Imine (-CH=N-) brings selectivity for silver enhanced fluorescence

Metal enhanced fluorescence (MEF) from organic molecules has been the focus of plasmonic/photonic research. In this regard, silver and gold particle stimulated fluorescence has received much attention. However, the involvement of small organic molecules in solution has not been accounted for in MEF. We have reported here that the aqueous alkaline solution of salicylaldehyde exhibits highly enhanced fluorescence at room temperature (λ(em) ∼ 420 nm, Stokes shift 120 nm, stability > a year) in the presence of Ag(I) or Au(III) after simple ageing for two days. The increased scattering cross-section of the in situ produced aggregated metal particles and the lightening rod effect of the metal aggregates that concentrate the electric field around the fluorophore (i.e., alkaline solutions of salicylaldehyde) are ascribed to such extraordinary fluorescence enhancement caused by Ag and Au particles. Finally, selective fluorescence enhancement due to silver particles alone has been made possible in the presence of ammonia or primary amines due to imine bond formation that eliminates Au enhanced fluorescence quantitatively. This finding allows us to design a highly selective Ag(I) sensor in the solution phase with a cheap and commercially available compound with LOD far below EPA-permissible levels. The imine induced selective silver enhanced fluorescence phenomenon becomes a general matter in our studies even with different Schiff bases.