The optical properties of the alumina/ZnO nanotubes

To observe emission at 600-700 nm, a 400 nm optical pass was used, resulting in the appearance of 410 nm emission. Fig. 7.10 shows the PL spectra of the ANZTs under different annealing conditions. As annealed in oxygen ambient, it was found that the PL spectra of the ANZTs exhibit a very broad peak from 425 nm to 750 nm as shown in Fig. 7.10(a).

Furthermore, it was also observed that the PL spectra are also changed with annealing atmosphere. As seen in Fig. 7.10(b), ANZTs annealed in nitrogen at different temperatures displayed one strong blue and one weak green-yellow peak. The broad green-yellow peak dominated at low annealing temperatures (below 600°C), and a strong blue emission peak at

~450 nm occurred at 600^oC. The blue emission was primarily due to singly ionized oxygen vacancies in alumina. Large numbers of oxygen vacancies in the ANZT alumina nanoparticle shell were produced by annealing in nitrogen at high temperature. The above-mentioned results indicated that the PL spectra of the alumina-coated ZnO nanotubes (ANZTs) could emit different colors by varying the annealing conditions. Fig. 7.10(c) illustrates this with blue, green and white emissions.

The PL spectra of the ANZTs annealed in oxygen at different temperatures were further analyzed by Gaussian curve fitting as shown in Fig. 7.11. The curve fitting of the PL spectra revealed the competition between the UV, blue, blue-green, green and yellow band emissions associated with various defects. The fit using four peaks schema can be used to compare the relative intensity of the PL emission at the different spectral bands and only for this purpose.

Such fit cannot describe the spectrum as a whole. When the samples were annealed at 200^oC in oxygen, the yellow emission dominated due to OH bond absorption on the surface of ANZTs. Increasing the annealing temperature to 400^oC increased the blue emission and decreased the yellow emission. The blue emission was probably related to phase transformation from the boehmite phase (octahedrally coordinated) to the γ-phase (tetrahedrallly coordinated), which starts to appear at this temperature. During phase transform, pentahedrally coordinated aluminum was produced, and singly ionized oxygen vacancies at the F⁺ centers were responsible for the blue emission. For a detailed discussion, refer to our previous study. In general, the oxygen affinity in alumina is stronger than that in ZnO. Hence, the oxygen interstitials in the ZnO core diffused into the alumina shell, reducing the number of oxygen interstitials in ZnO. The number of oxygen vacancies in the ZT core nanotubes increased, leading to a strong green emission. In addition, as shown in Fig. 7.8, an interaction occurred at the interface between the ZnO nanotube and the alumina layer. As the Zn atoms compounded with Al, the concentration of Zn atoms in the nanotubes decreased, and the concentration of VZn increased. Since the emission of VZn is located at ~496 nm, this created blue-green light. As a result, the relative intensity ratio of the four color peaks (blue : blue-green : green : yellow) was estimated to be 1.3 : 1.7 : 1.2 : 1 for the ANZTs annealed at 400^oC in oxygen.

Annealing ANZTs at a higher temperature (600^oC) compensated for the F⁺ centers and reduced the blue emission. According to the report by Yang et al., spinel structure might have been formed above 600^oC.[90] The HRTEM image marked with an arrow in Fig. 7.9(d) shows that an intercompound structure, possibly a spinel structure, composed of Zn and Al appeared in the interface between the ZnO nanotubes and the alumina layer. This made the blue-green emission peak strong. In addition, annealing increased the number of ZT surface defects and thus strengthened the yellow emission. At the same time, the green emission was also enhanced because the interaction on the interface induced a large number of oxygen

vacancies in ZnO. Therefore, the relative intensity ratio of the four color peaks (blue:

blue-green: green: yellow) changed to 0.86 : 1.23 : 1.11 :1, leading to white emission.

7.6 Summary

In summary, we have developed alumina nanoparticles-coated ZnO nanotubes (ANZTs) on ZnOf/Si substrates buffered with a ZnO film by combining simple chemical solution growth and annealing. The ANZTs emitted different light emissions as the annealing temperatures and atmospheres were changed. A white-light emission, consisting of blue, blue-green, green and yellow band emissions, was obtained. Photoluminescence measurements indicated a blue emission peak at ~450 nm, a blue-green emission at ~496 nm, a green emission peak at ~525 nm and a yellow emission peak at ~600nm. This work not only demonstrates a novel method for preparing white-light emission alumina-coated ZnO nanotubes, but also suggests that the defect structure and transition mechanisms of samples could be modified by different defect species.

Fig. 7.1 SEM image of the (a) ZnO nanorods synthesized for 5 hours (b) ZnO nanotubes synthesized for 4 days.

100nm (a)

100nm (b)

Fig. 7.2 SEM image of the (a) ZnO nanorods synthesized for 12 hours (b) ZnO nanotubes synthesized for 24 hours.

100 nm 100 nm (a)

(b)

0 5 10 15 20 25 30 35

Fig. 7.3 (a) pH value varying with growth time as a function of solution concentrations. (b) XRD analysis of the ZnO nanotubes for different growth time.

30 31 32 33 34 35 36 37 38 39 40

Fig. 7.4 (a) TEM images of a single ZnO nanotube. (b) High- resolution TEM image of a single ZnO nanotube.

0.52nm [0002]

(b)

2nm 10nm (a)

Fig. 7.5 TEM image of ZnO nanotubes annealed in oxygen at 400^oC at (a) low magnification and (b) high magnification and at 600^oC at (c) low magnification and (d) high magnification.

Fig. 7.6 (a) Room-temperature PL spectra of ZnO nanotubes after rapid thermal annealling at various temperatures in O2 atmosphere.

Fig. 7.7 (a) SEM and (b) TEM images of the alumina nanoparticles-coated ZnO nanotubes. (c) High magnification TEM images of the samples. (d) Intensity profile of Al, Zn, and O across and along one tube diameter.

Fig. 7.8 (a) High magnification TEM images of the (a) interface and (b) core of a sample annealed at 400^oC in ambient oxygen.

Fig. 7.9 (a) TEM overview of samples. (b) Magnified mid-section images from samples annealed at 600^oC in oxygen. (c) Corresponding ED pattern. (d) High-resolution TEM images of interface of alumina and ZnO nanotubes.

Fig. 7.10 Room-temperature PL spectra of ZnO nanotubes after rapid thermal annealling at various temperatures in (a) O2 and (b) N2. (c) Blue, green and white photograph.

Fig. 7.11 The Gaussian curve fit of PL spectra for samples annealled in oxygen at (a) 200 ^oC, (b) 400 ^oC, and (c) 600 ^oC.

Chapter 8

Smart ZnO nanotube for Controlled Drug Release

8.1 Introduction

Precise drug delivery process has attracted considerable attention. This device requires low power consumption, biocompatible, bio-imaging and low-cost. Among many bio-compatible materials, Zinc oxide (ZnO) nanotubes are of considerable interest for a variety of biomedical applications. ZTs have large surface area-volume ratio and porous structure to be an exceptional nano-template. ZTs own high sensitive properties due to the large surface area-volume ratio in nanostructures. Wang and co-workers used in situ TEM to provide the mechanical resonance behavior of ZnO nanobelts induced by an alternative electric field. [91]

Furthermore, they demonstrated a piezoelectric ZnO nanowire as a direct-current nanogenerator which could convert nanoscale mechanical energy into electric energy by ultrasonic waves. [30]

In general, ZnO are believed to be nontoxic, biosafe, and possibly biocompatible. Brayner et al. [47] reported preliminary studies of biocidal effects and cellular internalization of ZnO nanoparticles on Escherichia coli bacteria and showed that ZnO nanoparticles did not induce any damage, indicating ZnO is a nontoxic and biocompatible material. Similar observation was reported by Wang et. al.[49] These studies indicate that ZnO are benefit to be applied in bio-medicals.

Therefore, in this chapter, a study on the drug release of ZnO nanostructure will be investigated to demonstrate how ZnO can be used in biomedical applications. The ZnO nanotubes (ZTs) were used as a template since it can provide more surface area. The ZnO

nanotubes were grown on the ITO/PET substrate with ZnO film as a catalyst by chemical solution method. After grown, the surface of the ZTs was modified to become hydrophilic by using oxygen plasma and then put into the solution to absorb drugs. An electric field was applied to study the drug release from drug-loaded ZnO nanotubes. This device possesses low power consumption, biocompatible, bio-imaging and low-cost characteristic.

8.2 Morphology of the dye-ZnO nanotubes

As-grown ZnO nanotubes are synthesized in solution at 95^oC for 24 h. A low-magnification SEM image in Fig. 8.1(a) shows that the nanotubes are several (2~5 μm) micrometers in length. The corresponding high-magnification SEM image in Fig. 8.1(b) shows that each nanotube has a uniform width over its entire length. The typical diameter is in the range of 20-30 nm. The ZTs are open-headed tubular structures with wall thicknesses of about 5~7 nm and inner diameters of about 15~20 nm, as shown in the Fig. 8.1(c). Furthermore, the highly preferential growth of ZnO nanotubes was observed along the c-axis orientation (0002) with a lattice constant of ~ 0.52 nm. Fig. 8.1(d) shows the dark field (DF) TEM images of the 002 reflection and proved that the (002) plane of the ZTs disappeared.

Fig. 8.2(a) is the low magnification TEM image of the FITC-ZnO nanotubes with the diameter 30~40 nm. FITC was considered as a model drug to simulate the drug release behavior because it was a fluorescence dye. From the image, it can be clearly observed that the dye is absorbed on the ZTs to form core/shell structure. The high-resolution transmission electron microscopy (HRTEM) image in Figure 8.2(b) confirms FITC-ZnO nanotubes with a core tube structure and a thin amorphous shell. No observable crevices or cracks are visually detectable at the interface of these core/shell nanospheres, suggesting excellent physical integrity between the two dissimilar phases. The thickness of the shell is about 5~10 nm on average and the shell demonstrates a relatively dense nanoarchitecture. Fig. 8.2(c) further

reveals that the dye with the not only the surface but also the head of ZTs have been completely surrounded by dye. Due to capillarity, it could be supposed that the dye was encapsulated into the ZTs.

8.3 Release behavior of the dye-ZnO nanotubes

Figure 8.3 schematically illustrates the hypothesis of the release behavior of the dye-ZnO nanotubes. After the ZTs were synthesized on the substrate, the substrate was coated with Au electrode on the two ends. Then, the device was put into the neutral condition, such as 100 ml D.I water and actuated by applying a positive voltage. When the device was applied with an electric field, it was observed that the dye loaded on the ZTs would be released into the water.

The mechanical resonance of a single ZnO nanobelt, induced by an alternative electric field, was proved by in situ transmission electron microscopy. Due to the rectangular cross section of the nanowires, two fundamental resonance modes have been observed corresponding to two orthogonal transverse vibration directions. Therefore, a single dye-loaded nanotube would release the dye when applying the electric field. In addition, tune the release rate could be tuned and controlled by using various frequencies in this study.

Figure 8.4(a) shows the current versus voltage (I-V) characteristics for a pure ZTs device.

This result reveals that the ZTs present the characteristic similar to Ohmic contact between electrode and ZTs. The same condition was used to examine the dye-loaded ZTs. Before release process, the I-V curve is very rough. According to Yang’s report [44], a better electron transport in these nanowire films is a product of their excellent crystallinity and a radial electric field within each nanowire that assists carrier collection by repelling photoinjected electrons from the surrounding electrolyte. When something were absorbed on the surface of the ZTs, a phenomenon of the band bending would reduce the surface recombination velocity of the majority-carrier electrons regardless of the speed at which the electrons move.

Recombination may remain diffusion limited, but the rate which electrons transport at the oxide surface is determined by the magnitude of the surface field rather than the diffusion constant for electrons in the wire cores. Therefore, the electron could not transport continuously on the surface of the ZTs because the dye was absorbed on the surface of the ZTs, indicating that it would make the I-V characteristic rough. After the dye was completely released, it was observed that the I-V characteristic was restored to approach pure ZTs state, as shown in Fig. 8.4(b).

A kinetic analysis was performed using photoluminescence (PL) spectroscopy to monitor the release of dye molecules from the core-shell nanotubes in different time periods under exposure to high-frequency electric field. Fig. 8.5(a) shows the chemical structure of the FITC.

Fig. 8.5(c) and (d) show the resulting PL spectra of the released dye at different time periods under applied electric frequencies at 463 kHz and 100 kHz, respectively, where the peak intensity of the fluorescence spectra increases with time period independent of electric frequencies. The peak is corresponding to the green emission which is emitted by the FITC. It is noted that the peak intensity at 463 Hz is stronger compared to that at 100 kKz. This indicates that the cumulative released amount is much increased over 18 min-stimulus at 463 kHz compared to that at 100 kHz. The ZTs will be vibrated by the high frequency electric field. Moreover, the amplitude of vibration would increase with the frequency increasing.

Therefore, when the ZTs were vibrated strongly, the dye would drop from the surface of the ZTs. It would be assigned that vibration could be reduced the interforce between dye and ZTs with time increasing.

Fig. 8.5(b) shows that the release profile of dye for dye-loaded ZnO nanotubes is recorded by applying different external high-frequency electric field at three specific times. After electric excitation, a significant increase in the released amount of drug was observed. The result implied that the drug molecules in the nanotubes exactly followed the signals switching from burst to slow release for each operation. While applying an electric field, the amounts of

drug release can increase instantly, and under suitable control, the drug concentration can reach the therapeutic window within a short time period. A control experiment was also carried out to display that the model drug did not exhibit any obvious release if no electric field was applied. This indicates that a repeat drug release can be controlled through the ZnO nanostructure and the released amounts of the drug can be also tuned through the operation time.

8.4 Cell incubation with zinc oxide nanotubes (ZTs)

HeLa (human cervical cancer) cells were maintained in DMEM (Dulbecco’s modified Eagle’s medium) containing 10% fetal bovine serum, 100 units/mL penicillin, and 100 μg/mL streptomycin. Cells were cultured with complete medium at 37 °C in a humidified atmosphere of 5% CO2 in air. For all of the experiments, cells were harvested from subconfluent cultures by use of trypsin and were resuspended in fresh complete medium before plating.

Cellular uptake of the zinc oxide nanotubes (ZTs) was investigated by PL microscopy.

After 24 hours of HeLa cell incubation, which is employed as a cancer cell model, cells treated with zinc oxide nanotubes (ZTs) were monitored. The ZTs under the PL microscopy can be observed like dark dots. As the Figure X shown, the cells exhibited the normal morphologies even under the high concentration of the ZTs (>100 roots/cm²). The cells surrounded by the ZTs can still highly attach on the plates, suggesting the ZTs showed low efficiency to affect the cell growth although the ZTs were not highly uptaken by the cells.

Furthermore, the normal cells, MRC-5 cell lines, were incubated with the substrate covered the ZTs. After 12 hours, the cells can rapid attach on the substrate, indicating the good biocompatible of ZTs substrate. The cytoskeleton also can be seen clear on the substrate after the dyeing, and maintain well and strong structure. This finding indicates that the ZTs substrate showed little cytotoxicity to the cancerous and normal cells, suggesting excellent biocompatible character and should accordingly be highly compatible to healthy cells. As a critical requirement for drug delivery strategy, reduction of the cytotoxicity of drug carrier

itself can be satisfied and a further minimize the cytotoxicity of the drug can be accompanied.

8.5 Summary

We have developed smart dye-ZnO nanotubes devices. The dye is absorbed on the surface of the ZTs. Furthermore, the dye release behavior can be controlled by the high frequency electric field. Furthermore, by the cell survive experimental, the biocompatible properties of the ZTs would be proved excellently.

Fig 8.1 (a) SEM images of ZnO nanotubes. (b) High-magnification SEM image of ZnO nanotubes. (c) TEM image of a single ZnO nanotube. (d) Dark-field TEM image of a single ZnO nanotube.

(a) (b)

(c) (d)

50 nm

10 nm

5 nm

Fig 8.2 (a) TEM image of dye-ZnO nanotubes. (b) High- resolution TEM image of the dye-ZnO nanotubes (b) on the side part, (c) on the end part.

(a)

(b)

(c)

V V

Single NanoTubes

Drug loaded nanotubes Drug release out

Vibration Apply electric field

Flexible matrix ZnO Nanotubes

Apply Electric field

Fig. 8.3 Schematic illustration of the controlled release of dye-ZnO nanotubes.

Fig 8.4 I-V characteristics of the (a) ZTs. (b) dye-ZTs with electric field applied.

0 4 8 12 16 20

Fig 8.5 (a) FITC structure diagram (b) Cumulative dye release profiles of dye-ZnO nanotubes from different frequency in electric field. Emission spectra of the dye-ZnO nanotubes for applying (c) 463 kHz (d) 100 kHz electric field.

(a) (b)

(c) (d)

Fig. 8.6 (a) Fluorescence microscopy images of Hela cells absorbed on the (a) large amounts (b) small amounts of the ZTs after 24h incubation.

(a)

(b)

Fig. 8.7 Fluorescence microscopy images of MRC-5 cells absorbed on the ZTs after 36h incubation.

Chapter 9

Conclusions

9.1 Tunable growth and Growth behavior of ZnO nanorods synthesized in the aqueous solution

The growth of patterned ZnO nanorods can be controlled by changing the annealing conditions of the ZnOf/Si substrates. When the ZnOf/Si substrate was annealed above a critical temperature to promote the crystallization of ZnO phase, both ZNs and ZnOf on Si substrate were found to become crystallographically matched. The ZNs seem to preferentially nucleate from the cup tip near the grain boundary between two ZnO grains in the ZnO film. A higher annealing temperature may lead to the formation of a larger ZnO crystal due to coplanar coalescence behavior of several individual ZnO nanorods. The scattered ZNs present a two-stage growth mechanism with a self-assembly process of ZNs in the later growth stage.

Aligned ZNs were directly grown along the [0 0 0 2] direction from the ZnO film on Si.

9.2 Synthesis and Optical Properties of the Alumina/ZnO nanostructure

It is demonstrated that well-aligned arrays of Al2O3/ZnO nanocables on ZnOf/Si substrate substrates buffered with a ZnO film by combining a simple chemical solution with a low-temperature treatment. Al2O3/ZnO nanocables show a strong blue emission peak at ~450 nm appears at 400^oC and 600^oC in O2 and N2 atmospheres, respectively. With the increase of annealing temperature, the OH band in pseudo-boehmite structure tends to be broken and Al

－O^• would be formed during the phase transformation from pseudo-boehmite phase (octahedrally coordinated structure) to γ-phase aluminum oxide (tetrahedrallly coordinated

structure), favoring pentahedrally coordinated aluminum and the occurrence of singly ionized oxygen vacancies which are regarded to the F⁺ center. The alumina nanoparticles-coated ZnO nanotubes emitted different light emissions as the annealing temperatures and atmospheres were changed. According to the Kirkendall effect, Zn atoms diffused into the alumina layer and several voids were formed in the core of the tubes. When ANZTs were annealing at a higher temperature (600^oC), the samples demonstrated white-light emission. A white-light emission, consisting of blue, blue-green, green and yellow band emissions, was obtained from alumina nanoparticles-coated ZnO nanotubes.

9.3 Smart ZnO nanotube for Controlled Drug Release

Smart dye-ZnO nanotubes devices are synthesized. The dye is absorbed on the surface of the ZTs. Furthermore, the dye release behavior can be controlled by the high frequency electric field. The biocompatible properties of the ZTs were proved excellently.

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