Regulating Arrhenius Activation Energy and Fluorescence Quantum Yields of AuNCs-MOF to Achieve High Temperature Sensitivity in a Wide Response Window.

Luminescent thermometry affords remote measurement of temperature and shows huge potential in future technology beyond those possible with traditional methods. Strategies of temperature measurement aiming to increase thermal sensitivity in a wide temperature response window would represent a pivotal step forward, but most thermometers cannot do both of them. Herein, we propose a balancing strategy to achieve a trade-off between high Arrhenius activation energy (Ea), which could stretch the temperature response windows, and fluorescence quantum yields (QYs) in a manner that will increase thermal sensitivity in a wide response window. In particular, a luminescent thermometer composed of AuNCs-MOF is prepared via a facile impregnation process to enhance QYs and Ea, responsible for high relative sensitivity (Sr) (15.6% K-1) and ultrawide temperature linearity range (from 83 to 343 K), respectively. Taking fluorescence intensity and lifetime as multiple parameters, the maximum Sr can reach 22.4% K-1 by multiple linear regression. The maximum Sr and temperature response range of the proposed thermometer outperform those of the most recent luminescent thermometers. The strategy of balancing Sr and thermal response range by regulating Ea and QYs enables the construction of ultra-accurate thermal sensors in the age of artificial intelligence.

Fabrication of a two-dimensional bi-lanthanide metal-organic framework as a ratiometric fluorescent sensor based on energy competition.

Bimetallic lanthanide metal-organic frameworks (bi-Ln-MOFs) exhibit great appeal for ratiometric luminescent sensors due to their unique advantages. Specially, the low-lying energy of the empty 4f band of Ce4+ ions benefits Ce-MOFs with robust and broad fluorescent emission. Therefore, constructing ratiometric sensors based on Ce-MOFs is of significance but remains a challenge. Here, a two-dimensional (2D) bi-Ln-MOF is fabricated using Eu3+/Ce4+ and 5-boronoisophthalic acid (5-bop) via a crystal phase transformation strategy to construct a ratiometric luminescent Hg2+ sensor. Due to the lower energy gap of Ce4+ compared to Eu3+ and the corresponding stronger energy-absorption ability, the Ce4+ in bi-Ln-MOF shows a stronger and broader fluorescent emission than that of Eu3+. The substitution of the boric acid group in the bi-Ln-MOF by Hg2+ amplifies the difference between the two lanthanide ions. Therefore, the fluorescence intensity of Ce4+ increases whereas that of Eu3+ decreases accordingly, a behavior distinct from individual Eu-MOF or Ce-MOF performance. This novel bi-Ln-MOF sensor not only achieves a wide linear response range from 0.5 to 120 µM with a low detection limit of 167 nM for Hg2+, but also demonstrates exceptional selectivity and stability. The intriguing sensing mechanism of energy competition and the novel synthesis approach for 2D bi-Ln-MOF are anticipated to broaden the application possibilities of bi-Ln-MOFs for designing ratiometric sensors.

Metal-organic framework-based ratiometric point-of-care testing for quantitative visual detection of nitrite.

Nitrite (NO2-) is categorized as a carcinogenic substance and is subjected to severe limitations in water and food. To safeguard the public's health, developing fast and convenient methods for determination of NO2- is of significance. Point-of-care testing (POCT) affords demotic measurement of NO2- and shows huge potential in future technology beyond those possible with traditional methods. Here, a novel ratiometric fluorescent nanoprobe (Ru@MOF-NH2) is developed by integrating UiO-66-NH2 with tris(2,2'-bipyridyl)ruthenium(II) ([Ru(bpy)3]2+) through a one-pot approach. The special diazo-reaction between the amino group of UiO-66-NH2 and NO2- is responsible for the report signal (blue emission) with high selectivity and the red emission from [Ru(bpy)3]2+ offers the reference signal. The proposed probe shows obviously distinguishable color change from blue to red towards NO2- via naked-eye. Moreover, using a smartphone as the detection device to read color hue, ultra-sensitive quantitative detection of NO2- is achieved with a low limit of detection at 0.6 µΜ. The accuracy and repeatability determined in spiked samples through quantitative visualization is in the range of 105 to 117% with a coefficient of variation below 4.3%. This POCT sensing platform presents a promising strategy for detecting NO2- and expands the potential applications for on-site monitoring in food and environment safety assessment.

Naphthalimide Derivative-Functionalized Metal-Organic Framework for Highly Sensitive and Selective Determination of Aldehyde by Space Confinement-Induced Sensitivity Enhancement Effect.

Facile and sensitive determination of formaldehyde (FA) in indoor environments still remains challenging. Herein, a fluorescent probe, termed PHN@MOF, was synthesized by embedding the fluorescent molecule of N-propyl-4-hydrazine-naphthalimide (PHN) into a metal-organic framework (MOF) for sensitive and visual monitoring of FA. The hydrazine group of PHN acts as the specific reaction group with FA based on the condensation reaction. The host of MOF (UiO-66-NH2) offers the surrounding confinement space required for the reaction. Owing to the enrichment effect and molecular sieve selection of UiO-66-NH2 to FA, PHN@MOF, compared with free PHN, exhibits very high sensitivity and selectivity based on space confinement-induced sensitivity enhancement (SCISE). Moreover, the fluorescence of UiO-66-NH2 offers a reference signal for FA detection. Using this ratiometric fluorescent PHN@MOF probe, a colorimetric gel plate and test paper were developed and used to visually monitor FA in air.

A Boric Acid-Functionalized Lanthanide Metal-Organic Framework as a Fluorescence "Turn-on" Probe for Selective Monitoring of Hg2+ and CH3Hg.

Mercury detection remains an important task because of its high toxicity. Herein a new dual-signal probe based on a boric acid (BA)-functionalized lanthanide metal-organic framework (BA-Eu-MOF) was developed for the detection of Hg2+ and CH3Hg+ ions for the first time. The BA-Eu-MOF was synthesized by coordination of Eu3+ with 5-boronobezene-1, 3-dicarboxylic acid (5-bop) through a one-pot method. The 5-bop ligand not only acted as the "antenna" to sensitize the luminescence of Eu3+ but also provided reaction sites for Hg2+ and CH3Hg+. Owing to the electron-withdrawing effect of the BA group, the "antenna" effect of the ligand was passivating and the BA-Eu-MOF showed weak red emission in water. Upon addition of Hg2+ or CH3Hg+ into the system, a transmetalation reaction took place, i.e., BA groups were replaced by Hg2+ or CH3Hg+; therefore, the "antenna" effect of the ligand was triggered, leading to the enhancement of red emission. As Hg2+ or CH3Hg+ concentration increased, the red emission was gradually enhanced, and the color change was also observed with the naked eye under 365 nm ultraviolet light. Owing to the porous characteristics and the surface effect of the MOF, as well as the unique transmetalation reaction between the BA group and Hg2+ or CH3Hg+, the developed nanoprobe showed excellent characteristics for simultaneous detection of Hg2+ and CH3Hg+, such as simple preparation, convenient operation, "turn-on" signal output, high sensitivity, and selectivity. The unique features of the BA-Eu-MOF make it an attractive probe for monitoring Hg2+ and CH3Hg+.

The role of l-histidine as molecular tongs: a strategy of grasping Tb3+ using ZIF-8 to design sensors for monitoring an anthrax biomarker on-the-spot.

In this study, a novel lanthanide-doped nanoprobe for monitoring dipicolinic acid (DPA), a unique biomarker of Bacillus anthracis, was constructed by coordination of Tb3+ with l-histidine (His) functionalized ZIF-8 (His@ZIF-8). After being functionalized with His, the resultant His@ZIF-8 had abundant carboxyl and amino groups, which like tongs help His@ZIF-8 "grasp" Tb3+ firmly to form a stable lanthanide-doped nanoparticle (His@ZIF-8/Tb3+). Owing to the unsaturated coordination of Tb3+ with the amino acid group, the resultant His@ZIF-8/Tb3+ showed reserved response sites of Tb3+ to DPA because of its unique molecular structure. After the His@ZIF-8/Tb3+ coordination with DPA, the intrinsic fluorescence emission of the Tb3+ ions was triggered through energy transfer, leading to bright yellow green luminescence owing to the antenna role of DPA. Benefitting from the His functionalization and the characteristics of ZIF-8, especially the high porosity and large surface area, the developed His@ZIF-8/Tb3+ sensing platform exhibited attractive features as a fluorescent sensor for monitoring DPA such as fast response kinetics (10 s), high sensitivity and selectivity, and being portable, easy to operate, economical and secure. This sensor platform showed a satisfactory linear relationship (R 2 = 0.999) ranging from 0.08 to 10 µmol L-1 and an ultralow limit of detection (LOD) of 0.02 µmol L-1. This strategy for the design of functionalized MOFs to construct sensing probes and the resultant His@ZIF-8/Tb3+ would provide a potential strategy for the exploitation of other functionalized materials used in other research fields and promising fluorescence platforms for the detection of other targets.