Tony E. Karam

Fluorescence, Phosphorescence, and Chemiluminescence

Noureen Siraj,† Bilal El-Zahab,‡ Suzana Hamdan,† Tony E. Karam,† Louis H. Haber,† Min Li, Sayo O. Fakayode, Susmita Das, Bertha Valle, Robert M. Strongin, Gabor Patonay, Herman O. Sintim, Gary A. Baker, Aleeta Powe, Mark Lowry, Jan O. Karolin, Chris D. Geddes, and Isiah M. Warner*,† †Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States ‡Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33174, United States Process Development Center, Albemarle Corporation, Baton Rouge, Louisiana 70805, United States Department of Chemistry, Winston-Salem State University, Winston-Salem, North Carolina 27110, United States Department of Civil Engineering, Adamas Institute of Technology, Barasat, Kolkata 700126, West Bengal India Department of Chemistry, Texas Southern University, Houston, Texas 77004, United States Department of Chemistry, Portland State University, Portland, Oregon 97207, United States Department of Chemistry, Georgia State University, Atlanta, Georgia 30302-4098, United States Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States Department of Chemistry, University of Missouri Columbia, Columbia, Missouri 65211-7600, United States Department of Chemistry, University of Louisville, Louisville, Kentucky 40208, United States Institute of Fluorescence, University of Maryland Baltimore County, Baltimore, Maryland 21202, United States

Excited-state dynamics of size-dependent colloidal TiO2-Au nanocomposites

The ultrafast excited-state dynamics of size-dependent TiO2-Au nanocomposites synthesized by reducing gold nanoclusters to the surface of colloidal TiO2 nanoparticles are studied using pump-probe transient absorption spectroscopy with 400 nm excitation pulses. The results show that the relaxation processes of the plasmon depletion band, which are described by electron-phonon and phonon-phonon scattering lifetimes, are independent of the gold nanocluster shell size surrounding the TiO2 nanoparticle core. The dynamics corresponding to interfacial electron transfer between the gold nanoclusters and the TiO2 bandgap are observed to spectrally overlap with the gold interband transition signal, and the electron transfer lifetimes are shown to significantly decrease as the nanocluster shell size increases. Additionally, size-dependent periodic oscillations are observed and are attributed to acoustic phonons of a porous shell composed of aggregated gold nanoclusters around the TiO2 core, with frequencies that decrease and damping times that remain constant as the nanocluster shell size increases. These results are important for the development of improved catalytic nanomaterial applications.

Ultrafast and nonlinear spectroscopy of brilliant green-based nanoGUMBOS with enhanced near-infrared emission

The synthesis, characterization, ultrafast dynamics, and nonlinear spectroscopy of 30 nm nanospheres of brilliant green-bis(pentafluoroethylsulfonyl)imide ([BG][BETI]) in water are reported. These thermally stable nanoparticles are derived from a group of uniform materials based on organic salts (nanoGUMBOS) that exhibit enhanced near-infrared emission compared with the molecular dye in water. The examination of ultrafast transient absorption spectroscopy results reveals that the overall excited-state relaxation lifetimes of [BG][BETI] nanoGUMBOS are longer than the brilliant green molecular dye in water due to steric hindrance of the torsional degrees of freedom of the phenyl rings around the central carbon. Furthermore, the second harmonic generation signal of [BG][BETI] nanoGUMBOS is enhanced by approximately 7 times and 23 times as compared with colloidal gold nanoparticles of the same size and the brilliant green molecular dye in water, respectively. A very clear third harmonic generation signal is observed from the [BG][BETI] nanoGUMBOS but not from either the molecular dye or the gold nanoparticles. Overall, these results show that [BG][BETI] nanoGUMBOS exhibit altered ultrafast and nonlinear spectroscopy that is beneficial for various applications including nonlinear imaging probes, biomedical imaging, and molecular sensing.