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

Phase-shift nanodroplets as an emerging sonoresponsive nanomaterial for imaging and drug delivery applications

Nanodroplets - emerging phase-changing sonoresponsive materials - have attracted substantial attention in biomedical applications for both tumour imaging and therapeutic purposes due to their unique response to ultrasound. As ultrasound is applied at different frequencies and powers, nanodroplets have been shown to cavitate by the process of acoustic droplet vapourisation (ADV), causing the development of mechanical forces which promote sonoporation through cellular membranes. This allows drugs to be delivered efficiently into deeper tissues where tumours are located. Recent reviews on nanodroplets are mostly focused on the mechanism of cavitation and their applications in biomedical fields. However, the chemistry of the nanodroplet components has not been discussed or reviewed yet. In this review, the commonly used materials and preparation methods of nanodroplets are summarised. More importantly, this review provides examples of variable chemistry components in nanodroplets which link them to their efficiency as ultrasound-multimodal imaging agents to image and monitor drug delivery. Finally, the drawbacks of current research, future development, and future direction of nanodroplets are discussed.

PLGA-Based Composites for Various Biomedical Applications

Polymeric materials have been extensively explored in the field of nanomedicine; within them, poly lactic-co-glycolic acid (PLGA) holds a prominent position in micro- and nanotechnology due to its biocompatibility and controllable biodegradability. In this review we focus on the combination of PLGA with different inorganic nanomaterials in the form of nanocomposites to overcome the polymer’s limitations and extend its field of applications. We discuss their physicochemical properties and a variety of well-established synthesis methods for the preparation of different PLGA-based materials. Recent progress in the design and biomedical applications of PLGA-based materials are thoroughly discussed to provide a framework for future research.

In vivo spectroscopic photoacoustic imaging and laser-induced nanoparticle vaporization for anti-HER2 breast cancer

This study reports on the development and application of theragnostic agents targeting the HER2 receptors in breast tumors. The agent was constructed by loading silica-coated gold nanorods (GNRs) and a perfluorohexane liquid into PLGA-PEG nanoparticles, followed by surface conjugation with antibody Herceptin. The particle uptake in human breast cancer MDA-MB-231 (HER2-negative) and BT474 (HER2-positive) cell lines was tested. A proof of principle in vivo study was also performed using a xenograft mouse bilateral tumor model (16 mice, 32 tumors). Photoacoustic imaging was performed using a VevoLAZR device at 720/750/850 nm illuminations and 21 MHz central frequency. The relative concentrations of GNRs in the tumor were quantified using a linear spectral unmixing technique. The therapeutic efficacy of these nanoparticles was evaluated through optical droplet vaporization, and cell damage was confirmed using tissue immunofluorescence and histology. Our results demonstrate the potential of PLGA-GNRs as theragnostic agents for anti-HER2 breast cancer therapy.

Ultrasonic technologies in imaging and drug delivery

Ultrasonic technologies show great promise for diagnostic imaging and drug delivery in theranostic applications. The development of functional and molecular ultrasound imaging is based on the technical breakthrough of high frame-rate ultrasound. The evolution of shear wave elastography, high-frequency ultrasound imaging, ultrasound contrast imaging, and super-resolution blood flow imaging are described in this review. Recently, the therapeutic potential of the interaction of ultrasound with microbubble cavitation or droplet vaporization has become recognized. Microbubbles and phase-change droplets not only provide effective contrast media, but also show great therapeutic potential. Interaction with ultrasound induces unique and distinguishable biophysical features in microbubbles and droplets that promote drug loading and delivery. In particular, this approach demonstrates potential for central nervous system applications. Here, we systemically review the technological developments of theranostic ultrasound including novel ultrasound imaging techniques, the synergetic use of ultrasound with microbubbles and droplets, and microbubble/droplet drug-loading strategies for anticancer applications and disease modulation. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theranostic tool.

Multifunctional nanoparticles as theranostic agents for therapy and imaging of breast cancer

Over the last decade, there has been significant developments in nanotechnology, in particular for combined imaging and therapeutic applications (theranostics). The core or shell of nanoemulsions (NEs) can be loaded with various therapeutic agents, including drugs with low solubility for effective treatment, or various imaging agents for specific imaging modalities (e.g., MRI, fluorescence). In this work, perfluorohexane (PFH) NEs were synthesized for theranostic applications and were coupled to silica coated gold nanoparticles (scAuNPs) to increase the generation of PFH bubbles upon laser induced vaporization (i.e., optical droplet vaporization). The localized heat generated from the absorption properties of these nanoparticles (used to provide photoacoustic signals) can also be used to treat cancer without significantly damaging nearby healthy tissues. The theranostic potential of these PFH-NEs for contrast imaging of tumors and as a drug-delivery vehicle for therapeutic purposes were demonstrated for both in vitro and in vivo systems using a combination of photoacoustic, ultrasound and fluorescence imaging modalities. The ability of PFH-NEs to couple with scAuNPs, attach to the membranes of cancer cells and internalize within cancer cells, are encouraging for targeted chemotherapeutic applications for directly inducing cancer cell death via vaporization in clinical settings.

Anti-HER2 PLGA-PEG polymer nanoparticle containing gold nanorods and paclitaxel for laser-activated breast cancer detection and therapy

Phase-transition nanoparticles have been identified as effective theragnostic, anti-cancer agents. However, non-selective delivery of these agents results in inaccurate diagnosis and insufficient treatment. In this study, we report on the development of targeted phase-transition polymeric nanoparticles (NPs) for the imaging and treatment of breast cancer cell lines over-expressing human epidermal growth factor receptor 2 (HER2). These NPs contain a perfluorohexane liquid interior and gold nanorods (GNRs) stabilized by biodegradable and biocompatible copolymer PLGA-PEG. Water-insoluble therapeutic drug Paclitaxel (PAC) and fluorescent dye were encapsulated into the PLGA shell. The NP surfaces were conjugated to HER2-binding agent, Herceptin, to actively target HER2-positive cancer cells. We evaluated the potential of using these NPs as a photoacoustic contrast agent. The efficacy of cancer cell treatment by laser-induced vaporization and stimulated drug release were also investigated. The results showed that our synthesized PLGA-PEG-GNRs (mean diameter 285 ± 29 nm) actively targeted HER2 positive cells with high efficacy. The laser-induced vaporization caused more damage to the targeted cells versus PAC-only and negative controls. This agent may provide better diagnostic imaging and therapeutic potential than current methods for treating HER2-positive breast cancer.

Design and Synthesis of Luminescent Lanthanide-Based Bimodal Nanoprobes for Dual Magnetic Resonance (MR) and Optical Imaging

Current biomedical imaging techniques are crucial for the diagnosis of various diseases. Each imaging technique uses specific probes that, although each one has its own merits, do not encompass all the functionalities required for comprehensive imaging (sensitivity, non-invasiveness, etc.). Bimodal imaging methods are therefore rapidly becoming an important topic in advanced healthcare. This bimodality can be achieved by successive image acquisitions involving different and independent probes, one for each mode, with the risk of artifacts. It can be also achieved simultaneously by using a single probe combining a complete set of physical and chemical characteristics, in order to record complementary views of the same biological object at the same time. In this scenario, and focusing on bimodal magnetic resonance imaging (MRI) and optical imaging (OI), probes can be engineered by the attachment, more or less covalently, of a contrast agent (CA) to an organic or inorganic dye, or by designing single objects containing both the optical emitter and MRI-active dipole. If in the first type of system, there is frequent concern that at some point the dye may dissociate from the magnetic dipole, it may not in the second type. This review aims to present a summary of current activity relating to this kind of dual probes, with a special emphasis on lanthanide-based luminescent nano-objects.

Laser activatable perfluorocarbon bubbles for imaging and therapy through enhanced absorption from coupled silica coated gold nanoparticles

Nanoparticles have extensively been used for cancer therapy and imaging (i.e., theranostics) using various imaging modalities. Due to their physical and chemical properties (e.g., absorption, fluorescence, and magnetic properties) they have been used for image guided therapy for cancer treatment monitoring. There are various limitations that make many theranostic agents unable to be used for the extended periods of time required for enhancing theranostic capabilities. Some of these are due to inherent characteristics (e.g., change and/or breakdown of structure) present upon continuous irradiation and others are due to environmental (i.e., physiological) conditions that can lead to physical instability (i.e., in terms of size) affecting the amount of particles that can accumulate at the target site and the overall contrast that can be achieved. In this study, perfluorohexane (PFH) nanoemulsions (NEs) were synthesized with silica coated gold nanoparticles (PFH-NEs-scAuNPs) in order to give both stable and enhanced signals for cancer imaging by increasing vaporization of the emulsions into bubbles through the process of optical droplet vaporization (ODV). The resulting perfluorohexane bubbles could be imaged using nonlinear ultrasound (NL US) which significantly increases the signal to noise ratio due to the nonlinear scattering properties of oscillating bubbles. The NL US signals from PFH bubbles were found to be more stable compared to conventional bubbles used for contrast imaging. In addition, the vaporization of PFH NEs into bubbles was shown to cause significant cancer cell death reflecting the theranostic capabilities of the formed PFH bubbles. Since cell death is initiated with laser excitation of PFH-NEs-scAuNPs, these nanoparticles can specifically target cancer cells once they have accumulated at the tumor region. Due to the type of theranostic agent and imaging modality used, the PFH-NEs-scAuNPs can be used to provide higher specificity compared to other agents for locating the tumor region by minimizing tissue specific signals while at the same time being used to treat cancer.

PFH-NEs from PFH-NEs-scAuNPs can vaporize upon laser excitation leading to formation of PFH bubbles that can be used for contrast enhanced US imaging and therapy.

A fluorous biphase drug delivery system triggered by low frequency ultrasound: controlled release from perfluorous discoidal porous silicon particles

Conventional drug delivery systems face unsatisfactory loading efficiency, poor biological bypass, and uncontrollable release, which are great barriers for improving the treatment of many diseases. Herein, a proof-of-concept of a fluorous biphase drug delivery system (FB-DDS) trigged by low frequency ultrasound (LFUS) is proposed for the first time, where promoted incorporation and stabilization of therapeutic agents in nanocarriers was achieved through fluorine-fluorine interactions, and the encapsulated drugs were controllably released by external sources, resulting in minimized nonspecific toxicity and enhanced therapeutic efficacy. The FB-DDS was constructed from monodisperse, discoidal porous silicon particles (PSP) and was functionalized with 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS17) for loading perfluoropentane (PFP) and fluorinated drugs through fluorine-fluorine interactions. This delivery system was demonstrated by utilizing model compounds including a fluorous-tagged fluorescein and a fluorine containing antibiotic ciprofloxacin. Loading of the model molecules into fluorocarbon-coated carriers was facilitated by fluorous interactions, whereas ejection of the model molecules was promoted by applying LFUS to rapidly evaporate PFP. In the in vitro test, these carriers loaded with fluorine containing ciprofloxacin exhibited excellent antimicrobial activity against Pseudomonas aeruginosa biofilm formation. Overall, this innovative stimulus-responsive fluorous biphase drug delivery system will be a promising candidate for practical applications as well as encouraging further investigation of drug delivery and controlled release strategies.

Polyelectrolyte Coatings Can Control Charged Fluorocarbon Nanodroplet Stability and Their Interaction with Macrophage Cells

Fluorocarbon nanodroplets, ∼100 to ∼400 nm in diameter, are of immense interest in a variety of medical applications including the imaging and therapy of cancer and inflammatory diseases. However, fluorocarbon molecules are both hydrophobic and lipophobic; therefore, it is challenging to synthesize fluorocarbon nanodroplets with the optimal stability and surface properties without the use of highly specialized surfactants. Here, we hypothesize that we can decouple the control of fluorocarbon nanodroplet size and stability from its surface properties. We use a simple, two-step procedure where standard, easily available anionic fluorosurfactants are used to first stabilize the fluorocarbon nanodroplets, followed by electrostatically attaching functionalized polyelectrolytes to the nanodroplet surfaces to independently control their surface properties. Herein, we demonstrate that PEGylated polyelectrolyte coatings can effectively alter the fluorocarbon nanodroplet surface properties to reduce coalescence and its uptake into phagocytic cells in comparison with non-PEGylated polyelectrolyte coatings and uncoated nanodroplets, as measured by flow cytometry and fluorescence microscopy. In this study, perfluorooctyl bromide (PFOB) was used as a representative fluorocarbon material, and PEGylated PFOB nanodroplets with diameters between 250 and 290 nm, depending on the poly(ethylene glycol) block length, were prepared. The PEGylated PFOB nanodroplets had superior size stability in comparison with uncoated and non-PEGylated polyelectrolyte nanodroplets in saline and within macrophage cells. Of significance, non-PEGylated nanodroplets were rapidly internalized by macrophage cells, whereas PEGylated nanodroplets were predominantly colocalized on the cell membrane. This suggests that the PEGylated-polyelectrolyte coating on the charged PFOB nanodroplets may afford adjustable shielding from cells of the reticuloendothelial system. This report shows that using the same fluorosurfactant as a base layer, modularly assembled PFOB nanodroplets tailored for a variety of end applications can be created by selecting different polyelectrolyte coatings depending on their unique requirements for stability and interaction with phagocytic cells.

Photothermal-Enhanced Phase-Transition Nanodroplets for Ultrasound-Mediated Diagnosis and Gene Transfection

Gene therapy is one of the promising solutions in cancer therapeutics. Ultrasound-mediated gene delivery showed great potential as a noninvasive strategy for gene therapy. However, the efficiency of gene transfection and incorporation of multiple functions remain key challenges in the development of gene delivery systems. In this study, we developed perfluoropentane (PFP) and gold nanorods (AuNRs) loading nanodroplets for photothermal-enhanced ultrasound-mediated imaging and gene transfection. The nanodroplet theranostic system was formulated with fluorinated cationic poly(aspartamide) based polymer that encapsulated PFP, AuNRs, and plasmid DNA and was stabilized with a negatively charged poly(glutamic acid)-g-MeO-poly(ethylene glycol) (PGA-g-mPEG) coating. The nanodroplets presented good stability, biocompatibility, and DNA binding stability. Upon treatment with both near-infrared and ultrasound energy, the photothermal and ultrasound-responsive system exerted a synergistic effect, in which strong adsorption of light induced hyperthermia that promoted the phase transition of PFP and the following ultrasound irradiation, generating strong acoustic cavitation and sonoporation, thus leading to enhanced ultrasound contrast imaging and gene transfection efficiency both in vitro and in vivo.

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.

Ultrasound-responsive droplets for therapy: A review

The last two decades have seen the development of acoustically activated droplets, also known as phase-change emulsions, from a diagnostic tool to a therapeutic agent. Through bubble effects and triggered drug release, these superheated agents have found potential applications from oncology to neuromodulation. The aim of this review is to summarise the key developments in therapeutic droplet design and use, to discuss the current challenges slowing clinical translation, and to highlight the new frontiers progressing towards clinical implementation. The literature is summarised by addressing the droplet design criteria and by carrying out a multiparametric study of a range of droplet formulations and their associated vaporisation thresholds.

Synthesis of mesoporous-silica-coated Gd2O3:Eu@silica particles as cell imaging and drug delivery agents

Mesoporous-silica-coated Gd2O3:Eu/silica nanoparticles were synthesized by a multistep chemical process and characterized by XRD, TEM and N2 adsorption/desorption isotherms in terms of size, morphology and porosity. The core Gd2O3:Eu obtained by this method was highly luminescent upon excitation, giving the function of cell imaging upon incubation with the human cervical carcinoma (HeLa) cells. The outer porous silica shell is able to load the anticancer drug with a relatively high loading efficiency and release the loaded drugs at a sustained rate. The HeLa cells can be killed effectively on incubation with the core-shell porous particles loaded with the anticancer drug DOX. Meanwhile, the accumulation of mesoporous nanoparticles loaded with drugs in the target location could be monitored via fluorescence imaging. Therefore, the core-shell hybrid nanoparticles presented in this work are potential multifunctional biomaterials for smart detection or diagnosis and therapy in future biomedical engineering.

18F-Labeled perfluorocarbon droplets for positron emission tomography imaging

INTRODUCTION

Nanoscale perfluorocarbon (PFC) droplets have been used to create imaging agents and drug delivery vehicles. However, development and characterization of new formulations of PFC droplets are hindered because of the lack of simple methods for quantitative and sensitive assessment of whole body tissue distribution and pharmacokinetics of the droplets. To address this issue, a general-purpose method for radiolabeling the inner core of nanoscale perfluorocarbon droplets with a hydrophobic and lipophobic fluorine-18 compound was developed, so that positron emission tomography (PET) and quantitative biodistribution studies can be employed to evaluate PFC nanodroplets in vivo.

METHODS

A robust method to produce [¹⁸F]CF₃(CF₂)₇(CH₂)₃F from a tosylate precursor using [¹⁸F]F^- was developed. The product's effectiveness as a general label for different PFCs and its ability to distinguish the in vivo behavior of different PFC droplet formulations was evaluated using two types of PFC nanodroplets: fluorosurfactant-stabilized perfluorohexane (PFH) nanodroplets and lipid-stabilized perfluorooctylbromide (PFOB) nanodroplets. In vivo assessment of the ¹⁸F-labeled PFH and PFOB nanodroplets were conducted in normal mice following intravenous injection using small animal PET imaging and gamma counting of tissues and fluids.

RESULTS

[¹⁸F]CF₃(CF₂)₇(CH₂)₃F was produced in modest yield and was stable with respect to loss of fluoride in vitro. The labeled fluorocarbon was successfully integrated into PFH nanodroplets (~175 nm) and PFOB nanodroplets (~260 nm) without altering their mean sizes, size distributions, or surface charges compared to their non-radioactive analogues. No leakage of the radiolabel from the nanodroplets was detected after droplet formation in vitro. PET imaging and biodistribution data for the two droplet types tested showed significantly different tissue uptake and clearance patterns.

CONCLUSION

A convenient method for producing ¹⁸F-labeled PFC droplets was developed. The results highlight the potential utility of the strategy for pre-clinical evaluation of different PFC droplet formulations through direct PFC core labeling using a fluorinated radiolabel.

Methods of Generating Submicrometer Phase-Shift Perfluorocarbon Droplets for Applications in Medical Ultrasonography

Continued advances in the field of ultrasound and ultrasound contrast agents have created new approaches to imaging and medical intervention. Phase-shift perfluorocarbon droplets, which can be vaporized by ultrasound energy to transition from the liquid to the vapor state, are one of the most highly researched alternatives to clinical ultrasound contrast agents (i.e., microbubbles). In this paper, part of a special issue on methods in biomedical ultrasonics, we survey current techniques to prepare ultrasound-activated nanoscale phase-shift perfluorocarbon droplets, including sonication, extrusion, homogenization, microfluidics, and microbubble condensation. We provide example protocols and discuss advantages and limitations of each approach. Finally, we discuss best practice in characterization of this class of contrast agents with respect to size distribution and ultrasound activation.

Biodegradable polymeric nanoparticles containing gold nanoparticles and Paclitaxel for cancer imaging and drug delivery using photoacoustic methods

In this study, optical-triggered multifunctional theranostic agents for photoacoustic/fluorescent imaging and cancer therapy have been developed. This system consists of a perfluorohexane liquid and gold nanoparticles (GNPs) in the core, stabilized by a Poly (lactide-co-glycolic acid) (PLGA) polymer shell. When cancer cells containing PLGA-GNPs were exposed to laser pulses, cell viability decreased due to the vaporization of the particles in and around the cells. The particle chemo drug loading and delivery capacity was also investigated in vitro experiments. These particles show potential as photoacoustic imaging and therapy agents for future clinical translation in cancer therapy.

Preparation of Fluorous Solvent-Dispersed Fe3O4 Nanocrystals: Role of Oxygen in Ligand Exchange

Perfluorinated ligand-passivated Fe3O4 nanocrystals were prepared through a biphasic ligand exchange method. It was found that dissolved oxygen in the reaction media predominantly determined the degree of ligand exchange and the resultant dispersion property of nanocrystals in a fluorous solvent. X-ray photoelectron spectroscopic analyses revealed that dissolved oxygen molecules bind to the surface iron species of nanocrystals in competition with the carboxylate moiety of ligands during the exchange reaction, lowering the degree of ligand exchange and colloidal stability significantly. Reducing the oxygen content of the fluorous phase by N2 bubbling was found to result in a highly stable dispersion of phase-transferred Fe3O4 nanocrystals with a single-particle size distribution maintained for a few months.

Size reduction of cosolvent-infused microbubbles to form acoustically responsive monodisperse perfluorocarbon nanodroplets

Perfluorocarbon (PFC) nanodroplet agents are exciting new biomaterials that can be remotely vapourized by ultrasound or light to change into micron-scale gas bubbles in situ. After PFC nanodroplet vapourization, the micron-scale gas bubble can interact strongly with ultrasound radiation, such that the bubbles can be used for cancer imaging and therapy. For these phase-change agents to be useful, however, PFC nanodroplets must be produced in the range of 100 to 400 nm in diameter with high size control and monodispersity, restrictions that remain a challenge. Here, we address this challenge by taking advantage of the size control offered by microfluidics, in combination with the size reduction provided by cosolvent-infused PFC bubbles through both condensation and cosolvent dissolution. In this approach, PFC bubbles with a high percentage of cosolvent (in this study, diethyl ether, DEE) are produced using microfluidics at a temperature above the boiling point. After synthesis, these bubbles become much smaller through both condensation of the gas into liquid droplets and from dissolution of the DEE into the continuous phase. This approach demonstrates that monodisperse, cosolvent-incorporated PFC bubbles can directly form monodisperse PFC nanodroplets a factor of 24 times smaller than the precursor bubbles. We also demonstrate that these nanoscale droplets can be converted to echogenic microbubbles after exposure to ultrasound, showing that these PFC nanodroplets are viable for the in situ production of ultrasound contrast agents. We show that this system can overcome the minimum droplet size limit of standard microfluidics, and is a powerful new tool for generating monodisperse, PFC phase-change ultrasound contrast agents for treating and imaging cancer.

Direct incorporation of lipophilic nanoparticles into monodisperse perfluorocarbon nanodroplets via solvent dissolution from microfluidic-generated precursor microdroplets

Multifunctional medical agents based on imaging or therapy nanoparticles (NPs) incorporated into perfluorocarbon (PFC) droplets are promising new agents for cancer detection and treatment. For the first time, monodisperse PFC nanodroplets labeled with NPs have been produced. Lipophilic, as-synthesized, hydrocarbon-stabilized NPs are directly miscibilized into lipophobic PFCs using a removable cosolvent, diethyl ether (DEE), which eliminates the need of the typical time-consuming and expertise-specific NP surface modification steps previously required for NP incorporation into PFCs. This NP-DEE/PFC solution is then used to synthesize monodisperse, micrometer-scale, DEE-infused NP-PFC precursor droplets in water using microfluidics. After precursor microdroplet generation, the DEE cosolvent is removed by dissolution and evaporation, resulting in dramatically smaller, monodisperse, NP-labeled nanodroplets, with final droplet sizes far smaller than the minimum droplet size limit of the microfluidic system, and easily controlled by the amount of DEE mixed in the PFC phase prior to precursor droplet synthesis. Using this technique, unmodified lipophilic quantum dot (QD) NPs were integrated into monodisperse and PFC nanodroplets 165 times smaller in volume than the precursor microdroplets, with dimensions down to 470 nm. The final droplet sizes scaled with the PFC concentrations in the precursor microdroplets, and the QDs remain localized within the droplets after DEE is removed from the system. This method is robust and versatile, and it comprises a platform technology for other unmodified lipophilic NPs and molecules to be incorporated into different types of PFC droplets for the production of new NP-PFC hybrid agents for medical imaging and therapy applications.

Photoacoustic and ultrasound imaging using dual contrast perfluorocarbon nanodroplets triggered by laser pulses at 1064 nm

Recently, a dual photoacoustic and ultrasound contrast agent-named photoacoustic nanodroplet-has been introduced. Photoacoustic nanodroplets consist of a perfluorocarbon core, surfactant shell, and encapsulated photoabsorber. Upon pulsed laser irradiation the perfluorocarbon converts to gas, inducing a photoacoustic signal from vaporization and subsequent ultrasound contrast from the resulting gas microbubbles. In this work we synthesize nanodroplets which encapsulate gold nanorods with a peak absorption near 1064 nm. Such nanodroplets are optimal for extended photoacoustic imaging depth and contrast, safety and system cost. We characterized the nanodroplets for optical absorption, image contrast and vaporization threshold. We then imaged the particles in an ex vivo porcine tissue sample, reporting contrast enhancement in a biological environment. These 1064 nm triggerable photoacoustic nanodroplets are a robust biomedical tool to enhance image contrast at clinically relevant depths.

Assessment of the biodistribution of an [(18) F]FDG-loaded perfluorocarbon double emulsion using dynamic micro-PET in rats

Perfluorocarbon (PFC) double emulsions loaded with a water-soluble, therapeutic agent can be triggered by ultrasound in a process known as acoustic droplet vaporization. Elucidating the stability and biodistribution of these sonosensitive vehicles and encapsulated agents is critical in developing targeted drug delivery strategies using ultrasound. [(18) F]fluorodeoxyglucose (FDG) was encapsulated in a PFC double emulsion and the in vitro diffusion of FDG was assessed using a Franz diffusion cell. Using dynamic micro-positron emission tomography and direct tissue sampling, the biodistribution of FDG administered as a solution (i.e. non-emulsified) or as an emulsion was studied in Fisher 344 rats (n = 6) bearing subcutaneous 9L gliosarcoma. Standardized uptake values (SUVs) and area under the curve of the SUV (AUCSUV ) of FDG were calculated for various tissues. The FDG flux from the emulsion decreased by up to a factor of 6.9 compared with the FDG solution. FDG uptake, calculated from the AUCSUV , decreased by 36% and 44% for brain and tumor, respectively, when comparing FDG solution vs FDG emulsion (p < 0.01). Decreases in AUCSUV in highly metabolic tissues such as brain and tumor demonstrated retention of FDG within the double emulsion. No statistically significant differences in lung AUCSUV were observed, suggesting minimal accumulation of the emulsion in the pulmonary capillary bed. The liver AUCSUV increased by 356% for the FDG emulsion, thus indicating significant hepatic retention of the emulsion.

Towards a nanoscale mammographic contrast agent: development of a modular pre-clinical dual optical/x-ray agent

Contrast-enhanced digital mammography (CEDM) can provide improved breast cancer detection and characterization compared to conventional mammography by imaging the effects of tumour angiogenesis. Current small-molecule contrast agents used for CEDM are limited by a short plasma half-life and rapid extravasation into tissue interstitial space. To address these limitations, nanoscale agents that can remain intravascular except at sites of tumour angiogenesis can be used. For CEDM, this agent must be both biocompatible and strongly attenuate mammographic energy x-rays. Nanoscale perfluorooctylbromide (PFOB) droplets have good x-ray attenuation and have been used in patients for other applications. However, the macroscopic scale of x-ray imaging (50-100 µm) is inadequate for direct verification that PFOB droplets localize at sites of breast tumour angiogenesis. For efficient pre-clinical optimization for CEDM, we integrated an optical marker into PFOB droplets for microscopic assessment (≪50 µm). To develop PFOB droplets as a new nanoscale mammographic contrast agent, PFOB droplets were labelled with fluorescent quantum dots (QDs). The droplets had mean diameters of 160 nm, fluoresced at 635 nm and attenuated x-ray spectra at 30.5 keV mean energy with a relative attenuation of 5.6 ± 0.3 Hounsfield units (HU) mg(-1) mL(-1) QD-PFOB. With the agent loaded into tissue phantoms, good correlation between x-ray attenuation and optical fluorescence was found (R(2) = 0.96), confirming co-localization of the QDs with PFOB for quantitative assessment using x-ray or optical methods. Furthermore, the QDs can be removed from the PFOB agent without affecting its x-ray attenuation or structural properties for expedited translation of optimized PFOB droplet formulations into patients.

Surface-functionalized nanoparticles for biosensing and imaging-guided therapeutics

In this article, the very recent progress of various functional inorganic nanomaterials is reviewed including their unique properties, surface functionalization strategies, and applications in biosensing and imaging-guided therapeutics. The proper surface functionalization renders them with stability, biocompatibility and functionality in physiological environments, and further enables their targeted use in bioapplications after bioconjugation via selective and specific recognition. The surface-functionalized nanoprobes using the most actively studied nanoparticles (i.e., gold nanoparticles, quantum dots, upconversion nanoparticles, and magnetic nanoparticles) make them an excellent platform for a wide range of bioapplications. With more efforts in recent years, they have been widely developed as labeling probes to detect various biological species such as proteins, nucleic acids and ions, and extensively employed as imaging probes to guide therapeutics such as drug/gene delivery and photothermal/photodynamic therapy.

Characterization of submicron phase-change perfluorocarbon droplets for extravascular ultrasound imaging of cancer

Because many tumors possess blood vessels permeable to particles with diameters of 200 nm, it is possible that submicron perfluorocarbon droplets could constitute a novel extravascular ultrasound contrast agent capable of selectively enhancing tumors. Under exposure to bursts of ultrasound of sufficient rarefactional pressure, droplets can undergo vaporization to form echogenic microbubbles. In this study, phase-change thresholds of 220-nm-diameter droplets composed of perfluoropentane were studied in polyacrylamide gel phantoms maintained at temperatures of 21-37°C, exposed to high-pressure bursts of ultrasound with frequencies ranging from 5-15 MHz and durations of 1 μs to 1 ms. The thresholds were found to depend inversely and significantly (p < 0.001) on ultrasound frequency, pulse duration, and droplet temperature, ranging from 9.4 ± 0.8 MPa at 29°C for a 1-μs burst at 5 MHz to 3.2 ± 0.5 MPa at 37°C for a 1-ms burst at 15 MHz. The diameters of microbubbles formed from droplets decreased significantly as phantom stiffness increased (p < 0.0001), and were independent of pulse duration, although substantially more droplets were converted to microbubbles for 1-ms pulse durations compared with briefer exposures. In vivo experiments in a mouse tumor model demonstrated that intravenously injected droplets can be converted into highly echogenic microbubbles 1 h after administration.

Superparamagnetic iron oxide and drug complex-embedded acoustic droplets for ultrasound targeted theranosis

Ultrasound-triggered acoustic droplet vaporization (ADV) has been reported as a mechanical and chemical theranostic strategy for tumor treatment. However, targeting of sufficient amounts of droplets to solid tumors to direct effective mechanical force toward tumor cells remains a major challenge. In this study, we incorporated superparamagnetic iron oxide (SPIO) nanoparticles into acoustic droplets to allow both magnetism-assisted targeting and magnetic resonance (MR)-guided ultrasound-triggered ADV. The multi-functionality of these droplets was further increased by co-encapsulation of the chemotherapeutic drug doxorubicin (DOX) and surface conjugation of anti-vascular endothelial growth factor receptor 2 antibody, to serve as an additional targeting moiety. Maximum loading capacities of 7.69 mg SPIO and 1.53 mg DOX per mL were achieved, and magnetic properties were characterized by determination of magnetic hysteresis curves and transverse relaxation rates. In vitro and in vivo MR imaging demonstrated the feasibility of dual modal imaging of SPIO-embedded droplets. Finally, a vessel-mimicking phantom model with live C6 glioma cells was used to demonstrate a 5.4-fold improvement in targeting efficacy by magnetism-assisted targeting of the SPIO-embedded droplets, and effective disruption of cells by insonation-induced ADV, suggesting the potential of developing this system for future clinical applications.

Intracellular growth of nanoscale perfluorocarbon droplets for enhanced ultrasound-induced phase-change conversion

Perfluorocarbon (PFC) nanodroplets (NDs) have been proposed as phase-change contrast agents for ultrasound imaging. Since the ultrasound energy required to convert PFC droplets to microbubbles is inversely related to size, the conversion of PFC NDs at clinically-relevant pressures is challenging. We propose that if PFC NDs can accumulate in a close-packed configuration and grow in size in situ, phase-change conversion can occur at lower ultrasound pressures compared with isolated NDs. In this article, we show that PFC NDs can be designed to grow in size after loading in cells, from 0.26 ± 0.09 μm to 1.7 ± 0.6 μm after 2 h. This growth allowed for a substantial decrease in the ultrasound conversion threshold (to 1.4 MPa and 4.8 MPa at 1 MHz and 18 MHz, respectively), whereas non-coalesced NDs in cells and NDs alone were not converted up to the maximum applied pressure (2.1 MPa and 6.3 MPa at 1 MHz and 18 MHz, respectively). These results indicate that PFC NDs with conversion pressures too high for diagnostically feasible conversion can be used as phase-change agents if they can be induced to grow in size in situ.