Femtosecond time-resolved energy transfer from CdSe nanoparticles to phthalocyanines
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The first real-time observation of the early events during energy transfer from a photoexcited CdSe nanoparticle to an attached phthalocyanine molecule are presented in terms of a femtosecond spectroscopic pump–probe study of the energy transfer in conjugates of CdSe nanoparticles (NPs) and silicon phthalocyanines (Pcs) with 120 fs time resolution. Four different silicon phthalocyanines have been conjugated to CdSe NPs. All of these have proven potential for photodynamic therapy (PDT). In such NP-Pc conjugates efficient energy transfer (ET) from CdSe NPs to Pcs occurs upon selective photoexcitation of the NP moiety. Spectral analysis as well as time-resolved fluorescence up-conversion measurements revealed the structure and dynamics of the investigated conjugates. Femtosecond transient differential absorption (TDA) spectroscopy was used for the investigation of the non-radiative carrier and ET dynamics. The formation of excitons, trapped carriers states, as well as stimulated emission was monitored in the TDA spectra and the corresponding lifetimes of these states were recorded. The time component for energy transfer was found to be between 15 and 35 ps. The ET efficiencies are found to be 20-70% for the four Pc conjugates, according to fluorescence quenching experiments. Moreover, as a result of the conjugation between NP and the Pcs the photoluminescence efficiency of the Pc moieties in the conjugates do not strictly follow the quantum yields of the bare phthalocyanines.
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- 2.A.C.S. Samia, S. Dayal, C. Burda, Photochem. Photobiol. (2006), unpublishedGoogle Scholar
- 3.R.L. Morris, K. Azizuddin, M. Lam, J. Berlin, A.L. Nieminen, M.E. Kenney, A.C.S. Samia, C. Burda, N.L. Oleinick, Cancer Res. 63, 5194 (2003)Google Scholar
- 8.W.M.C. Sharman, M. Allen, J.E. van Lier, Therapeutic focus (DDT) 4, 507 (1999)Google Scholar
- 13.Prior to experiments the NPs were purified twice by precipitating them with methanol. The solvents were purchased from Fisher Scientific. Prior to RET measurements the toluene solutions of CdSe NPs and Pcs were mixed and kept in the dark for 48 h. Absorption and photoluminescence were measured in quartz cuvettes, using a Varian Cary 50 Bio UV-VIS and Varian Cary Eclipse fluorescence spectrometers. Femtosecond measurements were performed employing a pump–probe system driven by a 1 kHz CPA laserGoogle Scholar
- 14.J.C. Berlin, Ph.D. Thesis (Case Western Reserve University, Cleveland, OH 2006)Google Scholar
- 15.The apparatus function was determined by measuring a kinetic trace of the stimulated emission spectrum of a hexane solution of coumarin 153 at 475 nm. The measurements yielded a Gaussian curve with 220 fs FWHM. This function was later used in a deconvolution program for assessment of ultrafast kinetics. After the deconvolution procedure, a 100 fs time resolution could be achieved. The laser experiments were performed using a 2 mm cell. The concentrations of CdSe in the studied samples were 3.7 × 10-6 M. The total concentration of photons in the TA experiments was <3 × 10-6 M corresponding to <1 photons per one NP-Pc conjugate. No photodegradation of the samples was observedGoogle Scholar
- 16.J.R. Lakowicz, Principles of Fluorescence Spectroscopy 2nd edn., (Kluwer Academic/Plenum Publishers, New York, 1999)Google Scholar
- 17.The quantum yields of CdSe nanoparticles and Pcs were determined using rhodamine 6 G (ϕfl,EtOH=0.94) and cresyl violet (ϕfl,MeOH=0.54), respectively, as fluorescence standards . The correction for the refractive index of the solvent and the correction for the percent of light absorbed by the samples were taken into account.Google Scholar