Letter pubs.acs.org/NanoLett
Dumbbell-like PtPd−Fe3O4 Nanoparticles for Enhanced Electrochemical Detection of H2O2 Xiaolian Sun,§ Shaojun Guo,§ Yi Liu, and Shouheng Sun* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: Dumbbell-like PtxPd100−x−Fe3O4 nanoparticles (NPs) were synthesized and studied for electrocatalytic reduction and sensing of H2O2. In 0.1 M phosphate buffered saline (PBS) solution, the 4−10 nm PtxPd100−x−Fe3O4 NPs showed the Pt/Pd compositiondependent catalysis with Pt48Pd52−Fe3O4 NPs having the best activity. The Pt48Pd52−Fe3O4 NPs were tested for H2O2 detection, and their H2O2 detection limit reached 5 nM, which was suitable for monitoring H2O2 generated from Raw 264.7 cells. These dumbbell-like PtPd− Fe3O4 NPs are the most sensitive probe ever reported and can be used to achieve real-time quantitative detection of H2O2 in biological environment for biological and biomedical applications. KEYWORDS: PtPd alloy, dumbbell nanoparticles, electrocatalysis, H2O2 detection
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and of Pt−Fe3O4 on oxygen reduction reaction in alkaline solution.25 More recently, Au−Fe3O4 NPs were found to be more active than either single component Au or Fe3O4 NPs for H2O2 reduction in PBS.26 The enhanced activity is believed to come from the partial charge transfer between Au and Fe3O4 at the nanoscale interface. If this is the case in general, then due to the intrinsic high activity of Pt-based alloys, the PtPd−Fe3O4 NPs should have even higher activity for the reduction reaction and therefore have higher sensitivity for H2O2 detection. Our experiments show that PtPd−Fe3O4 is indeed a much better catalyst for H2O2 reduction reaction than either PtPd or Fe3O4. The electrochemical sensing based on the optimized PtPd− Fe3O4 can reach as low as 5 nM H2O2 and is successfully used to detect H2O2 released from Raw 264.7 (mouse leukemic monocyte macrophage) cells. Two-step procedure was employed to synthesize PtxPd100−x− Fe3O4 NPs. In the first step, monodisperse PtxPd100−x NPs were synthesized through coreduction of platinum acetylacetonate, Pt(acac)2, and palladium acetylacetonate, Pd(acac)2 by boranemorpholine in oleyamine (OAm) according to the previous report.27 Then, PtxPd100−x−Fe3O4 NPs were obtained by controlled nucleation and growth of Fe on PtxPd100−x NPs followed by air oxidation (Supporting Information).28 The size of the Fe3O4 NPs was controlled by the molar ratio of Fe(CO)5 to PtPd seeds. More Fe(CO)5 produced larger Fe3O4. Starting from different PtxPd100−x NP seeds, PtxPd100−x−Fe3O4 NPs with x = 33, 48, 67, and 100 were synthesized. Figure 1A shows the typical transmission electron microscopy (TEM) image of representative 4 nm Pt48Pd52 NPs synthesized as reported.27 Figure 1B,C is the TEM images of
eveloping a quantitative assay of hydrogen peroxide (H2O2) in biological solutions is of broad interest in biology and biomedicine. H2O2 is produced by almost all oxidases in mitochondria and can diffuse out freely through membranes to reach various cellular compartments. 1−3 Maintaining H2O2 at an appropriate level is essential for intracellular signaling transduction and normal cell functions.4−6 But the presence of excess of H2O2 can induce various kinds of biological damages, leading to aging, neurodegeneration, and cancer.7−11 To better understand the biological effects of H2O2, it is critically important to monitor H2O2 level in biological environment, especially in cellular environment. Conventionally, H2O2 is detected by colorimetric methods.12−15 Recently, electrochemical methods based on catalytic reduction of H2O2 by a natural enzyme, horseradish peroxidase (HRP), were found to be more direct, rapid, sensitive, and selective than the optical methods due to the specific high activity of HRP toward H2O2 reduction.16−18 However, as a natural enzyme, HRP is not readily available for such a detection purpose, and under electrochemical conditions used for the detection, it can lose its activity quickly. Nanoparticles (NPs) of metals and oxides have been explored as alternative electrochemical catalysts for H2O2 detection.19−22 Among various NPs reported, Pt and its alloy NPs showed much enhanced catalytic activity for H2O2 reduction reaction with the H2O2 detection limit reaching 2 μM level.20,21 Despite this increased activity and sensitivity, probes made from these single component NPs are still not sensitive enough for detecting H2O2 in cells at nM level.23 Herein, we report that dumbbell-like PtPd−Fe3O4 NPs can serve as an efficient catalyst for H2O2 reduction and sensitive detection. Dumbbell-like NPs have shown some interesting catalytic properties due to the interfacial interactions between two different nanostructures. This is seen in enhanced catalytic power of Au−Fe3O4 on CO oxidation to CO2 in gas phase24 © 2012 American Chemical Society
Received: June 25, 2012 Revised: August 15, 2012 Published: August 27, 2012 4859
dx.doi.org/10.1021/nl302358e | Nano Lett. 2012, 12, 4859−4863
Nano Letters
Letter
isopropanol + 5% Nafion (volume ratio: 4/1/0.025) to reach a concentration of 2 mg/mL. Twenty μL of this dispersion was deposited on the surface of glassy carbon (GC) electrode, and the catalysts were fixed onto the electrode by Nafion once the solvent was evaporated. Figure 3A,B shows the typical cyclic votammograms (CVs) of H2O2 reduction by Pt48Pd52, Fe3O4, and Pt48Pd52−Fe3O4 NPs in 0.1 M PBS (pH 7.4) containing 4 mM H2O2. We can see that the Pt48Pd52−Fe3O4 NPs generate a much higher reduction current than either Fe3O4 or Pt48Pd52 NPs based on the same Fe3O4 weight (Figure 3A) or Pt48Pd52 weight (Figure 3B), respectively. As a comparison, there is no current change without H2O2 (Figure 3C), further confirming that the current is generated via catalytic reduction of H2O2. Figure 3D shows the reduction current from the physical mixture of Pt48Pd52 and Fe3O4 NPs, and dumbbell-like Pt48Pd52−Fe3O4 NPs, as well as the calculated current addition of single Pt48Pd52 and Fe3O4 NPs from Figure 3A, B (black curve). Despite the fact that each of the NP amount is controlled to be equal to the corresponding NP in the dumbbell NPs, the reduction current generated from the mixture is much lower than that from the Pt48Pd52−Fe3O4 NPs. This infers that Pt48Pd52−Fe3O4 NPs as a composite catalyst can greatly promote electrochemical reduction of H2O2. We further studied Pt/Pd composition-dependent H2O2 reduction activity of the PtPd−Fe3O4 NPs. Figure 4 shows the typical CVs of different 4−10 nm PtxPd100−x−Fe3O4 NPs with x = 33, 48, 67, and 100 in the N2-saturated 0.1 M PBS (pH 7.4) plus 4 mM H2O2 at a scan rate of 50 mV s−1. We can see that each of the PtxPd100−x−Fe3O4 with x = 33, 48, and 67 has a higher reduction peak current than Pt−Fe3O4 NPs. This is consistent with the previous study that PtPd NPs have better activity than Pt NPs for the electrochemical reduction of H2O2.21 Among PtxPd100−x−Fe3O4 NPs tested, Pt48Pd52−Fe3O4 NPs show the lowest onset reduction potential and highest current peak at −0.456 V for H2O2 reduction. It is known that coupling Pt NPs with Fe3O4 NPs in the dumbbell-like Pt− Fe3O4 NPs increased the catalytic activity of Pt NPs for oxygen reduction reaction. This catalytic enhancement is proposed to arise from the partial electron transfer from Fe3O4 to Pt at the nanoscale interface, improving O2 absorption and activation on Pt surface adjacent to Fe3O4.25 In the present PtPd−Fe3O4 system, alloying Pt with Pd may further facilitate H2O2 absorption and activation. Thus, the PtxPd100−x−Fe3O4 NPs showed a Pt/Pd composition-dependent catalytic activity with Pt48Pd52−Fe3O4 NPs being the best catalyst for electrochemical reduction of H2O2. The H2O2 detection sensitivity of the Pt48Pd52−Fe3O4 NPs was examined by a current−time (i−t) technique at a constant potential. Figure 5A,B and the inset of Figure 5A show the typical amperometric responses of the Pt48Pd52−Fe3O4 NP modified GC electrode upon the addition of H2O2 into the stirring 0.1 M PBS (pH 7.4) at −0.25 V. As H2O2 was added, the modified electrode responded rapidly to the substrate, achieving the maximum steady-state current within 2 s. This fast electrode reduction response can be attributed to the fact that H2O2 is rapidly absorbed and activated on the surface of the Pt48Pd52−Fe3O4 NPs. The current vs H2O2 concentration is drawn in Figure 5C. These Pt48Pd52−Fe3O4 NPs show two similar linear responses in the ranges of 20−100 nM and >2 μM (up to 14 mM tested in this paper), but a slow current increase in 100 nM−2 μM region. It looks as though at high (>2 μM) and extremely low (
