The tuning of metal enhanced fluorescence for sensing applications

Abstract

Stable coinage metal nanoparticles (NPs) have been synthesized individually in an aqueous alkaline solution from the corresponding metal salts as precursors using the condensation product (CP) of salicylaldehyde and triethylenetetramine as a reagent. Silver and gold NPs are obtained with and without light illumination but UV irradiation is essential for Cu(0)NP formation. During nanoparticle formation the CP is oxidized to OCP which eventually becomes a fluorophore and also a stabilizer for the in situ produced NPs. It has been observed that silver and gold particle formation kinetics is accelerated by UV exposure. Thus the ease of evolution of coinage metal NP formation relates to their nobility. The as prepared OCP solutions containing coinage metals exhibit a fluorescence contrast behaviour (fluorescence enhancement by Cu and Ag; quenching by AuNP) due to the match and mismatch of wave vectors. The electric field evident from the FDTD simulation abreast of the scattering cross section of the NPs governed from Mie theory as a consequence of surface plasmon coupled emission (SPCE), near field electromagnetic intensity enhancement and lightening rod effect concentrating the electric field around the fluorophore are responsible for the Cu and AgNPs stimulated fluorescence. Again, lossy surface waves are anticipated for efficient quenching by the AuNPs. The most unprecedented observation is ‘Turn On’ fluorescence which is reported here as a result of the substitution of Au(0) or Cu(0) by Ag(0). Finally, the preferential fluorescence enhancement helps the selective detection of Ag(I) and Cu(II) well below the US Environmental Protection Agency (EPA) permissible level by tuning the experimental conditions.