## 4 Discussion  
The organometallic complexes used as model dopants are reported in Figure 1. Starting from our previous study on centric _Thalassioira weissflagii_ diatoms, we firstly selected [Ir(ppy)\(\downarrow\)bpyl]\({}^{+}\)[PF\({}_{6}\)]\({}^{-}\) to explore its suitability for doping another microalgal species such as the _Phaeodactylum tricornutum_ pennate diatom. Moreover, [Ru(bpy)\({}_{3}\)]\({}^{2+}\) 2[PF\({}_{6}\)]\({}^{-}\) was selected with the aim of exploring the possibility to extend the incorporation protocol to another complex with analogous cationic octahedral structure, but with a different nature of transition metal ion. We further investigated the suitability of the doping method for other neutral complexes, such as AlQ\({}_{3}\) and ZnQ\({}_{2}\), selected as commercially available organometallic dopants [26, 27, 31] for applications in optoelectronics. Emission spectra recorded for complexes in f/2 Guillard medium show an orange luminescence peaked at 587 nm (\(\lambda_{\text{exc}}\) = 375 nm) for [Ir(ppy)\({}_{3}\)]\({}^{+}\), whereas a typical red photoluminescence was observed for [Ru(bpy)\({}_{3}\)]\({}^{2+}\) (\(\lambda_{\text{emi}}\) = 607 nm, \(\lambda_{\text{exc}}\) = 463 nm). Yellow and green emissions were recorded for AlQ\({}_{3}\) (\(\lambda_{\text{emi}}\) = 491 nm and 531 nm, \(\lambda_{\text{exc}}\) = 380 nm) and ZnQ\({}_{2}\) (\(\lambda_{\text{emi}}\) = 489, 512, 535 nm, \(\lambda_{\text{exc}}\) = 380 nm), respectively (Figure 2a). The photostability of emissions of the complexes in the cells culture medium was initially demonstrated in the absence of the cells, in an 8 h time period (Figure 2c), long enough to let in vivo incorporation start in diatom microalgae. In fact, complexes mainly started to penetrate inside diatoms in the first day of their addition to cultures. Profiles of emission spectra were retained during the photostability study, with quenching percentages of luminescence intensities after 8 h of 40 \(\pm\) 12 for [Ir(ppy)\({}_{3}\)]\({}^{+}\), 5 \(\pm\) 3 for [Ru(bpy)\({}_{3}\)]\({}^{2+}\), 19 \(\pm\) 6 for AlQ\({}_{3}\), and 15 \(\pm\) 3 for ZnQ\({}_{2}\) (Figure 2c). Considering the limited reduction in photoluminescence intensity and retention of emission spectra profiles, all complexes were explored for feeding experiments. Diatoms were therefore exposed to solutions of organometallic emitters at different concentrations (10, 15 and 20 \(\upmu\)M) to evaluate incorporation of the complexes. However, the precipitation of emitters at 20 \(\upmu\)M and the reduced photoluminescence at 10 \(\upmu\)M limited the study to 15 \(\upmu\)M as the suitable value of concentration of all complexes as feeding agents. Incorporation of the complexes was investigated via bidimensional fluorescence microscopy of bare and doped _Phaeodactylum tricornutum_ cells at 1, 4, and 7 days after the addition of the organometallic emitters (Figure 3). As expected, chloroplasts in untreated cells of the control sample appeared as strongly red emitting mottles that increase in density from day 1 to day 4 of detection, with organization of cells in clusters on day 7 (Figure 3a). Red emitting chloroplasts were also evident in all dispersions of cells treated with organometallic complexes, even 7 days after their addition, thus showing the healthy state of diatoms during the complex incorporation (Figure 3b-e). Figure 3b shows that [Ir(ppy)\({}_{2}\)bpyl]\({}^{+}\) started to penetrate in the cells during the first day, and the yellow-orange photoluminescence of the complex was still observed both in protoplasm and in frustles 7 days after its addition (Figure 3 bii,biii). Both [Ru(bpy)\({}_{3}\)]\({}^{2+}\) and AlQ\({}_{3}\) were found to penetrate only in frustles (Figure 3c,d and insets). Moreover, in the case of doping with [Ru(bpy)\({}_{3}\)]\({}^{2+}\), an increase in doped cell density was observed passing from 1 to 7 days (Figure 3c). Insets of Figure 3b-d show incorporation of Ir, Ru, and Al emitters into biosilica shells.