Rui Hu

A Rapid Aqueous Fluoride Ion Sensor with Dual Output Modes

The development of sensors and receptors for biologically important anions is currently of great interest because of their indispensable roles in vital (or physiological) processes. Among the anions, fluoride ions are one of the most attractive targets because of their considerable significance for health and environmental issues. As an essential element of the body, the U.S. Public Health Service affirmed the optimal level to be 1 mg of consumed fluoride per day. On the other hand, unnecessary and inappropriate fluoride ingestion can result in fluorosis, urolithiasis, or even cancer. The EPA (United States Environmental Protection Agency) gives an enforceable drinking water standard for fluoride of 4 mg L 1 to prevent osteofluorosis and a secondary fluoride standard of 2 mgL 1 to protect against dental fluorosis. Hence, the accurate determination of the levels of fluoride in drinking water is necessary. To date, the ion-selective electrode, ion chromatography, and standard Willard and Winter methods are generally used for quantitative fluoride analysis. However, all these methods involve disadvantages, such as complicated procedures, high costs, or low mobility. Therefore, it is important to develop highly selective, sensitive, convenient, and rapid fluoride detection methods. Fluorescent chemosensors with high specificity and sensitivity, ease, and safety of handling have received considerable attention, and a number of fluorescence sensors have been reported that are capable of detecting fluoride ions. The recognition proceeded mostly through hydrogen bonding or Lewis acid coordination, and the sensors could only be operated in organic solvents to detect tetrabutylammonium (TBA) fluoride rather than inorganic fluoride salts. This incompatibility with aqueous environments is one of the main drawbacks that restrict the application of these sensors. Unavoidable interference from H2PO4 , AcO , or CN ions is the other disadvantage. To improve the performance of fluoride sensors, another strategy based on the chemical affinity between fluoride and silicon was developed. tertButyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl (TBDPS) were chosen as additional substituents for the dye molecule, and rendered the dye unreactive to potentially interfering compounds and thus sensitive only to fluoride ions. This approach was pioneered by Kim and Swager, who developed a fluorescent fluoride receptor in organic solvents. Subsequently, several research groups reported different chemodosimeters that can probe NaF in mixtures of organic solvents and water or even pure water. However, several tens of minutes or even hours are needed to complete the detection process because low concentrations of these chemodosimeters severely reduce the reaction rate between the fluoride ions and the silyl moieties. However, the low concentration is necessary for general organic dyes to avoid fluorescence quenching induced by concentration effects, such as self-absorption and self-quenching. To date, most fluorescent sensors were based on the specific fluoride ion dependence of the emission intensity and could be significantly influenced by the excitation power and the detector sensitivity. More importantly, fluorescence intensity changes are not suitable for direct observation with the naked eye. To develop a more convenient fluorescence sensor, it is essential to make use of other fluoridedependent measurable signals other than intensity. The fluorescence color change, which can be measured directly with a colorimeter or even distinguished easily by eye, is thus a good choice. Herein we describe a rapid and portable sensor for fluoride ions in aqueous solution. The sensor, which has a high sensitivity, operates through the special affinity between fluoride ions and silicon, and provides two independent modes of signal transduction based on fluoride-dependent changes of fluorescence color (color metric mode) or intensity (power metric mode), respectively. For the design of the sensor, we chose N-(3-(benzo[d]thiazol-2-yl)-4-(hydroxyphenyl) benzamide (3-BTHPB) as an excited-state intramolecular proton transfer (ESIPT) compound. Just like other ESIPT compounds, 3-BTHPB shows two emission bands, which originate from the enol and keto forms at 418 and 560 nm, respectively. The ratio of the two bands is determined by the number of molecules that could undergo ESIPT reactions. We coupled the tert-butyldiphenylchlorosilane with the sodium salt of 3-BTHPB to afford a derivative N-(3(benzo[d]thiazol-2-yl)-4-(tert-butyldiphenyl silyloxy)phenyl)benzamide (BTTPB). As expected, BTTPB shows only blueviolet fluorescence, which was almost identical to the emission of the enol form of 3-BTHPB (see the Supporting [*] R. Hu, J. Feng, D. H. Hu, Dr. S. Q. Wang, Dr. S. Y. Li, Prof. Dr. G. Q. Yang Beijing National Laboratory for Molecular Sciences Key Laboratory of Photochemistry, Institute of Chemistry Chinese Academy of Sciences Beijing 100190 (China) Fax: (+ 86)10-8261-7315 E-mail: shayuli@iccas.ac.cn gqyang@iccas.ac.cn

Understanding Solvent Effects on Luminescent Properties of a Triple Fluorescent ESIPT Compound and Application for White Light Emission

A triple fluorescent compound, N-salicylidene-3-hydroxy-4-(benzo[d]thiazol-2-yl)phenylamine (SalHBP), was dispersed in solid polymers and was developed as a white-light-emitting source in LED by using it as the first simple single compound with different configurations. The CIE coordinates were at (0.29, 0.35), close to those of pure white light. To explore speciation mechanisms in this single compound white light, SalHBP was dissolved in protic, nonpolar, and moderate polar solvent, respectively. Upon excitation, blue, green, and yellowish green emissions were observed from the three solutions at various temperatures. The conformation of SalHBP at room temperature was described by a Car-Parrinello molecular dynamics simulation. With the aid of hybrid density functional theory at the B3LYP/TZVP and PBE0/TZVP levels, three observed emission bands of SalHBP were assigned from the five most probable excited state conformations that were derived from four ground state conformations. The effect of solvent on the emission of SalHBP was summarized as a possibility for forming intermolecular hydrogen bonds between solvent and SalHBP molecules and competition between intra- and intermolecular hydrogen bonds.

Sensing in 15 s for Aqueous Fluoride Anion by Water-Insoluble Fluorescent Probe Incorporating Hydrogel

Anion recognition and sensing via artificial receptors have attracted a great deal of attention since they play a fundamental and important role in chemical, biological, medical, and environmental processes. Fluoride, as one of the smallest anions, is of particular interest because of its role in dental care and the analysis of drinking water. Herein, we invented a new method for F(-) detection by adopting the hydrogel as the supporter of reaction between a water insoluble fluorescent probe and F(-) in the water environment. This method is highly rapid, selective, and sensitive, which can determine F(-) levels in 15 s at the drinking water standard. A novel compound N-(3-(benzo[d]thiazol-2-yl)-4-(tert-butyldiphenylsilyloxy)phenyl) acetamide (BTBPA) was synthesized as the fluorescent probe because of the significant fluorescent color change from blue to green after the reaction with F(-). This method does not require the probe substances to be water-soluble, which greatly expands the range of the specific fluorescent molecules used in ion detection. Additionally, just a few microliter samples were required in the analysis procedures with this method.

Intracellular Fluorescent Temperature Probe Based on Triarylboron Substituted Poly N-Isopropylacrylamide and Energy Transfer

A novel hydrophilic fluorescence temperature probe (PNDP) based on polarity-sensitive triarylboron compound (DPTB) and PNIPAM is designed and synthesized. In order to overcome the shortcomings of the single-intensity-based sensing mechanism and obtain more robust signals, ratiometric readout is achieved by designing an efficient FRET system (PNDP-NR) between DPTB and Nile Red (NR). PNDP-NR possesses some excellent features, including wide temperature range, good linear relationship, high temperature resolution, excellent reversibility, and stability. Within a sensing temperature range of 30-55 °C, the fluorescence color of PNDP-NR experiences significant change from red to green-blue. PNDP-NR is also introduced into NIH/3T3 cells to sense the temperature at the single-cell level. It gave excellent photostability and low cytotoxicity in vivo.

Molecular Engineering of Aqueous Soluble Triarylboron-Compound-Based Two-Photon Fluorescent Probe for Mitochondria H2S with Analyte-Induced Finite Aggregation and Excellent Membrane Permeability.

Hydrogen sulfide (H2S) is a multifunctional signaling molecule that participates in many important biological processes. Herein, by functionalizing triarylboron with cyclen and diphenylamine, we synthesized TAB-1, TAB-2, and TAB-3 for H2S recongnization by rational design of molecular structures. Among them, aqueous soluble TAB-2 possesses excellent properties, including large two-photon action cross section, membrane permeability and can effectively complex with Cu(2+). The complex of TAB-2-Cu(2+) can selectively detect H2S with an instant response and mitochondria targeted. Moreover, the H2S-induced finite aggregation of indicators enhances their photostability and causes variation of the fluorescence lifetime. TAB-2-Cu(2+) has also been successfully applied for the mitochondria H2S imaging in NIH/3T3 fibroblast cells by TPM and FLIM.

Third-order nonlinear optical properties of a series of porphyrin-appended europium(III) bis(phthalocyaninato) complexes.

The third-order nonlinear optical properties of a series of phthalocyanine-porphyrin complexes containing a sandwich-type bis(phthalocyaninato) europium(III) core and one or two zinc(II) porphyrin unit(s) were investigated by Z-scan techniques at 1064 nm. The second-order molecular hyperpolarizabilities of these complexes are in the order of 10(-30) esu. The enhanced nonlinear properties are induced by the high delocalization of the electrons in the sandwich molecular structure, which may result in charge transfer between the central metal and the phthalocyanine ligands.

Multiple fluorescence ΔCIE and ΔRGB codes for sensing volatile organic compounds with a wide range of responses

A highly luminescent compound, stilbene-2,4-dimethyl-6-(1,2,2,4-tetramethyl-1,2-dihydroquinolin-6-yl)-1,3,5-s-triazine (MQT), exhibits solvent polarity-induced emission enhancement. A fluorescent sensor array was fabricated with MQT and porous polymer substrates. The colorimetric changes of the array exposed to VOCs can be readily distinguished by the naked eye. Post-processing procedures proved the high selectivity of the array for VOCs.

A colorimetric and ratiometric fluorescence sensor for sensitive detection of fluoride ions in water and toothpaste

Fluoride is a well-known anion that plays a significant physiological role. Sensitive and quantitative sensing of fluoride is of great importance to public health investigation. A colorimetric and ratiometric fluorescence sensor for fluoride ions based on silyl capped hydroxylpyrenealdehyde is designed and synthesized. The sensor detects fluoride ions through the desilylation mediated by fluoride ions and the consequent spectral change of the pyrene derivative. A significant absorption change from 420 to 523 nm in the visual region and a fluorescence shift from 492 to 603 nm can be observed upon addition of fluoride ions of tens of ppb, enabling direct observation with the bare eye. Based on the ratiometric fluorescence with up to 255-fold enhancement, the sensor can rapidly and selectively detect F− in water with a limit as low as 2.7 ppb, and furthermore, the sensor is successfully applied for determining the levels of F− in commercially available toothpaste.

Intramolecular aggregation and optical limiting properties of triazine-linked mono-, bis- and tris-phthalocyanines.

A series of triazine-linked mono-, bis- and tris-phthalocyanines are synthesized, intramolecular aggregation is found in bis- and tris-phthalocyanines via π-π stacking interaction. Theoretical and experimental studies reveal the formation of the intramolecular aggregation. The spectrographic, photophysical and nonlinear optical properties of these compounds are adjusted for the formation of the intramolecular aggregation. The bis-phthalocyanine dimer presents smaller fluorescence quantum yield, lower triplet formation yield and the triplet-minus-ground state extinction coefficient, which causes poorer optical limiting performance. It is interesting that the tris-phthalocyanine is composed of a mono-phthalocyanine part and a bis-phthalocyanine part, the optical limiting property of the tris-phthalocyanine is similar to that of mono-phthalocyanine.

Novel fluorescent probes based on intramolecular charge- and proton-transfer compounds*

Excited-state intramolecular proton-transfer (ESIPT) compounds and intramolecular charge-transfer (ICT) compounds have attracted attention due to their interesting and even unique emission properties. The intense and environment-sensitive emission showed by some members of the two families has been exploited in fluorescent probes for various forms of environmental sensing. In this paper, we summarize our recent study on the utilization of novel ESIPT and ICT compounds with intense emission as fluorescent probes after an introduction to the ESIPT and/or ICT processes and related photophysics mechanism.