Chemically bound gold nanoparticle arrays on silicon: assembly, properties and SERS study of protein interactions

Abstract

A highly reproducible and facile method for formation of ordered 2 dimensional arrays of CTAB protected 50 nm gold nanoparticles bonded to silicon wafers is described. The silicon wafers have been chemically modified with long-chain silanes terminated with thiol that penetrate the CTAB bilayer and chemically bind to the underlying gold nanoparticle. The silicon wafer provides a reproducibly smooth, chemically functionalizable and non-fluorescent substrate with a silicon phonon mode which may provide a convenient internal frequency and intensity calibration for vibrational spectroscopy. The CTAB bilayer provides a potentially biomimetic environment foranalyte, yet allows a sufficiently small nanoparticle separation to achieve a significant electric field enhancement. The arrays have been characterized using SEM and Raman spectroscopy. These studies reveal that the reproducibility of the arrays is excellent both between batches (< 10% RSD) and across a single batch (< 5% RSD). The arrays also exhibit good stability, and the effect of temperature on the arrays was also investigated. The interaction of protein and amino acid with the nanoparticle arrays was investigated using Raman microscopy to investigate their potential in bio-SERS spectroscopy. Raman of phenylalanine and the protein bovine pancreatic trypsin inhibitor, BPTI were studied using 785 nm excitation, coincident with the surface plasmon absorbance of the array. The arrays exhibit SERS enhancements of the order of 2.6 x 10 4 for phenylalanine, the standard deviation on the relative intensity of the 1555 cm -1 mode of phenylalanine is less than 10% for 100 randomly distributed locations across a single substrate and less than 20% between different substrates. Significantly, comparisons of the Raman spectra of the protein and phenlyalanine in solution and immobilized on the nanoparticle arrays indicates that the protein is non-randomly orientated on the arrays. Selective SERS enhancements suggest that aromatic residues penetrate through the bilayer inducing conformational changes in the protein.3

Optimizing the immobilization of gold nanoparticles on functionalized silicon surfaces: amine- vs thiol-terminated silane

“…We also changed the chain length for thiol-terminated layers through covalent grafting of mercaptoundecanoic acid (MUA) on APTES-modified layers. This latter functionalization procedure was successfully used by Kaminska et al to immobilize CTAB-covered nanoparticles on silicon and reach a densely packed layer [22]. X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used to characterize the formed layers and the protocol of nanoparticle deposition was modified, particularly using ultrasonication during the deposition of the spherical gold nanoparticles.…”

Site selective surface enhanced Raman on nanostructured cavities

“…14 The surface enhancement of the Raman spectra of [Ru(bpy) 2 (Qbpy)] 2+ on the FTO coated gold array were reported previously to be approximately 10 7 under 514 nm excitation and comparable enhancement is anticipated here. 6 As described by Bartlett and Baumberg et al, Mie and Bragg plasmon distributions are influenced by excitation angle to different degrees. 7 Their studies of SER of benzenethiol were performed using silver nanocavites formed on a glass substrate.…”

“…Such nanocavity array substrates can be made cost-effectively and reproducibly using a wide range of materials. [6][7][8] The resulting voids or cavities have been shown to enable better energy confinement than metallic particle nanostructures, resulting in better surface enhancements. 7,8 The optical properties of these substrates can be readily controlled through sphere diameter and thickness of the electrodeposited film.…”

“…Further improvement in understanding the way in which such substrates interact with adsorbates and incident and emitted light is needed so that their commercial (bio)analytical application potential can be fully realised. 6 In the case of nanocavity arrays, understanding the optimal region of plasmonic enhancement for luminescence and Raman spectroscopy and understanding how the angle of incident excitation affects enhancement are both of key importance.…”

“…7,8 The optical properties of these substrates can be readily controlled through sphere diameter and thickness of the electrodeposited film. 6,9 Metallic sphere segment void substrates sustain surface plasmons, they generate intense electric fields under illumination and consequently show huge enhancements of SERS in air and in contact with solution. 7,10 To date, while studies have been performed in regard to the Raman properties of these substrates, few studies have been performed on luminescence enhancements by these substrates.…”