Source: http://www.separationsnow.com/details/ezine/154a53804e3/For-efficient-extraction-get-knitting-SPME-with-knitting-aromatic-polymers.html?tzcheck=1
Published: May 16, 2016
Author: Jon Evans

Knit one, pearl one

In developing novel absorbent materials, scientists have taken inspiration from many different sources, of which perhaps the most unusual is knitting. In a 2011 paper in Macromolecules, a group of chemists from Huazhong University of Science and Technology in Wuhan, China, reported producing a novel microporous polymer by knitting together rigid aromatic building blocks with a cross-linker.

Admittedly, they were actually more influenced by the synthesis process for metal-organic frameworks (MOFs), in which metal-containing molecules are linked together by long, rigid organic groups, than by the creation of chunky sweaters from woollen balls of yarn. Nevertheless, the aromatic building blocks are certainly knitted together, just like woollen yarn in a chunky sweater, and so the subsequent materials are termed knitting aromatic polymers (KAPs).

These KAPs have very similar properties to MOFs: they are highly porous with a large surface area, and the size and distribution of the pores can be varied by simply using different building blocks. But KAPs are easier and cheaper to produce than MOFs, and are also more stable and robust, whereas MOFs can fall apart when exposed to certain solvents. So KAPs are being investigated for many of the same applications as MOFs, including now as an absorbent material for solid phase microextraction (SPME).

From particles to pores

Another group of Chinese researchers, led by Gangfeng Ouyang at Sun Yat-sen University in Guangzhou, decided to explore the ability of four different KAPs at extracting four benzene compounds and five polycyclic aromatic hydrocarbons (PAHs) from water. Both the benzene compounds and PAHs are common pollutants of bodies of water, produced by the incomplete burning of fuels and organic waste, but are also often used to test the abilities of novel SPME materials (see Deep down bound: Extracting pollutants from water with a deep eutectic solvent).

To produce their KAPs, Ouyang and his team reacted each of four different aromatic molecules – benzene, 1,3,5-triphenylbenzene, chlorobenzene and phenol – with a cross-linker in the presence of iron(III) chloride as a catalyst. They then coated the resultant KAPs onto a stainless steel wire to produce SPME fibers.

Studying the four KAPs with a scanning electron microscope revealed that they did indeed differ in the size and distribution of their pores, giving them different surface areas. Three of the KAPs were made up of a network of nanoparticles, with the KAP derived from benzene (KAP-B) possessing the highest surface area. In contrast, the fourth KAP, derived from phenol (KAP-P), was made up of much larger spherical particles, and so had the lowest surface area.

Down by the lake

Testing these KAPs on solutions of the benzene compounds and PAHs, Ouyang and his team found that all the KAPs were generally more effective at extracting the pollutants than a commercially-available SPME fiber made from polydimethylsiloxane (PDMS). The only exception being KAP-P with two of the PAHs – fluoranthene and pyrene – where PDMS performed better. The KAP derived from 1,3,5-triphenylbenzene (KAP-triPB) proved most effective at extracting the benzene compounds, while KAP-B proved most effective at extracting the PAH compounds. When the extracted pollutants were analysed by gas chromatography, they could all be detected at concentrations of 1ng/L or below, representing an increase in detection sensitivity of up to 27 times compared with PDMS.

KAP-triPB and KAP-B also performed very well with actual water samples. Using KAP-triPB, Ouyang and his team were able to detect the benzene compounds toluene, ethylbenzene and m-xylene in lake water, and using KAP-B they were able to detect the PAHs naphthalene and phenanthrene in river water. All of which shows that there is a lot more to knitting than awful Christmas sweaters.

Journal of Chromatography A (Article in Press): "Knitting aromatic polymers for efficient solid-phase microextraction of trace organic pollutants"