Lithium sensors based on photophysical changes of 1-aza-12-crown-4 naphthalene derivatives synthesized via Buchwald-Hartwig amination.

Lithium detection is of great significance in many applications. Lithium-sensing compounds with high selectivity are scarce and, if any, complicated to synthesize. We herein report a novel yet simple compound that can detect lithium ions in an organic solvent through changes in absorbance and fluorescence. Naphthalene functionalized with 1-aza-12-crown-4 (1) was synthesized via one step from commercially available 1-bromonaphthalene through Buchwald-Hartwig amination. In order to obtain a structure-property relationship, we also synthesized two other compounds that are structurally similar to 1, wherein the compounds 2 and 3 include an imide moiety (an electron acceptor) and do not include a 1-aza-12-crown-4 unit, respectively. Upon the addition of lithium ions, compound 1 displayed a clear isosbestic point in the absorption spectra and a new peak in the fluorescence spectra, whereas the compounds 2 and 3 indicated miniscule and no spectroscopic changes, respectively. 1H NMR titration studies and the calculated optimized geometry from density functional theory (DFT) indicated the lithium binding on the aza-crown. The calculated limit of detection (LOD) was 21 µM. The lithium detection with 1 is selective among other alkali metals (Na+, K+, and Cs+). DFT calculation indicated that the lone pair electrons in the nitrogen atom of 1 is delocalized yet available to bind lithium, whereas the nitrogen lone pair electrons of 2 showed significant intramolecular charge transfer to the imide acceptor, resulting in a high dipole moment, and thus were unavailable to bind lithium. This work elucidates the key design parameters for future lithium sensors.

A Water-Soluble Organic Photocatalyst Discovered for Highly Efficient Additive-Free Visible-Light-Driven Grafting of Polymers from Proteins at Ambient and Aqueous Environments.

Since the pioneering discovery of a protein bound to poly(ethylene glycol), the utility of protein-polymer conjugates (PPCs) is rapidly expanding to currently emerging applications. Photoinduced energy/electron-transfer reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization is a very promising method to prepare structurally well-defined PPCs, as it eliminates high-cost and time-consuming deoxygenation processes due to its oxygen tolerance. However, the oxygen-tolerance behavior of PET-RAFT polymerization is not well-investigated in aqueous environments, and thereby the preparation of PPCs using PET-RAFT polymerization needs a substantial amount of sacrificial reducing agents or inert-gas purging processes. Herein a novel water-soluble and biocompatible organic photocatalyst (PC) is reported, which enables visible-light-driven additive-free "grafting-from" polymerizations of a protein in ambient and aqueous environments. Interestingly, the developed PC shows unconventional "oxygen-acceleration" behavior for a variety of acrylic and acrylamide monomers in aqueous conditions without any additives, which are apparently distinct from previously reported systems. With such a PC, "grafting-from" polymerizations are successfully performed from protein in ambient buffer conditions under green light-emitting diode (LED) irradiation, which result in various PPCs that have neutral, anionic, cationic, and zwitterionic polyacrylates, and polyacrylamides. It is believed that this PC will be widely employed for a variety of photocatalysis processes in aqueous environments, including the living cell system.

Site-Selective Protein Immobilization on Polymeric Supports through N-Terminal Imidazolidinone Formation.

Protein immobilization techniques on polymeric supports have enabled many applications in biotechnology and materials science. Attaching the proteins with controlled orientations has inherent advantages, but approaches for doing this have been largely limited to cysteine or noncanonical amino acid targeting. Herein, we report a method to attach the N-terminal positions of native proteins to polymer resins site-specifically through the use of 2-pyridinecarboxyaldehyde (2PCA) derivatives. For high protein loadings and practical synthesis, we initiated this work by preparing highly reactive 2PCA derivatives using Pd-catalyzed cross-coupling amination. The resulting compounds were attached to amine-containing polyethylene glycol acrylamide resin (PEGA-NH2), which subsequently reacted with the N-termini of proteins to produce linkages that were stable over the long term but could be reversed through the addition of hydroxylamine. We envision that this site-selective, 2PCA-based protein immobilization can provide a simple and generalizable immobilization protocol.

Distinct Interfacial Fluorescence in Oil-in-Water Emulsions via Exciton Migration of Conjugated Polymers.

Commercial dyes are extensively utilized to stain specific phases for the visualization applications in emulsions and bioimaging. In general, dyes emit only one specific fluorescence signal and thus, in order to stain various phases and/or interfaces, one needs to incorporate multiple dyes and carefully consider their compatibility to avoid undesirable interactions with each other and with the components in the system. Herein, surfactant-type, perylene-endcapped fluorescent conjugated polymers that exhibit two different emissions are reported, which are cyan in water and red at oil-water interfaces. The interfacially distinct red emission results from enhanced exciton migration from the higher-bandgap polymer backbone to the lower-bandgap perylene endgroup. The confocal microscopy images exhibit the localized red emission exclusively from the circumference of oil droplets. This exciton migration and dual fluorescence of the polymers in different physical environments can provide a new concept of visualization methods in many amphiphilic colloidal systems and bioimaging.

Interfacial Pressure/Area Sensing: Dual-Fluorescence of Amphiphilic Conjugated Polymers at Water Interfaces.

Exciton migration to emissive defects in π-conjugated polymers is a robust signal amplification strategy for optoelectronic sensors. Herein we report end-capped conjugated polymers that show two distinct emissions as a function of interpolymer distances at the air-water and hydrocarbon-water interfaces. Amphiphilic poly(phenylene ethynylene)s (PPEs) end-capped with perylene monoimides display two distinct emission colors (cyan from PPE and red from perylene), the relative intensity of which depends on the surface pressure applied on the Langmuir monolayers. This behavior produces a ratiometric interfacial pressure indicator. Relative quantum yields are maintained at the different surface pressures and hence display no sign of self-quenching of the excitons in an aggregated state. These polymers can be organized at the micelle-water interface in lytropic liquid crystals, thereby paving the way for potential applications of end-capped amphiphilic conjugated polymers in biosensors and bioimaging.

Highly Emissive Excimers by 2D Compression of Conjugated Polymers.

Interactions between π-conjugated polymers are known to create ground-state aggregates, excimers, and exciplexes. With few exceptions, these species exhibit decreased fluorescence quantum yields relative to the isolated polymers in liquid or solid solutions. Herein, we report a method to assemble emissive conjugated polymer excimers and demonstrate their applicability in the detection of selected solvent vapors. Specifically, poly(phenylene ethynylene)s (PPEs) with amphiphilic side chains are organized in a Langmuir monolayer at the air-water interface. Compression of the monolayer results in the reversible conversion from a face-on organization of the π-system relative to the water to what appears to be an incline-stack conformation. The incline-stack organization creates a bright yellow emissive excimeric state with increases of 28% in relative fluorescence quantum yields to the face-on monolayer conformation. Multilayers can be transferred onto the glass substrate via a Langmuir-Blodgett method with preservation of the excimer emission. These films are metastable and the fluorescence reverts to a cyan color similar to the spectra obtained in solution and spin-cast films after exposure to selected solvent vapors. This behavior has practical utility as a fluorescence-based indicator for selected volatile organic compounds.

Functionalized Poly(3-hexylthiophene)s via Lithium-Bromine Exchange.

Poly(3-hexylthiophene) (P3HT) is one of the most extensively investigated conjugated polymers and has been employed as the active material in many devices including field-effect transistors, organic photovoltaics and sensors. As a result, methods to further tune the properties of P3HT are desirable for specific applications. Herein, we report a facile postpolymerization modification strategy to functionalize the 4-position of commercially available P3HT in two simple steps-bromination of the 4-position of P3HT (Br-P3HT) followed by lithium-bromine exchange and quenching with an electrophile. We achieved near quantitative lithium-bromine exchange with Br-P3HT, which requires over 100 thienyl lithiates to be present on a single polymer chain. The lithiated-P3HT is readily combined with functional electrophiles, resulting in P3HT derivatives with ketones, secondary alcohols, trimethylsilyl (TMS) group, fluorine, or an azide at the 4-position. We demonstrated that the azide-modified P3HT could undergo Cu-catalyzed or Cu-free click chemistry, significantly expanding the complexity of the structures that can be appended to P3HT using this method.

A new hematite photoanode doping strategy for solar water splitting: oxygen vacancy generation.

The enhancement of the electrical conductivity by doping is important in hematite (α-Fe(2)O(3)) photoanodes for efficient solar water oxidation. However, in spite of many successful demonstrations using extrinsic dopants, such as Sn, Ti, and Si, the achieved photocurrent is still lower than the practical requirement. There is still lack of our understanding of how intrinsic oxygen defects can change the photocurrent and interact with the extrinsic dopants. In this study, we systematically investigate the interplay of oxygen vacancies and extrinsic Sn dopants in the context of photoanodic properties. As a result, we demonstrate that the controlled generation of oxygen vacancies can activate the photoactivity of pure hematite remarkably and further enhance the Sn doping effects synergistically. Furthermore, the correlated behavior of oxygen vacancies and Sn dopants is closely linked to the variation of electrical conductance and results in the optimum concentration region to show the high photocurrent and low onset voltage.