However, the probe sonicator does not fit this requirement and has not been tested. In the safety research of nanomaterials, contamination is always avoided. This not only saves time but ensures that all the samples are treated equally, which makes the results among samples more reliable and comparable. Several vials are sonicated at the same time and at the same intensity. Similarly, an ultrasonic probe fitted with a vial tweeter is advantageous over the direct probe due to the above-mentioned contamination risks as well as the operation friendliness of the equipment. Although probe ultrasonication is known to perform better than bath sonication because of high localized intensity 5, bath sonication is often preferred over probe-type for the preparation of toxicological test suspensions because of the possible contamination risk through the tip, erosion of titanium probe tip after prolonged use, and probe immersion depth discrepancies. All types of sonication are available in a range of intensity and output power settings, sometimes adapted with a different type of sonotrode for specific processes or requirements, and are suitable for liquid volumes ranging from 2 to 250 mL.
Sonication is commonly carried out by either using a probe-type (direct) or an ultrasonic bath, or ultrasonic probe with a vial tweeter (indirect sonication). This in turn will ultimately affect the interaction of particles with test media and outcome of various in vitro and in vivo experiments, in order to deduce the potential hazards of nanomaterials. In the field of nanotoxicology, the ability to have control over dispersion quality is very important, as the dispersion step will determine key physicochemical properties, such as particle size/size distribution, shape, aggregation/agglomeration, surface charge, etc. Another is to measure the characteristic absorption of nanoparticles in the UV spectral range 4. This includes estimation of zeta potential (ZP) through measurement of electrophoretic mobility of particles. There are several ways to measure dispersion stability. Here, the stability of the suspension is defined as where the particles do not settle or sediment down in their dispersed state and the average hydrodynamic diameter measurements do not vary by more than 10% between the five repeated measurements taken during that time (around 10 min) 2, 3.
An important aspect to consider in the dispersion process is stability of the final dispersion. By applying sonication, sample homogeneity can improve, potentially achieving a much narrower particle size distribution. There are different sonicators, all having the general function of de-agglomerating particles, which disperse in a liquid medium as individual (or primary) particles. In a laboratory setting, the sonication method is carried out using a sonicator. Sonication is the process of generating cavitations, which involves the creation, growth, and collapse of bubbles (often called hot spots) formed in liquid due to the irradiation of high intensity ultrasound 1. Such a guideline is instrumental in ensuring dispersion quality repeatability in the nanoscience community, particularly in the field of nanotoxicology. Our goal is to offer a harmonized approach that can control the quality of the final, produced dispersion. This is necessary to identify the time points as well as other above-mentioned conditions during the sonication process in which there may be undesirable changes, such as damage to the particle surface thus affecting surface properties. The approach records the dispersion process in detail. However, with any change in either the nanomaterial type or dispersing medium, there needs to be optimization of the basic protocol by adjusting various factors such as sonication time, power, and sonicator type as well as temperature rise during the process. This approach has been adopted and shown to be suitable for several nanomaterials (cerium oxide, zinc oxide, and carbon nanotubes) dispersed in deionized (DI) water.
In this study, a systematic step-wise approach is carried out to identify optimal sonication conditions in order to achieve a stable dispersion. The sonication process is commonly used for de-agglomerating and dispersing nanomaterials in aqueous based media, necessary to improve homogeneity and stability of the suspension.
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