A CONTRIBUTION TO THE DESIGN OF FLUOROPHORE-METAL NANOPARTICLE HYBRIDS

LAURA C. ESTRADA; OSCAR E. MARTÍNEZ; PEDRO F. ARAMENDÍA

Campinas Brasil

Workshop; International workshop on nanomaterials and functional materials; 2009

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:595.3pt 841.9pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:35.4pt; mso-footer-margin:35.4pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Much research work has been devoted recently to the study of the interaction of fluorophores with metal nanoparticles (MNP). This research field has applications in high sensitivity sensing and tracing MNP enhance the electric field on nearby molecules. This effect increases absorption cross section and emission probability, while also increasing the non-radiative decay. As a consequence, depending on MNP-fluorophore distance, quenching or emission enhancement can be observed. In this work, we analyze the influence of molecular intrinsic emission quantum yield (ff), excitation and emission wavelength, and distance to the MNP surface on the gain in brightness and on the gain in total photons emitted by a fluorophore interacting with a MNP. We also analyze the consequences of the field enhancement in fluorescence fluctuation spectroscopy (FFS). This is a widespread technique for analyzing systems under equilibrium conditions to infer diffusion, concentration, and molecular dynamics. Typical conditions for FFS are: 1 fL observation volume and up to 100 molecules in the analyzed volume. This limits the concentration to the range 0.01 to 100 nM and imposes restrictions for the study of aggregation, for example. We present a new scheme for Near-Field FFS that, using the field enhancement by 40 nm radius gold NP, allows the reduction of the observation volume by 4 orders of magnitude. This allows a similar increase in concentration in the experiments, while it improves the spatial resolution of the dynamics under study. Finally, dimmer probes have a greater dynamic range for emission enhancement than do probes with high fluorescence quantum yields. For example, the enhancement ratio in the neighborhood of a MNP compared to the bulk is ca. 2 for ff = 1; 17 for ff = 0.1; and 70 for ff = 0.01; allowing on one side a better contrast and on the other, avoiding saturation of the detector. These enabled FFS experiments to be performed using individual gold NP and a 150mM Rose Bengal solution in glycerol.