Article pubs.acs.org/ac
In Situ Mass Spectrometric Monitoring of the Dynamic Electrochemical Process at the Electrode−Electrolyte Interface: a SIMS Approach Zhaoying Wang,†,‡,§ Yanyan Zhang,†,‡,§ Bingwen Liu,∥ Kui Wu,‡ Suntharampillai Thevuthasan,§ Donald R. Baer,§ Zihua Zhu,*,§ Xiao-Ying Yu,*,∥ and Fuyi Wang*,‡ ‡
Beijing National Laboratory for Molecular Sciences, National Centre for Mass Spectrometry in Beijing, CAS Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Environmental Molecular Sciences Laboratory and ∥Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ABSTRACT: The in situ molecular characterization of reaction intermediates and products at electrode−electrolyte interfaces is central to mechanistic studies of complex electrochemical processes, yet a great challenge. The coupling of electrochemistry (EC) and mass spectrometry (MS) has seen rapid development and found broad applicability in tackling challenges in analytical and bioanalytical chemistry. However, few truly in situ and real-time EC-MS studies have been reported at electrode−electrolyte interfaces. An innovative EC-MS coupling method named in situ liquid secondary ion mass spectrometry (SIMS) was recently developed by combining SIMS with a vacuum compatible microfluidic electrochemical device. Using this novel capability, we report the first in situ elucidation of the electro-oxidation mechanism of a biologically significant organic compound, ascorbic acid (AA), at the electrode−electrolyte interface. The short-lived radical intermediate was successfully captured, which had not been detected directly before. Moreover, we demonstrated the power of this new technique in real-time monitoring of the formation and dynamic evolution of electrical double layers at the electrode−electrolyte interface. This work suggests further promising applications of in situ liquid SIMS in studying more complex chemical and biological events at the electrode−electrolyte interface.
for EC-MS coupling in monitoring of electrochemical processes is how to interface the electrochemical cell with a mass spectrometer.8 Most recently, an exciting advancement in this field was reported by Zare and Chen et al., which is the invention of a rotating “waterwheel” working electrode setup combined with DESI.15−17 As the working electrode rotates, a thin film of electrolyte solution develops on the surface of the electrode. After it is hit by spray droplets, active transient intermediates generated on the electrode surface in several previously proposed pathways have been successfully identified,15−17 leading to a great impact on the electrochemistry field. However, due to the intrinsic ionization mechanism, this ECDESI coupling strategy has several limitations for truly in situ and real-time studies at the electrode−electrolyte interface. First, the thickness of the film on the electrode surface is about 1 mm, while the thickness of electrical double layer in the aqueous solution is typically in nanometer scale,18 limiting the
In situ molecular characterization of reaction intermediates and products at solid−liquid interfaces is central to mechanistic studies of complex chemical processes, for example, electrochemical redox reactions and heterogeneous catalytic reactions,1 yet a great challenge. For electrochemical redox reactions, traditional electrochemical methods such as cyclic voltammetry (CV),2,3 polarography,4 and chronoamperometry,5 as well as bulk electrolysis,6 have been used to study reaction mechanisms and kinetics; however, they do not provide direct chemical identification. In this regard, mass spectrometry (MS) can serve as a sensitive detector providing molecular information on intermediates and products during electrochemical reactions. The successful marriage of electrochemistry (EC) and MS was first reported by Bruckenstein et al. in 1971 for determination of volatile electrode reaction products.7 Thereafter, EC-MS coupling has developed quickly and found broad applicability in analytical and bioanalytical chemistry.8−10 In the past few years, increasing interest and efforts have been focused on probing electrochemical intermediates and elucidating electrochemical reaction mechanisms, particularly using electrospray ionization (ESI) and desorption electrospray ionization (DESI) MS with the combination of EC.11−14 The critical issue © 2016 American Chemical Society
Received: October 26, 2016 Accepted: December 12, 2016 Published: December 12, 2016 960
DOI: 10.1021/acs.analchem.6b04189 Anal. Chem. 2017, 89, 960−965
Article
Analytical Chemistry ability to collect information at electrode−electrolyte interfaces. Second, the transmission of secondary microdroplets from the surface to the gas phase for MS analysis takes milliseconds,15 leading to the challenge in detecting electrochemical intermediates with shorter lifetime (e.g., microseconds). Additionally, perturbation of intermediates may take place due to increased collision frequencies of charged microdroplets.19,20 Furthermore, to the best of our knowledge, the potential applied on the working electrode in previously reported EC-MS studies merely varied between a chosen value and an off state in a period of intervals instead of a dynamic range.15−17 Since dynamics are essential properties of electrochemistry, innovative EC-MS techniques capable of real-time monitoring the whole electrochemical processes are in great need to provide a more comprehensive understanding of potential dependent changes at the interfaces. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is based on analysis of emitted secondary ions induced by bombardment of primary ions under high vacuum environment. It is very surface sensitive with its information depth less than a few nanometers. In addition, the primary ion beam can be focused down to submicrometer size (e.g., 100 nm) for nanoscale chemical imaging analysis.21 Recently, an innovative EC-MS coupling method named in situ liquid SIMS was developed in our group at Pacific Northwest National Laboratory (PNNL) by combining ToF-SIMS and a vacuum compatible microfluidic electrochemical device named as the System for Analysis at the Liquid−Vacuum Interface (SALVI).22,23 The basic concept uses the primary ions to drill an aperture of 2 μm in diameter through a thin silicon nitride (SiN) membrane window and a gold film working electrode beneath. Then, the liquid adjacent to the electrode is exposed to the primary ion beam and secondary ions from the electrode−electrolyte interface are analyzed by the ToF mass analyzer. In our initial effort, a simple classic inorganic electrochemical model system, the electro-oxidation of potassium iodide at a gold film electrode surface, has been studied.22,23 This in situ liquid SIMS approach not only provides the potential to investigate the mechanism of electrochemical reactions similar to EC-ESI and EC-DESI approaches, but may also have the capability to overcome some of the limitations mentioned above. Here, we used the in situ liquid SIMS to investigate the electrochemical oxidization mechanism of ascorbic acid (AA) at the electrode−electrolyte interface. AA is an important antioxidant vitamin and a common electroactive biological compound. A proposed electrochemical oxidation pathway24 of AA in acid or neutral solutions is shown in Scheme 1. This process was first verified by polarographic analysis in acidic medium using a mercury capillary as the working electrode,25 and later by cyclic voltammetry (CV).26 However, these studies provided little direct molecular evidence of the reaction intermediates during the oxidation process. In the present study, we have successfully identified the short-lived intermediate that was previously proposed as a radical ion yet without any direct unambiguous molecular evidence. Moreover, the real-time ToF-SIMS analysis shows dynamic variations of key species on the top surface of the diffusion layer that correspond to the potential changing during CV scanning, chemically monitoring the formation, and evolution of electrical double layers at the electrode−electrolyte interface.
Scheme 1. Proposed Electrochemical Oxidation Pathway of Ascorbic Acid24,a
a
The radical ion (2) is the proposed reaction intermediate.
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EXPERIMENTAL SECTION Fabrication of the Electrochemical SALVI Cell. The vacuum compatible microfluidic electrochemical reaction cell was fabricated by using soft lithography, as described previously22 with appropriate modification (Figure 1a). In brief, the counter electrode (CE) and reference electrode (RE), made of Pt and Ag/AgCl, respectively, were connected with
Figure 1. (a) Schematic diagram of the microfluidic electrochemical reaction cell in a side view. (b) Depth profiles of representative ions in 10 mM HCl solution containing 0.5 mM AA during drilling a ∼2 μm hole through the SiN membrane and the Au film electrode. The quick increase in the signal intensity of O−, Cl−, and [AA-H]− (at ∼65−70 s) indicated punching through of the SiN/Au layers. The twodimensional images of H−, [AA-H]−, and total ions are also shown in the left. 961
DOI: 10.1021/acs.analchem.6b04189 Anal. Chem. 2017, 89, 960−965
Article
Analytical Chemistry copper metal wires and fixed in epoxy. The epoxy block was then placed onto a SU-8 template containing the inlet and outlet ports for electrolyte solutions, and polydimethylsiloxane (PDMS) prepolymer mixture was cast and cured. An Au film working electrode (WE, ∼30 nm thick) was sputter-coated on the backside of a 100 nm thick SiN window (1.5 × 1.5 mm2) centered on 200 μm thick silicon (7.5 × 7.5 mm2). The PDMS piece and the SiN piece were bonded using oxygen plasma, enclosing the flow reservoir. After the electrolyte was filled in the channel, the microfluidic cell was mounted onto the sample holder of ToF-SIMS and transferred into the instrument analysis chamber under high vacuum. Cyclic Voltammetry (CV). An electrochemical workstation (model 824 electrochemical detector, CH Instruments, Inc., Austin, TX, U.S.A.) was employed for CV measurements at a scan rate of 100 or 10 mV s−1 at 298 K. The experiments were carried out with a three-electrode system. The working electrode was the Au film underlying the SiN film, while a Pt electrode and an Ag/AgCl electrode served as the counter and reference electrodes, respectively. When performing step potential scanning accompanied by ToF-SIMS analysis, the potential was stepped from 0 to 0.8 V at 0.1 V increments (except 0.1 V, because very slight signal change was expected), being held constant for 2 min at each step. Dynamic CV scans at a rate of 10 mV s−1 were performed when real-time ToFSIMS data was acquired. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Measurements were conducted using a ToFSIMS V instrument (ION-TOF GmbH, Münster, Germany) with a 10 kHz 25 keV Bi3+ primary ion beam focused to ∼200 nm in diameter. The mass spectra were calibrated by H−, C−, O−, and OH−. An aperture of ∼2 μm in diameter was drilled through the SiN membrane and Au film electrode by a 1000 ns Bi3+ pulse. The penetration of the SiN/Au film was indicated by a sharp increase in signal intensity of the species in solution within the microchannel of the microfluidic cell (Figure 1b). After the aperture was made, the Bi3+ pulse width was changed from 1000 to 50 ns for better mass resolution. The vacuum pressure in the main chamber during measurements was 2−5 × 10−7 mbar. The integration time for each measurement at a constant potential scanning was about 120 s. More than 10 apertures could be drilled at different locations in a SiN window, and the device could still sustain a reasonably high vacuum (
