News
Dendritic probes boost biosensors
QCM biosensor, the three-dimensional hyers say that use of the surfaced-confined bridization surface yielded a 10-fold gain in DNA dendrimers expands the possibilities sensitivity and an 8-fold improvement in for hybridization biosensors, and they exdetection limit pect to improve sensitivity further by using When it comes to nucleic acid probes, dendrimers with more arms and by using a more is better. That's what Joseph Wang Wang and colleagues noted that the higher resonant frequency QCM. (J. Am. and colleagues at New Mexico State Unithree-dimensional dendrimers have more Chem. Soc. 1998,120,8281-82) versity and the Philadelphia-based comhybridization sites, and thus more detecpany Polyprobe have found. They have tion capability than the increased the sensitivity of DNA biosenDNA dendrimers or sors by replacing the usual singlebranched DNA strucstranded oligonucleotide probes with tures that have been three-dimensional DNA dendrimers. used previously for The researchers created structures with nucleic acid blot as—30 single-stranded arms of Cryptosporid- says. However, surface confinement of the ium parvum DNA and immobilizzd these three-dimensional dendendrimers on the surface of a quartzdrimers seems to crystal microbalance (QCM). Steady-state cause some steric hinsignals were obtained quickly, indicating drance which lowers that immobilization did not interfere with accessibility hybridization kinetics, and the dendrimerdegree and has not QCM biosensor responded rapidly to been observed in the changes in target concentration in the blot protocols None5-30-ug/mL and 25-200-ug/mL ranges. Schematic drawing of hybridization detection using a dendrimer— Compared with a traditional ollgonucleotide- theless the researchQCM biosensor. For simplicity, monolayer coverage is illustrated. assembly of the structures. Type A hexagons assembled around drops of water in heptane, whereas type B rings formed structures around droplets of organic solThe two-dimensional nature of photolithovents (in this case, pergraphic microfabrication can fall fluorodecalin, chloroform, flat, especially because some apor chlorobenzene) susplications cry out for threepended in water. Several dimensional microfabrication types of defects are possitechniques. George M. Whiteble, including overlapping sides and his co-workers at Harhexagons and holes due to vard University step into the fupoor packing. The spheres ture by constructing threecould be welded together dimensional structures with hexby plating silver onto them. agonal rings. Self-assembly of Although the resulting the rings can be manipulated by structures fragile controlling the surface hydrophobicity and hydrophilicity. they were strong enough to The rings were formed by withstand the capillary electroplating gold into cin array of forces associated with hexagonal molds made by standraining the droplet and dard photolithography Silicon the surrounding solvent was evaporated onto the rings and to stand on their own allowing the formation of hvdroin the air philic surface oxide layers The Constructing the threenumber of hydroohilic surfaces dimensional microstrucwas controlled bv changing when tures is only the first step, the photoresist was removed Sursay the Harvard scientists. faces without an oxide layer were Before they will become made h drnnhohic by adding useful, ways must be found h d th' 1 The researchers to endow them with optical, electronic, or magnetic funcfabncated two types of rings. In tions and to connect the one type (A), all the sides and one individual components. hexagonal face were hydrophobic, whereas the remaining hexagonal Schematic outline of the process used to fabricate the hexagonal rings. (J. Am. Chem. Soc. 1998, 120,8267-68) face was hydrophilic. In the sec(A) Mostly hydrophobic surfaces. (B) Mostly hydrophilic surfaces.
Microfabrication in 3-D
700 A
ond type (B), the sides and one face were hydrophilic and the remaining face was hydrophobic. Liquid drops formed a template for the
Analytical Chemistry News & Features, November 1, 1998
