Formation and Structure of Self-Assembled Monolayers

Capacitive monitoring of protein immobilization and antigen-antibody reactions on monomolecular alkylthiol films on gold electrodes

Self-assembled monolayers of omega-mercaptohexadecanoic acid and omega-mercaptohexadecylamine on gold electrodes are stable at neutral pH and display pure capacitive behavior at frequencies around 20 Hz. Different methods of covalent immobilization of proteins on these monolayers are compared. Various reagents including succinimides, thionylchloride, p-nitrophenol and carbodiimides were used to activate the carboxy groups of the adsorbed monolayer of omega-mercaptohexadecanoic acid. Glutaraldehyde, cyanuric chloride and phenylene diisocyanate were used to activate the amino groups of the monolayer of omega-mercaptohexadecylamine. The immobilization of albumin on the activated surface was studied by capacitive measurements. The N-hydroxysuccinimide and carbodiimide methods were identified as most suitable for protein immobilization in that they did not compromise the insulating properties of the alkylthiol layer and led to maximal increase of its dielectric thickness. These approaches were used for a layer-by-layer preparation of a capacitive immunosensor. Specifically, antibodies to human serum albumin were immobilized on the alkylthiol mono-layer. Binding of the antigen led to a decrease of the electrode capacitance. The detection limit of the immunosensor is as low as 15 nM (1 mg/l).

The synthesis of neoglycophospholipid conjugates via reductive amination of omega-oxoalkylglycosides and phosphatidylethanolamines

Phospholipid conjugates of mono- and disaccharides tethered with an n-decanyl spacer were efficiently synthesized via an improved reductive amination of deprotected omega-oxodecanyl beta-glycosides and phosphatidylethanolamines with or without alkenyl groups. The omega-oxodecanyl beta-glycosides were prepared by stereoselective glycosidation of glycosyl halides with 1, 10-decanediol followed by pyridinium dichromate oxidation. The acetyl groups of the omega-oxodecanyl beta-glycosides were removed with sodium methoxide prior to their conjugation with phosphatidylethanolamines.

Human Growth Hormone Adsorption Kinetics and Conformation on Self-Assembled Monolayers

The adsorption process of the recombinant human growth hormone on organic films, created by self-assembly of octadecyltrichlorosilane, arachidic acid, and dipalmitoylphosphatidylcholine, is investigated and compared to adsorption on silica and methylated silica substrates. Information on the adsorption process of human growth hormone (hGH) is obtained by using total internal reflection fluorescence (TIRF). The intensity, spectra, and quenching of the intrinsic fluorescence emitted by the growth hormone's single tryptophan are monitored and related to adsorption kinetics and protein conformation. For the various alkylated hydrophobic surfaces with differences in surface density and conformational freedom it is observed that the adsorbed amount of growth hormone is relatively large if the alkyl chains are in an ordered structure while the amounts adsorbed are considerably lower for adsorption onto less ordered alkyl chains of fatty acid and phospholipid layers. Adsorption on methylated surfaces results in a relatively large conformational change in the growth hormone's structure, as displayed by a 7 nm blue shift in emission wavelength and a large increase in the effectiveness of fluorescence quenching. Conformational changes are less evident for hGH adsorption onto the fatty acid and phospholipid alkyl chains. Adsorption kinetics on the hydrophilic head groups of the self-assembled monolayers are similar to those on solid hydrophilic surfaces. The relatively small conformational changes in the hGH structure observed for adsorption on silica are even further reduced for adsorption on fatty acid head groups.

Neoglycophospholipids with alkyl spacers: synthesis via an improved reductive amination and monolayer properties

An efficient synthesis of neoglycophospholipids with variable length alkyl spacer chains is described. Neoglycophospholipids tethered by alkyl chains of 3, 5, 7, 10, and 16 methylene units were synthesized in good overall yields in four steps. The key intermediates, omega-oxoalkyl glycopyranosides, were synthesized in two steps by glycosidation of chloro (or ethylthio) glycosides with a diol followed by oxidation of the remaining hydroxy group to an aldehyde functionality. Conjugation of the omega-glycoalkyl aldehyde with distearoylphosphatidylethanolamine via an improved reductive amination procedure significantly enhanced efficiency and yields with respect to those from traditional procedures. The amphiphilic properties of the neoglycophospholipids were characterized at the air-water interface. While the carbohydrate head group had relatively little effect, the length of the alkyl spacer profoundly influenced surface area-pressure isotherms.

Bioreactive self-assembled monolayers on hydrogen-passivated Si(111) as a new class of atomically flat substrates for biological scanning probe microscopy

This is the first report of bioreactive self-assembled monolayers, covalently bound to atomically flat silicon surfaces and capable of binding biomolecules for investigation by scanning probe microscopy and other surface-related assays and sensing devices. These monolayers are stable under a wide range of conditions and allow tailor-made functionalization for many purposes. We describe the substrate preparation and present an STM and SFM characterization, partly performed with multiwalled carbon nanotubes as tapping-mode supertips. Furthermore, we present two strategies of introducing in situ reactive headgroup functionalities. One method entails a free radical chlorosulfonation process with subsequent sulfonamide formation. A second method employs singlet carbenemediated hydrogen-carbon insertion of a heterobifunctional, amino-reactive trifluoromethyl-diazirinyl crosslinker. We believe that this new substrate is advantageous to others, because it (i) is atomically flat over large areas and can be prepared in a few hours with standard equipment, (ii) is stable under most conditions, (iii) can be modified to adjust a certain degree of reactivity and hydrophobicity, which allows physical adsorption or covalent crosslinking of the biological specimen, (iv) builds the bridge between semiconductor microfabrication and organic/biological molecular systems, and (v) is accessible to nanopatterning and applications requiring conductive substrates.

Gold Clustering at the Terminal Functions of Long-Chain Thiols and Amines

Treatment of 1,6-hexanedithiol with (4)/(3) mol equiv of tris[(triphenylphosphino)aurio(I)]oxonium tetrafluoroborate and sodium tetrafluoroborate affords (n-hexane-1,6-dithiolato)tetrakis-[(triphenylphosphine)gold(I)] bis(tetrafluoroborate) (1). The analogous reactions with beta-mercaptoethylamine, HS(CH(2))(2)NH(2), 1,4-diaminobutane, H(2)N(CH(2))(4)NH(2), and n-butyl- and n-octylamine, CH(3)(CH(2))(n)()NH(2) (n = 3 or 7), give the corresponding penta- (2), hexa- (3), and trinuclear (4, 5) complexes, respectively. The crystal structures of compounds 3 and 5 have been determined by single-crystal X-ray diffraction studies. In the hexanuclear complex 3, three gold atoms are bonded to each nitrogen, putting each of these atoms at the apex of an NAu(3) pyramid. There are no intra- or intermolecular interactions between the gold centers of different nitrogen atoms. The trinuclear complex 5 features the unfolded aliphatic chain at the apex of such a pyramid. In both compounds the gold atoms show close contacts of 3.0 +/- 0.1 Å, indicating significant bonding, which is probably the main driving force for the clustering of seemingly closed-shell (d(10)) gold(I) metal atoms.