Ultrasound-Assisted Levitation

The interest aroused by micro and nanotechnology in the scientific, chemical, biochemical, pharmaceutical, biological, biomedical and biotechnological fields, among others. 

In theory, the previous advantages could make miniaturization a panacea in practice, however, they do not. Thus, when a system is scaled down, some characteristics such as lengths, areas and volume ratios can differ so markedly from those of macroscopic systems as to affect the development of the process concerned. The new behaviour will be dominated by material confinement in small structures, a large interfacial volume fraction and various unique properties, phenomena and processes. In fact, many current theories of matter at the microscale level have critical gaps for nanometer dimensions and fail to describe the new phenomena observed at the nanoscale level accurately [66]. Also, scaling-down can be problematic with samples containing low analyte concentrations as their determination will require larger amounts of sample. 

Recent breakthroughs in miniaturized analytical instrumentation include fully integrated lab-on-a-chip and micro total analysis systems. The former have had only moderate success as many analytical chemists have been reluctant to accept them [67]. At present, chip-based analytical systems are subject to major shortcomings such as the risk of analyte adsorption on walls and at interfaces — which is important especially in low-volume analytical systems — and optical interference at the walls of the chips hampering detection. Further research in this field is required in order to effectively circumvent these shortcomings [68]. 

Levitation of small amounts of sample can be used to avoid contact with solid walls around the sample in a gas-surrounding medium (air in most cases). Levitation provides advantages similar to those of miniaturization in chip-based methods (basically, low reagent and sample consumption). In addition, levitation avoids contamination between samples and external objects, and also adverse effects of sample-wall contact on detection [69]. 

Sample levitation can be accomplished in different ways, one of which is by using ultrasonic energy. The phenomenon by which small samples of solids, liquids or suspensions can be levitated at the nodal points of a standing ultrasonic wave was first described by Bucks and Muller in 1933 [70]. The flexibility and potential of acoustic levitation in various fields are widely documented, mainly by studies in the analytical and bioanalytical fields [71-73]. Therefore, levitation can be considered a mature technique. Its development warrants inclusion of a dedicated section in this chapter to describe its fundamentals and compare the advantages and limitations of acoustic levitation with other levitation modes. The devices used for this purpose and the potential applications of each mode are also discussed. 

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Finally, Chapter 8 disousses some well-established detection techniques such as electroanalysis, whioh has been greatly improved by US assistance and led to the development of sonoeleotroanalysis, and others in which ultrasound has provided a more efficient way of placing or transporting the sample — or the solution resulting from a given treatment — to the deteotion point (e.g. ultrasound-assisted levitation or nebulization). 

Priego-Capote, F., de Castro, L. (2006). Ultrasound-assisted levitation Lab-on-a-drop. TrAC, Trends in Analytical Chemistry, 25, 856-867. 

Ultrasounds have been used to assist in the insertion of samples into solid-liquid analytical systems for some time. The earliest ultrasonic nebulizers and automatic slurry sampling systems were reported in the 1980s. However, the actual potential of ultrasounds for increasing the efficiency of sampling systems remains to be explored. Thus, a recently developed technique based on acoustic levitation has been found to substantially... 

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