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Spectrophotometric investigations on immobilized acid-base indicators
Analytica Chimica Acta, 207 (1988) 343-348
343
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Short C o m m u n i c a t i o n
S P E C T R O P H O T O M E T R I C I N V E S T I G A T I O N S ON IMMOBILIZED ACID-BASE INDICATORS
MAURO BACCI*, FRANCESCO BALDINI and ANNA MARIA SCHEGGI Istituto di Ricerca sulle Onde Elettromagnetiche del CNR, Via Panciatichi 64, 50127 Firenze (Italy)
(Received 14th October 1987) Summary. Diffusereflectance spectra of four acid-base indicators immobilizedon Amberlite XAD-
2 resin are reported as a function of pH. Adsorption modifies the acid-base properties of the indicator by broadening the pH intervals over which the dyes change colour and by shifting these intervals to higher pH values. The different behaviour in the liquid and immobilized phases may be explained by multilayered adsorption of some indicators. The occurrence of hysteresis during a pH cycle is reported. In recent years, the use of fibre-optic sensors for measuring chemical parameters has been studied extensively [1-6 ]. Particular attention has been given to the development of pH-sensitive fibre-optic devices, which could be competitive with traditional glass electrodes because of the absence of electromagnetic interference, ease of handling, small size and the possibility of measuring strongly acidic or alkaline samples [ 7-14 ]. The use of an acid-base indicator solution, in which the tip of the optical fibre is dipped, is not practical [9], so that the immobilization of indicators on suitable supports is needed. Some studies on the adsorption of indicators on Amberlite XAD-2, a non-ionic styrene/divinylbenzene copolymer, have been reported [15], b u t the acid-base properties of the immobilized indicators are modified [ 16 ]. To obtain better understanding of the behaviour of immobilized indicators for use in fibre-optic p H sensors, a thorough investigation of the spectroscopic properties of several indicators immobilized on XAD-2 in the visible range (400-800 n m ) was undertaken, and the spectral behaviour of the tested indicators in liquid and immobilized phases was compared. Experimental
The indicators and Amberlite XAD-2 polymer beads (20-50 mesh) were obtained from B D H , and the reagents for the buffer solutions from Farmitalia Carlo Erba (Milan).
0003°2670/88/$03.50
© 1988 Elsevier Science Publishers B.V.
344 The indicators investigated were bromothymol blue (BTB), phenol red, chlorophenol red (CPR) and alizarin. Solutions of these indicators at different concentrations were prepared conventionally [ 17 ]. The dyes were immobilized on XAD-2 as described by Kirkbright et al. [ 15 ], but with different concentrations of the dye (0.1-1%). Phosphate and glycine buffers were used to prepare solutions at constant pH in the ranges 6-8 and 8-12.5, respectively. The pH of the solutions was adjusted by adding 0.5 M sodium hydroxide and was measured with a Hanna HI-8418 pH meter. Spectrophotometric measurements were made with a Perkin-Elmer spectrophotometer (Model 552). Diffuse reflectance spectra of the adsorbed indicators were recorded by using an external integrating sphere connected to the spectrophotometer via optical fibres. The XAD-2 beads with the immobilized indicator and the buffer solution were placed in a Petri dish, which was placed on the hole of the integrating sphere and covered with a black cloth to keep light out. In order to investigate the behaviour of the same sample at different pH values, the buffer was changed with a syringe with a needle (i.d. 0.50 m m ) through which the resin beads could not pass. Checks on the temperature variations usually encountered in a laboratory (15-30 ° C ) showed that their effects were negligible. Results and discussion Figure 1 (a) shows the reflectance spectra of B T B immobilized on XAD-2 for different pH in the range 6.76-12.76. These spectra can be compared with the absorption spectra of aqueous solutions of BTB (1.5 × 10 -5 M) in the pH range 6.02-7.52 (Fig. lb). Corresponding spectra for phenol red are displayed in Fig. 2. For easier comparison, the reflectance measurements in Figs. 1 (a) and 2 (a) are expressed in an analogous way to absorbance, i.e., A' = log ( l / R ) , where R is the diffuse reflectance. From Figs. 1 and 2, a relationship can be derived between A' (4) at the peak wavelengths, and pH (Fig. 3 ). The first noticeable difference is the behaviour of the band characteristic of the alkaline species (at the longer wavelength). As has already been noted [ 16 ], the range over which a nearly linear relationship can be observed, is shifted towards higher pH values for immobilized indicators (cf. Table 1) only for chlorophenol red is the shift of the indicator interval negligible on immobilization. In solution, as the absorbance of one indicator species decreases, the other increases, depending on their concentration. This does not occur in the immobilized phase. In fact, log ( l / R ) for the acidic form (Fig. 3d-f) remains almost constant up to the pH values at which log (1/R) of the alkaline species tends to a maximum (Fig. 3a-c), although a constant value for the alkaline form is never obtained even in highly alkaline media. This last fact has been interpreted [ 16] by assuming desorption of the more polar alkaline species
345
7.52
,
f
7.45
276
0.9
~ 0.3 u m
0.6
0.2
O
4o0
sbo
6do
(nm)
7do
O
400
sot
,
6do
A
nm)
700
Fig. 1. Reflectance spectra of BTB immobilized on XAD-2 (a) and absorption spectra of BTB in aqueous solution ( 1.5 × 10- 5 M ) (b) for different pH values. For reflectance spectra, A' -- log ( 1/R ), where R is the reflectance of the sample.
758
10.4
09
~o~ I
.... 735
O2
I OI
L
4o0
I 56o
6do 7,(,~m~ 750
o
400
5;0
600
~ , ( ~ 700
Fig. 2. Reflectance (a) and absorption (b) spectra of phenol red. Detail as in Fig. 1.
from the hydrophobic polymer. If this were so, however, extensive bleeding from the resin should be evident for all the indicators, particularly at high pH; in the present work, it was observed that the behaviour of the indicators varied from extensive bleeding (even at nearly neutral pH) for phenol red to negligible bleeding at any pH for BTB. Moreover, structurally different dyes, like alizarin and phenol red, behaved very similarly, so that the observed effect must be closely related to the adsorption mechanism on the polymeric support. A slight modification of the hypothesis previously reported [ 16 ] may ac-
346 1.5 A'(625)
(a)
A'(570)
(b)
A~(550)
(c)
/
1.O-
O.5-
0
r 5
i 7
i
'
;
'
1'1
1.5- A'(435)
' 13 pH
5
(d)
'
7
'
;
' 1'1 pH
A'(443)
'
(e).
5
I
7
9
pH
A'(434)
11
(f)
1.O-
O . 5
,
5
,
7
,
,
9
,
,
11
,
pH
f
13
~
,
5
"/
'
;
'
PH
11
'
"
7
'
<9
'
pFt
1'1
Fig. 3. A' [ =log(i/R) ] vs. pH for BTB (a,d), phenol red (b,e), and alizarin (c,f) immobilized on XAD-2. A' (4) represents log ( l / R ) at the peak wavelengths (in nm) for the alkaline (a-c) and acidic (d-f) forms.
TABLE 1 Approximate pH intervals for the indicators tested Form
Aqueous soln. [17] Immobilized
pH interval BTB
Phenol red
CPR
Alizarin
6.0-7.6 7.2-9.7
6.6-8.0 6.9-9.7
5.2-6.8 5.3-6.7
5.6-7.2 6.3-9.1
347
count for these experimental data. The permanency of a well-defined isosbestic point in the spectra of the immobilized indicators (In) supports the occurrence of an equilibrium involving only two species (HIn and I n - ) [16]. It seems reasonable to assume, at least in most cases, that the contribution from the species in solution is negligible. A plausible explanation of the phenomena observed is that the polymer beads are coated with two or more layers of the dye: the inner layers are essentially constituted by undissociated molecules {HIn ), while ionic species ~I n - ) predominate in the outer layer exposed to the solvent. Accordingly, the inner layers act as a reservoir of undissociated HIn species, so that a nearly constant concentration of HIn can be "seen" at the surface of the beads; only when that reservoir disappears can a concentration decrease of the undissociated species be perceived. Another point to note is the broadening of the pH interval over which most of the adsorbed indicators change colour (Fig. 3 and Table 1). This can be explained by a distribution of the dye molecules over slightly different sites, whereas in solution all the molecules are equivalent. A further point, which should be emphasized because it is of primary importance for applications to fibre-optic sensors, is the presence of hysteresis for some indicators (Fig. 3) during a pH cycle. The effect is particularly evident for alizarin and chlorophenol red but is also significant for BTB, whereas no hysteresis for phenol red was observed. If the multilayer hypothesis is correct, the hysteresis could be caused by slow diffusion within the inner layers. In such a case, the presence of hydrophilic groups and steric hindrance would enhance the phenomenon.
Conclusions The technique of dye immobilization on a polymeric matrix seems to be promising for the development of fibre-optic pH sensors. However, as demonstrated above, the adsorption on the polymeric matrix can affect the acid-base properties of the indicator. Moreover, the occurrence and the extent of bleeding and hysteresis are factors that must be accurately examined before a definitive choice of the most suitable indicator can be made. This work was supported by the CNR Finalized Program on Material and Devices for Solid-State Electronics.
REFERENCES 1 2 3 4 5 6
S.A. Borman, Anal. Chem., 53 (1981) 1616. W.R. Seitz, Anal. Chem., 56 (1984) 16A. A.M. Scheggi, Int. J. Opt. Sens., 2 (1987) 3. C. Nylander, J. Phys. E, 18 (1985) 736. F.P. Milanovich, T.B. Hirschfeld and F.T. Wang, Proc. SPIE Int. Soc. Opt. Eng. (USA), 494 (1984) 18. J.I. Peterson and G.G. Vurek, Science, 224 (1984) 123.
348 7 G.F. Kirkbright, R. Narayanaswamy and N.A. Welti, Analyst, 109 (1984) 1025. 8 A.M. Scheggi and F. Baldini, Opt. Acta, 33 (1986) 1587. 9 M. Bacci, F. Baldini, M. Brenci, G. Conforti, R. Falciai and A.G. Mignani, Proc. 4th Int. Conf. Optical Fiber Sensors, Tokyo, 1986, p. 323. 10 Z. Zhujun and W.R. Seitz, Anal. Chim. Acta, 160 (1984) 47. 11 J.L. Gehrich, D.W. Lubbers, N. Opitz, D.R. Hansmann, W.W. Miller, J.K. Tusa and M. Yafuso, IEEE Trans. Biomed. Eng., 33 (1986) 117. 12 C. Munkholm, D.R. Walt, F.P. Milanovich and S.K. Klainer, Anal. Chem., 58 (1986) 1427. 13 Y. Kawabata, T. Imasaka and N. Ishibashi, Proc. 4th Int. Conf. Optical Fiber Sensors, Tokyo, 1986, p. 139. 14 H. Offenbacher, O.S. Wolfbeis and E. Furlinger, Sens. Actuat., 9 (1986 } 73. 15 G.F. Kirkbright, R. Narayanaswamy and N.A. Welti, Analyst, 109 (1984) 15. 16 R. Narayanaswamy and F. Sevilla, III, Anal. Chim. Acta, 189 (1986) 365. 17 R.C. Weast (Ed.), Handbook of Chemistry and Physics, 51st edn., The Chemical Rubber Company, Cleveland, OH, 1970, p. D-150.
343
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Short C o m m u n i c a t i o n
S P E C T R O P H O T O M E T R I C I N V E S T I G A T I O N S ON IMMOBILIZED ACID-BASE INDICATORS
MAURO BACCI*, FRANCESCO BALDINI and ANNA MARIA SCHEGGI Istituto di Ricerca sulle Onde Elettromagnetiche del CNR, Via Panciatichi 64, 50127 Firenze (Italy)
(Received 14th October 1987) Summary. Diffusereflectance spectra of four acid-base indicators immobilizedon Amberlite XAD-
2 resin are reported as a function of pH. Adsorption modifies the acid-base properties of the indicator by broadening the pH intervals over which the dyes change colour and by shifting these intervals to higher pH values. The different behaviour in the liquid and immobilized phases may be explained by multilayered adsorption of some indicators. The occurrence of hysteresis during a pH cycle is reported. In recent years, the use of fibre-optic sensors for measuring chemical parameters has been studied extensively [1-6 ]. Particular attention has been given to the development of pH-sensitive fibre-optic devices, which could be competitive with traditional glass electrodes because of the absence of electromagnetic interference, ease of handling, small size and the possibility of measuring strongly acidic or alkaline samples [ 7-14 ]. The use of an acid-base indicator solution, in which the tip of the optical fibre is dipped, is not practical [9], so that the immobilization of indicators on suitable supports is needed. Some studies on the adsorption of indicators on Amberlite XAD-2, a non-ionic styrene/divinylbenzene copolymer, have been reported [15], b u t the acid-base properties of the immobilized indicators are modified [ 16 ]. To obtain better understanding of the behaviour of immobilized indicators for use in fibre-optic p H sensors, a thorough investigation of the spectroscopic properties of several indicators immobilized on XAD-2 in the visible range (400-800 n m ) was undertaken, and the spectral behaviour of the tested indicators in liquid and immobilized phases was compared. Experimental
The indicators and Amberlite XAD-2 polymer beads (20-50 mesh) were obtained from B D H , and the reagents for the buffer solutions from Farmitalia Carlo Erba (Milan).
0003°2670/88/$03.50
© 1988 Elsevier Science Publishers B.V.
344 The indicators investigated were bromothymol blue (BTB), phenol red, chlorophenol red (CPR) and alizarin. Solutions of these indicators at different concentrations were prepared conventionally [ 17 ]. The dyes were immobilized on XAD-2 as described by Kirkbright et al. [ 15 ], but with different concentrations of the dye (0.1-1%). Phosphate and glycine buffers were used to prepare solutions at constant pH in the ranges 6-8 and 8-12.5, respectively. The pH of the solutions was adjusted by adding 0.5 M sodium hydroxide and was measured with a Hanna HI-8418 pH meter. Spectrophotometric measurements were made with a Perkin-Elmer spectrophotometer (Model 552). Diffuse reflectance spectra of the adsorbed indicators were recorded by using an external integrating sphere connected to the spectrophotometer via optical fibres. The XAD-2 beads with the immobilized indicator and the buffer solution were placed in a Petri dish, which was placed on the hole of the integrating sphere and covered with a black cloth to keep light out. In order to investigate the behaviour of the same sample at different pH values, the buffer was changed with a syringe with a needle (i.d. 0.50 m m ) through which the resin beads could not pass. Checks on the temperature variations usually encountered in a laboratory (15-30 ° C ) showed that their effects were negligible. Results and discussion Figure 1 (a) shows the reflectance spectra of B T B immobilized on XAD-2 for different pH in the range 6.76-12.76. These spectra can be compared with the absorption spectra of aqueous solutions of BTB (1.5 × 10 -5 M) in the pH range 6.02-7.52 (Fig. lb). Corresponding spectra for phenol red are displayed in Fig. 2. For easier comparison, the reflectance measurements in Figs. 1 (a) and 2 (a) are expressed in an analogous way to absorbance, i.e., A' = log ( l / R ) , where R is the diffuse reflectance. From Figs. 1 and 2, a relationship can be derived between A' (4) at the peak wavelengths, and pH (Fig. 3 ). The first noticeable difference is the behaviour of the band characteristic of the alkaline species (at the longer wavelength). As has already been noted [ 16 ], the range over which a nearly linear relationship can be observed, is shifted towards higher pH values for immobilized indicators (cf. Table 1) only for chlorophenol red is the shift of the indicator interval negligible on immobilization. In solution, as the absorbance of one indicator species decreases, the other increases, depending on their concentration. This does not occur in the immobilized phase. In fact, log ( l / R ) for the acidic form (Fig. 3d-f) remains almost constant up to the pH values at which log (1/R) of the alkaline species tends to a maximum (Fig. 3a-c), although a constant value for the alkaline form is never obtained even in highly alkaline media. This last fact has been interpreted [ 16] by assuming desorption of the more polar alkaline species
345
7.52
,
f
7.45
276
0.9
~ 0.3 u m
0.6
0.2
O
4o0
sbo
6do
(nm)
7do
O
400
sot
,
6do
A
nm)
700
Fig. 1. Reflectance spectra of BTB immobilized on XAD-2 (a) and absorption spectra of BTB in aqueous solution ( 1.5 × 10- 5 M ) (b) for different pH values. For reflectance spectra, A' -- log ( 1/R ), where R is the reflectance of the sample.
758
10.4
09
~o~ I
.... 735
O2
I OI
L
4o0
I 56o
6do 7,(,~m~ 750
o
400
5;0
600
~ , ( ~ 700
Fig. 2. Reflectance (a) and absorption (b) spectra of phenol red. Detail as in Fig. 1.
from the hydrophobic polymer. If this were so, however, extensive bleeding from the resin should be evident for all the indicators, particularly at high pH; in the present work, it was observed that the behaviour of the indicators varied from extensive bleeding (even at nearly neutral pH) for phenol red to negligible bleeding at any pH for BTB. Moreover, structurally different dyes, like alizarin and phenol red, behaved very similarly, so that the observed effect must be closely related to the adsorption mechanism on the polymeric support. A slight modification of the hypothesis previously reported [ 16 ] may ac-
346 1.5 A'(625)
(a)
A'(570)
(b)
A~(550)
(c)
/
1.O-
O.5-
0
r 5
i 7
i
'
;
'
1'1
1.5- A'(435)
' 13 pH
5
(d)
'
7
'
;
' 1'1 pH
A'(443)
'
(e).
5
I
7
9
pH
A'(434)
11
(f)
1.O-
O . 5
,
5
,
7
,
,
9
,
,
11
,
pH
f
13
~
,
5
"/
'
;
'
PH
11
'
"
7
'
<9
'
pFt
1'1
Fig. 3. A' [ =log(i/R) ] vs. pH for BTB (a,d), phenol red (b,e), and alizarin (c,f) immobilized on XAD-2. A' (4) represents log ( l / R ) at the peak wavelengths (in nm) for the alkaline (a-c) and acidic (d-f) forms.
TABLE 1 Approximate pH intervals for the indicators tested Form
Aqueous soln. [17] Immobilized
pH interval BTB
Phenol red
CPR
Alizarin
6.0-7.6 7.2-9.7
6.6-8.0 6.9-9.7
5.2-6.8 5.3-6.7
5.6-7.2 6.3-9.1
347
count for these experimental data. The permanency of a well-defined isosbestic point in the spectra of the immobilized indicators (In) supports the occurrence of an equilibrium involving only two species (HIn and I n - ) [16]. It seems reasonable to assume, at least in most cases, that the contribution from the species in solution is negligible. A plausible explanation of the phenomena observed is that the polymer beads are coated with two or more layers of the dye: the inner layers are essentially constituted by undissociated molecules {HIn ), while ionic species ~I n - ) predominate in the outer layer exposed to the solvent. Accordingly, the inner layers act as a reservoir of undissociated HIn species, so that a nearly constant concentration of HIn can be "seen" at the surface of the beads; only when that reservoir disappears can a concentration decrease of the undissociated species be perceived. Another point to note is the broadening of the pH interval over which most of the adsorbed indicators change colour (Fig. 3 and Table 1). This can be explained by a distribution of the dye molecules over slightly different sites, whereas in solution all the molecules are equivalent. A further point, which should be emphasized because it is of primary importance for applications to fibre-optic sensors, is the presence of hysteresis for some indicators (Fig. 3) during a pH cycle. The effect is particularly evident for alizarin and chlorophenol red but is also significant for BTB, whereas no hysteresis for phenol red was observed. If the multilayer hypothesis is correct, the hysteresis could be caused by slow diffusion within the inner layers. In such a case, the presence of hydrophilic groups and steric hindrance would enhance the phenomenon.
Conclusions The technique of dye immobilization on a polymeric matrix seems to be promising for the development of fibre-optic pH sensors. However, as demonstrated above, the adsorption on the polymeric matrix can affect the acid-base properties of the indicator. Moreover, the occurrence and the extent of bleeding and hysteresis are factors that must be accurately examined before a definitive choice of the most suitable indicator can be made. This work was supported by the CNR Finalized Program on Material and Devices for Solid-State Electronics.
REFERENCES 1 2 3 4 5 6
S.A. Borman, Anal. Chem., 53 (1981) 1616. W.R. Seitz, Anal. Chem., 56 (1984) 16A. A.M. Scheggi, Int. J. Opt. Sens., 2 (1987) 3. C. Nylander, J. Phys. E, 18 (1985) 736. F.P. Milanovich, T.B. Hirschfeld and F.T. Wang, Proc. SPIE Int. Soc. Opt. Eng. (USA), 494 (1984) 18. J.I. Peterson and G.G. Vurek, Science, 224 (1984) 123.
348 7 G.F. Kirkbright, R. Narayanaswamy and N.A. Welti, Analyst, 109 (1984) 1025. 8 A.M. Scheggi and F. Baldini, Opt. Acta, 33 (1986) 1587. 9 M. Bacci, F. Baldini, M. Brenci, G. Conforti, R. Falciai and A.G. Mignani, Proc. 4th Int. Conf. Optical Fiber Sensors, Tokyo, 1986, p. 323. 10 Z. Zhujun and W.R. Seitz, Anal. Chim. Acta, 160 (1984) 47. 11 J.L. Gehrich, D.W. Lubbers, N. Opitz, D.R. Hansmann, W.W. Miller, J.K. Tusa and M. Yafuso, IEEE Trans. Biomed. Eng., 33 (1986) 117. 12 C. Munkholm, D.R. Walt, F.P. Milanovich and S.K. Klainer, Anal. Chem., 58 (1986) 1427. 13 Y. Kawabata, T. Imasaka and N. Ishibashi, Proc. 4th Int. Conf. Optical Fiber Sensors, Tokyo, 1986, p. 139. 14 H. Offenbacher, O.S. Wolfbeis and E. Furlinger, Sens. Actuat., 9 (1986 } 73. 15 G.F. Kirkbright, R. Narayanaswamy and N.A. Welti, Analyst, 109 (1984) 15. 16 R. Narayanaswamy and F. Sevilla, III, Anal. Chim. Acta, 189 (1986) 365. 17 R.C. Weast (Ed.), Handbook of Chemistry and Physics, 51st edn., The Chemical Rubber Company, Cleveland, OH, 1970, p. D-150.