Research On The Application Of Core/Shell Metal Nanomaterials In Colorimetric Sensing For Inorganic Anions

The development of colorimetric sensors with visual detection,rapid response,and simple operation is always the hotspot in the field of analytical chemistry.On-site detection puts forward higher requirements to colorimetric sensors in terms of sensitivity,accuracy,and portability.Therefore,portable smartphone with high-resolution camera has become a new platform for colorimetric detection.Noble metal nanoparticles（such as Au,Ag,etc.）show great performance in colorimetry with unique optical and electrical properties.Among them,nanomaterials with core-shell structure are richer in color changes and more sensitive to changes in composition,making them a better choice for colorimetric probes.In this paper,several core-shell noble metal nanoprobes were prepared by a simple and green synthesis method,which were used for the detection of specific inorganic anions.Furthermore,the accuracy was improved by the smartphone platform.Au@Ag nanorods were synthesized for the detection of I^-.Au nanorods were firstly prepared as templates,which were then coated with Ag shell by reducing Ag NO₃.Cu²⁺was added to the probe solution as an oxidant for oxidizing I^-to be iodine when the mixed probe was exposed to the water sample containing I^-.The iodine further etched Ag shell,thereby changing the core-shell ratio of the probe,resulting in the shift of the UV absorption peak and the color change of probe.Therefore,a detection method for I^-based on Au@Ag nanorods was established,which had a good linear response correlation in the range of 3-50μmol·L^-1,the correlation coefficient was 0.982.Furthermore,this method could be used for the quantitative detecteion of I^-in real samples.For the detection of CN^-,Au nanoparticles were firstly prepared,which were then coated with Ag shell by reducing Ag NO₃,and further coated with another Au layer by reducing HAu Cl₄.The last step of HAu Cl₄ reduction brings the galvanic replacement between HAu Cl₄ and Ag,which accounts for the formation of the Au@Au-Ag yolk-shell nanostructure.After the probe was exposed to the water with CN^-,CN^-gradually etched the Au-Ag shell and Au core through the complex reaction between CN^-and noble metal,which changed the composition and core-shell ratio of the probe,resulting in the changes of the UV absorption peaks.During the etching process,the Au@Au-Ag yolk-shell nanoparticles also showed abundant color changes from blue,orange to pink.Therefore,the semi-quantitative detection of CN^-could be accomplished by the naked eye.By means of TEM,UV and EDX,the size,UV-vis spectrum and element distribution of the synthesized nanoparticles were characterized and explored to confirm their size,morphology and structure.The influence of reaction temperature,time and other conditions on the detection effect was explored respectively for each colorimetric sensor system.Furthermore,Au@Au-Ag yolk-shell nanoparticles are more adjustable than Au@Ag nanoparticles and Ag@Au nanoparticles.Three different sizes of Au@Au-Ag yolk-shell nanoprobes were prepared by adjusting the thickness of each layer,which showed different color changes for CN^–detection.The three colorimetry results verified and supplemented each other,and were combined to constitute a color matrix,which could be used for the quantitative detection of cyanide.Furthermore,we developed a computation program and a smartphone application,facilitating a rapid and objective readout of results,which were finally applied to real water samples with high accuracy.