# Interface Engineering Strategies for Fabricating Nanocrystal-Based Organic-Inorganic Nanocomposites
## 2 Overview of Ligand Exchange
### Purifications  
The synthetic procedures for high-quality NCs frequently rely on an excess of surfactants to provide enough colloidal stability and to control the morphology [34]. However, the remaining insulating ligands and by-products generated by the decomposition of reagent compounds have negative effect on the physical and chemical properties of the as-synthesized NCs [35]. Accordingly, effective means for eliminating these byproducts and excess insulating ligands are greatly required. The most common and simple purification method is a precipitation and re-dissolution protocol, which involves the precipitation of colloidal nanocrystals from the solution by a non-solvent. Specifically, anti-solvents (i.e., polar solvents) are first added to induce the flocculation of NCs dispersed in non-polar solvents by increasing the solvent polarity. The supernatant solution, which contains the by-products and excess surfactants, is then discarded and the purified NCs are re-dispersed in clean solvents [36]. Toluene or chloroform as the anti-solvent and methanol or acetone as the solvent are the most commonly used combinations this far [37; 38; 39]. An alternative purification technique involving extraction uses immiscible solvents. Metal complex by-products and excess surfactants are extracted via phase transfer from a non-polar to a polar solvent, because they are more soluble in the latter. For example, a chloroform/methanol mixture (1:1 ratio) was used to extract both cadmium oleate and oleic acid from a CdS NCs/octadec (ODE) solution [34; 40]. The transfer of by-products and excess ligands can be confirmed by Fourier transform infrared (FTIR) and ultraviolet (UV)-visible (vis) spectroscopy results. For example, [PERSON] et al. used UV-vis measurements to confirm that the amount of oleic acid in a CdSe NCs/ODE phase decreased upon stirring with extracting solvents (i.e., 1:1 chloroform/methanol) (Figure 2) [40]. Recent studies reveal that the use of protein solvents (e.g., methanol) as anti-solvents sometimes results in the detachment of covalently bound ligands and the introduction of methoxide functionalities on NC surfaces via proton exchange [41; 42]. This adversely influences the optical properties of the synthesized semiconducting nanocrystals, such as photoluminescence (PL) quenching due to the formation of mid-gap trapping sites [17; 43]. In this context, an aprotic anti-solvent, i.e., acetonitrile, has been successfully used to prevent the formation of alkoxy functional moieties on NC surfaces [41]. Oleic acid-capped CdSe QDs were subjected to repeated precipitation and  
Figure 2: Absorption spectra for the separation of CdS nanocrystals from cadmium oleate and oleic acid. Reprinted with permission from [40]. Copyright 2002, John Wiley & Sons. with methanol and acetonitrile, respectively. Interestingly, in a another study, \({}^{1}\)H NMR observation of a proton signal from the alkene moiety of oleic acid revealed that the use of acetonitrile as a precipitant leaves the bound surfactants untouched, while the employment of short-chain alcohols (e.g., methanol, ethanol, and isopropanol) induced the release of carboxylate ligands (Figure 3a,b) [33]. Notably, the broadening of this peak corresponds to bound-state ligands (i.e., oleic acid (OA)), while the peak sharpening indicates free-state ones. Clearly, the precipitation/resuspension with methanol produced a sharp signal around 5.8 ppm, strongly suggesting that the detachment of OA (free-state ligands) is due to proton transfer from methanol to OA [33]. In stark contrast, the proton signal remained broad when acetonitrile was used during the precipitation/resuspension procedure [44], further confirmed using PL measurements. The emission intensity can be used as evidence for the replacement of ligands because the loss of OA ligands is strongly correlated with PL quenching (Figure 3c,d). The photoluminescence spectrum of OA-capped CdSe QDs was recorded after successive purifications using methanol and acetonitrile.