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The Sponsorship Programme for the Testing of Manufactured Nanomaterials (further referred to as “the Testing Programme”) was concluded in March 2013, and the publication of the dossiers via the OECD website (www.oecd.org/science/nanosafety) started in June 2015. As indicated in the “Guidance manual for the testing of manufactured nanomaterials: OECD’s sponsorship programme”, after conclusion of the Testing Programme a next step is to consider “the status, need for, and coordination of further test development”. Parallel to concluding the final stages of the Testing Programme, a series of workshops have taken place, in which for different topics the applicability of existing OECD test guidelines for nanomaterials was discussed and the need for new ones analysed. One workshop focussed on physico-chemical methods, addressing in detail the relevance of each physico-chemical endpoint proposed in the Testing Programme for the regulation of nanomaterials. The methods were discussed in more general terms. However, as most of the proposed endpoints are new to the OECD Test Guidelines Programme, a much more detailed evaluation of the applied methods would
be highly relevant.
To this extent,the Netherlands volunteered to lead an initial detailed evaluation of the applicability of the test methods applied to determine the physico-chemical properties of different types of nanomaterials in the Testing Programme. This initial focus on physico-chemical properties was prompted by the essential need for an adequate and complete characterisation of nanomaterials to enable a further evaluation of their (toxicological) properties.
A number of experts from several delegations volunteered to review and evaluate the methods applied to determine the physico-chemical properties of the nanomaterials in the Testing Programme.
The Organization for Economic Co-operation and Development (OECD) established the Working Party on Manufactured Nanomaterials (WPMN) in 2006. The OECD-WPMN initiated the "Sponsorship Programme for the Testing of Manufactured Nanomaterials", where a defined set of manufactured nanomaterials is tested for their physical/chemical properties, their toxicological potential and their environmental fade. The results of this program have been declassified in June 2015. Having a big set of independent measurements for one endpoint/measurand, has the advantage of really independent comparision of methods. It is surprising what these few data sets already delivered. Nevertheless the amount of data in the OECD-dossiers is still not enough. While we have a lot of data for titanium dioxide, we have only one data set for zinc oxide and very weak data sets for silver nanoparticles. This limits the useablility of the data sets.
Regarding the quality of the data, we have seen a very good agreement of TEM and SMPS. This leads to the advice that we should have results for these two methods for each nanomaterial in the sponsorship programme.
Another outcome of this draft review is, that currently we cannot compare measurements of agglomerates. This also prohibits the use of DLS as a standard method for size determination.
Coating, stabilization layers, functionalization of particles or simple contamination are common variants of a core-shell system. For smaller nanoparticles this is of major importance. A particle with 16 nm diameter and a usual surface layer of 2 nm will have the same volume for the core as for the shell. In this case the material of the particle doesn’t have a clear definition. It is a common case that a particle consists of four different layers: Core, shell, stabilization layer and contamination. The properties of the particles differ according to this structure. For example silver particles might have a different dissolution rate for pure particles and for particles which are grown on top of a core.
Different solubility or defined other properties of materials is a common reason for producing core-shell systems. Gold cores are surrounded by silica to stabilize them or to get a defined distance between the cores. Silica might be surrounded by gold and the silica dissolved afterwards. This delivers hollow shells. Another important example for core-shell systems are quantum dots. A small core is surrounded by a different material for increasing the photoluminescence. Furthermore there a stabilization layer is needed. The smallest part of the final particles is the initial core. The photoluminescence is based on this core, but the shells contain much more material. Categorization should address this.
Core-shell systems are not covered by most of the existing decision trees for grouping. They are either regarded as special case or a singular layer. This disqualifies core-shell systems for grouping within the common models. There might be a very easy way to avoid this problem and even to combine some of the different decision trees. Starting the decision tree with the solubility of the outer shell and subsequently addressing the inner layers will be a pragmatic approach to solve the problem. If there is no shell, the categorization can start with a tiered approach or with the proposed “stawman” chemical categorization. If a shell is covering the surface there is a need to check if the shell is stable. If it is stable, the particle can be categorized based on this shell. If it is soluble, the ions need to be addressed as in the classic case. Furthermore the shell might increase the uptake by the cells. If the ions and the uptake are not critical the categorization can continue with the next layer.
With this not perfect but pragmatic approach, the surface layers can be addressed with very limited additional efforts. Most criteria are based on classically tabulated data. Including a rating system like the precautionary matrix approach might even address the fact that some parameters are not always Yes/No, e.g. solubility, ion toxicity and uptake.
Projekt Nano-TG110
(2017)
Die OECD-TG110 mit dem Titel "Particle Size Distribution Fibre Length and Diameter Distribution" wurde 1981 erstellt und seither nicht überarbeitet. Nanomaterialien, wie Nanopartikel oder Nanofasern werden nicht adressiert, genausowenig wie moderne Messverfahren. Das hier vorgestellte Projekt hat zur Aufgabe, die Größenmessung von Nanopartikeln und Nanofasern in einer neuen Technical Guideline der OECD zu standardisieren. Es wird die Projekthistorie dargestellt, die Anforderungen, der aktuelle Projekt-Bearbeitungsstand wird beleuchtet und die weiteren Arbeiten werden kurz skizziert.