Controlled Silanization Using Functional Silatrane for Thin and Homogeneous Antifouling Coatings

Sulfobetaine-Siloxanes: A Class of Self-Destructive Surfactants

The hydrolytic susceptibility of sulfobetaine-siloxane surfactants is investigated by comparison of a homologous series in this subclass of surfactants (R-(CH2)3N+(Me)2(CH2)3SO3-; R = (Me3SiO)3Si-, (Me3SiO)2Si(Me)-, (Me2SiO)3-Si(Me)-) with an analogue series of oxyethylene-siloxane surfactants (R-(CH2)3(OCH2CH2)10.2OH; R = (Me3SiO)3Si-, (Me3SiO)2Si(Me)-, (Me2SiO)3-Si(Me)-). Nuclear magnetic resonance (NMR) monitoring of these surfactants in an aqueous solution shows that the presence of the sulfobetaine head structure greatly enhances the hydrolysis rate of the siloxane tail as compared with oxyethylene-siloxane analogue control experiments. This sulfobetaine effect is confirmed by adding a model compound, (Me)3N+(CH2)3SO3-, to the oxyethylene-siloxane surfactants and observing the large hydrolysis enhancement. Measurements of pH indicate the sulfobetaine presence greatly enhances acidity, but rigorous analysis could discover no source of acid other than the presence of the sulfobetaine structure. Titration measurements confirmed the presence of a tightly bound hydration layer of 4-7 water molecules per sulfobetaine group. It is speculated that the source of acidity may originate from an aqueous exclusion zone nucleated by the hydrated sulfobetaine at the interface of a sulfobetaine-siloxane surfactant bilayer aggregate. Hydrolysis prevention is investigated by addition of a pH 7 phosphate buffer, of an alkyl polyglycoside cosurfactant, and of a combination of both, with a finding of very significant but not complete suppression of the hydrolysis.

Novel Antifouling Coatings by Zwitterionic Silica Grafting on Glass Substrates

Zwitterionic silica coatings for surface functionalization are greatly prominent because of their simple and fast preparation, high availability, and effective antifouling properties. In this work, two zwitterionic sulfobetaine silane coatings, i.e., mono-SBSi and tris-SBSi, were deposited on glass surfaces and tested for antifouling of biological material and biofilm using human cancer cell and seawater, respectively. The used zwitterionic precursors mono-SBSi and tris-SBSi differ by the number of hydrolyzable silane groups: mono-SBSi contains one trimethoxysilane group, whereas tris-SBSi contains three of these functions. First, X-ray photoelectron spectroscopy indicates the successful grafting of zwitterionic coatings onto a glass surface. Characterization using atomic force microscopy shows the different morphologies and roughness of the two coatings. The glass surface became more hydrophilic after the grafting of zwitterionic coatings than the bare glass substrate. The antifouling properties of two coatings were evaluated via human cancer cell adsorption. Interestingly, the tris-SBSi coating displays a significantly lower level of cell adsorption compared to that of both mono-SBSi coating and the non-modified control surface. The same trend was observed for biofilm formation in seawater. Finally, the toxicity of mono-SBSi and tris-SBSi coatings was evaluated on zebrafish embryos, indicating the good biocompatibility of both coatings. Our results indicate interesting antifouling properties of zwitterionic coatings. The chemical constitution of the used precursor has an impact on the antifouling properties of the formed coating: the tris-SBSi-based zwitterionic silica coatings display improved antifouling properties compared to those of the mono-SBSi-based coating. Besides, the use of trisilylated precursors should result in the formation of more resistant and robust coatings due to the higher number of grafting functions. For all these reasons, we anticipate that tris-SBSi coatings will open new perspectives for antifouling applications for biological environments and implants.

Solvent-Free and Efficient Synthesis of Silatranes via an Organocatalytic Protocol under Mild Conditions

The organocatalytic approach to the formation of silatranyl cages permitted the design of a solvent-free and efficient protocol for the preparation of various organosilatranes. We discovered that amidine derivatives efficiently catalyze the conversion of trialkoxysilanes into organosilatranes, and their catalytic activity is related to the pKBH+ values. NMR studies of equimolar reactions of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) with selected substrates allowed proposing a reliable scheme for the transesterification process and silatranyl cage formation. In addition, green chemistry metrics for the scaled-up synthesis of vinylsilatrane (3k) were appointed. Finally, a scheme for the industrial production of silatrane derivatives with DBU and solvent regeneration was proposed, supported by a catalyst recycling experiment.

Dipodal Silanes Greatly Stabilize Glass Surface Functionalization for DNA Microarray Synthesis and High-Throughput Biological Assays

Glass is by far the most common substrate for biomolecular arrays, including high-throughput sequencing flow cells and microarrays. The native glass hydroxyl surface is modified by using silane chemistry to provide appropriate functional groups and reactivities for either in situ synthesis or surface immobilization of biologically or chemically synthesized biomolecules. These arrays, typically of oligonucleotides or peptides, are then subjected to long incubation times in warm aqueous buffers prior to fluorescence readout. Under these conditions, the siloxy bonds to the glass are susceptible to hydrolysis, resulting in significant loss of biomolecules and concomitant loss of signal from the assay. Here, we demonstrate that functionalization of glass surfaces with dipodal silanes results in greatly improved stability compared to equivalent functionalization with standard monopodal silanes. Using photolithographic in situ synthesis of DNA, we show that dipodal silanes are compatible with phosphoramidite chemistry and that hybridization performed on the resulting arrays provides greatly improved signal and signal-to-noise ratios compared with surfaces functionalized with monopodal silanes.

Polymer Brushes on Silica Nanostructures Prepared by Aminopropylsilatrane Click Chemistry: Superior Antifouling and Biofunctionality

In nanobiotechnology, the importance of controlling interactions between biological molecules and surfaces is paramount. In recent years, many devices based on nanostructured silicon materials have been presented, such as nanopores and nanochannels. However, there is still a clear lack of simple, reliable, and efficient protocols for preventing and controlling biomolecule adsorption in such structures. In this work, we show a simple method for passivation or selective biofunctionalization of silica, without the need for polymerization reactions or vapor-phase deposition. The surface is simply exposed stepwise to three different chemicals over the course of ∼1 h. First, the use of aminopropylsilatrane is used to create a monolayer of amines, yielding more uniform layers than conventional silanization protocols. Second, a cross-linker layer and click chemistry are used to make the surface reactive toward thiols. In the third step, thick and dense poly(ethylene glycol) brushes are prepared by a grafting-to approach. The modified surfaces are shown to be superior to existing options for silica modification, exhibiting ultralow fouling (a few ng/cm2) after exposure to crude serum. In addition, by including a fraction of biotinylated polymer end groups, the surface can be functionalized further. We show that avidin can be detected label-free from a serum solution with a selectivity (compared to nonspecific binding) of more than 98% without the need for a reference channel. Furthermore, we show that our method can passivate the interior of 150 nm × 100 nm nanochannels in silica, showing complete elimination of adsorption of a sticky fluorescent protein. Additionally, our method is shown to be compatible with modifications of solid-state nanopores in 20 nm thin silicon nitride membranes and reduces the noise in the ion current. We consider these findings highly important for the broad field of nanobiotechnology, and we believe that our method will be very useful for a great variety of surface-based sensors and analytical devices.

3-Aminopropylsilatrane and Its Derivatives: A Variety of Applications

Silatranes arouse much research interest owing to their unique structure, unusual physical–chemical properties, and diverse biological activity. The application of some silatranes and their analogues has been discussed in several works. Meanwhile, a comprehensive review of the wide practical usage of silatranes is still absent in the literature. The ability of silatranes to mildly control hydrolysis allows them to form extremely stable and smooth siloxane monolayers almost on any surface. The high physiological activity of silatranes makes them prospective drug candidates. In the present review, based on the results of numerous previous studies, using the commercially available 3-aminopropylsilatrane and its hybrid derivatives, we have demonstrated the high potential of 1-organylsilatranes in various fields, including chemistry, biology, pharmaceuticals, medicine, agriculture, and industry. For example, these compounds can be employed as plant growth biostimulants, drugs, optical, catalytic, sorption, and special polymeric materials, as well as modern high-tech devices.

Study of microheterogeneity of silatrane-based silica surface bonding chemistry and its optimization for the synthesis of chiral stationary phases for enantioselective liquid chromatography

The present work systematically investigates the chemical microheterogeneity as part of the optimization of a single-step surface bonding chemistry of 3-mercaptopropylsilatrane (MPS) on mesoporous silica gel in comparison to the state-of-the-art silane chemistry with 3-mercaptopropyltrimethoxysilane (MPTMS). MPS functionalization turns out to be a favourable chemistry for the further use in thiol-ene click reactions such as the immobilization of chiral selectors, herein tert-butylcarbamoylquinine (tBuCQN), for the synthesis of chiral stationary phases (CSPs). MPS has higher reactivity than MPTMS and prefers the formation of trifunctional siloxane bondings unlike MPTMS which favours difunctional siloxane bonds to silica, as investigated by solid-state cross-polarization/magic angle spinning (CP/MAS) NMR (²⁹Si and ¹³C nuclei). Reaction conditions (ternary mixtures of methanol, water and toluene; with and without acid; prewetting of silica; HCl pretreatment of silica) were evaluated with the aim to find conditions which promote the formation of a horizontal siloxane polymer layer on top of the silica surface. Silanization reaction times could be reduced to 2 h. The ²⁹Si NMR signal corresponding to trifunctional siloxane bonding could be increased to 60% with no T1 signal that refers to monofunctional siloxane bonding in spite of water in the ternary reaction mixture. Furthermore, no significant disulfide bridges were formed in this approach, leading to high selector loadings. The thiol and selector coverage reached up to 4.6 and 1.4 µmol/m², respectively. With the preferred CSP, the enantioselectivity could be increased for a chiral probe (FMOC-Phe) and the mass transfer resistance (C-term) bisected compared to the corresponding CSP prepared from benchmark MPTMS-modified silica (2.54 vs 5.72 ms). It is demonstrated that the fine-tuning of the microstructure on the silica surface can have a significant influence on enantioselectivity and mass transfer kinetics of the resultant CSPs.

Fabrication of Polysulfobetaine Gradient Coating via Oxidation Polymerization of Pyrogallol To Modulate Biointerfaces

A surface with a gradient physical or chemical feature, such as roughness, hardness, wettability, and chemistry, serves as a powerful platform for high-throughput investigation of cell responses to a biointerface. In this work, we developed a continuous antifouling gradient surface using pyrogallol (PG) chemistry. A copolymer of a zwitterionic monomer, sulfobetaine methacrylate, and an amino monomer, aminoethyl methacrylate, were synthesized (pSBAE) and deposited on glass slides via the deposition of self-polymerized PG. A gradient of pSBAE was fabricated on glass slides in 7 min in the presence of an oxidant, ammonium persulfate, by withdrawing the reaction solution. The modified glass slide showed a wettability gradient, determined by measuring the water contact angle. Cell adhesion and protein adsorption were well correlated with surface wettability. We expect that this simple and faster method for the fabrication of a continuous chemical gradient is applicable for high-throughput screening of surface properties to modulate biointerfaces.

Optimization of Surface Loading of the Silatrane Anchoring Group on TiO2

Anchoring groups are usually needed for the attachment of small molecules to metal oxide surfaces such as in water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs). Here, we optimize the surface loading onto titanium dioxide surfaces of the silatrane anchoring group, a triethanolamine-protected trialkoxysilane. This anchoring group is not yet widely used because prior protocols afforded low surface coverage, but it has the advantage of high stability over a wide pH range and at both oxidizing and reducing potentials when bound. A new and improved method for estimating surface coverage is described here and used to determine that loading using previously reported binding protocols is very low. However, we were able to uncover several factors contributing to this low loading, which has allowed us to develop methods to greatly improve surface coverage for a variety of silatranes. Most notably, we were able to increase the loading of a model arylsilatrane by 145% through use of a benzoic acid additive. This is not general acid catalysis because alkylsilatranes are not similarly affected and 4-t-butylbenzoic acid, having a similar pK_a to benzoic acid, is not effective. Because the bulky t-butyl group of the latter additive is not expected to pi-stack with our arylsilatrane, we have tentatively assigned this enhancement to aromatic stacking between the aromatic additive and the arylsilatrane.

Investigating Thin Silicone Oil Films Using Four-Wave Mixing Spectroscopy and Sum Frequency Generation Vibrational Spectroscopy

This article applies four-wave mixing (FWM) spectroscopy, a third-order nonlinear optical spectroscopic technique which is not intrinsically surface- or interface-sensitive, to study silicone oil thin films, supplemented by second-order nonlinear-optical sum frequency generation (SFG) vibrational spectroscopy. Although studies of thin organic films using coherent antistokes Raman spectroscopy (CARS), a special case of FWM, have been reported previously, in this study we demonstrate the feasibility of using a more general FWM process which involves three independent excitation laser beams to investigate silicone oil thin films. The results show that the FWM method has the potential to detect and provide molecular-level information on ultrathin silicone oil layers, down to a film thickness of 1 nm. This developed FWM methodology is widely applicable and can be utilized to study important issues in the biopharmaceutical field, e.g., to examine the distribution of silicone oil on syringe glass surfaces with subnanometer sensitivity. It can also be used to study the potentially slow reactions between silicone oil and glass surfaces as proposed in the literature but without direct molecular-level information.

Controllable organosilane monolayer density of surface bonding using silatranes for thiol functionalization of silica particles for liquid chromatography and validation of microanalytical method for elemental composition determination

The present work systematically investigates a new strategy for the functionalization of silica gel using alkyl silatrane chemistry instead of alkylsilanes for synthesis of chromatographic stationary phases. In this work, silica was chemically modified for further functionalization by a thiol-ene click reaction. Thus, 3-mercaptopropylsilatrane (MPS) was used which is capable to form self-assembled monolayers (SAM) on top of silanol surfaces in a controlled manner as previously shown for silicon wafers. The utility of this chemistry for stationary phase synthesis in liquid chromatography was not evaluated yet. Hence, silica surface modifications using MPS were studied in comparison to established 3-mercaptopropyltrimethoxysilane (MPTMS) chemistry. First, the employed elemental analysis method was validated and it showed excellent intra-day and inter-day precisions (typically less than 5% RSD). It could be shown that the reaction kinetics of MPS was roughly 35-times faster than with MPTMS. After 30 min reaction time with MPS, the thiol content reached 74% of the maximal coverage. Due to controlled chemistry with MPS, which does not lead to oligomeric siloxane network at the silica surface, the ligand coverage was lower. However, multiple silanization cycles with MPS led to a dense surface coverage (around 4 µmol m^-2). ²⁹Si cross polarization/magic angle spinning (CP/MAS) solid-state NMR revealed distinct T¹/T²/T³ ratios for MPS and MPTMS materials with up to 80% T³ (indicative for trifunctional siloxane linkage) for MPS and around 20% T³ for MPTMS. This indicates a more homogeneous, thinner monolayer film of MPS on the silica surface, as compared to an irregular thick oligomeric siloxane network with MPTMS. Bonding of quinine carbamate as chiral selector afforded an efficient chiral stationary phase (CSP) for chromatographic enantiomer separation. Separation factors were comparable to MPTMS-bonded CSP, however, chromatographic efficiency was much better for the MPS-bonded CSP. H/u curves indicated a reduced mass transfer resistance by roughly factor 3 for MPS- compared to MPTMS-bonded CSP. This confirms better chromatographic performance of surfaces with homogeneous monolayer compared to network structures on the silica surface which suffer from poor stationary phase mass transfer.

Anticlogging Hemofiltration Device for Mass Collection of Circulating Tumor Cells by Ligand-Free Size Selection

A new hemofiltration system was developed to continuously capture circulating tumor cells (CTCs) from a large volume of whole blood using a column that was packed with antifouling zwitterionized silica microspheres. The silica microspheres were modified with sulfobetaine silane (SBSi) to inhibit fouling, resist clogging, and give a high surface wettability and prolonged operation time. Packed microspheres with different diameters formed size-controllable interstitial pores that effectively captured CTCs by ligand-free size selection. For optimized performance of the hemofiltration system, operational factors, including the size of microspheres, flow rate, and cross-sectional area of the column, were considered with respect to the removal rate for colorectal cancer cells and the retention rate for white blood cells and red blood cells. The captured CTCs were collected from the column by density sedimentation. A large quantity of colorectal cancer cells was spiked into sheep blood, and the sample was circulated for 5 h with a total operational volume of 2 L followed by collection and culture in vitro. The results showed that the proposed hemofiltration device selectively removed abundant CTCs from in vitro circulatory blood. The viable cells were harvested for amplification and potential applications for precision medicine.

Antifouling Coatings Generated from Unsymmetrical Partially Fluorinated Spiroalkanedithiols

Biofouling negatively impacts modern society on a daily basis, especially with regard to the important industries of medicine, oil, and shipping. This manuscript describes the preparation and study of model antifouling coatings generated from the adsorption of unsymmetrical partially fluorinated spiroalkanedithiols on gold. The antifouling properties of the self-assembled monolayers (SAMs) derived from the spiroalkanedithiols were compared to SAMs derived from analogous monodentate partially fluorinated and nonfluorinated alkanethiols. The antifouling properties were evaluated using in situ surface plasmon resonance spectroscopy (SPR), ex situ electrochemical quartz crystal microbalance (QCM) measurements, and ex situ ellipsometric thickness measurements. The resistance to nonspecific protein adsorption of the SAMs was evaluated with proteins having a wide range of properties and applications including protamine, lysozyme, bovine serum albumin, and fibrinogen. The results from the SPR and the QCM measurements demonstrated that in most cases, the SAM coatings derived from the partially fluorinated spiroalkanedithiols having mixed hydrocarbon and fluorocarbon tail groups exhibited better antifouling performance when compared to the SAMs derived from their single-component monodentate counterparts. The studies also revealed that while the SPR and the QCM measurements in most cases were able to distinguish the adsorption trends for the SAMs and proteins examined, the ellipsometric thickness measurements were markedly less discriminating. On the whole, these studies validate the use of unsymmetrical partially fluorinated spiroalkanedithiols for generating effective antifouling coatings on metal substrates.

A Precious-Metal-Free Hybrid Electrolyzer for Alcohol Oxidation Coupled to CO2 -to-Syngas Conversion

Electrolyzers combining CO2 reduction (CO2 R) with organic substrate oxidation can produce fuel and chemical feedstocks with a relatively low energy requirement when compared to systems that source electrons from water oxidation. Here, we report an anodic hybrid assembly based on a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) electrocatalyst modified with a silatrane-anchor (STEMPO), which is covalently immobilized on a mesoporous indium tin oxide (mesoITO) scaffold for efficient alcohol oxidation (AlcOx). This molecular anode was subsequently combined with a cathode consisting of a polymeric cobalt phthalocyanine on carbon nanotubes to construct a hybrid, precious-metal-free coupled AlcOx-CO2 R electrolyzer. After three-hour electrolysis, glycerol is selectively oxidized to glyceraldehyde with a turnover number (TON) of ≈1000 and Faradaic efficiency (FE) of 83 %. The cathode generated a stoichiometric amount of syngas with a CO:H2 ratio of 1.25±0.25 and an overall cobalt-based TON of 894 with a FE of 82 %. This prototype device inspires the design and implementation of nonconventional strategies for coupling CO2 R to less energy demanding, and value-added, oxidative chemistry.

Controlled Silanization: High Molecular Regularity of Functional Thiol Groups on Siloxane Coatings

A comparative study on deposition and molecular regularity of two organosilanes, i.e., commercially available (3-mercaptopropyl)trimethoxysilane (MPTMS) and newly developed mercaptopropylsilatrane (MPS), was conducted in this work. MPTMS and MPS were applied to modify silicon surfaces to characterize their deposition kinetics, surface morphology, thickness, and elemental composition and the reactivity of thiol end groups based on gold-thiol and thiol-ene chemistries. MPS possesses a tricyclic caged structure and a transannular N → Si dative bond, making it chemically stable and controllable to avoid fast hydrolysis and aggregation in solution. The results indicate that MPS allows faster deposition and better formation of thin and homogeneous films than MPTMS. More importantly, the functional thiol groups on MPS coatings enable immobilization of a large amount of gold nanoparticles and effective thiol-ene photopolymerization with zwitterionic sulfobetaine acrylamide. Postmodification on silanized surfaces with MPS endows excellent plasmonic and antifouling properties, potentially leading to valuable applications to biosensing and biomaterials. The work demonstrated the feasibility and applicability of the functional silatrane molecule for surface silanization in a controlled manner.

Synthesis of Mesoporous/Macroporous Microparticles Using Three-Dimensional Assembly of Chitosan-Functionalized Halloysite Nanotubes and Their Performance in the Adsorptive Removal of Oil Droplets from Water

Halloysite nanotubes (HNTs) were assembled into mesoporous/macroporous microparticles (c-g-HNTs MPs) using Pickering template-assisted approach. To unravel the stabilization mechanism in Pickering emulsion form, several emulsions and microparticles were prepared at various conditions and visualized using confocal laser scanning microscopy. The prepared c-g-HNTs MPs were used to treat emulsified oil solutions resulting in a maximum removal efficiency of 94.47%. The kinetics data of oil adsorption onto c-g-HNTs MPs was best fitted by the pseudo-second-order kinetic model ( R² = 0.9983). The maximum monolayer adsorption capacity of oil onto c-g-HNTs MPs as predicted by the multilayer Brunauer-Emmett-Teller model was found to be 788 mg/g. Compared with pristine HNTs, c-g-HNTs MPs exhibited higher self-settleability rates in aqueous solutions as well as in emulsified oil solutions, demonstrating their candidacy for practical water treatment applications. The c-g-HNTs MPs were repeatedly used for five adsorption-desorption cycles with minimal losses noticed in their performance.