| Since the advent of graphene,two-dimensional(2D)crystalline materials have attracted widespread attention due to their excellent physical and chemical properties.The exploration of new 2D materials has become an important research direction.In recent years,transition metal carbides,nitrides or carbonitrides(MXenes)have also attracted attention as a new type of 2D crystal material,and have been widely used in many fields such as energy storage,electromagnetic shielding,biomedicine,seawater desalination,etc.The adjustable structure and rich element composition endow it with excellent physical and chemical properties,such as excellent electrical conductivity,hydrophilicity,and biocompatibility.In addition,the low-dimensional characteristics of MXene perfectly meet the needs of current intelligent and miniaturized devices.Among them,Ti3C2Tx,as a typical MXene material,has been most widely studied and used.However,the delaminated Ti3C2Tx has poor environmental stability,and is easily oxidized and degraded by the attack of water and oxygen,thereby losing its excellent performance.At present,the oxidation degradation mechanism of Ti3C2Tx is still unclear,and there is a lack of effective solutions.This severely limits the durability of its excellent performance and hinders its industrial conversion and practical application.Moreover,as a new material,Ti3C2Tx still has many potential properties undiscovered,and further exploration is needed.Based on the above problems,this paper focuses on the research of Ti3C2Tx's environmental instability.On the one hand,our purpose is to research and solve the environmental unstability problem of Ti3C2Tx.Based on our study,we first proposed the oxidative degradation mechanism of Ti3C2Tx and gave a reasonable proof.Moreover,based on the understanding of its oxidative degradation mechanism,the ultra-stable Ti3C2Tx was achieved successfully.The stability is great significance for future research and application of MXene.On the other hand,our goal is to take advantage of the instability of Ti3C2Tx.The non-oxidized Ti3C2Tx-MXene quantum dots(NMQDs-Ti3C2Tx)were successfully prepared for the first time through the self-designed "micro-explosion method".It is also found that the quantum dots have Fenton-like reaction characteristics and can react with hydrogen peroxide to generate strong oxidizing hydroxyl radicals(·OH).Its Fenton-like reaction activity is higher than the well-known iron-based Fenton reaction.Happily,we found that the quantum dots have specific effect in killing of tumor cells,while maintaining good biocompatibility to normal cells.This research has achieved a breakthrough in using titanium-based materials with good biocompatibility to kill cancer cells.When the NMQDs-Ti3C2Tx was applied to tumor treatment,a good anti-tumor effect is obtained.Moreover,due to its instability,NMQDs-Ti3C2Tx will oxidize and degrade by itself after exerting its effects in the organism,making it safer and more efficient.This research has achieved a breakthrough in using titanium-based materials with good biocompatibility to kill cancer cells.This paper is divided into six chapters,and the main research contents of each chapter are as follows:Chapter 1 gives an overview on the physical and chemical properties,preparation methods and applications of Ti3C2Tx materials are systematically introduced.It fully demonstrated the application potential of Ti3C2Tx materials and pointed out the scientific problems that need to be solved at present.Accordingly,the research motives and goals of this paper are proposed.In Chapter 2,the various factors that affect the stability of Ti3C2Tx materials are systematically explored,and the oxidative degradation mechanism of Ti3C2Tx material is proposed for the first time.We found that the instability of Ti3C2Tx material is mainly due to the inherent reducibility.The reducibility comes from the inherent existence of Ti3+and Ti2+,which are susceptible to attack by water or oxygen.However,the abundant functional groups on the material surface can play a certain protective role,making it difficult for water and oxygen to contact the reducing titanium atoms.Therefore,Ti3C2Tx materials usually prepared by etching are in a metastable state.Once the material surface is defective or damaged,water and oxygen will threaten the existence of reducing Ti3+and Ti2+inside the material.Moreover,the reaction of titanium atoms with water and oxygen is an acidification process,that is,a process of continuously releasing hydrogen ions.In addition,we also found another feature of Ti3C2Tx material,that is,the fluorine functional groups will be easily desorbed from on the material surface under the action of the hydrogen bond of water molecules after the adsorption of hydrogen ions.After desorption,the reducing Ti element inside the material will lose the protection of the functional group,and "weak spots" will appear on the surface of the material and be attacked by water or oxygen.Based on the above-discovered factors that affecting stability,we propose the oxidation degradation mechanism of TiC2Tx.Since the preparation process of TiC2Tx includes violent acid etching and continuous centrifugal cleaning,it will inevitably cause defects on the material surface.Water or oxygen will undergo a redox reaction with the reducing titanium atoms at the defects,which is the beginning of the oxidative degradation of Ti3C2Tx materials.Since this reaction produces hydrogen ions,which will be adsorbed by the highly electronegative fluorine functional groups on the material surface.The fluorine functional group after the adsorption of hydrogen ions will desorb to the material under the action of the hydrogen bond of water molecules,which will produce new "defects" on the surface of the material,exposing new Ti2+ nd Ti3+,which once again become the attack point of water and oxygen.In this way,under the continuous vicious circle,the material undergoes point-like oxidation,which leads to material fracture and faster oxidative degradation.In the end,all the materials are oxidized and degraded into a mixture mainly containing titanium dioxide.In Chapter 3,based on the oxidative degradation process and mechanism of Ti3C2Tx,we designed a new alkaline solution etching method for preparing Ti3C2Tx focused on the key factors affecting its stability,and successfully prepared TiC2Tx without fluorine for the first time.The preparation method involves relatively mild reactions,avoids the previous violent reaction of acid etching,and realizes fluorine-free preparation.The prepared stable Ti3C2Tx has high quality,few defects,large two-dimensional size,and its surface functional group is O or-OH.The fluorine-free ultra-stable Ti3C2Tx has greatly improved environmental stability.In contrast,the previously reported Ti3C2Tx was completely oxidized and degraded in just a few days in an aqueous solution.The newly prepared fluorine-free ultra-stable Ti3C2Tx can be stored in aqueous solution for a long time without being oxidized and degraded.This research has realized a breakthrough to solve the instability of Ti3C2Tx,and has important significance for its application.In Chapter 4,we designed a simple and ultra-fast low-temperature"micro-explosion method" and successfully prepared NMQDs-Ti3C2Tx.The raw material used for preparing the quantum dots is Ti3C2Tx prepared by the traditional hydrofluoric acid etching method.Because the Ti3C2Tx obtained by this etching method is more likely to have a typical "acordion-like" microstructure.Using this microstructure,liquid nitrogen is used for intercalation,and then hot deionized water is added,resulting in a large temperature difference and a "water sealing" effect.The liquid nitrogen between the layers will quickly vaporize under the temperature difference,and a miniature explosion will occur between the layers under the effect of water sealing,thereby producing quantum dots.This method is simple,fast,green,does not introduce impurities,and avoids high temperatures,so that the prepared quantum dots are in a completely non-oxidized state.We used X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy and transmission electron microscopy to systematically and comprehensively characterize the obtained non-oxidized quantum dots.The quantum dot has a regular disc shape,with an average lateral size of about 7.23 nm and a thickness of about 5 nm.Its elemental composition and surface functional groups are the same as the raw material Ti3C2Tx,which is related to its preparation method only involving physical changes.For the characterization of the excellent performance of NMQDs-Ti3C2Tx,the Fenton-like reaction characteristics of the quantum dots were double-tested by the electron spin resonance test method and the TMB method.And by comparing it with Fe2+ Fenton reaction activity,it is found that the Fenton-like reaction activity of the prepared NMQDs-Ti3C2Tx is significantly stronger than Fe2+.Secondly,NMQDs-Ti3C2Tx has good dispersibility and dispersion stability due to the abundant hydrophilic functional groups on its surface,and can be self-dispersed in water without stirring and ultrasonic treatment.Finally,we also characterized the luminescence characteristics of NMQDs-Ti3C2Tx.The prepared NMQDs-Ti3C2Tx has photoluminescence properties,and its absolute quantum yield is?9.62%.Moreover,its photoluminescence spectrum(PL)has the general excitation-dependent behavior of conventional two-dimensional material-derived quantum dots.In addition,NMQDs-Ti3C2Tx has good biocompatibility and will not cause cell death after being cultured with normal cells.Combined with its ultra-small size and light-emitting characteristics,it is suitable for cell imaging.By labeling human adipose stem cells,it is proved that NMQDs-Ti3C2Tx can be used as a cell imaging probe,indicating the great potential of NMQDs-Ti3C2Tx in biomedical and optical applications.In Chapter 5,the application of NMQD-Ti3C2Tx in tumor treatment is systematically studied.Following our study,we found that the prepared quantum dots NMQDs-Ti3C2Tx have specific killing function to tumor cells,but maintain excellent biocompatibility to normal cells.When the NMQDs-Ti3C2Tx was applied to tumor treatment in mice,a favorable anti-tumor effect was obtained.In addition,the instability of the material also prevents the material from remaining in the body,and will degrade in the body after exerting its effect,so it will not cause serious side effects.This research has achieved a breakthrough in using Ti-based materials with good biocompatibility to kill cancer cells.In in vitro experiments,the cytotoxicity was detected by the Cell Counting Kit-8(CCK-8),and it was found that NMQDs-Ti3C2Tx is generally toxic to cancer cells(cervical cancer cells and breast cancer cells).In the cell live/dead experiment,the killing effect of NMQDs-Ti3C2Tx on cancer cells can be observed more intuitively.After culturing with 150?g/mL NMQDs-Ti3C2Tx for 48 hours,most breast cancer(MCF-7)cells were killed,while cervical cancer(HeLa)cells died in a large number only after treatment 24 hours.On the contrary,NMQDs-Ti3C2Tx has good biocompatibility to representative normal human adipose stem cells(ADSCs).Even at a concentration of 150 ?g/mL,the cells hardly die after 48 hours of culture.Therefore,NMQDs-Ti3C2Tx has high selectivity for the cytotoxicity of cancer cells.At the same time,we also evaluated the role of NMQDs-Ti3C2Tx in colony formation through clone formation experiments.Compared with the control group,the inhibitory effect of NMQDs-Ti3C2Tx at the concentration of 50 and 100 ?g/mL gradually increased,and the inhibitory effect was the best at the concentration of 150 ?g/mL.Statistical analysis shows that NMQDs-Ti3C2Tx has an excellent inhibitory effect on the colony formation of cancer cells.Regarding the mechanism of NMQDs-Ti3C2Tx killing cancer cells,we found that NMQDs-Ti3C2Tx can effectively produce a large amount of ·OH through Fenton-like reaction in cancer cells.Compared with normal cells,cancer cells have a large amount of H2O2 in the cell microenvironment due to their strong metabolism.Ti3+in NMQDs-Ti3C2Tx will have a Fenton-like reaction with H2O2,and the produced·OH has strong oxidizing properties,which can cause mitochondrial dysfunction,DNA damage and protein peroxidation,and ultimately lead to cancer cell apoptosis.Moreover,we found that NMQDs-Ti3C2Tx can be localized in mitochondria after entering cancer cells,which is a new target for cancer treatment.Considering that mitochondria are the main place for cells to produce H2O2,we speculate that NMQDs-Ti3C2Tx can accumulate in the mitochondria of cancer cells,thereby amplifying the oxidative stress response.In mice in vivo experiments,NMQDs-Ti3C2Tx has a high inhibitory and killing effect on xenograft HeLa tumors,but has almost no side-effects on normal tissues or organs,providing a safer strategy for effective tumor treatment.Due to the recognized biocompatibility of Ti-based materials and discovered that Ti3+has the ability to selectively kill cancer cells,the Ti-based Fenton reaction provides a safer new path for the catalytic treatment of tumors,with great potential.In Chapter 6,we have made a comprehensive summary of the main content,conclusions,innovations and further work that needs to be carried out in this paper. |
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