Geometry Factor of Plasmon-Inducing Metal Surface and Its Use in SEIRA Studies

Effect of Plasmonic Enhancement of Infrared Transitions Near the Metal Surface and Its Experimental Application

The enhancement of optical processes of the order of up to 102 near the rough metal surface (Au, Ag, Cu, etc.) has been known for the last 30 years for both optical transitions in adsorbed molecules (Raman scattering, luminescence, infrared (IR) absorption) [162-164] and for processes that do not depend on the presence of molecules on the metal surface (for example, second harmonic generation) [165]. The effect of amplification occurs on high-conductive surfaces in the case when the light wavelength far exceeds the value of the radius of curvature at the peaks of surface roughness or the value of the radius of the metal nano particle. The phenomenon of optical processes enhancement consists in a significant increase in the intensity of the optical transition (for example, the effective cross section of the Raman scattering increases by 102, for IR absorption by 10-102 times) or the efficiency of the process near the metal surface. The explanation of the effect is not straightforward and includes several mechanisms, such as an increase in the intensity of the electromagnetic field near the rough metal surface, island metal films or nanoparticles, the growth of the transition dipole moment of adsorbed molecules, etc. According to Ref. [165], the coefficient of amplification of an electric field is a function of the dielectric constant of a metal. However, it should be noted that the influence of metal under certain conditions and characteristics of the surface and adsorbed particles may, in addition to enhancement, also result in the quenching of optical transitions of molecules near the metal surface [166]. The theoretical interpretation of the amplification effect is given in Ref. [165] in general terms and includes two possible ways: the amplification of the electromagnetic field due to interaction with localized (surface) plasmon oscillations arising on inhomogeneities of the metal surface and the specific growth of the transition dipole moments of the adsorbed molecules on the metal surface.

The electric field generated by a plane electromagnetic wave at the point r of the space near the metal particle can be written as:

where со is the angular frequency and t is time. The additive £(r,(o) is associated with the excitation of the localized surface plasmon oscillations and has a local character, which is asymptotically fading, then:

where gap{r,oS) is the coefficient of the electrical field enhancement.

The effective cross section of the light interaction with a molecule adsorbed on the metal surface can be represented as follows:

where cr^0) is the cross section without metal [d = d0, E= E0), /—>/is the optical transition from state i (with energy E,) to state/ cE02/8ns the incident energy flow density, cis the speed of light, ^ fH[r,(o)i'j is the Hamiltonian matrix element of molecule interactions with field, /i(r) is the dipole moment amplification coefficient of the adsorbed molecule.

In the general case, the amplification of the optical transition is proportional to the field intensity. The calculation of the ratio of the real and imaginary parts of the complex dielectric constant at the corresponding wavelength is also used to determine the magnitude of the surface enhancement [165].

Upon irradiation of the rough metal surface, energy from the photon is transmitted to localized surface plasmon oscillations. Plasmonic oscillations can be in the state of resonances, the frequency of which is determined by the geometric parameters of surface roughness. In this case, energy from plasmonic oscillations is transmitted to the adsorbed molecules, which causes increased light absorption by molecules.

Enhancement Efficiency for Various Experimental Implementations of the SEIRA Technique

There are several experimental implementations of the surface- enhanced infrared absorption method. The main one is the SEIRA technique in reflection geometry, when the amplification takes place due to the increase in the electric field intensity when the localized surface plasmons are excited on the roughness of the metal film. A series of studies [17, 21, 167] connected with the proper use of the amplification effect by the metal surface of infrared absorption of nucleic acids and lipids deposited on the gold substrate were performed in experiments with reflection geometry. Glass plates with a deposited gold layer having thickness of 200-500 A served as SEIRA substrates (Fig. 3.24). The roughness of the surface of the manufactured samples was different, in the range of 20-100 A.

Infrared absorption spectra were recorded on the Fourier spectrometer IFS-48 (Bruker, Germany) in the region of 380-5300 cm'1 (Fig. 3.25). Spectra processing and the decomposition of complex bands on components were performed using the Opus-2.2 software.

DNA samples were deposited onto the surface of a gold film from an aqueous solution with a concentration of about 1 mg/ml and lyophi- lized.

AFM 3D image of the gold surface used in SIERA studies

Figure 3.24 AFM 3D image of the gold surface used in SIERA studies.

The obtained results show that due to the use of the SEIRA method for the study of infrared absorption of nucleic acids (analogous to lipids [168]), there is a different amplification of signal magnitude (depending on the geometry of the surface roughness of gold) compared to the spectrum on the neutral CaF2 substrate (Fig. 3.25). In this case, there was no deformation of the absorption bands contour present in experiments with commonly used CaF2 substrates. It should be noted that the presented SEIRA technique allows the registration of spectral features of molecules that are not available in usual IR spectroscopy due to the possibility of observing new absorption bands that appear in the enhanced SEIRA spectrum [17].

Studies have shown that signal enhancement in most cases increases with the presence of the characteristic roughness of the gold film surface in the range of 50-100 A. The IR signal increases by a factor of 3-5 for different vibration frequencies (Fig. 3.25, Table 3.1). The greatest amplification was obtained for the thickness of the gold film of 200 A, which is probably due to the greater correspondence of the geometry of the relief for film of such a thickness to the 3D geometry of DNA molecule compared with the geometry of the relief for gold films with other thickness. To calculate the possible magnitude of the amplification by a metal surface, the calculation of the ratio of the real and imaginary parts of the complex dielectric permittivity of the gold substrate at the corresponding frequency was used. For example, in the case when the calculated value for this ratio was 16 [s' = 566, e" = 35.7 at 2800 cm'1), in the experiment this value reached only 4.2, indicating the need for a more detailed study of amplification mechanisms due to localized surface plasmons in this case. Comparison of the obtained SEIRA spectra with the ones measured on the CaF2 substrate shows, in most cases, the coincidence of the wave positions of the bands (Table 3.1). Some frequency shifts are present, but for most bands they are insignificant (1-2 cm"1). This allows concluding that this method can be used for such tasks. The amplification coefficients of integral intensity of absorption bands, which correspond to the vibrations of different functional groups of DNA molecules, were obtained in the experiment, and their value was 3 to 5 depending on the thickness of the deposited gold layer.

SEIRA spectra of DNA

Figure 3.25 SEIRA spectra of DNA: 1 — on a glass substrate with a 200 A gold film thickness, 2 — 300 A gold film thickness, 3 — 400 A gold film thickness, 4 — on a CaF2 substrate. Adapted with permission from Ref. [17], Copyright 2002, John Wiley and Sons.

DNA on CaF2

DNA on gold film with thickness 200 A

DNA on gold film with thickness 300 A

Band position, cm"1

Band position, cm1

Enhancement factor

Enhancement factor

Exp.52

Calc.

Band position, cm1

Exp.

a2

Calc.

Band

assignment

a

a2

3

a2

3350

3359

4.3

6.9

47.61

3354

2.0

6.9

47.61

0-H, N-H, C-H

1655

1653

2.9

3.8

14.44

1649

1.4

3.7

13.69

C=0, C=N, N-H

1528

1530

3.0

3.5

12.25

1529

1.1

3.5

12.25

C=N

1419

1418

2.8

3.3

10.89

1418

1.0

3.3

10.89

C-H

1238

1230

2.4

3.0

9.00

1230

1.0

2.9

8.41

P02

1089

1084

2.1

2.7

7.29

1084

0.8

2.7

7.29

P02

964

963

2.4

2.4

5.76

964

0.9

2.4

5.76

C-C, C-0

892

889

1.7

2.3

5.29

892

0.9

2.3

5.29

C-C, C-0

833

831

2.0

2.2

4.84

829

0.8

2.2

4.84

C2

Source: Reproduced with permission from Ref. [17], Copyright 2002, Wiley Periodicals, Inc.

Experimental and estimated amplification coefficients of the absorption bands integral intensity of different DNA functional groups are given in Table 3.1. Experimentally, the amplification coefficients are determined by the ratio of the absorption bands integral intensities of the corresponding DNA functional groups.

The following method implements an enhancement in the attenuated total reflection (ATR) geometry. There is a possibility of resonance amplification of the electric field near the flat surface of the metal by nonradiative surface plasma waves (surface plasmon-polaritons), which are excited by the ATR method. Consideration of the electric field amplification mechanism by the surface plasmon-polariton shows substantially smaller values than the amplification of the electric field by localized plasmon oscillations [162]. Theoretically, the amplification magnitude of an electric field g[(Or) ~ 0)p/y ~ e'/e" at resonance is characterized by competition between the processes of excitation and decay of surface electromagnetic waves (SEW) or modes.

Therefore, the pumping level of SEW (in a given external field E0) is limited only by the processes of their decay. As shown in Ref. [169], when describing the SEW based on the dielectric function of a metal, the parameter 1/r describes the complete decay rate of the SEW:

Here, the following channels of SEW decay are allocated:

  • • Dissipative (relaxation time rd), associated with the transition of SEW energy into heat;
  • • Radiation (decay into photons per time Tr);
  • • Surface (scattering of SEW into other SEW per time rs).

According to this, it should be noted that the observed efficiency of the electric field amplification by localized surface plasmon oscillations and surface plasmon electromagnetic waves may be associated with different effects of roughness in relation to the mentioned surface plasmon processes. In fact, when the surface has roughness with a scale smaller than the wavelength of light, the probability of formation of localized surface plasmon oscillations increases, but the lifetime and electric field enhancement by surface electromagnetic waves sharply decreases, determined by the processes of their radiation decay and scattering on surface roughness [165].

SEIRA spectra of (1) DNA, (2) colloidal gold, and (3) DNA-colloidal gold solution on SPR chip. Adapted by permission from Ref. [21], Copyright 2004, Springer Nature

Figure 3.26 SEIRA spectra of (1) DNA, (2) colloidal gold, and (3) DNA-colloidal gold solution on SPR chip. Adapted by permission from Ref. [21], Copyright 2004, Springer Nature.

In the following approach, a study was carried out to investigate the effect of amplification on colloidal gold nanoparticles placed on the surface of the SPR chip. Studies of the spectral features of the condensed state of DNA upon interaction with colloidal gold were conducted using IR spectroscopy. Colloidal gold was added to aqueous DNA solution, which was placed on the surface of the SPR chip. For control, a DNA sample without colloidal gold was used. As a result of the experiment, changes in the spectrum of DNA with colloidal gold were recorded in comparison with spectra without nanoparticles (Fig. 3.26). There was a decrease in the intensity of the bands at certain frequencies, an increase in the half-width of some bands, and other features that are not specific to any of the A, B, Z forms of DNA. These results indicate that colloidal gold probably modified the DNA structure with its new state creation. In fact, the use of arrays of high-conducting structures in the SEIRA method, together with colloidal nanoparticles, requires studies on the effect of the substrate on the processes of amplification of optical transitions.

This makes a comprehensive analysis of the enhancement processes of optical transitions based on simpler models an actual task.

 
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