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Development of electrochemical sensor for selective recognition of PO43− ions using organic nanoparticles of dipodal receptor in aqueous medium
Electrochimica Acta 182 (2015) 1112–1117
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Development of electrochemical sensor for selective recognition of PO43 ions using organic nanoparticles of dipodal receptor in aqueous medium Beant Kaur Billinga,1, Jasminder Singha,1, Prabhat K. Agnihotria,* , Narinder Singhb,* a b
School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology Ropar (IIT Ropar), Rupnagar, Punjab, India, 140001 Department of Chemistry, Indian Institute of Technology Ropar (IIT Ropar), Rupnagar, Punjab, India, 140001
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 May 2015 Received in revised form 28 July 2015 Accepted 19 September 2015 Available online 28 September 2015
Dipodal Schiff's base based receptor (R1) was developed and characterized using 1H NMR, 13C NMR and mass spectroscopy and optimized its structure using DMol3 package. In order to evaluate the sensor activity in aqueous medium, R1was processed into organic nanoparticles (N1) using re-precipitation method. The prepared N1were characterized using techniques like dynamic light scattering (DLS) and transmission electron microscopy (TEM). To portray N1 as sensor, it was screened with various anions and found to respond selectively for PO43 ions with detection limit of 8.6 nM having a regression coefficient of 0.98912. The proposed sensor was successfully tested for real time analysis of biologically important phosphate molecules, with accuracy more than 95%. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Schiff’s base based dipodal receptor Organic Nanoparticles Electrochemical sensor Phosphate ions Real time application
1. Introduction Supramolecular chemistry has progressed to clear, coherent and lively body of concepts and has been incorporated in innovative era of investigation [1]. Among various research areas, recognition processes have come up as one of the most attractive zones to work these days and are made more interesting by coupling it to a specific action. Anion recognition is one such lively part of exploration, because of their essential role in biology, medicine, catalysis and environmental sciences [2–9]. However, the synthesis and development of molecular receptor for anion recognition is comparatively challenging task due to numerous reasons. For instance the anions are larger than isoelectronic cations. Thus, possess a lower charge to radius ratio, which means that electrostatic binding interactions are less effective in anions than their compatriot cations [10–12]. Moreover, anions are sensitive to pH values [13], as they get protonated at low pH, releasing their negative charge. Therefore, it is necessary that receptors must function within the pH frame of their target anion. Additionally anionic species have a wide range of geometries such
* Corresponding author. Tel.: +91 1881242176. E-mail address: [email protected] (N. Singh). Both authors have equal contributions..
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http://dx.doi.org/10.1016/j.electacta.2015.09.114 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
as, halides are spherical, SCN is linear, NO3 is planer, HPO42 is tetrahedral and PF6 is octahedral [14–17]. Consequently a higher degree of scheme is requisite to make receptors complementary to their anionic guest. Among several anions, phosphate ions are the chief constituent of living systems. Along with heterocyclic bases and sugars, phosphates are vital constituents of genes, which are the hereditary units in living systems [18]. Moreover, phosphate ions also play pervasive roles in signal transduction and energy storage in biological systems [19–20]. The significance of a phosphate-ion also lies in the fields of electroplating of metals, dyeing process, food flavourings, and bio-related process [21]. On the other hand, it has been found that phosphate is the main contaminant of ground water, as it causes eutrophication of lakes and coastal waterways [22]. Phosphate pollution of drinking water arise a severe concern in regard to public health [23] and also causes various medical syndromes such as hypertension, deficiency of vitamin D, mineral and bone ailment and Franconia syndrome, if found in excess [24– 25]. As recommended by the World Health Organization, the maximum permissible phosphate concentration in drinking water is 1 mg L1. In Australia 0.046 mg L1 is the maximum permissible level of phosphate in drinking water [26]. Therefore it is very crucial to determine and regulate the amount of phosphate ions in
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natural water bodies for sustaining good water quality. Therefore, for the safeguard of global environment, finding the precise amount of phosphate-ion has turn out to be very noteworthy. Several attempts have been made in recent years to arouse the utilization of phosphate-free detergent and drop the use of phosphate based fertilizers in agriculture, nevertheless still very high percentile of phosphate is observed in natural waters and its deposits [27]. The inherent tetrahedral structure of phosphate ions poses a challenging goal for designing an effective receptor. A wide range of quantitative analytical strategies have been reported in literature to detect anions in different sources which includes conventional malachite green assay method [28], chromogenic [29–31], and fluorescent chemo sensor [32–34]. Large time-consumption, low sensitivity and confound experimental process are some of the short comings of above stated methods. However, electrochemical analysis is perfect alternate for phosphate determination due to its simplicity, high sensitivity, fast response and quite low cost [35–37]. Therefore we focused our research on developing an electrochemical sensor for phosphate determination. Coupling recognition with specific act of signalling, i.e. change in cathodic and anodic currents. In continuation to our research for the development of nanoparticle based sensors, the present study deals with the selective and sensitive detection of phosphate anion through electro chemical mechanism using a Schiff’s base based receptor. Fig. 1. (A) DLS histogram of N1showing average size and (B) TEM image of N1.
2. Experimental 2.1. Materials and methods All chemicals used were of analytical grade and were purchased from Sigma-Aldrich Co. and solvents were purchased from SD Fine Chemicals Inc. and were used without further purification. 1H and 13C NMR spectra were recorded on Avance-II (Bruker) instrument, which operated at 400 MHz for 1H NMR and 100 MHz for 13C NMR (chemical shifts are expressed inppm). The elemental analyses were performed on a Flash EA 1112 elemental analyzer. The average particle size of nano-aggregates was determined using dynamic light scattering (DLS) using external probe feature of Metrohm Microtrac Ultra Nanotrac Particle Size Analyzer. TEM images were recorded on Hitachi (H-7500) instrument worked at 120 kV. This instrument has the resolution of 0.36 nm (point to point) with 40–120 kV operating voltage. A 400-mesh formvar carboncoated copper grid was used for sample preparation. Mass spectra were recorded on Waters Micromass Q-Tof model of Mass Spectrometer. CV and DPV studies were performed on an Epsilon BASi instrument. R1was fully characterized with elemental analysis.
was added. The resulting mixture was refluxed for 2 hours at a temperature of 120 C. The obtained ester was cooled down. In another round bottom flask, ester (6 g) was dissolved in 50 ml of THF and then 10 ml of hydrazine hydrate was added drop by drop. The solution was refluxed for 5 hours at 130 C. The progress of the reaction was monitored using thin layer chromatography (TLC). The precipitates thus obtained were separated out and cooled down. Further to a solution of isophthalohydrazide (100 mg) in MeOH (10 ml), 2-pyridinecarboxaldehyde (165.4 ml) was added at room temperature. The reaction mixture was stirred overnight; the liquid product (Schiff's base) was obtained. The solvent was removed under reduced pressure and white coloured crystals were obtained. The precipitates thus formed were filtered off and washed using MeOH. Product was characterized using 1H, 13C NMR and mass spectroscopy (Fig. S1-S3). 1H NMR (400 MHz, DMSO) d: 7.41 (t, 2H, Ar-H), 7.70 (t, 1H, Ar-H), 7.87 (t, 2H, Ar-H), 7.97 (d, 2H, ArH), 8.11 (d, 2H, Ar-H), 8.46 (s, 2H, HC=N), 12.18 (s, 2H, NH) and 13C
2.2. Synthesis of Receptor (R1) Synthesis and development of R1is a three-step process. In a solution of isophthalic acid (10 g) in 50 ml MeOH, 4 drops of HCl
Scheme 1. Synthetic procedure applied for synthesis of R1.
Fig. 2. Optimized geometry of R1 using DMol3 package.
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thorough addition of solution, the mixture was kept under further sonication for 15 minutes. Finely suspended organic nanoparticles of R1were obtained with a final concentration of 0.1 mM. 2.4. Electrochemical studies Cyclic voltammetric studies (CV) were carried out at a scan rate of 50.0 mV/s and Differential Pulse Voltammetry (DPV) studies were done with 20.0 mV/s scan rate using Ag/AgCl reference electrode, Pt wire used as counter electrode, Pt disc as working electrode and NaClO4 as supporting electrolyte. All studies were performed at 25 1 C. To ensure the stability and uniformity, the solution were shaken and kept for sufficient period of time before recording any spectrum. For binding studies, a stock solution of receptor was prepared in DMSO. Tetrabutylammonium anion salts (CH3COO, ClO4, HSO4, NO3, Cl, Br, I, and F) were employed for binding studies by electrochemical method. To perform this experiment, volumetric flasks of 5 mL were taken, containing an analyte solution along with 10 mM of ligand solution and TBA salt solution. The titrations were performed in 10 mL volumetric flasks that containing a 10 mM solution of receptor and successive addition of analyte (0-100 mM) was made to it. The interference studies were performed in 5 mL volumetric flasks, containing a solution of receptor along with PO43 and an equal molar of particular anion as a competitive anion. 2.5. Theoretical calculations The computational study was carried out by using density functional theory (DFT) using DMol3 package [39–40] using double numerical plus polarization (DNP) function (including a polarized d-function for all non-hydrogen atoms and p-function for all hydrogen atoms) as basic set (DNP). The local function for the exchange-correlation potential was the BLYP [41–42]. All electrons of the system were treated (Fig. 2). Fig. 3. A) Changes in CV profile of N1 (10 mM) upon addition of 100 mM of various tetrabutyl ammonium salts of anions; B) Changes in DPV profile of N1 (10 mM) upon addition of 100 mM of various tetrabutyl ammonium salts of anions.
NMR (100 MHz, DMSO) d: 120.56, 125.10, 127.61, 129.53, 131.65, 134.07, 137.50, 149.00, 150.12, 153.68, 163.29; Elemental analysis: calculated: C: 65.99%; H: 5.03%; N: 20.99%; Found: C: 65.87%; H: 5.06%; N: 21.08%. 2.3. Preparation of organic nanoparticles (N1) Single step re-precipitation method [38] was tracked to prepare N1. This approach is based on the dissolution of the synthesized ligand in an organic solvent with a successive addition into an aqueous medium using micro syringe under vigorous stirring. In a typical procedure, 4.0 mg of R1were dissolved in 1.0 mL of DMSO. The as prepared solution was then injected drop wise into 100 mL of double distilled water under continuous sonication. After the
2.6. Real time analysis For real time application of proposed sensor, various samples of biomolecules like Adenosine diphosphate (ADP), Adenosine triphosphate (ATP) and Nicotinamide adenine dinucleotide phosphate (NADP) has been prepared by adding known concentration of them in water and subjected them to analysis using proposed sensor. To further test the applicability of the proposed sensor, urine samples (U1 & U2) were collected from a local pathology lab along with the reports of phosphate present. The acquired samples were scrutinized using proposed sensor. 3. Results and Discussion 3.1. Synthesis R1 was synthesized by adding isophthalic acid in methanol with few drops of acid to form ester. Resulting ester was then
Fig. 4. Electrochemical behaviour of N1 under the influence of applied potential.
B.K. Billing et al. / Electrochimica Acta 182 (2015) 1112–1117
dissolved in THF and hydrazine hydrate. Isophthalohydrazide thus formed, was further added to 2-pyridine carboxaldehyde to form R1 (Scheme 1). The precipitates obtained were filtered off and washed with MeOH. The receptor 1 was fully characterized with 1 H, 13C NMR, mass spectroscopy and elemental analysis [Fig. S1S3]. 3.2. Fabrication of ONP (N1) N1 were obtained using re-precipitation method [38] which involves slow injection of the solution of 1 mL of R1 (10 mM dissolved in DMSO) to double distilled water (99 mL) under sonication for 15–20 minutes. During the progression of solution mixing, the size of N1 was analysed with DLS (differential light scattering) till desired size distribution of 20–30 nm was obtained [Fig. 1A] for N1. The formation of N1was further confirmed with the help of transmission electron microscopy (TEM) [Fig. 1B]. The TEM image of N1showed well organized structure. It also revealed that the particles are evenly scattered and possesses size less than 20 nm. 3.3. Geometry optimization The geometry optimization of synthesized receptor 1 was done using DMol3 [39–40] package with DFT (density functional theory). It is quite evident from the optimized structure (Fig. 2) that two pods are extended forming a cavity large enough to engulf an
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anion. The cavity has sufficient number of heteroatoms to hold an anion through hydrogen bonding. 3.4. Electrochemical sensing of phosphate For the evaluation of anion recognition behaviour of N1in aqueous medium, the interaction between tetrabutylammonium salts of various anions and N1was observed using electrochemical studies. Complex N1displayed minor changes upon addition of 100 mM of tetrabutylammonium (TBA) salts of CH3COO, ClO4, HSO4, NO3, Cl, Br, I, and F. However, a considerable shift in electrochemical profile of N1upon addition of PO43 (100 mM) ions in both CV [Fig. 3A] and DPV [Fig. 3B]. In case of CV, a shift was observed in cathodic peak, as it changed its position from 0.283 mV to 0.106 mV on addition of PO43 ions. Similarly, anodic peak showed a change from 0.706 mV to 0.584 mV and a new peak at 0.628 mV emerged in DPV profile of N1. The electrochemical behaviour of N1 can be explained on the basis that on application of potential oxygen atom loses an electron and changes to a cation radical, which is further stabilized by lone pair of adjacent nitrogen atom through resonance [Fig. 4]. The positive charge produced on the pods of N1 attracts the negatively charged phosphate ions towards the cavity [43]. The selectivity of N1 can be explained on the basis of perfectly fitting cavity formed by the two pods of N1, which is further stabilized by hydrogen bonding between hydrogen atom present on nitrogen and oxygen atoms of phosphate ions.
Fig. 5. A) Changes in CV profile of N1 (10 mM) upon successive addition of PO43 (0–100 mM); B) changes in DPV profile of N1 (10 mM) upon successive addition of PO43 (0– 100 mM).
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It was observed that phosphate ions respond linearly with N1, having linear regression coefficient of 0.98912. The detection limit of 8.6 nM was obtained using IUPAC 3s method [44–45]. DL ¼
Fig. 6. The normalized response of DPV profile of N1at potential of -0.628 mV on subsequent addition of phosphate ions, with a regression of 0.98912.
In order to verify the interaction of N1with PO43, titration studies were carried out with subsequent addition of small aliquots of PO43 ions (0–100 mM) in N1 (100 mM). It is fairly apparent from the CV profile [Fig. 5A] of N1that interaction of PO43 cause decrease in concentration of N1, which can be further related with decrease in parent peak of N1 at 0.106 mV. The process is followed by subsequent increase in peak intensity at -0.44 mV on further addition. This origin of new peak can be attributed to formation of complex between N1 and PO43 i.e. altogether a new species. In Similar way current decrease at 0.628 mV upon increase in concentration of PO43 in DPV profile [Fig. 5B]. The results indicate the formation of N1.PO43 complex, that modified the original redox properties of N1 i.e. disappearance of parent peak and origin of new peak. N1 is clearly electropositive losing an electron on application of potential Interaction of positively charged podands of N1 and negatively charged phosphate ions causes the change in electronic clouds of N1 leading to change in electrochemical profile of N1, further confirming the formation of complex between N1 and phosphate ions. Calibration curves were plotted at potential of 0.628 mV [Fig. 6] using DPV titration data for further use of the proposed sensor in real time application, for analysis of phosphate ions having unknown concentration of phosphate ions in it.
ð3sÞ m
Where s is the standard deviation got from titration and m is the slope of calibration plot. To extend the study for real sample utility of proposed sensor, it is quite necessary for sensor to work in complex compositions and should possess selectivity towards the said anion. To check the selectivity, interference studies were carried out using mixture of two anions i.e. PO43 as primary anion (100 mM) and other anions CH3COO, ClO4, HSO4, NO3, Cl, Br, I, and F (100 mM) as secondary or interfering anions [Fig. 7] at 0.628 mV. To check further extend the utility of N1; its response at different pH values in presence of PO43 was also evaluated [Fig. S4]. The current decreased as the pH of the solution was changed below pH 6, which was likely due to formation of large aggregates due to agglomeration. No change in the current of N1was observed in the pH range 6–12. Therefore, N1were favorable for application in a wide pH range (6-12). Perturbation of high ionic strength was ruled out by comparison of spectra of N1with phosphate ions with spectra recorded upon addition of 0– 200 equiv. of tetra butyl ammonium perchlorate [Fig. S5]. It is quite evident from the [Fig. S5] that binding of N1with phosphate ions is quite stable and does not get affected even in presence of higher concentration of salt. The comparison of the prepared sensor with recently reported sensors were made (Table S1). It is quite evident from the table that presented sensor shows a clear progression in electrochemical determination of phosphate ions in pure aqueous media. Mostly reported sensors were based on changes in photophysical properties or having partial aqueous system [46– 61]. However, the present sensor made use of electrochemical studies of N1 for determination of phosphate ions in pure aqueous system, without any interference from any other potential interferent anions, with detection in lower nanomolar range. 3.5. Real Time Analysis To test the workability of the proposed sensor for determination of phosphate ions in real time samples, some samples of biologically important phosphates like Adenosine diphosphate (ADP), Adenosine triphosphate (ATP) and Nicotinamide adenine dinucleotide phosphate (NADP) were prepared by dissolving their known concentration in water. The solutions of these biomolecules were subjected to analysis using proposed sensor. The samples of urine collected from local pathological laboratory (U1 &U2) were also tested using proposed sensor to check practical workability of it in real time sample analysis and obtained results were compared with the test results obtained from the lab (Table 1). It is quite evident from the results obtained that proposed sensor was able to determine the biologically important molecules
Table 1 Real time sample analysis of artificially prepared samples of phosphate containing biomolecules and their percentage recovery using proposed sensor (N1).
Fig. 7. Influence of various potential interfering anions on DPV profile of N1.PO43. It is quite evident from studies that proposed sensor show no interference in determination of PO43 even in presence of other potential interfering ions, as no substantial change in DPV profile of N1.PO43 is observed.
S. No.
Sample Conc. Added/ Reported
Conc. of Phosphate recovered a
%age Recovery
1. 2. 3. 4. 5.
ADP ATP NADP U1 U2
11.42 0.05 nM 14.27 0.03 nM 23.96 0.05 nM 19.85 0.05 mM 63.76 0.05 mM
95.16% 95.13% 95.8% 97.3% 97.5%
a
12 nM 15 nM 25 nM 20.4 mM 65.38 mM
Mean of three determinations.
B.K. Billing et al. / Electrochimica Acta 182 (2015) 1112–1117
and phosphate in urine samples with accuracy more than 95% and the results thus obtained are in complete accord with the results obtained from the laboratory. Hence, it can be said that the prepared receptor on subjecting to organic nanoparticles can be used for determination of biologically important phosphates in real time samples. 4. Conclusion The synthesis and characterization of dipodal Schiff's base based receptor was successfully done using spectroscopic techniques (NMR and MS). For sensor application, receptor 1 was subjected to organic nanoparticles using re-precipitation method and electrochemical behaviour was examined in an aqueous medium. Organic nanoparticles showed a considerable shift with the addition of PO43. It showed high selectivity for PO43 ions over other TBA salts of anions in aqueous medium, with 8.6 nM detection limit, which makes their utility in phosphate supplement analysis. The proposed sensor was also compared with the recent reports of phosphate sensing, clearly showing the presented work as progression in field of phosphate determination in total aqueous samples. The proposed sensor was successfully employed for determination of phosphate in various biomolecules and urine samples having phosphate moiety. Acknowledgement B. K. Billing is thankful to IIT Ropar for fellowship. N.S. is thankful to DST for research funding. 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.electacta.2015 .09.114. References [1] A.M. Costero, M.J. Banuls, M.J. Aurell, L.E. Ochando, A. Domenech, Tetrahedron 61 (10) (2005) 309–10320. [2] J. Wang, F.Q. Bai, B.H. Xia, L. Sun, H.X. Zhang, J. Phys. Chem. A 115 (2011) 1985–1991. [3] D. Nieto, A.M.G. Vadillo, S. Bruna, C.J. Pastor, A.E. Kaifer, I. Cuadrado, Chem. Commun. 47 (2011) 10398–10400. [4] Y. Kang, K. Gwon, J.H. Shin, H. Nam, M.E. Meyerhoff, G.S. Cha, Anal. Chem. 83 (2011) 3957–3962. [5] R.M. Duke, E.B. Veale, F.M. Pfeffer, P.E. Kruger, T. Gunnlaugsson, Chem. Soc. Rev. 39 (2010) 3936–3953. [6] S. Guha, S. Saha, J. Am. Chem. Soc. 132 (2010) 17674–17677. [7] K. Kikuchi, S. Hashimoto, S. Mizukami, T. Nagano, Org. Lett. 11 (2009) 2732–2735. [8] S. Hu, Y. Guo, Y. Xu, S. Shao, Org. Biomol. Chem. 6 (2008) 2071–2075. [9] A. Bianchi, K.B. James, E.G. Espana, Supramolecular Chemistry of Nanomaterials, Wiley-VCH, New York, 1997. [10] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. Engl. 40 (2001) 486. [11] J.J. Park, Y.H. Kim, C. Kim, J. Kang, Tetrahedron Lett. 52 (2011) 3361. [12] X.P. Bao, Y.H. Zhou, J.H. Yu, J. Lumin. 130 (2010) 392.
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Development of electrochemical sensor for selective recognition of PO43 ions using organic nanoparticles of dipodal receptor in aqueous medium Beant Kaur Billinga,1, Jasminder Singha,1, Prabhat K. Agnihotria,* , Narinder Singhb,* a b
School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology Ropar (IIT Ropar), Rupnagar, Punjab, India, 140001 Department of Chemistry, Indian Institute of Technology Ropar (IIT Ropar), Rupnagar, Punjab, India, 140001
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 May 2015 Received in revised form 28 July 2015 Accepted 19 September 2015 Available online 28 September 2015
Dipodal Schiff's base based receptor (R1) was developed and characterized using 1H NMR, 13C NMR and mass spectroscopy and optimized its structure using DMol3 package. In order to evaluate the sensor activity in aqueous medium, R1was processed into organic nanoparticles (N1) using re-precipitation method. The prepared N1were characterized using techniques like dynamic light scattering (DLS) and transmission electron microscopy (TEM). To portray N1 as sensor, it was screened with various anions and found to respond selectively for PO43 ions with detection limit of 8.6 nM having a regression coefficient of 0.98912. The proposed sensor was successfully tested for real time analysis of biologically important phosphate molecules, with accuracy more than 95%. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Schiff’s base based dipodal receptor Organic Nanoparticles Electrochemical sensor Phosphate ions Real time application
1. Introduction Supramolecular chemistry has progressed to clear, coherent and lively body of concepts and has been incorporated in innovative era of investigation [1]. Among various research areas, recognition processes have come up as one of the most attractive zones to work these days and are made more interesting by coupling it to a specific action. Anion recognition is one such lively part of exploration, because of their essential role in biology, medicine, catalysis and environmental sciences [2–9]. However, the synthesis and development of molecular receptor for anion recognition is comparatively challenging task due to numerous reasons. For instance the anions are larger than isoelectronic cations. Thus, possess a lower charge to radius ratio, which means that electrostatic binding interactions are less effective in anions than their compatriot cations [10–12]. Moreover, anions are sensitive to pH values [13], as they get protonated at low pH, releasing their negative charge. Therefore, it is necessary that receptors must function within the pH frame of their target anion. Additionally anionic species have a wide range of geometries such
* Corresponding author. Tel.: +91 1881242176. E-mail address: [email protected] (N. Singh). Both authors have equal contributions..
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http://dx.doi.org/10.1016/j.electacta.2015.09.114 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
as, halides are spherical, SCN is linear, NO3 is planer, HPO42 is tetrahedral and PF6 is octahedral [14–17]. Consequently a higher degree of scheme is requisite to make receptors complementary to their anionic guest. Among several anions, phosphate ions are the chief constituent of living systems. Along with heterocyclic bases and sugars, phosphates are vital constituents of genes, which are the hereditary units in living systems [18]. Moreover, phosphate ions also play pervasive roles in signal transduction and energy storage in biological systems [19–20]. The significance of a phosphate-ion also lies in the fields of electroplating of metals, dyeing process, food flavourings, and bio-related process [21]. On the other hand, it has been found that phosphate is the main contaminant of ground water, as it causes eutrophication of lakes and coastal waterways [22]. Phosphate pollution of drinking water arise a severe concern in regard to public health [23] and also causes various medical syndromes such as hypertension, deficiency of vitamin D, mineral and bone ailment and Franconia syndrome, if found in excess [24– 25]. As recommended by the World Health Organization, the maximum permissible phosphate concentration in drinking water is 1 mg L1. In Australia 0.046 mg L1 is the maximum permissible level of phosphate in drinking water [26]. Therefore it is very crucial to determine and regulate the amount of phosphate ions in
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natural water bodies for sustaining good water quality. Therefore, for the safeguard of global environment, finding the precise amount of phosphate-ion has turn out to be very noteworthy. Several attempts have been made in recent years to arouse the utilization of phosphate-free detergent and drop the use of phosphate based fertilizers in agriculture, nevertheless still very high percentile of phosphate is observed in natural waters and its deposits [27]. The inherent tetrahedral structure of phosphate ions poses a challenging goal for designing an effective receptor. A wide range of quantitative analytical strategies have been reported in literature to detect anions in different sources which includes conventional malachite green assay method [28], chromogenic [29–31], and fluorescent chemo sensor [32–34]. Large time-consumption, low sensitivity and confound experimental process are some of the short comings of above stated methods. However, electrochemical analysis is perfect alternate for phosphate determination due to its simplicity, high sensitivity, fast response and quite low cost [35–37]. Therefore we focused our research on developing an electrochemical sensor for phosphate determination. Coupling recognition with specific act of signalling, i.e. change in cathodic and anodic currents. In continuation to our research for the development of nanoparticle based sensors, the present study deals with the selective and sensitive detection of phosphate anion through electro chemical mechanism using a Schiff’s base based receptor. Fig. 1. (A) DLS histogram of N1showing average size and (B) TEM image of N1.
2. Experimental 2.1. Materials and methods All chemicals used were of analytical grade and were purchased from Sigma-Aldrich Co. and solvents were purchased from SD Fine Chemicals Inc. and were used without further purification. 1H and 13C NMR spectra were recorded on Avance-II (Bruker) instrument, which operated at 400 MHz for 1H NMR and 100 MHz for 13C NMR (chemical shifts are expressed inppm). The elemental analyses were performed on a Flash EA 1112 elemental analyzer. The average particle size of nano-aggregates was determined using dynamic light scattering (DLS) using external probe feature of Metrohm Microtrac Ultra Nanotrac Particle Size Analyzer. TEM images were recorded on Hitachi (H-7500) instrument worked at 120 kV. This instrument has the resolution of 0.36 nm (point to point) with 40–120 kV operating voltage. A 400-mesh formvar carboncoated copper grid was used for sample preparation. Mass spectra were recorded on Waters Micromass Q-Tof model of Mass Spectrometer. CV and DPV studies were performed on an Epsilon BASi instrument. R1was fully characterized with elemental analysis.
was added. The resulting mixture was refluxed for 2 hours at a temperature of 120 C. The obtained ester was cooled down. In another round bottom flask, ester (6 g) was dissolved in 50 ml of THF and then 10 ml of hydrazine hydrate was added drop by drop. The solution was refluxed for 5 hours at 130 C. The progress of the reaction was monitored using thin layer chromatography (TLC). The precipitates thus obtained were separated out and cooled down. Further to a solution of isophthalohydrazide (100 mg) in MeOH (10 ml), 2-pyridinecarboxaldehyde (165.4 ml) was added at room temperature. The reaction mixture was stirred overnight; the liquid product (Schiff's base) was obtained. The solvent was removed under reduced pressure and white coloured crystals were obtained. The precipitates thus formed were filtered off and washed using MeOH. Product was characterized using 1H, 13C NMR and mass spectroscopy (Fig. S1-S3). 1H NMR (400 MHz, DMSO) d: 7.41 (t, 2H, Ar-H), 7.70 (t, 1H, Ar-H), 7.87 (t, 2H, Ar-H), 7.97 (d, 2H, ArH), 8.11 (d, 2H, Ar-H), 8.46 (s, 2H, HC=N), 12.18 (s, 2H, NH) and 13C
2.2. Synthesis of Receptor (R1) Synthesis and development of R1is a three-step process. In a solution of isophthalic acid (10 g) in 50 ml MeOH, 4 drops of HCl
Scheme 1. Synthetic procedure applied for synthesis of R1.
Fig. 2. Optimized geometry of R1 using DMol3 package.
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thorough addition of solution, the mixture was kept under further sonication for 15 minutes. Finely suspended organic nanoparticles of R1were obtained with a final concentration of 0.1 mM. 2.4. Electrochemical studies Cyclic voltammetric studies (CV) were carried out at a scan rate of 50.0 mV/s and Differential Pulse Voltammetry (DPV) studies were done with 20.0 mV/s scan rate using Ag/AgCl reference electrode, Pt wire used as counter electrode, Pt disc as working electrode and NaClO4 as supporting electrolyte. All studies were performed at 25 1 C. To ensure the stability and uniformity, the solution were shaken and kept for sufficient period of time before recording any spectrum. For binding studies, a stock solution of receptor was prepared in DMSO. Tetrabutylammonium anion salts (CH3COO, ClO4, HSO4, NO3, Cl, Br, I, and F) were employed for binding studies by electrochemical method. To perform this experiment, volumetric flasks of 5 mL were taken, containing an analyte solution along with 10 mM of ligand solution and TBA salt solution. The titrations were performed in 10 mL volumetric flasks that containing a 10 mM solution of receptor and successive addition of analyte (0-100 mM) was made to it. The interference studies were performed in 5 mL volumetric flasks, containing a solution of receptor along with PO43 and an equal molar of particular anion as a competitive anion. 2.5. Theoretical calculations The computational study was carried out by using density functional theory (DFT) using DMol3 package [39–40] using double numerical plus polarization (DNP) function (including a polarized d-function for all non-hydrogen atoms and p-function for all hydrogen atoms) as basic set (DNP). The local function for the exchange-correlation potential was the BLYP [41–42]. All electrons of the system were treated (Fig. 2). Fig. 3. A) Changes in CV profile of N1 (10 mM) upon addition of 100 mM of various tetrabutyl ammonium salts of anions; B) Changes in DPV profile of N1 (10 mM) upon addition of 100 mM of various tetrabutyl ammonium salts of anions.
NMR (100 MHz, DMSO) d: 120.56, 125.10, 127.61, 129.53, 131.65, 134.07, 137.50, 149.00, 150.12, 153.68, 163.29; Elemental analysis: calculated: C: 65.99%; H: 5.03%; N: 20.99%; Found: C: 65.87%; H: 5.06%; N: 21.08%. 2.3. Preparation of organic nanoparticles (N1) Single step re-precipitation method [38] was tracked to prepare N1. This approach is based on the dissolution of the synthesized ligand in an organic solvent with a successive addition into an aqueous medium using micro syringe under vigorous stirring. In a typical procedure, 4.0 mg of R1were dissolved in 1.0 mL of DMSO. The as prepared solution was then injected drop wise into 100 mL of double distilled water under continuous sonication. After the
2.6. Real time analysis For real time application of proposed sensor, various samples of biomolecules like Adenosine diphosphate (ADP), Adenosine triphosphate (ATP) and Nicotinamide adenine dinucleotide phosphate (NADP) has been prepared by adding known concentration of them in water and subjected them to analysis using proposed sensor. To further test the applicability of the proposed sensor, urine samples (U1 & U2) were collected from a local pathology lab along with the reports of phosphate present. The acquired samples were scrutinized using proposed sensor. 3. Results and Discussion 3.1. Synthesis R1 was synthesized by adding isophthalic acid in methanol with few drops of acid to form ester. Resulting ester was then
Fig. 4. Electrochemical behaviour of N1 under the influence of applied potential.
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dissolved in THF and hydrazine hydrate. Isophthalohydrazide thus formed, was further added to 2-pyridine carboxaldehyde to form R1 (Scheme 1). The precipitates obtained were filtered off and washed with MeOH. The receptor 1 was fully characterized with 1 H, 13C NMR, mass spectroscopy and elemental analysis [Fig. S1S3]. 3.2. Fabrication of ONP (N1) N1 were obtained using re-precipitation method [38] which involves slow injection of the solution of 1 mL of R1 (10 mM dissolved in DMSO) to double distilled water (99 mL) under sonication for 15–20 minutes. During the progression of solution mixing, the size of N1 was analysed with DLS (differential light scattering) till desired size distribution of 20–30 nm was obtained [Fig. 1A] for N1. The formation of N1was further confirmed with the help of transmission electron microscopy (TEM) [Fig. 1B]. The TEM image of N1showed well organized structure. It also revealed that the particles are evenly scattered and possesses size less than 20 nm. 3.3. Geometry optimization The geometry optimization of synthesized receptor 1 was done using DMol3 [39–40] package with DFT (density functional theory). It is quite evident from the optimized structure (Fig. 2) that two pods are extended forming a cavity large enough to engulf an
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anion. The cavity has sufficient number of heteroatoms to hold an anion through hydrogen bonding. 3.4. Electrochemical sensing of phosphate For the evaluation of anion recognition behaviour of N1in aqueous medium, the interaction between tetrabutylammonium salts of various anions and N1was observed using electrochemical studies. Complex N1displayed minor changes upon addition of 100 mM of tetrabutylammonium (TBA) salts of CH3COO, ClO4, HSO4, NO3, Cl, Br, I, and F. However, a considerable shift in electrochemical profile of N1upon addition of PO43 (100 mM) ions in both CV [Fig. 3A] and DPV [Fig. 3B]. In case of CV, a shift was observed in cathodic peak, as it changed its position from 0.283 mV to 0.106 mV on addition of PO43 ions. Similarly, anodic peak showed a change from 0.706 mV to 0.584 mV and a new peak at 0.628 mV emerged in DPV profile of N1. The electrochemical behaviour of N1 can be explained on the basis that on application of potential oxygen atom loses an electron and changes to a cation radical, which is further stabilized by lone pair of adjacent nitrogen atom through resonance [Fig. 4]. The positive charge produced on the pods of N1 attracts the negatively charged phosphate ions towards the cavity [43]. The selectivity of N1 can be explained on the basis of perfectly fitting cavity formed by the two pods of N1, which is further stabilized by hydrogen bonding between hydrogen atom present on nitrogen and oxygen atoms of phosphate ions.
Fig. 5. A) Changes in CV profile of N1 (10 mM) upon successive addition of PO43 (0–100 mM); B) changes in DPV profile of N1 (10 mM) upon successive addition of PO43 (0– 100 mM).
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It was observed that phosphate ions respond linearly with N1, having linear regression coefficient of 0.98912. The detection limit of 8.6 nM was obtained using IUPAC 3s method [44–45]. DL ¼
Fig. 6. The normalized response of DPV profile of N1at potential of -0.628 mV on subsequent addition of phosphate ions, with a regression of 0.98912.
In order to verify the interaction of N1with PO43, titration studies were carried out with subsequent addition of small aliquots of PO43 ions (0–100 mM) in N1 (100 mM). It is fairly apparent from the CV profile [Fig. 5A] of N1that interaction of PO43 cause decrease in concentration of N1, which can be further related with decrease in parent peak of N1 at 0.106 mV. The process is followed by subsequent increase in peak intensity at -0.44 mV on further addition. This origin of new peak can be attributed to formation of complex between N1 and PO43 i.e. altogether a new species. In Similar way current decrease at 0.628 mV upon increase in concentration of PO43 in DPV profile [Fig. 5B]. The results indicate the formation of N1.PO43 complex, that modified the original redox properties of N1 i.e. disappearance of parent peak and origin of new peak. N1 is clearly electropositive losing an electron on application of potential Interaction of positively charged podands of N1 and negatively charged phosphate ions causes the change in electronic clouds of N1 leading to change in electrochemical profile of N1, further confirming the formation of complex between N1 and phosphate ions. Calibration curves were plotted at potential of 0.628 mV [Fig. 6] using DPV titration data for further use of the proposed sensor in real time application, for analysis of phosphate ions having unknown concentration of phosphate ions in it.
ð3sÞ m
Where s is the standard deviation got from titration and m is the slope of calibration plot. To extend the study for real sample utility of proposed sensor, it is quite necessary for sensor to work in complex compositions and should possess selectivity towards the said anion. To check the selectivity, interference studies were carried out using mixture of two anions i.e. PO43 as primary anion (100 mM) and other anions CH3COO, ClO4, HSO4, NO3, Cl, Br, I, and F (100 mM) as secondary or interfering anions [Fig. 7] at 0.628 mV. To check further extend the utility of N1; its response at different pH values in presence of PO43 was also evaluated [Fig. S4]. The current decreased as the pH of the solution was changed below pH 6, which was likely due to formation of large aggregates due to agglomeration. No change in the current of N1was observed in the pH range 6–12. Therefore, N1were favorable for application in a wide pH range (6-12). Perturbation of high ionic strength was ruled out by comparison of spectra of N1with phosphate ions with spectra recorded upon addition of 0– 200 equiv. of tetra butyl ammonium perchlorate [Fig. S5]. It is quite evident from the [Fig. S5] that binding of N1with phosphate ions is quite stable and does not get affected even in presence of higher concentration of salt. The comparison of the prepared sensor with recently reported sensors were made (Table S1). It is quite evident from the table that presented sensor shows a clear progression in electrochemical determination of phosphate ions in pure aqueous media. Mostly reported sensors were based on changes in photophysical properties or having partial aqueous system [46– 61]. However, the present sensor made use of electrochemical studies of N1 for determination of phosphate ions in pure aqueous system, without any interference from any other potential interferent anions, with detection in lower nanomolar range. 3.5. Real Time Analysis To test the workability of the proposed sensor for determination of phosphate ions in real time samples, some samples of biologically important phosphates like Adenosine diphosphate (ADP), Adenosine triphosphate (ATP) and Nicotinamide adenine dinucleotide phosphate (NADP) were prepared by dissolving their known concentration in water. The solutions of these biomolecules were subjected to analysis using proposed sensor. The samples of urine collected from local pathological laboratory (U1 &U2) were also tested using proposed sensor to check practical workability of it in real time sample analysis and obtained results were compared with the test results obtained from the lab (Table 1). It is quite evident from the results obtained that proposed sensor was able to determine the biologically important molecules
Table 1 Real time sample analysis of artificially prepared samples of phosphate containing biomolecules and their percentage recovery using proposed sensor (N1).
Fig. 7. Influence of various potential interfering anions on DPV profile of N1.PO43. It is quite evident from studies that proposed sensor show no interference in determination of PO43 even in presence of other potential interfering ions, as no substantial change in DPV profile of N1.PO43 is observed.
S. No.
Sample Conc. Added/ Reported
Conc. of Phosphate recovered a
%age Recovery
1. 2. 3. 4. 5.
ADP ATP NADP U1 U2
11.42 0.05 nM 14.27 0.03 nM 23.96 0.05 nM 19.85 0.05 mM 63.76 0.05 mM
95.16% 95.13% 95.8% 97.3% 97.5%
a
12 nM 15 nM 25 nM 20.4 mM 65.38 mM
Mean of three determinations.
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and phosphate in urine samples with accuracy more than 95% and the results thus obtained are in complete accord with the results obtained from the laboratory. Hence, it can be said that the prepared receptor on subjecting to organic nanoparticles can be used for determination of biologically important phosphates in real time samples. 4. Conclusion The synthesis and characterization of dipodal Schiff's base based receptor was successfully done using spectroscopic techniques (NMR and MS). For sensor application, receptor 1 was subjected to organic nanoparticles using re-precipitation method and electrochemical behaviour was examined in an aqueous medium. Organic nanoparticles showed a considerable shift with the addition of PO43. It showed high selectivity for PO43 ions over other TBA salts of anions in aqueous medium, with 8.6 nM detection limit, which makes their utility in phosphate supplement analysis. The proposed sensor was also compared with the recent reports of phosphate sensing, clearly showing the presented work as progression in field of phosphate determination in total aqueous samples. The proposed sensor was successfully employed for determination of phosphate in various biomolecules and urine samples having phosphate moiety. Acknowledgement B. K. Billing is thankful to IIT Ropar for fellowship. N.S. is thankful to DST for research funding. 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.electacta.2015 .09.114. References [1] A.M. Costero, M.J. Banuls, M.J. Aurell, L.E. Ochando, A. Domenech, Tetrahedron 61 (10) (2005) 309–10320. [2] J. Wang, F.Q. Bai, B.H. Xia, L. Sun, H.X. Zhang, J. Phys. Chem. A 115 (2011) 1985–1991. [3] D. Nieto, A.M.G. Vadillo, S. Bruna, C.J. Pastor, A.E. Kaifer, I. Cuadrado, Chem. Commun. 47 (2011) 10398–10400. [4] Y. Kang, K. Gwon, J.H. Shin, H. Nam, M.E. Meyerhoff, G.S. Cha, Anal. Chem. 83 (2011) 3957–3962. [5] R.M. Duke, E.B. Veale, F.M. Pfeffer, P.E. Kruger, T. Gunnlaugsson, Chem. Soc. Rev. 39 (2010) 3936–3953. [6] S. Guha, S. Saha, J. Am. Chem. Soc. 132 (2010) 17674–17677. [7] K. Kikuchi, S. Hashimoto, S. Mizukami, T. Nagano, Org. Lett. 11 (2009) 2732–2735. [8] S. Hu, Y. Guo, Y. Xu, S. Shao, Org. Biomol. Chem. 6 (2008) 2071–2075. [9] A. Bianchi, K.B. James, E.G. Espana, Supramolecular Chemistry of Nanomaterials, Wiley-VCH, New York, 1997. [10] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. Engl. 40 (2001) 486. [11] J.J. Park, Y.H. Kim, C. Kim, J. Kang, Tetrahedron Lett. 52 (2011) 3361. [12] X.P. Bao, Y.H. Zhou, J.H. Yu, J. Lumin. 130 (2010) 392.
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