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Dual emission and pH based naphthalimide derivative fluorescent sensor for the detection of Bi3+
Sensors and Actuators B 247 (2017) 632–640
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Dual emission and pH based naphthalimide derivative fluorescent sensor for the detection of Bi3+ Ramasamy Kavitha b , Thambusamy Stalin a,∗ a b
Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, Tamilnadu, India Department of Chemistry, Mahendra College of Engineering, Salem 636 106, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 10 August 2016 Received in revised form 9 March 2017 Accepted 10 March 2017 Available online 14 March 2017 Keywords: Fluorescent chemosensor Bi3+ -Cyclodextrin Naphthalimide pH probe
a b s t r a c t This paper describes a novel dual emission N-hydroxy 1,8-naphthalimide (NHN) sensor for selective determination of Bi3+ ion and pH. This chemosensor displayed a red shift in absorption spectra and fluorescence enhancement towards highly selective Bi3+ ion in neutral aqueous solution. The binding stoichiometry of NHN with Bi3+ have been determined to be 1:1 by the job’s plot and the binding constant is calculated by Benesi-Hildebrand relation. The effect of -cyclodextrin (-CD) on NHN is also studied at pH ∼ 7. The binding mode of NHN with Bi3+ through carbonyl and the hydroxyl group of NHN is confirmed by FT-IR spectra. The influence of hydroxyl group in naphthalimide as a function of pH and metal ions was studied by UV–vis and fluorescence spectroscopy. Fluorescence emission of naphthalimide varied in a narrow range of pH (5–9) and was only slightly influenced by the addition of metal ions in aqueous solution. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, much attention has been paid to supramolecular fluorescent systems for the sensing and reporting of cations which may be of importance for chemical, biological and environmental sciences [1–5]. The determination of pH has great interest in different areas such as environmental monitoring, clinical analysis, control and monitoring of biochemical reactions, industrial processes, development of new pharmaceutical products and scientific researches [6–10]. Different methods have been developed to monitor and to determine pH values, including electrochemistry [11], nuclear magnetic resonance [12], absorbance spectroscopy [13] and fluorescence probe analysis [14]. Among all of these methods, measurement of pH by fluorescence-based techniques has important implications in analytical and biological chemistry. Development of fluorescent pH sensing probes for both imaging and sensing applications has received much attention in the current literature [15]. A handful of detection schemes exploiting proton binding induced perturbations of de-excitation pathways such as photo induced electron transfer (PET) [15a,b], intramolecular charge transfer (ICT) [15e], Forster resonance energy transfer (FRET) [15c,d] have been used for the development of pH sensing
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Thambusamy). http://dx.doi.org/10.1016/j.snb.2017.03.043 0925-4005/© 2017 Elsevier B.V. All rights reserved.
probes. The photo induced electron transfer (PET) process has been widely exploited for the sensing of cations as well as anions [16]. Generally, PET sensors based on nitrogen donors are highly sensitive to environmental pH stimuli, because the degree of nitrogen protonation is strongly pH dependent. On the other hand, the heightened concern for environmentally and biologically relevant species such as Bi3+ , Al3+ , Pb2+ , Hg2+ and Cu2+ , has been active research on the potential impact of their toxic effects. In particular, bismuth compounds have application in semiconductors, cosmetic preparations, metallurgy and alloy industry, iron castings, electronics, lubricating oils and greases, pigments, medicines for treatment of helicobacter pyloric- induced gastritis, contact lens cleaning solution, nuclear reactor cooling fluids and reagent for purification of sugar [17–19]. On the other hand, a number of toxic effects in humans have been attributed to bismuth compounds such as nephropathy, osteoarthropathy, hepatitis and neuropathology [20]. As the use of bismuth in medicine increases, it has spread in the environment and the chance of exposure of organisms to bismuth has increased. Various methods have been evolved for the determination of bismuth. These include hydride generation inductively-coupled plasma (HG-ICPAES), electro thermal vaporization ICP mass spectrometry (ETV-ICP-MS), atomic absorption spectrometry (AAS), potentiometric stripping analysis (PSA), anodic stripping voltammetry and cathodic stripping voltammetry (CSV) [21–26]. However, most of these methods suffer from either widespread availability of instrumentation, prohibitive cost,
K. Ramasamy, S. Thambusamy / Sensors and Actuators B 247 (2017) 632–640
or technical. Because of these considerations, spectrofluorometric seems attractive for the determination of bismuth in several samples. The number of special fluorophores sensitive to pH or metal ions with dual excitation or emission wavelength is limited. Derivatives of naphthalimides have attracted the attention for this purpose by their fluorescent characteristics. Naphthalimides and their derivatives are well-known, and they demonstrate a wide variety of applications due to properties such as fluorescence, electro activity and photo stability. The 1,8-naphthalimide moiety has also been used to design fluorescent molecular probe sensitive to variation in pH or metal ion concentration [27]. Meanwhile, the photoreactions of naphthalimide have been investigated in the past decade by Kubo and coworkers [28]. Now a day the new sensors based on naphthalimide have been reported for the determination of cations [29–31], metal ions and protons [32], pH [33–35] and anions [36–39]. Sensors for the determination of metal ions and pH based on mono and bis-1,8-naphthalimides showing PET (photo induced electron transfer) effect also have been developed [40,41]. To our knowledge, there are no reports on the development of Nhydroxy1,8-naphthalimide sensor based on the measurement of fluorescence intensity in function of pH and metal ion. Based on these considerations, we present in this article a novel dual emission sensor towards pH and metal ion based on PET mechanism. The pH detection with this kind of probe in the literature are generally troubled with the insufficient water solubility and the inference from the coexist cations. To overcome the poor solubility of the probe, it is used along with -Cyclodextrin, which enhances the solubility of the probe. The precise structure of -cyclodextrin (CD) makes the probe readily soluble in aqueous solution and prepares it to undergo host-guest interaction with -Cyclodextrin [42]. This important property of CD is utilized in our research to convert the probe to operate in an aqueous system by the formation of the inclusion complex with -CD. The results show that the inclusion complex displays a highly selective and sensitive response towards Bi3+ cation in neutral/aqueous solution. The binding properties of the compound NHN with Bi3+ cation were investigated by UV–vis and fluorescence spectroscopy. A novel spectrofluorometric method for the determination of Bi3+ with high selectivity and sensitivity was proposed.
2. Experimental
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Table 1 Various Prototropic maxima (Absorption & emission spectra) and pKa, pKa* values of NHN with -CD medium. Species (pH/pKa)
Without -CD max
Neutral (pH ∼ 1–7) Monoanion (>pH ∼ 7)
pKa/pKa*
(nm)
342.5 233.0 406.5 338.0 234.5 6.9
With -CD Flu(nm)
max
383.0
340.0 237.0 406.5 338.0 235.5 6.8
382.5
4.8
(nm)
Flu(nm) 383.0 382.5
4.9
DMSO-d6 was used as a solvent, relaxation delay of 1 s and mixing time 300 ms under the spin lock conditions. 2.2. Preparation of stock solutions for UV–vis and fluorescence studies The stock solution of -CD (12 × 10−3 mol dm−3 ) was prepared using pH ∼ 7 (0.1 M KH2 PO4 + 0.1 M NaOH) buffer solutions. From the stock solution 2, 4, 6, 8, 10 and 12 × 10−3 M of -CD were prepared using pH ∼ 7 buffers. 1 × 10−2 M concentration of stock solution of NHN was prepared using ethanol and appropriate volume of this stock solution were added into 10 ml volumetric flasks and made up to the mark using following concentrations of CD solutions 0, 2, 4, 6, 8, 10 and 12 × 10−3 M respectively and shaken thoroughly. So, the final concentration of NHN in each flask becomes 2 × 10−5 M. Solutions in the pH range 1.0–12.0 were prepared by adding the appropriate amount of NaOH and H3 PO4 . A modified Hammett’s acidity scale (H0 ) [43] for the solutions below pH ∼ 2 (using a H2 SO4 –H2 O mixture) and Yagil basicity scale (H− ) [44] for solutions above pH ∼ 12 (using a NaOH–H2 O mixture) were employed. The solutions were prepared just before taking measurements. The recognition between Naphthalimide and different metal cations were investigated by UV–vis spectroscopy in aqueous solution. The stock solution of NHN and metal ions were in a concentration of 1 × 10−2 M. All absorption spectral studies were carried out in pure triply distilled water at room temperature. For colorimetric titrations, the stock solutions (5 × 10−5 M) of all metal ions were prepared in aqueous solution and used. To analyze the effect of metal through UV–vis and Fluorescence spectra, various concentrations of Bi3+ (5 × 10−9 –5 × 10−5 M) were prepared in 10 ml volumetric flask and shaken thoroughly. All the absorption and emission spectra were recorded at 30 ± 1 ◦ C.
2.1. Chemicals and instruments 3. Results and discussion -Cyclodextrin {-CD, were obtained from Hi-media chemical Company} and used without further purification. N-hydroxy1,8naphthalimide (NHN) purchased from Hi-media Chemical Reagents Company. Solutions of metal ions were prepared from perchlorate salts of metal ions used in this study and were dissolved in distilled water. Triply distilled water was used to prepare all solutions. All reactants were commercially available and used without further purification. The UV–vis spectra (absorption spectral measurements) were carried out with Shimadzu UV-2401PC double-beam spectrophotometer (range 1100–200 nm) with scan speed at 400 nm min-1, Fluorescence measurements were made using a Jasco FP-880 spectrofluorimeter and the pH values in the range 1.0–12.0 were measured on Elico pH meter LI-120, 1 H NMR spectra were taken by BRUKER-NMR 500 MHz in DMSO-d6 solvent, FT-IR was recorded using Nicolet 380 and solids of the metal complex grown from the aqueous solution are used. Two-dimensional rotating-frame Overhauser effect spectroscopy (ROESY) experiments were performed using BRUKER-NMR 400 MHz instrument operating at 300 K and the standard Bruker program was used,
3.1. Effect of pH Influence of pH on the absorbance and fluorescence behavior of naphthalimide was recorded in a pH range 1.0–12.0. The absorption and fluorescence spectra of different prototropic species of naphthalimide were recorded and the relevant data are given in Table 1 and Fig. 1. In absorption spectra, the two bands of NHN at 342.5 nm and 233.0 nm are due to n-* and -* transition [45]. It can be observed from the UV–vis absorption spectra that the absorption intensity of two bands increased with an increase in pH value from 1.0 to 7.0. In the pH range 1.0–7.0, observed absorption maxima resembles the spectra observed in aqueous solvent (e.g. in Water at ∼342.5 nm) and thus can be assigned to neutral species. The absorption maximum at 342.5 nm was observed, could be due to exists of the neutral form of NHN. In alkali medium, i.e. above pH ∼ 7. 0 to pH ∼ 12. 0, NHN gives a newly red shifted peak with the maxima at 406.5 nm and the peak at 342.5 nm is slightly blue shifted to 338.0 nm and besides the color of the solution is changed
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K. Ramasamy, S. Thambusamy / Sensors and Actuators B 247 (2017) 632–640 Table 2 Absorption, fluorescence maxima (nm) and log of NHN at different concentrations of -CD in pH ∼ 7 solution. S.No
Concentration of -CD(M)
1
Without -CD
2
0.002
3
0.004
4
0.006
5
0.008
6
0.010
7
0.012
Binding Constant (M−1 )
Fig. 1. (a) Absorption and (b) Emission spectra of NHN (Conc. 2 × 10−5 M) in various pH solutions.
pH ∼ 7 max (nm)
log
Flu(nm)
342.5 235.5 342.5 234.0 343.0 237.5 343.0 237.5 343.0 236.0 343.0 236.5 343.5 236.5 103.0
3.96 4.25 3.97 4.22 3.98 4.22 3.99 4.22 4.01 4.23 4.02 4.23 4.03 4.23
383.0 383.5 383.5 383.5 383.5 383.5 383.5 105.8
at 383.0 nm increases with increase in pH of the solution, whereas in the pH range 5.0–12, it is observed that the fluorescence intensity decreases as the pH value increases. Fluorescence spectra of NHN in aqueous solution at different pH values are presented in Fig. 1b. As can be seen, there is a variation of the fluorescence intensity in a pH range of 5.0–12.0. The influence of pH in the fluorescence intensity could be explained by considering the effect of the electron withdrawing inductive of the two adjacent C O groups leading a delocalization of the negative charge in the two oxygen atoms (Scheme 1). In order to identify a simple fluorescent probe for metal cations, NHN is used along with -Cyclodextrin (CD). When the compound NHN is used with CD, encapsulation of NHN by CD is occurring and CD increases the solubility and improving the shelf life of encapsulated substrate. Use of -CD, the probe (NHN) can be operated in an aqueous medium. Obviously, the chemosensor (NHN) is stable and hardly influenced while the pH value is greater than 7.0, hence all of the detection of metal ions are operated in solution of pH ∼ 7.0. 3.2. Effect of ˇ-cyclodextrin
Scheme 1. Prototropic equilibria (neutral-monoanion) of NHN in aqueous medium.
from colorless to yellow color. This clearly suggested that a stable monoanion is formed by the deprotonation of the OH group. It is a well-known fact that deprotonation of hydroxyl group gives red shifted absorption maxima [46]. Furthermore, the observed red shifted spectrum is due to the negative charge on the oxygen atom delocalized over the amide ring and formed a new intermediate with a positive charge on nitrogen atom and negative charge on both oxygen atoms. The band at 406.5 nm was clearly indicated the formation of 1,3 dipole compound with good stability. The prototropic equilibria is shown in Scheme 1. The pKa value for such equilibria determined spectrophotometrically is given in Table 1. Similarly, the fluorescence behavior of NHN was also studied in the pH range 1.0–12.0. In pH ∼ 1.0 to 5.0, the fluorescence intensity
Table 2 and Fig. 2 show the absorption and emission spectrum of NHN (2 × 10−5 mol dm−3 ) in pH ∼ 7.0 buffer solutions containing different concentration of -CD. The absorption peak of NHN in pH ∼ 7.0 aqueous solutions appears around 342.5 nm and upon the addition of -CD, all the absorption intensities are gradually enhanced at the same wavelength. The absorption spectra of NHN in -CD solution show little change upon increasing the concentration of -CD and molar extinction coefficient slightly increases at the same wavelength. In the presence of -CD, no significant change is observed in the absorption maxima, but the absorbance value is kept on increases. This behavior has been attributed to the enhanced dissolution of guest (NHN) molecule through the hydrophobic interaction of -CD cavity. Since this indicates the formation of host–guest inclusion complex of NHN with -CD. At pH ∼ 7.0, the compound NHN exists in neutral form and NHN does not show any change in absorption maxima, which indicating that unionized OH group is present in the hydrophobic part of -CD. Even though the compound NHN located inside the CD cavity, a hydroxyl group (OH) is located on the rims which will unable acid-base equilibrium. The absorbance changes on inclusion of guest molecules in -CD have been employed to determine the corresponding binding constant “K” and the stoichiometric ratio of the inclusion complex according to the Benesi–Hildebrand [47]
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635
Fig. 3. Fluorescence spectrum of NHN (5 × 10−5 M) in the presence of various cations in -CD aqueous Solutions.
NHN is entrapped into the -CD cavity to form 1:1 (NHN:-CD) inclusion complex. The binding constant for the formation of the complex has been determined by analyzing the changes in the intensity of emission maxima with the -CD concentration using the Benesi- Hildebrand relation [48] assuming the formation of a 1:1 host–guest complex. 1 1 1 = + (I − I0 ) (I − I0 ) (I − I0 ) K[ − CD]0
Fig. 2. (a) Absorption and (b) Fluorescence spectra of NHN (Conc. 2 × 10−5 M) in different -CD Concentrations (1) 0.0 M, (2) 0.002 M, (3) 0.004 M, (4) 0.006 M, (5) 0.008 M, (6) 0.010 M and (7) 0.012 M. Inset: Benesi-Hildebrand Plot for NHN with increasing concentration of -CD in pH ∼ 7.
relation. In the case of the inclusion complex formed between NHN and -CD, the equilibrium can be written as: K
NHN + -CDNHN : -CD
(1)
Where -CD and NHN: -CD represents -CD and the 1:1 inclusion complex of -CD with NHN respectively, K is the binding constant for the formation of -CD: NHN. The binding constant (K) is determined by using Benesi-Hildebrand relation, indicates 1:1 complex formed between -CD and NHN. The Benesi-Hildebrand relation for such equilibrium is given in Eq. (2). 1 1 1 + = (A − A0 ) (ε1 − ε0)[NHN]0 (ε1 − ε0)[NHN]0k[ − CD]0
(3)
Where I and I0 are the fluorescence intensities of the NHN in the presence and absence of -CD, [-CD]0 is the initial concentration of -CD and I is the limiting intensity of fluorescence. For both molecules, a plot of 1/(I–I0 ) versus 1/[-CD] gives a straight line as shown in Fig. 2 (Inset Figures of b), from the slope values of this plot, binding constant (K) is evaluated as 105.8 M−1 . The fluorescence intensity of free NHN is very low when compared to the inclusion complex. Moreover, the inclusion provides a rigid confirmation and restricts non-radiative disintegration and results in enhanced fluorescence intensity. The complete study of -CD effect on NHN using UV–vis and fluorescence methods revealed that the inclusion complex is formed between NHN and -CD. The chemosensor NHN is encapsulated with -CD and OH group is projected in the rim of -CD. This phenomenon of inclusion complexes of NHN motivated us to come across the analytical applications as chemosensor to detect metal ions in aqueous solutions and the experiments described below resulted that the inclusion complex of NHN showed good sensing behavior towards Bi3+ .
(2)
Where A and A0 are the absorbance of the aqueous solutions of NHN in the presence and absence of  CD, 1 and 0 are the molar absorption coefficients of inclusion complex (NHN:-CD) and NHN, [NHN]0 and [-CD]0 are the initial concentrations of NHN and CD respectively. A plot of 1/(A − A0 ) versus 1/[-CD] gives a straight line as shown in Fig. 2 (Inset figure of a), from the slope values of this plot, binding constant (K) is evaluated as 103.5 M−1 . The effect of -CD on the fluorescence spectra of NHN shown in Fig. 2b is more pronounced than the corresponding effect on the absorption spectra. In the fluorescence spectra, emission intensity at 383.0 nm increases with the increase in concentration of -CD from 0 to 0.012 M. This enhancement in emission intensity has been attributed to the enhanced dissolution of the guest molecule through the hydrophobic action of -CD. The results indicated that
3.3. Spectral response of NHN towards different metal ions 3.3.1. Fluorescence emission studies The cation binding interaction of sensor NHN is investigated by using fluorescence spectroscopy. The photo physical properties of NHN in the presence of different metal cations such as Cu+ , Ni2+ , Cd2+ , Co2+ , Hg2+ , Zr4+ , Zn2+ , Cu2+ , Bi3+ , Pb2+ and Mg2+ are studied in aqueous pH ∼ 7.0 buffer solutions in -CD medium (Fig. 3), showed a selective response with Bi3+ ion alone. The addition of other metal cations to NHN in aqueous solution causes no variation in the fluorescence intensity, while NHN shows fluorescence enhancement upon the addition of Bi3+ ion. A different response is obtained upon the addition of Bi3+ to the solution of NHN in -CD medium, not only causes greater enhancement of fluorescence intensity but selective behavior towards Bi3+ metal ions. The fluorescence spectrum of the
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Fig. 4. Fluorescence spectrum of NHN (5 × 10−5 M) with various concentration of Bi3+ ; [Bi3+ ] (10−5 M) = (1–10) 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5. Inset: BenesiHildebrand Plot for NHN with increasing concentration of Bi3+ .
chemosensor NHN is recorded in the range 350.0–550.0 nm upon the excitation at 320.0 nm. The emission peak at 383.0 nm in the spectrum of NHN enhanced upon the addition of Bi3+ in aqueous medium. The selectivity of NHN for Bi3+ was further followed by fluorescence titration studies under similar conditions. To appraise the sensitivity of chemosensor, varying concentration of Bi3+ (5 × 10−8 –5 × 10−4 M) was added to the solution of NHN in the presence of -CD (Fig. 4). The intensity of emission peak increases with increase in Bi3+ concentration. When 0.1 equivalent of Bi3+ is added to a solution of NHN in pH ∼ 7.0 medium, the emission peak at 383.0 nm increased significantly. These results clearly demonstrate the sensing ability of NHN for Bi3+ cation in water. In pH ∼ 7.0, the compound NHN is exists in neutral form and it forms a 1,3-dipole structure containing the positive charge on the nitrogen atom [48]. Since, it has 1,3 dipole nature, may coordinate effectively with the divalent and trivalent metal ions. The increase in emission intensity can be attributed to the coordination of Bi3+ ion with N atom of NHN, inhibiting the PET process and favoring the excimer formation [49]. A sensing mechanism for the fluorescence OFF–ON of chemosensor (NHN) is based on a PET process. When metal ion is absent in the host solution, the fluorescence emission of chemosensors is quenched by a PET process, which takes place through electron transfer from the nitrogen lone pair (donor) of the central chain to the naphthalimide (acceptor). By the strong coordination of Bi3+ to the naphthalimide (NHN) the PET effect is blocked, and as a result, fluorescence revives (fluorescence-ON). The simultaneously coordination process leads to the formation of intramolecular excimer. Generally, naphthalamides are effective fluorophore for sensing of metal cations. In which the metal Bi3+ producing different results than the other metal cations. When Bi3+ is added to the solution of chemosensor NHN, the metal ion Bi3+ interacts with the three donor atoms of naphthalimide such as OH group, Nitrogen and carbonyl group. The compound NHN has four donors such as three oxygen atoms and one nitrogen atom. In neutral form the complex formation with metal is confirmed through Job’s plot and the binding constant is calculated by Benesi-Hildebrand relation assuming the formation of a 1:1 complex with Bi3+ . Job’s method for the emission is then employed for the determination of the binding stoichiometry of NHN and Bi3+ . The total concentration of NHN and Bi3+ is fixed at a constant M, and then
Fig. 5. (a) Job’s plot of NHN and Bi3+ , which indicated the stoichiometry of NHN:Bi3+ complex is 1:1 (b) Fluorescence responses of NHN: -CD chemosensor to 10−5 M Bi3+ in presence of other selected metal cations (10−5 M) [concentration of NHN:10−5 M, -CD:12 × 10−3 M].
molar fraction of Bi3+ varied continuously. As shown in Fig. 5a, the NHN-Bi3+ complex exhibits a maximum fluorescence emission when the molecular fraction of Bi3+ is about 0.5, indicating that a 1:1 stoichiometry most possible for the binding mode of NHN and Bi3+ . Assuming 1:1 complex formation, the binding constant (K) of the NHN-Bi3+ complex is calculated to be 311 M−1 using Benesi-Hildebrand plot of the fluorometric titration data according to (Inset of Fig. 4). To further explore the use of NHN as an ion-selective fluorescent probe for Bi3+ , selectivity and competition experiments are examined with the above metal ions. The competition experiments are further conducted in the presence of Bi3+ followed by the addition of other metal ions respectively (Fig. 5b). The competition experiments reveal that the Bi3+ induced fluorescence response is unaffected in the presence of all the tested metal ions. The selectivity experiments show that these coexistent metal ions have negligible effects on the NHN-Bi3+ complex, even if the concentration of the co-existent metal ions higher than that of Bi3+ . The results demonstrate that the chemosensor NHN is able to discriminate between Bi3+ and chemically close ions. The fluorescence titration data are used to calculate the limit of detection (LOD) of Bi3+ metal cations for the NHN, and eventually, to explore its selective sensing. The calibration graph was plotted between the relative fluorescence intensity of NHN and Bi3+ concentration that showed the linearity with the correlation coefficient R2 = 0.991. The detection limit based on 3 SD/S (whereas SD is the standard deviation of the response and S is the slope of the calibration curve) was 0.58 g mL−1 . The precision of the method was studied by processing 10 replicate standard solutions of bismuth and the relative standard deviation was ±8%. Thus, the chemosensor NHN is useful for the detection of Bi3+ ions in solution of aqueous medium in the range of micro molar level. The LOD value was
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637
Fig. 6. UV–vis spectrum of NHN (5 × 10−5 M) in the presence of various cations in -CD aqueous Solutions Inset: Benesi-Hildebrand Plot for NHN with increasing concentration of Bi3+ . Table 3 Comparison of the proposed method with other reported methods for determination of Bi3+ . Analysis method
R.S.D (%)
LODa (g mL−1 )
Reference
Flow injection potentiometry Spectrophotometry Spectrophotometry Spectrophotometry Spectrophotometry Fluorescence quenching Fluorescence enhancement
<2.0 0.7 & 1.6 0.40 1.2 – 0.498
2.5 0.6 0.25 0.05 0.2 0.05 0.58
[51] [52] [53] [54] [55] [56] This work
a
Limit of detection.
compared with those of the previously reported Bi3+ determination with other methods [50–55]. The results are summarized in Table 3. The sensitivity of NHN for Bi3+ exceeds that reported in the literature for the selective fluorogenic detection of Bi3+ . Further, to confirm the applicability of the proposed method, the determination of microgram amounts of Bi(III) in real water samples has been performed. For the water samples, the samples were acidified with hydrochloric acid. The bismuth content was analyzed according to the general procedure. An AAS method was used as a reference method [56], and the results are shown in Table S1. 3.4. UV–vis spectroscopy studies The binding and recognition abilities of NHN in aqueous medium (5 × 10−5 M) against Bi3+ metal cation is also studied by UV–vis spectroscopy. The colorimetric sensing ability of NHN was monitored by the naked eye and visible spectroscopy. In the colorimetric analysis, the chemosensor showed no specific color change in the presence of Bi3+ ion. Similarly, other cations also did not show any significant color change even when added in excess. The absorption spectral changes of the sensor NHN (5 × 10−5 M) in the presence of Bi3+ ion are displayed in Fig. 6. The sequential addition of Bi3+ ion (5 × 10−8 –5 × 10−4 M) to the solution of chemosensor showed a gradual increase in absorbance at 342.5 nm. The corroborative evidence for 1:1 binding is also obtained from UV–vis analysis of
Fig. 7. FT-IR Spectra of (a) NHN and (b) NHN–Bi3+ complex. Note: Solids of the NHN–Bi3+ complex grown from aqueous solution are used.
the complex. The binding constant (K) of the newly formed complex was determined as 332 M−1 using Benesi-Hildebrand plot. 3.5. FT-IR and 1 H NMR spectroscopy We measured FT-IR and 1 H NMR spectra of N-hydroxy naphthalimide and its complex with metal cations to further examine the binding site. Solids of the NHN–Bi3+ complex grown from the aqueous solution are used. The newly formed band in the spectra of the complex at 652.1 cm−1 (Fig. 7a) is assigned to Bi O vibrations respectively. It confirmed the formation of complex between metal and chemosensor (NHN). In addition to the new band, the characteristic peaks of NHN are shifted to higher frequency value. Thus, in accordance with the 1;1 stoichiometry, NHN is likely to chelate Bi3+ via its C O, N atom and OH group [57]. In 1 H NMR spectra, the
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chemical shift of naphthyl protons (Hc and Hd ) of NHN are shifted to the up-field region (Fig. S1). In NHN, the doublet of doublet peak of Hb and Hd protons appeared as separate doublets upon inclusion, due to the H3 and H5 protons of -CD and in particular Hb protons of NHN shifted to downfield region in the metal complex (Scheme S1). It clearly suggests that, these protons are located in the hydrophobic cavity of -CD. In NHN, the intensity of these proton peaks is also decreased and the chemical shift of OH-proton is shifted to up field region, which confirms the co-ordination of Bi3+ towards OH group. In all the cases, upfield proton shift NHN molecules were observed because of the shielding effect. These results support the results obtained from FT-IR spectra. To verify this observation, 2D 1 H NMR (ROESY) spectra were recorded. The presence of NOE cross-peaks between protons of two different species in 2D 1 H NMR spectrum is an indication that they are in spatial contact through space within the cavity of -CD. Fig. S2 shows the 2D spectrum of NHN: -CD, two groups of intermolecular NOE cross-peaks were observed. The first NOE peak belongs to the interaction between the H3 protons of
-CD with the Hc protons of NHN and other NOE peak was assigned the interaction between the H5 protons of -CD with the Hd proton of NHN. We confirmed that the NHN is included in to the -CD cavity via wider rim. From the above fact the NHN interacts with -CD through space contact, not by bonding. Based on the spectroscopic data (UV–vis, Fluorescence, FT-IR and 1 H NMR spectra), it is confirmed that the N atom of NHN coordinates with Bi3+ ions. The proposed structure of the complex is shown in Scheme 2. 3.6. Fluorescence turn-off response to pH With the fluorescent sensory materials in hand, we explored their use as fluorescent pH sensing probes. The ion H+ is one of the most important targets among the species of interest in the biomedical field. The changes in UV–vis absorption spectra for NHN as a function of pH are shown in Fig. 1. The UV–vis absorption spectra showed that NHN underwent ground state deprotonation
Fig. 8. pH effect on the emission spectra (ex = 340 nm) of NHN (10 M) (a) pH = 1.00–5.40 and (b) pH = 5.20–11.00. Arrows show spectral changes upon increasing pH.
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HO
OH O
N
O
O
N
Bi3+
O
Bi3+
NHN
639
the coexisting of other cations. The results implied that NHN could respond to pH changes exclusively in the presence of various metal ions usually present in biological systems and therefore it could be considered as a selective fluorescence probe for pH monitoring. 4. Conclusions
NHN:Bi3+ complex
Scheme 2. The proposed reaction mechanism of the sensor NHN with Bi3+ .
process over the pH region studied. The deprotonation process occurred upon increasing the pH from 7.0 to 12.0, resulting in new red shifted peak at 406.5 nm, and is assigned to the phenolic hydroxyl proton dissociation. Due to the deprotonation in OH group, the intramolecular hydrogen bond between phenolic hydroxyl proton and nitrogen is wrecked. The emission spectra of NHN are strongly pH dependent as shown in Fig. 1. The emission intensities versus pH profiles of NHN were composed of two sigmoidal curves of opposite gradients which are due to two separate excited state protonation/deprotonation process over the pH region from 1.0 to 12.0 (Fig. 8). As pH increased from 1.0 to 5.0, the fluorescence intensities increased sharply. Conversely, further increases in pH from 5.0 to 12.0 elicit reductions in the emission intensities. These two processes constitute a typical off-on-off emission switching. The spectral changes observed here were also due to the dissociation of one proton on the OH group of NHN. Thus, over pH 1.0–12.0, NHN acted as an “off–on–off” emission switch with impressive emission intensity “on–off” ratios. The probe NHN exists in either the neutral phenolic form or the anionic phenolate form, the equilibrium of which depends on medium pH. These two forms exhibit distinct absorption and fluorescence characteristics. The absorption intensity of the anionic form increases with the increase in pH from 5.0 to 12.0, but in the excited state the fluorescence quenching was observed in the pH range 5.0–12.0 (Fig. 1). The probe NHN was stable enough and displayed sensitive fluorescent emission band at 383.0 nm when excited at 340.0 nm. With the gradual increase of pH from 5 to 12, the fluorescence intensity of NHN at 383.0 nm was quenched obviously (Fig. 8). The fluorescent quenching was recovered when the pH was re-adjusted from 12.0 to 5.0 and the color of NHN also changed from pale yellow to colorless. It is clear that the fluorescence is on in acidic media and off in alkaline media. Hence, the results indicated that NHN is a sensitive turn-off pH probe within the pH range from 5.0 to 12.0. The fluorescent quenching is probably induced by the deprotonation of NHN as pH increased. The pH sensitivity of the system is due to the pH dependent ionization of the hydroxyl group present in NHN. The changes in the fluorescence intensity as a function of pH are the result of the photo induced electron transfer process (PET) from the hydroxyl anion to the naphthalimide ring. To define the application scope of this pH probe, its response to other common metal ions should be examined. To investigate this phenomenon, addition of common metal ions i.e., Cu2+ , Zn2+ , Ca2+ , K+ , Hg2+ and Pb2+ (100 M) separately into NHN solution, did not cause any adverse effects on the emission intensities of the probe (Fig. S3). The interference results imply that NHN can selectively measure pH in the presence of various metal ions usually present in biological systems. All cases showed very similar quenching as that performed by the free probe itself. When Bi3+ is added the opposite effect occurs, i.e. increased in emission intensity. The fluorescence intensity changes caused by H+ are not obviously influenced by
The effects of -cyclodextrin (-CD) on NHN were studied and the results showed that NHN forms 1:1 inclusion complex with -CD. The photophysical ability of NHN to determinate cations in neutral aqueous solutions was investigated. The results have shown clearly that only protons and Bi3+ ions could be efficiently detected. This chemosensor demonstrated satisfactory in selectivity and reproducibility as well as sufficient stability as pH sensor. It covers a broad dynamic range of pH 5.0–12.0. The fluorescent chemosensor also demonstrated high selectivity toward Bi3+ ions even in the presence of other metal ions. The sensor promoted fluorescence on response in the presence of metal ions based on the photoinduced electron transfer (PET). Stoichiometry of the complex was found to be 1:1 by fluorescence and UV–vis spectroscopy. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.03.043. References [1] F. John, A.P. De Silva, C.M. David, Tetrahedron 61 (2005) 8551–8588. [2] T. Gunnlaugsson, M. Glynn, G. Tocci, P. Kruger, F. Pfeffer, Coord. Chem. Rev. 250 (2006) 3094–3117. [3] A.P. De Silva, T. Vance, M. West, G. Wright, Org. Biomol. Chem. 6 (2008) 2468–2481. [4] H. Kim, Z. Guo, W. Zhu, J. Yoon, H. Tian, Chem. Soc. Rev. 40 (2011) 79–93. [5] M. Jun, B. Roy, K. Ahn, Chem. Commun. 47 (2011) 7583–7601. [6] S.A. Grant, K. Bettencourt, P. Krulevitch, J. Hamilton, R. Glass, Sens. Actuators B Chem. 72 (2001) 174–179. [7] M. Ganter, A. Zollinger, J. Anaesth. 91 (2003) 397–409. [8] G.T. John, D. Goelling, I. Klimant, H. Schneider, E. Heinzle, J. Dairy Res. 70 (2003) 327–333. [9] A.S. Jeevarajan, S. Vani, T.D. Taylor, M.M. Anderson, Biotechnol. Bioeng. 78 (2002) 467–472. [10] J. Lin, TrACTrendAnal. Chem. 19 (2000) 541–552. [11] S.M. Al-Hilli, M. Willander, A. Ost, P. Stralfors, J. Appl. Phys. 102 (2007) 084304–084305. [12] R.J. Gillies, K. Ugurbil, J.A. Denhollander, R.G. Shulman, Proc. Natl. Acad. Sci. U. S. A. 78 (1981) 2125–2129. [13] J.R. Chaillet, K. Amsler, W.F. Boron, Proc. Natl. Acad. Sci. U. S. A. 83 (1986) 522–526. [14] J.Y. Han, K. Burgess, Chem. Rev. 110 (2010) 2709–2728. [15] (a) D. Urano, Y. Asanuma, Y. Hama, T. Koyama, M. Barrett, T. Kamiya, T. Nagano, A. Watanabe, P. Hasegawa, Nat. Med. 15 (2009) 104–109; (b) B. Tang, F. Yu, P. Li, L. Tong, X. Duan, T. Xie, X. Wang, J. Am. Chem. Soc. 131 (2009) 3016–3023; (c) J. Lei, L. Wang, Zhang, J. Chem. Commun. (2010) 8445–8447; (d) X. Zhou, F. Su, H. Lu, P. Senechal-Willis, Y. Tian, R.H. Johnson, D.R. Meldrum, Biomaterials 33 (2012) 171–180; (e) J.T. Hutt, J. Jo, A. Olasz, C.H. Chen, D. Lee, Z.D. Aron, Org. Lett. 14 (2012) 3162–3165. [16] A.P. De Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515–1566. [17] P.J. Sadler, J. Inorg. Biochem. 67 (1997) 4–6. [18] L.B. Yang, J. Eshraghi, R. Fassihi, J. Control Release 57 (1999) 215–218. [19] D.W. Thomas, in: E. Merin (Ed.), VCH Weinheim (1991) p. 789. [20] S. Itoh, S. Kaneco, K. Ohta, T. Mizuno, Anal. Chim. Acta 379 (1999) 169. [21] M.V. Hinds, D.C. Gregoie, E. Ozaki, J. Anal. At. Spectrom. 12 (1997) 131–135. [22] S. Tokalioglu, S. Kartal, L. Elci, Microchim. Acta 127 (1997) 281–286. [23] G.E.M. Hall, A.T. Maclaurin, J.C. Pelchat, G. Gauthier, Chem. Geol. 137 (1997) 79–86. [24] P. Ostapczuk, Anal. Chim. Acta 273 (1993) 35–39. [25] R.D. Ye, S.B. Khoo, Electroanalysis 9 (1997) 481–489. [26] C.M. Wang, Q.Y. Sun, H.L. Li, Electroanalysis 9 (1997) 645–649. [27] L.M. Daffy, A.P. De Silva, H.Q.N. Gunaratne, C. Huber, P.M. Lynch, T. Werner, O. Wolfbeis, Chem. Eur. J. 4 (1998) 1810–1815. [28] Y. Kubo, M. Suto, S. Tojo, T. Araki, J. Chem. Soc. Perkin Trans. 1 (1986) 771–779. [29] C. Hou, A.M. Urbanec, H. Cao, Tetrahedron Lett. 52 (2011) 4903–4910.
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Biographies Ramasamy Kavitha received her Ph.D. in Chemistry from Alagappa University, Karaikudi, India in 2015 under the guidance of Dr. T. Stalin. She is currently working as an Asst. Professor in Department of Chemistry, Mahendra College of Engineering, Salem-636 106, Tamilnadu, India; Her current research interest involves the development of chemosensor materials for environmental applications. Thambusamy Stalin is an Assistant Professor of Chemistry in the Department of Industrial Chemistry at Alagappa University, Tamilnadu, India (From 2009 to till date). He received his Ph.D degree from the Annamalai University, India in 2008. During his Ph.D, he focused on the preparation and characterization of the inclusion complexes in order to aqueous and solid phases using the cyclodextrin molecule. He is recipient of Department of Science and Technology (DST INDIA) Fast Track Young Scientist Award and also he received prestigious award UGC Raman Fellowship for Post-Doctoral Research for Indian Scholars in United States of America (2016). He is a Life member of the Indian science congress Association, Solid State Chemistry and Indian society for Radiation and Photochemical Sciences. His Current research interest involves the development of chemosensor materials for environmental and biological applications.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Dual emission and pH based naphthalimide derivative fluorescent sensor for the detection of Bi3+ Ramasamy Kavitha b , Thambusamy Stalin a,∗ a b
Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, Tamilnadu, India Department of Chemistry, Mahendra College of Engineering, Salem 636 106, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 10 August 2016 Received in revised form 9 March 2017 Accepted 10 March 2017 Available online 14 March 2017 Keywords: Fluorescent chemosensor Bi3+ -Cyclodextrin Naphthalimide pH probe
a b s t r a c t This paper describes a novel dual emission N-hydroxy 1,8-naphthalimide (NHN) sensor for selective determination of Bi3+ ion and pH. This chemosensor displayed a red shift in absorption spectra and fluorescence enhancement towards highly selective Bi3+ ion in neutral aqueous solution. The binding stoichiometry of NHN with Bi3+ have been determined to be 1:1 by the job’s plot and the binding constant is calculated by Benesi-Hildebrand relation. The effect of -cyclodextrin (-CD) on NHN is also studied at pH ∼ 7. The binding mode of NHN with Bi3+ through carbonyl and the hydroxyl group of NHN is confirmed by FT-IR spectra. The influence of hydroxyl group in naphthalimide as a function of pH and metal ions was studied by UV–vis and fluorescence spectroscopy. Fluorescence emission of naphthalimide varied in a narrow range of pH (5–9) and was only slightly influenced by the addition of metal ions in aqueous solution. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, much attention has been paid to supramolecular fluorescent systems for the sensing and reporting of cations which may be of importance for chemical, biological and environmental sciences [1–5]. The determination of pH has great interest in different areas such as environmental monitoring, clinical analysis, control and monitoring of biochemical reactions, industrial processes, development of new pharmaceutical products and scientific researches [6–10]. Different methods have been developed to monitor and to determine pH values, including electrochemistry [11], nuclear magnetic resonance [12], absorbance spectroscopy [13] and fluorescence probe analysis [14]. Among all of these methods, measurement of pH by fluorescence-based techniques has important implications in analytical and biological chemistry. Development of fluorescent pH sensing probes for both imaging and sensing applications has received much attention in the current literature [15]. A handful of detection schemes exploiting proton binding induced perturbations of de-excitation pathways such as photo induced electron transfer (PET) [15a,b], intramolecular charge transfer (ICT) [15e], Forster resonance energy transfer (FRET) [15c,d] have been used for the development of pH sensing
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Thambusamy). http://dx.doi.org/10.1016/j.snb.2017.03.043 0925-4005/© 2017 Elsevier B.V. All rights reserved.
probes. The photo induced electron transfer (PET) process has been widely exploited for the sensing of cations as well as anions [16]. Generally, PET sensors based on nitrogen donors are highly sensitive to environmental pH stimuli, because the degree of nitrogen protonation is strongly pH dependent. On the other hand, the heightened concern for environmentally and biologically relevant species such as Bi3+ , Al3+ , Pb2+ , Hg2+ and Cu2+ , has been active research on the potential impact of their toxic effects. In particular, bismuth compounds have application in semiconductors, cosmetic preparations, metallurgy and alloy industry, iron castings, electronics, lubricating oils and greases, pigments, medicines for treatment of helicobacter pyloric- induced gastritis, contact lens cleaning solution, nuclear reactor cooling fluids and reagent for purification of sugar [17–19]. On the other hand, a number of toxic effects in humans have been attributed to bismuth compounds such as nephropathy, osteoarthropathy, hepatitis and neuropathology [20]. As the use of bismuth in medicine increases, it has spread in the environment and the chance of exposure of organisms to bismuth has increased. Various methods have been evolved for the determination of bismuth. These include hydride generation inductively-coupled plasma (HG-ICPAES), electro thermal vaporization ICP mass spectrometry (ETV-ICP-MS), atomic absorption spectrometry (AAS), potentiometric stripping analysis (PSA), anodic stripping voltammetry and cathodic stripping voltammetry (CSV) [21–26]. However, most of these methods suffer from either widespread availability of instrumentation, prohibitive cost,
K. Ramasamy, S. Thambusamy / Sensors and Actuators B 247 (2017) 632–640
or technical. Because of these considerations, spectrofluorometric seems attractive for the determination of bismuth in several samples. The number of special fluorophores sensitive to pH or metal ions with dual excitation or emission wavelength is limited. Derivatives of naphthalimides have attracted the attention for this purpose by their fluorescent characteristics. Naphthalimides and their derivatives are well-known, and they demonstrate a wide variety of applications due to properties such as fluorescence, electro activity and photo stability. The 1,8-naphthalimide moiety has also been used to design fluorescent molecular probe sensitive to variation in pH or metal ion concentration [27]. Meanwhile, the photoreactions of naphthalimide have been investigated in the past decade by Kubo and coworkers [28]. Now a day the new sensors based on naphthalimide have been reported for the determination of cations [29–31], metal ions and protons [32], pH [33–35] and anions [36–39]. Sensors for the determination of metal ions and pH based on mono and bis-1,8-naphthalimides showing PET (photo induced electron transfer) effect also have been developed [40,41]. To our knowledge, there are no reports on the development of Nhydroxy1,8-naphthalimide sensor based on the measurement of fluorescence intensity in function of pH and metal ion. Based on these considerations, we present in this article a novel dual emission sensor towards pH and metal ion based on PET mechanism. The pH detection with this kind of probe in the literature are generally troubled with the insufficient water solubility and the inference from the coexist cations. To overcome the poor solubility of the probe, it is used along with -Cyclodextrin, which enhances the solubility of the probe. The precise structure of -cyclodextrin (CD) makes the probe readily soluble in aqueous solution and prepares it to undergo host-guest interaction with -Cyclodextrin [42]. This important property of CD is utilized in our research to convert the probe to operate in an aqueous system by the formation of the inclusion complex with -CD. The results show that the inclusion complex displays a highly selective and sensitive response towards Bi3+ cation in neutral/aqueous solution. The binding properties of the compound NHN with Bi3+ cation were investigated by UV–vis and fluorescence spectroscopy. A novel spectrofluorometric method for the determination of Bi3+ with high selectivity and sensitivity was proposed.
2. Experimental
633
Table 1 Various Prototropic maxima (Absorption & emission spectra) and pKa, pKa* values of NHN with -CD medium. Species (pH/pKa)
Without -CD max
Neutral (pH ∼ 1–7) Monoanion (>pH ∼ 7)
pKa/pKa*
(nm)
342.5 233.0 406.5 338.0 234.5 6.9
With -CD Flu(nm)
max
383.0
340.0 237.0 406.5 338.0 235.5 6.8
382.5
4.8
(nm)
Flu(nm) 383.0 382.5
4.9
DMSO-d6 was used as a solvent, relaxation delay of 1 s and mixing time 300 ms under the spin lock conditions. 2.2. Preparation of stock solutions for UV–vis and fluorescence studies The stock solution of -CD (12 × 10−3 mol dm−3 ) was prepared using pH ∼ 7 (0.1 M KH2 PO4 + 0.1 M NaOH) buffer solutions. From the stock solution 2, 4, 6, 8, 10 and 12 × 10−3 M of -CD were prepared using pH ∼ 7 buffers. 1 × 10−2 M concentration of stock solution of NHN was prepared using ethanol and appropriate volume of this stock solution were added into 10 ml volumetric flasks and made up to the mark using following concentrations of CD solutions 0, 2, 4, 6, 8, 10 and 12 × 10−3 M respectively and shaken thoroughly. So, the final concentration of NHN in each flask becomes 2 × 10−5 M. Solutions in the pH range 1.0–12.0 were prepared by adding the appropriate amount of NaOH and H3 PO4 . A modified Hammett’s acidity scale (H0 ) [43] for the solutions below pH ∼ 2 (using a H2 SO4 –H2 O mixture) and Yagil basicity scale (H− ) [44] for solutions above pH ∼ 12 (using a NaOH–H2 O mixture) were employed. The solutions were prepared just before taking measurements. The recognition between Naphthalimide and different metal cations were investigated by UV–vis spectroscopy in aqueous solution. The stock solution of NHN and metal ions were in a concentration of 1 × 10−2 M. All absorption spectral studies were carried out in pure triply distilled water at room temperature. For colorimetric titrations, the stock solutions (5 × 10−5 M) of all metal ions were prepared in aqueous solution and used. To analyze the effect of metal through UV–vis and Fluorescence spectra, various concentrations of Bi3+ (5 × 10−9 –5 × 10−5 M) were prepared in 10 ml volumetric flask and shaken thoroughly. All the absorption and emission spectra were recorded at 30 ± 1 ◦ C.
2.1. Chemicals and instruments 3. Results and discussion -Cyclodextrin {-CD, were obtained from Hi-media chemical Company} and used without further purification. N-hydroxy1,8naphthalimide (NHN) purchased from Hi-media Chemical Reagents Company. Solutions of metal ions were prepared from perchlorate salts of metal ions used in this study and were dissolved in distilled water. Triply distilled water was used to prepare all solutions. All reactants were commercially available and used without further purification. The UV–vis spectra (absorption spectral measurements) were carried out with Shimadzu UV-2401PC double-beam spectrophotometer (range 1100–200 nm) with scan speed at 400 nm min-1, Fluorescence measurements were made using a Jasco FP-880 spectrofluorimeter and the pH values in the range 1.0–12.0 were measured on Elico pH meter LI-120, 1 H NMR spectra were taken by BRUKER-NMR 500 MHz in DMSO-d6 solvent, FT-IR was recorded using Nicolet 380 and solids of the metal complex grown from the aqueous solution are used. Two-dimensional rotating-frame Overhauser effect spectroscopy (ROESY) experiments were performed using BRUKER-NMR 400 MHz instrument operating at 300 K and the standard Bruker program was used,
3.1. Effect of pH Influence of pH on the absorbance and fluorescence behavior of naphthalimide was recorded in a pH range 1.0–12.0. The absorption and fluorescence spectra of different prototropic species of naphthalimide were recorded and the relevant data are given in Table 1 and Fig. 1. In absorption spectra, the two bands of NHN at 342.5 nm and 233.0 nm are due to n-* and -* transition [45]. It can be observed from the UV–vis absorption spectra that the absorption intensity of two bands increased with an increase in pH value from 1.0 to 7.0. In the pH range 1.0–7.0, observed absorption maxima resembles the spectra observed in aqueous solvent (e.g. in Water at ∼342.5 nm) and thus can be assigned to neutral species. The absorption maximum at 342.5 nm was observed, could be due to exists of the neutral form of NHN. In alkali medium, i.e. above pH ∼ 7. 0 to pH ∼ 12. 0, NHN gives a newly red shifted peak with the maxima at 406.5 nm and the peak at 342.5 nm is slightly blue shifted to 338.0 nm and besides the color of the solution is changed
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K. Ramasamy, S. Thambusamy / Sensors and Actuators B 247 (2017) 632–640 Table 2 Absorption, fluorescence maxima (nm) and log of NHN at different concentrations of -CD in pH ∼ 7 solution. S.No
Concentration of -CD(M)
1
Without -CD
2
0.002
3
0.004
4
0.006
5
0.008
6
0.010
7
0.012
Binding Constant (M−1 )
Fig. 1. (a) Absorption and (b) Emission spectra of NHN (Conc. 2 × 10−5 M) in various pH solutions.
pH ∼ 7 max (nm)
log
Flu(nm)
342.5 235.5 342.5 234.0 343.0 237.5 343.0 237.5 343.0 236.0 343.0 236.5 343.5 236.5 103.0
3.96 4.25 3.97 4.22 3.98 4.22 3.99 4.22 4.01 4.23 4.02 4.23 4.03 4.23
383.0 383.5 383.5 383.5 383.5 383.5 383.5 105.8
at 383.0 nm increases with increase in pH of the solution, whereas in the pH range 5.0–12, it is observed that the fluorescence intensity decreases as the pH value increases. Fluorescence spectra of NHN in aqueous solution at different pH values are presented in Fig. 1b. As can be seen, there is a variation of the fluorescence intensity in a pH range of 5.0–12.0. The influence of pH in the fluorescence intensity could be explained by considering the effect of the electron withdrawing inductive of the two adjacent C O groups leading a delocalization of the negative charge in the two oxygen atoms (Scheme 1). In order to identify a simple fluorescent probe for metal cations, NHN is used along with -Cyclodextrin (CD). When the compound NHN is used with CD, encapsulation of NHN by CD is occurring and CD increases the solubility and improving the shelf life of encapsulated substrate. Use of -CD, the probe (NHN) can be operated in an aqueous medium. Obviously, the chemosensor (NHN) is stable and hardly influenced while the pH value is greater than 7.0, hence all of the detection of metal ions are operated in solution of pH ∼ 7.0. 3.2. Effect of ˇ-cyclodextrin
Scheme 1. Prototropic equilibria (neutral-monoanion) of NHN in aqueous medium.
from colorless to yellow color. This clearly suggested that a stable monoanion is formed by the deprotonation of the OH group. It is a well-known fact that deprotonation of hydroxyl group gives red shifted absorption maxima [46]. Furthermore, the observed red shifted spectrum is due to the negative charge on the oxygen atom delocalized over the amide ring and formed a new intermediate with a positive charge on nitrogen atom and negative charge on both oxygen atoms. The band at 406.5 nm was clearly indicated the formation of 1,3 dipole compound with good stability. The prototropic equilibria is shown in Scheme 1. The pKa value for such equilibria determined spectrophotometrically is given in Table 1. Similarly, the fluorescence behavior of NHN was also studied in the pH range 1.0–12.0. In pH ∼ 1.0 to 5.0, the fluorescence intensity
Table 2 and Fig. 2 show the absorption and emission spectrum of NHN (2 × 10−5 mol dm−3 ) in pH ∼ 7.0 buffer solutions containing different concentration of -CD. The absorption peak of NHN in pH ∼ 7.0 aqueous solutions appears around 342.5 nm and upon the addition of -CD, all the absorption intensities are gradually enhanced at the same wavelength. The absorption spectra of NHN in -CD solution show little change upon increasing the concentration of -CD and molar extinction coefficient slightly increases at the same wavelength. In the presence of -CD, no significant change is observed in the absorption maxima, but the absorbance value is kept on increases. This behavior has been attributed to the enhanced dissolution of guest (NHN) molecule through the hydrophobic interaction of -CD cavity. Since this indicates the formation of host–guest inclusion complex of NHN with -CD. At pH ∼ 7.0, the compound NHN exists in neutral form and NHN does not show any change in absorption maxima, which indicating that unionized OH group is present in the hydrophobic part of -CD. Even though the compound NHN located inside the CD cavity, a hydroxyl group (OH) is located on the rims which will unable acid-base equilibrium. The absorbance changes on inclusion of guest molecules in -CD have been employed to determine the corresponding binding constant “K” and the stoichiometric ratio of the inclusion complex according to the Benesi–Hildebrand [47]
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635
Fig. 3. Fluorescence spectrum of NHN (5 × 10−5 M) in the presence of various cations in -CD aqueous Solutions.
NHN is entrapped into the -CD cavity to form 1:1 (NHN:-CD) inclusion complex. The binding constant for the formation of the complex has been determined by analyzing the changes in the intensity of emission maxima with the -CD concentration using the Benesi- Hildebrand relation [48] assuming the formation of a 1:1 host–guest complex. 1 1 1 = + (I − I0 ) (I − I0 ) (I − I0 ) K[ − CD]0
Fig. 2. (a) Absorption and (b) Fluorescence spectra of NHN (Conc. 2 × 10−5 M) in different -CD Concentrations (1) 0.0 M, (2) 0.002 M, (3) 0.004 M, (4) 0.006 M, (5) 0.008 M, (6) 0.010 M and (7) 0.012 M. Inset: Benesi-Hildebrand Plot for NHN with increasing concentration of -CD in pH ∼ 7.
relation. In the case of the inclusion complex formed between NHN and -CD, the equilibrium can be written as: K
NHN + -CDNHN : -CD
(1)
Where -CD and NHN: -CD represents -CD and the 1:1 inclusion complex of -CD with NHN respectively, K is the binding constant for the formation of -CD: NHN. The binding constant (K) is determined by using Benesi-Hildebrand relation, indicates 1:1 complex formed between -CD and NHN. The Benesi-Hildebrand relation for such equilibrium is given in Eq. (2). 1 1 1 + = (A − A0 ) (ε1 − ε0)[NHN]0 (ε1 − ε0)[NHN]0k[ − CD]0
(3)
Where I and I0 are the fluorescence intensities of the NHN in the presence and absence of -CD, [-CD]0 is the initial concentration of -CD and I is the limiting intensity of fluorescence. For both molecules, a plot of 1/(I–I0 ) versus 1/[-CD] gives a straight line as shown in Fig. 2 (Inset Figures of b), from the slope values of this plot, binding constant (K) is evaluated as 105.8 M−1 . The fluorescence intensity of free NHN is very low when compared to the inclusion complex. Moreover, the inclusion provides a rigid confirmation and restricts non-radiative disintegration and results in enhanced fluorescence intensity. The complete study of -CD effect on NHN using UV–vis and fluorescence methods revealed that the inclusion complex is formed between NHN and -CD. The chemosensor NHN is encapsulated with -CD and OH group is projected in the rim of -CD. This phenomenon of inclusion complexes of NHN motivated us to come across the analytical applications as chemosensor to detect metal ions in aqueous solutions and the experiments described below resulted that the inclusion complex of NHN showed good sensing behavior towards Bi3+ .
(2)
Where A and A0 are the absorbance of the aqueous solutions of NHN in the presence and absence of  CD, 1 and 0 are the molar absorption coefficients of inclusion complex (NHN:-CD) and NHN, [NHN]0 and [-CD]0 are the initial concentrations of NHN and CD respectively. A plot of 1/(A − A0 ) versus 1/[-CD] gives a straight line as shown in Fig. 2 (Inset figure of a), from the slope values of this plot, binding constant (K) is evaluated as 103.5 M−1 . The effect of -CD on the fluorescence spectra of NHN shown in Fig. 2b is more pronounced than the corresponding effect on the absorption spectra. In the fluorescence spectra, emission intensity at 383.0 nm increases with the increase in concentration of -CD from 0 to 0.012 M. This enhancement in emission intensity has been attributed to the enhanced dissolution of the guest molecule through the hydrophobic action of -CD. The results indicated that
3.3. Spectral response of NHN towards different metal ions 3.3.1. Fluorescence emission studies The cation binding interaction of sensor NHN is investigated by using fluorescence spectroscopy. The photo physical properties of NHN in the presence of different metal cations such as Cu+ , Ni2+ , Cd2+ , Co2+ , Hg2+ , Zr4+ , Zn2+ , Cu2+ , Bi3+ , Pb2+ and Mg2+ are studied in aqueous pH ∼ 7.0 buffer solutions in -CD medium (Fig. 3), showed a selective response with Bi3+ ion alone. The addition of other metal cations to NHN in aqueous solution causes no variation in the fluorescence intensity, while NHN shows fluorescence enhancement upon the addition of Bi3+ ion. A different response is obtained upon the addition of Bi3+ to the solution of NHN in -CD medium, not only causes greater enhancement of fluorescence intensity but selective behavior towards Bi3+ metal ions. The fluorescence spectrum of the
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Fig. 4. Fluorescence spectrum of NHN (5 × 10−5 M) with various concentration of Bi3+ ; [Bi3+ ] (10−5 M) = (1–10) 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5. Inset: BenesiHildebrand Plot for NHN with increasing concentration of Bi3+ .
chemosensor NHN is recorded in the range 350.0–550.0 nm upon the excitation at 320.0 nm. The emission peak at 383.0 nm in the spectrum of NHN enhanced upon the addition of Bi3+ in aqueous medium. The selectivity of NHN for Bi3+ was further followed by fluorescence titration studies under similar conditions. To appraise the sensitivity of chemosensor, varying concentration of Bi3+ (5 × 10−8 –5 × 10−4 M) was added to the solution of NHN in the presence of -CD (Fig. 4). The intensity of emission peak increases with increase in Bi3+ concentration. When 0.1 equivalent of Bi3+ is added to a solution of NHN in pH ∼ 7.0 medium, the emission peak at 383.0 nm increased significantly. These results clearly demonstrate the sensing ability of NHN for Bi3+ cation in water. In pH ∼ 7.0, the compound NHN is exists in neutral form and it forms a 1,3-dipole structure containing the positive charge on the nitrogen atom [48]. Since, it has 1,3 dipole nature, may coordinate effectively with the divalent and trivalent metal ions. The increase in emission intensity can be attributed to the coordination of Bi3+ ion with N atom of NHN, inhibiting the PET process and favoring the excimer formation [49]. A sensing mechanism for the fluorescence OFF–ON of chemosensor (NHN) is based on a PET process. When metal ion is absent in the host solution, the fluorescence emission of chemosensors is quenched by a PET process, which takes place through electron transfer from the nitrogen lone pair (donor) of the central chain to the naphthalimide (acceptor). By the strong coordination of Bi3+ to the naphthalimide (NHN) the PET effect is blocked, and as a result, fluorescence revives (fluorescence-ON). The simultaneously coordination process leads to the formation of intramolecular excimer. Generally, naphthalamides are effective fluorophore for sensing of metal cations. In which the metal Bi3+ producing different results than the other metal cations. When Bi3+ is added to the solution of chemosensor NHN, the metal ion Bi3+ interacts with the three donor atoms of naphthalimide such as OH group, Nitrogen and carbonyl group. The compound NHN has four donors such as three oxygen atoms and one nitrogen atom. In neutral form the complex formation with metal is confirmed through Job’s plot and the binding constant is calculated by Benesi-Hildebrand relation assuming the formation of a 1:1 complex with Bi3+ . Job’s method for the emission is then employed for the determination of the binding stoichiometry of NHN and Bi3+ . The total concentration of NHN and Bi3+ is fixed at a constant M, and then
Fig. 5. (a) Job’s plot of NHN and Bi3+ , which indicated the stoichiometry of NHN:Bi3+ complex is 1:1 (b) Fluorescence responses of NHN: -CD chemosensor to 10−5 M Bi3+ in presence of other selected metal cations (10−5 M) [concentration of NHN:10−5 M, -CD:12 × 10−3 M].
molar fraction of Bi3+ varied continuously. As shown in Fig. 5a, the NHN-Bi3+ complex exhibits a maximum fluorescence emission when the molecular fraction of Bi3+ is about 0.5, indicating that a 1:1 stoichiometry most possible for the binding mode of NHN and Bi3+ . Assuming 1:1 complex formation, the binding constant (K) of the NHN-Bi3+ complex is calculated to be 311 M−1 using Benesi-Hildebrand plot of the fluorometric titration data according to (Inset of Fig. 4). To further explore the use of NHN as an ion-selective fluorescent probe for Bi3+ , selectivity and competition experiments are examined with the above metal ions. The competition experiments are further conducted in the presence of Bi3+ followed by the addition of other metal ions respectively (Fig. 5b). The competition experiments reveal that the Bi3+ induced fluorescence response is unaffected in the presence of all the tested metal ions. The selectivity experiments show that these coexistent metal ions have negligible effects on the NHN-Bi3+ complex, even if the concentration of the co-existent metal ions higher than that of Bi3+ . The results demonstrate that the chemosensor NHN is able to discriminate between Bi3+ and chemically close ions. The fluorescence titration data are used to calculate the limit of detection (LOD) of Bi3+ metal cations for the NHN, and eventually, to explore its selective sensing. The calibration graph was plotted between the relative fluorescence intensity of NHN and Bi3+ concentration that showed the linearity with the correlation coefficient R2 = 0.991. The detection limit based on 3 SD/S (whereas SD is the standard deviation of the response and S is the slope of the calibration curve) was 0.58 g mL−1 . The precision of the method was studied by processing 10 replicate standard solutions of bismuth and the relative standard deviation was ±8%. Thus, the chemosensor NHN is useful for the detection of Bi3+ ions in solution of aqueous medium in the range of micro molar level. The LOD value was
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Fig. 6. UV–vis spectrum of NHN (5 × 10−5 M) in the presence of various cations in -CD aqueous Solutions Inset: Benesi-Hildebrand Plot for NHN with increasing concentration of Bi3+ . Table 3 Comparison of the proposed method with other reported methods for determination of Bi3+ . Analysis method
R.S.D (%)
LODa (g mL−1 )
Reference
Flow injection potentiometry Spectrophotometry Spectrophotometry Spectrophotometry Spectrophotometry Fluorescence quenching Fluorescence enhancement
<2.0 0.7 & 1.6 0.40 1.2 – 0.498
2.5 0.6 0.25 0.05 0.2 0.05 0.58
[51] [52] [53] [54] [55] [56] This work
a
Limit of detection.
compared with those of the previously reported Bi3+ determination with other methods [50–55]. The results are summarized in Table 3. The sensitivity of NHN for Bi3+ exceeds that reported in the literature for the selective fluorogenic detection of Bi3+ . Further, to confirm the applicability of the proposed method, the determination of microgram amounts of Bi(III) in real water samples has been performed. For the water samples, the samples were acidified with hydrochloric acid. The bismuth content was analyzed according to the general procedure. An AAS method was used as a reference method [56], and the results are shown in Table S1. 3.4. UV–vis spectroscopy studies The binding and recognition abilities of NHN in aqueous medium (5 × 10−5 M) against Bi3+ metal cation is also studied by UV–vis spectroscopy. The colorimetric sensing ability of NHN was monitored by the naked eye and visible spectroscopy. In the colorimetric analysis, the chemosensor showed no specific color change in the presence of Bi3+ ion. Similarly, other cations also did not show any significant color change even when added in excess. The absorption spectral changes of the sensor NHN (5 × 10−5 M) in the presence of Bi3+ ion are displayed in Fig. 6. The sequential addition of Bi3+ ion (5 × 10−8 –5 × 10−4 M) to the solution of chemosensor showed a gradual increase in absorbance at 342.5 nm. The corroborative evidence for 1:1 binding is also obtained from UV–vis analysis of
Fig. 7. FT-IR Spectra of (a) NHN and (b) NHN–Bi3+ complex. Note: Solids of the NHN–Bi3+ complex grown from aqueous solution are used.
the complex. The binding constant (K) of the newly formed complex was determined as 332 M−1 using Benesi-Hildebrand plot. 3.5. FT-IR and 1 H NMR spectroscopy We measured FT-IR and 1 H NMR spectra of N-hydroxy naphthalimide and its complex with metal cations to further examine the binding site. Solids of the NHN–Bi3+ complex grown from the aqueous solution are used. The newly formed band in the spectra of the complex at 652.1 cm−1 (Fig. 7a) is assigned to Bi O vibrations respectively. It confirmed the formation of complex between metal and chemosensor (NHN). In addition to the new band, the characteristic peaks of NHN are shifted to higher frequency value. Thus, in accordance with the 1;1 stoichiometry, NHN is likely to chelate Bi3+ via its C O, N atom and OH group [57]. In 1 H NMR spectra, the
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chemical shift of naphthyl protons (Hc and Hd ) of NHN are shifted to the up-field region (Fig. S1). In NHN, the doublet of doublet peak of Hb and Hd protons appeared as separate doublets upon inclusion, due to the H3 and H5 protons of -CD and in particular Hb protons of NHN shifted to downfield region in the metal complex (Scheme S1). It clearly suggests that, these protons are located in the hydrophobic cavity of -CD. In NHN, the intensity of these proton peaks is also decreased and the chemical shift of OH-proton is shifted to up field region, which confirms the co-ordination of Bi3+ towards OH group. In all the cases, upfield proton shift NHN molecules were observed because of the shielding effect. These results support the results obtained from FT-IR spectra. To verify this observation, 2D 1 H NMR (ROESY) spectra were recorded. The presence of NOE cross-peaks between protons of two different species in 2D 1 H NMR spectrum is an indication that they are in spatial contact through space within the cavity of -CD. Fig. S2 shows the 2D spectrum of NHN: -CD, two groups of intermolecular NOE cross-peaks were observed. The first NOE peak belongs to the interaction between the H3 protons of
-CD with the Hc protons of NHN and other NOE peak was assigned the interaction between the H5 protons of -CD with the Hd proton of NHN. We confirmed that the NHN is included in to the -CD cavity via wider rim. From the above fact the NHN interacts with -CD through space contact, not by bonding. Based on the spectroscopic data (UV–vis, Fluorescence, FT-IR and 1 H NMR spectra), it is confirmed that the N atom of NHN coordinates with Bi3+ ions. The proposed structure of the complex is shown in Scheme 2. 3.6. Fluorescence turn-off response to pH With the fluorescent sensory materials in hand, we explored their use as fluorescent pH sensing probes. The ion H+ is one of the most important targets among the species of interest in the biomedical field. The changes in UV–vis absorption spectra for NHN as a function of pH are shown in Fig. 1. The UV–vis absorption spectra showed that NHN underwent ground state deprotonation
Fig. 8. pH effect on the emission spectra (ex = 340 nm) of NHN (10 M) (a) pH = 1.00–5.40 and (b) pH = 5.20–11.00. Arrows show spectral changes upon increasing pH.
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HO
OH O
N
O
O
N
Bi3+
O
Bi3+
NHN
639
the coexisting of other cations. The results implied that NHN could respond to pH changes exclusively in the presence of various metal ions usually present in biological systems and therefore it could be considered as a selective fluorescence probe for pH monitoring. 4. Conclusions
NHN:Bi3+ complex
Scheme 2. The proposed reaction mechanism of the sensor NHN with Bi3+ .
process over the pH region studied. The deprotonation process occurred upon increasing the pH from 7.0 to 12.0, resulting in new red shifted peak at 406.5 nm, and is assigned to the phenolic hydroxyl proton dissociation. Due to the deprotonation in OH group, the intramolecular hydrogen bond between phenolic hydroxyl proton and nitrogen is wrecked. The emission spectra of NHN are strongly pH dependent as shown in Fig. 1. The emission intensities versus pH profiles of NHN were composed of two sigmoidal curves of opposite gradients which are due to two separate excited state protonation/deprotonation process over the pH region from 1.0 to 12.0 (Fig. 8). As pH increased from 1.0 to 5.0, the fluorescence intensities increased sharply. Conversely, further increases in pH from 5.0 to 12.0 elicit reductions in the emission intensities. These two processes constitute a typical off-on-off emission switching. The spectral changes observed here were also due to the dissociation of one proton on the OH group of NHN. Thus, over pH 1.0–12.0, NHN acted as an “off–on–off” emission switch with impressive emission intensity “on–off” ratios. The probe NHN exists in either the neutral phenolic form or the anionic phenolate form, the equilibrium of which depends on medium pH. These two forms exhibit distinct absorption and fluorescence characteristics. The absorption intensity of the anionic form increases with the increase in pH from 5.0 to 12.0, but in the excited state the fluorescence quenching was observed in the pH range 5.0–12.0 (Fig. 1). The probe NHN was stable enough and displayed sensitive fluorescent emission band at 383.0 nm when excited at 340.0 nm. With the gradual increase of pH from 5 to 12, the fluorescence intensity of NHN at 383.0 nm was quenched obviously (Fig. 8). The fluorescent quenching was recovered when the pH was re-adjusted from 12.0 to 5.0 and the color of NHN also changed from pale yellow to colorless. It is clear that the fluorescence is on in acidic media and off in alkaline media. Hence, the results indicated that NHN is a sensitive turn-off pH probe within the pH range from 5.0 to 12.0. The fluorescent quenching is probably induced by the deprotonation of NHN as pH increased. The pH sensitivity of the system is due to the pH dependent ionization of the hydroxyl group present in NHN. The changes in the fluorescence intensity as a function of pH are the result of the photo induced electron transfer process (PET) from the hydroxyl anion to the naphthalimide ring. To define the application scope of this pH probe, its response to other common metal ions should be examined. To investigate this phenomenon, addition of common metal ions i.e., Cu2+ , Zn2+ , Ca2+ , K+ , Hg2+ and Pb2+ (100 M) separately into NHN solution, did not cause any adverse effects on the emission intensities of the probe (Fig. S3). The interference results imply that NHN can selectively measure pH in the presence of various metal ions usually present in biological systems. All cases showed very similar quenching as that performed by the free probe itself. When Bi3+ is added the opposite effect occurs, i.e. increased in emission intensity. The fluorescence intensity changes caused by H+ are not obviously influenced by
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Biographies Ramasamy Kavitha received her Ph.D. in Chemistry from Alagappa University, Karaikudi, India in 2015 under the guidance of Dr. T. Stalin. She is currently working as an Asst. Professor in Department of Chemistry, Mahendra College of Engineering, Salem-636 106, Tamilnadu, India; Her current research interest involves the development of chemosensor materials for environmental applications. Thambusamy Stalin is an Assistant Professor of Chemistry in the Department of Industrial Chemistry at Alagappa University, Tamilnadu, India (From 2009 to till date). He received his Ph.D degree from the Annamalai University, India in 2008. During his Ph.D, he focused on the preparation and characterization of the inclusion complexes in order to aqueous and solid phases using the cyclodextrin molecule. He is recipient of Department of Science and Technology (DST INDIA) Fast Track Young Scientist Award and also he received prestigious award UGC Raman Fellowship for Post-Doctoral Research for Indian Scholars in United States of America (2016). He is a Life member of the Indian science congress Association, Solid State Chemistry and Indian society for Radiation and Photochemical Sciences. His Current research interest involves the development of chemosensor materials for environmental and biological applications.