ISO/TR 27165:2012 — Nanotechnologies — Guidance on the Measurement of Nanoparticle Size

Particle size is arguably the most critical parameter in nanomaterial characterisation, directly influencing properties ranging from colloidal stability and solubility to biological interactions and catalytic activity. ISO/TR 27165:2012 provides comprehensive guidance on the measurement of nanoparticle size, addressing the fundamental challenges posed by the nanoscale regime where classical measurement assumptions break down. This article examines the technical depth of the standard, compares the strengths and limitations of different measurement techniques, and offers practical engineering recommendations for reliable nanoparticle size analysis.

Core Principles of Nanoparticle Size Metrology

A central theme of ISO/TR 27165 is that “particle size” is not a single, unambiguous quantity for non-spherical particles. Different measurement techniques probe different physical equivalents of size: electron microscopy measures the geometric projected area diameter, DLS reports the hydrodynamic diameter (including the electrical double layer and any adsorbed surface coating), SAXS yields the radius of gyration, and centrifugal sedimentation provides the Stokes equivalent diameter. Each of these diameters can differ significantly for the same particle population, and the standard emphasises that results from different methods should not be directly compared without understanding the underlying physical principles.

The standard introduces the concept of “equivalent spherical diameter” as a unifying framework, defining the diameter of a sphere that would behave identically to the particle under the same measurement conditions. This conceptual tool enables cross-method comparison but also highlights the inherent ambiguity: a 100 nm equivalent spherical diameter by DLS does not guarantee the same value by TEM. The practical recommendation is to select the measurement technique that most directly probes the property relevant to the application — for example, using DLS for characterising particles in suspension for biomedical applications, and TEM for dry powder morphology assessment.

Method Selection and Data Interpretation

ISO/TR 27165 provides a decision framework for selecting appropriate measurement techniques based on the material system, the information required, and the regulatory context. For regulatory compliance under REACH or similar frameworks, the standard recommends using at least two orthogonal techniques — one imaging-based (TEM/SEM) and one ensemble-based (DLS or SAXS) — to provide complementary information. The imaging technique establishes the primary particle size and morphology, while the ensemble technique provides statistically robust size distributions averaged over millions of particles.

A critical aspect covered by the standard is the treatment of multimodal distributions. Many nanomaterial dispersions contain multiple populations: primary particles, aggregates, and agglomerates. DLS, being intensity-weighted, is particularly susceptible to masking of small particles by larger ones — a 1% mass fraction of 200 nm particles can dominate the DLS signal from a 99% mass fraction of 20 nm particles. The standard recommends using multiple scattering angle measurements or combining DLS with fractionation techniques (e.g., analytical ultracentrifugation or field-flow fractionation) to resolve complex distributions. Mathematical deconvolution algorithms, including CONTIN and non-negative least squares (NNLS), are discussed with guidance on their appropriate application and limitations.

Practical Engineering Recommendations

Implementing reliable nanoparticle size measurement in an industrial QC environment requires careful attention to sample preparation, instrument calibration, and data analysis protocols. ISO/TR 27165 provides detailed guidance on these aspects. Sample preparation is often the dominant source of variability: insufficient dispersion leads to agglomerate measurement rather than primary particle size, while excessive sonication can fracture fragile particles or alter surface chemistry. The standard recommends systematic optimisation of dispersion parameters (sonication power, time, surfactant concentration) using zeta potential and size monitoring to identify the dispersion endpoint.

Instrument calibration is another critical consideration. Certified reference materials (CRMs) with traceable particle sizes — such as NIST SRM 1964 (polystyrene latex spheres) or IRMM-304 (gold nanoparticles) — should be used to validate instrument performance periodically. For DLS, the standard specifies acceptance criteria for the instrument response using monodisperse standards: the measured Z-average should be within ±2% of the certified value, and the polydispersity index (PDI) should be below 0.1 for a nominally monodisperse standard. Regular inter-laboratory comparisons are recommended to ensure ongoing measurement quality and to identify systematic biases.

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