Enhanced selective photocatalytic properties of a novel magnetic retrievable imprinted ZnFe₂O₄/PPy composite with specific recognition ability

Abstract

A novel magnetic retrievable imprinted photocatalyst with the antigen/antibody-like function was obtained by coupling the imprinted polymer onto ZnFe₂O₄ nanocrystals. The morphology, structure and properties of the imprinted ZnFe₂O₄/PPy composite were characterized and revealed that a tight junction was formed between ZnFe₂O₄ and the imprinted polymer layer. Compared with non-imprinted ZnFe₂O₄/PPy composite, imprinted ZnFe₂O₄/PPy composite not only had high photocatalytic efficiency, but also possessed the strong selective ability to specifically recognize and preferentially degrade ciprofloxacin. It was worth noting that the selectivity of the imprinted ZnFe₂O₄/PPy composite eliminated the interference of the spatial structure of different pollutants and truly realized the function of specific recognition. This work provided a new idea for preparing an imprinted photocatalyst and realized specific recognition.

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1. Introduction

Ciprofloxacin (CIP), the third-generation artificial synthetic fluoroquinolone antibacterial drug which has a strong function of broad-spectrum bacteriostasis, sterilization, is widely used for treatment of diseases in humans and animals.^1,2 In recent years, however, the rampant use of ciprofloxacin has caused large amounts of antibiotic residues in food-producing animals and water resources, which lead to drug resistance of pathogenic bacteria (such as superbugs) and have severely threatened human health as well as the sustainable development of society.^3,4 These kinds of pollution are generally the forms of trace/ultra trace presence in the environment compared with common contaminations. Therefore, it is urgent to find an effective approach to preferentially remove highly-toxic contaminants from the environment.

Nowadays, there are many conventional treatment methods to solve the problem, such as adsorption, microbial degradation and electrolysis.^5–7 Compared with these methods, photocatalysis technology has drawn intensive research attention due to its attractive applications in environmental cleaning and energy conversion.^8,9 Unfortunately, owing to the big band gap, it is impossible to maximize the use of solar energy, such as TiO₂, ZnO, BiVO₄ and Bi₂WO₆ photocatalyst.^10–13 Besides, it's also can't achieve rapid recovery and reuse at the end of the experiment, which limits its further application. ZnFe₂O₄ semiconductor has now received much attention in the photocatalysis field because of its narrow-band-gap semiconductor, excellent photochemical stability and rapid magnetic separation.^14–16

As single-phase photocatalysts, their activity is hindered by a high recombination rate of photo-induced electron–hole pairs.^17–20 Organic conducting polymers have aroused great interest to the researchers because of their unique electrical and optical properties.^21–23 Polypyrrole (PPy), as a traditional conducting polymer, with an extended π-conjugated electron system. It has been reported recently that PPy can improve the photocatalytic performance of Bi₂VO₆, TiO₂ and g-C₃N₄ (ref. 24–26) by promoted the separation of the electron–hole pairs effectively and prolonged charge carrier life. The modified photocatalyst can improve the effects of pollutants decomposition, but the selective degradation of highly-toxic contamination is still very poor, this is mainly because of the difficult to distinguish highly hazardous contaminants from large amounts of low toxicity pollutant. Hence, molecular imprinting is a promising technique for the preparation of polymeric materials as specific molecular recognition receptors and these molecularly imprinted polymers (MIPs) can offer desirable properties like specific recognition, stability, low cost and ease of mass preparation.^27–30 Meanwhile, the previous literature have attested that the pyrrole can serve as a functional monomer of MIPs,^28,31 therefore, pyrrole itself can act both as an effective catalytic active substance and as a functional monomer of MIPs in our investigation.

Imprinted photocatalysts, which integrate both advantages of photocatalysis technology and molecular imprinting technology, namely, the target pollutant was selective recognition by the antigen/antibody-like function of imprinted cavities; afterwards, it will be the contact with the photo-generated active radicals and then decomposition into inorganic small molecules. As far as the imprinted cavities size are concerned, when the volume of contaminant is bigger than the template molecules, which will certainly cause the degradation rate of contaminant decreased because of it cannot access imprinted cavities. Conversely, the volume is smaller than the template molecules can enter into imprinted cavities. But, the key to reach specific recognition is antigen/antibody-like function of imprinted photocatalyst. Therefore, two kinds of pollutants (enrofloxacin and 5-sulfosalicylic acid) with different size and shapes were selectivity investigated.

In this study, in order to solve the above mentioned problems, it is crucial to obtain the imprinted photocatalysts with highly photocatalytic efficiency and truly realized the specific recognition of target contaminant. Imprinted ZnFe₂O₄/PPy composite with antigen/antibody-like function was synthesized via a facile surface imprinting technique method by using CIP as the molecular template, pyrrole as the functional monomer, N,N′-methylenebisacrylamide as cross-linking agent, ammonium persulfate as initiator and ZnFe₂O₄ as the supporter. The selective degradation of the imprinted ZnFe₂O₄/PPy composite was researched and the correlation between the structure and the property was discussed based on the results of some systematic characterizes. Finally, the photocatalytic degradation mechanism of CIP was also discussed.

2. Experimental

2.1. Materials

The raw materials such as zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O), anhydrous ethanol, ethyleneglycol, pyrrole, ammonium persulfate (APS), N,N′-methylenebisacrylamide (MBAA) and dimethyl sulfoxide were purchased from Beijing Chemicals Co. Ltd. of China. Ciprofloxacin (C₁₇H₁₈FN₃O₃, CIP), enrofloxacin (C₁₉H₂₂FN₃O₃, ENR) and 5-sulfosalicylic acid (C₇H₆O₆S, SSA) were supplied by National Institutes for Food and Drug Control, and their chemical structures were presented in Table 1. All the reagents were of commercially available analytical grade and used as received.

Table 1 Chemical structures of different organic pollution

Compounds	Structure	Molecular weight (g mol^−1)
Ciprofloxacin		331.4
Enrofloxacin		359.4
5-Sulfosalicylic acid		254.2

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2.2. Synthesis of ZnFe₂O₄ nanoparticles

The ZnFe₂O₄ nanoparticles were prepared with a solvothermal process. Namely 1 mmol Zn(NO₃)₂·6H₂O and 2 mmol Fe(NO₃)₃·9H₂O were completely dissolved into a 40 mL ethanol–ethyleneglycol mixed solution (the volume ratio of ethanol and ethyleneglycol is 1 : 9) under continuous magnetic stirring, and then the homogeneous solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained the temperature of 473 K for 24 h. After cooling down to room temperature, the obtained precipitate were collected by magnet and then washed several times with deionized water and ethanol alternately before being dried at 333 K for 12 h.

2.3. Synthesis of imprinted photocatalyst

The imprinted ZnFe₂O₄/PPy composite were synthesized by surface imprinting technique method. Firstly, 0.02 g CIP and certain amount of pyrrole (0.025 mL, 0.050 mL, 0.075 mL, 0.100 mL, and 0.150 mL) were dissolved in 30 mL dimethyl sulfoxide, well mixed, ultrasonicated for 5 min and then the mixed solution was stirred for 5 h at ambient temperature. This step was prepared to preassemble solution. Subsequently, 0.3 g ZnFe₂O₄ were finely dispersed on the preassemble solution and purged with N₂. Secondly, a mixed solution of 0.05 g MBAA, 0.05 g APS and 10 mL dimethyl sulfoxide was added dropwise to it, after that, the suspension solution was refluxed polymerization for a certain amount of time (6 h, 9 h, 12 h, 15 h, and 18 h) at 348 K under a nitrogen atmosphere. Finally, the obtained precipitates were added to 200 mL deionized water, and the CIP (template molecular) was removed under UV light irradiation. The products were collected, washed with deionizer water and ethanol in turn and dried at 323 K. The imprinted photocatalysts were obtained. The route of fabrication of the imprinted photocatalyst is shown in Scheme 1. Similarly, the non-imprinted ZnFe₂O₄/PPy composite were synthesized in the same way but without the addition of CIP.

Scheme 1 Schematic preparation of imprinted ZnFe₂O₄/PPy composite.

2.4. Characterization

XRD patterns were recorded on a D8 ADVANCE X-ray diffractometer (Bruker AXS Co., Germany). Fourier transform infrared spectra (FT-IR) were recorded on a Nicolet Nexus 470 FT-IR (America thermo-electricity Company) with 2 cm⁻¹ resolution in the range 4000–400 cm⁻¹, using KBr pellets. The scanning electron microscope (SEM) images and X-ray energy diffraction spectrum (EDS) were examined with JSM-7001F scanning electron microscopy (JEOL Ltd., Japan) while the transmission electron microscope (TEM) images were examined with JEM-2100 transmission electron microscopy (JEOL, Japan). UV-vis diffuse reflectance spectra (DRS) were obtained using a Shimadzu UV-3600 spectrometer by using BaSO₄ as a reference. Photoluminescence (PL) spectra were obtained on a fluorescence spectrophotometer (Cary Eclipse Spectrophotometer, VARIAN, USA). Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI 660B (Chenhua, China) electrochemical workstation with a standard three-electrode cell at room temperature. Specific surface areas were measured by using a NOVA 2000 e analytical system (Quantachrome Co., USA). Magnetic measurements were carried out by using a vibrating sample magnetometer (VSM) (HH-15, Nanjing University). Ultraviolet-visible Spectrophotometer (UV-2450, Shimadzu, Japan) were used for spectrophotometric determination of contaminants.

2.5. Photocatalytic activity

The photocatalytic activities were evaluated by the degradation of CIP under simulated solar light (two hundred watt tungsten lamp (320 nm < λ < 780 nm)). The experiments were performed at 303 K and the aeration rate was 2 mL min⁻¹ as follows: 100 mg of photocatalyst was suspended in 100 mL of CIP aqueous solution (20 mg L⁻¹). Prior to irradiation, the suspension were stirred for 1 h in dark to ensure the establishment of an adsorption–desorption equilibrium, the initial concentration was determined. Afterwards, the sampling for light irradiation was conducted in the interval of 20 min for two hour, isolated by an external magnetic field; the concentration of CIP was measured with the UV-vis spectrophotometer in the wavelength of 276 nm. The degradation rate (Dr) was calculated by using the following formula. C₀ and C are the initial concentration and residual concentration of the antibiotic solution, respectively.

2.6. Selective photodegradation of contaminants

For the selective photocatalytic test, enrofloxacin (fluoroquinolone antibacterial drug, 20 mg L⁻¹) and 5-sulfosalicylic acid (a kind of pharmaceutical intermediate, 20 mg L⁻¹) were also photodegraded by imprinted ZnFe₂O₄/PPy composite and non-imprinted ZnFe₂O₄/PPy composite. This process was analogous to Section 2.5. The concentration of ENR and SSA were measured by UV-vis spectrophotometer in the wavelength of 224 nm and 256 nm, respectively.

3. Result and discussion

3.1. Characterization

The phase structures of samples were identified by XRD characterization in Fig. 1. It can be easily found that the diffraction peaks on all samples at 2θ = 29.908°, 35.225°, 42.806°, 53.095°, 56.595° and 62.139° are ascribed to (220), (311), (400), (422), (511) and (440), respectively, indexed to the structure of spinel-type ZnFe₂O₄ (PDF#74-2397).¹⁵ This indicated that the modification of imprinted polymer did not change the lattice structure of ZnFe₂O₄. Meanwhile, it should be noted that there are not observed the diffraction peaks of other crystalline phase such as wurtzite structured ZnO and hematite phase Fe₂O₃, indicating that the obtained composite may be only composed of ZnFe₂O₄ and imprinted polymer. Besides, with the introduction of the imprinted polymer, the intensity of the diffraction peaks originating from ZnFe₂O₄ decreased, but, there are no detected any signal peak that are ascribable to imprinted polymer over imprinted ZnFe₂O₄/PPy composite (Fig. 1b) and non-imprinted ZnFe₂O₄/PPy composite (Fig. 1c). It is plausible that the polymer is amorphous (Fig. S1†),^32,33 or the content of polymer may be too low to determine its existence.

Fig. 1 The XRD spectra of ZnFe₂O₄ (a), imprinted ZnFe₂O₄/PPy composite (b) and non-imprinted ZnFe₂O₄/PPy composite (c) (standard ZnFe₂O₄ pattern PDF#74-2397).

FTIR spectra of all nanoparticles were shown in Fig. 2. For pure ZnFe₂O₄, the bands at 3415 cm⁻¹, 572 cm⁻¹ and 452 cm⁻¹ can be assigned to the stretching vibration mode of O–H surface organic solvent, Fe–O functional group and Zn–O functional group,^34,35 respectively. The characteristic bands at 1627 cm⁻¹ and 1480 cm⁻¹ were attributed to the stretching vibration of C=O in α,β-unsaturated ketones of cross-linking agent and C=C in the PPy ring, respectively. The obvious peak at 1309 cm⁻¹ was ascribed to the C–N stretching in pyrrole rings of the PPy backbones.³⁶ The band at about 1182 cm⁻¹ reflect the C–H stretching of pyrrole rings while the peaks of =C–H in-plane vibration appear at 1045 cm⁻¹, besides, ring deformation at 898 cm⁻¹ was also observed.^37–39 While the C–N stretching vibration and C–C out-of-plane ring deformation vibrations were found at 1127 cm⁻¹ and 953 cm⁻¹,³⁷ respectively. Compared with the pure ZnFe₂O₄ and PPy, it can be found that the imprinted ZnFe₂O₄/PPy composite were obtained.

Fig. 2 The FT-IR spectra of PPy (a), ZnFe₂O₄ (b), non-imprinted ZnFe₂O₄/PPy composite (c) and imprinted ZnFe₂O₄/PPy composite (d).

Morphological characterizations of ZnFe₂O₄ and imprinted ZnFe₂O₄/PPy composite were done by the SEM, TEM and EDS mappings analyses. The monodispersed ZnFe₂O₄ nanospheres with narrow size distributions are demonstrated by SEM (Fig. 3a), the size of the resulting spheres is about 150 nm. The high resolution TEM image of ZnFe₂O₄ (Fig. 3b) shown that the microspheres were formed through the agglomeration of numerous small nanoparticles with diameters of about several nanometers. Compared with the pure ZnFe₂O₄, Fig. 3c and d illustrates the SEM and magnified TEM image of the imprinted ZnFe₂O₄/PPy composite, which displays a perfect blend between ZnFe₂O₄ and imprinted polymer film, the nanometer microspheres are tightly wrapped by the imprinted polymer film. The unique structure is advantageous to the separation of photo-generated electrons and holes, and further enhances its photocatalytic activity. Meanwhile, as shown in Fig. 4, the EDS mapping results exhibit uniform distribution of Zn, Fe, O, N and C elements throughout the imprinted ZnFe₂O₄/PPy composite, which further confirmed that the imprinted ZnFe₂O₄/PPy composite were successfully obtained.

Fig. 3 SEM images and TEM images of ZnFe₂O₄ (a and b) and imprinted ZnFe₂O₄/PPy composite (c and d).

Fig. 4 (a) SEM images of the imprinted ZnFe₂O₄/PPy composite and the corresponding EDS mapping of (b) Zn, (c) Fe, (d) O, (e) N and (f) C elements.

The UV-vis absorption spectra of ZnFe₂O₄, PPy, imprinted ZnFe₂O₄/PPy composite and non-imprinted ZnFe₂O₄/PPy composite were illustrated in Fig. 5a. Evidently, compared with the ZnFe₂O₄, the composite materials shows stronger light absorption in both ultraviolet and visible light regions, this may be due to the unique optical properties of polypyrrole for extending the photoresponse region of ZnFe₂O₄.²³ Furthermore, the plots obtained by the transformation based on the Tauc plot, it is calculated by the following equation:

(αhv) = A(hv − E_g)² (2)

where α, h, v, A and E_g indicate absorption coefficient, Planck constant, light frequency, proportionality and band gap energy, respectively, were displayed in Fig. 5b. The roughly estimated band gap values are 2.15 eV and 1.25 eV corresponding to ZnFe₂O₄ and PPy, respectively. These results indicated that the energy levels of the ZnFe₂O₄ and the PPy are staggered, and the photo-induced electron–hole pairs could be more effectively separated. Therefore, improved photocatalytic activity of the composites as expected.

Fig. 5 UV-vis absorption spectra (a) and plots of the (αhv)² versus the energy of light (hv) (b) of PPy (A), non-imprinted ZnFe₂O₄/PPy composite (B), imprinted ZnFe₂O₄/PPy composite (C) and ZnFe₂O₄ (D).

Nitrogen adsorption and desorption measurements and Barrett–Joyner–Halenda (BJH) pore diameter distribution were investigated, as shown in Fig. 6. According to IUPAC classification, both of them exhibited typical type IV isotherm curves with a H3 hysteresis loop,^40,41 indicating the presence of mesopores in accordance with the corresponding pore diameter distribution (the insert of Fig. 6). In addition, the Brunauer–Emmett–Teller (BET) specific surface area of imprinted ZnFe₂O₄/PPy composite is 173.92 m² g⁻¹, which is larger than that of non-imprinted ZnFe₂O₄/PPy composite (141.61 m² g⁻¹), mainly because of the introduced of unique three-dimensional imprinted cavities. However, non-imprinted ZnFe₂O₄/PPy composite is also mesopores composites; the reason may be that the cross-linking agent cannot encapsulate ZnFe₂O₄ totally. From the TEM images (Fig. 3b), ZnFe₂O₄ nanoparticles through the agglomeration of numerous small nanoparticles and formed the mesopores spherical structure, which are consistent with the pore diameter distributions of ZnFe₂O₄ in Fig. S2.† Above all, the unique imprinted cavities structure may provide more active sites for absorption of CIP molecules, and then preferentially degrade CIP.

Fig. 6 Nitrogen adsorption–desorption isotherms and pore diameter distributions (insert) of imprinted ZnFe₂O₄/PPy composite (a) and non-imprinted ZnFe₂O₄/PPy composite (b).

Furthermore, the magnetic hysteresis (M–H) loops for the ZnFe₂O₄ nanoparticles and imprinted ZnFe₂O₄/PPy composite were shown in Fig. 7. These curves are typical for a soft magnetic material and indicate hysteresis ferromagnetism in the field ranges of 10 KOe.⁴² The room temperature saturation magnetization values of ZnFe₂O₄ and imprinted ZnFe₂O₄/PPy composite were obtained to be ∼25.15 emu g⁻¹ and ∼20.21 emu g⁻¹, respectively. The weakened magnetism of imprinted ZnFe₂O₄/PPy composite may be caused by the content of ZnFe₂O₄ per unit weight decreased. However, it could be clearly observed from the photograph (inset) that the imprinted ZnFe₂O₄/PPy composite were easily separated using a magnet, indicated that it possesses a good magnetic separation performance and avoid secondary pollution of catalyst.

Fig. 7 Hysteresis loops of ZnFe₂O₄ nanoparticles (a) and imprinted ZnFe₂O₄/PPy composite (b). Inset images are of imprinted ZnFe₂O₄/PPy composite suspended in water (left) and attracted by a magnetic field (right).

3.2. Photocatalytic activity

3.2.1. The influence of different polymerization time. The polymerization time affect the performance of imprinted ZnFe₂O₄/PPy composite directly. In order to investigate the optimal photo-induced polymerization time, five different polymerization times were used to synthesize imprinted ZnFe₂O₄/PPy composite in accordance with Section 2.3 (the content of pyrrole was 0.100 mL). As showed in Fig. 8a, the photocatalytic activities of imprinted ZnFe₂O₄/PPy composite were not raised monotonously with the increasing of polymerization time. When the polymerization time shorter, the imprinted layer cannot be fully formed and produce PPy with low molecular weight and poor conductivity, which cannot separate photogenerated e⁻–h⁺ pairs effectively. Conversely, when the polymerization time was greater than 12 h, the length of PPy chains increased and then defects in the PPy chains also increased, which may be destroyed the π-conjugated structure of PPy and hindered the migration of electrons, and then the photodegradation rate rapidly decreased. What's more, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of PPy were influenced by different degrees of polymerization,⁴³ which also influences the degradation rate of imprinted ZnFe₂O₄/PPy composite. Hence, 12 h was chosen as the optimum polymerization time to prepare the imprinted ZnFe₂O₄/PPy composite in the subsequent experiments.

Fig. 8 The processes of photocatalytic degradation of CIP with different polymerization time (a) and different addition dose of pyrrole (b).

3.2.2. The influence of different adding dose of pyrrole. Fig. 8b showed that the degradation rate of different content of pyrrole. As can be seen from the graph, the photocatalytic activity of imprinted ZnFe₂O₄/PPy composite initially increased and then decreased with increase of content of pyrrole from 0.025 mL to 0.150 mL, the optimum doping content of pyrrole was 0.100 mL and the degradation rate reached to 82.76%. The content of pyrrole was lower than the optimal, template molecular and functional monomer cannot be completely bonding which can affect the quantity of imprinted cavities, and it also leads to the photogenerated e⁻–h⁺ pairs separation and mass transfer ability to be insufficient. Whereas if the content of pyrrole was too much, the thickness of the imprinted layer increased which will lead to the absorption of light for ZnFe₂O₄ decreased as well as the relative contents of ZnFe₂O₄ in the imprinted photocatalyst decreased. Therefore, in the subsequent experiments, the dose of pyrrole was 0.100 mL.

3.2.3. Photocatalytic activity of different catalysts. Fig. 9a shows the photodegradation rates of CIP using different catalysts under simulated solar irradiation. Prior to irradiation, the suspension were stirred for 1 h in dark to ensure the establishment of an adsorption–desorption equilibrium, to eliminate the effects of the adsorption (Fig. S3†). The study found the photolysis experiment without photocatalyst, CIP was almost not degraded. However, under identical experimental conditions, imprinted ZnFe₂O₄/PPy composite showed the highest photocatalytic activities (82.76%) for CIP compared with pure ZnFe₂O₄ (36.53%) and PPy photocatalysts (33.28%), which may be ascribed to the efficient inhibition of charge recombination and raises the separation efficiency of photo generated e⁻–h⁺ pairs, as expected. Fig. 9b shows the changes in the absorbance of CIP solution in the presence of imprinted ZnFe₂O₄/PPy composite at different irradiation time. Thus, now the question may rise: why this composite exhibited so high efficiency?

Fig. 9 (a) The processes of photocatalytic degradation of CIP under simulated solar irradiation, (b) changes in UV-vis spectra of CIP with irradiation time in the presence of novel imprinted ZnFe₂O₄/PPy composite.

As we know that a weaker PL intensity represents a lower recombination probability of photo-generated electron and hole under light irradiation. Fig. 10a shows a comparison of the PL spectra of ZnFe₂O₄-based nanoparticles, and the PL emission spectra of the different samples with excitation at 285 nm. PL spectrum of ZnFe₂O₄ shows a broad peak centre around 370 nm. This has been attributed to the Zn vacancies existing in the ferrite samples.^44,45 It can be noted that a drastic quenching of photoluminescence intensity of imprinted ZnFe₂O₄/PPy composite and non-imprinted ZnFe₂O₄/PPy composite after introduced of polymer layer, indicating that the recombination of the photo-generated carriers was greatly reduced in the hybrid. Also, the EIS technique was widely used to investigate the charge transfer at semiconductor/electrolyte interface.⁴⁶ The arc radius on EIS Nyquist plots of FTO/composites electrode were significantly smaller than that of the FTO/ZnFe₂O₄ (Fig. 10b), revealing that the charge separation efficiency for imprinted ZnFe₂O₄/PPy composite and non-imprinted ZnFe₂O₄/PPy composite were much higher than that for pure ZnFe₂O₄. This result is in accordance with that of PL spectra. In addition, the degradation mechanism of catalyst will be further analyzed about how to improve catalytic efficiency.

Fig. 10 PL (a) and EIS (b) spectra of ZnFe₂O₄ (A), imprinted ZnFe₂O₄/PPy composite (B) and non-imprinted ZnFe₂O₄/PPy composite (C).

3.2.4. Recyclability of the photocatalyst. The lifetime was an important parameter of the catalyst for practical application. Therefore, the stability of imprinted ZnFe₂O₄/PPy composite was investigated. As shown in Fig. 11, it can be seen that a marginal fall in the degradation rate after four consecutive cycles experiment, but, still reach up to 76.83%, the phenomenon is due to a slight loss of the catalyst after each cycle rather than the deactivation of the catalyst, which indicated that the imprinted ZnFe₂O₄/PPy composite had excellent photocatalytic stability in environmental remediation.

Fig. 11 Cyclic photodegradation of CIP by imprinted ZnFe₂O₄/PPy composite for 4 times.

3.3. Selectivity experiment

The degradation experiments of the different catalyst about three kinds of contaminants under the same test conditions, and results are shown in Fig. 12a. The degradation rate of CIP over the imprinted ZnFe₂O₄/PPy composite reaches up to 82.76%, but that of the non-imprinted ZnFe₂O₄/PPy composite declined significantly to 57.11%. In contrast, the degradation rate of SSA over the non-imprinted ZnFe₂O₄/PPy composite could reach 69.89%, but that of the imprinted ZnFe₂O₄/PPy composite shows an anomaly decrease, only 63.60%. Besides, ENR has nearly the same structure as CIP except more than one ethyl groups, the degradation trend of ENR over different catalysts are also similar to that of ciprofloxacin over the corresponding catalysts, but the degradation rates of ENR over imprinted ZnFe₂O₄/PPy composite (73.78%) and non-imprinted ZnFe₂O₄/PPy composite (65.34%) are still reduced compared with those of CIP. The results indicated imprinted photocatalyst had preferentially selectivity degrade target contaminant.

Fig. 12 (a) Photocatalytic degradation of ciprofloxacin (A and B), enrofloxacin (C and D) and 5-sulfosalicylic acid (E and F), and (b) the corresponding to kinetics fitting of imprinted ZnFe₂O₄/PPy composite (A, C and E) and non-imprinted ZnFe₂O₄/PPy composite (B, D and F), respectively.

To further illustrate the selective degradation, the linear profile indicated that photocatalytic degradation kinetics of contaminants could be approximated by the pseudo-first-order rate law and were presented in Fig. 12b. The apparent rate constant (k) (min⁻¹) were calculated by the eqn (3):

where C₀ (mg L⁻¹) and C (mg L⁻¹) are the concentration of CIP, ENR or SSA at t = 0 (min) and instant t (min), respectively. The selectivity coefficients (α) and the relative selectivity coefficient (α′) of the imprinted ZnFe₂O₄/PPy composite and non-imprinted ZnFe₂O₄/PPy composite relative to the competition species were obtained from eqn (4) and (5).

The k, α and α′ values were summarized in Table 2, the selectivity coefficients of imprinted ZnFe₂O₄/PPy composite for CIP relative to ENR and SSA are higher, 1.31 and 1.71, respectively. The selectivity coefficients of non-imprinted ZnFe₂O₄/PPy composite for CIP relative to ENR and SSA are lower, 0.792 and 0.682, respectively. This implies that the imprinted ZnFe₂O₄/PPy composite had higher photocatalyst selectivity for CIP over ENR and SSA. The relative selectivity coefficients of imprinted ZnFe₂O₄/PPy composite for CIP relative to ENR and SSA are 1.65 and 2.51, respectively, which are greater than 1 and showed the imprinted ZnFe₂O₄/PPy composite had preferentially selectivity oxidation target contaminants more than that of the non-imprinted ZnFe₂O₄/PPy composite.

Table 2 The selectivity coefficient (α) and the relative selectivity coefficient (α′) data of imprinted ZnFe₂O₄/PPy composite and non-imprinted ZnFe₂O₄/PPy composite

	Imprinted ZnFe2O4/PPy composite			Non-imprinted ZnFe2O4/PPy composite			α′
	k (min^−1)	R^2	α	k (min^−1)	R^2	α
CIP	0.0145	0.9765	—	0.00696	0.9676	—	—
ENR	0.0111	0.9723	1.31	0.00879	0.9898	0.792	1.65
SSA	0.00849	0.9944	1.71	0.0102	0.9988	0.682	2.51

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Obviously, imprinted photocatalyst had the ability to degrade target contaminant is mainly due to the imprinted cavities on the surface of catalyst. CIP is used as the template molecule, which can be specifically recognized by the imprinted cavities of imprinted ZnFe₂O₄/PPy composite. The photodegradation reaction of CIP can take place both on the outside surface and in the imprinted cavities. The enlarged specific surface area indicates that the effective active sites increased and the degradation rate of CIP also increases sharply. For large volume of pollutants (ENR), owing to the similar template molecule structure, it can be degraded not only on the surface of imprinted ZnFe₂O₄/PPy composite, but also in some imprinted cavities, occasionally. Thereby, the relative selectivity coefficient of imprinted ZnFe₂O₄/PPy composite for CIP relative to ENR is only 1.65. However, with regards to SSA, it has an entirely different molecular structure to CIP, resulting in it cannot be identified (even if the volume of SSA is smaller than the template molecules). The photodegradation reaction of SSA only occurs on the outside surface. For imprinted ZnFe₂O₄/PPy composite, because of the existence of imprinted cavities, this behavior leads to significantly reduce effective number of active sites on the outside surface. Therefore, the degradation rate of SSA over the non-imprinted ZnFe₂O₄/PPy composite are higher than the imprinted ZnFe₂O₄/PPy composite, and the relative selectivity coefficient reaches up to 2.51. From the above, the selectivity of imprinted ZnFe₂O₄/PPy composite eliminated the interference of the spatial structure of different pollutants and truly realized the function of specific recognition.

3.4. Photodegradation mechanism

Previously reported that a large number of main active species, including holes (h⁺), electrons (e⁻), hydroxyl radical (˙OH) and superoxide radical (˙O₂⁻), were involved in the photocatalytic degradation.^47–49 Therefore, the effects of different scavengers on the degradation of CIP were investigated in an attempt to illuminate the degradation mechanism. Ethylenediaminetetraacetic acid disodium salt (EDTA) was added as a scavenger of h⁺, silver nitrate (AgNO₃) as an e⁻ scavenger, tert-butyl alcohol (t-BuOH) as a scavenger of ˙OH and N₂ was adopted to quench ˙O₂⁻. Note that the photocatalytic activity will be suppressed, and the extent of decrease in the CIP degradation efficiency that was induced by the scavenger indicated the importance of the corresponding active species.

As shown in Fig. 13. With the addition of AgNO₃, the photocatalytic degradation efficiency of CIP was markedly decreased to 32.59%. At this point, it is not difficult to derive the effective number of photo-generated holes (h⁺) increased and ˙O₂⁻ is decreased in the system, which indicated that the latter one's function is more important (e⁻, as a reducing agent and can't direct oxidation organic contaminant). Meanwhile, the degradation of CIP slightly retarded with about 38.66% reduction under de-aerated conditions, this confirmed that ˙O₂⁻ play a major role in the reaction system. Then, the degradation rate of CIP was decreased to 48.39% by the injection of EDTA, which implied that the key role of holes can act either as the oxidant or the origin of the ˙OH radicals. As t-BuOH was added, an efficient ˙OH radicals quencher, the degradation efficiency of CIP reach up to 56.28%, which revealed that ˙OH radicals contributed to a lesser extent in CIP degradation. Therefore, the degree of influence was ˙O₂⁻ > h⁺ > ˙OH for the imprinted ZnFe₂O₄/PPy composite.

Fig. 13 Photodegradation efficiencies of CIP over imprinted ZnFe₂O₄/PPy composite in the presence of different scavengers (the concentration of t-BuOH, AgNO₃, N₂ and EDTA, are 1 mmol L⁻¹).

Based on the above research, a possible reaction mechanism for the high selective photocatalytic activity of imprinted ZnFe₂O₄/PPy composite was proposed as illustrated in Scheme 2. First, the CIP molecules can be adsorbed onto the imprinted cavities of imprinted ZnFe₂O₄/PPy composite by antigen/antibody-like function. For ZnFe₂O₄, the valence band (VB) and conduction band (CB) matches well with the LUMO and the HOMO of PPy. Under simulation solar irradiation, PPy was excited and produce holes in its HOMO orbital. The holes then migrated into the VB of ZnFe₂O₄, and the electrons were born in the CB of ZnFe₂O₄ and transferred to LUMO of PPy at the same time. It will be helpful to the efficient separation of photo-generated carriers and hinders the recombination of e⁻–h⁺ pairs. The resulting cascade of photo-generated electrons migrate easily to the imprinted cavities to react with oxygen to generate ˙O₂⁻ and ˙OH, and the positive charged h⁺ can react with OH⁻ or H₂O to yield ˙OH. There are extremely strong oxidants for the mineralization of CIP to inorganic molecular compound gradually.

Scheme 2 The possible mechanism of the photocatalytic degradation of target template CIP over imprinted ZnFe₂O₄/PPy composite.

The entire radical reaction processes are presented as follows:

PPy + hv → PPy (e⁻) + PPy (h⁺) (6)

ZnFe₂O₄ + hv → ZnFe₂O₄ (e⁻) + ZnFe₂O₄ (h⁺) (7)

e⁻ + O₂ → ˙O₂⁻ (10)

h⁺ + H₂O/OH⁻ → ˙OH (11)

CIP + ˙O₂⁻ + ˙OH + h⁺ → inorganic molecules (13)

4. Conclusions

Imprinted ZnFe₂O₄/PPy composite with antigen/antibody-like function was successfully prepared by a facile surface imprinting technique, which has a high photodegradation activity under mild condition. The imprinted cavities on the surface of imprinted ZnFe₂O₄/PPy composite eliminated the interference of the spatial structure of different pollutants and truly realized the function of specific recognition. Therefore, compared with non-imprinted ZnFe₂O₄/PPy composite, imprinted ZnFe₂O₄/PPy composite possessed the strong selective ability to specifically recognize and preferentially degrade CIP, and the relative selectivity coefficients of imprinted ZnFe₂O₄/PPy composite for CIP relative to ENR and SSA up to 1.65 and 2.51, respectively. In addition, the photocatalytic reaction mechanism proved that the major decomposition process of CIP molecules on the surface of imprinted ZnFe₂O₄/PPy arise from the chief contribution of ˙O₂⁻, h⁺ and ˙OH. Meanwhile, the imprinted ZnFe₂O₄/PPy composite possesses excellent separation ability and satisfactory photocatalytic stability. Finally, as research continues, I believe that this technique has wide application prospect.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21306068 and 21546013), the Natural Science Foundation of Jiangsu Province (No. K20130487 and BK20140532), the Postdoctoral Science Foundation of Jiangsu Province (No. 1501102B), the Innovation Programs Foundation of Jiangsu Province (No. SJZZ_0136 and SJLX15_0504).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07132e

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