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.

Etching-resistant silver nanoprisms by epitaxial deposition of a protecting layer of gold at the edges.

The protection of silver nanoprisms against etching by the epitaxial deposition of a thin layer of gold in solution has been investigated. It has been found that at low Au/Ag ratios (approximately 0.08 to 0.17) a thin layer of gold is deposited on the edges of the nanoprisms as expected, but without the structural damage typically associated with galvanic replacement. Furthermore, this layer of gold provides robust protection against etching of the nanoprisms by chloride and is strong evidence that etching by chloride is face-selective and does not take place at the flat {111} faces of the nanoprisms. Additionally, the deposition of a protecting layer of gold results in only a small red shift in the position of the main plasmon resonance. We have investigated the sensitivity of the localized surface plasmon resonance (LSPR) to changes in the bulk refractive index of the solution and find that the gold-protected silver nanoprisms are promising candidates for the development of new refractive index-based biosensors.