Fabrication of Crescent Shaped Microparticles for Particle Templated Droplet Formation.

Crescent-shaped hydrogel microparticles are shown to template uniform volume aqueous droplets upon simple mixing with aqueous and oil media for various bioassays. This emerging "lab on a particle" technique requires hydrogel particles with tunable material properties and dimensions. The crescent shape of the particles is attained by aqueous two-phase separation of polymers followed by photopolymerization of the curable precursor. In this work, the phase separation of poly(ethylene glycol) diacrylate (PEGDA, Mw 700) and dextran (Mw 40 000) for tunable manufacturing of crescent-shaped particles is investigated. The particles' morphology is precisely tuned by following a phase diagram, varying the UV intensity, and adjusting the flow rates of various streams. The fabricated particles with variable dimensions encapsulate uniform aqueous droplets upon mixing with an oil phase. The particles are fluorescently labeled with red and blue emitting dyes at variable concentrations to produce six color-coded particles. The blue fluorescent dye shows a moderate response to the pH change. The fluorescently labeled particles are able to tolerate an extremely acidic solution (pH 1) but disintegrate within an extremely basic solution (pH 14). The particle-templated droplets are able to effectively retain the disintegrating particle and the fluorescent signal at pH 14.

Ultrasensitive Picomolar Detection of Aqueous Acids in Microscale Fluorescent Droplets.

We report on a fluorescent-droplet-based acid-sensing scheme that allows limits of detection below 100 pM for weak acids. The concept is based on a strong partitioning of acid from an aqueous phase into octanol droplets. Using salicylic acid as a demonstration, we show that at a high concentration, the acid partitions into the organic phase by a factor of 260, which is approximately consistent with literature values. However, at lower concentrations, we obtain a partition coefficient as high as 106, which is partly responsible for the excellent sensing performance. The enhanced equilibrium partitioning is likely due to the interaction of the dissociated acid phase with the sensor dye employed for this work. The effect of droplet size was determined, after which we derived a simple model to predict the time dependence of the color change as a function of droplet size. This work shows that color-change fluorescent-droplet-based detection is a promising avenue that can lead to exceptional sensing performance from an aqueous analyte.

A Nanometric Probe of the Local Proton Concentration in Microtubule-Based Biophysical Systems.

We show a double-functional fluorescence sensing paradigm that can retrieve nanometric pH information on biological structures. We use this method to measure the extent of protonic condensation around microtubules, which are protein polymers that play many roles crucial to cell function. While microtubules are believed to have a profound impact on the local cytoplasmic pH, this has been hard to show experimentally due to the limitations of conventional sensing techniques. We show that subtle changes in the local electrochemical surroundings cause a double-functional sensor to transform its spectrum, thus allowing a direct measurement of the protonic concentration at the microtubule surface. Microtubules concentrate protons by as much as one unit on the pH scale, indicating a charge storage role within the cell via the localized ionic condensation. These results confirm the bioelectrical significance of microtubules and reveal a sensing concept that can deliver localized biochemical information on intracellular structures.

An Ultrasensitive Fluorescent Paper-Based CO2 Sensor.

We demonstrate a versatile and easily fabricated paper-based CO2 sensor. The sensor consists of a specially designed fluorescent color-shift chromophore infused into standard filter paper. The emission color of the resulting fluorescent paper changes upon exposure to CO2 due to the formation of carbonic acid, which underlies the sensing mechanism. By using a ratiometric method, the undesirable effects of photobleaching can be eliminated, leading to a stable and repeatable sensor performance. These multiuse sensors have a response time on the order of 1 min and feature low detection limits for a paper-based CO2 gas sensor, suggesting possible low-cost applications in smart buildings or other facilities in which CO2 levels are required to be continuously monitored.

Studies on the Biocompatibility of Poly(diethyl vinyl-phosphonate) with a New Fluorescent Marker.

Utilization of group transfer polymerization for the synthesis of poly(diethyl vinylphosphonate) (PDEVP) allows its controlled end-group functionalization. Thus, a new fluorescent chromophore/PDEVP conjugate is prepared and subjected to biocompatibility tests on two different human cell lines. In contrast to the previous studies, the tagged polymer is not absorbed by cells from the solution and has nearly no impact on cell mortality rate.

Gated Channels and Selectivity Tuning of CO2 over N2 Sorption by Post-Synthetic Modification of a UiO-66-Type Metal-Organic Framework.

The highly porous and stable metal-organic framework (MOF) UiO-66 was altered using post-synthetic modifications (PSMs). Prefunctionalization allowed the introduction of carbon double bonds into the framework through a four-step synthesis from 2-bromo-1,4-benzenedicarboxylic acid; the organic linker 2-allyl-1,4-benzenedicarboxylic acid was obtained. The corresponding functionalized MOF (UiO-66-allyl) served as a platform for further PSMs. From UiO-66-allyl, epoxy, dibromide, thioether, diamine, and amino alcohol functionalities were synthesized. The abilities of these compounds to adsorb CO2 and N2 were compared, which revealed the structure-selectivity correlations. All synthesized MOFs showed profound thermal stability together with an increased ability for selective CO2 uptake and molecular gate functionalities at low temperatures.

Mechanistic Investigations of the Stereoselective Rare Earth Metal-Mediated Ring-Opening Polymerization of ß-Butyrolactone.

Poly(3-hydroxybutyrate) (PHB) is produced by numerous bacteria as carbon and energy reserve storage material. Whereas nature only produces PHB in its strictly isotactic (R) form, homogeneous catalysis, when starting from racemic (rac) ß-butyrolactone (BL) as monomer, can in fact produce a wide variety of tacticities. The variation of the metal center and the surrounding ligand structure enable activity as well as tacticity tuning. However, no homogeneous catalyst exists to date that is easy to modify, highly active, and able to produce PHB with high isotacticities from rac-ß-BL. Therefore, in this work, the reaction kinetics of various 2-methoxyethylamino-bis(phenolate) lanthanide (Ln=Sm, Tb, Y, Lu) catalysts are examined in detail. The order in monomer and catalyst are determined to elucidate the reaction mechanism and the results are correlated with DFT calculations of the catalytic cycle. Furthermore, the enthalpies and entropies of the rate-determining steps are determined through temperature-dependent in situ IR measurements. Experimental and computational results converge in one specific mechanism for the ring-opening polymerization of BL and even allow us to rationalize the preference for syndiotactic PHB.

Concerning the deactivation of cobalt(III)-based porphyrin and salen catalysts in epoxide/CO2 copolymerization.

Functioning as active catalysts for propylene oxide (PO) and carbon dioxide copolymerization, cobalt(III)-based salen and porphyrin complexes have drawn great attention owing to their readily modifiable nature and promising catalytic behavior, such as high selectivity for the copolymer formation and good regioselectivity with respect to the polymer microstructure. Both cobalt(III)-salen and porphyrin catalysts have been found to undergo reduction reactions to their corresponding catalytically inactive cobalt(II) species in the presence of propylene oxide, as evidenced by UV/Vis and NMR spectroscopies and X-ray crystallography (for cobalt(II)-salen). Further investigations on a TPPCoCl (TPP = tetraphenylporphyrin) and NaOMe system reveal that such a catalyst reduction is attributed to the presence of alkoxide anions. Kinetic studies of the redox reaction of TPPCoCl with NaOMe suggests a pseudo-first order in cobalt(III)-porphyrin. The addition of a co-catalyst, namely bis(triphenylphosphine)iminium chloride (PPNCl), into the reaction system of cobalt(III)-salen/porphyrin and PO shows no direct stabilizing effect. However, the results of PO/CO2 copolymerization by cobalt(III)-salen/porphyrin with PPNCl suggest a suppressed catalyst reduction. This phenomenon is explained by a rapid transformation of the alkoxide into the carbonate chain end in the course of the polymer formation, greatly shortening the lifetime of the autoreducible PO-ring-opening intermediates, cobalt(III)-salen/porphyrin alkoxides.

Regarding initial ring opening of propylene oxide in its copolymerization with CO2 catalyzed by a cobalt(III) porphyrin complex.

Cobalt(III) tetraphenylporphyrin chloride (TPPCoCl) was experimentally proved to be an active catalyst for poly(propylene carbonate) production. It was chosen as a model catalyst in the present work to investigate the initiation step of propylene oxide (PO)/CO2 copolymerization, which is supposed to be the ring opening of the epoxide. Ring-opening intermediates (1-7) were detected by using (1) H NMR spectroscopy. A first-order reaction in TPPCoCl was determined. A combination of monometallic and bimetallic ring-opening pathways is proposed according to kinetics experiments. Addition of onium salts (e.g., bis(triphenylphosphine)iminium chloride, PPNCl) efficiently promoted the PO ring-opening rate. The existence of axial ligand exchange in the cobalt porphyrin complex in the presence of onium salts was suggested by analyzing collected (1) H NMR spectra.

Functionalization of metal-organic frameworks through the postsynthetic transformation of olefin side groups.

For the first time, the adaptability of the C=C double bond as a versatile precursor for the postsynthetic modification (PSM) of microporous materials was extensively investigated and evaluated. Therefore, an olefin-tagged 4,4'-bipyridine linker was synthesized and successfully introduced as pillar linker within a 9,10-triptycenedicarboxylate (TDC) zinc paddle-wheel metal-organic framework (MOF) through microwave-assisted synthesis. Different reactions, predominately used in organic chemistry, were tested, leading to the development of new postsynthetic reactions for the functionalization of solid materials. The postsynthetic oxidation of the olefin side groups applying osmium tetroxide (OsO4) as a catalyst led to the formation of a microporous material with free vicinal diol functionalities. The epoxidation with dimethyldioxirane (DMDO) enabled the synthesis of epoxy-functionalized MOFs. In addition to that, reaction procedures for a postsynthetic hydroboration with borane dimethyl sulfide as well as a photoinduced thiol-ene click reaction with ethyl mercaptan were developed. For all of these PSMs, yields of more than 90% were obtained, entirely maintaining the crystallinity of the MOFs. Since the direct introduction of the corresponding groups by means of pre-synthetic approaches is hardly possible, these new PSMs are useful tools for the functionalization of porous solids towards applications such as selective adsorption, separation, and catalysis.

Tandem post-synthetic modification for functionalized metal-organic frameworks via epoxidation and subsequent epoxide ring-opening.

A sequence of two new post-synthetic modifications, epoxidation using dimethyldioxirane and subsequent epoxide ring-opening, was developed, maintaining the MOF network. A kinetic study shows that the functionalization of metal-organic frameworks with synthetically versatile and reactive oxirane groups is even possible for small pore diameters.

Conformationally flexible dimeric salphen complexes for bifunctional catalysis.

Appropriate modification of the salphen ligand allows an easy modular design of flexibly linked dimeric salphen species and their complexes, which can act as bifunctional catalysts. A series of chromium salphen systems including monomeric compound and dimers with different spacer lengths were tested for their catalytic performance in ß-butyrolactone polymerization and CO(2)/propylene oxide copolymerization toward biodegradable materials. The results clearly show an enhancement in activity upon dimerization, thus underlining the role of bifunctional catalysis in the studied processes and extending the possible strategies for improvement of catalysts in these reactions.

Synthesis and properties of (triptycenedicarboxylatio)zinc coordination networks.

(Triptycenedicarboxylato)zinc metal-organic frameworks (MOFs) based on paddle wheel secondary building units (SBUs) with different axial ligands have been prepared. The reproducible formation of the layered paddle-wheel structures from triptycenedicarboxylic acid (H(2)TDC) and zinc nitrate under various conditions seems to be characteristic of this acid and is utilized for the construction of 3D frameworks by a pillaring approach. We attempted to bring additional functionalities into MOFs by employing the appropriate pillaring ligands, for example, bis(4-pyridyl)-s-tetrazine and bis(4-pyridyl)-dimethoxy-p-phenylenedivinylene, and investigated certain properties of some MOF materials, such as guest-exchange behavior, luminescence, microporosity, and stability.

Reactivity of dehydrometallophthalocyanines and -porphyrazines.

The zinc dehydrophthalocyanine 2 and zinc dehydrobenzoporhyrazine 8 a were generated from the 1N-aminobenzotriazole-annulated zinc phtalocyanine 1 and zinc benzoporhyrazine 8, respectively, by oxidation with Pb(OAc)(4) in different solvents, for example, diethyl ether, tetrahydrofuran, acetic acid, and benzene. The reactivity of 2 and 8 a was studied in detail. These species not only easily undergo Diels-Alder additions with dienes, but also the used solvents can be added. Among the addition products with solvents ethoxy-, acetoxy-, acetoxybutyloxy-substituted and barrelen-fused phthalocyanines and benzoporpyrazines were isolated. No products resulting from the dimerization of two dehydro species were observed either for 2 or 8 a. Analysis of the reaction products in comparison with those obtained by oxidation of 1-aminobenzotriazole 15 under similar conditions proves a higher reactivity (electrophilicity) of the dehydro-PcZn 2 and dehydro-PzZn 8 a in comparison with unsubstituted benzyne towards the solvents used, such as diethyl ether and benzene.

First generation and trapping of a dehydrometallophthalocyanine starting from triazole-functionalized zinc phthalocyanine.

Direct 1N-amination of the triazole-fused zinc phthalocyanine 2 and oxidation of the formed amino derivative 3 resulted in the generation of the very reactive intermediate, the dehydrometallophthalocyanine 4, which was not known previously. The latter was trapped in situ with different dienes, for example, furan, tetraphenylcyclopentadienone, and anthracene to form the corresponding Diels-Alder adducts. The products were characterized by 1H and 13C-dept135 NMR, and UV/Vis spectroscopy, MALDI-TOF mass spectrometry, and elemental analysis, which are fully in agreement with their structure. The developed synthetic procedure opens a simple and versatile pathway towards unsymmetrical peripheral modification of phthalocyanines, which is readily applicable to the micromol scale and is important for the design of new interesting Pc-based systems.