Colorimetric assay of lead using unmodified gold nanorods

Lead poisoning in adults can affect the peripheral and central nervous systems, the kidneys, and blood pressure. Thus, the development of environment-friendly and simple methods for Pb2+ detection is of great importance. Herein, a label-free colorimetric method has been developed for the detection of Pb2+ based on the conformational switch from single-stranded DNA to G-quadruplex. The electrostatic interactions between DNA probe and gold nanorods (GNRs) induce GNRs to space closely. However, the electrostatic interaction is not strong enough to change the suspension state of GNRs. In the presence of Pb2+, the formation of G-quadruplexes increases the surface charge density around DNA, which is expected to strengthen the electrostatic interaction between the GNRs and the DNA. Therefore, the longitudinal absorption of GNRs decreased because the stronger interaction induced aggregation of GNRs. Importantly, the decrease in longitudinal absorption is proportional to concentration of Pb2+. By monitoring the change of absorbance, Pb2+ can be detected at a level of 3 nM with a linear range from 5 nM to 1 μM. The overall test only takes a few minutes and very little interference is observed from other metal ions. The major advantages of this method are its low cost, convenience, simplicity, sensitivity, and specificity.