Solvent extraction is an inverse process of crystallization that takes advantage of the good solubility of target NCs to isolate them from the crude product and transfer them to the solution, usually in conjunction with recrystallization. The negative Au25(SR)18 shows exceptional solubility in acetonitrile, and thus the Murray and Jin groups extracted pure Au2s(SR)18 from the crude product and successfully grew the single crystals [76, 77]. Generally, the solvents used for extraction were acetonitrile, acetone, dichloromethane, toluene, and so on. By the same method, Jin's group obtained a series of pure Au NCs, including Au24 , Au38 , Au44 , Au130 , Au144 , and Au246 . Negishi et al. successfully extracted Au25.nAg„(SC12H25)18 with acetone from the crude product . A special example is the isolation of Au30(S-Adm)ls, which is dissolvable only in benzene and even insoluble in dichloromethane, a widely used solvent for oleophilic clusters. This peculiar solubility of Au30(SAdm)18 is indeed important as it allows one to purify NC by removing some by-products with dichloromethane first and then subject the extract from the resulting residue . Besides, hydrophilic ligand-protected Au NCs can also be extracted by a suitable solvent such as methanol .
Polyacrylamide Gel Electrophoresis
For subtle separation of the NCs mixture with close polarity, the above-mentioned methods generally do not work well. An earliest solution for that is PAGE, which requires two kinds of gels: a stacking gel and a separating gel. Normally, sample solutions are loaded onto the stacking gel and then eluted for a long time at a constant voltage to get sufficient separation. The NCs are separated based on their size and aqueous solubility, and PAGE often works well for those NCs that are protected by hydrophilic ligands, such as glutathione (GSH), mercaptobenzoic acid (MBA), and captopril (Cap). Separation can easily be observed by looking at the gels with different colors, which are then cut and extracted in appropriate solvents to get the purified NC.
In 1998, Whetten and co-workers fractionated Au NCs protected by monolayers of GSH by using PAGE and identified the most abundant species as Au28(SG)16 by mass spectrometry, which was lately corrected as Au25(SG)18 . After that, a series of giant Au NCs composed of a gold core and a glutathione (GSH) adsorbate layer have been prepared from Au(I)SG polymers and separated by gel electrophoresis . Tsukuda's group isolated a series of magic-numbered Au NCs . In 2005, they significantly improved the PAGE separation of the aqueous Au-SG NCs and obtained high-purity Au„(SG)m NCs. Compared to the earlier electrospray ionization mass spectrometry (ESI-MS) work, very clean ESI- MS spectra were obtained for the first time by suppressing the fragmentation of Au„(SG)m NCs in ESI-MS (Fig. 1.10) . Through the precise determination of masses of the isolated species, distinct NCs were identified unequivocally, including Au15(SG)13, Au18(SG)14, AU22(SG)16, AU22(SG)17, AU2s(SG)18, AU29(SG)2o, AU33(SG)22, and Au39(SG)24. Xie and co-workers isolated a red-emitting Au NC with the precise molecular formula of Au22(SG)18 (Fig. 1.11) . Other research groups have also used this method to obtain isolated gold NCs [86-91].
Figure 1.10 (a) Separation of Au„(SG)m NCs by PAGE, (b) Low-resolution ESI mass spectra of the fractionated Au:SG clusters (left). The mass spectra reproduced from the most intense peaks in the high-resolution spectra (right). The calculated spectra for Au„(SG)m are shown by the colored peaks with the corresponding n-m values. Reprinted with permission from Ref. , Copyright 200S, American Chemical Society.
Figure l.ll (a) UV-Vis absorption spectra of Au NCs separated from bands 1-4 in the native PAGE gel (insets), (b) ESI mass spectra of species 4. The red lines are the simulated isotope pattern of [Au22(SG)18°-5H]5“. Reprinted with permission from Ref. , Copyright 2014, American Chemical Society.
In addition to Au NCs, Ag NCs were also isolated by PAGE. For example, Kumar et al. synthesized glutathione-stabilized silver NCs by a modified Brust method and isolated them by PAGE using homemade polyacrylamide gels. There were at least 16 naked eye discernible PAGE bands with different colors, strongly indicating the multiple species involved in the as-synthesized Ag:SG NCs. Importantly, they found that the positions and colors of the PAGE bands were independent of the reaction conditions, which suggested that only the magic-numbered silver NCs were formed under different experimental conditions . It should be noted that the preparation of polyacrylamide is very important for the successful separation of NCs. By changing acrylamide concentration and other gel conditions, different core sizes can be separated.