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ANALYTICAL CURRENTS
Cell sorting by optical forces
Samp input Sample
Philippe Marchand and colleagues at
isn’t turned on and the cell goes into the
Genoptix are using optical switch con-
waste channel.
trols to move cells through a microfluidic
Marchand and colleagues sorted a
device. The investigators report that the
mixture of cells containing a constitutively
new cell-sorting approach is rapid (2–4
expressed histone-green fluorescent pro-
ms) and that the optical forces do not
tein and non-expressing cells in a micro-
damage the cells.
fluidic device. They demonstrated that as
After cells are introduced into a microfluidic device, they are concentrated into a narrow region by hydrodynamic
B Buffer
Buffer
A Analysis region Optical switch
few as 1000 and as many as 280,000 cells could be sorted in the device in 104/ bacterium. Compared with bulk Raman spectra, the bacterial surface-enhanced Raman scattering (SERS) spectra were less spectrally congested and showed greater differentiation; these properties allowed the researchers to clearly distinguish between species and strains. In all, they obtained SERS spectra for six different species of bacteria, including Gram-negative and Gram-positive bacteria. The spectra were obtained with ~2-mW incident laser power and a 10-s data accumulation time. Each species had a unique SERS vi-
(b) (a)
~500 nm
brational fingerprint in the ~2 m 400–1700 cm–1 region. Even closely related species, such as Bacillus anthracis Sterne, Bacillus cereus, and Bacillus thuringiensis, had unique SERS signatures. In addition, two different strains of the same bacteria had distinctly different SERS spectra. (a) A scanning electron microscope (SEM) image of a twoThe researchers also cell chain of Bacillus anthracis Sterne bacteria on a goldobtained bacterial SERS nanoparticle-covered SiO2 SERS chip. (b) An SEM image of spectra at the single-cell the SERS substrate at ~10-fold enhanced magnification, level using their goldshowing clusters of gold nanoparticles. coated substrates. The ent from SERS spectra of the same bachigh sensitivity of the approach allows teria obtained with previously reported for the identification of individual components in a mixture of cell types, which SERS substrates. The results suggest that the SERS signature of bacteria is could be important for identifying danspecific to the morphology of the SERSgerous pathogens in mixtures. SERS spectra obtained using the new promoting metal surface. (J. Phys. Chem. B 2005, 109, 312–320) gold-coated substrates were very differ-
Predicting single-molecule reaction rates Predicting reaction rates at the single-mol-
both unsubstituted thiophe-
ecule level is extremely difficult and typi-
nols and thiophenols with
cally involves the use of quantum mechan-
various chemical groups
ics principles. Now Ludwig Bartels and
placed at different positions
colleagues at the University of California,
on the aromatic ring. While
Riverside, have found an easier way to pre-
an STM probe applied elec-
dict single-molecule reaction rates by ap-
trical pulses to the mole-
plying a linear free energy relationship that
cules, the researchers
is similar to the Hammett equation used in
measured the rate of hydro-
organic chemistry. The researcher mea-
gen removal from the thio-
sured a single-molecule chemical reaction
phenols.
STM images of unsubstituted and para-halosubstituted thiophenols on a copper surface. (Adapted with permission. Copyright 2004 National Academy of Sciences.)
rate with a scanning tunneling microscope
The researchers discov-
(STM) and found that it was in good agree-
ered that the rate of hydro-
ment with that predicted using a Hammet-
gen removal depended on the chemical
gy relationship resembling the Hammett
like equation.
nature of groups or atoms attached to the
equation that describes the effect of sub-
aromatic ring and their relative positions on
stituents on aromatic rings on the rates of
nols, molecules that are simple aromatic
the ring. Their results support the theoreti-
chemical reactions. (Proc. Nat. Acad. Sci.
rings with an –SH group. They analyzed
cal predictions made by a linear free ener-
U.S.A. 2004, 101, 17,920–17,923)
Bartels and colleagues studied thiophe-
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