The Research On Characteristic Of Soil Humic Acid And Chemical Sensor

Soil is not only the loose surface in which material cycle and energy exchange occurs between the biological and environmental materials those have a life activity, but also the biogeochemical materials that be able to grow complex plants. Soil organic matter are the basic materials with biological properties and structures, not only the conditions but also the products of life activity. Humic substances (HS), a complex mixture of partially "decomposed" and otherwise transformed organic materials, are widely distributed in soil, natural water (e.g. lake, river, stream, sea and groundwater) and sediments, and contain a lot of functional groups such as carboxylic, phenolic, and methoxy groups. The size, chemical composition, structure, functional groups, and polyelectrolytic characteristics of HS may vary greatly, depending on the origin and age of the material.According to the solubility in acid and alkaline solution, HS can generally be divided into three main components:(1) Humic acid, HA, also known as brown rot acid, is soluble in dilute alkali solution, but flocculating in acidified alkaline extract. (2) Fulvic acid, FA, can dissolve in acid and alkaline solution. (3) Humin, Hu, the humus components, can not be extracted from soil samples in alkali and acid.HA, an important member of HS, plays a critical role in the fate and transportation of many toxic organic or inorganic chemicals and in nutrient cycling throughout the environment, which also has a great effect on the transport and transformation of both heavy metals and organic contaminants. The study on structural characteristic of HA will be helpful for us to know these reaction and processes due to the fact that the formation process of HA is quite complex. A better understanding of the structural and functional properties of HA may further improve our understanding of the intrinsic mechanisms of its complexation, reduction, mobilization or immobilization with heavy metals, radionuclides, pesticides, and other toxic chemicals. This, in turn, may improve our predictive capabilities of the behavior of HA and environmental pollutants in natural ecosystems.No single analytical tool, however, can provide definitive structural or functional information about HA because of its heterogeneous and ill-defined nature. The knowledge of the chemistry and reactions of HS may be possibly obtained only by the combination of a variety of spectroscopic and wet-chemical techniques. Many instrumental methods have been used to study the characteristics of HA, such as the nuclear magnetic resonance (NMR), Fourier-transform infrared spectrum (FTIR), surface enhanced Raman spectroscopy (SERS), Ultra Violet/Visible Spectrophotometry (UV-Vis), electron paramagnetic resonance (EPR), immobilized metal ion affinity chromatography (IMAC), High-pressure size exclusion chromatography (HPSEC), etc. Among analytical characterization methods, nondestructive spectroscopic techniques appear to be most useful, and they are directly applicable to both solid and liquid sample analysis. These methods have a number of attractive advantages including:(a) they are nondestructive; (b) only small amounts of samples are needed; (c) they are experimentally simple and do not require special manipulative sample preparation; and, perhaps most importantly, (d) they provide valuable information on molecular structure and chemical or functional properties of HA.At present, various kinds of fluorescence spectrum technology such as fluorescence excitation spectrum, fluorescence emission spectrum, synchronous fluorescence spectrum (SFS) and three-dimensional fluorescence spectrum (or three-dimensional excitation emission matrix fluorescence spectroscopy,3DEEM) are being used for determining the nature and physical chemical characteristic of HA. 3DEEM is a very useful spectrum fingerprint technology, because it can provide information of fluorescence intensity about HA when the excitation and emission wavelength are changed at the same time. By using of this technology and other related technologies, the more detailed properties about red soil humic acid and black soil humic acid were researched in this paper, and the similarities and differences between them were given. Meanwhile, in order to study the interaction between humic acid and heavy ions in soils and environment or biological molecules conveniently, photoeletrochemical sensors were prepared for detecting the heavy ions in soil and bio-molecules.Chemical sensor, combining the e-science, chemical sciences and materials science in one, is the high-tech products which belongs to the State 95 scientific and technological research and encouraged foreign investment projects. It can transform the sensitive device composed of chemical substance atfer molecular design, with quantitative and high selectivity, into monitorable electro-optical signals which then be analyzed and processed for environmental information about the chemical substances.Chemical sensors, because of its very high sensitivity, good selectivity and easy to carry, easy miniaturization, on-site analysis and monitoring, can be more and more used as prediction and automatic control devices for flammable, explosive, toxic and harmful gas detection in mining, petrochemical, bio-medicine and everyday lives. Chemical sensors are also used for determining a variety of substances with very low concentration, and even for ions concentration in cells. In medicine, people use the chemical sensors for diagnosis and treatment process automation. The widespread demand for chemical sensors in all areas lead to chemical sensor's research and development be very active and which has shown a very broad application prospects.This first chapter in this paper gives a overview about the definition, classification, elemental composition, functional groups and structure, nature and role of humic acids. Chapter 2 briefly describes the distribution, features and soil formation processes of two typical soils(red and black soils) in China.In chapter 3, Humic Acids (HAs) were extracted from two typical soils in China including red soil and black soil. The HAs were characterized jointly with elemental analysis (EA), scanning electron microscopy (SEM), ultra voilet (UV) spectroscopy, FTIR and 3DEEM. The results showed that HAs extracted from different soils were composed of C, H,O and N, and they have almost the same chemical groups as carboxyl, hydroxyl and much aromatic unsaturated substances, but the content of these groups is varied. The content of aromatic unsaturated substances and polysaccharides or polysaccharide-like substances of HAs from red soil is relatively high, while the content of aromatic or polyphenolic functional groups of HAs from black soil is larger. The SEM images indicate that two HAs are sheet-like regular amorphous aggregates, the surface of HAs extracted from black soil is smooth, while those of HAs from red soil is rough and many small particles can be observed on it. The 3DEEM showed that the concentration of soil HAs affected the numbers of fluorescence peak, and arose the red shift of the Ex/Em maximum wavelength. The effect of ionic strength on the 3DEEM of soil HA was neglectable. The effect of pH on the fluorescence intensity of black soil humic acid (BHA) was greater than that of red soil humic acid (RHA). The fluorescence quantum yields and fluorescence index of RHA were bigger than that of BHA at the same condition, which can be used to distinguish BHA and RHA.In chapter 4, the fluorescence characteristics of the complexes of both RHA and BHA with heavy metals were studied. The results indicate that ionic strength over the range from 0 to 0.05 mol/L NaNO3 did not affect the 3DEEMs of RHA and BHA. The concentration of HAs and pH of the test solution had obvious effects on the 3DEEM. The complexation abilities of the Cu-RHA and Cu-BHA are all stronger. The average complex ratio of Cu-RHA is 1:1.22, the complex constant is 1.6×106, and the average complex ratio of Cd-RHA is 3.2:1, the complex constant is 1.5×107. The average complex ratios of Cu-BHA and Cd-BHA are 1:1.1,1:1.5, and the complex constants of Cu-BHA and Cd-BHA are 1.25×105 and 3.5×103, respectively.In chapter 5,p-aminothiophenol (PATP) and humic acids (HA or HAs) were applied jointly as the electron transfer accelerants of redox reactions of cytochrome c (Cyt c) on Au electrodes. The electrochemical properties of the modified electrodes were studied carefully by field emission scanning electron microscope (FESEM), UV-Vis, electrochemical impedance spectroscopy (EIS), Raman spectroscopy (RS) and cyclic voltammetry (CV). The immobilized Cyt c displayed a couple of stable and well-defined redox peaks with a formal potential of -0.101 V (vs. SCE) in pH 7.0 c). Cyt c adsorption is of a monolayer with average surface coverage of 5.28 pmol/cm2. The electron transfer rate constant was calculated to be 2.14 s-1. It indicated that the HA film acted as a good adsorption matrix for Cyt c and an excellent accelerant for the redox of Cyt c.In chapter 6, PATP and dsDNA were applied jointly as the electron transfer accelerants of redox reactions of cytochrome c (Cyt c) on Au electrodes. The electrochemical properties of the modified electrodes were studied carefully by field emission scanning electron microscope (FESEM), ultraviolet-visible spectroscopy (UV-Vis), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The immobilized Cyt c displayed a couple of stable and well-defined redox peaks with a formal potential of -0.106 V (vs. SCE) in pH 7.0 phosphate buffer solution (PBS). Cyt c adsorption is of a monolayer with average surface coverage of 3.70 pmol cm-2. The electron transfer rate constant was calculated to be 2 s-1. It indicated that the ssDNA film acted as a good adsorption matrix for Cyt c and an excellent accelerant for the redox of Cyt c after hybridization with the target sequence DNA. The PATP-ssDNA-Cyt c modified gold electrode will probably be an alternative method of detecting hybridization of target DNA.In chapter 7, an amperometric immunosensor was constructed by dispersing graphite, schistosoma-japonicum antigen (SjAg) and silica sol-gel at low temperature. The performance characteristics of the prepared immunosensor. were examined in the buffer solution of o-aminophenol (o-AP) used as a substrate. It exhibited excellent physical and electrochemical stability with a renewable external surface. A competitive binding assay was employed to determine schistosoma-japonicum antibody (SjAb) with the aid of horseradish peroxidase labeled SjAb (HRP-SjAb). The experimental parameters for SjAb assay were optimized, including the amount of labeled SjAb in incubation solution, incubation time, temperature and the pH of solution. The use of o-AP substrate and amperometric detection at-250 mV (vs. SCE) results in a determination limit of 0.32μg/ml and a linear range extending up to 0.18μg/ml. The results of SjAb assay in serum samples demonstrate the feasibility of using the proposed immunosensor for clinical analysis.In chapter 8, an optical chemical sensor has been developed for determination of chromium(VI) based on reversible fluorescence quenching of a conductor wire compound 2-(2-(4-(2-(2,5-diheptyl-4-(2-thiophenyl-2-)ethinyl)phenyl)ethinyl)phenyl) ethinyl)2,5-diheptophenyl)ethinyl)thiophene (FDB) immobilized in a PVC membrane. The sensing membrane consisted of 55.0 mg PVC,105.0 mg diisooctyl (o-)phathalate (DIOP) and 5.0 mg NaTPB. The maximum response of the sensing membrane for chromium(VI) was obtained in 0.6 mol/L sulfuric acid when the maximum EX/EM wavelength is 371/407 nm respectively. Under optimum conditions the method showed good linearly for 3.78×10-6～1.26×10-3 mol/L Cr(Ⅵ) with detection limit of 5.16×10-7 mol/L. The response time was less than 1 min. Common cations and anions did not interfere the measurements of Cr(Ⅵ). The method was verified by the determination of trace Cr(Ⅵ) in environmental water with satisfactory results.