Fused bis[2-(2′-hydroxyphenyl)benzoxazole] derivatives for improved fluoride sensing: The impact of regiochemistry and competitive hydrogen bonding

Development of fluorescent chemical sensors for fluoride is important due to increased use of fluoride in environment. A fused bis[2-(2′-hydroxyphenyl)benzoxazole] 5, which is capable of giving ESIPT emission, is found to be a useful fluorescent sensor for fluoride detection. Upon binding to fluoride, bis(HBO) 5 shows a large spectral shift in both fluorescence (from ～490 nm to ～440 nm) and absorption (from 353 nm to 392 nm). In comparison with the isomeric 4, bis(HBO) 5 dramatically improves the sensitivity in fluoride binding (by an order of magnitude), revealing a large impact of regiochemistry on the sensor performance. ¹H NMR has been used to study the fluoride binding, and to correlate the intramolecular hydrogen bonding with the fluoride response. Sensitivity of 5 towards fluoride is as low as 10^?5 M. Bis(HBO) 5 also showed excellent selectivity towards fluoride while being silent to other anions (Cl^?, Br^?, HS^? and PO₄^3?), thus making 5 a potentially useful probe.     

A sensitive tetraphenylethene-based fluorescent probe for Zn2+ ion involving ESIPT and CHEF processes

A new tetraphenylethene-based fluorescent probe 2-(quinolin-8-yliminomethyl)-4-triphenylvinyl-phenol (HL) for detecting Zn²⁺ ion through the excited state intramolecular proton transfer (ESIPT) and chelation enhanced fluorescence (CHEF) processes has been designed and synthesized. The results show that HL emits relatively strong blue fluorescence at 460 nm without Zn²⁺ ion, however, probe HL displays highly pink fluorescent emission at 600 nm when adding Zn²⁺ ion. The fluorescent emission of HL appears an extremely large Stokes shift, which effectively reduces the interference of background signal. The limit of detection of HL for Zn²⁺ ion can reach to 9.0 × 10^–8 M.     

A fluorescent sensor for Al3+ based on a photochromic diarylethene with a hydrazinobenzothiazole Schiff base unit

A new fluorescent turn-on chemosensor for Al³⁺ based on a diarylethene unit was designed and synthesized. Photochromism, fluorescence switch, and metal ion recognition behaviors of this diarylethene derivative were investigated by absorption and fluorescence emission spectra. It shows an outstanding fluorometric sensing ability toward Al³⁺ ion, and the detection limit was measured to be 9.3 × 10^?8 mol L^?1 via fluorescence methods. Based on these interesting properties, a combinational logic circuit was constructed successfully.     

Red-emitting salicylaldehyde Schiff base with AIE behaviour and large Stokes shift

Three salicylaldehyde Schiff base (SSB) with AIE behavior were designed and facilely synthesized through a condensation reaction. In solid and aggregation states, these SSB dyes exhibited yellow to red emission with large Stokes shift. One of SSB dyes could specifically stain lipid droplets in living cells.     

Toward a highly sensitive and selective indole-rhodamine-based light-up probe for Hg2+ and its application in living cells

We herein designed and synthesized a light-up fluorescent probe L1 for Hg²⁺ species, which is based on indole derivative and Rhodamine fluorophore. The new probe can show a linear response to Hg²⁺ with high sensitivity and selectivity. As the Hg²⁺ concentration changed from 0 to 450 μM, the fluorescence intensity of L1 at 575 nm changed from 50 to 6181 (～120-fold). The detection limit of the probe was 5.0 × 10^?8 M. Besides, we have successfully applied L1 to monitor Hg²⁺ species in living MCF-7 cells by way of fluorescence imaging.     

Ovalbumin-stabilized gold nanoclusters with ascorbic acid as reducing agent for detection of serum copper

Ovalbumin-stabilized gold nanoclusters(OVA@AuNCs) were prepared with ascorbic acid as a reducing agent. This strategy could realize the synthesis of water-soluble OVA@AuNCs within 20 min. The asprepared fluorescent probe showed a red fluorescence emission at 630 nm. Moreover, the properties of the OVA@AuNCs were characterized by transmission electron microscope, dynamic light scattering,ultraviolet-visible spectroscopy, fluorescent spectroscopy. Based on the surface electron density decrease-induced fluorescence quenching mechanism, the OVA@AuNCs provided high sensitivity and selectivity for sensing copper ions. A good linear relationship was obtained between the fluorescence intensity of OVA@AuNCs and the concentration of copper ions in the range of 5.0-100.0 μmol/L(R~2=0.999) with a detection limit of 640 nmol/L Furthermore, the rat serum copper contents were determined by using the OVA@AuNCs based assay, indicating great potential of fluorescent probes for application in biological and clinical analysis.     

A borondipyrrolemethene-based turn-on fluorescent probe for silver ion with high sensitivity and selectivity and its application in water samples and living cells

A borondipyrrolemethene-based compound (1) is synthesized and used as a “turn-on” fluorescent probe for silver ions (Ag⁺). The probe displays highly sensitive fluorescence response toward Ag⁺ with a 40-fold fluorescence enhancement when 60 μM of Ag⁺ is added. The fluorescence intensity of the probe is linearly dependent on Ag⁺ concentration ranging from 0.05 to 60 μM. And the detection limit (LOD) can reach 0.02 μM, which complies with the standard of World Health Organization (WHO) for drinking water (0.9 μM). Moreover, the probe shows remarkable selectivity for Ag⁺ over other metal ions. Furthermore, the response behavior of 1 toward Ag⁺ is pH independent in the neutral range from 6.0 to 8.0. The response of 1 toward Ag⁺ is fast (response time is less than 2 min) and reversible chemically. What’s more, the sensing mechanism of probe 1 toward Ag⁺ is verified by mass spectra (MS) and density functional theory (DFT) calculations. In particular, the probe is applied for detection of Ag⁺ in water samples and living cells successfully.     

Highly sensitive fluorescent sensor targeting CuCl2 based on thiophene attached anthracene compound (TA)

A novel thiophene attached anthracene (TA) based fluorescent compound was designed and synthesized. The TA showed a high quantum yield (Qy = 0.34) in regard to fluorescence. We applied this TA compound to detect specific metal compound and found that it could identify CuCl₂ from other metals through dramatic fluorescence change at λ_max = 460 nm. It showed strong quenching fluorescence property with CuCl₂ while with other metal compounds it exhibited strong blue fluorescence emission. UV/Vis absorption spectroscopy clearly demonstrated that the quenching property of TA at λ_max = 460 nm was due to overlapping of the fluorescence peak of TA at λ_max = 460 nm and the absorption band of CuCl₂ (from 190 nm to 525 nm). Binding constant (K′), which was 0.0895 mM^?2, indicated a complexation ratio between TA and CuCl₂ as 1:2 and this interaction induced quenching property.     

Imidazo[1,5-α]pyridine-derived fluorescent turn-on probe for cellular thiols imaging with a large Stokes shift

We presented the design, synthesis and preliminary evaluation of the 2,4-dinitrobenzenesulfonate (DNBS) of imidazo[1,5-α]pyridine derivative, NIPY-DNBS, as a turn-on fluorescent probe for the detection of thiols in aqueous solution. The reaction mechanism was confirmed by means of fluorescence, absorption and HRMS. The large Stokes shift (201 nm), high sensitivity (the detection limit for Cys was calculated to be as low as 0.17 μM) and fast response (10 min) of NIPY-DNBS make it a practical and reliable method for fluorescence imaging. Furthermore, application of NIPY-DNBS for the selective detection of intracellular thiols has been successfully demonstrated in living A549 cells.     

An ESIPT-based fluorescent probe for sensitive and selective detection of Cys/Hcy over GSH with a red emission and a large Stokes shift

An ESIPT-based fluorescent probe (Probe 1) using acrylate as recognition group for the selective and sensitive detection of cysteine/homocysteine (Cys/Hcy) has been developed. In the presence of Cys/Hcy, this probe was transformed into 1,3-bis(bispyridin-2ylimino)isoindolin-4-ol (dye 4) which displayed red fluorescence with a large Stokes shift (217 nm) when excited. The detection limits are as low as 5.4 nM and 7.0 nM for Cys and Hcy respectively (based on S/N = 3). Importantly, this probe has been successfully demonstrated for the detection of intracellular Cys/Hcy in living cells.     

A simple and highly selective fluorescent sensor for palladium based on benzofuran-2-boronic acid

Benzofuran-2-boronic acid could be used as a fluorescent sensor for the detection of Pd²⁺ because it was rapidly converted to highly fluorescent derivative after mixing with Pd²⁺ under basic condition at room temperature. We found that dimerization of benzofuran was occurred to form fluorescent derivative by the catalytic activity of palladium. The fluorescence intensity at 360 nm increased with increasing the concentration of Pd²⁺. The excellent selectivity for Pd²⁺ was demonstrated among other metal ions. Based on this findings, we successfully applied benzofuran-2-boronic acid to develop a microplate-based assay for high-throughput measurement of Pd²⁺. The detection limit (blank + 3SD) for Pd²⁺ of the proposed assay was 9.8 nM.     

A rapid method for the detection of humic acid based on the poly(thymine)-templated copper nanoparticles

A label-free fluorescent method for sensitive detection of humic acid based on poly(thymine)-templated copper nanoparticles is reported.     

Fluorescence “turn-on” chemosensor for the selective detection of zinc ion based on Schiff-base derivative

N,N′-phenylenebis(salicylideaminato) (L) has been used to detect trace amounts of zinc ion in acetonitrile–water solution by fluorescence spectroscopy. The fluorescent probe undergoes fluorescent emission intensity enhancement upon binding to zinc ions in MeCN/H₂O (1:1, v/v) solution. The fluorescence enhancement of L is attributed to the 1:1 complex formation between L and Zn(II), which has been utilized as the basis for selective detection of Zn(II). The linear response range for Zn(II) covers a concentration range of 1.6 × 10^?7 to 1.0 × 10^?5 mol/L, and the detection limit is 1.5 × 10^?7 mol/L. The fluorescent probe exhibits high selectivity over other common metal ions, and the proposed fluorescent sensor was applied to determine zinc in water samples and waste water.     

A benzothizole-based fluorescent probe for Hg2+ recognition utilizing ESIPT coupled AIE characteristics

A new benzothizole-based fluorescent probe 1 for Hg²⁺ recognition utilizing “ESIPT+AIE” strategy has been developed. In THF/H₂O (1:1, v/v, PBS 20 mM, pH = 8.5) mixed solution, probe 1 displays rapid fluorescence responses to Hg²⁺ ions with high selectivity and sensitivity through Hg²⁺-triggered releasing of a compound possessing “ESIPT+AIE” characteristics. Cell imaging investigations indicate that probe 1 is cell permeable with low toxicity to MCF-7 cells, and applicable to detect Hg²⁺ ions in living MCF-7 cells.     

A lysosomal targeting fluorescent probe and its zinc imaging in SH-SY5Y human neuroblastoma cells

A fluorescent probe based PET mechanism was designed, and the probe could image endogenous release of Zn²⁺ upon H₂O₂ stimulation in SH-SY5Y cells.     

Cu2+-selective turn-on fluorescence signaling based on metal-induced hydrolysis of pyrenecarbohydrazide

We developed a simple Cu²⁺-selective turn-on fluorescence signaling probe based on the hydrolysis of 1-pyrenecarbohydrazide (1) to 1-pyrenecarboxylic acid. Probe 1 exhibited prominent fluorescence signaling of Cu²⁺ ions in a 10% aqueous Tris-buffered (pH 7.0) DMSO solution with a detection limit of 5.93 × 10^?8 M. Signaling with control compounds derived from pyreneacetic acid and pyrenebutyric acid showed that the fluorescence signal became less pronounced as the distance between the hydrazide functionality and the pyrene fluorophore increased. As a practical application, this probe was employed for the determination of Cu²⁺ in a simulated semiconductor wastewater.     

Quinoline-based highly selective and sensitive fluorescent probe specific for Cd2+ detection in mixed aqueous media

Quinoline-based fluorescent probe as a recognition unit was designed and synthesized in this study. The probe R1 displayed excellent selectivity and sensitivity for cadmium ions (Cd²⁺) over a wide range of metal ions in acetonitrile-water (MeCN-H₂O) mixed solution. In order to better understand the recognition mechanism between probe and Cd²⁺, the density functional theory calculations were performed. Finally, the colorimetric experiment result was observed and conveniently monitored by the naked eye, and a visual detection limit of 4 × 10^?6 mol L^?1 was achieved. These experimental results indicated the promising potential of the probe to detect Cd²⁺ in biological system. Furthermore, the probe R1 was successfully used for the highly sensitive detection of Cd²⁺ in living cells.     

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.     

A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe 1 was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm (I₄₅₄/I₃₆₀) increases linearly with fluoride ion concentration in the range 0.3-8.0 μmol L⁻¹ and the detection limit is 0.19 μmol L⁻¹. The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples.     

Highly fluorescence quenching graphene oxide-based oligodeoxynucleotide hairpin systems for probing CNG DNA repeat sequences

In this study we synthesized a novel graphene-oxide (GO) based CNG repeat hairpin probing system capable of detecting target CAG and CTG DNA repeat sequences. The fluorescence of the 30-mer CNG repeat hairpin structure was quenched dramatically by GO in the absence of the target sequence, with a high quenching constant [K = 0.030 (mg/mL)^?1]. We optimized the quenching behavior of this probing system by using graphene oxide (GO) to induce a high degree of discrimination factor (44.6 times) between the fluorescence of the target sequence and that of other non-target sequences. All detection process is explained by displacement mechanism using adsorption, desorption, and hybridization of probe with target DNA sequence on the GO. Graphene-oxide (GO) based CNG repeat hairpin probing system exhibited high sensitivity and selectivity to the target CNG repeat sequence and its detection process is so simple and quick.