- Email: [email protected]
Chemical switch based reusable dual optoelectronic sensor for nitrite
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
Chemical switch based reusable dual optoelectronic sensor for nitrite V. Vishnuvardhan, R. Kala, T. Prasada Rao ∗ Inorganic and Polymer materials Group (CSTD), National Institute for Interdisciplinary Science & Technology (NIIST), CSIR, Trivandrum 695019, India
a r t i c l e
i n f o
a b s t r a c t
Article history:
An optical sensor was developed for sensing of nitrite based on the monotonous decrease in
Received 27 March 2008
absorbance of Rhodamine 6G at 525 nm (the absorbance maximum of dye) with increasing
Received in revised form
concentration of nitrite. This sensor also permits naked eye detection. Various parameters
22 May 2008
like concentrations of sulphuric acid and Rhodamine 6G, response time and stability were
Accepted 24 May 2008
varied and optimal conditions are reported. Under these conditions, the developed sen-
Published on line 8 June 2008
sor enables the determination of nitrite in the concentration range 0–12.18 mol L−1 . The nitrite response is selective as 60–2.5 × 105 fold amounts of several anions and cations have
Keywords:
no deleterious effect. The addition of nitrite to Rhodamine 6G dye causes hypsochromic
Dual optical sensor
shift from 525 to 385 nm while several other anions like I− , SCN− , ClO4 − , [HgI4 ]2− and [Zn
Nitrite
(SCN)4 ]2− showed a bathochromatic shift from 525 to 575 nm. The sequential addition of
Chemical switch
nitrite and sulphamic to Rhodamine 6G in 0.75 mol L−1 sulphuric acid solution results in
Natural waters
switching of “ON” and “OFF” absorbance. The time elapse and concentration of sulphamic
Food materials
acid required for chemical switching was also established. Similar “ON” and “OFF” switching behaviour was observed in fluorescence studies also. This enabled the design and development of reusable chemical switch based dual optoelectronic sensor, for monitoring of traces of nitrite in environmental and food samples. The plausible mechanism for above switching behaviour is also proposed. © 2008 Published by Elsevier B.V.
1.
Introduction
The determination of nitrite is of general importance because of its harmful impact on human health. Nitrite is the most ubiquitous chemical contaminant in food, industrial and physiological systems. This ion can easily interact with amines to form toxic and carcinogenic nitrosoamines. Another danger that occurs after nitrite ingestion is that it can convert oxyhemoglobin to methemoglobin in our blood stream, thereby interfering with oxygen transport in the blood [1]. A limit of 45 g mL−1 has been proposed for the nitrite present in drinking water
∗
since excessive amounts lead to methemoglobinalmia in infants. Chemical switches find wide interest in science and technology due to their ability to undergo structural changes in response to a respective stimulation [2–7]. However, a search for a chromophore, whose changes in color can be used to sense nitrite both qualitatively and quantitatively in presence of several other coexisting anions, is highly desirable. Such reagents would be especially valuable if they are commercially marketed without resorting to dedicated synthetic protocols. Also extraction of these reagents into toxic organic solvents has to be avoided. Here, we have focused on Rhodamine 6G
Corresponding author. Tel.: +91 471 2515317; fax: +91 471 2491712. E-mail addresses: [email protected], [email protected] (T. Prasada Rao). 0003-2670/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.aca.2008.05.075
54
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
(Rh 6G) as a potential off-the-shelf chromoionophore that can sense nitrite. Rh 6G, a xanthane dye is an important analytical reagent which is stable, pH independent and has been applied to resonance scattering, spectrophotometric, fluorimetric and polarographic analysis. Optical nitrite sensor on chemical modification of a polymer film with gallo cyanine [8] and Safranine O [9] has been reported. However, chemical switch behaviour is not attempted in these works. Jiang et al. [10] have reported a resonance scattering method for the determination of traces of nitrite in water with Rh 6G in HCl–KI medium. This is an indirect method for the determination of nitrite where nitrite ion reacts with I− to form I3 − in dilute HCl and I3 − combines with Rh 6G+ to form (Rh 6G.I3 )n association particles with resonance scattering at 320, 400 and 595 nm. Similarly, Jiao et al. [11] have reported a reversible chemosensor for nitrite based on the fluorescence quenching of a carbazole derivative, which is also based on the reaction between nitrite ion and excess I− to form I3 − . These reports have a drawback that the selectivity of nitrite sensing could be influenced by the presence of several other species coexisting with nitrite which will also react with I− to form I3 − . We now propose Rh 6G based system for the sensing of nitrite capable of switching absorbance ‘ON’ and ‘OFF’ in aqueous solutions itself.
2.
Experimental
2.1.
Reagents
Analytical reagent grades were used and demineralized water was used for preparing the solutions. Stock solution of sodium nitrite was prepared by dissolving 0.1500 g of sodium nitrite (E Merck, Mumbai, India) in 100 mL of water. A small amount of chloroform was added as stabilizer [12]. Working standards were prepared by appropriate dilution of stock solution. Sulphuric acid (E Merck, Mumbai, India) and Rhodamine 6G (Aldrich, Milwaukee, WI, USA) were also used in the current study.
2.2.
2.4.
Analysis of natural waters
Appropriate aliquots of tap, river and pond water samples were taken for quantification by following the above mentioned procedure.
2.5.
Analysis of food materials
About 1 g amounts of table salt and sugar were dissolved in water and appropriate aliquots of this solution was taken for quantification by following the above mentioned procedure.
3.
Results
3.1.
Chemical switching – absorbance
A systematic study of addition of 5 mol L−1 nitrite to various 1 mL of 0.01% water soluble triphenylmethane cationic dyes was undertaken by both naked eye detection and spectrophotometric measurements after adjusting the overall acidity to pH 5.0 and 0.1 and 0.75 mol L−1 H2 SO4 . The cationic dyes chosen for this study are Rh 6G, Rh B, methylene blue, crystal violet, brilliant green, xylenol orange, catechol violet, ethyl violet and malachite green. Such studies indicated that Rh 6G alone gives a decrease in visual color intensity and consequent lowering of absorbance on addition of nitrite at acidities higher than 0.1 mol L−1 in H2 SO4 . On the other hand in case of other cationic dyes, there is no visual color change or change in absorbance spectra. The addition of nitrite to Rh 6G acidified to 0.75 mol L−1 (overall) sulphuric acid results in a decrease in absorbance at max of Rh 6G i.e. 525 nm, which had formed the basis of colorimetric determination of nitrite described briefly in our earlier report [12]. However, it is clear from Curve b of Fig. 1 that the addition of nitrite to Rh 6G results in an additional peak at 385 nm: a blue
Apparatus
A Shimadzu computer controlled UV–vis spectrophotometer UV-2401 PC (Shimadzu, Kyoto, Japan) SPEX Fluorolog 3 Spectrofluorometer (Horiba Jobin-Yvon, Edison, NJ, USA) and Digital pH meter LI-120 (ELICO, Hyderabad, India) were used for absorbance, fluorimetric and pH studies, respectively.
2.3.
Procedure for determination of nitrite
An aliquot of sample solution (up to 20 mL) containing 0–10 g of nitrite was taken into a 25 mL standard volumetric flask. 0.75 mL of 12.5 mol L−1 sulphuric acid was added to the above solution with mixing followed by 1 mL of 0.01% Rhodamine 6G solution. This was then diluted to the mark with deionized water and the absorbance was measured at 525 nm against a reagent blank. The concentration of nitrite was established by reference to a calibration graph.
Fig. 1 – UV–vis spectrum of the switch ‘on–off’ absorbance (a) Rh 6G, (b) Rh 6G + nitrite (6.96 mol L−1 ) and (c) Rh 6G + nitrite (6.96 mol L−1 ) + sulphamic acid (24.74 mol L−1 ) (the visual color change of Rh 6G (a), Rh 6G + nitrite (b) and Rh 6G + nitrite + sulphamic acid (c) shown in the inset). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
55
Fig. 2 – Absorbance spectra of Rh 6G with increasing amounts of 0, 0.87, 1.74, 3.48, 6.09, 8.70 and 12.18 mol L−1 of nitrite (a to g) and corresponding calibration graph (inset ‘b’) and plot showing incremental addition of 0, 2.18, 4.36, 6.54 and 8.70 mol L−1 of nitrite (i–v) (inset ‘a’), respectively.
shift of Rh 6G dye. The absorbances at this max with different concentrations of nitrite obeys Beer–Lambert’s law with a molar extinction coefficient (ε) of 3.5 × 103 L mol−1 cm−1 . The color change is vividly seen by the naked eye on addition of NO2 − to Rh 6G solution in presence of H+ ions, as the orange red color of Rh 6G changes to yellow (see inset of Fig. 1). Fig. 2 shows the changes in the UV–vis spectrum of Rh 6G with increasing concentration of nitrite. In the absence of nitrite, the spectrum of Rh 6G is characterized by a peak at max = 525 nm, the intensity of which goes on decreasing with an increase in the concentration of nitrite and another peak appears at 385 nm, whose color intensity increases. The absorbance change on incremental addition of nitrite to Rh 6G solution (inset a) and calibration plot at 525 nm (inset b) with a linear range of 0.87–12.18 mol L−1 of nitrite are shown in Fig. 2. The addition of 24.74 mol L−1 of sulphamic acid to a solution of Rh 6G containing nitrite (6.96 mol L−1 ) reverts the decrease in absorbance of the dye noticed on addition of nitrite (see inset of Fig. 1). Successive cyclic addition of 0.87 or 3.48 or 6.96 mol L−1 of nitrite and 24.74 mol L−1 of sulphamic acid in each case to Rh 6G in 0.75 mol L−1 H2 SO4 solution resulted in absorbance changes as shown in Fig. 3. It is clear from the figure that successive addition of nitrite and sulphamic acid results in “CHEMICAL SWITCHING” of Rh 6G cationic dye in absorbance mode allowing the possibility of reusing the sensor. The absorbance of the Rh 6G dye was switched ‘off’ on addition of nitrite ion in the presence of H+ ions. However, chemical switching to ‘ON’ absorbance occurs upon addition of sulphamic acid. The process is irreversible and it does not revert back to ‘on’ mode unless sulphamic acid is added. Hence, the amount of sulphamic acid required for complete reversible switching of 0.87, 3.48 and 6.96 mol L−1 nitrite was found to be 24.74 mol L−1 of sulphamic acid. The addition of lower (8.24 and 16.48 mol L−1 ) amounts of sulphamic acid results in partial reversal but for 32.98 and 41.23 mol L−1 of sul-
phamic acid results in lesser reduction in absorbance after the second cycle addition of nitrite. Similar switching trend has been noticed with other reductants such as hydroxylamine hydrochloride (115.9 mol L−1 ) and hydrazine sulphate (4.60 mol L−1 ).
3.2.
Chemical switching – fluorescence
The emission spectrum of 1 mL of 0.01% Rh 6G present in 25 mL adjusted to an overall acidity of 0.75 mol L−1 in H2 SO4 with excitation at 355 nm is shown in Curve a of Fig. 4. The addition of 6.96 mol L−1 nitrite results in significant QUENCHING as seen from the Curve b of Fig. 4 at emission maximum
Fig. 3 – Absorbance change during the cyclic addition of 0.87, 3.48 and 6.96 mol L−1 of nitrite and 24.74 mol L−1 of sulphamic acid and corresponding calibration plots for I and II cycle addition of nitrite (see inset).
56
SCN− 2.5 × 102 PO4 3− 2.5 × 105 NO3 − 2.5 × 105 Br− 2.5 × 105 Interferent Tolerance limit
Table 1 – Interference studies (6.96 mol L−1 of nitrite)
I− 62.5
ClO4 − 1.25 × 103
Cd2+ 2.5 × 105
Pb2+ 1.25 × 102
Hg2+ 6.25 × 103
Zn2+ 6.25 × 105
Cu2+ 1.25 × 103
NaCl 1.25 × 105
KCl 1.25 × 105
MgCl2 1.25 × 105
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
Fig. 4 – Fluorescence spectrum of the switch ‘on–off’ fluorescence (a) Rh 6G, (b) Rh 6G + nitrite (6.96 mol L−1 ) and (c) Rh 6G + nitrite (6.96 mol L−1 ) + sulphamic acid (24.74 mol L−1 ).
of Rh 6G i.e. 565 nm. Again, as in case of absorbance studies, the addition of 24.74 mol L−1 sulphamic acid results in retrieving the original fluorescence intensity of Rh 6G. This cycle is repeated once again on subsequent sequential addition of nitrite and sulphamic acid. Thus, a CHEMICAL SWITCHING behaviour is once again noticed in case of fluorescence studies also as in case of absorbance studies described above.
3.3.
Analytical optimization studies
The influence of acidity on the change in absorbance during the reaction of Rh 6G with nitrite was studied. The decrease in absorbance was found to be constant and maximum in the range 0.50–1.25 mol L−1 H2 SO4 (overall). The effect of the concentration of Rh 6G on the maximum change in absorbance was obtained as 1 mL of 0.01% Rh 6G in a total volume of 25 mL. The minimum time required for the complete reaction of nitrite with Rh 6G was 5 min and remained stable up to 30 min, indicating that the species formed by the reaction of nitrite with dye was relatively stable. The time required for the reaction of sulphamic acid with the species formed by the reaction between nitrite and Rh 6G is greater than 30 min i.e. the time required to regain the color intensity nearly equal to that of the free dye is greater than 30 min. The variation in absorbance with change in concentration of sulphamic acid added to the reaction mixture containing 8.7 mol L−1 of nitrite and Rh 6G indicates that a minimum of 24.74 mol L−1 of sulphamic acid is needed to switch the color system. Under these conditions, the determination of nitrite is possible in the concentration range 0–12.18 mol L−1 with a detection limit of 0.87 mol L−1 by absorbance mode. There was no dramatic enhancement in sensitivity in fluorescence mode since the quantification was by quenching of fluorescence which leads to higher blank values.
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
Table 2 – Analysis of environmental samples and food samplesa Sample
Tap water (g L−1 ) River water (g L−1 ) Pond water (g L−1 ) Table salt (g g−1 ) Sugar (g g−1 ) a
Nitrite sensor 9.1 21.0 56.8 1.20 1.05
± ± ± ± ±
p-Nitroaniline–quinoline-8-ol based spectrophotometric method [15]
2.4 1.4 3.2 0.02 0.13
9.2 19.8 64.1 1.20 0.83
± ± ± ± ±
1.7 1.7 5.8 0.20 0.15
Deduced from standard addition plot drawn from the analytical values obtained by spiking real sample with four different concentrations of nitrite.
3.4.
Effect of other anionic species
With the addition of other anions like I− , SCN− , ClO4 − , [HgI4 ]2− and [Zn(SCN4 )]2− to Rh 6G solution, the intensity of the peak at 525 nm decreases as in case of nitrite anion while a new peak appears at 578 nm – a sort of red shift of Rh 6G dye. The changes in absorbance at 525 nm was less dramatic and required greater amounts of anions (unlike nitrite) in case of I− , SCN− and ClO4 − compared to [HgI4 ]2− and [Zn(SCN4 )]2− to effect a commensurate change. Here, the observed red shift in the absorbance peak could be possibly due to the exchange of anions with Cl− of the quaternary ammonium part of the dye which is well documented in literature [13,14]. However, the above mentioned spectral changes on addition of anions either individually or in admixtures are completely annulled by addition of nitrite, thus enabling selective sensing of nitrite. The qualitative changes of absorbance with other anions are reflected in more quantifiable terms also. 2.5 × 105 fold amounts of NO3 − , Br− and PO4 3− and 2.5 × 102 , 62.5 and 1.25 × 103 fold amounts of SCN− , I− and ClO4 − ions with respect to nitrite did not interfere during nitrite sensing. In order to check the applicability of the system for sensing nitrite in complex real samples, studies were carried out in terms of selectivity of the sensor for other anions, cations and neutral salts also. Table 1 shows the tolerance limits of various coexisting anions, cations and neutral salts with respect to 6.96 mol L−1 nitrite. The quantification of nitrite was unaffected even in the presence of a multi-component mixture of PO4 3− /NO3 − /Br− (1 × 10−3 %), SCN− (4 × 10−3 %), ClO4 − (2 × 10−3 %) and I− (1 × 10−6 %). Moreover, the color reaction of nitrite with Rh 6G is highly specific as none of the anions, cations and their admixtures at 105 times to that of nitrite have any deleterious effect.
3.5.
Application to real samples
The applicability of the developed reusable sensor was tested for analysis of natural waters and selected food materials. The analysis was carried out by “Standard addition method” in which the values obtained from standard addition plot drawn from the analytical values obtained on spiking real samples with four known concentrations of nitrite. The results obtained for various real samples are shown in Table 2. Furthermore, the results obtained by present
57
method are compared with standard UV–vis spectrophotometric method based on “p-nitroaniline–quinoline-8-ol” as chromogenic reagent [15]. The analytical values obtained by the newly developed sensor compares favorably with standard spectrophotometric method indicating the efficacy of developed reusable sensor for reliable and routine monitoring of nitrite in various environmental samples and food materials.
4.
Discussion
The formation of ternary ion associate by interaction of charged binary anionic complexes of various metal cations like Hg, Cd, Pb, Zn, etc. with cationic dye Rh 6G in presence of iodide or thiocyanate results in bathochromic shift from 525 to 578 nm. This has been advantageously utilized by various research groups for sensitive determination of metal cations directly in aqueous phase itself thus precluding the use of toxic organic solvents. In all these cases, the concentration of metal cation increases the formation of ternary associate of [Hg I4 ]2− and [Zn(SCN)4 ]2− with Rh 6G and results in increase in absorbance at 578 nm with concomitant decrease in absorbance at max of the dye i.e. at 525 nm. On the other hand, the addition of nitrite into Rh 6G in 0.75 mol L−1 H2 SO4 solutions results in hypsochromic shift to 385 nm (see Fig. 1) wherein the increase in absorbance at this max is proportional to nitrite. Again, as seen from Fig. 2, the decrease in absorbance at max of the Rh 6G is also proportional to nitrite. The magnitude of absorbance change in latter case is high. Other anions show a similar decrease at 525 nm but at much larger amounts i.e. 102 –105 to that of nitrite and that too in absence of nitrite (see Table 1). The observed bathochromic shift noticed in presence of anions other than nitrite and binary anionic complexes of metal ions could possibly be due to the exchange of anions with Cl− of the quaternary ammonium part of the dye. The addition of traces of nitrite results in alteration in conjugation of Rh 6G (see Scheme 1) thus upsetting the bathochromic shift observed in case of other anions (even at higher amounts). This enables the sensing of nitrite in presence of large excess of extraneous anions or binary anionic complexes of metal cations or their admixtures. In addition, several cations do not have any deleterious effect as they do not interact with Rh 6G in the absence of suitable anionic ligand. In order to explore the cause of the decrease in absorbance of Rh 6G, the experiment was carried out at pH 5.0. We did not notice any change in the absorbance of Rh 6G on addition of NO2 − . On the other hand, in solutions of pH 2.0 or lower, it can significantly reduce the absorbance of Rh 6G at its max . Based on this observation, one could envisage a plausible mechanism as shown in Scheme 1. Nitrite ions in the presence of H+ ions produce nitrosyl cation, which is electron deficient and can attack the secondary amine group of Rh 6G (1), resulting in the formation of N-nitrosamine derivative (2) [16]. In the second step, sulphamic acid reacts with N-nitrosamine derivative to give back the original dye. As seen from Scheme 1, the extent of conjugation in the dye is lost when the N-nitrosamine derivative is formed and this results in a blue shift to 385 nm.
58
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
Scheme 1 – Mechanism involved in the chemical switch based nitrite sensor.
A similar “on–off” switch behaviour observed in case of fluorescence measurements can also be explained by the structural changes in Rh 6G on addition of nitrite and subsequent reversal by the addition of sulphamic acid. As we could not isolate the oxidized products formed on addition of nitrite, we were unable to get spectral evidences other than UV–vis and fluorescence studies.
5.
Conclusions
A chemical switch based reusable optical sensor for nitrite has been designed using the off-the-shelf readily available chromoionophore, i.e. Rh 6G. To the best of our knowledge, such a chemical switch system is the first of its kind, which can discriminate nitrite from very large amounts of several anions either individually or in admixtures due to blue and red shifts of the absorbance maximum of Rh 6G with nitrite and other anions, respectively. The developed sensor has been successfully demonstrated for its applicability in analysis of natural waters and food materials. A plausible mechanism involved in the chemical switching process has been proposed (see Scheme 1).
Acknowledgements This work was financially supported by Council of Scientific and Industrial Research (CSIR), New Delhi through Network project on “Specialty Inorganic Materials for Diverse Appli-
cations(NWP0010) and Senior Research Fellowship granted to Ms. R. Kala.
references
[1] Q. Yue, Z. Song, Microchem. J. 84 (2006) 10. [2] V. Amendola, I. Fabbrizzi, F. Foti, M.C. Licchelli, P. Pallavicini, D. Poggi, A. Sacchi, A. Taglietti, Coord. Chem. Rev. 250 (2006) 273. [3] R.A. Illos, E. Harlev, S. Bittner, Tetrahedran Lett. 46 (2005) 8427. [4] R.A. Illos, D. Shamir, L.J.W. Shimon, I. Zilbumann, S. Bittner, Tetrahedran Lett. 47 (2006) 5543. [5] K. Rurack, Spectrochim. Acta Part A 57 (2001) 2161. [6] K. Szacilowski, Z. Stasicka, Coord. Chem. Rev. 229 (2002) 17. [7] K. Szacilowski, W.C.R. Macyk, Chimie 9 (2006) 315. [8] A. Kazemzadeh, S. Daghighi, Spectrochim. Acta Part A 61 (2005) 1871. [9] A.A. Ensafi, A. Kazemzadeh, Microchem. J. 72 (2002) 193. [10] Z.L. Jiang, S.J. Sun, C.Y. Kang, X. Lu, J. Lan, Anal. Bioanal. Chem. 381 (2005) 896. [11] C.X. Jiao, C.G. Niu, S.Y. Huan, Q. Shen, Y. Yang, G.L. Shen, R.Q. Yu, Talanta 64 (2004) 637. [12] S. Kartikeyan, T. Prasadarao, C.S.P. Iyer, A.D. Damodaran, Mikrochim. Acta 111 (1993) 193. [13] R. Lobinski, Z. Marczenko, Crit. Rev. Anal. Chem. 23 (1992) 55. [14] T. Prasada Rao, M.L.P. Reddy, A.R. Pillai, Talanta 46 (1998) 765. [15] J. Nair, V.K. Gupta, Anal. Chim. Acta 111 (1979) 311. [16] W.H. Brown, C.S. Foote, B.L. Iverson (Eds.), Organic Chemistry, Brooke/Cole, 2004.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
Chemical switch based reusable dual optoelectronic sensor for nitrite V. Vishnuvardhan, R. Kala, T. Prasada Rao ∗ Inorganic and Polymer materials Group (CSTD), National Institute for Interdisciplinary Science & Technology (NIIST), CSIR, Trivandrum 695019, India
a r t i c l e
i n f o
a b s t r a c t
Article history:
An optical sensor was developed for sensing of nitrite based on the monotonous decrease in
Received 27 March 2008
absorbance of Rhodamine 6G at 525 nm (the absorbance maximum of dye) with increasing
Received in revised form
concentration of nitrite. This sensor also permits naked eye detection. Various parameters
22 May 2008
like concentrations of sulphuric acid and Rhodamine 6G, response time and stability were
Accepted 24 May 2008
varied and optimal conditions are reported. Under these conditions, the developed sen-
Published on line 8 June 2008
sor enables the determination of nitrite in the concentration range 0–12.18 mol L−1 . The nitrite response is selective as 60–2.5 × 105 fold amounts of several anions and cations have
Keywords:
no deleterious effect. The addition of nitrite to Rhodamine 6G dye causes hypsochromic
Dual optical sensor
shift from 525 to 385 nm while several other anions like I− , SCN− , ClO4 − , [HgI4 ]2− and [Zn
Nitrite
(SCN)4 ]2− showed a bathochromatic shift from 525 to 575 nm. The sequential addition of
Chemical switch
nitrite and sulphamic to Rhodamine 6G in 0.75 mol L−1 sulphuric acid solution results in
Natural waters
switching of “ON” and “OFF” absorbance. The time elapse and concentration of sulphamic
Food materials
acid required for chemical switching was also established. Similar “ON” and “OFF” switching behaviour was observed in fluorescence studies also. This enabled the design and development of reusable chemical switch based dual optoelectronic sensor, for monitoring of traces of nitrite in environmental and food samples. The plausible mechanism for above switching behaviour is also proposed. © 2008 Published by Elsevier B.V.
1.
Introduction
The determination of nitrite is of general importance because of its harmful impact on human health. Nitrite is the most ubiquitous chemical contaminant in food, industrial and physiological systems. This ion can easily interact with amines to form toxic and carcinogenic nitrosoamines. Another danger that occurs after nitrite ingestion is that it can convert oxyhemoglobin to methemoglobin in our blood stream, thereby interfering with oxygen transport in the blood [1]. A limit of 45 g mL−1 has been proposed for the nitrite present in drinking water
∗
since excessive amounts lead to methemoglobinalmia in infants. Chemical switches find wide interest in science and technology due to their ability to undergo structural changes in response to a respective stimulation [2–7]. However, a search for a chromophore, whose changes in color can be used to sense nitrite both qualitatively and quantitatively in presence of several other coexisting anions, is highly desirable. Such reagents would be especially valuable if they are commercially marketed without resorting to dedicated synthetic protocols. Also extraction of these reagents into toxic organic solvents has to be avoided. Here, we have focused on Rhodamine 6G
Corresponding author. Tel.: +91 471 2515317; fax: +91 471 2491712. E-mail addresses: [email protected], [email protected] (T. Prasada Rao). 0003-2670/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.aca.2008.05.075
54
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
(Rh 6G) as a potential off-the-shelf chromoionophore that can sense nitrite. Rh 6G, a xanthane dye is an important analytical reagent which is stable, pH independent and has been applied to resonance scattering, spectrophotometric, fluorimetric and polarographic analysis. Optical nitrite sensor on chemical modification of a polymer film with gallo cyanine [8] and Safranine O [9] has been reported. However, chemical switch behaviour is not attempted in these works. Jiang et al. [10] have reported a resonance scattering method for the determination of traces of nitrite in water with Rh 6G in HCl–KI medium. This is an indirect method for the determination of nitrite where nitrite ion reacts with I− to form I3 − in dilute HCl and I3 − combines with Rh 6G+ to form (Rh 6G.I3 )n association particles with resonance scattering at 320, 400 and 595 nm. Similarly, Jiao et al. [11] have reported a reversible chemosensor for nitrite based on the fluorescence quenching of a carbazole derivative, which is also based on the reaction between nitrite ion and excess I− to form I3 − . These reports have a drawback that the selectivity of nitrite sensing could be influenced by the presence of several other species coexisting with nitrite which will also react with I− to form I3 − . We now propose Rh 6G based system for the sensing of nitrite capable of switching absorbance ‘ON’ and ‘OFF’ in aqueous solutions itself.
2.
Experimental
2.1.
Reagents
Analytical reagent grades were used and demineralized water was used for preparing the solutions. Stock solution of sodium nitrite was prepared by dissolving 0.1500 g of sodium nitrite (E Merck, Mumbai, India) in 100 mL of water. A small amount of chloroform was added as stabilizer [12]. Working standards were prepared by appropriate dilution of stock solution. Sulphuric acid (E Merck, Mumbai, India) and Rhodamine 6G (Aldrich, Milwaukee, WI, USA) were also used in the current study.
2.2.
2.4.
Analysis of natural waters
Appropriate aliquots of tap, river and pond water samples were taken for quantification by following the above mentioned procedure.
2.5.
Analysis of food materials
About 1 g amounts of table salt and sugar were dissolved in water and appropriate aliquots of this solution was taken for quantification by following the above mentioned procedure.
3.
Results
3.1.
Chemical switching – absorbance
A systematic study of addition of 5 mol L−1 nitrite to various 1 mL of 0.01% water soluble triphenylmethane cationic dyes was undertaken by both naked eye detection and spectrophotometric measurements after adjusting the overall acidity to pH 5.0 and 0.1 and 0.75 mol L−1 H2 SO4 . The cationic dyes chosen for this study are Rh 6G, Rh B, methylene blue, crystal violet, brilliant green, xylenol orange, catechol violet, ethyl violet and malachite green. Such studies indicated that Rh 6G alone gives a decrease in visual color intensity and consequent lowering of absorbance on addition of nitrite at acidities higher than 0.1 mol L−1 in H2 SO4 . On the other hand in case of other cationic dyes, there is no visual color change or change in absorbance spectra. The addition of nitrite to Rh 6G acidified to 0.75 mol L−1 (overall) sulphuric acid results in a decrease in absorbance at max of Rh 6G i.e. 525 nm, which had formed the basis of colorimetric determination of nitrite described briefly in our earlier report [12]. However, it is clear from Curve b of Fig. 1 that the addition of nitrite to Rh 6G results in an additional peak at 385 nm: a blue
Apparatus
A Shimadzu computer controlled UV–vis spectrophotometer UV-2401 PC (Shimadzu, Kyoto, Japan) SPEX Fluorolog 3 Spectrofluorometer (Horiba Jobin-Yvon, Edison, NJ, USA) and Digital pH meter LI-120 (ELICO, Hyderabad, India) were used for absorbance, fluorimetric and pH studies, respectively.
2.3.
Procedure for determination of nitrite
An aliquot of sample solution (up to 20 mL) containing 0–10 g of nitrite was taken into a 25 mL standard volumetric flask. 0.75 mL of 12.5 mol L−1 sulphuric acid was added to the above solution with mixing followed by 1 mL of 0.01% Rhodamine 6G solution. This was then diluted to the mark with deionized water and the absorbance was measured at 525 nm against a reagent blank. The concentration of nitrite was established by reference to a calibration graph.
Fig. 1 – UV–vis spectrum of the switch ‘on–off’ absorbance (a) Rh 6G, (b) Rh 6G + nitrite (6.96 mol L−1 ) and (c) Rh 6G + nitrite (6.96 mol L−1 ) + sulphamic acid (24.74 mol L−1 ) (the visual color change of Rh 6G (a), Rh 6G + nitrite (b) and Rh 6G + nitrite + sulphamic acid (c) shown in the inset). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
55
Fig. 2 – Absorbance spectra of Rh 6G with increasing amounts of 0, 0.87, 1.74, 3.48, 6.09, 8.70 and 12.18 mol L−1 of nitrite (a to g) and corresponding calibration graph (inset ‘b’) and plot showing incremental addition of 0, 2.18, 4.36, 6.54 and 8.70 mol L−1 of nitrite (i–v) (inset ‘a’), respectively.
shift of Rh 6G dye. The absorbances at this max with different concentrations of nitrite obeys Beer–Lambert’s law with a molar extinction coefficient (ε) of 3.5 × 103 L mol−1 cm−1 . The color change is vividly seen by the naked eye on addition of NO2 − to Rh 6G solution in presence of H+ ions, as the orange red color of Rh 6G changes to yellow (see inset of Fig. 1). Fig. 2 shows the changes in the UV–vis spectrum of Rh 6G with increasing concentration of nitrite. In the absence of nitrite, the spectrum of Rh 6G is characterized by a peak at max = 525 nm, the intensity of which goes on decreasing with an increase in the concentration of nitrite and another peak appears at 385 nm, whose color intensity increases. The absorbance change on incremental addition of nitrite to Rh 6G solution (inset a) and calibration plot at 525 nm (inset b) with a linear range of 0.87–12.18 mol L−1 of nitrite are shown in Fig. 2. The addition of 24.74 mol L−1 of sulphamic acid to a solution of Rh 6G containing nitrite (6.96 mol L−1 ) reverts the decrease in absorbance of the dye noticed on addition of nitrite (see inset of Fig. 1). Successive cyclic addition of 0.87 or 3.48 or 6.96 mol L−1 of nitrite and 24.74 mol L−1 of sulphamic acid in each case to Rh 6G in 0.75 mol L−1 H2 SO4 solution resulted in absorbance changes as shown in Fig. 3. It is clear from the figure that successive addition of nitrite and sulphamic acid results in “CHEMICAL SWITCHING” of Rh 6G cationic dye in absorbance mode allowing the possibility of reusing the sensor. The absorbance of the Rh 6G dye was switched ‘off’ on addition of nitrite ion in the presence of H+ ions. However, chemical switching to ‘ON’ absorbance occurs upon addition of sulphamic acid. The process is irreversible and it does not revert back to ‘on’ mode unless sulphamic acid is added. Hence, the amount of sulphamic acid required for complete reversible switching of 0.87, 3.48 and 6.96 mol L−1 nitrite was found to be 24.74 mol L−1 of sulphamic acid. The addition of lower (8.24 and 16.48 mol L−1 ) amounts of sulphamic acid results in partial reversal but for 32.98 and 41.23 mol L−1 of sul-
phamic acid results in lesser reduction in absorbance after the second cycle addition of nitrite. Similar switching trend has been noticed with other reductants such as hydroxylamine hydrochloride (115.9 mol L−1 ) and hydrazine sulphate (4.60 mol L−1 ).
3.2.
Chemical switching – fluorescence
The emission spectrum of 1 mL of 0.01% Rh 6G present in 25 mL adjusted to an overall acidity of 0.75 mol L−1 in H2 SO4 with excitation at 355 nm is shown in Curve a of Fig. 4. The addition of 6.96 mol L−1 nitrite results in significant QUENCHING as seen from the Curve b of Fig. 4 at emission maximum
Fig. 3 – Absorbance change during the cyclic addition of 0.87, 3.48 and 6.96 mol L−1 of nitrite and 24.74 mol L−1 of sulphamic acid and corresponding calibration plots for I and II cycle addition of nitrite (see inset).
56
SCN− 2.5 × 102 PO4 3− 2.5 × 105 NO3 − 2.5 × 105 Br− 2.5 × 105 Interferent Tolerance limit
Table 1 – Interference studies (6.96 mol L−1 of nitrite)
I− 62.5
ClO4 − 1.25 × 103
Cd2+ 2.5 × 105
Pb2+ 1.25 × 102
Hg2+ 6.25 × 103
Zn2+ 6.25 × 105
Cu2+ 1.25 × 103
NaCl 1.25 × 105
KCl 1.25 × 105
MgCl2 1.25 × 105
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
Fig. 4 – Fluorescence spectrum of the switch ‘on–off’ fluorescence (a) Rh 6G, (b) Rh 6G + nitrite (6.96 mol L−1 ) and (c) Rh 6G + nitrite (6.96 mol L−1 ) + sulphamic acid (24.74 mol L−1 ).
of Rh 6G i.e. 565 nm. Again, as in case of absorbance studies, the addition of 24.74 mol L−1 sulphamic acid results in retrieving the original fluorescence intensity of Rh 6G. This cycle is repeated once again on subsequent sequential addition of nitrite and sulphamic acid. Thus, a CHEMICAL SWITCHING behaviour is once again noticed in case of fluorescence studies also as in case of absorbance studies described above.
3.3.
Analytical optimization studies
The influence of acidity on the change in absorbance during the reaction of Rh 6G with nitrite was studied. The decrease in absorbance was found to be constant and maximum in the range 0.50–1.25 mol L−1 H2 SO4 (overall). The effect of the concentration of Rh 6G on the maximum change in absorbance was obtained as 1 mL of 0.01% Rh 6G in a total volume of 25 mL. The minimum time required for the complete reaction of nitrite with Rh 6G was 5 min and remained stable up to 30 min, indicating that the species formed by the reaction of nitrite with dye was relatively stable. The time required for the reaction of sulphamic acid with the species formed by the reaction between nitrite and Rh 6G is greater than 30 min i.e. the time required to regain the color intensity nearly equal to that of the free dye is greater than 30 min. The variation in absorbance with change in concentration of sulphamic acid added to the reaction mixture containing 8.7 mol L−1 of nitrite and Rh 6G indicates that a minimum of 24.74 mol L−1 of sulphamic acid is needed to switch the color system. Under these conditions, the determination of nitrite is possible in the concentration range 0–12.18 mol L−1 with a detection limit of 0.87 mol L−1 by absorbance mode. There was no dramatic enhancement in sensitivity in fluorescence mode since the quantification was by quenching of fluorescence which leads to higher blank values.
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
Table 2 – Analysis of environmental samples and food samplesa Sample
Tap water (g L−1 ) River water (g L−1 ) Pond water (g L−1 ) Table salt (g g−1 ) Sugar (g g−1 ) a
Nitrite sensor 9.1 21.0 56.8 1.20 1.05
± ± ± ± ±
p-Nitroaniline–quinoline-8-ol based spectrophotometric method [15]
2.4 1.4 3.2 0.02 0.13
9.2 19.8 64.1 1.20 0.83
± ± ± ± ±
1.7 1.7 5.8 0.20 0.15
Deduced from standard addition plot drawn from the analytical values obtained by spiking real sample with four different concentrations of nitrite.
3.4.
Effect of other anionic species
With the addition of other anions like I− , SCN− , ClO4 − , [HgI4 ]2− and [Zn(SCN4 )]2− to Rh 6G solution, the intensity of the peak at 525 nm decreases as in case of nitrite anion while a new peak appears at 578 nm – a sort of red shift of Rh 6G dye. The changes in absorbance at 525 nm was less dramatic and required greater amounts of anions (unlike nitrite) in case of I− , SCN− and ClO4 − compared to [HgI4 ]2− and [Zn(SCN4 )]2− to effect a commensurate change. Here, the observed red shift in the absorbance peak could be possibly due to the exchange of anions with Cl− of the quaternary ammonium part of the dye which is well documented in literature [13,14]. However, the above mentioned spectral changes on addition of anions either individually or in admixtures are completely annulled by addition of nitrite, thus enabling selective sensing of nitrite. The qualitative changes of absorbance with other anions are reflected in more quantifiable terms also. 2.5 × 105 fold amounts of NO3 − , Br− and PO4 3− and 2.5 × 102 , 62.5 and 1.25 × 103 fold amounts of SCN− , I− and ClO4 − ions with respect to nitrite did not interfere during nitrite sensing. In order to check the applicability of the system for sensing nitrite in complex real samples, studies were carried out in terms of selectivity of the sensor for other anions, cations and neutral salts also. Table 1 shows the tolerance limits of various coexisting anions, cations and neutral salts with respect to 6.96 mol L−1 nitrite. The quantification of nitrite was unaffected even in the presence of a multi-component mixture of PO4 3− /NO3 − /Br− (1 × 10−3 %), SCN− (4 × 10−3 %), ClO4 − (2 × 10−3 %) and I− (1 × 10−6 %). Moreover, the color reaction of nitrite with Rh 6G is highly specific as none of the anions, cations and their admixtures at 105 times to that of nitrite have any deleterious effect.
3.5.
Application to real samples
The applicability of the developed reusable sensor was tested for analysis of natural waters and selected food materials. The analysis was carried out by “Standard addition method” in which the values obtained from standard addition plot drawn from the analytical values obtained on spiking real samples with four known concentrations of nitrite. The results obtained for various real samples are shown in Table 2. Furthermore, the results obtained by present
57
method are compared with standard UV–vis spectrophotometric method based on “p-nitroaniline–quinoline-8-ol” as chromogenic reagent [15]. The analytical values obtained by the newly developed sensor compares favorably with standard spectrophotometric method indicating the efficacy of developed reusable sensor for reliable and routine monitoring of nitrite in various environmental samples and food materials.
4.
Discussion
The formation of ternary ion associate by interaction of charged binary anionic complexes of various metal cations like Hg, Cd, Pb, Zn, etc. with cationic dye Rh 6G in presence of iodide or thiocyanate results in bathochromic shift from 525 to 578 nm. This has been advantageously utilized by various research groups for sensitive determination of metal cations directly in aqueous phase itself thus precluding the use of toxic organic solvents. In all these cases, the concentration of metal cation increases the formation of ternary associate of [Hg I4 ]2− and [Zn(SCN)4 ]2− with Rh 6G and results in increase in absorbance at 578 nm with concomitant decrease in absorbance at max of the dye i.e. at 525 nm. On the other hand, the addition of nitrite into Rh 6G in 0.75 mol L−1 H2 SO4 solutions results in hypsochromic shift to 385 nm (see Fig. 1) wherein the increase in absorbance at this max is proportional to nitrite. Again, as seen from Fig. 2, the decrease in absorbance at max of the Rh 6G is also proportional to nitrite. The magnitude of absorbance change in latter case is high. Other anions show a similar decrease at 525 nm but at much larger amounts i.e. 102 –105 to that of nitrite and that too in absence of nitrite (see Table 1). The observed bathochromic shift noticed in presence of anions other than nitrite and binary anionic complexes of metal ions could possibly be due to the exchange of anions with Cl− of the quaternary ammonium part of the dye. The addition of traces of nitrite results in alteration in conjugation of Rh 6G (see Scheme 1) thus upsetting the bathochromic shift observed in case of other anions (even at higher amounts). This enables the sensing of nitrite in presence of large excess of extraneous anions or binary anionic complexes of metal cations or their admixtures. In addition, several cations do not have any deleterious effect as they do not interact with Rh 6G in the absence of suitable anionic ligand. In order to explore the cause of the decrease in absorbance of Rh 6G, the experiment was carried out at pH 5.0. We did not notice any change in the absorbance of Rh 6G on addition of NO2 − . On the other hand, in solutions of pH 2.0 or lower, it can significantly reduce the absorbance of Rh 6G at its max . Based on this observation, one could envisage a plausible mechanism as shown in Scheme 1. Nitrite ions in the presence of H+ ions produce nitrosyl cation, which is electron deficient and can attack the secondary amine group of Rh 6G (1), resulting in the formation of N-nitrosamine derivative (2) [16]. In the second step, sulphamic acid reacts with N-nitrosamine derivative to give back the original dye. As seen from Scheme 1, the extent of conjugation in the dye is lost when the N-nitrosamine derivative is formed and this results in a blue shift to 385 nm.
58
a n a l y t i c a c h i m i c a a c t a 6 2 3 ( 2 0 0 8 ) 53–58
Scheme 1 – Mechanism involved in the chemical switch based nitrite sensor.
A similar “on–off” switch behaviour observed in case of fluorescence measurements can also be explained by the structural changes in Rh 6G on addition of nitrite and subsequent reversal by the addition of sulphamic acid. As we could not isolate the oxidized products formed on addition of nitrite, we were unable to get spectral evidences other than UV–vis and fluorescence studies.
5.
Conclusions
A chemical switch based reusable optical sensor for nitrite has been designed using the off-the-shelf readily available chromoionophore, i.e. Rh 6G. To the best of our knowledge, such a chemical switch system is the first of its kind, which can discriminate nitrite from very large amounts of several anions either individually or in admixtures due to blue and red shifts of the absorbance maximum of Rh 6G with nitrite and other anions, respectively. The developed sensor has been successfully demonstrated for its applicability in analysis of natural waters and food materials. A plausible mechanism involved in the chemical switching process has been proposed (see Scheme 1).
Acknowledgements This work was financially supported by Council of Scientific and Industrial Research (CSIR), New Delhi through Network project on “Specialty Inorganic Materials for Diverse Appli-
cations(NWP0010) and Senior Research Fellowship granted to Ms. R. Kala.
references
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