Photophysical and structural characterisation of in situ formed quantum dots.

Conjugated polymer-semiconductor quantum dot (QD) composites are attracting increasing attention due to the complementary properties of the two classes of materials. We report a convenient method for in situ formation of QDs, and explore the conditions required for light emission of nanocomposite blends. In particular we explore the properties of nanocomposites of the blue emitting polymer poly[9,9-bis(3,5-di-tert-butylphenyl)-9H-fluorene] together with cadmium sulphide (CdS) and cadmium selenide (CdSe) precursors. We show the formation of emissive quantum dots of CdSe from thermally decomposed precursor. The dots are formed inside the polymer matrix and have a photoluminescence quantum yield of 7.5%. Our results show the importance of appropriate energy level alignment, and are relevant to the application of organic-inorganic systems in optoelectronic devices.

Engineering thiophene-based nanoparticles to induce phototransduction in live cells under illumination.

We report that nanoparticles prepared from appropriately functionalized polythiophenes once administered to live cells can acquire phototransduction properties under illumination, becoming photoactive sites able to absorb visible light and convert it to an electrical signal through cell membrane polarization. Amine-reactive fluorescent nanoparticles with pendant N-succinimidyl-ester groups (NPs-NHS) are prepared from polythiophenes alternating unsubstituted and 3-(2,5-dioxopyrrolidin-1-yl-8-octanoate)-substituted thiophenes by a nanoprecipitation method. By 1H NMR of nanoparticles prepared using THF-d8/D2O (solvent/non-solvent) we demonstrate that the hydrolysis of the N-succinimidyl-ester group to free N-hydroxysuccinimide takes place slowly over several hours. NPs-NHS reactivity towards primary amine groups is tested towards the NH2 of d- and l-enantiomers of tryptophan. We show that the formation of a tryptophan-nanoparticle amidic bond creates a chiral shell displaying opposite CD signals for the nanoparticles bound to d or l enantiomers. The interaction of NPs-NHS with live HEK-293 cells is monitored via LSCM. We show that the NPs-NHS are not internalized but remain docked on the cell membrane. We assume that this is mainly the result of the reaction of the NHS groups in the external layer with NH2 groups present in cell membrane proteins, although the contribution of alternative mechanisms cannot be excluded. To support this assumption LSCM experiments show that nanoparticles of comparable size obtained from poly(3-hexylthiophene), NPs-P3HT, are rapidly internalized by live HEK-293 cells. Finally, using the whole-cell current clamp technique under light illumination we demonstrate that NPs-NHS can polarize the cell membrane upon light irradiation while NPs-P3HT cannot.

Tracking graphene by fluorescence imaging: a tool for detecting multiple populations of graphene in solution.

Most methods used for the characterization of graphene produced by liquid phase exfoliation require the deposition of the liquid sample on a substrate and subsequent drying. Because of this or other post-synthetic treatments, the reliability of the data in describing the actual features of the graphene particles in the pristine solution becomes questionable. Hence there is a need for new methods that permit the study of graphene directly in solution. Fluorescence imaging is at present the most convenient and sensitive method to visualize nanosized objects in solution. Here we report the development of a new method for visualizing and tracking exfoliated graphene directly in solution using a conventional set-up for fluorescence microscopy. We functionalized a fluorescent surfactant typically used for exfoliating graphite in aqueous phase (Pluronic P123) with two different fluorophores, in order to make graphene detectable by fluorescence microscopy. The photophysical interactions between the fluorescent surfactant and graphene were investigated at the bulk level. Finally, fluorescence microscopy allowed us to track the carbon particles produced and to identify two different populations of particles with sizes of 265 ± 25 and 1100 ± 200 nm respectively. The correlation of these results with TEM and DLS data is discussed.