|Main authors:||Susanne Klages, Nicolas Surdyk, Christophoros Christophoridis, Birgitte Hansen, Claudia Heidecke, Abel Henriot, Hyojin Kim, Sonja Schimmelpfennig|
|FAIRWAYiS Editor:||Jane Brandt|
|Source document:||Klages, S. et al. 2018. Review report of Agri-Drinking Water quality Indicators and IT/sensor techniques, on farm level, study site and drinking water source. FAIRWAY Project Deliverable 3.1, 180 pp
|1. Sensors for pesticide measurement in water|
|2. Sensors for nitrate measurement in water|
|3. Automatic sampler techniques for pesticide and nitrate measurement in (soil-) water|
Optical sensors provides a facile, rapid and low-cost approach for sensitive detection of pesticide based on FL, UVevis, Raman, SPR or chemiluminescence signal variations. Generally, an optical sensor contains recognition unit that can interact specially with desired target pesticide and transducer component that is employed for signaling the binding event. Recognition elements including enzyme, antibody, molecularly-imprinted polymers, aptamer, and host-guest recognizer, draw increasing attention of scientific researcher to improve analytical performance of sensor. By combining the recognition units-assisted target response, the current well-established optical probes can be divided into four broad categories based on signal output formats:
- fluorescence (FL),
- colorimetric (CL),
- surface-enhanced Raman scattering (SERS),
- surface plasmon resonance (SPR),
The optical sensors for pesticide detection based on various optical detection modes are fully described in a recent review (Yan et al., 2018) and are outlined below.
Fluorescence sensing strategy: With high sensitivity and simplification, fluorescence-based sensors as one of the most commonly used sensing candidate, have been widely applied in broad fields, including environmental monitoring ((Li et al., 2016, Guo et al., 2015, Wu et al., 2014), as the signal change can be collected vis spectrofluorophotometer and observed by naked eye on-site (Paterson and de la Rica, 2015, Wu et al., 2016, Zhang et al., 2011). As the development of advancing technologies, various kinds of materials have been widely employed for the fabrication of FL sensing platform, including fluorescent dyes (Strobl et al., 2017), semiconductors nanomaterials (Wu et al., 2013), metal nanomaterials (Chen et al., 2015, Wang et al., 2017), carbon materials (Yuan et al., 2016), and rare earth materials (Li et al., 2015). Meanwhile, it is very critical to choose and design a proper recognition unit that combined with FL probe for responding the fluorescent “turn off”, “turn on”, or “ratiometric” signal. Nsibande and Forbes reviewed the development of quantum dots-based FL probe for pesticide detection in terms of enzyme, molecularly-imprinted polymers (MIPs) and host-guest recognizer (Nsibande and Forbes, 2016). On the basis of the application of recognition elements, FL sensing strategies can be typically classified into several types: enzyme-mediated methods, antibody-assisted methods, MIPsbased methods, aptamer-based methods, host-guest complexes probe and other approach (see Yan et al., 2018 for details).
Colorimetric sensing strategy: Owing to its convenience and simplicity, colorimetric (CL) sensing strategy has proven to be a powerful analytical approach for the analysis of variety of analyte, including ions (Wang et al., 2014), chemical warfare agents (Yue et al., 2016), small organic molecules (Liu et al., 2011) and biomarkers (Sun et al., 2014). A prominent merit of CL sensing is that their direct visualization output makes them promising candidates for point-of-care assays. Therefore, the key challenge for fabricating CL platform is transforming response behavior into visual color change. Reviewed the remarkable achievements of nanomaterials, AuNPs as fascinate signal transducer have been widely utilized to design CL sensors for pesticide detection. Xu et al. developed AuNPs-based probe for the directly monitoring of acetamiprid based on the strong affinity between cyano group and gold (Sun et al., 2011). The sensing mechanism was based on the state change of AuNPs from dispersion to aggregation. The concentration of acetamiprid can be qualitatively estimated from the color change (red to blue). The color change during nanoparticle aggregation is highly dependent on their distance and concentration. Chen et al. (2018) used citrate-stabilized AuNPs for the rapid detection of terbuthylazine and dimethoate by visualizing the color change. This AuNPs-based CL sensor showed high selectivity and good sensitivity for pesticide detection in real environment samples. Recently, a CL sensor array was constructed for identifying five OPs based on the dispersion-aggregation behavior of AuNPs by Fahimi-Kashani and Hormozi-Nezhad (Fahimi-Kashani and Hormozi-Nezhad, 2016). Apart from unmodified AuNPs, functionalised AuNPs have been utilized to improve selectivity for CL detection of pesticide as well. Sun et al. displayed p-amino benzenesulfonic acid functionalised AuNPs as signal reporter for detecting carbaryl (Sun et al., 2013). Based on the similar protocol, Kim et al. (2015) introduced imidazole into AuNPs-based probe to improve the sensitivity and shorten the detection time for quantitative analysis of diazinon. In addition, melamine (Liu et al., 2015), p-nitroaniline dithiocarbamate (Rohit et al., 2016) and guanidine acetic acid (Bhamore et al., 2016) were also served as ligand to decorate AuNPs for selective CL detection of pesticide. Despite many advantages of those aggregate sensors including easy-to-use and cost-effective, more endeavors are still needed to improve the sensitivity and selectivity. The combination of recognition elements is preferred as they address the above limitations. Thus, numerous efforts have been devoted to integrating the specific affinity of recognition units with the optical properties of metal nanoparticles for realizing pesticide analysis in a sensitive, selective and accurate manner. From perspective of recognition elements, CL sensing strategies can be typically summarised as four types: enzyme strategies, antibody assays, aptamer-based methods and other approaches (see Yan et al., 2018 for details).
Surface enhanced Raman scattering strategies: Raman spectroscopy can identify the chemical content of different molecular species via the collection of molecular vibrations, that is, Raman spectroscopy possess the capability of molecular “fingerprint” recognition for distinct molecule/analyte. Surface enhanced Raman scattering strategy (SERS) essentially integrated the molecular specificity of Raman spectroscopy with optical properties of plasmonic nanostructures (Gruenke et al., 2016). Owing to optical resonance properties of coinage-metal nanostructures, the local electromagnetic field can be significantly enhanced, accompanying the improvement of the SERS signal. Taking advantages of ultrafast analysis capabilities, label-free, high stability and nondestructive characterization, the application of SERS received numerous concern in the field from biomedical diagnosis to environmental monitoring (Cialla-May et al., 2017, Henry et al., 2016, Ali et al., 2016). By means of coinage-metal nanostructures, SERS can even achieve an ultra-sensitivity down to the single-molecule level, which offered new opportunities toward obtaining single molecule recognition (Ding et al., 2016, Zrimsek et al., 2017). Recently, the development of SERS technique for pesticide detection in the aspect of sensitivity, reproducibility, selectivity and portability was recently reviewed (Pang et al., 2016). The following are recent achievements in pesticide SERS strategy as a powerful analytical tool that have focused on the development of metal nanostructures-enhanced amplification. In this section, according to the coinage metal nanoparticles-based solid substrates, SERS nanoprobes are typically designed as gold substrate, silver substrate and Au@Ag bimetallic substrate (see Yan et al. 2018 for details).
Other detection strategies: Other detection techniques, such as surface plasmon resonance (SPR) strategy and chemiluminescence strategy, have also gained strong driving forces in the detection of pesticide due to their convenient manipulation and high efficiency. By taking advantage of the outstanding distinguish ability provided by recognition unit, SPR and chemiluminescence strategy possessed excellent sensitivity and selectivity for real-time monitoring (see Yan et al. 2018 for details on some of the attractive research on SPR and chemiluminescence strategy).
Outcome and perspectives: Continuous concerns over pesticide residues have provided a long-driven force to develop novel techniques. In the past decade years, thousands of research literatures have been published for the routine and convenient monitoring of pesticide to meet increasing market and social requirements. Yan et al. (2018) have recently reviewed various kinds of optical strategy that were ingeniously designed and successfully applied for the detection of pesticide, with a specific focus on the fluorescence, colorimetric and surface-enhanced Raman scattering sensing strategies. With the emergence of high affinity of recognition elements, as well as various novel signal transduction approaches, optical assay reveal good performance to quantify pesticide residues in complex environment and food matrices, especially in the simplification and visualization design, making them ideally suitable for on-site application.
On the basis of the discussed research, the stability, accuracy, sensitivity and selectivity of optical sensor can be improved as follows: (1) the development of recognition units with excellent distinguish capacity to offer selectivity and sensitivity toward targeted analytes. For example, bi-enzyme cascade catalytic format has the merit of multi-signal amplification, greatly improving the sensitivity. (2) the utilization of novel nanomaterials that employ as signal reporters, substrates and catalysts. Ratiometric probe with dual-emission can provide built-in correction to eliminate environmental effects, exhibiting advantage in terms of enhanced sensitivity and accuracy. Nanozymes possess lower cost, higher stability, and excellent recyclability in comparison with natural enzymes, which improved the stability of sensor. Furthermore, the integration of optical strategy into paper-based analytical devices can be constructed in simplicity and miniaturization, further promoting the commercialization of devices.
Even though optical sensor has a promising future in pesticide determination, there are sustainable challenges to be addressed in the field. Particularly, most optical sensors still retain at laboratory level of testing and verifying proof-of-concept, which have not been exploited in practical applications. In the aspect of recognition events, the stability of recognition units (such as enzymes, antibody and aptamer) can be easily influenced by environmental conditions, such as temperature and pH. Furthermore, the integration of recognition event into the analytical system is a vital step in the fabrication of a successful sensor. The conjugation between recognition elements and functionalised nanomaterials will inevitably increase the complexity, cost and time of optical sensor, especially suppress the distinguish ability of recognition elements. From the perspective of nanomaterials, nanomaterials-based analytical platforms are in the starting stage of development. The specificity and catalytic activity of current nanozymes are lower than that of natural enzymes, in turn impeding the use of nanozymes. The synthesis of functional materials/nanomaterials with relatively narrow size distribution will seriously influence the performance of sensors, because inhomogeneous distribution of nanoprobe can reduce analysis accuracy. Thus, future endeavors should directly focus on addressing above obstacles.
While remarkable progress has been made toward the design of optical sensor for pesticide detection, tremendous opportunities and new trends are emerging. Coupling newly developed recognition elements (nanobodies, peptide aptamers and so on) with functional materials/nanomaterials will afford exciting opportunities for the monitoring of pesticide, which can improve the performance of sensors. On the other hands, the integration of field-deployable devices with optical sensor perform promising on-site applications, with the aid of 3D printing technologies, improving the reproducibility and stability of sensors. By taking advantage of miniaturized device and wire-less networking, the recognition event of pesticide can be transformed into a measurable digital signal by hand-held devices, such as smartphone, then the detection results can deliver to the servers. Thus, the portable detecting platforms can be carried out outside of laboratory setting with minimal user involvement, paving the way for a new generation of analytical devices in real-time detection. Yan et al. (2018) envision that, therefore, optical sensors will assuredly act significant roles in future on-site monitoring of pesticide.
Electrochemical sensors based on carbon nanotubes: In Table 10.1, the most relevant works related to pesticide electrochemical monitoring using carbon nanotubes-based electrochemical sensors reported in recent years are summarised. From that, Wong et al. (2017) have made a general overview of the current scenario related to this research topic, and, as can be seen, a number of works have been reported using different electrode architectures for the detection of various target analytes. Electrochemical sensors designed with pristine carbon nanotubes or combinations of carbon nanotubes with other modifiers can be found amongst these. There are modified electrodes consisting of CNTs and ionic liquids (ILs), porphyrin, phthalocyanines, metallic nanoparticles, and others. Thus, Wong et al. (2017) discussed the technical issues and the main analytical features, as well as the future challenges of these reports in specific subsections, which were classified according to the type of electrode modifier.
The review after Wong et al. (2017) demonstrated that carbon nanotubes provided electrochemical sensors with relatively good analytical performance in pesticide determination. Pesticides from different classes were electrochemically quantified using carbon nanotubes-based electrochemical sensors. The main electrode modification strategies consisted of the incorporation of carbon nanotubes within the composition of carbon paste electrodes and the modification of the surface of glassy carbon electrodes using the classical dropping cast method. Carbon nanotubes were used alone or in combination with different types of modifiers, including conductive polymers, phthalocyanines, porphyrins, metallic nanoparticles, ionic liquids, and graphene, among others. In general, a typical result achieved from the modification of carbon paste or glassy carbon electrodes is the very high increment of the analytical signal and the displacement of the working potential closer to zero. Both of these effects are desired to ensure high sensitivity and good analytical selectivity. The revised works demonstrated the construction of analytical curves with good linear concentration ranges (typically two concentration decades or more) and low detection limits (at least at the micromolar level). Moreover, in most cases, a good stability of response, precision of measurement, and accuracy in the recovery of spiked environmental samples are proved. Therefore, the positive effects of the use of carbon nanotubes as electrode modifiers for the preparation of electrochemical sensors dedicated to pesticide monitoring is very well illustrated and demonstrated. From the well-established electrochemical sensing performance of carbon nanotubes-based sensors toward pesticides, a set of challenges should be investigated and overcome for the advance of this important research topic.
Table 10.1: Electrochemical sensors based on carbon nanotubes for the detection of pesticides (after Wong et al., 2017)
An interesting approach for future investigations is the possibility of designing multiplexed arrays using microfluidic devices, with which different analytes could be simultaneously determined in different sensing points. This challenge is linked with a current and relevant trend in (electro)analytical chemistry, which is the miniaturization of the analytical devices, with minimization of the consumption of chemical reagents and waste generation, as well as the proposition of portable instrumentation for analysis in the field (outside of the lab doors). From an analytical point-of-view, the amperometric and voltammetric methods dedicated to the sensing of pesticides should to be subjected to more rigorous analytical tests in order to verify the selectivity and reproducibility (and improve them if necessary), long-term stability, and applicability in diversified matrice samples, once most of the electroanalytical methods are employed in an analysis of spiked water samples using bulk electrodes. The robustness of the electroanalytical methods must also be evaluated from the analysis of a great number of environmental samples. In terms of sensor architecture material, a current trend is the preparation of composites of carbon nanotubes with another allotropic carbon forms, such as carbon black, graphene, or diamond. These classes of carbon composite electrodes are very promissory for electroanalysis purposes, and future electrochemical investigations should be carried out on the sensing and biosensing of pesticides.
A biosensor is an analytical device, used for the detection of an analyte, combining a biological component (bioreceptor represented by biomolecules or synthetic molecules obtained using biological scaffolds) with a physico-chemical detector, as well as an associated electronic system, which amplifies, process and display the detected signal. A successful biosensor must use a highly specific biocatalyst, stable in various stirring, pH, and temperature conditions (most often enzymes) (Schöning and Poghossian, 2002), give a dose-dependent and, desirably, real-time response, be costeffective, portable, easy to use (Grieshaber et al., 2008).
Biosensors are used in a wide range of applications for the quick and easy detection of pesticides and water contaminants. Gheorghe et al. (2017) present an extensive review of biosensors including:
- Electrochemical biosensing techniques used for pesticides detection
- Optical and imaging biosensing methods
- Whole-cell based biosensors
Considering the electrochemical biosensing techniques, Ramnani et al. (2016) report that biosensors based on carbon nanostructures are suitable for the design of portable and point-of-use/field –deployable assay kits. Carbon allotropes such as graphene and carbon nanotubes, have indeed been incorporated in electrochemical biosensors for highly sensitive and selective detection of various analytes, due to their many advantages for such applications, like high carrier mobility, ambipolar electric field effect, high surface area, flexibility and compatibility with microfabrication techniques. A simple and sensitive electroanalytical method for cyclic voltammetry and differential pulse voltammetry determination using a magnetic nickel ferrite (NiFe2O4)/MWCNTs nanohybrid-modified GCE has been developed. The method was used to detect benomyl in real samples with satisfactory results (Wang et al., 2015).
Piezoelectric Biosensors, as immunosensors based on acoustic waves, are of emerging interest because of their good sensitivity, real-time monitoring capability, and experimental simplicity (Jia et al., 2012). Piezoelectric systems have emerged as ones of the most attractive biosensing assays for the biopesticides detection due to their simplicity, low instrumentation costs, possibility for real-time and label-free detection and generally high sensitivity. Piezoelectric crystals such as quartz vibrate with characteristic resonant frequency depending on their thickness and cut under the influence of an electric field. The resonant frequency will modify when different molecules adsorb or desorb from the surface of the crystal, and the induced changes are detected by an electronic circuit. Biosensors based on the quartz crystal microbalance have been reported in the literature for organophosphate and carbamate pesticide analysis (Marrazza, 2014).
As a part of developing new systems for continuously monitoring the presence of pesticides in groundwater, a microfluidic amperometric immunosensor was developed for detecting the herbicide residue 2,6-dichlorobenzamide (BAM) in water. A competitive immunosorbent assay served as the sensing mechanism and amperometry was applied for detection. Both the immunoreaction chip (IRC) and detection (D) unit are integrated on a modular microfluidic platform with in-built microflow-injection analysis (μFIA) function. The immunosorbent, immobilized in the channel of the IRC, was found to have high long-term stability and withstand many regeneration cycles, both of which are key requirements for systems utilized in continuous monitoring. Detection of BAM standard solutions was performed in the concentration range 0.0008-62.5 μg/L, which demonstrate the potential of the constructed μFIA immunosensor as an atline monitoring system for controlling the quality of groundwater supply (Uthuppu et al., 2015).
Busa et al. (2016) present a review of paper-based analytical devices (μPADs) that incorporate different detection methods such as colorimetric, electrochemical, fluorescence, chemiluminescence, and electrochemiluminescence techniques for food and water analysis. In Table 10.2., different paper-based platforms are presented.
Table 10.2: Summary of pesticides and insecticides for food and water analyses on paper-based platforms (after Busa et al., 2016)
 Wang, S.; Ge, L.; Li, L.; Yan, M.; Ge, S.; Yu, J. Molecularly imprinted polymer grafted paper-based multi-disk micro-disk plate for chemiluminescence detection of pesticide. Biosens. Bioelectron. 2013, 50, 262–268.
 Sicard, C.; Glen, C.; Aubie, B.; Wallace, D.; Jahanshahi-Anbuhi, S.; Pennings, K.; Daigger, G.T.; Pelton, R.; Brennan, J.D.; Filipe, C.D.M. Tools for water quality monitoring and mapping using paper-based sensors and cell phones. Water Res. 2015, 70, 360–369.
 Nouanthavong, S.; Nacapricha, D.; Henry, C.; Sameenoi, Y. Pesticide analysis using nanoceria-coated paper-based devices as a detection platform. Analyst 2016, 141, 1837–1846.
 Liu, W.; Kou, J.; Xing, H.; Li, B. Paper-based chromatographic chemiluminescence chip for the detection of dichlorvos in vegetables. Biosens. Bioelectron. 2014, 52, 76–81.
 Liu, W.; Guo, Y.; Luo, J.; Kou, J.; Zheng, H.; Li, B.; Zhang, Z. A molecularly imprinted polymer based a lab-on-paper chemiluminescence device for the detection of dichlorvos. Spectrochim. Acta. A Mol. Biomol. Spectrosc. 2015, 141, 51–57.
 Badawy, M.E.I.; El-Aswad, A.F. Bioactive paper sensor based on the acetylcholinesterase for the rapid detection of organophosphate and carbamate pesticides. Int. J. Anal. Chem. 2014, 2014, 536823.
 Sun, G.; Wang, P.; Ge, S.; Ge, L.; Yu, J.; Yan, M. Photoelectrochemical sensor for pentachlorophenol on microfluidic paper-based analytical device based on the molecular imprinting technique. Biosens. Bioelectron. 2014, 56, 97–103
 Su, Y.; Ma, S.; Jiang, K.; Han, X. CdTe-paper-based Visual Sensor for Detecting Methyl Viologen. Chin. J. Chem. 2015, 33, 446–450.
With the goal to devise portable and easy measuring techniques and considering the increasing use of smartphones, the number of μPAD strategies that incorporate mobile or smartphones for target measurements is increasing. For instance, Sincard et al. (2015) describe a combination of paper-based sensors as an ultra-low cost approach for large-scale monitoring of water quality. The paper-based analytical device (mPAD) produces a colorimetric signal that is dependent on the concentration of a specific target, including organophosphate pesticides in water. A mobile phone equipped with a camera for capturing images of two mPADs e one tested with a water sample and the other tested with clean water that is used as a control, and an on-site image processing app that uses a novel algorithm for quantifying color intensity and relating this to contaminant concentration (Figure 10.1). The mobile phone app utilizes a pixel counting algorithm that performs with less bias and user subjectivity than the typically used lab-based software, ImageJ. The use of a test and control strip reduces bias from variations in ambient lighting, making it possible to acquire and process images on-site. The cell phone is also able to GPS tag the location of the test, and transmit results to a newly developed website, WaterMap.ca™, that displays the quantitative results from the water samples on a map. We demonstrate our approach using a previously developed mPAD that detects the presence of organophosphate pesticides based on the inhibition of immobilized acetylcholinesterase by these contaminants. The objective of this paper is to highlight the importance and potential of developing and integrated monitoring system consisting of mPADs, cell-phones and a centralised web portal for low-cost monitoring environmental contaminants at a large-scale.
In their upcoming review on spectroscopic methods for determination of nitrite and nitrate in environmental samples, Singh et al. (2019) extensively described the different laboratory methods referring 229 publications on the topic.
According to Azmi et al. (2017), many researchers in the field of potentiometry, electrochemical, and biosensors have focused on miniaturising their detection systems to enhance the capability of nitrate in-situ measurement. The performance of miniaturised sensor systems is comparable to that of conventional systems.
Basically, the conventional architecture of the system consists of two electrodes known as the working electrode and the reference electrode; a salt bridge, and a voltmeter. Figure 10.2(a) illustrates the architecture of the conventional potentiometry system. Meanwhile Figure 10.2(b) illustrates that of the miniaturised potentiometry system.
The advantages of this technique are its low cost (Hassan et al., 2007, Zhang et al., 2015, Mendoza et al., 2014, Paczosa-bator et al., 2014), non-destruction of sample, portable device (Zhang et al., 2015, Mendoza et al., 2014, Chang et al., 2013, Hassan et al., 2007, Santos et al., 2004) with fast response/feedback (Zhang et al., 2015, Li and Li, 2010, Paczosa-bator et al., 2014, Chang et al., 2013, Santos et al., 2004) and the requirement of minimum sample pretreatment.
Research of the potentiometry system has followed several avenues. Early work by Hassan (1976) was concerned with organic nitrate ions and nitramine determination based on the reaction with mercury sulphuric acid mixture. Mendoza et. al. (2014) characterised a nanobiocomposite as Ion Selective Electrodes (ISE) for nitrate ion determination in water. Mahajan et. al. (2007) developed a polymeric membrane by means of Zn (||) complex-based electrodes that work as anion carriers for nitrate anion determination in water. Li and Li (2010) and Nuñez et al. (2013) predict the nitrate contamination level in water based on an artificial neural network (ANN) algorithm.
Azmi et al. (2017) remind that the use of a membrane helps the potentiometry system to be selective to nitrate ions and is one of the factors that affects the system’s limit of detection (LOD). Bendikov and Harmon (2005) mentioned that doped polypyrrole (PPy(NO3-)) is a highly selective membrane in an ISE system for nitrate determination in water. They revealed that conductive polymer polypyrrole is widely used due to its high conductivity ability and it being relatively stable. As a result, Zhang et al. (2015) took the initiative to apply doped polypyrrole as a sensitive membrane material for the potentiometry system. The polypyrrole could improve selectivity, simplify the recipe procedure, and reduce toxicity compared to the conventional non-porous polyvincyl chloride (PVC) ISE (Zhang et al., 2015). Moreover, this study successfully demonstrated that the use of carbon nanostructure materials between the membrane and the substrate layer in the electrode structure of potentiometric system could prevent the water formation that led to instability. Meanwhile, Mahajan et. al. (2007) developed a polymeric membrane that was made of zinc (||) complex for selective nitrate determination in water. The finding demonstrates that the output of a potentiometry system using zinc (||) complex membrane exhibits better selectivity for nitrate ions than for other inorganic anions. They highlighted the advantages of zinc (||) complexes, such as stable detection reproducibility and being highly sensitive to nitrate. Wardak (1976) developed an active membrane component using trihexyltetradecylphosphonium chloride (THTDPCl) for polymeric membrane. THTDPCl could enhance the PVC membrane sensitivity by reducing electrical resistance.
The majority of potentiometric nitrate sensors that integrated either true-liquid or liquid polymeric membranes are bulky due to the tubular design with internal reference electrode and internal reference electrolyte solutions. Thus, a micro-fabricated planar potentiometric sensor was introduced (Hassan et al., 2007, Calvo-lópez et al., 2013). The micro-scale sensor could provide several advantages such as small size, simple design, low cost and mass production. Various materials are introduced to produce a micro-scale potentiometric sensor chip. Such materials are screen-printed thick film, silicon transducer chip, silicon nitride base chip and metal printed flexible polyimide film. Current miniaturised micro scale sensors for nitrate detection demonstrate a good response towards nitrate ions.
The miniaturisation of ISEs, while maintaining their selectivity and sensitivity, is a crucial step in the next phase of ISE evolution. Traditionally, in so-called coated-wire ISEs, the ion-selective membrane is placed directly on a solid electronically conductive support, thereby removing the need for an inner solution. However, in these devices, it was observed that the long-term potential stability was quite limited, and they were useful only in specific applications such as capillary electrophoresis or in flow-injection analysis. An important breakthrough in ISE design was achieved by the application of conducting polymers (CPs) as a solid contact layer, i.e. a mediating layer between the electronically conducting substrate and ionically conducting ISE membrane, which was possible due to the mixed conductivity of CPs. Various conductive polymers have been examined as possible internal contact materials that could simultaneously stabilise the overall electrode potential and remove the need for an inner filling solution.
Basically, the conventional architecture of the system consists of two electrodes known as the working electrode and the reference electrode, a salt bridge, and a voltmeter.
The electrochemical detection of nitrate and nitrite can be divided into a number of categories. Fortunately, these can be broadly grouped within the distinctions of voltammetric and potentiometric systems. Electrochemical systems have the ability to convert the measurement of nitrite ions into the current signal, potential difference and impedance, respectively. In electrochemical systems, various types of electrode were introduced for nitrate detection. Table 10.4 summarises the system performance based on different types of material.
The electrochemical method is widely used due to its high sensitivity to nitrate, simple operation, easy to miniaturise and low power-consumption. However, the conventional electrochemical cell is too massive to be a portable and durable device. Research into electrochemical systems for nitrate detection has followed several avenues (Andreoli et al., 2011, Bhansali and Bhansali, 2013, Can et al., 2013). This is due to the demand for portable devices for continuous monitoring of nitrate concentration in aqueous solutions.
Several researchers have developed a microfludic base associated with electrochemical sensors for miniaturisation and portable purposes (Li et al., 2011, Li et al., 2012, Li et al., 2013). This combination has promoted many advantages such as the small configuration of electrodes that can be integrated within a microfluidic platform, requiring a minimum instrumentation, small volume of sample, fast response time, and low cost. Moreover miniaturised electrochemical detection is reliable, selective, and highly sensitive to the measured sample. The current architecture of miniaturised electrochemical sensor is designed based on the planar form or flatten of structure. According to Azmi et al. (2017), the performance of miniaturised electrochemical sensor demonstrated good LOD that is comparable to the conventional size of electrochemical system. Figure 10.3(a) illustrates the conventional electrochemical system architecture. Meanwhile, Figure 10.3(b) illustrates the miniaturised electrochemical system architecture.
A biosensor is one of the direct methods used for nitrate detection in water. In a biosensor system, the concentration of targeted ion in an analyte solution can be determined by employing the biological material, detection system and signal conditioning circuit. The analyte solution is directly exposed to a biological material. The biological material interacts with the targeted ion in the analyte solution. Information on the interaction process is then translated into an electrical signal such as voltage or current by a detection system. The signal is harvested by the signal conditioning circuit in the biosensor system. The signal conditioning circuit such as a digital data acquisition system will recondition the acquired data before being analysed. The concentration of nitrate ion is estimated based on the output signal of the proposed detection system.
Over the last decade, the miniaturisation of biosensor system has been carried out to characterize and quantify the bio molecules. The reduction size of the sensor system can promote lower material cost, lower the power consumption and the system weight. In most biosensors and also chemical and gas sensors, the trace of detection reversible redox species should be implemented by using very small amounts of samples, to descend upon the nanolitre or picolitre range.
Nitrate biosensors have been developed over the last two decades considering the advantage of enzymes that are strongly substrate-selective. Nitrate reductase (NR) is used in the fabrication of nitrate biosensors. However, its multiredox centre responsible for the biological conversion of nitrate to nitrite is generally not very active, and is deeply embedded in the protein structure, thus preventing the direct electron transfer with the electrode.
Carbon nanotubes (CNTs) have emerged as a new class of nanomaterials that are receiving considerable interest owing to their ability to promote electron transfer reactions with enzymes showing low electroactivity. The high conductivity of this carbon material has led to improving electrochemical signal transduction, while its nano architecture imposes an electron contact between the redox centres. CNTs can donate and accept electrons in a wide range of potentials and could therefore be used as mediators in biosensor systems. As a result, Can et al. (2012) investigated the performance of carbon nanotube/polypyrrole/nitrate reductase biofilm electrodes for nitrate detection.
Table 10.5 summarises the different types of biological material, detection systems, LOD and applications of biosensor systems for nitrate ion detection.
Table 10.5: Types of biological materials, detection systems, LOD (a review by Azmi et al., 2017)
Paper-based sensors, so-called paper-based analytical devices (PADs), are a new alternative technology for fabricating simple, low-cost, portable and disposable analytical devices for many application areas environmental monitoring. The unique properties of paper which allow passive liquid transport and compatibility with chemicals/biochemicals are the main advantages of using paper as a sensing platform.
Current paper-based sensors are focused on microfluidic delivery of solution to the detection site whereas more advanced designs involve complex 3-D geometries based on the same microfluidic principles (Figure 10.4). Although paper-based sensors are very promising, they still suffer from certain limitations such as accuracy and sensitivity (Liana et al., 2012). However, it is anticipated that in the future, with advances in fabrication and analytical techniques, that there will be more new and innovative developments in paper-based sensors. In the Netherlands, a monitoring tool based on this technology is tested (Nitrate-app), the measurement is paper based, A phone application scans and analyzes nitrate strips on the paper.
Traditional electrochemical sensors often suffer from the effects of fouling due to the adsorption of oxidation products on the electrode surface. That is why paper-based, inexpensive, disposable electrochemical sensors have been developed for nitrite analysis. For example, Wang et al. (2017) present a system based on a simple and efficient vacuum filtration system. Taking advantage of the physicochemical properties of graphene nanosheets and gold nanoparticles, the mass transport regime of nitrite at the paper-based electrode was thin layer diffusion rather than planar diffusion. In comparison with the electrochemical responses of commercial gold electrodes and glassy carbon electrodes (GCE), a considerably larger current signal is seen at the paper-based sensing interface, which significantly improved its sensitivity for nitrite detection. In particular, the paper-based electrode was a disposable sensing device, so that it effectively avoided the fouling effect arising from the adsorption of oxidation products. According to Wang et al. (2017), the paper-based sensing platform made it possible to determine nitrite in environmental and food samples in an accurate, convenient, inexpensive, and reproducible way, indicating that the proposed system is promising for practical applications in environmental monitoring and public health.
Automatic water sampling systems exist for pesticides and nitrates measured in water samples from ground- or surface waters or extracted from soils.. It is essentially a pump controlled by a clock or other automatic trigger, so that water samples can be pumped from a water source into a bottle at some pre-determined time or event and later collected for analysis (Figure 10.5). Such devices can be settled to collect water in the satured zone (piezometer), in streams, rivers and lakes. They can be portable or require an indoor environment. Experimental systems have also been designed to sample percolating water through the vadose/unsaturated zone. These in-house systems mainly consist of sucion cups connected to a controlled pump (Hamon et al., 2006; Farsad et al., 2012).
Classically, water samples should be stored in solvent-washed or brand-new (amber) glass bottles verified as uncontaminated, sealed with aluminium foil or Teflon, fitted with new plastic screw-caps and chilled immediately to less than 4°C in a refrigerator (Kennedy et al., 1998). Organic solvent (e. g. dichloromethane) can be added immediately where convenient to limit volatilisation or hydrolysis, although care to prevent leakage is essential. Extraction of water samples with organic solvent should be made within 48 hours and immediately on receipt. Even so, it can be anticipated that samples containing endosulfan isomers will loose chemicals by volatilisation if jars are not properly sealed, ideally with Teflon. A loss of chemicals can also occur by hydrolysis if the pH of the water is above 8.
Due to its instability in over time, automatic water sampling devices require regular human intervention, e. g. to collect water samples (every 24 hours), to fit in new sampling bottles/lysimeters, to change device batteries and for other maintenance work.
Another way consist to directly sample the analytes of interest (pesticides and nitrates) rather than water. This is the aim of passive sampling technologies, which have been developed to monitor pollutants in the aquatic environment. The advantage of passive samplers is, that they sample in situ without disrupting the environment. They can be used in ground- and surface water. Thanks to their phase or selective membrane, these devices allow to integrate sampling over the time and as a result to concentrate molecules.(Figure 10.6:). Their capacity of accumulation allows to improve the sensibility of the analytical process and so to detect concentrations of micropollutants in concentrations measured in µg/L or even ng/L. Sampling proceeds without the need for any energy sources other than this chemical potential difference. Several types of devices are used depending on targeted compounds. Pesticides can for example be sampled by SPME, SLM, sorbant devices, SPMD, PDBS, POCIS, TRIMPS, dialyse membranes, Chemcatcher, TLC and PISCES devices. These tools require to remain submerged and do not respond well to dry episodes. They are considered from now on as complementary tools with the discrete water sampling techniques.
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