Ca no3 2

We used commercially available silicon microcantilevers (Mikro Masch Co., Sunnyvale, CA) in all of our experiments. The dimensions were 350 μm in length, 35 μm in width, and 1 μm in thickness. One side of the cantilever was covered with a thin film of chromium (20 nm) followed by a 20-nm layer of gold, and the other side was silicon with a thin naturally grown oxide layer.
All the chemicals were of analytical grade. The chemicals including Pb(NO3)2, Mg(NO3)2, Fe(NO3)3, Cd(NO3)2, Zn(NO3)2, and Ca(NO3)2 were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. To tether the oligonucleotide to the gold surface of microcantilevers by Au-S bond, the 3′ end was tailored with an alkanethiol linker X (5′-GGGTGG-GTGGGTGGGT-3′-(CH2)6SH), 20 the thiolated G-rich singlestranded oligonucleotide Pb 2+ -dependent DNAzyme X and a single-stranded oligonucleotide Y (3′-C-ACCCTCCCAC-5′, which is partially complementary to the DNAzyme X); these were obtained from Sangon Biotechnology Co., Ltd. (Shanghai, PRC). Also, 3-(N-morpholino)propanesulfonic acid (MOPS) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) were purchased from Sigma-Aldrich. All reagents were used as received without further purification. The stock solution of oligonucleotide (100 μM) was prepared in 10 mM Tris-HAc buffer (pH 8.0).