Label-free fluorescent sensor for lead ion detection based on lead(II)-stabilized G-quadruplex formation

A label-free fluorescent DNA sensor for the detection of lead ions (Pb²⁺) based on lead(II)-stabilized G-quadruplex formation is proposed in this article. A guanine (G)-rich oligonucleotide, T30695, was used as a recognition probe, and a DNA intercalator, SYBR Green I (SG), was used as a signal reporter. In the absence of Pb²⁺, the SG intercalated with the single-stranded random-coil T30695 and emitted strong fluorescence. While in the presence of Pb²⁺, the random-coil T30695 would fold into a G-quadruplex structure and the SG could barely show weak fluorescence, and the fluorescence intensity was inversely proportional to the involving amount of Pb²⁺. Based on this, a selective lead ion sensor with a limit of detection of 3.79 ppb (parts per billion) and a detection range from 0 to 600 ppb was constructed. Because detection for real samples was also demonstrated to be reliable, this simple, low-cost, sensitive, and selective sensor holds good potential for Pb²⁺ detection in real environmental samples.     

Carbon nanoparticle for highly sensitive and selective fluorescent detection of mercury(II) ion in aqueous solution

In this article, carbon nanoparticles (CNPs) were used as a novel fluorescent sensing platform for highly sensitive and selective Hg(2+) detection. To the best of our knowledge, this is the first example of CNPs obtained from candle soot used in this type of sensor. The general concept used in this approach is based on that adsorption of the fluorescently labeled single-stranded DNA (ssDNA) probe by CNP via π-π stacking interactions between DNA bases and CNP leads to substantial dye fluorescence quenching; however, in the presence of Hg(2+), T-Hg(2+)-T induced hairpin structure does not adsorb on CNP and thus retains the dye fluorescence. A detection limit as low as 10nM was achieved. The present CNP-based biosensor for Hg(2+) detection exhibits remarkable specificity against other possible metal ions. Furthermore, superior selectivity performance was observed when Hg(2+) detection was carried out in the presence of a large amount of other interference ions. Finally, in order to evaluate its potential practical application, Hg(2+) detection was conducted with the use of lake water other than pure buffer and it is believed that it holds great promise for real sample analysis upon further development.     

A deep-red xanthene-based highly sensitive fluorescent probe for detection of hypochlorite

As an oxidant, deodorant and bleaching agent, the hypochlorous acid (HClO) and hypochlorite (ClO⁻) are widely used in corrosion inhibitors, textile dyes, pharmaceutical intermediates and in our daily lives. However, excess usage or aberrant accumulation of ClO⁻ leads to tissue damage or some diseases and even cancer. Therefore, it is necessary to develop a fluorescent probe that specifically identifies ClO⁻. In this article, we synthesized a deep-red xanthene-based fluorescent probe (XA-CN). The strong electron deficient group dicyano endows the probe XA-CN deep-red fluorescent emission with high solubility, selectivity and sensitivity for ClO⁻ detection. Studies showed that the probe demonstrated turn-off fluorescence (643 nm) at the presence of ClO⁻ in dimethylsulfoxide/phosphate-buffered saline 1:1 (pH 9) solution with a limit of detection of 1.64 μM. Detection mechanism investigation revealed that the electron deficient group -CN and the hydroxyl group was oxidized into aldehyde or carbonyl groups at the presence of ClO⁻, resulting ultraviolet-visible absorption of the probe blue shifted and turned-off fluorescence. Furthermore, XA-CN was successfully used for the detection of ClO⁻ in tap water samples.     

Long pepper (Piper longum) derived carbon dots as fluorescent sensing probe for sensitive detection of Sudan I

In this piece of work, microwave-assisted conversion of a natural precursor in to high-valued nano-scale material was carried out by a completely greener method. The fluorescent carbon dots prepared, designated as long pepper derived carbon dots (LPCDs), have been thoroughly characterized to explore the physical and chemical properties. The system exhibits excitation dependent emission behavior and from the optimal studies the excitation and emission wavelength of the system was found to be 330 nm and 455 nm respectively. On account of the superior fluorescent behavior of the LPCDs, it was successfully employed as a fluorescent sensing probe to detect Sudan I with good level of selectivity and sensitivity. This carcinogenic dye extensively used as food adulterant can impart several health issues. Food product safety is of high concern, therefore a simple facile and economical analytical method was proposed based on the fluorescence of LPCDs for this dye detection with satisfactory statistical parameters. A linear relationship was maintained in the range of 0 to 27.27 μM Sudan I with limit of detection of 0.92 μM. The quenching mechanism was studied and finally attributed to Förster resonance energy transfer (FRET) mechanism. In addition, the probe was effectively implemented for Sudan I detection in commercial chili powder samples with good level of recovery parameters.     

MOF@Au NPs/aptamer fluorescent probe for the selective and sensitive detection of thiamethoxam

The luminescence performance of fluorescent reagents plays a crucial role in fluorescence analysis. Therefore, in this study, a novel bi-ligand Zn-based metal–organic framework, Au nanoparticle (NP) fluorescent material was synthesized using a hydrothermal method with Zn as the metal source. Simultaneously, a DNA aptamer was introduced as a molecular recognition element to develop a Zn-based MOF@Au NPs/DNA aptamer fluorescent probe for the ultra-trace detection of thiamethoxam residues in agricultural products. The probe captured different concentrations of the target molecule, thiamethoxam, through the DNA aptamer, causing a conformational change in the DNA aptamer and bursting the fluorescence of the probe, therefore establishing a fluorometric method for thiamethoxam detection. This method is highly sensitive due to the excellent luminescence properties of the Zn-based MOF@Au NPs, and the DNA aptamer can specifically recognize thiamethoxam, offering high selectivity. The linear range of the method was 2.5–6000 × 10⁻¹¹ mol L⁻¹, with a detection limit of 8.33 × 10⁻¹² mol L⁻¹. This method was applied to the determination of actual samples, such as bananas, and the spiked recovery rate was found to be in the range 84.05–109.07%. Overall, the proposed probe has high sensitivity, high selectivity, and easy operation for the detection of thiamethoxam residues in actual samples.     

Opto-electrochemical planar wave-guide sensor for copper (II) ion

An opto-electrochemical sensor for copper (II) ion was developed. The sensor consists of a planar conductive indium-tin-oxide (ITO) glass support, which was coated by a polymeric copper (II) sensitive membrane. The sensing membrane is made of plasticized polyurethane matrix containing Zincon colorimetric reagent immobilized as ion pair with tetraoctylammonium ion. The instrumentation used commonly for wave-guide sensor development was extended with a potentiostat that made possible the control of the electric potential of the conductive surface. In this way the transport rate of ionic species through the membrane-sample interface could be influenced. In the presence of copper (II) ions the color of the membrane turned from red to blue, which was monitored optically. By applying positive potential to the conductive surface the direction of the ion diffusion at the membrane/sample interface was changed. As a result the sensing layer was regenerated within 2 minutes and was ready for further measurement. The sensor measured Cu(2+) ion in a concentration range of 1-200 microM.     

A versatile and highly sensitive probe for Hg(II), Pb(II) and Cd(II) detection individually and totally in water samples

The detection of heavy metal ions using enzyme-linked immunosorbent assays (ELISA) has been reported by several research groups. However, highly sensitive and selective detection of total heavy metal ions using ELISA is a major technical limitation. Here we describe the development of a versatile and highly sensitive probe combining goat anti-mice IgG, colloidal gold nanoparticles (AuNPs) and horseradish peroxidase (HRP). We demonstrate the utility of this probe using three kinds of heavy metal complete antigens and three monoclonal antibodies (McAbs) in one ELISA system to establish a high-throughput screening protocol. The procedure was successfully applied to analysis of Hg(II), Pb(II) and Cd(II) individually and totally from different water samples. The sensitivities for the detection of Hg(II), Pb(II) and Cd(II) individually and totally are 27.4, 3.9, 15.8 and 18.2 nM, respectively. And all limit of detection (LODs) are lower than 1.2 nM. The recovery results obtained from the developed technique showed a good correlation (R² = 0.983) with those from ICP-MS. The major advantage of the probe is the versatility and high sensibility. The probe could be potentially used, upon demand, as a sensitive and versatile detector for a broad range of applications.     

DNA‐based chemiluminescent nanoprobes for highly sensitive and selective detection of mercury(II) ion

A simple and sensitive DNA‐stablized gold nanoparticle (AuNP)‐based chemiluminescent (CL) probe for detecting mercury ion (Hg²⁺) in aqueous solution has been developed. The CL strategy relies upon the catalytic activity of unmodified AuNPs on the luminol–H₂O₂ CL reaction, and the interaction of unmodified AuNPs with DNA. The unmodified AuNPs can effectively differentiate unstructured and folded DNA. The DNA desorbs from AuNPs in the presence of Hg²⁺, leading to the increase in CL signal. By rationally varying the number of thymine in single‐strand oligonucleotides, the detection range could be tuned. Employing single‐strand oligonucleotides with 14 thymine in the detecting system, a sensitive linear range for Hg²⁺ ions from 5.0 × 10^–10 to 1.0 × 10^–7 mol/L and a detection limit of 2.1 × 10^–10 mol/L are obtained. Changing the number of thymine to 10 and 6, it leads to a narrow detection range but a high sensitivity. Besides, DNA‐based CL nanoprobes exhibit a remarkable selectivity for Hg²⁺ ions over a variety of competing metal ions. Copyright © 2012 John Wiley & Sons, Ltd.     

Carnosine as molecular probe for sensitive detection of Cu(II) ions using localized 1H NMR spectroscopy

Complex formation of carnosine (Csn) with Cu(II) is suspected to be of significant biochemical importance and can be detected by NMR via ion-induced paramagnetic relaxation of Csn signals. Here, we present quantification of the sensitivity achieved with localized (1)H NMR spectroscopy at physiological pH and high ligand-to-metal ratios. While characterizing the highly effective relaxation transfer onto a huge Csn pool due to fast ligand exchange, it is demonstrated that a metal-to-ligand ratio of approximately 100 ppm suffices to reduce Csn signals by approximately 50% in vitro, thus making the dipeptide a sensitive probe for such ions. Variation of the donor accessibility reveals that the paramagnetic effect is transferred onto a approximately 1370-fold donor abundance for a given ion concentration. A method is presented to characterize such effective ligand exchange relaxation transfer. These studies focus on the monomer formation since comparison with (1)H NMR data of human calf muscle demonstrates that the dimer complex is insignificant in vivo. Observed line broadening in living tissue yields an upper limit of ca. 195 ppm for the Csn-related copper concentration in human skeletal muscle.     

Discriminative detection of mercury (II) and hydrazine using a dual‐function fluorescent probe

A dual‐function fluorescent probe (Probe 1 ) was developed for discriminative detection of Hg²⁺ and N₂H₄. Probe 1 could discriminatively detect Hg²⁺ and N₂H₄ through two different reaction sites, with the mechanism for Probe 1 for Hg²⁺ depending on a desulfurization reaction and for N₂H₄ depending on the Schiff‐base reaction. N₂H₄ had minimal effect on Hg²⁺ detection in dimethyl sulfoxide (DMSO)/H₂O solution, but Hg²⁺ could interfere with N₂H₄ detection in DMSO/buffer solution. Different concentrations of Hg²⁺ and N₂H₄ resulted in different blue shades of Probe 1 test strips, and the shade of blue was different with the same concentration of Hg²⁺ or N₂H₄, as observed under ultraviolet light at 365 nm wavelength.     

A highly sensitive,cell-membrane-permeable fluorescent probe for glutathione

Glutathione (GSH) is a primary intracellular antioxidant. Here, we developed a novel, highly sensitive fluorescent probe for GSH, designated DNs-HMRG, whose fluorescence is regulated by two distinct switching mechanisms, intramolecular spirocyclization and photo-induced electron transfer (PeT). DNs-HMRG showed good cell permeability, and a rapid increase in fluorescence intensity was observed when it was applied to living cells. Further, taking advantage of the fact that the intracellular GSH level in tumor tissue is higher than that in normal tissue, we employed this probe for rapid (within a few tens of seconds) in vivo detection of tiny tumor nodules (less than 1 mm in diameter) in tumor-bearing mice. This probe is expected be a powerful tool in various biological applications, especially studies on redox status.     

Determination of tetracycline, chlortetracycline and oxytetracycline by flow injection with inhibitory chemiluminescence detection using copper(II) as a probe ion.

This paper reports an indirect flow-injection (FI) method for the determination of the tetracycline drugs (TCs), tetracycline (TC), chlortetracycline (CTC) and oxytetracycline (OTC), using copper(II) as a probe ion. The method was based on the inhibition caused by these TCs to the copper(II)-catalysed chemiluminescence (CL) reaction between luminol and H(2)O(2). The CL reaction was induced on-line and injection of the sample produced negative peaks as a result of the copper(II) complexation or displacement by the analytes. The height of the peaks was proportional to the drug concentration in the sample. The choice of the catalyst ion, the concentration of luminol, H(2)O(2) and copper(II) are discussed. The linear range was 3.6 x 10(-8)-1.0 x 10(-5), 1.1 x 10(-7)-1.0 x 10(-5) and 1.9 x 10(-7)-1.0 x 10(-5) mol/L for TC, CTC and OTC, respectively. The detection limit was 5.0 x 10(-9) mol/L for TC, 1.0 x 10(-8) mol/L for CTC and 2.0 x 10(-8) mol/L for OTC (3sigma), respectively. The method was applied to the determination of TCs in pharmaceutical preparations and human urine with recoveries in the range 95-105%.     

Ruthenium(II) tris(bipyridyl) ion as a luminescent probe for oxygen uptake

The present work describes an alternative technique for following the rate of oxygen uptake by chemical and enzymatic systems. This method is based on spectrofluorometric monitoring of the well-known quenching effect of molecular oxygen on the emission of the photoexcited [Ru(bpy)3]2+ ion, added to the reaction mixtures. The rate of oxygen consumption determined using the present method agrees with that obtained by conventional polarographic techniques in all of the following systems: ascorbate/Cu2+, glucose/glucose oxidase (EC 1.1.3.4), and propanal/horseradish peroxidase (EC 1.11.1.7); in the last case, agreement was observed both in the presence and absence of serum albumin and of chloroplasts. Spectrofluorometric data for amphotericin autoxidation in dimethyl sulfoxide are in accord with the rate of decay of the ESR signal of a spin trap added to the reaction mixture. The advantages and limitations of the present spectrofluorometric technique relative to conventional polarographic monitoring of dissolved oxygen are discussed.     

Label-free optical bifunctional oligonucleotide probe for homogeneous amplification detection of disease markers

Oligonucleotide-based detection schemes that avoid chemical modification possess significant advantages, including simplified design, intrinsic affinity for targets, low cost and ease to extend applications. In this contribution, we developed a label-free self-locked bifunctional oligonucleotide probe (signaling probe) for the detection of different disease markers in parallel. Two signal enhancement techniques based on isothermal circular strand-displacement polymerization reaction, cyclical nucleic acid strand-displacement polymerization (CNDP) and cyclical common (nonnucleic acid) target-displacement polymerization (CCDP), were employed to implement the amplification assay for p53 gene and PDGF-BB, respectively. The attractive assay properties confirmed the effectiveness of isothermal polymerization in common biosensing systems without evolving any chemical modification: PDGF could be detected down to 0.87ng/mL, and a dynamic response range of 8-5000ng/mL was achieved; The capability to screen the p53 gene was also considerably improved, including the detection limit, sensitivity, dynamic range and so on. Moreover, because no any chemical modification of the signaling probe was acquired and different targets were separately detected in homogeneous solution. This interrogating platform exhibits the design flexibility, convenience, simplicity and cost-effectiveness. The success achieved here is expected to serve as a significant step toward the development of robust label-free oligonucleotide probes in biomarker profiling and disease diagnostics.     

Nitrogen-doped carbon dots as a fluorescent probe for the highly sensitive detection of bilirubin and cell imaging

Nitrogen-doped carbon dots (NCDs) with bright blue fluorescence were constructed by a hydrothermal method using sucrose and l- proline as raw materials. The NCDs were characterized by transmitted electron microscopy, X-ray diffraction, Fourier-transform infrared spectrometry, X-ray photoelectron spectroscopy, and ultraviolet-visible absorption and fluorescence spectroscopy to investigate the morphology, elemental composition, and optical properties. The NCDs had good water solubility, high dispersibility with an average diameter of only 1.7 nm, and satisfactory optical properties with a fluorescence quantum yield of 23.4%. The NCDs were employed for the detection of bilirubin. A good linear response of the NCDs in the range 0.35–9.78 μM was obtained for bilirubin with a detection limit of 33 nM. The NCDs were also applied to the analysis of real samples, serum and urine, with a recovery of 95.34% to 104.66%. The low cytotoxicity and good biocompatibility of the NCDs were indicated by an MTT assay and cell imaging of HeLa cells. Compared with other detection systems, using NCDs for bilirubin detection was a facile and efficient method with good selectivity and sensitivity.     

Terbium (III) coordination polymer–copper (II) compound as fluorescent probe for time‐resolved fluorescence ‘turn‐on’ detection of hydrogen sulfide

With recognition of the biological importance of hydrogen sulfide (H₂S), we present a simple and effective fluorescent probe for H₂S using a Tb³⁺ coordination polymer–Cu²⁺ compound (DPA/Tb/G–Cu²⁺). Dipicolinic acid (DPA) and guanosine (G) can coordinate with Tb³⁺ to form a macromolecular coordination polymer (DPA/Tb/G). DPA/Tb/G specifically binds to Cu²⁺ in the presence of coexisting cations, and obvious fluorescence quenching is observed. The quenched fluorescence can be exclusively recovered upon the addition of sulfide, which is measured in the mode of time‐resolved fluorescence. The fluorescence intensities of the DPA/Tb/G–Cu²⁺ compound enhance linearly with increasing sulfide concentrations from 1 to 30 μM. The detection limit for sulfide in aqueous solution is estimated to be 0.3 μM (at 3σ). The DPA/Tb/G–Cu²⁺ compound was successfully applied to sense H₂S in human serum samples and exhibited a satisfactory result. It displays some desirable properties, such as fast detection procedure, high selectivity and excellent sensitivity. This method is very promising to be utilized for practical detection of H₂S in biological and environmental samples.     

Study of Ni(II) ion-DNA interactions with methylene blue as fluorescent probe

Spectroscopic studies of the interactions of Ni(II) ion with herring sperm DNA have been performed in this contribution using methylene blue (MB) as the fluorescent probe molecule. MB binds to double helical DNA via the intercalative mode, and its fluorescence is efficiently quenched by the DNA nucleobases. The fluorescence intensity of the probe molecule increases distinctly when Ni(II) ion is added to the MB-DNA solution system. These results indicate that the Ni(II) ions may be complexed with the DNA helix and probably bind at N-7 of adenine or guanine; consequently, some intercalated MB molecules are released due to the binding of Ni(II) ions to N-7 nitrogen of the purines. The Ni(II) ion-DNA interactions are further investigated by performing the photobleaching experiments of the MB-DNA complex in the presence and absence of Ni(II) ion. The bi-exponential decay functions of the fluorescence intensity have been observed in both cases and the shortening of the slow decay component when added Ni(II) ion also agrees with the release of MB from the DNA duplex.     

A rhodamine‐based fluorescent probe for Cu(II) determination in aqueous solution

An ‘off–on’ rhodamine‐based fluorescence probe for the selective detection of Cu(II) has been designed, exploiting the guest‐induced structure transform mechanism. This system shows a sharp Cu(II)‐selective fluorescence enhancement response in an aqueous system under physiological pH, and possesses high selectivity against a background of environmentally and biologically relevant metal ions. Under optimum conditions, the fluorescence intensity enhancement of this system is linearly proportional to the Cu(II) concentration from 50 nM to 6.0 μM with a detection limit of 29 nM. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of a fluorescent probe hydrolysis-insulated isothermal PCR for rapid and sensitive on-site detection of African swine fever virus

Highlights
1. A probe-based insulated isothermal PCR (iiPCR) assay was developed for rapid and onsite detection of ASFV.
2. The developed iiPCR showed similar sensitivity and specificity with OIE recommended real-time PCR.
3. Blood samples could be directly applied as PCR template in iiPCR without DNA extraction.