News Bacteria and integrated circuits make tiny sensors Two groups that might not appear at first glance to have much in common have joined forces to develop miniaturized sensors based on bioluminescent bacteria and integrated circuits on silicon chips. Gary Sayler, head of the Center for Environmental Biotechnology at the University of Tennessee, had already used bioluminescent bacteria in environmental monitoring devices, but they were connected by fiber optics to detectors and computers that were "cumbersome and expensive" and limited the number of compounds they could monitor. Michael Simpson an electrical engineer in the Instrumentation and Controls Division of Oak Ridge National Laboratory had been developing ways to put electrooptical detectors on silicon microchips The two researchers united their technologies to create what thev have dubbed "Critters on a Chio" The critters chip is a 2.2 mm x 2.2 mm x 0.5 mm silicon chip on which PseudomonasfluorescensHK44 has been placed. HK44 is a genetically engineered bacterial strain that fluo-
Sensors for divalent zinc Fluorescent chemosensors have improved the detection of metal cations in aqueous media, including biological samples, but sensors continue to have problems with cation selectivity. Biosensors made with proteins have been one way to approach this problem, but the large biomolecules can impose design constraints not present
resces at 490 nm as it degrades naphthalene. HK44's fluorescence response to naphthalene has a linear relationship to concentration until the bacteria are saturated. For the device to be quantitative, Simpson says that it is necessary to ensure that the bacteria population is stable and that the light measurement has a constant efficiency. HK44 is only one of several bacterial strains. Sayler says that they also have a strain that is senSimpson holds a critters chip. The integrated sitive to toluene, for example. "If circuit is magnified in the background. (Photo we can make this practical, it courtesv of Oak Ridae National Laboratory.) would be possible to implement chemical arrays of environmental sensors.' They have shown that the device can integrated circuit process used to manwork in laboratory-based experiments; ufacture computer chips. Simpson exSimpson says that at this point it's still pects that the device will be inexpensomething of a "laboratory curiosity". sive to manufacture, possibly less than a dollar apiece. The next step is figuring out how to make the critters chip robust. "If we put Currently, the two groups do not the bacteria on a chip," says Simpson, have joint funding to develop the crit"they have a finite lifetime. We need to ters chip, which hinders their profind a way to keep them moist tnd to gee gress. They continue to develop their nutrients to them." He says that they will own portions of the sensor—Sayler, have to develop the necessary microfluidthe biological, and Simpson, the elecics. To protect the electronics from the tronic. They hope to obtain joint fundbacteria, the chip will have to be encapsuing that will speed development of the lated in a thinfilmof silicon nitride. chip. Celia Henry
with an abiotic sensing molecule. Barbara Imperiali and Grant K. Walkup of California Institute of Technology have synthesized a family of zinc finger peptides that they use as fluorescent chemosensors with enhanced oxidative stability for divalent zinc. Each chemosensor consists of a synthetic peptide backbone and a fluorescent reporter that is sensitive to changes in the peptide conformation. Fluorophores may
Chemosensors for divalent zinc based on the zinc finger region, (a) The fluorophores used in the sensors. (Top) DMB, (middle) CMN, and (bottom) DNS. (b) A scheme representing the signal transduction mechanism. 338 A
Analytical Chemistry News & Features, June 1, 1997
be introduced at any point in the polypeptide by incorporating a special residue that takes the place of the hydrophobic cluster in the native zinc fingers. After the peptide is synthesized but before it is cleaved, the side chain can be unprotected and coupled to any of a number of fluorophores. This study used the fluorophores DMB (4-(dimethylamino)-benzamide), DNS (5-(dimethylamino) naphthalenesulfonamide), and CMN (3-carboxamido coumarin). The microenvironment of the fluorophore-bearing residue changes when the peptide complexes with Zn2+, resulting in enhanced fluorescence. DNS was determined to be the fluorophore with the best fluorescence and zinc-binding properties. The fourth-generation DNS peptide, which had the fluorophore in the middle of the peptide sequence, was responsive to submicromolar concentrations of Zn2+ in the presence of redox-active metal cations (Cu2+ and Fe2+) and vast excesses of the competing divalent cations Mg2* and Ca2+) (J. Am. Chem. Soc. 1199 119 3443-50)
