Py-PW11

Research Note pubs.acs.org/IECR

Adsorption of Human Serum Albumin (HSA) by SWNTs/Py-PW11 Nanocomposite Yuanchun Ji,† Tengfei Li,† and Yu-Fei Song* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, People’s Republic of China S Supporting Information *

ABSTRACT: Covalently grafted pyrene moieties onto the K7[PW11O39]·13H2O cluster (K7-PW11) results in the formation of a new organic/inorganic hybrid with the molecular formula of (Bu4N)4{(PW11O39)[O(Si(CH2)3NH−COOCH2C16H9)2]} (PyPW11), which has been immobilized onto single-walled carbon nanotubes (SWNTs) homogeneously via π−π stacking and electrostatic interactions. The resulting SWNTs/Py-PW11 nanocomposite material exhibits excellent adsorption of human serum albumin (HSA), as evidenced by high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS) studies. This work paves a new pathway for the development of polyoxometalate (POM)-based biomaterials. PW11 nanocomposite via π−π stacking and electrostatic interactions, and the nanocomposite material shows excellent adsorption of human serum albumin (HSA), as evidenced by high-resolution transmission electron microscopy (HR-TEM) (Scheme 1) and XPS studies, etc.

1. INTRODUCTION Polyoxometalates (POMs), which represent a class of early transition metals of V, Mo, W, Nb, etc. linked by oxygen bridges, show wide application in a diverse range of fields such as electronics, catalysis, and medicine.1,2 For example, their promising biological activities make them attractive candidates for various human diseases. It is worth noting that investigation of the molecular interactions between POMs and proteins are one of the important steps toward a better understanding of their biofunctions and application.3 Recently, Proust et al. has successfully achieved protein immobilization onto surface nanostructure by binding (NBu4)3[PW11O39{(SiC6H4NH2)2O}] covalently onto planar gold surface,4 in which the protein immobilization is between specific rabbit immunoglobulin (rIgGs) and antirabbit immunoglobulin (antirIgGs). Moreover, it has been pointed out that the regular organization of POMs on the surface enables the ordered arrays of proteins. Single-walled carbon nanotubes (SWNTs), with their unique one-dimensional structures, exhibit distinct electrical and spectroscopic properties, and SWNTs have been widely used in energy storage, medicine, and biology.5,6 By making use of SWNTs’ biocompatibility, Dai and his colleagues have developed biocompatible, highly selective SWNT-immunoglobulin G conjugates for in vitro protein detection.7 Recently, the investigation of POMs-modified carbon nanotubes (CNTs) has attracted wide interest, because the combination of CNTs and POMs could exhibit advantages from both components.8 Compared with the large number of POMs-containing functional materials in the literature, only a few POMs-based biofunctional materials have been reported so far. To take advantage of both POMs and CNTs, in this paper, we report a new organic/inorganic hybrid material with the molecular formula of (Bu 4 N) 4 {(PW 11 O 39 )[O(SiCH 2 CH 2 CH 2 NH− COOCH2C16H9)2]} (denoted as Py-PW11) by covalently grafting pyrene moieties onto the monolacunary POM cluster of K7[PW11O39]·13H2O (K7-PW11). Moreover, the fabrication of Py-PW11 with SWNTs leads to the formation of SWNTs/Py© 2014 American Chemical Society

Scheme 1. Schematic Representation of the Application of SWNTs/Py-PW11 Nanocomposite for Adsorption of Human Serum Albumin (HSA)

2. RESULTS AND DISCUSSION Reaction of the pyren-1-ylmethanol (Py-CH2OH) with 3(triethoxysilyl)-propyl isocyanate in the presence of triethylamine (Et3N) leads to the formation of the pyren-1-ylmethyl(3-(triethoxysilyl)propyl)-carbamate (Py-Si(OEt) 3 ) (see Scheme S1 in the Supporting Information). Further reaction of Py-Si(OEt)3 with K7[PW11O39]·13H2O (K7-PW11)9 affords new inorganic/organic hybrid (Bu4N)4(Py-PW11), which has been fully characterized by Fourier transform infrared (FT-IR) Received: Revised: Accepted: Published: 11566

May 5, 2014 June 8, 2014 July 1, 2014 July 1, 2014 dx.doi.org/10.1021/ie501839q | Ind. Eng. Chem. Res. 2014, 53, 11566−11570

Industrial & Engineering Chemistry Research

Research Note

Figure 1. (a) Fourier transform infrared (FT-IR) spectroscopy spectra of the pristine SWNTs, SWNTs/Py-PW11, and Py-PW11. (b) TGA measurements of pristine SWNTs, SWNTs/Py-PW11, and Py-PW11 in N2 (10 °C min−1). (c) Raman spectra of the purified SWNTs and SWNTs/Py-PW11, respectively (excitation λ = 633 nm). (d) Fluorescence spectra of Py-PW11 and SWNTs/Py-PW11 in dimethylformamide (DMF) (λex = 344 nm). Figure 3. HR-TEM images of (a and b) the mixture of Py-PW11 and HSA, (c and d) the mixture of pristine SWNTs and HSA, and (e and f) adsorption of HSA by the SWNTs/Py-PW11 nanocomposite.

nanocomposite, indicating the interactions between Py-PW11 and SWNTs. Thermogravimetric analysis (TGA) results of SWNTs, PyPW11, and SWNTs/Py-PW11 have been presented in Figure 1b. The pristine SWNTs do not exhibit any decomposition before 525 °C, while the Py-PW11 and SWNTs/Py-PW11 nanocomposite show the weight loss of 34.9% and 15.2% before that, respectively. For Py-PW11, a weight loss of ∼27.8% between 230.2 °C and 430.7 °C is due to the decomposition of the TBA+ cations. After that, the decomposition of organic groups dominates between 430.7 °C and 698.8 °C. If a similar weight-loss behavior is assumed to take place,10,11 the Py-PW11 content in the SWNTs/Py-PW11 nanocomposite can be roughly determined to be 43.6 wt %. Raman spectra (excited at 633 nm) of SWNTs and SWNTs/ Py-PW11 nanocomposite have been shown in Figure 1c, the tangential vibration mode (G band)11 at 1586 cm−1 is clearly observed for both SWNTs and SWNTs/Py-PW11 nanocomposite. Meanwhile, the disorder-induced D-band shifts slightly from 1326 in SWNTs to 1329 cm−1 in SWNTs/PyPW11. Contrast experiment shows that no Raman absorption can be observed at ∼1300 cm−1 for Py-PW11 alone. Moreover, it is noted that the Raman spectra of SWNTs/Py-PW11 exhibit a D-mode with similar intensity to that in pristine SWNTs, but the intensity of the G-mode in SWNTs/Py-PW11 is slightly higher than that in pristine SWNTs. Such results might be due to the formation of a few surface defects on the SWNTs after modification by Py-PW11 under the experimental conditions. As shown in Figure 1d, the fluorescence spectroscopy provides proof for the interactions between Py-PW11 and SWNTs. Pyrene is a well-known luminophore and its dilute and concentrated solutions exhibit monomer and excimer emissions in the UV and visible regions, respectively. The steady-state fluorescence emission spectrum of Py-PW11 shows two

Figure 2. SEM images of (a) the pristine SWNTs and (b) the SWNTs/Py-PW11 nanocomposite; (c) TEM image and (d−i) HRTEM images of the SWNTs/Py-PW11 nanocomposite.

spectroscopy, 1H NMR, 13C NMR, and 31P NMR (see Figure 1a, as well as Figures S1−S3 in the Supporting Information). The SWNTs/Py-PW11 nanocomposite shows π−π interactions between pyrene moieties and SWNTs, and electrostatic interactions between SWNTs and Py-PW11 (see Figure 1). As shown in Figure 1a, the formation of Py-PW11-modified SWNTs nanocomposite is evident from the FT-IR spectrum. For example, the FT-IR spectrum of pristine SWNTs displays two weak C−H stretching bands at 2919 and 2848 cm−1, and the aromatic CC vibration bands at 1653, 1439 cm−1, while the corresponding C−H stretching bands at 2925 and 2870 cm−1, and the CC vibration at 1629 and 1451 cm−1, can be observed in the SWNTs/Py-PW11 nanocomposite. The CO and P−O−P vibration bands shift from 1716 and 1044 cm−1 in the Py-PW11 to 1704 and 1053 cm−1 in the SWNTs/Py-PW11 11567

dx.doi.org/10.1021/ie501839q | Ind. Eng. Chem. Res. 2014, 53, 11566−11570


Research Note

Figure 4. XPS survey spectra of the SWNTs/Py-PW11 nanocomposite (a) before and (b) after HSA adsorption; deconvoluted (c) C 1s spectrum and (d) N 1s spectrum after HSA adsorption. Also shown are deconvoluted W 4f spectra (e) before and (f) after HSA adsorption.

(PBS). Then, the SWNTs/Py-PW11 nanocomposite is dissolved in water, to which the HSA solution is added. The resulting solution is kept steady for 2 weeks. As shown in Figure 3, HR-TEM images (Figures 3e and 3f) of the adsorption of HSA by the SWNTs/Py-PW11 nanocomposite show that the protein is adsorbed firmly on the Py-PW11 cluster. In comparison to the size of Py-PW11 with a diameter of ∼1−1.3 nm, the adsorbed cluster has a diameter of ∼50−80 nm. As shown in Figures 3a and 3b, contrast experiments by mixing Py-PW11 and HSA as a control indicate that HSA is mixed with Py-PW11 randomly. Moreover, it can be seen that some crystallization is produced that can be attributed to PyPW11. HR-TEM images (Figure 3c and 3d) of the sample by mixing pristine SWNTs and HSA exhibit that HSA is either formed in cluster detached from SWNTs, or covered on SWNTs with a membrane-like morphology. As a result, it can be concluded that SWNTs reveal a poor adsorption capacity for HSA, whereas the SWNTs/Py-PW11 nanocomposite shows very strong adsorption ability for HSA. To further substantiate the HSA adsorption, X-ray photoelectron spectroscopy (XPS) is used to investigate the surface composition of the SWNTs/Py-PW11 nanocomposite after adsorption experiment. As such, after washing in deionized water, the SWNTs/Py-PW11 nanocomposite that immersed in protein solution is dried by freezing dryer. The survey scan XPS spectra of SWNTs/Py-PW11 nanocomposite before and after

monomer emissions, at 381 and 395 nm, and one excimer emission, at 478 nm. In contrast, the emission spectrum of the SWNTs/Py-PW 11 nanocomposite is almost completely quenched, which indicates that the decay of singlet excited pyrene moieties is affected by their binding to SWNTs. In other words, the emission quenching of the SWNTs/Py-PW11 nanocomposite may be caused by the pronounced electron and/or energy transfer between Py-PW11 and SWNTs, since it is well-known that SWNTs are nonemissive. Figure 2 shows the scanning electron microscopy (SEM) images of SWNTs/Py-PW11 nanocomposite with the pristine SWNTs as a control. It can be seen that the agglomeration of SWNTs is clearly visible in the SEM images of pristine SWNTs. In contrast, the SWNTs bundles are found to be loosened upon treatment with Py-PW11, which suggests the improved dispersion of SWNTs in the nanocomposite sample. Transmission electron microscopy (TEM, Figure 2c) and HR-TEM (Figures 2d−2i) images of SWNTs/Py-PW11 nanocomposite show the presence of homogeneously distributed dark spots with diameter of ∼1 nm that can be safely assigned to be PyPW11 in combination with the EDX measurement (Figure S5 in the Supporting Information). In order to test the property of the SWNTs/Py-PW11 nanocomposite in adsorbing the protein molecule under the physiologic condition, the solution of human serum albumin (HSA) (8 μg/mL) is prepared in phosphate buffer solution 11568



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HSA adsorption are presented in Figures 4a and 4b. The presence of Na, Cl, P, O photoelectron peaks is due to the remaining NaCl, Na2HPO4, NaH2PO4 from the phosphate buffer solution (PBS), and that of N and O possibly comes from the HSA protein, which contains the elements C, N, O, and H. Meanwhile, the intensities for W 4f are diminished dramatically after HSA adsorption, which indicate that the protein is adsorbed on the surface of the SWNTs/Py-PW11 nanocomposite. After adsorption, the deconvoluted C 1s photoelectron peaks can be well-fitted with components at 284.9 eV of C−C and C−H bonds, 286.5 eV of C−O bonds, and 288.3 eV of CO bonds, which are characteristic carbon bonds in protein (Figure 4c).12 The peak at 284.9 eV corresponds to the aliphatic carbons of the amino acids; the peak at 286.5 eV can be assigned to NH−CHR−CO carbons of the protein backbone; and the peak at 288.3 eV is the −CO−NH− carbons.12 These assignment are in good agreement with literature reports.12 As shown in Figure 4d, three deconvoluted N 1s peaks can be observed at 399.7 eV of C−N, 401.7 eV of N−CO and N−H bond, and 402.8 eV of the positively charged amines NH3+.13 Contrast experiments indicate that, before HSA adsorption, the XPS-deconvoluted C 1s spectrum shows the main peak at 284.6 eV corresponding to the C−C and C−H of the amino acids, the peak at 286.5 eV by the C−O of the protein backbone and the peak at 288.3 eV by CO of the peptide bonds (see Figure S6 in the Supporting Information), while the XPS-deconvoluted N 1s spectrum exhibits the binding energies centered at 399.8 eV by C−N and 400.2 eV by N−CO and N−H. The positions of these peaks remain almost the same, suggesting that, after HSA adsorption, the chemical environments of C and N in HSA remain unchanged. In another words, the HSA structure is stable after adsorption. The deconvoluted XPS spectra of W 4f5/2 and W 4f 7/2 show the binding energies at 36.28 and 38.49 eV before HSA adsorption (Figure 4e), which have been shifted to 35.58 and 37.8 eV after adsorption, respectively. This is another proof for the adsorption of HSA onto the SWNTs/Py-PW11 nanocomposite.

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-10-64431832. E-mail addresses: songyufei@ hotmail.com, [email protected]. Author Contributions †

These authors have the same contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Basic Research Program of China (No. 2014CB932104), National Science Foundation of China (No. 21222104), the Fundamental Research Funds for the Central Universities (No. RC1302) and the Program for Changjiang Scholars and Innovative Research Team in University.

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3. CONCLUSION The noncovalent sidewall functionalization of single-walled carbon nanotubes (SWNTs) by a covalently modified polyoxometalate−pyrene (POM−pyrene) hybrid of Py-PW11 has been carried out and fully characterized. The Py-PW11modified SWNTs results in the quench of the emission spectrum, which might be due to the electron and energy transfer between Py-PW11 and SWNTs. The SWNTs/Py-PW11 nanocomposite exhibits excellent adsorption capability of human serum albumin (HSA), which has been confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and X-ray photoelectron spectroscopy (XPS) studies. Further investigation of the SWNTs/Py-PW11 nanocomposite as a biosensor and/or biochip is currently ongoing in the laboratory.

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Research Note

ASSOCIATED CONTENT

* Supporting Information S

Supporting Information includes an experimental section, 1H NMR spectra, the 13C NMR spectrum, the 31P NMR spectrum, and UV-vis absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org/. 11569



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