Photoacoustic Vaporization of Endoskeletal Droplets Loaded with Zinc Naphthalocyanine.

Vaporizable endoskeletal droplets are solid hydrocarbons in liquid fluorocarbon droplets in which melting of the hydrocarbon phase leads to the vaporization of the fluorocarbon phase. In prior work, vaporization of the endoskeletal droplets was achieved thermally by heating the surrounding aqueous medium. In this work, we introduce a near-infrared (NIR) optically absorbing naphthalocyanine dye (zinc 2,11,20,29-tetra-tert-butyl-2,3-naphthalocynanine) into the solid hydrocarbon (eicosane, n-C20H42) core of liquid fluorocarbon (C5F12) drops suspended in an aqueous medium. Droplets with a uniform diameter of 11.7 ± 0.7 µm were formed using a flow-focusing microfluidic device. The solid hydrocarbon formed a crumpled spherical structure within the liquid fluorocarbon droplet. The photoactivation behavior of these dye-containing endoskeletal droplets was investigated using NIR laser irradiation. When exposed to a pulsed laser of 720 nm wavelength, the dye-containing droplets vaporized at an average laser fluence of 65 mJ/cm2, whereas blank droplets without the dye did not vaporize at any fluence up to 100 mJ/cm2. Furthermore, dye-loaded droplets with a smaller, polydisperse size distribution were prepared using a simple shaking method and studied in a flow phantom for their photoacoustic signal and ultrasound contrast imaging. These results demonstrate that dye-containing endoskeletal droplets can be made to vaporize by externally applied optical energy. Such droplets may be useful for a variety of photoacoustic applications for sensing, imaging, and therapy.

Enhanced visibility through microbubble-induced photoacoustic fluctuation imaging.

A photoacoustic contrast mechanism is presented based on the photoacoustic fluctuations induced by microbubbles flowing inside a micro-vessel filled with a continuous absorber. It is demonstrated that the standard deviation of a homogeneous absorber mixed with microbubbles increases non-linearly as the microbubble concentration and microbubble size is increased. This effect is then utilized to perform photoacoustic fluctuation imaging with increased visibility and contrast of a blood flow phantom.

Photoacoustic Impulse Response of Lipid-Coated Ultrasound Contrast Agents.

The utility of ultrasound imaging and therapy with microbubbles may be greatly enhanced by determining their impulse-response dynamics as a function of size and composition. Prior methods for microbubble characterization utilizing high-speed cameras, acoustic transducers and laser-based techniques typically scan a limited frequency range. Here, we report on the use of a novel photoacoustic technique to measure the impulse response of single microbubbles. Individual microbubbles are driven with a broadband photoacoustic wave generated by a nanosecond-pulse laser illuminating an optical absorber. The resulting microbubble oscillations were detected by following transmission of a second laser as it passes twice through the microbubble. The system could even resolve oscillations resulting from a single-shot. As a proof-of-concept study, the size-dependent, linear impulse response of lipid-coated microbubbles was characterized using this technique. This unique method of microbubble characterization with exceptional spatiotemporal resolution opens new avenues for capturing and analyzing microbubble system dynamics.

Hydrophobically Modified Silica-Coated Gold Nanorods for Generating Nonlinear Photoacoustic Signals.

In this work, we report that gold nanorods coated with hydrophobically-modified mesoporous silica shells not only enhance photoacoustic (PA) signal over unmodified mesoporous silica coated gold nanorods, but that the relationship between PA amplitude and input laser fluence is strongly nonlinear. Mesoporous silica shells of ~14 nm thickness and with ~3 nm pores were grown on gold nanorods showing near infrared absorption. The silica was rendered hydrophobic with addition of dodecyltrichlorosilane, then re-suspended in aqueous media with a lipid monolayer. Analysis of the PA signal revealed not only an enhancement of PA signal compared to mesoporous silica coated gold nanorods at lower laser fluences, but also a nonlinear relationship between PA signal and laser fluence. We attribute each effect to the entrapment of solvent vapor in the mesopores: the vapor has both a larger expansion coefficient and thermal resistance than silica that enhances conversion to acoustic energy, and the hydrophobic porous surface is able to promote phase transition at the surface, leading to a nonlinear PA response even at fluences as low as 5 mJ cm-2. At 21 mJ cm-2, the highest laser fluence tested, the PA enhancement was >12-fold over mesoporous silica coated gold nanorods.

Sub-acoustic resolution optical focusing through scattering using photoacoustic fluctuation guided wavefront shaping.

Focusing light through turbid media using wavefront shaping generally requires a noninvasive guide star to provide feedback on the focusing process. Here we report a photoacoustic guide star mechanism suitable for wavefront shaping through a scattering wall that is based on the fluctuations in the photoacoustic signals generated in a micro-vessel filled with flowing absorbers. The standard deviation of photoacoustic signals generated from random distributions of particles is dependent on the illumination volume and increases nonlinearly as the illumination volume is decreased. We harness this effect to guide wavefront shaping using the standard deviation of the photoacoustic response as the feedback signal. We further demonstrate sub-acoustic resolution optical focusing through a diffuser with a genetic algorithm optimization routine.

Water use efficiency and productivity of habanero pepper (Capsicum chinense Jacq.) based on two transplanting dates.

Habanero pepper production was assessed with drip irrigation and plastic mulch, based on two transplanting dates. The objectives of the study were: (i) to evaluate the effect of two transplanting dates and the use of plastic mulch on water productivity and habanero pepper fruit yield under drip irrigation conditions; and (ii) to determine the profitability and economic viability of the product in the regional market. The work was conducted in the municipality of Huimanguillo, state of Tabasco, Mexico, in loam soils classified as Eutric Fluvisol. The Jaguar variety of habanero pepper, developed by INIFAP and possessing better genetic and productive characteristics, was used. Two transplanting dates were studied, (i) 30 January 2013 and (ii) 15 February 2013, with and without plastic mulch. The conclusions were: (i) application of irrigation depths based on crop evapotranspiration (ETc) and plastic mulch transplanted on 30 January increased the fruit yield of the crop and improved the benefit-cost ratio of the production system; and (ii) water use efficiency based on the 30 January transplanting date was 8.68 kg m⁻³ of water applied with plastic mulch, 6.51 kg m⁻³ without plastic mulch, and 3.65 kg m⁻³ for the 15 February transplanting date with plastic mulch.