Direct Drawing Method of Graphite onto Paper for High Performance

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Direct Drawing Method of Graphite onto Paper for High Performance Flexible Electrochemical Sensors Murilo Santhiago, Mathias Strauss, Mariane Peres Pereira, Andréia S. Chagas, and Carlos César Bof Bufon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15646 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

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ACS Applied Materials & Interfaces

Direct Drawing Method of Graphite onto Paper for High Performance Flexible Electrochemical Sensors Murilo Santhiago†, Mathias Strauss†, Mariane P. Pereira†, Andréia S. Chagas†, Carlos C.B. Bufon†,‡,§* †

Dr. Murilo Santhiago, Dr. Mathias Strauss, Mariane P. Pereira, Andréia S. Chagas, Dr. Carlos

C.B. Bufon Brazilian Nanotechnology National Laboratory (LNNano), CNPEM, 13083-970, Campinas, Brazil. ‡

Dr. Carlos C. B. Bufon

Institute of Chemistry (IQ), UNICAMP, 13083-970, Campinas, Brazil. §

Dr. Carlos C. B. Bufon

Institute of Physics "Gleb Wataghin" (IFGW), UNICAMP, 13083-859, Campinas, Brazil.

Keywords: paper-based devices, carbon electrodes, direct transfer method, nano-debris, electrocatalytic detection

Abstract: A simple and fast fabrication method to create high performance pencil-drawn electrochemical sensors is reported for the first time. The sluggish electron transfer observed on bare pencil-drawn surfaces was enhanced using two electrochemical steps: first oxidizing the surface and then reducing it in a subsequent step. The heterogeneous rate constant was found to be 5.1×10-3 cm s-1, which is the highest value reported so far for pencil-drawn surfaces. We have mapped the origin of such performance by atomic force microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Our results suggest that the oxidation process leads to chemical and structural transformations on the electrode surface. As a proof-of-concept, we have modified the pencil-drawn surface with Meldola’s blue to electrocatalytic detect nicotinamide 1 ACS Paragon Plus Environment


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adenine dinucleotide (NADH). The electrochemical device exhibited the highest catalytic constant (1.7×105 L mol-1 s- 1) and the lowest detection potential for NADH reported so far in paper-based electrodes.

1. Introduction Paper-based electronic and electrochemical devices have been extensively studied in the last years. Practical applications including transistors,1 capacitors,2 batteries,3 touch buttons,4 solar cells,5 energy harvesting systems6 and electrochemical sensors7–9 have been successfully developed. A common point shared by the aforementioned applications is the demand for conductive materials on paper.10–13 In such context, carbon-based conductive layers are very attractive for the construction of a variety of hybrid devices including low-cost electrochemical sensors and biosensors with broad operation window.7,8,14 Consequently, carbon-based electrodes are highly suitable to investigate the electrochemical properties of different compounds within a wide range of potentials. Recently, the increasing demand for simpler and faster patterning routes has driven the field toward printing and drawing techniques.15,16 Screen-printing is one of the most common routes to prepare carbon conductive current pathways on paper.17,18 For transfer/printing purposes, the carbon structures are dispersed in liquids and parameters such as viscosity, surface tension and density of the ink needs to be carefully evaluated before deposition.19 The dispersion of carbon nanotubes and graphene can be achieved by adding polymers to avoid aggregation. Graphite, for instance, can be oxidized by chemical routes to form stable dispersions of graphene oxide in water. The later process is a type of chemical exfoliation process, where graphite sheets are separated from the starting material and remains stable due to electrostatic repulsion.20 This route have been employed by many authors to fabricate carbon-based devices. However, chemical exfoliation and purification 2 ACS Paragon Plus Environment

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processes are laborious while solvent-free approaches, such as direct mechanical transfer of graphite/graphene layers to flexible substrates, may represent a significant step towards straightforward transfer methods. Pencil-drawing is by far the simplest and easy way to transfer/exfoliate carbon films to paper.21 Particularly, pencils are portable, cheap and can create a variety of patterns in large areas. Additionally, carbon tracks can be fabricated directly from commercial pencils on worldwide available printing and office papers. Those papers, typically, have a surface roughness around 5 µm, leading to an appropriate condition to exfoliate graphite particles from pencil.21,22 As the drawing process is repeated or cycled, conductive carbon films are obtained on the top of the substrate. In addition to strain23 and gas sensors,24 some electrochemical devices22,25–28 have been constructed using the pencil drawing technique. While strain and gas sensors exhibits excellent performance on paper, the electrochemical applications for sensing redox species are still at the early stage and very limited. The main reasons for such limitations are not well understood so far, even knowing that surface effects may play a substantial role on that. For instance, we have observed that pencil-drawn electrodes exhibits a sluggish heterogeneous electron transfer to inner-sphere redox probes. Thus, highly active carbon surfaces on flexible substrates, fabricated by simple routes are immensely desirable. Herein, we report a simple and straightforward transfer of graphite onto paper followed by an electrochemical treatment which results in a carbon-based platform for sensing applications with unprecedented performance. The mechanical exfoliation method guarantees the direct transfer of conductive carbon films to paper substrates, while

an in situ electrochemical

treatment ensures the outstanding performance of the devices by improving the heterogeneous electron transfer process. Here, we have combined fast anodic and cathodic electrochemical processes to achieve the optimum performance. Our results indicate that the improvement in the 3 ACS Paragon Plus Environment


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electrochemical response may be due to removal of non-carbon materials present in pencil formulation combined with structural changes on the graphite surface. Chemical and structural surface changes were mainly tracked by atomic force microscopy, Raman and X-ray photoelectron spectroscopies, as well as by electrochemical experiments. Furthermore, the treated surfaces modified by Meldola’s blue, have shown one of the highest electrocatalytic constants towards nicotinamide adenine diclucleotide (NADH). It is worth to mention that NADH is involved in more than 300 biological processes, supporting the broadness and relevance of our results.

2. Experimental 2.1 Materials and chemicals All chemicals were analytical graded. Conventional office paper was acquired from Gimba, SP, Brazil. Potassium chloride (99%), sodium phosphate dibasic (99%), sodium phosphate monobasic (99%), potassium ferricyanide (99%), potassium ferrocyanide trihydrate (99%) and nicotinamide adenine dinucleotide (NADH – 97%) were acquired from Sigma-Aldrich, SP, Brazil. Sodium hydroxide (99%) was acquired from Synth, SP, Brazil. Graphite pencil (hardness 4B) was obtained from General´s Pencil Company, NJ, USA. Cartridge-free ColorQube ink (108R00940) from Xerox was used to print the wax patterns on paper. Biopsy punches of 1.4 and 6 mm in diameter from Kolplast were used to pattern tapes and paper. Single and double-sided tape from 3M were used to delimitate the area of the electrodes. Sodium hypochlorite (2.5 % (w:v)) was acquired from Bufalo, SP, Brazil. Meldola´s Blue was acquired from Acros Organics, NJ, USA.

2.2 Construction of paper-based electrodes 4 ACS Paragon Plus Environment

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Conventional office paper was selected for the fabrication of all devices. Black patterns of (20.5 x 27 mm) were printed on paper using a Xerox ColorQube 8570 printer. Then, the wax patterned paper received a thermal treatment on a hot plate from Tecnal (120 °C during 120 s). After this step, the paper was allowed to cool at room temperature for 5 minutes before the construction of the electrodes. The reference and counter electrodes were fabricated on the same wax-treated paper. In one side, 150 nm of gold was deposited using a thermal deposition system (AJA International, MA, USA). A gold electrode was used as counter electrode. On the back face of the gold patterned paper we have used silver ink (SPI supplies, PA, USA) to fabricate the reference electrode. The detailed fabrication process is illustrated in Figure S1. The working electrode was fabricated by drawing the structure with the pencil (hardness 4B) directly on the surface of the wax-treated paper. Silver ink was used to pattern a conductive track on paper. Next, a piece of single sided tape was punched (1 mm in diameter) and attached to the electrode. The working (WE) and reference/counter electrodes (RE/CE) were assembled together using doublesided tape. We have used Ag/AgCl and a gold layer on paper in order to precisely track the electrochemical properties on the graphite film surface after each electrochemical treatment. These electrodes have been intensively studied by us and others, and their electrochemical properties are well stablished.8,14,29–31

2.3 Electrochemical treatment For the electrochemical treatment we have used the pencil-drawn surface as working electrode, a saturated calomel electrode (SCE) as reference and a platinum wire as counter electrode. The working electrode was positioned horizontally while the SCE and Pt wire were positioned about 2 mm from the WE active area. Next, a 0.1 M H2PO4-/HPO42- pH 7.0 solution was dropped at the active area in such way that all three electrodes were immersed. This 5 ACS Paragon Plus Environment


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procedure was used for both oxidation and reduction processes. The oxidation of the pencildrawn electrode was done by applying +1.8 V vs. SCE during 180 s. For the reduction process, we have scanned the potential in the cathodic direction at 50 mV s-1 from 0 V to -1.5 V. A single cycle was used in this step. The conventional reference (SCE) and counter electrodes (Pt wire) were just used for the electrochemical treatment step. The cyclic voltammograms and chronoamperograms were obtained using the previously described three-dimensional (3D) paperbased electrochemical cell. The electrochemical measurements were done using a PGSTAT-204 potentiostat model, from AUTOLAB (Eco Chemie, Netherlands), interfaced with a computer and controlled by the NOVA 1.11 software.

2.4 Meldola´s blue immobilization The redox mediator (Meldola´s blue) was immobilized on the surface of the working electrode after the oxidation/reduction process. A typical immobilization procedure consisted of adding 100 µL of 50 µM Meldola´s blue solution, prepared by using DI water (18.2 MΩ.cm) from Elga Veolia model Purelab Option-Q, UK, on the working electrode for 15 minutes. After this period, the working electrode was washed with copious amounts of DI water. Next, the working electrode was assembled with the paper-based RE/CE for the electrocatalytic studies. An analytical balance from Shimadzu (model AUW220D) with 5-digits precision was used to weight all the reagents and the values of the pH of all the solutions were determined by an pHmeter from Marconi model MA522.

2.5 Characterization A 3D Laser Scanning Confocal Microscope (LSCM) from Keyence, model VK-X200 series, Osaka, Japan was used to map the surface roughness of the paper-based devices. A 6 ACS Paragon Plus Environment

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Tensiometer from Attension THeTa L, Finland was used to measure the contact angle on the samples. Advancing (CAad) and receding (CArec) contact angles were measured using the sessile drop method. In order to measure the CAad, the liquid was pumped inside the drop. To measure CArec the liquid was pumped out from the drop.32 AFM topography images were obtained using the intermittent contact mode in a FlexAFM Nanosurf microscope with a NCHR NanoWorld silicon tip (resonance frequency = 320 kHz and force constant = 42N/m). The Raman spectroscopy was obtained using a confocal Raman T64000 from Horiba Scientific with 633 nm laser and a 100x objective mounted on an Olympus optical microscope. Raman spectra were acquired for the maps on areas of 15×15 µm, using 1 µm as distance between the reading points. The spectra were depicted and baseline corrections were performed with LapSpec 5 software. Xray photoelectron spectroscopy (XPS) were performed with a Thermo Scientific K-Alpha spectrometer, UK. All spectra were taken using an Al Ka microfocused monochromatized source with a resolution of 0.100 eV, pass energy of 50 eV and a spot size of 400 µm. All the characterizations were performed on freshly prepared samples.

3. Results and Discussion 3.1. Electrochemical characterization Graphite films were prepared on paper by hand drawing procedure. Figure 1(a) shows the fabrication process of the working electrode on wax treated paper. In order to obtain reproducible surfaces, the drawing process was performed using the sheet resistance (Rs) of the working electrodes as a control parameter. In our case, Rs was set to 45 (± 2) Ω sq-1 (Figure S2). Figures 1(b-c) shows the scanning electron microscopy (SEM) images of both the wax treated paper and the graphite film, respectively. In Figure 1(b) we exhibit randomly arranged cellulose fibers 7 ACS Paragon Plus Environment


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coated by a wax layer. Note that the graphite film covers the paper fibers creating a uniform carbon film (Figure 1(c)). Such film has a thickness between 2 – 2.5 µm, as indicated in Figure S3. In order to study the electrochemical properties of these carbon surfaces, we have fabricated paper-based electrochemical cells using a three-electrode configuration. Figure S1 describes the fabrication process of the RE/CE electrodes on paper. In order to attach the WE to RE/CE a paper based spacer was used, as illustrated in Figure 1d. A picture of the flexible 3D paper-based electrochemical cell can be visualized in Figure 1e.

Figure 1. (a) Schematic representation of the fabrication of pencil-drawn electrodes. SEM images of the wax treated paper (b) before and (c) after the pencil drawing procedure. The scale bars are 100 µm. (d) Assembly of the 3D paper-based electrochemical cell. The paper-based spacer is composed by a piece of wax-treated paper containing double sided tape on both sides with a punched hole of 4 mm. (e) Picture of the assembled 3D paper-based electrochemical cell. (f) Representative cyclic voltammograms in the presence of 5 mM Fe(CN)63-/4- in KCl 0.5 M at 30 mV s-1. (g) Cyclic voltammogram obtained in 0.1 M H2PO4-/HPO42- pH 7.0 at 50 mV s-1 after the oxidation process. The arrow indicates the scan starting point. 8 ACS Paragon Plus Environment

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Cyclic voltammograms were obtained in the presence of Fe(CN)63-/4- as redox probe (Figure 1f). The redox couple ferri-ferrocyanide has been used by many authors to investigate carbon-based interfaces due to its capability of providing chemical and structural information.33 The cyclic voltammogram for the bare graphite (G) electrodes shows a very large peak-to-peak separation (∆Ep = 415 mV), suggesting a sluggish electron transfer. Such aspect is one of the main features compromising the use of pencil-drawn electrodes for electrochemical applications. After the in situ electrochemical treatment a substantial enhancement of the performance was observed. As shown in the Figure 1f, ∆Ep decreases to 119 mV indicating that the interfacial redox process was significantly improved. It is important to highlight here that no significant improvement was achieved by simply soaking the electrode in buffer solution. In this experiment, the immersion time was the same one used to perform both the oxidation and reduction processes (Figure S4). During the oxidation step, oxygen bubbles at the interface, due to water electrolysis, are formed (Figure S5). It has been reported that electrochemical oxidation can cause the formation of oxygen-rich functionalities on carbon electrodes.34 In order to support this observation, a reduction peak at ~ - 800 mV is found in the cathodic direction of the cyclic voltammogram (Figure 1g). In addition, we have noticed that the reduction step enhances the electrochemical response by increasing peak currents (G-red), as illustrated in Figure 1f. Electrochemical treatment on carbon materials have been used to prepare reproducible surfaces since 1974.35 We have observed that the response of our paper-based devices follow, at some extend, similar trends when compared to electrochemically treated conventional carbon electrodes.36,37 For instance, preanodization and precathodization resulted in a cathodic shift of the half-wave potentials toward ferrocyanide oxidation for glassy carbon surfaces. Moreover, a large reduction peak was observed when the potential was scanned in the cathodic direction after preanodization of glassy carbon, at potentials higher than 1.5 V vs. SCE.37 It is important to 9 ACS Paragon Plus Environment


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mention that some combinations of electrolytes and potentials should be avoided when thin pencil-drawn films were deposited on paper. It is well know that electrochemical oxidation of carbon using potentials higher than 1.8 V in the presence of concentrated sulfuric acid leads to the formation of a graphite oxide layer.38 Such layer has poor electrical properties and depending on the oxidation time can detach from the electrode’s surface.38,39 In order to investigate the effects of the electrochemical treatment on the electron transfer kinetics, we have performed cyclic voltammograms at different scan rates (Figure S6). It is important to highlight that microcracks are not trapping the electroactive species at the graphite interface. Since the redox process is diffusion controlled, we have used Nicholson´s method40 to calculate the standard heterogeneous rate constant (ks). Here, to the best of our knowledge, the obtained constant of 5.1×10-3 cm s-1 is among the highest values reported so far for fully integrated flexible carbon electrodes on paper (see Table in S6).

3.2 Scanning electron microscopy, laser scanning confocal microscopy and contact angle measurements We systematically investigated the graphite interface after each electrochemical treatment by different characterization techniques. Figure 2a-c displays SEM images for the graphite surfaces under investigation. Figure 2a shows a crack-free surface where in some regions (green arrows) overlapped areas between graphite particles can be noticed. These overlapped areas are expected to be seen on pencil-drawn surfaces due to the layered nature of graphite. For the oxidized surface we have noticed the presence of few microcracks on the electrode surface (red arrows in Figure 2b). The microcracks are probably formed due to the mechanical stress caused by oxygen bubbles forcing the graphite flakes separation. However, the well-defined cyclic voltammogram (Figure 1f) indicates that after electrochemical oxidation the film’s resistance 10 ACS Paragon Plus Environment

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does not increase substantially. Most likely, the microcracks formation occur at the graphite/electrolyte interface and do not affect the bulk of the conductive film (at least under the timescale of our experiments). After the electrochemical reduction process, the microcracks are still present on the electrode’s surface (Figure 2c), indicating irreversible structural changes. High-magnification SEM images of the microcracks can be observed in Figure S7. Figure 1d shows a schematic illustration of the direct observation of oxygen bubbles and microcracks during the electrochemical steps used to improve the redox process. Next, we performed additional characterization experiments by using laser scanning confocal microscopy and contact angle measurements.

Figure 2. SEM images for (a) bare graphite surface (G), (b) oxidized (G-ox) and (c) reduced (Gred). The green and red arrows shows the presence of overlapped graphite particles and microcracks, respectively. All scale bars are 50 µm. (d) Schematic illustration of the direct observation of oxygen bubbles and microcracks during the electrochemical process.

Figure 3a shows the 3D laser scanning images for each step of the fabrication process. We have added the image of the wax-treated surface for comparison (the surface roughness was found 3.2 (±0.5) µm). We have observed that after the drawing process, the roughness of the bare 11 ACS Paragon Plus Environment


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graphite surface decreases to 1.5 (±0.1) µm. During the drawing process, the graphite particles covers both the fibers and the regions between them, as demonstrated in Figure 3a and Figures 1b-c. Significant changes on the graphite surface were noticed after the electrochemical oxidation process. The surface roughness of the G-ox electrode was found 3.1(±0.1) µm. This result is consistent with the SEM images presented in Figure 2. The formation of microcracks and the detachment of weakly adsorbed graphite particles can increase the surface roughness. After the electrochemical reduction process the roughness of the surface was found 2.9(±0.2) µm, which suggests that the impact caused on the surface by the oxidation process is irreversible. In addition, weakly attached carbon particles (micron size) may locally increase the contact resistance, resulting in a resistive behavior for the untreated electrodes. However, after the detachment of such particles under the electrochemical treatment, the G-ox and G-red voltammograms are less resistive (see Figure 1f).

Figure 3. (a) Surface characterization by confocal laser scanning microscopy. (b) Static (CAst), advancing (CAad) and receding (CArec) contact angle measurements for each step of the fabrication process. The scale bars in row (a) are 100 µm long and the red lines represent the regions on the top of a fiber were the linear roughness was measured.

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Static, advancing and receding contact angle measurements reveals changes on the wettability of the surfaces during the fabrication of the electrodes (See S8). Figure 3b shows the static contact angles (CAst) for each step of the construction process. It is important to mention that paper is an anisotropic material with a rough surface.41 In our case, the fibers can be very long (up to 500 µm in length) and display a variable diameter (1−20 µm), as can be view in Figure 1b. The effect of the topography in such type of material have to be considered together with the surface free energy. Wenzel and Cassie models are commonly used to describe wetting properties on rough surfaces, including paper substrates. For the former model, the liquid follows the roughness of the surface, while for the latter, the liquid drop resides on the top of roughness crests. The Cassie model also considers air pockets inside the texture. The two models include a roughness parameter (r) (See equations in S9), where higher contact angles for hydrophobic surfaces area expected when r increases.41–43 In the case of our construction process (Figure 3), the roughness and hydrophobicity are changing simultaneously. Also, microcracks and different oxygenated functional groups are being formed during the electrochemical treatment. Despite these complex surface transformations involving more than one type of material, we have observed some trends after each fabrication step. For instance, after drawing the contact angles (CAst, CAad and CArec) decreases in comparison with wax-treated paper. Next, the oxidation process promotes a second change on the wettability of the surface by decreasing the contact angles (mainly CAst and CAad). Furthermore, the oxidation leads to two major changes: the formation of microcracks and oxygenated surface groups. While the oxygenated groups are reduced by the electrochemical treatment, the microcracks remains after this process. It is worth mentioning that structural changes and microcracks may work as surface microchannels, promoting a better spreading of the water droplet. In fact, after the formation of the microcracks, it is difficult to distinguish 13 ACS Paragon Plus Environment


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between oxidized (G-ox) and reduced (G-red) surfaces by evaluating the contact angles alone. So far, our results clearly indicate that the first electrochemical treatment is the main responsible for the changes on the graphite surface.

3.3 Surface mapping: Atomic force microscopy and Raman spectroscopy Figure 4a-b shows AFM images of a bare graphite film (a) before and (b) after the electrochemical oxidation step. By comparison, some of the weakly adsorbed particles present in (a) are detached from the surface after the oxidation process. Such effect may be related to both the formation of oxygenated functional groups and the oxygen bubbles at the interface. For instance, during the growth of the bubble, the particles may be pushed away from the surface. The particles (Figure 4a) may be both carbon nano-debris and inorganic binders present in pencil formulation. We have observed that aluminum is present on the bare graphite surface (G) but is not detected after the oxidation process (See table in S10). Also, we consider that the oxidation process may promote changes in the type of carbon present on the surface. In order to support this hypothesis, we have mapped the film’s surface by using confocal Raman spectroscopy. Figure 4(c-d) shows the Raman maps of a bare graphite film (c) before and (d) after the electrochemical oxidation step. We have monitored the relative intensity of the D band (ID/IG ratio) on both cases in order to follow the impact of the electrochemical treatment. Figure 4c exhibit well-ordered graphitic carbon structures with low content of defects, where the ID/IG ratio is found low (royal blue regions). The low relative intensity of the D band was previously reported for basal plane highly oriented pyrolytic graphite electrodes.44 Therefore, the low defect density regions are most likely basal-plane oriented particles spread on the film’s surface. The Raman map in Figure 4c shows, in addition, several small islands with high ID/IG ratio (red/orange regions), suggesting the presence of highly defective structures.45,46 It is important to 14 ACS Paragon Plus Environment

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mention that such defective structures are made out of carbon. The Raman spectra shows a typical fingerprint that is unique for carbon-based materials (See S11). We believe that carbon nano-debris may also be detached from the surface by the same way that large carbon particles were removed (Figure 3a). However, further investigation is needed to specifically address this question. Furthermore, these defective structures were not verified on bare pencil but they can be formed during an electrochemical treatment (see Figure S8). After the electrochemical oxidation step (Figure 4d), the surface becomes more homogeneous in terms of the type of carbon structure (ID/IG ~1 predominates). We suggest that the improvement in the electrochemical response may be due to the removal of non-carbon materials present in pencil formulation combined with structural changes on the graphite surface.

Figure 4. AFM images of the (a) bare and (b) oxidized graphite. Raman maps showing ID/IG ratios of (c) bare and (d) oxidized graphite films. Figures (a-d) are 15×15 µm. (e) C1s X-ray 15 ACS Paragon Plus Environment


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photoelectron spectra. (f) Relative abundance of the chemical species after each electrochemical step.

3.4 Chemical composition of the interface The oxidation step causes the formation of oxygen functional groups that were evaluated by X-ray photoelectron spectroscopy (XPS). Figure 4e shows the C1s XPS spectra of the bare and electrochemically treated graphite films. As can be verified in the figure, the electrochemical oxidation enhances the amount of C−O and C=O groups. After the second electrochemical treatment, such oxygen rich groups decreases, suggesting that the large reduction peak observed in Figure 1g is regarded to the electrochemical reduction of these species at the interface. The relative abundance of the functional groups present at the interface can be better visualized in Figure 4f. For instance, the bare graphite film has ~35% of sp3 carbon on its surface. Its origin may be justified by both the presence of organic modifiers in pencil formulation (See S12) and/or highly defective carbon structures. After the oxidation step, the electrochemical performance was brought to an unprecedented level. The electrochemical reduction treatment step also alters the chemical composition of the surface by significantly reducing oxygen rich groups. Consequently, the excellent electrochemical characteristics is a result of the combination of structural and chemical modifications at the interface. The fact that the electrochemically treated surfaces becomes more homogeneous, its functionalization with redox-active molecules to detect biologically relevant species is a straightforward development.

3.5 Electrocatalytic detection As a proof-of-concept, we have investigated the electrocatalytic properties of pencildrawn surfaces towards nicotinamide adenine dinucleotide (NADH) oxidation. NADH is involved in more than 300 biological processes and can be considered as a relevant redox-active 16 ACS Paragon Plus Environment

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biomolecule when sensing and biosensing is aimmed.47 The electrochemical detection of NADH is a very challenging task due to its high detection potential and slow electrode kinetics.48 Therefore, we proceed describing the immobilization of a redox mediator on patterned pencildrawn surfaces. The electrochemically treated surface was also modified with Meldola’s blue to mediate the electron transfer. Figure 5a shows the cyclic voltammograms as the concentration of NADH increases.

Figure 5. (a) Cyclic voltammograms obtained as the concentration of NADH increases (green line). The blue trace refers to the background electrolyte. The measurements were done at 10 mV s-1. (b) Chronoamperograms for NADH concentrations between 10 and 1000 µM. The potential was fixed at -150 mV. The background electrolyte in Figures (a) and (b) is 0.1 M H2PO4-/ HPO42pH 7.0.

Both anodic and cathodic peaks, at -0.35 V and -0.4 V, respectively, are present in the background electrolyte. They can be ascribed to Meldola’s blue redox couple which was adsorbed on the electrochemically treated surface. In the presence of NADH, a drastic decrease 17 ACS Paragon Plus Environment


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of the oxidation potential of approximately 775 mV was observed when compared to the bare pencil-drawn surface (see S13). The detection principle is based on the electrochemical process of the redox mediator followed by a chemical reaction, as previously reported (see S14).49 It is important to highlight that no significant differences were noticed in the cyclic voltammograms in the presence or absence of dissolved oxygen (S15). Thus, no purging was necessary to remove oxygen from the background electrolyte. The electrocatalytic constant, calculated from the chronoamperograms shown in Figure 5b, was found to be 1.7 (± 0.4) × 105 M-1 s-1 (S16). Here, to the best of our knowledge, such a large decrease of the overpotential, associated with the high electrocatalytic performance was not reported so far (see table in S17). Also, the paper-based electrochemical cell has a great potential to detect NADH in biological conditions (S18). In summary, we reported a simple method to fabricate carbon conductive pathways on paper, with outstanding interfacial properties, for the development of electrochemical sensors and biosensors. The method consists of a direct mechanical exfoliation process of graphite onto paper, followed by a fast electrochemical treatment. As a model of carbon-based materials, we have employed commercially available soft pencils to fabricate the conductive films. A combination of chemical and structural changes on the surface was responsible for increasing the reversibility of the redox probe and to improve the homogeneity of the carbon-based material. We demonstrate that the modification of the electrochemically treated surface, by attaching Meldola’s blue on the carbon film, allowed us to detect NADH at very low potentials with a high electrocatalytic constant. Since drawing techniques can be scalable by using x-y plotters, for instance, the procedure presented here can be employed to create highly active surfaces to be integrated into a variety of electronic/electrochemical systems on paper substrates. Our work paves the way towards the fabrication of more reliable and efficient electrochemical sensors and biosensors using a straightforward approach to prepare carbon films on paper. 18 ACS Paragon Plus Environment

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Supporting Information: Supporting Information available with experimental details (construction of the RE/CE electrode), SEM image, electrical characterization, Raman maps and additional cyclic voltammograms.

Corresponding Author *Dr. Carlos C. B. Bufon e-mail: [email protected], phone +55(19)35175098

Acknowledgements The authors acknowledge CNPq (Project 483550/2013-2) and FAPESP (Project 2013/22127-2 and 2014/25979-2) for the financial support. We would like to thank National Center for Research in Energy and Materials (CNPEM) and Brazilian Nanotechnology National Laboratory (LNNano). We also thank Davi H. S. de Camargo, Cátia C. Côrrea and Evandro M. Lanzoni (LCS) for their valuable help. The authors would like to thank the Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ-UNICAMP/FAPESP Grant no. 2009/54066-7) for the Raman analysis. All authors contributed equally to this work.

Abbreviations NADH – Nicotinamide adenine dinucleotide (reduced form) G – Bare graphite film G-ox – Electrochemically oxidized graphite film G-red – Electrochemically reduced graphite film

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Direct Drawing Method of Graphite onto Paper for High Performance

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