Rationally Designed Multifunctional CarbonâPalladium Nanohybrids

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Rationally Designed Multifunctional Carbon-Palladium Nanohybrids for Wide Applications: From Electrochemical Catalysis/Nonenzymatic Sensor to Photothermal Tumor Therapy Yuchu He, Weiwei Cao, Cong Cong, Xuwu Zhang, Liyao Luo, Lei Li, Hongxia Cui, and Dawei Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06090 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Sustainable Chemistry & Engineering

Rationally Designed Multifunctional Carbon-Palladium Nanohybrids for Wide Applications: From Electrochemical Catalysis/Nonenzymatic Sensor to Photothermal Tumor Therapy

Yuchu He1,2, Weiwei Cao1, Cong Cong1, Xuwu Zhang1, Liyao Luo1, Lei Li1, Hongxia Cui1,3, Dawei Gao1,2*

1Applying

Chemistry Key Lab of Hebei Province, Department of Bioengineer, Yanshan

University, No.438 Hebei Street, Qinhuangdao, 066004, China.

2State

Key Laboratory of Metastable Materials Science and Technology, Yanshan University,

Qinhuangdao 066004, P. R. China.

3Hebei

Province Asparagus Industry Technology Research Institute, Qinhuangdao, 066004,

China.

*Corresponding author: Prof. Dawei Gao, Tel: (+86)13930338376. E-mail: [email protected]

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Yuchu He and Weiwei Cao contributed equally to this work.

ABSTRACT: Palladium nanomaterials have been widely investigated in many areas due to their high activity of catalysis and surface plasmon resonance (SPR) resulting from their special configuration of outer electron. Herein, the novel nanoparticles, reduced graphene oxide modified with palladium nanoflowers (rGO/PdNFs), were designed and synthesized by seeded growth. Their application potentials in three areas were explored, including electrochemical ethanol catalysis, nonenzymatic glucose sensor and photothermal tumor therapy. The hybrids of PdNFs and rGO increased the conductivity and active sites of PdNFs, and then enhanced the activity of catalysis. Interestingly, through control of morphologies, the absorption of PdNFs in near infrared region was enhanced compared with common palladium nanoparticles, which showed excellent potential in photothermal tumor therapy. These results indicated the stronger activity of catalysis to ethanol of rGO/PdNFs compared with the commercial Pd/C catalyst, superior sensitivity and selectivity of glucose, and effective photothermal antitumor efficacy. Overall, it is demonstrated that the multifunctional rGO/PdNFs nanohybrids could possess more application potentials.

KEYWORDS: Palladium nanoflowers, Reduced graphene oxide, Electrochemical catalysis, Nonenzymatic sensor, Photothermal tumor therapy


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Introduction Palladium nanomaterials exhibit remarkable catalytic effect in various fields due to their special configuration of outer electron1. For example, palladium catalysts transfer the chemical energy of ethanol to electric energy, which are applied in direct ethanol fuel cells (DEFCs)2,3. Furthermore, palladium catalysts solve the problems of exorbitant cost and CO poisoning resulting from common platinum catalysts as well4,5. In addition, palladium catalysts play an important role in clinical diagnostics and biotechnology because of their effect of glucose detection6. Traditional enzymatic sensors show excellent selectivity and susceptibility in the field of glucose detection7,8. Nevertheless, some long-standing challenges may limit their deep applications8. At present, some studies about nonenzymatic sensor seems to solve above mentioned drawbacks9. Palladium nanomaterials based nonenzymic sensors have now been proven for glucose10,11. Moreover, the applications of palladium nanoparticles have also been explored in the medicine and biology fields12, where multiple palladium-based nanomedicine has been focused resulting from their property of surface plasmon resonance (SPR)13. As the bare absorption of near-infrared (NIR) light in the major tissue14, NIR laser usually is used to induce the photothermal conversion of medicine in photothermal tumor therapy (PTT)15–17. However, for common palladium nanoparticles (PdNPs), their bare absorption in NIR window limits their applications in PTT. Thus, it is urgently needed to enhance the absorption of PdNPs in NIR window. It should be noted that the sizes and morphologies of nanomaterials are related to their function18, therefore, controllable synthesis different shapes of PdNPs and modification with functional groups can make PdNPs to be potential photothermal agents. Based on the same thought, Song et al19 synthesized porous hollow palladium nanomaterials to enhance the photothermal effect of palladium nanoparticles. Tang et al20 designed sub-10-nm Pd nanosheets, which were applied in near-infrared photothermal cancer therapy. Hence, through design and controllable synthesis, palladium nanomaterials could show not only the superior effect of ethanol catalysis and glucose detection, but also excellent biocompatibility and photothermal conversion in tumor therapy. Graphene oxide (GO) has attracted tremendous attention in past few years21–23. Because of the unique physical chemistry properties, GO provides a prospective platform for electronics24,25 and



electrochemical sensors26. Additionally, due to its excellent biocompatibility, GO has been taken advantage of nano-drug carriers27,28. Recently, a large number of researches have been investigated to the hybrid of graphene with other potential nanoparticles29. The hybridization of GO with other materials could enhance the effect of each component via cooperative interaction30– 32.

In this study, it is originally discovered that palladium nanoflowers (PdNFs) attached on reduced graphene oxide (rGO/PdNFs) possessed various properties including electrochemical ethanol catalysis, nonenzymatic glucose sensor and photothermal tumor therapy (Scheme 1). The hybrids of PdNFs and reduced GO (rGO) exhibited excellent activity of ethanol catalysis and glucose detection. Furthermore, the structure of like-flower provided the enhanced effect of photothermal conversion compared with PdNPs, which lead to the rGO/PdNFs as a potential photothermal agent. To the best of our knowledge, we designed the rGO/PdNFs and explored three applications of them for the first time. We believe that the continuous study of one specific nanomaterial to multiple applications promote the functionalization of more nanomaterials.

Results and Discussion Synthesis and Characterization. Due to the active groups of oxygen (hydroxy and carboxyl) of GO33, palladium ions (Pd2+) were combined on the surface of GO (GO-Pd2+) through the band of CO-Pd2+. The hydride ions (H-) ionized from sodium borohydride (NaBH4) attacked to the electron deficient 4d orbit of Pd2+ to form palladium atoms (Pd0). The palladium nanoparticles (PdNPs) attached on the reduced GO (rGO/PdNPs) were obtained through the nucleation of Pd0. With adding ascorbic acid (AA) into rGO/PdNPs, extra Pd2+ were reduced by transformation of enol and ketone on the surface of PdNPs. Because of the diverse of lattice energy on the surface, the flower-like nanoparticles were synthesized with continuous growth. Thus, palladium nanoflowers attached on the rGO to form rGO/PdNFs. According to Figure 1a, TEM images of GO, GO-Pd2+, rGO/PdNPs and rGO/PdNFs were shown. The combination between palladium nanoparticles with rGO and the changes of morphology were exhibited obviously. EDS spectra proved the element palladium (Figure 1b). As shown in Figure 1c, XRD analysis showed the characteristic peaks of rGO (004) and PdNFs


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(111). UV-vis spectra exhibited the enhanced absorption of rGO/PdNFs in the NIR window comparing with rGO, which indicated the high potential photothermal conversion of rGO/PdNFs (Figure 1d). A wide size distribution of rGO/PdNFs with average of 158 ± 4.7 nm was shown in Figure 1e. Zeta potential of the rGO/PdNFs was -13.24 ± 3.15 mV (Supporting Information, Figure S1). Furthermore, the chemical state and components of the PdNFs on the surface of rGO were characterized via XPS. The results showed the presence of Pd, O and C elements (Figure 1f). Two characteristic peaks of palladium 3d at 335.5 and 341.0 eV should contributed to Pd 3d5/2 and Pd 3d3/2, respectively, which is in good agreement with the literature values of Pd0. Another two characteristic peaks at 337.5 and 342.5 eV of element palladium indicated the divalent palladium in the solution (Figure 1g). Therefore, the XPS data proved the transformation between Pd2+ to Pd0 attached on the surface of rGO.

Electrocatalytic Activity in DEFCs. Prepared rGO/PdNFs and the commercial Pd/C catalysts modified electrode of palladium were used by same conditions. The CV was studied for the rGO/PdNFs and Pd/C in the 1.0 M KOH solution from -1.0 to 0.4 V at a scan rate of 50 mV s-1 (Figure 2a). The peaks obtained in the CV curves might be the adsorption of OH-, formation and reduction of Pd-O layer. For rGO/PdNFs, during the negative scan, the maximum current of the peak of Pd-O centers at -0.4 V was -113.86 μA, while the Pd/C was -20.42 μA. This result might be attributed to the great surface area of the rGO/PdNFs. Figure 2b showed the electrochemical catalytic activity of the rGO/PdNFs for ethanol catalysis in 1.0 KOH + 1.0 M C2H5OH solution. The maximum current of rGO/PdNFs was 520.68 μA, which is larger than the 23.98 μA of Pd/C catalyst. The results indicated the better effect of ethanol catalysis of prepared rGO/PdNFs compared with the commercial Pd/C catalyst. Furthermore, the mechanism of ethanol catalysis using rGO/PdNFs was also investigated. The catalytic process included diffusion of ethanol, adsorption of ethanol and hydroxyl, formation of Pd-O layer and intermediate products of carbonyl, desorption and diffusion of acetic acid34. The stability of rGO/PdNFs was also investigated by the chronoamperometry experiment. Figure 2c showed that the current density of rGO/PdNFs decreased slowly than the commercial Pd/C catalyst. The current densities of Pd/C and rGO/PdNFs at 8000 s are 0 mA and 0.05 mA indicating that the resistivity to poisoning of rGO/PdNFs was better. As shown in Figure 2d, with



the increase of scan rate, the peak current density increased linearly. This result exhibited that the good linear relationship between the current density and scan rate. Meanwhile, the measured Pd/C, rGO and rGO/PdNFs were also observed by TEM (Supporting Information, Figure S2) indicating the good dispersibility and stability of prepared rGO/PdNFs. The above results suggested the superior electrochemical catalysis and stability, which showed potential in the field of DEFCs.

Nonenzymatic Glucose Sensor. Semicircle diameter of impedance equals the electron transfer resistance35. Figure 3a presents the representative impedance spectrum of the bare GC electrode, rGO-GC electrode and rGO/PdNFs-GC electrode in 5.0 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) containing 0.1 M KCl. With the modified of rGO and rGO/PdNFs on the GC electrode, the Nyquist semicircle decreased. Figure 3b showed the CV curves of bare GC electron, rGO-GC electrode and rGO/PdNFs-GC electrode in 0.1 M NaOH solution. Figure 3c exhibited the CV of the rGO/PdNFs in 0.1 M NaOH solution with -0.6 V to 0.2 V in a scan rate of 50 mV s-1 with and without glucose. Through adding into glucose, an apparent anodic peak was observed, which indicated that the rGO/PdNFs showed superior activity of glucose detection. The CV curves of rGO/PdNFs at various scan rates were studied as well (Figure 3d). The current of the anodic peak enhanced through the increase of the scan rate from 10 to 200 mV s-1, which suggested the reaction was reversible and surface-confined. Figure 3e showed the typical current-time dynamic response of the rGO/PdNFs towards glucose. The amperometric signal exhibited linear correlation to glucose concentration in the range from 10 nM to 90 nM with a correlation coefficient of 0.9982, which covered three orders of magnitude of glucose concentrations. The detection limit was 82.2 nM for glucose, which was lower than those of Pt-Pb nanowire array electrode and Ni powder modified electrode36,37. Based on the previously report on oxidation mechanisms for glucose, a possible reason of the rGO/PdNFs for the glucose oxidation was studied as well38. The interference study of rGO/PdNFs was carried out as well. Glutathione (GSH), uric acid (UA) and cholesterol (Chol) present in physiological fluids are the most important interferences for direct electrochemical oxidation of glucose on different electrodes especially nonenzymatic biosensors39. As shown in Figure 3f, the rGO/PdNFs exhibited excellent selectivity to glucose in


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the presence of GSH, UA and Chol. Furthermore, the stability of the rGO/PdNFs as the nonenzymatic glucose biosensor was also explored. After 30 days, the response of the rGO/PdNFs decreased only 3.5% compared to the initial response indicating its long-term stability (Supporting Information Figure S4). The results indicated the excellent sensitivity, selectivity and stability of rGO/PdNFs as a nonenzymatic sensor to glucose.

Photothermal Tumor Therapy. The antitumor efficacy of rGO/PdNFs was investigated as well. The rough surface of rGO/PdNFs reduced the scattering of light and adsorbed more energy, which gave the potential of rGO/PdNFs in photothermal cancer therapy. As shown in Figure 4a, the temperature of rGO/PdNFs rose about 70 ℃ under laser irradiation (808 nm, 1.5 W cm-2, 5min). The results indicated that the rGO/PdNFs showed excellent photothermal conversion efficiency compared with rGO alone. To study the cellular uptake of the nanoparticles, HeLa cells were incubated with rGO/PdNFs/Cour6 and Cour6 (as positive control) for 24 hours. According to Figure 4b, FCM analysis showed the rGO/PdNFs/Cour6 was uptaken into HeLa cells obviously compared with the control group. Furthermore, the representative fluorescence images were given as well. As shown in Figure 4d, the cell nucleus of HeLa cells was stained by Hoechst, and the rGO/PdNFs/Cour6 were displayed in cytoplasm of HeLa cells, which indicated that the rGO/PdNFs were uptaken into cells successfully. After cellular uptake of rGO/PdNFs for 24 hours, the cytotoxicity of rGO/PdNFs was investigated. When incubated with rGO and rGO/PdNFs for 24 hours, the cells of control, rGO, rGO with laser irradiation and rGO/PdNFs groups exhibited indistinctive cytotoxicity, which suggested good biocompatibility of rGO and rGO/PdNFs. After laser irradiation of rGO/PdNFs, about 80% of cells were killed (Figure 4c). Additionally, the cells were co-incubated using fluorescein diacetate (FDA) and propidium iodide (PI) to stain the live or dead cells. The inverted fluorescence microscope images showed that the cell death was noticeable after laser irradiation in the rGO/PdNFs treated cells, which confirmed an excellent antitumor efficacy of rGO/PdNFs in vitro (Figure 4e). To suggest the photothermal performance of the rGO/PdNFs in vivo, we utilized the infrared thermal camera to show the temperature in mice. As shown in Figure 5c, after 3 min of the



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irradiation, the tumors treated with rGO/PdNFs showed a high temperature (65℃). Correspondingly, saline-treated mice suggested no apparent temperature rise (30℃). Moreover, the volumes of tumors were monitored in the all mice as well (Figure 5a). The tumor volume of the saline group increased about 30 times. After 10 days of treatment, the tumors volume decreased to 3.5 times in the group of rGO/PdNFs with laser irradiation. At the same time, the photos of the treated mice in every group were obtained after 10 days of treatment (Figure 5d). The results suggested superior antitumor efficacy of rGO/PdNFs in vivo. Additionally, the biodistribution of rGO/PdNFs was studied as well. Dominating organs and tumor tissues were taken away at different time (8 h, 24 h and 10 days) after tail vein injection. Efficient Pd accumulation of rGO/PdNFs in tissue was analyzed via ICP-MS (Figure 5b). The majority of rGO/PdNFs accumulated in tumor tissues at 8 hours, and the amount of palladium obtained to 28.2 μg/g. In addition, the palladium was found in liver and spleen at 24 hours mainly and then swept from the body after 10 days. The results showed the safety of the rGO/PdNFs for the living body. Biosafety is a pivotal factor in the biomedical field, which needs us to further research the in vivo toxicity. In our experiments, all the formulations did not have apparent weight loss (Supporting

Information,

Figure

S3).

To

investigate

the

possible

toxicology

of

prepared-nanomedicine, the major indexes of blood chemistry were examined. Figure 6a showed no obvious change of BUN for all therapeutic groups, which confirmed no side effect of all therapeutic groups on kidneys. According to Figure 6b, c, the ALT and AST kept in a natural range in all therapeutic groups indicating no apparent hepatotoxicity for the samples. Furthermore, to investigate the influence of rGO/PdNFs as nanomedicine on immune systems of living mice, the important immune factors (CD4, CD8 and IL-6) were also studied. Compared with saline, the group treated with rGO/PdNFs plus laser irradiation showed no significant change on above immune factors, which indicated the rGO/PdNFs have no effect on the immune system of mice (Figure 6d-f). The images of hematoxylin and eosin (H&E) staining were shown in Figure 6g. Relative to control groups, the rGO/PdNFs plus laser treated group showed the evident tumor necrosis. Overall, rGO/PdNFs were an excellent agent for efficient photothermal tumor therapy without biological toxicity.


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Conclusion In summary, multifunctional carbon-palladium nanohybrids were designed for wide applications in electrochemical ethanol catalysis, nonenzymatic glucose sensor and photothermal tumor therapy. We synthesized palladium nanoflowers attached on the reduced graphene oxide to appeal the extensive applications for one specific nanomaterial, which could motivate us to discover more scientific applications. Palladium nanomaterials as catalysts overcome the exorbitant cost and CO poisoning of platinum catalysts as well as excellent catalytic activity in direct ethanol fuel cells and high sensitivity in glucose detection. Furthermore, the hybrids between palladium nanomaterials and reduced graphene oxide enhanced the conductibility and dispersibility of palladium nanomaterials in catalysis, which indicated superior electrochemical catalytic activity and glucose detection. Not only that, we controlled the morphologies of palladium nanoparticles to palladium nanoflowers on the surface of reduced graphene oxide to enhance the absorption of NIR window. Hence, the increased photothermal conversion of rGO/PdNFs provides the possible in further photothermal tumor therapy. In this study, the prepared nanohybrids showed excellent effect of electrochemical catalysis compared with the commercial Pd/C catalysis, high sensitivity of glucose nonenzymatic sensing and photothermal therapy. We focused on not only the performance of the nanomaterials in the particular field, but also the additional function in the other scientific filed according to one specific nanomaterial. To the best of our knowledge, our study designed the rGO/PdNFs and explored three application areas of them for the first time. We believe that the continuous study of one specific nanomaterial to multiple applications promote the functionalization of more nanomaterials.

Experimental Section Chemicals. Palladium chloride (PdCl2) was obtained from Chengdu Xiya Reagent Co., Ltd. The Pd/C catalyst was obtained from Sigma Co., Ltd. Graphene oxide (GO) was purchased from Shanghai yuanye Biotechnology Co., Ltd. Glucose was purchased from Tianjin Guangfu Technology Development Fine Co., Ltd. All the chemicals and solvents used in the study were analytical and chromatographic grade and deionized water was used throughout the experiments. Synthesis of rGO/PdNFs. 5 mg of GO was mixed into deionized water under ultrasound for 12



hours. Subsequently, 20 mM PdCl2 solution was added into GO solution, which makes the Pd2+ combine with active groups on the GO. Then, 100 mM of NaBH4 was added into GO/Pd2+ solution in ice-bath. The rGO/PdNPs were prepared. Finally, 50 mM of ascorbic acid (AA) was added into rGO/PdNPs solution to promote the Pd nanoparticles continuous growth to form rGO/PdNFs. Electrochemical measurements. To investigate the electrochemical catalytic and biosensing activities of rGO/PdNFs, the cyclic voltammetry (CV) test was conducted on a CHI660E electrochemical workstation. 10 μL of rGO, rGO/PdNFs and commercial Pd/C catalyst was modified on the electrode surface and dried at room temperature. In vitro and in vivo photothermal tumor therapy and safety evaluation. The in vitro antitumor efficacy of rGO/PdNFs with laser irradiation was evaluated by HeLa cells line using the MTT assay. Kunming mice were inoculated subcutaneously with U14 cells. Under ether anesthesia, the mice were sacriﬁced at the day 10. At the same time, the tumor and the main organs (heart, liver, spleen, lung, and kidney) of mice were dissected for histological analysis. After 8 hours, 24 hours and 10 days, the experimental animals were sacrificed, and major organs (heart, liver, spleen, lung and kidney) and tumors were removed. After the tissues were digested with aqua regia for 1 day, the tissue debris was removed by centrifugation at 10000 rpm for 5 min, and then the Pd contents in different samples were determined using ICP-MS. The amount of Pd was expressed as the Pd mass per gram of tissue (µg/g).

Acknowledgments This work was supported by the National Natural Science Foundation (No. 21476190, 21776238), the Hebei province key basic research Foundation (No. 15961301D), Hebei education department key project (No. ZD2017084).

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Scheme 1. Schematic illustration of the synthetic route of rGO/PdNFs nanohybrids (top) and the wide applications in electrochemical ethanol catalysis/nonenzymatic glucose sensor and photothermal tumor therapy in vivo (bottom).


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Figure 1. Characterization. (a) TEM images of (i) GO, (ii) GO-Pd2+, (iii) rGO/PdNPs and (iv) rGO/PdNFs. (b) EDS and (c) XRD of rGO/PdNFs. (d) UV-vis spectra of rGO and rGO/PdNFs. (e) Size distribution of rGO/PdNFs. (f) XPS broad survey spectrum of rGO/PdNFs. (g) XPS narrow scan spectra of Pd 3d.



Figure 2. Electrochemical ethanol catalysis. Cyclic voltammetry curves of the commercial Pd/C and rGO/PdNFs modified electrodes in the (a) 1.0 M KOH solution and (b) 1.0 M KOH + 1.0 M C2H5OH solution. (c) Chronoamperometric curves of the rGO/PdNFs and Pd/C modified electrodes in 1.0 M KOH + 1.0 M C2H5OH solutions by applying a constant potential of -0.2V, and (d) cyclic voltammetry curves of rGO/PdNFs modified electrodes in the 1.0 M KOH + 1.0 M C2H5OH solutions at different scan rates.


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Figure 3. Electrochemical nonenzymatic glucose sensor. (a) The impedance spectrum of the bare GCE, rGO/GCE and rGO/PdNFs/GCE in 5.0 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) containing 0.1 M KCl. (b) Cyclic voltammograms of bare GCE, rGO/GCE and rGO/PdNFs/GCE in 0.1 M NaOH solution. (c) CV curves of the rGO/PdNFs without and with 100 nM glucose in 0.1 M NaOH solution at 50 mV s-1. (d) CV curves of the rGO/PdNFs in 0.1 M NaOH solution at different scan rates (10, 20, 30, 40, 50, 100, 200 mV s-1). (e) Typical current-time dynamic response of the rGO/PdNFs towards various concentrations of glucose (insert: calibration curve for glucose detection). (f) Interference study of rGO/PdNFs for electrochemical biosensor.



Figure 4. Cellular uptake and cytotoxicity. (a) Photothermal performance of rGO and rGO/PdNFs. (b) Cour6 uptake after treatment with rGO/PdNFs/Cour6 by FCM analysis of HeLa cells. (c) Cell viability of HeLa cells treated with rGO without/with laser irradiation and rGO/PdNFs without/with laser irradiation (808 nm, 1.5 W cm-2). (d) Representative fluorescence images of HeLa cells internalization of rGO/PdNFs/Cour6 and Cour6 after incubation for 24 hours. Cell nucleus was stained in blue (Hoechst) and Cour6 was shown in green. (e) Fluorescence micrographs of HeLa cells treated with different formulations (stained by FDA/PI, green for live and red for dead). The data are presented as the mean ± S.E. (n=3).


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Figure 5. In vivo photothermal tumor therapy. (a) Tumor growth profiles in different groups of the mice after various treatments. (b) Biodistribution of rGO/PdNFs in mice at different times after intratumor injection. The accumulation of rGO/PdNFs in tissues was confirmed by measuring the Pd content in the major organs and tumor tissues via ICP-MS. (c) Infrared thermal images in the tumor region of tumor-bearing mice treated with saline and rGO/PdNFs followed by laser irradiation (1.5 W cm-2 for 3 min). (d) Representative photographs of tumor-bearing mice and tumors after 10 days of treatments. (*P < 0.05, **P < 0.01, ***P

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