Amperometric detection of lactose using β-galactosidase immobilized in layer-by-layer films

ZIF-8 encapsulated-enzymes integrated nanozyme cascade biocatalysis platform for the colorimetric sensing of glucose and lactose in milk

Cascade biocatalytic reactions have a wide range of applications, especially in the filed of food analysis. Herein, a multi-enzyme composite (ZGGPC) was prepared by in-situ synthesis of Zeolite imidazole framework-8 (ZIF-8) on Prussian blue (PB) modified carbon cloth (CC). The composite encapsulated both glucose oxidase and β-galactosidase simultaneously during the synthesis process. CC and ZIF-8 showed high loading capacity for PB and natural enzymes, respectively. And ZIF-8 also displayed excellent tolerance in protecting enzyme activity under extreme conditions. Based on the cascade biocatalysis, ZGGPC was used to detect glucose and lactose by colorimetric method with detection limits of 1.2 μM and 1.7 mM, respectively. Benefiting from the merits of low cost, easy preparation, and good stability, the sensing system was used to successfully determine glucose and lactose in different milk samples. The present cascade biocatalysis system is hopeful to develop simple and efficient sensing platforms for food analysis.

Performance of a Flow-Through Enzyme Reactor Prepared from a Silica Monolith and an α-Poly(D-Lysine)-Enzyme Conjugate

Horseradish peroxidase (HRP) is covalently bound in aqueous solution to polycationic α-poly(D-lysine) chains of ≈1000 repeating units length, PDL, via a bis-aryl hydrazone bond (BAH). Under the experimental conditions used, about 15 HRP molecules are bound along the PDL chain. The purified PDL-BAH-HRP conjugate is very stable when stored at micromolar HRP concentration in a pH 7.2 phosphate buffer solution at 4 °C. When a defined volume of such a conjugate solution of desired HRP concentration (i.e., HRP activity) is added to a macro- and mesoporous silica monolith with pore sizes of 20-30 µm as well as below 30 nm, quantitative and stable noncovalent conjugate immobilization is achieved. The HRP-containing monolith can be used as flow-through enzyme reactor for bioanalytical applications at neutral or slightly alkaline pH, as demonstrated for the determination of hydrogen peroxide in diluted honey. The conjugate can be detached from the monolith by simple enzyme reactor washing with an aqueous solution of pH 5.0, enabling reloading with fresh conjugate solution at pH 7.2. Compared to previously investigated polycationic dendronized polymer-enzyme conjugates with approximately the same average polymer chain length, the PDL-BAH-HRP conjugate appears to be equally suitable for HRP immobilization on silica surfaces.

Nanomaterial-Based Electrochemical Nanodiagnostics for Human and Gut Metabolites Diagnostics: Recent Advances and Challenges

Metabolites are the intermediatory products of metabolic processes catalyzed by numerous enzymes found inside the cells. Detecting clinically relevant metabolites is important to understand their physiological and biological functions along with the evolving medical diagnostics. Rapid advances in detecting the tiny metabolites such as biomarkers that signify disease hallmarks have an immense need for high-performance identifying techniques. Low concentrations are found in biological fluids because the metabolites are difficult to dissolve in an aqueous medium. Therefore, the selective and sensitive study of metabolites as biomarkers in biological fluids is problematic. The different non-electrochemical and conventional methods need a long time of analysis, long sampling, high maintenance costs, and costly instrumentation. Hence, employing electrochemical techniques in clinical examination could efficiently meet the requirements of fully automated, inexpensive, specific, and quick means of biomarker detection. The electrochemical methods are broadly utilized in several emerging and established technologies, and electrochemical biosensors are employed to detect different metabolites. This review describes the advancement in electrochemical sensors developed for clinically associated human metabolites, including glucose, lactose, uric acid, urea, cholesterol, etc., and gut metabolites such as TMAO, TMA, and indole derivatives. Different sensing techniques are evaluated for their potential to achieve relevant degrees of multiplexing, specificity, and sensitivity limits. Moreover, we have also focused on the opportunities and remaining challenges for integrating the electrochemical sensor into the point-of-care (POC) devices.

Development of amperometric biosensor in modified carbon paste with enzymatic preparation based on lactase immobilized on carbon nanotubes

Abstract

The variety of products derived from milk, without or with lactose, encourages the development of more effective analytical techniques that can be applied to the quality control of both the production line and the final products. Thus, in this work an efficient and minimally invasive method for the detection of lactose was proposed, using a biosensor containing the enzyme lactase (LAC) immobilised on carbon nanotubes (CNTs) that, when reacting with lactose, emit an electrochemical signal. This biosensor was connected to a potentiostat, and its electrochemical cell was composed of the following three electrodes: reference electrode (Ag/AgCl), auxiliary electrode (platinum wire), and working electrode (biosensor) on which graphite (carbon) paste (CP), CNTs, and LAC were deposited. The transmission electron microscopy and scanning electron microscopy were used in the characterisation of the composite morphology, indicating excellent interactions between the CNTs and LAC. The sensitivity of the CP/LAC/CNT biosensor was determined as 5.67 μA cm^-2.mmol^-1 L and detection limits around 100 × 10^-6 mol L^-1 (electrode area = 0.12 cm²) and an increase in the stability of the system was observed with the introduction of CNTs because, with about 12 h of use, there was no variation in the signal (current). The results indicate that the association between the CNTs and LAC favoured the electrochemical system.

Graphic Abstract

Nanosensors-Assisted Quantitative Analysis of Biochemical Processes in Droplets

Here, we present a miniaturized lab-on-a-chip detecting system for an all-electric and label-free analysis of the emulsion droplets incorporating the nanoscopic silicon nanowires-based field-effect transistors (FETs). We specifically focus on the analysis of β-galactosidase e.g., activity, which is an important enzyme of the glycolysis metabolic pathway. Furthermore, the efficiency of the synthesis and action of β-galactosidase can be one of the markers for several diseases, e.g., cancer, hyper/hypoglycemia, cell senescence, or other disruptions in cell functioning. We measure the reaction and reaction kinetics-associated shift of the source-to-drain current I_sd in the system, which is caused by the change of the ionic strength of the microenvironment. With these results, we demonstrate that the ion-sensitive FETs are able to sense the interior of the aqueous reactors; thus, the conjunction of miniature nanosensors and droplet-based microfluidic systems conceptually opens a new route toward a sensitive, optics-less analysis of biochemical processes.

A Nucleic Acid Nanostructure Built through On-Electrode Ligation for Electrochemical Detection of a Broad Range of Analytes

For an assay to be most effective in point-of-care clinical analysis, it needs to be economical, simple, generalizable, and free from tedious workflows. While electrochemistry-based DNA sensors reduce instrumental costs and eliminate complicated procedures, there remains a need to address probe costs and generalizability, as numerous probes with multiple conjugations are needed to quantify a wide range of biomarkers. In this work, we have opened a route to circumvent complicated multiconjugation schemes using enzyme-catalyzed probe construction directly on the surface of the electrode. With this, we have created a versatile DNA nanostructure probe and validated its effectiveness by quantification of proteins (streptavidin, anti-digoxigenin, anti-tacrolimus) and small molecules (biotin, digoxigenin, tacrolimus) using the same platform. Tacrolimus, a widely prescribed immunosuppressant drug for organ transplant patients, was directly quantified with electrochemistry for the first time, with the assay range matching the therapeutic index range. Finally, the stability and sensitivity of the probe was confirmed in a background of minimally diluted human serum.

Conjugated Polymers Blended with Lipids and Galactosidase as Langmuir-Blodgett Films To Control the Biosensing Properties of Nanostructured Surfaces

The structure of enzymes must be conserved when incorporated in nanoelectronic devices because their activity determines the function of the device as sensors. Among the systems that can retain their conformational structures, Langmuir-Blodgett (LB) films can be useful to exploit the construction of bioelectronic devices organized at the molecular level because biological and polymeric materials can be coupled as ultrathin films for biosensors and actuators. In this paper, we immobilized a β-galactosidase enzyme in the LB films of stearic acid and the conjugated polymer poly[(9,9-dioctylfluorene)- co-thiophene]. After the characterization of the floating films using tensiometry, vibrational spectroscopy, and Brewster angle microscopy, they were transferred to solid supports as LB films, and the catalytic activity of the enzyme could be preserved as analyzed using UV-vis spectroscopy. We noted that the presence of a supramolecular structure formed in the LB films not only conserved the enzyme activity but also exhibited regular and distinctive output signals in all molecular architectures employed in this work. These results are related to the synergism between the compounds on the active layer associated with a surface morphology that facilitated the analyte diffusion because of an adequate molecular accommodation of all components. This work then demonstrates the viability of employing LB films composed of lipids, enzymes, and synthetic polymers as devices for biosensing applications.

Discrimination of Milks with a Multisensor System Based on Layer-by-Layer Films

A nanostructured electrochemical bi-sensor system for the analysis of milks has been developed using the layer-by-layer technique. The non-enzymatic sensor [CHI+IL/CuPcS]₂, is a layered material containing a negative film of the anionic sulfonated copper phthalocyanine (CuPcS) acting as electrocatalytic material, and a cationic layer containing a mixture of an ionic liquid (IL) (1-butyl-3-methylimidazolium tetrafluoroborate) that enhances the conductivity, and chitosan (CHI), that facilitates the enzyme immobilization. The biosensor ([CHI+IL/CuPcS]₂-GAO) results from the immobilization of galactose oxidase on the top of the LbL layers. FTIR, UV⁻vis, and AFM have confirmed the proposed structure and cyclic voltammetry has demonstrated the amplification caused by the combination of materials in the film. Sensors have been combined to form an electronic tongue for milk analysis. Principal component analysis has revealed the ability of the sensor system to discriminate between milk samples with different lactose content. Using a PLS-1 calibration models, correlations have been found between the voltammetric signals and chemical parameters measured by classical methods. PLS-1 models provide excellent correlations with lactose content. Additional information about other components, such as fats, proteins, and acidity, can also be obtained. The method developed is simple, and the short response time permits its use in assaying milk samples online.

Layer-by-layer films containing emodin or emodin encapsulated in liposomes for transdermal applications

Dermal drug release systems are an important area of research because they can be applied to the skin in a non-invasive procedure using a lower concentration of drugs. In this study, we have developed two types of Layer-by-Layer (LbL) films for releasing emodin (EM). In one system, EM was intercalated with poly(ethylenimine) PEI and poly(vinyl sufonate) (PVS) polyelectrolytes, forming (PEI/PVS)₂(PEI/EM)₇; in another, EM was incorporated in liposomes obtained by mixing dipalmitoyl phosphatidyl glycerol (DPPG) and palmitoyl oleoyl phosphatidyl glycerol (POPG) lipids, forming (PEI/PVS)₂(PEI/DPPG-POPG-EM)₇. UV-vis and FTIR spectroscopies were used to characterize the LbL films. These showed that the depositions of material by LbL were efficient, with increases in the absorbance of each bilayer evidencing the presence of EM in the film. The (PEI/PVS)₂(PEI/EM)₇ and (PEI/PVS)₂(PEI/DPPG-POPG-EM)₇ films released EM in three and five days, respectively. The cyclic voltammetry (CV) assay of the (PEI/PVS)₂(PEI/EM)₇ results are in agreement with UV-vis measurements, which suggest that EM was protonated in acid environments, while the CV of (PEI/PVS)₂(PEI/DPPG-POPG-EM)₇ demonstrated distinct protonation behaviour for EM within the inner liposome structure, even in acid solutions. Therefore, this study presents two systems based on LbL films and provides additional details about the release of EM from these films to create a viable alternative for transdermal applications.

Periodically Patterned Au-TiO2 Heterostructures for Photoelectrochemical Sensor

Periodically patterned Au nanorods in TiO₂ nanocavities (Au NRs@TiO₂) were fabricated via magnetron sputtering followed by a thermal dewetting process. This innovative Au NRs@TiO₂ heterostructure was used as a plasmonic sensing platform for photoelectrochemical detection of glucose and lactose. This Au NRs@TiO₂ patterned heterostructure possesses superior sensing properties to other Au nanoparticle-based sensors because (i) localized surface plasmon resonance (LSPR) generated at Au/TiO₂ interfaces enhanced sensitivity of glucose (lactose) amperometric detection; (ii) periodic Au nanocrystals in TiO₂ nanocavities accelerated charge separation and transfer rate, especially under monochromatic blue light irradiation; (iii) discrete planar architectures comprising Au NRs immobilized on TiO₂ substrates significantly improved stability and reusability of the sensors. A low detection limit of 1 μM (10 μM) and a high sensitivity of 812 μA mM^-1 cm^-2 (270 μA mM^-1 cm^-2) were achieved on the Au NRs@TiO₂ heterostructures for glucose (lactose) detection without the addition of enzymes. Good selectivity and superb stability over more than 8 weeks was also demonstrated using these Au NRs@TiO₂ heterostructures for glucose (lactose) detection. Additionally, this cost-efficient technique can be easily extended to other photoelectrochemical sensing systems when considering the combination of sensing and visible or infrared light source enhancement.

Direct Electron Transfer of Cellobiose Dehydrogenase on Positively Charged Polyethyleneimine Gold Nanoparticles

Efficient conjugation between biomolecules and electrode materials is one of the main challenges in the field of biosensors. Cellobiose dehydrogenase (CDH) is a monomeric enzyme, which consists of two separate domains: one catalytic dehydrogenase domain (DH_CDH ) carrying strongly bound flavin adenine dinucleotide (FAD) in the active site and a cytochrome domain (CYT_CDH ) carrying a b-type heme connected by a flexible linker region. Herein, we report on the development of a lactose biosensor, based on direct electron transfer (DET) from CDH from Phanerochaete sordida (PsCDH) electrostatically attached onto polyethyleneimine-stabilized gold nanoparticles (PEI@AuNPs) used to cover a conventional polycrystalline solid gold disk electrode. PEI@AuNPs were synthesized in aqueous solution using PEI as reducing agent for Au^III and as stabilizer for the nanoparticles. The heterogeneous electron-transfer (ET) rate (k_s ) for the redox reaction of immobilized PsCDH at the modified electrodes was calculated based on the Laviron theory and was found to be (39.6±2.5) s^-1 . The proposed lactose biosensor exhibits good long term stability as well as high and reproducible sensitivity to lactose with a response time less than 5 s and a linear range from 1 to 100 μm.

Electrochemical Characterization of Layer-By-Layer Assembled Ferrocene-Modified Linear Poly(ethylenimine)/Enzyme Bioanodes for Glucose Sensor and Biofuel Cell Applications

Ferrocenylhexyl- and ferrocenylpropyl-modified linear poly(ethylenimine) (Fc-C6-LPEI, Fc-C3-LPEI) were used with periodate-modified glucose oxidase (p-GOX) in the layer-by-layer assembly of enzymatic bioanodes on gold. Fc-C6-LPEI/p-GOX and Fc-C3-LPEI/p-GOX films of 16 bilayers were capable of generating up to 381 ± 3 and 1417 ± 63 μA cm(-2), respectively, in response to glucose. These responses are greater than those of analogous bioanodes fabricated using conventional cross-linking techniques and are extremely high for planar, low surface area, single-enzyme electrodes. (Fc-C3-LPEI/p-GOX)8 films generated 86 ± 3 μW cm(-2) at pH 7.0 and 149 ± 7 μW cm(-2) at pH 5.0, when poised against an air-breathing platinum cathode in a compartment-less biofuel cell. An increase in power output with decreasing pH was shown to be a result of increases in the platinum cathode performance, indicating it is the rate-limiting electrode in the biofuel cells. The effect of fabrication wash time on the buildup of material at the electrode's surface was probed using cyclic voltammetry (CV) and constant potential amperometry. The use of electrochemical techniques as a diagnostic tool for studying the material deposition process is discussed. CV peak separation (ΔE), surface coverage of the electroactive ferrocene (ΓFc), and amperometric sensitivity of the enzyme to glucose (Jmax), studied as a function of numbers of bilayers, showed that physisorption of materials onto the surface results from initial patchy deposition, rather than in distinctly uniform layers.

Recent Progress in Lectin-Based Biosensors

This article reviews recent progress in the development of lectin-based biosensors used for the determination of glucose, pathogenic bacteria and toxins, cancer cells, and lectins. Lectin proteins have been widely used for the construction of optical and electrochemical biosensors by exploiting the specific binding affinity to carbohydrates. Among lectin proteins, concanavalin A (Con A) is most frequently used for this purpose as glucose- and mannose-selective lectin. Con A is useful for immobilizing enzymes including glucose oxidase (GOx) and horseradish peroxidase (HRP) on the surface of a solid support to construct glucose and hydrogen peroxide sensors, because these enzymes are covered with intrinsic hydrocarbon chains. Con A-modified electrodes can be used as biosensors sensitive to glucose, cancer cells, and pathogenic bacteria covered with hydrocarbon chains. The target substrates are selectively adsorbed to the surface of Con A-modified electrodes through strong affinity of Con A to hydrocarbon chains. A recent topic in the development of lectin-based biosensors is a successful use of nanomaterials, such as metal nanoparticles and carbon nanotubes, for amplifying output signals of the sensors. In addition, lectin-based biosensors are useful for studying glycan expression on living cells.

Monoamine oxidase B layer-by-layer film fabrication and characterization toward dopamine detection

In this work nanostructured film composites of the monoamine oxidase B (MAO-B) enzyme, free or encapsulated in liposomes, were fabricated by the layer-by-layer (LbL) self-assembly technique, employing polyethylene imine (PEI) as polycation. Initially, the MAO-B enzyme was incorporated into liposomes in order to preserve its enzymatic structure ensuring their activity and catalytic stability. The LbL film growth was monitored by surface plasmon resonance (SPR) by gold resonance angle shift analysis after each bilayer deposition. Subsequently, the films were applied as amperometric biosensors for dopamine detection using Prussian Blue (PB) as the electron mediator. The biosensor fabricated by MAO-B incorporated into liposomes composed of DPPG:POPG in the ratio (1:4) (w/w) showed the best performance with a sensitivity of 0.86 (μA cm(-2))/(mmol L(-1)) and a detection limit of 0.33 mmol L(-1).

Bioresorbable polyelectrolytes for smuggling drugs into cells

There is ample evidence that biodegradable polyelectrolyte nanocapsules are multifunctional vehicles which can smuggle drugs into cells, and release them upon endogenous activation. A large number of endogenous stimuli have already been tested in vitro, and in vivo research is escalating. Thus, the interest in the design of intelligent polyelectrolyte multilayer (PEM) drug delivery systems is clear. The need of the hour is a systematic translation of PEM-based drug delivery systems from the lab to clinical studies. Reviews on multifarious stimuli that can trigger the release of drugs from such systems already exist. This review summarizes the available literature, with emphasis on the recent progress in PEM-based drug delivery systems that are receptive in the presence of endogenous stimuli, including enzymes, glucose, glutathione, pH, and temperature, and addresses different active and passive drug targeting strategies. Insights into the current knowledge on the diversified endogenous approaches and methodological challenges may bring inspiration to resolve issues that currently bottleneck the successful implementation of polyelectrolytes into the catalog of third-generation drug delivery systems.