Studying the photothermal activation of polydopamine-shelled, phase-change emulsion droplets into microbubbles using small- and ultra-small-angle neutron scattering.

Polydopamine-shelled perfluorocarbon (PDA/PFC) emulsion droplets are promising candidates for medical imaging and drug delivery applications. This study investigates their phase transition into microbubbles under near-infrared (NIR) illumination in situ using small- and ultra-small-angle neutron scattering (SANS and USANS) and contrast variation techniques. Supported by optical microscopy, thermogravimetric analysis, and ultrasound imaging, SANS and USANS results reveal rapid phase transition rates upon NIR illumination, dependent on PFC content and droplet size distribution. Specifically, perfluoropentane droplets rapidly transform into bubbles upon NIR irradiation, whereas perfluorohexane droplets exhibit greater resistance to phase change (bulk boiling points = 30 °C and 60 °C, respectively). Furthermore, smaller emulsion droplets with unimodal distribution resist NIR-triggered phase changes better than their bimodal counterparts. This observation is attributable to the lower boiling points of large emulsion droplets (lower Laplace pressure than smaller droplets) and the faster photothermal heating rates due to their thicker polydopamine shells. The insights gained from these techniques are crucial for designing phase-change emulsions activated by NIR for photothermal therapies and controlled drug delivery.

Mesoporous, anisotropic nanostructures from bioinspired polymeric catecholamine neurotransmitters and their potential application as photoacoustic imaging agents.

Mesoporous polydopamine (PDA) nanobowls, which can be prepared using Pluronic® F-127, ammonia, and 1,3,5-trimethylbenzene (TMB), are one of the most studied anisotropic nanoparticle systems. However, only limited reports on polymerised analogues polynorepinephrine (PNE) and polyepinephrine (PEP) exist. Herein, we present modifications to a one-pot, soft template method, originally applied to make PDA nanobowls, to fabricate new shape-anisotropic nanoparticles (mesoporous nanospheres or "nano-golf balls" and nanobowls) using PNE and PEP for the first time. These modifications include the use of different oil phases (TMB, toluene and o-xylene) and ammonia concentrations to induce anisotropic growth of PDA, PNE, and PEP particles. Moreover, this work features the application of oddly shaped PDA, PNE, and PEP nanoparticles as intravascular photoacoustic imaging enhancers in Intralipid®-India ink-based tissue-mimicking phantoms. Photoacoustic imaging experiments showed that mesoporous nanobowls exhibit stronger enhancement, in comparison to their mesoporous nano-golf ball and nanoaggregate counterparts. The photoacoustic enhancement also followed the general trend PDA > PNE > PEP due to the differences in the rates of polymerisation of the monomers and the optical absorption of the resulting polymers. Lastly, about two- to four-fold enhancement in photoacoustic signals was observed for the mesoporous nanostructures, when compared to smooth nanospheres and their nano-aggregates. These results suggest that shape manipulation can aid in overcoming the inherently lower performance of PNE and PEP as photoacoustic imaging agents, compared to PDA. Since nanomaterials with mesoporous and anisotropic morphologies have significant, unexplored potential with emerging applications, these results set the groundwork for future studies on photoacoustically active oddly shaped PNE- and PEP-based nanosystems.

Next-Generation Colloidal Materials for Ultrasound Imaging Applications.

Ultrasound has important applications, predominantly in the field of diagnostic imaging. Presently, colloidal systems such as microbubbles, phase-change emulsion droplets and particle systems with acoustic properties and multiresponsiveness are being developed to address typical issues faced when using commercial ultrasound contrast agents, and to extend the utility of such systems to targeted drug delivery and multimodal imaging. Current technologies and increasing research data on the chemistry, physics and materials science of new colloidal systems are also leading to the development of more complex, novel and application-specific colloidal assemblies with ultrasound contrast enhancement and other properties, which could be beneficial for multiple biomedical applications, especially imaging-guided treatments. In this article, we review recent developments in new colloids with applications that use ultrasound contrast enhancement. This work also highlights the emergence of colloidal materials fabricated from or modified with biologically derived and bio-inspired materials, particularly in the form of biopolymers and biomembranes. Challenges, limitations, potential developments and future directions of these next-generation colloidal systems are also presented and discussed.

Enhanced photoacoustic imaging in tissue-mimicking phantoms using polydopamine-shelled perfluorocarbon emulsion droplets.

The current work features process parameters for the ultrasound (25 kHz)-assisted fabrication of polydopamine-shelled perfluorocarbon (PDA/PFC) emulsion droplets with bimodal (modes at 100-600 nm and 1-6 µm) and unimodal (200-600 nm) size distributions. Initial screening of these materials revealed that only PDA/PFC emulsion droplets with bimodal distributions showed photoacoustic signal enhancement due to large size of their optically absorbing PDA shells. Performance of this particular type of emulsion droplets as photoacoustic agents were evaluated in Intralipid®-India ink media, mimicking the optical scattering and absorbanceof various tissuetypes. From these measurements, it was observed that PDA/PFC droplets with bimodal size distributions can enhance the photoacoustic signal of blood-mimicking phantom by up to five folds in various tissue-mimicking phantoms with absorption coefficients from 0.1 to 1.0 cm-1. Furthermore, using the information from enhanced photoacoustic images at 750 nm, the ultimate imaging depth was explored for polydopamine-shelled, perfluorohexane (PDA/PFH) emulsion droplets by photon trajectory simulations in 3D using a Monte Carlo approach. Based on these simulations, maximal tissue imaging depths for PDA/PFH emulsion droplets range from 10 to 40 mm, depending on the tissue type. These results demonstrate for the first time that ultrasonically fabricated PDA/PFC emulsion droplets have great potential as photoacoustic imaging agents that can be complemented with other reported characteristics of PDA/PFC emulsion droplets for extended applications in theranostics and other imaging modalities.

Tracking the heat-triggered phase change of polydopamine-shelled, perfluorocarbon emulsion droplets into microbubbles using neutron scattering.

Perfluorocarbon emulsion droplets are hybrid colloidal materials with vast applications, ranging from imaging to drug delivery, due to their controllable phase transition into microbubbles via heat application or acoustic droplet vapourisation. The current work highlights the application of small- and ultra-small-angle neutron scattering (SANS and USANS), in combination with contrast variation techniques, in observing the in situ phase transition of polydopamine-shelled, perfluorocarbon (PDA/PFC) emulsion droplets with controlled polydispersity into microbubbles upon heating. We correlate these measurements with optical and transmission electron microscopy imaging, dynamic light scattering, and thermogravimetric analysis to characterise these emulsions, and observe their phase transition into microbubbles. Results show that the phase transition of PDA/PFC droplets with perfluorohexane (PFH), perfluoropentane (PFP), and PFH-PFP mixtures occur at temperatures that are around 30-40 °C higher than the boiling points of pure liquid PFCs, and this is influenced by the specific PFC compositions (perfluorohexane, perfluoropentane, and mixtures of these PFCs). Analysis and model fitting of neutron scattering data allowed us to monitor droplet size distributions at different temperatures, giving valuable insights into the transformation of these polydisperse, emulsion droplet systems.

Exploring the transition of polydopamine-shelled perfluorohexane emulsion droplets into microbubbles using small- and ultra-small-angle neutron scattering.

Perfluorocarbon emulsion droplets are interesting colloidal systems with applications, ranging from diagnostics and theranostics to drug delivery, due to their controllable phase transition into microbubbles via heat application or acoustic droplet vapourisation. This work highlights the application of small- and ultra-small-angle neutron scattering (SANS and USANS, respectively), in combination with contrast variation techniques, in observing the in situ phase transition of polydopamine-stabilised perfluorohexane (PDA/PFH) emulsion droplets into microbubbles during heating. Results show peak USANS intensities at temperatures around 90 °C, which indicates that the phase transition of PDA/PFH emulsion droplets occurs at significantly higher temperatures than the bulk boiling point of pure liquid PFH (56 °C). Analysis and model fitting of the SANS and USANS data allowed us to estimate droplet sizes and interfacial properties at different temperatures (20 °C, 90 °C, and 20 °C after cooling), giving valuable insights about the transformation of these polydisperse emulsion droplet systems.

Ultrasound-assisted fabrication of acoustically active, erythrocyte membrane "bubbles".

In this communication, we report an ultrasound-assisted method, utilising human red blood cell (RBC) or erythrocyte membranes, to produce acoustically active "bubbles", intended for vasculature imaging. The resulting RBC membrane bubbles have an average size of 1.5 µm with a generally spherical morphology, altered internal aqueous compartment contents, and small gas-containing protrusions or "pockets" in between the membrane bilayer. We also found that this method produced some nanobubbles (200-400 nm diameter), due to the shedding of lipid components from the RBC membranes to compensate for the membrane structural changes. In vitro ultrasound imaging showed that RBC membrane bubbles had comparable ultrasound contrast enhancement as the standard DEFINTYTM microbubble preparation (~13% v/v) and lower concentrations of this standard contrast agent. This current technology demonstrate a new and important application of ultrasound and of RBC membranes, having inherent biocompatibility, as potential material for the development of new types of ultrasound imaging agents, without the use of additional lipid components and pre-made microbubbles.

Singularimetry of local phase gradients using vortex lattices and in-line holography.

We have developed a differential form of singularimetry, which utilizes phase vortices or intensity gradient singularities as topological fiducial markers in a structured illumination context. This approach analytically measures phase gradients imparted by refracting specimens, yielding quantitative information that is both local and deterministic. We have quantified our phase gradient experiments to demonstrate that lattices of wave field singularities can be used to detect subtle phase gradients imparted by a spherical specimen and fiber optic cylinders.

Local determination of thin liquid film profiles using colour interferometry.

We explore theoretically the interference of white light between two interfaces as a function of the optical conditions, using separately: a) idealised conditions where the light is composed of three discrete wavelengths; b) a more typically experimentally realisable case where light comprises a sum of three Gaussian wavelength distributions; and c) unfiltered white light from a broadband source comprising a broad distribution of wavelengths. It is demonstrated that the latter case is not only optically simple to arrange, but also provides unambiguous absolute separation information over the range 0-1µm --a useful range in studies of cell adhesion, thin liquid films and lubrication-- when coupled to detection using a typical colour camera. The utility of this technique is verified experimentally by exploring the air film between a cylinder and surface, as well as arbitrary liquid films beneath air bubbles that are interacting with solid surfaces.

Argand-plane vorticity singularities in complex scalar optical fields: an experimental study using optical speckle.

The Cornu spiral is, in essence, the image resulting from an Argand-plane map associated with monochromatic complex scalar plane waves diffracting from an infinite edge. Argand-plane maps can be useful in the analysis of more general optical fields. We experimentally study particular features of Argand-plane mappings known as "vorticity singularities" that are associated with mapping continuous single-valued complex scalar speckle fields to the Argand plane. Vorticity singularities possess a hierarchy of Argand-plane catastrophes including the fold, cusp and elliptic umbilic. We also confirm their connection to vortices in two-dimensional complex scalar waves. The study of vorticity singularities may also have implications for higher-dimensional fields such as coherence functions and multi-component fields such as vector and spinor fields.

Phase measurement using an optical vortex lattice produced with a three-beam interferometer.

A new phase-measurement technique is proposed, which utilizes a three-beam interferometer. Three-wave interference in the interferometer generates a uniform lattice of optical vortices, which is distorted by the presence of an object inserted in one arm of the interferometer. The transverse displacement of the vortices is proportional to the phase shift in the object wave. Tracking the vortices permits the phase of the object to be reconstructed. We demonstrate the method experimentally using a simple lens and a more complex object, namely the wing of a common house fly. Since the technique is implemented in real space, it is capable of reconstructing the phase locally.

Mask-assisted deterministic phase-amplitude retrieval from a single far-field intensity diffraction pattern: two experimental proofs of principle using visible light.

We recently developed a simple closed-form algorithm, which allows one to reconstruct the complex scalar wavefield at the exit surface of a sample, from the intensity of its far-field coherent diffraction pattern which is obtained in the presence of a suitable object-plane mask. In the first variant of this algorithm, the sample is contained within a uniformly illuminated sharp rectangular aperture in which at least one transverse dimension is at least twice that of the object. In the second variant, the sample is uniformly illuminated and is transversely displaced from an opaque rectangular mask in the object plane. For both variants, the far-field diffraction pattern is first Fourier transformed and then differentiated with respect to both transverse coordinates, in order to deterministically yield a series of independent reconstructions of the sample. Here we give an experimental demonstration of each of these two variants of our technique, using visible light.

Narrow-band coherent Rayleigh scattering in a flame.

We report on the application of coherent Rayleigh scattering (CRS) to the measurement of temperature in a flame using narrow bandwidth pump and probe fields. High resolution measurements of the line shape were used to derive flame temperature based on fits to the line shape. An uncertainty in the temperature of 3% was determined utilizing a CRS model that includes scattering from a multicomponent gas for the first time. This model was validated at room temperature for a mixture of atomic and molecular species.

OH concentration and temperature measurements by use of near-resonant holographic interferometry.

Near-resonant holographic interferometry is demonstrated to measure temperature and species concentration in a two-dimensional steady premixed air-acetylene flame. A peak temperature of (2600 +/- 100) K and a peak OH number density of (9.6 +/- 0.3) x 10(22) m(-3) are obtained, consistent with the expected values for such a flame. These values are determined by recording interferograms with a laser assumed sufficiently detuned from line center so that pressure and temperature broadening can be ignored. The results are thus obtained without making prior assumptions on the temperature or pressure of the flame beyond the existence of thermal equilibrium.

Optical microrheology using rotating laser-trapped particles.

We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.

Near-resonant holographic interferometry and absorption measurements of seeded atomic species in a flame.

The development of near-resonant holographic interferometry techniques for use on flows seeded with atomic species is described. A theoretical model for the refractivity that is due to the seed species is outlined, and an approximation to this model is also described that is shown to be valid for practical regimes of interest and allows the number density of the species to be determined without knowledge of line-broadening effects. The details of quantitative number density experiments performed on an air-acetylene flame are given, and a comparison with an alternative absorption-based experiment is made.

Enhanced flow visualization with near-resonant holographic interferometry.

Holographic interferometry measurements have been performed on high-speed, high-temperature gas flows with a laser output tuned near a resonant sodium transition. The technique allows the detection and quantification of the sodium concentration in the flow. By controlling the laser detuning and seeded sodium concentration, we performed flow visualization in low-density flows that are not normally detectable with standard interferometry. The technique was also successfully used to estimate the temperature in the boundary layer of the flow over a flat plate.