Highly efficient fluorescence resonance energy transfer in co-encapsulated BODIPY nanoparticles

Fluorescence resonance energy transfer (FRET) is a powerful tool used in a wide range of applications due to its high sensitivity and many other advantages. Co-encapsulation of a donor and an acceptor in nanoparticles is a useful strategy to bring the donor-acceptor pair in proximity for FRET. A highly efficient FRET system based on BODIPY-BODIPY (BODIPY: boron-dipyrromethene) donor-acceptor pair in nanoparticles was synthesized. Nanoparticles were formed by co-encapsulating a green emitting BODIPY derivative (FRET donor, lmax = 501 nm) and a red emitting BODIPY derivative (FRET acceptor, lmax = 601 nm) in an amphiphilic polymer using the precipitation method. Fluorescence measurements of encapsulated BODIPY in water following 501 nm excitation caused a 3.6 fold enhancement of the acceptor BODIPY emission at 601 nm indicating efficient energy transfer between the green emitting donor BODIPY and the red emitting BODIPY acceptor with a 100 nm Stokes shift. The calculated FRET efficiency was 96.5%. Encapsulated BODIPY derivatives were highly stable under our experimental conditions.

Low-temperature luminescence in organic-inorganic lead iodide perovskite single crystals

We present temperature and laser-power dependent photoluminescence (PL) study of methylammonium lead iodide (CH3NH3PbI3) single crystals in the orthorhombic phase. At temperatures below 140 K, we revealed the multi-component PL emission. In addition to a free exciton with an energy of 1.65 eV, we found emission bands with peaks approximately equal to 1.6 eV, 1.52 eV, and 1.48 eV. Analysis of the thermal evolution of the intensities, peak positions, and linewidths of all the PL bands allowed one to determine their origin. We attributed the PL peak with the energy of 1.6 eV to a bound exciton, while the free exciton-bound exciton splitting energy is 50-60 meV. The PL emission with an energy of 1.52 eV can be explained by the donor-acceptor pair (DAP) recombination, where donor and acceptor defects have a depth of about 12 meV and 120 meV, respectively. MA (CH3NH3) interstitials (MA+i ) and lead vacancies (V2-Pb) are the most suitable for the DAP transition to occur in CH3NH3PbI3 crystals. The 1.48 eV PL emission is consistent with the recombination of self-trapped excitons, and interstitial iodine is likely to be an active trap source. We found the variation of the self-trapped depth from 15 meV (at T<80 K) to 53 meV (at T>80 K) with increasing the temperature. Although the multi-component PL emission in CH3NH3PbI3 single crystals appears at low temperatures, defects and excitonic traps that cause this emission can affect the photophysics of hybrid perovskites at higher temperatures.

Donor-Acceptor Pair Quantum Emitters in Hexagonal Boron Nitride

Quantum emitters are needed for a myriad of applications ranging from quantum sensing to quantum computing. Hexagonal boron nitride (hBN) quantum emitters are the most promising solid-state platform to date due to its high brightness, stability, and the possibility of spin photon interface. However, the understanding of the physical origins of the single-photon emitters (SPEs) is still limited. Here, we present concrete and conclusive evidence that the dense SPEs in hBN, across entire visible spectrum, can be well explained by donor-acceptor pairs (DAPs). Based on the DAP transition generation mechanism, we have calculated their wavelength fingerprint, matching well with the experimentally observed photoluminescence spectrum. Our work serves as a step forward for the physical understanding of SPEs in hBN and their applications in quantum technologies.

Effect of Metal (Cu, Mn) Doping on the Structural, Morphological, Optical, Photoluminescence, Electrical and Photocatalytic Properties of In2s3 Nanoparticles

Pure and metal (Cu, Mn) doped In2S3 nanoparticles were prepared by the standard co-precipitation technique. This work aims to study the influence of metal (Cu, Mn) on the synthesized nanoparticles' structural, morphological, optical, photoluminescence, and electrical properties (NPs). The XRD pattern reveals that the metal (Cu, Mn) doped In2S3 NPs were polycrystalline with a cubic phase whose particle size varies between 36.2 and 49 nm. A significant decrease in the crystallite size was observed after metal doping. It is noted from the EDS spectra the elements like In, Cu, Mn and S are present in the NPs. The optical studies reveal that the high transmittance (80%) was observed for pure In2S3 NPs makes them an efficient window layer for solar cell applications. Also, the calculated bandgap energy was found to increase ( 2.8 – 3.28 e V) for metal doping. Photoluminescence measurements reveal that the photoemission is mainly due to the donor-acceptor pair transitions. The electrical studies indicate that the electrical conductivity decreases significantly with doping, and maximum electrical conductivity of 1.28 x 10-6 (Ω m)-1 was obtained for pure In2S3 NPs. A photocatalytic study reveals that high photocatalytic activity is observed for Mn-doped NPs under Methylene blue (84%).

Inorganic luminescent pigments

Inorganic luminescent pigments (luminescent materials, luminophores, phosphors) as synthetically generated crystalline compositions absorb energy followed by emission of light with lower energy, respectively, longer wavelengths. The light emission occurs often in the visible spectral range. External energy is necessary to enable luminescent materials to generate light. Luminescent pigments are divided into fluorescent and phosphorescent pigments. This classification goes back to different energy transitions. Emission based on allowed optical transitions, with decay times in the order of µs or faster is defined as fluorescence. Emission with longer decay times is called phosphorescence. The occurrence of fluorescence or phosphorescence as well as the decay time depend on structure and composition of a specific luminophore. There are four luminescence mechanisms discussed for inorganic luminescent materials: center luminescence, charge-transfer luminescence, donor–acceptor pair luminescence, and long-afterglow phosphorescence. The emission of luminescent light can have its origin in different excitation mechanisms such as optical excitation (UV radiation or even visible light), high-voltage or low-voltage electroluminescence and excitation with high energy particles (X-rays, γ-rays). Inorganic luminescent pigments are used mainly in fluorescent lamps, cathode-ray tubes, projection television tubes, plasma display panels, light-emitting diodes (LEDs) and for X-ray and γ-ray detection. The pigment particles are dispersed for the applications in specific binder systems. They are applied in form of thin layers and by means of luminophore/solvent suspensions, containing adhesive agents, on a substrate.

Shallow distance-dependent triplet energy migration mediated by endothermic charge-transfer

Conventional wisdom posits that spin-triplet energy transfer (TET) is only operative over short distances because Dexter-type electronic coupling for TET rapidly decreases with increasing donor acceptor separation. While coherent mechanisms such as super-exchange can enhance the magnitude of electronic coupling, they are equally attenuated with distance. Here, we report endothermic charge-transfer-mediated TET as an alternative mechanism featuring shallow distance-dependence and experimentally demonstrated it using a linked nanocrystal-polyacene donor acceptor pair. Donor-acceptor electronic coupling is quantitatively controlled through wavefunction leakage out of the core/shell semiconductor nanocrystals, while the charge/energy transfer driving force is conserved. Attenuation of the TET rate as a function of shell thickness clearly follows the trend of hole probability density on nanocrystal surfaces rather than the product of electron and hole densities, consistent with endothermic hole-transfer-mediated TET. The shallow distance-dependence afforded by this mechanism enables efficient TET across distances well beyond the nominal range of Dexter or super-exchange paradigms.

FRET-Based Detection and Quantification of HIV-1 Virion Maturation

HIV-1 infectivity is achieved through virion maturation. Virus particles undergo structural changes via cleavage of the Gag polyprotein mediated by the viral protease, causing the transition from an uninfectious to an infectious status. The majority of proviruses in people living with HIV-1 treated with combination antiretroviral therapy are defective with large internal deletions. Defective proviral DNA frequently preserves intact sequences capable of expressing viral structural proteins to form virus-like particles whose maturation status is an important factor for chronic antigen-mediated immune stimulation and inflammation. Thus, novel methods to study the maturation capability of defective virus particles are needed to characterize their immunogenicity. To build a quantitative tool to study virion maturation in vitro, we developed a novel single virion visualization technique based on fluorescence resonance energy transfer (FRET). We inserted an optimized intramolecular CFP-YPF FRET donor-acceptor pair bridged with an HIV-1 protease cleavage sequence between the Gag MA-CA domains. This system allowed us to microscopically distinguish mature and immature virions via their FRET signal when the FRET donor and acceptor proteins were separated by the viral protease during maturation. We found that approximately 80% of the FRET labeled virus particles were mature with equivalent infectivity to wild type. The proportion of immature virions was increased by treatment of virus producer cells with a protease inhibitor in a dose-dependent manner, which corresponded to a relative decrease in infectivity. Potential areas of application for this tool are assessing maturation efficiency in different cell type settings of intact or deficient proviral DNA integrated cells. We believe that this FRET-based single-virion imaging platform will facilitate estimating the impact on the immune system of both extracellular intact and defective viruses by quantifying the Gag maturation status.

Origin of photoluminescence of water-soluble CuInS2 quantum dots prepared via a hydrothermal method

The donor–acceptor pair emission is the underlying mechanism of the PL of the hydrothermally-synthesized water-soluble CIS QDs.

The first 2,6-di(1,6-naphthyridin-2-yl)pyridine-based redox photochromic coordination polymer platform with selective vapochromism for trolamine

Design and fabrication of crystalline photoswitable materials play highly crucial roles for the promisingly multidisciplinary applications. Well optimizations of the self-consistent donor-acceptor pair and skillful utilizations of weak intercomponent interactions...