Ferroelectric Texture of Individual Barium Titanate Nanocrystals.

Ferroelectric materials display exotic polarization textures at the nanoscale that could be used to improve the energetic efficiency of electronic components. The vast majority of studies were conducted in two dimensions on thin films that can be further nanostructured, but very few studies address the situation of individual isolated nanocrystals (NCs) synthesized in solution, while such structures could have other fields of applications. In this work, we experimentally and theoretically studied the polarization texture of ferroelectric barium titanate (BaTiO3, BTO) NCs attached to a conductive substrate and surrounded by air. We synthesized NCs of well-defined quasicubic shape and 160 nm average size that conserve the tetragonal structure of BTO at room temperature. We then investigated the inverse piezoelectric properties of such pristine individual NCs by vector piezoresponse force microscopy (PFM), taking particular care to suppress electrostatic artifacts. In all of the NCs studied, we could not detect any vertical PFM signal, and the maps of the lateral response all displayed larger displacement amplitude on the edges with deformations converging toward the center. Using field phase simulations dedicated to ferroelectric nanostructures, we were able to predict the equilibrium polarization texture. These simulations revealed that the NC core is composed of 180° up and down domains defining the polar axis that rotate by 90° in the two facets orthogonal to this axis, eventually lying within these planes forming a layer of about 10 nm thickness mainly composed of 180° domains along an edge. From this polarization distribution, we predicted the lateral PFM response, which was revealed to be in very good qualitative agreement with the experimental observations. This work positions PFM as a relevant tool to evaluate the potential of complex ferroelectric nanostructures to be used as sensors.

Structure and Photonic Properties of a Perylenediimide Monolayer Assembled by the Langmuir-Blodgett Technique.

The photonic responses of densely packed dye molecule assemblies are strongly dependent on their organization and environment. The precise control of molecular orientations and distances relative to the substrate and to each other is thus a key point in the design of photonic molecular materials. Herein, we report the preparation of a homogeneous and well-organized single monolayer of the perylenediimide (PDI) derivative by means of the Langmuir-Blodgett technique. Its optical properties disclose an intense charge-transfer excitonic absorption band related to important intermolecular coupling. Furthermore, an important immunity to photobleaching is observed for such a molecular assembly. The dipolar orientations of the molecules along the substrate have been unambiguously determined by angle-of-incidence-resolved polarized absorption and back-focal-plane fluorescence mapping. In addition, time-resolved spectroscopy reveals a fast two-dimensional diffusion of excitons consistent with strong π-stacking of adjacent PDI molecules.

Electronic effects of the Bernal stacking of graphite on self-assembled aromatic adsorbates.

We compare by Scanning Tunneling Microscopy (STM) self-organized honeycomb monolayers of aromatic molecules formed either on graphite or on graphene. A differential contrast between the adsorption sites observed exclusively on graphite evidences the electronic effects of the symmetry breaking by the staggered atomic layers forming this substrate.

From plasmon-induced luminescence enhancement in gold nanorods to plasmon-induced luminescence turn-off: a way to control reshaping.

Two-photon luminescence (TPL) turn-off in small single gold nanorods (GNRs) exposed to increased resonant femtosecond laser excitation (800 nm wavelength, pulse energy density varying from 125 µJ cm-2 to 2.5 mJ cm-2) is investigated. The origin is shown to be a photo-induced decrease of the rod aspect ratio. This aspect ratio reduction could reasonably be assigned to gold atom diffusion away from the rod tips, where hot spots are localized. The two-photon luminescence signal can be recovered after a blue-shift of the incident excitation wavelength. No change in the excitation wavelength results in an out of resonance excitation of the rods and thus a reduced absorption, acting as feedback to stabilize the GNR shape and size. A theoretical analysis is presented evidencing limited thermal effects in the femtosecond regime for small nanoparticles, in good agreement with complementary topographic characterizations using scanning electron microscopy (SEM) and atomic force microscopy (AFM). We show finally that TPL reveals itself as a highly sensitive tool to follow tiny changes resulting from the photo-induced reshaping of GNRs.

Oriented Gold Nanorods and Gold Nanorod Chains within Smectic Liquid Crystal Topological Defects.

We show that the use of oriented linear arrays of smectic A defects, the so-called smectic oily streaks, enables the orientation of gold nanorods (GNRs) for a large range of GNR diameters, ranging from 7 to 48 nm, and for various ligands. For the small GNRs it enables oriented end-to-end small chains of GNRs when the density is increased from around 2 GNRs/µm2 to around 6 GNRs/µm2. We have characterized the orientation of single GNRs by spectrophotometry and two-photon luminescence (TPL). A strongly anisotropic absorption of the composites and an on-off switching of GNR luminescence, both controlled by incident light polarization, are observed, revealing an orientation of the GNRs mostly parallel to the oily streaks. A more favorable trapping of GNRs by smectic dislocations with respect to ribbon-like defects is thus demonstrated. The dislocations appear to be localized at a specific localization, namely, the summit of rotating grain boundaries. Combining plasmonic absorption measurements, TPL measurements, and simulation of the plasmonic absorption, we show that the end-to-end GNR chains are both dimers and trimers, all parallel to each other, with a small gap between the coupled GNRs, on the order of 1.5 nm, thus associated with a large red-shift of 110 nm of the longitudinal plasmonic mode. A motion of the GNRs along the dislocations appears as a necessary ingredient for the formation of end-to-end GNR chains, the gap value being driven by the balance between the attracting van der Waals interactions and the steric repulsion between the GNRs and leading to interdigitation of the neighboring ligands. We thus obtain electromagnetic coupling of nanorods controlled by light polarization.

A Highly Selective Potassium Sensor for the Detection of Potassium in Living Tissues.

The development of highly selective sensors for potassium is of great interest in biology. Two new hydrosoluble potassium sensors (Calix-COU-Alkyne and Calix-COU-Am) based on a calix[4]arene bis(crown-6) and an extended coumarin were synthesized and characterized. The photophysical properties and complexation studies of these compounds have been investigated and show high molar extinction coefficients and high fluorescence quantum yields. Upon complexation with potassium in the millimolar concentration range, an increase of one- and two-photon fluorescence emission is detected. A twofold fluorescence enhancement is observed upon excitation at λ=405 nm. The ligands present excellent selectivity for potassium in the presence of various competitive cations in water and in a physiological medium. The photophysical properties are not affected by the presence of a large amount of competing cations (Na+ , Ca2+ , Mg2+ , etc.). Ex vivo measurements on mouse hippocampal slices show that Calix-COU-Alkyne accumulates extracellularly and does not alter the neuronal activity. Furthermore, the sensor can be utilized to monitor slow extracellular K+ increase induced by inhibition of K+ entry into the cells.

Influence of the oxazole ring connection on the fluorescence of oxazoyl-triphenylamine biphotonic DNA probes.

On the basis of our previous work on DNA fluorophores derived from vinylpyridinium-triphenylamine, we explored the structure space around the electron-rich triphenylamine (TP) core by changing the vinyl bond to an oxazole ring. As 2,5-diaryloxazoles are known to be highly fluorescent and efficient two photon absorbers, we synthesized analogues with two different connections of the oxazole to the triphenylamine core: TP-Ox2Py and TP-Ox5Py sets. Since the benzimidazolium group was proven to be more effective in the TP series than the pyridinium, we also synthesized a TP-Ox5Bzim set. The TP-Ox5Py series retains the TP-Py properties: on/off behavior on DNA, good two-photon cross-section and bright staining of nuclear DNA by microscopy under both one or two-photon excitation. On the other hand, the TP-Ox2Py series does not display fluorescence upon binding to DNA. The TP-Ox5Bzim set is fluorescent even in the absence of DNA and displays lower affinity than the corresponding TP-Ox5Py. CD experiments and docking were performed to understand these different behaviors.

Formation of hydroxyl-functionalized stilbenoid molecular sieves at the liquid/solid interface on top of a 1-decanol monolayer.

Specific molecular tectons can be designed to form molecular sieves through self-assembly at the solid-liquid interface. After demonstrating a model tecton bearing apolar alkyl chains, we then focus on a modified structure involving asymmetric functionalization of some alkyl chains with polar hydroxyl groups in order to get chemical selectivity in the sieving. As the formation of supramolecular self-assembled networks strongly depends on molecule-molecule, molecule-substrate and molecule-solvent interactions, we compared the tectons' self-assembly on graphite for two types of solvent. We demonstrate the possibility to create hydroxylated stilbenoid molecular sieves by using 1-decanol as a solvent. Interestingly, with this solvent, the porous network is developed on top of a 1-decanol monolayer.

Surface-confined self-assembled Janus tectons: a versatile platform towards the noncovalent functionalization of graphene.

A general strategy for simultaneously generating surface-based supramolecular architectures on flat sp(2) -hybridized carbon supports and independently exposing on demand off-plane functionality with controlled lateral order is highly desirable for the noncovalent functionalization of graphene. Here, we address this issue by providing a versatile molecular platform based on a library of new 3D Janus tectons that form surface-confined supramolecular adlayers in which it is possible to simultaneously steer the 2D self-assembly on flat C(sp(2))-based substrates and tailor the external interface above the substrate by exposure to a wide variety of small terminal chemical groups and functional moieties. This approach is validated throughout by scanning tunneling microscopy (STM) at the liquid-solid interface and molecular mechanics modeling studies. The successful self-assembly on graphene, together with the possibility to transfer the graphene monolayer onto various substrates, should considerably extend the application of our functionalization strategy.

Synthesis and characterization of glycoconjugated porphyrin triphenylamine hybrids for targeted two-photon photodynamic therapy.

In order to avoid side effects at the time of cancer eradication to the patients, the selectivity of treatments has become of strategic importance. In the case of photodynamic therapy (PDT), two-photon absorption combined with active targeting of tumors could allow both spatial and chemical selectivity. In this context, we present the synthesis, spectroscopic, and biological properties of a series of porphyrin-triphenylamine hybrids with excellent singlet oxygen production capacities and good two-photon absorption.

Highly cohesive dual nanoassemblies for complementary multiscale bioimaging.

Innovative nanostructures made of a high payload of fluorophores and superparamagnetic nanoparticles (NPs) have simply been fabricated upon self-assembling in a two-step process. The resulting hybrid supraparticles displayed a dense shell of iron oxide nanoparticles tightly attached through an appropriate polyelectrolyte to a highly emissive non-doped nanocore made of more than 105 small organic molecules. Cooperative magnetic dipole interactions arose due to the closely packed magnetic NPs at the nanoarchitecture surface, causing enhanced NMR transverse relaxivity. Large in vivo MRI T2 contrast was thus obtained with unusually diluted solutions after intravenous injection in small rodents. Two-photon excited fluorescence imaging could be performed, achieving unprecedented location resolution for agents combining both magnetic nanoparticles and fluorescence properties. Finally, TEM imaging of the sectioned mouse tissue succeeded in isolating the core-shell structures, which represents the first image of intact complex magnetic and fluorescent nanoassemblies upon in vivo injection. Such highly cohesive dual nanoarchitectures should open great horizons toward the assessment with high spatial resolution of the drug or labeled stem cell biodistribution.

DNA switches on the two-photon efficiency of an ultrabright triphenylamine fluorescent probe specific of AT regions.

We report on the design and synthesis of two-photon fluorescent triphenylamines bearing two or three vinyl branches terminated by a N-methyl benzimidazolium moiety. The new compounds (TP-2Bzim, TP-3Bzim) are light-up fluorescent DNA probes with a long wavelength emission (>580 nm). Compared to their pyridinium models, the TP-Bzim dyes exhibit a remarkable improvement of both their DNA affinity and fluorescence quantum yield, especially for the two-branch derivative (TP-2Bzim: ΦF = 0.54, Ka = 10(7) M(-1)), resulting in a large fluorescence emission turn-on ratio of up to 140. Concomitantly, the two-photon absorption cross-section of TP-2Bzim is dramatically enhanced upon DNA binding (δ = 1080 vs 110 GM for the free form). This effect of the DNA matrix on the nonlinear absorption is uncovered for the first time. This is attributed to a tight fit of the molecule inside the minor groove of AT-rich DNA which induces geometrical rearrangements in the dye ground state as supported by circular dichroism and molecular modeling data. Consequently, TP-2bzim displays an exceptional two-photon molecular brightness (δ×ΦF = 583 GM), a value unrivalled for a small biofluorophore. These properties enable to image nuclear DNA in fixed cells at submicromolar concentration ([TP-2Bzim] = 100 nM) and to visualize ultrabright foci of centromeric AT-rich chromatin. Finally TP-2Bzim exhibits a high photostability, is live-cell permeant, and does not require RNase treatment. This outstanding combination of optical and biological properties makes TP-2Bzim a bioprobe surpassing the best DNA stainers and paves the way for studying further nonlinear optical processes in DNA.

An optimized alkyl chain-based binding motif for 2D self-assembly: a comprehensive crystallographic approach.

Taking into account substrate crystallographic constraints, an overarching molecular binding motif has been designed to allow transferable self-assembling patterns on different substrates. This optimized clip demonstrates robust and equivalent self-assembled architectures on both highly oriented pyrolitic graphite (HOPG) and reconstructed Au(111) surfaces.

Vinyl-triphenylamine dyes, a new family of switchable fluorescent probes for targeted two-photon cellular imaging: from DNA to protein labeling.

On the basis of our previous work on vinyl-triphenylamine derived DNA fluorophores we explored the structure space around this core by coupling it to diverse cationic, anionic and zwitterionic groups in the aim of targeting different classes of biomolecules. In parallel core modifications were performed to optimize the photophysical properties (quantum yield, two-photon absorption). The resulting water soluble π-conjugated molecules are called TP dyes and display an exceptional combination of optical properties: high two-photon absorption cross-section, high photostability, no self-quenching, and switchable fluorescence emission when bound to a biopolymer matrix. The linear and nonlinear optical properties of the TP dyes were studied in vitro in presence of DNA and in presence of a model protein (human serum albumin) using complementary absorption and fluorescence spectroscopy characterization tools. Structure modifications enabled to switch from DNA probes (cationic TP-pyridinium series) to protein probes (anionic TP-rhodanine series) without affecting the optical properties. Finally most TP compounds appear cell-permeant and show an intracellular localization consistent with their in vitro target specificity.

Self-templating polythiophene derivatives: electronic decoupling of conjugated strands through staggered packing.

Whereas molecular electronics needs well-controlled 3D geometries for decoupling or interconnecting individual molecules, conjugated polymers form disordered structures when deposited on a substrate. We show that this trend can be overcome in polythiophene derivatives designed so as to exploit weak sulfur-bromine interactions. A self-template effect follows, leading to staggered organizations of well-aligned electronically decoupled conjugated strands, as observed in situ by scanning tunneling microscopy and spectroscopy on graphite.

Solution-growth kinetics and thermodynamics of nanoporous self-assembled molecular monolayers.

The temperature and concentration dependences of the self-assembly onto graphite from solution of a series of molecular building blocks able to form nanoporous structures are analyzed experimentally by in situ scanning tunneling microscopy. It is shown that the commonly observed coexistence of dense and nanoporous domains results from kinetic blockades rather than a thermodynamic equilibrium. The ripening can be favored by high densities of domain boundaries, which can be obtained by cooling the substrate before the nucleation and growth. Then ripening at higher-temperature yields large defect-free domains of a single structure. This thermodynamically stable structure can be either the dense or the nanoporous one, depending on the tecton concentration in the supernatant solution. A sharp phase transition from dense to honeycomb structures is observed at a critical concentration. This collective phenomenon is explained by introducing interactions between adsorbed molecules in the thermodynamic description of the whole system.

Trinaphthylamines as robust organic materials for two-photon-induced fluorescence.

The synthesis of a novel pi-conjugated trinaphthylamines series is described. These original push-pull octupolar systems exhibit large two-photon action cross section (sigma phi up to 510 GM) increased by a factor of 2-3 as compared to their triphenylamines analogues. This substantial improvement of the two-photon absorption properties is attributed to the stronger donor character of the trinaphthyl core.

Synthetic strategies to derivatizable triphenylamines displaying high two-photon absorption.

A versatile synthetic strategy to access a set of highly fluorescent pi-conjugated triphenylamines bearing a functional linker at various positions on one phenyl ring is described. These compounds were designed for large two-photon absorption (2PA) and in particular for labeling of biomolecules. The monoderivatized trisformylated or trisiodinated intermediates described herein allow introduction of a large variety of electron-withdrawing groups required for large 2PA as well as a panel of chemical functions suitable for coupling to biomolecules. The monoderivatized three-branched compounds and in particular the benzothiazole (TP-3Bz) series show remarkable linear (high extinction coefficients and high quantum yield) and nonlinear (high 2-photon cross sections) optical properties. Interestingly the presence of functional side chains does not disturb the two-photon absorption. Finally, monoderivatized two-branched derivatives also appear to be valuable candidates. Altogether the good optical properties of the new derivatizable pi-conjugated TPA combined with their small size and their compatibility with bioconjugation protocols suggest that they represent a new chemical class of labels potentially applicable for the tracking of biomolecules using two-photon scanning microscopy.