Surface tension of water in the presence of perfluorocarbon vapors

Nanodroplet vaporization with pulsed-laser excitation repeatedly amplifies photoacoustic signals at low vaporization thresholds

Nanodroplets' explosive vaporization triggered by absorption of laser pulses produces very large volume changes. These volume changes are two orders of magnitude higher than those of thermoelastic expansion generated by equivalent laser pulses, and should generate correspondingly higher photoacoustic waves (PAW). The generation of intense PAWs is desirable in photoacoustic tomography (PAT) to increase sensitivity. The biocompatibility and simplicity of nanodroplets obtained by sonication of perfluoropentane (PFP) in an aqueous solution of bovine serum albumin (BSA) containing a dye make them particularly appealing for use as contrast agents in clinical applications of PAT. Their usefulness depends on stability and reproducible vaporization of nanodroplets (liquid PFP inside) to microbubbles (gaseous PFP inside), and reversible condensation to nanodroplets. This work incorporates porphyrins with fluorinated chains and BSA labelled with fluorescent probes in PFP nanodroplets to investigate the structure and properties of such nanodroplets. Droplets prepared with average diameters in the 400-1000 nm range vaporize when exposed to nanosecond laser pulses with fluences above 3 mJ cm-2 and resist coalescence. The fluorinated chains are likely responsible for the low vaporization threshold, ∼2.5 mJ cm-2, which was obtained from the laser fluence dependence of the photoacoustic wave amplitudes. Only ca. 10% of the droplets incorporate fluorinated porphyrins. Nevertheless, PAWs generated with nanodroplets are ten times higher than those generated by aqueous BSA solutions containing an equivalent amount of porphyrin. Remarkably, successive laser pulses result in similar amplification, indicating that the microbubbles revert back to nanodroplets at a rate faster than the laser repetition rate (10 Hz). PFP nanodroplets are promising contrast agents for PAT and their performance increases with properly designed dyes.

Dynamic surface tension probe for measuring the concentration of extracellular vesicles

The concentration of extracellular vesicles (EVs) is an essential attribute of biofluids and EV preparations. EV concentration in body fluids was correlated with health status. The abundance of EV secreted by cultured cells into growth medium is vital in signaling studies, tissue and disease models, and biomanufacturing of acellular therapeutic secretome. A limited number of physical principles sensitive to EV concertation have been discovered so far. Particle-by-particle counting methods enumerate individual particles scattering light, modulating the Coulter current, or appearing in EM images. The available ensemble techniques in current use rely on the concentration-dependent signal intensity, as in the case of ELISA. In this study, we propose for the first-time the ensemble-based characterization of EV concentration by dynamic surface tension (DST) probe and demonstrate its implementation. We show that DST measurements agree with the widely used NTA measurements of EV concertation. The proposed method is low-cost and requires only basic laboratory equipment for implementation.

Investigation of the different parameters contributing to bubble sticking inside physiological bifurcations

Gas embolotherapy (GE) is a developing medical method which can be utilized either as an autonomous therapeutic method to treat vascularized solid tumors, or it can be combined with other medical procedures-such as high-intensity focused ultrasound-to improve their efficiency. This paper is dedicated to investigating the different parameters which influence bubble lodging inside human vasculature via 2D-modeling of bubble dynamics in arteries' and arterioles' bifurcations which are potential sticking positions. Values used in the simulations are in accordance with the non-dimensional physiological numbers. It is found out that inlet pressure plays a decisive role in bubble lodging; the lower the value, the higher the possibility of bubble sticking. On the other hand, gravity has a counteracting effect on bubble lodging in arteries, but not on arterioles. The initial length of the bubble is not a determining factor in sticking behavior, even though it affects the flow rate behavior. Surface tension, another critical factor, has a semi-linear impact on bubble resisting power; lowering the surface tension will reduce bubble resistance to the flow, diminishing the possibility of bubble lodging. Finally, it is shown that lower values for the static contact angle impose higher resistance to the flow.

Optoacoustic/Fluorescent/Acoustic Imaging Probe Based on Air-Filled Bubbles Functionalized with Gold Nanorods and Fluorescein Isothiocyanate

Liquid/surfactant/gas interfaces are promising objects for nanoengineered multimodal contrasts, which can be used for biomedical imaging in preclinical and clinical applications. Microbubbles with the gaseous core and shell made of lipids/proteins have already acted as ultrasound (US) contrast agents for angiography. In the present work, microbubbles with a shell composed of Span 60 and Tween 80 surfactants functionalized with fluorescein isothiocyanate and gold nanorods to achieve a multimodal combination of US, fluorescence, and optoacoustic imaging are described. Optimal conditions for microbubble generation by studying the surface tension of the initial solutions and analyzing the size, stability, and charge of the resulting bubbles were found. By controlling and modifying bubbles' surface properties, an increase in stability and storage time can be achieved. The functionalization of bubbles with gold nanoparticles and a dye by using an optimally selected sonication protocol was performed. The biomedical application's potential in imaging modalities of functionalized microbubbles using a medical US device with a frequency of 50 MHz, fluorescence tomography, and raster-scanning optoacoustic mesoscopy measurements was evaluated. The obtained results are important for optimum stabilization and functionalization of gas/liquid interfaces and the following applications in the multimodal biomedical imaging.

Air-Filled Bubbles Stabilized by Gold Nanoparticle/Photodynamic Dye Hybrid Structures for Theranostics

Microbubbles have already reached clinical practice as ultrasound contrast agents for angiography. However, modification of the bubbles’ shell is needed to produce probes for ultrasound and multimodal (fluorescence/photoacoustic) imaging methods in combination with theranostics (diagnostics and therapeutics). In the present work, hybrid structures based on microbubbles with an air core and a shell composed of bovine serum albumin, albumin-coated gold nanoparticles, and clinically available photodynamic dyes (zinc phthalocyanine, indocyanine green) were shown to achieve multimodal imaging for potential applications in photodynamic therapy. Microbubbles with an average size of 1.5 ± 0.3 μm and concentration up to 1.2 × 10⁹ microbubbles/mL were obtained and characterized. The introduction of the dye into the system reduced the solution’s surface tension, leading to an increase in the concentration and stability of bubbles. The combination of gold nanoparticles and photodynamic dyes’ influence on the fluorescent signal and probes’ stability is described. The potential use of the obtained probes in biomedical applications was evaluated using fluorescence tomography, raster-scanning optoacoustic microscopy and ultrasound response measurements using a medical ultrasound device at the frequency of 33 MHz. The results demonstrate the impact of microbubbles’ stabilization using gold nanoparticle/photodynamic dye hybrid structures to achieve probe applications in theranostics.

Repetitive optical coherence elastography measurements with blinking nanobombs

Excitation of dye-loaded perfluorocarbon nanoparticles (nanobombs) can generate highly localized axially propagating longitudinal shear waves (LSW) that can be used to quantify tissue mechanical properties without transversal scanning of the imaging beam. In this study, we used repetitive excitations of dodecafluoropentane (C5) and tetradecafluorohexane (C6) nanobombs by a nanosecond-pulsed laser to produce multiple LSWs from a single spot in a phantom. A 1.5 MHz Fourier-domain mode-locked laser in combination with a phase correction algorithm was used to perform elastography. Multiple nanobomb activations were also monitored by detecting photoacoustic signals. Our results demonstrate that C6 nanobombs can be used for repetitive generation of LSW from a single spot for the purpose of material elasticity assessment. This study opens new avenues for continuous quantification of tissue mechanical properties using single delivery of the nanoparticles.

On the influence of surfactant on the coarsening of aqueous foams

We review the coarsening process of foams made with various surfactants and gases, focusing on physico-chemical aspects. Several parameters strongly affect coarsening: foam liquid fraction and foam film permeability, this permeability depending on the surfactant used. Both parameters may evolve with time: the liquid fraction, due to gravity drainage, and the film permeability, due to the decrease of capillary pressure during bubble growth, and to the subsequent increase in film thickness. Bubble coalescence may enhance the bubble's growth rate, in which case the bubble polydispersity increases. The differences found between the experiments reported in the literature and between experiments and theories are discussed.

Effects of Substrate Heating and Wettability on Evaporation Dynamics and Deposition Patterns for a Sessile Water Droplet Containing Colloidal Particles

Effects of substrate temperature, substrate wettability, and particle concentration are experimentally investigated for evaporation of a sessile water droplet containing colloidal particles. Time-varying droplet shapes and temperature of the liquid-gas interface are measured using high-speed visualization and infrared thermography, respectively. The motion of the particles inside the evaporating droplet is qualitatively visualized by an optical microscope and the profile of the final particle deposit is measured by an optical profilometer. On a nonheated hydrophilic substrate, a ring-like deposit forms after the evaporation, as reported extensively in the literature, while on a heated hydrophilic substrate, a thinner ring with an inner deposit is reported in the present work. The latter is attributed to Marangoni convection, and recorded motion of the particles as well as measured temperature gradient across the liquid-gas interface confirms this hypothesis. The thinning of the ring scales with the substrate temperature and is reasoned to stronger Marangoni convection at larger substrate temperature. In the case of a nonheated hydrophobic substrate, an inner deposit forms due to very early depinning of the contact line. On the other hand, in the case of a heated hydrophobic substrate, the substrate heating as well as larger particle concentration helps in the pinning of the contact line, which results in a thin ring with an inner deposit. We propose a regime map for predicting three types of deposits-namely, ring, thin ring with inner deposit, and inner deposit-for varying substrate temperature, substrate wettability, and particle concentration. A first-order model corroborates the liquid-gas interface temperature measurements and variation in the measured ring profile with the substrate temperature.