Demonstration of a probe-sonicator calibration protocol for harmonizaton of batch dispersions used for toxicological testing

International audienceAn accurate evaluation of the possible toxic effects of manufactured nanomaterials (NM) on human health and the environment requires adoption of appropriate ways of performing the toxicological tests. In most assays, eco- and in vitro toxicological testing require dispersion of solid particles into liquid test mediums. Unless the producer provides the test substance in the form of a stock dispersion, the first step in a test is to prepare such a stock dispersion by dispersing dry NM powder into a suitable aqueous medium (dependent upon the test in question). Experimental studies show that the exact dispersion preparation methods can play an important role on the outcome of toxicity tests. Since guidance on this issue is still lacking, the wealth of data and information in the literature and still produced is difficult to evaluate and interpret. Even-though, other factors may also play an important role on the outcome, the accurate standardization of the dispersion procedures is the first indispensable step towards establishing reliable toxicological testing. Therefore, it was decided in the EU FP7 NANoREG project that a demonstrated standard should be developed for the project to produce comparable batch dispersions for all (eco-)toxicological studies. Probe-sonication was selected at the most suitable method to reach comparable results. Testing of an existing probe-sonicator calibration method using calorimetry from NIST showed that probe sonicators cannot be calibrated using this approach alone. Therefore, a two-step approach was developed which consist of 1) pre-calibration using a calorimetric method and 2) performance calibration where the dispersion efficiency is calibrated using a benchmark material (here NM-200 synthetic amorphous silica from the OECD WPMNM sponsorship program). The protocol was circulated in the project for performance testing. Results from 26 partners in the project conformed the suitability of the probacalibration protocol and the ability to produce comparable results across the project. The probe-calibration protocol was further applied to establish benchmark values for 19 NM in the NANoREG project using the NANOGENOTOX dispersion protocol. Based on 3-4 partner interlaboratory comparisons, harmonization through the probe-calibration protocol enables establishment of comparable dispersions for granular NM as measured by dynamic light scattering (DLS). However, some variations were observed for a few granular NM, which is generally ascribed to sample variability. DLS results are generally poor for carbon nanotubes and nanocellulose samples and comparability between laboratories was generally poor. Based on optical microscopy screening of dispersions made with the NANOGENOTOX dispersion protocol, only NM-411 (SWCNT) was poorly dispersed among all 19 NANoREG core test materials