Elsevier

Advanced Powder Technology

Volume 22, Issue 6, November 2011, Pages 766-770
Advanced Powder Technology

Original Research Paper
Validation of dynamic light scattering and centrifugal liquid sedimentation methods for nanoparticle characterisation

https://doi.org/10.1016/j.apt.2010.11.001Get rights and content

Abstract

A variety of techniques exists to analyse the size and size distribution of nanoparticles in a suspension. However, these nanoparticle characterisation methods have been rarely fully validated and appropriate reference materials with properly assigned SI traceable values are not easily found. This paper presents results of in-house validation studies of Dynamic Light Scattering (DLS) and Centrifugal Liquid Sedimentation (CLS) methods. During these studies, a silica nanoparticle reference material was tested under repeatability and intermediate precision conditions. The trueness of the DLS and CLS methods was investigated by measuring gold and polystyrene nanoparticle reference materials. Furthermore, for each method, an uncertainty budget has been established. Both method validation and estimation of reliable measurement uncertainties are prerequisites for the certification of new nanoparticle reference materials.

Introduction

Nanotechnology contributes to the improvement of many aspects of our daily life. Numerous commercial applications of nanomaterials already exist and new generations of nanomaterials are expected to follow in the near future. The development of new applications is related to the ability to accurately measure and understand the different properties of nanomaterials (e.g., nanoparticles). Since most properties are size-dependent, size and size distribution measurements can therefore be regarded as being crucial prerequisites. Different techniques are nowadays commonly used to measure the size and size distribution of nanoparticles. However, due to the lack of validated methods and appropriate reference materials, accurate interpretation of these results is very challenging [1].

The Institute for Reference Materials and Measurements (IRMM) of the Joint Research Centre (JRC) of the European Commission has released its first nanoparticle reference material IRMM-304 in 2008. This reference material (RM) has been produced and characterised in a transparent, detailed and metrologically rigorous manner. It is primarily intended for quality control in daily laboratory practice, method development and interlaboratory comparisons, operations which are important, especially for laboratories working under ISO/IEC 17025 [2] accreditation. IRMM-304 consists of silica nanoparticles suspended in an aqueous solution and has been characterised by means of DLS and CLS. The reference material should be used as-received, since its particle concentration has been optimised for both DLS and CLS. This avoids sample preparation steps which may have an erroneous effect on the final measurement result [3], [4]. The RM has now been used to validate two particle sizing methods, namely DLS and CLS [5], [6], [7]. The precision of the two methods was investigated by testing IRMM-304 under both repeatability and intermediate precision conditions. Furthermore, the trueness of the DLS and CLS methods was assessed by measuring gold and polystyrene latex nanoparticle RMs, respectively.

Based on these validation studies, uncertainty budgets have been established in a metrologically sound manner [8]. In general, uncertainty budgets are established via the so called top-down approach and/or the bottom-up approach. The bottom-up approach starts from a measurement equation and combines all relevant individual uncertainty sources (e.g., uncertainties that are associated with physical parameters, etc.), whereas, the top-down approach is based on type A uncertainties (e.g., repeatability, intermediate precision, trueness, etc.) that are estimated from experimental data, i.e., from method validation studies or interlaboratory comparisons. In our study, we have used the top-down approach to establish the uncertainty budget for the DLS method, whereas a combination of top-down and bottom-up approach was applied to the CLS method. Ideally, for a single method, no significant differences should be found between uncertainty budgets that have been estimated by the bottom-up approach and the top-down approach. However, in reality, it can be observed that both approaches often differ from each other. The often occurring problem for the bottom-up approach is that uncertainty sources, due to the sample heterogeneity and sample handling, are neglected. Since these uncertainty sources are often very significant, the final uncertainty budget is easily underestimated. Through method validation studies, the uncertainty related to sample handling and preparation is estimated in a much more reliable manner. Takahashi et al. investigated the uncertainty sources of the DLS method using the bottom-up approach [9].

The aim of this paper is to report and discuss the results of the in-house DLS and CLS method validation studies, including the estimation of measurement uncertainties.

Section snippets

Materials

The precision (repeatability and intermediate precision) of the methods was investigated by using IRMM-304 (IRMM, Geel, BE) [3], [4]. This RM consists of silica nanoparticles suspended in an aqueous solution. The assigned values and standard uncertainties were established by means of DLS frequency method (46 ± 2 nm), DLS cumulants analysis (43 ± 2 nm) and CLS (35 ± 1 nm). In addition to IRMM-304, three other particle size RMs were utilised in the CLS validation study. A first RM (PVC latex particle size

Method validation results

The CLS and DLS method validation studies were based on measuring the reference material IRMM-304 under repeatability and intermediate precision conditions. For CLS, a series of 15 measurements on IRMM-304 were spread over five days (three measurements per day). The instrument was calibrated with a PVC particle size RM. For DLS, 30 measurements on IRMM-304 were spread over five different days. During each measurement day, two different aliquots were analysed, each aliquot was measured three

Results and discussion

For the DLS and CLS methods, an overview of the individual relative standard uncertainty components, estimated from the method validation data (repeatability, intermediate precision and trueness), from literature (particle density) and from the calibrant certificate, is given in Table 4. As can be seen in the Table, intermediate precision and the uncertainty of trueness estimation dominate the uncertainty for DLS measurements. Lower uncertainties can be achieved if the intermediate precision is

Conclusions

This paper describes the validation of a DLS (cumulants methods) and a CLS (line-start) method for the determination of the particle size of silica nanoparticle suspensions. Both the DLS and the CLS method have proven to be robust and suitable for the particle size in the range of about 35–50 nm.

The estimated measurement uncertainties can be considered as fit-for-purpose in the certification of nanoparticle reference materials. Extension of the validity of the estimated uncertainty to other

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