Molecularly Imprinted Polymers/Metal–Organic Framework (MIL-53) for Fluorescent Sensing of Ciprofloxacin in Water ^†

Abstract

The contamination of water and food with antibiotics residues poses a severe risk to human health and aquatic environments. The excessive and uncontrolled use of antibiotics is one of the major causes of their presence in the environment. Their continuous consumption willingly or un-willingly can result in severe health issues such as allergy, headache, hypertension, muscle pain, and hormonal dysfunction. Beside these, the development of antimicrobial resistance (AMR) can make the situation more critical. Therefore, advanced analytical approaches over conventional techniques are required to detect antibiotic residues in a facile and cost-effective manner. The present work deals with the design of fluorescent nanostructures as sensing probes for the detection of ciprofloxacin. Here, we have synthesized NH₂-MIL-53(Al) using a hydrothermal approach. This fluorescent metal–organic framework (MOF) was further combined with molecular imprinted polymers (MIPs) for the selective and specific detection of ciprofloxacin in aqueous solutions. The use of MIPs over other biomolecules (such as antibody, enzymes, and others) is highly promising as it avoids any kind of pre-treatment of the sample. The MIP@NH₂-MIL-53(Al) nanostructure formation is confirmed by performing different characterization techniques involving both spectroscopy and microscopy. The performance of the developed fluorescent composite promotes its applicability for the highly sensitive and specific detection of ciprofloxacin in practical applications.

1. Introduction

All over the world, the emergence of antibiotics in the environment has been recognized as a serious threat to both humans and the ecosystem. Their accumulation in water resources can lead to several health issues and endanger aquatic life. Prolonged exposure to antibiotics in the environment can cause the development antimicrobial resistance (AMR). In such a condition, the treatment of simple bacterial infections will no longer be possible. As per the World Health Organization (WHO) report [1], AMR is among the top global major public health threats that require urgent multisectoral action. Therefore, it is highly desirable to develop facile strategies for monitoring the presence of antibiotics in aqueous solutions. Among different antibiotics, ciprofloxacin (with broad-spectrum antibacterial activity and lower side effects) is extensively used to treat urinary tract infections, lung infections, cell carcinoma, and cystic fibrosis [2,3]. However, unintentional exposure and further accumulation of this in the human body can cause serious health issues such as hematuria, gastrointestinal complaints, skin reactions, and liver damage [4,5]. Among the different conventional techniques such as chromatography, spectrofluorimetry, electrophoresis, electrochemical, etc., the fluorescence approach is a promising method due to its ease of operation, quick response, low cost, and high sensitivity [6,7,8].

Nowadays, different fluorescent materials, especially metal–organic frameworks (MOFs), have been developed to replace the fluorescent dyes. MOFs offer several unique photophysical properties such as large surface area, structure tunability, photostability, high porosity, and narrow fluorescence emission. Several research reports explored using fluorescent MOFs for the detection of ciprofloxacin. For instance, An et al. [9] reported the Tb-based coordination polymer (CP) for the fluorescent detection of ciprofloxacin. Here, ciprofloxacin offered a sensitization effect on the luminescence of Tb³⁺. Beside water and milk samples, the Tb-based complexes have also been reported for ciprofloxacin detection in urine samples [10]. However, the toxic nature of lanthanides can limit their application for real-field monitoring. Liu et al. reported the Zr-based MOF functionalized with citrate (C-MOF-808) for the fluorescent detection of ciprofloxacin in aqueous solutions [11]. Beside the high sensitivity of these fluorescent sensors, their combination with molecularly imprinted polymers (MIPs) can offer specific target detection capabilities [12,13]. The incorporation of MIPs into porous MOFs can support the excellent selective detection capabilities. The high surface area and availability of abundant pores in MOFs is highly favorable for the synthesis of imprinting sites. However, there are very few examples in the literature on the use of this combination of MOFs and MIPs for the detection of antibiotics. Herein, Al-based MOFs were developed first via a hydrothermal approach and later were combined with ciprofloxacin-specific MIPs. The fluorescent properties of both NH₂-MIL-53(Al) and its composite with MIPs were examined for the specific detection of ciprofloxacin.

2. Materials and Method

2.1. Materials

The aluminium(III) chloride hexahydrate (AlCl₃·6H₂O), N,N-dimethylformamide (DMF), 2-aminoterephthalic acid (NH₂-BDC), and ciprofloxacin were purchased from Sigma-Aldrich (St. Louis, MO, USA). The urea, tetraethoxysilane (TEOS), aminopropltriethoxysilane (APTES), and ethanol were procured from HiMedia Pvt. Ltd. (Bengaluru, India). The ammonia solution was purchased from E. Merck Ltd. (Darmstadt, Germany). All these analytical grade chemicals were utilized as received without any kind of purification. The distilled water (DW) was prepared in a laboratory using distillation unit.

2.2. Synthesis of NH₂-MIL-53(Al)

The Al-based MOF was synthesized via a hydrothermal approach, as earlier reported in [14]. In detail, 0.3 M AlCl₃·6H₂O was dissolved in 10 mL DW and was mixed in a NH₂-BDC solution (0.2 M, 15 mL DMF) under continuous stirring. After 10 min, 5 mL aqueous solution of urea (1 M) was dropped in the above solution and the mixture was stirred for another 10–15 min. Thereafter, the solution was poured into Teflon-lined autoclave and heated to 150 °C for 6 h. The collection of the resulting yellow precipitates was performed with the help of centrifugation at 9000 rpm for 15 min and they were further washed with DW. For activation, the synthesized product was redispersed in 20 mL methanol and DMF solution under dark stirring overnight. The final produce was centrifuged and dried at 80 °C. The resultant product was collected for further characterization.

2.3. Synthesis of NH₂-MIL-53(Al)/MIP

The NH₂-MIL-53(Al)/MIP nanocomposite was prepared using a sol–gel approach [15,16]. Briefly, the aqueous solution of the above synthesized MOF (30 mg) and ethanol (10 mL) were added in a reagent bottle. Then, 0.1 mL APTES was added in the above solution under magnetic stirring for self-assembly of the APTES over the MOF structure. Further, a template ciprofloxacin solution (1 mg/mL) was added in the above solution and allowed to stir for 20 min. Next, a drop of ammonia hydroxide was added, followed by dropwise addition of TEOS (1 mL) and ethanol (10 mL). After overnight stirring of this reaction mixture at room temperature, the final product was collected via centrifugation. The product was washed thoroughly with DW and ethanol in subsequent steps up to five times. The final product was dried and stored for further characterization.

2.4. Fluorescence Detection of Ciprofloxacin

In this experiment, the PL of the synthesized material (0.02 mg·mL⁻¹) was examined with varied excitation from 290 to 390 nm. The emission was recorded between 350 and 450 nm at a fixed excitation of 330 nm. The PL of the materials was also studied in the presence of ciprofloxacin (100 µM and 1000 µM) with a 3:1 ratio of ciprofloxacin and MOF or MOF/MIP.

2.5. Characterization of NH₂-MIL-53(Al) and NH₂-MIL-53(Al)/MIP

The crystalline structure of NH₂-MIL-53(Al) was studied using an X-ray diffractometer (Rigaku Ultima diffractometer, USA). The analysis of the porosity and surface area of synthesized MOF was performed with the help of Belsorp Max system (Microtrac, Osaka, Japan). The hydrodynamic diameter and zeta potential of the synthesized structure were also analyzed using a Zeta Sizer (Malvern Instruments, Malvern, UK). The UV-Vis absorption characteristics of NH₂-MIL-53(Al) and its composite with MIP were recorded using a Shimadzu UV-3600 spectrometer. The functional groups over the surface of the synthesized materials were studied using Fourier transform infrared (FTIR; Perkin Elmer, Waltham, MA, USA) spectroscopy. The morphology and topography of the materials were examined using field emission-scanning electron microscopy (FESEM; JSM6100, Jeol, Akishima, Tokyo) along with energy-dispersive X-ray (EDX) spectroscopy. The fluorescence spectra of the materials were recorded using a SpectraMax^® system (Molecular Devices, San Jose, CA, USA).

3. Results and Discussion

The synthesized NH₂-MIL-53(Al) is well crystallized as confirmed using the XRD pattern, Figure 1a. All the prominent characteristic peaks are in accordance with [17]. Using the Debye Scherrer equation, the crystallize size of NH₂-MIL-53(Al) was calculated as 24.34 nm with a lattice strain of 0.0186. The N₂ adsorption/desorption isotherms of the synthesized MOF are presented in Figure 1b. The surface area of the MOF using the BET method was found to be 630.24 m²·g⁻¹ along with an average pore diameter of 4.126 nm and a total pore volume of 0.6501 cm³·g⁻¹. The hydrodynamic diameter of the MOF was found to be ~483 nm with a polydispersity index of 0.573. The highly positive zeta potential (20.2 mV) of NH₂-MIL-53(Al) confirms its aqueous stability, which tends to avoid agglomeration.

In the UV-Vis spectra (Figure 2a), two absorption peaks were found for NH₂-MIL-53(Al) at 270 and 380 nm. The absorption peak at 380 nm changes to 330 nm after the interaction of NH₂-MIL-53(Al) with the MIPs. Figure 2b shows the fluorescence spectra of both compounds at a fixed excitation of 330 nm. The interaction of MIPs with the MOF structure results in the small quenching of the fluorescence of MOF. The fluorescence of both compounds is also studied as a function of the varied excitation (refer to Figure 2c). In the FTIR spectra (Figure 2d), the broad adsorption band centered at 3462.05 cm⁻¹ confirms the presence of the OH group in the framework. The peak at 1628.18 cm⁻¹ can be allotted to N–H bending [18]. The typical vibration, which was due to the benzene linker, was observed at 1450.09 cm⁻¹ and can be attributed to the aromatic ring stretch amines. A significant impact of the MIPs can be seen over the presence of functional groups of MOF. The size of as synthesized MOF was found within the 100 nm range with the shape symmetry (refer to Figure 3a). The uniform distribution of the MOF with MIP can be seen in Figure 3b. The EDS spectra of both compounds are presented in Figure 3c,d.

The effect of the concentration of ciprofloxacin on the fluorescence properties of Al-based MOF and its composite with MIPs is shown in Figure 4. Here, it is clear that there is no significant change in the fluorescence properties of NH₂-MIL-53(Al) at lower (100 µM) and higher concentrations (1000 µM) of ciprofloxacin (refer to curve a and b, respectively). However, a significant change in the fluorescence of NH₂-MIL-53(Al)/MIP can be observed with the increasing concentration of ciprofloxacin. As the concentration of ciprofloxacin increases from 100 µM to 1000 µM, a significant increase in the fluorescence emission at 430 nm was observed with an excitation of 330 nm (refer to curve c and d, respectively). This significant change in the fluorescence emission of the composite can be potentially utilized for the detection of ciprofloxacin.

4. Conclusions

Nowadays, the screening of antibiotics is of utmost importance for the safety of ecosystems and human health. Here, the synthesis of fluorescent NH₂-MIL-53(Al) was carried out using a hydrothermal approach. After confirmation of its crystalline nature and high surface area, its composite with MIP was developed via a sol–gel approach. Ciprofloxacin-specific MIP has a significant impact on the optical properties of NH₂-MIL-53(Al), i.e., UV-vis absorbance and fluorescence properties. The significant change in the fluorescence emission from NH₂-MIL-53(Al)/MIP (at 430 nm with an excitation at 330 nm) with the increasing concentration of ciprofloxacin can be effectively utilized for its detection.