Sabyasachi Chakrabortty

Ultralow-threshold two-photon pumped amplified spontaneous emission and lasing from seeded CdSe/CdS nanorod heterostructures

Ultralow-threshold two-photon pumped amplified spontaneous emission (2ASE) and lasing in seeded CdSe/CdS nanodot/nanorod heterostructures is demonstrated for the first time. Such heterostructures allow the independent tunability of the two-photon absorption (2PA) cross-section (σ(2)) through varying the CdS rod size, and that of the emission wavelength through varying the CdSe dot size. With an enhanced σ(2), 2ASE in these heterostructures is achieved with an ultralow threshold fluence of ~1.5 mJ/cm(2), which is as much as one order less than that required for spherical semiconductor NCs. Importantly, by exploiting this unique property of the seeded nanorods exhibiting strong quantum confinement even at relatively large rod sizes, a near reciprocal relation between the 2ASE threshold and the 2PA action cross-section (σ(2)η) (where η is the quantum yield) was found and validated over a wide volume range for II-VI semiconductor nanostructures. Ultrafast optical spectroscopy verified that while the Auger processes in these heterostructures are indeed suppressed, ASE in these samples could also be strongly affected by a fast hole-trapping process to the NR surface states. Lastly, to exemplify the potential of these seeded CdSe/CdS nanodot/nanorod heterostructures as a viable gain media for achieving two-photon lasing, a highly photostable microsphere laser with an ultralow pump threshold is showcased.

pH-Responsive quantum dots via an albumin polymer surface coating.

Multifunctional peptide-polymer hybrid materials have been applied as efficient and biocompatible quantum-dot coating materials. Significant pH responsiveness (e.g., an influence of the pH on the quantum yields of the peptide-polymer/QDs) was found and is attributed to conformational rearrangements of the peptide backbone.

Mitochondria Targeted Protein-Ruthenium Photosensitizer for Efficient Photodynamic Applications

Organelle-targeted photosensitization represents a promising approach in photodynamic therapy where the design of the active photosensitizer (PS) is very crucial. In this work, we developed a macromolecular PS with multiple copies of mitochondria-targeting groups and ruthenium complexes that displays highest phototoxicity toward several cancerous cell lines. In particular, enhanced anticancer activity was demonstrated in acute myeloid leukemia cell lines, where significant impairment of proliferation and clonogenicity occurs. Finally, attractive two-photon absorbing properties further underlined the great significance of this PS for mitochondria targeted PDT applications in deep tissue cancer therapy.

Dual Wavelength Electroluminescence from CdSe/CdS Tetrapods

We fabricated a single active layer quantum dot light-emitting diode device based on colloidal CdSe (core)/CdS (arm) tetrapod nanostructures capable of simultaneously producing room temperature electroluminesence (EL) peaks at two spectrally distinct wavelengths, namely, at ∼500 and ∼660 nm. This remarkable dual EL was found to originate from the CdS arms and CdSe core of the tetrapod architecture, which implies that the radiative recombination of injected charge carriers can independently take place at spatially distinct regions of the tetrapod. In contrast, control experiments employing CdSe-core-seeded CdS nanorods showed near-exclusive EL from the CdSe core. Time-resolved spectroscopy measurements on tetrapods revealed the presence of hole traps, which facilitated the localization and subsequent radiative recombination of excitons in the CdS arm regions, whereas excitonic recombination in nanorods took place predominantly within the vicinity of the CdSe core. These observations collectively highlight the role of morphology in the achievement of light emission from the different material components in heterostructured semiconductor nanoparticles, thus showing a way in developing a class of materials which are capable of exhibiting multiwavelength electroluminescence.

Enhanced tunability of the multiphoton absorption cross-section in seeded CdSe/CdS nanorod heterostructures

We present a method to separately tune the multiphoton absorption (MPA) and multiphoton excited photoluminescence using semiconductor core/enlarged-shell quantum dots (QDs), where the enlarged shell greatly enhances the MPA cross-sections while varying the core size facilitates emission wavelength selectivity. Following two-photon absorption (2PA) primarily in the shell and ultrafast charge-carrier localization to the core, luminescence occurs. We exemplify the validity of this method with CdSe/CdS nanorod heterostructures and find that the 2PA cross-section is enlarged to ∼1.4×106 GM for 180 nm nanorods (with 800 nm, 150 fs laser pulse excitation) which is two to four orders larger than that of CdSe QDs.

Engineering fluorescence in Au-tipped, CdSe-seeded CdS nanoheterostructures

Signifi cant advances in the wet chemical synthesis of colloidal semiconductor nanoparticles have afforded exquisite control over their size, shape, and composition, facilitating the development of multiple components within the same nanostructure and potentially multifarious functionalities. Metaltipped, seeded core–shell semiconductor nanorods exemplify such multicomponent heterostructures, where the metal tips can serve as an electrical contact [ 1–4 ] or provide an anchor point for self-assembly, [ 5 ] while the core–shell semiconductor can be engineered to exhibit unique optical properties. [ 6 , 7 ]

Observation of an Excitonic Quantum Coherence in CdSe Nanocrystals.

Recent observations of excitonic coherences within photosynthetic complexes suggest that quantum coherences could enhance biological light harvesting efficiencies. Here, we employ optical pump-probe spectroscopy with few-femtosecond pulses to observe an excitonic quantum coherence in CdSe nanocrystals, a prototypical artificial light harvesting system. This coherence, which encodes the high-speed migration of charge over nanometer length scales, is also found to markedly alter the displacement amplitudes of phonons, signaling dynamics in the non-Born-Oppenheimer regime.

3D Time-lapse Imaging and Quantification of Mitochondrial Dynamics

We present a 3D time-lapse imaging method for monitoring mitochondrial dynamics in living HeLa cells based on photothermal optical coherence microscopy and using novel surface functionalization of gold nanoparticles. The biocompatible protein-based biopolymer coating contains multiple functional groups which impart better cellular uptake and mitochondria targeting efficiency. The high stability of the gold nanoparticles allows continuous imaging over an extended time up to 3000 seconds without significant cell damage. By combining temporal autocorrelation analysis with a classical diffusion model, we quantify mitochondrial dynamics and cast these results into 3D maps showing the heterogeneity of diffusion parameters across the whole cell volume.

Gene Detection in Complex Biological Media Using Semiconductor Nanorods within an Integrated Microfluidic Device

The salient optical properties of highly luminescent semiconductor nanocrystals render them ideal fluorophores for clinical diagnostics, therapeutics, and highly sensitive biochip applications. Microfluidic systems allow miniaturization and integration of multiple biochemical processes in a single device and do not require sophisticated diagnostic tools. Herein, we describe a microfluidic system that integrates RNA extraction, reverse transcription to cDNA, amplification and detection within one integrated device to detect histidine decarboxylase (HDC) gene directly from human white blood cells samples. When anisotropic semiconductor nanorods (NRs) were used as the fluorescent probes, the detection limit was found to be 0.4 ng of total RNA, which was much lower than that obtained using spherical quantum dots (QDs) or organic dyes. This was attributed to the large action cross-section of NRs and their high probability of target capture in a pull-down detection scheme. The combination of large scale integrated microfluidics with highly fluorescent semiconductor NRs may find widespread utility in point-of-care devices and multitarget diagnostics.

NIR-emitting and photo-thermal active nanogold as mitochondria-specific probes

We report a bioinspired multifunctional albumin derived polypeptide coating comprising grafted poly(ethylene oxide) chains, multiple copies of the HIV TAT derived peptide enabling cellular uptake as well as mitochondria targeting triphenyl-phosphonium (TPP) groups. Exploring these polypeptide copolymers for passivating gold nanoparticles (Au NPs) yielded (i) NIR-emitting markers in confocal microscopy and (ii) photo-thermal active probes in optical coherence microscopy. We demonstrate the great potential of such multifunctional protein-derived biopolymer coatings for efficiently directing Au NP into cells and to subcellular targets to ultimately probe important cellular processes such as mitochondria dynamics and vitality inside living cells.