Synthesis Of Carbon Materials Based On Lignin And Humic Acid And Their Applications In Biochemical Analysis

Lignin is known as the second most abundant natural and renewable raw material after cellulose. It is also the main by-product in the pulp and paper industries. Black liquor pouring into the rivers from paper industries contains a great deal of lignin, causing water pollution and the increase of chemical oxygen demand(COD) in aquatic environments。Due to the rather complex chemical structure, lignin is not easily degraded to useful materials. Although some applications have been researched, the rather complex chemical structure of lignin limited its wide applications. Considering the three dimensional structure of lignin, it may have the advantages to prepare porous carbon materials. Humic acid(HA) is one of the major components of humic substances, which is commonly present in soils, brown and brown-black coals, natural waters, river, lake and sea sediments and other natural materials as a product of degradation of plant, algal, and microbial material. HA consists of carbon, oxygen, hydrogen and sometimes small amounts of nitrogen and occasionally phosphorous and sulphur, and contains many chemical functional groups such as carboxylic, carbonyl, phenolic, and hydroxyl groups connected with the aliphatic or aromatic carbons in the macromolecules structures. Therefore, HA can be used as carbon source to prepare all kinds of new functional materials. It is worth mentioning that the methods to extract HA from straw and industrial waste liquid(such as pulping waste liquor and fermentation wastewater) have made a great progress in recent years, which provides the new renewable, easily available and very cheap resources for the production of HA.In this paper, we preliminarily discussed the synthesis of humic acid by oxidation of the common oxidant H2O2 using lignin extracted from alkaline black liquor as raw material. After that, we prepared the new functional materials using inexpensive and earth-abundant materials of lignin and HA. Based on these materials, the simple, sensitive and selective colorimetric methods for the determination of cholesterol in serum samples and xanthine in urine samples were developed. In addition, the prepared functional material was applied on bioimaging. The main contents could be divided into four aspects as follow:(1) Humic acid has excellent chemical and biological activities, widely used in medical, agricultural, industrial and other fields. This paper preliminarily discussed the synthesis of humic acid by oxidation of the common oxidant H2O2 using lignin extracted from alkaline black liquor as raw material. The products were characterized by fourier transform infrared spectrum(FT-IR), ultraviolet-visible(UV-Vis) spectra, photoluminescence(PL) spectra and.high-performance liquid chromatography(HPLC). Results indicated that there are also other oxidizing substances in addition to the main product of humic acid. Meanwhile, the concentration of H2O2 on the influence of product was also discussed, the optimal concentration of H2O2 was 6%.(2) Lignin obtained from black liquor and urea have been successfully used as the precursors to prepare honeycomb-like nitrogen-doped porous carbon(PCN). Then Pt nanoparticles were uniformly dispersed on honeycomb-like PCN by glycol reduction to form Pt nanoparticles/PCN(Pt/PCN). The obtained Pt/PCN was characterized by scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray powder diffraction(XRD) and X-ray photoelectron spectroscopy(XPS), which showed that Pt nanoparticles with an average size of 4 nm were uniformly dispersed on the surface of honeycomb-like PCN. The prepared Pt/PCN was found to have much higher catalytic activity than individual Pt nanoparticles and PCN, which can catalyze oxidation of the peroxidase substrate 3, 3′, 5, 5′-tetramethylbenzidine(TMB) in the presence of H2O2 to produce a blue color reaction, the detection limit of H2O2 can be reached to 1.7×10-6 mol?L-1. Cholesterol oxidase(COx) is the enzyme responsible for oxidizing cholesterol to H2O2, therefore, a rapid, selective and sensitive colorimetric method for the cholesterol detection was developed with the detection limit of 8.3×10-6 mol?L-1. The method used lignin as the main raw material to prepare Pt/PCN is facile, low cost and rich in resource, and the prepared Pt/PCN was successfully applied as enzyme mimic for colorimetric detection of cholesterol sensitively. It is believed that this work not only provides a potential way to utilize the black-liquor lignin, but also puts forward a novel method for the detection of cholesterol in human body.(3) Pd nanoparticles supported on nitrogen, sulfur-doped three-dimentional hierarchical nanostructures(Pd/N-S-CS) was successfully prepared by glycol reduction of Pd on nitrogen, sulfur-doped three-dimentional hierarchical nanostructures(N-S-CS) using humic acid(HA) and thiourea as the precursors. Transmission electron microscopy(TEM), X-ray diffraction(XRD), energy-dispersive X-ray spectra(EDX) and X-ray photoelectron spectra(XPS) were employed to confirm the characterizations of Pd/N-S-CS. The results showed that the obtained Pd/N-S-CS had interesting three-dimensional hierarchical nanostructures which were composed of nanosheets, and Pd nanoparticles with average diameter of 6 nm were distributed on the surface of nanosheets. Furthermore, the Pd/N-S-CS exhibited excellent peroxidase-like activity and it can effectively catalyze the classical peroxidase substrate 3,3′,5,5′-tetramethylbenzidine(TMB) by H2O2 to produce a color reaction. Based on this finding, a simple, sensitive, and selective colorimetric method for H2O2 and xanthine detection was developed and applied to detect xanthine in urine samples. The detection limit of H2O2 and xanthine were 3.3 × 10-6 M and 2.9 × 10-7 mol?L-1, respectively. This work is expected to provide a novel and efficient method for the detection of xanthine in human body.(4) Humic acid as a raw material was applied for the preparation of graphene quantum dots(GQDs) by a simple one-step hydrothermal method. The transmission electron microscopy(TEM) images show that GQDs are well-crystalline with an average size of 4 nm. Fourier transform infrared spectrum(FT-IR) and X-ray photoelectron spectra(XPS) suggest the existence of functional groups such as hydroxyl and carboxylic groups on GQDs surface which makes the synthesized GQDs water-soluble. Optical properties of GQDs were characterized by photoluminescence(PL) spectra and ultraviolet-visible(UV-Vis) spectroscopy. It turns out that GQDs show a stable, excellent excitation-dependent photoluminescence. Therefore, the as-prepared GQDs can be applied as fluorescent materials for bioimaging. The cellular imaging studies indicate that GQDs can obviously imaging the cells. This work provides a novel low-cost raw material for the synthesis of fluorescent quantum dots by a facile method, which may have wide applications in the biomedical fields.