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ANALYTICAL CURRENTS Sensing DNA in a flash Here’s a new twist on the hybridization of complementary single strands to detect target DNA. Itamar Willner and colleagues at the Hebrew University of Jerusalem (Israel) describe the formation of a cross-linked DNA–CdS nanoparticle array on the surface of an electrode. A pulse of light activates the array, and the amount of target DNA present is determined by the amount of photocurrent flowing through the array. Two short oligonucleotides (oligos)—one that is complementary to the 3´ end of the target DNA, and one that is complementary to the 5´ end—are synthesized. Copies of one oligo are attached to the surface of an electrode; copies of the second oligo are attached to CdS nanoparticles. When the functionalized electrode is exposed to a solution containing the target DNA, and the functionalized CdS nanoparticles are added, the result is an array of threecomponent DNA duplexes. Repeating this process can expand the array. A pulse of light switches the array on or off—that is, starts or stops the flow of
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DNA-modified colloids Au surface
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Schematic diagram showing the assembly of the DNA–CdS nanoparticle array. One oligonucleotide (1) is attached to the gold surface of an electrode. A second oligonucleotide (2) is attached to CdS nanoparticles. In the presence of a target DNA sequence (3), a cross-linked array, which conducts photocurrent to the electrode, forms. (Adapted with permission. Copyright 2001 Wiley-VCH Verlag GmbH.)
photocurrent. The researchers suggest that the photocurrent is generated by the photoejection of conductance-band electrons from the CdS nanoparticles. [Ru(NH3)6]3+ enhances the electron
transfer from any inactive CdS particles. The amount of photocurrent increases with the amount of target DNA present. (Angew. Chem., Int. Ed. 2001, 40, 1861–1864)
C6H13
Golden cages O
Advances in nanotechnology will require the adaptation of current microfabrication technologies to an even smaller scale, and assembling three-dimensional structures on solid supports calls for special attention. For this application, selfassembly shows promise over covalent synthesis because it is a reversible process that allows self-repair of potential deficiencies. Self-assembly was previously limited to solution chemistry, but Enrico Dalcanale, David Reinhoudt, and colleagues at the University of Twente (The Netherlands) and the Università degli Studi di Parma
(Italy) have demonstrated a way to generate coordination cages directly on gold surfaces by using self-assembled monolayers (SAMs) as molecular platforms. Monolayers were prepared according to published procedures. Atomic force microscopy (AFM) acts as a “molecular ruler” to determine the thickness of the monolayers and show that metal coordinates to the gold surfaces to form cages. The formation of cages was confirmed by contact angle measurements, electrochemistry, and X-ray spectroscopy, in addition to AFM. (Angew.
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2 + [M(dppp)(OTf)2] CH2Cl2 7: 40 °C 6: RT
S S Au Au Au Au Au Au Au Au
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Representation of the reaction that leads to the formation of cages on a self-assembled monolayer. (Adapted with permission. Copyright 2001 Wiley-VCH Verlag GmbH.)
Chem., Int. Ed. 2001, 40 (10), 1892– 1895)
A U G U S T 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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ANALYTICAL CURRENTS Using NSOM to study surfaces The promise of near-field scanning optical microscopy (NSOM)—a technique that bypasses the Rayleigh diffraction limit and provides resolution on a nanometer scale—is demonstrated in two recent surface studies. In the first, Shinzaburo Ito and Hiroyuki Aoki of Kyoto University (Japan) investigate the phase separation of a polymer blend monolayer by NSOM. Understanding the properties and morphology of such monolayers is important in constructing welldefined layered structures. The polymer blend was a mixture of dye-labeled poly(octadecyl methacrylate) (PODMA) and poly(isobutyl methacrylate), which form a stable monolayer at the air/water interface. The phase-separated structure of the polymer monolayer
was imaged by fluorescence NSOM. In monolayers that were completely phaseseparated, the phase boundary at which some components mixed was mapped by energy-transfer emission NSOM. Timeresolved measurements were obtained with a temporal resolution of
