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ANALYTICAL CURRENTS Quantum-dot electrochemistry Because nanocrystal (NC) quantum dots have large ratios of surface area to volume, they are prone to engage in heterogeneous redox chemistry with the surrounding environment. Although this reactivity can degrade the NCs, Allen Bard, Brian Korgel, and colleagues at the University of Texas–Austin have discovered that it also offers useful properties, such as the emission of visible light. The researchers demonstrate that silicon NCs are chemically stable upon electron and hole injection. Furthermore, electron–hole annihilation, which results from electron-transfer reactions between NCs (or between NCs and redox-active coreactants), produces electrochemiluminescence with a peak maximum of 640 nm; photoluminescence occurs at a peak maximum of 420 nm. The researchers note that the NCs are efficient emitters (quantum yields of 5–20%), which is surprising because the indirect band gap of bulk silicon makes the material a poor candidate for light emission. The origin of the light emission is unknown.
Measurements of the electrochemical properties of freely diffusing silicon NCs in nonaqueous organic solvents suggested a large central gap between the onset of oxidation and reduction. This gap, which is related to the highest occupied and lowest unoccupied molecular or-
Simple, new optical microchamber Thanks to John Murray, Eustratios Bana-
a volume of ~4 µL—the contents are incu-
nis, and Allan Wolkoff at Albert Einstein
bated, washed, and labeled with fluores-
College of Medicine, fluorescence mi-
cent dyes. Drops of glycerol can be used
croscopy has its own equivalent of duct
to seal the chamber ends.
tape. It’s double-sided tape, which the re-
The researchers found that cellular
searchers use to make inexpensive, dis-
membranes routinely stick to the untreat-
posable optical microchambers.
ed glass coverslips, making the micro-
In the hands of these scientists, two
membrane-bound pro-
double-sided tape serve
teins. The simultaneous
as the walls of a micro-
detection of the liver-spe-
chamber on a large micro-
cific asialoglycoprotein re-
scope coverslip. A smaller
ceptor, its ligand asialooro-
piece of glass cut from a
somucoid, and the tumor
microscope slide is the
suppressor protein cave-
solution flows into the mi-
Energy
ECL
crochamber—which has
Image not available for use on the Web
olin-1 indicated that the receptor and ligand were often associated with each other, whereas many
Si core Surface
CO–2
The mechanisms for electrochemiluminescence (ECL) and photoluminescence (PL) of silicon clusters. (Adapted with permission. Copyright 2002 American Association for the Advancement of Science.) 408 A
chamber ideal for trapping and studying
long, narrow pieces of
chamber’s cover. After a PL
bitals, grew as the NC size decreased. In addition, the differential pulse voltammetry peak potentials above the onset for electron injection roughly corresponded to the expected Coulomb blockade or quantized double-layer charging energies. (Science, 2002, 296, 1293–1297)
Fluorescent dyes label microtubules (red), the liverspecific asialoglycoprotein receptor (green) and its ligand asialoorosomucoid (bright red), and the tumor suppressor protein caveolin-1 (blue). Yellow, aqua, or white spots indicate places where the dyes overlap partially or completely. (Adapted with permission. Copyright 2002 Elsevier Science USA.)
A N A LY T I C A L C H E M I S T R Y / A U G U S T 1 , 2 0 0 2
of the caveolin-1 structures were independent. In other experiments, the researchers monitored modifications to endocytic vesicles and established that the amount of protein could be quantified using this technique. (Anal. Biochem. 2002, 305, 55–67)
