The use of solid explosives to destroy ATMs has increased in recent years. In particular, pyrotechnic kits and homemade explosives have been used, with a total of 399 cases accounting for over 60% of physical ATM attacks in Germany in 2022. The use of these substances not only causes significant damage to property, but also poses a high risk to bystanders, as the perpetrators often lack adequate control over the substances used. According to the Federal Criminal Police Office Germany, the threat level is therefore still considered to be high, due to the ruthlessness of the perpetrators and the danger to the general public.[1]

The labelling of explosives is therefore an important means of uncovering distribution routes of legal professional pyrotechnics that are potentially being misused in ATM attacks and support the crime investigation. One possibility for labelling is the use of nanoparticles, which can be clearly identified by various characteristics such as their magnetic or optical properties or elemental composition. The use of lanthanide-based upconversion nanoparticles (UCNPs), which are capable of converting multiple low-energy photons into a single higher-energy photon, offers a combination of labelling properties. Fluorescence, elemental composition and different stoichiometries of the 15 lanthanides can be used in (individual) labels. Such labels can be decoded using inorganic mass spectrometry techniques such as ICP-MS. In particular, the use of single particle ICP-MS (spICP-MS) allows the characterization of UCNPs in terms of size and concentration in addition to elemental composition.[2]

To investigate the use of UCNP in explosives, pyrotechnic devices based on aluminum and potassium perchlorate were mixed with 2-4 % (w/w) freeze-dried UCNP (NaY_0.78Yb_0.20Er_0.02F₄ and NaYb_0.997Tm_0.003F₄). A pentaerythritol tetranitrate explosive charge was also wrapped in paper loaded with 1.5% (w/w) UCNP. Targets of copper, aluminum and quartz glass were placed at various distances around the charge. After detonation of the explosives, the plates and residues were collected as samples. The explosive residues were eluted from the plates using an ultrasonic bath and immediately characterised for concentration and size distribution by spICP-MS. In addition, the samples were acid digested and the elemental composition was determined. It can be shown that it is possible to reproducibly find different UCNPs after the explosion. The size distribution shows no significant change compared to non-blasted UCNP and the concentration of the detected particles decreases with decreasing distance from the blast site.

References
[1] Angriffe auf Geldautomaten, Bundeslagebild 2022, Bundeskriminalamt, 2023.
[2] M.D. Montaño., Anal Bioanal Chem. 2016, 408, 5053–5074