Ratiometric visualization of folic acid with a smartphone-assisted fluorescence paper device based on gadolinium and nitrogen co-doped CDs

In this work, a facial ratiometric fluorescence nanorpobe (RFP) based on gadolinium and nitrogen co-doped CDs (Gd,N-CDs) was for the first time proposed for visual detection of folic acid (FA). The Gd,N-CDs were prepared by hydrothermal heating of Gd(NO3)3⋅6H2O, o-phenylenediamine (OPD), 4-aminobenzoic acid (PABA). The as-prepared Gd,N-CDs had an average particle size of 10.27 ± 0.35 nm and exhibited two emission bands at 396 and 548 nm, respectively upon excitation at 340 nm. In the presence of FA, the 560 nm emission of Gd,N-CDs was reduced while the fluorescence signal at 457 nm was enhanced, and the quantitatively determination of FA was based on the fluorescence intensity ratio (F457/F560). The as-constructed RFP achieved a linear range of 0.0–30.0 μM and a low detection limit (LOD) of 3.43 nM. In particular, a distinguishable change in fluorescence colour from bright orange to blue enabled the development of a smartphone-assisted visual sensing platform and a paper-based analytical device for FA detection with LODs of 3.06 μM and 0.83 μM, respectively. The proposed sensing platform was successfully utilised for onsite detection of FA in vegetable, fruit and yolk samples with satisfactory results were achieved. This work highlights the application potential of smartphone-integrated CDs-based RFP as a promising portable method for on-site analysis of FA.