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ANALYTICAL CURRENTS Thin-film vapor responses small, irregular nanodomains, 20 0.00 which probably 15 helped vapors –0.05 on penetrate into the 10 –0.10 material. Films 5 grown at 120 °C –0.15 or higher were 0 more regular and 20 0 60 40 80 lamellar in detail Time (s) and did not respond as well to pentanol vapors. Color-coded changes in the current of a thin-film “electronic nose” On the other sensor to 25 exposures of 1-pentanol. hand, the response to octanonitrile was independent of film believe that vapor analyte adsorption is controlled by favorable hydrophobic morphology. interactions between the semiconducOther factors investigated included tor and analyte, intercalation to fill the film’s thickness, the chain length defect vacancies, and surface binding. in a series of alkyl-substituted hexathio(J. Phys. Chem. B 2002, 106, 12,563– phenes, and the behavior of a short 12,568) oligomer thiophene. The researchers Run number
Whether they are called electronic noses, tongues, or simply sensors, olfactory-mimicking detectors have become a major research arena. L. Torsi, A. Dodabalapur, H. Katz, and co-workers at Lucent Technologies and Università degli Studi di Bari (Italy) provide some framework for this research by investigating the relationship between thinfilm molecular structure and morphology and device response. Using 1-pentanol as the analyte, they found that the response is greater when the number of grain boundaries of the film increased. However, the relationship is more complicated with octanonitrile as the analyte. Various oligothiophene thin films on silicon wafers were studied in this report. For example, thin films of ,-dihexyl--hexathiophene grown at room temperature were found to consist of
Colorful Li+ detector Optical detection schemes for Li+ are relatively rare compared with its Group I and II element neighbors. The problem is not lack of interest in + Li —it has key roles in medicine and batteries, for example—but the CATHERINE MURPHY, UNIVERSITY OF SOUTH CAROLINA
selective organic chromophores available for this metal ion are not very soluble in aqueous solution. Catherine Murphy and her co-workers at the University of South Carolina show that this obstacle can be overcome by using gold nanoparticles. In this work, the researchers synthesized a 1,10-phenanthroline com+ pound that binds Li in a 2:1 ligand:metal ratio with added thiol groups
to latch onto the gold nanoparticles. The ligand was coated onto 4-nm + particles, yielding a dark orange color. Addition of Li did not precipitate
particles, but caused a red-shift in the absorption peak and a color + change to gray. The response was linear with an Li concentration in + the range of ~10–100 mM, and it did not suffer from Na interference.
(Langmuir 2002, 18, 10,407–10,410)
Easy to see. Gold nanoparticles (4 nm) coated with a lithiumbinding ligand change from dark orange on the left to gray in the presence of lithium ions. The color change was linear with metal concentration. M A R C H 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
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ANALYTICAL CURRENTS Quantum dots where no dots have gone before A blind spot exists in biological labeling technologies: Organic fluorescent labels have a limited range of colors and can
NEC Research Institute (USA) and the University of Minnesota get around this problem by enclosing quantum dots in
Figure Not Available for Use on the Web
A comparison of (A–C) conventional rhodamine green dextran and (D–F) micelle-covered quantum dots in labeling frog blastomeres. (Adapted with permission. Copyright 2002 American Association for the Advancement of Science.)
photobleach; and quantum dots, while resistant to photobleaching, are covered with hydrophobic organic ligands, which makes it difficult to use them in vivo. Benoit Dubertret and David Norris at Rockefeller University and colleagues at
phospholipid micelles. The researchers determined that unmodified ZnS-over-coated CdSe quantum dots could be surrounded by a micelle made of n-poly(ethylene glycol) phosphatidylethanolamine (PEG-PE) and
phosphatidylcholine. It turns out that quantum dots slightly larger than the micelle core (~3 nm) further stabilize the micelle by providing a solid hydrophobic surface for the PEG-PE. Micelles containing 4-nm quantum dots were stable for months, even in 1 M of salt. To make the micelle-covered dots into labels, half of the PEG-PE was replaced with an amino PEG-PE, which, when used to form a micelle, exposed a primary amine on the micelle surface. The primary amine was coupled to thiol-modified DNA. These DNAlabeled quantum dot micelles were added to frog embryos and remained in the injected cells until they divided and were found in the cells’ descendants. Only at higher concentrations, >5 109 quantum dots per cell, did cells form abnormalities. In addition, the fluorescent signal was observed until late tadpole stages. Even after four days, the quantum dots did not aggregate, as is common with conventional quantum dots. (Science 2002, 298, 1759–1762)
Sanford Simon and colleagues at Rockefeller University and the U.S. Naval Research Laboratory propose two new approaches for using luminescent quantum dots (QDs) to noninvasively label live cells. They show that the QDs can be used for multicolor imaging of live cells for more than a week without affecting cell growth or development. Their first approach is based on the ability of cells to capture “large” molecules by endocytosis. The researchers incubated mammalian (HeLa) cells or Dictyostelium discoideum cells with 400–600 nM dihydrolipoic acid-capped QDs for 2–3 h, and found that the QDs ended up in many of these cells’ vesicles. 92 A
The researchers report that QDs in the labeled HeLa cells were stable for more than a week, with no detectable effects on cell morphology or physiology. Even after continuous growth for 12 days—beyond the point that the cells became too crowded to grow further—the cells remained labeled. In the second method, Simon’s team selectively labeled cell surface proteins with QDs conjugated to Compared with traditional fluorescent probes, such as the enhanced green fluorescent protein shown here (green), antibody-conjugated QDs (red) offer the same labeling specificity to a multi-drug transporter protein in live cells.
A N A LY T I C A L C H E M I S T R Y / M A R C H 1 , 2 0 0 3
SANFORD SIMON, ROCKEFELLER UNIVERSITY
QDs offer long-term imaging of live cells
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antibodies. At first, Simon’s team observed the label only on the surface cells, but when the cells were maintained for 2 h at 37 °C, the label was predominantly found in a juxtanuclear pool of vesicles. Some residual label
on the membrane and many endocytic, labeled vesicles moving inward from the cell surface were reported by the researchers. This, they say, showed that the QDs were internalized by endocytosis and not by the compromised perme-
ability of the plasma membrane. They add that their approach shows how these inorganic fluorophores resist photochemical and metabolic damage in living cells. (Nat. Biotechnol. 2003, 21, 47–51)
ESI-FTMS determines protein structure Gary Kruppa and colleagues at Sandia National Laboratories show that chemical cross-linking followed by a one-step cleanup and electrospray ionization-FTMS (ESI-FTMS) process that uses “gas-phase purification” can determine protein structure without time-consuming chromatographic steps and proteolysis. Although MS has been important in pro-
Figure Not Available for Use on the Web
teomics for identifying proteins from proteolytic digests, it has been less successful in determining protein tertiary structures. Researchers have shown that MS analysis of proteins that were covalently modified by intramolecular bifunctional cross-linking reagents and digested proteolytically yields low-resolution, estimated distances between atoms or chemical groups (distance constraints). The key is that the cross-linking molecules have variable lengths and act like a “measuring” bar.
Researchers use chemical cross-linking and ESI-FTMS to study protein structures. Expanded regions of the MS/MS spectrum show the region around the y37 ion for (a) cross-linked ubiquitin and (b) unmodified ubiquitin; and the region around the b52 ion for (c) cross-linked ubiquitin and (d) unmodified ubiquitin. (Adapted with permission. Copyright 2002 John Wiley & Sons.)
However, the reagent can also form intermolecular cross-links with amino
residues within the protein and yield the
lated only the internally cross-linked species
residues. Thus, size-exclusion chromatog-
distance constraints.
in the mass spectrometer’s analyzer cell. The
raphy has been needed to remove these
resolution and mass accuracy of FTMS then
Kruppa and his team avoid the lengthy
protein dimers and higher-order multimers
separation steps, proteolysis, LC/MS, and
unambiguously identified cross-linked frag-
before the MS analysis. Computer software
computational analyses of the protein cross-
ments, and the researchers used in-house
predictions of the masses of the monomer-
link positions. Using ubiquitin as an example,
software with a fragmentation library to as-
ic cross-linked proteolytic peptides can
they directly injected the cross-linked protein
sign the spectra. (Rapid Commun. Mass
then help identify the attached amino acid
mixture into an ESI-FTMS instrument and iso-
Spectrom. 2002, 17, 155–162)
M A R C H 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
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