Application of graphene-based materials for detection of nitrate and nitrite in water—a review

Nitrite and nitrate are widely found in various water environments but the potential toxicity of nitrite and nitrate poses a great threat to human health. Recently, many methods have been developed to detect nitrate and nitrite in water. One of them is to use graphene-based materials. Graphene is a two-dimensional carbon nano-material with sp2 hybrid orbital, which has a large surface area and excellent conductivity and electron transfer ability. It is widely used for modifying electrodes for electrochemical sensors. Graphene based electrochemical sensors have the advantages of being low cost, effective and efficient for nitrite and nitrate detection. This paper reviews the application of graphene-based nanomaterials for electrochemical detection of nitrate and nitrite in water. The properties and advantages of the electrodes were modified by graphene, graphene oxide and reduced graphene oxide nanocomposite in the development of nitrite sensors are discussed in detail. Based on the review, the paper summarizes the working conditions and performance of different sensors, including working potential, pH, detection range, detection limit, sensitivity, reproducibility, repeatability and long-term stability. Furthermore, the challenges and suggestions for future research on the application of graphene-based nanocomposite electrochemical sensors for nitrite detection are also highlighted

Bio-Inspired Materials for Electrochemical Sensors

Electrochemical biosensors are a rapidly growing research area that has greatly improved its specificity, accuracy, and precision in the detection of biomolecules in contemporary literature and industry alike. Typically, these systems exist in a three-electrode conformation with a working electrode functioning as the anode, a counter electrode functioning as the cathode, and a reference electrode allowing for the control of potential in the system. The method by which these sensors work is through the sharing of electrons via redox reactions with the target molecule and the working electrode or modifications on its surface. By exploiting the function of biomaterials that participate in natural substrate-binding redox phenomena, new opportunities for detecting critical molecules in complex situations can be created. In this dissertation, three distinct electrochemical biosensors were created by mimicking natural phenomena and implementing materials that directly or indirectly participate in the corresponding reactions. First, a dopamine sensor was created via a composite of lignin-derived graphene oxide and the marine algae-derived polysaccharide kappa carrageenan. Different ratios of GO, a known electrooxidizing catalyst of dopamine, with kappa carrageenan were used to create a binder-free film for dropcasting on the working electrode. It was designed on the principle of its interactions with the nervous system when injected in rats to induce analgesia, interfering with standard dopamine behavior. The system demonstrated a linear range of 1 - 250 μmol L-1 and a limit of detection of 0.14 μmol L-1 (s/n=3). In the second chapter, a sensor for the human and animal health hazard nitrite was constructed using the transition metal sulfide NiS. Transition metal sulfides are the catalytic center for nitrite oxidation to nitrate in nitrogen fixing bacteria found in the environment. This section utilized a novel electrodeposition method for creating a binderfree layer of NiS on the surface of the glassy carbon electrode. This system demonstrated a linear range of 0.04 – 1 μM, 1 – 5.3 μM and a detection limit of 0.01 μM. For the final chapter, a novel sensor was created for the cryoprotective sugar trehalose, an indicator of bacterial contamination in meat and produce without any electrochemical assay precedent. This system utilized the interactions found between alkali earth metal ions and trehalose in which the two molecules form complexes. Magnesium phthalocyanine, which is a commercially available dye, as well as synthesized magnesium tetraphenylporphyrin and calcium tetraphenylporphyrin were implemented as drop-casted coatings on the working electrode to electrodeposit trehalose on the surface and detect its oxidation via squarewave anodic stripping voltammetry in the complex media Luria-Bertani broth. The system was also used to gauge fluctuations in E. coli in broth by autoclaving the cultures and directly testing the media containing lysed bacteria. The system demonstrated a linear range of 0.25 mM – 100 mM, with magnesium mesotetraphenylporphyrin exhibiting the highest repeatability

Towards deployable analytical systems for nutrient monitoring in natural waters

The freshwater environment is intrinsically linked to human, animal and plant life and is an indispensable resource for the economy. Effective water quality monitoring is therefore one of the cornerstones of environmental protection and this importance is reflected within both European and global legislation. Nutrient pollution in water bodies can be seen as one of the largest global problems which effects the freshwater environment. Current legislation and policies governing water quality depend on grab sampling techniques, providing only instantaneous data which can result in a non-representative estimate of the nutrient pollution load status of a water body. In order to fully satisfy the water sectors need for comprehensive analysis, management and protection, effective portable in-situ nutrient monitoring systems are required. The focal point of this research was based around the current need which exists for inexpensive, robust in-situ nutrient monitoring solutions for the freshwater environment. The primary goal was to develop a low-cost, field deployable, automated IC system for nutrient anion analysis. Complimentary to this work, portable systems based on colorimetry for nutrient analysis were also explored. Through this research, a portable low-cost nitrate test kit has been developed which is based on a modified version of the Griess assay and employs zinc as a reducing agent. The developed method was validated according to ISO17025 accreditation guidelines and reliably detected nitrate in a range of freshwater samples. A portable, lightweight capillary IC system for anion analysis in water was also developed and demonstrated in a laboratory setting. The IC uses low-cost, miniaturised components and through a modular design enables flexible system modification. Progressing from this capillary system, a new low-cost, UV absorbance detector incorporating a 235 nm light emitting diode (LED) was developed for portable ion chromatography. The detector enabled selective, fast determination of nitrite and nitrate in a range of natural waters. In an attempt to develop a portable system for ammonium analysis, a multi-material 3D printed microfluidic reactor with integrated heating was fabricated and used with colorimetry to facilitate fast ammonium determination. Although the analytical range for ammonium xxii determination was narrow, the developed 3D printed heater represents a novel contribution in the area of 3D printed analytical systems. Finally, an IC which is low-cost, automated and fully deployable was developed which allows for in-situ analysis of nitrite and nitrate in a wide variety of natural waters. The system employed 3D printed pumps for eluent delivery and the 235 nm LED based optical detector which was developed during the course of the research. The system was deployed at various locations around the world and achieved an analytical performance comparable to accredited benchtop instrumentation

Carbon-based nanostructures in hybrid materials for detection and removal of water pollutants

The thesis is mainly focused on the better understanding of carbon dots (C-dots) formation in bottom-up syntheses, by identifying the key chemical processes and correlating them to the observed fluorescence. Therefore, several types of C-dots were studied, by systematically varying the used (molecular) precursor ratios and reaction times. Selected samples were surface functionalized by organosilanes to reveal the role of the C-dots surface functional groups in the overall photoluminescence. As better understanding of the ongoing processes finally achieved, the synthesized C-dots were applied in photocatalysis experiments by combining them with titania and an appropriate C-dot was tested as a nitrite ion sensor

Recent advances in chemical sensors for soil analysis: a review

The continuously rising interest in chemical sensors' applications in environmental monitoring, for soil analysis in particular, is owed to the sufficient sensitivity and selectivity of these analytical devices, their low costs, their simple measurement setups, and the possibility to perform online and in-field analyses with them. In this review the recent advances in chemical sensors for soil analysis are summarized. The working principles of chemical sensors involved in soil analysis; their benefits and drawbacks; and select applications of both the single selective sensors and multisensor systems for assessments of main plant nutrition components, pollutants, and other important soil parameters (pH, moisture content, salinity, exhaled gases, etc.) of the past two decades with a focus on the last 5 years (from 2017 to 2021) are overviewed

Rational Design of Advanced Functional Materials for Electrochemical Devices

In recent years, there has been a fast-growing trend in developing urea (CO(NH2)2) as a substitute H2 carrier in energy conversion due to its high energy density, nontoxicity, stability, and nonflammability. Urea, a byproduct in the metabolism of proteins and a frequent contaminant in wastewater, is an abundant compound that has demonstrated favorable characteristics as a hydrogen-rich fuel source with 6.7 wt % gravimetric hydrogen content. Also, there is 2-2.5 wt % urea from mammal urine; therefore, 0.5 million ton of additional fuels will be produced per year just from human urine (240 million ton each year). Electrochemical oxidation has been recognized as an efficient strategy for urea conversion and wastewater remediation. Thus, the chemical energy harvested from urea/urine can be converted to electricity via urea oxidation reaction (UOR). Moreover, the removal of urea from water is a priority for improving drinking water quality and presents an opportunity for UOR. However, the transition of UOR from theory and laboratory experiments to real-world applications is largely limited by the conversion efficiency, catalyst cost, and feasibility of wide-spread usage. Therefore, utilization of urea using electrochemical method is a ‘two birds with one stone’ strategy which convert wastewater to electricity via anodic urea oxidation reaction (Seen in Chapter 2). Developing efficient and low-cost urea oxidation reaction (UOR) catalysts is a promising but still challenging task for environment and energy conversion technologies such as wastewater remediation and urea electrolysis. NiO nanoparticles that incorporated graphene as the NiO@Graphene composite were constructed to study the UOR process in terms of density functional theory. The single-atom model, which differed from the previous used heterojunction model (Chapter 2), was employed for the adsorption/desorption of urea and CO2 in the alkaline media. As demonstrated from the calculated results, NiO@Graphene prefers to adsorb the hydroxyl group than urea in the initial stage due to the stronger adsorption energy of the hydroxyl group. After NiOOH@Graphene was formed in the alkaline electrolyte, it presents excellent desorption energy of CO2 in the rate-determining step. Electronic density difference and the d band center diagram further confirmed that the Ni(III) species is the most favorable site for urea oxidation while facilitating charge transfer between urea and NiO@Graphene. Moreover, graphene provides a large surface for the incorporation of NiO nanoparticles, enhancing the electron transfer between NiOOH and graphene and promoting the mass transport in the alkaline electrolyte. Notably, this work provides theoretical guidance for the electrochemical urea oxidation work (As presented in Chapter 3). In addition, urea oxidation reaction (UOR) has been known as a typical energy conversion reaction but is also a viable method for renal/liver disease diagnostic detection. Here, we reported the three-dimensional nickel oxide nanoparticles decorated on the carbonized eggshell membrane (3D NiO/c-ESM) as a modified electrode toward urea detection. The electrocatalysts are characterized by XRD, SEM, and EDX to confirm its structural and morphological information. NiO/c-ESM modified electrode exhibits an outstanding performance for urea determination with a linear range from 0.05 to 2.5 mM, and limit detection of ~20 μM (3σ). This work offered a green approach for introducing 3D nanostructure through employing biowaste ESMs as templates, providing a typical example for producing new value-added nanomaterials with urea detection (Presented in Chapter 4). Generally, urea oxidation reaction happens on the anode, less attention is paid on the cathode. In fact, hydrogen evolution reaction happens on cathode during water/urea electrolysis. Therefore, in this chapter (Chapter 5), we focus our attention on the cathodic reaction, as follows: Transition metal oxides (TMOs), especially nickel oxide (NiO), are environmentally benign and cost-effective materials, and have recently emerged as potential hydrogen evolution reaction (HER) electrocatalysts for future industrial scale water splitting in alkaline environment. However, their applications in HER electrocatalysts remain challenging because of poor electronic conductivity and unsatisfactory activity. Besides, the disposal of eggshell waste is also an environmentally and economically challenging problem because of food industry. Here, we report the synthesis of NiO nanoparticles (NPs) encapsulated in the carbonization of eggshell membrane via a green and facile approach for HER application. Noteworthy to mention here that the active carbon was made from the waste, eggshell membrane (ESM), meanwhile, the eggshell was used as a micro-reactor for preparation of electrocatalyst, NiO/C nanocomposite. Then, the as-prepared NiO/C nanocomposite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS). The SEM, EDS and TEM images reveal that NiO nanoparticles distributed on the carbon support, and XRD patterns confirm the presence of the nanoparticles are NiO and C hybrids. The catalytic activity and durability of NiO/C nanocomposite was examined for HER in 1 M KOH solution. It has been observed that NiO/C nanocomposite showed the better catalytic activity with the smallest Tafel slope of 77.8 mV dec−1 than single component\u27s result, NiO particles (112.6 mV dec−1) and carbonization of ESM (94.4 mV dec−1). It indicates that the HER performance of electrocatalyst can be enhanced by synergistic effect between NiO particles and carbonization of ESM, with better durability after 500 CV cycles. Furthermore, such design principle for developing interfaces between TMOs and C by a green and facile method can offer a new approach for preparing more efficient electrocatalysts (Seen in Chapter 5). Differed from other chapters, Chapter 4 focuses on the electroanalytical application of advanced nanomaterials. In this chapter, the sweep wave voltammetry (SWV) method was used for molecule detection. It is noted that we also developed several methods to detect small molecules, including differential pulse voltammetry (DPV) and chronopotentiometry (i-t). Therefore, several novel nanomaterials like gold nanoparticles and ZIF-8, two-dimensional nickel phthalocyanine-based metal-organic framework compounds were synthesized, respectively, and then used for the electroanalytical application, listed as Appendix A and B avoiding breaking the logistic of the whole manuscript

Polyvinylpyrrolidone, graphene oxide and their composites as potential fluorescence sensing materials for nitrate and nitrite ions

The existence of toxic nitrate (NO3 -) and nitrite (NO2 -) ions above the permissible level causes environmental pollution and human health hazard. Therefore, many studies have been carried out to improve sensitivity and selectivity of sensors for the ion detections. In this study, polyvinylpyrrolidone (PVP), graphene oxide (GO), and polyvinylpyrrolidonegraphene oxide (PVP-GO) were prepared, characterized, and tested for their ability to detect nitrate and nitrite ions. A series of PVP with concentration of 1-10% was prepared by dissolvation in deionized water. The PVP has –C=O and –N–C sensing sites, with excitation wavelength of 285 nm and 330 nm and emission wavelength of 408 nm and 410 nm, respectively. All the PVP sensing materials showed greater sensitivity towards nitrite than nitrate. It was found that 7% PVP showed the best sensitivity for the analytes detection at both sensing sites. Nitrate preferred the –C=O site, while nitrite preferred the –N–C site. The selectivity tests demonstrated that 7% PVP gave great selectivity towards analytes even in the presence of SO4 2-, HCO3 -, or Cl-, but not OH- ion. The high interference from OH- could be due to hydrogen bond formation. Computational simulation for PVP and analytes was investigated using B3LYP/6-311G(d,p). The simulation studies revealed that PVP formed greater interaction with nitrite than nitrate. Characterization results showed that the GO was successfully prepared by the improved Hummers’ method. GO showed greater sensitivity for the detection to nitrite than nitrate in the range of 0-100 mM. Selectivity tests found that GO showed great selectivity tawards analytes even in the presence of SO4 2- or Cl-, but low selectivity in the presence of HCO3 - or OH- ion, due to the formation of hydrogen bond. Simulation results demonstrated that GO formed greater interaction with nitrite compared to nitrate. The high binding energies between hydroxyl or carboxyl site and the analytes showed that they might be the possible sensing sites in GO. A series of PVP-GO(x) composite was prepared by mixing and sonication treatment of 7% PVP (100 mL) and various amounts of GO (x = 0.0075-0.03 g). The characterizations supported the successful formation of the composites. All composites showed superior sensitivity towards nitrite than nitrate. Among the composites, the PVP-GO(0.01) showed the highest sensitivity for the detection of both analytes. For the selectivity tests, PVP-GO(0.01) showed great selectivity for the detection of analytes even in the presence of SO4 2-, HCO3 -, or Cl-, but not for OH- ion. The simulation tests exhibited that the –C=O site of PVP interacted with hydroxyl site of GO to form PVP-GO composite. The PVP-GO showed greater interaction with nitrite compared to nitrate. All computational results matched with the experimental results. The addition of GO to the PVP was found to increase the sensitivity and selectivity for nitrate detection, but not for nitrite detection. However, the composite gave better limit of detection (LOD) than the 7% PVP and GO. This study showed that among all the investigated materials, 7% PVP was the most potential fluorescence sensor for nitrate detection with LOD of 4.00 mM at –C=O site, while PVP-GO(0.01) was the most potential one for nitrite detection with LOD of 0.26 mM at –N–C site. Real sample testing using UTM lake water demonstrated the potential application of 7% PVP as a fluorescence sensor

Polyvinylpyrrolidone, graphene oxide and their composites as potential fluorescence sensing materials for nitrate and nitrite ions

The existence of toxic nitrate (NO3 -) and nitrite (NO2 -) ions above the permissible level causes environmental pollution and human health hazard. Therefore, many studies have been carried out to improve sensitivity and selectivity of sensors for the ion detections. In this study, polyvinylpyrrolidone (PVP), graphene oxide (GO), and polyvinylpyrrolidonegraphene oxide (PVP-GO) were prepared, characterized, and tested for their ability to detect nitrate and nitrite ions. A series of PVP with concentration of 1-10% was prepared by dissolvation in deionized water. The PVP has –C=O and –N–C sensing sites, with excitation wavelength of 285 nm and 330 nm and emission wavelength of 408 nm and 410 nm, respectively. All the PVP sensing materials showed greater sensitivity towards nitrite than nitrate. It was found that 7% PVP showed the best sensitivity for the analytes detection at both sensing sites. Nitrate preferred the –C=O site, while nitrite preferred the –N–C site. The selectivity tests demonstrated that 7% PVP gave great selectivity towards analytes even in the presence of SO4 2-, HCO3 -, or Cl-, but not OH- ion. The high interference from OH- could be due to hydrogen bond formation. Computational simulation for PVP and analytes was investigated using B3LYP/6-311G(d,p). The simulation studies revealed that PVP formed greater interaction with nitrite than nitrate. Characterization results showed that the GO was successfully prepared by the improved Hummers’ method. GO showed greater sensitivity for the detection to nitrite than nitrate in the range of 0-100 mM. Selectivity tests found that GO showed great selectivity tawards analytes even in the presence of SO4 2- or Cl-, but low selectivity in the presence of HCO3 - or OH- ion, due to the formation of hydrogen bond. Simulation results demonstrated that GO formed greater interaction with nitrite compared to nitrate. The high binding energies between hydroxyl or carboxyl site and the analytes showed that they might be the possible sensing sites in GO. A series of PVP-GO(x) composite was prepared by mixing and sonication treatment of 7% PVP (100 mL) and various amounts of GO (x = 0.0075-0.03 g). The characterizations supported the successful formation of the composites. All composites showed superior sensitivity towards nitrite than nitrate. Among the composites, the PVP-GO(0.01) showed the highest sensitivity for the detection of both analytes. For the selectivity tests, PVP-GO(0.01) showed great selectivity for the detection of analytes even in the presence of SO4 2-, HCO3 -, or Cl-, but not for OH- ion. The simulation tests exhibited that the –C=O site of PVP interacted with hydroxyl site of GO to form PVP-GO composite. The PVP-GO showed greater interaction with nitrite compared to nitrate. All computational results matched with the experimental results. The addition of GO to the PVP was found to increase the sensitivity and selectivity for nitrate detection, but not for nitrite detection. However, the composite gave better limit of detection (LOD) than the 7% PVP and GO. This study showed that among all the investigated materials, 7% PVP was the most potential fluorescence sensor for nitrate detection with LOD of 4.00 mM at –C=O site, while PVP-GO(0.01) was the most potential one for nitrite detection with LOD of 0.26 mM at –N–C site. Real sample testing using UTM lake water demonstrated the potential application of 7% PVP as a fluorescence sensor

A review of nanocomposite-modified electrochemical sensors for water quality monitoring

Electrochemical sensors play a significant role in detecting chemical ions, molecules, and pathogens in water and other applications. These sensors are sensitive, portable, fast, inexpensive, and suitable for online and in-situ measurements compared to other methods. They can provide the detection for any compound that can undergo certain transformations within a potential window. It enables applications in multiple ion detection, mainly since these sensors are primarily non-specific. In this paper, we provide a survey of electrochemical sensors for the detection of water contaminants, i.e., pesticides, nitrate, nitrite, phosphorus, water hardeners, disinfectant, and other emergent contaminants (phenol, estrogen, gallic acid etc.). We focus on the influence of surface modification of the working electrodes by carbon nanomaterials, metallic nanostructures, imprinted polymers and evaluate the corresponding sensing performance. Especially for pesticides, which are challenging and need special care, we highlight biosensors, such as enzymatic sensors, immunobiosensor, aptasensors, and biomimetic sensors. We discuss the sensors’ overall performance, especially concerning real-sample performance and the capability for actual field application