Letter pubs.acs.org/ac
Efficient Reaction Based Colorimetric Probe for Sensitive Detection, Quantification, and On-Site Analysis of Nitrite Ions in Natural Water Resources Nagappanpillai Adarsh, Madhesh Shanmugasundaram, and Danaboyina Ramaiah*,‡ Photosciences and Photonics, Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum, Kerala, 695 019 India S Supporting Information *
ABSTRACT: We have developed a novel aza-BODIPY probe for the sensitive colorimetric detection of the nitrite ions in the aqueous medium by a simple and direct method. This probe selectively recognizes the nitrite ions through a distinct visual color change from bright blue to intense green with a sensitivity of 20 ppb. Uniquely, this probe can be coated on a glass surface to fabricate a simple solid-state dipstick device that can be used for the visual detection of the nitrite ions in the presence of other competing anions in distilled as well as natural water resources like a sea, lake, and river. Furthermore, this probe can be used for the sensitive detection of the nitrate ions when coupled to a reduction step. Our results demonstrate that this probe not only can be used for the on-site analysis and quantification but also can replace the conventional spot test carried out for the nitrite ions in the laboratory practical experiments. itrite (NO2−) ions are considered to be type A inorganic contaminants in drinking water and have proven to be of a great threat to human health.1 Nitrite ions, upon interaction with proteins, act as important precursors for the generation of highly carcinogenic N-nitrosamines.2 Excessive consumption of nitrites can lead to a number of medical issues such as esophageal cancer, infant methemoglobinemia (blue baby syndrome), spontaneous abortion, and birth defects in the central nervous system.3 The maximum contaminant level (MCL), legislated by the U.S. Environmental Protection Agency (EPA), of nitrite is 1 ppm (21.7 μM), and analogous guideline values set by the World Health Organization (WHO) is 3 ppm.4 Therefore, determination of nitrite levels is extremely important in monitoring drinking water quality and clinical diagnosis. Several methods have been reported for the detection of nitrite ion concentrations, including those based on chemical and optical, ion chromatography, and electrochemical methods.5−7 Therefore, a simple and sensitive test for the detection of nitrite ions could greatly aid individuals in evaluating the quality of the drinking water source. Of the optical methods reported, the spectrophotometric technique is simple and can be used for the visual on-site analysis. The commercially used nitrite sensor, Griess reagent, is based on this technique, which involves the diazotization of sulfanilamide by nitrous acid under acidic conditions, and coupling of the in situ generated diazonium ion with N-(1naphthyl)ethylenediamine affords azochromophore.8 However, special attention is required for the preparation and storage of this reagent because of the usage of high concentrations of
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© 2013 American Chemical Society
three different components. Besides, the way of detection makes it difficult in the fabrication of a simple strip or dipstick for the practical applications. Herein, we report a novel azaBODIPY probe 1 which acts as a sensitive probe for nitrite ions and signals the event through visible color change. As far as we know, this is the simplest probe which can be used in the form of strips or dipsticks and can be used for the on-site analysis of the nitrite ions in the aqueous medium and replace the conventional spot test carried out in the laboratory practical experiments.
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RESULTS AND DISCUSSION
The synthesis of the probe 1 was achieved through a facile three-step route starting from the corresponding amino chalcone.9 Addition of nitromethane to the chalcone in presence of potassium carbonate gave the nitro derivative (ca. 80%). Subsequently, aza-dipyrromethene was synthesized by refluxing the nitromethane adduct with ammonium acetate in ethanol (ca. 50%). The aza-dipyrromethene thus formed was on reaction with boron trifluoride diethyl etherate in the presence of diethylisopropylamine (DIEA) affording the targeted aza-BODIPY probe 1 in ca. 80% yield (Scheme 1). All these compounds were unambiguously characterized on the Received: September 4, 2013 Accepted: October 17, 2013 Published: October 17, 2013 10008
dx.doi.org/10.1021/ac4031303 | Anal. Chem. 2013, 85, 10008−10012
Analytical Chemistry
Letter
monomer state and ruling out the formation of the aggregates under these conditions (Supporting Information, Figure S4). The blue color of the protonated species turned to green in the presence of nitrite ions (Figure 1D), which indicated the formation of hydroxyl derivative 1-OH, thereby demonstrating its potential as the probe for the colorimetric detection of nitrite ions.13 To investigate the potential of aza-BODIPY derivative 1 as the molecular probe, we have studied its interactions with various anions in the aqueous medium. The successive addition of the nitrite ions to the protonated form of the probe 1 resulted in a decrease in absorbance at 570 nm along with a gradual red shift in maximum. With the addition of 2 ppm of the nitrite ions, we observed ca. 70% hypochromicity at 570 nm with a concomitant bathochromic shift of 60 nm and a change in color from blue to green (Figure 2). Analysis of the
basis of spectroscopic and analytical evidence (Supporting Information, Figures S1−S3). Scheme 1. Synthesis of the aza-BODIPY Probe 1
The aza-BODIPY probe 1 exhibited good solubility in common organic solvents, high photostability, and excellent shelf life. It showed strong absorption in the NIR region (λmax @750 nm) having high molar extinction coefficient value of 4.8 ± 0.04 × 104 M−1cm−1 (Figure 1A), while fluorescence
Figure 2. Changes in absorption spectra of the aza-BODIPY probe 1 in 1N HCl (20 μM, protonated) by the successive addition of nitrite ions in water (a) 0 ppm to (j) 2 ppm. Inset shows the linear plot for the estimation of limit of detection. Data points represent the mean of more than three independent experiments (±SD).
concentration dependent absorbance revealed a linear response of the probe 1 for nitrite ions in the range of 0−2 ppm (inset of Figure 2). The limit of detection (LOD) of the probe 1 toward the nitrite ions was found to be 20 ppb (0.5 μM), which is more sensitive than the maximum allowable level of contamination of nitrite ions in water. The deep green colored species generated in the assay was highly visible, demonstrating the utility of the probe 1 for the on-site visual detection of the nitrite ions. Similar titration experiments were carried out with other biologically relevant anions such as SO42−, Cl−, HSO3−, CO32−, CH3COO−, NO3−, S2O32−, and N3−. We observed negligible changes in the absorption of the probe 1 even at 100-fold higher concentrations of these ions when compared to the nitrite ions (Supporting Information, Figures S5−S6). These results revealed that the probe 1 showed excellent selectivity toward the nitrite ions even in the presence of the other competitive anions (Figure 3) and thus can be used for the efficient detection of nitrite ions in samples of different origins. The possible mechanism for the detection can be due to the diazotization reaction between the protonated form of the probe 1 and the nitrite ions present in the water. The diazonium salt thus formed in situ undergoes hydrolysis in the aqueous medium to give green hydroxyl substituted product, 1OH (Scheme 2), which was characterized on the basis of spectral and analytical evidence (Supporting Information for the spectral details). The IR spectrum of the hydroxy derivative 1-OH showed a broad band at 3298 cm−1, and the FAB-MS
Figure 1. Normalized (A) absorption and (B) emission spectra of the probe 1 (2 × 10−4 M) in methanol (λex = 740 nm). Absorption spectra of (C) the protonated form of the probe 1 (pH = 0.2) and (D) the hydroxy derivative 1-OH.
maximum was observed at 815 nm with a quantum yield of ca. 0.02 (Figure 1B, IR-125 is used as the reference).10 Notably, the presence of electron rich amino groups in the aza-BODIPY dye bestowed an enhanced bathochromic shift in the absorption and emission spectra compared to those reported for the BODIPY and aza-BODIPY derivatives.11,12 The addition of dilute acid (1N HCl) to the probe 1 afforded the protonated species, which was characterized by the blue-shifted absorption, and the pH of the medium was kept constant (pH =
