Enhanced Infrared LSPR Sensitivity of Cap-Shaped Gold Nanoparticles Coupled to a Metallic Film

Abstract

We report on optical properties of gold deposited on SiO2 nanospheres randomly adsorbed on a thin gold layer. Extinction peaks with optical density of more than 2 are observed in the visible as well as near-IR regimes. The peak wavelength of the latter was affected exquisitely by the thickness of the top layer. A helium ion microscope (HIM) was used for careful observation of morphological transformation accompanying the change in the deposition thickness. Growth of grain structures into a capped-dimer structure was accompanied by slight blue-shift of the visible peak and significantly greater red-shift of the near-IR peak. Our finite-difference time-domain (FDTD) calculations show that these peaks in the visible and near-IR can be respectively attributed to dipole modes associated with transverse and longitudinal oscillations of free electrons in the gold-capped dimer. To investigate the refractive index sensitivity of these peaks, we used two approaches: immersion in solutions of varying refractive index and coating with an organic layer. With the first approach that characterizes the bulk sensitivity, the visible peak shows sensitivity of 122 nm/RIU, while the near-IR peak shifts at the rate of 506 nm/RIU. With the second approach that reflects the local sensitivity, the surface was saturated with alkaline phosphatase (ALP), whose subsequent reaction led to formation of a thin insoluble organic layer, causing a relatively small blue-shift, under 7 nm, of the visible peak and much larger red-shift, over 50 nm, of the near-IR peak when measured in buffer. When the same reaction was measured at end points in the air, the shift was as large as 444 nm for the near-IR peak.

Morphology Effects of Cap-shaped Silver Nanoparticle Films as a SERS Platform

“…At this stage, we can only speculate what model would predict such behaviors. In the past, we showed that a model based on dimers playing a dominant role could describe extinction spectra of cap-shaped gold nanoparticles in the visible and near-IR regions; 27 currently we are trying to find out whether more complicated clusters such as trimer or tetramer are more appropriate. At the same time, further work on characterizing gap junction areas within a cluster is needed.…”

Protection of silver and gold LSPR biosensors in corrosive NaCl environment by short alkanethiol molecules; characterized by extinction spectrum, helium ion microscopy and SERS

“…Our LSPR sensor need to be prepared with the top layer in this thickness range. 45 While we do not delve into details here, the same system exhibits another peak in the near-IR region. 45 In accordance with the general observation that nanostructures with red- shied extinction peaks are characterized by improved gures of merits, 24,46,47 the near-IR peak is characterized by gures of merit of 506 nm per RIU.…”

“…45 While we do not delve into details here, the same system exhibits another peak in the near-IR region. 45 In accordance with the general observation that nanostructures with red- shied extinction peaks are characterized by improved gures of merits, 24,46,47 the near-IR peak is characterized by gures of merit of 506 nm per RIU. 45 Overall, the prominent extinction peak, respectable gures of merit, and ease of preparing samples with areas greater than cm 2 with high uniformity make random MFON LSPR sensor a good candidate for practical application.…”

“…Moreover, the gure of merit, peak shi in nm per refractive index unit (nm per RIU), is in the range of 130-150 nm per RIU, which is close to the upper limit for LSPR sensors operating in the visible region. [43][44][45] When comparison is limited to other surfacebound sensors which tend to show slightly lower gures of merit due to the interference from the underlying solid surface, its advantage becomes great. Incidentally, as the top layer thickness is increased from under 17 to over 22 nm, adjacent Au nanoparticles become connected as seen in the insets.…”

Electrochemical synthesis of nanostructured gold film for the study of carbohydrate–lectin interactions using localized surface plasmon resonance spectroscopy

“…An alternate strategy involves depositing gold caps on SiO 2 nanospheres randomly arranged on a gold surface, for which a good LSPR response was found. 27 Electron beam lithography can make nanostructures precisely without any defects; 28 however, this technique is expensive and requires more time and expertise. 24 Evaporating a thin layer of metal on glass surface followed by annealing is also a cheaper and simpler technique; 29 however, the nanostructures produced are polydisperse.…”