Highly sensitive and selective label-free optical detection of mercuric ions using photon upconverting nanoparticles

We demonstrate a fluorescence-based, label-free detection scheme that reports the presence of Hg(II) ion using photon upconverting nanoparticles. A single-stranded DNA containing a number of thymine bases to be used as the Hg2+-capturing element is covalently attached to the photon upconverting NaYF4:Yb3+,Tm3+ nanoparticles. Under the illumination of 980 nm laser, energy transfer takes place between the NaYF4:Yb3+,Tm3+ nanoparticles as the donor and SYBR Green I, a DNA intercalating dye, as the acceptor. By monitoring the ratio of the acceptor emission to the donor emission, we can quantitatively detect the presence of the mercuric ions with a directly observed detection limit of 0.06 nM. The remarkably high signal-to-noise ratio of photon upconverting particles leads to very high sensitivity and specificity without the need of fluorophore labeling. The sensor also does not suffer from photobleaching.