The Triple Roles of Glutathione for a DNA-Cleaving DNAzyme and Development of a Fluorescent Glutathione/Cu2+-Dependent DNAzyme Sensor for Detection of Cu2+ in Drinking Water

Recent developments of (bio)-sensors for detection of main microbiological and non-biological pollutants in plastic bottled water samples: A critical review

The importance of water in all biological processes is undeniable. Ensuring access to clean and safe drinking water is crucial for maintaining sustainable water resources. To elaborate, the consumption of water of inadequate quality can have a repercussion on human health. Furthermore, according to the instability of tap water quality, the consumption rate of bottled water is increasing every day at the global level. Although most people believe bottled water is safe, it can also be contaminated by microbiological or chemical pollution, which can increase the risk of disease. Over the last decades, several conventional analytical tools applied to analyze the contamination of bottled water. On the other hand, some limitations restrict their application in this field. Therefore, biosensors, as emerging analytical method, attract tremendous attention for detection both microbial and chemical contamination of bottled water. Biosensors enjoy several facilities including selectivity, affordability, and sensitivity. In this review, the developed biosensors for analyzing contamination of bottled water were highlighted, as along with working strategies, pros and cons of studies. Challenges and prospects were also examined.

DNA-modified Prussian blue nanozymes for enhanced electrochemical biosensing

Prussian blue nanoparticles exhibit the potential to be employed in bioanalytical applications due to their robust stability, peroxidase-like catalytic functionality, straightforward synthesis, and biocompatibility. An efficient approach is presented for the synthesis of nucleic acid-modified Prussian blue nanoparticles (DNA-PBNPs), utilizing nanoparticle porosity to adsorb nucleic acids (polyT). This strategic adsorption leads to the exposure of nucleic acid sequences on the particle surface while retaining catalytic activity. DNA-PBNPs further couple with functional nucleic acid sequences and aptamers through complementary base pairing to act as transducers in biosensors and amplify signal acquisition. Subsequently, we integrated a copper ion-dependent DNAzyme (Cu2+-DNAzyme) and a vascular endothelial growth factor aptamer (VEGF aptamer) onto screen-printed electrodes to serve as recognition elements for analytes. Significantly, our approach leverages DNA-PBNPs as a superior alternative to traditional enzyme-linked antibodies in electrochemical biosensors, thereby enhancing both the efficiency and adaptability of these devices. Our study conclusively demonstrates the application of DNA-PBNPs in two different biosensing paradigms: the sensitive detection of copper ions and vascular endothelial growth factor (VEGF). These results indicate the promising potential of DNA-modified Prussian blue nanoparticles in advancing bioanalytical sensing technologies.

Symmetrical Ligand's Fabricated Porous Silicon Surface Based Photoluminescence Sensor for Metal Detection and Entrapment

This study was based on the development of surface-based photoluminescence sensor for metal detection, quantification, and sample purification employing the solid sensory chip having the capability of metal entrapment. The Co(II), Cu(II) and Hg(II) sensitive fluorescence sensor (TP) was first synthesized and characterized its sensing abilities towards tested metal ions by using fluorescence spectral investigation while the synthesis and complexation of the receptor was confirmed by the chromogenic, optical, spectroscopic and spectrometric analysis. Under optical investigation, the ligand solution exhibited substantial chromogenic changes as well as spectral variations upon reacting with copper, cobalt, and mercuric ions, while these behaviors were not seen for the rest of tested metallic ions i.e., Na+, Ag+, Ni2+, Mn2+, Pd2+, Pb2+, Cd2+, Zn2+, Sn2+, Fe2+, Fe3+, Cr3+, and Al3+. These colorimetric alterations and spectral shifting could potentially be employed to detect and quantify these specific metal ions. After the establishment of the ligand's selective complexation ability towards selected metals, it was fabricated over the substituted porous silicon surface (FPS) keeping in view of the development of surface-based photoluminescence sensor (TP-FPS) for the selected metal sensation and entrapment to purify the sample just be putting off the metal entrapped sensory solid chip. Surface characterization and ligand fabrication was inspected by plan and cross sectional electron microscopic investigations, vibrational and electronic spectral analysis. The sensitivity of the ligand (TP) in the solution phase metal discrimination was determined by employing the fluorescence titration analysis of the ligand solution after progressive induction of Co2+, Cu2+, and Hg2+, which afford the detection limit values of 2.14 × 10- 8, 3.47 × 10- 8 and 3.13 × 10- 3, respectively. Concurrently, photoluminescence titration of the surface fabricated sensor (TP-FPS) revealed detection limit values of 3.14 × 10- 9, 7.43 × 10- 9, and 8.21 × 10- 4, respectively, for the selected metal ions.

Ratiometric Fluorescence and Chromogenic Probe for Trace Detection of Selected Transition Metals

The design and development of a fluorescence sensor aimed at detecting and quantifying trace amounts of toxic transition metal ions within environmental, biological, and aquatic samples has garnered significant attention from diagnostic and testing laboratories, driven by the imperative to mitigate the health risks associated with these contaminants. In this context, we present the utilization of a heterocyclic symmetrical Schiff Base derivative for the purpose of fluorogenic and chromogenic detection of Co2+, Cu2+ and Hg2+ ions. The characterization of the ligand involved a comprehensive array of techniques, including physical assessments, optical analyses, NMR, FT-IR, and mass spectrometric examinations. The mechanism of ligand-metal complexation was elucidated through the utilization of photophysical parameters and FT-IR spectroscopic analysis, both before and after the interaction between the ligand and the metal salt solution. The pronounced alterations observed in absorption and fluorescence spectra, along with the distinctive chromogenic changes, following treatment with Co2+, Cu2+ and Hg2+, affirm the successful formation of complexes between the ligands and the treated metal ions. Notably, the receptor's complexation response exhibited selectivity towards Co(II), Cu(II), and Hg(II), with no observed chromogenic changes, spectral variations, or band shifts for the various tested metal ions, including Na+, Ag+, Ni2+, Mn2+, Pd2+, Pb2+, Cd2+, Zn2+, Sn2+, Fe2+, Fe3+, Cr3+ and Al3+. This absence of interaction between these metal ions and the ligand could be attributed to their compact or inadequately conducive conduction bands for complexation with the ligand's structural composition. To quantify the sensor's efficacy, fluorescence titration spectra were employed to determine the detection limits for Co2+, Cu2+ and Hg2+, yielding values of 2.92 × 10-8, 8.91 × 10-8, and 4.39 × 10-3 M, respectively. The Benesi-Hildebrand plots provided association constant values for the ligand-cobalt, ligand-copper, and ligand-mercury complexes as 0.74, 2.52, and 13.89 M-1, respectively.

A Eu-MOF-Based Fluorescent Sensing Probe for the Detection of Tryptophan and Cu2+ in Aqueous Solutions

Abnormal tryptophan (Trp) metabolism can be used as an important indicator of chronic hepatitis, paranoia, Parkinson's disease and other diseases. Deficiency or excessive accumulation of Cu2+ can cause diseases such as Wilson's disease and Alzheimer's disease. Eu-based metal-organic framework (Eu-MOF) was successfully prepared for fluorescence sensing of Trp and Cu2+ in an aqueous solution (pH = 7.4). Eu-MOF showed high selectivity and sensitivity for Trp and Cu2+ with detection limits of 0.22 µM and 0.09 µM and Ksv of 6.17 × 103 M- 1 and 2.37 × 104 M- 1 respectively. Trp and Cu2+ had overlapped UV absorption spectra with that of Eu-MOF and competed for the excitation light source. Trp also attenuated the antennae effect of organic ligands on Eu-MOF, thus quenching the red fluorescence of Eu-MOF. This study provides insights into the application of MOFs in bioanalysis and diagnostics.

Aggregation-induced Emission Active Naphthalimide Derived Schiff Base for Detecting Cu2+ and Its Applications

Herein, an aggregation-induced emission (AIE) active Schiff base (NHS) was synthesized by condensing naphthalimide hydrazide with salicylaldehyde. The non-fluorescent solution of NHS in DMSO turned to emissive NHS upon increasing the HEPES fraction in DMSO from 70 to 95%. The UV-Vis absorption and DLS studies supported the self-aggregation of NHS that restricted the intramolecular rotation and activated the ESIPT process. The blue fluorescence of AIE luminogen NHS in DMSO:HEPES (5:95, v/v, pH = 7.4) was examined by adding different metal ions (Al3+, Ca2+, Cd2+, Co2+, Cu2+, Cr2+, Fe2+, Fe3+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+ and Zn2+). NHS showed a selective fluorescence switch-off response for Cu2+ due to the chelation enhancement quenching effect (CHEQ). The quenching of NHS by Cu2+ was explored by using density functional theory (DFT) and Stern-Volmer plot. The practical utility of NHS was examined by quantitative and qualitative analysis of Cu2+ in real water samples.

A Bifunctional Fluorescence Probe Based on AIE-ICT Strategy for Visual Detection of Cu2+/Co2+ in Complex Matrix

A novel fluorescence chemical sensor-based probe 1-{[(E)-(2-aminophenyl)azanylidene]methyl}naphthalen-2-ol (AMN) was designed and synthesized, which performed a "naked eye" detection ability toward Cu²⁺ and Co²⁺ based on aggregation-induced emission (AIE) fluorescence strategy. It has sensitive detection ability for Cu²⁺ and Co²⁺. In addition, the color changed from yellow-green to orange under the sunlight, realizing the rapid identification of Cu²⁺/Co²⁺, which has the potential of on-site visual detection under the "naked eye". Moreover, different "on" and "off" fluorescence expressions were exhibited under excessive glutathione (GSH) in AMN-Cu²⁺ and AMN-Co²⁺ systems, which could be employed to distinguish Cu²⁺ from Co²⁺. The detection limits for Cu²⁺ and Co²⁺ were measured to be 8.29 × 10^-8 M and 9.13 × 10^-8 M, respectively. The binding mode of AMN was calculated to be 2:1 by Jobs' plot method analysis. Ultimately, the new fluorescence sensor was applied to detect Cu²⁺ and Co²⁺ in real samples (tap water, river water, and yellow croaker), and the results were satisfying. Therefore, this high-efficiency bifunctional chemical sensor platform based on "on-off" fluorescence detection will provide significant guidance for the advance development of single-molecule sensors for multi-ion detection.

Carbon dots fabricated by solid-phase carbonization using p-toluidine and l-cysteine for sensitive detection of copper

In this study, a label-free "turn off" fluorescent sensor has been resoundingly fabricated using carbon dots (CDs) for ultrasensitive detection of copper ions (Cu²⁺). CDs are prepared by solid phase carbonization method using p-toluidine and l-cysteine as the precursors. The synthesized CDs exhibited the highest fluorescence intensity with excitation and emission wavelengths set at 300 nm and 400 nm, respectively. The CDs were selective and sensitive to Cu²⁺ due to the static quenching mechanism. The concentration of CDs, and solution pH and incubation time were important parameters for the developed sensor. The experimental results showed that 20 mgL^-1 was enough for the analysis. As the solution pH was concerned, it was apparent that the sensor was endowed with an excellent response signal to Cu²⁺ and provided high sensitivity at pH 12. The interaction occurred very quickly, and the incubation time could be set at 1 min. The sensor provided a two-stage calibration curve to Cu²⁺ in the range of 0.05-0.7 and 0.7-4 μM with a limit of detection of 47 nM. The obtained results clearly demonstrated that this facile method was fast, reliable and selective for detecting Cu²⁺, which would explore a prospective strategy for developing effective and low-cost sensors for monitoring metal ions in aqueous environments.

Hydrogen Peroxide Production from Water Oxidation on a CuWO4 Anode in Oxygen-Deficient Conditions for Water Decontamination

Hydrogen peroxide, an environmentally benign oxidant, is an effective chemical agent for water purification. On-site production of H₂O₂ is considered economical because it avoids the cost of storage and transportation. Traditional generation of H₂O₂ from oxygen reduction, as a heterogeneous electrochemical reaction, suffers from mass transfer problems because of the limited solubility and low diffusion rate of oxygen in water. These limitations can be overcome if H₂O₂ is formed by water oxidation. Herein, conversion of water to hydrogen peroxide was achieved efficiently on a CuWO₄ anode. This water oxidation strategy can generate H₂O₂ at a rate of ∼11.8 μmol min^-1 cm^-2 at 3.0 V versus reversible hydrogen electrode. Importantly, this on-site H₂O₂ production shows high efficiency in water purification in O₂-deficient conditions. This water oxidation anode offers a feasible way to provide a green purification agent with only water as the final byproduct, avoiding toxic intermediates and residues during the production and application.

A novel ratiometric fluorescent and colorimetric sensor based on a 1,8-naphthalimide derivative for nanomolar Cu2+ sensing: smartphone and food applications

A novel 1,8-naphthalimide-based chemical sensor with ratiometric fluorescence behavior, as well as “naked-eye” response was developed for the sensitive and specific determination of Cu2+ at nanomolar levels.

Interfacially Super-Assembled Benzimidazole Derivative-Based Mesoporous Silica Nanoprobe for Sensitive Copper (II) Detection and Biosensing in Living Cells

Mesoporous silica nanoparticles (MSNs) functionalized with benzimidazole-derived fluorescent molecules (DHBM) are fabricated via a feasible interfacial superassembly strategy for the highly sensitive and selective detection of Cu²⁺ . DHBM-MSN exhibits an obvious quenching effect on Cu²⁺ in aqueous solutions, and the detection limit can be as low as 7.69×10^-8  M. The DHBM-MSN solid-state sensor has good recyclability, and the silica framework can simultaneously improve the photostability of DHBM. Two mesoporous silica nanoparticles with different morphologies were specially designed to verify that nanocarriers with different morphologies do not affect the specific detectionability. The detection mechanism of the fluorescent probe was systematically elucidated by combining experimental results and density function theory calculations. Moreover, the detection system was successfully applied to detect Cu²⁺ in bovine serum, juice, and live cells. These results indicate that the DHBM-MSN fluorescent sensor holds great potential in practical and biomedical applications.

Ultrasensitive glucose detection from tears and saliva through integrating a glucose oxidase-coupled DNAzyme and CRISPR-Cas12a

The accurate and sensitive detection of glucose from secretory clinical samples, such as tears and saliva, remains a great challenge. In this research, a novel ultrasensitive glucose detection method consisting of a glucose oxidase (GOx), pistol-like DNAzyme (PLDz), and CRISPR-Cas12a system is proposed. First, the oxidation of glucose catalyzed by GOx leads to the production of H₂O₂; the self-cleavage activity of PLDz is activated after recognition of the produced H₂O₂. The two procedures triggered by COx and PLDz play an important role in accurately identifying glucose and converting glucose signals to nucleic acids. The obtained PLDz fragments can be recognized by the Cas12 enzyme and thus activate the trans-cleavage activity of the Cas12a enzyme. Finally, the surrounding reporter probes are cut by the Cas12a enzyme to produce fluorescence signals. In summary, an ultra-sensitive and specific fluorescence method has been developed for glucose detection from secretory clinical samples, which could potentially contribute to the noninvasive diagnosis of diabetes mellitus.

Rapid and visual detection of Cd2+ based on aza-BODIPY near infrared dye and its application in real and biological samples for environmental contamination screening

Cadmium highly toxic and hazardous, and it can adversely affect human health leading to serious disorders. Herein, a water-soluble near-infrared sensor based on aza-BODIPY (1) was developed for dual determination of Cd²⁺ in environmental and biological media. This sensor exhibited color change from colorless to green along with a fluorescence enhancement in the near-infrared (NIR) region via photoinduced electron transfer (PET) after complexation with Cd²⁺. Sensor 1 can be employed in aqueous media at physiological pH for quantitative monitoring. It shows rapid response with high sensitivity (detection limit of 2.8 ppb; linear correlation over [Cd²⁺] 1.33 - 6.67 µM) and selectivity over potentially interfering ions. NIR sensor 1 can be used to determine [Cd²⁺] in living cells and environmental samples.

A selective and sensitive near-infrared fluorescent probe for real-time detection of Cu(i)

The disruption of copper homeostasis (Cu+/Cu2+) may cause neurodegenerative disorders. Thus, the need for understanding the role of Cu+ in physiological and pathological processes prompted the development of improved methods of Cu+ analysis. Herein, a new near-infrared (NIR) fluorescent turn-on probe (NPCu) for the detection of Cu+ was developed based on a Cu+-mediated benzylic ether bond cleavage mechanism. The probe showed high selectivity and sensitivity toward Cu+, and was successfully applied for bioimaging of Cu+ in living cells.

Europium(III) complex fluorescent sensor for dual channel recognition of Sn2+ and Cu2+ ions in water

A new europium(III) complex Eu(tta)₃L₁ (1a) (L₁ = (2-(3,5-dimethoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline), tta = 2-thenoyltrifluoroacetone) has been prepared and synthesized. The structure of complex was completely determined by several different analytical techniques including single-crystal X-ray diffraction, ¹H and ¹³C NMR. The crystal structure of the complex 1a belonged to monoclinic system with the space group P2₁/n. Its fluorescent properties were systematically studied in details by adding different metal ions in deionized water. Upon addition of Sn²⁺, its fluorescence intensity was strengthened and centered at 460 nm. And when Cu²⁺ was added, its fluorescence emission intensity was quenched quickly. The LODs for Sn²⁺ and Cu²⁺ were calculated to be 4.52 × 10^-7 mol L^-1 and 1.11 × 10^-7 mol L^-1, respectively. Furthermore, this sensor was successfully employed to monitor Sn²⁺ and Cu²⁺ in practical samples.

Zn2+-dependent DNAzymes that cleave all combinations of ribonucleotides

Although several DNAzymes are known, their utility is limited by a narrow range of substrate specificity. Here, we report the isolation of two zinc-dependent DNAzymes, ZincDz1 and ZincDz2, which exhibit compact catalytic core sequences with highly versatile hydrolysis activity. They were selected through in vitro selection followed by deep sequencing analysis. Despite their sequence similarity, each DNAzyme showed different Zn²⁺-concentration and pH-dependent reaction profiles, and cleaved the target RNA sequences at different sites. Using various substrate RNA sequences, we found that the cleavage sequence specificity of ZincDz2 and its highly active mutant ZincDz2-v2 to be 5'-rN↓rNrPu-3'. Furthermore, we demonstrated that the designed ZincDz2 could cut microRNA miR-155 at three different sites. These DNAzymes could be useful in a broad range of applications in the fields of medicine and biotechnology.

Highly selective fluorescence detection of Cu2+ based on Schiff base functionalized periodic mesoporous organosilicas

A novel copper ion sensing periodic mesoporous organosilica (SCN-PMO) was obtained by incorporating a Schiff base-based fluorescent receptor into the pore walls of mesoporous silica, which exhibited a well ordered mesoporous structure and excellent optical properties demonstrated by various characterization results. SCN-PMO possessed high selectivity and sensitivity towards Cu²⁺ based on its specific fluorescence response. The detection limit of SCN-PMO could be as low as 6.7 × 10^-7 M. Due to protection of the silica network, SCN chromophores in PMOs exhibited higher photostability and the resulting material possessed great repeatability. Additionally, the fluorescence changes of SCN-PMO towards copper ions in vivo (zebrafish) showed that SCN-PMO has potential application as a nanoprobe in biological fields.

Characterization of a DNA-hydrolyzing DNAzyme for generation of PCR strands of unequal length

I-R3 DNAzyme is a small, highly active catalytic DNA for DNA hydrolysis. In here, we designed two cis-structure DNAzymes (I-R3N and I-R3S) based on the different locates of the joint linker between I-R3 and its substrate. Data demonstrated that both DNAzymes were highly dependent on Zn²⁺, and worked at a narrow range around pH 7.0. They exhibited strong anti-interference with Mg²⁺ and Ca²⁺, but inhibited by Na⁺ and K⁺. Moreover, single and multiple-site mutations were generated within the catalytic core to carry out a comprehensive mutational study of I-R3 motif, in which most nucleotides were highly conserved and the nucleotides A5, T11 and T8 were identified as the mutational hotspots. Furthermore, an efficient variant A5G was obtained and its reaction condition was optimized. Finally, we constructed A5G to the 3' end of a single-stranded DNA (ssDNA) and applied it for asymmetrical PCR amplification to produce a single and double-stranded DNA mixture, in which A5G within ssDNA can self-cleave to generate a shorter desired ssDNA by denaturing gel separation. This would provide a new non-chemical modification approach for preparation of the expected ssDNA for in vitro selection of DNAzymes.

Insight into an Oxidative DNA-Cleaving DNAzyme: Multiple Cofactors, the Catalytic Core Map and a Highly Efficient Variant

An oxidative DNA-cleaving DNAzyme (PL) employs a double-cofactor model “X/Cu²⁺” for catalysis. Herein, we verified that reduced nicotinamide adenine dinucleotide (NADH), flavin mononucleotide, cysteine, dithiothreitol, catechol, resorcinol, hydroquinone, phloroglucinol, o-phenylenediamine, 3,3′,5,5'-tetramethylbenzidine, and hydroxylamine acted as cofactor X. According to their structural similarities or fluorescence property, we further confirmed that reduced nicotinamide adenine dinucleotide phosphate (NADPH), 2-mercaptoethanol, dopamine, chlorogenic acid, resveratrol, and 5-carboxyfluorescein also functioned as cofactor X. Superoxide anions might be the commonality behind these cofactors. We subsequently determined the conservative change of individual nucleotides in the catalytic core under four different cofactor X. The nucleotides A4 and C5 are highly conserved, whereas the conservative levels of other nucleotides are dependent on the types of cofactor X. Moreover, we observed that the minor change in the PL's secondary structure affects electrophoretic mobility. Finally, we characterized a highly efficient variant T3G and converted its double-cofactor NADH/Cu²⁺ to sole-cofactor NADH.

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An oxidative cleavage DNAzyme works with various cofactor X

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Catalytic nucleotide conservation fluctuates with different cofactor X

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The PL DNAzyme's minor secondary structure change affects electrophoretic mobility

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Double-cofactor model of the variant T3G can be converted to sole-cofactor model

Visible and Reversible Restrict of Molecular Configuration by Copper Ion and Pyrophosphate

Molecular configuration strongly impacts on its functions; however, due to complicated and diverse configuration as well as easy and rapid conversion among various configurations, research of molecular configuration is extremely difficult. If the free rotation of a molecule could be "slowed down" or even "frozen" by an external stimulus, such as ultralow temperature, then one configuration of the molecule could be captured and characterized relatively easily. Here, we show that the rotation of a hemicyanine-labeled 2-(2'-hydroxyphenyl)-4-methyloxazole (H-HPMO) molecule could be specifically and reversibly restricted by sequential additions of copper ion (Cu²⁺) and pyrophosphate (P₂O₇^4-), reflecting as remarkable fluorescence quenching and recovery, which could be directly observed by naked eyes. Binding affinity tests and cryogenic ¹H NMR indicate that Cu²⁺ forms intensive coordinate bonds with phenolic hydroxyl, oxazole, and methoxyl groups of HPMO, which strongly restricts the free rotations of these groups and blocks charge transfer. This study provides a precise, rapid, visible, reversible, and low-cost method to monitor the molecular configuration, indicating the broad application prospects of near-infrared fluorescent sensors in configuration analysis, biosensing, and drug-substrate complexation.

Fluorescence "Turn On-Off" Sensing of Copper (II) Ions Utilizing Coumarin-Based Chemosensor: Experimental Study, Theoretical Calculation, Mineral and Drinking Water Analysis

Herein, we report the preparation of a fluorescent sensor based on coumarin derivative for copper (II) ion sensing in CH₃CN/HEPES media. 6,7-dihydroxy-3-(4-(trifluoro)methylphenyl)coumarin (HMAC) sensor was fabricated and analyzed by spectroscopic techniques. The sensor demonstrates "turn on-off" fluorescence quenching in the presence of copper (II) ions at 458 nm. A clear complex between the chemosensor HMAC and copper (II) ions was characterized by ESI-MS as well as the Job's method. Also, the limit of detection (LOD, 3σ/k) value was determined as 24.5 nM in CH₃CN/HEPES (95/5, v/v) buffer media (pH = 7.0). This value is lower than the admissible level of copper (II) ions in drinking water (maximum 31.5 μM) reported by EU Water Framework Directive (WFD) and World Health Organization (WHO) guidelines. The theoretical calculations (density functional theory, DFT) have been performed for the geometric optimized structures. As a final stage, real sample analyses have successfully been performed by using HMAC, as well as ICP-OES method. The relative standard deviation for copper (II) in mineral and drinking water samples has been determined to be below 0.15% and recovery values are in the range of 95.48-109.20%.

Integration of Cd:ZnS QDs into ZIF-8 for enhanced selectivity toward Cu2+ detection

Cd:ZnS QDs were integrated into ZIF-8 that could specifically interact with Cu²⁺ for enhanced selectivity toward Cu²⁺ detection.

An ultrasensitive and selective fluorescent nanosensor based on porphyrinic metal–organic framework nanoparticles for Cu2+ detection

A fluorescent nanosensor based on ultrasmall MOF-525 NPs was proposed for the monitoring of Cu²⁺ in aqueous solution and living cells.

Hydrogel Interferometry for Ultrasensitive and Highly Selective Chemical Detection

Developing ultrasensitive chemical sensors with small scale and fast response through simple design and low-cost fabrication is highly desired but still challenging. Herein, a simple and universal sensing platform based on a hydrogel interferometer with femtomol-level sensitivity in detecting (bio)chemical molecules is demonstrated. A unique local concentrating effect (up to 10⁹ folds) in the hydrogel induced by the strong analyte binding and large amount of ligands, combined with the signal amplification effect by optical interference, endows this platform with an ultrahigh sensitivity, specifically 10^-14 m for copper ions and 1.0 × 10^-11 mg mL^-1 for glycoprotein with 2-4 order-of-magnitude enhancement. The specific chemical reactions between selected ligands and target analytes provide high selectivity in detecting complex fluids. This universal principle with broad chemistry, simple physics, and modular design allows for high performance in detecting wide customer choices of analytes, including metal ions and proteins. The scale of the sensor can be down to micrometer size. The nature of the soft gel makes this platform transparent, flexible, stretchable, and compatible with a variety of substrates, showing high sensing stability and robustness after 200 cycles of bending or stretching. The outstanding sensing performance grants this platform great promise in broad practical applications.

On/Off Fluorescent Chemosensor for Selective Detection of Divalent Iron and Copper Ions: Molecular Logic Operation and Protein Binding

Here, naphthalene diamine-based β-diketone derivative (compound LH) was successfully used as a dual signaling probe for divalent cations, Fe2+ and Cu2+ ions, in bimodal methods (colorimetric and fluorometric). It showed fluorescent enhancement for Fe2+ ion by photoinduced electron transfer mechanism and fluorescence quenching for Cu2+ ion by charge-transfer process. Binding stoichiometry for [LH-(Fe2+)2] and [LH-(Cu2+)2] was found to be 1:2 by Job's plot method and, the binding constants were calculated as 1.6638 × 1010 and 9.22929 × 108 M-1, respectively. Compound LH exhibited OR and XOR logic gate behavior with H+, Fe2+, and Cu2+ as inputs. Further, the compound LH and bovine serum albumin binding interaction showed quenching of fluorescence by Förster resonance energy-transfer mechanism.

An Ultrasensitive Fluorescence Sensor with Simple Operation for Cu2+ Specific Detection in Drinking Water

Whether short-term or long-term, overexposure to an abnormal amount of copper ion does significant harm to human health. Considering its nonbiodegradability, it is critical to sensitively detect copper ion. Herein, a novel fluorescent strategy with a "turn-on" signal was developed for highly sensitive and specific detection of copper ion (Cu2+). In the present investigation, we found that Cu2+ exhibits excellent peroxidase-like catalytic activity toward oxidizing the nonfluorescent substrate of Amplex Red into the product of resofurin with outstanding fluorescence emission under the aid of H2O2. Thus, an enzyme-free and label-free sensing system was constructed for copper ion detection with quite simple operation. To ensure the detection sensitivity and reproducibility, the amount of H2O2 and incubation time were optimized. The limit of detection can reach as low as 1.0 nM. In addition, the developed assay demonstrated excellent specificity and could be utilized to detect copper ion in water samples including tap water and bottled purified water without standing recovery.

Barbituric acid-modified graphitic carbon nitride nanosheets for ratiometric fluorescent detection of Cu2+

A ratiometric and visual fluorescent sensor for Cu²⁺ ions based on barbituric acid-modified graphitic carbon nitride nanosheets was designed.

Gly–Gly–His tripeptide- and silver nanoparticle-assisted electrochemical evaluation of copper(ii) ions in aqueous environment

A sensitive and selective electrochemical sensor for Cu²⁺ assay is developed using tripeptide-based recognition and silver nanoparticle-modified electrode.

Imaging Endogenous Metal Ions in Living Cells Using a DNAzyme-Catalytic Hairpin Assembly Probe

DNAzymes are a promising platform for metal ion detection, and a few DNAzyme-based sensors have been reported to detect metal ions inside cells. However, these methods required an influx of metal ions to increase their concentrations for detection. To address this major issue, the design of a catalytic hairpin assembly (CHA) reaction to amplify the signal from photocaged Na+ -specific DNAzyme to detect endogenous Na+ inside cells is reported. Upon light activation and in the presence of Na+ , the NaA43 DNAzyme cleaves its substrate strand and releases a product strand, which becomes an initiator that trigger the subsequent CHA amplification reaction. This strategy allows detection of endogenous Na+ inside cells, which has been demonstrated by both fluorescent imaging of individual cells and flow cytometry of the whole cell population. This method can be generally applied to detect other endogenous metal ions and thus contribute to deeper understanding of the role of metal ions in biological systems.

Cu2+-catalyzed and H2O2-facilitated oxidation strategy for sensing copper(ii) based on cysteine-mediated aggregation of gold nanoparticles

A novel colorimetric Cu²⁺ sensor using polyethyleneglycol-stabilized gold nanoparticles has been developed based on a cysteine/H₂O₂ system.