1P082
Vibrational SFG Spectroscopy of Organic Monolayers Fabricatedon Silica-deposited CaF2 Substrates(Center for Quantum Life Sciences and Graduate School of Science, Hiroshima University1, Institute for
Nanoscience Design, Osaka University2, Graduate School of Engineering Science, Osaka University3)!Adchara Padermshoke1, Shouta Konishi1, Toshiki Maeda1, Masato Ara2, Hirokazu Tada3, Taka-aki
Ishibashi1

Introduction. Infrared (IR)-visible/UV sum-frequency generation (SFG) spectroscopy is amethod for measuring vibrational spectra of interface molecules. Because only surface orinterface species give a resonant contribution to SFG signal, the technique is highly surfacespecific and has been developed into a very powerful surface analytical tool. Biomolecularinterfaces play a vital role in biological processes. An insight into the chemical composition
and molecular structure of functional interfaces in nature and also in artificial devices istherefore of great importance.

As a model system for biomolecular sensors, we areinvestigating organic monolayers silanized on solid substrates by SFG technique.
(1)

VISIR

SF signalAir(2)

Aqueous solution

Monolay ers Monolay ers
Monolay ers
SubstrateIRAirVIS

SF signal

Fig. 1 SFG measurement geometries in air (geometry 1) and in aqueous
solution (geometry 2).

Thesilanized monolayers arerobust, chemically stable, andcan serve to surfaceimmobilize various types of
biological molecules. Sincebiomolecular interfaces exist
physiologically in aqueous

media, it is crucial to conduct the SFG study under such condition in order to gather themolecular information closely reflects that in nature. However, unlike in air, the probing
geometry in which the SFG probe reaches the monolayers from the non-substrate side(geometry 1) (see Figure 1) is not applicable in aqueous solution because water does absorb
most of the IR probe light. As a consequence, the probing is only possible via anothergeometry in which light impinges on the monolayers through the substrate (geometry 2).
Unfortunately, conventional substrates generally used for silanization, i.e., fused silica (SiO2)
plates, are opaque throughout the IR region where the fingerprints of many organic molecules
are embedded. A very thin SiO2 layer, however, is transparent over that region. Therefore,we have developed a new substrate composed of a calcium fluoride (CaF2) window and a thin
SiO2 layer, which allows silanization and is transparent over a broad range of the IR probe.
Silica-deposited CaF2 Substrate Development. We have made composite substrates thatcomprise a SiO2 (500 nm) layer overlying a CaF2 (1 mm) plate using an electron-beamdeposition process and found that the strength of the SiO2 layer is not reproducible. However,the presence of a very thin silicon (Si) (3 nm) layer at the SiO2 and CaF2 interface

significantly improves the strength of the SiO2 layer. Thus, the composite substrates being
used in our study are composed of SiO2/Si/CaF2 (500 nm/3 nm/1 mm). We have ensured thatthe substrate transparency in the UV/visible, near-IR, and IR spectral regions is sufficient for
SFG measurements (see Figure 2).
Trans mittance

1.0
0.8
0.6
0.4
0.2
0.0
200

300

400

500

600

700

750

Wavelength / nm

1250

1750

2250 4000

Wavelength / nm

3000

20001000

Wavenumber / cm-1Fig. 2 Transmission spectra of a SiO2/Si/CaF2 (500 nm/3 nm/1mm) composite substrate in the UV/visible, nearIR and IR spectral regions (left to right).

Monolayer Fabrication. In this study, SFG measurements of phenyltriethoxysilane (PTES)(C6H5Si(OC2H5)3), have been carried out. The SFG spectra of PTES on SiO2 substrates andthose of PTES on the SiO2/Si/CaF2 ones were compared in order to validate the efficiency ofthe newly developed substrates. A 2% (v/v) solution of PTES was prepared in a mixture ofacetone and water (45:10 v/v) and adjusted to pH 4.5-5.5 with acetic acid. Silanization ofPTES on the SiO2 and SiO2/Si/CaF2 substrates was performed by immersing the ozonepretreated substrates in the silane solution at 37 oC for 120 hr with stirring. The substrates
were subsequently removed from the solutions, rinsed with acetone, and heated in air at 95 oCfor 20 min.
Results and Discussion. SFG spectra of the PTES
monolayer fabricated on the SiO2 substrate andthose of the monolayer fabricated on the composite
one were recorded with ppp (SFG,visible,IR) andssp polarization combination. Figure 3 shows thenormalized SFG spectra of PTES on the two typesof substrates in ppp polarization collected with theprobing geometry 1. The spectrum obtained fromthe composite substrate well reproduces thatobtained from the SiO2 one. A band attributed tothe phenyl ring vibration of PTES is clearly seen at
1600 cm-1 in both spectra.
Fig. 3 Normalized SFG spectra of PTES on
the SiO2 (top) and SiO2/Si/CaF2 substrates(bottom) in ppp polarization.

We are going tomeasure SFG spectra of PTES on the composite
substrates with the probing geometry 2 in air andthen in aqueous environment. Detailed analyses ofthe observed SFG spectra will be presented. The efficiency and usefulness of theSiO2/Si/CaF2 substrates will be verified, and possible improvement of the substrates and/ormonolayer fabrication will be discussed.