An Overview of the Design of Metal-Organic Frameworks-Based Fluorescent Chemosensors and Biosensors

Simple fluorochromic detection of chromium with ascorbic acid functionalized luminescent Bio-MOF-1

The bioaccumulation of various heavy metals in the environment and agriculture is posing serious hazards to human health. Hexavalent chromium is one of the most encountered heavy metal pollutants. The routine monitoring of Cr(VI) via simple methods assumes great analytical significance in sectors like environmental safety, food quality, etc. This study reports a novel biocompatible and luminescent metal-organic framework (ascorbic acid functionalized Bio-MOF-1) based "Turn-on" nanoprobe for rapid and sensitive optical detection of Cr(VI). Bio-MOF-1 has been synthesized, functionalized with ascorbic acid (AA), and then comprehensively characterized for its key material properties. The presence of Cr(VI) results in the photoluminescence recovery of Bio-MOF-1/AA. Using the above approach, Cr(VI) is detected over a wide concentration range of 0.02 to 20 ng mL-1, with the limit of detection being 0.01 ng mL-1. The nanoprobe is capable of detecting Cr(VI) in real water as well as in some spiked food samples. Hence, the ascorbic acid functionalized Bio-MOF-1 nanoprobe is established as a potential on-field detection tool for Cr(VI).

HOF-101-based dual-mode biosensor for photoelectrochemical/electrochemiluminescence detection and imaging of oxytetracycline

A unique hydrogen-bonded organic frameworks (HOF-101)-based photoelectrochemical (PEC) and electrochemiluminescence (ECL) dual-mode biosensor using polydopamine nanoparticles (PDAs) as quencher was constructed for ultrasensitive detection and imaging of oxytetracycline (OXY). In particular, HOF-101 was a superior ECL material and can be observed with the naked eye. Furthermore, it also had outstanding PEC signal, so HOF-101 was a new dual-signal material with excellent performance, thus it was explored to realize dual-mode detection. As the main component of natural melanin, PDAs not only had good biocompatibility, but also contained rich functional groups on the surface. Additionally, PDAs had excellent light absorption ability and poor conductivity, which made it the excellent photoquencher. In this work, PDAs were introduced on the surface of HOF-101 to quench its ECL and PEC signals by using the dual-aptamer sandwich method, achieving ultrasensitive detection of antibiotic OXY. Particularly for ECL detection, HOF-101 was firstly used to visually detect OXY. The detection range can reach 0.1 pM-100 nM, and the limit of detection (LOD) can reach 0.04 pM. This work showed a great contribution to the development of new ECL-PEC materials and ECL visualization analysis, which had outstanding application potential in the fields of food safety and biochemical analysis.

Microcrystalline Luminescent (Eu1-xLnx)2bdc3·nH2O (Ln = La, Gd, Lu) Antenna MOFs: Effect of Dopant Content on Structure, Particle Morphology, and Luminescent Properties

In this work, three series of micro-sized heterometallic europium-containing terephthalate MOFs, (Eu1-xLnx)2bdc3·nH2O (Ln = La, Gd, Lu), are synthesized via an ultrasound-assisted method in an aqueous medium. La3+ and Gd3+-doped terephthalates are isostructural to Eu2bdc3·4H2O. Lu3+-doped compounds are isostructural to Eu2bdc3·4H2O with Lu contents lower than 95 at.%. The compounds that are isostructural to Lu2bdc3·2.5H2O are formed at higher Lu3+ concentrations for the (Eu1-xLux)2bdc3·nH2O series. All materials consist of micrometer-sized particles. The particle shape is determined by the crystalline phase. All the synthesized samples demonstrate an "antenna" effect: a bright-red emission corresponding to the 5D0-7FJ transitions of Eu3+ ions is observed upon 310 nm excitation into the singlet electronic excited state of terephthalate ions. The fine structure of the emission spectra is determined by the crystalline phase due to the different local symmetries of the Eu3+ ions in the different kinds of crystalline structures. The photoluminescence quantum yield and 5D0 excited state lifetime of Eu3+ are equal to 11 ± 2% and 0.44 ± 0.01 ms, respectively, for the Ln2bdc3·4H2O structures. For the (Eu1-xLux)2bdc3·2.5H2O compounds, significant increases in the photoluminescence quantum yield and 5D0 excited state lifetime of Eu3+ are observed, reaching 23% and 1.62 ms, respectively.

A Coordination Polymer for the Fluorescence Turn-On Sensing of Saccharin, 2-Thiazolidinethione-4-carboxylic Acid, and Periodate

A luminescent 1D coordination polymer (CP) [Zn2L2(H2O)4]·H2O (1, H2L = 1-(4-carboxyphenyl)-1H-pyrazole-3-carboxylic acid) was prepared by a solvothermal method. 1 shows excellent fluorescence properties and has an obvious fluorescence "turn-on" phenomenon for saccharin (SAC), 2-thiazolidinethione-4-carboxylic acid (TTCA), and periodate (IO4-). Between 0 and 60 μM concentration range of SAC, the fluorescence enhancement efficiency (KEC) of 1 reaches 1.00 × 105 M-1 with the limit of detection (LOD) of 90 nM. 1 is the first CP-based sensing material for SAC detection. For TTCA detection, the KEC is 2.73 × 105 M-1 at the 25-80 μM concentration range, and the LOD is 33 nM, the lowest LOD among the sensors that detect TTCA at present. For IO4- ion detection, when the IO4- ion concentration ranges from 0 to 10 μM, the KEC is 2.34 × 105 M-1 and the LOD is as low as 39 nM. In order to better understand the sensing phenomenon, we also discuss in detail the sensing mechanisms for SAC, TTCA, and IO4- ions.

Sensitive Current Sensor Based on a Lanthanide Framework with Lewis Basic Bipyridyl Sites for Cu2+ Detection

A new Yb-based three-dimensional metal-organic framework with free Lewis basic sites, [Yb₂(ddbpdc)₃(CH₃OH)₂] (referred to as ACBP-6), from YbCl₃ and (6R,8R)-6,8-dimethyl-7,8-dihydro-6H-[1,5]dioxonino[7,6-b:8,9-b']dipyridine-3,11-dicarboxylic acid (H₂ddbpdc) was synthesized by a conventional solvothermal method. Two Yb³⁺ are connected by three carboxyl groups to form the [Yb₂(CO₂)₅] binuclear unit, which is further bridged by two carboxyl moieties to produce a tetranuclear secondary building unit. With further ligation of the ligand ddbpdc^2-, a 3-D MOF with helical channels is constructed. In the MOF, Yb³⁺ only coordinates with O atoms, leaving the bipyridyl N atoms of ddbpdc^2- unoccupied. The unsaturated Lewis basic sites make this framework possible to coordinate with other metal ions. After growing the ACBP-6 in situ into a glass micropipette, a novel current sensor is formed. This sensor shows high selectivity and a high signal-to-noise ratio toward Cu²⁺ detection with a detection limit of 1 μM, due to the stronger coordination ability between the Cu²⁺ and the bipyridyl N atoms.