Arindam Chowdhury

Colloidal CsPbBr3 Perovskite Nanocrystals: Luminescence beyond Traditional Quantum Dots

Traditional CdSe-based colloidal quantum dots (cQDs) have interesting photoluminescence (PL) properties. Herein we highlight the advantages in both ensemble and single-nanocrystal PL of colloidal CsPbBr3 nanocrystals (NCs) over the traditional cQDs. An ensemble of colloidal CsPbBr3 NCs (11 nm) exhibits ca. 90 % PL quantum yield with narrow (FWHM=86 meV) spectral width. Interestingly, the spectral width of a single-NC and an ensemble are almost identical, ruling out the problem of size-distribution in PL broadening. Eliminating this problem leads to a negligible influence of self-absorption and Förster resonance energy transfer, along with batch-to-batch reproducibility of NCs exhibiting PL peaks within ±1 nm. Also, PL peak positions do not alter with measurement temperature in the range of 25 to 100 °C. Importantly, CsPbBr3 NCs exhibit suppressed PL blinking with ca. 90 % of the individual NCs remain mostly emissive (on-time >85 %), without much influence of excitation power.

Sensing Hg(II) in Vitro and in Vivo Using a Benzimidazole Substituted BODIPY

A multisignaling Hg(II) sensor based on a benzimidazole substituted BODIPY framework was designed, which displays excellent selectively toward Hg(II) in vitro and in vivo. Optical and fluorogenic measurements in solution reveal that the sensor can detect mercury ions at submicromolar concentrations, with high specificity. The detection of Hg(II) is associated with a blue-shift in optical spectra and a simultaneous increase in the fluorescence quantum yield of the sensor, which is attributed to a decrease in charge delocalization and inhibition of photoinduced electron transfer upon binding to Hg(II). Using several spectroscopic measurements, it is shown that the binding mechanism involves two sensor molecules, where lone pairs of the benzimidazole nitrogen coordinate to a single mercury ion. The utility of this BODIPY sensor to detect Hg(II) in vivo was demonstrated by fluorescence imaging and spectroscopy of labeled human breast adenocarcinoma cells. While average emission intensity of the sensor over a large number of cells increases with incubated mercury concentrations, spatially resolved fluorescence spectroscopy performed on individual cells reveals clear spectral blue-shifts from a subensemble of sensors, corroborating the detection of Hg(II). Interestingly, the emission spectra at various submicrometer locations within cells exhibited considerable inhomogeneity in the extent of blue-shift, which demonstrates the potential of this sensor to monitor the local (effective) concentration of mercury ions within various subcellular environments.

Self healing hydrogels composed of amyloid nano fibrils for cell culture and stem cell differentiation

Amyloids are highly ordered protein/peptide aggregates associated with human diseases as well as various native biological functions. Given the diverse range of physiochemical properties of amyloids, we hypothesized that higher order amyloid self-assembly could be used for fabricating novel hydrogels for biomaterial applications. For proof of concept, we designed a series of peptides based on the high aggregation prone C-terminus of Aβ42, which is associated with Alzheimers disease. These Fmoc protected peptides self assemble to β sheet rich nanofibrils, forming hydrogels that are thermoreversible, non-toxic and thixotropic. Mechanistic studies indicate that while hydrophobic, π-π interactions and hydrogen bonding drive amyloid network formation to form supramolecular gel structure, the exposed hydrophobic surface of amyloid fibrils may render thixotropicity to these gels. We have demonstrated the utility of these hydrogels in supporting cell attachment and spreading across a diverse range of cell types. Finally, by tuning the stiffness of these gels through modulation of peptide concentration and salt concentration these hydrogels could be used as scaffolds that can drive differentiation of mesenchymal stem cells. Taken together, our results indicate that small size, ease of custom synthesis, thixotropic nature makes these amyloid-based hydrogels ideally suited for biomaterial/nanotechnology applications.

Pseudohalide (SCN(-))-Doped MAPbI3 Perovskites: A Few Surprises.

Pseudohalide thiocyanate anion (SCN(-)) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI3) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN(-) pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI3-x(SCN)x as compared to MAPbI3, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI3-x(SCN)x microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI3-x(SCN)x reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI3-x(SCN)x microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI3 crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials.

Quantum-confined stark effect in localized luminescent centers within InGaN/GaN quantum-well based light emitting diodes

The nature of the polarization-field in disorder induced nanoscale potential fluctuations (radiative traps) within (In,Ga)N based quantum-well (QW) heterostructures remains ambiguous. Spectrally resolved photoluminescence microscopy has been utilized to probe the local polarization field by monitoring the extent of quantum-confined Stark effect (QCSE) in radiative trap centers spontaneously formed within an (In,Ga)N QW based light emitting diode. Interestingly, two distinct categories of nanoscale radiative domains, which arise from indium compositional and interface-morphology related fluctuations of the active layers, are found to have very different degree of built-in polarization fields. Screening of QCSE in indium-rich emission centers results in blue-shift of transition energies by up to 400 meV, significantly higher than that reported previously for group III-nitride based semiconductor heterostructures. A lack of correlation between the extent of QCSE and local indium mole-fractions suggests that ...

Optoelectronic behaviors and carrier dynamics of individual localized luminescent centers in InGaN quantum-well light emitting diodes

Spatially, spectrally, and temporally resolved photoluminescence (PL) microscopy was performed on InGaN quantum-well light emitting diodes to probe individual localized luminescent centers arising from disorder induced potential fluctuations. Two energetically distinct localization centers were identified where the photoemission quantum-efficiency (QE) are correlated to the transition energies. PL lifetime measurements on emission centers suggest that activation barrier for non-radiative recombination (NR) processes determines their QE. The disparity in carrier dynamics not only substantiate two diverse mechanisms for localization processes, but also indicate the presence of multiple NR channels even within the trap centers implying their lateral dimensions to span several nanometers.

Synthesis of rare-earth doped ZnO nanorods and their defect–dopant correlated enhanced visible-orange luminescence

We report the synthesis of size controlled ZnO and rare-earth doped ZnO nanorods in the sub-10 nm diameter regime. The preferential anisotropic growth of the nanostructures along the polar c-axis leads to the formation of wurtzite phase ZnO nanorods. Photoluminescence measurements reveal enhancement of visible luminescence intensity with increasing RE3+ concentrations upon excitation of host ZnO into the band gap. The broad visible luminescence originates from multiple intrinsic or extrinsic defects. The luminescence from RE3+ is enabled by energy transfer from defect centers of the host nanocrystal lattice to dopant sites. Host–guest energy transfer facilitates efficient intra-4f orbital transitions (5D4 → 7Fj for Tb3+ and 5D0 → 7Fj for Eu3+) related characteristic green or red emission. Interestingly, different decay rates of host defects and RE3+ emission transition also allow temporal control to achieve either pure green or red color. This study suggests that manipulation of defects through bottom-up techniques is a viable method to modulate the energy transfer dynamics, which may help enable the future applications of ZnO-based phosphor materials in optoelectronic and multicolor emission displays.

Fluorescent probes for the detection of cyanide ions in aqueous medium: cellular uptake and assay for β-glucosidase and hydroxynitrile lyase

A chemodosimteric reagent (1) for the efficient detection of cyanide species (CN- and/or HCN) in aq. medium as well as under physiological conditions has been described. Selective reaction of the cyanide species with this reagent in the presence of all common interfering anions, amino acids and glutathione (GSH) led to the generation of the corresponding cyanohydrin derivative. The formation of the cyanohydrin derivative of the probe is associated with a visually detectable change in solution fluorescence in aq. buffer medium with 1.9 μM NaCN, the threshold limit set by WHO for the safe drinking water and this makes this fluorogenic sensor an ideal candidate for in-field applications. An apparent switch on the luminescence response, ultralow detection limit, low response time, cell membrane permeability and insignificant toxicity are key features of a probe molecule, which gives it a distinct edge over previously reported chemodosimetric reagents for the detection of cyanide species (CN- or HCN) in an aqueous environment. This methodology could be used for developing a generalized and efficient fluorescence-based assay for crucial enzymes like β-glucosidase and hydroxynitrile lyase. Furthermore, spectrally-resolved fluorescence microscopy measurements on single-cells revealed that this sensor molecule could also be used for imaging the cellular uptake of cyanide species from aq. solution contaminated with NaCN. Our results confirmed that statistical analysis of integrated intensity and transition energy obtained from the emission spectra collected over various microscopic sub-cellular regions can potentially be used to discriminate the effects of local cellular environments and that due to cyanide detection.

An approach to estimate spatial distribution of analyte within cells using spectrally-resolved fluorescence microscopy.

While fluorescence microscopy has become an essential tool amongst chemists and biologists for the detection of various analyte within cellular environments, non-uniform spatial distribution of sensors within cells often restricts extraction of reliable information on relative abundance of analytes in different subcellular regions. As an alternative to existing sensing methodologies such as ratiometric or FRET imaging, where relative proportion of analyte with respect to the sensor can be obtained within cells, we propose a methodology using spectrally-resolved fluorescence microscopy, via which both the relative abundance of sensor as well as their relative proportion with respect to the analyte can be simultaneously extracted for local subcellular regions. This method is exemplified using a BODIPY sensor, capable of detecting mercury ions within cellular environments, characterized by spectral blue-shift and concurrent enhancement of emission intensity. Spectral emission envelopes collected from sub-microscopic regions allowed us to compare the shift in transition energies as well as integrated emission intensities within various intracellular regions. Construction of a 2D scatter plot using spectral shifts and emission intensities, which depend on the relative amount of analyte with respect to sensor and the approximate local amounts of the probe, respectively, enabled qualitative extraction of relative abundance of analyte in various local regions within a single cell as well as amongst different cells. Although the comparisons remain semi-quantitative, this approach involving analysis of multiple spectral parameters opens up an alternative way to extract spatial distribution of analyte in heterogeneous systems. The proposed method would be especially relevant for fluorescent probes that undergo relatively nominal shift in transition energies compared to their emission bandwidths, which often restricts their usage for quantitative ratiometric imaging in cellular media due to strong cross-talk between energetically separated detection channels.

Single-Particle Tracking To Probe the Local Environment in Ice-Templated Crosslinked Colloidal Assemblies

We use single-particle tracking to investigate colloidal dynamics in hybrid assemblies comprising colloids enmeshed in a crosslinked polymer network. These assemblies are prepared using ice templating and are macroporous monolithic structures. We investigate microstructure-property relations in assemblies that appear chemically identical but show qualitatively different mechanical response. Specifically, we contrast elastic assemblies that can recover from large compressive deformations with plastic assemblies that fail on being compressed. Particle tracking provides insights into the microstructural differences that underlie the different mechanical response of elastic and plastic assemblies. Since colloidal motions in these assemblies are sluggish, particle tracking is especially sensitive to imaging artifacts such as stage drift. We demonstrate that the use of wavelet transforms applied to trajectories of probe particles from fluorescence microscopy eliminates stage drift, allowing a spatial resolution of about 2 nm. In elastic and plastic scaffolds, probe particles are surrounded by other particles-thus, their motion is caged. We present mean square displacement and van Hove distributions for particle motions and demonstrate that plastic assemblies are characterized by significantly larger spatial heterogeneity when compared with the elastic sponges. In elastic assemblies, particle diffusivities are peaked around a mean value, whereas in plastic assemblies, there is a wide distribution of diffusivities with no clear peak. Both elastic and plastic assemblies show a frequency independent solid modulus from particle tracking microrheology. Here too, there is a much wider distribution of modulus values for plastic scaffolds as compared to elastic, in contrast to bulk rheological measurements where both assemblies exhibit a similar response. We interpret our results in terms of the spatial distribution of crosslinks in the polymer mesh in the colloidal assemblies.