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ANALYTICAL CURRENTS Gas ionization sensors from carbon nanotubes Nikhil Koratkar, Pulickel Ajayan, and colleagues at Rensselaer Polytechnic Institute have fabricated a compact nanotube device that detects gases. The new sensor can be powered by batteries and is unaffected by temperature, humidity, and gas flow. Preliminary tests using these sensors as detectors in GC looked promising, the researchers say. Other groups have explored gas sensors based on the electrical conductance changes of semiconducting nanotubes. Although the method has high sensitivity, it can’t identify gases with low adsorption energies, poor diffusion kinetics, or poor charge transfer with nanotubes. In addition, nanotube conductance is very sensitive to changes in environmental conditions. In the new device, gases are identified on the basis of their breakdown voltages. The device’s anode is a vertically aligned multiwalled nanotube
(MWNT) film, the cathode is an aluminum sheet, and a dc voltage is applied between them. Because the nanotube tips have such small radii (~15 nm), they create very high nonlinear electric fields, which together form a “corona” of highly ionized gas that accelerates the breakdown of gases. For a fixed electrode spacing, each gas could be “fingerprinted” this way; for example, at an electrode spacing of 150 µm, helium had a breakdown voltage of 164 V, and ammonia’s was 430 V. Furthermore, the discharge current indicated the concentration of the gas. The high discharge currents of the nanotubes led to high sensitivities. The researchers obtained data over gas concentrations of 10–7–
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10–1 M. In a simulated GC test using He as the mobile phase, detection of analytes at ~25 ppm appeared feasible. (Nature 2003, 424, 171–174)
Simultaneous on-chip detection of glucose and insulin Joseph Wang and colleagues at New Mex-
munoassay for insulin and an enzymatic
ico State University have developed a
assay for glucose in a shared reaction cham-
novel microfluidic device to simultaneously
ber. The products of the reactions are then
measure glucose and insulin concentra-
electrophoretically separated and detected.
tions, both of which are used to diagnose
Wang and co-workers observe three
and manage diabetes. The device’s detec-
well-resolved peaks on the electrophero-
tion limits are physiologically relevant: 10
gram. One of the peaks corresponds to the
pM for insulin and 100 µM for glucose.
complex of anti-insulin bound to insulin,
Although insulin and glucose levels are
and another peak indicates the presence
routinely assayed in clinical laboratories, re-
of NADH. The third peak corresponds to
searchers have never combined the two into
unbound anti-insulin. Under optimized condi-
a single test until now. The new biochip de-
tions, the dual assay took 110 s to complete.
vice enables researchers to perform an im-
(J. Am. Chem. Soc. 2003, 125, 8444–8445)
Biochip schematic: (a) insulin antibody and glucose dehydrogenase, (b) reaction chamber, and (c) insulin and glucose. (d) p-Nitrophenyl phosphate is added postcolumn for the detection of anti-insulin.
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ANALYTICAL CURRENTS
Effectiveness of newborn screening by tandem MS Bridget Wilcken and co-workers at the Children’s Hospital at Westmead and the University of Sydney (both in Australia) evaluated tandem MS screening and found it valuable for diagnosing rare metabolic disorders. Although hospitals routinely use tandem MS to screen newborns’ blood for metabolic disorders, no one has conducted a randomized clinical trial to evaluate its effectiveness. Such a trial would include a control group of babies who would be denied tandem MS screening, which is a health risk few physicians are willing to take. A trial would also have to include a very large sample size to provide meaningful data about rare metabolic disorders. Instead, Wilcken and co-workers compared the records of newborns diagnosed with metabolic disorders during 1998–2002, when tandem MS was newly instituted as a routine clinical screening test, with the records of diagnoses made by physicians based on the appearance of newborns’ symptoms during the previous 24 years. Because symptoms do not appear until later in childhood for some disorders, the researchers also examined the records of all the children as they grew older. The researchers say that more cases of metabolic disorders in newborns are diagnosed by tandem MS screening than by symptoms alone, but the increase in sensitivity is only seen for a few disorders. For instance, the greatest increase in detection is observed for medium-chain acyl-CoA dehydrogenase deficiency. The researchers report that 23 cases were spread out over the entire 24-year period before screening, whereas 17 cases were identified during the 4 years in which babies were screened. (N. Engl. J. Med. 2003, 348, 2304–2312)
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Observing structural changes during ultrafast reactions resolved FSRS spectrum, which was Richard Mathies and colleagues at the compared with continuous-wave Raman University of California, Berkeley, have and picosecond resonance Raman specdeveloped a new vibrational spectrostra. Unlike the latter two spectra, the copy tool for studying structural changes FSRS spectrum of -carotene showed during ultrafast chemical and biological that the vibrational relaxation of the reactions. Femtosecond stimulated first excited singlet state (S1) occurs Raman spectroscopy (FSRS) is capable in a two-step process. In addition, the of capturing broad, high-resolution FSRS data allowed the researchers to vibrational spectra with
