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Polydopamine fluorescent organic nanomaterials present unique physicochemical and biological properties, which have great potential application in bio-imaging and chemical sensors. Here, folic acid (FA) adjustive polydopamine (PDA) fluorescent organic nanoparticles (FA-PDA FONs) were prepared by a facile one-pot self-polymerization strategy using dopamine (DA) and FA as precursors under mild conditions. The as-prepared FA-PDA FONs had an average size of 1.9 ± 0.3 nm in diameter with great aqueous dispersibility, and the FA-PDA FONs solution exhibit intense blue fluorescence under 365 nm UV lamp, and the quantum yield is ~8.27%. The FA-PDA FONs could be stable in a relatively wide pH range and high ionic strength salt solution, and the fluorescence intensities are constant. More importantly, here we developed a method for rapidly selective and sensitive detection of mercury ions (Hg2+) within 10 s using FA-PDA FONs based probe, the fluorescence intensities of FA-PDA FONs presented a great linear relationship to Hg2+ concentration, the linear range and limit of detection (LOD) were 0–18 µM and 0.18 µM, respectively. Furthermore, the feasibility of the developed Hg2+ sensor was verified by determination of Hg2+ in mineral water and tap water samples with satisfactory results.

A near-infrared (NIR) fluorescence probe for copper(Ⅱ) ions (Cu2+) has been synthesized and demonstrated that it could selectively and sensitively react with Cu2+ through specific coordination reaction. ESI-Mass and Job’s plot confirmed a stoichiometric ratio of 2–1 between the probe molecules and Cu2+. This NIR fluorescence probe shows a fast response, high sensitivity and anti-interference performance for Cu2+ and has been successfully applied for the detection of Cu2+ in aqueous solution. The detection limit was calculated as low as 14 nM. It has been further demonstrated that the probe exhibited good cell permeability and could be applied for monitoring the fluctuation of Cu2+ in living cells by NIR fluorescence imaging.

Analysis of molecular species is needed for applications in diagnosis of infections and genetic diseases. Herein, we demonstrate a target DNA-responsive ultrahigh fluorescence signal-on DNA amplification system via periodically programmed building and collapse of DNA networks. In this system, a pair of oligonucleotides of padlock probe (PP) and palindromic hairpin probe (PHP) are utilized. The presence of target DNA firstly hybridizes with PP, allowing the occurrence of rolling circle amplification (RCA) to produce RCA products with tandem repeats in abundance to bind and unfold numbers of PHPs. The conformational change of PHPs enables the building of DNA networks via the intermolecular palindrome pairing, but then makes the DNA networks collapsed via the palindrome-induced strand displacement polymerization. The displaced RCA products are dynamically reused to undergo periodically programmed multiple rounds of DNA network building and collapse. Depend on the bidirectional DNA assembly and disassembly, a strikingly amplified fluorescence can be collected to ultrasensitive and specific detection of target DNA. The practicability has been demonstrated by evaluating target-spiked human serum, saliva, and urine samples with acceptable recoveries and reproducibility. Therefore, this newly explored method opens a promising avenue for the detection of nucleic acids with low abundance in biochemical analysis and diseases diagnosis.

The present work reports a sensitive and selective fluorescent sensor for the detection of mercury ion, Hg(II), by hybridizing carbon nanodots (C-dots) and gold nanoclusters (Au NCs) through intrinsic interactions of the two components. The C-dots serve as the reference signal and the Au NCs as the reporter. This method employs the specific high affinity metallophilic Hg2+–Au+ interactions which can greatly quench the red fluorescence of Au NCs, while the blue fluorescence of C-dots is stable against Hg(II), leading to distinct ratiometric fluorescence changes when exposed to Hg(II). A limit of detection of 28 nM for Hg(II) in aqueous solution was estimated. Thus we applied the sensor for the detection of Hg(II) in real water samples including tap water, lake water and mineral water samples with good results. We further demonstrated that a visual chemical sensor could be manufactured by immobilizing the nanohybrid probe on a cellulose acetate circular filter paper. The paper-based sensor immediately showed a distinct fluorescence color evolution from pink to blue after exposure to a drop of the Hg(II) solution.

This work reports a novel dual-emissive fluorescent probe based on dye hybrid silica nanoparticles for ratiometric measurement of the hydroxyl radical (˙OH). In the probe sensing system, the blue emission of coumarin dye (coumarin-3-carboxylic acid, CCA) immobilized on the nanoparticle surface is selectively enhanced by ˙OH due to the formation of a coumarin hydroxylation product with strong fluorescence, whereas the emission of red fluorescent dye encapsulated in the silica nanoparticle is insensitive to ˙OH as a self-referencing signal, and so the probe provides a good quantitative analysis based on ratiometric fluorescence measurement with a detection limit of 1.65 μM. Moreover, the probe also shows high selectivity for ˙OH determination against metal ions, other reactive oxygen species and biological species. More importantly, it exhibits low cytotoxicity and high biocompatibility in living cells, and has been successfully used for cellular imaging of ˙OH, showing its promising application for monitoring of intracellular ˙OH signaling events.

This paper describes the synthesis of fluorescent copper nanoclusters (CuNC) with a fluorescence quantum yield as high as 2.3% after modification with cysteamine. The modified CuNC are shown to be viable probes for the determination of picric acid (PA). Fluorescence drops with increasing concentration of PA which can be detected fluorometrically with a 0.14 μM limit of detection. This is much lower than required by the People's Republic of China Surface Water Environmental Quality Standard (2.2 μM). The probe was successfully applied to the determination of PA in spiked tap water, lake water and river water. Graphical abstract Copper nanoclusters (CuNC) have weak fluorescence but after the modification with cysteamine, the fluorescence of CuNC is strongly enhanced. The fluorescence of such cysteamine-coated copper nanoclusters (CuNC-CA) is reduced upon the addition of picric acid (PA) through an inner filter effect (IFE).

The TiO2 nanosheets with exposed (010) facets were synthesized via facile surfactant self-assembly. Then, the TiO2 sheets were employed as scaffolding and deposited Ag/AgBr particles via improved oil/water self-assembly method. The TEM results suggested that Ag/AgBr particles well dispersed on the surface of TiO2 sheets with diameter 1–4 nm. The modified of Ag/AgBr enhanced the absorption in the visible light region, which was due to surface plasmonic resonance (SPR) of the metallic Ag clusters formed on the surface of AgBr particle. The as-prepared hierarchical composites photocatalyst Ag/AgBr/TiO2 exhibited excellent photocatalytic activity under nature solar-light irradiation and degraded ∼91.7% of Rhodamine (RhB) within 60 min. The degradation of colorless Bisphenol (BPA) further confirmed the photocatalytic abilities of Ag/AgBr/TiO2. The quenching tests exhibited that the Ag/AgBr/TiO2 composites photocatalytic degradation organic pollutions were mainly dominated by O2− oxidation reactions, while the h+ and OH have a minor contribution. The improved photocatalytic activities can be attributed to the suitable size of Ag/AgBr particles, abundant surface active sites, effective photogenerated electron-hole pair's separation, which significantly improve the photocatalytic activity.

Visible-light sensitive photocatalyst Ag/AgBr/Bi2WO6 was successfully synthesized by oil/water self-assembly method. The crystalline structure, morphologies and optical properties of the as-prepared samples were investigated. The results revealed that the Ag/AgBr successfully introduced into the surface of Bi2WO6. The modified of Ag/AgBr enhanced the absorption in the visible-light region, due to surface plasmonic resonance of the metallic Ag. The activities of as-prepared samples were investigated by the degradation of RhB and phenol under the visible-light. Experimental results revealed that the Ag/AgBr/Bi2WO6 composites exhibit high visible-light photocatalytic activity compared with that of pure Ag/AgBr and Bi2WO6.

A novel nanohybrid ratiometric fluorescence probe for on-site and visual determination of nitrogen dioxide (NO2) has been designed. The hybrid probe comprises blue-colored fluorescence carbon nanodots (CDs) and red-colored emission CdTe quantum dots (QDs). Such hybridized probe exhibits dual emission bands centered at 460 and 665 nm, respectively. The blue fluorescence of CDs is insensitive to the analyte, whereas the red emission of QDs is specifically quenched by the analyte, resulting in a distinguishable color change from orange-red to blue upon exposure to NO2. The limit of detection for nitrogen dioxide is estimated to be 19 nM in aqueous solution. More importantly, the nanohybrid probe has been successfully applied in visual detection of gaseous NO2 with a detection limit of 1 ppm, suggesting its potential application for NO2 sensing.

Among various fluorescence nanomaterials, the II-VI semiconductor nanocrystals (usually called quantum dots, QDs) should be very promising in sensing application because of their high quantum yields, capability for surface property manipulation, and unlimited possible chemical reactions. Herein, we present a fluorescence probe for hydrogen sulfide, which was prepared by first encapsulating inorganic cadmium telluride (CdTe) QDs in silica nanospheres, and subsequently engineering the silica surface with functional molecules azidocoumarin-4-acetic acid reactive to hydrogen sulfide. The nanohybrid probe exhibited two fluorescence bands centered at 452 and 657 nm, respectively. The red fluorescence at 657 nm of the nanohybrid probe is stable against H2S, while the blue fluorescence is specifically sensitive to H2S. The probe showed a distinct fluorescence color evolution from light magenta to blue upon exposure to different amounts of H2S, and a detection limit of 7.0 nM was estimated in aqueous solution. We further applied the nanohybrid probe for visual detection of gaseous H2S with a low concentration of 0.5 ppm using glass indicating spots sensors, suggesting its potential application for gaseous H2S sensing. Such an efficient on-site visual determination of gaseous hydrogen sulfide (H2S) is highly demanded in on-site environmental monitoring and protection.

Hydrogen sulfide is a toxic and flammable gaseous pollutant often emitted to air as a by-product of water supply, chemical, petroleum and coal industries. It can be transferred into sulfur dioxide in the air under some meteorologic conditions. Herein, we report a novel ratiometric fluorescence method for hydrogen sulfide based on silver nanoclusters and quantum dots. The silver nanoclusters have been self-assemblied onto the surface of silica spheres, which are embedded with red fluorescent quantum dots, to form a dual-emissive nanohybrid. Such dual-emissive nanohybrid has been applied for hydrogen sulfide detection on the basis of the interfacial interaction between silver nanoclusters and sulfide ions. The blue-emission of Ag NCs is specifically prone to hydrogen sulfide due to surface binding and etching, but the red-emission of QDs within the silica nanospheres is inert against hydrogen sulfide. The different response of the two components to hydrogen sulfide results in fluorescence color variation from violet to red when the blue fluorescence is gradually quenched. This nanohybrid has been successfully demonstrated for the application in sensitive and selective detection of hydrogen sulfide in aqueous solution and gaseous state.

The highly fluorescent graphene oxide modified by hexylenediamine was fabricated to determinate the amount of hypochlorous acid (HOCl) in aqueous solution. The fluorescence of graphene oxide was quenched by HOCl based on the intramolecular charge-transfer (ICT) between grapheme oxide and chloramines, which formed through the oxidation of amino groups at the lateral edge of graphene oxide by HOCl. This method shows low interference and high selectivity for HOCl with a limit of detection of 3.5 μM in water. Additionally, the fluorescence graphene oxide with a quantum yield of 11.2% showed a good spike and recovery test for detection of HOCl in tap water, suggesting that the fluorescent probe is a potential strategy for the accurate determination of HOCl in realistic condition.

Functional quantum dots (QDs) grafted with ferric dithiocarbamate complex layers (QDs-Fe(III)(DTC)3) were fabricated and demonstrated to be selectively reactive to nitric oxide. The dithiocarbamate (DTC) was covalently conjugated to the amine-coated QDs by a condensation reaction of the carboxyl in DTC and the amino polymer in surface of QDs. The weak fluorescence of QDs-Fe(III)(DTC)3 was attributed to the energy transfer between CdSe/ZnS and Fe(III)(DTC)3 complex at the surface of the functionalized quantum dots. Nitric oxide could greatly switch on the fluorescence of QDs-Fe(III)(DTC)3 by displacing the DTC in the Fe(III)(DTC)3 accompanied by reducing Fe(III) to Fe(II), thus shutting off the energy transfer way. The limit of detection for nitric oxide was estimated to be 3.3 μM and the specific detection was not interfered with other reactive oxygen species. Moreover, the probe was demonstrated for the sensing of gaseous nitric oxide, and the visual detection limit was as low as 10 ppm, showing the potential for sensing nitric oxide by the naked eye.

The sensitive and selective determination of fluoride ions is particularly significant in environmental protection, food safety, and health care products. In this work, a highly selective turn-on fluorescent probe for fluoride ions has been synthesized by simply functionalizing fluorescent isophthalaldehyde with silicone-oxygen bonding. The selectivity of the probe is based on the specific reactivity of the silyl group toward fluoride ions in aqueous solution. The nucleophilic substitution reaction of fluoride ions triggers the cleavage of the Si-O bond to release a strongly fluorescent product, which can be used for the determination of fluoride ions by fluorescence intensity enhancement. The probe molecules are specifically responsive and highly selective for the fluoride anion over other relevant anions and cations. This fluorescent probe also shows high photostability and exhibits good sensitivity for fluoride ions, and the limit of detection is as low as 67 ppb. We have demonstrated its application for on-site sensitive determination of fluoride ions for environmental monitoring and protection.

In this paper, we report a benzothiazole-functionalized cyanine fluorescence probe and demonstrate that it is selectively reactive to bisulfite, an intermediate indicator for oxidative stress. The selective reaction can be monitored by distinct ratiometric fluorescence variation favorable for cell imaging and visualization. The original probe can be regenerated in high yield through the elimination of bisulfite from the product by peroxides such as hydrogen peroxide, accompanied by fluorescence turning on at 590 nm, showing a potential application for the detection of peroxides. We successfully applied this probe for fluorescence imaging of bisulfite in cancer cells (MCF-7) treated with bisulfite and hydrogen peroxide as well as a selective detection limit of 0.34 μM bisulfite in aqueous solution.

Herein, we report a novel approach to the rapid visual detection of gaseous sulfur dioxide (SO2) by manipulating the surface chemistry of 3-aminopropyltriethoxysilane (APTS)-modified quantum dots (QDs) using fluorescent coumarin-3-carboxylic acid (CCA) for specific reaction with SO2. The CCA molecules are attached to the surface amino groups of the QDs through electrostatic attraction, thus the fluorescence of CCA is greatly suppressed because of the formation of an ion-pair complex between the ATPS-modified QDs and CCA. Such an interaction is vulnerable to SO2 because SO2 can readily react with surface amino groups to form strong charge-transfer complexes and subsequently release the strongly fluorescent CCA molecules. The mechanism has been carefully verified through a series of control experiments. Upon exposure to different amounts of SO2, the fluorescent color of the nanoparticle-based sensor displays continuously changes from red to blue. Most importantly, the approach owns high selectivity for SO2 and a tolerance of interference, which enables the sensor to detect SO2 in a practical application. Using this fluorescence-based sensing method, we have achieved a visual detection limit of 6 ppb for gaseous SO2.

The development of convenient methods for sulfur dioxide and its derivatives analysis is critically important because SO2 causes worldwide serious environmental problems and human diseases. In this work, we show an unprecedented example of an energy-transfer-based fluorescence nanoprobe for selective and quantitative detection of SO2, through molecular engineering of the fluorescent carbon nanodots by a cyanine dye which have a unique reactivity to bisulfite, achieving a detection limit of 1.8 μM with a linear relationship (R(2) = 0.9987). The specific detection was not interfered with other potential coexisted species. In addition, the probe is demonstrated for the determination of SO2 gas in aqueous solution as well as for visually monitoring of SO2 gas in air. This nanomaterial based probe is easily prepared, fast responding, and thus potentially attractive for extensive application for the determination of SO2 and other similar air pollutants.