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Cellulose-based sensor containing phenanthroline for the highly-selective and rapid detection of Fe2+ ions with naked-eye and fluorescent dual-modes Haq Nawaz, Weiguo Tian, JinMing Zhang, Ruonan Jia, Zhangyan Chen, and Jun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17342 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Applied Materials & Interfaces

Cellulose-based sensor containing phenanthroline for the highly-selective and rapid detection of Fe2+ ions with naked-eye and fluorescent dual-modes Haq Nawaz1, Weiguo Tian1, Jinming Zhang1,*, Ruonan Jia1,2, Zhangyan Chen1,2, Jun Zhang1,2,* 1

CAS Key Laboratory of Engineering Plastics, CAS Research/Education Center for Excellence in

Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing, 100190, China. 2

University of Chinese Academy of Sciences, Beijing 100049, China.

Abstract Iron ions play a vital role in many biological processes, and their concentrations are responsible for human health. Therefore, it is essential to detect the concentrations of iron ions by a rapid, accurate, highly-selective and practical method. Herein, we have synthesized a cellulose-based fluorescent sensor (Phen-MDI-CA) for the highly-selective and rapid detection of Fe2+ ions, via chemically bonding 1,10-phenanthroline-5-amine (Phen) onto cellulose acetate (CA) using 4,4’-methylene diphenyl diisocyanate (MDI) as a crosslinker. Benefiting from the anchoring and diluting effect of cellulose skeleton, the resultant Phen-MDI-CA displays excellent fluorescence properties in both solution and solid state. More interestingly, cellulose based polymer chain significantly improves the sensitivity of phenanthroline to Fe2+ ions. Upon meeting Fe2+ ions, a red, insoluble and non-fluorescent Fe-(Phen-MDI-CA) complex appears immediately; thus, Phen-MDI-CA can work as a multimode chromogenic-sensor for the highly selective, sensitive and rapid detection of Fe2+ ions. In the instrument-free visual mode, the detection limit for Fe2+ ions is 50 ppb, and in fluorescence mode, the detection limit is 2.6 ppb. To our knowledge, this is the first time that such a low detection limit for Fe2+ ions in aqueous media has been observed by the naked-eye. In addition, Phen-MDI-CA has good solubility and processability in common organic solvents, which facilitates its use in different material forms, e.g., printing ink, coating, and film. Therefore, the Fe2+-responsive and chromogenic Phen-MDI-CA exhibits a huge potential in the detection and extraction of Fe2+ ions.

Keywords: Solid fluorescent materials; Fe2+ detection; cellulose-based sensor; phenanthroline; dual-modes detection

1. Introduction Industrially influenced areas such as the textiles, metallurgical, agrochemical and iron industries have a high concentration of heavy metals in ground water. These metals not only contaminate drinking water but also leech into the soil and accumulate in plants and animals causing serious health risks in human beings. Since metal ions are of great significance for human health and the environment, the development of novel materials and facile methods with high selectivity and ∗

Corresponding authors.

E-mail address: [email protected] (J.M. Zhang); [email protected] (J. Zhang). Mailing address: Zhongguancun North First Street 2,100190 Beijing, PR China

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sensitivity for detecting metal ions have received much attention. Among all of the metal ions, iron ions are the most important and abundant transition metal ions in the human body, where they exist as ferrous (Fe2+) and ferric (Fe3+) ions under physiological conditions.1,2 These ions play an essential role in many chemical and biological processes, such as storage metabolism, oxygen delivery, mitochondrial respiration and electron transfer.3-5 Appropriate concentrations of iron ions are crucial for health. The excessive accumulation of iron ions in the human body has serious consequences, including kidney and liver damage6, dysfunction of vital organs, cancer, hemochromatosis and hepatitis.7-10 A disruption of iron homeostasis potentially gives rise to neurodegenerative diseases11, e.g., Alzheimer's disease and Parkinson's disease. Compared with stable Fe3+, labile Fe2+ is of special significance, because of the propensity of Fe2+ to oxidize to the Fe3+ form under aerobic aqueous environments. Moreover, Fe2+ is a potential catalyst of the Fenton reaction that causes severe cellular damage through the production of hydroxyl radicals, which are the most harmful reactive oxygen species in biological systems. Therefore, an accurate and fast determination of iron ions, especially Fe2+ ions, is a significant issue in both biological system and the environment. Currently, there are several analytical techniques available for detecting iron ions, including atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, flame atomic absorption spectroscopy and electrochemical detection. Although these methods are sensitive and provide accurate results, they suffer from non-portability, time-consuming sample processing, sophisticated operation, and high cost, which limits their prosperous applications. Chemo-sensors offer considerable advantages over these methods, because they allow for the conversion of the ions into signals which are read by widely available instruments or even by an untrained observer, especially if the signal is a color change (chromogenic sensor). A sensor is a self-contained analytical device that converts the concentration of chemical species into a signal which is read by an observer or instrument. Particularly, polymer based chemo-sensors have been attracting increasing attention because they exhibit superior performance to small organic molecules. One important advantage is signal amplification. Conjugated polymer chains contain multiple recognition elements that increase the recognition selectivity and the binding efficiency for the particular analyte.12 Additionally, polymer based film sensors are easily fabricated into devices. However, few studies are available for the development of chemo-sensors that are specific to the detection of Fe2+ ions. One possible reason might be that it is difficult to achieve high specificity and selectivity for detecting Fe2+/Fe3+ ions.13 Thus, it is attractive and challenging to fabricate highly-selective, fast-response, convenient and low-cost chemo-sensors to probe iron ions, particularly those with a simple and visual detection mode. In this work, a novel polymer-based fluorescent chemo-sensor (Phen-MDI-CA) has been developed from biopolymer cellulose by chemically bonding 1,10-phenanthroline-5-amine (Phen) onto cellulose acetate (CA) using 4,4’-methylene diphenyl diisocyanate (MDI) as a crosslinker


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under homogeneous reaction conditions (Fig. 1A). Phen-MDI-CA works as a multimode chromogenic sensor for the highly selective, visual and rapid detection of Fe2+ ions. The detection limit for Fe2+ ions in the instrument-free visual mode is 50 ppb, and the detection limit in the fluorescence mode is 2.6 ppb. Compared with the small molecular sensor phenanthroline, the obtained polymer-based fluorescent chemo-sensor has several important advantages, such as higher sensitivity, signal amplification, excellent formability, flexible material forms, and a combination of different output modes.

Fig. 1 (A) Schematic representation for the synthesis of Phen-MDI-CA (B) 1H-NMR spectrum of Phen-MDI-CA in DMSO-d6. (C) FTIR spectra of (a) Phen, (b) cellulose acetate, (c) Phen-MDI-CA with DS = 0.06 and (d) Phen-MDI-CA with DS = 0.93.

2. Results and discussion 2.1 Homogeneous synthesis and characterization of Phen-MDI-CA

The defined structure of the obtained Phen-MDI-CA has been characterized clearly by 1H-NMR and FTIR spectroscopy. In the 1H-NMR spectrum (Fig. 1B), there are new peaks of 6.8-9.2 ppm, which are the characteristic peaks of aromatic ring protons of MDI and Phen.14,15 Other new peaks in the range of 3.2-4.0 ppm are attributed to the alkyl protons in the MDI group. In the FTIR spectra (Fig. 1C), there is clear evidence of successfully anchoring Phen onto the cellulose chain by the appearance of 1400-1600 cm-1 for aromatic C=C stretching and 700-980 cm-1 for aromatic C-H out-of-plane bending vibration. In addition, in the FTIR spectra of Phen-MDI-CA, the intensity of the peaks at 3300-3600 cm-1 for the hydroxyl group O-H stretching vibration obviously decreases. These phenomena confirmed that the derivatization of cellulose with Phen was successfully accomplished.



By employing different molar ratios of Phen/CA, a series of Phen-MDI-CA samples were synthesized with degree of substitution (DS) of Phen ranging from 0.0020 to 0.93 (Table S1). The molar ratio of Phen/CA directly affects the DS of products. The results show a linear relationship between molar ratio of Phen/CA and DS of Phen (Fig. S1) because the whole process is completed during a homogeneous condition. The conversion rate of Phen is approximately 70%, which is apparently higher than those of heterogeneous reactions of cellulose. 2.2 Fluorescence properties of Phen-MDI-CA in solution and solid state

The resultant Phen-MDI-CA exhibits excellent fluorescence properties in both solution and a solid state. In DMSO solutions, Phen-MDI-CA displays a blue fluorescence with an emission peak at 446 nm (Fig. 2A and S2), which is different from original Phen with an emission peak at 501 nm, due to an intramolecular charge transfer in Phen-MDI-CA. Just like a typical fluorescence molecule, Phen-MDI-CA shows an aggregation caused quenching (ACQ) phenomenon. That is, as the concentration of Phen-MDI-CA increases, the emission intensity of Phen-MDI-CA increases initially, reaches a critical point at 2.50 x 10-5 M and then starts to decrease (Fig. S3). A similar phenomenon is observed in the effect of DS of Phen on the emission intensity, as shown in Fig. 2A and 2B, because the increase of DS means the concentration of fluorophore Phen in solution increases. Phen-MDI-CA with DS = 0.73 gives the strongest fluorescence emission. In general, ACQ molecules favor aggregation; thus, the fluorescence of ACQ molecules utterly quenches in a solid state. However, after covalently immobilizing fluorophore Phen onto cellulose chains, the anchoring and diluting effect of the polymer skeleton inhibit the aggregation and self-quenching of luminogens to a certain extent;16 consequently, all Phen-MDI-CA powders with different DS values emit bright blue fluorescence (Fig. 2C and 2D). An optimum DS of Phen related to the strongest fluorescent emission is 0.73 both in solution and solid state (Table S2). When the DS is less than 0.73, the emission intensity rises with an increase in DS of Phen; when the DS is higher than 0.73, the fluorescence of Phen-MDI-CA powders inevitably quenches gradually. This result suggests that sufficient cellulose skeletons are required to achieve excellent solid fluorescent materials from the ACQ luminogens. Based on this phenomenon, we mixed Phen-MDI-CA (DS = 0.73) with cellulose acetate (CA) to obtain a series of polymer blends with a tunable fluorescence intensity (Fig. 2E and 2F).


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Fig. 2 (A) Photographs of Phen-MDI-CA with different DS in DMSO at a concentration of 7.23 x10 M under visible light (top) and 365 nm UV light (bottom). (B) Dependence of the emission intensity of Phen-MDI-CA/DMSO solutions on the DS of Phen. (C) Photographs of Phen-MDI-CA powders with different DS values under visible light (top) and 365 nm UV light (bottom). (D) Dependence of the emission intensity of Phen-MDI-CA powders on DS. (E) Photographs of Phen-MDI-CA/CA with different contents of Phen-MDI-CA (DS = 0.73) under visible light (top) and 365 nm UV light (bottom). (F) Dependence of the emission intensity of Phen-MDI-CA (DS = 0.73)/CA on the content of Phen-MDI-CA.

2.3 Solubility and processability of Phen-MDI-CA

The synthesized Phen-MDI-CA is easily soluble in DMSO and DMF; thus, it can be processed into different material forms, including fluorescent inks, flexible films, coatings and microspheres, as



presented in Figs. 3 and S4. All materials display bright blue fluorescence. The mechanical properties of the cellulose-based materials are directly related to their industrial applications. Hence, the tensile strength of the prepared film was assessed. The Phen-MDI-CA film reveals enough strength of 60 MPa (Fig. S5), which is much better than the commercial polyolefin films with a tensile strength of 20-40 MPa.17 In addition, via an electrospray method, Phen-MDI-CA can form microspheres (Fig. S5). These microspheres have potential applications in blood purification and detoxification systems.18

Fig. 3 Photographs of Phen-MDI-CA in different material forms under visible light (top) and 365 nm UV light (bottom): (a) fluorescent printing on filter paper, (b) flexible fluorescent film, and (c) coatings on glass and steel.

2.4 Detection of Fe2+ ions by Phen-MDI-CA

Phen-MDI-CA is extremely sensitive to Fe2+ ions. Once Phen-MDI-CA meets Fe2+ ions, a red and non-fluorescent precipitate forms immediately, and the fluorescence emission intensity of Phen-MDI-CA decreases accordingly. The resultant precipitate is insoluble in almost all organic solvents, except dimethyl sulfoxide (DMSO). XPS curve of the precipitate indicates that a Fe-(Phen-MDI-CA) complex forms due to the appearance of a Fe2p3 orbital peak with a binding energy of 710.5 eV (Fig. S6).19,20 Through a titration test, we found that the optimal molar ratio of Phen in Phen-MDI-CA and Fe2+ ions was 2:1. Therefore, the Fe-(Phen-MDI-CA) complex shows a “CA-MDI-‘Phen--Fe2+--Phen’-MDI-CA” structure, as presented in Fig. 4. The formation of the Fe-(Phen-MDI-CA) complex probably causes the electrons of two nitrogen atoms at the juxta position on the Phen ring to transfer to the half-filled 3d orbitals of the Fe2+ ion; thus, there is a non-radiative electron or energy transfer process and the fluorescence of the Fe-(Phen-MDI-CA) complex is quenched.21 Moreover, due to the strong ″metal-to-ligand charge-transfer absorption″ of the Fe-(Phen-MDI-CA) complex22-24, an absorption peak is exhibited at approximately 529 nm (Fig. S7); therefore, a red color is displayed. Based on the above


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phenomenon, the fluorescent Phen-MDI-CA was employed to rapidly and sensitively detect Fe2+ ions for multimode detection, as shown in Fig. 4.

2+

Fig. 4 Detection of Fe ions in multimode. Detection mode 1: Fluorescence quenching phenomenon upon adding 1-2 drops of the Fe2+ aqueous solution (50 ppb) into Phen-MDI-CA (7.23 x 10-6 M in DMSO). Detection mode 2: Visual color change phenomenon upon adding 1-2 drops of Phen-MDI-CA -6

(7.23 x 10 M in DMSO) into a metal ion aqueous solution. Detection mode 3: Visual color change phenomenon of test papers by adding 2 drops of metal ions in an aqueous solution (1 ppm).

2+

2.4.1 Detection mode 1: Detection of Fe ions by fluorescence spectroscopy



The sensing approach of Detection mode 1 was further supported by using fluorescence measurement. The results are presented in Fig. 5. With an increase in the concentration of the Fe2+ aqueous solution, the emission intensity of Phen-MDI-CA decreases gradually. There is a linear relationship between the emission intensity of Phen-MDI-CA and the concentration of Fe2+ ions. The change in the emission intensity of Phen-MDI-CA (ΔF) as a function of Fe2+ concentration is depicted in Fig. 5C. The change (ΔF) is initially quite fast and slows as the concentration of Fe2+ ions increases. The detection limit for Fe2+ is 2.6 ppb (46 nM), which was determined by 3 δ/S where δ is the standard deviation of the lowest signal and S is the slope of the linear calibration plot. Furthermore, with the addition of other metal ions into a standard solution of Phen-MDI-CA, there was almost no change in the fluorescence emission intensity (Fig. 5D). Hence, the selectivity of Phen-MDI-CA for Fe2+ ions over other metal ions, including Fe3+ ions, is observably high. In contrast, UV-Vis absorption was also employed to detect the metal ions. The results indicate that Phen-MDI-CA responds to Fe2+ ions with high selectivity in the UV-Vis mode (Fig. S7). The detection limit of the UV-Vis method is 4.0 ppb, which is worse than the fluorescence method. In addition, in most cases, the detection process must be taken in DMSO solutions. Therefore, for Phen-MDI-CA, the fluorescence mode is a highly-sensitive and more convenient method to detect Fe2+ ions.

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Fig. 5 (A) Fluorescence spectra of Phen-MDI-CA (7.23 x 10-6 M in DMSO) with the addition of different 2+

concentration of Fe ions (0.01, 0.05, 0.089, 0.17, 0.89, 1.78, 3.57, 5.35, 7.14, 8.92, 10.7, 12.5, 14.2 and 17.8 µM). The inset shows the photographs of Phen-MDI-CA/DMSO solutions (a) before and (b)


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after addition of Fe2+ ions under UV-light. (B) Relationship between the emission intensity and Fe2+ ion concentration. (C) Effect of Fe2+ concentration on the change in emission intensity. (D) Fluorescent response of Phen-MDI-CA with the addition of various metal aqueous solutions (0.045 µM). The samples excitation wavelength was fixed at 365 nm for all measurements.

2+

2.4.2 Detection mode 2: Naked eye detection of Fe ions by color change

Except for the fluorescence method, the direct observation of the appearance of the red precipitate is employed to detect Fe2+ ions by the naked eye. A red precipitate forms immediately, when Phen-MDI-CA meets Fe2+ ions, even if there is a tiny amount of Fe2+ ions in aqueous solutions (Fig. 6). Other metal ions, including Fe3+ ions, produce white precipitates, when Phen-MDI-CA/DMSO solution is added into metal ion aqueous solutions (Fig. 6A). The appearance of the red color is because the Fe-Phen-MDI-CA complex has strong metal-to-ligand charge-transfer (MLCT) absorption in the visible region of the UV-Vis spectrum22-24; thus, an absorption peak appears at approximately 529 nm, whereas the complexes of other metal ions only absorb very weakly in this area (Fig. S7). Therefore, an instrument-free visual and highly-selective method is provided to detect Fe2+ ions by using the Phen-MDI-CA/DMSO solution as an indicator. In the visual mode, the detection limit is 50 ppb (850 nM), which is much lower than the recommended lower concentration of Fe2+ (5.36 µM) in drinking water by the World Health Organization.25 To our knowledge, this is the first time that this low detection limit for Fe2+ ions in aqueous media has been observed by the naked-eye.

Fig. 6 Naked eye detection of Fe2+ ions in aqueous media. (A) Addition of 1-2 drops of Phen-MDI-CA 2+

solution into various metal ions aqueous solutions (100 ppm). (B) Photographs of Fe ions aqueous solutions (0.05-70 ppm) after the addition of 1-2 drops of Phen-MDI-CA/DMSO solution (7.23 x 10

-6

M).

2.4.3 Detection mode 3: Detection of Fe2+ ions by test papers

Benefiting from the good processability and solid fluorescence property of Phen-MDI-CA, Phen-MDI-CA coatings fabricated by solution-casting and electrospray methods were used to



detect Fe2+ ions. Via a solution-casting method, Phen-MDI-CA was coated on filter papers to obtain the test papers. After placing 1-2 drops of Fe2+ ion aqueous solutions (1 ppm) onto these test papers, an instant red color appeared in < 2 s (Figs. 4 and S8). Among all of the tested metal ions, only Fe2+ ions produce a red colored clear spot, because of the formation of a red and non-fluorescent Fe-(Phen-MDI-CA) complex. The visual detection limit for Fe2+ was found to be 0.5 ppm, which is much lower than the recommended lower concentration of Fe2+ (5.36 μM) in drinking water by the World Health Organization.25 The Fe2+ ions can be detected by test papers in a fluorescence mode as well. We tipped 1-2 drops of various metal ion aqueous solutions (10 ppm) onto test papers with a Phen-MDI-CA coating and then observed them under UV-irradiation lamp at 365 nm. Among all of the tested metal ions, only Fe2+ ions produce a brunet but clear spot, owing to the formation of a red and non-fluorescent Fe-(Phen-MDI-CA) complex (Fig. S9). So, after coating Phen-MDI-CA onto a substrate, the resultant materials are expected to be used as a test paper to provide a qualitative analysis of metal ions. 2.5 Comparison of Phen-MDI-CA with phenanthroline and other Fe2+ sensors

It is well-known that phenanthroline can detect Fe2+ ions by UV-Vis absorption; therefore, in this work, we used 1,10-phenanthroline-5-amine as the small molecular sensor to detect metal ions by UV-vis, visual and fluorescence modes. The results reveal that the detection limit for Fe2+ ions is 156 ppb in the UV-Vis mode (Fig. S10), 1000 ppb in the visual mode (Fig. S11), and 50 ppb in the fluorescence mode (Fig. S12). A comparison with that of the polymer based sensor Phen-MDI-CA, it is obvious that Phen-MDI-CA is a better sensor to detect Fe2+ ions. That is, the cellulose polymer chain significantly improves the sensitivity of phenanthroline to Fe2+ ions. On the other hand, Fe2+ ions sensing materials from the previous literatures claimed that titania film based terpyridine had Fe2+ sensing properties with a visible detection limit of 300-5000 ppb and a UV detection limit of 200 ppb;24 hydrogels based Fe2+ sensing materials had a visual detection limit of 100 ppm and a UV detection limit of 10 ppb;26 the RhoNox-1 based fluorescent sensor developed for Fe2+ ion sensing had a fluorescent limit of detection of 0.2 µM;27 and the arene based fluorescent sensor had a detection limit of 8.54 µM.28 These results indicate a lower sensitivity than that of the cellulose-based sensor (Phen-MDI-CA) developed in this work, where the visual detection limit is 50 ppb, the UV detection limit is 4 ppb and the fluorescent detection limit is 2.6 ppb. The Fe2+ based sensors are shown in Table 1 with their detection limit and response time. The use of polymer based chemo-sensors have numerous advantages over small organic molecules such as high sensitivity and selectivity, signal amplification, multiple recognition sites that increase the recognition selectivity, binding efficiency for the particular analyte and ease of fabrication into devices.12 In summary, Phen-MDI-CA can be used as a multimode sensor for the selective and rapid detection of Fe2+ ions. In the fluorescence mode (Detection mode 1), the detection limit for Fe2+ ions is 2.6 ppb (46 nM), and in the instrument-free visual mode (Detection mode 2) , the


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detection limit is 50 ppb (890 nM). Notably, Phen-MDI-CA can immediately respond to Fe2+ ions. The response time is less than 2 s in all three test modes. Obviously, a comparison with some recently developed chemo-sensors (Table 1) indicate that Phen-MDI-CA has several advantages, such as a low detection limit, a short response time, instrument-free visual detection and a multimode response. Moreover, in consideration of excellent biodegradability, non-toxicity, low-cost and good processability of cellulose-based derivatives,29-31 the Fe2+-responsive and chromogenic Phen-MDI-CA exhibits a huge potential in the detection and extraction of Fe2+ ions. 2+

Table 1 The detection limits and response time for Fe ions on various sensors.

Sensors

Detection limit (µM)

Response time

References

Rhodamine B

0.2

1h

27

Terpy-functionalized TiO2

300 ppb

30 s

23

Carbon dots

0.02

2 min

32

MoS2/OPD/H2O2

0.007

30 s

33

N-aryl-O-acylhydroxylamine

0.5

1 min

13

N-doped carbon dots

10.98

few seconds

34

Arene based fluorescent probes

8.54

few seconds

35

Benzimidazolyl pyridine

0.28

-

22

Cellulose-Based Sensor Containing ... - ACS Publications

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