The sol-gel encapsulation of enzymes

Hydrophobic, organically-modified silica gels enhance the secondary structure of encapsulated apomyoglobin

Insertion of hydrophobic groups in a silica matrix, by addition of propyl- or trifluoropropyltrimethoxysilane, leads to a surprising increase in the helical content of apomyoglobin following encapsulation by the sol-gel technique.

Three-dimensional immobilization of β-galactosidase on a silicon surface

Many alternative strategies to immobilize and stabilize enzymes have been investigated in recent years for applications in biosensors. The entrapment of enzymes within silica-based nanospheres formed through silicification reactions provides high loading capacities for enzyme immobilization, resulting in high volumetric activity and enhanced mechanical stability. Here we report a strategy for chemically associating silica nanospheres containing entrapped enzyme to a silicon support. beta-galactosidase from E. coli was used as a model enzyme due to its versatility as a biosensor for lactose. The immobilization strategy resulted in a three-dimensional network of silica attached directly at the silicon surface, providing a significant increase in surface area and a corresponding 3.5-fold increase in enzyme loading compared to enzyme attached directly at the surface. The maximum activity recovered for a silicon square sample of 0.5 x 0.5 cm was 0.045 IU using the direct attachment of the enzyme through glutaraldehyde and 0.16 IU when using silica nanospheres. The immobilized beta-galactosidase prepared by silica deposition was stable and retained more than 80% of its initial activity after 10 days at 24 degrees C. The ability to generate three-dimensional structures with enhanced loading capacity for biosensing molecules offers the potential to substantially amplify biosensor sensitivity.

Matrix-free methods for laser desorption/ionization mass spectrometry

Matrix assisted laser desorption/ionization (MALDI) is a soft ionization mass spectrometric method that has become a preeminent technique in the analysis of a wide variety of compounds including polymers and proteins. The main drawback of MALDI is that it is difficult to analyze low molecular weight compounds (<1,000 m/z) because the matrix that allows MALDI to work interferes in this mass range. In recent years there has been considerable interest in developing laser desorption/ionization (LDI) techniques for the analysis of small molecules. This review examines the approaches to matrix-free LDI mass spectrometry including desorption/ionization on silicon (DIOS), sol-gels, and carbon-based microstructures. For the purposes of this review matrix-free methods are defined as those that do not require matrix to be mixed with the analyte and therefore does not require co-crystallization. The review will also examine mechanisms of ionization and applications of matrix-free LDI-MS.

Chemically surface modified gel (CSMG): An excellent enzyme-immobilization matrix for industrial processes

Invertase from S. cerevisiae has been immobilized on porous silica matrix, formed using sol-gel chemistry, with surface area of approximately 650 m(2)/g. The co-condensation of silica sol with 3-aminopropyl(triethoxy)silane produced an amino-chemically surface modified silica gel (N-CSMG) with a very high ligand loading of 3.6 mmol/g SiO(2); significantly higher than commercially available matrices. Surface amine groups were activated with glutaraldehyde to produce GA-N-CSMG, and invertase covalently attached by the aldehyde. Invertase was used as a model enzyme to measure the immobilizing character of the GA-N-CSMG material. Using an optimized immobilization protocol, a very high loading of 723 mg invertase per gram GA-N-CSMG is obtained; 3-200-fold higher than values published in literature. The reproducible, immobilized activity of 246,000 U/g GA-N-CSMG is also greater than any other in literature. Immobilized invertase showed almost 99% retention of free enzyme activity and no loss in catalytic efficiency. The apparent kinetic parameters K(M) and V(M) were determined using the Michealis-Menten kinetic model. K(M) of the free invertase was 1.5 times greater than that of the immobilized invertase--indicating a higher substrate affinity of the immobilized invertase. These findings show considerable promise for this material as an immobilization matrix in industrial processes.

Selective sample preparation with bioaffinity columns prepared by the sol–gel method

Bioaffinity materials prepared by entrapping highly selective bioligands in the pores of a sol-gel glass offer unique advantages for the clean-up of complex sample matrices. The inclusion of sol-gel bioaffinity columns frequently allows the efficient elimination of interfering matrix components and enrichment of analytes in a simple step. After introducing the basic principles the paper reviews the methods for the production of protein-doped sol-gel materials and discusses the characteristics of sol-gel affinity columns by comparing their stability, selectivity, binding capacity and reusability. The potential of sample clean-up with sol-gel affinity columns is demonstrated giving applications in environmental, food and clinical analysis.

Use of fluorescence to monitor the incorporation of horseradish peroxidase into a sol–gel derived medium

The solvatochromic dye nile red has been employed to monitor the incorporation of an enzyme (horseradish peroxidase) into a sol-gel derived medium. The fluorescence spectrum of the dye, when incorporated into the enzyme, was analysed as the sum of Gaussian component spectra and relative changes between these component spectra were monitored upon encapsulation of the dye-enzyme system within the host matrix. Activity of the confined enzyme was verified and the effect of temperature was also investigated, through the examination of nile red fluorescence in the sol-gel derived matrix, where a stabilising effect was noted.

Twenty years research in cholinesterase biosensors: From basic research to practical applications

Over the last decades, cholinesterase (ChE) biosensors have emerged as an ultra sensitive and rapid technique for toxicity analysis in environmental monitoring, food and quality control. These systems have the potential to complement or replace the classical analytical methods by simplifying or eliminating sample preparation protocols and making field testing easier and faster with significant decrease in costs per analysis. Over the years, engineering of more sensitive ChE enzymes, development of more reliable immobilization protocols and progress in the area of microelectronics could allow ChE biosensors to be competitive for field analysis and extend their applications to multianalyte screening, development of small, portable instrumentations for rapid toxicity testing, and detectors in chromatographic systems. In this paper, we will review the research efforts over the last 20 years in fabricating AChE biosensors and the recent trends and challenges encounter once the sensor is used outside research laboratory for in situ real sample applications. The review will discuss the generations of cholinesterase sensors with their advantages and limitations, the existing electrode configurations and fabrication techniques and their applications for toxicity monitoring. We will focus on low-cost electrochemical sensors and the approaches used for enzyme immobilization. Recent works for achieving high sensitivity and selectivity are also discussed.

Ion-exchange properties and electrochemical characterization of quaternary ammonium-functionalized silica microspheres obtained by the surfactant template route

Porous silica spheres functionalized with quaternary ammonium groups have been prepared by co-condensation of N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride (TMTMAC) and tetraethoxysilane (TEOS) in the presence of cetyltrimethylammonium as a template and ammonia as a catalyst. The physicochemical characteristics of the resulting ion exchangers have been analyzed by various techniques and discussed with respect to the amount of organofunctional groups in the materials. For comparison purposes, both an ordered MCM-41 type mesoporous silica and two silica gels of different pore size have been grafted with TMTMAC. The ion-exchange capabilities were first evaluated from batch experiments (determination of anion-exchange capacities) and then by ion-exchange voltammetry at carbon paste electrodes modified with these hybrid materials. Effective concentration of Fe(CN)(6)(3)(-) species in the anion exchangers was pointed out, while no significant accumulation of Ru(NH(3))(6)(3+) was observed. The preconcentration efficiency was discussed on the basis of the organic group content in the materials as well as their structure and porosity. A second series of materials displaying zwitterionic surfaces was finally prepared and characterized with respect to their physicochemical properties and ion-exchange voltammetric behavior. They consisted of sulfonic acid-functionalized mesoporous silica samples resulting from the oxidation of thiol-functionalized silica spheres obtained by co-condensation of mercaptopropyl-trimethoxysilane (MPTMS) and TEOS, which were then grafted with TMTMAC at various functionalization levels. Possible interactions between the ammonium and sulfonate moieties in the confined medium were pointed out from X-ray photoelectron spectroscopy. The competitive accumulation-rejection of Fe(CN)(6)(3)(-) and Ru(NH(3))(6)(3+) redox probes was finally studied by cyclic voltammetry.

Advantages of the Pre-Immobilization of Enzymes on Porous Supports for Their Entrapment in Sol−Gels

In this work, we have compared the entrapment of free or previously immobilized glucose oxidase using a sol-gel technique. The preimmobilization was carried out on Sepabeads (a porous support) derivatized with glutaraldehyde as the functional group. The prior immobilization of the enzyme permitted to maintain the enzyme activity intact after the formation of the sol-gel. In fact, only 10% of the enzyme activity was lost whereas the soluble enzyme lost 60% of its initial activity. Additionally, enzyme leakage from the sol-gel matrix was avoided, which was relatively high when entrapping the soluble enzyme (39% of the enzyme activity was released after 16 h of incubation in a buffered solution). Moreover, the immobilized enzyme, inside the porous support, cannot be in contact with the sol-gel, and, therefore, it maintained the stability achieved by means of the multipoint covalent attachment on the Sepabeads support.