3D printed graphene electrodes modified with Prussian blue

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Functional Inorganic Materials and Devices

3D printed graphene electrodes modified with Prussian blue: Emerging electrochemical sensing platform for peroxide detection Vera Katic, Pãmyla Layene dos Santos, Matheus Fernandes dos Santos, Bruno Morandi Pires, Hugo Campos Loureiro, Ana Paula Lima, Julia Queiroz, Richard Landers, Rodrigo A. A. Munoz, and Juliano A. Bonacin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09305 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

3D printed graphene electrodes modified with Prussian blue: Emerging electrochemical sensing platform for peroxide detection

Vera Katica,†, Pãmyla L. dos Santosa,†, Matheus F. dos Santosa, Bruno M. Piresa, Hugo C. Loureiroa, Ana P. Limac, Júlia C. M. Queirozc, Richard Landersb, Rodrigo A. A. Muñoz c, Juliano A. Bonacina* aInstitute

of Chemistry, University of Campinas, P. O. Box 6154, 13083-970, Campinas, SP, Brazil.

bInstitute

of Physics Gleb Wataghin, University of Campinas, P. O. Box 6165, 13083-859, Campinas, SP,

Brazil. cInstitute

of Chemistry, Federal University of Uberlândia, 38408-100, Uberlândia, MG, Brazil.

ACS Paragon Plus Environment

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ABSTRACT: 3D printing technologies have been considered an important technology due to the ease manufacturing of objects, freedom of design, waste minimization and fast prototyping. In chemistry, this technology potentializes the fabrication of conductive electrodes in large-scale for sensing applications. Herein, we reported the modification of 3D printed graphene electrode with Prussian blue. The modified electrode (3DGrE/PB) was characterized by microscopy (SEM and AFM), spectroscopic techniques and its electrochemical properties were compared to the traditional electrodes: glassy carbon, gold, and platinum. The 3DGrE/PB was used in the sensing of hydrogen peroxide in real-world samples of milk and mouthwash and the results obtained according to the technique of batch-injection analysis showed satisfactory for the concentration range typically found in such samples. Thus, 3DGrE/PB can be used as a new platform for sensing of molecular targets. Keywords: 3D printed graphene electrode; electrodeposition; Prussian blue; electrocatalysis; H2O2 detection.


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INTRODUCTION 3D printing, also known as additive manufacturing, constructs three dimensional objects by depositing layer by layer build material, digitally coordinated1. This emerging technology has proven to ease manufacturing of materials and is more advantageous compared to the subtractive manufacturing providing freedom of design, waste minimization and fast prototyping2. In contrast to traditional fabrication processes where predetermined molds are used and are optimized for largevolume production of identical objects, 3D printing provides significant flexibility with possibility to create unique objects tailored to a specific application3. Therefore, 3D printing has proven to be a useful technology for specialized applications in industry and for scientific research. Especially in the field of electrochemistry, since this technology allows the fabrication of electrochemical cells and electrodes in large-scale for sensing applications2,4–8. In this sense, 3D printed electrodes have been produced from conductive filaments based on carbon materials, which are reported as good substrates for electroanalytical application, since they offer great stability to the electrode and broad potential window9. Among these materials, graphene is highlighted and extensively studied in electrochemistry due to its fascinating properties such as high surface area, conductivity and tunable surface chemistry10. Thus, functionalization of graphene sheets with oxygenated functional groups can enhance the performance of sensors due to the interaction with some analytes. However, it is important a balance between the presence of functional groups and the conductivity of graphene9,11,12. Recently we have reported a simple electrochemical strategy to produce oxygenated functionalities on 3D printed graphene electrodes (3DGrEs) and enhance their performance for detection of dopamine. In this approach, the number of functional groups and structural defects can be modulated by application of different potentials, affecting the electrochemical behavior of the 3DGrEs and their electrocatalytic properties. Moreover, the proposed activation method can remove


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the blocking layer of polylactic acid (PLA) from the surface of the electrodes in order to improve their electron-transfer kinetics7. The functionalization of graphene-based materials also allows creating spots to anchor and stabilize surface modifiers with electrocatalytic properties. In this regard, Prussian blue (PB) (FeIII[FeII(CN)6]3) is a remarkable material used as a modifier, mainly due to its spectroscopic and electrochemical properties13–16. Its basic structure is a 3D network containing alternated Fe (II) and Fe (III) atoms connected by cyanide bridges that can be either oxidized or reduced states according to the applied potentials. Its structure allows the transfer of cations and anions from the material to solution and vice-versa. In addition, due to its high catalytic activity and good selectivity towards the reduction of hydrogen peroxide, Prussian blue is usually considered as “artificial peroxidase”17. The good selectivity towards the reduction of hydrogen peroxide can be attributed to its zeolitic structure, which only allows small molecule (such as H2O2) to diffuse into the lattice and to be reduced, thus decreasing the number of potential molecules that could interfere16,18. However, low stability in neutral media and poor conductivity of Prussian blue are obstacles for its efficient use in sensing applications19. To overcome these obstacles, a series of conductive supports have been used to enhance its conductivity. Among support materials, carbon-based materials have demonstrated to provide better electrochemical stability and electrical conductivity14. In this work, Prussian blue films were electrodeposited onto the surface of 3D printed graphene electrode and their electrochemical behavior was compared with conventional electrodes. The bare and modified 3DGrE were characterized by spectroscopic and microscopies techniques in order to confirm the modification and evaluate the morphology. In addition, the 3D printed modified electrode has been also investigated in the electrocatalytic reduction of H2O2 and its sensing properties were demonstrated in the amperometric determination of H2O2 in a real-world sample of mouthwash and milk using a 3D printed compact electrochemical cell.


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EXPERIMENTAL SECTION Chemicals and samples. Iron (III) chloride hexahydrate (FeCl3∙6H2O), potassium chloride (KCl) and hydrogen peroxide aqueous solution (H2O2, 30.0 wt%) were purchased from Synth. Potassium dihydrogen phosphate (KH2PO4) and potassium hydroxide (KOH) were purchased from Dinâmica and Synth, respectively, and used to prepare phosphate buffer solution (0.1 mol L−1 PBS, pH = 7.4). Potassium hexacyanidoferrate (III) (K3[Fe(CN)6]) was purchased from Sigma-Aldrich. Conductive Graphene 3D Printing PLA Filament was purchased from Black Magic 3D (volume resistivity: 0.6 ·cm). All chemicals were of analytical grade and used without any further purification. The aqueous solutions were prepared with ultrapure water (>18 MΩ cm) obtained from a Milli-Q Plus system (Millipore). A mouthwash sample containing H2O2 was acquired from a local drug store. Milk samples were obtained from a local market. Modification of the electrodes. Prior to the modification, glassy carbon, gold and platinum electrodes were carefully polished successively with 1.0, 0.5 and 0.3 m alumina, sonicated in ethanol: water for 10 min and rinsed with Milli-Q water. 3D printed graphene electrodes were fabricated and submitted to an electrochemical pre-treatment, based on our previous report7. Briefly, the surface of working electrode was oxidized (applying potential of 1.8 V vs. Ag/AgCl for 15 minutes), and then reduced (by applying potential of -1.8 V vs. Ag/AgCl for 50 seconds). Prussian blue films were electrodeposited by chronoamperometry, based on the method proposed by Karyakin20,21, under applied potential of 0.4 V vs. Ag/AgCl for 600 s in a solution containing 1.0 x 10-3 mol L-1 [Fe(CN)6]3-, 1.0 x 10-3 mol L-1 FeCl3, 0.01 mol L-1 HCl and 0.1 mol L-1 KCl. Then, the films were dried in an oven at 60 °C for 1 h. Finally, the modified electrodes were electrochemically activated by applying 25 voltammetric cycles from -0.2 to 0.6 V vs. Ag/AgCl in a solution of 0.1 mol L-1 HCl containing 0.1 mol L-1 KCl at 50 mV s-1.


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Characterization. The bare and modified 3D printed graphene electrode were characterized by Infrared, Raman, X-ray photoelectron spectroscopies. Fourier-transform infrared (FTIR) spectra were recorded on a portable Agilent Cary 630 ATR-FTIR analyzer, in the range of 450 to 4000 cm-1. Raman spectra of the electrodes were performed on a Confocal T64000 spectrometer from Jobin Yvon with a solid-state sapphire laser (532 nm, 10 mW), using an integration time of 20 s and a 100x LWD objective in the range of 1000-2400 cm-1. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VSW HA-100 spherical analyzer and Al Kα radiation (h = 1486.6 eV). The high-resolution spectra were measured with constant analyzer pass energies of 44 eV producing a full width at half-maximum (FWHM) linewidth of 1.6 eV for the Au (4f7/2) line. The pressure during the measurements was always less than 6 × 10-8 bar. The sample was fixed to a stainless-steel sample holder with double-faced conducting tape and analyzed without further preparation. Surface charging was corrected shifting all spectra so that the highest binding energy component of the PLA was at 289.06 eV22. Curve fitting was performed using Gaussian line shapes, and a Shirley background was subtracted from the data. The morphologies of the 3D printed graphene electrodes before and after the electrodeposition of Prussian blue were investigated by FEI Quanta 250 field emission scanning electron microscope (FE-SEM) from FEI Co., USA, operating at 2.0 kV. AFM and KPFM images were obtained simultaneously using Easyscan 2 Flex AFM from Nanosurf. The KPFM images were obtained in a dry atmosphere and an electrical frequency of 15 kHz with an amplitude of 5 V was applied. A Pt/Ir-coated tip with a resonance frequency of 75 kHz and a force constant of 2.8 N m -1 was used. Electrochemical characterization of the electrodes. Cyclic voltammetry (CV) measurements were performed on an AUTOLAB modular electrochemical system (ECO Chemie, Utrecht, Netherlands) (PGSTAT12) and driven by Nova 2.1 software in conjunction with a conventional three-electrode electrochemical cell and a personal computer for data storage and processing. Glassy carbon, gold, platinum or 3D printed graphene electrodes were employed as working electrode (bare


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or modified); Ag/AgCl (in KCl 3.0 mol L-1) as reference electrode, a Pt wire as the counter electrode. Cyclic voltammetric analyzes were performed in 0.1 mol L-1 KCl containing 5.0 mmol L-1 of [Ru(NH3)6]2+/3+ at different scan rates in order to determine the electrochemical surface areas and the heterogeneous electron transfer rates (k0) of the bare and modified electrodes. Chronoamperometry Chronoamperometry measurements were performed at applied potential of -0.15 V vs. Ag/AgCl in the presence of different concentrations hydrogen peroxide in order to calculate the diffusion coefficient of H2O2 rate constant of catalytic hydrogen peroxide reduction reaction. Phosphate buffer solution (0.1 mol L-1, pH=7.4) containing 0.1 M KCl was used as supporting electrolyte. Before each experiment, the electrolyte solution was purged with N2 for at least 10 min to remove dissolved O2 and kept under N2 atmosphere during measurements. All electrochemical measurements were performed at room temperature. The electroactive areas (Ae) of the electrodes were determined by cyclic voltammetry in 5.0 mmol L-1 of [Ru(NH3)6]Cl3 and 0.1 mol L-1 KCl at different scan rates, according to the RandlesŠevčíck equation, Equation 1: 𝑖𝑝 = 2.69 × 105 𝑛3/2𝐴𝑒𝐷1/2𝐶𝜐1/2

(1)

where ip is the peak current, n is the number of electrons transferred in the electrochemical process, Ae is electrode area, D is the diffusion coefficient (9.10 x 10-6 cm2 s-1 for [Ru(NH3)6]3+ in 0.1 mol L-1 KCl), C is the concentration of the redox probe and  is the applied voltammetric scan rate. In order to determinate the heterogeneous rate constants (k0obs) between the electrodes and [Ru(NH3)6]3+/4+ probe, cyclic voltammograms in 5.0 mmol L-1 of [Ru(NH3)6]Cl3 and 0.1 mol L-1 KCl at different scan rates were obtained, the constants were calculated using the Koch method23 by the Equation 2:

[ (

𝑘0𝑜𝑏𝑠 = 2.18

𝐷 ∝ 𝑛𝐹𝜈 1/2 𝑅𝑇

)

]𝑒𝑥𝑝[ ― ( )]𝛥𝐸 𝑎2𝑛𝐹 𝑅𝑇


𝑃

(2)

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where D is the diffusion coefficient (9.10 x 10-6 cm2 s-1 for [Ru(NH3)6]3+),  is assumed to correspond to 0.5, n is the number of electrons transferred in the electrochemical process, F is the Faraday constant,  is the applied voltammetric scan rate, R is the gas constant, and T is the temperature of the solution and Ep is peak-to-peak separation. The electroactive surface coverage () and the thickness (l) of the PB film can be calculated according to Equations 3 and 424, using the anodic current peak of the PW(Fe2+-Fe2+)/PB(Fe2+-Fe3+) redox process, Figures S2-S5. A plot of the anodic current peak as a function of the scan rate provides a linear correlation , which slope provides the value of the  in mol m-2 𝑛2𝐹2

𝑖𝑝𝑎 = 4𝑅𝑇 𝛤𝜈𝐴

𝑙=Γ

(3)

𝑎3𝑁𝐴

( ) 4

(4)

where ipa is the anodic peak current, n is the number of electrons involved in the redox process, F is the Faraday constant, R is the gas constant, T is the temperature,  is the electroactive surface coverage, 𝜈 is the scan rate and A the electrode area, a is the unit cell parameter of PB (1.01 nm), NA is the Avogadro number. The diffusion coefficient of hydrogen peroxide was calculated by Cottrell equation (Equation 5)25, under diffusion control, using the data presented in Figure S12. 1

𝑖=

nFA𝐷2𝐶 1 1

(5)

π2 𝑡2

where ip is the peak current (A), n is the number of electrons transferred in the electrochemical process (n=2), A is electrode area (0.468 cm2), D is the diffusion coefficient (cm2 s-1), C is the concentration of H2O2, F is the Faraday constant, t is the elapsed time (s).


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The rate constant of catalytic hydrogen peroxide reduction reaction, kcat, can be estimated by chronoamperometry13, according to the Equation 6. 𝑖𝑐𝑎𝑡 𝑖L

1 2

1 2

1 2

1

= 𝛾 𝜋 = 𝜋 (𝑘𝑐𝑎𝑡𝐶𝑡)2

(6)

where icat is the catalytic current of 3DGrE/PB in the presence H2O2, iL is the limiting current in the absence of hydrogen peroxide and 𝛾 = 𝑘𝑐𝑎𝑡𝐶𝑡 (C is the bulk concentration of H2O2 and t is the elapsed time). H2O2 detection and analysis of real samples. Electrochemical detection of H2O2 was carried out in the same equipment with a conventional three-electrode electrochemical cell. Cyclic voltammetry measurements were performed from -0.2 V to 0.5 V vs Ag/AgCl, at a scan rate of 50 mV s-1, upon successive addition of 5.0 mmol L-1 H2O2 stock solution, under N2 atmosphere. Chronoamperometry measurements were performed at applied potential of -0.15 V vs. Ag/AgCl under successive addition of 10 (5x). 20 (5x). 50 (5x). 100 (5x) and 500 L (4x) of 0.5 mmol L-1 H2O2 stock solution. Phosphate buffer solution (0.1 mol L-1, pH 7.4) containing 0.1 mol L-1 KCl or 0.1 mol L-1 was used as supporting electrolyte. The analyses of real samples and interferent tests were performed using a batch-injection analysis (BIA)26 system working with amperometric detection of H2O2 at a constant applied potential. Briefly, the system works with an electronic micropipette assembled on a 3D printed (using ABS filament) compact electrochemical cell of 50 mL at which the three electrodes are inserted. Reference (miniaturized Ag/AgCl) and counter (Pt wire) were positioned at the top of the cell besides the tip of the micropipette and the working electrode (3DGrE/PB) at the bottom of cell in such a way that the tip is 2 mm distant of the electrode surface. Scheme and image of cell can be seen in a recent report5. The micropipette is responsible to inject standard solutions and samples at a controlled injection rate and volume directly over the working electrode surface. The three-electrode


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system and tip were immersed in supporting electrolyte (40 mL) and each injected aliquot is immediately dispersed and diluted in the electrolyte. A magnetic stirring bar is placed inside the cell for continuous stirring during measurements which facilitates the removal of analyte solution from the working electrode to perform another injection quickly. Experiments using BIA with amperometric detection were performed in the presence of dissolved oxygen to show its feasibility for routine analysis.

RESULTS AND DISCUSSION Electrochemical deposition and morphology of Prussian blue on 3D printed graphene electrodes. 3D printed have been reported as a promising platform for sensing of molecular targets. Moreover, the electrochemical pre-treatment creates oxygen functionalities7, which allows anchoring of materials such as Prussian blue. Additionally, the performance of PB modified electrodes increases when this material is combined with partially reduced graphene oxide, due to the complementary effect of the electrocatalytic activity of PB and the fast electron transfer rate kinetics of graphene14. Thus, considering many advantages cited above, we have electrodeposited PB on the pre-treated 3D printed graphene electrode. The morphologies of the pre-treated 3D printed graphene electrode and the 3DGrE/PB are presented in Figure 1 A and B. As it can be seen in Figure 1B the surface of the pre-treated electrode shows a more homogeneous and smoother surface than the as printed electrode, as a result of the electrochemical treatment which removed a few layers of the polymeric surface exposing the bulk material, as already reported7. On electrochemical pre-treated graphene electrode, the electrodeposition of PB produced nanocubes with an average length of 138 ± 73 nm that appear to be anchored on the surface of the electrode. The similar behavior was observed in our previous report on the photochemical synthesis of PB nanocubes - reduced graphene oxide nanocomposites14. This


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result indicates that the oxygenated functional groups formed in either the polymeric surface and the graphene sheets due to the electrochemical treatment7, were responsible for the anchoring of PB nanoparticles. The growth and anchoring of PB nanoparticles on graphene sheets has already been reported27–29 and could be ascribed to the interaction of Fe3+ cations with negatively charged oxygen functionalities.

Figure 1. Field emission gun scanning electron microscopy (FEG-SEM) micrographs of (A) the as printed 3D graphene electrode (B) the 3D graphene electrode submitted to oxidation (for 900 seconds at 1.8 V vs. Ag/AgCl) and reduction (for 50 seconds at -1.8 V vs. Ag/AgCl ) (C) the 3D graphene electrode submitted to oxidation (for 900 seconds at 1.8 V vs. Ag/AgCl) and reduction (for


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50 seconds at -1.8 V vs. Ag/AgCl ) and afterward modified with Prussian blue and (D) the histogram of PB nanoparticle size obtained from approximately 1000 measurements.

AFM and KPFM images were obtained to better understand the effects of the electrochemical deposition of Prussian blue nanoparticle on the pre-treated electrode morphology and surface potential distribution and the results are presented in Figure 2.

Figure 2. AFM images of (A) the bare and (B) PB modified 3D printed graphene electrode and (C, D) the corresponding KPFM images, respectively.


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From the KPFM images in Figure 2C and D the average surface potential (SP) value of the electrodes were calculated and the SP value obtained for the pre-treated electrode was 0.5 V, whereas for the PB electrode was of 0.3 V. The reduction of the SP value could be related to the anchoring of Prussian blue nanoparticles on the oxygen functionalities or defects on the surface of the electrode and it is an indication of an increase in the charge dispersion capacity of the electrode due to the electrochemical deposition of Prussian blue. It is possible to identify from Figure 2C regions with higher charge dispersion capacity that could be ascribed to reduced graphene oxide sheets that were exposed to the surface after the electrochemical treatment, as previously reported7. The AFM and KPFM images of the PB modified electrode show that the regions with higher charge accumulation (Figure 2D) are assigned to the Prussian blue nanoparticles. However, as it can be observed only the PB nanoparticles with greater heights (brightest spots in Figure 2B) appeared in the KPFM image (Figure 2D) as regions of higher charge accumulation. The PB nanoparticles located closer to the surface of the electrode interact strongly with the exposed reduced graphene oxide sheets, which dissipate the charge. Spectroscopic characterization. XPS analysis provides information related to the chemical composition of the material produced. XPS spectra of bare and modified 3D electrode are presented in Figure S1. The elemental percentage obtained from deconvoluted spectra are shown in TableS1. In comparison to the spectrum of 3DGrEs before and after the modification, the presence of N and Fe elements clearly indicates the formation of Prussian blue, Figure S1 (A and B). The peak located at 707.8 eV can be attributed to the Fe2p3/2 of [Fe(CN)6]4-. The N1s spectrum has two main peaks located at 397.04 eV and 398.38 eV corresponding to C-N of [Fe(CN)6]4-30,31. A complete description of the XPS analysis can be found in the supporting information section. FTIR and Raman spectra of the as printed (3DGrE_As printed), pre-treated (3D_Bare) and PB modified 3D printed graphene electrodes (3DGrE/PB) are presented in Figure 3. A characteristic band of PB, attributed to (C≡N), can be observed close to 2150 cm-1 in both spectra. Figure 3A also ACS Paragon Plus Environment

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presented bands associated to PLA, the complete attribution of these bands is presented in Table S.3. The bands D and G in Figure 3B are assigned to vibrational modes of the graphene network and the ratio of their intensities (ID/IG) is proportional to the number of structural defects in the graphene sheet11,32. The calculated ID/IG ratios of 3DGrE_As printed, 3DGrE_Bare and 3DGrE/PB were 0.64, 1.53 and 0.73, respectively, which means that the pre-treatment increases the amount of defects/disorder due to the insertion of oxygenated groups. Moreover, after the modification, the number of defect sites decreases, which is different of the results reported in the literature. According to our previous work14, the presence of Prussian blue does not induce defects or structural disorder in the reduced graphene network. Similar results were reported by Chen et al.19, who showed that PB promotes slight changes in the graphitic structure of graphene aerogel. Thus, the decrease of ID/IG could be associated to the hydrolysis of the PLA during the activation step of the PB film in HCl, which exposes the graphene sheets from the bulk to the surface of the electrode. Since these sheets were not oxidized/reduced, they should present lower number of oxygenated functional groups (See Figure S1 and Table S1). These results corroborate the increase of the charge dispersion capacity after the modification with PB as showed in Figure 2D.


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Figure 3: (A) FTIR and (B) Raman spectra and ID/IG ratio of the 3D printed electrodes: as printed, bare and modified with PB.

Electrochemical properties. Electrochemical properties of the electrochemically deposited PB films on 4 different substrates were studied by cyclic voltammetry. The cyclic voltammograms in 0.1 mol L-1 HCl/KCl (pH=1.0) of modified 3DGrE, Au, GCE and Pt electrodes are presented in Figure 4.


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Figure 4: Comparison between the cyclic voltammograms of the PB modified electrodes in 0.1 mol L-1 HCl/KCl at 50 mV s−1.

As shown in Figure 4, the redox pair in the potentials near to 0.20 V vs. Ag/AgCl is ascribed to the redox processes of Prussian blue (Fe3+–Fe2+) / Prussian white (Fe2+–Fe2+). Equations 7 and 8 describe the possible electrochemical reactions of “soluble” and “insoluble” forms of PB, respectively33. KFeIII[FeII(CN)6] + K + + e ― ⇆K2FeII[FeII(CN)6] II + ― II II FeIII 4 [Fe (CN)6]3 + 4K + 4e ⇆K4Fe4 [Fe (CN)6]

(7) (8)

Figures from S2 to S5 show the cyclic voltammograms of the modified electrodes in 0.1 mol L-1 HCl/KCl at different scan rates. The electrochemical parameters obtained from these CVs are presented in Table S4. According to these results, the redox process described above is surface confined at Au, GCE and Pt, due to the linear correlation between the peak current density and the scan rate. For the 3DGrE/PB, linearity was observed from the plot of jp vs. square root of the scan rate (Figure S2C), which can be associated to the diffusional dependence. This is reasonable if one considers that the redox processes that occur on PB and its analogs involve intercalation of cations and anions on the material framework18. As the roughness of the surface of the 3D electrodes is


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higher in comparison with other materials, this effect is maximized in this situation, since it is more difficult for the electrolyte to reach some regions of the electrode. The stabilities of the PB on different substrates were investigated by cyclic voltammetry in 0.1 mol L-1 HCl/KCl (pH 1.0) and in 0.1 mol L-1 PBS containing 0.1 mol L-1 KCl (pH 7.4), according to Figures S6 and S7. The percentages of PB on the surface of 3DGrE, Au, GCE and Pt electrodes as function of the number of scans are presented in Figure 5.

Figure 5: Comparison between the percentages of films in function of the number of scans at pH 1.0 (0.1 mol L-1 HCl/KCl) and 7.4 (0.1 mol L-1 PBS/KCl) of the modified electrodes with PB: (A) 3D printed, (B) gold, (C) glassy carbon and (D) platinum.


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Most of them presented acceptable stabilities at pH=1 (the current decrease 10 %, 32 %, 2.9 %, and 1.0 %, respectively, on 3D printed, gold, glassy carbon and platinum after 100 cycles), except on gold electrode (current decrease was higher than 10%), and a decrease in the amount of film at pH=7.4 over all electrodes. The lower stability of Prussian blue in neutral or alkaline solution is well known, due to the formation of iron hydroxide, breaking the Fe2+−(CN)−Fe3+ bond17. However, as presented in Figure S8, the PB film showed good stability at pH = 7.4 when applying a constant potential of -0.15 V vs Ag/AgCl, indicating that degradation issues may be attributed to potential cycling instead of just interaction with the electrolyte. According to a previous report, the stability of PB films can be increased by electrochemical activation in HCl/KCl, which converts the “insoluble” PB in its most stable form, the “soluble” one.

13

The method consists in performing

successive voltammetric cycles of the modified substrates in the aforementioned acid solution, which reportedly enhanced stability of PB up to pH 9.0 electrolytes34. The stabilization process occurs on the interface electrode/solution, by the insertion of K+ into the interstitial sites of PB. Recently, we have reported a comparation between the stabilities of PB_SPE (screen printed electrode), using a bulk and surface modification methods. The best stability was achieved using the last one, due to the interfacial activation13. Therefore, the stability of PB film depends on its thickness, thin films tend to be more stable due to the higher conversion in “soluble” PB and the better interaction with the substrate. The values of film thickness (l), estimated from the electroactive surface coverages () are presented in Table 1. Table 1: Electroactive surface coverage, film thickness, electroactive area, heterogeneous electron transfer rate constant and number edge sites of the bare and modified electrodes. HCl/KCl [Ru(NH3)6]3+ l Ae k0obs x 10-3  edge -1 2 -2 Electrode (nm) (cm ) (cm s ) (mol m ) (%) PB 3DGrE Au GCE

4.55 x 10-5 1.46 x 10-1 5.96 x 10-5

7.03 22500 92.1

Bare

PB

Bare

PB

Bare

PB

0.2220 0.0051 0.0102

0.4680 0.0153 0.0256

0.83 1.34 3.58

0.12 1.09 1.21

0.21 0.90

0.03 0.30


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Pt

7.12 x 10-4

440

0.0045

0.0148

1.26

0.99

-

-

Comparing the behaviors of the modified electrodes in PBS, PB showed the best adherence on the Pt substrate. Additionally, the PB was more stable at 3DGrE than at GCE and Au electrodes. The increased stability of the 3DGrE/PB can be attributed to the electrostatic interactions between iron cations of PB and the oxygen functionalities, created by the electrochemical pre-treatment. Moreover, the formation of a thin PB film, in the order of few nanometers, allows its conversion into “soluble” PB. The lowest stability of Au/PB can be associated to highest film thickness, in the order of micrometers. It has been reported that the rate of chemical and electrochemical depositions of PB is higher on Au electrodes than on Pt or GCE substrates35,36. Therefore, using the same experimental conditions, such as electrodeposition time, thicker PB films are formed on Au. Thin PB films are obtained at 3DGrE due to its high surface area. The electrochemical behaviors of the bare and modified electrodes were also analyzed in the presence of [Ru(NH3)6]3+ by cyclic voltammetry (Figures S9 and S10) in order to study the electron transfer kinetics and determine the electroactive areas (Table 1). The electron transfer kinetics of the metallic substrates in the presence of [Ru(NH3)6]2+/3+ were similar. Since this outer sphere redox probe is sensitive to the electronic structure of the electrode material37, and the modifier is not covalently attached to Pt or Au substrates, k0obs did not alter significantly after the modification. However, the electron transfer rates of GCE and 3DGrE are dependent on the ratio between the basal and edge planes exposed on the surface of the carbonaceous support38–40.Whereas the origin of the electron transfer in carbon-based materials is the edge planes, the number edge sites on the surface (edge) can be estimated from Equation 9, proposed by Hallam et al41. 𝑘0𝑜𝑏𝑠 = 𝑘0𝑒𝑑𝑔𝑒(𝜃𝑒𝑑𝑔𝑒)


(9)

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were k0obs is the observed electron transfer rate; k0edge the electron transfer rate for edge plane (0.4 cm s-1) 42; edge is the number of edge sites on the surface of the electrode. The decrease of edge, from 0.90 (GCE) to 0.30 (GCE/PB) and from 0.21 (3DGrE_Bare) to 0.03 (3DGrE/PB), can be directly associated the coverage of the edge planes with PB. These results corroborated with those ones presented in Raman spectra. The reduction of the number of structural defects/disorders, after the modification of 3DGrE_Bare with PB, affects its electrochemical properties. Detection of H2O2. The performances of the bare and 3DGrE/PB towards H2O2 were evaluated by cyclic voltammetry, Figure 6. As it can be seen in Figure 6A, there was no response for H2O2 reduction at the bare 3D electrode. However, at the 3DGrE/PB (Figure 6B), the profile of the voltammogram changed after successive addition of H2O2, indicating the electrocatalytic activity of the modifier material. The Figure 6C shows plots of the current response of the modified 3D electrodes with PB as a function of the H2O2 concentration in different potentials.


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Figure 6: Cyclic voltammograms of (A) bare 3D and (B) modified 3D with PB in 0.1 mol L-1 PBS (pH 7.4) containing 0.1 mol L-1 KCl, with successive addition of 0.5 mmol L-1 H2O2 stock solution. Scan rate: 50 mV s-1. The additions of H2O2 were performed after the 3DGrE_PB was cycled in 0.1 mol L-1 PBS/0.1 mol L-1 KCl, 100 cycles. (C) Plots of the current response as a function of H2O2 concentration in different potentials of the voltammograms presented in B.

To better understanding the electrocatalytic process, cyclic voltammetry at different scan rates (Figure S11A) was performed in the presence of 0.05 mmol L-1. Figure S11B shows the linearity between the catalytic reduction peak current (ipa) and the square root of the scan rate (1/2). Therefore, the reduction of the analyte by the 3DGrE/PB is controlled by its diffusion at the studied scan rate range, as can be expressed by Equation (10). Moreover, the plot of the scan rate normalized


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current (ipa/1/2) versus scan rate (Figure S11C) presented a characteristic shape of an electrochemical-catalytic mechanism (ECcat). 1

1

𝐼(𝜇𝐴) = 9.5 ― 9.4 𝜈2(𝑚𝑉 𝑠 ―1)2

(10)

The diffusion coefficient of hydrogen peroxide calculated by Cottrell equation from the current-time curves (Figure S12) was 1.0 × 10-5 cm2 s-1, which is similar to those ones reported in the literature43,44. Although the mechanism of electrocatalytic reduction of hydrogen peroxide by Prussian blue is still not completely clear, the catalytic cycle involves oxidation of Prussian white by H2O2 and production of OH-, according to Eq. (11-13). 2KFeIII[FeII(CN)6] + 2K + + 2e ― →2K2FeII[FeII(CN)6] (11) 2K2FeII[FeII(CN)6] →2KFeIII[FeII(CN)6] + 2K + + 2e ― (12) H2O2 + 2e -

𝑘𝑐𝑎𝑡

2 OH ―

(13)

The rate constant (kcat) of catalytic hydrogen peroxide reduction reaction at the 3DGr/PB was determined from chronoamperometry measurements (Figure S13) and was found to be 1.0 × 103 L mol-1 s-1. The kcat of hydrogen peroxide for the proposed sensor was in the same order of magnitude of that one observed for a PB modified glassy carbon electrode (3.0 × 103 L mol-1 s-1) and close to kcat of the peroxidase enzyme (2.0 × 104 L mol-1 s-1)21. The decrease in peak currents of Prussian blue after successive addition of H2O2 can be associated to the formation of iron hydroxide and destruction of the Fe2+–(CN)–Fe3+ bond, Equation (14)11. Fe4[Fe(CN)6] + 12 OH - →4 Fe(OH)3 + 3 [Fe(CN)6]4 -

(14)

The results obtained from Figure 6C were used in the chronoamperometric measurements. The reduction potential of -0.15 V vs. Ag/AgCl was selected for the sensing of H2O2 due to the higher sensibility. The electrochemical response of a freshly prepared 3DGrE/PB in the


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electroreduction of H2O2 can be found in Figure S14. The novel 3DGrE/PB electrodes showed analytical performance similar to the substrates reported in the literature17,31,45–48. As presented in Table S5, the obtained linear range, detection limit and working potential were similar to values obtained by other systems for amperometric determination of hydrogen peroxide. Beyond the analytical performance, it was demonstrated the use of a low-cost electrode with possibility of largescale production, which is surely an advantage over the traditional electrodes also reported herein. Next we demonstrate an additional advantage of this sensor which is its stability and accuracy even when applied to real-world samples. Real sample analysis. Next, the 3DGrE/PB sensor was demonstrated for the analysis of H2O2 in real samples using the BIA system. Some parameters were evaluated to obtain the highest current signal for H2O2 (consequently higher detectability), response time (time to the signal returns to the baseline) and precision (RSD values). Initial experiments using amperometric detection with the BIA system revealed the need of stirring of electrolyte solution inside the cell to a faster reestablishment of the baseline current after an injection of standard solution of H2O2.For this reason, all experiments were performed keeping the electrolyte solution under constant stirring. The selection of the applied potential was performed based on a hydrodynamic voltammogram (Figure S15) which was constructed using an amperometric recording obtained for triplicate injection of 50 μmol L-1 H2O2 under the application of different applied potentials (from +0.2 to -0.3 V vs. Ag/AgCl). Figure S15 shows the current variation as function of the applied potential. The highest current values were obtained between +0.05 and 0.0 V vs. Ag/AgCl, which was expected due to the catalytic activity of PB. The lower responses obtained at more negative potentials can be because of dissolved oxygen (not removed in these experiments) which also responds to the electrode. Based on this experiment, the applied potential of 0.0 V vs. Ag/AgCl, at which minimal interference is verified (low potential detection avoids the oxidation or reduction of interfering electroactive species).


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The BIA parameters, injection volume and dispensing rate, were evaluated and their effects over the H2O2 signal are shown in Figure S16. The current response increased as the injection volume increased probably due to the higher amount of H2O2 being reduced at the 3DGrE/PB electrode. Although 200 µL provided a higher current response, the injection volume of 100 µL was selected due to the much faster responses obtained for this volume (response time was reduced from 16 ± 2 s to 21 ± 2 s (n=3), to reduce the amount of waste sample and to enhance the electrode activity towards H2O2 detection. The dispensing rate (injection rate) effect on the current response is shown in Figure S16B and high current variation was observed for lower rates of injection. The best compromise between current response and precision was obtained for injections at 277 µL s-1, which was selected in further experiments. An analytical curve using the selected conditions for H2O2 detection is shown in Figure 7. From this curve, the values of limit of detection (LOD) and limit of quantification (LOQ), sensitivity (slope), and analytical frequency (AF) were obtained and are listed in Table 2.

Figure 7: (A) Amperometric responses obtained in BIA, using 3DGrE/PB, for triplicate injections of standard solutions of hydrogen peroxide 1-1000 μmol L-1 (a-m) in 0.1 mol L-1 PBS (pH = 7.4)/0.1


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mol L-1 KCl. Potential: 0.0 V vs. Ag/AgCl , injection volume: 100 μL; dispensing rate 277 μL s-1. (B) Corresponding calibration curve and linear range obtained (1-700 μmol L-1). The inter-electrode precision was estimated based on the RSD value of the current for 100 μmol L-1 of H2O2 obtained on three different electrodes (also in Table 2). The low RSD value (< 5%) indicates a high precision of the electrode modification. A wide linear range was obtained for H2O2 sensing with a submicromolar LOD value, which is relevant for the detection of H2O2 residues in food sample. Table 2. Analytical parameters obtained from amperometric calibration curves for hydrogen peroxide in the BIA system using 3DGrE/PB electrode. Electrode

Linear range µmol L-1

Slope µA L µmol-1

R

LOD µmol L-1

LOQ µmol L-1

RSD (%)

AF (h-1)

3DGrE/PB

1 - 700

0.0407

0.9982

0.11

0.37

4.23

87

RSD: inter-electrode (n = 3) deviation for 100 μmol L-1 H2O2; AF: analytical frequency.

After selecting the conditions for the amperometric detection of H2O2 using the BIA system, two samples (mouthwash and milk) were analyzed after simple dilution in supporting electrolyte. The first sample contains H2O2 as bleaching agent for teeth whitening while the second sample may contain residues of H2O2 originated from the washing processes during milk production and storage49. To evaluate the accuracy of the proposed sensor, both samples were spiked and analyzed. Table 3 resumes the concentration values found in both samples before and after spiking, and the calculated recovery value used to estimate the accuracy of the sensor. Figure S17 shows the amperometric recording for the injection of all samples and standard solutions used to construct an analytical curve. The results in Table 3 shows recovery values between 97 and 120% which indicates an adequate accuracy of the sensor for H2O2 determination in both samples. Moreover, the milk sample showed a micromolar amount of H2O2 which indicates the presence of H2O2 residues in milk49. The mouthwash sample presented a value within the concentration range of H2O2 added to this kind of product. ACS Paragon Plus Environment

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Table 3: Concentrations of H2O2 obtained by the BIA system with amperometric detection, recovery values for the spiked samples and respective standard deviation values (n=3). Sample

Found (mmol L-1)

Mouthwash (A)

24.7 ± 0.2

Milk (B)

0.010 ± 0.002

Spiked (mmol L-1) 12.7 25.4 0.25 0.50 1.0

Recovery (mmol L-1) 13.3 ± 0.8 30.5 ± 1.2 0.29 ± 0.01 0.54 ± 0.02 0.98 ± 0.03

Recovery (%) 105 ± 6 120 ± 4 112 ± 3 106 ± 4 97 ± 3

Considering the analysis of biological samples using the proposed sensor combined with the BIA system, the interference study of dopamine, ascorbic acid and uric acid in the chronoamperometric measurements was evaluated. Figure S18 shows that the response to these molecules can be neglected. CONCLUSION As conclusion, it was possible to observe that the 3D printed graphene electrodes have the electrochemical behavior similar to the conventional ones (Au, GCE and Pt). The electrochemical activation process makes these electrodes conductive and allows it modification with electroactive materials like Prussian blue. The 3D printed graphene electrodes modified with Prussian blue were used in the sensing of hydrogen peroxide in real samples of milk and mouthwash and results using the technique of batch-injection analysis showed satisfactory for this concentration range. Thus, the obtained results suggest that 3D printed graphene electrodes can be easily modified and used in sensing of molecular targets.

AUTHOR INFORMATION Corresponding Author Prof. Juliano Alves Bonacin


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*E-mail: [email protected] Tel: +55 (19) 3521 3103; Fax: +55 (19) 3521 3023

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †V. Katic and P. L dos Santos contributed equally. Notes The authors declare that there is no conflict of interests regarding the publication of this study. ACKNOWLEDGEMENTS The authors are grateful and acknowledge the financial support of Brazilian Funding Agencies. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001, Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq (grant# 427731/2018-6 and grant# 307271/2017-0) and Fundação de Amparo à Pesquisa do Estado de São Paulo, FAPESP (grant#2013/22127-2 and grant#2017/239600). The authors would like to thank the LNNano for technical support during scanning Kelvin probe force microscopy work. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website. Additional

data

such

as

X-ray

photoelectron

spectra,

cyclic

voltammograms,

chronoamperometric measurements, real sample and interference studies.


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Table of Contents


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3D printed graphene electrodes modified with Prussian blue

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