Size-exclusion chromatography (SEC) was used to evaluate gold nanoparticles (Au NPs) for variations in their sizes after microwave (MW) irradiation, with the eluted NPs monitored through diode array detection to reveal their surface plasmon absorptions. The sizes of citrate-capped Au NPs decreased upon increasing the MW irradiation temperature, consistent with digestive ripening of these NPs under the operating conditions. In contrast, Au NPs capped with sodium dodecyl sulfate increased in size upon increasing the MW irradiation temperature, consistent with Ostwald ripening. When the Au NPs were capped with 3A-amino-3A-deoxy-(2AS,3AS)- -cyclodextrin (H2N- -CD), however, their dimensions were barely affected by the MW irradiation temperature, confirming that H2N- -CD is a good stabilizer against MW irradiation. Therefore, SEC—with its short analysis times, low operating costs, automated operation, and in situ analysis—has great potential for use in the rapid monitoring of NPs subjected to treatment under various MW irradiation conditions. 1. Introduction Metal nanoparticles (NPs) are attracting a great deal of attention from practitioners in a wide variety of scientific fields [1–13] because their physical and chemical properties are related to their chemical compositions, sizes, and surface structural characteristics [14–17]. Among all metal NPs, Au NPs are especially attractive for research in nanotechnology [18], for example, in the detection of cancer [19], DNA [20], and even single molecules through surface-enhanced Raman spectroscopy [21, 22]. Because the chemical or physical properties of Au NPs are size dependent [9, 23, 24], monitoring the sizes of Au NPs is a critical step toward understanding how their functions are related to their dimensions [25, 26]. Several research articles have reported the reaction kinetics of Au NP formation using the citrate method [27–31]. Both Ostwald ripening [28, 29], whereby particles increase in size during the reaction, and digestive ripening, where the average particle size decreases, have been observed [30, 31]—with the latter being used to break colloids into smaller particles through the addition of ligands. As a result, many strategies have been developed for varying the sizes of Au NPs [25]. Herein, we wished to establish a rapid and efficient strategy for determining the size-dependent chemical or physical properties of Au NPs prepared using a chosen treatment method. Many methods are currently available for particle size analysis [32], including transmission electron microscopy (TEM), dynamic light
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