Solution-solid Interface Research Articles

Deciphering the factors influencing electric field mediated polymerization and depolymerization at the solution–solid interface

Strong and oriented electric fields are known to influence structure as well as reactivity. The strong electric field (EF) between the tip of a scanning tunneling microscope (STM) and graphite has been used to modulate two-dimensional (2D) polymerization of aryl boronic acids where switching the polarity of the substrate bias enabled reversible transition between self-assembled molecular networks of monomers and crystalline 2D polymer (2DP) domains. Here, we untangle the different factors influencing the EF-mediated (de)polymerization of a boroxine-based 2DP on graphite. The influence of the solvent was systematically studied by varying the nature from polar protic to polar aprotic to non-polar. The effect of monomer concentration was also investigated in detail with a special focus on the time-dependence of the transition. Our experimental observations indicate that while the nucleation of 2DP domains is not initiated by the applied electric field, their depolymerization and subsequent desorption, are a consequence of the change in the polarity of the substrate bias within the area scanned by the STM tip. We conclude that the reversible transition is intimately linked to the bias-induced adsorption and desorption of the monomers, which, in turn, could drive changes in the local concentration of the monomers.

Self-Assembly of Discrete Oligomers of Naphthalenediimides in Bulk and on Surfaces.

Here, we report on the synthesis of discrete oligomers of alkyl-bridged naphthalenediimides (NDIs) and study their molecular nanostructures both in bulk, in solution, and at the liquid-solid interface. Via an iterative synthesis method, multiple NDI cores were bridged with short and saturated alkyl-diamines (C3 and C12 ) or long and unsaturated alkyl-diamines (u2 C33 to u8 C100 ) at their imide termini. The strong intermolecular interaction between the NDI cores was observed by probing their photophysical properties in solution. In bulk, the discrete NDI oligomers preferentially ordered in lamellar morphologies, irrespective of whether a saturated or unsaturated spacer was employed. Moreover, both the molecular architecture as well as the crystallization conditions play a significant role in the nanoscale ordering. The long unsaturated alkyl chains lead preferably to folded-chain conformations while their saturated analogues form stretched arrangements. At the solution-solid interface, well-defined lamellar regions were observed. These results show that precision in chemical structure alone is not sufficient to reach well-defined structures of discrete oligomers, but that it must be combined with precision in processing conditions.

Kinetics of Oxygen Coordination to a Graphite Supported Cobalt Porphyrin

Single molecule level time-dependent binding of O2 to cobalt(II) octaethylporphyrin [CoOEP] supported on graphite at the solution-solid interface was observed with scanning tunneling microscopy (STM). Using STM movies of the surface and a stochastic kinetic analysis it was possible to extract both the binding and dissociation rate constants for this reversible equilibrium process. This study was the first of its kind and has opened the way for kinetic analysis if reversible surface reactions at the single molecule level by STM methodology. These results are transformative for STM based studies because they demonstrate both the possibility of, and the methodology for, performing kinetic binding studies at the solution solid interface at room temperature.

Adsorption of a photoresponsive surfactant at the water–quartz interface: The role of UV irradiation and cucurbit[7]uril

It is a challenge to develop a method of controlling the adsorption of small molecules at different interfaces. Here, we report the photocontrolled adsorption of a surfactant containing an azobenzene group (AZO) at the aqueous solution–quartz interface. The adsorption of the surfactant decreases at the interface and the corresponding adsorption layer becomes more rigid after UV irradiation. In addition, the adsorption of the surfactant at the interface is also decreased by addition of cucurbit[7]uril (Q[7]) due to the host–guest interaction between Q[7] and the surfactant. Adsorption of surfactant is stronger at the aqueous solution–quartz interface than at the aqueous solution–air one. The aqueous solution–quartz interfacial tension (γsl) of the surfactant is decreased by using UV irradiation or Q[7]. The adsorption mechanisms of the surfactant were investigated. Our study provides the methods to control the adsorption of the small molecules at the aqueous solution–solid interface.

Plasmonic Scattering Imaging of Surface-Bonded Nanoparticles at the Solution-Solid Interface.

Imaging nanoscale objects at interfaces is essential for revealing surface-tuned mechanisms in chemistry, physics, and life science. Plasmonic-based imaging, a label-free and surface-sensitive technique, has been widely used for studying the chemical and biological behavior of nanoscale objects at interfaces. However, direct imaging of surface-bonded nanoscale objects remains challenging due to uneven image backgrounds. Here, we present a new surface-bonded nanoscale object detection microscopy that eliminates strong background interference by reconstructing accurate scattering patterns at different positions. Our method effectively functions at low signal-to-background ratios, allowing for optical scattering detection of surface-bonded polystyrene nanoparticles and severe acute respiratory syndrome coronavirus 2 pseudovirus. It is also compatible with other imaging configurations, such as bright-field imaging. This technique complements existing methods for dynamic scattering imaging and broadens the applications of plasmonic imaging techniques for high-throughput sensing of surface-bonded nanoscale objects, enhancing our understanding of the properties, composition, and morphology of nanoparticles and surfaces at the nanoscale.

Spontaneous and scanning-assisted desorption-adsorption dynamics in porous supramolecular networks at the solution-solid interface.

Self-assembled molecular networks (SAMNs) are formed by the spontaneous assembly of molecules on surfaces. On conductive atomically flat surfaces, and also at the liquid-solid interface, scanning tunneling microscopy (STM) can follow their growth dynamics. Desorption and adsorption dynamics are difficult to probe through the liquid-solid interface. Porous molecular networks are of particular interest because they may act as platforms for sensing and host-guest chemistry. Very little is known though about their stability, particularly in a liquid environment. To this end, we have investigated the desorption/adsorption dynamics of supramolecular porous monolayers of alkoxylated dehydrobenzo[12]annulene (DBA) derivatives at the interface between highly oriented pyrolytic graphite, the substrate, and 1-phenyloctane, the liquid. To trace the dynamics, structurally analogous chiral DBA derivatives were used as marker molecules, which co-assemble with the achiral ones forming the supramolecular network. This approach reveals the time scales of the adsorption/desorption dynamics, the significance of temperature, and the important role of the STM tip in inducing dynamics.

Single-Particle Electrochemical Imaging Provides Insights into Silver Nanoparticle Dissolution at the Solution-Solid Interface.

Dissolution of nanoparticles is an environmental interfacial process that affects the transformation of nanoparticles. Understanding the dissolution processes of nanoparticles is important to predict their fate in the aquatic environment. However, studying nanoparticle dissolution kinetics is still challenging since dissolution is usually coupled with nanoparticle aggregation. Here, we probed the dissolution process of Ag nanoparticles at the single-particle level by surface plasmon resonance microscopy. The single-particle imaging capability enabled us to classify Ag nanoparticles, measure the dissolution dynamics of single nanoparticles, and correlate the aggregation size with oxidation activity. Moreover, we studied the dual effect of natural organic matter on the dissolution of Ag nanoparticles and validated this result with real natural freshwater. Our study provides new insights into the dissolution of Ag nanoparticles, and this technique can be extended for other nanomaterials to evaluate their fate in aquatic environments.

Insight into ions adsorption at the C-S-H gel-aqueous electrolyte interface: From atomic-scale mechanism to macroscopic phenomena

Ions adsorption at the solution-solid interface is ubiquitous, and accordingly, much interest has been generated to pursue an intrinsic understanding of fundamental yet elusive adsorption law. Herein, general mechanisms of the ion adsorption by C-S-H gel were given from the perspective of electrostatic force. By means of molecular dynamics, a thermal-equilibrated electrical double layer (EDL) with the Stern layer resembling parallel-plates capacitor can be naturally produced at the interface. Aqueous Na+ partly dehydrated and penetrated into the channel between silicate chains, while Cl- was electrostatically attracted and yet repulsed by the substrate and adsorbed as a fully hydrated species. Comprehensive interpretation was given in terms of adsorption/desorption dynamics, atomic arrangement, and structure stability, etc. Adsorption behavior of C-S-H was controlled by the electrostatic interaction between charged species, rather than ions/solid’s hydration shell. Water molecules just played roles in filling the vacancy of ions/solid’s coordinated sites. Simulated and experimental results linked the atomic-scale mechanism to macroscopic phenomena, providing a clear picture of ions adsorption at the interface and also a general interpretation to electric potential results.

STM Investigation of Y[C6s-Pc]2 and Y[C4o-Pc]2 Complexes at the Solution/Solid Interface: Substrate Effects, Sub-Molecular Resolution, and Covalently Saturated Sulfur

Substituents present on a molecule are known to significantly control the assembly, adsorption, and orientation behavior on solid surfaces. Using scanning tunneling microscopy (STM), self-assembly of the Y[C6S-Pc]2and Y[C4O-Pc]2 double-decker complexes were investigated at a solution-solid interface. At concentrations above 1 µM, Y[C6S-Pc]2 formed well-defined monolayers with low defect density on highly oriented pyrolytic graphite (HOPG). On Au(111), on the other hand, it formed dense groups of small islands with some isolated molecules. There was a clear preference for multiples of 15 degrees between the orientations of the islands. A clear visualization of the Y[C6S-Pc]2 inner molecular structure including the sulfur atoms was achieved. At concentrations below 1 µM, stable isolated single molecules were observed only on Au(111), not on HOPG. The thiol linked side chains favor strong adsorption on Au(111) surface, with isolated single molecules being easily visualized and stable over several image scans. On both HOPG and Au(111) isolated molecules of Y[C4O-Pc]2 are never observed, Even at very low solution concentrations small rafts of molecules on an otherwise open substrate surfaces are observed. On HOPG the structure of the Y[C4O-Pc]2 monolayer is complex with both an open and filled cubic structure occurring as a mixed structure. We suggest that core moieties with peripheral S-linked alkanes may generally be particularly strong adsorbates on gold. Density functional calculations suggest that the S-linked alkane substituent system may be as much as 1 eV more stable on Au than a similar O-linked system. This work demonstrates the important role of molecule-substrate interactions, bias voltage, tunneling current, and STM tip on controlling and stabilizing molecular assembly.

Growth of Corrole Films from Solution: A Nanometer-Scale Study at the Real Solid–Liquid Interface

The self-assembly of corrole molecules dissolved in ethanol and then adsorbed onto an immersed Au(111) surface was investigated by scanning tunneling microscopy (STM) at the solution–solid interface. The physisorption and the subsequent organization of the molecules in multilayers were followed as a function of time at two concentrations, differing by about a factor of 500. In both cases, molecules adsorb flat on the gold surface. For the low concentration, we demonstrate that the first disordered phase is followed, rather abruptly, by a more ordered one, essentially composed of extremely smooth layers of molecules. Even the sharp and winding steps, which mark the layers, describe an evolution toward order. In fact, their rims tend to flatten out, as testified by the decrease in time of their fractal dimension. In the case of high concentration, we have monitored the kinetics of coverage, showing that its evolution for the uppermost layer can be described by Langmuirian kinetics, which provides the adsorption and desorption rate constants, ka = (20.7 ± 1.6) L/(mol s) and kd = (6.5 ± 0.1) × 10–4 s–1.

On-Surface Self-Assembled Structural Transformation Induced by Schiff Base Reaction and Hydrogen bonds.

By utilizing scanning tunneling microscopy (STM), the self-assembled nanostructures of three characteristic aldehydes have been examined at the solution-solid interface. By introducing the active reactant 5-aminoisophthalic acid (5-AIPA), we succeeded in changing the self-assembled molecular structures through the condensation reaction and obtained the information on structural transformation in real time. The corresponding carboxyl conjugated derivatives were formed in situ and developed into the closely packed and ordered molecular architectures via hydrogen bonds at the solution-solid surface. The relevant simulations have been utilized to interpret the mechanisms of forming the nanostructures. The corresponding theoretical calculation is used to explain the reaction mechanism. Compared with the traditional ways, the on-surface condensation reaction in situ could not only provide a more convenient method for regulating the self-assembled architectures but also offer a promising strategy for building functional nanostructures and devices.

STM Investigation of the Y[C6S-Pc]2 and Y[C4O-Pc]2Complex at the Solution–Solid Interface: Substrate Effects, Submolecular Resolution, and Vacancies

Substituents present on a molecule are known to significantly control the assembly, adsorption, and orientation behavior on solid surfaces. Using scanning tunneling microscopy (STM), self-assembly ...

The manner and extent to which the hydration shell impacts interactions between hydrated species.

The hydration shell (HS) has a critical impact on every contact between hydrated species, which is a prerequisite for a great many physical and chemical processes, such as ion adsorption at the solution-solid interface. This paper reveals the extent and manner to which the HS interferes with ion adsorption utilizing molecular dynamics. The single-layer HS is the smallest unit that maintains the ionic hydration structure and the force on it. The energy penalty incurred by partial dehydration upon adsorption is one of the approaches through which HS influences ion adsorption, yet the collision of water molecules in HS may be the critical one. The repulsive force during dehydration is, to great extent, neutralized by HS collision. The index for estimating the extent of the influence of the HS is not the hydration energy, but the quantification of the contest between HS' collision and the binding of adsorption sites. The hydration energy is larger for charged functional groups, but the HS' impact is much smaller, as compared with electroneutral group cases. As a result, the order of the adsorption capacity for different ionic species may be quite different between charged and electroneutral cases.

Growth of a self-assembled monolayer decoupled from the substrate: nucleation on-command using buffer layers.

Structural polymorphism is ubiquitous in physisorbed self-assembled monolayers formed at the solution–solid interface. One of the ways to influence network formation at this interface is to physically decouple the self-assembled monolayer from the underlying substrate thereby removing the influence of the substrate lattice, if any. Here we show a systematic exploration of self-assembly of a typical building block, namely 4-tetradecyloxybenzoic acid at the 1-phenyloctane–graphite interface in the presence and in the absence of a buffer layer formed by a long chain alkane, namely n-pentacontane. Using scanning tunneling microscopy (STM), three different structural polymorphs were identified for 4-tetradecyloxybenzoic acid at the 1-phenyloctane–graphite interface. Surprisingly, the same three structures were formed on top of the buffer layer, albeit at different concentrations. Systematic variation of experimental parameters did not lead to any new network in the presence of the buffer layer. We discovered that the self-assembly on top of the buffer layer allows better control over the nanoscale manipulation of the self-assembled networks. Using the influence of the STM tip, we could initiate the nucleation of small isolated domains of the benzoic acid on-command in a reproducible fashion. Such controlled nucleation experiments hold promise for studying fundamental processes inherent to the assembly process on surfaces.

Tracking Interfacial Dynamics of a Single Nanoparticle Using Plasmonic Scattering Interferometry.

The ability to track interfacial dynamics of a single nanoparticle at the solution-solid interface is crucial for understanding physical, chemical, and biological processes, but it remains a challenge. Here, we demonstrated a plasmonic imaging technique that can track unlabeled nanoparticles at the solution-solid interface with high spatial and temporal resolutions. This technique is based on particle-induced interferometric scattering of a surface plasmonic wave, which results in a high vertical sensitivity. Using this ability, we tracked the trajectories of a single nanoparticle interacting with a surface, measured the hydrodynamically hindered diffusion of nanoparticles, and revealed the surface chemistry-dependent behavior of nanoparticles at the interface. The application for tracking formation of membranes from a lipid vesicle was demonstrated, indicating the potential for investigating a broad range of nano-objects at interfaces in a complex environment.

Kinetics of Adsorption and Desorption of Non-Covalent Self-Assembling Systems

Self-Assembling systems that rely on non-covalent forces to drive ordering are the subject of intense study. Of particular interest are redox active systems like porphyrins and phthalocyanines that can generate self-assembled monolayers (SAMs) at the solution-solid interface. These offer the potential of creating highly ordered films from the simple process of solution dipping. In recent years, it has become clear that kinetics plays a significant role in determining the structure of these films. However, very little is known about the rates and mechanisms of the formation and stability of these interesting surface layers. In this talk we will present new data and models for the adsorption and desorption kinetics of non-covalent SAMs on gold surfaces. As might be suspected, the adsorption and desorption rates do not follow simple Langmuir behavior. Also, the potential for multilayer growth must be considered.

Electrostatically Governed Debye Screening Length at the Solution-Solid Interface for Biosensing Applications.

Biosensors based on field-effect devices (bioFETs) offer numerous advantages over current technologies and therefore have attracted immense research over the decades. However, short Debye screening length in highly ionic physiological solutions remains the main obstacle for bioFET realization. This challenge becomes considerably more acute at the electrolyte-oxide interface of the sensing area due to high ion concentration induced by the charged amphoteric sites, which prohibits any attempt to employ the field-effect mechanism to "sense" any charged biomolecules. In this work, we present an electrostatic approach by which the double layer (DL) excess ion concentration is removed, thus forcing the DL ion concentration to match the bulk concentration, which subsequently forces bulk screening length at the DL, thereby "exposing" target biomolecules to the underlying bioFET. To this end, we employ local tunable surface electric fields, introduced to the DL using surface passivated-metal electrodes. We examine numerically and analytically the effect of these electric fields on the DL ion distribution. We also numerically demonstrate the feasibility of the proposed approach for a fully depleted silicon-on-insulator based bioFET and show how the threshold voltage shift induced by the presence of target molecules increases by almost two orders of magnitude upon the removal of the surface excess ion population.

Polymorphic self-assembly of pyrazine-based tectons at the solution-solid interface.

Exploring the surface self-assembly of small molecules that act as building blocks (tectons) for complex supramolecular structures is crucial for realizing surface-supported functional molecular devices. Here, we report on the synthesis and surface self-assembly of a new pyrazine-derived molecule with pyridine pendants. Ambient scanning tunneling microscopy investigation at the solution–solid interface reveals polymorphic self-assembly of these molecules on a HOPG substrate. Two different molecular packing structures with equal distribution are observed. Detailed analysis of the STM images emphasizes the crucial role of weak intermolecular hydrogen bonding, and molecule–substrate interactions in the formation of the observed polymorphs. Such weak hydrogen bonding interactions are highly desirable for the formation of modular supramolecular architectures since they can provide sufficiently robust molecular structures and also facilitate error correction.

Amphiphile self-assembly dynamics at the solution-solid interface reveal asymmetry in head/tail desorption.

Amphiphilic alkoxybenzonitriles (ABNs) of varying chain length are studied at the solution/graphite interface to analyze dynamics of assembly. Competitive self-assembly between ABNs and alkanoic acid solvent is shown by scanning tunneling microscopy (STM) to be controlled by concentration and molecular size. Molecular dynamics (MD) simulations reveal key roles of the sub-nanosecond fundamental steps of desorption, adsorption, and on-surface motion. We discovered asymmetry in desorption-adsorption steps. Desorption starting from alkyl chain detachment from the surface is favored due to dynamic occlusion by neighbouring chains. Even though the nitrile head has a strong solvent affinity, it more frequently re-adsorbs following a detachment event.

Kinetic and Thermodynamic Control in Porphyrin and Phthalocyanine Self-Assembled Monolayers.

Porphyrins and phthalocyanines are ubiquitous in modern science and technology. Their stability, redox properties, and photoresponse make them candidates for numerous applications. Many of these applications rely on thin films, and these are critically dependent on the first monolayer. In this article, we focus on noncovalently bound self-assembled monolayers of porphyrins and phthalocyanines at the solution-solid interface with special emphasis on the kinetic and thermodynamic processes that define the films and their reaction chemistry. We first discuss the difference between film-formation kinetics and desorption kinetics from fully formed films. We then present evidence that many of these monolayers are controlled by adsorption kinetics and are not in thermodynamic equilibrium. Measurement of the solution-solid interface desorption energy by scanning tunneling microscopy is discussed, and data is presented for cobalt, nickel, and free base octaethylporphyrin. The activation energy for the desorption of these compounds into phenyloctane is about half of the computed desorption energy in vacuum, and this is discussed in terms of the role of the solvent. Preexponential factors are very low compared to desorption into vacuum, and this is attributed to a reduction in the entropy of activation due to the participation of solvent in the transition state. An example of the use of relative desorption kinetics to create a new binary surface structure is given. It is suggested that this is a synthesis route that may have been missed because of the large difference in solution concentrations required to drive binary film formation. Attention then turns to the axial reaction chemistry of metalloporphyrins and metallophthalocyanines supported on conducting surfaces. We show several examples of chemistry unique to the supported complexes: cases where the metal binds ligands more readily and cases where the substrate induces ligand loss. Understanding this new axial coordination chemistry is of great importance in catalysis, sensing, and the growth of 3D materials from a self-assembled template.