Hybrid Self-assembled Monolayer Research Articles

An electrochemical immunosensor based on hybrid self-assembled monolayers for rapid detection of Bombyx mori nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is highly contagious and poses a serious threat to sericulture production. Because there are currently no effective treatments for BmNPV, a rapid and simple detection method is urgently needed. This paper describes an electrochemical immunosensor for the detection of BmNPV. The immunosensor was fabricated by covalently immobilizing anti-BmNPV, a biorecognition element, onto the surface of the working gold electrode via 11-mercaptoundecanoic acid (MUA)/β-mercaptoethanol (ME) hybrid self-assembled monolayers. Electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM) were used to characterize the electrochemical performance and morphology of the immunosensor, respectively. Under optimum conditions, the developed immunosensor exhibited a linear response to BmNPV polyhedrin in the range of 1 × 102–1 × 108 fg/mL, with a low detection limit of 14.54 fg/mL. The immunosensor also exhibited remarkable repeatability, reproducibility, specificity, accuracy, and regeneration. Normal silkworm blood was mixed with BmNPV polyhedrin and analyzed quantitatively using this sensor, and the recovery was 92.31 %–100.61 %. Additionally, the sensor was used to analyze silkworm blood samples at different time points after BmNPV infection, and an obvious antigen signal was detected at 12 h post infection. Although this result agreed with that provided by the conventional polymerase chain reaction (PCR) method, the electroanalysis method established in this study was simpler, shorter in detection period, and lower in material cost. Furthermore, this innovative electrochemical immunosensor, developed for the ultra-sensitive and rapid detection of BmNPV, can be used for the early detection of virus-infected silkworms.

(Invited, Digital Presentation) Low Voltage Operation of CMOS Inverter Based on WSe2 n/p FETs

Transition metal dichalcogenides (TMDC) have remarkable properties for the next generation of electronic devices [1]. A complementary metal-oxide-semiconductor (CMOS) inverter consisting of pairs of p-type and n-type field-effect transistor (FET) is a fundamental building block in modern digital electronics [2]. Therefore, realization of both p-type and n-type FETs based on semiconducting TMDC is of particular importance. Among various semiconducting TMDC, tungsten diselenide (WSe2) is a promising candidate for constructing a CMOS inverter because of its high mobility, symmetric electron and hole effective mass and ambipolar transport [3]. These prominent features of WSe2 motivates the fabrication and characterization of a CMOS inverter. One of the significant challenges is to develop a high-gain CMOS inverter for operation at a low power supply voltage (Vdd) for future low power digital electronics. Previous studies on a WSe2 CMOS inverter were limited to operation with a low gain of 3 or a high Vdd of more than 1 V under conditions of relatively low gate capacitance and degraded interfacial properties [3-5]. This study addressed these issues by developing a doping technique and gate stack technology to demonstrate the high-gain with low-Vdd operation of a WSe2 CMOS inverter [6]. The focus is on the doping technique of both electrons and holes to a WSe2 layer by simple spin-coating. Of particular interest in this study is the impact of gate stack technology on the CMOS inverter characteristics in low-Vdd operation. In this study, a hybrid self-assembled monolayer (SAM)/aluminum oxide (AlOx) gate dielectric is applied to a WSe2 CMOS inverter for high-gain low-Vdd operation since superior interfacial properties with high gate capacitance have been reported in molybdenum disulfide (MoS2) FETs.For p-type WSe2 FETs, the fluoropolymer CYTOP (AGC, CTX-809A) was applied since polarization in CYTOP due to the difference in electronegativity between C–F bonds generates holes in WSe2. On the other hand, poly(vinyl alcohol) (PVA) was utilized for n-type WSe2 FETs because positive charges in PVA accumulates electrons in WSe2. The resin solutions of both CYTOP and PVA were purchased from a commercial supplier. A heavily doped p-type silicon substrate (p+-Si) was thermally oxidized to form a 60 nm thick silicon dioxide (SiO2) layer for the global back-gate architecture. Palladium (Pd) and gold (Au) as the source and drain contacts for p-type and n-type WSe2 FETs were fabricated by the lift-off process, respectively. An aluminum (Al) gate was prepared and a hybrid SAM/AlOx gate dielectric was formed by oxygen (O2) plasma and an immersion process. After that, mechanically exfoliated WSe2 was transferred with a poly(dimethylsiloxane) (PDMS) stamp. Finally, CYTOP and PVA were deposited by spin-coating for p-type and n-type WSe2 FETs, respectively.Figures 1 (a) and (b) show the Id–Vg characteristics of fabricated FETs. For the p-type WSe2 FET, a small SS of 78 mV/dec and an on/off ratio of about 106 order were observed. On the other hand, the Id–Vg characteristics of the n-type WSe2 FET were degraded compared with those of the p-type WSe2 FET because of the Schottky contacts.Figures 2 (a) and (b) show the transfer characteristics and gain as a function of Vdd in CMOS inverter. The gain of the CMOS inverter increased to as high as 9 at Vdd of 0.5 V.Figure 3 shows the benchmark of this study. Our proposed concepts enabled the operation with high gain at a low Vdd as low as 0.5 V for the WSe2 CMOS inverter, which had not been realized in previous studies. The dangling-bond-free and ultrathin SAM/AlOx gate dielectric has an important role in the low-Vdd operation of the WSe2 CMOS inverter. This study opens up interesting directions for the research and development of TMDC-based devices and circuits.

Surface modification of Ti6Al4V by forming hybrid self-assembled monolayers and its effect on collagen-I adsorption, osteoblast adhesion and integrin expression

Silanization is a widely explored chemical technique to modify surfaces for tailoring physio-chemical properties like surface chemistry and wettability but its impact on bio-interfaces interaction i.e. protein adsorption and cellular responses is not well understood. This work focusses on modulating surface properties of Ti6Al4V to study their behavior towards collagen-I (Col-I) adsorption and subsequently on osteoblast adhesion and integrin expression. The moderate hydrophobic Ti6Al4V surfaces (θ = 78 ± 3°) were prepared by forming hybrid self-assembled monolayers (SAMs), which contain both hydrophobic and hydrophilic moieties on the same SAM molecule and were characterized using different techniques. The experimental adsorbed amounts of Col-1 were compared with the predicted ones using the Gibbs equation, which were overestimated but linearly related. AFM and secondary structure analyses of the adsorbed Col-I revealed globular morphology with significant change only in helical content. Cells adhered to hybrid surface pre-adsorbed with Col-1 exhibited better cell adhesion (~100%) and spreading area (1127 µm2) as compared to cells adhered to hybrid surface without (adhesion of 69% and cell area of 509 µm2) and with (adhesion of 94% and cell area of 908 µm2) FBS in cell culture media. Further, integrin expression was analyzed using ELISA based integrin assay to elucidate the cellular adhesion. We observed higher expression of α1 and α2 integrins on surfaces with pre-adsorbed Col-I and were correlated with the increase in nuclei area indicating α1 and α2 mediated cell adhesion promoted the cell proliferation. Overall, cell-surface interactions were improved on hybrid surfaces with pre-adsorbed Col-I, which designated the potential applications of the modified Ti6Al4V especially in metallic orthopedic implants.

Direct stimulation of cellular immune response via TLR2 signaling triggered by contact with hybrid glyco-biointerfaces composed of chitohexaose and cellohexaose.

Recognition of carbohydrates by cell receptors activates various cell signaling processes. In this report, hybrid self-assembled monolayers (SAMs) of chitohexaose (βGlcNAc6) and cellohexaose (βGlc6) are prepared to induce a highly sensitive, glyco-density-dependent inflammatory response by HEK293 cells expressing toll-like receptor 2 (TLR2), which recognizes oligo-βGlcNAc moieties. Thiosemicarbazide-labeled βGlcNAc6 and βGlc6, as a bio-signal inducer and a spacer molecule, respectively, were immobilized on a gold substrate via spontaneous chemisorption to control the βGlcNAc6 density on the SAM surface. The βGlcNAc6 density ranged from 0 to 0.65 chains nm-2. The inflammatory response of HEK293 cells varied as a function of the surface glyco-density, and the hybrid SAM with 0.146 chains nm-2 of βGlcNAc6 induced a stronger response than pure βGlcNAc6-SAM with 0.654 chains nm-2. Thus, direct activation of TLR2-mediated cellular stimulation triggered by as-designed glyco-biointerfaces was possible through a mutual interaction between the fittingly βGlcNAc6 moieties assembled on hybrid glyco-SAMs and TLR2 on the surface of HEK293 cells.

Fabrication of hybrid self-assembled monolayer/hafnium oxide gate dielectric by radical oxidation for molybdenum disulfide field-effect transistors

In this study, radical oxidation is applied to the fabrication of a hybrid self-assembled monolayer (SAM)/hafnium oxide (HfOx) gate dielectric in molybdenum disulfide (MoS2) field-effect transistors. The fabrication process involves radical oxidation to form HfOx at the surface of metallic HfN, SAM formation by immersion, and the deterministic transfer of MoS2 flakes. A subthreshold slope of 75 mV/dec and small hysteresis were demonstrated, indicating superior interfacial properties. Cross-sectional transmission electron microscopy revealed the uniform formation of the HfOx layer at the surface of HfN. The SAM is indispensable for the superior interfacial properties in MoS2 field-effect transistors. The radical oxidation is not restricted to the oxidation of silicon and germanium substrates and was also found to be applicable to the fabrication of a high-k gate dielectric. This study opens up interesting possibilities of radical oxidation for research on functional electronic devices.

Probing the Dynamic Interaction between Damaged DNA and a Cellular Responsive Protein Using a Piezoelectric Mass Biosensor.

The binding events between damaged DNA and recognition biomolecules are of great interest for understanding the activity of DNA-damaging drugs and the related DNA repair networks. Herein, a simple and sensitive sensor system was tailored for real-time probing of the dynamic molecular recognition between cisplatin-damaged-DNA (cisPt-DNA) and a cellular responsive protein, high-mobility-group box 1 (HMGB1). By integration of flow injection analysis (FIA) with quartz crystal microbalance (QCM), the interaction time-course of cisPt-DNA and HMGB1 domain A (HMGB1a) was investigated. The highly specific sensing interface was carefully designed and fabricated using cisPt-DNA as recognition element. A hybrid self-assembled monolayer consisting of cysteamine and mercaptohexanol was introduced to resist nonspecific adsorption. The calculated kinetic parameters (kass and kdiss) and the dissociation constant (KD) demonstrated the rapid recognition and tight binding of HMGB1a toward cisPt-DNA. Molecular docking was employed to simulate the complex formed by cisPt-DNA and HMGB1a. The tight binding of such a DNA-damage responsive complex is appealing for the downstream molecular recognition event related to the resistance to DNA repair. This continuous-flow QCM biosensor is an ideal tool for studying specific interactions between drug-damaged-DNAs and their recognition proteins in a physiological-relevant environment, and will provide a potential sensor platform for rapid screening and evaluating metal anticancer drugs.

Properties of Adsorbed Bovine Serum Albumin and Fibrinogen on Self-Assembled Monolayers

We have studied kinetics of adsorption and properties of adsorbed bovine serum albumin (BSA) and fibrinogen (Fb) on a hydrophobic octyl surface, a hydrophilic amine surface, and a mixture of octyl and amine self-assembled monolayer (SAM) and newly synthesized hybrid SAM by using quartz crystal microbalance (QCM). In addition, we have proposed a combined kinetic and mass transfer constrained protein adsorption model. The model is fitted to a change in resonance frequency, ΔFn/n versus time data obtained from QCM to get the kinetic rate constants, mass transfer coefficient, and spreading of adsorbed proteins. Initial rate of adsorption increases with a decrease in surface energy of the substrate. The equilibrium adsorbed amount of BSA on the hybrid surface is less than that on the mixed surface and lies in between that on octyl and amine surfaces and that of Fb is the least on hybrid surface. The analysis of variation of the dissipation factor, ΔD, with ΔFn/n indicates that BSA is more flexible than Fb and the adsorbed layer of both proteins is softest on the hybrid surface. The relaxation times of adsorbed proteins are the slowest on the octyl surface, while those on the hybrid surface are the fastest. The analysis of secondary structures of proteins using ATR-FTIR suggests secondary structures of the proteins change during adsorption. The content of α-helix of the proteins increases due to adsorption on the amine surface, while that decreases on all other surfaces. The total content of α-helix and β-sheet strongly depends on the adsorbed mass of the proteins and is weakly dependent related to elasticity and viscosity of the adsorbed proteins, respectively.

Properties of competitively adsorbed BSA and fibrinogen from their mixture on mixed and hybrid surfaces

We have studied the adsorption of BSA and fibrinogen from their mixture onto surfaces with mixed self-assembled monolayer (SAM) of amine and octyl (ratio 1:1) and hybrid SAM. The properties of adsorbed proteins obtained from individual protein solution differ considerably from the properties of the adsorbed proteins obtained from mixture of proteins at same total concentration. The adsorbed amount of proteins is lesser and the adsorbed protein is more elastic if it is adsorbing from mixture of proteins. It is found that with increasing total protein concentration, adsorbed amount increases and elasticity of the adsorbed proteins decreases. The apparent displacements of BSA with Fb are observed on the graphs of change in frequency with time, which are obtained from quartz crystal microbalance.

Hybrid immobilization of galactosyl lactose and cellobiose on a gold substrate to modulate biological responses

Bioactive O-β-d-galactopyranosyl-(1→4)-O-β-d-galactopyranosyl-(1→4)-d-glucopyranose (4′-galactosyl lactose) was site-selectively modified at a reducing end with thiosemicarbazide (TSC). As-synthesized 4′-galactosyl lactose-TSC was immobilized on a gold substrate with cellobiose-TSC as a spacer through spontaneous self-assembly chemisorption via SAu bonding. Quartz crystal microbalance analysis suggested the successful formation of self-assembled monolayers (SAMs) of 4′-galactosyl lactose-TSC and/or cellobiose-TSC. Galactose-binding lectin exhibited the highest affinity for hybrid SAMs with an equimolar ratio of the two oligosaccharide-TSCs, while glucose-binding lectin showed decreasing adsorption with a decrease in cellobiose-TSC ratios. Human hepatocellular carcinoma cells, which recognize galactose residues, efficiently adhered to the hybrid SAMs. Higher enzymatic deethoxylation of ethoxyresorufin via cytochrome P450 appeared on hybrid SAMs. These results suggested that clustering of the bioactive sugars was involved in the cellular responses, possibly via biological carbohydrate–protein interactions. This approach to designing carbohydrate-based scaffolds should provide a basis for the functional development of glyco-decorated biointerfaces for cell culture applications.

Fabrication of a hydrophobic/hydrophilic hybrid-patterned microarray chip and its application to a cancer marker immunoassay

In this work, we report on a simple process for fabricating a hydrophobic/hydrophilic hybrid-patterned microarray chip for a fast and sensitive immunoassay. Two different types of self-assembled monolayers (SAMs) were used in the fabrication of hydrophilic well patterns and hydrophobic substrates. The hydrophilic/hydrophobic hybrid SAM pattern generates a clear-cut boundary between the sample and the background. A change in the precursor molecules allows for many different types of SAMs to be employed in the fabrication process. Fluorescence image-based detection has previously been used for the quantitative immune-analysis of a specific cancer marker. Here, a titanium-coated glass substrate was utilized to suppress auto-fluorescence signals from substrate backgrounds. Angiogenin (ANG), a small polypeptide implicated in both angiogenesis and tumor growth, was used as a target cancer marker for its validation. Assay results demonstrate that the hybrid-patterned array chip yields a narrower error deviation and a lower coefficient variation than in a conventional 96-well plate ELISA. Furthermore, the sample requirement (1 μL) for the hybrid-patterned chip is about 50 times less than that required in an ELISA (at least 50 μL). The proposed hydrophobic/hydrophilic hybrid-patterned microarray chip is expected to be a highly efficient tool that can be applied to a high throughput immunoassay of a specific cancer marker.

Hybrid surface from self-assembled layer and its effect on protein adsorption

We synthesized hybrid self-assembled monolayer (SAM) with short chain hydrophobic and hydrophilic groups on the same molecule. The physical characteristics such as surface roughness and surface energy of the synthesized hybrid SAM were compared with mono SAMs of amine, octyl and mixed amine-octyl SAM. We also compared the response of the surfaces towards adsorption of bovine serum albumin (BSA) using quartz crystal microbalance (QCM). We determined adsorbed amount (Γ) of BSA on the various surfaces from its various bulk concentrations. It follows the Langmuir adsorption isotherm in the concentration range of our study. The strength of adsorbed protein was characterized from the dissipation factor (ΔD). The highest ΔD value of adsorbed BSA was observed for the adsorption on hybrid surface. The arrangement of BSA on hybrid surface such that it leaded to soft layer, corresponded to the highest ΔD value. These findings suggest that the hybrid surface is a potential surface modifying agent of biomaterials.

INFLUENCE OF CONSTITUTION OF HYBRID FERROCENYL ALKANETHIOLS SELF-ASSEMBLED MONOLAYERS TO INTERFACIAL CHARACTERISTICS ON GOLD SURFACE

The interfacial characteristics of self-assembled monolayer (SAM), that are applied in miniaturized sensor design, were strongly influenced by the SAM constitution. The electrochemical and interfacial characteristics of hybrid SAMs that constituted with ferrocenyl octanethiol ( FcC 8 SH ) and unsubstituted octanethiol ( C 8) or 2-mercaptopyrimidine (2-MPD) were investigated in this study. The peaks of anodic oxidation were observed at around 0.49 V (vs. Ag/AgCl ) in the FcC 8 SH/C 8 hybrid SAM. However, the oxidation potentials descended from 0.42–0.38 V as the molar fraction of FcC 8 SH was decreased from 0.8–0.2 in the FcC 8 SH /2-MPD hybrid SAM. On the other hand, the interfacial hydrophilicity of FcC 8 SH /2-MPD hybrid SAM was measured to be higher than the FcC 8 SH/C 8 hybrid SAM due to the random arrangement of SAM caused by the shape of 2-MPD. Then, the surface roughness of hybrid SAM and chain length of ferrocenyl alkanethiols were all considered to be the parameters of interfacial characteristics. In summary, doping of 2-MPD in SAMs might reduce the rigidity of ferrocenyl alkanethiol SAMs and increase the interfacial hydrophilicity. This result will be useful in the development of the low power consumption portable bioanalytical system. And a miniaturized bioanalytical system based on ferrocenyl SAMs would be developed in the future.

Tribological investigation of MPTS‐MPTES SAMs

PurposeThis paper's aim is to study the tribological properties of hyrid monolayer composited by γ‐mercapto‐propyl trimethoxysilane (MPTS) and γ‐methacryloxy propyltrimethoxysilane (MPTES), and estimated the action mechanism.Design/methodology/approachMPTS‐MPTES were self‐assembled on a hydroxylated silicon substrate to form a two‐dimensional hybrid monolayer. Atomic force microscope (AFM), X‐ray photoelectron spectrometry and contact angle measurement were used to characterize the MPTS‐MPTES hybrid self‐assembled monolayer (SAM). The macrofriction and wear behaviors of the film sliding against an AISI‐52100 steel ball were examined on a unidirectional friction and wear tester, and the worn surface morphologies were observed on an AFM.FindingsThe tribological results show that the friction coefficient of silicon substrate reduces from 0.86 to 0.18 after the formation of the MPTS‐MPTES hybrid SAM on its surface, and the thin film has a long wear life (2,620 sliding pass). It is demonstrated that the MPTS‐MPTES SAM exhibited good wear resistant property with a low friction coefficient, and the superior friction reduction and wear life were attributed to the low surface energy and the characteristics of the hybrid SAM.Research limitations/implicationsThe film's tribological performance under dynamic load is not estimated.Practical implicationsA hybrid monolayer with superior tribological property was synthesized on a silicon substrate by self‐assembly process, and maybe it is the potential solution for micro‐electromechanical‐system lubrication.Originality/valueThis paper provides a study way of hybrid SAM on a silicon substrate as lubricating coating.

Gold Nanoparticles Surface-confined by Hybrid Self-Assembled Monolayers of Dendrimer and Dodecanethiol

Gold nanoparticles surface-confined by hybrid self-assembled monolayers (SAMs) were synthesized by replacement of dendrimer-passivated nanoparticles by dodecanethiol. Transmission electron microscopy, small-angle X-ray scattering (SAXS), infrared absorption spectroscopy, 1H NMR and elemental analysis were performed to characterize hybrid-layer-passivated nanoparticles. The hybrid particles were separated into domains of small and large spheres. This indicates that the replacement may be dependent on the particle size.

The Interplay of Lateral and Tiered Interactions in Stratified Self-Organized Molecular Assemblies

Hybrid self-assembled monolayers (SAMs) containing well-defined strata of different polarity enable insight into how fundamental interactions lead to higher order structure and may provide useful analogies for self-assembled multilayers, new hybrid materials, and functional biological interfaces. We report amide-containing alkanethiol SAMs with internal polar sublayers that are two amide groups thick and nonpolar overlayers comprising either dodecyl or hexadecyl chains. The assemblies have been characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and external reflective infrared spectroscopy (FTIR-ERS). XPS demonstrates the SAMs are of monolayer thickness, chemisorbed to the gold substrate, and anisotropically oriented. Contact angle data show the methyl surface for n = 16 is highly ordered, but the surface for n = 12 is less well ordered. FTIR-ERS reveals that the alkyl chains for n = 16 are close packed, but that those for n = 12 are disordered. FTIR-ERS also shows that, although the two-amide sublayers are compositionally identical, they are well ordered and assume polyglycine-II-like conformations for n = 16, but they are poorly ordered for n = 12. Comparison of these two SAMs to each other in the context of previously reported one- and three-amide SAMs leads to two conclusions. (1) The threshold n for alkyl chain length ordering in two-amide SAMs is 12 ≤ n ≤ 16. Thus, in SAMs with internal amide sublayers both one and two amide groups thick, the threshold number of methylenes required to form ordered alkyl regions is significantly increased compared to alkanethiol SAMs, demonstrating destructive interference of the amide region with the hydrocarbon ordering process. (2) In two-amide SAMs the formation of a well-ordered amide region depends on the ordering of an overlying hydrocarbon region. This behavior differs with that previously demonstrated for one- and three-amide SAMs, in which the amide groups assume characteristic conformations regardless of hydrocarbon region thickness and order. For two-amide SAMs, the apparent dependence of amide ordering on complementary ordering in the alkyl region provides evidence of an energetic interplay between the two sublayers, manifested as a “reverse ordering” effect. The previously unobserved elastic−elastic character of the buried interface in two-amide SAMs is contrasted with the rigid−elastic interface found in the one-amide SAMs.