Selective determination and detection of Pb2+in the environment is of importance. Pure and functionalized gold nanoparticles -AuNPs- have an interesting ability for selective sensing of different analytes, especially, heavy metal ions such as Pb2+ . Therefore, in this article by employing molecular dynamic -MD- simulations and quantum mechanic -QM- calculations, the aggregation mechanism of pure AuNPs and glutathione functionalized gold nanoparticles -Glu-AuNPs- for selective sensing of heavy metal ions was investigated. On the basis of obtained results, pure AuNPs are not stable in water, while Glu groups on the AuNPs surface protect gold nanoparticles from aggregation, therefore Glu-AuNPs have a considerable stability in comparison with AuNPs. Due to aggregation process in the presence of Pb 2+ , Glu-AuNPs can be used as a selective and sensitive sensor for detection of this ion. Indeed, Pb 2+ eliminates the repulsive interactions of the Glu-AuNPs and induces their aggregation, while other metal ions such as Ba 2+ ,Mg 2+,Ca2+ ,Cd2+ ,Co 2+ ,Fe 2+ ,Hg2+ ,Ni2+ , and Zn 2+ are not able to aggregate Glu-AuNPs. QM calculations reveal that Pb 2+ has the maximum electrostatic and donor-acceptor interactions with Glu groups, which efficiently reduces the Glu band gap more than that of other metal ions. A good agreement between the theoretical results and experimental data confirm the interesting ability of theoretical methods, not only for understanding the unknown aspects of the molecular mechanisms, but also for modeling of new sensors based on the functionalized AuNPs.

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