Synthesis Of Amino Acid-modified Gold Nanoparticles And Their Detection Of Metal Ions By Molecular Spectroscopic

With the rapid development of economy,heavy metal ion pollution has become more and more serious in recent years.The heavy metal ions that cannot degrade by themselves seriously threaten the health of organisms and the ecological balance of the environment.Traditional atomic spectroscopy methods for heavy metal ions have good accuracy and precision,but there are disadvantages such as high cost,complicated and time-consuming sample pre-treatment process.The molecular spectroscopy methods such as UV-visible absorption spectroscopy（UV-vis）and Raman spectroscopy have the advantages of low cost,easy operation and instant detection,and are of further research value.Many metal ions can cause shifts in the dispersion and aggregation states of functionalized nanogold solutions,allowing the absorbance and color of the mixed solutions to change with increasing metal ion concentrations.Therefore,researchers have established various UV-vis based colorimetric detection methods for heavy metal ions.Surface-enhanced Raman spectroscopy（SERS）is widely used in biomedical,environmental monitoring and food safety fields due to its ultra-high sensitivity,short response time,good selectivity,simple operation and rich spectral fingerprinting.Near-field interactions couple individual plasmonic oscillations,and this coupling strongly affects the electric field distribution around the nanostructure,especially in the gaps between nanoparticles,the"hot spot"region.When molecules are close to such a high electric field"hot spot"region,their Raman signal will be significantly enhanced.Since gold nanoparticles（Au NPs）have high stability and strong plasmonic effect,and amino acids have the advantages of good stability and strong affinity,the aggregation response of Au NPs modified with amino acids is analyzed and detected by molecular spectroscopy for heavy metal ions in this paper.In this paper,two main parts of work are carried out based on the above:In the first part,two amino acid-modified Au NPs were prepared.Ligand interaction of amino acids with metal ions leads to the aggregation of Au NPs.We used this principle to construct a molecular spectroscopy method for the quantitative detection of Hg²⁺using UV-vis.Specifically,Au NPs were prepared by the reduction of sodium citrate.The Au NPs were functionalized with two ligands,glutamine（Gln）and histidine（His）,which have a strong binding capacity to Au NPs.In this paper,the functionalized Au NPs were characterized in terms of morphology and structure by UV-vis,TEM,FT-IR,DLS,and XRD techniques.The functional ligand has a strong coordination effect on Hg²⁺,so the addition of Hg²⁺makes the functionalized Au NPs aggregate close to each other.Then the resulting coupling interactions lead to the redshift and broadening of the surface plasma absorption band.It exhibits a change in absorbance on the UV-vis and a change in color from red to blue.This series of changes allowed us to monitor the aggregation state of Au NPs and the change of Hg²⁺concentration in the sample.Therefore,a UV-vis analytical assay for the detection of Hg²⁺by Gln-His-Au NPs was constructed.Its analytical performance was examined.The experimental results showed that the method is sensitive,rapid,convenient and efficient.In the second part of this paper,a molecular spectroscopy method for the quantitative detection of Al³⁺using SERS was constructed.Amino acid-modified Au NPs were added to the raman reporter molecule.In addition,an analytical method was established by portable SERS test strips.Specifically,His which has coordination with metal ions,and raman signal molecule 4-mercaptobenzoic acid（4-MBA）were co-modified on Au NPs.The functionalized Au NPs were characterized by UV-vis,TEM,FT-IR,DLS,XRD and other techniques for morphology and structure.A SERS sensor was constructed for monitoring trace Al³⁺in environmental water samples.Since His has a strong coordination effect on Al³⁺,the addition of Al³⁺makes the functionalized Au NPs aggregate close to each other.The near-field interaction couples individual plasma oscillations.The Raman signal intensity of the 4-MBA is significantly enhanced when the 4-MBA is near the"hot spot"region with high electric field.Therefore,a SERS method for the analysis of His-4-MBA-Au NPs against Al³⁺was constructed.In order to make the assay more portable and practical,the portable SERS test strips were established.The physical agglomeration caused by the fiber structure of the test strips provided better sensitivity and improved the analytical performance of Al³⁺detection.The experimental results showed that the method has good sensitivity,selectivity,anti-interference ability and practicality.The aggregation response of the material and the"hot spot"region formed by the metal nanoparticles in close proximity to each other has great potential for other aspects of environmental monitoring.