Contamination of the environment with heavy metal ions has been an important concern throughout the world for decades. Driven by the need to detect trace amounts of mercury in environmental samples, this article demonstrates for the first time that nonlinear optical (NLO) properties of MPA-HCys-PDCA-modified gold nanoparticles can be used for rapid, easy and reliable screening of Hg(II) ions in aqueous solution, with high sensitivity (5 ppb) and selectivity over competing analytes. The hyper Rayleigh scattering (HRS) intensity increases 10 times after the addition of 20 ppm Hg 2+ ions to modified gold nanoparticle solution. The mechanism for HRS intensity change has been discussed in detail using particle size-dependent NLO properties as well as a two-state model. Our results show that the HRS assay for monitoring Hg(II) ions using MPAHCys-PDCA-modified gold nanoparticles has excellent selectivity over alkali, alkaline earth (Li + , Na + , K + , Mg 2+ , Ca 2+ ), and transition heavy metal ions (Pb 2+ , Pb + , Mn 2+ , Fe 2+ , Cu 2+ , Ni 2+ , Zn 2+ , Cd 2+ ).
We report size- and distance-dependent surface-energy transfer (SET) properties of gold nanoparticles for recognizing hepatitis C virus (HCV) RNA sequence sensitively and selectively (single-base mutations) in a homogeneous format. We have demonstrated that quenching efficiency increases by three orders of magnitude, as the particle size increases from 5 to 70 nm. Due to this extraordinarily high K(SV), nanoparticle SET (NSET) detection limit can be as low as 300 fM concentration of RNA, depending on the size of gold nanoparticle. We have shown that the distance-dependent quenching efficiency is highly dependent on the particle size and the distance at which the energy-transfer efficiency is 50 %, ranges all the way from 8 nm, which is very close to the accessible distance of conventional Förster resonance energy transfer (FRET), to about 40 nm by choosing gold nanoparticles of different diameters. Our result points out that dipole-to-metal-particle energy transfer and NSET models provide a better description of the distance dependence of the quenching efficiencies for 8 nm gold nanoparticle, but agreement is poor for 40 and 70 nm gold nanoparticles, for which the measured values were always larger than the predicted ones.
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