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+ ).

Contamination of the environment with mercury has been an important concern throughout the world for decades. Exposure to high Hg levels can be harmful to the brain, heart, kidneys, lungs, and immune system of humans of all ages. Driven by the need to detect trace amounts of mercury in environmental samples, here we present a miniaturized, inexpensive, and battery-operated ultrasensitive gold nanoparticle-based nanomaterial surface energy transfer probe for screening mercury levels in contaminated soil, water, and fish which has excellent sensitivity (2 ppt) and selectivity for Hg(II) over competing analytes, with the largest fluorescence enhancement to date for sensing Hg(II) in environmental samples (1100-fold). The sensitivity of our probe to detect mercury level in soil, water, and fish is about 2-3 orders of magnitude higher than the EPA standard limit. We demonstrate that our probe is suitable to screen the amount of mercury in different fish, shellfish, and water samples from various commercial sources.

Infectious diseases caused by the human immunodeficiency virus (HIV) remain the leading killers of human beings worldwide, and function to destabilize societies in Africa, Asia, and the Middle East. Driven by the need to detect the presence of HIV viral sequence, here we demonstrate that the second-order nonlinear optical (NLO) properties of gold nanorods can be used for screening HIV-1 viral DNA sequence without any modification, with good sensitivity (100 pico-molar) and selectivity (single base-pair mismatch). The hyper-Rayleigh scattering (HRS) intensity increases 45 times when a label-free 145-mer, ss-gag gene DNA, was hybridized with 100 pM target DNA. The mechanism of HRS intensity change has been discussed with experimental evidence for higher multipolar contribution to the NLO response of gold nanorods.

We report gold nanoparticle based FRET assay to monitor the cleavage of DNA by nucleases. Fluorescence signal enhancement is observed by a factor of 120 after the cleavage reaction in the presence of S1 nuclease. The mechanism of distant dependent fluorescence quenching has been discussed. Our experimental results on distance dependent fluorescence quenching match quite well with theoretical findings obtained from the fluorescence quenching model by Gersten and Nitzan (Surf. Sci. 1985, 158, 165). Our experimental observation paradigm for the design of optical based molecular ruler strategies at distances more than double the distances achievable using traditional dipole-dipole Columbic energy transfer based methods.

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