Charged Silica Research Articles

Study of Fluorescence Quenching Ability of Graphene Oxide with a Layer of Rigid and Tunable Silica Spacer.

The fluorescence quenching property of graphene oxide (GO) has been newly demonstrated and applied for fluorescence imaging and biosensing. In this work, a new nanostructure was designed for effectively studying the quenching ability of GO. The key element in this design is the fabrication of a layer of rigid and thickness adjustable silica spacer for manipulating the distance between the GO and fluorophores. First, a silica core modified with organic dye molecules was prepared, followed by the formation of a silica shell with a tunable thickness. Afterward, the GO was wrapped around silica nanoparticles based on the electrostatic interaction between the negatively charged GO and positively charged silica. The quenching efficiency of GO to different dye molecules was studied at various spacer thicknesses and varying concentrations of GO. Fluorescence lifetime of fluorophores was measured to determine the quenching mechanism. We found that the quenching efficiency of GO was still around 30% when the distance between dyes and GO was increased to more than 30 nm, which indicated the long-distance quenching ability of GO and confirmed the previous theoretical calculation. The quenching mechanisms were proposed schematically based on our experimental results. We expected that the proposed nanostructure could act as a feasible model for studying GO quenching property and shed light on designing GO-based fluorescence sensing systems.

Layer-by-Layer Engineered Polymer Capsules for Therapeutic Delivery.

Polymer capsules fabricated via layer-by-layer (LbL) assembly have emerged as promising carriers for therapeutic delivery. The versatile assembly technique allows an extensive choice of materials to be incorporated as constituents of the multilayers, which therefore endow capsules with specific properties and functionalities. This chapter describes protocols for fabrication of LbL-engineered poly(methacrylic acid) (PMA) capsules for applications in gene delivery, including (1) synthesis of building blocks, (2) cargo encapsulation, (3) multilayer film formation, (4) surface modification, and (5) cross-linking of multilayer films and dissolution of particle templates. DNA is adsorbed onto positively charged silica particle templates, followed by formation of polymer films via hydrogen-bonded multilayers of thiol-functionalized PMA and poly(N-vinylpyrrolidone) (PVP). The outer polymer membranes can be surface modified with copolymers of PMA and poly(ethylene glycol) (PEG). Upon film stabilization and dissolution of particle templates, disulfide-cross-linked DNA-loaded PMA capsules are obtained, which serve as therapeutic carriers that can degrade and facilitate cargo release in intracellular reducing environment.

Interactions in reentrant phase behavior of a charged nanoparticle solution by multivalent ions.

The interactions following a reentrant phase transition of charged silica nanoparticles from one phase to two phases and back to one phase by varying the concentration of multivalent counterions have been examined. The observations are far beyond the framework of Debye-Hückel or even nonlinear Poisson-Boltzmann equations and demonstrate the universal behavior of multivalent counterion-driven charge inversion. We show that the interplay of multivalent counterion-induced short-range attraction and long-range electrostatic repulsion between nanoparticles results in reentrant phase behavior.

The effect of surfactant adsorption on surface wettability and flow resistance in slit nanopore: A molecular dynamics study

Hypothesis: Due to large surface/volume ratio, fluid flow resistance in nanopores is affected dramatically by surfactants adsorption, which dictates wettability and friction. Surfactants with different aggregated morphology and molecular alignment at solid/water interface are expected to affect friction and mobility of surfactant adsorption layer, both of which should be possible to contribute to surfactant drag reduction mechanism in nanopores.Simulations: Molecular dynamics (MD) simulations were adopted to study the morphology of the adsorption layer of different types of surfactants on negatively charged hydrophilic silica surface and their effect on flow resistance in slit nanopores.Findings: Flow resistance differs as surfactant adsorption morphologies vary. Adsorption layer composed of hemimicelles with “head on” orientation in low adsorption amount exhibits low flow resistance. As adsorption amount increases, adsorption layer evolves into the double layer in which less polar surfactant composition is found to be beneficial for improving hydrophobicity of pore wall and interfacial water diffusion. More ordered and tight intermolecular packing and high mobility of the adsorption layer are found to be propitious to reduce the flow friction.

Ultrasensitive Ratiometric Homogeneous Electrochemical MicroRNA Biosensing via Target-Triggered Ru(III) Release and Redox Recycling.

A new label-free and enzyme-free ratiometric homogeneous electrochemical microRNA biosensing platform was constructed via target-triggered Ru(III) release and redox recycling. To design the effective ratiometric dual-signal strategy, [Ru(NH3)6]3+ (Ru(III)), as one of the electroactive probes, was ingeniously entrapped in the pores of the positively charged mesoporous silica nanoparticle (PMSN), and another electroactive probe, [Fe(CN)6]3- (Fe(III)), was selected to facilitate Ru(III) redox recycling due to its distinctly separated reduction potential and different redox properties. Owing to the liberation of the formed RNA-ssDNA complex from PMSN, the target miRNA triggered the Ru(III) release and was quickly electroreduced to Ru(II), and then, the in-site-generated Ru(II) could be chemically oxidized back to Ru(III) by Fe(III). Thus, with the release of Ru(III) and the consumption of Fe(III), a significant enhancement for the ratio of electroreduction current [Ru(NH3)6]3+ over [Fe(CN)6]3- (IRu(III)/IFe(III)) value was observed, which was dependent on the concentration of the target miRNA. Consequently, a simple, accurate, and ultrasensitive method for the miRNA assay was readily realized. Furthermore, the limit of detection (LOD) of our method was down to 33 aM (S/N = 3), comparable or even superior to other approaches reported in literature. More importantly, it also exhibited excellent analytical performance in the complex biological matrix cell lysates. Therefore, this homogeneous biosensing strategy not only provides an ingenious idea for realizing simple, rapid, reliable, and ultrasensitive bioassays but also has a great potential to be adopted as a powerful tool for precision medicine.

Enzymatic Fuel Cell-Based Self-Powered Homogeneous Immunosensing Platform via Target-Induced Glucose Release: An Appealing Alternative Strategy for Turn-On Melamine Assay.

Enzymatic fuel cell (EFC)-based self-powered biosensors have attracted considerable attention because of their unique feature of no need for extra power sources during the entire detection process, which endows them with the merits of simplicity, rapidness, low cost, anti-interference, and ease of use. Herein, we proposed, for the first time, an EFC-based self-powered homogeneous immunosensing platform by integrating the target-induced biofuel release and bioconjugate immunoassay for ultrasensitive melamine (ME) detection. In this design, the biofuel, i.e., glucose molecules, was entrapped in the pores of positively charged mesoporous silica nanoparticles and capped by the biogate AuNPs-labeled anti-ME antibody (AuNPs-Ab). The presence of the target ME triggered the entrapped glucose release due to the removal of the biogate via immunoreaction, which resulted in the transfer of electrons produced by glucose oxidation at the bioanode to the biocathode, and thus, the open-circuit voltage of the EFC-based self-powered immunosensor dramatically increased, realizing the ultrasensitive turn-on assay for ME. The limit of detection for ME assay was down to 2.1 pM (S/N = 3), superior to those previously reported in the literature. Notably, real milk samples need no special sample pretreatment for the detection of ME because of the good anti-interference ability of EFC-based self-powered biosensors and the excellent selectivity of the homogeneous immunoassay. Therefore, this appealing self-powered homogeneous immunosensing platform holds great promise as a successful prototype of portable and on-site bioassay in the field of food safety.

Charged polymer brushes-coated mesoporous silica nanoparticles for osteoarthritis therapy: a combination between hydration lubrication and drug delivery

Charged polymer brushes-coated mesoporous silica nanoparticles for osteoarthritis therapy: a combination between hydration lubrication and drug delivery

Fouling behavior of silica nanoparticle-surfactant mixtures during constant flux dead-end ultrafiltration

The increasing use of engineered nanoparticles in customer products results in their accumulation in water sources. In this experimental study, we investigated the role of surfactant type (cationic, anionic and non-ionic) and concentration on fouling development, nanoparticle rejection and fouling irreversibility during dead-end ultrafiltration of model silica nanoparticles. Our work demonstrates that the type of surfactant influences the nanoparticle stability, which in turn is responsible for differences in fouling behavior of the nanoparticles. Moreover, the surfactant itself interacts with the PES-PVP membrane and contributes to the fouling as well. We have shown that anionic SDS (sodium dodecylsulfate) does not interact extensively with the negatively charged silica nanoparticles and does not change significantly the surface charge and size of these nanoparticles. Adsorption of the cationic CTAB (cetyltrimethylammonium bromide) onto the silica nanoparticles causes charge transition and nanoparticle aggregation, whereas non-ionic TX-100 (Triton X-100) neutralizes the surface charge of the nanoparticles but does not change significantly the nanoparticle size. The most severe fouling development was observed for the silica nanoparticle – TX-100 system, where nanoparticles in the filtration cake formed exhibited the lowest repulsive interactions. Rejection of the nanoparticles was also highest for the mixture containing silica nanoparticles and TX-100.

Polyelectrolyte-assisted preparation of gold nanocluster-doped silica particles with high incorporation efficiency and improved stability

In this paper, we reported an approach for efficient incorporation of glutathione-capped gold nanoclusters (GSH-Au NCs) into silica particles with the assistance of a polyelectrolyte, poly-diallyldimethyl-ammoniumchloride (PDDA). In this approach, the negatively charged GSH-Au NCs were firstly mixed with the positively charged PDDA to form PDDA–Au NC complexes. Then, the complexes were added into a pre-hydrolyzed Stober system to get the Au NCs-doped silica particles. With increased ratio of PDDA in the complexes, the negative charges on surface of the Au NCs were neutralized gradually and finally reversed to positive in presence of excess PDDA, which facilitated the incorporation of the Au NCs into the negatively charged silica matrix. Under the optimal amount of PDDA in the complexes, the incorporation efficiency of Au NCs could be as high as 88%. After being incorporated into the silica matrix, the Au NCs become much robust against pH and heavy metal ions attributed to the protection effect of silica and PDDA. This approach was also extendable to highly efficient incorporation of other negatively charged metal nanoclusters, such as bovine serum albumin-capped Cu nanoclusters, into silica matrix.

Immunomodulatory Effects of Nanoparticles on Skin Allergy

In recent years there has been considerable effort to understand the interaction of nanomaterials with the skin. In this study we use an in vivo mouse model of allergic contact dermatitis to investigate how nanoparticles (NPs) may alter allergic responses in skin. We investigate a variety of NPs that vary in size, charge and composition. Results show that small (<200 nm) negative and neutral charged NPs exhibit an immunosuppressive effect but that positively charged NPs do not. Confocal imaging suggests positively charged NPs may penetrate skin to a lesser extent and thereby are less able interact with and alter the local immune responses. Interestingly, negatively charged silica (20 nm) NPs suppress allergic response to two chemically distinct sensitizers; 1-fluoro-2, 4-dinitrobenzene and 2-deoxyurushiol. Skin wiping and NP application time studies suggest that the immunomodulatory mechanism is not due solely to the blocking of sensitizer adduct formation in skin. Results suggest that NPs modulate early immune events that impact mast cell degranulation. Our study shows for the first time the potential to modulate the elicitation phase of the allergic response which depends on the NP charge and composition. These finding can be used to inform the design topical therapeutics to mitigate allergic responses in skin.

Biomolecules at the amorphous silica/water interface: Binding and fluorescence anisotropy of peptides

We investigate binding of the tripeptides Lys-Trp-Lys (KWK) and Glu-Trp-Glu (EWE) to the amorphous silica surface using atomistic simulations. These peptides were chosen because they were previously utilized in experiments measuring binding affinity and steady-state fluorescence anisotropy from the indole chromophore of the tryptophan residue. Our simulations were performed using silica with surface change density of −0.8 elementary charges per square nanometer, which is expected at neutral pH. Even though positive charged KWK binds more strongly to the negatively charged silica surface, EWE also forms bound complexes with the surface that are stable for at least 15ns of simulation, in agreement with the experiments which revealed evidence for binding of both KWK and EWE to silica. Binding mechanisms include a wide variety of electrostatic interactions, as well as hydrophobic interactions between the indole group and hydrophobic areas of the heterogeneous silica surface. The long-time limit of the fluorescence anisotropy of tryptophan is calculated from the simulations in order to help interpret the recent experiments. We identify several factors which control the magnitude of the fluorescence anisotropy for each binding configuration.

Optimized selection of liquid chromatography conditions for wide range analysis of natural compounds

Plant secondary metabolites are an almost unlimited reservoir of potential bioactive compounds. In view of the wide chemical space covered by natural compounds, their comprehensive analysis requires multiple and complementary approaches. In this study, numerous chromatographic conditions were tested for the analysis of a set of 120 representative natural compounds covering a wide polarity range (18 log P units). The experiments were performed on 59 different conditions involving 29 RPLC and HILIC dedicated stationary phases, as well as more exotic mixed mode columns. The best RPLC and HILIC conditions were determined using Derringer’s desirability functions, based on various criteria (i.e. retention, peak shape, distribution of compounds during the gradient…). After this first selection, only the most promising conditions were kept (19 in RPLC and 11 in HILIC). The selectivity complementarity among each chromatographic mode was assessed by principal component analysis (PCA) and hierarchical cluster analysis (HCA). In RPLC, a pentabromobenzyl (PBrBz) stationary phase was identified as particularly versatile and could constitute an elegant first intention screening column. Two additional conditions allowed to extend the range of natural compounds space that can be analyzed, while offering better selectivity for basic analytes (hybrid silica graft with C8 moiety operated at pH 9 (Hyb C8)) and acidic compounds (positively charged hybrid silica graft with pentafluorophenyl moiety (Hyb+ PFPh). Although less generic in terms of amenable compounds, an ion exchange/RP mixed mode stationary phase (MM TriP1) offered notably enhanced retention of more polar analytes under RPLC conditions. With these four conditions, 89% of the natural substances were detected by LC–MS with acceptable retentions and peak shapes. In HILIC, four acceptable and complementary conditions were also highlighted. Both Syncro-Z (zwitterionic HILIC phase) and Diol columns were found to offer balanced retention and selectivity for most of the polar compounds (log DpH3<1.0). These two columns could be advantageously complemented by hybrid Amide column operated at pH 3 and Amino stationary phase at pH 5, to further enhance both retention and selectivity of polar basic and acidic species, respectively.

Sensitive surface enhanced Raman spectroscopy (SERS) detection of methotrexate by core-shell-satellite magnetic microspheres

An effective approach for fabricating core-shell-satellite magnetic microspheres was developed to form a sub-10nm positively charged mesoporous silica (mSiO2) interlayer, which was applied to enrich samples and create sufficient hotspots between the inner Fe3O4@Ag core and outer assembled gold nanoparticles (AuNPs) satellites. Based on the prepared Fe3O4@Ag/mSiO2/AuNPs (FASA) as SERS substrates, a sensitive and label-free detection of methotrexate in serum was successfully realized. The positively charged mSiO2 interlayer exhibited strong capability in enriching methotrexate through the electrostatic interaction. The intraparticle plasmonic coupling between the inner Ag shell and the outside AuNPs satellites, and magnetism-induced aggregation greatly enhanced the SERS activity of methotrexate. The diluted serum containing different concentrations of methotrexate was employed as real samples and the minimum detected concentration of methotrexate was as low as 1nM with a linear response range from 1 to 200nM. This sensitive and label-free SERS detection of methotrexate based on FASA offers great potential for practical applications in medicine and biotechnology.

Electrostatic assembly of zwitterionic and amphiphilic supraparticles

We demonstrate the electrostatic assembly of oppositely charged silica particles into an ensemble of well-defined core-satellite supraparticles, which are a type of patchy particle. To achieve controlled heteroaggregation, we used oppositely charged silica particles with different sizes ranging from 5nm to 150nm at several concentrations. The assembly works best with larger particles, resulting in a fairly low polydispersity and a low amount of bridging between the individual clusters. Using smaller particles produces high polydispersity, large clusters and uncontrolled aggregation and bridging. Furthermore, even with controlled aggregation into well-defined clusters in the case of bigger particles, we observe an uneven covering of the central particles with around 1–6 satellite particles adsorbed to the central particle. This behavior is not predicted by simple pairwise DLVO potentials which would anticipate an even spacing of the satellite particles on the core. We explain these observations by taking into account the interactions of the adsorbing particles within the ionic cloud of the central particle. We hypothesize that when the adsorbing satellite particles are small compared to the diameter of the ion cloud of the core particle, they aggregate within the ion cloud and therefore do not create a well-defined monolayer on the surface of the core particle, instead forming small agglomerates during adsorption.Finally, both the assembled zwitterionic clusters and clusters that were partially hydrophobized were tested for their capabilities as Pickering emulsifiers. The zwitterionic clusters showed a strongly increased surface activity compared to the individual particles, while the hydrophobized particles changed the emulsion type from w/o to o/w. Interfacial dilatational rheological tests supported the observations from the emulsion tests. With this, we demonstrate that a relatively unordered ensemble of supraparticles is able to show well-defined functionality at a higher hierarchical level as Pickering emulsifiers.

Relating Silica Scaling in Reverse Osmosis to Membrane Surface Properties

We investigated the relationship between membrane surface properties and silica scaling in reverse osmosis (RO). The effects of membrane hydrophilicity, free energy for heterogeneous nucleation, and surface charge on silica scaling were examined by comparing thin-film composite polyamide membranes grafted with a variety of polymers. Results show that the rate of silica scaling was independent of both membrane hydrophilicity and free energy for heterogeneous nucleation. In contrast, membrane surface charge demonstrated a strong correlation with the extent of silica scaling (R2 > 0.95, p < 0.001). Positively charged membranes significantly facilitated silica scaling, whereas a more negative membrane surface charge led to reduced scaling. This observation suggests that deposition of negatively charged silica species on the membrane surface plays a critical role in silica scale formation. Our findings provide fundamental insights into the mechanisms governing silica scaling in reverse osmosis and highlight the potential of membrane surface modification as a strategy to reduce silica scaling.

Bovine Serum Albumin Adsorption at a Silica Surface Explored by Simulation and Experiment.

Molecular details of BSA adsorption on a silica surface are revealed by fully atomistic molecular dynamics (MD) simulations (with a 0.5 μs trajectory), supported by dynamic light scattering (DLS), zeta potential, multiparametric surface plasmon resonance (MP-SPR), and contact angle experiments. The experimental and theoretical methods complement one another and lead to a wider understanding of the mechanism of BSA adsorption across a range of pH 3-9. The MD results show how the negatively charged BSA at pH7 adsorbs to the negatively charged silica surface, and reveal a unique orientation with preserved secondary and tertiary structure. The experiments then show that the protein forms complete monolayers at ∼ pH6, just above the protein's isoelectric point (pH5.1). The surface contact angle is maximum when it is completely coated with protein, and the hydrophobicity of the surface is understood in terms of the simulated protein conformation. The adsorption behavior at higher pH > 6 is also consistently interpreted using the MD picture; both the contact angle and the adsorbed protein mass density decrease with increasing pH, in line with the increasing magnitude of negative charge on both the protein and the surface. At lower pH < 5 the protein starts to unfold, and the adsorbed mass dramatically decreases. The comprehensive picture that emerges for the formation of oriented protein films with preserved native conformation will help guide efforts to create functional films for new technologies.

Nanomechanics of pH-Responsive, Drug-Loaded, Bilayered Polymer Grafts.

Stimuli-responsive polymer films play an important role in the development of smart antibacterial coatings. In this study, we consider complementary architectures of polyelectrolyte films, including a thin chitosan layer (CH), poly(acrylic acid) (PAA) brushes, and a bilayer structure of CH grafted to PAA brushes (CH/PAA) as possible candidates for targeted drug delivery platforms. Atomic force microscopy (AFM) was employed to study the structure-mechanical property relationship for these mono- and bi-layered polymer grafts at pH 7.4 and 4.0, corresponding to physiological and biofilm formation conditions, respectively. Herein, the surface interactions between polymer grafts and the negatively charged silica colloid attached to an AFM lever are considered as representative interactions between the antibacterial coating and a bacteria/biofilm. The bilayered structure of CH/PAA showed significantly reduced adhesive interactions in comparison to pure CH but slightly higher interactions in comparison to PAA films. Among PAA and CH/PAA films, upon grafting CH over the PAA brushes, the normal stiffness increased by 10-fold at pH 7.4 and 20-fold at pH 4.0. Notably, the study also showed that the addition of an antibiotic drug such as multicationic Tobramycin (TOB) impacts the mechanical properties of the antibacterial coatings. Competition between TOB and water molecules for the PAA chains is shown to determine the structural properties of PAA and CH/PAA films loaded with TOB. At high pH (7.4), the TOB molecules, which remain multicationic, strongly interact with polyanionic PAA, thereby reducing the film's compressibility. On the contrary, at low pH (4.0), the water molecules preferentially interact with TOB in comparison to uncharged PAA chains and, upon TOB release, results in a stronger film collapse together with an increase in adhesive interactions between the probe, the surface, and the elastic modulus of the film. The bacterial proliferation on these platforms when compared to the measured mechanical properties shows a direct correlation; hence, understanding nanomechanical properties can provide insights into designing new antibacterial polymer coatings.

Confined Assemblies of Colloidal Particles with Soft Repulsive Interactions.

We investigate the microconfinement of charged silica nanoparticles dispersed in refractive index matching monomers in poly(dimethylsiloxane) (PDMS) porous membrane. Here, the silica colloidal particles interact with each other and the pore wall via electrostatic double layer forces. Different from the hard sphere systems where the assembled morphologies are prescribed by the diameter ratio between the cylindrical confinement and the nanoparticles, here we observe a much richer variety of assemblies that are highly sensitive to both bulk and local nanoparticle concentration with fixed particle size and channel size. The experimentally observed assembly morphologies are consistent with theoretical predictions from the literature, based on Yukawa potential in the low packing density regime. Also, most of the configurations found in the experiment are well described by computer simulations using pairwise additive long-range repulsive interactions, demonstrating the ability to control the system to obtain a desired structure.

Scrutinizing Electro-Osmosis and Surface Conductivity with Molecular Dynamics

We present a simulation and modeling study of electro-osmotic flow of an aqueous cesium chloride solution confined in a charged amorphous silica slot. Contrasting traditional models of the electric double layer, molecular dynamics simulations indicate that there is no stagnant layer, no Stern layer conduction, and no outer Helmholtz layer. The description of the interface requires two considerations. First, a distinction has to be made between free and surface-bonded ions. The latter do not form a physical layer but rather a set of ion–surface contact pairs. Second, the mobility of the free ions is reduced relative to their bulk value. This hydrodynamic effect needs to be included. These two concepts, coupled to simple macroscopic equations, are sufficient to describe surface conductivity and electro-osmotic flow in the frame of classical mean-field treatment. We show that surface conduction is negative at high concentration, and the Bikerman formula is only valid at low concentration.

Fabrication of Ellipsoidal Silica Yolk-Shell Magnetic Structures with Extremely Stable Au Nanoparticles as Highly Reactive and Recoverable Catalysts.

A novel strategy was reported for the fabrication of yolk-shell magnetic MFSVmS-Au nanocomposites (NCs) consisting of double-layered ellipsoidal mesoporous silica shells, numerous sub-4 nm Au nanoparticles (NPs), and magnetic Fe central cores. The hierarchical FSVmS NCs with ellipsoidal α-Fe2O3@mSiO2/mSiO2 as yolks/shells were first prepared through the facile sol-gel template-assisted method, and plenty of extremely stable ultrafine Au NPs were postencapsulated within interlayer cavities through the unique deposition-precipitation method mediated with Au(en)2Cl3 compounds. Notably, ethylenediamine ligands were used to synthesize the stable cationic complexes, [Au(en)2]3+, that readily underwent the deprotonation reaction to chemically modify negatively charged mesoporous silica under alkaline conditions. The subsequent two-stage programmed hydrogen annealing initiated the in situ formation of Au NPs and the reduction of α-Fe2O3 to magnetic Fe, where the synthesized Au NPs were highly resistant to harsh thermal sintering even at 700 °C. Given its structural superiority and magnetic nature, the MFSVmS-Au was demonstrated to be a highly efficient and recoverable nanocatalyst with superior activity and reusability toward the reduction of 4-nitrophenol to 4-aminophenol, and the pristine morphology was retained after six recycling tests.