Enhanced photocatalytic degradation of tetracycline hydrochloride by molecular imprinted film modified TiO2 nanotubes

To enhance the photocatalytic activity of TiO2 nanotubes, tetracycline hydrochloride (TC) molecularly imprinted titania modified TiO2 nanotubes (MIP-TiO2) was prepared by liquid phase deposition, which improved the molecular recognition ability of the photocatalyst toward template molecules. This MIP-TiO2 photocatalyst was characterized by ESEM and XRD, which showed that the imprinted titania was deposited on the nanotube uniformly and was of well-crystalized anatase-type. In the adsorption experiments, MIP-TiO2 exhibited a high adsorption capacity (about 1.6 times higher than that of TiO2 nanotubes) for TC mainly because of its imprinted sites and high surface area. Under UV irradiation MIP-TiO2 showed enhanced photocatalytic activity with an apparent first-order rate constant 1.9-fold that of TiO2 nanotubes.

TiO 2 has been widely studied for environmental protection applications because of its high photocatalytic (PC) activity, chemical stability, nontoxicity and relatively low price. Many organic pollutants such as persistent toxic substances (PTS), dyes, pesticides and herbicides are degraded by TiO 2 catalysts under UV light irradiation [1][2][3][4]. Titania nanotubes can improve the photocatalytic properties compared with other forms of titania because of its excellent PC activity [5,6]. In previous studies, TiO 2 nanotubes prepared by electrochemical anodic oxidation exhibited high PC and photoelectrocatalytic activity during organic pollutant degradation [7]. Their well-formed nanotube array architecture offers the ability to influence the absorption and propagation of light through the architecture and the closely packed vertically aligned crystalline tubes also offer excellent electron percolation pathways for vectorial charge transfer between the interfaces [8][9][10]. However, PC efficiency still does not fulfill the requirements for practical application in the treatment of low-level harmful pollutants. The efficiency of TiO 2 photocatalysts for application in wastewater treatment needs to increase.
Molecular imprinting is a promising technique for creating specific molecular recognition sites in solid materials by employing target molecule as a template [11,12]. Since Vlatakis et al. [13] reported theophylline and diazepam molecularly imprinted polymers (MIPs) for mimicking antibody combining sites, interest in the molecularly imprinting technique has increased because of its unique properties of predetermination, selectivity and specific affinity for templates [14][15][16]. Recently, Shen et al. [17,18] developed a new way to enhance the PC efficiency of TiO 2 nanoparticles by imprinting the target molecule onto a photocatalyst's surface. In our previous work, we found that the photoelectrocatalysis of TiO 2 nanotubes could also be improved and selectivity could be realized by modification with organic imprinted polymers on the substrate of the TiO 2 nanotubes [19]. With the development of molecular imprinting technology, inorganic materials have been preferred for applica-tions because of their stability. TiO 2 molecularly imprinted thin layers prepared by liquid phase deposition (LPD) or solgel methods show good sensitivity and selectivity for the detection or electrocatalysis of template molecules [20][21][22]. The preparation procedure of these materials is simple and they are more stable than organic materials. Accordingly, surface molecularly imprinted titania coated onto TiO 2 nanotubes is accessible for the construction of novel hybrid photocatalysis, which retains the specific adsorption ability of the imprinted network and the high PC activity of the nanotubes.
In this study, surface molecularly imprinted titania was prepared on TiO 2 nanotubes by the liquid phase deposition method. Tetracycline hydrochloride (TC, Figure 1) was used as a template and it is a type of broad-spectrum antibiotics that is used in pharmaceuticals and personal care products (PPCPs). The abuse of TC and other antibacterials in aquaculture and farming industry negatively affects the environment and antibiotic residues in water and food results in allergies in humans. The photocatalytic activity of the fabricated MIP-TiO 2 was found to be higher than that of TiO 2 nanotubes.

Chemicals and materials
Titanium sheets were purchased from Tianjin Gerui Co. Ltd. Tetracycline hydrochloride (TC) was obtained from Johnson Matthey. (NH 4 )TiF 6 was from Tianjin Fuchen Chemical Reagent Co. H 3 BO 3 was from Shenyang NO.5 Chemical Reagent Factory. All the other chemicals were of analytical grade and used as received. Ultrapure water was used for the preparation of all aqueous solutions.

Preparation of the TiO 2 nanotubes
The TiO 2 nanotubes were prepared by anodic oxidation according to literature in a two-electrode electrochemical cell [7]. To induce crystallinity, the initially amorphous asanodized layers were annealed at 500°C for 2 h at a rate of 3°C/min during the heating processes and then cooled to room temperature at a rate of 3°C/min.

Synthesis of imprinted titania modified TiO 2 nanotubes
The liquid phase deposition of imprinted titania onto the TiO 2 nanotubes was carried out by mixing 5 mL 0.1 mol/L (NH 4 ) 2 TiF 6 with 5 mL 0.3 mol/L H 3 BO 3 followed by adding 10 mg of TC. The TiO 2 nanotube substrate was placed vertically into the deposition solution and the temperature was maintained at 20°C in a water-bath for a certain deposition time. Molecularly imprinted titania modified TiO 2 nanotubes (MIP-TiO 2 ) were thus prepared after the template molecule was removed by thoroughly rinsing with water. Non-imprinted TiO 2 nanotubes (LPD-TiO 2 ) were used as a control sample and were prepared by the same method but in the absence of TC in the deposition solution and the same procedures were used as for MIP-TiO 2 .

Characterization and photocatalytic experiments
The morphological characterization of the samples was carried out using an environmental scanning electron microscope (ESEM; Quanta 200 FEG). X-ray diffraction (XRD) analysis was carried out using a diffractometer with Cu K radiation (Shimadzu LabX XRD-6000) to determine the crystalline structure of the samples.
The adsorption of TC onto LPD-TiO 2 and MIP-TiO 2 was performed by placing the sample in solutions containing 20 mg/L TC. The whole process was carried out with continuous magnetic stirring for 2 h at 20°C in a water bath. The amount of adsorbed TC was determined by measuring the difference between the total amount of TC and the residual amount in solution.
The photocatalytic oxidation of TC was carried out in a single quartz photochemical reactor. The lamp was placed vertically outside the reactor with an illumination intensity of 1.0 mW/cm 2 . All experiments were carried out with continuous magnetic stirring. The TC aqueous solution with an initial concentration of 20 mg/L was sampled periodically during the reaction and the absorbance was measured at 275 nm as a function of reaction time using a UV-7504 spectrophotometer.

Mechanism of the LPD procedure
Liquid phase deposition is a soft-wet chemical and selfassembling process for the preparation of oxide films. The principle of the LPD process is the formation of oxide thin films by the slow hydrolysis of a metal-fluoro complex by fluoride scavengers [6,21,23]. [TiF 6 ] 2 is hydrolyzed to [TiF 6−n (OH) n ] 2 groups and HF molecules. The hydrolysis accelerates after HF is captured by H 3 BO 3 . A mechanism for the preparation of the LPD-TiO 2 film can be expressed as the reactions (1) For the preparation of MIP-TiO 2 , the template can interact with [TiF 6−n (OH) n ] 2 groups and form noncovalent bonds with TiO 2 molecules (Figure 2). To remove the loose imprinted material and the adsorbed HBF 4 and NH 4 + , MIP-TiO 2 was washed with water. MIP-TiO 2 was then be used to rebind the TC molecules. Figure 3 shows top-view ESEM images of TiO 2 nanotubes that were fabricated using MIP-TiO 2 over different deposited times and upon LPD-TiO 2 liquid phase deposition over 5 h. As shown in Figure 3(a), the diameter range of the TiO 2 nanotubes is about 60 to 90 nm with wall thicknesses of about 20 nm. After liquid phase deposition, the molecular imprinted titania film was uniformly deposited onto the surface of the TiO 2 nanotubes. The film thickness increased with an increase in deposition time and simultaneously the inner radius decreased gradually (Figure 3(b)-(d)). For a deposition time of up to 21 h the whole TiO 2 nanotubes surface was covered by the molecular imprinted titania film, and an obvious crack appeared because of the over-covered molecular imprinted titania film. Therefore, in this study the imprinted titania that was deposited for 5 h was chosen for further investigation to determine the requirements for a high photocatalysis efficiency. Based on a comparison between the morphology of LPD-TiO 2 (Figure 3(f)) and MIP-TiO 2 (Figure 3(c)), which was deposited over 5 h it is reasonable to deduce that the template molecules that were added during the imprinting process had little influence on the morphology of the MIP-TiO 2 .   Figure 4 shows the XRD patterns obtained for the TiO 2 nanotubes, MIP-TiO 2 and LPD-TiO 2 . Well-crystalized anatase-type TiO 2 films without any other phase characteristics were formed on the TiO 2 nanotubes after annealing at 500°C. Both MIP-TiO 2 and LPD-TiO 2 are in anatase form. Neither contains the rutile phase, which is a polymorph that is less photoactive than anatase [24,25]. The strength of the peaks in the MIP-TiO 2 and LPD-TiO 2 samples are a little weaker than that for TiO 2 nanotubes, which is due to the loose density of the deposited samples. The XRD patterns indicate that a modification of the deposited layer on the TiO 2 nanotubes does not affect its crystallization and its photocatalytic ability is retained.

Adsorption properties of the TiO 2 nanotubes, LPD-TiO 2 and MIP-TiO 2
The adsorption of TC onto LPD-TiO 2 and MIP-TiO 2 was performed by placing the sample in solutions containing 20 mg/L TC. The amount of adsorbed TC was determined by measuring the difference between the total amount of TC and the residual amount in solution. The adsorption ability of the TiO 2 nanotubes for TC was determined under the same conditions. As shown in Table 1 the imprinting effect was confirmed by the fact that MIP-TiO 2 showed a higher adsorption capacity for TC than the TiO 2 nanotubes and LPD-TiO 2 . The maximum adsorption capacity under equilibrium conditions was about 1.3 ng for the 20 mg/L TC solution, which was 1.6 times higher than that of the TiO 2 nanotubes and 1.2 times of LPD-TiO 2 . The effective imprinted sites of the MIP-TiO 2 that was produced at the wall or the inner surface of the nanotubes contributed to the improved adsorption capacity for the target molecule. It is expect that MIP-TiO 2 will have a high TC photodegradation efficiency.

Photocatalytic activities
The photocatalytic degradation of TC by TiO 2 nanotubes, LPD-TiO 2 and MIP-TiO 2 was determined under UV light irradiation. The results are shown in Figure 5. The PC removals of TC for LPD-TiO 2 and MIP-TiO 2 are much higher than that for the TiO 2 nanotubes ( Figure 5(a)). The kinetics of TC degradation in this study for the direct photolysis (DP) and photocatalytic degradation by the TiO 2 nanotubes, LPD-TiO 2 and MIP-TiO 2 follows pseudo first order kinetics, as shown in Figure 5(b). The apparent first-order rate constant k was evaluated according to ln(C 0 /C)=kt, where C represents the TC concentration at time t and C 0 is the initial concentration. The corresponding kinetic constants and regression coefficients of TC by direct photocatalysis and photocatalysis by TiO 2 , LPD-TiO 2 and MIP-TiO 2 are also given in Figure 5(b). The presence of LPD-TiO 2 and MIP-  TiO 2 photocatalysts increases the rates of TC removal giving apparent constants of 0.163 and 0.218 h 1 , respectively. The rate constant for TC photodegradation by MIP-TiO 2 is about 1.9 times higher than that for the TiO 2 nanotubes.

Conclusions
In conclusion, using TC as a template and TiO 2 nanotubes as a support, a molecularly imprinted titania modified TiO 2 photocatalyst was prepared by liquid phase deposition. A MIP layer provided specific molecular recognition sites leading to a high absorption ability towards the target pollutant. Photodegradation experiments confirmed that the photocatalytic activity of the TiO 2 photocatalyst can be enhanced by combining molecularly imprinted titania with TiO 2 nanotubes. This is an alternative way to increase the PC activity of TiO 2 by a molecularly imprinted film fabricated by liquid phase deposition.