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New deposition process for nanoelectrodes
Current/nA
Carbon electrodes with radii as small as 1 nm have been fabricated, but how well do they work? Anthony Kucernak and Shengli Chen at Imperial College (United Kingdom) tackle that question by characterizing such nanoelectrodes with steady-state voltammetry using a range of redox probes. The researchers fabricated electrodes that have exposed apexes yet are free of pinholes along the main fiber. Kucernak and Chen then examined the voltammetric responses of these electrodes for the reduction of Ru(NH3)63+, IrCl 62 –, and Fe(CN)63– and the oxidation of Fe(CN) 64–. The researchers report that well-defined, steady-state voltammetric responses were obtained in the presence of a supporting electrolyte. Without the supporting electrolyte, however, they observed significant deviations from ideal voltammetric behavior that depended on the specific properties of the redox system and the electrode size. They propose that the dynamic diffuse double-layer effect of solid–liquid interfaces—not the Frumkin effect, which is based on analytical assumptions that there is a stationary and equilibrium charge distribution in the diffuse double layer—is responsible for these deviations in nanometer-sized electrodes. (J. Phys. Chem. B 2002, 106, 9396–9404)
0.0 (a) –0.1 reff = 0.11 µm i/id = 0.04 –0.2 –0.3 –0.4 –0.5 0.0 –0.5
(b)
–1.0 reff = 0.52 µm i/id = 0.27 –1.5 –2.0 –2.5 0.0 (c) reff = 3.5 µm
–5.0 i/id = 0.71 –10.0 –15.0
–0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 Potential/(V vs SCE)
0.4
0.5
Steady-state cyclic voltammograms for the reduction of 0.01 M K3Fe(CN)6 on carbon electrodes of various sizes (a–c) with (solid line) and without (dotted line) supporting electrolyte.
Nanoparticle multiplex—now in “technicolor” “Color” transforms a formerly gray-scale technique into a multiplex, quantitative analysis system. By tacking Raman-active dyes onto gold nanoparticles and determining fingerprint patterns for probe–nucleotide complexes, Chad Mirkin, YunWei Cao, and Rongchao Jin of Northwestern University distinguish among dissimilar DNA targets or RNA strands with single-nucleotide polymorphisms and achieve a 20-fmol detection limit. The new approach builds on earlier work by Mirkin’s lab with silver-enhanced gold nanoparticle labels for detecting the hybridization of oligonucleotides (oligos). “Scanometric” detection—an inherently gray-scale method that uses a flatbed scanner—and electrochemical detection for arrays of oligos have been described previously. In the new method, Raman-active dyes are added to the oligos. The silver coating provides a substrate for surface-enhanced Raman spectroscopy, resulting in strong, reproducible Raman signals that are not observed otherwise, yet it avoids the background signal seen with scanometric detection. The researchers consider this surface-enhanced Raman method equivalent to a “multicolor” system because the spectra are unique to the labels present, and they note that it may be more amenable to multiplexing than fluorescent dyes, which may exhibit indistinguishable spectra. The researchers tested the new nanoparticles with six different Raman dyes and various 30- to 36-nucleotide samples taken from viruses, including those that cause hepatitis A, HIV,
(I) Gray-scale scan of microarray spots (1) before and (2) after a stringency wash and (3) after silver enhancing. (II) Raman spectra of the same spots at (a–g) different ratios of two RNA strands, with (inset) Raman intensity ratios (I2 /I1) vs target ratios (T2 /T1). (Adapted with permission. Copyright 2002 American Association for the Advancement of Science.)
Ebola, and smallpox. Extending the approach to a mixture of two RNA targets that differed by a single-base mutation in the probe-binding region, the group found that the signal intensities of the two probes were proportional to the ratio of the target concentrations. (Science 2002, 297, 1536–1540) N O V E M B E R 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
563 A
