Shi Gang Liu

Water-Soluble Nonconjugated Polymer Nanoparticles with Strong Fluorescence Emission for Selective and Sensitive Detection of Nitro-Explosive Picric Acid in Aqueous Medium

Water-soluble nonconjugated polymer nanoparticles (PNPs) with strong fluorescence emission were prepared from hyperbranched poly(ethylenimine) (PEI) and d-glucose via Schiff base reaction and self-assembly in aqueous phase. Preparation of the PEI-d-glucose (PEI-G) PNPs was facile (one-pot reaction) and environmentally friendly under mild conditions. Also, PEI-G PNPs showed a high fluorescence quantum yield in aqueous solution, and the fluorescence properties (such as concentration- and solvent-dependent fluorescence) and origin of intrinsic fluorescence were investigated and discussed. PEI-G PNPs were then used to develop a fluorescent probe for fast, selective, and sensitive detection of nitro-explosive picric acid (PA) in aqueous medium, because the fluorescence can be easily quenched by PA whereas other nitro-explosives and structurally similar compounds only caused negligible quenching. A wide linear range (0.05-70 μM) and a low detection limit (26 nM) were obtained. The fluorescence quenching mechanism was carefully explored, and it was due to a combined effect of electron transfer, resonance energy transfer, and inner filter effect between PA and PEI-G PNPs, which resulted in good selectivity and sensitivity for PA. Finally, the developed sensor was successfully applied to detection of PA in environmental water samples.

Carbon quantum dots prepared with polyethyleneimine as both reducing agent and stabilizer for synthesis of Ag/CQDs composite for Hg2+ ions detection

A stable silver nanoparticles/carbon quantum dots (Ag/CQDs) composite was prepared by using CQDs as reducing and stabilizing agent. The CQDs synthesized with polyethyleneimine (PEI) showed an extraordinary reducibility. When Hg2+ was presented in the Ag/CQDs composite solution, a color change from yellow to colorless was observed, accompanied by a shift of surface plasmon resonance (SPR) band and decrease in absorbance of the Ag/CQDs composite. On the basis of the further studies on TEM, XPS and XRD analysis, the possible mechanism is attributed to the formation of a silver-mercury amalgam. Hence, a two dimensional sensing platform for Hg2+ detection was constructed upon the Ag/CQDs composite. Based on the change of absorbance, a good linear relationship was obtained from 0.5 to 50μM for Hg2+. And the limit of detection for Hg2+ was as low as 85nM, representing high sensitivity to Hg2+. More importantly, the proposed method also exhibits a good selectivity toward Hg2+ over other metal ions. Besides, this strategy demonstrates practicability for the detection of Hg2+ in real water samples with satisfactory results.

pH-Mediated Fluorescent Polymer Particles and Gel from Hyperbranched Polyethylenimine and the Mechanism of Intrinsic Fluorescence.

We report that fluorescence properties and morphology of hyperbranched polyethylenimine (hPEI) cross-linked with formaldehyde are highly dependent on the pH values of the cross-linking reaction. Under acidic and neutral conditions, water-soluble fluorescent copolymer particles (CPs) were produced. However, under basic conditions, white gels with weak fluorescence emission would be obtained. The water-soluble hPEI-formaldehyde (hPEI-F) CPs show strong intrinsic fluorescence without the conjugation to any classical fluorescent agents. By the combination of spectroscopy and microscopy techniques, the mechanism of fluorescence emission was discussed. We propose that the intrinsic fluorescence originates from the formation of a Schiff base in the cross-linking process between hPEI and formaldehyde. Schiff base bonds are the fluorescence-emitting moieties, and the compact structure of hPEI-F CPs plays an important role in their strong fluorescence emission. The exploration on fluorescence mechanism may provide a new strategy to prepare fluorescent polymer particles. In addition, the investigation shows that the hPEI-F CPs hold potential as a fluorescent probe for the detection of copper ions in aqueous media.

A new fluorescent sensor for detecting p-nitrophenol based on β-cyclodextrin-capped ZnO quantum dots

In this work, the fluorescent β-cyclodextrin-capped ZnO quantum dots (β-CD@ZnO QDs) with a good water-solubility have been synthesized via a facile method. And the synthesized β-CD@ZnO QDs with a diameter ∼3.64 nm and a yellow fluorescence have been characterized by high resolution transmission electron microscope, UV-visible absorption, photoluminescence, and Fourier transform infrared spectroscopy. The β-CD@ZnO QDs were used as a fluorescent sensor for the detection of p-nitrophenol. A linear relationship to the concentration of the p-nitrophenol over the range of 1.0–40 μM with the detection limit of 0.34 μM was obtained. The sensing property of the β-CD-capped ZnO QDs has been successfully examined in real water samples. And the quenching mechanism was proposed, which showed that the quenching effect may be caused by the inclusion complexation between β-CD and p-nitrophenol and electron transfer.

A Sensitive “Turn-On” Fluorescent Sensor for Melamine Based on FRET Effect between Polydopamine-Glutathione Nanoparticles and Ag Nanoparticles

In this work, Ag nanoparticles (AgNPs) were synthesized quickly by a one-step method utilizing polydopamine-glutathione nanoparticles (PDA-GNPs) as a reducing agent. The PDA-GNPs and the generated AgNPs acted as the energy donor and acceptor, respectively. Accordingly, the fluorescence of PDA-GNPs was quenched on the basis of fluorescence resonance energy transfer (FRET). In the presence of melamine, the preferential combination of Ag(I) and melamine to form Ag(I)-melamine complex prevents Ag(I) from forming AgNPs, together with fluorescence enhancement compared with the absence of melamine. Under the optimal conditions including the concentration of AgNO3, reaction time, reaction temperature, and pH, the fluorescence enhancement efficiency has a linear response to the concentration of melamine from 0.1 to 40 μM with a detection limit of 23 nM for melamine. The proposed method is simple, time-saving, and low-cost, which was further applied to detect melamine in real milk products with satisfactory results.

Size-dependent modulation of fluorescence and light scattering: a new strategy for development of ratiometric sensing

Simultaneous response of fluorescence and light scattering can be obtained by using nanomaterials with size- and shape-dependent physiochemical properties. On the basis of this principle, we report a new strategy to design ratiometric sensors by combining fluorescence and light scattering, two different and independent signals. To obtain fluorescence and scattering signals simultaneously under the same excitation, two signal collection strategies are proposed based on the principles of fluorescence, light scattering and diffraction. One is to collect normal (down-conversion) fluorescence and second-order scattering (SOS) signals, and the other is to record the fluorescence excited by the second-order diffraction light of excitation wavelength λ/2 (SODL-fluorescence) and first-order scattering (FOS) or frequency doubling scattering (FDS) signals. A proof of concept study has been performed by using a carbon dots (CDs) and cobalt oxyhydroxide (CoOOH) nanoflakes system for ascorbic acid (AA) sensing. Apart from construction of ratiometric sensors, the combined fluorescence and scattering can also act as a useful technique to monitor aggregation-induced fluorescence quenching or enhancement.

Turn-on fluorescence detection of pyrophosphate anion based on DNA-attached cobalt oxyhydroxide

The turn-on fluorescence of pyrophosphate anion (PPi) was detected using DNA-attached cobalt oxyhydroxide. The fluorescence signal was quenched when carboxyfluorescein-labeled ssDNA was adsorbed by CoOOH nanoflakes through electrostatic interaction. With the addition of target PPi, PPi competed with DNA for CoOOH nanoflakes, resulting in turn-on fluorescence.

Facile synthesis of multicolor photoluminescent polymer carbon dots with surface-state energy gap-controlled emission

The facile and effective synthesis of truly multicolor photoluminescent (PL) polymer carbon dots (PCDs) is still a considerable challenge. Here, a mild and facile synthetic method to prepare multicolor emissive PCDs by self-oxidation and autopolymerization using hydroquinone and ethylenediamine as precursors is reported. The resulting PCDs were separated via silica-gel column chromatography, which gave three types of PCDs emitting bright and stable yellow, green, and blue fluorescence. The three types of PCDs exhibited attractive features such as excellent solubility, excitation wavelength-independent photoluminescence, high stability, monoexponential fluorescence lifetimes, and homogeneous optical properties. The uniform optical properties of the as-prepared PCDs helped us to explore their multicolor emissive PL mechanism. Detailed characterization and investigation of the PCDs revealed that their surface state was primarily responsible for the observed multicolor emissive photoluminescence. A PL mechanism that involved control of the energy gap by the surface state was proposed to explain the effect of the surface functional groups on the PL properties of the PCDs. The emission of the PCDs was strongly associated with the CN functional groups on their surface. The band gap narrowed with increasing content of CN on the PCD surface, resulting in the red shift of the PL emission peak.

A fluorescence and colorimetric dual-mode assay of alkaline phosphatase activity via destroying oxidase-like CoOOH nanoflakes

Nanozymes are increasingly exploited as components in bioanalysis and diagnostics. Here, we report a fluorescence and colorimetric dual-mode assay for alkaline phosphatase (ALP) activity employing oxidase-like cobalt oxyhydroxide (CoOOH) nanoflakes. Colorless o-phenylenediamine (OPD), a substrate for oxidase, can be oxidized to a product (OxOPD) with yellow color and orange fluorescence in a CoOOH nanoflake solution. But, ascorbic acid (AA) is able to reduce CoOOH to cobalt ion (Co2+), which causes the decomposition and collapse of the CoOOH nanoflakes, and thereby the CoOOH nanoflakes are deprived of the oxidase-like property. Based on this principle, the fluorescence and colorimetric dual-mode detection of AA was achieved. Alkaline phosphatase (ALP) can make l-ascorbic acid-2-phosphate (AAP) hydrolyze to yield AA. As a result, with the help of AAP, the selective and sensitive dual-mode assay of ALP activity has been realized successfully by using fluorescence and UV-vis absorption spectroscopies. Quantitative analysis of ALP in human serum samples and an ALP inhibitor investigation were performed using this sensing system. Given the economical and sensitive properties, the proposed method based on CoOOH nanoflakes has great potential for not only probing ALP activity in biological systems but also screening potential ALP inhibitors. Meanwhile, in this study, a new insight has been provided into the application of CoOOH nanoflakes in the development of sensitive and convenient sensors.

A Label-free, Highly Sensitive and Selective Detection of Hemin Based on the Competition between Hemin and Protoporphyrin IX Binding to G-Quadruplexes.

Herein is reported a simple and label-free fluorescent detection method for hemin based on using protoporphyrin IX (PPIX) as a fluorescent signal reporter. PPIX emits weak fluorescence in an aqueous solution. When PPIX binds to G-quadruplexes, the fluorescence intensity of PPIX is greatly increased. While in the presence of target hemin, hemin competes with PPIX toward G-quadruplexes because its affinity to G-quadruplexes is higher than that of PPIX. With the formation of the hemin-G-quadruplex complex, PPIX is released to the solution from the G-quadruplex accompanied by quenching of the fluorescence of the system. This fluorescence change of the system can be used to monitor hemin with a low detection limit of 36 nM. In addition, the possible binding sites for PPIX binding to the G-quadruplex are discussed based on competition between hemin and PPIX. What is more, this method might pave the way for applying G-quadruplexes and PPIX to more sensing systems.