Spatially Confined Corner Defects Induce Chemical Functionality of TiO₂ Nanorods

Abstract

The appeal of nanoscience and the allure of nanotechnology have been fostered by the promise of unique properties originating from structural features on the nanoscale. Many literature examples exist in which exceptional electronic, optical, chemical, and biological activities stem from nanoscale materials. [1][2][3][4][5][6][7] The ubiquitous attribute of these examples is their convergence from a "bottom-up" design strategy into pioneering discoveries and devices. In an effort towards the production of functional nanomaterials, we have combined the successes of nanoparticle synthesis with the selective surface reactivity of organic compounds and the self-assembly of biological molecules. Our group has previously reported on the synthesis of TiO 2 having varied physical morphologies [8][9][10] and interesting surface properties and reactivity. [11] Other investigations with TiO 2 have shown that flat surfaces reconstruct to possess lattice sites in which surface titanium atoms can adopt a non-complete coordinate character. [12,13] In addition, recent reports have suggested that particular surfaces of TiO 2 particles, such as the 001 surface, have enhanced sitespecific reaction affinities; [14][15][16] however, a specific mechanism involving the undercoordinate atomic character and enhanced surface reactivity found in TiO 2 particles has not been concluded. Here, in an effort to explain this reactivity, we describe an atomic-resolution transmission electron microscopy (TEM) investigation of surface defect sites present at the tips of high-aspect-ratio ellipsoid nanoparticles of anatase TiO 2 . We contend that the ability to chemically modify the surface of nanostructured titanium dioxide resides in specific undercoordinated "corner" defect sites that constitute the surface of high-curvature nanoparticles. By tailoring particle shape, these defect sites are spatially positioned to isolated regions of the nanoparticles. Modification of the nanorod tip surfaces with the enediol molecule dopamine resulted in relaxation of the tip defect sites to their bulklike octahedral symmetry. The reactivity of surface defect sites was used for the controlled, site-specific biotinylation of the nanorod tips. The biotinylated rods were then conjugated through a "lock and key" interaction with avidin and assembled to produce tip-connected "chainlike" structures. This approach, in addition to the abundance of titanium dioxide in a myriad of technological applications (ranging from paint fillers, to biomedical applications, to catalysis), confirms TiO 2 as a material of choice for development of inexpensive, nanoscale biocompatible devices. Determination of the chemical environment of atoms in nanoparticles, particularly at surfaces, is an exceedingly difficult challenge and has been investigated with spectroscopy techniques and microscopy such as diffuse-reflectance IR Fourier-transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and near-field scanning optical microscopy (NSOM). [17][18][19][20] Additionally, est...

Photocatalytic probing of DNA sequence by using TiO2/dopamine-DNA triads

“…We have found that surface modification of TiO 2 nanoparticles with enediol ligands leads to the bidentate chelation of surface undercoordinated Ti atoms. The forces that are involved in binding the ligands are large enough to induce structural deformation and heal surface defect sites to bulk-like octahedral geometry [18]. As a result of this strong interaction between nanoparticle surface atoms and organic ligands, the surface trapping sites are removed [19] and highly delocalized bands of metal oxide nanoparticles are electronically coupled to organic linkers, making the optical properties of metal oxide nanoparticles tunable throughout the entire visible and near IR region [20].…”

“…The main drawback for using TiO 2 itself for biological applications is that it absorbs light energy in the UV part of the spectrum. It has been found that surface atoms of anatase TiO 2 are undercoordinated [17,18] and provide an opportunity for alternating the effective band gap of nanoparticles. We have found that surface modification of TiO 2 nanoparticles with enediol ligands leads to the bidentate chelation of surface undercoordinated Ti atoms.…”

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