## 2 Ecofriendly Strategies to Synthesize High-Quality PNCs  
### Introduction of Biogenic Organic Ligands  
The successful use of PNCs in optoelectronic and photovoltaic technologies has attracted attention from biological systems, where they can be useful for generating luminescent signals, pivotal for bioimaging and optical encoding applications [54]. However, considering the soft nature of the perovskite structure [55; 56], these materials are prone to decompose in a biological atmosphere, quenching their PL properties via eventual interaction between the environment and protective organic layers from the nanocrystal's surface. In this context, [PERSON] and coworkers [57] have developed the encapsulation of CsPbBr\({}_{3}\) PNCs with phospholipids (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) through the hydration method. Attending to phospholipids presents a hydrophilic head and a hydrophobic tail, and a micelle-type nanoparticle is formed. This CsPbBr\({}_{3}\)@DOPC micelle is water resistant, with improved PL emission stable for \(\sim\)27 days (Figure 2a,a'), and highly resistant to acid/minimum essential medium (MEM), and base aqueous solutions for a time \(\geq\)96 h (Figure 2b,c). Considering the biocompatibility of the phospholipid layer, the biofunctionalization of the composite material with folate-conjugated polyethylene glycol is favored. This composite is suitable to be incorporated into HeLa cells with the aim of performing multicolor bioimaging (Inset of Figure 2a). The preparation of this kind of material is one step forward in tumor recognition taking advantage of the optical encoding of PNCs by studying their PL properties. On the other hand, peptides can also be integrated into PNCs, giving potential applications in biomedicine, optical sensing, and imaging. By mixing a peptide based on 12-aminododecanoic acid (12-AA) with MAPbBr\({}_{3}\) PNCs [58], a partial ligand exchange is promoted. Here, the carboxylate moiety (-COO\({}^{-}\)) from 12-AA can link to metallic Pb, decreasing the density of structural defects produced by halide vacancies and the removal of OA. Meanwhile, the ammonium moiety (-NH\({}_{3}\)+) from the peptide shows a high affinity for Br- species from the perovskite surface, mediating a suitable ligand coverage [59]. This can also be confirmed by the blueshift observed in the PL emission peak from the PNCs in the presence of a higher concentration of 12-AA. This is ascribed to the instant capping effect, which restrains the nanocrystal particle size growth [60]. Figure 2: (**a**) PL spectra and (**a’**) corresponding relative intensity of CsPbBr\({}_{3}\) PNCs@DOPC micelles immersed in aqueous solutions under 27 days of analysis. Inset of Figure 2a (depicted as **a-2**) shows the typical PL image of HeLa cells immersed into CsPbBr\({}_{3}\)@DOPC micelle. Relative PL intensity of the PNCs@DOPC micelles obtained in (**b**) acid medium/MEM and (**c**) alkaline medium for a time \(\geq\)96 h of analysis. Inset of Figure 1a shows the fluorescence images of the PNCs@DOPC micelles introduced to incubated HeLa cells. Reproduced with permission [57]. Copyright 2019, American Chemical Society. (**d**) PLQY stability in water for 1-pristine CsPbBr\({}_{3}\) and 2-CsPbBr\({}_{3}\)-Pfd-L-Ser filters monitored for 6 months. Inset of Figure 1d exhibits the appearance of the corresponding PNCs filters. (**e**) Luminous efficiency in function of the applied current for 1-pristine CsPbBr\({}_{3}\) and 2-CsPbBr\({}_{3}\)-Pfd-L-Ser filters. Inset of Figure 1e depicts the typical green pc-LED configuration using the PNCs color filters. Reproduced under the terms of the CC-BY license [61]. Copyright 2022, The Authors. John Wiley & Sons, Inc.  
Another important contribution concerning the use of biogenic ligands has been shown by [PERSON] and coworkers [61], combining an amphiphilic capping agent such as phosphatidyl-L-serine (Ptd-L-Ser) with CsPbBr\({}_{3}\) PNCs.