Poly Ethylene Glycol Self-assembled Monolayers Research Articles

Measurements of interactions between fluorescent molecules and polyethylene glycol self-assembled monolayers.

Blocking the non-specific binding of fluorescent biomolecules to substrates is one of the most important approaches to minimize the background noise in single-molecule fluorescence detection. Polyethylene glycol (PEG) and its derivatives are the most frequently used self-assembled monolayers (SAMs) for surface passivation because they are particularly effective to reduce the adsorption of a majority of biomolecules. Most studies related to PEG SAMs focus only on the interactions between biomolecules and substrates, while few reports exist in which the interactions between fluorophores and organosilane SAMs are directly examined. The objective of this study is to try to clarify the interactions between fluorescein isothiocyanate (FITC) and PEG SAMs at different ionic strengths. Total internal reflection microscopy (TIRM) was utilized for quantitative analysis of the interactions. At low ionic strength, long-range attractions between FITC-modified polystyrene-silica particles and PEG SAM grafting substrates were observed, even though both of them had an ensemble-averaged negative charge. The origin of this attraction could be correlated to their nonuniformly charged surfaces. At high ionic strength, van der Waals attraction at short distances was measured as the electrostatic interactions were completely screened. Due to the polarizability of the FITC molecule, the van der Waals attractions increased with the thickness of the PEG SAMs. This phenomenon is explained by the hydration shell of the PEG SAMs.

Protein Adsorption on Self-Assembled Monolayers Induced by Surface Water Molecule

Three types of self-assembled monolayers (SAMs) are employed for the investigation of protein adsorption. An octadecylsilane (ODS) SAM, a fluoroalkylsilane (FAS) SAM, and a polyethylene glycol (PEG) SAM are selected as examples. The amount of adsorbed protein is measured by fluorescent microscopy. A molecular dynamics (MD) simulation is carried out for modeling both the molecular structures of the SAMs and the water structure on the SAMs. A hydrophilic PEG SAM prevents protein adsorption, while a large amount of adsorption is observed on a hydrophobic ODS SAM. In spite of the hydrophobicity, an FAS SAM prevents protein adsorption as well as a PEG SAM. MD calculation suggests that the existence of surface-bound water on an ODS SAM induces protein adsorption. In the case of the FAS SAM, owing to the electrostatic interaction and the flexibility of the precursor, the water molecule is not bound to the surface and protein adsorption is suppressed.

Dendrimer‐Scaffold‐Based Electron‐Beam Patterning of Biomolecules

Microarrays have revolutionized the field of genomics and more recently proteomics and have proven to be an asset in high-throughput screening. As the demand for improved sensitivity and throughput of biomolecular assays increases, considerable research effort has been put into developing microelectronic and nanophotonic biosensors, which are presumably more sensitive than conventional fluorescencebased assays and have a faster response time. The lithographic technology for making the densely packed microelectronic devices in a high-throughput manner is already quite advanced and may be integrated to form biosensors. This is expected to increase the pace of research in early detection of disease biomarkers, discovering cell-signal transduction pathways, and in drug discovery. Denser arrays are also important for reducing reagent volume consumption and to improve sensitivity. Successful biomolecule patterning on sensor chip circuitry requires a number of important steps. First, selective immobilization of the probe and reduction in non-specific binding should be achieved for higher signal-tonoise ratios. Second, reduction in the sensor size reduces the background, enhancing the signal-to-noise ratio, and therefore biomolecule patterns on the same order as the sensors are desirable. Third, the biomolecule pattern should be aligned with the sensor circuitry, which becomes more difficult as sensor size decreases. Finally, a fabrication process should be formulated to ensure that the biomolecules are intact and functional, which is a challenge given the harsh microor nanofabrication processing steps. Here, we have demonstrated an electron-beam (e-beam)-based approach fulfilling the above requirements for patterning biological macromolecules that does not involve the use of resist, hence eliminating the exposure of these biomolecules to harsh resist-stripping processes that are normally employed to remove the resist. A non-biofouling poly(ethylene glycol) self-assembled monolayer (PEG-SAM) was selectively removed by e-beam and patterned with aldehyde-terminated polyamidoamine dendrimer (ald-PAMAM-SAM) in a layer-by-layer (LbL) manner to covalently immobilize the aminated oligonucleotide, which bind only to their complementary sequence targets and can be stripped and reprobed. The Generation-6 (G-6) PAMAM molecule, terminated with 256 primary amine groups and 6.7 nm in diameter, was used to increase the surface density of aldehyde functional groups to increase the oligonucleotide-immobilization efficiency. Current techniques for patterning biomolecules involve the use of polymer-based templates, which can be removed mechanically without the use of organic solvents after biomolecule immobilization. However, serious limitations exist in each case. Poly(dimethylsiloxane) (PDMS)-based soft-lithographic techniques cannot be used to create high-resolution patterns, as aligning the PDMS pattern with sub-micrometer features has been shown to work in a mix-and-match manner with an accuracy of only 2 lm. Although alignment is not an issue for biomolecule patterning based on polymer liftoff, as it is an integrated process, this method involves extra steps of deposition and etching a polymer film, which increases the complexity of the process. Challenges are also encountered as the size of the photolithographic patterns decrease due to the increase in line-edge roughness (LER) and the isotropic nature of oxygen plasma etch. Patterned gold has been used for creating protein patterns using thiolbased linkers, but gold surfaces cannot be tolerated in some biosensors as gold interferes with the optical signal or conductivity of the sensor. This technique also includes extra photolithographic and lift-off processing steps for patterning gold. Protein patterning using fluorescence-tagged proteins C O M M U N IC A IO N S