Credit: © 2008 ACS

Electronic noses for the detection of small molecules are attractive for their portability. However, the selectivity of current models is somewhat limited, unless large arrays of sensor libraries are operated combinatorily, producing varying responses to different analytes. James Heath and colleagues at the California Institute of Technology have now designed1 a sensor in which the analytes are detected from their individual molecular interaction with the sensor elements, enabling specificity to given target molecules.

Learning from other attempts to mimic the mammalian olfactory system, in which the sensed molecules bind to receptor proteins, Heath and colleagues covalently coupled oligopeptides to the surfaces of silicon nanowires, which themselves are sensitive gas sensors. They first formed an array of nanowires into a film, and then sectioned it into individual sensor elements using photolithography and etching. Peptides were prepared using a solid-state method, in which amino acid residues were linked on a bead support, cleaved using acid, and then purified by HPLC. Amide coupling was then used to immobilize the peptides onto the nanowire surface. Three-component sensors were prepared, in which one sensor element contained a peptide sequence that recognizes acetic acid, one with a peptide that recognizes ammonia and one with no modification.

Heath and co-workers found that their sensor was able to detect the target molecules at low concentrations and to discriminate between them in mixtures. Theory and experiment indicate that acid/base reactivity and structural interactions are both equally important in the selectivity. With further development, possible future applications include detection of molecular disease indicators by breath monitoring, and of chemical threats.