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.

The presence of E coli in foodstuffs and drinking water is a chronic worldwide problem. The worldwide food production industry is worth about U.S. $578 billion, and the demand for biosensors to detect pathogens and pollutants in foodstuffs is growing day by day. Driven by the need, we report for the first time that two-photon Rayleigh scattering (TPRS) properties of gold nanorods can be used for rapid, highly sensitive and selective detection of Escherichia coli bacteria from aqueous solution, without any amplification or enrichment in 50 Colony Forming Units (cfu)/mL level with excellent discrimination against any other bacteria. TPRS intensity increases 40 times, when anti E. coli antibody-conjugated nanorods were mixed with various concentrations of Escherichia coli O157:H7 bacterium. The mechanism of TPRS intensity change has been discussed. This bionanotechnology assay could be adapted in studies using antibodies specific for various bacterial pathogens for the detection of a wide variety of bacterial pathogens used as bioterrorism agents in food, clinical samples, and environmental samples.

The hepatitis C virus (HCV) is a single-stranded (ss) RNA virus that is responsible for chronic liver diseases, such as cirrhosis, end-stage liver disease, and hepatocellular carcinoma. Driven by the need to detect the presence of the HCV viral sequence, herein it is demonstrated for the first time that the nonlinear optical (NLO) properties of gold nanoparticles can be used for screening and quantifying HCV RNA without any modification, with excellent detection limit (80 pM) and selectivity (single base-pair mismatch). The hyper-Rayleigh scattering (HRS) intensity increases 25 times when label-free, 145-mer, HCV ss-RNA is hybridized with 400 pM target RNA. The mechanism of HRS intensity change is discussed with experimental evidence for a higher multipolar contribution to the NLO response of gold nanoparticles.

A compact, highly specific, inexpensive and user friendly optical fibre laser-induced fluorescence (LIF) sensor based on fluorescence quenching by nanoparticles has been developed to detect single-strand (ss-) DNA hybridization at femtomolar level. The fluorescence of fluorophore-tagged ss-DNA increases by a factor of 80 when it binds to a complimentary DNA, while the addition of single-base mismatch DNA had no effect on the fluorescence efficiency. We present theoretical and experimental results on dye fluorescence quenching induced by gold nanoparticles having different particle sizes. Fluorescence spectra clearly show that the quenching efficiency decreases with increasing size of the gold nanoparticles and increasing the distance between dye and nanoparticles. The mechanism of size- and distant-dependent fluorescence quenching has been discussed. Effects of various influential experimental parameters and configurations were investigated in order to optimize and miniaturize the sensor performance.

Understanding the mechanism of how RNA molecules fold into their native structures are vital to their functional properties. Here we report for the first time that gold nanoparticle based NSET can be used for probing the transition states of an RNA unfolding reaction. Our result shows that time-dependent NSET can clearly distinguish structural transitions between unfolded to folded states. Our experimental observation point out that NSET can be used for the design of an optical based molecular ruler to track RNA folding transition states at distances more than double the distances achievable using traditional dipole-dipole Coulombic energy transfer based methods.