Anal. Chem. 1994,66, 1731-1735
This Research Contribution is in Commemoration of the Life and Science of
I. M. Kolthoff (1894- 1993).
Single Fiber-optic pH Sensor Based on Changes in Reflection Accompanying Polymer Swelling Zlad Shakhsher and W. Rudolf Seitz' Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824 Kenneth D. Legg Polysense, Inc., 29 Jefferson Road, Wellesley, Massachusetts 02 18 1
We have prepared fiber-opticpH sensors consisting of a small drop of aminated polystyrene on the tip of a single optical fiber with a core diameter of 100 pm. The sensor is prepared by dipcoating a partially polymerized solution and then completing the polymerizationon the fiber. This is followed by amination with diethanolamine. The polymer formulationincludesxylene/ dodecane to introduce porosity and Kraton 61652, a styreneethylene,butylene-styrene,triblock copolymer as a toughening agent. The polymer swells as the amine groups are protonated. This is accompanied by an increase in clarity of the polymer and a decrease in the intensity of light reflected back into the optical fiber. Intensity decreases by over a factor of 2 as the pH is decreased from 8.0 to 6.5. The resulting sensor is small and mechanically stable with a responsetime of severalminutes. We are interested in developing fiber-optic chemical sensors based on polymer swelling because we anticipate that they will combine robustness and calibration stability with low cost.I4 In previous work directed at demonstrating the feasibility of ion detection, we prepared pH-sensitive beads by aminating poly(vinylbenzy1chloride) with diethanolamine.4 Electrostatic repulsion between protonated amine groups causes the polymer to swell when exposed to acid. Because size increases continuously over the pH range from 6.5 to 8.0, this polymer is potentially suitable for sensing. In our earlier work, we found that adding small amounts of Kraton G1652, a styrene-ethylene,butylene-styrenetriblock copolymer, toughened the aminated polymers to the point where they could undergo multiple swelling and shrinking cycles without mechanical d e g r a d a t i ~ n .Toughening ~ occurs because the middle aliphatic block of Kraton G1652 forms small elastic domains within the polymer. According to the commonly accepted mechanism for toughening, microcracks that form in the rigid aminated polystyrene matrix terminate when they reach the elastic domains rather than propagating f~rther.~ (1) McCurley, M. F.; Seitz, W. R. Anal. Chim. Acta 1992, 249, 273-280. (2) Bai, M.; Seitz, W.R. SPIE Proc. 1993, 1794, 343-348. (3) Seitz, W. R. J. Mol. Struct. 1993, 292, 105-1 14. (4) Pan, S.; Conway, V.;Shakhshcr, Z. M.; Emerson, S.;Bai, M.; Seitz, W. R.; Legg, K. D. Anal. Chim. Acta 1993, 279, 195-202. (5) Kinloch, A. J. In Rubber Toughened Plastics; Riew, C. K., Ed.; Advances in Chemistry Series 222; American Chemical Society: Washington, DC, 1989; pp 67-91.
0003-2700/94/0366-173 1$04.50/0 0 1994 American Chemical Society
Because the refractive index of the aliphatic domains is considerably less than the refractive index of aminated polystyrene, polymers with added Kraton G1652 are good diffuse reflectors. In appearance they are white. It was, however, observed that upon swelling, the beads tended to become clearer. This suggested to us the possibility of developing a sensor based on changes in the extent to which the polymer reflects light as a function of pH. Here we report a pH sensor based on this principle. The transduction element consists of a single 100-Fm core diameter optical fiber coated on the end with a small drop of the polymer. The optical system includes an LED as the light source, a photodiode detector, and a fiber-optic coupler which serves as a beamsplitter. The resulting device is small, mechanically stable, and inexpensive with relatively rapid response.
EXPERIMENTAL SECTION Reagents. Vinylbenzyl chloride (98% pure, 30% para/ 70% meta) was purchased from Dow Chemical Co. The divinylbenzene from Polyscience, Inc., contained 55% divinylbenzene (meta and para), 42% ethyl vinylbenzene, and 3% other impurities. Kraton G1652, a styrene-ethylene,butylene-styrene triblock copolymer with a 2917 1 styrene/ ethylene,butylene ratio was donated to us by Shell Chemical Co. Dichloromethylvinylsilane was from Huls America. Xylene (mixture of ortho, meta, and para), dodecane, hydroxybutyl methyl cellulose, and xanthan gum were from Aldrich. Other reagents were from Fisher Scientific. Apparatus. Scattering was measured with a photometer shown schematically in Figure 1. This instrument was custom built by Optikos, Inc. (Cambridge, MA) and includes provisions for setting the initial intensity to zero. The source for all measurements was an LED with a maximum emission at 660 nm. The detector was a silicon photodiode. The 1 X 2 couplers with loo/ 140 glass-on-glass fiber were obtained from Gould, Inc. The polymer was attached to the end of the single port side of the coupler. SMA connectors coupled the other two fibers to the source and the detector as shown in Figure 1. Both the LED source and the photodiode detector are in SMA bushings, which hold them immediately adjacent to the end of the optical fiber. Preliminary experiments as well as the experiment to evaluate the effect of wavelength on reflected intensity ratios AnalyticalChetnistry, Vol. 66, No. 10, May 15, 1994 1731
Table 1. Sensor to Sensor Variability relative reflected intensity Digital Readout
2x1 coupler
Photodiode
100/140
Fibers Polymer
-1
Fiber
H+
k
Polymer
(b) Figure 1. Diagram of sensing system: (a) overall system; (b) closeup of polymer.
were performed using the 450-W xenon arc lamp source, excitation monochromator, and detection module of an SLM 8000 spectrofluorometer. An Orion 701A/digital ionanalyzer was used to measure pH. A Fisher Stereomaster I1 microscope was used to observe the polymer after it was formed at the end of the optical fiber and to measure the ratio of polymer diameters in acid and base. Procedures. Sensor Preparation. The first step in preparing a sensor was to modify the end of the single port fiber. The end of the fiber was polished until it was shiny. Then, it was activated with 2 M HC1 for 4 h, washed, and dried overnight. The surface was then silanized in 40 mL of 25% (v/v) dichloromethylvinylsilane in distilled toluene for 6 h. This step greatly improves adhesion of the polymer to the fiber. However, it is not clear whether the vinyl group actually participates in the polymerization or whether the surface treatment merely enhances the ability of the polymer solution to wet the fiber. The polymer is formed by free-radical polymerization of a solution containing Kraton G1652, vinylbenzyl chloride, and divinylbenzene. This solution also contains a free radical initiator, benzoyl peroxide, and pore-forming solvents, usually a mixture of xylene and dodecane. To get a significant amount of polymer to adhere to the end of the fiber, it is first necessary to partially polymerize this solution to increase its viscosity. This was accomplished by placing the solution in a tightly closed bottle containing the solution in a water bath at 85 "C until it became noticeably viscous. The end of the fiber was then dipped into the viscous mixture several times to build up the total amount of polymer on the end of the fiber. While this was done, hot air was blown on the end of the fiber to accelerate solid formation. Finally, the coated end of the fiber was placed in a solution of xanthan gum (0.080 g/L) and hydroxybutyl methyl cellulose (0.056 g/L) in distilled water at 85 "C for 4-5 h to allow the reaction to go to completion. Typically, the drop of polymer is initially about 1732 AnalyticalChemistry, Vol. 66,No.
in base
intensity ratio
4.2 5.4 8.7 5.4 14.9
5.2 6.0 11.2
1.24 1.11 1.29 1.07 1.33
5
5.8 19.8
+
Optical
\
in acid
1 2 3 4
-+
sensor no.
IO, May 15, 1994
100 pm wide. It was observed that the polymer tended to shrink to about 80 pm wide when the polymerization reaction was complete. We believe that some of the pore-forming solvent and possibly also the monomer is volatilizing or dissolving in water as the polymer forms. The final step in sensor preparation was to react the chloromethyl group on the polymer with diethanolamine to introduce a pH-sensitive functional group. This involved preswelling the polymer in 1,4-dioxane and then immersing it in diethanolamine at room temperature for 2 days. The derivatized polymer was then preconditioned in an acidic buffer for 1 day. Sensor Eualuation. The first step in evaluating a sensor was to measure the difference in reflected intensity when the sensor was cycled between pH 2 and pH 9. The mechanical stability of the polymer was investigated by cycling the sensor between these two pHs many times and observing the reproducibility of the optical signal. This was followed by visual inspection under a microscope. Sensors that were mechanically stable and showed a large change in intensity with pH were tested further. The optical signal was measured as a function of pH in phosphate and tris-maleate buffers with an ionic strength of 0.10 M. Response times were measured by cycling the sensor between pH 4.0 and pH 8.0 tris-maleate buffer. RESULTS AND DISCUSSION Polymer Formulation. Formulations were chosen based on observations with beads prepared by suspension polymerization. Some of these results have been reported in the l i t e r a t ~ r e .Polystyrene ~ was chosen as a polymer substrate because it is thermally stable and amenable to a wide variety of derivatization chemistries. Primary and secondary amines can be coupled to polystyrene substrates by chloromethylation followed by amination. We chose to form poly(vinylbenzy1 chloride) directly to avoid a separate chloromethylation step. Porosity can be introduced by including an inert solvent in the polymer form~lation.~9~ The pore size depends on the affinity of the inert solvent for polystyrene. Large pores are obtained in poor polystyrene solvents because the polymer partitions into a separate phase as it forms. Pore size can be varied using solvent mixtures.8 We chose to use a mixture of xylene and dodecane as the inert solvent because it yields pore sizes that are large enough to allow rapid diffusion of ions into the polymer without compromising the mechanical properties of the beads. We found it difficult to prepare sensors reproducibly. This is illustrated by the data in Table 1, which shows relative (6) Sederel, W. L.; DeJong, G. J. J. Appl. Poly. Sci. 1973,17, 2835-2846. (7) Millar, J. R.;Smith, D. G.; Kressman, T. R. E. J. Chem. SOC.1965,304-310. (8) Moore, J. C.J. Poly. Sci. A 1964,2, 835-843.
Table 2. Effect of Formulatlon on Polymer Propertler
% % mechanical acid/base response DVB Kraton diluent strength diameter ratio time (min) %
5 10 20 5 10
20 5 10 20 5 10 20 5 10
20
2 2 2 4 4 4 8 8 8 2 2 2 4 4 4
33 33 33 33 33 33 33 33 33 45 45 45 45 45 45
OK
OK OK
slow 1.75 1.27
5-7 5
1.3
7
crack
OK OK
0
crack
OK OK crack OK crack crack OK crack
1.23 1.07
9
1.4
1-2
28
-
1I
24 -
I:
20
2% kraton 4% kraton
8% kraton
1
-
8
4-
o
m
&
, do' b ' A ' & ' A ' do' b ' A ' & ' A ' d o '
PH Flgure 2. Relative reflected intensity vs pH with polymers containing 2, 4, and 8% Kraton. The Initial signal was adjusted to zero.
reflected intensities in acid and base for five different sensors all prepared using 2% Kraton G1652, 10% divinylbenzene, and 33% 1:l xyleneldodecane as the pore-forming diluent. Both the absolute reflected intensities and the ratio of intensities in acid and base vary considerably. The large variation in absolute intensities indicates that differences in the amount of polymer on the fiber are the major source of variability. Variations in the reproducibility of the fiber-tospectrometer coupling may contribute to the overall variability but are much smaller. It is interesting to note that the ratio of reflected intensities in acid and base is larger for the higher absolute intensities. It is likely that theobservedsignalconsists of a constant reflection at the fiberlpolymer interface and a variable signal due to changes in polymer optical properties with pH. Increasing the amount of polymer increases the relative magnitude of the variable signal. Other factors that may vary from sensor to sensor include the completeness of the derivatization reaction and the extent to which pore-forming solvent is lost during polymerization. Table 2 summarizes the characteristics of sensors prepared with a variety of polymer formulations. Even allowing for the high degree of sensor to sensor variability, several conclusions may be drawn from thesedata. Polymers prepared with 5% divinylbenzene (DVB) tend to be fragile, particularly when swollen, and usually crack. On the other hand, polymers prepared with 20%DVB do not swell very much in acid. The high degree of cross-linking reduces the degree of swelling and may also interfere with the amine derivatization reaction by reducing access to the interior of the polymer. Increasing the diluent level in the polymer from 33% to 45% leads to a sharp reduction in response time, but at the expense of mechanical stability. The higher level of diluent leads to larger pore sizes which improve hydrogen ion access to the polymer interior but also leave the polymer more vulnerable to cracking. Increasing Kraton levels do not seem to affect the mechanical properties of the bead. Thedata in Table 2 suggest that the diameter ratio decreases with increasing Kraton. However, other experiments with beads prepared by suspension polymerization show that this is a relatively small effect. The best sensors were prepared using 2% Kraton, 10% divinylbenzene, and 45%1:1 xylene/dodecane as the diluent. The rest of the data are for this formulation. Most of the
sensors prepared with this formulation responded reproducibly to pH without cracking. Polymer Optical Properties. Visually, amine-derivatized polymers containing 2%Kraton appear white in the unswollen state. When these polymers swell in acid, there is a noticeable decrease in bead opacity. We do not know why the opacity changes. It is likely that several effects contribute including refractive index changes as the solvent enters the polymer and changes in the size and structure of scattering domains, either Kraton domains or pores or both. Changes in the refractive index within the polymer will affect the intensity reflected at the optic fiber-polymer interface in addition to affecting scattered intensity within the polymer. Beads prepared to contain 4%or more Kraton appear white in both the swollen and unswollen state. The visual observations of bead optical properties are paralleled by observations with the sensor. Figure 2 shows intensity vs pH for sensors prepared with 2,4, and 8% Kraton in otherwise identical polymer formulations (10% DVB, 45% 1:1 xyleneldodecane as the diluent). The change in scattered intensity with pH decreases sharply with increasing Kraton level. We suspect that the variation in bead optical properties with Kraton levels may reflect changes in the size and not just the number of aliphatic domains in the polymer. These domains form dynamically as polymerization consumes monomer and reduces the solubility of the aliphatic domains in the polymer formulation. In previous work using toluene as the pore-forming diluent, we observed dramatic changes in morphology at higher Kraton levelsO4 Figure 3 shows that the ratio of scattered light intensities in base and acid has a significant wavelength dependence, decreasing at longer wavelength. This could reflect differences in the wavelength dependences of the refractive indices of the aliphatic and polystyrene phases in the polymer. Alternatively, it could mean that the size of the scattering domains is on the order of a wavelength leading to wavelength-dependent scattering. Polymerization Monitoring. Polymerization can be monitored by measuring reflected intensity through the fiber optic coupler. The reflected intensity drops when the partially polymerized polymer solution is first attached to the end of AnalyticalChemistry, Vol. 66,No. 10, May 15, 1994
1733
Varlatlonr reflected intensity
Table 4. Day-WDry Sen-
t
.-0
ii
U
.3 tn c B c
-
l.* ',l
t t
350
450
550
650
750
850
day no.
at pH 8.5
at pH 6.0
1 2 3 4 5 6 7 8 9 10 11 12 13
14.4 14.5 14.5 14.5 14.5 14.3 14.4 15.3 14.5 14.4 14.4 14.4 14.4
11.4 11.1 11.0 10.7 10.1 9.8 9.8 10.3 9.5 10.0 9.4 9.8 10.0
Wavelength (nm) Figure 3. Base (pH 8.5) to acid (pH 8.0) intensity ratio vs wavelength. I
I
d
Table 3. Sensor CondHlonlng
reflected intensity measurement no.
at pH 4.0
at pH 8.0
1 2 3 4 5 6 7
17.3 15.4 14.9 14.4 14.4 14.4 14.4
12.7 12.2 12.1 12.0 11.5 11.4 11.4 14k. Q
the optical fiber. This signal is due to reflection at the interface between the end of the fiber and the external medium. It decreases when the polymer solution replaces air at the end of the fiber because the polymer refractive index is closer to the refractive index of the fiber core. The reflected intensity then increases gradually as polymerization proceeds and scattering heterogeneities form. For example, in one experiment theintensity dropped from 8.52 to 3.25 when the polymer was first immersed in the polymer. As polymerization proceeded, the intensity rose to a final value of 9.73. Note, however, that reflection from the fiber-polymer interface is still a significant fraction of the total observed signal. Sensor Characteristics. Stability. When newly formed sensors are cycled between acid and base, intensities initially change, only reaching constant values after several cycles. Data illustrating this effect are shown in Table 3. A similar effect has been observed with Kraton-modified beads designed to respond to P H . ~We attribute it to microcrack formation due to swelling and shrinking induced stresses during the first few cycles. Once the microcracks have formed, the polymer is more able to accommodate the stresses due to size changes and does not undergo further changes in mechanical properties. Table 4 shows relative reflected intensities at pH 6.0 and pH 8.5 for one sensor that was tested daily for 2 weeks. These data show that the reflected intensity at pH 8.5 varies much less than the intensity at pH 6.0 which trends down for the first 6 days and then shows significant variability. Some of the day-to-day variation in intensity may arise from the instrument rather than the sensor. For many sensors, preconditioning at pH 4.0 for 24 h was sufficient to achieve a stable response. However, other sensors tended to vary even after preconditioning. This problem was 1734
AnatyticalChemistry, Vol. 66, No. 10, May 15, 1994
4.4' ; ' A ' & ' 4,4';'A' h' A 4'4'd3!
9
Q
Q
Q
h
h
h
b
h
Q
Q
O
J
W
O
J
PH Figure 4. Relative reflected intensity vs pH.
observed when the polymer formulation contained 33% diluent but not when it contained 45% diluent. Higher diluent percentages lead to larger pore volumes and improved access to the interior of the polymer. Response to p H . Figure 4 shows reflected intensity vs pH for a sensor prepared with 2% Kraton, 10% divinylbenzene, and 45% 1:l xylene/dodecane diluent. This is typical of the best sensors we prepared. The intensity changes by more than a factor of 2 as the polymer goes from fully protonated to fully deprotonated. Figure 5 shows reflected intensity vs pH for the same sensor on 4 consecutive days. These results show that, once the sensor has been conditioned, response does not change with time. The shape of the intensity vs pH curves are similar for all sensors shown, including not just the data in Figures 4 and 5 but also data for many other sensors. Superficially, they resemble the theoretical curves for acid-base indicators with a single pKa. The extent to which they match curves for normal indicators was checked by plotting log (1 - Zacid)/(lbase - I) vs pH where Z is the observed intensity, and zacid and zbase are the intensity extremes observed at low and high pH, respectively. These curves fit a linear model, but with slopes considerably larger than 1.OO. With a normal acid-base indicator, the fraction of the indicator in a particular form goes from 10 to 90% over a 2 pH unit interval (pH = pK, i - 1 to pKa -1). In the data in Figures 4 and 5, the signal goes from 10 to 90% of the range over a much smaller pH interval. The apparent pKa, Le., the pH at the point where reflected intensity is halfway between the maximum and minimum
GI
23
o
PH Flgure 5. Relative reflected intensity vs pH on four consecutive days. The Initial signal was adjusted to zero.
values, varies between 7.1 and 7.3 for different sensor preparations. This represents a large shift from the solution pKa value of 9.0 for diethanolamine. A similar result has been observed with beads formed by suspension polymeri~ a t i o n .The ~ shift means that the polymer matrix favors the uncharged form of the amine, probably because water activity in the polymer is less than in solution so that water is not available to fully solvate the protonated amine. Response Time. Figure 6 shows reflected intensity vs time for a sensor alternately exposed to pH 4.0 and pH 8.0 buffer. Swelling in acid is essentially complete after 3 min. However, shrinking in base takes longer. After 9 min, the intensity has still not completely stabilized with time. The polymer swells from the outside in. This means that the outside shrinks first. A shrunken outside layer may retard diffusion of hydroxide into the polymer resulting in a relatively slow response time for shrinking. This explanation is consistent with the observation that the change in intensity for the first 2 min is quite large followed by a slow approach to a constant value. The response times are considerably longer than expected based on response times observed with beads prepared by suspensionpolymerization. Polymer beads (0.7-mm unswollen diameter) with formulations similar to those used in the sensor shrink to constant diameters in approximately 20 minor lessn4 Since response time is expected to vary with the square of the polymer diameter? this would suggest that the sensor response (9) Tanaka, T.; Fillmore, D. J. J . Chem. Phys. 1979, 70, 1214-1218.
Shrinking Swelling
1
2
3
4
5
6
7
a
io
9
Time (min) Flgure 6. Relative intensity vs time for swelling and shrinking.
times should be much shorter. We believe that this seeming discrepancy is evidencethat we are losing pore-forming solvent during sensor fabrication. Effect ofscatterem insolution. A starch suspension with a turbidance of 0.1 1 in a 1.OO-cm cuvette was found to increase the observed intensity from a fully swollen polymer by approximately 3%. In the fully swollen state, the polymer is translucent such that a significant fraction of the source intensity goes through the polymer into the solution. In the unswollen state, response from the polymer was not affected by the starch suspension. Mechanical Stability. Sensors that reached stable intensities when cycled between pH 2.0 and 9.0 remained stable. Some sensors were used for several days and more than 100 swelling and shrinking cycles.
CONCLUSIONS We have described a new approach to optical sensing based on polymer swelling. The resulting sensors are robust and inexpensive. The concept can be applied with a variety of polymers and functional groups. Our immediate goal is to improve the fabrication process so that we can prepare sensors more reproducibly. ACKNOWLEDGMENT Partial support for this research was provided by SBIR N I H Grant 1 R43 HL48445 to Polysense, Inc. Received for review October 19, 1993. Accepted February 15, 1994." *Abstract published in Aduance ACS Absrracrs. April 1 , 1994.
AnalyticalChemistty, Vol. 66, No. 10, May 15, 1994
1735
