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ANALYTICAL CURRENTS
Sensitive detection of anthrax spores An optical method developed by Cagri
To demonstrate specificity, Savran and
The new method detected whole
Savran and colleagues at Purdue Universi-
spores, not their contents. The spores
colleagues carried out a competition bind-
ty can rapidly detect as few as 34 Bacillus
bound to short peptides immobilized on a
ing assay with B. subtilis spores. One well
anthracis spores. The investigators say the
glass sensor array, which consisted of
contained the B. anthracis-specific pep-
technique, when combined with an aerosol
gold-coated rings. The inner circle acted
tide, and the other had a peptide for B. sub-
capture unit, could test for anthrax spores
as a transparent well. When spores were
tilis. The investigators showed that the well
circulating in air.
bound to the peptides inside the well, a
modified with the B. subtilis peptide cap-
small part of the laser beam was blocked.
tured 46 B. subtilis spores and 1 spore of B.
This decreased the transmission intensity.
anthracis after a 30-minute incubation; the
Current detection methods for B. an-
thracis include MALDI MS and ELISA, which have a high degree of specificity but
The array was first modified with a pep-
well coupled with the B. anthracis peptide
require sophisticated equipment, extraction
tide specific for B. anthracis. The system
captured 43 B. anthracis spores and 1
and amplification of DNA, and isolation and
detected 34–140 B. anthracis spores from
spore of B. subtilis. (J. Am. Chem. Soc.
labeling of biomarkers.
2-µL suspensions in 4 wells in 35 minutes.
2007, 129, 732–733)
Gaseous ions likely formed by ion evaporation surface while the droplet still contains cule, form. Then, the gas-phase ion is Among enthusiasts of ESI, one lingermultiple solute molecules. released. In the ion-evaporation model, ing question has been how gaseous ions Fenn and Nguyen added a vapor of however, gaseous ions desorb from the are formed during the evaporation of polar solvents to a bath gas solvent from charged that contained charged droplets of solution. Now, Evaporation droplets of an analyte soluJohn Fenn and Steve Subdivision tion. Various analytes were Nguyen of Virginia ComDesorption + studied. The increase in the monwealth University pro+ + + + + + ++ + + + + + rate at which the solute ions vide evidence that the ion++ ++ + + + + + + + + + + + desorbed correlated directly evaporation model is + + + + + + + ++ + ++ with the number of charges applicable in most cases. + + + + + + + + + + + on the solute ion. In effect, The charged-residue and + + + + + + + + + the researchers say, solute ion-evaporation models have + + + ++ + + ions were “sputtered” into been discussed for years. In + + + + + + + + + + + + the ambient gas; this is conboth cases, the original + + + + ++ ++ + + + + + sistent with the ion-evaporadroplet breaks into smaller + + + + + + + + + + + + tion model. The exception, ones as the ions on the sur++ + ++ + + + + + + in which the charged-residue face are pushed closer to+ + model seems a better fit, was gether by evaporation of the for very large molecules solvent. In the charged(e.g., polyethylene glycols residue model, this process A schematic illustrating the charged-residue model (top) and the ionthat are ~5,000,000 Da). continues until “ultimate evaporation model (bottom) of ion formation from a charged droplet of (Proc. Natl. Acad. Sci. U.S.A. droplets”, each of which liquid. (Adapted with permission. Copyright 2007 National Academy of 2007, 104, 1111–1117) contains one solute moleSciences, U.S.A.) © 2007 AMERICAN CHEMICAL SOCIETY
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ANALYTICAL CURRENTS Nanodiamonds are a cell’s best friend neutral nitrogen-vacancy centers—defects that have strong absorption at ~560 nm and emission at ~700 nm. They first isolated individual 35-nm FNDs on a glass surface and tested their resistance to photobleaching by exposing them to high-intensity excitation (8 103 W/cm2) at 532 nm. The FNDs showed no loss of fluorescence emission intensity during a 300-second exposure, whereas a control fluorophore, Alexa Fluor 546, photobleached within 12 seconds. To test the biocompatibility of FNDs, the researchers incubated the nanodiamonds with HeLa cells and imaged them with epifluorescence microscopy. A single FND could be tracked in the cytoplasm of a live cell, and separate in vitro studies showed that the surfaces of the nanodiamonds could readily be derivatized to interact with biomolecules without loss of fluorescence inten-
Detection of bacteria in one step J. Manuel Perez and colleagues at the Uni-
number of bacteria in-
versity of Central Florida have developed a
creased, more bacterial
method to quickly detect bacteria in com-
epitopes would compete
plex samples with superparamagnetic iron
for the available nanopar-
oxide nanoparticles. The detection relies
ticles. This would cause
on the ability of the nanoparticles to switch
the nanoparticles to bind
between dispersed and assembled states
to the bacteria in a more
when they interact with a target, with a si-
disperse-like state with
multaneous change in the spin–spin relax-
small T2 values.
ation time (T2) of the solution’s water pro-
Dispersed state
10 µm
Superposition of bright-field and epifluorescence images of a HeLa cell after the uptake of FNDs. (Adapted with permission. Copyright 2007 National Academy of Sciences, U.S.A.)
sity. The researchers note that, in addition to their potential as probes, fluorescent nanodiamonds may also be used in the future as gene carriers or devices for tumor targeting. (Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 727–732)
Assembled state
Quasi-dispersed state
T2
The holy grail of biomolecular imaging is a fluorescent tag that avoids interfering with cellular autofluorescence (i.e., emits at wavelengths >600 nm), has high photostability, is not cytotoxic, and is easily conjugated to a specific biomolecular target. Various organic dyes, fluorescent proteins, and semiconductor nanocrystals have been used, but each has its own set of drawbacks. Now, Wunshain Fann, Huan-Cheng Chang, and colleagues at Academia Sinica (Taiwan) and the National Taiwan University have shown that fluorescent nanodiamonds (FNDs) possess all of the desirable properties of a fluorescent tag and show exceptional promise as cellular probes. FNDs are insulator-based nanoparticles whose fluorescence stems from point defects in the crystal lattice. The researchers used type Ib diamonds with a combination of negatively charged and
Target
The investigators test-
Dispersed nanoparticles assemble on the bacterium surface and induce large T2 values. As the number of bacteria increases, the T2 values become smaller.
tons. Changes in T2 (T2) have been used
ed their hypothesis on
previously to detect nucleic acids, proteins,
Mycobacteriumavium
and viruses, but larger targets hadn’t been
spp. paratuberculosis
attempted.
(MAP), a pathogen that
proportional to the MAP concentration in
affects cattle and is linked to Crohn’s dis-
milk, supporting their proposed model for
if the number of bacteria in solution was
ease in humans. Anti-MAP antibodies were
detection. They were also able to quantify
low, the ratio of nanoparticles available to
attached to the nanoparticles, and MAP
MAP in a concentration-dependent manner
bind on the bacterium surface would be
was detected in a mixture of bacteria. The
in whole-blood samples. (Nano Lett. 2007,
high, resulting in higher T2 values. As the
investigators found that T2 was indirectly
doi 10.1021/nl062553z)
Perez and colleagues hypothesized that
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news
Microfabricated Fabry–Pérot interferometer with nanochannels As nanochannels become thinner than ~20
nm. Ethanol filled the chan-
nm, optical microscopy can’t distinguish
nels at a reduced speed,
between gas and liquid phases because
whereas water filled the
the differences in the optical path length
channels at speeds faster
become negligible. Fluorescent solutes can
than anticipated.
be used, but the spatial and temporal infor-
The investigators specu-
mation from them doesn’t always reflect
lated that nanobubbles were
the fluid dynamics.
created during filling. In the
Ethanol meniscus (dark) fills a 12-nm channel (10 µm wide) from bottom to top. Dotted lines indicate the channel walls. The meandering motion leads to the entrapment of a gas bubble and its subsequent dissolution.
To overcome these challenges, Jan
case of water, these nano-
Eijkel and colleagues at the University of
bubbles could dramatically
Twente (The Netherlands) designed a
reduce the viscous drag. The investigators
they got closer, the air was increasingly
miniaturized Fabry–Pérot interferometer
verified that gas bubbles could exist by fill-
compressed. The menisci slowed down
with micromachined channels that had sil-
ing a channel with ethanol from both ends
and eventually stopped. The only way the
ver mirrors embedded in both channel
simultaneously. Air became trapped inside
gas could escape from the channel was to
walls. The investigators observed the be-
the channel, in part because of the rough-
dissolve in the liquid; the investigators
havior of ethanol and water as they filled
ness of the channel walls.
could observe this. (Nano Lett. 2007, doi
channels with heights between 6 and 20
The ethanol menisci meandered, and as
10.1021/nl062447x)
Imaging bacterial infections in live mice Bradley Smith and colleagues at the (a) (d) control fluorophore into the thigh 2211 muscles of nude mice. The entire University of Notre Dame, Phillip 1658 1106 animal was irradiated with filtered Morris USA, and Washington Uni553 light, and the emission intensity was versity School of Medicine have de0 imaged with a CCD camera. Only veloped a new probe that traces the location of bacteria in the body of a (b) (e) those mice that received the ZnDPA fluorophore showed high fluolive animal. They chose a NIR fluorescence at the injection site. rophore because signals from dyes Finally, the researchers tested with emission wavelengths in the whether conjugation of the probe to 650–900-nm region can propagate through ≥2 cm of tissue—a proper(c) (f) bacteria could occur in vivo. The bacteria were injected without prety necessary for imaging in a live treatment, and the fluorophores animal. were introduced into the bloodThis work builds on the authors’ stream via tail-vein injection. In recent discovery that zinc(II) dipicomice treated with the Zn-DPA fluolylamine (Zn-DPA) coordination rophore, the fluorescence signal complexes selectively stain the anFluorescent images of a mouse with S. aureus injected ionic surfaces of bacteria and apopinto the left thigh muscle (a) before and (b) immediately contrast reached its maximum after ~18 hours and was ~4 higher than totic animal cells. In this paper, the after injection of Zn-DPA fluorophore and at (c) 6, (d) in control injections. The researchers authors couple their bacteria-target- 12, (e) 18, and (f) 21 hours postinjection. note that the probe was well tolerating coordination complexes with a ed by the mice during the course of the carbocyanine dye to create their probe. coccus aureus cells. The authors then inexperiment. (J. Am. Chem. Soc. 2006, The researchers first showed that the jected S. aureus NRS11 preincubated 128, 16,476–16,477) fluorophore effectively stained Staphylowith either Zn-DPA fluorophore or M A R C H 1 , 2 0 0 7 / A N A LY T I C A L C H E M I S T R Y
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ANALYTICAL CURRENTS Droplet trains encode and decode signals A microfluidic device fabricated by George Whitesides and Michael Fuerstman at Harvard University and Piotr Garstecki at the Polish Academy of Sciences generates fluid flows that exhibit both nonlinear behavior and reversibility. The system also encrypts and decrypts signals that are encoded as a function of the time between droplets. In the new device, droplets of one fluid are suspended in a second, immiscible fluid and travel through a bifurcated channel. As each droplet enters one branch or the other, it affects the resistance of that branch and, thus, influences the path that subsequent droplets will follow. Depending on the interval between droplets, the patterns of droplets can be regular or irregular (i.e., nonlinear). Nevertheless, the behavior of the droplets remains reversible—the direction of travel reverses if the force is reversed. In addition, the researchers controlled the flow of droplets to manipulate the time between droplets. In this way, they encoded both analog signals (by adjusting the height of fluid in a reservoir) and digital signals (by adjusting the pressure), and they successfully decoded the signals farther down the channel. All of this was achieved without valves or switches. (Science 2007, doi 10.1126/science.1134514)
Microarray-based cancer studies are flawed Alain Dupuy and Richard M. Simon of the U.S. National Cancer Institute have analyzed 42 literature reports linking cancer outcomes with DNA microarray analysis and found that half of the published studies contained serious flaws in statistical analysis. Their inquiry revealed that several categories of error were common, depending on the type of study. For example, when researchers attempted to correlate outcome (e.g., response to a certain treatment) with differential gene expression, they frequently used a statistical analysis in which the p value did not adequately control the number of false positives. The authors suggest that, in the future, the objective of a study be clearly defined before the experimental design and statistical analysis are carried out on a particular data set. To guard against future errors, they also provide a checklist of dos and don’ts for statistical analysis of microarray studies. (J. Natl. Cancer Inst. 2007, 99, 147–157)
(a)
(b)
Time between droplets (s)
Time between droplets (s)
1.2
Input signal Encoded signal
1.1
1.2
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1 0.9
0.8 0.7 0.6 0.5
0.8 0.7 0.6 0.5
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0.4
(c) 0.5
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5 10 15 20 25 30 35 40 Input signal Encoded signal
Input signal Decoded signal
1.1
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Getting a handle on R-thalidomide Although implicated in serious birth defects decades ago, thalidomide has attracted attention recently as a possible treatment for HIV/AIDS and some cancers. To aid the study of this drug, Hiroaki Sawai and colleagues at Gunma Uni0
5 10 15 20 25 30 35 40
versity (Japan) and the Japan Science and Technology Agency have developed a DNA aptamer that selectively
Input signal Decoded signal
binds the (R ) isomer of thalidomide. The aptamer is based on a DNA sequence that bears a modified deoxyuridine triphosphate in place of a thymidine triphosphate. The aptamer folds into a three-way junction, and the binding site is in a hairpin bulge in one of the arms. The researchers note that the modified deoxyuridine group
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5 10 15 20 25 30 35 40 Interval number
0 5 10 15 20 25 30 35 40 Interval number
(a) Input and encoded signals and (b) the corresponding input and decoded signals in analog mode. (c) Input and encoded signals and (d) the corresponding input and decoded signals in digital mode. Missing bits are indicated by arrows. (Adapted with permission. Copyright 2007 American Association for the Advancement of Science.) 1770
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is crucial to the binding of thalidomide. The aptamer’s enantioselectivity is important because researchers still have not determined whether a single enantiomer is responsible for the drug’s action or its side effects. (J. Am.
Chem. Soc. 2007, 129, 1456–1464)
