Protein adsorption on nanoporous TiO2 films: a novel approach to studying photoinduced protein/electrode transfer reactions

Adsorption of Native Amino Acids on Nanocrystalline TiO2: Physical Chemistry, QSPR, and Theoretical Modeling

The affinity of biomolecules, such as peptides and proteins, with inorganic surfaces, is a fundamental topic in biotechnology and bionanotechnology. Amino acids are often used as "model" bits of peptides or proteins for studying their properties in different environments and/or developing functional surfaces. Despite great demand for knowledge about amino acid interactions with metal oxide surfaces, studies on the issue represent a fragmentary picture. In this paper, we describe amino acid adsorption on nanocrystalline anatase systematically at uniform conditions. Analysis of the Gibbs free adsorption energy indicated how the aliphatic, aromatic, polar, and charged side chain groups affect the binding affinity of the amino acids. Thermodynamic features of the l-amino acid adsorption receive thorough interpretation with calculated molecular descriptors. Theoretical modeling shows that amino acids complex with TiO₂ nanoparticles as zwitterions via ammonium group.

Transport-Limited Adsorption of Plasma Proteins on Bimodal Amphiphilic Polymer Co-Networks: Real-Time Studies by Spectroscopic Ellipsometry

Traditional hydrogels are commonly limited by poor mechanical properties and low oxygen permeability. Bimodal amphiphilic co-networks (β-APCNs) are a new class of materials that can overcome these limitations by combining hydrophilic and hydrophobic polymer chains within a network of co-continuous morphology. Applications that can benefit from these improved properties include therapeutic contact lenses, enzymatic catalysis supports, and immunoisolation membranes. The continuous hydrophobic phase could potentially increase the adsorption of plasma proteins in blood-contacting medical applications and compromise in vivo material performance, so it is critical to understand the surface characteristics of β-APCNs and adsorption of plasma proteins on β-APCNs. From real-time spectroscopic visible (Vis) ellipsometry measurements, plasma protein adsorption on β-APCNs is shown to be transport-limited. The adsorption of proteins on the β-APCNs is a multistep process with adsorption to the hydrophilic surface initially, followed by diffusion into the material to the internal hydrophilic/hydrophobic interfaces. Increasing the cross-linking of the PDMS phase reduced the protein intake by limiting the transport of large proteins. Moreover, the internalization of the proteins is confirmed by the difference between the surface-adsorbed protein layer determined from XPS and bulk thickness change from Vis ellipsometry, which can differ up to 20-fold. Desorption kinetics depend on the adsorption history with rapid desorption for slow adsorption rates (i.e., slow-diffusing proteins within the network), whereas proteins with fast adsorption kinetics do not readily desorb. This behavior can be directly related to the ability of the protein to spread or reorient, which affects the binding energy required to bind to the internal hydrophobic interfaces.

Functionalized ceramics for biomedical, biotechnological and environmental applications

Surface functionalization has become of paramount importance and is considered a fundamental tool for the development and design of countless devices and engineered systems for key technological areas in biomedical, biotechnological and environmental applications. In this review, surface functionalization strategies for alumina, zirconia, titania, silica, iron oxide and calcium phosphate are presented and discussed. These materials have become particularly important concerning the aforementioned applications, being not only of great academic, but also of steadily increasing human and commercial, interest. In this review, special emphasis is given to their use as biomaterials, biosensors, biological targets, drug delivery systems, implants, chromatographic supports for biomolecule purification and analysis, and adsorbents for toxic substances and pollutants. The objective of this review is to provide a broad picture of the enormous possibilities offered by surface functionalization and to identify particular challenges regarding surface analysis and characterization.

Assessment of the transferability of a protein force field for the simulation of peptide-surface interactions

In order to evaluate the transferability of existing empirical force fields for all-atom molecular simulations of protein adsorption behavior, we have developed and applied a method to calculate the adsorption free energy (DeltaG(ads)) of model peptides on functionalized surfaces for comparison with available experimental data. Simulations were conducted using the CHARMM program and force field using a host-guest peptide with the sequence TGTG-X-GTGT (where G and T are glycine and threonine amino acid residues, respectively, with X representing valine, threonine, aspartic acid, phenylalanine or lysine) over nine different functionalized alkanethiol self-assembled monolayer (SAM) surfaces with explicitly represented solvent. DeltaG(ads) was calculated using biased-energy replica exchange molecular dynamics to adequately sample the conformational states of the system. The simulation results showed that the CHARMM force-field was able to represent DeltaG(ads) within 1 kcal/mol of the experimental values for most systems, while deviations as large as 4 kcal/mol were found for others. In particular, the simulations reveal that CHARMM underestimates the strength of adsorption on the hydrophobic and positively charged amine surfaces. These results clearly show that improvements in force field parameterization are needed in order to accurately represent interactions between amino acid residues and functional groups of a surface and they provide a means for force field evaluation and modification for the eventual development and validation of an interfacial force field for the accurate simulation of protein adsorption behavior.

Characterizing the adsorption of peptides to TiO2 in aqueous solutions by liquid chromatography

The interactions between titanium oxide (TiO(2)) and flexible peptides, decorated by amine, carboxyl, and phosphoserine functional groups, were characterized using analytical liquid chromatography with various loading and eluting solutions. This approach enabled discernment of the type of intermolecular interactions generated between the peptides and the metal oxide surfaces in addition to unraveling more subtle effects, specific ions, and oxide phase may have on the adsorption. The peptide presenting Lys residues adsorbed to the oxide surface in the presence of Tris buffer and eluted under conditions that indicated its binding via electrostatic interactions at physiological pH values. Upon adsorption to the oxide in the presence of phosphate buffer, the same peptide exhibited stronger electrostatic interactions with the surface, mediated by the buffer phosphate ions. In Tris-buffered saline (TBS), pH 7.4, as the adsorption medium, the peptide with the phosphoserine residues exhibited affinity indicative of coordinative binding to the titanium oxide, whereas a similar peptide decorated by carboxylate groups failed to adsorb. On the basis of differences in the interactions of these peptides with the TiO(2), the efficient separation of the two peptides was demonstrated. A basic amphiphilic peptide, composed mostly of Lys and Leu residues, was found to strongly adsorb to TiO(2) while in helical conformation only, demonstrating the strong impact the secondary structure may have on adsorption to the surface. The methodology presented in this study allows the elucidation of in situ binding mechanism and relative strengths to titanium oxide surfaces at conditions which resemble biologically relevant environments.

SECM characterization of immobilised enzymes by self-assembled monolayers on titanium dioxide surfaces

SECM in generator-collector mode was used to detect the presence of immobilised enzymes on titanium dioxide layers which were chemically or electrochemically generated with possible application as chemical sensors and biosensors. Glucose oxidase (GOx) and horseradish peroxidase (HRP) were immobilised by SAM generation using aminopropyltriethoxysilane (APTES) and ascorbic acid. The enzymes were successfully immobilised on two different TiO(2) surfaces. A simple test of durability of the system was made and a model of SAM organisation is presented.

Fundamental studies of the cytochrome c immobilization by the potential cycling method on nanometer-scale nickel oxide surfaces

This work describes the performance of cytochrome c/nickel oxide nanoparticles/glassy carbon electrode, prepared by the electrochemical deposition of the nickel oxide nanoparticles (NiO NPs) on the glassy carbon (GC) electrode surface and the cytochrome c immobilization on the nickel oxide nanoparticle surfaces. An extensive sample examination with the help of the SEM and AFM presented the existence of different geometrical shapes of the nickel oxide particles. These geometrical structures could lead to the better immobilization of proteins on their surfaces. The resulting electrode displayed an excellent behavior for the redox of the cytochrome c. Also, the resulting heme protein exhibited a direct electrical contact with the electrode because of the structural alignment of the heme protein on the nanometer-scale nickel oxide surfaces. This method could be suitable for applications to nanofabricated devices. In the end, it was concluded that the cytochrome c could be tethered to the nanometer-scale nickel oxide surfaces.

Hemoglobin-based hydrogen peroxide biosensor tuned by the photovoltaic effect of nano titanium dioxide

The photovoltaic effect of titanium dioxide (TiO2) nanoparticles, induced by ultraviolet light, can greatly improve the catalytic activity of hemoglobin as a peroxidase, with the sensitivity increased nearly 3-fold and the detection limit lowered 2 orders, in contrast to the catalytic reactions in the dark, which indicates a possible method to tune the properties of proteins for development of photocontrolled protein-based biosensors.

Electrochemical study of photovoltaic effect of nano titanium dioxide on hemoglobin

Nano titanium dioxide (TiO2) and hemoglobin (Hb) were co-modified on pyrolytic graphite (PG) electrode to study the photovoltaic effect of TiO2 nanoparticles (NPs) on the electron transfer reactivity and catalytic activity of the protein. By means of cyclic voltammetry (CV) and FTIR measurements, the study was characterized in both aerobic and anaerobic environments. Experimental results revealed that the factors which mainly interacted with Hb were electron/hole pairs and reactive oxygen species (ROS) generated by the photovoltaic effect when TiO2 NPs were irradiated under ultraviolet (UV) light. The electron/hole pairs generated on the surface of TiO2 would influence the structure of Hb gently, so the electron transfer reactivity and catalytic ability of the protein slightly changed. In contrast, ROS interacted with Hb intensively, which brought in much conformational change to Hb and its active centers, and even cause some damage. Consequently, the electron transfer reactivity and catalytic activity of Hb changed with a process of increasing initially and decreasing afterwards.

Protein structural deformation induced lifetime shortening of photosynthetic bacteria light-harvesting complex LH2 excited state

Photosynthetic bacterial light-harvesting antenna complex LH2 was immobilized on the surface of TiO(2) nanoparticles in the colloidal solution. The LH2/TiO(2) assembly was investigated by the time-resolved spectroscopic methods. The excited-state lifetimes for carotenoid-containing and carotenoidless LH2 have been measured, showing a decrease in the excited-state lifetime of B850 when LH2 was immobilized on TiO(2). The possibility that the decrease of the LH2 excited-state lifetime being caused by an interfacial electron transfer reaction between B850 and the TiO(2) nanoparticle was precluded experimentally. We proposed that the observed change in the photophysical properties of LH2 when assembled onto TiO(2) nanoparticles is arising from the interfacial-interaction-induced structural deformation of the LH2 complex deviating from an ellipse of less eccentric to a more eccentric ellipse, and the observed phenomenon can be accounted by an elliptical exciton model. Experiment by using photoinactive SiO(2) nanoparticle in place of TiO(2) and core complex LH1 instead of LH2 provide further evidence to the proposed mechanism.

Using silver nanoparticle to enhance current response of biosensor

In this paper, we present a simple procedure to increase the sensitivity of a glucose biosensor. The feasibility of an amperometric glucose biosensor based on immobilization of glucose oxidase (GOx) in silver (Ag) sol was investigated for the first time. GOx was simply mixed with Ag nanoparticles and cross-linked with a polyvinyl butyral (PVB) medium by glutaraldehyde. Then a platinum electrode was coated with the mixed solution. The effects of the amount of the Ag particles used, with respect to the current response for enzyme electrodes, were studied. A set of experimental results indicate that the current response for the enzyme electrode containing hydrophobic Ag sol increased from 0.531 to 31.17 microA in the solution of 10 mmol/L beta-D glucose. The time reaching the steady-state current response reduced from 60 to 20s, three times less than those without Ag particles involved.

Electrochemical control of protein monolayers at indium tin oxide surfaces for the reagentless optical biosensing of nitric oxide

Cytochrome c has been immobilized onto functionalized, optically transparent indium tin oxide (ITO) electrodes by covalent and electrostatic techniques. Covalent immobilization was achieved by the formation of a disulfide bond between N-succinimidyl 3-(2-pyridyldithio)propionate-(SPDP-) modified cytochrome c and SPDP-silanized ITO. Additionally, ITO electrodes have been modified with the bifunctional reagent 1,12-dodecanedicarboxylic acid (DDCA), resulting in formation of a carboxylic acid-terminated monolayer. Covalent protein attachment to the DDCA-functionalized ITO was achieved with the cross-linker 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride. Electrostatic attachment of the protein involved ion-pair and hydrogen-bond interactions between the terminating carboxylic acid groups of the DDCA-functionalized ITO and the primary amine groups of the lysine residues of cytochrome c. The electrostatic interaction between the cytochrome c and the functionalized ITO resulted in greater rotational mobility of the protein at the electrode surface, leading to ca. 63% electroactivity, as compared to ca. 41% electroactivity for the covalently immobilized protein. The redox state of the electrostatically bound cytochrome c monolayers could be electrochemically switched between ferric and ferrous forms. Electrochemical control of the bound protein was used to regenerate the biosensing surface following binding of nitric oxide (NO). Ligation of NO with the cytochrome c was monitored by measurement of the change of absorbance intensity at 416 nm. Through application of a negative potential, the cytochrome c was reduced from the ferric to the ferrous form, which led to the removal of the ligated NO. Application of a positive potential regenerated the ferric cytochrome c, enabling multiple repeat measurements of NO. Such electrochemical control of proteins immobilized on transparent electrodes enables the optical biosensing of analyte targets without recourse to exogenous reagents.

Functionalizing Nanocrystalline Metal Oxide Electrodes With Robust Synthetic Redox Proteins

De novo designed synthetic redox proteins (maquettes) are structurally simpler, working counterparts of natural redox proteins. The robustness and adaptability of the maquette protein scaffold are ideal for functionalizing electrodes. A positive amino acid patch has been designed into a maquette surface for strong electrostatic anchoring to the negatively charged surfaces of nanocrystalline, mesoporous TiO(2) and SnO(2) films. Such mesoporous metal oxide electrodes offer a major advantage over conventional planar gold electrodes by facilitating formation of high optical density, spectroelectrochemically active thin films with protein loading orders of magnitude greater (up to 8 nmol cm(-2)) than that achieved with gold electrodes. The films are stable for weeks, essentially all immobilized-protein display rapid, reversible electrochemistry. Furthermore, carbon monoxide ligand binding to the reduced heme group of the protein is maintained, can be sensed optically and reversed electrochemically. Pulsed UV excitation of the metal oxide results in microsecond or faster photoreduction of an immobilized cytochrome and millisecond reoxidation. Upon substitution of the heme-group Fe by Zn, the light-activated maquette injects electrons from the singlet excited state of the Zn protoporphyrin IX into the metal oxide conduction band. The kinetics of cytochrome/metal oxide interfacial electron transfer obtained from the electrochemical and photochemical data obtained are discussed in terms of the free energies of the observed reactions and the electronic coupling between the protein heme group and the metal oxide surface.