Surface modification of silicon oxide with trialkoxysilanes toward close-packed monolayer formation

Optimizing Plasmonic Gold Nanorod Deposition on Glass Surfaces for High-Sensitivity Refractometric Biosensing

Owing to high surface sensitivity, gold nanorods (AuNRs) are widely used to construct surface-based nanoplasmonic biosensing platforms for label-free molecular diagnostic applications. A key fabrication step involves controlling AuNR deposition onto the target surface, which requires maximizing surface density while minimizing inter-particle aggregation, and is often achieved by surface functionalization with a self-assembled monolayer (SAM) prior to AuNR deposition. To date, existing studies have typically used a fixed concentration of SAM-forming organic molecules (0.2-10% v/v) while understanding how SAM density affects AuNR deposition and resulting sensing performance would be advantageous. Herein, we systematically investigated how controlling the (3-aminopropyl)triethoxysilane (APTES) concentration (1-30% v/v) during SAM preparation affects the fabrication of AuNR-coated glass surfaces for nanoplasmonic biosensing applications. Using scanning electron microscopy (SEM) and UV-visible spectroscopy, we identified an intermediate APTES concentration range that yielded the highest density of individually deposited AuNRs with minimal aggregation and also the highest peak wavelength in aqueous solution. Bulk refractive index sensitivity measurements indicated that the AuNR configuration had a strong effect on the sensing performance, and the corresponding wavelength-shift responses ranged from 125 to 290 nm per refractive index unit (RIU) depending on the APTES concentration used. Biosensing experiments involving protein detection and antigen-antibody interactions further demonstrated the high surface sensitivity of the optimized AuNR platform, especially in the low protein concentration range where the measurement shift was ~8-fold higher than that obtained with previously used sensing platforms.

Solution Chemistry to Control Boron-Containing Monolayers on Silicon: Reactions of Boric Acid and 4-Fluorophenylboronic Acid with H- and Cl-terminated Si(100)

The reactions of boric acid and 4-fluorophenylboronic acid with H- and Cl-terminated Si(100) surfaces in solution were investigated. X-ray photoelectron spectroscopy (XPS) studies reveal that both molecules react preferentially with Cl-Si(100) and not with H-Si(100) at identical conditions. On Cl-Si(100), the reactions introduce boron onto the surface, forming a Si-O-B structure. The quantification of boron surface coverage demonstrates that the 4-fluorophenylboronic acid leads to ∼2.8 times higher boron coverage compared to that of boric acid on Cl-Si(100). Consistent with these observations, density functional theory studies show that the reaction of boric acid and 4-fluorophenylboronic acid is more favorable with the Cl- versus H-terminated surface and that on Cl-Si(100) the reaction with 4-fluorophenylboronic acid is ∼55.3 kJ/mol more thermodynamically favorable than the reaction with boric acid. The computational studies were also used to demonstrate the propensity of the overall approach to form high-coverage monolayers on these surfaces, with implications for selective-area boron-based monolayer doping.

Fabrication of Biosensing Interface with Monolayers

Surface modification is recognized as one of the fundamental techniques to fabricate biosensing interfaces. This review focuses on the surface modification of carbon substrates (GC and HOPG) and silica with a close-packed monolayer, in particular. In the cases of carbon substrates, GC and HOPG, it was demonstrated that surface modification of carbon substrates with diazonium derivatives could create a close-packed monolayer similar to the self-assembled monolayer (SAM) formation with mercapto derivatives. Similarly, the potential of trialkoxysilanes to form a close-packed monolayer was evaluated, and modification with a close-packed monolayer tended to occur under milder conditions when the trialkoxysilanes had a longer alkyl chain. In these studies, we synthesized surface modification materials having ferrocene as a redox active moiety to explore features of the modified surfaces by an electrochemical method using cyclic voltammetry, where surface concentrations of immobilized molecules and blocking effect were studied to obtain insight for density leading to a close-packed layer. Based on those findings, fabrication of a biosensing interface on the silica sensing chip of the waveguide-mode sensor was carried out using triethoxysilane derivatives bearing succinimide ester and oligoethylene glycol moieties to immobilize antibodies and to suppress nonspecific adsorption of proteins, respectively. The results demonstrate that the waveguide-mode sensor powered by the biosensing interface fabricated with those triethoxysilane derivatives and antibody has the potential to detect several tens ng/mL of biomarkers in human serum with unlabeled detection method.

Silanization of a Metal-Polyphenol Coating onto Diverse Substrates as a Strategy for Controllable Wettability with Enhanced Performance to Resist Acid Corrosion

Wettability is a crucial characteristic of materials that plays a vital role in surface engineering. Surface modification is the key to changing the wettability of materials, and a simple and universal modification approach is being extensively pursued by researchers. Recently, metal-phenolic networks (MPNs) have been widely studied because they impart versatility and functionality in surface modification. However, an MPN is not stable for long periods, especially under acidic conditions, and is susceptible to pollution by invasive species. Spurred by the versatility of MPNs and various functionalities achieved by silanization, we introduce a general strategy to fabricate functionally stable coatings with controllable surface wettability by combining the two methods. The formation process of MPN and silane-MPN coatings was characterized by spectroscopic ellipsometry (SE), UV-visible-near-infrared (UV-vis-NIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle (WCA), etc. We found that the stability of the MPN was greatly enhanced after silanization, which is attributed to the cross-linking effect that occurs between silane and the MPN, namely, the cross-linking protection produced in this case. Additionally, the wettability of an MPN can be easily changed through our strategy. We trust that our strategy can further extend the applications of MPNs and points toward potential prospects in surface modification.

Immobilizing a π-Conjugated Catecholato Framework on Surfaces of SiO2 Insulator Films via a One-Atom Anchor of a Platinum Metal Center to Modulate Organic Transistor Performance

Chemical modification of insulating material surfaces is an important methodology to improve the performance of organic field-effect transistors (OFETs). However, few redox-active self-assembled monolayers (SAMs) have been constructed on gate insulator film surfaces, in contrast to the numerous SAMs formed on many types of conducting electrodes. In this study, we report a new approach to introduce a π-conjugated organic fragment in close proximity to an insulating material surface via a transition metal center acting as a one-atom anchor. On the basis of the reported coordination chemistry of a catecholato complex of Pt(II) in solution, we demonstrate that ligand exchange can occur on an insulating material surface, affording SAMs on the SiO₂ surface derived from a newly synthesized Pt(II) complex containing a benzothienobenzothiophene (BTBT) framework in the catecholato ligand. The resultant SAMs were characterized in detail by water contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and cyclic voltammetry. The SAMs served as good scaffolds of π-conjugated pillars for forming thin films of a well-known organic semiconductor C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), accompanied by the engagements of the C8-BTBT molecules with the SAMs containing the common BTBT framework at the first layer on SiO₂. OFETs containing the SAMs displayed improved performance in terms of hole mobility and onset voltage, presumably because of the unique interfacial structure between the organic semiconducting and inorganic insulating layers. These findings provide important insight into creating new elaborate interfaces through installing coordination chemistry in solution to solid surfaces, as well as OFET design by considering the compatibility between SAMs and organic semiconductors.

Ferrocene on Insulator: Silane Coupling to a SiO2 Surface and Influence on Electrical Transport at a Buried Interface with an Organic Semiconductor Layer

A silane coupling-based procedure for decoration of an insulator surface containing abundant hydroxy groups by constructing redox-active self-assembled monolayers (SAMs) is described. A newly synthesized ferrocene (Fc) derivative containing a triethoxysilyl group designated FcSi was immobilized on SiO₂/Si by a simple operation that involved immersing the substrate in a toluene solution of the Fc silane coupling reagent and then rinsing the resulting substrate. X-ray photoelectron spectroscopy (XPS) measurements confirmed that the Fc group was immobilized on SiO₂/Si in the Fe(II) state. Cyclic voltammetry measurements showed that the Fc groups were electrically insulated from the Si electrode by the SiO₂ layer. The FcSi on SiO₂/Si structures were found to serve as a good scaffold for formation of organic semiconductor thin films by vacuum thermal evaporation of C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), which is well-known as an organic field-effect transistor (OFET) material. The X-ray diffraction profile indicated that the conventional standing-up conformation of the C8-BTBT molecules perpendicular to the substrates was maintained in the thin films formed on FcSi@SiO₂/Si. Further vacuum thermal evaporation of Au provided an FcSi-based OFET structure with good transfer characteristics. The FcSi-based OFET showed pronounced source-drain current hysteresis between the forward and backward scans. The degree of this hysteresis was varied reversibly via gate bias manipulation, which was presumably accompanied by trapping and detrapping of hole carriers at the Fc-decorated SiO₂ surface. These findings provide new insights into application of redox-active SAMs to nonvolatile OFET memories while also creating new interfaces through junctions with functional thin films, in which the underlying redox-active SAMs play supporting roles.

Surface Modification of PDMS and Plastics with Zwitterionic Polymers

Surface modification of PDMS, polycarbonate, and acrylic resin was examined using various methacryl polymers bearing sulfobetaine, phosphoryl choline, and oligoethylene glycol units. We have found that zwitterionic polymers are adsorbed on the PDMS surface treated with plasma. The surface of PDMS is stable to keep high hydrophilicity after a month of the modification. On the other hand, one of sulfobetaine polymers showed distinguished adsorption behavior in the case of polycarbonate surface treated with plasma. Suppression effect for nonspecific adsorption of BSA was evaluated using polycarbonate and acrylic resin modified with the polymers. The modified surfaces showed suppression effect for nonspecific adsorption of BSA compared with the surface only treated with plasma.

Electrical Double-Layer and Ion Bridging Forces between Symmetric and Asymmetric Charged Surfaces in the Presence of Mono- and Divalent Ions

An atomic force microscope, employing the colloidal probe technique, was used to study the interactions between six different combinations of silane-functionalized silica surfaces in NaCl and CaCl₂ solutions. The surfaces consisted of monolayers of the apolar trimethoxy(octyl)silane, the positively charged (3-aminopropyl)trimethoxysilane, and the negatively charged (3-mercaptopropyl)trimethoxysilane. The interactions between the three symmetric systems, as well as between the three asymmetric combinations of surfaces, were measured and compared to calculated electrical double-layer forces. The results demonstrated that the long-range interactions between the surfaces in all cases were dominated by double-layer forces, while short-range interactions, including adhesion, were dominated by ion bridging forces in the cases where both interaction surfaces favored adsorption of calcium ions. The study thus also demonstrates how surface force studies in mono- and divalent salt solutions can be used as an analytical tool for probing specific functional groups on heterogeneous surfaces.

Surface etching, chemical modification and characterization of silicon nitride and silicon oxide--selective functionalization of Si3N4 and SiO2

The ability to selectively chemically functionalize silicon nitride (Si3N4) or silicon dioxide (SiO2) surfaces after cleaning would open interesting technological applications. In order to achieve this goal, the chemical composition of surfaces needs to be carefully characterized so that target chemical reactions can proceed on only one surface at a time. While wet-chemically cleaned silicon dioxide surfaces have been shown to be terminated with surficial Si-OH sites, chemical composition of the HF-etched silicon nitride surfaces is more controversial. In this work, we removed the native oxide under various aqueous HF-etching conditions and studied the chemical nature of the resulting Si3N4 surfaces using infrared absorption spectroscopy (IRAS), x-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and contact angle measurements. We find that HF-etched silicon nitride surfaces are terminated by surficial Si-F and Si-OH bonds, with slightly subsurface Si-OH, Si-O-Si, and Si-NH2 groups. The concentration of surficial Si-F sites is not dependent on HF concentration, but the distribution of oxygen and Si-NH2 displays a weak dependence. The Si-OH groups of the etched nitride surface are shown to react in a similar manner to the Si-OH sites on SiO2, and therefore no selectivity was found. Chemical selectivity was, however, demonstrated by first reacting the -NH2 groups on the etched nitride surface with aldehyde molecules, which do not react with the Si-OH sites on a SiO2 surface, and then using trichloro-organosilanes for selective reaction only on the SiO2 surface (no reactivity on the aldehyde-terminated Si3N4 surface).

Conjugation of A and B Blood Group Structures to Silica Microparticles for the Detection of Antigen-Specific B Cells

Silica microparticles were functionalized with A and B blood group carbohydrate antigens (A type I, A type II, B type I, and B type II) to enable the detection and monitoring of ABO antigen-specific B cells. Microparticles were prepared via the Stöber synthesis, labeled with an Alexafluor fluorescent dye, and characterized via TEM and fluorescence microscopy. The silica microparticles were functionalized with (3-aminopropyl)trimethoxysilane (APTMS), followed by the use of an established fluorenylmethyloxycarbonyl (Fmoc)-protected PEG-based linker. The terminal Fmoc moiety of the PEG-based linker was then deprotected, yielding free amino groups, to which the A and B antigens were coupled. The carbohydrate antigens were synthesized with a p-nitrophenol ester to enable conjugation to the functionalized silica microparticles via an amide bond. The number of free amine groups available for coupling for a given mass of PEG-functionalized silica microparticles was quantified via reaction with Fmoc-glycine. The antigen-functionalized microparticles were then evaluated for their specificity in binding to A and B antigen-reactive B-cells via flow cytometry, and for blocking of naturally occurring antibodies in human serum. Selective binding of the functionalized microparticles to blood group-reactive B cells was observed by flow cytometry and fluorescence microscopy. The modular approach outlined here is applicable to the preparation of silica microparticles containing any carbohydrate antigen and alternative fluorophores or labels. This approach therefore comprises a novel, general platform for screening B cell populations for binding to carbohydrate antigens, including, in this case, the human A and B blood group antigens.

Impact of plasma pretreatment and pore size on the sealing of ultra-low-k dielectrics by self-assembled monolayers

Self-assembled monolayers (SAMs) from an 11-cyanoundecyltrichlorosilane (CN-SAM) precursor were deposited on porous SiCOH low-k dielectrics with three different pore radii, namely, 1.7, 0.7, and lower than 0.5 nm. The low-k dielectrics were first pretreated with either O2 or He/H2 plasma in order to generate silanol groups on the hydrophobic pristine surface. Subsequently, the SAMs were chemically grafted to the silanol groups on the low-k surface. The SAMs distribution in the low-k films depends on the pore diameter: if the pore diameter is smaller than the size of the SAMs precursors, the SAM molecules are confined to the surface, while if the pore diameter exceeds the van der Waals radius of the SAMs precursor, the SAMs molecules reach deeper in the dielectric. In the latter case, when the pore sidewalls are made hydrophilic by the plasma treatment, the chemical grafting of the SAM precursors follows the profile of the generated silanol groups. The modification depth induced by the O2 plasma is governed by the diffusion of the oxygen radicals into the pores, which makes it the preferred choice for microporous materials. On the other hand, the vacuum ultraviolet (VUV) light plays a critical role, which makes it more suitable for hydrolyzing mesoporous materials. In addition to the density of the surface -OH groups, the nanoscale concave curvature associated with the pores also affects the molecular packing density and ordering with respect to the self-assembly behavior on flat surfaces. A simple model which correlates the low-k pore structure with the plasma hydrophilization mechanism and the SAMs distribution in the pores is presented.

Design and fabrication of biosensing interface for waveguide-mode sensor

In order to develop a biosensing system with waveguide-mode sensor, fabrication of a biosensing interface on the silica surface of the sensing chip was carried out using triethoxysilane derivatives with anti-leptin antibody. Triethoxysilane derivatives bearing succinimide ester and oligoethylene glycol moieties were synthesized to immobilize the antibody and to suppress nonspecific adsorption of proteins, respectively. The chip modified with triethoxysilane derivatives bearing oligoethylene glycol moiety suppressed nonspecific adsorption of proteins derived from human serum effectively by rinse with PBS containing surfactant (0.05% Tween 20). On the other hand, it was confirmed that antibody was immobilized on the chip by immersion into antibody solution to show response of antigen-antibody reaction, where the chip was modified with triethoxysilane derivatives bearing succinimide ester moiety. When the interface was fabricated with antibody and a mixture of triethoxysilane derivatives bearing succinimide ester and oligoethylene glycol moieties, the response of antigen-antibody reaction depended on composition of the mixture and enhanced with the increase of ratio for triethoxysilane derivatives bearing succinimide ester moiety reflecting the antibody concentration immobilized on the chip. While introduction of excess triethoxysilane derivatives bearing succinimide ester moiety induced nonspecific adsorption of proteins derived from human serum, the immobilized antibody on the chip kept its activity after 1-month storage in a refrigerator. Taking into consideration those factors, the biosensing interface was fabricated using triethoxysilane derivatives with anti-leptin antibody to examine performance of the waveguide-mode sensor. It was found that the detection limits for human leptin were 50 ng/mL in PBS and 100 ng/mL in human serum. The results demonstrate that the waveguide-mode sensor powered by the biosensing interface fabricated with those triethoxysilane derivatives and antibody has potential to detect several tens of nanograms per milliliter of biomarkers in human serum with an unlabeled detection method.