## 2 Applications of Chiral Nanoparticles
### Biosensors  
Chiral NPs and their nanocomposites have been shown to possess countless capabilities as tools for fabricating functional materials, such as non-linear optics and biological sensors and detectors [80]. The main findings about their exploitation as chiral biosensors are summarized in Table 4.  
For example, ZnO NPs functionalized with L-cysteine were used for detecting dopamine in urine samples with high selectivity and sensitivity [81]. The system exploited the variation in photoluminescence of the chiral NPs that was quenched upon binding to the neurotransmitter. On the other hand, a stereoselective electrochemiluminescence (ECL) interface was prepared for quantifying Pen enantiomers by using hemoglobin (Hb) and Au NP-functionalized graphite-like carbon nitride nanosheet composite attached to glassy carbon electrodes (Hb/Au-g-C\({}_{3}\)N\({}_{4}\)/GCE) [82]. Different ECL peaks were recorded from Pen enantiomers on the chiral interface although a larger response was obtained from the D-Pen derivatives, with a limit of detection of \(3.3\times 10^{-5}\) M. As a proof of principle, this stereoselectivity was also exploited for measuring D-Pen in spiked tablet samples. Surprisingly, [PERSON] et al. discovered that unmodified citrate-capped Au NPs (about 13 nm in diameter) could display chiral-selective response upon addition of L(D)-Trp [83]. However, a considerable red-to-blue color change of the gold solution could be detected in the presence \begin{table}
\begin{tabular}{c c c c} \hline
**NPs** & **Surface Functionalization** & **Molecule Detected** & **Reference** \\ \hline ZnO & L-cysteine & dopamine & [81] \\ Au & hemoglobin & D-penicillamine & [82] \\ Au & - & L(D)-triptophan & [83] \\ Poly(vinyl chloride) & - & \(S\)-citalopram & [84] \\ Au & - & \(S\)-citalopram & [85] \\ Au & - & L-alanine & [86] \\ Au@CoFe2O4 & \(\beta\)-cyclodextrintrin & L(D)-triptophan & [87] \\ Ag & chitosan & L(D)-triptophan & [88] \\ Ag & - & D-cysteine & [89] \\ Au & mAb & tropomyosin & [90] \\ Au & - & L-Leu & [91] \\ Au & cysteine & L(D)-Tyr, L(D)-Trp, L(D)-Glu & [92] \\ Ag & (R)-mandelic acid-derived & \(N\)-FmoC-L(D)-aspartic acid & [93] \\  & calix[4]arene & \(L\)(D)-mandelic acid & [94] \\ \hline \end{tabular}
\end{table} 
Table 4: Main applications of chiral NPs as biosensors and their principal characteristics. of D-Trp, while no color variation was observed when L-Trp was added. This process was used to quantify D-Trp in the range of 0.2-10 \(\upmu\)M, with a remarkable limit of detection of 0.1 \(\upmu\)M. Furthermore, the Au NPs could selectively adsorb D-Trp on their surface, and gentle centrifugation allowed the precipitation of D-Trp-bound Au NPs, while leaving in solution the other enantiomer, thus allowing separation of the racemate (Figure 8). [PERSON] et al. exploited molecularly imprinted chiral polymeric particles to create a novel enantioselective sensor able to recognize \(S\)-citalopram, an antidepressant drug, used as a template in the synthetic step [84]. The sensor was able of precisely quantify \(S\)-citalopram in urine samples with a linear range and with very short response time in the interval range of concentration of clinically administered drug. Citalopram was also detected by [PERSON] and colleagues by using citrate-capped Au NPs, reporting a simple and reliable colorimetric method for the quantification of its enantiomers in aqueous solution, with detection limits of 2.1 \(\upmu\)M [85]. \(S\)-citalopram, but not the other enantiomer, induced the aggregation of the Au NPs, probably because of electrostatic interactions and hydrogen bonding between the host and the guest, thus causing a reduction of the plasmon absorption peak at 520 nm and the development of a widened surface plasmon band around 600-700 nm. The method was effectively employed for the determination of citalopram in commercially available tablets, showing good precision and accuracy.