Because they measure different things. TEM images the metal core of dried particles and returns the true core diameter. Organic layers such as citrate, PEG or antibodies are essentially invisible to it. DLS measures how quickly a particle diffuses in liquid, so it reports a hydrodynamic diameter that includes the ligand shell, the solvation layer and the electrical double layer. A 40 nm core carrying a PEG coating can legitimately read 55–60 nm by DLS. DLS is also intensity-weighted, so a small number of aggregates shifts the average upward. Use the TEM core diameter for specifications and for surface-area or particle-number calculations, and use DLS as a comparative signal to track changes after conjugation or storage.

Both, because they come from different measurements. CoV is the coefficient of variation of the core diameter measured by TEM, and it tells you how uniform the particles themselves are; NanoBrand gold nanoparticles of 10-200nm size are produced at CoV ≤ 5%.

PDI is the polydispersity index derived from DLS and describes the width of the hydrodynamic size distribution, including aggregates and coating effects; our specification for the same nanoparticles is PDI < 0.1.

Zeta potential describes the effective surface charge of the particle in its medium and indicates how strongly particles repel one another. For citrate-capped gold nanoparticles in low-ionic-strength water, a strongly negative value is normal and is what keeps the suspension dispersed.

What matters most in practice is the change rather than the absolute figure: a drift toward zero after buffer exchange, conjugation or storage, signals that electrostatic stabilization is being lost, and aggregation typically follows. Note also that Zeta potential is medium-dependent, and pH, ionic strength and added protein all shift it, so a value measured in water does not predict behavior in your assay buffer. We recommend measuring it in a medium close to your working conditions, and read it alongside DLS rather than on its own.