Fluorescence detection of pesticides using quantum dot materials – A review

High pesticide use, especially in agriculture, can lead to environmental pollution and potentially adverse health effects. As result, pesticide residues end up in different media, including water and food products, which may serve as direct routes for human exposure. There is thus a continuous drive to develop analytical methods for screening and quantification of these compounds in the different environmental media in which they may occur. Development of quantum dot (QD) based sensors for monitoring pesticides has gained momentum in recent years. QD materials have excellent and unique optical properties and have high fluorescence quantum yields compared to other fluorophores. They have thus been used in numerous studies for the development of probes for organic pollutants. In this paper we specifically review their application as fluorescence probes for pesticide detection in different media including water and in fruits and vegetables. The low detection limits reported demonstrate the potential use of these methods as alternatives to expensive and time-consuming conventional techniques. We also highlight potential limitations that these probes may present when it comes to routine application. Finally we discuss possible future improvements to enhance the selectivity and robustness of these sensors. We note that there is still a need for researchers to develop standardized QD based sensors which could lead to their commercialization and routine application.     

Hard biaxial ellipsoids revisited: Numerical results

Monte Carlo simulations are undertaken for hard ellipsoids for a number of values of its semi-axes in the range c/a∈{0.1,10}$c/a\in \{0.1\text{,}10\}$. The isotropic phase results are compared to the Vega isotropic phase equation of state for hard biaxial ellipsoids [C. Vega, Virial coefficients and equation of state of hard ellipsoids, Mol. Phys. 92 (1997) 651]. The Vega equation of state is found to perform exceedingly well for all of the models studied. The biaxial phase is seen to form only after the previous formation of a discotic phase.     

碘和硫增感剂在卤化银颗粒表面的分布

碘在卤化银微晶中的含量和分布对感光度的影响引起乳剂配方设计者的广泛注意和重视.颗粒中的碘引起其内部结构和表面结构的改变,导致微晶颗粒表面化学及物理性质的改变,从而引起增感敏化斑的位置、分布以及随后潜影斑、灰雾斑的形成等一系列的变化并最终影响乳剂的感光性能.如果我们想有效地控制卤化银乳剂的感光度、灰雾、反差等一系列感光性能,必须调整卤化银乳剂颗粒的表面结构.     

检测活性氧的荧光探针新进展

活性氧对于人体是十分重要的。然而,过量的活性氧是相当有害的,它们会对人体产生氧化损伤,导致细胞死亡。活性氧现在已经引起了化学、生物、医药等多个领域学者的浓厚兴趣,它们被认为和多种病理条件有密切的联系。由于活性氧寿命短、反应活性高,并且大部分都存在于体内很难被捕获,因此它们的分析测定是一项国际性难题。荧光探针作为活性氧的高灵敏的检测分析物,已经得到越来越广泛和深入的研究。由于每一种活性氧都有它独特的生理学活性,因此设计高选择性的,能够检测具体一种活性氧的荧光探针分子就显得十分重要。本文主要对近三年来检测单线态氧、过氧化氢、超氧阴离子和羟自由基这四种活性氧的荧光探针的研究进展进行综述,关注这类荧光探针的检测机理以及具体应用。     

Preparation of barium hexaferrite nanopowders by the sol-gel method, using goethite

A series of barium hexaferrite nanoparticles (BaO·nFe₂O₃) with different n values were prepared by the sol-gel method, using goethite and Ba carbonate as raw materials. Phase identification of the samples was investigated by X-ray diffraction (XRD). XRD investigations show that the samples with n=5 and calcined at temperatures higher than 875 °C are single-phase Ba ferrite. An average crystallite size of 22 nm was obtained for the single-phase sample with minimum calcining temperature of 875 °C, using the Scherrer's formula. The morphology of the samples was checked by transmission electron microscope (TEM) and magnetic properties were measured by a sensitive permeameter. The results show that the samples have nonzero coercivities, which shows the particle size are not less than the critical size of Ba ferrite and then are not superparamagnet.     

Effect of hydrogen reduction on microstructure and magnetic properties of mechanochemically synthesized Fe-16.5Ni-16.5Co nano-powder

Most recent findings on structural and magnetic properties of Fe-Ni-Co nano-powders produced by mechanical alloying and subsequent low-temperature hydrogen reduction are presented in this paper. At 300 rpm, with ball to powder weight ratio of 20, single phase nickel-cobalt ferrite is mechanically synthesized for 50 h. The as-milled powder is then subjected to 1 h hydrogen reduction at 700 °C. Hydrogen reduction results in the formation of Fe-16.5%Ni-16.5%Co nano-powders. The phases of the powders are identified by X-ray diffraction (XRD) utilizing Cu Kα radiation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to study the morphology and the average size of the nano-powder particles. Chemical analysis of the phases present in the reduced sample is determined by electron dispersive spectrometry (EDS). The magnetic properties of the powders are measured by a vibration sample magnetometer (VSM). Results indicate a noticeable change in the magnetic properties of the samples due to Ni_0.5Co_0.5Fe₂O₄ compositional change into Fe-16.5Ni-16.5Co nano-powder.     

Synthesis and magnetic properties of La-substituted M-type Sr hexaferrites

Single-phase M-type hexagonal ferrites Sr_1−xLa_xFe₁₂O₁₉ (0≤x≤1) were prepared by a ceramic route. The stability limits of the ferrite phases were determined with a combination of various microscopy techniques, electron-probe micro-analysis, powder X-ray diffraction and thermal analysis. SrFe₁₂O₁₉ (x=0) is stable up to 1420 °C, whereas LaFe₁₂O₁₉ (x=1) exists between 1360 and 1400 °C only. The lattice parameters of Sr_1−xLa_xFe₁₂O₁₉ exhibit a linear variation with x, i.e. a₀ slightly increases and c₀ decreases with x, leading to a decrease of the unit cell volume with x. The saturation magnetization at T=5 K decreases with increasing La concentration. Room temperature Mössbauer analysis shows that the Fe³⁺/Fe²⁺ valence change occurs in the 2a sites for the whole composition range.     

Relative electron inelastic mean free paths for diamond and graphite at 8 keV and intrinsic contributions to the energy-loss

We used hard X-ray photoelectron spectroscopy (HAXPES) with 8 keV X-rays to investigate the 1s emission of carbon. We recorded spectra extending from the peak of the C 1s electrons (“elastic” line) to electrons with up to 110 eV energy-loss. Using two samples side by side, we could compare the inelastic mean free paths (IMFPs) of the electrons of almost 8 keV in diamond and graphite and find them to be practically identical despite about 50% difference in densities. Published extrapolations of their IMFP calculations at lower energies are in good agreement with this result. We show that information from the almost structureless region of overlapping multiple extrinsic energy-losses can be used to quantify the fraction of photoelectrons experiencing intrinsic energy-losses (those due to the sudden creation of the hole). We find that this fraction is 58% of the primary excited C 1s electrons for diamond and is practically the same for graphite. This is at first sight an unexpected result since hole-screening should differ in a semimetal from that in an insulator. The observation can be accounted for by dynamic screening in contrast to static screening.     

Modeling of failure characteristics of rectangular hard rock influenced by sample height-to-width ratios: A finite/discrete element approach

In the present study, a combined finite/element approach is used to analyze failure characteristics of a rectangular hard rock with different sample height-to-width ratios based on the uniaxial compression test. The objective of this paper is to illustrate the transitional mechanisms from shear fracture to extension failure of brittle rocks and the occurrence conditions of surface-parallel slabbing. Numerical results show that failure modes have experienced the shear, tension with shear and surface-parallel slabbing failure with the decrease of sample height-to-width ratios. It is seen from the simulation results that the shear band in larger height-to-width ratios is composed of an amount of extension meso-cracks that are parallel to the loading direction, and the slabbing failure tends to be the result of the accessible propagation of extension meso-cracks due to short sample height, which validate the essence of extension failure in hard rocks. The good agreement between simulation and laboratory testing also shows the practicability and advancement of a combined finite/discrete element approach.     

Emerging quasi-0D states at vanishing total entropy of the 1D hard sphere system: A coarse-grained similarity to the car parking problem

A coarse-grained system of one-dimensional (1D) hard spheres (HSs) is created using the Delaunay tessellation, which enables one to define the quasi-0D state. It is found from comparing the quasi-0D and 1D free energy densities that a frozen state due to the emergence of quasi-0D HSs is thermodynamically more favorable than fluidity with a large-scale heterogeneity above crossover volume fraction of ${\varphi }_c=e/(1+e)=0.731\cdots$ , at which the total entropy of the 1D state vanishes. The Delaunay-based lattice mapping further provides a similarity between the dense HS system above ${\varphi }_c$ and the jamming limit in the car parking problem.