Oxidative stress

Scientists have experimented with a whole range of advanced materials for conducting solid-phase microextraction (SPME), including graphane, metal-organic frameworks and aromatic polymers. But now a team of Chinese scientists has shown that SPME can also be conducted using a far-from-advanced material: paper.

This discovery is in keeping with the increasing use of paper in analytical science; many research groups have successfully constructed various cheap analytical devices from paper. Up to now, though, paper hasn’t been used much for conducting SPME, even though it is obviously naturally absorbent. But when Xiangying Meng and his colleagues from the University of Chinese Academy of Sciences in Beijing were looking for an absorbent material to extract a well-known biomarker for oxidative stress from urine, they thought that normal filter paper might make an ideal choice. This is because the biomarker, known as 8-hydroxy-2’-deoxyguanosine (8-OHdG), should readily form hydrogen bonds with the cellulose molecules in the paper.

Pieces of paper

Meng and his colleagues were looking for such an absorbent material to try to improve a method they had developed a few years later for detecting 8-OHdG in urine based on capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). Because 8-OHdG isn’t naturally fluorescent, this method involves reacting the 8-OHdG with the fluorescent compound 4-chloro-7-nitrobenzofurazan before separation and detection. Although their method worked, it suffered from interference from other compounds in the urine, hampering its sensitivity. Hence the need for an absorbent material that could remove the 8-OHdG from the interfering compounds prior to CE-LIF.

To conduct SPME with filter paper, Meng and his colleagues cut the paper up into small pieces, which they deposit into the urine sample. This sample is then shaken for one hour in an ice bath, after which the paper pieces are removed and dried. Finally, the pieces are placed into a solution of 4-chloro-7-nitrobenzofurazan held at an alkaline pH, in order to release the extracted 8-OHdG and make it fluorescent before the solution is analyzed by CE-LIF.

Disease biomarker

As a first test of this new and improved method, the scientists used it to extract known concentrations of 8-OHdG spiked into urine samples. They found that the extraction step could increase the concentration of 8-OHdG by a factor of more than three and significantly decrease the interference from other compounds in the urine. As a result, Meng and his colleagues could detect 8-OHdG at concentrations as low as 5nM and recover the various spiked concentrations with efficiencies of 99.8–103.5%. They then showed how this new and improved method could successfully determine actual 8-OHdG concentrations in five urine samples collected from college students.

Seeing as a CE microchip is one of the many analytical devices that have been developed with paper, this new study raises the possibility of developing an ultra-cheap, paper-based technique for detecting 8-OHdG in urine. This kind of technique could be highly useful for medical diagnosis; as a general biomarker of oxidative stress, elevated levels of urinary 8-OHdG have been linked with a range of different diseases, including cancer, diabetes, cardiovascular diseases and Alzheimer’s disease.

We’re all used to reading the results of medical tests on paper, but soon they could be produced with paper as well.