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ANALYTICAL CURRENTS

M A R C H 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y 7 9 A© 2 0 0 4 A M E R I C A N C H E M I C A L S O C I E T Y

Researchers at the University of Chicagohave taken sensory protein binding reac-tions from nature and coupled them to afluorescent signal to create highly sensi-tive and selective biosensors. The newsensors, which are built on a platform ofMerR proteins, can detect a wide rangeof metal ions and small molecules.

In nature, the MerR family proteins haveevolved to sense and control metal ionsdown to femtomolar levels with high selec-tivity. MerR proteins bind to specific DNAsequences, leaving them intact until a tar-get analyte is present. When the proteinbinds with its target analyte, it unwinds theDNA and breaks two central base pairs.

Chuan He and Peng Chen created onesensor with E. coli CueR, a protein thatregulates intracellular Cu+ concentrations.Pyrrolo-C, which tightly pairs with G in aduplex DNA, was used as the fluorescentreporter. When pyrrolo-C binds with G, thefluorescence is quenched, and when thetwo unpair, it is enhanced.

The researchers synthesized DNA con-taining the CueR binding sequence withpyrrolo-C incorporated in the middle of thesequence. In the presence of Cu+, the fluo-rescence intensity increased within sec-onds. The enhanced fluorescence was di-rectly proportional to the concentration ofCu+ in solution down to nanomolar levels.

The CueR sensor exhibited only weak fluo-rescence in the presence of divalent metalions; however, it did respond to Ag+ and Au+.

Applying the same strategy, the re-searchers also developed a sensor with theE. coli MerR protein, which has a high affini-ty for Hg2+. Although the MerR sensor washighly selective for Hg2+ over other metalions, it also responded to Cd2+. It should bepossible to design sensors for different ana-lytes simply by replacing the protein with aMerR-type protein that regulates the analyteof interest. Nature has created such proteinsfor a broad range of metal ions as well assome toxic organic chemicals. (J. Am. Chem.Soc. 2004, 126, 728–729)

MerR protein-based biosensors

A shift in wind direction is all it takes tomake air-monitoring instruments go frommeasuring high levels of an air pollutantto background levels. To get such a largedynamic range, multiple instruments areoften needed, particularly when indirectmethods are used.

Field-portable instruments for measur-ing H2S, a prominent sulfur gas found inreduced environments, suffer from a limit-ed dynamic range and cannot differentiatebetween H2S and methanethiol (CH3SH),a reduced sulfur gas commonly foundwith H2S. Instead of operating several in-struments in parallel, Kei Toda and col-leagues at Kumamoto University (Japan)and Purnendu “Sandy” Dasgupta of TexasTech University designed a portable sys-tem with a large dynamic range that canmonitor both H2S and CH3SH simulta-neously. As a bonus, the device is coupledto an SO2 analyzer for continuous mea-surement of all three sulfur compounds.

To differentiate between H2S andCH3SH, the instrument contains twodiffusion scrubbers—one made of Nafionthat selectively collects H2S and anothermade of porous poly(tetrafluoroethylene)that collects both H2S and CH3SH. Bothgases are measured by the decrease influorescence that occurs when they reactwith fluorescein mercuric acetate.

To overcome the limited dynamicrange, two scrubber units are used—one with a long scrubber and one witha short, less efficient scrubber. The twoscrubber units are aligned in series. Theless efficient one is positioned upstream toreduce reagent consumption, and a fluo-rescence detector is placed after each unit.

The researchers envision using the in-strument to survey geochemical and bio-genic sources of sulfur gases, such as vol-canoes and septic tanks. They demonstratethe portability and large dynamic rangeof the instrument for H2S and SO2 near

the crater of Mt. Aso, a volcano in Kuma-moto, Japan. In addition, they test thesystem by monitoring the headspace of alarge septic tank. (Environ. Sci. Technol.2004, 38, 1529–1536)

Detecting sulfur gases near the source

A field-portable sulfur analyzer is tested nearthe crater of Mt. Aso, a volcano in Japan.