Anal. Chem. 1995, 67, 4446-4451
In Situ Interferometric Microscopy of Temperatureand Potential-Dependent Volume Changes of a Redox Gel Kazutake Takada, Terumitsu Haseba, Tetsu Tatsuma, and Noboru Oyama*
Department of Applied Chemistty, Faculty of Technology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo 184, Japan Qiangmin Li and Henry S. White
Department of Chemistry, Henry Eyring Building, University of Utah, Salt Lake City, Utah 841 12
Temperature-and potential-controlledphase transitions of a thin film of a redox gel prepared from copolymerization of N-isopropylacrylamide, vinylferrocene, and NJV'-methylenebisacrylamide at a Au electrode, were monitored by in situ phase measurement interferometric microscopy (PMIM). The thickness and width of the gel film in a NaC104 solution decreased from 10 and -160 pm, respectively,at temperatures below 20 "C,to -4 and -120 pm, respectively, above 30 "C. Electrochemical oxidation of the ferrocene groups in the polymer gel (to positively charged ferrocenium) is shown to result in a -5 "C increase in the temperature at which the volume changes occurred. A reversible potential-dependentvolume change at constant temperature was also observed using PMIM. The results are discussed in terms of the dependency of hydrophobic and hydrophilic interactions of the polymer chains on the temperature and oxidation state of the gel film.
-
A number of polymer gels undergo reversible volume phase transitions in response to changes in temperature, solution composition, and electric Such gels can be further tailored to respond to physical and chemical stimuli by introducing functional groups on the polymer chains. For instance, Irie and Kunwatchakun4demonstrated that a polyacrylamide gel modified with a triphenylmethane leuco derivative exhibits a volume phase transition induced by ultraviolet light. Similarly, Suzuki and Tanaka5synthesized a copolymer of N-isopropylacrylamide(NIPA) and a porphyrin derivative which undergoes a volume change when exposed to visible light. In a previous report6 we prepared a redox-active gel from copolymerization of NIPA and vinylferrocene 0and demonstrated that a volume phase transition could be controlled by oxidation of the ferrocene moieties. In analogous fashion, the electrochemical behavior of the N I P W gel is strongly dependent on the temperature, displaying a shift (1) Tanaka, T. Phys. Rev. Lett. 1978, 40, 820. (2) Dusek, IC;Patterson, D. J. Polym. Sci. Part A-2 1968, 6, 1209. (3) Tanaka, T.; Fillmore, D.; Sun, S.-T.; Nishio, I.; Swislow, G.; Shah, A Phys. Rev. Lett. 1980, 45, 1636. (4) Irie, M.; Kunwatchakun, D. Macromolecules 1 9 8 6 , 19. 2476. (5) Suzuki, A,; Tanaka. T. Nature 1990, 346, 345. (6) Tatsuma. T.;Takada, K; Matsui, H.; Oyama, N. Macromolecules 1994,27, 6687.
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Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
in E" and increased charge transport rates above the phase transition temperature. Thin films of the NIPAJVF copolymer cast on a metal surface exhibit similar characteristics? Our h d i n g that the electrochemical response is a function of the volume state of the gel suggests a means for transduction of a variety of physical and chemical stimuli (light, temperature, solution composition) into electrical signals. The development of analytical applications of redox gels requires a fundamental understanding of how various chemical stimuli affect the volume state of the gel, as well as an understanding of how volume state affects the electrochemical behavior. In previous investigations of the NIPA/VF,6 the gel films employed were so thin (-1 pm) that the temperature- and potentialdependent volume changes could not be readily observed using optical microscopy. However, phase transitions were monitored using a quartz resonator coated with the NIPANF gel. The resistance component of the electromechanical impedance of the resonator reflects losses of the oscillation energy and is thus sensitive to the thickness and viscosity of the film.6-12Although the quartz resonator is a powerful method for detecting phase transitions in gels, it does not provide a direct measure of the film dimensions. In the present report, we describe the use of phase measurement interferometric microscopy (F'MIM)13-16 for in situ monitoring of volume changes of NIPAJVF gel films as a function of temperature and electrode potential. PMIM is a noncontacting laser interference microscopy which utilizes computerized phase measurements to determine the optical path length difference between the sample and an optical reference surface. In the simplest case of imaging an uncoated surface, the optical path length measured from point to point across the surface can be Oyama. N.; Tatsuma, T.; Takahashi, IC J. Phys. Chem. 1993, 97, 10504. Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. Borjas, R.; Buttry, D. A. J. Electroanal. Chem. 1990, 280, 73. Muramatsu. H.; Kimura, K. Anal. Chem. 1992, 64, 2502. Takada, IC;Tatsuma, T.; Oyama, N. J. Chem. Soc., Faraday Trans. 1995, 91, 1547. Tatsuma, T.; Hioki, Y.; Oyama, N. J. Electroanal. Chem.. in press, Kragt. H. J.; Earl, D. J.; Norton, J. D.; White, H.S. J. Electrochem. Soc. 1989, 136, 1752. Smyrl, W. H.; Atanaoski. R T.; Atanasoska, L.; Hartshom, L.; Lien, M.; Fletcher, IC; Fletcher, E. A. J. Electroanal. Chem. 1989, 264, 301. White, H. S.; Earl, D. J.; Norton, J. D.; Kragt, H. J. Anal. Chem. 1990, 62, 1130. Smith, C. P.; Fritz, D. C.; Tirrell, M.; White, H. S. Thin Solid Films 1 9 9 1 , 198. 369.
0003-2700/95/0367-4446$9.00/0 0 1995 American Chemical Society
used to reconstruct a three-dimensional image of the surface topography. Although the horizontal resolution is a function of the objective magnification and is limited by optical diffraction to -0.25 pm, the vertical resolution is better that 1nm. In imaging a smooth substrate that is partially covered by a transparent thin film, the optical image primarily reflects lateral variations in the optical thickness of the film. However, a PMIM optical image can be directly converted to an image of the true physical dimensions of the redox gel film provided that the refractive index of the redox gel is spatially uniform. A detailed theoretical description of the analysis of PMIM images of transparent thin films has been presented by Smith et a1.I6 Analyses of PMIM images of thin NIPA/VF gel films demonstrate that the volume change accompanying the reversible thermally induced phase transition is in reasonably good.agreement with volume changes measured for bulk NIPA/VF samples. However, the results suggest that adhesion of the gel to the substrate creates a spatially anisotropic contraction/expansion of the polymer gel during the phase transition. In addition, the transient response of electrcchemical-inducedvolume changes of the NIPA/VF gel have been monitored using PMIM. EXPERIMENTAL SECTION Materials. N-Isopropylacrylamidewas purchased from Tokyo Kasei and recrystallized from a mixed medium of toluene and petroleum ether (-1:l). Vinylferrocene from Aldrich, N,N'methylenebisacrylamide (BE), and 2,2'-azobisisobutyronitrile (AIBN) from Kanto Chemical were used as received. Other reagents of analytical grade were used as purchased. Aucoated glass plates were used as electrodes. Electrolyte solutions were prepared using doubly distilled water. Preparationof the NIF'A/VF GeL6J7 Prior to the preparation of the NIPA/VF gel-mod%ed electrode, Au electrodes were treated with a 10% toluene solution of methoxydimethylvinylsilane to introduce the vinyl group onto the electrode surface. The electrode surface was covered with a Teflon sheet having an indented zone (depth, -0.1 mm), which defines where the gel is polymerized. Dimethyl sulfoxide containing 9.4 M NIPA, 0.31 M VF, 0.094 M BIS, and 0.34 M AIBN (polymerization initiator) was introduced to the gap between the electrode and the Teflon sheet. The electrode was left for 8 h at 60 "C under nitrogen. A bulk gel was obtained similarly in a beaker. The gels were thoroughly washed with dimethyl sulfoxide and cold water (below 10 "C). Apparatus and Procedure. A Zygo Maxim 3D laser Model 5700 interferometricmicroscope with l o x Mirau objective (Middlefield, was used to image the NIPA/VF gel immobilized on the Au-coated glass plate. PMIM measures the phase, 4, of coherent light with wavelength 1 (A = 632.8 nm) reflected from the test surface relative to light reflected from an optical reference surface. Differences in phase, A4, between two locations on the surface are proportional to the difference in optical height, h, between the two locations (eq 1). An optical image of the surface A@ = 47chh/il
(1)
can thus be constructed by measuring the phase from point to point across the surface. A detailed description of the PMIM is presented elsewhere.16 (17) Tatsuma, T.; Saito, IC; Oyama, N. J. Chem. SOC.,Chem. Commun. 1994, 1853.
All PMIM measurements were performed in a temperaturecontrolled 0.1 M NaC104 aqueous solution. The temperature of the solution was monitored using a thermocouple. To avoid any evaporation of water from the cell, the microscope was enclosed in plastic sheets and the humidity was kept at -100%. A glass plate (1 mm thick) was placed on the solution surface to avoid condensation of HzO on the lens. The electrode potential was controlled with a potentiostat/function generator (Model 173, Princeton Applied Research Co.). An &/AgCl and platinum wire were used as a reference and counter electrode, respectively. The refractive indexes of the NIPA/VF gel and the solution were measured using an Abbe refractometer (Model lT, Atago, Japan). The temperature of the refractometer was controlled using a thermostated bath. RESULTS AND DISCUSSION
Volume Changesof a Bulk NIPA/VF Gel. The temperaturedependent volume of a bulk NIPA/VF gel sample (5 x 5 x 5 mm3 in the shrunken state) was measured in a 0.1 M NaC104 aqueous solution. A volume change occurred between 10 and 35 "C, with the volume below 10 "C being -$fold larger than that above 35 "C. At lower temperature, the gel is swollen due to hydrogen bonding between water and the amine or carbonyl group, as well as to hydrophobic hydration of the hydrophobic groups. In the hydrophobic hydration, water molecules form cagelike structures around the hydrophobic groups. At higher temperature, this hydrophobic hydration becomes weaker because of thermal movement of water, and further, hydrophobic interaction between the hydrophobic groups becomes stronger. Consequently, H20 is expelled from the gel resulting in a decrease in v ~ l u m e . ~ , ~ , ~ ~ PMIM Images of a N I P W Gel. PMIM measurements were carried out for a NIPA/VF gel immobilized on a Au electrode at open circuit in a 0.1 M NaC104 aqueous solution at various temperature. Figure 1 shows optical images of the as-prepared gel (reduced state) at (A) 10 and (B) 40 "C. In both figures, the flat areas on the left and right sides are the Au surface and the depressed region is the gel. The optical image shows the gel as a depression in the Au surface as a result of the longer optical path traveled in the gel, a consequence of the refractive index of the gel being larger than that of the solution. For the shrunken gel, some data points are missing. This reflects that the reflected light does not reach a corresponding chargecoupled device owing to diffused reflections. That is, the surface of the shrunken gel may be rougher than the swollen gel. This behavior, often observed for the bulk gels, may arise from uneven tension in the shrunken gel. If light is reflected from the solution/gel interface, the optical height of the gel h is given by16
h = n,d
(2)
where nl is the refractive index of the solution and d is the physical thickness of the gel. On the other hand, if light is reflected from the gel/electrode interface, the optical height id6
h = (nl - n,)d
(3)
where t t 2 is the refractive index of the gel. The refractive indexes (18) Inomata, H.; Goto, S.; Saito. S. Macromolecules 1990,23,4887
Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
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(A) 10 OC
(B) 40 O
C
(6)40 "C in a 0.1 M
of the 0.1 M NaCIOl aqueous solution and the NIPANF gel are 1.335 and 1.362, respeaively. at 10 "C and 1.331 and 1.476, respectively, at 40 "C. Therefore, it is reasonable that the optical height h of the gel has a negative value, resulting in inversion of the image. F i r e 2 shows sectional images of the gel computed on the basis of eq 3. Since the error of the refractive indexes is less than 0.001,the resulting errors in parts A and B of F i r e 2 are less than 0.4 pm and 0.03pm, respectively. However, if the 4440 Analylical Chemistty, Vol. 67, No. 24, December 15, 1995
reflection at both solntion/gel and gel/elechode interfaces should be taken into account" the theoretical relationship between h and d is nonlinear (Figure 3,dots). The sinusoidal oscillations are the result of multiple reflections within the As can be seen, though the use of eq 3 ( F i r e 3, solid line) causes errors of (65 nm, which are negligible in comparison to the gel thickness. It is apparent from F i r e 2 that the N I F " gel is swollen at 10 "C and shrunken at 40 "C in the aqueous solution. The
-0.2I
(A) 10°C
0
-
0
Reduced
0
50 pn
0 0.3
e
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(B) 40°C
-08
I 0
10
20
30
40
50
Temperature / 'C
,
I
50 pn
Figure 2. Corrected sectional PMIM images (physical height-based) of the NIPAfVF gel at (A) 10 and (6)40 "C (calculated from data of Figure 1 on the basis of eq 3).
0
Reduced
6 -
0
4 -
I 0
10
20
30
40
50
Temperature / "C Figure 4. Dependencies on temperature of the (A) optical height of the NIPANF gel at (0)-30 and (0)+500 mV vs Ag/AgCI and (B) physical height calculated from eq 3 at -30 mV vs Ag/AgCI in a 0.1 M NaCIO4 aqueous solution. -0.2
9.5
10.0
10.5
11.0
11.5
12.0
12.5
Physical height / pm
,
-0.4
5 -0.5. E .-0, a,
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.-8 4-
0"
-0.6
I
(B)
Relationships between the Gel Height and Temperature.
-
-0.7-
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swelling ratio. Thus we conclude that immobilizing the gel onto a solid support results in anisotropic swelling/shrinking behavior.
I
I
3.0
3.5
4.0
4.5
5.0
5.5
Physical height / pm Figure 3. Theoretical plots of optical vs physical height at (A) 10 and (B) 40 "C taking into account (dots) or ignoring (solid line) multiple light reflections occurring at the Au/gel and gekiolution interfaces. The solid lines were calculated from eq 3 using n,,~= 1.362 (10 "C) and 1.476 (40 "C) and nsoln= 1.335 (10 "C) and 1.331 (40 "C). The points were calculated according the procedure in ref 16 using n ~ ,= 0.13 13.2.
+
cross-sectional area of the swollen gel is 5.9-fold larger than that of the shrunken gel. This ratio is larger than the square of the cube root of the volumetric swelling ratio of the bulk gel (= g2I3 = 4.3). As seen in Figure 1, the gel is not lengthened with swelling. This must be because the present gel is immobilized on the electrode surface. Tension is therefore generated by the phase transition and probably increases the sectional
PMIM images of the NIPA/VF gel in a 0.1 M NaC104 aqueous solution were used to evaluate the relationships between the optical height of the top of the gel (relative to the Au surface) and temperature (Figure 4A). Since the redox potentials of the swollen and shrunken NIPA/VF gels are +200 and f255 mV vs Ag/AgCl, respectively? a potential of +500 or -30 mV was applied to the gel-modified electrode to oxidize or reduce the ferrocene sites, respectively. The temperature was scanned from 10 to 40 "C (-0.3 "C min-l). The optical height of the gel could not be measured above 40 "C because the surface of the gel was too rough for PMIM imaging. As shown in Figure 4, a large decrease in the optical height was observed above 14 "C for the reduced gel and above 20 "C for the oxidized gel. Similar results have been obtained in our previous measurements using quartz crystal resonators;6 viscoelastic properties of the reduced gel changed at 23-32 "C, while those of the oxidized one changed at 27-38 "C. The observed difference between the reduced and oxidized gels can be explained as follows: As described above, a balance of hydrophilicity and hydrophobicity of a gel determines the temperature at which the volume changes.18 Ferrocene (reduced form) is electrically neutral, but ferricinium ion (oxidized form) is cationic. Therefore,the increased hydrophilicity of the oxidized ferrocene must be responsible for the higher volume change temperature of the oxidized gel. Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
4449
...
180 0
.e..
1.45 NIPANF gel
U
.-C Q) .-c >
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0
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0
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140
0
OO
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0.1 M NaCIO, 0000000000000000
100
10
0
20
30
40
50
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10
20
30
40
50
Temperature / "C
Figure 6. Dependencies of the width of the NIPANF gel on temperature at (0) -30 and (0) +500 mV vs Ag/AgCI in a 0.1 M NaC104 aqueous solution.
Figure 5. Dependencies on temperature of the refractive indexes of the NIPANF gel film (5 x 15 x 1 mm3) and a 0.1 M NaC104 aaueous solution.
As mentioned above, the refractive indexes of the gel and the solution are required to convert the optical height into the physical height. These were measured using an Abbe refractometer. Temperature was scanned from 10 to 40 "C at -0.6 "C min-I. A bulk gel (reduced form, -5 x 15 x 1 mm3 in the swollen state) was used for the refractive index measurements. Unfortunately, refractive indexes of the electrochemically oxidized gel cannot be measured because a film coated on a solid substrate is not suitable as a sample for our equipment. The refractive index of the reduced gel displayed a large change between 20 and 36 "C, corresponding to the volume phase transition, while that of the 0.1 M NaC104 aqueous solution decreased only slightly over the same temperature range. Figure 4B shows the physical height d of the reduced NIPM VF gel as a function of temperature, evaluated from the optical height (Figure 4A) and the refractive indexes (Figure 5) using eq 3. A change in the physical height was observed in the range from 20 to 30 "C. This temperature is very close to the temperature where the viscoelastic change was observed in the quartz crystal measurements (23-32 0C).6 The height below 20 "C was -2.5fold larger than that above 30 "C. As mentioned above, the swelling ratio of the bulk gel was -9. Since the cube root of 9 is 2.1, the ratio of 2.5 is somewhat large. This can also be explained in terms of the tension generated by the phase transition because the gel is immobilized (see above).
Relationships between the Gel Width and Temperature. Figure 6 shows relationships between width of the gel on the electrode and temperature. The width were measured simultaneously with the optical height (Figure 4A) in the PMIM experiments. Large changes in the width of the gel were observed at 10-30 "C for the reduced gel and at 15-35 "C for the oxidized gel. The width of the swollen gel was 1.4 (reduced) to 1.5fold (oxidized) larger than that of the shrunken gel. These ratios are smaller than the cube root of the swelling ratio of the bulk gel (=2.1). The swelling/shrinking behavior of the gel in the lateral direction is restricted to some extent because the present gel is immobilized. ElectrochemicalControl of the Volume Change of the Gel. As mentioned above, the temperature at which the N I P m gel 4450 Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
I
-0.30 I -50
-30mV
1
+500 mV
1
I
I
0
50
-30 mV
1
+500 mVI -30mV
I
1 100
i
I
150
200
I1
I
Time I min 130
I
I
I
I
5.
__
!
90 -50
0
50
100
150
200
Time / min Figure 7. Time courses of the (A) optical height and (B)width of the NIPANF gel immobilized on a Au electrode during potential step experiments (between -30 and f500 mV vs Ag/AgCI) in a 0.1 M NaCIO4 aqueous solution.
changes its volume is raised by -5 "C upon oxidation of the ferrocene groups. Therefore, the volume of the gel can be electrochemically controlled. We have indirectly confirmed this by monitoring viscoelasticity changes of the gel immobilized on a quartz crystal resonator.6 Here the volume changes of the gel were observed by means of PMIM. Figure 7 shows the time courses of the optical height (A) and width (B) of the gel upon potential steps between +500 and -30 mV vs Ag/AgCl at 22 "C. As can be seen, the width increased upon oxidation and decreased upon reduction, as expected. The optical height also increased upon oxidation. This indicates that a decrease in In1 - nz/was larger than an increase in d (see eq 3); upon swelling, the refractive index of the gel decreases. Thus, the electrochemically controlled volume changes were monitored in situ by means of
PMIM. To improve a degree of the volume change, we should prepare a redox gel with a larger swelling ratio or that exhibiting a clear and discrete volume phase transition. Charge transfer rates in the gel should also be improved to shorten the response time. Work is currently under way toward these ends. CONCLUSIONS
The thermally controlled volume change and electrochemically controlled volume change were successfully monitored in situ by
means of PMIM. PMIM was demonstrated to be a powerful technique to observe volume changes of thin films in situ. Received for review August 3, 1995. Accepted September 28, 1995.@ AC950782S @Abstractpublished in Advance ACS Abstracts, November 1, 1995.
Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
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