Catalase-like and Peroxidase-like Catalytic Activities of Silicon

Letter pubs.acs.org/Langmuir

Catalase-like and Peroxidase-like Catalytic Activities of Silicon Nanowire Arrays Hongwei Wang,*,† Wenwen Jiang,† Yanwei Wang, Xiaoli Liu, Jianlin Yao, Lin Yuan, Zhaoqiang Wu, Dan Li, Bo Song, and Hong Chen* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren’ai Road, Suzhou 215123, PR China S Supporting Information *

ABSTRACT: Silicon nanowire arrays (SiNWAs) were found to have catalytic activities similar to those of biological enzymes catalase and peroxidase. Thus not only can these materials catalyze the decomposition reaction of H2O2 into water and oxygen, but they can also catalyze the oxidation of ophenylenediamine (OPD), a common substrate for peroxidases, by H2O2. The presence of Si−H bonds and the morphology of the SiNWAs are found to be crucial to the occurrence of such catalytic activity. When the SiNWAs are reacted with H2O2, the data from Raman spectroscopy suggests the formation of (Si−H)2···(O species) ((Si−H)2···Os), which is presumably responsible for the catalytic activity. These findings suggest the potential use of SiNWAs as enzyme mimics in medicine, biotechnology, and environmental chemistry.

1. INTRODUCTION Hydrogen peroxide (H2O2) is a byproduct of oxidative metabolism in organisms,1−3 which may cause molecular damage and, over time, cellular and tissue dysfunction. These will ultimately accelerate aging and increase the risk of a number of diseases including cancer, atherosclerosis, diabetes, Alzheimer’s disease, and Parkinson’s disease.4−6 It is therefore generally believed that H2O2 is toxic in vivo and if generated should be removed immediately. The scavenging of H2O2 in cells relies on the enzymes in peroxisomes known as catalases and peroxidases.7,8 However, because H2O2 is able to cross cell membranes readily and relocate to other cell structures or surrounding tissues,9−11 the peroxisomes in the cells alone cannot thoroughly remove all of the H2O2 generated in a timely manner. To eliminate the redundant H2O2, some catalase and peroxidase mimetics are synthesized by many researchers and used for the treatment of cancer, Alzheimer’s disease, and Parkinson’s disease.12−14 These drugs are designed as compounds of metals, such as gold, platinum, copper, and cobalt. However, the metals themselves can be cytotoxic and thus inevitably give rise to side effects.15−17 Silicon-based nanostructured materials are biocompatible and have a wide range of applications in biomedical research.18 Silicon nanowire arrays (SiNWAs), for example, are used in biological detection, cell growth regulation, protein adsorption, and DNA transformation.19−25 An unanticipated phenomenon of the SiNWAs possessing dual enzyme activities similar to those of catalases and peroxidases was observed in our laboratory. To the best of our knowledge, such enzymelike catalytic activities of SiNWAs have not been reported. © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of SiNWAs and Surface Treatment. SiNWAs were prepared by the chemical etching of an n-type Si[100] wafer (5 mm × 5 mm) in a AgNO3/HF aqueous solution following the same procedures as in previous studies.22,25 Briefly, silicon wafers were cleaned in a freshly prepared piranha solution (7:3 v/v H2SO4/H2O2) at 90 °C for 30 min and were then rinsed with deionized water and dried in a stream of nitrogen. The cleaned silicon wafers were immersed in an etching solution containing 5.0 mol·L−1 HF and 20 mmol·L−1 AgNO3 at 50 °C for 5, 10, or 30 min to prepare silicon nanowire arrays with nanowire lengths of 4, 9, and 24 μm. The resulting surfaces were immersed in 20% nitric acid for 1 min and then rinsed extensively with deionized water. To prepare hydrogenized silicon nanowire arrays and silicon wafer, the samples were soaked in HF solution (5% v/v in deionized water) for 5 min, rinsed with distilled water, and dried under N2. For piranha treatment, the samples were soaked in piranha solution (7:3 v/v H2SO4/H2O2) at 90 °C for 2 h, rinsed with distilled water, and dried in a stream of argon. The surface morphology of as-prepared silicon nanowire arrays (SiNWAs) was observed using a field-emission scanning electron microscope (FESEM, S-4800, Japan). The chemical compositions of the silicon wafer and SiNWAs were determined by XPS with an Escalab MK II (XPS) (VG Scientific Ltd.). 2.2. Scavenging of Hydrogen Peroxide. SiNWA samples were soaked in 200 μL of H2O2 solution (4% m/v in deionized water) at room temperature for different times. Concentrations of H2O2 were measured by absorbance at 240 nm (Varioskan Flash, Thermo Scientific, USA). Reactions were carried out in the dark or under natural illumination. Oxygen evolution was detected by gas chromatography. Received: July 21, 2012 Published: December 17, 2012 3

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Figure 1. Effect of silicon nanowire arrays on the decomposition of H2O2 into water and oxygen: (A) Decrease in H2O2 concentration after 5 min via an untreated silicon wafer (referred to as Si), an HF-treated silicon wafer (referred to as H−Si), untreated silicon nanowire arrays (referred to as SiNWAs), HF-treated silicon nanowire arrays (referred to as H-SiNWAs), and piranha-treated silicon nanowire arrays (referred to as P-SiNWAs); the control referred to the case with only H2O2 solution. (B) Decrease in H2O2 concentration via an HF-treated silicon wafer (referred to as H−Si) and silicon nanowire arrays (referred to as H-SiNWAs) as a function of time. (C) (Bottom) Decrease in H2O2 concentration after 10 min via HSiNWAs with different nanowire lengths. (Top) SEM images of arrays of different nanowire lengths (scale bar 20 μm); the etching time is also shown. 2.3. Oxidation of o-Phenylenediamine (OPD). OPD solution was prepared by dissolving 10 mg of OPD in 10 mL of deionized water. Different quantities of hydrogen peroxide were added to the OPD solutions to prepare the reaction mixtures. HF-treated SiNWAs (H-SiNWAs) were then incubated with the reaction mixtures at 37 °C for different times. Reactions were carried out in the dark or under natural illumination. H2SO4 (2 mol·L−1) was added to stop the reaction. The absorbance spectrum of the reaction mixture was measured from 350 to 800 nm (Varioskan Flash, Thermo Scientific, USA). 2.4. In Situ Raman Spectroscopy. Raman spectra were recorded on a confocal Raman system (Horiba Jobin Yvon HR800, France) during the scavenging of H2O2 in the presence of H-SiNWAs. A 632.8 nm He−Ne laser was employed as the excitation source. The laser beam was focused on the samples through a 50× objective with a long working distance. The slit and pinhole were 200 μm in width and 800 μm in diameter, respectively. The data acquisition time was 60 s.

Si−H bonds onto the surface, showed extremely high activity in decomposing H2O2 whereas after treatment with piranha solution, which introduces Si−OH bonds onto the surface, the nanowires had no catalytic activity (Figure 1B). HF-treated and piranha-treated SiNWAs have similar morphologies and roughness values, but they had totally different catalytic activities. We thus concluded that the catalytic activity of the untreated SiNWAs comes from the residual Si−H bonds formed during sample preparation. In addition, the nanowire structures are also important to the catalytic activity. As shown in Figure 1B, the concentration of H2O2 barely changed in the presence of an HF-treated silicon wafer whereas in the presence of hydrogenized SiNWAs the decrease in the decreased H2O2 concentration reached about 90% in 10 min, and the process was accompanied by the release of oxygen (Supporting Information). In this experiment, the amount of H2O2 consumption (about 211.7 μmol) was at least 2 orders of magnitude larger than that of the Si−H bonds estimated theoretically for a hydrogenized SiNWA sample (Supporting Information). Therefore, the decomposition of H2O2 was not due to the consumption of Si−H bonds on HSiNWAs. Furthermore, the rate of H2O2 decomposition was found to increase with increasing nanowire length (Figure 1C), which suggested that the morphology of SiNWAs, especially the length of the nanowires (and therefore the total area), greatly affects its activity in decomposing H2O2.

3. RESULTS AND DISCUSSION From scanning electron microscopy images (Supporting Information), we estimated the diameter and number density of nanowires on SiNWAs to be ∼70 nm and ∼1010 cm−2, respectively, in agreement with our previous studies.22,25 We first observed that SiNWAs can catalyze the decomposition of H2O2 to water and oxygen. Such catalase-like catalytic activity of SiNWAs was related to the chemical composition of the surfaces. More specifically, as shown in Figure 1A, SiNWAs after treatment with HF, which introduces 4

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Figure 2. Oxidation of o-phenylenediamine (OPD) by H2O2. (A) Color evolution of the reaction mixtures in the presence of HF-treated silicon nanowire arrays (referred to as H-SiNWAs). The control referred to the case with only OPD and H2O2 solutions. (B, C) UV−vis spectra as a function of time for a reaction mixture treated or not treated with H-SiNWAs, respectively. The reaction mixture includes 1.5% m/v H2O2 and 0.5 mg mL−1 OPD. The effect of H2O2 concentration on this reaction is shown in the Supporting Information.

Figure 3. (A) Raman spectra recorded for HF-treated silicon nanowire arrays (referred to as H-SiNWAs) in air (referred to as dry), in water (referred to as water), in H2O2 (i.e., during the disproportional reaction; referred to as H2O2), and after the removal of H2O2 from the system by washing with water (referred to as washed). Os indicates the oxygen species. (B) Our proposed mechanisms for the herein reported catalytic activity of H-SiNWAs in scavenging H2O2.

2A. When H-SiNWAs were used instead of peroxidase, oxidation was apparent after 5 min and the solution turned brown. Moreover, the color deepened with increasing reaction time (Figure 2A). This process was also confirmed by data from absorption spectra (Figure 2B,C). Although we deliberately carried out the reactions in the absence of light, we have verified that the herein reported catalase- and peroxidase-like

We also observed that H-SiNWAs can catalyze the oxidation of OPD by H2O2. OPD, a common substrate for peroxidases, is transparent in aqueous solution.26,27 In the presence of peroxidase and H2O2, OPD is oxidized to 2,3-diaminophenazine (DAP) by H2O2, resulting in a yellow-orange solution. Without the addition of peroxidase (i.e., with H2O2 alone), the oxidation of OPD was slow, as shown by the control in Figure 5

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catalytic activities of H-SiNWAs can also be observed under natural illumination conditions. Therefore, we hypothesize that the mechanism of this catalytic activity is different from that of the photocatalytic activity proposed previously.28 To shed light on the catalytic mechanism, we employed in situ Raman spectroscopy to monitor the progress of the reaction. As shown in Figure 3A, the Raman spectra of HSiNWAs in either air or water showed an obvious band at 2110 cm−1 corresponding to Si−H vibrations.29−31 Upon addition of H2O2, the band was red shifted to 2060 cm−1, which we attributed to the formation of (Si−H)2···(O species) ((Si− H)2···Os, possibly as (Si−H)2···2O2H) when Si−H bonds react with H2O2. The formation of (Si−H)2···O structures weakened the relative intensity of the Si−H band, causing the red shift. Interestingly, after the removal of H2O2 from the system, the Raman spectrum of cleaned H-SiNWAs in air again showed the 2110 cm−1 band whereas the band at 2060 cm−1 disappeared, indicating that the reaction of (Si−H)2···O formation was reversible. On the basis of these results, we propose in Figure 3B a mechanism for the herein reported catalase- and peroxidase-like catalytic activities of SiNWAs, which is different from the mechanism previously proposed for the photocatalytic activity of silicon nanowires.28

ASSOCIATED CONTENT

S Supporting Information *

Details of chemicals, surface characterization (XPS results and a theoretical estimation of the number of Si−H bonds on HSiNWAs), oxygen evolution from the scavenging of H2O2, and effect of H2O2 concentration on the oxidation of OPD in the presence of H-SiNWAs. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS Hydrogenized silicon nanowire arrays were found to possess catalase-like activity (release of oxygen) as well as peroxidaselike activity (oxidation of substrate). The presence of Si−H bonds and the nanostructured morphology are found to be crucial to such catalytic activities. The data from Raman spectroscopy suggests the formation of activated complexes, (Si−H)2···Os, in the presence of H2O2, which is responsible for the catalytic activity. Because the Si−H bonds can be easily made in large quantities and can also be easily reactivated by HF treatment, our findings may stimulate studies of silicon nanowires as enzyme mimetics in the nanocatalysis field.

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars (21125418), the National Natural Science Foundation of China (20920102035, 20974086, 21074083 and 21104055) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors thank Dr. J. Brash of McMaster University for helpful discussions. 6

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