Screening the Surface Structure-Dependent Action of a Benzotriazole Derivative on Copper Electrochemistry in a Triple-Phase Nanoscale Environment.

Copper (Cu) corrosion is a compelling problem in the automotive sector and in oil refinery and transport, where it is mainly caused by the action of acidic aqueous droplets dispersed in an oil phase. Corrosion inhibitors, such as benzotriazole (BTAH) and its derivatives, are widely used to limit such corrosion processes. The efficacy of corrosion inhibitors is expected to be dependent on the surface crystallography of metals exposed to the corrosion environment. Yet, studies of the effect of additives at the local level of the surface crystallographic structure of polycrystalline metals are challenging, particularly lacking for the triple-phase corrosion problem (metal/aqueous/oil). To address this issue, scanning electrochemical cell microscopy (SECCM), is used in an acidic nanodroplet meniscus|oil layer|polycrystalline Cu configuration to explore the grain-dependent influence of an oil soluble BTAH derivative (BTA-R) on Cu electrochemistry within the confines of a local aqueous nanoprobe. Electrochemical maps, collected in the voltammetric mode at an array of >1000 points across the Cu surface, reveal both cathodic (mainly the oxygen reduction reaction) and anodic (Cu electrooxidation) processes, of relevance to corrosion, as a function of the local crystallographic structure, deduced with co-located electron backscatter diffraction (EBSD). BTA-R is active on the whole spectrum of crystallographic orientations analyzed, but there is a complex grain-dependent action, distinct for oxygen reduction and Cu oxidation. The methodology pinpoints the surface structural motifs that facilitate corrosion-related processes and where BTA-R works most efficiently. Combined SECCM-EBSD provides a detailed screen of a spectrum of surface sites, and the results should inform future modeling studies, ultimately contributing to a better inhibitor design.

Nanoscale electrochemistry in a copper/aqueous/oil three-phase system: surface structure-activity-corrosion potential relationships.

Practically important metal electrodes are usually polycrystalline, comprising surface grains of many different crystallographic orientations, as well as grain boundaries. In this study, scanning electrochemical cell microscopy (SECCM) is applied in tandem with co-located electron backscattered diffraction (EBSD) to give a holistic view of the relationship between the surface structure and the electrochemical activity and corrosion susceptibility of polycrystalline Cu. An unusual aqueous nanodroplet/oil (dodecane)/metal three-phase configuration is employed, which opens up new prospects for fundamental studies of multiphase electrochemical systems, and mimics the environment of corrosion in certain industrial and automotive applications. In this configuration, the nanodroplet formed at the end of the SECCM probe (nanopipette) is surrounded by dodecane, which acts as a reservoir for oil-soluble species (e.g., O2) and can give rise to enhanced flux(es) across the immiscible liquid-liquid interface, as shown by finite element method (FEM) simulations. This unique three-phase configuration is used to fingerprint nanoscale corrosion in a nanodroplet cell, and to analyse the interrelationship between the Cu oxidation, Cu2+ deposition and oxygen reduction reaction (ORR) processes, together with nanoscale open circuit (corrosion) potential, in a grain-by-grain manner. Complex patterns of surface reactivity highlight the important role of grains of high-index orientation and microscopic surface defects (e.g., microscratches) in modulating the corrosion-properties of polycrystalline Cu. This work provides a roadmap for in-depth surface structure-function studies in (electro)materials science and highlights how small variations in surface structure (e.g., crystallographic orientation) can give rise to large differences in nanoscale reactivity.

Fluorescent Block Copolymer Micelles That Can Self-Report on Their Assembly and Small Molecule Encapsulation.

Block copolymer micelles have been prepared with a dithiomaleimide (DTM) fluorophore located in either the core or shell. Poly(triethylene glycol acrylate)-b-poly(tert-butyl acrylate) (P(TEGA)-b-P(tBA)) was synthesized by RAFT polymerization, with a DTM-functional acrylate monomer copolymerized into either the core forming P(tBA) block or the shell forming P(TEGA) block. Self-assembly by direct dissolution afforded spherical micelles with Rh of ca. 35 nm. Core-labeled micelles (CLMs) displayed bright emission (Φf = 17%) due to good protection of the fluorophore, whereas shell-labeled micelles (SLMs) had lower efficiency emission due to collisional quenching in the solvated corona. The transition from micelles to polymer unimers upon dilution could be detected by measuring the emission intensity of the solutions. For the core-labeled micelles, the fluorescence lifetime was also responsive to the supramolecular state, the lifetime being significantly longer for the micelles (τAv,I = 19 ns) than for the polymer unimers (τAv,I = 9 ns). The core-labeled micelles could also self-report on the presence of a fluorescent hydrophobic guest molecule (Nile Red) as a result of Förster resonance energy transfer (FRET) between the DTM fluorophore and the guest. The sensitivity of the DTM fluorophore to its environment therefore provides a simple handle to obtain detailed structural information for the labeled polymer micelles. A case will also be made for the application superiority of core-labeled micelles over shell-labeled micelles for the DTM fluorophore.

Aminomaleimide fluorophores: a simple functional group with bright, solvent dependent emission.

Amino-substituted maleimides form a new class of highly emissive compounds, with large Stokes shifts (>100 nm) and high quantum yields (up to ∼60%). Emission is responsive to the maleimide's environment with both a red-shift, and quenching, observed in protic polar solvents. Aminomaleimides are easily functionalised, providing a versatile fluorescent probe.

Effect of Micellization on the Thermoresponsive Behavior of Polymeric Assemblies.

The chain density of polymer micelles, dictated by their aggregation number (Nagg), is an often overlooked parameter that governs the macroscopic behavior of responsive assemblies. Using a combination of variable-temperature light scattering, turbidimetry, and microcalorimetry experiments, the cloud point and thermal collapse of micellar poly(N-isopropylacrylamide) (pNIPAM) corona chains at lower temperatures than the cloud point were found to be largely independent of the micelle's Nagg. By controlling the core composition, the degree of hysteresis associated with the thermal transition was found to increase as a function of core hydrophobicity. We performed this study on well-characterized micelles with tunable Nagg values, composed of a thermoresponsive corona (pNIPAM) and a nonresponsive core block poly(n-butyl acrylate-co-N,N-dimethylacrylamide) (p(nBA-co-DMA)), which were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. This allowed for a distinction to be made between thermoresponsive behavior at both the molecular and macroscopic level. The study of the subtle differences between these behaviors was made possible using a combination of complementary techniques. These results highlight the critical need for consideration of the effect that self-assembly plays on the responsive behavior of polymer chains when compared with free unimers in solution.

Dual effect of thiol addition on fluorescent polymeric micelles: ON-to-OFF emissive switch and morphology transition.

The morphology transition from micelles to vesicles of a solution-state self-assembled block copolymer, containing a fluorescent dye at the core-shell interface, has been induced by an addition-elimination reaction using a thiol, and has been shown to be coupled to a simultaneous ON-to-OFF switch in particle fluorescence.

The analysis of solution self-assembled polymeric nanomaterials.

There has been much interest in the construction of soft nanomaterials in solution due to a desire to emulate the exquisite structure and function of Nature's equivalents (e.g. enzymes, viruses, proteins and DNA). Nature's soft nanomaterials are capable of selectivity, precision and efficiency in areas such as information storage and replication, transportation and delivery, and synthesis and catalysis. To this end, the use of small molecules, amphiphiles, colloids, and polymers have been investigated for the development of advanced materials in myriad fields of biomedicine and synthetic chemistry. Two major challenges are faced in this area of research: the reproducible, scalable and precise synthesis of such constructs and the reliable, accurate and in-depth analysis of these materials. This tutorial review will focus on this second aspect and provide a guide for the characterisation and analysis of soft nanomaterials in solution using scattering and microscopic techniques.

New functional handle for use as a self-reporting contrast and delivery agent in nanomedicine.

The synthesis and photophysical characterization of a chromophore-bridged block copolymer system is presented. This system is based on a dithiomaleimide (DTM) functional group as a highly emissive functionality which can readily be incorporated into polymeric scaffolds. A key advantage of this new reporter group is its versatile chemistry, ease of further functionalization, and notably small size, which allows for ready incorporation without affecting or disrupting the self-assembly process critical to the formation of core-shell polymeric contrast and drug delivery agents. We demonstrate the potential of this functionality with a diblock system which has been shown to be appropriate for micellization and, when in the micellar state, does not self-quench. The block copolymer is shown to be significantly more emissive than the lone dye, with a concentration-independent emission and anisotropy profile from 1.5 mM to 0.15 µM. An emission lifetime and anisotropy decay comparison of the block copolymer to its micelle displays that time-domain fluorescence lifetime imaging (FLIM) is able to rapidly resolve differences in the supramolecular state of this block-dye-block polymer system. Furthermore, the ability to resolve these differences in the supramolecular state means that the DTM micelles are capable of self-reporting when disassembly occurs, simply by monitoring with FLIM. We demonstrate the great potential for in vitro applications that this system provides by using FLIM to observe micelle disassembly in different vascular components of rat hippocampal tissue. In total this system represents a new class of in-chain emitter which is appropriate for application in quantitative imaging and the tracking of particle degradation/disassembly events in biological environments.

Conjugation-induced fluorescent labeling of proteins and polymers using dithiomaleimides.

Dithiomaleimides (DTMs) with alkyl substituents are shown to be a novel class of highly emissive fluorophores. Variable solubility and further functionalization can easily be tailored through the choice of N and S substituents. Inclusion of a DTM unit into a ROP/RAFT initiator or insertion into the disulfide bond of salmon calcitonin (sCT) demonstrates the utility for fluorescent labeling of polymers and proteins. Simultaneous PEGylation and fluorescent labeling of sCT is also demonstrated, using the DTM unit as both a linker and a fluorophore. It is anticipated that DTMs will offer an attractive alternative to commonly used bulky, planar fluorophores.

Hollow block copolymer nanoparticles through a spontaneous one-step structural reorganization.

The spontaneous one-step synthesis of hollow nanocages and nanotubes from spherical and cylindrical micelles based on poly(acrylic acid)-b-polylactide (P(AA)-b-P(LA)) block copolymers (BCPs) has been achieved. This structural reorganization, which occurs simply upon drying of the samples, was elucidated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). We show that it was necessary to use stain-free imaging to examine these nanoscale assemblies, as the hollow nature of the particles was obscured by application of a heavy metal stain. Additionally, the internal topology of the P(AA)-b-P(LA) particles could be tuned by manipulating the drying conditions to give solid or compartmentalized structures. Upon resuspension, these reorganized nanoparticles retain their hollow structure and display significantly enhanced loading of a hydrophobic dye compared to the original solid cylinders.

Dibromomaleimide End Functional Polymers by RAFT Polymerization Without the Need of Protecting Groups.

Polymers bearing the dibromomaleimide (DBM) group as a functional chain end have been synthesized by RAFT polymerization. A DBM functional chain transfer agent (CTA) was utilized to afford well-defined PtBA, PMA, and PTEGA, without the requirement of protecting group chemistry. It was found that RAFT polymerization of NIPAM and styrene with this CTA was severely retarded/inhibited which is ascribed to their relatively low propagation rate constants compared to acrylates. This observation is accounted for by a reversible trapping of propagating radicals by the DBM group in RAFT polymerizations using a monomer with low kp. However, further attempts to synthesize DBM-terminated PtBA and PMA by ATRP using an analogous initiator were unsuccessful, and broad PDI were observed. Furthermore, highly efficient postpolymerization functionalization of DBM-terminated PMA produced by RAFT, with the model compound thiophenol was also demonstrated.