Porous Silica Particles Research Articles

Towards Achieving Circular Economy in the Production of Silica from Rice Husk as a Sustainable Adsorbent

The growing concern over water pollution and waste management requires innovative solutions that promote resource efficiency within a circular economy. This study aims to utilize rice husk (RH) as a sustainable feedstock to develop highly porous silica particles and generate valuable by-products, addressing the dual challenges of waste reduction and water contamination. We hypothesize that optimizing the production of amorphous silica from acid-washed RH will enhance its adsorptive properties and facilitate the concurrent generation of bio-oil and syngas. Amorphous silica particles were extracted from acid-washed RH with a yield of 15 wt% using a combination of acid washing at 100 °C, pyrolysis at 500 °C, and calcination at 700 °C with controlled heating at 2 °C/min. The optimized material (RH2-SiO2), composed of small (60–200 nm) and large (50–200 µm) particles, had a specific surface area of 320 m2/g, with funnel-shaped pores with diameters from 17 nm to 4 nm and showed a maximum cadmium adsorption capacity of 407 mg Cd/g SiO2. Additionally, the pyrolysis process yielded CO-rich syngas and bio-oil with an elevated phenolic content, demonstrating a higher bio-oil yield and reduced gas production compared to untreated RH. Some limitations were identified, including the need for bio-oil upgrading, further research into the application of RH2-SiO2 for wastewater treatment, and the scaling-up of adsorbent production. Despite the challenges, these results contribute to the development of a promising adsorbent for water pollution control while enhancing the value of agricultural waste and moving closer to a circular economy model.

Layer-by-Layer Assembling and Capsule Formation of Polysaccharide-Based Polyelectrolytes Studied by Whispering Gallery Mode Experiments and Confocal Laser Scanning Microscopy

The layer-by-layer (LbL) assembling of oppositely charged polyelectrolytes was studied using semi-synthetic polysaccharide derivatives, namely the polycations 6-aminoethylamino-6-deoxy cellulose (ADC) and cellulose (2-(ethylamino)ethylcarbamate (CAEC), as well as the polyanion cellulose sulfate (CS). The synthetic polymers poly(allylamine) (PAH) and poly(styrene sulfonate) (PSS) were employed as well for comparison. The stepwise adsorption process was monitored by whispering gallery mode (WGM) experiments and zeta-potential measurements. Distinct differences between synthetic- and polysaccharide-based assemblies were observed in terms of the quantitative adsorption of mass and adsorption kinetics. The LbL-approach was used to prepare µm-sized capsules with the aid of porous and non-porous silica particle templates. The polysaccharide-based capsule showed a switchable permeability that was not observed for the synthetic polymer materials. At ambient pH values of 7, low-molecular dyes could penetrate the capsule wall while no permeation occurred at elevated pH values of 8. Finally, the preparation of protein-loaded LbL-capsules was studied using the combination of CAEC and CS. It was shown that high amounts of protein (streptavidin and ovomucoid) can be encapsulated and that no leaking or disintegration of the cargo macromolecules occurred during the preparation step. Based on this work, potential use in biomedical areas can be concluded, such as the encapsulation of bioactive compounds (e.g., pharmaceutical compounds, antibodies) for drug delivery or sensing purposes.

A symmetric chiral aromatic polyamide modified silica material combining rigid-flexible and application in liquid chromatography

The development of new separation materials is an effective way to improve the separation efficiency of chiral and achiral analytes. Here, a C3-symmetric chiral tricarboxylic acid compound was prepared and successfully immobilized onto the surface of porous silica particles by a one-pot synthesis strategy. The resulting material (TPHs-SiO2) was composed of symmetric chiral aromatic polyamide with a combination of rigid-flexibility, which gave it both chiral and hydrophilic interaction/ion exchange (HILIC/IEC) characteristics. TPHs-SiO2 as a new stationary phase for liquid chromatography can separate 1-phenyl-ethanol, 1-phenyl-propanol, 1-phenyl-1,2-ethylene glycol, 4-chloro-a-methylbenzyl alcohol, ofloxacin and mandelic acid in normal phase chromatography (NPLC) with resolutions of 2.69, 5.89, 4.49, 1.04, 1.73 and 5.47, respectively, thus indicating that TPHs-SiO2 has chiral resolution performance. TPHs-SiO2 can also achieve the separation of achiral compounds, such as nucleosides, nucleotides, and aniline compounds in HILIC/IEC mode. Therefore, the prepared stationary phase can be used as an ideal multifunctional separation material with chiral separation capability and mixed-mode separation capability (HILIC/IEC), realizing the separation of chiral compounds and different types of compounds. This work provides a new idea and method for developing multifunctional separation materials with multi-selectivity and high efficiency, that has important theoretical and application value.

Retention mechanisms of dipeptides on superficially porous particle vancomycin- and teicoplanin-based chiral stationary phases

Chromatographic behavior of new chiral stationary phases (CSPs) Chiral-T and Chiral-V with teicoplanin and vancomycin antibiotics grafted onto superficially porous silica particles was studied in relation to dipeptide (DP) stereoisomers. The unbuffered water–methanol solutions were used as mobile phases (MPs). The effects of physical properties and molecular structure of analytes and selectors on retention and separation of DP stereoisomers are discussed herein. Chiral-T was evinced to exhibit high enantioselectivity, with highest α values attaining 16.5, 18.8 and 20.4 for Gly-Leu, dd/ll-Phe-Leu and ld/dl-Ala-Ala. At this point, Chiral-V did not exhibit enantioselectivity towards DP stereoisomers. The effect of MP composition on retention and enantioseparation of DPs was investigated. Lipophilicity of DPs was found to be an essential factor in the dependence of their retention vs. methanol concentration in МPs. Lipophobic DPs were eluted more quickly by water-rich solvents, with lipophilic DPs exhibiting an asymmetric U-shaped, or a descending dependence of retention factor vs. the methanol percentage on Chiral-T or Chiral-V, respectively. A theoretical model taking into account interaction of both solvents of a binary MP with both an analyte and adsorption sites was successfully applied so as to approximate and interpret the dependences of DP retention (monotonic and U-shaped) vs. a modifier content in MP. Water molecules were evinced to predominantly participate in competitive adsorption with DP molecules. The model predicted better solvation of lipophilic DPs by methanol and better solvation of lipophobic DPs by water. An attempt was made to verify the possibility of modeling by molecular docking the processes occurring during interaction between DP stereoisomers and CSPs, including consideration of the influence of competitive binding of eluent molecules in selector cavity.

Feasibility and Advantages of Continuous Synthesis of Bioinspired Silica Using CO2 as an Acidifying Agent.

In this work, we present a method for the continuous synthesis of bioinspired porous silica (BIS) particles using carbon dioxide (CO2) as an acidifying agent. Typical BIS synthesis uses strong mineral acids (e.g., HCl) to initiate the hydrolysis and subsequent condensation reactions. The use of strong acids leads to challenges in controlling the reaction pH. The synthesis approach proposed in this work offers for the first time CO2 as an attractive alternative for the synthesis of BIS and demonstrates the continuous process. The developed method leverages the mild acidic and the self-buffering nature of the CO2 combined with additional options for controlling mass transfer rates to facilitate enhanced control of pH, which is crucial for controlling the properties of synthesized BIS. Proof of concept experiments conducted in continuous mode demonstrated a yield of over 70% and a surface area exceeding 500 m2/g. These results indicate the successful synthesis of BIS using CO2 with properties in the desired range. The enhanced pH control offered by this CO2-based process will facilitate the implementation of a sustainable and robust continuous process for BIS synthesis.

Analytical method development and validation for the simultaneous estimation of montelukast and Acebrophylline by RP- HPLC method

High performance liquid chromatography is at present one of the most sophisticated tool of the analysis. The estimation of Montelukast and Acebrophylline was done by RP-HPLC. The Phosphate buffer was pH 3.0 and the mobile phase was optimized with consists of Methanol: Phosphate buffer mixed in the ratio of 70:30 % v/ v. Inertsil C18 column C18 (4.6 x 150mm, 5mm) or equivalent chemically bonded to porous silica particles was used as stationary phase. The detection was carried out using UV detector at 260 nm. The solutions were chromatographed at a constant flow rate of 0.8 ml/min. the linearity range of Montelukast and Acebrophylline were found to be from 100-500 mg/ml of Montelukast and 1-5mg/ml of Acebrophylline . Linear regression coefficient was not more than 0.999.The values of % RSD are less than 2% indicating accuracy and precision of the method. The percentage recovery varies from 98-102% of Montelukast and acebrophylline. LOD and LOQ were found to be within limit. The results obtained on the validation parameters met ICH and USP requirements .it inferred the method found to be simple, accurate, precise and linear. The method was found to be having suitable application in routine laboratory analysis with high degree of accuracy and precision.

Surface-Area Enhanced Raman Spectroscopy of DNA in Porous Silica: A Quantitative and Reproducible Alternative to Plasmonic-Based SERS.

Despite the success of surface-enhanced Raman spectroscopy (SERS) for detecting DNA immobilized on plasmonic metal surfaces, its quantitative response is limited by the rapid falloff of enhancement with distance from the metal surface and variations in sensitivity that depend on orientation and proximity to plasmonic "hot spots". In this work, we assess an alternative approach for enhancing detection by immobilizing DNA on the interior surfaces of porous silica particles. These substrates provide over a 1000-fold greater surface area for detection compared to a planar support. The porous silica substrate is a purely dielectric material with randomly oriented internal surfaces, where scattering is independent of proximity and orientation of oligonucleotides relative to the silica surface. We characterize the quantitative response of Raman scattering from DNA in porous silica particles with sequences used in previous SERS investigations of DNA for comparison. The results show that Raman scattering of DNA in porous silica is independent of distance of nucleotides from the silica surface, allowing detection of longer DNA strands with constant sensitivity. The surface area enhancement within particles is reproducible (<4% particle-to-particle variation) owing to the uniform internal pore structure and surface chemistry of the silica support. DNA immobilization with a bis-thiosuccinimide linker provides a Raman-active internal standard for quantitative interpretation of Raman scattering results. Despite the high (30 mM) concentrations of immobilized DNA within porous silica particles, they can be used to measure nanomolar binding affinities of target molecules to DNA by equilibrating a very small number of particles with a sufficiently large volume of low-concentration solution of target molecules.

Exploring the performance of cellulose tris-3,5-dichlorophenylcarbamate as a stationary phase for the chiral electro-chromatographic separation of azole antifungals

In this study, the enantiorecognition and separation performance of cellulose tris-3,5-dichlorophenylcarbamate (CDCPC) as the chiral selector for the enantioseparation of some chiral antifungal compounds was investigated. The CDCPC was bonded to 5 µm porous silica particles and the stationary phase packed into fused silica capillaries (250 mm x 75 µm I.D.) using the "slurry method". The column was used for the separation of twelve selected azole compounds in their enantiomers by capillary electrochromatography (CEC). To study chiral recognition, the mobile phase was modified changing the organic solvent, water content, buffer type, concentration, and pH. Each change resulted in the enhancement of specific interactions at the expense of others, leading to chiral recognition. Their effects on retention factors, enantiomeric resolution, and peak broadening are critically discussed. The developed methodology was used as an appropriate test bench to evaluate the stability of the stationary phase under different mobile phase conditions, demonstrating a remarkable enantioselective capacity even after several tens of hours of continuous use. The optimized CEC-UV method was very effective in the enantiomeric separation of seven out of twelve azole fungicides, totaling fourteen separated peaks in a single 30 min run analysis.

Amine-functionalized porous silica production via ex- and in-situ method using silicate precursors as a selective adsorbent for CO2 capture applications

A comparison of the amine-modified silica particle’s characteristics via ex- and in-situ routes and their application as a CO2 gas adsorbent is reported. Modifying silica particles via ex-situ involves two separate steps: forming porous silica particles with sodium lauryl sulfate (SLS) as a template and impregnation using ultrasound assistance. In contrast to ex-situ modification, in-situ modification of silica particles is carried out in one step by mixing directly between the silica source and the modifying agent. Controlling the characteristics of modified silica particles via in-situ is carried out by adding an SLS template removed simultaneously with particle formation to increase the surface area and porosity. Increasing the SLS template concentration shows a linear relationship between increasing particle surface area and amine loading. However, two different modification routes exert a direct influence on aminopropyl distribution. Silanization via in-situ which involves a simultaneous condensation reaction produces a higher amine loading reaching 1.2845 mmol/g of silica than via ex-situ which is only 0.9610 mmol/g of silica. The amount of aminopropyl that can be grafted on the silica surface shows a linear relationship to the quantity of CO2 gas adsorption capacity. Amine-modified silica particles obtained the highest adsorption capability via the in-situ route with an SLS 3 CMC template of 2.32 mmol/g silica at an operating pressure of 6 bar.

In situ carbonization metamorphoses porous silica particles into biodegradable therapeutic carriers of lesser consequence on TGF-β1 mediated fibrosis.

Extensive modifications have been made to the synthesis protocol for porous silica particles to improve the shape, size and yield percentage, but problems associated with improvement in biodegradability and decrease in chances to induce side effects still remain a concern. To circumvent these limitations, a facile modification strategy has been employed through in situ carbonization of porous silica particles. Herein, carbon particles were integrated within porous silica core-shell particles (Si-P-CNPs) during the synthesis process and found to preserve the ordered structural morphology. Curcumin was used as a model drug for loading in prepared Si-P-CNPs whereas lung cancer cells were used as a model system to study the in vitro fate. These Si-P-CNPs showed improved drug loading, drug effectivity, biodegradability and avoidance of interaction with transforming growth factor β1 (TGF-β1) indicating the possibility of reducing the chances of lung fibrosis and thereby enhancing the safety profile over conventional porous silica particles.

Scaling-up continuous production of mesoporous silica particles at kg scale: design &amp; operational strategies

This paper demonstrates a continuous flow pilot-scale production of highly porous mesoporous silica particles (MSPs) via a soft template based technique.

Method Development and Validation of Indigenously Isolated Caspofungin Acetate and its related Substances using RP-HPLC Method

Caspofungin is prepared from a precursor PneumocandinB0 derived from a fungus Glarea lozoyensis. Prepared crude caspofungin acetate has many impurities, namely, A, B, D, E and PneumocandinB0, and these impurities vary slightly in chemical or structural composition and need to be eliminated by purification. The present study involves the preparation of caspofungin acetate from PneumocandinB0, followed by purification and method development for detecting caspofungin and its related substances.”We used a high-tech liquid chromatograph system, which included components like the Agilent 1260 infinity quaternary pump module, a sophisticated MWD detector, and the Empower 3 data handling system. To separate and analyze substances, we employed an analytical column made of stainless steel. This column was 150 mm long and had an internal diameter of 3.0 mm. It was filled with octadecyl silane chemically bonded to porous silica particles, each with a tiny diameter of 3 micrometers. The test method underwent validation to ensure it could accurately identify specific elements, detect minimal quantities, establish the lowest measurable amounts, maintain linearity, demonstrate precision, achieve accuracy, and suitably handle sample solutions and the system.” and the developed test method is suitable and can be introduced into the routine use for the detection of caspofungin acetate and its related substances.

Smart Composite Coatings for Corrosion Protection

Corrosion is a major problem faced by the automobile industry. To overcome corrosion, the industry relies on surface coatings in which metallic (Zn, Cr etc.) and polymeric coatings (polyurethane, epoxy, etc.) are widely recommended. However, currently smart, self-healing coatings are considered to be the state of the art option to inhibit cut-edge corrosion and to enhance the life-time. The direct introduction of inhibitors into any matrix creates undesirable effects, majorly the integrity of the inhibitor with the matrix and spontaneous release. Hence, loading inhibitors into a suitable container and subsequent release of the same when the coating is damaged favors the self-healing property of a coating, which results in high durability.Such a smart coating can be achieved by the synthesis of porous silica particles, which can be an effective corrosion inhibitor container. They possess exceptional properties like nontoxicity, high surface area, and capability to be filled with different types of active materials, including organic, inorganic, and hybrid inhibitors. Hence, these can be the equivalent and non-toxic alternative to chromates which are toxic and carcinogenic to the environment and the living system.Among the different porous containers reported in the literature, mesoporous silica nanoparticles have large surface area which is beneficial to load the required quantity of corrosion inhibitors. The corrosion inhibitor-loaded porous silica containers are compatible with different matrices, including metals, polymers, and ceramics, to prepare the composite materials. To control the instant release of the loaded inhibitors, encapsulation with a polyelectrolyte is required. As a result, improved self-healing properties can be achieved as compared to the particles without encapsulation.Among the various types of porous silica, Santa Barbara Amorphous-15 (SBA-15) has several benefits, including the ease of preparation, nontoxic precursors, hexagonal structure to carry a high quantity of material, size range from micro to nanometer, controllable pore size (20-500Å), large specific surface area (∼1000m2/g), porosity, surface morphology and huge pore volume. This work describes the synthesis of mesoporous silica (SBA-15) using a three-step solvo-thermal process. The process includes template preparation (synthesis of mesoporous silica), loading with benzotriazole (BTA) an organic corrosion inhibitor, followed by Poly(diallyldimethylammonium chloride) (PDDA) encapsulation.The synthesised mesoporous silica was characterised for morphological features, porosity, and surface charge. Based on the BET analysis, successful modification of the mesoporous silica particles at different stages shows a reduction in the surface area and pore volume. The zeta (ζ)-potential measurements showed a significant variation in surface charge after the modification of the mesoporous silica particles. The releasing nature of BTA before and after the PDDA encapsulation was studied using UV-Visible technique at different pH levels. Based on the characterisation of the PDDA encapsulated inhibitor-loaded mesoporous silica particles, appropriate coating was prepared by incorporating these particles into eg. metallic (Zn coating) and/or polymeric (epoxy coating) matrices. The release and corrosion inhibition performance of these coatings were studied using electrochemical techniques. The multi-functional performance of these coatings would be highlighted.

Hierarchically porous silica particles: One-pot synthesis, tunable hydrophilic/hydrophobic properties, prospects for selective oil adsorption

Submicron spherical silica particles were synthesized via base hydrolysis of tetraethoxysilane (TEOS) and phenyltriethoxysilane (PTEOS) in a presence of cetyltrimethylammonium bromide. It was shown that the replacement of a part of TEOS with PTEOS in the reaction mixture led to formation of particles containing hierarchical micro-mesoporous structure with specific surface area of up to ∼ 1170 m2 g−1. The amount of PTEOS leads to the increase of water contact angle of the particles from 10° to 155° which causes a gradual change in their surface properties from hydrophilic to superhydrophobic. The static adsorption capacity of the particles was measured for water, toluene and methyl isobutyl ketone and was found to be affected by both the amount of OH- or C6H5-groups on the surface and the characteristics of porosity. In particular, the adsorption capacity for non-polar volatile organics increases with the increase of the hydrophobicity of particles. It was found that synthesized superhydrophobic silica particles efficiently and quickly absorb oil from water surface and then can be easily removed mechanically.

Analytical Method Development and Validation of Lycopene Present in Multivitamin Tablets

Lycopene is an organic compound belonging to tetraterpene and a carotene category. Lycopene is a brick red carotenoid hydrocarbon found in tomatoes and other red fruits and vegetables. Lycopene is extracted from red fruit &amp; vegetables and found in two physical form Oily liquid &amp; granular solid powder form. Lycopene is also used as supplement with multivitamin formulated as tablets. An RP-HPLC method was developed and validated for the Lycopene (Powder form or water suspendable form) in Multivitamin Tablets. The chromatographic system was equipped with C18 a stainless steel column 30 cm x 4.0 mm, packed with octadecylsilane chemically bonded to porous silica or ceramic micro particles (5µm) and wavelength set at 475 nm, in conjunction with a mobile phase of Methanol, Water and Tetrahydrofuran in the ratio of 66:4:30 % v/v at a flow rate of 1.5 ml/min. The retention time of Lycopene was found to be 6 min ±1 min. The separation was performed at ambient temperature. Linearity was observed in the concentration range of 80-120% with correlation coefficient 0.9999 and slope 75145.63 Percentage recovery obtained 99.06-101.83 %. The percentage Assay was found to be 100.25 to 102.61 %.The proposed method is precise, accurate, selective and rapid for the determination of lycopene (Powder form) in Multivitamin tablet. The proposed method is optimized and validated as per guidelines of WHO TRS 937 &amp; the International Conference on Harmonization (ICH) guidelines.

Novel Machine Learning-Based Method for Estimation of the Surface Area of Porous Silica Particles

Novel Machine Learning-Based Method for Estimation of the Surface Area of Porous Silica Particles

B-182 Advantage of Core-Shell Particle Morphology Over Fully Porous HPLC Column for the Analysis of Vitamin K Compounds by LC-MS/MS

Abstract Background Vitamin K’s are a group of structurally similar, fat-soluble compounds. There are two natural forms K1 (Phylloquinone) and K2 (Menaquinone), where the later has short and longer chain homologues called Vitamin K2-MK-4 and Vitamin K2-MK-7 respectively. Due to the extremely hydrophobic nature and poor ionization characteristics of these compounds, LC-MS/MS analysis at low concentrations becomes challenging. In this communication we make parallel comparisons of three HPLC columns packed with fully porous and core-shell particles to showcase the superiority of core-shell columns for sensitive detection of vitamin K analytes by LC-MS/MS. Method We compared fully porous (Gemini and Luna, Phenomenex, Torrance, CA) and core-shell (Kinetex EVO, Phenomenex) C18 columns for this study. All columns evaluated were 50 × 2.1 mm with a 5 μm particle size to maintain low back pressure for the viscous organic modifier utilized. Deionized water and a combination of isopropanol and acetonitrile (1:1; v/v) were used as mobile phase A and B respectively for chromatographic separation. An Agilent 1290 Infinity series LC system was employed with a SCIEX 6500 QTRAP mass spectrometer under APCI for MS detection. Analyst software 1.7.2 used for data analysis. Results The LC-MS/MS method had a 4-minute runtime for chromatographic separation of the analytes including 1.5 min of equilibration time. The comparison data revealed the Gemini C18 column shows some higher selectivity and resolution values, whereas both EVO and Luna C18 columns were somewhat comparable. However, the core-shell EVO C18 column displayed much higher efficiency with narrower and compressed peak widths, which dramatically increased the peak height for all 3 analytes exhibiting the highest signal/noise ratio. Conclusion The core-shell Kinetex 5 μm, EVO C18 column with its unique organo-silica grafting shows much higher efficiency and sensitive detection capability for vitamin K analysis by LC-MS/MS compared to that of conventional fully porous silica particles.

Synthesis of Pore-Size-Tunable Porous Silica Particles and Their Effects on Dental Resin Composites.

The filler/resin matrix interface interaction plays a vital role in the properties of dental resin composites (DRCs). Porous particles are promising fillers due to their potential in constructing micromechanical interlocking at filler/resin matrix interfaces, therefore improving the properties of the resulting DRCs, where the pore size is significantly important. However, how to control the pore size of porous particles via a simple synthesis method is still a challenge, and how their pore sizes affect the properties of resulting DRCs has not been studied. In this study, porous silica (DPS) with a dendritic structure and an adjustable pore size was synthesized by changing the amounts of catalyst in the initial microemulsion. These synthesized DPS particles were directly used as unimodal fillers and mixed with a resin matrix to formulate DRCs. The results showed that the DPS pore size affects the properties of DRCs, especially the mechanical property. Among various DPS particles with different pore sizes, DPS6 resulted in 19.5% and 31.4% improvement in flexural strength, and 24.4% and 30.7% enhancement in compression strength, respectively, compared to DPS1 and DPS9. These DPS particles could help to design novel dental restorative materials and have promising applications in biomedicine, catalysis, and adsorption.

Effect of Magnetite Content and Specificity of Nickel(II) Ions on Electrokinetic Properties of Composites Based on Porous Silica Particles

Effect of Magnetite Content and Specificity of Nickel(II) Ions on Electrokinetic Properties of Composites Based on Porous Silica Particles

Selective and high capacity recovery of aqueous Ag(I) by thiol functionalized mesoporous silica sorbent

A simplified sol–gel method was developed for the preparation of thiol functionalized porous silica particles using (3-mercaptopropyl)-trimethoxy-silane (MPTMS) for co-gelation in the presence of micellar templates. The resulting short range ordered porous silica microparticles are exceptionally effective sorbents of aqueous Ag(I). The binding of aqueous Ag(I) is almost stoichiometric in the broad pH range between 4.0 and 9.0 even at low Ag(I) concentrations until reaching the limiting 238 mg g−1 sorption capacity. The sorbent displays high selectivity towards Ag(I), which was tested in the simultaneous presence of several different metal compounds. The practically complete recovery of Ag(I) and the regeneration of the sorbent was achieved by washing with a 10 mM Na2S2O3 solution. X-ray photoelectron spectroscopy (XPS) revealed that silver is not reduced on the surface of the sorbent. The reason for the strong and selective binding of Ag(I) is the formation of layered thiolate coordination compounds on the functionalized silica surface that display characteristically high stability even in the presence of other competing metal ions.