Ivan Gorelikov

Vaporization of perfluorocarbon droplets using optical irradiation.

Micron-sized liquid perfluorocarbon (PFC) droplets are currently being investigated as activatable agents for medical imaging and cancer therapy. After injection into the bloodstream, superheated PFC droplets can be vaporized to a gas phase for ultrasound imaging, or for cancer therapy via targeted drug delivery and vessel occlusion. Droplet vaporization has been previously demonstrated using acoustic methods. We propose using laser irradiation as a means to induce PFC droplet vaporization using a method we term optical droplet vaporization (ODV). In order to facilitate ODV of PFC droplets which have negligible absorption in the infrared spectrum, optical absorbing nanoparticles were incorporated into the droplet. In this study, micron-sized PFC droplets loaded with silica-coated lead sulfide (PbS) nanoparticles were evaluated using a 1064 nm laser and ultra-high frequency photoacoustic ultrasound (at 200 and 375 MHz). The photoacoustic response was proportional to nanoparticle loading and successful optical droplet vaporization of individual PFC droplets was confirmed using photoacoustic, acoustic, and optical measurements. A minimum laser fluence of 1.4 J/cm(2) was required to vaporize the droplets. The vaporization of PFC droplets via laser irradiation can lead to the activation of PFC agents in tissues previously not accessible using standard ultrasound-based techniques.

Room-temperature amplified spontaneous emission at 1300 nm in solution-processed PbS quantum-dot films

We report room-temperature amplified spontaneous emission and spectral narrowing at infrared wavelengths in solution-processed films made up of PbS quantum-dot nanocrystals. The results are relevant to optical amplification and lasing integrated upon a variety of substrates. The active optical medium operates at room temperature without any additional matrix material, providing an optical gain of 260 cm(-1) and a pump threshold of 1 mJ/cm(-2). Nanocrystals synthesized in an aqueous solution and stabilized by use of short ligands result in high quantum-dot volume fractions in solid films and in a redshift emission relative to absorption.

Microfluidic Assembly of Monodisperse, Nanoparticle-Incorporated Perfluorocarbon Microbubbles for Medical Imaging and Therapy

New medical imaging contrast agents that permit multiple imaging and therapy applications using a single agent can result in more accurate diagnosis and local treatment of diseased tissue. Solid nanoparticles (NPs) (5-150 nm in size) have emerged as promising imaging and therapy agents, as have micrometer-scale, perfluorocarbon gas-filled microbubbles (MBs) used in patients as intravascular ultrasound contrast agents. We propose that the modular combination of small, solid NPs and larger, highly compressible MBs into a single agent is an effective way to attain the desired complementary and hybrid properties of two very different agents. Presented here is a new strategy for the simple and robust incorporation of various medical NPs with monodisperse MBs based upon the controlled pH-based regulation of the electrostatic attraction between NPs and the MB shell. Using this simple approach, microfluidic-generated, protein-lipid-coated, perfluorobutane MBs (with size control down to 3 microm) were incorporated with silica-coated NPs, including CdSe/ZnS quantum dots, gold nanorods, iron oxide NPs, and Gd-loaded mesoporous silica NPs. The silica interface permits NP inclusion within MBs to be independent of NP composition, morphology, and size. Significantly, the NP-incorporated MBs (NP-MBs) diluted in saline were detectable using low-pressure ultrasound, and the monodisperse MB platform can be produced at high-throughput, sufficient for in vivo usage (10(6) MB/sec). The modular synthesis of a variety of NP-MBs can facilitate flexible, user-defined, multifunctional imaging and therapy agents tailored for specific applications and disease types.

Acoustic and photoacoustic characterization of micron-sized perfluorocarbon emulsions

Abstract. Perfluorocarbon droplets containing nanoparticles (NPs) have recently been investigated as theranostic and dual-mode contrast agents. These droplets can be vaporized via laser irradiation or used as photoacoustic contrast agents below the vaporization threshold. This study investigates the photoacoustic mechanism of NP-loaded droplets using photoacoustic frequencies between 100 and 1000 MHz, where distinct spectral features are observed that are related to the droplet composition. The measured photoacoustic spectrum from NP-loaded perfluorocarbon droplets was compared to a theoretical model that assumes a homogenous liquid. Good agreement in the location of the spectral features was observed, which suggests the NPs act primarily as optical absorbers to induce thermal expansion of the droplet as a single homogenous object. The NP size and composition do not affect the photoacoustic spectrum; therefore, the photoacoustic signal can be maximized by optimizing the NP optical absorbing properties. To confirm the theoretical parameters in the model, photoacoustic, ultrasonic, and optical methods were used to estimate the droplet diameter. Photoacoustic and ultrasonic methods agreed to within 1.4%, while the optical measurement was 8.5% higher; this difference decreased with increasing droplet size. The small discrepancy may be attributed to the difficulty in observing the small droplets through the partially translucent phantom.

Silica-coated quantum dots for optical evaluation of perfluorocarbon droplet interactions with cells.

There has been recent interest in developing new, targeted, perfluorocarbon (PFC) droplet-based contrast agents for medical imaging (e.g., magnetic resonance imaging, X-ray/computed tomography, and ultrasound imaging). However, due to the large number of potential PFCs and droplet stabilization strategies available, it is challenging to determine in advance the PFC droplet formulation that will result in the optimal in vivo behavior and imaging performance required for clinical success. We propose that the integration of fluorescent quantum dots (QDs) into new PFC droplet agents can help to rapidly screen new PFC-based candidate agents for biological compatibility early in their development. QD labels can allow the interaction of PFC droplets with single cells to be assessed at high sensitivity and resolution using optical methods in vitro, complementing the deeper depth penetration but lower resolution provided by PFC droplet imaging using in vivo medical imaging systems. In this work, we introduce a simple and robust method to miscibilize silica-coated nanoparticles into hydrophobic and lipophobic PFCs through fluorination of the silica surface via a hydrolysis-condensation reaction with 1H,1H,2H,2H-perfluorodecyltriethoxysilane. Using CdSe/ZnS core/shell QDs, we show that nanoscale, QD-labeled PFC droplets can be easily formed, with similar sizes and surface charges as unlabeled PFC droplets. The QD label can be used to determine the PFC droplet uptake into cells in vitro by fluorescence microscopy and flow cytometry, and can be used to validate the fate of PFC droplets in vivo in small animals via fluorescence microscopy of histological tissue sections. This is demonstrated in macrophage and cancer cells, and in rabbits, respectively. This work reveals the potential of using QD labels for rapid, preclinical, optical assessment of different PFC droplet formulations for their future use in patients.

Modeling photoacoustic spectral features of micron-sized particles

The photoacoustic signal generated from particles when irradiated by light is determined by attributes of the particle such as the size, speed of sound, morphology and the optical absorption coefficient. Unique features such as periodically varying minima and maxima are observed throughout the photoacoustic signal power spectrum, where the periodicity depends on these physical attributes. The frequency content of the photoacoustic signals can be used to obtain the physical attributes of unknown particles by comparison to analytical solutions of homogeneous symmetric geometric structures, such as spheres. However, analytical solutions do not exist for irregularly shaped particles, inhomogeneous particles or particles near structures. A finite element model (FEM) was used to simulate photoacoustic wave propagation from four different particle configurations: a homogeneous particle suspended in water, a homogeneous particle on a reflecting boundary, an inhomogeneous particle with an absorbing shell and non-absorbing core, and an irregularly shaped particle such as a red blood cell. Biocompatible perfluorocarbon droplets, 3-5 μm in diameter containing optically absorbing nanoparticles were used as the representative ideal particles, as they are spherical, homogeneous, optically translucent, and have known physical properties. The photoacoustic spectrum of micron-sized single droplets in suspension and on a reflecting boundary were measured over the frequency range of 100-500 MHz and compared directly to analytical models and the FEM. Good agreement between the analytical model, FEM and measured values were observed for a droplet in suspension, where the spectral minima agreed to within a 3.3 MHz standard deviation. For a droplet on a reflecting boundary, spectral features were correctly reproduced using the FEM but not the analytical model. The photoacoustic spectra from other common particle configurations such as particle with an absorbing shell and a biconcave-shaped red blood cell were also investigated, where unique features in the power spectrum could be used to identify them.

Nanoparticle-loaded perfluorocarbon droplets for imaging and therapy

Nanoscale perfluorocarbon droplets that are in the liquid phase at physiological temperatures, but which can be converted to gas using ultrasound, offer potential as a contrast agent for the detection and therapy of solid tumours. Nanoparticles such as quantum dots can also be encapsulated within PFC droplets, enabling multi-modal imaging and controlled nanoparticle release. In this work, experiments were conducted to investigate the impact of nanoparticle incorporation on droplet conversion at low and high ultrasound frequencies. It was found that incorporation of quantum dots lowered the inertial cavitation threshold at 1 MHz by 20%. In contrast, quantum dot nanoparticles did not significantly alter the conversion threshold of perfluorohexane or perfluoropentane droplets at 18 MHz. It was also shown that perfluoropentane droplets could be converted to gas and imaged at high frequency in hepatomas in mice, using brief high pressure bursts to achieve the phase conversion. Finally, optically fluorescent quantum dots incorporated within droplets were used to demonstate the feasibility of assessing biodistribution in rabbits using fluorescence histology.

Optical droplet vaporization (ODV): Photoacoustic characterization of perfluorocarbon droplets

Optical droplet vaporization (ODV) of nanoscale and micron-sized liquid perfluorocarbon (PFC) droplets via a 1064 nm laser is presented. The stability and laser fluence threshold were investigated for PFC compounds with varying boiling points. Using an external optical absorber to facilitate droplet vaporization, it was found that droplets with boiling points at 29°C and 56°C were consistently vaporized upon laser irradiation using a fluence of 0.7 J/cm2 or greater, while those with higher boiling points did not, up to a maximum laser fluence of 3.8 J/cm2. Upon vaporization, the droplet rapidly expanded to approximately 10–20× the original diameter, then slowly and continuously expanded at a rate of up to 1 µm/s. Lead sulphide (PbS) nanoparticles were incorporated into perfluoropentane (PFP) droplets to facilitate vaporization. The fluence threshold to induce vaporization ranged from 0.8 to 1.6 J/cm2, the wide range likely due to variances of the PbS concentration within the droplets. Prior to vaporization, the photoacoustic spectral features of individual droplets 2–8 µm in diameter measured at 375 MHz agreed very well with the theoretical prediction using a liquid sphere model. In summary, the use of liquid droplets for photoacoustic imaging and cancer therapy has been demonstrated.

Optical Droplet Vaporization of Micron-sized Perfluorocarbon Droplets and their Photoacoustic Detection

An acoustic and photoacoustic characterization of micron-sized perfluorocarbon (PFC) droplets is presented. PFC droplets are currently being investigated as acoustic and photoacoustic contrast agents and as cancer therapy agents. Pulse echo measurements at 375 MHz were used to determine the diameter, ranging from 3.2 to 6.5 μm, and the sound velocity, ranging from 311 to 406 m/s of nine droplets. An average sound velocity of 379 ± 18 m/s was calculated for droplets larger than the ultrasound beam width of 4.0 μm. Optical droplet vaporization, where vaporization of a single droplet occurred upon laser irradiation of sufficient intensity, was verified using pulse echo acoustic methods. The ultrasonic backscatter amplitude, acoustic impedance and attenuation increased after vaporization, consistent with a phase change from a liquid to gas core. Photoacoustic measurements were used to compare the spectra of three droplets ranging in diameter from 3.0 to 6.2 μm to a theoretical model. Good agreement in the spectral features was observed over the bandwidth of the 375 MHz transducer.

Photoacoustic spectral characterization of perfluorocarbon droplets

Perfluorocarbon droplets containing optical absorbing nanoparticles have been developed for use as theranostic agents (for both imaging and therapy) and as dual-mode contrast agents. Droplets can be used as photoacoustic contrast agents, vaporized via optical irradiation, then the resulting bubbles can be used as ultrasound imaging and therapeutic agents. The photoacoustic signals from micron-sized droplets containing silica coated gold nanospheres were measured using ultra-high frequencies (100-1000 MHz). The spectra of droplets embedded in a gelatin phantom were compared to a theoretical model which calculates the pressure wave from a spherical homogenous liquid undergoing thermoelastic expansion resulting from laser absorption. The location of the spectral features of the theoretical model and experimental spectra were in agreement after accounting for increases in the droplet sound speed with frequency. The agreement between experiment and model indicate that droplets (which have negligible optical absorption in the visible and infrared spectra by themselves) emitted pressure waves related to the droplet composition and size, and was independent of the physical characteristics of the optical absorbing nanoparticles. The diameter of individual droplets was calculated using three independent methods: the time domain photoacoustic signal, the time domain pulse echo ultrasound signal, and a fit to the photoacoustic model, then compared to the diameter as measured by optical microscopy. It was found the photoacoustic and ultrasound methods calculated diameters an average of 2.6% of each other, and 8.8% lower than that measured using optical microscopy. The discrepancy between the calculated diameters and the optical measurements may be due to the difficulty in resolving the droplet edges after being embedded in the translucent gelatin medium.