Hu Cang

Gold nanocages as contrast agents for spectroscopic optical coherence tomography

We describe gold nanocages as a new class of potential contrast agent for spectroscopic optical coherence tomography (OCT). Monodispersed gold nanocages of an approximately 35 nm edge length exhibit strong optical resonance, with the peak wavelength tunable in the near-infrared range. We characterized the optical properties of the nanocage by using OCT experiments along with numerical calculations, revealing an absorption cross section approximately 5 orders of magnitude larger than conventional dyes. Experiments with tissue phantoms demonstrated that the nanocages provide enhanced contrast for spectroscopic as well as conventional intensity-based OCT imaging.

Probing the electromagnetic field of a 15-nanometre hotspot by single molecule imaging

When light illuminates a rough metallic surface, hotspots can appear, where the light is concentrated on the nanometre scale, producing an intense electromagnetic field. This phenomenon, called the surface enhancement effect, has a broad range of potential applications, such as the detection of weak chemical signals. Hotspots are believed to be associated with localized electromagnetic modes, caused by the randomness of the surface texture. Probing the electromagnetic field of the hotspots would offer much insight towards uncovering the mechanism generating the enhancement; however, it requires a spatial resolution of 1–2 nm, which has been a long-standing challenge in optics. The resolution of an optical microscope is limited to about half the wavelength of the incident light, approximately 200–300 nm. Although current state-of-the-art techniques, including near-field scanning optical microscopy, electron energy-loss spectroscopy, cathode luminescence imaging and two-photon photoemission imaging have subwavelength resolution, they either introduce a non-negligible amount of perturbation, complicating interpretation of the data, or operate only in a vacuum. As a result, after more than 30 years since the discovery of the surface enhancement effect, how the local field is distributed remains unknown. Here we present a technique that uses Brownian motion of single molecules to probe the local field. It enables two-dimensional imaging of the fluorescence enhancement profile of single hotspots on the surfaces of aluminium thin films and silver nanoparticle clusters, with accuracy down to 1.2 nm. Strong fluorescence enhancements, up to 54 and 136 times respectively, are observed in those two systems. This strong enhancement indicates that the local field, which decays exponentially from the peak of a hotspot, dominates the fluorescence enhancement profile.

Orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments are used to study the orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM+NO3−) over time scales from ∼1 ps to ∼2 ns, and the temperatures range from 410 to 295 K. The temperatures cover the normal liquid state and the weakly supercooled state. The orientational dynamics exhibit characteristics typical of normal organic glass-forming liquids. The longest time scale portion of the data decays as a single exponential and obeys the Debye–Stokes–Einstein relation. The decay of the OHD-OKE signal begins (∼1 ps) with a temperature independent power law, t−z, z=1.02±0.05, the “intermediate power law.” The power law decay is followed by a crossover region, modeled as a second power law, the von Schweidler power law. The longest time scale decay is the exponential α relaxation. The intermediate power law decay has been observed in van der Waals supercooled liquids previously. These are the first such obse...

Confocal three dimensional tracking of a single nanoparticle with concurrent spectroscopic readouts

We present an apparatus that noninvasively tracks a moving nanoparticle in three dimensions while providing concurrent sequential spectroscopic measurements. The design, based on confocal microscopy, uses a near-infrared laser and a dark-field condenser for illumination of a gold nanoparticle. By monitoring the scattered light from the nanoparticle and using a piezoelectric stage, the system was able to continuously bring the diffusive particle in a glycerol/water solution back to the focal volume with spatial resolution and response time of less than 210nm and a millisecond, respectively.

Logarithmic decay of the orientational correlation function in supercooled liquids on the Ps to Ns time scale

Dynamics of supercooled ortho-terphenyl, salol, benzophenone, 2-biphenylmethanol, and dibutylphthalate have been studied using optical heterodyne detected optical Kerr effect experiments over a broad range of time, <1 ps to tens of ns. On time scales longer than those influenced by intramolecular vibrational dynamics, “intermediate” power law decays with temperature independent exponents close to −1 have been observed from ∼2 ps to 1–10 ns in all five samples. The intermediate power law decays occur over a wide range of temperatures from well above to somewhat below Tc, the mode-coupling theory (MCT) critical temperature. The intermediate power law corresponds to approximately a logarithmic decay of the polarizability–polarizability (orientational) correlation function. The amplitude of the intermediate power law increases with increasing temperature as [(T−Tc)/Tc]1/2. The intermediate power law decay is followed by a second longer time scale power law, and the final portion of the decay is exponential. A...

Dynamics in supercooled liquids and in the isotropic phase of liquid crystals: A comparison

A comparison is made of the dynamics observed over wide ranges of time and temperature between five supercooled liquids and four isotropic phase liquid crystals that have been previously studied separately. Optical-heterodyne-detected optical Kerr effect (OHD–OKE) measurements were employed to obtain the orientational relaxation dynamics over time scales from sub-ps to tens of ns. For the supercooled liquids, the temperatures range from above the melting point down to ∼Tc, the mode coupling theory critical temperature. For the liquid crystals, the temperatures range from well above the isotropic-to-nematic phase transition temperature TNI down to ∼TNI. For time scales longer than those dominated by intramolecular vibrational dynamics (≳1 ps), the fundamental details of the dynamics are identical. All nine liquids exhibit decays of the OHD–OKE signal that begin (>1 ps) with a temperature-independent power law t−z, where z is somewhat less than or equal to 1. The power law decay is followed in both the supe...

Guiding a confocal microscope by single fluorescent nanoparticles

Confocal optical microscopes offer unparalleled high sensitivity and three-dimensional (3D) imaging capability but require slow point-by-point scanning; they are inefficient for imaging moving objects. We propose a more efficient solution. Instead of indiscriminate scanning, we let the focus of the microscope pursue the object of interest such that no time is wasted on uninformative background, allowing us to visualize 3D trajectories of fluorescent nanoparticles in solution with millisecond temporal and ~200 nm spatial resolution.

Adding an unnatural covalent bond to proteins through proximity-enhanced bioreactivity

Natural proteins often rely on the disulfide bond to covalently link side chains. Here we genetically introduce a new type of covalent bond into proteins by enabling an unnatural amino acid to react with a proximal cysteine. We demonstrate the utility of this bond for enabling irreversible binding between an affibody and its protein substrate, capturing peptide-protein interactions in mammalian cells, and improving the photon output of fluorescent proteins.

Liquid crystal dynamics in the isotropic phase

The dynamics in the isotropic phase of the liquid crystal 1-isothiocyanato-(4-propylcyclohexyl) benzene (3-CHBT) are investigated from very short time (∼1 ps) to very long time (>100 ns) as function of temperature. The data decay exponentially only on the longest time scale (>10 ns). The temperature dependence of the long time scale exponential decays is described well by the Landau–de Gennes theory of the randomization of pseudo-nematic domains that exist in the isotropic phase of liquid crystals near the isotropic to nematic phase transition. Over the full range of times, the data are fit with a model function that contains a short time power law. The power law exponent is temperature independent over a wide range of temperatures. Integration of the function gives the empirical polarizability–polarizability (orientational) correlation function. A preliminary theoretical treatment of collective motions yields a correlation function that indicates that the data can decay as a power law at short times. The...

Short time dynamics in the isotropic phase of liquid crystals: the aspect ratio and the power law decay

Optical heterodyne detected optical Kerr effect (OHD-OKE) experiments are used to study the orientational dynamics of the liquid crystal 4 0 -octyl-4-biphenylcarbonitrile (8CB) in the isotropic phase near the isotropic to nematic phase transition. The results are compared to those for three other liquid crystals. The 8CB data display a short time scale temperature independent power law decay and a long time scale exponential decay with a temperature dependence described by Landau–de Gennes theory. The power law exponent is )0.56. Combining this result with previous results for three other liquid crystals [J. Chem. Phys. 116 (2002) 6339; J. Chem. Phys. 116 (2002) 360], it is found that the power law exponent depends linearly on the aspect ratio of the liquid crystal. 2002 Elsevier Science B.V. All rights reserved.