Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 218

composition may have a direct infl uence on the toxicity potential of the material.
For example, certain metal-containing materials are made of heavy metals that are
toxic at low concentrations of their dissolved ions. This has lead to the importance
of measuring the dissolution rates of NPs (Section 6.2.1.11).
But the chemical composition of NPs may not necessarily be homogeneously
distributed, either in the particle population or even within each particle. Certain
NPs (e.g. quantum dots) have different chemical composition in the core, shell and
surface layer. The variations in chemical composition can lead to drastically differ-
ent reactivities, so analysis of both the whole particle and the different parts may
be important but diffi cult. Certain NPs, for example carbon nanotubes, have high
levels of catalyst metal contamination which needs to be quantifi ed.

6.2.1.7 Surface Area

It is often stated that NPs are important as catalysts because they have a much
higher specifi c surface area (SSA) than their larger counterparts, and this is of
course true, but not the full picture (Chapter 3). The importance of surface area
for chemical reactivity can probably be translated to explain some of the nanotoxi-
cology effects too. Still only a few studies exist that test this hypothesis. One
example showed a correlation of human lung toxicity to SSA of the NP (Donaldson
et al. , 2004 ).

6.2.1.8 Surface Charge

When NPs are dispersed in water they rapidly interact with the protons and
hydroxyl ions of the water and gain or lose protons depending on the nature of the
material. This proton exchange leads to charged groups on the nanoparticle surface.
The charged groups will attract oppositely charged ions (counter ions) that will
associate strongly close to the surface, and these counter ions will dominate over
other ions further away ( 3 -100 nm from surface) in the so-called diffuse layer or

electrical double layer (EDL) of the NP. The thickness of this layer is called the
Debye length and is inversely proportional to the ionic strength of the water.
Nanoparticles of the same surface charge will repel each other. When this happens,
NPs are electrostatically stabilized. When ionic strength increases the Debye length
decreases and the effective charge in the EDL decreases. This brings the nanopar-
ticles into suffi ciently close contact with each other that attractive van der Waals'
forces become dominant and aggregation occurs. The most appropriate metric
when studying the specifi c binding of metals or other solutes to the surface is the
surface proton charge (by acid-base titrations). But when studying nanoparticle
stability and agglomeration it is more relevant to measure the charge in the EDL.
This charge or potential at the hydrodynamic slipping plane is called zeta potential
(Section 6.2.5.7 ).
In environmental transport studies, surface charge is a key parameter (Tiller and
O' Melia, 1993 ; Guzman et al. , 2006). It has been shown that the coating of natural
organic matter (especially humic substances) provides all surfaces with a negative
surface charge (Hunter and Liss, 1979; Beckett and Lee, 1990; Ledin et al. , 1993 )
that makes them more immune to agglomeration due to electrostatic stabilization

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