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A novel lanthanide MOF thin film: The highly performance self-calibrating luminescent sensor for detecting formaldehyde as an illegal preservative in aquatic product
Accepted Manuscript Title: A novel lanthanide MOF thin film: The highly performance self-calibrating luminescent sensor for detecting formaldehyde as an illegal preservative in aquatic product Authors: Yi Wang, Gaowei Zhang, Feng Zhang, Tianshu Chu, Yangyi Yang PII: DOI: Reference:
S0925-4005(17)30884-5 http://dx.doi.org/doi:10.1016/j.snb.2017.05.063 SNB 22347
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
27-9-2016 7-5-2017 13-5-2017
Please cite this article as: Yi Wang, Gaowei Zhang, Feng Zhang, Tianshu Chu, Yangyi Yang, A novel lanthanide MOF thin film: The highly performance self-calibrating luminescent sensor for detecting formaldehyde as an illegal preservative in aquatic product, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.05.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel lanthanide MOF thin film: The highly performance self-calibrating luminescent sensor for detecting formaldehyde as an illegal preservative in aquatic product
Yi Wang, Gaowei Zhang, Feng Zhang, Tianshu Chu and Yangyi Yang*
School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, P.R. China
*Corresponding Author E-mail: [email protected].
1
Graphical abstract
Research Highlights
A new luminescent thin film of Eu-NDC@HPAN have been fabricated on hydrolyzed polyacrylonitrile through layer by layer method.
The dual emissive Eu luminescent film is developed as a self-calibrating sensor for formaldehyde (0.05-1%) in aqueous solution.
The thin film of Eu-NDC@HPAN is applied to detect formaldehyde in sleeve-fish aqueous solution.
The lanthanide complex film is first used as a luminescent self-calibrating sensor for small molecules.
ABSTRACT A continuous and well attached thin film of Eu3+ MOF with dual-emission was directly grown on hydrolyzed polyacrylonitrile (HPAN) using layer by layer approach. The structure and morphology of the luminescent Eu-NDC@HPAN thin film was investigated by XRD, FT-IR and SEM. The film was utilized as a self-calibrating luminescent sensor for detecting formaldehyde in aqueous solution. The emission intensity ratio (I453/I616) increased linearly with increasing formaldehyde content in the range from 0.05 to 1%. The sensing mechanism was also discussed. For practical purpose, the thin film has been applied to detect formaldehyde as an illegal 2
preservative in aquatic product, This is first demonstrated that lanthanide complex luminescent film is application in self-calibrating small molecule sensors in biological systems successfully. Keywords：Lanthanide metal organic framework; dual emission; luminescent film; self-calibrating sensor; formaldehyde detection.
1. Introduction Metal organic frameworks (MOFs) are a class of crystalline hybrid materials made from an assembly of metal ions coordinated to organic linkers, and very promising as multifunctional materials [1-3]. Luminescence is one of the most attracting properties of the MOFs [4, 5]. Especially, luminescent lanthanide MOFs attracted considerable interest due to the unique optical properties of lanthanide ions and have been emerging as interesting materials [6-12]. In the past decade, studies on luminescent Ln-MOFs for sensing metal ions [13, 14], anions [15, 16], organic molecules [17, 18] and temperature [19, 20] have been widely reported. However, luminescent applications are usually based on lanthanide MOFs in the powder state. Few investigations concerning the fabrication of Ln-MOF films were studied so far. As the micrometer MOF materials are usually insoluble and heterogeneous in solution, the suspension-state luminescent experiment is helpful to the qualitative test of the luminescent spectra for the lanthanide MOFs. Nevertheless, in the studies of the quantificational luminescent spectra, the unstabilities of the suspensions would cause larger uncertainty in measuring luminescence intensity [16]. In addition, the fabrication of film into devices has several obvious advantages: (1) films with any shape and size (dependent on the substrate) can be fabricated for diverse needs; (2) they are easily stored and transported; (3) the preparation of films into devices is required for important industrial applications [21-23]. These MOF films are almost based on transition metal MOFs and cannot satisfy all the applications in sensing functionality. As many of lanthanide complexes have poor film forming ability and the lanthanide(III) ions can cause cluster formation in high concentration, It is always a great challenge to fabricate Ln-MOF films on supports in a simple way [23, 24]. Among the reported lanthanide MOF sensors, most of them are based on single emission luminescence quenching, which is usually influenced by environmental interferences like fluctuations of the source intensity, sensor concentration, etc. These would lead to inaccuracy in quantification. A simple ratiometric sensing approach is one of the breakthroughs because it provides a self-calibrating mechanism for comparing the luminescence intensities of different luminescent centers without requiring any external reference [25-27]. Studies on MOF sensors with dual luminescent centers are very limited up to now [17, 28]. Lanthanide ions such as terbium and europium can emit the characteristic luminescence by antenna effect. Naphthalic derivatives are effective sensitizers of the luminescent lanthanide ions and exhibit interesting optical properties[29, 30]. In this work, we study lanthanide metal organic framework with dual emission detecting analytes via interaction between analyte and ligand. Recognition of aldehydes can draw public attention for a long time owing to their biological toxicity [31-33]. Among aldehydes, formaldehyde (HCHO) is widely used in construction, and furniture, particle board. Formaldehyde is very harmful to human being and can result in watery eyes, asthma and respiratory irritation [34]. In addition, some illegal vendors are driven by economic interests often add HCHO to the soaking solution to extend the keeping life of 3
waterishlogged food products. Thus, it is vitally necessary to monitor in light of their importance not only as air contaminant but also as indicators of food quality [35]. HCHO-detection is usually performed by high performance liquid chromatography (HPLC), gas chromatography, etc. Nevertheless, their daily utilization has been limited by relatively slow, time consuming and expensive. It is necessary to develop easy, high-sensitive and low-cost luminescent sensors. The wide variety of luminescent LnMOFs and their inherent synthetic versatility seems to make them ideal for analyte recognition. However, in recent years, there is no report about lanthanide MOFs as self-calibrated luminescent sensors for formaldehyde in waterishlogged product as illegal preservative or formaldehyde vapor [36, 37]. Based on above considerations and our experience of fabricating Ln-MOF films [38-40], the right choice of the substrates for the target is the requirement to ensure the growth of film. If we want to take advantage of the low cost, high processing ability of the polymer, polyacrylonitrile (PAN) is a good candidate to meet both requirements. Moreover, it possesses lots of nitrile groups (–CN) which can be hydrolyzed into carboxyl groups (–COO) in the presence of an alkaline solution, which can provide nucleation sites for MOF growth and ensure the adhesion between the MOF layer and the PAN [41, 42]. Herein, we design and fabricate a new luminescent thin film of Eu-NDC@HPAN with dual-emission on HPAN via layer by layer strategy. The Eu-NDC@HPAN was developed as a self-calibrating luminescent sensor targeting formaldehyde in aqueous solution, and exhibited an excellent ratiometric luminescence response to HCHO (0.05-1%). The advantages of the thin film, such as high selectivity, aquo solution stability, low-cost and convenient detection will expand its application in biological system (scheme 1).
2． Experimental 2.1 Materials and methods 2,6-naphthalenedicarboxylate (H2NDC) was purchased from TCI (Shanghai) Development Co. Ltd. Polyacrylonitrile (PAN) were purchased from Spectrum. All chemicals were commercially available and used without further purification. Sleeve-fish was purchased from a seafood market in Guangzhou (Guangdong, China). Europium nitrate was prepared by dissolving Eu2O3 in excess nitric acid (67.5%) followed by evaporation and crystallization. All the luminescence measurements were recorded by Himadzu RF 5301 PC spectrofluorophotometer in the range of 350–800 nm with excitation wavelength of 360 nm. Mass spectra were carried out on LCMS-2010A. Powder X-ray diffraction (PXRD) measurements were studied on a D/MAX 2200VPC diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a voltage of 40 kV and a current of 26 mA/cm 2. The surface morphologies of the film were investigated by a scanning electron microscope (SEM, S4800, Hitachi). UV-Vis spectra of NDC were studied using a Shimadzu UV 3101PC spectrophotometer. FT-IR spectra were recorded from KBr pellets in the range of 4000–400 cm-1 on a Nicolet 330 FT-IR spectrometer. 2.2 Hydrolization of the PAN The PAN (1.5 g) dissolved in 15 mL DMF and heated to 80℃for 72 h and then cooled to room temperature forming the PAN film. The PAN film was hydrolyzed by immersing in 2 mol/L NaOH aqueous solution at 60℃. After 2 h, the PAN film was taken out and rinsed with deionized water until the pH value of the rinsing water reached about 7.0. 2.3 Preparation of the Eu-NDC@HPAN film 4
H2NDC (0.065g, 0.2 mmol) were dissolved in 20 mL deionized water (stirring with sodium hydroxide to adjust the pH value of 7). Eu(NO3)3·6H2O (0.090g, 0.2 mmol) was dissolved in 20 mL deionized water. MOF was grown by repeating cycles consisting of five individual immersing steps. Firstly, the hydrolyzed polyacrylonitrile (HPAN) film was immersed in Eu(NO3)3·6H2O solution, which causes the Eu 3+ ions to bind with the –COO at the HPAN surface. Secondly, the HPAN film was rinsed with the deionized water to remove unreacted Eu3+ ions, and then put into bind with the NDC and rinsed with the deionized water. By subsequently repeating and alternating the immersion of HPAN in the Eu(NO3)3·6H2O solution and in the NDC solution, Eu-NDC@HPAN were grown on HPAN by layer-by-layer (LBL) method. At last, the film was washed with deionized water and dried at 60 ℃ for 3–4 h.
3. Results and discussion 3.1 Characterization Fig. 1 presents PXRD patterns of the simulated Eu-NDC and Eu-NDC@HPAN. The PXRD patterns of the Eu-NDC@PAN correspond closely to simulated Eu-NDC, indicating that the material is crystalline and constitutes the same phase as simulated Eu-NDC[43]. In the unit, there are four Eu3+ ions, six NDC anions, five coordinated and three lattice water molecules. This complex exhibits the same porous honeycomb-like architecture along the a-axis. The phase of the substrate is not observed in the XRD due to the amorphous structure of the HPAN. As can be seen in Fig. 2 (a) and 2 (b), the images describe that these particles possess an interesting square architecture assembled by Eu-NDC with the thickness of less 200 nm. Numerous particles with the size of 2–4 μm are attached to the surface of HPAN homogeneously. The FT-IR spectra of PAN, HPAN and Eu-NDC@HPAN were presented in Fig. 3. A sharp, obvious adsorption band at 2243 cm −1 in PAN, attributed to C≡N group. In the spectrum of HPAN and Eu-NDC@HPAN, the –CN groups at around 2243 cm-1 decrease sharply after hydrolysis, and the adsorption bands at around 1470 cm -1 and 3300 cm-1 attributed to –OH group in the carboxyl. This provides evidence that -COOH occur hydrolysis in alkaline solution. 3.2 Photoluminescence properties As seen in Fig. 4, Eu-NDC@HPAN exhibits characteristic emission of the Eu 3+ ion upon excitation at 360 nm. Eu-NDC@HPAN exhibits the peaks at 590, 612 700 nm, which are attributed to 5D0→7FJ (J = 1, 2, 4) transitions of the Eu3+ ion, respectively. 3.3 Sensing for formaldehyde In light of the excellent luminescence property and Eu-NDC@HPAN (Supporting information Fig. S1), we Eu-NDC@HPAN for detecting small molecules. The Eu-NDC@HPAN toward various small molecules (Propanol, EtOH and HCHO) are shown in Fig. 5, most small molecules 5
good water stability of examine the potential of luminescent responses of acetone, DMF, H2O MeOH, show slight influence on the
emission intensity of Eu-NDC@HPAN at 616nm and 453 nm, however, HCHO shows a significant quenching effect on the luminescence intensity of the emission at 616 nm, meanwhile, luminescence intensity of the emission at 453 nm becomes conspicuous (Supporting information Fig. S2). The luminescence properties of Eu-NDC and deprotonated NDC were investigated in aqueous solution at room temperature, as shown in Fig.6, It was found that deprotonated NDC displays an intense emission band at 410 nm upon excitation at 360 nm. Compared to the deprotonated NDC, the deprotonated NDC immersed in HCHO (40%) showed a large red shift (40 nm) for maximum emission, The red-shift should be contributed from the hydrogen bond (HCHO-NDC) between formaldehyde and generated NDC, the results of UV-Vis spectra were used to verify the existence of interaction, (Supporting information Fig. S3). Eu-NDC treated with HCHO displays an intense emission band at 453 nm upon excitation at 360 nm，which is consistent with deprotonated NDC immersed in HCHO, the results demonstrate that the sensing of HCHO was attributed to the collapse of the crystal structure of Eu-NDC and the ligand regenerated after adding HCHO. The luminescence property of Eu-NDC@HPAN was investigated when the HCHO content was gradually increased. Upon an increase in the concentration of HCHO, the intensity of the emission at 616 nm dramatically decreases, while simultaneously the emission of NDC at 453 nm becomes conspicuous. On the basis of these results, in Fig. 7, it is obvious that the Eu-NDC@HPAN thin film can rationally detect HCHO in aqueous solution through a ratiometric luminescence approach. The relative ratio of the luminescence intensities at 453 and 616 nm exhibit 3.2-fold increase when the percentage of added HCHO reaches 1%. The results show a linear relationship between the emission ratios (I453/I616) and the concentrations of HCHO from 0.05 to 1%, suggesting that the Eu-NDC@HPAN is potentially useful for the quantitative determination of HCHO (Supporting information Fig. S4). The anti-interference sensing ability to HCHO was further studied by the competing experiments. Eu-NDC@HPAN was immersed in HCHO aqueous solution in the presence of the same concentration of other small molecules (2.5%). As shown in Fig. 8, relatively low interference was observed for the detection of HCHO when different small molecules existed, which validates anti-interference capabilities of HCHO. The results demonstrate that Eu-NDC@HPAN was a high selective sensor for HCHO detection. HCHO is considered to be cancerigenic, which is harmful to human health, so it cannot be directly used as a food preservative. Some unscrupulous peddlers add HCHO in aquatic products as an illegal food preservative to remain the keeping life of waterishlogged product during storage or transport. Therefore, it is important to check HCHO in aquatic product. The film of Eu-NDC@HPAN was applied to detect HCHO in sleeve-fish samples (Supporting information Fig. S5). HCHO is also one of the typical indoor air pollutants. It is mainly released from building and indoor decorative materials. Such a colorless, volatile and deleterious gas with a pungent and suffocating 6
odor poses a serious health hazard to human beings by causing central nervous system damage, blood and respiratory disease, etc. Therefore, indoor HCHO detection is of great necessity and in huge demand for human health. The film Eu-NDC@HPAN was also applied to detect HCHO vapor (Supporting information Fig. S6). The results show that the film can detect HCHO in aquatic product and HCHO vapor. The HCHO sensor does not require any additional calibration of luminescence intensity and it can be low cost and accurate, the HCHO sensor is superior to other lanthanide MOF sensing of HCHO [44, 45]. These unique features together with the higher sensitivity and our HCHO sensor might enable it to be applied in practical applications. 3.4 Quenching mechanism As seen in Fig. 9, PXRD of the residues of suspensions (Eu-NDC and HCHO) were employed to study the structure changes during HCHO aqueous solution treatment. A few new peaks come out in the XRD pattern besides the pattern of Eu-NDC, which indicates that a new structure may be formed after a long immersion time (12 hours). Meanwhile, the XRD pattern of some new peaks is consistent with the pattern of H 2NDC. As seen in Fig. 10，Fourier transform infrared spectra show that the vibration bands at 1685, 1294, and 831 cm−1 For H2NDC were determined. Compared to Eu-NDC, in infrared spectra of Eu-NDC treated with HCHO, new peaks at 1685 and 1294 cm −1 which are consistent with the peaks of H2NDC, thus, the new structure may be assigned as H2NDC. For proving the above speculation, as shown in Fig. 11, electrospray ionization mass spectrometry spectra (the residues from Eu-NDC before and after immersed in HCHO) were studied. As shown in Fig. 11b, the spectra of the residues from Eu-NDC after addition of HCHO exhibited an intense peak at m/z 215.3, which could be belonged to the NDC. All of these results suggest that the sensing of HCHO by Eu-NDC was ascribed to the collapse of the crystal structure of Eu-NDC and regeneration of the ligand after adding HCHO, thus resulting in the disappearance of the emission at 616 nm and the emergence of the NDC emission at 453 nm.
4. Conclusions In summary, a new dual emissive luminescent thin film of Eu-NDC@HPAN is designed and fabricated on HPAN by Layer by layer strategy at mild condition. The film can be used as a self-calibrating luminescent sensor for detecting formaldehyde in aqueous solution. The ratio of emission intensity (I453/I616) increases linearly with increasing HCHO content in the range from 0.05 to 1%. The excellent selectivity, sensitivity and water stability enable this film to realize the applications in the sensing HCHO as an illegal preservative in aquatic product successfully. This is first demonstrated that the lanthanide complex thin film is used as a self-calibrating luminescent sensor for small molecules in life science.
Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (51472275, 20973203 and 91022012) and Guangdong Natural Science Foundation (2014A030313207). 7
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Author Biographies Yi Wang is a graduate student major in physical chemistry in the School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, China. He is currently studying luminescent lanthanide coordination polymer sensors. Gaowei Zhang is a graduate student major in chemistry engineering in School of Materials Science and Engineering, Sun Yat-Sen University. Feng Zhang is a graduate student major in materials physics and chemistry in the School of Chemistry and Chemical Engineering, Sun Yat-Sen University. He is currently studying luminescent lanthanide coordination polymer sensors. Tianshu Chu is a graduate student major in physical chemistry in the School of Chemistry and Chemical Engineering, Sun Yat-Sen University. He is currently studying luminescent lanthanide coordination polymer sensors. Yangyi Yang is an associate professor in the School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou, China. He received his Ph.D. degree from the School of Chemistry and Chemical Engineering, Sun Yat-Sen University in 2001. His main research interests are lanthanide coordination polymers and their applications.
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Fig. 1 PXRD patterns of the simulated Eu-NDC, Eu-NDC@HPAN and HPAN.
(b)
(a)
Fig. 2 SEM of Eu-NDC@HPAN (a) Top view and (b) Enlarged.
Fig. 3 FT-IR spectra of PAN, HPAN and Eu-NDC@HPAN.
12
Fig. 4 The excitation (dashed) and emission (solid) spectra of the Eu-NDC@HPAN.
Fig. 5 Luminescence spectra of Eu-NDC@HPAN treated with different small molecules (λex = 360 nm).
Fig. 6 Luminescence spectra of deprotonated NDC and Eu-NDC (0.25mg/mL) (before and after treated with formaldehyde) (λex = 360 nm).
13
(a)
(b)
Fig. 7 (a) Plot of I453/I616 versus the formaldehyde content in aqueous solution (λex = 360 nm). (b) Luminescence spectra of Eu-NDC@HPAN as a function of the formaldehyde concentration (from top: 1, 0.5, 0.25, 0.1, 0.05%) in aqueous solution
Fig. 8 Luminescence responses of Eu-NDC@HPAN to various small molecules. The red bars represent the relative ratio (I453/I616) of the emission intensities of Eu-NDC@HPAN in the presence of small molecules. The blue bars represent a change of the relative ratio (I453/I616) of the emission intensities that occurs upon the subsequent addition of formaldehyde to the above solution (λex = 360 nm).
Fig. 9 PXRD patterns of simulated Eu-NDC, as-synthesized Eu-NDC and Eu-NDC treated with formaldehyde for 12 hours.
14
Fig. 10 FT-IR spectra of as-prepared Eu-NDC, Eu-NDC (treated with formaldehyde) and NDC.
(a)
(b)
Fig. 11 The ESI-MS of the residues from Eu-NDC before (a) and after (b) immersed in formaldehyde. Retreating condition: soaking the residues in DMF.
15
Scheme 1 Schematic representation of the synthesis process of Eu-NDC@HPAN and the luminescence quenching phenomenon of HCHO to Eu-NDC@HPAN.
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S0925-4005(17)30884-5 http://dx.doi.org/doi:10.1016/j.snb.2017.05.063 SNB 22347
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
27-9-2016 7-5-2017 13-5-2017
Please cite this article as: Yi Wang, Gaowei Zhang, Feng Zhang, Tianshu Chu, Yangyi Yang, A novel lanthanide MOF thin film: The highly performance self-calibrating luminescent sensor for detecting formaldehyde as an illegal preservative in aquatic product, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.05.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel lanthanide MOF thin film: The highly performance self-calibrating luminescent sensor for detecting formaldehyde as an illegal preservative in aquatic product
Yi Wang, Gaowei Zhang, Feng Zhang, Tianshu Chu and Yangyi Yang*
School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, P.R. China
*Corresponding Author E-mail: [email protected].
1
Graphical abstract
Research Highlights
A new luminescent thin film of Eu-NDC@HPAN have been fabricated on hydrolyzed polyacrylonitrile through layer by layer method.
The dual emissive Eu luminescent film is developed as a self-calibrating sensor for formaldehyde (0.05-1%) in aqueous solution.
The thin film of Eu-NDC@HPAN is applied to detect formaldehyde in sleeve-fish aqueous solution.
The lanthanide complex film is first used as a luminescent self-calibrating sensor for small molecules.
ABSTRACT A continuous and well attached thin film of Eu3+ MOF with dual-emission was directly grown on hydrolyzed polyacrylonitrile (HPAN) using layer by layer approach. The structure and morphology of the luminescent Eu-NDC@HPAN thin film was investigated by XRD, FT-IR and SEM. The film was utilized as a self-calibrating luminescent sensor for detecting formaldehyde in aqueous solution. The emission intensity ratio (I453/I616) increased linearly with increasing formaldehyde content in the range from 0.05 to 1%. The sensing mechanism was also discussed. For practical purpose, the thin film has been applied to detect formaldehyde as an illegal 2
preservative in aquatic product, This is first demonstrated that lanthanide complex luminescent film is application in self-calibrating small molecule sensors in biological systems successfully. Keywords：Lanthanide metal organic framework; dual emission; luminescent film; self-calibrating sensor; formaldehyde detection.
1. Introduction Metal organic frameworks (MOFs) are a class of crystalline hybrid materials made from an assembly of metal ions coordinated to organic linkers, and very promising as multifunctional materials [1-3]. Luminescence is one of the most attracting properties of the MOFs [4, 5]. Especially, luminescent lanthanide MOFs attracted considerable interest due to the unique optical properties of lanthanide ions and have been emerging as interesting materials [6-12]. In the past decade, studies on luminescent Ln-MOFs for sensing metal ions [13, 14], anions [15, 16], organic molecules [17, 18] and temperature [19, 20] have been widely reported. However, luminescent applications are usually based on lanthanide MOFs in the powder state. Few investigations concerning the fabrication of Ln-MOF films were studied so far. As the micrometer MOF materials are usually insoluble and heterogeneous in solution, the suspension-state luminescent experiment is helpful to the qualitative test of the luminescent spectra for the lanthanide MOFs. Nevertheless, in the studies of the quantificational luminescent spectra, the unstabilities of the suspensions would cause larger uncertainty in measuring luminescence intensity [16]. In addition, the fabrication of film into devices has several obvious advantages: (1) films with any shape and size (dependent on the substrate) can be fabricated for diverse needs; (2) they are easily stored and transported; (3) the preparation of films into devices is required for important industrial applications [21-23]. These MOF films are almost based on transition metal MOFs and cannot satisfy all the applications in sensing functionality. As many of lanthanide complexes have poor film forming ability and the lanthanide(III) ions can cause cluster formation in high concentration, It is always a great challenge to fabricate Ln-MOF films on supports in a simple way [23, 24]. Among the reported lanthanide MOF sensors, most of them are based on single emission luminescence quenching, which is usually influenced by environmental interferences like fluctuations of the source intensity, sensor concentration, etc. These would lead to inaccuracy in quantification. A simple ratiometric sensing approach is one of the breakthroughs because it provides a self-calibrating mechanism for comparing the luminescence intensities of different luminescent centers without requiring any external reference [25-27]. Studies on MOF sensors with dual luminescent centers are very limited up to now [17, 28]. Lanthanide ions such as terbium and europium can emit the characteristic luminescence by antenna effect. Naphthalic derivatives are effective sensitizers of the luminescent lanthanide ions and exhibit interesting optical properties[29, 30]. In this work, we study lanthanide metal organic framework with dual emission detecting analytes via interaction between analyte and ligand. Recognition of aldehydes can draw public attention for a long time owing to their biological toxicity [31-33]. Among aldehydes, formaldehyde (HCHO) is widely used in construction, and furniture, particle board. Formaldehyde is very harmful to human being and can result in watery eyes, asthma and respiratory irritation [34]. In addition, some illegal vendors are driven by economic interests often add HCHO to the soaking solution to extend the keeping life of 3
waterishlogged food products. Thus, it is vitally necessary to monitor in light of their importance not only as air contaminant but also as indicators of food quality [35]. HCHO-detection is usually performed by high performance liquid chromatography (HPLC), gas chromatography, etc. Nevertheless, their daily utilization has been limited by relatively slow, time consuming and expensive. It is necessary to develop easy, high-sensitive and low-cost luminescent sensors. The wide variety of luminescent LnMOFs and their inherent synthetic versatility seems to make them ideal for analyte recognition. However, in recent years, there is no report about lanthanide MOFs as self-calibrated luminescent sensors for formaldehyde in waterishlogged product as illegal preservative or formaldehyde vapor [36, 37]. Based on above considerations and our experience of fabricating Ln-MOF films [38-40], the right choice of the substrates for the target is the requirement to ensure the growth of film. If we want to take advantage of the low cost, high processing ability of the polymer, polyacrylonitrile (PAN) is a good candidate to meet both requirements. Moreover, it possesses lots of nitrile groups (–CN) which can be hydrolyzed into carboxyl groups (–COO) in the presence of an alkaline solution, which can provide nucleation sites for MOF growth and ensure the adhesion between the MOF layer and the PAN [41, 42]. Herein, we design and fabricate a new luminescent thin film of Eu-NDC@HPAN with dual-emission on HPAN via layer by layer strategy. The Eu-NDC@HPAN was developed as a self-calibrating luminescent sensor targeting formaldehyde in aqueous solution, and exhibited an excellent ratiometric luminescence response to HCHO (0.05-1%). The advantages of the thin film, such as high selectivity, aquo solution stability, low-cost and convenient detection will expand its application in biological system (scheme 1).
2． Experimental 2.1 Materials and methods 2,6-naphthalenedicarboxylate (H2NDC) was purchased from TCI (Shanghai) Development Co. Ltd. Polyacrylonitrile (PAN) were purchased from Spectrum. All chemicals were commercially available and used without further purification. Sleeve-fish was purchased from a seafood market in Guangzhou (Guangdong, China). Europium nitrate was prepared by dissolving Eu2O3 in excess nitric acid (67.5%) followed by evaporation and crystallization. All the luminescence measurements were recorded by Himadzu RF 5301 PC spectrofluorophotometer in the range of 350–800 nm with excitation wavelength of 360 nm. Mass spectra were carried out on LCMS-2010A. Powder X-ray diffraction (PXRD) measurements were studied on a D/MAX 2200VPC diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a voltage of 40 kV and a current of 26 mA/cm 2. The surface morphologies of the film were investigated by a scanning electron microscope (SEM, S4800, Hitachi). UV-Vis spectra of NDC were studied using a Shimadzu UV 3101PC spectrophotometer. FT-IR spectra were recorded from KBr pellets in the range of 4000–400 cm-1 on a Nicolet 330 FT-IR spectrometer. 2.2 Hydrolization of the PAN The PAN (1.5 g) dissolved in 15 mL DMF and heated to 80℃for 72 h and then cooled to room temperature forming the PAN film. The PAN film was hydrolyzed by immersing in 2 mol/L NaOH aqueous solution at 60℃. After 2 h, the PAN film was taken out and rinsed with deionized water until the pH value of the rinsing water reached about 7.0. 2.3 Preparation of the Eu-NDC@HPAN film 4
H2NDC (0.065g, 0.2 mmol) were dissolved in 20 mL deionized water (stirring with sodium hydroxide to adjust the pH value of 7). Eu(NO3)3·6H2O (0.090g, 0.2 mmol) was dissolved in 20 mL deionized water. MOF was grown by repeating cycles consisting of five individual immersing steps. Firstly, the hydrolyzed polyacrylonitrile (HPAN) film was immersed in Eu(NO3)3·6H2O solution, which causes the Eu 3+ ions to bind with the –COO at the HPAN surface. Secondly, the HPAN film was rinsed with the deionized water to remove unreacted Eu3+ ions, and then put into bind with the NDC and rinsed with the deionized water. By subsequently repeating and alternating the immersion of HPAN in the Eu(NO3)3·6H2O solution and in the NDC solution, Eu-NDC@HPAN were grown on HPAN by layer-by-layer (LBL) method. At last, the film was washed with deionized water and dried at 60 ℃ for 3–4 h.
3. Results and discussion 3.1 Characterization Fig. 1 presents PXRD patterns of the simulated Eu-NDC and Eu-NDC@HPAN. The PXRD patterns of the Eu-NDC@PAN correspond closely to simulated Eu-NDC, indicating that the material is crystalline and constitutes the same phase as simulated Eu-NDC[43]. In the unit, there are four Eu3+ ions, six NDC anions, five coordinated and three lattice water molecules. This complex exhibits the same porous honeycomb-like architecture along the a-axis. The phase of the substrate is not observed in the XRD due to the amorphous structure of the HPAN. As can be seen in Fig. 2 (a) and 2 (b), the images describe that these particles possess an interesting square architecture assembled by Eu-NDC with the thickness of less 200 nm. Numerous particles with the size of 2–4 μm are attached to the surface of HPAN homogeneously. The FT-IR spectra of PAN, HPAN and Eu-NDC@HPAN were presented in Fig. 3. A sharp, obvious adsorption band at 2243 cm −1 in PAN, attributed to C≡N group. In the spectrum of HPAN and Eu-NDC@HPAN, the –CN groups at around 2243 cm-1 decrease sharply after hydrolysis, and the adsorption bands at around 1470 cm -1 and 3300 cm-1 attributed to –OH group in the carboxyl. This provides evidence that -COOH occur hydrolysis in alkaline solution. 3.2 Photoluminescence properties As seen in Fig. 4, Eu-NDC@HPAN exhibits characteristic emission of the Eu 3+ ion upon excitation at 360 nm. Eu-NDC@HPAN exhibits the peaks at 590, 612 700 nm, which are attributed to 5D0→7FJ (J = 1, 2, 4) transitions of the Eu3+ ion, respectively. 3.3 Sensing for formaldehyde In light of the excellent luminescence property and Eu-NDC@HPAN (Supporting information Fig. S1), we Eu-NDC@HPAN for detecting small molecules. The Eu-NDC@HPAN toward various small molecules (Propanol, EtOH and HCHO) are shown in Fig. 5, most small molecules 5
good water stability of examine the potential of luminescent responses of acetone, DMF, H2O MeOH, show slight influence on the
emission intensity of Eu-NDC@HPAN at 616nm and 453 nm, however, HCHO shows a significant quenching effect on the luminescence intensity of the emission at 616 nm, meanwhile, luminescence intensity of the emission at 453 nm becomes conspicuous (Supporting information Fig. S2). The luminescence properties of Eu-NDC and deprotonated NDC were investigated in aqueous solution at room temperature, as shown in Fig.6, It was found that deprotonated NDC displays an intense emission band at 410 nm upon excitation at 360 nm. Compared to the deprotonated NDC, the deprotonated NDC immersed in HCHO (40%) showed a large red shift (40 nm) for maximum emission, The red-shift should be contributed from the hydrogen bond (HCHO-NDC) between formaldehyde and generated NDC, the results of UV-Vis spectra were used to verify the existence of interaction, (Supporting information Fig. S3). Eu-NDC treated with HCHO displays an intense emission band at 453 nm upon excitation at 360 nm，which is consistent with deprotonated NDC immersed in HCHO, the results demonstrate that the sensing of HCHO was attributed to the collapse of the crystal structure of Eu-NDC and the ligand regenerated after adding HCHO. The luminescence property of Eu-NDC@HPAN was investigated when the HCHO content was gradually increased. Upon an increase in the concentration of HCHO, the intensity of the emission at 616 nm dramatically decreases, while simultaneously the emission of NDC at 453 nm becomes conspicuous. On the basis of these results, in Fig. 7, it is obvious that the Eu-NDC@HPAN thin film can rationally detect HCHO in aqueous solution through a ratiometric luminescence approach. The relative ratio of the luminescence intensities at 453 and 616 nm exhibit 3.2-fold increase when the percentage of added HCHO reaches 1%. The results show a linear relationship between the emission ratios (I453/I616) and the concentrations of HCHO from 0.05 to 1%, suggesting that the Eu-NDC@HPAN is potentially useful for the quantitative determination of HCHO (Supporting information Fig. S4). The anti-interference sensing ability to HCHO was further studied by the competing experiments. Eu-NDC@HPAN was immersed in HCHO aqueous solution in the presence of the same concentration of other small molecules (2.5%). As shown in Fig. 8, relatively low interference was observed for the detection of HCHO when different small molecules existed, which validates anti-interference capabilities of HCHO. The results demonstrate that Eu-NDC@HPAN was a high selective sensor for HCHO detection. HCHO is considered to be cancerigenic, which is harmful to human health, so it cannot be directly used as a food preservative. Some unscrupulous peddlers add HCHO in aquatic products as an illegal food preservative to remain the keeping life of waterishlogged product during storage or transport. Therefore, it is important to check HCHO in aquatic product. The film of Eu-NDC@HPAN was applied to detect HCHO in sleeve-fish samples (Supporting information Fig. S5). HCHO is also one of the typical indoor air pollutants. It is mainly released from building and indoor decorative materials. Such a colorless, volatile and deleterious gas with a pungent and suffocating 6
odor poses a serious health hazard to human beings by causing central nervous system damage, blood and respiratory disease, etc. Therefore, indoor HCHO detection is of great necessity and in huge demand for human health. The film Eu-NDC@HPAN was also applied to detect HCHO vapor (Supporting information Fig. S6). The results show that the film can detect HCHO in aquatic product and HCHO vapor. The HCHO sensor does not require any additional calibration of luminescence intensity and it can be low cost and accurate, the HCHO sensor is superior to other lanthanide MOF sensing of HCHO [44, 45]. These unique features together with the higher sensitivity and our HCHO sensor might enable it to be applied in practical applications. 3.4 Quenching mechanism As seen in Fig. 9, PXRD of the residues of suspensions (Eu-NDC and HCHO) were employed to study the structure changes during HCHO aqueous solution treatment. A few new peaks come out in the XRD pattern besides the pattern of Eu-NDC, which indicates that a new structure may be formed after a long immersion time (12 hours). Meanwhile, the XRD pattern of some new peaks is consistent with the pattern of H 2NDC. As seen in Fig. 10，Fourier transform infrared spectra show that the vibration bands at 1685, 1294, and 831 cm−1 For H2NDC were determined. Compared to Eu-NDC, in infrared spectra of Eu-NDC treated with HCHO, new peaks at 1685 and 1294 cm −1 which are consistent with the peaks of H2NDC, thus, the new structure may be assigned as H2NDC. For proving the above speculation, as shown in Fig. 11, electrospray ionization mass spectrometry spectra (the residues from Eu-NDC before and after immersed in HCHO) were studied. As shown in Fig. 11b, the spectra of the residues from Eu-NDC after addition of HCHO exhibited an intense peak at m/z 215.3, which could be belonged to the NDC. All of these results suggest that the sensing of HCHO by Eu-NDC was ascribed to the collapse of the crystal structure of Eu-NDC and regeneration of the ligand after adding HCHO, thus resulting in the disappearance of the emission at 616 nm and the emergence of the NDC emission at 453 nm.
4. Conclusions In summary, a new dual emissive luminescent thin film of Eu-NDC@HPAN is designed and fabricated on HPAN by Layer by layer strategy at mild condition. The film can be used as a self-calibrating luminescent sensor for detecting formaldehyde in aqueous solution. The ratio of emission intensity (I453/I616) increases linearly with increasing HCHO content in the range from 0.05 to 1%. The excellent selectivity, sensitivity and water stability enable this film to realize the applications in the sensing HCHO as an illegal preservative in aquatic product successfully. This is first demonstrated that the lanthanide complex thin film is used as a self-calibrating luminescent sensor for small molecules in life science.
Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (51472275, 20973203 and 91022012) and Guangdong Natural Science Foundation (2014A030313207). 7
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Author Biographies Yi Wang is a graduate student major in physical chemistry in the School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, China. He is currently studying luminescent lanthanide coordination polymer sensors. Gaowei Zhang is a graduate student major in chemistry engineering in School of Materials Science and Engineering, Sun Yat-Sen University. Feng Zhang is a graduate student major in materials physics and chemistry in the School of Chemistry and Chemical Engineering, Sun Yat-Sen University. He is currently studying luminescent lanthanide coordination polymer sensors. Tianshu Chu is a graduate student major in physical chemistry in the School of Chemistry and Chemical Engineering, Sun Yat-Sen University. He is currently studying luminescent lanthanide coordination polymer sensors. Yangyi Yang is an associate professor in the School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou, China. He received his Ph.D. degree from the School of Chemistry and Chemical Engineering, Sun Yat-Sen University in 2001. His main research interests are lanthanide coordination polymers and their applications.
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Fig. 1 PXRD patterns of the simulated Eu-NDC, Eu-NDC@HPAN and HPAN.
(b)
(a)
Fig. 2 SEM of Eu-NDC@HPAN (a) Top view and (b) Enlarged.
Fig. 3 FT-IR spectra of PAN, HPAN and Eu-NDC@HPAN.
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Fig. 4 The excitation (dashed) and emission (solid) spectra of the Eu-NDC@HPAN.
Fig. 5 Luminescence spectra of Eu-NDC@HPAN treated with different small molecules (λex = 360 nm).
Fig. 6 Luminescence spectra of deprotonated NDC and Eu-NDC (0.25mg/mL) (before and after treated with formaldehyde) (λex = 360 nm).
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(a)
(b)
Fig. 7 (a) Plot of I453/I616 versus the formaldehyde content in aqueous solution (λex = 360 nm). (b) Luminescence spectra of Eu-NDC@HPAN as a function of the formaldehyde concentration (from top: 1, 0.5, 0.25, 0.1, 0.05%) in aqueous solution
Fig. 8 Luminescence responses of Eu-NDC@HPAN to various small molecules. The red bars represent the relative ratio (I453/I616) of the emission intensities of Eu-NDC@HPAN in the presence of small molecules. The blue bars represent a change of the relative ratio (I453/I616) of the emission intensities that occurs upon the subsequent addition of formaldehyde to the above solution (λex = 360 nm).
Fig. 9 PXRD patterns of simulated Eu-NDC, as-synthesized Eu-NDC and Eu-NDC treated with formaldehyde for 12 hours.
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Fig. 10 FT-IR spectra of as-prepared Eu-NDC, Eu-NDC (treated with formaldehyde) and NDC.
(a)
(b)
Fig. 11 The ESI-MS of the residues from Eu-NDC before (a) and after (b) immersed in formaldehyde. Retreating condition: soaking the residues in DMF.
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Scheme 1 Schematic representation of the synthesis process of Eu-NDC@HPAN and the luminescence quenching phenomenon of HCHO to Eu-NDC@HPAN.
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