Fabrication of dense, uniform aminosilane monolayers: A platform for protein or ligand immobilization

Patterned Carboxymethyl-Dextran Functionalized Surfaces Using Organic Mixed Monolayers for Biosensing Applications

The deposition of biomolecules on biosensing surface platforms plays a key role in achieving the required sensitivity and selectivity for biomolecular interactions analysis. Controlling the interaction between the surface and biomolecules is increasingly becoming a crucial design tool to modulate the surface properties needed to improve the performance of the assay and the detection outcome. Carboxymethyl-dextran (CMD) coating can be exploited to promote chemical grafting of proteins, providing a hydrophilic, bioinert, nonfouling surface and a high surface density of immobilized proteins. In the present work, we developed and optimized a technique to produce a cost-effective CMD-based patterned surface for the immobilization of biomolecules to be used on standard protocols optimization. They consist of silicon or glass substrates with patterned bioactive areas able to efficiently confine the sampling solution by simply exploiting hydrophilic/hydrophobic patterning of the surface. The fabrication process involves the use of low-cost instruments and techniques, compatible with large scale production. The devices were validated through a chemiluminescence assay we recently developed for the analysis of binding of DNA nanoassemblies modified with an affinity binder to target proteins immobilized on the bioactive areas. Through this assay we were able to characterize the chemical reactivity of two target proteins toward a dextran matrix on patterned surfaces and to compare it with model CMD-based surface plasmon resonance (SPR) surfaces. We found a high reproducibility and selectivity in molecular recognition, consistent with results obtained on SPR sensor surfaces. The suggested approach is straightforward, cheap, and provides the means to assess patterned functionalized surfaces for bioanalytical platforms.

Smartphone-based competitive immunoassay for quantitative on-site detection of meat adulteration

Rapid, sensitive, and portable analytical methods for on-site inspection of food fraud are now an urgent requirement to ensure food quality and satisfy the ethnic considerations of consumers. Hence, for the first time, a colorimetric smartphone-based immunoassay was developed for the on-site detection of pork adulteration in meat. In detail, the immunoassay was based on a competitive strategy in which immobilized standard porcine IgG competed with the target porcine IgG extracted in a single step from meat samples. The parameters involved in each step of the immunoassay conception and the digital colorimetric detection were carefully investigated and optimized. Using polystyrene microplates as ready-to-use stable and portable immunoplatforms, TMB as chromogenic substrate, smartphone as signal readout, and Image J software for image processing; the developed immunoassay was able to detect as low as 0.01% of pork in meat mixtures in a total assay time of 30 min. The selectivity of the immunoassay was evaluated for different meat species, and it was shown to selectively respond only to pork. Furthermore, excellent stability of the prepared immunological platform was demonstrated under extreme temperature conditions (50 °C), which confirms its high portability potential for in situ quantification of pork, while being relatively cost effective and non-laborious. The developed method also provides great precision (RSD < 6%) and accuracy (relative error< 6%). Given the universal use of smartphones as portable and affordable devices, such format of immunoassay could be a promising approach for rapid and sensitive real-time monitoring of food fraud.

Impact of Silanization Parameters and Antibody Immobilization Strategy on Binding Capacity of Photonic Ring Resonators

Ring resonator-based biosensors have found widespread application as the transducing principle in “lab-on-a-chip” platforms due to their sensitivity, small size and support for multiplexed sensing. Their sensitivity is, however, not inherently selective towards biomarkers, and surface functionalization of the sensors is key in transforming the sensitivity to be specific for a particular biomarker. There is currently no consensus on process parameters for optimized functionalization of these sensors. Moreover, the procedures are typically optimized on flat silicon oxide substrates as test systems prior to applying the procedure to the actual sensor. Here we present what is, to our knowledge, the first comparison of optimization of silanization on flat silicon oxide substrates to results of protein capture on sensors where all parameters of two conjugation protocols are tested on both platforms. The conjugation protocols differed in the chosen silanization solvents and protein immobilization strategy. The data show that selection of acetic acid as the solvent in the silanization step generally yields a higher protein binding capacity for C-reactive protein (CRP) onto anti-CRP functionalized ring resonator sensors than using ethanol as the solvent. Furthermore, using the BS3 linker resulted in more consistent protein binding capacity across the silanization parameters tested. Overall, the data indicate that selection of parameters in the silanization and immobilization protocols harbor potential for improved biosensor binding capacity and should therefore be included as an essential part of the biosensor development process.

The Surface Science of Microarray Generation-A Critical Inventory

Microarrays are powerful tools in biomedical research and have become indispensable for high-throughput multiplex analysis, especially for DNA and protein analysis. The basis for all microarray processing and fabrication is surface modification of a chip substrate and many different strategies to couple probe molecules to such substrates have been developed. We present here a critical assessment of typical biochip generation processes from a surface science point of view. While great progress has been made from a molecular biology point of view on the development of qualitative assays and impressive results have been obtained on the detection of rather low concentrations of DNA or proteins, quantitative chip-based assays are still comparably rare. We argue that lack of stable and reliable deposition chemistries has led in many cases to suboptimal quantitative reproducibility, impeded further progress in microarray development and prevented a more significant penetration of microarray technology into the diagnostic market. We suggest that surface-attached hydrogel networks might be a promising strategy to achieve highly sensitive and quantitatively reproducible microarrays.

Extracellular vesicles nanoarray technology: Immobilization of individual extracellular vesicles on nanopatterned polyethylene glycol-lipid conjugate brushes

Arraying individual extracellular vesicles (EVs) on a chip is expected one of the promising approaches for investigating their inherent properties. In this study, we immobilized individual EVs on a surface using a nanopatterned tethering chip-based versatile platform. A microfluidic device was used to ensure soft, reproducible exposure of the EVs over the whole chip surface. The device is incorporated with a high-density nanoarray chip patterned with 200-nm diameter nanospots composed of polyethylene glycol (PEG)-lipid conjugate brushes. We present a procedure adopted for fabricating high-density PEG-lipid modified nanospots (200 nmϕ, 5.0 × 105 spots/mm2 in 2 × 2 mm2 area). This procedure involves nanopatterning using electron beam lithography, followed by multistep selective chemical modification. Aqueous treatment of a silane coupling agent, used as a linker between PEG-lipid molecules and the silicon surface, was the key step that enabled surface modification using a nanopatterned resist film as a mask. The nanoarray chip was removed from the device for subsequent measurements such as atomic force microscopy (AFM). We developed a prototype device and individually immobilized EVs derived from different cell lines (Sk-Br-3 and HEK293) on tethering nanospots. We characterized EV's morphology using AFM and showed the possibility of evaluating the deformability of EVs using the aspect ratio as an indicator.

Neutralized chimeric DNA probe for the improvement of GC-rich RNA detection specificity on the nanowire field-effect transistor

Silicon nanowire (SiNW) field-effect transistors (FETs) is a powerful tool in genetic molecule analysis because of their high sensitivity, short detection time, and label-free detection. In nucleic acid detection, GC-rich nucleic acid sequences form self- and cross-dimers and stem-loop structures, which can easily obtain data containing signals from nonspecific DNA binding. The features of GC-rich nucleic acid sequences cause inaccuracies in nucleic acid detection and hinder the development of precision medicine. To improve the inaccurate detection results, we used phosphate-methylated (neutral) nucleotides to synthesize the neutralized chimeric DNA oligomer probe. The probe fragment originated from a primer for the detection of hepatitis C virus (HCV) genotype 3b, and single-mismatched and perfect-matched targets were designed for single nucleotide polymorphisms (SNP) detection on the SiNW FET device. Experimental results revealed that the HCV-3b chimeric neutralized DNA (nDNA) probe exhibited better performance for SNP discrimination in 10 mM bis-tris propane buffer at 25 °C than a regular DNA probe. The SNP discrimination of the nDNA probe could be further improved at 40 °C on the FET device. Consequently, the neutralized chimeric DNA probe could successfully distinguish SNP in the detection of GC-rich target sequences under optimal operating conditions on the SiNW FET device.

Template mediated protein self-assembly as a valuable tool in regenerative therapy

The assembly of natural proteinaceous biopolymers into macro-scale architectures is of great importance in synthetic biology, soft-material science and regenerative therapy. The self-assembly of protein tends to be limited due to anisotropic interactions among protein molecules, poor solubility and stability. Here, we introduce a unique platform to self-immobilize diverse proteins (fibrous and globular, positively and negatively charged, low and high molecular weight) using silicon surfaces with pendant -NH₂ groups via a facile one step diffusion limited aggregation (DLA) method. All the experimental proteins (type I collagen, bovine serum albumin and cytochrome C) self-assemble into seaweed-like branched dendritic architectures via classical DLA in the absence of any electrolytes. The notable differences in branching architectures are due to dissimilarities in protein colloidal sub-units, which is typical for each protein type, along with the heterogeneous distribution of surface -NH₂ groups. Fractal analysis of assembled structures is used to explain the underlying route of fractal deposition; which concludes how proteins with different functionality can yield similar assembly. Further, the nano-micro-structured surfaces can be used to provide functional topographical cues to study cellular responses, as demonstrated using rat bone marrow stem cells. The results indicate that the immobilization of proteins via DLA does not affect functionality, instead serving as topographical cues to guide cell morphology. This indicates a promising design strategy at the tissue-material interface and is anticipated to guide future surface modifications. A cost-effective standard templating strategy is therefore proposed for fundamental and applied particle aggregation studies, which can be used at multiple length scales for biomaterial design and surface reformation.

Photoconductivity, pH Sensitivity, Noise, and Channel Length Effects in Si Nanowire FET Sensors

Silicon nanowire (NW) field-effect transistor (FET) sensors of various lengths were fabricated. Transport properties of Si NW FET sensors were investigated involving noise spectroscopy and current-voltage (I-V) characterization. The static I-V dependencies demonstrate the high quality of fabricated silicon FETs without leakage current. Transport and noise properties of NW FET structures were investigated under different light illumination conditions, as well as in sensor configuration in an aqueous solution with different pH values. Furthermore, we studied channel length effects on the photoconductivity, noise, and pH sensitivity. The magnitude of the channel current is approximately inversely proportional to the length of the current channel, and the pH sensitivity increases with the increase of channel length approaching the Nernst limit value of 59.5 mV/pH. We demonstrate that dominant 1/f-noise can be screened by the generation-recombination plateau at certain pH of the solution or external optical excitation. The characteristic frequency of the generation-recombination noise component decreases with increasing of illumination power. Moreover, it is shown that the measured value of the slope of 1/f-noise spectral density dependence on the current channel length is 2.7 which is close to the theoretically predicted value of 3.

Manipulating the adhesion of electroless nickel-phosphorus film on silicon wafers by silane compound modification and rapid thermal annealing

In this study, the effect of 3-2-(2-aminoethylamino) ethylamino propyl trimethoxysilane (ETAS) modification and post rapid thermal annealing (RTA) treatment on the adhesion of electroless plated nickel-phosphorus (ELP Ni-P) film on polyvinyl alcohol-capped palladium nanoclusters (PVA-Pd) catalyzed silicon wafers is systematically investigated. Characterized by pull-off adhesion, atomic force microscopy, X-ray spectroscopy and water contact angle, a time-dependent, three-staged ETAS grafting mechanism including islandish grafting, a self-assembly monolayer (SAM) and multi-layer grafting is proposed and this mechanism is well correlated to the pull-off adhesion of ELP Ni-P film. In the absence of RTA, the highest ELP Ni-P film adhesion occurs when ETAS modification approaches SAM, where insufficient or multi-layer ETAS grafting fails to provide satisfactory results. On the other hand, if RTA is applied, the best ELP Ni-P film adhesion happens when ETAS modification is islandish owing to the formation of nickel silicide, where SAM or multi-layer ETAS modification cannot provide satisfactory adhesion because the interaction between ETAS and PVA-Pd has been sabotaged during RTA. Evidenced by microstructural images, we also confirmed that ETAS can act as an efficient barrier layer for nickel diffusion to bulk silicon.

Enzyme immobilization on metal oxide semiconductors exploiting amine functionalized layer

The immobilization of glucose oxidase (GOx) on indium-gallium zinc oxide (IGZO) thin films is studied in order to fabricate a high performance biosensor.

Optimizing the silanization of thermally-decomposed iron oxide nanoparticles for efficient aqueous phase transfer and MRI applications

A facile silanization method allows to efficiently stabilise in aqueous solution iron oxide NPs synthesized by thermal decomposition for MRI applications.

Antibacterial coatings on titanium surfaces: a comparison study between in vitro single-species and multispecies biofilm

Dental plaque is a biofilm that causes dental caries, gingivitis, and periodontitis. Most of the studies in antibacterial coatings have been conducted by in vitro single-species biofilm formation, but oral biofilm involves more than 700 different bacterial species that are able to interact. Therefore, new studies are focused on in vitro multispecies biofilm models that mimic in vivo biofilms. The aim of the present work was to study different antibacterial coatings onto titanium surfaces and evaluate the in vitro antimicrobial properties of the surfaces on two different bacterial species and an oral biofilm. The lactate dehydrogenase assay determined that treated samples did not affect fibroblast viability. In addition, the viability of microorganisms on modified samples was evaluated by a LIVE/DEAD BacLight bacterial viability kit. Although a decrease in viable bacteria onto treated samples was obtained, the results showed differences in effectiveness when single-biofilm and oral plaque were tested. It confirms, as we expected, the distinct sensitivities that bacterial strains have. Thus, this multispecies biofilms model holds a great potential to assess antibacterial properties onto samples for dental purposes.

Covalent immobilization of hLf1-11 peptide on a titanium surface reduces bacterial adhesion and biofilm formation

Bacterial infection represents a major cause of implant failure in dentistry. A common approach to overcoming this issue and treating peri-implant infection consists in the use of antibiotics. However, the rise of multidrug-resistant bacteria poses serious concerns to this strategy. A promising alternative is the use of antimicrobial peptides due to their broad-spectrum activity against bacteria and reduced bacterial resistance responses. The aim of the present study was to determine the in vitro antibacterial activity of the human lactoferrin-derived peptide hLf1-11 anchored to titanium surfaces. To this end, titanium samples were functionalized with the hLf1-11 peptide either by silanization methods or physical adsorption. X-ray photoelectron spectroscopy analyses confirmed the successful covalent attachment of the hLf1-11 peptide onto titanium surfaces. Lactate dehydrogenase assay determined that hLf1-11 peptide did not affect fibroblast viability. An outstanding reduction in the adhesion and early stages of biofilm formation of Streptococcus sanguinis and Lactobacillus salivarius was observed on the biofunctionalized surfaces compared to control non-treated samples. Furthermore, samples coated with the hLf1-11 peptide inhibited the early stages of bacterial growth. Thus, this strategy holds great potential to develop antimicrobial biomaterials for dental applications.

Kinetics of (3-aminopropyl)triethoxylsilane (APTES) silanization of superparamagnetic iron oxide nanoparticles

Silanization of magnetic ironoxide nanoparticles with (3-aminopropyl)triethoxylsilane (APTES) is reported. The kinetics of silanization toward saturation was investigated using different solvents including water, water/ethanol (1/1), and toluene/methanol (1/1) at different reaction temperature with different APTES loading. The nanoparticles were characterized by Fourier transform infrared spectroscopy, vibrating sample magnetometry, transmission electron microscopy, and thermal gravimetric analysis (TGA). Grafting density data based on TGA were used for the kinetic modeling. It is shown that initial silanization takes place very fast but the progress toward saturation is very slow, and the mechanism may involve adsorption, chemical sorption, and chemical diffusion processes. The highest equilibrium grafting density of 301 mg/g was yielded when using toluene/methanol mixture as the solvent at a reaction temperature of 70 °C.

Application of catalyst-free click reactions in attaching affinity molecules to tips of atomic force microscopy for detection of protein biomarkers

Atomic force microscopy (AFM) has been extensively used in studies of biological interactions. Particularly, AFM based force spectroscopy and recognition imaging can sense biomolecules on a single molecule level, having great potential to become a tool for molecular diagnostics in clinics. These techniques, however, require affinity molecules to be attached to AFM tips in order to specifically detect their targets. The attachment chemistry currently used on silicon tips involves multiple steps of reactions and moisture sensitive chemicals, such as (3-aminopropyl)triethoxysilane (APTES) and N-hydroxysuccinimide (NHS) ester, making the process difficult to operate in aqueous solutions. In the present study, we have developed a user-friendly protocol to functionalize the AFM tips with affinity molecules. A key feature of it is that all reactions are carried out in aqueous solutions. In summary, we first synthesized a molecular anchor composed of cyclooctyne and silatrane for introduction of a chemically reactive function to AFM tips and a bifunctional polyethylene glycol linker that harnesses two orthogonal click reactions, copper free alkyne-azide cycloaddition and thiol-vinylsulfone Michael addition, for attaching affinity molecules to AFM tips. The attachment chemistry was then validated by attaching antithrombin DNA aptamers and cyclo-RGD peptides to silicon nitride (SiN) tips, respectively, and measuring forces of unbinding these affinity molecules from their protein cognates human α-thrombin and human α5β1-integrin immobilized on mica surfaces. In turn, we used the same attachment chemistry to functionalize silicon tips with the same affinity molecules for AFM based recognition imaging, showing that the disease-relevant biomarkers such as α-thrombin and α5β1-integrin can be detected with high sensitivity and specificity by the single molecule technique. These studies demonstrate the feasibility of our attachment chemistry for the use in functionalization of AFM tips with affinity molecules.