In the Laboratory
Making and Using a Sensing Polymeric Material for Cu2+
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An Introduction to Polymers and Chemical Sensing Jean R. Paddock, Anne T. Maghasi, William R. Heineman, and Carl J. Seliskar* Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172; *
[email protected] With polymers and chemical sensors having become such integral components of everyday life and wide-ranging scientific research, it is important that introduction to both topics occur early in the academic experience, supplying educated researchers and sustaining interest in both fields. A well-defined sensor is typically anchored in the basic principles of chemistry, biology, and physics. There may be some introduction to sensors and polymers in a lecture-based chemistry course (e.g., instrumental analysis), but there is a lack of simple, hands-on laboratory experiences, as evidenced by the availability of few innovative sensor experiments, a fraction of which are geared towards education (1–3). This experiment fills that gap, providing a simple chemical sensor-related experiment rooted in the synthesis of polymeric materials for use in either an advanced high-school or undergraduate collegiate laboratory. Students are introduced to and combine the concepts of the chemical sensor, polymer chemistry, spectroscopy, metal chelates, and quantitative analytical methods. Experimental Overview In this two-day experiment, students synthesize a porous, crosslinked polymer network doped with a spectroscopi-
H2 C
H2 C
H2 C
CH
CH
CH
OH
OH
OH
H2 C
+ O
PVA
H
O
CH2
glutaraldehyde
2.5
+
CH C H2
C H2
glutaraldehyde crosslink
CH C H2
M2ⴙ O
OH C H2
N
OH
CH C H2
C H2
CH
N
2.0
O
CH
+ OH
N
2 H2O
CH O
O
C H2
OH
CH2
C
CH
O
CH2
CH2
C H2
CH
CH2
H 3 Oⴙ
CH2
OH
PAN CuSO4 PAN ⴙ CuSO4
H2 C
CH CH
C
H
H2 C
H2 C CH
Absorbance
H2 C
cally-active chelating agent that act together as a component of a chemical sensor. Poly(vinyl alcohol), PVA, is crosslinked with glutaraldehyde in the presence of the catalyst H3O+ (Scheme I). A polyelectrolyte, poly(acrylic acid), PAA (Figure 1), acts as an ion-exchange medium for cations and is entrapped in the network, which functions as a semipermeable hydrogel. The chelating agent, 1-(2-pyridylazo)-2napthol, PAN (structure in Figure 2), is then incorporated into the network, adding to the matrix a binding site or selective element for a 2+ metal ion. On the first day, students prepare copper(II) sulfate, CuSO4, solutions of different concentrations via serial dilution and prepare the network polymer. Students also assess the PAN–copper ion interaction via absorption spectroscopy. On the second day, students introduce their polymer network to the prepared solutions of aqueous Cu2+ solutions of varying concentration, and after a fixed exposure time, observe and spectroscopically determine the relationship between the quantity of PAN–Cu complex formed in the network polymer and Cu2+ solution concentration. A calibration plot is generated from these data, and students are then able to identify the concentration of an unknown Cu2+ solution based on this plot.
H
1.5
1.0
0.5
PVA Scheme I. Crosslinking of PVA with glutaraldehyde. Network entrapped PAA (protonated form) is shown at bottom.
0.0 400
H2 C
H2 C CH
H2 C CH
CH
Figure 1. Network entrapped PAA (protonated form).
Journal of Chemical Education
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700
800
900
1000
Wavelength / nm
H2 C
COOH COOH COOH
1370
500
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Figure 2. Absorption spectra of 8 x 10-5 M PAN, 0.05 M CuSO4, and resultant PAN–Cu complex. PAN–Metal2+ complex is shown at right.
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In the Laboratory
Hazards
2.0
Results and Discussion Aqueous solutions of PAN and Cu2+ interact, yielding a 1:1 PAN–Cu complex (λmax = 555 nm). The absorbance spectrum of the complex is shown in Figure 2 along with the spectra of the components. It has been reported that PAN and Cu2+ can form a stable complex in a 2:1 ratio (4), but we did not observe its formation (5). Absorbance of PAN contributes greatly below 500 nm (ε465 = 1.8 × 104 M᎑1 cm᎑1) (6), but does not interfere with the absorbance maximum for PAN– Cu found at 555 nm. Note also in the PAN–Cu spectrum that the Cu2+ peak at 810 nm (ε810 = 12 M᎑1 cm᎑1) (7) remains relatively unchanged, thus showing an insignificant quantity of the excess Cu2+ being used to form the PAN–Cu complex. Student synthesis of the PVA–PAA兾PAN doped network is straightforward with hydrogel curing occurring over several days during which the thick, monolithic gel is drained of excess water. Equilibration of this dried polymer network in water is necessary prior to Cu2+ solution exposure to avoid any effect that network swelling may have on the uptake of Cu2+ into the matrix. The Cu2+ uptake is time dependent with longer exposure time resulting in greater concentration of ions into the matrix. An upper limit is reached around three hours of exposure, with spectra generated from these network sections representing a Cu2+ concentration that is too high to accurately determine spectrophotometrically. In this laboratory it is recommended that spectra be obtained approximately one hour following initial network exposure to CuSO4 solutions, as shown in Figure 3. Noise below 500 nm is attributed to highly absorbent PAN and anomalous behavior at 651 nm is an artifact of the Hewlett-Packard spectrophotometer. Matrix effects do not seem to affect PAN– Cu complex formation. The spectrum profile remains unchanged upon chelation in the polymer network versus in solution with λmax at 555 nm in both cases (8). Increased Cu2+ concentration present in CuSO4 solutions results in increased concentration of PAN–Cu complex in the network polymer. At λmax = 555 nm an absorbance maximum corresponding to each concentration may be obtained and a calibration plot of absorbance versus CuSO4 solution concentration may be generated (Figure 4). Students are then able to identify the concentration of an unknown Cu 2+ solution. Based on this linear relationship, PVA– PAA兾PAN doped networks could be a useful component of a chemical sensor geared towards 2+ metal ions such as Cu2+. Supplemental Material Instructions for the students and notes for the instructor are available in this issue of JCE Online. W
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(e)
Absorbance
1.5
(d) 1.0
(c) 0.5
(b) (a) 0.0 400
450
500
550
600
650
700
Wavelength / nm Figure 3. Absorption spectra of PVA–PAA/PAN network sections soaked for 1 hour in CuSO4 solution at a concentration of (a) 0.025 mM, (b) 0.05 mM, (c) 0.10 mM, (d) 0.15 mM, and (e) 0.20 mM.
2.0
1.5
Absorbance
Chemicals used in this experiment are used as dilute solutions in water ( ≤ 0.5 M) and warrant National Fire Protection Association (NFPA) ratings of 0 to 3. Of note is glutaraldehyde, holding a health rating of 3 and having been investigated as a mutagen, tumorigen, and reproductive effector. Students should handle this chemical in a ventilated hood, wearing safety gloves and goggles at all times. Additionally, PAN is a dye-type material and caution should be taken to avoid contact with work surfaces, clothing, or skin, as staining can occur.
1.0
0.5
0.0 0.00
0.05
0.10
0.15
0.20
Concentration / (mmol/L) Figure 4. Calibration plot for the PAN–Cu complex at λmax = 555 nm (data from spectra shown in Figure 3).
Literature Cited 1. Sommerdijk, N. A. J. M.; Poppe, A.; Gibson, C. A.; Wright, J. D. J. Mater. Chem. 1998, 8, 565–567. 2. Javaid, M. A.; Keay, P. J. J. Sol-Gel Sci. Technol. 2000, 17, 55– 59. 3. Martinez-Faregas, E.; Alegret, S. J. Chem. Educ. 1994, 71, A67. 4. Powell, H. K. J.; Town, R. M. Anal. Chim. Acta 1991, 248, 95–102. 5. Shtoyko, T.; Zudans, I.; Richardson, J. N.; Seliskar, C. J.; Heineman, W. R. J. Chem. Educ. 2004, 81, 1617–1619. 6. Hojo, M.; Ueda, T.; Inoue, A. Bull. Chem. Soc. Jpn. 2002, 75, 2629–2636. 7. Prenesti, E.; Daniele, P. G.; Toso, S. Anal. Chim. Acta 2002, 459, 323–336. 8. Lazaro, F.; De Castro, L.; Valcarcel, M. Anal. Chim. Acta 1998, 214, 217–227.
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