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

Dye-induced photoluminescence quenching of quantum dots: role of excited state lifetime and confinement of charge carriers

We investigate the role of quantum confinement and photoluminescence (PL) lifetime of photoexcited charge carriers in semiconductor core/shell quantum dots (QDs) via PL quenching due to surface modification. Surface modification is controlled by varying the number of dye molecules adsorbed onto the QD shell surface forming QD-dye nanoassemblies. We selected CuInS₂/ZnS (CIS) and InP/ZnS (InP) core/shell QDs exhibiting relatively weak (664 meV) and strong (1194 meV) confinement potentials for the conduction band electron. Moreover, the difference in the emission mechanism gives rise to a long and short excited state lifetime of CIS (ca. 290 ns) and InP (ca. 37 ns) QDs. Dye molecules of different ionic characters (rhodamine 575: zwitterionic and rhodamine 560: cationic) are used as quenchers. A detailed analysis of Stern-Volmer data shows that (i) quenching is generally more pronounced in CIS-dye assemblies as compared to InP-dye assemblies, (ii) dynamic quenching is dominating in all QD-dye assemblies with only a minor contribution from static quenching and (iii) the cationic dye shows a stronger interaction with the QD shell surface than the zwitterionic dye. Observations (i) and (ii) can be explained by the differences in the amplitude of the electronic component of the exciton wavefunction near the dye binding sites in both QDs, which results in the breaking up of the electron-hole pair and favors charge trapping. Observation (iii) can be attributed to the variations in electrostatic interactions between the negatively charged QD shell surface and the cationic and zwitterionic dyes, with the former exhibiting a stronger interaction. Moreover, the long lifetime of CIS QDs facilitates us to easily probe different time scales of the trapping processes and thus differentiate the origins of static and dynamic quenching components that appear in the Stern-Volmer analysis.

All-in-One Process for Color Tuning and Patterning of Perovskite Quantum Dot Light-Emitting Diodes

Although post-synthetic anion exchange allows halide perovskite quantum dots to easily change the optical bandgap of materials, additional exchange of shorter ligands is required to use them as active materials in optoelectronic devices. In this study, a novel all-in-one process exchanging ligands and halide anions in film-state for facile color tuning and patterning of cesium lead halide perovskite colloidal quantum dot (PeQD) light-emitting diodes (LEDs) is proposed. The proposed all-in-one process significantly enhances the performances of PeQD LEDs by passivating the PeQD with shorter ligands. In addition, the all-in-one process is repeated more stably in the film state. Red, green, and blue LEDs with extremely narrow emission spectra using cesium lead bromide PeQDs and appropriate butylammonium halide solutions are fabricated. Furthermore, the proposed all-in-one process in film-state facilitated rapid color change in localized areas, thereby aiding in realizing fine patterns of narrow widths (300 µm) using simple contact masks. Consequently, various paint-over red/green/blue patterns in PeQD LEDs by applying halide solutions additively are fabricated.

Energy/Charge Transfer Modulation with Spacer Ligands for Highly Emissive Quantum Dot-Polymer Blend

A blend of perovskite quantum dots (QDs) and a hole transport layer (HTL) is a feasible candidate to solve the long-standing issues in light-emitting diodes (LEDs) such as charge injection, energy state matching, and defect passivation. However, QD:HTL blend structures for QD-based LEDs suffer from fast charge and energy transfers due to an inhomogeneous distribution of QDs and the HTL matrix. Here we report new cross-linkable spacer ligands between QDs and TFB that result in a highly emissive QD:TFB-blended LED device. We synthesize three representative spacer ligands to control the charge and energy transfers between QDs and the HTL. The first spacer ligand is used for controlling the molecular distance between QDs and TFB, and the second spacer ligand is designed to investigate how molecular interaction between QDs and the spacer ligand affects the optical property of the QD:TFB blend. Subsequently, the best spacer ligand, a 10-((2-benzoylbenzoyl)oxy)decanoic acid, is designed to anchor TFB (via a benzophenone group) and simultaneously bond to QDs (with a carboxylic acid functional group). The carboxylic acid group strongly interacts with QDs, dramatically improving the cross-linking rate between QDs and TFB. Due to the direct interaction between QDs and TFB, hole carriers can be effectively injected to perovskite QDs through the conductive backbone of TFB, resulting in the highest luminance values of 10917 cd/m² at 7.4 V due to carrier injection balance. This is at least 10 times better LED performance compared with a pristine QD device.

A phenol phosphorescent microsensor of mesoporous molecularly imprinted polymers

Based on the optical quenching phenomenon, a smart mesoporous phosphorescent microsensor was built. It is a phenol microsensor, which inherits a high selectivity of molecularly imprinted polymers (MIPs) and room-temperature phosphorescence (RTP) properties of Mn-doped ZnS quantum dots (QDs). On the surface of silane-modified Mn-doped ZnS QDs, the phenol microsensor was synthesized by a sol–gel process. Because of the presence of a porogenic agent, a mesoporous structure played an important role in increasing the detection sensitivity. The MPTS-modified Mn-doped ZnS QDs were used as solid supports and auxiliary monomers. Under optimal conditions, the experiment for the detection of phenol had a linear range of 5.0 to 50 μmol L⁻¹ with a correlation coefficient of 0.9983 and a high imprinting factor (IF) of 3.28. In addition, the as-prepared Mn-doped ZnS QD@ms-MIPs were successfully applied for phenol determination and selectivity in water samples. Therefore, this study provides a highly selective and sensitive mesoporous phosphorescent microsensor for the detection of phenol.

Based on the optical quenching phenomenon, a smart mesoporous phosphorescent microsensor was built.

A fluorescent microsensor for the selective detection of bifenthrin

Based on the fluorescence quenching phenomenon, a smart fluorescent microsensor was synthesized. The bifenthrin (BI) microsensor inherited the high selectivity of molecular imprinted polymers (MIPs) and the excellent fluorescence properties of aqueous CdTe quantum dots (QDs). Aqueous CdTe QDs are functionalized by octadecyl-4-vinylbenzyl-dimethyl-ammonium chloride (OVDAC). A type of functional monomer, 4-vinylphenylboronic acid (VPBA), was used and its boronic acid groups could covalently combine with a cis-diol compound for direct imprinting polymerization. The OVDAC-functionalized aqueous CdTe QDs were used as solid supports and auxiliary monomers. Under optimal conditions, experimentation showed that BI had a linear detection range of 10 to 300 μmol L⁻¹ with a correlation coefficient of 0.9968 and a high imprinting factor (IF) of 4.53. In addition, the prepared MIP-OVDAC/CdTe QDs were successfully used to detect BI in water samples. Therefore, this work provided a highly selective and sensitive fluorescence probe for the detection of BI. In addition, the fluorescence probe could be used to detect other targets by changing the functional monomers.

Based on the fluorescence quenching phenomenon, a smart fluorescent microsensor was synthesized.

Energy Transfer from Perovskite Nanocrystals to Dye Molecules Does Not Occur by FRET

Single formamidinium lead bromide (FAPbBr₃) perovskite nanocubes, approximately 10 nm in size, have extinction cross sections orders of magnitude larger than single dye molecules and can therefore be used to photoexcite one single dye molecule within their immediate vicinity by means of excitation-energy transfer (EET). The rate of photon emission by the single dye molecule is increased by 2 orders of magnitude under excitation by EET compared to direct excitation at the same laser fluence. Because the dye cannot accommodate biexcitons, NC biexcitons are filtered out by EET, giving rise to up to an order-of-magnitude improvement in the fidelity of photon antibunching. We demonstrate here that, contrary to expectation, energy transfer from the nanocrystal to dye molecules does not depend on the spectral line widths of the donor and acceptor and is therefore not governed by Förster's theory of resonance energy transfer (FRET). Two different cyanine dye acceptors with substantially different spectral overlaps with the nanocrystal donor show a similar light-harvesting capability. Cooling the sample from room temperature to 5 K reduces the average transition line widths 25-fold but has no apparent effect on the number of molecules emitting, i.e., on the spatial density of single dye molecules being photoexcited by single nanocrystals. Narrow zero-phonon lines are identified for both donor and acceptor, with an energetic separation of over 40 times the line width, implying a complete absence of spectral overlap-even though EET is evident. Both donor and acceptor exhibit spectral fluctuations, but no correlation is apparent between the jitter, which controls spectral overlap, and the overall light harvesting. We conclude that the energy transfer process is fundamentally nonresonant, implying effective energy dissipation in the perovskite donor because of strong electron-phonon coupling of the carriers comprising the exciton. The work highlights the importance of performing cryogenic spectroscopy to reveal the underlying mechanisms of energy transfer in complex donor-acceptor systems.

Two-Component Analysis of Photoluminescence Bands for Semiconductor Quantum Dots in Solutions

We present the quantitative analysis of the photoluminescence (PL) obtained for semiconductor TOPO-capped CdSe/ZnS QDs in solutions at 77–293[Formula: see text]K. The PL bands are approximated more accurately when assuming the superposition of at least two Gaussian components differing considerably in the linewidth (FWHM) and having different nature.

Temperature Dependence of Photoluminescence for Spin-Coated Semiconductor Quantum Dots and Quantum Dot-Dye Nanoassemblies on Quartz Substrate

The attachment of pyridyl substituted porphyrin molecule to the surface of CdSe/ZnS quantum dots in solutions is realized in the competition with capping ligand TOPO molecules resulting in the specific change of photoluminescence for the quantum dots across the temperature range of 77–290[Formula: see text]K. We have shown that fixation of alone quantum dots or quantum dot-porphyrin nanoassemblies on quartz substrate changes significantly temperature dependence of photoluminescence. In contrast to the samples in a glass-forming solution no phase transition of the TOPO capping layer was observed upon removal of the capping layer.

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.