Abstract
Plasmonic gold nanorods play important roles in nowadays state-of-the-art plasmonic sensing techniques. Most of the previous studies and applications focused on gold nanorods with relatively small aspect ratios, where the plasmon wavelengths are smaller than 900 nm. Gold nanorods with large aspect ratios are predicted to exhibit high refractive-index sensitivity (Langmir 2008, 24, 5233–5237), which therefore should be promising for the development of high-performance plasmonic chemical- and bio-sensors. In this study, we developed gold nanorods with aspect ratios over 7.9, which exhibit plasmon resonances around 1064 nm. The refractive index (RI) sensitivity of these nanorods have been evaluated by varying their dielectric environment, whereby a sensitivity as high as 473 nm/RIU (refractive index unit) can be obtained. Furthermore, we have demonstrated the large-aspect-ratio nanorods as efficient substrate for surface enhanced Raman spectroscopy (SERS), where an enhancement factor (EF) as high as 9.47 × 108 was measured using 4-methylbenzenethiol (4-MBT) as probe molecule. Finally, a type of flexible SERS substrate is developed by conjugating the gold nanorods with the polystyrene (PS) polymer. The results obtained in our study can benefit the development of plasmonic sensing techniques utilized in the near-infrared spectral region.
Highlights
Noble metal nanostructures exhibit strong localized surface plasmon resonances (LSPR), the electromagnetic modes associated with the collective oscillations of the free electrons confined to the nanoscale [1], which have gained tremendous attention in the past decades
We showed that the gold nanorods can be integrated with the polystyrene (PS) substrates to form a flexible
The gold nanorods fabricated by binary surfactant seed-mediated growth method were dispersed in aqueous solution of cetyltrimethyl ammonium bromide (CTAB) and NaOL
Summary
Noble metal nanostructures exhibit strong localized surface plasmon resonances (LSPR), the electromagnetic modes associated with the collective oscillations of the free electrons confined to the nanoscale [1], which have gained tremendous attention in the past decades. Gold nanorods elongated along their longitudinal directions have received particular interests due to that the operation wavelengths of the longitudinal LSPR, i.e., plasmon mode associated with the electron oscillations along the length axis, can be synthetically tuned across the visible to the near-infrared region by changing their aspect ratios, i.e., the ratio between their lengths and diameters [2]. In such a manner, the localized electromagnetic field enhancements associated with the plasmon resonances can be tailored to match the excitation wavelengths by adjusting the nanorod geometries. From the LSPR sensing point of view, nanorod with large aspect ratios, which exhibit longer plasmon resonance wavelengths, were shown to exhibit higher
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