The rate of singlet fission in a dimer is known to be dependent on the coupling between the two chromophores that make up the dimer. A series of ethynyltetraceme dimers have been synthesized with the chromophore units at ortho-, meta- and para- configurations. Broadband femtosecond time-resolved pump-probe spectroscopy and time-correlated single photon counting has been used to observe the singlet state, the multiexciton state and the separated triplet states in these dimers in both solution and neat film phases. In solution, only the para-dimer forms separated triplets while the meta-dimer does not undergo singlet fission at all. In neat films, inter-dimer chromophore interactions lead to singlet fission in both the meta-dimer and the para-dimer. Target analysis using compartmental kinetic models allows for the spectral features of the different excited states in singlet fission to be resolved, along with the rate constants of the excited state processes involved.
Dense inorganic nanoparticles have recently been identified as promising radiosensitizers. In addition to dose enhancement through increased attenuation of ionizing radiation relative to biological tissue, scintillating nanoparticles can transfer energy to coupled photosensitizers to amplify production of reactive oxygen species, as well as provide UVvisible emission for optical imaging. Lanthanum fluoride is a transparent material that is easily prepared as nanocrystals, and which can provide radioluminescence at a number of wavelengths through simple substitution of lanthanum ions with other luminescent lanthanides. We have prepared lanthanum fluoride nanoparticles doped with cerium, terbium, or both, that have good spectral overlap with chlorine6 or Rose Bengal photosensitizer molecules. We have also developed a strategy for stable conjugation of the photosensitizers to the nanoparticle surface, allowing for high energy transfer efficiencies on a per molecule basis. Additionally, we have succeeded in making our conjugates colloidally stable under physiological conditions. Here we present our latest results, using nanoparticles and nanoparticle-photosensitizer conjugates to demonstrate radiation dose enhancement in B16 melanoma cells. The effects of nanoparticle treatment prior to 250 kVp x-ray irradiation were investigated through clonogenic survival assays and cell cycle analysis. Using a custom apparatus, we have also observed scintillation of the nanoparticles and conjugates under the same conditions that the cell samples are irradiated.
Broadband pump-probe spectroscopy over the entire time range (200 fs to 500 ns) relevant to monitor the polaron
generation and recombination dynamics were performed on the bulk heterojunction composites of poly (3-
hexylthiophene) (P3HT) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) with [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. The modeling of the polaron dynamics with the Debye-
Smoluchowski diffusion model provides the charge separation length at the polymer:fullerene interface. Furthermore, the
computed polaron yield using the polaron cross-section over the entire time range reveals the amount of photo-generated
charges in P3HT:PCBM and P3HTT-DPP-10%:PCBM bulk-heterojunction thin films.
We report progress towards combining radiation therapy (RT) and photodynamic therapy (PDT) using scintillating nanoparticle (NP)-photosensitizer conjugates. In this approach, scintillating NPs are excited by clinically relevant ionizing radiation sources and subsequently transfer energy to conjugated photosensitizers via FRET, acting as an energy mediator between ionizing radiation and photosensitizer molecules. The excited photosensitizers generate reactive oxygen species that can induce local damage and immune response. Advantages of the scheme include: 1) Compared with traditional radiation therapy, a possible decrease of the total radiation dose needed to eliminate the lesion; 2) Compared with traditional PDT, the ability to target deeper and more highly pigmented lesions; 3) The possibility of additional photosensitizing effects due to the scintillation of the nanoparticles. In this work, the photosensitizer molecule chlorin e6 was covalently bound to the surface of LaF3:Ce NPs. After conjugation, the photoluminescence intensity of NPs decreased, and fluorescence lifetime of conjugated chlorin e6 became sensitive to excitation wavelength, suggesting rapid FRET. In addition, scintillation spectra of nanoparticles were measured. Preliminary calculations suggest that the observed scintillation efficiencies are sufficient to enhance RT. In vitro cancer cell studies suggest conjugates are taken up by cells. Survival curves with radiation exposure suggest that the particles alone cause radiosensitization comparable to that seen with gold nanoparticles.
Ultrafast transient absorption spectroscopy is used in conjunction with spectroelectrochemistry and chemical doping experiments to study the photogeneration of charges in hybrid bulk heterojunction (BHJ) thin films composed of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and CdSe nanocrystals. Chemical doping experiments on hybrid and neat PCPDTBT:CdSe thin films are used to deconvolute the spectral signatures of the transient states in the near infrared. We confirm the formation and assignment of oxidized species in chemical doping experiments by comparing the spectral data to that from spectroelectrochemical measurements on hybrid and neat PCPDTBT:CdSe BHJ thin films. The deconvolution procedure allows extraction of the polaron populations in the neat polymer and hybrid thin films.
Ultrafast transient absorption spectroscopy is used in conjunction with chemical doping experiments to study
the photo-generation of charges in hybrid thin films composed of PCPDTBT and CdSe quantum dots. We show
how we use chemical doping experiments to de-convolute the spectral signatures of the transient states in the
The response of solubilized quantum dot solutions to visible or UV irradiation is highly variable, and contradictory
reports exist in the literature. Using several different preparations of core CdSe, core-shell CdSe/ZnS, and CdTe
quantum dots (QDs), we investigated the time-resolved photoluminescence as a function of 400 nm irradiation. We
found that photoenhancement and photodegradation were highly dependent upon irradiation power, with the QDs being
highly stable at fluences of < 2 mW. However, great variability was seen among independent preparations of QDs, with
fresher dots showing greater photostability than those that had been aged in organic solvent. Conjugation of dopamine to
the QDs also led to variable effects, with some batches showing lifetime enhancement upon conjugation and others
suppression. In all cases, QD-dopamine conjugates showed increased lifetimes upon irradiation, up to a maximum effect
at ~ 5 min post irradiation at 2.4 mW. The antioxidant beta-mercaptoethanol also affected different batches of QDs
differently; it prevented photoenhancement with certain batches but not others. We propose a mechanism of
photoenhancement and surface oxidation that relates the variability to the number of solubilising groups on the QD
surface. The potential of photoenhancement as a sensing mechanism in cells is proposed.
Semiconductor quantum dots (QDs) possess highly reactive electrons and holes after photoexcitation. The energy of these electrons and holes can be deliberately modulated by attaching the QD to an electron donor or acceptor. This eliminates (quenches) QD fluorescence, as well as affecting the ability of the QD to oxidize or reduce common biomolecules such as glutathione and DNA. This greatly alters the fluorescent properties and toxicity of such QDs
inside cells. In this work, we show that a specific electron donor, the neurotransmitter dopamine, yields redox-sensitive conjugates when attached to at least some colors of CdSe/ZnS QDs. The potential for the use of such conjugates as sensors, and the implications for enhanced toxicity in such conjugates are discussed.
The interaction between semiconductor nanocrystals (quantum dots) and biological structures and cells is strongly influenced by nanoparticle size, exposure to light, and surface cap or conjugate. Hydrophobic particles can insert into lipid bilayers, often resulting in membrane leakage. Oxidizing and reducing agents can photosensitize the quantum dot, yielding greater potential for cell damage; however, penetration into cells is seen only if the particle is specifically targeted to a receptor or antigen on the cell. When particles interact with DNA, oxidation can occur as measured by the presence of hydroxyguanine, preventing cellular replication. In this paper, several cell-free and whole-cell systems are presented to investigate the mechanisms for nanoparticle entry across lipid bilayers, the evolution of their surface composition with light and oxygen exposure, and their potential as targeted cytotoxic drugs and/or environmental hazards. Uptake into mammalian cells, Gram positive and Gram negative bacteria were compared and contrasted in order to identify important factors in nanoparticle uptake and toxicity. Simultaneously, Fourier transform infrared spectroscopy (FTIR) and time-correlated single photon counting (TCSPC) were used to track quantum dot surface degradation with time. Finally, a lipid bilayer system was used to investigate nanoparticle-membrane interactions. The advantages of this system are that its composition is fully known, so that the role of cell-surface receptors is eliminated, and recordings may be performed in the dark. These studies allowed for the formulation of preliminary models of quantum dot binding and entry that consider novel variables.