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ANALYTICAL CURRENTS An attractive new nanobioassay
New chromogenic nitrate sensor Sensors that visually change color are
aqueous solutions in the presence of other
handy, especially for fieldwork, because
anions. The sensor uses a metal in complex
they require no additional, possibly ex-
with a ligand—an aza-oxa-thia macro-
pensive equipment. However, it has been
cyclic ring that is bonded to an azobenzene dye. Different metal ions
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impart varying levels of specificity to the receptors. Sensors containing Cu(II) or Fe(III) are sensitive to differ-
Vials containing the chromogenic Hg(II)–ligand complex show that it is specific for NO3– (yellow), whereas the color of the solution remains unchanged from its pure state (red, left vial) in the presence of (left to right): F –, Cl–, Br–, I¯, H2PO4–, and HSO4–. (Adapted with permission. Copyright 2001 Wiley-VCH Verlag GmbH.)
ent sets of anions, and their solutions in acetonitrile change color from red to yellow. But only the Hg(II) complex is specific for nitrate. Because the Hg(II) complex is insensitive to many metal ions, it can
difficult to make a chromogenic sensor
competitively analyze nitrate in water
specific for nitrate because many other
samples containing common cations and
anions coordinate more readily with re-
anions in a typical concentration range of
ceptor sites. Ramón Martínez-Máñez and
1 10–3 to 5 10–4 M. Nitrate concentra-
co-workers at the Universidad Politécnica
tions of 20–200 ppm were found in 50-µL
de Valencia (Spain) have not only accom-
aqueous samples mixed with 10 mL of
plished this feat, but extend its application
acetonitrile. (Angew. Chem., Int. Ed. 2002,
to determine concentrations of nitrate in
41, 1416–1418)
(b)
(a) Hybridization
(c) Ag+/HQ
Au label
M (e) 362 A
(d)
A N A LY T I C A L C H E M I S T R Y / J U LY 1 , 2 0 0 2
The Human Genome Project has lured many researchers to develop DNA sensors and high-density DNA arrays. And Joseph Wang and colleagues at New Mexico State University are among those caught in the attraction. They have created a nanoparticle-based protocol for detecting DNA hybridization that uses magnetically induced, solidstate electrochemical stripping detection of metal tags. Their new bioassay begins with the hybridization of a target oligonucleotide to magnetic beads coated with biotinylated DNA probes. The researchers then allow streptavidin-coated gold nanoparticles to bind to the captured target DNA. Next, they catalytically deposit silver on the gold-nanoparticle tags and dry the aggregate sample. At this stage, most of the 3-D, DNA-linked aggregate is covered with silver. Wang and his team were able to magnetically collect and anchor such a DNA-linked particle assembly onto a thick-film electrode. This, they say, led to a direct contact of the silver tag with the surface and allowed solid-state electrochemical transduction using constantcurrent chronopotentiometry to detect oxidative dissolution of the silver tracer. Transmission electron microscopy observations confirmed that the DNA hybrid “bridges” the metal nanoparticles to the magnetic beads. On the basis of initial experiments, the researchers estimated a detection limit of ~150 pg/mL (1.2 fmol) of target DNA. Repeated measurements using a 100-ng/mL target DNA concentration yielded reproducible signals with a standard deviation of 7%. (J. Am. Chem. Soc. 2002, 124, 4208–4209)
Schematic of the magnetically induced solid-state electrochemical detection of DNA showing the (a) introduction of probe-coated magnetic beads; (b) hybridization of target DNA; (c) capture of gold nanoparticles; (d) deposition of silver on the gold nanoparticles; and (e) the magnet used to collect the assemblies.
