Selective determination of Ag+ in the presence of Cd2+, Hg2+ and Cu2+ based on their different interactions with gold nanoclusters

In this work, a fluorescence method was developed for selective detection of Ag⁺ in the presence of Cd²⁺, Hg²⁺, and Cu²⁺ based on gold nanoclusters (AuNCs). That is, bovine serum albumin (BSA) templated AuNCs with double emission peaks were synthesized using BSA as a protective agent. AuNCs with uniform distribution and average size between 2.0 and 2.2 nm were synthesized using a green and simple method, and showed bright orange-red fluorescence under ultraviolet light. AuNCs have two emission peaks at 450 nm and 630 nm with an excitation wavelength of 365 nm. Under alkaline conditions, Cd²⁺ can combine with the surface sulfhydryl groups of BSA–AuNCs to form Cd–S bonds, which cause AuNCs to aggregate, resulting in an increase in fluorescence intensity at 630 nm. Conversely, due to the d¹⁰–d¹⁰ metal affinity interaction, the addition of Hg²⁺ can reduce the fluorescence peak at 630 nm. Ag⁺ was reduced to Ag⁰ by gold nuclei in AuNCs, forming a stable hybrid Au@ AgNCs species with blue-shifted and enhanced fluorescence. Finally, the paramagnetic behavior of Cu²⁺ combined with BSA causes the excited electrons of the gold cluster to lose their energy via ISC, eventually leading to simultaneous quenching of the two emission peaks. The results show that the limit of detection (LOD) of Ag⁺, Hg²⁺, Cd²⁺ and Cu²⁺ is 1.19 μM, 3.39 μM, 1.83 μM and 5.95 μM, respectively.

A fluorescence method was developed for selective detection of Ag⁺ in the presence of Cd²⁺, Hg²⁺, and Cu²⁺ based on gold nanoclusters. The limit of detection for Ag⁺, Hg²⁺, Cd²⁺ and Cu²⁺ is 1.19 μM, 3.39 μM, 1.83 μM and 5.95 μM, respectively.     

Porous BMTTPA–CS–GO nanocomposite for the efficient removal of heavy metal ions from aqueous solutions

In this study, a stable, cost-effective and environmentally friendly porous 2,5-bis(methylthio)terephthalaldehyde–chitosan–grafted graphene oxide (BMTTPA–CS–GO) nanocomposite was synthesized by covalently grafting BMTTPA–CS onto the surfaces of graphene oxide and used for removing heavy metal ions from polluted water. According to well-established Hg²⁺–thioether coordination chemistry, the newly designed covalently linked stable porous BMTTPA–CS–GO nanocomposite with thioether units on the pore walls greatly increases the adsorption capacity of Hg²⁺ and does not cause secondary pollution to the environment. The results of sorption experiments and inductively coupled plasma mass spectrometry measurements demonstrate that the maximum adsorption capacity of Hg²⁺ on BMTTPA–CS–GO at pH 7 is 306.8 mg g⁻¹, indicating that BMTTPA–CS–GO has excellent adsorption performance for Hg²⁺. The experimental results show that this stable, environmentally friendly, cost-effective and excellent adsorption performance of BMTTPA–CS–GO makes it a potential nanocomposite for removing Hg²⁺ and other heavy metal ions from polluted water, and even drinking water. This study suggests that covalently linked crucial groups on the surface of carbon-based materials are essential for improving the adsorption capacity of adsorbents for heavy metal ions.

Novel porous BMTTPA–CS–GO nanocomposites are prepared by covalently grafting BMTTPA–CS onto GO surfaces, and used for efficient removal of heavy metal ions from polluted water.     

Nanochannel-based sensor for the detection of lead ions in traditional Chinese medicine

Lead ions (Pb²⁺) are used in the quality control of traditional Chinese medicine (TCM) preparations because they are highly toxic to human health. At present, sophisticated analytical instrumentation and complicated procedures for sample analysis are needed for the determination of Pb²⁺. Herein, a simple, fast, and sensitive peptide-modified nanochannel sensor to detect Pb²⁺ in TCM is reported, which is based on a Pb²⁺-specific peptide modified porous anodized aluminum membrane (PAAM). This peptide-based nanochannel clearly has the highest selectivity for Pb²⁺ when compared to other heavy metal ions, including As²⁺, Cd³⁺, Co²⁺, Cr²⁺, Cu²⁺, Fe³⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, and Zn²⁺. Based on linear ranges from 0.01 to 0.16 μM and 10 to 100 μM, the detection limit was calculated to be 0.005 μM. Moreover, this peptide-based nanochannel sensor was successfully used to detect Pb²⁺ in complex TCM samples. In addition, when compared with the gold standard atomic absorption spectrophotometry (AAS) method, the recovery of the peptide-modified nanochannel sensor was between 87.7% and 116.8%. The experimental results prove that this new sensor is able to achieve accurate detection of Pb²⁺ in TCM samples. Thus, this sensor system could provide a simple assay for sensitive and selective detection of Pb²⁺ in TCM, thereby showing great potential in the practical application for the quality control of heavy metals in TCM.

The nanochannel-based sensor is able to achieve detection of Pb²⁺ in TCM samples.     

Role of graphene oxide in mitigated toxicity of heavy metal ions on Daphnia magna

Graphene oxide (GO) is increasingly used and inevitably released into aquatic environments, facilitating its interaction with traditional pollutants such as heavy metal ions. However, the potential effect of GO on the toxicity of heavy metal ions to aquatic animals is unknown. This work aims to assess the toxicity of heavy metal ions (Cu(ii), Cd(ii), and Zn(ii)) on Daphnia magna (D. magna) in the presence of GO. GO nanoparticles remarkably reduced the concentrations of heavy metal ions by adsorption and decreased the metal accumulation in D. magna. The maximum desorption rate of heavy metal ions from metal-adsorbed GO was below 5%. At pH 7.8, with addition of 2 mg L⁻¹ GO, the 72 h median lethal concentration (LC₅₀) values of Cu(ii), Cd(ii), and Zn(ii) were increased from 14.3, 38, and 780 μg L⁻¹ to 36.6, 72, and 1010 μg L⁻¹, respectively. The analyses of oxidative stress indicators suggested that the oxidative damage to D. magna by heavy metal ions was reduced after addition of GO at pH 7.8. Moreover, a higher pH level in the growing range (6.5 to 8.5) of D. magna led to weaker toxicity of metal ions with GO addition due to more adsorption and less bioaccumulation. The results revealed the role of GO nanoparticles in the mitigated toxicity of heavy metal ions in the aquatic environment.

Graphene oxide nanoparticles mitigates the biotoxicity of heavy metal ions (Cu(ii), Cd(ii), and Zn(ii)) on aquatic animals (Daphnia magna).     

Sulfhydryl functionalized carbon quantum dots as a turn-off fluorescent probe for sensitive detection of Hg2+

Mercury ion (Hg²⁺) is one of the most toxic heavy metal ions and lowering the detection limit of Hg²⁺ is always a challenge in analytical chemistry and environmental analysis. In this work, sulfhydryl functionalized carbon quantum dots (HS-CQDs) were synthesized through a one-pot hydrothermal method. The obtained HS-CQDs were able to detect mercury ions Hg²⁺ rapidly and sensitively through fluorescence quenching, which may be ascribed to the formation of nonfluorescent ground-state complexes and electron transfer reaction between HS-CQDs and Hg²⁺. A modification of the HS-CQD surface by –SH was confirmed using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The HS-CQDs sensing system obtained a good linear relationship over a Hg²⁺ concentration ranging from 0.45 μM to 2.1 μM with a detection limit of 12 nM. Delightfully, the sensor has been successfully used to detect Hg²⁺ in real samples with satisfactory results. This means that the sensor has the potential to be used for testing actual samples.

Schematic presentation of the synthesis of HS-CQDs and the application as a “turn-off” fluorescent probe for Hg²⁺ detection.     

Metal ion size profoundly affects H3glyox chelate chemistry

The bisoxine hexadentate chelating ligand, H₃glyox was investigated for its affinity for Mn²⁺, Cu²⁺ and Lu³⁺ ions; all three metal ions are relevant with applications in nuclear medicine and medicinal inorganic chemistry. The aqueous coordination chemistry and thermodynamic stability of all three metal complexes were thoroughly investigated by detailed DFT structure calculations and stability constant determination, by employing UV in-batch spectrophotometric titrations, giving pM values (pM = −log[Mⁿ⁺]_free when [Mⁿ⁺] = 1 μM, [L] = 10 μM at pH 7.4 and 25 °C) – pCu (25.2) > pLu (18.1) > pMn (12.0). DFT calculated structures revealed different geometries and coordination preferences of the three metal ions; notable was an inner sphere water molecule in the Mn²⁺ complex. H₃glyox labels [^52gMn]Mn²⁺, [⁶⁴Cu]Cu²⁺ and [¹⁷⁷Lu]Lu³⁺ at ambient conditions with apparent molar activities of 40 MBq μmol⁻¹, 500 MBq μmol⁻¹ and 25 GBq μmol⁻¹, respectively. Collectively, these initial investigations provide insight into the effects of metal ion size and charge on the chelation with the hexadentate H₃glyox and indicate that further investigations of the Mn²⁺–H₃glyox complex in ^52g/55Mn-based bimodal imaging might be worthwhile.

The bisoxine hexadentate chelating ligand, H₃glyox was investigated for its affinity for Mn²⁺, Cu²⁺ and Lu³⁺ ions; all three metal ions are relevant with applications in nuclear medicine and medicinal inorganic chemistry.     

Single-atom niobium doped BCN nanotubes for highly sensitive electrochemical detection of nitrobenzene

Herein, single-atom niobium-doped boron–carbon–nitrogen nanotubes (SANb-BCN) were synthesized and utilized to fabricate an electrochemical sensor for the detection of nitrobenzene (NB), an environmental pollutant. SANb-BCN were characterized through scanning transmission electron microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, and Raman spectroscopy. The Nb-BNC material modified on a glassy carbon electrode (GCE) showed an excellent electrochemical response behavior toward NB. The SANb-BCN-modified GCE (SANb-BCN/GCE) gave rise to a prominent NB reduction peak at −0.6 V, which was positively shifted by 120 mV from the NB reduction peak of the bare GCE. Furthermore, the NB peak current (55.74 μA) obtained using SANb-BCN/GCE was nearly 42-fold higher than that using the bare GCE (1.32 μA), indicating that SANb-BCN/GCE is a highly sensitive electrochemical sensor for NB. An ultralow limit of detection (0.70 μM, S/N = 3) was also achieved. Furthermore, the SANb-BCN/GCE sensor was found to possess favorable anti-interference ability during NB detection; thus, the presence of various organic and inorganic coexisting species, including Mg²⁺, Cr⁶⁺, Cu²⁺, K⁺, Ca²⁺, NH⁴⁺, Cd²⁺, urea, 1-bromo-4-nitrobenzene, 3-hydroxybenzoic, terephthalic acid, 1-iodo-4-nitrobenzene, and toluene, minimally affected the NB detection signal. Notably, the SANb-BNC sensor material exhibited high sensitivity and specificity toward detection of NB in environmental samples. Thus, the use of the proposed sensor will serve as an effective alternative method for the identification and treatment of pollutants.

Herein, single-atom niobium-doped boron–carbon–nitrogen nanotubes (SANb-BCN) were synthesized and utilized to fabricate an electrochemical sensor for the detection of nitrobenzene (NB), an environmental pollutant.     

Metal oxide QD based ultrasensitive microsphere fluorescent sensor for copper,chromium and iron ions in water

Herein we developed a rapid, cheap, and water-soluble ultra-sensitive ZnO quantum dot (QD) based metal sensor for detecting different hazardous metal ions up to the picomolar range in water. Various spectroscopic and microscopic techniques confirmed the formation of 2.15 ± 0.46 μm of ZnO QD conjugated CMC microspheres (ZCM microspheres) which contain 5.5 ± 0.5 nm fluorescent zinc oxide (ZnO) QDs. Our system, as a promising sensor, exhibited excellent photostability and affinity towards various heavy metal ions. The detection limits were calculated to be 16 pM for Cu²⁺ and 0.18 nM for Cr⁶⁺ ions which are better than previously reported values. The simple fluorescence ‘turn off’ property of our ZCM microsphere sensor system can serve a two-in-one purpose by not only detecting the heavy metals but also quantifying them. Nonetheless, pattern recognition for different heavy metals helped us to detect and identify multiple heavy metal ions. Finally, their practical applications on real samples also demonstrated that the ZCM sensor can be effectively utilized for detection of Cr⁶⁺, Fe³⁺, Cu²⁺ present in the real water samples. This study may inspire future research and design of target fluorescent metal oxide QDs with specific functions.

Herein we developed a rapid, cheap, and water-soluble ultra-sensitive ZnO quantum dot (QD) based metal sensor for detecting different hazardous metal ions up to the picomolar range in water.     

Studies on a glutathione coated hollow ZnO modified glassy carbon electrode; a novel Pb(ii) selective electrochemical sensor

Herein, we report the electrochemical detection of heavy metal ions such as Pb(ii), Cd(ii) and Hg(ii) ions while using glutathione coated hollow ZnO modified glassy carbon electrode (Glu-h-ZnO/GCE). An excellent voltammetric response of the modified electrode towards these metal ions was observed by different voltammetric techniques. Among the different target metal ions, a selective electrochemical response (sensitivity = 4.57 μA μM⁻¹) for the detection of Pb(ii) ions was obtained with differential pulse voltammetric (DPV) measurements. Besides, under optimal experimental conditions and in the linear concentration range of 2–18 μM, a very low detection limit of 0.42 μM was obtained for Pb(ii) ion. The observed electrochemical behaviour of Glu-h-ZnO/GCE towards these metal ions is in conformity with the band gap of the composite in the presence of various test metal ions. The band gap studies of the composite and various “Composite-Metal Ion” systems were obtained by reflectance as well as by computational methods where results are in close agreement, justifying the observed electrochemical behaviour of the systems. The lowest band gap value of the “Composite-Pb” system may be the reason for the excellent electrochemical response of the Glu-h-ZnO modified GCE towards the detection of Pb(ii) ion.

Decrease in the band gap of the "Composite-Metal" systems in comparison to pure composite is a key factor in the electrochemical detection of heavy metal ions such as Pb(ii), Cd(ii) and Hg(ii) ions while using glutathione coated hollow ZnO modified glassy carbon electrode (Glu-h-ZnO/GCE).     

Ionic liquid-immobilized silica gel as a new sorbent for solid-phase extraction of heavy metal ions in water samples

In the current study, we have developed a solid-phase extraction (SPE) method with novel C18-alkylimidazolium ionic liquid immobilized silica (SiO₂–(CH₂)₃–Im–C₁₈) for the preconcentration of trace heavy metals from aqueous samples as a prior step to their determination by inductively coupled plasma mass spectrometry (ICPMS). The material was characterized by Fourier-transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), Energy-Dispersive X-ray Spectroscopy (EDS), and Brunauer–Emmett–Teller (BET) analysis. A mini-column packed with SiO₂–(CH₂)₃–Im–C₁₈ sorbent was used for the extraction of the metal ions complexed with 1-(2-pyridylazo)-2-naphthol (PAN) from the water sample. The effects of pH, PAN concentration, length of the alkyl chain of the ionic liquid, eluent concentration, eluent volume, and breakthrough volume have been investigated. The SiO₂–(CH₂)₃–Im–C₁₈ allows the isolation and preconcentration of the heavy metal ions with enrichment factors of 150, 60, 80, 80, and 150 for Cr³⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺, respectively. The limits of detection (LODs) for Cr³⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺ were 0.724, 11.329, 4.571, 0.112, and 0.819 μg L⁻¹, respectively with the relative standard deviation (RSD) in the range of 0.941–1.351%.

Novel C18-alkylimidazolium ionic liquid immobilized silica (SiO₂–(CH₂)₃–Im–C₁₈) was synthesized through a four-step procedure. It showed high efficiency for the separation/preconcentration of trace heavy metal ions from aqueous samples.     

A bromine-catalysis-synthesized poly(3,4-ethylenedioxythiophene)/graphitic carbon nitride electrochemical sensor for heavy metal ion determination

In this paper, poly(3,4-ethylenedioxythiophene)/graphitic carbon nitride (PEDOT/g-C₃N₄) composites were prepared by the bromine catalysed polymerization (BCP) method with varying weight ratios of monomer to g-C₃N₄. For comparison, solid-state polymerization (SSP) and metal oxidative polymerization (MOP) methods were also used for the synthesis of PEDOT/g-C₃N₄ composites. Electrochemical determination of heavy metal ions (Cd²⁺ and Pb²⁺) was carried out by differential pulse voltammetry (DPV) on composite-modified glass carbon electrodes (GCEs), which were prepared by different methods. The obtained composites were analysed by Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible absorption spectroscopy (UV-vis), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that the bromine catalysed polymerization (BCP) method is an effective way to prepare the PEDOT/g-C₃N₄ composite, and the combination of PEDOT with g-C₃N₄ can improve the electrochemical activity of electrode materials. And, the composite from the BCP method modified electrode (PEDOT/10 wt% g-C₃N₄/GCE) exhibited the widest linear responses for Cd²⁺ and Pb²⁺, ranging from 0.06–12 μM and 0.04–11.6 μM with detection limits (S/N = 3) of 0.0014 μM and 0.00421 μM, respectively.

The PEDOT/g-C₃N₄ composite prepared by a Br₂-catalyzed polymerization method exhibited the widest linear electrochemical responses for Cd²⁺ and Pb²⁺.     

Bis-BODIPY linked-triazole based on catechol core for selective dual detection of Ag+ and Hg2+

Herein, we introduced a new chemosensor, bis-BODIPY linked-triazole based on catechol (BODIPY-OO) prepared by bridging two units of BODIPY fluorophore/triazole binding group with a catechol unit. A solution of this compound displayed 4- and 2-fold enhancements in fluorescence intensity after adding a mole equivalent amount of Ag⁺ and Hg²⁺ ions in methanol media, respectively. ¹H NMR titrations of BODIPY-OO with Ag⁺ and Hg²⁺ suggested that the triazole was involved in the recognition process. BODIPY-OO showed high sensitivity toward Ag⁺ and Hg²⁺ over other metal ions with detection limits of 0.45 μM and 1 μM, respectively. It can also distinguish Hg²⁺ from Ag⁺ by addition of an EDTA. This compound can therefore be employed as practical fluorescent probe for monitoring the presence of Ag⁺ and Hg²⁺ ions.

BODIPY–triazole–catechol combination serves as a “turn-on” fluorescent probe for dual detection and differentiation of Hg²⁺ and Ag⁺ ions.     

Super rapid removal of copper,cadmium and lead ions from water by NTA-silica gel

Copper (Cu²⁺), cadmium (Cd²⁺) and lead ions (Pb²⁺) are toxic to human beings and other organisms. In this study, a silica gel material modified with nitrilotriacetic acid (NTA-silica gel) was sensibly designed and prepared via a simple amidation procedure for the removal of Cu²⁺, Cd²⁺ and Pb²⁺ from water. The NTA-silica gels showed rapid removal performances for the three metal ions (Pb²⁺ (<2 min), Cu²⁺ and Cd²⁺ (<20 min)) with relatively high adsorption capacities (63.5, 53.14 and 76.22 mg g⁻¹ for Cu²⁺, Cd²⁺ and Pb²⁺, respectively). At the same concentration of 20 mg L⁻¹, the removal efficiencies of the three metals by the adsorbent ranged from 96% to 99%. The Freundlich and Langmuir models were utilized to fit the adsorption isotherms. The adsorption kinetics for the three metal ions was pseudo-second-order kinetics. The removal performance of the NTA-silica gels increased in a wide pH range (2–9) and maintained in the presence of competitive metal ions (Na⁺, Mg²⁺, Ca²⁺ and Al³⁺) with different concentrations. In addition, the NTA-silica gels were easily regenerated (washed with 1% HNO₃) and reused for 5 cycles with high adsorption capacity. This study indicates that the NTA-silica gel is a reusable adsorbent for the rapid, convenient, and efficient removal of Cu²⁺, Cd²⁺, and Pb²⁺ from contaminated aquatic environments.

A silica gel material modified with nitrilotriacetic acid (NTA-silica gel) was sensibly designed and prepared via a simple method for the super rapid removal of Cu²⁺, Cd²⁺ and Pb²⁺ from water.     

Visual artificial tongue for identification of various metal ions in mixtures and real water samples: a colorimetric sensor array using off-the-shelf dyes

A three-unit colorimetric sensor array in aim of detecting heavy metal ions has been constructed with two off-the-shelf dyes. Multivariate data analysis is performed using LDA and HCA to recognize colour change patterns based on both absorption spectra and RGB values from image scans. The sensor array is able to differentiate 15 metal ions not only in separate solutions, but also mixtures of 3, 5, and 7 different metal ions and real water samples.

A colorimetric sensor array was constructed to detect metal ions by pattern recognition based on image analysis and absorption spectra.

Pollution of heavy metal ions in water and soil has become a worldwide environmental concern, due to their great harm to animal and human health even at low concentrations. Hence, to identify and distinguish heavy metal ions both qualitatively and quantitatively has been a field of constant research interest. Traditional methods for heavy metal ion detection such as inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS) are, although accurate and most common, very unpractical for on-site testing. A more convenient technique is continuously under demand.Inspired by mammalian olfactory and gustatory systems, array-based sensing platforms, which use a series of cross-reactive sensors instead of specific probes, have emerged as promising alternatives due to their simple fabrication, flexibility, convenient data collection and analysis. Sensor arrays have been reported to distinguish combinations of many analytes and mixtures,^1,2 including physical stimuli (temperature, humidity, light), volatile organic compounds, explosives, toxic industrial chemicals, metal ions, flavonoids, biomolecules (biothiols, phosphates), pesticides, proteins, pathogens (virus, bacteria, fungi), cells, food and beverages (liquors, teas, milks, red wines, coffees, whiskies), herbal medicines, and disease biomarkers.^3–7For metal ions discrimination and quantification, there are different analytical methods to construct response patterns, including electrochemical methods,⁸ steady-state and time-resolved fluorescence spectroscopy,^9–14 UV-visible absorption spectroscopy,^15,16 and digital imaging and colour calibration.^17–21 Although most reported work successfully distinguished samples of single metal ions or mixtures of up to 3 metal ions, applicable methods for analysis of mixtures consisting of more than 3 metal ions as well as real water samples are still of great challenge.Herein, we developed a simple and cost-effective three-unit colorimetric sensor array (Scheme 1) using dithizone in two solvent conditions and resazurin (structures shown in Fig. 1), to distinguish 15 different metal ions, their mixtures, as well as real water samples. Pattern recognition based on data from absorption spectra as well as scanned-images was realized using linear discriminant analysis (LDA) and hierarchical clustering analysis (HCA).Open in a separate windowScheme 1Schematic illustration of colorimetric sensor array for detection of metal ions.Open in a separate windowFig. 1Structures of the two off-the-shelf dyes used in our sensor array.Dithizone was reported to show different colour responses towards diverse metal ions, including Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Mn²⁺ Ag⁺, Cd²⁺, and Hg²⁺.^22–24 Yet its application as a practical sensing molecule for metal ions is limited by its poor solubility in aqueous solution. CTAB was found to improve dithizone''s sensitivity for metal ions detection by favouring dissolution and interaction of dithizone with metal ions.^25,26 Moreover, the pH value of the solution was also reported to influence the property of dithizone, which causes distinct absorption spectral change and colour alteration.²⁷ Thus, we introduced NaOH to the second unit to generate multidimensional information. These different solvent conditions were screened with varying amounts of the surfactant CTAB and NaOH in HEPES for the optimization of dithizone sensing. Based on the results of preliminary experiments (Fig. S1^†), dithizone/CTAB (unit 1: 30 μM of dithizone and 90 μM of CTAB in HEPES buffer) and dithizone/CTAB/NaOH (unit 2: 30 μM of dithizone, 10 mM of CTAB, and 6 mM of NaOH in HEPES buffer) were used as two sensor units in the array. Resazurin was reported capable of discriminating different metal ions based on its voltammetric behaviour.²⁸ Thus we also included it in our sensor array as unit 3 (unit 3: 24.5 μM of resazurin in water).To test the proof-of-concept of the proposed three-unit sensor array, 15 metal ions (Pb²⁺, Ag⁺, Cr³⁺, Cd²⁺, Fe³⁺, As(iii), Zn²⁺, Ni²⁺, Cu²⁺, Mn²⁺, Ba²⁺, Al²⁺, Co²⁺, Sn²⁺, Hg²⁺, each at 5 μM), were selected as analytes. In our study, data were acquired by two different methods, using microplate reader to generate absorption spectral measurement, as well as using flatbed scanner to obtain RGB values in images. Compared to common colorimetric methods which require complex instrumentation, the latter method above demonstrated its convenience by its simple operation, straightforward visualization effect, and good sensitivity. Firstly, the real digital images of the three-unit sensor array upon adding single metal ions at the concentration of 5 μM were recorded by a commercially available and cheap flatbed scanner (Fig. 2A). For each well on the plate, the red, green, and blue (RGB) values from the centre were extracted using our program written in Python (version 3.7.3). The changes of RGB values with and without metal ions, ΔR, ΔG, and ΔB, were used to generate the false-colour images (Fig. 2B) and the corresponding heat map (Fig. 2C). Obvious colour change can be seen in the presence of Ni²⁺, As(iii), Hg²⁺, Co²⁺, Cu²⁺, Zn²⁺, Fe³⁺, Cr³⁺, while only subtle changes take place in the presence of Sn²⁺, Al³⁺, Cd²⁺, Ba²⁺. These different responses presumably result from the different interactions between the sensor units and metal ions.Open in a separate windowFig. 2Performance of sensor array to 15 metal ions single solutions based on colour image change pattern (A) colour image of sensor array recorded by flatbed scanner upon addition of metal ions. (B) Colour difference map of the sensor array. For purposes of visualization, the colour range of the difference map was expanded from 4 to 8 bits per colour (RGB range of 0–58 expanded to 0–255). (C) Heatmap derived from the difference map. (D) LDA 2D plot and (E) HCA dendrogram derived from the colour response pattern of sensor array. Concentration of all used metal ions is 5 μM.LDA was then applied to further digitize and visualize the colour change patterns. LDA is a statistical analysis method that can visually differentiate two or more kinds of objects or events based on their linear combination of features, selected so as to maximize the ratio of inter-class variance to intra-class variance. All metal ions at the concentration of 5 μM were tested using four replicate measurements to provide a training matrix of sixteen samples (fifteen metal ions + one control) × three sensors × three colour channels × four replicates. The resulting training data were analysed and processed through LDA using Sci-kit learn package (version 0.21.2) in Python and transformed into nine canonical scores, which account for 58.23%, 21.32%, 15.40%, 3.53%, 0.82%, 0.38%, 0.16%, 0.10%, 0.04% of variations, respectively. The first two factors accounted for 79.55% of the total variance and were used to construct the two-dimensional (2-D) discrimination plot (Fig. 2D). All the metal ions can be separately grouped with clarity, despite the relatively close between clusters of Al³⁺, Sn²⁺, Fe³⁺ and the high proximity between clusters of Pb²⁺ and Ba²⁺ (as shown in the magnified inset of Fig. 2D).HCA is a common method to build a hierarchy of clusters according to their similarity characterized by the Euclidean distance. Unlike LDA, in which only a few most significant factors are used to do the visualization (in this case, the first three factors accounted for the 91.12% variance of the total), HCA takes all the features into consideration when computing the Euclidean distance among samples. As shown in Fig. 2E, HCA resulted in clear discrimination of 15 metal ions and the control with no misclassification, confirming that the proposed sensor array has strong discrimination ability of these 15 metal ions.Then “leave-one-out” cross-validation was used to evaluate the prediction ability of the LDA classifier. The training set was prepared by removing each sample one at a time, and the LDA model was built on the “leave-one-out” training set. Then the removed sample was reclassified using the LDA model. According to the classification result, the percentage of correct classification by the LDA model would be calculated. In this study, leave-one-out cross-validation for the LDA classification mode showed 100% accuracy for prediction of 15 metal ions at the concentration of 5 μM.As a comparison, UV-vis absorption spectra of the array upon adding these metal ions at the same concentrations were measured using a microplate reader and the response patterns were analysed as well. The patterns of the absorbance change were obtained as (A₀ − A)/A₀, in which A₀ and A are the absorbance without and with metal ions, respectively, in the following equation in specific wavelength range for each sensor unit. As shown in Fig. S2,^† three units responded differently to addition of different metal ions. In order to maximize the spectral information as well as minimize the influence of noise, 61 wavelengths (350–650 nm, every 5 nm) of the first unit, and 55 wavelengths (350–620 nm, every 5 nm) of the other two units were selected for further analysis respectively. Similarly, LDA, “leave-one-out” cross validation, and HCA were applied to characterize the absorbance change patterns. All samples were completely separated into different clusters (Fig. S3^†), including the clusters of Pb²⁺ and Ba²⁺, which were in high proximity in the RGB data analysis. The improved classification performance presumably arose from the higher sensitivity of the microplate reader than that of ordinary flatbed scanner. The image analysis makes a practical alternative to the absorption analysis, with 100% accuracy in both “leave-one-out” cross-validation and HCA in our study. To the best of our knowledge, this three-unit sensor array is by far the simplest one, which is able to discriminate 15 metal ions at such a low concentration. In addition, to evaluate the robustness of our proposed array, double-blind test was carried out to identify 30 unknown metal ion samples. The results (Table S1^†) showed that identification accuracy of 93.33% was achieved, which confirms the feasibility of this sensor array to identify unknown metal ions.Further exploration of the potential application of this array in quantitative analysis was carried out using both image analysis and absorption analysis. Among these fifteen metal ions, Ni²⁺, Cu²⁺, Hg²⁺ (Fig. 3) and Co²⁺ (Fig. S4^†) showed good correlations in LDA 2-D figures. As shown in Fig. 3(A, C and E) for Ni²⁺, Cu²⁺ and Hg²⁺, the 2-D plots using the first two factors displayed clear separations for different concentrations. Plotting factor 1 (the most significant factor) vs. concentrations of these three metal ions showed a good correlation, with the linear detection ranges for Ni²⁺, Cu²⁺ and Hg²⁺ at 0–4 μM, 0–8 μM and 0.5–3 μM, respectively. The results highly suggest that our proposed sensor array might find its potential application in quantitative analysis of some metal ions. Similar results from absorption spectral data analysis were also obtained with higher sensitivity (Fig. S5^†).Open in a separate windowFig. 3Discrimination of Ni²⁺, Cu²⁺ and Hg²⁺ at different concentrations. (A, C and E) LDA plots for the detection of metal ions at different concentrations. (B, D and F) The relationship between factor 1 and different concentrations of metal ions.Distinguishing mixture samples with different composition of metal ions is a great challenge for sensor arrays. Inspired by the strategy of mixture preparation in Bushdid et al. work.²⁹ We prepared 15 pairs of mixtures (referred to as “mixture A” and “mixture B”) that consist of 3, 5 and 7 components drawn from the collection of 15 metal ions (Fig. S6 and Table S2^†). To generate each mixture, we combined these components together at equal ratios. The most apparent difference between two pairs of mixtures with the same number of components is the percentage of components they share which varied from 0 to (N − 1/N) (N represents the number of components of the mixture pair). The components in each mixture were randomly selected by a written python program, and the concentration of each component in all mixtures was fixed at 5 μM. Fig. S7^† and 4(A, C and E) shows that different pairs displayed distinct colour change patterns, which arose from the different components of the mixtures. LDA 2-D plots demonstrated that all pairs of mixtures with the same number of components were completely separated (Fig. 4B, E and F). The “leave-one-out” cross-validation reached 100% accuracy and all the samples were clustered correctly in HCA (Fig. S8^†). The discrimination capability of image analysis was comparable to that of absorption spectral data analysis (Fig. S9^†). These results exhibited the potential of the proposed array as an advanced sensor array which can provide discrimination of highly similar complex mixtures.Open in a separate windowFig. 4Colour change profile of mixture A (left) and mixture B (right) of 15 different metal ions mixture pairs including (A) 3 metal ions, (C) 5 metal ions and (E) 7 metal ions. For purposes of visualization, the colour range of the difference map was expanded from 4 to 8 bits per colour (RGB range of 0–77 expanded to 0–255). LDA plot of sensor array against metal ions mixture pairs consists of exactly (B) 3 components, (D) 5 components and (F) 7 components using RGB channels.Detection of metal ions in real environmental water source are of greater practical significance than lab-prepared samples.To explore the capacity of our array in practical application, real water samples were tested in our study. 7 real water samples including super pure water (SPW), deionized water (DW), tap water (TW), lake water (LW), artificial lake water (ALW), river water (RW) and sea water (SW) were collected and directly tested without intentionally adding any metal ions. LDA 2-D plot demonstrated that all samples were correctly clustered and completely separated (Fig. 5A), while HCA showed clear discrimination of all real water samples with no misclassification (Fig. 5B), which was in accordance with the results of absorption spectral data analysis (Fig. S10^†). The successful differentiation of real water samples revealed the potential for on-site analysis.Open in a separate windowFig. 5Performance of the sensor array on distinguishing real water samples. (A) 2D LDA plot and (B) HCA dendrogram using RGB change values.In conclusion, we fabricated a new three-unit colorimetric sensor array using two commercially available low-cost dyes for detection of heavy metal ions in aqueous solution. In addition to commonly used absorption spectral measurements, colorimetric change patterns were also successfully constructed by imaging analysis of RGB values. LDA and HCA results proved that this array could achieve 100% accuracy in discriminating 15 metal ions solutions at 5 μM. Quantitative analysis of several ions was achieved at sub-micromolar range. Mixtures of 3, 5 and 7 metal ions as well as 7 real water samples without additional spiked metal ions were also accurately differentiated by imaging analysis, suggesting that this array has potential for on-site metal ions detection.     

Microwave-assisted synthesis of glutathione-coated hollow zinc oxide for the removal of heavy metal ions from aqueous systems

Glutathione has tremendous binding potential with metal ions present in water. However, the solubility of glutathione in water limits its productivity in the removal of these toxic ions from aqueous systems. The removability of heavy ions with glutathione and the associated adsorption capability are enhanced; for this purpose, glutathione is coated over hollow zinc oxide particles. Glutathione-coated hollow zinc oxide (Glu@h-ZnO) was successfully synthesized under microwave (MW) conditions using polystyrene (PS) as the template. The as-synthesized material was characterized by Fourier transform infrared (FTIR) spectroscopy, and the results were supported by X-ray diffraction crystallography (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), dynamic light scattering (DLS), Brunauer–Emmett–Teller (BET) studies and zeta potential (ζ) analysis. The sorption performance of Glu@h-ZnO towards the uptake of Hg²⁺, Cd²⁺ and Pb²⁺ ions from an aqueous medium under non-competitive batch conditions was investigated and the material was found to have the maximum affinity for Hg²⁺ ions with a maximum adsorption (q_m) capacity of 233 mg g⁻¹. The adsorption kinetics for Hg²⁺ ions and the effects of pH and ζ on the adsorption properties were also studied in detail. Finally, the experimental data were correlated with theoretical data obtained from density functional theory (DFT) studies and good agreement between the two was obtained.

Environmentally benign Glu@h-ZnO possesses good affinity for heavy metal ions, with enhanced adsorption capacity due to its high specific surface area.     

Novel single excitation dual-emission carbon dots for colorimetric and ratiometric fluorescent dual mode detection of Cu2+ and Al3+ ions

In this study, dual-emission carbon dots (D-CDs) are synthesized via a simple one-step solvothermal treatment of red tea. The obtained D-CDs are characterized by XPS, IR, TEM, XRD, fluorescence and UV-vis spectroscopy techniques. It is found that D-CDs present a strong red fluorescence emission peak at 671 nm and weak blue fluorescence emission peak at 478 nm under the excitation wavelength of 410 nm. The unique dual-emission properties of D-CDs provide great opportunities in ratiometric fluorescence sensing applications. The results show that Cu²⁺ ions can quench the fluorescence of the red emission band of D-CDs effectively, resulting in the disappearance of red fluorescence ultimately. Upon the addition of Al³⁺ ions, the fluorescence of blue emission band at 478 nm grows apparently, and the fluorescence color transforms gradually from red to orange, then to yellow-green. Based on these findings, a novel ratiometric fluorescence and colorimetric dual mode nanosensor is developed for simultaneous detection of Cu²⁺ and Al³⁺ ions. Regarding Cu²⁺ ions, the fluorescent detection linear range is 0.1–50 μM with detection limit of 0.1 μM, and the colorimetric detection limit is estimated as 25 μM. With regard to Al³⁺ ions, the fluorescent detection linear range is 0–20 μM and 25–100 μM with detection limit of 0.5 μM, and the colorimetric detection limit is 20 μM. Furthermore, the fluorescence response mechanisms of Cu²⁺ and Al³⁺ ions were discussed detailed. To the best of our current knowledge, this will be the first research work on the simultaneous determination of Cu²⁺ and Al³⁺ using D-CDs as fluorescent probes.

D-CDs with strong red emission and weak blue emission as an effective colorimetric and ratiometric fluorescence sensing probe are employed to realize the simultaneous detection of Cu²⁺ and Al³⁺ ions without any interference effect.     

Fluorescence detection of malachite green and cations (Cr3+, Fe3+ and Cu2+) by a europium-based coordination polymer

Due to remarkable fluorescence characteristics, lanthanide coordination polymers (CP) have been widely employed in fluorescence detection, but it is rarely reported that they act as multifunctional luminescent probes dedicated to detecting malachite green (MG) and various metal ions. A europium-based CP fluorescent probe, Eu(PDCA)₂(H₂O)₆ (PDCA = 2,6-pyridinedicarboxylic acid), has been synthesized and exhibited excellent recognition ability for malachite green and metal cations (Cr³⁺, Fe³⁺ and Cu²⁺) among 11 metal cations, 13 anions and six other compounds. The recognition was achieved by fluorescence quenching when MG, Cr³⁺, Fe³⁺ and Cu²⁺ were added to a suspension of Eu(PDCA)₂(H₂O)₆ respectively. Eu(PDCA)₂(H₂O)₆ is a multifunctional luminescent probe, and displayed high quenching efficiencies K_sv (2.10 × 10⁶ M⁻¹ for MG; 1.46 × 10⁵ M⁻¹ for Cr³⁺; 7.26 × 10⁵ M⁻¹ for Fe³⁺; 3.64 × 10⁵ M⁻¹ for Cu²⁺), and low detection limits (MG: 0.039 μM; Cr³⁺: 0.539 μM; Fe³⁺: 0.490 μM; Cu²⁺: 0.654 μM), presenting excellent selectivity and sensitivity, especially for MG. In addition, Eu(PDCA)₂(H₂O)₆ was also made into fluorescent test strips, which can rapidly and effectively examine trace amounts of MG, Cr³⁺, Fe³⁺ and Cu²⁺ in aqueous solutions. This work provides a new perspective for detecting malachite green in fish ponds and heavy metal ions in waste water.

A europium-based CP fluorescent sensor was synthesized and exhibited excellent recognition ability for malachite green (MG) and metal cations (Cr³⁺, Fe³⁺ and Cu²⁺).     

Utilization of a plasticized PVC optical sensor for the selective and efficient detection of cobalt(ii) in environmental samples

A novel sensitive, selective, and reversible cobalt(ii) ion optical sensor was prepared by the incorporation of 5-[o-carboxyphenylazo]2,4-dihydroxybenzoic acid [CPDB] and sodium tetraphenylborate (NaTPB) in a plasticized polyvinyl chloride (PVC) membrane containing dioctyl adipate (DOA) as a plasticizer. The influence of several parameters such as pH, base matrix, solvent mediator and reagent concentration was optimized. A comparison of the obtained results with those of previously reported sensors revealed that the proposed method, in addition to being fast and simple, provided a good linear range (0.05–45.20 μM) and low detection limit (0.015 μM). Low detection and quantification limits and excellent selectivity in the presence of interfering ions such as Fe³⁺, Cu²⁺, Ni²⁺, Ag⁺, Au³⁺, Cr³⁺, Cd²⁺, Zn²⁺, Hg²⁺, and SO₄²⁻ make it feasible to monitor Co²⁺ ion content accurately and repeatedly in environmental samples with complicated matrices. The optode was regenerated successfully using 0.3 M nitric acid (HNO₃) solution while its response was reversible with a relative standard deviation (RSD) lower than 1.9% for seven replicate determinations of 20 μM Co²⁺ in various membranes. The optode was stable and was stored for at least 15 days without observing any change in its sensitivity.

A novel sensitive, selective, and reversible cobalt(ii) ion optical sensor was prepared by the incorporation of [CPDB] and (NaTPB) in a (PVC) membrane containing (DOA) as a plasticizer.     

Synthesis and characterization of TiO2/graphene oxide nanocomposites for photoreduction of heavy metal ions in reverse osmosis concentrate

In this study, graphene oxide (GO), titanium dioxide (TiO₂) and TiO₂/GO nanocomposites were synthesized as the catalysts for photoreduction of endocrine disrupting heavy metal ions in reverse osmosis concentrates (ROC). The morphology, structure and chemical composition of these catalysts were characterized by scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Brunauer–Emmett–Teller analysis, Barrett–Joyner–Halenda, Fourier transform infrared spectroscopy and Raman spectroscopy. The photocatalytic experiments showed that TiO₂/GO nanocomposites exhibit a higher photoreduction performance than pure TiO₂ and GO. Under the optimal conditions, the removal rates of Cd²⁺ and Pb²⁺ can reach 66.32 and 88.96%, respectively, confirming the effectiveness of photoreduction to reduce the endocrine disrupting heavy metal ions in ROC resulted from the combined adsorption–reduction with TiO₂/GO nanocomposites.

TiO₂/GO nanocomposites were synthesized successfully and exhibited an excellent ability to reduce heavy metal ions in reverse osmosis concentrate.     

A solvent-dependent chemosensor for fluorimetric detection of Hg2+ and colorimetric detection of Cu2+ based on a new diarylethene with a rhodamine B unit

A novel solvent-dependent chemosensor 1o based on a diarylethene containing a rhodamine B unit has been designed. It could be used as a dual-functional chemosensor for selective detection of Hg²⁺ and Cu²⁺ by monitoring the changes in the fluorescence and UV-vis spectral in different solvents. A striking fluorescence enhancement at 617 nm was observed in DMSO upon the addition of Hg²⁺. However, 1o showed a remarkable absorption band appeared with maximum absorption at 555 nm after the addition of Cu²⁺ in THF. The results of ESI-MS spectra and Job''s plot confirmed a 1 : 1 binding stoichiometry between 1o and the two ions. The limits of detection of Hg²⁺ and Cu²⁺ were determined to be 0.14 μM and 0.51 μM, respectively. A 1 : 2 demultiplexer circuit was constructed by using UV light as data input, Cu²⁺ as the address input, and the absorbance at 555 nm and the absorbance ratio of (A₆₀₃/A₂₇₄) as the dual data outputs.

A novel solvent-dependent chemosensor based on a diarylethene derivative for fluorescent “turn-on” recognition of Hg²⁺ and colorimetric detection of Cu²⁺ was synthesized, its multi-controllable photoswitchable behaviors with light and chemical stimuli were investigated.