Label-free electrochemical monitoring of concentration enrichment during bipolar electrode focusing.

We show that a label-free electrochemical method can be used to monitor the position of an enriched analyte band during bipolar electrode focusing in a microfluidic device. The method relies on formation of a depleted buffer cation region, which is responsible for concentration enrichment of the charged analyte. However, this depletion region also leads to an increase in the local electric field in the solution near a bipolar electrode (BPE), and this in turn results in enhanced faradaic reactions (oxidation and reduction of water) at the BPE. Therefore, it is possible to detect the presence of the concentrated analyte band by measuring the current passing through the BPE used for concentration enrichment, or the concentrated band can be detected at a secondary BPE dedicated to that purpose. Both experiments and simulations are presented that fully elucidate the underlying phenomenon responsible for these observations.

Bipolar electrodes: a useful tool for concentration, separation, and detection of analytes in microelectrochemical systems.

Over the past decade, bipolar electrochemistry has emerged from relative obscurity to provide a promising new means for integrating electrochemistry into lab-on-a-chip systems. This article describes the fundamental operating principles of bipolar electrodes, as well as several interesting applications.

Organometallic electrodes: modification of electrode surfaces through cathodic reduction of cyclopentadienyldiazonium complexes of cobalt and manganese.

Two organometallic complexes having cyclopentadienyldiazonium ligands have been isolated and characterized by spectroscopy, X-ray crystallography, and electrochemistry. Both CoCp(η(5)-C(5)H(4)N(2))(2+) (2(2+)) and Mn(CO)(3)(η(5)-C(5)H(4)N(2))(+) (3(+)) undergo facile cyclopentadienyldiazonium ligand-based one-electron reductions which liberate dinitrogen and result in strong binding of the cyclopentadienyl ligand to a glassy carbon surface, similar to the processes well established for organic aryldiazonium salts. The organometallic-modified electrodes are robust and have a thickness of approximately one monolayer (Γ = (2-4) × 10(-10) mol cm(-2)). Their voltammetric responses are as expected for a cobaltocenium-modified electrode, [CoCp(η(5)-C(5)H(4)-E)](+), where Cp = cyclopentadienyl and E = electrode, and a "cymantrene"-modified electrode Mn(CO)(3)(η(5)-C(5)H(4)-E). The cobaltocenium electrode has two cathodic surface waves. The first (E(1/2) = -1.34 V vs ferrocene) is highly reversible, whereas the second (E(pc) = -2.4 V) is not, consistent with the known behavior of cobaltocenium. The cymantrene-substituted electrode has a partially chemically reversible anodic wave at E(1/2) = 0.96 V, also consistent with the behavior of its Mn(CO)(3)Cp parent. Many of the properties of aryl-modified electrodes, such as "blockage" of the voltammetric responses of test analytes, are also seen for the organometallic-modified electrodes. Surface-based substitution of a carbonyl group by a phosphite ligand, P(OR)(3), R = Ph or Me, was observed when the cymantrene-modified electrode was anodically oxidized in the presence of a phosphite ligand. The successful grafting of organometallic moieties by direct bonding of a cyclopentadienyl ligand to electrode surfaces expands the chemical and electrochemical dimensions of diazonium-based modified electrodes.

Bipolar electrode focusing: the effect of current and electric field on concentration enrichment.

Bipolar electrode focusing at discontinuous bipolar electrodes (BPEs) provides new insight into the faradaic current and electric field characteristics associated with the technique and allows for the controlled transport of a focused anionic tracer in a microfluidic channel. The findings corroborate our previously reported simulation results, which describe the formation of an extended electric field gradient leading to concentration enrichment. This gradient has been attributed to the passage of faradaic current through a BPE affixed to the floor of the microchannel. Our results demonstrate that the onset of faradaic current is coincident with the onset of concentration enrichment. Utilizing an array of microband electrodes, the tracer may be passed from one stationary position to another by rapidly relocating the BPE. However, the tracer movement is limited to one direction, confirming that the electrophoretic velocity of the analyte exceeds the electroosmosis-driven bulk fluid flow velocity at only the cathodic edge of the BPE.

Bipolar electrode focusing: simultaneous concentration enrichment and separation in a microfluidic channel containing a bipolar electrode.

A method for simultaneously concentrating and separating analytes in a buffer-filled microfluidic channel is reported. The approach is based on modulation of the local electric field within the channel and the corresponding opposition of electrophoretic and electroosmotic flow (EOF) velocities. Dye molecules having different electrophoretic mobilities are focused at different locations within the channel where concentration takes place. At least three species, all small dye molecules, can be simultaneously concentrated and separated, with localized enrichment factors up to approximately 600 achieved within 400 s. The enrichment zones affect the electric field profile, as evidenced by significant differences in focusing of single versus multiple analytes. The EOF could be modulated by modifying the channel walls with an appropriate polymer, and this had the effect of increasing both the enrichment factors and resolution of the separation. Numerical simulations provide insights into the underlying fundamental principles for the experimental findings.

Cymantrene radical cation family: spectral and structural characterization of the half-sandwich analogues of ferrocenium ion.

The anodic one-electron oxidation of three members of the half-sandwich family of piano-stool compounds MnCp (gamma)(CO) 3, where Cp (gamma) is a generic cyclopentadienyl ligand, has been studied in a CH 2Cl 2/[NBu 4][TFAB] electrolyte (TFAB = [B(C6F5) 4] (-)). The long-sought 17 e (-) radical cation of the parent complex MnCp(CO) 3 (cymantrene, 1, E 1/2 = 0.92 V vs ferrocene) has been shown to be persistent in solutions that use weakly coordinating anions in place of more nucleophilic traditional electrolyte anions. Spectroscopically characterized for the first time, 1 (+) was shown to absorb in the visible (530 nm), near-IR (2066 nm), and IR (2118, 1934 cm (-1)) regions. It was ESR-active at low temperatures (g parallel = 2.213, g perpendicular = 2.079, A parallel (Mn) = 79.2 G, A perpendicular (Mn) = 50 G) and NMR active at room temperature (delta = 22.4 vs TMS). The radical cations of the Cp-functionalized analogues, Mn(eta (5)-C5H 4NH2)(CO) 3, 2, E 1/2 = 0.62 V, and MnCp*(CO) 3 (Cp*= eta (5)-C 5Me 5, 3), E 1/2 = 0.64 V, were generated electrochemically as well by the chemical oxidant [ReCp(CO) 3] (+). The structures of 2 (+) and 3 (+) were determined by X-ray crystallographic studies of their TFAB salts. Compared to the structures of the corresponding neutral compounds, the cations showed elongated Mn-C(O) bonds and shortened C-O bonds, displaying the effect of diminished metal-to-CO backbonding. The bond-length changes in the Mn(CO) 3 moiety were much larger in 3 (+) (avg changes, Mn-C(O) = + 0.142 A, C-O = -0.063 A) than in 2 (+) (avg changes, Mn-C(O) = + 0.006 A, C-O = -0.003 A). Although there were only minor changes in the metal-to-center ring distances upon oxidation of either 2 or 3, there was decidedly less bending of the C(N) atom out of the cyclopentadienyl plane in 2 (+) compared to 2. The optical, vibrational, and magnetic resonance spectra of radicals 2 (+) and 3 (+) were also observed. The spectral data argue for the SOMOs of the 17-electron species being largely located on the Mn(CO) 3 moiety, having 40-50% Mn d-orbital character, with the ground states of the radicals, most likely (2)A'', lying close in energy (within about 6000 cm (-1)) to excited states that are responsible for their rapid electronic relaxations. The cymantrenyl moiety is proposed as an anodic redox tag (or label) having physical and chemical properties that are significantly different from those of its ferrocenyl analogue.

[Re(eta(5)-C5H5)(CO)3]+ family of 17-electron compounds: monomer/dimer equilibria and other reactions.

The anodic electrochemical oxidations of ReCp(CO)3 (1, Cp = eta(5)-C5H5), Re(eta(5)-C5H4NH2)(CO)3 (2), and ReCp*(CO)3 (3, Cp* = eta(5)-C5Me5), have been studied in CH2Cl2 containing [NBu4][TFAB] (TFAB = [B(C6F5)4]-) as supporting electrolyte. One-electron oxidations were observed with E(1/2) = 1.16, 0.79, and 0.91 V vs ferrocene for 1-3, respectively. In each case, rapid dimerization of the radical cation gave the dimer dication, [Re2Cp(gamma)2(CO)6]2+ (where Cp(gamma) represents a generic cyclopentadienyl ligand), which may be itself reduced cathodically back to the original 18-electron neutral complex ReCp(gamma)(CO)3. DFT calculations show that the SOMO of 1+ is highly Re-based and hybridized to point away from the metal, thereby facilitating the dimerization process and other reactions of the Re(II) center. The dimers, isolated in all three cases, have long metal-metal bonds that are unsupported by bridging ligands, the bond lengths being calculated as 3.229 A for [Re2Cp2(CO)6]2+ (1(2)2+) and measured as 3.1097 A for [Re2(C5H4NH2)2(CO)6]2+ (2(2)2+) by X-ray crystallography on [Re2(C5H4NH2)2(CO)6][TFAB]2. The monomer/dimer equilibrium constants are between K(dim) = 10(5) M(-1) and 10(7) M(-1) for these systems, so that partial dissociation of the dimers gives a modest amount of the corresponding monomer that is free to undergo radical cation reactions. The radical 1+ slowly abstracts a chlorine atom from dichloromethane to give the 18-electron complex [ReCp(CO)3Cl]+ as a side product. The radical cation 1+ acts as a powerful one-electron oxidant capable of effectively driving outer-sphere electron-transfer reactions with reagents having potentials of up to 0.9 V vs ferrocene.