Rabeka Alam

Designing quantum rods for optimized energy transfer with firefly luciferase enzymes

The bioluminescence resonance energy transfer (BRET) between firefly luciferase from Photinus pyralis (Ppy) with core/shell semiconductive quantum rods (QRs) has been studied as a function of QR aspect ratio and internal microstructure. The QRs were found to be ideal energy acceptors, and Ppy-to-core distances were optimized using rod-in-rod microstructures that were achieved by the synthetic control of rod morphology, surface chemistry, and Ppy:QR loading. The BRET ratios of >44 measured are the highest efficiencies to date.

Novel multistep BRET-FRET energy transfer using nanoconjugates of firefly proteins, quantum dots, and red fluorescent proteins

Sequential bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) from firefly luciferase to red fluorescent proteins using quantum dot or rod acceptor/donor linkers is described. The effect of morphology and tuned optical properties on the efficiency of this unique BRET-FRET system was evaluated.

Modulation of Cu2–xS Nanocrystal Plasmon Resonance through Reversible Photoinduced Electron Transfer

Copper sulfide (Cu(2-x)S) nanocrystals with nonstoichiometric composition exhibit plasmon resonance in the near-infrared region. Compositional changes and varying electron density markedly affect the position and intensity of the plasmon resonance. We report a photochemically induced phenomenon of modulating the plasmon resonance in a controlled fashion. As photogenerated reduced methyl viologen radicals transfer electrons to Cu(2-x)S in inert solutions, we observe a decrease in localized surface plasmon resonance (LSPR) absorbance at 1160 nm. Upon exposure to air, the plasmon resonance band recovers as stored electrons are scavenged away by oxygen. This cycle of electron charge and discharge of Cu(2-x)S nanocrystals is reversible and can be repeated through photoirradiation in N2 saturated solution followed by exposure of the suspension to air. The spectroscopic studies that provide mechanistic insights into the reversible charging and discharging of plasmonic Cu(2-x)S are discussed.

Near infrared bioluminescence resonance energy transfer from firefly luciferase?quantum dot bionanoconjugates

The bioluminescence resonance energy transfer (BRET) between firefly luciferase enzymes and semiconductive quantum dots (QDs) with near infrared emission is described. The QD were phase transferred to aqueous buffers using a histidine mediated phase transfer route, and incubated with a hexahistidine tagged, green emitting variant of firefly luciferase from Photinus pyralis (PPyGRTS). The PPyGRTS were bound to the QD interface via the hexahistidine tag, which effectively displaces the histidine layer and binds directly to the QD interfaces, allowing for short donor-acceptor distances (∼5.5 nm). Due to this, high BRET efficiency ratios of ∼5 were obtained. These PPyGRTS-QD bio-nano conjugates were characterized by transmission electron microscopy, thermal gravimetric analysis, Fourier transform infrared spectroscopy and BRET emission studies. The final optimized conjugate was easily observable by night vision imaging, demonstrating the potential of these materials in imaging and signaling/sensing applications.

Attenuating surface plasmon resonance via core/alloy architectures.

The layer-by-layer processing of Au/Au(x)Pd(1-x) core/alloy nanoparticles via microwave irradiation (MWI) based hydrothermal heating is described. Alloy shell growth was monitored by the attenuation of surface plasmon resonance (SPR) as a function of shell thickness and composition. Discrete dipole approximation (DDA) correlated the SPR to particle morphology.

Probing Bioluminescence Resonance Energy Transfer in Quantum Rod-Luciferase Nanoconjugates

We describe the necessary design criteria to create highly efficient energy transfer conjugates containing luciferase enzymes derived from Photinus pyralis (Ppy) and semiconductor quantum rods (QRs) with rod-in-rod (r/r) microstructure. By fine-tuning the synthetic conditions, CdSe/CdS r/r-QRs were prepared with two different emission colors and three different aspect ratios (l/w) each. These were hybridized with blue, green, and red emitting Ppy, leading to a number of new BRET nanoconjugates. Measurements of the emission BRET ratio (BR) indicate that the resulting energy transfer is highly dependent on QR energy accepting properties, which include absorption, quantum yield, and optical anisotropy, as well as its morphological and topological properties, such as aspect ratio and defect concentration. The highest BR was found using r/r-QRs with lower l/w that were conjugated with red Ppy, which may be activating one of the anisotropic CdSe core energy levels. The role QR surface defects play on Ppy binding, and energy transfer was studied by growth of gold nanoparticles at the defects, which indicated that each QR set has different sites. The Ppy binding at those sites is suggested by the observed BRET red-shift as a function of Ppy-to-QR loading (L), where the lowest L results in highest efficiency and furthest shift.

Keplerate cluster (Mo-132) mediated electrostatic assembly of nanoparticles.

The electrostatic assembly between a series of differently charged Mo-132-type Keplerates present in the compounds (NH4)42[{(Mo(VI))Mo(VI)5O21(H2O)6}12 {Mo(V)2O4(CH3COO)}30].ca. {300 H2O+10 CH3COONH4} (Mo-132a), (NH4)72-n[{(H2O)81-n+(NH4)n} {(Mo(VI))Mo(VI)5O21(H2O)6}12 {Mo(V)2O4(SO4)}30].ca. 200 H2O (Mo-132b), and Na10(NH4)62[{(Mo(VI))Mo(VI)5O21(H2O)6}12 {Mo(V)2O4(HPO4)}30]. ca. {300H2O+2Na(+)+2NH4(+)+4H2PO4(-)} (Mo-132c) with cationic gold nanoparticles (AuNPs) was investigated for the first time. The rapid electrostatic assembly from nanoscopic entities to micron scale aggregates was observed upon precipitation, which closely matched the point of aggregate electroneutrality. Successful assembly was demonstrated using UV-vis, DLS, TEM, and zeta-potential analysis. Results indicate that the point at which precipitation occurs is related to charge balance or electroneutrality, and that counterions at both the Mo-132 and AuNP play a significant role in assembly.