Stefania Impellizzeri

Supramolecular strategies to construct biocompatible and photoswitchable fluorescent assemblies.

We designed and synthesized an amphiphilic copolymer with pendant hydrophobic decyl and hydrophilic poly(ethylene glycol) chains along a common poly(methacrylate) backbone. This macromolecular construct captures hydrophobic boron dipyrromethene fluorophores and hydrophobic spiropyran photochromes and transfers mixtures of both components in aqueous environments. Within the resulting hydrophilic supramolecular assemblies, the spiropyran components retain their photochemical properties and switch reversibly to the corresponding merocyanine isomers upon ultraviolet illumination. Their photoinduced transformations activate intermolecular electron and energy transfer pathways, which culminate in the quenching of the boron dipyrromethene fluorescence. As a result, the emission intensity of these supramolecular constructs can be modulated in aqueous environments under optical control. Furthermore, the macromolecular envelope around the fluorescent and photochromic components can cross the membrane of Chinese hamster ovarian cells and transport its cargo unaffected into the cytosol. Indeed, the fluorescence of these supramolecular constructs can be modulated also intracellularly by operating the photochromic component with optical inputs. In addition, cytotoxicity tests demonstrate that these supramolecular assemblies and the illumination conditions required for their operation have essentially no influence on cell viability. Thus, supramolecular events can be invoked to construct fluorescent and photoswitchable systems from separate components, while imposing aqueous solubility and biocompatibility on the resulting assemblies. In principle, this simple protocol can evolve into a general strategy to deliver and operate intracellularly functional molecular components under optical control.

Photoinduced enhancement in the luminescence of hydrophilic quantum dots coated with photocleavable ligands.

In search of strategies to photoactivate the luminescence of semiconductor quantum dots, we devised a synthetic approach to attach photocleavable 2-nitrobenzyl groups to CdSe-ZnS core-shell quantum dots coated with hydrophilic polymeric ligands. The emission intensity of the resulting nanostructured constructs increases by more than 60% with the photolysis of the 2-nitrobenzyl appendages. Indeed, the photoinduced separation of the organic chromophores from the inorganic nanoparticles suppresses an electron-transfer pathway from the latter to the former and is mostly responsible for the luminescence enhancement. However, the thiol groups anchoring the polymeric envelope to the ZnS shell also contribute to the photoinduced emission increase. Presumably, their photooxidation eliminates defects on the nanoparticle surface and promotes the radiative deactivation of the excited quantum dots. This effect is fully reversible but its magnitude is only a fraction of the change caused by the photocleavage of the 2-nitrobenzyl groups. In addition, these particular quantum dots can cross the membrane of model cells and their luminescence increases by ~80% after the intracellular photocleavage of the 2-nitrobenzyl quenchers. Thus, photoswitchable luminescent constructs with biocompatible character can be assembled combining the established photochemistry of the 2-nitrobenzyl photocage with the outstanding photophysical properties of semiconductor quantum dots and the hydrophilic character of appropriate polymeric ligands.

Photoswitchable Fluorescent Dyads Incorporating BODIPY and [1,3]Oxazine Components

We designed and synthesized three compounds incorporating a BODIPY fluorophore and an oxazine photochrome within the same molecular skeleton and differing in the nature of the linker bridging the two functional components. The [1,3]oxazine ring of the photochrome opens in less than 6 ns upon laser excitation in two of the three fluorophore-photochrome dyads. This process generates a 3H-indolium cation with a quantum yield of 0.02-0.05. The photogenerated isomer has a lifetime of 1-3 μs and reverts to the original species with first-order kinetics. Both photochromic systems tolerate hundreds of switching cycles with no sign of degradation. The visible excitation of the dyads is accompanied by the characteristic fluorescence of the BODIPY component. However, the cationic fragment of their photogenerated isomers can accept an electron or energy from the excited fluorophore. As a result, the photoinduced transformation of the photochromic component within each dyad results in the effective quenching of the BODIPY emission. Indeed, the fluorescence of these photoswitchable compounds can be modulated on a microsecond time scale with excellent fatigue resistance under optical control. Thus, our operating principles and choice of functional components can ultimately lead to the development of valuable photoswitchable fluorescent probes for the super-resolution imaging of biological samples.

Structural and Size Effects on the Spectroscopic and Redox Properties of CdSe Nanocrystals in Solution: The Role of Defect States

Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid-sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron-transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron-hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle-in-a-box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size-dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth.

Anti-proliferative and anti-cancer properties of Achyranthes aspera: specific inhibitory activity against pancreatic cancer cells.

AIMS OF THE STUDY Achyranthes aspera (Family: Amaranthacea) is a medicinal plant used as an anti-cancer agent in ayurveda, a traditional system of medicine practiced in subcontinental India. The aim of the study was to systematically investigate the anti-proliferative properties of Achyranthes aspera leaves extracted in methanol (LE) on human cancer cells in vitro. MATERIALS AND METHODS We tested time, dose dependent and specific anti-proliferative activity of LE by clonogenic cell survival assay on human cancer and normal epithelial cell lines in vitro. We further investigated its effect on the expression of metastatic and angiogenic genes by real time polymerase chain reaction. On silica gel column, we carried out initial fractionation analysis. RESULTS LE exhibited time and dose dependent cytotoxicity on several tumor cells. Compared to cancer cells of colon, breast, lung and prostate origin, pancreatic cancer cells were significantly more sensitive to LE. Preliminary mechanistic studies suggested that LE selectively suppressed the transcription of metalloproteases (MMP-1 and -2), inhibitors of MMPs (TIMP-2) and angiogenic factors (VEGF-A and VEGF-B). Fractionation of LE on methanol equilibrated silica gel column resolved into three fractions of which fraction (F 3) was found to be enriched with anti-proliferative activity. CONCLUSION Methanolic extract of Achyranthes aspera contains potent anti-proliferative compound with specific activity against pancreatic cancer. Further studies are needed to confirm the in vivo anti-tumorigenicity and subsequent chemical characterization of the active molecule(s).

Redox properties of CdSe and CdSe-ZnS quantum dots in solution

Semiconductor quantum dots (QDs) are inorganic nanoparticles which, because of their unique size-dependent electronic properties, are of high potential interest for the construction of functional nanodevices. Photoinduced electron transfer is a versatile mechanism used to implement light-induced functionalities in multicomponent (supra)molecular assemblies. Indeed, QDs can be employed as active components in new generations of these systems. The rational design of the latter, however, requires prior knowledge of the photo-physical properties and redox potentials of the nanocrystals. Here we discuss the results of recent systematic electrochemical investigations aimed at understanding the structural factors that regulate the redox properties of CdSe core and CdSe–ZnS core–shell QDs.

Synthesis, acid properties and catalysis by niobium oxide nanostructured materials

Several forms of niobium oxide were prepared, including nanostructured mesoporous materials, and their acidity properties were comprehensively investigated and compared with commercially available materials. The composites were characterized by a variety of techniques, including XRD, TEM, N2 adsorption and Hammett acid indicator studies. The acidity of the niobium oxide derivatives was also investigated by the ability of the materials to successfully promote the halochromic ring-opening of an oxazine-coumarin probe that was specifically designed for use in fluorescence imaging studies. The ring-opening reaction was easily monitored using UV-visible, fluorescence and NMR spectroscopy. Single molecule microscopy was employed to gain a more in-depth understanding of the niobium oxide acid catalysis pathway. Using this technique, the rate of niobium oxide mediated protonation was estimated to be 1.8 × 10−13 mol m−2 s−1. Single molecule analysis was also used to obtain a detailed map of Bronsted acid sites on the niobium oxide surface. The active sites, located by multiple blinking events, do not seem to be localized on any area of the material, but rather randomly distributed throughout the solid state surface. As the reaction proceeds, the sites with the highest acidity and accessibility are gradually consumed, making the next tier of acid sites available for reaction. The phenomenon was more closely characterized by using time lapsed reactivity maps.

Photochemical synthesis and characterization of novel samarium oxide nanoparticles: toward a heterogeneous Brønsted acid catalyst

Samarium oxide nanoparticles (Sm2O3NP) were prepared photochemically for the first time. Characterization shows spherical, polydisperse Sm2O3NP stabilized by 4-HEBA, a substituted benzoic acid. The Sm2O3NP also possess Bronsted acidity. This new material may prove to be a potent heterogeneous acid catalyst.

Mechanistic insights into the Nb2O5 and niobium phosphate catalyzed in situ condensation of a fluorescent halochromic assembly

Solid niobium oxides (Nb2O5·nH2O) and niobium phosphate were used as heterogeneous acid catalysts to promote the condensation between a switchable oxazine and a fluorescent coumarin in an aprotic solvent. The catalysts were found to promote the generation of an active methylene from the enamine-based portion of the oxazine, which was followed by a nucleophilic attack on the aldehyde functionality of the coumarin reagent. In the resulting system, the emission of the conjugated fluorophore can be observed at 670 nm and, thus, the processes occurring at the catalyst surface can be monitored in real time by total internal reflection fluorescence microscopy (TIRFM).

Two-Photon Excitation of a Plasmonic Nanoswitch Monitored by Single-Molecule Fluorescence Microscopy.

Visible-light excitation of the surface plasmon band of silver nanoplates can effectively localize and concentrate the incident electromagnetic field enhancing the photochemical performance of organic molecules. Herein, the first single-molecule study of the plasmon-assisted isomerization of a photochrome-fluorophore dyad, designed to switch between a nonfluorescent and a fluorescent state in response to the photochromic transformation, is reported. The photochemistry of the switchable assembly, consisting of a photochromic benzooxazine chemically conjugated to a coumarin moiety, is examined in real time with total internal reflection fluorescence microscopy in the presence of silver nanoplates excited with a 633 nm laser. The metallic nanostructures significantly enhance the visible light-induced performance of the photoconversion, which normally requires ultraviolet excitation. The resulting ring-open isomer is strongly fluorescent and can also be excited at 633 nm. These stochastic emission events are used to monitor photochromic activation and show quadratic dependence on incident power. The utilization of a single laser wavelength for both photochromic activation and excitation effectively mimics a pseudo two-colours system.