198
B. P. Majee and A. K. Mishra
4 Examples of Different SERS Substrates
Different metallic and semiconducting SERS substrates have been investigated in
last few decades for the detection of organic impurities, biomolecules and/or drugs.
Researchers have observed and analyzed the effect of shape, size, surface roughness
etc. of nanomaterials substrates for SERS detection. Here, we provide some of the
examples of metallic and semiconducting SERS substrates.
4.1 Metal SERS Substrates
The metal nanostructures (Au, Ag, Cu etc.) are still known as the best candidates
for high-efficiency SERS applications. There have been several studies especially
on Au and Ag nanostructures for SERS detection of organic impurities and drugs.
Tian et al. studied different shapes and size of Au nanostructures for SERS detection.
They synthesized three different shapes of Au nanostructures- sphere, triangle and
star shapes [37]. In this work, Au nanospheres were synthesized via the seed-mediated
growth method and nanotriangles were prepared by chemical reduction method while
nanostars were synthesized via surfactant-directed, seed-mediated growth method.
The SEM images of prepared Au nanospheres, nanotriangles and nanostars in this
work are shown in Fig. 7. Researchers used these nanostructures for the detection of
Rhodamine 6G (R6G) molecules. The SERS spectra of R6G molecule in suspensions
of Au nanostars, nanotriangles and aggregated nanospheres are shown in Fig. 7d.
They observed that the enhancement increases as shape varies from nanospheres <
nanotriangles < < nanostars. The different enhancement occurs due to the difference
in number of intrinsic hotspots per particle. The number of hotspots per particle
increases in the following order- nanospheres < nanotriangles < nanostars. In SERS
enhancement, hotspots are the locations in very close to the nanostructures, in which
the local electric (E loc ) field enhanced largely due to the external electric field as
compared to its surroundings electric field [38]. As a result, if any molecule present
in a SERS-active hotspot then huge enhancement in the signal is observed.
In another study on flower-like Au nanostructured array for SERS detection,
Kim et al. prepared such structure with varying thickness from 5.1 to 49.6 nm
using photolithography (top-down) and electro-deposition (bottom-up) methods
[24]. Figure 8a shows the SEM image of the synthesized flower like Au nanostructure. The Authors detected two different analytes, Benzenethiol (BT) and Brilliant
cresyl blue (BCB) and showed that rougher Au nanostructure exhibit higher SERS
enhancement. Fig. 8b shows the SERS spectrum of BT indicating strong signal due
to the hotspots at the surface of the SERS substrate. Figure 8c shows the SERS
spectra of BCB molecule of different concentrations and the inset image shows
the zoomed view of the lowest concentrations (1 nM). The main reason behind the
SERS detection is electromagnetic enhancement due to the sharp tips and valleys
on the Au nanostructures. The Ag nanostructures are also known as suitable SERS
B. P. Majee and A. K. Mishra
4 Examples of Different SERS Substrates
Different metallic and semiconducting SERS substrates have been investigated in
last few decades for the detection of organic impurities, biomolecules and/or drugs.
Researchers have observed and analyzed the effect of shape, size, surface roughness
etc. of nanomaterials substrates for SERS detection. Here, we provide some of the
examples of metallic and semiconducting SERS substrates.
4.1 Metal SERS Substrates
The metal nanostructures (Au, Ag, Cu etc.) are still known as the best candidates
for high-efficiency SERS applications. There have been several studies especially
on Au and Ag nanostructures for SERS detection of organic impurities and drugs.
Tian et al. studied different shapes and size of Au nanostructures for SERS detection.
They synthesized three different shapes of Au nanostructures- sphere, triangle and
star shapes [37]. In this work, Au nanospheres were synthesized via the seed-mediated
growth method and nanotriangles were prepared by chemical reduction method while
nanostars were synthesized via surfactant-directed, seed-mediated growth method.
The SEM images of prepared Au nanospheres, nanotriangles and nanostars in this
work are shown in Fig. 7. Researchers used these nanostructures for the detection of
Rhodamine 6G (R6G) molecules. The SERS spectra of R6G molecule in suspensions
of Au nanostars, nanotriangles and aggregated nanospheres are shown in Fig. 7d.
They observed that the enhancement increases as shape varies from nanospheres <
nanotriangles < < nanostars. The different enhancement occurs due to the difference
in number of intrinsic hotspots per particle. The number of hotspots per particle
increases in the following order- nanospheres < nanotriangles < nanostars. In SERS
enhancement, hotspots are the locations in very close to the nanostructures, in which
the local electric (E loc ) field enhanced largely due to the external electric field as
compared to its surroundings electric field [38]. As a result, if any molecule present
in a SERS-active hotspot then huge enhancement in the signal is observed.
In another study on flower-like Au nanostructured array for SERS detection,
Kim et al. prepared such structure with varying thickness from 5.1 to 49.6 nm
using photolithography (top-down) and electro-deposition (bottom-up) methods
[24]. Figure 8a shows the SEM image of the synthesized flower like Au nanostructure. The Authors detected two different analytes, Benzenethiol (BT) and Brilliant
cresyl blue (BCB) and showed that rougher Au nanostructure exhibit higher SERS
enhancement. Fig. 8b shows the SERS spectrum of BT indicating strong signal due
to the hotspots at the surface of the SERS substrate. Figure 8c shows the SERS
spectra of BCB molecule of different concentrations and the inset image shows
the zoomed view of the lowest concentrations (1 nM). The main reason behind the
SERS detection is electromagnetic enhancement due to the sharp tips and valleys
on the Au nanostructures. The Ag nanostructures are also known as suitable SERS