Superparamagnetic photonic crystals with DNA probes for rapid visual detection of mercury

A novel superparamagnetic photonic crystal DNA probe (Fe₃O₄@SiO₂@amino@DNA SPC) was developed to enable rapid visual detection of Hg²⁺. This unique photonic crystal (PC) was synthesized by combining superparamagnetic nanospheres with DNA probes. The DNA probe, rich in thymine (T), detects mercury ions through base mismatch, resulting in the formation of T-Hg²⁺-T loop hairpin structures. With the binding of Hg²⁺ to the probe attached to superparamagnetic nanospheres, the PC structure assembled by these nanospheres, formed by the magnetic field, was changed. This change enhanced the reflection intensity; it could be quantified using a fiber optic spectrometer and was visible to the naked eye. The Fe₃O₄@SiO₂@amino@DNA SPC, specific to Hg²⁺, exhibited a reflection peak at 679 nm, which intensified with increasing Hg²⁺ concentration. The reflection intensity increased by 132.58 a.u., and the PC color shifted from red to yellow as the Hg²⁺ concentration increased from 0.1 μg/L to 1 mg/L.