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ANALYTICAL CURRENTS
Luke P. Lee and colleagues at the University of California, Berkeley, and the University of California, San Francisco, have developed an optical method for manipulating liquid flow in microfluidic devices. The approach could lead to the development of automated and multiplexed microfluidic systems for biomolecular and cellular applications, as well as nanoscale, optically powered machines in aqueous environments. Lee and colleagues introduced photothermal nanoparticles (PNPs), in the form of gold nanocrescents, into a millimeter-scale droplet sitting on a substrate. The PNPs accumulated near the droplet’s liquid–air interface. When a focused light beam illuminated the PNPs, the particles generated heat and transferred it to the surrounding liquid. The heat transfer accelerated evaporation at the interface and produced a vapor, which almost immediately condensed on the substrate as tiny droplets near, or even in contact with, the liquid–air interface of the original droplet.
As the tiny droplets coalesced with the original droplet, the contact line of the original droplet was drawn forward. The liquid movement could be precisely controlled by turning the laser beam on and off. Lee and colleagues also demonstrated that the PNPs could achieve laminar, unidirectional flow in microfluidic channels. Higher flow speeds were possible because the vapor When illuminated by laser beams, PNPs precisely control and droplets were limited the flow of liquids in microfluidic devices. to the close confines of the direction of the cell transport could be microchannels. The investigators conprecisely controlled, and the PNPs didfirmed that liquid movement wasn’t n’t have any detectable effects on the vionly limited to straight channels— ability of the cells. The technique could it could also be guided around the lead to the fabrication of integrated micorners of T-junctions. crofluidic devices that don’t require In addition, Lee and colleagues pumps, valves, surface-chemistry modifishowed that three different cell types cations, or patterned electrodes. (Nat. could be optically transported in media that contained the PNPs. The speed and Mater. 2006, 5, 27–32)
In-gel stable isotope labeling Differences in protein expression are typi-
In the new approach, proteins are ex-
in gel at the protein level after visualization
cally visualized in stained 1-D and 2-D gels.
tracted from any source, including tissues,
with gel electrophoresis offers researchers
However, abundant proteins often mask
cell lines, and immunoprecipitates, and
a new strategy for the relative quantifica-
less-abundant proteins, and quantification
treated under two experimental conditions.
tion of protein expression as well as many
of posttranslational modifications is diffi-
The proteins are resolved in separate lanes
posttranslational modifications. Any organ-
cult. To overcome these limitations, John
by gel electrophoresis, and the regions of in-
ism or protein source can be used with
Asara and colleagues at Beth Israel Dea-
terest are reacted separately with light- and
ISIL, and almost all commonly used labeling
coness Medical Center, Purdue University,
heavy-isotope-labeled reagents. The labeled
reagents are compatible with the strategy.
and Harvard Medical School have devel-
proteins are then mixed together, digested
In addition, the method is expected to work
oped a method called in-gel stable isotope
with proteases, and analyzed by LC/MS.
with all types of polyacrylamide gels.
labeling (ISIL).
© 2006 AMERICAN CHEMICAL SOCIETY
The ability to incorporate isotopic labels
(J. Proteome Res. 2006, 5, 155–163)
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LUKE P. LEE
Optical control of microfluidic flow
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ANALYTICAL CURRENTS Lipid microarrays identify autoimmune antibodies William Robinson and colleagues at the Stanford University School of Medicine, Palo Alto VA Health Care System, and the University of Alberta (Canada) have developed large-scale lipid microarrays to study antibodies present in multiple sclerosis patients and mice with experimental autoimmune encephalomyelitis (EAE). The analysis of autoimmune responses to lipids is complicated because of the large number of potential lipid antigens, the hydrophobic character of lipids, and the technical problems of detecting antibody responses to lipids. Multiple sclerosis is thought to occur when antibodies destroy the protective myelin sheath that surrounds nerves in the central nervous system. Previous studies have demonstrated antibody attacks on myelin sheath proteins. However, the myelin sheath is also rich in lipids, which could also be targets of the antibodies. Robinson and colleagues created lipid arrays containing duplicate spots of 50 different brain, myelin, and microbial lipids, as well as glycolipids. All the lipids represented potential targets in multiple sclerosis. They incubated dilute serum or cerebrospinal fluid from multiple sclerosis patients or EAE mice on the lipid arrays and detected antibody binding by chemiluminescence. The assays with the lipid arrays were found to be 5–25 more sensitive than conventional ELISA for detecting the activity of lipid-specific antibodies. A statistical analysis of the data revealed lipid-specific antibodies against myelin, oxidized lipids, and microbial lipids in the cerebrospinal fluid of multiple sclerosis patients. The data also suggested lipid-specific antibody responses in the EAE mice. In both the multiple sclerosis patients and the EAE mice, numerous common lipid targets were identified, including sulfatide, oxidized cholesterol, and sphingomyelin. Robinson and colleagues thus suggested that autoimmune attacks against these common lipid targets could be causes for diseases that involve a loss of the myelin sheath. (Nat. Med. 2006, 12, 138–143) 1374
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Sizing up nanoparticles The ability to measure nanoparticle dimensions inside micro- and nanoscale reactors is necessary to optimize reaction conditions during particle synthesis. To meet this need, Daniel Chiu and colleagues at the University of Washington have developed a strategy for measuring the sizes of nanoparticles in real time within microfluidic channels. Chiu and colleagues used confocal correlation spectroscopy (CCS) to measure the dimensions of fluorescent and nonfluorescent nanoparticles. CCS is amenable to studying low-concentration samples in small volumes (~0.3 fL). The technique usually measures the diffusion of fluorescent particles undergoing Brownian motion. However, the investigators were able to measure the backscattering of nonfluorescent nanoparticles and obtain the dimensions of the particles by CCS. The investigators measured particles ranging from 11-nm-diam quantum dots to 300-nm-diam fluorescent beads in fluorescence mode. For the nonfluorescence mode, they were limited to a range from 40-nm-diam gold colloids to 300nm-diam latex beads because of low signals. In order to measure the dimensions of nanoparticles in real time inside microfluidic channels, Chiu and colleagues introduced an L-shaped chamber into a channel. Nanoparticles flowing through the channel were collected in a deadvolume region of the L-shaped chamber where their motion became solely Brownian. The investigators demonstrated that a variety of particles, both fluorescent and nonfluorescent, could be detected by CCS in the dead-volume region. The measured radii of the particles ranged from 5.5 to 150 nm and agreed well with the certified values provided by the manufacturers. (J. Am. Chem. Soc. 2006, 128, 730–731)
50 µm
Nanoparticles are collected in an L-shaped chamber within a microchannel to be sized in real time by CCS.
news
Titan’s hazy atmosphere
NASA
and several other institutions examThe hazy atmosphere that surrounds ined the physical characteristics of Saturn’s largest moon, Titan, is Titan’s environment. They reportcomposed of mainly nitrogen and ed temperature and density profiles methane. The Huygens probe, refrom an altitude of 1400 km down cently launched from the Cassini to the surface, as determined by spacecraft, will help scientists to the Huygens Atmospheric Strucbetter understand the origin of ture Instrument (HASI). The denthese constituents. Data from that sity of the upper atmosphere was probe are now beginning to appear determined directly by HASI, and in the literature, providing the first the temperature was derived from direct observations of Titan’s surthe density measurements. In the face and lower atmosphere. lower atmosphere and on Titan’s Hasso Niemann and colleagues surface, HASI measured temperaat NASA and several institutions ture and pressure directly. Electrical around the world have published activity was also monitored during some of the first GC/MS data from the descent of the probe, in hopes the Huygens probe, including altiof finding evidence of lightning tude profiles of the major atmoactivity. spheric constituents, isotopic abunData from the Huygens probe are beginning to shed The researchers found higher dances, and trace organic species. light on the composition of Titan’s atmosphere and the than expected temperatures and The team confirmed the presence nature of its aerosols. densities in the upper atmosphere of nitrogen and methane. Although of Titan. In addition, they observed a they did not detect any noble gases also detected trace organic species on other than argon, they found both Titan’s surface, including cyanogens and lower ionospheric layer between 140 and 40 km. Electrical conductivity primordial 36Ar and radiogenic 40Ar, ethane. In another paper, Francesca Ferri and peaked at ~60 km. (Nature 2005, 438, which provided important clues about colleagues at Università di Padova (Italy) 779–784; 785–791) the outgassing history of Titan. They
Profiling depth in MALDI imaging MS In MALDI imaging MS, it is generally as-
peared to be evenly distributed throughout
for brain tissue, the signal decreased; this
sumed that only analytes on the surface
the tissue, the MALDI signal intensity varied
suggests that the matrix effect is depend-
are detected. Exactly what constitutes a
with the thickness of tissue samples. Chen
ent on the tissue type.
surface analyte, however, is unclear. Are
and colleagues speculated that the matrix
molecules within 1 nm of the surface de-
solvent must access the interior of the tissue.
The results indicate that matrix solvent penetrates tissue as deeply as 40 µm. In
To test their hypothesis, the researchers
addition, analytes in the interior of the tis-
from the surface? To answer these ques-
covered the liver slices with blank rat brain
sue can be extracted to the surface in situ
tions, Jiwen Chen and colleagues at the
tissue slices. Prazosin was readily detected
and detected by MALDI MS. For the high-
Schering-Plough Research Institute studied
in the liver slices even when they were
est extraction efficiency, the researchers
the profiling depth of MALDI imaging MS.
covered with two layers of brain tissue
recommend using conditions that decrease
The researchers analyzed slices of liver
measuring 40 µm in thickness. No signal
the matrix solvent evaporation during ma-
tissue from mice treated with the hyperten-
was obtained for the brain tissue alone.
trix application. (Rapid Commun. Mass
sion drug prazosin. Although the drug ap-
When blank kidney tissue was substituted
Spectrom. 2006, 20, 284–290)
tected? How about those further than 1 nm
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ANALYTICAL CURRENTS Detecting marine toxins Properly oriented antibody-binding proteins Nadezhda Kulagina and colleagues at the Naval Research Laboratory and the National Oceanic and Atmospheric Administration have developed a new method for monitoring the presence of brevetoxin-3 (PbTx-3) and saxitoxin (STX) in seawater. PbTx-3 and STX are potent neurotoxins produced by harmful algal blooms.
By adding a layer of protein A, G, or L (ProA, ProG, or ProL), researchers have improved the binding capacity, sensitivity, and stability of antibody arrays. Getting the proteins in a proper orientation, however, remains a challenge. Wilfred Chen, Ashok Mulchandani, and colleagues at the University of California, Riverside, have found one solution to the problem. They have developed a fabrication method that takes advantage of the thermally tunable hydrophobic properties of elastin (ELP)-based biopolymers. The method provides a universal way to immobilize antibodies in a proper orientation without covalent modifications. In the new approach, ELP fusions with ProA, ProG, or ProL are used to immobilize antibodies onto hydrophobic surfaces. Antibodies are conjugated to ProA, ProG, or ProL in the fusion proteins, and the complexes are immobilized via a temperature-triggered interaction between ELP and the
A spinal-cord neuronal network cultured over a microelectrode array substrate is used to detect neurotoxins in seawater.
hydrophobic surface. To demonstrate the method’s multiplexing capability, the researchers created arrays with rabbit, goat, and mouse antibodies, each labeled with
The method uses a neuronal network biosensor, which relies on cultured mammalian neurons grown over microelectrode arrays. The inherent bioelectrical activity of the network is monitored noninvasively in a portable recording system. The researchers prepared neuronal networks from spinal cord tissue of embryonic mice. Cells were seeded on a microelectrode array surface with 64 recording sites. Extracellular action potentials across the network were analyzed both before and during exposure to purified toxins or algal cell solutions. Toxin-producing algal cultures induced changes in the neuronal network activity that were consistent with the toxin concentration. The extracellular action potentials were sensitive to PbTx-3 and STX in complex matrixes, including algal growth medium and natural seawater. Detection limits for PbTx-3 and STX were in the nanomolar range. (Environ. Sci. Technol. 2006, 40, 578–583)
different fluorophores. They demonstrated the method’s utility by fabricating an antibody array for the detection of cancer antigen 19-9, a tumor marker used in the diagnosis of liver cancer. The new fabrication method could provide a simple way to immobilize antibodies for use in a variety of microarray sensors. (J. Am. Chem. Soc. 2006, 128, 676–677)
NIST upgrades MS database The National Institute of Standards and Technology (NIST), in collaboration with the U.S. Environmental Protection Agency (EPA) and the National Institutes of Health (NIH), has released a new version of the NIST/EPA/NIH Mass Spectral Library. The new edition, called NIST 05, adds ~20,000 MS spectra to the database, bringing the total number of spectra to >163,000. Each one has been critically evaluated to ensure the data are current. The upgraded database now includes >2000 MS/MS spectra, which could be of use to proteomics and metabolomics researchers. Gas-phase retention index data for >25,000 volatile organic compounds have also been added to the library for use with GC. The NIST 05 library is available with version 2.0d of the NIST MS Search Program for Windows. More information can be obtained from the NIST Standard Reference Data Program at www.nist.gov/srd/nist1a.htm.
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