Effective van der Waals surface energy of self-assembled monolayer films having systematically varying degrees of molecular fluorination.

The systematic variation of the van der Waals surface energy with fluorination for a series of self-assembled monolayers (SAMs) generated by the adsorption of partially fluorinated alkanethiols onto the surface of gold is examined experimentally and theoretically. The surface energy is elucidated on the basis of an effective Hamaker constant, which is obtained as a combination of the respective Hamaker constants of fluorocarbons and hydrocarbons; the fraction depends on the degree of fluorination. The good agreement between experiment and theory is discussed. In addition, the Hamaker constants of various liquids contacted on the well-defined hydrophobic surfaces are interpreted using modified Lifshitz theory.

DNA Loading and Release Using Custom-Tailored Poly(l-lysine) Surfaces.

This Article describes the generation and study of surfaces modified with custom-crafted poly(l-lysine) (PLL) coatings for use in the loading and delivery of single-stranded DNA (ssDNA). The experimental strategy utilizes bidentate dithiol adsorbates to generate stably bound azide-terminated self-assembled monolayers (SAMs) on gold possessing an oligo(ethylene glycol) (OEG) spacer. Consequent to the molecular assembly on gold, the azide termini are covalently attached to a maleimide linker moiety via a copper-catalyzed azide-alkyne "click" reaction. Functionalization with maleimide provides a platform for the subsequent attachment of cysteine-terminated poly(l-lysine) (PLL), thus forming a suitable surface for the loading of ssDNA via electrostatic interactions. In efforts to maximize DNA loading, we generate SAMs containing mixtures of short and long PLL segments and explore the DNA-loading capability of the various PLL SAMs. We then use thermal increases to trigger the release of the ssDNA from the surface. By examining the loading and release of ssDNA using these new two-dimensional systems, we gain preliminary insight into the potential efficacy of this approach when using three-dimensional gold nanostructure systems in future gene-delivery and biosensing applications.

A Nanoparticle-Decorated Biomolecule-Responsive Polymer Enables Robust Signaling Cascade for Biosensing.

To meet the increasing demands for ultrasensitivity in monitoring trace amounts of low-abundance early biomarkers or environmental toxins, the development of a robust sensing system is urgently needed. Here, a novel signal cascade strategy is reported via an ultrasensitive polymeric sensing system (UPSS) composed of gold nanoparticle (gNP)-decorated polymer, which enables gNP aggregation in polymeric network and electrical conductance change upon specific aptamer-based biomolecular recognition. Ultralow concentrations of thrombin (10-18 m) as well as a low molecular weight anatoxin (165 Da, 10-14 m) are detected selectively and reproducibly. The biomolecular recognition induced polymeric network shrinkage responses as well as dose-dependent responses of the UPSS are validated using in situ real-time atomic-force microscopy, representing the first instance of real-time detection of biomolecular binding-induced polymer shrinkage in soft matter. Furthermore, in situ real-time confocal laser scanning microscopy imaging reveals the dynamic process of gNP aggregation responses upon biomolecular binding.

Patterned networks of mouse hippocampal neurons on peptide-coated gold surfaces.

Patterned networks of hippocampal neurons were generated on peptide-coated gold substrates prepared by microscope projection photolithography and microcontact printing. A 19 amino acid peptide fragment of laminin A (PA22-2) that includes the IKVAV cell adhesion domain was used to direct patterns of cell adhesion in primary culture. Microscale grid patterns of peptide were deposited on gold-coated glass cover slips by soft lithography using "stamps" fashioned from polydimethylsiloxane. Strong coordination bonding between gold atoms on the surface and the sulfur atoms of the N-terminal cysteine residues supported stable adhesion of the peptide, which was confirmed by immunofluorescence using anti-IKVAV antiserum. Dispersed hippocampal cells isolated from neonatal mouse pups were grown on peptide-patterned gold substrates for 7 days. Neurons preferentially adhered to peptide-coated regions of the gold surface and restricted their processes to the peptide patterns. Whole cell recordings of neurons grown in patterned arrays revealed an average membrane potential of -50 mV, as well as the presence of voltage-gated ion conductances. Peptide-modified gold surfaces serve as convenient and effective substrates for growing ordered neural networks that are compatible with existing multi-electrode array recording technology.

Bioinspired Zwitterionic Surface Coatings with Robust Photostability and Fouling Resistance.

Great care has been paid to the biointerface between a bulk material and the biological environment, which plays a key role in the optimized performance of medical devices. In this work, we report a new superhydrophilic adsorbate, called L-cysteine betaine (Cys-b), having branched zwitterionic groups that give rise to surfaces and nanoparticles with enhanced chemical stability, biofouling resistance, and inertness to environmental changes. Cys-b was synthesized from the amphoteric sulfur-containing amino acid, L-cysteine (Cys), by quaternization of its amino group. Gold surfaces modified with Cys-b exhibited prominent repellence against the nonspecific adsorption of proteins, bacteria, and fibroblast cells. In addition, Cys-b existed in zwitterionic form over a wide pH range (i.e., pH 3.4 to 10.8), and showed excellent suppression in photoinduced oxidation on gold substrates. Furthermore, the modification of hollow Ag@Au nanoshells with Cys-b gave rise to nanoparticles with excellent colloidal stability and resistance to coordinative interaction with Cu(2+). Taken together, the unique features of Cys-b offer a new nanoscale coating for use in a wide spectrum of applications.

Gold nanoshell-decorated silicone surfaces for the near-infrared (NIR) photothermal destruction of the pathogenic bacterium E. faecalis.

Catheter-related infections (CRIs) are associated with the formation of pathogenic biofilms on the surfaces of silicone catheters, which are ubiquitous in medicine. These biofilms provide protection against antimicrobial agents and facilitate the development of bacterial resistance to antibiotics. The application of photothermal agents on catheter surfaces is an innovative approach to overcoming biofilm-generated CRIs. Gold nanoshells (AuNSs) represent a promising photothermal tool, because they can be used to generate heat upon exposure to near-infrared (NIR) radiation, are biologically inert at physiological temperatures, and can be engineered for the photothermal ablation of cells and tissue. In this study, AuNSs functionalized with carboxylate-terminated organosulfur ligands were attached to model catheter surfaces and tested for their effectiveness at killing adhered Enterococcus faecalis (E. faecalis) bacteria. The morphology of the AuNSs was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while the elemental composition was characterized by energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Furthermore, optical and photothermal properties were acquired by ultraviolet-visible (UV-vis) spectroscopy and thermographic imaging with an infrared camera, respectively. Bacterial survival studies on AuNS-modified surfaces irradiated with and without NIR light were evaluated using a colony-formation assay. These studies demonstrated that AuNS-modified surfaces, when illuminated with NIR light, can effectively kill E. faecalis on silicone surfaces.

Can cyclopropyl-terminated self-assembled monolayers on gold be used to mimic the surface of polyethylene?

This paper describes the formation of a new series of monolayer films generated by the self-assembly of omega-cyclopropylalkanethiols, CyPr(CH(2))(n)SH (n = 9-13), onto the surface of gold. Procedures used to prepare the omega-cyclopropylalkanethiol adsorbates are also reported. Methyl-, vinyl-, and isopropyl-terminated self-assembled monolayers (SAMs) were also prepared and used as reference films to evaluate the structure and properties of the new cyclopropyl-terminated films. Ellipsometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) were used to examine the structure of the SAMs. A small but systematically lower thickness of the new films compared to that of analogous methyl-terminated SAMs was observed. Also, the orientation of the ring with respect to the surface normal was observed to vary systematically with the number of methylene groups in the adsorbate backbone (i.e., odd vs even chain lengths). Measurements of wettability by contact angle goniometry also revealed a small but reproducible "odd-even" effect for all contacting liquids used, except hexadecane, which almost completely wet the surfaces (theta(a) = 10-13 degrees ). When compared to the wettability data obtained from methyl- and isopropyl-terminated SAMs, the wettability data obtained from the cyclopropyl-terminated SAMs suggest that these films offer an increased density of atomic contacts per unit area across the surface, and thus enhanced attractive interactions with contacting liquids. Comparison of the wettabilities of vinyl-terminated and cyclopropyl-terminated films is complicated by dipole-induced dipole interactions and/or pi-pi interactions between the surfaces and the probe liquids. Furthermore, the significantly similar wettabilities of the cyclopropyl-terminated SAMs and the surface of polyethylene suggests that these SAMs (and perhaps other SAMs with judiciously designed tailgroups) can be used to mimic the interfacial properties of polymeric materials without complications arising from surface reconstruction.

Laser-scanning lithography (LSL) for the soft lithographic patterning of cell-adhesive self-assembled monolayers.

We report the development of laser-scanning lithography (LSL), which employs a laser-scanning confocal microscope to pattern photoresists that can be utilized, for example, in the fabrication of masters for use in soft lithography. This convenient technique provides even exposure across the entire view field and facilitates accurate alignment of successive photoresist exposures. Features on the scale of 3 microm have been achieved to date with a 10x objective (NA 0.45). Virtual masks, instructions for laser irradiation, were drawn using the Region of Interest (ROI) function of a Zeiss LSM 510 microscope. These regions were then exposed to a 458 nm argon laser for 32 micros (0.9 mW/microm(2)). Differential interference contrast (DIC) imaging was utilized with a non-destructive 514 nm argon laser as an immediate quality check of each exposure, to align successive exposures, and to reduce chromatic aberration between imaging and exposure. Developed masters were replica-molded with poly(dimethylsiloxane) (PDMS); these masters were then utilized for microcontact printing of cell-adhesive self-assembled monolayers (SAMs) to demonstrate the utility of this process. Initial studies confirmed that human dermal fibroblast adhesion and spreading were limited to cell-adhesive SAM areas. LSL is a rapid, flexible, and readily available technique that will accelerate master design and preparation; moreover, it can be applied to additional forms of photolithography and photopolymerization for studies in cell biology, biomaterials design and evaluation, materials science, and surface chemistry.