The SERS enhancement factor (SERS-EF) is one of the most important parameters that characterizes the ability of a given substrate to enhance the Raman signal for SERS applications. The comparison of SERS intensities and SERS-EF values across different substrates is a common practice to unravel the performance of a given substrate. In this study, it is shown that such a comparison may lack significance if we compare substrates of very distinct nature and optical properties. It is specifically shown that the SERS-EF values for static substrates (e.g. immobilized metallic nanostructures) cannot be compared to those of dynamic ones (e.g. colloidal metal nanoparticle solutions), and that the optical properties for the latter show strong dependence on the metal-molecule interaction dynamics. The most representative experimental results concerning the dynamic substrates have been supported by generalized Mie theory simulations, which are tools used to describe the substrate complexity and the microscopic information not usually taken into account.
Associating polyaniline and metallic
nanoparticles may result in
the combination/synergism of properties, which have been attracting
great attention. This paper describes an investigation that verified
the effectiveness of several gold nanostructures acting as surface
enhanced Raman scattering (SERS) substrates using the emeraldine salt
form of PANI (PANI-ES) as a probe-molecule. The surface enhanced resonance
Raman scattering (SERRS) spectra of PANI-ES at 1064 nm results from
two enhancement effects: one from the PANI-ES resonance Raman and
the other from localized surface plasmon resonance of the gold nanostructures
when the plasmon wavelength approaches the NIR region. A considerable
SERRS intensity for PANI-ES at the 10 nM level could be obtained using
a gold nanoplates colloidal suspension as substrates. The SERRS spectra
of PANI-ES and the high correlation between SERRS intensity and LSPR
extinction of Au nanostructures at 1064 nm indicate that the enhancement
was mostly due to the electromagnetic mechanism. These results indicate
that PANI-ES can be employed as a general probe-molecule for the evaluation
of SERRS substrates performance at 1064 nm excitation.
Molecular wires of
the oligophenyleneimine (OPI) families were
used as bridging gaps between gold flat surface and gold nanorods,
forming molecular junction systems such as (AuFlat|OPI|AuNR). Systems
with different gap sizes were synthesized from 2.2 nm for the OPI-1p
to 9.9 nm for the OPI-13p (where n is equal to the
number of phenylene groups), and the intensities of the SERS bands
at 1078 cm–1
v(CS) and 1168 cm–1 β(CH) were obtained for each gap length. Our
results showed an unusual behavior for the bands 1078 cm–1
v(CS) and 1168 cm–1 β(CH)
as a function of OPI (gap) size. To address these results electromagnetic
field simulations by the discrete dipole approximation (DDA) method
for the systems (AuFlat|Gap|AuNR) were performed. Nevertheless the
high SERS intensities observed for (AuFlat|OPI|AuNR) with large gap
sizes for excitation at 785 nm indicated that there is a strong dependence
on the electronic properties of the molecular wire, which supersedes
the electromagnetic contribution of the plasmonic coupling. The experimental
and simulated results indicated that both electromagnetic (dipole–image
interaction and surface plasmon resonance) and molecular properties
are contributing to the SERS intensity behavior. Additionally, it
has been noticed that the length of the molecular wire that resulted
in a decrease in SERS intensity is coincident with the reported length
in which the transition from tunneling to hoping conduction occurs
for OPI molecular wires.
Materials science has observed a continuous increase in the use of metal nanoparticles in a wide range of studies, from fundamental physics to technological applications such as photocatalysis and optical communication devices. This broad scope has the same fundamental origin, the localized surface plasmons, whose excitation leads to strong light confinement, especially in the vicinity of closely spaced nanoparticles, the hot spots. The field amplification may be used to amplify the Raman scattering of adsorbed molecules, which is known as surface-enhanced Raman scattering (SERS). A crucial and limiting characteristic of SERS hot spots is their very localized nature, that influences the SERS intensity reproducibility as well as the probabilities of observation of single-molecule SERS signals. In this paper we discuss the correlation between SERS performance and gold nanorod cluster structures using transmission electron microscopy, SERS spectra and numerical simulations. The experimental data showed interesting behavior for the combination of end-to-end and side-by-side interactions, revealing the possibility of creating strong hot spots with a more extended spatial distribution. The results give insights into the development of high-performance SERS substrates.
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