Isolation of Gold Nanoclusters
When a product is generated, the isolation (separation) of the product from the reaction mixture may be problematical or even challenging, especially for those cases in which multiple products with similar chemical and physical properties coexist. In this section, we will mainly review the physical techniques and methods commonly used in the gold NCs isolation (separation).
Fractionated precipitation is an effective, facile separation method for those NCs with obvious solubility difference [63-66]. The larger or relatively solubility-poor NCs will be precipitated out first after the addition of poor solvent into the NCs mixture solution. By such a principle, Whetten and co-workers isolated a series of Au NPs in 1.5-3.5 nm range, with the core masses to be 93-92 k, ~57 k, 46-45 k, and 29-27 к (k = 1000 amu or kDa) determined by laser desorption/ ionization mass spectrometry (LDIMS), containing from ~1300 to ~100 Au atoms . The separations are performed starting from nearly saturated toluene solutions of NPs mixtures (typically 10 mL of 20 mg/mL concentration) and slowly adding a miscible poor solvent (usually acetone) by passive vapor transfer until the solution volume is increased by a predetermined amount. The solution is then isolated, allowed an extended equilibration period while stirring, and finally centrifuged and decanted to remove the soluble fraction from the precipitated fraction, each of which is analyzed by MS. The procedure is then repeated separately on both fractions, and so on, to generate many further fractions, until a handful of highly purified fractions are obtained. In the subsequent work, even smaller Au NCs were obtained through repeated fractional crystallization, such as species with core masses of 15-14 к (1.3 nm, ~75 atoms) and 8 к (1.1 nm, ~38 atoms) .
In addition to the separation of oil-soluble NCs reported by the Whetten group, the isolation of water-soluble NCs and NPs can be also conducted by this method. For example, Wu et al. isolated three glutathione (-SG) capped gold nanoparticles, including Au2s(SG)18 NC, 2- and 4-nm NPs in one pot by precipitation with different amounts of methanol . Larger NPs are readily precipitated out with the addition of only a small amount of methanol, leaving smaller particles in the supernatant. By controlling the amount of methanol added, the three predominant species (1-3, in order of the decreasing particle size) were separated (Fig. 1.8). Yang and co-workers separated water-soluble gold NCs stabilized with penicillamine ligands in aqueous medium by sequential size- selective precipitation. Au NCs were precipitated out and separated successively from larger to smaller ones by progressively increasing the concentration of acetone in the aqueous Au NCs solution .
The Dass group even used the fractionated precipitation method to isolate the Au-Ag alloy NCs, such as Au38.„Ag„(SR)24  and Au25-/Agn(SR)24 .
Figure 1.8 One-pot synthesis of Au-SG nanoparticles and separation of three major species by methanol-induced precipitation. Reprinted with permission from Ref. , Copyright 2010, John Wiley and Sons.
Recrystallization, which means repeated crystallization, is widely used in industry and laboratory, and it is also used in the purification of NCs. For instance, Schaaff et al. carried out the recrystallization of 29 kDa Au:SR compounds (confirmed as Au144(SR)60 lately) and obtained the microcrystals from slow precipitation out of a dilute solvent-nonsolvent interaction . Specifically, the Au:SR compounds were precipitated out of concentrated toluene solutions by the addition of excess ethanol and allowed to stand at room temperature for 12-15 h. The precipitate was then filtered, redissolved in toluene, and reprecipitated twice to ensure that the excess disulfide (RSSR) and unreacted thiol (RSH) were removed and the finely crystalline black powder was obtained (Fig. 1.9). The Murray group prepared Au-Pd alloy NCs and purified them by recrystallization from the acetonitrile/methanol mixture for several times ; Wu and co-workers obtained pure Au2s(SG)18 by recrystallization in water-methanol solution [35, 40, 41, 74]; Quinn's group utilized precipitation/dispersion cycles to purify Au38 NCs .
Figure 1.9 (a) Optical micrograph and (b) SEM micrograph show the typical
crystal of 29 kDa Au:SR compounds that is grown from slow precipitation out of a dilute solvent-nonsolvent interaction. Reprinted with permission from Ref. , Copyright 2001, American Chemical Society.