Perfluorocarbon Nanodroplets Research Articles

Phase-Transition Temperature of Gold-Nanorod-Coated Nanodroplets to Microbubbles by Pulsed Laser.

Previously, gold-nanorod-coated perfluorocarbon nanodroplets have been developed as light-activated on-demand drug delivery carriers. When gold nanorods on the perfluorocarbon nanodroplets resonate with a laser wavelength, plasmonic heat is generated and vaporizes the nanodroplets to gas bubbles. Optimal laser parameters such as pulse duration, pulse repetition frequency, and average power are critical to effectively trigger the phase transition of nanodroplets and allow for drug release. This study focused on determining the temperature of a gold-nanorod-coated perfluorocarbon nanodroplet during phase transition to a gas bubble using a femtosecond laser. Two integrated experimental and theoretical methods were explored. First, the theoretical temperature was determined by the Arrhenius equation and the time it took for the phase transition to occur, assuming the phase-transition process followed a first-order kinetic model. The activation energy and Arrhenius constant of the phase-transition process were obtained via light transmittance through a nanodroplet suspension at different temperatures. The time required for phase transition by a femtosecond laser was measured using an optical microscope. The second approach used a classical heat diffusion model. When the pulse peak energy was considered in the model, the temperature predicted matched the experimental observation of phase-transition temperature threshold, while the total energy value failed to predict the temperature threshold. The results suggest that the phase-transition mechanism is triggered by the vaporization of the nanodroplets via photothermal heating, which is influenced by the peak energy of the laser. It also indicates that optimal laser parameters can be determined by a simple calculation using the classical heat diffusion model and peak energy to control phase transition.

Phase Change Ultrasound Contrast Agents with a Photopolymerized Diacetylene Shell.

Phase change contrast agents for ultrasound (US) imaging consist of nanodroplets (NDs) with a perfluorocarbon (PFC) liquid core stabilized with a lipid or a polymer shell. Liquid ↔ gas transition, occurring in the core, can be triggered by US to produce acoustically active microbubbles (MBs) in a process named acoustic droplet vaporization (ADV). MB shells containing polymerized diacetylene moiety were considered as a good trade off between the lipid MBs, showing optimal attenuation, and the polymeric ones, displaying enhanced stability. This work reports on novel perfluoropentane and perfluorobutane NDs stabilized with a monolayer of an amphiphilic fatty acid, i.e. 10,12-pentacosadiynoic acid (PCDA), cured with ultraviolet (UV) irradiation. The photopolymerization of the diacetylene groups, evidenced by the appearance of a blue color due to the conjugation of ene-yne sequences, exhibits a chromatic transition from the nonfluorescent blue color to a fluorescent red color when the NDs are heated or the pH of the suspension is basic. An estimate of the molecular weights reached by the polymerized PCDA in the shell, poly(PCDA), has been obtained using gel permeation chromatography and MALDI-TOF mass spectrometry. The poly(PCDA)/PFC NDs show good biocompatibility with fibroblast cells. ADV efficiency and acoustic properties before and after the transition were tested using a 1 MHz probe, revealing a resonance frequency between 1 and 2 MHz similar to other lipidic MBs. The surface of PCDA shelled NDs can be easily modified without influencing the stability and the acoustic performances of droplets. As a proof of concept we report on the conjugation of cyclic RGD and PEG chains of the particles to support targeting ability toward endothelial cells.

Lipid Shell Composition Plays a Critical Role in the Stable Size Reduction of Perfluorocarbon Nanodroplets

Perfluorocarbon nanodroplets (PFCnDs) are phase-change contrast agents that have the potential to enable extravascular contrast-enhanced ultrasound and photoacoustic (US/PA) imaging. Producing consistently small, monodisperse PFCnDs remains a challenge without resorting to technically challenging methods. We investigated the impact of variable shell composition on PFCnD size and US/PA image properties. Our results suggest that increasing the molar percentage of PEGylated lipid reduces the size and size variance of PFCnDs. Furthermore, our imaging studies revealed that nanodroplets with more PEGylated lipids produce increased US/PA signal compared with those with the standard formulation. Finally, we highlight the ability of this approach to facilitate US/PA imaging in a murine model of breast cancer. These data indicate that, through a facile synthesis process, it is possible to produce monodisperse, small-sized PFCnDs. Novel in their simplicity, these methods may promote the use of PFCnDs among a broader user base to study a variety of extravascular phenomena.

Moving toward Noninvasive, Focused Ultrasound Therapeutic Delivery of Drugs in the Brain: Prolonged Opening of Blood-Brain Barrier May Not Be Needed.

Moving toward Noninvasive, Focused Ultrasound Therapeutic Delivery of Drugs in the Brain: Prolonged Opening of Blood-Brain Barrier May Not Be Needed.

Light, sound, nanobubbles: New approach to contrast-enhanced ultrasound and photoacoustic imaging

To overcome the most significant deficiencies of conventional and contrast-enhanced ultrasound imaging—low contrast and large size of microbubbles, we introduced new class of contrast agents—nanometer scale particles that are capable of escaping vascular compartments, penetrating into tissue, and then, once they reach the target site, generating sufficient ultrasound and photoacoustic contrast upon user-controlled optical activation. These multimodal contrast agents—phase-change perfluorocarbon nanodroplets and plasmonic nanoparticles covered by azide compounds—are stable at physiological temperatures, biocompatible, and monodisperse in size. Given the unique properties of the particles, our approach to image these particles is drastically different and is based on ultrasound read-out of the optically induced temporal changes. Specifically, time-varying ultrasound signals exhibited by the nanoparticles versus the static background are used to reconstruct a high contrast, background-free image of the contrast agent. Furthermore, these particles allow for multiplexed molecular imaging by permitting user-controlled triggering of distinct color-coded populations of contrast agents via tuning of the incident laser irradiation to match peak optical absorption of the particles. Finally, nanoparticles may also contain therapeutic cargo and thus can be used for controlled drug delivery and release. This presentation, via examples, will discuss diagnostic imaging and image-guided therapy using the gas-generating nanoparticles.

Selective intracellular delivery of perfluorocarbon nanodroplets for cytotoxicity threshold reduction on ultrasound-induced vaporization.

Phase-change nanodroplets (PCNDs), which are liquid perfluorocarbon nanoparticles, have garnered much attention as ultrasound-responsive nanomedicines. The vaporization phenomenon has been employed to treat tumors mechanically. However, the ultrasound pressure applied to induce vaporization must be low to avoid damage to nontarget tissues. Here, we report that the pressure threshold for vaporization to induce cytotoxicity can be significantly reduced by selective intracellular delivery of PCNDs into targeted tumors. In vitro experiments revealed that selective intracellular delivery of PCNDs induced PCND aggregation specifically inside the targeted cells. This close-packed configuration decreased the pressure threshold for vaporization to induce cytotoxicity. Moreover, following ultrasound exposure, significant decrease was observed in the viability of cells that incorporated PCNDs (35%) but not in the viability of cells that did not incorporate PCNDs (88%). Intracellular delivery of PCNDs reduced ultrasound pressure applied for vaporization to induce cytotoxicity. Confocal laser scanning microscopy and flow cytometry revealed that prolonged PCND-cell incubation increased PCND uptake and aggregation. This aggregation effect might have contributed to the cytotoxicity threshold reduction effect.

Color-coded perfluorocarbon nanodroplets for multiplexed ultrasound and photoacoustic imaging

Laser-activated perfluorocarbon nanodroplets are an emerging class of phase change, dual-contrast agents that can be utilized in ultrasound and photoacoustic imaging. Through the ability to differentiate subpopulations of nanodroplets via laser activation at different wavelengths of near-infrared light, optically-triggered color-coded perfluorocarbon nanodroplets present themselves as an attractive tool for multiplexed ultrasound and photoacoustic imaging. In particular, laser-activated droplets can be used to provide quantitative spatiotemporal information regarding distinct biological targets, allowing for their potential use in a wide range of diagnos tic and therapeutic applications. In the work presented, laser-activated color-coded perfluorocarbon nanodroplets are synthesized to selectively respond to laser irradiation at corresponding wavelengths. The dynamic ultrasound and photoacoustic signals produced by laser-activated perfluorocarbon nanodroplets are evaluated in situ prior to implementation in a murine model. In vivo, these particles are used to distinguish unique particle trafficking mechanisms and are shown to provide ultrasound and photoacoustic contrast for up to 72 hours within lymphatics. Overall, the conducted studies show that laser-activated color-coded perfluorocarbon nanodroplets are a promising agent for multiplexed ultrasound and photoacoustic imaging.

Facile fabrication of uniform nanoscale perfluorocarbon droplets as ultrasound contrast agents

Perfluorocarbon (PFC) nanodroplets have emerged as a novel phase-change contrast agent, remotely triggered by ultrasound in situ, which holds great potential for ultrasound imaging, early cancer diagnosis, and ultrasound-mediated cancer treatment. As the characteristics of PFC nanodroplets are highly size dependent, it is crucial to tightly control the size and uniformity of PFC nanodroplets, which remain a challenge using current available emulsification techniques. To meet this niche, we developed a novel method to produce monodisperse PFC nanodroplets by the Ouzo effect using a staggered herringbone micromixer. With this method, we fabricated PFC nanodroplets from ~ 200 to ~ 700 nm in diameter with a narrow size distribution (PDI < 0.1) and the throughput of nanodroplet production could be as high as 0.48 ml/h. The stability and biocompatibility of the prepared PFC nanodroplets were verified. Finally, the acoustic characterization of PFC nanodroplets was conducted to demonstrate the feasibility of PFC nanodroplets as ultrasound contrast agents via acoustic direct vaporization.

Perfluorocarbon nanodroplets can reoxygenate hypoxic tumors in vivo without carbogen breathing.

Nanoscale perfluorocarbon (PFC) droplets have enormous potential as clinical theranostic agents. They are biocompatible and are currently used in vivo as contrast agents for a variety of medical imaging modalities, including ultrasound, computed tomography, photoacoustic and 19F-magnetic resonance imaging. PFC nanodroplets can also carry molecular and nanoparticulate drugs and be activated in situ by ultrasound or light for targeted therapy. Recently, there has been renewed interest in using PFC nanodroplets for hypoxic tumor reoxygenation towards radiosensitization based on the high oxygen solubility of PFCs. Previous studies showed that tumor oxygenation using PFC agents only occurs in combination with enhanced oxygen breathing. However, recent studies suggest that PFC agents that accumulate in solid tumors can contribute to radiosensitization, presumably due to tumor reoxygenation without enhanced oxygen breathing. In this study, we quantify the impact of oxygenation due to PFC nanodroplet accumulation in tumors alone in comparison with other reoxygenation methodologies, in particular, carbogen breathing.Methods: Lipid-stabilized, PFC (i.e., perfluorooctyl bromide, CF3(CF2)7Br, PFOB) nanoscale droplets were synthesized and evaluated in xenograft prostate (DU145) tumors in male mice. Biodistribution assessment of the nanodroplets was achieved using a fluorescent lipophilic indocarbocyanine dye label (i.e., DiI dye) on the lipid shell in combination with fluorescence imaging in mice (n≥3 per group). Hypoxia reduction in tumors was measured using PET imaging and a known hypoxia radiotracer, [18F]FAZA (n≥ 3 per group).Results: Lipid-stabilized nanoscale PFOB emulsions (mean diameter of ~250 nm), accumulated in the xenograft prostate tumors in mice 24 hours post-injection. In vivo PET imaging with [18F]FAZA showed that the accumulation of the PFOB nanodroplets in the tumor tissues alone significantly reduced tumor hypoxia, without enhanced oxygen (i.e., carbogen) breathing. This reoxygenation effect was found to be comparable with carbogen breathing alone.Conclusion: Accumulation of nanoscale PFOB agents in solid tumors alone successfully reoxygenated hypoxic tumors to levels comparable with carbogen breathing alone, an established tumor oxygenation method. This study confirms that PFC agents can be used to reoxygenate hypoxic tumors in addition to their current applications as multifunctional theranostic agents.

Magnetic nanodroplets for targeted drug delivery

A limitation of microbubbles for both diagnostic and therapeutic applications is their relatively rapid clearance times, typically Notwithstanding their significant potential, the development of nanodroplets still poses some considerable challenges. The conversion efficiency, i.e., the proportion of droplets undergoing a phase change for a given set of ultrasound exposure conditions is often very low. Thus either very high concentrations of nanodroplets or potentially damaging ultrasound intensities are required. We therefore investigated the effect of loading perfluorocarbon nanodroplets with superparamagnetic solid nanoparticles to act as nucleation agents to promote phase transition thereby improving conversion efficiency. Using iron oxide particles also provides a means of imaging the droplets using magnetic resonance imaging (MRI) and manipulating the droplets using an external magnetic field which has been shown in previous work to be highly advantageous for drug delivery. We first determined the effect upon the physical properties of the nanodroplets in terms of their size, surface charge, and stability under physiological conditions. Their response to ultrasound exposure was then examined to determine conversion efficiency, change in size and potential for image contrast enhancement under both ultrasound and MRI. Finally, delivery of a small molecule chemotherapy drug, paclitaxel, and small interfering ribonucleic acid (siRNA) to different cancer cell lines was investigated. The nanodroplets were stable at 37°C over 10 days and showed a significantly higher conversion efficiency than a control droplet formulated without nanoparticles at 1 MHz and peak negative pressures Magnetic nanodroplets for targeted drug delivery

Hyaluronic acid and polydopamine functionalized phase change nanoparticles for ultrasound imaging-guided photothermal-chemotherapy.

In this study, we successfully prepared polydopamine (PD) and hyaluronic acid (HA) coated liquid perfluorocarbon nanoparticles, which integrated the good photothermal conversion effect of PD, the ultrasound imaging properties of the liquid fluorocarbon and active tumor-targeting of HA. Liquid perfluorocarbon nanodroplets significantly improved in terms of their physical stability after being coated with PD and HA as the shell. Photothermal and ultrasound imaging experiments showed that PD could effectively convert near-infrared (NIR) laser energy into heat for photothermal therapy, which could induce a phase change of the liquid perfluorocarbon, thereby generating microbubbles, enhancing ultrasound imaging signals and promoting drug release. In vitro cellular experiments showed that HA on the surface of the nanoparticles could actively target 4T1 breast cancer cells by CD44 receptor-mediated endocytosis. Furthermore, the novel nanosystem has shown effective inhibition of tumor growth by synergistic photothermal-chemotherapy in tumor-bearing mice. In conclusion, the novel nanosystem we prepared in this study is expected to provide a new strategy for the diagnosis and treatment of cancer.

Design and fabrication of multi-functional targeted photosensitizer-chemo drug-encapsulated perfluorocarbon nanodroplets for cancer treatment

Design and fabrication of multi-functional targeted photosensitizer-chemo drug-encapsulated perfluorocarbon nanodroplets for cancer treatment

Development of Rifampicin-Indocyanine Green-Loaded Perfluorocarbon Nanodroplets for Photo-Chemo-Probiotic Antimicrobial Therapy.

Acne vulgaris, generally resulted from overgrowth of Propionibacterium acnes (P. acnes), is one of the most difficult-to-treat facial dermatoses and more than 90% of adolescents experienced the disease worldwide. Because the innate non-lymphoid immune system cannot effectively eliminate excessive P. acnes from the skin surface, so far the therapy of acne vulgaris is still mainly dependent on antibiotic treatment. However, long-term or overdose of antibiotics may initiate microbial drug resistance and/or generate unexpected side effects that seriously hamper the use of antibiotics in the clinic. To overcome the aforementioned challenges, the novel rifampicin (RIF)-indocyanine green (ICG)-loaded perfluorocarbon (PFC) nanodroplets (RIPNDs) that may offer combined photo-, chemo-, and probiotic efficacies to P. acnes eradication were developed in this study. The RIPND was first characterized as a sphere-like nanoparticle with surface charge of −20.9 ± 2.40 mV and size of 240.7 ± 6.73 nm, in which the encapsulation efficiencies of RIF and ICG were 54.0 ± 10.5% and 95.0 ± 4.84%, respectively. In comparison to the freely dissolved ICG, the RIPNDs conferred an enhanced thermal stability to the entrapped ICG, and were able to provide a comparable hyperthermia effect and markedly increased production of singlet oxygen under near infrared (NIR; 808 nm, 6 W/cm2) exposure. Furthermore, the RIPNDs were able to induce fermentation of S. epidermidis but not P. acnes, indicating that the RIPNDs may serve as a selective fermentation initiator for the target probiotics. Based on the microbial population index analyses, P. acnes with 1 × 106 cells/mL can be completely eradicated by 12-h co-culture with S. epidermidis fermentation products followed by treatment of RIPNDs (≥20-μM ICG/3.8-μM RIF) + NIR for 5 min, whereby the resulted microbial mortality was even higher than that caused by using 16-fold enhanced amount of loaded RIF alone. Overall these efforts show that the RIPNDs were able to provide improved ICG stability, selective fermentability to S. epidermidis, and enhanced antimicrobial efficacy compared to equal dosage of free RIF and/or ICG, indicating that the developed nanodroplets are highly potential for use in the clinical anti-P. acne treatment with reduced chemotoxicity.

Laser-activated perfluorocarbon nanodroplets: a new tool for blood brain barrier opening.

A major obstacle in the monitoring and treatment of neurological diseases is the blood brain barrier (BBB), a semipermeable barrier that prevents the delivery of many therapeutics and imaging contrast agents to the brain. In this work, we explored the possibility of laser-activated perfluorocarbon nanodroplets (PFCnDs) to open the BBB and deliver agents to the brain tissue. Specifically, near infrared (NIR) dye-loaded PFCnDs comprised of a perfluorocarbon (PFC) core with a boiling point above physiological temperature were repeatedly vaporized and recondensed from liquid droplet to gas bubble under pulsed laser excitation. As a result, this pulse-to-pulse repeated behavior enabled the recurring interaction of PFCnDs with the endothelial lining of the BBB, allowing for a BBB opening and extravasation of dye into the brain tissue. The blood brain barrier opening and delivery of agents to tissue was confirmed on the macro and the molecular level by evaluating Evans Blue staining, ultrasound-guided photoacoustic (USPA) imaging, and histological tissue analysis. The demonstrated PFCnD-assisted pulsed laser method for BBB opening, therefore, represents a tool that has the potential to enable non-invasive, cost-effective, and efficient image-guided delivery of contrast and therapeutic agents to the brain.

Laser-Activated Bioprobes with High Photothermal Conversion Efficiency for Sensitive Photoacoustic/Ultrasound Imaging and Photothermal Sensing.

Laser-activated bioprobes with high photothermal conversion efficiency (IRPDA@PFH NDs) based on biocompatible IR-780 doped polydopamine perfluorocarbon nanodroplets (NDs) were developed. When protected by gelatin microspheres, their near-spherical morphologies can be easily observed with transmission electron microscope. Doping IR-780 (3 w/w % of added dopamine hydrochloride) can significantly enhance near-infrared (NIR) absorption and photothermal conversion efficiency to 57.7%. The enhanced NIR absorption and nonradiative relaxation are preferred to stronger photoacoustic (PA) signals and higher PA imaging definition; ultrasound (US) signals also increase more than 2.5 times because of easier phase change of NDs. These bioprobes had sensitive PA/US imaging capability with highly effective substitute utilizations, in which polydopamine was used either as a PA contrast or a photothermal agent. Perfluorocarbon can be used as an US contrast agent and temperature indicator. More importantly, the gray value increments of US increase with temperature in a general range from 35 to 55 °C. Especially, an approximate linear increasing of gray value in the optimized photothermal therapy (PTT) range from 35°C to 50°C could be used for the temperature monitoring and control of PTT. During PTT, the heated regions and the extent of photothermal heating can be visualized by US imaging. These findings indicate their great potential for biosensing and PTT monitoring.

Nanobomb optical coherence elastography.

Wave-based optical elastography is rapidly emerging as a powerful technique for quantifying tissue biomechanical properties due to its noninvasive nature and high displacement sensitivity. However, current approaches are limited in their ability to produce high-frequency waves and highly localized mechanical stress. In this Letter, we demonstrate that the rapid liquid-to-gas phase transition of dye-loaded perfluorocarbon nanodroplets ("nanobombs") initiated by a pulsed laser can produce highly localized, high-frequency, and broadband elastic waves. The waves were detected by an ultra-fast line-field low-coherence holography system. For comparison, we also excited waves using a focused micro-air-pulse. Results from tissue-mimicking phantoms showed that the nanobombs produced elastic waves with frequencies up to ∼9 kHz, which was much greater than the ∼2 kHz waves excited by the air-pulse. Consequently, the nanobombs enabled more accurate quantification of sample viscoelasticity. Combined with their potential for functionalization, the nanobombs show promise for accurate and highly specific noncontact all-optical elastography.

Efficient and controllable thermal ablation induced by short-pulsed HIFU sequence assisted with perfluorohexane nanodroplets

A HIFU sequence with extremely short pulse duration and high pulse repetition frequency can achieve thermal ablation at a low acoustic power using inertial cavitation. Because of its cavitation-dependent property, the therapeutic outcome is unreliable when the treatment zone lacks cavitation nuclei. To overcome this intrinsic limitation, we introduced perfluorocarbon nanodroplets as extra cavitation nuclei into short-pulsed HIFU-mediated thermal ablation. Two types of nanodroplets were used with perfluorohexane (PFH) as the core material coated with bovine serum albumin (BSA) or an anionic fluorosurfactant (FS) to demonstrate the feasibility of this study. The thermal ablation process was recorded by high-speed photography. The inertial cavitation activity during the ablation was revealed by sonoluminescence (SL). The high-speed photography results show that the thermal ablation volume increased by ∼643% and 596% with BSA-PFH and FS-PFH, respectively, than the short-pulsed HIFU alone at an acoustic power of 19.5 W. Using nanodroplets, much larger ablation volumes were created even at a much lower acoustic power. Meanwhile, the treatment time for ablating a desired volume significantly reduced in the presence of nanodroplets. Moreover, by adjusting the treatment time, lesion migration towards the HIFU transducer could also be avoided. The SL results show that the thermal lesion shape was significantly dependent on the inertial cavitation in this short-pulsed HIFU-mediated thermal ablation. The inertial cavitation activity became more predictable by using nanodroplets. Therefore, the introduction of PFH nanodroplets as extra cavitation nuclei made the short-pulsed HIFU thermal ablation more efficient by increasing the ablation volume and speed, and more controllable by reducing the acoustic power and preventing lesion migration.

Organic Semiconducting Photoacoustic Nanodroplets for Laser-Activatable Ultrasound Imaging and Combinational Cancer Therapy.

Combination of photoacoustic (PA) and ultrasound (US) imaging offers high spatial resolution images with deep tissue penetration, which shows great potential in applications in medical imaging. Development of PA/US dual-contrast agents with high contrast and excellent biocompatibility is of great interest. Herein, an organic semiconducting photoacoustic nanodroplet, PS-PDI-PAnD, is developed by stabilizing low-boiling-point perfluorocarbon (PFC) droplet with a photoabsorber and photoacoustic agent of perylene diimide (PDI) molecules and coencapsulating the droplet with photosensitizers of ZnF16Pc molecules. Upon irradiation, the PDI acts as an efficient photoabsorber to trigger the liquid-to-gas phase transition of the PFC, resulting in dual-modal PA/US imaging contrast as well as photothermal heating. On the other hand, PFC can serve as an O2 reservoir to overcome the hypoxia-associated resistance in cancer therapies, especially in photodynamic therapy. The encapsulated photosensitizers will benefit from the sustained oxygen release from the PFC, leading to promoted photodynamic efficacy regardless of pre-existing hypoxia in the tumors. When intravenously injected into tumor-bearing mice, the PS-PDI-PAnDs show a high tumor accumulation via EPR effect. With a single 671 nm laser irradiation, the PS-PDI-PAnDs exhibit a dual-modal PA/US imaging-guided synergistic photothermal and oxygen self-enriched photodynamic treatment, resulting in complete tumor eradication and minimal side effects. The PS-PDI-PAnDs represents a type of PFC nanodroplets for synergistic PDT/PTT treatment upon a single laser irradiation, which is expected to hold great potential in the clinical translation in dual-modal PA/US imaging-guided combinational cancer therapy.

PHOTOACOUSTIC CAVITATION OF NANODROPLETS: HIGH SPEED OBSERVATION AND ACOUSTIC SIGNAL ANALYSIS

Perfluorocarbon (PFC) nanodroplet has a longer lifespan and small volume to circulate freely in blood microcirculation. Its phase transition has great potential in localized therapy. Photoacoustic cavitation, a new approach involving synergetic laser and ultrasound to achieve the phase transition of PFC nanodroplets, could improve the spatial and temporal resolution and reduce the energy threshold compared to individual use of either modalities. In this study, we present a confocal microscopic system to induce photoacoustic cavitation of nanodroplets and study its characteristics by high speed observation and acoustic detection. Upon excitation by laser and ultrasound pulses, the liquid–gas phase transition of PFC nanodroplets and subsequent bubble dynamics were demonstrated. Two distinct bubble formation patterns were observed. It is associated with the size distribution of the droplets within the confocal volume of laser and ultrasound. Moreover, the detected acoustic signals suggested the presence of stable cavitation. Due to the reduced energy threshold of phase transition, the proposed method enables the theranostic applications of PFC nanodroplets at lower and safer laser/ultrasound energies in tissues or cells. Detection of high amplitude signals also revealed the potential of this method to generate strong photoacoustic signals for the photoacoustic imaging.

Copper Sulfide Perfluorocarbon Nanodroplets as Clinically Relevant Photoacoustic/Ultrasound Imaging Agents.

We have developed laser-activated perfluorocarbon nanodroplets containing copper sulfide nanoparticles (CuS NPs) for contrast-enhanced ultrasound and photoacoustic imaging. As potential clinical contrast agents, CuS NPs have favorable properties including biocompatibility, biodegradability, and enhance contrast in photoacoustic images at clinically relevant depths. However, CuS NPs are not efficient optical absorbers when compared to plasmonic nanoparticles and therefore, contrast enhancement with CuS NPs is limited, requiring high concentrations to generate images with sufficient signal-to-noise ratio. We have combined CuS NPs with laser-activated perfluorocarbon nanodroplets (PFCnDs) to achieve enhanced photoacoustic contrast and, more importantly, ultrasound contrast while retaining the favorable clinical characteristics of CuS NPs. The imaging characteristics of synthesized CuS-PFCnD constructs were first tested in tissue-mimicking phantoms and then in in vivo murine models. The results demonstrate that CuS-PFCnDs enhance contrast in photoacoustic (PA) and ultrasound (US) imaging. Upon systemic administration in vivo, CuS-PFCnDs remain stable and their unique vaporization provides sufficient PA/US contrast that can be further exploited for contrast-enhanced background-free imaging. The conducted studies provide a solid foundation for further development of CuS-PFCnDs as PA/US diagnostic and eventually therapeutic agents for clinical applications.