Dmitry V. Guzatov

Photoacoustic imaging of living mouse brain vasculature using hollow gold nanospheres

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 approximately 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 approximately 100 mum 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.

Nonresonant Surface-Enhanced Raman Scattering of ZnO Quantum Dots with Au and Ag Nanoparticles

Pronounced 10(4)-fold enhancement of Raman scattering has been obtained for ZnO nanocrystals on substrates coated with 50 nm Ag nanoparticles under nonresonant excitation with a commercial red-emitting laser. This makes feasible beyond 10(-18) mole detection of ZnO nanocrystals with a commercial setup using a 0.1 mW continuous wave laser and can be purposefully used in analytical applications where conjugated nanocrystals serve as Raman markers. For Au-coated surfaces the enhancement is much lower and the heating effects in the course of Raman experiments are pronounced.

Acoustic signals generated by laser-irradiated metal nanoparticles

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 Zipfs law for low concentrations of nanoparticles, while increasing their concentration brings the pressure magnitude distribution into conformance with the Gaussian law.

Optical properties of a plasmonic nano-antenna: an analytical approach

The optical properties of a plasmonic nano-antenna made of two metallic nanospheroids (prolate or oblate) were investigated analytically in quasi-static approximation. It is shown that in clusters of two nanospheroids, three types of plasmonic modes can be present. Two of them can be effectively excited by a plane electromagnetic wave, while the third one can be effectively excited only by a nanolocalized light source (an atom, a molecule or a quantum dot) placed in the gap between the nanoparticles. Analytical expressions for the absorption cross-section, the enhancement of local fields and the radiative decay rate of an excited atom placed near such a nano-antenna are presented and analyzed.

Engineering of radiation of optically active molecules with chiral nano-meta-particles

The radiation of an optically active (chiral) molecule placed near a 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 e and μ negative (double negative material (DNG)) or negative μ and positive e (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.

The influence of chiral spherical particles on the radiation of optically active molecules

Within the framework of nonrelativistic quantum electrodynamics, a theory of spontaneous emission of a chiral molecule located near a chiral (bi-isotropic) spherical particle is developed. It is shown that the structure of photons in the presence of chiral spherical particles differs significantly from the structure of usual transverse electric and transverse magnetic photons. Exact analytical expressions for a spontaneous emission decay rate of a chiral molecule with arbitrary electric and magnetic dipole moments of transition located near a chiral spherical particle with arbitrary parameters, are obtained and analyzed in detail. Simple asymptotes for the case of a nanoparticle are obtained. Substantial influence of even small chirality on a sphere made from dielectric or double negative metamaterial is found. It is shown that by using chiral spherical particles it is possible to effectively control the radiation of a given enantiomer of optically active molecules.

Plasmon-enhanced fluorescence of labeled biomolecules on top of a silver sol-gel film

Abstract. A simple chemical technique was devised for the fabrication of silver nanostructured substrates which can be used for plasmonic enhancement of labeled proteins fluorescence. For bovine serum albumin labeled with fluorescein isothiocyanate, the obtained enhancement factor ranges from three to seven, depending on metal-luminophore spacing and silver nanoparticle size. For excitation with linear polarized light, the enhancement factor increases noticeably for p-polarization and decreases for s-polarization. The experimental results were interpreted in terms of the theoretical model in which the enhancement factor depends on incident light polarization, luminophoremetal spacing and silver nanoparticle size. Proposed plasmonic substrates can be considered as an affordable replacement of standard ones in different types of fluorescent assays for the purpose of increasing sensitivity.

Spontaneous emission of an atom placed near a nanobelt of elliptical cross-section

Spontaneous emission of an atom (molecule) placed near a nanocylinder of elliptical cross-section of an arbitrary composition is studied. The analytical expressions have been obtained for the radiative and nonradiative channels of spontaneous decay and investigated in details.

Plasmonic enhancement of electroluminescence

Here plasmonic effect specifically on electroluminescence (EL) is studied in terms of radiative and nonradiative decay rates for a dipole near a metal spherical nanoparticle (NP). Contribution from scattering is taken into account and is shown to play a decisive role in EL enhancement owing to pronounced size-dependent radiative decay enhancement and weak size effect on non-radiative counterpart. Unlike photoluminescence where local incident field factor mainly determines the enhancement possibility and level, EL enhancement is only possible by means of quantum yield rise, EL enhancement being feasible only for an intrinsic quantum yield Q0 < 1. The resulting plasmonic effect is independent of intrinsic emitter lifetime but is exclusively defined by the value of Q0, emission spectrum, NP diameter and emitter-metal spacing. For 0.1< Q0 < 0.25, Ag nanoparticles are shown to enhance LED/OLED intensity by several times over the whole visible whereas Au particles feature lower effect within the red-orange rang...

Colloidal Photoluminescent Refractive Index Nanosensor Using Plasmonic Effects

Abstract Fluorescence enhancement by metal nanostructures which is sensitive to refractive index n of an ambient medium is suggested as an operation principle of a novel refractive index sensor for liquids. Calculations are made for spherical and spheroidal Ag particles, and potential feasibility of sensitivity of the order of Δn=10−4 is demonstrated. Sensors of this type can be made fully colloidal with metal bodies deposited on a substrate or comprising a metal layer covering colloidal assembly of dielectric particles to serve as a test strip as well as placed on a fiber tip end to get local probing of refractive index in the tip-enhanced refractometry mode. Colloidal core-shell semiconductor nanocrystals may become the best candidates for this type of sensors whereas molecular probes may be affected by chemical properties of tested liquids.