Retention of heavy metal ions on SBA-15 mesoporous silica functionalised with carboxylic groups

Three samples of SBA-15 functionalised with -(CH(2))(3)COOH groups have been prepared by co-condensation, starting from solutions of TEOS and 4-(triethoxysilyl)butyronitrile, acting as -(CH(2))(3)COOH precursor, of different molar compositions. Materials were characterised by X-ray diffraction, nitrogen adsorption, and FT-IR spectroscopy. The pK(a) and the acidic capacity were measured for all samples by potentiometric titration. The acidic capacity increases with increasing amount of -COOH precursor in the synthesis mixture only up to 10% molar of total alkoxysilane. The value for the pK(a)(4.75) is independent of the acidic capacity of the material. The sample prepared starting from an amount of -COOH precursor equal to 10% molar of total alkoxysilane was chosen to test selective interactions with heavy metals of environmental importance (Ag(+), Cd(2+), Fe(3+), Cu(2+), Zn(2+)) at different pH values and ionic strengths. The significant and selective adsorption exhibited by the material has been exploited in a preliminary cation-exchange chromatographic application showing the possibility of eluting the metal ions at different retention times.     

Development of nanomaterial chemosensors for toxic metal ions sensing

Heavy metal ions are harmful to aquatic life and humans owing to their high toxicity and non‐biodegradability, so their removal from wastewater is an important task. Therefore, this work focuses on designing suitable, simple and economical nanosensors to detect and remove these metal ions with high selectivity and sensitivity. Based on this idea, different types of mesoporous materials such as hexagonal SBA‐15, cubic SBA‐16 and spherical MCM‐41, their chloro‐functionalized derivatives, as well as 4‐(4‐nitro‐phenylazo)‐naphthalen‐1‐ol (NPAN) azo dye have been synthesized, with the aim of designing some optical nanosensors for metal ions sensing applications. The mentioned azo dye has been anchored into the chloro‐functionalized mesoporous materials. The designed nanosensors were characterized using scanning and transmission electron microscopy as well as Fourier transform infrared and UV–visible spectral analysis, nitrogen adsorption–desorption isotherms, low‐angle X‐ray diffraction and thermogravimetric analyses. Their optical sensing to various toxic metal ions such as Cd (II), Hg (II), Mn (II), Fe (II), Zn (II) and Pb (II) at different values of pH (1.1, 4.9, 7 and 12) was investigated. The optimization of experimental conditions, including the effect of pH and metal ion concentration, was examined. The experimental results showed that the solution pH had a major impact on metal ion detection. The optical nanosensors respond well to the tested metal ions, as reflected by the enhancement in both absorption and emission spectra upon adding different concentrations of the metal salts and were fully reversible on adding ethylene diamine tetra acetic acid or citric acid to the formed complexes. High values of the binding constants for the designed nanosensors were observed at pHs 7 and 12, confirming the strong chelation of different metals to the nanosensor at these pHs. Also, high binding constants and sensitivity were observed for NPAN‐MCM‐41 as a nanosensor to detect the different metal ions. From the obtained results, we succeeded in transforming the harmful azo dye into an environmentally friendly form via designing of the optical nanosensors used to detect toxic metal ions in wastewater with high sensitivity.     

Smart nano-architectures as potential sensing tools for detecting heavy metal ions in aqueous matrices

The discharge of heavy metal ions into water resources as a result of human activities has become a global issue. Contamination with heavy metal ions poses a major threat to the environment and human health. Therefore, there is a dire need to probe the presence of heavy metal ions in a more selective, facile, quick, cost-effective and sensitive way. Conventional sensors are being utilized to sense heavy metal ions; however, various challenges and limitations like interference, overlapping of oxidation potential, selectivity and sensitivity are associated with them that limit their in-field applicability. Hence, nanomaterial based chemical sensors have emerged as an alternative substitute and are extensively employed for the detection of heavy metal ions as a potent analytical tool. The incorporation of nanomaterials in sensors increases their sensitivity, selectivity, portability, on-site detection capability and device performance. Nanomaterial based electrodes exhibit enhanced performance because surface of electrode at nano-scale level offers high catalytic potential, large active surface area and high conductivity. Therefore, this review addresses the recent progress on chemical sensors based on different nanomaterials such as carbon nanotubes (CNTs), metal nanoparticles, graphene, carbon quantum dots and nanocomposites for sensing heavy metals ions using different sensing approaches. Furthermore, various types of optical sensors such as fluorescence, luminescence and colorimetry sensors have been presented in detail.     

Cyanide sensing via metal ion removal from a fluorogenic BODIPY complex

We report a highly selective and sensitive reversible cyanide sensor operating in the ‘turn-off-on’ mode via decomplexation of Cu(II) ions from a brightly fluorescent boradiazaindacene derivative. The sensor is reversible as the emission signal originates from the dynamic equilibration of dipicolylamine-Cu(II) and tetracyanocuprate complex ions.     

Silicas with thiourea grafted groups for the concentration of heavy metal ions

Two methods for the chemical bonding of thiourea and its derivatives on dispersed silica have been developed as a result of a detailed study of the process of the thiourea chemisorption on silica surface. Obtained S-containing silicas possess a high extraction efficiency for heavy metal ions.     

Amide-nitrophenyl based colorimetric receptors for selective sensing of fluoride ions

New chromogenic receptors containing 2-nitrophenyl or 3,5-dinitrophenyl groups appended to the amide or in secondary amine positions have been synthesized and characterized. Upon addition of fluoride to two of the receptors in acetonitrile, the solution acquired a yellow colour. The third receptor showed an intense purple colour with fluoride in acetonitrile and the appearance of the purple colour can be detected by the naked eye at parts per million level. The addition of chloride, bromide and iodide to the receptors did not induce any colour. Thus the receptors can act as fluoride ion sensors even in the presence of other halide ions.     

Fluorescence sensing of ammonium and organoammonium ions with tripodal oxazoline receptors

A new class of fluorescence sensors for ammonium and organoammonium ions has been disclosed. One of the sensors, an alaninol-derived tripodal oxazoline (1a) shows significant fluorescence enhancement upon binding NH(4)(+) but little response toward K(+), Na(+), and Mg(2+) ions. Owing to its chiral environment, a phenylglycinol-derived tripodal oxazoline (1b) shows chiral discrimination in fluorescence upon binding enantiomeric guests. [reaction: see text]     

Luminescent dendrimers as ligands for metal ions

Suitably designed luminescent dendrimers can play a role of ligands for luminescent and non-luminescent metal ions. This combination leads to species capable of exhibiting interesting and unusual properties, including (i) shielding excited states from quenching processes, (ii) light harvesting, (iii) conversion of incident UV light into visible or infrared emission, and (iv) metal ions sensing with signal amplification.     

Well dispersed single-walled carbon nanotubes with strong visible fluorescence in water for metal ions sensing

A novel and simple method to prepare well dispersed single-walled carbon nanotubes with strong visible fluorescence in water is reported. The visible fluorescence was found to be responsive to pH value and metal ions, and tunable emission ability of oxidized SWCNTs depending on the excitation wavelength and a novel self-excitation and emission process were found.     

Synthesis and recognition behavior of multi-point receptors with binding sites for different metal ions

Novel multi-point receptors which have a calix[4]arene skeleton, two esters, and two bipyridine moieties with a polyether chain were designed and synthesized. The host with shorter polyethers recognizes Na⁺ and Ag⁺ cooperatively, and the host with longer polyethers captures them independently.     

Chromophoric thin film based on cellulose triacetate blends for sensing metal ions

Chromophoric sensors were made based on 8-hydroxyquinoline immobilized onto a thin film of a polymer blend matrix. The thin films were made by the solution casting method using cellulose triacetate and polyethylene glycol (PEG 600) as plasticizer and pore-forming agent. Different contents of PEG 600 additive were investigated. The prepared films were characterized by FTIR and thermal analysis. The absorption and fluorescence spectra of different films were dependent on the content of PEG 600 with clear quenching of the fluorescence of the film that contains PEG 600 compared to that with zero content. This behavior was attributed to the collective effect of hydrogen bonding (intra- and intermolecular hydrogen bonding) that enhances the process of excited-state proton transfer. This result is favorable to a responsive sensor that shows fluorescence off in the absence of metal ions and fluorescence on upon metal ion chelation. The detection of 5 × 10⁻⁵ M of Al³⁺, Zn²⁺ and thallium (I) in aqueous solution has been observed with the fluorescence method. The result obtained is consistent with the enhancing effect of PEG 600 in the detectability of metal ions. Compared with the detection of Al³⁺ and Zn²⁺, the sensor shows better detection of thallium (I), with clear fluorescence spectra.     

Oxidic glasses as hosts for migrating metal ions

The results of experiments where Tl⁺ and Pb²⁺ ions are electrolysed into a sodium borate glass (35 mol% Na₂O and 50 °C) are brought up to date in order to take into account recent developments in the chemistry of borate glasses. It is first necessary to consider the unique chemistry of the oxide(-II) species in terms of its electronegativity, electronic polarisability and acid–base properties, and the significant relationship between these is discussed. It is described how the Lewis basicity of oxidic materials such as glasses can be expressed quantitatively on the optical basicity scale and how determinations are made by various experimental methods. These methods include optical spectroscopy of ‘probe’ ions such as Tl⁺ or Pb²⁺, measurement of electronic polarisability and far-infrared spectroscopic ‘rattling’ frequencies of constituent metal ions. When Pb²⁺ ions are electrolysed into the sodium borate glass, it is found that there is migration of Na⁺ ions away from and of O^2? ions towards the (lead) anode with formation of PbO. There is almost complete depletion of Na₂O in the anode region so that the composition of the glass approximates to B₂O₃. A similar process occurs to a limited degree in the case of thallium, but the Tl⁺ ions are able to penetrate more deeply into the glass. Their ultraviolet ¹S₀?→?³P₁ frequency indicates that the sites they occupy have much greater basicity than the bulk glass. The two-site model of Kamitsos proposes that in borate glasses, there are higher and lower basicity sites, and the measured optical basicity of Tl⁺ indicates occupation of the higher sites. Since it has been shown that BO₄ groups in the glass are weakly basic, it is unlikely that they are involved in the higher sites. It is discussed how the higher site basicity implies greater covalency in the interaction of the Tl⁺ ion with the oxide(-II)s that constitute the pathway, and it is suggested that this is an important factor in the electromigration process.     

Cyclic hexapeptides with free carboxylate groups as new receptors for monosaccharides

[structure: see text]. Cyclic hexapeptides composed of alternating L-proline and 3-aminobenzoic acid subunits with substituents on the aromatic subunits that contain free carboxylate groups are able to bind monosaccharides in 4% CD3OD/CDCl3. The binding selectivity of these peptides depends on the structure of the substituents on the aromatic subunits.     

Using amino acids for the chromatofocusing of metal ions on silica with bonded tetraethylenepentamine groups

Amino acid-based eluents are used for the chromatofocusing of metal ions on Tetren-SiO₂ chelating sorbent (silica with bonded tetraethylenepentamine groups) for the first time. The smoothest quasilinear pH gradients form for eluents based on glutamic and aspartic acids. The separation of Mn²⁺, Cr³⁺, Co²⁺, Ni²⁺, and Cu²⁺ is achieved.     

Development of water-soluble far-red fluorogenic dyes for enzyme sensing

A series of Nile Blue analogs, 5-amino-9-dialkylamino benzo[a]phenoxazine dyes, 7-9, were prepared by condensation of N-alkyl or N-sulfo-propyl 4-arylazo-substituted 3-hydroxyaniline with 4-arylazo-substituted 1-naphthlamines or 8-amino-2-naphthalenesulfonic acid in the presence of perchloric acid. These fluorochromes have excitation and emission maxima near 640 and 680 nm, respectively. The fluorescence intensity in aqueous solution increases as additional sulfonate groups are added to the benzo[a]phenoxazine core. Compound 7, which has two sulfonate groups, is ten-times brighter than Nile Blue. Fluorogenic substrates containing this hydrophilic far-red dye were synthesized and applied to detect enzymatic activities of two model proteases, trypsin and leucine aminopeptidase. Given their excellent fluorogenic property, these novel far-red dyes should be useful for enzyme sensing in biological assays.     

Solvent-dependent chromogenic sensing for Cu and fluorogenic sensing for Zn and Al: a multifunctional chemosensor with dual-mode

A new multifunctional chemosensor 1 was synthesized and characterized by spectroscopic tools along with a single crystal X-ray crystallography. It can exhibit selective recognition responses toward Cu²⁺, Zn²⁺ and Al³⁺ in different solvent systems with bimodal methods (colorimetric and fluorescence). This sensor 1 detected Cu²⁺ ions through a distinct color change from colorless to yellow in aqueous solution. Interestingly, the receptor 1 was found to be reversible by EDTA. The detection limit (11 μM) of 1 for Cu²⁺ is much lower than WHO guideline (30 μM) in drinking water. In addition, the sensor 1 showed significant fluorescence enhancements in the presence of Zn²⁺ ion and Al³⁺ ion in two different organic solvents (DMF and MeCN), respectively. The binding modes of the three complexes were determined to be a 1:1 complexation stoichiometry through Job plot, ESI-mass spectrometry analysis, and ¹H NMR titration.     

Selective co-aggregation of gold nanoparticles functionalised with complementary hydrogen-bonding groups

Molecular recognition between two species of Au55 clusters bearing complementary hydrogen-bonding groups provides a facile route to accomplish solution-based assembly of two-component nanoparticle aggregates.