Noteworthy Chemistry

September 2, 2013

 

Make a super-absorbent from magnesium carbonate. Magnesium, the eighth most abundant element in the Earth’s crust, is rarely found in its anhydrous carbonate form, MgCO3 (magnesite). Chemists have found that MgCO3 is difficult to produce, especially at low temperatures. J. Forsgren, A. Mihranyan, M. Strømme, and co-workers at Uppsala University (Sweden) however, not only developed a template-free, low-temperature protocol for synthesizing nano-sized MgCO3 (which they call Upsalite), but also set new records for surface area and water absorption in alkaline earth metal carbonates.

The authors pressurized a reaction vessel containing MgO and MeOH with CO2 (1–3 bar). The reactor was kept at 50 °C for 3 h and then cooled to room temperature for 4 days. Upsalite was obtained by air-drying the rigid gel (HOMgOCO2Me) in the vessel at 70 °C. The resulting anhydrous MgCO3 has a highly porous nanostructure, with <6-nm pore sizes. The detailed reaction sequence is shown in the figure.

Reaction sequence that produces Upsalite

Upsalite features an exceptionally large specific surface area (800 m2/g), much greater than those of common desiccants such as fumed silica (196 m2/g), hydromagnesite (38 m2/g), and zeolite Y (600 m2/g). Upsalite also has extraordinary water adsorption capability, comparable with or higher than that of hydrophilic zeolite Y when the relative humidity (RH) is <60%.

At room temperature, Upsalite retains >75% of its adsorbed water when the RH decreases from 95% to 5%. The robust structure of Upsalite remains intact upon hydration, and its anhydrous form can be regenerated by heating the hydrate to ≈100 °C. (PLoS ONE 2013, 8, No. e68486; Xin Su)

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Electrogenerated chemiluminescence detects peroxide explosives. The white solid triacetone triperoxide (TATP, at left in the figure) is one of the most sensitive explosives known—it can be set off by friction, heat, or impact. It was used in 2005 in the attack on the London public transportation system that killed 52 people and injured more than 700. Organic peroxide explosives, including TATP and hexamethylene triperoxide diamine (HMTD, at right in the figure), can be made from off-the-shelf ingredients, and they easily pass through common screening devices designed to detect nitrogen-based explosives.

Peroxide explosives TATP and HMTD

Several reliable methods exist for detecting peroxide-based explosives. Some, like MS, are very sensitive, but they are expensive and not well-suited to field testing. Ion mobility spectroscopy results are affected by temperature and moisture, and luminophore impurities hamper fluorescence testing. IR and UV–vis spectroscopy often require extensive sample preparation and are not sensitive to trace amounts of explosives.

S. Parajuli and W. Miao* of the University of Southern Mississippi (Hattiesburg) set out to develop a method for detecting trace amounts of organic peroxides that would be suitable for use at security checkpoints in mass-transit facilities and in forensic and environmental testing applications. They chose electrogenerated chemiluminescence (ECL) because of its selectivity, sensitivity, and rapid results.

The authors tested ECL's ability to detect and quantify TATP and to distinguish this compound from HMTD in a H2O–MeCN solution containing a phosphate buffer and Ru(bpy)32+ as an ECL emitter. TATP is sparingly soluble in water, but water is needed to produce hydroxyl radicals from the peroxide functional groups in TATP. MeCN is a better solvent, and it enhances the ECL intensity by stabilizing hydroxyl radicals in solution.

Cathodic potential scanning produces hydroxyl radicals. These oxidize electrogenerated Ru(bpy)3+ cations to form excited-state Ru(bpy)32+*, which quickly reverts to Ru(bpy)3+, accompanied by photon emission.

This method’s TATP detection limit (2.5 μM) is 3 times lower than that of UV−vis detection and 400 times lower than that attainable with LC-IR. Results can be obtained in as little as 5 min.

Oxygen-based bleaches commonly found in laundry detergents could leave trace amounts of H2O2 on a traveler's clothing, creating false positives for TATP. Pretreating samples with a catalase enzyme, followed by enzyme deactivation with NaN3, effectively suppresses ECL from this source.

TATP produces ECL upon cathodic potential scanning only, whereas HMTD produces ECL upon both cathodic and anodic potential scanning because of its tertiary amine functional groups. This difference can be used to distinguish between the two compounds. (Anal. Chem. 2013, 85, 8008–8015; Nancy McGuire)

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Use a two-vessel system to resolve conglomerates. J. E. Hein, R. M. Kellogg, and co-workers at the University of California, Merced, and Syncom BV (Groningen, The Netherlands) describe the resolution of a nonracemizable conglomerate salt that uses coupled preferential crystallization. This technique uses two vessels (flasks), one that contains enantiopure material and the other, racemic material.

The liquid phases from the two flasks are circulated through each other; a filter prevents any solid transfer. The flask containing racemic material is subjected to physical attrition; alternatively, the two flasks can be maintained at different temperatures. Over time, the flask containing the racemate becomes depleted in the enantiomer that is present in the other flask, so that at the end of the process each flask contains only one enantiomer.

The authors exemplify their technique by using the potassium salt of racemic esomeprazole diethanol solvate. (Esomeprazole [Nexium] is a widely used proton-pump inhibitor.) The method gives an 87% recovery of enantiopure (S)-esomeprazole, the desired enantiomer. The rate of enantio-enrichment depends on the liquid’s circulation rate between the two flasks. (Org. Process Res. Dev. 2013, 17, 946–950; Will Watson)

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Fluorogen nanoaggregates detect aluminum selectively and sensitively. Aluminum is the most abundant metal in the Earth’s crust and the most widely used nonferrous metal. Al3+ toxicity, however, is a major impediment to crop production in acidic soil. It is also linked to serious illnesses such as osteomalacia and Parkinson’s disease. Detecting Al3+ is therefore important for protecting human health and the environment.

Only a few fluorescent probes for Al3+ have been developed. X.-B. Zhang, Z.-J. Ran, and co-workers at Hunan University (Changsha, China) and China Agricultural University (Beijing) report a sensitive, selective fluorescent probe (1) for Al3+.

Al3+ probe

Probe 1 contains tetraphenylethylene and diethylenetriamine units; they act as fluorogen reporters and recognition ligands, respectively. Free 1 is nonfluorescent, but its emission is turned on by interacting with Al3+.

The probe is highly sensitive, with a detection limit as low as 0.5 μM and a dynamic range as wide as 2.0–11 μM. The probe is specific to Al3+ and resists interference by other metal ions. The authors verified that the sensing mechanism of 1 is associated with the formation of a 1–Al3+ complex and the resulting aggregation-induced emission. (Anal. Methods 2013, 5, 3909–3914; Ben Zhong Tang)

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Detect and quantify caffeine easily and visibly. Despite its presence in many beverages and medicines, caffeine detection and quantification rely largely on complicated processing and expensive instrumentation, including TLC, HPLC-MS, and immunoassays. Y.-K. Cho, Y.-T. Chang, and colleagues at the National University of Singapore; Ulsan National Institute of Science and Technology (Korea); and the Agency for Science, Technology and Research (Singapore) may have solved this problem by developing an automated boron-dipyrromethene (BODIPY) dye–based system for detecting and quantifying caffeine.

The authors screened diversity-oriented fluorescence libraries to design and synthesize a BODIPY dye (1) that they named Caffeine Orange (CO). The dye exhibits as much as 250-fold fluorescence enhancement when it binds with caffeine in aqueous solution. They found that CO binds with caffeine via hydrogen bonding and π–π interactions, with a dissociation constant of 50 μM. CO is highly selective toward caffeine in the presence of caffeine analogues such as theophylline and theobromine.

Caffeine Orange

The authors used CO-based microfluidics techniques to construct a lab-on-a-disc device for automated caffeine detection and quantification. Samples are extracted with a centrifuge before they are passed through a reverse-phase C4 column. Caffeine concentration is measured by an optical fiber–coupled spectrophotometer. The authors demonstrate that this device accurately quantifies the caffeine contents of coffee, espresso, and Red Bull energy drink. (Sci. Rep. 2013, 3, No. 2255; Xin Su)

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Run a continuous Heck coupling with a homogeneous catalyst. L. Peeva, J. Arbour, and A. Livingston* at Imperial College London describe the development of a continuous Heck coupling reaction. The one-pot reaction is run at 80 ºC in DMF solvent and uses an organic solvent nanofiltration membrane to retain the homogeneous catalyst, which is prepared in situ from Pd(OAc)2 and 1,3-bis(diphenylphosphino)propane (dppp).

Preliminary investigations involved testing the membranes and backing material (polypropylene) to determine whether they have any effect on the Heck reaction between PhI and methyl acrylate. Three membranes were studied: PBI (polybenzimidazole cross-linked with 1,4-dibromobutane), APTS (polyimide cross-linked with aminopropyltrimethyoxysilane), and PEEK [poly(ether ether ketone)]. The PEEK membrane preformed best, giving an 85% yield of methyl cinnamate over 1000 h with palladium contamination levels 20% lower than found in a conventional batch reaction. The other two membranes ruptured during testing. (Org. Process Res. Dev. 2013, 17, 967–975; Will Watson)

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