Preparation And Photothermal Properties Of Lignin-based Nitrogen-doped Carbon Nanotubes

As a kind of natural aromatic polymer,lignin has the advantages of low cost,high carbon content and being renewable.Because the carbon to oxygen ratio of lignin is higher than that of cellulose,lignin is often used as raw material to prepare high-value carbon products.Nitrogen dopants can destroy the electron and spin properties of the carbon skeleton,thereby improving the overall performance.Therefore,it is of great significance to develop an environmentally friendly and economical preparation technology for nitrogen-doped carbon nanotubes using lignin as carbon source.In this paper,nitrogen-doped lignin-based carbon nanotubes(L-NCNTs)were prepared by modifying lignin with small molecular nitrogen sources(urea,melamine,etc.).In addition,ibuprofen(IBU)-loaded nanoparticles(L-NCNTs/NML@IBU)were prepared by adding L-NCNTs as a photothermal agent to lignin micelles(NML),and their applications in the field of photothermal response drug carriers were preliminarily explored.(1)Preparation and characterization of nitrogen-doped Lignin-based carbon nanotubes.Nitrogen-doped lignin-based carbon nanotubes L-MNCNTs and L-UNCNTs were prepared by in-situ doping method using melamine modified lignin and urea modified lignin,respectively.In addition,lignin-based carbon nanotubes were prepared using pure lignin and then nitrogen-doped lignin-based carbon nanotubes L-HNCNTs were prepared by post-doping method.The effects of doping methods and types of nitrogen source on the morphology,size and nitrogen content of nitrogen-doped carbon nanotubes were investigated.The structure of modified lignin was tested by FT-IR,and strong hydrogen bonding was found between lignin and small nitrogen-containing molecules.FE-SEM and TEM results showed that L-MNCNTs and L-UNCNTs prepared by in-situ doping with modified lignin had longer length and more wall.TEM mapping tests showed that nitrogen atoms were evenly distributed on the surface of L-NCNTs.XPS analysis showed that the nitrogen contents of L-MNCNTs and L-UNCNTs prepared by in-situ doping were higher than that of L-HNCNTs,and the nitrogen content of L-UNCNTs prepared with urea-modified lignin was the highest(3.6%).The results of multi-peak resolution for XPS suggested that L-MNCNTs and L-UNCNTs had high pyridine-N contents,which were 49.4%and 46.9%respectively.L-HNCNTs had a high pyrrolic-N content of 48.2%.(2)Photothermal properties and drug release of nitrogen-doped lignin-based carbon nanotubes.The photothermal experiments showed that L-MNCNTs,L-UNCNTs and L-HNCNTs had good photothermal properties.The photothermal efficiency of L-MNCNTs was as high as 58.8%,while that of undoped carbon nanotubes was only 39.6%.After nitrogen doping,the photothermal efficiency of carbon nanotubes was improved to varying degrees.And the obtained L-NCNTs showed good photothermal properties at different light power and concentration.By comparing the photothermal data of L-MNCNTs,L-UNCNTs and L-HNCNTs,it was proved that the photothermal conversion efficiency of pyridine-N was higher than that of pyrrolic-N.The data of photothermal stability showed that L-CNTs,L-MNCNTs,L-UNCNTs and L-HNCNTs had good photothermal stability.In addition,two drug-loaded nanoparticles L-UNCNTs/NML@IBU and L-MNCNTs/NML@IBU were prepared by solvent exchange method.TEM analysis showed that the morphology of L-UNCNTs/NML@IBU and L-MNCNTs/NML@IBU were spherical at pH of 7.2～7.4.The diameter of the L-UNCNTs/NML@IBU nanoparticles was about 150 nm,and that of the L-MNCNTs/NML@IBU nanoparticles was about 200 nm.The photothermal data showed that the L-UNCNTs/NML@IBU and the L-MNCNTs/NML@IBU showed good photothermal responsiveness and photothermal stability under 808 nm laser irradiation.The drug-loading and releasing performance of the nanoparticles were systematically studied.The drug release rate of the L-MNCNTs/NML@IBU nanosystem increased significantly after 808nm laser irradiation,reaching 53.52%in the 10th hour,while that of the group without light was only 38.59%.In vitro experiments showed that the L-NCNTs/NML@IBU nanoparticles had little toxicity to normal cells.