Study of surface modification strategies to create glassy carbon-supported, aptamer-based sensors for continuous molecular monitoring

Electrochemical Sensors for Antibiotic Detection: A Focused Review with a Brief Overview of Commercial Technologies

Antimicrobial resistance (AMR) poses a significant threat to global health, powered by pathogens that become increasingly proficient at withstanding antibiotic treatments. This review introduces the factors contributing to antimicrobial resistance (AMR), highlighting the presence of antibiotics in different environmental and biological matrices as a significant contributor to the resistance. It emphasizes the urgent need for robust and effective detection methods to identify these substances and mitigate their impact on AMR. Traditional techniques, such as liquid chromatography-mass spectrometry (LC-MS) and immunoassays, are discussed alongside their limitations. The review underscores the emerging role of biosensors as promising alternatives for antibiotic detection, with a particular focus on electrochemical biosensors. Therefore, the manuscript extensively explores the principles and various types of electrochemical biosensors, elucidating their advantages, including high sensitivity, rapid response, and potential for point-of-care applications. Moreover, the manuscript investigates recent advances in materials used to fabricate electrochemical platforms for antibiotic detection, such as aptamers and molecularly imprinted polymers, highlighting their role in enhancing sensor performance and selectivity. This review culminates with an evaluation and summary of commercially available and spin-off sensors for antibiotic detection, emphasizing their versatility and portability. By explaining the landscape, role, and future outlook of electrochemical biosensors in antibiotic detection, this review provides insights into the ongoing efforts to combat the escalating threat of AMR effectively.

Self-assembled monolayer of poly(o-phenylenediamine)/silver core-shell hybrid-based enzyme-free impedimetric glucose sensor for blood samples

A core-shell hybrid of poly(o-phenylenediamine)/silver was prepared by a simple single-step process and self-assembled on a glassy carbon electrode to design an enzyme-free electrochemical glucose sensor. The working electrode was fabricated by self-assembling a dimethyl sulfoxide (DMSO) solution of the prepared hybrid on a glassy carbon electrode. The electrochemical properties of the fabricated electrode were analyzed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in an aqueous potassium chloride electrolyte containing the [Fe(CN)6]3-/4- redox couple. The electrochemical sensing responses towards varying concentrations of glucose were studied by CV and EIS in the same redox electrolyte medium. The poly(o-phenylenediamine)/silver core-shell hybrid-based sensor was found to show a reliable response in the EIS experiment over the CV experiment. By analyzing the EIS sensing responses, the experimental limit of detection was determined to be 10 μL of ∼80 mg per dL glucose solution in 25 mL of 1 (M) KCl electrolyte containing the 10-3 M [Fe(CN)6]3-/4- redox couple with a sensitivity of 298 Ω mg-1 dL cm-2. Excellent selectivity towards glucose was confirmed over various bioactive interfering biomolecules like ascorbic acid, dopamine, uric acid, lactose, fructose and sucrose. The glucose content in the human blood sample was verified by the designed sensor with almost 100% recovery.

Utilization of Spontaneous Alkyne-Gold Self-Assembly Chemistry as an Alternative Method for Fabricating Electrochemical Aptamer-Based Sensors

Electrochemical aptamer-based (E-AB) sensors are a promising class of biosensors which use structure-switching redox-labeled oligonucleotides (aptamers) codeposited with passivating alkanethiol monolayers on electrode surfaces to specifically bind and detect target analytes. Signaling in E-AB sensors is an outcome of aptamer conformational changes upon target binding, with the sequence of the aptamer imparting specificity toward the analyte of interest. The change in conformation translates to a change in electron transfer between the redox label attached to the aptamer and the underlying electrode and is related to analyte concentration, allowing specific electrochemical detection of nonelectroactive analytes. E-AB sensor measurements are reagentless with time resolutions of seconds or less and may be miniaturized into the submicron range. Traditionally these sensors are fabricated using thiol-on-gold chemistry. Here we present an alternate immobilization chemistry, gold-alkyne binding, which results in an increase in sensor lifetimes under ideal conditions by up to ∼100%. We find that gold-alkyne binding is spontaneous and supports efficient E-AB sensor signaling with analytical performance characteristics similar to those of thiol generated monolayers. The surface modification differs from gold-thiol binding only in the time and aptamer concentration required to achieve similar aptamer surface coverages. In addition, alkynated aptamers differ from their thiolated analogues only by their chemical handle for surface attachment, so any existing aptamers can be easily adapted to utilize this attachment strategy.

Beyond the Gold-Thiol Paradigm: Exploring Alternative Interfaces for Electrochemical Nucleic Acid-Based Sensing

Nucleic acid-based electrochemical sensors (NBEs) use oligonucleotides as affinity reagents for the detection of a variety of targets, ranging from small-molecule therapeutics to whole viruses. Because of their versatility in molecular sensing, NBEs are being developed broadly for diagnostic and biomedical research applications. Benchmark NBEs are fabricated via self-assembly of thiol-based monolayers on gold. Although robust for rapid prototyping, thiol monolayers suffer from limitations in terms of stability under voltage modulation and in the face of competitive ligands such as thiolated molecules naturally occurring in biofluids. Additionally, gold cannot be deployed as an NBE substrate for all biomedical applications, such as in cases where molecular measurements coupled to real-time, under-the-sensor tissue imaging is needed. Seeking to overcome these limitations, the field of NBEs is pursuing alternative ligands and electrode surfaces. In this perspective, I discuss new interface fabrication strategies that have successfully achieved NBE sensing, or that have the potential to allow NBE sensing on conductive surfaces other than gold. I hope this perspective will provide the reader with a fresh view of how future NBE interfaces could be constructed and will serve as inspiration for the pursuit of collaborative developments in the field of NBEs.

Controlling Gold Morphology Using Electrodeposition for the Preparation of Electrochemical Aptamer-Based Sensors

Nanostructuring of gold surfaces to enhance electroactive surface area has proven to significantly enhance the performance of electrochemical aptamer-based (E-AB) sensors, particularly for electrodes on the microscale. Unlike for sensors fabricated on polished gold surfaces, predicting the behavior of E-AB sensors on surfaces with varied gold morphologies becomes more intricate due to the effects of surface roughness and the shapes and sizes of surface features on supporting a self-assembled monolayer. In this study, we explored the impact of gold morphology characteristics on sensor performance, evaluating parameters such as signal change in response to the addition of the target analyte, aptamer probe packing density, and continuous sensing ability. Our findings reveal that surface area enhancement can either enhance or diminish sensor performance for gold nanostructured E-AB sensors, contingent upon the surface morphology. In particular, our results indicate that the aptamer packing density and target analyte signal change results are heavily dependent on gold nanostructure size and features. Sensing surfaces with larger nanoparticle diameters, which were prepared using electrodeposition at a constant potential, had a reduced aptamer packing density and exhibited diminished sensor performance. However, the equivalent packing density of polished electrodes did not yield the equivalent signal change. Other surfaces that were prepared using pulsed waveform electrodeposition achieved optimal signal change with a deposition time, tdep, of 120 s, and increased deposition time with enhanced electroactive surface area resulted in minimized signal changes and more rapid sensor degradation. By investigating sensing surfaces with varied morphologies, we have demonstrated that enhancing the electroactive surface does not always enhance the signal change of the sensor, and aptamer packing density alone does not dictate observed signal change trends. We anticipate that understanding how electrodeposition techniques enhance or diminish sensor performance will pave the way for further exploration of nanostructure-aptamer relationships, contributing to the future development of optimized, miniaturized electrochemical aptamer-based sensors for continuous, in vivo sensing.

A pendulum-type electrochemical aptamer-based sensor for continuous, real-time and stable detection of proteins

Continuous detection of proteins is crucial for health management and biomedical research. Electrochemical aptamer-based (E-AB) sensor that relies on binding affinity between a recognition oligonucleotide and its specific target is a versatile platform to fulfill this purpose. Yet, the vast majority of E-AB sensors are characterized by voltammetric methods, which suffer from signal drifts and low-frequency data acquisition during continuous operations. To overcome these limitations, we developed a novel E-AB sensor empowered by Gold nanoparticle-DNA Pendulum (GDP). Using chronoamperometric interrogation, the developed sensor enabled drift-resistant, high-frequency, and real-time monitoring of vascular endothelial growth factor (VEGF), a vital signaling protein that regulates angiogenesis, endothelial cell proliferation and vasculogenesis. We assembled VEGF aptamer-anchored GDP probes to a reduced graphene modified electrode, where a fast chronoamperometric current transient occurs as the GDP rapidly transport to the electrode surface. In the presence of target molecules, longer and concentration-dependent time decays were observed because of slower motion of the GDP in its bound state. After optimizing several decisive parameters, including composition ratios of GDP, probe density, and incubation time, the GDP empowered E-AB sensor achieves continuous, selective, and reversible monitoring of VEGF in both phosphate buffered saline (PBS) solutions and artificial urine with a wide detection range from 13 fM to 130 nM. Moreover, the developed sensor acquires signals on a millisecond timescale, and remains resistant to signal degradation during operation. This study offers a new approach to designing E-AB architectures for continuous biomolecular monitoring.

Editors' Choice-Review-The Future of Carbon-Based Neurochemical Sensing: A Critical Perspective

Carbon-based sensors have remained critical materials for electrochemical detection of neurochemicals, rooted in their inherent biocompatibility and broad potential window. Real-time monitoring using fast-scan cyclic voltammetry has resulted in the rise of minimally invasive carbon fiber microelectrodes as the material of choice for making measurements in tissue, but challenges with carbon fiber's innate properties have limited its applicability to understudied neurochemicals. Here, we provide a critical review of the state of carbon-based real-time neurochemical detection and offer insight into ways we envision addressing these limitations in the future. This piece focuses on three main hinderances of traditional carbon fiber based materials: diminished temporal resolution due to geometric properties and adsorption/desorption properties of the material, poor selectivity/specificity to most neurochemicals, and the inability to tune amorphous carbon surfaces for specific interfacial interactions. Routes to addressing these challenges could lie in methods like computational modeling of single-molecule interfacial interactions, expansion to tunable carbon-based materials, and novel approaches to synthesizing these materials. We hope this critical piece does justice to describing the novel carbon-based materials that have preceded this work, and we hope this review provides useful solutions to innovate carbon-based material development in the future for individualized neurochemical structures.

Stability of N-Heterocyclic Carbene Monolayers under Continuous Voltammetric Interrogation

N-Heterocyclic carbenes (NHCs) are promising monolayer-forming ligands that can overcome limitations of thiol-based monolayers in terms of stability, surface functionality, and reactivity across a variety of transition-metal surfaces. Recent publications have reported the ability of NHCs to support biomolecular receptors on gold substrates for sensing applications and improved tolerance to prolonged biofluid exposure relative to thiols. However, important questions remain regarding the stability of these monolayers when subjected to voltage perturbations, which is needed for applications with electrochemical platforms. Here, we investigate the ability of two NHCs, 1,3-diisopropylbenzimidazole and 5-(ethoxycarbonyl)-1,3-diisopropylbenzimidazole, to form monolayers via self-assembly from methanolic solutions of their trifluoromethanesulfonate salts. We compare the electrochemical behavior of the resulting monolayers relative to that of benchmark mercaptohexanol monolayers in phosphate-buffered saline. Within the -0.15 to 0.25 V vs Ag|AgCl voltage window, NHC monolayers are stable on gold surfaces, wherein they electrochemically perform like thiol-based monolayers and undergo similar reorganization kinetics, displaying long-term stability under incubation in buffered media and under continuous voltammetric interrogation. At negative voltages, NHC monolayers cathodically desorb from the electrode surface at lower bias (-0.1 V) than thiol-based monolayers (-0.5 V). At voltages more positive than 0.25 V, NHC monolayers anodically desorb from electrode surfaces at similar voltages to thiol-based monolayers. These results highlight new limitations to NHC monolayer stability imposed by electrochemical interrogation of the underlying gold electrodes. Our results serve as a framework for future optimization of NHC monolayers on gold for electrochemical applications, as well as structure-functionality studies of NHCs on gold.

Revolutionizing Precision Medicine: Exploring Wearable Sensors for Therapeutic Drug Monitoring and Personalized Therapy

Precision medicine, particularly therapeutic drug monitoring (TDM), is essential for optimizing drug dosage and minimizing toxicity. However, current TDM methods have limitations, including the need for skilled operators, patient discomfort, and the inability to monitor dynamic drug level changes. In recent years, wearable sensors have emerged as a promising solution for drug monitoring. These sensors offer real-time and continuous measurement of drug concentrations in biofluids, enabling personalized medicine and reducing the risk of toxicity. This review provides an overview of drugs detectable by wearable sensors and explores biosensing technologies that can enable drug monitoring in the future. It presents a comparative analysis of multiple biosensing technologies and evaluates their strengths and limitations for integration into wearable detection systems. The promising capabilities of wearable sensors for real-time and continuous drug monitoring offer revolutionary advancements in diagnostic tools, supporting personalized medicine and optimal therapeutic effects. Wearable sensors are poised to become essential components of healthcare systems, catering to the diverse needs of patients and reducing healthcare costs.

Explaining the Decay of Nucleic Acid-Based Sensors under Continuous Voltammetric Interrogation

Nucleic acid-based electrochemical sensors (NBEs) can support continuous and highly selective molecular monitoring in biological fluids, both in vitro and in vivo, via affinity-based interactions. Such interactions afford a sensing versatility that is not supported by strategies that depend on target-specific reactivity. Thus, NBEs have significantly expanded the scope of molecules that can be monitored continuously in biological systems. However, the technology is limited by the lability of the thiol-based monolayers employed for sensor fabrication. Seeking to understand the main drivers of monolayer degradation, we studied four possible mechanisms of NBE decay: (i) passive desorption of monolayer elements in undisturbed sensors, (ii) voltage-induced desorption under continuous voltammetric interrogation, (iii) competitive displacement by thiolated molecules naturally present in biofluids like serum, and (iv) protein binding. Our results indicate that voltage-induced desorption of monolayer elements is the main mechanism by which NBEs decay in phosphate-buffered saline. This degradation can be overcome by using a voltage window contained between -0.2 and 0.2 V vs Ag|AgCl, reported for the first time in this work, where electrochemical oxygen reduction and surface gold oxidation cannot occur. This result underscores the need for chemically stable redox reporters with more positive reduction potentials than the benchmark methylene blue and the ability to cycle thousands of times between redox states to support continuous sensing for long periods. Additionally, in biofluids, the rate of sensor decay is further accelerated by the presence of thiolated small molecules like cysteine and glutathione, which can competitively displace monolayer elements even in the absence of voltage-induced damage. We hope that this work will serve as a framework to inspire future development of novel sensor interfaces aiming to eliminate the mechanisms of signal decay in NBEs.

Continuous Square Wave Voltammetry for High Information Content Interrogation of Conformation Switching Sensors

Square wave voltammetry (SWV) is a voltammetric technique for measuring Faradaic current while minimizing contributions from non-Faradaic processes. In square wave voltammetry, the potential waveform applied to a working electrode and the current sampling protocols followed are designed to minimize contributions from non-Faradaic processes (i.e., double layer charging) to improve voltammetric sensitivity. To achieve this, the current is measured at the end of each forward and reverse potential pulse after allowing time for non-Faradaic currents to decay exponentially. A consequence of sampling current at the end of a potential pulse is that the current data from the preceding time of the potential pulse are discarded. These discarded data can provide information about the non-Faradaic contributions as well as information about the redox system including charge transfer rates. In this paper, we introduce continuous square wave voltammetry (cSWV), which utilizes the continuous collection of current to maximize the information content obtainable from a single voltammetry sweep eliminating the need for multiple scans. cSWV enables acquiring a multitude of voltammograms corresponding to various frequencies and, thus, different scan rates from a single sweep. An application that benefits significantly from cSWV is conformation switching, functional nucleic acid sensors. We demonstrate the utility of cSWV on two representative small molecules targeting electrochemical, aptamer-based sensors. Moreover, we show that cSWV provides comparable results to those obtained from traditional square wave voltammetry, but with cSWV, we are able to acquire dynamic information about the sensor surfaces enabling rapid calibration and optimization of sensing performance. We also demonstrate cSWV on soluble redox markers. cSWV can potentially become a mainstay technique in the field of conformation switching sensors.

Os(II/III) complex supports pH-insensitive electrochemical DNA-based sensing with superior operational stability than the benchmark methylene blue reporter

DNA-based electrochemical sensors use redox reporters to transduce affinity events into electrical currents. Ideally, such reporters must be electrochemically reversible, chemically stable for thousands of redox cycles, and tolerant to changing chemical environments. Here we report the first use of an Os(II/III) complex in DNA-based sensors, which undergoes pH-insensitive electron transfer with 35% better operational stability relative to the benchmark methylene blue, making it a promising reporter for continuous molecular monitoring applications where pH fluctuates with time.