Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 82

that coatings must be considered carefully when predicting the environmental
chemistry of a nanoparticle.
3.3.2 Redox Chemistry in Other Nanoparticle Systems
3.3.2.1 Precious Metal Nanoparticles
The properties and applications of precious metal nanoparticles (gold, silver, plati-
num and palladium) are a very popular subject of study in nanoscience; materials
such as platinum have long been used for commercial catalysis. Here the focus is
upon gold as the primary example, as its behaviour arguably has many commonali-
ties with other precious metal nanoparticles. Nanoparticulate gold is also particu-
larly relevant as it is popular for application in nanomedicine, biolabelling and
sensing (Hayat, 1989 , El - Sayed et al. , 2005, 2007; Huang et al. , 2006, 2007a, 2007b;
Oyelere et al. , 2007 ; Sonnichsen and Alivisatos, 2005 ).
Unlike materials such as platinum, gold in bulk form is chemically fairly inert.
Nevertheless, in nanoparticulate form, it can display signifi cant catalytic activity,
both in solution and on solid supports. Gold nanoparticles can participate in many
redox reactions. These include reactions of potential environmental interest, such
as the oxidation of carbon monoxide (Ketchie et al. , 2007 ; Valden et al. , 1998 ) and
the degradation of organic pollutants (Deng et al. , 2005, 2007; Panigrahi et al. , 2007 ).
Precious metal nanoparticles can behave as electron transfer mediators between
molecules or other species (e.g. between semiconductor nanocrystals and mole-
cules) (Kiwi and Gratzel, 1979; Miller et al. , 1981 ; Cozzoli et al. , 2004 ). Reported
reaction rates are signifi cant, often comparable to the rates for reactions promoted
by commonly studied catalysts (Somorjai, 1994).
Size dependence of catalytic redox behaviour has been observed in colloidal gold
reactions in aqueous solutions. Reports of such behaviour in the literature vary
widely. For example, Sau and co-workers examined the gold nanoparticle catalysis
of eosin dye reduction with sodium borohydride (NaBH 4 ) and found two distinct
size dependence regimes (Sau et al. , 2001). Surface area normalized rates decreased
for nanoparticle diameters from 10 to 15 nm, then increased from 15 to 46 nm. In
a different study monitoring the hydrogenation of anthracene, a polycyclic aro-
matic hydrocarbon pollutant, nanoparticles ranging from 4.1 to 24.7 nm displayed
an increasing turnover frequency (number of anthracenes reduced per surface
atom per second) with decreasing size (Deng et al. , 2005, 2007). Similar variation
has been observed for other precious metal nanoparticle systems (Sharma et al. ,
2003 ; Duan et al. , 2007 ).
The reported variation in size dependent properties complicates attempts to
predict the behaviour of precious metal nanoparticles in the environment.
Nevertheless, additional studies can potentially rectify this problem. Recent studies
on platinum nanoparticle catalysis indicate that nanoparticle morphology could be
important in determining reaction rates (Narayanan and El-Sayed, 2005). In the
aforementioned studies on colloidal precious metal nanoparticle catalysis, mostly
size characterization is reported (particles are generally assumed to be approxi-
mately spherical). It is possible that preparations leading to batches with different