Depolymerization Of Lignin And Its Application In The Preparation Of Carbon Dots

Lignin is a kind of cheap and easily available renewable natural material,belonging to aromatic polymers.Lignin is a network of amorphous biomolecules connected by ether bonds and carbon-carbon bonds.Lignin contains abundant active groups,which provides the possibility for its modification,so lignin can be used as the precursor of various functional materials.carbon dots（CDs）is a new class of"zero-dimensional"fluorescent carbon nanomaterial,which is based on sp² and sp³ hybrid carbon as its basic skeleton,and has the advantages of low toxicity,photostability and biocompatibility on the surface containing rich oxygen-containing functional groups,and has been widely concerned in fluorescence analysis and imaging.Since both lignin structure and CDs use carbon structure as the skeleton,it is possible to prepare CDs using lignin as the precursor material,and lignin contains rich active functional groups,which provides the basis for the subsequent regulation of fluorescence properties of CDs.Based on this,this paper mainly carries out the following work:Chapter One:Introduction.Firstly,the structure and classification of lignin were briefly described.Then,the lignin research status was reviewed,including the depolymerization method of lignin,the modification method and the research progress of biomass based CDs.Finally,the research content of this paper was introduced.Chapter Two:Depolymerization of Kraft lignin（KL）with lithium bromide as catalyst.The experimental results showed that the aryl ether bond was broken on the Lignin and along with local demethylation reaction,Depolymerization lignin（DKL）with good solubility and low molecular weight was obtained.The content of hydroxyl group in DKL increased,the methoxy group decreased,and the thermal stability increased.Using KL and DKL as raw materials,the KL matrix carbon point（K-CDs）and the depolymerized lignin carbon point（D-CDs）were successfully prepared by hydrothermal method.The surface of the two CDs contains carboxyl,carboxyl and hydroxyl groups,which show strong absorption characteristics in the wavelength range of 200～380 nm.K-CDs and D-CDs show wavelength-dependent excitation at 260～420 nm.Compared with K-CDs,D-CDs has longer emission wavelength and higher fluorescence quantum yield.Chapter Three:Nitrogen modified lignin（N-DKL）was obtained by microwave reaction of DKL with guanidine hydrochloride in anhydrous ethanol.Hydrothermal synthesis of N-CDs doped with N-DKL as precursor.The experimental results show that N-CDs has good dispersion,the size distribution is between 1.5～2.5 nm,the surface of N-CDs contains a large number of oxygen-containing functional groups such as-COOH and-OH,N-CDs has obvious absorption at 200～380 nm,and at 260～420nm,The behavior of N-CDs solution is dependent on the excitation wavelength.The optimal fluorescence emission wavelength of N-CDs is 515 nm,which is attributed to the nitrogen doping,which is conducive to the emission of CDs in the long-wave direction.Chapter Four:Based on the fluorescence properties of lignin-based CDs,a CDs-Mn O₂ nano fluorescence sensor was designed to filtration effect（IFE）,which was highly sensitive and rapid detection of Glutathione（GSH）.Lignin-based CDs were used as fluorescence emitters,and manganese dioxide（Mn O₂）nanosheets were used as fluorescence absorbent.Their emission wavelengths overlapped to a certain extent,leading to the decrease of fluorescence intensity of CDs.The addition of GSH led to the decomposition of Mn O₂ nanosheets,and the fluorescence performance of CDs was restored.Based on IFE,To achieve a simple,highly sensitive and selective detection of GSH.