Reusable fluorescent photocrosslinked polymeric sensor for determining lead ions in aqueous media

    Reusable fluorescent photocrosslinked polymeric sensor for determining lead ions in aqueous media Soner C¸ubuk, Nes¸e Tas¸ci, Memet Vezir Kahraman, G¨ulay Bayramo˘glu, Ece K¨ok Yetimo˘glu PII: DOI: Reference:

S1386-1425(16)30050-6 doi: 10.1016/j.saa.2016.01.050 SAA 14260

To appear in: Received date: Revised date: Accepted date:

19 October 2015 11 January 2016 23 January 2016

Please cite this article as: Soner C ¸ ubuk, Ne¸se Ta¸sci, Memet Vezir Kahraman, G¨ ulay Bayramo˘ glu, Ece K¨ ok Yetimo˘glu, Reusable fluorescent photocrosslinked polymeric sensor for determining lead ions in aqueous media, (2016), doi: 10.1016/j.saa.2016.01.050

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lead ions in aqueous media

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Reusable fluorescent photocrosslinked polymeric sensor for determining

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Soner Çubuka*, Neşe Taşcia, Memet Vezir Kahramana*, Gülay Bayramoğlub, Ece Kök

Marmara University, Faculty of Art and Science, Chemistry Department, 34722 Istanbul, Turkey

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Yetimoğlua

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Yalova University, Faculty of Engineering, Department of Polymer Engineering, 77200 Yalova , Turkey

* Corresponding Authors:

Dr. Soner Çubuk

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Assoc. Prof. Dr. Memet Vezir Kahraman

E-mails: [email protected] [email protected]

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ACCEPTED MANUSCRIPT ABSTRACT In this study, 1-vinylimidazole units bearing photocured films were prepared as fluorescent

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sensors towards Pb2+ in aqueous solutions. The influence of experimental parameters such as

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pH, time and foreign ions concentrations were investigated. Sensor response was linear over a

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concentration range of 4. 83× 10-8 to 4. 83× 10-7 mol L-1. The sensor was highly sensitive with a detection limit as low as 1.87 × 10−8 mol L-1, and having a selectivity of over four thousands folds. The response time of the sensor was found to be 5 min. When stored in a desiccator at

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room temperature the sensor showed good stability after 5 months period. The fluorescence

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sensors were successful in the determination of Pb2+ in water samples as well as in the determination of the quantitative amount of lead and the results were satisfying. Compared

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with previously reported literature, the prepared new sensor is highly sensitive and selective.

Keywords: Fluorimetric sensor, photo-curing, selective lead (II) determination, vinyl

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imidazole.

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ACCEPTED MANUSCRIPT 1. Introduction Heavy metal ions such as lead (II) have long been identified as major toxic environmental

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pollutants [1]. Lead contamination of the environment, especially of drinking water and food

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chain, lead to serious problems due to its high toxicity and non-biodegradability. It is well

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known that low levels of lead exposure cause severe risks to human health such as convulsions, central brain damages, miscarriages, kidney damages, and even death [2, 3].

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Basic sources of these ions are; natural sources, mining and industrial applications which include pigments, anticorrosion coatings, batteries, alloys, etc. [4]. The maximum

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concentration of lead (II) is strictly regulated by European Union which is 1 mg L-1 for food and the level of Pb2+ in drinking water should be below 10 mg L-1 according to World Health

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Organization [5]. Because of these regulations and risks, detection of lead (II) in

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environmental or biological samples has become a crucial matter for researchers. Atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry (ICP-

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MS), fluorescence spectrometry and electrochemical techniques (such as ion-selective potentiometry and anodic stripping voltammetry) have been widely used methods for the

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determination of trace amount of metal ions [6]. However, the need of sufficient chemistry knowledge, the difficulty in learning the instruments, complicated pretreatment procedures, the high costs of the instruments and the long analysis times can be counted among the drawbacks for these methods. Therefore, there is a great demand for molecular recognition and sensing systems for the detection of small amounts of metal ions. Among these systems, fluorescent chemical sensors have attracted considerable attention for the detection of metal cations because of their good selectivity and high sensitivity [7]. For this reason, the fluorescent polymers have become very attractive. These polymers usually contain covalently bonded fluorophores units such as indole, imidazole groups on their backbone which behave similar to their monomeric sensor units [8, 9]. 3

ACCEPTED MANUSCRIPT Hereby, we report the preparation and analytical properties of imidazole groups bearing UV cured polymeric film, designed to act as a fluorescence chemo sensor for lead (II) analysis.

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The influence of pH value and time on the fluorescence intensity of the sensor have also been

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investigated and discussed.

2. Experimental

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2.1 Materials

1-vinylimidazole (VIM), poly(ethylene glycol) diacrylate (PEGDA) (Mn= 575 g mol-1) and

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trimethylolpropane triacrylate (TMPTA) were purchased from Sigma Aldrich. The photoinitiator, DMPA (2,2-dimethoxy-2-phenylacetophenone) was obtained from Across. All

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other reagents were purchased from Merck and were used without further purification. The

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water used in the experiments was purified by using a Milli Q-water purification system

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(Millipore, Incekaralar-Turkey). The resulting purified water has a resistivity of 18.2 MΩcm.

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2.2 Preparation of lead sensitive fluorescence sensor A polymeric fluorescence sensor was prepared by photopolymerization method. While poly(ethylene glycol) diacrylate was used as a base resin, trimethylolpropane triacrylate was used as a crosslinker. Due to the photopolymerizable vinylic group on the molecule, 1vinylimidazole was used as a reactive fluorophore. Besides, DMPA was used as a photoinitiator. 10% VIM, 70% PEGDA, 20% TMTPA and photoinitiator (3% of whole formulation) were mixed in a small beaker. To eliminate the dissolved oxygen, nitrogen gas was purged with formulation about 15 minutes. Then, formulation was poured into Teflon® molds (W × L × D: 12 mm × 50 mm × 1 mm) and cured under high pressure UV lamp (OSRAM 300 W,  max = 365 nm) for 3 min and then UV cured films were removed from

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ACCEPTED MANUSCRIPT the molds. In order to remove the unreacted monomers, cross-linked films were kept inside

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deionized water for 24 hours, then dried in a vacuum oven at 30 ºC to a constant weight.

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2.3 Characterization

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FT-IR spectrum was performed on Perkin Elmer Spectrum100 ATR-FTIR spectrophotometer between the range of 4000–650 cm−1 at room temperature with the 4 cm−1 resolution mode

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and 20 scans. Morphology of the polymeric films was investigated by scanning electron microscopy (SEM). Prior to the SEM analysis, dried sample was coated with platinum having

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a thickness of approximately 300 Å using an Edwards S 150 B sputter coater and the surface was observed by using a Philips XL30 ESEM-FEG/EDAX system. Fluorescence

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measurements were carried out by using Varian Cary Eclipse Spectrofluorometer. The pH

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values of the solutions were checked using a digital pH meter (WTW) calibrated with

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standard buffer solutions from Merck. All the experiments were carried out at room temperature: 25 ± 1 ºC.

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3. Results and Discussion 3.1. Characterization

3.1.1. FTIR spectroscopy

1-vinylimidazole (VIM), poly(ethylene glycol) diacrylate and trimethylolpropane triacrylate were cross-linked in the presence of photoinitiator by using a UV lamp. Scheme 1 demonstrates the preparation and structure of polymeric film. The structure of the obtained polymeric films was investigated by ATR-FTIR spectrometer. The ATR-FTIR spectrum of the film is given in Figure 1. The absorption peak at 1720 cm-1 is attributed to the stretching vibration of carbonyl groups of PEGDA and TMPTA. The disappearance of the vinyl stretching at 1040 cm-1 was taken as an indication of successful crosslinking of VIM. The 5

ACCEPTED MANUSCRIPT peak around 1635 cm-1 can be attributed to the C=C double bonds of acrylates, stating that not all of the acrylate groups have participated in the crosslinking process [10].

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Scheme 1

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Figure 1

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3.1.2. SEM Measurements

The surface morphology of the polymeric film is an important factor for the design of an

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efficient sensor. Homogeneity, porosity and surface conditions are the most important parameters, which influence the fluorescence efficiency. After the UV curing process, a

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macroscopically uniform semi-transparent film was obtained. Figure 2 demonstrates the SEM images of the Pb2+ sensing film. SEM images showed smooth surface without any phase

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separation, indicating the formation of good structural integrity.

3.2. Spectral Characterization

Fluorimetric experiments were carried out on a Varian Eclipse spectrofluorometer. The

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excitation and emission bandwidth slits were set at 5 nm. The fluorescence excitation and emission spectra of the sensor were carried out in the absence and presence of lead (II). The fluorescence spectra were recorded at excitation and emission peaks which were found at 375nm, 420 nm, respectively (Figure 3).

Figure 3

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3.3. Effect of pH

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The effects of pH on the fluorescence intensity in the presence of Pb 2+ were determined

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within a pH range of 3.0 to 6.0 with the concentration of Pb2+ fixed at 2.42×10-7 mol L-1. The

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effect of pH on the fluorescence intensity was shown in Figure 4. The polymeric sensor was pH sensitive because of the imidazole groups in the cross-linked structure. This is due to the

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fact that the imidazole ring is being known as amphoteric [10]. The results confirmed that there were considerable changes in their fluorescence intensity with respect to pH. The

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fluorescence intensity increased by increasing pH up to pH 4.5 where the maximum intensity was observed which may be caused by protonation of imidazole ring attached to the

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polymeric sensor. On the other hand, fluorescence intensity decreased when pH value was

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higher than pH 4.5 which reduced the complexation of Pb2+ with the sensor. These results

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indicate that the proposed polymeric sensor is convenient for applications of Pb2+ determination for acidic conditions. Therefore, pH 4.5 was chosen for all further

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experiments since the highest fluorescence intensity was obtained at this pH.

Figure 4

3.4. Response Time The sensor response time can be defined as the time required for a sensor output to change from its previous state to a final settled value within a tolerance band of the correct new value [11]. Since the response of the sensor is affected by pH and Pb2+ concentration, response time measurements were conducted at pH 4.5 in the presence of 2.42×10-7 mol L-1 Pb2+ at room temperature for a period of 600 seconds, and the fluorescence intensity was measured every 15 seconds. As shown in Figure 5, the relative fluorescence intensity first increased 7

ACCEPTED MANUSCRIPT constantly with a time. After reaching the maximum intensity at 300th second, a gradual

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decrease of fluorescence intensity was observed and finally reached a steady-state level.

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Figure 5

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3.5. Calibration range and detection limit

The influence of the Pb2+ concentration on the fluorescence intensities of the sensor is

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presented in Figure 6. The fluorescence quenching efficiencies (I-I0) of sensor was plotted (II0, where I and I0 are fluorescence intensity in the presence and absence of Pb2+, respectively)

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versus functions of the logarithm of Pb2+ concentrations at pH 4.5. The studied concentration

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range was 4.83× 10-8 - 4.83× 10-7 mol L-1 Pb2+. The solutions were prepared by the addition

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of appropriate amounts of standard Pb2+ solution to the 0.1 mol L-1 acetate buffer at pH 4.5. Fluorescence spectral changes of the sensor for a wide range of Pb2+ concentration are also presented in Figure 6. The fluorescence intensity of the sensor was clearly increased as a

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function of increasing concentration of Pb2+. Moreover, the detection limit of the method was found to be 1.87 × 10−8 mol L-1 (n = 7).

Figure 6

3.6. Interference of coexisting foreign ions

Since a sensor must recognize only targeted species in the presence of other substances, the recognition behavior of the sensor has become an important criteria. Therefore, a systematic study of the interferences of foreign ions in the selectivity of Pb2+ was conducted. Selectivity

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ACCEPTED MANUSCRIPT of the sensor was evaluated from the changes of the fluorescence intensity of the Pb2+ solution adjusted to 2.42×10-7 mol L-1, after adding foreign metal ions at various concentration levels.

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The results are presented in Table 1. The concentrations of the foreign ions are found to be

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between 461 and 4958 folds which are higher than Pb2+ ions. The Cd2+, Sb2+, Ni2+ and Hg2+

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can be tolerated at relatively high concentrations, but Ag+, Cr3+, Co2+ and Au+ can be allowed only at relatively low concentrations. The results revealed that the fluorescence sensor

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exhibits the exceptional specification for Pb2+ against most of the metal ions.

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Table 1

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3.7. Sample Determination

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The developed method was successfully applied to the direct fluorimetric determination of Pb2+ in tap water as a real sample. Table 2 shows that the analysis results from our method are

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compatible with those detected by ICP-MS method in the determination of Pb2+ in the tap water. Compared with the method mentioned above, it can be said that the proposed method

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for Pb2+ determination is easier, cheaper and more rapid.

Table 2

3.8. Reversibility, reproducibility, short term stability and life time The reproducibility and reversibility of any sensor have critical importance in evaluating the accuracy and suitability of its use in any measurements for the purpose of selective ions determination. The reversibility of the sensor was studied by fluorescence intensity measurements of the same sensor when it was exposed to 2.42×10-7 mol L-1 Pb2+ in buffer solution of pH 4.5, repeatedly. The results indicated that the sensor possess full reversibility. 9

ACCEPTED MANUSCRIPT It was also found that; sensor can be regenerated completely within 2 minutes after soaking in distilled water. After regeneration, for the next measurement, the membrane should be kept in

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the buffer solution of pH 4.5 for 2 min.

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The short-term stability of the sensor was evaluated by its fluorescence intensity in contact

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with a 2.42×10-7 mol L-1 Pb2+ in the buffer solution of pH 4.5 over a period of 10 hours. From the fluorescence intensities taken every 30 min (n= 7), it was found that the response is almost complete with a relative standard deviation of ±1.6 during 10 h monitoring. In addition, it was

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found that same sensor can be used without any significant change for at least 5 months,

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which implies that the proposed fluorescence sensor is very stable. To determine the repeatability of the sensor differences in the fluorescence response, 50 successive runs were made using a same gel. It was found that the response was almost the same between the first

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and fiftieth cycles with a low standard deviation (± 1.3). The results implied that the reproducibility of the sensor is satisfactory.

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The reproducibility of the sensor was investigated by using seven different polymeric films. All films were prepared under the same conditions with the same composition. As for

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reversibility and stability measurement, the fluorescence intensities of each sensor were measured in 2.42×10-7 mol L-1 Pb2+ in buffer solution of pH 4.5. According to the test results, the standard deviation of the fluorescence intensity of the sensors was found to be ±1.4.

3.9. Comparison of the proposed Pb (II) sensing film with other fluorescence sensors and other spectroscopic techniques Fluorescence sensors have been under development for many years and most reported ones are used for the detection of small amounts of cations such as Li+ , Na+ , K+ , Ca2+, Mg2+, Zn2+, Al3+, etc. Among other cations, detection of heavy metals such as Cd2+, Sb2+, Ni2+, Pb2+ and Hg2+ is known to be essential because of serious health issues. Since the development of 10

ACCEPTED MANUSCRIPT new methods that can easily detect Pb2+, high sensitivity and selectivity became the main interest and many researches have been performed. In literature there are many various

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fluorimetric methods that can be used for Pb2+ detection. The summarized data in Table 3,

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include the λ ex/em, working ranges, pHs, limits of detection, and response times of the lead

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sensors. According to the examples in Table 3, proposed Pb2+ sensitive sensor has relatively low detection limit value compared to the other fluorescence sensors in the literature [12-20]. Also the sensor can compete with the other previously reported fluorimetric methods with its

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response time and reusability.

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Other Pb2+ ion sensors based on spectroscopic [19], reflectance spectroscopic [20], voltammetric [21], cyclic voltammetric [22], potentiometric [23] have also been developed. When compared with the above mentioned works, the proposed sensor provides an improved

4. Conclusion

Table 3

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performance.

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A fully reversible fluorescence sensor has been developed. Highly selective fluorescent polymeric sensor for lead ions based on 1-vinyl imidazole was prepared by photopolymerization technique. The cross-linked polymeric sensor gained nondisposable characteristics and is fully reversible, as it can be easily regenerated with immersing in distillated water. The polymeric sensor showed high precision at wide Pb (II) concentrations (4.83× 10-8- 4.83× 10-7 mol L-1) and a detection limit of 1.87 × 10−8 mol L-1 (n = 7). The response time of the polymeric sensor was 300 seconds at pH 4.5. The sensor can be successfully applied to the determination of trace amounts of lead ions in aqueous solutions.

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crops grown on soils with elevated heavy metals content, Ecotoxicol. Environ. Saf. 118

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[2] C.D. Carrington, P.M. Bolger, Toxic Metals: Lead, in: Y. Motarjemi, G. Moy, E. Todd (Eds.), Encyclopedia of Food Safety Volume 2: Hazards and Diseases, Academic Press,

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Michigan, 2014, pp. 349–351.

[3] G. Flora, D. Gupta, A. Tiwari, Toxicity of lead: a review with recent updates, Interdiscip.

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lead using non-living biomass of marine algae, J. Hazard. Mater. 92 (2002) 253–262

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Geneva, 1993, pp.49-50.

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[5] J.K. Fawel, Guidelines for drinking water quality, second ed., vol.1: recommendations,

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ACCEPTED MANUSCRIPT [11] J. J. Carr, J.M. Brown, Introduction to biomedical equipment technology, fourth ed., Prentice Hall, Upper Saddle River NJ, 2001.

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[12] L. Marbella, B. Serli- Mitasev, P. Basu, Development of a fluorescent Pb2+ sensor,

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Angew. Chem. 48 (2009) 3996- 3998.

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[13] N. Aksuner, Development of a new fluorescent sensor based on a triazolo-thiadiazin derivative immobilized in polyvinyl chloride membrane for sensitive detection of lead(II) ions, Sens. Actuators, B 157 (2011) 162-168.

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Lippolis, A novel fluorimetric bulk optode membrane based on 5,8-bis((5′-chloro-8′hydroxy-7′-quinolinyl)methyl)-2,11-dithia-5,8-diaza-2,6-pyridinophane for selective detection of lead(II) ions, Talanta 80 (2010) 2023–2033.

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[21] Z.A. Tagar, Sirajuddin, N. Memon, M.H. Agheem, Y. Junejo, S.S. Hassan, N.H. Kalwar,

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with a long lifetime, Anal. Chem. 84 (2012) 134-140.

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Scheme.1. Chemical structure of the polymeric sensor.

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Fig. 1. FTIR spectrum of lead sensitive polymeric film.

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Fig. 2. SEM micrograph of lead sensitive polymeric film at 2500x (left), at 5000x (central)

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and at 10000x (right) magnifications.

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Fig. 3. Excitation and emission spectra of optical sensor in the (a) absence (line) and the (b)

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presence of a 2.42×10-7 mol L-1 Pb(II) (dot line); (λex= 375 nm, λem= 420 nm).

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Fig. 4. Effect of pH on fluorescence intensity of the Pb(II) sensing film.

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Fig. 5. Effect of time on fluorescence intensity of Pb(II) sensor during 10 min. (C= 2.42×10-7

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mol L-1 Pb(II), pH=4.5).

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Fig. 6. Fluorescence spectra of the sensing membrane in the presence of different concentration of Pb(II) at pH 4.5. a) 0 , (b) 4.8×10-8 mol L-1, (c) 1.2×10-7 mol L-1, (d) 2.4×10-7

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mol L-1, (e) 3.6×10-7 mol L-1, and (f) 4.8x10-7 mol L-1. The inset shows the calibration curve

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for determination of Pb(II) (λex/em = 375/420); (I– I0), where I0 and I are the fluorescence

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intensities before and after Pb(II) was added, respectively).

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Tolerance levela or upper limit tested (mol L-1)

Na+

2.54× 10-3

K+

2.12× 10-3

Ca2+

1.43× 10-3

Mg2+

1.55× 10-3

Cd2+

1.07× 10-3

Au3+

2.44× 10-4

Ag+

1.11× 10-4

Sb3+

9.88× 10-4

Ni2+

1.03× 10-3

Co2+

2.04× 10-4

Hg2+

1.20× 10-3

Cu2+

1.76× 10-3

Fe3+

1.21× 10-4

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1.38× 10-4

Zn2+

2.01× 10-4

Cr3+

1.16× 10-4

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Mn2+

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Species

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Table 1. Effect of foreign ions on the fluorescence intensity of 2.42×10-7 mol L-1 Pb(II) solution at optimum conditions.

Less than ±5% relative error.

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Pb(II) added

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ICP-MS

RSD

Recovery

samples

( mol L-1)

( mol L-1)

( mol L-1)

(%)

(%)

Tap water 1

5.00.10-8

(1.24±0.04).10-8

(1.22±0.05)10-8

1.61

102.7

Tap water 2

1.00.10-7

(2.47±0.04).10-7

(2.43±0.02).10-7

1.46

101.5

Tap water 3

1.50.10-7

(3.70±0.03).10-7

(3.65±0.04).10-7

1.19

101.2

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Water

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Table 2. Determination of Pb(II) ions in tap water samples with the proposed sensor.

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Table 3. Selected examples of fluorimetric lead sensors. λ ex/em (nm)

Ref (12)

389/ 423

4.83×10-9 to 2.42×10-7 mol L-1

Ref (13)

330/ 430

Ref (14)

pH or Medium

Interference

2.5% MeOH/ water NM

4.83×10-8 mol L-1

No interference

5×10-8 to 3.8×10-4 mol L-1

pH 5.5 BS

3 min

2.2 × 10−8 mol L-1

No interference

388/ 515

3.0× 10−7 to 2.5×10−2 mol L-1

pH 5.5 BS

< 5 min

2.0× 10−7 mol L−1

No interference

Ref (15)

298/ 355

1.9× 10−7 to 1.9×10−4 mol L-1

pH 9.00 BS

15 min

8.3 × 10−8 mol L−1

No interference

Ref (16)

280/ 359

5× 10-6 to 6× 10-5 mol L-1

pH 7.4 HEPES

NM

5 × 10−7 mol L-1

No interference

Ref (17)

312/ 562

0 to 0.6× 10-6 mol L-1 and

pH 7.2 BS

2 min

6× 10-8 mol L-1

No interference

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0.6× 10-6 to 2× 10-4 mol L-1

Response time

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Linear range

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LOD

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Fluorescence sensor

368/ 396

4.6× 10-5 to 8× 10-5 mol L-1

pH:7.4 HEPES

5 min

9.4× 10−7 mol L-1

No interference

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375/ 420

4.83× 10-8 to 4.83× 10-7 mol L-1

pH 4.50 BS

5 min

1.87 × 10−8 mol L-1

No interference

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BS: Buffer solution, NM: Not mentioned.

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Ref (18)

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Graphical abstract

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Highlights  A novel reusable polymeric fluorescence sensor for Pb(II) analysis was developed. Detection limit for Pb(II) was 1.87x10-8 mol L-1.

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This sensor was successfully employed to determine Pb2+ in aqueous media.

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Preparation of lead ions sensor is simple and quick.

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