Introduction

The persistence of cells and DNA on surfaces is influenced by many different factors, including substrate types and their physicochemical properties [1–5]. In a forensic context, the topography of a substrate, its roughness, wettability and chemical surface interactions play roles in the deposition, adherence and persistence of biological material. Rougher surfaces with increased surface area and microscopic crevices tend to trap more cellular material, potentially enhancing cell and DNA preservation [6–8]. Conversely, highly hydrophobic surfaces may repel aqueous components of biological fluids, affecting the initial deposition of DNA-containing material [9]. Additionally, the strength of molecular interactions between the biological material and the surface can promote stronger adhesion [1, 10, 11]. Further, post-deposition, environmental conditions such as temperature and humidity and physical actions with the surface will either enhance DNA preservation or accelerate its degradation, or loss from, the substrate surface [8, 12–17]. The preservation of DNA over time is crucial in forensic investigations where a crime scene may not be processed for days and samples may be stored for months or years, depending on case circumstances, laboratory practices and priorities.

To maximise the chances of successful DNA recovery, it is not only important to understand DNA persistence over time but also to accurately localise and visualise the biological material on evidence items before sampling, to optimise DNA recovery. Recently, the use of Diamond Nucleic Acid Dye™ (DD) has been described as possible means of visualising touch deposits on items of evidence [18–21]. This fluorescent dye has also demonstrated utility in shedder status assessment [22]. Currently, there are limited studies investigating the use of DD on different substrates in light of possible persistence issues [23–26]. Generally, when DD is used, the stained cells are visualised and counted soon [19, 21] after exposure. During the course of conducting case work, there may be times when DD- exposed items will need to be re-examined sometime after the initial examination. It is currently unknown how long after the initial exposure of cells to DD they remain fluorescent and visible. Further, if the fluorescence intensity declines over time, is this dependent on the deposit substrate and if so, is it possible to effectively re-apply DD to visualise the cells.

This study aimed to investigate the most effective method of applying DD, and the persistence of touch cells on six different surfaces (glass, plastic, melamine, aluminium, leather and cotton) over a twelve-month period assessed using DD staining and imaging. Additionally, we aimed to explore DD fluorescence retention across the surfaces at 20 different time points over 6 months and a further 6 months after respraying the samples, including the assessment of the utility of DD respraying, at two different time points (6 and 12 months), thereby contributing to a more comprehensive understanding of the factors affecting persistence and fluorescence of biological material on these regularly encountered substrates.