Nanoplasmonics and surface enhanced spectroscopy

Effect of plasmon resonance of Ag and Au nanoparticles on spectral-luminescent properties of polymethine dye 
Ibrayev N.Kh.1, Seliverstova E.V. 1, Zhumabay N.D. 1, Ishchenko A.A. 2
1 Institute of Molecular Nanophotonics, Buketov KarSU, Universitetskaya str. 28, Karaganda 100028, Kazakhstan.
E-mail: niazibrayev@mail.ru, evgnseliverstova@gmail.com

2 Institute of Organic Chemistry, Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanskaya str. 5, 02660 Kiev-94, Ukraine 
E-mail: alexish@i.com.ua

The spectral-luminescent properties of the cationic polymethine dye (PD) with the addition of gold and silver nanoparticles (NPs) were studied in ethanol solutions. The concentration of PD in all solutions was constant and equal to 10-5 mol/l. The concentration of silver or gold NPs was varied in the range of 10-10 – 10-14 mol/l. The Ag and Au NPs in ethanol were obtained by laser ablation method. The average sizes of the Ag and Au Nps were determined with the Zetasizer Nano ZS analyzer and were equal to 60 nm for Au and 90 nm for Ag. The absorption spectrum of silver NPs exhibits as a wide band with a maximum of about 405 nm. The maximum absorption band of gold NPs exhibits at 530 nm. It was shown that the absorption spectrum of Au nanoparticles has a greater degree of overlap with the absorption and fluorescence spectra of the dye than the absorption spectrum of Ag.
Measurements have shown that both the absorption and fluorescence of the polymethine were increased in the presence of plasmon NPs. The position of the maxima of the absorption and the fluorescence bands of the dye and their half-width were not changed. The maximum enhancement of fluorescence in 4 times was recorded for the concentration of Ag NPs, equal to 3*10-14 mol/l, and of 11 times – for the Au NPs with the concentration, equal to 7*10-11 mol/l.
The reason for the enhancement of the fluorescence intensity of the dye and the decrease in its lifetime can be associated with an increase in the rate of electronic transitions in dye molecules that are located in the near field of metallic NPs with excited plasmons. The decrease in the intensity of the luminescence of the dye at high concentrations of metal NPs can be the result of nonradiative energy transfer from the PD molecules to NPs, which leads to a decrease in the probability of radiative decay of excited molecules.
The results obtained can be used in sensors, laser technology, photovoltaics, as well as for recording and storing information.