The process for photoluminescent lead detection in gunshot residue. [Image: AMOLF]
Photoluminescent semiconductors haven’t historically played starring roles in mystery novels and true-crime stories, but they may in the future.
Researchers in the Netherlands have developed a new method for turning lead-based gunshot residue into light-emitting perovskites (Forensic Sci. Int., doi:10.1016/j.forsciint.2025.112415 ). The reagent-based conversion makes even small amounts of residue glow under ultraviolet light for more rapid and precise tracing of bullet trajectories and on-scene testing of potential evidence.
Detecting lead out of the lab
At the scene of a violent crime, forensic investigators look for particles of lead and other heavy metals that result from the fragmentation of bullets. The “gold standard” for analyzing this residue is scanning electronic microscopy with energy-dispersion spectroscopy (SEM-EDS), but that method requires expensive laboratory equipment and specialized knowledge, and it can delay timely evidence needed in legal processes. Furthermore, on-scene tests involving an organic solvent sometimes produce false positives because the chemical can react with components other than the lead and barium from the gunshot residue.
Several years ago, Wim Noorduin and colleagues at AMOLF and the University of Amsterdam developed a general lead-detection technique that uses a halide solution to convert lead into perovskite, a substance under great investigation for use in LEDs, solar cells and other optoelectronic devices. Illumination with UV light makes the newly formed perovskite glow green to the naked eye.
In 2021, the team established a startup to commercialize its technology with a practical lead-detection kit, which has been successful for lead-testing everyday items like dishes and paint dust. Now, they have developed a new version of their method that works especially well with the lead atoms in gunshot residue.
The experimental results (B-H) from testing the regions denoted on the subject (A). [Image: AMOLF]
Solving forensics problems
The researchers recognized that smaller particles of perovskite can actually shine brighter than larger ones and leveraged this advantage to detect the particles of lead left behind from gunshot residue. They demonstrated the potential of this technique for forensic-specific applications including shooting distance estimation, rapid prescreening of shooting incident suspects and crime scene reconstruction.
In their experiments, the Dutch researchers prepared a reagent containing methylammonium bromide, which is a known precursor to perovskite. Using two different brands of pistols, they fired 9-mm rounds into cotton cloth backdrops at distances ranging from 0 to 200 cm. Spraying the cloths with the reagent not only showed the bright green luminescence under UV irradiation, but also revealed that without the UV light, the cloth showed yellowing consistent with the presence of lead perovskite and lead bromide.
The team also tested the reagent on the shooter’s hands. Even after repeated handwashing, the method still picked up traces of lead, particularly on the shooter’s fingernails and in skin creases. Even bystanders at the shooting scene had picked up trace levels of gunshot residue that the photoluminescent technique could detect.
In addition to use at crime scenes, the researchers suggest that the perovskite method could help limit health risks for soldiers and police officers who regularly use shooting ranges. They say that their study also highlights that in future, perovskite-based lead detection may be useful for detecting low concentrations of lead in water and biological samples like blood, which will help mitigate lead poisoning.