Preparation Of MXene Quantum Dots And Their Application In Fluorescence And Electrochemical Sensing

Objective:Environmental and human health issues have always been of great concern,and rapid quantitative monitoring of environmental pollutants and biomarkers in humans is of great importance.Among the many analytical methods,fluorescence and electrochemical methods have received a lot of attention from scholars in the field of analysis for their high sensitivity,good selectivity,low sampling,simplicity and rapidity.MXene is a class of metal carbides or nitrides with a two-dimensional（2D）layered structure.MXene quantum dots（MQDs）prepared from MXene materials not only inherits the advantage of MXene material including excellent hydrophilicity and biocompatibility,but also excellent optoelectronic properties due to quantum confinement and edge effects.These advantages make MQDs as an ideal material for the preparation of fluorescent and electrochemical sensors.（1）In the first part of this paper,we developed a new nitrogen-doped MQDs（N-MQDs）fluorescent sensor using covalent modifications,and applied it to the detection of metal ions in the real river water.Meanwhile,the detection intrinsic principles was investigated;（2）In the second part,we developed a novel electrochemical sensor（MQDs@3DE）by drop-casting the prepared MQDs onto a three-dimensionally（3D）printed electrode（3DE）.It was applied to the detection of the biomarker of dopamine and its intrinsic principles was investigated.Methods:（1）First,we synthesized amino-rich N-MQDs by modifying the surface of titanium carbide（Ti₃C₂）nanosheets with3-aminopropyltriethoxysilane（APTES）and then cutting the functional nanosheets into small-sized quantum dots by hydrothermal reaction.The N-MQDs were then characterized by UV-Vis spectrophotometry,fluorescence spectrophotometry,Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）and transmission electron microscopy（TEM）.N-MQDs can realize the sensitive detection of iron(Fe³⁺)and copper(Cu²⁺)ions in water by fluorescence quenching.In the presence of Cu²⁺and Fe³⁺,the exclusive detection of Cu²⁺can also realize using N-MQDs by the addition of Fe³⁺masking agent.In addition,the quantification concentration range and detection limit of the N-MQDs for the two metal ions and the fluorescence quenching selectivity for metal ions of the N-MQDs were investigated.Then,other optical properties of the MQDs synthesized using the reported method and the prepared N-MQDs were also compared including the fluorescence quantum yields and the sensitivity for the detection of metal ions.The fluorescence quenching mechanism of the N-MQDs was determined by examining the change of fluorescence lifetime and UV absorption spectra in the presence of Fe³⁺or Cu²⁺ions.Finally,the prepared N-MQDs was applied to the detection of Cu²⁺in real water.（2）Firstly,MQDs were prepared using reported method and the optical properties of the MQDs were characterized by UV-Vis spectrophotometry and fluorescence spectrophotometry.Afterwards,3D printed electrodes（3DE）were printed using graphene/polylactic acid（PLA）material by using fused deposition molding（FDM）method.Finally,the synthesized MQDs were modified on the activated 3DE（Activated-3DE）using drop casting to construct a new electrochemical sensor（MQDs@3DE）.The surface physical and chemical properties of the prepared 3DEs were measured by scanning electron microscopy（SEM）,XPS and contact angle test.Cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）methods were also applied to examine the changes in the electrochemical properties of the prepared 3DE.The synthesized MQDs@3DE were used for the detection of the biologically active substance dopamine（DA）.In addition,the anti-interference ability,the stability and the optimum p H for DA detection by using the synthesized electrochemical sensor were also examined.The mechanism underlying the detection of DA by MQDs@3DE was further investigated by varying the scan rate parameters.Finally,the MQDs@3DE electrochemical sensor was further applied to the selective detection of DA in real samples.Results:（1）The UV spectra of the prepared N-MQDs indicated the strong UV absorption property and the fluorescence spectra showed the strongest emission peak at approximately 427 nm with an excitation wavelength of 330nm.The fluorescence quantum yield of the N-MQDs was calculated to be15.4%.FT-IR spectra showed typical absorption bands of N-MQDs.The stretching vibration at 3215 cm^-1 was attributed to the N-H bond in APTES,and the Si-O peaks at 1131 cm^-1 and 1035 cm^-1 and the Si-O-Ti peak at 946 cm^-1indicated that APTES has been successfully grafted to the N-MQDs.XPS images showed that the characteristic peaks of N 1s and Si 2p at 400 e V and102 e V,respectively,were attributed to the successful bonding of APTES.Subsequently,N-MQDs was used to detect Fe³⁺and Cu²⁺at quantitative concentrations ranging from 0.5 to 500μM.The LODs for Fe³⁺and Cu²⁺were calculated to be 0.17 and 0.15μM respectively.It was also demonstrated that the addition of the Fe³⁺masking agent SHPP can successfully eliminate the interference of Fe³⁺,allowing the exclusive detection of Cu²⁺by N-MQDs.（2）UV absorption and fluorescence spectra of the prepared MQDs was consistent with the previously reported optical properties of MQDs.SEM images showed a smooth surface of untreated 3DE（Bare-3DE）with conductive graphene nanofibres covered by PLA.After removal of the non-conductive PLA by etching with N,N-dimethylformamide（DMF）,the graphene nanofibres were exposed.Subsequently,the successful covering of the MQDs roughened the exposed graphene nanofibres on the surface of the Activated-3DE.As seen in the XPS spectra,the main elements of Bare-3DE and Activated-3DE were C and O.Compared to Activated-3DE,the peaks for F 1s,N 1s and Ti 2p appeared on the XPS spectra of MQDs@3DE.The results of the contact angle test showed the modification of MQDs further reduced the contact angle（12.50°）.CV results showed that the current of the redox peak of Bare-3DE increased significantly after activation,and the electron transfer capacity of Activated-3DE increased further after adsorption of MQDs.The EIS results showed that modification of MQDs significantly reduced the charge transfer resistance（92.59Ω）.The prepared MQDs@3DE sensor can be quantified DA ranging from 0.01 to 20μM with an LOD of 3 n M and a good anti-interference ability.Finally,the prepared electrochemical sensor was successfully applied to the determination of DA in real DA-injected samples.Conclusion:（1）In this study,the N-MQDs fluorescent sensor was capable of sensitive detection of Fe³⁺and Cu²⁺with good optical properties,metal ion selectivity and stability.The addition of Fe³⁺masking agent enabled the exclusive detection of Cu²⁺by N-MQDs in the presence of Fe³⁺.The sensor was also successfully used for the detection of Cu²⁺in river water.Covalent doping of heteroatoms into MXene QDs materials opens up new horizons,and the new N-MQDs with abundant active amino groups for surface modification,can expand the application of MQDs in other analytical fields such as chromatography and electrochemistry.（2）In this work,MQDs@3DE were constructed by modifying MQDs on the surface of 3DE by a simple casting method to construct an electrochemical sensor for the sensitive detection of DA.A series of electrochemical results showed that the MQDs can endow higher conductivity and sensitivity to Activated-3DE for the detection of DA.At the same time,MQDs@3DE had a good interference resistance and stability.In addition,the prepared sensor has been successfully applied to the determination of DA in real injection.This work opens up a new way to enhance the electrochemical performance of 3DE using MQDs materials.