## 2 Applications of Chiral Nanoparticles
For example, [PERSON] and colleagues functionalized Au NPs with either D- or L-cysteine, which were used as enantioselective adsorbents of racemic propylene oxide that caused an increase in the optical rotation [23]. The authors managed to quantitatively determine the enantiospecific ratios of the adsorbed species by measuring the enantiospecific equilibrium constants, \(K_{\text{L}}\)\({}^{\text{S}}\)/\(K_{\text{D}}\)\({}^{\text{S}}\) = \(K_{\text{D}}\)\({}^{\text{R}}\)/\(K_{\text{L}}\)\({}^{\text{R}}\) (where the subscript represents the chirality of the ligand-NP complex and the superscript that of the adsorbed species), without the need to know the surface area of the NPs (Figure 1). They later prepared tetrahedral Au NPs, again capped with D- or L-cysteine, for the enantioselective separation of the chiral pharmaceutical propranolol, an antihypertensive drug [24]. Over the past decade, production of superparamagnetic nanoparticles has been extensively investigated, not only for its essential scientific interest but also for many technological applications such as medical imaging [35; 36], magnetic field supported transport [37; 38; 39], and separations [40; 41]. Similarly, they were also employed for separating racemic mixtures. Benefits of magnetic separation over more traditional methods are related to its rapidity, easiness, and cost-effectiveness. [PERSON] et al. synthetized silica-encapsulated magnetic nanoparticles (MNPs) functionalized on their surface with carboxymethyl-\(\beta\)-cyclodextrintrin (CD) for the enantioselective recognition of aromatic amino acids [25]. The Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\)/CD MNPs, with the chiral properties of the \(\beta\)-CD and the biocompatibility \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**NPs** & **Surface Functionalization** & **Application** & \begin{tabular}{c} **Molecule** \\ **Recognized** \\ \end{tabular} & **Reference** \\ \hline Au/SiO\({}_{2}\) & \(\beta\)-cyclodextrintrin & \begin{tabular}{c} chromatographic \\ separation \\ \end{tabular} & \begin{tabular}{c} metrazol \\ \end{tabular} & [30] \\ \hline Au Janus NPs & \begin{tabular}{c} hexanethiolates \\ (hydrophobic side) and \\ 3-mercapto-1,2-propandiol \\ (hydrophilic side) \\ \end{tabular} & \begin{tabular}{c} enantioselective \\ recognition \\ \end{tabular} & D-cysteine & [31] \\ \hline Ag nanohelices & \begin{tabular}{c} benzenacyclotricosaphanec- \\ (1,2-dithiolan-3-yl)pentanoate \\ derivative 1 \\ \end{tabular} & \
\begin{tabular}{c} enantioselective \\ recognition \\ \end{tabular} &
\begin{tabular}{c} chiral binapthlyl \\ chromophore \\ \end{tabular} & [32] \\ \hline Ag & cysteine & self-assembly & - & [33] \\ \hline Ag and Au & C16-threonine dipeptide & self-assembly & - & [34] \\ \hline \hline \end{tabular}  
* Complete chemical name: 3.6,9,12,14,17,20,23-octaoxa-1,2(1,2)-dinphthalena-13(1,3)-benzenacyclotricosaphanec-135-ylmethyl5-(1,2-dithiolan-3-yl)pentanoate.  
\end{table} 
Table 1: _Cont._  
Figure 1: Recognition system developed by [PERSON] and colleagues. Reprinted with permission from [23], Copyright Elsevier, 2014.  
and superparamagnetism of iron oxide, could selectively adsorb amino acid enantiomers (L-Trp \(>\) L-Trp and with much stronger binding force as compared to the corresponding D enantiomers) through host-guest interactions. The authors also showed that the CD MNPs selectively adsorbed the L enantiomers from racemic Trp, Phe, and Tyr with enantiomeric excess (e.e.) of 94%, 73%, and 58%, respectively, as determined by chromatographic method [26]. Enantioselectivity was shown to be mainly due to hydrogen bonding between the amine in \(\alpha\) to the chiral center of the amino acid and the secondary hydroxyl groups of CD. Silica spheres were also functionalized on their surface with \(\beta\)-cyclodextrintrin-capped Au NPs (about 60 nm in diameter) by [PERSON] and colleagues, and they were used as chiral selectors for the enantioseparation of several chiral pesticides and drugs by HPLC [27].