# Interface Engineering Strategies for Fabricating Nanocrystal-Based Organic-Inorganic Nanocomposites
## 2 Overview of Ligand Exchange
### Characterization  
Characterization techniques used thus far to investigate the surface properties of NCs include X-ray photoelectron spectroscopy [45; 46], UV-vis spectroscopy [47], infrared spectroscopy [48], photoluminescence spectroscopy [49; 50; 51], and nuclear magnetic resonance (NMR) spectroscopy [52; 53]. To determine the properties of surface ligands or the interfacial properties between organic and inorganic elements, various characterization techniques must be combined [54]. Among these, NMR spectroscopy is one of most powerful and common characterization techniques for identifying ligands on the nanocrystal surface. It can effectively verify the type of ligands by detecting a unique spectral signature without any destruction of samples. Moreover, it enables discrimination between bound ligands and free ones by observing the broadened NMR resonance of protons within surface-bound  
Figure 3: Alkene region of the proton NMR spectra after precipitation/resuspension with (**a**) methanol and (**b**) acetonitrile. Photoluminescence characterization of CdSe QDs after purification with (**c**) methanol and (**d**) acetonitrile. Reprinted with permission from [33]. Copyright 2012, American Chemical Society. species [55]. Based on this broadening effect, one-dimensional (1D) \({}^{1}\)H NMR spectroscopy can be employed to monitor the behavior of ligands on NC surfaces. The binding nature of carboxylic acid ligands with CdSe QDs has been studied using \({}^{1}\)H NMR measurements in several studies [56; 57; 58]. The comparison of the different proton signals from the free-state OA ligands and purified OA-capped CdSe QDs is shown in Figure 4a,b [59]. The addition of excess OA ligands to a purified OA-QDs solution resulted in the generation of a new and separate set of sharp resonance peaks next to those due to bound OA, along with broadening of the alkene proton resonance peaks (Figure 4c). Intriguingly, the intensity of bound OA ligands remains constant, indicating that the number of tightly bound OA ligands is not affected despite the addition of excess OA [56]. This, however, does not mean that there is no adsorption-desorption reaction. In order to verify the existence of an equilibrium transfer, deuterated OA (OA(9,10)-d\({}_{2}\)), i.e., OA that has its alkene hydrogen replaced by deuterium, was added to OA-CdSe QDs in a toluene-d\({}_{8}\) solution. Interestingly, the intensity of the broadened resonance peak at 5.65 ppm, ascribed to the alkene proton of bound OA, decreased, while the sharpened signal, attributed to free OA, increased (Figure 4d). The decrease in intensity of the alkene resonance is also proportional to the amount of OA(9,10)-d\({}_{2}\) added. These observations indicate that all the bound OA ligands are involved in the exchange [59]. Diffusion-ordered NMR spectroscopy (DOSY) is a useful and powerful tool for distinguishing between surface-bound and free ligands. It measures the diffusion coefficients of species associated with proton resonances, determining their hydrodynamic radii (using the Stokes-Einstein relation). For example, DOSY was used to measure the hydrodynamic radii of free ligands (e.g., OA) and that of ligands bound to NCs (i.e., OA-CdSe QDs) in an OA/CdSe QDs system. Their estimated values were 0.56 nm and 4.2 nm, respectively, in a THF solvent [32]. Clearly, the hydrodynamic radius is higher for OA-capped QDs than for free OA. Nuclear Overhauser spectroscopy (NOESY) is also a well-known technique for assigning \({}^{1}\)H NMR signals to free and bound ligands [44]. It measures the enhanced cross-coupling NMR signals originating from dipole-dipole interactions among protons within the same molecules when they are immobilized on slowly tumbling objects, such as NCs. An alternative technique based on radioactive labeling was developed by [PERSON] et al. to effectively detect ligands in minute amounts. The technique uses tritium (\({}^{3}\)H) as a radioactive tracer for the quantification of ligands bound to the QDs [60].