Gold nanorods emit strong photoluminescence under two photon excitation; the efficient two photon lumininescence (TPL) arises from the local field enhancement assisted by surface plasmons. The surface plasmon effects on the TPL efficiency and spectrum are investigated by measuring the TPL of gold nanorods with various aspect ratios. A large TPL efficiency is found when incident light wavelength coincides with the longitudinal surface plasmon mode of the gold nanorods. However, the emission spectra of nanorods with various aspect ratios look similar and exhibit modest surface plasmon features, which implies a major non-radiative decay of excited surface plasmons.

Cobra cardiotoxin, a cytotoxic beta-sheet basic polypeptide, is known to cause membrane leakage in many cells including human erythrocytes. Herein, we demonstrate that the major cobra cardiotoxin from Naja atra, CTX A3, can cause leakage of vesicle contents in phosphatidylglycerol (PG) and phosphatidylserine containing, but not in pure phosphatidylcholine (PC), membrane bilayers. By the combined polarized attenuated total reflection infrared spectroscopy and computer simulation studies, CTX A3 is shown to peripherally bind to both zwitterionic and anionic monolayers in a similar edgewise manner with a tilted angle of approximately 48 +/- 20 degrees between the beta-sheet plane of the CTX molecule and the normal of the membrane surface. The average surface area expansion induced by CTX A3 binding to the PG monolayer, however, is two times larger than that of the PC monolayer as determined by the Langmuir minitrough method. Interaction energy considerations of CTX A3 on neutral and negatively charged membrane surfaces suggests that the electrostatic interaction between anionic lipid and cationic CTXs plays a role in modulating the penetration depth of CTX molecules on the initial peripheral binding mode and reveals a pathway leading to the formation of an inserted mode in negatively charged membrane bilayers.

We experimentally and theoretically study the density-dependent optical properties of 20 nm gold nanoparticle monolayers on silicon at 530 nm. Both results indicate a density-dependent resonance behavior. The density-dependent resonance suggests that the optical properties of the gold nanoparticle monolayers can be drastically modulated by changing the volume ratio V(r). It also implies, in this composite system, that the total surface plasmon resonance is maximized at a certain V(r).

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A tip nanobiosensor for monitoring DNA replication was presented. The effects of excitation power and polarization on tip-enhanced fluorescence (TEF) were assessed with the tip immersed in fluorescein isothiocyanate solution first. The photon count rose on average fivefold with radially polarized illumination at 50 mW. We then used polymerase-functionalized tips for monitoring loop-mediated isothermal amplification on Hepatitis C virus cDNA. The amplicon-SYBR® Green I complex was detected and compared to real-time loop-mediated isothermal amplification. The signals of the reaction using 4 and 0.004 ng∕μl templates were detected 10 and 30 min earlier, respectively. The results showed the potential of TEF in developing a nanobiosensor for real-time DNA amplification.