Multidentate adsorbates for self-assembled monolayer films

Phosphonic acid anchored tripodal molecular films on indium tin oxide

Whereas monopodal self-assembling monolayers (SAMs) are most frequently used for surface and interface engineering, tripodal SAMs are less popular due to the difficulty in achieving a reliable and homogeneous bonding configuration. In this context, in the present study, the potential of phosphonic acid (PA) decorated triptycene (TripPA) for formation of SAMs on oxide substrates was studied, using indium tin oxide (ITO) as a representative and application-relevant test support. A combination of several complementary experimental techniques was applied and a suitable monopodal reference system, benzylphosphonic acid (PPA), was used. The resulting data consistently show that TripPA forms well-defined, densely packed, and nearly contamination-free tripodal SAMs on ITO, with the similar parameters and properties as the monopodal reference system. Modification of wetting properties and work function of ITO by non-substituted and cyano-decorated TripPA SAMs was demonstrated, showing a potential of this tripodal system for surface engineering of oxide substrates.

Self-Assembled Monolayers of Push-Pull Chromophores as Active Layers and Their Applications

In recent decades, considerable attention has been focused on the design and development of surfaces with defined or tunable properties for a wide range of applications and fields. To this end, self-assembled monolayers (SAMs) of organic compounds offer a unique and straightforward route of modifying and engineering the surface properties of any substrate. Thus, alkane-based self-assembled monolayers constitute one of the most extensively studied organic thin-film nanomaterials, which have found wide applications in antifouling surfaces, the control of wettability or cell adhesion, sensors, optical devices, corrosion protection, and organic electronics, among many other applications, some of which have led to their technological transfer to industry. Nevertheless, recently, aromatic-based SAMs have gained importance as functional components, particularly in molecular electronics, bioelectronics, sensors, etc., due to their intrinsic electrical conductivity and optical properties, opening up new perspectives in these fields. However, some key issues affecting device performance still need to be resolved to ensure their full use and access to novel functionalities such as memory, sensors, or active layers in optoelectronic devices. In this context, we will present herein recent advances in π-conjugated systems-based self-assembled monolayers (e.g., push-pull chromophores) as active layers and their applications.

Ultrasensitive Detection of Hydrogen Peroxide Using Methylene Blue Grafted on Molecular Wires as Nanozyme with Catalase-like Activity

In this study, we present the development of an electrochemical sensor designed for ultrasensitive detection of endogenous H2O2. This sensor relies on signal amplification achieved through nanozyme activity exhibited by methylene blue (MB) grafted onto a peptide support. The sensor exhibited excellent selectivity and sensitivity, with a limit of detection of 18 nM and a linear detection range of 20-200 nM. Thus, we have validated the concept of the MB-peptide system, serving as both an electroactive label and a catalyst for H2O2 decomposition under electrochemical conditions. The implemented signal amplification system enables the rapid detection of H2O2, with an overall assay time of 1-2 min, a significant improvement compared to amperometric detection using surface-immobilized enzymes.

Tunable physics through coordination chemistry: formation on oxide surface of Ti and Al chelates with 3-hydroxyflavone capable of electron injection and light emission

The optoelectronic features of 3-hydroxyflavone (3HF) self-assembled on the surface of an n-type semiconducting metal oxide (TiO₂) and an insulator (Al₂O₃) are herein investigated. 3HF molecules use the coordinatively unsaturated metal ions present on the oxide surface to form metal complexes, which exhibit different behaviors upon light irradiation, depending on the nature of the metal ion. Specifically, we show that the photoluminescence of the surface species can be modulated according to the chemical properties of the complex (i.e. the binding metal ion), resulting in solid-state emitters in a high quantum yield (about 15%). Furthermore, photoinduced charge injection can be promoted or inhibited, providing a multifunctional hybrid system.

Porous Honeycomb Self-Assembled Monolayers: Tripodal Adsorption and Hidden Chirality of Carboxylate Anchored Triptycenes on Ag

Molecules with tripodal anchoring to substrates represent a versatile platform for the fabrication of robust self-assembled monolayers (SAMs), complementing the conventional monopodal approach. In this context, we studied the adsorption of 1,8,13-tricarboxytriptycene (Trip-CA) on Ag(111), mimicked by a bilayer of silver atoms underpotentially deposited on Au. While tripodal SAMs frequently suffer from poor structural quality and inhomogeneous bonding configurations, the triptycene scaffold featuring three carboxylic acid anchoring groups yields highly crystalline SAM structures. A pronounced polymorphism is observed, with the formation of distinctly different structures depending on preparation conditions. Besides hexagonal molecular arrangements, the occurrence of a honeycomb structure is particularly intriguing as such an open structure is unusual for SAMs consisting of upright-standing molecules. Advanced spectroscopic tools reveal an equivalent bonding of all carboxylic acid anchoring groups. Notably, density functional theory calculations predict a chiral arrangement of the molecules in the honeycomb network, which, surprisingly, is not apparent in experimental scanning tunneling microscopy (STM) images. This seeming discrepancy between theory and experiment can be resolved by considering the details of the actual electronic structure of the adsorbate layer. The presented results represent an exemplary showcase for the intricacy of interpreting STM images of complex molecular films. They are also further evidence for the potential of triptycenes as basic building blocks for generating well-defined layers with unusual structural motifs.

Substrate Materials for Biomolecular Immobilization within Electrochemical Biosensors

Electrochemical biosensors have potential applications for agriculture, food safety, environmental monitoring, sports medicine, biomedicine, and other fields. One of the primary challenges in this field is the immobilization of biomolecular probes atop a solid substrate material with adequate stability, storage lifetime, and reproducibility. This review summarizes the current state of the art for covalent bonding of biomolecules onto solid substrate materials. Early research focused on the use of Au electrodes, with immobilization of biomolecules through ω-functionalized Au-thiol self-assembled monolayers (SAMs), but stability is usually inadequate due to the weak Au-S bond strength. Other noble substrates such as C, Pt, and Si have also been studied. While their nobility has the advantage of ensuring biocompatibility, it also has the disadvantage of making them relatively unreactive towards covalent bond formation. With the exception of Sn-doped In2O3 (indium tin oxide, ITO), most metal oxides are not electrically conductive enough for use within electrochemical biosensors. Recent research has focused on transition metal dichalcogenides (TMDs) such as MoS2 and on electrically conductive polymers such as polyaniline, polypyrrole, and polythiophene. In addition, the deposition of functionalized thin films from aryldiazonium cations has attracted significant attention as a substrate-independent method for biofunctionalization.

Formation of HCN+ in collisions of N+ and N2+ with a self-assembled propanethiol surface on gold

Collisions of N⁺ and N₂⁺ with C3 hydrocarbons, represented by a self assembled monolayer of propanethiol on a polcrystalline gold surface, were investigated by experiments over the incident energy range between 5 eV and 100 eV. For N⁺, formation of HCN⁺ is observed at incident energies of projectile ions as low as 20 eV. In the case of N₂⁺ projectile ions, the yield of HCN⁺ increased above zero only at incident energies of about 50 eV. This collision energy in the laboratory frame corresponds to an activation energy of about 3 eV to 3.5 eV. In the case of N⁺ projectile ions, the yield of HCN⁺ was large for most of the incident energy range, but decreased to zero at incident energies below 20 eV. This may indicate a very small energy threshold for the surface reaction between N⁺ and C3 hydrocarbons of a few tenths of an eV. Such a threshold for the formation of HCN⁺ may exist also for collisions of N⁺ with an adsorbed mixture of hydrocarbon molecules.

Silanization of a Metal-Polyphenol Coating onto Diverse Substrates as a Strategy for Controllable Wettability with Enhanced Performance to Resist Acid Corrosion

Wettability is a crucial characteristic of materials that plays a vital role in surface engineering. Surface modification is the key to changing the wettability of materials, and a simple and universal modification approach is being extensively pursued by researchers. Recently, metal-phenolic networks (MPNs) have been widely studied because they impart versatility and functionality in surface modification. However, an MPN is not stable for long periods, especially under acidic conditions, and is susceptible to pollution by invasive species. Spurred by the versatility of MPNs and various functionalities achieved by silanization, we introduce a general strategy to fabricate functionally stable coatings with controllable surface wettability by combining the two methods. The formation process of MPN and silane-MPN coatings was characterized by spectroscopic ellipsometry (SE), UV-visible-near-infrared (UV-vis-NIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle (WCA), etc. We found that the stability of the MPN was greatly enhanced after silanization, which is attributed to the cross-linking effect that occurs between silane and the MPN, namely, the cross-linking protection produced in this case. Additionally, the wettability of an MPN can be easily changed through our strategy. We trust that our strategy can further extend the applications of MPNs and points toward potential prospects in surface modification.

Antifouling Coatings Generated from Unsymmetrical Partially Fluorinated Spiroalkanedithiols

Biofouling negatively impacts modern society on a daily basis, especially with regard to the important industries of medicine, oil, and shipping. This manuscript describes the preparation and study of model antifouling coatings generated from the adsorption of unsymmetrical partially fluorinated spiroalkanedithiols on gold. The antifouling properties of the self-assembled monolayers (SAMs) derived from the spiroalkanedithiols were compared to SAMs derived from analogous monodentate partially fluorinated and nonfluorinated alkanethiols. The antifouling properties were evaluated using in situ surface plasmon resonance spectroscopy (SPR), ex situ electrochemical quartz crystal microbalance (QCM) measurements, and ex situ ellipsometric thickness measurements. The resistance to nonspecific protein adsorption of the SAMs was evaluated with proteins having a wide range of properties and applications including protamine, lysozyme, bovine serum albumin, and fibrinogen. The results from the SPR and the QCM measurements demonstrated that in most cases, the SAM coatings derived from the partially fluorinated spiroalkanedithiols having mixed hydrocarbon and fluorocarbon tail groups exhibited better antifouling performance when compared to the SAMs derived from their single-component monodentate counterparts. The studies also revealed that while the SPR and the QCM measurements in most cases were able to distinguish the adsorption trends for the SAMs and proteins examined, the ellipsometric thickness measurements were markedly less discriminating. On the whole, these studies validate the use of unsymmetrical partially fluorinated spiroalkanedithiols for generating effective antifouling coatings on metal substrates.

Olefin-Bridged Bidentate Adsorbates for Generating Self-Assembled Monolayers on Gold

A series of custom-designed olefin-bridged bidentate adsorbates composed of an olefin group linking symmetrical hydrocarbon moieties of varying chain lengths was synthesized and used for the preparation of self-assembled monolayers (SAMs) on gold. The structures of the adsorbates are in the form Z-[CH₃(CH₂)_m]₂(C═C)[CH₂SH]₂ (OBCnSH) where m = 12-15 and n = m + 3 (OBC15SH, OBC16SH, OBC17SH, and OBC18SH). The influence of the olefin linker on the structural and interfacial properties of the SAMs was investigated and compared to SAMs formed from analogous n-alkanethiols. Characterization techniques included ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization modulation-infrared reflection-adsorption spectroscopy (PM-IRRAS), and contact angle measurements. The OBCnSH SAMs exhibited ellipsometric thicknesses that were similar to their monodentate counterparts, suggesting that the new olefin-bridged adsorbates pack similarly to the monodentate analogs. Characterization by PM-IRRAS revealed that the OBCnSH SAMs were as conformationally ordered as those derived from the reference n-alkanethiols with the exception of the adsorbate with the shortest chain length OBC15SH, which exhibited low coverage and a liquid-like structure. Unlike the SAMs derived from the n-alkanethiols, the OBCnSH SAMs failed to exhibit "odd-even" effects. However, the OBCnSH SAMs displayed similar hexadecane contact angles as their n-alkanethiol counterparts with the exception of OBC15SH, which exhibited markedly diminished hexadecane contact angles. The similar structural and interfacial properties of the OBCnSH SAMs, when compared to analogous n-alkanethiol SAMs, render the molecular architecture of the olefin-bridged dithiol as a robust platform for the synthesis of adsorbates with two chemically distinct tailgroups for use in the preparation and study of phase-incompatible "conflicted" interfaces.

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

The societal impact of the electronics industry is enormous—not to mention how this industry impinges on the global economy. The foreseen limits of the current technology—technical, economic, and sustainability issues—open the door to the search for successor technologies. In this context, molecular electronics has emerged as a promising candidate that, at least in the short-term, will not likely replace our silicon-based electronics, but improve its performance through a nascent hybrid technology. Such technology will take advantage of both the small dimensions of the molecules and new functionalities resulting from the quantum effects that govern the properties at the molecular scale. An optimization of interface engineering and integration of molecules to form densely integrated individually addressable arrays of molecules are two crucial aspects in the molecular electronics field. These challenges should be met to establish the bridge between organic functional materials and hard electronics required for the incorporation of such hybrid technology in the market. In this review, the most advanced methods for fabricating large-area molecular electronic devices are presented, highlighting their advantages and limitations. Special emphasis is focused on bottom-up methodologies for the fabrication of well-ordered and tightly-packed monolayers onto the bottom electrode, followed by a description of the top-contact deposition methods so far used.

Tetrapodal Diazatriptycene Enforces Orthogonal Orientation in Self-Assembled Monolayers

Conformationally rigid multipodal molecules should control the orientation and packing density of functional head groups upon self-assembly on solid supports. Common tripods frequently fail in this regard because of inhomogeneous bonding configuration and stochastic orientation. These issues are circumvented by a suitable tetrapodal diazatriptycene moiety, bearing four thiol-anchoring groups, as demonstrated in the present study. Such molecules form well-defined self-assembled monolayers (SAMs) on Au(111) substrates, whereby the tetrapodal scaffold enforces a nearly upright orientation of the terminal head group with respect to the substrate, with at least three of the four anchoring groups providing thiolate-like covalent attachment to the surface. Functionalization by condensation chemistry allows a large variety of functional head groups to be introduced to the tetrapod, paving the path toward advanced surface engineering and sensor fabrication.

Radical polymerization as a versatile tool for surface grafting of thin hydrogel films

The surface of solid substrates is the main part that interacts with the environment.

Raman reporters derived from aryl diazonium salts for SERS encoded-nanoparticles

A new generation of surface-enhanced Raman scattering encoded-nanoparticles has been designed by combining aryl diazonium salt chemistry and gold nanoparticles.

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.

Highly Reproducible Formation of a Polymer Single-Molecule Junction for a Well-Defined Current Signal

Single-molecule devices attract much interest in the development of nanoscale electronics. Although a variety of functional single molecules for single-molecule electronics have been developed, there still remains the need to implement sophisticated functionalization toward practical applications. Given its superior functionality encountered in macroscopic materials, a polymer could be a useful building block in the single-molecule devices. Therefore, a molecular junction composed of polymer has now been created. Furthermore, an automated algorithm was developed to quantitatively analyze the tunneling current through the junction. Quantitative analysis revealed that the polymer junction exhibits a higher formation probability and longer lifetime than its monomer counterpart. These results suggest that the polymer provides a unique opportunity to design both stable and highly functional molecular devices for nanoelectronics.

Self-Assembled Bilayers as an Anchoring Strategy: Catalysts, Chromophores, and Chromophore-Catalyst Assemblies

Anchoring strategies for immobilization of molecular catalysts, chromophores, and chromophore-catalyst assemblies on electrode surfaces play an important role in solar energy conversion devices such as dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. They are also important in interfacial studies with surface-bound molecules including electron-transfer dynamics and mechanistic studies related to small molecule activation catalysis. Significant progress has been made in this area, but many challenges remain in terms of stability, synthetic complexity, and versatility. We report here a new anchoring strategy based on self-assembled bilayers. This strategy takes advantage of noncovalent interactions between long alkyl chains chemically bound to a metal-oxide electrode surface and long alkyl chains on the molecule being anchored. The new methodology is applicable to the heterogenization of both catalysts and chromophores as well as to the in situ "synthesis" of chromophore-catalyst assemblies on the electrode surface.

Probabilistic mapping of single molecule junction configurations as a tool to achieve the desired geometry of asymmetric tripodal molecules

Four molecules containing identical tripodal anchors and p-oligophenylene molecular wires of increasing length were used to demonstrate tuning of the asymmetric molecular junction to the desired geometry by probabilistic mapping of single molecule junction configurations in a scanning tunnelling microscopy break junction experiment.

EIS-Based Biosensors in Foodborne Pathogen Detection with a Special Focus on Listeria monocytogenes

In this chapter methods and protocols for surfaces adapted to electrochemical impedance detection, antibody binding, electrolyte couples used, and instrumentation for EIS Biosensing are presented. Various technical bottlenecks have been overcome in recent years. Other limitations still present in this technique are discussed. We present the most recent applications in food pathogen detection based on EIS methods, as well as using other antibody-based platforms.

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.

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

A custom-designed series of unsymmetrical spiroalkanedithiols having tailgroups comprised of a terminally fluorinated chain and a hydrocarbon chain of varying lengths were synthesized and used to prepare self-assembled monolayers (SAMs) on gold substrates. The specific structure of the adsorbates was of the form [CH₃(CH₂)n][CF₃(CF₂)₇(CH₂)₈]C[CH₂SH]₂, where n = 7, 9, and 15 (designated as F8H10-C10, F8H10-C12, and F8H10-C18, respectively). The influence of the length of the hydrocarbon chain in the bidentate dithiol on the structure and interfacial properties of the monolayer was explored. A structurally analogous partially fluorinated monodentate alkanethiol and the corresponding normal alkanethiols were used to generate appropriate SAMs as reference systems. Measurements of ellipsometric thickness showed an unexpectedly low film thickness for the SAMs derived from the bidentate adsorbates, possibly due to disruptions in interchain packing caused by the fluorocarbon chains (i.e., phase-incompatible fluorocarbon-hydrocarbon interactions), ultimately giving rise to loosely packed and disordered films. Analysis by X-ray photoelectron spectroscopy (XPS) were also consistent with a model in which the films were loosely packed; additionally, the XPS spectra confirmed the attachment of the sulfur headgroups of the bidentate adsorbates onto the gold substrates. Studies of the SAMs by polarization modulation-infrared reflection-adsorption spectroscopy (PM-IRRAS) suggested that as the length of the hydrocarbon chain in the adsorbates was extended, a more ordered surface was achieved by reducing the tilt of the fluorocarbon segment. The wettability data indicated that the adsorbates with longer alkyl chains were less wettable than those with shorter alkyl chains, likely due to an increase in interchain van der Waals forces in the former.

Nanoporous Anodic Alumina Photonic Crystals for Optical Chemo- and Biosensing: Fundamentals, Advances, and Perspectives

Optical sensors are a class of devices that enable the identification and/or quantification of analyte molecules across multiple fields and disciplines such as environmental protection, medical diagnosis, security, food technology, biotechnology, and animal welfare. Nanoporous photonic crystal (PC) structures provide excellent platforms to develop such systems for a plethora of applications since these engineered materials enable precise and versatile control of light⁻matter interactions at the nanoscale. Nanoporous PCs provide both high sensitivity to monitor in real-time molecular binding events and a nanoporous matrix for selective immobilization of molecules of interest over increased surface areas. Nanoporous anodic alumina (NAA), a nanomaterial long envisaged as a PC, is an outstanding platform material to develop optical sensing systems in combination with multiple photonic technologies. Nanoporous anodic alumina photonic crystals (NAA-PCs) provide a versatile nanoporous structure that can be engineered in a multidimensional fashion to create unique PC sensing platforms such as Fabry⁻Pérot interferometers, distributed Bragg reflectors, gradient-index filters, optical microcavities, and others. The effective medium of NAA-PCs undergoes changes upon interactions with analyte molecules. These changes modify the NAA-PCs' spectral fingerprints, which can be readily quantified to develop different sensing systems. This review introduces the fundamental development of NAA-PCs, compiling the most significant advances in the use of these optical materials for chemo- and biosensing applications, with a final prospective outlook about this exciting and dynamic field.

Unconventional Single-Molecule Conductance Behavior for a New Heterocyclic Anchoring Group: Pyrazolyl

Electrical conductance across a molecular junction is strongly determined by the anchoring group of the molecule. Here we highlight the unusual behavior of 1,4-bis(1H-pyrazol-4-ylethynyl)benzene that exhibits unconventional junction current versus junction-stretching distance curves, which are peak-shaped and feature two conducting states of 2.3 × 10^-4 G₀ and 3.4 × 10^-4 G₀. A combination of theory and experiments is used to understand the conductance of single-molecule junctions featuring this new anchoring group, i.e., pyrazolyl. These results demonstrate that the pyrazolyl moiety changes its protonation state and contact binding during junction evolution and that it also binds in either end-on or facial geometries with gold contacts. The pyrazolyl moiety holds general interest as a contacting group, because this linkage leads to a strong double anchoring of the molecule to the gold electrode, resulting in enhanced conductance values.

N-Heterocyclic-Carbene-Treated Gold Surfaces in Pentacene Organic Field-Effect Transistors: Improved Stability and Contact at the Interface

N-Heterocyclic carbenes (NHCs), which react with the surface of Au electrodes, have been successfully applied in pentacene transistors. With the application of NHCs, the charge-carrier mobility of pentacene transistors increased by five times, while the contact resistance at the pentacene-Au interface was reduced by 85 %. Even after annealing the NHC-Au electrodes at 200 °C for 2 h before pentacene deposition, the charge-carrier mobility of the pentacene transistors did not decrease. The distinguished performance makes NHCs as excellent alternatives to thiols as metal modifiers for the application in organic field-effect transistors (OFETs).

Ultrastable Gold Nanoparticles Modified by Bidentate N-Heterocyclic Carbene Ligands

Highly stable gold nanoparticles (Au NPs) functionalized by bidentate N-heterocyclic carbene (NHC) ligands have been synthesized by top-down and bottom-up approaches. A detailed study of the effect of alkylation, denticity, and method of synthesis has led to the production of NHC-stabilized nanoparticles with higher thermal stability than bi- and tridentate thiol-protected Au NPs and than monodentate NHC-stabilized NPs. Importantly, bidentate NHC-protected NPs also displayed unprecedented stability to external thiol, which has been an unsolved problem to date with all nanoparticles. Thus, multidentate NHC ligands are an important, and as yet unrecognized, step forward for the preparation of high stability nanomaterials.

Green and Facile Synthesis of Highly Stable Gold Nanoparticles via Hyperbranched Polymer In-Situ Reduction and Their Application in Ag⁺ Detection and Separation

The development of a green and facile strategy for synthesizing high stable gold nanoparticles (AuNPs) is still highly challenging. Additionally, the main problems regarding AuNPs based colorimetric sensors are their poor selectivity and low sensitivity, as well their tendency to aggregate during their synthesis and sensing process. Herein, we present an in-situ reduction strategy to synthesize thermoresponsive hyperbranched polymer (i.e., Hyperbranched polyethylenimine-terminal isobutyramide (HPEI-IBAm)) functionalized AuNPs. The HPEI-IBAm-AuNPs show excellent thermal stability up to 200 °C, high tolerance of a wide range of pH value (3⁻13), and high salt resistance. HPEI-IBAm acted as the template, the reducing agent, and the stabilizing agent for the preparation of AuNPs. The HPEI-IBAm-AuNPs can be used as colorimetric sensors for the detection of Ag⁺. In the detecting process, HPEI-IBAm serves as a trigger agent to cause an unusual color change from red to brown. This new non-aggregation-based colorimetric sensor showed high stability (maintaining the color lasting without fading), high selectivity, and high sensitivity with an extremely low detection limit of 7.22 nM and a good linear relationship in a wide concentration range of 0⁻2.0 mM (R² = 0.9921). Significantly, based on the thermoresponsive property of the HPEI-IBAm, the AuNPs/Ag composites can be separated after sensing detection, which can avoid secondary pollutions. Therefore, the green preparation and the applications of the unusual colorimetric sensor truly embody the concepts of energy saving, environmental protection, and sustainable development.

Fluorescence-Free Biosensor Methods in Detection of Food Pathogens with a Special Focus on Listeria monocytogenes

Food pathogens contaminate food products that allow their growth on the shelf and also under refrigerated conditions. Therefore, it is of utmost importance to lower the limit of detection (LOD) of the method used and to obtain the results within hours to few days. Biosensor methods exploit the available technologies to individuate and provide an approximate quantification of the bacteria present in a sample. The main bottleneck of these methods depends on the aspecific binding to the surfaces and on a change in sensitivity when bacteria are in a complex food matrix with respect to bacteria in a liquid food sample. In this review, we introduce surface plasmon resonance (SPR), new advancements in SPR techniques, and electrochemical impedance spectroscopy (EIS), as fluorescence-free biosensing technologies for detection of L. monocytogenes in foods. The application of the two methods has facilitated L. monocytogenes detection with LOD of 1 log CFU/mL. Further advancements are envisaged through the combination of biosensor methods with immunoseparation of bacteria from larger volumes, application of lab-on-chip technologies, and EIS sensing methods for multiplex pathogen detection. Validation efforts are being conducted to demonstrate the robustness of detection, reproducibility and variability in multi-site installations.

N-Heterocyclic Carbene Self-assembled Monolayers on Copper and Gold: Dramatic Effect of Wingtip Groups on Binding, Orientation and Assembly

Self-assembled monolayers of N-heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N-substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl- or ethyl-substituted NHCs lie flat. Temperature-programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes =152±10 kJ mol-1 . NHCs that bind upright desorb cleanly, while flat-lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2 Cu species by extraction of a copper atom from the surface after annealing.

N-Heterocyclic Carbene Self-Assembled Monolayers on Gold as Surface Plasmon Resonance Biosensors

Surface plasmon resonance (SPR)-based biosensing is a powerful tool to study the recognition processes between biomolecules in real-time without need for labels. The use of thiol chemistry is a critical component in surface functionalization of various SPR biosensor surfaces on gold. However, its use is hampered by the high propensity for oxidation of the gold-thiol linkage even in ambient atmosphere, resulting in a short lifetime of SPR sensor chips unless strict precautions are taken. Herein, we describe an approach to overcome this limitation by employing highly robust self-assembled monolayers (SAMs) of alkylated N-heterocyclic carbenes (NHCs) on gold. An alkylated NHC sensor surface was developed and its biosensing capabilities were compared to a commercial thiol-based analogue-a hydrophobic association (HPA) chip-in terms of its ability to act as a reliable platform for biospecific interaction analysis under a wide range of conditions. The NHC-based SPR sensor outperforms related thiol-based sesnsors in several aspects, including lower nonspecific binding capacity, better chemical stability, higher reproducibility, shorter equilibration time, and longer life span. We also demonstrate that the NHC-based sensor can be used for rapid and efficient formation of a hybrid lipid bilayer for use in membrane interaction studies. Overall, this work identifies the great promise in designing NHC-based surfaces as a new technology platform for SPR-based biosensing.

Homogeneously Mixed Monolayers: Emergence of Compositionally Conflicted Interfaces

The ability to manipulate interfaces at the nanoscale via a variety of thin-film technologies offers a plethora of avenues for advancing surface applications. These include surfaces with remarkable antibiofouling properties as well as those with tunable physical and electronic properties. Molecular self-assembly is one notably attractive method used to decorate and modify surfaces. Of particular interest to surface scientists has been the thiolate-gold system, which serves as a reliable method for generating model thin-film monolayers that transform the interfacial properties of gold surfaces. Despite widespread interest, efforts to tune the interfacial properties using mixed adsorbate systems have frequently led to phase-separated domains of molecules on the surface with random sizes and shapes depending on the structure and chemical composition of the adsorbates. This feature article highlights newly emerging methods for generating mixed thin-film interfaces, not only to enhance the aforementioned properties of organic thin films, but also to give rise to interfacial compositions never before observed in nature. An example would be the development of monolayers formed from bidentate adsorbates and other unique headgroup architectures that provide the surface bonding stability necessary to allow the assembly of interfaces that expose mixtures of chains that are fundamentally different in character (i.e., either phase-incompatible or structurally dissimilar), producing compositionally "conflicted" interfaces. By also exploring the prior efforts to produce such homogeneously blended interfaces, this feature article seeks to convey the relationships between the methods of film formation and the overall properties of the resulting interfaces.

Spatial and Lateral Control of Functionality by Rigid Molecular Platforms

Surface mounted molecular devices have received significant attention in the scientific community because of their unique ability to construct functional materials. The key involves the platform on which the molecular device works on solid substrates, such as in solid-liquid or solid-vacuum interfaces. Here, we outline the concept of rigid molecular platforms to immobilize active functionality atop flat surfaces in a controllable manner. Most of these (multipodal) platforms have at least three anchoring groups to control the spatial arrangement of the protruding functional moieties and form mechanically stable and electronically tuned contacts to the underlying substrate. Another approach is based on employing of flat aromatic scaffolds bearing perpendicular functionalities that form stable lateral assemblies on various surfaces. Emphasis is placed on the need for controllable assembly and separation of these tailor-made molecules that expose functionalities at the molecular scale. The discussions are focused on the different molecular designs realizing functional 3D architectures on surfaces, the role of various anchoring strategies to control the spatial arrangement, and structural considerations controlling physical features like the coupling to the surface or the available space for sterically demanding molecular operations.

Nanophase Segregation of Self-Assembled Monolayers on Gold Nanoparticles

Nanophase segregation of a bicomponent thiol self-assembled monolayer is predicted using atomistic molecular dynamics simulations and experimentally confirmed. The simulations suggest the formation of domains rich in acid-terminated chains, on one hand, and of domains rich in amide-functionalized ethylene glycol oligomers, on the other hand. In particular, within the amide-ethylene glycol oligomers region, a key role is played by the formation of interchain hydrogen bonds. The predicted phase segregation is experimentally confirmed by the synthesis of 35 and 15 nm gold nanoparticles functionalized with several binary mixtures of ligands. An extensive study by transmission electron microscopy and electron tomography, using silica selective heterogeneous nucleation on acid-rich domains to provide electron contrast, supports simulations and highlights patchy nanoparticles with a trend toward Janus nano-objects depending on the nature of the ligands and the particle size. These results validate our computational platform as an effective tool to predict nanophase separation in organic mixtures on a surface and drive further exploration of advanced nanoparticle functionalization.

Bidentate Aromatic Thiols on Gold: New Insight Regarding the Influence of Branching on the Structure, Packing, Wetting, and Stability of Self-Assembled Monolayers on Gold Surfaces

A series of 2-phenylpropane-1,3-dithiol derivatives with single (R1ArDT), double (R2ArDT), and triple (R3ArDT) octadecyloxy chains substituted at the 4-, 3,5-, and, 3,4,5-positions, respectively, on the aromatic ring were synthesized and used to form self-assembled monolayers (SAMs) on gold. Insight into the relationship between the surface chain and headgroup packing densities was investigated by varying the number of surface chains for the bidentate adsorbates in these monolayers. Characterization of the resulting SAMs using ellipsometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection-absorption spectroscopy, and contact angle goniometry revealed that the tailgroups become more comformationally ordered and more densely packed as the number of alkyl chains per adsorbate was increased. Conversely, the molecular packing density (i.e., number of molecules per unit area) decreased as the number of alkyl chains per adsorbate was increased. Of particular interest, the desorption profiles obtained in isooctane at 80 °C suggested that the bidentate adsorbate with the most densely packed alkyl chains, R3ArDT, was significantly more stable than the other SAMs, producing the following relative order for thermal stability for the dithiolate SAMs: R3ArDT > R2ArDT > R1ArDT.

Rapid Surface Functionalization of Hydrogen-Terminated Silicon by Alkyl Silanols

Surface functionalization of inorganic semiconductor substrates, particularly silicon, has focused attention toward many technologically important applications, involving photovoltaic energy, biosensing and catalysis. For such modification processes, oxide-free (H-terminated) silicon surfaces are highly required, and different chemical approaches have been described in the past decades. However, their reactivity is often poor, requiring long reaction times (2–18 h) or the use of UV light (10–30 min). Here, we report a simple and rapid surface functionalization for H-terminated Si(111) surfaces using alkyl silanols. This catalyst-free surface reaction is fast (15 min at room temperature) and can be accelerated with UV light irradiation, reducing the reaction time to 1–2 min. This grafting procedure leads to densely packed organic monolayers that are hydrolytically stable (even up to 30 days at pH 3 or 11) and can display excellent antifouling behavior against a range of organic polymers.

Structure, Wettability, and Thermal Stability of Organic Thin-Films on Gold Generated from the Molecular Self-Assembly of Unsymmetrical Oligo(ethylene glycol) Spiroalkanedithiols

Organic thin-films on gold were prepared from a set of new, custom-designed bidentate alkanethiols possessing a mixture of normal alkane and methoxy-terminated tri(ethylene glycol) chains. The new unsymmetrical spiroalkanedithiol adsorbates were of the form [CH₃O(CH₂CH₂O)₃(CH₂)₅]-[CH₃(CH₂)_n+1]C[CH₂SH]₂ where n = 3 and 14; designated EG3C7-C7 and EG3C7-C18, respectively. Their corresponding self-assembled monolayers (SAMs) on gold were characterized and compared with monothiol SAMs derived from an analogous normal alkanethiol (C18SH) and an alkanethiol terminated with an oligo(ethylene glycol) (OEG) moiety (i.e., EG3C7SH). Ellipsometric data revealed reduced film thicknesses for the double-chained dithiolate SAMs, which perhaps arose from the phase-incompatible merger of a hydrocarbon chain with an OEG moiety, contributing to disorder in the films and/or an increase in chain tilt. The comparable wettabilities of the SAMs derived from EG3C7SH and EG3C7-C7, using water as the contacting liquid, are consistent with exposure of the OEG moieties at both interfaces, whereas the lower wettability of the SAM derived from EG3C7-C18 is consistent with exposure of hydrocarbon chains at the interface. The data collected by X-ray photoelectron spectroscopy confirmed the formation of the new OEG-terminated dithiolate SAMs, and also revealed them as less densely packed monolayers due in part to the large molecular cross section of the OEG moieties and to their double-chained structure with dual surface bonds. Mixed SAMs formed from pairs of monothiols having chain compositions analogous to those of the chains of the new dithiols showed that an EG3C7SH/heptanethiol-mixed SAM and the EG3C7-C7 SAM produced almost identical characterization data, revealing the favorable film formation dynamics for adsorbate structures where the alkyl chains can assemble beneath the phase-incompatible OEG termini. For the mixed SAM formed from EG3C7SH/C18SH, the data indicate that the EG3C7SH component failed to incorporate in the film, demonstrating that the blending of phase-incompatible chains is sometimes best accomplished when both chains exist on a single adsorbate structure. Furthermore, the results of solution-phase thermal desorption tests revealed that the OEG-terminated films generated from the bidentate EG3C7-C7 and EG3C7-C18 adsorbates exhibit enhanced thermal stability when compared to the film generated from monodentate EG3C7SH. In a brief study of protein adsorption, the multicomponent SAMs showed a greater ability to resist the adsorption of fibrinogen on their surfaces when compared to the SAM derived from C18SH, but not better than the monolayer derived from EG3C7SH.

Effect of phosphonate monolayer adsorbate on the microwave photoresponse of TiO2 nanotube membranes mounted on a planar double ring resonator

In this study, the effects of a phosphonate molecular monolayer adsorbed on the surface of a free-standing self-organized TiO2 nanotube membrane, on the microwave photoresponse of the membrane are presented. This phenomenon is monitored using planar microwave sensors. A double ring resonator is utilized to monitor the permittivity and conductivity variation on the monolayer coated membrane and the sensor environment separately. It is shown that the rise time and subsequent decay of the amplitude (A), resonance frequency (f 0) and quality factor (Q) of the resonator depend on the existence and the type of the monolayer coating the membrane. Three different monolayers of n-decylphosphonic acid (DPA), 1H, 1H', 2H, 2H'-perfluorodecyl phosphonic acid (PFDPA) and 16-phosphonohexadecanoic acid adsorbed on the titania nanotube membrane are investigated while monitoring their microwave properties during the illumination time period and in the relaxation period, which demonstrate different behavior in comparison to each other and to the bare nanotube membrane layer. The effect of humidity on the TiO2 nanotube membrane with and without different monolayers is also studied and the results demonstrate distinguishable microwave responses. While each of the monolayer-coated membranes exhibited an attenuation of the photo-induced change in A, f 0 and Q with respect to the bare membrane, PFDPA-coated membranes showed the smallest relative change in the monitored microwave parameters upon ultraviolet illumination and upon the introduction of different levels of humidity. These effects are explained on the basis of surface trap passivation by the monolayers as well as the hydrophobicity of the monolayers. Our work also shows how the interactions of self-assembled monolayers with charge carriers and surface states on metal oxides may be used to indirectly sense their presence through measurement of the microwave response.

Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing

The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol⁻¹) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.

Self-assembled monolayers (SAMs) have shown tremendous number of applications but can suffer from low stability. Here, the authors report air and bench stable carbene precursors allowing facile SAM formation, and furthermore demonstrate an application in biosensing