Quantification of transcription factor binding in cell extracts using an electrochemical, structure-switching biosensor

Strategy, Design, and Fabrication of Electrochemical Biosensors: A Tutorial

Advanced healthcare requires novel technologies capable of real-time sensing to monitor acute and long-term health. The challenge relies on converting a real-time quantitative biological and chemical signal into a desired measurable output. Given the success in detecting glucose and the commercialization of glucometers, electrochemical biosensors continue to be a mainstay of academic and industrial research activities. Despite the wealth of literature on electrochemical biosensors, reports are often specific to a particular application (e.g., pathogens, cancer markers, glucose, etc.), and most fail to convey the underlying strategy and design, and if it is transferable to detection of a different analyte. Here we present a tutorial review for those entering this research area that summarizes the basic electrochemical techniques utilized as well as discusses the designs and optimization strategies employed to improve sensitivity and maximize signal output.

Thermodynamic Allosteric Switch-Actuated 3D DNA Nanomachine for Ultrasensitive Electrochemical/Fluorescent Dual-Mode Biosensing of a Transcription Factor

Herein, we reported an innovative thermodynamic allosteric switch-actuated 3D DNA nanomachine for selective, sensitive, and accurate electrochemical (EC)/fluorescent (FL) dual-mode biosensing of a microphthalmia-associated transcription factor (MITF). The thermodynamic allosteric switch was ingeniously customized as a hairpin probe (HP) that was in dynamic equilibrium but rapidly interconverting conformations. At the "inactive state", the MITF-binding region and the switch part were "sequestered". Upon the introduction of MITF, an MITF-HP complex promptly formed, and the equilibrium of HP thermodynamically inclined from the "inactive state" toward the "active state" conformation. Immediately, the exposed switch on HP effectively actuated the 3D DNA nanomachine and synchronously produced the restriction site for Nb.BbvCI nicking endonuclease. After the autonomous conveying of the 3D DNA nanomachine by means of the high-efficiency circularly nicking endonuclease signal amplification (NESA), not only was MB-S1 in the supernatant used for FL measurements but also MB-SP/MNs/S2 in the precipitate was adapted for EC analysis, significantly improving the utilization of output products derived from the 3D DNA nanomachine. Accordingly, benefiting from the efficient DNA nanomachine signal amplification manner and the self-calibration function of a dual-mode bioassay, the constructed biosensor exhibits superior sensitivity and accuracy for MITF determination.

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

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

Rapid and Quantitative Detection of Lung Cancer Biomarker ENOX2 Using a Novel Aptamer in an Electrochemical DNA-Based (E-DNA) Biosensor

To overcome early cancer detection challenges, diagnostic tools enabling more sensitive, rapid, and noninvasive detection are necessary. An attractive cancer target for diagnostic blood tests is human Ecto-NOX disulfide-thiol exchanger 2 (ENOX2), expressed in most human cancer types and regularly shed into blood sera. Here, we developed an electrochemical DNA-based (E-DNA) biosensor that rapidly detects physiologically relevant levels of ENOX2. To identify ENOX2-binding aptamers that could potentially be used in a biosensor, recombinantly expressed ENOX2 was used as a binding target in an oligonucleotide library pull-down that generated a highly enriched ENOX2-binding aptamer. This candidate aptamer sensitively bound ENOX2 via gel mobility shift assays. To enable this aptamer to function in an ENOX2 E-DNA biosensor, the aptamer sequence was modified to adopt two conformations, one capable of ENOX2 binding, and one with disrupted ENOX2 binding. Upon ENOX2 introduction, a conformational shift to the ENOX2 binding state resulted in changed dynamics of a redox reporter molecule, which generated a rapid, significant, and target-specific electrical current readout change. ENOX2 biosensor sensitivity was at or below the diagnostic range. The ENOX2 E-DNA biosensor design presented here may enable the development of more sensitive, rapid, diagnostic tools for early cancer detection.

Tailoring electrode surface charge to achieve discrimination and quantification of chemically similar small molecules with electrochemical aptamers

Electrochemical biosensors based on structure-switching aptamers offer many advantages because they can operate directly in complex samples and offer the potential to integrate with miniaturized electronics. Unfortunately, these biosensors often suffer from cross-reactivity problems when measuring a target in samples containing other chemically similar molecules, such as precursors or metabolites. While some progress has been made in selecting highly specific aptamers, the discovery of these reagents remains slow and costly. In this work, we demonstrate a novel strategy to distinguish molecules with miniscule difference in chemical composition (such as a single hydroxyl group) - with cross reactive aptamer probes - by tuning the charge state of the surface on which the aptamer probes are immobilized. As an exemplar, we show that our strategy can distinguish between DOX and many structurally similar analytes, including its primary metabolite doxorubicinol (DOXol). We then demonstrate the ability to accurately quantify mixtures of these two molecules based on their differential response to sensors with different surface-charge properties. We believe this methodology is general and can be extended to a broad range of applications.

A Folding-Based Electrochemical Aptasensor for the Single-Step Detection of the SARS-CoV-2 Spike Protein

Efficient and timely testing has taken center stage in the management, control, and monitoring of the current COVID-19 pandemic. Simple, rapid, cost-effective diagnostics are needed that can complement current polymerase chain reaction-based methods and lateral flow immunoassays. Here, we report the development of an electrochemical sensing platform based on single-walled carbon nanotube screen-printed electrodes (SWCNT-SPEs) functionalized with a redox-tagged DNA aptamer that specifically binds to the receptor binding domain of the SARS-CoV-2 spike protein S1 subunit. Single-step, reagentless detection of the S1 protein is achieved through a binding-induced, concentration-dependent folding of the DNA aptamer that reduces the efficiency of the electron transfer process between the redox tag and the electrode surface and causes a suppression of the resulting amperometric signal. This aptasensor is specific for the target S1 protein with a dissociation constant (K_D) value of 43 ± 4 nM and a limit of detection of 7 nM. We demonstrate that the target S1 protein can be detected both in a buffer solution and in an artificial viral transport medium widely used for the collection of nasopharyngeal swabs, and that no cross-reactivity is observed in the presence of different, non-target viral proteins. We expect that this SWCNT-SPE-based format of electrochemical aptasensor will prove useful for the detection of other protein targets for which nucleic acid aptamer ligands are made available.

An Allosteric Transcription Factor DNA-Binding Electrochemical Biosensor for Progesterone

We describe an electrochemical strategy to transduce allosteric transcription factor (aTF) binding affinity to sense steroid hormones. Our approach utilizes square wave voltammetry to monitor changes in current output as a progesterone (PRG)-specific aTF (SRTF1) unbinds from the cognate DNA sequence in the presence of PRG. The sensor detects PRG in artificial urine samples with sufficient sensitivity suitable for clinical applications. Our results highlight the capability of using aTFs as the biorecognition elements to develop electrochemical point-of-care biosensors for the detection of small-molecule biomarkers and analytes.

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

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

Paving the way towards continuous biosensing by implementing affinity-based nanoswitches on state-dependent readout platforms

Combining affinity-based nanoswitches with state-dependent readout platforms allows for continuous biosensing and acquisition of real-time information about biochemical processes occurring in the environment of interest.

Binding mediated MNAzyme signal amplification strategy for enzyme-free and label-free detection of DNA-binding proteins

A novel MNAzyme signal amplification strategy was developed for enzyme-free and label-free detection of DNA-binding proteins. This strategy relied on the binding-mediated MNAzyme cleavage and G-quadruplex-based light-up fluorescence switch. Three DNA sequences were designed to construct the MNAzyme in which DNA1 (including half binding site of the target protein and a toehold sequence) and DNA2 (including another half binding site of the target protein and one MNAzyme partzyme) firstly hybridized. The target protein recognized the binding sites on DNA1-DNA2 hybrid to form a stable protein-DNA1-DNA2 conjugates. Then, the MNAzyme was assembled with the presence of DNA3 which contained another MNAzyme partzyme and the complementary sequence of DNA1. The active MNAzyme cleaved DNA4 to release the G-quadruplex that was locked in the stem of DNA4. Finally, N-methyl mesoporphyrin IX (NMM) was inserted into the released G-quadruplex structure and the fluorescence signal was turned on. Taking nuclear factor-κB p50 (NF-κB p50) as the model, the limit of detection was low to 0.14 nM. Furthermore, the sequence-specific recognition of NF-κB p50 with DNA displayed excellent selectivity and specificity. The results in present work showed that this strategy will be a promising tool for DNA-binding proteins analysis in biomedical exploration and clinical diagnosis.

Electrochemical DNA Biosensor That Detects Early Celiac Disease Autoantibodies

Although it is estimated that more than one million Americans have celiac disease (CD), it remains challenging to diagnose. CD, an autoimmune and inflammatory response following the ingestion of gluten-containing foods, has symptoms overlapping with other diseases and requires invasive diagnostics. The gold standard for CD diagnosis involves serologic blood tests followed by invasive confirmatory biopsies. Here, we propose a less invasive method using an electrochemical DNA (E-DNA) biosensor for CD-specific autoantibodies (AABs) circulating in blood. In our approach, CD-specific AABs bind a synthetic neoepitope, causing a conformational change in the biosensor, as well as a change in the environment of an attached redox reporter, producing a measurable current reduction. We assessed the biosensor’s ability to detect CD-specific patient-derived AABs in physiological buffer as well as buffer supplemented with bovine serum. Our biosensor was able to detect AABs in a dose-dependent manner; increased signal change correlated with increased AAB concentration with an apparent dissociation constant of 0.09 ± 0.03 units/mL of AABs. Furthermore, we found our biosensor to be target-specific, with minimal off-target binding of multiple unrelated biomarkers. Future efforts aimed at increasing sensitivity in complex media may build upon the biosensor design presented here to further improve CD AAB detection and CD diagnostic tools.

An SPR investigation into the therapeutic drug monitoring of the anticancer drug imatinib with selective aptamers operating in human plasma

An ss-DNA aptamer-based biosensor was devised to detect the anticancer drug imatinib by means of surface plasmon resonance.

Switching the activity of Taq polymerase using clamp-like triplex aptamer structure

In nature, allostery is the principal approach for regulating cellular processes and pathways. Inspired by nature, structure-switching aptamer-based nanodevices are widely used in artificial biotechnologies. However, the canonical aptamer structures in the nanodevices usually adopt a duplex form, which limits the flexibility and controllability. Here, a new regulating strategy based on a clamp-like triplex aptamer structure (CLTAS) was proposed for switching DNA polymerase activity via conformational changes. It was demonstrated that the polymerase activity could be regulated by either adjusting structure parameters or dynamic reactions including strand displacement or enzymatic digestion. Compared with the duplex aptamer structure, the CLTAS possesses programmability, excellent affinity and high discrimination efficiency. The CLTAS was successfully applied to distinguish single-base mismatches. The strategy expands the application scope of triplex structures and shows potential in biosensing and programmable nanomachines.

Protein-Controlled Actuation of Dynamic Nucleic Acid Networks by Using Synthetic DNA Translators*

Integrating dynamic DNA nanotechnology with protein-controlled actuation will expand our ability to process molecular information. We have developed a strategy to actuate strand displacement reactions using DNA-binding proteins by engineering synthetic DNA translators that convert specific protein-binding events into trigger inputs through a programmed conformational change. We have constructed synthetic DNA networks responsive to two different DNA-binding proteins, TATA-binding protein and Myc-Max, and demonstrated multi-input activation of strand displacement reactions. We achieved protein-controlled regulation of a synthetic RNA and of an enzyme through artificial DNA-based communication, showing the potential of our molecular system in performing further programmable tasks.

Determination of the concentration of transcription factor by using exonuclease III-aided amplification and gold nanoparticle mediated fluorescence intensity: A new method for gene transcription related enzyme detection

Herein, we report a new probe for the determination of the concentration of NF-κB p50, one kind of DNA-binding transcription factors (TFs), by using Exonuclease III (Exo III)-aided amplification and gold nanoparticle mediated fluorescence intensity. Since TFs play critical roles in various biological processes, the detection of TFs can provide a lot of useful biological information for studding gene expression regulation related disease. In our system, in the presence of transcription factor, Exo III based amplification reaction was trigged. This enzymatic digestion results in the release of intermediate DNA and ultimately liberating the fluorophore (which, separated from the quencher of AuNP and BHQ2, now fluoresces). The released intermediate DNA then hybridizes with another strand3, whence the cycle starts anew. So, the fluorescence intensity reflects the NF-κB p50 concentration with a detection limit of 1.32 pM. Importantly, this method might be further extended to selectively detect various dsDNA-binding proteins by simply changing the binding-site sequences of strand1/strand2 duplex probes.

Recent development of nucleic acid nanosensors to detect sequence-specific binding interactions: From metal ions, small molecules to proteins and pathogens

DNA carries important genetic instructions and plays vital roles in regulating biological activities in living cells. Proteins such as transcription factors binds to DNA to regulate the biological functions of DNA, and similarly many drug molecules also bind to DNA to modulate its functions. Due to the importance of protein-DNA and drug-DNA binding, there has been intense effort in developing novel nanosensors in the same length scale as DNA, to effectively study these binding interactions in details. In addition, aptamers can be artificially selected to detect metal ions and pathogens such as bacteria and viruses, making nucleic acid nanosensors more versatile in detecting a large variety of analytes. In this minireview, we first explained the different types and binding modes of protein-DNA and drug-DNA interactions in the biological systems, as well as aptamer-target binding. This was followed by the review of five types of nucleic acid nanosensors based on optical or electrochemical detection. The five types of nucleic acid nanosensors utilizing colorimetric, dynamic light scattering (DLS), surface-enhanced Raman spectroscopy (SERS), fluorescence and electrochemical detections have been recently developed to tackle some of the challenges in high-throughput screening technology for large scale analysis, which is especially useful for drug development and mass screening for pandemic outbreak such as SARS or COVID-19.

Bioapplications of DNA nanotechnology at the solid-liquid interface

DNA nanotechnology engineered at the solid-liquid interface has advanced our fundamental understanding of DNA hybridization kinetics and facilitated the design of improved biosensing, bioimaging and therapeutic platforms. Three research branches of DNA nanotechnology exist: (i) structural DNA nanotechnology for the construction of various nanoscale patterns; (ii) dynamic DNA nanotechnology for the operation of nanodevices; and (iii) functional DNA nanotechnology for the exploration of new DNA functions. Although the initial stages of DNA nanotechnology research began in aqueous solution, current research efforts have shifted to solid-liquid interfaces. Based on shape and component features, these interfaces can be classified as flat interfaces, nanoparticle interfaces, and soft interfaces of DNA origami and cell membranes. This review briefly discusses the development of DNA nanotechnology. We then highlight the important roles of structural DNA nanotechnology in tailoring the properties of flat interfaces and modifications of nanoparticle interfaces, and extensively review their successful bioapplications. In addition, engineering advances in DNA nanodevices at interfaces for improved biosensing both in vitro and in vivo are presented. The use of DNA nanotechnology as a tool to engineer cell membranes to reveal protein levels and cell behavior is also discussed. Finally, we present challenges and an outlook for this emerging field.

Target-induced diffusivity enhancement for rapid and highly sensitive homogeneous electrochemical detection of BLM in human serum

A simple, rapid, and sensitive homogeneous electrochemical bleomycin (BLM) bioassay has been successfully developed through the target-induced specific/efficient cleavage reaction. The designed probe, denoted as MB-DNA, contains both methylene blue (MB) and target recognizable sequences, and presents relatively low electrochemical signal. Upon the addition of BLM, the recognition/cleavage reaction occurs and leads to the in-situ generation of MB tag (MB-DNA-1), leading to the reduced electrostatic repulsive force. As a result, an obvious enhancement in differential pulse voltammetry (DPV) current is determined, which is relied on the amount of BLM. Thus, a turn on homogeneous electrochemical method for BLM is really achieved, and exhibits high sensitivity of 33 pM, and the shortest response time of 20 min. Furthermore, this electrochemical bioassay presents excellent sensing performance in the analysis of BLM in real samples. Comparing with other sensing strategies for BLM, this proposed electrochemical platform is just consisted of one DNA probe alone, and affords a really rapid and sensitive strategy for BLM analysis.

Electrochemical Aptamer-Based Sensors for Rapid Point-of-Use Monitoring of the Mycotoxin Ochratoxin A Directly in a Food Stream

The ability to measure the concentration of specific small molecules continuously and in real-time in complex sample streams would impact many areas of agriculture, food safety, and food production. Monitoring for mycotoxin taint in real time during food processing, for example, could improve public health. Towards this end, we describe here an inexpensive electrochemical DNA-based sensor that supports real-time monitor of the mycotoxin ochratoxin A in a flowing stream of foodstuffs.

Electrochemical Aptamer Scaffold Biosensors for Detection of Botulism and Ricin Proteins

Electrochemical DNA (E-DNA) biosensors enable the detection and quantification of a variety of molecular targets, including oligonucleotides, small molecules, heavy metals, antibodies, and proteins. Here we describe the design, electrode preparation and sensor attachment, and voltammetry conditions needed to generate and perform measurements using E-DNA biosensors against two protein targets, the biological toxins ricin and botulinum neurotoxin. This method can be applied to generate E-DNA biosensors for the detection of many other protein targets, with potential advantages over other systems including sensitive detection limits typically in the nanomolar range, real-time monitoring, and reusable biosensors.

Allosterically regulated DNA-based switches: From design to bioanalytical applications

DNA-based switches are structure-switching biomolecules widely employed in different bioanalytical applications. Of particular interest are DNA-based switches whose activity is regulated through the use of allostery. Allostery is a naturally occurring mechanism in which ligand binding induces the modulation and fine control of a connected biomolecule function as a consequence of changes in concentration of the effector. Through this general mechanism, many different allosteric DNA-based switches able to respond in a highly controlled way at the presence of a specific molecular effector have been engineered. Here, we discuss how to design allosterically regulated DNA-based switches and their applications in the field of molecular sensing, diagnostic and drug release.

A one-step structure-switching electrochemical sensor for transcription factor detection enhanced with synergistic catalysis of PtNi@MIL-101 and Exo III-assisted cycling amplification

We present a new type of PtNi@MIL-101 electrocatalyst and Exo III-assisted cycling amplification for one-step electrochemical detection of NF-κB p50.

Target binding protection mediated rolling circle amplification for sensitive detection of transcription factors

Transcription factors (TFs) play central roles in the regulation of gene expression by binding with specific DNA sequences. As a potential diagnostic marker, sensitive detection of TFs is essential for pharmacological research development and clinical disease diagnosis. Here, a new fluorescent method based on target binding protection mediated rolling circle amplification (RCA) was developed for TFs detection. A hairpin probe with recognition site for target binding, cleavage site for Nt.BbvCI digestion and two hanging DNA strands with part of G-quadruplex complementary sequences for signal output was designed. Moreover, the hairpin probe could serve as template of RCA after being ligated. Firstly, TFs bound with hairpin probes and protected signal complementary sequences against cleavage by Nt.BbvCI due to space hindrance effect, while the excess hairpin probes were effectively digested to avoid false positive signal. Then, TFs and Nt.BbvCI were dissociated from hairpin probes by heating, complete hairpin probes being preserved. Next, protected hairpin probes were specifically connected to dumbbell templates under the action of T4 DNA ligase. Subsequently, dumbbell templates hybridized with primer to initiate the RCA reaction, obtaining numerous G-quadruplex sequences. Finally, N-methyl-mesoporphyrin IX (NMM) bound with G-quadruplex to generate enhanced fluorescence signal. The proposed assay system achieved excellent specificity and sensitivity toward TATA-binding protein (TBP) with a detection limit as low as 88pM, and with a linear range from 100pM to 40nM. The strategy proposed here was looking forward to offer a powerful tool for TFs related bioanalysis and disease diagnostics.

Ultrasensitive fluorescence detection of transcription factors based on kisscomplex formation and the T7 RNA polymerase amplification method

Herein, we report a kisscomplex based protein fluorescence assay (KPFA) method, which employed the formation of a kisscomplex and the T7 RNA polymerase amplification method, for the assay of transcription factors with high sensitivity. The detection limits of MITF and NF-κB p65 are 0.23 pM and 0.496 pM, respectively.

Maximizing the Signal Gain of Electrochemical-DNA Sensors

Electrochemical DNA (E-DNA) sensors have emerged as a promising class of biosensors capable of detecting a wide range of molecular analytes (nucleic acids, proteins, small molecules, inorganic ions) without the need for exogenous reagents or wash steps. In these sensors, a binding-induced conformational change in an electrode-bound "probe" (a target-binding nucleic acid or nucleic-acid-peptide chimera) alters the location of an attached redox reporter, leading to a change in electron transfer that is typically monitored using square-wave voltammetry. Because signaling in this class of sensors relies on binding-induced changes in electron transfer rate, the signal gain of such sensors (change in signal upon the addition of saturating target) is dependent on the frequency of the square-wave potential pulse used to interrogate them, with the optimal square-wave frequency depending on the structure of the probe, the nature of the redox reporter, and other features of the sensor. Here, we show that, because it alters the driving force of the redox reaction and thus electron transfer kinetics, signal gain in this class of sensors is also strongly dependent on the amplitude of the square-wave potential pulse. Specifically, we show here that the simultaneous optimization of square-wave frequency and amplitude produces large (often more than 2-fold) increases in the signal gain of a wide range of E-DNA-type sensors.

A novel dinuclear iridium(III) complex as a G-quadruplex-selective probe for the luminescent switch-on detection of transcription factor HIF-1α

A novel dinuclear Ir(III) complex 5 was discovered to be specific to G-quadruplex DNA, and was utilized in a label-free G-quadruplex-based detection platform for transcription factor activity. The principle of this assay was demonstrated by using HIF-1α as a model protein. Moreover, this HIF-1α detection assay exhibited potential use for biological sample analysis.

Amplified binding-induced homogeneous assay through catalytic cycling of analyte for ultrasensitive protein detection

Amplified binding-induced homogeneous assay through catalytic cycling of analytes for ultrasensitive protein detection.

Recent advances in transcription factor assays in vitro

We review the recent advances in transcription factor assaysin vitroand highlight the emerging trends as well.

A Modular, DNA-Based Beacon for Single-Step Fluorescence Detection of Antibodies and Other Proteins

A versatile platform for the one-step fluorescence detection of both monovalent and multivalent proteins has been developed. This system is based on a conformation-switching stem-loop DNA scaffold that presents a small-molecule, polypeptide, or nucleic-acid recognition element on each of its two stem strands. The steric strain associated with the binding of one (multivalent) or two (monovalent) target molecules to these elements opens the stem, enhancing the emission of an attached fluorophore/quencher pair. The sensors respond rapidly (<10 min) and selectively, enabling the facile detection of specific proteins even in complex samples, such as blood serum. The versatility of the platform was demonstrated by detecting five bivalent proteins (four antibodies and the chemokine platelet-derived growth factor) and two monovalent proteins (a Fab fragment and the transcription factor TBP) with low nanomolar detection limits and no detectable cross-reactivity.

A regenerated electrochemical biosensor for label-free detection of glucose and urea based on conformational switch of i-motif oligonucleotide probe

Improving the reproducibility of electrochemical signal remains a great challenge over the past decades. In this work, i-motif oligonucleotide probe-based electrochemical DNA (E-DNA) sensor is introduced for the first time as a regenerated sensing platform, which enhances the reproducibility of electrochemical signal, for label-free detection of glucose and urea. The addition of glucose or urea is able to activate glucose oxidase-catalyzed or urease-catalyzed reaction, inducing or destroying the formation of i-motif oligonucleotide probe. The conformational switch of oligonucleotide probe can be recorded by electrochemical impedance spectroscopy. Thus, the difference of electron transfer resistance is utilized for the quantitative determination of glucose and urea. We further demonstrate that the E-DNA sensor exhibits high selectivity, excellent stability, and remarkable regenerated ability. The human serum analysis indicates that this simple and regenerated strategy holds promising potential in future biosensing applications.

Effect of structure variation of the aptamer-DNA duplex probe on the performance of displacement-based electrochemical aptamer sensors

Electrochemical aptamer-based (E-AB) sensors employing electrode-immobilized, redox-tagged aptamer probes have emerged as a promising platform for the sensitive and quick detection of target analytes ranging from small molecules to proteins. Signal generation in this class of sensor is linked to change in electron transfer efficiency upon binding-induced change in flexibility/conformation of the aptamer probe. Because of this signaling mechanism, signal gains of these sensors can be improved by employing a displacement-based recognition system, which links target binding with a large-scale flexibility/conformation shift from the aptamer-DNA duplex to the single-stranded DNA or the native aptamer. Despite the relatively large number of displacement-based E-AB sensor samples, little attention has been paid to the structure variation of the aptamer-DNA duplex probe. Here we detail the effects of complementary length and position of the aptamer-DNA duplex probe on the performance of a model displacement-based E-AB sensor for ATP. We find that, greater background suppression and signal gain are observed with longer complementary length of the aptamer-DNA duplex probe. However, sensor equilibration time slows monotonically with increasing complementary length; and with too many target binding sites in aptamer sequence being occupied by the complementary DNA, the aptamer-target binding does not occur and no signal gain observed. We also demonstrate that signal gain of the displacement-based E-AB sensor is strongly dependent on the complementary position of the aptamer-DNA duplex probe, with complementary position located at the electrode-attached or redox-tagged end of the duplex probe, larger background suppression and signal increase than that of the middle position are observed. These results highlight the importance of rational structure design of the aptamer-DNA duplex probe and provide new insights into the optimization of displacement-based E-AB sensors.

Electrochemical aptamer scaffold biosensors for detection of botulism and ricin toxins

Protein toxins present considerable health risks, but detection often requires laborious analysis. Here, we developed electrochemical aptamer biosensors for ricin and botulinum neurotoxins, which display robust and specific signal at nanomolar concentrations and function in dilute serum. These biosensors may aid future efforts for the rapid diagnosis of toxins.

Nanoscale characterization of DNA conformation using dual-color fluorescence axial localization and label-free biosensing

Quantitative determination of the density and conformation of DNA molecules tethered to the surface can help optimize and understand DNA nanosensors and nanodevices, which use conformational or motional changes of surface-immobilized DNA for detection or actuation. We present an interferometric sensing platform that combines (i) dual-color fluorescence spectroscopy for precise axial co-localization of two fluorophores attached at different nucleotides of surface-immobilized DNA molecules and (ii) independent label-free quantification of biomolecule surface density at the same site. Using this platform, we examined the conformation of DNA molecules immobilized on a three-dimensional polymeric surface and demonstrated simultaneous detection of DNA conformational change and binding in real-time. These results demonstrate that independent quantification of both surface density and molecular nanoscale conformation constitutes a versatile approach for nanoscale solid-biochemical interface investigations and molecular binding assays.