Abstract
Due to the enhanced electromagnetic field at the tips of metal nanoparticles, the spiked structure of gold nanostars (AuNSs) is promising for surface-enhanced Raman scattering (SERS). Therefore, the challenge is the synthesis of well designed particles with sharp tips. The influence of different surfactants, i.e., dioctyl sodium sulfosuccinate (AOT), sodium dodecyl sulfate (SDS), and benzylhexadecyldimethylammonium chloride (BDAC), as well as the combination of surfactant mixtures on the formation of nanostars in the presence of Ag+ ions and ascorbic acid was investigated. By varying the amount of BDAC in mixed micelles the core/spike-shell morphology of the resulting AuNSs can be tuned from small cores to large ones with sharp and large spikes. The concomitant red-shift in the absorption toward the NIR region without losing the SERS enhancement enables their use for biological applications and for time-resolved spectroscopic studies of chemical reactions, which require a permanent supply with a fresh and homogeneous solution. HRTEM micrographs and energy-dispersive X-ray (EDX) experiments allow us to verify the mechanism of nanostar formation according to the silver underpotential deposition on the spike surface in combination with micelle adsorption.
Highlights
A characteristic feature of the AuNSs synthesized in the concentrated anionic surfactant dioctyl sodium sulfosuccinate (AOT) template phase is the core/spike ratio of about 1.5 : 1
Raman spectrum of 4-NTP without the gold stars displayed a very weak signal showing the direct in uence of surface-enhanced Raman scattering (SERS)
To give an estimation of the enhancement factor, the intensity of the strongest vibrational mode (NO2) of the 4-NTP is further compared according to the following formula; EF 1⁄4
Summary
As a result of their special chemical and physical properties, the attention given to gold nanoparticles (AuNPs) expanded with the enhanced potential of their applications in catalysis, photonics, biological sensing, and nanomedicine.[1,2,3,4,5,6,7,8] the shape- and size dependent plasmon resonance of the particles and the related enhanced electromagnetic eld is of special interest in electroanalysis and surface enhanced Raman scattering (SERS).[9,10,11,12,13,14,15]The extraordinary role of sharp tips and spikes in surface enhanced Raman scattering leads to the synthesis of starshaped nanoparticles, i.e., nanostars and nano owers, with well-de ned tips and spikes.[16,17,18,19,20,21,22,23,24] In general protocols of synthesis of gold nanostars (AuNSs) can be divided into seed mediated methods and one-pot procedures.[25]. By adding ascorbic acid to gold clusters, incorporated into catanionic surfactant-based vesicles, nano owers can be synthesized, too.[24] In that case interactions between the anionic surfactant dioctyl sodium sulfosuccinate (AOT) and the cationic surfactant CTAB are of special relevance for the seed mediated gold cluster formation, and the nal nano ower formation. Inspired by these results and our molecular dynamics (MD) simulations about AOT micelle and bilayer adsorption on gold surfaces,[29,30] we started to investigate the formation of gold nanoparticles in presence of mixed surfactants. As anionic surfactants we have used AOT and sodium dodecyl sulfate (SDS), and as cationic surfactant benzylhexadecyldimethylammonium chloride (BDAC), due to the fact that AOT and BDAC build up mixed micelles, already shown experimentally as well as by MD simulations.[31]
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