In recent years, heavy metal pollution has become increasingly serious. Heavy metals exist in an environment mainly in the form of ions (heavy metal ions, HMs). They can contaminate food, water, soil, and the atmosphere, leading to serious harm to plants and animals. With high bioavailability and nonbiodegradability, HMs can accumulate through biomagnification. Consequently, heavy metal pollution has become the cause of many fatal diseases threatening human health and ecological environment. Therefore, the accurate detection of HMs is vital and necessary. In this paper, the harm and limit standards of heavy metals were systematically summarized and the common analysis methods were overviewed and compared. Specifically, the latest research progress of ratiometric photoelectric sensor, including optical and electrical sensor, were mainly described. The research status and advantages and disadvantages of a photoelectric sensor were summarized. Furthermore, the future directions were proposed, which provided the reference for the further research and application of the ratiometric photoelectric sensor.

Detection of Heavy Metal Ions by Ratiometric Photoelectric Sensor.

An efficient donor triphenylamine aldehyde Schiff-base with p-toluene sulfonyl hydrazine fluorescent probe (TPA-PTH) was synthesized and characterized by various spectroscopic techniques. The probe TPA-PTH displayed drastic color changes with Cu2+ ions over metal ions in aqueous environment with a fluorescence-off mechanism.The detection limit of Cu2+ was found to be 1.25 × 10-8 M by using the equation LOD = K × SD/S. The 2:1 binding stoichiometric mode of TPA-PTH with Cu2+ ion was further tested by HR mass and Job’s plot analyses. Furthermore, the proposed probe has been effectively applied to determine the trace amount of Cu2+ ions in different water samples with 96%∼98% recoveries. The sensing mechanism of TPA-PTH was further investigated by Density Functional Theory (DFT) calculations using a Gaussian 16 program. DFT calculations were theoretically supported the experimental findings. 

A simple triphenylamine based turn-off fluorescent sensor for copper (II) ion detection in semi-aqueous solutions

Fluorometric/Colorimetric chemosensors have played a compelling role in the precise detection of metal ions in recent years owing to benefits such as specificity, economy, rapid reaction to analytes, ease of operation, naked-eye screening and low detection limit. In this study, we focus on pyrene fluorophore-derived fluorescent chemosensors, especially for detecting d-transition metal ions such as Cu2+, Hg2+, Fe2+/Fe3+, and Zn2+ ions, as well as their applications in environmental and biological samples. Pyrene is a very important polycyclic aromatic hydrocarbon that acts as a fluorophore with distinct photophysical behavior in solution when compared to other well-known scaffolds. It can also be used to create chemosensors for specific detection of cations, anions, and other analytes that could be used in practical applications. We have outlined pyrene-based chemosensors, their conceivable mechanisms and applications. Energy transfer (ET), ligand to metal charge transfer (LMCT), aggregation induced emission (AIE), excimer and exciplex formation, excited state intramolecular proton transfer (ESIPT), photoinduced electron transfer (PET), intramolecular charge transfer (ICT), and chelation enhanced fluorescence (CHEF) are among the mechanisms used in these developed chemosensors. In terms of applications, pyrene-based fluorescent derivatives are used efficiently in the examination of live cells, actual water, and food samples in the chemical, biological, environmental, and medical domains. The synthesis and design of adaptable pyrene fluorophore-based derivatives for specific detection on d-transition metal ions, binding processes, detection limits, and diverse applications were specifically covered in recent studies 2018–2022. 

A Review on Pyrene Based Chemosensors for the Specific Detection on d-transition Metal Ions and Their Various Applications

The formation or destruction of pyrene excimer with strong emission change make it a versatile fluorophore to construct excimer emission-based fluorescence probes for various analytes detection. The distinct difference between...

Pyrene-based monomer-excimer dual response organosilicon polymer for selective detection of 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP)

In this study, a unique Schiff base (SP) was synthesized from 1-aminopyrine (1-AP) and salicylaldehyde (SA) compounds. The obtained oligomer of this Schiff base was carried out by the oxidative polymerization method. The structural characterizations of 2-((pyren-1-yl-imino)methyl)phenol (SP) and oligo-2-((pyren-1-yl-imino)methyl)phenol (OSP) by FT-IR, 1H NMR and 13C NMR measurements, electrochemical properties by cyclic voltammetry (CV); optical properties by UV/Vis studies and photoluminescence (PL) spectrophotometry; thermal stability were performed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The molecular weight of the obtained oligomer was determined using gel permeation chromatography (GPC). Accordingly, Mn and Mw values  of OSP were found to be 2800 and 3150 Da, respectively. The imine bond provided the structures with high thermal stability, and the maximum mass loss temperatures (Tmax) of SP and OSP were found to be 350 0 and 343 °C, respectively. PL study showed that SP could be used as a Cr3+ sensor among a series of metals. The results revealed that the limit of detection (LOD) value of SP was 1.03 × 10−9 M and was not affected by other ions in the environment.

Synthesis and characterization of a pyrene-based Schiff base and its oligomer: Investigation of fluorescent Cr3+ probe

A highly selective and sensitive fluorescent ratiometric probe (1) based on pyrene-containing Schiff base for the dection of Cu2+ was synthesized. In acetonitrile–HEPES buffer solution (20.0 mM HEPES, 8:2, v/v, pH 7.4), only Cu2+, out of tested common cations, could induce the aggregation of probe 1. Fluorescence analysis showed that the excimer emission band of pyrene groups of probe 1 appeared and gradually increased with the addition of Cu2+. The fluorescent intensity ratio of excimer and monomer (I470/I409) of probe 1 increased 27.8-fold upon the addition of one equiv. of Cu2+ with a detection limit of 20.2 nM (1.28 ppb). The addition of Cu2+ also induced obvious color change and the characteristic peaks of pyrene excimer in UV–vis spectrometra of the relutant solution. Results from the Job plot, metal-binding fluorescent titration, mass spectra experiments and density functional theory calculations indicated a binding ratio of 2:1 (probe 1: Cu2+). In addition, probe 1 showed good potential as a fluorescent visualizing probe for the detection of Cu2+ in living GS cells of epinepheluscoioides.

A pyrene-containing Schiff base fluorescent ratiometric probe for the detection of Cu2+ in aqueous solutions and in cells

A dual-pKa values pH fluorescence probe based on pyrene, N-(2′-morpholinoethyl)-pyrene-1-carboxamide (MPCA), was fabricated through facile one-step reaction. Interestingly, MPCA showed a pKa1 of 1.88 under extremely acidic conditions and a pKa2 of 5.81 under weak acidic conditions in deionized water, which indicate that MPCA has a high sensitivity to change of pH value. When the pH value was decreased further from 4.05 to 0.52, the fluorescence response of MPCA was quenched by 93.8% due to quenching of fluorescence emission of the pyrene monomer and apparent conversion of monomer/excimer of pyrene group. The results show that pyrene excimer of MPCA can be produced in extremely acidic environment. When the pH value of the solution was decreased from 7.03 to 4.05, the fluorescence response of MPCA was enhanced due to the inhibition of photoinduced electron transfer (PET) from N atom on morpholine ring to pyrene fluorophore. In addition, MPCA has a short response time, good repeatability and anti-cation interference for pH monitoring. MPCA was then applied successfully to monitor the acidity of environmental water systems (tap water and Pearl River water) and the living GS cells of epinepheluscoioides. Finally, a fluorescence pH test paper based on the colorful changes of MPCA at different pHs was developed, which can conveniently monitor the pHs of aqueous solutions.

A pyrene-based pH fluorescence probe with continuous multiple responses under acidic conditions and its application for environmental water systems and cells

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A pyrene-containing Schiff base fluorescent ratiometric probe for the detection of Cu2+ in aqueous solutions and in cells

A pyrene-based pH fluorescence probe with continuous multiple responses under acidic conditions and its application for environmental water systems and cells