A potentiometric formaldehyde biosensor based on immobilization of alcohol oxidase on acryloxysuccinimide-modified acrylic microspheres

Design, construction and optimization of formaldehyde growth biosensors with broad application in biotechnology

Formaldehyde is a key metabolite in natural and synthetic one-carbon metabolism. To facilitate the engineering of formaldehyde-producing enzymes, the development of sensitive, user-friendly, and cost-effective detection methods is required. In this study, we engineered Escherichia coli to serve as a cellular biosensor capable of detecting a broad range of formaldehyde concentrations. Using both natural and promiscuous formaldehyde assimilation enzymes, we designed three distinct E. coli growth biosensor strains that depend on formaldehyde for cell growth. These strains were engineered to be auxotrophic for one or several essential metabolites that could be produced through formaldehyde assimilation. The respective assimilating enzyme was expressed from the genome to compensate the auxotrophy in the presence of formaldehyde. We first predicted the formaldehyde dependency of the biosensors by flux balance analysis and then analysed it experimentally. Subsequent to strain engineering, we enhanced the formaldehyde sensitivity of two biosensors either through adaptive laboratory evolution or modifications at metabolic branch points. The final set of biosensors demonstrated the ability to detect formaldehyde concentrations ranging approximately from 30 μM to 13 mM. We demonstrated the application of the biosensors by assaying the in vivo activity of different methanol dehydrogenases in the most sensitive strain. The fully genomic nature of the biosensors allows them to be deployed as "plug-and-play" devices for high-throughput screenings of extensive enzyme libraries. The formaldehyde growth biosensors developed in this study hold significant promise for advancing the field of enzyme engineering, thereby supporting the establishment of a sustainable one-carbon bioeconomy.

Recent Advances in Electrochemical Sensors for Formaldehyde

Formaldehyde, a ubiquitous indoor air pollutant, plays a significant role in various biological processes, posing both environmental and health challenges. This comprehensive review delves into the latest advancements in electrochemical methods for detecting formaldehyde, a compound of growing concern due to its widespread use and potential health hazards. This review underscores the inherent advantages of electrochemical techniques, such as high sensitivity, selectivity, and capability for real-time analysis, making them highly effective for formaldehyde monitoring. We explore the fundamental principles, mechanisms, and diverse methodologies employed in electrochemical formaldehyde detection, highlighting the role of innovative sensing materials and electrodes. Special attention is given to recent developments in nanotechnology and sensor design, which significantly enhance the sensitivity and selectivity of these detection systems. Moreover, this review identifies current challenges and discusses future research directions. Our aim is to encourage ongoing research and innovation in this field, ultimately leading to the development of advanced, practical solutions for formaldehyde detection in various environmental and biological contexts.

Optical enzymatic formaldehyde biosensor based on alcohol oxidase and pH-sensitive methacrylic-acrylic optode membrane

Optical biosensor for the detection of formaldehyde has been developed based on the transparent enzymatic stacked membranes system on the glass substrate, and employing optical absorption transducer with H⁺ ion-selective Nile Blue chromoionophore (NBCM) dye-doped methacrylic acrylic (MB28) copolymer membrane as the optode membrane. Alcohol oxidase (AOx) enzymes were entrapped within the biocompatible sol-gel matrix and deposited on top of the pH optode membrane. As the uppermost catalytic membrane catalyzes the oxidative conversion of formaldehyde to formic acid and hydrogen peroxide, the immobilized NBCM undergoes protonation reaction and forms HNBCM⁺, the dark blue ion-chromoionophore complex via H⁺ ion transfer reaction within the soft and flexible MB28 polymeric membrane. This rendered the enzymatic optode membrane absorbed a high yellow light intensity from the light source and exhibited maximum absorption peaks at 610 and 660 nm. Optical evaluation of formaldehyde by means on UV-vis absorption transduction of the enzymatic stacked membranes demonstrated rapid response time of 10 min with high sensitivity, good linearity and high reproducibility across a wide formaldehyde concentration range of 1 × 10^-3-1 × 10³ mM (R² = 0.9913), and limit of detection (LOD) at 1 × 10^-3 mM, which could be useful for formaldehyde assay in industrial, agricultural, environmental, food and beverages as well as medical samples. The formaldehyde concentration in snapper fish, pomfret fish and threadfin fish samples determined by the proposed optical enzymatic biosensor were very much close to the formaldehyde concentration values determined by the UV-vis spectrophotometric NASH standard method based on the statistical t-test. This suggests that the optical biosensor can be used as a reliable method for quantitative determination of formaldehyde levels in food samples.

Industrial applications of immobilized nano-biocatalysts

Immobilized enzyme-based catalytic constructs could greatly improve various industrial processes due to their extraordinary catalytic activity and reaction specificity. In recent decades, nano-enzymes, defined as enzyme immobilized on nanomaterials, gained popularity for the enzymes' improved stability, reusability, and ease of separation from the biocatalytic process. Thus, enzymes can be strategically incorporated into nanostructured materials to engineer nano-enzymes, such as nanoporous particles, nanofibers, nanoflowers, nanogels, nanomembranes, metal-organic frameworks, multi-walled or single-walled carbon nanotubes, and nanoparticles with tuned shape and size. Surface-area-to-volume ratio, pore-volume, chemical compositions, electrical charge or conductivity of nanomaterials, protein charge, hydrophobicity, and amino acid composition on protein surface play fundamental roles in the nano-enzyme preparation and catalytic properties. With proper understanding, the optimization of the above-mentioned factors will lead to favorable micro-environments for biocatalysts of industrial relevance. Thus, the application of nano-enzymes promise to further strengthen the advances in catalysis, biotransformation, biosensing, and biomarker discovery. Herein, this review article spotlights recent progress in nano-enzyme development and their possible implementation in different areas, including biomedicine, biosensors, bioremediation of industrial pollutants, biofuel production, textile, leather, detergent, food industries and antifouling.

A new electroanalytical methodology for the determination of formaldehyde in wood-based products

A new electroanalytical methodology was developed for the sensitive and selective determination of formaldehyde in wood-based products (WBPs), featuring an extraction process using a Headspace Liquid Acceptor System (HLAS), and detection by square-wave voltammetry (SWV) on unmodified screen-printed carbon electrodes (SPCEs). HLAS, here presented for the first time, captures and derivatizes formaldehyde released from the sample by using the acetylacetone reagent as acceptor solution. The product of formaldehyde with acetylacetone, in the presence of ammonium salt, is 3,5-diacetyl-1,4-dihydrolutidine (DDL) which we have found to be electrochemically active at unmodified SPCEs, generating a selective oxidation peak at +0.4 V. Detection and quantification limits of 0.57 and 1.89 mg kg^-1 were obtained, together with intra- and inter-day precisions below 10% (as relative standard deviation, RSD). The methodology was used to determine formaldehyde content in seven WBPs, with similar results being obtained by the developed HLAS-SPCE method and the European standard method EN 717-3, with a profound reduction of total analysis time. The developed HLAS-SPCE combines the use of a new sample preparation procedure for volatiles with, as far as we know, the first determination of formaldehyde (as the derivative product, DDL) on unmodified SPCEs, offering a promising alternative for the determination of formaldehyde in WBPs and other samples.

Enzyme Immobilization on Nanomaterials for Biosensor and Biocatalyst in Food and Biomedical Industry

Enzymes exhibit a great catalytic activity for several physiological processes. Utilization of immobilized enzymes has a great potential in several food industries due to their excellent functional properties, simple processing and cost effectiveness during the past decades. Though they have several applications, they still exhibit some challenges. To overcome the challenges, nanoparticles with their unique physicochemical properties act as very attractive carriers for enzyme immobilization. The enzyme immobilization method is not only widely used in the food industry but is also a component methodology in the pharmaceutical industry. Compared to the free enzymes, immobilized forms are more robust and resistant to environmental changes. In this method, the mobility of enzymes is artificially restricted to changing their structure and properties. Due to their sensitive nature, the classical immobilization methods are still limited as a result of the reduction of enzyme activity. In order to improve the enzyme activity and their properties, nanomaterials are used as a carrier for enzyme immobilization. Recently, much attention has been directed towards the research on the potentiality of the immobilized enzymes in the food industry. Hence, the present review emphasizes the different types of immobilization methods that is presently used in the food industry and other applications. Various types of nanomaterials such as nanofibers, nanoflowers and magnetic nanoparticles are significantly used as a support material in the immobilization methods. However, several numbers of immobilized enzymes are used in the food industries to improve the processing methods which not only reduce the production cost but also the effluents from the industry.

Stability and Stabilization of Enzyme Biosensors: The Key to Successful Application and Commercialization

Fifty-five years have passed and more than 100,000 articles have been published since the first report of an electrochemical enzyme biosensor. However, very few biosensors have reached practical application and commercialization. The bulk of the research effort has been on increasing sensitivity and selectivity. In contrast, the number of publications dealing with stability or stabilization of enzyme biosensors is very small. Here, we critically review enzyme stabilization strategies as well as the progress that has been done in the past 20 years with respect to enzyme biosensor stabilization. Glucose oxidase, lactate oxidase, alcohol oxidase, and xanthine oxidase are the focus of this review because of their potential applications in food. The inconsistency in reporting biosensor stability was identified as a critical hurdle to research progress in this area. Fundamental questions that remain unanswered are outlined.

A Self-Powered Biosensor with a Flake Electrochromic Display for Electrochemical and Colorimetric Formaldehyde Detection

The formaldehyde biosensors with the features of cost effectiveness, high specificity, easy operation, and simplicity are urgently desired in routing and field detection of formaldehyde. Here, we report a new design of an enzymatic self-powered biosensor (ESPB) toward formaldehyde detection. The ESPB involves a formaldehyde dehydrogenase/poly-methylene green/buckypaper bioanode as the sensing electrode and a Prussian blue/Au nanoparticles/carbon fiber paper cathode as the electrochromic display. Formaldehyde acts as the fuel to drive the ESPB, relying on that the concentration of formaldehyde can be determined with the ESPB by both directly measuring the variance in short circuit current and observing the color change of the cathode. By measuring the variance in short circuit current, a linear detection range from 0.01 to 0.35 mM and a calculated detection limit of 0.006 mM are obtained, comparable to or better than those reported before. The color change of the cathode can be distinguished easily and exactly via the naked eye after immersing the ESPB in formaldehyde solution for 90 s with the concentration up to 0.35 mM, covering the permissive level of formaldehyde in some standards associated with environmental quality control. Specially, the formaldehyde concentration can be precisely quantified by analyzing the color change of the cathode digitally using the equation of B/(R + G + B). In the following test of real spiked samples of tap water and lake water, the recovery ratios of formaldehyde with the concentrations from 0.010 to 0.045 mM are tested to be between 95 and 100% by both measuring the variance in short circuit current and analyzing the color change of the cathode digitally. In addition, the ESPB exhibits negligible interference from acetaldehyde and ethanol and can be stored at 4 °C for 21 days with a loss of less than 8% in its initial value of short circuit current. Therefore, the ESPB with the capability of working like disposable test paper can be expected as a sensitive, simple, rapid, cost-effective colorimetric method with high selectivity in routing and field formaldehyde detection.

Development of electrochemical biosensor based on CNT-Fe3O4 nanocomposite to determine formaldehyde adulteration in orange juice

An electrochemical biosensor was developed to determine formaldehyde (HCHO) adulteration commonly found in food. The current responses of various electrodes based on multiwalled carbon nanotubes (CNTs) and synthesized nanocomposite (CNT-Fe3O4) were measured using cyclic voltammetry. The nanocomposite based biosensor shows comparatively high sensitivity (527 µA mg/L-1 cm-2), low detection limit (0.05 mg/L) in linear detection range 0.05-0.5 mg/L for formaldehyde detection using formaldehyde dehydrogenase (FDH) enzyme. In real sample analysis, the low obtained RSD values (less than 1.79) and good recovery rates (more than 90%) signify an efficient and precise sensor for the selective quantification of formaldehyde in orange juice. The developed biosensor has future implications for determining formaldehyde adulteration in citrus fruit juices and other liquid foods in agri-food chain to further resolve global food safety concerns, control unethical business practices of adulteration and reduce the widespread food borne illness outbreaks.

Microfluidic Preconcentration Chip with Self-Assembled Chemical Modified Surface for Trace Carbonyl Compounds Detection

Carbonyl compounds in water sources are typical characteristic pollutants, which are important indicators in the health risk assessment of water quality. Commonly used analytical chemistry methods face issues such as complex operations, low sensitivity, and long analysis times. Here, we report a silicon microfluidic device based on click chemical surface modification that was engineered to achieve rapid, convenient and efficient capture of trace level carbonyl compounds in liquid solvent. The micro pillar arrays of the chip and microfluidic channels were designed under the basis of finite element (FEM) analysis and fabricated by the microelectromechanical systems (MEMS) technique. The surface of the micropillars was sputtered with precious metal silver and functionalized with the organic substance amino-oxy dodecane thiol (ADT) by self-assembly for capturing trace carbonyl compounds. The detection of ppb level fluorescent carbonyl compounds demonstrates that the strategy proposed in this work shows great potential for rapid water quality testing and for other samples with trace carbonyl compounds.

Electrochemical Biosensor for Nitrite Based on Polyacrylic-Graphene Composite Film with Covalently Immobilized Hemoglobin

A new biosensor for the analysis of nitrite in food was developed based on hemoglobin (Hb) covalently immobilized on the succinimide functionalized poly(n-butyl acrylate)-graphene [poly(nBA)-rGO] composite film deposited on a carbon-paste screen-printed electrode (SPE). The immobilized Hb on the poly(nBA)-rGO conducting matrix exhibited electrocatalytic ability for the reduction of nitrite with significant enhancement in the reduction peak at −0.6 V versus Ag/AgCl reference electrode. Thus, direct determination of nitrite can be achieved by monitoring the cathodic peak current signal of the proposed polyacrylic-graphene hybrid film-based voltammetric nitrite biosensor. The nitrite biosensor exhibited a reproducible dynamic linear response range from 0.05–5 mg L⁻¹ nitrite and a detection limit of 0.03 mg L⁻¹. No significant interference was observed by potential interfering ions such as Ca²⁺, Na⁺, K⁺, NH₄⁺, Mg²⁺, and NO₃⁻ ions. Analysis of nitrite in both raw and processed edible bird’s nest (EBN) samples demonstrated recovery of close to 100%. The covalent immobilization of Hb on poly(nBA)-rGO composite film has improved the performance of the electrochemical nitrite biosensor in terms of broader detection range, lower detection limit, and prolonged biosensor stability.

An Amperometric Biosensor for the Determination of Bacterial Sepsis Biomarker, Secretory Phospholipase Group 2-IIA Using a Tri-Enzyme System

A tri-enzyme system consisting of choline kinase/choline oxidase/horseradish peroxidase was used in the rapid and specific determination of the biomarker for bacterial sepsis infection, secretory phospholipase Group 2-IIA (sPLA2-IIA). These enzymes were individually immobilized onto the acrylic microspheres via succinimide groups for the preparation of an electrochemical biosensor. The reaction of sPLA2-IIA with its substrate initiated a cascading enzymatic reaction in the tri-enzyme system that led to the final production of hydrogen peroxide, which presence was indicated by the redox characteristics of potassium ferricyanide, K₃Fe(CN)₆. An amperometric biosensor based on enzyme conjugated acrylic microspheres and gold nanoparticles composite coated onto a carbon-paste screen printed electrode (SPE) was fabricated and the current measurement was performed at a low potential of 0.20 V. This enzymatic biosensor gave a linear range 0.01–100 ng/mL (R² = 0.98304) with a detection limit recorded at 5 × 10⁻³ ng/mL towards sPLA2-IIA. Moreover, the biosensor showed good reproducibility (relative standard deviation (RSD) of 3.04% (n = 5). The biosensor response was reliable up to 25 days of storage at 4 °C. Analysis of human serum samples for sPLA2-IIA indicated that the biosensor has potential for rapid bacterial sepsis diagnosis in hospital emergency department.

A New Laccase Based Biosensor for Tartrazine

Laccase enzyme, a commonly used enzyme for the construction of biosensors for phenolic compounds was used for the first time to develop a new biosensor for the determination of the azo-dye tartrazine. The electrochemical biosensor was based on the immobilization of laccase on functionalized methacrylate-acrylate microspheres. The biosensor membrane is a composite of the laccase conjugated microspheres and gold nanoparticles (AuNPs) coated on a carbon-paste screen-printed electrode. The reaction involving tartrazine can be catalyzed by laccase enzyme, where the current change was measured by differential pulse voltammetry (DPV) at 1.1 V. The anodic peak current was linear within the tartrazine concentration range of 0.2 to 14 μM (R² = 0.979) and the detection limit was 0.04 μM. Common food ingredients or additives such as glucose, sucrose, ascorbic acid, phenol and sunset yellow did not interfere with the biosensor response. Furthermore, the biosensor response was stable up to 30 days of storage period at 4 °C. Foods and beverage were used as real samples for the biosensor validation. The biosensor response to tartrazine showed no significant difference with a standard HPLC method for tartrazine analysis.

Effects of Temperature and pH on Immobilized Laccase Activity in Conjugated Methacrylate-Acrylate Microspheres

Immobilization of laccase on the functionalized methacrylate-acrylate copolymer microspheres was studied. Poly(glycidyl methacrylate-co-n-butyl acrylate) microspheres consisting of epoxy groups were synthesized using facile emulsion photocuring technique. The epoxy groups in poly(GMA-co-nBA) microspheres were then converted to amino groups. Laccase immobilization is based on covalent binding via amino groups on the enzyme surface and aldehyde group on the microspheres. The FTIR spectra showed peak at 1646 cm−1 assigned to the conformation of the polymerization that referred to GMA and nBA monomers, respectively. After modification of the polymer, intensity of FTIR peaks assigned to the epoxy ring at 844 cm−1 and 904 cm−1 was decreased. The results obtained from FTIR exhibit a good agreement with the epoxy content method. The activity of laccase-immobilized microspheres increased upon increasing the epoxy content. Furthermore, poly(GMA-co-nBA) microspheres revealed uniform size below 2 µm that contributes to large surface area of the microspheres to be used as a matrix, thus increasing the enzyme capacity and enzymatic reaction. Immobilized enzyme also shifted to higher pH and temperature compared to free enzyme.

Development of Formaldehyde Biosensor for Determination of Formalin in Fish Samples; Malabar Red Snapper (Lutjanus malabaricus) and Longtail Tuna (Thunnus tonggol)

Electrochemical biosensors are widely recognized in biosensing devices due to the fact that gives a direct, reliable, and reproducible measurement within a short period. During bio-interaction process and the generation of electrons, it produces electrochemical signals which can be measured using an electrochemical detector. A formaldehyde biosensor was successfully developed by depositing an ionic liquid (IL) (e.g., 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][Otf])), gold nanoparticles (AuNPs), and chitosan (CHIT), onto a glassy carbon electrode (GCE). The developed formaldehyde biosensor was analyzed for sensitivity, reproducibility, storage stability, and detection limits. Methylene blue was used as a redox indicator for increasing the electron transfer in the electrochemical cell. The developed biosensor measured the NADH electron from the NAD⁺ reduction at a potential of 0.4 V. Under optimal conditions, the differential pulse voltammetry (DPV) method detected a wider linear range of formaldehyde concentrations from 0.01 to 10 ppm within 5 s, with a detection limit of 0.1 ppm. The proposed method was successfully detected with the presence of formalin in fish samples, Lutjanus malabaricus and Thunnus Tonggol. The proposed method is a simple, rapid, and highly accurate, compared to the existing technique.

A ready-to-use, versatile, multiplex-able three-dimensional scaffold-based immunoassay chip for high throughput hepatotoxicity evaluation

Hydrogel as three-dimensional (3D) substrate has been employed in miniaturized high throughput protein detection platforms to increase the number of effective antibodies and signal augmentation. However, the high water content of the hydrogel can dilute samples and create barrier to mass transfer, limiting hydrogel height to several microns in most platforms. Moreover, these platforms cannot achieve widespread use in common laboratories as they usually rely heavily on expensive robotic liquid handlers and custom-built components. Here we developed a ready-to-use, easy to store and handle, versatile and multiplex-able 3D scaffold-based immunoassay chip (3D immunoChip) possible for high throughput protein quantification using bench-top equipment in common laboratories. Sample dilution, mass transfer, signal scattering and storage problems can be avoided by using dry scaffolds that regain transparency upon rehydration. When combined with hydrophilic-hydrophobic patterned reagent loading slides, manual high throughput handling of samples can be achieved. As these micro-scaffolds are patterned without barriers in between, simultaneous and effortless washing of all the reaction zones is possible in a Petri dish. Such features aid the 3D immunoChip in saving up to 100 times reagent and about 6 times labour. The 3D immunoChip is able to detect albumin (ALB), as a model analyte, from 5 ng mL(-1) to 1000 ng mL(-1), making it comparable to the commercialized ELISA kit based on a 96-well plate (0.22-400 ng mL(-1)). This thus enables the 3D immunoChip to directly detect ALB secreted by HepaRG cells cultured in a 3D cell culture array chip for high throughput drug hepatotoxicity evaluation, which could potentially accelerate drug screening.

Diverse applications of electronic-nose technologies in agriculture and forestry

Electronic-nose (e-nose) instruments, derived from numerous types of aroma-sensor technologies, have been developed for a diversity of applications in the broad fields of agriculture and forestry. Recent advances in e-nose technologies within the plant sciences, including improvements in gas-sensor designs, innovations in data analysis and pattern-recognition algorithms, and progress in material science and systems integration methods, have led to significant benefits to both industries. Electronic noses have been used in a variety of commercial agricultural-related industries, including the agricultural sectors of agronomy, biochemical processing, botany, cell culture, plant cultivar selections, environmental monitoring, horticulture, pesticide detection, plant physiology and pathology. Applications in forestry include uses in chemotaxonomy, log tracking, wood and paper processing, forest management, forest health protection, and waste management. These aroma-detection applications have improved plant-based product attributes, quality, uniformity, and consistency in ways that have increased the efficiency and effectiveness of production and manufacturing processes. This paper provides a comprehensive review and summary of a broad range of electronic-nose technologies and applications, developed specifically for the agriculture and forestry industries over the past thirty years, which have offered solutions that have greatly improved worldwide agricultural and agroforestry production systems.

Preparation of a Cu(II)-PVA/PA6 composite nanofibrous membrane for enzyme immobilization

PVA/PA6 composite nanofibers were formed by electrospinning. Cu(II)-PVA/PA6 metal chelated nanofibers, prepared by the reaction between PVA/PA6 composite nanofibers and Cu²⁺ solution, were used as the support for catalase immobilization. The result of the experiments showed that PVA/PA6 composite nanofibers had an excellent chelation capacity for Cu²⁺ ions, and the structures of nanofibers were stable during the reaction with Cu²⁺ solution. The adsorption of Cu(II) onto PVA/PA6 composite nanofibers was studied by the Langmuir isothermal adsorption model. The maximum amount of coordinated Cu(II) (q_m) was 3.731 mmol/g (dry fiber), and the binding constant (K_l) was 0.0593 L/mmol. Kinetic parameters were analyzed for both immobilized and free catalases. The value of V_max (3774 μmol/mg·min) for the immobilized catalases was smaller than that of the free catalases (4878 μmol/mg·min), while the K_m for the immobilized catalases was larger. The immobilized catalases showed better resistance to pH and temperature than that of free form, and the storage stabilities, reusability of immobilized catalases were significantly improved. The half-lives of free and immobilized catalases were 8 days and 24 days, respectively.