Chemical sensors for environmental analysis

Chemical sensors for environmental monitoring are under active development but at the moment are of only limited value. However some fiber-optical sensor systems combined with opto-electronics and lasers have the potential to fulfil the legal requirements in monitoring environmentally relevant analytes in the ppt-range. The principles of sensors, and new developments reported in the literature or from our own research are discussed.     

Electrochemical sensors for monitoring environmental pollutants

Stricter environmental controls on the emission and discharge of chemical pollutants are creating an increased demand for the development of improved chemical sensor devices. Although electrochemical sensors show great promise for this task, their utility has been constrained by a number of practical problems, the most serious being the effect of surface adsorption of impurities leading to non-reproducible response. This review presents a survey of recent advances in electrochemical sensor technology which have attempted to improve the performance of these devices. Three main areas of development have been addressed; advances in sensor design and measurement techniques, novel approaches to conferring electrode selectivity and the use of microminiaturization and microelectronics fabrication techniques. Recent applications and future prospects for the measurement of toxic metals, organics and gases including volatile organic compounds are surveyed.     

Electrochemical sensors for environmental gas analysis

The application of electrochemical sensors for measurement of concentration of pollutant gases in air in the part-per-billion (10⁹) range is reviewed. Performance-limiting factors, particularly the effects of extremes and of relatively rapid changes in ambient temperature and humidity, are noted. Variations in composition of the electrolyte in the meniscus at the electrode–gas interface and instability of the solid–liquid–gas contact line, causing important variations in current due to background electrode reactions, are deduced and suggested as the reason for the performance limitations. Suggestions are made for mitigation through instrument design.     

Nanoparticles in electrochemical sensors for environmental monitoring

We review the state-of-the-art application of nanoparticles (NPs) in electrochemical analysis of environmental pollutants. We summarize methods for preparing NPs and modifying electrode surfaces with NPs. We describe several examples of applications in environmental electrochemical sensors and performance in terms of sensitivity and selectivity for both metal and metal-oxide NPs. We present recent trends in the beneficial use of NPs in constructing electrochemical sensors for environmental monitoring and discuss future challenges.NPs have promising potential to increase competitiveness of electrochemical sensors in environmental monitoring, though research has focused mainly on development of methodology for fabricating new sensors, and the number of studies for optimizing the performance of sensors and the applicability to real samples is still limited.     

Developments in microscale and nanoscale sensors for biomedical sensing

The widespread use of point of care testing in biomedical and clinical applications is a major aim of the electrochemical field. A large number of groups are working on lab-on-a-chip systems or sensor arrays which are underpinned by electrochemical detection methodologies. Miniaturized transducers have the potential to be adopted in such systems for diagnosis of a range of diseases in both clinical and nonclinical settings. In this review, we will present the current trends and state of the art for a selection of miniaturized sensing elements (microelectrodes, nanoelectrodes, and field-effect transistors) and provide an impression of current technologies, their associated performance characteristics, and also considering the major barriers to adoption and how they might be surmounted in future so these technologies can fulfil their early promise.     

Recent progress on MOF-based optical sensors for VOC sensing

The raising apprehension of volatile organic compound (VOC) exposures urges the exploration of advanced monitoring platforms. Metal–organic frameworks (MOFs) provide many attractive features including tailorable porosity, high surface areas, good chemical/thermal stability, and various host–guest interactions, making them appealing candidates for VOC capture and sensing. To comprehensively exploit the potential of MOFs as sensing materials, great efforts have been dedicated to the shaping and patterning of MOFs for next-level device integration. Among different types of sensors (chemiresistive sensors, gravimetric sensors, optical sensors, etc.), MOFs coupled with optical sensors feature distinctive strength. This review summarized the latest advancements in MOF-based optical sensors with a particular focus on VOC sensing. The subject is discussed by different mechanisms: colorimetry, luminescence, and sensors based on optical index modulations. Critical analysis for each system highlighting practical aspects was also deliberated.

MOF-based optical sensors can achieve volatile organic compound sensing via different mechanisms: colorimetric sensing, luminescent sensing and optical-index modulation sensing.     

Quantitative SERS sensors for environmental analysis of naphthalene

In the investigation of chemical pollutants, such as PAHs (Polycyclic Aromatic Hydrocarbons) at low concentration in aqueous medium, Surface-Enhanced Raman Scattering (SERS) stands for an alternative to the inherent low cross-section of normal Raman scattering. Indeed, SERS is a very sensitive spectroscopic technique due to the excitation of the surface plasmon modes of the nanostructured metallic film. The surface of quartz substrates was coated with a hydrophobic film obtained by silanization and subsequently reacted with polystyrene (PS) beads coated with gold nanoparticles. The hydrophobic surface of the SERS substrates pre-concentrates non-polar molecules such as naphthalene. Under laser excitation, the SERS-active substrates allow the detection and the identification of the target molecules localized close to the gold nanoparticles. The morphology of the SERS substrates based on polystyrene beads surrounded by gold nanoparticles was characterized by scanning electron microscopy (SEM). Furthermore, the Raman fingerprint of the polystyrene stands for an internal spectral reference. To this extent, an innovative method to detect and to quantify organic molecules, as naphthalene in the range of 1 to 20 ppm, in aqueous media was carried out. Such SERS-active substrates tend towards an application as quantitative SERS sensors for the environmental analysis of naphthalene.     

Novel sol–gel reversible thermochromic materials for environmental sensors

Anhydrous cobalt(II) chloride has been encapsulated in several inorganic sol–gel matrices with different solvent/water ratios. Sols were cast into cuvettes and hermetically closed. Such sol–gel materials were found to be sensitive to temperature in the 10–50 °C range showing a change of colour. General characterisation of the sensitive materials was made by immersion into a thermostatic water bath and recording of the corresponding visible spectra. The optical response consisted of a change in colour from light pink to deep blue as the temperature increases. These temperature detectors behave as sensors showing good optical sensitivity in the range mentioned above and reversibility for more than 30 cycles. The sensors response time is at about 15 min and their lifetime is 2 months at least. These sol–gel materials have been designed to be applied for preservation and conservation purposes. High temperatures and cyclical temperature changes can yield severe consequences for the correct preservation of cultural heritage materials (textiles, archaeological ceramics and other remains, metallic objects and statues, stained glass windows, etc.) both in museums and outdoors.     

Free radical oxidation reaction for selectively solvatochromic sensors with dynamic sensing ability

A novel BODIPY (boradiazaindacene) dye denoted as BODIPY-DT containing terpyridine unit has been designed and characterized. The dye is found to be selective and visual solvatochromic sensor toward DMF among test organic solvents. The sensing process displays time-controllable, dynamic signal outputs in the emission colors including red, purple, yellow and even white emission colors. It is presented that selective free radical oxidation reaction happens during the recognition process.     

Dissolvable membranes as sensing elements for microfluidics based biological/chemical sensors

We demonstrate a chemical and biological sensing mechanism in microfluidics that transduces chemical and biological signals to electrical signals with large intrinsic amplification without need for complex electronics. The sensing mechanism involves a dissolvable membrane separating a liquid sample chamber from an interdigitated electrode. Dissolution of the membrane (here, a disulfide cross-linked poly(acrylamide) hydrogel) in the presence of a specific target (here, a reducing agent-dithiothreitol) allows the target solution to flow into contact with the electrode. The liquid movement displaces the air dielectric with a liquid, leading to a change (open circuit to approximately 1 kOmega) in the resistance between the electrodes. Thus, a biochemical event is transduced into an electrical signal via fluid movement. The concentration of the target is estimated by monitoring the difference in dissolution times of two juxtaposed sensing membranes having different dissolution characteristics. No dc power is consumed by the sensor until detection of the target. A range of targets could be sensed by defining membranes specific to the target. This sensing mechanism might find applications in sensing targets such as toxins, which exhibit enzymatic activity.     

Imaging fiber microarray fluorescent ion sensors based on bulk optode microspheres

Optical imaging fibers with micrometer-sized wells were used as a sensing platform for the development of microarray optical ion sensors based on selective bulk extraction principles established earlier for optodes. Uniform 10 μm sized microspheres based on plasticized poly(vinyl chloride) containing various combinations of ionophores, fluoroionophores and lipophilic ion-exchangers were prepared for the detection of sodium, potassium, calcium and chloride, and deposited onto the wells of etched fiber bundles. Specifically, sodium sensing particles were based on tert-butylcalix[4]arene tetraacetic acid tetraethylester, potassium particles on 2-dodecyl-2-methyl-1,3-propanediyl bis[N-[5′-nitro(benzo-15-crown-5)-4′-yl]carbamate] (BME-44), calcium particles on an acrylic derivative of ETH 129 (AU-1) covalently attached to a methacrylic polymer, and chloride particles based on the anticrown ionophore [9]mercuracarborand-3 (MC-3). The fluorescence emission characteristics of individual microspheres were observed from the backside of the fibers and were found to selectively and rapidly change as a function of the sample composition. The optical characteristics of the particles were found to be comparable to that of corresponding thin optode films and particles deposited onto microscope glass slides. The measuring ranges (logarithmic molar concentrations) at pH 7.0 were found as −3 to 0 for sodium, −3.5 to −0.5 for potassium, −7 to −2 for calcium, and −5 to 0.5 for chloride. Selectivities were determined over other common electrolytes and found to be sufficient for physiological applications. The simultaneous deposition of sodium and chloride sensing particles was successfully performed, demonstrating that such microarray sensors are capable of simultaneously sensing multiple analytes. This technology is compatible with other microsphere-based fluorescent sensing principles, forming a promising total analysis platform for a variety of applications.     

Halide-containing organic persistent luminescent materials for environmental sensing applications

Great progress has been made in the development of various organic persistent luminescent (OPL) materials in the past few years, and increasing attention has been paid to their interesting applications in environmental sensing due to their long emission lifetimes and high sensitivity. Especially, the introduction of different halogen elements facilitates highly efficient OPL emission with distinct lifetimes and colours. In this review, we summarize the current status of the halide-containing OPL materials for environmental sensing applications. To begin with, the photophysical processes and luminescence mechanisms of OPL materials are expounded in detail to better understand the relationship among molecular structures, OPL properties, and sensing applications. Then, representative halide-containing material systems, such as small molecules, polymers, and doping systems, are summarized with their interesting applications in sensing temperature, oxygen, H₂O, UV light and organic solvents. In addition, several challenges and future research opportunities in this field are discussed. This review aims to provide some reasonable guidance on the material design of OPL sensors and their practical applications, and tries to provide a new perspective on the application direction of organic optoelectronics.

This review presents a summary of the molecular design of halide-containing organic persistent luminescent materials, and their environmental sensing applications.     

Understanding signal amplification strategies of nanostructured electrochemical sensors for environmental pollutants

Nanomaterials used in electrochemical sensors can significantly improve the analytical performance to environmental pollutants. This review mainly discusses the strategies for signal amplification by the rational design of nanoelectrode materials from the perspective of mass and electron transfer processes of electrode/solution interface. First, the advantages and features of nanostructured electrochemical sensors for environmental pollutants are summarized. Then, the detailed discussions are focused on the signal amplification strategies by regulating dimensionality, atomic arrangement, and composition of electrode materials. This review gives a unique insight about the influences of electrode material design on mass and electron transfer processes of electrochemical sensors. Finally, on the basis of the current achievements in the field of nanomaterials, the perspectives on the challenges and opportunities for the exploration of nanostructured electrochemical sensors are put forward.     

Responsive photonic crystal for the sensing of environmental pollutants

This review covers the concepts of photonic crystal (PhC) and its usage for the sensing of environmental pollutants. PhCs are composed of periodic and ordered nanostructures which can manipulate the diffraction or reflection of light propagation through the structures. If the light spectra locate in the visible range, the color of materials can be observed by naked eye. The optical properties of PhCs are determined by the lattice constant of the crystal or by the refractive index contrast between the colloids and the surrounding medium. Based on these features, responsive PhCs can be designed to detect the environmental pollutants. In this review, we primarily described the photonic crystals for the sensing of volatile organic compounds (VOCs), organophosphates (OPs), heavy metal ions and endocrine disrupting chemicals (EDCs), and these sensors exhibited excellent sensitivity and are promising for the on-site monitoring of pollutants.     

Micro- and nanomechanical sensors for environmental, chemical, and biological detection

Micro- and nanoelectromechanical systems, including cantilevers and other small scale structures, have been studied for sensor applications. Accurate sensing of gaseous or aqueous environments, chemical vapors, and biomolecules have been demonstrated using a variety of these devices that undergo static deflections or shifts in resonant frequency upon analyte binding. In particular, biological detection of viruses, antigens, DNA, and other proteins is of great interest. While the majority of currently used detection schemes are reliant on biomarkers, such as fluorescent labels, time, effort, and chemical activity could be saved by developing an ultrasensitive method of label-free mass detection. Micro- and nanoscale sensors have been effectively applied as label-free detectors. In the following, we review the technologies and recent developments in the field of micro- and nanoelectromechanical sensors with particular emphasis on their application as biological sensors and recent work towards integrating these sensors in microfluidic systems.     

Dopamine sensing and selectivity of Nafion-coated plant tissue powder sensors

Dopamine tissue sensors were prepared using biofilms containing pretreated tissue powder of potato, banana, mushroom and apple. These sensors showed good stability, reproducibility and service life span of more than 200 days involving 300 to 500 measurements. Overlaying the sensors with a Nafion coating considerably improved the sensor selectivity for dopamine by effectively precluding neutral and anionic solutes from entering the biofilm thus removing the interference of most of the 23 solutes examined in this study. The response of the Nafion-coated apple powder sensor in a mixture containing dopamine and the three strong interferents (arterenol, histidine and tyrosine), at concentrations within their respective linear calibration range, was linear with respect to the concentration of each of the solutes. The sensitivity of each of the components in the mixture was higher than the corresponding single component linear range sensitivity.     

Organic vapor sensing properties of copolymer Langmuir-Blodgett thin film sensors

In this study, a novel poly[Styrene (ST)-co-Glycidyl Methacrylate (GMA)] copolymer material is used to fabricate Langmuir-Blodgett (LB) thin films and investigate organic vapor sensing properties. Quartz Crystal Microbalance (QCM) system is used to investigate gas sensing performance of copolymer LB films during exposure to Volatile Organic Compounds (VOCs). The poly[Styrene (ST)-co-Glycidyl Methacrylate (GMA)] LB thin film sensor sensitivities are determined to be between 0.12 and 0.25 Hz ppm^?1. Detection limits of the copolymer LB thin film are found to be between 23 and 49 ppm against organic vapors. The copolymer LB thin films are more sensitive to chloroform than other vapors used in this study. The results demonstrated that the poly[Styrene (ST)-co-Glycidyl Methacrylate (GMA)] copolymer material is promising as a organic vapor sensing device at room temperature.     

Ion-selective potentiometric sensors with silicone sensing membranes: A review

Ion-selective electrodes (ISEs) are used widely in mainframe analyzers for clinical chemistry, but there is also an increasing interest in the development of paper-based devices, wearable and implantable sensors, and other miniaturized ISEs. This trend is spurring much research in developing solid contact materials that enable miniaturization. The development of suitable polymeric matrixes for such sensors has only received less attention. In particular, in spite of lifetime limitations and toxicity concerns, polymeric matrixes comprising plasticizers are still commonly used. To that end, we note the benefits of silicone materials as alternative polymeric matrixes and, in particular, their promise for enhanced biocompatibility. While there has been steady progress in the development of ISEs with silicone membranes, this topic has not been reviewed for many years. This review critically discusses key fundamental characteristics of ISEs with silicone sensing and reference membranes, including their biocompatibility, adhesion to device substrates, water uptake, polarity, common impurities, and commercial availabilities. This is followed by a discussion of specific types of silicones and their use in ISEs, with the goal to inform and stimulate future research efforts into such devices.