Interrelated Roles of Multiple Key Structure-Directing Agents in Seed-Mediated Growth of Monodisperse and Multibranched Au Nanoparticles.

Detailed mechanistic investigations of the interrelated roles of multiple key structure-directing agents in the growth solution of Au nanoparticles (AuNPs) is required for the optimization of synthetic protocols. Here, we report a robust seed-mediated growth strategy for synthesizing multibranched NPs (MB-AuNPs) with monodispersed size distribution, and investigate the roles of Ag ions and 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES) based on an overgrowth synthesis approach. The intertwining roles of Ag+ , surface-capping stabilizers, and reducing agents were elucidated, and used to control the morphology of MB-AuNPs. The overgrowth of MB-AuNPs involves two distinct underlying pathways, namely, directional and anisotropic growth of Au branches on specific facets of Au seeds as well as an aggregation and growth mechanism governed by HEPES. In addition to Ag ions and HEPES, morphology tunability can also be achieved by pre-modification of the Au seeds with molecular probes. Optimized probe-containing MB-AuNPs prove to be excellent surface-enhanced Raman scattering (SERS) substrates and nanozymes. Taken together, the results of this work reveal the mechanistic evolution of nanocrystal growth which should stimulate the development of new synthetic strategies, improve the capabilities of tuning the optical, catalytic, and electronic properties of NPs, and further advance their applications in biolabeling, imaging, biosensing, and therapy.

Monolayer Arrays of Nanoparticles on Block Copolymer Brush Films.

Two-dimensional arrays of nanoparticles (NPs) have widespread applications in optical coatings, plasmonic sensors, and nanocomposites. Current bottom-up approaches that use homogeneous NP templates, such as silane self-assembled monolayers or homopolymers, are typically plagued by NP aggregation, whereas patterned block copolymer (BCP) films require specific compositions for specific NP distributions. Here, we show, using polystyrene- b-poly(4-vinylpyridine) (PS- b-P4VP) and gold NPs (AuNPs) of various sizes, that a nanothin PS- b-P4VP brushlike coating (comprised of a P4VP wetting layer and a PS overlayer), which is adsorbed onto flat substrates during their immersion in very dilute PS- b-P4VP tetrahydrofuran solutions, provides an excellent template for obtaining dense and well-dispersed AuNPs with little aggregation. These non-close-packed arrays have similar characteristics regardless of immersion time in solution (about 10-120 s studied), solution concentration below a critical value (0.1 and 0.05 mg/mL studied), and AuNP diameter (10-90 nm studied). Very dilute BCP solutions are necessary to avoid deposition, during substrate withdrawal, of additional material onto the adsorbed BCP layer, which typically leads to patterned surfaces. The PS brush coverage depends on immersion time (adsorption kinetics), but full coverage does not inhibit AuNP adsorption, which is attributed to PS molecular rearrangement during exposure to the aqueous AuNP colloidal solution. The simplicity, versatility and robustness of the method will enable applications in materials science requiring dense, unaggregated NP arrays.

Taming Macromolecules with Light: Lessons Learned from Vibrational Spectroscopy.

Exciting new applications, from large-area nanopatterning and templating to soft light-powered robotics, are emerging from the fundamental research on light-triggered changes in macromolecular systems upon photoisomerization of azobenzene-based molecular photoswitches. The understanding of how the initial molecular-scale photoisomerization of azobenzene, a complex photochemical event in itself, is translated into the response of macromolecules and even into macroscopic-scale motion of illuminated azomaterials is an enormous task. The focus here is on how this knowledge has advanced by applying different vibrational spectroscopy techniques that provide rich molecular insight into the photoresponse of chemically specific molecular moieties. In particular, infrared and Raman spectroscopy studies are highlighted, in the context of phototriggered perturbation of self-assembled structures and photoinduced linear and circular anisotropy, as well as photoinduced surface patterning, with the objective of offering a perspective on how vibrational spectroscopy can help in answering an array of essential yet unsettled questions.

Submolecular Plasticization Induced by Photons in Azobenzene Materials.

We demonstrate experimentally for the first time that the illumination of azobenzene derivatives leads to changes in molecular environment similar to those observed on heating but that are highly heterogeneous at the submolecular scale. This localized photoplasticization, which can be associated with a free volume gradient, helps to understand the puzzling phenomenon of photoinduced macroscopic material flow and photoexpansion upon illumination far below the glass transition temperature (T(g)). The findings stem from the correlation of infrared (IR) spectral band shifts measured upon illumination with those measured at controlled temperatures for two amorphous DR1-functionalized azo derivatives, a polymer, pDR1A, and a molecular glass, gDR1. This new approach reveals that IR spectroscopy can be used as an efficient label-free molecular-scale thermometer that allows the assignment of an effective temperature (T(eff)) to each moiety in these compounds when irradiated. While no band shift is observed upon illumination for the vibrational modes assigned to backbone moieties of pDR1A and gDR1 and a small band shift is found for the spacer moiety, dramatic band shifts are recorded for the azo moiety, corresponding to an increase in T(eff) of up to nearly 200 °C and a molecular environment that is equivalent to thermal heating well above the bulk T(g) of the material. An irradiated azo-containing material thus combines characteristic properties of amorphous materials both below and above its bulk T(g). The direct measurement of T(eff) is a powerful probe of the local environment at the submolecular scale, paving the way toward better rationalization of photoexpansion and the athermal malleability of azo-containing materials upon illumination below their T(g).

Nonphospholipid fluid liposomes with switchable photocontrolled release.

We created novel nonphospholipid photosensitive liposomes from a mixture of a monoacylated azobenzene amphiphile (AzoC10N(+)) and cholesterol sulfate (Schol). This system belongs to the family of sterol-enriched nonphospholipid liposomes that were shown to form stable large unilamellar vesicles (LUVs) with enhanced impermeability. Fluid bilayers were successfully prepared from AzoC10N(+)/Schol (25/75 molar ratio) mixtures, and LUVs could be derived at room temperature using standard extrusion methods. The isomerization process of the bilayer-inserted AzoC10N(+) was characterized. Leakage from these liposomes could be induced by the photoconversion of AzoC10N(+) from its trans form to its cis form. This photocontrolled release from fluid liposomes contrasts with the case of phospholipid-based azo-containing liposomes, which are generally required to be in the gel phase to be photosensitive. It is proposed that the very high degree of conformational order of the monoalkylated amphiphile and the tight packing of the hydrophobic core of the AzoC10N(+)/Schol liposomes make them responsive to the presence of the bulky cis azo isomer. Interestingly, the liposome impermeability could be fully restored by the photoisomerization of the cis form back to the trans form, providing a sharp on-and-off control of payload release. In addition, these nonphospholipid liposomes display a very limited passive release. Therefore, it is shown that AzoC10N(+)/Schol LUVs can be used as nanocontainers, whose content can be released by light in a controlled and switchable manner.

Understanding and controlling morphology formation in Langmuir-Blodgett block copolymer films using PS-P4VP and PS-P4VP/PDP.

This contribution offers a comprehensive understanding of the factors that govern the morphologies of Langmuir-Blodgett (LB) monolayers of amphiphilic diblock copolymers (BCs). This is achieved by a detailed investigation of a wide range of polystyrene-poly(4-vinyl pyridine) (PS-P4VP) block copolymers, in contrast to much more limited ranges in previous studies. Parameters that are varied include the block ratios (mainly for similar total molecular weights, occasionally other total molecular weights), the presence or not of 3-n-pentadecylphenol (PDP, usually equimolar with VP, with which it hydrogen bonds), the spreading solution concentration ("low" and "high"), and the LB technique (standard vs "solvent-assisted"). Our observations are compared with previously published results on other amphiphilic diblock copolymers, which had given rise to contradictory interpretations of morphology formation. Based on the accumulated results, we re-establish early literature conclusions that three main categories of LB block copolymer morphologies are obtained depending on the block ratio, termed planar, strand, and dot regimes. The block composition boundaries in terms of mol % block content are shown to be similar for all BCs having alkyl chain substituents on the hydrophilic block (such as PS-P4VP/PDP) and are shifted to higher values for BCs with no alkyl chain substituents (such as PS-P4VP). This is attributed to the higher surface area per repeat unit of the hydrophilic block monolayer on the water surface for the former, as supported by the onset and limiting areas of the Langmuir isotherms for the BCs in the dot regime. 2D phase diagrams are discussed in terms of relative effective surface areas of the two blocks. We identify and discuss how kinetic effects on morphology formation, which have been highlighted in more recent literature, are superposed on the compositional effects. The kinetic effects are shown to depend on the morphology regime, most strongly influencing the strand and, especially, planar regimes, where they give rise to a diversity of specific structures. Besides film dewetting mechanisms, which are different when occurring in structured versus unstructured films (the latter previously discussed in the literature), kinetic influences are discussed in terms of chain association dynamics leading to depletion effects that impact on growing aggregates. These depletion effects particularly manifest themselves in more dilute spreading solutions, with higher molecular weight polymers, and in composition regimes characterized by equilibrium degrees of aggregation that are effectively infinite. It is by understanding these various kinetic influences that the diversity of structures can be classified by the three main composition-dependent regimes.

Photophysical and electrochemical investigations of the fluorescent probe, 4,4'-bis(2-benzoxazolyl)stilbene.

In solution, 4,4'-bis(2-benzoxazolyl)stilbene (BBS) was found to exhibit consistently high absolute fluorescence quantum yields (Φ(fl) ≥ 0.88) and a monoexponential lifetime, both independent of BBS concentration. The BBS steady-state and time-resolved photophysics were investigated by different techniques to understand the various deactivation pathways. Nonradiative deactivation of BBS singlet excited state by intersystem crossing was found to be negligible. Other than fluorescence, the excited state of BBS was found to be deactivated by trans-cis photoisomerization. At low concentrations (≈5 µg/mL), UV spectroscopy and laser flash photolysis showed concordant results that the photoinduced cis isomer gradually replaced the original absorption spectrum of the pure trans isomer. However, at high concentrations (≈0.2 mg/mL), (1)H NMR and DOSY measurements confirmed that irradiating BBS at 350 nm induced a conversion from the trans-BBS into its cis isomer by photoisomerization. It was further found that the stilbene moiety of both isomers was photocleaved. The resulting photoproduct was an aldehyde that was oxidized under ambient conditions to its corresponding carboxylic acid, i.e., 4-(1,3-benzoxazol-2-yl)benzoic acid. The structure of the photoproduct was unequivocally confirmed by X-ray diffraction. Spectroscopic investigation of BBS showed a limited photoisomerization after irradiation at 350 nm of a trans solution. The BBS electrochemistry showed irreversible oxidation, resulting in an unstable and highly reactive radical cation. Similarly, the cathodic process was also found to be irreversible, giving rise to a radical anion and showing its n-doping character.

Pressure-induced order transition in nanodot-forming diblock copolymers at the air/water interface.

Understanding and controlling the processes in block copolymer (BC) monolayers at the air/water interface during surface area compression is a key issue for producing ultrathin films of predetermined morphology with well-defined order and known dimensions. Langmuir isotherms of nanodot-forming BC monolayers generally display a plateau indicative of a 2D phase transition, which has been the subject of various interpretations in the literature. Here, based on investigations of Langmuir-Blodgett and Langmuir-Schaefer nanodot films of PS-P4VP mixed with 3-n-pentadecylphenol (PDP), we show by atomic force microscopy (AFM) that it involves a change in nanodot packing order (from quasi-hexagonal to quasi-square), argued to be a general phenomenon for nanodot BC monolayers. It is accompanied by system-specific conformational changes (as discussed in previous literature), which, in the present case, implicate PDP alkyl chain ordering, as deduced previously from in situ infrared data and indirectly supported here by AFM imaging.

Molecular-Level Photo-Orientation Insights into Macroscopic Photo-Induced Motion in Azobenzene-Containing Polymer Complexes.

As part of continuing efforts to deepen the understanding of photo-induced mass transport in azo-containing polymers, we compared the diffraction efficiency (DE) during surface-relief grating (SRG) inscription, photo-induced molecular orientation (), and thermal stability in two sets of supramolecular azopolymer complexes, namely, hydrogen-bonded (H-bonded) and ionically bonded (i-bonded) complexes, both as a function of the polymer degree of polymerization (DP). To that end, poly(4-vinylpyridine) (P4VP) polymers with DPs of 41, 480, and 1900 were H-bonded at an equimolar ratio with 4-hydroxy-4'-dimethylaminoazobenzene (azoOH), and the fully quaternized derivatives of the three P4VPs (P4VPMe) were i-bonded via ion exchange to sodium 4-[(4-dimethylamino)-phenylazo]benzene sulfonate (azoSO3), also known as methyl orange, where the OH functionality of azoOH is replaced by a sulfonate group. The i-bonded complexes show much better DE performances and levels than those of H-bonded complexes, which we relate to the liquid crystal structure of the former complexes. Fitting the curves by a biexponential equation leads to two parameters associated with a fast trans-cis or angular hole burning (AHB) process and a slow angular redistribution (AR) process of the azo, respectively. It is found that AHB is predominant in the H-bonded complexes, whereas the AR contribution is much greater in the i-bonded complexes, assuring their superior SRG efficiency that is enabled by the anisotropic free volume created mainly by the AR process. In each set of complexes, the SRG efficiency is much better for the lowest DP complex, while the AR contribution is constant (and low) for the H-bonded complexes and increases roughly linearly with the decrease in DP for the i-bonded complexes. The latter difference might be related to the presence of entanglements in the complexes with DPs 480 and 1900, which slow down the macroscopic movement during SRG inscription but not the molecular-scale movement in photo-orientation.

Surface-Enhanced Raman Scattering Optophysiology Nanofibers for the Detection of Heavy Metals in Single Breast Cancer Cells.

Mercury(II) ions (Hg2+) and silver ions (Ag+) are two of the most hazardous pollutants causing serious damage to human health. Here, we constructed surface-enhanced Raman scattering (SERS)-active nanofibers covered with 4-mercaptopyridine (4-Mpy)-modified gold nanoparticles to detect Hg2+ and Ag+. Experimental evidence suggests that the observed spectral changes originate from the combined effect of (i) the coordination between the nitrogen on 4-Mpy and the metal ions and (ii) the 4-Mpy molecular orientation (from flatter to more perpendicular with respect to the metal surface). The relative intensity of a pair of characteristic Raman peaks (at ∼428 and ∼708 cm-1) was used to quantify the metal ion concentration, greatly increasing the reproducibility of the measurement compared to signal-on or signal-off detection based on a single SERS peak. The detection limit of this method for Hg2+ is lower than that for the Ag+ (5 vs 100 nM), which can be explained by the stronger interaction energy between Hg2+ and N compared to Ag+ and N, as demonstrated by density functional theory calculations. The Hg2+ and Ag+ ions can be masked by adding ethylenediaminetetraacetate and Cl-, respectively, to the Hg2+ and Ag+ samples. The good sensitivity, high reproducibility, and excellent selectivity of these nanosensors were also demonstrated. Furthermore, detection of Hg2+ in living breast cancer cells at the subcellular level is possible, thanks to the nanometric size of the herein described SERS nanosensors, allowing high spatial resolution and minimal cell damage.

Multiplexed SERS Detection of Microcystins with Aptamer-Driven Core-Satellite Assemblies.

We describe surface-enhanced Raman spectroscopy (SERS) aptasensors that can indirectly detect MC-LR and MC-RR, individually or simultaneously, in natural water and in algal culture. The sensor is constructed from nanoparticles composed of successive layers of Au core-SERS label-silver shell-gold shell (Au@label@Ag@Au NPs), functionalized on the outer Au surface by MC-LR and/or MC-RR aptamers. These NPs are immobilized on asymmetric Au nanoflowers (AuNFs) dispersed on planar silicon substrates through DNA hybridization of the aptamers and capture DNA sequences with which the AuNFs are functionalized, thereby forming core-satellite nanostructures on the substrates. This construction led to greater electromagnetic (EM) field enhancement of the Raman label-modified region, as supported by finite-difference time-domain (FDTD) simulations of the core-satellite assembly. In the presence of MC-LR and/or MC-RR, the aptamer-functionalized NPs dissociate from the AuNFs because of the stronger affinity of the aptamers with the MCs, which decreases the SERS signal, thus allowing indirect detection of the MCs. The improved SERS sensitivity significantly decreased the limit of detection (LOD) for separate MC-LR detection (0.8 pM) and for multiplex detection (1.5 pM for MC-LR and 1.3 pM for MC-RR), compared with other recently reported SERS-based methods for MC-LR detection. The aptasensors show excellent selectivity to MC-LR/MC-RR and excellent recoveries (96-105%). The use of these SERS aptasensors to monitor MC-LR production over 1 week in a culture medium of M. aeruginosa cells demonstrates the applicability of the sensors in a realistic environment.

(E)-4,4'-Bis(1,3-benzoxazol-2-yl)stilbene at 150 and 375 K.

The title compound, a chromophore of formula C(28)H(18)N(2)O(2), crystallizes with the molecule lying on an inversion centre to give one-half of a crystallographically independent molecule in the asymmetric unit. The molecule is almost planar, with slight deviation of the benzene rings from the mean molecular plane. The structure is characterized by a herringbone packing arrangement arising from C-H...pi and pi-pi intermolecular interactions. The benzoxazole group is disordered between two orientations, with occupancy factors of 0.669 (10) and 0.331 (10) at 150 K [0.712 (7) and 0.288 (7) at 375 K].

In Situ Growth of AuNPs on Glass Nanofibers for SERS Sensors.

It is challenging to fabricate plasmonic nanosensors on high-curvature surfaces with high sensitivity and reproducibility at low cost. Here, we report a facile and straightforward strategy, based on an in situ growth technique, for fabricating glass nanofibers covered by asymmetric gold nanoparticles (AuNPs) with tunable morphologies and adjustable spacings, leading to much improved surface-enhanced Raman scattering (SERS) sensitivity because of hotspots generated by the AuNP surface irregularities and adjacent AuNP coupling. First, nanosensors covered with uniform and well-dispersed citrate-capped spherical AuNPs were constructed using a polystyrene-b-poly(4-vinylpyridine) (PS-P4VP, with 33 mol % P4VP content and 61 kg/mol total molecular weight) block copolymer brush-layer templating method, and then, the deposited AuNPs were grown to asymmetric AuNPs. AuNP morphologies and hence the optical characteristics of AuNP-covered glass nanofibers were easily controlled by the choice of experimental parameters, such as the growth time and growth solution composition. In particular, tunable AuNP average diameters between about 40 and 80 nm with AuNP spacings between about 50 and 1 nm were achieved within 15 min of growth. The SERS sensitivity of branched AuNP-covered nanofibers (3 min growth time) was demonstrated to be more than threefold more intense than that of the original spherical AuNP-covered nanofibers using a 633 nm laser. Finite-difference time-domain simulations were performed, showing that the electric field enhancement is highest for intermediate AuNP diameters. Furthermore, SERS applications of these nanosensors for H2O2 detection and pH sensing were demonstrated, offering appealing and promising candidates for real-time monitoring of extra/intracellular species in vitro and in vivo.

Branched Au Nanoparticles on Nanofibers for Surface-Enhanced Raman Scattering Sensing of Intracellular pH and Extracellular pH Gradients.

The development of plasmonic-active nanosensors for surface-enhanced Raman scattering (SERS) sensing is important for gaining knowledge on intracellular and extracellular chemical processes, hypoxia detection, and label-free detection of neurotransmitters and metabolites, among other applications in cell biology. The fabrication of SERS nanosensors for optophysiology measurements using substrates such as nanofibers with a uniform distribution of plasmonic nanoparticles (NPs) remains a critical hurdle. We report here on a strategy using block copolymer brush-layer templating and ligand exchange for fabricating highly reproducible and stable SERS-active nanofibers with tip diameters down to 60 nm and covered with well-dispersed and uniformly distributed branched AuNPs, which have intrinsic hotspots favoring inherently high plasmonic sensitivity. Among the SERS sensors investigated, those with Au nanostars with short branches [AuNS(S)s] exhibit the greatest SERS sensitivity, as verified also by COMSOL Multiphysics simulations. Functionalization of the AuNS(S)s with the pH-sensitive molecule, 4-mercaptobenzoic acid, led to SERS nanosensors capable of quantifying pH over a linear range of 6.5-9.5, covering the physiological range. These pH nanosensors were shown to be able to detect the intracellular pH as well as extracellular pH gradients of in vitro breast cancer cells with minimal invasiveness and improved SERS sensitivity, along with a high spatial resolution capability.

Polymer-Templated Gold Nanoparticles on Optical Fibers for Enhanced-Sensitivity Localized Surface Plasmon Resonance Biosensors.

Dense arrays of well-dispersed gold nanoparticles (AuNPs) on optical fibers are shown to bridge the gap in sensitivity and sensing performance between localized surface plasmon resonance (LSPR) and classical SPR sensing. A simple self-assembly method relying on a poly(styrene- b-4-vinylpyridine) (PS- b-P4VP) block copolymer brush layer was used to immobilize AuNPs of different diameters from 10 to 92 nm on optical fibers. In comparison with standard AuNP deposition methods using (3-aminopropyl)trimethoxysilane (APTMS) and polyelectrolytes, the sensitivity with the PS- b-P4VP templating method was found to be 3-fold better, a consequence of the smaller gap between particles and the presence of fewer AuNP aggregates. Hence, the sensitivity of the LSPR sensor for IgG was comparable to a classical SPR, also on optical fibers, and about 68% of that for a prism-based wavelength-interrogation SPR instrument. The reproducibility and the detection limit of the LSPR sensor were about the same as the SPR sensor. The enhanced performance of the LSPR sensors using the PS- b-P4VP block copolymer fabrication method paves the way for use of these LSPR biosensors in a smaller and more cost-effective platform.

Block Copolymer Brush Layer-Templated Gold Nanoparticles on Nanofibers for Surface-Enhanced Raman Scattering Optophysiology.

A nanothin block copolymer (BCP) brush-layer film adsorbed on glass nanofibers is shown to address the long-standing challenge of forming a template for the deposition of dense and well-dispersed nanoparticles on highly curved surfaces, allowing the development of an improved nanosensor for neurotransmitters. We employed a polystyrene- block-poly(4-vinylpyridine) BCP and plasmonic gold nanoparticles (AuNPs) of 52 nm in diameter for the fabrication of the nanosensor on pulled fibers with diameters down to 200 nm. The method is simple, using only solution processes and a plasma cleaning step. The templating of the AuNPs on the nanofiber surprisingly gave rise to more than 1 order of magnitude improvement in the surface-enhanced Raman scattering (SERS) performance for 4-mercaptobenzoic acid compared to the same AuNPs aggregated on identical fibers without the use of a template. We hypothesize that a wavelength-scale lens formed by the nanofiber contributes to enhancing the SERS performance to the extent that it can melt the glass nanofiber under moderate laser power. We then show the capability of this nanosensor to detect the corelease of the neurotransmitters dopamine and glutamate from living mouse brain dopaminergic neurons with a sensitivity 1 order of magnitude greater than with aggregated AuNPs. The simplicity of fabrication and the far superior performance of the BCP-templated nanofiber demonstrates the potential of this method to efficiently pattern nanoparticles on highly curved surfaces and its application as molecular nanosensors for cell physiology.

In Situ Photocontrol of Block Copolymer Morphology During Dip-Coating of Thin Films.

We demonstrate a unique combination of simultaneous top-down and bottom-up control of the morphology of block copolymer films by application of in situ optical irradiation during dip-coating. A light-addressable and block-selective small molecule, 4-butyl-4'-hydroxyazobenzene (BHAB), is introduced into a diblock copolymer of polystyrene and poly(4-vinylpyridine) (PS-P4VP) of 28.4 wt % P4VP via supramolecular chemistry, notably by hydrogen bonding to P4VP. We show that the spherical morphology of thin films dip-coated from a THF solution at slow withdrawal rates in the dark convert to cylindrical morphology when dip-coated under illumination. This is attributed to volume expansion of the P4VP/BHAB phase due to trans-cis photoisomerization combined with a light-induced increase in BHAB uptake in the film. The demonstrated photocontrol highlights the potential of dip-coating as a scalable film preparation method that can be easily coupled with external stimuli to direct nanostructured self-assembly in the films as solvent evaporates.

Double-striped metallic patterns from PS-b-P4VP nanostrand templates.

A new nanometallic pattern, characterized by randomly disposed double or twin one-dimensional stripes and that adds to the nanotechnology toolbox, has been obtained from a unique template possessing the nanostrand morphology. This morphology had previously been shown to form in Langmuir-Blodgett films made from a polystyrene-poly(4-vinylpyridine) (PS-P4VP) diblock copolymer blended with 3-n-pentadecylphenol (PDP). The nanostrand backbone is composed of PS, and it is bordered along both sides by a P4VP monolayer, visualized for the first time by high resolution atomic force microscopy. The exposed P4VP alongside the nanostrands serves as sites for depositing compounds attracted selectively to P4VP. Here, both gold ions (HAuCl4·3H2O) and gold nanoparticles (AuNP, 12 nm in diameter, stabilized with sodium citrate) were complexed to the P4VP. Plasma treatment of the gold ions led to double stripes of monolayer metallic gold. To obtain dense deposition of AuNP in double rows, it was necessary to acidify the AuNP aqueous solution (pH 5.2 here). The achievement of the metallic double-stripe patterns also confirms the composition of the nanostrand morphology, which up to now had been deduced indirectly. The double-stripe pattern has possible applications for plasmonic lasers, energy transport, and biosensors.

Morphology, Thickness, and Composition Evolution in Supramolecular Block Copolymer Films over a Wide Range of Dip-Coating Rates.

Dip-coating, an important industrial technique, has been underexploited for preparing block copolymer (BC) thin films, such that the knowledge regarding their general characteristics is limited. Here, we present an overview of the crucial factors that determine how BC film morphology evolves as a function of dip-coating rate (withdrawal speed) over a wide range, illustrated using THF solutions of a polystyrene-b-poly(4-vinyl pyridine) (PS-P4VP) diblock copolymer mixed with two small molecules, naphthol and naphthoic acid, which are hydrogen-bonders with P4VP. Key factors in determining the film morphology are the systematic variation in film thickness and, for supramolecular BCs, in film composition with dip-coating rate. The former shows a general V-shaped dependence, related to the so-called capillarity and draining regimes identified previously for dip-coated sol-gel films. The relative small molecule content in the films studied is shown to increase in the capillarity regime from low to that of the dip-coating solution and thereafter to remain constant. Together, these changes, in addition to solvent and other effects, determine the film morphology and its evolution with dip-coating rate.

Photophysical, electrochemical and crystallographic investigations of the fluorophore 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene.

The photophysics of 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBT) were investigated for assessing its limitations for use as a universal fluorophore and as a viable sensor for both polymeric and solution studies. This is of importance given the limitations of currently used materials. BBT's steady-state and time-resolved fluorescence were additionally investigated to correlate its solid-state features, observed by fluorescence spectroscopy when mixed in poly(1,4-butylene succinate) (PBS) films, with its single crystal characteristics. The conjugated fluorophore was found to be highly fluorescent, with absolute quantum yields of (Φ(fl)) ≥ 0.60. The Φ(fl) values were high, regardless of solvent polarity and proticity and whether alone or in polymeric films. The major competitive fluorescence quenching pathway was found to occur by intersystem crossing to the triplet state. This was confirmed by laser flash photolysis in which the BBT triplet absorbed at 500 nm. The triplet transient was confirmed by quenching studies with 1,3-cyclohexadiene. Meanwhile, nonradiative deactivation of BBT's singlet excited state by internal conversion was found to be negligible. In solution and especially when distributed in semicrystalline PBS, BBT exhibits spectral changes and a bathochromic shift as a function of concentration due to aggregation of ground state molecules, which is present even at low BBT concentrations. Consistent monoexponential lifetimes on the order of ∼2 ns were observed regardless of solvent and independent of both the excitation wavelength and concentration. The constant excited state kinetics confirm the absence of a singlet excited state deactivation by excimer formation. The electrochemistry of BBT demonstrated that it is irreversibly oxidized and the resulting radical cation is unstable. Conversely, the cathodic process, resulting in the radical anion, is reversible, confirming its n-doping character. Crystallographic studies revealed that the planes described by the benzoxazolyl moieties are twisted from the plane described by the central thiophene. Several weak C-H···π and π-π intermolecular interactions were also observed. BBT's high solubility in common solvents combined with its measured enhanced optoelectronic properties make it a candidate as a universal fluorophore reference and smart material for both polymeric and solution studies.