This work describes methods for the fabrication of calcium phosphate films on crystalline silicon substrates, with the long-term goal of employing such materials in either drug delivery or sensing applications. The emphasis of this particular account encompasses both film fabrication as well as surface modification strategies relevant to the sensing of bio-molecular species. For the former, a new process for calcium phosphate film formation is described entailing the use of a high energy DC spark onto silicon surfaces in the presence of Ca 10 (PO 4 ) 6 (OH) 2 . For sensing purposes, routes to modification of the calcium phosphate/porous silicon surface have been achieved via the use of bifunctional bisphosphonate derivatives, with proof-of-concept demonstrated by the covalent attachment of fluorescent fluorescein tags.

A method for fabrication of calcium phosphate films on crystalline silicon substrates which employs a high energy dc spark on silicon surfaces in the presence of Ca 10 ͑PO 4 ͒ 6 ͑OH͒ 2 is described. Such a process kinetically traps the desired calcium phosphate on the substrate with concomitant formation of a porous layer. Scanning electron micrographs of the silicon substrate show a nanoporous surface morphology for those areas exposed to spark processing. Energy dispersive X-ray analysis and infrared vibrational spectroscopy of the spark-processed regions of the Si substrate are clearly consistent with the presence of the desired biologically relevant material. In conjunction with an automated stage, facile formation of deliberate micrometer-size patterns is demonstrated.While long considered as the paradigm of semiconductor microelectronics, crystalline silicon ͑Si͒ has, with few exceptions, received minimal attention for its potential as a biomaterial. 1-4 Recent efforts by Canham and co-workers have clearly demonstrated that one approach to induce bioactivity in Si is to electrochemically form nanoporous Si films followed by the cathodic growth of calcium phosphate ͑the common inorganic mineral phase of bone͒. 5-7 Such materials are attracting attention as possible drug delivery vehicles 8,9 and a host of other applications. 10 As an alternative to the anodic etch process, previous studies have shown that the high-frequency arc from a Tesla coil can ablate the surface of crystalline silicon, producing nanoporous surfaces. [11][12][13] It has also been subsequently demonstrated that if a rare earth salt is deposited on the silicon surface and then spark processed, the resulting Si porous layer is doped with ''clusters'' of rare earth ions coordinated to oxygen. 14,15 These latter studies suggest that it should be possible to kinetically trap a variety of useful, thermally stable materials as the ablated Si coalesces onto the surface. Thus, this paper describes the use of a spark ablation process to produce calcium phosphate films anchored to Si. These materials are characterized via scanning electron microscopy ͑SEM͒, energy dispersive X-ray analysis ͑XEDS͒, and infrared vibrational spectroscopy ͑FTIR͒. These results confirm that the porous spark-processed regions of the substrate are clearly consistent with the presence of the desired biologically relevant material. The impact of electrode gap and spark duration, parameters that most sensitively determine film thickness and composition, are also discussed.Square pieces (15 ϫ 15 mm) of p-type, ͗100͘, boron-doped Czochralski ͑CZ͒ Si ͑6-8 ⍀ cm͒ were employed in these experiments. Electrical contact was achieved using Ni wire and silver epoxy. An excess of Ca 10 ͑PO 4 ͒ 6 ͑OH͒ 2 ͑Aldrich͒ is deposited on the wafer surface by either a slurry in acetone ͑or water͒ or alternatively by direct application of the dry calcium phosphate powder onto the surface with compression by a glass rod. The sample was allowed to dry, producing a layer of calcium phosphate covering...

The growth of known biologically-relevant mineral phases on semiconducting surfaces is one strategy to explicitly induce bioactivity in such materials, either for sensing or drug delivery applications. In this work, we describe the use of a spark ablation process to fabricate deliberate patterns of Ca10(PO4)6(OH)2 on crystalline Si (calcified nanoporous silicon). These patterns have been principally characterized by scanning electron microscopy in conjunction with elemental characterization by energy dispersive x-ray analysis. This is followed by a detailed comparison of the effects of fibroblast adhesion and proliferation onto calcified nanoporous Si, calcified nanoporous Si derivatized with alendronate, as well as control samples of an identical surface area containing porous SiO2. Fibroblast adhesion and proliferation assays demonstrate that a higher density of cells grow on the Ca3(PO4)2 /porous Si/ SiO2 structures relative to the alendronate-modified surfaces and porous Si/SiOM2 samples.

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