Stable Encapsulated Air Nanobubbles in Water

The dispersion into water of nanocapsules bearing a highly hydrophobic fluorinated internal lining yielded encapsulated air nanobubbles. These bubbles, like their micrometer‐sized counterparts (microbubbles), effectively reflected ultrasound. More importantly, the nanobubbles survived under ultrasonication 100‐times longer than a commercial microbubble sample that is currently in clinical use. We justify this unprecedented stability theoretically. These nanobubbles, owing to their small size and potential ability to permeate the capillary networks of tissues, may expand the applications of microbubbles in diagnostic ultrasonography and find new applications in ultrasound‐regulated drug delivery.     

Effect of ultrasonication on the flotation of fine graphite particles: Nanobubbles or not?

It has been reported that nanobubbles can be produced by ultrasonication. However, it remains unclear whether part of the contribution of ultrasonication on flotation performance can be attributed to the generation of nanobubbles. In this work, we systematically studied this point of ultrasonication by combining a series of techniques including flotation testing, AFM (atomic force microscope) measurement, and settling testing. AFM imaging showed that no surface nanobubbles were found at the HOPG-water interface before and after subjection to ultrasonication. Further, surface nanobubbles were generated with solution exchange before ultrasonciation and then they were subjected to ultrasonication. It was found that ultrasonication did not destroy the pre-existing surface nanobubbles at the HOPG (highly oriented pyrolytic graphite) -water interface. Settling tests and flotation tests indicate that ultrasonication has a negligible influence on the interaction between graphite particles and thus flotation performance. Nanobubbles were not one of the outcomes for ultrasonication.     

Plasmonics for pulsed-laser cell nanosurgery: Fundamentals and applications

This review describes the fundamental aspects of pulsed laser interaction with plasmonic nanostructures, and its applications to cell nanosurgery, including the destruction, modification or manipulation of molecular, sub-cellular and cellular structures. The review assumes no prior knowledge of the field of plasmonics and begins with a short review of the basic theory of plasmon excitation and optical properties of nanoscale metallic structures. Fundamentals of short and ultrashort laser pulse interaction with plasmonic nanostructures in a water environment are then discussed. Special emphasis is put on the consequences of the irradiation on the surrounding environment of the nanostructure, including heating, low-density plasma generation, pressure wave release and formation of vapor bubbles. The paper is concluded with a review of different applications of pulsed-laser interaction with plasmonic nanostructures for cell nanosurgery, including photothermal therapy, plasmonic enhanced cell transfection, molecular surgery and drug delivery.     

Research progress on bulk nanobubbles

There are still fundamental problems lying in the basic research of bulk nanobubbles. Are the bulk nanobubbles reported in the literature nano scale bubbles or contaminants in fact? At present, there is not yet sufficient experimental evidence to show that the bulk nanoparticles are only gas bubbles but not other nano scale contaminants. If they are indeed nanobubbles, what causes the bulk nanobubbles observed in the literature to be much more stable than being predicted by the Epstein-Plesset theory?This paper firstly discusses the contradiction between the traditional theory prediction and the observed lifetime of the bulk nanobubbles, and then discusses whether the so-called nanobubbles are gas aggregates. We review the existing typical models, and the influence of different conditions on the stability of bulk nanobubbles, for paving the road to a clear understanding of the stability mechanism of bulk nanobubbles. In addition, the representative production methods and characterization methods of bulk nanobubbles are discussed in order to offer some guidance to their wide range of commercial applications.     

Imaging interfacial micro‐ and nano‐bubbles by scanning transmission soft X‐ray microscopy

Synchrotron‐based scanning transmission soft X‐ray microscopy (STXM) with nanometer resolution was used to investigate the existence and behavior of interfacial gas nanobubbles confined between two silicon nitride windows. The observed nanobubbles of SF₆ and Ne with diameters smaller than 2.5 µm were quite stable. However, larger bubbles became unstable and grew during the soft X‐ray imaging, indicating that stable nanobubbles may have a length scale, which is consistent with a previous report using atomic force microscopy [Zhang et al. (2010), Soft Matter, 6 , 4515–4519]. Here, it is shown that STXM is a promising technique for studying the aggregation of gases near the solid/water interfaces at the nanometer scale.     

Superhydrophobic silica nanoparticles as ultrasound contrast agents

Microbubbles have been widely studied as ultrasound contrast agents for diagnosis and as drug/gene carriers for therapy. However, their size and stability (lifetime of 5–12 min) limited their applications. The development of stable nanoscale ultrasound contrast agents would therefore benefit both. Generating bubbles persistently in situ would be one of the promising solutions to the problem of short lifetime. We hypothesized that bubbles could be generated in situ by providing stable air nuclei since it has been found that the interfacial nanobubbles on a hydrophobic surface have a much longer lifetime (orders of days). Mesoporous silica nanoparticles (MSNs) with large surface areas and different levels of hydrophobicity were prepared to test our hypothesis. It is clear that the superhydrophobic and porous nanoparticles exhibited a significant and strong contrast intensity compared with other nanoparticles. The bubbles generated from superhydrophobic nanoparticles sustained for at least 30 min at a MI of 1.0, while lipid microbubble lasted for about 5 min at the same settings. In summary MSNs have been transformed into reliable bubble precursors by making simple superhydrophobic modification, and made into a promising contrast agent with the potentials to serve as theranostic agents that are sensitive to ultrasound stimulation.     

Interfacial nanobubbles produced by long-time preserved cold water

Interfacial gaseous nanobubbles which have remarkable properties such as unexpectedly long lifetime and significant potential applications, are drawing more and more attention. However, the recent dispute about the contamination or gas inside the nanobubbles causes a large confusion due to the lack of simple and clean method to produce gas nanobubbles.Here we report a convenient and clean method to effectively produce interfacial nanobubbles based on a pure water system.By adding the cold water cooled at 4℃ for more than 48 h onto highly oriented pyrolytic graphite(HOPG) surface, we find that the average density and total volume of nanobubbles are increased to a high level and mainly dominated by the concentrations of the dissolved gases in cold water. Our findings and methods are crucial and helpful for settling the newly arisen debates on gas nanobubbles.     

Investigating the Existence of Bulk Nanobubbles with Ultrasound

Nanobubbles are expected to dissolve in milliseconds. Experimental evidence of nanobubbles that were stable for days had thus been first received with circumspection. If the large number of experimental confirmations has now made clear that surface nanobubbles could exist, bulk nanobubbles are still subject to debate. When observations are reported, the main problem is to make sure the observed particles are really made of gas. We show that ultrasound is an ideal tool for investigating the existence of bulk nanobubbles: 1) it is sensitive to minute quantities of gas, 2) it allows one to determine the bubble size distribution, 3) it discriminates unambiguously between gaseous and solid/liquid inclusions. To illustrate the efficiency of ultrasonic detection, we performed size measurements of bubbles produced by a commercial nano‐/microbubble generator. No nanobubble was detected with this device. It would be insightful to use ultrasonic detection in experimental situations for which stable nanobubbles were reported.     

Study on Nanobubble Generation: Saline Solution/Water Exchange Method

In a recently introduced method for nanobubble generation, water is replaced with NaCl solution. It has the same mechanism as alcohol/water exchange: a liquid of higher gas solubility is used to replace one of lower gas solubility. Herein, the opposite process is realized by replacement of saline solutions with water. Interestingly, nanobubbles are also observed by AFM when different concentrations and valences of saline liquids are employed.