DETERMINATION OF OXALATE ION DOPANT LEVEL IN POLYPYRROLE USING FT-IR

Abstract

A pellet method using standard addition and FT-IR was used to estimate oxalate ion doping levels in electrosynthesized polypyrrole. The method is useful for materials where removal of analyte from an insoluble material is problematic. Here, electrosynthesized oxalate doped polypyrrole is dispersed in potassium bromide. Spikes of sodium oxalate are added and the mixtures pressed into pellets. The oxalate carbonyl absorption peak is then used to quantify the amount of oxalate present in the polypyrrole. The mass fraction of oxalate dopant in polypyrrole was determined to be 0.4 ± 0.1 % and coincides with the original synthesis solution composition.

Introduction

The conducting polymer polypyrrole (PPY), has been of interest due to its high electrical conductivity, ease of preparation, stability, and good mechanical properties. Many possible applications from sensors, electrode materials, to neural scaffolding have made polypyrrole the subject of much research in recent years (1,2). Our group has been investigating polypyrrole as a neural interface material in artificial retina structures.

The chemical structure of intrinsic PPY is shown in Figure 1 (a). It can be synthesized electrochemically via the oxidation of pyrrole whereby it forms an electrodeposit on the working electrode. The conductivity of PPY is increased by adding an anionic dopant to the synthesis solution which incorporates itself into the film. Due to charge balance, the negative dopantions cause compensating positive charges to be incorporated into the conjugated pi orbital system, increasing conductivity by a hole conduction mechanism. Dopants including common anions such as chloride and perchlorate as well as less common dopants such as, 1,5-naphthalene disulfonic acid, 5-sulfosalicylic acid (3), poly m-aminobenzene sulfonic acid, and functionalized single-walled nanotubes (4) have been investigated. Our laboratory has been interested in avoiding oxidizers and potential mobile ionic contaminants, while being interested in a more biocompatible dopant. One possible candidate is the oxalate anion. In addition we have been interested in determining if the oxalate anion may have favorable conjugated pi system interactions with PPY. The structure of the doped PPY with oxalate anion is shown in Figure 1 (b). Notice each oxalate anion is doubly charged with two positive charges compensated in the PPY chain. As more dopant is added, the conductivity increases. In our experiments, we are interested in determining how much oxalate dopant the polypyrrole can uptake and therefore need a way to quantify it.

Figure 1.

Chemical structure of (a) intrinsic polypyrrole and (b) oxalate doped polypyrrole.

Oxalate has been found in renal stones and has been studied qualitatively using FT-IR (5). Quantitative methods such colorimetry, high performance liquid chromatography, gas chromatography, enzymatic digestion, and ion chromatography with conductiometric detection have been used to determine the amount of oxalate inside renal stones (6). These methods are adequate as long as the matrix can be completely dissolved without chemically changing the oxalate in unpredictable ways. However, we have found that polypyrrole is quite insoluble in solvents that would solubilize the oxalate ion, in particular, any aqueous solvent. We therefore cannot be confident in completely releasing the oxalate ion from the matrix for isolated analysis. This leaves the common method candidates suitable for solid state chemical analysis such as X-ray and infra-red spectrometry. In the case of Scanning Electron Microscopy-Energy Dispersive Spectrometry (SEM-EDS) which we have available in our laboratory, the detection limits are around the 1% level and so, may not be able to detect our dopant at lower levels. SEM-EDS would also only pick up an atomic oxygen signal which may come from other chemical sources such as an over oxidized PPY film. Many labs have available a Fourier Transform Infra-Red spectrophotometer (FT-IR) available which if the material is sampled appropriately can give a bulk estimation of oxalate quantity based on a unique chemical signal. For these reasons we chose to develop an FT-IR analysis method.

A common method for FT-IR analysis of solid state materials is to mix (grind together) the sample with potassium bromide and then press into pellets. The FT-IR spectrum in our case will then contain the polypyrrole absorptions as well as the oxalate absorptions present inside it. By using standard additions of oxalate to a series of pellets of constant mass, quantitative information can be determined. Since potassium bromide has water absorption signals near the analytical oxalate absorption signal, a blank determination will be required to remove this absorption. Nevertheless, since the method presented here is done in room atmosphere, it should be regarded as semi-quantitative.

Experimental

Preparation of Polypyrrole

In a 50 mL beaker 0.100 g of ammonium oxalate (AmOx, Mallinckrodt, Analytical Reagent), 0.5 mL of pyrrole (Aldrich, 98 % Reagent Grade), and 24.5 mL of water are mixed together until the AmOx has dissolved. A tungsten working electrode (Alfa Aesar, 99.95 % m), glassy carbon counter electrode (Alfa Aesar, Type 1), and platinum pseudo-reference electrode (Aldrich, 99.9 % m) is placed into the solution. These electrodes are then connected to a bi-potentiostat (Pine Instruments Model AFRDE5) which is set at a constant voltage of 800 mV and run for 16 hours. The PPY deposit is then sonicated with deionized water, removed from the tungsten electrode, and allowed to air dry overnight.

PPY KBr Stock Mixture

A 2.0 % PPY/KBr stock solid solution mixture is prepared by first weighing the PPY sample. The amount of dried and dessicated KBr (International Crystal Labs, Spectrograde) needed is calculated and added to the PPY. The total mass is recorded. After thoroughly grinding both KBr and PPY together, the stock mixture was then stored in an oven at 120°C until needed. The 2.0 % mixture was found to give a linear response in the concentration range of interest here.

Standard addition sample pellets

Pellets for a series of standard additions are prepared as follows. A solution of 0.010 M sodium oxalate (NaOx, Mallinckrodt, Analytical Reagent) is prepared. Five aluminum boats are pre-weighed and filled with 0.020 g of stock mixture. Four boats are spiked with 1, 5, 10, and 15 μL of NaOx solution. The boats are placed into the oven to drive off the water for approximately 3 hours until constant weight. Each sample is then ground with a mortar and pestle and 0.01500g is placed into pelletizing die and pressed into a pellet at 34 Nm for 10–15 minutes. The pellet mass is kept as constant as possible in order to keep the path length constant. After pressing, the pellet dies are carefully and slowly pulled apart so there is no accidental cracking or holes created. Each pellet is analyzed by FT-IR spectrometry (Thermo Nicolet Avatar 360 E.S.P. running OMNIC software). Before and after of each use, the pellet die, mortar, and pestle are rinsed with distilled water, wiped, rinsed with acetone, and then placed in the oven until needed.

Results and Discussion

Analysis of Standard addition sample pellets

The infrared spectrum of polypyrrole shows N-H stretching at 3435 cm⁻¹ with several additional peaks between 3000–2800 cm⁻¹ due to conjugated aromatic C-H stretching. Additionally, multiple peaks appearing between 1600–1300 cm⁻¹ are due to ring stretching. Finally, absorptions showing between 840–740 cm⁻¹ are most likely due to out of plane C-H bending (7). The infrared spectrum of oxalate features absorption between 1757 – 1585 cm⁻¹ due to carbonyl stretching (5) and also displays a peak showing near 1400 cm⁻¹ due to symmetric carbonyl stretching (7).

Figure 2 shows the overlaid spectra of the 1, 5,10, and 15 (μL NaOx/PPY/KBr spiked sample pellets and the unspiked PPY/KBr stock solution sample pellet. The peak centered at wave number 1639 cm⁻¹ is due to the oxalate ion carbonyl stretching and will be used as the quantitative analytic peak for oxalate. As can be seen in Figure 2, there is a regular progression of increasing absorbance at the 1639 cm⁻¹ peak. All other peaks are of no quantitative importance. Using the Omnic software (Thermo Nicolet), each peak centered at 1639 cm⁻¹ is isolated and baseline corrected. Using the peak area tool, the peak area vertical markers are set between wave numbers 1710.550 cm⁻¹ and 1579.414 cm⁻¹. Omnic is allowed to set the area baseline automatically for each peak. The peak area outlined by the markers is then computed for each peak.

Figure 2.

Overlaid FT-IR absorbance spectra of spiked and unspiked oxalate doped NaOx/PPY/KBr sample pellets. Increasing absorbance signals at 1639 cm⁻¹ correspond to the 0, 1,5,10, and 15 μL NaOx spikes. Peak areas were calculated from each individual baseline.

The oxalate mass percentage for each spiked sample is calculated from the molarity and volume of the oxalate spike, converting from mole to grams of oxalate (using the molecular weight of oxalate ion), then, dividing by the total pellet mass. These are then plotted as peak area vs. oxalate mass percent in Figure 3. According to the standard addition method, the x-intercept in Figure 3 is the mass percent of the unspiked sample. Least squares analysis was used to determine the x-intercept and found it to be 101 ± 33 ppm oxalate.

Figure 3.

Standard addition plot of oxalate ion concentration versus peak area for the oxalate doped sample pellets.

Analysis of Standard addition KBr blank pellets

Since potassium bromide itself absorbs water and has a water IR absorbance peak which overlaps with the carbonyl absorbance peak in sodium oxalate, a blanking correction is necessary. For this, the procedure above is repeated on samples that contain no PPY. Figure 4 shows the results of the overlaid 0,1,5,10, and 15 (μLNaOx/KBr spiked blank sample pellets. As shown in Figure 4, there is a slight shift in the IR absorbance peak from 1629 cm⁻¹ in the unspiked KBr pellet (which is entirely due to water) to 1639 cm⁻¹ for the spiked pellets (corresponding to the oxalate). As before, the spectra exhibit a regular increase in absorbance as more sodium oxalate is spiked into the samples. The peak area vs. oxalate concentration plot for the blanks is shown in Figure 5. Least squares analysis on this data gives a result of 21 ± 11 ppm oxalate. This result is the contribution of the matrix to the analyte signal and is to be subtracted from the PPY sample determination.

Figure 4.

Overlaid FT-IR absorbance spectra of oxalate spiked and unspiked KBr blank sample pellets. Increasing absorbance signals correspond to the 0,1, 5,10, and 15 μL NaOx spikes. Peak areas were determined from each individual baseline.

Figure 5.

Standard addition plot of oxalate ion concentration versus peak area for the KBr blank sample pellets.

Determination of Oxalate in Polypyrrole

Subtracting the blank determination (21 ± 11 ppm) from the sample determination (101 ± 33 ppm) results in 80 ± 30 ppm oxalate contained in the PPY sample pellet. Since the amount of oxalate doped PPY in all pellets is 2.0 % and the total mass of the unspiked pellet is known (0.01500 g), the product of these gives the mass of doped PPY in the unspiked pellet (3.0 × 10⁻⁴ g). The product of the oxalate mass fraction in the unspiked pellet ((80 ± 30) × 10⁻⁶), and total unspiked pellet mass (0.01500 g) gives the mass of oxalate in the unspiked pellet ((1.2 ± .4) × 10⁻⁶ g). The ratio of this oxalate mass to the mass of doped PPY yields the mass percent of oxalate in the synthesized polypyrrole and was determined to be 0.4 ±0.1 %. This roughly coincides with the original PPY synthesis solution composition which was approximately 0.30 % oxalate.

Conclusion

In conclusion, we have been able to approximately quantify the amount of oxalate uptake in electrosynthesized polypyrrole and that this amount is approximately the same amount as present in the polypyrrole synthesis solution. The method illustrated here is accessible to many labs, and offers an approach that is minimally disturbing to the original sample whereby alteration of analyte levels may occur under dissolving conditions necessary to remove it for analysis using other methods.