Immobilization of enzymes into nanocavities for the improvement of biosensor stability

Micro additive manufacturing of glucose biosensors: A feasibility study

Flexible electrochemical sensors for measurement and quantification of biomarkers are attracting a great deal of attention in non-invasive medical applications, due to their high mechanical compatibility and conformability with the human body. Realization of the full potential of such novel systems relies heavily on their effective manufacturing. Particularly, there is a need for manufacturing techniques that can realize complex designs, consisting of multiple functional materials which are required for sensor functionality. Among emerging additive manufacturing techniques, Direct-Ink-Writing (DIW), where polymer nanocomposite inks are dispensed through nozzles and deposited with high spatial control, carries a great potential to address this need. Here, we introduce a 3D printed flexible electrochemical biosensor for glucose detection. We show that our biosensor works linearly in glucose solution with a concentration range between 100 and 1000 μM. The sensitivity of glucose biosensor is estimated to be 17.5 nA μM^-1, and the calculated value of the detection limit (S/N = 3) is 6.9 μM. The demonstrated electrochemical performance and surface properties of the printed sensors show the promising advantages of using this technique over the conventional screen printing method. These advantages include higher sensitivity and specificity and, reduced material consumption.

Nano-in-Nano Approach for Enzyme Immobilization Based on Block Copolymers

We set up a facile approach for fabrication of supports with tailored nanoporosity for immobilization of enzymes. To this aim block copolymers (BCPs) self-assembly has been used to prepare nanostructured thin films with well-defined architecture containing pores of tailorable size delimited by walls with tailorable degree of hydrophilicity. In particular, we employed a mixture of polystyrene-block-poly(l-lactide) (PS-PLLA) and polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymers to generate thin films with a lamellar morphology consisting of PS lamellar domains alternating with mixed PEO/PLLA blocks lamellar domains. Selective basic hydrolysis of the PLLA blocks generates thin films, patterned with nanometric channels containing hydrophilic PEO chains pending from PS walls. The shape and size of the channels and the degree of hydrophilicity of the pores depend on the relative length of the blocks, the molecular mass of the BCPs, and the composition of the mixture. The strength of our approach is demonstrated in the case of physical adsorption of the hemoprotein peroxidase from horseradish (HRP) using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) with H₂O₂ as substrate. The large surface area, the tailored pore sizes, and the functionalization with hydrophilic PEO blocks make the designed nanostructured materials suitable supports for the nanoconfinement of HRP biomolecules endowed with high catalytic performance, no mass-transfer limitations, and long-term stability.

Temperature-Invariant Aqueous Microgels as Hosts for Biomacromolecules

Immobilization of enzymes on solid supports has been widely used to improve enzyme recycling, enzyme stability, and performance. We are interested in using aqueous microgels (colloidal hydrogels) as carriers for enzymes used in high-temperature reactions. These microgels should maintain their volume and colloidal stability in aqueous media up to 100 °C to serve as thermo-stable supports for enzymes. For this purpose, we prepared poly(N-hydroxyethyl acrylamide) (PHEAA) microgels via a two-step synthesis. First, we used precipitation polymerization in water to synthesize colloidal poly(diethylene glycol-ethyl ether acrylate) (PDEGAC) particles as a precursor. PDEGAC forms solvent swollen microgels in organic solvents such as methanol and dioxane and in water at temperatures below 15 °C. In the second step, these PDEGAC particles were transformed to PHEAA microgels through aminolysis in dioxane with ethanolamine and a small amount of ethylenediamine. Dynamic laser scattering studies confirmed that the colloidal stability of microgels was maintained during the aminolysis in dioxane and subsequent transfer to water. Characterization of the PHEAA microgels indicated about 9 mol % of primary amino groups. These provide functionality for bioconjugation. As proof-of-concept experiments, we attached the enzyme horseradish peroxidase (HRP) to these aqueous microgels through (i) N-(3-(dimethylamino)propyl)-N'-ethylcarbodiimide hydrochloride (EDC) coupling to the carboxylated microgels or (ii) bis-aryl hydrazone (BAH) coupling to microgels functionalized with 6-hydrazinonicotinate acetone (PHEAA-HyNic). Our results showed that HRP maintained its catalytic activity after covalent attachment (87% for EDC coupling, 96% for BAH coupling). The microgel enhanced the stability of the enzyme to thermal denaturation. For example, the residual activity of the microgel-supported enzyme was 76% after 330 min of annealing at 50 °C, compared to only 20% for the free enzyme under these conditions. PHEAA microgels in water show great promise as hosts for enzymatic reaction, especially at elevated temperatures.

Enzyme Kinetics in Liquid Crystalline Mesophases: Size Matters, But Also Topology

Lyotropic liquid crystalline systems (LLCs) are excellent immobilizing carriers for enzymes, due to their biocompatibility and well-defined pore nanostructure. Here we show that the liquid crystalline mesophase topology can greatly influence the enzymatic activity in a typical peroxidase (Horseradish peroxidase, HRP) enzymatic reaction. Enzyme kinetics was investigated in different LLC mesophases based on monolinolein, with varying symmetries and dimensions such as the 1D cylindrical inverse hexagonal phase (HII), the 2D planar lamellar phase (Lα), and two 3D bicontinuous cubic phases of double diamond (Pn3m) and gyroid (Ia3d) space groups. As expected, the mesophase with largest water channel size shows highest activity, regardless of the topology. Interestingly, however, when mesophases with different topologies have the same water channel size, then the topology plays the dominant role, and the enzyme showed the highest activity in the 3D tetra-fold connected Pn3m, followed by the Ia3d with trifold connectivity, and finally the 1D HII phase. This study demonstrates that the enzymatic activity in LLC mesophases depends on both the water channel size and the topology of the mesophase.

Controlling enzymatic activity and kinetics in swollen mesophases by physical nano-confinement

Bicontinuous lipid cubic mesophases are widely investigated as hosting matrices for functional enzymes to build biosensors and bio-devices due to their unique structural characteristics. However, the enzymatic activity within standard mesophases (in-meso) is severely hindered by the relatively small diameter of the mesophase aqueous channels, which provide only limited space for enzymes, and restrict them into a highly confined environment. We show that the enzymatic activity of a model enzyme, horseradish peroxidase (HRP), can be accurately controlled by relaxing its confinement within the cubic phases' water channels, when the aqueous channel diameters are systematically swollen with varying amount of hydration-enhancing sugar ester. The in-meso activity and kinetics of HRP are then systematically investigated by UV-vis spectroscopy, as a function of the size of the aqueous mesophase channels. The enzymatic activity of HRP increases with the swelling of the water channels. In swollen mesophases with water channel diameter larger than the HRP size, the enzymatic activity is more than double that measured in standard mesophases, approaching again the enzymatic activity of free HRP in bulk water. We also show that the physically-entrapped enzymes in the mesophases exhibit a restricted-diffusion-induced initial lag period and report the first observation of in-meso enzymatic kinetics significantly deviating from the normal Michaelis-Menten behaviour observed in free solutions, with deviations vanishing when enzyme confinement is released by swelling the mesophase.

Nanoporous Gold Electrodes and Their Applications in Analytical Chemistry

Nanoporous gold prepared by dealloying Au:Ag alloys has recently become an attractive material in the field of analytical chemistry. This conductive material has an open, 3D porous framework consisting of nanosized pores and ligaments with surface areas that are 10s to 100s of times larger than planar gold of an equivalent geometric area. The high surface area coupled with an open pore network makes nanoporous gold an ideal support for the development of chemical sensors. Important attributes include conductivity, high surface area, ease of preparation and modification, tunable pore size, and a bicontinuous open pore network. In this paper, the fabrication, characterization, and applications of nanoporous gold in chemical sensing are reviewed specifically as they relate to the development of immunosensors, enzyme-based biosensors, DNA sensors, Raman sensors, and small molecule sensors.

Enzyme immobilization: an overview on techniques and support materials

The current demands of the world’s biotechnological industries are enhancement in enzyme productivity and development of novel techniques for increasing their shelf life. These requirements are inevitable to facilitate large-scale and economic formulation. Enzyme immobilization provides an excellent base for increasing availability of enzyme to the substrate with greater turnover over a considerable period of time. Several natural and synthetic supports have been assessed for their efficiency for enzyme immobilization. Nowadays, immobilized enzymes are preferred over their free counterpart due to their prolonged availability that curtails redundant downstream and purification processes. Future investigations should endeavor at adopting logistic and sensible entrapment techniques along with innovatively modified supports to improve the state of enzyme immobilization and provide new perspectives to the industrial sector.

Temperature-independent porous nanocontainers for single-molecule fluorescence studies

In this work, we demonstrate the capability of using lipid vesicles biofunctionalized with protein channels to perform single-molecule fluorescence measurements over a biologically relevant temperature range. Lipid vesicles can serve as an ideal nanocontainer for single-molecule fluorescence measurements of biomacromolecules. One serious limitation of the vesicle encapsulation method has been that the lipid membrane is practically impermeable to most ions and small molecules, limiting its application to observing reactions in equilibrium with the initial buffer condition. To permeabilize the barrier, Staphylococcus aureus toxin α-hemolysin (aHL) channels have been incorporated into the membrane. These aHL channels have been characterized using single-molecule fluorescence resonance energy transfer signals from vesicle-encapsulated guanine-rich DNA that folds in a G-quadruplex motif as well as from the Rep helicase-DNA system. We show that these aHL channels are permeable to monovalent ions and small molecules, such as ATP, over the biologically relevant temperature range (17-37 °C). Ions can efficiently pass through preformed aHL channels to initiate DNA folding without any detectable delay. With addition of the cholesterol to the membrane, we also report a 35-fold improvement in the aHL channel formation efficiency, making this approach more practical for wider applications. Finally, the temperature-dependent single-molecule enzymatic study inside these nanocontainers is demonstrated by measuring the Rep helicase repetitive shuttling dynamics along a single-stranded DNA at various temperatures. The permeability of the biofriendly nanocontainer over a wide range of temperature would be effectively applied to other surface-based high-throughput measurements and sensors beyond the single-molecule fluorescence measurements.

Degradation of polycyclic aromatic hydrocarbons by free and nanoclay-immobilized manganese peroxidase from Anthracophyllum discolor

Manganese peroxidase (MnP) produced by Anthracophyllum discolor, a Chilean white rot fungus, was immobilized on nanoclay obtained from volcanic soil and its ability to degrade polycyclic aromatic hydrocarbons (PAHs) compared with the free enzyme was evaluated. At the same time, nanoclay characterization was performed. Nanoclay characterization by transmission electronic microscopy showed a particle average size smaller than 100 nm. The isoelectric points (IEP) of nanoclay and MnP from A. discolor were 7.0 and 3.7, respectively, as determined by micro electrophoresis migration and preparative isoelectric focusing. Results indicated that 75% of the enzyme was immobilized on the nanoclay through physical adsorption. As compared to the free enzyme, immobilized MnP from A. discolor achieved an improved stability to temperature and pH. The activation energy (Ea) value for immobilized MnP (51.9 kJ mol(-1)) was higher than that of the free MnP (34.4 kJ mol(-1)). The immobilized enzyme was able to degrade pyrene (>86%), anthracene (>65%), alone or in mixture, and to a less extent fluoranthene (<15.2%) and phenanthrene (<8.6%). Compared to free MnP from A. discolor, the enzyme immobilized on nanoclay enhanced the enzymatic transformation of anthracene in soil. Overall results indicate that nanoclay, a carrier of natural origin, is a suitable support material for MnP immobilization. In addition, immobilized MnP shows an increased stability to high temperature, pH and time storage, as well as an enhanced PAHs degradation efficiency in soil. All these characteristics may suggest the possible use of nanoclay-immobilized MnP from A. discolor as a valuable option for in situ bioremediation purposes.

Coulometric D-fructose biosensor based on direct electron transfer using D-fructose dehydrogenase

This paper describes a batch-type coulometric d-fructose biosensor based on direct electron transfer reaction of d-fructose dehydrogenase (FDH) adsorbed on a porous carbon electrode surface. The adsorbed-FDH electrodes catalyzed the electrochemical two-electron oxidation of d-fructose to 5-keto-d-fructose without a mediator. Nanostructured carbon particle-modified electrodes were used for the coulometric d-fructose biosensor to enhance the catalytic current density. The electric charge for the d-fructose oxidation gained by the biocoulometric measurement was in good agreement with the theoretical value corresponding to d-fructose amount in the range from 1 to 100 mM with a sample volume of 1 muL. This method is also applicable to the determination of several oligo/polysaccharides containing the d-fructose unit, in combination with specific hydrolases to yield d-fructose. An example was demonstrated by sucrose determination in which the electrode modified with FDH and invertase was used as a working electrode. To address the problem of electroactive interferences such as ascorbate, the electric charge at the FDH-free electrode was subtracted from the total charge obtained at the FDH-adsorbed electrode. The d-fructose concentrations in several beverages were successfully determined with this method.

Enzyme-modified nanoporous gold-based electrochemical biosensors

On the basis of the unique physical and chemical properties of nanoporous gold (NPG), which was obtained simply by dealloying Ag from Au/Ag alloy, an attempt was made in the present study to develop NPG-based electrochemical biosensors. The NPG-modified glassy carbon electrode (NPG/GCE) exhibited high-electrocatalytic activity toward the oxidation of nicotinamide adenine dinucleotide (NADH) and hydrogen peroxide (H(2)O(2)), which resulted in a remarkable decrease in the overpotential of NADH and H(2)O(2) electro-oxidation when compared with the gold sheet electrode. The high density of edge-plane-like defective sites and large specific surface area of NPG should be responsible for the electrocatalytic behavior. Such electrocatalytic behavior of the NPG/GCE permitted effective low-potential amperometric biosensing of ethanol or glucose via the incorporation of alcohol dehydrogenase (ADH) or glucose oxidase (GOD) within the three-dimensional matrix of NPG. The ADH- and GOD-modified NPG-based biosensors showed good analytical performance for biosensing ethanol and glucose due to the clean, reproducible and uniformly distributed microstructure of NPG. The stabilization effect of NPG on the incorporated enzymes also made the constructed biosensors very stable. After 1 month storage at 4 degrees C, the ADH- and GOD-based biosensors lost only 5.0% and 4.2% of the original current response. All these indicated that NPG was a promising electrode material for biosensors construction.

Na⁺,K⁺-ATPase as the Target Enzyme for Organic and Inorganic Compounds

This paper gives an overview of the literature data concerning specific and non specific inhibitors of Na⁺,K⁺-ATPase receptor. The immobilization approaches developed to improve the rather low time and temperature stability of Na⁺,K⁺-ATPase, as well to preserve the enzyme properties were overviewed. The functional immobilization of Na⁺,K⁺-ATPase receptor as the target, with preservation of the full functional protein activity and access of various substances to an optimum number of binding sites under controlled conditions in the combination with high sensitive technology for the detection of enzyme activity is the basis for application of this enzyme in medical, pharmaceutical and environmental research.