Selected Metal Ions Research Articles

Cover Feature: Unfolding the Role of Building Units of MOFs with Mechanistic Insight Towards Selective Metal Ions Detection in Water (Chem. Eur. J. 21/2022)

The role of the organic building units of MOFs towards selective detection of a targeted metal cation (Fe3+) has been demonstrated by systematic screening by varying the number of Lewis basic (pyridyl-N atoms) sites in a series of isostructural metal–organic frameworks (MOFs). All three fluorescent MOFs are seen to present a quenching response towards Fe3+ ions in water; however, only UiO-67@N exhibits highly selective and sensitive response, whereas the emission of both UiO-67 and UiO-67@NN is quenched by several metal ions. More information can be found in the Research Article by S. K. Ghosh et al. (DOI: 10.1002/chem.202104175).

Self‐Catalyzed Rechargeable Lithium‐Air Battery by in situ Metal Ion Doping of Discharge Products: A Combined Theoretical and Experimental Study

Lithium‐air battery has emerged as a viable electrochemical energy technology; yet a substantial overpotential is typically observed, due to the insulating nature of the discharge product Li2O2 that hinders the reaction kinetics and device performance. Furthermore, finite solid–solid/–liquid interfaces are formed between Li2O2 and catalysts and limit the activity of the electrocatalysts in battery reactions, leading to inadequate electrolytic efficiency. Herein, in‐situ doping of Li2O2 by select metal ions is found to significantly enhance the lithium‐air battery performance, and Co2+ stands out as the most effective dopant among the series. This is ascribed to the unique catalytic activity of the resulting Co‐Ox sites towards oxygen electrocatalysis, rendering the lithium‐air battery self‐catalytically active. Theoretical studies based on density functional theory calculations show that structural compression occurs upon Co2+ doping, which lowers the energy barrier of Li2O2 decomposition. Results from this study highlight the significance of in situ electrochemical doping of the discharge product in enhancing the performance of lithium‐air battery.

Unfolding the Role of Building Units of MOFs with Mechanistic Insight Towards Selective Metal Ions Detection in Water.

The potential emergence of fluorescence-based techniques has propelled research towards developing probes that can sense trace metal ions specifically. Although luminescent metal-organic frameworks (MOFs) are well suited for this application, the role of building blocks towards detection is not fully understood. In this work, a systematic screening by varying number of Lewis basic (pyridyl-N atoms) sites is carried out in a series of isostructural, robust UiO-67 MOFs, and targeting a model metal ion-Fe3+ . All the three fluorescent MOFs are seen to present quenching response towards Fe3+ ions in water. However, UiO-67@N exhibits highly selective and sensitive response, whereas emission of both UiO-67 and UiO-67@NN is quenched by several metal ions. Detailed experimental and theoretical mechanistic investigation is carried out in addition to demonstration of UiO-67@N being able to sense trace amount of Fe3+ ions in synthetic biological water sample. Further, UiO-67@N based mixed-matrix membrane (MMM) has been prepared and employed to mimic the real time Fe3+ ions detection in water.

Recovery of rare earth elements from NdFeB magnet by mono- and bifunctional mesoporous silica: Waste recycling strategies and perspectives

Rare earth elements (REEs) such as Nd3+ and Dy3+ were recovered from simulated and real leaching solution of the NdFeB magnet via solid phase extraction (SPE). Extraction of REEs from simulated solutions was investigated using silica functionalized with NH2-, EDTA and/or phosphonic groups. The effects of several experimental factors (pH, efficiency of adsorbents, selectivity, and elution of metal ions) on extraction of REEs were investigated. Exploiting specific affinities of adsorbents toward REEs in the presence of competing ions, selective separation of REEs was achieved successfully. The affinity of phosphorous/nitrogen containing adsorbents had the descending order of adsorption: Fe3+ > Dy3+ > Nd3+ > Ni2+ > Al3+ for multi-component systems. In the first approach, a procedure based on precipitation of Fe3+ ions combined with SPE was evaluated for the recovery of REEs in two steps. Most of the Fe3+ ions (85%) were efficiently separated by adding an ammonia solution (pH ~ 2.5). Chemical analysis of precipitate showed, though, a high content of REEs. Extraction of REEs from supernatant liquid via bi-functional mesoporous silica with EDTA and/or phosphonic groups recovered ~97.0% of Nd3+ with Ni2+ and Al3+ ions as impurities. Non-ordered silica functionalized with phosphonic groups, showed economical superiority over other mesoporous adsorbents studied for REEs extraction. Complete recovery (97.8%) of REEs was achieved after several stages of SPE with adsorbents functionalized by P/N-containing groups. The in-advance removal of Fe3+ ions was a prerequisite for successful implementation of this approach. Finally, separation of Nd3+ and Dy3+ was performed from nitric acid media using the gradient elution to obtain 98.4% purity Nd3+. The proposed step-by-step SPE procedure by P/N-containing functionalized silica was successfully applied for extraction of REEs (95.5%) in an industrial magnet. This study opens up possibilities for application of the developed approach for the End-of-Life materials recycling.

Selective Metal Ion Irradiation Using Bipolar HIPIMS: A New Route to Tailor Film Nanostructure and the Resulting Mechanical Properties

This manuscript introduces and experimentally demonstrates a novel concept of selective metal ion irradiation by combining bipolar HIPIMS with conventional DC magnetron sputtering operation and simple DC biasing. The addition of the positive pulse to a conventional HIPIMS discharge accelerates the predominantly metal ions created during the negative HIPIMS phase with an energy proportional to the positive pulse amplitude and ionization state. Two distinct metal elements with large difference in atomic mass (Cr and Nb) are used on this work to irradiate a TiAlN matrix which is being deposited by conventional DCMS. The positive acceleration voltages used for both Cr and Nb discharges were varied between 0 to +200 V to analyze the influence of Nb and Cr metal ion irradiation on the mechanical and microstructural properties of TiAlN films. Even though the total metal ion incorporation into the TiAlN matrix for both Cr and Nb is less than 10% at%, strong effects are observed on the resulting film properties. It was observed that use of the lighter metal ion Cr is more beneficial than the heavier metal ion Nb. The Cr bombardment allows a hardness improvement from 7 to 22 GPa as well as a reduced film accumulated stress at the highest positive acceleration voltage. From the XRD measurements it is observed that the Cr atoms are inserted into the TiAlN cubic matrix maintaining its crystalline structure. However, the bombardment with the high-mass metal ion (Nb) promotes the deformation of the cubic TiAlN matrix, resulting in a spinodal decomposition and further degradation of the crystalline structure with the appearance of the hexagonal wurtzite-type Al-rich phase. This is also translated to the resulting film mechanical properties, as hardness rapidly decreases from 25 to 10 GPa and stress increases linearly with the positive voltage acceleration.

Ethylenediaminetetraacetate functionalized ordered Santa Barbara Amorphous‐15 mesoporous silica as an effective adsorbent for preconcentration of some heavy metals followed by inductively coupled plasma atomic emission spectrometry

AbstractA simple and sensitive method was developed for preconcentration of the several transition metal ions using Santa Barbara Amorphous‐15 mesoporous silica functionalized with the ethylenediaminetetraacetate functional group. The determination of the metal ions was performed by inductively coupled plasma atomic emission spectrometry. The effective parameters on the extraction efficiency of the ions were investigated. The optimized conditions were: the sample solution pH 4, adsorbent amount = 10 mg, adsorption time = 30 min, elution solvent = 1.5 M of HCl, and desorption time = 5 min. Under the optimum conditions, limits of detection of 50, 5, 10, 50, 10, and 500 ng/L were obtained for Co2+, Mn2+, Cu2+, Cd2+, Zn2+, and Fe3+ respectively. The relative standard deviations for determination of the selected metal ions at concentration level of 50 μg/L (n = 4) were in the range of 1.7–5.8%. Linear ranges for Co2+ and Cd2+ (0.1–250 μg/L), Cu2+ and Zn2+ (0.05–250 μg/L), Fe3+ (1–250 μg/L), and Mn2+ (0.01–25 and 50–500 μg/L) were obtained with perfect correlation coefficients (R = 0.9958–0.9996, n = 8). The applicability of the method was evaluated by the extraction and determination of the heavy metal ions in different water samples.

Tailoring Escherichia coli Chemotactic Sensing towards Cadmium by Computational Redesign of Ribose-Binding Protein.

ABSTRACTPeriplasmic binding proteins such as ribose-binding proteins (RBPs) are involved in the bacterial chemotaxis two-component system. RBP selectively identifies and interacts with ribose to induce a conformational change that leads to chemotaxis. Here, we report the development of an engineered Escherichia coli (E. coli) strain expressing a redesigned RBP that can effectively sense cadmium ions and regulate chemotactic movement of cells toward a cadmium ion gradient. RBP was computationally redesigned to bind cadmium ions and produce the conformational change required for chemoreceptor binding. The successful design, CdRBP1, binds to cadmium ions with a dissociation constant of 268 nM. When CdRBP1 was expressed in the periplasmic space of E. coli, the bacteria became live cadmium ion hunters with high selectivity over other divalent metal ions. This work presents an example of making cadmium ions, which are toxic for most organisms, as an attractant to regulate cells movement. Our approach also demonstrates that RBP can be precisely designed to develop metal-detecting living systems for potential applications in synthetic biology and environmental studies.IMPORTANCE Cadmium pollution is one of the major environmental problems due to excessive release and accumulation. New technologies that can auto-detect cadmium ions with good biocompatibility are in urgent need. In this study, we engineered the bacterial chemotaxis system to positively sense cadmium ions by redesigning ribose-binding protein (RBP) to tightly bind cadmium ion and produce the right conformational change for receptor binding and signaling. Our engineered E. coli cells can auto-detect and chase cadmium ions with divalent metal ion selectivity. Many attempts have been carried out to redesign RBP at the ribose binding site with little success. Instead of the ribose binding site, we introduced the cadmium binding site in the opening of the ribose binding pocket by a specially developed computational algorithm. Our design strategy can be applied to engineer live bacteria with autonomous detection and remediation abilities for metal ions or other chemicals in the future.

“Turn-on” fluorescence probe for Al (III) and Hg (II) ions in aqueous medium: Synthesis, characterization, cytotoxicity, visual results in solution and cancer cells

In this study, An efficient OFF–ON fluorescent compound integrated with nitrobenzofurazan and phenolphthalein (NBFP) has been prepared for the detection of metals ions. NBFP was characterized with 1H NMR, 13C NMR, COSY-NMR, FTIR, and Mass spectrometry. The fluorescent response towards metal ions displayed that NBFP is selective for Hg (II) and Al (III), with a detection limit of 14.7 µM and 16.3 µM for Hg(II) and Al(III), respectively. Job’s plot for NBFP + Hg (II) complex, and NBFP + Al (III) complex confirmed the 1:2 ligand–metal stoichiometry. Binding constant (Ksv) from Stern-Volmer equation was found to be 1.60 × 102 M−1 for NBFP + Hg (II) complex and 9.25 × 102 M−1 for NBFP + Al (III) complex respectively, indicating a strong interaction between ligand and selective metal ions in the presence of other ions. The quantum yield of NBFP was found to be ΦL = 0.028, exhibiting a weak and wide emission band. Cytotoxicity of NBFP has been evaluated before application in living cell fluorescent imaging for the detection of metal ions. The result showed that NBFP exhibits good biocompatibility in living cells and displayed a good response for the detection of Al (III) and Hg (II) ions in living cells using the confocal microscope.

Structure-based thermodynamics of ion selectivity (Mg2+versus Ca2+ and K+versus Na+) in the active site of the eukaryotic lariat group II intron from algae Pylaiella littoralis.

Group II introns are metalloenzymes that can catalyze self-splicing. Recently, the crystal structures of the eukaryotic group IIB lariat intron from the brown algae Pylaiella littoralis have been reported for two intermediate states (pre-hydrolytic (2s) and post-hydrolytic) along the self-splicing pathway. Three characteristic metal-ion binding sites (M1 and M2 sites for catalytic Mg2+ ions, and K1 site for K+) in the catalytic pocket of the lariat intron have been identified and proposed to be crucial for self-splicing. Using the X-ray structures as a template, we quantitatively estimated the energetics of divalent (Mg2+versus Ca2+) and monovalent (K+versus Na+) ion selectivity and established a direct link between the energetics and structures of this lariat intron (bound to cognate and near-cognate metal ions). Molecular dynamics (MD) free energy simulations showed that the lariat intron was strongly selective between divalent metal ions. The strength of divalent metal-ion selectivity was noticeably high in the post-hydrolytic state (ΔΔG ≈ 20 kcal mol-1) relative to its pre-hydrolytic (2s) state (ΔΔG ≈ 13 kcal mol-1). Quantum chemical calculations ensured that the sign of the estimated divalent metal-ion selectivity was correct. The M1-binding pocket was less solvent-exposed in the case of the post-hydrolytic state relative to the pre-hydrolytic (2s) state, which boosted the metal-ion selectivity of the former. Surprisingly, in contrast to the bacterial linear group II intron, the lariat intron was found to be non-selective between monovalent ions (K+versus Na+). The interaction network in the first coordination shell of Ca2+ in the M1-binding pocket was different relative to Mg2+. Mg2+ substitution by Ca2+ resulted in the substitution of a single M1-RNA interaction by the M1-water interaction. In the pre-hydrolytic (2s) state, Ca2+ substitution completely disrupted the M1⋯5'-exon interaction; thus, the nature of the divalent metal ion is critical for catalysis. The interaction network in the M2 site was independent of the nature of the divalent metal ions (Mg2+ or Ca2+). The monovalent ion was loosely bound in the wet binding pocket (K1 site) of the lariat intron; thus, the substitution of K+ by Na+ could not significantly alter the free energy of the complex. The metal ion selectivity was dependent on the solvent accessibility of the metal-ion-binding-pocket, dry pocket enhanced the selectivity.

Super-assembly of freestanding graphene oxide-aramid fiber membrane with T-mode subnanochannels for sensitive ion transport

Biomimetic nacre-like membranes composed of two-dimensional lamellar sheets and one-dimensional nanofibers exhibit high mechanical strength and excellent stability. Thus, they show substantial application in the field of membrane science and water purification. However, the limited techniques for the assembly of two-dimensional lamellar membranes and one-dimensional nanofibers hamper their development and application. Herein, we developed a nacre-like and freestanding graphene oxide/aramid fiber membrane with abundant T-mode subnanochannels by introducing aramid fibers into graphene oxide interlamination via the super-assembly interaction between graphene oxide and aramid fibers. Benefiting from the presence of stable and adjustable sub-nanometer-size ion transport channels, the graphene oxide/aramid fiber composite membrane exhibited excellent mono/divalent ion selectivity of 3.51 (K+/Mg2+), which is superior to that of the pure graphene oxide membrane. The experimental results suggest that the mono/divalent ion selectivity is ascribed to the subnanochannels in the graphene oxide/aramid fiber composite membrane, electrostatic repulsion interaction and strong interaction between the divalent metal ion and carboxyl groups. Moreover, the composite membrane exhibited remarkable charge selectivity with a K+/Cl- ratio of up to ∼158, indicating that this graphene oxide/aramid fiber composite membrane has great potential for application in energy conversion. This study provides an avenue to prepare freestanding and nacre-like composite membranes with abundant T-mode ion transport channels for ion recognition and energy conversion, which also shows great application prospects in the field of membrane science and water purification.

Chain length effect of spiro-ring N-alkylation on photophysical signalling parameters in Fe(III) selective rhodamine probes.

Manifestation of photophysical signalling parameters in rhodamine derivatives exhibiting complexation induced spiro-ring opening is crucial for the realization of selective metal ion detection at trace levels. Substitution of various functional groups, such as alkylation to the core architecture, modulates the physico-chemical properties of such molecular probes. Despite a few studies, relationships between the extent of photophysical signal modulations and the chain lengths of n-alkyl substituents are still elusive. In this investigation, a few molecular probes based on the rhodamine B (1-5) and rhodamine 6G (6-10) platform were synthesized by their derivatization with n-alkyl substituents of varying chain lengths at the amino-donor of their spiro-ring end, which exhibited Fe(III)-selective absorption and fluorescence 'off-on' signal transduction along with colorization of solution. The Fe(III)-selectivity in these probes remained the same despite their structural distinctions through varied n-alkyl chain lengths of the substituents; however, the quantifiable signalling parameters such as spectroscopic enhancement factors, sensitivity, the kinetics of spiro-ring opening and effectiveness of probe-Fe(III) interactions were analyzed. These parameters were also correlated in terms of the influence of different chain lengths of n-alkyl substituents that efficiently contributed to their inter-componential interactive stereo-electronic environment.

Deracemisation and stereoinversion by a nanoconfined bidirectional enzyme cascade: dual control by electrochemistry and selective metal ion activation.

The unique ability of the 'electrochemical leaf' (e-Leaf) to drive and control nanoconfined enzyme cascades bidirectionally, while directly monitoring their rate in real-time as electrical current, is exploited to achieve deracemisation and stereoinversion of secondary alcohols using a single electrode in one pot. Two alcohol dehydrogenase enzymes with opposing enantioselectivities, from Thermoanaerobacter ethanolicus (selective for S) and Lactobacillus kefir (selective for R) are driven bidirectionally via coupling to the fast and quasi-reversible interconversion of NADP+/NADPH catalysed by ferredoxin NADP+ reductase - all enzymes being co-entrapped in a nanoporous indium tin oxide electrode. Activity of the Lactobacillus kefir enzyme depends on the binding of a non-catalytic Mg2+, allowing it to be switched off after an oxidative half-cycle, by adding EDTA - the S-selective enzyme, with a tightly-bound Zn2+, remaining fully active. Racemate → S or R → S conversions are thus achieved in high yield with unprecedented ease.

Cd2+-selective fluorescence response of TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) derivatives bearing ether oxygen-binding sites.

Moderate Zn2+ selectivity over Cd2+ (IZn/ICd = 1.6) in the fluorescence enhancement of TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) was changed to Cd2+ preference via the introduction of a methoxymethyloxy (MOMO) substituent at the 8-position of one of the four quinoline rings (IZn/ICd = 0.2). Thus, 8-MOMOTQEN (N-(8-methoxymethyloxy-2-quinolylmethyl)-N,N',N'-tris(2-quinolylmethyl)ethylenediamine) showed not only high Cd2+-selectivity but also an enhanced fluorescence quantum yield upon Cd2+ binding and high sensitivity for Cd2+ detection as shown by ϕCd = 0.065 and LOD (limit of detection) = 19 nM. The two oxygen atoms of the MOMO group in 8-MOMOTQEN play a crucial role in the fluorescent metal-ion selectivity because the corresponding hydroxy (8-OHTQEN) and methoxy (8-MeOTQEN) derivatives resulted in a poor fluorescent response and metal selectivity, respectively. Another N6O2 ligand, N,N'-bis(8-methoxy-2-quinolylethyl)-N,N'-bis(2-quinolylmethyl)ethylenediamine ((8-MeO)2TQEN) exhibited a Zn2+-selective fluorescence enhancement (IZn/ICd = 2.2), indicating the superiority of the MOMO group for the selective sensing of Cd2+.

Highly detection of Zn (II) ion sensing and photocatalytic activities of biosynthesized AgNPs using NilgirianthusCiliatus leaf extract and its properties

In the present work, the highly selective approaches of silver nanoparticles (AgNPs) are biologically synthesized from Nilgirianthusciliatus leaves extract for colorimetric detection of Zn (II) ions to development of nanosensors. The synthesized AgNPs were characterized by UV-visible, FT-IR, XRD, SEM-EDX, TEM-SAED, and BET. The well-dispersed spherical-like particles with sizes are found to be 15 nm. The Ag NPs were able to detect Zn2+ions in the nano-molar level with sensing mechanism of Zn2+ ions in label-free AgNPs were characterized and exhibited good precise metal ion selectivity.

Stimulation of Sulfonamides Antibacterial Drugs Activity as a Result of Complexation with Ru(III): Physicochemical and Biological Study.

Antibiotic resistance is a global problem, and one promising solution to overcome this issue is using metallodrugs, which are drugs containing metal ions and ligands. These complexes are superior to free ligands in various characteristics including anticancer properties and mechanism of action. The pharmacological potential of metallodrugs can be modulated by the appropriate selection of ligands and metal ions. A good example of proper coordination is the combination of sulfonamides (sulfamerazine, sulfathiazole) with a ruthenium(III) ion. This work aimed to confirm that the activity of sulfonamides antibacterial drugs is initiated and/or stimulated by their coordination to an Ru(III) ion. The study determined the structure, electrochemical profile, CT-DNA affinity, and antimicrobial as well as anticancer properties of the synthesized complexes. The results proved that Ru(III) complexes exhibited better biological properties than the free ligands.

Two Faces of the Same Coin: Coupling X-Ray Absorption and NMR Spectroscopies to Investigate the Exchange Reaction Between Prototypical Cu Coordination Complexes.

The satisfactory rationalization of complex reactive pathways in solution chemistry may greatly benefit from the combined use of advanced experimental and theoretical complementary methods of analysis. In this work, we combine X-Ray Absorption and 1 H NMR spectroscopies with state-of-the-art Multivariate Curve Resolution and theoretical analyses to gain a comprehensive view on a prototypical reaction involving the variation of the oxidation state and local structure environment of a selected metal ion coordinated by organic ligands. Specifically, we investigate the 2-cyano-2-phenylpropanoic acid reduction of the octahedral complex established by the Cu2+ ion with terpyridine to the tetrahedral complex formed by Cu+ and neocuproine. Through our interdisciplinary approach we gain insights into the nature, concentration time evolution and structures of the key metal (XAS measurements) and organic (1 H NMR measurements) species under reaction. We believe our method may prove to be useful in the toolbox necessary to understand the mechanisms of reactive processes of interest in solution.

Selectivity of transition metal ions by a layered tin phosphate

A unique layered ammonium tin phosphate (NH4+-SnP) with unusually large d-spacing (16.57 Å) exhibited high uptake of transition metal ions compared with other common ion-exchangers from solutions containing large concentration of competing ions of Na+, K+, Mg2+ and Ca2+. The ion-exchange capacity (C) sequence of NH4+-SnP was found to be: CCu > CZn > CNi ≈ CMn. This NH4+-SnP material is potentially useful for removal of transition metal ions from wastewater for remediation or recovery.

Synthesis, characterization and applications: Novel terpolymer and its composite

A new approach for the treatment of effluent streams has been developed by synthesizing an innovative chitosan based terpolymer composite gaining a considerable attention in the wastewater treatment community. The present research article reports on the synthesis of terpolymer composite from 4,4′ – oxydianiline, carbohydrazide with formaldehyde and chitosan for the recovery of toxic and heavy metals from aqueous solutions. The synthesized adsorbents have been characterized by elemental analysis, spectral studies and surface morphology by scanning electron microscopy. Batch separation technique has been used in the ion-exchange process for the removal of selective metal ions like Co2+, Cd2+, Pb2+, Hg2+ by terpolymer and its composite. The results revealed that both the adsorbents are good ion-exchanger however, the composite act as better ion exchanger than the terpolymer. It may be due to the more active sets of amine and carbonyl groups present in terpolymer ligand and the size of particle, high porous and greater surface area, this result investigated by SEM analysis. The results of recovery of toxic metals were matched with the earlier reported terpolymer resins. The adsorption isotherm models were investigated and the results were found well fitted with Langmuir isotherm compared to Freundlich isotherm and follows first order kinetics.

Cationic Effects on the Structural Dynamics of the Metal Ion-Crown Ether Complexes Investigated by Ultrafast Infrared Spectroscopy.

It is usually believed that the binding affinity and selectivity of an alkali metal ion with crown ether are defined by the size matching model. However, the underlying mechanism of the specific host-guest interactions and the structural dynamics of the metal ions confined in the cavity of the crown ethers in the solutions are still not clear. In this report, a series of alkali thiocyanate salts (XSCN; X = Li, Na, K, and Cs) complexed with 18-crown-6 (a typical crown ether) in the chloroform solutions were studied by the polarization-selective infrared pump-probe spectroscopy and the ultrafast two-dimensional infrared (2D IR) spectroscopy. The SCN- counteranions were employed as the local vibrational probe to reveal the specific host-guest interactions in the crown ether complexes. The rotational dynamics and spectral diffusion of SCN- vibration were both measured by ultrafast IR spectroscopy, and it was found that the metal cations hosted by the crown ethers can have a pronounced effect on the rotational dynamics of the counteranions. The reorientational time constants of the SCN- vibration in the complexation follow the order Li+ > Na+ > K+ ≃ Cs+. More importantly, the spectral diffusion dynamics of SCN-, which quantifies the decay of the correlation of the frequency fluctuations in the complexation, was also affected by the metal ions but showed a different order of cationic effect. A detailed analysis of the 2D IR data showed that the spectral diffusion of SCN- counteranion clearly decayed with two different time scales in the complex of 18-crown-6 with K+. The 3-4-fold slowdown in spectral diffusion indicated that the fluctuation of SCN- vibrational transition frequency was strongly affected by the K+ cation due to the geometric constraint imposed by the crown ether. The results should help the researchers to unravel the specific host-guest interactions and further reveal the origination of the binding selectivity of crown ether for metal cations in the condensed phases from the perspective of structural dynamics.

Sensing of Transition Metals by Top-Down Carbon Dots

Carbon quantum dots (CQDs) are a new class of carbon-rich materials with a range of unique optical and structural properties. They can be defined as carbon nanoparticles, with sizes in the range of 1–10 nm, displaying absorption and emission activities in the UV-VIS range. Depending on the structure, CQDs display a wide variability of properties, which provides the possibility of finely tuning them for several applications. The great advantages of CQDs are certainly the ease of synthesis, non-toxicity, and the strong interactions with the surrounding environment. Based on this, CQDs are especially promising as selective chemosensors. The present study reports on carbon quantum dots synthesized with a top-down (TD) approach, and characterized by different optical, spectroscopic, and morphological techniques to identify the selectivity for metal ions belonging to the first transition series. In particular, the study focuses on the interaction between two samples, namely TD and TDA, featuring different surface functionalization, and heavy metal ions. Their sensing towards Co2+, Cu2+, Fe3+, Zn2+, and Ni2+ has been tested by fluorescence (PL), steady state absorption spectroscopy, and time-resolved PL spectroscopy, in order to determine the fluorescence quenching. We found a PL quenching in the presence of concentrations of metal salts starting from 0.5 µM, and a selectivity towards the interacting ions, depending on CQDs’ surface features paving the way for their use for sensing.