Detection of latent DNA

Touch DNA is one of the most common sample types submitted for DNA profiling. There is currently no process to visualise the presence of such DNA deposited when a person makes direct contact with items of forensic relevance. This report demonstrates the effective use of Diamond Dye to bind to DNA and allow visualisation of deposited cellular material using a mini-fluorescence microscope. Volunteers made contact with a range of items typical of those submitted as part of a forensic investigation. Contact was for less than 5 s and occurred either 15 min after hands were washed to remove any traces of DNA, and therefore under controlled conditions, or at an undefined time post handwashing to mimic real-world scenarios. Diamond Dye bound to cellular material on all the items used and in all cases it was clear where the volunteers had made this brief contact. It was also clear where no contact had been made. DNA profiling was performed on a sub-set of samples to confirm that the cellular material viewed under the microscope was human in origin and deposited by the person contacting the item; this was the result obtained in every sample tested. Diamond dye is relatively inexpensive, simple to apply, binds to the DNA in 3 s or less, has no mutagenic effects at the concentrations used, does not affect subsequent DNA profiling, and does not bind effectively to bacterial DNA. In combination with a mini-fluorescence microscope, this proof-of-concept study shows that otherwise invisible DNA deposited by touch can be visualised. The position and amount of cellular material deposited during even brief contact can be recorded allowing targeted sampling in any further DNA typing of forensically-significant items.