Size and metal composition characterization of nano- and microparticles in tattoo inks by a combination of analytical techniques

Abstract

We have applied different analytical techniques for studying the size and metal composition of nanoparticles (NPs) and microparticles in tattoo inks. The morphology and size distribution of particles were determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). These techniques allowed us to obtain size information on all particles in tattoo mixtures. Higher resolution characterization of the size distributions was achieved by means of asymmetric flow field fractionation (AF4) combined with multi-angle light scattering (MALS) detection, which allowed for particle fraction separation and sizing. The high sensitivity and multi-elemental capability of inductively coupled plasma mass spectrometry (ICP-MS) enabled the off-line determination of the metal concentration of the particles in the AF4 fractions. Counting and sizing of particles were also performed by single particle (SP)-ICP-MS. The TEM images of the tattoo inks indicated the presence of individual particles and of aggregates and agglomerates of different shapes; carbon black was recognized in black inks and TiO₂ in lighter shades. The DLS measurements showed that the inks were polydispersed with particles continuously distributed from tens to hundreds of nanometers (the Z-average hydrodynamic diameter range was 31–421 nm). The use of AF4 revealed the presence of two different fractions of particles in colored inks and one fraction in black inks, with mean sizes ranging from ca. 50 nm to ca. 400 nm. AF4 and off-line ICP-MS analyses of these fractions showed micronic and sub-micronic Al and Ti particles and Cu at the nanometer level (<100 nm). SP-ICP-MS analysis confirmed these latter findings: the inks contained nanosized materials of Cu (such as CuO or Cu-phthalocyanine), while Al (as Al₂O₃) and Ti (as TiO₂) were detected as larger sized particles. Overall, this work confirms the power of different analytical techniques in detecting and characterizing NPs and microparticles in tattoo inks. We show that tattoo ink suspensions unquestionably contain NPs, suggesting the need for an assessment of how they interact with human skin as well as if they can penetrate the derma to reach the bloodstream.