Preparation of saline-stable, silica-coated triangular silver nanoplates of use for optical sensing.

Triangular silver nanoplates (TSNPs) may find application in next generation optical bio-sensors owing to the high sensitivity of the spectral position of their main plasmon band to changes in local refractive index. Unfortunately, etching of the anisotropic nanoplates to spherical particles occurs upon exposure to chloride ions from salt, with a concomitant decrease in optical sensitivity. Herein are detailed two general methods for the silica coating of TSNPs, with the aim of forming a protective barrier against chloride etching. It has been necessary to modify literature approaches for the coating of spherical Ag nanoparticles, since these are either ineffective for anisotropic nanoplates or lead to their degradation. The first method is a modified Stöber approach using tetraethylorthosilicate (TEOS) as the alkoxide precursor and dimethylamine in low concentration as the basic catalyst, with prior priming of the nanoplate surfaces by diaminopropane. The thickness of the silica layer can be tuned between 7 and 20nm by varying the primer and alkoxide concentrations. The second method involves deposition of a thin dense layer of silica from sodium silicate solution onto mercaptopropyltriethoxysilane (MPTES) or mercaptopropyltrimethoxysilane (MPTMS) primed TSNPs. This latter method offers protection against anion etching - experiments suggest that the adsorbed MPTES provides much of the barrier to chloride ions, while the silica shell serves to prevent particle aggregation. It was found that the silica coated particles substantially retained the sensitivity to refractive index of the as-grown TSNPs while being able to withstand salt concentrations typical of bio-testing conditions.

Versatile solution phase triangular silver nanoplates for highly sensitive plasmon resonance sensing.

Solution phase triangular silver nanoplates (TSNP) with versatile tunability throughout the visible-NIR wavelengths are presented as highly sensitive localized surface plasmon refractive index sensors. A range of 20 TSNP solutions with edge lengths ranging from 11 to 200 nm and aspect ratios from 2 to 13 have been studied comprehensively using AFM, TEM, and UV-vis-NIR spectroscopy. Studies of the localized surface plasmon resonance (LSPR) peak's sensitivity to refractive index changes are performed using a simple sucrose concentration method whereby the surrounding refractive index can solely be changed without variation in any other parameter. The dependence of the TSNP localized surface plasmon resonance (LSPR) peak wavelength lambda(max) and its bulk refractive index sensitivity on the nanoplate's structure is determined. LSPR sensitivities are observed to increase linearly with lambda(max) up to 800 nm, with the values lying within the upper limit theoretically predicted for optimal sensitivity, notwithstanding any diminution due to ensemble averaging. A nonlinear increase in sensitivity is apparent at wavelengths within the NIR region with values reaching 1096 nm.RIU(-1) at lambda(max) 1093 nm. Theoretical studies performed using a simple aspect ratio dependent approximation method and discrete dipole approximation methods confirm the dependence of the LSPR bulk refractive index sensitivity upon the TSNP aspect ratio measured experimentally. These studies highlight the importance of this key parameter in acquiring such high sensitivities and promote these TSNP sols for sensing applications at appropriate wavelengths for biological samples.