Volume 96, Issue 5, 01 September 2004
Index of content:
- APPLIED BIOPHYSICS (PACS 87)
Conductivity of DNA probed by conducting–atomic force microscopy: Effects of contact electrode, DNA structure, and surface interactions96(2004); http://dx.doi.org/10.1063/1.1769606View Description Hide Description
We studied the electrical conductivity of DNA molecules with conducting–atomic force microscopy as a function of the chemical nature of the substrate surfaces, the nature of the electrical contact, and the number of DNA molecules (from a few molecules to ropes and large fibers containing up to molecules). Independent of the chemical nature of the surface(hydrophobic or hydrophilic, electrically neutral or charged), we find that DNA is highly resistive. From a large number of current-voltage curves measured at several distances along the DNA, we estimate a conductivity of about per DNA molecule. For single DNA molecules, this highly resistive behavior is correlated with its flattened conformation on the surface (reduced thickness, , compared to its nominal value, ). We find that intercalating an organic semiconductor buffer film between the DNA and the metal electrode improves the reliability of the contact, while direct metal evaporation usually destroys the DNA and prevents any current measurements. After long exposure under vacuum or dry nitrogen, the conductivity strongly decreases, leading to the conclusion that water molecules and ions in the hydration shell of the DNA play a major role.
Electrophoresis of long deoxyribonucleic acid in curved channels: The effect of channel width on migration dynamics96(2004); http://dx.doi.org/10.1063/1.1776625View Description Hide Description
We investigated the dynamics of long deoxyribonucleic acid (DNA) migrating through curved channels under electric fields. Long DNA exhibits large conformational changes in the curved channels because of the inhomogeneity of the electric fields around curves. Two kinds of channel shapes were used for the examination. One (type I) has the same width in the curved region as in the straight region. The other (type II) is wider in the curved region than in the straight region. The difference in migration rates between long DNA and short DNA was larger in type II than in type I chips. We discuss the separation mechanism of the type II chip.
Laser trapping and patterning of protein microcrystals: Toward highly integrated protein microarrays96(2004); http://dx.doi.org/10.1063/1.1777400View Description Hide Description
Some insect virus infections occlude into a crystalline matrix consisting of a protein named polyhedrin. The shape of the matrix is a cubic polyhedron of the size of a few micrometers. Recently it was shown that these polyhedra could immobilize various functional proteins within them. Therefore, the polyhedron is interesting as an element in a protein chip. In this work, individual polyhedra were arrayed and bonded under a microscope by focused laser beams, with the aim of fabricating a highly integrated protein chip. The polyhedron was trapped and transferred to a suitable position on a polymer substrate by optical trapping with a (YAG, yttrium aluminum garnet) laser. To bond the polyhedron on the substrate, the polymer surface was mechanically and chemically modified by multiphoton absorption of a , femtosecond: sapphire laser, which results in strong adhesion of the polyhedron to the substrate. The arraying and bonding of polyhedra were successful, to a precision of about , with this procedure. The biological activity of polyhedra after these laser irradiations was confirmed by the fluorescence of green fluorescent protein occluded in the polyhedrin matrix.
Glucose biosensors based on organic light-emitting devices structurally integrated with a luminescent sensing element96(2004); http://dx.doi.org/10.1063/1.1778477View Description Hide Description
A platform for photoluminescence based biosensing is demonstrated for glucose. The sensor is structurally integrated, i.e., individually addressable organic light-emitting device pixels (serving as the light source) and the sensing element are fabricated on glass or plastic substrates attached back-to-back. This results in a very compact, potentially miniaturizable sensor, which should strongly impact -based biosensor technology. The sensing element is an oxygen-sensitive dye coembedded with glucose oxidase in a thin film or dissolved in solution. The glucose biosensor is demonstrated for two ∕dye pairs: [blue ]∕[ dye] and [green ]∕[ dye]. Both -intensity and -lifetime modes are demonstrated for each pair; the lifetime mode eliminates the need for frequent sensorcalibration. The sensor performance is evaluated in terms of design, dynamic range, limit of detection, and stability. The use of the glucose biosensor in conjunction with an oxygen sensor is also discussed.