Manganese Phosphate Self-assembled Nanoparticle Surface and Its application for Superoxide Anion Detection

www.nature.com/scientificreports OPEN received: 02 February 2016 accepted: 08 June 2016 Published: 30 June 2016 Manganese Phosphate Selfassembled Nanoparticle Surface and Its application for Superoxide Anion Detection Xiaohui Shen, Qi Wang, Yuhong Liu, Wenxiao Xue, Lie Ma, Shuaihui Feng, Mimi Wan, Fenghe Wang & Chun Mao Quantitative analysis of superoxide anion (O2·−) has increasing importance considering its potential damages to organism. Herein, a novel Mn-superoxide dismutase (MnSOD) mimics, silica-manganous phosphate (SiO2-Mn3(PO4)2) nanoparticles, were designed and synthesized by surface self-assembly processes that occur on the surface of silica-phytic acid (SiO2-PA) nanoparticles. The composite nanoparticles were characterized by fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electronic microscopy (SEM), electron diffraction pattern, energy dispersive spectroscopy (EDS) and elemental mapping. Then the electrochemical measurements of O2·− based on the incorporation of SiO2-Mn3(PO4)2 onto the surface of electrodes were performed, and some satisfactory results were obtained. This is the first report that manganous phosphate (Mn3(PO4)2) nanoparticles with shape-controlled, but not multilayer sheets, were utilized for O2·− detection. The surface self-assembly technology we proposed will offer the ideal material to construct more types biosensor and catalytic system for its nanosized effect. Active reactive oxygen species (ROS) containing oxygen atoms are the substances with strong oxidizing ability, which can cause or aggravate cancer, cardiovascular diseases, asthma, cataract, ulcer disease, Alzheimer’s disease, Parkinson’s disease and other diseases. O2·−, the critical important part of the so-called ROS, is implicated in many physiological and pathological processes1–3. Under normal physiological conditions, O2·− maintains the relatively balanced level in vivo. Once the cell produces excessive O2·− in response to external stimulus or pathological changes, it will lead to etiology of aging, cancer, and progressive neurodegenerative diseases such as Parkinson’s disease. Thus, real-time analysis and detection of O2·− have great significance. A variety of approaches have been tried to measure O2·− concentration, such as electron spin resonance 4–6, spectrophotometry7, chemiluminescence8, colorimetry9,10, chromatograph11,12 and fluorescence13–15. However, these methods cost much and usually occupy too much space. In comparison with other methods, the electrochemical method has recently attracted a great deal of attention owing to its advantages including high sensitivity, low detection limit, simplicity, direct, real-time detection and so on. Up to date, the commonly used electrochemical enzyme sensors are fabricated by immobilizing superoxide dismutase (SOD) and cytochrome (cyt c) onto the electrode surface. However, the enzymatic O2·− sensors are easily affected by pH and temperature changes, which limit their practical applications due to the poor stability of nature enzyme. Nanozymes, possessing enzymatic activities with nanostructure, have attracted particular attention as emerging natural enzyme mimics, they offer the possibility of lowered cost, improved stability, and excellent recyclability16–18. Meanwhile, bionic concept has gained more and more attention19–21. Mn-superoxide dismutase (MnSOD) mimics, manganous phosphate (Mn3(PO4)2), manganous pyrophosphate (Mn2P2O7) and manganese (II) complexes are usually used to fabricate biosensors for O2·− detection22,23. Cabelli have studied the antioxidant mechanism of aggregated Mn3(PO4)2 particles in organic vivo24. Li used DNA as a template to National and Local Joint Engineering Research Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210023, China. Correspondence and requests for materials should be addressed to M.W. (email: ) or C.M. (email: ) Scientific Reports | 6:28989 | DOI: 10.1038/srep28989 1 www.nature.com/scientificreports/ Figure 1. FTIR spectra of (a) SiO2 NPs, and (b) SiO2-PA NPs. Figure 2. (a) SiO2 NPs, (b) SiO2-NH2 NPs, (c) SiO2-PA NPs, and (d) SiO2-Mn3(PO4)2 NPs of Zeta potential analysis. produce Mn3(PO4)2 nanosheets and decorated this biomimetic enzyme onto the electrode surface for sensitive in-situ detection of O2·− 25. However, the intrinsic drawbacks of DNA, including high cost, instability, and storage difficulty, may limit their widely applications of electrochemical sensors. Dai also reported the high efficient catalysis of Mn2P2O7, which was used as a SOD mimic for O2·− detection26. There is a serious problem in dealing with the preparation of these reported MnSOD mimics. It is that the conventional synthesized MnSOD mimics that reported in the previous literatures have multilayer sheet structure with uncontrolled shape, thickness and size. This approach will bring resources waste and low catalytic efficiency. We wonder how it is possible to utilize surface self-assembly technology and nanotechnology to construct a more efficient MnSOD mimic for promoting analytical properties. In this paper, SiO2-Mn3(PO4)2 NPs were synthesized by surface self-assembly processes that occur on the surface of SiO2-phytic acid (SiO2-PA). To the best of our knowledge, there are no reports employing surface coating technique to immobilize Mn3(PO4)2 onto the surface of NPs for O2·− detection. The SiO2-Mn3(PO4)2 NPs have many advantages, like controllable shape with nanoscale, high specificsurface area than that of nano-sheet structure, low cost, simple preparation process, non-toxic, and so on. This novel MnSOD mimic we prepared is utilized to fabricate biosensors, and the electrochemical measurements of O2·− based on the incorporation of SiO2-Mn3(PO4)2 onto the electrodes surface are performed. Results and Discussion Figure 1 showed the fourier transform infrared (FTIR) spectroscopy of SiO2 NPs (a) and SiO2-PA NPs (b). For curve (a), the appearance of characteristic peak at 1106 cm−1 and 957 cm−1 were attributed to the O-Si-O bonds stretching vibration, indicating that SiO2 NPs were successfully synthesized27. Compared with unmodified SiO2 NPs, the SiO2-PA NPs illustrated three extra peaks at 2928, 1552 and 695 cm−1, which should be attributed to -C-NH2 stretching, symmetric -NH2 stretching, and the bending vibrations of -NH in APTES, respectively28. The results indicated that APTES was successfully modified onto the surface of SiO2 NPs29. More importantly, an adsorption peak at 1092 cm−1 was observed due to the overlap of the characteristic peak of phosphate group (PO43−) and the peak of asymmetric O-Si-O stretching30. The results confirmed that the SiO2 NPs were successfully modified by APTES and PA. Scientific Reports | 6:28989 | DOI: 10.1038/srep28989 2 www.nature.com/scientificreports/ Figure 3. TEM images of (A) SiO2 (...truncated)