Label-free detection of protein-protein interactions at the GaAs/water interface through surface infrared spectroscopy: discrimination between specific and nonspecific interactions by using secondary structure analysis

Galvanic Displacement Reaction Enabled Specific and Sensitive Detection of Bacteria with a Digital Photocorrosion GaAs/AlGaAs Biosensor

The conjugation of ionic gold with bacterial antibodies makes it possible to induce a specific interaction between targeted bacteria and the surface of a GaAs/AlGaAs biochip. The process of immobilization is based on a galvanic displacement reaction (GDR) involving electron transfer between GaAs and Au3+ ions that leads to the formation of a Au-Ga alloy anchoring bacteria to the biochip surface. The GDR-based immobilization of Escherichia coli on biochips comprising a stack of GaAs/AlGaAs nanolayers (dGaAs = 12 nm, dAlGaAs = 10 nm) was confirmed by X-ray photoelectron spectroscopy and atomic force microscopy-based infrared experiments. We report the successful application of this approach for highly sensitive detection of E. coli with a digital photocorrosion (DIP) biosensor. The photoluminescence (PL) monitored DIP of GaAs/AlGaAs nanolayers results in the formation of a PL intensity maximum whose temporal appearance depends on the electric charge transfer between bacteria and the biochip. The formation of a robust bacteria-biochip interface achieved with the GDR process allowed us to observe the role of bacteria on the temporal position of a PL intensity maximum related to the etching of two pairs of GaAs/AlGaAs nanolayers extending up to 24 nm below the biochip surface. We demonstrate the attractive detection of E. coli at 250 CFU/mL, and we discuss the potential of this approach for designing a family of biosensors addressing the quasi-continuous monitoring of a water environment for the presence of pathogenic bacteria.

Superior performance of novel chitosan-based flocculants in decolorization of anionic dyes: Responses of flocculation performance to flocculant molecular structures and hydrophobicity and flocculation mechanism

To achieve economical and efficient decolorization, two novel flocculants, weakly hydrophobic comb-like chitosan-graft-poly (N, N-Dimethylacrylamide) (CSPD) and strongly hydrophobic chain-like chitosan-graft-L-Cyclohexylglycine (CSLC) were synthesized in this study. To assess the effectiveness and application of CSPD and CSLC, the impacts of factors, including flocculant dosages, initial pH, initial dye concentrations, co-existing inorganic ions and turbidities, on the decolorization performance were explored. The results suggested that the optimum decolorizing efficiencies of the five anionic dyes ranged from 83.17% to 99.40%. Moreover, for accurately controlling flocculation performance, the responses to flocculant molecular structures and hydrophobicity in flocculation using CSPD and CSLC were studied. The Comb-like structure gives CSPD a wider dosage range for effective decolorization and better efficiencies with large molecule dyes under weak alkaline conditions. The strong hydrophobicity makes CSLC more effective in decolorization and more suitable for removing small molecule dyes under weak alkaline conditions. Meanwhile, the responses of removal efficiency and floc size to flocculant hydrophobicity are more sensitive. Mechanism studies revealed that charge neutralization, hydrogen bonding and hydrophobic association worked together in the decolorization of CSPD and CSLC. This study has provided meaningful guidance for developing flocculants in the treatment of diverse printing and dyeing wastewater.

FTIR based approach to study EnaC mechanosensory functions

The pulmonary epithelial sodium ion channel (ENaC) is gaining importance for its sodium gating and mechanosensitive roles. The mechano functional studies on ENaC suggest direct molecular interactions between the ENaC protein with cytoskeleton microtubules and other extracellular matrix components. Also, in few mechanotransduction studies, ENaC was shown to respond both to membrane stretch as well as cell volume changes. However, the conformational characteristic of ENaC during sodium and mechano gating are yet to be fully elucidated. Thus obtaining ENaC protein conformational spectrum based on Fourier Transform Infrared Radiation (FTIR) spectroscopy in solution will be useful in predicting the nature of conformational changes occurring during any cell volume changes in an epithelial cell. The conformational spectrum looks promising in studying the disease biology of cystic fibrosis (CF) and CF like conditions that arise due to abnormal ion conductance membrane proteins and subsequent frequent fluid retentions. This review article presents the basics of epithelial ENaC protein as a gated mechanosensor and FTIR for developing fluid dynamics of ENaC protein. This can be applied to develop an ENaC based quantum mechanosensor for the prognosis as well as diagnosis of cystic fibrosis (CF) and allied lung diseases.

Detection of the Chilli Leaf Curl Virus Using an Attenuated Total Reflection-Mediated Localized Surface-Plasmon-Resonance-Based Optical Platform

The development of a nanoparticle-based optical platform has been presented as a biosensor for detecting target-specific plant virus DNA. The binding dynamics of gold nanoparticles has been studied on the amine-functionalized surface by the attenuated total reflection (ATR)-based evanescent wave absorption method monitoring the localized surface plasmon resonance (LSPR). The developed surface was established as a refractive index sensor by monitoring the LSPR absorption peak of gold nanoparticles. This nanoparticle-immobilized surface was explored to establish as a biosensing platform with target-specific immunoglobulin (IgG) antibody-antigen interaction. The IgG concentration-dependent variation of absorbance was correlated with the refractive index change. After successfully establishing this ATR configuration as an LSPR-based biosensor, the single-stranded DNA of the chilli leaf curl virus was detected using its complementary DNA sequence as a receptor. The limit of detection of this sensor was determined to be 1.0 μg/mL for this target viral DNA. This ATR absorption technique has enormous potential as an LSPR based nano-biosensor for the detection of other begomoviruses.

Label-free monitoring of immuno-specific interactions of adsorbed multilayer of proteins

Protein-protein interactions in adsorbed multilayer of an immuno-specific system of proteins that include staphylococcal protein A (SpA), bovine serum albumin (BSA), anti-chicken immunoglobulin Y (ac-IgG), chicken serum IgG (cs-IgG), and rabbit serum IgG (rs-IgG) on polystyrene (PS) were studied using attenuated total reflection-Fourier transform infrared spectroscopy. A systematic analysis allowed a direct qualitative and quantitative determination of protein interactions at each step of specific and nonspecific binding conditions at the molecular level. The study also provided information about (1) the adsorption behavior of the proteins, (2) the role of SpA in enabling correct orientation of the adsorbed IgG and maintaining the stability of the adsorbed SpA/ac-IgG system on the PS surface, (3) the function of BSA as both blocking reagent and promoter of specific and selective binding, and (4) the bioactivity conserved accommodation of SpA molecules on the PS surface. Furthermore, the unique characteristics of cs-IgG such as passive toward SpA adsorption and exposure of the multivalence state at nonspecific binding conditions was revealed spectroscopically. The present investigation provides a platform for further extension of the adopted methodology to a more complex system of immuno-detection for highly sensitive and rapid diagnostics.

Aptamer-based sensor for quantitative detection of mercury (II) ions by attenuated total reflection surface enhanced infrared absorption spectroscopy

A sensing platform based on the attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) technique and immobilized aptamer has been proposed herein for the selective detection of mercury ions (Hg²⁺). In the proposed platform, 5' thiolated 32-mer DNA probes with methylene blue at the 3' end were immobilized on a thin gold (Au) surface layer. Following Hg²⁺ ions interacting with T bases of the aptamer, T-Hg-T bonds are formed; resulting in a hairpin-shaped formation of the DNA and a detectable change in the IR absorbance of the sensing interface. Notably, the background noise produced by external molecules (e.g., water, non-specific binding molecules and bulk solution) is reduced to a negligible level by means of the ATR detection mode. It is shown that the proposed sensor has a linear response (R² = 0.986) with high sensitivity and good selectivity over the Hg²⁺ range of 0.01 μM-50 μM.

Immobilized enzymes: understanding enzyme – surface interactions at the molecular level

Interactions between immobilized enzymes and supporting surfaces are complex and context-dependent and can significantly alter enzyme structure, stability and activity.

Mid-infrared spectroscopy for protein analysis: potential and challenges

Mid-infrared (MIR) spectroscopy investigates the interaction of MIR photons with both organic and inorganic molecules via the excitation of vibrational and rotational modes, providing inherent molecular selectivity. In general, infrared (IR) spectroscopy is particularly sensitive to protein structure and structural changes via vibrational resonances originating from the polypeptide backbone or side chains; hence information on the secondary structure of proteins can be obtained in a label-free fashion. In this review, the challenges for IR spectroscopy for protein analysis are discussed as are the potential and limitations of different IR spectroscopic techniques enabling protein analysis. In particular, the amide I spectral range has been widely used to study protein secondary structure, conformational changes, protein aggregation, protein adsorption, and the formation of amyloid fibrils. In addition to representative examples of the potential of IR spectroscopy in various fields related to protein analysis, the potential of protein analysis taking advantage of miniaturized MIR systems, including waveguide-enhanced MIR sensors, is detailed.

Influence of a Thiolate Chemical Layer on GaAs (100) Biofunctionalization: An Original Approach Coupling Atomic Force Microscopy and Mass Spectrometry Methods

Widely used in microelectronics and optoelectronics; Gallium Arsenide (GaAs) is a III-V crystal with several interesting properties for microsystem and biosensor applications. Among these; its piezoelectric properties and the ability to directly biofunctionalize the bare surface, offer an opportunity to combine a highly sensitive transducer with a specific bio-interface; which are the two essential parts of a biosensor. To optimize the biorecognition part; it is necessary to control protein coverage and the binding affinity of the protein layer on the GaAs surface. In this paper; we investigate the potential of a specific chemical interface composed of thiolate molecules with different chain lengths; possessing hydroxyl (MUDO; for 11-mercapto-1-undecanol (HS(CH₂)₁₁OH)) or carboxyl (MHDA; for mercaptohexadecanoic acid (HS(CH₂)₁₅CO₂H)) end groups; to reconstitute a dense and homogeneous albumin (Rat Serum Albumin; RSA) protein layer on the GaAs (100) surface. The protein monolayer formation and the covalent binding existing between RSA proteins and carboxyl end groups were characterized by atomic force microscopy (AFM) analysis. Characterization in terms of topography; protein layer thickness and stability lead us to propose the 10% MHDA/MUDO interface as the optimal chemical layer to efficiently graft proteins. This analysis was coupled with insitu MALDI-TOF mass spectrometry measurements; which proved the presence of a dense and uniform grafted protein layer on the 10% MHDA/MUDO interface. We show in this study that a critical number of carboxylic docking sites (10%) is required to obtain homogeneous and dense protein coverage on GaAs. Such a protein bio-interface is of fundamental importance to ensure a highly specific and sensitive biosensor.

Recent applications of ATR FTIR spectroscopy and imaging to proteins

Attenuated Total Reflection (ATR) Fourier Transform Infrared (FTIR) spectroscopy is a label-free, non-destructive analytical technique that can be used extensively to study a wide variety of different molecules in a range of different conditions. The aim of this review is to discuss and highlight the recent advances in the applications of ATR FTIR spectroscopic imaging to proteins. It briefly covers the basic principles of ATR FTIR spectroscopy and ATR FTIR spectroscopic imaging as well as their advantages to the study of proteins compared to other techniques and other forms of FTIR spectroscopy. It will then go on to examine the advances that have been made within the field over the last several years, particularly the use of ATR FTIR spectroscopy for the understanding and development of protein interaction with surfaces. Additionally, the growing potential of Surface Enhanced Infrared Spectroscopy (SEIRAS) within this area of applications will be discussed. The review includes the applications of ATR FTIR imaging to protein crystallisation and for high-throughput studies, highlighting the future potential of the technology within the field of protein structural studies and beyond.

Electrochemical functionalization of gold and silicon surfaces by a maleimide group as a biosensor for immunological application

In the present study we investigated the preparation of biofunctionalized surfaces using the direct electrochemical grafting of maleimidophenyl molecules with subsequent covalent immobilization of specific peptide to detect target antibody, thereby extending the application of the biosensing systems towards immunodiagnostics. Para-maleimidophenyl (p-MP) functional groups were electrochemically grafted on gold and silicon surfaces from solutions of the corresponding diazonium salt. A specially synthesized peptide modified with cysteine (Cys-peptide) was then immobilized on the p-MP grafted substrates by cross-linking between the maleimide groups and the sulfhydryl group of the cysteine residues. Accordingly, the Cys-peptide worked as an antigen that was able to bind specifically the target antibody (anti-GST antibody), while it was non-sensitive to a negative contrast antibody (i.e. anti-Flag β). The immobilization of both specific and non-specific antibodies on the Cys-peptide-modified surfaces was monitored by infrared spectroscopic ellipsometry, a quartz crystal microbalance integrated in flow injection analysis system and potentiometric response. The results obtained clearly demonstrated that the direct modification of a surface with maleimidophenyl provides a very simple and reliable way of preparing biofunctionalized surfaces suitable for the construction of immunological biosensors.

Label-free strategy for in-situ analysis of protein binding interaction based on attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS)

A versatile ATR-SEIRAS methodology is described herein for highly sensitive analysis of immunoglobulin (IgG) recognition. This strategy allows in situ tracking of specific protein binding at the liquid-solid interface. Most importantly, interferential signal from environmental molecules (e.g., water, nonspecific binding molecules, and bulk molecules) can be eliminated to negligible levels by using the ATR analysis mode, and the sensitive IR structural information of target proteins is obtained simultaneously. A simplified numerical model has been established to quantitatively describe the kinetics and thermodynamics of protein recognition processes at surfaces. Compared with conventional label-free methods for protein binding study, experimental results obtained from IR spectroscopic information are more reliable. The presented ATR-SEIRAS method is powerful in studying surface limited protein binding reactions.

Bifunctional oligo(ethylene glycol) decorated surfaces which permit covalent protein immobilization and resist protein adsorption

In this article, surface coatings derived from homo-bifunctional tri(ethylene glycol) (EG(3)) and hexa(ethylene glycol) (EG(6)) molecules which have two terminal aldehyde groups are reported. These homo-bifunctional molecules can be used to functionalize amine-terminated surfaces through crosslinking one aldehyde group to surface amine groups, while leaving the other aldehyde group available for covalent immobilization of proteins. Best of all, after reducing remaining aldehyde groups on the surface with a reducing agent, sodium borohydride, the surface becomes oligo(ethylene glycol) (OEG)-terminated. The OEG-terminated surface can resist nonspecific protein adsorption, a feature that is often required for many biosensors and biomedical devices. Although some mixed self-assembled monolayers formed from two different organothiols also permit covalent protein immobilization and resist nonspecific protein adsorption, the procedure reported herein requires only one type of homo-bifunctional molecule and can be applied to both silicon and gold surfaces.

Surface functionalization of germanium ATR devices for use in FTIR-biosensors

Biosensors based on intrinsic detection methods have attracted growing interest. The use of Fourier transform infra-red (FTIR) spectroscopy with the attenuated internal total reflection (ATR) mode, in the biodetection context, requires appropriate surface functionalization of the ATR optical element. Here, we report the direct grafting of a thin organic layer (about 20 A depth) on the surface of a germanium crystal. This covering, constructed with novel amphiphilic molecules 2b (namely, 2,5,8,11,14,17,20-heptaoxadocosan-22-yl-3-(triethoxysilyl) propylcarbamate), is stable for several hours under phosphate buffered saline (PBS) flux and features protein-repulsive properties. Photografting of molecule 5 (namely, O-succinimidyl 4-(p-azidophenyl)butanoate) affords the activated ATR element, ready for the covalent fixation of receptors, penicillin recognizing proteins BlaR-CTD for instance. The different steps of the previous construction have been monitored by water contact angle (theta(w)) measurements, spectroscopic ellipsometry (covering depth), X-ray photoelectron spectroscopy (XPS) by using a fluorinated tag for the control of surface reactivity, and FTIR-ATR spectroscopy for the structural analysis of grafted molecules. Indeed, contrarily to silicon device, germanium device offers a broad spectral window (1000-4000 cm(-1)) and thus amide I and II absorption bands can be recorded. This work lays the foundations for the construction of novel FTIR biosensors.