Water we waiting for? Evaluating a greener, safer, more cost-effective strategy for applying 1,2-indanedione to fingermarks on paper

Developing fingermarks on paper-based evidence with 1,2-indanedione and zinc chloride is a fundamental evidence gathering methodology; however, the most widely-used carrier solvents to effect it, HFE-7100, will be phased out after 2025, necessitating new 1,2-indanedione formulations. While flammable hydrocarbon solvents have been suggested as suitable replacements for HFE-7100, the compromise to safety makes them unappealing long-term alternatives. This work evaluates a fundamentally different approach; a water-based, non-flammable xanthan gum gel medium hosting 1,2-indanedione that allows for detection of natural fingermarks. The optimised xanthan gum hydrogel formulation preserved fingermark ridge detail whereas the equivalent free-flowing ethanol/water-based solution did not. A natural fingermark depletion study demonstrated that hydrogels could be effective on simulated casework evidence, particularly 24 h after gel application. A validation study (n = 1920) comparing the 1,2-indanedione hydrogel formulation against a petroleum ether formulation found that, while the latter retained significantly better performance at this early stage of development, from a holistic frugal forensics perspective, hydrogels offer major benefits to cost and safety. This work also identified that refining the gel application method will likely improve the gel's performance, which will guide further development to deliver a safer, longer-term alternative to HFE-7100.