A nanobubble is a bubble having a diameter of less than one micrometre (10-6 m). Nanobubbles are cavities (bubbles) which contain gas, but present in aqueous solutions.
The size of a microbubble is less than one millimetre but greater than one micrometre. When a microbubble undergoes shrinking, its size decreases to less than a micrometre, thus forming a nanobubble. But, this formation of a naturally-occurring nanobubble is usually temporary and transient due to the physically labile nature of the bubbles.
However, under the right conditions of pressure as well as in the presence of suitable electrolytes, nanobubbles can be formed which are highly stable in nature. Electrolytes contribute to the formation of a counter-ion screen surrounding the nanobubbles. This prevents the nanobubbles from coming into contact with atmospheric gases and atmospheric pressure. Thus, the nanobubble attains stability. 
Formation of nanobubble with comparison to macrobubble and microbubble 
Nanobubbles are produced under tremendous amounts of pressure, since greater pressure increases surface tension. In order to counter the increasing surface tension, the surface area and volume of these bubbles become extremely low. The actual size of a nanobubble varies, depending upon the saturation of the surrounding aqueous environment.
The main reason put forward for the long-lasting nature of a nanobubble is that the interface between the nanobubble gaseous surface and the surrounding liquid environment is charged. This charge opposes the increasing surface tension (caused due to increasing pressure which is responsible for the initial formation of the nanobubble). This opposing counter-interaction between the increasing charged interface and increasing internal surface tension is regarded to be mainly responsible for the overall stability of a nanobubble.
Nanobubbles possess the property to aggregate and organize themselves in aqueous solutions, as in the case of oil droplets suspended in water. 
Applications of Nanobubbles
1. Malaria detection - Hemozoin is a product that is formed by digestion of blood consumed by blood-sucking mosquitoes.  This is also referred to as “malaria pigment” in mosquitoes. 
Formation and presence of hemozoin is extremely critical for the survival of blood-sucking mosquitoes. Therefore, hemozoin is seen as a target for the development of new and upcoming drugs for malaria treatment.
Nanobubbles have been used to develop a non-invasive method of malaria detection. Hemozoin nanoparticles are subjected to "safe" levels of laser pulse radiation, having near-infrared wavelength, for a short duration of around a picosecond. This exposure to radiation creates a bubble of hemozoin in vapour form around the hemozoin nanoparticles. These vapour-covered hemozoin nanoparticles generate acoustic signals, which when coming into contact with the dermal tissue, detect the presence of malarial infection as low as 0.00034% in animals. 
This novel method of detection of malarial infection is extremely rapid, simple, inexpensive, safe, and completely non-invasive as it does not involve time-consuming processes such as drawing blood or using other medical reagents for testing.
2. Gene delivery - Nanobubbles can be used as an effective means for gene delivery without the use of viruses.
When nanobubbles are subjected to ultrasound treatment, they are able to oscillate and resonate at various frequencies. This property of resonance of nanobubbles can be applied in the field of gene delivery. Owing to the small size, nanobubbles carrying the gene of interest can be subjected to extravasation from blood vessels. Extravasation allows the nanobubbles to freely enter the surrounding tissue, and deliver the gene carried by them in an extremely specific manner, without any invasiveness. 
Another method developed for gene delivery using nanobubbles is the use of chitosan. Chitosan is a substance that possesses low toxicity and low immunogenicity. Therefore, chances of occurrence of hypersensitive reactions and/or anaphylaxis are extremely low. Chitosan nanobubble is bound to plasmid DNA and transfected using ultrasound. 30 seconds of exposure to ultrasound has been found to achieve moderate rate of transfection, hence gene delivery inside the cell of interest. 
3. Drug delivery to cancer cells - Plasmonic nanobubbles are those nanobubbles which are excited due to exposure to laser pulse radiation. As a result, a thin layer of vapourized liquid is formed on the cell surface. 
Gold nanobubbles are used for the purpose of drug delivery inside cancer cells. Chemotherapeutic drugs are embedded inside these nanobubbles.
When chemotherapeutic drugs are required to be administered for cancer therapy, these nanobubbles can be introduced inside the tumour with the help of laser. The tumour membranes open up, and these nanobubbles are absorbed. The chemotherapeutic drugs are then released inside the tumour.
It has been found out that the use of nanobubbles for administration of chemotherapeutic drugs for cancer therapy had 30 times greater effect as compared to conventional methods of chemotherapeutic drug administration. Also, delivery of chemotherapeutic drugs through nanobubbles reduces the overall drug dosage to less than one-tenth of the normal dosage. 
4. Prevention of suffocation - Patients suffering from breathing or respiratory problems or, those individuals who suffer from low oxygen levels can be treated by the use of oxygen nanobubbles which are directly injected into the bloodstream. This method of direct oxygen introduction bypasses breathing, and can be administered in emergency medical situations, such as stroke, brain haemorrhage, or heart attack.
It has been found that patients who are administered oxygen nanobubble therapy can be kept alive and stable for a period of 15 minutes, following which hospitalization is required.  It is, therefore, extremely important to note that this method of direct oxygen administration through nanobubbles is only a form of emergency treatment, and cannot be regarded as an alternative to hospitalization in emergency medical situations.
5. Ultrasound Molecular Imaging - The small size of nanobubbles would render them extremely redundant to be used for imaging purposes. However, lipid nanobubbles have found to be passively involved for the purpose of tumour tissue imaging through ultrasound. Tumour tissues are highly permeable. Therefore, the nanobubbles are able to enter the tissue easily, and be retained for a long time. The tumours can be stained using red fluorescence dyes, which indicates the presence of lipid nanobubbles in that particular area of the affected tissue. 
6. Wastewater treatment - Nanobubbles can be used as an effective means to treat wastewater.
Working of a nanobubble wastewater plant -
♦ Wastewater is introduced in a tank containing nanobubbles.
♦ This mixture of wastewater containing nanobubbles is then introduced into the mixing tank which contains sludge rich in microorganisms.
♦ Microorganisms present in the sludge are activated by the nanobubbles, thus beginning a three-stage treatment process -
• Biological treatment - Activated microorganisms present in the sludge decompose the biological waste present in wastewater.
• Chemical treatment - This biologically-treated wastewater is then subjected to oxidation reaction in the contact oxidation tank. In the presence of hydroxyl radicals (OH-), peroxide, ozone, etc., chemical organic impurities, heavy metals, etc. are broken down. The end product of this chemical treatment is the formation of water.
• Physical treatment - This chemically-treated wastewater is introduced in an adsorption tank containing charcoal which removes the remaining solid impurities through the process of adsorption. 
7. Removal of viruses from oysters - Ozone nanobubbles can be used as a means for virus removal from oysters.
Norovirus is a virus that infects oysters. It releases a toxin which leads to discolouration, foul odour, and makes oysters unsuitable for consumption. Oysters infected by norovirus can be soaked in ozone nanobubbles, and the virus can be eliminated.
The same method may prove to be useful to suppress infections caused by legionella bacteria and carp herpes virus. 
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