p-nitrophenol complex as a colorimetric sensor for phosphate and pyrophosphate anions in water

Sensors and Actuators B 155 (2011) 909–914

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Per-6-ammonium-␤-cyclodextrin/p-nitrophenol complex as a colorimetric sensor for phosphate and pyrophosphate anions in water Ismail Abulkalam Azath, Palaniswamy Suresh, Kasi Pitchumani ∗ School of Chemistry, Madurai Kamaraj University, Madurai 625021, India

a r t i c l e

i n f o

Article history: Received 19 October 2010 Received in revised form 6 January 2011 Accepted 21 January 2011 Available online 1 February 2011 Keywords: Colorimetric anion sensor Phosphate Pyrophosphate anions Per-6-ammonium-␤-cyclodextrin p-Nitrophenol Aqueous medium

a b s t r a c t Using per-6-ammonium-␤-cyclodextrin (per-6-NH3 + -␤-CD) as an anion binding site and p-nitrophenol as a spectroscopic probe, a colorimetric sensor is developed for phosphate and pyrophosphate anions in water. Per-6-NH3 + -␤-CD forms a 1:2 inclusion complex with p-nitrophenol as characterized by NOESY and ESI-MS spectra and it undergoes a distinct color change from colorless to intense yellow upon exposure to phosphate or pyrophosphate anions over other anions including perchlorate, ATP2− , ADP2− and AMP2− . The seven ammonium groups of 1, bind phosphate (characterized by ESI-MS) or pyrophosphate anions specifically by electrostatic interaction. This naked eye sensing is significant for very low concentration (5 × 10−5 M) of anion with 1:2 ratio of host and guest. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Selective recognition and sensing of phosphate and pyrophosphate ions have been the focus of intense research, as these ions are among the most important constituents of living systems. In combination with heterocyclic bases and sugars, phosphates are essential components of genes, which are the hereditary units in living systems. In addition, phosphate ions and their derivatives play pivotal roles in signal transduction and energy storage in biological systems [1,2]. The inherent tetrahedral structure of phosphate ions poses a challenging goal for the design of effective receptors. Though numerous materials have been developed to sense phosphate and pyrophosphate ions in water, most of them use metal ion complexes for sensing [3–7]. Also many of the reported sensors need either an organic solvent as the sensing medium [8–10] or 3–100 equivalents of phosphate for effective sensing [11–19]. Aminocyclodextrins are known to bind strongly to phosphate anions and nucleotides [20–25]. Per-6-ammonium-␤-cyclodextrin (per-6-NH3 + -␤-CD) is a good receptor for anionic analytes which, in contrast to the natural or unmodified cyclodextrins, offers to the guest, an “amphiphilic” cavity lined by hydrophobic functionalities and terminating in a crown of positive charges on the narrow opening of the cavity. Kaifer et al., used inclusion complexes of per6-NH3 + -␤-CD with ferrocene derivatives to detect phosphate anion

∗ Corresponding author. Tel.: +91 452 2456614; fax: +91 452 2459181. E-mail addresses: [email protected], [email protected] (K. Pitchumani). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.041

by voltammetric methods, where 26 equivalents of phosphate ions were required [26]. We have reported that the complexes of per-6amino-␤-cyclodextrin (per-6-NH2 -␤-CD) with p-nitrophenol can be used as a selective colorimetric and ratiometric sensor for Fe3+ and Ru3+ cations [27]. In this article, we demonstrate the use of per-6-NH3 + -␤-CD as an anion binding site and p-nitrophenol as a spectroscopic probe for the selective sensing of phosphate and pyrophosphate anions (one equivalent) in water. 2. Materials and methods 2.1. Methods Absorption spectra were recorded using a JASCO V-550 double beam spectrophotometer with PMT detector. UV analyses were done using JASCO-Spectral manager and the calculations were done in Microsoft Excel 2003 software. Electrospray ionisation mass spectrometry (ESI-MS) analysis was performed in the positive ion mode on a liquid chromatography-ion trap mass spectrometer (LCQ Fleet, Thermo Fisher Instruments Limited, US). The samples were introduced into the ion source by infusion method at flow rate 1 ␮L/min. The capillary voltage of the mass spectrometer was 33 V, with source voltage 4.98 kV for the mass scale (m/z 150–2000). Energy minimized structures of the complexes were obtained by molecular mechanics calculations using Insight II/Discover programs on Silicon Graphics IRIS workstation. Calculations were done in vacuum and structures were minimized by using AMBER force field and RMS derivative 0.001 is achieved in each case.

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Fig. 1. Positive-ion mode ESI-MS spectrum of sodium adduct of per-6-NH3 + -␤-CD/p-nitrophenolate anion.

2.2. Synthesis of aminocyclodextrins

All the measurements were carried out in double distilled water which was free from ions. The stock solution of per-6-NH2 -␤-CD (1 × 10−3 M) was prepared by dissolving 0.282 g of per-6-NH2 -␤CD in 250 mL water. Stock solutions of p-nitrophenol (1 × 10−3 M) and hydrochloric acid (0.001 M) were prepared in water. Solutions (5 × 10−4 M) of K3 PO4 , KF, KCl, KBr, KI, KNO3 , K[PF6 ], NaClO4 , and Na4 P2 O7 were prepared by dissolving them in water. Adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) were added in the form of disodium salt. For UV titration, per-6-NH2 -␤-CD (0.5 mL of stock), p-nitrophenol (1.0 mL of stock), hydrochloric acid (2.5 mL of stock) and anions (0.1–1.0 mL of stock) were taken in 10 mL SMF and analysed.

nitrophenolate anion (max 401 nm) by abstraction of a proton from phenolic hydroxyl group by the amino group [27]. Upon careful and controlled addition of hydrochloric acid (see Supplementary data, Fig. S1), the remaining amino groups of per-6-NH2 -␤-CD are protonated and become per-6-ammonium-␤-CD (per-6-NH3 + ␤-CD) (pH 6), and the yellow color gets disappeared due to the formation of per-6-NH3 + -␤-CD:p-nitrophenolate anion complex, 1. A 1:2 inclusion complex [binding constant 1.2 × 104 M−2 calculated by Benesi–Hildebrand method (see Supplementary data, Fig. S2)] is formed as evident from Job’s plot of continuous variation (see Supplementary data, Fig. S3). Inclusion complex, 1 is also characterized by NOESY and ESI-MS spectra. The cross peaks between H6 and H5 protons of per-6-NH3 + -␤-CD and p-nitrophenol aromatic protons in NOESY spectrum clearly confirm the formation of inclusion complex, 1 (see Supplementary data, Fig. S4). Moreover, in ESI-MS spectrum, an intense peak at m/z 1434.33 corresponding to the sodium adduct of per-6-NH3 + -␤-CD and p-nitrophenolate anion is observed which indicates the presence of 1:2 complex (Fig. 1). Addition of anion causes the displacement of p-nitrophenolate anion from the complex 1 that regenerating the yellow color, which can be detected by naked eye. These interesting features prompted us to develop a simple colorimetric sensor for anions.

3. Results and discussion

3.2. Study of anion sensing by 1

3.1. Preparation and characterization of per-6-NH3 + -ˇ-CD:p-nitrophenolate anion complex, 1

The anion sensing ability of 1 is evaluated by colorimetric titration against increasing concentrations (5 × 10−6 to 5 × 10−5 M) of each anion (as potassium salt except ATP2− , ADP2− , AMP2− , pyrophosphate and perchlorate which are added as sodium salts) in water. In the case of phosphate anion we have observed a strong absorbance enhancement against the complex, 1. As the phos-

Per-6-NH2 -␤-CD [28] and mono-6-NH2 -␤-CD [29] were prepared by following the literature procedures. The products were dried for 24 h under vacuum over phosphrous pentoxide at 60 ◦ C and then stored in the same phosphorus pentoxide vacuum desiccator. 2.3. Preparation of stock solutions and UV titration

p-Nitrophenol [10 × 10−5 M, max 318 nm, pKa 7.14 [30] is added to per-6-NH2 -␤-CD [5 × 10−5 M, pKa value 6.6–8.5] [31]. The solution becomes yellow in color, due to the formation of p-

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F− , Cl− , Br− , NO3 − , ClO4 − and PF6 − display only weak absorbance enhancements (for perchlorate, chloride, sulphate and ATP dianion see Supplementary data, Figs. S6–S9).

3.3. Mechanism of anion sensing

Fig. 2. Absorption spectra of 1 upon addition of phosphate anion (5 × 10−6 M to 5 × 10−5 M).

Fig. 3. Absorption spectra of 1 upon addition of various anions (5 × 10−5 M).

phate concentration increases from 5 × 10−6 to 5 × 10−5 M, the absorbance at 318 nm decreases with a concomitant increase in the absorbance at 401 nm. A clear isobestic point at 350 nm indicates the presence of only two absorbing species (Fig. 2). Similar observations, as in phosphate, are also obtained in addition of pyrophosphate anion (see Supplementary data, Fig. S5). The selectivity and sensitivity of the complex 1 towards various anions are presented in Fig. 3. As can be seen ATP2− , ADP2− , AMP2− , CO3 2− ,

A plausible mechanism for this anion sensing is given in Scheme 1. Addition of two equivalents of p-nitrophenol and five equivalents of hydrochloric acid to per-6-NH2 -␤CD results in the formation of 1. Addition of phosphate or pyrophosphate anion to 1 results in the stronger binding to the ammonium ions in the narrower side of 1. The consequent weakening of interactions between p-nitrophenolate ions and ammonium ions pushes the former back into CD cavity and regenerates its intense yellow color (Scheme 1). The proposed mechanism finds strong support from the observation of an intense peak at m/z. 1505.92 in ESI-MS, which corresponds to the strong 1:1 complex formed between 1 and phosphate anion (Fig. 4). This observation also indicates that the non-covalent binding between 1 and phosphate anion is fairly strong, (as it still forms an inclusion complex in the gas phase even though excess kinetic energy has been given to the inclusion complex during electron spray ionization process). Thus, the significant absorbance enhancement may be attributed to the electrostatic interaction between phosphate and 1 in which the oxygen atoms of phosphate coordinates to the –NH3 + of per-6-NH3 + -␤-CD. The high affinity of receptor 1, for phosphate and pyrophosphate among other anions is attributed to the high negative charges which bind electrostatically to the ammonium groups of per-6-NH3 + -␤-CD. This view finds strong support from the binding constant values, determined by Benesi-Hildebrand method. Binding affinity of phosphate (6.2 × 104 M−1 ) and pyrophosphate anion (6.8 × 104 M−1 ) with complex 1 is more than that of other anions (see Supplementary data, Table S1). This particular strong interaction of phosphate with amines is attributed to the ability of phosphate species to behave as both acceptor and donor in hydrogen bond formation. The polyammoniumanion complex formation in water is reported to follow the order phosphate > carbonate  nitrate ∼ halide ions [32–34]. At pH of the present study, the dominant forms of phosphate anions are likely to be H2 PO4 − and HPO4 2− (for H3 PO4 pKa1 = 2.1, pKa2 = 7.2, pKa3 = 12.3) [35,36]. This mono and di protonation of PO4 3− may also have contributed to its efficient binding to 1. To confirm the presence and need for seven ammonium ions in 1, essential for efficient binding of a phosphate anion, control experiments are carried out with native ␤-CD/p-nitrophenol complex in the presence of an external base, ethylenediamine. Addition of phosphate to the above causes only very small enhancement in absorbance (see Supplementary data, Fig. S10). We have also

Scheme 1. Mechanism of anion sensing in water.

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Fig. 4. Positive-ion mode ESI-MS spectrum of mixture of 1 with phosphate anion.

synthesized and compared the changes in absorbance of mono-6ammonium-␤-cyclodextrin (mono-6-NH3 + -␤-CD) upon addition of phosphate with per-6-NH3 + -␤-CD. It is found that the relative changes in absorbance of mono-6-NH3 + -␤-CD are lesser than that in per-6-NH3 + -␤-CD (see Supplementary data, Fig. S10).

3.4. Molecular modelling studies

Fig. 5. Energy minimized structures of mixture of per-6-NH3 + -␤-CD (shown as lines) and p-nitrophenoxide ion (shown as sticks) with phosphate anion (PO4 3− ) (shown as dots).

Fig. 6. Energy minimized structures of mixture of per-6-NH3 + -␤-CD (shown as lines) and p-nitrophenoxide ion (shown as sticks) with pyrophosphate anion (P2 O7 4− ) (shown as dots).

The structure of per-6-NH3 + -␤-CD and its binding behavior can also be predicted from molecular mechanics–molecular

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Fig. 7. Ratiometric response (A401 /A318 ) of 1 (5 × 10−5 M) with various anions (5 × 10−5 M).

Fig. 8. Color change induced upon addition of various anions (5 × 10−5 M) to receptor 1 (5 × 10−5 M) in water. From left to right (1) without anion; (2) Cl− , (3) I− , (4) Br− , (5) F− , (6) AMP2− , (7) NO3 − , (8) ADP2− , (9) ClO4 − , (10) [PF6 ]− , (11) CO3 2− , (12) SO4 2− , (13) ATP2− , (14) PO4 3− , (15) P2 O7 4− . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

dynamics calculations. In per-6-NH3 + -␤-CD, the NH3 + group side is expanded because of electrostatic repulsion and the distances between adjacent (C-1 and C-2) and opposite (C-1 and C-4) –NH3 + groups are 8.32 and 12.96 A˚ respectively. When pnitrophenoxide ion is included (E = − 137.56 kcal molg−1 ) the distances decrease to 4.86 and 5.48 A˚ and two of the ammonium groups are visualized to interact with p-nitrophenoxide ion (see Supplementary data, Fig. S11). When phosphate anion is added to 1 (E = − 833.45 kcal mol−1 ), the –NH3 + groups come close together and the above distances decrease further to 3.84 and 4.14 A˚ respectively. During docking, phosphate is initially positioned outside of the primary side of the cyclodextrin cavity and upon energy minimization, the phosphate group moves towards the ammonium ions of the cavity. Out of seven ammonium groups of the receptor only three ammonium groups interact with phosphate anion (Fig. 5) and four ammonium groups interact with pyrophosphate anion (E = − 858.18 kcal mol−1 , Fig. 6). Energy minimized structure shows that the amino group of per-6-NH3 + -␤CD binds to phosphate and pyrophosphate by strong electrostatic interaction. The present sensor system exhibits an excellent selectivity and ratiometric response towards phosphate and pyrophosphate anions over other anions including ATP2− , ADP2− , AMP2− , fluoride and perchlorate ions (Fig. 7). That the present sensor system can detect analyte by naked eye at equimolar ratio of receptor 1 and phosphate, the color change from colorless to yellow when phosphate and pyrophosphate ions are added to the aqueous solution of 1 and other anions fail to show any color change, is of particular interest (Fig. 8).

4. Conclusion In summary, we have developed a novel colorimetric sensor for phosphate ions using per-6-NH3 + -␤-CD as a supramolecular host and anion binding site and p-nitrophenolate anion as a spectroscopic probe, which has high selectivity and sensitivity towards phosphate and pyrophosphate over a variety of other anions. It can visually detect phosphate ions in aqueous solution spectrophotometrically and, in the concentration range of 10−5 M. Moreover, the experiments described in this work may be developed into useful analytical methodologies for the selective determination of phosphate in biological samples. Acknowledgement AKA thanks University Grants Commission (UGC), New Delhi, INDIA for Junior Research Fellowship (Award No. F. No. 102(5)/2007(i)-E.U.II). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2011.01.041. References [1] R.L.P. Adams, J.T. Knower, D.P. Leader (Eds.), The Biochemistry of Nucleic Acids, 10th edn., Chapman and Hall, New York, 1986. [2] W. Saenger, Principles of Nucleic Acid Structure, Springer, New York, 1998.

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Biographies Ismail Abulkalam Azath (1984) received his B.Sc. (Chemistry) degree from Sadakathullah Appa College (Manonmaniam Sundaranar University), Palayamkottai in 2004, M.Sc. (Chemistry) from Manonmaniam Sundaranar University, Tirunelveli, India in 2006, M. Phil. (Chemistry) from Madurai Kamaraj University, Madurai, India in 2007 and doing Ph.D., in the same university. His research interests are cyclodextrin based supramolecular catalysts and sensors and synthesis of modified cyclodextrins. Palaniswamy Suresh (1980) received his B.Sc. (Industrial Chemistry) degree from Periyar Arts College (University of Madras), Cuddalore in 2001 and M.Sc. (Chemistry) from Madurai Kamaraj University, Madurai, India in 2003. He has received PhD from the same university in 2010. He did his postdoctoral research with Prof. Raphael G. Raptis, University of Puerto Rico, USA. Now he has joined as Assistant Professor in Natural Products Chemistry at Madurai Kamaraj University. His research interests are synthesis of modified cyclodextrins, cyclodextrin based supramolecular catalysts and sensors. Kasi Pitchumani (1954) received his M.Sc. (Chemistry) from Madurai Kamaraj University, Madurai, India. He has received PhD from the same university in 1981 and was appointed as Professor in Organic Chemistry in 1996 to till present. He did his postdoctoral research with Prof. V. Ramamurthy, University of Miami, USA and Prof. Akihiko Ueno, Tokyo Institute of Technology, Japan. He has 29 years of teaching experience in Organic Chemistry and published 115 research articles in peer reviewed journals. His research interests are supramolecular photochemistry and chemistry in confined media like clays, zeolites, hydrotalcites and cyclodextrins. Currently he is also involved in synthesis of modified cyclodextrins and newer nanomaterials for developing sensors.