In the first study, it was shown that Tween 80/limonene/nisin with little agitation produced droplets of just 20nm, but that Tween 80 and limonene without nisin also made a nano emulsion. There is a discussion also of how it gets mixed which I haven't digested yet. But it seems that with the Tween 80 and limonene mix it may not be a problem since it apparently mixes easily without little agitation. "In addition, the presence of some bioactive compounds may have minimal effect on particle size. A small increase in particle diameter was observed when nisin was added to a nanoemulsion made with 4% oil (D‐limonene) and 6% surfactant." So, only a small increase with the added nisin. Tween 80 and 40 are the best surfactants for particle diameter as Figure 6 shows. Hence, 90% water, 4% D-limonene, and 6% Tween 40 as a formula for a nano emulsion of fisetin is my theory.
I think you need to know the solubility of Fisetin in the Tween 40 + D-limonene mixture (the self-microemulsifying drug delivery system) to achieve Fisetin loaded microemulsions. Do you have idea about it?