Direct observation of mixed-parity excited states in surface-passivated PbS nanocrystals

Photoluminescent emission is observed from surface-passivated PbS nanocrystals following the two-photon excitation of high-energy excitonic states. The emission appears directly at the excitation energy with no detectable Stokes-shift for a wide range of excitation energies. The observation of direct emission from states excited by two-photon absorption indicates that the parity of the excited states of surface-passivated PbS nanocrystals is partially mixed.

Growing semiconductor nanocrystals directly in a conducting polymer

We describe a single step method to synthesise lead sulphide (PbS) nanocrystals directly in the conjugated polymer poly (2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV). This method allows size control of the nanocrystal via co-solvent ratios. We find good agreement between nanocrystal sizes determined by transmission electron microscopy and sizes theoretically determined from the absorption edge of the nanocrystals. Finally we show that this synthesis technique is not restricted to MEH-PPV and demonstrate that nanocrystals can be grown in Poly(3-hexylthiophene-2,5-diyl) (P3HT). (C) 2005 Elsevier B.V. All rights reserved.

Esterified astaxanthin levels in lobster epithelia correlate with shell colour intensity: Potential role in crustacean shell colour formation

Carotenoids, particularly astaxanthin, are the primary pigment in crustacean shell colour. Sub-adults of the western rock lobster, Panulirus cygnus, moult from a deep red colour (termed the red phase) to a much paler colour (the white phase) at sexual maturation. We observe a 2.4-fold difference in the amount of total carotenoid present in the shell extracts of reds compared to whites, as might be expected. However, analysis of the underlying epithelium shows that there is no correlation with shell colour and the amount of free (unesterified) astaxanthin-the level of free astaxanthin in reds and whites is not significantly different. Instead, we observe a correlated two-fold difference in the amount of esterified astaxanthin present in the epithelium of red versus white individuals. These data suggest a role for esterified astaxanthin in regulating shell colour formation and suggest that esterification may promote secretion and eventual incorporation of unesterified astaxanthin into the exoskeleton. (c) 2005 Elsevier Inc. All rights reserved.

Synthesis of gadolinium-labeled shell-crosslinked nanoparticles for magnetic resonance imaging applications

Shell-crosslinked knedel-like nanoparticles (SCKs; knedel is a Polish term for dumplings) were derivatized with gadolinium Shell chelates and studied as robust magnetic-resonance-imaging-active structures with hydrodynamic diameters of 40 +/- 3 nm. SCKs possessing an amphiphilic core-shell morphology were produced from the aqueous assembly of diblock copolymers of poly(acrylic acid) (PAA) and poly(methyl acrylate) (PMA), PAA(52)-b-PMA(128), and subsequent covalent crosslinking by amidation upon reaction with 2,2'-(ethylenedioxy)bis(ethylamine) throughout the shell layer. The properties of these materials, including non-toxicity towards mammalian cells, non-immunogenicity within mice, and capability for polyvalent targeting, make them ideal candidates for utilization within biological systems. The synthesis of SCKs derivatized with Gd-III and designed for potential use as a unique nanometer-scale contrast agent for MRI applications is described herein. Utilization of an amino-functionalized diethylenetriaminepentaacetic acid-Gd analogue allowed for direct covalent conjugation throughout the hydrophilic shell layer of the SCKs and served to increase the rotational correlation lifetime of the Gd. In addition, the highly hydrated nature of the shell layer in which the Gd was located allowed for rapid water exchange; thus, the resulting material demonstrated large ionic relaxivities (39 s(-1) mM(-1)) in an applied magnetic field of 0.47 T at 40 degrees C and, as a result of the large loading capacity of the material, also demonstrated high molecular relaxivities (20 000 s(-1) mM(-1)).

Surface morphology dependent photoluminescence from colloidal silicon nanocrystals

Monodisperse 1-2 nm silicon nanocrystals are synthesized in reverse micelles and have their surfaces capped with either allylamine or 1-heptene to produce either hydrophilic or hydrophobic silicon nanocrystals. Optical characterization (absorption, PL, and time-resolved PL) is performed on colloidal solutions with the two types of surface-capped silicon nanocrystals with identical size distributions. Direct evidence is obtained for the modification of the optical properties of silicon nanocrystals by the surface-capping molecule. The two different surface-capped silicon nanocrystals show remarkably different optical properties.

Time-resolved photoluminescence spectroscopy of ligand-capped PbS nanocrystals

PbS nanocrystals are synthesized using colloidal techniques and have their surfaces capped with oleic acid. The absorption band edge of the PbS nanocrystals is tuned between 900 and 580 nm. The PbS nanocrystals exhibit tuneable photoluminescence with large non-resonant Stokes shifts of up to 500 mcV. The magnitude of the Stokes shift is found to be dependent upon the size of PbS nanocrystals. Time-resolved photoluminescence spectroscopy of the PbS nanocrystals reveals that the photouminescence has an extraordinarily long lifetime of 1 mus. This long fluorescence lifetime is attributed to the effect of dielectric screening similar to that observed in other IV-VI semiconductor nanocrystals.

Non-linear photoluminescence from purified aqueous PbS nanocrystals

A simple and effective method for purifying photoluminescent water-soluble surface passivated PbS nanocrystals has been developed. Centrifuging at high speeds removes PbS nanocrystals that exhibit strong red band edge photoluminescence from an original solution containing multiple nanocrystalline species with broad photoluminescence spectra. The ability to purify the PbS nanocrystals allowed two-photon photoluminescence spectroscopy to be performed on water-soluble PbS nanocrystals and be attributed to band edge recombination. (c) 2006 Elsevier B.V. All rights reserved.

Highly efficient luminescence from a hybrid state found in strongly quantum confined PbS nanocrystals

We report that high quality PbS nanocrystals, synthesized in the strong quantum confinement regime, have quantum yields as high as 70% at room temperature. We use a combination of modelling and photoluminescence up-conversion to show that we obtain a nearly monodisperse size distribution. Nevertheless, the emission displays a large nonresonant Stokes shift. The magnitude of the Stokes shift is found to be directly proportional to the degree of quantum confinement, from which we establish that the emission results from the recombination of one quantum confined charge carrier with one localized or surface-trapped charge carrier. Furthermore, the surface state energy is found to lie outside the bulk bandgap so that surface-related emission only commences for strongly quantum confined nanocrystals, thus highlighting a regime where improved surface passivation becomes necessary.

A rapidly evolving secretome builds and patterns a sea shell

Background: Instructions to fabricate mineralized structures with distinct nanoscale architectures, such as seashells and coral and vertebrate skeletons, are encoded in the genomes of a wide variety of animals. In mollusks, the mantle is responsible for the extracellular production of the shell, directing the ordered biomineralization of CaCO3 and the deposition of architectural and color patterns. The evolutionary origins of the ability to synthesize calcified structures across various metazoan taxa remain obscure, with only a small number of protein families identified from molluskan shells. The recent sequencing of a wide range of metazoan genomes coupled with the analysis of gene expression in non-model animals has allowed us to investigate the evolution and process of biomineralization in gastropod mollusks. Results: Here we show that over 25% of the genes expressed in the mantle of the vetigastropod Haliotis asinina encode secreted proteins, indicating that hundreds of proteins are likely to be contributing to shell fabrication and patterning. Almost 85% of the secretome encodes novel proteins; remarkably, only 19% of these have identifiable homologues in the full genome of the patellogastropod Lottia scutum. The spatial expression profiles of mantle genes that belong to the secretome is restricted to discrete mantle zones, with each zone responsible for the fabrication of one of the structural layers of the shell. Patterned expression of a subset of genes along the length of the mantle is indicative of roles in shell ornamentation. For example, Has-sometsuke maps precisely to pigmentation patterns in the shell, providing the first case of a gene product to be involved in molluskan shell pigmentation. We also describe the expression of two novel genes involved in nacre (mother of pearl) deposition. Conclusion: The unexpected complexity and evolvability of this secretome and the modular design of the molluskan mantle enables diversification of shell strength and design, and as such must contribute to the variety of adaptive architectures and colors found in mollusk shells. The composition of this novel mantle-specific secretome suggests that there are significant molecular differences in the ways in which gastropods synthesize their shells.