International Journal of Environmental Research and Public Health

Review Disposable and Low-Cost Colorimetric Sensors for Environmental Analysis

Giancarla Alberti 1,*, Camilla Zanoni 1, Lisa Rita Magnaghi 1,2 and Raffaela Biesuz 1,2

1 Department of Chemistry, University of Pavia, Via Taramelli 12, 27100 Pavia, Italy; [email protected] (C.Z.); [email protected] (L.R.M.); [email protected] (R.B.) 2 Unità di Ricerca di Pavia, INSTM, Via G. Giusti 9, 50121 Firenze, Italy * Correspondence: [email protected]



Received: 18 October 2020; Accepted: 9 November 2020; Published: 11 November 2020


Abstract: Environmental contamination affects human health and reduces the quality of life. Therefore, the monitoring of water and air quality is important, ensuring that all areas are acquiescent with the current legislation. Colorimetric sensors deliver quick, naked-eye detection, low-cost, and adequate determination of environmental analytes. In particular, disposable sensors are cheap and easy-to-use devices for single-shot measurements. Due to increasing requests for in situ analysis or resource-limited zones, disposable sensors’ development has increased. This review provides a brief insight into low-cost and disposable colorimetric sensors currently used for environmental analysis. The advantages and disadvantages of different colorimetric devices for environmental analysis are discussed.

Keywords: colorimetric sensors; disposable devices; low-cost sensors; environmental analysis; in loco analysis

1. Introduction Current attention to environment systems derives from the evidence that people are disrupting the natural processes, and the degree of life quality, or life itself, is being threatened. For example, the rapid population growth and the significant increase in energy consumption are signals of contingent problems. Emissions from automobiles, toxic particulate matter derived from combustion and incineration processes, waste production, or massive use of pesticides are the principal alarms. Pollution is frequently associated with the diffusion of chemical toxics into the environment. Commonly, problems derive from an increase in the concentration of substances, which are naturally present in the environment due to their diffusion above the “natural” levels [1]. Many pollutants, such as organic compounds, drugs, dyes, pesticides, heavy metal ions, and anions can seriously affect air, water, and soils [2]. Moreover, small amounts of toxic substances can persuade ecological damage and irreversibly hurt people [3,4]. Attention in environmental monitoring continues to expand, thus the need for chemical analysis of environmental samples is growing. A significant percentage of the present monitoring is devoted to agreeing with legislation, and environmental analysis also provides scientific indication for further developments in the regulation matter [1]. Environmental monitoring provides interesting challenges to sensing technology: The development of quick, low-cost, sensitive, and selective methods for analyzing toxic pollutants is remarkable [5]. Usual classical instrumental methods are expensive and time-consuming; besides, restrictions in sampling and pretreatment standard techniques occur [6].

Int. J. Environ. Res. Public Health 2020, 17, 8331; doi:10.3390/ijerph17228331 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2020, 17, 8331 2 of 23

Classical and standard methods, especially for traces analysis, require some costly instruments, such as for toxic metal ions and inorganic contaminants: Atomic absorption spectroscopy (AAS), atomic fluorescence spectrometry (AFS) [7], inductively-coupled plasma-mass spectrometry (ICP-MS) [8], optical emission spectroscopy (OES) [9], X-ray fluorescence spectrometry [10]; or for organic pollutants: High-performance liquid chromatography (HPLC) [11], liquid chromatography- mass spectrometry (LC-MS), and gas chromatography-mass spectrometry (GC-MS) [12]. Trained personnel is also required to carry out operational procedures [13]. On the other hand, a necessity occurs for accurate, cheap, and on-site environmental pollutants monitoring. This goal can be achieved using sensing devices. Nevertheless, the development and use of emerging sensors are tentative due to technological and cultural obstacles [6]. In any case, with the progress of nanotechnology and materials science, several sensors and biosensors, based on nanomaterials such as nanoparticles (NPs) [14–17], quantum dots (QDs) [18], carbon nanotubes, and nanofibers (CNTs/CNFs) [19–21], nanowires [22], graphenes [23,24], etc., have been developed for environmental monitoring. For example, glassy carbon electrodes modified with nanomaterials are applied as electrochemical sensors [25–29]. Fluorescent NPs and QDs-based platforms allowed determinations, thanks to fluorescent quenching [30]. Metal-NP-based active materials are used for surface-enhanced Raman scattering analysis [31]. Unfortunately, this kind of sensor shows some limitations due to the time-consuming analysis, which is not useful for in situ determinations. In contrast, colorimetric sensors seem promising for identifying pollutants in environmental samples thanks to their easy preparation, naked-eye sensing, fast detection, and pretty good sensitivity [32–35]. In this direction, the development of disposable and low-cost colorimetric sensors is key for a quick and in situ detection of environmental pollutants. Disposable sensors are cheap and easy-to-use devices. They do not experience the memory effects, do not need pretreatment before their usage or cleaning between measurements. Due to the growing demand for in-situ analysis, the disposable sensors’ global market is rapidly increasing, particularly for medical diagnostics and environmental monitoring [13,36]. This paper comprehensively reviews disposable colorimetric sensors, recently described in the literature, providing a critical discussion about the advantages and disadvantages in their applications for practical, in situ, quick, and reproducible environmental analysis.

2. Detection and Image Analysis Different kinds of detectors can be used for optical sensing, from the most current and low-cost devices (i.e., scanners or camera phones), to more specialized instruments, such as spectrophotometers and fluorimeters. The office scanner is a shared device that provides fair resolution and guarantees the digitalized image; moreover, the image intensity is not affected by the external light. Scanners are widely employed since they can be portable (for example, pen scanners) and suitable for unskilled personnel. Digital and cell phone cameras can also be used as detectors, with the advantage that they do not require any particular skills. One shortcoming is that digital and cell phone cameras cannot focus on too close objects (around 20 cm). After digitalization, images can be analyzed using software or apps such as Adobe Photoshop, Microsoft Paint, DigitalColor Meter, Corel Photo-Paint, GIMPs. Digitalized images are then converted to color spaces: RGB, CMYK, grayscale, HSV, or CIE L*a*b* [37]. Depending on the color and hue of the image analyzed, the full information, or just one channel of the color space, can be used for data treatment. The last group of detectors includes simple photometers [38] and homemade devices. For example, the portable color identifier described by Li et al. [39] or the transmittance colorimeter described by Ellerbee et al. [40]; both devices were assembled with components readily available in any electronic store. Int. J. Environ. Res. Public Health 2020, 17, x 3 of 23 described by Ellerbee et al. [40]; both devices were assembled with components readily available in any electronic store. Int.In J. Environ.any case, Res. Publicbecause Health of2020 the, 17 rapid, 8331 expansion of the smartphone market, this approach for 3 image of 23 collection could replace all currently available detection techniques. Besides, smartphones are user friendly, low-cost, and do not require highly technical know-how. In any case, because of the rapid expansion of the smartphone market, this approach for image Some research groups developed apps to directly collect and process images with a smartphone collection could replace all currently available detection techniques. Besides, smartphones are user without the need for computers; moreover, RGB apps for color space analysis are offered free of friendly, low-cost, and do not require highly technical know-how. charge [41]. Some research groups developed apps to directly collect and process images with a smartphone without the need for computers; moreover, RGB apps for color space analysis are offered free of 3. Chemometric Tools for Colorimetric Data Analysis charge [41]. Colorimetric sensors produce large datasets with many variables, so for handling this wealth of 3. Chemometric Tools for Colorimetric Data Analysis data, various chemometric methods have been proposed. Mainly used are principal component analysisColorimetric (PCA), hierarchical sensors produce cluster analysis large datasets (HCA), with and many partial variables, least square so for regression handling this(PLS). wealth of data, various chemometric methods have been proposed. Mainly used are principal component 3.1.analysis PCA (PCA), hierarchical cluster analysis (HCA), and partial least square regression (PLS).

3.1.Principal PCA component analysis (PCA) is an unsupervised chemometric method that simultaneously analyzes the entire multidimensional data set. It is a technique for reducing datasets’ Principal component analysis (PCA) is an unsupervised chemometric method that simultaneously dimensionality, increasing interpretability, and preserving as much statistical information while analyzes the entire multidimensional data set. It is a technique for reducing datasets’ dimensionality, minimizing information loss. The PCA algorithm uses an orthogonal transformation to convert increasing interpretability, and preserving as much statistical information while minimizing information observable and related variables into a set of uncorrelated variables, called principal components loss. The PCA algorithm uses an orthogonal transformation to convert observable and related variables (PCs). These, even if few components are sufficient to describe the dataset, and in this way, patterns into a set of uncorrelated variables, called principal components (PCs). These, even if few components andare trends sufficient in the to describe data can the be dataset, easily and visualized in this way, and patterns described. and PCA trends has in the been data applied can be in easily many disciplines,visualized and and it described. has been PCA recognized has been as applied a useful in manytool to disciplines, interpret anddata it generated has been recognized by colorimetric as a sensorsuseful for tool environmental to interpret data analysis generated [41,42 by]. colorimetric sensors for environmental analysis [41,42]. AsAs an anexample, example, Zhou Zhou et al. et [ al.43][ developed43] developed a multichann a multichannelel colorimetric colorimetric sensor sensor array array based based on the pHon-induced the pH-induced etching of etchingsilver nanoprisms of silver nanoprisms(AgNPRs) suitable (AgNPRs) for sensing suitable five for thiols sensing (see five Figure thiols 1a): Cysteine(see Figure (Cys),1a): dimercaptosuccinic Cysteine (Cys), dimercaptosuccinic acid (DMSA), dithiothreitolacid (DMSA), dithiothreitol(DTT), glutathione (DTT), (GSH),glutathione and 3- mercaptopropionic(GSH), and 3-mercaptopropionic acid (MPA), at acid trace (MPA), concentrations at trace concentrations (low to 10 (low nM). to PCA 10 nM). is employed PCA is employed for data analysis.for data The analysis. PCA plots The indicated PCA plots the indicated possibility the possibilityto distinguish to distinguish and identify and three identify thiols three in mixtures thiols at indifferent mixtures concentratio at differentns concentrations of 10, 80, and of400 10, nM 80, (Figure and 400 1b), nM and (Figure all the1b), five and thiols all the at a five concentration thiols at of a400 concentration nM in diluted of 400 fetal nM bovine in diluted serum fetal (for bovine simulating serum a (for real simulating sample, Figure a real sample,1c). Other Figure unknown1c). samplesOther unknownare analyzed, samples and are the analyzed, five thiols and are the accurately five thiols identified, are accurately so the identified, array seems so the promising array seems for thiolpromising sensingfor in thiolbiological sensing and in environmental biological and environmental matrixes. matrixes.

FigureFigure 1. 1.AgNPRAgNPR-based-based colorimetric colorimetric sensor sensor for for thiols: (a)) sensing sensing mechanism, mechanism, ( b ()b PCA) PCA (Principal (Principal ComponentComponent Analysis Analysis)) plot plot of of the the data data matrix matrix of three thiols atat didifferentfferent concentrations, concentrations, and and (c )(c PCA) PCA plotplot of ofthe the data data matrix matrix of of five five thiols thiols at at a concentration ofof 400400 nM nM [ 43[43].].

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MoreInt. J. Environ. interesting Res. Public PCA Health applications 2020, 17, x to colorimetric arrays with smartphone detection4have of 23 been reported [44More,45 ].interesting In both cases,PCA applications the colorimetric to colorimetric analysis arrays is used with to smartphone quantify simultaneouslydetection have been diff erent analytes,reported and the[44,45 chemometric]. In both cases, approach the colorimetric permitted analysis a clear is used distinction to quantify between simultaneously the analytes different signals. Inanalytes, summary, and PCAthe chemometric has been recognized approach permitted as an effective a clear tool distinction for data between treatment the analytes of the results signals. obtained by colorimetricIn summary, sensor PCA devices, has beenas evidenced recognized by as the an numerous effective tool papers for data on thistreatment subject of [ the46 – results53]. obtained by colorimetric sensor devices, as evidenced by the numerous papers on this subject [46– 3.2. HCA53].

Hierarchical3.2. HCA cluster analysis (HCA) is a multivariate unsupervised analysis technique sometimes coupled to PCA for data treatment in colorimetric arrays [45,48,49,51,53,54]. Additionally known as Hierarchical cluster analysis (HCA) is a multivariate unsupervised analysis technique hierarchical clustering, HCA aims to group subjects with similar characteristics into clusters. In HCA, sometimes coupled to PCA for data treatment in colorimetric arrays [45,48,49,51,53,54]. Additionally two diknownfferent as strategies hierarchical can clustering, be used: agglomerative HCA aims to group and divisive. subjects withThe first similar case characteristics is an iterative into process; each dataclusters. point In isHCA, initially two considereddifferent strategies an individual can be used: cluster. agglomerative At each iteration, and divisive. the similar The first clusters case is merge with othersan iterative until process; one single each data cluster point is is formed. initially considered This strategy an individual is also called cluster. a At “bottom-up” each iteration, approach. the In thesimilar divisive clusters clustering merge (also with definedothers until top-down), one single a singlecluster clusteris formed. of allThis data strategy is divided is also routinelycalled a into two smallest“bottom similar-up” approach. clusters; In the d processivisive clu endsstering when (also only defined one top cluster-down), for a each single observation cluster of all is data obtained. The divisiveis divided algorithm routinely is into practically two smallest the inversesimilar clusters; of the agglomerative the process ends one. when only one cluster for each observation is obtained. The divisive algorithm is practically the inverse of the agglomerative A measure of dissimilarity between sets of observations is needed for deciding which clusters can one. be combinedA measure (for agglomerative) of dissimilarity or between when a sets cluster of observations should be splitis needed (for divisive).for deciding The which general clusters approach consistscan ofbeusing combined an appropriate(for agglomerative) metric or (a when measure a cluster of theshould distance be split between (for divisive). pairs The of observations)general and aapproach linkage criterion consists of that using specifies an appropriate the dissimilarity metric (a ofmeasure sets as of a function the distance of the between pairwise pairs distances of of observations.observations) and a linkage criterion that specifies the dissimilarity of sets as a function of the Thepairwise result distances of hierarchical of observations. clustering is often presented in a dendrogram (also called a “tree diagram”). The objectsThe are result organized of hierarchical in a row clustering according is to often their presented similarities in a [ dendrogram55]. (also called a “tree diagram”). The objects are organized in a row according to their similarities [55]. As an example, Figure2 shows the dendrogram reported by Bang et al. [ 48]. In this work, As an example, Figure 2 shows the dendrogram reported by Bang et al. [48]. In this work, colorimetriccolorimetric sensor sensor arrays arrays are are developed developed toto identifyidentify and and quantify quantify toxic toxic aliphatic aliphatic amines. amines. The HCA The HCA procedureprocedure permitted permitted the the complete complete identification identification ofof 11 structurally structurally similar similar amines. amines.

FigureFigure 2. Dendrogram 2. Dendrogram HCA HCA (Hierarchical (Hierarchical Cluster Cluster Analysis) Analysis for) for 11 structurally 11 structurally similar similar aliphatic aliphatic amines usingamines Ward’s usingmethod. Ward’s No misclassifications method. No misclassifications were observed were among observed the 36 among trials. the All 36 experiments trials. All were run inexperi triplicatements [ 48were]. run in triplicate [48].

3.3. PLS

The multivariate regression PLS (partial least squares regression) is a supervised technique that combines characteristics of the principal component analysis (PCA) and multiple regression. The work Int. J. Environ. Res. Public Health 2020, 17, 8331 5 of 23 of Gerlach and Kowalski [56] describes in detail the procedure of PLS. Thanks to its potential to extract information, PLS regression based on data obtained from colorimetric sensors is broadly used. PLS is commonly applied to the simultaneous examination of two datasets, such as spectra or image parameters (i.e., RGB, HSV, etc.) and concentration. Based on different factors (latent variables), PLS creates a linear model y = X b, allowing the prediction of the concentration (y) from spectra or × image parameters (X); b includes the regression coefficients obtained in a calibration process. In detail, X is the n m matrix of measured responses obtained from spectra or images, n is the × number of samples, and m is the number of wavelengths of the whole spectrum or all image parameters. y is the n c concentration matrix of c analytes. b is the n c vector of regression coefficients, which is × × obtained in the calibration. Knowing b, the calibrated model can predict new y concentrations from measured X spectra or image parameters of unknown samples. Unlike unsupervised methods, like PCA and HCA, supervised techniques result in predictive models that permit qualitative (classification techniques) or quantitative analyses (regression methods). Among the latter, PLS is definitely the most famous regression tool for sensing applications. This chemometric method is applied in a discrete number of works dealing with colorimetric sensors’ development in environmental analysis. Below are some examples: For the simultaneous analysis of multiple ions, Huang et al. [57] proposed a rapid and simple colorimetric method to detect metal ions and oxyanions based on gold nanoparticles functionalized with amino acids (amino acid-AuNPs) using PLS for data treatment. Taking advantage of the multivariate analysis by PLS, they can simultaneously quantify each ion in binary and ternary mixture of Cr(III), Cr(VI), and Fe(III). Ferri et al. [58] developed a colorimetric array containing twelve dyes to detect and discriminate several organophosphorus pesticides in water. PLS model is applied for evaluating the influence of the concentration on the chromogenic array response. The PLS model shows a good agreement between the measured and predicted values, suggesting that the array could determine all the organophosphorus pesticides investigated. Catechols are common substances used as pharmaceutical or intermediates in chemical synthesis. Despite the high value of catechols in these fields, the illegal discharge and accidental leak of catechols cause environmental pollution and harm human health. Wang et al. [59] proposed a colorimetric sensor array to identify and quantify 13 different catechols. The device is obtained using pH indicators and phenylboronic acid. In particular, PLS models are applied for the quantitative determination of each specific catechol’s concentrations. In different works of Alberti and Biesuz [47,60–62], the PLS method is successfully applied to quantify different analytes simultaneously; in these studies, low-cost, disposable colorimetric sensors are used.

4. Materials for Disposable Colorimetric Sensors Despite the development of research in material science, it is still impossible to produce a unique material meeting all the disposable colorimetric sensors’ requirements due to their wide range of applications. This section reports an overview of some materials used as support for disposable colorimetric sensors. They can be grouped into four categories:

- Cellulose-based materials; - Textiles and woven non-woven fabrics; - Synthetic polymeric supports; - Sol-gel materials.

The focus is on sustainability (recyclable, biodegradable, etc.), fields of application, cost, and other particular properties, reporting the different materials’ advantages and limitations. Int. J. Environ. Res. Public Health 2020, 17, 8331 6 of 23

4.1. Cellulose-Based Materials Cellulose is a chain of glucose residues, and it is the main constituent of the plant cell wall. Cellulose is the most abundant widespread, and sustainable biopolymer used for numerous applications. Paper is a smart, cellulose-based material for disposable sensors thanks to its properties: It is low-cost, lightweight, flexible, and biodegradable. It can be easily functionalized by chemicals and is well-suited with low-cost techniques of sensors fabrication like printing. Paper also can be used to develop microfluidic devices [63–65]. The choice is based mainly on the device’s fabrication steps and the particular application area since the variety of paper available materials. In the development of colorimetric sensors and microfluidic devices, filter papers have been widely used in recent years to produce paper-based sensors. Colorimetric paper-based sensors are among the most common, since this material gives a bright, high-contrast, colorless background for color change analyses [66]. For example, Mentele et al. [67] reported a paper-based device for three metal ions determination. Sensing is based on three dyes fixed on the paper: 1,10-phenanthroline for Fe(II), dimethylglyoxime for Ni(II), and bathocuproine for Cu(I). In particular, the wax printed device comprises of three zones: sampling, pretreatment, and detection. The pretreatment zone is the best feature of this device: masking reagents and reducing agents can complex interfering metals eventually present, and if necessary, change the pH. This device effectively detected the three cations with good sensitivity; moreover, it could be easily modified by introducing different dyes to determine other metal ions. Carpenter et al. [68] proposed a sensor for hydrogen sulfide gas. The device consists of a moist paper impregnated with a Cu(II)/PAN (1-(2-pyridylazo)-2-naphthol) complex and NaOH. Water and NaOH help to adsorb and convert H2S gas to sulfide, increasing the sensitivity. Of interest is the sensor’s coupling with a handheld colorimeter faced to a smartphone, obtaining the direct H2S quantification, and only measuring the degree of color change. The sensor can be applied for a wide range of concentrations for industrial monitoring and oral hygiene, with LOD of about 30 µg/L, so it can be suitable to detect the average concentration of bad breath (80 µg/L) and the US OSHA permissible exposure limit (10 mg/L). Alberti et al. [69] described a functionalized paper with deferoxamine (DFO) for sensing Fe(III) and V(V) in natural waters. DFO is selected thanks to the possibility of linking it on the paper and because its complexes with Fe(III) and V(V) are colored, thus a simple colorimetric sensor is prepared. The so-named DFO-papers are prepared following a method based on the procedure proposed by Takagai et al. [70,71]. It consists of a halogenation step, followed by introducing the DFO molecules (see Figure3). The colorimetric response of the sensory DFO–paper to the presence of Fe(III) or V(V) in solutions allowed the detection of these cations with the naked eye and their quantification. In particular, images acquired by a desktop scanner of the DFO–papers after exposure to aqueous solutions of Fe(III) or V(V) are used to quantify both metal ions. RGB parameters obtained by the scanner are processed using freeware software, followed by a simple chemometric procedure. This approach proved to be successfulInt. J. Environ. in Res. determining Public Health 20 Fe(III)20, 17, andx V(V) with a new ready-to-use and economical sensor. 7 of 23

Figure 3. Preparation of papers functionalized by deferoxamine (DFO-papers) [43,69]. Figure 3. Preparation of papers functionalized by deferoxamine (DFO-papers) [43,69].

Xu et al. [72] described devices obtained incorporating spirolactam rhodamine derivatives onto cellulose filter papers’ surface. They use these sensors to recognize Hg(II) ions in aqueous samples. The device showed an improved fluorescent emission intensity and a marked color change from colorless to pink after the complex formation with Hg(II) ions. The detection limit obtained is about 5 × 10−5 M; furthermore, this sensor displays high selectivity for mercury(II) over numerous environmentally critical metal ions. Chen et al. [46] present a paper-based device for colorimetric sensing of dissolved NH3 and CO2. They functionalized a paper platform with CO2 and NH3 sensitive dyes showing the noteworthy performance in dissolved gases’ determination. The sensitivity of the device is limited by the RGB read-out from the scanned images. The responses of CO2 and NH3 are distinguished by applying the chemometric tool of Principle Component Analysis (PCA) on the RGB data. It is crucial to highlight that the results are obtained starting from solutions containing one analyte at a time. For real-world applications to actual samples, it will be nice to test the device with both gases’ mixtures at different concentrations. Sicard et al. [73] proposed a tool for water quality monitoring. The sensor is a paper-based device (µPAD) that produced a colorimetric signal dependent on a specific analyte’s concentration (for example, the dynamic range for paraoxon and malathion is from 10−8 to 10−6 M). A cellphone equipped with a camera captured images of two µPADs: one put in contact with a water sample and the other, used as a control, with distilled water. The method has yet to be perfected, but it is promising and has a very high potential for improving water quality monitoring. In addition to papers, there is a wide choice of other cellulose-based materials. For example, cellophane can be used as a substrate for the fabrication of disposable and low-cost devices. Cellophane is a thin, transparent, renewed cellulose film obtained from sodium cellulose xanthate. It is transparent to visible and UV-light; it is not porous but encloses several capillaries filled with solutions during its swelling [74]. Pávai et al. [75] developed a curcumin-colored cellophane test strip for pH measurements. Strips are simply obtained by immersion of 2 × 2 cm pieces of cellophane in aqueous solutions of E100- curcumin, (1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) for 24 h, then washed with ultrapure water and dried at room temperature. The color of the sensor change from yellow at low pH to brown-red to the high values, as reported in Figure 4. Based on readily available materials, this sensor can provide an effective strategy for in situ pH determination.

Figure 4. Colors of the curcumin-cellophane sensors after exposure to standard pH solutions (Radelkis Kft., Budapest, Hungary; pH 2.10, 5.14, 7.12, 9.35, and 11.46 (±0.03 at 25°C)) [75].

Int. J. Environ. Res. Public Health 2020, 17, x 7 of 23

Int. J. Environ. Res. Public Health 2020, 17, 8331 7 of 23 Figure 3. Preparation of papers functionalized by deferoxamine (DFO-papers) [43,69].

Xu et al. [ 7722] described devices obtained incorporating spirolactam rhodamine derivatives onto cellulose filter filter papers papers’’ surface surface.. They They use use these these sensors sensors to to recognize recognize Hg(II) Hg(II) ions ions in in aqueous aqueous samples. samples. TheThe device showed an improved fluorescentfluorescent emission intensity and a marked color change from colorless to to pink pink after after the the complex complex formation formation with with Hg(II) Hg(II) ions. ions. The detection The detection limit obtained limit obtained is about is about 5 10 5 M; furthermore, this sensor displays high selectivity for mercury(II) over numerous 5 × 10−5× M; − furthermore, this sensor displays high selectivity for mercury(II) over numerous environmentally critical metal ions. Chen et al. [46] present a paper-based device for colorimetric sensing of dissolved NH and CO . Chen et al. [46] present a paper-based device for colorimetric sensing of dissolved NH33 and CO2. They functionalized a paper platform with CO and NH sensitive dyes showing the noteworthy They functionalized a paper platform with CO22 and NH3 sensitive dyes showing the noteworthy performance in dissolved gases gases’’ determination.determination. The The sensitivity sensitivity of of the the device device is is limited by the RGB read-out from the scanned images. The responses of CO and NH are distinguished by applying read-out from the scanned images. The responses of CO2 and2 NH3 are3 distinguished by applying the chemometricthe chemometric tool of tool Principle of Principle Component Component Analysis Analysis (PCA) (PCA)on the onRGB the data RGB. It is data. crucial It isto crucialhighlight to thathighlight the results that theare obtained results are starting obtained from starting solutions from containing solutions one containing analyte at onea time analyte. For real at- aworld time. applicationFor real-worlds to applicationsactual samples to actual, it will samples, be nice itto will testbe the nice device to test with the both device gases with’ mixtur both gases’es at different mixtures concentrationsat different concentrations.. Sicard et et al. al. [7 [373] ]proposed proposed a tool a tool for forwater water quality quality monitoring. monitoring. The sensor The sensor is a paper is a-based paper-based device (µPAD)device ( µ thatPAD) produced that produced a colorimetric a colorimetric signal signal dependent dependent on a onspecific a specific analyte analyte’s’s concentration concentration (for 8 6 (forexample example,, the dynamic the dynamic range range for paraoxon for paraoxon and malathion and malathion is from is 10 from−8 to 10 10−6− M)to. A 10 cellphone− M). A cellphoneequipped withequipped a camera with captured a camera images captured of imagestwo µPADs: of two oneµPADs: put in onecontact put inwith contact a water with sample a water and sample the other, and usedthe other, as a usedcontrol, as a with control, distilled with distilledwater. The water. method The method has yet hasto be yet perfected, to be perfected, but it butis promising it is promising and hasand a has very a veryhigh highpotential potential for improving for improving water water quality quality monitoring monitoring.. In addition to papers, there is a wide choice of other cellulose-basedcellulose-based materials.materials. For example, cellophane can be us useded as a substrate for the fabrication fabrication of disposable and low low-cost-cost devices. Cellophane Cellophane is is a a thin, thin, transparent, renewed cellulose film film obtained from sodium cellulose xanthate. ItIt isis transparenttransparent to to visible visible and and UV-light; UV-light; itis it not is n porousot porous but enclosesbut encloses several several capillaries capillaries filled withfilled solutionswith solutions during during its swelling its swelling [74]. [74]. PávaiPávai et et al. al. [7 [575] developed] developed a curcumin a curcumin-colored-colored cellophane cellophane test strip test stripfor pH for measurements. pH measurements. Strips Strips are simply obtained by immersion of 2 2 cm pieces of cellophane in aqueous solutions are simply obtained by immersion of 2 × 2 cm pieces× of cellophane in aqueous solutions of E100- curcumiof E100-curcumin,n, (1E,6E)- (1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione)1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) for 24 for h, 24then h, wasthenhed washed with withultrapure ultrapure water water and dried and driedat room at roomtemperature. temperature. The color The colorof the ofsensor the sensor change change from yellowfrom yellow at low at pH low to pH brown to brown-red-red to the to hig theh values, high values, as reported as reported in Figure in Figure 4. 4. Based on on readily readily available available materials, materials, this this sensor sensor can can provide provide an aneffective effective strategy strategy for forin situ in situpH pHdetermination. determination.

Figure 4.4. ColorsColors of of the the curcumin-cellophane curcumin-cellophane sensors sensors after exposureafter exposure to standard to standard pH solutions pH (Radelkissolutions (RadelkisKft., Budapest, Kft., Budapest, Hungary; Hungary; pH 2.10, 5.14, pH 2.10, 7.12, 5.14, 9.35, 7.1 and2, 11.469.35, (and0.03 11.46 at 25 (±0.03C)) at [75 25].°C)) [75]. ± ◦ The need for biodegradable films for packaging, sorbents, and sensors has encouraged the production of novel materials derived from natural sources, mostly agricultural byproducts. Sugarcane bagasse is cellulose-rich waste from the sugar industry, so it can be an excellent candidate for obtaining cellulose-based devices. Guo W. et al. [76] presented a novel cellulose-based colorimetric device for selective sensing of Ag+ and Cu2+. It is suitably obtained by grafting the 2,5-dithiourea (DTu) onto bagasse-pulp cellulose via a one-pot reaction. After exposure to metal-ion aqueous solutions, the cellulose-DTu device exhibits a white to yellow-red and white to light grey color change in the presence of Ag+ and Cu2+, respectively. Other cations, such as Al3+, Ba2+, Ca2+, Cd2+, Hg2+,K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Zn2+, do not interfere in the measurements since they do not produce any color change. Int. J. Environ. Res. Public Health 2020, 17, x 8 of 23

The need for biodegradable films for packaging, sorbents, and sensors has encouraged the production of novel materials derived from natural sources, mostly agricultural byproducts. Sugarcane bagasse is cellulose-rich waste from the sugar industry, so it can be an excellent candidate for obtaining cellulose-based devices. Guo W. et al. [76] presented a novel cellulose-based colorimetric device for selective sensing of Ag+ and Cu2+. It is suitably obtained by grafting the 2,5- dithiourea (DTu) onto bagasse-pulp cellulose via a one-pot reaction. After exposure to metal-ion aqueous solutions, the cellulose-DTu device exhibits a white to yellow-red and white to light grey Int. J. Environ.color change Res. Public in the Health presence2020, 17of, Ag 8331+ and Cu2+, respectively. Other cations, such as Al3+, Ba2+, Ca2+, Cd2+, 8 of 23 Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Zn2+, do not interfere in the measurements since they do not produce any color change. + 2+ FigureFigure5 shows 5 shows a possible a possible mechanism mechanism of of the the complexation complexation reactions between between Ag Ag+ or Cuor2+ Cu and and the cellulose-DTu.the cellulose-DTu. + ThisThis sensor sensor features features the the advantages advantages of of rapidity, rapidity, easiness,easiness, and and remarkable remarkable selectivity selectivity for Ag for+ Agand and Cu2+Cusensing.2+ sensing. An An evident evident color color change change from from whitewhite to yellow yellow-red-red (for (for Ag Ag+) +and) and light light grey grey (for (forCu2+ Cu) in2 +) in just 5just s is 5 demonstrated. s is demonstrated.

2+ FigureFigure 5. A 5 possible. A possible mechanism mechanism for for the the complexation complexation reaction of of Ag+ Ag +andand Cu Cu2+ withwith cellulose cellulose-DTu-DTu (2,5-dithiourea)(2,5-dithiourea [76) ].[76].

NanocelluloseNanocellulose made made from from nanofibers, nanofibers, nanocrystalline,nanocrystalline, or or bacterial bacterial can can be used be used in disposable in disposable sensorssensors as a as biodegradable a biodegradable substrate. substrate. CelluloseCellulose nanofibers nanofibers (CNFs) (CNFs) are are isolated isolated and andpurified purified using using differentdifferent chemical chemical or mechanicalor mechanical processes, processes, startingstarting from from cellulose cellulose fib fibersers of lignocellulosic of lignocellulosic resources resources,, suchsuch as wood as wo andod and agricultural agricultural residues residues [ 77[77].]. Chauhan et al. [78] described a one-step synthesis of a nanocellulose functionalized with a dye Chauhan et al. [78] described a one-step synthesis of a nanocellulose functionalized with a dye for for optical pH sensing. The nanocellulose is obtained by hydrolysis (with 64% sulfuric acid for 45 min optical pH sensing. The nanocellulose is obtained by hydrolysis (with 64% sulfuric acid for 45 min at 45 °C) of commercial microcrystalline cellulose from cotton fibers. The dye employed belongs to at 45the◦C) class of commercial of Remazol dyes microcrystalline [79]; it is covalently cellulose attached from to cottonthe nanocellulose fibers. The material dye employed according belongsto the to the classreaction of Remazol synthesis dyes shown [79 in]; Figure it is covalently 6. attached to the nanocellulose material according to the reactionInt. J. synthesis TheEnviron. thus Res. obtainedshown Public Health in nanoc Figure2020,ellulose 17, 6x. –dye product gave stable suspensions that change color from9 of 23 orange to purple, increasing the pH from acid to alkaline. A piece of the nanocellulose–dye film is taped on a plastic strip, thus obtaining a disposable stick that responds reversibly and rapidly to pH changes.

FigureFigure 6. Synthesis 6. Synthesis of the of the nanocellulose nanocellulose dye dye film: film: (i)(i) 96%96% H2SOSO4,4 45, 45 min, min, 45 45 °C,◦ C,yield: yield: 7.4%; 7.4%; (ii) NaOH (ii) NaOH 1 M until1 M until pH = pH10; = (iii)10; (iii) nanocellulose nanocellulose suspension suspension inin water, yield: yield: 37% 37% [7 [878]. ].

4.2. Textiles and Woven Non-Woven Fabrics Textiles represent an attractive class of substrates for fabricating smart chemical sensors. Chromic textiles are fabrics or fibers that change color depending on the type of external stimulus. Several works have been addressed to develop these textiles since their use as smart materials [80]. Of particular interest for environmental monitoring could be ionochromic textiles, but, until now, very few studies are reported for application in this field. A noteworthy paper [81] presents an ionochromic textile for the detection of Fe(II). This material is produced by dyeing 1,10—phenanthroline (PHE) on silk. The interaction with Fe(II) produces an evident color change from white to red, as shown in Figure 7. The best performances were obtained after contacted with 8 mg/L of Fe(II) solution at neutral pH for about 15 min. This ionochromic textile may find broad applications in different fields, such as Fe(II) detection, magic toys, anti- counterfeiting materials, and bionic silk flowers.

Figure 7. Fe(II)-sensitive color changes of silk fabric dyed with 1,10—phenanthroline (PHE) [81].

Hydrogen peroxide (H2O2) plays an essential role in chemistry, biology, environmental, food, and industrial analysis, thus detecting H2O2 concentration with high selectivity and sensitivity is crucial. An interesting study focuses on fabricating a flexible, textile-based, disposable biosensing device fabricated via a photolithography technique. It is applied to develop a biosensor for the colorimetric detection of H2O2 [82]. The analysis is based on the reaction of the dye 2,2′-Azino-bis(3- ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS), that, after oxidation by Horseradish peroxidase (POX) in the presence of H2O2, produced a purple colored product. After contact with

Int. J. Environ. Res. Public Health 2020, 17, x 9 of 23

Int. J. Environ. Res. Public Health 2020, 17, 8331 9 of 23

Figure 6. Synthesis of the nanocellulose dye film: (i) 96% H2SO4, 45 min, 45 °C, yield: 7.4%; (ii) NaOH The thus obtained nanocellulose–dye product gave stable suspensions that change color from 1 Morange until topH purple, = 10; (iii) increasing nanocellulose the pH suspension from acid to in alkaline. water, yield: A piece 37% of [7 the8]. nanocellulose–dye film is taped on a plastic strip, thus obtaining a disposable stick that responds reversibly and rapidly to 4.2. TextilespH changes. and Woven Non-Woven Fabrics

Textiles4.2. Textiles represent and Woven an Non-Woven attractive Fabrics class of substrates for fabricating smart chemical sensors. Chromic textiles are fabrics or fibers that change color depending on the type of external stimulus. Textiles represent an attractive class of substrates for fabricating smart chemical sensors. Chromic Severaltextiles works are have fabrics been or fibers addressed that change to develop color depending these textiles on the type since of external their use stimulus. as smart Several materials works [80]. Of particularhave been interest addressed for toenvironmental develop these textilesmonitoring since theircould use be as i smartonochromic materials textiles, [80]. Of but particular, until now, very fewinterest studies for environmentalare reported monitoringfor application could in be this ionochromic field. textiles, but, until now, very few studies A arenoteworthy reported for paper application [81] present in this field.s an ionochromic textile for the detection of Fe(II). This material is producedA by noteworthy dyeing 1,10 paper— [81phenanthroline] presents an ionochromic (PHE) on textile silk. forThe the interaction detection of with Fe(II). Fe(II) This material produces an evidentis color produced change by dyeing from 1,10—phenanthrolinewhite to red, as shown (PHE) in onFigure silk. The7. The interaction best performances with Fe(II) produces were obtained an evident color change from white to red, as shown in Figure7. The best performances were obtained after contacted with 8 mg/L of Fe(II) solution at neutral pH for about 15 min. This ionochromic textile after contacted with 8 mg/L of Fe(II) solution at neutral pH for about 15 min. This ionochromic textile may findmay find broad broad application applicationss inin di ff differenterent fields, fields, such as Fe(II) such detection, as Fe(II) magic detection, toys, anti-counterfeiting magic toys, anti- counterfeitingmaterials, materials and bionic, and silk flowers.bionic silk flowers.

Figure 7. Fe(II)-sensitive color changes of silk fabric dyed with 1,10—phenanthroline (PHE) [81]. Figure 7. Fe(II)-sensitive color changes of silk fabric dyed with 1,10—phenanthroline (PHE) [81]. Hydrogen peroxide (H2O2) plays an essential role in chemistry, biology, environmental, food, and industrial analysis, thus detecting H O concentration with high selectivity and sensitivity Hydrogen peroxide (H2O2) plays an essential2 2 role in chemistry, biology, environmental, food, is crucial. and industrial analysis, thus detecting H2O2 concentration with high selectivity and sensitivity is An interesting study focuses on fabricating a flexible, textile-based, disposable biosensing crucial. device fabricated via a photolithography technique. It is applied to develop a biosensor for the An interesting study focuses on fabricating a flexible, textile-based, disposable biosensing device colorimetric detection of H2O2 [82]. The analysis is based on the reaction of the dye 2,20-Azino-bis(3- fabricatedethylbenzthiazoline-6-sulfonic via a photolithography acid)technique. diammonium It is applied salt (ABTS), to develop that, after a biosensor oxidation byfor Horseradish the colorimetric detectionperoxidase of H2O (POX)2 [82 in]. the The presence analysis of H 2isO2 , based produced on a purplethe reaction colored product. of the After dye contact 2,2′-Azino with-bis(3- ethylbenzthiazolineH2O2, photographs-6-sulfonic of the colored acid) materialdiammonium are taken salt and (ABTS), the color that intensity, after analysis oxidation is done by using Horseradish the Image J program for H O quantification. The system could work until 50 C temperatures without peroxidase (POX) in the presence2 2 of H2O2, produced a purple colored ◦product. After contact with any loss in the signal and in a pH range between 3 to 7. The recognition of volatile gases in the environment is crucial for monitoring food spoilage or allergens, human health, and guarantee public and workplace safety. A method for obtaining a stable entrapment of dyes on a thread substrate has been proposed for gas sensing [83]. The dyes used in this work are 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride (MnTPP), methyl red (MR), and bromothymol blue (BTB). A smartphone is employed to extract the red (R), green (G), and blue (B) parameters from the acquired images of the thread before and after contact with the gaseous analyte. This sensor can determine from 50 to 1000 ppm of ammonia and hydrogen chloride vapors (i.e., components commonly found in cleaning supplies, fertilizer, and materials production). Examples of gas sensing are shown in Figure8, where optical images of BTB, MR, and MnTPP devices for different ammonia or HCl concentrations are reported. Int. J. Environ. Res. Public Health 2020, 17, x 10 of 23

H2O2, photographs of the colored material are taken and the color intensity analysis is done using the Image J program for H2O2 quantification. The system could work until 50 °C temperatures without any loss in the signal and in a pH range between 3 to 7. The recognition of volatile gases in the environment is crucial for monitoring food spoilage or allergens, human health, and guarantee public and workplace safety. A method for obtaining a stable entrapment of dyes on a thread substrate has been proposed for gas sensing [83]. The dyes used in this work are 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride (MnTPP), methyl red (MR), and bromothymol blue (BTB). A smartphone is employed to extract the red (R), green (G), and blue (B) parameters from the acquired images of the thread before and after contact with the gaseous analyte. This sensor can determine from 50 to 1000 ppm of ammonia and hydrogen chloride vapors (i.e., components commonly found in cleaning supplies, fertilizer, and materials production). Examples of gas sensing are shown in Figure 8, where optical images of BTB, MR, and MnTPP devices for different ammonia or HCl concentrations are reported. The use of these devices as washable gas sensors integrated into textiles represents a further exciting application, and they belong to the new generation of smart textiles for environmental Int. J. Environ. Res. Public Health 2020, 17, 8331 10 of 23 monitoring of various volatile gases.

Figure 8 8.. OpticalOptical images images of BTB of ( BTBbromothymol (bromothymol blue), MR blue), (methyl MR red (methyl), and red), MnTPP and (5,10, MnTPP15,20- (5,10,15,20-Tetraphenyl-21H,23H-porphineTetraphenyl-21H,23H-porphine manganese(III) manganese(III) chloride) chloride) devices devicesfor different for di ffconcentrationserent concentrations of (a)

ofNH (a3) or NH (b3) orHCl (b )[8 HCl3]. [83].

The useColour of theseCatcher devices® (CC as, Henkel washable Italia gas, Milano sensors, Italy integrated) and similar into textiles products represents have been a further in the excitingwashing application,powder market and for they several belong years. to theTheir new success generation is because of smarttheir use textiles in the for washing environmental machine monitoringcan prevent ofcolor various run problems. volatile gases. Each Colour Catcher® packet contains 12 to 20 sheets of 11 × 25 cm, madeThe of a Colour material Catcher that looks® (CC, relatively Henkel rigid, Italia, but Milano,, once Italy)wetted, and it becomes similar products soft, like havea textile. been The in theCC washingsheet express powder sequestration market for capability several years. towards Their ions success and ismolecules because theironce usereleased in the by washing clothes, machine even in canthe preventpresence color of surfactants run problems. and Eachfabric Colour softeners. Catcher Tissue® packet dyes containsare often 12 anionic, to 20 sheets so, in of some 11 25recen cm,t × madepapers of [60 a material–62], the that CC looks sheet relatively is tested rigid, as support but, once for wetted, preparing it becomes colorimetric soft, like sensors. a textile. The The study CC sheetreported express in [60 sequestration] describes an capability inexpensive towards and ions disposable and molecules device for once Fe(III) released and by Al(III) clothes, sensing, even inprepared the presence by dyeing of surfactants an aqueous and solution fabric softeners.of Alizarine Tissue Red dyesS on are2 × often2.5 cm anionic, pieces of so, CC in somefor 3 recenth. The papersobtained [60 sensor–62], the is CCcalled sheet Aliz is- testedCC@, asand support, when for kept preparing in water colorimetric solutions, it sensors. is stable The for study at least reported three indays. [60 ]Fe(III) describes and anAl(III) inexpensive interaction ands with disposable the Alizarin device Red for S Fe(III)-based and sensor Al(III) produce sensing, a change prepared in the by dyeingabsorption an aqueous spectra solutionof the anchored of Alizarine dye, Red enabli S onng 2 to2.5 observe cm pieces a naked of CC-ey fore color 3 h. The variation obtained (see sensor Figure is × called9). The Aliz-CC@, Aliz-CC@ and, device when can kept sorb in water both solutions,cations in it about is stable 1– for2 h. at By least this three sensor, days. it Fe(III) is possible and Al(III) the interactionssimultaneous with determination the Alizarin of Red Fe(III) S-based and Al(III) sensor applying produce athe change chemometric in the absorption multivariate spectra regression of the anchoredPLS (partial dye, least enabling squares to observe regression a naked-eye) to the color UV variation–vis spectra (see ofFigure the 9). Aliz The-CC@, Aliz-CC@ registered device after can sorbequilibration both cations of thein about sensor 1–2 in h. mixtures By this sensor, of Fe(III) it is possible and Al(III) the simultaneous at pH = 4.5. determination The models obtained of Fe(III) anddemonstrated Al(III) applying their ability the chemometric to predict multivariatethe concentration regression of both PLS cations (partial in least aqueous squares samples, regression) so the to thesensor UV–vis is promising spectra of for the the Aliz-CC@, simultaneous registered determination after equilibration of Fe(III) ofand the Al(III) sensor in in environmental mixtures of Fe(III) and andbiological Al(III) samples. at pH = 4.5. The models obtained demonstrated their ability to predict the concentration of both cations in aqueous samples, so the sensor is promising for the simultaneous determination of

Int.Fe(III) J. Environ. and Al(III)Res. Public in Health environmental 2020, 17, x and biological samples. 11 of 23

(a) (b) (c)

Figure 9 9.. ColourColour change change of of Aliz Aliz-CC@-CC@ before before ( (aa),), and and after after equilibration equilibration in in Al(III) Al(III) solution solution ( (bb)) or or Fe(III) Fe(III) solution ( c))[ [6060].

The same CCCC sheetsheet is is also also used used to to develop develop other other colorimetric colorimetric devices devices for for diff erentdifferent metal metal ions ions and andsulfur sul andfur thiols and thiols detection. detection In particular,. In particular, Eriochrome Eriochrome Black T Black (EBT) T and (EBT) 1-(2-pyridylazo)-2-naphthol and 1-(2-pyridylazo)-2- (PAN)naphthol were (PAN) employed, were theemployed, first for Ca(II)the first and for Mg(II), Ca(II) and and the Mg(II), last for Co(II),and the Ni(II), last andfor Co(II) Zn(II), sensingNi(II), [and61]. Zn(II) sensing [61]. These devices, respectively named EBT-CC and PAN, are simply obtained, placing portions of CC in aqueous solutions of the dyes. The best experimental conditions for each sensor’s preparation are found. Similarly to the previously described Aliz-CC@, PLS (partial least squares regression) is applied to correlate the device’s UV–vis spectra with the metal ion concentration. The PLS models obtained proved their ability to predict the investigated metal ions’ concentrations in synthetic and actual aqueous samples. Due to the biological and environmental relevance of thiols and sulfides, the scientific community drew the attention of their determination. In this field, a disposable and low-cost sensor is proposed [62]. With the same procedure reported above, the 5,50-dithiobis(2-nitrobenzoic acid) (Ellman’s reagent, ELL), a specific reagent for sulfides and thiols, is fixed on pieces of CC. The device obtained is named ELL-CC. Figure 10 shows a schematic of the work. As shown in the picture of Figure 10, the device assumes a yellow color after equilibration with solutions of sulfides and thiols. This property has been exploited to develop an optical sensor since the color intensity can be correlated with the analyte sorbed concentration. The PLS models obtained proved their predictive capabilities, so the device is promising to a fast and economical determination of sulfurs and thiols in actual environmental and biological samples.

Figure 10. Preparation and application of the ELL-CC sensor [62].

Int. J. Environ. Res. Public Health 2020, 17, x 11 of 23

(a) (b) (c)

Figure 9. Colour change of Aliz-CC@ before (a), and after equilibration in Al(III) solution (b) or Fe(III) solution (c) [60].

The same CC sheet is also used to develop other colorimetric devices for different metal ions Int. J. Environ. Res. Public Health 2020, 17, 8331 11 of 23 and sulfur and thiols detection. In particular, Eriochrome Black T (EBT) and 1-(2-pyridylazo)-2- naphthol (PAN) were employed, the first for Ca(II) and Mg(II), and the last for Co(II), Ni(II), and TheseZn(II) devices, sensing respectively[61]. These devices named, EBT-CCrespectively and PAN,named are EBT simply-CC obtained,and PAN placing, are simply portions obtained, of CC placingin aqueous portions solutions of CC of in the aqueous dyes. Thesolutions best experimentalof the dyes. T conditionshe best experimental for each sensor’s conditions preparation for each sensor’sare found. preparation Similarly toare the found. previously Similarly described to the Aliz-CC@,previously PLSdescribed (partial Aliz least-CC@ squares, PLS regression)(partial least is squaresapplied to regression) correlate the is device’s applied UV–visto correlate spectra the with device the metal’s UV ion–vis concentration. spectra with The the PLS metal models ion concentrationobtained proved. The their PLS ability models to obtained predict the proved investigated their ability metal to ions’ predict concentrations the investigated in synthetic metal ions and’ concentratactual aqueousions in samples. synthetic and actual aqueous samples. Due to to the the biological biological and and environmental environmental relevance relevance of thiols of and thiols sulfides, and the sul scientificfides, the community scientific communitydrew the attention drew the of their attention determination. of their determination. In this field, a disposableIn this field, and a disposable low-cost sensor and islow proposed-cost sensor [62]. Withis proposed the same [62 procedure]. With the reported same procedure above, the report 5,50-dithiobis(2-nitrobenzoiced above, the 5,50-dithiobis(2 acid)-nitrobenzoic (Ellman’s reagent, acid) (EllmanELL), a specific’s reagent, reagent ELL), for a specific sulfides reagent and thiols, for sul is fixedfides onand pieces thiols, of is CC. fixed The on device piecesobtained of CC. The is nameddevice obtainedELL-CC. Figureis named 10 showsELL-CC. a schematic Figure 10 ofshows the work. a schematic As shown of the in thework. picture As shown of Figure in 10the, thepicture device of Figureassumes 10 a, the yellow device color assumes after equilibrationa yellow color with after solutions equilibration of sulfides with solutions and thiols. of sul Thisfide propertys and thiols. has Thisbeen exploitedproperty to has develop been exploited an optical to sensor develop since an the optical color intensity sensor cansince be the correlated color intensity with the analyte can be correlatedsorbed concentration. with the analyte The PLS sorbed models concentration obtained proved. The PLS their models predictive obtained capabilities, proved their so the predictive device is promisingcapabilities, to so a fastthe device and economical is promising determination to a fast and of economical sulfurs and determination thiols in actual of environmental sulfurs and thiols and inbiological actual environmental samples. and biological samples.

Figure 10.10. Preparation and application of the ELL ELL-CC-CC sensor [6262].

4.3. Synthetic Polymeric Supports Synthetic polymers are usually cheap and enable fast mass production and possibly prototyping. Due to the wide assortment of polymers with different properties, such as flexibility, transparency, stretchability, etc., they can be adequate supports for disposable sensors. For example, thermoplastics are thermosoftening, thus they can be molded and reformed over a specific temperature (glass transition). Classical thermoplastics for disposable sensors are poly(methyl methacrylate) (PMMA), polypropylene (PP), polystyrene (PS), poly(ethylene terephthalate (PET), and poly(tetrafluoroethylene) (PTFE). Unlike other polymers, they offer chemical resistance to organic solvents, lower gas impermeability, a wide range of stiffness, and reduced biofouling. Thanks to these properties, thermoplastics are efficient substrates for disposable devices [36]. The following are some examples. Int. J. Environ. Res. Public Health 2020, 17, 8331 12 of 23

Halogenated organic compounds (HOCs) such as trihalomethanes (THMs) are known to cause adverse health effects, so novel methods can rapidly monitor the potential THMs concentration of source and finished drinking water, and could help mitigate potential health worries and improve the quality of waters. Wujcik et al. [84] proposed a colorimetric sensor based on nanoporous polypropylene fiber membranes for trihalomethanes determination at environmentally relevant levels (ppb-scale). In particular, a syndiotactic polypropylene (sPP) nanoporous electrospun fiber membrane is prepared and used as a sorbent for THMs in the headspace of a heated water sample. The colorimetric Fujiwara reaction [85] is applied as a detection reaction. The membrane is then tested for preconcentration and assay of THMs from contaminated water samples using image intensity analysis. Thanks to the positive results, this device holds potential as a portable tool for economic, on-site environmental analysis. The work of Salcedo and Sevilla [86] presents a Hg(0) sensor based on cuprous iodide (CuI)/polystyrene composite immobilized on a cellulosic substrate: This device showed a reddish color in the presence of mercury vapors. Images of the sensor are acquired by a smartphone camera and analyzed in RGB color space by ImageJ (open-source image processing program). The colorimetric measurement is based on the formation of a red complex between Hg and CuI, according to the following reaction: 4 CuI + Hg Cu  2[HgI4] + Cu. The method is validated by recovery tests, spiking an air sample with gas mixtures standard at different Hg concentrations: the recovery is satisfactory, ranging between 95 and 112%. The device is suitable for measuring gaseous mercury at ppb levels. Besides thermoplastics, other synthetic materials were developed to prepare colorimetric devices. For example, Deng et al. [87] reported a fibrous material obtained incorporating the dye 4-(2-pyridylazo)-resorcinol (PAR) onto polyacrylonitrile (PAN) fiber for the detection of some heavy metal ions, such as Cu(II), Hg(II), Ni(II), and Pb(II). The fiber is prepared using an MW-assisted method, and it is named PANMW-PAR. Figure 11 shows a scheme of fiber preparation and the sorption isotherms for the selected cations. Results demonstrated that PANMW-PAR fibers after contact with the metal ions solution show a color change from red to black, and the sensitivity is pretty high. The sorption capacity for the investigated metal ions, especially for Hg(II), is excellent. PANMW-PAR fibers are easily regenerated with EDTA solution and can be reused after 10 sorption cycles. No response is observed in solutions of other abundant cations, such as Ca(II), Mg(II), and Al(III), showing the remarkable selectivity of the fiber for heavy metal ions. Int. J. Environ. Res. Public Health 2020, 17, x 13 of 23

Figure 11.11. ScheSchememe of PANMW--PARPAR synthesis synthesis and and sorption isotherms for Cu(II), Hg(II), Ni(II),Ni(II), and Pb(II). [87].].

Exposure to VOCs (volatile organic compounds) can severely affect human health, so VOCs’ detection and identification are essential. The application of colorimetric techniques is a new strategy to overcome many limits of traditional gas sensing methods [88]. A chemosensor, obtained by a self- assembled polydiacetylene (PDA)/graphene stacked composite film, is developed to quantify VOCs [89]. Graphene is used as support for the PDA-based sensor since ordered monolayers of PDA are efficiently formed on the large surface of a graphene sheet; moreover, graphene is highly transparent in the UV–vis range, thus the signal of the visual chromatic change can be efficiently identified by spectroscopy. For testing the ability to detect VOCs, the PDA/graphene paper is exposed for 2 min to organic vapors in concentrations ranging from 0.01% to 20% (percent by volume). Figure 12 shows PDA/graphene film images after contact with the VOCs, demonstrating an evident color change detectable just by the naked eye, so without other instruments.

Figure 12. Photographs of the PDA/graphene films after exposure to dimethylformamide (DMF),

methanol (CH3OH), chloroform (CHCl3), and tetrahydrofuran (THF) vapors for 2 min [89].

Formaldehyde is widely applied as a base chemical to manufacture building materials and household products. Nevertheless, it is a dangerous air pollutant, and extended exposure to this contaminant can cause serious health problems. A new and simple strategy for colorimetric sensing of formaldehyde (HCHO) using a nanofiber-based functional strip is presented [90]. In particular, the polyethyleneimine (PEI) functionalized poly(methyl methacrylate) nanofiber membrane is fabricated by electrospinning method. The functionalized nanofiber showed a color change from white to yellow after interaction with formaldehyde. It is observed that this sensor could detect down to 75 ppm of formaldehyde at room temperature. On evaluating possible interferents, such as ethanol, DMF, DCM, acetone, acetic acid, and chloroform, it is shown that no color variation is verified, confirming the sensor’s selectivity for formaldehyde. The mechanism involves the nucleophilic

Int. J. Environ. Res. Public Health 2020, 17, x 13 of 23

Figure 11. Scheme of PANMW-PAR synthesis and sorption isotherms for Cu(II), Hg(II), Ni(II), and Pb(II). [87]. Int. J. Environ. Res. Public Health 2020, 17, 8331 13 of 23 Exposure to VOCs (volatile organic compounds) can severely affect human health, so VOCs’ detection and identification are essential. The application of colorimetric techniques is a new strategy Exposure to VOCs (volatile organic compounds) can severely affect human health, so VOCs’ to overcome many limits of traditional gas sensing methods [88]. A chemosensor, obtained by a self- detection and identification are essential. The application of colorimetric techniques is a new strategy assembledto overcome polydiacetylene many limits (PDA)/graphene of traditional gas stacked sensing composite methods [88 film,]. A is chemosensor, developed to obtained quantify by VOCs a [89]. self-assembledGraphene is used polydiacetylene as support (PDA)for the/graphene PDA-based stacked sensor composite since ordered film, is developed monolayer tos quantify of PDA are efficientlyVOCs form [89].ed Graphene on the large is used surface as support of a graphene for the PDA-based sheet; moreover, sensor since graphene ordered is highly monolayers transparent of in thePDA UV are–vis e ffirangeciently, thus formed the onsignal the largeof the surface visual of chromatic a graphene change sheet; moreover, can be efficiently graphene identified is highly by spectroscopy.transparent For in testing the UV–vis the ability range, thusto detect the signal VOCs, of the the PDA/graphene visual chromatic paper change is canexposed be effi forciently 2 min to organicidentified vapors by in spectroscopy. concentrations For testing ranging the from ability 0.01% to detect to VOCs,20% (percent the PDA by/graphene volume). paper Figure is exposed 12 shows PDA/graphenefor 2 min to organic film images vapors inafter concentrations contact with ranging the from VOCs, 0.01% demonstrating to 20% (percent an by evidentvolume). color Figure change 12 detectableshows PDAjust by/graphene the naked film eye images, so afterwithout contact other with instrument the VOCs, demonstratings. an evident color change detectable just by the naked eye, so without other instruments.

Figure 12. Photographs of the PDA/graphene films after exposure to dimethylformamide (DMF), Figure 12. Photographs of the PDA/graphene films after exposure to dimethylformamide (DMF), methanol (CH3OH), chloroform (CHCl3), and tetrahydrofuran (THF) vapors for 2 min [89]. methanol (CH3OH), chloroform (CHCl3), and tetrahydrofuran (THF) vapors for 2 min [89]. Formaldehyde is widely applied as a base chemical to manufacture building materials and householdFormaldehyde products. is widely Nevertheless, applied it as is a a dangerous base chemical air pollutant, to manufacture and extended building exposure materials to this and householdcontaminant products. can cause Nevertheless serious health, it problems.is a dangerous A new andair simplepollutant, strategy and for extended colorimetric exposure sensing toof this contaminantformaldehyde can cause (HCHO) serious using health a nanofiber-based problems. A functionalnew and simple strip is strateg presentedy for [90 colorimetric]. In particular, sensing of formaldehydethe polyethyleneimine (HCHO) using (PEI) a functionalized nanofiber-based poly(methyl functional methacrylate) strip is presented nanofiber [90]. membraneIn particular, is the fabricated by electrospinning method. The functionalized nanofiber showed a color change from polyethyleneimine (PEI) functionalized poly(methyl methacrylate) nanofiber membrane is fabricated white to yellow after interaction with formaldehyde. It is observed that this sensor could detect by electrospinning method. The functionalized nanofiber showed a color change from white to down to 75 ppm of formaldehyde at room temperature. On evaluating possible interferents, such as yellowethanol, after DMF,interaction DCM, acetone,with formaldehyde. acetic acid, and It chloroform, is observed it is that shown this that sensor no color could variation detect is verified,down to 75 ppmconfirming of formaldehyde the sensor’s at room selectivity temperature. for formaldehyde. On evaluating The mechanism possible interferents involves the, such nucleophilic as ethanol, DMF,addition DCM, reaction acetone, between acetic formaldehyde acid, and chloroform, and the primary it is amineshown of PEI.that The no nanofiber color variation format favoredis verified, confirmingthe reaction the due sensor to the’s high selectivity surface area.for formaldehyde. This sensor provides The low-cost mechanism sensing involves of formaldehyde the nucleophilic at low concentrations, and it is advantageous for environmental monitoring and food quality assessments. Among the most popular polymers in colorimetric sensors, acetylcellulose and polyvinyl chloride (PVC) are widely applied. The transparent triacetylcellulose is a completely acetylated cellulose polymer. It can be employed as a raw material to prepare plastics, fibers, and films such as photo films. Moreover, the triacetylcellulose film can be used as porous support for colorimetric sensors. Tavallali’s and Safavi groups of two Iranian Universities developed several triacetylcellulose-based disposable optical sensors for metal ions, using, as starter material, waste photographic film tapes, immobilizing on the membrane different dyes [69–73,91–95]. All these studies adopted the same procedure to prepare the sensors. The photographic films were previously treated with sodium hypochlorite to remove the colored layer. The obtained transparent films were contacted with a suitable dye solution in ethylene diamine for a few minutes at room temperature, then washed with double-distilled water for removing excess ethylene diamine and the loosely trapped dye. The prepared membranes were kept in water until their use. Int. J. Environ. Res. Public Health 2020, 17, 8331 14 of 23

The goal of these kinds of devices is to use waste material with excellent optical and mechanical properties as support for dyes immobilization. These membranes respond to metal ions by changing color, and the signal can be revealed by nacked-eyes or spectrophotometrically. By adapting the above-reported procedure, a colorimetric sensor for Cu(II), Cd(II), Zn(II), and Hg(II) sensing has been prepared by immobilization of dithizone (DTZ) on a triacetylcellulose film (mem-DTZ) [47,74]. The membrane responds to the divalent cations by changing color reversibly. The metal ions sorption on the mem-DTZ has been characterized by kinetics, isotherms, and profiles in the function of the pH. The paper highlights methods for assessing the cations’ concentrations, either individually or in a mixture, in unknown samples. After equilibration of the mem-DTZ with different cations solutions, UV–vis spectra, and RGB parameters of pictures obtained by a desktop scanner are acquired and correlated with the metal ions’ concentration in solutions. A single metal ion determination is performed by applying principal component analysis (PCA) to the pictures’ RGB parameters; otherwise, UV–vis spectra of a mixture of two cations are subjected to partial least squares (PLS) regression. The sensor’s applicability to real samples has been proved by analyzing the four metal cations in a certificate material (Sewage Sludge CC136A), drinking water, and white wine samples.

4.4. Sol-Gel Materials The sol-gel method allows for the formation of ceramics and glassy materials at temperatures much lower than those used in traditional melting techniques. The first study about sol-gels appeared about 150 years ago, but the real development of this technology and its applications occurred in the last few years. Sol-gel methods can produce different kinds of materials: monoliths, thin films, or fibers. The strict control of the experimental conditions during the production process yields materials with well-defined properties [96]. The sol-gel-based materials may provide excellent substrates for various devices such as colorimetric sensors; some examples are here summarized. Int. J. Environ.A Res. low-cost, Public Health colorimetric 2020, 17 array, x for sensing TICs (toxic industrial chemicals) has been developed [54].15 of 23 The colorimetric sensor array consisted of 36 different dyes printed on a nonporous PET (polyethylene particular,terephthalate) hierarchical film. The cluster indicator analysis classes for (HCA) this colorimetric using the sensor minimum array include variance metal ion-containing method (Ward’s dyes (e.g., metalloporphyrins), pH indicators, vapochromic/solvatochromic dyes, and redox-sensitive method). metal salts. The device is printed through an array of 36 floating slotted pins by dipping into the Theinkwell sensor and array transferring can discriminate to the PET film. among The sol-gel 20 obtained TICs at is their kept under permissible a slow nitrogen exposure stream limit for (PEL) concentrations,at least three with days limits before of any detection analysis. in Color the difewfference µg/L maps range are (well obtained below by subtracting the PEL). the Moreover, array’s the array isdigital not image affected before by exposure various from potential that after interfering exposure, so each agents, spot andis characterized it shows bygood its RGB stability color and reproducibility.values (see Figure 13). Chemometric analyses are carried out. In particular, hierarchical cluster analysis (HCA) using the minimum variance method (Ward’s method).

Figure 13. Images of the colorimetric array before exposure, after 2 min of exposure to ammonia at its Figure IDLH13. Images (immediately of the colorimetric dangerous to lifearray or health)before concentrationexposure, after (300 2 ppm)min of at exposure 298 K and 50%to ammonia relative at its IDLH (humidityimmediately and the dangerous generated colorto life di fforerence health map) concentration [54]. (300 ppm) at 298 K and 50% relative humidity and the generated color difference map [54].

Sensitive detection of trimethylamine (TMA) both in aqueous and gaseous phases is obtained using a heap colorimetric sensor array [97]. This device incorporates three different kinds of colorants: metal-containing dyes (i.e., Zn(II) metalloporphyrin), pH indicators, and dyes with large permanent dipoles (i.e., vapochromic dyes). Highly porous sol−gel formulations are used to better respond to gaseous analytes and the ideal hydrophobicity of the matrix, minimizing the colorants’ leaching during analysis. The 20-element arrays are linearized for improved gas flow path. It is printed and then attached to a polycarbonate cartridge, providing an ideal flow path for analytes and a flow volume of <180 μL (Figure 14). As a response, digital pictures of the arrays are acquired before and after exposure to aqueous solutions or gas mixtures; from changes in RGB values of each spot, color difference maps are produced. Characteristic color change patterns permitted easy discrimination among different trimethylamine concentrations. This array showed excellent reversibility with the possibility of distinguishing trimethylamine from similar amine odorants.

Figure 14. Colorimetric sensor array for trimethylamine (TMA) detection. (a) Linearized 20-element sensor array for vapor detection; (b) schematic of the experimental setup consisting of a closed Petri dish containing 10 mL of a buffered aqueous TMA solution, an array positioned in the solution, and an ordinary flatbed scanner for imaging [97].

Int. J. Environ. Res. Public Health 2020, 17, x 15 of 23 particular, hierarchical cluster analysis (HCA) using the minimum variance method (Ward’s method). The sensor array can discriminate among 20 TICs at their permissible exposure limit (PEL) concentrations, with limits of detection in the few µg/L range (well below the PEL). Moreover, the array is not affected by various potential interfering agents, and it shows good stability and reproducibility.

Figure 13. Images of the colorimetric array before exposure, after 2 min of exposure to ammonia at its IDLH (immediately dangerous to life or health) concentration (300 ppm) at 298 K and 50% relative humidity and the generated color difference map [54].

Sensitive detection of trimethylamine (TMA) both in aqueous and gaseous phases is obtained using a heap colorimetric sensor array [97]. This device incorporates three different kinds of colorantsInt. J. Environ.: metal Res.-containing Public Health 2020dyes, 17 (i.e., 8331, Zn(II) metalloporphyrin), pH indicators, and dyes 15with of 23 large permanent dipoles (i.e., vapochromic dyes). HighlyThe porous sensor arraysol−gel can formulations discriminate are among used 20 to TICs better at respond their permissible to gaseous exposure analytes limit and (PEL) the ideal hydrophobicityconcentrations, of with the limitsmatrix, of detectionminimiz ining the the few colorantsµg/L range’ leaching (well below during the PEL). analysis Moreover,. The the 20 array-element arraysis notare a fflinearizedected by various for improved potential gas interfering flow path. agents, It is and printed it shows and good then stability attached and to reproducibility. a polycarbonate cartridge,Sensitive providing detection an ideal of trimethylamineflow path for analytes (TMA) both and in a aqueousflow volume and gaseous of <180 phasesμL (Figure is obtained 14). usingAs a aresponse, heap colorimetric digital pictures sensor array of the [97]. arrays This device are acquired incorporates before three and diff erentafter kindsexposure of colorants: to aqueous solutionsmetal-containing or gas mixtures dyes (i.e.,; from Zn(II) changes metalloporphyrin), in RGB values pH indicators, of each and spot, dyes color with difference large permanent maps are dipoles (i.e., vapochromic dyes). produced. Highly porous sol-gel formulations are used to better respond to gaseous analytes and the ideal Characteristic color change patterns permitted easy discrimination among different hydrophobicity of the matrix, minimizing the colorants’ leaching during analysis. The 20-element trimethylaminearrays are linearized concentrations. for improved This gas array flow path. showed It is excellent printed and reversibility then attached with to a polycarbonate the possibility of distinguishingcartridge, providing trimethylamine an ideal flowfrom path similar for analytes amine odorants. and a flow volume of <180 µL (Figure 14).

Figure 14. Colorimetric sensor array for trimethylamine (TMA) detection. (a) Linearized 20-element Figuresensor 14. arrayColorimetric for vapor sensor detection; array (b) for schematic trimethylamine of the experimental (TMA) detection. setup consisting (a) Linearized of a closed 20 Petri-element sensordish array containing for vapor 10 mL detection; of a buffered (b) aqueousschematic TMA of solution,the experimental an array positioned setup consisting in the solution, of a closed and an Petri dishordinary containing flatbed 10 mL scanner of a forbuffered imaging aqueous [97]. TMA solution, an array positioned in the solution, and an ordinary flatbed scanner for imaging [97]. As a response, digital pictures of the arrays are acquired before and after exposure to aqueous

solutions or gas mixtures; from changes in RGB values of each spot, color difference maps are produced. Characteristic color change patterns permitted easy discrimination among different trimethylamine concentrations. This array showed excellent reversibility with the possibility of distinguishing trimethylamine from similar amine odorants. Chemical, biological, radiological, nuclear, and explosive agents pose significant threats in the twenty-first century, particularly for armed forces and first responders (i.e., police, firefighters, and other emergency personnel). Thus, quick, selective, and sensitive detection systems are required to identify and quantify specific compounds. In particular, there is a need for low-cost disposable sensors that are sensitive, stable, and reliable [41]. A couple of examples of sol-gel based sensors in this field are summarized in the following. A sensing device for selective detection of 2,4,6-trinitrotoluene (TNT) is obtained by functionalization of silica nanoparticles (NPs) with 3-aminopropyl-triethoxysilane (APTES) [98]. The amine anchored to the silica nanoparticles’ surface (SiO2-NH2) behaves like a probe for TNT molecules. A colorimetric change from the green of SiO2-NH2 nanoparticles towards red is observed after the reaction with TNT. As the TNT concentration increases, a gradual change to a dark red color can be observed, as shown in Figure 15. The naked eye’s detection range is limited to 100 µM, but using 12 optical instruments makes it possible to detect lower quantities (about 10− M). Thanks to the pretty good results, this device has the potential to further progress for easy-to-use and low-cost sensing applications. Int. J. Environ. Res. Public Health 2020, 17, x 16 of 23

Chemical, biological, radiological, nuclear, and explosive agents pose significant threats in the twenty-first century, particularly for armed forces and first responders (i.e., police, firefighters, and other emergency personnel). Thus, quick, selective, and sensitive detection systems are required to identify and quantify specific compounds. In particular, there is a need for low-cost disposable sensors that are sensitive, stable, and reliable [41]. A couple of examples of sol−gel based sensors in this field are summarized in the following. A sensing device for selective detection of 2,4,6-trinitrotoluene (TNT) is obtained by functionalization of silica nanoparticles (NPs) with 3-aminopropyl-triethoxysilane (APTES) [98]. The amine anchored to the silica nanoparticles’ surface (SiO2-NH2) behaves like a probe for TNT molecules. A colorimetric change from the green of SiO2-NH2 nanoparticles towards red is observed after the reaction with TNT. As the TNT concentration increases, a gradual change to a dark red color can be observed, as shown in Figure 15. The naked eye’s detection range is limited to 100 μM, but using optical instruments makes it possible to detect lower quantities (about 10−12 M). Thanks to the pretty good results, this device has the potential to further progress for easy-to- use Int.and J. Environ.low-cost Res. sensing Public Health applications.2020, 17, 8331 16 of 23

Figure 15. Functionalized Silica Nanoparticles for TNT detection. (a) The complex between the amine Figuregroup 15.and Functionalized TNT molecule Silica and (Nanoparticlesb) a gradual change for TNT in color detection. as the TNT (a) concentrationThe complex increasesbetween [98the]. amine group and TNT molecule and (b) a gradual change in color as the TNT concentration increases [98]. A colorimetric sensor is developed for the sensitive gas detection of cyclohexanone, a volatile marker of some explosives, such as RDX (1,3,5-trinitro-1,3,5-triazinane) and HMX (1,3,5,7-tetranitro- A colorimetric sensor is developed for the sensitive gas detection of cyclohexanone, a volatile 1,3,5,7-tetrazocane) [99]. It consists of silica-dye composite microspheres prepared using ultrasonic marker of some explosives, such as RDX (1,3,5-trinitro-1,3,5-triazinane) and HMX (1,3,5,7-tetranitro- and aerosol-gel synthesis, comprising the hydrolysis of ultrasonically sprayed organosiloxanes at 1,3,5,7-tetrazocane) [99]. It consists of silica-dye composite microspheres prepared using ultrasonic 150 ◦C, with the adding of the ketone-responsive indicator dye, pararosaniline. With this procedure, and nanoporousaerosol−gel microspheres synthesis, comprising with high surface the hydrolysis area (~300 mof2 /ultrasonicallyg) are prepared sprayed and used organosiloxanes as colorimetric at 150 inks.°C , with Indeed, the theadding sensor of inksthe ketone are dropped-responsive on cellulose indicator paper, dye, and pararosaniline. the so obtained colorimetricWith this procedure, strip nanoporousallowed formicrospheres a quick quantification with high of surface gaseous area cyclohexanone (~300 m2/g) inar aboute prepared 2 min, and with used a LOD as colorimetric around inks150. Indeed, ppb. The the sensor sensor displayed inks are highdrop specificityped on cellulose towards paper, cyclohexanone and the againstso obtained humidity colorimetric and other strip allowedcommon for a solvents quick quantification classes, such as acetonitrile,of gaseous ammonia,cyclohexanone ethanol, in ether, about and 2 min, ethyl with acetate. a LOD This kindaround of 150 ppb.device The could sensor be display an alternativeed high for sensingspecificity explosives. towards cyclohexanone against humidity and other commonA solvents similar approach classes, issuch applied as acetonitrile, to develop a ammonia, colorimetric ethanol, sensor array ether, to and identify ethyl aliphatic acetate amines,. This kind of devicepollutants could often be presentan alternative in industrial for sensing wastewater explosives. effluents, and agricultural drainage because of their extensive use in several applications [48]. A similar approach is applied to develop a colorimetric sensor array to identify aliphatic amines, Sixteen different nanoporous pigment microspheres are prepared by an ultrasonic-spray aerosol-gel pollutantssynthesis, often starting present from in dyes industrial and silica. wastewater effluents, and agricultural drainage because of their extensiveThe use colorimetric in several applications sensor arrays [48 are]. obtained by printing the prepared inks onto standard chromatographySixteen different paper. nanoporous The arrays are pigment successfully microspheres tested for theare identification prepared and by quantificationan ultrasonic of-spray aero11sol−gel toxic aliphaticsynthesis amines., starting As from evident dyes from and the silica. images registered by a scanner reported in Figure 16, theThe sensor colorimetric arrays showed sensor a di arraysfferent a colorre obtained change for by each printing amine. the Subtracting prepared the inks image onto obtained standard chromatographybefore and after paper. exposure The to arrays amines, a direff successfullyerence maps are tested produced. for the The identification RGB parameters and were quanti collectedfication of 11 toxicin the aliphatic center-half amines. of each As spot. evident Chemometric from the analysis images (i.e., registered principal by component a scanner analysis reported (PCA) in Figure and 16, the hierarchicalsensor arrays cluster showed analysis a different (HCA)) is color applied change on the for RGB each triplet. amine. Subtracting the image obtained before andThese after sensor exposure arrays demonstrated to amines, difference the ability maps to discriminate are produced. among The eleven RGB similar parameters aliphatic were amines and gaseous ammonia at different concentrations. The detection limits are always below the permissible exposure limit (50 ppm). As(V) poisoning provokes severe damage to human health and the environment; indeed, arsenic contamination in groundwater is a significant problem in several countries. The exposure of this pollutant can cause severe health hazards such as skin discoloration and thickening, lesions, nausea, diarrhea, paralysis, blindness, and lung and bladder cancer. Arsenic toxicity largely depends on its chemical forms: Arsenite and arsenate are the most toxic species. Int. J. Environ. Res. Public Health 2020, 17, x 17 of 23

collected in the center-half of each spot. Chemometric analysis (i.e., principal component analysis (PCA) and hierarchical cluster analysis (HCA)) is applied on the RGB triplet. These sensor arrays demonstrated the ability to discriminate among eleven similar aliphatic amines and gaseous ammonia at different concentrations. The detection limits are always below the Int.permissible J. Environ. Res. exposure Public Health limit2020 (50, 17 ppm)., 8331 17 of 23

FigureFigure 16. 16.Color Color profiles profiles of of 11 11 similar similar aliphatic aliphatic amines amines after after equilibration equilibrationwith withthe theaerosol-gel-based aerosol−gel-based colorimetriccolorimetric sensor sensor arrays arrays [ 48[48].].

AA low-cost,s(V) poisoning colorimetric, provokes hydrogel, severe damage dipstick to human device health for As(V) and isthe developed environment [100; ];indeed, the method arsenic consistscontamination of forming in groundwater a blue-colored is antimonyl–arseno–molybdate a significant problem in several complex. countries. The sensor The exposure is obtained of this by encapsulatingpollutant can ammoniumcause severe molybdate, health hazards potassium such as antimonyl skin discoloration tartrate, and thickening, ascorbic acid lesions, in a polymer nausea, hydrogeldiarrhea, of paralysis, acrylamide, blindness, polyvinyl and alcohol, lung and glutaraldehyde.bladder cancer. TheArsenic as-prepared toxicity hydrogellargely depends is dip-coated on its withchemical plastic forms: detector Arsenite strips. and arsenate are the most toxic species. InA particular, low-cost, the colorimetric, method is basedhydrogel on arsenate’s, dipstick reactiondevice for with As ammonium(V) is developed molybdate, [100] which; the method forms anconsists arseno–molybdate of forming a blueblue complex-colored antimonyl in the presence–arseno of– amolybdate reducing agent complex (for. example, The sensor L-ascorbic is obtained acid). by encapsulatiFigure 17ng shows ammonium a schematic molybdate, of the potassium whole process, antimo fromnyl the tartrate synthesis, and toascorbic the sensing. acid in a polymer 1 hydrogelThis device of acrylamide, has a pretty polyvinyl good LOD, alcohol lower, and than glutaraldehyde 10 µg L− of As(V);. The as moreover,-prepared it displayedhydrogel is good dip- 1 1 selectivitycoated with in p thelastic presence detector of strips<100 .µ g L− of phosphate and 3 mg L− of iron. This sensor is simple, portable,In particular, cheap, and the user-friendly method is for based on-site on determination.arsenate’s reaction The RGB with analysis ammonium gave impressivemolybdate results,, which sinceforms the an As(V) arseno concentration–molybdate blue can complex be simply in determinedthe presence by of digitala reducing analysis agent of (for the example, picture taken L-ascorbic by a smartphoneacid). camera, and without any instrument, power, or other utilities. AmongFigure all17 theshows transition a schematic metal of ions, theiron whole is an process, essential from element the synthesis in different to the biochemical sensing. processes, such asThis in device oxygen has transportation a pretty good and LOD cellular, lowermetabolism. than 10 μg L−1 On of theAs(V); other moreover, hand, iron it displayed deficiency good or overdoseselectivity results in the in presence abnormal of dangerous <100 μg L functions−1 of phosphate in the body’sand 3 mg system L−1 of and iron. induces This varioussensor is diseases, simple, suchportable, as Parkinson’s, cheap, and Alzheimer’s user-friendly cancer, for and on-site anemia. determination Therefore,. there The RGB is a need analysis for active gave sensors impressive for monitoringresults, since iron the content As(V) in concentration biological and can environmental be simply determined samples. by digital analysis of the picture taken by a smartphone camera, and without any instrument, power, or other utilities.

Int. J. Environ. Res. Public Health 2020, 17, 8331 18 of 23

Int. J. Environ. Res. Public Health 2020, 17, x 18 of 23

FigureFigure 17. 1Schematic7. Schematic diagram diagram of of the the whole whole procedure for for As( As(V)V) determination determination by theby thepolymer polymer hydrogel-basedhydrogel-based colorimetric colorimetric dipstick dipstick sensor sensor described described in [ [101000].].

To thisAmong end, all a the selective transition and metal reusable ions, colorimetriciron is an essential sensor element for visual in different detection biochemical of Fe(III) is processes, such as in oxygen transportation and cellular metabolism. On the other hand, iron developed [101]. The system consists of a hybrid polymeric film prepared by the sol-gel technique and deficiency or overdose results in abnormal dangerous functions in the body’s system and induces functionalized with rhodamine derivative (RB-UTES). various diseases, such as Parkinson’s, Alzheimer’s cancer, and anemia. Therefore, there is a need for Theactive sensor sensors film for exhibitedmonitoring high iron selectivitycontent in biological and sensitivity and environmental to Fe(III) and samples allowed. direct naked-eye detection,To since this the end, film’s a selective color changes and reusable from white colorimetric to pink. sensor The sensor for visual film returns detection to itsof originalFe(III) is color afterdeveloped cleaning with [101]. 0.1 The M system ethylenediamine. consists of a hybrid polymeric film prepared by the sol−gel technique Thanksand functi toonalized its analytical with rhodamine performance, derivative this (RB sensor-UTES). is promising for the environmental analysis of Fe(III).The sensor film exhibited high selectivity and sensitivity to Fe(III) and allowed direct naked-eye detection, since the film’s color changes from white to pink. The sensor film returns to its original 5. Conclusionscolor after cleaning with 0.1 M ethylenediamine. Thanks to its analytical performance, this sensor is promising for the environmental analysis of EvenFe(III). if there is an extensive range of low-cost, disposable, colorimetric sensors already available commercially or prepared in research labs, thanks to the expansion of smartphones, digital communication5. Conclusions networks, image analysis apps, and chemometric tools, the field of this kind of sensing still has muchEven roomif there to is grow.an extensive range of low-cost, disposable, colorimetric sensors already available Therecommercially is a need or to prepared develop in “zero-cost” research labs disposable, thanks sensors to the expansion interfaced of with smartphones, open-source digital software and hardware.communication networks, image analysis apps, and chemometric tools, the field of this kind of Futuresensing still trends has much should room comprise: to grow. New devices obtained using “green” materials; low-cost, sustainable,The andre isbiodegradable a need to develop sensors; “zero-cost” miniaturization disposable sensors and use interfaced of portable with analyzers open-source or smartphones;software and applicationand hardware of. functionalized nanomaterials. Moreover,Future smartphone-based trends should comprise: colorimetric New devices devices’ obtained development using “green” aims to strengthenmaterials; low more-cost, handy sustainable, and biodegradable sensors; miniaturization and use of portable analyzers or and practical environmental analytes testing in resource-limited zones. smartphones; and application of functionalized nanomaterials. Author Contributions:Moreover, smartphoneConceptualization,-based colorimetric G.A.; writing—original devices’ development draft preparation, aims to strengthen G.A.; writing—review more handy and editing,and R.B., practical C.Z., L.R.M.environmental All authors analytes have readtesting and in agreedresource to- thelimited published zones. version of the manuscript. Funding: This research received no external funding. Author Contributions: Conceptualization, G.A.; writing—original draft preparation, G.A.; writing—review and Conflictsediting, of Interest:R.B., C.Z.,The L.R.M authors. All authors declare have no read conflict and ofagreed interest. to the published version of the manuscript.

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