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Multiplex infrared spectroscopy imaging for monitoring spatially resolved redox chemistry Lucyano J. A. Macedo, and Frank Nelson Crespilho Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04438 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018
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eMultiplex infrared spectroscopy imaging for monitoring spatially rresolved redox chemistry Lucyano J. A. Macedo and Frank N. Crespilho* São Carlos Institute of Chemistry, University of São Paulo, São Paulo 13560-970, Brazil Supporting Information Placeholder ABSTRACT: IR spectroscopy is an excellent method for understanding surface redox chemistry. However, obtaining sufficient spatial resolution to analyze in situ surface redox reactions is difficult because the aqueous sampling environments provide some challenges for IR spectroscopy. These challenges arise because of the vibrational contribution of water. In this letter, we demonstrate a solution to this problem, where the key development enabling the coupling of spectromicroscopy with electrochemical measurements is a CaF2/electrolyte/Au sandwich IRsensitive sample holder that acts as an electrochemical cell. In this system, there is a very thin layer of aqueous electrolyte (ca. 10 µm), and it is possible to monitor, in real time, the vibrational maps and changes to the Au surface modified with iron(II, III) hexacyanoferrate(II, III) by varying the electrochemical potential. By selecting specific vibrational modes with a focal plane array detector, which allows the simultaneous collection of IR spectra from 4096 microscopic regions, chemical maps showing the surface changes were obtained and analyzed using color, providing new insights into how the charge transfer processes affect the chemical composition in specific 2D spatially resolved regions.
Fourier transform infrared (FTIR) spectromicroscopy or ‘micro-FTIR’ can be used to analyze specific areas of a complex sample from the vibrational signatures and has a spatial resolution in micrometric areas of samples such as polymers,1 formulations,2 and biological components.3-6 A branch of micro-FTIR called micro-ATR uses a micro-crystal to internally reflect the beam and gather information about the surface of the sample.7 Spatial resolution on the nanometer order has been reached (so-called nanoFTIR) by focusing the IR beam onto the sample with an atomic force microscope (AFM) tip.8-10 Both approaches provide useful information regarding the spatial distribution of heterogeneities in the composition of microstructures and surfaces. Several designs have been developed for the study of electrochemical processes through FTIR analysis, for instance, thin-layer reflectance IR spectroelectrochemistry.11-12 Furthermore, advances in sample preparation have allowed the coupling of micro-FTIR to electrochemical systems,13,14 and, recently, Ash and coworkers15 reported a micro-FTIR study of single crystals of a NiFe hydrogenase enzyme. In their experiments, by controlling the electrochemical potential, they observed a change in the vibrational signature of the metallic cluster of this protein. It is worth mentioning that that the immobilization of molecules onto thin metallic layer provides an alternative for gathering higher signals in FTIR analysis. The interaction of electrical field with the dipole momentum of the
vibration of molecules adsorbed on metal surfaces resonate and create an enhancement of IR radiation absorption, phenomenon known as Surface Enhanced Infrared Absorption (SEIRA). This approach has been widely used in electrochemical systems mainly to study the molecular orientation of thin films.16-18 Although there have been reports of these coupled techniques, to date, the simultaneous detection in different sample regions of the vibrational changes of a heterogeneous surface (multiplex imaging) under electrochemical control has not been reported. Here, we report a chemical imaging approach based on multiplex micro-FTIR coupled with an electrochemical cell to allow us to gather information concerning how reactants species change in a specific area of the electrode. In this case, we studied how the redox processes influence the vibrational signatures of an adsorbed thin film of iron(II, III) hexacyanoferrate(II, III) (Prussian blue, or PB) by monitoring a local heterogeneous region of the working electrode. A Hyperion 3000 microscope coupled to a Vertex 70v FTIR spectrometer (Bruker) was used to obtain both in situ optical and in situ vibrational information from the electrode. The microscope was equipped with a focal plane array (FPA) detector, which allows the simultaneous collection of IR spectra from 4096 regions within the microscope image, thus making it possible to build maps related to specific vibrations on the surface. For the electrochemical system, we performed the measurements using a potentiostat/galvanostat PGSTAT128N (Autolab). The key feature for the coupling of spectromicroscopy and electrochemical measurements is a sample holder that is also an electrochemical cell. A home-built polytetrafluoroethylene (PTFE) three-electrode electrochemical cell (Figure 1) allows the analysis of thins films adsorbed on flat electrodes. In this case, we used a flat gold mirror as the working electrode, whose surface was modified by electrodepositing a thin film of PB, as described in the Supporting Information. One of the main reasons against using FTIR spectroscopy for the analysis of systems in aqueous media lies in the interference caused by water, which plays a fundamental role as the electrolyte solvent and is in vast excess (~55 mol·L-1) of the electrolyte ions. Therefore, its characteristic absorptions νO-H (ca. 3650 cm-1) and δO-H (ca. 1590 cm-1) at such high concentration usually preclude the observation of vibrations from the molecules in these regions of the spectrum.19 A way to avoid this limitation is the use of deuterated water (D2O),20 whose vibrations shift to νO-D (ca. 2670 cm-1) and δO-D (ca. 1180 cm-1). Other kinds of interference also originate from the absorption of radiation that results in an exponential decrease in the reflected beam intensity as a function of the path length (eq 1): 1
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Figure 2. Effect of the electrolyte layer on the intensity of the IR signal. (a) Single channel spectrum of a polystyrene film using different height of electrolyte layer (●) No interference of the film; (●) 3 mm CaF2 window; (●) 3 mm CaF2 window + 9 µm of H2O (●) 3 mm CaF2 window + 18 µm of H2O; (●) 3 mm CaF2 window + 27 µm of H2O (●) 3 mm CaF2 window + 36 µm of H2O; and (●) 3 mm CaF2 window + 45 µm of H2O. (b) Exponential decay of the IR signal intensity as a function of the electrolyte layer thickness at 2926 cm-1.
Figure 1. EVSM (Electrochemical Coupled Vibrational Spectromicroscopy) instrumental schematic. (a) Representation of a micro-FTIR with a sample holder/microscope stage that works as an electrochemical cell. (b) Magnified region of a cross-section of the interior of the cell showing the positions of the system electrode, sample, electrolyte, and CaF2 window.
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By employing FPA detectors in FTIR spectromicroscopy instrumentation, it is possible to reach a point-to-point resolution as low as the optical diffraction limit.4, 23-24 For instance, we have obtained a pixel-to-pixel distance down to 2.7 µm with a 15× objective, where each pixel represents one collected spectrum (Figure S1 of the Supporting Information). Depending on the objective lens used in the experiment, an overlap of signals is possible, so the displayed spectrum may result from a sum of contributions from more than one analyzed region.
(1)
where I and I0 are the reflected and incident intensities of the radiation, respectively, αλ is the absorption coefficient of the medium at a certain wavenumber (cm-1), and L is the path length.21 To address this issue, we performed a series of measurements by stacking gaskets between a polystyrene (PS) film and the CaF2 window. Importantly, CaF2 also absorbs some of the radiation, even though it is transparent to IR radiation (cutoff at ca. 1100 cm-1). We found that the window absorbed ~53% of the radiation in comparison to the signal of the interferent-free PS film after the beam passes through the 3-mm thick CaF2 window (Figure 2a). This intensity loss by the presence of the CaF2 window comes probably from an angular disposition of the window to the sample, not being parallel to one another, causing reflection and diffraction the beam while passing through the cell. Stacking the gaskets between the CaF2 window and the PS film, an electrolyte layer is formed; thus, we could quantify the interference of the H2O on the IR measurements. Indeed, as predicted by eq 1, the intensity decays exponentially as a function of the radiation path length (Figure 2b), and an αλ of 624 cm-1 was observed, in agreement with data reported by Downing and Williams.22 Until the thickness of the electrolyte layer reached 18 µm, the analyzed signal at 2926 cm-1 from the PS film, which overlaps with the νO-H of water, was still detected, although 71% of the beam intensity was absorbed by the liquid layer. Using a thicker electrolyte layer, almost no signal was detected. Consequently, the data quality in the final FTIR spectrum is poor and, by practical means, useless in this region of the spectrum.
Figure 3. The identity of the surface. (a) Cyclic voltammogram of the synthesized PB film on gold (scan rate: 0.02 V·s-1). 3D plots of the absorbance intensity along the surface of a heterogeneous region of the electrode polarized at 0.4 V (vs. Ag/AgClsat) by selecting two distinct spectral regions: (b) 2100 and (c) 2150 cm-1. A microscopic heterostructure composed of gold and PB was used to probe the contrast in both visible and vibrational images because of its well-known electrochemical and vibrational behavior.25 The surface of working electrode was obtained by electrodepositing PB on gold. A region that had been lithographically passivated remained PB-free and could be used as control in the FTIR measurements The electrode underwent voltammetric analysis before the spectroelectrochemical measurements, and the electrochemical response showed a quasi-reversible redox process with E1/2 = 0.22 V, characteristic of the interconversion of Prussian white (PW) to PB (eq 2)26 (Figure 3a). K2[Fe2+Fe2+(CN)6] (PW) ⇌ K[Fe2+Fe3+(CN)6] (PB) + K+ + e- (2)
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Figure 4. Electrochemical SNIFTIR of PB-modified Au electrode. (a) Spectral change (SNIFTIR) of the PB film on gold subtracted from the spectrum taken at -0.2 V and (b) evolution of the bands attributed to PB vibrations upon the application of different potentials. (c) Optical microscopic images (left) and chemical image of the system by selecting specific regions of the SNFTIR spectra at (a): red, 2150 cm-1; blue, 2100 cm-1; and green, 2050 cm-1. Supporting electrolyte: KCl 1.0 mol·L-1, T = 20 °C. Scale bar: 50 µm. Figure 3b,c show high-resolution 3D maps of two distinct spectral regions (2100 and 2150 cm-1, respectively), obtained after the surface had been polarized to 0.4 V. The absorption intensity along the multiplex chemical images provides information about the chemical profile of the sample. In this boundary region, the image concerning the vibrational mode at 2100 cm-1 shows contrast between these two regions, whereas there is no differentiation for the one at 2150 cm-1 because neither of the two surface components have vibrational modes in this region. Using the same potential window as the cyclic voltammetry (from -0.2 to 0.4 V versus Ag/AgClsat), we monitored the PB-modified electrode as we modulated the potential applied to the gold electrode. Visually, we observed an electrochromic effect in this material, where the color changes were dependent on the applied potential. An almost transparent sample was observed at -0.2 V, but the color of the film drastically changed to blue when the potential reached 0.4 V (Figure 4c, left). Changes were also observed in the IR spectrum of PB (Figure 4a) at different potentials. By monitoring specific bands of the subtracted normalized intensity FTIR (SNIFTIR) spectra, such as those centered at ~2050 and ~2100 cm-1, which were assigned to the C≡N stretching vibration of the [Fe2+(high2+ 3+ 2+ 25 respectively, the spin)–C≡N–Fe ] and [Fe –C≡N–Fe ] units, chemical images concerning these bands follow the same trend. While the band at 2050 cm-1 increased with increasing potential from -0.2 to 0.3 V, it suddenly almost disappears at 0.4 V (Figure 4b) due to the complete oxidation of the [Fe2+(high-spin)–C≡N–Fe2+] unities to the [Fe3+–C≡N–Fe2+] species as expected from the oxidation process observed in the cyclic voltammogram of this PB film (Figure 3a). It is possible to see the same effect regarding the
color changes in the maps (Figure 4c, green). Similarly, the color change in the maps concerning the vibrational mode centered at 2100 cm-1 also follows the trend in the SNIFTIR spectra of the referred band (Figure 4c, blue). Unlike the band centered at 2050 cm-1, this band did not disappear and only intensified once the potential had reached 0.4 V. In these measurements, the CaF2 window was placed in direct contact with the PB film. The structural change response of the PB film is still possible even with such a small amount of electrolyte because the PB films are porous and allow the easy diffusion of the electrolyte and K+ cations through the crystalline structure of the film.27 Although higher IR signals are affected with electrolyte, we used thicker electrolyte layers also for these experiments and the maps obtained are consistent with the optical image when using up to 18 µm of electrolyte in this setup (Figure S2 of the Supporting Information), likewise the observed for the PS film. The requirement for a thin layer of aqueous electrolyte for successful spectral analysis represents a compromise for the quality of electrochemical behavior. The electrochemical performance can be affected when using small amounts of electrolyte. However, thin layers of electrolyte are necessary in micro-FTIR due to the characteristic IR absorption from water. Other challenge in the experimental setup is that all electrodes should be immersed in very thin thickness of electrolyte. Our results confirm that this experimental condition does not compromise the electrochemical control of the system and the molecules on the surface of the working electrode still respond as expected. We also monitored a randomly chosen region of the spectrum that did not present any vibrational modes that could be attributed
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to PB or the substrate. Unlike the abovementioned spectral regions whose vibrational modes are attributed to changes in the structure of PB, the one centered at 2150 cm-1 shows no change in terms of absorption along the potential sweep and, consequently, the chemical map of the same region does not show any contrast between the region containing the PB film and the region with just gold when the electrochemically modulated energetic state of the system was varied (Figure 4c, red). In summary, the optimization on the instrumental setup of micro-FTIR allows the analysis of the chemical change in a heterogeneously composed surface with microscopic resolution. In the presented example with a PB film, the direct energetic control of the system via electrochemistry allows the location on the electrode surface where the reaction is taking place to be determined. Furthermore, we can determine how the reaction is carried out. This was achieved by controlling the height of the liquid layer through which the radiation passes. In addition, we have presented an approach that allows the study of the behavior of electrochemical reactions of thin films at metal–electrolyte interfaces; in particular, this method allows not only high spatial resolution but also the selection of specific vibrations, which opens up the possibility to gather information on how charge transfer affects the chemical composition of redox heterostructures. Moreover, we catch a glimpse of a future in which IR mapping the surface in response to applied potential opens up the possibility for others applications, e.g. to gather information on how reactants and products diffuse within electrocatalytic surfaces. ASSOCIATED CONTENT
Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental methods: sample preparation, detailed EVSM (Electrochemical Coupled Vibrational Spectromicroscopy) parameters, and data collection (PDF).
AUTHOR INFORMATION Corresponding Author *
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT
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This material is based upon work supported by FAPESP (Projects: 2015/16672-3, 2013/14262-7) and CNPq (Project: 478525/2013-3).
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