Iron Oxide–Gold Composite Nanoparticles and Nano-Gap Junctions …
267
25]. It is based on the vibrational Raman signatures of molecules that are positioned
near metal nanoscale surfaces which are greatly enhanced in intensity compared to
the molecules alone [6]. Vibrational levels depend on kinds of atoms, their bond
strengths, and arrangements. Experimentally, surface-enhanced Raman scattering is
similar to normal Raman spectroscopy; however, enhancing the signal requires the
presence of gold, silver, or copper substrates with a size or roughness on the order
of 10–100 nm. The enhancement effect is due to a combination of two different
effects: electromagnetic and chemical effects. The electromagnetic effect is an
enhancement of the electromagnetic field incident in the vicinity of the adsorbed
molecule as a consequence of exciting the surface plasmon of the metal. The magnitude of electromagnetic enhancement is highly dependent on the plasmon absorption of the substrate. The chemical effect is due to the electronic resonance/charge
transfer between a molecule and a metal surface, which leads to an increase in the
polarizability of the molecule [4, 6, 25].
The molecules that are rich in electrons are the most effective platforms for evaluation of surface-enhanced Raman spectroscopy effects. The most studied model
analytes in our laboratories are 4-mercaptobenzoic acid or 4-mercaptophenol [3, 5, 6,
11]. For example, we previously reported surface-enhanced Raman scattering studies
in solutions of 4-mercaptobenzoic acid present on silver (Ag) nanowires, silver–
gold (Ag–Au) bimetallic nanowires, and gold (Au) nanotubes [6]. We demonstrated
a six order of magnitude enhancement effect on silver nanowires. More recently,
colloidal solutions of rhodium–silver bimetallic nanoparticles also demonstrated
surface-enhanced Raman scattering effects, with enhancement factor almost four
times higher than for silver nanoparticles [3]. A waveguide modified with nanoparticles for use with surface-enhanced Raman spectroscopy for detection of gas analytes
was also developed by us [25].
In this study, the analyte of interest, 4-mercaptophenol, was captured on all
substrates including gold–iron oxide composite nanostructures and gold nanoparticles. The 4-mercaptophenol binds to the gold nanoparticle through the thiol group,
and its ring-breathing modes are the ones evaluated in Raman experiments [3, 5,
6, 24]. The ring-breathing modes of the 4-mercaptophenol display changes in the
polarizability and can be observed in Raman scattering signals at approximately
1100, 1175, and 1260 cm
−1 [3, 5, 6, 11, 24, 25]. By using 4-mercaptophenol, Raman
enhancement can be monitored while maintaining a controlled environment of the
analyte of interest. A schematic of samples used in our studies is shown in Fig. 9.
The surface-enhanced Raman scattering analyte of interest, 4-mercaptophenol,
was immersed in the nanoparticle solutions and incubated for several hours before
analysis. To ensure that the thiol bond from the sulfide group to the nanoparticle
was formed, the surface charge of the nanoparticles was taken before and after the
addition of 4-mercaptophenol.
Zeta potential measurements were performed using phase analysis light scattering
to monitor the surface charge before and after the surface engineering experiments.
Table 1 outlines the change in zeta potential with both the addition of the SERS
analyte to gold spheres and iron oxide–gold multifunctional structures with each
becoming more positive as 4-mercaptophenol was added. The negative surface charge
267
25]. It is based on the vibrational Raman signatures of molecules that are positioned
near metal nanoscale surfaces which are greatly enhanced in intensity compared to
the molecules alone [6]. Vibrational levels depend on kinds of atoms, their bond
strengths, and arrangements. Experimentally, surface-enhanced Raman scattering is
similar to normal Raman spectroscopy; however, enhancing the signal requires the
presence of gold, silver, or copper substrates with a size or roughness on the order
of 10–100 nm. The enhancement effect is due to a combination of two different
effects: electromagnetic and chemical effects. The electromagnetic effect is an
enhancement of the electromagnetic field incident in the vicinity of the adsorbed
molecule as a consequence of exciting the surface plasmon of the metal. The magnitude of electromagnetic enhancement is highly dependent on the plasmon absorption of the substrate. The chemical effect is due to the electronic resonance/charge
transfer between a molecule and a metal surface, which leads to an increase in the
polarizability of the molecule [4, 6, 25].
The molecules that are rich in electrons are the most effective platforms for evaluation of surface-enhanced Raman spectroscopy effects. The most studied model
analytes in our laboratories are 4-mercaptobenzoic acid or 4-mercaptophenol [3, 5, 6,
11]. For example, we previously reported surface-enhanced Raman scattering studies
in solutions of 4-mercaptobenzoic acid present on silver (Ag) nanowires, silver–
gold (Ag–Au) bimetallic nanowires, and gold (Au) nanotubes [6]. We demonstrated
a six order of magnitude enhancement effect on silver nanowires. More recently,
colloidal solutions of rhodium–silver bimetallic nanoparticles also demonstrated
surface-enhanced Raman scattering effects, with enhancement factor almost four
times higher than for silver nanoparticles [3]. A waveguide modified with nanoparticles for use with surface-enhanced Raman spectroscopy for detection of gas analytes
was also developed by us [25].
In this study, the analyte of interest, 4-mercaptophenol, was captured on all
substrates including gold–iron oxide composite nanostructures and gold nanoparticles. The 4-mercaptophenol binds to the gold nanoparticle through the thiol group,
and its ring-breathing modes are the ones evaluated in Raman experiments [3, 5,
6, 24]. The ring-breathing modes of the 4-mercaptophenol display changes in the
polarizability and can be observed in Raman scattering signals at approximately
1100, 1175, and 1260 cm
−1 [3, 5, 6, 11, 24, 25]. By using 4-mercaptophenol, Raman
enhancement can be monitored while maintaining a controlled environment of the
analyte of interest. A schematic of samples used in our studies is shown in Fig. 9.
The surface-enhanced Raman scattering analyte of interest, 4-mercaptophenol,
was immersed in the nanoparticle solutions and incubated for several hours before
analysis. To ensure that the thiol bond from the sulfide group to the nanoparticle
was formed, the surface charge of the nanoparticles was taken before and after the
addition of 4-mercaptophenol.
Zeta potential measurements were performed using phase analysis light scattering
to monitor the surface charge before and after the surface engineering experiments.
Table 1 outlines the change in zeta potential with both the addition of the SERS
analyte to gold spheres and iron oxide–gold multifunctional structures with each
becoming more positive as 4-mercaptophenol was added. The negative surface charge