A manganese-based metal-organic framework electrochemical sensor for highly sensitive cadmium ions detection

https://doi.org/10.1016/j.jssc.2019.03.051Get rights and content

Highlights

  • A new electrochemical sensors for Cd2+ based on metal-organic framework ZJU-77.

  • Good water-stability and high affinity of N atoms on ZJU-77 to Cd2+.

  • Modified ZJU-77/Nafion/GCE electrodes for Cd2+ electrochemical detection.

  • High sensitivity with a detection limit of 0.12 ppb.

  • Excellent selectivity and anti-interference capacity in real environment.

Abstract

A designed manganese-based metal-organic framework (ZJU-77, ZJU = Zhejiang University) has been solvothermally synthesized and structurally characterized. Due to the good water-stability and high affinity of nitrogen atoms on the frameworks to Cd2+, ZJU-77 and Nafion composites were dropped onto glassy carbon electrodes (GCE) to fabricate a modified ZJU-77/Nafion/GCE electrode, which could be employed for Cd2+ electrochemical detection with a detection limit calculated to be 0.12 ppb, lower than the upper concentration limit of Cd2+ permitted in drinking water. Excellent selectivity and anti-interference capacity in the presence of other competing species were exhibited. Furthermore, this proposed sensor was successfully applied to detect Cd2+ ions in various water samples obtained from the real environment.

Graphical abstract

A manganese-based metal-organic framework was designed to modify electrodes as an electrochemical sensor for high sensitive cadmium ion detection.

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Introduction

Heavy metal ions were commonly regarded as a class of threatening species owing to their catholic usage and high poisonousness nature. Among various heavy metal ions, Cd2+ was notorious as one of the most dangerous pollutants, which was iatrically regarded to be related to cardiovascular damages, prostate and renal injuries, diabetes and cancers even exposed to trace level of Cd2+ [[1], [2], [3], [4], [5]]. Moreover, Cd2+ was not biodegradable, which could be easy absorbed and accumulated into human bodies from the environment with a long period of time. On this account, the World Health Organization (WHO) and the US Environmental Protection Agency (EPA) set up the upper concentration limit of Cd2+ allowed in the drinking water of approximate 3 and 5 ppb [[5], [6], [7], [8]]. Due to its noxious effects even at low concentrations, the effective detection for Cd2+ is especially demanded.

There are various methods applied for Cd2+ detection such as coupled plasma mass spectrometry, synchrotron radiation X-ray spectrometry and atomic emission spectrometry. Nevertheless, these analysis methods are always along with sophisticated instruments and high cost. Compared with other analysis techniques, electrochemical stripping voltammetry has been considered as an alternative method with convenience, low cost, high sensitivity and rapid response. It is worth mentioning that this electrochemical stripping voltammetry method includes a pre-concentration process before the electrochemical sensing measurement [9]. The pre-concentration process usually contributes to high sensitivity and fast response. However, there are some restrictions in most electrode materials for sensing, including low porosity and lack of chemical designability. As a consequence, exploring tunable chemical structures and porous materials for electrochemical detection are of high significance on improving the performances of Cd2+ sensors since the vital role it played in the process of pre-concentration.

In recent decades, metal-organic frameworks (MOFs) have attracted widespread attention as an emerging class of porous crystal materials, which are coordinated by metal ions/clusters and multifunctional organic ligands [[10], [11], [12]]. Their unique features of unsaturated metal active sites, defined periodic crystal structures and devisable multi-purpose frameworks endow them splendid potential as electrochemical sensors [[13], [14], [15], [16], [17], [18], [19], [20]]. Especially, MOFs have distinctive advantages of high surface areas and chemically tunable frameworks, which result in faster and more sufficient interactions with analytes and thus might lead to the improvement on the sensitivity. On the other hand, the adjustable porosity of MOFs might also enhance the selectivity by isolating other interferential species through the size effect [[21], [22], [23], [24]].

Inspired by these considerations, we designed a novel manganese-based metal-organic framework (ZJU-77, ZJU = Zhejiang University) for highly sensitive Cd2+ detection. In view of the large ionic radius of cadmium ions and the strong affinity to the bare nitrogen atoms, we introduced a pyridine unit into the framework, which provides pre-concentration sites for cadmium ions gathering. Hence, an individual organic ligand 3,3'-(pyridine-2,5-diyl)dibenzoic acid was selected to fabricate a manganese-based MOFs (ZJU-77) for Cd2+ detection. In this work, to explore the potential electrochemical sensing performance, ZJU-77 and Nafion composites were dropped on glassy carbon electrode to form a modified ZJU-77/Nafion electrode and this modified electrode was employed for cadmium ions detection. ZJU-77/Nafion exhibited high selectivity and sensitivity and fast response in the presence of interferential species. The sensory performance was further investigated with various real water samples including lake, tap and drinking water. Experimental results demonstrated the fine precision and repeatability of ZJU-77/Nafion, and this modified electrode material has potentialities in Cd2+ sensing in natural water environments.

Section snippets

Materials and apparatus

All reagents in this work were bought commercially and used as supplied. Powder X-ray diffraction (PXRD) data were obtained with the recording rate of 5°/min in two Theta = 3–50° on a SHIMADZU XRD-7000 X-ray diffractometer with Cu Kα (λ = 1.542 Å) radiation at room temperature. Thermogravimetric analysis (TGA) was researched on a Netzsch TG209F3 ranged from 40 °C to 800 °C in nitrogen atmosphere under the heating rate of 5 °C/min. Elemental analyses (EA) for C, H, and N were carried out on an

Crystal structure and characterization

The rod-like colorless crystal (Fig. S1) was hydrothermally synthesized by the reaction of MnCl2·4H2O and 3, 3'-(pyridine-2, 5-diyl)dibenzoic acid at 80 °C for three days. ZJU-77 crystallizes in the monoclinic space group C 1 2/c 1 (15) (Tab. S1). As shown in Fig. 1, the manganese atom was six-coordinated with two O atoms from two DMF molecules and four carboxylate oxygen atoms from four ligands according to the single crystal diffraction results. The frameworks of ZJU-77 were formed with

Conclusions

In conclusion, we synthesized a novel manganese-based metal-organic framework (ZJU-77) by simple solvothermal methods. ZJU-77 and Nafion composites were dropped on glassy carbon electrodes to fabricate a modified ZJU-77/Nafion/GCE electrode for electrochemical detection. By introducing recognized nitrogen atoms into framework, this well-designed sensor displayed highly selective and sensitive detection of Cd2+ using square wave anodic stripping voltammetry. The detection limit of this modified

Acknowledgment

This work was supported by the National Natural Science Foundation of China (No. 51472217, 51432001, 51632008, U1609219 and 51772268) and Zhejiang Provincial Natural Science Foundation of China (No. LZ15E020001).

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