Target-responsive DNAzyme hydrogel for portable colorimetric detection of lanthanide(Ⅲ) ions

Lanthanide elements(Ln)play an important role in industry and agriculture.As a result of the increasing consumption of lanthanides,environmental emission of Ln has become detrimental to the health of flora and fauna.Current methods for trace lanthanides detection mainly rely on sophisticated instruments.In this article,a Ln~(3+)dependent DNAzyme was incorporated into a hydrogel to generate Ln~(3+)sensitive DNAzyme hydrogel for portable colorimetric detection.The enzyme strand and its substrate strand act as crosslinker and functional unit of the hydrogel with polyacrylamide chains as the scaffold and gold nanoparticles(AuNPs)as the indicator of hydrogel stability.Any ions in the Ln~(3+)series can trigger the cleavage of substrate strand by activating the enzyme strand,thereby decreasing the crosslink ratio and leading to collapse of the hydrogel.The release of the encapsulated AuNPs turns the supernatant wine red.Using this colorimetric method,Ln~(3+)can be detected with high sensitivity,with a limit of detection(LOD)of 20 nM for Ce~(3+).The hydrogel responds specifically to any Ln~(3+)ion and works well with the spiked lake sample without the need of instruments and skilled operators.Our results suggest that the lanthanide responsive hydrogel can be used for portable and sensitive detection of Ln~(3+)contamination in the field.     

Ultrasensitive detection of lead ion based on target induced assembly of DNAzyme modified gold nanoparticle and graphene oxide

In this paper, we report a novel colorimetric strategy for the detection of small molecules by using Pb²⁺ ion as an example. In this strategy, DNAzyme duplex modified gold nanoparticles (GNPs) are designed to be unable to interact with graphene oxide (GO). However, in the presence of Pb²⁺, the substrate strand of the DNAzyme is cleaved at its cleavage site, resulting in the disassembly of the DNAzyme duplex modified GNPs into three parts, i.e., the 3′- and 5′-fragments of substrate strand and the DNAzyme strand modified GNPs. By taking advantage of the efficient cross-linking effect of ssDNA-GNPs to GO, colorimetric sensor for the detection of the metal ion can be fabricated with a detection limit of 100 pM, which is much lower than the previous reports. This colorimetric method has also been used for the determination of Pb²⁺ in the tap water of the local city and the water from a reservoir with satisfactory results, so it may have potential applications in the future.     

Dual Signal Amplification Electrochemical Biosensor for Lead Cation

Herein, a sensitive electrochemical Pb²⁺ sensor was developed which based on DNA‐functionalized Au nanoparticles(AuNPs) and nanocomposite modified electrode. The DNA‐functionalized AuNPs includes two types of DNA, namely a Pb²⁺‐mediated DNAzyme comprising a biotin labeled‐enzyme DNA and a substrate strand DNA with a typical stem‐loop structure, and a ferrocene‐labeled linear signal DNA. Without Pb²⁺, the hairpin loop impeded biotin binding to avidin on the electrode. However,when the goal Pb²⁺ exists, the substratum strand was divided into two fragments that lead to the enzyme strand was substratumed on the electrode and biotin was admited by avidin, bringing about DNA‐functionalized AuNP(AuNPs) deposition on the electrode surface.The differential pulse voltammetry (DPV) was used to measure electrochemical response signals connect to signal DNA.For the amplification characters of the DNA‐functionalized AuNPs and nanocomposite, the electrochemical detection signal of Pb²⁺ was greatly improved and revealed high specificity. Under optimum conditions, the resultant biosensor bringed out a high sensitivity and selectivity for the determination of Pb²⁺. The proposed method was able to detect as low as picomolar Pb²⁺ concentrations.     

G-quadruplex-assisted enzyme strand recycling for amplified label-free fluorescent detection of UO22+

A G-quadruplex-assisted enzyme strand recycling strategy was developed for amplified label-free fluorescent detection of uranyl ion (UO₂²⁺).     

Synthesis and Characteristics of Self-Assembled Multifunctional Ln3+-DNA Hybrid Coordination Polymers

Owing to the unique catalytic, optical and magnetic properties, lanthanides (Ln) as multicomponent biomarkers, are widely used in the field of optical sensing, mass spectrometry and magnetic resonance imaging. As ligands, DNA molecules have good biocompatibility, high stability, cost efficiency, programmability and biodegradability. Based on the coordination-driven self-assembly between Ln ions (Ln³⁺) and DNA molecules, a multifunctional Ln³⁺-DNA hybrid coordination polymers (CPs) were synthesized. Not only a series of different Ln³⁺ (single Ln³⁺) and DNA hybrid CPs were synthesized, but one hybrid CP contains two kinds of Ln³⁺ was obtained. Besides, the synthetic CPs in cell fluorescence imaging and miRNA sensing also exhibited high performance. This work provides a novel idea for the synthesis of DNA based nanomaterials, which is promising for biologically-related applications.     

Detection of Metal Ions (Cu2+, Hg2+) and Cocaine by Using Ligation DNAzyme Machinery

The Cu²⁺‐dependent ligation DNAzyme is implemented as a biocatalyst for the colorimetric or chemiluminescence detection of Cu²⁺ ions, Hg²⁺ ions, or cocaine. These sensing platforms are based on the structural tailoring of the sequence of the Cu²⁺‐dependent ligation DNAzyme for specific analytes. The tethering of a subunit of the hemin/G‐quadruplex DNAzyme to the ligation DNAzyme sequence, and the incorporation of an imidazole‐functionalized nucleic‐acid sequence, which acts as a co‐substrate for the ligation DNAzyme that is tethered to the complementary hemin/G‐quadruplex subunit. In the presence of different analytes, Cu²⁺ ions, Hg²⁺ ions, or cocaine, the pretailored Cu²⁺‐dependent ligation DNAzyme sequence stimulates the respective ligation process by combining the imidazole‐functionalized co‐substrate with the ligation DNAzyme sequence. These reactions lead to the self‐assembly of stable hemin/G‐quadruplex DNAzyme nanostructures that enable the colorimetric analysis of the substrate through the DNAzyme‐catalyzed oxidation of 2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonic acid), ABTS^2?, by H₂O₂ into the colored product ABTS^.?, or the chemiluminescence detection of the substrate through the DNAzyme‐catalyzed oxidation of luminol by H₂O₂. The detection limits for the sensing of Cu²⁺ ions, Hg²⁺ ions, and cocaine correspond to 1 nM , 10 nM and 2.5 μM , respectively. These different sensing platforms also reveal impressive selectivities.     

Signal-amplification detection of small molecules by use of Mg2+- dependent DNAzyme

Because small molecules can be beneficial or toxic in biology and the environment, specific and sensitive detection of small molecules is one of the most important objectives of the scientific community. In this study, new signal amplification assays for detection of small molecules based on Mg²⁺-dependent DNAzyme were developed. A cleavable DNA substrate containing a ribonucleotide, the ends of which were labeled with black hole quencher (BHQ) and 6-carboxyfluorescein (FAM), was used for fluorescence detection. When the small molecule of interest is added to the assay solution, the Mg²⁺-dependent DNAzyme is activated, facilitating hybridization between the Mg²⁺-dependent DNAzyme and the DNA substrate. Binding of the substrate to the DNAzyme structure results in hydrolytic cleavage of the substrate in the presence of Mg²⁺ ions. The fluorescence signal was amplified by continuous cleavage of the enzyme substrate. Ochratoxin A (OTA) and adenosine triphosphate (ATP) were used as model analytes in these experiments. This method can detect OTA specifically with a detection limit as low as 140 pmol?L^?1 and detect ATP specifically with a detection limit as low as 13 nmol?L^?1. Moreover, this method is potentially extendable to detection of other small molecules which are able to dissociate the aptamer from the DNAzyme, leading to activation of the DNAzyme.     

A novel colorimetric aptasensor for detection of chloramphenicol based on lanthanum ion–assisted gold nanoparticle aggregation and smartphone imaging

A label-free, rapid response colorimetric aptasensor for sensitive detection of chloramphenicol (CAP) was proposed, which was based on the strategy of ssDNA-modified gold nanoparticle (AuNP) aggregation assisted by lanthanum (La³⁺) ions. The AuNPs generated a color change that could be monitored in the red, green, and blue and analyzed by the smartphone imaging app. La³⁺, as a trigger agent, strongly combined with the phosphate groups of the surface of ssDNA-AuNPs probe, which helps create AuNP aggregation and the color change of AuNPs from red to blue. On the contrary, when mixing with CAP, the aptamer (Apt) bound to CAP to form a rigid structure of the Apt-CAP complex, and La³⁺ attached to the phosphate groups of the complex, which prevented the aptamer from binding to the surface of the AuNPs. As a result, the color of the AuNPs changed to violet-red. Finally, UV-vis absorption spectroscopy and the smartphone imaging app were employed to determine CAP with a lower detection limit of 7.65 nM and 5.88 nM, respectively. The proposed strategy featuring high selectivity and strong anti-interference ability for detection of CAP in practical samples was achieved. It is worth mentioning that the simple and portable colorimetric aptasensor will be used for facilitating on-site detection of food samples.

     

An Electrochemical Sensor for the Detection of Cu2+ Based on Gold Nanoflowers‐modifed Electrode and DNAzyme Functionalized Au@MIL‐101 (Fe)

In this study, we developed an electrochemical sensor for sensitive detection of Cu²⁺ based on gold nanoflowers (AuNFs)‐modified electrode and DNAzyme functionalized Au@MIL‐101(Fe) (MIL: Materials of Institute Lavoisier). The AuNFs‐modified indium tin oxide modified conductive glass electrode(AuNFs/ITO) prepared via electrodeposition showed improved electronic transport properties and provided more active sites to adsorb large amounts of oligonucleotide substrate (DNA1) via thiol‐gold bonds. The stable Au@MIL‐101(Fe) could guarantee the sensitivity because of its intrinsic peroxidase mimic property, while the Cu²⁺‐dependent DNA‐cleaving DNAzyme linked to Au@MIL‐101(Fe) achieved the selectivity toward Cu²⁺. After the DNAzyme substrate strand (DNA2) was cleaved into two parts due to the presence of Cu²⁺, the oligonucleotide fragment linked to MIL‐101(Fe) was able to hybridize with DNA1 adsorbed onto the surface of AuNFs/ITO. Due to the peroxidase‐like catalytic activity of MIL‐101(Fe) and the affinity recognition property of DNAzyme toward Cu²⁺, the electrochemical biosensor showed a sensitive detection range from 0.001 to 100 μM, a detection limit of 0.457 nM and a high selectivity, demonstrating its potential for Cu²⁺ detection in real environmental samples.     

Highly selective and sensitive visualizable detection of Hg2+ based on anti-aggregation of gold nanoparticles

For the widely used gold nanoparticles (AuNPs)-based colorimetric probes, AuNPs generally change from dispersion to aggregation state accompanying with corresponding color turning from red to blue. Although colorimetric probes based on the anti-aggregation of AuNPs show exceptional selectivity and sensitivity, few examples have been reported in literature. A facile but highly sensitive and selective colorimetric probe based on the anti-aggregation of AuNPs transferred from the deactivation of aggregation agent 4,4′-dipyridyl by Hg²⁺ was developed in this work. This reported probe is suitable for real-time detection of Hg²⁺ in water with a detection limit of 3.0 ppb for Hg²⁺, and exhibits a selectivity toward Hg²⁺ by two orders of magnitude over other metal ions. The dynamic range of this probe can be conveniently tuned by adjusting the amount of 4,4′-dipyridyl used.     

DNAzyme crosslinked hydrogel: a new platform for visual detection of metal ions

We propose the use of DNAzyme as a crosslinker of hydrogel to develop a catalytic platform for the sensing of metal ions. The DNAzyme crosslinked hydrogel can undergo gel-sol transition in response to Cu(2+) ions, which enables sensitive visual detection of Cu(2+) by observing the release of pre-trapped AuNPs.     

Paper-based scanometric assay for lead ion detection using DNAzyme

A facile and simple paper-based scanometric assay was developed to detect Pb²⁺ using GR5-DNAzyme. Magnetic beads (MBs) and gold nanoparticles (AuNPs) were used as a signal collector and a signal indicator, respectively. They were linked together by GR5-DNAzyme, comprising an enzyme and a substrate strand pairing up with each other. In the presence of Pb²⁺, the substrate strand is cut into two pieces, resulting in the disassembly of AuNPs from the MBs. These AuNPs were spotted on predefined areas on a chromatography paper, where signal is amplified through silver reduction. This sensing platform exhibits high sensitivity and selectivity toward Pb²⁺, giving a detection limit of 0.3 nM and a linear fitting range from 0.1 to 1000 nM. Testing of this biosensor in river water and synthetic urine samples also showed satisfying results. Besides offering simultaneous and multi-sample analysis, this paper-based sensing platform presented here could be potentially applied and served as a general platform for on-site, naked eyes, and low-cost monitoring of other heavy metal ions in environmental and body fluid samples.     

Ethylenediaminetetraacetic Acid Functionalized Gold Nanoparticles for Sensitive Colorimetric Detection of Chromium(III)

A cost‐effective and sensitive colorimetric method was described for the determination of chromium(III) ion (Cr³⁺) by using ethylenediaminetetraacetic acid functionalized gold nanoparticles (EDTA‐AuNPs) as a probe. The stable and dispersed EDTA‐AuNPs were prepared by reducing HAuCl4 with sodium borohydride in presence of EDTA as a capping agent. Upon the addition of Cr³⁺, the colour of EDTA‐AuNPs solution changed from red to violet, which was in response to the surface plasmon absorption of dispersed and aggregated EDTA‐AuNPs. The procedure allowed the determination of Cr³⁺ in the range of 0.1–1.0 mol/L. The limit of detection for Cr³⁺ was 0.08 mol/L. The relative standard deviation was 2.5 % for eight repeated measurements of 0.6 mol/L Cr³⁺ solution. The method was applied to the determination of Cr³⁺ in water samples.     

Single-Crystal-to-Single-Crystal Installation of Ln4(OH)4 Cubanes in an Anionic Metallosupramolecular Framework

Postsynthetic installation of lanthanide cubanes into a metallosupramolecular framework via a single-crystal-to-single-crystal (SCSC) transformation is presented. Soaking single crystals of K₆[Rh₄Zn₄O(l -cys)₁₂] (K₆[ 1 ]; l -H₂cys=l -cysteine) in a water/ethanol solution containing Ln(OAc)₃ (Ln³⁺=lanthanide ion) results in the exchange of K⁺ by Ln³⁺ with retention of the single crystallinity, producing Ln₂[ 1 ] ( 2_Ln ) and Ln_0.33[Ln₄(OH)₄(OAc)₃(H₂O)₇][ 1 ] ( 3_Ln ) for early and late lanthanides, respectively. While the Ln³⁺ ions in 2_Ln exist as disordered aqua species, those in 3_Ln form ordered hydroxide-bridged cubane clusters that connect [ 1 ]⁶⁻ anions in a 3D metal-organic framework through coordination bonds with carboxylate groups. This study shows the utility of an anionic metallosupramolecular framework with carboxylate groups for the creation of a series of metal cubanes that have great potential for various applications, such as magnetic materials and heterogeneous catalysts.     

Redox reactions in molten electrolytes containing chlorides of rare-earth metals

Solutions of rare-earth metals (REM, Ln) in molten chlorides, including mixtures MCl + LnCl₃, where M stands for an alkali metal, are studied by methods of potentiometry, voltammetry, and conductimetry in broad intervals of concentration and temperature. The results that had been obtained give sufficiently comprehensive and reliable enough information concerning the valence state of rare-earth metals, as well as the structure and composition of complex ions that make a substantial impact on the properties of electrolytes. It is demonstrated that the co-existence of ions of rare-earth metals in different oxidation states, which form as a result of possible redox reactions 2Ln³⁺ + Ln ? 3Ln²⁺, Ln²⁺ + Ln ? 2Ln⁺, and nM⁺ + Ln ? nM + Lnⁿ⁺, clearly manifests itself in the thermodynamic and transport properties of molten systems Ln-LnCl₃ and Ln-LnCl₃-MCl.     

Toehold-mediated strand displacement reaction-propelled cascade DNAzyme amplifier for microRNA let-7a detection

DNAzyme amplifiers have been extensively explored as a useful sensing platform, but single DNAzyme amplifier is limited in biosensing applications by its low sensitivity. Herein, a cascade DNAzyme amplifier was designed by exploiting concurrent amplification cycle principles of toehold-mediated strand displacement reaction (TSDR) and Zn²⁺-assisted DNAzyme cycle with lower cost and simpler procedures. Compared with single DNAzyme amplifier, the proposed TSDR-propelled cascade DNAzyme amplifier exhibited higher sensitivity by releasing more DNAzyme through TSDR to cleave substrate strand during the DNAzyme cycle. Base on this, let-7a could be sensitively detected in the range of 5–50 nmol/L with a detection limit of 64 pmol/L. Furthermore, the dual signal amplification strategy of the cascade DNAzyme amplifier exhibited excellent selectivity to distinguish single-base mismatched DNA strands, which has been successfully applied to the determination of let-7a in blood serum, showing high promise in early cancer diagnosis.     

Single‐Crystal‐to‐Single‐Crystal Installation of Ln4(OH)4 Cubanes in an Anionic Metallosupramolecular Framework

Postsynthetic installation of lanthanide cubanes into a metallosupramolecular framework via a single‐crystal‐to‐single‐crystal (SCSC) transformation is presented. Soaking single crystals of K₆[Rh₄Zn₄O(l ‐cys)₁₂] (K₆[ 1 ]; l ‐H₂cys=l ‐cysteine) in a water/ethanol solution containing Ln(OAc)₃ (Ln³⁺=lanthanide ion) results in the exchange of K⁺ by Ln³⁺ with retention of the single crystallinity, producing Ln₂[ 1 ] ( 2_Ln ) and Ln_0.33[Ln₄(OH)₄(OAc)₃(H₂O)₇][ 1 ] ( 3_Ln ) for early and late lanthanides, respectively. While the Ln³⁺ ions in 2_Ln exist as disordered aqua species, those in 3_Ln form ordered hydroxide‐bridged cubane clusters that connect [ 1 ]^6? anions in a 3D metal‐organic framework through coordination bonds with carboxylate groups. This study shows the utility of an anionic metallosupramolecular framework with carboxylate groups for the creation of a series of metal cubanes that have great potential for various applications, such as magnetic materials and heterogeneous catalysts.     

Sorption of microamounts of Ce3+, Pm3+, Eu3+, Gd3+ and Yb3+ on aluminium hydroxide

The sorption of microamounts of trivalent lanthanides (Ln³⁺) on freshly precipitated Al(OH)₃ has been measured in dependence on pH and the time of sorption. Also, the influence of organic complexing ligands and inorganic electrolytes on the sorption process has been investigated. The mechanism of sorption is discussed. Freshly precipitated Al(OH)₃ can be used for the preconcentration of microamounts of trivalent lanthanides. However, the preconcentration is not quantitative in the presence of high concentrations of complexing ligands (citrate and similar) which form strong complexes with Ln³⁺ ions.     

Assistant deoxyribonucleic acid recycling with Zn and molecular beacon for electrochemical detection of deoxyribonucleic acid via target-triggered assembly of mutated DNAzyme

A novel enzyme-free amplification strategy was designed for sensitive electrochemical detection of deoxyribonucleic acid (DNA) based on Zn²⁺ assistant DNA recycling via target-triggered assembly of mutated DNAzyme. A gold electrode was used to immobilize molecular beacon (MB) as the recognition probe and perform the amplification procedure. In the presence of target DNA, the hairpin probe 1 was opened, and the DNAzyme was liberated from the caged structure. The activated DNAzyme first hybridized and then cleaved the MB in the presence of cofactor Zn²⁺. After cleavage, the MB was cleaved into two pieces and the ferrocene (Fc) labeled piece dissociated from the gold electrode, thus obviously decreasing the Fc signal and forming a free DNAzyme strand. Finally, each target-induced activated DNAzyme underwent many cycles to trigger the cleavage of many MB substrates. Therefore, the peak current of Fc dramatically decreased to approximately zero. The strategy showed a detection limit at 35 fM levels, which was about 2 orders of magnitude lower than that of the conventional hybridization without Zn²⁺-based amplification. The Zn²⁺ assistant DNA recycling offers a versatile platform for DNA detection in a cost-effective manner, and has a promising application in clinical diagnosis.     

In Vitro Selection of Chromium‐Dependent DNAzymes for Sensing Chromium(III) and Chromium(VI)

Chromium is a very important analyte for environmental monitoring, and developing biosensors for chromium is a long‐standing analytical challenge. In this work, in vitro selection of RNA‐cleaving DNAzymes was carried out in the presence of Cr³⁺. The most active DNAzyme turned out to be the previously reported lanthanide‐dependent Ce13d DNAzyme. Although the Ce13d activity was about 150‐fold lower with Cr³⁺ than that with lanthanides, the activity of lanthanides and other competing metals was masked by using a phosphate buffer; this left Cr³⁺ as the only metal that could activate Ce13d. With 100 μm Cr³⁺, the cleavage rate is 1.6 h^?1 at pH 6. By using a molecular beacon design, Cr³⁺ was measured with a detection limit of 70 nm , which was significantly lower than the United States Environmental Protection Agency (EPA) limit (11 μm ). Cr⁴⁺ was measured after reduction by NaBH₄ to Cr³⁺, and it could be sensed with a similar detection limit of 140 nm Cr⁴⁺; this value was lower than the EPA limit of 300 nm . This sensor was tested for chromium speciation analysis in a real sample, and the results supported its application for environmental monitoring. At the same time, it has enhanced our understanding of the interactions between chromium and DNA.