# New Light on Phtyoremediation: The Use of Luminescent Solar Concentrators
## 2 Materials and Methods
### Photosynthetic Process and Selection of Luminescent Dye  
In the process of photosynthesis, plants absorb the solar radiation in the range 400-700 nanometers (this range is called Photosynthetically Active Radiation, PAR), primarily using the pigment Chlorophyll, the most abundant in the plant. In addition to chlorophyll, plants also use other pigments belonging to the carotenoid group.  
The chlorophyll exists in two forms: chlorophyll \(\mathbf{a}\) and chlorophyll \(\mathbf{b}\). Chlorophyll \(\mathbf{a}\) shows a strong light absorption in the blue zone of the solar spectrum as well in the red zone; chlorophyll \(\mathbf{b}\) absorbs mostly blue and orange light. In contrast, both absorb poorly the green and near-green light, which reflects the typical green color of the leaves. In the near-green zone, the absorption of carotenoids takes place (Figure 2).  
The photosynthetic efficiency (i.e., the fraction of light energy converted into chemical energy during photosynthesis) depends on the wavelength of light. The red light (600-700 nm) is the most efficient; the efficiency increases when coupled to an equal far-red light (700-800 nm). The violet-blue light, even if less efficient (efficiency = 0.65-0.75) [38], is  
Figure 2: Absorption spectrum for chlorophylls and carotenoids (based on data from [37]).  
necessary for photosynthesis because it promotes the development of chloroplasts, where photosynthesis takes place [39]. A low percent of violet-blue light (-7%) is enough to ensure plants good health [40]. Outside the visible spectrum, the UV light (200-400 nm) damages the chloroplasts hindering the photosynthetic process [41].  
Considering this, luminescent dye Lumogen F305 (perylene-based molecule by BASF) was chosen for its spectroscopic features, high quantum yield (100%) and good photostability (5% of degraded dye after 4600 h accelerated ageing). Indeed, the absorption spectrum of Lumogen is characterized by a strong band in the range 500-600 nm with maximum absorption at 576 nm and a weak band between 400 and 500 nm; the photoluminescence emission takes place between 600 and 750 nm with a maximum fluorescence peak at 615 nm (Figure 3).  
Green light of solar spectrum, which is largely captured by Lumogen, is not absorbed by chlorophylls and carotenoids. Therefore, a LSC panel doped with Lumogen does not interfere with the photosynthetic process.  
High intensity and correct distribution of transmitted sunlight are required to LSC plates for greenhouse application; these conditions allow high growth and good development of plants. Increased conversion of sunlight absorbed in electricity is also needed for making self-supporting greenhouses. In this respect, the amount of luminescent dye in the plate plays a key role.  
Different dye concentrations were evaluated to determine the best one in terms of quantity and quality of transmitted light and electricity production. Transmitted light and its distribution in PAR range were measured from the transmittance UV-Vis spectra by using a Perkin Elmer UV-Vis-NIR Lambda 950 spectrometer. Every spectrum was firstly weighted for the solar spectrum by the AM 1.5 Reference Solar Spectrum. Then, the integrated area of the solar weighted transmission spectrum (in the range 400-700 nm) of the sample was perceptualized using the AM 1.5 as reference.  
PAR attenuation (%) is the attenuation of the transmitted light (\(\mathrm{I_{TL}}\)) with respect to the incident light (\(\mathrm{I_{IL}}\)), as reported in Equation (1):  
\[\left(\mathrm{I_{TL}-I_{IL}}\right)/\mathrm{I_{IL}}\ ^{*}100 \tag{1}\]