We rationally designed an ultrasensitive and label-free sensing platform for determination of cadmium (Cd). The sensing platform contains G-quadruplex-Cd(II) specific aptamer (GCDSA) constructed by incorporating G-rich sequence at the end of 5′ and the critical domain of the Cd-4 aptamer. GCDSA designed act as both a special recognition sequence for Cd 2+ and a signal DNAzyme. In absence of Cd 2+ , GCDSA may mainly exist in a random coil sequence. Upon addition of Cd 2+ , GCDSA could probably be induced to fold into a G-quadruplex structure. The generation of plentiful active G-quadruplex interacts with hemin to form a peroxidase-like DNAzyme, leading to increased absorbance signal of the sensing system. ΔA was directly proportional to the two segments of concentrations for Cd 2+ , with the detection of limit of 0.15 nM. The proposed method avoids the labeled oligonucleotides and allows directly quantitative analysis of the samples by cheap instruments, with an excellent dynamic range.
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