Renewable Solid Electrodes in Microfluidics: Recovering the

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Renewable solid electrodes in microfluidics: recovering the electrochemical activity without treating the surface Carlos A Teixeira, Gabriela Furlan Giordano, Maisa B Beltrame, Luis Carlos Silveira Vieira, Angelo Luiz Gobbi, and Renato Sousa Lima Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03453 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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Analytical Chemistry

Renewable solid electrodes in microfluidics: recovering the electrochemical activity without treating the surface Carlos A. Teixeira, Gabriela F. Giordano, Maisa B. Beltrame, Luis C. S. Vieira, Angelo L. Gobbi, and Renato S. Lima* Laboratório de Microfabricação, Laboratório Nacional de Nanotecnologia, Centro Nacional de Pesquisa em Energia e Materiais, Campinas, São Paulo 13083-970, Brasil ABSTRACT: The contamination, passivation, or fouling of the detection electrodes is a serious problem undermining the analytical performance of electroanalytical devices. The methods to regenerate the electrochemical activity of the solid electrodes involve mechanical, physical, or chemical surface treatments that usually add operational time, complexity, chemicals, and further instrumental requirements to the analysis. In this paper, we describe for the first time a reproducible method for renewing solid electrodes whenever their morphology or composition are non-specifically changed without any surface treatment. These renewable electrodes are the closest analogue to the mercury drop electrodes. Our approach was applied in microfluidics, where the downsides related to non-specific modifications of the electrode are more critical. The renewal consisted in manually sliding metal-coated microwires across a channel with the sample. For this purpose, the chip was composed of a single piece of polydimethylsiloxane (PDMS) with three parallel channels interconnected to one perpendicular and top channel. The microwires were inserted in each one of the parallel channels acting as working, counter, and pseudoreference electrodes for voltammetry. This assembly allowed the renewal of all the three electrodes by simply pulling the microwires. The absence of any interfaces in the chips and the elastomeric nature of the PDMS allowed us to pull the microwires without the occurrence of leakages for the electrode channels even at harsh flow rates of up to 40.0 mL min-1. We expect this paper can assist the researchers to develop new microfluidic platforms that eliminate any steps of electrode cleaning, representing a powerful alternative for precise and robust analyses to real samples. Electrodes can be used to pump liquids, perform separations, or produce electroanalytical detectors/sensors acting such as sensing and conducting components. The interactions between the electrode surfaces and the samples are essential in all of these cases generating, e.g., high sensitive and selective electroanalytical determinations. Conversely, this interaction can lead to the contamination, passivation, or fouling of the electrodes. Such phenomena are a serious downside in electrochemical analyses because they can undermine the analytical performance by affecting both the peak current and potential. The methods for regenerating the active area of the solid electrodes whenever its morphology or composition are nonspecifically changed include: grinding, mechanical polishing, extrusion, acid washing, chemical oxidation, electrochemical treatment, flame etching, and photodegradation.1-13 To regenerate the electrodes with reproducible surface area is a challenge for these techniques. Such reproducibility is crucial by eliminating further calibration steps in analytical experiments. Furthermore, the foregoing regeneration methods generally add operational time, complexity, chemicals, and instruments to the analysis routine. For instance, the polishing of the electrodes with reproducibility can require the use of O2 plasma.6 With regard to the visible light-induced regeneration,13 the irradiation lasts 2 h and the cleaning efficiency for high molecular weight compounds is poor. The electrochemical activity recovery is especially challenging in microfluidics because of two reasons. First, the electrodes are commonly placed inside irreversibly bonded chips.

Second, such electrodes often rely on films with thicknesses ranging from tens of µm to 250 nm.14 Both of these aspects hinder more vigorous cleaning procedures such as polishing. Reversible chips exhibiting high adhesion strength are a potential alternative in the first case by ensuring access to the electrodes.15,16 Nonetheless, it should be noted that the surface of both the device and the film-based electrode can be damaged by the used cleaning protocol. Electrochemical techniques are especially attractive in microfluidics due to the easy integration into microscale channels, simple operation, fast response, reduced power requirements, and ability for improving figures of merit such as sensitivity, selectivity, separation efficiency, and repeatability. This ability is related to the diverse materials and surface modification methods that can be used for the electrodes.17,18 Taking into account the relevance of the electrochemical detectors in microfluidics and the previously stated drawbacks associated to the contamination, passivation, or fouling of the electrodes (especially when incorporated into microchannels), we describe in this paper the creation of renewable solid electrodes in polydimethylsiloxane (PDMS) devices. The electroactive area of microwire-based electrodes was reproducibly renewed without any surface treatment. Hence, our approach could be considered as the closest analogue to the mercury drop electrode. To our knowledge, this is the first report on describing the deployment of renewable solid-state electrodes assuming the absence of electrode treatment.

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MICROFABRICATION The renewal consisted of manually sliding metal-coated microwires across a perpendicular microchannel containing the sample. For this purpose, the microdevices were composed of a single piece of PDMS with three parallel channels interconnected to one perpendicular and top channel (sample channel). Three metal-coated microwires were mechanically inserted in each one of the parallel channels acting as working, counter, and reference electrodes for voltammetry. This assembly allowed the electrochemical activity renewal of all the three electrodes simply pulling the microwires. The absence of any interfaces in the chips and the elastomeric nature of the PDMS allowed us to pull the microwires without the occurrence of leakage in the electrode channels even at harsh flow rates. The bulky microfluidic chips (without interface) were fabricated by an unusual bondless method that relied on sequential steps of polymerization and scaffold removal (PSR).19 Briefly, a scaffold containing the template of the channel was initially shaped. Afterward, PDMS monomers were poured onto a pool covering the scaffold altogether. Succeeding the cure of the polymer, the scaffold was removed creating a single piece of PDMS with channels. Fig. 1 shows the main fabrication steps, the chip considering the presence of only one channel for sample, and the renewal mode. Initially, one nylon wire was inserted in the holes of an aluminum piece and stretched with the aid of metal connectors. Afterward, silica capillaries were placed in parallel to each other and perpendicularly under the nylon wire. Droplets of a positive resist were inserted on the intersections of the nylon with the silica. Then, the resist was pre-baked acting as a glue. This step was intended to ensure a contact area among nylon and silica by avoiding the filling of this region with PDMS monomers, which were subsequently poured onto the previous piece covering the scaffold altogether. After the cure of the polymer, the scaffold was mechanically withdrawn out from the PDMS network. In this case, a low tension was initially applied on the outside area of the nylon for a few seconds. A gradual detachment between the polymer walls and the scaffold was observed even with naked eye, decreasing the frictional resistance. Therefore, the nylon could be slide out from the device as translational move despite the presence of the glue in the silica-nylon junctions. Next, the resist on capillaries was removed by adding acetone into the sample channel with a wash bottle. Finally, the silica capillaries were removed using the same procedure used for nylon as described above. The microchannels exhibited circular cross section as recently shown by our group.19 The electrodes consisted of stainless steel microwires coated by thin-films of Gold (Au), Silver (Ag), or Nickel (Ni). Such microwires feature enough mechanical rigidity to allow their manual insertion in the electrode channels of the PSR chips as illustrated in Fig. 1. At this point, it is worthwhile to underline the contact area established between nylon and silica before adding PDMS monomers was important by defining the electroactive area of the thin-film electrodes into the microfluidic channel. The electrode renewal was made by manually pulling the microwires across the channel with the sample. Apart from the absence of surface treatment, this renewal can be conducted with reproducibility not only for the working electrode, but also for the reference and counter electrodes.

Figure 1. Fabrication, device, and procedure of electrode renewal. Aluminum piece with the scaffold of nylon wire and silica capillaries onto flat supports of metal and glass (a); pouring of PDMS monomers on the pool (b); final chip composed of a single piece of PDMS after the cure of the polymer and removal of the scaffold (c); device with microwires (electrodes) integrated in parallel (d); and amplified views of the junctions between the channel of the sample and the microwires presenting Au (e) and Ni (f) as working electrodes (in the middle). Inset in (a) exhibits the cross regions of the nylon wire (on the top) with silica capillaries. Herein, the compound in red is the resist that was applied like glue. To improve its visualization in the photos, such resist was added in excess. In practice, this glue was used in reduced quantities. The contacts between nylon and silica were kept before adding PDMS with the aid of scotch tapes as shown in (a). In (d), the microwires acted like pseudoreference, working, and counter electrodes from left to the right. The arrows in (e,f) indicate the renewal direction of the working electrode into the sample channel. The dimensions of the device were 34 mm x 56 mm x 10 mm.

PSR is a straightforward method to fabricate microfluidic devices eliminating the bonding step and conventional techniques for engraving the microchannels. This latter step usually needs clean room facilities and expensive instrumentation required in photolithography or lithography processes.20 Other hurdles include time-consuming operation, raise in channel’s roughness,21 poor aspect ratio,22 and use of aggressive chemicals.23 In terms of the bonding, such step normally leads to a

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substantial increase in the operational time, cost, and complexity of the fabrication of the microfluidic chips.24 Herein, further disadvantages are low adhesion strength and harsh conditions (e.g., temperature and O2 plasma) that can damage integrated functional units such as electrodes and anchored molecules.20 Other advantage of the PSR method in relation to the conventional techniques is the compatibility to integrate 3D functional elements.25-30 Apart from the advantages associated to the PSR fabrication, the construction and integration of the electrodes shows positive aspects compared with the conventional methods. The fabrication of thin-film electrodes in microfluidics is usually based on processes of photolithography, vacuum deposition,31,32 microfluidics-assisted molding, electroless deposition,33-35 or microcontact printing.36-38 As advantage, the electrodes herein shown eliminate the use of clean room facilities. Whilst the metallization of the microwires exhibits a high cost, the required instruments allow the film deposition on hundreds of substrate in a single run. Such aspect is ideal for ultra largescale integration. Alternatively, metal microwires could also be used as electrodes in accordance with works that described the application of this assembly in PDMS chips.39-46

EXPERIMENTAL SECTION Chemicals. Sodium hydroxide (NaOH), ethanol, potassium hexacyanoferrate(II) trihydrate (K4[Fe(CN)6].3H2O), potassium chloride (KCl), aniline, nitric acid (HNO3), hydrochloric acid (HCl), and sodium nitrate (NaNO3) were purchased from Merck (Darmstadt, Germany). Monoethylene glycol (MEG), nickel nitrate hexahydrate (Ni(NO3)2.6H2O), cobalt nitrate hexahydrate (Co(NO3)2.6H2O), 1-octadecanethiol, cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O), and acetone were provided by Sigma-Aldrich (St Louis, MO). Sylgard 184 silicone and AZ® 50XT resist were supplied from Dow Corning (Midland, MI) and Microchemicals (Ulm, Baden-Württemberg, Germany), respectively. Deionized water (Milli-Q, Millipore Corp., Bedford, MA) was obtained with resistivity no less than 18 MΩ cm Fabrication. PDMS was prepared by mixing the monomers with curing agent at 10:1 w/w ratio. Then, this mixture was degassed under vacuum for 30 min. In order to cure the resist (AZ® 50XT) that was added in the nylon-silica junctions, such scaffold was placed on laboratory oven (Blue M, Blue Island, IL) at 120 ºC for 10 min. After, the PDMS was dumped onto the aluminum piece and cured in two steps: 55 ºC for 10 min and then 95 ºC for 40 min. When the scaffold was withdrawn, a hard bake of the device was performed at 120 °C for 20 min to complete the crosslinking reactions. The dimensions of the microdevices were 34 mm x 56 mm x 10 mm. The scaffolds of nylon wire and silica capillary had diameters of approximately 800 and 670 µm, respectively. The gap between the electrode channels was 6 mm. Electrodes. The experiments were performed in a potentiostat/galvanostat (AUTOLAB, Metrohm AG PGSTAT302N, Herisau, Switzerland). The electrodes consisted of thin-films deposited on stainless steel microwires (720-µm diameter) by electron beam vapor deposition (Oerlikon Leybold Vacuum, UNIVEX 300, Cologne, Germany). The adhesion of the metal thin-films onto different materials is commonly poor. Alterna-

tives for this case are the substrate oxidation in O2 plasma and the use of adhesion promoters, typically a second metal on the substrate as Titanium (Ti) or Chromium (Cr).20 Herein, Cr (20 nm) was used like adhesion promoter. The counter electrode was composed of Cr/Au films (150 nm) in all of the assays. Cr/Au or Cr/Ag films (200 nm) were used as reference electrodes. In this later case, AgCl was in-situ electrochemically deposited on the Ag thin-film applying a current of 20 µA for 20 min in 0.5 mol L-1 HCl.47 Regarding the working electrode, Cr/Au or Cr/Ni (300 nm) films were employed. Handling the liquids. The solutions inside the PSR devices were replaced with the aid of syringe-pump (New Era Pump Systems Inc., NE-8000, Farmingdale, NY). The syringes were furnished with tygon tubes (1.52-mm i.d.) that were attached to polytetrafluoroethylene luer-lock connectors. These pieces were connected to the channels using stainless steel needles (580-µm i.d.). In all of the tests, the solutions were maintained at steady state during the acquisition of the signals. Renewal. The reproducibility of the renewal was initially investigated by assessing the values of peak current obtained for the Fe(CN)63-/4- redox couple in cyclic voltammetry. The analyses were realized to new surfaces pulling either only the working electrodes or all of the three electrodes. For renewal into the microfluidic channel, the microwires were pulled in the presence of the sample in the channel after a sequence of measurements (n = 3). Apart from the reproducibility tests of the renewal process, the Fe(CN)63-/4- probe was used for calculating the electroactive area of the working electrode through the Randles-Sevcik equation. Working electrodes coated by Au and solutions of 10.0 mmol L-1 Fe(CN)63-/4- in 100.0 mmol L-1 KCl (supporting electrolyte) were utilized in all of these cases. The potential range changed of -0.40 up to +0.40 V. To calculate the electroactive area, ten scan rates (ν) were applied ranging from 0.01 up to 0.10 V s-1. Otherwise, a value of 0.05 V s-1 was used in the reproducibility tests. The foregoing data were obtained using Au-based pseudoreference electrode. In addition, the renewal performance was further tested renewing only the working electrode with Ag/AgCl-coated microwire as reference electrode. This electrode was used only herein. The assays presented next were realized again with Au microwire as pseudoreference electrode. The reproducibility of the renewal herein proposed was also evaluated by cyclic voltammetry analyses to MEG. Ni-based working electrode and MEG solutions prepared in NaOH and Na2SO4 at 100.0 mmol L-1 were used. The potential range was of +0.10 up to +0.65 V with ν of 0.05 V s-1. Analytical curves (2.0 to 10.0 mmol L-1 of MEG) were constructed for each Ni surface. Only the working electrode was renewed. The analytical sensitivity obtained for the diverse curves were used for evaluating the renewal performance. The experiments for each concentration of MEG were performed with n = 5. Concerning the working electrode applied to analyze MEG, the Ni surface was in-situ modified with Ni(OH)2 nanoparticles before constructing the analytical curve for each renewed electrode. Such modification was based on the electrochemical deposition of hydroxides of Ni, Co, and Cd onto the surface of Ni thin-film in agreement with the literature.48 Briefly, a cathodic current density of -1.27 mA cm-2 was applied during 20 s in a solution with 10.0 mmol L-1 of Ni, Co, and Cd nitrates in the ratios: 77, 20, and 3% mol/mol, respectively. This solution

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was prepared in 20.0 mmol L-1 NaNO3. When the deposition was ended, the electrode inside the microchannel was exposed to 100.0 mmol L-1 NaOH medium for 15 min. After, 30 cycles of cyclic voltammetry were made in this solution of NaOH at the potential range of +0.10 to +0.65 V and ν of 0.05 V s-1. In relation to the analyses to MEG standards, the currents related to the oxidation of this glycol at 0.60 V were subtracted from the charging and electrolyte currents (analytical signals). Finally, we further assessed the renewal pulling three Aucoated microwires after an in-situ passivation of the electrodes with 50.0 mmol L-1 1-octadecanethiol in ethanol for 1 h. The peak currents recorded for 10.0 mmol L-1 Fe(CN)63-/4- in 100.0 mmol L-1 KCl were used again to study the renewal reproducibility. The experimental conditions were the same in relation to those described above to the Au working electrodes. In all of the situations shown in this paper, the confidence intervals were calculated for α = 0.05 and 95% confidence. Microscopy. Top views of the channels were achieved by stereoscopy (Leica M125, Wetzlar, Germany). Additionally, images of the modified Ni microwires were collected by fieldemission gun scanning electron microscopy (SEM-FEG, FEI Inspect F50, Hillsboro, OR).

RESULTS AND DISCUSSION Images of the PSR channels focusing on the junction between the sample and electrode channels are shown in Fig. 2. On basis of such top views, the PDMS’s walls showed welldefined edges and also low roughness. This good definition is imparted by the soft surface of the nylon and silica scaffolds. The PSR channels had satisfactory repeatability in accordance with the confidence intervals and relative standard deviations (RSD) that were calculated to the diameters. Their values were recorded at diverse points by stereoscopy. The global averages were 795 ± 6 µm (sample channel, Sc) and 671 ± 3 µm (elec-

Figure 2. Microchannels. Top views of the electrode channel (Ec) integrating microwire and sample channel (Sc) without fluid (a) and with dye at 1.0 (b), 20.0 (c), and 45.0 mL min-1 (d). The fluid leaked for Ec in (d) as indicated by the vertical arrows. Sc deformed to 820 and 838 µm at 20.0 and 40.0 mL min-1, respectively. The horizontal arrows indicate the flow direction in Sc.

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trode channel, Ec) to four chips (n = 40). The RSD as a function of the diameter for the same device (intra-chip precision) showed averages of 1.0% (Sc) and 0.6% (Ec) (n = 10). Furthermore, these values were 1.2% and 0.9%, respectively, for the different microchips (inter-chip precision) (n = 40). The microwire/channel ratio in diameter should meet two requirements. First, this fraction should be sufficiently low for decreasing the frictional resistance between PDMS and electrode. Under ideal conditions, the renewal of the microwire by simply pulling it would be possible. Second, such ratio should be high to avoid leakage of the fluids from the sample channel to the electrode channels. The diameters of the film-coated microwires and of the channels obtained by the silica scaffold were approximately 720 and 670 µm, respectively. The resulting ratio of 1.1 was great ensuring an easy manual sliding of the microwires and avoiding leakages even at harsh flow rates of concentrated aniline dye of up to 40.0 mL min-1. The sample channel elastically deformed to 838 µm at 45.0 mL min-1 as shown in Fig. 2. This condition lead to the leakage of fluid to the electrode channel. The data obtained up to 40.0 mL min1 are striking for the application of high flow rates in microfluidics. In general, the conventional bonding methods withstand values of up to units of mL min-1 only.20 Different assays depend upon how rapidly homogeneous mixings are created. For instance, the use of harsh flow rates in microfluidics improves the reaction yield in flash chemistry49 and decreases the size and polydispersity of the droplets in emulsification.50 The voltammograms and redox peak currents for Fe(CN)63/4couple recorded after successive renewals of the Au-coated microwires using Au pseudoreference electrode are shown in Fig. 3 (a,b). The separation between the anodic and cathodic peak potentials was roughly 0.06 V, supporting the expected reversible behavior of the redox probe. Additionally, the voltammograms were well defined and consistent with the presence of semi-infinite linear diffusion of the probe. Thus, the Au-coated microwire showed a macroelectrode behavior at the applied scan rate with absence of radial diffusion. The reproducibility of the electrochemical activity recovery was investigated by pulling either only the working electrode or the three electrodes. In both these situations, the responses were reproducibly recovered. The peak currents obtained for the 10 renewals pulling only the working electrode (Fig. 3 (a)) were in agreement on each other according to Student’s t tests at 95% confidence level. The global averages of the peak currents for oxidation and reduction of Fe(CN)63-/4- were 14.9 ± 0.5 µA and 15.2 ± 0.5 µA (n = 30), respectively. The RSDs were 5.0% for oxidation and 4.8% for reduction. When pulling the three electrodes (Fig. 3 (b)), the 10 renewals were reproducible at 95% confidence as well. The global averages of the peak currents for oxidation and reduction of Fe(CN)63-/4-were 13.5 ± 0.5 µA and 13.6 ± 0.5 µA (n = 30), respectively. The RSDs were 6.0% for oxidation and 6.1% for reduction. Finally, the currents for a same surface had a great intra-assay precision. In this regard, the most confidence intervals was lower than 0.1 µA and its maximum value was 0.7 µA. Concerning the use of Ag/AgCl-coated microwire as reference electrode, the voltammograms and redox peak currents for Fe(CN)63-/4- related to successive renewals of the working electrode (Cr/Au film) are shown in Fig. 3 (c). Here, the deviations in current were remarkably decreased. The global aver-

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The deviations in the currents recorded for the successive renewals are likely because of the changes in the electroactive area. The geometric electrode area is determined by the junctions between the channels for sample and electrode, which can be visualized in Fig. 2 (a). Such parameter was fixed in all of the renewed surfaces. Hence, the possible variations in area were related either to the non-uniform deposition of the films or to the uneven surface of the stainless steel microwires. Such aspects damage the renewal reproducibility by changing the electroactive area. This parameter was estimated to be 2.6 10-4 cm2 (Supporting Information) to one Au electrode surface into the sample channel. In relation to the Au-based pseudoreference electrode, the use of the Ag/AgCl as reference electrode decreased the deviations in the peak current for the successive renewals. We suppose such data is related to the variation of the electrical double layer capacitance of the working electrode (C Cd) with the cell potential (E E), affecting the charging current (ii) and then the peak current. In capacitors, the capacitance is not a function of the potential. Thus, the relationship between charge (q q) and potential is linear where the capacitance (slope) remains constant. Otherwise, such relationship is not linear in the electrode/solution interfaces because Cd changes with E. Mathematically:51

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(1)

with: σS = –σM

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where and are the total excess charge densities on double layer and metal, respectively. For a voltage ramp, Cd is a function of the charging current of an electrochemical cell as follows: σS

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Figure 3. Renewal reproducibility using Au film-coated microwire as working electrode. Peak currents and cyclic voltammograms for Fe(CN)63-/4- couple recorded after successive renewals of the working electrode only (a,c) and of the three electrodes (b). In (a,b), Au-coated microwire was used as pseudoreference electrode. In (c), we utilized Ag/AgCl microwire as reference electrode. The areas in red and blue represent the limits calculated by the Student’s t test. Experimental conditions as noted in the text. Ip, peak current and I, current.

ages of the peak currents for Fe(CN)63-/4- oxidation and reduction were 11.1 ± 0.2 µA and 10.9 ± 0.2 µA (n = 18), respectively. The RSDs were only 2.1% for oxidation and 1.8% for reduction. The confidence intervals ranged from 0.1 to 0.3 µA.

where ν is the scan rate as defined above and RS is the solution resistance. In this regard, a low stability of the reference electrode leads to alterations in Cd and i from non-systematic variations in the cell potential. In conclusion, the improvement in the renewal reproducibility by the use of Ag/AgCl as reference electrode was likely because a lower change in the cell charging current than the variation observed for the Au-based pseudoreference electrode. The renewal performance was also studied through different analytical curves that were obtained to four surfaces utilizing modified Ni film as working electrode. Its pulling for renewal was realized after the construction of an entire curve to MEG standards. The cyclic voltammograms are shown in Fig. 4 (a). Colorless Ni(OH)2 nanoparticles coated on working electrode are reversibly oxidized in alkaline solution to black NiOOH (it causes the electrocatalysis) in accordance to:52-54 Ni(OH)2 (colorless solid) + OH- → NiOOH (black solid) + H2O + e-

(4)

The change in color for Ni(OH)2 and NiOOH inside the microfluidic channel is shown in Fig. 4 (b). The anodic and cathodic peaks at 0.44 and 0.36 V, respectively, are attributed to the Ni(II)/Ni(III) couple.55 The oxidation of MEG occurred at 0.60 V.48 Besides, the NiOOH contents were remained constant in all of the experiments (anodic currents at 0.44 V). This result indicates the oxidation is essentially controlled by the electron

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Potential (V vs. Au) Figure 4. Data for MEG using modified Ni-coated microwire as working electrode. Voltammograms to MEG standards (a), photos of the Ni electrode into the sample channel (b), and SEM-FEG images of the non-modified (top) and modified (bottom) Ni surface (c). In (b), 1-2 and 3-4 are related to the gradual formation of black NiOOH (anodic run) and colorless Ni(OH)2 (cathodic run), respectively. Experimental conditions as noted in the text.

30

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CMEG (mmol L-1) Figure 5. Renewal reproducibility using microwire coated by modified Ni thin-film like working electrode. Analytical curves obtained to four electrochemical surfaces renewing only the working electrode. Inset: curve constructed from the global averages of the signals of all the renewals. Experimental conditions as noted in the text. Ip, peak current.

transfer from organic molecules (org; MEG in this case) that are adsorbed on the oxide film of the electrode as illustrated in equation:56 NiOOH + org → NiOOH(org) → NiOOH + products + H+ + e-

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(5)

Fig. 4 (c) shows microscopies of Ni-coated microwires without and with nanoparticles of Ni(OH)2. These nanostructures had an average diameter of 125 ± 8 nm (n = 15). The analytical curves obtained for MEG are depicted in Fig. 5. The renewal processes showed a satisfactory reproducibility again. Considering the four curves, the analytical sensibilities exhibited a global average of 3.8 ± 0.1 µA mmol-1 L (n = 4) with a RSD of 3.5%. The analyte contents obtained by the four renewals pulling only the working electrode would be reproducible according to Student’s t tests at 95% confidence level. For a response of 24 µA, e.g., the MEG concentrations determined by the curves would show an average of 6.3 ± 0.2 mmol L-1 (n = 4). In terms of the RSD, its value would be only 3.1%.

Figure 6. Renewal reproducibility after the electrode passivation with thiol using Au-coated microwire as working electrode. Peak currents for Fe(CN)63-/4- obtained to new surfaces after sequential steps of passivation and renewal. Inset: the voltammograms in red and gray were recorded for the same surfaces before and after exposing the electrodes to thiol, respectively. Blue voltammogram (similar to the red) was obtained after the subsequent passivation and renewal of the electrodes. The areas in red and blue represent the limits calculated through the Student’s t test. Experimental conditions as noted in the text. Ip, peak current and I, current. With regard to the evaluation of the renewal after passivating the electrodes, the voltammograms and currents to successive contaminations and renewals of the three Au microwires are shown in Fig. 6. The peak currents to Fe(CN)63-/4- after the thiol passivation were reduced in roughly 80% compared with the initial currents. This data was because the increase in resistance to electron transfers. Otherwise, the electrochemical activity was effectively and reproducibly recovered by pulling all of the three electrode according to Student’s t tests at confidence level of 95%. The global averages of the peak currents were 17.2 ± 0.5 µA (oxidation) and 17.9 ± 0.4 µA (reduction) (n = 12). The RSDs were 2.8% for oxidation and 2.1% for reduction. In terms of the intra-assay precision, the confidence intervals to a same surface changed of 0.2 to 0.6 µA only.

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CONCLUSIONS

ASSOCIATED CONTENT

Renewable solid electrodes are, to the best of our knowledge, described for the first time assuming the absence of any surface treatment. This process offers reproducibly recovering the electrochemical activity of the working, reference, and counter electrodes. The renewal relied on simply pulling metal filmcoated microwires across a top channel containing the sample. The electrochemical activity regeneration herein shown eliminates the necessity for constructing new analytical curves in order to achieve reproducible results of analyte concentration. Furthermore, the absence of any interfaces in the chips and the elastomeric nature of the PDMS allowed us to easily pull the microwires without leakage in the electrode channels even at a flow rate of 40 mL min-1. The construction and integration of the electrodes present positive aspects compared with the conventional techniques. Alternatively, metal microwires could be used as electrodes. In addition, the fabrication of the devices is a simple method to get microfluidic platforms, eliminating the bonding and conventional steps to engrave the channels. Nonetheless, the manual step applied to shape the scaffold (related to the microfluidic channel) exhibits crucial restrictions for complex, discontinuous, and 3D structures. In these cases, the channels could be shaped by hot bending, soldering, 3D impression, or direct writing from different scaffolds as shown in literature.19,25-30 Other important aspect of the approach concerns the maximum number of renewals for a single electrode. Considering the low diameter of the microfluidic channels, this parameter depends especially on the length of the microwire. In this paper, e.g., the microwires exceeding the sample channel (length available for the electrochemical regenerations) had approximately 50 mm in length. These electrodes allowed us to make from 10 to 15 renewals with reproducibility. The employment of longest microwires is a simple output to enhance the maximum number of renewals for a same electrode. In terms of the substrate, PDMS shows essential advantages for different analyses.57 This polymer is non-toxic, transparent down to 300 nm, and commercially supplied at accessible prices (~$170 per Kg). The PDMS still displays great elasticity, thus exhibiting ability for integrating functional components such as valves and pumps. Such aspect effectively contributes for the development of micro total analysis and microelectromechanical systems. In addition, this polymer has outstanding gas permeability, a necessary property to keep cells and bacteria alive for long time into the chip. Thereby, PDMS is ideal for assays in biology. Conversely, this elastomer presents restrictions for organic solvent-based tests because of the phenomenon of swelling.58 One potential alternative to attain resistance against organic media is the in-situ modification of the PDMS channel walls with perfluorinated ether.59 This article raises questions that encourage new scientific investigations, such as more detailed studies on the effect of the renewal-associated friction over the analytical performance of chemical sensors and biosensors, when molecules would be anchored onto the microwire surface. Other research concerns the fabrication of e-tongue platforms incorporating microwires coated by different metals. In this case, we expect an increase in the capacity to recognize patterns in multivariate analyses without performing surface chemical modifications.

Supporting Information Graphics the calculation of the electrode area, Figure S1 (PDF).

Notes The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author * Microfabrication Laboratory, Brazilian Nanotechnology Nation-

al Laboratory, Brazilian Center for Research in Energy and Materials, Campinas, Brazil. E-mail: [email protected].

ACKNOWLEDGMENT Financial support for this project was provided by Petrobras (Grant Nr. 2015/00301-6). Rafael Defavari from Brazilian Center for Research in Energy and Materials is thanked for taking the photos.

REFERENCES (1)

(2)

(3) (4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14) (15) (16)

(17)

(18)

(19)

Thornton, D. C.; Corby, K. T.; Spendel, V. A.; Jordan, J.; RobbatJr, A.; Rutstrom, D. J.; Gross, M.; Ritzler, G. Anal. Chem. 1985, 57, 150–155. Johnson, D. C.; LaCourse, W. R. Anal. Chem. 1990, 62, 589A–597A. Wang, J.; Kawde, A.; Sahlin, E. Analyst 2000, 125, 5–7. Strand, A. M.; Venton, B. J. Anal. Chem. 2008, 80, 3708– 3715. Takmakov, P.; Zachek, M. K.; Keithley, R. B.; Walsh, P. L.; Donley, C.; McCarty, G. S.; Wightman, R. M. Anal. Chem. 2010, 82, 2020–2028. Garrett, D. J.; Brooksby P. A.; Rawson, F. J.; Baronian, K. H. R.; Downard, A. J. Anal. Chem. 2011, 83, 8347–8351. Kiran, R.; Scorsone, E.; Mailley, P.; Bergonzo, P. Anal. Chem. 2012, 84, 10207–10213. Park, J.; Kim, M.; Kim, S. Sensor. Actuator. B Chem. 2014, 204, 197–202. Xu, J. Q.; Liu, Y. L.; Wang, Q.; Duo, H. H.; Zhang, X. W.; Li, Y. T.; Huang, W. H. Angew. Chem. Int. Ed. 2015, 54, 14402–14406. Baś, B.; Wegiel, K.; Jedlińska, K. Electrochim. Acta 2015, 178, 665–672. Temerk, Y. M.; Ibrahim, H. S. M.; Schuhmann, W. Electroanalysis 2016, 28, 372–379. Xu, J. Q.; Duo, H. H.; Zhang, Y. G.; Zhang, X. W.; Fang, W.; Liu, Y. L.; Shen, A. G.; Hu, J. M.; Huang, W. H. Anal. Chem. 2016, 88, 3789–3795. Duo, H. H.; Xu, J. Q.; Liu, Y. L.; Jin, Z. H.; Hu, X. B.; Huang, W. H. J. Electroanal. Chem. 2016, 10.1016/j.jelechem.2016.06.046. Xu, X.; Zhang, S.; Chen, H.; Kong, J. Talanta 2009, 80, 8–18. Wasay, A.; Sameoto, D. Lab Chip 2015, 15, 2749–2753. Shiroma, L. S.; Piazzetta, M. H. O.; Duarte-Junior, G. F.; Coltro, W. K. T.; Carrilho, E.; Gobbi, A.; Lima, R. S. Sci. Rep. 2016, 6, 26032 (12pp). Nge, P. N.; Rogers, C. I.; Woolley, A. T. Chem. Rev. 2013, 113, 2550–2583. Patabadige, D. E. W.; Jia, S.; Sibbitts, J.; Sadeghi, J.; Sellens, K.; Culbertson, C. T. Anal. Chem. 2016, 88, 320–338. Camargo, C. L.; Shiroma, L. S.; Giordano, G. F.; Gobbi, A. L.; Vieira, L. C. S.; Lima, R. S. Anal. Chim. Acta 2016, 940, 73–83.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(20)

(21)

(22)

(23)

(24)

(25)

(26)

(27) (28)

(29)

(30)

(31)

(32)

(33)

(34) (35)

(36)

(37)

(38)

(39) (40)

(41)

(42)

(43) (44)

(45)

(46)

(47)

(48)

(49)

(50)

Lima, R. S.; Leão, P. A. G. C.; Piazzetta, M. H. O.; Monteiro, A. M.; Shiroma, L. Y.; Gobbi, A.; Carrilho, E. Sci. Rep. 2015, 5, 13276 (15pp). Klank, H.; Kutter, J. P.; Geschke, O. Lab Chip 2002, 2, 242– 246. Mu, X. Liang, Q.; Hu, P.; Ren, K.; Wang, Y.; Luo, G. Lab Chip 2009, 9, 1994–1996. Zacheo, A. Zizzari, A.; Perrone, E.; Carbone, L.; Giancane, G.; Valli, L.; Rinaldi, R.; Arima, V. Lab Chip 2015, 15, 2395– 2399. Ren, K.; Zhou, J.; Wu, H. Acc. Chem. Res. 2013, 46, 2396– 2406. Verma, M. K. S.; Majumder, A.; Ghatak, A. Langmuir 2006, 22, 10291–10295. Song, S. H.; Lee, C. K.; Kim, T. J.; Shin, I. C.; Jun, S. C.; Jung, H. I. Microfluid. Nanofluid. 2010, 9, 533–540. Lee, J.; Paek, J.; Kim, J. Lab Chip 2012, 12, 2638–2642. Saggiomo, V.; Velders, A. H. Adv. Sci. 2015, 2, 1500125 (5pp). Hwang, R.; Paydar, O. H.; Candler, R. N. Sensor. Actuat. APhys. 2015, 226, 137–142. Parekh, D. P.; Ladd, C.; Panich, L.; Moussa, K.; Dickey, M. D. Lab Chip 2016, 16, 1812–1820. Kadowaki, M.; Yoshizawa, H.; Mori, S.; Suzuki, M. Thin Solid Films 2006, 506, 123–127. Lima, R. S.; Piazzetta, M. H. O.; Gobbi, A.; Segato, T. P.; Cabral, M. F.; Machado, S A. S.; Carrilho, E. Chem. Comm. 2013, 49, 11382–11384. Kenis, P. J. A.; Ismagilov, R. F.; Whitesides, G. M. Science 1999, 285, 83–85. Gao, Y. X.; Chen, I. W. Lab Chip 2008, 8, 1695–1699. Yan, J. L.; Du, Y.; Liu, J. F.; Cao, W. D.; Sun, S. H.; Zhou, W. H.; Yang, X. R.; Wang, E. K. Anal. Chem. 2003, 75, 5406–5412. Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654–7655. Atmaja, B.; Frommer, J.; Scott, J. C. Langmuir 2006, 22, 4734–4740. Lim, S. S.; Chang, W. J.; Koo, Y. M.; Bashir, R. Lab Chip 2006, 6, 578–580. Garcia, C. D.; Henry, C. S. Anal. Chem. 2003, 75, 4778–4783. Liu, Y.; Vickers, J. A.; Henry, C. S. Anal. Chem. 2004, 76, 1513–1517. Garcia, C. D.; Dressen, B. M.; Henderson, A.; Henry, C. S. Electrophoresis 2005, 26, 703–709. Vickers, J. A.; Henry, C. S. Electrophoresis 2005, 26, 4641– 4647. Ding, Y. S.; Garcia, C. D. Analyst 2006, 131, 208–214. Ding, Y. S.; Ayon, A.; Garcia, C. D. Anal. Chim. Acta 2007, 584, 244–251. Vickers, J. A.; Dressen, B. M.; Weston, M. C.; Boonsong, K.; Chailapakul, O.; Cropek, D. M.; Henry, C. S. Electrophoresis 2007, 28, 1123–1129. Holcomb, R. E.; Kraly, J. R.; Henry, C. S. Analyst 2009, 134, 486–492. Pedrotti, J. J.; Angnes, L.; Gutz, I. G. R. Electroanal. 1996, 8, 673–675. Giordano, G. F.; Vieira, L. C. S.; Gobbi, A. L.; Lima, R. S.; Kubota, L. T. Anal. Chim. Acta 2015, 875, 33–40. Ren, W.; Kim, H.; Lee, H. J.; Wang, J.; Wang, H.; Kim, D. P. Lab Chip 2014, 14, 4263–4269. You, J. B.; Kang, K.; Tran T. T.; Park, H.; Hwang, W. R.; Kim, J. M.; Im, S. G. Lab Chip 2015, 15, 1727–1735.

(51)

(52)

(53)

(54)

(55)

(56)

(57)

(58)

(59)

Page 8 of 9

Bard, A. L.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 2nd edition, 2001; Chapter 1, pp 11−18; Chapter 13, pp 540−554. Fleischmann, M.; Korinek, K.; Pletcher, D. J. Chem. Soc. Perk. T. 2 1972, 10, 1396–1403. Corrigan, D. E.; Bendert, R. M. J. Electrochem. Soc. 1989, 136, 723–728. Jia, L-P; Wang, H–S. Sensor. Actuat. B-Chem. 2013, 177, 1035–1042. Liu, Y.; Zhang, L.; Guo, Q.; Hou, H.; You, T. Anal. Chim. Acta 2010, 663, 153–157. Vértes, G.; Horányi, G. J. Electroanal. Chem. Interfacial Electrochem. 1974, 52, 47–53. Qin, D.; Xia, Y.; Whitesides, G. M. Nat. Protoc. 2010, 5, 491–502. Lee, J. N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 6544–6554. Thanawala, S. K.; Chaudhury, M. K. Langmuir 2000, 16, 1256–1260.

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Renewable Solid Electrodes in Microfluidics: Recovering the

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