Robust Maleimide-Functionalized Gold Surfaces and Nanoparticles Generated Using Custom-Designed Bidentate Adsorbates

Strategies for Surface Design in Surface Plasmon Resonance (SPR) Sensing

Surface plasmon resonance (SPR) comprises several surface-sensitive techniques that enable the trace and ultra-trace detection of various analytes through affinity pairing. Although enabling label-free, sensitive detection and real-time monitoring, several issues remain to be addressed, such as poor stability, non-specific adsorption and the loss of operational activity of biomolecules. In this review, the progress over sensor modification, immobilization techniques and novel 2D nanomaterials, gold nanostructures and magnetic nanoparticles for signal amplification is discussed. The advantages and disadvantages of each design strategy will be provided together with some of the recent achievements.

Subsurface-Carbon-Induced Local Charge of Copper for an On-Surface Displacement Reaction

Transition-metal carbides have sparked unprecedented enthusiasm as high-performance catalysts in recent years. Still, the catalytic properties of copper carbide remain unexplored. By introducing subsurface carbon to Cu(111), a displacement reaction of a proton in a carboxyl acid group with a single Cu atom is demonstrated at the atomic scale and room temperature. Its occurrence is attributed to the C-doping-induced local charge of surface Cu atoms (up to +0.30 e/atom), which accelerates the rate of on-surface deprotonation via reduction of the corresponding energy barrier, thus enabling the instant displacement of a proton with a Cu atom when the molecules adsorb on the surface. This well-defined and robust Cu^δ+ surface based on subsurface-carbon doping offers a novel catalytic platform for on-surface synthesis.

Quaternary Ammonium-Terminated Films Formed from Mixed Bidentate Adsorbates Provide a High-Capacity Platform for Oligonucleotide Delivery

The exposure of quaternary ammonium groups on surfaces allows self-assembled monolayers (SAMs) to serve as architectural platforms for immobilizing oligonucleotides. The current study describes the preparation of SAMs derived from four unique bidentate adsorbates containing two different ammonium termini (i.e., trimethyl- and triethyl-) and comparison to their monodentate analogs. Our studies found that SAMs derived from the bidentate adsorbates offered considerable enhancements in oligonucleotide binding when compared to SAMs derived from their monodentate analogs. The generated SAMs were analyzed using ellipsometry, X-ray photoelectron spectroscopy, contact angle goniometry, polarization modulation infrared reflection-absorption spectroscopy, and electrochemical quartz crystal microbalance. These analyses showed that the immobilization of oligonucleotides was affected by changes in the terminal functionalities and the relative packing densities of the monolayers. In efforts to enhance further the immobilization of oligonucleotides on these SAM surfaces, we explored the use of adsorbates having aliphatic linkers with systematically varying chain lengths to form binary SAMs on gold. Mixed monolayers with 50:50 ratios of adsorbates showed the greatest oligonucleotide binding. These studies lay the groundwork for oligonucleotide delivery using gold-based nanoparticles and nanoshells.

Inhibiting Reductive Elimination as an Intramolecular Disulfide Dramatically Enhances the Thermal Stability of SAMs on Gold Derived from Bidentate Adsorbents

The bidentate aromatic adsorbate, 5-(octadecyloxy)-1,3-benzenedimethanethiol (R1ArmDT), with a specific design of extended S-S distance and a geometric constraint to resist cyclic disulfide formation was synthesized. The film formation and thermal stability of self-assembled monolayers (SAMs) derived from R1ArmDT were investigated and compared to those of SAMs derived from an analogous bidentate dithiol 2-(4-(octadecyloxy)-phenyl)propane-1,3-dithiol (R1ArDT), in which the two sulfur atoms can readily form a cyclic disulfide upon reductive elimination from the surface. Although the SAMs derived from R1ArmDT were less densely packed than those derived from R1ArDT, as judged by the data obtained by X-ray photoelectron spectroscopy and polarization modulation infrared reflection absorption spectroscopy, the SAMs derived from R1ArmDT were markedly more thermally stable than those derived from R1ArDT. The greater thermal stability of the R1ArmDT SAMs can be rationalized on the basis of the structure of the bidentate R1ArmDT headgroup, in which the two pendant sulfur atoms cannot access each other intramolecularly to form a cyclic disulfide upon reductive elimination from the surface.

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.

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.