No abstract

Photoacoustic tomography (PAT) also referred to as optoacoustic tomography (OAT) is a hybrid imaging modality that employs nonionizing optical radiation and ultrasonic detection. Here, we describe the application of a new class of optical contrast agents based on mesoscopic hollow gold nanospheres (HAuNS) to PAT. HAuNS are ~40 nm in diameter with a hollow interior and consist of a thin gold wall. They display strong resonance absorption tuned to the near infrared (NIR) range, with an absorption peak at 800 nm, whose photoacoustic efficiency is significantly greater than that of blood. Following surface conjugation with thiolated poly(ethylene glycol), the pegylated HAuNS (PEG-HAuNS) had distribution and elimination half-lives of 1.38±0.38 and 71.82±30.46 h, respectively. Compared with PAT images based on the intrinsic optical contrast in nude mice, the PAT images acquired within 2 h after intravenous administration of PEG-HAuNS showed the brain vasculature with greater clarity and detail. The image depicted brain blood vessels as small as ~100 µm in diameter using PEG-HAuNS as contrast agents. Preliminary results showed no acute toxicity to the liver, spleen, or kidneys in mice following a single imaging dose of PEG-HAuNS. Our results indicate that PEG-HAuNS are promising contrast agents for PAT, with high spatial resolution and enhanced sensitivity.

Metal-enhanced fluorescence of molecular probes in plasmonic nanostructures offers highly sensitive chemical and biomedical analyses, but a comprehensive theory of the phenomenon is far from being complete. In this study, a systematic theoretical analysis is provided for overall luminescence enhancement/quenching for fluorophores near silver spherical nanoparticles. The approach accounts for local intensity enhancement, radiative and nonradiative rates modification, light polarization, molecule position, and its dipole moment orientation. Numerical modeling has been performed for fluorescein-based labels (e.g., Alexa Fluor 488) widely used in biomedical studies and development. The maximal enhancement exceeding 50 times is predicted for nanoparticle diameter 50 nm, the optimal excitation wavelength being 370 nm. For long-wave excitation, bigger particles are more efficient. The experiments with a fluorescein isothiocyanate conjugate of bovine serum albumin confirmed theoretical predictions. The results provide an extensive and promising estimate for simple and affordable silver-based nanostructures to be used in fluorescent plasmonic sensors.

We present a physical model that explains several sequential stages of the conversion of optical to acoustical energy when irradiating diluted suspensions of metal nanoparticles with laser pulses. Optical absorption and scattering of a single particle driven by plasmon resonance interactions in an aqueous medium are considered. Thermal effects produced by laser-irradiated nanoparticles, dynamics of vapor bubble formation, and acoustic signals from expanding bubbles formed around heated nanoparticles are calculated. Stochastic features of the pressure magnitude emitted as a result of low-fluence irradiation of suspensions are also discussed. The probabilistic distribution of pressure magnitude from individual bubbles was found to obey Zipf's law for low concentrations of nanoparticles, while increasing their concentration brings the pressure magnitude distribution into conformance with the Gaussian law.

Radiative decay rates of an atom placed near triaxial nanoellipsoid are investigated in long wavelength approximation. Analytical results are obtained in general case. It is shown that triaxial ellipsoid can be used for efficient control of decay rate of an atom, molecule or quantum dot. For example decay rate near silver ellipsoid can be enhanced by 5 orders of magnitude. It is also shown, that triaxial nanoellipsoid can be used for simultaneous efficient control of absorption and emission rates of fluorophores. * Electronic address: vklim@sci.lebedev.ru

The plasmon oscillations in a cluster of two metallic nanospheres are studied theoretically. Particular attention is paid to the case of nearly touching spheres. Simple analytical expressions have been found for the spectra of plasmon oscillations of different symmetry in this case. A new type of the plasmon oscillations, which are strongly localized between the spheres, and which totally disappear at separation of the spheres, has been discovered. The found plasmon oscillations have a dramatic effect on optical properties of an atom localized between the spheres.Much attention has been paid, of late, to the experimental and theoretical study of optical properties of the metallic nanoparticles. This interest is mainly due to a considerable enhancement of local fields near the nanoparticles. An especially high increase occurs in the case of plasmon polariton resonances ("plasmons") [1,2] or phonon polariton resonances ("phonons") [3]. On the basis of this effect one considers quite a number of possible applications. One of the most developed is the use of large local fields for enhancement of the Raman scattering cross-section [4]. Recent experiments have shown that such an increase may achieve 10-14 orders of magnitude, which may help to resolve separate molecules [5]- [7]. The local enhancement of the fields can also be used to increase the fluorescence intensity and to determine the structure of a single DNA strand without using the fluorescent labels [8,9]. By using the nanoparticles of complex configuration one can provide enhancement of both the absorption and the emission of light by natural and artificial fluorophores [10]. Of particular interest and promise are the studies of optical properties of the clusters of two and more metallic nanoparticles, because by changing the cluster's geometry one can effectively control the spectra of the plasmon oscillations. This effect makes it possible to produce, for example, new types of biosensors [11]-[13]. A whole series of experimental [14]-[17] and theoretical [18]-[27] studies have been devoted to the two-particle clusters.

The proposed paradigm of plasmonic atoms and plasmonic molecules allows one to describe and predict the strongly localized plasmonic oscillations in the clusters of nanoparticles and some other nanostructures in uniform way. Strongly localized plasmonic molecules near the contacting surfaces might become the fundamental elements (by analogy with Lego bricks) for a construction of fully integrated opto-electronic nanodevices of any complexity and scale of integration.

The radiation of optically active (chiral) molecule placed near chiral nanosphere is investigated. The optimal conditions for engineering of radiation of optically active (chiral) molecules with the help of chiral nanoparticles are derived. It is shown that for this purpose, the substance of the chiral particle must have both ε and µ negative (double negative material (DNG)) or negative µ and positive ε (µ negative material (MNG)). Our results pave the way to an effective engineering of radiation of "left" and "right" molecules and to creating pure optical devices for separation of drugs enantiomers.It is well known that nanoparticles influence substantially both fluorescence of molecules and Raman scattering of light by molecules. These effects are especially strong in the case of metallic nanoparticles where plasmon resonances can be excited. As a result, it is possible to detect radiation even from single molecule (surface enhanced Raman scattering SERS [1] and surface enhanced fluorescence SEF [2]) and to use this effect in various applications, in particular, for detection of certain proteins in disease diagnosis [3]. Recently it was shown that planar chiral metamaterials [4] and even nonchiral plasmonic nanoparticles [5] are able to dramatically (by several orders of magnitude) change the chiral dichroism of a chiral molecule and thus such metamaterials could form the basis for assaying technologies capable of detecting amyloid diseases and certain types of viruses.Interesting effects are also found in the study of influence of chiral nanoparticles [6] and nanoparticles with negative refraction index [7] on electric dipole radiations of atoms and molecules. Full quantum-electrodynamics theory of influence of nanospheres of various composition including media with negative refraction index on radiation of electric and magnetic dipoles was developed in [8]. However, the interference effects between these dipoles were not investigated there.Even more interesting problem is to investigate the possibility of controlling radiation of optically active molecules [9]. As far as we know this problem has not been investigated until now. The goal of present work is to investigate the influence of chiral nanoparticles on spontaneous emission of optically active molecules and to show that it is possible to discriminate drug enantiomers with pure optical methods. Geometry of the problem is shown in Fig. 1.As the model of an optically active molecule we assume FIG. 1:Geometry of the considered problem. An optically active (chiral) excited molecule is placed near chiral spherical nanoparticle having radius a. The total size of the region of molecule and nanobody is much less than the wavelength λ. Molecule is placed at the distance r0 from the center of co-ordinates, which coincides with the nanoparticle center, and characterized by its electric (d0) and magnetic (m0) dipole moments which induce corresponding dipole moments δd and δm of nanoparticle.that its radiation is of both electric dipole and magnetic dipole nature, ...