A ratiometric fluorescence platform for lead ion detection via RNA cleavage-inhibited self-assembly of three-arm branched junction.

Heavy metal pollution can pose a threat to food safety and human health, and accurate quantification of heavy metal ions is a vital requirement. Emerging DNA nanostructures-based biosensors offer attractive tools toward ultra-sensitive or rapid analysis of heavy metal ions. However, the problems including complex design, severe reaction conditions and undesirable reliability are inevitable obstacle in advancing their extension and application. Herein, a ratiometric fluorescent platform was established for monitoring lead ion (Pb2+) in food based on dual Förster resonance energy transfer (FRET) and RNA cleavage-inhibited self-assembly of three-arm branched junction (TBJ). GR-5 DNAzyme was employed for Pb2+ recognition, and enzyme-free amplification technique catalytic hairpin assembly (CHA) served to form FRET probes-carried TBJ. The substrate strand (S) of DNAzyme triggered the generation of CHA-TBJ, and Pb2+-responsive cleavage of S hindered the assembly of CHA-TBJ, causing opposite changes in the FRET states of FAM/BHQ1 and ROX/BHQ2 pairs. The fluorescence responses were recorded through synchronous fluorescence spectrometry to indicate Pb2+ concentration, allowing sensitive and reliable identification of Pb2+ in the linear range of 0.05-5 ng mL-1 with the detection limit of 0.03 ng mL-1. The Pb2+ detection can be achieved under conventional reaction conditions, simple mixing procedures and one-step measurement operation. The approach can afford excellent specificity for Pb2+ against competing metal ions, and can be applied to analyze Pb2+ in tea samples with satisfactory results. This facile fluorescence platform shows a capable method for Pb2+ detection, and provides new avenue in the development of ratiometric approaches and DNAzyme strategies for monitoring heavy metal pollution, facilitating the transformation of DNAzyme-based biosensors for food safety control.

Platform Formed from ZIF-8 and DNAzyme: "Turn-On" Fluorescence Assay for Simple, High-Sensitivity, and High-Selectivity Detection of Pb2.

Lead contamination has posed a potential threat to the environment and food safety, arousing extensive concern. In this work, we fabricated a novel fluorescent sensing platform based on zeolitic imidazolate framework-8 (ZIF-8) and DNAzyme for monitoring Pb2+ in water and fish samples. ZIF-8 was proposed as a fluorescence quencher with the advantages of simple synthesis, low cost, and high quenching efficiency. The Pb2+-dependent GR5 DNAzyme containing the large ssDNA loop can be adsorbed onto ZIF-8 accompanied by fluorescence quenching. Upon binding with Pb2+, GR5 DNAzyme was activated and cleaved, leading to the release of FAM-labeled 5-base ssDNA, which restored the fluorescence. The "turn-on" assay can detect Pb2+ through the one-pot procedure in the range of 0.01-10.0 nM with a detection limit of 7.1 pM. The platform is promising for on-site monitoring of Pb2+ owing to the excellent performance of high sensitivity, low background, strong anti-interference ability, and simple operation.

Facile detection of melamine by a FAM-aptamer-G-quadruplex construct.

The development of a novel method for melamine detection that uses a FAM-aptamer-G-quadruplex construct due to the efficient quenching ability of an aptamer-linked G-quadruplex is reported herein. The construct, which is labeled with the fluorescent dye 6-carboxyfluorescein (FAM), consists of two parts: a melamine-binding aptamer and a G-rich sequence that can form a G-quadruplex structure. Because of the specific recognition of melamine by the T-rich aptamer, this aptamer folds into a hairpin structure in the presence of melamine, which draws the G-quadruplex closer to the FAM fluorophore, leading to the quenching of the fluorescence of FAM. Thus, a highly sensitive and selective fluorescence strategy for assaying melamine was established. Under optimal conditions, the fluorescence quenching is proportional to the concentration of melamine within the range 10-90 nM, and the method has a detection limit of 6.32 nM. Further application of the method to plastic cup samples suggested that it permitted recoveries of between 97.15% ± 1.02 and 101.92% ± 2.07. The detected amounts of melamine spiked into the plastic cup samples and the corresponding amounts measured by HPLC were in good accordance, indicating that this fluorescent method is reliable and practical. Owing to its high sensitivity, excellent selectivity, and convenient procedure, this strategy represents a promising alternative method of melamine screening. Graphical abstract.

Improving the viability and versatility of the E × B probe with an active cooling system.

A thermostatic E × B probe is designed to protect the probe body from the thermal effect of the plasma plume that has a significant influence on the resolution of the probe for high-power electric thrusters. An active cooling system, which consists of a cooling panel and carbon fiber felts combined with a recycling system of liquid coolants or an open-type system of gas coolants, is employed to realize the protection of the probe. The threshold for the design parameters for the active cooling system is estimated by deriving the energy transfer of the plasma plume-probe body interaction and the energy taken away by the coolants, and the design details are explained. The diagnostics of the LIPS-300 ion thruster with a power of 3 kW and a screen-grid voltage of 1450 V was implemented by the designed thermostatic E × B probe. The measured spectra illustrate that the thermostatic E × B probe can distinguish the fractions of Xe+ ions and Xe2+ ions without areas of overlap. In addition, the temperature of the probe body was less than 306 K in the beam region of the plasma plume during the 200-min-long continuous test. A thermostatic E × B probe is useful for enhancing the viability and versatility of equipment and for reducing uneconomical and complex test procedures.

Highly Sensitive Aptamer-Based Colorimetric Detection of Melamine in Raw Milk with Cysteamine-Stabilized Gold Nanoparticles.

In this paper, a rapid and facile visual method for detecting melamine in raw milk was developed by using T-rich single-stranded DNA (ssDNA) aptamers as the recognition element and cysteamine-stabilized gold nanoparticles (CS-AuNPs) as the color indicator. The electrostatic interaction between the negatively-charged melamine-binding aptamers and the positively-charged CS-AuNPs induced the aggregation of the CS-AuNPs and a remarkable change in color from wine red to blue. The specific recognition between melamine and T-rich aptamers could cause the formation of melamine-aptamer complex through hydrogen bonding. Compared with the electrostatic interaction between CS-AuNPs and aptamers, the strongly specific recognition between melamine and aptamers played a dominant role, thus the presence of melamine could prevent the aptamers-induced aggregation and color change of CS-AuNPs. As a result of this, using CS-AuNPs as the colorimetric probe, a simple and sensitive aptasensor for melamine determination is developed. Under the optimum conditions, the absorbance change (ΔA 526) of CS-AuNPs was linearly proportional to melamine concentration in the range of 1­24 nM, with the detection limit of 0.389 nM. The application of the proposed method for the determination of melamine in raw milk samples spiked with different amounts of melamine suggested satisfactory recoveries between 92.8% and 112.2%, which are in full accordance with the results from HPLC. Due to the advantages of low cost, high sensitivity, simple operation procedure, reduced detection time, and practical application, this method would be expected for on-site and real-time screening of melamine in raw milk.

An outbreak of type π vaccine-derived poliovirus in Sichuan province, China: emergence and circulation in an under-immunized population.

BACKGROUND: During August 2011-February 2012, an outbreak of type Π circulating vaccine-derived poliovirus (cVDPVs) occurred in Sichuan Province, China. METHODS: A field investigation of the outbreak was conducted to characterize outbreak isolates and to guide emergency response. Sequence analysis of poliovirus capsid protein VP1 was performed to determine the viral propagation, and a coverage survey was carried out for risk assessment. RESULTS: One clinical compatible polio case and three VDPV cases were determined in Ngawa County, Ngawa Tibetan and Qiang Autonomous Prefecture, Sichuan Province. Case patients were unimmunized children, 0.8-1 years old. Genetic sequencing showed that the isolates diverged from the VP1 region of the type Π Sabin strain by 5-12 nucleotides (nt) and shared the same 5 nt VP1 substitutions, which indicate single lineage of cVDPVs. Of the 7 acute flaccid paralysis cases (all>6 months) reported in Ngawa Prefecture in 2011, 4 (57.1%) cases (including 2 polio cases) did not receive oral attenuated poliovirus vaccine. Supplementary immunization activities (SIAs) were conducted in February-May, 2012, and the strain has not been isolated since. CONCLUSION: High coverage of routine immunization should be maintained among children until WPV transmission is globally eradicated. Risk assessments should be conducted regularly to pinpoint high risk areas or subpopulations, with SIAs developed if necessary.