We have used short laser pulses to generate transient vapor nanobubbles around plasmonic nanoparticles. The photothermal, mechanical and optical properties of such bubbles were found to be different from those of plasmonic nanoparticle and vapor bubbles as well. This phenomena was considered as a new complex nanosystem -plasmonic nanobubble (PNB). Mechanical and optical scattering properties of PNB depended upon the nanoparticle surface and heat capacity, clusterization state, and the optical pulse length. The generation of the PNB required much higher laser pulse fluence thresholds than the explosive boiling level, and was characterized by the relatively high lower threshold of the minimal size (lifetime) of PNB. Optical scattering by PNB and its diameter (measured as the lifetime) has been varied with the fluence of laser pulse and this has demonstrated the tunable nature of PNB.Keywords vapor nanobubble; photothermal; scattering; pulsed laser; gold nanoparticle; plasmon resonance; vapor bubble Plasmonic nanoparticles (NP) are known for their outstanding photothermal [1][2][3][4][5][6] and optical scattering 1,[7][8][9][10][11][12] properties. These properties result from the interaction of NPs with optical radiation and the surrounding environment, and are characterized through their absorption and scattering cross-sections, respectively, which are well studied as a function of the parameters of NP, environment and optical radiation. [13][14][15][16][17][18][19] Since such interaction usually occurs in some medium (environment) the secondary thermal and hydrodynamic phenomena in such medium may significantly influence the properties of NPs as optical probes and heat sources. Short optical pulses and high-energy excitation of plasmonic NPs create transient and non-stationary thermal fields with vapor-liquid interfaces that significantly influence heat transfer from NP to its environment and create a strong gradient of the refractive index. In addition, vapor layers around NP cause blue shifts and attenuation of the extinction spectrum. 20 The multi-factor and non-stationary nature of the radiation-NP-vapor-liquid system complicates its experimental study and theoretical modeling. Available thermal models 6,17,[21][22][23][24] are applicable mainly for continuous excitation and stationary heat transfer at low temperatures or for very short laser pulses (femto-second range), for a limited number of shapes of NP, and for temperatures that assume constant plasmonic cross-sections and are *Rice University, Physics and Astronomy -MS 61, 6100 Main Street, Houston, TX 77005, dmitri.lapotko@rice.edu . NIH Public AccessAuthor Manuscript ACS Nano. Author manuscript; available in PMC 2011 April 27. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript below the boiling threshold for the environment. Measuring a high transient temperature around individual NPs in a two-phase medium can be quite difficult if not impossible. Experimental methods 8,[21][22][23][24][25] for monitoring the transien...

Noble metal nanoparticles exhibit sharp spectral extinction peaks at visible and near-infrared frequencies due to the resonant excitation of their free electrons, termed localized surface plasmon resonance (LSPR). Since the resonant frequency is dependent on the refractive index of the nanoparticle surroundings, LSPR can be the basis for sensing molecular interactions near the nanoparticle surface. However, previous studies have not yet determined whether the LSPR mechanism can reach the ultimate sensing limit: the detection of individual molecules. Here we demonstrate single molecule LSPR detection by monitoring antibody-antigen unbinding events through the scattering spectra of individual gold bipyramids. Both experiments and finite element simulations indicate that the unbinding of single antigen molecules results in small, discrete <0.5 nm blue-shifts of the plasmon resonance. The unbinding rate is consistent with antibody-antigen binding kinetics determined from previous ensemble experiments. According to these results, the effective refractive index of a single protein is approximately 1.54. LSPR sensing could therefore be a powerful addition to the current toolbox of single molecule detection methods since it probes interactions on long timescales and under relatively natural conditions.

Gold nanoparticles bound to substrates exhibit localized surface plasmon resonance (LSPR) in their optical extinction spectra at visible and near-infrared wavelengths. The LSPR wavelength is sensitive to the surrounding refractive index, enabling a simple, label-free immunoassay when capture antibodies are bound to the nanoparticles. Gold bipyramids are nanoparticles with a penta-twinned crystal structure, which have a sharp LSPR because of their high monodispersity. Bipyramid substrates were found to have a refractive index sensitivity ranging from 288 to 381 nm/RIU (-0.62 to -0.68 eV/RIU), increasing with the nanoparticle size and aspect ratio. In an immunoassay, the bipyramid substrates yielded higher sensitivity than nanorods and nanospheres. An immunoassay sensitivity constant which depends on both the optical properties of the nanoparticle and conjugation chemistry was found to be K(LSPR) = 0.01 nm x microm(2) for gold bipyramids.

Surface-enhanced Raman spectroscopy (SERS) of gold nanorods in cetyltrimethylammonium bromide solution has been used to analyze the interfacial surfactant structure based on the distance-dependent electromagnetic enhancement. The spectra were consistent with a surfactant bilayer oriented normal to the surface. As the surfactant concentration was reduced, a structural transition in the surfactant layer was observed through a sudden increase in the signal from the alkane chains. The structural transition was shown to influence the displacement of the surfactant layer by thiolated poly(ethylene glycol). The monodisperse and thoroughly characterized gold nanorod samples yield consistent enhancement factors that were compared to electromagnetic simulations.

Gold and silver nanoparticles and nanostructures exhibit plasmon resonances that result in strong scattering and absorption of light, as well as enhanced optical fi elds near the metal surface. The resonant fi eld enhancement dramatically enhances the weak Raman scattering signals from molecules near the metal surface. [ 1 ] This effect, called surface enhanced Raman scattering (SERS), has been widely pursued as a molecular sensing technology over the past decade. [ 2 , 3 ] SERS is non-destructive, suffi ciently sensitive for single molecule detection, and provides inherent molecular specifi city since it yields molecular vibrational spectra. While fi eld enhancement occurs over an entire nanostructure surface, SERS signals are strongest from small gaps between nanoparticles referred to as "hot spots" where the fi eld enhancement is maximal. [ 4 , 5 ] Most SERS work to date has focused on detection with substrates that are designed to maximize the density, sensitivity, and reproducibility of hot spots in order to give the strongest possible SERS signal. Many substrates have been developed which refl ect the variety of nanofabrication, synthesis, and assembly strategies that have emerged over the past decade. These include semiconducting nanowires, [ 6 ] aggregated colloids, [ 7 , 8 ] colloidal lithography, [ 9 , 10 ] soft lithography, [ 11 , 12 ] e-beam lithography, [ 13 ] and colloidal assembly. [ 14 , 15 ] Most of the substrates consist of nanostructured gold or silver on a fl at substrate. Some reports describe substrates with increased surface roughness to increase the number of hot spots, including aligned carbon nanotube substrates that support silver nanoparticles. [ 16 ] These substrates were found to provide highly sensitive detection. However, optical scattering and light collection occur in a three dimensional focal volume. Therefore, to maximize the quantity of scattered light generated and detected, SERS substrates should contain hot spots in a large three dimensional volume that is matched to the optics of the SERS instrumentation. Three dimensional substrates have been fabricated and tested based on porous silicon, microfabricated silicon, and porous gold. [17][18][19][20] These have indeed improved the detection limit for small molecules like trinitrotoluene (TNT). Therefore, it is desirable to create densely packed metal nanostructures with nanogaps to form plentiful hot spots for better SERS performance. [21][22][23] In the present study, we describe the fabrication of a structurally tunable 3D SERS substrate based on vertically aligned CNTs. Vertically aligned CNTs provide a new paradigm to realize three dimensional SERS substrates with high nanoparticle density. The vertically aligned CNTs were synthesized on a SiO 2 substrate by water-assisted chemical vapor deposition (CVD). The resulting vertically aligned CNTs were several millimeters long and were composed of a mixture of double-and triple-walled nanotubes. [ 24 , 25 ] A gold fi lm (50 nm thickness) was deposited on top of the CN...