Table 2.

Studies on the effect of NP agglomeration/aggregation on cellular particle uptake and cytotoxicity.

NP species	Cell type	Results	Ref.
Al, Al2O3, Ag, Cu, TiO2, SiO2 of different sizes	HEL-30 mouse keratinocytes	Aggregation of NPs increased cytotoxicity for some NP types, but not for others.	[31]
TiO2 of different sizes	THP-1 human monocytic cells, NCI-H292 human bronchial epithelial cells	No difference in cytotoxicity between small and large NP aggregates in THP-1 cells. In NCI-H292 cells large particle aggregates are more toxic.	[36]
Fe2O3 and CeO2 with various functionalizations	NIH/3T3 mouse fibroblasts	Degree of NP uptake is a function of aggregate size and type of surface functionalization.	[37]
PVA-coated SPION (superparamagnetic iron oxide NPs)	HeLa human cervix carcinoma cells	Increasing aggregation favours enhanced uptake. This effect was not driven by sedimentation as aggregates were stable in suspension.	[34]
Transferrin-coated Au NPs	HeLa, A549 human lung epithelial carcinoma cells, MDA-MB-435	Aggregation leads to decreased uptake in HeLa and A549 cells, but to an increase in MDA-MB-435 cells. Uptake is dependent on cell type and uptake mechanism used.	[38]
Ag and TiO2 NPs	HepG-2 human hepatoma cells, THP-1, A549	The least agglomerated particles are the most cytotoxic.	[39]
Ag NPs with various functionalizations	RAW-264.7 mouse macrophages, C-10 lung epithelial cells	NP cytotoxicity depends on the type of surface coating, particle charge, extent of aggregation and cell type used.	[40]
SiO2 NPs with various functionalizations	HaCaT human keratinocytes, primary skin cells	Degree of NP uptake is independent of aggregate size, but dependent on particle charge in HaCaT cells. Aggregation leads to a decrease in particle uptake in primary skin cells.	[41]
SiO2 NPs with various functionalizations	HeLa human cervix carcinoma cells	Efficient cellular NP uptake is favoured by high colloidal stability in cell culture medium and a high positive zeta potential. Aggregates bind to the cells, but are not internalized.	[32]
Monodisperse and aggregated silicone-based NPs	J774 mouse macrophages, BALB/c3T3 mouse fibroblasts	Cytotoxicity of NPs is dependent on particle surface area rather than aggregation status.	[42]
TiO2 anatase and anatase/rutile NPs	Caco-2 human colon adenocarcinoma cells	Cytotoxicity is determined by the specific surface area, crystallinity and phase composition of the particles.	[43]