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Highly selective silica-based fluorescent nanosensor for ferric ion (Fe3+) detection in aqueous media
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 218 (2019) 293–298
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Highly selective silica-based fluorescent nanosensor for ferric ion (Fe3+) detection in aqueous media Zeinab Salahshoor a, Jahan B. Ghasemi b, Afsaneh Shahbazi a,⁎, Alireza Badiei b a b
Environmental Sciences Research Institute, Shahid Beheshti University, G.C., Tehran 1983969411, Iran School of Chemistry, College of Science, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 14 January 2019 Received in revised form 20 March 2019 Accepted 31 March 2019 Available online 2 April 2019 Keywords: Fluorescence sensor Mesoporous silica Fe3+ Real samples
a b s t r a c t A highly selective fluorescence nanosensor was built based on the functionalized SBA-15 mesoporous silica with Dinitrophenylhydrazine (DNPH). The interaction of fabricated nanosensor was studied with metals ions in aqueous media. Under optimum conditions (time 5 min, pH 7, sensor dose 0.02 g dispersed in 1 L), SBA-15-DNPH was highly selective toward the Fe3+ ion in the presence of the different groups of interfering metal ions. The detection limit and linear concentration range of the nanosensor were 39 × 10−6 M and 5 × 10−5–375 × 10−5 M; respectively. The efficiency of nanosensor was independent on the pH range studied (6–9), which indicated that SBA-15-DNPH is a promising candidate for sensing and identification of Fe3+ ion in the environmental and other real matrix samples. SBA-15-DNPH showed good performance for Fe3+ ion detection and quantification in real samples. © 2019 Published by Elsevier B.V.
1. Introduction The development of fluorescence sensing for cations such as transition metal ions particularly needs consideration due to their possible analytical application in many fields such as chemistry, environment, biology and medicine [1]. Developing highly selective nanosensors with low detection limit for sensing ferric ion (Fe3+) in aqueous solutions has always been an attractive topic for those who value the significance of Fe3+ ion to the life sciences, mineral exploration and environmental sustainability. Fe3+ ion, as the most plentiful transition metal, is widely distributed in the environment and organism which plays noteworthy roles in oxygen uptake and metabolism, catalytic site in proteins and enzymes structure, electron transfer, and repair and synthesis of DNA. Hence, deficit or excess from the normal permissible limit can induce serious disorders and has a toxic and notable effect on organisms and environmental systems. Deficiency of Fe3+ ion leads to low oxygen delivery to cells, which cause diseases such as anemia, hemochromatosis and cancers. On the other hand, extra levels of Fe3+ ion will not only result in organ dysfunction, but also may leading to serious diseases, such as Parkinson's, Alzheimer's and Huntington's diseases [2,3]. Therefore, the development of effective sensing systems for qualitative and quantitative determination of Fe3+ ion is essential.
Fluorescent detection compared to other sensing methods, shows great privilege in its high selectivity, sensitivity, fast response and simplicity [4]. Literature is contained with significant number of fluorescent sensor and probes for Fe3+ ion detection [1–3,5–16]. However, many of these probes exhibit either one or two of the following features that limit their practical applications. In one hand, most of the reported Fe3+ probes cannot work in pure aqueous solution due to their hydrophobicity and low binding affinity. In the other hand, most of them usually have interference problems caused by other transition metal cations such as Cu2+, Co2+, Al3+ or Hg2+. Therefore, searching for highly selective fluorescent sensors for Fe3+ in 100% aqueous solutions are of great importance. Mesoporous silica is a form of silica which is recently used in nanotechnology. The most common type of mesoporous silica, SBA15, possesses large pore size, uniform channel and large surface area so it is an excellent support for constructing nanosensors. The straight channel of SBA-15 is beneficial for facilitating the entering and diffusion of the target ions [17,18]. Though SBA-15 by itself alone is not fluorescent, but it can supply substrate containing a lot of hydroxyl groups as serving the binding sites for grafting of fluorophore group [19]. Therefore, in the present work, DNPH-SBA-15 were successfully synthesized which were not reported before. DNPH-SBA-15 is also successfully applied to the determination of Fe3+ ion in real environmental samples. 2. Material and methods
⁎ Corresponding author. E-mail address: [email protected] (A. Shahbazi).
https://doi.org/10.1016/j.saa.2019.03.118 1386-1425/© 2019 Published by Elsevier B.V.
Mesoporous silica (SBA-15) was synthesized using our previous reported work [20,21]. Then the surface of SBA-15 was modified via
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Fig. 1. Synthetic route of the functionalization of SBA-15 with DNPH.
Fig. 2. (a) The SEM image of SBA-15; (b) powder XRD patterns of SBA-15, SBA-15-AAQ, and SBA-15-DNPH; (c) N2 adsorption–desorption isotherms of SBA-15, SBA-15-AAQ and SBA-15DNPH (inset, Panel c: the pore volume vs. pore diameter); (d) FT-IR spectra of as synthesized SBA-15, SBA-15-AAQ and SBA-15-DNPH.
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grafting method in two steps as outlined in Fig. 1 (more details of experiments are available in supporting information). The obtained SBA-15DNPH was characterized by different techniques such as XRD, N2 adsorption–desorption, SEM, and FTIR.
3. Results and discussion 3.1. Characterization The SEM images of SBA-15 (Fig. 2a) presented fairly uniform ropelike domains with an average size 1 μm which is in good agreement with the SBA-15 morphology reported in literature [22,23]. The XRD patterns of SBA-15, SBA-15-AAQ and SBA-15-DNPH are demonstrated in Fig. 2b. Three distinct reflections in the XRD pattern of the SBA-15, one intense peak at 2θ of 0.99 and two weak reflections at 2θ of 1.7 and 1.9, respectively indexed, as the (1 0 0), (1 1 0), and (2 0 0) which are related to the hexagonal structure of SBA-15 [24]. The XRD pattern was also preserved after modification with AAQ and DNPH which means the functionalization did not affect the crystal structural of the SBA-15. The intensities of the XRD peaks for SBA-15AAQ and SBA-15-DNPH are smaller than those measured for the SBA15, which is probably caused by the pore filling effect of the SBA-15 channels or the anchoring ligands on the outer surface of SBA-15 [25]. Fig. 2c shows pore size distributions and BET surface areas of nanosensor derived from the N2 adsorption–desorption isotherms data. It can be clearly observed that samples display a type IV nitrogen adsorption–desorption isotherm. According to the IUPAC nomenclature, type IV is characteristic of uniform mesoporous material [21,26]. Also, SBA-15-AAQ and SBA-15-DNPH showed a lower value for pore diameter and pore volume relative to SBA-15 (Fig. 2c, inset) which is due to the introduction of functional groups on the surface of SBA-15. The results of FT-IR spectroscopic analyses were shown in Fig. 2d. The characteristic bands at 480, 810 and 1080 cm−1 in all synthesis components are attributed to Si-O-Si vibration. The band at 1723 cm−1 is corresponding to C_O stretching. The significant bands appeared in the region of C_C at 1278 cm−1,\\CH at 2924 cm−1. The existence of 3430 cm−1 (N\\H and O\\H stretching), 1570 cm−1 (N\\H bending), 1505 cm−1 (N\\H bending) and 1569 cm−1 (aromatic C\\C vibration) in the FT-IR spectra of SBA-15 modified with AAQ reveal that this organic compound was successfully grafted on the surface of SBA-15. The presence of N\\O symmetric stretching (1324 cm−1) and NO2 groups (727 and 1395 cm−1) on the final product, was specified the SBA-15-AAQ was modified with DNPH.
3.2. Optimization of condition of fluorescence spectroscopy experiments The parameters including time (1–10 min), SBA-15-DNPH nanosensor amount (0.01–0.05 g), and pH of aqueous media (2–12) were optimized for the detection system. The stability of the fluorescence intensity of SBA-15-DNPH was confirmed by the results of time experiments. To ensure the rapid detection of metal ions, a time interval of 5 min is chosen as the optimum reaction time. This exposure time ensured that metal ions entered into the SBA-15-DNPH pores. The fluorescence intensity was increased from 250 to 765 when the amounts of the SBA-15-DNPH increased from 0.01 to 0.05 g. However, the sensitivity of detection decreased by increasing of the nanosensor amount. Hence, after a series of optimization steps, the amount of 0.02 g/L was chosen as the suitable nanosensor amount. The effect of pH on the fluorescence intensity showed that the fluorescence intensity was well related to the pH value of the media. The fluorescence intensity of the SBA-15-DNPH nanosensor reached the maximum value at the pH range of 6–9. According to the optimal pH of surface water, the optimum pH for subsequent studies was kept constant at 7.
Fig. 3. Fluorescence spectra (λmax = 300 nm) of SBA-15-DNPH (0.02 g) aqueous dispersion in the presence of different metal ions (Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd2+, Cs3+, Cu2+, Co2+, Al3+, Fe2+, Fe3+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+ (50 μL, 10−2 M)).
3.3. Study of metal ions sensing Detection of metal ions including Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd , Cs3+, Cu2+, Co2+, Al3+, Fe2+, Fe3+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+ (50 μL, 10−2 M) was accomplished by following of the changes in fluorescence intensity (PL signal) of an aqueous SBA-15DNPH suspension before and after addition of the each metal ion under optimum condition (time of 5 min, nanosensor amount 0.02 g and pH of 7). As shown in Fig. 3, upon the addition of various metal ions, no considerable changes in the position of fluorescence spectra of nanosensor SBA-15-DNPH were observed. However, the addition of Fe3+ ion creates significant quenching of SBA-15-DNPH fluorescence. Thus, the metal sensing studies indicated that SBA-15-DNPH was highly selective for Fe3+ ion which can be assigned to paramagnetic effect of Fe3+ ion compare to other metal ions [3,7]. The theoretical detection limit of SBA-15-DNPH, which is a significant index of merit of sensor applications, was evaluated according to the fluorescence titration curve (Fig. 4a). Upon stepwise addition of a Fe3+ solution to a 3 mL water suspension of SBA-15-DNPH (0.02 g/L), a reduction of the emission intensity was observed (Fig. 4). As it is shown in a Fig. 4b there is a good linear relationship between fluorescence intensity and molar concentration of Fe3+ ion in the range (5.00–375) × 10−5 M with a squared correlation coefficient of 0.99. The shape of the fluorescence spectra remained unchanged even in the presence of high Fe3+ ion concentration, which indicated that Fe3+ ion had not any transformation effect on the structure of SBA-15-DNPH [27]. Furthermore, the detection limit was estimated as 39 × 10−6 on the basis of DL = 3σ/m. The Binding constant of the Fe3+-SBA-15-DNPH complex was calculated by fitting of the fluorescence quenching data to a linear form of Benesi-Hildebrand in Eq. (2) [28]. 2+
I0 I I0 1 þ ¼ ðI0 −IÞ ½L ½LK S ½M
ð1Þ
where I0 is the fluorescence intensity of SBA-15-DNPH in the absence of Fe3+ ion, I is changing of fluorescence intensity in the presence of added Fe3+ ion, and KS is the stability constant. [L] and [M] are the molar concentration of SBA-15-DNPH and Fe3+ ion respectively. By plotting of 1/(I − I0) versus 1/[Fe3+], a linear relationship (Fig. 4c) was obtained
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Fig. 4. (a) Fluorescence response of SBA-15-DNPH (0.02 g) in the presence of various concentrations of Fe3+ ion (A: SBA-15-DNPH in free buffer, B: 5 × 10−5, C: 25 × 10−5, D: 50 × 10−5, E: 75 × 10−5, F: 100 × 10−5, G: 125 × 10−5, H: 150 × 10−5, I: 175 × 10−5, J: 225 × 10−5, K: 275 × 10−5, L: 325 × 10−5, M: 375 × 10−5 M of Fe3+ ion); inset: (b) relationship between fluorescence intensity upon gradual addition of Fe3+ ion; (c) Benesi–Hildebrand plots for SBA-15-DNPH in the presence of Fe3+ ion.
which indicates formation of a 1:1 host-guest complex [29]. The calculated binding constant between Fe3+ ion and SBA-15-DNPH was 1.25 × 102 M−1. 3.4. Fluorescence quenching mechanism The fluorescence quenching mechanism of SBA-15-DNPH in the presence of Fe3+ was further investigated by fluorometric titration data and corresponding plot according to the Stern-Volmer equation (Eq. (2)):
I0 ¼ 1 þ K q ½Q I
ð2Þ
where I0 and I are the fluorescence intensity in the absence and presence of quencher Fe3+ ion respectively, Kq is the Stern–Volmer constant representing the affinity between luminophore SBA-15-DNPH and quencher Fe3+ ion and [Q] is the molar concentration of Fe3+ ion. The Kq obtained from the slope of the plot of the fluorescence decreased efficiency (I0/I) vs. the concentration of Fe3+ ion. Plotting of the I0/I vs concentration of Fe3+ ion results in a linear plot (Fig. 5a). The linear plot indicated that the quenching mechanism is either purely static or dynamic. In order to understand the dominant mechanism of the interaction between SBA-15-DNPH nanosensor and Fe3+, the UV–Vis spectra of SBA-15-DNPH in the absence and presence of Fe3+ were investigated in Fig. 5b. The absorption spectrum of SBA-15-DNPH exhibits any band between 200 and 700 nm. The addition of Fe3+ produced pronounced hyperchromic effect, which may be ascribed to the newly formed
2.4
(b) SBA-15-DNPH
Absorbance
Io/I
1.7
1.4
1.1
0.8
0
0.1
0.2 [Fe3+]/10-4
0.3
SBA-15-DNPH+Fe3+
1.6
0.8
230
330
430 530 WaveLength(nm)
630
Fig. 5. (a) Stern-Volmer plot of SBA-15-DNPH in the presence of various concentration of Fe3+ ion; (b) absorption spectra of fluorescent nanosensor SBA-15-DNPH before and after addition of Fe3+ (100 μL of 10−2 M) in 3 mL of sensor probe in pH 7.
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complex between SBA-15-DNPH and Fe3+ and the absorption of Fe3+ in the UV–Vis region. Thus, the possible reason for the fluorescence quenching is the formation of a ground state complex between the nanosensor and Fe3+ and the mode of quenching is of static quenching [30]. The possible mode of Fe3+ binding is that Fe3+ coordinates with the functional groups from the SBA-15-DNPH, which lead to either an electron transfer or an electronic energy transfer [31]. 3.5. Effect of potential interfering metal ions To demonstrate the selectivity of SBA-15-DNPH nanosensor, the fluorescence response of SBA-15-DNPH to aliquots of Fe3+ ion (50 μL, 10−2 M) in the presence of other coexisting metal ions (50 μL, 10−2 M) including Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd2+, Cs3+, Cu2+, Co2+, Al3+, Fe2+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+ in aqueous media was investigated. As shown in Fig. 6, Fe3+ ion drastically decreases the fluorescence intensity of SBA-15-DNPH in the presence of other metal ions. This finding approves that not only the surface of SBA-15-DNPH nanosensor is capable to detect Fe3+ species but also it is highly selective toward Fe3+ ion over the other non-target metal ions. Fe3+ ion has a higher binding affinity and faster chelating kinetics with NO2 functional groups on the SBA-15-DNPH nanosensor than other metal ions [2]. The variations of the fluorescence intensity of SBA-15-DNPH nanosensor and Fe3+ ion interaction is not affected by the presence of the other metal ions which results in that the proposed nanosensor can easily be used in practical applications. 3.6. The effect of pH on the response of SBA-15-DNPH to Fe3+ ion Fluorescence response of nanosensor SBA-15-DNPH was investigated in the pH range of 2 to12 in the absence and presence of the Fe3 + ion by keeping the Fe3+ concentration at constant value of 50 μL of 10−2 M. As illustrated in Fig. 7 the proposed nanosensor exhibits the highest fluorescence intensity at region 6 b pH b 9. At the pH lower than 5, the fluorescence intensity of the SBA-15-DNPH decreased by decreasing the pH value, which might be due to the protonation of the SBA-15-DNPH at high acidity content.
Fig. 7. Effect of pH in range 2 to12 on the fluorescence intensity of the SBA-15-DNPH (0.02 g) and SBA-15-DNPH-Fe3+ (50 μL, 10−2 M).
As shown by Fig. 7 the nanosensor SBA-15-DNPH responds to Fe3+ ion over the pH range 2–12. Under the alkaline condition the partial precipitation of Fe(OH)3 occurred and apparent concentration of Fe3+ ion in the sample solution is decreased [30,32]. These results showed that SBA-15-DNPH is a pH-independent Fe3+-sensing in the range 6–9 of pH in aqueous media. Therefore, the proposed nanosensor can be successfully used in the environmental and the biological real samples. 3.7. Determination of Fe3+ ion in real samples To evaluate the practical applicability of SBA-15-DNPH nanosensor, it was used for the detection of Fe3+ ion in real aqueous samples. For this aim, two kinds of real aqueous and soil samples including spiked tap water and mineral samples were selected. The tap water sample was obtained from Tehran city, Iran and spiked with known concentration of Fe3+ ion without any pretreatment and mineral stones was obtained from Mahdiabad mine, Yazd, Iran. For preparation of mineral solution from stone samples, 0.1 g of powder was dissolved in HNO3 (3 mL, 85%) and HCl (9 mL, 37%) with stirring at 45 °C for 45 min. After cooling, the solution was transferred into a 50 mL volumetric flask and diluted to the mark with deionized water. Then it used for detection of Fe3+ ion under the optimum condition. The results (Table 1) were indicated that SBA-15-DNPH nanosensor is capable for determination of Fe3+ in the real environmental samples. 4. Conclusion The SBA-15-DNPH nanosensor, which can be dispersed in aqueous media, was synthesized via grafting approach. SBA-15-DNPH structure, physical properties and surface chemistry were studied by various characterization techniques including XRD, N2 adsorption-desorption, SEM and FTIR. Furthermore, the room temperature florescence response of SBA-15-DNPH was applied for detection of various metal ions in aqueous media. The SBA-15-DNPH showed high selectivity toward Fe3+ ion over the coexisting metal ions and the detection limit was obtained Table 1 Analytical results for detection of Fe3+ ion in real samples by the proposed method.
Fig. 6. Fluorescence spectra (λmax = 300 nm) of SBA-15-DNPH upon addition of Fe3+ ion in the presence of other coexisting metal ions (Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd2+, Cs3+, Cu2+, Co2+, Al3+, Fe2+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+) in aqueous media.
Sample
Spiked [×10−4 M]
Detected [×10−4 M]
Tap water Mineral stone
5 3
4.09 ± 0.02 3.8 ± 0.03
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39 × 10−6 M in aqueous media. Based on the Benesi-Hildebrand method, the binding constant of the formed complex between SBA15-DNPH and Fe3+ ion was 1.25 × 102 M−1. The quenching mechanism was studied and the complexing ratio between the SBA-15-DNPH and Fe3+was demonstrated to be 1:1. The experimental results revealed that the SBA-15-DNPH is insensitive against pH in the range 6–9. The experimental results showed the successful application of SBA-15DNPH for sensing and detection of ferric metal ion in real aqueous samples. Acknowledgements The authors gratefully acknowledge the financial support provided by the Shahid Beheshti University and the University of Tehran, Iran. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.03.118. References [1] L. Fu, et al., Selective and sensitive fluorescent turn-off chemosensors for Fe3+, Luminescence 28 (4) (2013) 602–606. [2] J. Afshani, et al., A single optical sensor with high sensitivity for detection of Fe3+ and CN− ions, J. Lumin. 179 (2016) 463–468. [3] N. Lashgari, A. Badiei, G.M. Ziarani, A novel functionalized nanoporous SBA-15 as a selective fluorescent sensor for the detection of multianalytes (Fe3+ and Cr2O72 −) in water, J. Phys. Chem. Solids 103 (2017) 238–248. [4] R. Atchudan, et al., Highly fluorescent nitrogen-doped carbon dots derived from Phyllanthus acidus utilized as a fluorescent probe for label-free selective detection of Fe3+ ions, live cell imaging and fluorescent ink, Biosens. Bioelectron. 99 (2018) 303–311. [5] A.J. Weerasinghe, et al., Single-and multiphoton turn-on fluorescent Fe3+ sensors based on bis(rhodamine), J. Phys. Chem. B 114 (29) (2010) 9413–9419. [6] S.-R. Liu, S.-P. Wu, New water-soluble highly selective fluorescent chemosensor for Fe(III) ions and its application to living cell imaging, Sensors Actuators B Chem. 171 (2012) 1110–1116. [7] G. Shiravand, A. Badiei, G.M. Ziarani, Carboxyl-rich g-C3N4 nanoparticles: synthesis, characterization and their application for selective fluorescence sensing of Hg2+ and Fe3+ in aqueous media, Sensors Actuators B Chem. 242 (2017) 244–252. [8] L. Tarazi, et al., Investigation of the spectral properties of a squarylium near-infrared dye and its complexation with Fe(III) and Co(II) ions, Spectrochim. Acta A Mol. Biomol. Spectrosc. 58 (2) (2002) 257–264. [9] J. Mao, et al., Tuning the selectivity of two chemosensors to Fe(III) and Cr(III), Org. Lett. 9 (22) (2007) 4567–4570. [10] S. Ghosh, R. Chakrabarty, P.S. Mukherjee, Design, synthesis, and characterizations of a series of Pt4 macrocycles and fluorescent sensing of Fe3+/Cu2+/Ni2+ through metal coordination, Inorg. Chem. 48 (2) (2008) 549–556. [11] B. Ma, S. Wu, F. Zeng, Reusable polymer film chemosensor for ratiometric fluorescence sensing in aqueous media, Sensors Actuators B Chem. 145 (1) (2010) 451–456. [12] Q. Meng, et al., Novel chitosan-based fluorescent materials for the selective detection and adsorption of Fe3+ in water and consequent bio-imaging applications, Talanta 97 (2012) 456–461.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Highly selective silica-based fluorescent nanosensor for ferric ion (Fe3+) detection in aqueous media Zeinab Salahshoor a, Jahan B. Ghasemi b, Afsaneh Shahbazi a,⁎, Alireza Badiei b a b
Environmental Sciences Research Institute, Shahid Beheshti University, G.C., Tehran 1983969411, Iran School of Chemistry, College of Science, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 14 January 2019 Received in revised form 20 March 2019 Accepted 31 March 2019 Available online 2 April 2019 Keywords: Fluorescence sensor Mesoporous silica Fe3+ Real samples
a b s t r a c t A highly selective fluorescence nanosensor was built based on the functionalized SBA-15 mesoporous silica with Dinitrophenylhydrazine (DNPH). The interaction of fabricated nanosensor was studied with metals ions in aqueous media. Under optimum conditions (time 5 min, pH 7, sensor dose 0.02 g dispersed in 1 L), SBA-15-DNPH was highly selective toward the Fe3+ ion in the presence of the different groups of interfering metal ions. The detection limit and linear concentration range of the nanosensor were 39 × 10−6 M and 5 × 10−5–375 × 10−5 M; respectively. The efficiency of nanosensor was independent on the pH range studied (6–9), which indicated that SBA-15-DNPH is a promising candidate for sensing and identification of Fe3+ ion in the environmental and other real matrix samples. SBA-15-DNPH showed good performance for Fe3+ ion detection and quantification in real samples. © 2019 Published by Elsevier B.V.
1. Introduction The development of fluorescence sensing for cations such as transition metal ions particularly needs consideration due to their possible analytical application in many fields such as chemistry, environment, biology and medicine [1]. Developing highly selective nanosensors with low detection limit for sensing ferric ion (Fe3+) in aqueous solutions has always been an attractive topic for those who value the significance of Fe3+ ion to the life sciences, mineral exploration and environmental sustainability. Fe3+ ion, as the most plentiful transition metal, is widely distributed in the environment and organism which plays noteworthy roles in oxygen uptake and metabolism, catalytic site in proteins and enzymes structure, electron transfer, and repair and synthesis of DNA. Hence, deficit or excess from the normal permissible limit can induce serious disorders and has a toxic and notable effect on organisms and environmental systems. Deficiency of Fe3+ ion leads to low oxygen delivery to cells, which cause diseases such as anemia, hemochromatosis and cancers. On the other hand, extra levels of Fe3+ ion will not only result in organ dysfunction, but also may leading to serious diseases, such as Parkinson's, Alzheimer's and Huntington's diseases [2,3]. Therefore, the development of effective sensing systems for qualitative and quantitative determination of Fe3+ ion is essential.
Fluorescent detection compared to other sensing methods, shows great privilege in its high selectivity, sensitivity, fast response and simplicity [4]. Literature is contained with significant number of fluorescent sensor and probes for Fe3+ ion detection [1–3,5–16]. However, many of these probes exhibit either one or two of the following features that limit their practical applications. In one hand, most of the reported Fe3+ probes cannot work in pure aqueous solution due to their hydrophobicity and low binding affinity. In the other hand, most of them usually have interference problems caused by other transition metal cations such as Cu2+, Co2+, Al3+ or Hg2+. Therefore, searching for highly selective fluorescent sensors for Fe3+ in 100% aqueous solutions are of great importance. Mesoporous silica is a form of silica which is recently used in nanotechnology. The most common type of mesoporous silica, SBA15, possesses large pore size, uniform channel and large surface area so it is an excellent support for constructing nanosensors. The straight channel of SBA-15 is beneficial for facilitating the entering and diffusion of the target ions [17,18]. Though SBA-15 by itself alone is not fluorescent, but it can supply substrate containing a lot of hydroxyl groups as serving the binding sites for grafting of fluorophore group [19]. Therefore, in the present work, DNPH-SBA-15 were successfully synthesized which were not reported before. DNPH-SBA-15 is also successfully applied to the determination of Fe3+ ion in real environmental samples. 2. Material and methods
⁎ Corresponding author. E-mail address: [email protected] (A. Shahbazi).
https://doi.org/10.1016/j.saa.2019.03.118 1386-1425/© 2019 Published by Elsevier B.V.
Mesoporous silica (SBA-15) was synthesized using our previous reported work [20,21]. Then the surface of SBA-15 was modified via
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Fig. 1. Synthetic route of the functionalization of SBA-15 with DNPH.
Fig. 2. (a) The SEM image of SBA-15; (b) powder XRD patterns of SBA-15, SBA-15-AAQ, and SBA-15-DNPH; (c) N2 adsorption–desorption isotherms of SBA-15, SBA-15-AAQ and SBA-15DNPH (inset, Panel c: the pore volume vs. pore diameter); (d) FT-IR spectra of as synthesized SBA-15, SBA-15-AAQ and SBA-15-DNPH.
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grafting method in two steps as outlined in Fig. 1 (more details of experiments are available in supporting information). The obtained SBA-15DNPH was characterized by different techniques such as XRD, N2 adsorption–desorption, SEM, and FTIR.
3. Results and discussion 3.1. Characterization The SEM images of SBA-15 (Fig. 2a) presented fairly uniform ropelike domains with an average size 1 μm which is in good agreement with the SBA-15 morphology reported in literature [22,23]. The XRD patterns of SBA-15, SBA-15-AAQ and SBA-15-DNPH are demonstrated in Fig. 2b. Three distinct reflections in the XRD pattern of the SBA-15, one intense peak at 2θ of 0.99 and two weak reflections at 2θ of 1.7 and 1.9, respectively indexed, as the (1 0 0), (1 1 0), and (2 0 0) which are related to the hexagonal structure of SBA-15 [24]. The XRD pattern was also preserved after modification with AAQ and DNPH which means the functionalization did not affect the crystal structural of the SBA-15. The intensities of the XRD peaks for SBA-15AAQ and SBA-15-DNPH are smaller than those measured for the SBA15, which is probably caused by the pore filling effect of the SBA-15 channels or the anchoring ligands on the outer surface of SBA-15 [25]. Fig. 2c shows pore size distributions and BET surface areas of nanosensor derived from the N2 adsorption–desorption isotherms data. It can be clearly observed that samples display a type IV nitrogen adsorption–desorption isotherm. According to the IUPAC nomenclature, type IV is characteristic of uniform mesoporous material [21,26]. Also, SBA-15-AAQ and SBA-15-DNPH showed a lower value for pore diameter and pore volume relative to SBA-15 (Fig. 2c, inset) which is due to the introduction of functional groups on the surface of SBA-15. The results of FT-IR spectroscopic analyses were shown in Fig. 2d. The characteristic bands at 480, 810 and 1080 cm−1 in all synthesis components are attributed to Si-O-Si vibration. The band at 1723 cm−1 is corresponding to C_O stretching. The significant bands appeared in the region of C_C at 1278 cm−1,\\CH at 2924 cm−1. The existence of 3430 cm−1 (N\\H and O\\H stretching), 1570 cm−1 (N\\H bending), 1505 cm−1 (N\\H bending) and 1569 cm−1 (aromatic C\\C vibration) in the FT-IR spectra of SBA-15 modified with AAQ reveal that this organic compound was successfully grafted on the surface of SBA-15. The presence of N\\O symmetric stretching (1324 cm−1) and NO2 groups (727 and 1395 cm−1) on the final product, was specified the SBA-15-AAQ was modified with DNPH.
3.2. Optimization of condition of fluorescence spectroscopy experiments The parameters including time (1–10 min), SBA-15-DNPH nanosensor amount (0.01–0.05 g), and pH of aqueous media (2–12) were optimized for the detection system. The stability of the fluorescence intensity of SBA-15-DNPH was confirmed by the results of time experiments. To ensure the rapid detection of metal ions, a time interval of 5 min is chosen as the optimum reaction time. This exposure time ensured that metal ions entered into the SBA-15-DNPH pores. The fluorescence intensity was increased from 250 to 765 when the amounts of the SBA-15-DNPH increased from 0.01 to 0.05 g. However, the sensitivity of detection decreased by increasing of the nanosensor amount. Hence, after a series of optimization steps, the amount of 0.02 g/L was chosen as the suitable nanosensor amount. The effect of pH on the fluorescence intensity showed that the fluorescence intensity was well related to the pH value of the media. The fluorescence intensity of the SBA-15-DNPH nanosensor reached the maximum value at the pH range of 6–9. According to the optimal pH of surface water, the optimum pH for subsequent studies was kept constant at 7.
Fig. 3. Fluorescence spectra (λmax = 300 nm) of SBA-15-DNPH (0.02 g) aqueous dispersion in the presence of different metal ions (Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd2+, Cs3+, Cu2+, Co2+, Al3+, Fe2+, Fe3+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+ (50 μL, 10−2 M)).
3.3. Study of metal ions sensing Detection of metal ions including Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd , Cs3+, Cu2+, Co2+, Al3+, Fe2+, Fe3+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+ (50 μL, 10−2 M) was accomplished by following of the changes in fluorescence intensity (PL signal) of an aqueous SBA-15DNPH suspension before and after addition of the each metal ion under optimum condition (time of 5 min, nanosensor amount 0.02 g and pH of 7). As shown in Fig. 3, upon the addition of various metal ions, no considerable changes in the position of fluorescence spectra of nanosensor SBA-15-DNPH were observed. However, the addition of Fe3+ ion creates significant quenching of SBA-15-DNPH fluorescence. Thus, the metal sensing studies indicated that SBA-15-DNPH was highly selective for Fe3+ ion which can be assigned to paramagnetic effect of Fe3+ ion compare to other metal ions [3,7]. The theoretical detection limit of SBA-15-DNPH, which is a significant index of merit of sensor applications, was evaluated according to the fluorescence titration curve (Fig. 4a). Upon stepwise addition of a Fe3+ solution to a 3 mL water suspension of SBA-15-DNPH (0.02 g/L), a reduction of the emission intensity was observed (Fig. 4). As it is shown in a Fig. 4b there is a good linear relationship between fluorescence intensity and molar concentration of Fe3+ ion in the range (5.00–375) × 10−5 M with a squared correlation coefficient of 0.99. The shape of the fluorescence spectra remained unchanged even in the presence of high Fe3+ ion concentration, which indicated that Fe3+ ion had not any transformation effect on the structure of SBA-15-DNPH [27]. Furthermore, the detection limit was estimated as 39 × 10−6 on the basis of DL = 3σ/m. The Binding constant of the Fe3+-SBA-15-DNPH complex was calculated by fitting of the fluorescence quenching data to a linear form of Benesi-Hildebrand in Eq. (2) [28]. 2+
I0 I I0 1 þ ¼ ðI0 −IÞ ½L ½LK S ½M
ð1Þ
where I0 is the fluorescence intensity of SBA-15-DNPH in the absence of Fe3+ ion, I is changing of fluorescence intensity in the presence of added Fe3+ ion, and KS is the stability constant. [L] and [M] are the molar concentration of SBA-15-DNPH and Fe3+ ion respectively. By plotting of 1/(I − I0) versus 1/[Fe3+], a linear relationship (Fig. 4c) was obtained
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Fig. 4. (a) Fluorescence response of SBA-15-DNPH (0.02 g) in the presence of various concentrations of Fe3+ ion (A: SBA-15-DNPH in free buffer, B: 5 × 10−5, C: 25 × 10−5, D: 50 × 10−5, E: 75 × 10−5, F: 100 × 10−5, G: 125 × 10−5, H: 150 × 10−5, I: 175 × 10−5, J: 225 × 10−5, K: 275 × 10−5, L: 325 × 10−5, M: 375 × 10−5 M of Fe3+ ion); inset: (b) relationship between fluorescence intensity upon gradual addition of Fe3+ ion; (c) Benesi–Hildebrand plots for SBA-15-DNPH in the presence of Fe3+ ion.
which indicates formation of a 1:1 host-guest complex [29]. The calculated binding constant between Fe3+ ion and SBA-15-DNPH was 1.25 × 102 M−1. 3.4. Fluorescence quenching mechanism The fluorescence quenching mechanism of SBA-15-DNPH in the presence of Fe3+ was further investigated by fluorometric titration data and corresponding plot according to the Stern-Volmer equation (Eq. (2)):
I0 ¼ 1 þ K q ½Q I
ð2Þ
where I0 and I are the fluorescence intensity in the absence and presence of quencher Fe3+ ion respectively, Kq is the Stern–Volmer constant representing the affinity between luminophore SBA-15-DNPH and quencher Fe3+ ion and [Q] is the molar concentration of Fe3+ ion. The Kq obtained from the slope of the plot of the fluorescence decreased efficiency (I0/I) vs. the concentration of Fe3+ ion. Plotting of the I0/I vs concentration of Fe3+ ion results in a linear plot (Fig. 5a). The linear plot indicated that the quenching mechanism is either purely static or dynamic. In order to understand the dominant mechanism of the interaction between SBA-15-DNPH nanosensor and Fe3+, the UV–Vis spectra of SBA-15-DNPH in the absence and presence of Fe3+ were investigated in Fig. 5b. The absorption spectrum of SBA-15-DNPH exhibits any band between 200 and 700 nm. The addition of Fe3+ produced pronounced hyperchromic effect, which may be ascribed to the newly formed
2.4
(b) SBA-15-DNPH
Absorbance
Io/I
1.7
1.4
1.1
0.8
0
0.1
0.2 [Fe3+]/10-4
0.3
SBA-15-DNPH+Fe3+
1.6
0.8
230
330
430 530 WaveLength(nm)
630
Fig. 5. (a) Stern-Volmer plot of SBA-15-DNPH in the presence of various concentration of Fe3+ ion; (b) absorption spectra of fluorescent nanosensor SBA-15-DNPH before and after addition of Fe3+ (100 μL of 10−2 M) in 3 mL of sensor probe in pH 7.
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complex between SBA-15-DNPH and Fe3+ and the absorption of Fe3+ in the UV–Vis region. Thus, the possible reason for the fluorescence quenching is the formation of a ground state complex between the nanosensor and Fe3+ and the mode of quenching is of static quenching [30]. The possible mode of Fe3+ binding is that Fe3+ coordinates with the functional groups from the SBA-15-DNPH, which lead to either an electron transfer or an electronic energy transfer [31]. 3.5. Effect of potential interfering metal ions To demonstrate the selectivity of SBA-15-DNPH nanosensor, the fluorescence response of SBA-15-DNPH to aliquots of Fe3+ ion (50 μL, 10−2 M) in the presence of other coexisting metal ions (50 μL, 10−2 M) including Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd2+, Cs3+, Cu2+, Co2+, Al3+, Fe2+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+ in aqueous media was investigated. As shown in Fig. 6, Fe3+ ion drastically decreases the fluorescence intensity of SBA-15-DNPH in the presence of other metal ions. This finding approves that not only the surface of SBA-15-DNPH nanosensor is capable to detect Fe3+ species but also it is highly selective toward Fe3+ ion over the other non-target metal ions. Fe3+ ion has a higher binding affinity and faster chelating kinetics with NO2 functional groups on the SBA-15-DNPH nanosensor than other metal ions [2]. The variations of the fluorescence intensity of SBA-15-DNPH nanosensor and Fe3+ ion interaction is not affected by the presence of the other metal ions which results in that the proposed nanosensor can easily be used in practical applications. 3.6. The effect of pH on the response of SBA-15-DNPH to Fe3+ ion Fluorescence response of nanosensor SBA-15-DNPH was investigated in the pH range of 2 to12 in the absence and presence of the Fe3 + ion by keeping the Fe3+ concentration at constant value of 50 μL of 10−2 M. As illustrated in Fig. 7 the proposed nanosensor exhibits the highest fluorescence intensity at region 6 b pH b 9. At the pH lower than 5, the fluorescence intensity of the SBA-15-DNPH decreased by decreasing the pH value, which might be due to the protonation of the SBA-15-DNPH at high acidity content.
Fig. 7. Effect of pH in range 2 to12 on the fluorescence intensity of the SBA-15-DNPH (0.02 g) and SBA-15-DNPH-Fe3+ (50 μL, 10−2 M).
As shown by Fig. 7 the nanosensor SBA-15-DNPH responds to Fe3+ ion over the pH range 2–12. Under the alkaline condition the partial precipitation of Fe(OH)3 occurred and apparent concentration of Fe3+ ion in the sample solution is decreased [30,32]. These results showed that SBA-15-DNPH is a pH-independent Fe3+-sensing in the range 6–9 of pH in aqueous media. Therefore, the proposed nanosensor can be successfully used in the environmental and the biological real samples. 3.7. Determination of Fe3+ ion in real samples To evaluate the practical applicability of SBA-15-DNPH nanosensor, it was used for the detection of Fe3+ ion in real aqueous samples. For this aim, two kinds of real aqueous and soil samples including spiked tap water and mineral samples were selected. The tap water sample was obtained from Tehran city, Iran and spiked with known concentration of Fe3+ ion without any pretreatment and mineral stones was obtained from Mahdiabad mine, Yazd, Iran. For preparation of mineral solution from stone samples, 0.1 g of powder was dissolved in HNO3 (3 mL, 85%) and HCl (9 mL, 37%) with stirring at 45 °C for 45 min. After cooling, the solution was transferred into a 50 mL volumetric flask and diluted to the mark with deionized water. Then it used for detection of Fe3+ ion under the optimum condition. The results (Table 1) were indicated that SBA-15-DNPH nanosensor is capable for determination of Fe3+ in the real environmental samples. 4. Conclusion The SBA-15-DNPH nanosensor, which can be dispersed in aqueous media, was synthesized via grafting approach. SBA-15-DNPH structure, physical properties and surface chemistry were studied by various characterization techniques including XRD, N2 adsorption-desorption, SEM and FTIR. Furthermore, the room temperature florescence response of SBA-15-DNPH was applied for detection of various metal ions in aqueous media. The SBA-15-DNPH showed high selectivity toward Fe3+ ion over the coexisting metal ions and the detection limit was obtained Table 1 Analytical results for detection of Fe3+ ion in real samples by the proposed method.
Fig. 6. Fluorescence spectra (λmax = 300 nm) of SBA-15-DNPH upon addition of Fe3+ ion in the presence of other coexisting metal ions (Hg2+, Zn2+, Ni2+, Cr3+, Pb2+, Cd2+, Cs3+, Cu2+, Co2+, Al3+, Fe2+, Na+, K+, Ca2+, Li+, Ba2+, Mg2+, Sr2+) in aqueous media.
Sample
Spiked [×10−4 M]
Detected [×10−4 M]
Tap water Mineral stone
5 3
4.09 ± 0.02 3.8 ± 0.03
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39 × 10−6 M in aqueous media. Based on the Benesi-Hildebrand method, the binding constant of the formed complex between SBA15-DNPH and Fe3+ ion was 1.25 × 102 M−1. The quenching mechanism was studied and the complexing ratio between the SBA-15-DNPH and Fe3+was demonstrated to be 1:1. The experimental results revealed that the SBA-15-DNPH is insensitive against pH in the range 6–9. The experimental results showed the successful application of SBA-15DNPH for sensing and detection of ferric metal ion in real aqueous samples. Acknowledgements The authors gratefully acknowledge the financial support provided by the Shahid Beheshti University and the University of Tehran, Iran. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.03.118. References [1] L. Fu, et al., Selective and sensitive fluorescent turn-off chemosensors for Fe3+, Luminescence 28 (4) (2013) 602–606. [2] J. Afshani, et al., A single optical sensor with high sensitivity for detection of Fe3+ and CN− ions, J. Lumin. 179 (2016) 463–468. [3] N. Lashgari, A. Badiei, G.M. Ziarani, A novel functionalized nanoporous SBA-15 as a selective fluorescent sensor for the detection of multianalytes (Fe3+ and Cr2O72 −) in water, J. Phys. Chem. Solids 103 (2017) 238–248. [4] R. Atchudan, et al., Highly fluorescent nitrogen-doped carbon dots derived from Phyllanthus acidus utilized as a fluorescent probe for label-free selective detection of Fe3+ ions, live cell imaging and fluorescent ink, Biosens. Bioelectron. 99 (2018) 303–311. [5] A.J. Weerasinghe, et al., Single-and multiphoton turn-on fluorescent Fe3+ sensors based on bis(rhodamine), J. Phys. Chem. B 114 (29) (2010) 9413–9419. [6] S.-R. Liu, S.-P. Wu, New water-soluble highly selective fluorescent chemosensor for Fe(III) ions and its application to living cell imaging, Sensors Actuators B Chem. 171 (2012) 1110–1116. [7] G. Shiravand, A. Badiei, G.M. Ziarani, Carboxyl-rich g-C3N4 nanoparticles: synthesis, characterization and their application for selective fluorescence sensing of Hg2+ and Fe3+ in aqueous media, Sensors Actuators B Chem. 242 (2017) 244–252. [8] L. Tarazi, et al., Investigation of the spectral properties of a squarylium near-infrared dye and its complexation with Fe(III) and Co(II) ions, Spectrochim. Acta A Mol. Biomol. Spectrosc. 58 (2) (2002) 257–264. [9] J. Mao, et al., Tuning the selectivity of two chemosensors to Fe(III) and Cr(III), Org. Lett. 9 (22) (2007) 4567–4570. [10] S. Ghosh, R. Chakrabarty, P.S. Mukherjee, Design, synthesis, and characterizations of a series of Pt4 macrocycles and fluorescent sensing of Fe3+/Cu2+/Ni2+ through metal coordination, Inorg. Chem. 48 (2) (2008) 549–556. [11] B. Ma, S. Wu, F. Zeng, Reusable polymer film chemosensor for ratiometric fluorescence sensing in aqueous media, Sensors Actuators B Chem. 145 (1) (2010) 451–456. [12] Q. Meng, et al., Novel chitosan-based fluorescent materials for the selective detection and adsorption of Fe3+ in water and consequent bio-imaging applications, Talanta 97 (2012) 456–461.
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