Fluorimetric Detection of Phosphates in Water Using a Disassembly Approach: A Comparison of Fe III ‐, Zn II ‐, Mn II ‐ and Mn III ‐salen Complexes

"Covalent-Disassembly"-Based Approaches For Sensing Applications

The detection of analytes with small molecular probes is crucial for the analysis and understanding of chemical, medicinal, environmental and biological situations as well as processes. Classic detection approaches rely on the concept of molecular recognition and bond formation reactions. Bond breakage reactions have been less explored in similar contexts. This concept article introduces metal-salen and metal-imine complexes as "covalent-disassembly"-based (DB)-probes for detecting polyoxophosphates, thiols, amino acids, HCN and changes in pH. It discusses the roles, importance and combinations of structurally functionalized molecular building blocks in the construction of DB-probes. Applications of optimized DB-probes for analyte detection in live cells and foodstuff are also discussed. Furthermore, the mechanism of the disassembly of a Fe(III)-salen probe upon pyrophosphate binding is presented. Extraordinary selectivity for this analyte was achieved by a multistep disassembly sequence including an unprecedented structural change of the metal complex (i. e. "induced-fit" principle). Design principles of probes for sensing applications following the "covalent-disassembly" approach are summarized, which will help improving current systems, but will also facilitate the development of new DB-probes for challenging analytic targets.

A DFT Study of Phosphate Ion Adsorption on Graphene Nanodots: Implications for Sensing

The optical properties of graphene nanodots (GND) and their interaction with phosphate ions have been investigated to explore their potential for optical sensing applications. The absorption spectra of pristine GND and modified GND systems were analyzed using time-dependent density functional theory (TD-DFT) calculation investigations. The results revealed that the size of adsorbed phosphate ions on GND surfaces correlated with the energy gap of the GND systems, leading to significant modifications in their absorption spectra. The introduction of vacancies and metal dopants in GND systems resulted in variations in the absorption bands and shifts in their wavelengths. Moreover, the absorption spectra of GND systems were further altered upon the adsorption of phosphate ions. These findings provide valuable insights into the optical behavior of GND and highlight their potential for the development of sensitive and selective optical sensors for phosphate detection.

A CuII -Salicylidene Glycinato Complex for the Selective Fluorometric Detection of Homocysteine over 20 Proteinogenic Amino Acids

Homocysteine (Hcy) is a sulfur‐containing α‐amino acid that differs by one methylene (CH₂) subunit from homologous cysteine (Cys). Elevated levels of Hcy are diagnostic markers of cardiovascular disease and other medical conditions. We present a new Cu^II‐salicylidene glycinato complex 1 for the selective fluorometric detection of Hcy in water. In the presence of this analyte, the non‐fluorescent copper‐complex demetallates and disassembles into its building blocks. This process liberates a 3‐chloro‐5‐sulfosalicylaldehyde signaling unit and is accompanied by a 51‐fold turn‐on fluorescence at 485 nm (λ_ex=350 nm). Out of twenty proteinogenic amino acids, only histidine (12‐fold turn‐on fluorescence) and Cys (8‐fold turn‐on fluorescence) trigger some disassembly of probe 1. In comparison with important pioneering work on the detection of biothiols, this study strikingly demonstrates that structural modifications of chelate core structures steer substrate selectivity of metal‐based probes. Importantly, probe 1 has proven suitable for the detection of Hcy in artificial urine.

A simple epoxy resin screen-printed paper-based analytical device for detection of phosphate in soil

This study develops a simple and low-cost 3D paper-based analytical device (3D PAD) for the detection of available phosphate in soil. Epoxy resin is presented as a new hydrophobic material for low-cost mass production of PADs using the screen-printing method. An optimized concentration of epoxy resin solution is screen printed onto Whatman filter paper no. 1 in an easy one-step process to create hydrophobic patterns on PADs. The epoxy resin is air dried at room temperature, without heating or UV curing. This method delivers high reproducibility, resolution, and stability, and the epoxy resin barrier is compatible with both organic solvents and aqueous solutions. The molybdenum blue method is used in this PAD to measure phosphate in a colorimetric assay. The developed 3D PAD attains a linear range of 0.5-40 mg L^-1, with a limit of detection (LOD) of 0.25 mg L^-1, and a limit of quantitation (LOQ) of 0.83 mg L^-1. The relative standard deviation of intra-day measurements is 1.52-2.46%, and the inter-day standard deviation is 1.89-2.74%, indicating satisfactory reproducibility. This 3D PAD was tested for its ability to detect phosphate in a variety of actual soil samples and the results were validated against spectrophotometric analysis using a paired t-test, which showed high accuracy. In short, the new analytical device described in this study is simple, fast, and inexpensive to make and use, providing a versatile phosphate detection tool for many soil types, even in situations when resources are limited.

Induced fit activity-based sensing: a mechanistic study of pyrophosphate detection with a "flexible" Fe-salen complex

Activity-based sensing of biological targets is attracting increasing attention. In this work, we report detailed UV-Vis and fluorescence mechanistic studies on an Fe-salen based probe, [Fe^III{salenMeCl₂(SO₃)₂}OH₂]⁻ for pyrophosphate (PPi) detection. In the presence of PPi as an analyte, the probe disassembles into its molecular subunits and releases a fluorescent signal. Our studies illustrate that the aqua form of the complex (1-OH2) is the active species and that upon substitution of Fe-coordinated H₂O and an initial end-on coordination of HP₂O₇³⁻, the “trapped” pyrophosphate species switches from a monodentate to a bidentate coordination mode (i.e. linkage isomerism) via a probable equilibrium process. The elusive intermediate is further stabilized by a hydrogen bonding interaction that activates the probe for the subsequent final irreversible rate-limiting step, and allows selective discrimination between the other pyrophosphate (H₂P₂O₇²⁻ and P₂O₇⁴⁻) species in favour of the HP₂O₇³⁻. The flexible mode of molecular recognition and binding of HP₂O₇³⁻ by the tetradentate probe 1-OH2 is unexpected and most effective at physiological pH, and has precedence in enzymatic catalysis (i.e. induced fit principle). These binding properties explain the previously observed outstanding selectivity of 1-OH2 for pyrophosphate over other (poly)oxophosphates and potentially competing analytes.

A detailed mechanistic study of pyrophosphate (PPi) detection with a fluorometric Fe-salen based probe unravels the key structural switch of the Fe-bound PPi (“induced fit principle”) explaining the novel selectivity over other competing analytes.