Noteworthy Chemistry

June 23, 2014

Noteworthy Chemistry is on vacation. Here are some significant items from the past year. 

Make “green” silver nanoparticles with orange-peel extract. 
Silver nanoparticles (AgNPs) with <30 nm diam have drastically enhanced properties over bulk silver and therefore are being used more frequently in consumer products. There is increasing evidence, however, that AgNPs have toxic effects on humans and animals, especially laboratory animals used to mimic bio- and neurochemistry functions in humans. Research also shows that AgNPs are harmful in aquatic environments.

It is difficult to analyze the toxicity of AgNPs because the synthesis processes require harsh or toxic chemicals, which are difficult to handle safely and may confound the sensitivity testing. Most AgNPs are shipped as dry powders that are handled differently by researchers.

J. E. Owens and coauthors at the University of Colorado (Colorado Springs) and Colorado State University (Ft. Collins) developed a safe, cost- and time-efficient method to synthesize AgNPs that uses green chemistry methods. In 2011, S. Kaviya et al. reported the use of navel orange–peel extract to synthesize AgNPs from AgNO3(Spectrochim. Acta, Part A 2011, 79, 594–598). The nanoparticles were capped by compounds found in the orange-peel extract. Numerous authors have tried other plant-based extracts, including mushrooms, tea leaves, aloe vera, and more.

Owens and her coauthors hypothesized that any citrus peel could be used to synthesize and cap highly dispersed AgNPs, so they tested extracts of navel orange, ruby red grapefruit, Minneola tangelo, lemon, and lime peels with AgNO3. They used a laboratory-size microwave synthesizer in their experiments because microwave heating is instrumental in creating high-quality nanoparticles. The high temperatures also accelerate the reductive action of aldehydes in the extracts.

Once the microwaved mixture cooled, the researchers conducted several tests to verify their results. Samples derived from lemon and lime extracts were dark gray to black and had to be diluted for analysis. All of the citrus-peel extracts were compared with the orange-peel extract to evaluate the AgNP synthesis.

Certain aldehydes were found only in the orange peel extract, and the extract had a higher abundance of some alcohols. The authors believe that the aldehydes are responsible for reducing the AgNO3 and capping the AgNPs. Most of the AgNPs produced by this process were very small; 94.5% were <30 nm and 77.7% were <10 nm. Overall, the size varied from 2–4 to 56.1 nm. Further study of the mechanisms of this process is needed to explain the large size variance.

This study shows that AgNPs can be produced in a benign environment in a short time. This makes it possible to test for AgNP toxicity without using toxic reagents and capping agents. (ACS Sustainable Chem. Eng. 20142 (3), 367–376; Beth Ashby Mitchell)

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It’s all about location for light-activated colloidal particles. A. J. C. Kuehne* and co-workers at RWTH Aachen University (Germany) synthesized photoswitchable, monodisperse colloidal particles by using the Suzuki–Miyaura dispersion polymerization of a fluorenediboronic acid ester with 3,5-dibromoazobenzene units.

The authors examined the effects of placing the azobenzene unit along the polymer main chain (P1), perpendicular to the main chain (P2), or delocalized from the main chain via a hexyl linker (P3). Precipitation of the conjugated polymeric particles occurs at a critical molecular weight, which varies from flat platelets with columnar packing (P1) to amorphous, spherical colloids (P2 and P3) and depends on side chain flexibility and the placement of the azobenzene derivative. Below a threshold concentration, the particle sizes of the P1 and P2 polymers are tunable; and low polydispersity is maintained.

Radiation-induced switchability between the trans and cis conformations of the azo groups is considerably faster for P1 compared with P2 and P3 because the azo functionalities are incorporated along the backbone. Light stimulation also elongates the P1 platelets irreversibly. In contrast, the spherical P2 and P3 colloids maintain their shapes upon irradiation. The conformational switching of P3 occurs at higher energies than the others because of its decoupled electronic state.

The authors believe that their findings have applications in such diverse fields as coatings, imaging, and electronics. (ACS Macro Lett. 2013, 2, 766–769LaShanda Korley)   

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Capture nanoparticle and virus images with a smart phone. Detecting single nanoparticles, microbes, and tagging-agent molecules in the field could be useful in biomedicine, environmental and food inspection, forensic analysis, epidemiology, and detection of counterfeit and contraband products. Optical imaging and spectroscopy of single nanosized objects typically require complex, expensive experimental setups in a controlled laboratory environment. A field instrument for this type of work would open new applications and allow single-object detection and analysis when access to large laboratory facilities is not available or affordable.

As a first step toward this goal, A. Ozcan and colleagues at the University of California, Los Angeles, built a fluorescence imaging system that can be mounted on a smart phone. It uses a diode laser source and a lens coupled with a long-pass thin-film interference filter to collect the fluorescence signal from a 0.6 mm x 0.6 mm area of the sample, which is inserted into the device on a sliding tray.

For larger, strongly fluorescing objects that are relatively insensitive to imaging and focus conditions, the field of view can cover the entire 3 mm x 3 mm sample area. The angle between the laser beam and the sample is ≈15°, which reduces the background noise at the detector. The smart phone's camera serves as a detector, and a translation stage allows focus adjustment. The researchers used a 3-D laser printer to fashion a lightweight holder for mounting the optical components onto the smart phone.

Smart phone that images tiny amounts of particles

This version of the device has a spatial resolution of ≈1.5 μm and the ability to detect objects labeled with a few hundred fluorophores. The authors demonstrated their device by using 100-nm dye-doped polystyrene beads and dye-labeled human cytomegaloviruses. They validated their results by using conventional optical and electron microscopy.

S. Khatua and M. Orrit at the Leiden Institute of Physics (The Netherlands) wrote a perspective (ACS Nano 7, 8340–8343) on this device, in which they explored potential applications and recommended directions for future development. (ACS Nano 2013, 7,9147–9155; Nancy McGuire)

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Researchers make a profit from a pilot plant reaction. R. H. Harris and co-workers at GlaxoSmithKline Research and Development (Stevenage, UK) developed a “fit-for-purpose” method for scaling up the synthesis of a sphingosine 1-phosphate receptor agonist. They shortened the route to the 5-hydroxytetrahydroisoquinoline intermediate from eight to two steps by carrying out a Robinson annulation on N-Boc-4-piperidone followed by aromatization of the cyclohexane ring. (Boc is tert-butoxycarbonyl.)

The authors found, however, that only a Saegusa oxidation that uses stoichiometric quantities of Pd(OAc)2 catalyst gives good conversion in the aromatization. Optimizing the workup by adding HCO2K at the end of the reaction to reduce the Pd(II) and precipitate the palladium as Pd(0) made it possible to recover 10.3 kg of the 10.5kg of palladium used in the pilot plant.

The price of palladium doubled during the campaign, so GlaxoSmithKline sold the palladium back to supplier Johnson Matthey at a profit of UK£62,500. Subsequently, the authors developed a more economical CuBr2-mediated aromatization reaction. (Org. Process Res. Dev. 2013, 17, 1239–1246Will Watson)

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