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ANALYTICAL CURRENTS Multiple-layer MALDI Ulrich Schubert and Michael Meier at the Eindhoven University of Technology and the Dutch Polymer Institute (both in The Netherlands) have developed a new method for preparing polymer samples for MALDI TOFMS that yields reproducible spectra that are highly accurate. Instead of pre-mixing the analyte, salt additive, and matrix and then spotting this mixture on the MALDI sample target, the researchers use a “multiple-layer approach”. Each component is spotted separately, one after the other, onto the MALDI target. In this work, the researchers optimized the multiple-layer approach for poly(ethylene glycol) (PEG), a polymer useful for applications such as drug delivery systems and batteries. Each layer was spotted onto the MALDI target, allowed to dry, then
covered with another layer, thereby eliminating the need to consider miscibility of the components. Using five PEG standards, they tested various matrices, matrix solvents, and doping salt solutions. The best results were obtained using the matrix dithranol in chloroform and sodium iodide as the doping salt. The researchers also found that the order of spotting was important. The best spectra were observed when the first spot on the MALDI target was the matrix, followed by the doping salt solution, then the analyte. Some of the multiplelayer combinations resulted in a higher signal intensity and S/N than the conventional mixing approach. PEGs up to 35 kDa could be detected using multiple-layer spotting, which is a
Figure Not Available for Use on the Web
Spectrum of PEG 3 using the multiple-layer method. (Adapted with permission. Copyright 2003 Wiley.)
vast improvement upon known techniques. The method is faster than conventional methods, and may be useful for high-throughput screening. (Rapid Commun. Mass Spectrom. 2003, 17, 713–716)
Detection of isomers by chromogenic receptors Several chromogenic receptors have been
late anions initially tested,
described for the detection of metal ions,
only oxalate, malonate, and
and until now, none existed for the deter-
maleate, which have car-
mination of anion isomers. Ramón Martínez-
boxylate groups close to
Máñez and colleagues at Universidad
each other, appeared magen-
Politécnica de Valencia (Spain) report they
ta or red-magenta when in
have synthesized the first known supramo-
solution with one of the 1,3,5-
lecular chromogenic receptors for discrim-
triarylpent-2-en-1,5-diones.
inating between anion isomers in aqueous
The receptors also changed
solutions.
color when in solution with
The researchers synthesized a series of yellow 1,3,5-triarylpent-2-en-1,5-dione re1 8 ceptors, abbreviated L –L , with absorption
Figure Not Available for Use on the Web
In the presence of L5, a solution containing maleate (left) but not fumarate (right) turns magenta. (Adapted with permission. Copyright 2003 Wiley–Verlag GMbH.)
phthalate anions (ortho) but not isophthal-
hydrogen bonds with the receptor, which
ate (meta) or terephthalate (para) anions.
leads to nucleophilic attack of the receptor’s
These results led Martinez-Máñez to
C1 carbon by its hydroxylic oxygen, resulting
peaks at 370–380 nm. In the presence of par-
conclude that only diacids with “tweezer-
in cyclization and formation of the magenta
ticular carboxylates, the yellow receptors
like” arrangements of carboxylate groups
cation. Understanding this mechanism, the
1
8
undergo cyclization to produce a magenta
cause the cyclization of L –L receptors.
researchers believe, will assist in the devel-
pyrylium cation with an absorption peak at
They say two carboxylate groups in close
opment of additional chromogenic sensors.
550 nm. Of the carboxylate and dicarboxy-
proximity to each other can both form strong
(Angew. Chem., Int. Ed. 2003, 42, 647–650)
M A Y 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
199 A
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ANALYTICAL CURRENTS Carbon nanotube-coated electrodes trodes with the nanotube mixture. In a 5 10–3 M H2O2 solution, cyclic voltammograms with the carbon nanotube/Nafion electrode generated a peak of ~0.20 V, but the same experiment with a bare glassy carbon electrode showed no response over a wide potential range. Other nanotube-coated types of electrodes yielded a similar reduction in the H2O2 overvoltage. The researchers tested the modified nanotube electrode with well-known detection schemes for glucose and catecholamine neurotransmitters. The combination of the lower redox potentials and the antifouling and discriminative properties of Nafion films gave good results, even in the presence of such interferences as acetaminophen, ascorbic
Electrodes coated with carbon nanotubes solubilized in the perfluorosulfonated polymer Nafion exhibit a marked decrease in the overvoltage for the H2O2 redox reaction. This discovery opens the door to using lower potential detection ranges for oxidase-based amperometric biosensors, such as those for glucose, and avoiding problems with interfering compounds. Working with carbon nanotubes has been difficult because of their poor solubility. Joseph Wang and co-workers at New Mexico State University and Pacific Northwest National Laboratory found that they could suspend single- and multiwalled carbon nanotubes in Nafion solutions; optimal results were with 0.5% wt Nafion in a buffer solution. Using this approach, they coated glassy carbon elec-
Solubility test. Photographs of single-walled carbon nanotubes in (a) phosphate buffer, (b) 98% ethanol, (c) 10% ethanol in buffer, (d) 0.1% Nafion in buffer, (e) 0.5% Nafion in buffer, and (f) 5% Nafion in ethanol.
acid, and uric acid. (J. Am. Chem. Soc. 2003, 125, 2408–2409)
Shedding light on luminescence patterning Luis M. Liz-Marzán, Nicholas Kotov, and col-
tential applications for their NP patterning
tion to the tunability of the particle diame-
leagues at Oklahoma State University and
are in cryptography and cell tagging.
ters, the system may provide a versatile
Universidade de Vigo (Spain) introduce a a
c
The researchers were able to increase
b
platform for multicolor images similar to the
the photoluminescence intensity of citrate-
high-resolution images produced by scan-
stabilized NPs by irradiating them with am-
ning probe microscopy. Using 200-mesh
bient oxygen. They prepared solid films of
transmission electron microscopy (TEM)
CdSe/CdS on glass slides and Si wafers by
copper grids placed on top of the NP film
standard LBL procedures. The researchers
as lithography masks, the researchers cre-
report that exposure of the CdSe or CdSe/
ated highly luminescent squares in the irra-
CdS dispersions to ambient light for several
diated areas and dark lines in regions shield-
days dramatically enhanced the lumines-
ed by the TEM grid. They were able to make
cence intensity, with a quantum yield (QY)
patterns of different colors by using NPs of
as high as 25–45%. They say no obvious in-
different diameters.
d [nm] 500 00 100 0000 00 2 300 000 40 0000 0 5 000 0 6 000 0 7 000 0 8 0 900 nm
[
10 2 000 30 0000 40 000 50 000 60 000 7000000 0 80 90 000 ] [nm 000
]
(a–c) Fluorescence and confocal (insets) microscopy images of light-induced 200mesh grid patterns made for NPs of different diameters. (d) Topography image of patterned film, scale bar 200 µm.
crease in photoluminescence was observed
Unlike standard photolithography or
for the same samples stored in the dark. In
conventional photography, their optical
another example, exposing the CdSe/CdS/
patterning technique doesn’t require chem-
ZnS nanorods to laser light overnight re-
ical functionalization of the surfaces and
new approach to functional patterning of
sulted in a QY increase of luminescence
subsequent processing. The luminescence
multicolor, optically active nanoparticle (NP)
yield from 1–16%.
patterns also provide a high S/N, making
films that combines some of the advantages
The group says that because of the high
them ideal for optical devices, information
of layer-by-layer assembling (LBL) and pho-
contrast in luminescent intensity between
processing, and biophotonics. (J. Am. Chem.
tolithography. The researchers say that po-
dark and bright states of the NPs, in addi-
Soc. 2003, 125, 2830–2831)
200 A
A N A LY T I C A L C H E M I S T R Y / M A Y 1 , 2 0 0 3
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Microsecond separations Nanoscale optical biosensor If Guinness included a section for analytical chemists in its book of world records, Matthew Plenert and Jason Shear would get the nod for the fastest chromatographic separation. The University of Texas–Austin researchers demonstrate a baseline separation of a
Richard Van Duyne and colleagues at Northwestern Uni-
a
b O
versity introduce a new fabrica-
HS
tion method that improves the
HS
C OH O CH3 HN
adhesion of Ag nanotriangles 100 nm
for a real-time immunoassay sensor. They say their results
H2N
under real-world conditions.
O
H N
NH
S O
c
show that their biosensors are reusable and could work in vivo
O
Ka,surf +
Ag Mica
Ag
Antibiotin
Mica
The researchers revamped the substrate of a localized
Figure Not Available for Use on the Web
surface plasmon resonance (LSPR) nanobiosensor by substituting mica for glass in the
LSPR nanobiosensor. (a) Tapping mode atomic force microscopy of the Ag nanoparticles. (b) Surface chemistry of the Ag nanobiosensor. (c) Schematic representation of antibiotin binding to a biotinylated Ag nanobiosensor fabricated by NSL.
substrate and using a surfactant in the nanosphere lithography (NSL) fabrication process. Through the NSL fabrication process, they created massively parallel arrays of Ag nanoparticles on the mica substrate. Atomic force microscopy showed that the nanotriangles were ~90 nm wide and ~50 nm high. To demonstrate the sensor, the researchers covalently attached biotin to the surface of the Ag nanotriangles and then incubated the particles in an antibiotin solution for 3 h. Using the shift in the extinction maximum (max) for the LSPR spectrum of the triangular (a) Schematic of the high-speed CE separation system with optical gating used to detect analytes in 0.1 MV/cm to get this high-speed separation with CE. To improve separation efficiency and lower separation times, they further narrowed the capillary by pulling it into an hourglass shape. For example, they describe a
Ag nanoparticles, they were able to monitor the interaction between biotin (the surfaceconfined antigen) and antibiotin (the solution-phase antibody). They observed a red shift in the LSPR max of ~38 nm, and fits to the experimental data revealed a surface-binding affinity of 4.5 107 M–1. The changes in the LSPR max were monitored by UV–vis spectroscopy. The researchers also determined that the biosensor had a detection limit of
