Light–Dependent, Covalent Immobilization of Biomolecules on ‘Inert’ Surfaces

A Nanoporous Cytochrome c Film with Highly Ordered Porous Structure for Sensing of Toxic Vapors

Creating well-ordered nanoporosity in biomolecules promises stability and activity, offering access to an even wider range of application possibilities. Here, the preparation of nanoporous protein films containing cytochrome c protein molecules is reported through a soft-templating strategy using polystyrene (PS) spheres of different sizes as templates. The stability of the cytochrome c film is demonstrated through electrochemistry studies to show a reusable nature of these films over a long period of time. The size of the PS spheres is varied to tune the pore diameter and the thickness of the cytochrome c films, which are quite stable and highly selective for sensing toxic acidic vapors. The fusion of the templating strategy and the self-assembly of biomolecules may offer various possibilities by generating a new series of porous biomolecules including enzymes with different molecular weights and diameters, peptides, antibodies, and DNA with interesting catalytic, adsorption, sensing, and electronic properties.

General Dialdehyde Click Chemistry for Amine Bioconjugation

The development of methods for conjugating a range of molecules to primary amine functional groups has revolutionized the fields of chemistry, biology, and material science. The primary amine is a key functional group and one of the most important nucleophiles and bases used in all of synthetic chemistry. Therefore, tremendous interest in the synthesis of molecules containing primary amines and strategies to devise chemical reactions to react with primary amines has been at the core of chemical research. In particular, primary amines are a ubiquitous functional group found in biological systems as free amino acids, as key side chain lysines in proteins, and in signaling molecules and metabolites and are also present in many natural product classes. Due to its abundance, the primary amine is the most convenient functional group handle in molecules for ligation to other molecules for a broad range of applications that impact all scientific fields. Because of the primary amine's central importance in synthetic chemistry, acid-base chemistry, redox chemistry, and biology, many methods have been developed to efficiently react with primary amines, including activated carboxylic acids, isothiocyanates, Michael addition type systems, and reaction with ketones or aldehydes followed by in situ reductive amination. Herein, we introduce a new traceless, high-yield, fast click-chemistry method based on the rapid and efficient trapping of amine groups via a functionalized dialdehyde group. The click reaction occurs in mild conditions in organic solvents or aqueous media and proceeds in high yield, and the starting dialdehyde reagent and resulting dialdehyde click conjugates are stable. Moreover, no catalyst or dialdehyde-activating group is required, and the only byproduct is water. The initial dialdehyde and the resulting conjugate are both straightforward to characterize, and the reaction proceeds with high atom economy. To demonstrate the broad scope of this new click-conjugation strategy, we designed a straightforward scheme to synthesize a suite of dialdehyde reagents. The dialdehyde molecules were used for applications in cell-surface engineering and for tailoring surfaces for material science applications. We anticipate the broad utility of the general dialdehyde click chemistry to primary amines in all areas of chemical research, ranging from polymers and bioconjugation to material science and nanoscience.

Perfluorophenyl azides: new applications in surface functionalization and nanomaterial synthesis

A major challenge in materials science is the ongoing search for coupling agents that are readily synthesized, capable of versatile chemistry, able to easily functionalize materials and surfaces, and efficient in covalently linking organic and inorganic entities. A decade ago, we began a research program investigating perfluorophenylazides (PFPA) as the coupling agents in surface functionalization and nanomaterial synthesis. The p-substituted PFPAs are attractive heterobifunctional coupling agents because of their two distinct and synthetically distinguishable reactive centers: (i) the fluorinated phenylazide, which is capable of forming stable covalent adducts, and (ii) the functional group R, which can be tailored through synthesis. Two approaches have been undertaken for material synthesis and surface functionalization. The first method involves synthesizing PFPA bearing the first molecule or material with a functional linker R and then attaching the resulting PFPA to the second material by activating the azido group. In the second approach, the material surface is first functionalized with PFPA via functional center R, and coupling of the second molecule or material is achieved with the surface azido groups. In this Account, we review the design and protocols of the two approaches, providing examples in which PFPA derivatives were successfully used in material surface functionalization, ligand conjugation, and the synthesis of hybrid nanomaterials. The methods developed have proved to be general and versatile, and they are applicable to a wide range of materials (especially those that lack reactive functional groups or are difficult to derivatize) and to various substrates of polymers, oxides, carbon materials, and metal films. The coupling chemistry can be initiated by light, heat, and electrons. Patterned structures can be generated by selectively activating the areas of interest. Furthermore, the process is easy to perform, and light activation occurs in minutes, greatly facilitating the efficiency of the reaction. PFPAs indeed demonstrate many benefits as versatile surface coupling agents and offer opportunities for further exploration.

Construction of oligonucleotide microarrays (biochip) using heterobifunctional reagents

A number of hetero- and homobifunctional reagents have been reported to immobilize biomolecules on a variety of supports. However, efforts are on to search for a method, which is relatively simple, involving minimum of steps, cost effective, easy to reproduce, and that produces stable oligonucleotide arrays. Two new reagents, viz., [N-(2-trifluoroethanesulfonatoethyl)-N-(methyl)-triethoxysilylpropyl-3-amine], and [N-(3-trifluoroethanesulfonyloxypropyl)anthraquinone-2-carboxamide] have been designed considering the above points. These reagents contain different functional groups at their two ends. In [N-(2-trifluoroethanesulfonatoethyl)-N-(methyl)-triethoxysilylpropyl-3-amine], one end (triethoxysilyl) is capable of binding to the virgin glass surface and the other one consists of trifluoroethanesulfonate (tresyl) function specific toward aminoalkyl and mercaptoalkyl functionalities, which are easy to introduce at the 3'- or 5'-end of oligonucleotides. Likewise, in [N-(3-trifluoroethanesulfonyloxypropyl)anthraquinone-2-carboxamide], one end consists of photoactivatable moiety (anthraquinone) capable of reacting to a C-H containing surface and the tresyl function at the other end reacts specifically with aminoalkyl and mercaptoalkyl functionalities in modified oligonucleotides. These reagents have successfully been utilized to construct a number of oligonucleotide arrays and subsequently used for the detection of mismatches.

Choice of polymer matrix, its functionalization and estimation of functional group density for preparation of biochips

Oligonucleotide microarray has become an important and powerful tool for various genomic analyses, where, unlike conventional methods, one can identify simultaneously a large number of targets in a given sample. Postsynthesis immobilization, the most widely used method, involves the noncovalent and covalent fixing of suitably modified oligonucleotides on the solid supports. Among the various functional groups aminoalkyl, hydroxyalkyl, mercaptoalkyl, aldehyde, epoxy, and carboxyl the most frequently used functional groups on the polymeric surfaces. Because glass and polypropylene, the most widely used materials, are nonporous in nature, the functional groups density on the surface remains very low. In order to know the exact concentration of a ligand to be immobilized, it is essential to estimate the accessible functional groups on these surfaces. For this purpose, sensitive methods are required to estimate exact density of available functional groups on the surfaces. Apart from physical methods, a number of sensitive chemical methods, by making use of high extinction coefficient of 4,4'-dimethoxytrityl cation (epsilon(498) = 70,000 L mol-1 cm-1), have been reported to estimate accessible functional groups on the glass based polymer supports. In this chapter, use of these reagents for spectrophotometric determination of functional group density on glass microslides and polypropylene film has been discussed.

Covalent bound sensing layers on surface acoustic wave (SAW) biosensors

This paper reports on the development of immunosensors based on commercially available surface acoustic wave (SAW) devices working at 380 MHz. Approaches for coating the sensor surface with a sensing layer of receptive biomolecules are presented and discussed. It was found that the sensitivity strongly relates to the immobilization method. Additionally, the sensitivity can be influenced by the density of accessible biomolecules on the active sensing area. Usually, by most of the standard immobilization procedures, two-dimensional layers of receptive biomolecules are obtained. We present a three-dimensional layer, which provides a higher absolute amount of recognition molecules. A dextran layer is photoimmobilized to the sensor surface and the recognition molecules are covalently embedded into the dextran matrix. The feasibility of specific immunosensing is investigated using SAW sensors connected to a fluid handling system.

Light-induced activation of an inert surface for covalent immobilization of a protein ligand

A simple and mild procedure is developed for the preparation of an activated polymer surface, used for immobilization of a protein ligand through a covalent linkage. Activation of the polymer surface is carried out by attaching an active functional group through 1-fluoro-2-nitro-4-azidobenzene (FNAB). UV irradiation of FNAB transforms its azido group into a highly reactive nitrene, which binds with the inert polymer surface, whereas the active fluoro group of FNAB, now part of the polymer, remains intact. Covalent linkage between the ligand and the inert surface is established through this active fluoro group in a thermochemical reaction. The photochemical step is carried out under dry conditions to exclude the possibility of undesirable reactions between the solvent and the highly reactive nitrene. The method can be used for activation of different inert polymer surfaces having carbon hydrogen bonds. The efficacy of our method is demonstrated by immobilizing horseradish peroxidase on an activated polystyrene surface. The enzyme, immobilized through the photolinker, is found to give a twofold increase in absorbance with the substrate as compared to the directly adsorbed enzyme. The method may have many applications in the preparation of bioreactors, biostrips, and biosensors, and in diagnostic tests involving the ELISA technique.

Photochemical immobilization of anthraquinone conjugated oligonucleotides and PCR amplicons on solid surfaces

Ligand immobilization on solid surfaces is an essential step in fields such as diagnostics, bio sensor manufacturing, and new material sciences in general. In this paper a photochemical approach based on anthraquinone as the chromophore is presented. Photochemical procedures offer special advantages as they are able to generate highly reactive species in an orientation specific manner. As presented here, anthraquinone (AQ) mediated covalent DNA immobilization appears to be superior to currently known procedures. A synthetic procedure providing AQ-phosphoramidites is presented. These reagents facilitate AQ conjugation during routine DNA synthesis, thus enabling the AQ-oligonucleotides to be immobilized in a very convenient and efficient manner. AQ-conjugated PCR primers can be used directly in PCR. When the PCR is performed in solution, the amplicons can be immobilized after the PCR. Moreover, when the primers are immobilized prior to the PCR, a solid-phase PCR can be performed and the amplicons are thus produced directly on the solid support.

Covalent photolinker-mediated immobilization of an intermediate dextran layer to polymer-coated surfaces for biosensing applications

A new method is presented for the covalent binding of dextran as an intermediate layer on surface acoustic wave (SAW) devices. For biosensing applications in aqueous media commercially available SAW devices require surface passivation to prevent corrosion of the aluminum device structures in electrolytes. Thin films of polyimide and parylene revealed exceptional passivation properties. They were used as a base for dextran immobilization. Covalent binding of dextran to polymer-coated surfaces was achieved by photoimmobilization. Aryldiazirine-functionalized bovine serum albumin served as the multifunctional light-activable linking agent (photolinker polymer). Dextran and photolinker polymer were mixed and photobonded to sensor surfaces. Essential photoimmobilization parameters were optimized. The binding of proteins to dextran applying carbodiimide chemistries was exemplified with antiurease antibodies and the feasibility of specific immunosensing was investigated on SAW sensors connected to a fluid handling system.

Platelet interactions with plasma-polymerized ethylene oxide and N-vinyl-2-pyrrolidone films and linear poly(ethylene oxide) layer

Dimethyldichlorosilane (DDS)-treated glass (DDS-glass) was modified with either poly(ethylene oxide) (PEO) films or poly(N-vinyl-2-pyrrolidone) (PNVP) films by plasma polymerization. The thickness of the plasma polymerized films was varied between 40 and 700 nm. The results showed that the hydrophilic plasma polymerized PEO and PNVP films on DDS-glass did not prevent platelet adhesion and activation. The film thickness had only marginal influence on the prevention of platelet activation. In contrast, platelet adhesion was prevented on DDS-glass absorbed with a PEO-containing block copolymer (Pluronic F-108 surfactant) even at a calculated thickness of the PEO layer of less than 40 nm. This study shows that surface hydrophilization is not sufficient for prevention of platelet adhesion and activation. The contrasting results in platelet adhesion between cross-linked plasma polymers and linear PEO-containing block copolymers may be explained qualitatively by a steric repulsion mechanism that is achieved by the conformational freedom of the linear PEO chains interacting with water.

Immobilization of proteins to lipid bilayers

Phospholipid bilayers deposited on sensor surfaces are excellent substrates for immobilizing proteins via a molecular anchor. An integrated optics sensing device was used to accurately measure the binding kinetics of proteins thus anchored. By comparing the results with measurements using proteins from which the anchor had been enzymatically removed, it was shown that the anchor accounts for essentially all the irreversible binding. The insertion of the anchor into the lipid bilayer is a spontaneous process. This method of immobilization should be widely applicable to many soluble protein molecules, to which an anchor can be attached by routine methods of molecular engineering.

Photochemical linkage of antibodies to silicon chips

Antibodies and antigen binding fragments thereof were photochemically immobilized on surface-modified silicon chips of 5 x 5 mm size. Silicon surface-grafted diazirines and benzophenones formed covalent bonds with the immunoreagents on light activation. Photolithographic immobilization of monoclonal antibodies in aqueous media was achieved on silicon chips by activating surface-grafted benzophenones. The presence of bovine serum albumin during irradiation reduced nonspecific adsorption of the immunoreagents and retained the immunoactivity of the photoimmobilized molecules.