Self-ordered TiO2 quantum dot array prepared via anodic oxidation
© Drbohlavova et al; licensee Springer. 2012
Received: 5 November 2010
Accepted: 14 February 2012
Published: 14 February 2012
The template-based methods belong to low-cost and rapid preparation techniques for various nanostructures like nanowires, nanotubes, and nanodots or even quantum dots [QDs]. The nanostructured surfaces with QDs are very promising in the application as a sensor array, also called 'fluorescence array detector.' In particular, this new sensing approach is suitable for the detection of various biomolecules (DNA, proteins) in vitro (in clinical diagnostics) as well as for in vivo imaging.
The paper deals with the fabrication of TiO2 planar nanostructures (QDs) by the process of titanium anodic oxidation through an alumina nanoporous template on a silicon substrate. Scanning electron microscopy observation showed that the average diameter of TiO2 QDs is less than 10 nm. Raman spectroscopic characterization of self-organized titania QDs confirmed the presence of an anatase phase after annealing at 400°C in vacuum. Such heat-treated TiO2 QDs revealed a broad emission peak in the visible range (characterized by fluorescence spectroscopy).
Keywordsquantum dots biosensing TiO2 template methods nanoporous mask
Semiconductor quantum dots [QDs] with exceptional physical and optical properties are favorable fluorescent markers in medicine, where they can serve as biosensors and labels in biological imaging [1–3]. Commonly, QDs are used in colloidal form (in aqueous solution) [4, 5]. Nowadays, there is a demand for QDs deposited on various solid supports. Nevertheless, directly grown QDs in planar form (so-called lab-on-chip) for in situ biosensing purposes are studied very rarely . Occasionally, the scientists try to encapsulate the colloidal QDs in some matrix, e.g., from polymeric compounds .
The deposited nanostructures are mostly fabricated through traditional top-down patterning methods like epitaxy or lithographic techniques (mainly photolithography and e- beam lithography), which are expensive and time-consuming. Contrary to these methods, the template-based technique seems to be more convenient for nanostructured material synthesis since it is affordable and provides reproducible results. Concerning the template, nanoporous alumina belongs to the most extensively studied materials .
The biocompatibility of QDs used for medicinal purposes is usually a problematic issue because most of them are toxic (due to the cadmium ion content), which poses a potential danger especially for future medical applications. Using QDs prepared from titanium dioxide prevents this difficulty because TiO2 is a non-toxic material. Nevertheless, there are no scientific papers about the preparation and application of deposited TiO2 QDs (only nanodots without the quantum confinement effect). Final quantum confinement of TiO2 QDs strongly depends on the anatase/rutile phase composition [9, 10]. The phase development of TiO2 nanodots is much different from that of TiO2 thin films and powders. Generally, the nanodots are polycrystalline and consist of a mixed phase of anatase and rutile after high temperature annealing. However, Chen et al. revealed that TiO2 nanodots with a single anatase phase can be prepared from an epitaxial Al/TiN bilayered film on a sapphire substrate by electrochemical anodization of the TiN layer using a nanoporous anodic alumina film as the template .
In this work, we modified the experimental method of Chen et al. for the fabrication of TiO2 nanodots to prepare a TiO2 QD array on silicon substrate with dimensions required to reach the quantum size effect. The key point in our preparation process was to determine the appropriate anodizing potential and deposition time to ensure the optimal nanostructure dimensions, which is necessary from the quantum confinement point of view, as mentioned above. The choice of optimal electrolyte is also significant in order to avoid the possible contamination of the product. The physical, chemical, and luminescence properties of TiO2 QDs are discussed as well.
All chemicals such as sulfuric acid (97%, pro analysi [p.a.]), phosphoric acid (84%, p.a.), isopropanol (99.8%, p.a.), and chromium trioxide (99%, p.a.) were purchased from Penta (Prague, Czech Republic). Deionized water underwent demineralization by reverse osmosis using the instrument Aqua Osmotic 02 (Aqua Osmotic, Tisnov, Czech Republic) and subsequent purification using the Millipore RG system and Milli Q water (18.2 MΩ; Millipore Corp., Billerica, MA, USA).
Titanium and aluminum layer preparation
Prior to titanium deposition, the wafers were degreased and cleaned in isopropanol, rinsed out with deionized water, and finally treated with plasma. A high-purity titanium layer (99.995%, Safina, Jesenice, Czech Republic) with a thickness of 100 nm was prepared by tetrode sputtering on a 4-inch silicon wafer previously coated with a SiO2 layer (prepared by thermal CVD). Subsequently, a high-purity aluminum layer (99.99+%) with a thickness of 1 μm was deposited by thermal evaporation (PVD).
Ordered arrays of titania QDs were achieved by successive anodization of aluminum and titanium layers using the utility model equipment for electrochemical post-processing deposition fabricated in our laboratory (a detailed description of the tool is reported by Hubalek et al. ). Thanks to the different anodizing behavior of these layers, the same electrolyte can be applied during the whole process. A concentration of 3 M sulfuric acid was chosen as electrolyte in a constant potential mode (4 V) at 11°C. This acid provides a smaller pore diameter in the alumina template compared to other commonly used electrolytes (oxalic or phosphoric acids). An aqueous solution of H3PO4 (50 ml L-1) and CrO3 (30 g L-1) was used for chemical etching of the alumina template (5 min at 60°C).
Characterization of physical and chemical properties
The size of the QDs was estimated using scanning electron microscopy Mira II MLU (Tescan Mira, Brno, Czech Republic). The topography of the pure Ti layer was analyzed by atomic force microscopy [AFM] (Agilent 5500, Agilent Technologies, Santa Clara, CA, USA) with a 10-nm SiC tip. The phase composition of TiO2 QDs was characterized by Raman spectroscopy (Renishaw, Wotton-under-Edge, UK) with a NIR laser operating at 745 nm.
Results and discussion
The cheap, rapid, and easy reproducible template-based method was used successfully for the synthesis of titania QD sensor array suitable for various biomolecule (DNA, proteins) sensing in vitro. The anodization process through the highly ordered alumina nanoporous template provided less than 10-nm-sized TiO2 QDs densely covering the titanium surface and showing the strong photoluminescence peak in the visible range. The usage of this fluorescence array detector may be of great importance for clinical diagnostics and in vivo imaging.
The financial support from the grants GACR P102/10/P618, KAN 208130801 and CEITEC CZ.1.05/1.1.00/02.0068 is highly acknowledged.
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