Toxicological Effects of Metal Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 193

cysteine surface modifiers, or with a reduced gold surface (up to 25 µM) to be non-
toxic. Gold nanomaterials are often synthesized using seed-mediated methods com-
prising the cationic surfactant cetyltrimethylammonium bromide (CTAB) in growth
solution, leaving an original coating of assembled CTAB molecules on the particle
surface (Takahashi et al. 2006; Hauck, Ghazani, and Chan 2008; Wang et al. 2013a).
Several studies reported that the modified surface of gold nanomaterials by capping
with the surfactant CTAB has a negative impact on cellular functions. However, after
a washing procedure, gold nanoparticles with surface-bound CTAB did not cause
a change in cellular metabolic activity, whereas the surfactant CTAB in solution
was found to be highly cytotoxic. The removal of excess soluble CTAB by washing
completely abolished the cytotoxicity that was demonstrated for the initial prepara-
tion of CTAB-containing gold nanoparticles (Connor et al. 2005). Similar results
of further studies illustrated that human colon carcinoma cells show significantly
reduced viability when exposed to 0.4 nM of CTAB-capped gold nanorods (Alkilany
et al. 2009). The authors concluded that these effects were mainly caused by excess
CTAB that desorbs from the surface of the capped rods or by residual free contami-
nations, rather than by the CTAB-capped nanorods themselves. Overcoating of the
CTAB-capped gold nanorods with two different polyelectrolytes, either with the net
negatively charged polyacrylic acid (PAA), thereby converting the surface charge, or
a further layering with the positively charged poly(allylamine) hydrochloride, was
found to be relatively nontoxic, implicating that the initially observed cytotoxicity is
not correlated to the particle surface charge. Moreover, supernatants of the CTAB-
capped gold nanorod preparations were shown to induce nearly the same cytotoxic-
ity as the original particle-containing dispersions (Alkilany et al. 2009).
A recently published study reported that CTAB-capped 30 nm gold nanorods
exposed in serum-free culture-medium generate defects in the cell membranes and
induce cell death (Wang et al. 2013). The authors attributed these effects mainly to
the bilayer structure of CTAB on the surface of the rods rather than to their sur-
face charge. Other positively charged surface modifications using polyelectrolytes
exhibited minor adverse effects to the cells (Wang et al. 2013b). An extraction of
the CTAB surface capping and replacement with phospholipids like phosphatidyl-
choline was able to reduce the cytotoxicity for HeLa cells (Takahashi et al. 2006).
Coating with polyethylene glycol (PEG) has also been described as a useful method
to confer protective effects against cytotoxicity to gold nanorods. PEGylated gold
nanorods have been found to reveal superior biocompatibility, compared to polysty-
rene sulfonate-coated nanorods, which facilitated cellular uptake and substantially
reduced cell viability, demonstrated in four cell lines of different origin (Rayavarapu
et al. 2010). These and other data again demonstrate that surface modifications, coat-
ings, and surface charge, indeed play substantial roles for cytotoxic properties of
gold nanomaterials, as well as for their cellular uptake (Hauck, Ghazani, and Chan
2008). Furthermore, in a physiological environment the nanomaterials will change
their physiochemical properties by surface interaction and coatings with biomole-
cules. For example, the binding of serum proteins has been shown to alter the surface
charge of gold nanorods, and a serum albumin protein corona on CTAB-coated gold
nanorods was found to confer improved biocompatibility to the nanomaterials by
reducing their potentially destructive effects for the cell membrane. A serum protein