Leila Zeiri

Poly(methyl methacrylate)-Supported Polydiacetylene Films: Unique Chromatic Transitions and Molecular Sensing

Polydiacetylenes (PDAs) constitute a family of conjugated polymers exhibiting unique colorimetric and fluorescence transitions, and have attracted significant interest as chemo- and biosensing materials. We spin-coated PDA films upon poly(methyl methacrylate) (PMMA), and investigated the photophysical properties and sensing applications of the new PDA configuration. Specifically, the as-polymerized blue PDA layer underwent distinct transformations to purple, red, and yellow phases, which could be quantified through conventional color scanning combined with application of image analysis algorithms. Furthermore, we recorded a reversible red-purple PDA transition that was induced by ultraviolet irradiation, a phenomenon that had not been reported previously in PDA film systems. We show that distinct color and fluorescence transitions were induced in the PMMA-supported PDA films by amphiphilic substances-surfactants and ionic liquids-and that the chromatic transformations were correlated to the analyte structures and properties. Overall, this study presents a new chromatic PDA film system in which noncovalent interactions between the PMMA substrate and spin-coated PDA give rise to distinct chromatic properties and molecular sensing capabilities.

Synthesis and properties of nanocrystalline π-SnS – a new cubic phase of tin sulphide

We report on the synthesis of the newly discovered cubic phase of tin sulfide π-SnS and compare its properties to the well-known phase of tin sulfide, α-SnS. Shape control was achieved by the variation of synthesis parameters, resulting in cubic, rhombic dodecahedral and tetrahedral shapes of the π-SnS nanoparticles. X-ray diffraction provided authentication of the proposed model and refined determination of the lattice parameter a = 11.595 A. Raman spectroscopy showed a substantial shift towards higher energies and peak splitting for π-SnS. Optical absorption spectroscopy indicated an indirect band gap of 1.53 eV, in good agreement with density functional theory (DFT) calculations indicating a band gap greater than that of α-SnS. DFT total energy calculations show that the π-SnS phase is energetically similar to α-SnS, and is significantly more stable than the hypothetical ideal rocksalt structure of SnS.

Transparent, conductive, and SERS-active Au nanofiber films assembled on an amphiphilic peptide template

The use of biological materials as templates for functional molecular assemblies is an active research field at the interface between chemistry, biology, and materials science. We demonstrate the formation of gold nanofiber films on β-sheet peptide domains assembled at the air/water interface. The gold deposition scheme employed a recently discovered chemical process involving spontaneous crystallization and reduction of water-soluble Au(SCN)4(1-) upon anchoring to surface-displayed amine moieties. Here we show that an interlinked network of crystalline Au nanofibers is readily formed upon incubation of the Au(iii) thiocyanate complex with the peptide monolayers. Intriguingly, the resultant films were optically transparent, enabled electrical conductivity, and displayed pronounced surface enhanced Raman spectroscopy (SERS) activity, making the approach a promising avenue for construction of nano-structured films exhibiting practical applications.

A new nanocrystalline binary phase: synthesis and properties of cubic tin monoselenide

A new nanometric cubic binary phase of the tin mono-selenide system, π-SnSe, was obtained as cube shaped nanoparticles. Its structure and atomic positions were adopted from previously reported π-SnS (P213, a0 = 11.7 A). The proposed structure model of π-SnSe, with 64 atoms per unit cell, was refined against experimental X-ray diffraction using Rietveld method (a0 = 11.9702(9) A; Rp = 1.65 Rwp = 2.11). The optical properties of this new cubic SnSe phase were characterized by Raman and optical absorption spectroscopies. The optical band gap was assessed to be indirect, with Eg = 1.28 eV (in the near infrared), compared to Eg = 0.9 eV (indirect) and 1.3 eV (direct) for the conventional orthorhombic phase of α-SnSe. Raman spectroscopy indicated significant phonon restraining, which is likely to be beneficial for thermoelectric applications. Since the new cubic phase belongs to a class of non-centrosymmetric crystals, interesting and potentially useful properties may arise. Density functional theory calculations have been applied in order to validate phase stability and evaluate the energy bandgap. These results, together with the recently discovered cubic phase of π-SnS, confirm the existence of a new class of nanoscale materials in the tin chalcogenide system.

Lipid/polydiacetylene films for colorimetric protein surface-charge analysis.

The distribution and organization of charges on a protein surface are fundamental properties which affect protein functions and interactions. We demonstrate a new approach for protein surface-charge analysis through modulating protein interactions with chromatic lipid/polydiacetylene (PDA) films. We show that visible and easily quantifiable blue-red transitions, induced on the film surface through electrostatic interactions between the negatively charged PDA and positive soluble species, constitute an effective means for characterizing protein surface charge. Specifically, protein-film interactions can be significantly modulated by complexation between the tested macromolecules and lipid-embedded multivalent calixarene ligands displaying charged residues, making possible protein discrimination based upon the abundance and organization of surface charge. The lipid/PDA film system, in conjunction with the calixarene-derived ligands, facilitates characterization of protein surface charges and identification of anomalous protein electrostatic properties.

Zinc modified polydiacetylene Langmuir films

Polydiacetylene (PDA) Langmuir films (LF) containing Zn2+ cations were prepared in different experimental procedures. The incorporation of zinc cations resulted in new morphological and spectral properties. We found that zinc cations stabilize the blue phase of PDA, and inhibit its transformation into the red phase, withstanding prolonged exposure to UV irradiation. The increased stability was accompanied with the loss of the linear strand morphology and appearance of a new, transient purple phase before the final red phase stage. It was found that the most significant differences in film properties occur when the zinc cations are already present in solution during the polymerization stage at sufficient concentration. In such case zinc ions penetrate into the headgroup interlayer in the trilayer organization, and are implicated in the films chromatic properties. In cases where polymerization was carried out prior to introduction of the cations, the properties of PDA/Zn2+ films are similar to films produced on a pure water subphase.

Effect of metal cations on polydiacetylene Langmuir films.

Polydiacetylene (PDA) Langmuir films (LFs) are a unique class of materials that couple a highly aligned conjugated backbone with tailorable pendant side groups and terminal functionalities. The films exhibit chromatic transitions from monomer to blue polymer and finally to a red phase that can be activated optically, thermally, chemically, and mechanically. The properties of PDA LFs are strongly affected by the presence of metal cations in the aqueous subphase of the film due to their interaction with the carboxylic head groups of the polymer. In the present study the influence of divalent cadmium, barium, copper, and lead cations on the structural, morphological, and optical properties of PDA LFs was investigated by means of surface pressure-molecular area (π-A) isotherms, atomic force microscopy, optical absorbance, and Raman spectroscopy. The threshold concentrations for the influence of metal cations on the film structure, stability, and phase transformation were determined by π-A analyses. It was found that each of the investigated cations has a unique influence on the properties of PDA LFs. Cadmium cations induce moderate phase transition kinetics with reduced domain size and fragmented morphology. Barium cations contribute to stabilization of the PDA blue phase and enhanced linear strand morphology. On the other hand, copper cations enhance rapid formation of the PDA red phase and cause fragmented morphology of the film, while the presence of lead cations results in severe perturbation of the film with only a small area of the film able to be effectively polymerized. The influence of the metal cations is correlated with the solubility product (K(sp)), association strength, and ionic-covalent bond nature between the metal cations and the PDA carboxylic head groups.

Two-photon polymerization of polydiacetylene.

We show that visible light can polymerize diacetylene monomers into polydiacetylene (PDA) in a two-photon process. We monitor the process by measuring Raman intensities of PDA using a Raman laser at 633 nm with variable intensity I and show that the Raman cross section at short times increases as I3, corresponding to a two-photon process. The process generates a relatively stable blue phase PDA, in contrast with UV polymerization that leads to a fast blue to red phase transformation.

Surface-Enhanced Raman Scattering (SERS) of Microorganisms

Normal Raman (NR) spectra and surface-enhanced Raman scattering (SERS) spectra were obtained for the bacterium Escherichia coli and were compared with those of two other microorganisms, Haloferax volcanii and Thiobacillus neapolitanus. It was found that at 514 nm the SERS of E. coli was similar to that of flavin adenine dinucleotide (FAD). Upon increasing the excitation wavelength, contributions from other cell components became evident, and they were attributed to nicotinamide adenine dinucleotide (NAD) or other adenine-containing molecules in the bacterium. A comprehensive study of FAD, riboflavin (RF), NAD, and adenine under various experimental conditions was thus performed to shed light on the features in the SERS obtained for E. coli. Comparison of NR and SERS measurements of the various samples enabled a better understanding of the SERS spectra and their sensitivity to the specific experimental conditions (excess metal ion concentration and laser excitation wavelengths and intensity). It was concluded that SERS is a highly sensitive technique and that careful examination of the spectra can provide important chemical information.

Bifunctional Carbon‐Dot‐WS2 Nanorods for Photothermal Therapy and Cell Imaging

Multifunctional nanoparticles have attracted significant interest as biomedical vehicles, combining diagnostic, imaging, and therapeutic properties. We describe herein the construction of new nanoparticle conjugates comprising WS2 nanorods (NRs) coupled to fluorescent carbon dots (C-dots). We show that the WS2 -C-dot hybrids integrate the unique physical properties of the two species, specifically the photothermal activity of the WS2 NRs upon irradiation with near-infrared (NIR) light and the excitation-dependent luminescence emission of the C-dots. The WS2 -C-dot NRs have been shown to be non-cytotoxic and have been successfully employed for multicolour cell imaging and targeted cell killing under NIR irradiation, pointing to their potential utilization as effective therapeutic vehicles.