Unimolecular Catalytic DNA Biosensor for Amplified Detection of l-Histidine via an Enzymatic Recycling Cleavage Strategy

DNAzyme-Based Microscale Thermophoresis Sensor

Microscale thermophoresis (MST) technology has emerged as a powerful growing method in a molecular interaction study by measuring fluorescence responses of molecules inside a capillary to infrared (IR) laser heating with the benefits of rapid ratiometric measurement, separation-free, no immobilization, and low sample consumption. Combining the advantages of RNA-cleaving DNAzymes in target recognition and enzymatic catalysis and the strength of MST technology for fluorescence signaling, here, we reported a DNAzyme-based MST method for sensitive target detection. We introduced a fluorescein terminal label at the RNA-cleaving DNAzyme, and the substrate was linked to DNAzyme together with a poly-T sequence in a unimolecular design or not conjugated with DNAzyme in a bimolecular design. The presence of the cofactor activated DNAzyme to catalytically cleave the substrate, causing molecular structure alteration and significant changes in MST signals. This DNAzyme MST sensor enabled sensitively detecting activator targets Pb²⁺ and l-histidine, with a detection limit of 49 pM Pb²⁺ and 3.9 μM l-histidine. This biosensing strategy is universal and promising for wide applications.

Recent Advances in Self-Assembled DNA Nanostructures for Bioimaging

DNA nanotechnology has been proven to be a powerful platform to assist the development of imaging probes for biomedical research. The attractive features of DNA nanostructures, such as nanometer precision, controllable size, programmable functions, and biocompatibility, have enabled researchers to design and customize DNA nanoprobes for bioimaging applications. However, DNA probes with low molecular weights (e.g., 10-100 nt) generally suffer from low stability in physiological buffer environments. To improve the stability of DNA nanoprobes in such environments, DNA nanostructures can be designed with relatively larger sizes and defined shapes. In addition, the established modification methods for DNA nanostructures are also essential in enhancing their properties and performances in a physiological environment. In this review, we begin with a brief recap of the development of DNA nanostructures including DNA tiles, DNA origami, and multifunctional DNA nanostructures with modifications. Then we highlight the recent advances of DNA nanostructures for bioimaging, emphasizing the latest developments in probe modifications and DNA-PAINT imaging. Multiple imaging modules for intracellular biomolecular imaging and cell membrane biomarkers recognition are also summarized. In the end, we discuss the advantages and challenges of applying DNA nanostructures in bioimaging research and speculate on its future developments.

A Convenient and Label-Free Colorimetric Detection for L-Histidine Based on Inhibition of Oxidation of 3,3',5,5'-Tetramethylbenzidine-H2O2 System Triggered by Copper Ions

L-Histidine (L-His) is an essential amino acid, which is used to synthesize proteins and enzymes. The concentration of L-His in the body is controlled to regulate tissue growth and repair of tissues. In this study, a rapid and sensitive method was developed for colorimetric L-his detection using Cu²⁺ ions to inhibit the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂ system. H₂O₂ can oxidize TMB to oxTMB in the presence of copper, and the change in color from colorless (TMB) to blue (oxTMB) is similar to that observed in the presence of peroxidase. However, because the imidazole ring and carboxyl group of L-His can coordinate with Cu²⁺ ions to form stable L-His-Cu²⁺ complexes, the color of the TMB-H₂O₂ solution remains unchanged after the addition of L-His. Therefore, because L-His effectively hinders the colorimetric reaction of TMB with H₂O₂, this assay can be used to quantitatively determine the concentration of L-His in samples. Under optimized conditions, our colorimetric sensor exhibited two linear ranges of 60 nM to 1 μM and 1 μM to 1 mM for L-His detection and a detection limit of 50 nM (S/N = 3); furthermore, the assay can be performed within 20 min. Moreover, the proposed assay was used to determine the concentration of L-His in urine samples, suggesting that this convenient and label-free colorimetric method presents promising applications in bioanalytical chemistry and clinical diagnosis.

A Practical Electrochemical Nanotool for Facile Quantification of Amino Acids in Single Cell

Though significant advances are made in the arena of single-cell electroanalysis, quantification of intracellular amino acids of human cells remains unsolved. Exemplified by l-histidine (l-His), this issue is addressed by a practical electrochemical nanotool synergizing the highly accessible nanopipette with commercially available synthetic DNAzyme. The fabricated nanotools are screened before operation of a single-use manner, and the l-His-provoked cleavage of the DNA molecules can be sensibly transduced by the ionic current rectification response, the intrinsic property of nanopipette governed by its interior surface charges. Regional distribution of cytosolic l-His level in human cells is electrochemically quantified for the first time, and time-dependent drug treatment effects are further revealed. This work unveils the possibility of electrochemistry for quantification of cytosolic amino acids of a spatial- and time-based manner and ultimately enables a better understanding of amino acid-involved events in living cells.

A Bimetallic Metal-Organic Framework Encapsulated with DNAzyme for Intracellular Drug Synthesis and Self-Sufficient Gene Therapy

Although chemotherapy is one of the most widely used cancer treatments, there are serious side effects, drug resistance, and secondary metastasis. To address these problems, herein we designed a bimetallic metal-organic framework (MOF) encapsulated with DNAzyme for co-triggered in situ cancer drug synthesis and DNAzyme-based gene therapy. Once in cancer cells, MOFs would disassemble and liberate copper ions, zinc ions, and DNAzyme under the acidic environment of lysosomes. Copper ions can catalyze the synthesis of the chemotherapeutic drug through copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction after being reduced to Cu^I ; zinc ions act as the cofactor to activate the cleavage activity of DNAzyme. The anticancer drug is synthesized intracellularly and can kill cancer cells on site to minimize side effects to normal organisms. The activated DNAzyme starts gene therapy to inhibit tumor proliferation and metastasis by targeting and cleaving oncogene substrates.

DNAzyme-powered nucleic acid release from solid supports

Here, we demonstrate use of a Mg²⁺-dependent, site-specific DNA enzyme (DNAzyme) to cleave oligos from polyacrylamide gel beads, which is suitable for use in drop-based assays. We show that cleavage efficiency is improved by use of a tandem-repeat cleavage site. We further demonstrate that DNAzyme-released oligos function as primers in reverse transcription of cell-released mRNA.

Entropy-driven DNA logic circuits regulated by DNAzyme

The catalytic DNA circuits play a critical role in engineered biological systems and molecular information processing. Actually, some of the natural or synthetic DNA circuits were triggered by covalent modifications, where conformational changes were induced to facilitate complex DNA engineering functions and signal transmissions. However, most of the reported artificial catalytic DNA circuits were regulated by the toehold-mediated reaction. Therefore, it is significant to propose a strategy to regulate the catalytic DNA circuit not only by the toehold-mediated mechanism, but also by involving the conformational changes induced by the covalent modification. In this study, we developed the catalytic DNA logic circuits regulated by DNAzyme. Here, a regulation strategy based on the covalent modification was proposed to control the DNA circuit, combing two reaction mechanisms: DNAzyme digestion and entropy-driven strand displacement. The DNAzyme and DNA catalyst can participate into the reactions alternatively, thus realizing the cascading catalytic circuits. Using the DNAzyme regulation, a series of logic gates (YES, OR and AND) were constructed. In addition, a two-layer cascading circuit and a feedback self-catalysis circuit were also established. The proposed DNAzyme-regulated strategy shows great potentials as a reliable and feasible method for constructing more complex catalytic DNA circuits.

Silver-ion-mediated Mg2+-dependent DNAzyme activity for amplified fluorescence detection of cysteine

Schematic illustration of a DNAzyme-based fluorescent biosensor for amplified Cys detection.

A simple mix-and-read bacteria detection system based on a DNAzyme and a molecular beacon

A simple improved mix-and-read method for the detection of bacteria is developed based on a DNAzyme and a molecular beacon.

A DNA-Mediated Chemically Induced Dimerization (D-CID) Nanodevice for Nongenetic Receptor Engineering To Control Cell Behavior

Small-molecule regulation is a powerful switching tool to manipulate cell signal transduction for a desired function; however, most available methods usually require genetic engineering to endow cells with responsiveness to user-defined small molecules. Herein, we demonstrate a nongenetic approach for small-molecule-controlled receptor activation and consequent cell behavior manipulation that is based on DNA-mediated chemically induced dimerization (D-CID). D-CID uses a programmable chemical-responsive DNA nanodevice to trigger DNA strand displacement and induce the activation of c-Met, a tyrosine kinase receptor cognate for hepatocyte growth factor, through dimerization. Through the use of various functional nucleic acids, including aptamers and DNAzymes, as recognition modules, the versatility of D-CID in inducing c-Met signaling upon addition of various small-molecular or ionic cues, including ATP, histidine, and Zn²⁺ , is demonstrated. Moreover, owing its multi-input properties, D-CID can be used to manipulate the behaviors of multiple cell populations simultaneously in a selective and programmable fashion.

RNA splicing process analysis for identifying antisense oligonucleotide inhibitors with padlock probe-based isothermal amplification

We report a highly sensitive method for quantifying the splicing products in different steps, enabling us to analyze the splicing process and identify ASO inhibitors.

RNA splicing, which mainly involves two transesterification steps, is a fundamental process of gene expression and its abnormal regulation contributes to serious genetic diseases. Antisense oligonucleotides (ASOs) are genetic control tools that can be used to specifically control genes through alteration of the RNA splicing pathway. Despite intensive research, how ASOs or various other factors influence the multiple processes of RNA splicing still remains obscure. This is largely due to an inability to analyze the splicing efficiency of each step in the RNA splicing process with high sensitivity. We addressed this limitation by introducing a padlock probe-based isothermal amplification assay to achieve quantification of the specific products in different splicing steps. With this amplified assay, the roles that ASOs play in RNA splicing inhibition in the first and second steps could be distinguished. We identified that 5′-ASO could block RNA splicing by inhibiting the first step, while 3′-ASO could block RNA splicing by inhibiting the second step. This method provides a versatile tool for assisting efficient ASO design and discovering new splicing modulators and therapeutic drugs.

Ag nanocluster-based label-free catalytic and molecular beacons for amplified biosensing

By employing DNAzyme as a recognition group and amplifier, and DNA-stabilized silver nanoclusters (DNA/AgNCs) as signal reporters, we reported for the first time a label-free catalytic and molecular beacon as an amplified biosensing platform for highly selective detection of cofactors such as Pb(2+) and L-histidine.

Dendritic DNA-porphyrin as mimetic enzyme for amplified fluorescent detection of DNA

In this work, a novel dendritic DNA-porphyrin superstructure was designed as mimetic enzyme for the amplified fluorescent detection of DNA. The dendritic DNA superstructure was in situ assembled with three auxiliary DNAs via hybridization chain reaction. With groove interaction between iron porphyrin (FeTMPyP) and double-stranded DNA, the dendritic DNA superstructure is capable to gather abundant FeTMPyP molecules to form dendritic DNA-FeTMPyP mimetic enzyme. Using tyramine as a substrate, the dendritic DNA-FeTMPyP demonstrated excellent peroxidase-like catalytic oxidation of tyramine into fluorescent dityramine in the presence of H2O2. Based on an amplified fluorescence signal, a signal on strategy is proposed for DNA detection with high sensitivity, good specificity and practicability. The assembly of porphyrin with dendritic DNA not only provided the new avenue to construct mimetic enzyme but also established label-free sensing platform for a wide range of analytes.

Highly selective ratiometric fluorescent recognition of histidine by tetraphenylethene–terpyridine–Zn(ii) complexes

Fluorescent detection of histidine is achieved with distinctive color change from yellow to blue by using the Zn(ii) complexes of the terpyridine–tetraphenylethene conjugates.

Isothermal Amplification of Nucleic Acids

Isothermal amplification of nucleic acids is a simple process that rapidly and efficiently accumulates nucleic acid sequences at constant temperature. Since the early 1990s, various isothermal amplification techniques have been developed as alternatives to polymerase chain reaction (PCR). These isothermal amplification methods have been used for biosensing targets such as DNA, RNA, cells, proteins, small molecules, and ions. The applications of these techniques for in situ or intracellular bioimaging and sequencing have been amply demonstrated. Amplicons produced by isothermal amplification methods have also been utilized to construct versatile nucleic acid nanomaterials for promising applications in biomedicine, bioimaging, and biosensing. The integration of isothermal amplification into microsystems or portable devices improves nucleic acid-based on-site assays and confers high sensitivity. Single-cell and single-molecule analyses have also been implemented based on integrated microfluidic systems. In this review, we provide a comprehensive overview of the isothermal amplification of nucleic acids encompassing work published in the past two decades. First, different isothermal amplification techniques are classified into three types based on reaction kinetics. Then, we summarize the applications of isothermal amplification in bioanalysis, diagnostics, nanotechnology, materials science, and device integration. Finally, several challenges and perspectives in the field are discussed.

Target-fueled DNA walker for highly selective miRNA detection

We report a DNA walking biosensor that can realize the detection of let-7a with a detection limit of 58 fM and high selectivity for resolving one nucleotide variation.

Artificial DNA motifs as architectural scaffolds have been widely used to assemble a variety of nanoscale devices. Synthetic DNA nanostructures have accomplished mechanical switching in response to external stimuli, suggesting the promise of constructing a walking device that is being used in the field of biosensors. Here, we design a novel miRNA-responsive DNA walker biosensor based on strand displacement cascades and an enzymatic recycling cleavage strategy. By using miRNA as a driving force, the DNA walkers can be activated to move along the track and generate specific signals for let-7a with a high signal-to-noise ratio. This biosensor exhibits excellent analytical performance toward the sensing of let-7a with great specificity for resolving one nucleotide variation and a detection limit of 58 fM. Such an ultraselective sensor shows that DNA nanostructures have great potential in providing platforms for applications in the fields of biosensing, clinical diagnostics and environmental sample analysis.

Functional nucleic acid-based sensors for heavy metal ion assays

Heavy metal contaminants such as lead ions (Pb(2+)), mercury ions (Hg(2+)) and silver ions (Ag(+)) can cause significant harm to humans and generate enduring bioaccumulation in ecological systems. Even though a variety of methods have been developed for Pb(2+), Hg(2+) and Ag(+) assays, most of them are usually laborious and time-consuming with poor sensitivity. Due to their unique advantages of excellent catalytic properties and high affinity for heavy metal ions, functional nucleic acids such as DNAzymes and aptamers show great promise in the development of novel sensors for heavy metal ion assays. In this review, we summarize the development of functional nucleic acid-based sensors for the detection of Pb(2+), Hg(2+) and Ag(+), and especially focus on two categories including the direct assay and the amplification-based assay. We highlight the emerging trends in the development of sensitive and selective sensors for heavy metal ion assays as well.

Synthetic biology and biomimetic chemistry as converging technologies fostering a new generation of smart biosensors

Biosensors are powerful tunable systems able to switch between an ON/OFF status in response to an external stimulus. This extraordinary property could be engineered by adopting synthetic biology or biomimetic chemistry to obtain tailor-made biosensors having the desired requirements of robustness, sensitivity and detection range. Recent advances in both disciplines, in fact, allow to re-design the configuration of the sensing elements - either by modifying toggle switches and gene networks, or by producing synthetic entities mimicking key properties of natural molecules. The present review considered the role of synthetic biology in sustaining biosensor technology, reporting examples from the literature and reflecting on the features that make it a useful tool for designing and constructing engineered biological systems for sensing application. Besides, a section dedicated to bioinspired synthetic molecules as powerful tools to enhance biosensor potential is reported, and treated as an extension of the concept of biomimetic chemistry, where organic synthesis is used to generate artificial molecules that mimic natural molecules. Thus, the design of synthetic molecules, such as aptamers, biomimetics, molecular imprinting polymers, peptide nucleic acids, and ribozymes were encompassed as "products" of biomimetic chemistry.

Ultra-Low Level Detection of L-Histidine Using Solution-Processed ZnO Nanorod on Flexible Substrate

This work demonstrates a novel label free and sensitive approach for the detection of L-histidine. This is a simple and reliable method for ultra-low level detection of L-histidine. All solution processed synthesizing technique was utilized to develop such type of detection scheme. Silicon substrate was replaced by normal transparent sheet to make it more facile and cost-effective detection technique. Fabricated device for L-histidine detection works upon the variation of current through the ZnO nanorod with L-histidine concentration. Operation principle strongly depends upon the electron charge transfer between metal cation and L-histidine inside the chelating complex. Morphological, structural and optical characterization of solution processed synthesized ZnO nanorod (ZnO NR) was carried out prior to sensor device fabrication. Our sensor device exhibits the sensitivity around 0.86 nA/fM and lower limit of detection (LOD) ∼ 0.1 fM(S/N=3).

Highly selective and sensitive recognition of histidine based on the oxidase-like activity of Cu2+ions

A simple, highly sensitive and selective PL sensing platform for histidine has been developed, based on the oxidase-like activity of Cu²⁺ions.

A novel glutathione-stabilized silver–gold nano-alloy/Cu2+ combination as a fluorescent switch probe for l-histidine

This paper reports the synthesis of glutathione-stabilized silver–gold nano-alloys and their use as a fluorescent switch probe for the detection of l-histidine.

Versatile biosensing platform for DNA detection based on a DNAzyme and restriction-endonuclease-assisted recycling

On the basis of a DNAzyme and a restriction-endonuclease-assisted target recycling strategy using Pd-Au alloy nanocrystals to immobilize probe DNA on an electrode and catalyze the reduction of H2O2 which amplified signal and promoted the detection sensitivity, a versatile biosensing platform for DNA detection was proposed. Using p53 and oral cancer genes as models, hemin/G-quadruplex simultaneously acted as a reduced nicotinamide adenine dinucleotide (NADH) oxidase and a horseradish peroxidase (HRP)-mimicking DNAzyme, and a versatile DNA biosensor was designed for the first time based on the good electrocatalytic activity of Pd-Au alloy nanocrystals. Hemin/G-quadruplex catalyzed the reduction of H2O2, which was generated from NADH in the presence of O2, to produce an electrochemical signal when thionine functioned as the electron mediator. Moreover, the nicking endonuclease N.BstNB I caused the target DNA to cycle for multiple rounds and further amplified the electrochemical response. This versatile DNA biosensor exhibited linear ranges for the detection of p53 and oral cancer genes from 0.1 fmol L(-1) to 0.1 nmol L(-1) and 0.1 fmol L(-1) to 1 nmol L(-1), respectively. The detection limits, established as 3σ, were estimated to be 0.03 and 0.06 fmol L(-1) for the p53 and oral cancer genes, respectively. The as-prepared biosensor could discriminate mismatched sequences, indicating a satisfactory selectivity and validating the feasibility of the proposed strategy. More importantly, simply by changing the helper DNA, this versatile DNA biosensor could detect different target DNA species, which could create a new avenue for the potential diagnosis of cancer.

Cleaved DNAzyme substrate induced enzymatic cascade for the exponential amplified analysis of L-histidine

A novel strategy of cleaved DNAzyme substrate induced enzymatic cascade has been devised for the exponential amplified detection of L-histidine. The enzyme strand carries out hydrolytic cleavage of the substrate strand in the presence of L-histidine. The cleaved DNAzyme substrates introduce the polymerase/endonuclease reaction cycles as primers. The L-histidine acts as the activator for enzymatic cascade amplification generating a distinguishable fluorescence enhancement. A good nonlinear correlation (R=0.9994) between fluorescence intensity and the logarithm of the L-histidine concentration is obtained over the range from 50 nM to 1.0 mM. The detection limit was estimated as 30 nM. This efficient amplification of the fluorescence signal is attributed to the L-histidine induced cooperation of Klenow Fragment polymerase (exo(-)) and Nb.BbvCI endonuclease reaction. The activation of such enzymatic cascades through analyte-DNAzyme interactions has a substantial impact on the development of exponential amplified DNAzyme sensors.

Self-phosphorylating deoxyribozyme initiated cascade enzymatic amplification for guanosine-5'-triphosphate detection

The self-phosphorylating deoxyribozymes identified by in vitro selection can catalyze their own phosphorylation by utilizing phosphate donor guanosine-5'-triphosphate (GTP) which plays a critical role in a majority of cellular processes. On the basis of the unique properties of self-phosphorylating deoxyribozymes, we report a novel GTP sensor coupled with λ exonuclease cleavage reaction and nicking enzyme assisted fluorescence signal amplification process. The deoxyribozymes with special catalytic and structural characteristics display good stability compared to protein and RNA enzymes. We combined these properties with enzymatic recycling cleavage strategy to build a sensor which produced enhanced fluorescence signal. Sensitive and selective detection of GTP was successfully realized with the well-designed deoxyribozyme-based sensing platform by taking advantage of the self-phosphorylating ability of the kinase deoxyribozyme, efficient digestion capacity of λ exonuclease, and enzymatic recycling amplification of nicking enzyme. The method not only provides a platform for detecting GTP but also shows great potential in analyzing a variety of targets by combining deoxyribozymes with signal amplification strategy.

DNA dendrimer: an efficient nanocarrier of functional nucleic acids for intracellular molecular sensing

Functional nucleic acid (FNA)-based sensing systems have been developed for efficient detection of a wide range of biorelated analytes by employing DNAzymes or aptamers as recognition units. However, their intracellular delivery has always been a concern, mainly in delivery efficiency, kinetics, and the amount of delivered FNAs. Here we report a DNA dendrimer scaffold as an efficient nanocarrier to deliver FNAs and to conduct in situ monitoring of biological molecules in living cells. A histidine-dependent DNAzyme and an anti-ATP aptamer were chosen separately as the model FNAs to make the FNA dendrimer. The FNA-embedded DNA dendrimers maintained the catalytic activity of the DNAzyme or the aptamer recognition function toward ATP in the cellular environment, with no change in sensitivity or specificity. Moreover, these DNA dendrimeric nanocarriers show excellent biocompatibility, high intracellular delivery efficiency, and sufficient stability in a cellular environment. This FNA dendrimeric nanocarrier may find a broad spectrum of applications in biomedical diagnosis and therapy.

A DNA–scaffolded silver nanocluster/Cu2+ ensemble as a turn-on fluorescent probe for histidine

A new type of rapid, sensitive, and selective fluorescence turn-on assay was developed for detection of histidine using a DNA–scaffolded silver nanocluster/Cu²⁺ ensemble (DNA–AgNC/Cu²⁺).

Target-triggered cyclic assembly of DNA-protein hybrid nanowires for dual-amplified fluorescence anisotropy assay of small molecules

Aptamer-based fluorescence anisotropy (FA) assays have attracted great interest in recent years. However, a key factor that determines FA value is molar mass, thus limiting the utility of this assay for the detection of small molecules. To solve this problem, streptavidin, as a molar mass amplifier, was used in a hybridization chain reaction (HCR) to construct a target-triggered cyclic assembly of DNA-protein hybrid nanowires for highly sensitive detection of small molecules by fluorescence anisotropy. In this assay, one blocking DNA strand is released by target-aptamer recognition. The DNA then serves as an initiator to trigger enzyme-free autonomous cross-opening of hairpin probes via HCR to form a DNA nanowire for further assembly of streptavidin. Using adenosine triphosphate (ATP) as the small molecule target, this novel dual-amplified, aptamer-based FA assay affords high sensitivity with a detection limit of 100 nM. This limit of detection (LOD) is much lower than that of the disassembly approach without HCR amplification or the assembly strategy without streptavidin. In contrast to the previous turn-off disassembly approaches based on nonspecific interactions between the aptamer probe and amplification moieties, the proposed aptamer-based FA assay method exhibits a turn-on response to ATP, which can increase sensing reliability and reduce the risk of false hits. Moreover, because of its resistance to environmental interferences, this FA assay has been successfully applied for direct detection of 0.5 μM ATP in complex biological samples, including cell media, human urine, and human serum, demonstrating its practicality in real complex biological systems.

A portable chemical sensor for histidine based on the strategy of click chemistry

A novel portable chemical sensor is developed in combination of the personal glucose meters (PGM) with click chemistry for sensitive and selective determination of histidine. Invertase-labeled alkynyl-DNA can be modified onto the surfaces of Streptavidin Magnespheres Paramagnetic Particles (PMPs) through copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction and formed invertase-functionalized PMPs, which can be separated easily. The presence of invertase can convert sucrose to glucose and can be monitored by the PGM easily. The presence of histidine can inhibit the CuAAC, so the read-out signal of PGM decreased. The difference in signals from the PGM before and after addition of histidine has a good linear correlation with the logarithm of the histidine concentrations in the range of 0.01~100 μM with a detection limit of 3.4 nM, which is lower than those of many other chemical sensors. Moreover, the assay of histidine in milk samples is demonstrated with satisfactory results.

A label-free G-quadruplex-based luminescent switch-on assay for the selective detection of histidine

A label-free G-quadruplex-based luminescent switch-on assay has been developed for the selective detection of micromolar histidine in aqueous solution. In this study, an iridium(III) complex was employed as a G-quadruplex-specific luminescent probe while a guanine-rich oligonucleotide (Pu27, 5'-TG4AG3TG4AG3TG4A2G2-3')/cupric ion (Cu(2+)) ensemble was employed as a recognition unit for histidine. The initial luminescence of the iridium(III) complex in the presence of G-quadruplex DNA is effectively quenched by Cu(2+) ions due to the Cu(2+)-mediated unfolding of the G-quadruplex motif. The addition of histidine sequesters Cu(2+) ions from the ensemble, thereby restoring the luminescence of the system. The assay could detect down to 1 μM of histidine in aqueous media, and also exhibited good selectivity for histidine over other amino acids with the use of the cysteine, masking agent N-ethylmaleimide. Furthermore, the application of the assay for the detection of histidine in diluted urine samples was demonstrated.

Catalytic DNA-based fluorescence polarization chiral sensing platform for L-histidine detection at trace level

This paper reports a novel fluorescence polarization (FP) chiral sensor approach based on a catalytic DNA. This platform involves an enzyme module (E), which was able to trigger the L-histidine-dependent cleavage of an RNA phosphoester bond of a substrate domain (S), whereas it did not accept the D-enantiomer as cofactor. Two assay formats were proposed, based on bi- and unimolecular strategies. The bimolecular design was related to the use of separate E and fluorescently labelled S* sequences. The two oligonucleotide strands were pre-assembled via complementary regions at their extremities. As the result of the large molecular volume of the formed assembly, the S* probe displayed a high fluorescence anisotropy signal. Upon addition of the L-histidine, the DNAzyme cleaved the phosphoester bond of the S* component, leading to the loss of stem stability and the release of single-stranded products of lower size. This was accompanied by a significant decrease in the fluorescence anisotropy response. As a simpler alternative, the unimolecular design, where E and S sequences are linked together through a loop to form a single fluorescent probe E-S*, was also investigated. It was found that the unimolecular approach provided an improved FP response relative to the bimolecular one. Under optimized operating conditions, such a chiral sensing platform allowed the detection of as low as 0.05% of the L-histidine enantiomer in a non-racemic mixture.

An aptasensing platform for simultaneous detection of multiple analytes based on the amplification of exonuclease-catalyzed target recycling and DNA concatemers

In the present study, an ultrasensitive electrochemical aptasensor for the simultaneous detection of thrombin (TB) and ochratoxin A (OTA) was fabricated by using exonuclease-catalyzed target recycling and DNA concatemers for signal amplification. The previously hybridized double-stranded DNAs (SH-cTBA/TBA and SH-cOBA/OBA) were self-assembled on a gold electrode. In the presence of targets, the formation of aptamer-target complexes would lead to not only the dissociation of aptamers (TBA and OBA) from the double-stranded DNAs but also the transformation of the complementary DNAs (SH-cTBA and SH-cOBA) into hairpin structures. Subsequently, owing to employment of RecJf exonuclease, which is a single-stranded DNA-specific exonuclease to selectively digest the appointed DNAs (TBA and OBA), the targets could be liberated from the aptamer-target complexes for recycling of the analytes. Thereafter, probe DNAs (T1 and T2) were employed to hybridize with SH-cTBA and SH-cOBA respectively to provide primers for the concatemer reaction. After that, when four auxiliary DNA strands S1, anthraquinone-2-carboxylic acid (AQ)-labeled S2, S3, S4, as well as hemin were introduced, extended dsDNA polymers with lots of AQ moieties and hemin-G-quadruplex complexes could form on the electrode surface. Then, based on the signal of the AQ and hemin-G-quadruplex complex, an electrochemical aptasensor for the simultaneous detection of TB and OTA was successfully fabricated.