Analysis of Nanobubbles using Nanoparticle Tracking Analysis
It was initially believed that pressures within bubbles of such small size and radius of curvature, such as nanobubbles, would be too great to allow the formation of stable structures. However, research into this field has produced an increasing body of evidence which suggests that, under the right conditions, these bubbles are readily formed and may remain stable. It has been suggested that nanobubbles only form in the presence of salts, which may be part of the reason researchers are interested in investigating the role of layers of counter-ions formed at the surface of the bubble may have on stability.
Kaneo Chiba and Masayoshi Takahashi of Japan’s famous AIST research centre have shown that in the presence of electrolytes and with the correct physical stimulus, stable nanobubbles can be formed from conventional microbubbles. Microbubbles tend to coalesce to form large buoyant bubbles which either float away or collapse under intense surface tension-derived pressure to the point that they vanish, as predicted by theory. However, the addition of salt (electrolytes) is thought to cause the formation of a counter-ion screen around the nanobubbles which effectively blocks the ability of gases within them to diffuse out.
Furthermore, Prof. William Drucker of the University of Melbourne has also used infra-red spectroscopy to show that the pressure of gas within such nanobubbles is not significantly higher than atmospheric pressure, perhaps explaining their stability and resistance to collapse.
Figure 1: The formation of nanobubbles.
In their 2001 study Kikuchi et al showed that the formation of hydrogen nanobubbles was related to the influence of electrolysis conditions on the hydrogen content and the diameter distribution of hydrogen nanobubbles. In this study, Dynamic Light Scattering (DLS) was used to analyse the nanobubbles. However, a recent study by a major pharmaceutical company in Japan showed that the concentration of nanobubbles in a mechanically formed suspension was very low (<107/ml). This is below the optimal concentration required in order to obtain a meaningful analysis using DLS. Analysis of nanobubbles using conventional Electron Microscopy is difficult owing to the vacuum required.
NanoSight’s Nanoparticle Tracking Analysis (NTA) methodology has been applied to the analysis of nanobubbles in liquid suspension.In a blind experiment in which three samples of nanobubble suspensions containing high, low and zero numbers of nanobubbles were tested in duplicate, NanoSight results were found to match exactly those predicted. The graph shows the results in which sample A (series 1 and 6) contained a high concentration of nanobubbles, sample B (series 3 and 5) contained a low concentration of nanobubbles and sample C (Series 2 and 4) were control blanks. It should be noted that NanoSight measures concentration of nanobubbles per unit volume as well as size and size distribution.
Figure 2. NanoSight results.
More recently, Ichiro Otsuka (2008) of Ohu University, Japan has studied the possible role of nanobubbles in ultra-high diluted samples of active agents in which the phenomenon of succussion is considered relevant. He used NanoSight technology to examine nanobubble formation and concentration in more detail than was possible using an electrozone (Coulter) method or conventional DLS techniques.
Although this is still an emerging field, there exists a wide range of proposed nanobubble applications and interest in their usage is growing rapidly.
Utilizing electrolyte stabled ozone nanobubbles, researchers have been able to sterilize materials for many months. This has great potential in both the preservation of foodstuffs and in the medical field as an alternative to traditional chlorine based methodologies.
Oxygen nanobubbles have been implicated in the prevention of arteriosclerosis by the inhibition of mRNA expression induced by cytokine stimulation in rat aorta cell lines.
When formed in liquids in capillaries, nanobubbles have been shown to greatly improve liquid flow characteristics. They have also been proposed as contrast agents in scanning techniques as well as cleaning agents in silicon manufacturing processes.
A new field of drug delivery applications is being actively researched in which nanobubbles play an active part, though details of this highly secretive field are hard to come by. However, as reported in Reuters, Natalya Rapoport of the University of Utah's Department of Bioengineering is using nanobubbles with the chemotherapy drug doxorubicin to seek out and congregate at cancer tumors when injected into the bloodstream. "These nanobubbles don't penetrate normal blood vessels but they do penetrate blood vessels in the tumor," said Rapoport, whose study appears in the Journal of the National Cancer Institute. Once in the tumor, the nanobubbles combine to form larger microbubbles which can be seen on an ultrasound. "When these bubbles accumulate, I give strong ultrasound radiation to the tumor to blow them up. The drug then gets out of these bubbles locally at the tumor site."
The NanoSight methodology for direct visualization, sizing and counting of nanoscale materials is ideally suited for the analysis of nanobubbles. The concentration ranges common in this application frequently match those required for ideal analysis conditions, and the addition of Zeta Potential capability provides researchers with an insight into the charge properties of their materials.
Kenji Kikuchi, Hiroko Takeda, Beatrice Rabolt, Takuji Okaya, Zempachi Ogumi, Yasuhiro Saihara and Hiroyuki Noguchi (2001) Hydrogen particles and supersaturation in alkaline water from an alkali–ion–water electrolyzer, Journal of Electroanalytical Chemistry, p1–6 Ichiro Otsuka (2008) Effect of 1:2 aqueous dilution on O2 nanobbubles in a 0.1 M Na2CO3 solution, Proc The 59th Annual Meeting of the International Society of Electrochemistry, September 7 to 12, 2008, Seville, Spain; s10-P-062, p139