Molecular sensors confined on SiOx substrates

Metal-free platforms for molecular thin films as high-performance supercapacitors

Controlling chemical functionalization and achieving stable electrode-molecule interfaces for high-performance electrochemical energy storage applications remain challenging tasks. Herein, we present a simple, controllable, scalable, and versatile electrochemical modification approach of graphite rods (GRs) extracted from low-cost Eveready cells that were covalently modified with anthracene oligomers. The anthracene oligomers with a total layer thickness of ∼24 nm on the GR electrode yield a remarkable specific capacitance of ∼670 F g-1 with good galvanostatic charge-discharge cycling stability (10 000) recorded in 1 M H2SO4 electrolyte. Such a boost in capacitance is attributed mainly to two contributions: (i) an electrical double-layer at the anthracene oligomer/GR/electrolyte interfaces, and (ii) the proton-coupled electron transfer (PCET) reaction, which ensures a substantial faradaic contribution to the total capacitance. Due to the higher conductivity of the anthracene films, it possesses more azo groups (-N[double bond, length as m-dash]N-) during the electrochemical growth of the oligomer films compared to pyrene and naphthalene oligomers, which is key to PCET reactions. AC-based electrical studies unravel the in-depth charge interfacial electrical behavior of anthracene-grafted electrodes. Asymmetrical solid-state supercapacitor devices were made using anthracene-modified biomass-derived porous carbon, which showed improved performance with a specific capacitance of ∼155 F g-1 at 2 A g-1 with an energy density of 5.8 W h kg-1 at a high-power density of 2010 W kg-1 and powered LED lighting for a longer period. The present work provides a promising metal-free approach in developing organic thin-film hybrid capacitors.

Metal-organic Frameworks in Semiconductor Devices

Metal-organic frameworks (MOFs) are a specific class of hybrid, crystalline, nano-porous materials made of metal-ion-based 'nodes' and organic linkers. Most of the studies on MOFs largely focused on porosity, chemical and structural diversity, gas sorption, sensing, drug delivery, catalysis, and separation applications. In contrast, much less reports paid attention to understanding and tuning the electrical properties of MOFs. Poor electrical conductivity of MOFs (~10-7-10-10 S cm-1), reported in earlier studies, impeded their applications in electronics, optoelectronics, and renewable energy storage. To overcome this drawback, the MOF community has adopted several intriguing strategies for electronic applications. The present review focuses on creatively designed bulk MOFs and surface-anchored MOFs (SURMOFs) with different metal nodes (from transition metals to lanthanides), ligand functionalities, and doping entities, allowing tuning and enhancement of electrical conductivity. Diverse platforms for MOFs-based electronic device fabrications, conductivity measurements, and underlying charge transport mechanisms are also addressed. Overall, the review highlights the pros and cons of MOFs-based electronics (MOFtronics), followed by an analysis of the future directions of research, including optimization of the MOF compositions, heterostructures, electrical contacts, device stacking, and further relevant options which can be of interest for MOF researchers and result in improved devices performance.

Thickness-Dependent Charge Transport in Three Dimensional Ru(II)- Tris(phenanthroline)-Based Molecular Assemblies

We describe here the fabrication of large-area molecular junctions with a configuration of ITO/[Ru(Phen)3]/Al to understand temperature- and thickness-dependent charge transport phenomena. Thanks to the electrochemical technique, thin layers of electroactive ruthenium(II)-tris(phenanthroline) [Ru(Phen)3] with thicknesses of 4-16 nm are covalently grown on sputtering-deposited patterned ITO electrodes. The bias-induced molecular junctions exhibit symmetric current-voltage (j-V) curves, demonstrating highly efficient long-range charge transport and weak attenuation with increased molecular film thickness (β = 0.70 to 0.79 nm-1). Such a lower β value is attributed to the accessibility of Ru(Phen)3 molecular conduction channels to Fermi levels of both the electrodes and a strong electronic coupling at ITO-molecules interfaces. The thinner junctions (d = 3.9 nm) follow charge transport via resonant tunneling, while the thicker junctions (d = 10-16 nm) follow thermally activated (activation energy, Ea ∼ 43 meV) Poole-Frenkel charge conduction, showing a clear "molecular signature" in the nanometric junctions.

Bioactive Coatings on Titanium: A Review on Hydroxylation, Self-Assembled Monolayers (SAMs) and Surface Modification Strategies

Titanium (Ti) and its alloys have been demonstrated over the last decades to play an important role as inert materials in the field of orthopedic and dental implants. Nevertheless, with the widespread use of Ti, implant-associated rejection issues have arisen. To overcome these problems, antibacterial properties, fast and adequate osseointegration and long-term stability are essential features. Indeed, surface modification is currently presented as a versatile strategy for developing Ti coatings with all these challenging requirements and achieve a successful performance of the implant. Numerous approaches have been investigated to obtain stable and well-organized Ti coatings that promote the tailoring of surface chemical functionalization regardless of the geometry and shape of the implant. However, among all the approaches available in the literature to functionalize the Ti surface, a promising strategy is the combination of surface pre-activation treatments typically followed by the development of intermediate anchoring layers (self-assembled monolayers, SAMs) that serve as the supporting linkage of a final active layer. Therefore, this paper aims to review the latest approaches in the biomedical area to obtain bioactive coatings onto Ti surfaces with a special focus on (i) the most employed methods for Ti surface hydroxylation, (ii) SAMs-mediated active coatings development, and (iii) the latest advances in active agent immobilization and polymeric coatings for controlled release on Ti surfaces.

Electrochemical Potential-Driven High-Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications

Molecules are fascinating candidates for constructing tunable and electrically conducting devices by the assembly of either a single molecule or an ensemble of molecules between two electrical contacts followed by current-voltage (I-V) analysis, which is often termed "molecular electronics". Recently, there has been also an upsurge of interest in spin-based electronics or spintronics across the molecules, which offer additional scope to create ultrafast responsive devices with less power consumption and lower heat generation using the intrinsic spin property rather than electronic charge. Researchers have been exploring this idea of utilizing organic molecules, organometallics, coordination complexes, polymers, and biomolecules (proteins, enzymes, oligopeptides, DNA) in integrating molecular electronics and spintronics devices. Although several methods exist to prepare molecular thin-films on suitable electrodes, the electrochemical potential-driven technique has emerged as highly efficient. In this Review we describe recent advances in the electrochemical potential driven growth of nanometric various molecular films on technologically relevant substrates, including non-magnetic and magnetic electrodes to investigate the stimuli-responsive charge and spin transport phenomena.

Surface chemical reactions on self-assembled silane based monolayers

In this review, we aim to update our review "Chemical modification of self-assembled silane-based monolayers by surface reactions" which was published in 2010 and has developed into an important guiding tool for researchers working on the modification of solid substrate surface properties by chemical modification of silane-based self-assembled monolayers. Due to the rapid development of this field of research in the last decade, the utilization of chemical functionalities in self-assembled monolayers has been significantly improved and some new processes were introduced in chemical surface reactions for tailoring the properties of solid substrates. Thus, it is time to update the developments in the surface functionalization of silane-based molecules. Hence, after a short introduction on self-assembled monolayers, this review focuses on a series of chemical reactions, i.e., nucleophilic substitution, click chemistry, supramolecular modification, photochemical reaction, and other reactions, which have been applied for the modification of hydroxyl-terminated substrates, like silicon and glass, which have been reported during the last 10 years.

Versatile electrochemical approaches towards the fabrication of molecular electronic devices

We highlight state-of-the-art electrochemical approaches for diazonium electroreduction on various electrodes that may be suitable for flexible molecular electronic junctions.

Redox-Active Ferrocene grafted on H-Terminated Si(111): Electrochemical Characterization of the Charge Transport Mechanism and Dynamics

Electroactive self-assembled monolayers (SAMs) bearing a ferrocene (Fc) redox couple were chemically assembled on H-terminated semiconducting degenerate-doped n-type Si(111) substrate. This allows to create a Si(111)|organic-spacer|Fc hybrid interface, where the ferrocene moiety is covalently immobilized on the silicon, via two alkyl molecular spacers of different length. Organic monolayer formation was probed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS) measurements, which were also used to estimate thickness and surface assembled monolayer (SAM) surface coverage. Atomic force microscopy (AFM) measurements allowed to ascertain surface morphology and roughness. The single electron transfer process, between the ferrocene redox probe and the Si electrode surface, was probed by cyclic voltammetry (CV) measurements. CVs recorded at different scan rates, in the 10 to 500 mV s⁻¹ range, allowed to determine peak-to-peak separation, half-wave potential, and charge-transfer rate constant (K_ET). The experimental findings suggest that the electron transfer is a one electron quasi-reversible process. The present demonstration of surface engineering of functional redox-active organometallic molecule can be efficient in the field of molecular electronics, surface-base redox chemistry, opto-electronic applications.

A ternary Fe(ii)-terpyridyl complex-based single platform for reversible multiple-ion recognition

Multiple ion-recognition activity by a ternary Fe(ii)-terpyridyl complex, [Fe(PhT)(PT)]2+ (1) (PhT = 4'-phenyl-2,2':6',2''-terpyridine; PT = 4'-pyridyl-2,2':6',2''-terpyridine), is demonstrated for cyanide (CN-), fluoride (F-) and hydroxide (OH-) ions in an aqueous medium with sufficient sensitivity, fast response, reproducibility and selectivity with a dual optical read-out. The sensing event was reversible with the "by-eye" visualization of back and forth colour changes. Three cyanide ions replaced PT from 1, as observed from the crystal structure of the 1 + CN- couple. Fluoride and hydroxide ions appeared to show multivariate interactions with 1. Observed structural and spectral changes correlated well with theoretical calculations. A string of cations at quantitative levels (Ag+/Hg2+/Fe2+/Fe3+) was used to decouple the 1 + anion complex to yield 1, which enabled the recognition of these cations while permitting the reuse of 1 for at least five set-reset cycles.

Non-contact identification and differentiation of illicit drugs using fluorescent films

Sensitive and rapid identification of illicit drugs in a non-contact mode remains a challenge for years. Here we report three film-based fluorescent sensors showing unprecedented sensitivity, selectivity, and response speed to the existence of six widely abused illicit drugs, including methamphetamine (MAPA), ecstasy, magu, caffeine, phenobarbital (PB), and ketamine in vapor phase. Importantly, for these drugs, the sensing can be successfully performed after 5.0 × 10⁵, 4.0 × 10⁵, 2.0 × 10⁵, 1.0 × 10⁵, 4.0 × 10⁴, and 2.0 × 10² times dilution of their saturated vapor with air at room temperature, respectively. Also, presence of odorous substances (toiletries, fruits, dirty clothes, etc.), water, and amido-bond-containing organic compounds (typical organic amines, legal drugs, and different amino acids) shows little effect upon the sensing. More importantly, discrimination and identification of them can be realized by using the sensors in an array way. Based upon the discoveries, a conceptual, two-sensor based detector is developed, and non-contact detection of the drugs is realized.

Sensitive and rapid identification of illicit drugs in a non-contact mode remains a challenge. Here, the authors report three film-based fluorescent sensors showing remarkable sensitivity, selectivity and response speed to six widely abused illicit drugs in vapor phase.

Covalently Assembled Monolayers of Homo- and Heteroleptic FeII -Terpyridyl Complexes on SiOx and ITO-Coated Glass Substrates: An Experimental and Theoretical Study

Well-defined Fe^II -terpyridyl monolayers were fabricated on SiO_x and conductive ITO-coated glass substrates through covalent-bond formation between the metallo-organic complexes and a preassembled coupling layer. Three different homo- and heteroleptic complexes with terminal pyridyl, amine, and phenyl groups were tested. All the films were found to be densely packed and homogeneous, and consist of molecules standing upright. They exhibited high thermal (up to ≈220 °C) and temporal (up to 5 h at 100 °C) stability. The UV/Vis spectra of the monolayers showed pronounced metal-to-ligand charge-transfer bands with a significant redshift compared with the solution spectra of the metallo-ligands with a pendant pyridyl group quaternized with the coupling layer, whereas the shift was significantly smaller when the coupling layer was bonded to the primary amine (-NH₂ ) group of the complex. Cyclic voltammograms of the monolayers showed reversible, one-electron redox behavior and suggested strong electronic coupling between the confined molecules and the underlying substrate. Analysis of the electrochemistry data allowed us to estimate the charge-transfer rate constant between the metal center and the substrate. Additionally, detailed quantum-chemical calculations were performed to support and rationalize the experimentally observed photophysical properties of the Fe^II -terpyridyl complexes both in the solution state and when bound to a SiO_x -based substrate.

Nanometric Assembly of Functional Terpyridyl Complexes on Transparent and Conductive Oxide Substrates: Structure, Properties, and Applications

Over the last few decades, molecular assemblies on solid substrates have become increasingly popular, challenging the traditional systems and materials in terms of better control over molecular structure and function at the nanoscale. A variety of such assemblies with high complexity and adjustable properties was generated on the basis of organic, inorganic, organometallic, polymeric, and biomolecular building blocks. Particular versatile elements in this context are terpyridyls due to their wide design flexibility, ease of functionalization, and ability to coordinate to a broad variety of transition-metal ions without forming diastereoisomers, which facilitates tuning of their optical and electronic properties. Specifically, metal-terpyridyl complexes are worthy building blocks for generating optoelectronically active assemblies on technologically relevant transparent and conductive oxide substrates. In this context, the present Account summarizes our recent results on the preparation, characterization, and applications of nanometric (2-10 nm) surface-confined molecular assemblies of Cu²⁺, Fe²⁺, Ru²⁺, and Os²⁺-terpyridyl complexes on SiO_x-based substrates (glass, quartz, silicon, and ITO-coated glass). These assemblies rely on covalent bond formation between the iodo-/chloro-terminated functionalized SiO_x substrates and the pendant group (mostly pyridyl) hosted on the terpyridyl complexes. Such an anchoring provides excellent thermal, temporal, radiative, and electrochemical stability to the assemblies as needed for technological applications. The functional, covalently assembled monolayers were extended to fabricate molecular dyads (bilayers), triads (trilayers), and oligomers by an established layer-by-layer procedure using suitable metallolinkers such as Cu²⁺, Ag⁺, and Pd²⁺. The chemical, optical, and electrochemical properties of these assemblies could be precisely adjusted by selection of proper metal-terpyridyl complexes and/or metallolinkers, so that the resulting systems served, relying on the specific design, as sensors, catalysts, molecular logic gates, and photochromic devices. For instance, a Cu-terpyridyl-based assembly on a glass substrate showed "turn on" detection of ascorbic acid. In another example, heterometallic molecular triads were exposed to redox-active NO⁺ for selective oxidation of the metal ions, and the optical readout was utilized for configuring multiple-input-based molecular logic gates. Furthermore, bias-driven (+0.6 to +1.6 V vs Ag/AgCl) optical properties of the heteroleptic Ru²⁺/Os²⁺-terpyridyl monolayers were modulated and "read out" by spectro-electrochemical techniques demonstrating high charge/information density (3-4 × 10¹⁴ electrons/cm²). Moreover, the manipulation of the M^2+/3+ (M = Fe, Ru, and Os) redox wave in the assembly provided the possibility to create mixed-valence redox-states paving the way toward the fabrication of "multi-bit" memory systems. We truly believe that due to these intriguing characteristics and excellent stability, terpyridyl-based molecular assemblies have the potential to become a versatile platform for the next generation of smart optoelectronic devices.

Metal-ions linked surface-confined molecular dyads of Zn-porphyrin–metallo-terpyridine: an experimental and theoretical study

Solid state molecular engineering is performed on SiOx-substrates by combining transition metal ions and metallo-porphyrins and terpyridyl complexes.