lulose film. A 14-ug/mL IgG solution was placed directly on the film, and unbound antibody was rinsed away. The probe was then immersed in H20 or plasma solu­tions containing SNX-III acetate (an a>conotoxin), which is a 25 amino acid peptide. The detection limit for the method is 0.2 fmol/uLfor H20 solution; the detection limit for plasma solution is still under investigation.

They said that the time-consuming covalent immobilization procedure limits the use of on-probe immunoassay for real applications; however, the nitrocellulose film provides a simple and efficient ap­proach to on-probe immobilization. The nitrocellulose film facilitates the analysis of complex biological solutions, because the suppression effect is reduced relative to a bare probe tip. Direct extraction from solution also helps to overcome the sup­pression effect and concentrates the ana-lyte on the probe.

Climbing an electron staircase Single-molecule detection has attracted great interest recently, and single-molecule electrochemistry is no exception. The usual vision of a voltammogram is as a smooth continuum, but small enough electrodes make it possible to watch the effects of sin­gle electrons. Allen J. Bard and Fu-Ren Fan of the University of Texas at Austin have observed a coulomb staircase in which each step is the result of a single electron.

They used nanometer-scale electrodes that acted as faradaic capacitors. Because the electrodes were so small, the charge for each additional electron represented a volt­age change of ~ 0.1 V, which could easily be observed. Bard says that the steps were smeared somewhat by thermal effects, be­cause the experiments were conducted at room temperature rather than at liquid he­lium temperatures. The derivative of the coulomb staircase yielded regularly spaced maxima, substantiating the fact that each step was the result of the addition of a sin­gle electron. Larger electrodes did not dis­play the staircase effect.

In a separate experiment they injected charge into the electrode and observed the relaxation back to equilibrium. With a 10-um electrode, they observed a smooth decay, whereas with a 17-nm electrode the current-time curve showed discrete steps as it relaxed back to equilibrium.

Bard says that it is still too early to know whether this method will eventually have use as an analytical technique.

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Analytical Chemistry News & Features, June 1, 1997 343 A