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Free-standing palladium modified reduced graphene oxide paper based on one-pot co-reduction and its sensing application
Chemical Physics Letters 712 (2018) 71–77
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Free-standing palladium modified reduced graphene oxide paper based on one-pot co-reduction and its sensing application
T
Zhan-Hong Lia,b, Xue-Ling Zhaoa,b, Run-Min Songa, Cheng Chena,b, Peng-Ju Weia, ⁎ Zhi-Gang Zhua,b, a b
School of Environmental and Materials Engineering, College of Engineering, Shanghai Polytechnic University, Shanghai 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai 201209, China
H I GH L IG H T S
facile palladium functionalized reduced graphene oxide paper was prepared via one-pot co-reduction method. • AEvenly-distributed Pd particles show to decorate graphene paper surface and interpose between graphene sheets. • As-prepared freestanding Pd/rGOP can potentially be used as a glucose sensor. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemistry Catalysis Glucose sensor Non-enzymatic
A free-standing palladium/reduced graphene oxide paper (Pd/rGOP) based on one-pot co-reduction was prepared. The as-prepared Pd/rGOPs contain evenly distributed Pd particles interposing between the rGOP layers. The structure and composition of Pd/rGOP were characterized by SEM, EDX, XRD and Raman spectra. We investigated the glucose sensing performance of Pd/rGOP by chronoamperometry, the sensitivity reach as high as 6.70 µA mM−1 cm−2 with the linear range between 0.5 mM and 8 mM. The sensor also exhibits a good antiinterference ability and detection selectivity toward the interference. The proposed sensor has great potential to become a reliable tool in point-of-care medical devices.
1. Introduction In recent years, portable and wearable devices have attracted great interest because of their great convenience. However, the development of an ideal functionalized, low-cost, and lightweight material for such devices is still challenging. Paper-like conductive materials may be an alternative because of their applications in sensors [1], Li-ion batteries [2], supercapacitors [3], fuel cells [4], hydrogen storage [5] and so on. Graphene is a two-dimensional (2D) atom-thin sheet with sp2-hybridized carbons that has been intensively studied because of its unique thermal, mechanical, chemical and electric properties [6]. Graphene oxide (GO) contains oxygen functional groups on its basal planes and edges, resulting in its easy dispersibility in water. On the other hand, graphene oxide has poor electrical conductivity due to the disruption of its sp2 bonding networks by oxygen functional groups. Thus, the poor electrical conductivity may limit the use of GO in electrical or electrochemical applications such as supercapacitors [7,8], fuel cells [9–11], Li-ion batteries [12,13] and sensors [14,15]. To recover the
⁎
Corresponding author. E-mail address: [email protected] (Z.-G. Zhu).
https://doi.org/10.1016/j.cplett.2018.09.047 Received 22 June 2018; Accepted 21 September 2018 Available online 22 September 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
electrical conductivity of GO, most of the oxygen functional groups must be removed, thus obtaining fully reduced GO. However, the reduced graphene oxide (rGO) is difficult to disperse in water due to its tendency to aggregate. Recently, GO or rGO was selected to assemble into well-ordered and free-standing paper-like materials [16,17]. Compared to other paper-like materials (carbon nanotubes or graphite), GO or rGO paper is superior due to its mechanical strength [18]. The preparation of GO paper (GOP) involves vacuum filtration [16,19–23], evaporation [24–26], mechanical compression [27,28], speed vacuum centrifugation [29], mold-casting [30] and other techniques. Vacuum filtration is a rapid, simple, low-cost procedure that is most commonly used to obtain GOP. To prepare rGO paper (rGOP) or its functionalized materials, normally, a three-step method has been selected based on vacuum filtration [19,21–23]. To facilitate the preparation of metalfunctionalized rGOP, the metal salt and GO could be co-reduced by reductant, and then, the vacuum filtration is conducted to obtain the metal particle-functionalized rGOP. Correspondingly, the functionalized rGOP made by metal particles interposing between graphene
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3. Results and discussion
sheets can be prepared. As one of the transition metals, palladium is widely used as an electrocatalytic active center. In this work, a free-standing palladium/ reduced graphene oxide paper (Pd/rGOP) based on one-pot co-reduction was prepared using vacuum filtration. We co-reduced PdCl2 and GO by hydrazine in the presence of polyvinyl pyrrolidone (PVP) in a one-pot reaction followed by vacuum filtration, thus obtaining a freestanding Pd/rGOP. The one-pot co-reduction facilitates the preparation of functionalized rGOP. Hydrazine acts as the reductant and promoting the dispersion of rGO; PVP plays an important role in effectively reducing the aggregation of rGO during the process. The as-prepared Pd/ rGOP showed its potential application in glucose sensing.
Using simple vacuum filtration, GO can be easily assembled into well-ordered and free-standing GOP due to its easy dispersibility in water. For rGO, its loss of oxygen functional groups will reduce its dispersibility, and it will tend to aggregate in water, which will hamper the assembly of rGO into a high-quality paper-like material. In other words, good dispersibility in water is critical for the assembly of rGO into a well-ordered paper-like material. The reducibility of hydrazine will remove oxygen functional groups from GO to form rGO. At the same time, the excess hydrazine will lead to the rGO, which carries a negative charge to be surrounded by the N2H4+ counter ions, and the charge stabilization will lead to a good dispersion of rGO in water [32]. Overall, in this work, we used the excess hydrazine not only to reduce GO to form rGO but also to provide a good dispersibility for rGO. Fig. 2 shows images of the different solutions prepared by various recipes. As shown in Fig. 2A, the uniformly dispersed solution was obtained by the reduction of excess GO by hydrazine (GO + N2H4), and it can be stable for several months. As discussed above, the good dispersion of rGO benefits from the charge stabilization of the counter ions. Fig. 2B shows that after the addition of PdCl2, the reduction of GO by excess hydrazine (GO + N2H4 + PdCl2) will lead to aggregation. This is because the addition of PdCl2 will destroy the charge balance established in the early stage, leading to rGO aggregation. PVP is a water-soluble polymer, according to our experimental experience, the addition of PVP can effectively reduce the aggregation of rGO. As shown in Fig. 2C, with participation of PVP (GO + N2H4 + PdCl2 + PVP), the resultant solution becomes uniformly dispersed again. Since PVP is a poor conductor, the amount of PVP must be carefully controlled. The more PVP added, the better rGO dispersion produced with poorer electrical conductivity of the as-prepared Pd/ rGOP obtained. The surface morphologies of the as-prepared GOP, rGOP and Pd/ rGOP were characterized using SEM, as shown in Fig. 3. As illustrated from Fig. 3A, the wrinkle and folds of GOP can be clearly observed. After the addition of PVP, rGOP assembled from the N2H4-reduced GO also shows similar wrinkles and folds to GOP (Fig. 3B). Compared to GOP, rGOP shows improved electron transfer ability and metallic gloss on both sides. Fig. 3C indicates that the as-prepared Pd/rGOP with Pd particles anchored on both sides of the graphene sheet has fewer wrinkles and folds compared to GOP and rGOP, which is due to the introduction of the Pd particles reducing the restacking or agglomeration of the rGO sheets [33]. Additionally, Pd particles are well-distributed on the rGOP surface. The inset of Fig. 3C shows the image of the as-prepared Pd/rGOP. Fig. 3D shows the cross-section of the asprepared Pd/rGOP, where the well-packed layered structure of the rGO sheets can be clearly observed, and the thickness of the Pd/rGOP can be easily controlled by the amount of the solution during filtration. At the same time, image of Pd particle found in the cross-section of Pd/rGOP is shown in the inset of Fig. 3D. Fig. 4A shows the enlarged view of Fig. 3C, and it can be seen that the Pd particles have irregular shapes, which may be due to the aggregation of smaller Pd nanoparticles during the synthesis. At the same time, we observe that Pd particles are distributed in different layers of the rGO sheet. For example, examination of the framed area (1) shows that the Pd particle is located right on the surface of Pd/rGOP, whereas for the framed area (2), the Pd particle is located on the surface several layers below. Fig. 4B–D are EDX characterizations of the corresponding areas marked in Fig. 4A with carbon and palladium as the constituent elements. The amounts of the Pd and C elements are similar. The C element is mainly from rGOP, and the Pd element is from the reduced PdCl2. The only found element C and Pd but no O or Cl may indicate the successful co-reduction of GO and PdCl2 in one pot. XRD was used to characterize the crystallinity, as shown in Fig. 5A. The XRD patterns of rGOP show a main diffraction peak at 22.6° which is indexed to the C(2 0 0) of the short range order in graphene sheet,
2. Materials and methods 2.1. Reagents and chemicals Graphite powder, PdCl2, and glucose were purchased from SigmaAldrich and used as received. PVP (K30) and hydrazine hydrate (85%) were purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. and used without further purification. Deionized water from a Milli-Q system (18.2 MΩ cm at 25 °C) was used throughout the experiments.
2.2. Apparatus and electrochemical measurements The electrochemical experiments were carried out using an electrochemical workstation (760e, CHI, China). A three-electrode system was applied, using rGOP or Pd/rGOP (punched into 3 mm in diameter) as the working electrode, a Pt net (1 × 1 cm) as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. The morphology and microstructure were characterized by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) carried out at the acceleration voltage of 10.0 kV. The elemental composition data were obtained using an energy dispersive X-ray spectrometer (EDX) attached to the SEM instrument. The phase and crystallinity of the samples were analyzed by X-ray diffraction using a monochromatized Cu target radiation source (λ = 1.5418 Å) at the 2θ in the 20–80° range at a scanning rate of 8°/min, (XRD, D8-Advance, Bruker, Germany). The intrinsic structures of GOP, rGOP and Pd/rGOP were characterized by Raman spectroscopy (Senterra, Germany). To make the solution disperse evenly, the sample was ultrasonicated by an ultrasonic cleaning machine (360 W, 15L, Jiemeng, China).
2.3. Fabrication of palladium/reduced graphene oxide paper GO was synthesized on the basis of our previous report [31], and then, a 1 mg/mL GO aqueous solution was prepared by dissolving GO in deionized water with the aid of ultrasonication. To obtain Pd/rGOP, PdCl2 and GO were co-reduced in one pot: 50 mg PdCl2, 50 mg PVP and 5 mL 1 mg/mL GO were mixed together, and the mixture was then diluted to 70 mL by deionized water. The mixture was heated to 90 °C under magnetic stirring. Then, 10 mL of 85% hydrazine hydrate was added dropwise. The resulting solution was kept at 90 °C for 1 h. Prior to vacuum filtration, the solution was ultrasonicated for 1 h to make the resulting solution disperse evenly. The obtained aqueous dispersion was then filtered with a membrane (mixed cellulose ester, 5 cm in diameter and 0.22 μm in pore size). The as-prepared Pd/rGOP was washed several times with deionized water to remove the excess reagent. For comparison, rGOP was also fabricated without PdCl2 added in the aforementioned preparation process, and GOP was fabricated by vacuum filtering of 5 mL of 1 mg/mL GO. After filtration, the prepared papers were kept in room temperature conditions overnight, and the asprepared papers were peeled off from the membrane. A schematic illustration of the preparation process for the Pd/rGOP is shown in Fig. 1. 72
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Fig. 1. Schematic illustration of the preparation process for the Pd/rGOP.
Pd/rGOP were calculated as 1.09, 2.32 and 1.35, respectively, as indicated in Fig. 5B. Compared to GOP, rGOP shows an increased ID/IG value because the reduction of GO by hydrazine will introduce more defects into the rGO paper structure. However, Pd/rGOP had a decreased ID/IG value compared to rGOP because the participation of Pd may reduce the defect density by filling the holes after the removal of the oxygen and iodine species. To the best of our knowledge, the reduction of the ID/IG value of Pd/rGOP may be used as evidence for the metal particles being modified on rGOP [19]. Furthermore, without the metal particles interposed between the rGOP layers (just on rGOP surface), the ID/IG values are almost the same [37,38]. Therefore, we could infer from the comparison of the Raman spectra of rGOP and Pd/ rGOP that Pd particles interpose between the rGOP layers, in agreement with SEM characterizations. For electrochemical studies, cyclic voltammetry (CV) experiments were conducted to evaluate the electrocatalytic activity of the as-prepared electrodes toward glucose electro-oxidation in an alkaline solution. As shown in Fig. 6, the comparisons of the CV plots between rGOP and Pd/rGOP in a 0.1 M NaOH solution, both in the absence and presence of 30 mM glucose, were derived, and the potential was changed from −0.8 V to 0.8 V at a scan rate of 50 mV s−1. This indicates that rGOP had no obvious peak before or after the addition of 30 mM glucose. Moreover, in the absence of glucose, CV of Pd/rGOP shows the characteristic features of Pd in a 0.1 M NaOH solution. Specifically, in the CV curve for Pd/rGOP in 0.1 M NaOH solution without glucose, the anodic wave between −0.3 V and 0.4 V should be ascribed to the formation of Pd(OH)x [39,40]. The reaction can be described as following:
Fig. 2. Images of the dispersions of (A) GO + N2H4, (B) GO + N2H4 + PdCl2 and (C) GO + N2H4 + PdCl2 + PVP.
indicating the successful reduction of GO during the reaction process [34]. According to Bragg’s law, the C(2 0 0) diffraction peak at 22.6° corresponds to the d-spacing of 0.40 nm. In addition to the C(2 0 0) diffraction peak, Pd/rGOP shows diffraction peaks at 40.2°, 46.7° and 68.3°, which are fitted to the (1 1 1), (2 0 0) and (2 2 0) planes of the Pd (ICDD-JCPDS card no. 46-1043), respectively, indicating the existence of crystalline Pd. To summarize, both GO and PdCl2 are co-reduced successfully by hydrazine, which is consistent with the conclusion obtained by SEM characterization. The structural defects and graphite structure of the graphene-based paper-like materials can be characterized using the Raman spectrum. As shown in Fig. 5B, the Raman spectrum of GOP, rGOP and Pd/rGOP presents two obvious peaks. The peak at approximately 1345 cm−1 is the so-called D band related to the vibrations of the carbon atoms with the sp3 electronic configuration, which reflects the degree of defects in the lattice structure [35]. The peak at approximately 1600 cm−1 is the so-called G band related to the first-order scattering of the E2g mode of GO and corresponds to the in-plane vibration of the sp2-bonded carbon atoms in a 2D hexagonal lattice [35]. The intensity ratio of D and G (ID/ IG) is widely used to estimate the density of defects in graphitic structures: the larger the value of ID/IG, the higher the defect density [36]. This is because the D band intensity depends on the amount of the defects in the graphene sheet [19]. The ID/IG values for GOP, rGOP and
Pd + xOH− → Pd(OH)x + xe
(1)
As the anodic scan moves more positive than 0.4 V, Pd oxide would be formed and the anodic current increased, the series reactions can be described as following: Pd(OH)x + xOH− → Pd-O + xH2O + xe− 73
(2)
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Fig. 3. SEM images of as-prepared (A) GOP, (B) rGOP and (C) Pd/rGOP with (D) its cross-section view. The inset in (C) is an image of Pd/rGOP; the inset in (D) is an image of Pd particle found in cross-section.
Fig. 4. Characterizations of as-prepared Pd/rGOP: (A) SEM images, and EDX of its corresponding areas framed of (B) 1, (C) 2 and (D) 3. 74
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Fig. 5. (A) XRD patterns of rGOP and Pd/rGOP and (B) Raman spectrum of GOP, rGOP and Pd/rGOP, with their corresponding ID/IG values indicated.
scan should be ascribed to the reduction of Pd oxide to Pd. Upon the addition of 30 mM glucose, an anodic peak at −0.07 V appeared in the CV curve, which should be ascribed to the electro-absorption of glucose on Pd to form an incompletely oxidized adsorbed intermediate (Pdglucoseads) [40,41]. The reaction can be described as following: Pd + glucose → Pd-glucoseads + H+ + e−
(4)
As the potential scans more over than −0.07 V, the accumulation of the intermediate may block the electroactive sites of Pd and lead to a decline of the anodic current. As the potential becomes more positive during the scan, Pd(OH)x species will be produced (reaction (1)) which is considered to occur at the potential of approximately 0.3 V [42–44]. The Pd(OH)x species will oxidize the adsorbed intermediates, and reaction can be described as following: (5)
glucolactone + H2O → gluconic acid
(6)
The produced gluconolactone is rapidly hydrolyzed to form gluconic acid in solution [45]. As the anodic scan moves more positive than 0.4 V, Pd oxide would be formed (reaction (2) and (3)). Then, as potential scans negatively from 0.8 V, the reduction of Pd oxides will proceed, renewing the Pd surface and recovering the electroactive sites. The re-formation of the active Pd(OH)x will occur and lead to the direct glucose electro-oxidation, the reaction can be described as following:
Fig. 6. Cyclic voltammograms of rGOP and Pd/rGOP in 0.1 M NaOH in the absence (dashed curves) and presence (solid curves) of 30 mM glucose. Scanning rate: 50 mV s−1.
Pd(OH)x + Pd(OH)x → Pd-O + xH2O
Pd(OH)x + Pd-glucoseads → Pd + glucolactone
(3)
The cathodic peak observed at −0.46 V in the negative potential
Fig. 7. Investigation of the optimal potential for glucose sensing. (A) Chronoamperometric curves recorded for responses of Pd/rGOP toward 2 mM glucose at different applied potential. (B) Effect of applied potential on the amperometric response of 2 mM glucose. 75
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Fig. 8. Chronoamperometric curves of Pd/rGOP in 0.1 M NaOH solution with successive addition of different concentrations of glucose at 0.4 V. The inset shows the corresponding calibration curve.
Fig. 9. Chronoamperometric curves of Pd/rGOP in 0.1 M NaOH solution with successive addition of different species at 0.4 V.
prepared Pd/rGOP glucose sensor against other Pd/GO or Pd/rGO nonenzymatic glucose sensors reported previously is listed in Table 1. It is commonly known that some easily oxidative species like ascorbic acid (AA) and uric acid (UA) co-exist with glucose in human bodily fluids (blood, saliva, tears, sweat, etc.). To study the detection selectively of Pd/rGOP for glucose, the amperometric response of the sensor upon addition of different species was obtained, as shown in Fig. 9. Considering the normal physiological concentration of glucose is 4.4–6.6 mM [49] and the interfering species are about 0.1 mM [46–47], the interference test was undergone with 1 mM of glucose and 0.1 mM of the interferences. As it shown, remarkable responses of the Pd/rGOP toward the first and second injections of 1 mM glucose were observed. In comparison, the responses of the Pd/rGOP toward 0.1 mM AA and UA are negligible. The result indicates that the Pd/rGOP sensor exhibits good anti-interference ability and selectivity.
Table 1 The performance comparison of the as-prepared Pd/rGOP glucose sensor against other Pd/GO or Pd/rGO nonenzymatic glucose sensors reported previously. Electrode
Potential (V)
LOD (µM)
Linear range (mM)
Reference
Pd/graphene Pd/graphene oxide Pd/graphene-wrapped carbon nanotubes Pd/graphite spherereduced graphene oxide Pd/reduced graphene oxide paper
0.4 0.4 0.05
1 N/A 1
0.01–5 0.2–10 to 19.5
[46] [42] [47]
0.3
N/A
1–12
[48]
0.4
30
0.5–8
This work
Pd(OH)x + glucose → Pd + glucolactone + H2O
(7)
4. Conclusions
Therefore, an anodic peak for the oxidation of glucose at the potential of −0.46 V is observed in Fig. 6. To investigate the optimal potential for glucose sensing, chronoamperometry was used to record the amperometric response of Pd/ rGOP toward 2 mM glucose at different applied potentials of 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 V, as shown in Fig. 7. As the potential increased from 0 V to 0.4 V, the amperometric responses of the electrode toward glucose increased, and the response reached the maximum at 0.4 V; as potential continually increased from 0.4 V to 0.6 V, the amperometric responses of the electrode toward glucose decreased. The Pd oxide may be formed once the potential is greater than 0.4 V, which will block the electroactive sites for glucose oxidation. The optimal potential at 0.4 V was chosen for further examination of the glucose sensing performance. For glucose sensing, chronoamperometric responses of Pd/rGOP were recorded quantitatively by applying a constant potential at 0.4 V with the successive addition of different concentrations of glucose into a 0.1 M NaOH solution. The stair-like current-time plot is shown in Fig. 8, and the arrows show the concentration of injected glucose. The results clearly indicate that the current can reach a dynamic equilibrium within 5 s of the addition of the glucose solution, generating a near steady-state current signal. The calibration plot for glucose determination was linear over a wide range between 0.5 mM and 8 mM with the correlation coefficient of 0.9820, as shown in the inset of Fig. 8. The error bars in the calibration plot were obtained from three separate experiments. The sensitivity of the Pd/rGOP can reach as high as 6.70 µA mM−1 cm−2. The performance comparison of the as-
In summary, a free-standing palladium/reduced graphene oxide paper based on one-pot co-reduction was prepared using vacuum filtration. The one-pot co-reduction strategy could effectively facilitate the preparation of functionalized rGOP without multiple steps of GO reduction and metal particles modification. The as-prepared Pd/rGOP with Pd particles interposed between the rGOP layers and distributed evenly. Finally, we investigated the glucose sensing performance of the as-prepared Pd/rGOP, and it is found that this sensor can reach as high as 6.70 µA mM−1 cm−2 with the linear range between 0.5 mM and 8 mM. The sensor exhibits a good anti-interference ability and selectivity toward AA and UA. The proposed sensor shows great potential to become a reliable tool in biosensor and point-of-care medical devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (61471233, 21504051), the Sailing Project from the Science and Technology Commission of Shanghai Municipality (17YF1406600), the Young Teacher Training Program of Shanghai Universities (ZZEGD15036), Gaoyuan Discipline of Shanghai–Environmental Science and Engineering Resource Recycling Science and Engineering).
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Free-standing palladium modified reduced graphene oxide paper based on one-pot co-reduction and its sensing application
T
Zhan-Hong Lia,b, Xue-Ling Zhaoa,b, Run-Min Songa, Cheng Chena,b, Peng-Ju Weia, ⁎ Zhi-Gang Zhua,b, a b
School of Environmental and Materials Engineering, College of Engineering, Shanghai Polytechnic University, Shanghai 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai 201209, China
H I GH L IG H T S
facile palladium functionalized reduced graphene oxide paper was prepared via one-pot co-reduction method. • AEvenly-distributed Pd particles show to decorate graphene paper surface and interpose between graphene sheets. • As-prepared freestanding Pd/rGOP can potentially be used as a glucose sensor. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemistry Catalysis Glucose sensor Non-enzymatic
A free-standing palladium/reduced graphene oxide paper (Pd/rGOP) based on one-pot co-reduction was prepared. The as-prepared Pd/rGOPs contain evenly distributed Pd particles interposing between the rGOP layers. The structure and composition of Pd/rGOP were characterized by SEM, EDX, XRD and Raman spectra. We investigated the glucose sensing performance of Pd/rGOP by chronoamperometry, the sensitivity reach as high as 6.70 µA mM−1 cm−2 with the linear range between 0.5 mM and 8 mM. The sensor also exhibits a good antiinterference ability and detection selectivity toward the interference. The proposed sensor has great potential to become a reliable tool in point-of-care medical devices.
1. Introduction In recent years, portable and wearable devices have attracted great interest because of their great convenience. However, the development of an ideal functionalized, low-cost, and lightweight material for such devices is still challenging. Paper-like conductive materials may be an alternative because of their applications in sensors [1], Li-ion batteries [2], supercapacitors [3], fuel cells [4], hydrogen storage [5] and so on. Graphene is a two-dimensional (2D) atom-thin sheet with sp2-hybridized carbons that has been intensively studied because of its unique thermal, mechanical, chemical and electric properties [6]. Graphene oxide (GO) contains oxygen functional groups on its basal planes and edges, resulting in its easy dispersibility in water. On the other hand, graphene oxide has poor electrical conductivity due to the disruption of its sp2 bonding networks by oxygen functional groups. Thus, the poor electrical conductivity may limit the use of GO in electrical or electrochemical applications such as supercapacitors [7,8], fuel cells [9–11], Li-ion batteries [12,13] and sensors [14,15]. To recover the
⁎
Corresponding author. E-mail address: [email protected] (Z.-G. Zhu).
https://doi.org/10.1016/j.cplett.2018.09.047 Received 22 June 2018; Accepted 21 September 2018 Available online 22 September 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
electrical conductivity of GO, most of the oxygen functional groups must be removed, thus obtaining fully reduced GO. However, the reduced graphene oxide (rGO) is difficult to disperse in water due to its tendency to aggregate. Recently, GO or rGO was selected to assemble into well-ordered and free-standing paper-like materials [16,17]. Compared to other paper-like materials (carbon nanotubes or graphite), GO or rGO paper is superior due to its mechanical strength [18]. The preparation of GO paper (GOP) involves vacuum filtration [16,19–23], evaporation [24–26], mechanical compression [27,28], speed vacuum centrifugation [29], mold-casting [30] and other techniques. Vacuum filtration is a rapid, simple, low-cost procedure that is most commonly used to obtain GOP. To prepare rGO paper (rGOP) or its functionalized materials, normally, a three-step method has been selected based on vacuum filtration [19,21–23]. To facilitate the preparation of metalfunctionalized rGOP, the metal salt and GO could be co-reduced by reductant, and then, the vacuum filtration is conducted to obtain the metal particle-functionalized rGOP. Correspondingly, the functionalized rGOP made by metal particles interposing between graphene
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3. Results and discussion
sheets can be prepared. As one of the transition metals, palladium is widely used as an electrocatalytic active center. In this work, a free-standing palladium/ reduced graphene oxide paper (Pd/rGOP) based on one-pot co-reduction was prepared using vacuum filtration. We co-reduced PdCl2 and GO by hydrazine in the presence of polyvinyl pyrrolidone (PVP) in a one-pot reaction followed by vacuum filtration, thus obtaining a freestanding Pd/rGOP. The one-pot co-reduction facilitates the preparation of functionalized rGOP. Hydrazine acts as the reductant and promoting the dispersion of rGO; PVP plays an important role in effectively reducing the aggregation of rGO during the process. The as-prepared Pd/ rGOP showed its potential application in glucose sensing.
Using simple vacuum filtration, GO can be easily assembled into well-ordered and free-standing GOP due to its easy dispersibility in water. For rGO, its loss of oxygen functional groups will reduce its dispersibility, and it will tend to aggregate in water, which will hamper the assembly of rGO into a high-quality paper-like material. In other words, good dispersibility in water is critical for the assembly of rGO into a well-ordered paper-like material. The reducibility of hydrazine will remove oxygen functional groups from GO to form rGO. At the same time, the excess hydrazine will lead to the rGO, which carries a negative charge to be surrounded by the N2H4+ counter ions, and the charge stabilization will lead to a good dispersion of rGO in water [32]. Overall, in this work, we used the excess hydrazine not only to reduce GO to form rGO but also to provide a good dispersibility for rGO. Fig. 2 shows images of the different solutions prepared by various recipes. As shown in Fig. 2A, the uniformly dispersed solution was obtained by the reduction of excess GO by hydrazine (GO + N2H4), and it can be stable for several months. As discussed above, the good dispersion of rGO benefits from the charge stabilization of the counter ions. Fig. 2B shows that after the addition of PdCl2, the reduction of GO by excess hydrazine (GO + N2H4 + PdCl2) will lead to aggregation. This is because the addition of PdCl2 will destroy the charge balance established in the early stage, leading to rGO aggregation. PVP is a water-soluble polymer, according to our experimental experience, the addition of PVP can effectively reduce the aggregation of rGO. As shown in Fig. 2C, with participation of PVP (GO + N2H4 + PdCl2 + PVP), the resultant solution becomes uniformly dispersed again. Since PVP is a poor conductor, the amount of PVP must be carefully controlled. The more PVP added, the better rGO dispersion produced with poorer electrical conductivity of the as-prepared Pd/ rGOP obtained. The surface morphologies of the as-prepared GOP, rGOP and Pd/ rGOP were characterized using SEM, as shown in Fig. 3. As illustrated from Fig. 3A, the wrinkle and folds of GOP can be clearly observed. After the addition of PVP, rGOP assembled from the N2H4-reduced GO also shows similar wrinkles and folds to GOP (Fig. 3B). Compared to GOP, rGOP shows improved electron transfer ability and metallic gloss on both sides. Fig. 3C indicates that the as-prepared Pd/rGOP with Pd particles anchored on both sides of the graphene sheet has fewer wrinkles and folds compared to GOP and rGOP, which is due to the introduction of the Pd particles reducing the restacking or agglomeration of the rGO sheets [33]. Additionally, Pd particles are well-distributed on the rGOP surface. The inset of Fig. 3C shows the image of the as-prepared Pd/rGOP. Fig. 3D shows the cross-section of the asprepared Pd/rGOP, where the well-packed layered structure of the rGO sheets can be clearly observed, and the thickness of the Pd/rGOP can be easily controlled by the amount of the solution during filtration. At the same time, image of Pd particle found in the cross-section of Pd/rGOP is shown in the inset of Fig. 3D. Fig. 4A shows the enlarged view of Fig. 3C, and it can be seen that the Pd particles have irregular shapes, which may be due to the aggregation of smaller Pd nanoparticles during the synthesis. At the same time, we observe that Pd particles are distributed in different layers of the rGO sheet. For example, examination of the framed area (1) shows that the Pd particle is located right on the surface of Pd/rGOP, whereas for the framed area (2), the Pd particle is located on the surface several layers below. Fig. 4B–D are EDX characterizations of the corresponding areas marked in Fig. 4A with carbon and palladium as the constituent elements. The amounts of the Pd and C elements are similar. The C element is mainly from rGOP, and the Pd element is from the reduced PdCl2. The only found element C and Pd but no O or Cl may indicate the successful co-reduction of GO and PdCl2 in one pot. XRD was used to characterize the crystallinity, as shown in Fig. 5A. The XRD patterns of rGOP show a main diffraction peak at 22.6° which is indexed to the C(2 0 0) of the short range order in graphene sheet,
2. Materials and methods 2.1. Reagents and chemicals Graphite powder, PdCl2, and glucose were purchased from SigmaAldrich and used as received. PVP (K30) and hydrazine hydrate (85%) were purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. and used without further purification. Deionized water from a Milli-Q system (18.2 MΩ cm at 25 °C) was used throughout the experiments.
2.2. Apparatus and electrochemical measurements The electrochemical experiments were carried out using an electrochemical workstation (760e, CHI, China). A three-electrode system was applied, using rGOP or Pd/rGOP (punched into 3 mm in diameter) as the working electrode, a Pt net (1 × 1 cm) as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. The morphology and microstructure were characterized by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) carried out at the acceleration voltage of 10.0 kV. The elemental composition data were obtained using an energy dispersive X-ray spectrometer (EDX) attached to the SEM instrument. The phase and crystallinity of the samples were analyzed by X-ray diffraction using a monochromatized Cu target radiation source (λ = 1.5418 Å) at the 2θ in the 20–80° range at a scanning rate of 8°/min, (XRD, D8-Advance, Bruker, Germany). The intrinsic structures of GOP, rGOP and Pd/rGOP were characterized by Raman spectroscopy (Senterra, Germany). To make the solution disperse evenly, the sample was ultrasonicated by an ultrasonic cleaning machine (360 W, 15L, Jiemeng, China).
2.3. Fabrication of palladium/reduced graphene oxide paper GO was synthesized on the basis of our previous report [31], and then, a 1 mg/mL GO aqueous solution was prepared by dissolving GO in deionized water with the aid of ultrasonication. To obtain Pd/rGOP, PdCl2 and GO were co-reduced in one pot: 50 mg PdCl2, 50 mg PVP and 5 mL 1 mg/mL GO were mixed together, and the mixture was then diluted to 70 mL by deionized water. The mixture was heated to 90 °C under magnetic stirring. Then, 10 mL of 85% hydrazine hydrate was added dropwise. The resulting solution was kept at 90 °C for 1 h. Prior to vacuum filtration, the solution was ultrasonicated for 1 h to make the resulting solution disperse evenly. The obtained aqueous dispersion was then filtered with a membrane (mixed cellulose ester, 5 cm in diameter and 0.22 μm in pore size). The as-prepared Pd/rGOP was washed several times with deionized water to remove the excess reagent. For comparison, rGOP was also fabricated without PdCl2 added in the aforementioned preparation process, and GOP was fabricated by vacuum filtering of 5 mL of 1 mg/mL GO. After filtration, the prepared papers were kept in room temperature conditions overnight, and the asprepared papers were peeled off from the membrane. A schematic illustration of the preparation process for the Pd/rGOP is shown in Fig. 1. 72
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Fig. 1. Schematic illustration of the preparation process for the Pd/rGOP.
Pd/rGOP were calculated as 1.09, 2.32 and 1.35, respectively, as indicated in Fig. 5B. Compared to GOP, rGOP shows an increased ID/IG value because the reduction of GO by hydrazine will introduce more defects into the rGO paper structure. However, Pd/rGOP had a decreased ID/IG value compared to rGOP because the participation of Pd may reduce the defect density by filling the holes after the removal of the oxygen and iodine species. To the best of our knowledge, the reduction of the ID/IG value of Pd/rGOP may be used as evidence for the metal particles being modified on rGOP [19]. Furthermore, without the metal particles interposed between the rGOP layers (just on rGOP surface), the ID/IG values are almost the same [37,38]. Therefore, we could infer from the comparison of the Raman spectra of rGOP and Pd/ rGOP that Pd particles interpose between the rGOP layers, in agreement with SEM characterizations. For electrochemical studies, cyclic voltammetry (CV) experiments were conducted to evaluate the electrocatalytic activity of the as-prepared electrodes toward glucose electro-oxidation in an alkaline solution. As shown in Fig. 6, the comparisons of the CV plots between rGOP and Pd/rGOP in a 0.1 M NaOH solution, both in the absence and presence of 30 mM glucose, were derived, and the potential was changed from −0.8 V to 0.8 V at a scan rate of 50 mV s−1. This indicates that rGOP had no obvious peak before or after the addition of 30 mM glucose. Moreover, in the absence of glucose, CV of Pd/rGOP shows the characteristic features of Pd in a 0.1 M NaOH solution. Specifically, in the CV curve for Pd/rGOP in 0.1 M NaOH solution without glucose, the anodic wave between −0.3 V and 0.4 V should be ascribed to the formation of Pd(OH)x [39,40]. The reaction can be described as following:
Fig. 2. Images of the dispersions of (A) GO + N2H4, (B) GO + N2H4 + PdCl2 and (C) GO + N2H4 + PdCl2 + PVP.
indicating the successful reduction of GO during the reaction process [34]. According to Bragg’s law, the C(2 0 0) diffraction peak at 22.6° corresponds to the d-spacing of 0.40 nm. In addition to the C(2 0 0) diffraction peak, Pd/rGOP shows diffraction peaks at 40.2°, 46.7° and 68.3°, which are fitted to the (1 1 1), (2 0 0) and (2 2 0) planes of the Pd (ICDD-JCPDS card no. 46-1043), respectively, indicating the existence of crystalline Pd. To summarize, both GO and PdCl2 are co-reduced successfully by hydrazine, which is consistent with the conclusion obtained by SEM characterization. The structural defects and graphite structure of the graphene-based paper-like materials can be characterized using the Raman spectrum. As shown in Fig. 5B, the Raman spectrum of GOP, rGOP and Pd/rGOP presents two obvious peaks. The peak at approximately 1345 cm−1 is the so-called D band related to the vibrations of the carbon atoms with the sp3 electronic configuration, which reflects the degree of defects in the lattice structure [35]. The peak at approximately 1600 cm−1 is the so-called G band related to the first-order scattering of the E2g mode of GO and corresponds to the in-plane vibration of the sp2-bonded carbon atoms in a 2D hexagonal lattice [35]. The intensity ratio of D and G (ID/ IG) is widely used to estimate the density of defects in graphitic structures: the larger the value of ID/IG, the higher the defect density [36]. This is because the D band intensity depends on the amount of the defects in the graphene sheet [19]. The ID/IG values for GOP, rGOP and
Pd + xOH− → Pd(OH)x + xe
(1)
As the anodic scan moves more positive than 0.4 V, Pd oxide would be formed and the anodic current increased, the series reactions can be described as following: Pd(OH)x + xOH− → Pd-O + xH2O + xe− 73
(2)
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Fig. 3. SEM images of as-prepared (A) GOP, (B) rGOP and (C) Pd/rGOP with (D) its cross-section view. The inset in (C) is an image of Pd/rGOP; the inset in (D) is an image of Pd particle found in cross-section.
Fig. 4. Characterizations of as-prepared Pd/rGOP: (A) SEM images, and EDX of its corresponding areas framed of (B) 1, (C) 2 and (D) 3. 74
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Fig. 5. (A) XRD patterns of rGOP and Pd/rGOP and (B) Raman spectrum of GOP, rGOP and Pd/rGOP, with their corresponding ID/IG values indicated.
scan should be ascribed to the reduction of Pd oxide to Pd. Upon the addition of 30 mM glucose, an anodic peak at −0.07 V appeared in the CV curve, which should be ascribed to the electro-absorption of glucose on Pd to form an incompletely oxidized adsorbed intermediate (Pdglucoseads) [40,41]. The reaction can be described as following: Pd + glucose → Pd-glucoseads + H+ + e−
(4)
As the potential scans more over than −0.07 V, the accumulation of the intermediate may block the electroactive sites of Pd and lead to a decline of the anodic current. As the potential becomes more positive during the scan, Pd(OH)x species will be produced (reaction (1)) which is considered to occur at the potential of approximately 0.3 V [42–44]. The Pd(OH)x species will oxidize the adsorbed intermediates, and reaction can be described as following: (5)
glucolactone + H2O → gluconic acid
(6)
The produced gluconolactone is rapidly hydrolyzed to form gluconic acid in solution [45]. As the anodic scan moves more positive than 0.4 V, Pd oxide would be formed (reaction (2) and (3)). Then, as potential scans negatively from 0.8 V, the reduction of Pd oxides will proceed, renewing the Pd surface and recovering the electroactive sites. The re-formation of the active Pd(OH)x will occur and lead to the direct glucose electro-oxidation, the reaction can be described as following:
Fig. 6. Cyclic voltammograms of rGOP and Pd/rGOP in 0.1 M NaOH in the absence (dashed curves) and presence (solid curves) of 30 mM glucose. Scanning rate: 50 mV s−1.
Pd(OH)x + Pd(OH)x → Pd-O + xH2O
Pd(OH)x + Pd-glucoseads → Pd + glucolactone
(3)
The cathodic peak observed at −0.46 V in the negative potential
Fig. 7. Investigation of the optimal potential for glucose sensing. (A) Chronoamperometric curves recorded for responses of Pd/rGOP toward 2 mM glucose at different applied potential. (B) Effect of applied potential on the amperometric response of 2 mM glucose. 75
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Fig. 8. Chronoamperometric curves of Pd/rGOP in 0.1 M NaOH solution with successive addition of different concentrations of glucose at 0.4 V. The inset shows the corresponding calibration curve.
Fig. 9. Chronoamperometric curves of Pd/rGOP in 0.1 M NaOH solution with successive addition of different species at 0.4 V.
prepared Pd/rGOP glucose sensor against other Pd/GO or Pd/rGO nonenzymatic glucose sensors reported previously is listed in Table 1. It is commonly known that some easily oxidative species like ascorbic acid (AA) and uric acid (UA) co-exist with glucose in human bodily fluids (blood, saliva, tears, sweat, etc.). To study the detection selectively of Pd/rGOP for glucose, the amperometric response of the sensor upon addition of different species was obtained, as shown in Fig. 9. Considering the normal physiological concentration of glucose is 4.4–6.6 mM [49] and the interfering species are about 0.1 mM [46–47], the interference test was undergone with 1 mM of glucose and 0.1 mM of the interferences. As it shown, remarkable responses of the Pd/rGOP toward the first and second injections of 1 mM glucose were observed. In comparison, the responses of the Pd/rGOP toward 0.1 mM AA and UA are negligible. The result indicates that the Pd/rGOP sensor exhibits good anti-interference ability and selectivity.
Table 1 The performance comparison of the as-prepared Pd/rGOP glucose sensor against other Pd/GO or Pd/rGO nonenzymatic glucose sensors reported previously. Electrode
Potential (V)
LOD (µM)
Linear range (mM)
Reference
Pd/graphene Pd/graphene oxide Pd/graphene-wrapped carbon nanotubes Pd/graphite spherereduced graphene oxide Pd/reduced graphene oxide paper
0.4 0.4 0.05
1 N/A 1
0.01–5 0.2–10 to 19.5
[46] [42] [47]
0.3
N/A
1–12
[48]
0.4
30
0.5–8
This work
Pd(OH)x + glucose → Pd + glucolactone + H2O
(7)
4. Conclusions
Therefore, an anodic peak for the oxidation of glucose at the potential of −0.46 V is observed in Fig. 6. To investigate the optimal potential for glucose sensing, chronoamperometry was used to record the amperometric response of Pd/ rGOP toward 2 mM glucose at different applied potentials of 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 V, as shown in Fig. 7. As the potential increased from 0 V to 0.4 V, the amperometric responses of the electrode toward glucose increased, and the response reached the maximum at 0.4 V; as potential continually increased from 0.4 V to 0.6 V, the amperometric responses of the electrode toward glucose decreased. The Pd oxide may be formed once the potential is greater than 0.4 V, which will block the electroactive sites for glucose oxidation. The optimal potential at 0.4 V was chosen for further examination of the glucose sensing performance. For glucose sensing, chronoamperometric responses of Pd/rGOP were recorded quantitatively by applying a constant potential at 0.4 V with the successive addition of different concentrations of glucose into a 0.1 M NaOH solution. The stair-like current-time plot is shown in Fig. 8, and the arrows show the concentration of injected glucose. The results clearly indicate that the current can reach a dynamic equilibrium within 5 s of the addition of the glucose solution, generating a near steady-state current signal. The calibration plot for glucose determination was linear over a wide range between 0.5 mM and 8 mM with the correlation coefficient of 0.9820, as shown in the inset of Fig. 8. The error bars in the calibration plot were obtained from three separate experiments. The sensitivity of the Pd/rGOP can reach as high as 6.70 µA mM−1 cm−2. The performance comparison of the as-
In summary, a free-standing palladium/reduced graphene oxide paper based on one-pot co-reduction was prepared using vacuum filtration. The one-pot co-reduction strategy could effectively facilitate the preparation of functionalized rGOP without multiple steps of GO reduction and metal particles modification. The as-prepared Pd/rGOP with Pd particles interposed between the rGOP layers and distributed evenly. Finally, we investigated the glucose sensing performance of the as-prepared Pd/rGOP, and it is found that this sensor can reach as high as 6.70 µA mM−1 cm−2 with the linear range between 0.5 mM and 8 mM. The sensor exhibits a good anti-interference ability and selectivity toward AA and UA. The proposed sensor shows great potential to become a reliable tool in biosensor and point-of-care medical devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (61471233, 21504051), the Sailing Project from the Science and Technology Commission of Shanghai Municipality (17YF1406600), the Young Teacher Training Program of Shanghai Universities (ZZEGD15036), Gaoyuan Discipline of Shanghai–Environmental Science and Engineering Resource Recycling Science and Engineering).
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Chemical Physics Letters 712 (2018) 71–77
Z.-H. Li et al.
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