Fluorescent Carbon Dots as Cost-Effective and Facile Probes for Caffeic Acid Sensing via a Fluorescence Quenching Process.

Caffeic acid (CA), a familiar color stabilizing reagent, has aroused general concern due to its uncontrolled addition, and thus the detection of CA is increasingly important. In our report, the bright carbon dots (CDs) were prepared via hydrothermal treatment with urea and citric acid act as raw material and their characteristics were discussed through X-ray diffraction (XRD), transmission electron microscopy (TEM) and so on. Impressively, the strong emission of the as-prepared CDs (Quantum Yield: 24.3%) decreased sharply upon a full reaction with the added CA. Hence, we first present an improved strategy for determining CA based upon the quenching of the strong emission of CDs. In this strategy, 0.79-100.0 µmol L- 1 caffeic acid could be simply detected, and a detection limit of 0.24 µmol L- 1 was allowed. Additionally, CA in red wine samples can be successfully detected by this method and the exploration of the quenching mechanism of the CA-CDs system was done.

Carbon dot-based fluorescent probes for sensitive and selective detection of luteolin through the inner filter effect.

As a flavonoid, luteolin (LTL) has been universally studied due to its many advantages. Luteolin is a component in drugs and accurate detection is important to determine their LTL content and estimate their disposal effects. In our study, we found that the strong fluorescence of CDs made using ammonium citrate with the quantum yield (QY) of 8.8%, could be quenched sharply by LTL through the inner filter effect (IFE), and therefore a fluorescence assay for LTL detection using CDs as probes was devised. In this fluorescence assay, the limit of detection (LOD) was 0.07 µmol L-1 with a dynamic range of 0.23-50.0 µmol L-1 . The CDs also possessed excellent selectivity and achieved acceptable recovery and relative standard deviation (RSD). Importantly, for use of environmentally friendly and nontoxic CDs as fluorescent probes to detect LTL, its operation, instruments and materials are cheap and easy. Therefore, this study improved the ease of LTL detection, and can be used widely.

Multiresponsive Color-Changing and Tough Hydrogels Enabled by Self-Assembled Epoxy Oligomer Microspheres.

The fabrication process of hydrogels often incorporates various strategies to achieve multiple responses and enhance strength, which always make the procedure complex and even hinder the incorporation. Here, we develop a facile and flexible method to simultaneously achieve multiresponsive color-changing and tough properties in hydrogels by introducing epoxy oligomer microspheres (DEPMS) to hydrophobic association (HA) hydrogels. DEPMS is responsive to both pH and solvents, showing color changes due to conversion to a conjugated structure. The obtained DEPMS composite hydrogels could demonstrate diverse color-changing patterns by simply adjusting the components and pH of the solvents. Meanwhile, amphiphilic DEPMS helps to disperse hydrophobic regions of the HA hydrogel, resulting in more uniform cross-linking and thereby contributing to the enhanced mechanical properties. The tensile strength and toughness of the composite hydrogels could be easily adjusted and reach 1.00 MPa and 11.18 MJ m-3, respectively. This work provides an approach to the design of multiple responsive and tough hydrogels while offering insights into the recycling of waste epoxy resins.

Water-solvent regulation on complete hydrolysis of thermosetting polyester and complete separation of degradation products.

As one of the largest productions of thermosetting plastics, unsaturated polyester resin (UPR) is difficult to be effectively chemcycled after it is discarded due to its dense network structure. Herein, we demonstrate a mild method for efficient alkaline hydrolysis of UPR into useful feedstocks in mixed solvents of polar aprotic solvent and a small amount of H2O by utilizing the fragmentation effect of the solvent on the UPR and the swelling effect of H2O on the subsequent partially hydrolyzed UPR respectively. The mixed solvents also play a key role in the aggregation and solubility of the degradation products. It is worth noting that the tetrahydrofuran (THF)-H2O system achieved 100 % separation of degradation products in an energy-efficient way taking advantage of the insolubility of the carboxylate-containing products in THF and the low boiling point of THF. The participation of non-reactive mixed solvents greatly promotes both the degradation and the separation process of thermosetting polymers.

Multifunctional Materials from Aerobic Degradation of Epoxy Thermoset via Phosphotungstic Acid-Involved Activation and Bonding.

The growing scale of production of wind turbines represents a big challenge for chemical recycling of amine-cured epoxy resin (EP) to achieve high-efficiency degradation and high-value utilization of degradation products. Here, H2 O2 /phosphotungstic acid (HPW) catalytic oxidation system is demonstrated to completely degrade EP thermoset with the solid recovery rate of 96% at a reaction temperature of 80 °C for 4 h. Owing to protonation and bonding effect of HPW to the amine groups, the degradation products had a weight-average molecular weight of 4285 with narrow molecular weight distribution. They were used as dye adsorption blend films and supramolecular adhesives based on hydrogen bonding and coordination bonding respectively. The work demonstrates a feasible and promising method to recover the EP thermoset into high-performance materials.

Constructing ultrathin FeS/FeOxH@Fe nano-sheets for highly efficient oxygen evolution reaction.

Exploring earth-abundant catalysts with ultra-high activity and durability are the decisive challenges for oxygen evolution reaction. This work prepared the FeS/FeOxH@Fe nanosheets as the efficient and stable electrocatalysts of oxygen evolution reaction (OER) through a simple one-step co-deposition method. The FeS/FeOxH@Fe exhibited small overpotentials of 245, 376 and 482 mV at the current density of 10, 500 and 1000 mA cm-2 without iR-compensations in 1.0 M KOH solution, respectively. Constructing amorphous structure and the interface between amorphous and crystal can obviously improve the conductivity of FeOxH, which is beneficial to the improvement of catalytic performance. This work may provide an effective and controlled strategy to design highly active OER catalysts with an interface structure between amorphous and crystal by a well-designed co-deposition.

Synthesis of high-performance sodium carboxymethyl cellulose-based adsorbent for effective removal of methylene blue and Pb (II).

In this work, a biobased adsorbent (CMC-PAMA) consisting of sodium carboxymethyl cellulose, polyacrylic acid and polyacrylamide was prepared by a simple thermal crosslinking method for the removal of methylene blue (MB) and Pb (II) from aqueous solutions. The CMC-PAMA showed excellent static adsorption capacities for MB and Pb (II) of 1611.44 mg·g-1 840.11 mg·g-1, respectively, indicating its excellent performance on dyes dynamic removal. The adsorption performance of the as-prepared CMC-PAMA could be fitted well with pseudo-second-order and Langmuir isothermal model. The adsorption mechanism between CMC-PAMA and Pb (II) was confirmed by FT-IR, XPS and ion-exchange behavior. Interestingly, CMC-PAMA showed high efficiency for separation of MB/methyl orange (MO) mixtures, low equilibrium swelling ratio (0.52 g·g-1) in distilled water, good stability and reusability. With those combined advantages, CMC-PAMA could find potential application in wastewater treatment.

Fluorescent carbon dots synthesized by microwave-assisted pyrolysis for chromium(VI) and ascorbic acid sensing and logic gate operation.

Herein, nitrogen, sulfur co-doped fluorescent carbon dots (N,SCDs) were synthesized by simple and facile microwave-assisted pyrolysis using ammonium citrate and cysteamine hydrochloride as precursors. The obtained N,SCDs exhibited outstanding photostability, excitation-wavelength independence, excellent fluorescence properties with fluorescence quantum yield (FLQY) up to 54.8%. Significantly, the fluorescence of N,SCDs was effectively quenched by Cr(VI) based on inner filter effect (IFE). Subsequently, the fluorescence of the N,SCDs-Cr(VI) system successfully recovered with addition of ascorbic acid (AA) owing to redox reaction between Cr(VI) and AA. Therefore, N,SCDs could be employed as an efficient fluorescent "turn-off" probe for highly sensitive detection of Cr(VI), with a liner detection range from 0.35 to 126.0 µmol L-1 and a detection limit of 0.11 µmol L-1. Meanwhile, the N,SCDs-Cr(VI) system could be used as an fluorescent "turn-on" sensor for detection of AA with a detection limit of 0.17 µmol L-1 and a liner detection range of 0.57-264.0 µmol L-1. Interestingly, a molecular logic gate was constructed based on the fluorescence recover of the N,SCDs-Cr(VI) system in the presence of AA. Most importantly, this fluorescent probe has been applied for determination of Cr(VI) and AA in environmental and biological fields with satisfactory results.

Carbon dots synthesized by hydrothermal process via sodium citrate and NH4HCO3 for sensitive detection of temperature and sunset yellow.

In this work, the carbon dots (CDs) were synthesized by an one-pot portable hydrothermal method from sodium citrate and NH4HCO3. The obtained CDs were found to show high fluorescence quantum yield (31%), excitation-wavelength independence, outstanding water-soluble, excellent chemical and optical stability. Interestingly, it was discovered that sunset yellow (SY) was sensitive and effective in quenching the fluorescence of CDs. Based on this observation, CDs could be applied as a novel fluoescent probe for detection of SY. The analytical potential of this probe was evaluated and illustrated a good linear relationship at the concentration range from 0.25 to 56.0 µmol L-1 with a detection limit (3σ/k) of 74.49 nmol L-1. Meanwhile, the practicality of this nanosensor for SY determination in real samples was validated with good accuracy and precision. In addtion, long time UV radiation and high ionic strength hardly influenced on the fluorescence intensity of CDs. However, the fluorescence intensity of CDs decreased at higher temperature and the fluorescence was recovered when temperature back to 5 °C. Therefore, CDs could be also employed as an fluorescent temperature sensor with significant sensitivity, linearity and remarkable reversibility.