Fluorescence detection of lead(II) ions through their induced catalytic activity of DNAzymes

Quinoline-quinoline schiff-base as an effective chromogenic, fluorogenic, and smartphone assisted RGB detection of Pb2+ ion in near aqueous medium

A novel multimode colorimetric and fluorescent chemosensor was developed using an 8-hydroxy quinoline carbaldehyde Schiff base with a quinoline hydrazide probe (E)-2-((2-(quinolin-2-yl)hydrazineylidene)methyl)quinolin-8-ol (L). NMR (1H & 13C), FTIR, and HR-mass spectral characterization techniques confirmed the probe L structural conformation. As Probe L contacts Pb2+ ions, a color change and turn-off emission can be visually detected in EtOH:H2O (1:1, v/v, pH = 7.21) medium. The probe displays a good emission at 440 nm due to the combined ESIPT and ICT process. The Pb2+ ion interacts with the probe and selectively quenches fluorescence by inhibiting ESIPT and >CN- isomerization. As per Job's plot, L-Pb2+ complex formation occurred in a 1:1 stoichiometric ratio, with association constant (Ka) and quenching constant (Ksv) estimated at 1.52 × 105 M-1 and 4.12 × 105 M, respectively. The detection limits of Pb2+ by spectrophotometric and spectrofluorometric were 1.99 μM (41 ppb) and 23.4 nM (485 ppt), respectively. Additionally, the test paper kit and RGB tool were used to monitor the color changes of L with Pb2+ and the LOD was found to be 5.99 μM (125 ppb). Its recognition mechanism has been verified by 1H NMR, ESI-mass, and theoretical studies.

Ratiometric Fluorescence Sensing System for Lead Ions Based on Self-Assembly of Bioprobes Triggered by Specific Pb2+-Peptide Interactions

Lead is one of the most toxic substances. However, there are few ratiometric fluorescent probes for sensing Pb2+ in aqueous solution as well as living cells because specific ligands for Pb2+ ions have not been well characterized. Considering the interactions between Pb2+ and peptides, we developed ratiometric fluorescent probes for Pb2+ based on the peptide receptor in two steps. First, we synthesized fluorescent probes (1-3) based on the tetrapeptide receptor (ECEE-NH2) containing hard and soft ligands by conjugation with diverse fluorophores that showed excimer emission when they aggregated. After investigation of fluorescent responses to metal ions, benzothiazolyl-cyanovinylene was evaluated as an appropriate fluorophore for ratiometric detection of Pb2+. Next, we modified the peptide receptor to decrease the number of hard ligands and/or to replace Cys with disulfide bond and methylated Cys for improving selectivity and cell permeability. From this process, we developed two fluorescent probes (3 and 8) among the probes (1-8) that exhibited remarkable ratiometric sensing properties for Pb2+ including high water solubility (≤2% DMF), visible light excitation, high sensitivity, selectivity for Pb2+, low detection limits (<10 nM), and fast response (<6 min). The binding mode study revealed that specific Pb2+-peptide interactions of the probes caused nanosized aggregates in which the fluorophores of the probes came close each other, exhibiting excimer emission. In particular, 8 based on tetrapeptide bearing a disulfide bond and two carboxyl groups with a good permeability successfully quantified intracellular uptake of Pb2+ in live cells through ratiometric fluorescent signals. The ratiometric sensing system based on specific metal-peptide interactions and excimer emission process could provide a valuable tool to quantify Pb2+ in live cells and pure aqueous solutions.

Ratiometric fluorescent detection of lead ions in aquatic environment and living cells using a fluorescent peptide-based probe

Ratiometric fluorescent detection using dual emission bands is highly necessary to quantify Pb(II) in aquatic environment and live cells. We synthesized a ratiometric fluorescent peptidyl probe (1) by conjugation of a peptide receptor for Pb(II) with an excimer-forming benzothiazolylcyanovinylene fluorophore. The peptidyl probe dissolved well in aqueous solution and displayed an emission band at 538 nm (λ_ex = 460 nm). Upon addition of Pb(II) (0-20 μM), the emission maximum shifted from 538 nm to 575 nm and the emission intensity ratio (I₅₇₅ /I₅₃₈) increased significantly from 0.40 to 2.26. 1 exhibited a selective ratiometric response to Pb(II) over other metal ions. 1 with a low detection limit (1.2 ppb) of Pb(II) detected nanomolar concentrations (0-500 nM) of Pb(II) ions in groundwater and tap water. The cell-permeable probe detected intracellular Pb(II) by ratiometric fluorescent images. The binding mode study using NMR, IR and CD spectroscopy, and TEM revealed that the probe formed a 1:1 complex with Pb(II) and then formed red-emissive nanoparticles and fibrils. The probe exhibited desirable detection properties such as ratiometric detection, high solubility in water, visible light excitation, high selectivity and sensitivity for Pb(II), cell-permeability, and rapid response (< 6 min).

Recent Advances in Chemical Sensors for Soil Analysis: A Review

The continuously rising interest in chemical sensors’ applications in environmental monitoring, for soil analysis in particular, is owed to the sufficient sensitivity and selectivity of these analytical devices, their low costs, their simple measurement setups, and the possibility to perform online and in-field analyses with them. In this review the recent advances in chemical sensors for soil analysis are summarized. The working principles of chemical sensors involved in soil analysis; their benefits and drawbacks; and select applications of both the single selective sensors and multisensor systems for assessments of main plant nutrition components, pollutants, and other important soil parameters (pH, moisture content, salinity, exhaled gases, etc.) of the past two decades with a focus on the last 5 years (from 2017 to 2021) are overviewed.

Inhibition of CRISPR-Cas12a trans-cleavage by lead (II)-induced G-quadruplex and its analytical application

In this work, the inhibition of clustered regularly interspaced short palindromic repeats (CRISPR) - CRISPR associated protein (Cas) trans-cleavage by Pb²⁺-induced G-quadruplex has been firstly explored to detect Pb²⁺ in tea beverage and milk. In absence of Pb²⁺, the Na⁺-induced G-quadruplex can be cleaved by CRISPR-Cas12a. In contrast, Pb²⁺ can competitively combine with G-quadruplex, resulting in its conformational changes and resistance to trans-cleavage of CRISPR-Cas12a. Therefore, the fluorescence resonance energy transfer can happen. Pb²⁺ can be detected in a linear range from 100 nM to 5 µM with a lowest detection limit of 2.6 nM and a relative standard deviation of 4.32%. In summary, this work not only provides a new method for Pb²⁺ detection based on its induced G-quadruplex inhibition on CRISPR-Cas12a trans-cleavage, but also broadens the application of CRISPR-Cas system for heavy metal analysis in the field of food safety.

Designing, fabrication and evaluation of a rapid, point-of-care and noninvasive system for the detection of lead (Pb2+)

Non-invasive collection of biological sample such as sweat, urine, saliva, hairs and, stool and onsite detection of anlaytes in those samples is an interesting and viable approach for rapid screening of various toxicants in body. Environmental exposure/presence of lead (₈₂Pb) and its rapid detection provide one such opportunity. A chemical spot based colorimetric method and a transdermal patch device based on this spot test, is developed for rapid and qualitative assessment of inorganic lead (Pb²⁺) in non-coloured biological or environmental liquid samples. The transdermal patch system contains two important parts, a chemical spot prepared on a thin glass sheet and, an absorbent paper (11 µm pore size). A one step colour development reaction is able to identify the presence or absence of Pb²⁺. In-vitro evaluation for sensitivity and cut-off value determination, within run and between run precision testing, specificity testing were done. In-vivo evaluation of the developed patch system was performed in occupationally lead-exposed subjects and in control volunteers. In-vivo field testing results were further validated with gold standard test for lead detection. Blood lead levels and patch lead levels were found to be positively correlated (r = 0.57, P < 0.0001). In addition, the sensitivity and specificity of device in identification of Pb²⁺ was found to be 75.93% (95% CI = 62.36%-86.51%) and 95.24% (95% CI = 76.18%-99.88%). The developed system appears as a reliable, non-invasive rapid test with minimum step involve for identification of Pb²⁺ in a given system.

On-Strand Knoevenagel Insertion of a Hemicyanine Molecular Rotor Loop Residue for Turn-On Fluorescence Detection of Pb-Induced G-Quadruplex Rigidity

We demonstrate the ability to distinguish Pb2+ from K+ within the central cavity of the antiparallel G-quadruplex (GQ) DNA produced by the thrombin binding aptamer (TBA) using an internal molecular rotor fluorescent probe. An indole-aldehyde containing an acyclic N-glycol group was first employed in the on-strand Knoevenagel condensation with five different heterocyclic quaternary cationic acceptors to assess the molecular rotor character of the resulting cyanine-styryl dyes within duplex DNA. An indole-pyridinium (4PI) nucleobase surrogate displayed the greatest turn-on emission response to duplex formation and was thus inserted into the loop residues of TBA to monitor GQ-folding in the presence of Pb2+ versus K+. TBA-4PI exhibits turn-on emission upon Pb2+-binding with a brightness (ε·Φfl) of 9000 cm-1 M-1 compared to K+-binding (ε·Φfl ∼ 2000 cm-1 M-1) due to Pb2+-induced GQ rigidity with 4PI-G-tetrad stacking interactions. The Pb2+-bound TBA-4PI GQ also provides energy-transfer (ET) fluorescence with a diagnostic excitation at 310 nm for distinguishing Pb2+ from K+ within the antiparallel GQ. The TBA-4PI GQ affords the desired turn-on fluorescence response for detecting Pb2+ ions with an apparent dissociation constant (Kd) of 63 nM and a limit of detection (LOD) of 19 nM in an aqueous buffer. It can also distinguish Pb2+ (230 nM) from K+ (1.5 mM, 6500-fold excess) in an antiparallel GQ recognition motif without topology twitching.

An aptasensor for the detection of Pb2+ based on photoinduced electron transfer between a G-quadruplex-hemin complex and a fluorophore

Due to the threat to health of heavy metal contamination, simple and rapid detection methods for heavy metals are an urgent needed in environment protection and food safety. In this work, we have developed a fluorescent aptasensor for the 'turn-off' model detection of Pb²⁺ . The key feature of the aptasensor is that the dye-labelled nucleic acid strand can be folded into a G-quadruplex structure in the presence of Pb²⁺ . This conformational change induces photoinduced electron transfer (PET) between a G-quadruplex-hemin complex and 6-carboxyrhodamine X (ROX), which results in fluorescence quenching of ROX. We systematically investigated the interaction mechanism between Pb²⁺ and the aptasensor and the effects of several environmental parameters were also studied. Under the optimum conditions, the proposed method exhibited a good liner relationship between the concentration of Pb²⁺ and fluorescence quenching efficiency in the range 25-500 nM and the limit of detection was 1.02 nM. In addition, this method also manifested good performance in spiked lettuce samples with satisfactory recoveries of 87.10-109.6%. This target-induced PET platform can be further expanded to other heavy metal analysis.

Novel Aminosilane (APTES)-Grafted Polyaniline@Graphene Oxide (PANI-GO) Nanocomposite for Electrochemical Sensor

Lead is a potentially toxic element (PTE) that has several adverse medical effects in humans. Its presence in the environment became prominent due to anthropogenic activities. The current study explores the use of newly developed composite materials (organic-inorganic hybrid) based on PANI-GO-APTES for electrochemical detection of Pb²⁺ in aqueous solution. The composite material (PANI-GO-APTES) was synthesized by chemical method and was characterized with SEM, XPS, XEDS, XRD, TGA, FTIR, EIS and CV. The result of characterization indicates the successful synthesis of the intended material. The PANI-GO-APTES was successfully applied for electrochemical detection of Pb²⁺ using cyclic voltammetry and linear sweep voltammetry method. The limit of detection of Pb²⁺ was 0.0053 µM in the linear range of 0.01 µM to 0.4 µM. The current response produced during the electrochemical reduction of Pb²⁺ catalyzed by PANI-GO-APTES was also very repeatable, reproducible and rapid. The application of PANI-GO-APTES-modified GCE in real sample analysis was also established. Therefore, PANI-GO-APTES is presented as a potential Pb²⁺ sensor for environmental and human health safety.

Fluorescence and electrochemical assay for bimodal detection of lead ions based on Metal-Organic framework nanosheets

The accurate measurement of heavy metal ions is essential for human health and environmental protection. Here, we report the design of a simple and convenient bimodal strategy for signal-on, label-free lead ion detection in environmental samples based on two-dimensional metal-organic framework (2D-MOF) nanosheets. 2D-MOFs have different affinities toward guanine-rich DNA (ssGDNA) and the G-quadruplex, allowing these structures to be distinguished. The nanosheets were also used as quenchers for fluorescent lead ion detection. Using lead ions to induce G-quadruplex formation from ssGDNA, a simple fluorescence resonance energy transfer (FRET) strategy was developed for lead ion detection; the detection limit was 3.3 nM. Based on changes in the GDNA configuration, the FRET system was converted into an electrochemical sensor for lead ion assays using an electrode modified with the 2D-MOF nanosheets. Electrochemical impedance spectroscopy showed a high sensitivity and a low limit of detection (i.e., 8.7 pM) of the electrode. The adaptability of the bimodal mechanism was verified through the successful detection of lead ions in tap water and fertilizer samples, and the method accuracy was demonstrated through inductively coupled plasma analysis. The developed bimodal device is cost-effective, highly sensitive, and allows for convenient operation, thereby rendering it a promising and reliable system for the detection of lead ions in environmental samples.

G-quadruplex: Flexible conformational changes by cations, pH, crowding and its applications to biosensing

G-quadruplex (G4) is a non-canonical structure that is formed in G-rich sequences of nucleic acids. G4s play important roles in vivo, such as telomere maintenance, transcription, and DNA replication. There are three typical topologies of G4: parallel, anti-parallel, and hybrid. In general, metal cations, such as potassium and sodium, stabilize G4s through coordination in the G-quartet. While G4s have some functions in vivo, there are many reports of developed applications that use G4s. As various conformations of G4s could form from one sequence depending on varying conditions, many researchers have developed G4-based sensors. Furthermore, G4 is a great scaffold of aptamers since many aptamers folded into G4s have also been reported. However, there are some challenges about its practical use due to the difference between practical sample conditions and experimental ones. G4 conformations are dramatically altered by the surrounding conditions, such as metal cations, pH, and crowding. Many studies have been conducted to characterize G4 conformations under various conditions, not only to use G4s in practical applications but also to reveal its function in vivo. In this review, we summarize recent studies that have investigated the effects of surrounding conditions (e.g., metal cations, pH, and crowding) on G4 conformations and the application of G4s mainly in biosensor fields, and in others.

Review of recent progress on DNA-based biosensors for Pb2+ detection

Lead (Pb) is a highly toxic heavy metal of great environmental and health concerns, and interestingly Pb²⁺ has played important roles in nucleic acids chemistry. Since 2000, using DNA for selective detection of Pb²⁺ has become a rapidly growing topic in the analytical community. Pb²⁺ can serve as the most active cofactor for RNA-cleaving DNAzymes including the GR5, 17E and 8-17 DNAzymes. Recently, Pb²⁺ was found to promote a porphyrin metalation DNAzyme named T30695. In addition, Pb²⁺ can tightly bind to various G-quadruplex sequences inducing their unique folding and binding to other molecules such as dyes and hemin. The peroxidase-like activity of G-quadruplex/hemin complexes was also used for Pb²⁺ sensing. In this article, these Pb²⁺ recognition mechanisms are reviewed from fundamental chemistry to the design of fluorescent, colorimetric, and electrochemical biosensors. In addition, various signal amplification mechanisms such as rolling circle amplification, hairpin hybridization chain reaction and nuclease-assisted methods are coupled to these sensing methods to drive up sensitivity. We mainly cover recent examples published since 2015. In the end, some practical aspects of these sensors and future research opportunities are discussed.

Metal-induced G-quadruplex polymorphism for ratiometric and label-free detection of lead pollution in tea

Lead pollution are critical concerns for food safety and human health. Herein, a ratiometric metal-induced G-quadruplex polymorphism was introduced to construct aptamer probes, enabling label-free and ratiometric detection of lead in tea, thus is promising for on-site detection of lead pollution. The key feature of the aptamer probe is the synergistic utilization of the dual-wavelength fluorescent signal outputs from a G-quadruplex specific dye and a DNA intercalation dye under a single-wavelength excitation, leading to a more stable and reliable recognition of Pb²⁺ than that of analyses based on single fluorescent reporter. The aptamer probe allowed to a mix-and-read, rapid, cost-effective detection of Pb²⁺ with high specificity and accuracy. Pb²⁺ analysis in tap water and tea exhibited good performance with recovery rates of 92.3%-109.0%. The adoption of ratiometric metal-induced G-quadruplex polymorphism would be a compelling design strategy for constructing robust aptasensor, facilitating the translation of aptamer for food safety control.

Recent advances on functional nucleic acid-based biosensors for detection of food contaminants

It has seen increasing development of reliable, robust, and flexible biosensors for rapid food-safety analysis in the past few decades. Recently, functional nucleic acid-based biosensors have attracted attention because of their programmability, bottom-up characteristics, and structural switches. However, few systematic reviews devoted to categorizing the potential of DNA nanostructures and devices were found for detecting food contaminants. Hence, the applications of functional nucleic acid-based biosensors were reviewed for analyzing food contaminants, including foodborne pathogen bacteria, biotoxins, heavy metals, and et al. In addition to categorizing the various biosensors, multiple signal readout strategies, such as optical, electrochemical, and mass-based signals were also examined. Finally, the future changes and potential opportunities, as well as practical applications of functional nucleic acid-based biosensors were discussed.

Novel formation of Au/Ag bimetallic nanoparticles from a mixture of monometallic nanoparticles and their application for the rapid detection of lead in onion samples

Characterization of gold, silver and gold/silver bimetallic nanoparticles for colorimetric detection of lead in onion samples.

The investigation of the G-quadruplex aptamer selectivity to Pb2+ ion: a joint molecular dynamics simulation and density functional theory study

The aptamers with the ability to form a G-quadruplex structure can be stable in the presence of some ions. Hence, study of the interactions between such aptamers and ions can be beneficial to determine the highest selective aptamer toward an ion. In this article, molecular dynamics (MD) simulations and quantum mechanics (QM) calculations have been applied to investigate the selectivity of the T30695 aptamer toward Pb²⁺ in comparison with some ions. The Free Energy Landscape (FEL) analysis indicates that Pb²⁺ has remained inside the aptamer during the MD simulation, while the other ions have left it. The Molecular Mechanics Poisson-Boltzmann Surface Area (MM-PBSA) binding energies prove that the conformational stability of the aptamer is the highest in the presence of Pb²⁺. According to the compaction parameters, the greatest compressed ion-aptamer complex, and hence, the highest ion-aptamer interaction have been induced in the presence of Pb²⁺. The contact maps clarify the closer contacts between the nucleotides of the aptamer in the presence of Pb²⁺. The density functional theory (DFT) results show that Pb²⁺ forms the most stable complex with the aptamer, which is consistent with the MD results. The QM calculations reveal that the N-H bonds and the O…H distances are the longest and the shortest, respectively, in the presence of Pb²⁺. The obtained results verify that the strongest hydrogen bonds (HBs), and hence, the most compressed aptamer structure are induced by Pb²⁺. Besides, atoms in molecules (AIM) and natural bond orbital (NBO) analyses confirm the results.Communicated by Ramaswamy H. Sarma.

Efficient DNA-Catalyzed Porphyrin Metalation for Fluorescent Ratiometric Pb2+ Detection

Developing biosensors for Pb²⁺ is an important analytical topic. DNA-based Pb²⁺ sensors have been designed mainly based on RNA-cleaving DNAzymes and Pb²⁺-induced folding of G-quadruplex (G4) DNA. Porphyrin metalation is a key reaction in biology and catalysis. Many enzyme mimics have been developed to catalyze this reaction, and some metalation DNAzymes were reported with a G4 structure. Inspired by the excellent G4 binding properties of certain divalent metal ions, we herein screened a few metals and G-rich DNA sequences. The metalation activity of a DNA named T30695 (sequence: (G₃T)₄) was significantly accelerated by Pb²⁺. The reaction of Cu²⁺ insertion into the mesoporphyrin IX had a k_cat of 0.89 min^-1 and a K_m of 9.8 μM, representing a catalytic efficiency similar to that of human ferrochelatase. The reason for the acceleration was attributed to Pb²⁺ binding of the G4 DNA and the catalytic activity of the large Pb²⁺ ion for this reaction. A ratiometric sensor for Pb²⁺ was developed by inserting Zn²⁺ with a detection limit of 23.5 nM Pb²⁺. This work has established a new DNA-based reaction that can be used for Pb²⁺ detection, and it also provides a highly efficient new DNAzyme for porphyrin metalation, which might be used for signal production for other biosensors.

Fluorescence covalent interaction enhanced sensor for lead ion based on novel graphitic carbon nitride nanocones

Novel graphitic carbon nitride nanocones (g-CNNCs) were synthesized for the first time in this study. The SEM, TEM, XPS and FT-IR were used to research the structure of the g-CNNCs. We found that the g-CNNCs showed high selective and sensitive for fluorescence enhancement detection of Pb²⁺ ion via covalent interaction. In addition, the g-CNNCs exhibit stable and specific concentration-dependent fluorescence intensity in the presence of Pb²⁺ ion in the range of 1-200 μmol·dm^-3, and the limit of detection was estimated to be 0.0438 μmol·dm^-3 (3S/k). More importantly, the g-CNNCs were used to detect practical samples with satisfactory results.

Colorimetric determination of lead(II) or mercury(II) based on target induced switching of the enzyme-like activity of metallothionein-stabilized copper nanoclusters

It is shown that metallothionein-stabilized copper nanoclusters (MT-CuNCs) display catalase-like activity. In the presence of either lead(II) or mercury(II), the catalase-like activity is converted to a peroxidase-like activity. On addition of Pb(II) or Hg(II), the inhibitory effect of MT-CuNCs on the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) with H₂O₂ is weakened. On the other hand, the catalytic effect of the nanoclusters on the chromogenic reaction is increased. The system MT-CuNCs-Pb(II)/Hg(II) exhibits high affinity for the substrates TMB and H₂O₂. Their catalytic behavior follows Michaelis-Menten kinetics. Based on these findings, a method was developed for visual detection (via the blue coloration formed) and spectrophotometric determination (at 450 nm) of Pb(II) and Hg(II). The linear range for Pb(II) extends from 0.7 to 96 μM, and the linear ranges for Hg(II) from 97 nM to 2.3 μM and from 3.1 μM to 15.6 μM. The detection limits are 142 nM for Pb(II) and 43.8 nM for Hg(II). Graphical abstract Metallothionein-stabilized copper nanoclusters (MT-CuNCs) display catalase-like activity. On addition of Pb(II) or Hg(II), the catalase-like activity is converted to a peroxidase-like activity. The latter catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂.

Ultrasensitive colorimetric detection of Hg2+ ions based on enhanced catalytic performance of gold amalgam dispersed in channels of rose petals

We herein present a simple, low-cost, and ultrasensitive colorimetric sensing strategy for the detection of mercury ions (Hg²⁺) that takes advantage of the natural pore structure in rose petals to encapsulate gold nanoparticles (AuNPs).

Supramolecularly Assembled Ratiometric Fluorescent Sensory Nanosystem for "Traffic Light"-Type Lead Ion or pH Sensing

The combination of functional nucleic acids and nanomaterials enables the continuous development of hybrid nanosystems that have found wide applications including chemo/biosensing. Herein, we report the supramolecular assembly of a "sesame biscuit"-like superstructural nanosystem based on aptamer, quantum dot (QD), and graphene oxide (GO), and its diverse applications in Pb²⁺ and pH sensing. The nanosystem was assembled via adsorbing silica-encapsulated green-emitting QD onto the edge of GO by ionic interaction, followed by absorbing aptamer-modified red-emitting QD onto the GO surface via the π-stacking interaction. The nanosystem showed the characteristic of the nonquenched green fluorescence due to silica encapsulation and quenched red fluorescence owing to nanomaterial surface energy transfer. The nanosystem responded to Pb²⁺/pH in ratiometric fluorescence: the red fluorescence varied upon analyte-driven conformational changes of the aptamer, whereas the green one remained constant. Under optimized conditions, the nanosystem was demonstrated to be capable of quantifying Pb²⁺ with a detection limit of 11.7 pM, as well as pH with a sensing resolution of 0.1 pH unit. More importantly, the ratiometric nanosystem facilitated visualization of analytes in a distinct "traffic light" manner, which was exemplified by semiquantification of exogenous Pb²⁺ in living cells. To demonstrate practicality, fluorescent test strips were fabricated by immobilizing the nanosystem on paper. The fluorescent test strips displayed traffic light-type fluorescence color changes, with the capacity for on-site, naked-eye detection of Pb²⁺ in real samples, as well as point-of-care pH testing in routine urinalysis.

Logic Catalytic Interconversion of G-Molecular Hydrogel

By incorporating hemin into G-quadruplex (G₄) during cation-templated self-assembly between guanosine and KB(OH)₄, we have constructed an artificial enzyme hydrogel (AEH)-based system for the highly sensitive and selective detection of Pb²⁺. The sensing strategy is based on a Pb²⁺-induced decrease in AEH activity. Because of the higher efficiency of Pb²⁺ for stabilizing G₄ compared with K⁺, the Pb²⁺ ions substitute K⁺ and trigger hemin release from G₄, thus giving rise to a conformational interconversion accompanied by the loss of enzyme activity. The Pb²⁺-induced catalytic interconversion endows the AEH-based system with high sensitivity and selectivity for detecting Pb²⁺. As a result, the AEH-based system shows an excellent response for Pb²⁺ in the range from 1 pM to 50 nM with a limit of detection of ∼0.32 pM, which is much lower than that of the previously reported G₄-DNAzyme. We also demonstrate that this AEH-based system exhibits high selectivity toward Pb²⁺ over other metal ions. Furthermore, two two-input INHIBIT logic gates have been constructed via switching of the catalytic interconversion induced by K⁺ and Pb²⁺ or K⁺ and pH. Given its versatility, this AEH-based system provides a novel platform for sensing and biomolecular computation.

Highly sensitive and selective detection of Pb2+ using a turn-on fluorescent aptamer DNA silver nanoclusters sensor

A novel turn-on fluorescent biosensor has been constructed using C-PS2.M-DNA-templated silver nanoclusters (Ag NCs) with an average diameter of about 1 nm. The proposed approach presents a low-toxic, simple, sensitive, and selective detection for Pb²⁺. The fluorescence intensity of C-PS2.M-DNA-Ag NCs enhances significantly in the presence of Pb²⁺, which is attributed to the special interaction between Pb²⁺ and its aptamer DNA PS2.M. Pb²⁺ induces the aptamer to form G-quadruplex and makes two darkish DNA/Ag NCs located at the 3' and 5' terminus close, resulting in the fluorescence light-up. Moreover, Pb²⁺ can be detected as low as 3.0 nM within a good linear range from 5 to 50 nM (R = 0.9862). Furthermore, the application for detection of Pb²⁺ in real water samples further demonstrates the reliability of the sensor. Thus, this sensor system shows a potential application for monitoring Pb²⁺ in environmental samples.

A suspending-droplet mode paper-based microfluidic platform for low-cost, rapid, and convenient detection of lead(II) ions in liquid solution

A paper-based microfluidic device based on unconventional principle was developed and used to detect lead ions through a two-step process including heated incubation and subsequent mixing. The device was made by generating a superhydrophobic pattern, which defines channel and reservoir barriers, on a water-impermeable paper substrate, followed by loading and drying the reagents in the defined reservoirs. Different from the conventional paper-based devices that are made of water-permeable paper, the as-prepared device holds water drops in discrete reservoirs, and the water drops will not move unless the device is titled along the direction of the predefined channels. In this way, the liquid samples applied onto the device are handled as individual drops and could be stored, transported, and mixed on demand. Different from the conventional paper-based devices that use capillary force to drive liquid, our new device uses wetting and gravity as driving force. We name this operation principle suspending-droplet mode paper-based device (SD-μPAD). The use of a Teflon contact-printing stamp makes the production of such devices rapid, cost efficient, and mass productive. Utilizing a G-quadruplex-based luminescence switch-on assay, we demonstrated rapid, convenient, highly sensitive, and low cost detection of lead(II) ions in water samples, using a custom made battery-powered portable device, and a smart phone as the detector.

A Sensitive and Label-Free Pb(II) Fluorescence Sensor Based on a DNAzyme Controlled G-Quadruplex/Thioflavin T Conformation

Pb(II) can cause serious damaging effects to human health, and thus, the study of Pb²⁺ detection methods to sensitively and selectively monitor Pb(II) pollution has significant importance. In this work, we have developed a label-free fluorescence sensing strategy based on a Pb(II) DNAzyme cleavage and the ThT/G-quadruplex complex. In the presence of Pb(II), a G-rich tail was cut and released from the substrate strand, which then would form a G-quadruplex structure by combination with ThT dye. The fluorescence signal increase was then measured for sensitive Pb(II) quantification with a limit of detection of 0.06 nM. Our sensor also demonstrated high selectivity against six different metal ions, which is very important for the analysis of complex samples.

Interaction of hemin with quadruplex DNA

A DNA enzyme with peroxidase activity is a G-quadruplex-based DNAzyme formed by hemin and G-quadruplex DNA. Activity of peroxide DNAzymes can be influenced by the structure of quadruplex DNA. In this investigation, the interaction of hemin with T30695 G-quadruplex DNA is evaluated. Molecular dynamic simulation indicates that the binding mode of hemin to G-quadruplex DNA is end-stacking, which is consistent with absorption spectroscopy. Based on fluorescence spectroscopy, hemin ejects thiazole orange from bases of four-strand DNA. Circular dichroism spectra showed that no alteration occurs in this type of DNA structure. Graphical Abstract Peroxidase DNAzyme is formed by hemin and G-quadruplex DNA.

Metal Cations in G-Quadruplex Folding and Stability

This review is focused on the structural and physicochemical aspects of metal cation coordination to G-Quadruplexes (GQ) and their effects on GQ stability and conformation. G-quadruplex structures are non-canonical secondary structures formed by both DNA and RNA. G-quadruplexes regulate a wide range of important biochemical processes. Besides the sequence requirements, the coordination of monovalent cations in the GQ is essential for its formation and determines the stability and polymorphism of GQ structures. The nature, location, and dynamics of the cation coordination and their impact on the overall GQ stability are dependent on several factors such as the ionic radii, hydration energy, and the bonding strength to the O6 of guanines. The intracellular monovalent cation concentration and the localized ion concentrations determine the formation of GQs and can potentially dictate their regulatory roles. A wide range of biochemical and biophysical studies on an array of GQ enabling sequences have generated at a minimum the knowledge base that allows us to often predict the stability of GQs in the presence of the physiologically relevant metal ions, however, prediction of conformation of such GQs is still out of the realm.

Phosphate-perylene modified G-quadruplex probes for the detection of Pb2+ using fluorescence anisotropy

A simple and universal phosphate-perylene modification strategy was applied in the development of G-quadruplex probes with thrombin binding aptamer (TBA) and [d(TGGGT)₄] (G4) sequences. A perylene moiety was inserted at different phosphate positions of oligonucleotides without a significant effect on the G-quadruplex structures. Upon binding with K⁺ or Pb²⁺, these probes showed different perylene fluorescence anisotropy responses due to the different labeling positions and G-quadruplex structures. Two probes (G4-9 and TBA-9) were successfully used in Pb²⁺ detection through fluorescence anisotropy. Once the complexes of Pb²⁺ with G4-9 or TBA-9 were formed, the rotational diffusion of the perylene moiety was limited, resulting in a significant increase in fluorescence anisotropy. Both probes showed good sensitivity to Pb²⁺ and their fluorescence anisotropy signals demonstrated good linear responses to the logarithm of Pb²⁺ concentrations and the detection limits were 24.5 nM and 30.0 nM for TBA-9 and G4-9, respectively.

Spinach RNA aptamer detects lead(II) with high selectivity

Spinach RNA aptamer contains a G-quadruplex motif that serves as a platform for binding and fluorescence activation of a GFP-like fluorophore. Here we show that Pb(2+) induces formation of Spinach's G-quadruplex and activates fluorescence with high selectivity and sensitivity. This device establishes the first example of an RNA-based sensor that provides a simple and inexpensive tool for Pb(2+) detection.