## 2 Applications of Chiral Nanoparticles
### Biosensors
The colorimetric quantification of L-alanine in human serum was investigated by [PERSON] and [PERSON], who used inherently chiral Au NPs (about 8 nm in diameter) embedded in a paper-based lab-in-a-syringe device [86]. The device was based on the enantioselective recognition of the amino acid and the subsequent precipitation of the aggregate. The variation of the color of the solution allowed for the determination of alanine with a limit of detection of 0.77 mM. [PERSON] et al. presented a novel, facile, and generic procedure that exploited the electrocatalytic properties of chiral magnetic nanobiofluids (mNBFs) and the electrochemical enantiobiosensing ability of a magneto nanocomposite graphene paste electrode (mNC-GPE) [87]. A specific chiral mNBF, based on an aqueous dispersion of cobalt ferrite loaded with \(\beta\)-cyclodextrinutrin coated Au NPs (\(\beta\)-CD-Au@CoFe\({}_{2}\)O\({}_{4}\) NPs), was exploited for the enantioselective recognition of tryptophan, although the procedure could be easily adapted to many other APIs, thus opening up new approaches for enantio(bio)sensing. Tryptophan was also determined by a novel, fast, and economical sensor by using chitosan-capped Ag NPs [88]. The sensor was based on the colorimetric measurement by scanometry and spectrophotometry of a reaction solution comprising chitosan-capped Ag NPs, phosphate buffer, and tryptophan. Figure 8: TEM images of the effect of Trp addition to a solution of citrate-capped Au NPs. Reprinted with permission from [83], Copyright Elsevier, 2014. A recent colorimetric sensing platform for the enantioselective recognition of chiral analytes also exploited Ag NPs that were embedded into translucent nanopaper [89]. The nanopaper was prepared by an environmentally friendly method that exploited bacterial cellulose made of nanofibers while the Ag NPs were synthesized in situ, thus allowing their direct encapsulation. The NPs presented a selective sensing reply toward D-cysteine with a limit of detection of 4.88 \(\upmu\)M (Figure 9). A biosensor based on chiral assemblies of Au NP coupled with CD spectroscopy was developed for the recognition and quantification of tropomyosin (TROP), considered to be the major allergen in shellfish allergy [90]. Monoclonal antibody-Au NP and TROP-Au NP probes could organize into terpolymers or aggregates of diverse oligomers, depending on the concentration of the free antigen. The biosensor was able to quantify the allergen in seven different shellfish species, with good repeatability and accuracy, remarkable specificity, and an impressive limit of detection of 21 nM. A similar approach, but with cube-shaped gold nanoparticles, was employed for the determination of leucine [91]. The colorimetric analysis exploited the ability of L-Leu, but not D-Leu, to induce rapid aggregation of Au NCs (average edge of about 46 nm), thus causing a color variation of the solution from red to transparent, yielding a modest, cheap, quick, and easy-to-function sensing system. Interestingly, the enantioselectivity was associated with the geometrical structure of the NPs because only the NCs, but not similar spherical or rod-shaped Au nanocrystals, were able to discriminate the amino acid. [PERSON] et al. successfully fabricated a device measuring both potential difference and current ratio that exploited left/right-handed double helix carbon nanotubes functionalized with polypyrrole-containing cysteine-capped Au NPs (L(D)-DHCNT@PPy@L(D)-Cys-Au) via a facile and environmentally friendly procedure [92]. This highly efficient electrochemical chiral sensor exploited the chiral structure of L(D)-Cys and L(D)-DHCNT, and the exceptional conductivity of PPy and Au NPs, thus being able to distinguish and quantify tyrosine (Tyr), tryptophan (Trp), and glutamic acid (Glu) enantiomers. By covering Ag NPs with a chiral (\(R\))-mandelic acid-derived calix[4]arene, [PERSON] et al. managed to quantify \(N\)-Fmoc-L(D)-aspartic acid in solution by measuring through dynamic light scattering (DLS) the assemblies formed with the NPs (Figure 10) [93].