A Multi-Functional Fluorescence Sensing Platform Based on a Defective UiO-66 for Tetracycline and Moxifloxacin

Abstract

In recent years, the excessive use and disordered discharge of antibiotics have had sustained adverse effects on ecological balance and human health. The convenient and effective detection of these “emerging pollutants” has become one of the research hotspots in the environmental field. In this study, a defective UiO-66 material, namely UiO-66-D, was constructed for the sensitive and selective sensing of tetracycline (TC) and moxifloxacin (MXF) in water. By utilizing a modulated synthesis approach with concentrated HCl, stable blue fluorescence at 400 nm was achieved for UiO-66-D. The as-prepared UiO-66-D could conduct the inner filter effect (IFE) within a short time (10 s) when sensing TC and MXF, and the fluorescence of the UiO-66-D was quenched. In particular, when sensing MXF, a ratiometric signal response was generated due to the combined effect of the IFE and the fluorescence of MXF itself. The sensitive and selective detection of TC and MXF using UiO-66-D was free from the interference of common anions and cations in water samples. The detection limit (LOD) for TC was determined to be 70.9 nM (0–115 μM), while for MXF, it was found to be 33.1 nM (0–24 μM). Additionally, UiO-66-D was successfully used to recognize TC and MXF in lake water with good recoveries, demonstrating that UiO-66-D exhibits substantial potential in the recognition of pollutants in environmental waters.

1. Introduction

Due to their broad-spectrum antibacterial activity, antibiotics are widely used in the treatment of protozoa or bacterial infections in humans and animals. Studies in recent years have shown that a large proportion of antibiotics (about 30–70%) cannot be metabolized and absorbed by humans and animals [1]. Moreover, because antibiotics are stable enough, the currently commonly used water treatment process cannot effectively remove antibiotics, resulting in antibiotics being discharged into the surrounding environment. Recent studies have revealed the presence of antibiotics in surface waters worldwide, with concentrations ranging from ng·L⁻¹ to mg·L⁻¹ [2,3]. Although low concentrations of antibiotics do not cause acute reactions in aquatic organisms, due to the difficult nature of antibiotics to degrade, they can exist stably in the water environment, thus continuously accumulating in aquatic organisms, destroying the microbial balance of the ecosystem, and enriching in the human body through the food chain. When exposed to antibiotic residues for a long period of time, the microorganisms in water will undergo adaptive evolution and induce the generation of resistance genes, which will cause irreversible damage and persistent pollution to the aquatic ecosystem [4]. Moreover, human health can also be directly or indirectly harmed, such as suffering from joint diseases, kidney diseases, internal secretion disorders, and the failure of the central axis meridian system. Among many antibiotics, tetracycline (TC) and moxifloxacin (MXF) are widely used in disease prevention and treatment and food processing because of their good antibacterial effect and growth promotion ability. Because of this, the development of new and efficient detection technology for the high efficiency, economy, and green detection of antibiotics such as TC and MXF has great application prospects in the field of environmental governance and remediation, and also has important practical significance for the stable development of society and harmony.

At present, the problem of antibiotic pollution in water environments is becoming more and more serious, and it is urgent to find practical and effective methods to quickly understand the status quo of antibiotic pollution to provide guidance for antibiotic control. To this end, researchers have invested a lot of energy and continue to develop a variety of detection technologies for antibiotics in water. Nowadays, high-performance liquid chromatography (HPLC) [5,6] and liquid chromatography–tandem mass spectrometry (LCMS/MS) [7] are the traditional antibiotic detection methods with high accuracy. However, these methods require the use of large, expensive instruments, often require long detection times, and have very high demands on the operator, which limit the efficient detection of antibiotics. Recently, various new technologies such as fluorescence [8,9,10] and electrochemical sensing [11,12,13] have been developed. Among them, fluorescence detection technology has been widely considered because of its simple operation, quick response, and high accuracy [14,15]. The selection of fluorescence sensing materials is crucial to fluorescence detection technology. In recent years, fluorescent materials such as quantum dots [16,17,18,19,20] and metal–organic frameworks (MOFs) [1,21,22,23] have been employed as fluorescent materials for the recognition of antibiotics in water. Among many fluorescent materials, fluorescent MOFs have become the dominant materials for constructing fluorescence sensing platforms with the following advantages: (i) the structure of MOFs can be accurately understood through single-crystal analysis, so as to clarify their structure–activity relationship; (ii) MOFs have the characteristics of a large specific surface area and high porosity, which can help pollutants diffuse to the active site and enrich the detected substances, so as to achieve the efficient detection of trace pollutants in water; (iii) they have diverse structures and rich active sites, so it is easy to obtain a high reaction efficiency; (iv) some characteristics can be improved by changing the structure or functional modification. The fluorescence change of MOFs is usually caused by the synergistic effect of host–guest electron transfer and energy transfer. Since the lowest unoccupied molecular orbital (LUMO) of most analytes is usually lower than the LUMOs or conduction bands (CBs) of MOFs, electrons are easily transferred from the MOFs to the analyte. The energy transfer between the analyte and the MOFs is also critical for sensing, which usually depends on the degree of overlap between the absorption of the analyte and the emission of the sensor. Zr-MOFs containing carboxylic acid ligands usually have high stability in water because of their high-strength Zr-O bond, and have great application prospects in the field of water treatment. Among the stable Zr-MOFs, UiO-66, first reported by Lillerud et al. at the University of Oslo [24], constitutes a zirconium oxide complex connected with 1,4-benzenedicarboxylic acid (BDC, which is also commonly called terephthalic acid) linkers. The UiO-66 MOF exhibits excellent physical and chemical properties, such as superior stabilities in aqueous and acidic conditions, as well as excellent mechanical properties, and has thus been widely used in water environmental remediation [25,26,27,28]. Furthermore, the framework defects can enhance porosity and provide numerous coordinatively unsaturated (CUS) Zr⁴⁺ sites, which are advantageous for reacting with water pollutants [29].

Inspired by the above considerations, concentrated HCl was employed in the preparation of UiO-66 to construct a defective framework called UiO-66-D in this work. The obtained UiO-66-D retains water stability and exhibits strong blue fluorescence in aqueous solutions. In addition, UiO-66-D can be used for the sensitive and accurate detection of TC and MXF with fluorescence signal quenching or ratiometric responses, and the LOD was calculated as 70.9 nM in the linear range of 0–115 μM for TC, and 33.1 nM in the linear range of 0–24 μM for MXF, respectively. It indicates, on the basis of a mechanism analysis, that the fluorescence quenching with TC was caused by IFE, while the ratiometric response with MXF was the combined effect of the IFE and the fluorescence signal of MXF itself. Significantly, UiO-66-D enabled the selective detection of TC and MXF, and the detection of both TC and MXF was undisturbed with the existence of common anions and cations in water. Furthermore, UiO-66-D had been successfully used for the recognition of TC and MXF in lake water samples, indicating significant potential in the practical application of the recognition of water environmental pollutants.

2. Experimental

2.1. Materials and Methods

All chemical agents used in this study were directly obtained commercially and were not further purified. Zirconium(IV) chloride (ZrCl₄, 98%), terephthalic acid (C₈H₆O₄, 99%), moxifloxacin (C₂₁H₂₄FN₃O₄, 98%), methanol (CH₃OH, 99.8%), N,N-Dimethylformamide (DMF, C₃H₇NO, 99.8%), tetracycline (C₂₂H₂₄N₂O₈, 98%), moxifloxacin (MXF, C₂₁H₂₄FN₃O₄·HCl, 98%), sodium sulfite (Na₂SO₃, 99%), sodium bisulfate (NaHSO₄, 99%), magnesium chloride (MgCl₂, 99%), cupric chloride anhydrous (CuCl₂, 99%), nickel(II) chloride hexahydrate (NiCl₂, 98%), barium chloride dihydrate (BaCl₂·2H₂O, 99.5%), sodium phosphate (Na₃PO₄, 99%), and iron trichloride (FeCl₃, 97%) were purchased from Aladdin industrial Co., Ltd. (Beijing, China). Hydrochloric acid (HCl, 36%), sodium hydroxide (NaOH, 96%), sodium fluoride (NaF, 99%), sodium nitrite (NaNO₂, 99%), sodium hydrogen sulfite (NaHSO₃, 99%), sodium bicarbonate (NaHCO₃, 99.5%), sodium carbonate (Na₂CO₃, 99%), sodium nitrate (NaNO₃, 99%), sodium sulfate (Na₂SO₄, 99%), sodium thiosulfate (Na₂S₂O₃, 98.5%), sodium persulfate (Na₂S₂O₈, 98.5%), potassium chloride (KCl, 99.8%), calcium chloride (CaCl₂, 96%), Iron(II) chloride tetrahydrate (FeCl₂·4H₂O, 99%), cadmium chloride hydrate (CdCl₂·2.5H₂O, 99%), zinc chloride (ZnCl₂, 98%), and aluminum trichloride (AlCl₃, 98.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water (18.2 MΩ⋅cm) was prepared with a Milli-Q purification system.

The powder X-ray diffraction (XRD) patterns used to identify the crystalline phase were documented on a Bruker D8 X-ray diffractometer with graphite monochromatized CuKα radiation (λ = 0.15418 Å) at a 40 mA current and 40 kV voltage in the range of 2θ = 5–60^o. Fourier transform infrared (FTIR) spectra in the range of 4000–400 cm⁻¹, used for analyzing the functional groups, were recorded on a Nicolet iS50 spectrometer with KBr pellets. The surface area was obtained with the N₂ adsorption and desorption isotherms of samples tested on a Micromeritics ASAP 2460 at 77 K, adopting the Brunauer–Emmett–Teller (BET) method. The pore size distributions were obtained based on nonlocal density functional theory (NLDFT). The fluorescence performance was analyzed using a Thermo Scientific (Waltham, MA, USA) Lumina fluorescence spectrometer equipped with a 150 W Xenon lamp. The fluorescence decay curve was measured on a FS5 Fluorescence Spectrometer (Edinburgh, UK), and the fluorescence lifetimes were calculated based on the luminescent decay fitting. A HACH DR-6000 UV–visible spectrophotometer was used for recording the UV-vis absorption spectra.

2.2. Preparation of Defective UiO-66 (UiO-66-D)

UiO-66-D was synthesized with the hydrothermal method according to reported procedures [29]. First, 3.2 mmol of ZrCl₄, 10 mL of HCl, and 40 mL of DMF were added in a 250 mL flask. After 5 min of sonication with a water bath at room temperature, the ZrCl₄ was dissolved. Then, 3.2 mmol of terephthalic acid was added to the mixture solution, and DMF was added to adjust the total volume to 100 mL. The mixture was treated with ultrasound for 30 min, then equally added to four 25 mL hydrothermal synthesis reactors and heated at 120 °C for 24 h. After being cooled to room temperature, the precipitate was obtained through filtration, followed by three washes with DMF and methanol. Subsequently, it was dried under vacuum conditions at 60 °C for a duration of 6 h.

2.3. Fluorescence Detection of TC and MXF

To assess the feasibility of UiO-66-D for sensing antibiotics in water, the fluorescence stability in water and the reaction time were first investigated. In detail, UiO-66-D (2 mg) was dispersed in the deionized water (2.5 mL), and the emission intensity of UiO-66-D at room temperature was recorded at different times over 24 h under a 320 nm excitation wavelength. Moreover, the fluorescence stability of UiO-66-D in water with various pH values was tested by dispersing 2 mg of UiO-66-D in 2.5 mL of deionized water in a wide pH range from 2 to 10 at room temperature, and recording the emission intensity under excitation at 320 nm. The reaction time for sensing TC and MXF was, respectively, studied by adding UiO-66-D (2 mg) in 2.5 mL TC (115 μM) or MXF solutions (67 μM) at different times at room temperature, and recording the fluorescence emission intensity under 320 nm excitation; the intensity at 400 nm was recorded for the TC sensing and the ratiometric fluorescence of I₅₀₉/I₄₀₀ was calculated for MFX sensing. During the experiment, the mixed solution of UiO-66-D and TC/MXF was stirred at a constant rate to maintain its uniformity.

After optimizing the above experimental parameters, the sensing performance towards TC and MFX was investigated. In a general procedure, 2 mg of UiO-66-D was dispersed in 2.5 mL TC or MFX solutions with different concentrations, and the fluorescence intensity at 400 nm and the ratiometric fluorescence of I₅₀₉/I₄₀₀ were recorded, respectively, in real time at room temperature. Furthermore, the selectivity and anti-interference capability of UiO-66-D toward TC and MFX was further investigated at room temperature. Common ions in water, including metal ions (115 μM, K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, Cu²⁺, Ni²⁺, Cd²⁺, Ba²⁺) and anions (115 μM, F⁻, Cl⁻, NO₂⁻, S₂O₃²⁻, SO₃⁻, HSO₃⁻, HCO₃⁻, HSO₄⁻, CO₃²⁻, NO₃⁻, SO₄²⁻, PO₄³⁻, S₂O₈²⁻), were added to replace TC and MFX or co-exist with TC and MFX, and the same assay procedure was performed.

2.4. Recognition of TC and MFX in Environmental Water Samples

Lake water from Changzhou University (Changzhou, China) was collected as the environmental water sample to evaluate the practical application. Then, the environmental water sample was preliminarily treated with a 0.45 μm pore size membrane, filtered, and used to prepare TC and MFX solutions at various concentrations. The sensing procedure of TC and MFX in the real water sample was the same as performed above.

3. Results and Discussion

3.1. Characterization

The crystal structure of UiO-66-D was characterized based on its XRD patterns. As depicted in Figure 1a, diffraction peaks at 2θ = 7.36°, 8.50°, 12.02°, 14.12°, 17.06°, 22.22°, 25.72°, and 33.12° of UiO-66-D were, respectively, ascribed to the (111), (002), (022), (113), (004), (115), (224), and (137) reflections of UiO-66, which showed accordance with the XRD pattern of UiO-66 [24], indicating structurally similar to fcu-phase UiO-66. FTIR spectra were determined to analyze the chemical composition of UiO-66-D. The adsorption peaks observed in the FTIR spectra of UiO-66-D (Figure 1b) correspond well to those of the fcu-phase UiO-66. Specifically, the peaks at 1580 and 1400 cm⁻¹ can be attributed to the stretching vibrations of ν_as (-COO⁻) and ν_s (-COO⁻) groups in BDC²⁻, while the peaks in the range of 600–800 cm⁻¹ are associated with Zr-O modes [29]. Furthermore, UiO-66-D exhibits a higher specific surface area (1273.8 m²/g) and pore volume (0.58 cm³/g) compared to UiO-66, which has a specific surface area of 799.8 m²/g and pore volume of 0.30 cm³/g (Figure 1c), indicating that the addition of concentrated HCl does cause structural defects. However, the pore size distribution in Figure 1d indicates that the larger openings of UiO-66-D were about 17 Å, and the defects’ type was the reo type (∼15 Å cavities), which may be randomly oriented rather than ordered and cannot be detected in XRD patterns [30].

3.2. Optical Properties of UiO-66-D

The fluorescence spectra were conducted at room temperature to evaluate the optical properties of UiO-66-D. As in Figure 2a, UiO-66-D showed emission spectra with an emission peak at 400 nm, and showed excitation spectra with an excitation peak obtained at 320 nm. Moreover, UiO-66-D showed a white color under natural light, and exhibited a bright blue color based on the π-π* transition of H₂DBC under UV light (Figure S1). Accordingly, as shown in the CIE chromaticity diagram (Figure 2b), the CIE coordinates of UiO-66-D are located at the point of a (X = 0.16, Y = 0.06) in the blue region.

The optical stability of UiO-66-D was further studied to evaluate the feasibility of UiO-66-D as a fluorescence senor for analyte monitoring in water environments. As described in Figure S2, the fluorescence emission intensity at 400 nm of UiO-66-D was not affected by the immersion time being prolonged to 24 h. Moreover, the fluorescence emission intensity at 400 nm of UiO-66-D remained almost unchanged in the pH range of 2~10 (Figure S3). The above results show that the fluorescence properties of UiO-66-D remain stable in water for a long time, and are not affected by pH changes, which verifies the feasibility of it being used as a water pollutant sensor.

3.3. Fluorescence Response of UiO-66-D towards TC and MXF

First, the response kinetics were explored based on the test of the emission intensity of UiO-66-D for TC and MXF sensing over different lengths of time. As shown in Figures S4 and S5, the fluorescence responses for both TC and MXF sensing occurred rapidly within 10 s and remained stable over 10 min; thus, all sensing tests for both TC and MXF with UiO-66-D in this study were performed in real time.

Furthermore, the fluorescence detection performance of UiO-66-D towards TC and MXF with various concentrations was studied. As shown in Figure 3a, with the increased addition of TC from 0 to 115 μM, the emission intensity at 400 nm under 320 nm excitation was decreased, and the quenching efficiency reached 87%, which is higher than that of UiO-66 (Table S1). Moreover, the color of the emission under UV light changed from bright blue to dark blue (Figure 3a inset). The relationship between the emission intensity at 400 nm and the TC concentration at 0–115 μM had a good, linear Stern–Volmer (S-V) relationship [31] (Equation (1)) (Figure 3b). The K_sv value of TC was 3.093 × 10⁴ M⁻¹ and the correlation coefficient (R²) was 0.994.

I₀/I = K_sv[M] + 1,(1)

where I₀ is the emission intensity of UiO-66-D; I is the emission intensity of the mixed system of UiO-66-D and TC; K_sv is the S-V constant (M⁻¹); and [M] represents the molar concentration of TC (μM).

Different from the detection of TC, with the addition of MXF, another emission peak at 509 nm appeared, and two emission peaks showed a reverse fluorescence effect, in which one weakened and another was enhanced (Figure 3c). When the concentration of MXF reached 67 μM, the emission intensity of UiO-66-D at 400 nm was reduced by 86.6%, which is higher than that of UiO-66 (Table S1), but the intensity of another emission peak at 509 nm was enhanced by 96.3%. Accordingly, the color of the emission under UV light changed from bright blue to bright green (Figure 3c inset). Moreover, the changes in the relative peak intensity (I₅₀₉/I₄₀₀) of UiO-66-D for sensing MXF are shown in Figure 3d, where no linear correlation between I₅₀₉/I₄₀₀ and the wide concentration range of MXF from 0 to 67 μM was found. However, it exhibited a strong linear correlation within the concentration range of 0 to 24 μM, with an R² value of 0.996 and a K_sv value of 6.64 × 10⁴ M⁻¹.

The LODs for TC and MXF were determined as 70.9 nM and 33.1 nM, respectively, based on the IUPAC criteria (3σ/slope), where σ represents the standard deviation of three blank measurements [32]. As shown in Table 1, the LOD for TC and MXF sensing in this work is better or comparable than most of other detection platforms.

Selectivity and anti-interference are crucial factors in assessing the applicability of sensors. Therefore, we investigated the selectivity and anti-interference capabilities of UiO-66-D towards TC and MXF sensing. As shown in Figure 4a,b, compared to the fluorescence signals obtained in the existence of other common ions in water, there was a decrease or change in emission intensity only when TC or MXF was present, indicating high selectivity towards TC and MXF sensing by UiO-66-D. Furthermore, when TC or MXF co-existed with other common ions in water, changes in fluorescence were consistent with those observed when only TC or MXF were present (Figure 4c–f), confirming that common anions and cations did not interfere with the detection process for TC or MXF.

Recyclability is another key factor in evaluating the performance of sensors in practical applications. Therefore, five-cycle experiments were taken to evaluate the recyclability of UiO-66-D in TC and MXF sensing. As described in Figure 5a,b, both the initial emission intensity and quenching percentage for TC sensing or the fluorescence ratio for the MXF sensing of UiO-66-D did not change significantly after five cycles, demonstrating good sensing performance stability and good reuse performance.

3.4. Supposed Mechanism of UiO-66-D for TC and MXF Sensing

The response mechanism of UiO-66-D for TC and MXF sensing was further studied by testing and analyzing the structure of the materials before and after detection, as well as the UV-vis absorption spectrum, fluorescence spectrum, and fluorescence lifetime. As seen in Figure 1a, UiO-66-D retained its original crystalline structure after the detection of TC and MXF, thereby indicating that the alteration in the fluorescent signal was not attributed to structural damage. The fluorescence lifetime of UiO-66-D before and after the detection of TC and MXF in aqueous solution was further studied. As shown in Figure 6a, the average fluorescence lifetime of UiO-66-D in water was measured as 6.91 ns. Subsequent to the detection of TC and MXF, the average fluorescence lifetime was 7.29 ns and 7.01 ns, respectively, exhibiting a difference within one order of magnitude from pre-detection values, rendering this change. During the sensing process, if there is an occurrence of fluorescence resonance energy transfer (FRET), then significant reduction would be observed in the fluorescence lifetime post-detection. However, it can be inferred from this experiment that there is almost no change between the pre- and post-detection fluorescence lifetimes, suggesting that the quenching process is static, with FRET not being the primary cause for the modulation of the fluorescence signal; instead, an inner filter effect (IFE) may occur. Furthermore, additional testing revealed non-overlapping UV-vis absorption spectra for both TC and MXF with emission spectra of UiO-66-D, but overlapping excitation spectra (Figure 6b), confirming the occurrence of IFEs for fluorescence quenching by UiO-66-D. Moreover, regarding MXF sensing specifically, it should be noted that the peak at 509 nm originates from MXF itself (Figure S6), wherein the increase in intensity corresponds to elevated concentration levels of MXF.

3.5. Sensing TC and MXF in Lake Water

The feasibility assessment involved verification through a standard addition method for determining concentrations of TC and MXF present within real water samples, thus validating the practical application potentiality associated with this fluorescent sensor technology utilizing the UiO-66-D compound. Different volumes (20, 40, and 60 μL) of TC (400 μM) and MXF (400 μM) were added to lake water samples, and recoveries of UiO-66-D for the detection of TC and MXF were obtained based on Equation (2).

Recovery = C_d/C_s × 100%(2)

where C_d (mg·L⁻¹) is the calculated concentration of TC or MXF based on the fluorescence detection of UiO-66-D, and C_s (mg·L⁻¹) is the concentration of TC or MXF added to the lake water samples.

As illustrated in Table 2, with the addition of various concentrations of TC or MXF to the lake water, the recoveries were calculated as 95.67~99.8% for TC and 100.3–101.8% for MXF, and the relative standard deviation (RSD) was less than 1.6% (n = 3). The results showed that UiO-66-D can accurately detect antibiotics in actual water samples, and has great practical application potential.

4. Conclusions

In conclusion, we have developed a multi-functional fluorescence sensing platform based on a defective UiO-66 (UiO-66-D) with stable blue fluorescence in response to the need of the current demand for an efficient and accurate detection of antibiotics in water environments. UiO-66-D can be used for TC sensing with fluorescence quenching and for MXF sensing with two fluorescence signals altered in ratios. UiO-66-D exhibited sensitivity and selectivity to detect TC and MXF, with LODs of 70.9 nM (0–115 μM) and 33.1 nM (0–24 μM), respectively. The fluorescence signal of UiO-66-D was greatly changed by TC owing to the IFE, and the ratiometric signal response for MXF sensing was generated by the combined effect of the IFE and the fluorescence signal of MXF itself. In addition, the potential practical application of UiO-66-D for pollutant recognition in environmental water was further verified by detecting TC and MXF in lake water, with recoveries of 95.67–99.8% for TC and 100.3%–101.8% for MXF. In all, this work can provide a new perspective for the construction of water pollutant sensing methods, provide a good reference for the workers in this field, and have great application prospects in the field of environmental governance.

Author Contributions

Conceptualization, Y.Z. and Y.L.; methodology, Y.Z. and Y.L.; software, M.S. and D.Z.; validation, Y.L.; formal analysis, M.S. and D.Z.; investigation, Y.L.; resources, Y.Z.; data curation, Y.L.; writing—original draft preparation, Y.Z.; writing—review and editing, Y.Z. and Y.L.; visualization, Y.L.; supervision, M.S. and D.Z.; project administration, Y.Z.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data will be made available on request.

Acknowledgments

We thank Changzhou University for their support in the field of experimental instruments.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. (a) XRD patterns of UiO-66, UiO-66-D, and UiO-66-D after detection of TC and MXF; (b) FTIR spectra of UiO-66 and UiO-66-D; (c) nitrogen adsorption and desorption isotherms of UiO-66 and UiO-66-D; (d) pore size distributions of UiO-66-and UiO-66-D.

Figure 2. (a) The excitation (black line) and emission (red line) spectra of UiO-66-D; (b) the chromaticity diagram of UiO-66-D excited at 320 nm.

Figure 3. (a) The fluorescent spectra of the mixed solution of UiO-66-D and different concentrations of TC under excitation at 320 nm; (b) the S-V plot of UiO-66-D toward TC. (c) The fluorescent spectra of the mixed solution of UiO-66-D and different concentrations of MXF under excitation at 320 nm; (d) the relationship between the I509/I400 value and the concentration of MXF (inset: the linear relationship between the I509/I400 value of UiO-66-D and the concentration of MXF).

Figure 4. Relative intensities of UiO-66-D upon the addition of (a) metal ions (115 μM) and (b) anions (115 μM) in aqueous solutions; competitive binding results of TC co-existing with metal ions (115 μM) (c) and anions (115 μM) (d) in aqueous solutions; competitive binding results of MXF co-existing with (e) metal ions (115 μM) and (f) anions (115 μM) in aqueous solutions.

Figure 5. Recovery tests of UiO-66-D for TC (a) and MXF (b) sensing.

Figure 6. (a) The fluorescence decay curve of UiO-66-D (black line) and the mixed solution with UiO-66-D and TC (red line) or MXF (blue line); (b) the UV-vis adsorption spectra of TC (purple line) and MXF (blue line), the excitation spectra (black line, Em = 400 nm), and emission spectra (red line, Ex = 320 nm) of UiO-66-D.

Table 1

Comparison of the sensing performance for sensing TC and MXF.

Material	Analytes	LOD (nM)	Linear Range (μM)	Ref.
NH2-MIL-53(Al)	TC	920	1.5–70	[33]
BSA-AuNCs		65	0.2–10	[34]
N,S-doped carbon nanodots		160	0.8–10	[35]
Nitrogen-doped durian shell-derived carbon dots		75	0–30	[36]
UiO-66-D		70.9	0–115	This work
Carbon quantum dots	MXF	2.59	0.33–2	[37]
[Co(apba)2(H2O)2]		43	0–100	[38]
UiO-66-D		33.1	0–24	This work

Table 2

Detection of TC and MXF in lake water.

Antibiotics	SpikedConcentration (μM)	Measured (μM)	Recovery (%)	RSD (%)
TC	3.175	3.114	98.1	1.6
	6.299	6.024	95.6	0.6
	9.375	9.053	96.6	0.9
MXF	3.175	3.231	101.8	1.1
	6.299	6.488	103.0	1.5
	9.375	9.407	100.3	0.6

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w16010145/s1: Figure S1: Optical photographs of UiO-66-D under natural and UV light (365 nm); Figure S2: The fluorescence intensity of UiO-66-D at 400 nm with different immersion times; Figure S3: The fluorescence intensity of UiO-66-D at 400 nm with different pH values; Figure S4: Fluorescence intensity of the mixed solution of UiO-66-D and TC at 400 nm (E_x = 320 nm) at different times; Figure S5: The ratio of fluorescence intensity (I₅₀₉/I₄₀₀) of the mixed solution of UiO-66-D and MXF at 400 nm (E_x = 320 nm) at different times; Figure S6: The fluorescence spectra of MXF at 320 nm excitation. Table S1. The fluorescence sensing performance comparison of UiO-66-D and UiO-66.

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