Sensing strategies of the chromophores containing responsive groups.
Hazardous gas and ion pollutants are the most serious environmental problems around the world. It is of great importance to develop devices for easy detection of these hazardous substances. Fluorescence technology with high resolution and operational simplicity has attracted a lot of attention in recent years. Organic fluorescent dyes absorb/emit lights within a broad wavelength range, which is suitable for various demands. Chromophores, such as perylene, cyanine dyes, spiropyran, and so on, are widely studied as fluorescent probes for gases and ions. The dyes could respond to external stimuli through structural changes of the conjugated chromophore itself or the attached functional groups, leading to detectable spectral changes. Organic dyes are incorporated into nanoscaled films and layers, which are portable and durable for effective sensing in complex environments. In this chapter, preparation and application of fluorescent films and layers (FFL) for gaseous/ionic detection are reviewed. We discuss the response mechanism of fluorescent dyes, the fabrication of nanoscaled FFL, and some examples of FFL for the detection of gas and ion pollutants.
- environmental pollutants
- nanoscaled films and layers
- gases/ions sensor
Human activities have introduced increasingly hazardous pollutants into the environment since the growth of industry, agriculture, and livestock. These pollutants including toxic gases and trace element ions caused pollutions of atmosphere, water, and soils, which severely harm the existence and development of human beings . Thus, effective and inexpensive systems for detection and quantification of environmental pollutants have been progressively more important. Currently, conventional detection methods are based on chromatography-mass spectrometry (GC-MS), electrochemical systems, and spectrophotometers [2–4]. In these processes, large stationary instruments and complex components are required, which limit their use in resource-limited fields. Thus, there is currently a global effort to develop new technologies to evaluate and detect these environmental pollutants.
In recent years, fluorescence technology (FT) with high resolution and operational simplicity has attracted a lot of attention . Taking advantage of FT, sensors based on chromophores have been considered promising alternatives for environmental measurements due to their simplicity, high sensitivity, and inexpensive nature. Particularly, organic fluorescent dyes are widely studied because they absorb/emit lights with a broad wavelength range, which could fulfill a variety of demands [6, 7]. Functionalized organic dyes could respond to external stimuli through structural or morphological change, which leads to the change of their apparent colors and fluorescence signals. It’s worth mentioning that the response is easy to check by naked eye or using ultraviolet (UV) lamp. Therefore, design strategies to chemosensors for effective detection of environmental pollutants have attracted significant attention over the past few years.
On the other hand, recent developments in the field of nanotechnology have triggered increased interest in using nanomaterials for environmental applications . Nanoscaled films and layers possess unique properties that can be used to improve the performance of existing sensors. Organic dyes are incorporated into self-supported films and layers as fluorescent films and layers (FFL) that are portable and durable for effective sensing in complex environments. Furthermore, taking advantage of their nanoscaled dimensions and controllable surface, FFL perform high sensitivity and selectivity to the analyte, which could be regarded as a promising sensing material especially for hazardous pollutants.
This chapter focuses on the recent developments of nanoscaled FFL for the detection of environmental pollutants. We first begin with the sensing mechanism of synthetic dyes and the fabrication of FFL based on these dyes. Then, some examples of FFL for the detection of gaseous pollutants (such as CO2, NH3, and HCl gas) and metal ions (such as Hg2+, Pb2+, and Cd2+) is discussed.
1.1. Sensing mechanism of synthetic dyes for environmental applications
The main hurdle for fluorescent techniques lies in the construction of fluorescent sensors with high selectivity, sensitivity, and stability. Taking advantage of their structural designability, easy synthesis/modification, and high fluorescence quantum yields , organic sensors have become a new multidisciplinary research field involving organic synthesis, analytical chemistry, and environmentology. For environmental applications, different chromophores with various response mechanisms have been developed for the detection of hazardous pollutants. Herein, we present a survey on recent progress in sensing strategies of organic chromophore for the detection of hazardous gases and ions.
Organic dyes with aromatic structure, such as rylene dye and cyanine dye, emit strong and stable fluorescence but easily aggregate due to strong π-π stacking interaction . For the modification of these dyes, gas/ion responsive groups are attached to the periphery of the conjugation structure. The structural changes of functional groups usually affect the electron distribution or aggregate state of the fluorophores, resulting in distinct changes in the absorption/fluorescence spectrum. Based on organic dyes, a number of photophysical signaling mechanisms for sensing have been developed, such as photoinduced electron transfer (PET) [11, 12], intramolecular charge transfer (ICT) [11, 12], fluorescence resonance energy transfer (FRET) , and aggregation-disaggregation effect. In addition, different mechanisms of aggregation-induced signal change including aggregation-caused quenching (ACQ)  and aggregation-induced emission (AIE)  are studied for sensing environmental pollutants. The sensing strategies of fluorescent dyes are concluded in Table 1.
PET with “fluorophore-spacer-receptor” format is the most commonly exploited approach for designing fluorescent sensors and switches. Herman et al. reported an approach for sensing carbon dioxide based on PET (Figure 1) . In this case, sensor
Methods based on energy transfer (ET), such as FRET, hold great promise for pollutant detection, allowing precise and quantitative analysis and imaging even in complicated systems. Kim et al. reported a calixcrown chemosensor
ACQ and AIE dyes are both developed as sensing materials with “turn-off” or “turn-on” models. Yin et al. reported a selective fluorescent chemosensor based on ACQ dye
Tang et al. reported a simple approach for quantitative detection of CO2 gas with an AIE chromophore hexaphenylsilole (
Another strategy taking advantage of the self-association of dyes is also investigated. The aggregation of dyes also strongly affects their spectroscopic characteristics, and these spectral changes can be attributed to aggregation of the dye molecules in water to form dimers and higher order aggregates in the “J-” or “H-” type aggregation state . Yin et al. developed a self-assembled fluorescent film by carboxylic acid group functionalized squarylium cyanine dyes
1.2. Fabrication of films with organic dyes
Pollutant detection is challenging in atmosphere, water, and soil because of the complex components and rigorous conditions. Although organic dyes are sensitive to specific analytes, their applications in real environment are still limited due to their unstable nature. Nanomaterials have many excellent properties, such as strong adsorption, fine and tunable nanostructure, and stability . The combination of organic dye and nanotechnology could maximize the functions of dye, while resulting in functional materials with higher performance. Nanomaterials with various architectures, such as particle, film, and layer, have been widely applied as sensors for environmental monitoring and pollutant detection. Film sensors have attracted enormous attention due to easy preparation, good applicability, and portable properties. In the chemosensory processes of films, the physical or chemical interactions occur at the surface or interface of the materials. Thus, controlling of the specific surface area, surface energy, and surface chemistry of films and layers through nanotechnology is of great importance. For example, compared with bulk materials, nanofibers with high surface area to volume ratio possess higher functional group density. Porous films also have high surface area and ordered porosity. These unique characters lead to higher sensitivity of the nanoscaled films and layers .
Advances in nanoscaled FFL are providing unprecedented opportunities to treat pollutants in environment. During the fabrication of FFL for pollutant detection, the following four precedent conditions should be met: (1) environment security, (2) reuse of sensors, (3) low cost, and (4) high detection efficiency. Based on these concerns, polymer films with dyes doped or embedded were widely used as sensing fluorescent films.
1.2.1. Casting films
Casting films were prepared by doping organic dyes into matrix polymers directly. The solvent mixture of dye and polymer was spread into a thin layer and then air-dried to prepare the film. The film is conveniently manufactured into devices and easily tailored according to practical need, which is widely adopted in the field of environment monitoring [24–27].
1.2.2. Cross-linking films
As polymer is an ideal supporting martial for organic sensing molecules, efforts have been made to further improve its stability and mechanical strength. Polymer networks could achieve stronger intermolecular interactions, closer molecular packing, and reduced polymer mobility by means of cross-linking using physical or chemical treatments . Therefore, cross-linking has been explored as a viable method to improve the properties of polymer films as well as to give a stable environment for the embedded dyes.
Yin et al. reported a series of cross-linked fluorescent polyvinyl pyrrolidone (PVP) films covalently attached with different perylene derivatives (PDA, 4Cl-PDA, and 4Cl-PDI) (Figure 6) [29–31]. The ring-opening reaction of anhydride end groups and aromatic nucleophilic substitution between the chlorine atom and the secondary amine in PVP both contributed to the PVP-chromophore conjugation. The obtained fluorescent film
1.2.3. Layer-by-layer (LbL) assembled films or layers
The LbL method has been reported to be a useful method for the preparation of molecularly assembled films. It was introduced in 1966 by Iler , and subsequently extended by Decher and coworkers to encompass the alternate adsorption of polycations and polyanions onto a surface . A versatile approach for preparing nanoscaled thin film by LbL method is shown in Figure 7 . A substrate with inherent charge is first exposed to an oppositely charged polyelectrolyte, followed by thorough rinsing. Reversal of the surface charge then facilitates further adsorption steps. The process is continued until the desired layer number (or thickness) is achieved.
The technique is still expanding its potential because of its versatility for the fabrication of ordered multilayers. Molecular self-assembly of macromolecules and nanoreactors is controlled by the interplay of intra- and intermolecular as well as interfacial interactions. Through tuning the intra- and intermolecular interactions, films and layers with varied surface properties could be obtained.
Serhiy et al. reported an LbL thin film
1.2.4. Electrospinning films
Ultrathin films, such as nanofiber films with different structures and functions, have been developed for environmental applications in recent years. Electrospinning was an easy but versatile method for various polymers to fabricate ultrathin films. Differing from normal fibers, the electrospun ultrathin fibers show nanoscale diameters, high lengths, and large surface area to volume ratios . By controlling the processing parameters, different fibers could be obtained with various morphologies, such as core-shell, hollow, and porous structures [37, 38]. Generally, the sensitivity of a sensor will increase with a growing surface area per unit mass, due to the detection carried out on the sensor surface . Therefore, the large surface area to volume ratio and good interconnectivity make electrospun ultrathin fibers highly attractive for sensor applications. It is worth noting that a porous nanostructured fiber can be generated directly by controlling phase separation during electrospinning. The enhanced surface to volume ratio of the porous nanofibers could improve the sensitivity of the sensors.
There are two main classifications of fabrication processes of ultrathin fiber sensors. One is to incorporate sensing substances onto the outer surface of electrospun fibers. Sensors fabricated by this method usually have higher sensitivity but are unstable in external environment. Lee et al. reported a dots-on-fibers (DoF) hybrid nanostructure
Another strategy is to fabricate nanofiber sensors directly by adding sensing elements into the spinning solution. Ding et al. reported an approach for colorimetric quantitative detection of Pb2+ by embedding newly synthesized polydiacetylenes (PDA) into the electrospun polyacrylonitrile nanofibrous membrane (PAN NFM, Figure 10) . Compared with the casting film-based PAN NFM, the electrospun film
2. FLL for the detection of environmental pollutants
2.1. Gaseous pollutant detection
2.1.1. Carbon dioxide (CO2) detection
Carbon dioxide is a major public concern with widespread discussion because of its role in global greenhouse warming. Moreover, CO2 is also quite critical to the modern agricultural, food, environmental, oil, and chemical industries. However, it is dangerous for living beings to stay at high concentration levels of CO2 . Therefore, the importance and prevalence of such a gas (CO2) provides an incentive for development of new methods that can be used for the rapid and selective detection and monitoring of this relatively inert gas both in gaseous and liquid phases. Some traditional detection methods, such as electrochemical and infrared spectroscopic techniques, are usually employed for CO2 sensing and detection; however, these methods are believed to be expensive and time consuming. Hence, the development of chemosensors for determining the concentration level of CO2 is of great interest.
The use of pH indicators is one strategy in the construction of fluorescent sensors for CO2 detection. The most famous pH indicator used for sensing CO2 is 1-hydroxypyrene-3,6,8-trisulfonate (HPTS,
Another strategy was developed based on chemical reaction between amine and CO2. CO2 is a weak electrophile that can react with an active basic amine to form corresponding carbamate salt and ammonium salt. Sijbesma et al. investigated the reaction between CO2 and optically pure chiral diamines
2.1.2. Ammonia gas detection
Ammonia (NH3) is widely employed in industrial and agricultural systems such as refrigeration, stock farming, fertilizer production, and food processing . In addition to its broad applications, ammonia is also toxic, irritating, and corrosive to human skin, eyes, and respiratory system . Recently, fluorescent sensors for ammonia detection have shown great promise with their low cost, easy use, and high sensitivity.
Gu and Huang reported the polyaniline
Colored sensing films have slight color variations especially under low concentrations of ammonia, which are not easily perceived by visual inspection. Yin et al. reported a flexible naked eye ammonia sensor mat
2.1.3. Hydrogen chloride gas (HCl) detection
Sensitive detection and monitoring of HCl gas have attracted a lot of attention . HCl is mainly released in the mass production of halogenated polymers. Therefore, it is of great importance to sense and regulate HCl concentration in the adsorbing towers of these factories, which benefits the emission monitoring and air quality control.
As shown in Figure 15, Wu and coworkers reported an HCl gas sensor
2.1.4. Volatile organic compound (VOC) detection
The detection of volatile organic compounds is an important aspect considering that VOCs are continuously released into the environment by different sources like industrial processes, transportation, agriculture, etc. . These VOCs not only cause environmental pollution but also directly affect human health. For example, alcohols and aromatic hydrocarbons are potentially hazardous to human health due to their capabilities to stimulate the mucous membranes and upper respiratory tracts . Various detection methods have been proposed based on changes in electrochemical, conducting, and chromism properties of the corresponding sensor matrices. Colorimetric sensor systems are of particular interest thanks to their effectiveness and simplicity.
Formaldehyde could be slowly released from many building materials such as paints, adhesives, wallboards, and ceiling tiles, which irritates the mucous membranes and can make a person irritated and uncomfortable. The reaction of primary amine with aldehyde is often utilized to detect formaldehyde. Suslick and coworkers utilized simple pH indicators to detect the change in basicity as a simple colorimetric system for gaseous formaldehyde (Figure 16) . Six different pH indicators, such as methyl red, bromocresol purple, 4-nitrophenol, alizarin, nitrazine yellow, and bromoxylenol blue, were introduced into a poly(ethylene glycol) (PEG) polymer containing five different amounts (0.24, 0.48, 0.58, 0.82, and 1.03 wt%) of PEG with amine termination. This polymer film
Synthetic amines are produced in millions of tons each year and have broad applications in agricultural, pharmaceutical, and food industries. The excess of organic amines in air would seriously damage the ecological environment and pose severe threats to human health. Selective detection of trace amines in the vapor phase has attracted intense attention because of the increasing concerns regarding air pollution monitoring, quality control of food, and even medical diagnosis. Lin and coworkers reported an AIE dye
2.2. Metal ion detection
During the last decades, increased contributions of contaminants caused drastic changes in ecosystems. Soil and water pollution with metals especially heavy metals is very frequently encountered. Metals are not biodegradable, and therefore they remain in ecological systems and in the food chain indefinitely, exposing top-level predators to very high levels of pollution. These interactions metals have toxicological and carcinogenic effects such as those affecting the central nervous system (Hg2+, Pb2+, As2+); the kidneys or liver (Cu2+, Cd2+, Hg2+, Pb2+); or skin, bones, or teeth (Ni2+, Cu2+, Cd2+, Cr2+) . Therefore, World Health Organization (WHO) and Environmental Protection Agency (EPA) have strictly defined the concentration limits of metal ions that are allowed in the drinking water. Especially, lead, cadmium, and mercury ions are banned in electrical and electronic equipment by the European Union’s Restriction on Hazardous Substances (RoHS) . As is well known, standard techniques for trace heavy-metal analysis (even in the ppt and ppq range) require sophisticated analytical techniques such as atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICPMS), mass spectroscopy (MS), X-ray fluorescence spectroscopy (R-FS), and potentiometric methods and specialized personnel to carry out the operational procedures . Efforts are ongoing to prepare rapid and inexpensive techniques for heavy-metal detection. Here, optical detections for heavy-metal ions based on fluorescent films are reviewed.
2.2.1. Mercury ion (Hg2+) detection
Mercury is widespread in air, water, and soil and is notoriously known for its high toxicity to human beings such as severe damage to the central nervous system. Moreover, accumulation of mercury in human body could lead to various kinds of cognitive and motor disorders as well as Minamata disease . Many sources are to blame for the generation of mercury, for example, coal plants, gold production, measuring instrument, and mercury lamps . Mercury is mainly uptaken by human beings through daily diet such as fish, and it is vital that considerable efforts should focus on the development and evolution of sensitive and selective detection methods.
Yin et al. reported a highly sensitive and selective fluorescent nanofibrous membrane
2.2.2. Lead ion (Pb2+) detection
As an abundant but toxic metal, lead is often left in the environment due to its application in batteries, gasoline, pigments, etc. . Lead pollution causes long-term damage to both human health and the environment, as most of the mined lead, 300 million tons to date, is still going back to soil and groundwater finally. Trace amounts of lead may cause neurological, reproductive, cardiovascular, and developmental disorders. Particularly, lead poisoning will introduce serious problems in children including slow motor responses, decreased IQs, and hypertension.
Kim et al. reported a rhodamine-based chemosensor
2.2.3. Cadmium ion (Cd2+) detection
Cadmium is a kind of heavy metal with severe toxicity and carcinogenicity. It is commonly seen in electric batteries, pigments in plastics, electroplated steel, and so on . Human being tends to uptake cadmium mainly through smoking and daily diet. Exposure to high concentration of cadmium would result in increased risk in cardiovascular diseases and cancer, and it may also cause damage to liver and kidneys .
Wei et al. reported a fluorescence layered double hydroxide (LDH) ultrathin films
This chapter reports the preparation of nanoscaled FFL and the detection of environmental pollutants. Several sensing strategies are brought up based on the different dyes. Nanoscaled FFL act as a promising tool for hazardous pollutant detection, including gaseous pollutants (such as CO2, NH3, HCl gas, and VOCs) and metal ions (such as Hg2+, Pb2+, Cd2+). However, although numerous chemosensors for gases/ions have been widely developed and applied, a big challenge remains to construct those that display high selectivities, sensitivities, and reusabilities.
This work was financially supported by the National Natural Science Foundation of China (21574009 and 51521062), Beijing collaborative innovative research center for cardiovascular diseases, and the Higher Education and High-quality and World-class Universities (PY201605).