Yury P. Rakovich

Fluorescent Quantum Dots as Artificial Antennas for Enhanced Light Harvesting and Energy Transfer to Photosynthetic Reaction Centers

The development of artificial photosynthetic systems that utilize solar energy is one of the most challenging goals of chemistry and material sciences. The straightforward way to construct an artificial photosynthetic device for practical solar fuel production for the practical use of solar energy is to mimic the structural and functional organization of the natural photosynthetic machinery. In photosynthetic organisms, light is initially absorbed by antenna protein–pigment complexes in which it induces an excited electronic state (exciton), and then excitons (or electron–hole pairs) are transferred by means of F rster resonance energy transfer (FRET) to specialist chlorophyll cofactors in specialized reaction centers (RCs); here, excitons dissociate into their constituent carriers which are used in chemical transformations for the synthesis of high-energy molecules that fuel the organism. An artificial device that mimics this process for solar energy conversion should include, among other components, an efficient light-harvesting antenna capable of transferring the excitation energy to the RC. Based on the principle of photosynthesis, a variety of artificial antenna systems have been developed using supramolecular chemistry in which dendrimers incorporate porphyrins or other organic fluorophores or organometallic complexes. Although efficient excitation-energy transfer was obtained in such systems, the use of organic fluorophores in light-harvesting systems is rather limited because of their narrow spectral windows for light-collecting and lack of photostability. Recently it was suggested that inorganic nanocrystals, which are able to collect light over a wide spectral window, may achieve significantly greater absorption than natural photosystems, thus enhancing and could thus be used to enhance the light-harvesting process. Simultaneously, these nanocrystals may also be very efficient in excitationenergy transfer. This has led us to contemplate the development of hybrid materials in which light energy harvested by the nanocrystals in the optical region may be transferred to the RC in order to enhance the efficiency of the photosynthetic process. The simplest and best understood photosynthetic RC is that found in purple bacteria (Rhodobacter sphaeroides, for example). Although RCs from different photosynthetic organisms vary in their structure and composition, they are always composed of complexes of pigments and proteins, and RC fromRb. sphaeroides is known to be a good model of all the photosynthetic RCs. Here, we demonstrate that photoluminescent quantum dots (QDs) of these selected photoluminescence (PL) wavelengths may be tagged with the RC of Rh. sphaeroides in such a way that FRET from the QD to the RC is realized (Figure 1). A nearly threefold increase in the rate of generation of excitons in the RC is demonstrated, and theoretical estimates predict even stronger enhancements, thus indicating that further optimization is possible. Advances in inorganic synthesis have resulted in the production of monodispersed QDs such as highly photoluminescent CdSe/ZnS core/shell and CdTe nanocrystals. The light absorption by these QDs appears as a quasicontinuous superposition of peaks with extinction coefficients orders of magnitude higher than those of organic molecules. QDs are ultrastable against photobleaching, and the quantum [*] Prof. I. Nabiev CIC NanoGUNE Consolider, 20018 San Sebastian (Spain) and EA n83798, Universit de Reims Champagne-Ardenne 51100 Reims (France) and Ikerbasque, Basque Foundation for Science 48011 Bilbao (Spain) Fax: (+34)943-574-001 E-mail: i.nabiev@nanogune.eu

Off-resonance surface plasmon enhanced spontaneous emission from CdTe quantum dots

Surface plasmon (SP) enhanced photoluminescence (PL) from CdTe quantum dots (QDs) on monolayers of Au nanoparticles is investigated under both resonant and nonresonant conditions. Enhancement of the QD PL intensity is observed when the emission spectrum is redshifted with respect to the SP absorption resonance. Coupling to the SPs results in a redshift and broadening of the PL spectrum, and an increase in the PL decay rate. The largest coupling is observed for QD monolayers with peak emission at 667nm, producing a ten fold increase in PL intensity. No change in PL intensity and decay rate is observed at the SP resonance.

Surface plasmon enhanced Förster resonance energy transfer between the CdTe quantum dots

Forster resonance energy transfer (FRET) between CdTe quantum dots (QDs) at nanoscale proximity to gold nanoparticle (Au NP) layers is investigated experimentally. We have observed the enhancement in the acceptor QDs’ photoluminescence lifetime intensities. The decrease in donor QDs’ exciton lifetime from 5.74to2.06ns, accompanied by an increase in acceptor QDs’ exciton lifetime from 3.38to7.52ns, provided evidence for enhanced FRET between the QDs near Au NPs. The Au NPs’ surface plasmon dipole fields are assisted to overcome the weak electronic coupling between the emitting (donor) and absorbing (acceptor) transition exciton dipoles in the homogeneous medium.Forster resonance energy transfer (FRET) between CdTe quantum dots (QDs) at nanoscale proximity to gold nanoparticle (Au NP) layers is investigated experimentally. We have observed the enhancement in the acceptor QDs’ photoluminescence lifetime intensities. The decrease in donor QDs’ exciton lifetime from 5.74to2.06ns, accompanied by an increase in acceptor QDs’ exciton lifetime from 3.38to7.52ns, provided evidence for enhanced FRET between the QDs near Au NPs. The Au NPs’ surface plasmon dipole fields are assisted to overcome the weak electronic coupling between the emitting (donor) and absorbing (acceptor) transition exciton dipoles in the homogeneous medium.

CdTe nanoparticles display tropism to core histones and histone-rich cell organelles.

The disclosure of the mechanisms of nanoparticle interaction with specific intracellular targets represents one of the key tasks in nanobiology. Unmodified luminescent semiconductor nanoparticles, or quantum dots (QDs), are capable of a strikingly rapid accumulation in the nuclei and nucleoli of living human cells, driven by processes of yet unknown nature. Here, it is hypothesized that such a strong tropism of QDs could be mediated by charge-related properties of the macromolecules presented in the nuclear compartments. As the complex microenvironment encountered by the QDs in the nuclei and nucleoli of live cells is primarily presented by proteins and other biopolymers, such as DNA and RNA, the model of human phagocytic cell line THP1, nuclear lysates, purified protein, and nucleic acid solutions is utilized to investigate the interactions of the QDs with these most abundant classes of intranuclear macromolecules. Using a combination of advanced technological approaches, including live cell confocal microscopy, fluorescent lifetime imaging (FLIM), spectroscopic methods, and zeta potential measurements, it is demonstrated that unmodified CdTe QDs preferentially bind to the positively charged core histone proteins as opposed to the DNA or RNA, resulting in a dramatic shift off the absorption band, and a red shift and decrease in the pholuminescence (PL) intensity of the QDs. FLIM imaging of the QDs demonstrates an increased formation of QD/protein aggregates in the presence of core histones, with a resulting significant reduction in the PL lifetime. FLIM technology for the first time reveals that the localization of negatively charged QDs to their ultimate nuclear and nucleolar destinations dramatically affects the QDs photoluminescence lifetimes, and offers thereby a sensitive readout for physical interactions between QDs and their intracellular macromolecular targets. These findings strongly suggest that charge-mediated QD/histone interactions could provide the basis for QD nuclear localization downstream of intracellular transport mechanisms.

CdTe Quantum Dot/Dye Hybrid System as Photosensitizer for Photodynamic Therapy.

We have studied the photodynamic properties of novel CdTe quantum dots—methylene blue hybrid photosensitizer. Absorption spectroscopy, photoluminescence spectroscopy, and fluorescence lifetime imaging of this system reveal efficient charge transfer between nanocrystals and the methylene blue dye. Near-infrared photoluminescence measurements provide evidence for an increased efficiency of singlet oxygen production by the methylene blue dye. In vitro studies on the growth of HepG2 and HeLa cancerous cells were also performed, they point toward an improvement in the cell kill efficiency for the methylene blue-semiconductor nanocrystals hybrid system.

Nanojets and directional emission in symmetric photonic molecules

Symmetric directional emission of light from multisphere photonic molecules is experimentally shown in this work. The photonic molecules are illuminated in the vertical direction with a defocused laser beam. The emission is attributed to photonic nanojets generated in the structure. Furthermore, spectral analysis exhibit whispering gallery mode resonances of coupled and uncoupled modes. A benzene molecule-like structure consisting of a 7-microspheres cyclic photonic molecule shows a field emission pattern similar to the spatial distribution of the orbitals of the benzene molecule.

Emission properties of colloidal quantum dots on polyelectrolyte multilayers

We present steady state and time-resolved photoluminescence (PL) characteristics of differently charged CdTe quantum dots (QDs) adsorbed onto a polyelectrolyte (PE) multilayer. The PE multilayer is built up using a layer-by-layer assembly technique. We find that the diffusion of the QDs into the PE multilayer is an important factor in the case of 3-mercapto-1, 2-propanediol stabilized QDs (neutral surface charge), resulting in a ∼31-fold enhancement in PL intensity accompanied by a blue shift in the PL spectra and an increase in decay lifetime from 3.74xa0ns to a maximum of 11.65xa0ns. These modified emission properties are attributed to the enhanced surface related emission resulting from the interaction of the QDs surface with the PE. We find that diffusion does not occur for thioglycolic acid (TGA) stabilized QDs (negative surface charge) or 2-mercaptoethylamine stabilized QDs (positive surface charge), indicating localization of the QDs on top of the PE multilayer. However, the PL lifetime of the TGA stabilized QDs decreases from 9.58 to 5.78xa0ns with increasing PE multilayer thickness. This provides evidence for increased intrinsic exciton recombination relative to surface related emission, which results in an overall reduction in the average lifetime. Our studies indicate the importance of the QD surface charge in determining the interaction with the PE multilayers and the subsequent modification of the QD emission properties.

Emerging applications of fluorescent nanocrystals quantum dots for micrometastases detection

The occurrence of metastases is one of the main causes of death in many cancers and the main cause of death for breast cancer patients. Micrometastases of disseminated tumour cells and circulating tumour cells are present in more than 30% of breast cancer patients without any clinical or even histopathological signs of metastasis. Low abundance of these cell types in clinical diagnostic material dictates the necessity of their enrichment prior to reliable detection. Current micrometastases detection techniques are based on immunocytochemical and molecular methods suffering from low efficiency of tumour cells enrichment and observer‐dependent interpretation. The use of highly fluorescent semiconductor nanocrystals, also known as “quantum dots” and nanocrystal‐encoded microbeads tagged with a wide panel of antibodies against specific tumour markers offers unique possibilities for ultra‐sensitive micrometastases detection in patients serum and tissues. The nanoparticle‐based diagnostics provides an opportunity for highly sensitive parallel quantification of specific proteins in a rapid and low‐cost method, thereby providing a link between the primary tumour and the micrometastases for early diagnosis.

Radiation-pressure-induced mode splitting in a spherical microcavity with an elastic shell

In this work, we present a novel method to reveal azimuthal whispering gallery modes (WGMs) in a spherical microcavity coated with a nano-meter thick polyelectrolyte shell and one monolayer of CdTe semiconductor quantum dots. The new approach in this experiment is based on the deformation of the spherical shape in a non-contact way using the radiation pressure from a laser beam, which causes the lifting of the degeneracy of the WGMs. The resonance peak linewidth and splitting parameters can be efficiently controlled by the strength of the radiation pressure and the elastic properties of the surface shell.

Whispering gallery modes in photoluminescence and Raman spectra of a spherical microcavity with CdTe quantum dots: anti-Stokes emission and interference effects

We have studied the photoluminescence and Raman spectra of a system consisting of a polystyrene latex microsphere coated by CdTe colloidal quantum dots. The cavity-induced enhancement of the Raman scattering allows the observation of Raman spectra from only a monolayer of CdTe quantum dots. Periodic structure with very narrow peaks in the photoluminescence spectra of a single microsphere was detected both in the Stokes and anti-Stokes spectral regions, arising from the coupling between the emission of quantum dots and spherical cavity modes.