Langmuir 1994,10,3023-3026
3023
Photophysical Studies of Poly(N-isopropylacrylamide)Microgel Structures S. Pankasem and J. K. Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
M. J. Snowdent and B. Vincent School of Chemistry, University of Bristol, Bristol BS8 ITS, U.K. Received April 7,1994. I n Final Form: June 28, 1994@ Microgels of poly(N-isopropylacrylamide) exhibit the interesting property of size contraction with increasing temperature. Fluorescent probes, pyrene and tris(bipyridine)ruthenium, are used to monitor the above changes, and the studies are correlated with independent light scattering data.
Introduction Photophysical studies have been widely used to investigate colloidal systems, including po1yelectrolytes.l The basic concept is to incorporate a molecule with selected photophysical properties into the system. This probe molecule, via its spectroscopic properties, reports back on the nature of its environment, and on the access of other quencher molecules, from various regions of the system, to the probe molecule.lb,c Thermosensitive colloidal microgel particles of poly(N-isopropylacrylamide)have been prepared according to the emulsion polymerization technique described by Pelton and Chibante.2 The microgel particles have an open sponge-like structure when dispersed in water a t 25 "C; however, when heated to 50 "C, they contract (reversibly) and force out the solvent molecules located within the interstitial spaces. This contraction is believed to be a consequence of a decrease in the polymer-solvent interaction with increasing temperature. Microgels of poly(N-isopropylacrylamide)absorb polymer molecules3and also small molecules and ions.4 At 25 "C,the microgels readily absorb ions to a typical absorption capacity of 20-50 mg of heavy metal nitrates per gram of m i ~ r o g e lAs . ~ the microgels are heated to 50 "C, a large proportion (approximately 80%)of the absorbed material is r e l e a ~ e d .This ~ process can be repeated a number of times simply by heating and cooling the dispersions. In this study, pyrene and tris(bipyridine)ruthenium are used as probe molecules to investigate the solution properties of the above N-isopropylacrylamide microgel.
Experimental Section Materials. Poly(N-isopropylacrylamide)particles were prePyrene (Alpared by single-stage emulsion polymeri~ation.~-~ drich) was purified by column chromatographywith silica gel + School of Biological and Chemical Sciences, University of Greenwich, Woolwich, London SE1 86PF,U.K. Abstract published inAdvance ACSAbstracts, August 15,1994. @
(1)(a) Fendler, J. H. Membrane Mimetics Chemistry; Wiley: New York, 1982. (b) Kalyanasundaram, K.Photochemistry in Microheterogeneous Media; Wiley: London, 1986. ( c ) Thomas, J. K. Chemistry of Excitation at Interfaces; ACS Monograph No. 184;Washington, DC, 1984. (d) Chu, D.;Thomas, J. K. In Photochemistry and Photophysics VolumeIll; Rabek, J.F.,Ed.; CRS Press: Boca Raton,FL, 1991;Chapter
2.
Chibante, P. Colloids Surf. 1991,58,271. (2)Pelton, R.H.; (3)Snowden, M. J.;Vincent,B.J. Chem.Soc.,Chem. Commun. 1992,
16,1103. (4) Snowden, M. J. J.Chem. Soc., Chem. Commun. 1992,11,803. (5)Snowden,M, J.;Thomas,D.;Vincent,B.Analyst 1993,118,1367.
0743-7463/94/2410-3023$04.50l0
and cyclohexane as adsorbent and solvent, respectively. Ru(bpy)&lz (Aldrich),potassium ferricyanide(J.T. Baker),cupric sulfate (Fisher),thallous chloride (A. D. Mackay), potassium iodide (Fisher), and oxygen (Mittler) were used as received. Nitromethane(Aldrich)and nitrobenzene (Aldrich)were distilled twice before use. Methods. Apulse laser system (PRANitromiteLN 100which produces 70 mJ/pulse of 337.1 nm-pulse with 120-ps width, Hamamatsu R1664 U multichannel plate photomultiplier,and a Tektronix 7912 AD 500-MHz waveform digitizer)was used to excite the molecules and rapid spectrophotometry to record the fluorescence lifetimes.lCLight scattering (Malvern 7027 dual LOGLIN correlator equipped with a krypton ion 530.9-nmlaser) was used to investigatethe hydrodynamic size of the particle^,^ while a spectrofluorometer (SLM - SPF 500 C) and an absorption spectrophotometer (Cary13)were used to measure spectroscopic properties of the system. Experiments were performed with a microgel concentration of 0.15 wt % unless stated otherwise. For pyrene experiments, M was used, and for all the a pyrene concentration of 2 x time-resolved fluorescence measurements, excitation and monitoring wavelengths were 337 and 400 nm, respectively. For Ru(bpy)32+experiments, a R ~ ( b p y ) , ~concentration + of 1 x M was used, and for all the time-resolved fluorescence measurements, excitation and monitoring wavelengths were 337 and 620 nm, respectively.
Results and Discussion Figure 1 shows the fluorescence spectra of pyrene in the microgel dispersion a t 21 and 50 "C. A significant difference in the spectrum is observed. In particular, the ratio of the 383.5-nm peak as compared to the 373-nm peak is changed; this is normally called the IIIA ratio. This ratio has been widely applied to monitor the nature of various microenvironments, and it usually decreases when the polarity of the environment increases.lC The large number of points recorded in taking the spectrum give a high degree of accuracy to this measurement. From the figure, the IIM ratio of pyrene fluorescence jumps from 0.53 a t 21 "C to 0.62 at 50 "C, indicating that the microenvironment for pyrene in the microgel becomes more hydrophobic a t higher temperatures. These same data, i.e., IIM ratio, are recorded versus temperature, in Figure 2, and show a continuous rise beyond 30 "C; the effect is not as marked for pyrene in water. The figure also shows that the molecular diameter of the colloid particle decreases continually at this temperature range. An interpretation of the present data is that increasing temperature closes the polymer coil and provides a more hydrophobic environment for pyrene, and a t higher
0 1994 American Chemical Society
Pankasem et al.
3024 Langmuir, Vol. 10, No. 9, 1994
P E u I
420
400
130
360
440
400
480
500
Wavmlangth (nm)
Figure 1. Fluorescence spectra of pyrene in polymer solution at 21 "C (lef't)and 50 "C (right). The spectrtum at 50 "C was moved 2 nm to the right in order to distinguish it from the 21 "C spectrum. Table 1. QuenchingRate Constants for Excited b e n e 21 "C (M-ls-l) 50 "C(M-l s-lP water polymer quencher water polymer I1.18 109 1.11 109 1.54 x lo9 kl, = 9.60 x lo7 kzq = 1.24 x lo9 Tl+ 5.97 109 5.99 109 9.19 x lo9 kl, = 0 kzq = 6.24 x lo9 C H ~ N O 5.79 ~ x 109 6.55 x 109 1.02 x 1O'O k1, = 0 k2, = 7.17 x log 0 2 8.90 109 8.37 109 1.87 x 10" k1, = 0 kzq = 1.85 x 10"
~~
~
a At 50 "C,a double-exponential fit was applied with a = 0.76, klo = 3.72 x lo6 s-l, and k2o = 5.93 x lo6 s-l.
h
1
10
20
30
40
50
60
70
Temperature ("C)
Figure 2. Relationship between III/I ratio of pyrene fluorescence in water x 0.1 ( 0 )and in microgel x 0.1 (0);decay rate constant of tris(bipyridine)ruthenium in water x lo6 s-l (m) and in micorgel x lo6 s-l (0); hydrodyaromic diameter of microgel x 100 nm (A) and temperature. In some cases where a double exponentialwas applied, average rate constants were used.
temperatures a significant amount of pyrene resides in the polymer compared to the aqueous phase. More will be said on this later. Figure 3a,b shows details which further amplify the information in this system. These figures show the effect of added quenchers to the polymerpyrene system: the pyrene fluorescence is quenched, leading to a more rapid decay, and the extent of quenching decreases with temperature; the figures show fluorescence quenching curves a t 21 and 50 "C. In homogeneous solution, fluorescence quenching always increases with temperature, in contrast to the data in the microgel system. It is also noteworthy that the fluorescence decay is a single exponential a t low temperature (21 "C),and the data shown in Figure 3a are the experimental data fitted with a single exponential. The reactions are
-
P* P + fluorescence It, P* + Q
-
quenching
It,
(1) (2)
where P* and Q represent excited pyrene and the quencher, respectively. From reactions 1 and 2, the concentration of excited pyrene, monitored by fluorescence decay, can be written as
where [P*]and [P*l0 are concentrations of P* a t time t and 0, respectively. The natural decay rate constant ko and the quenching rate constant k, are extracted from the fitting curves, and k, for various quenchers are summarized in Table 1. At higher temperatures, the decay curves (Figure 3b) are no longer single exponentials. The concept of two sites for pyrene solubilization is then applied, and the curves are fitted with a standard double-exponential fit,6
[P*I= [P*I,(ae-klt+ (1 - a)
(4)
where kl and kz are the decay rate constants for two distinct sites and a is a weighting factor. The interpretation is that P* resides both in the more hydrophobic gel coil (corresponding to the increased I I H ratio) and in the aqueous phase. The relative amounts in each phase are given by a,while k1 refers to the gel coil and 122 to the (6) Dellaguardia, R.; Thomas, J. K. J. Phys. Chem. 1983,87,990.
Langmuir, Vol. 10, No. 9, 1994 3025
Poly(N-isopropylacrylamide)Microgel Structures 1.00
= 20 1
1
I
>k
0.79
-
.
H
cn
I
I\\ \
Z
W I-
z H
/
15
0.50
0
2.5
W
>
5
7.5
10
KI (mM)
H
IQ
0.25
-I
-
w
a 0.00
t 0
200
BOO
400
TIME
800
1000
(nanoseconds)
1.00
>IH
0.7s
m
Z
W
-t
Z
H
@.(Io
0
2.5
5
7.5
10
W
> H
IQ: 0.91
1
W
a: 0.00 0
I
I
900
400
TIME
I 0 0
I
moo
1QoQ
(nanoseconds)
Figure 3. (a, Top) Decays for pyrene fluorescencein polymer at 21 "C with various concentrations of KI. From to to bottom: 0, 1.92,3.83,5.75,7.66,and 9.58 mM. Insert: relationship between k and KI concentration. (b,Bottom) Decays for pyrene fluorescence in polymer at 50 "C with various concentrations of KI. From top to bottom: 0, 1.92, 3.83, 5.75, 7.66, and 9.58 mM. Insert: relationship between k1 and k2 and KI concentration.
aqueous phase. Again, ko and k , for each phase are extracted from the fits, and K1, and Kzq are the slopes of the plots of K 1 and k:, versus [&I as shown on the insert of Figure 3a and 3b. The rate constants are collected in Table 1.
The concept of two sites of solubilization of pyrene a t 50 "C,on the gel coil and in the water phase, is borne out by the effect of [gel coil] on a. The following values of a are obtaind by the double-exponential gel: 0.45%polymer, a = 0.87; 0.15% polymer, a = 0.76; and 0.05% polymer,
3026 Langmuir, Vol. 10, No. 9, 1994
Pankasem et al.
Table 2. Quenching Rate Constants for Excited Ru(bpy)a2+ 21 "C (M-l s-l) 50 "C (M-l s-lIa quencher water polymer water polymer Fe(CN)& 3.29 x 1O'O 2.89 x 1 O l o 5.88 x 1O'O ki, = 0 kZq = 5.22 x 1O1O nitrobenzene 7.07 109 7.18 109 1.27 x 1O1O kiq = 0 k2q = 1.55 x 10" cu2+ 5.30 107 5.55 x 107 8.44 x 107 polymer precipitation a At 50 "C, a double-exponential fit was applied with a = 0.50, klo = 1.73 x lo6 s-l, and k2o = 1.82 x lo6 s-l.
a = 0.54. As [polymer] decreases, the pyrene resides more in the aqueous phase. In Table 1, it is noted that a t 21 "C the quenching of the pyrene is identical in the presence of polymer to that found in water. This indicates that the pyrene and the molecules are predominantly contained in the water phase. However, a t 50 "C the double-exponential behavior of the fluorescence and the increased III/I ratio show that 76% of the pyrene quenches more slowly than the remaining 24%,the former pyrene being in the polymer phase and the latter in the aqueous phase. Similar data are given in Table 2 and Figure 2 b for the quenching of tris(bipyridine)ruthenium by various quenchers in water and in the microgel system. Here, a t 50 "C the tris(bipyridine)ruthenium is incorporated into the
microgel system and the aqueous phase in a manner similar t o that for the pyrene system. The incorporation is indicated by the identical fluorescence lifetimes in the microgel and in water. These studies show that photophysical methods are good indicators of molecular changes of systems under various conditions, in this particular study, changes induced by temperature. The studies comment on how the systems incorporate molecules dissolved in the dispersion medium and the nature of their position between the colloid system and the aqueous phase.
Acknowledgment. S.P.and J.KT.thank the National Science Foundation for partial support of this work.
