Self-Diffusion Driven Ultrafast Detection of ppm-Level Nitroaromatic

Subscriber access provided by NEW YORK UNIV

Article

Self-diffusion Driven Ultrafast Detection of ppm-Level Nitro Aromatic Pollutants in Aqueous Media using a Hydrophilic Fluorescent Paper Sensor Wei Lu, Jiawei Zhang, Youju Huang, Patrick Theato, Qing Huang, and Tao Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08826 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Self-diffusion Driven Ultrafast Detection of ppm-Level Nitro Aromatic Pollutants in Aqueous Media using a Hydrophilic Fluorescent Paper Sensor Wei Lu, † Jiawei Zhang,* † Youju Huang, † Patrick Théato, ‡ Qing Huang† and Tao Chen*†,§ †

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine

Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China ‡

Institute for Technical and Macromolecular Chemistry, University of Hamburg, Bundesstrasse 45, D-

20146 Hamburg, Germany University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China

§

KEYWORDS: aqueous phase, fluorescence quenching, nitro aromatic pollutants, paper sensor, selfdiffusion, ultrafast detection

ABSTRACT: Development of fluorescent film probes for toxic nitro aromatic compound pollutants (NACs) such as 2,4,6-trinitrotoluene (TNT) in real water samples implies broad applications in environmental and industrial safety control. Despite many recent advances, there are still some difficult challenges facing this area, e.g., the restricted sensitivity and long response time caused by hindered and slow diffusion of aqueous NACs samples inside a dense, solid film. Hence, we report herein a robust fluorescent paper sensor with improved sensing abilities, which is prepared by absorbing hydrophilic pyrene-functionalized polymer uniformly into cellulose-based filter papers. Thanks to the numerous oxygen-containing groups grafted on cellulose papers, they allow passive and ultrafast capillary force driving diffusion of aqueous NACs

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 22

samples into their hydrophilic matrix. Specifically speaking, these paper sensors can offer efficient selfdiffusion paths inside the test stripes and immediately bring the pyrene fluorophores and NACs quenchers into close proximity. Therefore, the developed paper-based test stripes enable both naked-eye detection of low-ppm-level TNT and ultrafast fluorescence quenching with a response time of only a few seconds, which are difficult to be achieved by conventional film probes.

INTRODUCTION As a typical kind of highly toxic environmental pollutants, nitro aromatic compounds (NACs) such as 2,4,6trinitrotoluene (TNT) are nowadays heavily consumed for both military and civil applications. Despite being hydrophobic, TNT is to a certain extent water soluble and has been found in some industrial waste water samples in concentrations up to 80 ppm.1 Therefore, TNT detection in such “real-world” aqueous samples is of significant interest. Nowadays, an assortment of HPLC/MS-based instrumental analyses have been developed to measure NACs in aqueous phase.2~3 However, these methods, despite state-of-the-art, usually depend on specialized analytical devices and may suffer from complicated operation, long measurement time or high cost, which render them not suitable for real-time and field assays. Alternatively, fluorescence-based approaches are becoming increasingly attractive for NACs sensing because they have advantages in terms of high sensitivity, fast response, facile and low-cost operation. Over the past decade, great efforts have been devoted to the development of numerous powerful fluorescent probes based on various nanomaterials, molecular fluorophores, conjugated polymers and so on.4~12 However, many of these well-developed probes primarily work in solution phase. From the viewpoint of practical infield applications, solid-state luminescent sensory materials seem more appealing as they can provide portability, good stability and operational simplicity.13~17 Hence, great recent efforts have been devoted to fabricating fluorometric film probes for NACs.18~44 For example, Fang et al. prepared a powerful pyrene-containing conjugated polymer-based fluorescent films casted on glass plates and realized highly selective and sensitive sensing of TNT in aqueous medium.45 Tang and co-workers took advantage of 2 ACS Paragon Plus Environment

Page 3 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


aggregation-induced emission mechanism to successfully construct excellent tetraphenylethene-modified poly(acrylate)-based film probes on quartz slides for effective NACs detection.46 More recently, Lei and co-workers reported the development of electrospun pyrene-polyethersulfone nanofibrous films that allows fast and visual detection of TNT in aqueous medium.47 These impressive advances have moved solid-state fluorescent film probes much closer to practical NACs sensing in real water samples. Meanwhile, there has also been a growing interest in exploring paper-based fluorescent test stripes for analytes in aqueous medium due to the unique properties of cellulose papers.48~50 For instance, since billions of tons of cellulose are produced annually by photosynthesis, paper is generally low-cost, renewable and biodegradable. More importantly, thanks to the numerous grafted oxygen-containing groups, papers allow ultrafast and spontaneous capillary force driving diffusion of aqueous samples into its hydrophilic matrix. This property is of particular importance as it can address some widely-known difficult challenges facing solid-state sensory materials,51 that is, restricted sensitivity and long equilibrium response time caused by hindered diffusion of liquid analytes inside the dense solid films. It can be envisaged that the ultrafast selfdiffusion of sensory solutions inside paper sensors will significantly and quickly increase the binding sites for guest species. Therefore, cellulose-based papers can be an excellent functional scaffold for fast-response and sensitive test stripes. However, such portable and cost-effective fluorometric assays for aqueous samples polluted by NACs remain underdeveloped. Herein, we report a robust fluorescent paper sensor, which enables ultrafast and visual detection of ppmlevel TNT in aqueous phase. The probe is fabricated by bonding a specially designed hydrophilic but waterinsoluble fluorescent polymer (poly(2-hydroxyethyl methacrylate-co-pyrene-butyric acid hema, poly(HEMA-co-PyMA)) to the commercially available filter papers. Its design takes advantage of the widely-known electron deficient NACs-induced quenching of pyrene excimer emission of poly(HEMAco-PyMA), which is ascribed to the formation of charge-transfer complexes between the electron acceptor (nitro aromatic quenchers such as TNT) and the electron donor (pyrene fluorophores).52-54 As schemed in Figure 1, the desired fluorescent sensors can be facilely prepared by immersing the pristine filter papers

3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 22

into methanolic poly(HEMA-co-PyMA) solutions and subsequently drying in air, because the polymer solutions can spontaneously absorb uniformly onto filter papers to produce fluorescent papers. Upon exposure to aqueous NACs samples, their ultrafast capillary force driving diffusion into the hydrophilic test stripe matrix brings the quencher (NACs) and pyrene fluorophores into close spatial affinity. As a result, the highly cyan light-emitting test stripes immediately become dark, generating a fluorescence signal proportional to the concentration of NACs present in water samples with a response time of a few seconds.

Figure 1. (a-b) Paper-based fluorescent test stripes were prepared by first immersing the pristine rectangle-shape cellulose filter paper into the methanol solutions of poly(HEMA-co-PyMA) and then drying it at ambient conditions; (c-d) To record the fluorescence response of the test stripes to NACs, 5 μL NACs-polluted aqueous solution was transferred by the micropipette and dipped onto the test strips at controlled conditions (25 oC), immediately leading to visible fluorescence reduction. The probe design takes advantage of the widely-known electron deficient TNT-induced quenching of pyrene excimer emission of poly(HEMA-co-PyMA), which is ascribed to the formation of charge transfer complexes between the electron acceptor (TNT quencher) and the electron donor (pyrene fluorophores); (e) molecular structures of the fluorescent poly(HEMA-co-PyMA) polymer; (f) photos showing visible fluorescence response of the test stripe to aqueous TNT sample (25 ppm) taken under a hand-held UV lamp at 365 nm.

To the best of our knowledge, the developed paper sensor represents a notable advance towards the portable and effective analytical platform for NACs in aqueous medium. It is proved to be highly sensitive to even low-ppm-level TNT and exhibit a visible signal change in an unprecedented response time of only 5 seconds, which cannot be achieved in conventional film probes. It is also characterized with facile and scalable fabrication, low cost and quite good sensing selectivity. These promising results promoted us to further test their practical application in complex real water samples, including domestic and river water.


Page 5 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION Synthesis and Characterization of the Hydrophilic but Water-insoluble Poly(HEMA-co-PyMA). The synthetic procedure of the designed pyrene-functionalized hydrophilic but water-insoluble poly(HEMAco-PyMA) copolymer is illustrated in Figure 2(a). As shown below, the fluorescent monomer PyMA was first prepared via a simple one-step condensation reaction between 1-pyrenylbutyric acid and hydroxyethyl methacrylate (HEMA) according to the reported method.55 Its chemical structure was verified by 1H NMR spectroscopy (Figure S1), the results of which are consistent with that reported in the literature. The targeted poly(HEMA-co-PyMA) copolymer was then produced by radical copolymerization of PyMA with HEMA using azodiisobutyronitrile (AIBN) as the initiator. The obtained polymers were purified by precipitation in a large amount of diethyl ether (good solvent for PyMA) and subsequently dialyzing against deionized water (good solvent for HEMA) for three days in order to totally remove the unreacted monomers. The high-purity poly(HEMA-co-PyMA) polymers were then collected after evaporation of the solvent under vacuum.

Figure 2. (a) Synthetic procedures of the fluorescent monomer PyMA and pyrene-functionalized copolymer poly(HEMAco-PyMA); (b) 1H NMR spectra of two poly(HEMA-co-PyMA) copolymers with varying pyrene content recorded in DMSO-d6.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 22

On the basis of the possible modulation of the luminescent features via changes in the content of pyrene fluorophores, two poly(HEMA-co-PyMA) samples were synthesized by varying the feed ratio of PyMA to HEMA. Table S1 summarize the results of these polymerization reaction. Both of these polymers are readily soluble in common organic solvents (e.g. methanol, DMSO, DMF) but not in deionized water. Their chemical structures are confirmed by 1H NMR and FT-IR spectral analysis. Taking poly-2 with a higher pyrene content (13.5 mol%) as an example, the signals of the polymeric backbone protons resonate between 0.5 and 2.1 ppm (Figure 2(b)) in the 1H NMR spectrum. While aromatic protons of pyrene fluorophores appear as multiple signal peaks at 8.0~8.4 ppm, demonstrating the successful incorporation of pyrene moieties in the obtained polymer. The evident stretching vibrational modes in polycyclic aromatic hydrocarbons observed around 1500 cm-1 in its FT-IR spectrum further confirm the chemical structure of poly-2. Fluorescent Film Probes on Quartz Plates for NACs Sensing in Aqueous Medium. As a reference to the corresponding paper sensors, the detection performance of poly(HEMA-co-PyMA)-based fluorescent films casted on quartz plates was first investigated. To clearly describe their photo-luminescent features and sensing results for NACs, poly-1 with 0.67 mol% pyrene content was taken as a typical example and subject to systematical studies. As shown in Figure 3(a), the quality polymeric films (1.0 g/m2) can be easily casted on quartz plates from the methanol solutions of poly-1 according to the reported method.56~57 Thanks to the excellent film-forming property of poly(HEMA), the obtained films are quite transparent and thus facilely accessible to the excitation light. When excited at 341 nm, the prepared films exhibit typical fluorescence emission of monomeric pyrene from an excited singlet state (Figure 3(b)), which is quite similar to that recorded in the methanol solutions of poly-1. It is also noted that no enhanced fluorescent emission at longer wavelength was observed as mentioned in the literatures,58 indicating limited aggregation induced pyrene excimer emission for poly-1-based films. Remarkably, the water-insoluble films, despite highly hydrophilic, kept almost the same fluorescence spectra even after being immersed in deionized water under irradiation at 341 nm for 20 min (Figure 3(c)), which is sufficient long to conduct the whole measurement. These 6 ACS Paragon Plus Environment

Page 7 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


results demonstrate the quite good luminescence stability of poly-1-based films in water, suggesting their great potential applications as film probes for detecting analytes in aqueous medium.

Figure 3. (a) Photos of the hydrophilic but water-insoluble poly-1-based thin films casted on quartz plates, which were taken under room light (top) and UV light at 365 nm (down), respectively; (b) the normalized fluorescent spectra of poly1 in methanol (1 mg/mL) and its thin film casted on quartz plate recorded in water; (c) the time-dependent fluorescence spectra of poly-1-based thin films recorded after being immersed in deionized water and under irradiation at 341 nm; (d) the time-dependent fluorescent spectra of poly-1-based film probes at controlled temperature (25 oC) when exposed to aqueous solutions of 50 ppm TNT; (e) time-dependent emission intensity profiles of poly-1-based film probes in the presence of varying-concentration TNT (25, 50 and 75 ppm). Excitation was at 341 nm for fluorescence spectra measurement.

The sensing performance of the hydrophilic but water-insoluble poly-1-based films (1.0 g/m2) was then investigated by dipping the films in aqueous TNT solutions of different concentrations at controlled temperature (25 oC). As expected, remarkable fluorescence intensity reduction was observed after being treated with 50-ppm-level TNT solutions (Figure 3(d)). To our surprise, the signal response of poly-1-based film to TNT is proved to be quite fast. As summarized in Figure 3(e), its emission intensities around 380 nm rapidly fall within only 30 s and nearly level off at about 1 min for aqueous TNT samples of various concentrations. This finding is highly desired and represents a small but notable advance, because most of the reported solid-state NACs-sensing systems are characterized with the equilibrium response time ranging from several to tens of minutes. This uniquely fast spectroscopic response may be primarily attributed to the special chemical structure of the sensory polymers, where a large number of highly hydrophilic



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 22

hydroxyl (-OH) moieties are located as the pendant groups. Obviously, the grafted hydroxyl groups will definitely facilitate the fast diffusion of aqueous TNT solutions into the solid film matrix. As a result, the equilibrium response time is greatly shortened.

Figure 4. (a) Fluorescence spectra of poly-1-based film probes upon treatment with aqueous TNT solutions of increasing concentration; (b) their quenching efficiencies at 380 nm versus the concentration of TNT in the range from 25 to 125 ppm (the given quenching efficiencies are the average of three parallel measurements); (c) fluorescence quenching efficiency values of poly-1-based film probes upon treatment with aqueous solutions of common (nitro) aromatic compounds and volatile organic compounds as well as (d) many usually coexisting water-soluble cations and anions in real water samples. The concentrations of TNT, DNT and NB are 100 ppm, while the concentration of other species are 500 ppm. Excitation was at 341 nm for fluorescence spectra measurement.

Next, the TNT-concentration-dependent signal response of poly-1-based film probes was studied at controlled conditions (25 oC). It reveals that their fluorescence emission intensities gradually decrease with an increase in TNT concentration. Its fluorescence quenching efficiency (defined as (F0-F)/F0×100%, where F0 and F are the maximum emission intensity in the absence and presence of the analyte, respectively) versus TNT concentration is given in Figure 4(a). Similar to the results recorded in methanolic poly-1 8 ACS Paragon Plus Environment

Page 9 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solutions (Figure S2(a)), high-concentration TNT samples enhance the quenching efficiencies of the film probes (Figure 4(b)), suggesting its potential ability for quantitative measurement of TNT. This type of analyte-induced reduction in emission intensity is believed to be ascribed to the formation of charge-transfer complexes between TNT (the electron acceptor) and pyrene fluorophore (the electron donor).52-54 Furthermore, to indicate sensing specificity of poly-1-based film probes, their fluorescence response to other (nitro) aromatic compounds and many possibly coexisting water-soluble species in real water was also investigated. As summarized in Figure 4(c), the results highlight that the film probe is more sensitive to TNT than other nitro aromatic compounds such as 2,4-dinitrotoluene (DNT) and nitrobenzene (NB). Apart from this, no significant fluorescence intensity change is noticed in the presence of other common aromatic compounds and volatile organic compounds. These results indicate that electron-neutral and electron-rich aromatic species engendered little or no fluorescence change, while the most electrondeficient aromatic substances (TNT) induced the greatest quenching. Such a finding is fully consistent with the proposed mechanism,52-54 in which the analyte-induced emission intensity reduction is ascribed to the formation of charge-transfer complexes between the electron acceptor (nitro aromatic quenchers) and the electron donor (pyrene fluorophores). Therefore, the quenching performance of different nitro aromatic pollutants is primarily affected by the exergonicity of electron transfer between pyrene and nitro aromatic compounds. As is well known, TNT has a lower LUMO level than DNT and NB due to the larger number of nitro groups (NO2) which endows TNT with lower reduction potential and stronger electron accepting ability. Orbital energy matching of the LUMO levels of TNT and pyrene groups is thus much better than that of pyrene and DNT or NB, which can explain why TNT resulted in the highest fluorescence quenching efficiency of the polymeric probes. The quenching behavior of TNT is also characterized by the normalized fluorescence intensity (F0/F) and quenching constant using the Stern−Volmer (S−V) equation:59 F0/F=KSV[TNT]+1

(1)

where F0 and F represents the ﬂuorescence intensity of poly-1-based film probes at 380 nm in the absence and presence of TNT, respectively; [TNT] is the molar concentration of TNT and KSV is the quenching 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 22

constant (M−1). Figure S3 shows the Stern−Volmer plot for poly-1-based film probe with aqueous TNT solutions of increasing concentration. It is found that S−V plot for TNT shows quite good linearity through a large concentration range (0~125 ppm). The quenching constant is calculated to be about 0.80 ×104 M−1, which is much larger than that of DNT or NB. These results further demonstrate that the as-prepared film probe is sensitive and selective to the TNT analyte in aqueous solutions. Moreover, our sensory films are also not sensitive to many commonly found water-soluble inorganic species (Figure 4(d)), especially highconcentration chloride, magnesium and calcium ions (500 ppm) which usually exists in natural water. These above results clearly demonstrate the quite good sensing selectivity of poly(HEMA-co-PyMA)-based fluorescent films on quartz plates. However, although poly-1-based film probe is characterized with fast and selective fluorescence response to TNT, its luminescence bands mainly lie in the UV region (

Self-Diffusion Driven Ultrafast Detection of ppm-Level Nitroaromatic

Recommend Documents