A ratiometric fluorescent chemosensor for the detection of cysteine in aqueous solution at neutral pH

A ratiometric fluorescent chemosensor 2‐(2′‐hydroxy‐3′‐formyl‐5′‐methoxyphenyl)benzothiazole ( 1 ) was developed for the detection of cysteine (Cys). In aqueous solution at neutral pH, 1 exhibited a ratiometric fluorescent response to Cys with a remarkable red‐to‐green shift in the emission wavelength. This fluorescence change was attributed to the cyclization reaction between the formyl group in 1 and the amino and sulfhydryl group in Cys in a stoichiometry of 1: 1 according to the proposed mechanism. At neutral pH, 1 displayed a significant fluorescence ratio signal enhancement with the addition of Cys. Furthermore, 1 showed good selectivity toward Cys. The detection limit and linear range were 5.6 and 0–100 μmol/L, respectively, which demonstrated that 1 could recognize relatively low concentrations of Cys and is a good candidate for applications in detecting Cys.     

Reaction‐based probe for hydrogen sulfite: dual‐channel and good ratiometric response

We designed and synthesized a new series of intramolecular charge transfer (ICT) molecules (compounds T1, T2 and T3) by attaching various electron‐donating thiophene groups to the triphenylamine backbone with aldehyde group as the electron acceptor. Based on the nucleophilic addition reaction between hydrogen sulfite and aldehyde, all compounds could act as ratiometric optical probe for hydrogen sulfite and displayed efficient chromogenic and fluorogenic signaling. Upon the addition of hydrogen sulfite anions, probe T3 displayed apparent fluorescent color changes from yellowish‐green to blue, with a large emission wavelength shift (Δλ = 120 nm). T3 responded to hydrogen sulfite with high sensitivity and the detection limit was determined to be as low as 0.9 μM. At the same time, apparent changes in UV–vis spectra could also be observed. By virtue of the special nucleophilic addition reaction with aldehyde, T3 displayed high selectivity over other anions. Copyright © 2016 John Wiley & Sons, Ltd.     

A ratiometric fluorescent probe for alkaline phosphatase via regulation of excited‐state intramolecular proton transfer

A ratiometric fluorescent probe 2‐(benzimidazol‐2‐yl)phenyl phosphoric acid (1) for alkaline phosphatase (ALP) is designed and synthesized. The method employs the modulation of the excited‐state intramolecular proton transfer (ESIPT) process of 2‐(2'‐hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. Upon phosphorylated with POCl₃, HPBI shows only an emission peak at 363 nm due to the blockage of ESIPT. However, once selective enzymatic hydrolysis with alkaline phosphatase (ALP) in Tris–HCl buffer occurs, the probe 1 is returned to HPBI and the ESIPT process is switched on, which results in a decrease in the emission band at 363 nm and an increase in a new fluorescence peak around 430 nm. The fluorescence intensity ratio at 430 and 360 nm (I₄₃₀/I₃₆₀) increases linearly with the activity of ALP up to 0.050 U/mL and the detection limit is 0.0013 U/mL. The proposed probe shows excellent specificity toward ALP. Copyright © 2015 John Wiley & Sons, Ltd.     

A ratiometric fluorescent sensor for Zn2+ based on internal charge transfer (ICT)

A ratiometric fluorescent chemosensor 5 was designed and synthesized based on internal charge transfer (ICT). The indicator absorbs and emits light in the visible wavelength range. In acetonitrile, blue shifts in fluorescent emission upon zinc binding are due to the formation of a 1:2 metal/ligand complex, which induced a fluorescent emission at 616 nm at the expense of the fluorescent emission at 672 nm.     

Rapid and visual detection for hypochlorite of an AIE enhanced fluorescence probe

In this paper, a new ‘turn‐on' fluorescence probe for the rapid, sensitive, and visual detection of hypochlorite is reported. The push–pull type trianiline–tricyanofuran‐based fluorescent probe was prepared using a condensation reaction between tricyanofuran and the thiophene–trianiline derivative that had high quantum yields and showed aggregation‐induced emission enhanced properties. Upon exposure to hypochlorite, prominent fluorescence enhancement of the probe was observed via the release of the fluorophore from the probe. The probe showed a ratiometric absorption change at 315 nm and 575 nm. Importantly, the probe showed an excellent detection limit for hypochlorite at 1.2 × 10^?7 M in solution and it was successfully applied for monitoring hypochlorite in waste water by test strip. This work reports a new fluorescence analytical sensing method for hypochlorite that has potential practical value in environmental monitoring and biological discrimination.     

Characterization of a highly Al3+‐selective fluorescence probe based on naphthalimide‐Schiff base and its application to practical water samples

A new fluorescent Al³⁺‐probe, N‐allyl‐4‐[3,3′‐((2‐aminoethyl)azanediyl)‐bis(N´‐(2‐hydroxybenzylidene)propanehy‐drazide)]‐1,8‐naphthalimide ( L ), was designed and synthesized based on 1,8‐naphthalimide. The probe L contains 1,8‐naphthalimide moiety as the fluorophore and a Schiff base as the recognition group. The structure of L was determined by single crystal X‐ray. L emission at 526 nm increased on addition of Al³⁺ under excitation wavelength at 350 nm. L exhibited high selectivity and sensitivity fluorescence emission towards to Al³⁺ in ethanol/Tris–HCl buffer solution (1:1, v/v, pH = 7.2) as compared with other tested metal ions. A good linearity with a correlation coefficient (R²) of 0.99 was observed in the concentration range 2–10 μM. The binding constant and the detection limit of L for Al³⁺ were calculated to 2.6 × 10⁴ M^?1 and 0.34 μM, respectively. The results of experiments that including Job plot, ultraviolet–visible (UV–Vis) light titration, fluorescence titration, ESI‐MS and ¹H NMR titration, indicated a 1:1 stoichiometric complex between L and Al³⁺. L was highly effective in monitoring Al³⁺ in real‐life Yellow River and tap water samples.     

A ‘turn‐on’ fluorescent chemosensor for quantification of serum albumin in aqueous solution at neutral pH

A fluorescent chemosensor 1 (4‐diethylamino‐2′‐hydroxychalcone) for detecting serum albumin with long‐wavelength emission, good selectivity and facile synthesis was reported. Upon the addition of bovine serum albumin (BSA) to an aqueous solution of 1 at neutral pH, a ‘turn‐on’ fluorescence response was observed at 596 nm based on a hydrophobic binding mode between 1 and BSA. A linear range of 0.10–1.00 mg/mL and a detection limit of 9.1 µg/mL for BSA were obtained, respectively. Moreover, 1 was successfully applied to detect BSA in real bovine serum samples with satisfied recovery and accuracy, which suggested that 1 could serve as a valid and effective fluorescent chemosensor for quantification of BSA. Copyright © 2015 John Wiley & Sons, Ltd.     

Detection of mercury(II) by DNA templated gold nanoclusters based on forming thymidine–Hg2+–thymidine duplexes

We have successfully synthesized gold nanoclusters (AuNCs) templated with DNA (5′‐CCCCCCCCCCCCTTTTTT‐3′), and subsequently employed the fluorescent DNA‐AuNCs as a novel probe for sensitive detections of mercury ions (Hg²⁺). Basically, the procedure is due to the formation of thymidine–Hg²⁺–thymidine duplexes between DNA‐AuNCs and Hg²⁺, thus leading to aggregations of DNA‐AuNCs described here occurring, and facilitating their fluorescence decrease. Significantly, this decrease of fluorescent signals permitted sensitive detection of Hg²⁺ in a linear range of 0.1–100 µmol L^?1, with a detection limit of 0.083 µmol L^?1 at a signal‐to‐noise ratio of 3. Additionally, the practicality of this probe for assaying Hg²⁺ in human urine and lake water samples was further validated, and showed various advantages including simplicity, selectivity, sensitivity and low cost, demonstrating its potential to broaden ways for assaying Hg²⁺ in real samples. Copyright © 2014 John Wiley & Sons, Ltd.     

A highly efficient and selective turn‐on fluorescent sensor for Hg2+, Ag+ and Ag nanoparticles based on a coumarin dithioate derivative

Based on chelation‐enhanced fluorescence, a new fluorescent coumarin derivative probe 3(1‐(7‐hydroxy‐4‐methylcoumarin)ethylidene)hydrazinecarbodithioate for Hg²⁺, Ag⁺ and Ag nanoparticles is reported. Fluorescent probe acts as a rapid and highly selective “off–on” fluorescent probe and fluorescence enhancement by factors 5 to12 times was observed upon selective complexation with Hg²⁺, Ag⁺ and Ag nanoparticles. The molar ratio plots indicated the formation of 1:1 complexes between Hg²⁺ and Ag⁺ with the probe. The linear response range covers a concentration range 0.1 × 10^–5–1.9 × 10^–5 mol/L, 0.1 × 10^–5–2.3 × 10^–5 mol/L and 0.146 × 10^–12–2.63 × 10^–12 mol/L for Hg²⁺, Ag⁺ and Ag nanoparticles, respectively. The interference effect of some anions and cations was also tested. Copyright © 2013 John Wiley & Sons, Ltd.     

A fluorescent probe for biothiols based on the conjugate addition of thiols to α,β‐unsaturated ester

A sensitive fluorogenic probe 1 for biothiols was developed based on the Michael addition reaction. The probe 1 was readily synthesized via the reaction of 2‐(4′‐hydroxyphenyl) benzimidazole (HPBI) with acryloyl chloride and shows weak fluorescence emission. Upon mixing with biothiols, the fluorescence of 1 is significantly enhanced due to the conjugate addition of thiols to the α,β‐unsaturated carbonyl moiety, thus eliminating the photoinduced electron transfer (PET) quenching of the fluorophore by the intramolecular carbon–carbon double bond. Cysteine (Cys) was selected as the representative thiol in the spectral experiment. A good linear relationship was obtained from 1.0 to 30.0 µmol L^?1 for Cys and the detection limit was 0.17 µmol L^?1. Furthermore, probe 1 was highly selective for biothiols without the interference of some biologically relevant analytes and has been applied to detecting biothiols in human urine samples. Copyright © 2010 John Wiley & Sons, Ltd.     

Ratiometric fluorescent detection of Cu2+ based on dual‐emission ZIF‐8@rhodamine‐B nanocomposites

Zeolitic imidazolate framework‐8 (ZIF‐8) loading rhodamine‐B (ZIF‐8@rhodamine‐B) nanocomposites was proposed and used as ratiometric fluorescent sensor to detect copper(II) ion (Cu²⁺). Scanning electron microscopy, Fourier transform infrared spectroscopy, X‐ray powder diffraction, nitrogen adsorption/desorption isotherms and fluorescence emission spectroscopy were employed to characterize the ZIF‐8@rhodamine‐B nanocomposites. The results showed the rhodamine‐B was successfully assembled on ZIF‐8 based on the π‐π interaction and the hydrogen bond between the nitrogen atom of ZIF‐8 and –COOH of rhodamine‐B. The as‐obtained ZIF‐8@rhodamine‐B nanocomposites were octahedron with size about 150–200 nm, had good water dispersion, and exhibited the characteristic fluorescence emission of ZIF‐8 at 335 nm and rhodamine‐B at 575 nm. The Cu²⁺ could quench fluorescence of ZIF‐8 rather than rhodamine‐B. The ZIF‐8 not only acted as the template to assemble rhodamine‐B, but also was employed as the signal fluorescence together with the fluorescence of rhodamine‐B as the reference to construct a novel ratiometric fluorescent sensor to detect Cu²⁺. The resulted ZIF‐8@rhodamine‐B nanocomposite fluorescence probe showed good linear range (68.4 nM to 125 μM) with a low detection limit (22.8 nM) for Cu²⁺ combined with good sensitivity and selectivity. The work also provides a better way to design ratiometric fluorescent sensors from ZIF‐8 and other fluorescent molecules.     

Electrochemical detection of honokiol and magnolol in traditional Chinese medicines using acetylene black nanoparticle-modified electrode

Introduction – Honokiol and magnolol are the active components of Magnolia officinalis, which is a widely used traditional Chinese medicine. Their simultaneous analysis is, therefore, important for the quality control of the product. Objective – To establish a simple, sensitive and rapid electrochemical method for the simultaneous detection of honokiol and magnolol based on the remarkable enhancement effect of acetylene black nanoparticle (AB). Methodology – The AB‐modified electrode was prepared via solvent evaporation. The electrochemical response of honokiol and magnolol was investigated using cyclic voltammetry. The simultaneous detection was performed with differential pulse voltammetry. The method was validated in terms of linearity, sensitivity, precision and accuracy. Results – The linear range for honokiol is 0.5–300 µg/L, and the limit of detection (LOD) is 0.25 µg/L (9.4 × 10^?10 mol/L). For magnolol, the linear range is 10–250 µg/L, and the LOD is 5 µg/L (1.88 × 10^?8 mol/L). Conclusion – The new method was successfully used to determine honokiol and magnolol in a traditional Chinese medicine called Ageratum liquid. Copyright © 2010 John Wiley & Sons, Ltd.     

Microflow‐injection chemiluminescence of luminol and hypochlorite enhanced by phloxine B

We report for the first time that the sensitivity of the luminol–hypochlorite chemiluminescence (CL) reaction was enhanced approximately 10 times by the addition of phloxine B. The maximum wavelength of CL emission shifted from 431 to 595 nm in the absence and presence, respectively, of phloxine B, suggesting that an efficient chemiluminescence resonance energy transfer occurred between a luminol donor and a phloxine B acceptor in the luminol–hypochlorite–phloxine B system. Based on this observation, a simple, rapid and sensitive microflow injection CL method, using a microchip with spiral channel configurations, was developed for the determination of hypochlorite. Under optimized conditions, a linear calibration curve (R² = 0.9944) over the range 0.1–10.0 µmol/L was obtained, with a detection limit of 0.025 µmol/L (S:N = 3). The relative standard deviation (RSD) was found to be 4.2% (n = 10) for 2.5 µmol/L hypochlorite. The sample consumption was only 2 μL, with a sample throughput of 90/h. The method has been used for determining trace amounts of hypochlorite in water samples with satisfactory results. Copyright © 2012 John Wiley & Sons, Ltd.     

A new silver nanochain SERS analytical platform to detect trace hexametaphosphate with a rhodamine S molecular probe

Using AgNO₃ as the precursor, stable silver nanochain (AgNC) sols, orange‐red in color, were prepared using hydrazine hydrate. A strong surface plasmon resonance Rayleigh scattering (RRS) peak occurred at 420 nm plus two surface plasmon resonance (SPR) absorption peaks at 410 nm and 510 nm. Rhodamine S (RhS) cationic dye was absorbed on the as‐prepared AgNC substrate to obtain a RhS–AgNC surface‐enhanced Raman scattering (SERS) nanoprobe that exhibited a strong SERS peak at 1506 cm^–1 and a strong RRS peak at 375 nm. Upon addition of the analyte sodium hexametaphosphate (HP), it reacted with RhS, which resulted in a decrease in the SERS and RRS peaks that was studied in detail. The decreased SERS and RRS intensities correlated linearly with HP concentration in the range of 0.0125–0.3 µmol/L and 0.05–1.0 µmol/L, with a detection limit of 6 nmol/L and 20 nmol/L HP respectively. Due to advantages of high sensitivity, good selectivity and simple operation, the RhS molecular probes were used to determine HP concentration in real samples. Copyright © 2015 John Wiley & Sons, Ltd.     

Fluorimetric determination of proteins using 4‐chloro‐(2′‐hydroxylophenylazo)rhodanine–Ti(IV) complex as a spectral probe

A novel method for the determination of proteins was developed, based on the enhancement of fluorescence with 4‐chloro‐(2′‐hydroxylophenylazo)rhodanine–Ti(IV) [ClHARP–Ti(IV)] complex as a fluorescence probe. The excitation and emission wavelengths of the system were 335 nm and 376 nm, respectively. The presence of bis(2‐ethylhexyl)sulphosuccinate sodium salt (AOT) microemulsion greatly increased the sensitivity of the system. Under optimal conditions, four kinds of proteins, including bovine serum albumin (BSA), human serum albumin (HSA), egg albumin (Ova), and γ‐globin (γ‐G) were studied. The detection limits were 0.182 µg/mL for BSA, 0.0788 µg/mL for HSA, 0.216 µg/mL for Ova and 0.484 µg/mL for γ‐G. The linear ranges of the calibration were 0–12.0, 0–10.0, 0–18.0 and 0–18.0 µg/mL, respectively. The method possessed high sensitivity, good selectivity and was applied to the analysis of protein in milk powder and cornmeal with satisfactory results. Copyright © 2008 John Wiley & Sons, Ltd.     

Development of a new procedure for the determination of captopril in pharmaceutical formulations employing chemiluminescence and a multicommuted flow analysis approach

This paper describes a new technique for the determination of captopril in pharmaceutical formulations, implemented by employing multicommuted flow analysis. The analytical procedure was based on the reaction between hypochlorite and captopril. The remaining hypochlorite oxidized luminol that generated electromagnetic radiation detected using a homemade luminometer. To the best of our knowledge, this is the first time that this reaction has been exploited for the determination of captopril in pharmaceutical products, offering a clean analytical procedure with minimal reagent usage. The effectiveness of the proposed procedure was confirmed by analyzing a set of pharmaceutical formulations. Application of the paired t‐test showed that there was no significant difference between the data sets at a 95% confidence level. The useful features of the new analytical procedure included a linear response for captopril concentrations in the range 20.0–150.0 µmol/L (r = 0.997), a limit of detection (3σ) of 2.0 µmol/L, a sample throughput of 164 determinations per hour, reagent consumption of 9 µg luminol and 42 µg hypochlorite per determination and generation of 0.63 mL of waste. A relative standard deviation of 1% (n = 6) for a standard solution containing 80 µmol/L captopril was also obtained. Copyright © 2015 John Wiley & Sons, Ltd.     

Utilizing a nano‐sorbent for the selective solid‐phase extraction of vanillic acid prior to its determination by photoluminescence spectroscopy

Vanillic acid (VA) is a phenolic acid, and acts as a natural antioxidant in fruits, vegetables and plants. The extraction and determination of trace levels of VA in plants is important, because stimulation of protein synthesis and activation of antioxidant enzymes occur in the presence of phenolic acids at trace levels. In this research, a photoluminescence spectroscopic method was developed for the quantification of VA in plant samples after separation and pre‐concentration. Selective extraction of VA from aqueous solution was performed using a solid‐phase extraction column packed with nickel–aluminum layered double hydroxide as a nano‐sorbent. After elution of extracted analyte from the column using 3 mL of a 3 mol/L NaOH solution, its concentration was determined spectrofluorometrically at λ_em = 357 nm with excitation at λ_ex = 280 nm. The spectrofluorometry method gave a linear response for VA within the range 20.0–900.0 µg/L, with a correlation coefficient of 0.9982. The limit of detection and sorption capacity were 7.6 µg/L and 66.2 mg/g, respectively. The method was validated by comparing the obtained results with gas chromatographic data. This method was used to determine VA in Chenopodium album and Prangos asperula plants. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis of a novel fluorescent probe based on 7‐nitrobenzo‐2‐oxa‐1,3‐diazole skeleton for the rapid determination of vitamin B12 in pharmaceuticals

A new fluorescent probe, 4‐N,N‐di(2‐hydroxyethyl)imino‐7‐nitrobenzo‐2‐oxa‐1,3‐diazole (HINBD) was synthesized in a single step with reasonably good yield. The water‐soluble HINBD emits strongly in the visible region (λ_ex = 479 nm, λ_em = 545 nm) and is stable over a wide range of pH values. It was found that vitamin B₁₂ (VB₁₂) had the ability to quench the fluorescence of HINBD, and the quenched fluorescence intensity was proportional to the concentration of VB₁₂. A method for VB₁₂ determination based on the quenching fluorescence of HINBD was thus established. Interference effects of various substances, including sugars, vitamins, amino acids, inorganic cations and some organic substances have been studied. Under optimal conditions, the linear range is 0.0–2.4 × 10^–5 mol/L. The determination limit is 8.3 × 10^–8 mol/L. The method was applied to measure VB₁₂ in pharmaceutical preparations with satisfactory results. Copyright © 2013 John Wiley & Sons, Ltd.     

Core–shell structured CdTe/CdS@SiO2@CdTe@SiO2 composite fluorescent spheres: Synthesis and application for Cd2+ detection

Core–shell structured quantum dot (QD)–silica fluorescent nanoparticles have attracted a great deal of attention due to the excellent optical properties of QDs and the stability of silica. In this study, core–shell structured CdTe/CdS@SiO₂@CdTe@SiO₂ fluorescent nanospheres were synthesized based on the Stöber method using multistep silica encapsulation. The second silica layer on the CdTe QDs maintained the optical stability of nanospheres and decreased adverse influences on the probe during subsequent processing. Red‐emissive CdTe/CdS QDs (630 nm) were used as a built‐in reference signal and green‐emissive CdTe QDs (550 nm) were used as a responding probe. The fluorescence of CdTe QDs was greatly quenched by added S^2?, owing to a S^2?‐induced change in the CdTe QDs surface state in the shell. Upon addition of Cd²⁺ to the S^2?‐quenched CdTe/CdS@SiO₂@CdTe@SiO₂ system, the responding signal at 550 nm was dramatically restored, whereas the emission at 630 nm remained almost unchanged; this response could be used as a ratiometric ‘off–on’ fluorescent probe for the detection of Cd²⁺. The sensing mechanism was suggested to be: the newly formed CdS‐like cluster with a higher band gap facilitated exciton/hole recombination and effectively enhanced the fluorescence of the CdTe QDs. The proposed probe shows a highly sensitive and selective response to Cd²⁺ and has potential application in the detection of Cd²⁺ in environmental or biological samples.     

A ratiometric fluorescence sensor for Fe3+ based on FRET and PET processes

A rhodamine/coumarin‐based ratiometric fluorescent Fe³⁺ sensor has been designed and synthesized. The sensor exhibits a good response to Fe³⁺ ions with high sensitivity, selectivity and a large shift in the emission spectra (>100 nm), which shows Fe³⁺‐induced FRET OFF–ON and PET ON–OFF behavior. Copyright © 2015 John Wiley & Sons, Ltd.