Nitrate biosensors and biological methods for nitrate determination

Ultra-sensitive nitrate-ion detection via transconductance-enhanced graphene ion-sensitive field-effect transistors

Current potentiometric sensing methods are limited to detecting nitrate at parts-per-billion (sub-micromolar) concentrations, and there are no existing potentiometric chemical sensors with ultralow detection limits below the parts-per-trillion (picomolar) level. To address these challenges, we integrate interdigital graphene ion-sensitive field-effect transistors (ISFETs) with a nitrate ion-sensitive membrane (ISM). The work aims to maximize nitrate ion transport through the nitrate ISM, while achieving high device transconductance by evaluating graphene layer thickness, optimizing channel width-to-length ratio (RWL), and enlarging total sensing area. The captured nitrate ions by the nitrate ISM induce surface potential changes that are transduced into electrical signals by graphene, manifested as the Dirac point shifts. The device exhibits Nernst response behavior under ultralow concentrations, achieving a sensitivity of 28 mV/decade and establishing a record low limit of detection of 0.041 ppt (4.8 × 10-13 M). Additionally, the sensor showed a wide linear detection range from 0.1 ppt (1.2 × 10-12 M) to 100 ppm (1.2 × 10-3 M). Furthermore, successful detection of nitrate in tap and snow water was demonstrated with high accuracy, indicating promising applications to drinking water safety and environmental water quality control.

Screen-printed biosensor based on electro-polymerization of bio-composite for nitrate detection in aqueous media

Bacillus sp. possessing a periplasmic nitrate reductase was used as a recognition element to develop a nitrate biosensor. The bacteria was embedded within a polyaniline (PANI) electro-conductive matrix via electro-polymerization on miniaturized carbon screen-printed electrodes (SPE) at 100 mV/s and scan rate from -0.35 V to + 1.7 V. Surface medication of SPE was verified via Fourier Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The optimal bacterial density was OD600 1.2. To enhance the biosensors performance, Bacillus sp. was (1) grown in riboflavin (RF) inducing media as an endogenous redox mediator and (2) exposed to different gamma radiation doses as a physical method to increase electron transfer. Results show a link between exposing cells to gamma irradiation stress, this was evident by electron spin resonance (ESR) and changes in FTIR spectrum, in addition to the increase in catalase enzyme. The nitrate limit of detection (LOD) was 0.5-25 mg/L for non-irradiated RF induced immobilized cells and LOD was 0.5-75 mg/L nitrate for 2 kGy gamma irradiated cells. The prepared biosensor showed acceptable reproducibility and multiple usages after storage at 4°C over 3 months. Low cost and simple preparation allow the biosensor to be mass-produced as a disposable device. Bacillus sp. and its endogenous redox mediator immobilized within polyaniline are good candidates for the improvement of amperometric biosensors for the quantification of nitrate in aqueous solutions.

Advances in Technological Research for Online and In Situ Water Quality Monitoring—A Review

Monitoring water quality is an essential tool for the control of pollutants and pathogens that can cause damage to the environment and human health. However, water quality analysis is usually performed in laboratory environments, often with the use of high-cost equipment and qualified professionals. With the progress of nanotechnology and the advance in engineering materials, several studies have shown, in recent years, the development of technologies aimed at monitoring water quality, with the ability to reduce the costs of analysis and accelerate the achievement of results for management and decision-making. In this work, a review was carried out on several low-cost developed technologies and applied in situ for water quality monitoring. Thus, new alternative technologies for the main physical (color, temperature, and turbidity), chemical (chlorine, fluorine, phosphorus, metals, nitrogen, dissolved oxygen, pH, and oxidation–reduction potential), and biological (total coliforms, Escherichia coli, algae, and cyanobacteria) water quality parameters were described. It was observed that there has been an increase in the number of publications related to the topic in recent years, mainly since 2012, with 641 studies being published in 2021. The main new technologies developed are based on optical or electrochemical sensors, however, due to the recent development of these technologies, more robust analyses and evaluations in real conditions are essential to guarantee the precision and repeatability of the methods, especially when it is desirable to compare the values with government regulatory standards.

Combined SPRi Sensor for Simultaneous Detection of Nitrate and Ammonium in Wastewater

Water pollution is a serious problem in modern society. Agriculture, being responsible for the discharge of agrochemicals, organic matter, or drug residues, produces a huge amount of wastewater. Aquaponics has the potential to reduce both water consumption and the impact of water pollution on fish farming and plant production. In the aquatic environment, inorganic nitrogen is mostly present in the form of nitrate and ammonium ions. Nitrate, as a final product of ammonia mineralization, is the most common chemical contaminant in aquifers around the world. For continuous monitoring of nitrogen compounds in wastewater, we propose a sensor for the simultaneous detection of nitrate and ammonium. A surface plasmon resonance imaging method with enzyme-mediated detection was used. Active layers of nitrate reductase and glutamine synthetase were created on the gold surface of a biochip and tested for the sensing of nitrate and ammonium in water from an aquaponic system. The proposed sensor was applied in water samples with a concentration of NO3- and NH4+ in a range between 24-780 mg·L-1 and 0.26-120 mg·L-1, respectively, with minimal pretreatment of a sample by its dilution with a buffer prior to contact on a biochip surface.

Greenhouse gas emissions from advanced nitrogen-removal onsite wastewater treatment systems

Advanced onsite wastewater treatment systems (OWTS) designed to remove nitrogen from residential wastewater play an important role in protecting environmental and public health. Nevertheless, the microbial processes involved in treatment produce greenhouse gases (GHGs) that contribute to global climate change, including CO₂, CH₄, N₂O. We measured GHG emissions from 27 advanced N-removal OWTS in the towns of Jamestown and Charlestown, Rhode Island, USA, and assessed differences in flux based on OWTS technology, home occupancy (year-round vs. seasonal), and zone within the system (oxic vs. anoxic/hypoxic). We also investigated the relationship between flux and wastewater properties. Flux values for CO₂, CH₄, and N₂O ranged from -0.44 to 61.8, -0.0029 to 25.3, and -0.02 to 0.23 μmol GHG m^-2 s^-1, respectively. CO₂ and N₂O flux varied among technologies, whereas occupancy pattern did not significantly impact any GHG fluxes. CO₂ and CH₄ - but not N₂O - flux was significantly higher in the anoxic/hypoxic zone than in the oxic zone. Greenhouse gas fluxes in the oxic zone were not related to any wastewater properties. CO₂ and CH₄ flux from the anoxic/hypoxic zone peaked at ~22-23 °C, and was negatively correlated with dissolved oxygen levels, the latter suggesting that CO₂ and CH₄ flux result primarily from anaerobic respiration. Ammonium concentration and CH₄ flux were positively correlated, likely due to inhibition of CH₄ oxidation by NH₄⁺. N₂O flux in the anoxic/hypoxic zone was not correlated to any wastewater property. We estimate that advanced N-removal OWTS contribute 262 g CO₂ equivalents capita^-1 day^-1, slightly lower than emissions from conventional OWTS. Our results suggest that technology influences CO₂ and N₂O flux and zone influences CO₂ and CH₄ flux, while occupancy pattern does not appear to impact GHG flux. Manipulating wastewater properties, such as temperature and dissolved oxygen, may help mitigate GHG emissions from these systems.

Recent advances in sensors for electrochemical analysis of nitrate in food and environmental matrices

Nitrate is one of the most common contaminants in food and the environment and mainly arises from intense human activities. Electrochemical sensors have been considered as one of the most promising analytical tools for the rapid detection of nitrate in food and environmental matrices due to their quick response, high sensitivity, ease of operation and miniaturisation, and low sample and power consumption. In this review, we summarise advances in sensors for electrochemical analysis of nitrate over the past decade. We also discuss the application of electrochemical sensing systems for the determination of nitrate in the matrices of fresh water, seawater, food, soil and particulate matter.

Microorganisms and Their Metabolic Capabilities in the Context of the Biogeochemical Nitrogen Cycle at Extreme Environments

Extreme microorganisms (extremophile) are organisms that inhabit environments characterized by inhospitable parameters for most live beings (extreme temperatures and pH values, high or low ionic strength, pressure, or scarcity of nutrients). To grow optimally under these conditions, extremophiles have evolved molecular adaptations affecting their physiology, metabolism, cell signaling, etc. Due to their peculiarities in terms of physiology and metabolism, they have become good models for (i) understanding the limits of life on Earth, (ii) exploring the possible existence of extraterrestrial life (Astrobiology), or (iii) to look for potential applications in biotechnology. Recent research has revealed that extremophilic microbes play key roles in all biogeochemical cycles on Earth. Nitrogen cycle (N-cycle) is one of the most important biogeochemical cycles in nature; thanks to it, nitrogen is converted into multiple chemical forms, which circulate among atmospheric, terrestrial and aquatic ecosystems. This review summarizes recent knowledge on the role of extreme microorganisms in the N-cycle in extremophilic ecosystems, with special emphasis on members of the Archaea domain. Potential implications of these microbes in global warming and nitrogen balance, as well as their biotechnological applications are also discussed.

Heterologous and Homologous Expression of Proteins from Haloarchaea: Denitrification as Case of Study

Haloarchaea (halophilic microbes belonging to the Archaea domain) are microorganisms requiring mid or even high salt concentrations to be alive. The molecular machinery of these organisms is adapted to such conditions, which are stressful for most life forms. Among their molecular adaptations, halophilic proteins are characterized by their high content of acidic amino acids (Aspartate (Asp) and glumate (Glu)), being only stable in solutions containing high salt concentration (between 1 and 4 M total salt concentration). Recent knowledge about haloarchaeal peptides, proteins, and enzymes have revealed that many haloarchaeal species produce proteins of interest due to their potential applications in biotechnology-based industries. Although proteins of interest are usually overproduced in recombinant prokaryotic or eukaryotic expression systems, these procedures do not accurately work for halophilic proteins, mainly if such proteins contain metallocofactors in their structures. This work summarizes the main challenges of heterologous and homologous expression of enzymes from haloarchaea, paying special attention to the metalloenzymes involved in the pathway of denitrification (anaerobic reduction of nitrate to dinitrogen), a pathway with significant implications in wastewater treatment, climate change, and biosensor design.

Hydrochemical Characteristics and Water Quality Assessment for the Upper Reaches of Syr Darya River in Aral Sea Basin, Central Asia

Based on water sampling of the upper reaches of the Syr River and its tributaries from the parts of Aral Sea Basin in Kyrgyzstan, the chemical compositions of river waters were systematically analyzed for revealing the hydrochemical characteristics and evaluating the water quality. Research indicates that there are some differences in ion concentration between the low-flow season (LFS) and high-flow season (HFS), but the hydrochemical classification reflected that all water samples fall in the calcium bicarbonate category, except that only three samples fall in the not dominant category during the LFS. The water quality classification shows that the water samples fall in the excellent to good categories for irrigation use. The analysis shows that the main ions of river waters come from the weathering of rocks, and the dissolution of carbonates is higher than that of silicates. Human activities have had an impact on the waterbody, especially inferred from the indicators of NH4-N and fecal coliform (FC). FC groups were detected in some rivers, in which the detection rate at the high-water level increased. The contents of potentially toxic elements are lower than international drinking water standards, but there are clustering differences between the LFS and HFS. There may be anthropogenic intrusions of Cu, Pb, and Zn during the LFS period and of Cu, Pb, Zn, and Cd during the HFS period. The results fill the gaps in the study of the hydrochemical composition and water quality assessment in the Aral Sea Basin and will also provide a basis for water resource management and for the study of water quality evolution in the future.

Microliter Operation for Determination of Nitrate-Nitrogen via Simple Zinc Reduction and Color Formation in a Well Plate with a Smartphone

We propose a simple greener colorimetric method for the determination of nitrate-nitrogen by operating on a 96-well microplate and using a smartphone camera as a simple detector. A slurry containing 0.3 mg zinc was used for reduction of nitrate to nitrite, the reduction solution was transferred to a 96-well microplate to react with Griess reagent to form a pink azo dye product. The color product image was captured and processed by a smartphone camera and ImageJ software, respectively. The limit of detection and limit of quantitation were 0.04 and 0.10 mg/L nitrate-nitrogen, respectively, for the smartphone camera. Application to real samples was demonstrated. The proposed method results showed no significant difference (at 95% confidence) with the hydrazine reduction method. The proposed method could be used as an alternative method for on-site analysis due to the advantages of portability and rapidity; duplicate run of 20 samples could be carried out simultaneously in 12 min.

Direct Electron Transfer of Enzymes Facilitated by Cytochromes

The direct electron transfer (DET) of enzymes has been utilized to develop biosensors and enzymatic biofuel cells on micro- and nanostructured electrodes. Whereas some enzymes exhibit direct electron transfer between their active-site cofactor and an electrode, other oxidoreductases depend on acquired cytochrome domains or cytochrome subunits as built-in redox mediators. The physiological function of these cytochromes is to transfer electrons between the active-site cofactor and a redox partner protein. The exchange of the natural electron acceptor/donor by an electrode has been demonstrated for several cytochrome carrying oxidoreductases. These multi-cofactor enzymes have been applied in third generation biosensors to detect glucose, lactate, and other analytes. This review investigates and classifies oxidoreductases with a cytochrome domain, enzyme complexes with a cytochrome subunit, and covers designed cytochrome fusion enzymes. The structurally and electrochemically best characterized proponents from each enzyme class carrying a cytochrome, that is, flavoenzymes, quinoenzymes, molybdenum-cofactor enzymes, iron-sulfur cluster enzymes, and multi-haem enzymes, are featured, and their biochemical, kinetic, and electrochemical properties are compared. The cytochromes molecular and functional properties as well as their contribution to the interdomain electron transfer (IET, between active-site and cytochrome) and DET (between cytochrome and electrode) with regard to the achieved current density is discussed. Protein design strategies for cytochrome-fused enzymes are reviewed and the limiting factors as well as strategies to overcome them are outlined.

In Situ Metalorganic Deposition of Silver Nanoparticles on Gold Substrate and Square Wave Voltammetry: A Highly Efficient Combination for Nanomolar Detection of Nitrate Ions in Sea Water

The electro-reduction of nitrate ions in artificial sea water was investigated at a gold substrate (EAu) functionalized by silver nanoparticles (AgNPs). These AgNPs were generated in situ on the gold substrate by the direct decomposition of the metalorganic N,N′-diisopropylacetamidinate silver precursor [Ag(Amd)] in the liquid phase. Very small and well dispersed AgNPs were deposited on the gold electrode and then used as working electrode (EAu/AgNPs). Square wave voltammetry (SWV) was successfully employed to detect nitrate ions (NO3−) with a detection limit (LOD) of 0.9 nmol∙L−1 in artificial sea water (pH = 6.0) without pre-concentration or pH adjustment.

A review on spectroscopic methods for determination of nitrite and nitrate in environmental samples

Nitrate is an important pollutant found in environmental samples. Nitrate and nitrite pose various environmental as well as health hazards. Different methods of determining nitrate in various environmental samples developed during previous years include spectrophotometric, chemiluminescence, electrochemical detection, chromatographic, capillary electrophoretic, spectrofluorimetric methods. Out of these, methods based on spectroscopic detection of nitrate have been discussed in this review article due to their easy availability, high sensitivity, low detection limit, economical and facile nature. Methods based on spectrophotometry, Raman Spectroscopy, IR and FTIR Spectroscopy, atomic absorption spectroscopy (AAS), fluorescence spectroscopy, chemiluminescence, mass spectroscopy, molecular emission cavity analysis (MECA), electron paramagnetic resonance spectrometry (EPR) and nuclear magnetic resonance spectroscopy (NMR) have been reviewed. The basic principle, detection limits, detection range, RSD%, sample throughput/h, advantages and disadvantages have been discussed.

Fabrication of a robust and highly sensitive nitrate biosensor based on directly grown zinc oxide nanorods on a silver electrode

This article reports the fabrication of a robust and highly sensitive nitrate biosensor based on directly-grown ZnO NRs on an electrode surface.