Fluorescence energy transfer probes based on the guanine quadruplex formation for the fluorometric detection of potassium ion

Dual-labeled oligonucleotide derivative, FAT-0, carrying 6-carboxyfluorescein (FAM) and 6-carboxy-tetramethylrhodamine (TAMRA) labels at 5′- and 3′-termini of thrombin-binding aptamer (TBA) sequence 5′-GGTTGGTGTGGTTGG-3′ and its derivatives, FAT-n (n = 3, 5, and 7) were designed and synthesized. FAT-n derivatives contained a T_mA spacer (m = 2, 4, and 6, respectively) at 5′-end of TBA sequence. The probes were developed to estimate the spacer effect on FRET efficiency and to identify the best probe for sensing of K⁺. Circular dichroism (CD), UV-vis absorption, and fluorescence studies revealed that all FAT-n probes could form the intramolecular tetraplex structures after binding K⁺. Association constants of particular K⁺/FAT-n complexes were determined using different experimental approaches. Suitability of particular probes for sensitive monitoring of K⁺ in intra- and extracellular conditions was examined and discussed. Calibration graphs of fluorescence ratio were linear in the K⁺ concentration range of 2-10 mM for extracellular conditions showing sensitivity of 1.2% mM⁻¹ K⁺ and for intracellular conditions in the range of 100-200 mM with sensitivity of 0.49% mM⁻¹ K⁺.     

A sensitive fluorescence anisotropy method for detection of lead (II) ion by a G-quadruplex-inducible DNA aptamer

Sensitive and selective detection of Pb²⁺ is of great importance to both human health and environmental protection. Here we propose a novel fluorescence anisotropy (FA) approach for sensing Pb²⁺ in homogeneous solution by a G-rich thrombin binding aptamer (TBA). The TBA labeled with 6-carboxytetramethylrhodamine (TMR) at the seventh thymine nucleotide was used as a fluorescent probe for signaling Pb²⁺. It was found that the aptamer probe had a high FA in the absence of Pb²⁺. This is because the rotation of TMR is restricted by intramolecular interaction with the adjacent guanine bases, which results in photoinduced electron transfer (PET). When the aptamer probe binds to Pb²⁺ to form G-quadruplex, the intramolecular interaction should be eliminated, resulting in faster rotation of the fluorophore TMR in solution. Therefore, FA of aptamer probe is expected to decrease significantly upon binding to Pb²⁺. Indeed, we observed a decrease in FA of aptamer probe upon Pb²⁺ binding. Circular dichroism, fluorescence spectra, and fluorescence lifetime measurement were used to verify the reliability and reasonability of the sensing mechanism. By monitoring the FA change of the aptamer probe, we were able to real-time detect binding between the TBA probe and Pb²⁺. Moreover, the aptamer probe was exploited as a recognition element for quantification of Pb²⁺ in homogeneous solution. The change in FA showed a linear response to Pb²⁺ from 10 nM to 2.0 μM, with 1.0 nM limit of detection. In addition, this sensing system exhibited good selectivity for Pb²⁺ over other metal ions. The method is simple, quick and inherits the advantages of aptamer and FA.     

Highly sensitive determination of trace potassium ion in serum using the resonance light scattering technique with sodium tetraphenylboron

A resonance light scattering technique has been developed in order to determine potassium ion in serum. Potassium ion was found to bind the tetraphenylboronate anion [(C₆H₅)₄B]⁻ in acetate buffer (pH 8.0) in the presence of sodium dodecyl benzene sulfonate as a stabilizer, forming the B(C₆H₅)₄-K aggregate which produces intense resonance scattering light. Effects of factors such as acidity, ionic strength and interferents on the RLS of B(C₆H₅)₄-K were investigated. The solution pH close to neutral facilitates the production of RLS, and few biologically relevant species interfere in the determination of potassium ion. The resonance scattering light intensity at the maximum peak of 567 nm was linear to the concentration of potassium ion in the range of 0.2–2.0 μg mL⁻¹ with a detection limit of 20.0 ng mL⁻¹. The method was applied to determine trace amounts of potassium ion in serum and showed high sensitivity and accuracy compared with the clinically used ion-selective electrode method.     

An optical sensor for mercury ion based on the fluorescence quenching of tetra(p-dimethylaminophenyl)porphyrin

An optical sensor for mercury ion (Hg²⁺), based on quenching the fluorescence of the sensing reagent porphyrin immobilized in plasticized poly(vinyl chloride) (PVC) membrane, has been developed. The responses to mercury ion were compared for the sensors modified with three porphyrin compounds including 5,10,15,20-tetraphenylporphyrin (TPP), tetra(p-dimethylaminophenyl)porphyrin (TDMAPP) and tetra(N-phenylpyrazole) porphyrin (TPPP). Among them, TDMAPP showed the most remarkable response to Hg²⁺. The drastic decrease of the TDMAPP fluorescence intensity was attributed to the formation of a complex between TDMAPP and Hg²⁺, which has been utilized as the fabrication basis of a Hg²⁺-sensitive fluorescence sensor. The analytical performance characteristics of the TDMAPP modified sensor was investigated. The response mechanism, especially involving the response difference of three porphyrin compounds, was discussed in detail. The sensor can be applied to the quantification of Hg²⁺ with a linear range covering from 4.0 × 10⁻⁸ mol L⁻¹ to 4.0 × 10⁻⁶ mol L⁻¹. The limit of detection was 8.0 × 10⁻⁹ mol L⁻¹. The sensor exhibited excellent reproducibility, reversibility and selectivity. Also, the TDMAPP-based sensor was successfully used for the determination of Hg²⁺ in environmental water samples.     

Development of 1,3a,6a-triazapentalene-labeled enterobactin as a fluorescence quenching sensor of iron ion

1,3a,6a-Triazapentalene (TAP)-labeled enterobactin was developed as an iron ion sensor. 3-Acetylated-TAP was successfully introduced to the catechol ring of enterobactin, a well-recognized siderophore secreted by various Gram-negative bacteria. The fluorescence of TAP-labeled enterobactin decreased gradually as the amount of Fe³⁺ ion as an additive was increased, and 1.2 equiv of Fe³⁺ ion completely quenched the fluorescence. In clear contrast, when other metal ions were used, the fluorescence of TAP-labeled enterobactin remained even at 5.0 equiv.     

Simple and sensitive detection of dopamine in the presence of high concentration of ascorbic acid using gold nanoparticles as colorimetric probes

A highly sensitive and selective method is presented for colorimetric determination of dopamine using gold nanoparticles (AuNPs). Dopamine induces the aggregation of AuNPs, this resulting in a color change from red to blue or purple. Aggregation is accelerated by the presence of Cu(II), especially at low concentrations of dopamine. The concentration of dopamine can be quantified visually or using a UV-vis spectrometer. The detection limit is as low as 30 nM. The assay is simple, inexpensive, and highly sensitive. Ascorbic acid in even 100-fold molar excess does not interfere. The mechanism of the aggregation of the AuNPs is discussed.     

Colorimetric detection of sodium ion in serum based on the G-quadruplex conformation related DNAzyme activity

There has been a big challenge in developing the Na⁺ sensor that can be practically used in the physiological system with the interference of large amounts of K⁺. In this research, a novel Na⁺ sensor has been designed based on the G-quadruplex-conformation related DNAzyme activity. The sensor exhibits high selectivity and sensitivity with the detection limit of 0.6 μM, which enables the sensor to be practically used in determination of the Na⁺ level in serum. The research not only provides a simple Na⁺ sensor but also opens a new way for developing the detection technology of Na⁺.     

Highly sensitive fluorescence detection of chloride ion in aqueous solution with Ag-modified porous g-C3N4 nanosheets

The porous g-C₃N₄ (PCN) nanosheets are successfully synthesized and further modified with nano-sized Ag by a simple wet-chemical process. Interestingly, the Ag-modified porous g-C₃N₄ (Ag-PCN) nanosheets exhibit competitive fluorescence detection performance of chloride ion (Cl⁻) in aqueous solution. Under the optimized conditions, the concentration of Cl⁻ could be quantitative analyzed with the Ag-PCN in a wide detection range from 0.5 mmol/L to 0.1 mol/L, with a low detection limitation of 0.06 mmol/L. It is confirmed that the fluorescence of PCN could be effectively decayed by the photoinduced charge transfer via the adsorbed Cl⁻ for trapping holes, mainly by means of the time-resolved fluorescence and surface photovoltage spectra. The porous structure and modified Ag promote the adsorption of Cl⁻ on resulting Ag-PCN, leading to excellent fluorescence detection for Cl⁻. This work provides a feasible route to develop a fluorescence detection of Cl⁻ with g-C₃N₄ nanosheets in environment water.     

Cytidine-stabilized gold nanocluster as a fluorescence turn-on and turn-off probe for dual functional detection of Ag and Hg

In this study, we have developed a label-free, dual functional detection strategy for highly selective and sensitive determination of aqueous Ag⁺ and Hg²⁺ by using cytidine stabilized Au NCs and AuAg NCs as fluorescent turn-on and turn off probes, respectively. The Au NCs and AuAg NCs showed a remarkably rapid response and high selectivity for Ag⁺ and Hg²⁺ over other metal ions, and relevant detection limit of Ag⁺ and Hg²⁺ is ca. 10 nM and 30 nM, respectively. Importantly, the fluorescence enhanced Au NCs by doping Ag⁺ can be conveniently reusable for the detection of Hg²⁺ based on the corresponding fluorescence quenching. The sensing mechanism was based on the high-affinity metallophilic Hg²⁺–Ag⁺ interaction, which effectively quenched the fluorescence of AuAg NCs. Furthermore, these fluorescent nanoprobes could be readily applied to Ag⁺ and Hg²⁺ detection in environmental water samples, indicating their possibility to be utilized as a convenient, dual functional, rapid response, and label-free fluorescence sensor for related environmental and health monitoring.     

Optical sensor for berberine utilizing its intrinsic fluorescence enhanced by the formation of inclusion complex with butylated-β-cyclodextrin

An optical sensor for berberine, the basic ingredient of the widely used traditional Chinese medicine Coptis Chinensis, based on its intrinsic fluorescence enhanced by butylated-β-cyclodextrin (HDB-β-CD) immobilized in plasticized poly(vinyl chloride) (PVC) membrane has been developed. The drastic enhancement of fluorescence intensity of berberine was attributed to the formation of an inclusion complex between HDB-β-CD and berberine, which has been utilized as the basis of the fabrication of a berberine-sensitive fluorescence sensor. The proposed sensor was quite distinct from those fluorescent sensors for berberine reported so far which relied upon quenching the fluorescence of the sensing reagent immobilized on membrane by berberine. The response mechanism of optode membrane was discussed in detail from the view of molecular dynamics and the optimum steric configuration of the inclusion complex was presented by molecular dynamics simulation. The analytical performance characteristics of the proposed berberine-sensitive sensor were investigated. The sensor can be applied to the quantification of berberine with a linear range covering from 4.0×10⁻⁷ to 2.0×10⁻⁵ mol l⁻¹ with a detection limit of 8.0×10⁻⁸ mol l⁻¹. The sensor exhibits excellent reproducibility, reversibility and selectivity. The recommended method was successfully used for the determination of berberine in pharmaceutical preparations.     

Fluorescence sensor using a molecularly imprinted polymer as a recognition receptor for the detection of aluminium ions in aqueous media

This paper describes the investigation of a molecularly imprinted polymer (MIP) as a sensing receptor for Al³⁺ ion detection by using an optical approach. Al³⁺ ion was adopted as the template molecule and 8-hydroxyquinoline sulfonic acid ligand as the fluorescence tag. The polymer was synthesised using acrylamide as monomer, 2-hydroxyethyl methacrylate as co-monomer and ethylene glycol dimethracylate as cross-linker. The free radical polymerisation was performed in methanol and initiated by 2,2′-azobisisobutyronitrile at 70 °C. The imprinted polymer was fluorometrically characterised using a fibre optic attachment in a self-designed flow-cell. NaF was used to leach the Al³⁺ ion from the MIP. The optimum pH for the rebinding of Al³⁺ ion with the leached polymer was found to be pH 5 and the fluorescence response was found to be stable within the buffer strength range of 0.05–0.10 M. The fluorescence intensity during Al³⁺ ion rebinding was inversely dependent on temperature, and a low interference response (<3%) toward metal ions except for Cu²⁺ and Zn²⁺ ions was observed. The polymer rebinding repeatability study conducted over 9 cycles with Al³⁺ ion (0.8×10⁻⁴ M) was found to give an RSD value of 2.82% with a standard deviation of 0.53. The dynamic range of the system was found to be linear up to 1.0×10⁻⁴ M Al³⁺ ion with a limit of detection of 3.62 μM.     

CdTe quantum dots as fluorescence sensor for the determination of vitamin B6 in aqueous solution

A novel, rapid and simple CdTe quantum dots （QDs） based technology platform was established for selective and sensitive determination of vitamin B6 in aqueous solution. It can perform accurate and reproducible quantification of vitamin B6 in pharmaceutical with satisfactory results.     

Fabrication of an electrochemical sensor based on spiropyran for sensitive and selective detection of fluoride ion

In the past decades, numerous electrochemical sensors based on exogenous electroactive substance have been reported. Due to non-specific interaction between the redox mediator and the target, the instability caused by false signal may not be avoided. To address this issue, in this paper, a new electrochemical sensor based on spiropyran skeleton, namely SPOSi, was designed for specific electrochemical response to fluoride ions (F⁻). The breakage of Si–O induced by F⁻ based on the specific nucleophilic substitution reaction between F⁻ and silica would directly produce a hydroquinone structure for electrochemical signal generation. To improve the sensitivity, SPOSi probe was assembled on the single-walled carbon nanotubes (SWCNTs) modified glassy carbon electrode (GCE) through the π–π conjugating interaction. This electrode was successfully applied to monitor F⁻ with a detection limit of 8.3 × 10⁻⁸ M. Compared with the conventional F⁻ ion selected electrode (ISE) which utilized noncovalent interaction, this method displays higher stability and a comparable sensitivity in the urine samples.     

A functionalized gold nanoparticles and Rhodamine 6G based fluorescent sensor for high sensitive and selective detection of mercury(II) in environmental water samples

A gold-nanoparticles (Au NPs)-Rhodamine 6G (Rh6G) based fluorescent sensor for detecting Hg (II) in aqueous solution has been developed. Water-soluble and monodisperse gold nanoparticles (Au NPs) has been prepared facilely and further modified with thioglycolic acid (TGA). Free Rh6G dye was strongly fluorescent in bulk solution. The sensor system composing of Rh6G and Au NPs fluoresce weakly as result of fluorescence resonance energy transfer (FRET) and collision. The fluorescence of Rh6G and Au NPs based sensor was gradually recovered due to Rh6G units departed from the surface of functionalized Au NPs in the presence of Hg(II). Based on the modulation of fluorescence quenching efficiency of Rh6G-Au NPs by Hg(II) at pH 9.0 of teraborate buffer solution, a simple, rapid, reliable and specific turn-on fluorescent assay for Hg(II) was proposed. Under the optimum conditions, the fluorescence intensity of sensor is proportional to the concentration of Hg(II). The calibration graphs are linear over the range of 5.0 × 10⁻¹⁰ to 3.55 × 10⁻⁸ mol L⁻¹, and the corresponding limit of detection (LOD) is low as 6.0 × 10⁻¹¹ mol L⁻¹. The relative standard deviation of 10 replicate measurements is 1.5% for 2.0 × 10⁻⁹ mol L⁻¹ Hg(II). In comparison with conventional fluorimetric methods for detection of mercury ion, the present nanosensor endowed with higher sensitivity and selectivity for Hg(II) in aqueous solution. Mercury(II) of real environmental water samples was determined by our proposed method with satisfactory results that were obtained by atomic absorption spectroscopy (AAS).     

Developing a genetically encoded green fluorescent protein mutant for sensitive light-up fluorescent sensing and cellular imaging of Hg(II)

Hg(II) is well-known for quenching fluorescence in a distance dependent manner. Nevertheless, when we exposed the fluorophore of a green fluorescent protein (GFP) toward Hg(II), through H148C mutation, the GFP fluorescence could be “lighted up” by Hg(II) down to sub-nM level. The detection linear range is 0.5–3.0 nM for protein solutions at 8.0 nM. The GFPH148C protein displayed a promising selectivity toward Hg(II) and also the cellular imaging capacity. Spectra measurements suggested that the ground-state redistribution of protein contributed to the fluorescence enhancement, which was found not limited to Hg(II), and thus presented an opening for building a pool of GFP-based chemosensors toward other heavy metal ions.     

A dual-amplification aptasensor for highly sensitive detection of thrombin based on the functionalized graphene-Pd nanoparticles composites and the hemin/G-quadruplex

In this work, an advanced sandwich-type electrochemical aptasensor for thrombin was proposed by integrating hemin/G-quadruplex with functionalized graphene-Pd nanoparticles composites (PdNPs-RGs). The hemin/G-quadruplex formed by intercalating hemin into thrombin binding aptamer (TBA), firstly acted as a NADH oxidase, assisting the oxidation of NADH to NAD⁺ accompanying with the generation of H₂O₂ in the presence of dissolved O₂. Subsequently, the hemin/G-quadruplex acted as HRP-mimicking DNAzyme that rapidly bioelectrocatalyze the reduction of the produced H₂O₂. At the same time, the Pd nanoparticles supported on p-iodoaniline functionalized graphene were also adopted to catalyze the reduction of H₂O₂. Thus, with the dual catalysis, a dramatically amplified electrochemical signal could be obtained. Besides, the avidin–biotin system for binding aptamer sequences on electrodes not only improved the sensitivity of thrombin analysis but also obtained an acceptable repeatability of the aptasensor. With several factors mentioned above, a wide linear ranged from 0.1 pM to 50 nM was acquired with a relatively low detection limit of 0.03 pM (defined as S/N = 3). These excellent performances provided our approach a promising way for ultrasensitive assay in electrochemical aptasensors.     

Recent advances in fluorescence sensor for the detection of peroxide explosives

The detection of peroxide explosives (PEs) has attracted considerable attention all over the world in global security owing to their simple preparation, poor chemical stability and easy decomposition. In recent years, great efforts have been devoted to developing organic fluorescence sensors for detecting the PEs because of their fast response, high sensitivity and high selectivity. In this short review, we firstly discuss the sensing mechanisms for fluorescence based the PEs detection. Next, we reviewed recent progress of PE probes in the nearly 5 years and the design strategies of the material structures to enhance the sensitivity or selectivity, such as conjugated polymers and assembled nanoparticles.     

Polyethyleneimine-capped silver nanoclusters as a fluorescence probe for sensitive detection of hydrogen peroxide and glucose

In this work, we utilized polyethyleneimine-capped silver nanoclusters (PEI-Ag nanoclusters) to develop a new fluorometric method for the determination of hydrogen peroxide and glucose with high sensitivity. The PEI-Ag nanoclusters have an average size of 2 nm and show a blue emission at 455 nm. The photostable properties of the PEI-Ag nanoclusters were examined. The fluorescence of the PEI-Ag nanoclusters could be particularly quenched by H₂O₂. The oxidization of glucose by glucose oxidase coupled with the fluorescence quenching of PEI-Ag nanoclusters by H₂O₂ can be used to detect glucose. Under optimum conditions, the fluorescence intensity quenched linearly in the range of 500 nM–100 μM with high sensitivity. The detection limit for H₂O₂ was 400 nM. And a linear correlation was established between fluorescence intensity (F₀ − F) and concentration of glucose in the range of 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵ M and 1.0 × 10⁻⁵ to 1.0 × 10⁻³ M with a detection limit of 8.0 × 10⁻⁷ M. The method was used for the detection of glucose in human serum samples with satisfactory results. Furthermore, the mechanism of sensitive fluorescence quenching response of Ag nanoclusters to glucose and H₂O₂ has been discussed.     

Fluorescence enhancement of CdTe/CdS quantum dots by coupling of glyphosate and its application for sensitive detection of copper ion

A novel fluorescent probe for Cu²⁺ determination based on the fluorescence quenching of glyphosate (Glyp)-functionalized quantum dots (QDs) was firstly reported. Glyp had been used to modify the surface of QDs to form Glyp-functionalized QDs following the capping of thioglycolic acid on the core–shell CdTe/CdS QDs. Under the optimal conditions, the response was linearly proportional to the concentration of Cu²⁺ between 2.4 × 10⁻² μg mL⁻¹ and 28 μg mL⁻¹, with a detection limit of 1.3 × 10⁻³ μg mL⁻¹ (3δ). The Glyp-functionalized QDs fluorescent probe offers good sensitivity and selectivity for detecting Cu²⁺. The fluorescent probe was successfully used for the determination of Cu²⁺ in environmental samples. The mechanism of reaction was also discussed.     

Electrochemical aptamer sensor for highly sensitive detection of mercury ion with Au/Pt@carbon nanofiber‐modified electrode

In this paper, an electrochemical aptamer sensor was proposed for the highly sensitive detection of mercury ion (Hg²⁺). Carbon nanofiber (CNF) was prepared by electrospinning and high‐temperature carbonization, which was used for the loading of platinum nanoparticles (PtNPs) by the hydrothermal method. The Pt@CNF nanocomposite was modified on the surface of carbon ionic liquid electrode (CILE) to obtain Pt@CNF/CILE, which was further decorated by gold nanoparticles (AuNPs) through electrodeposition to get Au/Pt@CNF/CILE. Self‐assembling of the thiol‐based aptamer was further realized by the formation of Au‐S bond to get an electrochemical aptamer sensor (Aptamer/Au/Pt@CNF/CILE). Due to the specific binding of aptamer probe to Hg²⁺ with the formation of T‐Hg²⁺‐T structure, a highly sensitive quantitative detection of Hg²⁺ could be achieved by recording the changes of current signal after reacting with Hg²⁺ within the concentration range from 1.0 × 10^?15 mol/L to 1.0 × 10^?6 mol/L and the detection limit of 3.33 × 10^?16 mol/L (3σ). Real water samples were successfully analyzed by this method.