The unfolding of G-quadruplexes and its adverse effect on DNA-gold nanoparticles-based sensing system

The adsorption of DNAs in G-quadruplex solution onto 13 nm gold nanoparticles (AuNPs) was studied through monitoring of the localized surface plasmon resonance (LSPR) absorbance of 13 nm AuNPs at 520 and 650 nm (A650/A520) in the solutions of three widely studied guanine-rich sequences, TBA(5'-GGTTGGTGTGGTTGG-3'), PW17(5'-GGGTAGGGCGGGTTGGG-3'), and PSO (5'-GGGTTAGGGTTAGGGTTAGGG-3'). It was found that the degree of adsorption of DNAs in Pb(2+) stabilized G-quadruplex (G-Pb(2+)) solutions is up to 93% after more than 5h of incubation. Furthermore, the lead concentrations in the solutions containing G-quadruplex and AuNP were analyzed by an inductively coupled plasma atomic emission spectrometer. The results showed that Pb(2+) had been released from the G-quadruplexes, which means the G-quadruplexes may be unfolded in the presence of AuNP. This interaction between G-quadruplexes and AuNP demonstrated that long time incubation between DNAs and AuNPs would possibly make it unable to distinguish G-quadruplex from ssDNA. Thus, a biosensing system consisting of PW17 and AuNPs was developed to detect Pb(2+). It was found that the LSPR responses at A650/A520 were sensitive to [Pb(2+)]. However, the sensitivity of the system was interfered by the potential unfolding of PW17-Pb(2+) in the presence of AuNPs. This unexpected adverse effect of AuNPs on DNA-based biosensors should be taken into consideration in the future development of biosensing systems that are based on ssDNA aptamers and unmodified AuNPs.