| Soil is a good habitat for various microbial live and reproduction. Aspergillus flavus is one of the commonest fungi in soil, its detection rate can reach more than80%, especially in the soil which cultivates peanuts and corn. The soil Aspergillus flavus can produce aflatoxins in their metabolic process, which have adverse effects on plant growth and harvest. It should be noted that70%of the Aspergillus flavus isolated from the soil can produce aflatoxin.Aflatoxin is a kind of compounds containing similar chemical structure. Its molecular structure includes double furan rings and coumarin. Usually, there are six kinds of Aflatoxin. Among them, the AFB1are the most widely existed, the most toxic and the strongest carcinogenicity one. Iron oxide is a common mineral and distributes widely in soil, which has an important effect on the process of adsorption, migration and transformation of soil AFB1. Iron oxide is the highest levels among all metal oxide in soil. It contains different types such as goethite, hematite, lepidocrocite and magnetite. All kinds of iron oxide could interact with the soil humus, and hence form organic inorganic compounds which could further affect the adsorption and biodegradation of organic pollutants and fungal toxins in soil. Meanwhile, the heavy metal ions, e.g. Pb2+, Cu2+, Zn2+, can also cause an important effect on series of chemical process in soil. In order to understand the environmental behavior of biological toxins in the soil, systematic understand the adsorption, transformation and fate of AFB1in soil, and reduce the pollution and harm of AFB1to agricultural product, explore pollution control methods of AFT in the return to soil such as straw, We establish the study goal of adsorption of AFB1in iron oxide, iron oxide-humic acid and iron oxide-humic-Pb-+system in this paper.Four common iron oxides in soil, i.e. goethite, hematite, lepidocrocite and magnetite, synthesised by the hydrothermal method, are employed to investigate the influence of different reaction time on the crystallinity of targeted crystal. Humic acid and fulvic acid are used to study on the modification of synthesised iron oxides. The XRD spectrum showed that humic acid and fulvic acid can not change the crystalline structure of iron oxide, but there are some changes in the details of its XRD spectrum. Due to the effect of humic acid and fulvic acid, a reduced diffraction intensity and a wider diffraction peak emerged in the XRD spectrum of goethite, hematite and magnetite crystal, while an increased diffraction intensity and a wider diffraction peak appear in the XRD spectrum of lepidocrocite, indicating that the interaction between the iron oxides and humic acid and fulvic acid must exist, and the influence of humic acid on the nature of iron oxide is greater than that of the fulvic acid. The properties of lepidocrocite affected by humic acid are the largest among the four kinds of iron oxides, followed by goethite, and magnetite is the least one. The crystal shape, aggregation structure, and the surface change of the reaction product which is produced by the four kinds of synthesised iron oxides and humic acid and fulvic acid are characterized by SEM and AFM. The characterization of SEM and AFM displayed that neither humic acid nor fulvic acid can change the crystal original configurations of the iron oxide, but they can affect both the state of crystal aggregation and the surface properties of the crystal via the interaction with iron oxide crystal.In the present work we investigated the adsorption of AFB1by four different iron oxides and their compounds modified by humic acid and fulvic acid, and the system of "iron oxide-humus-Pb2-" through several aspects, such as adsorption capacity, adsorption dynamics, adsorption isotherm and the impacts of heavy metal ions to preliminarily identify the adsorption patterns of AFB1by iron oxides.The adsorption of AFB1by goethite, hematite, lepidocrocite and magnetite are similar, in which the adsorption increased with the increasing of initial concentration of AFB1in solution. At a lower concentration, the adsorption capacity were ranked as: goethite> lepidocrocite> hematite. At a higher concentration of AFB1, the adsorption capacity followed the series:goethite, magnetite>lepidocrocite>hematite. The AFB1adsorption capacity by four kinds of iron oxides was correlated with the surface area, to some extent, but is not completely positive correlated. Lepidocrocite has the largest surface area, however, the adsorption capacity was not the biggest at a lower initial concentration of AFB1. While magnetite had the smallest surface area but the adsorption capacity was higher than that of hematite. All above indicate that the AFB1adsorption by iron oxides is not only resulted from physical adsorption but more complicated considering some chemical processes, such as the reactions of coordination and complexion is involved.As to the compounds of iron oxides modified by humic acid and fulvic acid, the impact of the later one was smaller. The adsorption curves of AFB1by the four kinds of iron oxides and the compound modified by fulvic acid were similar, showing that the adsorption capacity increases with the reaction time. This result suited the features of zero-order kinetics in superficial adsorption. In the range of1-8h after the start of adsorption experiments, there exist both the adsorption and desorption phenomenon. The features of iron oxides changed intensely after being modified by humic acid, which brought a great change of the adsorption curve compared with that of the original iron oxides. Before the point of12h, the adsorption increased slowly with time. After that the adsorption rate increased sharply and cannot reach the adsorption equilibrium within the experimental period.The experimental data were well fitted by Langmuir and Freundlich adsorption isotherm. The adsorption processes of goethite, hematite, lepidocrocite and magnetite were all Freundlich-pattern. After modified by humic acid and fulvic acid, the adsorption isotherms changed dramatically. Among them, the iron oxides modified by humic acid were the most prominent one and their adsorption isotherms changed from Freundlich-pattern to Langmuir-pattern; while the fulvic-modified ones retain the Freundlich-pattern. Goethite, hematite, and magnetite modified by humic acid, and goethite modified by fulvic acid were Langmuir-pattern; while hematite, lepidocrocite and magnetite modified by fulvic acid and lepidocrocite modified by humic acid were Freundlich-pattern.When the Pb2+were added into the compound of iron oxide-humus, the "iron oxide-humus-Pb2+" system can be naturally formed. This study investigated the adsorption of AFB1in "iron oxide-humus-Pb2+"system, and also compared the effects of two kinds of heavy metals Cu2+ã€Zn2+on the adsorption of AFB1by the iron oxides. These three kinds of heavy metals all showed inhibition in the process of adsorption of AFB1, and the inhibition ability increased with the concentration increasing. Among the three heavy metals, the inhibition effects showed Zn2+> Cu2+>Pb2+. However, the inhibition effects differed among different iron oxides. Comparing the effects of heavy metals on iron oxides and their humus-modified compound, the inhibition serial is iron oxide>fulvic-modified>humic-modified.We found that the content of Pb2+and AFB1were quite low because of the strong adsorption of the compound of iron oxide-humus. This conclusion has also been confirmed by previous studies. However, as a strong toxic substance, it is necessary to detect the very lowest limit in soil and other system; even though their detection could be often disturbed by some other sunstance. Therefore, it is of great practical value to establish a new method to detect the Pb2+and AFB1sensitively, easily and fast in soil system. Compared with the traditional method, the modern immune analysis method, based on the special biochemical reaction, is a simple and efficient way for Pb2+and AFB1test. Consequently, in our present work we established a new method using biosensor to detect the Pb+and AFB1in the complicated soil system.In this work we synthesized a kind of Ag core Au shell nano-composite, which is used to tag anti-AFB1original antibody and served as the detection elements, and establish a new lateral flow immunoassay test strip for AFB1detection. The nitrocellulose membrane whose aperture is0.2μm is adopted as reaction pad for immunoassay test strip. BSA-AFB1is coated in the detection line and its optimal quantity is1μg·cm-1, with1μL·cm-1undiluted goat-anti-mouse IgG coated in the control line. The lower limit of detection of immunoassay test strip is0.1ng·mL-1, and the sensitivity of this method is10times higher than that of the traditional colloidal gold immunochromatographic assay. In the actual sample detection, compared with the classical ELISA method, the result of the detection which standard sample added is normal and good reproducibility and stability, is achieved in the new method. The assemly of the test strip is simple and no complicated mark and separation step is needed; Furthermore, the operation is easy, fast, without sophisticated instruments and equipments and skilled operators in the new method. The new method provides a simple, quick and secure approach of the screening of AFB1for the remediation of contaminated soil, the management of straw mulching, analysis and monitoring of food security, which is of great potential commercial value.Based on the modification of DNAzyme and the gold-nanoparticle labled probe as the strategy of signal amplification, the method to detect Pb2+in soil and environment based on the individual glucose meter was constructed. The glucose amylase was immobilized combining with gold probe via the cleavage of DNAzyme induced by Pb2+ specifically in the method. Thus, the method make the detection of Pb2+come true by the reading of the individual glucose meter according to the amount of starch hydrolyzed by glucose amylase. This method of high sensitivity tested by enzymatic catalysis can be used to detect the extremely small amount of Pb2+in the environment. Its lower limit of the detection is one thousand times as low as that reached with the traditional methods, which is especially useful in areas such as food safety. It is a selective method with high sensitivity, simple operation and lower cost, and is also suitable for primary laboratories and the general public. The establishment of this method has provided an alternative simple, sensitive and cheap detection scheme of Pb2+which can be applied to various fields such as soil safety, environmental testing, and food assessment and so on.In conclusion, the highlights in this study mainly contains:1) the humic acid-modified and fulvic acid-modified iron oxides showed differences by their XRD and SEM, which indicated the effects of humic acid on the superficial properties of iron oxide were more significant than that of fulvic acid; The adsorption of AFB1by iron oxide and their humus-modified compounds was investigated and the adsorption patterns were preliminarily specified. These results provide important basis for further investigation the fate of AFB1in soil and environment, and also shed a new light on on degradation and detoxification of AFBl adsorbed by agricultural products.2) based on the new kind of immunoassay test strip we developed in this study, a new kind of immunochromatography for AFB1test in soil and food was established;3) a quick and sensitive method using a portable blood-glucose meter to detect Pb2+in soil environment was constructed based on the modification of DNAzyma and the gold-nanoparticle labeled probe as the strategy of signal amplification. |
