Assembly of gold nanoparticles using turnip yellow mosaic virus as an in-solution SERS sensor

Abstract

A common challenge in nanotechnology is the conception of materials with well-defined nanoscale structure. In recent years, virus capsids have been used as templates to create a network to organize 3D nano-objects, building thus new functional nanomaterials and then devices. In this work, we synthetized 3D gold nanoclusters and we used them as Surface Enhanced Raman Scattering (SERS) sensor substrates in solution. In practice, gold nanoparticles (AuNPs) were grafted on turnip yellow mosaic virus (TYMV) capsid, an icosahedral plant virus. Two strategies were considered to covalently bind AuNPs of different sizes (5, 10 and 20 nm) to TYMV. After purification by agarose electrophoresis and digestion by agarase, the resulting nano-bio-hybrid AuNP-TYVM was characterized by different tools. Typically, dynamic light scattering (DLS) confirmed the grafting through the hydrodynamic size increase by comparing AuNPs alone to AuNP-TYMV (up to 33, 50 and 68 nm for 5, 10 and 20 nm sized AuNPs, respectively) or capsids alone (28 nm). Transmission electronic microscopy (TEM) observations revealed that AuNPs were arranged with 5-fold symmetry, in agreement with their grafting around icosahedral capsids. Moreover, UV-vis absorption spectroscopy showed a red-shift of the plasmon absorption band on the grafted AuNP spectrum (530 nm) compared to that of the non-grafted one (520 nm). Finally, by recording in solution the Raman spectra of a dissolved probe molecule, namely 1,2-bis(4-pyridyl)ethane (BPE), in the presence of AuNP-TYVM and bare AuNPs or capsids, a net enhancement of the Raman signal was observed when BPE is adsorbed on AuNP-TYVM. The analytical enhancement factor (AEF) value of AuNP-TYMV is 5 times higher than that of AuNPs. These results revealed that AuNPs organized around virus capsid are able to serve as in-solution SERS-substrates, which is very interesting for the conception of ultrasensitive sensors in biological media.

Fig. 1. Schematic representation of the two chemical strategies used to bond AuNP to TYVM.

Fig. 2. Variation of ζ potential against pH of TYMV (red) and TYMV coated with 6-aminohexane-1-thiol (blue).

Fig. 3. Absorption spectra of AuNP of 20 nm in the presence of increasing concentrations of 6-amino-hexane-1-thiol.

Fig. 4. XPS spectra of AuNP (blue) and AuNP@6-aminohexane-1-thiol (black): (a) survey spectra, (b) HR-Au 4f region, (c) HR-S 2p region and deconvolution (red) and (d) HR-N 1s region.

Fig. 5. Agarose electrophoresis of TYMV grafted with AuNPs in different experimental conditions before (a) and after (b) staining with Coomassie Brilliant Blue G. TYMV grafted with AuNP via peptide bond, lane 1: by EDC in MES 0.1 M pH 5.5, lane 2: EDC/NHS in phosphate buffer 10 mM pH 7.0, lane 3: EDC in phosphate buffer 10 mM pH 6.0, lane 4: EDC in acidic water pH 6.0, respectively; lane 5: free AuNP; lane 6: TYMV grafted with Au NPs via electrostatic bond; lane 7: ungrafted TYMV. Purple arrow: grafted TYMV. Red arrow: free AuNP. Blue arrow: ungrafted TYMV.

Fig. 6. Agarose electrophoresis of TYMV grafted with different ratios of AuNP : TYMV (a) lane 1–3: 20 nm-AuNP grafted onto 25 μg TYMV with the ratio of 10 : 1, 15 : 1, 20 : 1, respectively. lane 4: free 20 nm-AuNP. (b) Lane 1, 2 : 10 nm-AuNP grafted onto 25 μg and 50 μg of TYMV to achieve the ratio of 30 : 1 and 15 : 1, respectively. Lane 3: free 10 nm-AuNP. Purple arrow: grafted AuNP. Red arrow: free AuNP.

Fig. 7. Absorption spectra of 20 nm-AuNP in the presence of increasing concentrations of TYMV coated by 6-aminohexane-1-thiol.

Fig. 8. SDS PAGE. Lane 1 & 2: TYMV grafted with Au NP with concentration of TYMV (18 and 20 μg mL⁻¹ in 5 mL of grafting solution, respectively); lane 3: molecular weight markers: trypsinogen (24 000 Da), α-lactalbumin (14 200 Da); lane 4: TYMV. Green arrow: AuNP grafted TYMV capsid subunit. Blue arrow: ungrafted TYMV capsid subunit.

Fig. 9. Agarose electrophoresis of TYMV grafted with AuNP in optimum condition for (a) strategy 1 (lane 2), in comparison with free AuNP (lane 1) and (b) strategy 2 (lane 3). Purple arrow: grafted TYMV. Red arrow: free AuNP.

Fig. 10. (a) Picture of free AuNP (left side) and grafted to TYMV (right side) after gel purification. Absorption spectra of free AuNP (red) and grafted to TYMV for AuNP (violet) of (b) 20 nm; (c) 10 nm and (d) 5 nm.

Fig. 11. DLS experiments. Particles size distribution of TYMV (black), TYMV grafted with AuNP of 5 (green), 10 (blue) and 20 nm (red).

Fig. 12. TEM images of AuNP of 5 (a), 10 (b) and 20 nm (c) grafted to TYMV. (b) Model of TYMV grafted with AuNP.

Fig. 13. SERS experiments. Raman spectra of BPE at 10⁻¹ M in solution in ethanol (green) and at 10⁻⁵ M in the presence of TYMV (blue), 20 nm-AuNP (black) and TYMV grafted to 20 nm-AuNP (red).

Fig. 14. (A) Raman spectra of BPE at 10⁻⁷ (black), 10⁻⁶ (red), 10⁻⁵ (blue) and 10⁻⁴ M (green) in solution in the presence of AuNP-TYMV. The curves are shifted to better see the different curves. Plot of log of SERS intensity at 1600 cm⁻¹ (B) and 1200 cm⁻¹ (C) against log[BPE]. Errors bars correspond to standard deviations from 2–3 experiments. (B) Slope, (0.9 ± 0.1); intercept, (8.6 ± 0.8); r² = 0.89. (C) Slope, (0.9 ± 0.1); intercept, (8.7 ± 0.7); r² = 0.92.