Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold

An Arylazopyrazole-Based N-Heterocyclic Carbene as a Photoswitch on Gold Surfaces: Light-Switchable Wettability, Work Function, and Conductance

A novel photoresponsive and fully conjugated N-heterocyclic carbene (NHC) has been synthesized that combines the excellent photophysical properties of arylazopyrazoles (AAPs) with an NHC that acts as a robust surface anchor (AAP-BIMe). The formation of self-assembled monolayers (SAMs) on gold was proven by ToF-SIMS and XPS, and the organic film displayed a very high stability at elevated temperatures. This stability was also reflected in a high desorption energy, which was determined by temperature-programmed SIMS measurements. E-/Z-AAP-BIMe@Au photoisomerization resulted in reversible alterations of the surface energy (i.e. wettability), the surface potential (i.e. work function), and the conductance (i.e. resistance). The effects could be explained by the difference in the dipole moment of the isomers. Furthermore, sequential application of a dummy ligand by microcontact printing and subsequent backfilling with AAP-BIMe allowed its patterning on gold. To the best of our knowledge, this is the first example of a photoswitchable NHC on a gold surface. These properties of AAP-BIMe@Au illustrate its suitability as a molecular switch for electronic devices.

Self-assembled monolayer formation of pentamers-like molecules onto FCC(111) surfaces: the case of curcuminoids onto Au(111) surface

The adsorption of rigid straight electrically polarized pentamers over a FCC(111) surface is studied. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and simulated under the frame of the density functional theory. We now obtain and report the charge distribution of the molecule which allows to propose a deposition model followed by Monte Carlo simulations over an ad-hoc lattice gas model. We show that for a certain value of the chemical potential there exists an isotropic-nematic phase transition which can explain the formation of a self-assembled monolayer like the one observed in the transmission electron microscopy images. An order parameter is defined to characterize the transition which presents a step-like behavior at a critical chemical potential value. The possible nature of the nematic transition in conjunction with an ergodicity breakdown is discussed as future work by means of statistical physics techniques.

N-Heterocyclic Carbenes for the Self-Assembly of Thin and Highly Insulating Monolayers with High Quality and Stability

As an organic nanostructure, self-assembled monolayers (SAMs) play a central role in many aspects of nanotechnology, including molecular electronics. In this work, we show that SAMs based on N-heterocyclic carbenes on a Au(111) substrate offer a high level of crystallinity and also exhibit the highest possible packing density. As a result of this structural optimization, defect concentrations were reduced by 2-3 orders of magnitude and thermal stability was ∼100 K higher than those of any other SAMs on Au. The conductivity of these SAMs is ∼4 orders of magnitude lower than that of standard alkanethiols of comparable length, which together with very low defect concentration and high thermal stability makes them a highly interesting material for potential application in organic thin film transistors. The self-assembly of such dense, highly crystalline, and notably stable structures is associated with strong C-Au bonding and the rational design of assembled molecules, resulting in the high mobility of both adsorbate and substrate atoms, as confirmed by the size of the molecular domains and the adsorbate-driven modification of the Au(111) substrate, respectively.

The poly-thymine based DNA photolithography onto electrostatic coupling substrates

In order to develop a rapid and high fidelity process for DNA self-assembly with patterning, the pattern of thymine dimerization is presented onto electrostatically bound DNA substrate by photolithography. The ability of binding for the process, which is attenuated conditions such as contact with photomask and washing by solution buffer is evaluated by X-ray photoelectron spectroscopy (XPS). Through thymine dimerization and hybridization chain reaction (HCR), DNA patterns, including multi-patterns, are demonstrated. For expansion to protein molecular patterning, the target DNA is tethered to biotin, allowing patterning with streptavidin linked fluorophores such as Cy3-streptavidin and phyecocayine-streptavidin.

Hybrid Catalysts for Artificial Photosynthesis: Merging Approaches from Molecular, Materials, and Biological Catalysis

Increasing demand for sustainable energy sources continues to motivate the development of new catalytic processes that store intermittent energy in the form of chemical bonds. In this context, photosynthetic organisms harvest light to drive dark reactions reducing carbon dioxide, an abundant and accessible carbon source, to store solar energy in the form of glucose and other biomass feedstocks. Inspired by this biological process, the field of artificial photosynthesis aims to store renewable energy in chemical bonds spanning fuels, foods, medicines, and materials using light, water, and CO₂ as the primary chemical feedstocks, with the added benefit of mitigating the accumulation of CO₂ as a greenhouse gas in the atmosphere. As such, devising new catalyst platforms for transforming CO₂ into value-added chemical products is of importance. Historically, catalyst design for artificial photosynthesis has been approached from the three traditional fields of catalysis: molecular, materials, and biological. In this Account, we show progress from our laboratory in constructing new hybrid catalysts for artificial photosynthesis that draw upon design concepts from all three of these traditional fields of catalysis and blur the boundaries between them. Starting with molecular catalysis, we incorporated biological design elements that are prevalent in enzymes into synthetic systems. Specifically, we demonstrated that proper positioning of intramolecular hydrogen bond donors or addition of intermolecular multipoint hydrogen bond donors with classic iron porphyrin and nickel cyclam platforms can substantially increase rates of CO₂ reduction and break electronic scaling relationships. In parallel, we incorporated a key materials design element, namely, high surface area and porosity for maximizing active site exposure, into molecular systems. A supramolecular porous organic cage molecule was synthesized with iron porphyrin building blocks, and the porosity was observed to facilitate substrate and charge transport through the catalyst film. In turn, molecular design elements can be incorporated into materials catalysts for CO₂ reduction. First, we utilized molecular synthons in a bottom-up reticular approach to drive polymerization/assembly into a bulk framework material. Second, we established an organometallic approach in which molecular ligands, including chelating ones, are adsorbed onto a bulk inorganic solid to create and tune new active sites on surfaces. Finally, we describe two examples in which molecular, materials, and biological design elements are all integrated to catalyze the reduction of CO₂ into CH₄ using a hybrid biological-materials interface with sustainably generated H₂ as the reductant or to reduce CO into value-added C₂ products acetate and ethanol using a hybrid molecular-materials interface to construct a biomimetic, bimetallic active site. Taken together, our program in catalysis for energy and sustainability has revealed that combining more conventional design strategies in synergistic ways can lead to advances in artificial photosynthesis.

A Benchtop Method for Appending Protic Functional Groups to N-Heterocyclic Carbene Protected Gold Nanoparticles

The remarkable resilience of N-heterocyclic carbene (NHC) gold bonds has quickly made NHCs the ligand of choice when functionalizing gold surfaces. Despite rapid progress using deposition from free or CO₂ -protected NHCs, synthetic challenges hinder the functionalization of NHC surfaces with protic functional groups, such as alcohols and amines, particularly on larger nanoparticles. Here, we synthesize NHC-functionalized gold surfaces from gold(I) NHC complexes and aqueous nanoparticles without the need for additional reagents, enabling otherwise difficult functional groups to be appended to the carbene. The resilience of the NHC-Au bond allows for multi-step post-synthetic modification. Beginning with the nitro-NHC, we form an amine-NHC terminated surface, which further undergoes amide coupling with carboxylic acids. The simplicity of this approach, its compatibility with aqueous nanoparticle solutions, and its ability to yield protic functionality, greatly expands the potential of NHC-functionalized noble metal surfaces.

Thiol-free self-assembled oligoethylene glycols enable robust air-stable molecular electronics

Self-assembled monolayers (SAMs) are widely used to engineer the surface properties of metals. The relatively simple and versatile chemistry of metal-thiolate bonds makes thiolate SAMs the preferred option in a range of applications, yet fragility and a tendency to oxidize in air limit their long-term use. Here, we report the formation of thiol-free self-assembled mono- and bilayers of glycol ethers, which bind to the surface of coinage metals through the spontaneous chemisorption of glycol ether-functionalized fullerenes. As-prepared assemblies are bilayers presenting fullerene cages at both the substrate and ambient interface. Subsequent exposure to functionalized glycol ethers displaces the topmost layer of glycol ether-functionalized fullerenes, and the resulting assemblies expose functional groups to the ambient interface. These layers exhibit the key properties of thiolate SAMs, yet they are stable to ambient conditions for several weeks, as shown by the performance of tunnelling junctions formed from SAMs of alkyl-functionalized glycol ethers. Glycol ether-functionalized spiropyrans incorporated into mixed monolayers lead to reversible, light-driven conductance switching. Self-assemblies of glycol ethers are drop-in replacements for thiolate SAMs that retain all of their useful properties while avoiding the drawbacks of metal-thiolate bonds.

Reaction of chloroauric acid with histidine in microdroplets yields a catalytic Au-(His)2 complex

An aqueous solution containing histidine (His, 100 μM) and chloroauric acid (HAuCl4, 10 μM) is electrosprayed (-4.5 kV) from a capillary (50 μm in diameter) with N2 nebulizing gas (120 psi). The resulting microdroplets entered a mass spectrometer with a 2 cm flight path. The mass spectrum recorded in negative ion mode showed several peaks including the Au5 nanocluster with the major one being [Au + 2His-2H]-, which is a catalytically active species. The reaction time was less 1 ms, and the yield of [Au + 2His-2H]- was 76%. In contrast, the bulk reaction for the same concentration run at room temperature for 2 h did not produce this species but instead formed Au10 nanocluster. When a solution of water and acetonitrile (1 : 1) containing indoline (100 mM) and the phenylacetylene (200 mM) as well as histidine and chloroauric acid at the same concentrations as above was electrosprayed, the mass spectrum showed the formation of the intermediate [Au + 2His + phenylacetylene + H]+. Upon collecting the microdroplets, the 4-methyl-4,6-diphenyl-1,2-dihydro-4H-pyrrolo[3,2,1-ij] quinolone product was observed by 1H nuclear magnetic resonance and liquid chromatography with a yield of 44%. The microdroplet synthesis using the Au-(His)2 complex as a catalyst was scaled up using room-temperature ultrasonic nebulization to produce the product at the rate of 35 mg min-1, which is semi-preparative and demonstrates the promise of using microdroplet reactions for chemical synthesis.

Strong Metal-Adsorbate Interactions Increase the Reactivity and Decrease the Orientational Order of OH-Functionalized N-Heterocyclic Carbene Monolayers

Fundamental understanding of the correlation between the structure and reactivity of chemically addressable N-heterocyclic carbene (NHC) molecules on various surfaces is essential for the design of functional NHC-based self-assembled monolayers. In this work, we identified the ways by which the deposition of chemically addressable OH-NHCs on Au(111) or Pt(111) surfaces modified the anchoring geometry and chemical reactivity of surface-anchored NHCs. The properties of surface-anchored NHCs were probed by conducting X-ray photoelectron spectroscopy and polarized near-edge X-ray absorption fine structure measurements. While no preferred orientation was identified for OH-NHCs on Pt(111), the anchored molecules adopted a preferred flat-lying position on Au(111). Dehydrogenation and aromatization of the imidazoline ring along with partial hydroxyl oxidation were detected in OH-NHCs that were anchored on Au(111). The dehydrogenation and aromatization reactions were facilitated, along with partial decomposition, for OH-NHCs that were anchored on Pt(111). The spectroscopic results reveal that stronger metal-adsorbate interactions increase the reactivity of surface-anchored OH-NHCs while decreasing their molecular orientational order.

N-Heterocyclic carbene and thiol micropatterns enable the selective deposition and transfer of copper films

Microcontact printed patterns of N-heterocyclic carbenes (NHCs) and thiols were prepared on gold substrates and utilized as templates for the creation of metallic Cu structures using electroplating. The presence of the NHC in the pattern is essential to enable the transfer of the resulting copper microstructures to a second substrate.

Site-dependent selectivity in oxidation reactions on single Pt nanoparticles

Site-dependent selectivity in oxidation reactions on Pt nanoparticles was identified by conducting IR nanospectroscopy measurements while using allyl-functionalized N-heterocyclic carbenes (allyl-NHCs) as probe molecules.

Developing a toll-like receptor biosensor for Gram-positive bacterial detection and its storage strategies

Conditions to store toll-like receptor2/6 sensors and use them to detect bacterial analytes, including pathogen-associated molecular patterns and bacterial cultures.

Synthesis and properties of an Au6 cluster supported by a mixed N-heterocyclic carbene–thiolate ligand

The preparation of a novel Au₆ cluster bearing a bidentate mixed carbene–thiolate ligand is presented.

Flexible NO2 -Functionalized N-Heterocyclic Carbene Monolayers on Au (111) Surface

The formation of flexible self-assembled monolayers (SAMs) in which an external trigger modifies the geometry of surface-anchored molecules is essential for the development of functional materials with tunable properties. In this work, it is demonstrated that NO₂ -functionalized N-heterocyclic carbene molecules (NHCs), which were anchored on Au (111) surface, change their orientation from tilted into flat-lying position following trigger-induced reduction of their nitro groups. DFT calculations identified that the energetic driving force for reorientation was the lower steric hindrance and stronger interactions between the chemically reduced NHCs and the Au surface. The trigger-induced changes in the NHCs' anchoring geometry and chemical functionality modified the work function and the hydrophobicity of the NHC-decorated Au surface, demonstrating the impact of a chemically tunable NHC-based SAM on the properties of the metal surface.

Tuning the catalytic activity and selectivity of water-soluble bimetallic RuPt nanoparticles by modifying their surface metal distribution

Bimetallic ruthenium-platinum nanoparticles (RuPt NPs) of different surface distributions and stabilized by using a sulfonated N-heterocyclic carbene ligand (1-(2,6-diisopropylphenyl)-3-(3-potassium sulfonatopropyl)-imidazol-2-ylidene) were prepared from Ru(COD)(COT) (COD = cyclooctadiene and COT = cyclooctatriene), and platinum precursors having various decomposition rates (Pt(NBE)₃, NBE = norbornene, Pt(CH₃)₂(COD) and Pt₂(DBA)₃, DBA = dibenzylideneacetone). Structural and surface studies by FT-IR and solid-state MAS NMR, using carbon monoxide as a probe molecule, revealed the presence of different structures and surface compositions for different nanoparticles of similar sizes, which principally depend on the decomposition rate of the organometallic precursors used during the synthesis. Specifically, the slower the decomposition rate of the platinum precursor, the higher the number of Pt atoms at the NP surface. The different bimetallic RuPt NPs, as well as their monometallic equivalents (Pt and Ru NPs), were used in isotopic H/D exchange through C-H activation on l-lysine. Interestingly, the activity and selectivity of the direct C-H deuteration were dependent on the NP surface composition at the α position but not on that at the ε position. Chemical shift perturbation (CSP) experiments revealed that the difference in reactivity at the α position is due to a Pt-carboxylate interaction, which hinders the H/D exchange.

Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes

Gold superatom nanoclusters stabilized entirely by N-heterocyclic carbenes (NHCs) and halides are reported. The reduction of well-defined NHC-Au-Cl complexes produces clusters comprised of an icosahedral Au₁₃ core surrounded by a symmetrical arrangement of nine NHCs and three chlorides. X-ray crystallography shows that the clusters are characterized by multiple CH-π and π-π interactions, which rigidify the ligand and likely contribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0%, which is significantly greater than that of the most luminescent ligand-protected Au₁₃ superatom cluster. Density functional theory analysis suggests that clusters are 8-electron superatoms with a wide HOMO-LUMO energy gap of 2 eV. Consistent with this, the clusters have high stability relative to phosphine stabilized clusters.

Elucidating the Influence of Anchoring Geometry on the Reactivity of NO2-Functionalized N-Heterocyclic Carbene Monolayers

The development of chemically addressable N-heterocyclic carbene (NHC) based self-assembled monolayers (SAMs) requires in-depth understanding of the influence of NHC's anchoring geometry on its chemical functionality. Herein, it is demonstrated that the chemical reactivity of surface-anchored NO₂-functionalized NHCs (NO₂-NHCs) can be tuned by modifying the distance between the functional group and the reactive surface, which is governed by the deposition technique. Liquid deposition of NO₂-NHCs on Pt(111) induced a SAM in which the NO₂-aryl groups were flat-lying on the surface. The high proximity between the NO₂ groups and the Pt surface led to high reactivity, and 85% of the NO₂ groups were reduced at room temperature. Lower reactivity was obtained with vapor-deposited NO₂-NHCs that assumed a preferred upright geometry. The separation between the NO₂ groups in the vapor-deposited NO₂-NHCs and the reactive surface circumvented their surface-induced reduction, which was facilitated only after exposure to harsher reducing conditions.

Multidentate Anchors for Surface Functionalization

The bottom-up functionalization of solid surfaces shows increasing importance for a wide range of interdisciplinary applications. Multidentate anchors with more than two contact points can bind to solid surfaces with strong chemisorption, well-defined upright configuration, and tailored functionality. The surface functionalization using multidentate anchors with three (tripodal), four (quadripodal), or more binding points is summarized herein, with a focus on those beyond classical tripodal anchors. In particular, the molecular design on how to achieve multisite interaction between anchor and substrate and the introduction of functional groups to thin films are discussed.

Single molecule vs. large area design of molecular electronic devices incorporating an efficient 2-aminepyridine double anchoring group

When a molecule is bound to external electrodes by terminal anchor groups, the latter are of paramount importance in determining the electrical conductance of the resulting molecular junction. Here we explore the electrical properties of a molecule with bidentate anchor groups, namely 4,4'-(1,4-phenylenebis(ethyne-2,1-diyl))bis(pyridin-2-amine), in both large area devices and at the single molecule level. We find an electrical conductance of 0.6 × 10-4G0 and 1.2 × 10-4G0 for the monolayer and for the single molecule, respectively. These values are approximately one order of magnitude higher than those reported for monodentate materials having the same molecular skeleton. A combination of theory and experiments is employed to understand the conductance of monolayer and single molecule electrical junctions featuring this new multidentate anchor group. Our results demonstrate that the molecule has a tilt angle of 30° with respect to the normal to the surface in the monolayer, while the break-off length in the single molecule junction occurs for molecules having a tilt angle estimated as 40°, which would account for the difference in their conductance values per molecule. The bidentate 2-aminepyridine anchor is of general interest as a contact group, since this terminal functionalized aromatic ring favours binding of the adsorbate to the metal contact resulting in enhanced conductance values.

What Does Nanoparticle Stability Mean?

The term "nanoparticle stability" is widely used to describe the preservation of a particular nanostructure property ranging from aggregation, composition, crystallinity, shape, size, and surface chemistry. As a result, this catch-all term has various meanings, which depend on the specific nanoparticle property of interest and/or application. In this feature article, we provide an answer to the question, "What does nanoparticle stability mean?". Broadly speaking, the definition of nanoparticle stability depends on the targeted size dependent property that is exploited and can only exist for a finite period of time given all nanostructures are inherently thermodynamically and energetically unfavorable relative to bulk states. To answer this question specifically, however, the relationship between nanoparticle stability and the physical/chemical properties of metal/metal oxide nanoparticles are discussed. Specific definitions are explored in terms of aggregation state, core composition, shape, size, and surface chemistry. Next, mechanisms of promoting nanoparticle stability are defined and related to these same nanoparticle properties. Metrics involving both kinetics and thermodynamics are considered. Methods that provide quantitative metrics for measuring and modeling nanoparticle stability in terms of core composition, shape, size, and surface chemistry are outlined. The stability of solution-phase nanoparticles are also impacted by aggregation state. Thus, collision and DLVO theories are discussed. Finally, challenges and opportunities in understanding what nanoparticle stability means are addressed to facilitate further studies with this important class of materials.

One-step synthesis and XPS investigations of chiral NHC-Au(0)/Au(i) nanoparticles

Although N-heterocyclic carbenes (NHCs) have been demonstrated as suitable ligands for the stabilisation of gold nanoparticles (AuNPs) through a variety of methods, the manner in which such AuNPs form is yet to be fully elucidated. We report a simple and fast one-step synthesis of uniform chiral (l/d)-histidin-2-ylidene stabilised gold nanoparticles using the organometallic Au(i) complex as a well defined starting material. The resulting nanoparticles have an average size of 2.35 ± 0.43 nm for the L analog whereas an average size of 2.25 ± 0.39 nm was found for the D analog. X-ray photoelectron spectroscopy analyses reveal the presence of Au(i) and Au(0) in all NHC stabilised AuNPs. In contrast, measured X-ray photoelectron spectra of dodecanethiol protected gold nanoparticles showed only the presence of a Au(0) species. This observation leads us to postulate that AuNPs synthesised from organometallic NHC-Au(i) complexes exhibit a monolayer of Au(i) surrounding a Au(0) core. This work highlights the importance of synthetic method choice for NHC-stabilized AuNPs, as this could determine Au oxidation states and resulting AuNP properties such as catalytic activities and stabilities.

N-Heterocyclic Carbenes in Materials Chemistry

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

Non-chemisorbed gold-sulfur binding prevails in self-assembled monolayers

Gold-thiol contacts are ubiquitous across the physical and biological sciences in connecting organic molecules to surfaces. When thiols bind to gold in self-assembled monolayers (SAMs) the fate of the hydrogen remains a subject of profound debate-with implications for our understanding of their physical properties, spectroscopic features and formation mechanism(s). Exploiting measurements of the transmission through a molecular junction, which is highly sensitive to the nature of the molecule-electrode contact, we demonstrate here that the nature of the gold-sulfur bond in SAMs can be probed via single-molecule conductance measurements. Critically, we find that SAM measurements of dithiol-terminated molecular junctions yield a significantly lower conductance than solution measurements of the same molecule. Through numerous control experiments, conductance noise analysis and transport calculations based on density functional theory, we show that the gold-sulfur bond in SAMs prepared from the solution deposition of dithiols does not have chemisorbed character, which strongly suggests that under these widely used preparation conditions the hydrogen is retained.

Tunneling and thermoelectric characteristics of N-heterocyclic carbene-based large-area molecular junctions

Tunneling and thermoelectric characteristics of NHC-based large-area junctions were demonstrated for the first time.

Optimizing bidentate N-heterocyclic carbene ligands for the modification of late transition metal surfaces – new insights through theory

We identify key factors determining the adsorption behaviour of bidentate NHCs on noble metal surfaces.

Using SERS To Understand the Binding of N-Heterocyclic Carbenes to Gold Surfaces

Surface functionalization is an essential component of most applications of noble-metal surfaces. Thiols and amines are traditionally employed to attach molecules to noble-metal surfaces, but they have limitations. A growing body of research, however, suggests that N-heterocyclic carbenes (NHCs) can be readily employed for surface functionalization with superior chemical stability compared with thiols. We demonstrate the power of surface-enhanced Raman scattering combined with theory to present a comprehensive picture of NHC binding to gold surfaces. In particular, we synthesize a library of NHC isotopologues and use surface-enhanced Raman scattering to record the vibrational spectra of these NHCs while bound to gold surfaces. Our experimental data are compared with first-principles theory, yielding numerous new insights into the binding of NHCs to gold surfaces. In addition to these insights, we expect our approach to be a general method for probing the local surface properties of NHC-functionalized surfaces for their expanding use in sensing applications.

Robust gold nanorods stabilized by bidentate N-heterocyclic-carbene-thiolate ligands

Although N-heterocyclic carbenes (NHCs) have demonstrated outstanding potential for use as surface anchors, synthetic challenges have limited their application to either large planar substrates or very small spherical nanoparticles. The development of a strategy to graft NHCs onto non-spherical nanomaterials, such as gold nanorods, would greatly expand their utility as surface ligands. Here, we use a bidentate thiolate-NHC-gold(I) complex that is easily grafted onto commercial cetyl trimethylammonium bromide-stabilized gold nanorods through ligand exchange. On mild reduction of the resulting surface-tethered NHC-gold(I) complexes, the gold atom attached to the NHC complex is added to the surface as an adatom, thereby precluding the need for reorganization of the underlying surface lattice upon NHC binding. The resulting thiolate-NHC-stabilized gold nanorods are stable towards excess glutathione for up to six days, and under conditions with large variations in pH, high and low temperatures, high salt concentrations, or in biological media and cell culture. We also demonstrate the utility of these nanorods for in vitro photothermal therapy.

Modification of gold surface by electrosynthesized mono aza crown ether substituted catechol-terminated alkane dithiol and its application as a new electrochemical sensor for trace detection of cadmium ions

Among the toxic metals, cadmium is a very dangerous pollutant because it can extremely damage organs in humans and animals. This toxic metal is introduced into water from different industries such as metal plating, batteries, and alloys. Cadmium bioaccumulates in vital organs and unlike organic pollutants does not show any biological degradation. In this study, an electroactive self-assembled monolayer (SAM) was developed by covalent attachment of a novel mono aza-crown ether substituted catechol-terminated hexane dithiol onto the gold surface. The electrochemical behavior of the fabricated SAM electrode was investigated using voltammetry techniques and electrochemical impedance spectroscopy (EIS). The obtained results from voltammetric experiments revealed that the crown ether moiety of SAM forms a selective complex with cadmium ion. Under optimal conditions, Cd²⁺ could be detected in the range of 15 μM to 65 μM with a detection limit of 4.5 μM. Selectivity measurements reveal that the sensor is specific for Cd²⁺ even in the presence of high concentrations of other metal ions. The proposed sensor was applied to the determination of cadmium ion in water samples with high sensitivity and good selectivity.