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ANALYTICAL CURRENTS
Plasmon sensing in nanoscale holes Mikael Käll and colleagues at the Chalmers
To understand the properties of plas-
citation was highly localized and that the
University of Technology and Lund Univer-
mons in holes, Käll and colleagues fabri-
decay length of the plasmons was compa-
sity (both in Sweden) have analyzed the
cated a thin gold film containing 60-nm-
rable with that of the plasmons in metal
sensing capabilities of plasmons generated
diam cavities. The resonance of the
nanoparticles.
in individual nanoscale holes. The work
plasmons was within the red part of the
could lead to the development of highly
visible spectrum; this allowed the investi-
adsorption in a single nanoscale hole. Bi-
sensitive and label-free detection methods
gators to use dark-field spectroscopy for
otin conjugated to bovine serum albumin
for the analysis of biological molecules.
their analyses.
(BSA) was first introduced into the hole,
Käll and colleagues next studied protein
The investigators studied the effect of
followed by neutravidin, a deglycosylated
waves, are commonly generated either on
local changes in refractive index by attach-
form of the biotin binding partner, avidin.
a flat metal surface or within metal nano-
ing self-assembled monolayers of various
The investigators found that the plasmons
particles. Plasmons produced in nanoscale
polymer lengths to the metal surface within
in a single 60-nm-diam hole could sense
holes were recently demonstrated to be
the holes. By analyzing the response of the
the successive adsorptions of the ~2-nm
qualitatively similar to the plasmons in
plasmons to the monolayers, they quantita-
biotin–BSA and ~4.7-nm neutravidin layers.
metal nanoparticles.
tively determined that the hole plasmon ex-
(Nano Lett. 2005, 5, 2335–2339)
Plasmons, which are charged density
BIOTECHNOLOGY Molecular movements come to light A new microscope can track proteins that step along other molecules (Phys. Rev. Lett. 2005, doi 10.1103/PhysRevLett.95.208102). Steven Block and colleagues at Stanford University, in collaboration with Robert Landick at the University of Wisconsin, Madison, used the microscope to follow a single molecule of RNA polymerase in real time as it moved along a DNA template making RNA (Nature 2005, 438, 460 – 465). Because the microscope is so sensitive, they could deduce the enzyme’s position with angstrom resolution. “We can measure the position of something to within the diameter of a hydrogen atom,” Block says. The researchers attached a polystyrene bead to the RNA polymerase and then tethered the enzyme to another bead with a length of DNA. They trapped each bead with a focused IR laser beam. Instead of placing each bead near the center of its optical trap, how636
ever, they placed the first bead at the periphery. This kept the force on that bead constant even when the bead moved. Thus, “any stretching of the DNA or any other compliant linkage does not vary over time,” Block says. “These elastically compliant elements, therefore, do not contribute noise to the displacement signal.” When the researchers added nucleotide triphosphates, the polymerase moved along the DNA, taking the first bead with it—like an ant carrying a potato chip, Block explains. The size of the enzyme’s steps could be deduced directly from the bead’s displacement. The experiment showed that RNA polymerase ratchets along DNA base by base, 3.7 Å at a time. Other innovations also contributed to the microscope’s sensitivity. The researchers eliminated twinkling of the laser beam by sealing the microscope’s external optics in helium, which is less
A N A LY T I C A L C H E M I S T R Y / F E B R U A R Y 1 , 2 0 0 6
refractive than air. And because they didn’t need a computer to control the forces on the beads, they avoided time lags and feedback instability. The work could open the door to single-molecule DNA sequencing, Block says. “It will also allow us to look at the motions of virtually any macromolecule. Previously, work with optical traps was confined to molecules that made very large conformational changes.” Block suggests that almost any protein, including hemoglobin (which makes a conformational shift of 2–3 Å when it binds oxygen), could potentially be studied this way. Moreover, shifts in protein conformation could be analyzed in physiological environments to complement crystallography. “The promise of single-molecule biophysics is to be able to study molecules under normal conditions and at the reductionist limit, which is just one molecule,” he says. a —Linda Sage
