Elucidating the Mechanisms Underlying the Signal Drift of Electrochemical Aptamer-Based Sensors in Whole Blood

Direct detection of doxorubicin in whole blood using a hydrogel-protected electrochemical aptamer-based biosensor

Electrochemical aptamer-based biosensors (EABs) have been developed for multiple important biomarkers for their convenient and real-time features. However, the application of EABs in complex biological fluids has been limited by the rapid loss of sensitivity and selectivity due to inactivation and biofouling of aptamer probes and electrodes. To address this issue, we report the preparation of a simple hydrogel-protected aptamer-based biosensor (HP-EAB) for direct detection of Doxorubicin (DOX) in whole blood. The aptamer provides excellent selectivity for the electrochemical sensor, allowing the prepared sensor to accurately detect DOX in a 50-fold diluted whole blood sample. The agarose hydrogel coating on the electrode surface allows the passage of small molecules while hindering the adsorption of biomolecules from the whole blood matrix to the electrode surface. The experimental results show that the prepared HP-EAB has high stability compared with the unprotected EAB, and the HP-EAB maintains excellent detection performance after 7 days of storage. The hydrogel coating can effectively reduce the non-specific response to the whole blood matrix and prolong the life-time of the sensor. When used to detect DOX in rabbit whole blood, the HP-EAB exhibited excellent detection performance with a detection limit of 25.9 nM (S/N = 3) and a detection range of 0.1 μM-50 μM. The developed HP-EAB provides an excellent platform for the rapid and accurate determination of important analytes in complex biological fluids.

Carboxylate-Terminated Electrode Surfaces Improve the Performance of Electrochemical Aptamer-Based Sensors

Electrochemical aptamer-based (EAB) sensors are a molecular measurement platform that enables the continuous, real-time measurement of a wide range of drugs and biomarkers in situ in the living body. EAB sensors are fabricated by depositing a thiol-modified, target-binding aptamer on the surface of a gold electrode, followed by backfilling with an alkanethiol to form a self-assembled monolayer. And while the majority of previously described EAB sensors have employed hydroxyl-terminated monolayers, a handful of studies have shown that altering the monolayer headgroup can strongly affect sensor performance. Here, using 4 different EAB sensors, we show that the mixed monolayers composed of mixtures of 6-carbon hydroxyl-terminated thiols and varying amounts of either 6- or 8-carbon, carboxylate-terminated thiols lead to improved EAB sensor performance. Specifically, the use of such mixed monolayers enhances the signal gain (the relative change in the signal seen upon target addition) for all tested sensors, often by several fold, both in buffer and whole blood at room temperature or physiological temperatures. Moreover, these improvements in gain are achieved without significant changes in the aptamer affinity or the stability of the resulting sensors. In addition to proving a ready means of improving EAB sensor performance, these results suggest that exploration of the chemistry of the electrode surface employed in such sensors could prove to be a fruitful means of advancing this unique in vivo sensing technology.

Engineered Aptamer-Derived Fluorescent Aptasensor: the Label-Free, Single-Step, Rapid Detection of Vancomycin in Clinical Samples

Currently, the reported vancomycin (VCM) aptamers, including the 3- (Kd  =  9.13 × 10-6 m) and 4-truncated variants (Kd = 45.5 × 10-6 m), are engineered via stem truncation of the VCM parent aptamer, which inevitably compromises their affinities, thus affecting their clinical application within the VCM therapeutic window of 6.9-13.8 × 10-6 m. Herein, the binding pocket of the VCM parent aptamer is elucidated for the first time and we implemented the Post-SELEX modification strategy involving truncation and mutagenesis to refined the VCM parent aptamer. This yielded a VCM aptamer (ABC20-11) with an intramolecular G-triplex, an enhanced thioflavin T (ThT) fluorescence intensity, and an improved affinity (Kd = 0.591 × 10-6 m) and specificity (one-methyl level) for VCM. Utilizing a portable fluorescence detector specifically designed for rapidly detecting VCM concentration and leveraging the competitive binding between VCM and ThT to ABC20-11, a label-free fluorescent aptasensor is developed. This aptasensor exhibits exceptional analytical performances across various clinical samples (serum, cerebrospinal fluid, and joint fluid), with corresponding linear ranges of 0.5-50, 0.5-40, and 0.5-50 × 10-6 m and detection limits at 0.11, 0.12, and 0.16 × 10-6 m, respectively. Consequently, the proposed VCM aptasensor displays considerable clinical value and potential for use in rapid VCM detection.

Continuous Biosensing to Monitor Acute Systemic Inflammation, a Diagnostic Need for Therapeutic Guidance

Continuous monitoring of acute inflammation can become a very important next step for guiding therapeutic interventions in severely ill patients. This Perspective discusses the current medical need for patients with acute inflammatory diseases and the potential of continuous biosensing technologies. First, we discuss biomarkers that could help to monitor the state of a patient with acute systemic inflammation based on theoretical studies and empirical data. Then, based on the state of the art, we describe sensing strategies that could be applied for the continuous monitoring of acute inflammatory biomarkers, followed by challenges that must be overcome. Nanoswitch-based continuous biosensors enable suitable measurement frequencies but still lack sensitivity, while regeneration risks lower sensor reliability. Developments are still needed in bioreceptors and molecular architectures, regeneration techniques, combined with suitable sampling and sample pretreatment methods, for bringing continuous biosensing of inflammation closer to reality. Furthermore, collaborations between healthcare professionals and scientists, regulatory bodies, and biosensor engineers are needed for a successful translation of sensing technologies from the laboratory to clinical practice.

Wearable Electrochemical Biosensors for Advanced Healthcare Monitoring

Recent advancements in wearable electrochemical biosensors have opened new avenues for on-body and continuous detection of biomarkers, enabling personalized, real-time, and preventive healthcare. While glucose monitoring has set a precedent for wearable biosensors, the field is rapidly expanding to include a wider range of analytes crucial for disease diagnosis, treatment, and management. In this review, recent key innovations are examined in the design and manufacturing underpinning these biosensing platforms including biorecognition elements, signal transduction methods, electrode and substrate materials, and fabrication techniques. The applications of these biosensors are then highlighted in detecting a variety of biochemical markers, such as small molecules, hormones, drugs, and macromolecules, in biofluids including interstitial fluid, sweat, wound exudate, saliva, and tears. Additionally, the review also covers recent advances in wearable electrochemical biosensing platforms, such as multi-sensory integration, closed-loop control, and power supply. Furthermore, the challenges associated with critical issues are discussed, such as biocompatibility, biofouling, and sensor degradation, and the opportunities in materials science, nanotechnology, and artificial intelligence to overcome these limitations.

Ferrocenyl Compounds as Alternative Redox Labels for Robust and Versatile Electrochemical Aptamer-Based Sensors

This study explores the potential of seven ferrocenyl (Fc) compounds with cross-linking groups as alternative redox labels to methylene blue (MB) for electrochemical aptamer-based (E-AB) sensors. The cross-linking efficiency, formal potential (E0'), and electrochemical durability of these compounds were evaluated. Compound Fc1a-X exhibited superior performance, characterized by efficient cross-linking, a moderate and pH-insensitive E0', and enhanced durability during repeated potential scans. The attachment of Fc1a-X, which includes a 3-carbon chain spacer and an N-hydroxysuccinimide-ester cross-linking group, to an amine-terminated monolayer on a Au electrode demonstrated high cross-linking efficiency, which is critical for achieving high sensitivity. The E0' of Fc1a-X attached to the aptamer monolayer was 0.14 V, which is within the optimal range of -0.2 to 0.2 V vs Ag/AgCl. Square wave voltammetry showed that the peak potential and current of Fc1a-X are pH-insensitive, which is critical for versatile use. In serum, Fc1a-X maintained stable peak current levels without a gradual decrease after an initial rapid decrease during the first 2 h with considerably less reduction over 12 h compared to MB. Using Fc1a-X as the redox label, an E-AB sensor effectively detected doxorubicin in serum, covering the clinical range. These findings suggest Fc1a-X as a promising candidate for developing robust, versatile, and sensitive E-AB sensors.

Effects of Physiological-Scale Variation in Cations, pH, and Temperature on the Calibration of Electrochemical Aptamer-Based Sensors

Electrochemical aptamer-based (EAB) sensors are the first technology supporting high-frequency, real-time, in vivo molecular measurements that is independent of the chemical reactivity of its targets, rendering it easily generalizable. As is true for all biosensors, however, EAB sensor performance is affected by the measurement environment, potentially reducing accuracy when this environment deviates from the conditions under which the sensor was calibrated. Here, we address this question by measuring the extent to which physiological-scale environmental fluctuations reduce the accuracy of a representative set of EAB sensors and explore the means of correcting these effects. To do so, we first calibrated sensors against vancomycin, phenylalanine, and tryptophan under conditions that match the average ionic strength, cation composition, pH, and temperature of healthy human plasma. We then assessed their accuracy in samples for which the ionic composition, pH, and temperature were at the lower and upper ends of their physiological ranges. Doing so, we find that physiologically relevant fluctuations in ionic strength, cation composition, and pH do not significantly harm EAB sensor accuracy. Specifically, all 3 of our test-bed sensors achieve clinically significant mean relative accuracy (i.e., better than 20%) over the clinically or physiologically relevant concentration ranges of their target molecules. In contrast, physiologically plausible variations away from the temperature used for calibration induce more substantial errors. With knowledge of the temperature in hand, however, these errors are easily ameliorated. It thus appears that physiologically induced changes in the sensing environment are likely not a major impediment to clinical application of this in vivo molecular monitoring technology.

Cold-hot Janus electrochemical aptamer-based sensor for calibration-free determination of biomolecules

Real-time, high-frequency measurements of pharmaceuticals, metabolites, exogenous antigens, and other biomolecules in biological samples can provide critical information for health management and clinical diagnosis. Electrochemical aptamer-based (EAB) sensor is a promising analytical technique capable of achieving these goals. However, the issues of insufficient sensitivity, frequent calibration and lack of adapted portable electrochemical device limit its practical application in immediate detection. In response we have fabricated an on-chip-integrated, cold-hot Janus EAB (J-EAB) sensor based on the thermoelectric coolers (TECs). Attributed to the Peltier effect, the enhanced/suppressed current response can be generated simultaneously on cold/hot sides of the J-EAB sensor. The ratio of the current responses on the cold and hot sides was used as the detection signal, enabling rapid on-site, calibration-free determination of small molecules (procaine) as well as macromolecules (SARS-CoV-2 spike protein) in single step, with detection limits of 1 μM and 10 nM, respectively. We have further demonstrated that the J-EAB sensor is effective in improving the ease and usability of the actual detection process, and is expected to provide a universal, low-cost, fast and easy potential analytical tool for other clinically important biomarkers, drugs or pharmaceutical small molecules.

Integrated Electrochemical Aptamer Biosensing and Colorimetric pH Monitoring via Hydrogel Microneedle Assays for Assessing Antibiotic Treatment

Current methods for therapeutic drug monitoring (TDM) have a long turnaround time as they involve collecting patients' blood samples followed by transferring the samples to medical laboratories where sample processing and analysis are performed. To enable real-time and minimally invasive TDM, a microneedle (MN) biosensor to monitor the levels of two important antibiotics, vancomycin (VAN) and gentamicin (GEN) is developed. The MN biosensor is composed of a hydrogel MN (HMN), and an aptamer-functionalized flexible (Flex) electrode, named HMN-Flex. The HMN extracts dermal interstitial fluid (ISF) and transfers it to the Flex electrode where sensing of the target antibiotics happens. The HMN-Flex performance is validated ex vivo using skin models as well as in vivo in live rat animal models. Data is leveraged from the HMN-Flex system to construct pharmacokinetic profiles for VAN and GEN and compare these profiles with conventional blood-based measurements. Additionally, to track pH and monitor patient's response during antibiotic treatment, an HMN is developed that employs a colorimetric method to detect changes in the pH, named HMN-pH assay, whose performance has been validated both in vitro and in vivo. Further, multiplexed antibiotic and pH detection is achieved by simultaneously employing the HMN-pH and HMN-Flex on live animals.

An electrochemical proximity assay (ECPA) for antibody detection incorporating flexible spacers for improved performance

A clever approach for biosensing is to leverage the concept of the proximity effect, where analyte binding to probes can be coupled to a second, controlled binding event such as short DNA strands. This analyte-dependent effect has been exploited in various sensors with optical or electrochemical readouts. Electrochemical proximity assays (ECPA) are more amenable to miniaturization and adaptation to the point-of-care, yet ECPA has been generally targeted toward protein sensing with antibody-oligonucleotide probes. Antibodies themselves are also important as biomarkers, since they are produced in bodily fluids in response to various diseases or infections, often in low amounts. In this work, by using antigen-DNA conjugates, we targeted an ECPA method for antibody sensing and showed that the assay performance can be greatly enhanced using flexible spacers in the DNA conjugates. After adding flexible polyethylene glycol (PEG) spacers at two distinct positions, the spacers ultimately increased the antibody-dependent current by a factor of 4.0 without significant background increases, similar to our recent work using thermofluorimetric analysis (TFA). The optimized ECPA was applied to anti-digoxigenin antibody quantification at concentrations ranging over two orders of magnitude, from the limit of detection of 300 pM up to 50 nM. The assay was functional in 90% human serum, where increased ionic strength was used to counteract double-layer repulsion effects at the electrode. This flexible-probe ECPA methodology should be useful for sensing other antibodies in the future with high sensitivity, and the mechanism for signal improvement with probe flexibility may be applicable to other DNA-based electrochemical sensor platforms.

Label-free, real-time monitoring of cytochrome C drug responses in microdissected tumor biopsies with a multi-well aptasensor platform

Functional assays on intact tumor biopsies can complement genomics-based approaches for precision oncology, drug testing, and organs-on-chips cancer disease models by capturing key therapeutic response determinants, such as tissue architecture, tumor heterogeneity, and the tumor microenvironment. Most of these assays rely on fluorescent labeling, a semiquantitative method best suited for single-time-point assays or labor-intensive immunostaining analysis. Here, we report integrated aptamer electrochemical sensors for on-chip, real-time monitoring of cytochrome C, a cell death indicator, from intact microdissected tissues with high affinity and specificity. The platform features a multi-well sensor layout and a multiplexed electronic setup. The aptasensors measure increases in cytochrome C in the supernatant of mouse or human microdissected tumors after exposure to various drug treatments. Because of the sensor's high affinity, it primarily tracks rising concentrations of cytochrome C, capturing dynamic changes during apoptosis. This approach could help develop more advanced cancer disease models and apply to other complex in vitro disease models, such as organs-on-chips and organoids.

Tuning N-heterocyclic carbene wingtips to form electrochemically stable adlayers on metals

Self-assembled monolayers (SAMs) are employed in electrochemical biosensors to passivate and functionalize electrode surfaces. These monolayers prevent the occurrence of undesired electrochemical reactions and act as scaffolds for coupling bioaffinity reagents. Thiols are the most common adlayer used for this application; however, the thiol-gold bond is susceptible to competitive displacement by naturally occurring solvated thiols in biological fluids, as well as to desorption under continuous voltage interrogation. To overcome these issues, N-heterocyclic carbene (NHC) monolayers have been proposed as an alternative for electrochemical biosensor applications due to the strong carbon-gold bond. To maximize the effectiveness of NHCs for SAMs, a thorough understanding of both the steric effects of wingtip substituents and NHC precursor type to the passivation of electrode surfaces is required. In this study, five different NHC wingtips as well as two kinds of NHC precursors were evaluated. The best performing NHC adlayers can be cycled continuously for four days (over 30 000 voltammetric cycles) without appreciably desorbing from the electrode surface. Benchmark thiol monolayers, in contrast, rapidly desorb after only twelve hours. Investigations also show NHC adlayer formation on other biosensor-relevant electrodes such as platinum and palladium.

Simplified Electrochemical Approach for End-Point Yet Quantitative Detection of Nucleic Acids in Resource-Limited Settings

Nucleic acid detection plays a crucial role in various aspects of health care, necessitating accessible and reliable quantification methods, especially in resource-limited settings. This work presents a simplified electrochemical approach for end-point yet quantitative nucleic acid detection. By elevating the concentration of redox species and choosing potential as the signals, we achieved enhanced signal robustness, even in the presence of interfering substances. Leveraging this robustness, we accurately measured pH-induced redox potential changes in methylene blue solution for end-point nucleic acid detection after loop-mediated isothermal amplification (LAMP). Our method demonstrated quantitative detection of the SARS-CoV-2 N gene and human ATCB gene and successful discrimination of the human BRAF V600E mutation, comparable in sensitivity to commercial kits. The developed user-friendly electrochemical method offers a simplified and reliable approach for end-point yet quantitative detection of nucleic acids, potentially expanding the benefits of nucleic acid testing in resource-limited settings.

Rapid nanomolar detection of cocaine in biofluids by electrochemical aptamer-based sensor with low-temperature effect for drugged driving screening

Cocaine is one of the most abused illicit drugs, and its abuse damages the central nervous system and can even lead directly to death. Therefore, the development of simple, rapid and highly sensitive detection methods is crucial for the prevention and control of drug abuse, traffic accidents and crime. In this work, an electrochemical aptamer-based (EAB) sensor based on the low-temperature enhancement effect was developed for the direct determination of cocaine in bio-samples. The signal gain of the sensor at 10 °C was greatly improved compared to room temperature, owing to the improved affinity between the aptamer and the target. Additionally, the electroactive area of the gold electrode used to fabricate the EAB sensor was increased 20 times by a simple electrochemical roughening method. The porous electrode possesses more efficient electron transfer and better antifouling properties after roughening. These improvements enabled the sensor to achieve rapid detection of cocaine in complex bio-samples. The low detection limits (LOD) of cocaine in undiluted urine, 50% serum and 50% saliva were 70 nM, 30 nM and 10 nM, respectively, which are below the concentration threshold in drugged driving screening. The aptasensor was simple to construct and reusable, which offers potential for drugged driving screening in the real world.

Nucleic acid-based wearable and implantable electrochemical sensors

The rapid advancements in nucleic acid-based electrochemical sensors for implantable and wearable applications have marked a significant leap forward in the domain of personal healthcare over the last decade. This technology promises to revolutionize personalized healthcare by facilitating the early diagnosis of diseases, monitoring of disease progression, and tailoring of individual treatment plans. This review navigates through the latest developments in this field, focusing on the strategies for nucleic acid sensing that enable real-time and continuous biomarker analysis directly in various biofluids, such as blood, interstitial fluid, sweat, and saliva. The review delves into various nucleic acid sensing strategies, emphasizing the innovative designs of biorecognition elements and signal transduction mechanisms that enable implantable and wearable applications. Special perspective is given to enhance nucleic acid-based sensor selectivity and sensitivity, which are crucial for the accurate detection of low-level biomarkers. The integration of such sensors into implantable and wearable platforms, including microneedle arrays and flexible electronic systems, actualizes their use in on-body devices for health monitoring. We also tackle the technical challenges encountered in the development of these sensors, such as ensuring long-term stability, managing the complexity of biofluid dynamics, and fulfilling the need for real-time, continuous, and reagentless detection. In conclusion, the review highlights the importance of these sensors in the future of medical engineering, offering insights into design considerations and future research directions to overcome existing limitations and fully realize the potential of nucleic acid-based electrochemical sensors for healthcare applications.

Molecular Origins of Long-Term Changes in a Competitive Continuous Biosensor with Single-Molecule Resolution

Biosensing by particle motion is a biosensing technology that relies on single-molecule interactions and enables the continuous monitoring of analytes from picomolar to micromolar concentration levels. However, during sensor operation, the signals are observed to change gradually. Here, we present a comprehensive methodology to elucidate the molecular origins of long-term changes in a particle motion sensor, focusing on a competitive sensor design under conditions without flow. Experiments were performed wherein only the particles or only the surfaces were aged in order to clarify how each individual component changes over time. Furthermore, distributions of particle motion patterns and switching activity were studied to reveal how particle populations change over timespans of several days. For a cortisol sensor with anticortisol antibodies on the particles and cortisol analogues on the sensing surface, the leading hypotheses for the long-term changes are (i) that the particles lose antibodies and develop nonspecific interactions and (ii) that analogue molecules dissociate from the sensing surface. The developed methodologies and the acquired insights pave a way for realizing sensors that can operate over long timespans.

Evaluation of Transducer Elements Based on Different Material Configurations for Aptamer-Based Electrochemical Biosensors

The selection of an appropriate transducer is a key element in biosensor development. Currently, a wide variety of substrates and working electrode materials utilizing different fabrication techniques are used in the field of biosensors. In the frame of this study, the following three specific material configurations with gold-finish layers were investigated regarding their efficacy to be used as electrochemical (EC) biosensors: (I) a silicone-based sensor substrate with a layer configuration of 50 nm SiO/50 nm SiN/100 nm Au/30-50 nm WTi/140 nm SiO/bulk Si); (II) polyethylene naphthalate (PEN) with a gold inkjet-printed layer; and (III) polyethylene terephthalate (PET) with a screen-printed gold layer. Electrodes were characterized using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) to evaluate their performance as electrochemical transducers in an aptamer-based biosensor for the detection of cardiac troponin I using the redox molecule hexacyanoferrade/hexacyaniferrade (K3[Fe (CN)6]/K4[Fe (CN)6]. Baseline signals were obtained from clean electrodes after a specific cleaning procedure and after functionalization with the thiolate cardiac troponin I aptamers "Tro4" and "Tro6". With the goal of improving the PEN-based and PET-based performance, sintered PEN-based samples and PET-based samples with a carbon or silver layer under the gold were studied. The effect of a high number of immobilized aptamers will be tested in further work using the PEN-based sample. In this study, the charge-transfer resistance (Rct), anodic peak height (Ipa), cathodic peak height (Ipc) and peak separation (∆E) were determined. The PEN-based electrodes demonstrated better biosensor properties such as lower initial Rct values, a greater change in Rct after the immobilization of the Tro4 aptamer on its surface, higher Ipc and Ipa values and lower ∆E, which correlated with a higher number of immobilized aptamers compared with the other two types of samples functionalized using the same procedure.

Recent advances in graphene-based electroanalytical devices for healthcare applications

Graphene, with its outstanding mechanical, electrical, and biocompatible properties, stands out as an emerging nanomaterial for healthcare applications, especially in building electroanalytical biodevices. With the rising prevalence of chronic diseases and infectious diseases, such as the COVID-19 pandemic, the demand for point-of-care testing and remote patient monitoring has never been greater. Owing to their portability, ease of manufacturing, scalability, and rapid and sensitive response, electroanalytical devices excel in these settings for improved healthcare accessibility, especially in resource-limited settings. The development of different synthesis methods yielding large-scale graphene and its derivatives with controllable properties, compatible with device manufacturing - from lithography to various printing methods - and tunable electrical, chemical, and electrochemical properties make it an attractive candidate for electroanalytical devices. This review article sheds light on how graphene-based devices can be transformative in addressing pressing healthcare needs, ranging from the fundamental understanding of biology in in vivo and ex vivo studies to early disease detection and management using in vitro assays and wearable devices. In particular, the article provides a special focus on (i) synthesis and functionalization techniques, emphasizing their suitability for scalable integration into devices, (ii) various transduction methods to design diverse electroanalytical device architectures, (iii) a myriad of applications using devices based on graphene, its derivatives, and hybrids with other nanomaterials, and (iv) emerging technologies at the intersection of device engineering and advanced data analytics. Finally, some of the major hurdles that graphene biodevices face for translation into clinical applications are discussed.

Whole blood multiplex measurements using electrochemical aptamer-based biosensors

Simultaneous measurements of various molecules ("multiplex") using electrochemical biosensors typically require multiple electrode implementations, which for neonates, hemophiliacs, etc. is problematic. Here, we introduce the oxazine ATTO 700 into electrochemical aptamer-based biosensors to achieve "true" multiplex, continuous and real-time measurements of two different molecules in undiluted whole blood using a single electrode.

Structure-Switching Electrochemical Aptasensor for Rapid, Reagentless, and Single-Step Nanomolar Detection of C-Reactive Protein

C-reactive protein (CRP) is an acute-phase reactant and sensitive indicator for sepsis and other life-threatening pathologies, including systemic inflammatory response syndrome. Currently, clinical turn-around times for established CRP detection methods take between 30 min to hours or even days from centralized laboratories. Here, we report the development of an electrochemical biosensor using redox probe-tagged DNA aptamers, functionalized onto inexpensive, commercially available screen-printed electrodes. Binding-induced conformational switching of the CRP-targeting aptamer induces a specific and selective signal-ON event, which enables single-step and reagentless detection of CRP in as little as 1 min. The aptasensor limit of detection spans approximately 20-60 nM in 50% human serum with dynamic response windows spanning 1-200 or 1-500 nM (R = 0.97/R = 0.98 respectively). The sensor is stable for at least 1 week and can be reused numerous times, as judged from repeated real-time dosing and dose-response assays. By decoupling binding events from the signal induction mechanism, structure-switching electrochemical aptamer-based sensors provide considerable advantages over their adsorption-based counterparts. Our work expands on the retinue of such sensors reported in the literature and is the first instance of structure-switching electrochemical aptamer-based sensors (SS-EABs) for reagentless, voltammetric CRP detection. We hope this study inspires further investigations into the suitability of SS-EABs for diagnostics, which will aid translational R&D toward fully realized devices aimed at point-of-care applications or for broader use by the public.

Effects of storage conditions on the performance of an electrochemical aptamer-based sensor

The electrochemical aptamer-based (EAB) sensor platform is the only molecular monitoring approach yet reported that is (1) real time and effectively continuous, (2) selective enough to deploy in situ in the living body, and (3) independent of the chemical or enzymatic reactivity of its target, rendering it adaptable to a wide range of analytes. These attributes suggest the EAB platform will prove to be an important tool in both biomedical research and clinical practice. To advance this possibility, here we have explored the stability of EAB sensors upon storage, using retention of the target recognizing aptamer, the sensor's signal gain, and the affinity of the aptamer as our performance metrics. Doing so we find that low-temperature (-20 °C) storage is sufficient to preserve sensor functionality for at least six months without the need for exogenous preservatives.

Smartphone-Based Sensing of Cortisol by Functionalized Rhodamine Probes

During a stress condition, the human body synthesizes catecholamine neurotransmitters and specific hormones (called "stress hormones"), the most important of which is cortisol. The monitoring of cortisol levels should be extremely important to control the stress levels, and for this reason, it shows important medical applications. The common analytical methods (HPLC, GC-MS) cannot be used in real life, due to the bulky size of the instruments and the necessity of specialized personnel. Molecular probes solve these problems due to their fast and easy use. The synthesis of new fluorescent rhodamine probes, able to interact by non-covalent interactions with cortisol, the recognition properties in solution as well as in solid state by Strip Test, using a smartphone as detector, are here reported. DFT calculations and FT-IR measurements suggest the formation of supramolecular complexes through hydrogen bonds as main non-covalent interaction. The present study represents one of the first sensor, based on synthetical chemical receptors, able to detect cortisol in a linear range from 1 mM to 1 pM, based on non-covalent molecular recognition and paves the way to the realization of practical point-of-care device for the monitoring of cortisol in real live.

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.

The Use of Xenonucleic Acids Significantly Reduces the In Vivo Drift of Electrochemical Aptamer-Based Sensors

Electrochemical aptamer-based sensors support the high-frequency, real-time monitoring of molecules-of-interest in vivo. Achieving this requires methods for correcting the sensor drift seen during in vivo placements. While this correction ensures EAB sensor measurements remain accurate, as drift progresses it reduces the signal-to-noise ratio and precision. Here, we show that enzymatic cleavage of the sensor's target-recognizing DNA aptamer is a major source of this signal loss. To demonstrate this, we deployed a tobramycin-detecting EAB sensor analog fabricated with the DNase-resistant "xenonucleic acid" 2'O-methyl-RNA in a live rat. In contrast to the sensor employing the equivalent DNA aptamer, the 2'O-methyl-RNA aptamer sensor lost very little signal and had improved signal-to-noise. We further characterized the EAB sensor drift using unstructured DNA or 2'O-methyl-RNA oligonucleotides. While the two devices drift similarly in vitro in whole blood, the in vivo drift of the 2'O-methyl-RNA-employing device is less compared to the DNA-employing device. Studies of the electron transfer kinetics suggested that the greater drift of the latter sensor arises due to enzymatic DNA degradation. These findings, coupled with advances in the selection of aptamers employing XNA, suggest a means of improving EAB sensor stability when they are used to perform molecular monitoring in the living body.

Recent Advancements in Molecularly Imprinted Polymers Based Aptasensors: Critical Role of Nanomaterials for the Efficient Food Safety Analysis

Biosensors are being studied extensively for their ability to detect and analyze molecules. There has been a growing interest in combining molecular imprinted polymers (MIPs) and aptamers to create hybrid recognition elements that offer advantages such as target binding, sensitivity, selectivity, and stability. These hybrid elements have been successfully used in identifying a wide range of analytes in food samples. However, the application of MIP-based aptasensors in different sensing approaches is still challenging due to the low conductivity of MIPs-aptamers and limited adsorption capacity of MIPs. To address these limitations, researchers have been exploring the use of nanomaterials (NMs) to design efficient multiple-recognition systems that exploit the synergies between aptamers and MIPs. These hybrid systems can enhance the sensitivity and selectivity of MIP-based aptasensors in quantifying analytical samples. This review provides a comprehensive overview of recent advancements in the field of MIP-based aptasensors. It also introduces technologies that combine MIPs and aptamers to achieve higher sensitivity and selectivity in quantifying analytical samples. The review also highlights potential future trends and practical approaches that can be employed to address the limitations of MIP-based aptasensors, including the use of new NMs, the development of new fabrication techniques, and the integration of MIP-based aptasensors with other analytical tools.

Single Voltammetric Sweep Calibration-Free Interrogation of Electrochemical Aptamer-Based Sensors Employing Continuous Square Wave Voltammetry

Continuous square wave voltammetry (cSWV) is a technique that enables the continuous collection of current data (at 100 kHz) to maximize the information content obtainable from a single voltammetric sweep. This data collection procedure results in the generation of multiple voltammograms corresponding to different effective square wave frequencies. The application of cSWV brings significant benefits to electrochemical aptamer-based (E-AB) sensors. The E-AB sensor platform permits continuous real-time monitoring of small biological molecules. Traditionally, E-AB sensors report only on changes in analyte concentration rather than absolute quantification in matrices when basal concentrations are not known a priori. This is because they exhibit a voltammetric peak current even in the absence of a target. However, using a dual-frequency approach, calibration-free sensing can be performed effectively, eliminating the sensor-to-sensor variation by taking ratiometric current responses obtained at two different frequencies from two different voltammetric sweeps. In employing our approach, cSWV provides a great advantage over the conventionally used square wave voltammetry since the required voltammograms are collected with a single sweep, which improves the temporal resolution of the measurement when considering the current at multiple frequencies for improved accuracy and reduced surface interrogation. Moreover, we show here that using cSWV provides significantly improved concentration predictions. E-AB sensors sensitive to ATP and tobramycin were interrogated across a wide range of concentrations. With this approach, cSWV allowed us to estimate the target concentration, retaining up to an ±5% error of the expected concentration when tested in buffer and complex media.

Codeposition Enhances the Performance of Electrochemical Aptamer-Based Sensors

Electrochemical aptamer-based (EAB) sensors, a minimally invasive means of performing high-frequency, real-time measurement of drugs and biomarkers in situ in the body, have traditionally been fabricated by depositing their target-recognizing aptamer onto an interrogating gold electrode using a "sequential" two-step method involving deposition of the thiol-modified oligonucleotide (typically for 1 h) followed by incubation in mercaptohexanol solution (typically overnight) to complete the formation of a stable, self-assembled monolayer. Here we use EAB sensors targeting vancomycin, tryptophan, and phenylalanine to show that "codeposition", a less commonly employed EAB fabrication method in which the thiol-modified aptamer and the mercaptohexanol diluent are deposited on the electrode simultaneously and for as little as 1 h, improves the signal gain (relative change in signal upon the addition of high concentrations of the target) of the vancomycin and tryptophan sensors without significantly reducing their stability. In contrast, the gain of the phenylalanine sensor is effectively identical irrespective of the fabrication approach employed. This sensor, however, appears to employ binding-induced displacement of the redox reporter rather than binding-induced folding as its signal transduction mechanism, suggesting in turn a mechanism for the improvement observed for the other two sensors. Codeposition thus not only provides a more convenient means of fabricating EAB sensors but also can improve their performance.

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

Enhancing the sensitivity and stability of electrochemical aptamer-based sensors by AuNPs@MXene nanocomposite for continuous monitoring of biomarkers

Electrochemical aptamer-based (E-AB) sensors offer exciting potential for real-time tracking of various biomarkers, such as proteins and small molecules, due to their exceptional selectivity and adaptability. However, most E-AB sensors rely on planar gold structures, which inherently limit their sensitivity and operational stability for continuous monitoring of biomarkers. Although gold nanostructures have recently enhanced E-AB sensor performance, no studies have explored the combination of gold nanostructure with other types of nanomaterials for continuous molecular monitoring. To fill this gap, we employed gold nanoparticles and MXene Ti3C2 (AuNPs@MXene), a versatile nanocomposite, in designing an E-AB sensor targeted at vascular endothelial growth factor (VEGF), a crucial human signaling protein. Remarkably, the AuNPs@MXene nanocomposite achieved over thirty-fold and half-fold increases in active surface area compared to bare and AuNPs-modified gold electrodes, respectively, significantly elevating the analytical capabilities of E-AB sensors during continuous operation. After a systematic optimization and characterization process, the newly developed E-AB sensor, powered by AuNPs@MXene nanocomposite, demonstrated both enhanced stability and heightened sensitivity. Overall, our findings open new avenues for the incorporation of nanocomposites in E-AB sensor design, enabling the creation of more sensitive and durable real-time monitoring systems.

Cortisol sensing by optical sensors

During a stress condition, the human body synthesizes catecholamine neurotransmitters and specific hormones (called "stress hormones"), the most important of which is cortisol. The monitoring of cortisol levels is extremely important for controlling the stress levels. For this reason, it has important medical applications. Common analytical methods (HPLC, GC-MS) cannot be used in real life due to the bulkiness of the instruments and the necessity of specialized operators. Molecular probes solve this problem. This review aims to provide a description of recent developments in this field, focusing on the analytical aspects and the possibility to obtain real practical devices from these molecular probes.

Label-Free, Real-Time Monitoring of Cytochrome C Responses to Drugs in Microdissected Tumor Biopsies with a Multi-Well Aptasensor Platform

Functional assays on intact tumor biopsies can potentially complement and extend genomics-based approaches for precision oncology, drug testing, and organs-on-chips cancer disease models by capturing key determinants of therapeutic response, such as tissue architecture, tumor heterogeneity, and the tumor microenvironment. Currently, most of these assays rely on fluorescent labeling, a semi-quantitative method best suited to be a single-time-point terminal assay or labor-intensive terminal immunostaining analysis. Here, we report integrated aptamer electrochemical sensors for on-chip, real-time monitoring of increases of cytochrome C, a cell death indicator, from intact microdissected tissues with high affinity and specificity. The platform features a multi-well sensor layout and a multiplexed electronic setup. The aptasensors measure increases in cytochrome C in the supernatant of mouse or human microdissected tumors after exposure to various drug treatments. Since the aptamer probe can be easily exchanged to recognize different targets, the platform could be adapted for multiplexed monitoring of various biomarkers, providing critical information on the tumor and its microenvironment. This approach could not only help develop more advanced cancer disease models but also apply to other complex in vitro disease models, such as organs-on-chips and organoids.

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.

Wearing the Lab: Advances and Challenges in Skin-Interfaced Systems for Continuous Biochemical Sensing

Continuous, on-demand, and, most importantly, contextual data regarding individual biomarker concentrations exemplify the holy grail for personalized health and performance monitoring. This is well-illustrated for continuous glucose monitoring, which has drastically improved outcomes and quality of life for diabetic patients over the past 2 decades. Recent advances in wearable biosensing technologies (biorecognition elements, transduction mechanisms, materials, and integration schemes) have begun to make monitoring of other clinically relevant analytes a reality via minimally invasive skin-interfaced devices. However, several challenges concerning sensitivity, specificity, calibration, sensor longevity, and overall device lifetime must be addressed before these systems can be made commercially viable. In this chapter, a logical framework for developing a wearable skin-interfaced device for a desired application is proposed with careful consideration of the feasibility of monitoring certain analytes in sweat and interstitial fluid and the current development of the tools available to do so. Specifically, we focus on recent advancements in the engineering of biorecognition elements, the development of more robust signal transduction mechanisms, and novel integration schemes that allow for continuous quantitative analysis. Furthermore, we highlight the most compelling and promising prospects in the field of wearable biosensing and the challenges that remain in translating these technologies into useful products for disease management and for optimizing human performance.

Recent advances in aptamer-based platforms for cortisol hormone monitoring

The stressful conditions of today-life make it urgent the timely prevention and treatment of many physiological and psychological disorders related to stress. According to the significant progress made in the near future, rapid, accurate, and on-spot measurement of cortisol hormone as a dominant stress biomarker using miniaturized digital devices is not far from expected. With a special potency in the fields of diagnosis and healthcare monitoring, aptamer-mediated biosensors (aptasensors) are promising for the quantitative monitoring of cortisol levels in the different matrices (sweat, saliva, urine, cerebrospinal fluid, blood serum, etc.). Accordingly, this in-depth study reviews the superior achievements in the aptasensing strategies to detect cortisol hormone with the synergism of diverse two/three dimensional nanostructured materials, enzymatic amplification components, and antibody motifs. The represented discussions offer a universal perspective to achieve lab-on-chip aptasensing arrays as future user-friendly skin-patchable electronic gadgets for on-site and real-time quantification of cortisol levels.

Electrochemical Aptamer-Based Biosensors for Measurements in Undiluted Human Saliva

Although blood remains a gold standard diagnostic fluid for most health exams, it involves an unpleasant and relatively invasive sampling procedure (finger pricking or venous draw). Saliva contains many relevant and useful biomarkers for diagnostic purposes, and its collection, in contrast, is noninvasive and can be obtained with minimal effort. Current saliva analyses are, however, achieved using chromatography or lateral flow assays, which, despite their high accuracy and sensitivity, can demand expensive laboratory-based instruments operated by trained personnel or offer only semiquantitative results. In response, we investigated electrochemical aptamer-based (E-AB) biosensors, a reagentless sensing platform, to allow for continuous and real-time measurements directly in undiluted, unstimulated human whole saliva. As a proof-of-concept study, we developed E-AB biosensors capable of detecting low-molecular-weight analytes (glucose and adenosine monophosphate (AMP)). To our knowledge, we report the first E-AB sensor for glucose, an approach that is inherently independent of its chemical reactivity in contrast to home glucometers. For these three sensors, we evaluated their figures of merits, stability, and reusability over short- and long-term exposure directly in saliva. In doing so, we found that E-AB sensors allow rapid and convenient molecular measurements in whole saliva with unprecedented sensitivities in the pico- to nanomolar regime and could be regenerated and reused up to 7 days when washed and stored in phosphate-buffered saline at room temperature. We envision that salivary molecular measurements using E-AB sensors are a promising alternative to invasive techniques and can be used for improved point-of-care clinical diagnosis and at-home measurements.

DNA Chimeras as Electrochemical Biosensors for Host-Guest Measurements in Blood

Few sensing platforms have become ubiquitous to enable rapid and convenient measurements at the point-of-care. Those, however, are "one-off" technologies, meaning that they can only detect a single target and are hardly adaptable. In response, we plan to develop a sensing platform that can be extended to detect other classes of molecules and that affords rapid, convenient, continuous measurements directly in undiluted complex matrices. For this, we decided to rely on a host molecule that presents reversible interactions toward specific guest molecules to develop a new class of sensors that we coined "Electrochemical DNA-host chimeras". As a proof-of-concept for our sensor, we decided to use cyclobis(paraquat-p-phenylene) ("blue box") that we attached on an electrode-bound DNA to allow measurements of electron-rich guests such as dopamine and aspirin. Doing so allows to promote host-guest complex that could be quantified using blue box's electrochemistry. Because of this unique sensor architecture, we achieve, to our knowledge, the first reagentless, continuous and rapid (<5 min) host-guest measurements in undiluted whole blood. We envision that given the library of electroactive host molecules that this will allow the development of a sensing platform for measurements of several classes of molecules in complex matrices at the point-of-care.

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.

Opportunities and challenges in the diagnostic utility of dermal interstitial fluid

The volume of interstitial fluid (ISF) in the human body is three times that of blood. Yet, collecting diagnostically useful ISF is more challenging than collecting blood because the extraction of dermal ISF disrupts the delicate balance of pressure between ISF, blood and lymph, and because the triggered local inflammation further skews the concentrations of many analytes in the extracted fluid. In this Perspective, we overview the most meaningful differences in the make-up of ISF and blood, and discuss why ISF cannot be viewed generally as a diagnostically useful proxy for blood. We also argue that continuous sensing of small-molecule analytes in dermal ISF via rapid assays compatible with nanolitre sample volumes or via miniaturized sensors inserted into the dermis can offer clinically advantageous utility, particularly for the monitoring of therapeutic drugs and of the status of the immune system.

Highly Uniform DNA Monolayers Generated by Freezing-Directed Assembly on Gold Surfaces Enable Robust Electrochemical Sensing in Whole Blood

Assembling DNA on solid surfaces is fundamental to surface-based DNA technology. However, precise control over DNA conformation and organization at solid-liquid interfaces remains a challenge, resulting in limited stability and sensitivity in biosensing applications. We herein communicate a simple and robust method for creating highly uniform DNA monolayers on gold surfaces by a freeze-thawing process. Using Raman spectroscopy, fluorescent imaging, and square wave voltammetry, we demonstrate that thiolated DNA is concentrated and immobilized on gold surfaces with an upright conformation. Moreover, our results reveal that the freezing-induced DNA surfaces are more uniform, leading to improved DNA stability and target recognition. Lastly, we demonstrate the successful detection of a model drug in undiluted whole blood while mitigating the effects of biofouling. Our work not only provides a simple approach to tailor the DNA-gold surface for biosensors but also sheds light on the unique behavior of DNA oligonucleotides upon freezing on the liquid-solid interface.

Real-Time Monitoring of Antibiotics in the Critically Ill Using Biosensors

By ensuring optimal dosing, therapeutic drug monitoring (TDM) improves outcomes in critically ill patients by maximizing effectiveness while minimizing toxicity. Current methods for measuring plasma drug concentrations, however, can be challenging, time-consuming, and slow to return an answer, limiting the extent to which TDM is used to optimize drug exposure. A potentially promising solution to this dilemma is provided by biosensors, molecular sensing devices that employ biorecognition elements to recognize and quantify their target molecules rapidly and in a single step. This paper reviews the current state of the art for biosensors regarding their application to TDM of antibiotics in the critically ill, both as ex vivo point-of-care devices supporting single timepoint measurements and in vivo devices supporting continuous real-time monitoring in situ in the body. This paper also discusses the clinical development of biosensors for TDM, including regulatory challenges and the need for standardized performance evaluation. We conclude by arguing that, through precise and real-time monitoring of antibiotics, the application of biosensors in TDM holds great promise for enhancing the optimization of drug exposure in critically ill patients, offering the potential for improved outcomes.

Dual-Aptamer Drift Canceling Techniques to Improve Long-Term Stability of Real-Time Structure-Switching Aptasensors

This paper presents a dual-aptamer scheme to mitigate signal drifts caused by structure-switching aptamers during long-term monitoring. Electrochemical aptamer-based (E-AB) biosensors have recently shown great potential for continuous in vivo monitoring. However, the accuracy of detection is often limited by signaling drifts. Traditional approaches rely on kinetic differential measurements (KDM) coupled with square-wave voltammetry to eliminate these drifts. Yet, we have discovered that KDM does not apply universally to all aptamers, as their responses at different SWV frequencies heavily rely on their structure-switching characteristics and the electron transfer (ET) kinetics of the redox reporters. In light of this, we propose a "dual-aptamer" scheme that utilizes two aptamers, each responding differently to the same target molecule to cancel out drift. These paired aptamers are identified through (1) screening from an existing pool of aptamers and (2) engineering the signaling behavior of the redox reporters. We demonstrate the differential signaling of the aptamer pair in the presence of ampicillin and ATP molecules and show that the pair exhibits similar drifts in undiluted goat serum. By implementing drift cancelation, sensor drift is reduced by a factor of 370. Additionally, the differential signaling enables an increased recording throughput by leveraging differential readout electronics. The authors believe that the proposed technique holds significant benefits for long-term in vivo monitoring.

Using seconds-resolved pharmacokinetic datasets to assess pharmacokinetic models encompassing time-varying physiology

AIM

Pharmacokinetics have historically been assessed using drug concentration data obtained via blood draws and bench-top analysis. The cumbersome nature of these typically constrains studies to at most a dozen concentration measurements per dosing event. This, in turn, limits our statistical power in the detection of hours-scale, time-varying physiological processes. Given the recent advent of in vivo electrochemical aptamer-based (EAB) sensors, however, we can now obtain hundreds of concentration measurements per administration. Our aim in this paper was to assess the ability of these time-dense datasets to describe time-varying pharmacokinetic models with good statistical significance.

METHODS

We used seconds-resolved measurements of plasma tobramycin concentrations in rats to statistically compare traditional one- and two-compartmental pharmacokinetic models to new models in which the proportional relationship between a drug's plasma concentration and its elimination rate varies in response to changing kidney function.

RESULTS

We found that a modified one-compartment model in which the proportionality between the plasma concentration of tobramycin and its elimination rate falls reciprocally with time either meets or is preferred over the standard two-compartment pharmacokinetic model for half of the datasets characterized. When we reduced the impact of the drug's rapid distribution phase on the model, this one-compartment, time-varying model was statistically preferred over the standard one-compartment model for 80% of our datasets.

CONCLUSIONS

Our results highlight both the impact that simple physiological changes (such as varying kidney function) can have on drug pharmacokinetics and the ability of high-time resolution EAB sensor measurements to identify such impacts.

Calibration-Free, Seconds-Resolved In Vivo Molecular Measurements using Fourier-Transform Impedance Spectroscopy Interrogation of Electrochemical Aptamer Sensors

Electrochemical aptamer-based (EAB) sensors are capable of measuring the concentrations of specific molecules in vivo, in real time, and with a few-second time resolution. For their signal transduction mechanism, these sensors utilize a binding-induced conformational change in their target-recognizing, redox-reporter-modified aptamer to alter the rate of electron transfer between the reporter and the supporting electrode. While a variety of voltammetric techniques have been used to monitor this change in kinetics, they suffer from various drawbacks, including time resolution limited to several seconds and sensor-to-sensor variation that requires calibration to remove. Here, however, we show that the use of fast Fourier transform electrochemical impedance spectroscopy (FFT-EIS) to interrogate EAB sensors leads to improved (here better than 2 s) time resolution and calibration-free operation, even when such sensors are deployed in vivo. To showcase these benefits, we demonstrate the approach's ability to perform real-time molecular measurements in the veins of living rats.

Survey of oligoethylene glycol-based self-assembled monolayers on electrochemical aptamer-based sensor in biological fluids

The ability to monitor levels of endogenous markers and clearance profiles of drugs and their metabolites can improve the quality of biomedical research and precision with which therapies are individualized. Towards this end, electrochemical aptamer-based (EAB) sensors have been developed that support the real-time monitoring of specific analytes in vivo with clinically relevant specificity and sensitivity. A challenge associated with the in vivo deployment of EAB sensors, however, is how to manage the signal drift which, although correctable, ultimately leads to unacceptably low signal-to-noise ratios, limiting the measurement duration. Motivated by the correction of signal drift, in this paper, we have explored the use of oligoethylene glycol (OEG), a widely employed antifouling coating, to reduce the signal drift in EAB sensors. Counter to expectations, however, when challenged in 37 °C whole blood in vitro, EAB sensors employing OEG-modified self-assembled monolayers exhibit both greater drift and reduced signal gain, compared with those employ a simple, hydroxyl-terminated monolayer. On the other hand, when EAB sensor was prepared with a mix monolayer using MCH and lipoamido OEG 2 alcohol, reduced signal noise was observed compared to the same sensor prepared with MCH presumably due to improved SAM construction. These results suggest broader exploration of antifouling materials will be required to improve the signal drift of EAB sensors.

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.

Efforts toward the continuous monitoring of molecular markers of performance

OBJECTIVES

Technologies supporting the continuous, real-time measurement of blood oxygen saturation and plasma glucose levels have improved our ability to monitor performance status. Our ability to monitor other molecular markers of performance, however, including the hormones known to indicate overtraining and general health, has lagged. That is, although a number of other molecular markers of performance status have been identified, we have struggled to develop viable technologies supporting their real-time monitoring in the body. Here we review biosensor approaches that may support such measurements, as well as the molecules potentially of greatest interest to monitor.

DESIGN

Narrative literature review.

METHOD

Literature review.

RESULTS

Significant effort has been made to harness the specificity, affinity, and generalizability of biomolecular recognition in a platform technology supporting continuous in vivo molecular measurements. Most biosensor approaches, however, are either not generalizable to most targets, or fail when challenged in the complex environments found in vivo. Electrochemical aptamer-based sensors, in contrast, are the first technology to simultaneously achieve both of these critical attributes. In an effort to illustrate the potential of this platform technology, we both critically review the literature describing it and briefly survey some of the molecular performance markers we believe will prove advantageous to monitor using it.

CONCLUSIONS

Electrochemical aptamer-based sensors may be the first truly generalizable technology for monitoring specific molecules in situ in the body and how adaptation of the platform to subcutaneous microneedles will enable the real-time monitoring of performance markers via a wearable, minimally invasive device.

Perspective-The Feasibility of Continuous Protein Monitoring in Interstitial Fluid

Real-time continuous monitoring of proteins in-vivo holds great potential for personalized medical applications. Unfortunately, a prominent knowledge gap exists in the fundamental biology regarding protein transfer and correlation between interstitial fluid and blood. Additionally, technological sensing will require affinity-based platforms that cannot be robustly protected in-vivo and will therefore be challenged in sensitivity, longevity, and fouling over multi-day to week timelines. Here we use electrochemical aptamer sensors as a model system to discuss further research necessary to achieve continuous protein sensing.

Microneedle electrochemical aptamer-based sensing: Real-time small molecule measurements using sensor-embedded, commercially-available stainless steel microneedles

Microneedle sensors could enable minimally-invasive, continuous molecular monitoring - informing on disease status and treatment in real-time. Wearable sensors for pharmaceuticals, for example, would create opportunities for treatments personalized to individual pharmacokinetics. Here, we demonstrate a commercial-off-the-shelf (COTS) approach for microneedle sensing using an electrochemical aptamer-based sensor that detects the high-toxicity antibiotic, vancomycin. Wearable monitoring of vancomycin could improve patient care by allowing targeted drug dosing within its narrow clinical window of safety and efficacy. To produce sensors, we miniaturize the electrochemical aptamer-based sensors to a microelectrode format, and embed them within stainless steel microneedles (sourced from commercial insulin pen needles). The microneedle sensors achieve quantitative measurements in body-temperature undiluted blood. Further, the sensors effectively maintain electrochemical signal within porcine skin. This COTS approach requires no cleanroom fabrication or specialized equipment, and produces individually-addressable, sterilizable microneedle sensors capable of easily penetrating the skin. In the future, this approach could be adapted for multiplexed detection, enabling real-time monitoring of a range of biomarkers.

Week-Long Operation of Electrochemical Aptamer Sensors: New Insights into Self-Assembled Monolayer Degradation Mechanisms and Solutions for Stability in Serum at Body Temperature

Conventional wisdom suggests that widely utilized self-assembled alkylthiolate monolayers on gold are too unstable to last more than several days when exposed to complex fluids such as raw serum at body temperature. Demonstrated here is that these monolayers can not only last at least 1 week under such harsh conditions but that significant applied value can be captured for continuous electrochemical aptamer biosensors. Electrochemical aptamer biosensors provide an ideal tool to investigate monolayer degradation, as aptamer sensors require a tightly packed monolayer to preserve sensor signal vs background current and readily reveal fouling by albumin and other solutes when operating in biofluids. Week-long operation in serum at 37 °C is achieved by (1) increasing van der Waals interactions between adjacent monolayer molecules to increase the activation energy required for desorption, (2) optimizing electrochemical measurement to decrease both alkylthiolate oxidation and electric-field-induced desorption, and (3) mitigating fouling using protective zwitterionic membranes and zwitterion-based blocking layers with antifouling properties. This work further proposes origins and mechanisms of monolayer degradation in a logical stepwise manner that was previously unobservable over multiday time scales. Several of the observed results are surprising, revealing that short-term improvements to sensor longevity (i.e., hours) actually increase sensor degradation in the longer term (i.e., days). The results and underlying insights on mechanisms not only push forward fundamental understanding of stability for self-assembled monolayers but also demonstrate an important milestone for continuous electrochemical aptamer biosensors.

Real-Time Intracellular Analysis of Kanamycin Using Microaptasensors

With the emergence of multidrug-resistant bacteria, infection-related death toll is on the rise. Overuse of antibiotics and their leakage into waterways have transformed the environment into a sink, resulting in bacterial resistance permeating through all tiers of the food cycle. As one of the primary sources of food, fish and fish products such as fish eggs must be studied for their ability to accumulate relevant antibiotics. While the accumulation of these pharmaceuticals has previously been studied, there remains a need to analyze these processes in real time. Electrochemical aptamer-based sensor technology allows for selective, real-time monitoring of small molecules. Herein, we report the first use of miniaturized electrochemical aptamer-based sensors for the analysis of the passive uptake of the aminoglycoside antibiotic, kanamycin, in single salmon eggs. We use pulled platinum microelectrodes and increase the surface area at the electrode tip through dendritic gold deposition. These electrodes showed a 100-fold increase in DNA immobilization on the electrode surface as compared to bare microelectrodes. Additionally, the sensors showed improved stability in complex biological media over an extended period of time when compared to the more widely used macrosensors (r = 1 mm). The sensor range was determined to extend from nanomolar to micromolar concentrations of kanamycin in fish egg lysate and when used in a single salmon egg the μ-aptasensors were able to monitor the passive uptake of kanamycin over time. The accumulation kinetics were simulated using COMSOL Multiphysics software. This research presents the first reported record of passive uptake of a small molecule in a single cell in real-time using electrochemistry.