Correlative Voltammetric Microscopy: Structure–Activity Relationships

Jul 29, 2019 - Screen-printed carbon electrodes (SPCEs) are widely used for electrochemical sensors. However, little is known about their electrochemi...
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Correlative Voltammetric Microscopy: Structure-Activity Relationships in the Microscopic Electrochemical Behavior of Screen Printed Carbon Electrodes Daniel Martín-Yerga, Agustin Costa-García, and Patrick R. Unwin ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.9b01021 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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ACS Sensors

Correlative Voltammetric Microscopy: Structure-Activity Relationships in the Microscopic Electrochemical Behavior of Screen Printed Carbon Electrodes Daniel Martín-Yerga1,#, Agustín Costa-García1, Patrick R. Unwin2* 1Department

of Physical and Analytical Chemistry, University of Oviedo, Julián Clavería, Oviedo 33006, Spain.

2Department

of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K.

KEYWORDS: Screen-printed electrodes, electrochemical microscopy; carbon electrodes; sensing; dopamine oxidation

ABSTRACT: Screen-printed carbon electrodes (SPCEs) are widely used for electrochemical sensors. However, little is known about their electrochemical behavior at the microscopic level. In this work, we use voltammetric scanning electrochemical cell microscopy (SECCM), with dual-channel probes, to determine the microscopic factors governing the electrochemical response of SPCEs. SECCM cyclic voltammetry (CV) measurements are performed directly in hundreds of different locations of SPCEs, with high spatial resolution, using a sub-µm sized probe. Further, the localized electrode activity is spatially-correlated to co-located surface structure information from scanning electron microscopy and microRaman spectroscopy. This approach is applied to two model electrochemical processes: hexaammineruthenium (III/II) ([Ru(NH3)6]3+/2+), a well-known outer-sphere redox couple; and dopamine (DA) which undergoes a more complex electronproton coupled electro-oxidation, with complications from adsorption of both DA and side-products. The electrochemical reduction of [Ru(NH3)6]3+ proceeds fairly uniformly across the surface of SPCEs on the sub-µm scale. In contrast, DA electrooxidation shows a strong dependence on the microstructure of the SPCE. By studying this process at different concentrations of DA, the relative contributions of (i) intrinsic electrode kinetics and (ii) adsorption of DA are elucidated in detail, as a function of local electrode character and surface structure. These studies provide major new insights on the electrochemical activity of SPCEs and further position voltammetric SECCM as a powerful technique for the electrochemical imaging of complex, heterogeneous and topographically rough electrode surfaces.

Screen-printed electrodes are devices fabricated using thick-film technology that are widely used in the development of electrochemical (bio)sensors for point-ofcare testing,1 wearable2 and decentralized monitoring.3 They are well established in many laboratories and have been applied in a vast number of studies in fields such as heavy metal detection,4 nanoparticle characterization,5 spectroelectrochemistry6 and biosensing,7 with the low cost offering a disposable electrode philosophy. In a typical configuration, these devices are manufactured as a three-electrode arrangement (working, counter and reference electrodes) on a ceramic or plastic substrate. The conductive paths (electrodes and connections) are produced by an ink, which essentially consists of the conductor material, a binder and a solvent. An insulating material is overlaid to insulate the connections and produce the final electrode geometry. Graphite is the material employed most for the working electrode.8 Although the solvent is removed in a high temperature curing step, the binder (normally, a nonelectroactive polymer) becomes part of the electrode and could diminish electron transfer due to blocking of the conductive particles.9,10

Macroscale electrochemical characterization of different commercial and homemade screen-printed electrodes has been previously reported, for example, examining the effect of the carbon ink,11 the nature and amount of binder12,13 and other factors.14 However, macroscale studies may not be best suited for the analysis of the highly heterogeneous screen-printed electrode structure. Although there seems to be a consensus that much of the screen-printed surface shows low activity or slow electron transfer due to the presence of the non-electroactive binder,13,15 little is known about the behavior of screenprinted electrodes at the microscale, and there have been no direct measurements of microscale activity. Scanning electrochemical microscopy (SECM) has found wide application for mapping the electrochemical activity of electrode materials,16,17 but has not been used to investigate the localized activity at screen-printed electrodes. The most relevant report we could find examined screen-printed micro-array electrodes,18 but only at the resolution of an entire electrode in an array. Scanning electrochemical cell microscopy (SECCM) differs from SECM in that direct measurements of electrode activity are made via meniscus contact from an

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electrochemical cell contained in a micropipette or nanopipette.19,20 This technique has great versatility for studying a wide variety of different surfaces and reactions.21,22 It can be used in both a continuous scanning mode23 or hopping mode (approach-hold-retract),24 both of which map topography and activity synchronously, with the hopping mode being particularly powerful for the study of relatively rough samples,25,26 as considered herein. Voltammetric SECCM is a particularly powerful method for visualizing electrode activity as the full potentiodynamic behaviour is recorded at each pixel in a scan.24,27 This technique generates a vast amount of data that can be displayed in different ways (e.g. equipotential current maps and movies, spatially resolved CVs, maps of potential for a particular current criterion, Tafel analysis maps, etc.).28 In this work, we use hopping-mode voltammetric SECCM to study the localized electrochemical activity of screen printed carbon electrodes (SPCEs) with the aim of understanding the influence of the heterogeneous surface on the electrochemical activity. The [Ru(NH3)6]3+/2+ redox reaction, as an example of an outer sphere process, and dopamine electro-oxidation at different concentrations and scan rates, were used to evaluate the electrode behavior. The contact of the droplet with the SPCE surface at each pixel investigated left behind salt residue that could be used to correlate, exactly, the electrochemical measurement with the surface structure as determined by co-located scanning electron microscopy (SEM) and Raman micro-spectroscopy. These studies reveal major new perspectives on the behavior of SPCEs that will be of immense value for developing improved, tailored devices, as well as advance fundamental knowledge of electrochemistry at carbon electrodes. EXPERIMENTAL Solutions and reagents Solutions of hexaammineruthenium(III) ([Ru(NH3)6]3+) chloride and dopamine hydrochloride were freshly prepared daily. A 50 mM pH 7.2 phosphate buffer solution (PBS) with 25 mM potassium chloride was used as supporting electrolyte. All chemicals were of analytical grade, purchased from Sigma-Aldrich and used as received. Dichlorodimethylsilane (99%, Acros Organics) was employed for the silanization of the tips. Ultrapure water (SELECT-HP, Purite, 18.2 M cm resistivity at 25 ºC) was used throughout this work. Instrumentation SPCEs were purchased from DropSens (DRP-110). These devices incorporate a three-electrode cell printed on a ceramic substrate (3.4x1.0 cm). Both the working (diskshaped, 4 mm diameter) and the counter electrodes were made of graphite ink, whereas the quasi-reference electrode (QRE) and electric contacts were made of silver ink. A top insulating layer printed over the contacts ensured that they are insulated when covered by a solution to make the electrochemical cell. Macroscopic electrochemical experiments

Macroscopic CV experiments were performed in the traditional three-electrode setup, with the SPCEs used as received, and the device connected to a CH Instruments model 760 potentiostat through a DRP-DSC (DropSens) connector. A 40 µL drop of solution was positioned on the SPCE to cover the working, counter and quasi-reference electrodes. All macroscale potentials reported are versus the QRE on the SPCE. For [Ru(NH3)6]3+ in bulk solution, cyclic voltammograms (CVs) were recorded from -0.25 V to -0.6 V and for DA CVs were recorded from -0.5 V to +0.7 V. The scan rate was 100 or 500 mV/s. Voltammetric SECCM setup Voltammetric SECCM measurements on SPCE working electrodes were performed using a home built instrument, employing an FPGA card (PCIe 7852R, National Instruments) controlled through the LabVIEW 11.0 interface (National Instruments) and a lock-in amplifier (SR830, Stanford Research Systems).19 Dual barrel SECCM tips were prepared by pulling borosilicate theta capillaries (Harvard Apparatus) in a laser puller (P2000, Sutter Instruments). The diameter of the fabricated tips was ~760 nm. These tips were silanized on the outer walls by immersion in dichlorodimethylsilane solution with argon gas flowing through the tip. The probes were back-filled with the electrolyte of interest, and a AgCl-coated Ag wire was placed in each compart of the capillary, to act as a quasi-reference counter electrode (QRCE).29 The filled tip was then mounted on a 3-axis xyz piezoelectric positioning block (NanoCube, Physik Instruments) to control the movement of the tip. A home-built potentiostat was used to control the bias potential (V2, in Figure 1) between the QRCEs. This applied bias potential (150 mV in this work) generated an ion conductance current between the QRCEs across the liquid meniscus, with the response used to aid landing of the meniscus (vide infra). The entire setup was placed in a Faraday cage to minimize electronic noise. The potential of the surface (Vsurf) was controlled by V1 and V2 (fixed) (Figure 1) via the potentiostat. Vsurf is defined as Vsurf = - (V1 + V2/2).19 During the entire scan procedure, modulation of the tip z-position (123.8 Hz, 40 nm peak amplitude) was performed with the lock-in amplifier. This modulation generated an alternating component to the ion conductance current (iAC), with a threshold value being used to land the meniscus in each spot, without the pipet making contact. The hopping (approach-hold-retract) mode involved the following steps: approach of the probe to the SPCE surface (at a speed of 0.3 µm s-1), when contact of the meniscus with the surface was detected, probe movement ceased and a potential sweep was applied to the substrate (forward and reverse directions to make a CV). Then, the probe was retracted (speed of 0.8 µm s-1) from the surface (4 µm in the z axis) and moved at the same speed to the next point to repeat the approach-measureretract process, until a predefined array of grid points had been covered across the surface. In order to avoid any spatial overlap of the measurements, the hopping scans were performed with a separation of 3 µm between each point in the xy plane. Since this is a considerable hop distance and the resulting images were quite sparse in

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ACS Sensors terms of number of pixels, the data were fitted to a cubic spline to generate interpolated surface images. Some maps have also been plotted as raw data in the Supporting Information (vide infra). The interpolation and general data analysis were performed using a code written inhouse in Python language with the aid of specific SciPy30 libraries.

the SPCEs. Figure 2b shows a typical SECCM CV for the reduction of 250 µM [Ru(NH3)6]3+ on a SPCE (scan rate of 100 mV/s). The sigmoidal voltammogram shape, with a steady-state limiting current at the most cathodic potentials, indicates the expected quasi-radial diffusion of the redox-active species from the SECCM tip to the microscopic domain of the SPCE.19,32

Figure 2. Cyclic voltammograms for the reduction of 250 µM of [Ru(NH3)6]3+ at the macroscale (a) and in a SECCM setup (b). The specific area where the SECCM CV was measured is indicated in Figure 3c. Figure 1. Schematic of the two electrochemical methods for characterization of SPCEs: macroscale electrochemistry with a droplet covering all three electrodes (left part) and localized voltammetric hopping SECCM mode of the working electrode (right part). The blue circles indicate the sub-µm probed areas of the SPCE, and the arrows indicate the movement of the pipet. isurf is the electrochemical current of the substrate and iDC is the ionic current between the QRCEs. An SEM image of the end of a typical tip is also shown.

Structural characterization SEM was performed on a Zeiss SUPRA 55 FE-SEM instrument (at 10 keV). Electrolyte residues from the SECCM meniscus contact positions were observed with the SEM and revealed the droplet (i.e. working electrode) size and measurement location. The droplet sizes of each SECCM landing were measured using ImageJ software. Raman spectra and maps were recorded with a Renishaw InVia micro-Raman spectrometer, using a diode-pumped solid-state laser (Renishaw RL523C50) with an excitation wavelength of 532 nm. The spot size of the Raman laser was ~700 nm in diameter, and spectra were acquired at 0.5% power with an integration time of 30 s. RESULTS AND DISCUSSION Electrochemistry of [Ru(NH3)6]3+ at SPCEs Typical consecutive CVs for the reduction of 250 µM of [Ru(NH3)6]3+ at SPCEs at a scan rate of 100 mV/s are shown in the Figure 2a. These voltammograms show the characteristic shape of planar diffusion to the electrode. The peak potential difference (Ep) was in the range of 6070 mV, close to the theoretical value for a reversible oneelectron process (59 mV at 25 °C).31 There was little variation in the peak intensities or peak potential for consecutive CVs. Voltammetric SECCM experiments were performed in order to obtain localized electrochemical information on

Spatially resolved electrochemical behavior was recorded at an array of points and can be illustrated through activity maps of various types. For instance, Figure 3a shows the quartile potential difference across the scanned area of the electrode surface (Eq = E1/4 – E3/4, where E1/4 is the potential where the current is 25% of the limiting value and E3/4 is the potential for 75% of the limiting current). In most areas, the Eq is between 62-75 mV, relatively close to the theoretical value of 59 mV for a reversible process,31 and in agreement with the macroscale experiments. Similar information can be obtained from the half-wave potential, E1/2, map (Figure S1). Voltammetric SECCM can further be represented as a series of equipotential current maps as a function of spatial position at a series of different driving potentials, compiled as a movie as shown in the Movie S1 (SI), which illustrates the evolution of the normalized current (i/ilim) for a cycling potential sweep for the reduction of [Ru(NH3)6]3+ at SPCEs (111 frames in total). A frame from this movie at a potential close to the halfway potential (-0.26 V) is presented in Figure 3b, from which it can be seen that the electrochemical activity is relatively uniform over the entire surface. SECCM maps from raw data are shown in Figure S2. Each meniscus contact with the surface leaves an array of salt residues that serve as markers for the analysis of the electrode and correlation of electrochemical images and movies with other microscopic techniques. In particular, SEM images and Raman maps were obtained in the areas of the SECCM experiments and a significant heterogeneity in surface regions was found. For instance, SEM micrographs (Figure 3c) reveal a complex surface, further evident in Figure 3d, which shows a Raman map representing the ratio between the intensity of bands G/D (bands at ~1590 and ~1360 cm-1, respectively). This ratio reveals the crystallinity and defect density of graphite, as the G band is indicative of high quality sp2 carbon, and the D band is indicative of disordered carbon, including the

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presence of significant defects and step edges.33,34 Thus, the largest magnitudes of the G/D ratio highlights those areas with the most crystalline graphite material. In fact, the Raman map correlates very well with the structure observed in the SEM micrograph, showing that areas with a higher G/D ratio do indeed pinpoint areas with distinct graphite microstructures. Two typical spectra from the map, for these distinct regions, are shown in SI, Figure S3. Significantly, for the [Ru(NH3)6]3+/2+ couple, the structural heterogeneity of the graphite surface does not show any significant correlation with the local electron transfer kinetics as reflected in Eq (Figure 3a), because on the timescale of the electrochemical measurements the reaction is close to Nernstian.

Figure 3. Maps of Eq (a) and the i/ilim ratio (b) for the reduction of [Ru(NH3)6]3+ at -0.26 V. SEM image (c) and Raman map of the IG/ID ratio (d) for the scanned area.

Previous studies proposed that the amount of binder may play an important role in the electrochemical response of [Ru(NH3)6]3+ in SPCEs.13,15 However, that information was characteristic of the average electrode surface for electrodes with different amounts of binder. The SECCM results suggest that the local composition of the SPCEs studied here does not have a detrimental effect on the [Ru(NH3)6]3+/2+ process on the timescale of SECCM (which is faster than conventional voltammetry, compare current densities in Figure 2). Several authors have also reported that the electrochemical response of [Ru(NH3)6]3+ at SPCEs depends on the population of edge plane likedefects,13,35 although it has recently been demonstrated that basal plane graphite supports fast kinetics for the [Ru(NH3)6]3+ reduction.36–38 Indeed SI, Figure S4 shows no correlation between ET kinetics and the different graphite microstructure; rather, for [Ru(NH3)6]3+ reduction, a close to Nernstian response is observed. Dopamine electro-oxidation at SPCEs The electro-oxidation of DA is a complex process because different species may be involved, depending on the pH of the solution, adsorption of DA may play a role39,40 and side reactions lead to the formation of oligomers that

tend to block the electrode surface, inhibiting subsequent electron transfer.41,42 The effects of different carbon structures and microstructures is potentially very important and has received attention.42–44 A brief description of the electro-oxidation process is described in the SI, Figure S5. The electro-oxidation of DA has been studied at a macroscopic level with different types of screen-printed electrodes, but usually a fixed concentration within the 0.12 mM range has been used.45–48 Since the side reaction leading to oligomer formation and passivation is dependent on DA concentration and the timescale of the measurements,42,49 we employed concentrations and scan rates so as to isolate and characterize the effect of surface microstructure on: (i) intrinsic electrochemical kinetics, under conditions where passivation from products was greatly diminished (low [DA] and faster scan rate) and (ii) adsorption of electroactive DA itself (ultralow [DA]). The electro-oxidation of DA was first assessed at the macroscale. Five consecutive CVs were recorded at different concentrations of DA (30, 500 µM) and scan rates (100, 500 mV/s). Figure 4a-b shows typical CVs for 30 µM at 500 mV/s (4a) and 500 µM at 100 mV/s (4b). The CVs for the oxidation of 30 µM DA at 100 mV/s and 500 µM DA at 500 mV/s, are shown in the SI, Figure S6. Overall, the general electro-oxidation signature is similar to other carbon-based or metal electrodes.50 There is an oxidation wave with a peak potential ca. 0.20 V for the oxidation of DA to dopaminequinone (DAQ), and the reverse reduction process with a peak potential ca. 0 V. As mentioned in the SI, section S5, DAQ may react chemically to form leucodopaminechrome (LDAC), so not all of the generated DAQ may be available for the reduction, manifested as a diminished cathodic peak current. LDAC, generated chemically, can be oxidized at a potential close to -0.2 V,51 generating dopaminechrome (DAC), which can be reduced back to LDAC at a potential close to -0.4 V. Thus, the intensity of the peaks for the DAC/LDAC redox process is an indicator of the extent of any side reactions, although this is ascertained most readily from the magnitude of the DAQ reduction wave to the DA oxidation wave. Analysing the consecutive measurements, the peak current for DA oxidation becomes progressively smaller, and by the fifth CV is smaller by an amount in the range 5 to 35% compared to the first, attributed to blocking of the screen-printed electrode by oxidation products. The magnitude of the blocking effect depends on [DA] and the scan rate, being more significant at low scan rates and relatively high [DA]. In fact, the blocking effect is relatively low at a concentration of 30 µM and a scan rate of 500 mV/s, with a ratio close to 1 between the anodic and cathodic peak currents for all cycles. In light of the macroscopic measurements above, SECCM experiments were performed using a concentration of 30 µM of DA and a scan rate of 500 mV/s to have less surface blocking from products (vide supra) and the results obtained could be related mainly to the intrinsic electrochemical activity for the electro-oxidation

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ACS Sensors of DA. Most of the SECCM voltammograms had the form of a steady-state curve (Figure 4c), but in some specific areas of the electrode, voltammograms were obtained where a small sharp peak appeared at a potential just before the steady-state current plateau with the same feature on the reverse. This different electrochemical response may be related to the carbon microstructure as described in further detail below.

Figure 4. Consecutive macroscopic CVs for the oxidation of 30 µM DA at 500 mV/s (a) and 500 µM of DA at 100 mV/s (b) at SPCEs. (c-d) Two typical cyclic voltammograms obtained in the SECCM setup for the oxidation of 30 µM of DA at 500 mV/s at different areas of a SPCE CVs (indicated in the Figure 5c).

Since the scan rate is such that the SECCM diffusional response should be under close to steady-state conditions (Figure 4c), the peak is attributed to adsorbed DA in certain areas of the surface. A map representing the ratio between the maximum current, imax, and the final limiting current, ilim, for the oxidation of DA is shown in Figure 5a in order to visualize this effect over the entire scanned area. In most areas, this ratio is close to 1 (i.e. maximum current is the limiting current and the response is largely controlled by DA diffusion), but there are two areas where this ratio is appreciably larger. A complementary map of the Eq is shown in Figure 5b (and a map of the half-wave potential, E1/2, in Figure S7), which indicates spatial heterogeneity of kinetics. Some of these zones correspond to the assigned DA adsorption zones, but other areas show smaller Eq or overpotential (less positive E1/2) even when a classical sigmoidal curve is observed. SECCM maps from raw data are shown in Figure S8. This heterogeneity in the electrochemical activity can be observed dynamically in a video of the surface current density as a function of potential (Movie S2). Figure 5c shows an SEM image of the scanned area after the SECCM measurements. The droplet residues are fairly consistent in the different spots, and the droplet size was used to estimate the working electrode area in each of the SECCM measurements (Figure S9) and to calculate current density. Most striking is that areas with a higher

electrochemical activity (either faster kinetics or adsorption) correspond to areas where larger graphite particles are evident. Some of these large particles were also detected by the SECCM topographic map (Figure S10). Areas with more sluggish kinetics or no detectable DA adsorption, are those with a flat and smooth surface structure. This fact seems to agree with other studies using SPCEs, where, after an activation with plasma, bare graphite particles appeared on the electrode surface and faster DA oxidation kinetics was observed.45

Figure 5. Map representations for the oxidation of 30 µM DA at 500 mV/s in a SECCM setup: a) ratio between the maximum current and the limiting current, and b) half-wave potential. c) SEM image and d) Raman map of the IG/ID ratio for the scanned area. In part c the circled spots mark areas where the individual SECCM CVs in Figure 4 were taken (with corresponding colors).

Further evidence for the strong correlation between electrochemical activity and electrode microstructure was revealed from Raman maps of the scanned area. Figure 5d shows a map of the G/D band instensity ratio (bands at ~1590 and ~1360 cm-1, respectively). Areas of higher electrochemical activity and of adsorbed DA correspond to areas with a higher G/D ratio, indicating that graphite particles with fewer defects are responsible for the highest electrochemical response. Similar results were found in SECCM experiments at a scan rate of 100 mV/s (Figure S11). The influence of repetitive cycling was studied by performing 5 cycles at each spot in a voltammetric SECCM experiment at 500 mV/s. The ratio between the maximum current of the first and consecutive cycles was close to 1 (Figure S12), which indicates that the blocking of the surface by the accumulation of oligomeric side-products is insignificant under these conditions. The results obtained under these conditions reflect the intrinsic electrochemical activity of SPCEs. The adsorption of DA at SPCEs was studied further with voltammetric SECCM using [DA] = 1 µM. Since DA adsorption is fast (close to diffusion-controlled on timescales similar to these herein),40 meniscus contact with the electrode for 5 s was considered sufficient to allow

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Figure 6. a) Two typical cyclic voltammograms for the oxidation of 1 µM of DA in a SECCM setup obtained at different surface locations. Map representation of (b) the anodic peak potential (Epa) and (c) the surface coverage (ads) along the electrode surface. d) SEM image and e) Raman map of the IG/ID ratio for the scanned area. The SEM image shows the areas where the CVs in a) were performed (with corresponding colors and labels). The irregular shape of the residue patches may indicate slight droplet spreading due to a longer contact time compared to previous experiments reported above.

for DA adsorption, before application of the CV potential profile at a scan rate of 500 mV/s to detect electroactive adsorbed DA. Using this very low [DA] and relatively fast scan rate, the oxidation of adsorbed DA would dominate (vide infra). Example CVs are shown in Figure 6a and the specific surface locations where they were recorded are depicted in Figure 6d. In one case (red curve) there is a defined peak for the oxidation of DA with a fast decay to the baseline at a potential close to 0.0 V. The corresponding reduction process is also observed with peak current charge values close to the oxidation peak values. In other areas of the SPCE (blue curve), DA electrooxidation occurs at more positive potentials and without the corresponding reduction process on reversal of the applied potential. In this case, the electro-oxidation of DA is clearly slower and irreversible. In both example CVs, the values of the oxidation peak current (about 1x10-4 A cm-2) are much higher than would be expected for a purely diffusion-limited reaction (which would also be under steady-state conditions, e.g. Figure 4c), indicating that the oxidation and reduction signatures are mainly due to adsorbed DA. For example, scaling the diffusion-limited currents in Figure 4 to the lower concentration of DA, we would expect a steady-state limiting current density ca. 10-5 A cm-2 for a diffusional process, which is an order of magnitude lower than measured experimentally. An SECCM map representing the anodic peak potential (Epa) for DA oxidation (Figure 6b) shows the specific regions on the SPCE where the reaction is more facile. The total charge under the forward voltammetric peak (Qads) can be used to produce a map of electroactive DA surface coverage (ads) on the SPCE according to Qads = nFAads, where n is the number of electrons, F is the Faraday constant and A is the meniscus area (Figure 6c). Surface coverage values ca. 10-10 mol cm-2 were found across the electrode surface, which is in the same order of magnitude as calculated by previous studies of DA oxidation with carbon electrodes.40,52 SECCM maps from raw data are shown in Figure S13. Analysing the corresponding co-

located SEM and Raman images of the SPCE (Figures 6de), it is clear that electrode regions with a higher degree of crystallinity show more DA adsorption and facile kinetics. Interestingly, the SECCM experiments could also provide some topographic information about this heterogeneous electrode surface (Figure S14). CONCLUSIONS Correlative electrochemical multi-microscopy, combining data from voltammetric SECCM, with complementary co-located information on electrode structure from micro-Raman spectroscopy and SEM, has revealed considerable new perspectives on surface structure controls on electrochemical kinetics at SPCEs for both simple outer sphere processes ([Ru(NH3)6]3+/2+) and more complicated reactions, exemplified by DA electrooxidation. The rate of the [Ru(NH3)6]3+/2+ redox reaction was fairly homogeneous across the surface of SPCEs, showing close to reversible behavior on the SECCM timescale. In contrast, the oxidation of DA was clearly dependent on the location on the SPCE surface. Furthermore, by tuning the concentration of DA, the oxidation of DA from solution and adsorbed on the SPCE surface has been highlighted. Both processes are strongly influenced by the local structure of the SPCE. The most facile electro-oxidation has been observed in surface regions having graphite particles with a high degree of crystallinity. Such sites also promote fast electro-oxidation of adsorbed DA. This adsorbed layer may also play as an electrocatalyst for oxidation of solution-phase DA.53 Interestingly, this same pattern of activity has been found for the oxidation of NADH at HOPG, where defect-free (basal) sites were associated with fast electrochemistry, but were also the regions where HADH adsorption was strongest.54 The data presented herein unequivocally demonstrate the essential role of the heterogeneous microstructure of screen-printed electrodes in the electrochemical activity. These results may help the rational design of new SPCEs with enhanced activity.

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ASSOCIATED CONTENT Supporting Information. S1. SECCM map of the E1/2 for [Ru(NH3)6]3+ reduction; S2. SECCM maps as raw data for [Ru(NH3)6]3+ reduction. S3. Raman spectra of screen-printed carbon electrodes; S4. Correlation between activity of [Ru(NH3)6]3+ reduction and carbon microstructure; S5. Dopamine electro-oxidation mechanism; S6. Macroscale cyclic voltammetry of 30 µM DA oxidation at 100 mV/s; S7. SECCM map of the E1/2 for 30 µM DA electro-oxidation; S8. SECCM maps as raw data for 30 µM DA electro-oxidation; S9. Droplet contact areas for the 30 µM DA SECCM experiment; S10. Topographic map obtained for the 30 µM DA SECCM experiment; S11. Correlative activity-structure maps for 30 µM DA oxidation at 100 mV/s; S12. Consecutive voltammetric SECCM of 30 µM dopamine at 500 mV/s; S13. SECCM maps as raw data for 1 µM DA electro-oxidation; S14. Topographic map obtained for the 1 µM DA SECCM experiment; S15. Spatiallyresolved electrochemical activity movies for [Ru(NH3)6]3+ reduction and DA oxidation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding author: [email protected]

Present Addresses #Current

address: Department of Chemical Engineering, KTH Royal Institute of Technology, Teknikringen 42, Stockholm 100 44, Sweden

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources DMY thanks the Spanish Ministry of Economy for the FPI fellowships (BES-2012-054408; EEBB-I-15-10190). PRU thanks the Royal Society for a Wolfson Research Merit Award.

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