Acid-functionalized Silica Nanoparticles Research Articles

The preparation of a boronate affinity-based controlled oriented imprinting coating on a silica nanoparticle surface for the separation and purification of shikimic acid in herbal medicine.

Shikimic acid (SA) is one of the most effective drugs against the A (H1N1) virus and has high medicinal value. Additionally, it has the ability to generate non-toxic herbicides and antimicrobial medications. The extraction from plants has proven to be the main route of production of SA with economic benefits and environmental efficiency. Therefore, it is necessary to perform purification of SA from these herbal medicines before quantifying it. In this study, researchers employed a boronate affinity-based controlled oriented surface imprinting technique to produce molecularly imprinted polymers (MIPs) as highly effective solid phase extraction (SPE) adsorbents for the isolation and purification of SA. 3-Fluoro-4-formylphenylboronic acid functionalized silica nanoparticles were used as supporting materials for immobilizing SA. Poly(2-anilinoethanol) with a higher hydrophilic domain can be used as an effective imprinting coating. The prepared SA-imprinted silica nanoparticles exhibited several significant results, such as good specificity, high binding capacity (39.06 ± 2.24 mg g-1), moderate binding constant (6.61 × 10-4 M-1), fast kinetics (8 min) and low binding pH (pH 5.0) toward SA. The replication of SA-imprinted silica nanoparticles was deemed satisfactory. The SA-imprinted silica nanoparticles could be still reused after seven adsorption-desorption cycles, which indicated high chemical stability. In addition, the recoveries of the proposed method for SA at three spiked level analysis in star aniseed and meadow cranesbill were 96.2% to 109.0% and 91.6% to 103.5%, respectively. The SA-imprinted silica nanoparticles that have been prepared are capable of identifying the target SA in real herbal medicines. Our approach makes sample pre-preparation simple, fast, selective and efficient.

EDTA-functionalized silica nanoparticles as a conditioning agent for dentin bonding using etch-and-rinse technique

ObjectiveThis study investigated the possibility of using ethylenediaminetetraacetic acid functionalized silica nanoparticles (EDTA-SiO2) as a dentin-conditioning agent using etch-and-rinse technique to promote the durability of dentin bonding. MethodsThe SiO2-EDTA were synthesized by N- [(3- trimethoxysilyl) propyl] ethylenediamine triacetic acid (EDTA-TMS) and SiO2 (50 nm), then characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The capacity of SiO2-EDTA to chelate calcium ions from dentin was examined by inductively coupled plasma-optic emission spectrometry (ICP-OES). The dentin surfaces conditioned with SiO2-EDTA were detected by field emission scanning electron microscopy (SEM), TEM and microhardness testing. For dentin bonding, dentin surfaces were adopted wet- or dry-bonding technique and bonded with adhesive (AdperTM Single Bond2) and applied composite resin (Filtek Z350) on them. The durability of dentin bonding was evaluated by mircotensile bond strength test, in-situ zymography and nanoleakage testing. ResultsFTIR, TGA and XPS results showed that SiO2-EDTA contained N element and carboxyl groups. SEM, TEM and microhardness results indicated that SiO2-EDTA group created extrafibrillar demineralization and retained more intrafibrillar minerals within dentin surface. In the dentin bonding experiment, SiO2-EDTA group achieved acceptable bond strength, and reduced the activity of matrix metalloproteinase and nanoleakage along bonding interface. ConclusionIt was possible to generate a feasible dentin conditioning agent (SiO2-EDTA), which could create dentin extrafibrillar demineralization and improve dentin bond durability. Clinical significanceThis study introduces a new dentin conditioning scheme based on SiO2-EDTA to create extrafibrillar demineralization for dentin bonding. This strategy has the potential to be used in clinic to promote the life of restoration bonding.

Highly active organosulfonic aryl-silica nanoparticles as efficient catalysts for biomass derived biodiesel and fuel additives

Post-grafting methodologies were used to prepare aryl-organosulfonic acid functionalized silica nanoparticles (SO3H-aryl-SiO2NPs) as highly active heterogeneous acid catalysts for catalytic esterification of free fatty acids (FFA) and levulinic acid (LA), and aldol condensation of furfural with 3-methylfuran, well known renewable feedstocks for biodiesel and fuel additives production. The chemical and structural characterization of the SO3H-aryl-SiO2NPs confirmed the success of the functionalization. Complete conversion for all FFA was observed using SO3H-aryl-SiO2NPs after 2 h of reaction. SiO2NPs_CSPTMS showed to be the most active and stable catalyst with no significant loss of activity in the first 5 cycles. In the LA esterification in the presence of an alcohol, complete conversion and selectivity and high reusability were observed (e.g., 83.2% yield after 10 cycles in the presence of 1-propanol, SiO2NPs10_CSPTMS, at 120 °C and 2h reaction). Finally, aldol condensation of furfural and 3-methylfuran had been performed using SiO2NPs_CSPTMS catalysts with almost 100% yield after 10 min at 65 °C, high stability and reusability for 7 catalytic cycles.In general, the prepared SO3H-aryl-SiO2NPs have demonstrated promising catalytic performance for transformation of biomass-derived feedstocks into biodiesel and fuel additives.

Nanoparticles as vectors for antibiotic resistance: The association of silica nanoparticles with environmentally relevant extracellular antibiotic resistance genes

A relevant but yet unconsidered subset of particles that may alter the fate of extracellular antibiotic resistance genes (eARGs) are nano-scale particles (NPs), which are ubiquitous in natural environments and have unique properties. In this study, sorption isotherms were developed describing the association of linear DNA fragments isolated from widespread eARGs (blaI and nptII) with either micon-sized kaolinite or silica nanoparticles (SNPs), to determine if sorption capacity was enhanced at the nanoscale. For each isotherm, eARG fragments were added at five starting concentrations (5–40 μg/mL) to mixed batch systems with 0.25 g of particles and nuclease-free water. Sorption was quantified by the removal of DNA from solution, as detected by a Qubit fluorimeter. Isotherms were developed for eARGs of various fragment lengths (508, 680 and 861 bp), guanine-cytosine (GC) contents (34%, 47% and 54%) and both double and single stranded eARGs, to assess the impact of DNA properties on particle association. Sorption isotherms were also developed in systems with added humic acid and/or CaCl2, to assess the impact of these environmental parameters on sorption. FTIR analysis was performed to analyze the conformation of sorbed eARGs. Desorption of eARGs was studied by quantifying the removal of eDNA from washed and vortexed post-sorption particles. Statistically significant irreversible sorption of eARGs to environmentally relevant NPs (humic acid functionalized silica nanoparticles) was demonstrated for the first time. Nano-emergent properties did not increase sorption capacity of eARGs, but led to a unique compressed conformation of sorbed eARGs. The addition of humic acid, increased CaCl2 concentration and small DNA fragment size favored sorption. NPs showed a slight preference for the sorption of single-stranded DNA over double-stranded DNA. These findings suggest that NP association with eARGs may be a significant and unique environmental phenomenon that could influence the spread of antibiotic resistance.

Composite proton conducting membranes from chitosan, poly(vinyl alcohol) and sulfonic acid-functionalized silica nanoparticles

Composite proton conducting membranes were successfully synthesized from chitosan, poly (vinyl alcohol) and sulfonic acid-functionalized silica nanoparticles. Sulfosuccinic acid (SSA) and glutaraldehyde were used as double crosslinking agents, where the effect of SSA content on membrane properties, including water vapor absorption, water uptake, ion exchange capacity, and proton conductivity was investigated and were found to increase as a function of SSA loading. The most promising membrane was then formed into a composite with either silica nanoparticles containing poly (2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS-Si) or poly (styrene sulfonic acid) (PSSA-Si). PAMPS-Si and PSSA-Si were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The effects of nanoparticle loading and polymer shell on membrane properties were studied. Proton conductivity increased at higher nanoparticle loadings, and reached a maximum of 3.8–3.9 × 10−3 S/cm at 20% loading. The influence of polymer shell on membrane properties was not significantly observed.

PH-Responsive porphyrin-silica nanoparticles conjugate via ionic self-assembly

Ionic self-assembly (ISA) is a powerful tool that has been exploited to create various functional nanomaterials through the electrostatic interactions of different building blocks. Herein, we disclose for the first time the synthesis and characterization of benzoic acid functionalized silica nanoparticles (MSNs-Bn) and its subsequent ISA with meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP) to form a pH responsive hybrid material. The resulting pH responsive conjugate is an attractive candidate as a drug delivery vehicle in which the MSNs-Bn act as a drug carrier and the TMPyP act as a capping agent for the silica nanopores. The application of these pH-responsive interactions between the negatively charged carboxylate groups on the surface of mesoporous silica nanoparticles and the positively charged porphyrin as a drug delivery system was investigated by studying the loading and release of Hoechst 33342 dye (a water-soluble biological stain with similar size to those of therapeutic drugs) in response to a pH change. The reported approach enables the delivery of a chemotherapeutic drug in addition to the TMPyP, which is also well-known as a photodynamic therapy (PDT) agent, thus paving the way for synergistic chemo-photodynamic cancer therapy. The study shows that the resulting conjugate demonstrate the drug delivery mechanism which is responsive to changing the pH values from 7.5 to 4.5, and can deliver about 2 wt% of the chemotherapeutic drug tested.

Influence of casting substrate on bulk morphology and vanadium ion transport in ionomer nanocomposites

Perfluorosulfonic acid (PFSA) ionomer nanocomposites are a promising solution to address the poor ion selectivity of current membranes utilized in vanadium redox flow batteries. Herein, we investigate the impact of a casting substrate on the nanostructure and vanadium ion transport in bulk ionomer and ionomer nanocomposite membranes (i.e., films with thicknesses of ∼100 μm). Specifically, solution-cast ionomer nanocomposite membranes, containing either unfunctionalized (hydroxyl groups), amine-functionalized, or sulfonic acid-functionalized silica nanoparticles (SiNPs), were fabricated by casting on either a polished quartz or polytetrafluoroethylene (PTFE) substrates. Surprisingly, the choice of the casting substrate was seen to affect the bulk morphology of the PFSA ionomers, resulting in substrate-specific vanadium ion transport, where suppressed ion transport was observed for membranes cast on the polished quartz, when compared to their PTFE-cast counterparts. Additionally, the chemical composition of the substrate-adjacent surface was a function of both the substrate and the surface functionality of the SiNPs. Moreover, it was observed that both the chemical composition of the membrane surface and the substrate-induced changes to the bulk ionomer morphology governed vanadyl ion transport through the PFSA ionomers. Results from this work have direct implications for the design of next-generation ionomer nanocomposites, as the casting substrate used to fabricate these materials, and the orientation of these membranes inside the operating flow battery, can significantly influence transport of vanadium ions.

Phenylboronic acid functionalized silica nanoparticles with enlarged ordered mesopores for efficient insulin loading and controlled release

Mesoporous silica particles have been extensively used to construct various drug delivery systems. An optimized design of pore size, surface functionalization and pore structure, is still needed for further application, especially for use as biomacromolecules carriers. Herein, one intelligent glucose-responsive insulin delivery system Insulin@ZnO-P@G-MSP was constructed based on mesoporous silica nanoparticles with large ordered mesopores (>5 nm) and small particle sizes (∼150 nm), combined with phenylboronic acids as glucose-responsive units and zinc oxide nanoparticles as the capping agents. The release characteristics of this system showed a positive correlation with the species and concentration of the saccharides and could be switched on and off by adjusting the glucose concentration. The stable release rate in the serum and good biocompatibility with liver cells illustrated the feasibility of this system in further biological applications. This work demonstrates an innovative approach to the development of a biomolecules delivery system by introducing mesoporous silica particles with large ordered mesopores and nanometer-scale particle sizes.

Preparation and application of phosphinic acid functionalized nanosilica for the effective removal of mercury (II) in aqueous solutions

A novel adsorbent was synthesized by functionalizing nanosilica with phosphinic acid groups for the removal of mercury ions from aqueous solutions. The synthesized absorbent was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and thermogravimetric analysis (TGA). Meanwhile, the effects of initial pH, contact time and initial mercury ions concentration on the removal of mercury ions from aqueous solutions were investigated by batch adsorption experiments. The results showed that the removal rate reached 99.11% and the maximum adsorption capacity was up to 274.32 mg/g at pH 2 and the equilibrium time of adsorption was about 1 h. The absorbent presented good selectivity for mercury ions among Zn(II), Ni(II), Mn(II) and Ge(IIII). Moreover, the obtained adsorbent has good reusability and the absorbability decreased only from 98.76 to 94.75% after five cycles. Besides, the adsorption isotherms fitted well to the Langmuir isotherm model and adsorption kinetics followed the pseudo-second-order model. The removal mechanism of mercury via phosphinic acid-functionalized silica nanoparticles was chelation interaction. All the experimental data indicated that the phosphinic acid-functionalized silica nanoparticles are very promising in removal of mercury ions from aqueous solutions.

Branched polyethyleneimine-assisted boronic acid-functionalized silica nanoparticles for the selective enrichment of trace glycoproteins

Boronate affinity materials have attracted more and more attention in extraction, separation and enrichment of glycoproteins due to the important roles that glycoproteins take on in recent years. However, conventional boronate affinity materials suffer from low binding affinity mainly because of the use of single boronic acids. This makes the extraction of glycoproteins of trace concentration become rather difficult or impossible. Here we present a novel boronate avidity material, polyethyleneimine (PEI)-assisted boronic acid-functionalized silica nanoparticles (SNPs). Branched PEI was applied as a scaffold to amplify the number of boronic acid moieties. While 3-carboxybenzoboroxole, exhibiting high affinity and excellent water solubility toward glycoproteins, was used as an affinity ligand. Due to the PEI-assisted synergistic multivalent binding, the boronate avidity SNPs exhibited strong binding strength toward glycoproteins with dissociation constants of 10−7 M, which was the highest among reported boronic acid-functionalized materials that can be applied for glycoproteomic analysis. Such a high avidity enabled the selective extraction of trace glycoproteins as low as 0.4 pg/mL. This feature greatly favored the selective enrichment of trace glycoproteins from real samples. Meanwhile, the boronate avidity SNPs was tolerant of the interference of abundant sugars. In addition, the PEI-assisted boronate avidity SNPs exhibited high binding capacity and low binding pH. The feasibility for practical applications was demonstrated with the selective enrichment of trace glycoproteins in human saliva.

Sulfonic acid functionalized silica nanoparticles as catalysts for the esterification of linoleic acid

Sulfonic acid functionalized SiO2-nanoparticles showed excellent performance in linoleic acid esterification: 100% conversion, 2 h reaction time, TOF = 53–498 h−1.

Preparation, characterization and properties of PVDF-g-PAMPS/PMMA-co-PAMPS/silica nanoparticle as a new proton exchange nanocomposite membrane

In this study, preparation and characterization of PVDF/PMMA-co-PAMPS/silica nanocomposite membranes as a new proton exchange membrane were investigated. Polyvinylidene fluoride (PVDF) containing pendant sulfonic acid groups was synthesized by grafting through RAFT polymerization of 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) from vinyl pendent groups of already modified PVDF as a macro-monomer. The synthesized PVDF graft copolymer (PVDF-g-PAMPS) was blended with miscible copolymer containing sulfonated segments (PMMA-co-PAMPS) and sulfonic acid functionalized silica nanoparticles in order to achieve high performance nanocomposite proton exchange membrane. It was found that the addition of PMMA-co-PAMPS copolymer resulted in a significant increase in porosity and slight reduction in crystallinity of the nanocomposite membranes, which favored the water uptake and proton transport at ambient temperature. Both the water uptake and the ion-exchange capacity (IEC) of the nanocomposite membranes were increased by increasing silica nanoparticles loading. The maximum proton conductivity was 20mS/cm at 25°C for the nanocomposite membrane containing 10% silica nanoparticles. Also, this nanocomposite membrane showed a power density as high as 34.3mW/cm2 at peak current density of 140mA/cm2. The thermal and mechanical properties of these nanocomposite membranes were also studied.

Silica nanoparticles for the oriented encapsulation of membrane proteins into artificial bilayer lipid membranes.

An artificial bilayer lipid membrane system is presented, featuring the oriented encapsulation of membrane proteins in a functionally active form. Nickel nitrilo-triacetic acid-functionalized silica nanoparticles, of a diameter of around 25 nm, are used to attach the proteins via a genetically engineered histidine tag in a uniform orientation. Subsequently, the proteins are reconstituted within a phospholipid bilayer, formed around the particles by in situ dialysis to form so-called proteo-lipobeads (PLBs). With a final size of about 50 nm, the PLBs can be employed for UV/vis spectroscopy studies, particularly of multiredox center proteins, because the effects of light scattering are negligible. As a proof of concept, we use cytochrome c oxidase (CcO) from P. denitrificans with the his tag genetically engineered to subunit I. In this orientation, the P side of CcO is directed to the outside and hence electron transfer can be initiated by reduced cytochrome c (cc). UV/vis measurements are used in order to determine the occupancy by CcO molecules encapsulated in the lipid bilayer as well as the kinetics of electron transfer between CcO and cc. The kinetic data are analyzed in terms of the Michaelis-Menten kinetics showing that the turnover rate of CcO is significantly decreased compared to that of solubilized protein, whereas the binding characteristics are improved. The data demonstrate the suitability of PLBs for functional cell-free bioassays of membrane proteins.

A comparative photophysicochemical study of mono substituted phthalocyanines grafted onto silica nanoparticles

In this study, we report on the covalent linking of carboxylic acid functionalized silica nanoparticles with zinc phthalocyanine mono-substituted non-peripherally and peripherally with either a 4-amino phenoxy (1, peripheral and 2, non-peripheral) or an amino group (3 peripheral). The grafting is achieved via the formation of an amide bond between the carboxylic acid of the silica nanoparticles and the amino group of the phthalocyanine complexes. The hybrid nanoparticles retained the amorphous nature of silica nanoparticles after conjugation. A slight decrease in fluorescence and a general improvement in triplet quantum yields compared to free Pcs were observed. Triplet lifetimes for 2- SiNPs and 3- SiNPs also improved when compared to the free phthalocyanine. The changes in singlet oxygen quantum yields upon conjugation were minimal.

Fabrication of a novel laccase biosensor based on silica nanoparticles modified with phytic acid for sensitive detection of dopamine

The development of novel nanomaterials for the ultra-sensitive detection of biological species has received great attention. Herein we report the design and synthesis of phytic acid functionalized silica nanoparticles (SiO2-PA NPs) and their biomedical sensor application for sensitive detection of dopamine. The SiO2-PA NPs were characterized by transmission electron microscopy (TEM) and Energy dispersive X-ray spectrometer (EDS). The biocompatibility between SiO2-PA NPs and laccase was also investigated by circular dichroism (CD) spectra. Moreover, the novel laccase biosensor based on SiO2-PA NPs for sensitive detection of dopamine (DA) was fabricated, and the electrochemical properties of this laccase biosensor were investigated. The results showed the biosensor had good electrocatalytic activity toward DA with a wide linear range (0.99–103.10μM) and a low detection limit 0.26±0.003μM. It can be attributed to the good biocompatibility and nano-effect of SiO2-PA NPs. The biosensor was successfully tested for determination of DA in pharmaceutical and rabbit blood serum samples. The development of biomimetic materials science will offer a novel platform for application to substance detection.

Biocompatible phosphonic acid-functionalized silica nanoparticles for sensitive detection of hypoxanthine in real samples

A novel hypoxanthine biosensor fabricated by immobilizing the xanthine oxidase (XOD) onto the phosphonic acid-functionalized silica (SiO2-P) film on the surface of glassy carbon electrode (GCE) was designed and constructed in this work. A biomimetic platform was designed with the phosphonic acid-functionalized silica nanoparticles (SiO2-P NPs) synthesized by the method of reverse microemulsion and electrostatic binding. In such a platform, XOD was selected as model protein to fabricate hypoxanthine biosensor based on SiO2-P NPs. The nanocomposite was characterized with transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDS) and electrochemical impedance spectroscopy (EIS). Based on the advantageous functions of SiO2-P NPs, the entrapped XOD could preserve its bioactivity and exhibited an excellent electrochemical behavior with a formal potential of -0.37 V in phosphate buffer solution (PBS, pH=7). Response studies to hypoxanthine were carried out using current-time response curve. The biosensor exhibited a wide linear response ranging from 1.00×10(-6) to 2.61×10(-4) M. The detection limit of 2.33×10(-7) M at a signal-to-noise ratio of 3 was lower than that most reported previously. In addition, the electrode modified with XOD/(SiO2-P NPs) film also had a strong anti-interference ability in the presence of uric acid (UA) and ascorbic acid (AA). The assay results of hypoxanthine in fish samples were in a good agreement with the reference values.

A novel glucose biosensor based on phosphonic acid-functionalized silica nanoparticles for sensitive detection of glucose in real samples

An effective strategy for preparation amperometric biosensor by using the phosphonic acid-functionalized silica nanoparticles (PFSi NPs) as special modified materials is proposed. In such a strategy, glucose oxidase (GOD) was selected as model protein to fabricate glucose biosensor in the presence of phosphonic acid-functionalized silica nanoparticles (PFSi NPs). The PFSi NPs were first modified on the surface of glassy carbon (GC) electrode, then, GOD was adsorbed onto the PFSi NPs film by drop-coating. The PFSi NPs were characterized by transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectra. The interaction of PFSi NPs with GOD was investigated by the circular dicroism spectroscopy (CD). The results showed PFSi NPs could essentially maintain the native conformation of GOD. The direct electron transfer of GOD on (PFSi NPs)/GCE electrode exhibited excellent electrocatalytic activity for the oxidation of glucose. The proposed biosensor modified with PFSi NPs displayed a fast amperometric response (5s) to glucose, a good linear current–time relation over a wide range of glucose concentrations from 5.00×10−4 to 1.87×10−1M, and a low detection limit of 2.44×10−5M (S/N=3). Moreover, the biosensor can be used for assessment of the concentration of glucose in many real samples (relative error<3%). The GOD biosensor modified with PFSi NPs will have essential meaning and practical application in future that attributed to the simple method of fabrication and good performance.

Flowability and Mechanical and Thermal Properties of Nylon 6/Ethylene bis-Stearamide/Carboxylic Silica Composites

Nylon 6 (PA 6)/ethylene bis-stearamide (EBS)/SiO2- carboxylic acid-functionalized silica nanoparticles (COOH) composites were prepared by in-situ polymerization of caprolactam. SiO2-COOH was used to enhance the compatibility between SiO2 and PA 6 matrix. For comparison, pure PA 6 and PA 6/EBS composites were also prepared via the same method. The PA 6/EBS/SiO2-COOH composites with low content of EBS and SiO2-COOH had greater melt-flow index (MFI) (the value of MFI increased by 50%–80%) than the pure PA 6. The results of mechanical properties showed almost no decrease in the tensile strength of PA 6/EBS/SiO2-COOH composites, with the bending strength decreasing by 17%–21%. However, the Izod impact strength of the PA 6/EBS/SiO2-COOH composites was greatly improved compared with pure PA 6, which indicated that the toughness of PA 6/EBS/SiO2-COOH had been greatly improved. The morphology of Izod impacted fractured surfaces of PA 6/EBS/SiO2-COOH was observed by scanning electron microscopy. The results revealed that the PA 6/EBS/SiO2-COOH composites presented a typical ductile fracture behavior with large amounts of long and large strip-like cracks. When the content of SiO2-COOH was 0.2 wt%, the SiO2-COOH particles were uniformly dispersed over the entire body of the PA 6 matrix. The results from differential scanning calorimetry indicated that the melting point (Tm), degree of crystallinity (Xc), and crystallization temperatures (Tc) of PA 6/EBS/SiO2-COOH composites were lower than the pure PA 6.

Application of structural analogs of dimercaptosuccinic acid-functionalized silica nanoparticles (DMSA-[silica]) to adsorption of mercury, cadmium, and lead

In order to gain additional insight into mercury selectivity with nano-sized DMSA-[silica], we investigated a series of ligands related to DMSA, these are: monomercaptosuccinic acid; MMSA, 2-mercapto-4-methyl-5-thiazoleacetic acid; MCT, ortho-thiosalicylic acid; o-TSA and para-thiosalicylic acid; p-TSA. The MMSA chelate is structurally similar to DMSA except that it has only one thiol group. The chelates o-TSA and p-TSA each have one thiol and one carboxylic acid group. MCT includes neutral S and N atoms in close proximity to the thiolate binding site. MCT, o-TSA and p-TSA resemble each other in having equal numbers of carboxylic acid and thiol groups and formation of amide bonds with the linker on silica is expected to eliminate the carboxylate binding sites, making thiolates the only binding sites for Hg(II), Cd(II), and Pb(II) metals ions in the nano-sized; MMSA-[silica], MCT-[silica], o-TSA-[silica], and p-TSA-[silica]. Each of the nano-sized MMSA-[silica], MCT-[silica], o-TSA-[silica], and p-TSA-[silica], show a higher preference for Hg(II) over Cd(II) and Pb(II) compared to the same free chelates in solution, respectively. In addition, there are differences in the level of metal ion chelation for each functionalized silica nanoparticle. These differences in the degree of metal chelation for each functionalized silica nanoparticles surface are explained by the difference in thiolate/carboxylate ratio upon attachment to the surface and on steric reasons based on the orientation of the thiol groups on the surface. When attached via amide bond formation, the thiolate site in o-TSA-[silica] will face towards the silica surface, while for p-TSA-[silica], the thiolate site is expected to be pointed outwards and away from the silica surface. In comparing MMSA-[silica] to DMSA-[silica], the thiolate/carboxylate ratio decreases from 2/1 in DMSA-[silica] to 1/1 in MMSA-[silica] (assuming attachment via one amide bond in each case). This effect of increasing the ratio of thiolate to carboxylate upon attachment to the surface is believed to play a role in the selectivity enhancement towards Hg(II) over Cd(II) and Pb(II).

Preparation and self-assembly of carboxylic acid-functionalized silica

A simple method for the fabrication of silica nanoparticle film based on the covalent-bonding interaction between carboxylic acid-functionalized silica nanoparticles (SiO 2–COOH) and amino-terminated silicon wafer was developed. Prior to assembly, silica nanoparticles with an average diameter 80 nm were prepared using the Stöber method, amino-functionalized silica nanoparticles (SiO 2–NH 2) were prepared by a silanization with 3-aminopropyltriethoxysilane (APTES), while carboxylic acid-functionalized silica nanoparticles (SiO 2–COOH) were prepared by a ring opening linker elongation reaction of the amine functions with succinic anhydride, at the same time, amino-terminated silicon wafer (Si–NH 2) was obtained by self-assembling 3-aminopropyltriethoxysilane, then one layer relative close-packed carboxylic acid-functionalized silica nanoparticles (SiO 2–COOH) was arranged on silicon wafer through amidation reaction under DCC coupling agent.