1576
Anal. Chem. 1986, 58, 1576-1578
AIDS FOR ANALYTICAL CHEMISTS New Method for Obtainlng Transmission Infrared and Visible Spectra of Thin Polymeric Films Nigel A. S u r r i d g e a n d Thomas J. Meyer*
Department of Chemistry, Venable and Kenan Laboratories, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514 There is a continuing need for spectroanalytical techniques for the identification, characterization, and analysis of thin polymeric films on electrodes (I). In many cases a metallic substrate is required for the fabrication of the polymeric film. One example is electropolymerization, where the polymer is formed at the electrode surface (2,3).Another is the chemical modification of prefabricated polymers during which the integrity of the film could not be maintained without specific adsorption onto a solid support such as platinum (4).Some considerable work has been done by Mattson (5, 6) in developing carbon on germanium for IR spectroelectrochemistry, but only in a multiple reflection mode, and no visible spectra were obtained of the films. Metallic minigrids attached to IR transarent materials, such as that used by Laser and Ariel could be useful for obtaining transmission IR-vis spectra of polymeric films, although uniformity of spun-cast polymers on a minigrid may be difficult to control. Although specular reflectance techniques can be used to obtain electronic spectra of thin transparent films on metallic surfaces, light scattering and related problems can make this technique insensitive in the IR region for typical organic polymeric films on the order of 0.1 pm in thickness. It is also very difficult to quantify absorbances using the specular reflectance technique. We report here a procedure for obtaining both transmission IR and visible spectra of the same electropolymerized or spun-cast polymeric film on platinized sodium chloride plates. If the metallic layer is sufficiently thin, the salt plates remain semitransparent in both the IR and visible regions and yet are sufficiently conductive to use as electrodes.
and peeled from the sodium chloride plate beneath. Due to the high internal resistance of the electrode, and its large surface area, no waves for the ligand-localized vbpyO/-couple within the film could be resolved in the cyclic voltammogram. Rather, only broad, featureless faradaic currents were observed. Reasonably well-resolved visible spectra of the film were obtained on an HP8450A diode array spectrophotometer by subtracting out the spectrum of the sodium chloride/platinum prior to polymerization. Infrared spectra were obtained with a Nicolet 20DX FTIR spectrometer, again by subtracting the electrode base line. (b) The organic polymer, chlorosulfonated polystyrene (I) (8
(n,
EXPERIMENTAL SECTION Metallized NaCl Plates. Standard IR sodium chloride plates (4 mm thick) were hand polished with methanol and dried in a vacuum oven (50 "C) overnight. One side of the plates was coated A, as measured by step profilometry) of with a thin layer (~200 platinum using a Polaron E5100 SEM sputter coater with a platinum foil target and a sputter time of 4 min. The sputtering procedure rendered the plates conductive, with a surface resistance of =200 G/cm (two-probe measurement in air). Similar coatings of gold can be achieved by use of a gold target. The metallic films are not mechanically strong but are stable in most dry organic solvents. Preparation and Characterization of the Polymeric Films. (a) An electropolymerized film, poly-(vbpy)Re(CO),Cl (vbpy = 4-vinyl-4'-methyl-2,2'- bipyridine) was prepared using the sodium chloride/platinum plate as the working electrode in a standard three-electrode, single-compartment cell ( 2 ) . Electrical contact was made with a small piece of aluminum and an alligator clip. The solvent was degassed acetonitrile (Burdick and Jackson) stored over molecular sieves, with 0.1 M tetraethylammonium perchlorate as the supporting electrolyte. The electrode was cycled between 0 and -1.9 V at 200 mV/s for 5 min. Reduction of the complex in this potential range (at -1.3 V vs. saturated sodium calomel electrode) occurs at the a* levels of the vbpy group, which leads to polymerization. After the cycling period the electrode was removed from the cell and rinsed well with dry acetonitrile. A golden/green film could be seen on the electrode, although a small percentage of the underlying platinum film had cracked 0003-2700/86/0358-1576$01.50/0
b0,Cl
I
mg, average molecular weight = 4000), was dissolved in 250 p L of dry 2-butanone (Aldrich) in a drybox, and one drop (=lo pL) of the resulting stock solution (8 mM) was spun cast onto a salt/platinum plate using a Headway Research spin coater at 2000 rpm. After the polymer/substrate was vacuum dried for 12 h, a thin uniform polymeric film ~ 1 2 0 A 0 thick was obtained as measured with a Tencor Alpha-step 100 profiler. FTIR and visible spectra were obtained as above. Chemical modification of the film was carried out by soaking in a 5 mM solution of ( ~ ~ Y ) ~ R U ( ~ - A Pwhere ) ( P Fbpy ~ ) ~=,2,2'-bipyridine and 5-AP = 5-amino-l,lO-phenanthroline, for 20 min at room temperature. Under these conditions, a reaction occurs between the amino group on the chromophore and the SOzClgroup on the polymer to form a sulfonamide linkage as described elsewhere (4). The resulting films have provided a basis for a photoelectrochemical cell (49). The modification of the intact polymeric film was conveniently monitored and quantified by observing the appearance of the metal-to-ligand charge transfer (MLCT) absorption band in the visible spectrum of the polymer after soaking, e.g., [(bpy)zRull(5-AP)I2+A [ (bpy-)Ru"'(bpy)(5-AP)I2+,A,,
=
460 nm.
RESULTS A N D DISCUSSION The electropolymerization procedure involves initial reduction a t the T* levels of the vbpy group
followed by anionic polymerization. 0 1986 American Chemical Society
1577
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
70
0
8 m
0
w
0
0
P
8
WAVELENGTH (nm)
8
B
1500 WAVENUMBER (cm-1)
2000
E
1000
Figure 3. Infrared spectrum of chlorosulfonated polystyrene film after subtraction of substrate spectrum.
Figure 1. Absorption spectrum of poly-(vbpy)Re(CO),CIfilm after digkal subtraction of the sodlum chlorlde substrate spectrum.
"I
I
I
0 2000
1500
1000
500
xWAVELENGTH :
H
0
::
WAVENUMBER (cm-1)
Figure 2. Infrared spectrum of poly-(vbpy)Re(CO),CIfilm after digltal subtraction of substrate.
H
1
0
0
E
m
(nm)
Figure 4. Absorption spectrum of chlorosulfonated polystyrene film after subtraction of substrate spectrum.
The visible spectrum of the resulting poly-(vbpy)Re(CO)&l fiim is shown in Figure 1, where the well-documented (10) bpy a a* transition a t 292 nm is well-resolved, along with a shoulder at 370 nm due to the Re-based MLCT transition. In the IR spectrum shown in Figure 2 the facial carbonyl stretches are clearly seen as a single band a t 2020 cm-l and a broad band with a shoulder at 1889 cm-l. The sharp band at 1094 cm-I is due to perchlorate anion used as electrolyte, while the broad band at 1441 cm-l can be assigned to a series of unresolved bpy ring vibrations. A similar band is seen in reflectance IR spectra of electropolymerizedRu(vbpy),(PF&
-
(11).
In the case of the spun-cast chlorosulfonated polystyrene film the SO2 symmetric and asymmetric stretches are seen a t 1173 cm-l and 1376 cm-l, respectivly (Figure 3). In the visible region (Figure 4) a peak at 249 nm is seen due to the a* transition, with a featureless sloping base line ring a above 400 nm. Following acquisition of the spectra in Figures 3 and 4, the plate and film were soaked in the acetonitrile solution containing the chromophore (bpy),R~(fi-AP)~+. Following the soaking procedure and rinsing with copious quantities of clean acetonitrile, less than 5% loss of film coverage due to peeling of the platinum had occurred. The visible spectrum of the chromophore-impregnated film with the unsoaked plate and film subtracted (Figure 5 ) shows absorbances a t ,A, = 460 nm due to the chromophoric sites now linked to the polymeric backbone. There is also evidence for the presence of chromophoric sites that are simply trapped within the polymer matrix, but not covalently bound, by the appearance of an additional absorption band a t A,, = 370 nm (12). In conclusion, the technique described here may be generally applicable to the acquisition of IR and visible spectra
-
H
0
0
8 WAVELENGTH
H
B
0
8
(nm)
Flgure 5. Absorption spectrum of chlorosulfonated polystyrene film after soaking in ruthenium complex and subtraction of the spectrum in Figure 4.
of many thin polymeric films, either electrochemicallygrown or spun cast, and may prove to be of considerable value in monitoring subsequent chemical modification steps that affect the spectral properties of the polymers. It also appears that different metals (Pt, Au) can be used to render the surface conductive. A significant advantage of this approach is that it gives unstructured base lines in the visible spectral region, in contrast to other semitransparent electrodes such as Sn02 or TiOz. The absence of structured base lines makes possible the resolution of bands with absorbances as low as 0.01. In addition, we are also investigating the use of the metallized NaCl plates as a basis for an in situ spectroelectrochemical cell for use in both the visible and IR regions. eegistry 100243-95-6.
No. Poly-(vbpy)Re(CO)&1 (homopolymer),
1578
Anal. Chem. lQ86, 58,1578-1580
LITERATURE C I T E D (1) Murray, R. W., I n "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcei Dekker: New York, 1964, Vol. 13. (2) Denisevich, P.; Abruna, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1082, 2 1 , 2153. (3) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1083, 22, 2151. (4) Ellis, C. D.; Meyer, T. J. Inorg. Chem. 1984, 2 3 , 1748. (5) Mattson, J. S.;Smith, C. A. Anal. Chem. 1975, 4 7 , 1122. (6) Mattson, J. S.;Jones, T. T. Anal. Chem. 1878, 48, 2164. (7) Laser, D.; Ariel, M. J. Nectroanal. Chem. 1973, 4 1 , 381. (8) Hupp, J. T.; Otruba, J. P.; Parus, S. J.; Meyer, T. J. J. Electroanal. Chem. 1985, 190, 287.
(9) Hupp, J. T.; Meyer, T. J., in preparation.
(IO) Morse, D. L.; Wrighton, M. S . J. Am. Chem. SOC. 1974, 9 6 , 998. ( I l ) Surridge, N . A., unpublished results. (12) Hupp, J. T., unpublished results.
RECEIVED for review September 3,1985. Accepted December 12,1985. Acknowledgments for funding of this work are made to the Army Research office-Durham L d e r Grant DAAG29-82-K-0111.
Automated Mercury Film Electrode for Flow Injection Analysis and High-Performance Liquid Chromatography Detection Hari Gunasingham,* B. T. Tay, a n d K. P. Ang
Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 0511 The mercury electrode, in the form of the dropping mercury electrode or hanging mercury drop electrode, has not gained widespread use in flowing stream analysis because of its inherent instability and due to practical difficulties. Reductive detection a t solid electrodes is however limited by their cathodic potential range, particularly a t low pH. Moreover, reactions at solid electrodes are generally found to be sluggish compared to those a t the mercury electrode. This is particularly true for glassy carbon electrodes (GCE). Many reactions, such as the reduction of nitrobenzene (l),are more irreversible a t glassy carbon than say platinum or gold. However, glassy carbon is usually preferred, especially in continuous monitoring applications, on account of its relative immunity to passivation and poisoning. The use of the mercury film electrode (MFE) as an alternative to the mercury electrode has been well applied to anodic stripping voltammetry (ASV) in both discrete and flowing stream analysis. Its application to organic analysis is however relatively unexplored. In one of the few works on this aspect, Kublik showed by cyclic voltammetric studies that the MFE actually has a similar response to the HMDE (2). This work was, however, applied to static analysis, and the mercury film was prepared in situ. In this paper, we show the practical application of an automated MFE for reductive detection in flow injection analysis (FIA) and HPLC. The approach is to prepare the mercury film before each analysis, thereby presenting a fresh surface to the analyte. The use of microprocessor control in conjunction with the wall-jet electrode (WJE) enables the reproducible control of the film plating and stripping and the actual analyses. We also show that, compared with a bare glassy carbon electrode, the MFE is superior in regard to the reversibility of electrode reactions and the cathodic working range. The theory of the WJE was first described by Yamada and Matsuda (3). It has since been thoroughly investigated by several groups in its application to continuous-flow monitoring (3-11). The unique feature of the W J configuration is the requirement for a large geometric cell volume. Unlike other configurations, the effective dead volume is only of the order of the hydrodynamic boundary layer, which is unaffected by the geometric cell volume. Also, there is no cross-contamination of the active solution by the bulk volume (12).High sample throughput is thereby feasible. The exploitation of
these features in continuous monitoring ASV at a mercury film WJE was recently described (13). EXPERIMENTAL SECTION Electrochemical System. The large-volume WJ cell used in this work is identical to the one used in a previous work (6). The geometric cell volume is 20 mL. The working electrode was a 5.4-mm-diameter glassy carbon disk (Tokai, Tokyo, Japan), which was press fitted into a poly(tetrafluoroethy1ene) (PTFE) casing. A copper lead afforded contact between the GCE and the polarographic analyzer. Similarly a 3-mm-diameter platinum disk fitted into a PTFE casing, served as the counter electrode. The reference system used was a AglAgCl electrode with saturated KC1. All potentials quoted in this paper are with respect to this reference electrode. The 0.6-mm inlet diameter nozzle was positioned 5 mm away from the back wall of the cell so as not to interfere with the flow of the hydrodynamic boundary layer (6, 7). The WJ cell was controlled by a PAR Model 174A polarographic analyzer (Princeton Applied Research, NJ). The cyclic voltammograms and amperometric peaks were recorded on a Graphtec Model WX 2400 x-y recorder (Watanabe Instruments, Japan). Preparation of the GCE Surface. The GCE was polished to a mirror finish with a fine slurry of alumina that was prepared by grinding 1-pm particles with a pestle and mortar. The electrode was then polished further with a slurry of 0.05-pm alumina (BAS, Inc., IN) to produce a scratch-free surface. Instrumentation. A schematicdiagram of the instrumentation is shown in Figure 1. An Apple IIe microcomputer was used to automate the mercury film plating and stripping sequence at the MFWJE. In the plating step the computer controls the plating potential through a digital-analog converter connected to the external input of the PAR 174A polarographic analyzer (Princeton Applied Research, Princeton, NJ). The plating step commences when the computer causes the pneumatically actuated four-way valve (Rheodyne Model 5010) to switch from the buffer stream to the mercury stream. Once the plating is completed, the valve switches to the buffer or sample stream. On completion of a sample run, the mercury film is stripped off. This sequence may be repetitively employed for as many analyses as required. Figure 2 gives the flow chart of the control program for the sequencing operation. The user inputs the various parameters for the analysis sequence into a menu. When no more analyses have to be performed the program returns to the menu. The input-output and timing control is implemented in machine code, whereas the menu driver is written in Applesoft BASIC. A peristaltic pump (Eyela Model MP-3) and a syringe pump (Sage Instruments, MA) were used to deliver the blank buffer
0003-2700/68/0358-1578$01.50/00 1988 American Chemical Society
