Multidonor surface energy transfer from Alexa Fluor dyes to gold nanoparticles: a quest for innovative sensor applications

We report energy transfer (ET) from two dyes: Alexa Fluor 514 (AF514) and Alexa Fluor 532 (AF532) to gold nanoparticles (AuNPs) of three different sizes (10, 30, and 53 nm) employing steady-state and time-resolved fluorescence measurements. The results show that the fluorescence intensity and fluorescence lifetimes of donor (D) molecules AF514 and AF532 decrease with increase in the concentration of acceptor (A) AuNPs (2 to 10 μM) upon interaction with AuNPs, thereby confirming the occurrence of ET between D and A. This clearly suggests that these two Alexa Fluor molecules act as efficient donors and AuNPs as excellent acceptors. Interestingly, the Forster distance (Ro) determined for these dyes varies from 212 to 550 Å with increasing size of AuNPs and suggests that the ET from AF514 and AF532 to AuNPs is essentially obeying surface energy transfer (SET) process following 1 / d4 distance dependence. As is well known, Forster resonance energy transfer is efficient for separation distances of up to 100 Å, beyond which its efficiency decreases. Thus, the present results follow dipole-surface type ET from molecule dipole (AF514 and AF532) to nanometal (Au) surface. The influence of size and distance on the SET from AF514 and AF532 to AuNPs is discussed. Further, the quenching of donor fluorescence in the presence of AuNPs and nonradiative ET are analyzed using Stern–Volmer plots. Our study is an experimental quest to explore the potential of such dye–noble metal NPs pairs performing as sensitive chemical and biosensors.