Interview with Dr. Myriam Sohn | Senior Application Technology Scientist IP Manager Global Technical Center Sun Care, BASF

Dr. Myriam Sohn

Dr. Sohn, your article “Nano or Non-Nano: the Key Aspect of the Measurement Method” was awarded first place by the SOFW award jury. Congratulations!

Why is the question “Nano or Non-Nano” important for cosmetic formulators?

There are two main reasons why it is crucial to know if a particulate UV filter is nano or non-nano. For one thing, cosmetic particulate ingredients that exhibit a size in the nano range, as defined by the EC recommendation (2011/696/EU), require nano labelling obligation in the list of ingredients on the product packaging (EU cosmetic regulation 1223/2009, July 11th, 2013). In the EC Recommendation (new adoption of 10.06.2022), the term ‘nanomaterial’ means “a natural, incidental or manufactured material consisting of solid particles that are present, either on their own or as identifiable constituent particles in aggregates or agglomerates, and where 50 % or more of these particles in the number-based size distribution fulfil at least one of the following conditions: one or more external dimensions of the particle are in the size range 1 nm to 100 nm […].”

In addition to the labelling obligation, the identification is also highly relevant for the registration process. The safety and approval requirements of a particulate filter are different depending on whether the filter is nano or non-nano. In Europe, nano UV-filters go through a specific approval that requires additional risk assessment before they are authorized to be used. That explains why an accurate measurement of the size and a correct identification as nano or non-nano is very important. 

Who or what is the Nano Define Consortium?

The NanoDefine project was funded by the 7th EU Framework Program for research and was tasked with developing the implementation for the regulatory definition of nanomaterials. A consortium was created to work on this project, which included 29 European RTD (research or technological developments) performers, counting experts from metrology institutes, nanomaterial suppliers, instrument manufacturers, regulators as well as academics.

The consortium developed an approach that allows identification of whether a material is nano or not according to the definition of the EC recommendation. To do so, a high number of existing particle size measurement methods were evaluated and classified as Tier 1 (screening purpose methods) and Tier 2 (confirmatory purpose methods). The assessed techniques include only readily available, robust, and standardized methods to provide a reliable analysis of the number-based size distribution as required by the EC recommendation. The NanoDefine consortium established a decision support flow scheme to guide any operator in the characterization of the particulate material to be tested as well as in the choice of the most appropriate measurement method. 

The objective was to provide the industry, stakeholders and regulators the means (information and procedures) to identify whether a particulate material is nano or non-nano according to the definition of nanomaterials of the EC Recommendation.

Which methods are common for defining Nanoparticles?

Many techniques exist to measure the size of particulate materials. The difficulty lies in choosing the method that is most appropriate to the material to be evaluated. Indeed, the range of applicability of a technique depends not only on its measurement principle but also on the physico-chemical properties of the tested material. There is no single method suitable for measuring the size of every particulate material. Any operator can rely on the published NanoDefine Methods Manuals to help in the selection of the most suitable method(s) for the size characterization of its material. There are three NanoDefine Methods Manuals that provide insights into the NanoDefine framework, an overview of the measurement methods, their principles and capabilities, as well as information about standard operating procedures and tools developed by the consortium. All referenced methods belong to one of the four types: Counting, fractionating, spectroscopic ensemble and integral techniques. Typical spectroscopic ensemble techniques include dynamic light scattering. Common fractionating techniques include analytical centrifugation such as X ray disc centrifuge. Counting methods include electron microscopy techniques such as TEM and SEM.

In the sun protection field, particulate filters are either of an inorganic or organic nature. Based on the composition and the fact that the constituent particles need to be detected to comply with the EC Recommendation, the NanoDefine Methods Manual recommends suitable Tier 1 screening and Tier 2 confirmatory methods. 

What methods did you use in your study?

We used Tier 1 (screening purpose) and Tier 2 (confirmatory purpose) methods. The objective was to measure the median value of the particle size and number weighted distribution of different Titanium Dioxide and Zinc Oxide samples as well as the Zinc Oxide in a finished formulation using the decision support scheme developed by NanoDefine. We first used the Tier 1 fractionating methodasymmetrical flow field-flow fractionation (AF4) coupled with UV and IC-PMS (inductively coupled plasma mass spectroscopy) detectors. Then we also evaluated all samples with the Tier 2 transmission electron microscopy (TEM) method, which is a counting and sample visualization method.

What are your conclusions?

We obtained opposite outcomes for a same sample using Tier 1 and Tier 2 techniques. While the employed Tier 1 method showed the non-nano characteristic of a tested sample, the Tier 2 confirmatory method identified it as a nanomaterial.

The discrepancy between Tier 1 and Tier 2 techniques may suggest, for instance, the presence of agglomeration or aggregates in the sample that were not entirely dispersed despite careful sample preparation. This is a known issue. The Tier 1 method used is based on the motion of the particles and is unable to differentiate between individual large particles and agglomerates or aggregates composed of constituent particles.

Our study reveals that limiting the nano interpretation of a material to a Tier 1 methodology can lead to an erroneous identification of a material as non-nano. The non-nano feature of a material measured with a Tier 1 method should absolutely be confirmed by using an electron microscopy Tier 2 technique to avoid any uncertainty. With the specific materials and Tier 1 findings, the NanoDefine Method Manual also requires the user to proceed to a Tier 2 confirmatory method.

This work intended to underline the importance of the correct methodology selection and sample preparation to accurately define a particulate material as nano or non-nano according to the definition of the EC Recommendation.

Do you have anything else you would like to add? 

Yes, I would like to conclude to say that the work of nano define consortium was very great, because there exist no clear scale available for the industry, for suppliers and also for product manufacturers and regulatory bodies to measure the particle size properly of raw materials and also in finished products. So this is really a huge advance thanks to the nano define project. 

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