Microfluidic Channel Flow Cell for Simultaneous Cryoelectrochemical

Feb 2, 2007 - Rudolph Le Roux, Sinead Matthews, and Adrian C. Fisher* ... John D. Watkins , Stuart M. MacDonald , Paul S. Fordred , Steven D. Bull ...
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Anal. Chem. 2007, 79, 1865-1873

Microfluidic Channel Flow Cell for Simultaneous Cryoelectrochemical Electron Spin Resonance Andrew J. Wain and Richard G. Compton*

Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, OX1 3QZ, U.K. Rudolph Le Roux, Sinead Matthews, and Adrian C. Fisher*

Department of Chemical Engineering, University of Cambridge, New Museums Site, Pembroke Street, Cambridge, CB2 3RA, U.K.

A novel microfluidic electrochemical channel flow cell has been constructed for in situ operation in a cylindrical TE011 resonant ESR cavity under variable temperature conditions. The cell has a U-tube configuration, consisting of an inlet and outlet channel which run parallel and contain evaporated gold film working, pseudo-reference, and counter electrodes. This geometry was employed to permit use in conjunction with variable temperature apparatus which does not allow a flow-through approach. The cell is characterized qualitatively and quantitatively using the one-electron reduction of p-bromonitrobenzene in acetonitrile at room temperature as a model system, and the ESR signal-flow rate response is validated by use of three-dimensional digital simulation of the concentration profile for a stable electrogenerated radical species under hydrodynamic conditions. The cell is then used to obtain ESR spectra for a number of radical species in acetonitrile at 233 K, including the radical anions of mand p-iodonitrobenzene, o-bromonitrobenzene, and mnitrobenzyl chloride, the latter three being unstable at room temperature. Spectra are also presented for the radical anion of 2-chloranthraquinone and the crystal violet radical, which display improved resolution at low temperatures. It has long been recognized that the use of electron spin resonance (ESR) spectroscopy in conjunction with electrochemistry can be of great use in probing electrode processes, especially from a mechanistic perspective, permitting the identification of radical intermediates and products, in addition to the determination of radical lifetimes. Since the early experiments in this field in the late 1950s,1-3 a great deal of progress has been made, with particular advances being achieved in the area of in situ electrochemical cell design. Various approaches have been adopted, and * To whom correspondence should be addressed: (e-mail) richard.compton@ chemistry.oxford.ac.uk (R.G.C.), [email protected] (A.C.F.); (phone) +44 (0) 1865 275 413 (R.G.C.), +44 (0) 1223 763996 (A.C.F.); (fax) +44 (0) 1865 275 410 (R.G.C.), +44 (0) 1223 767407 (A.C.F.). (1) Austin, D. E. G.; Peover, M. E.; Ingram, M. H.; Ingram, D. J. Nature 1958, 181, 1784. (2) Maki, A. H.; Geske, D. H. J. Am. Chem. Soc. 1960, 82, 267. (3) Maki, A. H.; Geske, D. H. J. Chem. Phys. 1959, 30, 1356. 10.1021/ac061910n CCC: $37.00 Published on Web 02/02/2007

© 2007 American Chemical Society

many of the developments made have been reviewed.4 One strategy that has received much attention is the use of hydrodynamic flow, with channel and tubular geometries being of primary interest.5,6 Notable advantages in employing this methodology include the ability to sustain the delivery of electroactive material to the working electrode, leading to the injection of high and reproducible Faradaic currents and consequently a high level of sensitivity, while allowing quantitative kinetic and mechanistic information to be extracted due to the mathematically well-defined flow regime.5,7 The pertinent theory for such convective systems is well developed, which has enabled the digital simulation of concentration profiles and numerical modeling of mass transport limiting currents and ESR signal intensity data for both stable and unstable electogenerated radicals, making this a very powerful technique indeed.6,8,9 A critical issue in the optimization of cell design is the problem of dielectric loss. The sensitivity of an ESR cavity, characterized by its Q factor, can be significantly compromised by the absorption of microwaves by dipolar solvents, and so it is desirable to minimize electrolyte solution volume. However, since is the intensity of the ESR signal determined by the number of spins within the cavity, the magnitude of the ESR line is limited by the volume of sample employed. Consequently, this compromise of dielectric absorption versus filling factor leads to a trade off in sensitivity with sample size, the optimum volume being determined by the nature of the solvent employed. Furthermore, although the TE011 cylindrical cavity employed in this work has an inherently higher Q, by a factor of at least 3,10,11 than the TE102 (4) Wadhawan, J. D.; Compton, R. G. Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH Verlag GmbH and Co. kGaA: Weinheim, Germany, 2003; Vol. 2, Chapter 3. (5) Coles, B. A.; Compton, R. G. J. Electroanal. Chem. 1983, 144, 87. (6) Wain, A. J.; Thompson, M.; Klymenko, O. V.; Compton, R. G. Phys. Chem. Chem. Phys. 2004, 6, 4018. (7) Webster, R. D.; Bond, A. M.; Coles, B. A.; Compton, R. G. J. Electroanal. Chem. 1996, 404, 303. (8) Cooper, J. A.; Compton, R. G. Electroanalysis 1998, 10, 141. (9) Streeter, I.; Wain, A. J.; Thompson, M.; Compton, R. G. J. Phys. Chem. B 2005, 109, 12636. (10) Alger, R. S. Electron Paramagnetic Resonance: Techniques and Applications; John Wiley and Sons: New York, 1968. (11) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance, Elementary Theory and Practical Applications; John Wiley and Sons: New York, 1994.

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rectangular cavity typically used in conjunction with flat electrolysis cells, it is optimal to confine the cell to a region close to the central cavity axis, where maximum sensitivity is achieved. This clearly places a limitation on the size and design of electrochemical ESR cells. An alternative approach to electrochemical ESR is based on the loop-gap resonator introduced by Froncisz and Hyde.12 Loop-gap resonators have a number of advantages over conventional resonant cavities, notably their smaller size and greater observed signal intensity. Both of these features can be beneficial for electrochemical purposes, and the design of appropriate cells has been reported.13 The use of a tubular flow-through cell within a cylindrical cavity has been shown to demonstrate high sensitivity, since the tube could easily be restricted to the central cavity axis,6,9 but was found to be unsuitable for low-temperature studies due to a restriction imposed by the variable temperature apparatus employed. The widely used11 variable temperature adapter controls the cavity temperature by venting cooled or warmed nitrogen gas into a jacket which is inserted inside the cavity, but in doing so, blocks one of the two cavity apertures, preventing the flow-through approach. It is clear that the application of low temperatures has the potential to greatly broaden the range and lifetime of reactive systems accessible to electrochemical ESR, but it also offers the advantages of further improved cavity sensitivity and narrower line widths,11 and so an electrochemical hydrodynamic cell which is compatible with such low-temperature fixtures would be a highly valuable tool. Designing a flow cell in which the solution inlet and outlet are positioned at the same end presents a particular challenge, especially with regards to the limitations on solution volume and cell width described above. One solution that has been identified is the use of a tubular flow cell that is blocked at one end and fed with a narrow tube running down its center, and this approach was used successfully to detect the o-bromonitrobenzene radical anion in acetonitrile at 233 K.14 Another in situ design which is compatible with variable temperature apparatus is the wall jet cell proposed by Bagchi et al.,15 in which a jet of electrolyte impinges on a mercury electrode, but the hydrodynamics of this regime are not mathematically well defined. A refinement to this design made by Compton et al.16 allowed for more controlled (12) Froncisz, W.; Hyde, J. S. J. Magn. Reson. 1982, 47, 515. (13) Allendoerfer, R. D.; Froncisz, W; Felix, C. C. J. Magn. Reson. 1988, 76, 100. (14) Wain, A. J.; Compton, R. G. J. Electroanal. Chem. 2006, 587, 203. (15) Bagchi, R. N.; Bond, A. M.; Scholz, F. J. Electroanal. Chem. 1988, 252, 259. (16) Compton, R. G.; Greaves, C. R.; Waller, A. M. J. Electroanal. Chem. 1990, 277, 83. (17) Tabeling, P. Introduction to Microfluidics; Oxford University Press Inc.: New York, 2005. (18) Ehrfeld, W.; Hessel, V.; Lo ¨we, H. Microreactors; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2000. (19) Dietrich, T. R.; Ehrfeld, W.; Lacher, M.; Kra¨mer, M.; Speit, B. Microelectron. Eng. 1996, 30, 497. (20) Wang, J. Talanta 2002, 56, 223. (21) Daniel, D.; Gutz, I. G. R. Talanta 2005, 68, 429. (22) Amatore, C.; Belotti, M.; Chen, Y.; Roy, E.; Sella, C.; Thouin, L. J. Electroanal. Chem. 2004, 573, 333. (23) Yunus, K.; Marks, C. B.; Fisher, A. C.; Allsopp, D. W. E.; Ryan, T. J.; Dryfe, R. A. W.; Hill, S. S.; Roberts, E. P. L.; Brennan, C. M. Electrochem. Commun. 2002, 4, 579. (24) Yunus, K.; Fisher, A. C. Electroanalysis 2003, 15, 1782. (25) Fulian, Q.; Gooch, K. A.; Fisher, A. C.; Stevens, N. P. C.; Compton, R. G. Anal. Chem. 2000, 72, 3480. (26) Coles, B. A.; Compton, R. G.; Spackman, R. A. Electroanalysis 1993, 5, 41.

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convection, but attempts to further improve sensitivity when employing particularly lossy solvents, by reducing the tube diameter, were met with relatively little success.4 Recently there has been a large focus on the miniaturization of flow cells and reactors, particularly for lab-on-a-chip applications, and the fabrication of microfluidic devices is fast becoming a vastly sought and readily achievable goal.17-19 In particular, the use of microelectrochemical reactors is becoming more widespread with the development of photolithographic techniques for the deposition of well-defined electrode films and the availability of photoetchable materials.19-25 It is not unreasonable then to propose that such technologies might be extended to address the problem of a variable temperature electrochemical ESR flow cell. The consequences of solution volume minimization in such a cell have been discussed, though it is possible other advantages may ensue. For example, the use of small electrodes and cell dimensions may offer improved ease of potential control as compared to large scale devices,26 and the possibility of microwave power leakage out of the ESR cavity via electrode connections could be minimized by their small size and precise location within the cavity. Furthermore, modern fabrication procedures should allow the positioning of electrodes and channels to be a highly accurate and reproducible process, allowing for the testing of multiple prototypes. In this work we present the design, fabrication, and characterization of a novel microfluidic channel flow cell suitable for variable temperature (233-293 K) electrochemical ESR. The cell consists of parallel inlet and outlet channels running back-to-back, connected at one end to form a U-tube configuration, as depicted in Figure 1. Working and quasi-reference electrodes are positioned in the inlet channel, and two large counter electrodes are located in the outlet channel. The cell is employed for the study of a range of stable and reactive halonitrobenzene systems in acetonitrile at room temperature and 233 K, and by employing low temperatures we report the spectrum of the reactive m-nitrobenzyl chloride radical anion, electrogenerated in acetonitrile, for the first time. The use of electrolysis at low temperatures is also shown to yield improvements in spectral resolution for the 1-chloroanthraquinone radical anion and the crystal violet radical. Furthermore, the ESR signal-flow rate behavior of the cell is validated for the stable p-bromonitrobenzene radical anion by means of three-dimensional simulation for this experimental configuration. THEORY Numerical simulations were carried out using a threedimensional model of the mass transport and electrolysis reactions occurring within the electrochemical variable temperature ESR cell. A transport-limited one-electron reduction of A was simulated

A + e(m)- f B

at the working electrode where sufficient electrolyte has been assumed to be present so that migratory transport may be neglected. The three-dimensional mass transport equation of interest is then given by

D

dC dC d2C d2C d2C dC +v +w )0 +D 2 +D 2 +u 2 dx dy dz dx dy dz

(1)

Figure 1. (a) Cell schematic and (b) cross sections through the electrochemical ESR flow cell: (i) inlet tube, (ii) outlet tube, (iii) counter electrode connection points, (iv) working/reference electrode connection points, (v) Pt foil secondary counter electrode, (vi) Au film primary counter electrode, (vii) inlet channel (1 mm wide, 250 µm deep), (viii) outlet channel, (ix) Au film working and pseudoreference electrodes, (x) glass base plate, (xi) polymer resin sealant, (xii) hole connecting inlet and outlet channels, and (xiii) standard 5 mm diameter NMR tube.

where C is the concentration of a species, D is the diffusion coefficient, t is the time, and u, v, and w are the velocities of the solution in the x, y, and z direction, respectively. The solution procedure has been described previously,27,28 and this approach permits the concentration distribution to be calculated throughout the entire electrochemical cell. Calculations of the ESR signal intensity follow an analogous approach reported previously for two-dimensional problems,5 where the strength is calculated by convoluting the sin2 sensitivity of the TE011 cylindrical cavity with the number of spins and integrating the spins throughout the cavity such that

S ) S0

∫

xd

-xu

[

sin2

]∫

(x - xc)π ( l

2h

0

c(x, y) dy) dx

(2)

where S is the signal strength, S0 is the signal due to 1 mol of the radical species in the cavity, xd and xu are the respective x coordinates upstream and downstream of the cavity, l is the length of the cavity, xc is the x coordinate at the center of the cavity, and c is the concentration of the radical species in the x and y coordinates calculated through integrating with respect to the width 2h of the cavity. This approach permits the total ESR signal intensity at a fixed volume flow rate to be calculated, and the electrolysis current may be obtained using eq 3

i ) -nFDAJ

(3)

where i is the current, n is the number of electrons transferred

per reaction, F is the Faraday constant, D is the diffusion coefficient, A is the electrode area, and J is the concentration flux at the surface of the electrode. EXPERIMENTAL SECTION Cell Fabrication. The construction of the variable temperature cell has been achieved through the application of a two-stage photolithography approach. In the procedure a 1 mm thick glass substrate is used. A positive resist (Microposit S1828 resist) is spun out onto this substrate and then patterned using a suitable mask containing the electrode pattern through direct UV exposure. The exposed regions are then developed out using a developer solution (Microposit 351)(Shipley). The glass substrate beneath the newly exposed regions is then coated with layers of titanium and gold to an approximate thickness of 200 nm by thermal evaporation (BOC Edwards Auto 306). The electrode thickness proved sufficient to not require resistance compensation in the electrochemical measurements. Subsequent development of the unexposed resist layers in acetone yields a set of gold working and reference electrodes having a characteristic dimension of 3 mm length by 3 mm width. A counter electrode was fabricated downstream of the working and reference electrodes and positioned upstream the edge of the cavity on the alternate side of the glass substrate. The position of the working electrode was designed so that it is sited upstream of the edge of the cavity (27) Matthews, S. M.; Du, G. Q.; Fisher, A. C. J. Solid State Electrochem. 2006, 10, 817. (28) Wain, A. J.; Compton, R. G.; Le Roux, R.; Matthews, S.; Yunus, K.; Fisher, A. C. J. Phys. Chem., in press.

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Table 1. Hyperfine Coupling Constants Calculated from ESR Spectra of Halonitrobenzene- and Nitrobenzyl-Derived Radical Anions Electrogenerated in Acetonitrile Solution HFCC/mT ((0.005 mT) position

p-BNB

o-BNB

p-INB

m-INB

NB

m-NBCl

m-NT

N Ho Hm Hp Hmethyl Cl

0.996 0.341 0.114

0.972 0.326 0.116 0.402

0.993 0.326 0.114

0.945 0.34 0.104 0.418

1.114 0.336 0.109 0.386

1.033 0.339 0.133 0.398 0.057 0.057

1.087 0.335 0.109 0.38 0.109

approximately 17 mm from the maximum sensitive region of the cavity. The motivation behind this approach stems first from the need to be able to generate a great deal of radical and hence a greater signal intensity by using a large working electrode. The detection sensitivity of the amount of radical produced under flowthrough conditions would also be diminished due to reduced residence time from convectional dilution. This becomes a critical factor that needs to be considered in the design especially when it is desirable to work with radical intermediate species that decay via fast reaction kinetics. In the second stage of the fabrication procedure, two L-shaped microchannels were fabricated onto alternate sides of the glass substrate containing the newly embedded gold electrodes. This was achieved through the use of a negative resist (MicroChem SU-8 2100) that is spun onto both sides of the glass/electrode substrate. Subsequent exposure to UV under the mask containing the desired pattern transfers this pattern to the resist. After developing the unexposed regions in a suitable developer solution, (Microposit EC developer) two L-shaped channels having the characteristic dimensions of 250 µm height by 800 µm width by 180 mm length for the longest section of the L-shape were produced. In order to connect the two L-shaped channels, a 1 mm hole was fabricated at the base of the thinnest section of the glass substrate which measured approximately 3.5 mm end to end perpendicularly to the direction of solution flow. The two open faces of both microchannels were then sealed using a heat sealable thermosetting polymer, and the whole ensemble was then finally potted into a NMR tube using an epoxy resin (Struers Epofix resin) in order to impart greater mechanical strength and to support the thinnest part of the electrochemical cell during the experiment. Inlet and outlet tubes were inserted through the holes drilled in the base plate using a (Proxxon TBM 220 bench drill) base plate and glued into place using Araldite. Electrical contact between the connecting wires and the contact pads illustrated was achieved using a commercially available solder (RS Sn60 Pb40). The above procedure was optimized for solvents used in the experimental studies (e.g., acetonitrile and 1,2-dimethoxyethane); it should be noted that modifications to the surface treatments and fabrication materials may be necessary if alternative solvents such as dichloromethane are used. Chemical Reagents. Electrochemical experiments were carried out in acetonitrile purchased from Fisher with 0.1 M tetran-butylammonium perchlorate (TBAP, Fluka, puriss electrochemical grade) as supporting electrolyte. All reagents were purchased from Aldrich, were of the highest commercially available grade, and were used without further purification. All electrolyte solutions 1868 Analytical Chemistry, Vol. 79, No. 5, March 1, 2007

were purged with oxygen-free nitrogen (BOC Gases, Guildford, Surrey, U.K.) for at least 30 min prior to experimentation. Apparatus. Potential control was achieved using a computercontrolled PGSTAT30 potentiostat (Autolab, Eco Chemie, Utrecht, The Netherlands). Fluid motion was maintained using a gravity flow system, as used previously, employing glass capillaries to achieve slower flow rates. The volume flow rates employed were in the range of 0.25 × 10-3 to 40 × 10-3 cm3 s-1. These flow rates correspond to residence times in the cavity of 0.6-102 s. ESR spectra were obtained using a JEOL JES-FA100 X-band spectrometer with a cylindrical (TE011) cavity resonator. In all ESR experiments, a microwave power of 1 mW was used and regular tests were carried out to confirm that increasing the microwave power increased the ESR signal. Doing so ensured that the system was not power saturated such that the ESR signal intensity gave a direct measure of the number of electrogenerated spins in the cavity. A magnetic field modulation width of 0.02 mT was typically employed. Line widths employed in the spectral simulations were typically close to 0.05 mT, which is as expected for solution-phase radicals which show little inhomogeneous broadening. (Higher line widths were observed in the case of p-iodonitrobenzene and chloroanthraquinone, presumably as a consequence of quadrupolar relaxation). Temperature control within the ESR cavity was achieved using a JEOL ES-DVT4 temperature controller and cavity adaptor. This consists of a double-walled silica tube which fits inside the cavity and is fed with nitrogen gas evaporated from a liquid nitrogen source. The nitrogen gas is heated to the required temperature by a computer-controlled heater, and the temperature is monitored by a thermocouple at the base of the cavity. Employing this apparatus, the cavity temperature was controlled to within (0.5 K and experiments were conducted in the range of 233-293 K, the lower limit being restricted by the freezing point of the solvent/electrolyte. It is difficult to be sure if the temperature of the solution is the same as that of the cavity, particularly at high flow rates, though the small solution volumes employed ensure that this equilibration time is short. RESULTS AND DISCUSSION Cell Characterization: The Reduction of p-Bromonitrobenzene. Preliminary characterization of the channel flow cell was carried out using the one-electron reduction of p-bromonitrobenzene (p-BNB) in acetonitrile (MeCN) as a model system, since previous studies have shown the p-bromonitrobenzene radical anion to behave as stable on the experimental time scales employed in this work and its ESR spectrum has an easily interpretable hyperfine structure.14,29 Linear sweep voltammetry

Figure 2. (a) Linear sweep voltammetry of 2 mM p-bromonitrobenzene in acetonitrile using the channel cell at flow rates of 0.7 × 10-3, 1.5 × 10-3, and 3.2 × 10-3 cm3 s-1. (b) Limiting currents plotted as a function of volume flow rate for 1 mM p-bromonitrobenzene.

of 2 mM p-bromonitrobenzene in MeCN supported with 0.1M TBAP is shown in Figure 2a for flow rates of 0.7 × 10-3, 1.5 × 10-3, and 3.2 × 10-3 cm3 s-1. Plots of limiting current against the cube root of volume flow rate were found to be linear in the flow rate range of 0.25 × 10-3 to 40 × 10-3 cm3 s-1, in accordance with the Levich equation,5,6 as shown in Figure 2b. The flow cell was inserted into the ESR cavity with the variable temperature apparatus in place, and it was found that the cell could be easily centralized and its position (insertion depth) adjusted by use of a Teflon collar tightened around the NMR tube. Cavity tuning was achieved easily with the cell filled with acetonitrile/ electrolyte solution, and preliminary scans showed that the cell itself displayed a negligible ESR signal at the magnetic field modulation widths typically employed. In situ electrolysis of p-bromonitrobenzene at its first reduction potential, at a flow rate of 0.6 × 10-3 cm3 s-1, yielded the ESR spectrum shown in Figure 3a, where a modulation width of 0.02 mT was used. The hyperfine structure is as expected for the radical anion of p-bromonitrobenzene, and the coupling constants extracted, shown in Table 1, are in good agreement with the literature values.14,29 The high signalto-noise ratio of the observed spectrum gives an indication of the high sensitivity resulting from minimized dielectric loss. It was found that turning the cell through an angle of 90° had little effect on the sensitivity, though retuning of the cavity was necessary. Next the signal intensity was measured for a range of flow rates, and the data are plotted in Figure 3b. Digital Simulation. Digital simulations were carried out to predict the variation of the spectroscopic signal intensity as a

function of the volume flow rate for the experimental conditions outlined above. Figure 3b shows the variation of ratio of ESR signal intensity and current as a function of volume flow rate over 10 orders of magnitude of the solution flow rate. At low solution velocities the gradient tends toward -1; in this case almost complete conversion of the p-bromonitrobenzene occurs over the electrode, the current then becomes proportional to the volumetric flow rate, and the ESR signal intensity within the cell becomes constant. At the high flow rate range the gradient tends toward -2/3 in an analogous manner to that observed for straight channel and tubular electrode configurations. Although the variable temperature arrangement is considerably different in geometry to both these cases, it should be noted that the “U” section at the bottom of the cell is in a region of extremely low ESR sensitivity; hence, it essentially contributes nothing to the overall signal. Consequently, the straight inlet and outlet regions limit the signal variation in an analogous manner to that observed for straight single-channel geometries.5 Also shown on Figure 3b is the experimentally obtained data for the p-bromonitrobenzene case. It can be seen to lie close to the transition region of the S/ilim plots and shows good agreement with the predicted response of the system. Reduction of Halonitrobenzenes at 233 K. The cell was next used at low temperatures in order to investigate some systems which have been reported as unstable at room temperature. The electrochemical reduction of o-bromonitrobenzene (o-BNB) in acetonitrile occurs as a single voltammetric wave and is thought to proceed via the following ECE mechanism at room temperature:30,31

Br-C6H4-NO2 + e- f [Br-C6H4-NO2]•[Br-C6H4-NO2]•- f Br- + •C6H4-NO2 •

C6H4-NO2 + HS f C6H5-NO2 + S• C6H5-NO2 + e- f [C6H5-NO2]•-

The electrogenerated o-bromonitrobenzene radical anion undergoes the loss of bromide to yield a reactive nitrobenzyl radical which is believed to abstract a hydrogen atom from the solvent/ electrolyte (HS) to yield the electroactive nitrobenzene. The ultimate product is the nitrobenzene radical anion, and this has been observed using electrochemical ESR.30 The rate constant for the room-temperature debromination of the parent radical anion in acetonitrile has reported as 20 s-1,30 and it has been shown previously that this radical anion intermediate can be detected by lowering the temperature to 233 K,14 and so the new cell was tested with this system. Electrolysis of a 3 mM solution of o-bromonitrobenzene at its reduction potential at room temperature yielded no ESR spectra under flowing conditions. However, stopping the flow and allowing the buildup of radical ions led to the observation of the nitroben(29) Compton, R. G.; Dryfe, R. A. W. J. Electroanal. Chem. 1994, 375, 247. (30) Nelson, R. F.; Carpenter, A. K.; Seo, E. T. J. Electrochem. Soc. 1973, 120, 206. (31) Compton, R. G.; Wellington, R. G.; Dobson, P. J.; Leigh, P. A. J. Electroanal. Chem. 1994, 370, 129.

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Figure 4. ESR spectrum of electrogenerated o-bromonitrobenzene radical anion at 233 K.

Figure 3. (a) ESR spectrum of electrogenerated p-bromonitrobenzene radical anion at room temperature. (b) Plot of ESR signal intensity against volume flow rate for electrogenerated p-bromonitrobenzene radical anion, experimental (O) and numerical simulation (9).

zene (NB) radical anion spectrum (hyperfine coupling constants are reported in Table 1), as predicted by the ECE scheme. Upon lowering the temperature to 233 K, the spectrum shown in Figure 4 was observed under flowing conditions. The hyperfine structure is consistent with the o-bromonitrobenzene radical anion, and the extracted coupling constants, reported in Table 1, are close to those determined previously.14 Thus, the cell functioned well at low temperatures, displaying the expected behavior for this system and allowing the acquisition of high-quality ESR spectra for a radical ion that is unstable at room temperature. We now turn our attention to some spectra previously unreported in this media. The cathodic reduction of iodonitrobenzenes has received some attention in the literature, and the mechanism observed appears to follow the same behavior as that of the bromo derivative, though not unexpectedly with enhanced kinetics.30,32-38 (32) Compton, R. G.; Dryfe, R. A. W.; Eklund, J. C.; Nei, L.; Page, S. D. Electroanalysis 1996, 8, 214. (33) Compton, R. G.; Dryfe, R. A. W.; Eklund, J. C.; Page, S. D.; Hirst, J.; Nei, L.; Fleet, G. W. J.; Hsia, K. Y.; Bethell, D.; Martingale, L. J. J. Chem. Soc., Perkin Trans. 2 1995, 1673. (34) Coles, B. A.; Moorcroft, M. J.; Compton, R. G. J. Electroanal. Chem. 2001, 513, 87. (35) Kitagawa, T. Chem. Lett. 1973, 489. (36) Tsai, Y.-C.; Coles, B. A.; Compton, R. G.; Marken, F. J. Am. Chem. Soc. 2002, 124, 9784. (37) Kitagawa, T.; Nakashima, R. Rev. Polarogr. 1966, 13, 115. (38) Lawless, J. G.; Hawley, M. D. J. Electroanal. Chem. 1969, 21, 365.

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It would seem that the fast reactivity of the iodonitrobenzenederived radical anions in conventional aprotic solvents at room temperature has precluded the acquisition of interpretable ESR spectra using electrochemical means, and so it was envisaged that these systems might be investigated using the new cell at low temperatures. Nelson et al. reported a room-temperature first-order rate constant of 0.057 s-1 for the decomposition of the m-iodonitrobenzene (m-INB) radical anion, electrogenerated in acetonitrile at room temperature, and used a platinum gauze in situ ESR cell to measure the resulting spectra.30 The spectrum presented by these authors is that of the nitrobenzene radical anion, though it is mentioned that the primary m-iodonitrobenzene radical anion intermediate is observed on shorter time scales. Interestingly, Compton et al. observed only the nitrobenzene radical anion when studying this compound in acetonitrile using an in situ channel flow cell and determined a room-temperature rate constant of 0.3 s-1.33 When a solution of 3 mM m-iodonitrobenzene was electrolyzed in the new flow cell, it was found that the parent radical anion spectrum was observed at room temperature, though lowering the temperature to 233 K enhanced resolution and greatly improved signal-to-noise ratio (S/N at 233 K was 125, as compared to 34 at 293 K). The low-temperature spectrum is shown in Figure 5a. Hyperfine coupling constants determined from this spectrum are reported in Table 1, and a simulated spectrum based on these values is depicted in Figure 5b, which shows excellent agreement with the experimental spectrum. In contrast with previous reports,32 at no point was the nitrobenzene radical anion detected, which may be a consequence of the fast axial flow velocities resulting from the small channel dimensions (for the volume flow rates employed, the estimated axial flow velocities at the center of channel are in the range of 0.06-20 cm s-1, approximately an order of magnitude greater than previous ESR studies). The p-iodonitrobenzene (p-INB) radical anion is known to react faster than the meta derivative in acetonitrile, with a reported room-temperature rate constant of 0.64 s-1,30 and as such there appears to be no literature reports of its ESR spectrum in aprotic media. There are, however, reports of the radical anion being detected by ESR spectroscopy in mildly protic environments such as methylisobutylketone and DMF-water mixtures.35,37 Electrolysis of a 2 mM solution of p-iodonitrobenzene in acetonitrile, employing the new cell at room temperature, yielded no ESR

Figure 5. (a) ESR spectrum (modulation width 0.005 mT) of electrogenerated m-iodobenzene radical anion at 233 K. (b) Simulated spectrum for the m-iodobenzene radical anion. (c) ESR spectrum (modulation width 0.1 mT) of electrogenerated p-iodobenzene radical anion at 233 K. (d) Simulated spectrum for the p-iodobenzene radical anion.

Figure 6. Structure of (a) m-nitrobenzyl chloride, (b) 1-chloroanthraquinone, and (c) the crystal violet cation.

spectrum, as was the case with the o-bromo derivative, but upon lowering the temperature to 233 K the spectrum in Figure 5c was observed. The spectrum is significantly broadened, possibly as a result of the iodine nuclear quadrupole moment39 combined with the relatively high electron density in the para position. Attempts to improve resolution by lowering the iodonitrobenzene concen(39) Alonso, R. E.; Svane, A.; Rodrı´guez, C. O.; Christensen, N. E. Phys. Rev. B 2004, 69, 1.

tration and magnetic field modulation width were unsuccessful, and so it was not possible to accurately determine the proton hyperfine coupling constants. However, spectral simulation based on the proton splittings in the equivalent bromo derivative spectrum was found to be in good agreement with the experimental spectrum when a relatively high line width of 0.26 mT was employed (Figure 5d), suggesting that the primary radical anion is responsible for the observed spectrum. Kitagawa reported similar line broadening problems in methylisobutylketone, and although the coupling constants reported (1.414, 0.314, and 0.082 mT)35 appear notably different from those determined in this work (see Table 1), a direct comparison is difficult due to the likely operation of solvent effects.37 Attempts to observe the even more reactive o-iodonitrobenzene radical anion using the new cell at low temperatures proved unsuccessful, the room-temperature rate constant for its decay in acetonitrile being reported as greater than 100 s-1.30 Reduction of m-Nitrobenzyl Chloride. The cathodic reduction of m-nitrobenzyl chloride (m-NBCl) in acetonitrile has been investigated by numerous authors and is believed to proceed with an ECE-DISP scheme, similar to that for the halonitrobenzenes above.40-42 A single-electron reduction is followed by loss of chloride, to yield a radical which can abstract a hydrogen atom from the solvent system and undergo further reduction, to yield the m-nitrotoluene (m-NT) radical anion. Using fast scan voltammetry in acetonitrile, Wipf and Wightman41 measured a rate constant for the dehalogenation of the parent m-nitrobenzyl (40) Lawless, J. G.; Bartak, D. E.; Hawley, M. D. J. Am. Chem. Soc. 1969, 91, 7121. (41) Wipf, D. O.; Wightman, R. M. J. Phys. Chem. 1989, 93, 4286. (42) Peterson, P.; Carpenter, A. K.; Nelson, R. F. J. Electroanal. Chem. 1970, 27, 1.

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Figure 8. ESR spectra obtained for the electrogenerated radical anion of 1-chloroanthraquinone at (a) 293 and (b) 233 K.

Figure 7. (a) ESR spectrum obtained upon electroreduction of m-nitrobenzyl chloride in acetonitrile at 233 K, (b) simulated spectrum for m-nitrobenzyl chloride radical anion based on hyperfine coupling constants reported in Table 1, and (c) ESR spectrum of electrogenerated m-nitrotoluene radical anion at 233 K.

chloride radical anion as 10 s-1 at room temperature. Peterson et al.42 studied the electrochemical reduction of this compound using conventional in situ ESR electrolysis at room temperature but did not observe the parent radical anion; only spectra attributed to radical products of follow-up chemical and electrochemical reactions, such as the m-nitrotoluene and the m-nitrobenzaldehyde radical anion, were obtained. Electrolysis of a 2 mM acetonitrile solution of m-nitrobenzyl chloride at its reduction potential yielded no ESR spectra at room temperature. However, upon reducing the temperature to 233 K, the spectrum depicted in Figure 7a was observed. The spectrum was attributed to the parent m-nitrobenzyl chloride radical anion, and hyperfine coupling constants were extracted on this basis (see Table 1). With the use of these parameters the spectrum shown in Figure 7b was simulated, and this shows excellent agreement with the experimental spectrum. Despite the consistency between simulated and experimental spectra, the possibility that the parent radical anion had undergone a full ECE reduction to yield the m-nitrotoluene radical anion was nevertheless considered. An authentic sample of m-nitrotoluene was dissolved in acetonitrile 1872 Analytical Chemistry, Vol. 79, No. 5, March 1, 2007

and reduced using the flow cell in order to directly electrogenerate the m-nitrotoluene radical anion. The spectrum observed at 233 K is depicted in Figure 7c, and hyperfine coupling constants deduced are given in Table 1. It is clear that this spectrum, attributed to the m-nitrotoluene radical anion, bears little resemblance to that shown in 7a, confirming that the radical species observed in this work from the reduction of the benzyl chloride is not the two-electron ECE product, as reported previously from room-temperature studies.42 Reduction of 1-Chloroanthraquinone and Crystal Violet. Thus far we have focused on reactive electrochemical systems where the chemical kinetics of radical ion decay have been retarded by the lowering of temperature, allowing us to electrochemically obtain spectra which were hitherto inaccessible by this means. An additional advantage which may be observed when operating under low-temperature conditions is the reduction in ESR line width and the resulting improvements in spectral resolution.10 To highlight this fact, in this final section we turn to the electrochemical reduction of 1-chloroanthraquinone and the crystal violet cation in acetonitrile (see Figure 6). The one-electron reduction products of these compounds (namely, the 1-chloroanthraquinone radical anion and the crystal violet radical) have been studied by room-temperature electrochemical ESR methods in acetonitrile and are known to be stable, but the resulting ESR spectra were reported to be of an especially poor resolution.43,44 (43) Compton, R. G.; Coles, B. A.; Pikington, M. B. G.; Bethell, D. J. Chem. Soc., Faraday Trans. 1990, 86, 663. (44) Compton, R. G.; Coles, B. A.; Stearn, G. M.; Waller, A. M. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2357.

the hyperfine structure was not of a high enough definition to allow unambiguous determination of the associated coupling constants, especially as a result of the numerous proton inequivalencies.45,46 The resolution was great enough, however, to confirm that the spectrum is not consistent with that of the dehalogenated anthraquinone radical anion, a product which is known to be generated by an ECE process for the corresponding iodo derivative.43 Finally, a 2 mM solution of crystal violet (the chloride salt of the cation depicted in Figure 6) in acetonitrile was electrolyzed at its reduction potential to yield the corresponding neutral radical. The ESR spectrum observed at room temperature is shown in Figure 9a and is similar to that measured previously using a channel flow cell.44 The spectrum recorded at 233 K can be seen in Figure 9b, and although the broad shape remains, hyperfine structure is now visible as a result of reduced line width. The spectrum of this radical has been measured in 1,2-dimethoxyethane, by chemical reduction of crystal violet under highly controlled conditions, by Stanoeva et al.,47 who used ESR and ENDOR spectroscopy to extract the following coupling constants: N ) 0.113, Ho ) 0.09, Hm ) 0.239, and Hmethyl ) 0.113 mT. With the use of these values, the spectrum in Figure 9c was simulated, which, at a line width of 0.027 mT, shows good qualitative agreement with the experimental spectrum shown in Figure 9b. Although there was little question as to the identity of the radical responsible for the previously reported spectrum, this low-temperature spectrum serves to further support the singleelectron reduction mechanism proposed.44 In both of the above cases, attempts to further improve resolution by lowering the magnetic field modulation width, or by decreasing the concentration of the starting material, were met with little success. Figure 9. ESR spectra obtained for the electrogenerated crystal violet radical at (a) 293 and (b) 233 K. (c) Simulated spectrum for the crystal violet radical on the basis of coupling constants reported in ref 45.

In the case of chloroanthraquinone, the low resolution may be attributed to quadrupolar relaxation due to the presence of the chlorine nucleus (I ) 3/2). For the crystal violet, the large number of spectral lines over a small field width results in inhomogeneous broadening of the ESR spectrum. The new cryoelectrochemical ESR cell was therefore employed in an attempt to improve this resolution. An acetonitrile solution of 2 mM 1-chloroanthraquinone was electrolyzed at its reduction potential and at room temperature yielded the spectrum depicted in Figure 8a. The spectrum resembles that reported by Compton et al.,43 who studied this process using a channel electrode at room temperature and attributed the spectrum to the parent radical anion. Upon cooling the cell to 233 K, the resolution of the spectrum was notably improved, as shown in Figure 8b. Despite these improvements, (45) Eloranta, J.; Vatanen, V.; Gro¨nroos, A.; Vuolle, M.; Ma¨kela¨, R.; Heikkila¨, H. Magn. Reson. Chem. 1996, 34, 903. (46) Ma¨kela¨, R.; Vuolle, M. J. Chem. Soc., Faraday Trans. 1 1898, 85, 4011. (47) Stanoeva, T.; Neshchadin, D.; Gescheidt, G.; Ludvik, J.; Lajoie, B.; Batchelor, S. N. J. Phys. Chem. A 2005, 109, 11103.

CONCLUSIONS The design, fabrication, and application of a novel microfluidic electrochemical ESR channel flow cell for in situ operation within a variable temperature cylindrical TE011 resonant ESR cavity has been presented. With the use of the single-electron reduction of p-bromonitrobenzene in acetonitrile at room temperature as a model system the cell was shown to give a quantitative electrochemical response under hydrodynamic conditions, and the variation of ESR signal intensity with flow rate for the stable electrogenerated radical anion was validated using three-dimensional numerical simulation. Application of the cell at low temperatures has allowed access to a number of ESR spectra which have been previously inaccessible using this technique either due to the reactivity of the paramagnetic generated or as a result of line-broadening effects. ACKNOWLEDGMENT A.J.W. thanks the EPSRC and JEOL for funding; R.L.R. and S.M. thank the EPSRC for funding.

Received for review December 21, 2006.

October

10,

2006.

Accepted

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