Nanoassemblies Designed from Semiconductor Quantum Dots and Molecular Arrays

Hybrid Complexes of Photosensitizers with Luminescent Nanoparticles: Design of the Structure

Increasing the efficiency of the photodynamic action of the dyes used in photodynamic therapy is crucial in the field of modern biomedicine. There are two main approaches used to increase the efficiency of photosensitizers. The first one is targeted delivery to the object of photodynamic action, while the second one is increasing the absorption capacity of the molecule. Both approaches can be implemented by producing dye-nanoparticle conjugates. In this review, we focus on the features of the latter approach, when nanoparticles act as a light-harvesting agent and nonradiatively transfer the electronic excitation energy to a photosensitizer molecule. We will consider the hybrid photosensitizer-quantum dot complexes with energy transfer occurring according to the inductive-resonance mechanism as an example. The principle consisting in optimizing the design of hybrid complexes is proposed after an analysis of the published data; the parameters affecting the efficiency of energy transfer and the generation of reactive oxygen species in such systems are described.

Fluorescence biosensor based on silicon quantum dots and 5,5'-dithiobis-(2-nitrobenzoic acid) for thiols in living cells

An efficient and stable fluorescent sensor is described for the detection and imaging of thiols. It is making use of silicon quantum dots (SiQDs) which can be rapidly prepared. They were characterized by transmission electron microscopy, X-ray power diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectrometry. The SiQDs have an absorption maximum at 300 nm and displayed blue-green fluorescence with excitation/emission maxima at 410/480 nm. A mixture of SiQDs and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) exhibits strong fluorescence emission which however is quenched within 30 s of incubation with thiols. This is assumed to be due to an inner filter effect caused by the reaction of DTNB and thiols. The following thiols were tested: cysteine, homocysteine, and glutathione. The sensor has a linear response in the 3-100 μM thiol concentration range, and the LODs are between 0.80 and 0.96 μM. The sensor displays low cytotoxicity and was applied to fluorescence imaging of MCF-7 cells and Hela cells where it demonstrated excellent biocompatibility.

Impact of Quantum Dot Surface on Complex Formation with Chlorin e₆ and Photodynamic Therapy

Nanomaterials have permeated various fields of scientific research, including that of biomedicine, as alternatives for disease diagnosis and therapy. Among different structures, quantum dots (QDs) have distinctive physico-chemical properties sought after in cancer research and eradication. Within the context of cancer therapy, QDs serve the role of transporters and energy donors to photodynamic therapy (PDT) drugs, extending the applicability and efficiency of classic PDT. In contrast to conventional PDT agents, QDs’ surface can be designed to promote cellular targeting and internalization, while their spectral properties enable better light harvesting and deep-tissue use. Here, we investigate the possibility of complex formation between different amphiphilic coating bearing QDs and photosensitizer chlorin e₆ (Ce₆). We show that complex formation dynamics are dependent on the type of coating—phospholipids or amphiphilic polymers—as well as on the surface charge of QDs. Förster’s resonant energy transfer occurred in every complex studied, confirming the possibility of indirect Ce₆ excitation. Nonetheless, in vitro PDT activity was restricted only to negative charge bearing QD-Ce₆ complexes, correlating with better accumulation in cancer cells. Overall, these findings help to better design such and similar complexes, as gained insights can be straightforwardly translated to other types of nanostructures—expanding the palette of possible therapeutic agents for cancer therapy.

The nature of non-FRET photoluminescence quenching in nanoassemblies from semiconductor quantum dots and dye molecules

Nanoassemblies formed via self-assembly based on colloidal CdSe quantum dots (QDs) and porphyrin (H2P) dye molecules show Fluorescence Resonant Energy Transfer (FRET) and non-FRET quenching of QD photoluminescence (PL). We present a procedure to unravel and quantify these two relaxation pathways via dynamic and static PL quenching experiments. Accordingly, FRET amounts at maximum to 10% of the total quenching efficiency. Since the degree of ligand coverage is inhomogeneously distributed across the QD ensemble PL quantum yields vary broadly. The attachment of H2P molecules occurs preferentially to those QDs with low ligand coverage. Along with that, nanoassembly formation deviates strongly from Poisson statistics. Like FRET, non-FRET depends on the QD size. We assign non-FRET quenching to the formation of specific new Cd2+ trap states following depletion of several ligands by the spacious dye molecules. While FRET follows quantitatively the Förster model, non-FRET appears on time scales of 1-3 ns in new and enhanced non-radiative near-band-edge QD PL decay channels caused by a trapping of the electrons in long-lived intra-gap states which then manifests itself in a subsequent weak PL emission. We assign the related intra-band emission to a recombination of deep-trap electrons and shallow-trap holes.

Evaluation of Neuroimaging Findings of Central Nervous System Complications in Heart Transplant Recipients

OBJECTIVES

In this study, we presented neuroradiologic findings and diagnoses of neurologic complications in a series of heart transplant recipients.

MATERIALS AND METHODS

A retrospective review was conducted at Başkent University Hospital. We searched the hospital and radiology databases and identified 109 heart transplant recipients. Thirty-one of these recipients had neuroradiologic evaluations secondary to presentation of neurologic symptoms after heart transplant, with 18 patients evaluated with computed tomography and 22 patients evaluated with magnetic resonance imaging (overlap of imaging-defined groups occurred in 9 recipients). Computed tomography and magnetic resonance imaging studies were retrieved from the Picture Archiving and Communication System, with each type of imaging retrospectively evaluated on consensus by 2 radiologists.

RESULTS

Radiopathologic findings related to symptoms were detected in 12 of the 31 study patients. The most common abnormality was posterior reversible leukoencephalopathy syndrome (5 patients, 4.6%). The other abnormalities were ischemic stroke (3 patients, 2.8%), hemorrhagic stroke (1 patient, 0.9%), intracranial abscess (2 patients, 1.8%), and intracranial dissemination of sinusoidal fungal infection and related hemorrhagic infarct (1 patient, 0.9%). The other 19 heart transplant recipients who underwent computed tomography and/or magnetic resonance imaging for neurologic complaints showed no neuroradiologic findings related to neurologic symptoms.

CONCLUSIONS

Posterior reversible leukoencephalopathy syndrome and ischemic stroke were the most common neurologic complications in our heart transplant recipients. The other complications were hemorrhagic stroke, intracranial abscess, and intracranial dissemination of sinusoidal fungal infection. Neurologic complications are common in heart transplant recipients and should be identified promptly for early treatment. For the recognition of these complications, computed tomography should be performed for initial evaluation to rule out edema or hemorrhage. However, in the presence of serious neurologic symptoms that cannot be explained by computed tomography, magnetic resonance imaging should be indicated.

A study into the role of surface capping on energy transfer in metal cluster-semiconductor nanocomposites

Metal cluster-semiconductor nanocomposite materials remain a frontier area of research for the development of optoelectronic, photovoltaic and light harvesting devices because metal nanoclusters and semiconductor QDs are promising candidates for photon harvesting. Here, we have designed well defined metal cluster-semiconductor nanostructures using different surface capped negatively charged Au25 nanoclusters (Au NCs) and positively charged cysteamine capped CdTe quantum dots using electrostatic interactions. The main focus of this article is to address the impact of surface capping agents on the photophysical properties of Au cluster-CdTe QD hybrid nanocomposites. Steady state and time resolved spectroscopic studies reveal that photoluminescence quenching, radiative and nonradiative rate, and energy transfer between Au nanoclusters and CdTe QDs have been influenced by the nature of the capping agent. We have calculated the energy transfer related parameters such as the overlap integral, distance between donor and acceptor, Förster distance, efficiency of energy transfer and rate of energy transfer from CdTe QDs to three different Au NCs. Photoluminescence quenching varies from 73% to 43% when changing the capping agents from bovine serum albumin (BSA) to glutathione (GSH). The efficiency of the energy transfer from CdTe QDs to BSA-capped Au NCs is found to be 83%, for Cys-capped Au NCs it was 46% and for GSH-capped Au NCs it was 35%. The efficiency depends on the number of Au clusters attached per QD. This reveals that the nature of capping ligands plays a crucial role in the energy transfer phenomena from CdTe QDs to Au NCs. Interesting findings reveal that the efficient energy transfer in metal cluster-semiconductor nanocomposites may open up new possibilities in designing artificial light harvesting systems for future applications.

Ligand Binding to Distinct Sites on Nanocrystals Affecting Energy and Charge Transfer

Hybrid optoelectronic devices are attractive because they offer the promise of low-cost, roll-to-roll fabrication. Despite this, energy transfer between organic and inorganic interfaces is not well understood. Device engineering on this class of solution-processed materials generally focuses on replacing the long insulating ligands with short ones. Here, we show that energy and charge transfer between an inorganic nanocrystal (NC) donor and organic molecular acceptor is acutely sensitive to the chemical moiety linking the two species. Our results reveal that the CdS NCs have distinct binding sites for different chemical species because only resonance energy transfer (RET) is observed for the carboxylic-acid-functionalized ligand, while both RET and charge transfer are observed for the amine-functionalized ligand. We observe that the equilibrium constant for this static quenching term increases with decreasing particle size. This finding offers a new approach in the design of hybrid thin films for devices and NC probes based on RET used for imaging, sensing, signal transduction, and photon management.

Tuning electronic states of a CdSe/ZnS quantum dot by only one functional dye molecule

Self-assembly of only one functionalized porphyrin dye molecule with one CdSe/ZnS quantum dot (QD) not only modifies the photoluminescence (PL) intensity but also creates a few energetically clearly distinguishable electronic states, opening additional effective relaxation pathways. The related energy modifications are in the range of 10-30 meV and show a pronounced sensitivity to the specific nature of the respective dye. We assign the emerging energies to surface states. Time-resolved PL spectroscopy in combination with spectral deconvolution reveals that surface properties of QDs are a complex interplay of the nature of the dye molecule and the topography of the ligand layer across a temperature range from 77 to 290 K. This includes a kind of phase transition of trioctylphosphine oxide ligands, switching the nature of surface states observed below and above the phase transition temperature. Most importantly, our findings can be closely related to recent calculations of ligand-induced modifications of surface states of QDs. The identification of the optical properties emerged from a combination of spectroscopy on single QDs and QDs in an ensemble.

Energy and charge transfer in nanoscale hybrid materials

Hybrid materials composed of colloidal semiconductor quantum dots and π-conjugated organic molecules and polymers have attracted continuous interest in recent years, because they may find applications in bio-sensing, photodetection, and photovoltaics. Fundamental processes occurring in these nanohybrids are light absorption and emission as well as energy and/or charge transfer between the components. For future applications it is mandatory to understand, control, and optimize the wide parameter space with respect to chemical assembly and the desired photophysical properties. Accordingly, different approaches to tackle this issue are described here. Simple organic dye molecules (Dye)/quantum dot (QD) conjugates are studied with stationary and time-resolved spectroscopy to address the dynamics of energy and ultra-fast charge transfer. Micellar as well as lamellar nanostructures derived from diblock copolymers are employed to fine-tune the energy transfer efficiency of QD donor/dye acceptor couples. Finally, the transport of charges through organic components coupled to the quantum dot surface is discussed with an emphasis on functional devices.

Facilely controlling the Förster energy transfer efficiency of dendron encapsulated conjugated organic molecular wire–CdSe quantum dot nanostructures

On Den-OPE–CdSe nanostructures, as the size of the dendrimer increases, the energy transfer efficiency from Den-OPEs to CdSe QDs enhances.

Comparison of self-assembled and micelle encapsulated QD chemosensor constructs for biological sensing

Whereas a variety of covalent conjugation strategies have been utilized to prepare quantum dot (QD)-based nanosensors, supramolecular approaches of self-assembly have been underexplored. A major advantage of self-assembly is the ability to circumvent laborious synthetic efforts attendant to covalent conjugation of a chemosensor to functionalized QDs. Here, we combine a CdSe/ZnS core–shell QD with gold(iii) corroles using both self-assembly and micelle encapsulation to form QD nanosensors. Appreciable spectral overlap between QD emission and corrole absorption results in efficient Förster resonance energy transfer (FRET), which may be initiated by one- or two-photon excitation. The triplet state of the gold(iii) corroles is quenched by molecular oxygen, enabling these constructs to function as optical O₂ sensors, which is useful for the metabolic profiling of tumours. The photophysical properties, including QD and corrole lifetimes, FRET efficiency, and O₂ sensitivity, have been determined for each construct. The relative merits of each conjugation strategy are assessed with regard to their implementation as sensors.

Self-organization principles in the formation of multiporphyrin complexes and "semiconductor quantum dot-porphyrin" nanoassemblies

In this paper, we review several aspects of molecular recognition (based on non-covalent binding interactions) occurring between meso-pyridyl substituted tetrapyrrole extra-ligands and chemical dimers of tetrapyrrolic macrocycles containing central Zn ions and spacers of various nature and flexibility. Experimental results obtained by us earlier are analyzed using a novel approach (based on steady-state absorption/fluorescence measurements) for the evaluation of complexation constants KC for the formation of porphyrin triads. It was found that KC values [ KC ~ (0.5 – 70) × 106 M-1] show noticeable dependence on the structural parameters of the interacting subunits as well as on the solvent nature. The same self-assembly approach has been used to attach meso-pyridyl substituted porphyrins to the surface of semiconductor CdSe / ZnS quantum dots (QD). It was comparatively found that in contrast to self-assembled porphyrin triads, the formation of "QD-porphyrin" nanoassemblies takes place in competition with surface stabilizing tri-n-octyl phosphine oxide (TOPO) ligand molecules and attached porphyrin molecules. It manifests in a temporal dynamics of QD photoluminescence caused by ligand exchange, TOPO layer reorganization, QD surface reconstruction, solvent properties. It was shown that the sensitivity of QD surface morphology to attached organic ligands (e.g. porphyrins) provides an opportunity to control the dynamics and pathways of the exciton relaxation in "QD-dye" nanoassemblies by changing the structure and electronic properties of these ligands.

A non-covalent complex of quantum dots and chlorin e6: efficient energy transfer and remarkable stability in living cells revealed by FLIM

Non-covalent complex of lipid-coated CdSe/ZnS quantum dots and second-generation photosensitizer, chlorin e₆ can enter living HeLa cells with maintained integrity that ensures efficient FRET.

Two-photon oxygen sensing with quantum dot-porphyrin conjugates

Supramolecular assemblies of a quantum dot (QD) associated to palladium(II) porphyrins have been developed to detect oxygen (pO2) in organic solvents. Palladium porphyrins are sensitive in the 0-160 Torr range, making them ideal phosphors for in vivo biological oxygen quantification. Porphyrins with meso pyridyl substituents bind to the surface of the QD to produce self-assembled nanosensors. Appreciable overlap between QD emission and porphyrin absorption features results in efficient Förster resonance energy transfer (FRET) for signal transduction in these sensors. The QD serves as a photon antenna, enhancing porphyrin emission under both one- and two-photon excitation, demonstrating that QD-palladium porphyrin conjugates may be used for oxygen sensing over physiological oxygen ranges.

Covalent Linking Greatly Enhances Photoinduced Electron Transfer in Fullerene-Quantum Dot Nanocomposites: Time-Domain Ab Initio Study

Nonadiabatic molecular dynamics combined with time-domain density functional theory are used to study electron transfer (ET) from a CdSe quantum dot (QD) to the C60 fullerene, occurring in several types of hybrid organic/inorganic nanocomposites. By unveiling the time dependence of the ET process, we show that covalent bonding between the QD and C60 is particularly important to ensure ultrafast transmission of the excited electron from the QD photon-harvester to the C60 electron acceptor. Despite the close proximity of the donor and acceptor species provided by direct van der Waals contact, it leads to a notably weaker QD-C60 interaction than a lengthy molecular bridge. We show that the ET rate in a nonbonded mixture of QDs and C60 can be enhanced by doping. The photoinduced ET is promoted primarily by mid- and low-frequency vibrations. The study establishes the basic design principles for enhancing photoinduced charge separation in nanoscale light harvesting materials.

Size-Dependent Non-FRET Photoluminescence Quenching in Nanocomposites Based on Semiconductor Quantum Dots CdSe/ZnS and Functionalized Porphyrin Ligands

We review recent experimental work to utilize the size dependence of the luminescence quenching of colloidal semiconductor quantum dots induced by functionalized porphyrin molecules attached to the surface to describe a photoluminescence (PL) quenching process which is different from usual models of charge transfer (CT) or Foerster resonant energy transfer (FRET). Steady-state and picosecond time-resolved measurements were carried out for nanocomposites based on colloidal CdSe/ZnS and CdSe quantum dots (QDs) of various sizes and surfacely attached tetra-mesopyridyl-substituted porphyrin molecules (“Quantum Dot-Porphyrin” nanocomposites), in toluene at 295 K. It was found that the major part of the observed strong quenching of QD PL in “QD-Porphyrin” nanocomposites can neither be assigned to FRET nor to photoinduced charge transfer between the QD and the chromophore. This PL quenching depends on QD size and shell and is stronger for smaller quantum dots: QD PL quenching rate constants scale inversely with the QD diameter. Based on the comparison of experimental data and quantum mechanical calculations, it has been concluded that QD PL quenching in “QD-Porphyrin” nanocomposites can be understood in terms of a tunneling of the electron (of the excited electron-hole pair) followed by a (self-) localization of the electron or formation of trap states. The major contribution to PL quenching is found to be proportional to the calculated quantum-confined exciton wave function at the QD surface. Our findings highlight that single functionalized molecules can be considered as one of the probes for the complex interface physics and dynamics of colloidal semiconductor QD.

FRET and ligand related NON-FRET processes in single quantum dot-perylene bisimide assemblies

Nanoassemblies are formed via self-assembly of ZnS capped CdSe quantum dots (QD) and perylene bisimide dyes (PBI). Upon assembly formation with functionalized dye molecules the QD photoluminescence (PL) is quenched. Quenching has been assigned partly to FRET (fluorescence resonance energy transfer) and NON-FRET processes. By means of time resolved single particle spectroscopy of immobilized QD-dye assemblies, it is demonstrated that NON-FRET processes are due to new non-radiative decay channels caused by the assembly formation process itself. Immobilized (single) assemblies exhibit the same processes as ensembles of assemblies in toluene solution. Only one dye molecule on a QD quenches the PL up to 50%, which is much stronger than is expected when replacing a volume related number of ligands. NON-FRET processes are distinct from photoinduced charge and/or energy transfer. A combination of a Stern-Volmer and FRET analysis of ensemble experiments supports the investigation of the dynamics of assembly formation at extremely low concentration ratios of PBI to QD. This allows us to distinguish between the effects of PBI and ligands on PL quenching on a single molecule level which is not possible in conventional ligand dynamic experiments.

Identification of different donor-acceptor structures via Förster Resonance Energy Transfer (FRET) in quantum-dot-perylene bisimide assemblies

Nanoassemblies are formed via self-assembly of ZnS capped CdSe quantum dots (QD) and perylene bisimide (PBI) dyes. Upon assembly formation the QD photoluminescence is quenched, as can be detected both via single particle detection and ensemble experiments in solution. Quenching has been assigned to FRET and NON-FRET processes. Analysis of FRET allows for a distinction between different geometries of the QD dye assemblies. Time-resolved single molecule spectroscopy reveals intrinsic fluctuations of the PBI fluorescence lifetime and spectrum, caused by rearrangement of the phenoxy side groups. The distribution of such molecular conformations and their changed dynamics upon assembly formation are discussed in the scope of FRET efficiency and surface ligand density.

Biofunctional nanocomposite of carbon nanofiber with water-soluble porphyrin for highly sensitive ethanol biosensing

A biofunctional hybrid nanocomposite of carbon nanofiber (CNF) with water-soluble iron(III) meso-tetrakis(N-methylpyridinum-4-yl) porphyrin (FeTMPyP) was designed via non-covalent interaction for preparation of highly sensitive ethanol biosensor. The prepared nanocomposite showed good dispersion in water and was characterized with steady-state electronic absorption spectroscopy and scanning electron microscope. The nanocomposite combined the good conductivity of CNF and the excellent catalytic activity of both CNF and FeTMPyP toward the reduction of dissolved oxygen, producing a method for amperometric detection of oxygen ranging from 6.5 nM to 6.4 microM at a low overpotential. The nanocomposite modified electrode was further used for assembly of alcohol oxidase to construct an amperometric biosensor for ethanol. The biosensor showed rapid and highly sensitive response to ethanol with a linear range from 2.0 microM to 112 microM. The immobilized alcohol oxidase also showed its direct electrochemistry. The biofunctional nanocomposite provides a new way to not only construct the highly sensitive biosensors but also mimic the catalytic activity of enzyme in the life process.

Light harvesting and energy transfer in multiporphyrin-modified CdSe nanoparticles

Multiporphyrin-modified CdSe nanoparticles (CdSe-H2P) were prepared to elucidate the interaction between chromophores and luminescent semiconducting nanoparticles in the excited and ground states. The CdSe-H2P nanoparticles were obtained by place-exchange reactions of hexadecylamine-thiophenol-modified CdSe nanoparticles with porphyrin alkanethiols in toluene. The number of porphyrin molecules on the surface of a single CdSe nanoparticle increased with increasing reaction time to reach a saturated maximum of 21. The porphyrins as well as the core in CdSe-H2P can absorb UV/Vis radiation. Steady-state emission and emission-lifetime measurements reveal efficient energy transfer from the CdSe excited state to the porphyrins in the CdSe-H2P nanoparticles. The resulting porphyrin excited singlet state is not quenched by the CdSe core. These unique properties are in sharp contrast with those of multiporphyrin-modified metal and silica nanoparticles. Thus, semiconducting nanoparticle-multiporphyrin composites are highly promising as novel artificial photosynthetic materials.

Photoinduced Vibrational Coherence Transfer in Molecular Dimers

At short times that are faster than dephasing, photoinduced evolution of the vibrational subsystem in an electron-phonon molecular structure depends strongly on the electronic evolution. As the electronic population shifts between the donor and acceptor states, in the diabatic description the state with the largest population determines the equilibrium positions and frequencies of the vibrational modes, which oscillate continuously and without loss of coherence. The vibrational coherence transfer between the electronic states detected recently in a number of systems is described theoretically by application of the quantized Hamiltonian dynamics (QHD) formalism [J. Chem. Phys. 2000, 113, 6557] to the coupled electronic and vibrational degrees of freedom of a model heterodimer. The observed coherent modulation of the frequency of the probe signal is represented with simple analytic and numeric QHD models.