5.2 Ultrasensitive Spectroscopy with Plasmonic, Surface-Enhanced 2D IR
2D IR spectroscopy can nowadays be applied to routinely measure vibrational
dynamics of samples in bulk solution environments with low absorption coefficients
(\ 500 M
-1 cm
-1 ) and with low mM concentrations or even less [306]. Recently,
surface-sensitive variants of 2D IR spectroscopy have achieved to resolve signals
from only monolayer thin samples at solid-liquid/gas interfaces [15, 173]. It must be
noted, however, that in case of a fully covered surface, the number of molecules
contributing to the signal is still a factor of about 5 lower compared to a bulk
solution sample with, say, 1 mM concentration and a 50 lm path-length [15]. To be
able to record vibrational dynamics over multiple integers of the IR-label’s
vibrational relaxation time constant from monolayer thin samples with even the
smallest absorption coefficients (\ 100 M
-1 cm
-1 ), it is, therefore, desirable to
obtain a maximum sensitivity of the methods. One way to achieve that is to combine
femtosecond 2D IR spectroscopy with established methods from ultra-sensitive IR
spectroscopy that exploit the properties of plasmonic substrates [7, 307–310]. That
approach has recently been demonstrated by different groups based on controlled
nanoantennas [311], or heterogeneous plasmonic substrates [179, 243]. The
plasmonic substrates allow the generation of enhanced optical near-fields around
polarizable nanostructures, thereby concentrating the interaction volume of the light
and the sample. That signal enhancement factors of up to about 500 have been
characterized that are achievable under typical conditions for 2D IR spectroscopy
[243], and even stronger enhancement has been estimated [311]. These values are
generally obtained by comparison of the obtained signals to signals from analogous
samples that do not benefit from surface enhancement effects [179, 243, 312]. The
strong near-fields therefore open up a couple of exciting applications for ultrafast
spectroscopy that have partially been demonstrated very recently, as discussed in
the following.
A first and obvious possibility is the exploitation of the increased near-fields for
higher-order processes in samples that are either surface-bound, or within the
interaction length of the optical near-fields from the nanostructures (\ \ 10 nm).
This effect is generally referred to as ‘‘vibrational ladder-climbing’’
[242, 243, 313–329] and is schematically depicted in Fig. 30a. Taken a given
anharmonicity in an electronic ground state potential of an immobilized molecule
on a plasmonic substrate, a broadband laser pulse (bandwidth * 200 cm
-1 ) can
successively excite populations in higher-lying vibrational states by interacting
multiple times with the sample. In the first demonstration of this effect in surfaceenhanced 2D IR spectroscopy, Hamm et al. employed few nanometer thick, sputtercoated, heterogeneous Gold (Au) layers as plasmonic substrates (Fig. 30b) for 2D
ATR IR spectroscopy on p-mercaptobenzonitrile (p-PhCN) monolayers. Such metal
layers near the percolation threshold exhibit rather continuous areas of metal
patches (light regions), separated by small gaps (\ 10 nm in width, dark regions).
Polarization-controlled excitation of molecules inside the gaps [179, 243, 330]
allowed a dramatic signal enhancement and the clear observation of vibrational
ladder-climbing in the 2D ATR IR signals from the CN-stretch vibration of the
monolayer up to the 5–6 transition (Fig. 30c and d). In that way, more than
Top Curr Chem (Z) (2017) 375:86
123
182
Reprinted from the journal
2D IR spectroscopy can nowadays be applied to routinely measure vibrational
dynamics of samples in bulk solution environments with low absorption coefficients
(\ 500 M
-1 cm
-1 ) and with low mM concentrations or even less [306]. Recently,
surface-sensitive variants of 2D IR spectroscopy have achieved to resolve signals
from only monolayer thin samples at solid-liquid/gas interfaces [15, 173]. It must be
noted, however, that in case of a fully covered surface, the number of molecules
contributing to the signal is still a factor of about 5 lower compared to a bulk
solution sample with, say, 1 mM concentration and a 50 lm path-length [15]. To be
able to record vibrational dynamics over multiple integers of the IR-label’s
vibrational relaxation time constant from monolayer thin samples with even the
smallest absorption coefficients (\ 100 M
-1 cm
-1 ), it is, therefore, desirable to
obtain a maximum sensitivity of the methods. One way to achieve that is to combine
femtosecond 2D IR spectroscopy with established methods from ultra-sensitive IR
spectroscopy that exploit the properties of plasmonic substrates [7, 307–310]. That
approach has recently been demonstrated by different groups based on controlled
nanoantennas [311], or heterogeneous plasmonic substrates [179, 243]. The
plasmonic substrates allow the generation of enhanced optical near-fields around
polarizable nanostructures, thereby concentrating the interaction volume of the light
and the sample. That signal enhancement factors of up to about 500 have been
characterized that are achievable under typical conditions for 2D IR spectroscopy
[243], and even stronger enhancement has been estimated [311]. These values are
generally obtained by comparison of the obtained signals to signals from analogous
samples that do not benefit from surface enhancement effects [179, 243, 312]. The
strong near-fields therefore open up a couple of exciting applications for ultrafast
spectroscopy that have partially been demonstrated very recently, as discussed in
the following.
A first and obvious possibility is the exploitation of the increased near-fields for
higher-order processes in samples that are either surface-bound, or within the
interaction length of the optical near-fields from the nanostructures (\ \ 10 nm).
This effect is generally referred to as ‘‘vibrational ladder-climbing’’
[242, 243, 313–329] and is schematically depicted in Fig. 30a. Taken a given
anharmonicity in an electronic ground state potential of an immobilized molecule
on a plasmonic substrate, a broadband laser pulse (bandwidth * 200 cm
-1 ) can
successively excite populations in higher-lying vibrational states by interacting
multiple times with the sample. In the first demonstration of this effect in surfaceenhanced 2D IR spectroscopy, Hamm et al. employed few nanometer thick, sputtercoated, heterogeneous Gold (Au) layers as plasmonic substrates (Fig. 30b) for 2D
ATR IR spectroscopy on p-mercaptobenzonitrile (p-PhCN) monolayers. Such metal
layers near the percolation threshold exhibit rather continuous areas of metal
patches (light regions), separated by small gaps (\ 10 nm in width, dark regions).
Polarization-controlled excitation of molecules inside the gaps [179, 243, 330]
allowed a dramatic signal enhancement and the clear observation of vibrational
ladder-climbing in the 2D ATR IR signals from the CN-stretch vibration of the
monolayer up to the 5–6 transition (Fig. 30c and d). In that way, more than
Top Curr Chem (Z) (2017) 375:86
123
182
Reprinted from the journal