of detection (LOD) of 0.07 and 0.18 fM, respectively. In addition, the dual-mode PEC-EC biosensor also demonstrated remarkable selectivity, high stability, good reproducibility, and preferable regeneration ability, as well as acceptable applicability, for which the detected HIV-1 DNA in human serum showed good consistency with real concentrations. Thereby, the present work can open a new dual-mode PEC-EC platform for detecting HIV-1 DNA based on the porous-organic framework heterostructure.

A DNA Electrochemical Sensor via Terminal Protection of Small-Molecule-Linked DNA for Highly Sensitive Protein Detection

The qualitative and quantitative determination of marker protein is of great significance in the life sciences and in medicine. Here, we developed an electrochemical DNA biosensor for protein detection based on DNA self-assembly and the terminal protecting effects of small-molecule-linked DNA. This strategy is demonstrated using the small molecule biotin and its receptor protein streptavidin (SA). We immobilized DNA with a designed structure and sequence on the surface of the gold electrode, and we named it M1-Biotin DNA. M1-Biotin DNA selectively combines with SA to generate M1-Biotin-SA DNA and protects M1-Biotin DNA from digestion by EXO III; therefore, M1-Biotin DNA remains intact on the electrode surface. M1-Biotin-SA DNA was modified with methylene blue (MB); the MB reporter molecule is located near the surface of the gold electrode, which generates a substantial electrochemical signal during the detection of SA. Through this strategy, we can exploit the presence or absence of an electrochemical signal to provide qualitative target protein determination as well as the strength of the electrochemical signal to quantitatively analyze the target protein concentration. This strategy has been proven to be used for the quantitative analysis of the interaction between biotin and streptavidin (SA). Under optimal conditions, the detection limit of the proposed biosensor is as low as 18.8 pM, and the linear range is from 0.5 nM to 5 μM, showing high sensitivity. The detection ability of this DNA biosensor in complex serum samples has also been studied. At the same time, we detected the folate receptor (FR) to confirm that this strategy can be used to detect other proteins. Therefore, this electrochemical DNA biosensor provides a sensitive, low-cost, and fast target protein detection platform, which may provide a reliable and powerful tool for early disease diagnosis.

Hybridization Chain Reaction-Amplified Electrochemical DNA-Based Sensors Enable Calibration-Free Measurements of Nucleic Acids Directly in Whole Blood

Hybridization chain reaction (HCR) amplification strategy has been extensively explored for the application of electrochemical DNA-based sensors. Despite the enhancement in its sensitivity using the HCR, such sensor platform exhibited significant sensor-to-sensor variations in current due to variations in probe counts and lengths. To circumvent this, we are developing here a calibration-free "O-N" approach to generate a ratiometric, unitless value that is independent of these variations. Specifically, this approach employs two types of redox reporters, denoted as "One reporter" and "N reporters", with the former attached on the capture DNA and the latter on H1 and H2 strands. By optimizing the attachment sites of these reporters onto DNA strands, we demonstrate a significantly enhanced sensitivity of such sensor platform by four orders of magnitude, achieving accurate, calibration-free measurement of nucleic acids including ctDNA directly in undiluted whole blood without the requirement to calibrate each individual sensor.

Reagentless biomolecular analysis using a molecular pendulum

The development of reagentless sensors that can detect molecular analytes in biological fluids could enable a broad range of applications in personalized health monitoring. However, only a limited set of molecular inputs can currently be detected using reagentless sensors. Here, we report a sensing mechanism that is compatible with the analysis of proteins that are important physiological markers of stress, allergy, cardiovascular health, inflammation and cancer. The sensing method is based on the motion of an inverted molecular pendulum that exhibits field-induced transport modulated by the presence of a bound analyte. We measure the sensor's electric field-mediated transport using the electron-transfer kinetics of an attached reporter molecule. Using time-resolved electrochemical measurements that enable unidirectional motion of our sensor, the presence of an analyte bound to our sensor complex can be tracked continuously in real time. We show that this sensing approach is compatible with making measurements in blood, saliva, urine, tears and sweat and that the sensors can collect data in situ in living animals.

Strategies for Biomolecular Analysis and Continuous Physiological Monitoring

Portable devices capable of rapid disease detection and health monitoring are crucial to decentralizing diagnostics from clinical laboratories to the patient point-of-need. Although technologies have been developed targeting this challenge, many require the use of reporter molecules or reagents that complicate the automation and autonomy of sensors. New work in the field has targeted reagentless approaches to enable breakthroughs that will allow personalized monitoring of a wide range of biomarkers on demand. This Perspective focuses on the ability of reagentless platforms to revolutionize the field of sensing by allowing rapid and real-time analysis in resource-poor settings. First, we will highlight advantages of reagentless sensing techniques, specifically electrochemical detection strategies. Advances in this field, including the development of wearable and in situ sensors capable of real-time monitoring of biomarkers such as nucleic acids, proteins, viral particles, bacteria, therapeutic agents, and metabolites, will be discussed. Reagentless platforms which allow for wash-free, calibration free-detection with increased dynamic range are highlighted as a key technological advance for autonomous sensing applications. Furthermore, we will highlight remaining challenges which must be overcome to enable widespread use of reagentless devices. Finally, future prospects and potential breakthroughs in precision medicine that will arise as a result of further development of reagentless sensing approaches are discussed.

Interrogation of Electrochemical Aptamer-Based Sensors via Peak-to-Peak Separation in Cyclic Voltammetry Improves the Temporal Stability and Batch-to-Batch Variability in Biological Fluids

Electrochemical, aptamer-based (E-AB) sensors support continuous, real-time measurements of specific molecular targets in complex fluids such as undiluted serum. They achieve these measurements by using redox-reporter-modified, electrode-attached aptamers that undergo target binding-induced conformational changes which, in turn, change electron transfer between the reporter and the sensor surface. Traditionally, E-AB sensors are interrogated via pulse voltammetry to monitor binding-induced changes in transfer kinetics. While these pulse techniques are sensitive to changes in electron transfer, they also respond to progressive changes in the sensor surface driven by biofouling or monolayer desorption and, consequently, present a significant drift. Moreover, we have empirically observed that differential voltage pulsing can accelerate monolayer desorption from the sensor surface, presumably via field-induced actuation of aptamers. Here, in contrast, we demonstrate the potential advantages of employing cyclic voltammetry to measure electron-transfer changes directly. In our approach, the target concentration is reported via changes in the peak-to-peak separation, ΔE_P, of cyclic voltammograms. Because the magnitude of ΔE_P is insensitive to variations in the number of aptamer probes on the electrode, ΔE_P-interrogated E-AB sensors are resistant to drift and show decreased batch-to-batch and day-to-day variability in sensor performance. Moreover, ΔE_P-based measurements can also be performed in a few hundred milliseconds and are, thus, competitive with other subsecond interrogation strategies such as chronoamperometry but with the added benefit of retaining sensor capacitance information that can report on monolayer stability over time.

Conformational Changes of Immobilized Polythymine due to External Stressors Studied with Temperature-Controlled Electrochemical Microdevices

Conformational changes of single-stranded DNA (ssDNA) play an important role in a DNA strand's ability to bind to target ligands. A variety of factors can influence conformation, including temperature, ionic strength, pH, buffer cation valency, strand length, and sequence. To better understand the effects of these factors on immobilized DNA structures, we employ temperature-controlled electrochemical microsensors to study the effects of salt concentration and temperature variation on the conformation and motion of polythymine (polyT) strands of varying lengths (10, 20, 50 nucleotides). PolyT strands were tethered to a gold working electrode at the proximal end through a thiol linker via covalent bonding between the Au electrode and sulfur link, which can tend to decompose between a temperature range of 60 and 90 °C. The strands were also modified with an electrochemically active methylene blue (MB) moiety at the distal end. Electron transfer (eT) was measured by square wave voltammetry (SWV) and used to infer information pertaining to the average distance between the MB and the working electrode. We observe changes in DNA flexibility due to varying ionic strength, while the effects of increased DNA thermal motion are tracked for elevated temperatures. This work elucidates the behavior of ssDNA in the presence of a phosphate-buffered saline at NaCl concentrations ranging from 20 to 1000 mmol/L through a temperature range of 10-50 °C in 1° increments, well below the decomposition temperature range. The results lay the groundwork for studies on more complex DNA strands in conjunction with different chemical and physical conditions.

Detection of the SARS-CoV-2 spike protein in saliva with Shrinky-Dink© electrodes

Using the children's toy, Shrinky-Dink©, we present an aptamer-based electrochemical (E-AB) assay that recognizes the spike protein of SARS-CoV-2 in saliva for viral infection detection. The low-cost electrodes are implementable at population scale and demonstrate detection down to 1 ag mL-1 of the S1 subunit of the spike protein.

Building Biomimetic Potency Tests for Islet Transplantation

Diabetes is a disease of insulin insufficiency, requiring many to rely on exogenous insulin with constant monitoring to avoid a fatal outcome. Islet transplantation is a recent therapy that can provide insulin independence, but the procedure is still limited by both the availability of human islets and reliable tests to assess their function. While stem cell technologies are poised to fill the shortage of transplantable cells, better methods are still needed for predicting transplantation outcome. To ensure islet quality, we propose that the next generation of islet potency tests should be biomimetic systems that match glucose stimulation dynamics and cell microenvironmental preferences and rapidly assess conditional and continuous insulin secretion with minimal manual handing. Here, we review the current approaches for islet potency testing and outline technologies and methods that can be used to arrive at a more predictive potency test that tracks islet secretory capacity in a relevant context. With the development of potency tests that can report on islet secretion dynamics in a context relevant to their intended function, islet transplantation can expand into a more widely accessible and reliable treatment option for individuals with diabetes.

Detection of the SARS-CoV-2 spike protein in saliva with Shrinky-Dink© electrodes

Using the children's toy, Shrinky-Dink ©, we present an aptamer-based electrochemical (E-AB) assay that recognizes the spike protein of SARS-CoV-2 in saliva for viral infection detection. The low-cost electrodes are implementable at population scale and demonstrate detection down to 0.1 fg mL ^-1 of the S1 subunit of the spike protein.

Surpassing the detection limit and accuracy of the electrochemical DNA sensor through the application of CRISPR Cas systems

Robust developments of personalized medicine for next-generation healthcare highlight the need for sensitive and accurate point-of-care platforms for quantification of disease biomarkers. Broad presentations of clustered regularly interspaced short palindromic repeats (CRISPR) as an accurate gene editing tool also indicate that the high-specificity and programmability of CRISPR system can be utilized for the development of biosensing systems. Herein, we present a CRISPR Cas system enhanced electrochemical DNA (E-DNA) sensor with unprecedented sensitivity and accuracy. The principle of the E-DNA sensor is the target induced conformational change of the surface signaling probe (containing an electrochemical tag), leading to the variation of the electron transfer rate of the electrochemical tag. With the introduction of CRISPR cleavage activity into the E-DNA sensor, a more apparent signal change between with and without the presence of the target can be achieved. We compared the performance of Cas9 and Cas12a enhanced E-DNA sensor and optimized the chemical environment of CRISPR, achieving a femto-molar detection limit without enzymatic amplification. Moreover, we correlated the CRISPR cleavage signal with the original E-DNA signal as a strategy to indicate potential mismatches in the target sequence. Comparing with classic DNA electrochemistry based mutation detection strategy, CRISPR enhanced E-DNA sensor can determine the presence of a single mutation at an unknown concentration condition. Overall, we believe that the CRISPR enhanced E-DNA sensing strategy will be of especially high utility for point-of-care systems owing to the programmability, modularity, high-sensitivity and high-accuracy.

E-DNA scaffold sensors and the reagentless, single-step, measurement of HIV-diagnostic antibodies in human serum

The multiplexed, point-of-care measurement of specific antibodies could improve the speed with which diseases are diagnosed and their treatment initiated. To this end, we are developing E-DNA scaffold sensors, which consist of a rigid, nucleic acid "scaffold" attached on one end to an electrode and presenting both a redox reporter and an epitope on the other. In the absence of antibody, the reporter efficiently transfers electrons when interrogated electrochemically. Binding-induced steric hindrance limits movement, reducing electron transfer in a manner that is both easily measured and quantitatively related to target concentration. Previously we have used monoclonal antibodies to explore the analytical performance of E-DNA sensors, showing that they support the rapid, single-step, quantitative detection of multiple antibodies in small volume samples. Here, in contrast, we employ authentic human samples to better explore the platform's clinical potential. Specifically, we developed E-DNA sensors targeting three HIV-specific antibodies and then compared the analytical and clinical performance of these against those of gold standard serological techniques. Doing so we find that, although the multistep amplification of an ELISA leads to a lower detection limits, the clinical sensitivity of ELISAs, E-DNA sensors and lateral-flow dipsticks are indistinguishable across our test set. It thus appears that, by merging the quantitation and multiplexing of ELISAs with the convenience and speed of dipsticks, E-DNA scaffold sensors could significantly improve on current serological practice.

Polymer scaffold layers of screen-printed electrodes for homogeneous deposition of silver nanoparticles: application to the amperometric detection of hydrogen peroxide

A method is described for electrochemical oxidation of polymers on the surface of screen-printed electrodes (SPCE). These act as scaffold layers for homogeneous deposition of silver nanoparticles (AgNPs). Hexamethylenediamine (HMDA) and poly(ethylene glycol) were immobilized on the SPCE surface via electrochemical oxidation. AgNPs were then electrodeposited on the scaffolds on the SPCE. This type of different carbon chain containing materials like PEG and HMDA act as big tunnels for electron mobility and are useful for the homogenous deposition of AgNPs on the SPCE surface without agglomeration. The resulting sensor was applied to the determination of hydrogen peroxide (H₂O₂) as a model analyte. It is found to display favorable catalytic and conductive properties towards the reduction of H₂O₂. Cyclic voltammetry and amperometry revealed that the modified electrode performs better than other modified SPCEs. Best operated at a potential of around -0.61 V (vs Ag|AgCl), the amperometric response is linear in the 10-180 μM H₂O₂ concentration range and the detection limit is 1.5 μM. The sensor is stable and reproducible. The resultant sensor was appplied to toothpaste analysis, and good recovery values were gained. Graphical abstractSchematic representation of electropolymerization of poly(ethylene glycol) and hexamethylenediamine scaffold layers on screen-printed electrodes for homogeneous electrodeposition of silver nanoparticles. This electrode was applied for the amperometric determination of hydrogen peroxide.

Structural Changes of Mercaptohexanol Self-Assembled Monolayers on Gold and Their Influence on Impedimetric Aptamer Sensors

Despite a large number of publications describing biosensors based on electrochemical impedance spectroscopy (EIS), little attention has been paid to the stability and reproducibility issues of the sensor interfaces. In this work, the stability and reproducibility of faradaic EIS analyses on the aptamer/mercaptohexanol (MCH) self-assembled monolayer (SAM)-functionalized gold surfaces in ferri- and ferrocyanide solution were systematically evaluated prior to and after the aptamer-probe DNA hybridization. It is shown that the EIS data exhibited significant drift, and this significantly affected the reproducibility of the EIS signal of the hybridization. As a result, no significant difference between the charge transfer resistance (R_CT) changes induced by the aptamer-target DNA hybridization and that caused by the drift could be identified. A conditioning of the electrode in the measurement solution for more than 12 h was required to reach a stable R_CT baseline prior to the aptamer-probe DNA hybridization. The monitored drift in R_CT and double layer capacitance during the conditioning suggests that the MCH SAM on the gold surface reorganized to a thinner but more closely packed layer. We also observed that the hot binding buffer used in the following aptamer-probe DNA hybridization process could induce additional MCH and aptamer reorganization, and thus further drift in R_CT. As a result, the R_CT change caused by the aptamer-probe DNA hybridization was less than that caused by the hot binding buffer (blank control experiment). Therefore, it is suggested that the use of high temperature in the EIS measurement should be carefully evaluated or avoided. This work provides practical guidelines for the EIS measurements. Moreover, because SAM-functionalized gold electrodes are widely used in biosensors, for example, DNA sensors, an improved understanding of the origin of the observed drift is very important for the development of well-functioning and reproducible biosensors.

Open Source Software for the Real-Time Control, Processing, and Visualization of High-Volume Electrochemical Data

Electrochemical sensors are major players in the race for improved molecular diagnostics due to their convenience, temporal resolution, manufacturing scalability, and their ability to support real-time measurements. This is evident in the ever-increasing number of health-related electrochemical sensing platforms, ranging from single-measurement point-of-care devices to wearable devices supporting immediate and continuous monitoring. In support of the need for such systems to rapidly process large data volumes, we describe here an open-source, easily customizable, multiplatform compatible program for the real-time control, processing, and visualization of electrochemical data. The software’s architecture is modular and fully documented, allowing the easy customization of the code to support the processing of voltammetric (e.g., square-wave and cyclic) and chronoamperometric data. The program, which we have called Software for the Analysis and Continuous Monitoring of Electrochemical Systems (SACMES), also includes a graphical interface allowing the user to easily change analysis parameters (e.g., signal/noise processing, baseline correction) in real-time. To demonstrate the versatility of SACMES we use it here to analyze the real-time data output by (1) the electrochemical, aptamer-based measurement of a specific small-molecule target, (2) a monoclonal antibody-detecting DNA-scaffold sensor, and (3) the determination of the folding thermodynamics of an electrode-attached, redox-reporter-modified protein.

An electrochemical scaffold sensor for rapid syphilis diagnosis

The faster a disease can be diagnosed, the sooner effective treatment can be initiated, motivating a drive to replace standard laboratory techniques with point-of-care technologies that return answers in minutes rather than hours. Thus motivated, we describe the development of an E-DNA scaffold sensor for the rapid and convenient measurement of antibodies diagnostic of syphilis. To achieve this (and in contrast to previous sensors of this class, which relied on single, linear epitopes for detection), we utilized a near full-length antigen as the sensor's recognition element, allowing us to simultaneously display multiple epitopes. The resultant sensor is able to detect antibodies against Treponema pallidum pallidum, the causative agent of syphilis, at clinically relevant concentrations in samples in less than 10 min. Preliminary results obtained using sero-positive and sero-negative human samples suggest the clinical sensitivity and specificity of the approach compare well to current gold-standard tests, while being simple and rapid enough to deploy at the point of care.

A Nucleic Acid Nanostructure Built through On-Electrode Ligation for Electrochemical Detection of a Broad Range of Analytes

For an assay to be most effective in point-of-care clinical analysis, it needs to be economical, simple, generalizable, and free from tedious workflows. While electrochemistry-based DNA sensors reduce instrumental costs and eliminate complicated procedures, there remains a need to address probe costs and generalizability, as numerous probes with multiple conjugations are needed to quantify a wide range of biomarkers. In this work, we have opened a route to circumvent complicated multiconjugation schemes using enzyme-catalyzed probe construction directly on the surface of the electrode. With this, we have created a versatile DNA nanostructure probe and validated its effectiveness by quantification of proteins (streptavidin, anti-digoxigenin, anti-tacrolimus) and small molecules (biotin, digoxigenin, tacrolimus) using the same platform. Tacrolimus, a widely prescribed immunosuppressant drug for organ transplant patients, was directly quantified with electrochemistry for the first time, with the assay range matching the therapeutic index range. Finally, the stability and sensitivity of the probe was confirmed in a background of minimally diluted human serum.

Recent Advances on Electrochemical Biosensing Strategies toward Universal Point-of-Care Systems

A number of very recently developed electrochemical biosensing strategies are promoting electrochemical biosensing systems into practical point-of-care applications. The focus of research endeavors has transferred from detection of a specific analyte to the development of general biosensing strategies that can be applied for a single category of analytes, such as nucleic acids, proteins, and cells. In this Minireview, recent cutting-edge research on electrochemical biosensing strategies are described. These developments resolved critical challenges regarding the application of electrochemical biosensors to practical point-of-care systems, such as rapid readout, simple biosensor fabrication method, ultra-high detection sensitivity, direct analysis in a complex biological matrix, and multiplexed target analysis. This Minireview provides general guidelines both for scientists in the biosensing research community and for the biosensor industry on development of point-of-care system, benefiting global healthcare.

Guide to Selecting a Biorecognition Element for Biosensors

Biosensors are powerful diagnostic tools defined as having a biorecognition element for analyte specificity and a transducer for a quantifiable signal. There are a variety of different biorecognition elements, each with unique characteristics. Understanding the advantages and disadvantages of each biorecognition element and their influence on overall biosensor performance is crucial in the planning stages to promote the success of novel biosensor development. Therefore, this review will focus on selecting the optimal biorecognition element in the preliminary design phase for novel biosensors. Included is a review of the typical characteristics and binding mechanisms of various biorecognition elements, and how they relate to biosensor performance characteristics, specifically sensitivity, selectivity, reproducibility, and reusability. The goal is to point toward language needed to improve the design and development of biosensors toward clinical success.