Selective dual sensing strategy for free and vitamin D3 micelles in food samples based on S,N‑GQDs photoinduced electron transfer

A reliable nanotechnological sensing strategy, based on an S,N-co-doped graphene quantum dot (GQD) 
platform, has been developed to distinctly detect two key variants of vitamin D₃, specifically the 
free (VD₃) and the nanoencapsulated form (VD₃Ms). For this purpose, food-grade vitamin D₃ micelles 
were self-assembled using a low-energy procedure (droplet size: 49.6 nm, polydispersity index: 
0.34, ζ-potential: −33 mV, encapsulation efficiency: 90 %) with an innovative surfactant mixture 
(Tween 60 and quillaja saponin). Herein, four fluorescent nanoprobes were also synthesized and 
thoroughly char-acterized: S,N-co-doped GQDs, α-cyclodextrin-GQDs, β-cyclodextrin-GQDs, and 
γ-cyclodextrin-GQDs. The goal was to achieve a selective dual sensing strategy for free VD₃ and 
VD₃Ms by exploiting their distinctive quenching behaviors. Thus, the four nanosensors allowed the 
individual sensing of both targets to be performed (except α-CD-GQD for VD₃Ms), but S,N-GQDs were 
finally selected due to selectivity and sensitivity (quantum yield, QY= 0.76) criteria. This choice 
led to a photoinduced electron transfer (PET) mechanism associated with static quenching, where 
differentiation was evidenced through a displayed 13-nm hypsochromic (blue) shift when interacting 
with VD₃Ms. The reliability of this dual approach was demonstrated through an extensive evaluation 
of analytical performance characteristics. The feasibility and accuracy were proven in commercial 
food preparations and nutritional supplements containing declared nanoencapsulated and raw VD₃, 
whose results were validated by a paired Student’s t-test comparison with a UV-Vis method. To the 
best of our knowledge, this represents the first non-destructive analytical approach addressing the 
groundbreaking foodomic trend to distinctly detect different bioactive forms of vitamin D₃, while 
also preserving their native nanostructures as a chemical challenge, thus providing reliable 
information about their final stability and bioavailability.