- Email: [email protected]
Graphene and carbon quantum dots electrochemistry
Electrochemistry Communications 52 (2015) 75–79
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Graphene and carbon quantum dots electrochemistry Chee Shan Lim a, Katerina Hola b, Adriano Ambrosi a, Radek Zboril b, Martin Pumera a,⁎ a b
Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link 637371, Singapore Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacký University, Olomouc 77146, Czech Republic
a r t i c l e
i n f o
Article history: Received 20 January 2015 Received in revised form 28 January 2015 Accepted 28 January 2015 Available online 4 February 2015 Keywords: Quantum dots Graphene Carbon Electron transfer
a b s t r a c t Graphene and carbon quantum (QDs) dots exhibit interesting and well-defined properties owing to their quantum confinement. In this work, graphene QDs (G-QDs) and carbon QDs of size ~6 nm and ~2 nm, respectively, were prepared and their potential uses in electrochemistry and electrochemical sensing were subsequently investigated. It was discovered that the C-QDs surface displayed a faster electron transfer rate compared to the G-QDs following analyses with the ferro/ferricyanide redox probe. Studies were also carried out with redox biomarkers such as uric acid (UA) and ascorbic acid (AA), and it was found that while the C-QDs displayed electrocatalytic properties toward the oxidation of both UA and AA, the G-QDs seemed to only have an impact on AA, from the decrease in the oxidation peak potential. This work provides direct electrochemical comparison of the two latest frontiers of carbon nanomaterials and opens the way for their electrochemical sensing applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon and graphene quantum dots are among the latest frontiers of the research on carbon nanomaterials [1–3]. They attract huge scientific attention due to their interesting fluorescent properties, mainly their excitation wavelength-dependent emission [4]. Owing to their low toxicity, good photostability and chemical inertness, quantum dots have developed applications in various fields of science, e.g., in vitro and in vivo bioimaging [5,6], biosensing [7–9], photoelectrochemical water splitting [10,11], photocatalysis [12] and light-emitting diodes [6,13]. In many aspects, they can replace the well-known quantum dots based on metal chalcogenides as a result of their excellent biocompatibility and low-cost preparation techniques. Graphene quantum dots and carbon quantum dots, in general, are materials with sizes in the nanometers range. Graphene quantum dots (G-QDs) are typically derived from graphene/graphite or other graphitic 3D material by topdown synthetic approaches. They normally exist in few-layer structures, with lateral dimension up to 100 nm [6,14]. On the contrary, carbon dots (C-QDs) are mostly prepared by bottom-up synthetic strategies and have spherical shape up to 10 nm [6]. These particles are also known as carbogenic dots or carbon nanodots [15,16]. Enormous efforts have been devoted to reveal the origin of the photoluminescence (PL) of C-QDs and G-QDs currently. The determination of the PL was carried out through the investigations of several aspects, including the type and amount of functional groups [17,18], the size of the graphitic core [4] as well as the type of carbon backbone [15]. However, little attempt was carried out on the investigation of the role ⁎ Corresponding author. E-mail address: [email protected] (M. Pumera).
http://dx.doi.org/10.1016/j.elecom.2015.01.023 1388-2481/© 2015 Elsevier B.V. All rights reserved.
of these parameters in the electrochemistry of G-QDs and C-QDs despite multiple studies dedicated to the electrochemistry of other nanocarbon counterparts such as graphene [19], carbon nanotubes [20,21], nanodiscs [22] and carbon black (with size of particles in the order of tens of nm) [23–27]. Here, we explore the electrochemistry of a simple electrochemical redox marker, ferro/ferricyanide and two important biomarkers, uric acid and ascorbic acid, using basal-plane pyrolytic graphite electrodes modified with G-QDs and C-QDs. 2. Experimental section 2.1. Materials Lauryl gallate, graphite puriss N99.0%, uric acid, ascorbic acid, potassium phosphate monobasic, sodium phosphate dibasic, potassium chloride, sodium chloride, potassium ferrocyanide, Dowex® 50WX4 hydrogen form (20–50 mesh) and dialysis tubing (benzoylated, MW cut off 2,000) were purchased from Sigma–Aldrich, Singapore and Czech Republic. Basal-plane pyrolytic graphite (BPPG) electrodes with a diameter of 3 mm were obtained from Autolab, Japan. Ag/AgCl reference and Pt counter electrodes were from CH Instruments, TX, USA. 2.2. Syntheses G-QDs were prepared from the graphite using the known procedure of a two-step cutting process [18]. C-QDs were prepared according to our procedure which was reported earlier [4,28]. After which, these as-prepared C-QDs were treated in the mixture of acetone and sodium hydroxide (10 mg of C-QDs in 5 mL of acetone and 5 mL of 0.5 M NaOH) to obtain water-dispersible C-QDs, the excess of sodium
76
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79
hydroxide was removed by passing the sample through Dowex® 50WX4 in hydrogen form.
A
2.3. Apparatus TEM images were acquired using a JEOL 2010 HC operating at 100 kV. Raman spectra were acquired using a DXR Raman microscope with the 780 nm excitation line of a diode laser. X-ray photoelectron spectroscopy (XPS) was performed with a Phoibos 100 spectrometer and a Mg X-ray radiation source (SPECS, Germany). The florescent spectra of the samples were recorded using QuantaMaster 40 and LaserStrobe Spectrofluorometers (PTI, USA). All voltammetric experiments were performed on a μAutolab type III electrochemical analyser (Eco Chemie, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie).
B
2.4. Procedures Electrochemical experiments were performed at room temperature using a three-electrode configuration. A platinum electrode (Autolab) served as an auxiliary electrode, while an Ag/AgCl electrode (CH Instruments, USA) served as a reference electrode. The BPPG electrode surface was renewed by pressing the surface onto an adhesive tape and removing the tape, which removes the top few layers of graphite at the same time. This was repeated several times before rinsing the electrode surface with acetone to remove any residual adhesive glue [29]. Cyclic voltammetry experiments were performed at a scan rate of 100 mV s− 1, using a background electrolyte of phosphate buffer solution (50 mM, pH 7.2). G-QDs and C-QDs were placed at the surface of BPPG electrode by drop cast method, where 1 μL of the required suspension was deposited onto the electrode surface and allowed to evaporate at room temperature. Typical colloidal solution of QDs contained 1 mg/mL of G-QDs and 1 mg/mL of C-QDs. 3. Results and discussion Prior to electrochemical studies, a detailed characterization was carried out on the graphene and carbon quantum dots. Figs. 1A and B show the transmission electron microscopic (TEM) images of the G-QDs and C-QDs. It is apparent that the size of the graphene quantum dots is ~6–7 nm while that of C-QDs is ~2 nm. It is noteworthy that diameter of such quantum dots is highly monodisperse. The Raman spectra of these materials show two dominant peaks, at ~1350 cm−1 and ~1560 cm−1, as displayed in Fig. 1C. These Raman signals correspond to the defects in the graphene basal plane (D-band) and the pristine sp2 lattice (G-band) structures, respectively. Both the G-QDs and C-QDs exhibited significant defect-related Raman signals (D-band), which are likely to originate from the large density of defects for the amorphous C-QDs and from the material synthesis for the damaged structure of G-QDs [18]. Subsequently, XPS analysis was performed on the sample of C-QDs and G-QDs deposited on silicon oxide surface from a water dispersion. As exemplified by the high resolution C1s scan in Fig. 2B, the sample CQDs was characterized by the presence of a substantial amount of oxygen-containing groups. In particular, the signal at about 286 eV, which was assigned to the C–O groups (hydroxyl, epoxy), contributes to about 9% of the C 1 s signal while the signal at about 288 eV, corresponding to C = O groups (carbonyl), makes up about 17.5%. Finally, the signal at higher binding energies was attributed to the presence of carboxylic groups (290 eV). Around 4% of the C 1 s signal comprises of these carboxyl groups. Similarly, the G-QDs sample also contained a significant amount of oxygen groups (see Fig. 2A). The signal of C–O groups contributes to 7% and the signal of carbonyl groups to 10% of the C 1 s signal. Carboxylic groups, however, were not present in C 1 s high resolution XPS spectrum of the G-QDs. It is therefore apparent that C-QDs
C
Fig. 1. (A, B) Transmission electron micrographs of (A) G-QDs and (B) C-QDs. Scale bar: 50 nm. (C) Raman spectra of graphene and carbon quantum dots. G-QDs: black line; C-QDs: red line.
contained larger amount of oxygen groups, concurring with previous postulation. However, the maximum fluorescent intensity for both samples was located at 445 nm (see Figs. 2C, D), suggesting comparable quantum confinement to the band gap structure of the aromatic domains of the quantum dots. Following the chemical and structural characterization by the means of Raman and XPS spectroscopies, the focus was shifted toward the electrochemistry of the graphene and carbon quantum dots. The G-QDs and C-QDs were deposited at the surface of a basal plane pyrolytic graphite (BPPG) electrode, and their inherent electrochemistry was then studied. The study was carried out as graphene oxide [30] and various reduced graphenes [31] (even in nanoscale dimensions [32,33]) have been known to exhibit significant inherent electrochemistry with oxygencontaining groups being oxidized or reduced electrochemically [34]. Fig. 3A shows cyclic voltammograms at BPPG electrodes modified with the respective quantum dots in 50 mM phosphate buffer solution (pH 7.2). It is observed that the C-QDs generated a small inherent anodic signal at ~0.4 V (vs. Ag/AgCl); this oxidation signal is most likely attributed to the quinone-like functionalities at the surface of C-QDs [34]. G-QDs,
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79
77
Fig. 2. Top: high resolution C 1s XPS spectrum of (A) graphene quantum dots and (B) carbon quantum dots. The inset spectra show the survey XPS scans of the samples. Bottom: fluorescence spectra of graphene (C) and carbon quantum dots (D). The spectra were recorded for different excitation wavelenghts of 250–500 nm. The emission maximum for both samples is located at 445 nm.
on the other hand, exhibited featureless voltammograms, as denoted by the decomposition of electrolyte at ~ +1.1 V and the reduction of protons at potentials lower than −1.0 V. Subsequently, the heterogeneous electron transfer (HET) rates of the materials were then put in comparison against the bare BPPG surface using a redox probe, potassium ferro/ferricyanide. As exemplified in Fig. 3B, the electrode surface modified with C-QDs generated the largest signals, with the smallest peak-to-peak separation (ΔEp-p) of 826 mV. Using the Nicholson's method, the C-QDs-modified surface induced a HET rate of 2.24 × 10−7 cm s−1. As for the G-QDs-modified and bare BPPG surfaces, their ΔEp-p are 947 mV and 960 mV, corresponding to HET rates of 4.33 × 10−8 cm s−1 and 3.63 × 10−8 cm s−1, respectively, both slower than that of C-QDs. Coupled with the larger peak currents attained as compared to the other two surfaces, the C-QDs proved to be a more promising material in the parameters of material sensitivity and HET rate. It is also noteworthy that the G-QDs generated larger peak signals than the bare BPPG surface, deeming it a more sensitive material for electrochemical sensing despite their comparable HET
rates. It should be noted that the ferro/ferricyanide is surfacesensitive, enabling its use in distinguishing between different carbon (graphene) surfaces, which is not possible with outer-sphere probes such as ruthenium hexamine [35]. Following which, the electrochemical oxidation of uric acid and ascorbic acid were performed at the three surfaces, and the results are displayed in Fig. 4. The electrochemistry of these biomarkers involves the adsorption step, which is more complex than the straightforward one-electron transfer electrochemistry of ferri/ferrocyanide as studied earlier. The oxidation of uric acid on the electrode modified with G-QDs occurred at about 625 mV, identical to the peak potential measured using the bare BPPG electrode. However, the oxidation peak of uric acid at C-QDs was obtained at a lower potential of 584 mV. This could possibly be due to the interaction of uric acid with the surface functionalities of the C-QDs. On the other hand, the oxidation of ascorbic acid on G-QD and CQD surfaces (~695 mV) occurred at an earlier potential than that obtained using the bare BPPG electrode (784 mV). It is apparent that both carbon and graphene QDs demonstrated fine electrocatalytic properties toward
Fig. 3. Cyclic voltammograms of G-QDs and C-QDs compared against bare BPPG in (A) 50 mM phosphate buffer (pH 7.2) and (B) 10 mM potassium ferro/ferricyanide. Scan rate: 100 mV s−1.
78
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79
Fig. 4. Cyclic voltammograms of 5 mM of (A) uric acid and (B) ascorbic acid at G-QDs-, C-QDs-modified electrodes and bare BPPG electrode. Conditions, 50 mM phosphate buffer (pH 7.2); scan rate, 100 mV s−1.
the oxidation of ascorbic acid. The different activity toward oxidation of uric acid (UA) and ascorbic acid (AA) was also reported for the case of reduced graphene oxide nanoribbons recently [36]. It was suggested that this observation originates from the different types of hydrogen bonding between AA (or UA) and graphene oxide functional groups; in particular, that between O–H groups of AA and C=O of reduced graphene oxide nanoribbons, and between N–H and C=O groups of uric acid and C=O and O–H groups of reduced graphene oxide nanoribbons.
[6]
[7]
[8] [9]
4. Conclusion
[10]
In this work, we have synthesized and characterized two different types of nanocarbon materials, graphene and carbon quantum dots. From an electrochemical standpoint, a faster electron transfer rate was observed at the electrode surface modified with carbon quantum dots compared to that modified with graphene quantum dots using potassium ferro/ferricyanide, a surface-sensitive redox probe. The oxidation of uric acid and ascorbic acid on these surfaces, on the other hand, exhibited small but visible decreases in the peak potential as compared to the bare BPPG electrode. While C-QDs displayed electrocatalytic properties toward the oxidation of both uric acid and ascorbic acid, the G-QDs were only able to lower the oxidation potential of the ascorbic acid biomarker. As a result, the C-QDs proved to be a better choice of material than the G-QDs with its slightly superior electrochemical properties, although both nanocarbon materials have outperformed the bare BPPG surface.
[11]
[12] [13]
[14]
[15]
[16] [17]
[18]
Acknowledgments [19]
M.P. acknowledges Tier 2 grant (MOE2013-T2-1-056) from Ministry of Education, Singapore. R. Z. and K. H. gratefully acknowledge the support by project LO1305 of the Ministry of Education, Youth and Sports of the Czech Republic. Financial support from the Internal Student Grant (IGA) of Palacký University in Olomouc, Czech Republic (IGA_PrF_2014023) is also gratefully acknowledged.
[21]
References
[24]
[1] S.N. Baker, G.A. Baker, Luminescent carbon nanodots: emergent nanolights, Angew. Chem. Int. Ed. 49 (2010) 6726–6744. [2] F. Liu, M.-H. Jang, H.D. Ha, J.-H. Kim, Y.-H. Cho, T.S. Seo, Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin of blue and green luminescence, Adv. Mater. 25 (2013) 3657–3662. [3] A. Ambrosi, C.K. Chua, A. Bonanni, M. Pumera, Electrochemistry of graphene and related materials, Chem. Rev. 114 (2014) 7150–7188. [4] K. Hola, A.B. Bourlinos, O. Kozak, K. Berka, K.M. Siskova, M. Havrdova, et al., Photoluminescence effects of graphitic core size and surface functional groups in carbon dots: COO− induced red-shift emission, Carbon 70 (2014) 279–286. [5] K.K.R. Datta, O. Kozak, V. Ranc, M. Havrdova, A.B. Bourlinos, K. Safarova, et al., Quaternized carbon dots modified graphene oxide for selective cell labelling—
[20]
[22] [23]
[25] [26]
[27]
[28]
controlled nucleus and cytoplasm imaging, Chem. Commun. 50 (2014) 10782–10785. K. Hola, Y. Zhang, Y. Wang, E.P. Giannelis, R. Zboril, A.L. Rogach, Carbon dots—emerging light emitters for bioimaging, cancer therapy and optoelectronics, Nano Today 9 (2014) 590–603. C.X. Guo, J. Xie, B. Wang, X. Zheng, H. Bin Yang, C.M. Li, A new class of fluorescentdots: long luminescent lifetime bio-dots self-assembled from DNA at low temperatures, Sci. Rep. 3 (2013) 2957. T.-Y. Yeh, C.-I. Wang, H.-T. Chang, Photoluminescent C-dots@RGO for sensitive detection of hydrogen peroxide and glucose, Talanta 115 (2013) 718–723. A. Martin, A. Escarpa, Graphene: the cutting-edge interaction between chemistry and electrochemistry, TrAC Trends Anal. Chem. 56 (2014) 13–26. X. Yu, R. Liu, G. Zhang, H. Cao, Carbon quantum dots as novel sensitizers for photoelectrochemical solar hydrogen generation and their size-dependent effect, Nanotechnology 24 (2013) 335401. T.-F. Yeh, C.-Y. Teng, S.-J. Chen, H. Teng, Nitrogen-doped graphene oxide quantum dots as photocatalysts for overall water-splitting under visible light illumination, Adv. Mater. 26 (2014) 3297–3303. H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, et al., Water-soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434. X. Zhang, Y. Zhang, Y. Wang, S. Kalytchuk, S.V. Kershaw, Y. Wang, et al., Colorswitchable electroluminescence of carbon dot light-emitting diodes, ACS Nano 7 (2013) 11234–11241. J. Lu, J. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, One-pot synthesis of fluorescent carbon graphene by the exfoliation of graphite in ionic liquids, ACS Nano 3 (2009) 2367–2375. L. Wang, S. Zhu, H. Wang, S. Qu, Y. Zhang, J. Zhang, et al., Common origin of green luminescence in carbon nanodots and graphene quantum dots, ACS Nano 8 (2014) 2541–2547. A.B. Bourlinos, A. Stassinopoulos, D. Anglos, R. Zboril, M. Karakassides, E.P. Giannelis, Surface functionalized carbogenic quantum dots, Small 4 (2008) 455–458. Y.-M. Long, C.-H. Zhou, Z.-L. Zhang, Z.-Q. Tian, L. Bao, Y. Lin, et al., Shifting and nonshifting fluorescence emitted by carbon nanodots, J. Mater. Chem. 22 (2012) 5917–5920. S.H. Jin, D.H. Kim, G.H. Jun, S.H. Hong, S. Jeon, Tuning the photoluminescence of graphene quantum dots through the charge transfer effect of functional groups, ACS Nano 7 (2013) 1239–1245. M. Pumera, Electrochemistry of graphene, graphene oxide and other graphenoids: review, Electrochem. Commun. 36 (2013) 14–18. C.E. Banks, T.J. Davies, G.G. Wildgoose, R.G. Compton, Electrocatalysis at graphite and carbon nanotube modified electrodes: edge-plane sites and tube ends are the reactive sites, Chem. Commun. (2005) 829–841. C.E. Banks, R.G. Compton, New electrodes for old: from carbon nanotubes to edge plane pyrolytic graphite, Analyst 131 (2006) 15–21. J.G.S. Moo, M. Pumera, Electrochemical properties of carbon nanodiscs, RSC Adv. 2 (2012) 1565–1568. H.L. Poh, M. Pumera, Nanoporous carbon materials for electrochemical sensing, Chem. Asian. J. 7 (2012) 412–416. H.L. Poh, A. Bonanni, M. Pumera, Nanoporous carbon as a sensing platform for DNA detection: the use of impedance spectroscopy for hairpin-DNA based assay, RSC Adv. 2 (2012) 1021–1024. D. Lowinsohn, P. Gan, K. Tschulik, J.S. Foord, R.G. Compton, Nanocarbon paste electrodes, Electroanalysis 25 (2013) 2435–2444. P.T. Lee, D. Lowinsohn, R.G. Compton, Simultaneous detection of homocysteine and cysteine in the presence of ascorbic acid and glutathione using a nanocarbon modified electrode, Electroanalysis 26 (2014) 1488–1496. P. Gan, D. Lowinsohn, J.S. Foord, R.G. Compton, The measurement of the Gibbs energy of transfer between oil and water using a nano-carbon paste electrode, Electroanalysis 26 (2014) 351–358, http://dx.doi.org/10.1002/elan.201300478. A.B. Bourlinos, M.A. Karakassides, A. Kouloumpis, D. Gournis, A. Bakandritsos, I. Papagiannouli, et al., Synthesis, characterization and non-linear optical response of organophilic carbon dots, Carbon 61 (2013) 640–649.
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79 [29] C.H.A. Wong, M. Pumera, On reproducibility of preparation of basal plane pyrolytic graphite electrode surface, Electrochem. Commun. 13 (2011) 1054–1059. [30] M. Zhou, Y. Wang, Y. Zhai, J. Zhai, W. Ren, F. Wang, et al., Controlled synthesis of large-area and patterned electrochemically reduced graphene oxide films, Chem. Eur. J. 15 (2009) 6116–6120. [31] E.L.K. Chng, M. Pumera, Solid-state electrochemistry of graphene oxides: absolute quantification of reducible groups using voltammetry, Chem. Asian. J. 6 (2011) 2899–2901. [32] A. Bonanni, A. Ambrosi, C.K. Chua, M. Pumera, Oxidation debris in graphene oxide is responsible for its inherent electroactivity, ACS Nano 8 (2014) 4197–4204.
79
[33] A.Y.S. Eng, M. Pumera, Direct voltammetry of colloidal graphene oxides, Electrochem. Commun. 43 (2014) 87–90. [34] A.Y.S. Eng, A. Ambrosi, C.K. Chua, F. Saněk, Z. Sofer, M. Pumera, Unusual inherent electrochemistry of graphene oxides prepared using permanganate oxidants, Chem. Eur. J. 19 (2013) 12673–12683. [35] R.L. McCreery, Advanced carbon electrode materials for molecular electrochemistry, Chem. Rev. 108 (2008) 2646–2687. [36] A. Martin, J. Hernández-Ferrer, L. Vázquez, M.-T. Martinez, A. Escarpa, Controlled chemistry of tailored graphene nanoribbons for electrochemistry: a rational approach to optimizing molecule detection, RSC Adv. 4 (2014) 132–139.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Graphene and carbon quantum dots electrochemistry Chee Shan Lim a, Katerina Hola b, Adriano Ambrosi a, Radek Zboril b, Martin Pumera a,⁎ a b
Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link 637371, Singapore Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacký University, Olomouc 77146, Czech Republic
a r t i c l e
i n f o
Article history: Received 20 January 2015 Received in revised form 28 January 2015 Accepted 28 January 2015 Available online 4 February 2015 Keywords: Quantum dots Graphene Carbon Electron transfer
a b s t r a c t Graphene and carbon quantum (QDs) dots exhibit interesting and well-defined properties owing to their quantum confinement. In this work, graphene QDs (G-QDs) and carbon QDs of size ~6 nm and ~2 nm, respectively, were prepared and their potential uses in electrochemistry and electrochemical sensing were subsequently investigated. It was discovered that the C-QDs surface displayed a faster electron transfer rate compared to the G-QDs following analyses with the ferro/ferricyanide redox probe. Studies were also carried out with redox biomarkers such as uric acid (UA) and ascorbic acid (AA), and it was found that while the C-QDs displayed electrocatalytic properties toward the oxidation of both UA and AA, the G-QDs seemed to only have an impact on AA, from the decrease in the oxidation peak potential. This work provides direct electrochemical comparison of the two latest frontiers of carbon nanomaterials and opens the way for their electrochemical sensing applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon and graphene quantum dots are among the latest frontiers of the research on carbon nanomaterials [1–3]. They attract huge scientific attention due to their interesting fluorescent properties, mainly their excitation wavelength-dependent emission [4]. Owing to their low toxicity, good photostability and chemical inertness, quantum dots have developed applications in various fields of science, e.g., in vitro and in vivo bioimaging [5,6], biosensing [7–9], photoelectrochemical water splitting [10,11], photocatalysis [12] and light-emitting diodes [6,13]. In many aspects, they can replace the well-known quantum dots based on metal chalcogenides as a result of their excellent biocompatibility and low-cost preparation techniques. Graphene quantum dots and carbon quantum dots, in general, are materials with sizes in the nanometers range. Graphene quantum dots (G-QDs) are typically derived from graphene/graphite or other graphitic 3D material by topdown synthetic approaches. They normally exist in few-layer structures, with lateral dimension up to 100 nm [6,14]. On the contrary, carbon dots (C-QDs) are mostly prepared by bottom-up synthetic strategies and have spherical shape up to 10 nm [6]. These particles are also known as carbogenic dots or carbon nanodots [15,16]. Enormous efforts have been devoted to reveal the origin of the photoluminescence (PL) of C-QDs and G-QDs currently. The determination of the PL was carried out through the investigations of several aspects, including the type and amount of functional groups [17,18], the size of the graphitic core [4] as well as the type of carbon backbone [15]. However, little attempt was carried out on the investigation of the role ⁎ Corresponding author. E-mail address: [email protected] (M. Pumera).
http://dx.doi.org/10.1016/j.elecom.2015.01.023 1388-2481/© 2015 Elsevier B.V. All rights reserved.
of these parameters in the electrochemistry of G-QDs and C-QDs despite multiple studies dedicated to the electrochemistry of other nanocarbon counterparts such as graphene [19], carbon nanotubes [20,21], nanodiscs [22] and carbon black (with size of particles in the order of tens of nm) [23–27]. Here, we explore the electrochemistry of a simple electrochemical redox marker, ferro/ferricyanide and two important biomarkers, uric acid and ascorbic acid, using basal-plane pyrolytic graphite electrodes modified with G-QDs and C-QDs. 2. Experimental section 2.1. Materials Lauryl gallate, graphite puriss N99.0%, uric acid, ascorbic acid, potassium phosphate monobasic, sodium phosphate dibasic, potassium chloride, sodium chloride, potassium ferrocyanide, Dowex® 50WX4 hydrogen form (20–50 mesh) and dialysis tubing (benzoylated, MW cut off 2,000) were purchased from Sigma–Aldrich, Singapore and Czech Republic. Basal-plane pyrolytic graphite (BPPG) electrodes with a diameter of 3 mm were obtained from Autolab, Japan. Ag/AgCl reference and Pt counter electrodes were from CH Instruments, TX, USA. 2.2. Syntheses G-QDs were prepared from the graphite using the known procedure of a two-step cutting process [18]. C-QDs were prepared according to our procedure which was reported earlier [4,28]. After which, these as-prepared C-QDs were treated in the mixture of acetone and sodium hydroxide (10 mg of C-QDs in 5 mL of acetone and 5 mL of 0.5 M NaOH) to obtain water-dispersible C-QDs, the excess of sodium
76
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79
hydroxide was removed by passing the sample through Dowex® 50WX4 in hydrogen form.
A
2.3. Apparatus TEM images were acquired using a JEOL 2010 HC operating at 100 kV. Raman spectra were acquired using a DXR Raman microscope with the 780 nm excitation line of a diode laser. X-ray photoelectron spectroscopy (XPS) was performed with a Phoibos 100 spectrometer and a Mg X-ray radiation source (SPECS, Germany). The florescent spectra of the samples were recorded using QuantaMaster 40 and LaserStrobe Spectrofluorometers (PTI, USA). All voltammetric experiments were performed on a μAutolab type III electrochemical analyser (Eco Chemie, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie).
B
2.4. Procedures Electrochemical experiments were performed at room temperature using a three-electrode configuration. A platinum electrode (Autolab) served as an auxiliary electrode, while an Ag/AgCl electrode (CH Instruments, USA) served as a reference electrode. The BPPG electrode surface was renewed by pressing the surface onto an adhesive tape and removing the tape, which removes the top few layers of graphite at the same time. This was repeated several times before rinsing the electrode surface with acetone to remove any residual adhesive glue [29]. Cyclic voltammetry experiments were performed at a scan rate of 100 mV s− 1, using a background electrolyte of phosphate buffer solution (50 mM, pH 7.2). G-QDs and C-QDs were placed at the surface of BPPG electrode by drop cast method, where 1 μL of the required suspension was deposited onto the electrode surface and allowed to evaporate at room temperature. Typical colloidal solution of QDs contained 1 mg/mL of G-QDs and 1 mg/mL of C-QDs. 3. Results and discussion Prior to electrochemical studies, a detailed characterization was carried out on the graphene and carbon quantum dots. Figs. 1A and B show the transmission electron microscopic (TEM) images of the G-QDs and C-QDs. It is apparent that the size of the graphene quantum dots is ~6–7 nm while that of C-QDs is ~2 nm. It is noteworthy that diameter of such quantum dots is highly monodisperse. The Raman spectra of these materials show two dominant peaks, at ~1350 cm−1 and ~1560 cm−1, as displayed in Fig. 1C. These Raman signals correspond to the defects in the graphene basal plane (D-band) and the pristine sp2 lattice (G-band) structures, respectively. Both the G-QDs and C-QDs exhibited significant defect-related Raman signals (D-band), which are likely to originate from the large density of defects for the amorphous C-QDs and from the material synthesis for the damaged structure of G-QDs [18]. Subsequently, XPS analysis was performed on the sample of C-QDs and G-QDs deposited on silicon oxide surface from a water dispersion. As exemplified by the high resolution C1s scan in Fig. 2B, the sample CQDs was characterized by the presence of a substantial amount of oxygen-containing groups. In particular, the signal at about 286 eV, which was assigned to the C–O groups (hydroxyl, epoxy), contributes to about 9% of the C 1 s signal while the signal at about 288 eV, corresponding to C = O groups (carbonyl), makes up about 17.5%. Finally, the signal at higher binding energies was attributed to the presence of carboxylic groups (290 eV). Around 4% of the C 1 s signal comprises of these carboxyl groups. Similarly, the G-QDs sample also contained a significant amount of oxygen groups (see Fig. 2A). The signal of C–O groups contributes to 7% and the signal of carbonyl groups to 10% of the C 1 s signal. Carboxylic groups, however, were not present in C 1 s high resolution XPS spectrum of the G-QDs. It is therefore apparent that C-QDs
C
Fig. 1. (A, B) Transmission electron micrographs of (A) G-QDs and (B) C-QDs. Scale bar: 50 nm. (C) Raman spectra of graphene and carbon quantum dots. G-QDs: black line; C-QDs: red line.
contained larger amount of oxygen groups, concurring with previous postulation. However, the maximum fluorescent intensity for both samples was located at 445 nm (see Figs. 2C, D), suggesting comparable quantum confinement to the band gap structure of the aromatic domains of the quantum dots. Following the chemical and structural characterization by the means of Raman and XPS spectroscopies, the focus was shifted toward the electrochemistry of the graphene and carbon quantum dots. The G-QDs and C-QDs were deposited at the surface of a basal plane pyrolytic graphite (BPPG) electrode, and their inherent electrochemistry was then studied. The study was carried out as graphene oxide [30] and various reduced graphenes [31] (even in nanoscale dimensions [32,33]) have been known to exhibit significant inherent electrochemistry with oxygencontaining groups being oxidized or reduced electrochemically [34]. Fig. 3A shows cyclic voltammograms at BPPG electrodes modified with the respective quantum dots in 50 mM phosphate buffer solution (pH 7.2). It is observed that the C-QDs generated a small inherent anodic signal at ~0.4 V (vs. Ag/AgCl); this oxidation signal is most likely attributed to the quinone-like functionalities at the surface of C-QDs [34]. G-QDs,
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79
77
Fig. 2. Top: high resolution C 1s XPS spectrum of (A) graphene quantum dots and (B) carbon quantum dots. The inset spectra show the survey XPS scans of the samples. Bottom: fluorescence spectra of graphene (C) and carbon quantum dots (D). The spectra were recorded for different excitation wavelenghts of 250–500 nm. The emission maximum for both samples is located at 445 nm.
on the other hand, exhibited featureless voltammograms, as denoted by the decomposition of electrolyte at ~ +1.1 V and the reduction of protons at potentials lower than −1.0 V. Subsequently, the heterogeneous electron transfer (HET) rates of the materials were then put in comparison against the bare BPPG surface using a redox probe, potassium ferro/ferricyanide. As exemplified in Fig. 3B, the electrode surface modified with C-QDs generated the largest signals, with the smallest peak-to-peak separation (ΔEp-p) of 826 mV. Using the Nicholson's method, the C-QDs-modified surface induced a HET rate of 2.24 × 10−7 cm s−1. As for the G-QDs-modified and bare BPPG surfaces, their ΔEp-p are 947 mV and 960 mV, corresponding to HET rates of 4.33 × 10−8 cm s−1 and 3.63 × 10−8 cm s−1, respectively, both slower than that of C-QDs. Coupled with the larger peak currents attained as compared to the other two surfaces, the C-QDs proved to be a more promising material in the parameters of material sensitivity and HET rate. It is also noteworthy that the G-QDs generated larger peak signals than the bare BPPG surface, deeming it a more sensitive material for electrochemical sensing despite their comparable HET
rates. It should be noted that the ferro/ferricyanide is surfacesensitive, enabling its use in distinguishing between different carbon (graphene) surfaces, which is not possible with outer-sphere probes such as ruthenium hexamine [35]. Following which, the electrochemical oxidation of uric acid and ascorbic acid were performed at the three surfaces, and the results are displayed in Fig. 4. The electrochemistry of these biomarkers involves the adsorption step, which is more complex than the straightforward one-electron transfer electrochemistry of ferri/ferrocyanide as studied earlier. The oxidation of uric acid on the electrode modified with G-QDs occurred at about 625 mV, identical to the peak potential measured using the bare BPPG electrode. However, the oxidation peak of uric acid at C-QDs was obtained at a lower potential of 584 mV. This could possibly be due to the interaction of uric acid with the surface functionalities of the C-QDs. On the other hand, the oxidation of ascorbic acid on G-QD and CQD surfaces (~695 mV) occurred at an earlier potential than that obtained using the bare BPPG electrode (784 mV). It is apparent that both carbon and graphene QDs demonstrated fine electrocatalytic properties toward
Fig. 3. Cyclic voltammograms of G-QDs and C-QDs compared against bare BPPG in (A) 50 mM phosphate buffer (pH 7.2) and (B) 10 mM potassium ferro/ferricyanide. Scan rate: 100 mV s−1.
78
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79
Fig. 4. Cyclic voltammograms of 5 mM of (A) uric acid and (B) ascorbic acid at G-QDs-, C-QDs-modified electrodes and bare BPPG electrode. Conditions, 50 mM phosphate buffer (pH 7.2); scan rate, 100 mV s−1.
the oxidation of ascorbic acid. The different activity toward oxidation of uric acid (UA) and ascorbic acid (AA) was also reported for the case of reduced graphene oxide nanoribbons recently [36]. It was suggested that this observation originates from the different types of hydrogen bonding between AA (or UA) and graphene oxide functional groups; in particular, that between O–H groups of AA and C=O of reduced graphene oxide nanoribbons, and between N–H and C=O groups of uric acid and C=O and O–H groups of reduced graphene oxide nanoribbons.
[6]
[7]
[8] [9]
4. Conclusion
[10]
In this work, we have synthesized and characterized two different types of nanocarbon materials, graphene and carbon quantum dots. From an electrochemical standpoint, a faster electron transfer rate was observed at the electrode surface modified with carbon quantum dots compared to that modified with graphene quantum dots using potassium ferro/ferricyanide, a surface-sensitive redox probe. The oxidation of uric acid and ascorbic acid on these surfaces, on the other hand, exhibited small but visible decreases in the peak potential as compared to the bare BPPG electrode. While C-QDs displayed electrocatalytic properties toward the oxidation of both uric acid and ascorbic acid, the G-QDs were only able to lower the oxidation potential of the ascorbic acid biomarker. As a result, the C-QDs proved to be a better choice of material than the G-QDs with its slightly superior electrochemical properties, although both nanocarbon materials have outperformed the bare BPPG surface.
[11]
[12] [13]
[14]
[15]
[16] [17]
[18]
Acknowledgments [19]
M.P. acknowledges Tier 2 grant (MOE2013-T2-1-056) from Ministry of Education, Singapore. R. Z. and K. H. gratefully acknowledge the support by project LO1305 of the Ministry of Education, Youth and Sports of the Czech Republic. Financial support from the Internal Student Grant (IGA) of Palacký University in Olomouc, Czech Republic (IGA_PrF_2014023) is also gratefully acknowledged.
[21]
References
[24]
[1] S.N. Baker, G.A. Baker, Luminescent carbon nanodots: emergent nanolights, Angew. Chem. Int. Ed. 49 (2010) 6726–6744. [2] F. Liu, M.-H. Jang, H.D. Ha, J.-H. Kim, Y.-H. Cho, T.S. Seo, Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin of blue and green luminescence, Adv. Mater. 25 (2013) 3657–3662. [3] A. Ambrosi, C.K. Chua, A. Bonanni, M. Pumera, Electrochemistry of graphene and related materials, Chem. Rev. 114 (2014) 7150–7188. [4] K. Hola, A.B. Bourlinos, O. Kozak, K. Berka, K.M. Siskova, M. Havrdova, et al., Photoluminescence effects of graphitic core size and surface functional groups in carbon dots: COO− induced red-shift emission, Carbon 70 (2014) 279–286. [5] K.K.R. Datta, O. Kozak, V. Ranc, M. Havrdova, A.B. Bourlinos, K. Safarova, et al., Quaternized carbon dots modified graphene oxide for selective cell labelling—
[20]
[22] [23]
[25] [26]
[27]
[28]
controlled nucleus and cytoplasm imaging, Chem. Commun. 50 (2014) 10782–10785. K. Hola, Y. Zhang, Y. Wang, E.P. Giannelis, R. Zboril, A.L. Rogach, Carbon dots—emerging light emitters for bioimaging, cancer therapy and optoelectronics, Nano Today 9 (2014) 590–603. C.X. Guo, J. Xie, B. Wang, X. Zheng, H. Bin Yang, C.M. Li, A new class of fluorescentdots: long luminescent lifetime bio-dots self-assembled from DNA at low temperatures, Sci. Rep. 3 (2013) 2957. T.-Y. Yeh, C.-I. Wang, H.-T. Chang, Photoluminescent C-dots@RGO for sensitive detection of hydrogen peroxide and glucose, Talanta 115 (2013) 718–723. A. Martin, A. Escarpa, Graphene: the cutting-edge interaction between chemistry and electrochemistry, TrAC Trends Anal. Chem. 56 (2014) 13–26. X. Yu, R. Liu, G. Zhang, H. Cao, Carbon quantum dots as novel sensitizers for photoelectrochemical solar hydrogen generation and their size-dependent effect, Nanotechnology 24 (2013) 335401. T.-F. Yeh, C.-Y. Teng, S.-J. Chen, H. Teng, Nitrogen-doped graphene oxide quantum dots as photocatalysts for overall water-splitting under visible light illumination, Adv. Mater. 26 (2014) 3297–3303. H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, et al., Water-soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434. X. Zhang, Y. Zhang, Y. Wang, S. Kalytchuk, S.V. Kershaw, Y. Wang, et al., Colorswitchable electroluminescence of carbon dot light-emitting diodes, ACS Nano 7 (2013) 11234–11241. J. Lu, J. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, One-pot synthesis of fluorescent carbon graphene by the exfoliation of graphite in ionic liquids, ACS Nano 3 (2009) 2367–2375. L. Wang, S. Zhu, H. Wang, S. Qu, Y. Zhang, J. Zhang, et al., Common origin of green luminescence in carbon nanodots and graphene quantum dots, ACS Nano 8 (2014) 2541–2547. A.B. Bourlinos, A. Stassinopoulos, D. Anglos, R. Zboril, M. Karakassides, E.P. Giannelis, Surface functionalized carbogenic quantum dots, Small 4 (2008) 455–458. Y.-M. Long, C.-H. Zhou, Z.-L. Zhang, Z.-Q. Tian, L. Bao, Y. Lin, et al., Shifting and nonshifting fluorescence emitted by carbon nanodots, J. Mater. Chem. 22 (2012) 5917–5920. S.H. Jin, D.H. Kim, G.H. Jun, S.H. Hong, S. Jeon, Tuning the photoluminescence of graphene quantum dots through the charge transfer effect of functional groups, ACS Nano 7 (2013) 1239–1245. M. Pumera, Electrochemistry of graphene, graphene oxide and other graphenoids: review, Electrochem. Commun. 36 (2013) 14–18. C.E. Banks, T.J. Davies, G.G. Wildgoose, R.G. Compton, Electrocatalysis at graphite and carbon nanotube modified electrodes: edge-plane sites and tube ends are the reactive sites, Chem. Commun. (2005) 829–841. C.E. Banks, R.G. Compton, New electrodes for old: from carbon nanotubes to edge plane pyrolytic graphite, Analyst 131 (2006) 15–21. J.G.S. Moo, M. Pumera, Electrochemical properties of carbon nanodiscs, RSC Adv. 2 (2012) 1565–1568. H.L. Poh, M. Pumera, Nanoporous carbon materials for electrochemical sensing, Chem. Asian. J. 7 (2012) 412–416. H.L. Poh, A. Bonanni, M. Pumera, Nanoporous carbon as a sensing platform for DNA detection: the use of impedance spectroscopy for hairpin-DNA based assay, RSC Adv. 2 (2012) 1021–1024. D. Lowinsohn, P. Gan, K. Tschulik, J.S. Foord, R.G. Compton, Nanocarbon paste electrodes, Electroanalysis 25 (2013) 2435–2444. P.T. Lee, D. Lowinsohn, R.G. Compton, Simultaneous detection of homocysteine and cysteine in the presence of ascorbic acid and glutathione using a nanocarbon modified electrode, Electroanalysis 26 (2014) 1488–1496. P. Gan, D. Lowinsohn, J.S. Foord, R.G. Compton, The measurement of the Gibbs energy of transfer between oil and water using a nano-carbon paste electrode, Electroanalysis 26 (2014) 351–358, http://dx.doi.org/10.1002/elan.201300478. A.B. Bourlinos, M.A. Karakassides, A. Kouloumpis, D. Gournis, A. Bakandritsos, I. Papagiannouli, et al., Synthesis, characterization and non-linear optical response of organophilic carbon dots, Carbon 61 (2013) 640–649.
C.S. Lim et al. / Electrochemistry Communications 52 (2015) 75–79 [29] C.H.A. Wong, M. Pumera, On reproducibility of preparation of basal plane pyrolytic graphite electrode surface, Electrochem. Commun. 13 (2011) 1054–1059. [30] M. Zhou, Y. Wang, Y. Zhai, J. Zhai, W. Ren, F. Wang, et al., Controlled synthesis of large-area and patterned electrochemically reduced graphene oxide films, Chem. Eur. J. 15 (2009) 6116–6120. [31] E.L.K. Chng, M. Pumera, Solid-state electrochemistry of graphene oxides: absolute quantification of reducible groups using voltammetry, Chem. Asian. J. 6 (2011) 2899–2901. [32] A. Bonanni, A. Ambrosi, C.K. Chua, M. Pumera, Oxidation debris in graphene oxide is responsible for its inherent electroactivity, ACS Nano 8 (2014) 4197–4204.
79
[33] A.Y.S. Eng, M. Pumera, Direct voltammetry of colloidal graphene oxides, Electrochem. Commun. 43 (2014) 87–90. [34] A.Y.S. Eng, A. Ambrosi, C.K. Chua, F. Saněk, Z. Sofer, M. Pumera, Unusual inherent electrochemistry of graphene oxides prepared using permanganate oxidants, Chem. Eur. J. 19 (2013) 12673–12683. [35] R.L. McCreery, Advanced carbon electrode materials for molecular electrochemistry, Chem. Rev. 108 (2008) 2646–2687. [36] A. Martin, J. Hernández-Ferrer, L. Vázquez, M.-T. Martinez, A. Escarpa, Controlled chemistry of tailored graphene nanoribbons for electrochemistry: a rational approach to optimizing molecule detection, RSC Adv. 4 (2014) 132–139.