A novel luminescent sensor based on Tb@UiO-66 for highly detecting Sm³⁺ and teflubenzuron

Highlights

• Tb@UiO-66 is a dual-platform chemosensor.

• Tb@UiO-66 exhibited luminescence enhancement for detecting Sm³⁺ and teflubenzuron.

• Tb@UiO-66 could be used to detect teflubenzuron in real water samples.

Abstract

Background

A novel luminescent sensor with dual-emitting platforms, Tb@UiO-66, was used for highly detecting Sm³⁺ and teflubenzuron.

Methods

Tb@UiO-66 was synthesized on basis of the exchange of guest molecules, where Tb³⁺ ions were encapsulated into the cages of UiO-66.

Significant finds

The results of multiband fluorescence originated from the linker-to-cluster charge transfer and the characteristic emission of Tb^III. Tb@UiO-66 displayed luminescence enhancements or quenches according to small molecules. Especially for Sm³⁺ and teflubenzuron, the characteristic peaks of Tb@UiO-66 at 490, 545, 585, and 621 nm increased with the increasing of the concentration of the analytes, where Tb@UiO-66 exhibited the effect of luminescence enhancement. The detection limits for detecting Sm³⁺ and teflubenzuron were 0.09 μM and 0.21 μM, and the changes of luminescence intensities could be observed by naked eyes under the UV light at 365 nm. Tb@UiO-66 could also be used to detect teflubenzuron in real water samples. Tb@UiO-66 could be reused for at least 5 cycles for the detection of Sm³⁺ and teflubenzuron with the framework of Tb@UiO-66 uncollapsed. These results implied that the integration of Ln^III luminescent materials and Zr-MOFs by doping method provided a new strategy for fabricating novel luminescent sensors for detecting polluting small molecules.

Introduction

Lanthanides (Lns), which is one of the most important branches of rare earth elements (REEs), have attracted considerable attention because of their applications in various fields, such as permanent magnets [1], luminescence [2,3], electric cars and smartphones [4,5], gas adsorption and separation [6], [7], [8], and lasers [9], etc. Tb^III, Dy^III and Ho^III can be served as molecular coolers. Eu^III-based and Tb^III-based composites are the superior luminescent materials in the range of visible light. The main commercial application of Sm^III is samarium-cobalt magnets, which have permanent magnetization second only to neodymium magnets. While, the increasing demands for Ln^III salts are limited by the total amount in nature, which are insoluble in the presence of phosphate with the concentration of 10⁻¹³ M [10]. It is urgent to seek out environmentally friendly methods for detecting Ln^III ions without separated from the system.

Pesticides [11], [12], [13], which are kinds of toxic organic compounds, have played a prominent role in increasing agricultural production and income, and have become one of the most effective measures to solve the problems of food and clothing for the world's 6 billion people. However, with the long-term use of pesticides, the pesticide residues in agricultural products exceed the international standard, which not only cause serious pollution to ecosystems, but also cause great harm to human health [14], [15], [16]. The negative impact caused by pesticide residues has aroused people's attention. Pesticide residues analyses methods are divided into three main categories according to their principles: chromatography, spectroscopy and biochemical measurement methods [17,18]. Chromatography method has the characteristics of excellent separation ability, good selectivity, and high sensitivity. While the disadvantages of chromatography method are long pretreatment time, interferences in the detection process and high costs. Chromatography method must be also combined with a highly selective detector. Spectroscopy method is fast, while the detection limit is relatively high. The principle of biochemical measurement method for detecting pesticides is to inhibit acetylcholinesterase (AChE) [19,20]. The biochemical measurement method has the advantages of low price and rapid detection, etc. However, the enzymes and antibodies served as sensors are easily affected by complex detection conditions. The scope of applications of biochemical measurement method is limited. Herein, exploiting a more convenient, fast, efficient and accurate approach to detect pesticides is strongly urgent priority.

So far, many techniques and materials for detecting cations and pesticides have been developed, such as biosensors [21], luminescent sensors [22], electrochemical sensors [23], porous materials [24], inductively coupled plasma mass spectroscopy (ICP-MS) and atomic adsorption spectrometry (AFS), etc. However, complicated sample preparation, sophisticated instruments, time-consuming and high cost have hindered the usage of these techniques. Chemosensors, such as luminescent sensors and electrochemical sensors, have developed rapidly due to the merits of sensitivity and selectivity of sensors [25]. Compared with electrochemical sensors, luminescent sensors cannot only eliminate the disadvantages of irregular shape and template leakage from electrochemical sensors, but also have high sensitivity and precision [26].

Metal–organic frameworks (MOFs), a class of crystal materials, are fabricated by metal ions or clusters and rigid or flexible organic ligands, which have been extensively applied in various fields, for instance, catalysis [27,28], adsorption [29,30], chemosensors [31,32], and electrochemistry [33,34], etc. While, the frameworks of most MOFs are unstable in the air due to the reversible coordination bonds, so that the potential applications of MOFs are limited to a certain degree [35], [36], [37]. Many efforts have been made to directly promote the stabilities of MOFs. One of the most remarkable examples is the synthesis of UiO-66, Zr₆(μ₃-O)₄(μ₃-OH)₄(BDC)₆ by Cavka et al [38], which exhibits the property of extremely hydrothermal stability. After that, many Zr-based metal–organic frameworks were reported and the potential applications were explored. The reason for the stabilities of Zr-MOFs is that zirconium ion has high charge density and bond polarization, and zirconium ion has strong electron affinity once it is bonded with the oxygen atoms coming from carboxylate [39]. The construction of luminescent sensors based on MOFs should be considered from the following points. Firstly, the frameworks of MOFs should be stable in different solvents, such as Zr-MOFs. Secondly, luminescent ligands with typical electron-rich properties combined with luminescent guest molecules should be selected to prepare luminescent sensors based on MOFs, which can eliminate the drawback of a single-emitting platform of the material with low recognition and stability. In addition, the preparation of dual-emitting platforms luminescent sensors based on MOFs also provides the possibility for the construction of visible luminescent probes. As luminescent sensors, lanthanide-based MOFs (LnMOFs) have stimulated the scientists’ attention due to the advantage of tuned luminescence properties [40]. Usually, the syntheses of Ln^III luminescent sensors have been classified into two methods. One is that Ln^III ions, as nodes, linked to the organic ligands directly and the other one is that Ln^III ions were encapsulated into the channels or cages of MOFs [41]. On basis of the viewpoint, Ln^III ions and Zr-MOFs may be integrated together to establish a new type of luminescent sensors, namely Ln@Zr-MOFs, which have the advantages of luminescent sensors based on MOFs mentioned above.

In this work, 1,4-dicarboxybenzene (H₂BDC) and ZrCl₄ were utilized to synthesize UiO-66. Next, Tb^III ions were encapsulated into the tetrahedral and octahedral cages of UiO-66, namely Tb@UiO-66, which can be served as a multifunctional luminescent sensor for highly detecting cations and pesticides, especially for Sm³⁺ and teflubenzuron. A series of approaches were utilized to characterize Tb@UiO-66, for instance, inductively coupled plasma (ICP), thermogravimetric analyses (TGA), infrared radiation (IR) and powder X-ray diffraction (PXRD). Tb@UiO-66 showed the luminescence enhancements phenomena for the detection of Sm³⁺ and teflubenzuron observed by the naked eyes under the UV light at 365 nm. Meanwhile, Tb@UiO-66 exhibited superior recyclability and low LODs for detecting Sm³⁺ and teflubenzuron. Tb@UiO-66 could also be used to detect teflubenzuron in real water samples. The mechanisms for detecting Sm³⁺ and teflubenzuron were investigated.