DNA photography

It is a technology that is fast disappearing in the digital age. But black and white photography is more than an archaic art-form to a team of German chemists who have used it as the basis for a simple method of detecting labelled DNA molecules at attomole (10^-18 mol) levels.  

Thomas Carell and his team at Ludwigs-Maximilians-Universit?t M?nchen believe the ’photographs’ of DNA solutions may allow development of cheap genetic testing kits that don’t rely on fluorescence detectors or the DNA-amplifying polymerase chain reaction (PCR). They  report their findings in the journal Angewandte Chemie this week. 

Black and white photographic paper and film contain a layer of light-sensitive AgBr crystals. Photons cause these crystals to form clusters of a few atoms of silver metal, which in turn catalyze the reduction of the entire AgBr crystal. This leaves a black stain in the areas that have been exposed to light. AgBr is only sensitive to blue and UV light, however, so a dye is added to photopaper to capture red light. The dye then transfers energy to the AgBr which is then reduced as normal. 

Carell’s team took a common dye of this sort as the starting point for their work and attached it to a strand of DNA. Then, in a darkroom, they placed small drops of a solution of this dye-labelled DNA onto normal photopaper. When they exposed the paper to red light and developed it as normal, dark spots had formed where the dye-labelled DNA had been. The rest of the paper, including control spots of un-labelled DNA, was unchanged. 

To test the method for detection of health-threatening pathogens, Carell used a hairpin-shaped single-strand DNA sequence tagged with a fluorescent group at one end and a fluorescence quencher at the other. The ’loop’ of the hairpin was complementary to part of a gene belonging to the bacteria that cause plague. When mixed with the bacterial DNA the hairpin recognized it and opened up, separating its fluorescent beacon from the quencher. When this mixture was spotted onto photopaper the resultant fluorescence sensitized the AgBr giving the characteristic black spot down to femtomole  (10^-15 mol) levels of bacteria DNA. 

Carell said he was surprised by the sensitivity of the technique, particularly since they used ’ordinary’ photopaper in place of expensive fluorescence detection machines. He is working with manufacturers who believe a specially-designed paper could be even more sensitive. But he isn’t limiting the technique to DNA. ’In principle the technique can be used to detect anything as long as one has a molecule recognizing the target which is labeled with a dye’ he told Chemistry World. 

’This is a really nice piece of work’ said Duncan Graham, a bioanalytical chemist at the University of Strathclyde, Glasgow, UK. Graham points to the technique’s simplicity and ’phenomenal’ sensitivity. He suggests that it could find more application in protein analysis where there is no technique analogous to PCR.

Tom Westgate 

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