A surface plasmon resonance array biosensor based on spectroscopic imaging

We have developed a multi-element transduction system which combines conventional SPR spectroscopy with one-dimensional SPR microscopy to create an effective platform for monitoring binding events on macro- or micro-patterned receptor arrays created on disposable sensor chips. This creates an effective platform for monitoring simultaneous binding events on each of the regions patterned with the receptors. This system has been specifically designed with commercially available components to allow relatively easy duplication. Furthermore, this system can use a proven, simple method to compensate for changes in the bulk index of refraction of the solution containing the analytes due to changes in temperature or solute concentration with simple modifications to the sensor chips alone. Preliminary results demonstrate how this system can be used to monitor several independent biospecific binding events simultaneously.

Use of self-assembled monolayers of different wettabilities to study surface selection and primary adhesion processes of green algal (Enteromorpha) zoospores

We investigated surface selection and adhesion of motile zoospores of a green, macrofouling alga (Enteromorpha) to self-assembled monolayers (SAMs) having a range of wettabilities. The SAMs were formed from alkyl thiols terminated with methyl (CH(3)) or hydroxyl (OH) groups or mixtures of CH(3)- and OH-terminated alkyl thiols and were characterized by measuring the advancing contact angles and by X-ray photoelectron spectroscopy. There was a positive correlation between the number of spores that attached to the SAMs and increasing contact angle (hydrophobicity). Moreover, the sizes of the spore groups (adjacent spores touching) were larger on the hydrophobic SAMs. Video microscopy of a patterned arrangement of SAMs showed that more zoospores were engaged in swimming and "searching" above the hydrophobic sectors than above the hydrophilic sectors, suggesting that the cells were able to "sense" that the hydrophobic surfaces were more favorable for settlement. The results are discussed in relation to the attachment of microorganisms to substrata having different wettabilities.

Surface-grafted, environmentally sensitive polymers for biofilm release

Controlling bacterial biofouling is desirable for almost every human enterprise in which solid surfaces are introduced into nonsterile aqueous environments. One approach that is used to decrease contamination of manufactured devices by microorganisms is using materials that easily slough off accumulated material (i.e., fouling release surfaces). The compounds currently used for this purpose rely on low surface energy to inhibit strong attachment of organisms. In this study, we examined the possible use of environmentally responsive (or "smart") polymers as a new class of fouling release agents; a surface-grafted thermally responsive polymer, poly(N-isopropylacrylamide) (PNIPAAM), was used as a model compound. PNIPAAM is known to have a lower critical solubility temperature of approximately 32 degrees C (i.e., it is insoluble in water at temperatures above 32 degrees C and is soluble at temperatures below 32 degrees C). Under experimental conditions, >90% of cultured microorganisms (Staphylococcus epidermidis, Halomonas marina) and naturally occurring marine microorganisms that attached to grafted PNIPAAM surfaces during 2-, 18-, 36-, and 72-h incubations were removed when the hydration state of the polymer was changed from a wettability that was favorable for attachment to a wettability that was less favorable. Of particular significance is the observation that an organism known to attach in the greatest numbers to hydrophobic substrata (i.e., H. marina) was removed when transition of PNIPAAM to a more hydrated state occurred, whereas an organism that attaches in the greatest numbers to hydrophilic substrata (i.e., S. epidermidis) was removed when the opposite transition occurred. Neither solvated nor desolvated PNIPAAM exhibited intrinsic fouling release properties, indicating that the phase transition was the important factor in removal of organisms. Based on our observations of the behavior of this model system, we suggest that environmentally responsive polymers represent a new approach for controlling biofouling release.

Computer-controlled laser ablation: a convenient and versatile tool for micropatterning biofunctional synthetic surfaces for applications in biosensing and tissue engineering

This paper describes laser-based methods for preparing micropatterns of bioactive molecular species in self-assembled monolayers (SAMs) and micropatterns of proteins and other biological molecules immobilized on solid substrates. Applications of these micropatterned surfaces in multianalyte biosensing and tissue engineering are emphasized. The focus of the paper is on the use of a computer-controlled laser ablation system comprising a research-grade inverted optical microscope, a pulsed nitrogen-pumped dye laser emitting at 390 nm, a programmable sample stage, and the computerized control system. The laser system can be implemented in a typical biosensor or tissue culture laboratory to enable the facile and reproducible fabrication of micropatterned surfaces by several methods. Various methods for patterning are discussed with examples given and emphasis placed on (1) laser ablation in the fabrication of photolithography masks, (2) electrochemical patterning of SAMs, and (3) laser desorption of SAMs. The relative merits of each technique are discussed with respect to application in fabrication of active surfaces for biosensing and tissue culture applications.

Attachment of bacteria to model solid surfaces: oligo(ethylene glycol) surfaces inhibit bacterial attachment

Bacterial cell attachment to the surfaces of self-assembled monolayers formed by the adsorption of omega-substituted alkanethiols on transparent gold films has been studied under defined bacterial culture and flow conditions. Phase contrast microscopy was used to quantify the attachment of two organisms, one of medical (Staphylococcus epidermidis) and one of marine (Deleya marina) importance. Self-assembled monolayers terminated with hexa(ethylene glycol), methyl, carboxylic acid and fluorocarbon groups were investigated. Over the range of experimental conditions, self-assembled monolayers formed from HS(CH2)11(OCH2CH2)6OH were found to be uniformly resistant to bacterial attachment, with a 99.7% reduction of attachment for both organisms when compared to the most fouled surface for each organism. On other surfaces, S. epidermidis and D. marina were shown to exhibit very different attachment responses to the wettability of the substratum. While the attachment of S. epidermidis correlated positively with surface hydrophilicity, D. marina showed a preference for hydrophobic surfaces. This study suggests that surfaces incorporating high densities of oligo(ethylene glycol) are good candidates for surfaces that interact minimally with bacteria.

Imaging of features on surfaces by condensation figures

Condensation of a vapor to a liquid on a cold surface that is not wet completely by this liquid leads to the formation of an array of droplets. If the surface is heterogeneous in its physical properties (especially its interfacial free energy), the patterns of these arrays reflect this heterogeneity. The distribution of droplets of water (condensation figures or CFs) observed by optical microscopy on a surface can be correlated with the molecular structure of that surface. The substrates used to investigate the formation and morphology of the CFs were patterned, self-assembled monolayers of different alkanethiolates on gold and of alkyl siloxanes on glass. Analysis of CFs is a valuable nondestructive technique for characterizing heterogeneities in surfaces.

Glow discharge plasma deposition of tetraethylene glycol dimethyl ether for fouling-resistant biomaterial surfaces

The glow discharge plasma deposition (GDPD) of tetraethylene glycol dimethyl ether is introduced as a novel method for obtaining surfaces that are resistant to protein adsorption and cellular attachment. Analysis of films by x-ray photoelectron spectroscopy and several biological assays indicate the formation of a fouling-resistant, PEO-like surface on several substrata (e.g., glass, polytetrafluoroethylene, polyethylene). Adsorption of 125I-radiolabelled proteins (fibrinogen, albumin and IgG) from buffer and plasma was very low (typically less than 20 ng/cm2) when compared to the untreated substrata, which exhibited much higher levels of protein adsorption. Not all coated substrata adsorbed equal amounts of protein (e.g., coated glass samples typically adsorbed more protein than coated polyethylene or coated polytetrafluoroethylene samples), suggesting that the substratum used may affect the amount of protein adsorbed. Measurement of dynamic platelet adhesion, using epifluorescent video microscopy, and endothelial cell attachment further demonstrates the short-term nonadhesiveness of these surfaces.