Polylactic Acid Electrodes Promise High

Article Cite This: Anal. Chem. XXXX, XXX, XXX−XXX

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3D-Printed Graphene/Polylactic Acid Electrodes Promise High Sensitivity in Electroanalysis C. Lorena Manzanares Palenzuela, Filip Novotný, Petr Krupička, Zdeněk Sofer, and Martin Pumera* Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Additive manufacturing provides a unique tool for prototyping structures toward electrochemical sensing, due to its ability to produce highly versatile, tailored-shaped devices in a low-cost and fast way with minimized waste. Here we present 3D-printed graphene electrodes for electrochemical sensing. Ring- and disc-shaped electrodes were 3Dprinted with a Fused Deposition Modeling printer and characterized using cyclic voltammetry and scanning electron microscopy. Different redox probes K3Fe(CN)6:K4Fe(CN)6, FeCl3, ascorbic acid, Ru(NH3)6Cl3, and ferrocene monocarboxylic acid) were used to assess the electrochemical performance of these devices. Finally, the electrochemical detection of picric acid and ascorbic acid was carried out as proof-of-concept analytes for sensing applications. Such customizable platforms represent promising alternatives to conventional electrodes for a wide range of sensing applications.

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and potential window. Moreover, from a technological point of view, 3D metal printing requires highly expensive equipment (tens of thousands of dollars) while carbon/polymer 3D printers cost only a few hundred dollars. Therefore, the latter is more accessible for prototyping, making it an ideal platform for customizing electrochemical devices, as recently shown with graphite 3D-printed electrodes applied for Pb2+ detection.39 Here, we show the utility and versatility of 3D-printed graphene electrodes for sensing applications.

hree-dimensional (3D) printing, also known as additive manufacturing, has the potential to transform science and technology by creating customized devices that previously required dedicated facilities and complex procedures to make, while generating minimum waste.1,2 The technology has received extraordinary levels of attention from industry and research laboratories (academic and government use have been on the rise) given the amount of possibilities it provides in the development of advanced architectures and systems out of a wide range of materials, including metals, polymers, ceramics, and even living cells.2 These capabilities, together with the availability of affordable desktop-size printing machines, have extended the application spectrum of this technology to various research fields, i.e. energy,3−10 biomedical applications,11−18 labware fabrication,14,19,20 food engineering,21−23 microfluidics/ lab-on-chip and sensing systems,1,3,24−39 and more. With the advent of 3D printing, the development of sensing systems has been expanding at an increasing rate with the fabrication of functional interfaces and devices at reduced prices and with great versatility.1,35 Despite there being other cuttingedge technologies also complying with these features,40−43 3D printing is becoming of increasing interest for the electrochemistry community. More specifically, the field of electroanalysisparticularly electrical/electrochemical sensors and point-of-care devicescan benefit from this technology by customizing electrode design7,18,19,27,28,37−39 or simply by creating the “housing” of the electrodes in a way that improves sample handling and enables automation, i.e. channelling interfaces with fluidic control.25,29,32,44−48 Our group exploited the use of bare steel and gold-,27,28,37,38 bismuth-,37 or catalystplated3,10 (e.g., nickel, platinum, iridium) steel 3D-printed electrodes for sensing and energy applications. However, steel electrodes have limited pH and electrochemical potential windows.49 Carbon electrodes are preferred substrates for a wide range of applications given that they offer greater stability © XXXX American Chemical Society

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EXPERIMENTAL SECTION Reagents and Materials. Hexaammineruthenium(III) chloride, potassium chloride, ferrocene monocarboxylic acid (Fc-COOH), perchloric acid, and picric acid were purchased from Sigma-Aldrich (Czech Republic). Ascorbic acid was purchased from Fluorochem (Hadfield, UK). Iron chloride hexahydrate was acquired from Acros Organics (Geel, Belgium). Dimethylformamide (DMF) was purchased from Fisher Scientific (Leics, UK). Absolute ethanol and acetic acid glacial were obtained from Penta (Prague, Czech Republic). Potassium hexacyanoferrate (II), potassium hexacyanoferrate(III), and sodium acetate anhydrous were purchased from Lach-Ner (Neratovice, Czech Republic). Platinum and Ag/ AgCl electrodes were purchased from CH Instruments (Texas, USA). Deionized water (16 MΩ) was used in all experiments. 3D Printing of Electrodes and Activation. The graphene/polylactic acid filament was purchased from Black Magic 3D (New York, USA). Electrode design was first drawn using sketch-up 3D modeling open-source software. Fused deposition modeling was employed to fabricate the electroReceived: January 6, 2018 Accepted: April 5, 2018

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DOI: 10.1021/acs.analchem.8b00083 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

Figure 1. 3D-printed electrode dimensions and shapes.

Figure 2. Cyclic voltammograms of 3D-printed electrodes, ring- and disc-shaped (I and II, respectively), recorded for various redox probes (1 mM): Ru(NH3)6Cl3, Fc-COOH, K3Fe(CN)6:K4Fe(CN)6, FeCl3, and ascorbic acid (A to E, respectively) at a scan rate of 100 mV s−1. See experimental conditions for more details. Dashed lines: nonactivated electrodes. Full lines: activated electrodes.

Scanning Electron Microscopy. The morphology of both the filament and the 3D-printed objects was investigated using scanning electron microscope (SEM) with an FEG electron source and secondary electron detector (Tescan Lyra dual beam microscope). Elemental composition and mapping were performed using an energy dispersive spectroscopy (EDS) analyzer (X-MaxN) with a 20 mm2 SDD detector (Oxford instruments) and AZtecEnergy software. To conduct the measurements, the samples were placed on a carbon conductive tape. SEM imaging and SEM-EDS analysis were carried out using a 5 kV and 20 kV electron beam, respectively.

chemical structures with the graphene/polylactic acid composite filaments. The 3D printing was performed with 3D printer TRILAB (DeltiX, Czech Republic). The 3D-printed structures (ring and disc-shaped; see Figure 1) were immersed in DMF for 10 min, rinsed thoroughly with ethanol, and left to air-dry for 24 h. The graphene electrodes were directly held by a crocodile clip and placed in a 5 mL electrochemical cell. Electrochemical Measurements. Cyclic voltammetry (CV) experiments were performed at room temperature (25 °C) using a three-electrode configuration (KCl 1 M Ag/AgCl as the reference electrode and platinum wire as the counter electrode) at a scan rate of 100 mV s−1 in supporting electrolyte KCl 0.1 M for all redox probes (1 mM), except Fe3+ (1 mM of FeCl3), which was measured in HClO4 0.1 M. Picric acid was measured using acetate buffer 0.1 M pH 4.6 as the supporting electrolyte. Measurements were carried out using an Autolab PGSTAT 204 (Metrohm Autolab, Switzerland), controlled by Nova 2.1 software (Metrohm Autolab, Switzerland).

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RESULTS AND DISCUSSION Two different structure types were 3D-printed from the graphene/polylactic acid filament (Figure 1), ring- and discshaped, and applied for electrochemical sensing of picric acid and ascorbic acid. The ring comprised 2 mm cylinders incorporated in a semicircular fashion with one empty spot left for the connection in the electrochemical cell. B


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Figure 3. SEM images of the nonactivated and activated ring structure.

and graphene. Interestingly, the presence of iron and titanium is apparent. The latter has been also evidenced in previous work.7 The oxidation current intensity of [Fe(CN)6]4−/3− was used to calculate the electroactive area in both electrodes by means of the Randles−Ševčiḱ equation for quasi-reversible electrochemical systems, i.e. 3.7 and 1.3 cm2 for the ring and disc electrodes, respectively. These values were ∼2.6 and 1.9 times higher than the geometrical areas, due to the roughness/ porosity of the activated electrodes. The ring-shaped electrodes were chosen for subsequent sensing experiments. Picric acid detection is presented in Figure 4A. The nitro groups are reduced to the hydroxylamine form in a single 4e−/ 4H+ reaction occurring at negative potentials (a reduction current is seen evolving in Figure 4A from −0.35 V and −0.45 V for highest to lowest analyte levels, respectively). The arylhydroxylamine derivates are then oxidized to nitroso groups in a chemically reversible, electrochemically quasi-reversible reaction that occurs at slightly positive potentials (within the working potential window seen in Figure 4A). The observed voltammetric behavior is well-known,50,51 and it was also confirmed in this work using glassy carbon electrodes (Figure S5). The chemical pathway is illustrated in Figure S5. The explosive could be detected from 5 to 350 ppm by monitoring the hydroxylamine-nitroso reaction with the 3D-printed graphene electrodes. Ascorbic acid detection was also carried out with the 3Dprinted graphene electrodes (Figure 4B). The characteristic irreversible redox process is observed in the voltammograms, with increasing current intensity at the concentration range of 10 to 500 μM. The performance of these 3D-printed electrodes in the sensitive detection of the two model analytes herein presented reveals the potential of 3D printing technologies for electroanalytical applications. However, further characterization of printable materials is necessary to gain more insight into the field and to expand the versatility of the fabricated objects. It is clear that any postprinting procedure required to “activate” the

We have performed an electrochemical characterization using the following reagents (and their corresponding redox processes): K3Fe(CN)6:K4Fe(CN)6 ([Fe(CN)6]4−/3− ↔ [Fe(CN) 6 ] 3−/4− ), Ru(NH 3 ) 6 Cl 3 ([Ru(NH 3 ) 6 ] 3+ ↔ [Ru(NH3)6]2+), FeCl3 (Fe3+ ↔ Fe2+), ascorbic acid (C6H8O6 → C6H6O7), and ferrocene monocarboxylic acid (Fc-COOH ↔ Fc+-COOH) (Figure 2A−E, dashed lines). Except for [Ru(NH3)6]3+/2+, known for being surface-insensitive or outersphere, the electrochemical response of the other probes is basically nonexistent. We developed an activation step for the graphene 3D-printed electrodes consisting of exposing these structures to DMF to dissolve the fused polymer on the surface of the electrode. DMF-activated electrodes become highly electroactive, as seen in full lines in Figure 2. Background currents in the supporting electrolytes were recorded and are shown in Figure S1. Different activation times (1, 10, 20, and 60 min) were studied (Figure S2). After 1 min, considerable differences in the morphology of the material were evidenced compared to the nonactivated, as well as the appearance of redox activity of the ferro/ferricyanide pair. This favorable effect was enhanced after 10 min of DMF treatment, whereas longer times resulted in an impairment of [Fe(CN)6]4−/3− electrochemistry. In addition, the filament was damaged with cracks in the surface especially after 60 min of DMF immersion, compromising the structural integrity of the material (Figure 3S). Thus, immersing the 3Dprinted graphene/polylactic acid electrodes in DMF for 10 min was the optimum time to generate the desired activating effect. The morphology of the 3D-printed object (ring) was also evaluated with SEM (Figure 3) to assess changes before and after DMF treatment, after 3D printing. Figure 3 shows that the material becomes more conductive given the observed contrast after the 10 min activation, as seen previously with the filament. Further surface analysis was conducted utilizing EDS (Figure S4). Carbon and oxygen are the most predominant peaks, naturally from a combination of the polylactic acid structure C


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the implementation of 3D printing technologies for on-demand customizable sensor fabrication.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b00083. Background currents and working potential window in KCl 0.1 M and in HClO4 0.1 M for the activated 3Dprinted electrodes; effect of different immersion times of the graphene/polylactic acid filament on DMF; effect of 60 min immersion time of the graphene/polylactic acid filament in DMF on its structural integrity; EDS analysis of the activated graphene/polylactic acid filament; voltammetric behavior of picric acid at glassy carbon electrodes and chemical pathway (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +420 220444002. ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the project Advanced Functional Nanorobots (Reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). Z.S. was supported by Czech Science Foundation (GACR No. 16-05167S) and Neuron Foundation for Science support.

Figure 4. Cyclic voltammograms of 3D-printed graphene electrodes recorded for different concentration levels of (A) picric acid in acetate buffer 0.1 M pH 4.6 (inlet: calibration plot using anodic peak intensity) and (B) ascorbic acid in KCl 0.1 M (inlet: calibration plot). Dashed line: nonactivated electrodes in the presence of the highest concentration of analyte. Discontinuous line: blank current in the supporting electrolyte. Full lines from light gray to black: activated electrodes in the presence of increasing analyte level (5 to 360 ppm for picric acid and 10 to 500 μM for ascorbic acid).

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surface is not ideal when envisioning a “ready-to-use” protocol for electrode printing. However, we consider the activation procedure proposed herein to be a rather simpler treatment than the commonly used electrodeposition. 3D printing is still in early stages regarding the fabrication of active/electroconductive surfaces. Further research is highly encouraged in light of this promising cross-disciplinary field.

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CONCLUSIONS We have demonstrated a proof-of-concept for the straightforward fabrication and use of graphene-based electrodes for electrochemical sensing. An increase in electroactivity was attained through a simple activation procedure consisting of DMF-assisted partial dissolution of the insulating polymer polylactic acid. Two different geometries were 3D-printed and electrochemically characterized. Picric acid and ascorbic acid were detected with these electrodes with almost 2 orders of magnitude. We believe these results to be highly encouraging in D


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Polylactic Acid Electrodes Promise High

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