Green Synthesis And Application In Metel Ion Detection Of Fluorescent Gold Nanoclusters

Fluorescent gold nanoclusters（AuNCs）are emerging as novel fluorescent materials and have attracted great attentions in the field of ion detection and bioimaging owing to their unusual physicochemical properties,such a s long fluorescence lifetime,ultrasmall size,large Stokes shift,high fluorescence emission,as well as excellent biocompatibility.At present,several methods such as thermal reduction method,etching method,microwave-assisted method,ultrasonic method and electrochemical method have been developed to prepare AuNCs,while most of them need tedious procedures and a very long reaction time,which restrict their wide applications.In this paper,methionine was used as a template to synthesize AuNCs with different ways.Then,the AuNCs were used for the detection of metal ions and biological imaging.In the present study,the methionine capped gold nanoclusters（Met-AuNCs）were prepared by water heating method.The reaction conditions such as methionine concentration,Na OH concentration,time and temperature were optimized.Finally,0.18 M methionine,0.04 M NaOH,75 oC and 10 min were the best synthesis conditions,which showed a strong and stable orange red luminescence.Under photoexcitation at 370 nm,the Met-AuNCs possessed a maximum emission at 580 nm,a mean size of 2.4 nm and a quantum yield of 2.3%.Based on the fluorescence quenching effect,the Met-AuNCs could be used as a fluorescent probe to detect Cu²⁺ and Co²⁺.It was observed that the plot displayed a good linear relationship in a broad Cu²⁺ and Co²⁺ concentration range from 0.0001 to 6 μM and 0.001 to 2 μM,respectively.The detection limits for Cu²⁺ and Co²⁺ were around 47 pM and 0.42 nM,respectively.The effects were used to design a test paper for visual detection of Cu²⁺ and Co²⁺.Using this test paper,1 μM of Cu²⁺/Co²⁺ could be detect under the UV lamp.It is perceived to be a promising tool for the rapid on-site determination of Cu²⁺ and Co²⁺ in real water samples.We also synthesized Met-AuNCs using the methionine as a ligand with microwave irradiation and optimized the reaction conditions.Final conditions were 0.24 M methionine,800 W and 80 s.The maximal excitation and emission wavelengths of the as-prepared Met-AuNCs were 370 nm and 565 nm,respectively.The quantum yield of luminescent Met-AuNCs in aqueous solution was measured at 16.8%.And the average diameter was 3.5 nm.Based on the Schiff base formed by salicylaldehyde and Met-AuNCs,Zn²⁺ and Al³⁺ led to blue-shift and increase of the fluorescence.It was observed that the plot displayed a good linear relationship in a broad Zn²⁺ and Al³⁺ concentration range from 0.02 ～ 1.6 μM and 0.002 ～ 1.6 μM,respectively.The detection limits for Zn²⁺ and Al³⁺ were 1.2 nM and 2 pM,respectively.Further researches indicated that Met-AuNCs could be applied to cell imaging.