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In this paper, we report ultrasonically active nanoscale contrast agents that behave as thermometric sensors through phase change in their stabilizing phospholipid monolayer. Phospholipid-stabilized, hydrophobic mesoporous silica nanoparticles (P@hMSNs) are known to interact with high-intensity focused ultrasound (HIFU) to promote cavitation at their surfaces, which can be used for both imaging and therapy. We show that the lateral lipid phase behavior of the phosphocholine lipid dictates the acoustic contrast of the P@hMSNs. When the lipids are in the gel phase below their melting temperature, the P@hMSNs generate detectable microbubbles when exposed to HIFU. However, if the lipids exhibit a liquid expanded phase, the P@hMSNs cease to generate bubbles in response to HIFU insonation. We verify that the heating and subsequent transition of lipid coating the hMSN are associated with the loss of acoustic response by doping laurdan dye into the lipid monolayer and imaging lipid phase through red shifts in emission spectra. Similarly, cessation of cavitation was also induced by adding a fluidizing surfactant such as Triton X, which could be reversed upon washing away the excess surfactant. Finally, by controlling for the partial fluidization caused by the adsorption of protein, P@hMSNs may be used as thermometric sensors of the bulk fluid temperature. These findings not only impact the utilization of nanoscale agents as stimulus-responsive ultrasound contrast agents but also have broader implications for how cavitation may be initiated at surfaces coated by a surfactant.

While thermal ablation of various solid tumors has been demonstrated using high intensity focused ultrasound (HIFU), the therapeutic outcomes of this technique are still unsatisfactory because of common recurrence of thermally ablated cancers and treatment side effects due to the high ultrasound intensity and acoustic pressure requirements. More precise ablation of tumors can be achieved by generating cavitating bubbles in the tissue using shorter pulses with higher acoustic pressures, which induce mechanical damage rather than thermal. However, it has remained as a challenge to safely deliver the acoustic pressures required for mechanical ablation of solid tumors. Here, we report a method to achieve mechanical ablation at lower acoustic pressures by utilizing phospholipid-stabilized hydrophobic mesoporous silica nanoparticles (PL-hMSN). The PL-hMSNs act as seeds for nucleation of cavitation events and thus significantly reduce the peak negative pressures and spatial-average temporal-average HIFU intensities needed to achieve mechanical ablation. Substantial mechanical damage was observed in the red blood cell or tumor spheroid containing tissue mimicking phantoms at PL-hMSN concentrations as low as 10 μg mL -1 , after only 5 s of HIFU treatment with peak negative pressures ∼11 MPa and duty cycles ∼0.01%. Even the application of HIFU (peak negative pressure of 16.8 MPa and duty cycle of 0.017%) for 1 min in the presence of PL-hMSN (200 μg mL -1 ) did not cause any detectable temperature increase in tissue-mimicking phantoms. In addition, the mechanical effects of cavitation promoted by PL-hMSNs were observed up to 0.5 mm from the center of the cavitation events. This method may thus also improve delivery of therapeutics or nanoparticles to tumor environments with limited macromolecular transport.

Mutually reactive, fluorogenic molecules are presented as a simple and novel technique for in-solution biosensing. The hypothesis behind this work was that aggregating droplets into close proximity would cause rapid mixing of their contents. To take advantage of this effect, a novel pair of fluorogenic redox molecules were designed to remain in lipid-stabilized oil droplets but mix once aggregated. First, the hydrophobic cyanine dye 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI) was reduced with sodium borohydride to form a nonfluorescent analog (HDiI). Hydrophobic quinone derivatives were then screened as oxidizing agents, and it was found that p-fluoranil oxidized nonfluorescent HDiI back to fluorescent DiI. Next, HDiI and p-fluoranil were loaded into NEOBEE oil nanodroplets of average diameter 600 nm that were stabilized by a monolayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE)-polyethylene glycol (PEG), and DSPE-PEG-biotin. Addition of streptavidin caused aggregation of droplets and the appearance of red fluorescent aggregates within 30 min. Next, Nanoparticle Tracking Analysis was used to record the fluorescence of the droplets and their aggregates. By integrating the fluorescence emission of the tracked droplets, streptavidin could be detected down to 100 fM. Finally, the droplets were reformulated to sense for vascular endothelial growth factor (VEGF), a biomarker for tumor metastasis. Using anti-VEGF aptamers attached to DSPE-PEG incorporated into the nanodroplet monolayer, VEGF could also be detected down to 100 fM.