Modular Flow-Through Platform for Spectroelectrochemical Analysis

Publication Date (Web): April 18, 2017 .... The ECL response was detected using a Jazz Spectrometer (Ocean Optics) coupled with an Ocean Optics Vis-NI...
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Modular Flow-Through Platform for Spectroelectrochemical Analysis Tomer Noyhouzer, Michael E. Snowden, Ushula M. Tefashe, and Janine Mauzeroll* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec Canada H3A OB S Supporting Information *

ABSTRACT: A new type of flow platform for electrochemical and spectroelectrochemical measurements is presented. Finite element method simulations confirm that the hydrodynamic profile within the device is not turbulent and provides an analytical platform for the investigation of homogeneous kinetics, radical lifetimes, and reaction mechanisms. The modular “plug and play” configuration of the platform allows one to carry out electrochemistry and spectroscopy individually or simultaneously. Specific demonstrations of electroanalytical measurements using the flow system platform includes voltammetric analysis of organometallic compounds and quantitative analysis of ascorbic acid in commercial orange juice samples. Combined spectroelectrochemical demonstrations include electrochemical luminescence of ruthenium compounds and ligand exchange reactions of iron complexes using UV−vis spectroscopy. lectrochemical flow systems are broadly used in research and applied fields such as environmental, industrial, and medical monitoring1 because they cost-effectively determine concentrations, yield energy data (redox potential), and elucidate reaction mechanism via kinetic analysis.2 The hydrodynamics of such systems offer comparative advantages over diffusion controlled electrochemical techniques3,4 insofar as the increased mass transport allows access to higher kinetic rates in analytical studies5−8 and offers the potential for industrial scale up.9 The ability to control mass transport has also led to automated chemical processes,10 either by continuous variation of the flow rate or for batch processing.11 Automating the stages of liquid replacement and mixing increases the precision and accuracy of measurements.8,11−15 Flow systems also provide much needed design flexibility. Choosing the right flow pattern (laminar5,12,16 or turbulent,16−18) and format (continuous, stop-flow, quench-flow, and bypass flow19) is as critical as choosing the right electrochemical method to increase the collection efficiency and sensitivity. Overall, the majority of electrochemical flow systems rely on laminar flow patterns because they are stable, well characterized, and relatively easy to model.20−24 The combination of reaction-oriented electrochemical flow systems with species-focused spectroscopy enables more complete analysis of reactions involving multiple electron transfer steps, as well as unstable intermediates. In this work, we present a spectroelectrochemical (SEC) flow-cell (Figure 1) based on a flow-through methodology, which obtains a reliable hydrodynamic profile. Compared to commercially available flow systems that rely on flow injection analysis (FIA) combined with a wall-jet or flow-by (channel) methodology,25 the flow-through system increases the electroactive surface area to generate higher concentrations during voltammetry, enabling

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© XXXX American Chemical Society

a strong spectroscopic response. It is also resistant to the negative effects related to the presence of bubbles that can lead to signal loss. Overall, the modular cell design controls the rate of mass transport, electrochemical measurement, and spectroscopic methods, thus creating an easy to use, multipurpose SEC platform. Three different electroactive samples were selected to validate the electrochemical module response (Figure 1 Unit ABC1), which was developed for both batch measurements and for continuous monitoring. Ferrocenemethanol or hexacyanoferrate, two reversible redox systems, were introduced in unit A (Figure 1) and their voltammetric response under varying scan and flow rate conditions was measured in unit B before exiting through unit C1. The ABC1 flow cell configuration was also used in a quantitative electroanalytical method to monitor the concentration of ascorbic acid (AA), a redox species present in a complex matrix (orange juice). Substitution of unit C1 in Figure 1 by unit C2 or C3 provides SEC configurations capable of detecting electrogenerated chemiluminescence (ECL) and UV−vis absorbance, respectively. ECL involves the electrochemical generation of species with electrons in excited states that relax to emit light of a specific wavelength.26 This is a very powerful technique for the detection of biomolecules. Advantages over conventional fluorescence methods include enhanced signal-to-noise ratios (S/N), increased sensitivity, reduced self-quenching, and simplified equipment.27,28 Using the ABC2 flow cell configuration, ECL measurements were obtained using the tris(2,2′bipyridyl)dichlororuthenium(II) (Rubpy, the luminophore) Received: November 23, 2016 Accepted: April 18, 2017 Published: April 18, 2017 A

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Figure 1. Schematic drawing of the system backbone (Units A and B) and the individual units for standard electrochemical measurements (C1), ECL measurements (C2), and UV−vis measurements (C3) as well as the validation reactions that are associated with each of the units. WE, CE, and RE represent working, counter, and reference electrodes, respectively, while FO represents the fiber-optic connections.

Figure 2. (A) Effect of the flow rate on the CV for reversible oxidation and reduction of 2 mM FcMeOH at a scan rate of 100 mV s−1 (all other conditions and electrodes are as described in Figure 3). (B) The absolute reduction peak currents as a function of the cube root of the flow rate (νf).

and Tripropylamine (TPrA, the coreactant) system.26,29 When increasing the rate of mass transport, an increase in the ECL signal is expected, hence further improving the S/N and sensitivity of the measurement. The ABC2 SEC configuration is not limited to ECL and could be used to investigate other electrogenerated species that have a half-life of several seconds, such as the chloranil radical anion or methyl viologen dication.30,31 Finally, the flow cell configuration ABC3 yields an electrochemical/UV−vis unit. This unit is ideally suited to study homogeneous kinetics of multiple step reactions. To demonstrate the potential of this configuration a colorless Fe(II) aqueous complex is electrochemically oxidized into Fe(III) in the presence of potassium thiocyanate, followed by the spontaneous formation of the dark red [Fe(III)SCN]2+ complex, which is monitored by UV−vis spectroscopy.

(≥98%), potassium phosphate monobasic (≥99%), soluble starch (ACS reagent), ferrocenemethanol (97%, FcMeOH), potassium hexacyanoferrate(II) trihydrate (>99%), tripropylamine (>99%, TPrA), and tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (99.95%, Rubpy) were purchased from Sigma-Aldrich. Iron(II) sulfate heptahydrate (>99%) was purchased from Acros Organic, NJ. Hydrochloric acid (ACS grade) and potassium iodate were obtained from ACP Chemicals, while ascorbic acid (AA) was acquired from Fisher Chemicals. All solutions were prepared using Millipore Milli-Q 18.2 MΩ cm water. Instruments and Procedures. Electrochemical experiments were conducted using a three-electrode setup with either a Pt or Au Honeycomb electrode (Pine Instruments) as the working electrode, Pt Honeycomb electrode as the counter electrode, and Ag/AgCl QRE fabricated in house32 using a 1.0 mm diameter Ag wire (Goodfellow). Electrochemical measurements were conducted using a HEKA bipotentiostat (model ELP1, HEKA Electronik, Germany) controlled by Potmaster software. Fluid flow was controlled using a NE-1000 syringe

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MATERIALS AND METHODS Chemicals. Potassium chloride, potassium iodate (98%), potassium thiocyanate (>99.0%), potassium phosphate dibasic B

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Vis-NIR fiber optic cable positioned at a fixed position directly in front of the working electrode (Figure 1C) and recorded using Ocean Optics software. A solution containing 1 mM Rubpy and 0.2 M TPrA in 0.1 M phosphate buffer (pH 7.3) was passed through the system at different flow rates while the potential was cycled between −1 and 1.5 V. A series of ECL spectra was acquired at each potential and the integrated ECL light intensity from 500 to 800 nm was also measured. UV−vis Module (ABC3 Configuration). Different concentrations of Fe(II)SO4 (0−20 mM) were prepared in a solution containing 0.1 M KCl (as electrolyte), 1 mM HCl (pH 3), and 60 mM KSCN (as complexing agent). The Fe(II)SO4 solutions were introduced in the ABC3 configured flow through cell at a flow rate of 1.5 mL min−1. Spectra were acquired before (background) and after application of a constant oxidizing potential (0.8 V). In line with the reaction scheme presented in Figure 1, the absorption peak of [Fe(III)SCN]2+ at 470 nm was monitored.36 The UV−vis absorbance was recorded using a Maya LSL spectrometer (Ocean Optics) coupled with an Ocean Optics UV−vis fiber optic cable for the transmitted light (outlet light) and an Ocean Optics halogen lamp coupled with a vis-NIR fiber optic as a light source (inlet light). UV−vis spectra (300−800 nm) were recorded using Ocean Optics OceanView and treated with Origin 9.1 (OriginLab). Finite Element Simulations. Simulations to determine the hydrodynamics within the flow platform were performed using COMSOL Multiphysics 5.2 (COMSOL Inc., USA). The geometry of the model was created from the dimensions of the cell and the electrodes. For computational efficiency, the symmetry planes through opposite corners of the hexagonal electrode and through the center of the sides of the hexagon were used to reduce the domain size (Figure S-1). The hydrodynamics were calculated by solving the steady-state Navier−Stokes equations for incompressible fluids, consisting of the momentum balance (eq 1) and continuity (eq 2) equations,

pump (New Era) equipped with a 10 mL Hamilton syringe (Hamilton). Cell Design. The flow through platform design is modular and is demonstrated using three configurations (Figure 1, ABC1) electrochemistry, (Figure 1, ABC2) SEC-ECL, and (Figure 1, ABC3) SEC−UV−vis. Note that more than three modules can be combined together and in any order desired by the end user, providing a flexible experimental platform. Additionally, the modular design allows for rapid assembly, for example, replacing an entire module requires less than 2 min. In SEC-ECL configuration the optical fiber is positioned 20 mm downstream adjacent to the working electrode. In the SEC-UV−vis configuration, the light source and the optical fiber are positioned perpendicular to the flow. The working and counter electrodes used in the flow cell platform are screenprinted electrodes (SPE) consisting of a ceramic material with a metallic surface (Pt or Au) and 19 holes (arranged hexagonally) machined perpendicular to the electrode surface (Figure S-1). For these experiments the counter electrode is positioned between unit A and B and the working electrode is positioned between unit B and C, with the reference electrode mounted in a flangeless screw downstream from the outlet of the cell. It is worth mentioning that previous works12,33 showed the importance of positioning the reference electrode downstream in order to prevent fouling of the fluent by the reference. This issue can also be resolved using a commercially available fritted reference electrode. Flow Cell Measurements. The performance of the SEC flow cell was validated at 22.5 °C by using the following studies: Electrochemistry Module (ABC1 Configuration). (A) The electrochemical response of the cell was investigated using a solution of 1 mM FcMeOH in 0.1 M KCl. Two different experiments were conducted, measuring the effect of the scan rate (10−300 V s−1) during a constant flow of 0.1 mL min−1 and of the effect of the flow rate (0−2 mL min−1 in Figure 2 and 0−6 mL min−1 in Figure S-2) at a constant scan rate of 0.1 V s−1 on the reversible oxidation−reduction peaks. In both experiments the potential was cycled between −0.2 and 0.5 V vs a Ag/AgCl wire. Each experiment was repeated at least 3 times. (B) The analytical response was studied using solutions of 1−5 mM [Fe(II)(CN)6]4− in 0.1 M KCl that was scanned between −0.30 and 0.65 V vs Ag/AgCl at a fast flow rate of 6 mL min−1. Each measurement was repeated more than 3 times, and the oxidation current at 0.6 V was recorded. (C) For a real sample analysis, AA was measured in our system, and the results were compared to a titration method. (I) Standard addition: In agreement with previous reports,34 the oxidation peak at 0.590 V vs Ag/AgCl QRE was chosen as the applied potential for the detection of AA during linear sweep voltammetry (LSV), where the potential was scanned from 0 to 0.6 at 0.1 V s−1 using a flow rate of 0.2 or 0.3 mL min−1. Standard addition analysis was performed by adding five 1 mL aliquots of 10 mM AA in 0.1 M KCl to a 10 mL sample of commercial orange juice (Oasis classic no pulp, Oasis Canada). (II) Redox-titration: Commercial orange juice samples (20 mL) were diluted with 150 mL of water. Following additions of 5 mL of 0.6 M KI, 5 mL of 1 M HCl, and 1 mL of starch 0.5% solution (indicator), the diluted samples were titrated using 2 mM KIO3 until a color change (toward a green color) was observed.35 Electrochemical Luminescence (ECL) Module (ABC 2 Configuration). The ECL response was detected using a Jazz Spectrometer (Ocean Optics) coupled with an Ocean Optics

ρ V ·∇V = −∇(p + η(∇V + ∇V)T ))

(1)

∇·V = 0

(2)

where ρ is the density of water (1.00 g cm−3 under experimental conditions), V is the velocity vector which has components of u, v, and w in the x, y, and z, directions, respectively), p is pressure, η is the dynamic viscosity of water (1.00 mPa s), and T is the matrix transpose operator. Edges of the domain that represent the walls of the system (e.g., the tubules, the o-rings, and the flow system modules) were set as no-slip flow boundaries (eq 3). The symmetry planes were described by eq 4. The inlet and outlet boundaries were defined by eqs 5 and 6, respectively, No slip:

u = 0, v = 0, and w = 0

Symmetry:

Inlet: Outlet:

V=

V·n = 0

Vf πri 2

(3) (4)

·n (5)

p = p0 , η(∇V + (∇V)T )n = 0

(6)

where n is the inward normal vector, Vf is the volume flow rate, and ri is the radius of the inlet, herein, the radius of the flow cell module pipe (3 mm). C

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Figure 3. (A) Fluid velocity within the ceramic electrode, (B) velocity profile taken at the midpoint of the tubules along the plane of symmetry (as indicated in part A).

Figure 4. (A) Effect of the CV scan rate on the reversible oxidation and reduction (between −0.2 and 0.5 V vs Ag/AgCl QRE) of 1 mM FcMeOH in 0.1 M KCl using a Pt Honeycomb electrode as a working electrode and a constant flow of 0.1 mL min−1. (B) The height of reduction (red ●) and oxidation (black ■) peak currents as a function of the square root of the scan rate (ν).

Figure 5. (A) CV of [Fe(II)(CN)6]4− (0−5 mM, 3 repeats of each concentration) oxidation in 0.1 M KCl measured at a flow rate of 2 mL min−1 and 0.1 V s−1 scan rate. (B) The linear correlation between the anodic current at 0.5 V vs Ag/AgCl and the concentration of [Fe(II)(CN)6]4.

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within the ceramic electrode. Inside the tubules, the fluid velocity increases rapidly. At all the experimental velocities, laminar flow is developed rapidly, with all the tubules exhibiting velocity profiles with less than 1% variance. Figure 3B shows the velocity profile taken at the midpoint of the tubules along the planes of symmetry, showing all tubules have fully developed Poiseuille flow. Therefore, numerical simulations show that the hydrodynamics within the device are well-defined and nonturbulent.

RESULTS AND DISCUSSION Numerical Hydrodynamic Simulations. Finite element method simulations were performed to ensure the device would provide laminar flow over the practical fluid velocity range. For all points within the device and all flow rates, a Reynolds number of less than 2000 was obtained, indicating that the device provides laminar flow. As fluid enters it flows through the cell, and as the diameter of the cell decreases the fluid velocity increases. Figure 3A shows the change in velocity D

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Figure 6. (A) Standard addition calibration curve for the measurement of AA concentration in a commercial orange juice and (B) LSV response recorded during the standard addition (0−5 mM of ascorbic acid) experiments at a flow rate of 0.3 mL min−1.

greater than 2 mL min−1, caused by variations in the fluid velocity delivered by the syringe pump. This variation is predominantly due to the incremental turning of the worm gear, which is driven by a stepper motor. Additionally, the use of a Faraday cage around the experiment and localized shielding of the pump was used to minimize the noise from induced currents. This will help reduce the pump noise but not entirely eliminating it since some of the noise is systematic and associated with the pump’s mechanical operation. Plotting the anodic current at 0.5 V vs Ag/AgCl (approximately 300 mV more positive than oxidation peak potential) as a function of the [Fe(II)(CN)6]4− concentration (Figure 5B), exhibits a linear behavior (R2 = 0.99) with a limit of detection (LOD) of 0.1 mM. These results clearly indicate that our system is suitable for analytical purposes. Quantitation of an analyte in a complex matrix is demonstrated through the determination of the concentration of AA in commercial orange juice. As previously mentioned there is a systematic noise that originates from the mechanical operation of the pump. This noise is also proportional to the flow rate, therefore by increasing the flow rate we not only decrease the efficiency of the measurement12 but also increase the overall noise. To obtain a better S/N ratio, a lower flow rate of 0.3 mL min−1 is preferred for quantitative analysis. The 1 mL aliquots of 10 mM AA were added to 10 mL of the commercial orange juice sample. The standard addition method results, using linear scan voltammetry (LSV) at a scan rate of 0.1 V s−1, are presented in Figure 6 (and Figure S-3 for a flow rate of 0.2 mL min−1). These data were analyzed, providing concentrations of 2.98 ± 0.03 mM (at a flow rate of 0.3 mL min−1) and 3.18 ± 0.07 mM (at a flow rate of 0.2 mL min−1), which are in agreement with the concentration (3.1 mM) reported by the manufacturer. The results were crosschecked by redox (iodometric) titration, which yielded a concentration of 3.2 ± 0.1 mM. Interestingly, the AA standard addition method was performed at low flow rates (0.2 and 0.3 mL min−1), which highlights the ability of the flow platform to handle smaller sample volumes, a most desirable characteristic for applications to real world samples (e.g., blood monitoring or diluted samples). Overall, the redox calibration curve and standard addition methods highlight the analytical accuracy of the flowthrough platform for analytical measurements in a complex matrix. Spectroscopic Measurements. The combination of hydrodynamic control, electrochemical measurement, and

Hydrodynamic Stability Measurements. The stability of the hydrodynamics within the cell is exemplified by highly consistent electrochemical measurements while using the ABC1 (Figure 1). Cyclic voltammetry (CV) experiments, where the scan rate and the flow rate through the device are varied, provide consistent and predictable data. Figure 4A presents the effect of varying the CV scan rate on the FcMeOH redox current when using a Pt honeycomb-working electrode and 0.1 mL min−1 fixed flow rate. Both the reduction and oxidation peak currents were plotted as a function of the square root of the scan rate (Figure 4B), showing a clear linear correlation (R2 > 0.999). The anodic and cathodic branch are not symmetrical (Figure 4B) because the convective fluid flow removes material from the electrode surface prior to the cathodic sweep. In Figure 4A, the effect of increasing fluid flow rate of FcMeOH from 0.1 mL min−1 to 2 mL min−1 is investigated. Increasing the flow rate leads to a larger steady state oxidation current because of enhanced mass transport and a transition to a steady state response as the convective contribution becomes dominant (Figure 2A and Figure S-2). Conversely, the reduction peak, at 0.1 V, decreases when the flow rate increases due to the continuous removal of the oxidized FcMeOH+ from the electrode surface. When the oxidation peak was plotted versus the cube root of the flow rate (νf1/3) a linear trend (R2 = 0.988) is observed, which is in complete agreement with the developed theory for channel electrodes.20,21 The data presented is not corrected for the capacitive background current of the working electrode, hence the none zero intercept. Furthermore, as there is no observable ohmic drop in the system at the highest observed current (Figure 5, 850 mA), the system can be used for measurements which require an accurate working electrode potential.5,37 These results confirm that the flow system provides stable hydrodynamics as predicted by the simulation and a reproducible electrochemical response. Analytical Measurements. Analytical method development is readily achieved in the ABC1 configuration (Figure 1) using an Au honeycomb SPE as the working electrode. Two example experiments are presented herein; a concentration calibration curve for [Fe(II)(CN)6]4− (Figure 5) and the quantitation of AA concentration in orange juice by a standard addition method (Figure 6). In Figure 5A, the shape of the voltammograms recorded during [Fe(II)(CN)6]4− oxidation approaches a steady state current, as convection is the significant component of the mass transport (vide supra Figure 2). Fluctuations in the current are also observed for flow rates E

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Figure 7. (A) Reproducibility of the ECL signal (black line) and applied potential (dashed red line) as a function of time and (B) single CV scan that was recorded during the ECL experiment.

Figure 8. (A) Averaged ECL signal that was recorded with the SEC setup using a Pt honeycomb electrode at different flow rates (0.25−2.00 mL min−1) and (B) plot of the ECL signal that was obtained as a function of the flow rate.

the stronger (second) ECL signal is recorded as a function of the flow rate (Figure 8). As the flow rate increases (hence increasing the mass transport) an increase in the ECL signal is observed (Figure S-4). However, this increase is not completely linear, as at high flow rates (>1 mL min−1) a steady intensity value is reached (Figure 8B), likely due to the TPrA concentration limitation. Both of the ECL peaks rely on a reaction series that starts or ends with a TPrA radical species and at a fast flow rate the species that are generated at the electrode are rapidly removed from the surface. Finally, in the SEC-UV−vis configuration (Figure 1, ABC3) coupling between electrochemistry and UV−vis spectroscopy was demonstrated in a two-step experiment involving transition metal complexation reactions (Figure 1). First, the absorbance of the flowing solution containing Fe(II) and KSCN was measured at open circuit potential (OCP). In a second step, the electrode was held at 0.8 V vs Ag/AgCl to oxidize Fe(II) to Fe(III) in the presence of SCN−, thus changing the coordination number of the complex metal center and producing a color change from pale yellow to dark red. The absorbance of the electrogenerated species was measured simultaneously using the UV−vis detector that was positioned downstream of the working electrode. Given the large molar absorption coefficient of [Fe(III)SCN]2+ in the 400−500 nm region (9000−10000 cm−1 M−1),43 a significant increase in the absorbance44 was expected with increasing [Fe(III)SCN]2+ concentration (Figure 9A). The blank corrected results were ploted versus

spectroscopy is important for the investigation of electron transfer processes and redox characteristics of biological systems38 as well as radical reactions.39 In the SEC-ECL configuration (Figure 1, ABC3), the electrochemical and spectroscopic responses were monitored in a typical coreactant ECL experiment (Figure 7) involving 1 mM Rubpy and 0.2 M TPrA in 0.1 M phosphate buffer (pH 7.3) flowing through the system at 1 mL min−1. The potential was cycled 3 times between −1 and 1.5 V vs Ag/AgCl while ECL spectra were recorded every 32 ms, and the integrated intensity between 600 and 650 nm was measured. In Figure 7A, the reproducibility of the ECL responses (black line) during electrochemical cycling (dashed red line) is demonstrated. For each CV cycle, two ECL peaks are observed, the first at 1.0−1.1 V vs Ag/AgCl (smaller) and a second at 1.2−1.3 V vs Ag/AgCl (big). A stable and reproducible ECL signal is obtained due to the continual replacement of solution near the electrode surface. Figure 7B shows the acquired voltammogram that correlates to this scan, where two peaks at 1 and 1.3 V are generated from the ECL process.40−42 The first peak (small) is assigned to Ru(bpy)32+* generated from the reaction between TPrA+• and Ru(bpy)3+. The intensity of this peak depends on the concentration fluxes of TPrA+• and TPrA•.29 The stronger ECL signal is due to the generation of Ru(bpy)32+* from the reaction series that starts with the reaction of Ru(bpy)32+ and TPrA• at the electrode surface.29,40 Furthermore, we conducted another set of experiments where F

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Figure 9. (A) Absorbance spectra of electrogenerated [Fe(III)SCN]2+((aq) and (B) background subtracted absorbance of [Fe(III)SCN]2+(aq) at 470 nm as a function of the Fe(II)(aq) concentration (R2 0.988).

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[Fe(II)SCN]2+ (Figure 9B) and a linear trend (R2 = 0.988) was obtained, thus showing not only the ability of the system to measure the change in the oxidation state but also the significant potential of our platform for spectroelectrochemistry.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b04649. Scheme of the symmetry planes and model used for the simulation, effect of fast flow rates on the obtained CV, calibration curve of AA at 0.2 mL min−1, and an image of the ECL response (PDF) Video of the obtained ECL signal (AVI)

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CONCLUSIONS In this work we describe a flow platform for electrochemical and spectroelectrochemical measurements. The flow cell was designed in a modular configuration, where the simple replacement of one unit will enable the use of the system for different types of measurements, from pure electrochemical to SEC experiments. The electrochemical response and the hydrodynamics of the system are well characterized using both empirical data and finite element method modeling; both methods show that the hydrodynamics within the device are laminar, which is ideal for analytical measurements as well as fundamental SEC investigations. The analytical response of the flow through cell is validated by measuring different concentrations of [Fe(II)(CN)6]4− and AA quantitation in a complex matrix. We demonstrated the significant potential and versatility of the system for SEC measurements by performing advanced spectroscopic measurements such as ECL and UV−vis measurements of electrogenerated species. This easy to use and configure platform demonstrates the advantages of a flow system, such as the increase in the rate of mass transport that affects both the electrochemical and spectroscopic response, can be applied to various types of measurements, ranging from fundamental mechanistic studies to analytical measurements. Furthermore, by constructing a hyphenated system, for example, using Units ABC1 and ABC3, a dual activation process followed by a UV−vis measurement could be envisioned, to sustain combined herbicide and pesticide studies.45−47 Longterm use of this platform also could involve HPLC detection, taking advantage of the built in mass transport assembly and the linear flow pattern, which are already within the required HPLC flow rates (0.2−1 mL min−1).48 Furthermore, the ability to also perform ECL (Unit ABC2) can increase the overall sensitivity of analytical detection.49,50 Finally, the ability to carry out ECL opens several future avenues for the detection of biomolecules and advantages over conventional fluorescence methods include enhanced S/N, increased sensitivity, reduced self-quenching, and simplified equipment.27,28

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone (514) 398-3898. ORCID

Tomer Noyhouzer: 0000-0002-3638-6238 Janine Mauzeroll: 0000-0003-4752-7507 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare the following competing financial interest(s): Based on the results of this study, Patent WO 2016/145539 was submitted.

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ACKNOWLEDGMENTS We thank the NSERC, CFI, CSACS, and CQMF for financial support. We also acknowledge Dr. Samuel Perry for manuscript editing and Jean-Philippe Guay for technical assistance.

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REFERENCES

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DOI: 10.1021/acs.analchem.6b04649 Anal. Chem. XXXX, XXX, XXX−XXX