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  • Diamond is forever … and biocompatible

Jun 2007
Atomic force microscopy characterizes diamond surfaces for sensing applications

Gary Boas

Researchers are working to develop cell-based sensors on diamond because the material could serve as a signal transduction system for biological or chemical sensing. The biocompatibility of the material is not fully understood, however, especially with respect to its surface chemistry and topography. Studies have shown that its surface properties can be made hydrophobic or hydrophilic, which is important to cellular adhesion. However, the surface adhesion properties of diamond have yet to be characterized in detail.

As suggested by these fluorescence micrographs, the weakest cellular adhesion was on diamond that was treated with freshly preparedH-terminated surfaces (a), followed by surfaces functionalized with undecylenic acid (b). The strongest came from treatment with ultraviolet light (c). The top row shows microcrystalline diamond surfaces; the bottom row, ultrananocrystalline diamond surfaces.

A group with the National University of Singapore and Hartstoffbeschichtungs GmbH in Innsbruck, Austria, therefore investigated the adhesion properties of cells on photochemically functionalized diamond, reporting its findings in the May 8 issue of Langmuir. By correlating various surface conditions and topographies with the cell adhesion forces, cell attachment and cell viability, the scientists showed how these characteristics affect cell growth and, thus, determined the biocompatibility of diamond.

Strong cellular adhesion is necessary when developing a platform that uses diamond — or any material — to detect signal transduction for monitoring cellular activities, especially in continuous flow systems, where the cell and its adhesive components will be buffeted by external forces. Without strong adhesion, cells would not be able to sense, interpret, integrate or respond to extracellular signals.

The researchers measured the adhesion forces of cells on various diamond surfaces by attaching single cells to the cantilever of a multimode atomic force microscope made by Veeco Instruments Inc. of Woodbury, N.Y. The cantilevers were functionalized with concanavalin A.

They lowered the cell-cantilever system to the diamond surface until they achieved a predefined contact force. After 200 ms, they retracted the cantilever from the surface. During retraction, any adhesive interactions between the cell and the surface would pull the cantilever downward, which was followed by a rupture of the bond. Such ruptures appeared as sharp jumps in the retraction curve, each of which was considered a de-adhesion event.

The experiments revealed different adhesion and de-adhesion forces for cells on diamond, depending on the functional groups on the surface, according to Kian Ping Loh, the leader of the study. The strongest maximum de-adhesion forces were on diamond surfaces that were treated with ultraviolet radiation. The next strongest were on surfaces functionalized with undecylenic acid, and the weakest were on freshly prepared H-terminated surfaces.

Using atomic force microscopy, researchers have characterized diamond surfaces to determine the material’s potential for biosensing applications. The approach and retraction of single cells attached to an AFM cantilever provided important information about the cellular adhesion forces of various diamond surfaces: Strong cellular adhesionis necessary when developing any material as a signal transduction platform formonitoring cellular activities. The schematic shown here represents a typical approach-and-retraction force curve.

The researchers noted that functionalization with undecylenic acid increased the maximum de-adhesion forces by about twofold, with respect to the freshly prepared H-terminated surfaces; treatment with ultraviolet radiation increased the forces by about 2.5-fold.

The scientists also reported that cell adhesion on diamond is mediated by the electrostatic interactions, or hydrogen bond formations, between cell membrane proteins and the diamond’s surface. The relatively low adhesion forces on H-terminated surfaces, they added, resulted from the hydrophobic properties of such surfaces. The other surfaces, in contrast, had a high density of carbonyl or carboxylic acid groups, which contributed to strong electrostatic interactions as well as to hydrogen bonding with the cell membrane proteins.

Cell cultures

The investigators then performed a series of cell culture experiments to correlate the measured cell adhesion forces with subsequent cell growth. These findings were similar to the earlier ones: The ultraviolet-treated surfaces exhibited the highest cell density; next highest were the surfaces functionalized with undecylenic acid followed by the freshly prepared H-terminated surfaces.

Finally, they carried out cell viability studies to determine the percentages of live cells on the various diamond surfaces. Here, the ultraviolet-treated surfaces and the surfaces functionalized with undecylenic acid exhibited cell viability of 75 percent or more; the H-terminated surfaces, only 30 percent cell viability.

Such characterization of cellular adhesion on diamond will help advance the use of the material as a signal transduction platform. The researchers are exploring the potential of diamond for sensing applications.

“We are trying to grow algae cells on diamond for environmental sensing,” Loh noted, “by making use of the change in metabolism of the cells when chemical or biological agents are introduced. Preliminary results show that cell-based diamond sensing platform shows remarkable antifouling properties compared to other electrode materials.”

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