Disordered array of Au covered Silicon nanowires for SERS biosensing combined with electrochemical detection

Abstract

We report on highly disordered array of Au coated silicon nanowires (Au/SiNWs) as surface enhanced Raman scattering (SERS) probe combined with electrochemical detection for biosensing applications. SiNWs, few microns long, were grown by plasma enhanced chemical vapor deposition on common microscope slides and covered by Au evaporated film, 150 nm thick. The capability of the resulting composite structure to act as SERS biosensor was studied via the biotin-avidin interaction: the Raman signal obtained from this structure allowed to follow each surface modification step as well as to detect efficiently avidin molecules over a broad range of concentrations from micromolar down to the nanomolar values. The metallic coverage wrapping SiNWs was exploited also to obtain a dual detection of the same bioanalyte by electrochemical impedance spectroscopy (EIS). Indeed, the SERS signal and impedance modifications induced by the biomolecule perturbations on the metalized surface of the NWs were monitored on the very same three-electrode device with the Au/SiNWs acting as both working electrode and SERS probe.

Figure 1

Photograph of the Au/SiNWs grown on microscope slide (a). Tilted view (ca. 50°) SEM images of the as grown SiNWs (b) and after Au coverage (c). EDX spectra of the as grown SiNWs (d) and after Au coverage (e).

Figure 2

Total integrated reflectivity spectra of the Au/SiNWs (red line) compared with Au planar layer (blue line) (a). Relative variation of the reflectivity, , where R_Au and R_Au/SiNW are the reflectivity of the planar Au and the Au/SiNWs, respectively (b). In the inset the enlarged at smaller wavelengths.

Figure 3

Optical images obtained on the sample at different stages of the functionalization process: the pristine Au/SiNWs (a); treated with cysteamine (Au/SiNWs+Cys) (b); after NHS-biotin binding (Au/SiNWs+Cys+Btn) (d); finally after the immersion in solution with 1 μM content of avidin (Au/SiNWs+Cys+Btn+Avd) (e). The image obtained after control experiment: Au/SiNWs modified with sole cysteamine and after exposure to 1 μM content of avidin solution (Au/SiNW+Cyst+Avd) (c). Scale bar is 100 μm.

Figure 4

Raman spectra obtained at different stages of the functionalization process of the samples: the pristine Au/SiNW substrate (violet line), after cysteamine modification (cyan line), functionalized with NHS-biotin (green line) and finally treated with avidin at 1 μM content (red line) (a). Control tests performed by skipping the treatment with NHS-biotin and immersing only in avidin at 1 μM content (b) and on the reference Au planar film (blue line) functionalized with cysteamine + NHs-biotin (black line) and avidin at 1 μM content (red line) (c).

Figure 5. Raman analysis of the biosensor sensitivity.

Spectra collected on identical devices at different avidin concentrations, from 1 μM down to 1 nM. The spectrum before protein adsorption is shown as reference (black line). In the inset the result of the fitting procedure applied to the spectrum collected at 100 nM of avidin concentration. The band ascribed to protein adsorption is centered at about 1380 cm⁻¹ (blue line).

Figure 6

Photograph of the three-electrode device with Au/SiNW used WE and Raman probe (a). Bode plots of the impedance modulus, |Z|, for the biotin modified Au/SiNWs after the immersion in avidin solution with different concentration ranging from 10 pM up to 1 μM (b). Dependence of impedance variation, , on the avidin concentration obtained at the frequency of 10 KHz. The values correspond to the average measurements performed on a minimum of three samples, identically treated. The relative standard deviation is indicated by the error bars (c). Intensity of the Raman band at about 1380 cm⁻¹ ascribed to the binding of avidin on the biosensor surface, tuned as for the impedance measurements. The reported values represent the band area obtained by means of a fitting procedure applied to the collected spectra (d).