Study On Electrochemical Heavy Metal Sensors Based On Novel Porous Carbon-bismuth Composites And MXenes

Anthropogenic activities have the potential to alter the natural concentrations of a variety of heavy metals in groudwater,while toxic heavy metals at trace level can pose detrimental risks to both environment and human health.Thus,realizing the detection of toxic heavy metal at trace level in rapidity,high sensitivity and selectivity is significant to precisely evaluate the environmental pollution while it also receives great attention due to the challenges.Although conventional detection appliances including atomic absorption spectroscopy（AAS）,atomic fluorescence spectrometry（AFS）,inductively coupled plasma optical emission spectroscopy（ICP-OES）,and inductively coupled plasma mass spectrometry（ICP-MS）have provided highly reliable results and sensitivities,however,they often require well-trained technicians and complex instrumentation,which limit their onsite practical applications for detecting target contaminants.Electrochemical detection of heavy metals has attracted growing interests due to the advantages of portability,high sensitivity,simultaneous determination of analytes with simple instrumentation and low maintenance cost.The goal of this thesis is to develop several novel electrochemical platforms for ultrasensitive detection of heavy metal ions,which is based on the structure design and surface modification of the electrode materials.Hierarchical porous carbon and MXene were selected as the representive materials support for highly dispersed Bi micro-nanoparticles.The microstructure,morphology and chemical composition of these synthesized composites（Bi@BiOCl@C,Bi@BAC and Bi@d-Ti₃C₂）were controlled scientifically and rationally through a variety of approaches（such as NaCl template-assisted template method,microwave-assisted hydrothermal method,carbothermal reduction method and etc.）.Moreover,the connection between the material structure and the sensing performance of heavy metal ions were established.The surface modification of Ti₃C₂ was carried out through alkaline intercalation treatment,and the influence mechanism of surface functional groups on the sensing performance was revealed.Moreover,we design a new microgrid structure electrode through mechanical engraving technique besides using the conventional working electrode including glassy carbon electrode and carbon paste electrode.In summary,the research was aimed at providing novel mechanisim and tecnology support for the fast,convenient and effective electrochemical detection of heavy metal ions.The main parts of the results are summarized briefly as follows:（1）We reported a facile,cheap,environmentally friendly and scalable method to synthesize Bi@BiOCl nanoparticles embedded in 3D interconnected carbon nanosheets with meso-,macropores composites using NaCl as the template and citric acid as the carbon source by pyrolysis treatment at 700°C under N₂ atmosphere.The micrometerscale NaCl crystal acted as a recyclable skeleton to adsorb the precursors on its surfaces,which assisted the formation of micrometer-sized graphitic carbon nanosheets with nanometer thickness by the template effect during the pyrolysis.Besides,Bi@BiOCl nanoparticles were obtained through the in-situ confinement effects.The weight ratios of Bismuth nitrate to Citric acid plays an important role in determining the material properties,such as the molar ratio of Bi to BiOCl,and the particle size of Bi@BiOCl,the degree graphitization and the pore strcture.We obtained different analytical performance for the determination of Pb²⁺and Cd²⁺with four Bi@BiOCl@C nanocomposites modified GCE by square wave anodic stripping voltammetry（SWASV）.Under optimized conditions,Bi@BiOCl@C-3 modified GCE exhibited excellent electroanalytical performance for the simultaneous detection of Pb²⁺and Cd²⁺,with linear ranges in 1⁶0 ppb,sensitivity of analysis of 0.38 and 0.49μA ppb^-1 and correlation coefficient of 0.996 and 0.998,respectively.The developed protocol has shown a limit of detection（LOD）of about 0.2 and 0.4 ppb for Pb²⁺and Cd²⁺,respectively,which is much lower than that of the World Health Organization（WHO）limits.The superior electrochemical performance of the Bi@BiOCl@C composites can be attributed to the high electronic conductivity,hierarchical pore structure of supporting carbon sheets,abundant active sites along with uniform distribution of Bi@BiOCl nanoparticles,excellent structure stability and binding strength on electrode.（2）Inspired by the design concept of hierarchical porous natural materials,Platanus seed was used as both a template and a precursor for synthesizing a novel 3D lotus root-like hollow bimass activated carbon framework induced by the carbonization and KOH activation.By using a microwave solvothermal process,highly dispersed bismuth particles decorated bimass activated carbon（Bi@BAC）heterostructures were successfully fabricated through in situ pyrolyzed.The Bi@BAC composites was characterized by XRD,XPS,Raman,SEM,FTIR,and N₂ adsorption/desorption measurements.It was found that the as-prepared materials possessed a 3D hierarchical micro,meso-macroporous structure.As the excellent electron transinformation rate,high specific surface area,abundant mass transport channels and uniformly dispersed active sites along the surfaces of Bi@BAC,thus we obtained impressive analytical performance for the simultaneous determination of Pb²⁺and Cd²⁺at the fabricated Bi@BAC modified carbon paste electrode by SWASV.Under optimized conditions,Bi@BAC modified carbon paste electrode exhibited excellent electroanalytical performance for the simultaneous detection of Pb²⁺and Cd²⁺,a wide linear response ranges of 0.5⁵0 ppb,with an low detection limits of 0.13 and 0.08 ppb were observed,respectively.Moreover,the detection of lake water and tap water samples were realized with constructed sensors.（3）Two-dimensional accordion-like alk-Ti₃C₂ was prepared by acid etching and alkaline intercalation treatment,and demonstrated as a new platform for electrochemical detection toward multiple heavy metal ions using SWASV.The alk-Ti₃C₂ modified electrode exhibited significantly improved electrochemical response in comparison with the Ti₃C₂modified electrode due to the unique morphology and surface chemistry characteristics.Moreover,the alk-Ti₃C₂ modified electrode showed superior performances for both individual as well as simultaneous detections for the detection of Cd²⁺,Pb²⁺,Cu²⁺and Hg²⁺.Under the optimum conditions,it exhibited a high sensitivity superior to most of reported values and good linear correlations,with a detection limit of 0.098,0.041,0.032 and 0.130μM for Cd²⁺,Pb²⁺,Cu²⁺and Hg²⁺,respectively.The peak potentials of each heavy metal were well defined and sufficiently separated during the simultaneous voltammetric determination of four target heavy metal ions.Furthermore,mutual interference among the four target metal ions was explored,and the preferential deposition of Pb²⁺in the presence of other three metal ions together with an enhanced Hg²⁺sensitivity in the presence of Cd²⁺was discovered.（4）We proposed and fabricated a miniaturized and fully integrated electrochemical sensor incorporated with micro-patterned few-layer Ti₃C₂ and bismuth composites.The morphological and structural characterization,performed through TEM,XRD and XPS techniques,revealed the presence of Ti₃C₂ nanosheets structure,embedded with bismuth nanorods.In this nanocomposite material,the advantages of few layer Ti₃C₂ for accelerating mass and electron transfer rate,was combined with the excellent electroanalytical activity of the bismuth nanorod structure for highly sensitive heavy metal sensing.The operation parameters affecting the stripping current response were investigated and optimized.Under optimum conditions,the SWASV simultaneous determination of Pb²⁺,Cd²⁺and Zn²⁺showed good electroanalytical performance with linear ranges in 1²0 ppb,exhibited well-defined and separate stripping peaks,sensitivity of 0.98,0.84 and 0.60μA ppb^-1 and limit of detection of 0.2,0.4 and 0.7 ppb,respectively.The Bi@d-Ti₃C₂ modified microgrid electrode array demonstrated excellent reproducibility,stability and great resistance to interference.Furthermore,the application for real water analysis was demonstrated.