Abstract
The use of indium tin oxide (ITO) and focused monomode microwave heating for the ultra-rapid crystallization of L-alanine (a model amino acid) is reported. Commercially available ITO dots (< 5 mm) attached to blank poly(methyl)methacrylate (PMMA, 5 cm in diameter with 21-well silicon isolators: referred to as the iCrystal plates) were found to withstand prolonged microwave heating during crystallization experiments. Crystallization of L-alanine was performed at room temperature (a control experiment), with the use of two microwave sources: a 2.45 GHz conventional microwave (900 W, power level 1, a control experiment) and 8 GHz (20 W) solid state, monomode microwave source with an applicator tip that focuses the microwave field to a 5-mm cavity. Initial appearance of L-alanine crystals and on iCrystal plates with ITO dots took 47 ± 2.9 min, 12 ± 7.6 min and 1.5 ± 0.5 min at room temperature, using a conventional microwave and focused monomode microwave heating, respectively. Complete evaporation of the solvent using the focused microwaves was achieved in 3.2 ± 0.5 min, which is ~52-fold and ~172-fold faster than that observed at room temperature and using conventional microwave heating, respectively. The size and number of L-alanine crystals was dependent on the type of the 21-well iCrystal plates and the microwave heating method: 33 crystals of 585 ± 137 μm in size at room temperature > 37 crystals of 542 ± 100 μm in size with conventional microwave heating > 331 crystals of 311 ± 190 μm in size with focused monomode microwave. FTIR, optical microscopy and powder X-ray diffraction analysis showed that the chemical composition and crystallinity of the L-alanine crystals did not change when exposed to microwave heating and ITO surfaces. In addition, theoretical simulations for the binding of L-alanine molecules to ITO and other metals showed the predicted nature of hydrogen bonds formed between L-alanine and these surfaces.
Highlights
Crystallization is an important technique consistently utilized within the food, chemical and pharmaceutical and biotechnology industries [1, 2]
The size of indium tin oxide (ITO) films equal to the diameter of the iCrystal plates (5 cm) was found to be inefficient due to sustained damage occurred during microwave heating
The size of L-alanine crystals http://www.nanobe.org was observed to be depend on the type of the platform and the microwave heating method: largest size (585 ± 137 mm) at room temperature in 160 min > larger size (542 ± 100 mm) with conventional microwave oven in 55 min > smallest size (311 ± 190 mm) with focused monomode microwave in 3 min. These observations were attributed to the difference in the rate of evaporation of the solvent, where the slowest rate was observed for crystallization experiments carried out at room temperature and the fastest rate of evaporation was observed with focused monomode microwave heating
Summary
Crystallization is an important technique consistently utilized within the food, chemical and pharmaceutical and biotechnology industries [1, 2]. Evaporative crystallization, in which nucleation and crystal growth occurs when the solution evaporates, is a widely used chemical solid–liquid separation technique [4]. Evaporative crystallization is typically employed at small-scale processes where solvent can be removed in a costeffective manner to produce highly-desirable crystals. A drawback of evaporative crystallization is the requirement of highly soluble compounds to grow high quality of crystals in large quantities [5]. Despite the advantages afforded by the evaporative crystallization technique, such as solvent evaporation by heating and relatively simple instrumentation, there is still a need for improvements for the growth of crystals of desired products in a speedy, repeatable and high-throughput fash ion
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