Aggregation Induced Emissive Luminogens for Sensing of Toxic Elements

The major findings in the growing field of aggregation induced emissive (AIE) active materials for the detection of environmental toxic pollutants have been summarized and discussed in this Review article. Owing to the underlying photophysical phenomenon, fluorescent AIE active molecules show more impact on sensing applications. The major focus in current research efforts is on the development of AIE active materials such as TPE based organic fluorescent molecules, metal organic framework, and polymers that can be employed for the detection of toxic pollutants such as CN⁻, NO₂⁻, Hg²⁺, Cd²⁺, As³⁺, As⁵⁺, F⁻, Pb²⁺, Sb³⁺ ions.     

Promising hole-transporting materials for perovskite solar cells: Modulation of the electron-deficient units in triphenylamine derivative-based molecules

Modulation of the electron-deficient π-bridge units in 4-methoxy-N-(4-methoxyphenyl)-N-phenylbenzenamine (MeTPA)-based hole-transporting materials (HTMs) is a significant approach to improve hole mobility of HTMs for perovskite solar cells (PSCs). In this study, a class of simple MeTPA-based HTMs (H1-H4) with different π-bridged electron-deficient units were designed for the purpose of providing a theoretical model to obtain potential MeTPA-based HTMs. The results indicated that H2 to H4 exhibit better performance, such as larger Stokes shifts, smaller exciton-binding energy, better stability, good solubility, and higher hole mobility, in comparison with the parental material H1. H2 to H4 materials with high hole mobility (5.45 × 10⁻⁴, 2.70 × 10⁻¹, and 3.99 × 10⁻³ cm² V⁻¹ second⁻¹, respectively) may embody promising HTMs to yield good performance in PSCs. Therefore, the useful information obtained regarding control of the electron-deficient π-bridge units of MeTPA-based HTMs is an effective way to obtain excellent HTMs for PSC applications.     

Tuning Phonon Energies in Lanthanide-doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion

Optical applications of lanthanide-doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate the size-controlled synthesis of low-phonon-energy KPb₂X₅ (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm⁻¹. KPb₂Cl₅ nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb₂X₅ nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche-like emission from KPb₂Cl₅ : Nd³⁺ nanoparticles. The realization of nanoparticles with tunable, ultra-low phonon energies facilitates the discovery of nanomaterials with phonon-dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.     

Highly photoluminescent MoOx quantum dots: Facile synthesis and application in off-on Pi sensing in lake water samples

Molybdenum oxide (MoO_x) is a well-studied transition-metal semiconductor material, and has a wider band gap than MoS₂ which makes it become a promising versatile probe in a variety of fields, such as gas sensor, catalysis, energy storage ect. However, few MoO_x nanomaterials possessing photoluminescence have been reported until now, not to mention the application as photoluminescent probes. Herein, a one-pot method is developed for facile synthesis of highly photoluminescent MoO_x quantum dots (MoO_x QDs) in which commercial molybdenum disulfide powder and hydrogen peroxide (H₂O₂) are involved as the precursor and oxidant, respectively. Compared with current synthesis methods, the proposed one has the advantages of rapid, one-pot, easily prepared, environment friendly as well as strong photoluminescence. The obtained MoO_x QDs is further utilized as an efficient photoluminescent probe, and a new off-on sensor has been constructed for phosphate (Pi) determination in complicated lake water samples, attributed to the fact that the binding affinity of Eu³⁺ ions to the oxygen atoms from Pi is much higher than that from the surface of MoO_x QDs. Under the optimal conditions, a good linear relationship was found between the enhanced photoluminescence intensity and Pi concentration in the range of 0.1–160.0 μM with the detection limit of 56 nM (3σ/k). The first application of the photoluminescent MoO_x nanomaterials for ion photochemical sensing will open the gate of employing MoO_x nanomaterials as versatile probes in a variety of fields, such as chemi-/bio-sensor, cell imaging, biomedical and so on.     

Upconversion Luminescence through Cooperative and Energy-Transfer Mechanisms in Yb3+-Metal-Organic Frameworks

Lanthanide-doped metal–organic frameworks (Ln-MOFs) have versatile luminescence properties, however it is challenging to achieve lanthanide-based upconversion luminescence in these materials. Here, 1,3,5-benzenetricarboxylic acid (BTC) and trivalent Yb³⁺ ions were used to generate crystalline Yb-BTC MOF 1D-microrods with upconversion luminescence under near infrared excitation via cooperative luminescence. Subsequently, the Yb-BTC MOFs were doped with a variety of different lanthanides to evaluate the potential for Yb³⁺-based upconversion and energy transfer. Yb-BTC MOFs doped with Er³⁺, Ho³⁺, Tb³⁺, and Eu³⁺ ions exhibit both the cooperative luminescence from Yb³⁺ and the characteristic emission bands of these ions under 980 nm irradiation. In contrast, only the 497 nm upconversion emission band from Yb³⁺ is observed in the MOFs doped with Tm³⁺, Pr³⁺, Sm³⁺, and Dy³⁺. The effects of different dopants on the efficiency of cooperative luminescence were established and will provide guidance for the exploitation of Ln-MOFs exhibiting upconversion.     

Chemosensing hybrid materials: Chalcones‐functionalized cage‐like mesoporous SBA‐16 material for highly selective detection of toxic metal ions

Highly selective and low‐cost optical nanosensors of organic–inorganic hybrid materials for heavy metal ions detection have been prepared via the functionalization of mesoporous silica (SBA‐16) with chalcone fluorescent chromophores. The successful attachment of organic chalcone moieties and preservation of original structure of SBA‐16 after the anchoring process were confirmed by extensive characterizations using various techniques like Fourier transform infrared and UV–visible spectroscopies, transmission electron microscopy, nitrogen adsorption–desorption isotherms, low‐angle X‐ray diffraction and thermogravimetric analysis. The colorimetric behaviour, selectivity and sensitivity were also investigated. The optical nanosensors respond selectively to heavy metal ions, such as Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Hg²⁺, with observable colour changes in 0.01 M Tris–HCl aqueous buffer solution. Also, the optical sensing ability of the investigated nanosensors to the mentioned metal ions was investigated using steady‐state absorption and emission techniques. Significant increase in the absorption spectra and a static quenching in the emission spectra are observed upon adding various concentrations of the studied metal ions. The spectral changes as well as the observable colour changes suggest that the investigated nanosensors are suitable for simple, economic, online analysis and remote design of these toxic metal ions with fast kinetic responses. Finally, the low detection limits for all the studied metals are in good agreement with those recommended by both the US Environmental Protection Agency and World Health Organization, except for Hg²⁺ and Cd²⁺, indicating that the investigated nanosensors have hypersensitivity, selectivity and better recognition for all the studied metal ions.     

Cooperative Sensitization Upconversion in Solution Dispersions of Co-Crystal Assemblies of Mononuclear Yb3+ and Eu3+ Complexes

Lanthanide upconversion luminescence in nanoparticles has prompted continuous breakthroughs in information storage, temperature sensing, and biomedical applications, among others. Achieving upconversion luminescence at the molecular scale is still a critical challenge in modern chemistry. In this work, we explored the upconversion luminescence of solution dispersions of co-crystals composed of discrete mononuclear Yb(DBM)₃Bpy and Eu(DBM)₃Bpy complexes (DBM: dibenzoylmethane, Bpy: 2,2′-bipyridine). The 613 nm emission of Eu³⁺ was observed under excitation of Yb³⁺ at 980 nm. From the series of molecular assemblies studied, the most intense luminescence was obtained for a 1 : 1 molar ratio of Yb³⁺ : Eu³⁺, resulting in a high quantum yield of 0.67 % at 2.1 W cm⁻². The structure and energy transfer mechanism of the assemblies were fully characterized. This is the first example of an Eu³⁺-based upconverting system composed of two discrete mononuclear lanthanide complexes present as co-crystals in non-deuterated solution.     

Effect of Toxic Metal Ions on Photosensitized Singlet Oxygen Generation for Photodegradation of Polyaromatic Hydrocarbon Derivatives and Inactivation of Escherichia coli

Here, we report an experimental study of the effect of toxic metal ions on photosensitized singlet oxygen generation for photodegradation of PAH derivatives, Anthracene‐9,10‐dipropionic acid disodium salt (ADPA) and 1,5‐dihydroxynapthalene (DHN) and photoinactivation of Escherichia coli bacteria by using cationic meso‐tetra(N‐methyl‐4‐pyridyl)porphine tetrachloride (TMPyP) as a singlet oxygen photosensitizer. Three s‐block metals ions, such as Na⁺, K⁺ and Ca²⁺ and five toxic metals such as Cd²⁺, Cu²⁺, Hg²⁺, Zn²⁺ and Pb²⁺ were studied. The s‐block metal ions showed no change in the rate of photodegradation of ADPA or DHN by TMPyP, whereas a dramatic change in the photodegradation of ADPA and DHN was observed in the presence of toxic metals. The maximum photodegradation rate constants of ADPA and DHN were observed for Cd²⁺ ions [(3.91 ± 0.20) × 10^?3 s^?1 and (7.18 ± 0.35) × 10^?4 s^?1, respectively]. Strikingly, the photodegradation of ADPA and DHN was almost completely inhibited in the presence of Hg²⁺ ions and Cu²⁺ ions. A complete inhibition of growth of E. coli was observed upon visible light irradiation of E. coli solutions with TMPyP and toxic metal ions particularly, Cd²⁺, Hg²⁺, Zn²⁺ and Pb²⁺ ions, except for Cu²⁺ ions where a significantly slow inhibition of E. coli's growth was observed.     

Retention behavior of nickel, copper, cadmium and zinc ions from aqueous solutions on silico-titanate and silico-antimonate used as inorganic ion exchange materials

Silico-titanate (SiTi) and silico-antimonate (SiSb) have been synthesized and characterized using X-ray diffraction patterns, infrared and thermal analysis techniques. Divalent cations such as Ni²⁺, Cd²⁺, Zn²⁺ and Cu²⁺ in the pH range 2 to 8 have been exchanged with the exchangeable active sites of the exchangers using a batch technique. From the results obtained, the equilibrium capacities and distribution coefficient values were calculated indicating high selectivity values for Ni²⁺, Cd²⁺, Zn²⁺ and Cu²⁺ ions on silico-titanate and silico-antimonate compared to other titanates and antimonates. Also SiTi and SiSb show high chemical stability in H₂O, nitric and hydrochloric acids. All these results support the suitability of the prepared materials for the removal of the toxic metals concerned from waste waters. Based on the results obtained, practical separation experiments for the above mentioned cations on SiTi and SiSb columns from aqueous waste solutions were carried out.     

3,4‐Phenylenedioxythiophene (PheDOT) Based Hole‐Transporting Materials for Perovskite Solar Cells

Two new electron‐rich molecules based on 3,4‐phenylenedioxythiophene (PheDOT) were synthesized and successfully adopted as hole‐transporting materials (HTMs) in perovskite solar cells (PSCs). X‐ray diffraction, absorption spectra, photoluminescence spectra, electrochemical properties, thermal stabilities, hole mobilities, conductivities, and photovoltaic parameters of PSCs based on these two HTMs were compared with each other. By introducing methoxy substituents into the main skeleton, the energy levels of PheDOT‐core HTM were tuned to match with the perovskite, and its hole mobility was also improved (1.33×10^?4 cm² V^?1 s^?1, being higher than that of spiro‐OMeTAD, 2.34×10^?5 cm² V^?1 s^?1). The PSC based on MeO‐PheDOT as HTM exhibits a short‐circuit current density (J_sc) of 18.31 mA cm^?2, an open‐circuit potential (V_oc) of 0.914 V, and a fill factor (FF) of 0.636, yielding an encouraging power conversion efficiency (PCE) of 10.64 % under AM 1.5G illumination. These results give some insight into how the molecular structures of HTMs affect their performances and pave the way for developing high‐efficiency and low‐cost HTMs for PSCs.     

Preconcentration and Determination of Toxic Metals by A New Calix[4]arene Bonded HPLC Column

The present study deals with the preconcentration and determination of toxic metal ions using p-tetranitrocalix[4]arene (3) appended silica-based new HPLC column. The synthesized material was characterized using Fourier transform infrared and scanning electron microscopy techniques. The sorption characteristics of the HPLC column were investigated for three toxic metals (Cd²⁺, Hg²⁺ and Pb²⁺) in column agreement. The experiments were performed in five steps that were monitored using a UV–visible diode-array detector. However, all the HPLC experimental results were reconfirmed by using atomic absorption spectrophotometer. The effect of concentration on the sorption efficiency of the column was evaluated for all the three metals and the data obtained were investigated using Langmuir, Freundlich and Dubinin–Redushkevich (D–R) sorption isotherms. The value of coefficient of determination (R ²), i.e. 0.99, suggested that the Freundlich sorption isotherm was found to be the best-fit model for all the three toxic metal ions, whereas, mean free energy was calculated from the D–R isotherm as 5.3, 5.7, and 5.8 kJ/mol for Pb²⁺, Cd²⁺, and Hg²⁺, respectively; suggesting that physical electrostatic force is involved in the sorption process. The developed method was validated for system efficiency, accuracy, and precision.     

Preconcentration and Determination of Toxic Metals by A New Calix[4]arene Bonded HPLC Column

The present study deals with the preconcentration and determination of toxic metal ions using p-tetranitrocalix[4]arene (3) appended silica-based new HPLC column. The synthesized material was characterized using Fourier transform infrared and scanning electron microscopy techniques. The sorption characteristics of the HPLC column were investigated for three toxic metals (Cd²⁺, Hg²⁺ and Pb²⁺) in column agreement. The experiments were performed in five steps that were monitored using a UV–visible diode-array detector. However, all the HPLC experimental results were reconfirmed by using atomic absorption spectrophotometer. The effect of concentration on the sorption efficiency of the column was evaluated for all the three metals and the data obtained were investigated using Langmuir, Freundlich and Dubinin–Redushkevich (D–R) sorption isotherms. The value of coefficient of determination (R ²), i.e. 0.99, suggested that the Freundlich sorption isotherm was found to be the best-fit model for all the three toxic metal ions, whereas, mean free energy was calculated from the D–R isotherm as 5.3, 5.7, and 5.8 kJ/mol for Pb²⁺, Cd²⁺, and Hg²⁺, respectively; suggesting that physical electrostatic force is involved in the sorption process. The developed method was validated for system efficiency, accuracy, and precision.

     

A multifunctional upconversion nanoparticles probe for Cu2+ sensing and pattern recognition of biothiols

Lanthanide-doped upconversion nanoparticles (Ln-UCNPs) are a new type of nanomaterials with excellent fluorescence properties, which are well applied in fluorescent biosensing. Herein we developed a multifunctional probe based on the surface engineering of core-shell structure UCNPs with polyacrylic acid (PAA). The developed PAA/UCNPs probe could be highly selective to detect and respond to Cu²⁺ at different pH. Cu²⁺ could easily combine with the carboxylate anion of PAA to quench the fluorescence of UCNPs. Therefore, we creatively proposed a fluorescent array sensor (PAA/UCNPs-Cu²⁺), in which the same material acted as the sensing element by coupled with pH regulation for pattern recognition of 5 thiols. It could also easily identify the chiral enantiomer of cystine (l-Cys-and d-Cys), and distinguish their mixed samples with different concentrations, and more importantly, it could be combined with urine samples to detect actual level of homocysteine (Hcys) to provide a new solution for judging whether the human body suffers from homocystinuria.     

Non-conductive nanomaterial enhanced electrochemical response in stripping voltammetry: The use of nanostructured magnesium silicate hollow spheres for heavy metal ions detection

Nanostructured magnesium silicate hollow spheres, one kind of non-conductive nanomaterials, were used in heavy metal ions (HMIs) detection with enhanced performance for the first time. The detailed study of the enhancing electrochemical response in stripping voltammetry for simultaneous detection of ultratrace Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ was described. Electrochemical properties of modified electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The operational parameters which have influence on the deposition and stripping of metal ions, such as supporting electrolytes, pH value, and deposition time were carefully studied. The anodic stripping voltammetric performance toward HMIs was evaluated using square wave anodic stripping voltammetry (SWASV) analysis. The detection limits achieved (0.186 nM, 0.247 nM, 0.169 nM and 0.375 nM for Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺) are much lower than the guideline values in drinking water given by the World Health Organization (WHO). In addition, the interference and stability of the modified electrode were also investigated under the optimized conditions. An interesting phenomenon of mutual interference between different metal ions was observed. Most importantly, the sensitivity of Pb²⁺ increased in the presence of certain concentrations of other metal ions, such as Cd²⁺, Cu²⁺ and Hg²⁺ both individually and simultaneously. The proposed electrochemical sensing method is thus expected to open new opportunities to broaden the use of SWASV in analysis for detecting HMIs in the environment.     

Strong up conversion photoluminescence in Er3+ doped Bi4Ti3O12 ferroelectric materials prepared by sol–gel method

Bi₄Ti₃O₁₂ (BTO) doped with different concentrations of Er³⁺ was prepared using sol–gel method. Their structures and surface morphology were examined by X-ray diffraction, Raman spectroscopy and scanning electron microscopy, respectively. All XRD peaks can be indexed according to orthorhombic BTO phase, which is consistent with Raman measurement results. Strong upconversion green luminescence is observed in the Er³⁺ doped BTO powders pumped by 980 nm at room temperature. Three upconversion emission bands centered at about 525, 550 and 662 nm are due to the radiative relaxation of Er³⁺ from ²H_11/2, ⁴S_3/2 and ⁴F_9/2 to the ground level ⁴I_15/2, respectively. The upconversion emission mechanism of the samples were identified and analyzed. In addition, the doped BTO ceramics show obvious hysteresis loops, demonstrating the ferroelectric properties of the samples. These results illustrate the potential of this class of materials for photonic applications in optoelectronics devices.     

Single precursor-based luminescent nitrogen-doped carbon dots and their application for iron (III) sensing

Iron III (Fe³⁺) sensing is of great importance for monitoring its levels in different environmental and biological systems since its levels are tightly associated with many environmental concerns and serious diseases. Compared to the ordinary Fe³⁺ detection methods involving the sophisticated and expensive instruments, the use of fluorescent materials with short response time and low cost attracts much attention. Amongst a variety of fluorescent materials, a nitrogen-doped carbon dot (N-CD) is emerging as promising luminescent materials for biosensing due to their superior photoluminescent properties, good water solubility, and biocompatibility. Herein, N-CD prepared via a green and cost-effective one-pot hydrothermal method using a new single precursor N-(2-hydroxyethyl) ethylenediamine triacetic acid (HEDTA) as both the carbon and nitrogen sources is reported. The blue fluorescent emission of N-CDs is quenched by the addition of Fe³⁺, and the quenching intensity is concentration dependent in the wide range (0.76–400 μM) with a detection limit of 0.16 μM. The quantum yield for the as prepared N-CDs is 14.17%. The N-CDs also show a high selectivity for Fe³⁺ chelation amongst a range of biological metal ions. The fluorescent quenching is attributed to the formation of the coordinate covalent bonds between the Fe³⁺ and N-CDs, and the mechanism is proved to be a static type on the basis of the photoluminescence lifetime and the temperature-dependent fluorescent intensity change. Our eco-friendly and simple strategy will benefit the application of CDs in various fields.     

Synthesis and characterization of nanoporous NaYF4:Yb3+, Tm3+@SiO2 nanocomposites

Novel upconversion nanocomposites with nanoporous structure were presented in this paper. Silica-coated cubic NaYF₄:Yb³⁺, Tm³⁺ nanoparticles were first prepared. After annealing, monodisperse cubic/hexagonal mixed phases NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanoparticles were obtained, and the NaYF₄:Yb³⁺, Tm³⁺ cores became nanoporous. To the best of our knowledge, the nanoporous structure in NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanocomposites was observed for the first time. They demonstrate increased upconversion emission compared with unannealed dense NaYF₄:Yb³⁺, Tm³⁺ nanoparticles due to the appearance of the hexagonal NaYF₄:Yb³⁺, Tm³⁺. The silica shell not only makes the nanocomposites possess bio-affinity but also protects the NaYF₄:Yb³⁺, Tm³⁺ cores from aggregating and growing up. Thus the upconversion, nanoporous and bio-affinity properties were combined into one single nanoparticle. The nanocomposites have been characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), small angle X-ray diffraction (SAXRD) and emission spectroscopy. These multifunctional nanocomposites are expected to find applications in biological fields, such as biolabels, drug storage and delivery.     

Natural and Synthetic Polymer Scaffolds Comprising Upconversion Nanoparticles as a Bioimaging Platform for Tissue Engineering

Modern biocompatible materials of both natural and synthetic origin, in combination with advanced techniques for their processing and functionalization, provide the basis for tissue engineering constructs (TECs) for the effective replacement of specific body defects and guided tissue regeneration. Here we describe TECs fabricated using electrospinning and 3D printing techniques on a base of synthetic (polylactic-co-glycolic acids, PLGA) and natural (collagen, COL, and hyaluronic acid, HA) polymers impregnated with core/shell β-NaYF₄:Yb³⁺,Er³⁺/NaYF₄ upconversion nanoparticles (UCNPs) for in vitro control of the tissue/scaffold interaction. Polymeric structures impregnated with core/shell β-NaYF₄:Yb³⁺,Er³⁺/NaYF₄ nanoparticles were visualized with high optical contrast using laser irradiation at 976 nm. We found that the photoluminescence spectra of impregnated scaffolds differ from the spectrum of free UCNPs that could be used to control the scaffold microenvironment, polymer biodegradation, and cargo release. We proved the absence of UCNP-impregnated scaffold cytotoxicity and demonstrated their high efficiency for cell attachment, proliferation, and colonization. We also modified the COL-based scaffold fabrication technology to increase their tensile strength and structural stability within the living body. The proposed approach is a technological platform for “smart scaffold” development and fabrication based on bioresorbable polymer structures impregnated with UCNPs, providing the desired photoluminescent, biochemical, and mechanical properties for intravital visualization and monitoring of their behavior and tissue/scaffold interaction in real time.     

Intercrossed Carbon Nanorings with Pure Surface States as Low‐Cost and Environment‐Friendly Phosphors for White‐Light‐Emitting Diodes

As an important energy‐saving technique, white‐light‐emitting diodes (W‐LEDs) have been seeking for low‐cost and environment‐friendly substitutes for rare‐earth‐based expensive phosphors or Pd²⁺/Cd²⁺‐based toxic quantum dots (QDs). In this work, precursors and chemical processes were elaborately designed to synthesize intercrossed carbon nanorings (IC‐CNRs) with relatively pure hydroxy surface states for the first time, which enable them to overcome the aggregation‐induced quenching (AIQ) effect, and to emit stable yellow‐orange luminescence in both colloidal and solid states. As a direct benefit of such scarce solid luminescence from carbon nanomaterials, W‐LEDs with color coordinate at (0.28, 0.27), which is close to pure white light (0.33, 0.33), were achieved through using these low‐temperature‐synthesized and toxic ion‐free IC‐CNRs as solid phosphors on blue LED chips. This work demonstrates that the design of surface states plays a crucial role in exploring new functions of fluorescent carbon nanomaterials.     

Metal Ions as Activators of Hypoxia Inducible Factor

Activation of antihypoxic program under the action of a number of transition and heavy metals has been studied using cell-based HIF1 ODD-luc and HRE-luc reporters. It has been demonstrated that Au³⁺, Pb²⁺, Sn²⁺, Hg²⁺ are weak HIF1 ODD-luc activators, likely reflecting their weak competition for the ironbinding site in the active center of HIF prolyl hydroxylase. Metals capable of replacing iron–Mn²⁺, Zn²⁺, Cu²⁺ и Ni²⁺–activate at high submillimolar concentrations, which indicates low permeability of the cell membrane for transition metals. The highest activation is observed for Co²⁺ and Cd²⁺, however, Cd²⁺ is highly toxic even at 10 μM, in contrast to Co²⁺, which activates both reporters without toxicity signs up to 25 μM for 24 h. A significant activation by Co²⁺ is observed already in low micromolar range of concentrations, which can be recommended for use in hypoxia mimicking.