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ANALYTICAL CURRENTS Monolayer-protected gold clusters From nanotubes to nanoparticles, analytical chemists are just beginning to mine a wealth of new materials. Royce Murray and colleagues from the University of North Carolina go directly for the gold—in this case, investigating the electronic properties of coated gold nanoparticles that are linked together in a network. Labeled “monolayer-protected clusters”, polymer films of these nanoparticles show promise as chemiresistor-type sensors for vapors. This study used Au140 nanoparticles, which are coated with a mix of alkanethiolate and carboxyalkanethiolate ligands; the latter serve as linkers and stitch particles together as carboxylateCu2+-carboxylate couplings. Conductivity occurs by very fast electron hopping between nano-
particles. Changing the length of the nonlinking alkanethiolate coating from 4 to 12 carbons had the greatest effect on conductivity, resulting in a 1000fold variation. This and other observations are consistent with electron tunneling through the alkanethiolate chain and nonbonded contacts between chains on adjacent nanoparticles. Exposing polmer films of the protected gold clusters to organic liquids or vapor, such as ethanol or dichloromethane, lowers the conductivity, which then reversibly increases as the film dries. This indicates that the linker ligand is substantially folded in the dry film and stretches as the organic compound is absorbed. (J. Am. Chem. Soc. 2002, 124, 8958– 8964)
Au core
Carboxylate "linker" site
Alkanethiolate "nonlinker"
MPC assembled film
Cu2+
Au IDA finger
Au IDA finger
Au IDA finger
Glass
Future sensor? Monolayer-protected gold clusters in a film attached to interdigitated array electrodes.
Electrochemical study of arsenic removal James Farrell and colleagues at the Universi-
actions that occur on the surface of high-
pH, the absence of oxides on the iron sur-
ty of Arizona tackle the environmental debate
purity zerovalent iron particles in arsenic
face, and the effect of different concentra-
over whether iron filter media can remove
solutions.
tions. The electrochemical experiments
As(V) and As(III) in drinking water. Through
Tafel diagrams were used to determine
showed that as both the pH and the As(III)
electrochemical studies, the researchers
the corrosion rate for iron wire electrodes
to As(V) ratio near the iron surface increased,
found there is minimal or no reduction of
in anaerobic 3-mM CaSO4 electrolyte solu-
As(V) was less likely to undergo reduction
As(V) in iron media filters under conditions
tions with As(V) concentrations between
(i.e., the thermodynamics became more
relevant to potable water treatment.
100 and 20,000 µg/L. Compared with a blank
unfavorable).
Public and government concern over ar-
electrolyte solution, the iron corrosion rate
senic toxicity has prompted more research
dropped five-fold in the presence of As(III)
for developing new methods for removing
and As(V), regardless of arsenic concentra-
arsenic compounds from potable water. Re-
tion. This was the result, they say, of ar-
searchers have proposed zerovalent iron
senic compounds being chemisorbed to the
filings as a filter medium for removing As(V)
iron surface and blocking cathodic sites for
and As(III) compounds from potable water.
water reduction.
From their results, Farrell’s group concludes that the pH and potential conditions necessary for significant As(V) reduction will be difficult, if not impossible, to achieve in an open system under freely corroding conditions. Thus, without biological reduction, there will be little conversion of As(V)
There are even conflicting reports on whether
Chronoamperometry and chronopoten-
iron filter media can reduce As(V) to As(III).
tiometry were used to look at various fac-
to As(III) in zerovalent iron filter media.
Farrell and his group investigated redox re-
tors affecting the As(V) reduction, such as
(Environ. Sci. Technol. 2002, 36, 3188–3193)
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