Graphene-porphyrin composite synthesis through graphite exfoliation: The electrochemical sensing of catechol

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Research paper

Graphene-porphyrin composite synthesis through graphite exfoliation: The electrochemical sensing of catechol Maria Coros¸ a , Florina Pog˘acean a,∗ , Lidia M˘agerus¸an a , Marcela-Corina Ros¸u a , Alin Sebastian Porav a , Crina Socaci a , Attila Bende a , Raluca-Ioana Stefan-van Staden b,c , Stela Pruneanu a,∗ a

National Institute for Research and Development of Isotopic and Molecular Technologies,67-103 Donat Street, 400293 Cluj-Napoca, Romania Laboratory of Electrochemistry and PATLAB, National Institute of Research for Electrochemistry and Condensed Matter, 202 Splaiul Independentei Street, 060021, Bucharest-6, Romania c Faculty of Applied Chemistry and Material Science, Politehnica University of Bucharest, Bucharest, Romania b

a r t i c l e

i n f o

Article history: Received 6 June 2017 Received in revised form 28 September 2017 Accepted 28 September 2017 Available online xxx Keywords: Electrochemically exfoliated graphene TPyP Catechol Modified GC electrode

a b s t r a c t This paper reports for the first time a novel approach for the synthesis of graphene-porphyrin composite (EGr-TPyP) through the electrochemical exfoliation of graphite in the presence of 5,10,15,20-tetra(4pyridyl)porphyrin (TPyP), in a neutral electrolyte (0.2 M KCl). No exfoliation was observed in the absence of TPyP molecules. The XRD investigation proves that the EGr-TPyP composite consists in a mixture of few-layer (FLG) and multi-layer (MLG) graphene. The mean crystallites size, calculated from the fullwidth at half-maximum (FWHM) of the corresponding diffraction peak, was found to be 0.73 nm in case of FLG and 6.33 nm in case of MLG. Moreover, the interlayer distance for MLG (0.337 nm) is similar with that of bulk graphite (0.335 nm) while for FLG this number is larger (0.397 nm) suggesting the presence of more wrinkled or disordered graphene sheets. UV–vis and XPS spectroscopy prove that TPyP molecules remained attached to graphene sheets at the end of the exfoliation process. The performances of the EGrTPyP layer deposited on top of a glassy carbon (GC) electrode were evaluated during the catechol (CAT) detection. In case of EGr-TPyP/GC modified electrode, the CAT redox process is highly accelerated, so the peak currents are significantly higher than those obtained with bare GC substrate. The GC electrode has a low sensitivity towards CAT detection (0.185 A• M−1 ·cm−2 ), a narrow linear range (one decade, 10−5 − 10−4 M) and a relatively high detection limit (LOD = 3.03 × 10−6 M). In contrast, the EGr-TPyP/GC electrode has a larger sensitivity (3.22 A• M−1 ·cm−2 ), a considerably wider linear range (two decades, 10−6 − 10−4 M) and lower detection limit (LOD = 3.03 × 10−7 M). © 2017 Elsevier B.V. All rights reserved.

1. Introduction Phenols of anthropogenic origin exist in the environment due to the activity of chemical, pharmaceutical or petrolier industries. Most of the synthetic phenolic compounds are toxic and constitute pollutants in water, food, and soil, therefore their detection is essential. For example, catechol (CAT) is used in photography, cosmetic, dye, rubber, synthetic material and insecticide production [1] and has been identified as one of the most abundant organic products in tobacco smoke [2]. It has been reported that exposure to catechol induces: high-blood pressure and upper respiratory tract

∗ Corresponding authors. E-mail addresses: fl[email protected] (F. Pog˘acean), [email protected] (S. Pruneanu).

irritation, as well as kidney damage and convulsions in high-doses [3]. Due to its high toxicity and harmful effects on the environment and human health, the catechol detection has become of great importance. Many analytical techniques are used to determine catechol, including chemiluminescence [4], high performance liquid chromatography [5], fluorescence, and gas chromatography/mass spectrometry [6]. Conventional methods are expensive, time consuming and quite complicated. Recently, electrochemical methods have been employed for the detection of catechol [7,8]. Since the bare electrodes are not convenient for the detection of catechol due to their poor sensitivity and fouling of signals by the oxidized products, modified electrodes with different materials (hybrid materials, nanomaterials, conductive polymers and biological molecules) have been used for its sensing [9–11]. Among these, graphene modified electrodes were found to be very efficient in the determination of the phenolic compounds [12].

https://doi.org/10.1016/j.snb.2017.09.205 0925-4005/© 2017 Elsevier B.V. All rights reserved.

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Fig. 1. TEM images of EGr-TPyP sample; scale bar 200 nm (a); HR-TEM images of EGr-TPyP sample, showing MLG and FLG (inset); scale bar 10 nm (b); Topography image at the edge of a graphene sheet (≈ 250 × 300 nm) composed of three overlapped flakes (c) − the thickness of the flakes varies between 5 and 8 nm.

Graphene is currently making an important impact in the electrochemistry field due to its high surface area, high chemical stability, and unique electrical and mechanical properties [13–15]. Various fabrication techniques such as micromechanical cleavage, epitaxial growth, chemical vapor deposition, and thermal/chemical reduction of graphene oxide [16] have been used to obtain graphene nanosheets. Recently, electrochemical exfoliation of graphite has attracted particular attention being an easy, fast and environmentally friendly strategy to produce high quality graphene. Ionic liquids and acidic solutions were mainly used as electrolytes in the electrochemical exfoliation reaction [17]. In our previous work [18] we reported a simple, cost-effective electrochemical approach to produce graphene by electrochemical exfoliation of graphite rods, in acidic electrolytes. X-ray powder diffraction was used as the main technique for graphene structural characterization. Nevertheless, the production of graphene in mild (near neutral) solution by electrochemical exfoliation was rarely reported [19]. Nowadays, there are a greater number of studies focused on the electrochemical exfoliation of graphite in various electrolytes [20–25] compared to the electrochemical production of functional graphene [26,27]. The electrochemical production of functional graphene still remains a non-controllable and a non-optimized process. Aromatic molecules, such as porphyrins, proved to be efficient assistants in the graphite exfoliation producing new hybrids with interesting properties in a single step [28]. Porphyrins are a group of heterocyclic organic compounds composed of four modified pyrrole rings interconnected with methine bridges. They interact with carbon materials through ␲–␲ stacking interactions. For this reason, they have been widely used to decorate carbon nanotubes,

fullerenes and graphene [29,30]. Until now, graphene sheets with undisturbed networks were prepared via porphyrin exfoliation of graphite in N-methyl-pyrrolidone [31]. Another efficient fabrication of single-layer nanographene hybrid platelets was achieved by graphite exfoliation using a free base porphyrin [28]. Electrodes modified with graphene-porphyrin composites can be used in the field of electrochemical sensors, such as the detection of dopamine [32], glucose [33] or low traces of explosives [34]. In this work we report the assembly of electrochemically exfoliated graphene and 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) in aqueous media. To the best of our knowledge, there is no report about the electrochemical exfoliation of graphite in the presence of TPyP in KCl electrolyte. The synergic effect between graphene and porphyrin led to a highly efficient electrocatalytic activity for the catechol oxidation. The novelty of our method lies in the use of a cost effective, non-toxic and environmental friendly electrolyte (0.2 M solution of potassium chloride). Our prepared material consists in a mixture of few layer and multi layer graphene which presents excellent electrocatalytic properties towards catechol detection. In addition the method ensures the preservation of graphene 2D structure, good reproducibility in the number of layers and excellent adherence to the transducer surface (glassy carbon).

2. Experimental 2.1. Chemicals All reagents were of analytical grade and used without further purification. Catechol, hydroquinone, resorcinol, and ethanol were

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2.3. Preparation of EGr-TPyP modified glassy carbon electrode (EGr-TPyP/GC)

Scheme1. Experimental set-up for electrochemical exfoliation of graphite in the presence of TPyP.

purchased from Merck (Germany) and N,N-Dimethylformamide (DMF) from J.T. Baker (Holland). KCl was obtained from Alfa-Aesar (Germany). High purity graphite rods (6 mm diameter, 99.995%) were obtained from Sigma-Aldrich and 5,10,15,20-tetra(4-pyridyl) porphyrin (TPyP) was obtained from Fluka. The buffer solutions, either acetate buffer or phosphate-buffered saline (PBS), were prepared with double-distilled water. 2.2. Preparation of electrochemically exfoliated graphene-porphyrin composite (EGr-TPyP) The material was synthesized by electrochemical exfoliation of a graphite rod in a two-electrode cell (see Scheme 1) in the presence of TPyP. We generally observed that at low voltage (7 V) the exfoliation process is extremely slow while at high voltage (12 V) the process is faster and the final material contains a large amount of graphite. Under optimized conditions, the electrolyte was 100 mL aqueous solution of 0.2 M KCl containing 6 × 10−6 M TPyP, previously dissolved by sonication (3 min) in 1 mL ethanol. The voltage between the two electrodes was set at + 9 V (DC). The experiment was carried out at room temperature, for 7 h. Under those conditions, the anode exfoliation mediated by TPyP took place and a black precipitate appeared in the solution. The exfoliated material was thoroughly washed with double distilled water several times (to remove the KCl salt) and then sonicated for 2 h. Finally, the solution was filtered with a Whatman qualitative filter paper (5895 ) in order to separate the small graphene nanosheets from the large ones, which remained on the filter. During the exfoliation process, porphyrin molecules remain attached to the graphene sheets, through noncovalent ␲–␲ interaction. The obtained material was dried by lyophilisation, using a Christ Alpha 1–4 LSC Freeze Dryer. As a blank test, the exfoliation of graphite rod in 0.2 M KCl electrolyte without porphyrin was carried out, but no material was collected after 7 h. In the presence of TPyP the exfoliation process was greatly improved. This may be explained by the fact that porphyrin can penetrate inside the graphite rod, promoting the exfoliation. We did the optimization of TPyP concentration in the electrolyte by taking into account the quality and electrochemical performances of the obtained materials. Hence, we observed that at low TPyP concentration (< 10−6 M) the exfoliation process was very slow and the amount of graphene collected after 8 h was around few mg. In contrast, the usage of a high porphyrin concentration (e.g. 6.6 × 10−5 M) in the electrolyte was not favorable for the exfoliation process since the final product consisted only of multi-layer graphene (e.g. 26 layers or more). In comparison, under optimum conditions (6 × 10−6 M porphyrin; 9 V applied bias) the product consisted mainly of bi-layer graphene (2 layers; 67%) with a small amount of multi-layer graphene (19 layers; 33%). During the optimization process, we did not obtain only few-layer graphene as final product. From the electrochemical point of view, the material obtained using high TPyP concentration has lower electro-catalytic performances compared to the material obtained under optimum conditions (see Fig. S1). A schematic representation of graphite electrochemical exfoliation in the presence of TPyP is depicted in Scheme 1.

Glassy carbon electrode was initially polished on a felt cloth then ultrasonically cleaned with double-distilled water and dried at room temperature for several hours. The optimization of the EGr-TPyP amount deposited on GC electrode was also performed. Generally, the minimum volume of EGr-TPyP solution (concentration of 1 mg/mL in DMF) necessary to fully cover the electrode surface was 8 ␮L. The addition of a larger volume of solution on the electrode surface (10 or 15 ␮L) leads to the increase of the capacitive current and decrease of faradic current (peak current, see Fig. S2). Therefore, the optimum amount was selected to be 8 ␮L. After 24 h, the modified electrode (EGr-TPyP/GC) was used for the electrochemical investigation of catechol. 2.4. Instruments The morphological characteristics of the obtained material were analyzed by Transmission Electron Microscopy (H-7650 120 kV Automatic TEM, Hitachi, Japan). The Atomic Force Microscopy analysis of graphene-porphyrin morphology was carried out with a Keysight 9500 Scanning Probe Microscope. The system was equipped with a standard 90 ␮m scanner. Imaging mode: Acoustic AC mode (tapping); k = 7 N/m, f = 1500 kHz; all closed loop control imaging; Quickscan nose cone. The samples were dispersed by sonication in ethanol, drop-casted on a freshly cleaved mica surface and dried at room temperature. The X-ray powder diffraction (XRD) patterns were collected with a Bruker D8 Advance diffractometer, using CuK␣1 radiation (␭=1.5406 Å). In order to increase the resolution, a Ge (111) monochromator in the incident beam was used, to filter out the K␣2 radiation. Before plotting the experimental results, the spectra were background corrected. UV–vis spectra were recorded with a Specord 250 PLUS Spectrophotometer (Analytik Jena, Germany). The samples (EGr-TPyP and TPyP) were dispersed in ethanol, before being measured in the 200–800 nm wavelength range. X-Ray Photoelectron Spectroscopy (XPS) measurements were performed using a SPECS spectrometer, equipped with a dualanode X-ray source Al/Mg, a PHOIBOS 150 2DCCD hemispherical energy analyzer and a multi-channeltron detector. The pressure inside the measurement chamber was maintained constant at about 1 × 10−9 torr. The sample, as colloidal suspension in methanol, was dried in successive layers on indium foil, previously attached to the wolfram sample holder with carbon tape. Irradiation was made with an AlK␣ X-ray source (1486.6 eV) operated at 200 W. The XPS survey spectra were recorded at 30 eV pass energy, 0.5 eV/step. The high resolution spectra for individual elements were recorded by accumulating 10 − 15 scans at 30 eV pass energy and 0.1 eV/step. The surface cleaning was ensured through argon ion bombardment at 500 V for 5 min. Data analysis and experimental curve fitting of the C 1s and N 1s spectra was performed using Casa XPS software with a Gaussian-Lorentzian product function and a non-linear Shirley background correction. The electrochemical measurements (Electrochemical Quartz Crystal Microbalance-EQCM, Cyclic Voltammetry-CV and Square Wave Voltammetry-SWV), were recorded with an Autolab PGSTAT-302N Potentiostat/Galvanostat (Metrohm-Autolab B.V., Netherlands), using a three-electrode cell. EQCM measurements were performed with a 6 MHz AT-cut quartz crystal covered with gold, having the surface area of 0.35 cm2 . The measurements were normally run between − 0.2 and + 0.7 V vs Ag/AgCl, with a scan rate of 10 mV• s−1 . Molecular modeling - The equilibrium geometry of the 7 × 7 graphene sheet − TPyP supramolecular complex was obtained

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Fig. 2. XRD spectrum of electrochemically exfoliated graphene-porphyrin composite (gray line); the deconvolution of the measured spectrum into few-layer (red line) and multi-layer graphene (blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. The UV–vis spectrum of EGr-TPyP in ethanol (0.1 mg·mL−1 ); inset: the spectrum of TPyP in ethanol (10−6 M).

distance (d) was found using Bragg equation [39] and the average number of graphene layers (n) was obtained using Eq. (1): at DFT level of theory, considering the MN12-SX [35] exchangecorrelation functional and def2-SVP [36] basis set implemented in the Gaussian 09 [37] program package. No negative wave numbers were obtained for all optimization cases proving that true minima of the potential energy surfaces were found.

3. Results and discussions 3.1. Morphological and structural characterization of EGr-TPyP and molecular modeling The morphological characteristics of the electrochemically exfoliated graphene-porphyrin composite can be seen in Fig. 1. The TEM image (Fig. 1a) reveals thin graphene sheets with scrolled edges (the dark lines). HR-TEM image of the same sample confirms the presence of multi-layer (Fig. 1b) and few-layer graphene (inset − Fig. 1b). The two-dimensional structure is excellently preserved, which is beneficial to the investigation of various electrochemical processes. The AFM investigation of EGr-TPyP sample can be seen in Fig. 1c. The image reveals a large graphene sheet (≈250 × 300 nm) of about 18 nm thickness, composed of three thinner flakes each formed of stacked carbon layers. From the corresponding profile line it is clear that the thin flakes have the thickness between 5 and 8 nm, corresponding to multi-layer graphene. The values found for the flakes thicknesses are in excellent agreement with the crystallite size determined by X-ray powder Diffraction (6.33 nm for multilayer graphene). The presence of attached TPyP molecules could not be observed during the TEM/AFM investigation, so further analyses were needed. X-Ray powder diffraction analysis was used to characterize the phase purity and crystalline nature of the synthesized material (Fig. 2). XRD provides three types of structural information: the interlayer distance (d), the number of graphene layers (n), and the size of graphene crystallites (D). The asymmetric peak around 25◦ (corresponding to (002) reflections of graphite) was separated by fitting into two Gaussian shaped peaks (few- and multi-layer graphene, FLG and MLG respectively). The mean crystallites size (D) was calculated from the full width at half maximum (FWHM) of each peak using the Debye-Scherrer equation [38] and was found to be 0.73 nm in case of FLG and 6.33 nm in case of MLG. The interlayer

n=

D d

(1)

In the case of MLG, the interlayer distance (0.337 nm) is similar with that of bulk graphite (0.335 nm) while for FLG this number is larger (0.397 nm) suggesting the presence of more wrinkled or disordered graphene sheets. As a result of the electrochemical exfoliation process, the obtained powder is a mixture of few-layer (2 layers; 67%) and multi-layer graphene (19 layers; 33%). The FLG:MLG ratio is around 2. XRD results proved the successful exfoliation of graphite to graphene but gave no indication about the presence of TPyP molecules in the obtained material. In order to investigate this, UV–vis and XPS spectroscopy measurements were carried out. Generally, porphyrins have several characteristic absorption bands in the 200–800 nm wavelength range, therefore the usage of UV–vis spectroscopy may help to clarify if TPyP molecules are attached to graphene surface. The intensity and color of porphyrins are due to their highly conjugated ␲-electron systems. The UV–vis spectrum of TPyP (10−6 M in ethanol) is similar with that of other porphyrins and consists of two distinct regions: the near ultraviolet region (380–500 nm) where the Soret band appears (molar extinction coefficient of 105 M−1 ·cm−1 ) and the visible region (500–750 nm) which contain a set of weaker intensity Q bands (molar extinction coefficient of 104 M−1 ·cm−1 ) (inset, Fig. 3). The Soret band is due to the transition of ␲-electrons from the ground state to the second excited state (S0 → S2) while the Q bands appear due to the weaker transitions to the first excited state (S0 →S1). The UV–vis spectrum of EGr-TPyP can be seen in Fig. 3, and exhibits two broad bands. The first one is at 271 nm and is characteristic to ␲– ␲* transition of electrons within the aromatic double bonds in graphene sheets. The second band appears at 413 nm (circled region) and may be fairly attributed to the presence of porphyrin molecules, attached to graphene. Comparing with pure TPyP in ethanol, its intensity is very low. Several reasons may be invoked, such as the low concentration (6 × 10−6 M TPyP) used during the electrochemical exfoliation of graphite, and the repeated washing procedures that may remove the TPyP molecules before interacting with graphene sheets. X-Ray Photoelectron Spectroscopy also confirmed that porphyrin molecules remained attached to graphene sheets. Fig. 4 shows the survey XPS data of EGr-TPyP. As expected, one can clearly see the well defined C and O peaks, with no distinct evidence for

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Fig. 4. The XPS survey spectrum of EGr-TPyP; (inset): the N 1s high resolution core-level spectra.

Fig. 5. Core level high-resolution N 1s (a) and C 1s (b) XPS spectra of EGr-TPyP.

the N presence. The nitrogen was seen only while performing high resolution measurements, in the (390 − 412) eV binding energy (BE) range − as is presented in the inset of Fig. 4. The low N 1s spectrum resolution suggests that the EGr-TPyP material contains a small amount of TPyP as already indicated by UV–vis results. A closer examination of the N 1s region (Fig. 5a) shows that the obtained signal contains three distinct peaks located at: 396.4 eV − for the two iminic N atoms ( N-, green), 399.8 eV − associated to the four N atoms in the peripheral pyridyl group (Npyridyl , blue) and 402.3 eV − corresponding to the two pyrrolic N atoms (-NH-, magenta). These values are in good agreement with other results from literature [40,41]. In Fig. 5b, the experimental C 1s spectrum (dotted) is compared with the approximated one (continuous), obtained by summing all identified peak components. Since for EGr-TPyP material different types of C bond peaks are mixed together in the (280 − 296) eV binding energy range, the C 1s experimental data was deconvoluted into six different components, corresponding to different carboncontaining groups. The most intense contribution is assigned to graphite, aromatic carbon atoms (284.4 eV) and represents 55.2% from the total C peak. At 285.4 eV is located the aliphatic carbon contribution (18.1%). We were not able to distinguish between the C N and C O bonded atoms, therefore we assigned just one contribution with a maximum around 286.5 eV, representing 11.3% from the total carbon peak. In addition, the keto (C O) − 287.9 eV (9.9%) and carboxylic (O C O) − 289.2 eV (3.9%) groups are also present. In the high binding energy range, at 292.9 eV, a small plasmon loss contribution (1.6%), assigned to the ␲→␲* shake-up satellite band of graphitic carbons, was also observed. The observed chemical shifts account for the difference in BE values of electrons in one specific chemical state of the atom versus the value relevant to pure element. The obtained values are in good agreement with literature [42–44]. The C/N ratio was found to be 39.14, confirming once again

that only a small amount of TPyP molecules remained attached to the exfoliated graphene sheets. The molecular modeling proved that the interaction between graphene and TPyP is mainly ␲-␲ stacking interaction. The geometry configuration of graphene sheet-TPyP supramolecular complex, considering its top- (a) and side-view (b) orientation, is presented in Scheme 2. The interatomic distance between the graphene sheets and porphyrin is 3.44 Å. The BSSE corrected intermolecular interaction energy was calculated using the same level of theory, but considering the larger def-SVPP [36] basis set. The binding energy between the almost flat 7 × 7 graphene sheet and the TPyP molecule is −33.84 kcal/mol. Such energy is strong enough to keep the TPyP molecule attached to graphene surface. The intermolecular interaction energy analysis of the optimized complex shows that the bound molecular configuration is exclusively given by electron correlation effects which are specific to ␲-␲ stacking interactions. The complex in its MN12-SX/def2SVP equilibrium geometry displays repulsive interaction at the Hatree-Fock (HF) level, showing a +32.75 kcal/mol energy value, which indicates that the electron correlation effects give a very strong attractive contribution (more than −66.00 kcal/mol) to the final interaction energy. In addition, it is worth mentioning that the pyridyl rings have no planar conformation being twisted with about 60◦ in comparison with the porphyrin plane.

3.2. Electrochemical studies The ability of EGr-TPyP/GC modified electrode to enhance the signal generated during the electrochemical oxidation of catechol, one of the pollutants belonging to the phenol family, is also pursued. The pH influence on the CV response can be seen in Fig. S3. The highest anodic peak current (green curve) was obtained in pH 6 solution, which is the reason for choosing this pH for further investigations.

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Scheme 2. The geometry configuration of the 7 × 7 graphene sheet − TPyP supramolecular complex [(a) − top view; (b) − side view]. Table 1 The performances of several modified electrodes towards catechol detection. Nanomaterial/electrode

Detection limit (␮M)

Linear range (␮M)

Reference

graphene-like carbon nanosheets/GCE gold/Ni(OH)2 nanocomposites supported on reduced graphene oxide/GCE nitrogen-doped graphene/GCE graphene/screen printed electrode gold nanoparticles decorated on graphene oxide@polydopamine composite/GCE carbon nanocages-reduced graphene oxide composites/GCE EGr-TPyP/GC

0.05 0.13

0.5–50 0.4 − 33.8

[46] [47]

0.5 1.7 0.015

5 − 492 15 − 50 0.3 − 67.55

[48] [49] [50]

0.4

1 − 300

[51]

0.303

1 − 100

This work

In order to better understand the redox process, the EQCM technique was employed, using a 6 MHz AT-cut quartz crystal covered with gold (surface area 0.35 cm2 ). Hence, successive cyclic voltammograms were recorded in pH 6 PBS solution containing 10−4 M CAT (see Fig. 6a). One can observe that on gold substrate, the catechol oxidation takes place at + 0.52 V while the reduction occurs at considerably lower potential, around + 0.07 V (Ep ˜≈450 mV, quasi-reversible process). In addition, the peak current intensity constantly decreases with the number of scans, indicating the diminution of the electrode active area. The CVs result is in good agreement with the variation of crystal oscillation frequency (f) vs potential (E), as shown in Fig. 6b. One can observe that in the capacitive current region (from −0.2 to 0.2 V) the frequency variation is very small, since no oxidation occurs. Once the potential exceeds the onset of the oxidation process, the oscillation frequency starts to decrease. At the end of the reduction process the frequency does not reach the initial value (zero), indicating that the oxidation products remain attached at the crystal surface. The frequency variation (f) can be used to determine the mass change (m) generated by catechol oxidation, as shown by Sauerbrey equation [45]: f = −Cf · m,

(2)

where f is the change in frequency (Hz), Cf is the sensitivity factor of the crystal (Hz·g−1 ·cm−2 ) and m is the change in mass per unit area (g · cm−2 ). The sensitivity factor Cf is provided by Eq. (3), where n is the number of harmonics at which the crystal is driven (n = 1, by design), f is the resonant frequency of the fundamental mode of the loaded crystal (Hz), q is the quartz density (2.648 g · cm−3 ) and q is the quartz shear modulus (2.947 × 1011 g · cm−1 · s−2 ). 2nf 2 Cf = √ q q

(3)

Fig. 6. Successive CVs recorded in pH6 PBS solution containing 10−4 M CAT; scan rate 10 mV·s−1 (a); the corresponding change in the crystal oscillation frequency (f) vs potential (E), (b).

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In our case, Cf was equal with 0.0815 Hz·ng−1 ·cm2 , for a 6 MHz crystal. Taking into consideration the frequency change (˜ 1.5 Hz) during the oxidation process (from −0.2 to 0.8 V), the corresponding mass change was determined to be 18.4 × 10−9 g·cm−2 (or 6.44 × 10−9 g, for the whole crystal area of 0.35 cm2 ). By dividing this value with the molar mass of CAT (110.1 g·mol−1 ), we obtained a number of 5.8 × 10−11 mol that was oxidized at the crystal surface. Next, using the Avogadro’s number (6.023 × 1023 particles·mol−1 ) we determined that 3.49 × 1013 CAT molecules were involved in the oxidation process. The electrochemical behavior of catechol at bare GC electrode (0.028 cm2 − active area) is similar with that occurring at gold substrate (Fig. S4a). The oxidation wave is larger than the reduction wave and the corresponding peak potentials are at 0.46 and 0.1 V respectively, indicating also a quasi-reversible process (Ep ˜≈360 mV). We also tested the catalytic ability of GC electrode modified only with TPyP. As can be seen in Fig. S4a, the cathodic peak current is higher than that corresponding to bare GC electrode. This may be attributed to nitrogen atoms from the pyridyl groups that may donate electrons and improve the reduction process of catechol. However, the peak potential separation remains very large (≈400 mV, quasi-reversible process). In the case of EGr-TPyP modified GC electrode (Fig. S4b) the redox process is highly accelerated and the two redox waves are well defined and closer to each other (Ep ˜≈60 mV, reversible process). In addition, the peak currents are significantly higher than those obtained with bare GC or TPyP/GC. So, one can conclude that the high electro-catalytic ability of the layer deposited on top of GC electrode may be attributed both to graphene and porphyrin molecules. Next, the catechol detection was investigated by SWV in pH 6 PBS solution, containing increasing concentration of catechol (10−6 - 10−4 M). Fig. 7 (a, b) shows the current variation as a function of CAT concentration, for bare GC (a) and graphene-porphyrin composite modified electrode, EGr-TPyP/GC (b). As expected, at bare GC the oxidation signal is considerable lower than that recorded with EGr-TPyP/GC electrode. This can be clearly seen in Fig. 7(c), where the corresponding calibration plots (Ipeak vs CAT concentration) are presented. For bare GC electrode the sensitivity towards CAT is low (0.185 A• M−1 ·cm−2 ) and the linear range is narrow (10−5 − 10−4 M) with a limit of detection of 3.03 × 10−6 M. In contrast, the EGr-TPyP/GC electrode has a higher sensitivity (3.22 A• M−1 ·cm−2 ), a considerably wider linear range (10−6 − 10−4 M) and one order of magnitude lower detection limit (LOD = 3.03 × 10−7 M). The performance of EGr-TPyP/GC electrode was further tested in the presence of interfering species, hydroquinone (HQ) and resorcinol (REZ). Hence, the catechol signal was recorded by SWV in pH 6 PBS solutions containing increasing concentrations of catechol (from 10−6 to 10−4 M) and 5 × 10−5 M hydroquinone (Fig. S5a). One can see that the HQ signal is well defined and appears at lower potential (0.16 V) compared with that of CAT (0.26 V). However, its presence in the studied solutions leads to a significant decrease of CAT peak intensity and of electrode sensitivity (1.4 A• M−1 ·cm−2 ) along with the increase of LOD value (1.82 × 10−6 M) (Fig. S5b). Next, the influence of resorcinol as interfering specie was tested during catechol detection (Fig. S5c). Similarly, the signal was recorded by SWV in pH 6 PBS solutions containing catechol (from 10−6 to 10−4 M) and 5 × 10−5 M resorcinol. It is interesting to see that resorcinol oxidation signal appears at a more positive potential (0.65 V) comparing with that of catechol (0.27 V). This may be explained by the differences in reactivity of these isomers. The reactivity of the aromatic ring activated with an OH group is higher when the OH group is in the ortho- or para- position. Consequently, hydroquinone and catechol have the aromatic ring activated, while the resorcinol ring is not activated. Its presence in the solution

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Fig. 7. SWVs recorded with bare GC and EGr-TPyP/GC electrodes in pH 6 PBS solutions containing increasing concentrations of CAT (a,b); calibration curves corresponding to bare GC (blue) and EGr-TPyP/GC modified electrode (red) (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

shifts upwards the entire calibration plot: the electrode sensitivity was higher (3.74 A• M−1 ·cm−2 ) but the LOD value was similar (3.03 × 10−7 M) (see also Fig. S5b). The performances of EGr-TPyP/GC electrode towards the detection of catechol were compared with those of other types of modified electrodes, found in the literature (see Table 1). One can see that the limit of detection as well as the linear range is comparable to that of other types of graphene-modified electrodes previously reported.

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8 Table 2 Determination of catechol in mineral and tap water. EGr-TPyP/GC

Added (M)

Found (M)

RSD (%)

Mineral water

10−5 3 × 10−5 10−4 10−5 3 × 10−5 10−4

0.95 × 10−5 3.3 × 10−5 1.04 × 10−4 0.97 × 10−5 2.96 × 10−5 1.03 × 10−4

6.21 5.22 3.15 4.49 5.07 6.48

Tap water

3.3. Real sample analysis The potential application of the EGr-TPyP/GC modified electrode for the determination of catechol in real sample was tested in two drinking water sources: tap water (the pH was adjusted to pH 6) and a commercial mineral water (pH 5.9) containing known quantities (mg/L) of interfering species (23.59 Na+ ; 4.75 K+ ; 60.14 Mg2+ ; 191.2 Ca2+ ; 11.12 Cl− ; 13.57 SO4 2− ). At the first scanning of the sample no signal from catechol was found, therefore the standard addition method was used. The determination of three concentrations of catechol was carried out and the results can be found in Table 2. One can see that the determined values are very close to those added, proving the applicability of the EGr-TPyP/GC modified electrode in real sample analysis. 4. Conclusions A novel approach for the synthesis of graphene-porphyrin composite through electrochemical exfoliation of graphite in the presence of porphyrin (EGr-TPyP) in 0.2 M KCl is reported. The structural investigations (XRD; UV–vis; XPS) prove that the obtained material is a mixture of few-layer and multi-layer graphene, having also a small amount of TPyP molecules attached to the graphene sheets. Glassy-carbon electrode modified with EGr-TPyP material exhibits superior performances, in terms of sensitivity, linear range and limit of detection in comparison with bare GC electrode, towards the detection of catechol. Acknowledgement This work was supported by the Romanian National Authority for Scientific Research and Innovation, CNCS-UEFISCDI, Project No. PN-II-RU-TE-2014-4-0305 and Project No. PN-II-PT-PCCA-20134-1282 (230/2014). TEM/HRTEM measurements were partially supported through the infrastructure obtained in the Project: Research Center and Advanced Technologies for Alternative Energies − CETATEA − 623/11.03.2014. The authors are grateful to Dr. Gerald Kada (Keysight Technologies GmbH, Linz, Austria) for AFM investigation and to Dr. Ioan Ovidiu Pana and Dr. Cristian Leostean for performing the XPS measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.snb.2017.09.205. References [1] J. Michałowicz, W. Duda, Phenols − Sources and toxicity, Polish J. Environ. Stud. 16 (2007) 347–362. [2] E. Bermudez, K. Stone, K.M. Carter, W.A. Pryor, Environmental tobacco smoke is just as damaging to DNA as mainstream smoke, Environ. Health Perspect. 102 (1994) 870–874. [3] K.R. Olson, Emergency Evaluation and Treatment, The McGraw-Hill Companies, United States of America, 2004. [4] L. Zhao, B. Lv, H. Yuan, Z. Zhou, D. Xiao, A Sensitive chemiluminescence method for determination of hydroquinone and catechol, Sensors 7 (2007) 578–588.

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Biographies Maria Coros was born in Medias, Romania. She received her B.Sc. and Ph.D. degrees from Babes-Bolyai University, Faculty of Chemistry and Chemical Engineering, ClujNapoca, in 2004 and 2010, respectively. Her recent research interests are focused on the usage of carbon nanomaterials, especially graphene, in the fabrication of new types of modified electrodes and their applications. Florina Pogacean was born in Turda, Romania. In 2001 she received her B.Sc. in Chemistry from Babes-Bolyai University and in 2011 her Ph.D., from the same Institution. She is currently Researcher at INCDTIM Cluj-Napoca, working with graphene-modified electrodes for analytical applications (detection of pharmaceutical pollutants). Lidia Magerusan received her Master Degree in Solid State Physics in 2006 and her Ph.D. in Physics in 2010, from Babes-Bolyai University. Since 2009 she is Researcher at INCDTIM, Cluj-Napoca. Her current research activity is focused on the synthesis, characterization and electrochemical applications of graphene-based nanocomposites. Marcela-Corina Rosu is currently researcher at INCDTIM Cluj-Napoca, working in the field of graphene/TiO2 materials. In 2007 she received her B.Sc. in Chemical-Physics from Babes-Bolyai University and in 2012 her Ph.D., from the same Institution. Her main research interests are related with the preparation, characterization and testing of TiO2 -based photosensitive semiconductors as photoelectrodes/photo-catalysts for environmental applications. Alin Sebastian Porav received his B.Sc. in Biology and M.Sc. in Molecular Biotechnology from Babes-Bolyai University, Cluj-Napoca (2008–2013). He is currently PhD student in Biology and he is also working as a Research Assistant in the Integrated Electron Microscopy Laboratory (LIME) at INCDTIM Cluj-Napoca, Romania. His main research interest is focused on the determination of the 3D structure of molecular complexes, mainly the light harvesting antenna from cyanobacteria, using negative staining TEM and cryo-TEM. Crina Socaci graduated from Babes-Bolyai University (Cluj-Napoca, 1999) and obtained her Ph.D. in Chemistry in 2005 at the same University. She is Senior Researcher at INCDTIM Cluj-Napoca and her research interest is related with the synthesis of carbon-based nanomaterials and their applications. Attila Bende received the B.Sc. and M.Sc. degrees in Physics from Babes-Bolyai University, Cluj-Napoca, Romania in 1996 and 1997, respectively. His current research interest includes molecular modeling, especially the theoretical study of the intermolecular interactions in molecular clusters as well as electronic excited states in molecules. Prof Dr Habil. Raluca-Ioana van Staden is Head of the Laboratory of Electrochemistry and PATLAB of the National Institute of Research for Electrochemistry and Condensed Matter and Professor at the University “Politehnica” from Bucharest. Raluca is member of the Executive Committee of Sensor Division of Electrochemical Society from USA, and Chair of the International Romanian Chapter of American Chemical Society. She published more than 250 papers in journals and more than 12 books and chapters in books in the field of electrochemical sensors with high emphasis in bioanalysis and biomedical analysis. Stela Maria Pruneanu received her B.Sc. in 1987 and her Ph.D. in 1999, from BabesBolyai University, Cluj-Napoca, Romania. During 2004–2008, she worked as Post Doc at Teesside University and Newcastle University, UK. She is currently Senior Researcher at INCDTIM Cluj-Napoca and her interest is focused on carbon-based nanocomposites for electrochemical and analytical applications.

Please cite this article in press as: M. Coros¸, et al., Graphene-porphyrin composite synthesis through graphite exfoliation: The electrochemical sensing of catechol, Sens. Actuators B: Chem. (2017), https://doi.org/10.1016/j.snb.2017.09.205