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Hard and Soft Biofunctionalized Diamond.

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Highlights
DOI: 10.1002/anie.200602509
Functionalization of Diamond
Hard and Soft: Biofunctionalized Diamond**
Anke Krger*
Keywords:
diamond · DNA · nanostructures · surface chemistry ·
thin films
In recent years, the functionalization of
semiconductor and carbon materials has
developed into an active field of research at the border between organic
chemistry and materials science. Thus,
numerous studies on the surface modification of diamond have been published. Diamond, owing to its dopability,
biocompatibility, and mechanical and
chemical stability, is a versatile material
for many applications.
Diamond materials can be produced
by various established techniques.[1]
Methods exist for the synthesis of single-crystalline and polycrystalline diamond films, as well as the production of
nano- to microscopic diamond particles.
Depending on the chosen synthetic
method, diamond materials exhibit significantly different surface properties.
For example, hydrogenated diamond
films have a highly hydrophobic character, which hinders the nonspecific adsorption of polar biomolecules.[2] In
contrast, poly-l-lysine, enzymes, or antibodies can be adsorbed on polar diamond surfaces coated with oxygen-containing groups. These noncovalently
modified diamond materials are being
investigated for sensors and other biomedical applications.[3] The presence of
functional groups on the diamond surface also enables the covalent bonding
of various moieties. Depending on the
original surface coverage, different functionalization strategies, such as silaniza-
[*] Dr. A. Kr$ger
Otto-Diels-Institut f$r Organische Chemie
Christian-Albrechts-Universit+t zu Kiel
Otto-Hahn-Platz 3, 24098 Kiel (Germany)
Fax: (+ 49) 431-880-1558
E-mail: akrueger@oc.uni-kiel.de
[**] A.K. gratefully acknowledges the Fonds der
Chemischen Industrie for a Liebig habilitation scholarship.
6426
Dedicated to Professor Henning Hopf
on the occasion of his 65th birthday
tion, esterification, amidation, and direct C C coupling have been applied.
These techniques have been used for the
modification of hydrogenated, halogenated, carboxylated, and hydroxylated
diamond surfaces.[4]
The aim of many of these investigations is the grafting of biologically active
moieties, such as DNA oligonucleotides,
peptides, enzymes, or antibodies.[5] The
application of biofunctionalized diamond in, for example, bioassays or
“lab on the chip” devices has been
considered.[6] Recently, the inherent
fluorescence of defective diamond has
been used for the fluorescence marking
of drugs. The N-V centers (isolated
nitrogen defects with adjacent vacancies) of diamond emit a characteristic
red fluorescence,[7] which is suitable for
fluorescence microscopy, including the
examination of living cells.[8] The advantages of such a diamond marker, in
addition to its favorable emission wavelength, are the high stability of the
emission and the biocompatibility of
the material.
The covalent attachment of DNA
molecules to modified diamond surfaces
has been described by several groups.
Takahashi and co-workers used terminal
carboxyl groups to connect the DNA
molecules through the formation of
amide linkages.[9] Ando and co-workers
described the grafting of DNA molecules through thymidines immobilized
by ester bridges.[10] A much more stable
coupling can be achieved by the formation of direct C C bonds. Hamers and
co-workers introduced a very flexible
technique that uses hydrogenated diamond films as the starting material.[11] In
a photochemical reaction, the terminal
vinyl group of a bifunctional organic
moiety is connected to the diamond
surface. After the deprotection of a
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
terminal amino group, a subsequent
reaction with sulfosuccinimidyl-4-(Nmaleimidomethyl)cyclohexane-1-carboxylate (SSMCC) yields a modified
diamond surface that is ready to be
grafted with DNA. The grafting is then
carried out using thiol-modified DNA
(Scheme 1).
Recently, Nebel and co-workers reported the microscopic characterization
of DNA-modified diamond surfaces.[12]
Using the technique described above,
the authors produced a continuous, covalently bound DNA film on a structured, partly hydrogenated (100) diamond surface. Subsequent reaction with
fluorescence-labeled
complementary
oligonucleotides yielded functionalized
diamond films, which were characterized by fluorescence microscopy. In this
study, only the areas that were initially
hydrogenated exhibited a strong fluorescence, whereas oxidized areas exhibited only weak fluorescence, owing to
nonspecific adsorption of the marker.
The morphology of the DNA-modified diamond surface was studied using
atomic force microscopy (AFM). By
performing the measurements in a buffered solution, the natural conformation
of the DNA molecules was conserved.
Binding-force measurements demonstrated the covalent bonding of the
DNA strands (Figure 1). Information
on the thickness and orientation of the
DNA layer was also obtained: the
molecules form a pinhole-free film with
a thickness of 76 = and are arranged at
an angle of approximately 368 relative to
the diamond surface. The authors have,
thus, performed the first microscopic
characterization of a biofunctionalized
diamond film. This characterization
method is of crucial significance for the
development of diamond-based sensors,
as only a reliable and established anaAngew. Chem. Int. Ed. 2006, 45, 6426 – 6427
Angewandte
Chemie
Scheme 1. Functionalization of a diamond surface with DNA oligonucleotides. See text for
details.
Figure 1. a) Schematic representation of binding-force measurements on a DNA-functionalized
diamond surface using an AFM tip. b) In these measurements, the DNA layer is removed by
scratching with the tip at increasing applied force (11–56 nN). (Adapted, with kind permission,
from reference [12].)
lytical technique can ensure that surface
coverage is complete and that the orientation of grafted molecules is correct.
In addition to the sensor arrays
mentioned above, functionalized diamond materials are promising candidates for applications in targeted drug
delivery and fluorescence diagnostics.
Whether functionalized diamond will
replace other materials, such as gold,
silica, and quantum dots, in this field
remains to be seen, but without question, diamond represents an attractive
and complementary alternative.
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Published online: September 4, 2006
Angew. Chem. Int. Ed. 2006, 45, 6426 – 6427
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6427
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