Detection of Microenvironmental Changes Induced by Protein Adsorption onto Self-Assembled Monolayers using an Extended Gate-Field Effect Transistor

Label-Free Monitoring of Histone Acetylation Using Aptamer-Functionalized Field-Effect Transistor and Quartz Crystal Microbalance Sensors

Chemical and enzymatic modifications of amino acid residues in protein after translation contain rich information about physiological conditions and diseases. Histone acetylation/deacetylation is the essential post-translational modification by regulating gene transcription. Such qualitative changes of biomacromolecules need to be detected in point-of-care systems for an early and accurate diagnosis. However, there is no technique to aid this issue. Previously, we have applied an aptamer-functionalized field-effect transistor (FET) to the specific protein biosensing. Quantitative changes of target protein in a physiological solution have been determined by detecting innate charges of captured protein at the gate-solution interface. Moreover, we have succeeded in developing an integrated system of FET and quartz crystal microbalance (QCM) sensors for determining the adsorbed mass and charge, simultaneously or in parallel. Prompted by this, in this study, we developed a new label-free method for detecting histone acetylation using FET and QCM sensors. The loss of positive charge of lysine residue by chemically induced acetylation of histone subunits (H3 and H4) was successfully detected by potentiometric signals using anti-histone aptamer-functionalized FET. The adsorbed mass was determined by the same anti-histone aptamer-functionalized QCM. From these results, the degree of acetylation was correlated to the charge-to-mass ratio of histone subunits. The histone required for the detection was below 100 nM, owing to the high sensitivity of aptamer-functionalized FET and QCM sensors. These findings will guide us to a new way of measuring post-translational modification of protein in a decentralized manner for an early and accurate diagnosis.

Direct and label-free influenza virus detection based on multisite binding to sialic acid receptors

A system to discriminate human or avian influenza A remains a highly sought-after tool for prevention of influenza pandemics in humans. Selective binding of the influenza A viral hemagglutinin (HA) to specific sialic acid (SA) receptors (Neu5Acα(2-6)Gal in humans, Neu5Acα(2-3)Gal in birds) is determined by the genotype of the HA and neuraminidase (NA) segments, making it one of the key characteristics that distinguishes human or avian influenza A virus. Here we demonstrate the direct detection of whole H1N1 influenza A virus using 6′-sialyllactose (Neu5Acα(2-6)Galβ(1-4)Glc, 6SL)-immobilized gold electrodes as biosensing surfaces. The sensitivity was higher than that of conventional immunochromatographic technique (ICT) for influenza virus and not restricted by genetic drift. The label-free detection technology via direct attachment of a whole virus using a chemically modified electrode is a promising means to provide a simple and rapid diagnostic system for viral infections.

Electrical Chips for Biological Point-of-Care Detection

As the future of health care diagnostics moves toward more portable and personalized techniques, there is immense potential to harness the power of electrical signals for biological sensing and diagnostic applications at the point of care. Electrical biochips can be used to both manipulate and sense biological entities, as they can have several inherent advantages, including on-chip sample preparation, label-free detection, reduced cost and complexity, decreased sample volumes, increased portability, and large-scale multiplexing. The advantages of fully integrated electrical biochip platforms are particularly attractive for point-of-care systems. This review summarizes these electrical lab-on-a-chip technologies and highlights opportunities to accelerate the transition from academic publications to commercial success.

Dual aptamer-immobilized surfaces for improved affinity through multiple target binding in potentiometric thrombin biosensing

We developed a label-free and reagent-less potentiometric biosensor with improved affinity for thrombin. Two different oligomeric DNA aptamers that can recognize different epitopes in thrombin were introduced in parallel or serial manners on the sensing surface to capture the target via multiple contacts as found in many biological systems. The spacer and linker in the aptamer probes were optimized for exerting the best performance in molecular recognition. To gain the specificity of the sensor to the target, an antifouling molecule, sulfobeaine-3-undecanethiol (SB), was introduced on the sensor to form a self-assembled monolayer (SAM). Surface characterization revealed that the aptamer probe density was comparable to the distance of the two epitopes in thrombin, while the backfilling SB SAM was tightly aligned on the surface to resist nonspecific adsorption. The apparent binding parameters were obtained by thrombin sensing in potentiometry using the 1:1 Langmuir adsorption model, showing the improved dissociation constants (Kd) with the limit of detection of 5.5 nM on the dual aptamer-immobilized surfaces compared with single aptamer-immobilized ones. A fine control of spacer and linker length in the aptamer ligand was essential to realize the multivalent binding of thrombin on the sensor surface. The findings reported herein are effective for improving the sensitivity of potentiometric biosensor in an affordable way towards detection of tiny amount of biomolecules.

Current and emerging challenges of field effect transistor based bio-sensing

Field-effect-transistor (FET) based electrical signal transduction is an increasingly prevalent strategy for bio-sensing. This technique, often termed "Bio-FETs", provides an essentially label-free and real-time based bio-sensing platform effective for a variety of targets. This review highlights recent progress and challenges in the field. A special focus is on the comprehension of emerging nanotechnology-based approaches to facilitate signal-transduction and amplification. Some new targets of Bio-FETs and the future perspectives are also discussed.

Label-free and reagent-less protein biosensing using aptamer-modified extended-gate field-effect transistors

We have developed biosensors based on an aptamer-modified field-effect transistor (FET) for the detection of lysozyme and thrombin. An oligonucleotide aptamer as a sensitive and specific ligand for these model proteins was covalently immobilized on a gold electrode extended to the gate of FET together with thiol molecules to make a densely packed self-assembled monolayer (SAM). The aptamer-based potentiometry was achieved in a multi-parallel way using a microelectrodes array format of the gate electrode. A change in the gate potential was monitored in real-time after introduction of a target protein at various concentrations to the functionalized electrodes in a buffer solution. Specific protein binding altered the charge density at the gate/solution interface, i.e., interface potential, because of the intrinsic local net-charges of the captured protein. The potentiometry successfully determined the lysozyme and thrombin on the solid phase with their dynamic ranges 15.2-1040 nM and 13.4-1300 nM and the limit of detection of 12.0 nM and 6.7 nM, respectively. Importantly, robust signals were obtained by the specific protein recognition even in the spiked 10% fetal bovine serum (FBS) conditions. The technique herein described is all within a complementary metal oxide semiconductor (CMOS) compatible format, and is thus promising for highly efficient and low cost manufacturing with the readiness of downsizing and integration by virtue of advanced semiconductor processing technologies.

Competition of bovine serum albumin adsorption and bacterial adhesion onto surface-grafted ODT: in situ study by vibrational SFG and fluorescence confocal microscopy

The interaction of hydrophilic and hydrophobic ovococcoid bacteria and bovine serum albumin (BSA) proteins with a well ordered surface of octadecanethiol (ODT) self assembled monolayer (SAM) has been studied in different situations where proteins were either preadsorbed on ODT or adsorbed simultaneously with bacterial adhesion as in life conditions. The two situations lead to very different antimicrobial behavior. Bacterial adhesion on preadsorbed BSA is very limited, while the simultaneous exposure of ODT SAM to proteins and bacteria lead to a markedly weaker antimicrobial effect. The combination of sum frequency generation spectroscopy and fluorescence confocal microscopy experiments allow one to draw conclusions on the factors that govern the ODT SAM or BSA film interaction with bacteria at the molecular level. On the hydrophobic ODT surface, interaction with hydrophobic or hydrophilic biomolecules results in opposite effects on the SAM, namely, a flattening or a raise of the terminal methyl groups of ODT. On an amphiphilic BSA layer, the bacterial adhesion strength is weakened by the negative charges carried by both BSA and bacteria. Surprisingly, preadsorbed BSA that cover part of the bacteria cell walls increase the adhesion strength to the BSA film and reduce hydrophobic interactions with the ODT SAM. Finally, bacterial adhesion on a BSA film is shown to modify the BSA proteins in some way that change their interaction with the ODT SAM. The antimicrobial effect is much stronger in the case of a preadsorbed BSA layer than when BSA and bacteria are in competition to colonize the ODT SAM surface.

Interpretation of protein adsorption through its intrinsic electric charges: a comparative study using a field-effect transistor, surface plasmon resonance, and quartz crystal microbalance

We describe the highly sensitive detection of the nonspecific adsorption of proteins onto a 1-undecanethiol self-assembled monolayer (SAM)-formed gold electrode by parallel analysis using field effect transistor (FET), surface plasmon resonance (SPR), and quartz crystal microbalance (QCM) sensors. The FET sensor detects the innate electric charges of the adsorbed protein at the electrode/solution interface, transforming the change in charge density into a potentiometric signal in real time, without the requirement for labels. In particular, using the Debye-Huckel model, the degree of potential shift was proportional to the dry mass of adsorbed albumin and β-casein. A comparison of the FET signal with SPR and QCM data provided information on the conformation and orientation of the surface-bound protein by observing characteristic break points in the correlation slopes between the signals. These slope transitions reflect a multistage process that occurs upon protein adsorption as a function of protein concentration, including interim coverage, film dehydration, and monolayer condensation. The FET biosensor, in combination with SPR and QCM, represents a new technology for interrogating protein-material interactions both quantitatively and qualitatively.

The role of surface chemistry-induced cell characteristics on nonviral gene delivery to mouse fibroblasts

Background

Gene delivery approaches serve as a platform to modify gene expression of a cell population with applications including functional genomics, tissue engineering, and gene therapy. The delivery of exogenous genetic material via nonviral vectors has proven to be less toxic and to cause less of an immune response in comparison to viral vectors, but with decreased efficiency of gene transfer. Attempts have been made to improve nonviral gene transfer efficiency by modifying physicochemical properties of gene delivery vectors as well as developing new delivery techniques. In order to further improve and understand nonviral gene delivery, our approach focuses on the cell-material interface, since materials are known to modulate cell behavior, potentially rendering cells more responsive to nonviral gene transfer. In this study, self-assembled monolayers of alkanethiols on gold were employed as model biomaterial interfaces with varying surface chemistries. NIH/3T3 mouse fibroblasts were seeded on the modified surfaces and transfected using either lipid- or polymer- based complexing agents.

Results

Transfection was increased in cells on charged hydrophilic surfaces presenting carboxylic acid terminal functional groups, while cells on uncharged hydrophobic surfaces presenting methyl terminations demonstrated reduced transfection for both complexing agents. Surface–induced cellular characteristics that were hypothesized to affect nonviral gene transfer were subsequently investigated. Cells on charged hydrophilic surfaces presented higher cell densities, more cell spreading, more cells with ellipsoid morphologies, and increased quantities of focal adhesions and cytoskeleton features within cells, in contrast to cell on uncharged hydrophobic surfaces, and these cell behaviors were subsequently correlated to transfection characteristics.

Conclusions

Extracellular influences on nonviral gene delivery were investigated by evaluating the upregulation and downregulation of transgene expression as a function of the cell behaviors induced by changes in the cells’ microenvronments. This study demonstrates that simple surface modifications can lead to changes in the efficiency of nonviral gene delivery. In addition, statistically significant differences in various surface-induced cell characteristics were statistically correlated to transfection trends in fibroblasts using both lipid and polymer mediated DNA delivery approaches. The correlations between the evaluated complexing agents and cell behaviors (cell density, spreading, shape, cytoskeleton, focal adhesions, and viability) suggest that polymer-mediated transfection is correlated to cell morphological traits while lipid-mediated transfection correlates to proliferative characteristics.

A hairpin DNA aptamer coupled with groove binders as a smart switch for a field-effect transistor biosensor

We report here that a hairpin-structured DNA that possesses an anti-ATP aptamer sequence successfully detected target ATP or adenosine in a temperature-dependent manner by nanoscale intramolecular displacement on the surface of a gold electrode as an extended gate of a field-effect transistor (FET). The structural switching of the hairpin aptamer from closed loop to open-loop conformations was accompanied by the release of the preloaded DNA binder (DAPI) from the stem part of the hairpin aptamer into the solution phase. The loss of intrinsic positive charges of DAPI (2+) from the diffusion layer at the gate/solution nano-interface as a result of target capturing was responsible for generating a specific signal by the field-effect. We emphasize a new aspect of the structured DNA aptamer in combination with FET: the DAPI-loaded hairpin aptamer successfully detected even uncharged adenosine, which remains a major challenge for FET-based biosensors. Given the simplicity in design of the primary and secondary structures of oligonucleotide aptamers, it is easy to apply this technology to a wide variety of bio-analytes, irrespective of their electric charges. In view of these advantages, our findings may offer a new trend in the design of stimuli-responsive "smart" biomolecular switches for semiconductor-based biosensors.

Enhanced tissue penetration-induced high bonding strength of a novel tissue adhesive composed of cholesteryl group-modified gelatin and disuccinimidyl tartarate

The aim of this study was to evaluate the effect of cholesteryl group content on the bonding strength of a novel tissue adhesive composed of cholesteryl group-modified geletin (CholGltn) and disuccinimidyl tartarate (DST). The bonding strength of this tissue adhesive with fresh arterial media reached a maximum at a CholGltn content of 70% in the CholGltn/gelatin (Gltn) mixture, which then decreased with increasing CholGltn content with a fixed succinimidyl group:amino group ratio of 1:1. The maximum bonding strength obtained was 6-fold higher compared with that of the original Gltn. Furthermore, maximum peeling strength was also obtained at a CholGltn content of 70% in the CholGltn/Gltn mixture and at a similar succinimidyl group:amino group ratio. The highest peeling strength was 8-fold higher compared with Gltn and 6-fold higher compared with commercial aldehyde-based adhesive. After exposure of FITC-labeled Gltn or CholGltn to aortic smooth muscle cells (SMCs), which are abundant in arterial media, CholGltn integrated effectively with the surface of SMCs. This indicated that FITC-labeled CholGltn anchors into the cell membrane of SMCs. From these results, it was demonstrated that tissue adhesive composed of a CholGltn/Gltn mixture and DST showed improved penetration into arterial media compared with adhesive composed of Gltn and DST. This behavior supports the suggestion that the hydrophobic cholesteryl group in Gltn contributes to the enhanced bonding/peeling strength. This novel tissue adhesive may become a useful material in the clinical field for the treatment of aortic dissection.

Molecular analysis of blood with micro-/nanoscale field-effect-transistor biosensors

Rapid and accurate molecular blood analysis is essential for disease diagnosis and management. Field-effect transistor (FET) biosensors are a type of device that promise to advance blood point-of-care testing by offering desirable characteristics such as portability, high sensitivity, brief detection time, low manufacturing cost, multiplexing, and label-free detection. By controlling device parameters, desired FET biosensor performance is obtained. This review focuses on the effects of sensing environment, micro-/nanoscale device structure, operation mode, and surface functionalization on device performance and long-term stability.

Chemical-vapor-deposition-based polymer substrates for spatially resolved analysis of protein binding by imaging ellipsometry

Biomolecular interactions between proteins and synthetic surfaces impact diverse biomedical fields. Simple, quantitative, label-free technologies for the analysis of protein adsorption and binding of biomolecules are thus needed. Here, we report the use of a novel type of substrate, poly-p-xylylene coatings prepared by chemical vapor deposition (CVD) polymerization, for surface plasmon resonance enhanced ellipsometry (SPREE) studies and assess the reactive coatings as spatially resolved biomolecular sensing arrays. Prior to use in binding studies, reactive coatings were fully characterized by Fourier transform infrared spectroscopy, electrochemical impedance spectroscopy, and ellipsometry. As a result, the chemical structure, thickness, and homogeneous coverage of the substrate surface were confirmed for a series of CVD-coated samples. Subsequent SPREE imaging and fluorescence microscopy indicated that the synthetic substrates supported detectable binding of a cascade of biomolecules. Moreover, analysis revealed a useful thickness range for CVD films in the assessment of protein and/or antigen-antibody binding via SPREE imaging. With a variety of functionalized end groups available for biomolecule immobilization and ease of patterning, CVD thin films are useful substrates for spatially resolved, quantitative binding arrays.