Chemical Imaging of Microstructured Self-Assembled Monolayers with

May 22, 2007 - We report on the infrared spectroscopic characterization of microstructured self-assembled monolayers by scanning near-field infrared m...
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J. Phys. Chem. C 2007, 111, 8166-8171

Chemical Imaging of Microstructured Self-Assembled Monolayers with Nanometer Resolution Ilona Kopf,† Jean-Se´ bastien Samson,† Go1 tz Wollny,† Christian Grunwald,‡ Erik Bru1 ndermann,† and Martina Havenith*,† Physical Chemistry II, Ruhr-UniVersity Bochum, Germany, and Nanostructure Laboratory, ELETTRA Sincrotrone Trieste S.C.p.A., Italy ReceiVed: January 10, 2007; In Final Form: March 22, 2007

We report on the infrared spectroscopic characterization of microstructured self-assembled monolayers by scanning near-field infrared microscopy. Using a CO laser as the radiation source in the characteristic amide band (at 1711 cm-1 (λ ) 5.85 µm)), we were able to image self-assembled monolayers of thiols. The measurements were carried out on well-defined microcontact printed line patterns of monomolecular films of 1-octadecanethiolate and biotinylated alkylthiolate (BAT). The lateral resolution was ∼90 × 90 nm2, which is well below the Abbe limit. The detection limit was 5 × 10-20 mol corresponding to 27 attogram or 30 000 molecules of BAT. This demonstrates the high sensitivity of our setup, which allows recording IR spectra of a single monolayer.

I. Introduction Analytical tools that are able to image the molecular composition and orientation of surfaces with high lateral resolution are of importance in material science, microelectronics, and bioengineering. Established techniques are X-ray photoelectron spectroscopy (XPS), scanning Auger spectroscopy,3 imaging secondary ion mass spectroscopy, and fluorescence spectroscopy using chromophors as labels. Many of these techniques require special conditions such as vacuum or special sample preparation such as metallic coating. An experimental approach for the mapping of the chemical surface composition with a straight forward and label-free characterization of functional surface groups on a nanometer scale is still a challenge. We report here the setup of a scanning near-field infrared microscope (SNIM) which allowed imaging of a single monolayer with a lateral resolution of ∼90 × 90 nm2. The infrared (IR) spectral region is known as the chemical fingerprint region. IR microscopy5,6 has become an important tool for chemical identification due to its nondestructive nature and easy handling. In addition, many biomolecules, such as nucleic acids, proteins and lipids, have characteristic and welldefined IR-active vibrational modes in the amide or C-H and O-H bands.7 However, the spatial resolution of conventional microscopes such as Fourier transform infrared (FTIR) microscopes is restricted to about λ/2 by the Abbe’s diffraction limit. Typical lateral resolutions are several micrometers, implying that conventional FTIR microscopy averages about a larger area, thereby preventing the detection of smaller domains. A technique that is able to distinguish between different chemical compositions of nanodomains without the need of additional labels is highly desirable and would open new fields. The development of scanning near-field optical microscopy8-10 has enabled fluorescence detection with nanometer resolution. * To whom correspondence should [email protected]. † Ruhr-University Bochum. ‡ ELETTRA Sincrotrone Trieste S.C.p.A..

be

addressed.

E-mail:

As long as the probe is in the near-field (e.g., the distance between the light emitter and the probe is less than a wavelength) the resolution is no longer limited by the Abbe limit. In the infrared, the apertureless or scattering near-field microscopy (s-SNIM) has been reported in the literature,11-13 which is based on detection of scattered light from an oscillating antenna. With these techniques, a spatial resolution of less than λ/100 could be demonstrated.14 Infrared near-field microscopy allows a sensitive chemical and structural characterization with nanometer resolution for applications in biology, chemistry, and material science. We will demonstrate that SNIM combines the advantage of infrared contrast due to high chemical sensitivity with high topographical resolution. It offers the possibility to study heterogeneous sample surfaces by their chemical fingerprint (IR absorption) without the need for dyes or other sample modifications. Additionally, SNIM allows scanning under ambient conditions, which is important for biologically active substances. Different types of heterogeneous systems have been investigated and characterized by SNIM such as ion-implanted semiconductor surfaces,15-17 polymers,18-24 and organic materials like organic thin films, cells and tissues.25-29 Initially, the method required high-power lasers (g1 W) so that the first applications were carried out using fixed frequency IR gas lasers. Recently, we demonstrated SNIM with a tunable high-power tabletop optical parametric oscillator (>2 W) developed within our group.17 This allows us to record IR-images within the chemically important and characteristic C-H and O-H bands (2600-3400 cm-1). In the present work, we apply s-SNIM to image a thin selfassembled monomolecular organic film consisting of periodic 2 µm lines of 1-octadecanethiolates (ODT) and 1.5 µm lines biotinylated alkylthiolates (BAT). We have chosen a sample with structured thiolates for the following reasons: (i) thiols provide densely packed and highly ordered organic monolayers. This guarantees uniform conditions and allows a sensitive test because the IR absorption is (due to the surface-selection-rule) known to depend on the arrangement of the molecules at the

10.1021/jp070201q CCC: $37.00 © 2007 American Chemical Society Published on Web 05/22/2007

Chemical Imaging of Microstructured SAMs

Figure 1. Experimental setup of our s-SNIM. OM ) gold-coated offaxis parabolic mirror, M ) mirror, and MCT ) mercury cadmium telluride detector.

surface (e.g., upstanding molecules versus tilted molecules).30,31 (ii) self-assembled monolayers (SAMs) provide a good and easy to handle prototype system, which will allow a characterization of interactions of different biomolecules (e.g., proteins) with various surfaces in the future.32-36 Although biomolecule-surface interactions often play a key role in lifescience, they are still not completely understood. Controlling the interaction with synthetic surfaces is of general importance in biochemistry, biology, biotechnology, and medicine (e.g., for biosensors, implant, etc.). On the other hand, fundamental studies on the near-field optical properties of different biomolecules and heterogeneous systems of biomolecules (e.g., artificial membranes) can provide a reliable and interpretable characterization of biological samples. In this paper, we report on s-SNIM in the region of the amide bands, which provide fingerprint bands for secondary structures of proteins.37,38 II. Experimental Section Setup of the Scanning Near-Field Microscope. In Figure 1, our SNIM setup is shown. A commercial tapping-mode atomic force microscope (Nanotec Electronics) was modified to meet our specific applications. We use commercial goldcoated cantilevers (MikroMasch NSC16/Cr-Au) with a tip curvature radius of 40 nm. The cantilever is illuminated by a polarized beam of a home-built liquid nitrogen-cooled sealedoff CO laser39,40 that is focused by a parabolic mirror onto the tip. The laser is line tunable with a typical output power of g1 W. In our SNIM experiments, typically we use a power of 300 mW (measured at the laser output) that is coupled into the beamexpander telescope (Figure 1). The laser provides more than 100 lines with a typical frequency separation of 3 cm-1 between subsequent laser lines. The use of CO isotopes (12C16O, 13C16O, 12C18O) increases the number of available laser lines to about 400 in the spectral region from 1600 to 2100 cm-1 (4.8-6.3 µm). The average spacing between laser lines is 1.2 cm-1, which provides a good coverage of the amide region. The line width is about 150 kHz. The infrared radiation is scattered from the apex of an oscillating tip and focused by a CaF2 lens onto a liquid nitrogen-cooled mercury cadmium telluride detector (MCT, Judson Technologies). Our experimental setup uses a 90° scattering detection scheme (i.e., the detected light is collected from a solid angle centered 90° off the incident infrared light rays). Scanning the sample allows monitoring of the local near-field interaction between tip and sample, which depends on the dielectric constant of the tip material and specimen surface. Further details of our experimental setup are described in ref 17. The tip is fixed but oscillates vertically with a resonance frequency, f, of 170 kHz. Because of the oscillation of the tip,

J. Phys. Chem. C, Vol. 111, No. 23, 2007 8167 the scattered infrared light is amplitude modulated. We use phase-sensitive detection by a lock-in amplifier on the first harmonic (2f ∼ 340 kHz). Previously, it was demonstrated that background signals are more effectively suppressed for phase sensitive detection at higher harmonics (compared to 1f), thereby increasing the expected signal-to-noise ratio.41 Each image was recorded at a scan rate of 0.5 Hz per line. The time constant is 3 ms per point. Our setup allows simultaneous recording of the topography and the near-field infrared contrast of a sample. Sample Preparation. We used gold-coated n(100) silicon (Anfatec Instruments GmbH, Germany), which was cleaned with hot piranha solution (7:3 mixture of H2SO4 and H2O2) for approximately 15 min. Preparation of homogeneous SAMs was done by immersing the clean gold substrates into 1 mM ethanolic solutions of the pure thiols (Figure 2) for approximately 120 s at room temperature. Afterward, the substrates were rinsed with absolute ethanol and were dried in a stream of nitrogen. For the preparation of microstructured samples, microcontact printing42,43 was used to create periodic 2 µm lines of ODT on the gold. As a first step, an elastic stamp of polydimethylsiloxane (PDMS) was cleaned in 50% ethanol in an ultrasonic bath for 30 min. The stamp was then immersed for 10 min into a 5 mM ethanolic solution of ODT, subsequently predried in a stream of nitrogen, and left in an Eppendorf tube for 20 min to dry completely. Stamping was done by gently pressing the stamp to the gold surface by hand. After a contact time of 120 s, the stamp was carefully peeled off from the substrate. To fill the remaining pure gold areas with BAT, the substrate was incubated in a 1 mM ethanolic solution of BAT. After 80 min, the microstructured surface was rinsed with absolute ethanol and was dried in a stream of nitrogen. Electron microscopic images of the stamp (evaporated with a thin graphite layer) and the structured substrate are displayed in Figure 3. As a result, we obtained patterned surfaces with a lateral periodic structure of =2.5 µm lines of ODT separated by =1.5 µm wide lines of BAT. For the preparation of a BAT-KBr pellet, 200 mg of dry KBr was carefully pulverized before 2 mg of alkanthiol was added to the KBr powder. The solid was mixed well to ensure the homogeneous distribution of the thiol in the KBr. Finally, the KBr-doped powder was filled into a hydraulic press to form a KBr pellet. III. Experimental Results We have recorded FTIR spectra of homogeneous monomolecular films of ODT and BAT, respectively, as references and additionally a volume spectrum of a KBr pellet containing BAT. These experiments were carried out using a commercial FTIR spectrometer (Bio-Rad FTS-3000 Excalibur). Figure 5 shows the characteristic IR bands between 1600 and 1850 wavenumbers of both molecules. We note that the BAT-SAM spectrum in the investigated range from 1600 to 1850 cm-1 differs from the volume spectrum due to the specific surface selection rules. This a result of the regular orientation of the molecules in the monolayer, which is inclined by 30°, in combination with the highly conducting gold substrate that results in local electric fields normal to the surface. In contrast, in the volume sample (mixed in KBr pellet) a random distribution of molecules allows the excitement of all available normal modes. In contrast to ODT, both BAT spectra show an intense absorption peak at approximately 1711 cm-1 caused by the CdO stretching vibration of the ureido group (R-HN-(CdO)-NH-R). The amide I vibration of the ureido group is shifted to higher frequencies due to the second N-H group. The weaker shoulder

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Figure 2. Bond-line structures of the used thiols. (a) ) Biotinylated alkylthiol (BAT); (b) ) 1-octadecanethiol (ODT).

of the frequency and compared with the normalized far-field spectra of the homogeneous monolayers. We determined the near-field intensities by a histogram analysis of regions of interest and calculated from these intensities the near-field contrast (IODT - IBAT)/IODT, which was normalized to 1. The lateral resolution of the topographic and SNIM pictures is determined by a closer investigation of the slope of a stripe edge. The resulting line plots are shown in Figure 6. The data points were calculated by averaging over five rows. An error function line fit to the data points evaluates the step resolution. The topographic resolution is calculated as the lateral distance between the 10 and 90% position of the slope and amounts to ∼70 nm. The near-field resolution at a frequency of 1711 cm-1 (λ ) 5.85 µm) is ∼90 nm corresponding to approximately λ/60. IV. Discussion

Figure 3. Comparison of the stamp pattern and the structured substrate. (a) Electron microscope image of the dry PDMS stamp that was used to print ODT. The stamping bars are 2 µm broad with a spacing of 1.5 µm. (b) Electron microscope image of the structured substrate with 2.5 µm wide ODT stripes (bright lines) and 1.5 µm broad BAT stripes (dark lines). (c) Schematic cross-section of the structured substrate.

at 1650 cm-1 in the BAT surface spectrum is assigned to the two amide groups in the chain of the BAT. We recorded simultaneously the topography and the infrared near-field contrast of the micropatterned sample. Figure 4 shows a 9 × 5 µm2 topographic image (a) and the corresponding nearfield image (b) recorded at an IR frequency of 1711 cm-1, which corresponds to the center of the IR absorption band of the CdO stretching vibration of the ureido group (N-(CdO)-N) in BAT. We have chosen a linear gray scale in which white represents the maximum height and scattered near-field intensity. The line plots show profiles perpendicular to the stripe pattern as indicated by the dashed lines in the images. To improve the signal-to-noise ratio, we have averaged over 25 single lines. The topography profile (Figure 4a) shows a height difference of 1.3 nm between ODT and BAT. This agrees well with the difference of the calculated total heights of 2.5 nm for ODT and 3.7 nm for BAT, suggesting a highly ordered BAT film. The near-field image (Figure 4b) clearly reveals the structure of the stamped surface. The contrast of the infrared image is inverse to the topographical image. The stripes of BAT appear darker in comparison to ODT. This can be explained by a decrease of scattered light by IR absorption of the biotin monolayer in the near-field. To record the frequency dependence of the infrared nearfield contrast, we have tuned the laser to five distinct frequencies (1640, 1692, 1711, 1722, and 1800 cm-1). In Figure 5, the relative infrared near-field contrast is displayed in dependence

In Figure 3, we display an electron microscopic image of the microstructured surface pattern. We observe that the stripes of the stamp (Figure 3a) are smaller than that of the microcontact printed surface (Figure 3b). The stripes of the stamp are ∼2 µm wide and separated by ∼1.5 µm lines. The electron microscope (measured under vacuum conditions, Figure 3b) and the near-field and topography images (measured under laboratory air and humidity conditions, Figure 4) show microcontactprinted patterns consisting of =2.5 µm stamped ODT stripes and =1.5 µm broad BAT stripes. These differences are attributed to the swelling of the PDMS during cleaning and inking of the PDMS stamp in ethanolic solutions. In addition, broadening of the stamped stripes can occur during the stamping process (e.g., from diffusion of the molecular ink). More detailed analysis of diverse topography and near-field images reveals that the width of the stripes can vary by ( 0.5 µm. These variations are caused by imperfections of the stamp or uncontrolled adhesion and loading forces during stamping. In the present study, we demonstrate the high sensitivity of scanning near-field microscopy. In Figure 4b, a clear pattern contrast is seen when the frequency was set to the maximum of the absorption peak (1711 cm-1). The near-field contrast ratio taken at 1711 and 1800 cm-1 is 9. This result demonstrates that a single functional group (ureido) is sufficient to obtain a significant near-field contrast. The signal-to-noise is limited by the presence of low frequency (e50 Hz) acoustic vibrations due to several vacuum pumps in the laboratory setting. The acoustic waves lead to cantilever oscillations modulating the topography and near-field signal and reducing the image contrast (Figures 4 and 6). In Figure 5, the normalized far-field spectra are compared with the relative, frequency dependent near-field contrast. The near-field contrast follows the same frequency dependence as the far-field absorption of the BAT-SAM. For slightly lower (1692 cm-1) and higher frequencies (1722 cm-1), the near-field contrast is decreased in comparison to the resonance frequency at 1711 cm-1. Also at 1640 cm-1 close to the amide I absorption shoulder, a weak contrast can be detected. Tuning the laser far out of resonance to 1800 cm-1 still results in a very weak contrast. This could be due to the different heights of the

Chemical Imaging of Microstructured SAMs

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Figure 4. Topography (a) and infrared near-field (b) images (9 × 5 µm2) in gray scale. White represents the tallest features in the topography image and the maximum detected signal in the near-field image. The line plots show 16 term-moving average profiles perpendicular to the stripe pattern averaged over 25 lines as indicated by the dashed lines in the images. The near-field is measured at the resonance frequency (1711 cm-1) of the ureido group of BAT.

Figure 5. Normalized FTIR absorption spectra of pure homogeneous monolayer of ODT (gray line) and BAT (black line) and near-field infrared contrast (1-(IBAT/IODT)) (points). For better comparison, the near-field contrast was normalized to one. Additionally, a normalized FTIR absorption spectrum of a BAT-KBr pellet (dashed line) is displayed.

molecules assuming dipole-mirror-dipole theory41 or caused by the different reflection properties. However, at 1711 cm-1 the absorption component dominates and a good agreement between the spectral features of the far-field and near-field is seen. To quantify the near-field contrast, we have calculated the complex dielectric constant of BAT and ODT from the measured volume and SAM absorption spectra. We use the complex refractive index nˆ ) n + ık in which the real part, the refractive index, is obtained by the Kramers-Kronig relation using the infinite refractive index of 1.4336 for both thiols. The maximum

k for an ODT-SAM in the C-H range (2800-3100 cm-1) is about 0.07, while for a BAT-SAM the value is 0.04. In the investigated CO region (1600-1850 cm-1), we obtain for the BAT-SAM a maximum k value of 0.09 at 1711 cm-1. The corresponding value for the ODT-SAM is very small (0.008 at 1711 cm-1), which yields very small refractive index variations of 0.4%. In comparison, we note that for the BAT-SAM the refractive index varies by 3%. The tip is considered as a spherical dipole according to ref 14 with a radius R corresponding to the tip curvature with R ) 40 nm here. With the Clausius-Mosotti relation, the polariz-

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ability of the tip RTIP immediately follows as

Au - M RTIP ) 4πR3 Au + 2M

(4.1)

with the dielectric constant of the medium (in this case, air) M ) 1 and of gold Au ) -1240 + ı3484 at 1711 cm-1.44 Since the real and imaginary parts of Au are very large compared to the fraction, which is comparable to 1, we obtain RTIP ≈ 4πR3. We have used the mirror-dipole model45 to estimate the influence of the BAT dielectric constant. The effective polarizability of the system dipole-sample is

(

Reff ) MRTIP(1 + β) 1 -

RTIP β 16π(z + R)

)

-1

3

(4.2)

with

β)

SAM - M SAM + M

(4.3)

Because the infrared wavelength is larger than the tip diameter and the distance between the sample and the tip, we obtain the scattering and absorption cross-sections (analogous to ref 14) according to classical electrostatics as

σSCA )

k4 |R |2 6π eff

(4.4)

The imaginary component of the effective polarizability (σABS ) kIm[Reff]) describes the absorption. We find that the maximum absorption cross-sections σABS for our samples are 32 nm 2 (BAT) and 3 nm 2 (ODT) at 1711 cm-1 (for z ) 0). In contrast, σSCA does not exceed 0.03 nm2 for both BAT and ODT in the whole range from 1600 to 1850 cm-1, which is a factor of 1001000 times smaller than the contribution by absorption. The absorption cross-sections are so dominant that they determine the line shape of the near-field signal. This implies that the recorded spectra resemble very much the far-field spectra in their spectral response, however, with a nanometric lateral resolution of 90 nm defined by roughly twice the tip radius of 40 nm. The total cross-section for BAT is then 10 and 0.9 nm2 for ODT, resulting in a cross-section ratio for BAT compared to ODT monolayers of approximately 11. Harmonic demodulation was used that reduces the amplitude of the actual signal by about a factor of 2 for each harmonic but does not alter the expected ratio for both monolayers due to the dominance of the absorption cross-section in the near-field signal intensity. The predicted contrast of 11 agrees well with the experimental value of 9 as obtained when we compare the measured intensity outside the absorption with the maximum signal (see Figure 5). The investigation of the near-field cross-sections shows that in the case where the absorption dominates, the far-field spectrum reoccurs at the near-field level. The spectral response of near-field data is represented by the ratio of the far-field spectra because the ODT has very little absorption in the investigated region mainly by the far-field spectrum of the BATSAM. At 1800 cm-1, we measure still a contrast, even as this is outside the frequency range where we expect an absorption for BAT (Figure 5). We therefore have to consider an additional contrast mechanism. Although at this frequency the far-field absorption is nearly the same for both SAMs, we have to take into account that the BAT layer is 1.2 nm higher with a total

Figure 6. Topography (a) and infrared near-field (b) line plots. An error function line fit to the data points evaluates the step resolution. Five rows were averaged to obtain the data points. The dotted lines mark the 90 and 10% positions that were used to calculate the resolution. The height difference between the two thiolates is ∼1.3 nm.

height of 3.7 nm. The interaction of the tip dipole-layer gold substrate can be viewed as a capacitor at contact in which the thicker layer reduces the signal due to the (z + R)3 near-field dependence with z/R of 3%. We attribute the remaining contrast at 1800 cm-1 in part to this height difference. However, we want to point out that any artificial contrast based on differences in height should be independent of the wavelength. It can therefore only result in a constant off-set, independent of the observed frequency dependent IR near-field signal as observed in the present measurement. The latter corresponds to a spectral feature resembling the specific chemical signature. For the IR near-field, we obtain a lateral resolution of 90 nm (Figure 6) corresponding to twice the tip curvature radius (40 nm). The assumption is made that a cross-section of 21.4 Å2 per sulfur atom of an alkanthiolate35 corresponds to an amount of BAT of 7 × 10-24 mol/nm 2 corresponding to 4 molecules per nm2. A detection area of 902 nm2 leads to a lower detection limit of 5 × 10-20 mol corresponding to 3 × 104 molecules or 27 attogram substance. The BAT monolayer has a thickness of 3.7 nm. This leads to an estimate for the minimum detection volume of 90 × 90 × 3.7 nm3, corresponding to a volume of 3 × 10-20 liters. We could also argue that a cylinder with a diameter of 90 nm is probed giving π(90 nm/2)2 × 3.7 nm ) 0.7 × 10-20 liters. With a molar weight of 541.86 g/mol and a density of approximately 0.9 g/cm3, we obtain 1.2 × 10-20 mol. A similar number of 10-20 liters is stated by Brehm et al.,46 who studied a virus 18 nm in diameter with a length of about 400 nm. Both experiments demonstrate the high sensitivity of SNIM and its potential to

Chemical Imaging of Microstructured SAMs obtain label-free chemical imaging for very small amounts of substances. Most likely our sensitivity is even higher, because biotinylated alkylthiolates are less densely packed than alkanethiolates due to steric constraints. The spatial expansion of the biotin group at the end of the thiolates exceeds that of the methyl end group, which prevents dense packing. In summary, the results show that our CO laser-based scanning near-field infrared microscope allows us to obtain characteristic spectral IR features of 90 × 90 nm2 spot size for a single monolayer by vibrational spectroscopy. This demonstrates the high sensitivity of the method. The measurements yield a minimum detectable substance of 5 × 10-20 mol corresponding to 27 attogram or 30 000 molecules of BAT. The use of sharper tips (e.g., with a curvature radius of 5-10 nm) should lead to a localization of the near-field interaction between tip and monolayer that could further improve the detection limit to even less than 30 000 molecules. BAT can serve as a very simple, stable, and easy to handle model system for much more complex protein systems. In addition, the biotin group allows the immobilization of different biomolecules such as proteins47 or nucleic acids48 at the surface. SNIM can be considered as a promising new tool for various applications in surface science (e.g., analysis of the new generation of nanobiochips). Acknowledgment. The authors acknowledge A. Terfort for providing the PDMS stamps and the synthesis of the biotinylated alkylthiol. We thank Ch. Wo¨ll for access to a FTIR spectrometer. The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft under grant HA2394/12-1. References and Notes (1) Rossi, A.; Elsener, B.; Spencer, N. D. Spectrosc. Eur. 2005, 16 (6), 14-19. (2) Blomfield, C. J. J. Electron Spectrosc. Relat. Phenom. 2005, 143 (2-3), 241-249. (3) Prutton, M. Processing, Interpretation and Quantification of Auger Images. In Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy; IM Publications: Charlton, England, 2003; pp 705-732. (4) Guerquin-Kern, J.-L.; Wu, T.-D.; Quintana, C.; Croisy, A. Biochim. Biophys. Acta 2005, 1724 (3), 228-238. (5) Diem, M.; Romeo, M.; Boydston-White, S.; Miljkovic, M.; Mattha¨us, C. Analyst 2004, 129, 880-85. (6) Workman, J. J. Appl. Spectrosc. ReV. 1999, 34 (1-2), 1-89. (7) Mantsch, H. H.; Chapman, D. Infrared Spectroscopy of Biomolecules; Wiley-Liss: Wilmington, DE, 1996. (8) Lewis, A.; Isaacson, M.; Harootunian, A.; Muray, A. Ultramicroscopy 1983, 13 (3), 227-31. (9) Pohl, D. W.; Denk, W.; Lanz, M. Appl. Phys. Lett. 1984, 44 (651). (10) Synge, E. H. Philos. Mag. 1928, 6, 356-62. (11) Keilmann, F. Proc. SPIE-Int. Soc. Opt. Eng. 1991, 1576, 387-8. (12) Knoll, B.; Keilmann, F. Appl. Phys. A 1998, 66 (5), 477-481. (13) Lahrech, A.; Bachelot, R.; Gleyzes, P.; Boccara, A. C. Opt. Lett. 1996, 21, 1315-17. (14) Knoll, B.; Keilmann, F. Nature 1999, 399, 134-137. (15) Knoll, B.; Keilmann, F. Appl. Phys. Lett. 2000, 77 (24), 3980-82. (16) Lahrech, A.; Bachelot, R.; Gleyzes, P.; Boccara, A. C. Appl. Phys. Lett. 1997, 71, 575-77.

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