Showing papers by "Zhen Li published in 2012"
TL;DR: An inhomogeneous planar substrate (g-C(3)N(4)) promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron-hole puddle, on the supported graphene layer, which can potentially allow overcoming the graphene's band gap hurdle in constructing field effect transistors.
Abstract: Opening up a band gap and finding a suitable substrate material are two big challenges for building graphene-based nanodevices. Using state-of-the-art hybrid density functional theory incorporating long-range dispersion corrections, we investigate the interface between optically active graphitic carbon nitride (g-C(3)N(4)) and electronically active graphene. We find an inhomogeneous planar substrate (g-C(3)N(4)) promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron-hole puddle, on the supported graphene layer. The composite displays significant charge transfer from graphene to the g-C(3)N(4) substrate, which alters the electronic properties of both components. In particular, the strong electronic coupling at the graphene/g-C(3)N(4) interface opens a 70 meV gap in g-C(3)N(4)-supported graphene, a feature that can potentially allow overcoming the graphene's band gap hurdle in constructing field effect transistors. Additionally, the 2-D planar structure of g-C(3)N(4) is free of dangling bonds, providing an ideal substrate for graphene to sit on. Furthermore, when compared to a pure g-C(3)N(4) monolayer, the hybrid graphene/g-C(3)N(4) complex displays an enhanced optical absorption in the visible region, a promising feature for novel photovoltaic and photocatalytic applications.
532 citations
TL;DR: Water-soluble and well-crystallized graphene quantum dots with lateral size about 3.0 nm were fabricated by a hydrothermal cutting method and their photoluminescence (PL) properties as well as the potential for bioimaging were demonstrated as mentioned in this paper.
Abstract: Water-soluble and well-crystallized graphene quantum dots with lateral size about 3.0 nm were fabricated by a hydrothermal cutting method and their photoluminescence (PL) properties as well as the potential for bioimaging were demonstrated.
422 citations
TL;DR: ZnO nanowire networks featuring excellent charge transport and light scattering properties are grown in situ within TiO(2) films, which remarkably enhance the overall conversion efficiency of dye-sensitized solar cells (DSSCs) by 26.9%, compared to that of benchmarkTiO( 2) films.
Abstract: ZnO nanowire networks featuring excellent charge transport and light scattering properties are grown in situ within TiO2 films. The resultant TiO2/ZnO composites, used as photoanodes, remarkably enhance the overall conversion efficiency of dye-sensitized solar cells (DSSCs) by 26.9%, compared to that of benchmark TiO2 films.
223 citations
TL;DR: Characterization, and Microstructure Zhiwen Chen,*,§ Zheng Jiao,*,†,‡ Zhen Li,† Minghong Wu,*,‡ Chan-Hung Shek, C. M. Lawrence Wu, and Joseph K. Lai.
Abstract: Characterization, and Microstructure Zhiwen Chen,*,†,§ Zheng Jiao,*,†,‡ Dengyu Pan,‡ Zhen Li,† Minghong Wu,*,†,‡ Chan-Hung Shek, C. M. Lawrence Wu, and Joseph K. L. Lai †Shanghai Applied Radiation Institute and ‡Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, People’s Republic of China Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong
215 citations
TL;DR: Spring-like carbon nanotube ropes consisting of perfectly arranged loops are fabricated by spinning single-walled nanotubes films, and can sustain tensile strains as high as 285%.
Abstract: Spring-like carbon nanotube ropes consisting of perfectly arranged loops are fabricated by spinning single-walled nanotube films, and can sustain tensile strains as high as 285%.
192 citations
TL;DR: The in vivo results show that the liver signal from T1‐weighted MRI is positively enhanced 26%, and then negatively decreased 20% after injection of the iron oxide nanoparticles, which is stronger than those obtained from Gd‐DTPA using the same dosage.
Abstract: Monodispersed water-soluble and biocompatible ultrasmall magnetic iron oxide nanoparticles (UMIONs, D = 3.3 +/- 0.5 nm) generated from a high-temperature coprecipitation route are successfully used as efficient positive and negative dual contrast agents of magnetic resonance imaging (MRI). Their longitudinal relaxivity at 4.7 T (r1 = 8.3 mM-1 s-1) is larger than that of clinically used T1-positive agent Gd-DTPA (r1 = 4.8 mM-1 s-1), and three times that of commercial contrast agent SHU-555C (r1 = 2.9 mM-1 s-1). The transversal relaxivity (r2 = 35.1 mM-1 s-1) is six times that of Gd-DTPA (r2 = 5.3 mM-1 s-1), half of SHU-555C (r2 = 69 mM-1 s-1). The in vivo results show that the liver signal from T1-weighted MRI is positively enhanced 26%, and then negatively decreased 20% after injection of the iron oxide nanoparticles, which is stronger than those obtained from Gd-DTPA (<10%) using the same dosage. The kidney signal is positively enhanced up to 35%, similar to that obtained from Gd-DTPA. Under T2-weighted conditions, the liver signal is negatively enhanced ?70%, which is significantly higher than that from Gd-DTPA (?6%). These results demonstrate the great potential of the UMIONs in dual contrast agents, especially as an alternative to Gd-based positive contrast agents, which have risks of inducing side effects in patients.
174 citations
162 citations
TL;DR: In this article, two new TPE-based conjugated molecules, constructed using tetraphenylethene (TPE) and carbazole or spirofluorene moieties, have been successfully prepared.
Abstract: In this paper, two new TPE-based conjugated molecules, constructed using tetraphenylethene (TPE) and carbazole or spirofluorene moieties, have been successfully prepared. They exhibited aggregation induced emission (AIE) properties, high thermal and morphological stabilities, and low oxidation potential, making them promising properties for optoelectric materials. The fabricated non-doped multilayer OLEDs demonstrate that devices using these luminophors as the emitting layer show relatively good performance, and the device of SFTPE gives a maximum luminance and efficiency of 8196 cd m−2 and 3.33 cd A−1, respectively, with the maximum emission wavelength at about 466 nm, thanks to the adjusted molecular structure and AIE characteristic of TPE.
158 citations
TL;DR: CNT-Si junction solar cells with efficiencies reaching 15% are reported by coating a TiO2 antireflection layer and doping CNTs with oxidative chemicals, resulting in much enhanced short-circuit current and external quantum efficiency.
Abstract: Combining carbon nanotubes (CNTs), graphene or conducting polymers with conventional silicon wafers leads to promising solar cell architectures with rapidly improved power conversion efficiency until recently. Here, we report CNT-Si junction solar cells with efficiencies reaching 15% by coating a TiO2 antireflection layer and doping CNTs with oxidative chemicals, under air mass (AM 1.5) illumination at a calibrated intensity of 100 mW/cm2 and an active device area of 15 mm2. The TiO2 layer significantly inhibits light reflectance from the Si surface, resulting in much enhanced short-circuit current (by 30%) and external quantum efficiency. Our method is simple, well-controlled, and very effective in boosting the performance of CNT-Si solar cells.
152 citations
TL;DR: A new ratiometric colorimetric chemodosimeter toward cyanide anion was designed and synthesized by using intramolecular charge transfer (ICT) as a signaling mechanism and displayed apparent color changes from deep red to light yellow, which could be observed by naked eyes.
Abstract: Taking advantages of both the well-known azobenzene structure and the special nucleophilicity of cyanide toward the acceptor moiety, a new ratiometric colorimetric chemodosimeter (Azo-1) toward cyanide anion was designed and synthesized by using intramolecular charge transfer (ICT) as a signaling mechanism. Upon the addition of CN– anion, the probe displayed apparent color changes from deep red to light yellow, which could be observed by naked eyes. With the aid of the UV–vis spectrometer, the detection limit could be as low as 1.1 μM. The probe possessed high selectivity for cyanide with respect to other common anions. In addition, Azo-1 could serve as practical colorimetric probe for “in-the-field” measurements that would not require any additional equipment by virtue of “dip-sticks” approach.
150 citations
TL;DR: A new ratiometric fluorescent and colorimetric probe (Coum-1) was designed by tuning the π-conjugated bridge to affect the intramolecular charge transfer efficiency and successfully applied to the fluorescent microscopic imaging for the detection of CN(-) in HeLa cells.
Abstract: Taking advantage of the special nucleophilicity of cyanide, a new ratiometric fluorescent and colorimetric probe (Coum-1) was designed by tuning the π–conjugated bridge to affect the intramolecular charge transfer efficiency. Upon the addition of CN– anion, the probe displayed very large blue-shift in both fluorescence (90 nm) and absorption (90 nm) spectra, with the detection limit of 800 nM. Other anions gave nearly no interference. Furthermore, Coum-1 was successfully applied to the fluorescent microscopic imaging for the detection of CN– in HeLa cells.
TL;DR: This is the first report of the AIE activity of the TPE-based conjugated hyperbranched polymers, and their corresponding PLED devices also demonstrate good performance.
Abstract: With the aim to develop new tetraphenylethylene (TPE)-based conjugated hyperbranched polymer, TPE units, one famous aggregation-induced emission (AIE) active group, are utilized to construct hyperbranched polymers with three other aromatic blocks, through an "A2+B4" approach by using one-pot Suzuki polycondensation reaction. These three hyperbranched polymers exhibit interesting AIEE behavior and act as explosive chemsensors with high sensitivity both in the nanoparticles and solid states. This is the first report of the AIE activity of the TPE-based conjugated hyperbranched polymers. Their corresponding PLED devices also demonstrate good performance.
TL;DR: In this article, tetraphenylethylene (TPE) units were utilized to construct the hyperbranched polymer HP-TPE-Cz with carbazole moieties, a good hole-transporting and electroluminescent group.
Abstract: In this paper, tetraphenylethylene (TPE) units, a well-known aggregation-induced emission (AIE) active group, are utilized to construct the hyperbranched polymer HP-TPE-Cz with carbazole moieties, a good hole-transporting and electroluminescent group, through an “A2 + B4” approach by using a one-pot Suzuki polycondensation reaction. For comparison, its analog linear polymer LP-TPE-Cz, also constructed from these two moieties, was prepared. These two polymers exhibit interesting aggregation-induced emission enhanced (AIEE) behavior and act as explosive chemosensors with high sensitivity both as nanoparticles and in solid state, due to the presence of TPE units. Also, the HP-TPE-Cz PLED device exhibited a remarkably enhanced current efficiency (2.13 cd A−1) and luminescence efficiency (5914 cd m−2), compared with its analog linear polymer LP-TPE-Cz (1.04 cd A−1, 1654 cd m−2).
TL;DR: Water-soluble fluorescent copper, silver and gold nanoclusters with quantum yields of 2.2, 6.8 and 5.3%, respectively, are prepared by a robust photoreduction of their inorganic precursors in the presence of poly functionalized with pentaerythritol tetrakis 3-mercaptopropionate.
TL;DR: In this paper, a facile two-step method was developed to synthesize the α-MnS composites with nitrogen-doped carbon (NC) as anode materials for lithium-ion batteries.
Abstract: Coral-like α-MnS composites with nitrogen-doped carbon (NC) were designed as anode materials for lithium-ion batteries. A facile two-step method was developed to synthesize the composites. Hydrothermally obtained polyvinyl pyrrolidone (PVP) capped (NH4)2Mn2(SO4)3 was used as a precursor. The α-MnS–NC composites were attained by heating the precursor at different temperatures for an appropriate time in a N2 atmosphere. The microstructure and morphology were carefully investigated by means of field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and powder X-ray diffraction (XRD). As anode materials, the α-MnS–NC composites exhibit large reversible capacity, excellent cyclic stability and high rate capability. At a current density of 500 mA g−1, the discharge capacity reaches as high as 878 mA h g−1 at the first cycle and remains at 699 mA h g−1 even after 400 cycles.
TL;DR: By in situ nanoscaled composition of 3D CNT network with polyaniline (PANI), the thermoelectric performance of PANI was significantly improved, while the self-sustainable and flexible structure of the 3DCNT network has been retained, and it is hoped that as-fabricated 3D cNT network will contribute to the development of low-cost organic thermOElectric area.
Abstract: Electrical and thermal transportation properties of a novel structured 3D CNT network have been systematically investigated. The 3D CNT net work maintains extremely low thermal conductivity of only 0.035 W/(m K) in standard atmosphere at room temperature, which is among the lowest compared with other reported CNT macrostructures. Its electrical transportation could be adjusted through a convenient gas-fuming doping process. By potassium (K) doping, the original p-type CNT network converted to n-type, whereas iodine (I2) doping enhanced its electrical conductivity. The self-sustainable homogeneous network structure of as-fabricated 3D CNT network made it a promising candidate as the template for polymer composition. By in situ nanoscaled composition of 3D CNT network with polyaniline (PANI), the thermoelectric performance of PANI was significantly improved, while the self-sustainable and flexible structure of the 3D CNT network has been retained. It is hoped that as-fabricated 3D CNT network will contribut...
TL;DR: In this article, three benzene-cored fluorophors with TPE moiety peripheries (PhTPE, Ph2TPE and Ph3TPE) are synthesized and their thermal, optical and electronic properties are investigated.
Abstract: In this paper, three benzene-cored fluorophors with TPE moiety peripheries (PhTPE, Ph2TPE and Ph3TPE), are synthesized and their thermal, optical and electronic properties are investigated. All of them are nearly non-emissive when readily dissolved in solution but become highly emissive in the aggregate state, making them promising candidates for optoelectronic materials. Non-doped OLEDs with these luminogens as emissive layers exhibit sky-blue to deep blue emissions from 488 to 457 nm with Lmax, ηC, max and ηP, max up to 3966 cd m−2, 5.0 cd A−1 and 3.87 lm W−1, respectively, owing to the twisted conformation between the benzene core and TPE units.
TL;DR: A new near-IR "turn-on" fluorescent chemosensor with high selectivity for Hg(2+) ions was designed according to the twisted intramolecular charge transfer (TICT) mechanism, and this naphthalenedimide-based sensor with long wavelength absorption and emission can be used to image intracellular HG( 2+) ions in living Hela cells.
TL;DR: In this paper, a template-free and nontoxic one-pot solvothermal route for synthesizing submicrometer-sized yolk@shell hierarchical spheres (TiO2 YSHSs) was reported, which possess a permeable shell self-assembled by ultrathin anatase TiO2 nanosheets.
Abstract: We report a facile, template-free and nontoxic one-pot solvothermal route for synthesizing submicrometer-sized yolk@shell hierarchical spheres, which possess a permeable shell self-assembled by ultrathin anatase TiO2 nanosheets (NSs) with nearly 90% of exposed {001} facets and mesoporous inner sphere with a high specific surface area. Compared to the {001} faceted TiO2 NSs and standard Degussa P25, the anatase TiO2 yolk@shell hierarchical spheres (TiO2 YSHSs) were obtained with surface area up to 245.1 m2 g−1 and their submicrometer scale simultaneously promoted light scattering in the visible region. A light to electricity conversion efficiency (η) of 6.01% was achieved for the DSSCs with TiO2 YSHSs as its photoanode, under 100 mW cm−2 illumination, indicating 49.9% and 34.8% increases compared to the DSSCs with TiO2 NSs (4.01%) and the standard Degussa P25 (4.46%) as photoanodes, respectively. The enhancement can be mainly attributed to the higher dye loading on TiO2 YSHSs (4.35 × 10−5 mol cm−2) than that of TiO2 NSs (3.14 × 10−5 mol cm−2) and P25 (3.32 × 10−5 mol cm−2); longer lifetime of the injected electrons in TiO2 YSHSs film (65.79 ms) than that of in TiO2 NSs film (57.90 ms); and the good capability of light scattering of TiO2 YSHSs in visible light region, which are confirmed by UV-vis spectrophotometer and electrochemical impedance spectroscopy (EIS). The growth mechanism of the TiO2 YSHSs has also been investigated in detail.
TL;DR: This work explores how immobilization of lipases onto magnetite nanoparticles affects their biocatalytic performance under carefully controlled surface modification, finding the activity of immobilized lipases increased with increasing chain length of the alkyl silane.
Abstract: Background: Biocatalytic processes often require a full recycling of biocatalysts to optimize economic benefits and minimize waste disposal. Immobilization of biocatalysts onto particulate carriers has been widely explored as an option to meet these requirements. However, surface properties often affect the amount of biocatalysts immobilized, their bioactivity and stability, hampering their wide applications. The aim of this work is to explore how immobilization of lipases onto magnetite nanoparticles affects their biocatalytic performance under carefully controlled surface modification. Methodology/Principal Findings: Magnetite nanoparticles, prepared through a co-precipitation method, were coated with alkyl silanes of different alkyl chain lengths to modulate their surface hydrophobicity. Candida rugosa lipase was then directly immobilized onto the modified nanoparticles through hydrophobic interaction. Enzyme activity was assessed by catalytic hydrolysis of p-nitrophenyl acetate. The activity of immobilized lipases was found to increase with increasing chain length of the alkyl silane. Furthermore, the catalytic activities of lipases immobilized on trimethoxyl octadecyl silane (C18) modified Fe3O4 were a factor of 2 or more than the values reported from other surface immobilized systems. After 7 recycles, the activities of the lipases immobilized on C18 modified nanoparticles retained 65%, indicating significant enhancement of stability as well through hydrophobic interaction. Lipase immobilized magnetic nanoparticles facilitated easy separation and recycling with high activity retaining. Conclusions/Significance: The activity of immobilized lipases increased with increasing alkyl chain length of the alkyl trimethoxy silanes used in the surface modification of magnetite nanoparticles. Lipase stability was also improved through hydrophobic interaction. Alkyl silane modified magnetite nanoparticles are thus highly attractive carriers for enzyme immobilization enabling efficient enzyme recovery and recycling.
TL;DR: The zebrafish model demonstrates the potential of a hyperactivated epidermal growth factor receptor pathway in initiating heptocarcinogenesis and is thus a convenient model for further investigation of oncogene addiction and future anti-cancer drug screening.
TL;DR: In this paper, tetraphenylethylene (TPE) units were utilized to construct a new functional polyfluorene (PF) P1, which exhibited the exciting property of aggregation-induced emission enhancement (AIEE), instead of the aggregation-caused quenching of normal PFs, and could probe the explosive with high sensitivity both in the nanoparticles and solid state.
TL;DR: In this article, three metal-free organic sensitizers containing 11,12-bis(hexyloxy) dibenzo[a,c]phenazine (BPz) units were synthesized and used for dye-sensitized solar cells (DSSCs).
Abstract: Three new metal-free organic sensitizers containing 11,12-bis(hexyloxy) dibenzo[a,c]phenazine (BPz) units were synthesized and used for dye-sensitized solar cells (DSSCs). The broad absorption spectra indicate that the light harvesting abilities were enhanced by the introduction of the BPz unit in the π-conjugated space, which can also cause an anti-aggregation effect and the suppression of charge recombination. Among these sensitizers, LI-39 showed the best photovoltaic performance: a short-circuit photocurrent density (Jsc) of 14.40 mA cm−2, an open-circuit photovoltage (Voc) of 0.74 V, and a fill factor (ff) of 0.67, corresponding to an overall conversion efficiency of 7.18% under standard global AM 1.5 solar light conditions. The result shows that the organic sensitizers based on this bulky fused aromatic rings as well as the auxiliary acceptor are the promising candidates for improvement of the performance of DSSCs.
TL;DR: The results show that both HNO(3) and the PEDOT-PSS film could enhance the energy conversion efficiency of graphene/SPA solar cells.
Abstract: A single-layer graphene film was grown on copper foil by chemical vapor deposition and transferred onto a silicon-pillar-array (SPA) substrate to make a Schottky junction solar cell. The SPA substrate was specifically designed to suppress reflectance and enhance light absorption. The energy conversion efficiency of the prepared graphene/SPA solar cells achieved a maximum of 2.90% with a junction area of 0.09 cm2. HNO3 was employed to dope the graphene in the solar cells, and the time dependence of HNO3 treatment on the cell performance was studied. Poly(3,4-ethylenedioxythiophene) polystyrenesulfonic acid (PEDOT-PSS) was also introduced in graphene/SPA solar cells by spin coating on top of the graphene film, and its modification on the cell performance was characterized. The results show that both HNO3 and the PEDOT-PSS film could enhance the energy conversion efficiency of graphene/SPA solar cells.
TL;DR: A new series of intramolecular-charge-transfer molecules synthesized by attaching various electron-donating thiophenes groups to a triphenylamine backbone with an aldehyde group as the electron acceptor were revealed to be mediated by ICT, as confirmed by time-dependent DFT calculations.
Abstract: A new series of intramolecular-charge-transfer (ICT) molecules (compounds 1, 2, and 3) were synthesized by attaching various electron-donating thiophenes groups to a triphenylamine backbone with an aldehyde group as the electron acceptor. Based on the protection reaction between ethanethiol and aldehyde, the corresponding dithioacetals (compounds S1, S2, and S3) were prepared to serve as novel colorimetric and fluorescent chemosensors for Hg(2+) ions. Also, compound S1 was further utilized to construct the chemical-reaction-based conjugated polymer probe (PS1) towards Hg(2+) ions. In the presence of as little as 10 nM Hg(2+), compound PS1 displayed an apparent change in the fluorescent intensity. The sensing processes were revealed to be mediated by ICT, as confirmed by time-dependent DFT calculations. Furthermore, compound S1 was successfully applied to microscopic imaging for the detection of Hg(2+) in HeLa cells with ratiometric fluorescent methods.
TL;DR: In this paper, the structural, compositional and optical characterizations of the synthesized nanowires were characterized by X-ray diffraction (XRD) pattern, Fourier transform infrared (FTIR) and UV-vis spectroscopy.
TL;DR: In this paper, a series of dye sensitizers with pyrrole as the conjugated bridge were synthesized, in which two pieces of N-arylpyrrole-based organic dye moieties were linked together through various aromatic rings.
Abstract: A series of “H” type dye sensitizers with pyrrole as the conjugated bridge were synthesized, in which two pieces of N-arylpyrrole-based organic dye moieties were linked together through various aromatic rings. Interestingly, the introduced aromatic isolation group not only gave a new possibility to modify the construction structure, but also controlled the topological structure of the resultant dyes in some degree. As a result, their performance could be adjusted, and the configuration of “H”-type could suppress the aggregations on the TiO2 surface. The performance of the DSCs based on these dyes as the sensitizers demonstrated that the structure of these dyes was beneficial to the devices, and the conversion efficiency of the solar cell based on dye LI-22 with carbazole as the isolation group was as high as 5.22%.
TL;DR: For the first time, two bipolar AIE-active luminogens constructed from tetraphenylethene and oxadiazole were utilized as fluorescence host materials in sky-blue doped OLEDs and exhibited high efficiencies, broadening the scope for the utilization of AIE materials in the optoelectronic field.
TL;DR: In this article, global-like dendrimers with 9, 21 and 45 azobenzene chromophore moieties were prepared with high purity and satisfactory yields, through the combination of divergent and convergent approaches, coupled with the utilization of the powerful Sharpless "click" reaction.
Abstract: In this paper, global-like dendrimers, G1-TPA, G2-TPA and G3-TPA, bearing 9, 21 and 45 azobenzene chromophore moieties, respectively, were conveniently prepared with high purity and satisfactory yields, through the combination of divergent and convergent approaches, coupled with the utilization of the powerful Sharpless “click” reaction. Due to their perfect 3D structure and the isolation effect of the exterior benzene moieties and the interior triazole rings, these global-like dendrimers exhibited large d33 (a second harmonic generation coefficient) values, e.g., 246.0 pm V−1 for G3-TPA, which, to the best of our knowledge, is a new record reported so far for simple azo chromophore moieties.
TL;DR: In this paper, a colorimetric azobenzene based chemosensor was designed for the detection of hypochlorite by fully utilizing the oxidation property of hypochemicals and different coordinating properties of Cu+ and Cu2+ in pure aqueous solutions.
Abstract: Colorimetric azobenzene based chemosensor 1 was designed for the detection of hypochlorite by fully utilizing the oxidation property of hypochlorite and different coordinating properties of Cu+ and Cu2+ in pure aqueous solutions. In the presence of cuprous ions, the red solution of chemosensor 1 changed to yellow, upon the addition of hypochlorite. Other anions, including H2PO42−, H2PO4−, SO42−, Cl−, ClO4−, HSO3−, Br−, PO43−, CO32−, I−, HCO3−, SO32−, F−, AcO−, HSO4−, NO2−, ClO3−, IO3− and H2O2, nearly gave no influence to the probing behavior of chemosensor 1 towards hypochlorite, making chemosensor 1 a highly selective and sensitive hypochlorite chemosensor in pure aqueous solutions.