## 2 Applications of Chiral Nanoparticles
The presence of Au NPs improved the overall efficiency of the system compared to traditional \(\beta\)-CD-modified silica column and could be easily applied to the separation of additional racemic mixtures. [PERSON] and colleagues also exploited MNPs for separating chiral drugs, a fundamental step in the pharmaceutical industry [28]. Bovine serum albumin (BSA) anchored to the NPs surface was able to recognize ibuprofen and ofloxacin with an e.e. of 13% and 14% for single-stage adsorption, respectively, and e.e. values of 54% and 39% for multi-stage operation, respectively. A hybrid procedure combining the magnetic enantiomeric enrichment followed by crystallization allowed for the obtaining of an even higher optical purity (up to 99% e.e. for ibuprofen). Mesoporous silica MNPs functionalized on the outer surface with teicoplanin, an antibiotic glycoptide, showed excellent chiral selectivity and separation ability from racemic solutions of aromatic amino acids using an exterior magnetic field [29]. Much stronger interactions were stabilized with the (+)-enantiomers, compared to the (-)-enantiomers, of tryptophan and phenylalanine, and the system was also shown to be highly stable and reusable (Figure 2). Layer-by-layer self-assembly of cyclodextrin-coated Au NPs onto silica spheres was also exploited as stationary phase for the chiral recognition and separation of metpazinol from its three enantiomers by capillary electrochromatography [30]. The methodology was shown to possess good linearity (\(\geq\)0.999), recovery (92.0-94.5%), and repeatability, and it was efficiently employed for the enantioseparation of metpazinol in spiked urine samples. Utilizing a different approach, [PERSON] and colleagues prepared gold Janus nanoparticles covered by (hydrophobic) hexanethiolates and (hydrophilic) 3-mercapto-1,2-propandiol that displayed positive circular dichroism (CD) absorption in the visible range following the selective encapsulation of enantiomers into the emulsion cavities of the NP [31]. In particular, D-cysteine resulted as the favored enantiomer trapped into the emulsions, where the ligand exchange reaction occurred at the hydrophilic face of the Janus NPs. By controlling concentration and reaction time, chiral Janus nanoparticles could be also prepared by ligand exchange reactions utilizing a racemic mixture of ligands. Enantioselective recognition was also exploited for amplifying the optical activity of chiral molecules by using chiral ligands containing the binapthly chromophore anchored on the surface of  
Figure 2: Magnetic separation of (+)-Trp and (+)-Phe by means of teicoplanin-functionalized MNPs. Reprinted with permission from [29], Copyright Elsevier, 2013. Ag NPs with a sub-5 nm helical pitch (\(P\)) [32]. The amplification was attributed to the enantiospecific adsorption of the ligand, which caused a variation of the dihedral angle of the binaphthyl chromophore, thus transferring the chiral signal of the Ag NPs to the binaphthyl chromophore through the Ag-S bicontacts. This enantioselective amplification did not happen on the Ag NPs without chiral footprint or on the chiral Ag NPs with a nominal \(P>5\) nm because of the dimensional mismatch of the footprints and the Ag-S bicontacts. Chiral recognition can also lead to particular self-assembled tridimensional structures. For example, [PERSON] and colleagues prepared silver nanoparticles (\(\approx\)45 nm) functionalized with cysteine and \(N\)-, C-, and \(S\)-protected cysteine derivatives and observed the formation of an uncommon chiral architecture regardless of the enantiomer employed [33]. The authors noted that the assembly produced chiral electronic CD spectra, caused by optical molecular dipoles between the amino acids with a primary amine and the metallic core, and that the formation of the assembly depended on the concentration of the NPs and on the pH of the solution. These findings are in agreement with a model developed by [PERSON] et al.