Photoluminescent Sensor for Fe³⁺ Based on Calix[4]arene-Derivative-Modified UiO-66-NH₂

Abstract

Iron is an important ion that regulates many metabolic functions in the human body, and its deficiency or overload can cause serious health problems. In view of this, the development of tools that can quantify iron in aqueous media is quite valuable. A new material based on a metal-organic framework (MOF) and a calixarene derivative (CALIX), here named CALIX@UiO-66-NH₂, was successfully prepared and used as a luminescent sensor for iron ions in a water matrix containing various metal ions. The post-synthetic modification was demonstrated to be efficient on the MOF (UiO-66-NH₂) modification, leading to a hybrid platform. The effect of the combination of both components (MOF and CALIX) led to a platform more sensitive to iron detection than the independent compounds. The CALIX@UiO-66-NH₂ sensor shows a linear response toward Fe³⁺ ions with satisfactory linearity (R² = 0.994) within the concentration range of 6.74 × 10⁻⁶−1.72 × 10⁻⁵ mol L⁻¹. The limit of detection was determined to be as low as 1.6 mg L⁻¹. The data demonstrate that CALIX@UiO-66-NH₂ is an excellent probe for detecting Fe³⁺, as confirmed by almost complete fluorescence suppression.

Keywords:
Fe³⁺ detection; calixarene; metal-organic framework; post-synthetic modification; fluorescent sensor

Introduction

The amount of trace metals in humans plays a vital role due to their relation with human metabolism, and either deficiency or excess in a living organism can lead to biological system disorder.¹ Iron is an essential trace element for the human body and is crucial for many biological processes like deoxiribonucleic acid (DNA) synthesis, oxygen, and electron transport.², ³ It is well known that anemia occurs due to iron deficiency, and on the other side, high iron levels can lead to several health problems.⁴ Such diseases as endocrine problems, heart disease, diabetes, and other illnesses have the risk increased by the high levels of iron.⁵ Some international organizations, such as the World Health Organization (WHO), have established a sanitary security limit for iron in drinking water of 2.0 mg L⁻¹ (3.57 × 10⁻⁵ mol L⁻¹).⁶ Thus, developing rapid, low-cost, efficient, and simple methods for monitoring and detecting iron in the environment is very important. The most used method for determining metal ions is atomic absorption inductively coupled plasma optical emission spectrometry (AAS).⁴ However, this method requires a high-cost instrument, and its sensitivity for low concentration is insufficient for complex matrices requiring pre-concentration steps.

Metal-organic frameworks (MOFs), a class of porous materials based on a multidentate organic ligand and a metal unit, have recently been used for environment remediations.⁷, ⁸ The properties of MOFs, such as high surface area and adjustable pore size,⁹ enable their potential applications in many fields like catalysis,⁷ gas storage,¹⁰ drug delivery,¹¹ forensics,¹² environment recovery,¹³, ¹⁴ and fluorescent sensing.¹⁵ Among many studies involving this class of materials, the post-synthetic modification (PSM) of MOFs has become an attractive approach for developing new materials.¹⁶ One of these exciting and novel post-synthesis approaches is the decoration of MOFs with calixarene molecules.

Calixarenes are a class of macrocyclic compounds with an intrinsic cavity that enables the formation of an inclusion complex through its cone-shaped structure.¹⁷ The size of the calixarene is controlled by the number of phenolic units in its structure. The most well-known and stable calixarene is the p-terc-butylcalix[4]arene with four phenolic units. At ambient temperature, the p-terc-butylcalix[4]rene can adopt four conformational isomers, named 1,2-alternate, 1,3-alternate, partial cone, and cone. The difference between those isomers lies in the phenolic ring position.¹⁸ Chemically stable calixarenes can be easily modified to enrich their host-guest properties useful in many fields like adsorption of small molecules,¹⁹ complex chemistry,²⁰ catalysis,²¹ cations adsorption, sensing,²² and others. The upper and lower borders of calixarene can be modified with different types of functional groups, which opens the possibility of linking various materials to calixarene derivatives (CALIXs).²³ However, studies on the synthesis, characterization, and properties of MOFs decorated with CALIX (or any other macrocyclic compounds) are still rare. Du et al.²⁴ developed a fluorescent probe for hippuric acid in aqueous media. The probe was based on a calixarene derivative with carboxylic groups decorating MOF. The material showed a limit of detection of 3.7 μg mL⁻¹ over a concentration range of 0.005-3 mg mL⁻¹. Our research group has recently developed a coordination polymer based on CALIX and lanthanide ions, showing a great luminescent response to Fe³⁺ in aqueous media.²⁵ Thus, materials derived from calixarene have demonstrated potential as fluorescent probes for many analytes.

The efficacy of guest coordination is determined by the type and number of binding sites, with the calix[4] arene framework playing a critical role. The phenyl rings of the calix[4]arene structure enable interactions with cations through π-bonding.²⁶ Coordination polymers frequently display luminescent characteristics that can be controlled by the presence of certain metal ions. Variations in luminescence intensity, emission wavelength, and lifetime can be used to identify metal ions.²⁷ Another process by which coordination polymers interact with metal ions is fluorescence resonance energy transfer (FRET), which alters their emission characteristics and allows them to be used in sensing applications.²⁸ Furthermore, coordination polymers can create host-guest complexes with metal ions and molecules, which modify their spectroscopic properties. Moreover, MOFs and calixarenes are significant components in ion sensing due to their selective binding characteristics, luminescent behavior, and chemical structural diversity, enabling specialized sensing applications. Their capacity to form host-guest complexes with metal ions and perform fluorescence resonance energy transfer (FRET) makes them valuable instruments for detecting and measuring potentially dangerous ions in complicated settings.²⁷, ²⁹

This work aims to synthesize a fluorescent probe based on CALIX with four carboxylic acid groups and MOF, UiO-66-NH₂. CALIX was attached to MOF through an amide bond between the carboxylic acid group from CALIX and the amine group from MOF. The platform was tested as a fluorescent sensor for detecting Fe³⁺ in aqueous media.

Experimental

Materials

4-tert-Butylphenol (99%, C₁₀H₁₄O), 2-aminoterephthalic acid (99%, H₂NC₆H₃-1,4-(COOH)₂), zirconium(IV) chloride (99%, ZrCl₄), N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC) (≥ 99.0%, C₈H₁₇N₃.HCl), L-lysine (98%, H₂N(CH₂)₄CH(NH₂)CO₂H), melamine (99%, C₃H₆N₆), cobalt(II) nitrate hexahydrate (98%, Co(NO₃)₂.6H₂O), iron(II) nitrate nonahydrate (Fe(NO₃)₂.9H₂O), N-hydroxysulfosuccinime sodium salt (sulfo-NHS) (≥ 98%, C₄H₄NNaO₆S) and formaldehyde solution ( 37% m/v H₂O, CH₂O) were purchased from Sigma-Aldrich (São Paulo, Brazil). Hydrochloric acid (≥ 99.0%, HCl), ethyl bromoacetate (≥ 99.0%, C₄H₇O₂Br), potassium carbonate (≥ 99.0%, K₂CO₃), phenol (≥ 99.0%, C₂H₅OH), sodium hydroxide (≥ 99.0%, NaOH), potassium iodide (≥ 99.0%, KI), N,N-dimethylformamide-DMF (≥ 99.0%, C₃H₇NO), acetone (≥ 99.0%, C₃H₆O), chloroform (≥ 99.0%, CHCl₃), methanol (≥ 99.0%, CH3OH), hexane (≥ 99.0%, C₆H₁₄), ethyl acetate (≥ 99.0%, C₄H₈O₂) and copper(II) nitrate trihydrate (≥ 99.0%, Cu(NO₃)₂.H₂O) were purchased from Dinâmica (Indaiatuba, Brazil). Aluminum(III) nitrate nonahydrate (≥ 99.0%, Al(NO₃)₃.9H₂O), calcium(II) nitrate tetrahydrate (≥ 99.0%, Ca(NO₃)₂.4H₂O), magnesium(II) sulfate heptahydrate (≥ 99.0%, MgSO₄.7H₂O), and sodium nitrate (NaNO₃) were purchased from Vetec (Duque de Caxias, Brazil). The magnesium sulfate was kept at 150 °C for 3 h, while the other chemicals were used as received without further purifications.

Synthesis

Synthesis of the tetra-acid calixarene derivative

The tetra-acid calixarene derivative was synthesized in three steps. The first step was the synthesis of p-terc-butyl-calix[4]arene (1) according to the procedure described by Gutsche et al.³⁰ Tetraethylacetyl-p-tert-butyl-calix[4] arene (2) and tetra-acid-p-tert-butyl-calix[4]arene (3) were synthesized according to the method described by Arora et al.²³ and Arnaud-Neu et al.³¹ with modifications. The synthesis scheme of the tetra-acid derivative is represented in Figure 1 (full procedures of the calixarene compound syntheses can be found in the ^{Supplementary Information} (SI) section). Nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared spectroscopy (FTIR) spectra of the mentioned calixarenes can be found in ^SI section (Figures S1-S6).

Figure 1
Synthesis scheme of the tetra-acid calixarene derivative: (1) p-tert-butylcalix[4]arene, (2) tetra-ester-calixarene and (3) tetra-acid-calixarene.

Synthesis of UiO-66-NH₂

MOF UiO-66-NH₂ was synthesized according to the procedure described by Xu et al.³² In a beaker, zirconium(IV) chloride (149 mg, 0.643 mmol) and DMF (36.6 mL, 475.5 mmol) were added; the solution was stirred for 5 min. Then, 2-aminoterephthalic acid (114 mg, 0.643 mmol) was added, and the solution was sonicated for 10 min using a high-intensity ultrasonic processor at 50% of the total amplitude, used for all carried out experiments (500 W, 20 kHz, model VC505, Sonics & Materials Inc., USA). The obtained solution was transferred to a Teflon reactor (120 mL), placed in an autoclave, and kept at 120 °C for 24 h. After this time, the reactor was cooled to room temperature. The obtained solid was recovered by centrifugation (3 min, 5000 rpm) and washed with DMF (3 ×, 15 mL) and ethanol (3 ×, 15 mL). The powder was immersed in methanol (50 mL) under stirring for 24 h to remove unreacted reagents. Subsequently, the product was vacuum-dried at 80 °C for 24 h to remove the remaining methanol. A light-yellow powder with a 49% yield was obtained at the end of the process.

Synthesis of CALIX@UiO-66-NH₂

Tetra-acid-calixarene (50 mg) and water (50 mL) were added to a beaker, and the mixture was sonicated for 3 h (50% of the maximum amplitude) with a cooler water system at the temperature of 17 °C. Then, EDC (50 mg) and sulfo-NHS (20 mg) were added, and the suspension was stirred (15 min) to activate the carboxylic groups of calixarene. Soon after, 20 mg of UiO-66-NH₂ were added to the mixture to form a suspension, kept under stirring for 72 h. After filtration, the solid was washed with water (15 mL), and then, stirred (12 h) in methanol (50 mL). The solid was recovered by filtration and dried (60 °C) in a vacuum oven.

Photoluminescence experiments

The procedure adopted to prepare the samples was described by Du et al.²⁴ The experiments were carried out using suspensions prepared with 3 mg of powder samples (CALIX@UiO-66-NH2, UiO-66-NH2, or CALIX) dispersed in 10 mL of distilled water. Each suspension was previously sonicated (50% amplitude) for 15 min while the temperature was kept at 17 °C by a cooling system. After that, the temperature was increased to 26 °C and held for 24 h. Finally, the supernatant was recovered for photoluminescence analysis.

In a typical procedure, 2 mL of the suspension and 1 mL of the solution containing the analyte (1 mg mL⁻¹) were added to a cuvette. The resulting suspension was mixed for 10 s, and then, analyzed by photoluminescence spectroscopy. The excitation and emission spectra were acquired at room temperature using a Jobin Yvon Fluorolog-3 spectrofluorometer with a 450 W Xenon continuous lamp and a 150 W Xenon flash tube for excitation and a slit aperture of 3 nm (Horiba, Japan).

The design of the cation sensing procedure is based on the work of Ma et al.³³ Aqueous solutions of metal nitrates were prepared and used in the study of the response of the produced materials to the metal cations (Cr³⁺, Zn²⁺, Ni²⁺, Ca²⁺, Co²⁺, Cu²⁺, Pb²⁺, Na⁺, Cd²⁺, and Mn²⁺). In a typical experiment, 1 mL of aqueous nitrate solution (1 mg mL⁻¹) and 2 mL of a CALIX@UiO-66-NH₂ aqueous suspension (0.3 mg mL⁻¹) were homogenized adequately in a quartz cuvette, and then, the emission spectrum acquired at room temperature (λ_ex = 335 nm). Based on the obtained results, Fe³⁺ cation was selected for subsequent investigation on different concentrations. Thus, 2 mL of a CALIX@UiO-66-NH₂ aqueous suspension (0.3 mg mL⁻¹) were transferred to a quartz cuvette. Then, 0.100 mL of a Fe³⁺ solution (0.0125 mg mL¹) was added, followed by proper homogenization and acquisition of the emission spectrum at room temperature (λ_ex = 335 nm). After that, another 0.100 mL of the same Fe³⁺ solution was added, followed by homogenization and acquisition of the emission spectrum. This procedure was repeated nine times.

To study the interaction of the platform with Fe³⁺ ions in the presence of other metal ions (Cr³⁺, Zn²⁺, Ni²⁺, Ca²⁺, Co²⁺, Cu²⁺, Pb²⁺, Na⁺, Mn²⁺), 1.9 mL of the suspension containing the platform (0.3 mg mL⁻¹) were added to 0.100 mL of the solution (1 mg mL⁻¹) containing the interfering metal. Then, after stirring for 10 s, 0.300 mL of a solution containing Fe³⁺ ions (0.0125 mg mL⁻¹) and 0.700 mL of distilled water were added, and the emission spectra were recorded.

Stability test

To investigate the stability of CALIX, UiO-66-NH₂, and CALIX@UiO-66-NH₂ in high iron(III) concentration, 10 mg of a sample and 20 mg of iron nitrate nonahydrate were added to 10 mL of methanol and kept under stirring for 30 min at ambient temperature. The sample was recovered by centrifugation, dried at 60 °C for 24 h, and then analyzed by Fourier transform infrared microspectroscopy (μ-FTIR).

Material characterization

Single-point μ-FTIR transmittance measurements in reflection geometry were carried out using an FTIR spectromicroscope (Cary 620, Agilent Technologies, Santa Clara, USA). The choice for reflectance mode FTIR was based on the sensibility of this technique to this type of sample. The samples were deposited on an Au/Si substrate to improve the signal on the detector. As conventionally practiced in FTIR analysis, the measurements were produced by interferometry from a Michelson interferometer using a Globar source as the illumination source and a mercury-cadmium-telluride (MCT) detector. In this setup, the infrared (IR) light passes through the interferometer and then is directed to the microscope, which focuses on the sample surface using objective lens. A 25× objective lens were used producing a ca. 420 × 420 μm²⁺ spot size on the sample surface. The objective lens collected the reflected light in a confocal arrangement and sent it to the MCT detector. Then, FTIR spectra were obtained by computing the Fourier transforms of the acquired interferograms. In our measurements, the spectral resolution was 8 cm⁻¹, and the covered spectral range spanned from 4000-400 cm⁻¹. All presented μ-FTIR absorbance spectra corresponded to the average of 256 single spectra. All spectra were normalized to a clean gold surface spectrum taken as the reference background.

The absorption spectra in the IR region of the stability test samples were acquired in a Spectrum 400 equipment with an attenuated total reflectance (ATR) module and ZnSe crystal (PerkinElmer, USA). The range used to acquire the spectra was 4000-500 cm⁻¹. Thermogravimetric analysis was performed using a thermogravimetric analyzer model TGA2 (Mettler Toledo, Switzerland). The analyzes were performed in a temperature range between 30 and 700 °C using a heating rate of 5 °C min⁻¹ under an inert nitrogen atmosphere at a flow rate of 50 mL min⁻¹. The diffractograms were acquired on an X-ray diffraction 7000 Diffractometer, using Cu Kα radiation (λ 1.5406 Å) with a nickel filter, steps of 0.02º, current of 30 mA, and voltage of 30 kV, sweep speed of 2 degree min¹ in the range from 5°-80° (Shimadzu, Kyoto, Japan). The identification of crystalline phases was performed using the Cambridge Structural Database (CSD) and Crystallography Open Database (COD) databases. The crystallite size was calculated using the Scherrer equation.³⁴ To acquire the ¹H and ¹³C NMR spectra, a UNMRS/400 MHz nuclear magnetic resonance spectrometer was used (Varian, USA). The micrographs were acquired on a scanning electron microscope (SEM) model Mira 3 (Tescan, Czech Republic).

Results and Discussion

Fourier-transform infrared micro spectroscopy (μ-FTIR)

The infrared spectra of UiO-66-NH2, CALIX, and CALIX@UiO-66-NH₂ are shown in Figure 2. In the FTIR spectrum of calixarene derivative, the presence of the band at 3531 cm⁻¹, assigned to the stretching vibration of the O-H bond, suggests that the ethyl groups (calixarene-tetra-ester) were replaced by hydrogens, giving rise to the carboxylic acid groups. It is also possible to observe the stretching of the C=O and C–O bonds at 1734 cm¹ and 1184 cm⁻¹, respectively, which are characteristic of the carboxylic acid groups. The C–H methyl groups are evidenced by the presence of the band at 2958 cm⁻¹.

Figure 2
μ-FTIR spectra of UiO-66-NH₂, CALIX, and CALIX@UiO-66-NH₂.

The formation of MOF, UiO-66-NH2, can be suggested by the appearance of characteristic FTIR bands, such as those present at 1563 and 1388 cm⁻¹, corresponding to the coordination modes of COO– groups, indicating the coordination of the carboxylate group to the metallic center. The low-wavenumber region of the infrared spectrum contains two characteristic bands of the amine group at 1256 and 767 cm⁻¹, associated with the elongation of the C–N bond and the scissor-like vibration mode of the –NH₂ group, respectively. The results obtained agree with those reported in the literature.³⁵, ³⁶

In the spectrum of the CALIX@UiO-66-NH₂ platform, the C-H alkyl stretching vibration at 2958 cm⁻¹ still exists, indicating the presence of the CALIX molecule in the structure of the platform. As expected, the band at 1734 cm⁻¹, corresponding to the C–O (COOH) groups of CALIX disappears; instead, new bands appear at 1682, 1650, 1621, and 1542 cm⁻¹, indicating the formation of the amide and hydrogen bonds between –COOH from CALIX and the –NH₂ or –NH₃⁺ groups from UiO-66-NH₂ (see inset in Figure 2).³⁷ The stretching vibration of the N–H bond is observed in the platform spectrum at 761 cm⁻¹ with a slight decrease in relative intensity, suggesting that not all amine groups were linked to the COOH groups of the CALIX. These results agree with those published by Du et al.²⁴

X-ray powder diffraction (XRPD)

The XRPD patterns for the three prepared samples are shown in Figure 3. The diffractogram of UiO-66-NH₂ is in good agreement with the calculated diffraction pattern of MOF [C₂₈H₂₈O₃₂Zr₆]n (CCDC file No. 733458).³⁸ Furthermore, the average crystallite size for this sample was 87 nm, suggesting a nanocrystalline characteristic. The diffraction pattern of CALIX@UiO-66-NH2 sample comprises characteristic diffraction lines belonging to MOF, at the range of 5-10° (2θ) and around 25º (2θ), indicated with the vertical dashed line. In the proposed system, the calixarene bound to t MOF does not necessarily have an ordered and repetitive distribution and may only be attached to the surface of the particles. Therefore, it is not necessarily expected to visualize the diffraction peaks attributed to calixarene in the hybrid material. However, it is worth noting that the most intense peaks, observed above 30° (2θ), are not assigned to any of the starting materials (UiO-66-NH₂ and CALIX), suggesting the crystallization of a new phase, which was not identified despite our efforts.

Figure 3
Experimental XRD patterns of CALIX, CALIX@UiO-66-NH₂, and MOF-UiO-66-NH₂ compared to the simulated pattern for UiO-66.

Thermogravimetric analysis

The thermogravimetric (TG) curve of the samples CALIX, UiO-66-NH2, and CALIX@UiO-66-NH2 are shown in Figure 4. CALIX firstly presented a mass loss of 5% in the range from 30 to 110 °C, attributed to the solvent desorption from the surface of the particles and sequentially an unusual mass increase from 115 to 250 °C. Above 250 °C, the sample showed a progressive mass loss of 95% until the total decomposition at approximately 740 °C. The TG curve of the MOF UiO-66-NH₂ presents the first mass loss of 25% in the range of 30-100 °C attributed to the desorption of solvent molecules present in the material surface, such as DMF, ethanol, or methanol. The second mass loss in the range of 100-400 °C is due to the desorption of a coordinated solvent and adsorbed in structure pores, corresponding to 15% of the mass loss. The third mass loss is associated with the collapse of the structure and the decomposition of the organic part of the structure, observed from 400 to 700 °C, corresponding to 25%. The remaining mass comes from zirconium oxide formed from the MOF decomposition. The platform TG curve exhibits a mass loss of 10% from 30 to 100 °C, attributed to the solvent desorption from the surface and a mass increase in the same region observed for the CALIX, suggesting the presence of calixarene in the platform. The third mass loss of 60% observed in the range of 230-600 °C is associated with the collapse of the structure and decomposition of organic matter. However, there is an unexpected increase in the mass of the material in the range of 660-830 °C.

Figure 4
TG curves of CALIX, UiO-66-NH₂, and CALIX@UiO-66-NH₂.

Scanning electron microscopy (SEM)

The SEM micrographs of the obtained powders are shown in Figure 5. The CALIX sample presents plate-like particles with irregular sizes as shown in Figure 5a. The UiO-66-NH₂ sample has a microstructure characterized by submicrometric octahedron-like particles forming soft agglomerates (Figure 5b). A mean particle size histogram is shown in Figure 6. The average particle size is around 120 nm, which agrees with results reported in the literature (50-150 nm).³²

Figure 5
SEM micrographs of (a) CALIX, (b) MOF, and (c) CALIX@UiO-NH₂, in two different magnifications.

Figure 6
Histogram of the MOF particle size distribution (based on SEM).

The micrograph of CALIX@UiO-66-NH₂ is somehow similar to that of pure MOF, which consists of agglomerates of near-octahedron particles with different sizes (Figure 5c), suggesting the presence of MOF in the sample. Also, bigger agglomerates of the plate-like rounded plates can be visible, which possibly may correspond to the calixarene phase. After the platform synthesis, the material was washed with chloroform to remove any unreacted calixarene molecules, which excludes the possibility of the physical mixture of both components (pure CALIX and pure MOF).

Photoluminescent properties

The excitation and emission spectra of the synthesized materials, CALIX, UiO-66-NH_2, and the CALIX@UiO-66-NH2 were obtained at room temperature from the aqueous suspensions of these materials and are shown in Figure 7. The suspensions were prepared by dispersion of the material using ultrasound irradiation, as already mentioned. For each material, 3.0 mg were suspended in 10 mL of distilled water, followed by sonication. The CALIX excitation spectrum was obtained in the range of 250-410 nm by monitoring the emission at 430 nm. The spectrum presents a wide band with a maximum at 340 nm (Figure 7a).

Figure 7
Excitation and emission spectra of (a) CALIX, (b) UiO-66-NH₂, CALIX@UiO-66-NH₂, and (d) the comparison of the maximum emission intensity.

The excitation spectrum of UiO-66-NH₂ was obtained in the range of 250-430 nm by monitoring the emission band at 455 nm. The excitation and emission bands are centered at 395 and 455 nm, respectively (Figure 7b), agreeing with the literature data.³² The excitation and emission spectra of CALIX@UiO-66-NH₂ have a similar profile to the calixarene derivative, although a slight blue-shift is observed in both cases (see Figure 7c). Notably, the emission intensity of CALIX@UiO-66-NH₂ is nearly ten times higher than the fluorescence intensity of the precursor materials (Figure 7d) despite the lowest concentration used (0.14, 0.17 and 0.25 mg mL¹, for CALIX@UiO-66-NH₂, UiO-66-NH2, and CALIX, respectively).

Photoluminescent sensing of metal ions

The potential for sensing metal cations was explored using aqueous suspensions of CALIX@UiO-66-NH₂ in the presence of different metal salts. In this study, transition metal cations (Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Mn²⁺, Ni²⁺, Pb²⁺, Zn²⁺), as well as representatives of alkali metals (Na⁺) and alkaline earth metals (Ca²⁺), were investigated. As shown in Figure 8a, the photoluminescence intensity of CALIX@UiO-66-NH₂ in water decreases in the presence of metal cations. It is worth noting that in the presence of Pb²⁺+ and Cr³⁺, the characteristic emission band is also shifted to higher wavelengths. The quenching in the platform emission due to the cation presence can result from the turn-off mechanism. The turn-off mechanism is based on the quenching of emission properties resulting from introducing an analyte to the sensory material.

Figure 8
(a) Emission spectra and (b) relative emission intensity histogram of CALIX@UiO-66-NH₂ aqueous suspension in the presence of different metal cations at a concentration of 1 mg mL⁻¹.

The photoluminescence intensities of the suspension in the presence of different cations are compared in the histogram shown in Figure 8b. The photoluminescence suppression was noted for all cations used, but mainly for Fe³⁺+, with the intensity being reduced to only 0.1% of the photoluminescence intensity observed for the pure suspension of CALIX@UiO-66-NH₂. The intensity suppression was also expressive for Cu²⁺ (relative intensity of 4%). It is observed that the photoluminescence suppression in the presence of transition metals is greater than that observed for alkali and alkaline earth metals (relative intensity of 63 and 58% for Na⁺ and Ca²⁺, respectively).

Since the suspension showed greater sensitivity to Fe³⁺ cation, further experiments were carried out on CALIX, UiO-66-NH₂, and CALIX@UiO-66-NH₂, varying the concentration of this cation from 0 to 0.0125 mg mL⁻¹. The results are shown in Figure 9.

Figure 9
Photoluminescence emission intensity upon successive additions of Fe³⁺ (0.0125 mg mL⁻¹) for CALIX, UiO-66-NH₂, and CALIX@UiO-66-NH₂.

For this chosen concentration range, no successive drop in fluorescence intensity was observed for the tetra-acid-calixarene after subsequent aliquot additions. However, an increase was followed by a stabilization in intensity (Figure 9a). This fact suggests that the calixarene derivative has no significant response for Fe³⁺ ions in this concentration range. The UiO-66-NH₂ sample showed a slight increase in fluorescence intensity after adding the first aliquot, then decreased until the addition of the fourth aliquot. Subsequent additions kept the intensity constant; no appreciable increase or decrease was observed (Figure 9b).

The platform, however, showed a different response to Fe³⁺ in this concentration range (Figure 9c). As seen, after the addition of the first and second aliquots, there was an increase in the fluorescence intensity of the platform. This increase in intensity may be associated with the formation of complexes between the tetra-acid-calixarene present in the platform structure and Fe³⁺ cations, considering the availability of free carboxylic groups.

The data obtained for this concentration fit well to the linear response (R²⁺ = 0.994) (Figure 10).

Figure 10
Linear fitting for fluorescence intensity against Fe³⁺ concentrations for CALIX@UiO-66-NH₂.

The Stern-Volmer plot (Figure 11) agrees with the linear model with R²⁺ = 0.998, suggesting a single deactivation mechanism model. From the slope of the line and based on the Stern-Volmer equation, a value of approximately 2.3 × 10³⁺ mol L⁻¹ for Stern-Volmer constant (K_SV) was found.

Figure 11
Stern-Volmer plot for CALIX@UiO-66-NH₂ in the presence of Fe³⁺.

The limit of detection of ions in solution was 6.74 × 10⁻⁶ mol L⁻¹, below the sanitary security limit for iron in drinking water 2.0 mg L⁻¹ (3.57 × 10⁻⁵ mol L⁻¹). Below this concentration, an unexpected increase in fluorescence intensity was observed (Figure 9c). The value of the limit of detection is in the same order as this obtained for rhodamine (1.2 × 10⁻⁶ mol L⁻¹),³⁹ also in water, but higher than for other platforms with fluorophore/chromophore molecules in different non-aqueous or mixed media: rhodamine in tetrahydrofuran (0.16 × 10⁻⁶ mol L⁻¹),⁴⁰ pyrene in DMF (2.0 × 10⁻⁶ mol L⁻¹),⁴¹ fluorescein in dimethyl sulfoxide/water, 3:7, v/v (7.4 × 10⁻⁹ mol L⁻¹),⁴² quinoline (5.0 × 10⁻⁸ mol L⁻¹),⁴³ imidazole in acetonitrile/water 7:3, v/v.⁴⁴

To study the effect of interfering cations in the sensing of Fe³⁺, further tests using the platform were performed based on the procedure described by Ma et al.³⁺ The emission spectra of the platform in the presence of Fe³⁺/interfering cation are presented in Figure 12a. Based on these results, one can notice that Na⁺ ions do not change the emission intensity for the platform in the presence of Fe³⁺, suggesting that the platform prefers iron in the presence of sodium. Other cations that do not significantly interfere with the platform intensity are Ca²⁺, Ni²⁺, Co²⁺, and Zn²⁺ (causing less than 12% of emission drops for the platform). The Pb²⁺, Cr³⁺, and Cu²⁺ cations represent significant interference (causing more than 35% of emission drops for the platform), evidenced by the decrease in the platform/Fe³⁺ intensity. Cu²⁺+ cations show the second most significant suppression in platform intensity, suggesting a strong platform/Cu²⁺ interaction. Figure 12b shows the relative emission intensity of platform/Fe³⁺ in the presence of interfering cations.

Figure 12
(a) Emission spectra of CALIX@UiO-66-NH₂/Fe³⁺ in the presence of interfering cations, and (b) relative emission intensity of the platform in the presence of interfering cations.

The interaction between Fe³⁺ ions and solid materials can lead to the collapse of the structure. Given this effect, the materials were treated with a concentrated solution of Fe³⁺ ions (2:1 m/m, Fe³⁺/material in CH₃OH). The FTIR spectra of the samples (CALIX, UiO-66-NH2, and CALIX@UiO-66-NH₂) in the presence and absence of Fe³⁺ ions are shown in Figure 13.

Figure 13
FTIR-ATR spectra of CALIX, UiO-66-NH₂, and CALIX@UiO-66-NH₂ in high-concentrated Fe³⁺ solution.

Based on the FTIR spectra (Figure 13), it can be assumed that the calixarene derivative and MOF are stable in a highly concentrated Fe³⁺ solution. This can be observed by the presence of characteristic bands after contact with the iron solution, indicating that the material structure remains stable. However, in the case of the calixarene derivative, a decrease in the band intensity associated with the COOH groups can be observed, suggesting, as expected, the formation of the COO–Fe bonds (complex). A similar effect caused by the presence of Fe³⁺ may be seen in the spectrum of the platform. The expressive decrease in the intensity of the COOH band suggests Fe³⁺ bonding to the free carboxylic groups present in CALIX@UiO-66-NH₂. The other characteristic bands remain intact, indicating structural stability in an environment with high iron concentration.

Conclusions

A new fluorescent probe, CALIX@UiO-66-NH₂ was successfully prepared by a post-synthetic modification of MOF UiO-66-NH₂ with an acid calixarene derivative. The amide bond between the calixarene derivative and MOF was confirmed by μ-FTIR. The structure, stability, and fluorescent properties of the new material were evaluated. The combination of MOF and calixarene resulted in a material with increased fluorescence emission intensity and higher sensitivity to Fe³⁺ ions, making it a promising candidate for sensing applications. The preliminary fluorescent sensing studies indicated that the probe is a promising, rapid, and stable platform for detecting Fe³⁺ in an aqueous medium, especially within a concentration range of Fe³⁺ suitable for human consumption.

Supplementary Information

Supplementary information is available free of charge at http://jbcs.org.br as a PDF file.

Acknowledgments

This research used facilities of the Brazilian Synchrotron Light Laboratory (LNLS), and the IMBUIA beamline staff is acknowledged for their assistance during the experiments [20221618]. J. D. S. F. would like to thank CAPES for a scholarship granted. This work was supported by PRONEX/FACEPE/CNPq (grant No. APQ-0675-1.06/14). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Finance Code 001. We also acknowledge CEMENE and Central Analítica-DQF-UFPE for their support with the characterizations.

References

• ¹
  Şahin, Ç. A.; Tokgöz, Ilknur; Bektaç, S.; J. Hazard. Mater. 2010, 181, 359. [Crossref] [PubMed]
  » Crossref» PubMed
• ²
  Łukasik, N.; Wagner-Wysiecka, E.; J. Photochem. Photobiol., A 2017, 346, 318. [Crossref]
  » Crossref
• ³
  Łukasik, N.; Wagner-Wysiecka, E.; Małachowska, A.; Analyst 2019, 144, 3119. [Crossref] [PubMed]
  » Crossref» PubMed
• ⁴
  Kassem, M. A.; Amin, A. S.; Food Chem. 2013, 141, 1941. [Crossref] [PubMEd]
  » Crossref» PubMEd
• ⁵
  Niederau, C.; Fischer, R.; Porschel, A.; Stremmel, W.; Haussinger, D.; Strohmeyer, G.; Gastroenterology 1996, 110, 1107. [Crossref] [PubMed]
  » Crossref» PubMed
• ⁶
  World Health Organization (WHO); Guidelines for Drinking-Water Quality Health Criteria and other Supporting Information, vol. 2, 2^nd ed.; WHO: Geneva, 1996.
• ⁷
  Shamim, M. A.; Zia, H.; Zeeshan, M.; Khan, M. Y.; Shahid, M.; J. Environ. Chem. Eng. 2022, 10, 106991. [Crossref]
  » Crossref
• ⁸
  Barros, B. S.; de Lima Neto, O. J.; de Oliveira Frós, A. C.; Kulesza, J.; ChemistrySelect 2018, 3, 7459. [Crossref]
  » Crossref
• ⁹
  Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M.; Science 2013, 341, 1230444. [Crossref] [PubMed]
  » Crossref» PubMed
• ¹⁰
  Li, Y.; Yang, R. T.; Langmuir 2007, 23, 12937 [Crossref]
  » Crossref
• ¹¹
  Cao, J.; Li, X.; Tian, H.; Curr. Med. Chem. 2019, 27, 5949. [Crossref] [PubMed]
  » Crossref» PubMed
• ¹²
  Júnior, J. C. A.; dos Santos, G. L.; Colaço, M. V.; Barroso, R. C.; Ferreira, F. F.; dos Santos, M. V.; de Campos, N. R.; Marinho, M. V.; Jesus, L. T.; Freire, R. O.; Marques, L. F.; J. Phys. Chem. C 2020, 124, 9996. [Crossref]
  » Crossref
• ¹³
  Zhang, H.; Nai, J.; Yu, L.; Lou, X. W.; Joule 2017, 1, 77. [Crossref]
  » Crossref
• ¹⁴
  Monteiro, A. F. F.; Ribeiro, S. L. S.; Santos, T. I. S.; Fonseca, J. D. S.; Lukasik, N.; Kulesza, J.; Barros, B. S.; Microporous Mesoporous Mater. 2022, 337, [Crossref]
  » Crossref
• ¹⁵
  Zhu, X.; Zheng, H.; Wei, X.; Lin, Z.; Guo, L.; Qiu, B.; Chen, G.; Chem. Commun. 2013, 49, 1276. [Crossref] [PubMed]
  » Crossref» PubMed
• ¹⁶
  Cohen, S. M.; Chem. Sci. 2010, 1, 32. [Crossref]
  » Crossref
• ¹⁷
  Gutsche, C. D.; Dhawan, B.; No, K. H.; Muthukrishnan, R.; J. Am. Chem. Soc. 1981, 103, 3782. [Crossref]
  » Crossref
• ¹⁸
  Gutsche, C. D.; Top. Curr. Chem. 1984, 123, 1.
• ¹⁹
  de Oliveira Frós, A. C.; de Oliveira, M. A.; Macêdo Soares, A. A.; Hallwass, F.; Chojnacki, J.; Barros, B. S.; Júnior, S. A.; Kulesza, J.; Inorg. Chim. Acta 2019, 492, 161. [Crossref]
  » Crossref
• ²⁰
  Yamada, M.; Hamada, F.; CrystEngComm 2013, 15, 5703. [Crossref]
  » Crossref
• ²¹
  Homden, D. M.; Redshaw, C.; Chem. Rev. 2008, 108, 5086. [Crossref] [PubMed]
  » Crossref» PubMed
• ²²
  Wang, J.; Zhuang, S.; Nucl. Eng. Technol. 2020, 52, 328. [Crossref]
  » Crossref
• ²³
  Arora, L. S.; Chawla, H. M.; Shahid, M.; Pant, N.; Org. Prep. Proced. Int. 2017, 49, 228. [Crossref]
  » Crossref
• ²⁴
  Du, Y.; Li, X.; Zheng, H.; Lv, X.; Jia, Q.; Anal. Chim. Acta 2018, 1001, 134. [Crossref]
  » Crossref
• ²⁵
  Silva Lins, I. M.; Daniel da Silva Fonseca, J.; da Luz, L. L.; Chojnacki, J.; Júnior, S. A.; Barros, B. S.; Kulesza, J.; J. Solid State Chem. 2021, 295, [Crossref]
  » Crossref
• ²⁶
  Kumar, N.; Qui, P. X.; Roopa; Leray, I.; Ha-Thi, M. H.; Calixarene-Based Fluorescent Molecular Sensors, 2^nd ed.; Elsevier: Oxford, 2017.
• ²⁷
  Hu, Z.; Deibert, B. J.; Li, J.; Chem. Soc. Rev. 2014, 43, 5815. [Crossref] [PubMed]
  » Crossref» PubMed
• ²⁸
  Karmakar, A.; Samanta, P.; Dutta, S.; Ghosh, S. K.; Chem. Asian J. 2019, 14, 4506. [Crossref] [PubMed]
  » Crossref» PubMed
• ²⁹
  Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T.; Chem. Rev. 2012, 112, 1105. [Crossref] [PubMed]
  » Crossref» PubMed
• ³⁰
  Gutsche, C. D.; Iqbal, M.; Org. Synth. 2003, 68, 234. [Crossref]
  » Crossref
• ³¹
  Arnaud-Neu, F.; Barrett, G.; Cremin, S.; Deasy, M.; Ferguson, G.; Harris, S. J.; Lough, A. J.; Guerra, L.; McKervey, M. A.; Schwing-Weill, M. J.; Schwinte, P.; J. Chem. Soc., Perkin Trans. 2 1992, 1119. [Crossref]
  » Crossref
• ³²
  Xu, X.-Y.; Yan, B.; Sens. Actuators, B 2016, 230, 463. [Crossref]
  » Crossref
• ³³
  Ma, X.; Zhang, X.; Hao, Z.; Yang, K.; Han, L.; Microchem. J. 2021, 168, 106492. [Crossref]
  » Crossref
• ³⁴
  Scherrer, P.; Debye, P.; Nachr. Ges. Wiss. Göttingen, Math.-Physik. Klasse 1918, 2, 98.
• ³⁵
  Zhu, H.; Huang, J.; Zhou, Q.; Lv, Z.; Li, C.; Hu, G.; J. Lumin. 2019, 208, 67. [Crossref]
  » Crossref
• ³⁶
  Lv, G.; Liu, J.; Xiong, Z.; Zhang, Z.; Guan, Z.; J. Chem. Eng. Data 2016, 61, 3868. [Crossref]
  » Crossref
• ³⁷
  Chen, Y.-F.; Tan, L.-L.; Liu, J.-M.; Qin, S.; Xie, Z.-Q.; Huang, J.-F.; Xu, Y.-W.; Xiao, L.-M.; Su, C.-Y.; Appl. Catal., B 2017, 206, 426. [Crossref]
  » Crossref
• ³⁸
  Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P.; J. Am. Chem. Soc. 2008, 130, 13850. [Crossref]
  » Crossref
• ³⁹
  Goswami, S.; Das, S.; Aich, K.; Sarkar, D.; Mondal, T. K.; Quah, C. K.; Fun, H. K.; Daltan Trans. 2013, 42, 15113. [Crossref] [PubMed]
  » Crossref» PubMed
• ⁴⁰
  Vijay, N.; Wu, S. P.; Velmathi, S.; J. Photochem. Photobiol., A 2019, 384, 112060. [Crossref]
  » Crossref
• ⁴¹
  Guo, Y.; Wang, L.; Zhuo, J.; Xu, B.; Li, X.; Zhang, J.; Zhang, Z.; Chi, H.; Dong, Y.; Lu, G.; Tetrahedron Lett. 2017, 58, 3951. [Crossref]
  » Crossref
• ⁴²
  Gao, Y.; Liu, H.; Liu, Q.; Wang, W.; Tetrahedron Lett. 2016, 57, 1852. [Crossref]
  » Crossref
• ⁴³
  Shamsipur, M.; Sadeghi, M.; Garau, A.; Lippolis, V.; Anal. Chim. Acta 2013, 761, 169. [Crossref] [PubMed]
  » Crossref» PubMed
• ⁴⁴
  Ganesan, J. S.; Sepperumal, M.; Balasubramaniem, A.; Ayyanar, S.; Spectrochim. Acta, Part A 2020, 230, 117993. [Crossref] [PubMed]
  » Crossref» PubMed

Edited by

Editor handled this article: Fernando Carlos Giacomelli (Associate)

Publication Dates

• Publication in this collection
  23 Aug 2024
• Date of issue
  2025

History

• Received
  10 Jan 2024
• Accepted
  31 July 2024