Microfluidic paper-based analytical devices for environmental analysis of soil, air, ecology and river water

Sensors & Actuators: B. Chemical 301 (2019) 126855

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Microfluidic paper-based analytical devices for environmental analysis of soil, air, ecology and river water Chia-Te Kunga, Chih-Yao Houb, Yao-Nan Wangc, Lung-Ming Fud,e,

T

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a

Department of Emergency Medicine, Kaohsiung Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, 833, Taiwan Department of Seafood Science, National Kaohsiung University of Science and Technology, Kaohsiung 811, Taiwan c Department of Vehicle Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan d Department of Engineering Science, National Cheng Kung University, Tainan, 701, Taiwan e Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan b

ARTICLE INFO

ABSTRACT

Keywords: Microfluidic Paper-based devices Water Soil Air Ecology Pesticide Environment

Microfluidic paper-based analytical devices (μPADs) have experienced rapid growth over the past decade due to their simple design, low cost, minimal sample requirement, and good sensitivity, selectivity and accuracy. While designed originally for point-of-care medical diagnostics, biological, and food safety applications, μPADs are now used increasingly for environmental monitoring purposes. This review provides a detailed overview of the μPADs developed over the past ten years for the environmental analysis of soil, air, ecology (pesticides) and river water. The review commences by introducing the fabrication techniques and detection methods used in μPAD technology. A detailed description of the main μPAD frameworks proposed in the past decade for environmental monitoring is then provided. The review concludes by examining the challenges facing μPADs for environmental monitoring and identifying probable avenues of future research.

1. Introduction As technology has advanced, the environment has become increasingly polluted with heavy metals, nutrients, microorganisms, organic pollutants, pesticides, and so on. Many of these pollutants are extremely harmful to human health, and hence the problem of developing instrumented techniques capable of detecting analyte concentrations in diverse sample matrices on the scale of parts per million (ppm), or even parts per trillion (ppt), has attracted great interest in recent decades. To meet the needs of the monitoring community for high sensitivity and low detection limits, most analytic methods rely on expensive equipment, which requires a high level of training to operate reliably. However, there is a growing need for low-cost technologies to detect and monitor environmental contaminant concentrations quickly, easily and in-field in order to provide more timely data regarding the extent and magnitude of pollution. Microfluidic paper-based analytical devices (μPADs) off ;er an opportunity to address this need by increasing the frequency and geographic coverage of environmental monitoring, while simultaneously reducing the analytic costs and complexity of the measurement process. μPAD platforms enable the realization of low-cost, flexible, simple and portable analytical instruments [1–3]. μPADs typically consist of a

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small piece of patterned paper with a 2D or 3D structure capable of testing several tens of micro-liters of liquid sample within a relatively short period of time. During the past decade, numerous papers have been published in the μPAD field [4,5]. Typically, the devices proposed in these papers comprise an arrangement of microfluidic channels created by patterning hydrophobic materials on hydrophilic paper [6–8]. The testing process for biological substances (e.g., tears, urine, blood, sweat, saliva) [9–13], food (e.g., additives, bacteria, organic compounds) [14–17], and environmental reagents (e.g., heavy metal ions, nutrients, organic contaminants, microorganisms) [18–21] involves wicking the sample to the detection zone under the effects of capillary action without the need for an external pump [22–27]. Analytic detection is then facilitated by a chemical reaction process, which induces a change in color, electrochemical properties and / or light absorption or emission of the detection zone. Continuing industrialization and technological development has led to serious pollution of the air, soil and water in many regions of the world. Typically, these pollutants include nutrients, such as carbon, nitrogen and phosphorus, and many other contaminants, including heavy metals, organic contaminants, pharmaceuticals, herbicides, insecticides, disinfectants and industrial byproducts. Such pollution not only causes serious harm to the environment and surrounding

Corresponding author at: Department of Engineering Science, National Cheng Kung University, Tainan, 701, Taiwan. E-mail address: [email protected] (L.-M. Fu).

https://doi.org/10.1016/j.snb.2019.126855 Received 25 April 2019; Received in revised form 18 July 2019; Accepted 22 July 2019 Available online 08 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

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ecosystems, but also poses a major threat to human health and economic development. For a more detailed overview of the clinical presentations of some of the main environmental pollutants affecting humans nowadays, please refer to Supplementary Materials Table S1. Given the magnitude of the problem, the need for simple and cost-effective methods to enable the reliable monitoring of environmental pollution has emerged as a critical concern in recent years. This review provides a comprehensive overview of the main μPAD platforms developed over the past decade for environmental monitoring. The review commences by describing the various types of substrate material used in the fabrication of μPAD devices. Typical μPAD fabrication methods and detection techniques are then introduced. A detailed description of the main μPAD frameworks proposed over the past ten years for the analysis of soil, air, ecology (pesticides), and river water is then presented. Finally, the major challenges facing μPADs for environmental monitoring and analysis purposes are discussed, and likely directions of future research explored.

2.2. μPAD fabrication techniques Numerous approaches are available for fabricating μPADs using chemical modification or physical deposition techniques to alter the material characteristics of the cellulose matrix. In general, the treatment process changes the properties of specific areas of the cellulose paper from hydrophilic to hydrophobic; with parallel hydrophobic lines providing channels to guide the liquid flow as it permeates through the substrate under the effects of capillary action. The resulting μPADs can be broadly classified as either 2-dimensional (2D) [60–65] or 3-dimensional (3D) [66–70], depending on the direction (horizontal or horizontal / vertical) in which the fluid flows. Many methods have been proposed for fabricating μPADs, including lithography; 3D printing; wax printing; wax jetting; wax smearing; inkjet printing; screen printing; polydimethylsiloxane (PDMS) masking; laser cutting; hot embossing; printer cutting; sol-gel processing; scholar glue spraying; hydrophobic silanization; stamping; wet etching; and permanent marker pen, technical drawing pen, and eyeliner pencil writing. For a more detailed overview of the principles and comparisons of the main fabrication techniques, please refer to the review articles [71,72]. In recent years, a low-cost and efficient “origami” paperfolding method has also been proposed for the production of 3D μPADs based simply on the folding of the paper substrate [73–77]. For example, Xie et al. [78] developed a 3D μPAD, in which a paper mask coated with a thin layer of uncured PDMS mixture was folded with chromatographic paper to form a sandwich structure and was then heated to prompt the penetration of the PDMS into the chromatographic paper.

2. μPAD paper selection, fabrication and detection techniques 2.1. Paper selection Many different types of paper may be employed in μPADs depending on the fabrication method used and the particular analyte in question, including filter paper, chromatography paper, bioactive paper, glossy paper, nitrocellulose membrane paper, graphite paper, vegetal paper, and flexible paper [28–32]. Among these various types of paper, filter paper and chromatography paper (e.g., Whatman or Advantec) are among the most commonly used due to their relatively uniform thickness and superior pore size, which lead to enhanced adsorption and retention properties together with an improved wicking performance [33–38]. Furthermore, both papers undergo full bleaching following manufacturing, which removes virtually all of the impurities from the paper matrix. Glossy paper has also been proposed as a viable material for μPADs [39]. Glossy paper is composed of cellulose fibers and inorganic fillers blended into a paper matrix and has the advantage that its surface properties are more easily altered than those of traditional paper substrates. Bioactive paper has also been applied in μPADs [40,41]. In traditional paper substrates, only cationic molecules adsorb onto the wet cellulose fibers which make up the paper matrix. Furthermore, while proteins can be adsorbed on cellulose fibers, the rate and extent of this adsorption process is much lower than that of other hydrophilic surfaces. Consequently, the cellulose fibers must be modified in some way as to allow them to more readily absorb biomolecules. In bioactive paper, this is achieved by activating the surface of the paper using aldehyde or amide, for example, and then covalently conjugating biomolecules to the activated surface. Nitrocellulose membranes have also been used as μPAD substrates due to their chemical functional groups, which readily enable the covalent immobilization of biomolecules. Nitrocellulose additionally allows for charge-charge interactions, weak hydrogen bonds, and van der Waals interactions with protein-based substrates [42–44]. Due to their high protein-binding abilities, such membranes are commonly used in enzyme-linked immunosorbent assays (ELISA) and gold nanoparticle-based assays [45–50]. However, while nitrocellulose membranes can be easily patterned by wax printing, the penetration of the molten wax through the membrane thickness is slow compared to that in filter paper [51–53]. In graphite paper, the physically stripped graphene dispersion is further processed into layered graphene sheets by rapid filtration, high temperature annealing, and mechanical compression [54–59]. The resulting high-quality graphene and close contact between graphene sheets impart the graphene paper with ultrahigh conductive properties, which render the paper an ideal substrate material for electrical-based μPAD applications.

2.3. Detection techniques Many traditional techniques are available for the detection of toxic substances such as nutrients, heavy metals, microorganisms, organic contaminants and pesticides in environmental analyses. These methods include inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma-atomic/optical emission spectrometry (ICPAES/OES), energy dispersive X-ray fluorescence (EDXRF), electrochemical methods, electrothermal atomic adsorption spectrometry (ETAAS), flame atomic absorption spectrometry (FAAS), gas chromatography (GC), high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC–MS), and atomic absorption spectrophotometry (AAS) [79]. Generally speaking, these methods have high sensitivity, good specificity, and excellent precision. However, all of them require complex equipment, professional personnel, and laborious operations. Thus, there is an urgent requirement for detection methods which are simpler, more cost-effective, and portable. This need is particularly pressing in developing countries and areas of the world with a lack of infrastructure, professional experts, and appropriate environmental treatment. Most previous research on μPADs has focused mainly on the design of the microfluidic chip itself rather than the overall detection system [1]. Thus, while modern μPADs are typically very small, the associated detection and correlation devices are still rather large. Moreover, they are generally available only in well-equipped laboratories and inspection centers. Consequently, the developed μPADs cannot be used for onsite detection and still require extensive inspector training. To address these problems, recent research has attempted to realize truly portable μPAD platforms, in which not only the sampling process, but also the detection and analysis procedures, can be performed on-site. Typically, these platforms involve the use of such detection techniques as colorimetric detection, fluorescence detection, electrochemical detection, chemiluminescence (CL) detection, electrochemiluminescence (ECL) detection, nanoparticle-based detection, mass spectrometry detection, and Raman spectroscopy detection. Colorimetric detection is one of the most widely used techniques in μPAD platforms nowadays and is defined by the passive movement of 2

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(As3+, Nd3+, Br−, Mn2+, Cr3+, F−, Cu2+, Pb2+ and Zn2+), neodymium (Nd), soil-transmitted helminth (STH), and tetrabromobisphenol A [88–113].

the analyte solution to test zones by capillary action followed by a reaction process with precisely loaded reagents to prompt a quantifiable color change [80]. The colorimetric images are generally acquired by a scanner, smartphone camera, or digital/CMOS camera, and are then transferred to a PC or cell phone for subsequent analysis. The detection limit for such methods is generally of the order of a few ppm (or the same level unit). Fluorescence/luminescent detection methods [81] exploit the interaction between the target molecules and a fluorescent dye, which results in a measurable electrical signal under illumination by a light source with a specific wavelength. Fluorescence/luminescence detection methods can easily meet these requirements, so fluorescence-based sensors appear to be one of the most promising candidates for chemical sensing. Due to its unique advantages in sensitivity and selectivity, this technology is widely used for the identification of cations and anions, response time and on-site monitoring. The detection limit for such methods is usually of the order of ppb (or the same level unit). Electrochemical techniques for μPAD platforms include cyclic voltammetry, amperometry, coulometry, and potentiometry. Electrochemical methods [82] involve the use of microelectrodes, convective mass transfer, surface-modified electrodes, flow injection, signal amplification, ion-selective electrodes, and ion-exchange membranes. The detection limit for such methods is typically around a few fM (or the same level unit). Chemiluminescence methods [83], which measure the light intensity generated from a chemical reaction, have gained significant attention as a detection technique for μPADs in recent years due to their inexpensive reagents and high signal-to-noise ratio, which makes possible detection limits of a few ppm (or the same level unit). Electrochemiluminescence is a derivative of the CL process [84], in which the signal is generated by an electrochemical reaction rather than a pure chemical reaction. Compared to CL-based detection, wherein signal emission is initiated upon application of the relevant reagents, ECL-based detection has the practical advantage that temporally-controlled signal capture can be more readily achieved through user-timed potential application. Moreover, the detection limit is generally of the order of a few ppb (or the same level unit). Mass spectrometry detection, magnetic resonance sensors, Raman spectroscopy, and capacitive sensors have also begun to be applied in μPAD platforms nowadays [85,86]. Accordingly, the quantitative analysis of μPAD kits had been addressed in many fields such as diagnostic tests, environmental monitoring and food safety, etc. The idea of combining multiple methods in one μPAD is promising, because these methods can potentially complement each other and minimize draw backs of a single method. For example, Wu et al. [87] reported a threedimensional multi-layer paper-based device that combines a thin adhesive film and paper folding to enable automated and multiple ELISA for colorimetric and electrochemical quantification of troponin I. Those detections of limit is approximately a few ppb (or the same level unit). For a more detailed overview of the detection methods and the range of LOD concentrations of each method, please refer to the review article [1].

3.1.1. Explosive residue pollution [88–95] Wang et al. [89] developed an inkjet-printed silver nanoparticle (Ag NP) paper-based sensor combined with a Raman spectroscopic method for the detection of crystalline trinitrotoluene (TNT) crystals and residues in the open environment (see Fig. 1(a)). The proposed sensor was modified with p-aminobenzenethiol (PABT) in order to more efficiently collect the airborne TNT via a charge-transfer reaction and to enhance the Raman scattering effect. It was shown that the device achieved a sensitivity limit of 1.6 × 10−17 g/cm2 TNT and was capable of detecting the odors emitted from 1.4 ppm TNT in soil and 7.2, 2.9 and 5.7 ng/cm2 TNT on clothing, leather and envelopes, respectively, within 2 s. Ryan et al. [90] presented a paper-based platform for the trace analysis of TNT in soil samples using an electrochemical probe consisting of graphite electrodes, filter paper, and ethylene glycol and choline chloride as the solvent/electrolyte. The sensitivity and detection limit of the probe were shown to be 0.75 nA/ng and 100 ng, respectively. Ueland et al. [91] used a wax-printing technique to realize a capillary-driven μPAD for the lab-on-chip (LoC) screening of explosive residues in soil. The experimental results showed that the device achieved a TNT detection limit of 1.4 ∼ 5.6 ng with a recovery rate ranging from 12 ∼ 40%. Shriver-Lake et al. [94] developed a paper-based probe for the electrochemical determination of chlorate using screen-printed carbon electrodes impregnated with vanadium-containing polyoxometalate anions (see Fig. 1(b)). In the proposed device, filter paper embedded with catalyst was cut into small strips and placed on the surface of commercial screen-printed electrodes. A small quantity of buffer solution was then added to render the electrode electrochemically active. The device showed a linear cyclic voltammetry current response over the chlorate concentration range of 0.156 ∼ 1.25 mg/mL and achieved a limit of detection (LOD) of 0.083 mg/mL. Table 1 briefly summarizes several other μPADs reported in the recent literature for explosive residue in soil pollution analysis. 3.1.2. Heavy metal pollution [96–101] Xiao et al [96] developed an enhanced 3D paper-based biosensing device for Ag+ detection, which uses a personal glucose meter with high sensitivity and portability. In the proposed device, the 3D origami μPADs integrated a piece of reagent-loaded nanoporous membrane. The membrane combined analyte-triggered self-growth of silver nanoparticles to block membrane pores in situ and perform signal amplification efficiently with a handheld personal glucose meter. This developed device can detect the Ag+ concentration of real water examples, i.e., tap water, drinking water, pond water and soil water. The LOD was found to be 58.1 pM. Nam and An [98] proposed a paper-disc device that includes the growth zone of soil algae Chlorococcum infusionum, chlorophyll fluorescence and photosynthetic activity, which exposure to copper treated soil. The study showed that the paper-disc device method is an effective, user-friendly method for assessing the metal toxicity of copper (Cu) and nickel (Ni) to soil algae. Chlorophyll fluorescence and photosynthetic activity decreased with increasing concentrations of Cu+2 or Ni+2 contaminated soil. The algal growth zone was visually analyzed and showed similar results to chlorophyll fluorescence. Sutariya et al. [99] developed a paper-based sensor device based on a single-step fluorescence consisting of a pyrene group connected to calix [4]arene as a fluorescent unit (TDPC) for the analysis of Nd3+ industrial soil sample and As3+ and Br－ of industrial wastewater. In the proposed device, the paper-based sensor device comprises a luminescence sensing probe (CHEF-PET fluorescent probe) embedded in a cellulose matrix, wherein the resonance energy transfer appears to be a sensing mechanism, which is highly selective and sensitive for As3+,

3. Environmental pollution analysis and applications 3.1. Soil analysis Soil is the basic unit of the environment and is a valuable natural resource on which the entire world depends. However, large-scale industrialization and urbanization has resulted in significant soil pollution in many regions of the world. This pollution not only seriously degrades the soil fertility, but also impairs the capacity of the soil to retain nutrients and water. For the effects on these human bodies, please refer to Supplementary Material Table S1. Consequently, many μPAD platforms have been proposed for the analysis of common soil pollutants, including phosphate, chlorate, explosive residues, ammonia, copper (Cu), nickel (Ni), acid volatile sulfides (AVS), heavy metals 3

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Fig. 1. (a) Working principle of AgNP paper-based device to detect odor from TNT crystals using Raman spectra detection. (Reprinted from Ref. [89] with permission of American Chemical Society.) (b) Analysis method of paper-based electrochemical device for chlorate detection. (Reprinted from Ref. [94] with permission of MPDI (open access).) (c) Schematic illustration of paper-based spray ionization-MS device and calibration curve with isotopic internal standard for TBBPA analysis. (Reprinted from Ref. [108] with permission of Elsevier.).

Nd3+ and Br−. The results showed that the proposed system achieved detection limit of 11.53 nM, 0.65 nM and 11.25 nM for As3+, Nd3+ and Br−, respectively. The same group [100,101] also further performed a TAAC probe with a fluorescent paper-based device to enhance the analytical performance of fluorescent colorimetric detection. In soil, serum and industrial water samples, LOD can be enhanced to 0.88 nM for La3+, 0.19 nM for Cu2+, 0.15 nM for Br−, 11 nM for Mn2+, 4 nM for Cr3+ and 19 nM for F−. Table 1 briefly summarizes several other μPADs reported in the recent literature for heavy metal in soil pollution analysis.

with an isotopic internal standard method (see Fig. 1(c)). In the proposed device, soil and sediment extracts, spiked with 13C12 TBBPA (Ring-13C12), were loaded and separated on chromatographic paper and then sprayed and ionized by a high negative voltage. The paper substrate was then analyzed by mass spectrometry without purification and chromatography separation processes. The device showed a linear range of 0.1–100 μg/L, a LOD of 0.039 μg/L and a relative standard deviation of 5.3%. Naik et al. [110] preaented a loop-mediated isothermal DNA amplification (LAMP) paper-based device that combines one-step assay and thermal lysis to detect E. coli and M. smegmatis cells. The device combines thermal lysis and LAMP into a single reaction step on a paperbased device without any intermediate intervention and uses fluorescence from the DNA-binding dye PicoGreen to detect amplicons. The results showed that 100 CFU/mL of E. coli and M. smegmatis DNA amplification can be completed in 30 min. Similary, the authors in [111,112] used paper-based devices to detect E. coli in dust/soil or algae particle samples. Table 1 briefly summarizes several other μPADs reported in the recent literature for soil pollution analysis.

3.1.3. Other pollution [102–113] Wang et al. [105] presented a quantitative paper-based DNA reader (qPDR) for quantifying soil-transmitted helminth (STH) at the molecular level using “distance” as the readout. Since dsDNA depends on the concentration, the migration distance of SG-1 in the test region can be quantitatively determined by the concentration of dsDNA, so that dsDNA can be quantified by simply reading the migration distance of SG-1 in qPDR. The device exploited the unique interfacial interaction of SYBR Green I (a DNA intercalating dye) with the native cellulose matrix of chromatographic paper and performed polymerase chain reactions using a smartphone-controlled portable thermal cycler to detect minute amounts of genetic markers from adult worms expelled by STH-infected children. Liu et al. [108] developed a simple and rapid method for detecting tetrabromobisphenol A (TBBPA) in soils and sediments using a paper-based spray ionization mass spectrometry technique combined

3.2. Air analysis Air pollution has serious and wide-ranging effects on human health and the environment (See Supplementary Material Table S1). Air pollutants typically include solid particles, liquid droplets, or gases, and may be of either natural origin or manmade. Many μPADs have been 4

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Table 1 Summarizes of the μPADs developed for the determination of contamination in environmental analysis. Ref. and First author

years

Materials & structures

Fabrication methods

Detection methods

Target & (Sample matrices)

Detection limit

[91] Ueland

2016

Wax printing

Fluorescence

2015

Wax ink

Colorimetric

[95] Moram

2018

Soaking

SERS

Explosive residues (soil) Explosive residues (soil) Explosive residues

1.4 ng

[93] Peters

Filter paper, 2-D Chromatography paper, 2D Filter paper, 2-D Filter paper, 2-D Filter paper, 3-D Filter paper, 2-D Filter paper, 2-D Filter paper, 2-D Filter paper, 2-D Filter paper, 2-D Chromatography paper, 2D Chromatography paper, 2D Filter paper, 2-D Filter paper, 2-D Bare paper 2-D Photo paper 2-D Filter paper, 2-D N/R 2-D N/R

[99] Sutariya

2019

[103] Jayawardane

2014

[104] Jayawardane

2015

[106] Pellegrini

2018

[109] Basuri

2019

[113] Suaifan

2019

[115] Tang

2018

[118] Kuretake

2017

[119] Motooka

2018

[122] Alkasir

2015

[123] Colozz

2019

[127] Maity

2019

[129] Quddious

2016

[130] Petruci

2015

[131] Zhang

2015

[132] Kan

2019

[133] Sitanurak

2018

[138] Rattanarat

2014

[153] Yang

2016

[155] Nouanthavong

2016

[156] Ding

2016

[160] Wu

2017

[162] Ayazi

2018

[164] Mohammadi

2017

[166] Apilux

2015

[169] Evard

Wax printing Inkjet printing

Colorimetric (fluorescence) Colorimetric

Inkjet printing

Colorimetric

Cutting

Colorimetric

Cutting

SHPPSI MS

Wax printing

Colorimetric

N/R

Colorimetric

Cutting

Electrochemical

Screen- printing

Electrochemical

Inkjet printing

Colorimetric

Screen- printing

Electrochemical

Dip coating

Electrochemical

Inkjet printing

Electrochemical

Cutting

Fluorescence

Pencil-trace printing Spray-coated

Electrochemical

Filter paper, 2-D Filter paper, 2-D

N/R

Colorimetric

Wax printing

Colorimetric, Electrochemical

Filter 2-D Filter 2-D Filter 3-D Filter 2-D Filter 2-D

paper,

Inkjet printing

Electrochemical

paper,

Screen- printing

Colorimetric

paper,

Wax printing

Electrochemical

paper,

Inkjet printing

Colorimetric

paper,

Immersed

USA-TFME-GC-FID method

Filter paper, 2-D Filter paper, 3-D

Wax printing

Colorimetric

Punching

Colorimetric

2015

Filter paper, 2-D

Cutting

MS

[170] Lee

2018 2019

Cutting, calendering Dip-coating

SERS

[171] Zhang

Filter paper, 2-D Filter paper, 2-D

Electrochemical

3+

3+

As , Nd , Br (soil) Reactive phosphate (soil) Ammonia (sewage and soil water) Acid volatile sulfides (soil) Melamine (mike, soil) S. chartarum (soil) Methyl isothiocyanate (air) Ethanol (air) Ethanol (air) Bisphenol A (air) Sulfur mustard (air) NH3 (air) H2S gas (air) H2S gas (air) NO2 gas (air) NO2 gas (air) Hypochlorite (air) Cr, Cd and Pb (air) Trichlorfon (pesticide) Methyl- paraoxon, chlorpyrifos-oxon (pesticide) Methyl- parathion (pesticide) Paraoxon, trichlorfon (pesticide) Fenthion, chlorpyrifos, fenithrothion, phosalone, edifenphos, ethion (pesticide) －

Methiocarb (pesticide) Carbofuran, dichlorvos, carbaryl, paraoxon, pirimicarb (pesticide), Thiabendazole, aldicarb, imazalil, methomyl and methiocarb (pesticides) Thiram, ferbam (pesticide) Melamine, Thiram (pesticide)

SERS

0.39 μg 1.82 ng 11.53 nM, 0.65 nM, 11.25 nM 0.05 mg/L 0.8 mg N/L 0.1 μmoles/g 1.2 ppt (10 pM) 10 spores/mL 100 ppb 50 ppm 15 ppm 0.28 μg/g 0.019 g·min/m3 1 ppm 5 ppm 2 ppb 4.8 ppb 1 ppm 2 g Cl2/L 0.12 μg 0.25 ng 0. 89 μmol/L 8 ng/mL, 5.3 ng/mL 0.06 nM 0.01 ng/mL, 0.04 ng/mL 0.05 ng/mL, 0.05 ng/mL, 0.1 ng/mL, 0.3 ng/mL, 0.05 ng/mL, 0.3 ng/mL 5 ng/mL 0.003 ppm, 0.3 ppm, 0.5 ppm, 0.6 ppm, 0.6 ppm 5 mg/kg 0.46 nM, 0.49 nM 1 ppm

(continued on next page)

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Table 1 (continued) Ref. and First author

years

Materials & structures

Fabrication methods

Detection methods

Target & (Sample matrices)

Detection limit

[177] Jafry

2019

Filter paper, 2-D

Jet printing

Colorimetric

Methyl- paraoxon (pesticide)

10 μM

AChE: Acetylcholinesterase; MS: Mass spectrometry; N/R: No report; SERS: Surface-enhanced Raman scattering; SHPPSI MS: superhydrophobic preconcentration paper spray ionization mass spectrometry; USA-TFME-GC-FID method: ultrasound assisted thin film microextraction gas chromatography-flame ionization detection.

developed in recent years for the analysis of air pollution or air properties [114–148]. Typically, these devices are aimed at the detection of organic conpunds (volatile organic compounds (VOCs), ethylene (C2H4), bisphenol A (BPA), methyl isothiocyanate (MITC), ethanol vapor (C2H6O), sulfur mustard), inorginaic conpunds (ammonia gas (NH3), hydrogen sulfide (H2S), hypochlorite (ClO−), nitrogen dioxide (NO2),), and other substances (metal, oxygen (O2), humidity, pressure).

470 nm for the determination of hydrogen sulfide (H2S) in air (see Fig. 3(a)). The response time of the device was shown to be less than 60 s with a LOD of around 3 ppb. Kan et al. [132] developed a PbS nanowires paper-based sensor device for room-temperature NO2 gas detection. In the proposed device, PbS nanowires were spray coating onto apaper-based substrate at room temperature to enhance the sensitivity and flexibility of the gas sensor, because the PbS nanowires-on-paper sensor exhibits a porous network microstructure. The response of the paper-based sensor to 50 ppm of NO2 gas was 17.5 at room temperature, with the response and recovery time being 3 and 148 s, respectively. The device exhibited a linear response for NO2 concentrations in the range of 1–50 ppm with a detection limit of 1 ppm. Dhummakupt et al. [134] proposed a paper-based spray ionization device for the analysis of trimethyl phosphate, dimethyl methylphosphonate, and diisopropyl methylphosphonate collected on glass filter paper. In the detection process, a disposable cartridge containing a fiber filter is mounted on a specially designed 3D printing holder that effectively and reproducibly captures the aerosol onto the paper spray substrate. The device was capable of detecting aerosol concentrations as low as 1 × 10－6 mg/m3 and allowed for improved direct aerosol capture efficiency greater than 40%.

3.2.1. Organic compounds pollution [114–123] Soga et al. [114] developed a paper-based colorimetric sensor array for the detection of chemical compounds. The device was fabricated on standard copy paper using an inkjet printing technique and was patterned with dye encapsulated with polymer nanoparticles with different polarities to detect volatile primary amine (see Fig. 2(a)). It was shown that under the designed measurement conditions, the device achieved a high discrimination ability and good reproducibility for amine concentrations as low as 50 ppm. Tang and Sun [115] have developed a highly sensitive paper-based colorimetric sensor device for the detection of trace amounts of methyl isothiocyanate (MITC) in the air. The MITC paper-based sensor is based on detoxification reaction of glutathione (GSH) that binds to electrophilic toxicants during toxicantsduring under metabolic conditions and becomes unique when directly sensing the concentration of MITC. The LOD was found to be 100 ppb. Chen et al. [116] developed a paper-based sensor for the detection of volatile gases in air by means of an integrated optical and electronic sensor array patterned on the substrate surface through the direct handwriting of sensing materials (see Fig. 2(b)). The sensor was tested in ambient air for various volatile gases, including methanol, ammonia, toluene, acetone and ethanol. For each analyte, classification and detection were performed using a support-vector machine (SVM) technique. The results showed that the combined optical and electrical response provided a better discriminative power than that achieved using either sensing method alone. The authors in [117–120] used a paperbased cantilever array sensor and paper-based enzymatic biosensor, respectively, to detect volatile organic compounds (VOCs) and ethanol (breath analysis). Luo et al. [121] presented a paper-based plasma-assisted cataluminescence (PA-CTL) sensor for ethylene detection incorporating a substrate embedded with 0.320 wt% Mn-doped SiO2 nanomaterials (see Fig. 2(c)). The device showed a linear response in the range of 33∼6667 ppm and achieved a limit of detection of 10 ppm. Table 1 briefly summarizes several other μPADs reported in the recent literature for organic compounds in air pollution analysis.

3.2.3. Other substances analysis [135–148] Sun et al. [135] developed a paper-based microfluidic device for the on-site multiaxial quantification of airborne trace metals using an unmanned aerial vehicle (UAV) for sampling purposes and an integrated colorimetric detection method implemented on a smartphone (see Fig. 3(b)). The experimental results showed that the proposed system achieved limits of detection of 1.86 × 102, 45.84, 80.40, 8.16, 10.08 and1.52 × 102 ng for six metals commonly found in airborne PMs, namely Fe, Cu, Ni, Co, Mn, and Cr respectively. The same group [136] further integrated graphene oxide (GO) with a paper-based device to enhance the analytical performance of colorimetric detection. The LODs can be enhanced to 16.6, 5.1, and 9.9 ng for Fe, Cu, and Ni, respectively. The authors in [137,138] used μPAD-coated AgNPs aggregation and multilayer paper-based device for colorimetric and electrochemical quantification, respectively, to detect particulate matter (PM) pollution and six metals (Cd, Pb, Fe, Cu, Ni, and Cr) in air sample. A special paper-based device is used to detect ATP from airborne bacteria developed by Nguyen et al. [139]. The paper-based device can detect ATP extracted from purified E. coli as low as 1.17 × 103 CFU/mL. Gimenez et al. [140] developed a simple paper-based oxygen sensor consisting of a film of ZnO crystals dispersed over the paper surface. It was shown that when the device was illuminated by UV light, the current passing through the sensor underwent a small change under the effects of oxygen desorption. Moreover, the magnitude of this change was proportional to the concentration of oxygen adjacent to the sensor surface. The device was capable of detecting oxygen concentrations as low as 27.6 g/m3 even under low humidity conditions of 40% relative humidity (RH). Zhao et al. [142] presented a carbon-based humidity sensor in which the electrodes were written on the paper substrate by commercial pencils and the sensitive layer was drawn with oxidized multi-walled carbon nanotubes (o-MWCNTs) ink marker (see Fig. 3(c)). The response curve exhibited a high slope (0.41) and good linearity (R2 = 0.9976) in the 33∼95% RH range. The authors in [143,144]

3.2.2. Inorganic compounds pollution [124–134] Huang et al. [124] fabricated a paper-based sensor for the detection of ammonia gas (NH3) using silver and poly(m-aminobenzene sulphonic acid) functionalized single-walled carbon nanotubes (SWNT-PABS) deposited on photopaper using an inkjet printer. The sensor showed an excellent response, a short recovery time under NH3 concentrations at the ppm level, and good stability for at least several months. The authors in [125–127] used SWNTs and perovskite halide CH3NH3PbI3 (MAPI) deposited on paper to detect the presence of toxic NH3 gas. Petruci and Cardoso [128] developed a detection system incorporating a paper substrate infused with fluorescein mercury acetate (FMA), a light emitting diode (LED), and a spectrometer with a wavelength of 6

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Fig. 2. (a) Analysis principle of paper-based color sensor system and colorimetry results for principal component analysis. (Reprinted from Ref. [114] with permission of American Chemical Society.) (b) Schematic illustration of paper-based optoelectronic sensor (paper-nose) with colorimetric (optical) and chemiresistive (electronic) sensor array platform. (Reprinted from Ref. [116] with permission of Elsevier.) (c) Schematic illustration of paper-based PA-CTL system for ethylene detection. (Reprinted from Ref. [121] with permission of Elsevier.).

fabricated paper-based humidity sensors consisting of conductive molybdenum carbide–graphene (MCG) composites and polymers poly(3,4ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) deposited on paper substrates, respectively. Zang et al. [147] developed a paper-based sensor for ambient pressure determination incorporating a conducting polymer (CP)-coated paper substrate. The experimental results showed that the sensor achieved a LOD as low as 0.3 Pa and a sensitivity of 2 kPa−1 (P ≤ 75 Pa). Table 1 briefly summarizes several other μPADs reported in the recent literature for air pollution analysis.

animal pests. However, runoff can carry pesticides into aquatic environments, while wind can carry them to other fields, grazing areas, human settlements and undeveloped areas. Moreover, pesticides easily enter the human body through the inhalation of aerosols, dust and vapor; oral exposure by consuming contaminated food or water; and skin exposure by direct contact. As such, pesticides have wide-ranging adverse effects on both the environment and human health (See Supplementary Material Table S1). Accordingly, many μPAD platforms have been proposed for the analysis of pesticides in recent years [149–176], including insecticides (dichlorvos (DDV), trichlorfon, methiocarb, methyl parathion, carbaryl, nitenpyram, chlorpyrifos, methomyl, profenofos, chlorpyrifos, aldicarb), fungicides (thiram, thiabendazole, ferbam, carbamate, imazalil), and others (melamine,

3.3. Ecology analysis ― pesticide Pesticides are widely used throughout agriculture to kill fungal or 7

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Fig. 3. (a) Schematic illustration of paper-based sensing platform for H2S detection and sample concentration measurement using LED excitation and microfiber USB spectrometer. (Reprinted from Ref. [128] with permission of American Chemical Society.) (b) Integrated paper-based colorimetric detection and UAV for quantification of metals in airborne particulate matter. (Reprinted from Ref. [135] with permission of Elsevier.) (c) Paper-based chip based on o-MWCNTs for electrochemical detection of humidity and dynamic response of sensor. (Reprinted from Ref. [142] with permission of American Chemical Society.).

dimethyl methylphosphonate, indole-3-acetic acid (IAA), salicylic acid (SA))

tetraphenylethylene (TPE) and the addition reaction capability of maleimide for the rapid naked-eye detection of AchE activity and Ops (see Fig. 4(b)). The LODs for AchE and Ops were found to be 2.5 mU/ mL and 0.5 ng/mL, respectively. The authors in [158,159] similarly used paper-based devices to detect AchE activity and cholinesterase activity in human blood. For the CMs type, Zhang et al. [163] presented a molecularly imprinted polymers (MIPs) paper-based biomimetic enzyme-linked immunosorbent assay (BELISA) device for the detection of carbaryl. In the proposed device, the surface of the paper substrate was first modified with γ-MAPS by hydrolytic action and the MIP layer was then anchored on the modified surface by copolymerization. The device showed a linear response (R2 = 0.99) over the concentration range of 0.001∼1 mg/L. Moreover, the LODs of the half-inhibitory concentration (IC50) and IC15 were 0.116 mg/L and 0.007 mg/L, respectively. Wang et al. [165] developed a highly-sensitive SERS platform incorporating a silver dendrites-decorated superhydrophobic paper-based device and a Raman measurement system for the detection of nitenpyram pesticide (see Fig. 5(a)). It was shown that the layered fernlike silver dendrites (F-AgNDs) not only enhanced the hydrophobicity of the surface, but also improved the SERS detection performance. The LOD and enhancement factor were found to be 1 nM and 3.4 × 107, respectively. Table 1 summarizes several other μPADs reported in the recent literature for insecticide analysis.

3.3.1. Insecticide pollution [149–157] In the insecticide pollution, the literature can be divided into two categories, using the μPAD platform for detection, namely organophosphorus (Ops) and carbamates (CMs). For the Ops type, Liu et al. [149,150] developed a paper-based CL analytical device combined with paper chromatography to determine the concentration of DDV in vegetables. The device used a mixed solution of luminol and H2O2 to produce CL and exhibited a linear response for DDV concentrations in the range of 3.0 ng/mL to 1.0 μg/mL with a detection limit of 0.8 ng/ mL. Apilux et al. [152] presented a thioglycolic acid (TGA)-capped CdTe QD paper-based device based on fluorescence conversion for the detection of DDV, pirimicarb and carbaryl. The detection zone was impregnated with acetylcholinesterase (AchE) and choline oxidase (ChOx) enzymes and was coated with TGA-capped CdTe quantum dots (QDs). It was shown that in the assay process, the bi-enzyme reaction induced by the presence of insecticide caused a reduction of H2O2 and led to a quantifiable change in the fluorescence intensity of the TGAcapped CdTe QDs under black light. The LOD was found to be 0.01, 0.05 and 0.01 ppm for DDV, pirimicarb and carbaryl, respectively. Several μPAD platforms have been proposed for the detection of complex organophosphorus (Ops) compounds in environmental samples and AchE activity and Ops in human serum samples [153–162]. For example, Kim et al. [154] presented a multilayered paper-based device for the detection of chlorpyrifos pesticide (OP-type) using a colorimetric method (see Fig. 4(a)). The device comprised three paper layers for sample injection, reagent storage, and reaction / color development, respectively. In the detection process, the sample solution was dripped onto the paper-based device and subsequently underwent mixing with two deposited reagents (AchE and indoxyl acetate (IDA)) to produce a blue image in the reaction zone. The LOD was found to be 8.60 ppm. Chang et al. [157] proposed a paper-based fluorescent sensor based on the aggregation-induced emission effect of

3.3.2. Fungicide pollution [168–173] Fungicides are biocidal compounds or biological organisms used to kill parasitic fungi or spores thereof. Fungicides can be used in both agricultural and fungal infections against animals. Ma et al. [168] proposed an on-site detection system for thiram, thiabendazole and methyl parathion pesticides comprising a surface-enhanced Raman scattering (SERS) paper substrate patterned with graphene oxide (GO) and Ag NPs. The device showed an excellent SERS activity and was capable of extracting thiram, thiabendazole and methyl parathion residues in fruits and vegetables with LODs of 0.26 ng/cm2, 28 ng/cm2 8

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Fig. 4. (a) Schematic illustration of colorimetric paper-based sensor device and procedure for pesticide analysis using competitive-inhibiting reaction. (Reprinted from Ref. [154] with permission of Springer.) (b) Analysis principle and observation results for fluorescent paper-based assay for AChE activity in presence of diazinon with different concentrations. (Reprinted from Ref. [157] with permission of Elsevier.).

and 7.4 ng/cm2, respectively. Lee et al. [170] also developed a paperbased SERS device that can detect pesticides of the fungicide type by hydrophobically modifying the filter paper with high sensitivity and repeatability. The filter paper is subjected to a hydrophobic treatment of the alkyl ketene dimer to prevent the paper from absorbing AgNP and the sample solution. Unlike conventional filter papers, hydrophobically modified filter paper produces more SERS hot spots composed of AgNP clusters on the paper surface for more sensitive detection of pesticides. Fungicides such as thiram and ferbam can be detected at the nanomolar level using the proposed device and found to have an LOD of 0.46 nM and 0.49 nM, respectively. Identically, Deng et al. [172] also presented a paper-based SERS device for detection of malachite green residues in fish. The surface-enhanced Raman scattering signal intensity has a good linear relationship with the malachite green concentration between 1 × 10−7 and 1 × 10-5 mol/L, and LOD was be founded to 5 × 10-10 mol / L. Table 1 summarizes several other μPADs reported in the recent literature for fungicide analysis.

butyrylcholinesterase (BchE) enzyme activity towards butyrylthiocholine with and without exposure to nerve agent, respectively, and enabled an entirely reagent-free analysis. The sensitivity of the device was enhanced through the use of carbon black/Prussian blue nanocomposites as working electrodes. In a series of tests performed using paraoxon as a nerve agent simulant, the device exhibited a linear detection performance down to 3 μg/L. Lee et al. [176] developed a foldable paper-based device for detecting dimethyl methylphosphonate (DMMP) in a semi-quantitative manner using a colorimetric technique (see Fig. 5(b)). In the proposed device, the color of the paper channel changed to yellow in the presence of DMMP as a result of the hydrogen peroxide produced from the enzymatic reactions and the DMMP concentration was then quantified using an angle-based readout. A linear angle response was observed for DMMP concentrations in the range of 1 ∼ 4 M. Table 1 summarizes several other μPADs reported in the recent literature for pesticide analysis. 3.4. River water analysis

3.3.3. Other pollution [174–177] Cinti et al. [174] developed an integrated paper-based screenprinted electrochemical biosensor for the detection of nerve agents. The device was based on the double electrochemical measurement of

Water pollution is a major global concern nowadays and is observed in many water bodies, including rivers, oceans, lakes and aquifers. Water contamination is usually the result of human activities, and 9

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Fig. 5. (a) Photograph of fabrication process for flexible SERS substrate based on silver dendrites-decorated filter paper for nitenpyram detection and SERS spectra result. (Reprinted from Ref. [165] with permission of Elsevier.) (b) Schematic illustration of fabrication process for paper-based SERS swabs and detection principle for pesticide residues. (Reprinted from Ref. [176] with permission of Elsevier.).

varied in direct proportion to the concentration of Cu2+ ions in the sample. The experimental results showed that the device achieved a LOD of 0.96 ppm (15 μM). Liu et al. [195] developed a paper-based analytical device for the image-based colorimetric detection of Cu2+, in which an on-line reaction between polyethyleneimine (PEI) and Cu2+ was induced within a T-type microfluidic channel under the application of an external electric driving field. The device achieved a linear response (R2 = 0.991) over a Cu2+ concentration range of 0.1∼1.0 mM with a LOD equal to 30 μM. Chen et al. [197] presented a colorimetric paper-based analytical platform based on AuNPs and a smartphone for water source Hg2+ detection (see Fig. 7(a)). The presence of Hg2+ ions induced the aggregation of the AuNPs via thymine–Hg2+–thymine (T–Hg2+–T) coordination chemistry. The resulting color change of the reaction zone was imaged and analyzed by the smartphone. The experimental results obtained for spiked pond and river water samples showed that the device was capable of performing Hg2+ detection down to 50 nM. Faham et al. [202] used AuNPs modified with 2,2′-thiodiacetic acid (TDA) to determine the concentrations of Cr3+ and Cr6+ ions using a paper-based device coupled with UV–vis spectrophotometry, dynamic light scattering, energy dispersive spectroscopy and transmission electron microscopy (see Fig. 7(b)). The device was used to determine the concentration of chromium ions in spiked water, urine and human plasma, and was found to have a good linear correlation (R2 = 0.98) over the concentration range of 1.0 nM to 0.1 mM and a LOD equal to 0.64 nM (0.033 ppb). Asano and Shiraishi [205] proposed a μPAD for the colorimetric detection of iron in water samples using phenanthroline indicator. Briefly, a hydrophobic surface was produced via immersion in octadecyltrichlorodecane (OTS)-n-hexane and the surface was then exposed to UV light through a photomask to produce a hydrophilic region. The visible color change induced in the reaction zone following the introduction of the sample solution was captured by a high-resolution camera and used to determine the corresponding iron concentration. The LOD and limit of quantitation (LOQ) were found to be 3.96 and 13.8 mM, respectively. Li et al. [208] developed a colorimetric paperbased membrane sensor for the determination of Ni2+ ions using zincon organometallic reagent combined with hollow ZnSiO3 nanospheres as a mixed ionophore (see Fig. 7(c)). In the presence of Ni2+, the

includes a wide range of chemicals and pathogens. Water pollution is the leading cause of death and disease worldwide (See Supplementary Material Table S1), and hence many μPAD platforms for detecting water contaminants have been proposed. These devices are generally intended for the analysis of river water [178–232] and typically target analytes such as heavy metals (mercury (Hg2+), lead (Pb2+), cobalt (Co2+), zinc (Zn2+), chromium (Cr6+, Cr3+), iron (Fe3+), copper (Cu2+), nickel (Ni2+), cadmium (Cd2+), silver (Ag+), manganum (Mn2+), calcium (Ca2+), magnesium (Mg2+)), non-metal ions (nitrite, arsenic (As), chloride (Cl−), bromide (Br−), iodide (I−), sulfide (S2−), and organic contaminants and microorganisms (pesticides, cyanide, ethinylestradiol, phosphate, dipicolinic acid, hydrogel, trinitrophenol, bacterial). 3.4.1. Heavy metal ion pollution [183–212] Vijitvarasan et al. [183] developed a paper-based scanometric colorimetric sensing assay that coupled enzyme probes (PE), magnetic beads (MB), substrate probes (PS) and gold nanoparticles (AuNPs) to perform the detection of Pb2+. As shown in Fig. 6(a), the AuNPs-PS were spotted on a paper substrate and the signal induced in the presence of a magnetic field was then amplified by a silver nitrate enhancer solution. The device exhibited a linear response over the range of 0.1∼1000 nM and a LOD of 0.3 nM. The literature contains several other DNAzyme-based methods for detecting metal ions using colorimetric or ELC techniques [184,185], label-free fluorescence assays [186,187], or aptamer-based assays [188], Zhang et al. [190] presented a label-free oligonucleotide sequences-AuNPs paper device for the colorimetric sensing of Ag+. Label-free oligonucleotide sequences (S1) were attached to unmodified AuNPs by cytosine–Ag+–cytosine (C–Ag+–C) coordination chemistry; giving rise to an OR function. A logic gate was then constructed using S1 and Ag+ as inputs to the S2attached AuNP mixture. The detection limit of the proposed device was shown to be as low as 10 nM. Sadollahkhani et al. [192] proposed a ZnO@ZnS core-shell nanoparticle paper-based sensor for the detection of Cu2+ ions in aqueous solutions (see Fig. 6(b)). The difference in solubility between ZnS and CuS resulted in a cation exchange between the ZnS nanoparticle surface and the Cu2+ ions. The cation exchange prompted a color transformation of the test paper from colorless to yellow, where the intensity 10

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Fig. 6. (a) Schematic illustration of detection procedure for lead ions using paper-based scanometric assay device. (Reprinted from Ref. [183] with permission of Elsevier.) (b) Functional mechanism and colorimetric detection results for Cu2+ analysis using PAD sensor platform. (Reprinted from Ref. [192] with permission of American Chemical Society.).

competition with the Zn2+ ions resulted in a quantifiable fading of the blue color in the reaction zone. It was shown that the selectivity of the proposed device could be enhanced by using masking agent Na2-EDTA as a co-ionophore. The device exhibited a linear response over the range of 32 nM to 9.2 mM Ni2+ and achieved a LOD of 35.6 nM. Table 2 summarizes several other μPADs reported in the recent literature for heavy metal ion pollution analysis.

mL, respectively. Devi et al. [216] presented a detailed overview of the progress made in sensor materials for the optical detection of arsenic in water samples between 2013 and 2018. Cuartero et al. [217] developed an electrochemical PAD for halide ion detection in food supplements and water samples via cyclic voltammetry. As shown in Fig. 8(a), the device consisted of a cellulose paper substrate, a cation exchange Donnan exclusion membrane, and a silver-foil working electrode. Cyclic voltammetry was used to perform the sequential oxidation and plating of the halide at the silver electrode, and was followed by regeneration with an inverted potential. The feasibility of the proposed device was demonstrated by detecting Cl−, Br− and I− ions in concentrations ranging from 10−4.5 M to 0.6 M for Cl− and 10−4.8 M to 0.1 M for Br− and I−. It was shown that the device achieved a LOD of 10−5 M. Vasimalai et al. [219] developed a paperbased testing kit for the detection of S2– ions in water samples based on the quenching effect of S2– on 6-mercapto-s-triazolo(4,3-b)-s-tetrazingold nanodots (MT-AuNDs) (see Fig. 8(b)). The device showed a linear response over the range of 870 nM to 16 μM S2– and achieved a LOD of 2 nM. Table 2 summarizes some of the other μPADs reported in the recent literature for non-metal ion pollution analysis.

3.4.2. Other ion pollutants [213–219] Liu et al. [213] developed an Ag-based μPAD chemiresistor for the electrochemical detection of nitrite ions in river water. In the proposed device, the introduction of nitrite prompted a Griess reaction, and the resulting azo and sulphonamide products reacted with the Ag metal and Ag+ ions to cause a change in resistance of the response region. The sensor was shown to provide a linear response over two nitride concentration ranges of 10 nM to 5.0 μM and 10 μM to 3.2 mM, respectively, Moreover, the LOD was found to be 8.5 × 10−2 nM. Cardoso et al. [214] developed a μPAD platform combined with colorimetric detection and a modified Griess reaction for the determination of nitrite in clinical, food and river water samples. The reaction images were captured by a scanner, converted to a color scale, and then analyzed in the magenta channel. The device achieved a LOD of 5.6 μM and a sensitivity of 0.56 AU/mM. Pena-Pereira et al. [215] proposed an AgNO3-based paper device for arsenic (As) detection involving in situ arsine generation followed by transfer of volatiles to the headspace and a subsequent reaction with silver nitrate in the detection zone. Under optimal conditions, the LOD and LOQ were found to be 1.1 and 3.6 ng/

3.4.3. Organic contaminants and microorganism pollution [220–232] Arduini et al. [221] proposed a 3D origami paper-based device combining different enzyme-inhibition biosensors and office paper screen-printed electrodes (SPE) for the electrochemical detection of several types of pesticide in river water (see Fig. 9(a)). In the proposed device, the presence of pesticide was detected by monitoring the 11

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Fig. 7. (a) Photographs, functional mechanism and detection results obtained by μPAD treated with AuNPs for on-site Hg2+ assay. (Reprinted from Ref. [197] with permission of American Chemical Society.) (b) Mechanism and colorimetric detection results obtained by PAD sensor platform based on TDA-AuNPs for Cr3+ assay. (Reprinted from Ref. [202] with permission of Springer.) (c) Schematic illustration of Ni2+ ion analysis using colorimetric paper-based membrane sensor based on indicator displacement mechanism and corresponding colorimetric sensing results. (Reprinted from Ref. [208] with permission of Elsevier.).

(R2 = 0.917) for the river water samples. The LOD of the biosensor was found to be 2 ppb. The same group [222] also used a paper-based screen-printed electrochemical sensor to detect phosphate. Saraji and Bagheri [224] developed a paper-based headspace extraction device coupled with digital image processing for the detection of cyanide in mineral, tap, well, river, sea and wastewater samples (see Fig. 9(b)). In

enzymatic activity degree chrono-amperometrically in the absence and presence of pesticide, respectively, using a portable potentiostat. The device was tested using paraoxon, 2,4-dichlorophenoxyacetic acid and atrazine at ppb concentration levels in both standard solutions and river water samples. Two linear response ranges were observed, namely 2∼100 ppb (R2 = 0.907) for the control solutions and 10∼100 ppb 12

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Table 2 Summarizes of the μPADs developed for the determination of contamination in river water analysis and maximum contaminant level in drinking water. Ref. and First author

years

[184] Xu [185] Sun

2018 2018

[186] Li

2018

[187] Lin

2016

[188] Fakhri

2018

[189] Almeida

2012

[191] Dhavamani

2018

[193] Liu

2012

[194] Cui

2019

Materials & structures

Fabrication methods

Detection methods

Target 2+

Detection limit

MCL(mg/L)EPA

Chromatography paper, 3-D Art-paper, 2-D Filter paper, 2-D Filter paper, 2-D Filter paper, 2-D Chromatography paper, 2-D

Wax printing Microcontact printing

ECL Colorimetric

Pb Pb2+

0.5 nM 10 nM

0.015 0.015

Wax printing

Fluorescence

Pb2+

10 nM

0.015

N/R

ICP-MS

CO2 laser cutting

Colorimetric

Pb2+, Cd2+ Pb2+

0.69 mg/L, 0.51 mg/L 0.7 nM

0.015 0.005 0.015

N/R

SR-TXRF

paper,

Hand drawn

Colorimetric

6.8 μg/L, 4.4 μg/L, 3.9 μg/L, 3.8 μg/L, 3.9 μg/L, 9.1 μg/L, 10 μM

0.05 1.0 5.0 0.015

Filter 2-D Filter 2-D Filter 2-D Filter 2-D

Mn, Co, Ni, Cu, Zn, Pb Ag+

0.10

paper,

Cutting and soaking

Fluorescence

Cu2+

5 μM

1.0

paper,

Inkjet printing

Colorimetric

Cu2+

0.08 μM

1.0

Wax printing

Colorimetric

2+

Cutting and immersing

Fluorescence

4.8 mg/L, 1.6 mg/L 0.18 mg/L 0.14 nM

1.0 0.1

Filter paper, 2-D Filter paper, 2-D

Ni , Cu2+, Cr2+ Hg2+

Laser printer

Fluorescence

Cutting

Fluorescence

Hg2+, Pb2+, Cd2+, Fe3+, Cu2+ Cr6+

10 μM, 20 μM, 20 μM, 10 μM, 20 μM 0.24 μM

0.002 0.015 0.005 0.3 1.0 0.1

Wax printing Photo-lithography Wax and screen printing

CL Colorimetric Colorimetric

Cr3+ Cr6+ Ni2+, Cr6+, Hg2+ Cu, Fe, Zn

0.02 ppm 30 ppm 0.24 ppm, 0.18 ppm, 0.19 ppm 0.1 ppm

0.1 0.1 0.1 0.002

0.2 mM, 0.4mM 0.5 mM

0.3

[196] Sun

2018

paper,

[198] Anh

2017

[199] Abbasi-Moayed

2018

[200] Bu

2016

[201] Alahmad [203] Asano [204] Devad-hasan

2016 2018 2018

Filter paper, 2-D Chromatography paper, 2-D Chromatography paper, 2-D Chromatography paper, 2-D

[206] Hofstetter

2018

Chromatography paper, 2-D

Wax printing

Colorimetric

[207] Xu

2018

paper,

Laser printer and coating

Colorimetric

[209] Karita

2016

paper,

Wax printing

Colorimetric

[210] Ostad

2017

paper,

Hand drawn

Colorimetric

[212] Liu

2018

paper,

Immersed

Colorimetric

Fe3+, Ni2+ Ca2+, Mg2+ Ca, Mg Co2+

[218] Yakoh

2018

paper,

Wax printing

Colorimetric

Cl－

1.3 mg/L

[223] Petruci

2018

paper,

Wax printing

Colorimetric

HCN

10 ppb

[229] Rengaraj

2018

Filter 2-D Filter 2-D Filter 2-D Filter 2-D Filter 2-D Filter 2-D Filter 2-D

paper,

Screen printing

Electrochemical

Bacte- rial

1.9 × 103 CFU/mL

0.002

1.0 0.3 5

8.3 mg/L, 1.0 mg/L 10 mg/L 4.0

0

ECL: Electrochemiluminescence; ICP-MS: Inductively coupled plasma mass spectrometry; SR-TXRF: Synchrotron radiation-excited total reflection X-ray fluorescence; CL: Chemiluminescence; MCL: Maximum Contaminant Level in drinking water (National Primary Drinking Water Regulations, EPA).

the detection process, the cyanide was converted to cyanogen chloride via a chloramine-T/pyridine-barbituric acid reaction, and the cyanide concentration was then determined via an inspection of the resulting color change in the reaction region. A linear response was observed for cyanide concentrations in the range of 3.0 ∼ 100 μg/L. Moreover, the LOD was equal to 0.7 μg/L and the analyte recovery rate was 69 ∼ 111%. Scala-Benuzzi et al. [226,227] proposed an electrochemical paperbased immunocapture assay (EPIA) method for determining the concentration of ethinyl estradiol (EE2) in river water (see Fig. 9(c)). The EPIA method used paper microzones modified with silica nanoparticles

(SNs) and anti-EE2 specific antibodies to capture and pre-concentrate the EE2. The EE2 was then electrochemically detected by Osteryoung square wave voltammetry (OSWV), wherein the oxidation current was proportional to the EE2 concentration. The device response was found to vary linearly over the concentration range of 0.5 ∼ 120 ng/L with a LOD of 0.1 ng/L. Alcaine et al. [228] developed a phage amplification paper-based fluidic device combined with a colorimetric detection method for bacteria detection in river water samples. The device utilized bioengineered T7 phage strains to enhance the sensitivity of phage amplification-based lateral flow assays (LFA) by over 100-fold. The enhanced sensitivity enabled a LOD of 103 cfu/ml for E. coli after 7 h. 13

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Fig. 8. (a) (I) Schematic illustration of electrochemical cell with closing system on μPAD and corresponding voltammetric readout. (Reprinted from Ref. [217] with permission of American Chemical Society.) (b) Mechanism and fluorescence detection results for S2– using MTT-AuND-modified PAD sensor platform. (Reprinted from Ref. [219] with permission of American Chemical Society.).

look set to provide an invaluable contribution toward protecting and improving human and environmental health throughout the world for years to come. This article has provided a comprehensive review of the latest developments in μPAD technology in the environmental analysis and monitoring field. The review has focused particularly on the use of μPADs for the analysis of soil, air, pesticides, and river water. In general, the surveyed literature has confirmed that μPADs provide a cheap and practical alternative to the large-scale equipment used in traditional environmental monitoring applications. Many of the concepts and techniques adopted in the literature show significant potential for further development in the future. For example, μPADs combined with UAVs provide a promising approach for the detection of PM2.5 in the environment [128,129], and future monitoring of environmental sources is quite helpful. In addition, μPADs incorporating coated nanoparticles and graphene oxide (GO) also appear to have significant potential for the high-sensitivity, high-selectivity detection of pesticides, heavy metal ions, bacterial cells, allergens and toxins [129,157,233–242]. As a result, nanoparticles and GO seem likely to play a key role in enabling the development of sophisticated μPAD platforms in the future. Although μPAD technology has advanced rapidly in recent years, several key challenges still remain to be overcome. For example, the World Health Organization (WHO) and United States Environmental Protection Agency (US-EPA) have set permissible limits for Hg2+ in water (tap or drinking) of 0.001 and 0.002 ppm, respectively. However, existing colorimetric-based μPAD methods [168,175,243–246] achieve

Rong et al. [230] presented a ratiometric fluorescence paper-based device for anthrax biomarker detection based on ethylenediamine tetraacetic acid (EDTA) and Eu(III) ion functionalized manganese-doped carbon dots (FMn-CDs) (see Fig. 9(d)). It was shown that 2, 6-dipicolinic acid (DPA), an important biomarker of Bacillus anthracisspore, sensitized the Eu(III) combined on the FMn-CDs to produce bright red fluorescence under UV irradiation. The device exhibited a linear response over the range of 0.1–750 nM DPA and achieved a LOD of 0.1. Tawfik et al. [232] developed a paper-based sensor with a thiophene copolymer quantum dot (TCPQD)-doped chitosan film for determining the concentration of trace quantities of 2,4,6-trinitrophenol (TNP) explosive in tap water and river water samples using a fluorescence quenching technique. The LOD was found to be 2.29 pg TNP, while the recovery was 98.02∼107.50%. Table 2 briefly summarizes several other μPADs reported in the recent literature for the detection of organic contaminants and other microorganism pollutants. 4. Conclusions and outlook μPADs have attracted immense interest in recent decades for a wide variety of monitoring and analysis applications, including disease diagnosis, food safety, environmental monitoring, and others. μPADs are inexpensive, portable, highly sensitive, and straightforward to use. Consequently, they provide an ideal analysis tool for resource-constrained environments lacking highly-trained technicians and wellequipped laboratories. Future advances in μPAD technology, combined with a more extensive distribution and availability of μPAD devices, 14

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Fig. 9. (a) Schematic illustration of origami paper-based electrochemical biosensor for pesticide detection. (Reprinted from Ref. [221] with permission of Elsevier.) (b) Procedure, mechanism and analysis results for cyanide detection using colorimetric paper-based headspace extraction system. (Reprinted from Ref. [224] with permission of Elsevier.) (c) Schematic illustration of detection procedure for EE2 using electrochemical paper-based immunocapture assay device. (Reprinted from Ref. [226] with permission of American Chemical Society.) (d) Schematic illustration of colorimetric FMn-CD paper-based device for DPA detection using ratiometric fluorescence method. (Reprinted from Ref. [230] with permission of Elsevier.).

LODs of Hg2+ only of 0.002–10 ppm. In other words, the sensitivity of these methods is lower than the prescribed regulatory limit for drinking water. Nonetheless, paper-based fluorescent devices have many advantages, including high sensitivity, fluorescent probes providing a wide range of wavelengths, qualitative/quantitative analysis, dynamic wide linear range, and good repeatability. For example, QD fluorescence quenching, with a LOD limit in the range of 0.01 ∼ 10 μg/L (ppb) [81,153,199,247], appears to provide a possible solution for overcoming this problem, and thus merits further attention in future studies. Another problem to be addressed in the future is that of the detection system integrated with the μPAD. At present, many μPAD platforms use mass spectrometry to quantify the target analyte [127,248–250]. However, while mass spectrometry has the advantages of simplicity and reliability, the detection system is too large and expensive for in-situ monitoring and analysis. Therefore, the development of more portable and inexpensive detection systems also represents an important avenue of future μPAD research. At present, there are already μPAD related products for environmental pollution analysis in the world. For example, in the water quality testing, the home water test kit (Lamotte Co. USA; Bioeasy Biotechnology Co. China, et al.) has been introduced. Most of these rapid testing products are qualitative tests that quickly detect malachite green, Al, Cr, Fe, Cu and Cd in water. In the rapid detection of pesticides, there are also related μPAD products, such as rapid pesticide residue detection kits have also been developed (Neogen Co. USA; MRight Biomedical Co. Taiwan; Apex Biotechnology Co. Taiwan, etc.), most of which are used for insecticide analysis. Such insecticid, only Ops and CMs types, are also qualitative tests. In the rapid detection of

air and soil pollution, there are no commercially available μPAD products. Therefore, the application of μPAD in the rapid detection of environmental pollution is still worth developing, such as the development of various contaminated μPAD and the quantitative detection of μPAD platform. In conclusion, this review has distilled out the key concepts from the huge body of information available in the literature relating to μPAD platforms for environmental analysis and monitoring applications. μPAD technology has advanced dramatically over the past ten years or so and, provided that the challenges described above are overcome, appears to represent the analytical tool of choice for reliable and inexpensive environmental analysis into the foreseeable future. Acknowledgement This study was supported by the Ministry of Science and Technology of Taiwan (107-2622-B-006-007-CC2, 106-2221-E-006-253-MY3, and 106-2314-B-006-085-MY3). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126855. References [1] L.M. Fu, Y.N. Wang, Detection methods and applications of microfluidic paperbased analytical devices, Trends Anal. Chem. 107 (2018) 196–211.

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nanoparticles in a paper based analytical device, RSC Adv. 6 (2016) 69060–69066. [245] W.W. Chen, X.E. Fang, H. Li, H.M. Cao, J.L. Kong, A simple paper-based colorimetric device for rapid mercury(II) assay, Sci. Rep. 6 (2016) 7. [246] S.K. Patil, D. Das, A nanomolar detection of mercury (II) ion by a chemodosimetric rhodamine-based sensor in an aqueous medium: potential applications in real water samples and as paper strips Spectrochim, Acta A Mol. Biomol. Spectrosc. 210 (2019) 44–51. [247] M. Zhao, H. Li, W. Liu, W. Chu, Y. Chen, Paper-based laser induced fluorescence immunodevice combining with CdTe embedded silica nanoparticles signal enhancement strategy, Sens. Actuators B Chem. 242 (2017) 87–94. [248] Y. Yang, J. Wu, J. Deng, K. Yuan, X. Chen, N. Liu, T. Luan, Rapid and on-site analysis of amphetamine-type illicit drugs in whole blood and raw urine by slugflow microextraction coupled with paper spray mass spectrometry, Anal. Chim. Acta 1032 (2018) 75–82. [249] M. Yu, R. Wen, L. Jiang, S. Huang, Z. Fang, B. Chen, L. Wang, Rapid analysis of benzoic acid and vitamin C in beverages by paper spray mass spectrometry, Food Chem. 268 (2018) 411–415. [250] D. Kim, J. Lee, B. Kim, Optimization and application of paper-based spray ionization mass spectrometry for analysis of natural organic matter, Anal. Chem. 90 (2018) 12027–12034. Chia-Te Kung received the B.S. and M.S. degrees from the Department of Medicine and Public Health Institute, Kaohsiung Medical University, Kaohsiung, Taiwan, in 1991 and 2011, respectively. He has been with the Emergency Department, Kaohsiung Chang Gung Memorial Hospital, since 1991. He is currently an Associate Professor with the Department of Medicine, Chang Gung University. His specialties include emergency medicine, critical care medicine, microfluidics paper-based devices and their biomedical applications, and disaster medicine. Chih-Yao Hou received M.S. and Ph.D. degrees from the Department of Food Science from National Pingtung University of Science and Technology, Taiwan, in 2004 and 2008, respectively. He is currently an Assistant Professor in the Department of Seafood Science at National Kaohsiung University of Science and Technology. His current research involves food engineering, food protein chemistry, food inspection and analysis, and microfluidic paper-based devices and applications. Yao-Nan Wang received M.S. and Ph.D. degrees from the Department of Mechanical Engineering from National Cheng Kung University (NCKU), Taiwan, in 2003 and 2008, respectively. He is currently an Associate Professor in the Department of Vehicle Engineering at National Pingtung University of Science and Technology. His current research involves thermo-fluid engineering and integration of microdevices. Lung-Ming Fu received M.S. and Ph.D. degrees in Engineering Science from National Cheng Kung University (NCKU), Taiwan, in 1997 and 2001. He had his postdoc training in Department of Engineering Science at NCKU during 2002-2003. He is currently a Distinguished Professor of Engineering Science Department at National Cheng Kung University, Tainan, Taiwan. His research interests are in microfluidic systems, microfluidic paper-based devices and applications, MEMS fabrication technologies, microsensor and computational fluid dynamics.

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