DNA G-quadruplex-templated formation of the fluorescent silver nanocluster and its application to bioimaging

Herein, a novel kind of silver nanocluster is synthesized simply by mixing G-quadruplex template with silver ions and reduction reagent (NaBH4, here). AS1411 (a G-quadruplex that can bind nucleolin over-expressed in cancer cells) is used as the main model template to prove the synthesis protocol and its potential application. We used fluorescence assay, CD, MALDI TOF MS, and TEM to characterize the silver nanocluster. It is found that after formation of the silver nanocluster, AS1411 still keeps its structure and is able to bind with nucleolin in cancer cell. Meanwhile, this binding behavior can greatly enhance the fluorescence intensity of the silver nanocluster. This property can be directly employed into bioimaging HeLa cells. The cell toxicity (3-[4,5-dimethylthiazolyl-2]-2,5-diphenyltetrazolium bromide, MU) assay demonstrated that the silver nanocluster has only little affect on the cytotoxicity to the cells, which further proves the applicability of the method in tumor cell imaging. At last, the universality of the synthesis protocol is verified by using a series of other G-quadruplex sequences as templates. For a lot of functional nucleic acids, such as human telomeres and certain aptamers, are with G-rich sequences and can fold into G-quadruplexes in functioning conditions, our method displays a promising application space in future researches. (C) 2011 Elsevier B.V. All rights reserved.