Surface characterization, protein adsorption, and initial cell-surface reactions on glutathione and 3-mercapto-1,2,-propanediol immobilized to gold

Nanoliter Sensing for Infrared Bioanalytics

Nondestructive label-free bioanalytics of microliter to nanoliter sample volumes with low analyte concentrations requires novel analytic approaches. For this purpose, we present an optofluidic platform that combines surface-enhanced in situ infrared spectroscopy with microfluidics for sensing of surface-immobilized ultrathin biomolecular films in liquid analytes. Submonolayer sensitivity down to surface densities of few ng/cm² is demonstrated for the adsorption of the thiolate tripeptide glutathione and for the recognition of streptavidin on a biotinylated enhancement substrate. Nonfunctionalized and functionalized metal island films on planar oxidized silicon substrates are used for signal enhancement with quantifiable enhancement properties. A single-reflection geometry at an incidence angle below the attenuated-total-reflection (ATR) regime is used with ordinary planar, IR-transparent windows. The geometry circumvents the strong IR absorption of common polymer materials and of aqueous environments in the IR fingerprint region. This practice enables straightforward quantitative analyses of, e.g., adsorption kinetics as well as chemical and structural properties in dependence of external stimuli.

Cell adhesion and proliferation on hydrophilic dendritically modified surfaces

Dendritically modified, or "dendronized" surfaces are generated by modification of a substrate with perfectly branched polymers, known as dendrimers. Here, such dendronized surfaces were prepared by initial chemisorption of poly(ethylene glycol)-mono-thiol (HS-PEG(650)-OH) onto gold-coated silicon wafers, followed by divergent synthesis of aliphatic polyester dendrons, generation 1-4, starting from the terminal PEG OH- group. The adhesion and proliferation of human corneal epithelial cells (HCEC) and mouse 3T3 fibroblasts (M-3T3) as model cells on these hydroxyl-terminated dendronized surfaces were investigated. In addition, the effect of covalently attaching PEG mono-methyl ether (PEG-OMe) chains (M(n)=2000 Da) to the peripheral hydroxyl groups of G1- and G2-dendronized surfaces on adhesion and proliferation of the same cell lines was studied. Little or no HCEC adhesion was noted on gold surfaces modified with PEG mono-thiol (HO-PEG-SH) in serum-free medium. These cells showed a greater affinity for the dendronized surfaces compared to the control Au surfaces at early incubation stages (1 day). At longer incubation times, HCEC proliferation increased exponentially on the dendronized surfaces. However, when G1- and G2-dendronized surfaces were modified with PEG-OMe chains, adhesion of both HCEC and M-3T3 cells was significantly reduced. Cell studies with M-3T3 fibroblasts, carried out in serum-containing medium, showed that cell attachment was diminished for the PEG-grafted Au surfaces compared to the control Au and G1-G4 dendronized surfaces.

Probing chiral interfaces by infrared spectroscopic methods

Biological homochirality on earth and its tremendous consequences for pharmaceutical science and technology has led to an ever increasing interest in the selective production, the resolution and the detection of enantiomers of a chiral compound. Chiral surfaces and interfaces that can distinguish between enantiomers play a key role in this respect as enantioselective catalysts as well as for separation purposes. Despite the impressive progress in these areas in the last decade, molecular-level understanding of the interactions that are at the origin of enantiodiscrimination are lagging behind due to the lack of powerful experimental techniques to spot these interactions selectively with high sensitivity. In this article, techniques based on infrared spectroscopy are highlighted that are able to selectively target the chiral properties of interfaces. In particular, these methods are the combination of Attenuated Total Reflection InfraRed (ATR-IR) with Modulation Excitation Spectroscopy (MES) to probe enantiodiscriminating interactions at chiral solid-liquid interfaces and Vibrational Circular Dichroism (VCD), which is used to probe the structure of chirally-modified metal nanoparticles. The former technique aims at suppressing signals arising from non-selective interactions, which may completely hide the signals of interest due to enantiodiscriminating interactions. Recently, this method was successfully applied to investigate enantiodiscrimination at self-assembled monolayers of chiral thiols on gold surfaces. The nanometer size analogues of the latter--gold nanoparticles protected by a monolayer of a chiral thiol--are amenable to VCD spectroscopy. It is shown that this technique yields detailed structural information on the adsorption mode and the conformation of the adsorbed thiol. This may also turn out to be useful to clarify how chirality can be bestowed onto the metal core itself and the nature of the chirality of the latter, which is manifested in the metal-based circular dichroism activity of these nanoparticles.

Evaluation of alginate purification methods: effect on polyphenol, endotoxin, and protein contamination

Alginate, a polysaccharide extracted from brown seaweed, is widely used for the microencapsulation of islets of Langerhans, allowing their transplantation without immunosuppression. This natural polymer is known to be largely contaminated. The implantation of islets encapsulated using unpurified alginate leads to the development of fibrotic cell overgrowth around the microcapsules and normalization of the blood glucose is restricted to a very short period if it is achieved at all. Several research groups have developed their own purification method and obtained relatively good results. No comparative evaluation of the efficiencies of these methods has been published. We conducted an evaluative study of five different alginate preparations: a pharmaceutical-grade alginate in its raw state, the same alginate after purification according to three different published methods, and a commercially available purified alginate. The results showed that all purification methods reduced the amounts of known contaminants, that is, polyphenols, endotoxins, and proteins, although with varying efficiencies. Increased viscosity of alginate solutions was observed after purification of the alginates. Despite a general efficiency in decreasing contamination levels, all of the purified alginates contained relatively high residual amounts of protein contaminants. Because proteins may be immunogenic, these residual proteins may have a role in persisting microcapsule immunogenicity.

Adsorption kinetics of l-glutathione on gold and structural changes during self-assembly: an in situATR-IR and QCM study

The adsorption of L-glutathione (gamma-Glu-Cys-Gly) from ethanol on gold surfaces was studied in situ by both attenuated total reflection infrared (ATR-IR) spectroscopy and using a quartz crystal microbalance (QCM). The molecule is firmly anchored to the gold surface through the thiol group. Different IR signals of adsorbed L-glutathione, notably the amide I and nu(-COOH), show significantly different behavior with time, which reveals that their increase is not related to adsorption (mass uptake) alone. This indicates that structural transformations take place during the formation of the self-assembled monolayer (SAM). In particular, the intensity of the acid signal increases quickly only within the first couple of minutes. The complexity of the self-assembling process is confirmed by QCM measurements, which show fast mass uptake within about 100 s followed by a considerably slower regime. The structural change superimposed on the mass uptake is, based on the in situ time-resolved ATR-IR measurements, assigned to the interaction of the acid group of the Gly moiety with the surface. The latter group is protonated in ethanol but deprotonates upon interaction with the gold surface. The protonation-deprotonation equilibrium is sensitive to external stimuli, such as the presence of dissolved L-glutathione molecules. The interaction of the acid group with the surface and concomitant deprotonation proceeds via two distinguishable steps, the first being a reorientation of the molecule, followed by the deprotonation.

L-glutathione chemisorption on gold and acid/base induced structural changes: a PM-IRRAS and time-resolved in situ ATR-IR spectroscopic study

Adsorption of the tripeptide L-glutathione (gamma-glu-cys-gly) on gold surfaces was investigated by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and attenuated total reflection (ATR) infrared spectroscopy. PM-IRRAS was used to study ex situ the adsorbate layer prepared from aqueous solutions at different pH, whereas ATR-IR was applied to study in situ adsorption from ethanol in the presence and absence of acid and base. ATR-IR was furthermore combined with modulation spectroscopy in order to investigate the reversible changes within the adsorbate layer induced by acid and base stimuli, respectively. The molecule is firmly anchored on the gold surface via the thiol group of the cys part. However, the ATR-IR spectra in ethanol indicate a further interaction with the gold surface via the carboxylic acid group of the gly part of the molecule, which deprotonates upon adsorption. Hydrochloric acid readily protonates the two acid groups of the adsorbed molecule. During subsequent ethanol flow the acid groups deprotonate again, a process which proceeds in two distinct steps: a fast step associated with the deprotonation of the acid in the glu part of the molecule and a considerably slower step associated with deprotonation of the acid in the gly moiety. The latter process is assisted by the interaction of the corresponding acid group with the surface. The spectra furthermore indicate a rearrangement of the hydrogen bonding network within the adsorbate layer upon deprotonation. Depending on the protonation state during adsorption of l-glutathione, the response toward identical protonation-deprotonation stimuli is significantly different. This is explained by the ionic state-dependent shape of the molecule, as supported by density functional theory calculations. The different shapes of the individual molecules during layer formation thus influence the structure of the adsorbate layer.

Effect of synthesis temperature of PEO-grafted PU/PS IPNs on surface morphology and in vitro blood compatibility

When hydrophilic/hydrophobic polymers have a microdomain structure, platelet adhesion and activation are effectively suppressed by prohibition of the excessive assembly of glycoproteins and adenosine triphosphate (ATP) consumption of the platelets on the surface. In this study, poly(ethylene oxide)-grafted hydrophilic polyurethane (PU)/hydrophobic polystyrene (PS) interpenetrating polymer networks (IPNs) were synthesized by varying the synthesis temperature to control the phase separation and the microdomain surface structure, and the effect of the degree of phase separation on the in vitro blood compatibility. The size of the dispersed PS-rich domains in the PU-rich matrix decreased, and the hydrophilicity also decreased as the synthesis temperature of the PS network during the IPN synthesis was decreased, as the phase separation was suppressed during the synthesis. The amount of the adsorbed bovine plasma fibrinogens (BPF) on the PEO-grafted PU/PS IPNs decreased as the synthesis temperature was decreased, and the in vitro adhesion of the platelets was also suppressed on the PEO-grafted PU/PS IPNs prepared at lower temperature. The microdomain structure on the surface affected the adhesion and the activation of the adhered platelets, and the suppression of the phase separation resulted in the decrease of the domain size, which also enhanced the blood compatibility of the PEO-grafted PU/PS IPNs.

Cellular reactions and bone apposition to titanium surfaces with different surface roughness and oxide thickness cleaned by oxidation

Titanium surfaces with three different surface characteristics were exposed to an intraperitoneal milieu in mouse or rat, or inserted into rabbit bone. The cleaning regimen of the TiO2 surfaces in this study included oxidation by heat or acid and a final rinsing and storage in water. Intraperitoneal exposure ranged from 1 to 64 min and the healing period in bone was 6 weeks. Cell recruitment to the surfaces was quantified by acridine orange staining and specific antibodies directed against cell membrane antigens. Removal torque, bone-to-metal contact, total bone area and histological evaluations were used to evaluate fixture stability and the healing-in of the implants. After the healing period of 6 weeks only a transient significant difference was seen in the total number of cells adherent on the surfaces. No significant differences were observed between any of the surfaces for removal torque, bone-to-metal contact, or bone area. The areas lacking bone-to-metal contact were filled with normal vascularised connective tissue with no signs of fibrous capsule formation or giant cells. These findings differ from findings published earlier of Ti implants that underwent a cleaning regimen with alcohol as the final rinsing step. The tissues around the implants were richly vascularised and there was continued bone growth toward the surfaces. The bone-to-metal contact in this study was lower than that seen with alcohol-cleaned TiO2.

Cell adhesion peptide modification of gold-coated polyurethanes for vascular endothelial cell adhesion

Gold-coated polyurethanes were chemisorbed with three cell-adhesion peptides having an N-terminal cysteine: cys-arg-gly-asp (CRGD), cys-arg-glu-asp-val (CREDV), and the cyclic peptide cys-cys-arg-arg-gly-asp-try-leu-cys (CCRRGDWLC). The peptides were selected based on their presumed preferential interactions with the cell-surface integrins on vascular endothelial cells. The ability of the surfaces to support the preferential adhesion of human vascular endothelial cells was studied by comparing in vitro adhesion results for these cells with those from mouse 3T3 fibroblasts. Surface modification with the peptides was confirmed by water-contact angles and XPS. Surface morphology was determined by AFM and SEM. In vitro cell-culture studies in conjunction with plasma-protein adsorption and immunoblotting were performed on the various modified surfaces. The data suggest that peptide-modified surfaces have significant potential for supporting cell adhesion. Little or no cell adhesion was noted on gold- or cysteine-modified control surfaces. Human vascular endothelial cells showed the greatest adhesion to the CCRRGDWLC-modified surfaces, and the 3T3 fibroblasts adhered best to the CREDV-modified surfaces. Protein adsorption studies suggest that the preferential adsorption of the cell-adhesive proteins fibronectin and vitronectin is not likely mediating the differences noted. It is concluded that the cell-adhesive peptide-modified gold-coated polymers have significant potential for further development both as model substrates for fundamental studies and for use in biomaterials applications.

In vivo cell recruitment, cytokine release and chemiluminescence response at gold, and thiol functionalized surfaces

Hydroxylated and methylated surfaces were prepared by the self-assembled monolayer technique (SAM) of alkane thiols on gold. The surfaces were used to evaluate the influence of implant surface chemistry on protein deposition and inflammatory cell response. Implants were inserted subcutaneously in the rat for 3 and 24 h. The surface chemical properties influenced the in vitro rat plasma protein adsorption (ellipsometry/antibody) with few exceptions (albumin not found and fibrinogen always found). The number of recruited cells and their distribution (DNA from implant versus from exudate) was influenced by the different chemistries at 24 h, but not at 3 h. HIS48+, ED1+, ED2+ and small numbers of CD5+ cells were present in the exudate at both time periods (flow cytometry). The cellular oxidative metabolism was low, although cells on -OH surfaces responded with the highest phorbol ester-stimulated chemiluminescence (CL)/DNA. The levels of cytokines IL-1alpha, IL-1beta and TNFalpha (ELISA) were not influenced by material surface chemistry. Sham operated sites had a higher cytokine concentration/DNA compared with exudates from an implant milieu. The results of this study show that surface chemical functionalization modifies specific events in the inflammatory response around implants in soft tissues.