# Visible Light Irradiation-Mediated Drug Elution Activity of Nitrogen-Doped TiO2 Nanotubes

Authors: Seunghan Oh; Kyung-Suk Moon; Joo-Hee Moon; Ji-Myung Bae; Sungho Jin
Journal: Journal of Nanomaterials
 (2013)

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## Abstract

We have developed nitrogen-dopedTiO2 nanotubes showing photocatalytic activity in the visible light region and have investigated the triggered release of antibiotics from these nanotubes in response to remote visible light irradiation. Scanning electron microscopy (SEM) observations indicated that the structure of TiO2 nanotubes was not destroyed on the conditions of 0.05 and 0.1 M diethanolamine treatment. The results of X-ray photoelectron spectroscopy (XPS) confirmed that nitrogen, in the forms of nitrite (NO2-) and nitrogen monoxide (NO), had been incorporated into the TiO2 nanotube surface. A drug-release test revealed that the antibiotic-loaded TiO2 nanotubes showed sustained and prolonged drug elution with the help of polylactic acid. Visible light irradiation tests showed that the antibiotic release from nitrogen-doped TiO2 nanotubes was significantly higher than that from pure TiO2 nanotubes (P<0.05).


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## Body

## 1. Introduction

Development of TiO2 nanostructures has been a focus area in the fields of photocatalysis [1–3], solar cells [4], and biomedical applications [5–7]. TiO2 is well known as a highly efficient photocatalyst and has been widely used for degrading organic pollutants for air purification and sterilization [8, 9]. However, the band gap of TiO2 allows limited photocatalytic activity, which occurs solely in the narrow ranges of ultraviolet light. Many researchers have tried to develop visible light photocatalysts by modifying the structure of titanium dioxide. Metal doping is one of the typical modifications of TiO2, and several kinds of metal elements, such as Cr, Co, Mo, Mn, and V, have been used to adjust the band gap of TiO2 and promote its photocatalytic activity in the visible light range.The unique advantages of TiO2 nanostructures on Ti over microscale TiO2 structures have been reported in the field of solar cells and biomaterials [10–15]. In addition, the morphology and crystallinity of TiO2 nanotubes affect the adhesion, proliferation, and functionality of many types of cells [16–19]. For example, it is known that an array of TiO2 nanotubes on Ti promotes osseointegration into animal bone in vivo[14, 20, 21].Bacterial infection is one of the major reasons for the failure of orthopedic implants. An ideal solution to reduce bacterial infection is antibiotic drug therapy around 2 months after implant surgery. The delivery methods of antibiotics are generally systemic, intravenous, intramuscular, and topical. However, systemic antibiotic delivery is typically associated with certain side effects, including unwanted cytotoxicity. In recent years, various drug-delivery systems have been developed to facilitate drug effectiveness at the site of implantation [22, 23]. These drug-delivery devices release a proper dose of the drug at the site of action and thereby avoid undesirable side effects. However, these devices lack the ability to exert an on-off control over drug release. Consideration of release periods as well as the elution point of drugs is very important to reduce bacterial infection. Unwanted high doses of the drug lead to unavoidable toxic effects. Being able to trigger the antibiotic release, for example, by using light stimulation, would be highly desirable to minimize complications and side effects for the orthopaedic implants. However, ultra violet (UV) light triggering TiO2 photocatalytic activity is well known to be used for sterilization and to be harmful to mucous membrane in mouth. Therefore, the development of new TiO2 showing photocatalytic activity at visible light region is required to minimize the side effect of UV light irradiation and promote the effect of remote-controlled drug release. In terms of the photocatalytic activity of TiO2 nanotubes at visible light region, several studies reported that nitrogen or Fe-doped TiO2 nanotubes showed excellent photocatalytic activity and degree of dye degradation compared to pure TiO2 nanotubes by visible light irradiation [24–27].In this study, we developed nitrogen-doped 100 nm TiO2 nanotubes on Ti, performed surface analysis to examine the amounts and structure of nitrogen incorporated into Ti, and investigated the triggered release of antibiotic drug from nitrogen-doped TiO2 nanotubes in response to remote visible light irradiation.


## 2. Materials and Methods

### 2.1. Fabrication of Nitrogen-Doped TiO2 Nanotubes

As reported previously [28], a machined Ti sheet (0.2 mm thick, 99.5%; Hyundai Titanium Co., Republic of Korea) was electropolished by an electrochemical etching process and cleaned with acetone and deionized water. To prepare nitrogen-doped TiO2 nanotube arrays on a Ti sheet, 0.05, 0.1, or 0.2 M diethanolamine (DEA; Sigma, MO, USA) was added into 0.5 w/v% hydrofluoric acid (48 w/v%; Merck, NJ, USA) in a mixture of water and acetic acid (98 w/v%; JT Baker, NJ, USA) in the volumetric ratio of  7 : 1. Anodization voltage and time were 20 V and 30 min, respectively.Samples were then rinsed with deionized water, dried at 60°C for 24 h, and heat treated at 500°C for 2 h in an atmosphere of N2. Morphological and surface analyses of nitrogen-doped TiO2 nanotube arrays were performed by field emission scanning electron microscopy (FE-SEM, S4800; Hitachi/Horiba, Japan), transmission electron microscopy (TEM, Tecnai  G2; FEI Co., USA, power: 300 kV), and X-ray photoelectron spectroscopy (XPS, K-Alpha ESKA system; Thermo, USA), respectively. Also, contact angle of experimental specimen was measured by contact angle meter (Theta Optical Tensiometer, KSV, Finland). The solvent of contact angle measurement was D.I. water.


### 2.2. Drug Release Test

Three antibacterial drugs, tetracycline (Sigma, MO, USA), cetylpyridinium chloride (CPC; Sigma, MO, USA), and chlorhexidine (Sigma, MO, USA), were mixed with polylactic acid (PLA; Sigma, MO, USA) and loaded into TiO2 nanotubes. Tetracycline is an antibiotic drug that serves as a protein synthesis inhibitor. Chlorhexidine is a chemical antiseptic, while cetylpyridinium chloride is a strong bactericide. The amount of antibiotics released as a function of incubation time was measured by a microplate ELISA reader (Spectra Max 250; Thermo Electron Co., USA).The amount of antibiotics released in response to visible right irradiation was also measured by the microplate ELISA reader. The source of visible light was a dental light curing unit (intensity, 1000 mW/cm2; wavenumber of irradiated light, 470 nm; Elipar Free-Light 2; 3 M ESPE Co., USA).


### 2.3. Data Analysis

All data were expressed as mean± standard deviation values and analyzed statistically by one-way ANOVA (SPSS 12.0; SPSS GmbH, Germany) and post hoc Duncan’s multiple range test. Significant differences were considered if P values were less than 0.05.


## 2.1. Fabrication of Nitrogen-Doped TiO2 Nanotubes

As reported previously [28], a machined Ti sheet (0.2 mm thick, 99.5%; Hyundai Titanium Co., Republic of Korea) was electropolished by an electrochemical etching process and cleaned with acetone and deionized water. To prepare nitrogen-doped TiO2 nanotube arrays on a Ti sheet, 0.05, 0.1, or 0.2 M diethanolamine (DEA; Sigma, MO, USA) was added into 0.5 w/v% hydrofluoric acid (48 w/v%; Merck, NJ, USA) in a mixture of water and acetic acid (98 w/v%; JT Baker, NJ, USA) in the volumetric ratio of  7 : 1. Anodization voltage and time were 20 V and 30 min, respectively.Samples were then rinsed with deionized water, dried at 60°C for 24 h, and heat treated at 500°C for 2 h in an atmosphere of N2. Morphological and surface analyses of nitrogen-doped TiO2 nanotube arrays were performed by field emission scanning electron microscopy (FE-SEM, S4800; Hitachi/Horiba, Japan), transmission electron microscopy (TEM, Tecnai  G2; FEI Co., USA, power: 300 kV), and X-ray photoelectron spectroscopy (XPS, K-Alpha ESKA system; Thermo, USA), respectively. Also, contact angle of experimental specimen was measured by contact angle meter (Theta Optical Tensiometer, KSV, Finland). The solvent of contact angle measurement was D.I. water.


## 2.2. Drug Release Test

Three antibacterial drugs, tetracycline (Sigma, MO, USA), cetylpyridinium chloride (CPC; Sigma, MO, USA), and chlorhexidine (Sigma, MO, USA), were mixed with polylactic acid (PLA; Sigma, MO, USA) and loaded into TiO2 nanotubes. Tetracycline is an antibiotic drug that serves as a protein synthesis inhibitor. Chlorhexidine is a chemical antiseptic, while cetylpyridinium chloride is a strong bactericide. The amount of antibiotics released as a function of incubation time was measured by a microplate ELISA reader (Spectra Max 250; Thermo Electron Co., USA).The amount of antibiotics released in response to visible right irradiation was also measured by the microplate ELISA reader. The source of visible light was a dental light curing unit (intensity, 1000 mW/cm2; wavenumber of irradiated light, 470 nm; Elipar Free-Light 2; 3 M ESPE Co., USA).


## 2.3. Data Analysis

All data were expressed as mean± standard deviation values and analyzed statistically by one-way ANOVA (SPSS 12.0; SPSS GmbH, Germany) and post hoc Duncan’s multiple range test. Significant differences were considered if P values were less than 0.05.


## 3. Results and Discussion

As shown in Figures1(a), 1(b), and 1(c), SEM images show the differences in appearance among N-doped 100 nm TiO2 nanotubes with 0.05 M, 0.1 M, and 0.2 M of DEA. The micrographs of N-doped TiO2 nanotubes show somewhat randomly organized nanotube geometry with different concentrations of DEA, in contrast to the SEM image of undoped TiO2 nanotubes. However, the nanotubular structure was not formed on the Ti surface at a DEA concentration of 0.2 M. Therefore, we examined the characteristics of N-doped TiO2 nanotubes at a DEA concentration of 0.1 M. TEM image (Figure 1(d)) of N-doped 100 nm TiO2 nanotubes treated by 0.1 M DEA indicates that nanosized (100 nm thickness) porous layer was formed at top surface of TiO2 nanotubes. High-magnification TEM image (Figure 1(e)) illustrates that the lattice spacing of newly formed layer is 0.35 nm, and this spacing is corresponding to the (101) planes of anatase TiO2 as previously reported [29].SEM images of 100 nm TiO2 nanotubes with (a) 0.05 M, (b) 0.1 M, and (c) 0.2 M of DEA treatment. (d) TEM and (e) high-magnification TEM images of 100 nm TiO2 nanotubes with 0.1 M DEA treatment.
(a)
(b)
(c)
(d)
(e)Figure2 indicates X-ray diffraction (XRD) patterns of undoped and N-doped TiO2 nanotubes. As shown, XRD mainly detected anatase TiO2 and Ti crystalline phases. There was no dramatic difference in crystallinity between undoped and N-doped TiO2 nanotubes after heat treatment. Therefore, we expect that DEA treatment had essentially no effect on the crystallinity of TiO2 nanotubes in this study.Figure 2
X-ray diffraction patterns of undoped and nitrogen-doped TiO2 nanotubes treated by 0.1 M DEA.The XPS spectra of TiO2 nanotubes and N-doped TiO2 nanotubes are shown in Figure 3(a). In terms of pure TiO2 nanotubes, Ti, O, and C elements were detected at 459.6, 531.2, and 285.5 eV, respectively. Among these elements, carbon is supposed to be contaminant deposited at the surface of TiO2 nanotubes. The surfaces of N-doped TiO2 nanotubes were composed of Ti, O, N, and C contaminants, and a very weak N signal was detected at the surface of N-doped TiO2 nanotubes. The XPS analysis also resulted that the N amounts in undoped TiO2 nanotubes and N-doped TiO2 nanotubes were 0.57 and 3.39 atomic%, respectively. The atomic ratio of Ti to N of N-doped TiO2 nanotubes was 5.13. As previously reported, the photocatalytic effect of N dopant was affected by both the N content of N-doped TiO2 and the degree of nitrogen atoms reacting with TiO2 precursor [30]. Therefore, N-doped TiO2 nanotubes having high N amounts and the atomic ratio of Ti to N are supposed to result in enhanced photocatalytic activity by visible light irradiation.(a) XPS spectra of TiO2 nanotubes and N-doped TiO2 nanotubes. (b) High resolution XPS spectra of N 1s for TiO2 nanotubes and N-doped TiO2 nanotubes.
(a)
(b)Also, the N amount doped in TiO2 structure is related to the intensity of photocatalytic activity at visible light region, and N amount is affected by the reaction temperature of dopant and TiO2. Previous studies have reported 5–8 atomic% of nitrogen incorporation into the TiO2 surface by chemical treatment and excellent photocatalytic activity in the visible light region [30–32]. These studies involved heat treatment of N-doped TiO2 nanoparticles at temperatures of 800–900°C to maximize the concentration of nitrogen doping. However, we could not heat treat TiO2 nanotubes above 500°C because heat treatment above 500°C resulted in the formation of rutile structure destroying the nanotubular structure of TiO2. This limitation provides lower N amount of TiO2 nanotubes compared to that of other TiO2 nanoparticles. There are several researches obtaining highly N-doped TiO2 nanomaterials without conventional sintering process. Xiang et al. developed nitrogen-, sulfur- or carbon- doped TiO2 nanosheets with exposed (001) facets showing excellent photocatalytic activity at visible region by solvothermal process [29, 33, 34]. Also, solvothermal process is seemed to be one of the techniques enhancing N-doping into the structure of TiO2 nanotubes without the destruction of nanotubular structure as previously reported [24, 26]. Further experiment is required to investigate the comparison of the photocatalytic activity between sintering process and solvothermal process for doping nitrogen into TiO2 nanotubes.The N 1s peaks were detected at the surface of N-doped TiO2 nanotubes at 406.1 and 402.5 eV of binding energy, respectively (see Figure 3(b)). In terms of the location of nitrogen dopant in TiO2 structure, many researches have reported that nitrogen species are doped into TiO2 in different forms due to doping process, reaction technique, and nitrogen sources [24–27, 29, 33–38]. From the results of previous researches [35–37], typical N 1s peak of TiN species mainly in substitutional N was less than 397.5 eV, whilst interstitial N in TiN species showed above 400 eV of typical N 1s binding energy. From the results of XPS analysis, typical binding energies of 402.5 and 406.1 eV are assigned to NO and NO2- generating highest localized state for interstitial species, which are characteristics of interstitial N-doped TiO2 on the basis of previous studies [35–37, 39]. Thus, it is confirmed that nitrogen from DEA is effectively doped into TiO2 nanotubes, and the N 1s peaks obtained from this study are assigned to interstitial N-doped TiO2 nanotubes.Figure4(a) shows the cross-sectional views of water droplets on machined Ti, electropolished Ti, and undoped and N-doped TiO2 nanotubes. Electropolishing did not change the hydrophilicity of Ti surface dramatically, but nitrogen doping did change the wettability of TiO2 nanotubes by changing the hydrophilic surface to a super hydrophobic (>120°) surface. Therefore, we are investigating the effect of the super hydrophobicity of N-doped TiO2 nanotubes on the behavior and functionality of human mesenchymal stem cells.(a) Cross-sectional views of water droplets on Ti, electropolished Ti, 100 nm TiO2 nanotubes, and N-doped 100 nm TiO2 nanotubes. (b) Graph showing water droplet contact angles at the surface of the nanotubes.
(a)
(b)Presented in Figure5 is the effect of PLA on the release behavior of antibacterial drugs such as tetracycline, CPC, and chlorhexidine. As shown in Figure 5(a), the experimental group with 10% tetracycline shows only an initial burst of drug release in the incubation period. However, all experimental groups with 1% PLA show sustained release of drugs as a function of incubation time. Therefore, we confirmed that 1% PLA could alter the elution behavior of all drugs and allow sustained and prolonged drug release regardless of the drug type.The release amounts of antibacterial drugs as a function of incubation time. (a) 10% tetracycline mixed with/without 1% PLA. (b) 10% CPC and chlorhexidine mixed with 1% PLA.
(a)
(b)Figure6 shows the elution concentrations of the three antibacterial drugs loaded on the surface of undoped and N-doped TiO2 nanotubes, respectively, after 30 seconds of visible light irradiation with the dental curing unit. In the tetracycline and chlorhexidine elution tests, the release concentrations of drugs from N-doped TiO2 nanotubes were significantly higher than those from undoped TiO2 nanotubes (P<0.05). However, the total amount of CPC released was much lower than the amounts of tetracycline and chlorhexidine, and there was no significant difference between CPC release from undoped and N-doped TiO2 nanotubes.(A) Dental light curing unit and visible light irradiation. (B) The concentration of drug released by visible light irradiation. (a) 10% Tetracycline, (b) 10% chlorhexidine, and (c) 10% CPC with 1% PLA.On the basis of these results, we can summarize that nitrogen doping into the TiO2 nanotubular structure was performed successfully by DEA treatment, even though the amount of nitrogen doping was lower than reported in other studies because of the lower heat treatment temperature used in this study. Moreover, drugs stored in N-doped TiO2 nanotubes were released effectively by the visible light irradiation with the dental light curing unit.


## 4. Summary

The visible light irradiation-mediated drug elution activity of nitrogen-doped TiO2 nanotubes has been investigated in this study. We found that nitrogen was effectively doped into the TiO2 nanotubular structure, with the existence of NO2- (406.1 eV) detected by the XPS analysis playing an important role in the photocatalytic activity of TiO2 in the visible light region. The results of the drug release test showed that PLA facilitated sustained and prolonged elution of drugs. We conclude that N-doped TiO2 nanotubes are expected to overcome the limited usage of TiO2, which shows photocatalytic activity only within the UV region, thereby allows the development of novel fusion technologies in the field of implant materials.



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