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B. P. Majee and A. K. Mishra
the signal. Even if the excitation wavelength cannot match exactly to the resonance
condition, it can influence the SERS enhancement via an intensity borrowing mechanism [23]. Figure 4 depicts that the electromagnetic enhancement shows relatively
higher magnitude of enhancement among the others enhancement mechanism.
3.2 Enhancement Factor (EF)
Since the discovery of SERS, correct estimation for the “magnitude” of the enhancement remains a challenge for scientific community. The effect of SERS on detecting
a probe molecule is characterized by Enhancement factor (EF), which is a pure
number. The electromagnetic and chemical enhancement is contributed to the total
EF in SERS study. The EF depends on the SERS substrate, excitation source and
the analyte molecule and its value can be found anywhere in the range 10
3 –10
14
[31]. In case of SERS measurement, a diversity of situations can arise such as single
molecule, multiple molecules, distribution of analyte molecule on the surface and
the averages over time etc., which make a single general definition of the EF impossible in a SERS process. Hence, there have been different ways of calculating EF. As
an example, single-molecule enhancement factor (SMEF) is suitable for theoretical
estimations of the EF, while substrate specific enhancement factor (SSEF) is the most
useful and used definition [32]. Here, we will discuss different definitions for EF in
SERS detection process.
3.2.1 The Single Molecule Enhancement Factor
It is the increase of the conventional Raman scattering at certain localized position
for a given molecule at a specific point on the SERS substrate. It depends on the
Raman tensor and orientation of the analyte molecule on the SERS substrate. It also
depends upon the polarization and direction orientations of incident laser. Hence, to
avoid conflicts with factor like the orientation of the molecule, the single-molecule
enhancement factor (SMEF) can be expressed as follows
EF SM =
I
SM
SERS
I
SM
RS
(7)
where I
SM
SERS and I
SM
RS represent the intensities of SERS signal for single-molecule
and the average Raman signal per molecule in the absence of surface enhancement,
respectively [31].
B. P. Majee and A. K. Mishra
the signal. Even if the excitation wavelength cannot match exactly to the resonance
condition, it can influence the SERS enhancement via an intensity borrowing mechanism [23]. Figure 4 depicts that the electromagnetic enhancement shows relatively
higher magnitude of enhancement among the others enhancement mechanism.
3.2 Enhancement Factor (EF)
Since the discovery of SERS, correct estimation for the “magnitude” of the enhancement remains a challenge for scientific community. The effect of SERS on detecting
a probe molecule is characterized by Enhancement factor (EF), which is a pure
number. The electromagnetic and chemical enhancement is contributed to the total
EF in SERS study. The EF depends on the SERS substrate, excitation source and
the analyte molecule and its value can be found anywhere in the range 10
3 –10
14
[31]. In case of SERS measurement, a diversity of situations can arise such as single
molecule, multiple molecules, distribution of analyte molecule on the surface and
the averages over time etc., which make a single general definition of the EF impossible in a SERS process. Hence, there have been different ways of calculating EF. As
an example, single-molecule enhancement factor (SMEF) is suitable for theoretical
estimations of the EF, while substrate specific enhancement factor (SSEF) is the most
useful and used definition [32]. Here, we will discuss different definitions for EF in
SERS detection process.
3.2.1 The Single Molecule Enhancement Factor
It is the increase of the conventional Raman scattering at certain localized position
for a given molecule at a specific point on the SERS substrate. It depends on the
Raman tensor and orientation of the analyte molecule on the SERS substrate. It also
depends upon the polarization and direction orientations of incident laser. Hence, to
avoid conflicts with factor like the orientation of the molecule, the single-molecule
enhancement factor (SMEF) can be expressed as follows
EF SM =
I
SM
SERS
I
SM
RS
(7)
where I
SM
SERS and I
SM
RS represent the intensities of SERS signal for single-molecule
and the average Raman signal per molecule in the absence of surface enhancement,
respectively [31].