Platelet compatibility improvement by proper choice of acidic terminal functionality for mixed-charge self-assembled monolayers

Self-assembled monolayers of sulfonate-terminated alkanethiols investigated by frequency modulation atomic force microscopy in liquid

A molecular-scale understanding of self-assembled monolayers (SAMs) of sulfonate-terminated alkanethiols is crucial for interfacial studies of functionalized SAMs and their various applications. However, such an understanding has been difficult to achieve because of the lack of direct information on these molecular-scale structures in real space. In this study, we investigated the structures of sulfonate SAMs of sodium 11-mercapto-1-undecanesulfonate (MUS) by frequency modulation atomic force microscopy (FM-AFM) in liquid. The subnanometer-resolution FM-AFM images showed that the single-component MUS SAM prepared in pure water had random surface structures. In contrast, the MUS SAM prepared in a water-ethanol mixed solvent showed periodic striped structures with a flat-lying conformation. The results suggest a significant solvent effect on molecular-scale structures of long-chain sulfonate SAMs. In addition, we investigated the molecular-scale structures of mixed SAMs of MUS and 11-mercapto-1-undecanol (MUO) with alkane chains of the same length. The FM-AFM images of the mixed SAMs showed clear phase separation between MUS SAM and MUO SAM domains. In the MUO SAM domains, the incorporated MUS molecules appeared as protrusions. The results obtained in this study provide direct structural information on long-chain sulfonate and mixed SAMs.

Hydrolysis-controlled protein adsorption and antifouling behaviors of mixed charged self-assembled monolayer: A molecular simulation study

Understanding the mechanism of the antimicrobial and antifouling properties of mixed charged materials is of great significance. The interactions between human gamma fibrinogen (γFg) and mixed carboxylic methyl ether-terminated (COOCH3-) and trimethylamino-terminated (N(CH3)3(+)-) SAMs and the influence of hydrolysis were studied by molecular simulations. After hydrolysis, the mixed SAMs exhibit behaviors from antimicrobial to antifouling, since the COOCH3-thiols were translated into carboxylic acid (COO(-)-) terminated thiols, which carried a net charge of -1 e. Simulation results showed that the main differences between COOCH3-/N(CH3)3(+)-SAM and COO(-)-/N(CH3)3(+)-SAM are the charged property and the hydration layer above the surface. γFg could stably adsorb on the positively-charged COOCH3-/N(CH3)3(+)-SAM. The adsorption behavior is mainly induced by the strong electrostatic attraction. There is a single hydration layer bound to the surface, which is related to the N(CH3)3(+) groups. The van der Waals repulsion between γFg and the single hydration layer are not strong enough to compensate the strong electrostatic attraction. After hydrolysis, the positively-charged SAM was transferred to a neutral mixed charged surface, the electrostatic attraction between γFg and the surface disappears. Meanwhile, the SAM surface is covered by double hydration layers, which is induced by the N(CH3)3(+) and COO(-) groups; water molecules around COO(-) groups are obviously denser than that around N(CH3)3(+) groups. With the combined contribution from double hydration layers and the vanishment of electrostatic attraction, γFg is forced to desorb from the surface. After hydrolysis, the internal structure of mixed SAM appears more ordered due to the electrostatic interactions between charged groups on the top of SAMs.

STATEMENT OF SIGNIFICANCE

The antimicrobial and antifouling materials are of great importance in many biological applications. The strong hydration property of surfaces and the interactions between proteins and surfaces play a key role in resisting protein adsorption. The mixed SAMs, constructed from a 1:1 combination of COOCH3- and N(CH3)3(+)-terminated thiols, can induce protein adsorption mainly through the electrostatic interaction. When the COOCH3-terminated thiols were hydrolyzed to negatively charged COO(-)-terminated thiols, the mixed-charged SAMs switched from antimicrobial to antifouling. Due to the strong hydration property of the mixed charged SAMs, the adsorbed γFg moved away from the surface. Understanding the interactions between protein and mixed-charged SAMs in the atomistic level is important for the practical design and development of new antimicrobial and antifouling materials.

Surface-Enhanced Raman Spectroscopy as a Probe of the Surface Chemistry of Nanostructured Materials

Surface-enhanced Raman spectroscopy (SERS) is now widely used as a rapid and inexpensive tool for chemical/biochemical analysis. The method can give enormous increases in the intensities of the Raman signals of low-concentration molecular targets if they are adsorbed on suitable enhancing substrates, which are typically composed of nanostructured Ag or Au. However, the features of SERS that allow it to be used as a chemical sensor also mean that it can be used as a powerful probe of the surface chemistry of any nanostructured material that can provide SERS enhancement. This is important because it is the surface chemistry that controls how these materials interact with their local environment and, in real applications, this interaction can be more important than more commonly measured properties such as morphology or plasmonic absorption. Here, the opportunity that this approach to SERS provides is illustrated with examples where the surface chemistry is both characterized and controlled in order to create functional nanomaterials.

Surface modification of titanium substrate with a novel covalently-bound copolymer thin film for improving its platelet compatibility

Despite of its widely uses in various clinical applications, the titanium-based material still faces different challenges, such as hemocompatibility and anti-biofouling characteristics required in various situations. The objective of this investigation was to develop a novel surface modification strategy for titanium-based material to improve the platelet compatibility that is important in rigorous blood-contacting cardiovascular applications. In this work, a series of copolymers, which composed of novel 6-acryloyloxy hexyl phosphonic acid (AcrHPA) and sulfobetaine methacrylate (SBMA) was synthesized. The phosphonic acid group in these copolymers can impart covalent binding to the titanium substrate while the zwitterionic sulfobetaine functionality is considered being able to reduce the platelet adhesion and activation on the modified titanium substrate. NMR analyses suggested that copolymerization reaction is likely not an ideal statistical reaction but to add the monomers in a random order. Studies have shown that the composition of the monomers affected the surface characteristics and platelet compatibility of these covalent-bound AcrHPA-SBMA copolymers on titanium substrate. Contact angle analysis has shown the addition of SBMA can increase surface hydrophilicity of the spun-coated copolymers. In addition, AFM analyses have revealed that the surface roughness of the spun-coated copolymer layer were varied with the ratio of AcrHPA and SBMA. The most platelet compatible surface was noted on the one modified by the highest amount of SBMA added (i.e. 70 mol%) in copolymerization. In summary, the surface modification scheme presented here would be of potential as well as manufacturing process applicable for future development in blood-contacting titanium-based biomedical devices.

Surface tailoring of nanoparticles via mixed-charge monolayers and their biomedical applications

The recent convergence of nanomaterials and medicine has provided an expanding horizon for people to achieve encouraging advances in many biomedical applications such as cancer diagnosis and therapy. However, to realize desirable functions in the rather complex biological systems, a suitable surface coating is greatly in need for nanoparticles (NPs), regardless of the species. In this review, a recently developed surface modification strategy is highlighted--mixed-charge monolayers--with an emphasis on the nanointerfaces of inorganic NPs. Two typical mixed-charge gold NPs (AuNPs) prepared from surface modifications with different combinations of oppositely charged alkanethiols are shown as detailed examples to discuss how the mixed-charge monolayer can help NPs meet the criteria for in vitro and in vivo biomedical applications, including those critical issues like colloidal stability, nonfouling properties, and smart responses (pH-sensitivity) for tumor targeting.

Barnacle cyprid motility and distribution in the water column as an indicator of the settlement-inhibiting potential of nontoxic antifouling chemistries

Testing of new coatings to control fouling frequently involves single-species laboratory bioassays. Barnacle cyprids are among the most widely used model organisms in marine biofouling research, and surfaces that inhibit their settlement are considered to be promising candidates for new coating concepts. An analysis of motility parameters (mean velocity and swimming area coefficient) and distribution of cyprids of Balanus amphitrite in different swimming regions in the vicinity of model surfaces (self-assembled monolayers) is presented. The data are correlated with the settlement preferences of cyprids on these surfaces. Cyprids were predominantly found in interfacial regions and the transition frequencies between swimming regions of different depths were determined.

Surface-enhanced Raman scattering-active gold nanoparticles modified with a monolayer of silver film

As shown in the literature, electrochemical underpotential deposition (UPD) offers the ability to deposit up to a monolayer of one metal onto a more noble metal with a flat surface. In this work, we develop an electrochemical pathway to prepare more surface-enhanced Raman scattering (SERS)-active substrates with Ag UPD-modified Au nanoparticles (NPs) by using sonoelectrochemical deposition-dissolution cycles (SEDDCs). Encouragingly, the SERS of Rhodamine 6G (R6G) adsorbed on these Ag UPD-modified Au NPs exhibits a higher intensity by ca. 12-fold magnitude, as compared with that of R6G adsorbed on unmodified Au NPs. The prepared SERS-active substrate demonstrates a large Raman scattering enhancement for R6G with a detection limit of 2 × 10(-14) M and an enhancement factor of 5.0 × 10(8). Also, the strategy proposed in this work to improve the SERS effects by using UPD Ag based on SEDDCs has an effect on the smaller probe molecules of 2,2'-bipyridine (BPy).