Voltage-induced payload release and wettability control on TiO2 and TiO2 nanotubes.
Voltage-induced payload release and wettability control on TiO2 and TiO2 nanotubes.
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DOI：
10.1002/anie.200905111
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发表时间：
2010-01
影响因子：
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通讯作者：
Yanyan Song;Poulomi Roy;I. Paramasivam;P. Schmuki
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文献类型：
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作者：
Yanyan Song;Poulomi Roy;I. Paramasivam;P. Schmuki
自1972年Fujishima和本田的第一份报告以来，[1] TiO 2出色的光催化性能已经为5000多篇针对基本方面和应用的出版物[2-4]奠定了基础，例如光诱导水分解，[5，6]自清洁，[7]和有机污染物降解。[3]最近，高纵横比自组织TiO 2纳米管层[8]的开发甚至进一步拓宽了TiO 2作为非常有效的催化剂体系[9]的用途，该催化剂体系提供了高度限定的三维纳米结构。一些有趣的应用是基于附着在TiO 2表面的有机单层的断裂。例如，我们最近报道了TiO 2纳米管上有机单层的光诱导切割，用于精确调节表面润湿性[10，11]和控制药物释放。[12所有这些光催化反应的基本原理是UV光在TiO 2中产生电子-空穴对。所产生的电荷载流子被喷射到周围的电解质中，并产生高度反应性的氧化还原物质;即，转移到水电解质中的价带空穴（h+）具有足够的能量来产生OHC自由基。这些自由基可用于引发连接的烃链中的断链，从而释放末端有效载荷。在这里，我们表明，一个伪光催化反应，特别是断链，可以在没有光的情况下，通过使用施加的电压，预计将导致价带空穴的产生。如果施加大于带隙的偏置电压，则这可以在耗尽条件下对于充分掺杂的半导体实现。[14-17]这种电压诱导的载流子产生已被报道用于硅，其中它被用于电化学蚀刻或金属沉积反应。[14为了将该原理转移到TiO 2层，要求是可以建立材料的充分限定和高度掺杂的介电性质（1018-1019 cm ↑ [3]）。[16]为此，我们使用阳极形成的TiO 2纳米管层（和作为参考阳极形成的致密层），退火到掺杂水平约为1.0 × 1019 cm ↑ [3]的ZnO结构。[18]在这些二氧化钛结构，我们要么graftedOPDA（十八烷基膦酸）的润湿性控制实验，或附加的酶，即辣根过氧化物酶（HRP），通过维生素C连接器，以证明有效载荷释放。关于样品制备和表征的详细信息见支持信息。用这些样品，在黑暗中在两电极和三电极电化学电池中在不同的阳极电压下进行了电压诱导断链实验。图1显示了接枝在TiO 2表面上的OPDA的示例。对于OPDA改性的TiO 2纳米管表面，获得了超疏水性润湿行为（图1b;未改性的TiO 2纳米管表面是超亲水性的）。[10通过将超疏水样品浸入含水电解质中并施加足够的阳极极化，可以触发接触角的显著改变（图1b）。电压必须足够阳极化（大于3VAg/AgCl）以获得显著效果。为了通过XPS适当地研究电压诱导的表面化学改变，我们还对OPDA接枝的平坦（致密）TiO 2表面进行了类似的实验（图1c）。为了展示TiO 2的电子性质和结晶度对于成功触发反应的重要性，我们在无定形TiO 2表面上进行了相同的实验。从
Since the first report of Fujishima and Honda in 1972,[1] the outstanding photocatalytic properties of TiO2 have built the basis for over 5000 publications [2–4] targeting fundamental aspects and applications, such as photoinduced water splitting,[5, 6] self-cleaning,[7] and organic pollutant degradation.[3] Most recently, the development of high-aspect-ratio selforganized TiO2 nanotube layers [8] has even further widened the use of TiO2 as a very efficient catalyst system [9] that provides a highly defined three-dimensional nanoarchitecture. Some interesting applications of photocatalysis are based on the scission of organic monolayers attached to the TiO2 surface. For example, we recently reported the photoinduced cut of organic monolayers on TiO2 nanotubes for a precise adjustment of the surface wettability [10, 11] and for controlled drug release.[12, 13] The underlying principle of all these photocatalytic reactions is that UV light produces electron–hole pairs in the semiconductive TiO2. The generated charge carriers are ejected to the surrounding electrolyte and create highly reactive redox species; namely, valenceband holes (h+) transferred to water electrolytes have a sufficient energy to produce OHC radicals. These radicals can be used to trigger chain scission in attached hydrocarbon chains, and thus liberate terminal payloads. Herein we show that a pseudo photocatalytic reaction, and in particular chain scission, can be initiated in the absence of light by using an applied voltage that is expected to lead to valence-band hole generation. This may be achieved for sufficiently doped semiconductors under depletion conditions if a bias voltage is applied that is larger than the band gap.[14–17] Such voltage-induced carrier generation has been reported for silicon, where it was exploited for electrochemical etching or metal deposition reactions.[14, 15] To transfer the principle to TiO2 layers, a requirement is that a sufficiently defined and highly doped semiconductive nature of the material can be established (1018–1019 cmÀ3).[16] For this we used anodically formed TiO2 nanotube layers (and for reference anodically formed compact layers) that were annealed to an anatase structure with a doping level of approximately 1.0 1019 cmÀ3.[18] Onto these TiO2 structures we either graftedOPDA (octadecylphosphonic acid) for wettability control experiments, or attached an enzyme, namely horseradish peroxidase (HRP), via a vitamin C linker to demonstrate payload release. Details on sample preparation and characterization are given in the Supporting Information. With these samples, voltage-induced chain scission experiments were carried out in the dark at different anodic voltages in two-and three-electrode electrochemical cells. Figure 1 shows an example for OPDA grafted on a TiO2 surface. For OPDA-modified TiO2 nanotube surfaces, a superhydrophobic wetting behavior is obtained (Figure 1b; non-modified TiO2 nanotube surfaces are superhydrophilic).[10, 11] By immersing the superhydrophobic samples in an aqueous electrolyte and applying a sufficient anodic polarization, significant alterations in the contact angle can be triggered (Figure 1b). The voltage has to be sufficiently anodic (more than 3 VAg/AgCl) to obtain a significant effect. To properly study voltage-induced surface chemistry alterations by XPS, we additionally carried out similar experiments on OPDA-grafted flat (compact) TiO2 surfaces (Figure 1c). To show how crucial the electronic properties and crystallinity of TiO2 are to successfully trigger the reaction, we carried out identical experiments on an amorphous TiO2 surface. From