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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 10 — May. 12, 2008
  • pp: 7453–7459
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Nanoscale depth resolution in scanning near-field infrared microscopy

Götz Wollny, Erik Bründermann, Zoran Arsov, Luca Quaroni, and Martina Havenith  »View Author Affiliations


Optics Express, Vol. 16, Issue 10, pp. 7453-7459 (2008)
http://dx.doi.org/10.1364/OE.16.007453


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Abstract

We have recorded nanoscale topography and infrared chemical fingerprints of attomole layered lipids consisting of dimyristoylphosphatidylcholine on silicon and mica. Lipids deposited on mica built stacks consisting of up to 25 bilayers, each approximately 5 nm thick, spanning a range from 5–125 nm in height. Contrast evaluation as a function of layer thickness provides the near-field depth resolution.

© 2008 Optical Society of America

1. Introduction

State of the art microscopy of biological materials has to reveal structures with nanometer dimensions. Due to the diffraction limit (≈λ/2), conventional microscopy would require short wavelengths (λ) in the ultraviolet to X-ray involving potentially damaging photon energies. New imaging techniques using fluorophors have been developed to beat the diffraction limit such as 4Pi-microscopy. A resolution of less than 100 nm has been obtained, allowing the observation of mitochondrial compartments in a yeast cell [1

1. A. Egner, S. Jacobs, and St. Hell, “Fast 100-nm resolution three-dimensional microscope reveals structural plasticity of mitochondria in live yeast,” Proc. Natl. Acad. Sci. 99, 3370–3375 (2002). [CrossRef] [PubMed]

]. Fluorescence imaging techniques can also follow the path of a single virus penetrating into a cell [2

2. G. Seisenberger, M. U. Ried, Th. Endreß, H. Büning, M. Hallek, and Ch. Bräuchle, “Real-time single-molecule imaging of the infection pathway of an adeno-associated virus,” Science 294, 1929–1932 (2001). [CrossRef] [PubMed]

].

In many applications, a non-invasive, non-destructive label-free imaging technique is desired to obtain an unaltered view of molecular processes. Chemical microscopy identifies label-free molecules and materials by spectral, and therefore chemical, fingerprints in the infrared region (IR). IR spectroscopy allows the analysis of biochemical components and spectroscopic changes during distinct cell division cycles [3

3. M. Diem, M. Romeo, C. Matthäus, M. Miljkovic, L. Miller, and P. Lasch, “Comparison of Fourier transform infrared (FTIR)spectra of individual cells acquired using synchrotron and conventional sources,” Infrared Phys. Technol. 45, 331–338 (2004). [CrossRef]

]. To obtain nanometer chemical information with optical lasers (see Ref. [4

4. A. Hartschuh, H. Qian, A. J. Meixner, N. Anderson, and L. Novotny, “Tip-enhanced optical spectroscopy for surface analysis in biosciences,” Surf. Interface Anal. 381472–1480 (2006). [CrossRef]

] for sub-15 nm resolution using the Raman effect) and for infrared light sources with even longer wavelengths from 3 to 11 µm, near-field methods are needed.

Scanning near-field infrared microscopy (SNIM), with nanometer resolution, combines the advantages of IR spectroscopy with high resolution atomic force microscopy (AFM). Further in-depth information on the development of scanning near-field microscopy can be found in the following reviews by Courjon and Bainier [5

5. D. Courjon and C. Bainier, “Near field microscopy and near field optics,” Rep. Prog. Phys. 57, 989–1028 (1994). [CrossRef]

], Dragena and Leone [6

6. B. Dragnea and S. R. Leone, “Advances in submicron infrared vibrational band chemical imaging,” Int. Rev. Phys. Chem. 20, 59–92 (2001). [CrossRef]

], Keilmann and Hillenbrand [7

7. F. Keilmann and R. Hillenbrand, “Near-field microscopy by elastic light scattering from a tip,” Phil. Trans. R. Soc. Lond. A 362, 787–805 (2004). [CrossRef]

]. In addition, the feasibility of sub-surface nanoscale imaging was previously demonstrated [8

8. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13, 8893–8899 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-22-8893. [CrossRef] [PubMed]

, 9

9. N. Anderson, P. Anger, A. Hartschuh, and L. Novotny, “Subsurface Raman Imaging with Nanoscale Resolution,” Nano Lett. 6, 744–749 (2006). [CrossRef] [PubMed]

, 10

10. J. S. Samson, G. Wollny, E. Bründermann, A. Bergner, A. Hecker, G. Schwaab, A. D. Wieck, and M. Havenith, “Setup of a scanning near field infrared microscope (SNIM): Imaging of sub-surface nano-structures in gallium doped silicon,” Phys. Chem. Chem. Phys. 8, 753–758 (2006). [CrossRef] [PubMed]

]. Very recent work provides a theoretical formalism to model near-field contrast and line shapes [11

11. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy,” Opt. Express 15, 8550–8565 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-14-8550. [CrossRef] [PubMed]

, 12

12. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16, 1529–1545 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-3-1529. [CrossRef] [PubMed]

] (and references within).

Previously, using an all solid state, tunable, table-top opto-parametric oscillator (2500–4000 cm-1), we demonstrated chemical microscopy with 30 nm lateral resolution [10

10. J. S. Samson, G. Wollny, E. Bründermann, A. Bergner, A. Hecker, G. Schwaab, A. D. Wieck, and M. Havenith, “Setup of a scanning near field infrared microscope (SNIM): Imaging of sub-surface nano-structures in gallium doped silicon,” Phys. Chem. Chem. Phys. 8, 753–758 (2006). [CrossRef] [PubMed]

]. With a liquid nitrogen-cooled, tunable CO-laser (1500–2000 cm-1) [13

13. U. Merker, P. Engels, F. Madeja, M. Havenith, and W. Urban, “High-resolution CO-laser sideband spectrometer for molecular-beam optothermal spectroscopy in the 5-6.6 µm wavelength region,” Rev. Sci. Instrum. 70, 1933–1938 (1999). [CrossRef]

], we recorded the IR spectrum in the amid region of a biotin monolayer supported on a gold surface [14

14. I. Kopf, J. S. Samson, G. Wollny, Ch. Grunwald, E. Bründermann, and M. Havenith, “Chemical imaging of microstructured self-assembled monolayers with nanometer resolution,” J. Phys. Chem. C 111, 8166–8171 (2007). [CrossRef]

]. A detection limit of 30000 molecules corresponding to 5 attoliter was achieved.

In this paper, we present measurements of supported lipid bilayers and vesicles on silicon and mica. Especially, the sample preparation on mica led to controlled single bilayers and bilayer stacks of different heights. The contrast as a function of lipid height allowed to analyze the contributions of sample and substrate, lipids and mica, respectively.

2. Experimental

The set-up of our SNIM is shown in Fig. 1(a). The CO-laser power of up to 5 W is attenuated to approximately 100 mW at the entrance of the telescope. The IR light is focused by an off-axis parabolic mirror with a 30 µm diameter onto the metallized tip (Fig. 1(b)) of an AFM cantilever (curvature radius: R=40 nm). A topographic and near-field image is simultaneously recorded by lateral scanning of the sample while the tip is oscillating at a frequency of f=170 kHz. The amplitude modulated scatter light is collected with a CaF2 lens and focused on a mercury cadmium telluride (MCT) detector using phase-sensitive, lock-in, detection. The near-field signal increases non-linearly as a function of the sample-tip distance [11

11. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy,” Opt. Express 15, 8550–8565 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-14-8550. [CrossRef] [PubMed]

], whereas the scatter light, e.g. from the shaft of the cantilever, shows a weaker non-linearity. Therefore, we recorded the signal at the first harmonic of the oscillation frequency (2f=340 kHz) to considerably reduce topographic and background artifacts (cf. Ref. [7

7. F. Keilmann and R. Hillenbrand, “Near-field microscopy by elastic light scattering from a tip,” Phil. Trans. R. Soc. Lond. A 362, 787–805 (2004). [CrossRef]

]). Further details of our set-up can be found in the references [10

10. J. S. Samson, G. Wollny, E. Bründermann, A. Bergner, A. Hecker, G. Schwaab, A. D. Wieck, and M. Havenith, “Setup of a scanning near field infrared microscope (SNIM): Imaging of sub-surface nano-structures in gallium doped silicon,” Phys. Chem. Chem. Phys. 8, 753–758 (2006). [CrossRef] [PubMed]

, 14

14. I. Kopf, J. S. Samson, G. Wollny, Ch. Grunwald, E. Bründermann, and M. Havenith, “Chemical imaging of microstructured self-assembled monolayers with nanometer resolution,” J. Phys. Chem. C 111, 8166–8171 (2007). [CrossRef]

].

Fig. 1. (a) Set-up of the grating tunable CO-laser illuminated SNIM. The IR scatter light from the tip-sample system is collected with a CaF2-lens and MCT-detector. A HeNe-laser is used for optical alignment. (b) Illustration of the 5 nm thick bilayer and stack formation by fusion of multilamellar vesicles to a substrate. (c) Topography image of a lipid patch with 4 bilayers (20 nm height) and 3 additional bilayers in a smaller patch on top corresponding to 35 nm total height above the mica substrate.

Our preparation of dimyristoylphosphatidylcholine (DMPC) vesicles is detailed in Ref. [15

15. Z. Arsov and L. Quaroni, “Direct interaction between cholesterol and phosphatidylcholines in hydrated membranes revealed by ATR-FTIR spectroscopy,” Chem. Phys. Lipids 150, 35–48 (2007). [CrossRef] [PubMed]

]. The vesicles were diluted in distilled water and filtered with a 0.1 µm filter. The vesicle solution (5 µl of 0.16 mg/ml), including 5×1010/ml gold particles of 30 nm diameter, were deposited on either a 5×5 mm2 freshly cleaved mica substrate, an optically polished or Au-coated Si wafer. Mica is a typical AFM-substrate since it can be easily cleaved to provide a ultra-flat surface. Mica has a very low near-field signal while Au has a very high value due to its polarizability which we have confirmed by approach curves to the pure substrates. The near-field signals for each material change marginally in the investigated frequency range providing a minimum-maximum scale after normalization from 0 to 1. In-between we find the investigated DMPC as a function of frequency.

3. Results and discussion

We recorded IR spectra of DMPC multilamellar vesicles adsorbed on Au-coated Si and pure mica substrates by Fourier-transform infrared (FTIR) spectroscopy between 700 and 8000 cm-1 using either grazing incidence (GI) or attenuated total reflection (ATR). The spectra show characteristic absorption bands, the carbonyl stretching mode at 1739 cm-1 and absorption features due to incorporated water in vesicles (for GI). From the spectra, we derived [16

16. T. Buffeteau, B. Desbat, and D. Eyquem, “Attenuated total reflection Fourier transform infrared microspectroscopy: Theory and application to polymer samples,” Vib. Spectrosc. 11, 29–36 (1996). [CrossRef]

, 17

17. F. Picard, T. Buffeteau, B. Desbat, M. Auger, and M. Pézolet, “Quantitative orientation measurements in thin lipid films by attenuated total reflection infrared spectroscopy,” Biophysical J. 76, 539–551 (1999). [CrossRef]

] the components of the complex refractive index =n+ik (Fig. 2(d)). The imaginary part, k, was calculated from the FTIR absorbance of adsorbed multilamellar vesicles on Au and Ge which was used in ATR. In the GI case, the sample contained residual water similar to the samples studied by SNIM. A k-value of 0.12 at 1739 cm-1 was used for the calculation [16

16. T. Buffeteau, B. Desbat, and D. Eyquem, “Attenuated total reflection Fourier transform infrared microspectroscopy: Theory and application to polymer samples,” Vib. Spectrosc. 11, 29–36 (1996). [CrossRef]

]. The refractive index, n, varies for the different measurements and was estimated by using Kramers-Krönig relation with the DMPC refractive index at infinity n ≈1.5 [17

17. F. Picard, T. Buffeteau, B. Desbat, M. Auger, and M. Pézolet, “Quantitative orientation measurements in thin lipid films by attenuated total reflection infrared spectroscopy,” Biophysical J. 76, 539–551 (1999). [CrossRef]

].

Fig. 2. (a) Topography of DMPC bilayers stacks and vesicles on Si. (b) Marked section of (a) showing mainly the substrate Si (black). (c) Height, h, profile showing ≈1 µm thick lipid coverage. (d) Near-field contrast, experimental C and theoretical K, of DMPC on Si as a function of laser frequency, ν. The error bar represents the maximum error of the whole data set (connecting line as a guide to the eye). The FTIR absorption is shown as absorbance A with the refractive index n for both types of measurement (ATR in blue and GI in red).

With SNIM we investigatedDMPC bilayer stacks and vesicles on Si. The sample preparation led to thick lipid coverage up to 1 µm. Near-field data, evaluated in each image, was scaled to the values obtained in a region where we expect uncovered Si (Fig. 2(b)–(c)). The Si dielectric constant does not vary significantly in the investigated frequency range serving as constant reference. The contrast, C=(IDMPC-ISi)/ISi, was calculated from measured intensities of DMPC and uncovered Si, IDMPC and ISi, at each laser frequency. The frequency dependence of the near-field contrast is found to resemble mostly the dispersion of the carbonyl absorption at 1739 cm-1 (Fig. 2(d)) in accordance for thick samples (cf. Ref. [12

12. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16, 1529–1545 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-3-1529. [CrossRef] [PubMed]

]). We have calculated the contrast K similar to the formalism of Ref. [11

11. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy,” Opt. Express 15, 8550–8565 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-14-8550. [CrossRef] [PubMed]

] calculating the effective polarizability and demodulating the s 2 with Fourier decomposition. The result here takes not into account the Fresnel factors or complex multilayers as investigated recently [11

11. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy,” Opt. Express 15, 8550–8565 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-14-8550. [CrossRef] [PubMed]

, 12

12. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16, 1529–1545 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-3-1529. [CrossRef] [PubMed]

].

Other features are attributed to the residual water content within intact vesicles and bound water in the layers shown in the difference of the ATR and GI measurement. The contrast is due to the interplay of substrate, sample and tip geometry, and the materials dielectric constant as outlined by Aizpurua et al. [12

12. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16, 1529–1545 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-3-1529. [CrossRef] [PubMed]

]. Our set-up uses non-interferometric detection [18

18. N. Ocelic, A. J. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89, 101124-1–101124-3 (2006). [CrossRef]

], so may obtain a mixture of absorption and dispersion-like frequency dependence, therefore accounting for the broader peak.

Sample preparation of DMPC on mica led to controlled formation of single bilayers and stacks of up to 25 bilayers. Images were recorded at various frequencies (Fig. 3). A topographic image is shown in Fig. 3(b). Figure 3(c) shows the simultaneously recorded near-field image at 1744 cm-1, close to the maximum absorption of DMPC. The raw data in Fig. 3(b)–(c) was corrected by subtracting a linear function in each line, the “flatten function” in the imaging software WSxM [19

19. I. Horcas, R. Fernandez, J. M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero, and A. M. Baro, “WSxM: A software for scanning probe microscopy and a tool for nanotechnology,” Rev. Sci. Instrum. 78, 013705-1–013705-8 (2007). [CrossRef] [PubMed]

]. The smallest resolved bilayer stacks had lateral dimensions of approximately 100 nm. Single Au nanoparticles with a size of 30 nm (≈λ/200) were found at various positions, mostly at lipid patch edges. Figure 3(a) shows the contrast as a function of topographic height, h, at 1744 and 1754 cm-1. The contrast C=(IDMPC-IMICA)/(IAu-IMICA) is defined, as previously mentioned to form a frequency independent scale from 0 to 1, with the measured lock-in voltage amplitude of the lipids, mica and 30 nm Au particles, i.e. IDMPC, IMICA and IAu, respectively. The DMPC contrast varies from 0.1 to 0.6 (Fig. 3(a)) between the value of mica and Au, i.e. C(IIMICA)→0 and C(IIAu)→1. Selected lipid patches at different constant height were analyzed, and standard deviations in topography and near-field signal calculated. For simplicity, the size of the data points reflects only the maximum standard deviation of all points in the whole data set.

Fig. 3. (a) Near-field contrast, C, of DMPC referenced to Au and mica as a function of height, h, and bilayer number (each 5 nm thick) for different frequency. The contour line encloses the most frequent data pairs of topography height and near-field contrast at 1744 cm-1 (orange). The 2f-sensitivity limit (C≈0.15) corresponds to 3 bilayers. Selected bilayer patches are given as data points. The blue line represents the contrast calculated from the cross-section σ (see inset). The inset shows Au nanoparticles visible at 1658 cm-1 due to low DMPC contrast. (b) Topography of DMPC stacked bilayers on mica (5×5 µm2). In the color scale the maximum topography corresponds to 128 nm (noise level: 2 nm). (c) Near-field image measured at 1744 cm-1 with a maximum voltage of 138 mV (noise level: 20 mV). Selected image areas (1–10) are shown in a bar diagram.

The frequency of the near-field image data at 1744 cm-1 as a function of corresponding topography height was counted to obtain a two-dimensional histogram. The shown contour line encloses the most frequent data in the histogram (Fig. 3(a)). The measured 2f-lock-in voltages for areas, numbered from 1 to 10, are shown in a bar diagram (Fig. 3(c)) corresponding to selected 30 nm Au particles, DMPC patches and mica substrate. The measured voltages were used to calculate the contrast.

The near-field interaction can be approximated by considering the tip as a spherical dipole induced by the infrared electric light field [14

14. I. Kopf, J. S. Samson, G. Wollny, Ch. Grunwald, E. Bründermann, and M. Havenith, “Chemical imaging of microstructured self-assembled monolayers with nanometer resolution,” J. Phys. Chem. C 111, 8166–8171 (2007). [CrossRef]

, 20

20. B. Knoll and F. Keilmann, “Near-field probing of vibrational absorption for chemical microscopy,” Nature 399, 134–137 (1999). [CrossRef]

]. Recent work has extended the theoretical modeling to extended dipoles and multilayers [11

11. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy,” Opt. Express 15, 8550–8565 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-14-8550. [CrossRef] [PubMed]

, 12

12. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16, 1529–1545 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-3-1529. [CrossRef] [PubMed]

]. The inset in Fig. 3(a) shows the calculation of the near-field cross-section σ=σ(z)-σ(z→∞) at the carbonyl band as a function of the tip distance and subtracting the far-field contribution [20

20. B. Knoll and F. Keilmann, “Near-field probing of vibrational absorption for chemical microscopy,” Nature 399, 134–137 (1999). [CrossRef]

]. For a tip with R=40 nm, we expect significant near-field contributions up to a distance of approximately 2R, i.e. 80 nm. It was previously shown that for a lateral resolution of <120 nm, a material contrast can be obtained for objects >80 nm deep below the surface [8

8. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13, 8893–8899 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-22-8893. [CrossRef] [PubMed]

]. In this study, we were able to evaluate the contrast as a function of the depth variation of the dielectric constant due to layer by layer build-up of lipids into stacks. With increasing layer thickness, the penetration depth, 2R, probes an increasing number of lipid layers, and less and less of the mica substrate. We obtain a larger contrast for lipids relative to mica but a decreasing contrast relative to gold. The contrast tends to saturate at 0.6, at approximately 3R or 120 nm. We calculated the contrast from the theoretical cross-sections (σ) of DMPC, Au, and mica at 1744 cm-1 using an average dielectric constant

ε=εMICA(1+εMICAεLIPIDεLIPIDh2R)1
(1)

with weighing factors 2R-h and h, for mica and the respective DMPC layer height. The calculated curve is in good agreement with the main weight of the data (blue line in Fig. 3(a)).

4. Conclusion

We have demonstrated chemical imaging of lipid bilayer stacks. Patches of 100 nm have been resolved at the peak absorption of the carbonyl band with high contrast. The limit sensitivity was reached at 3 bilayers corresponding to 1 attomole. The penetration depth of more than 100 nm is determined by tip curvature and DMPC polarizability. The contrast of DMPC on various substrates was evaluated and the contributions of the substrate mica and the lipids were calculated and separated. The latter is especially important for future investigations of thin samples with a thickness of less than 120 nm or complex biological systems, like cell membranes.

Acknowledgement

We like to thank T. Taubner for helpful comments, I. Kopf and J. S. Samson for assisting in the experiments, D. Schmidt for carefully reading the manuscript. Z. Arsov acknowledges the financial support from the Sincrotrone Trieste and the Slovenian Research Agency (project No. Z1-9502). The work has been performed within UZMT of the Ruhr-Universität and EU programme INTCHEM. We acknowledge financial support by the Deutsche Forschungsge-meinschaft (HA2394/12–1) and the BMBF 05 KS7PC2 coordinated by PT-DESY (innovative instrumentation of the synchrotron ANKA, Karlsruhe, Germany).

References and links

1.

A. Egner, S. Jacobs, and St. Hell, “Fast 100-nm resolution three-dimensional microscope reveals structural plasticity of mitochondria in live yeast,” Proc. Natl. Acad. Sci. 99, 3370–3375 (2002). [CrossRef] [PubMed]

2.

G. Seisenberger, M. U. Ried, Th. Endreß, H. Büning, M. Hallek, and Ch. Bräuchle, “Real-time single-molecule imaging of the infection pathway of an adeno-associated virus,” Science 294, 1929–1932 (2001). [CrossRef] [PubMed]

3.

M. Diem, M. Romeo, C. Matthäus, M. Miljkovic, L. Miller, and P. Lasch, “Comparison of Fourier transform infrared (FTIR)spectra of individual cells acquired using synchrotron and conventional sources,” Infrared Phys. Technol. 45, 331–338 (2004). [CrossRef]

4.

A. Hartschuh, H. Qian, A. J. Meixner, N. Anderson, and L. Novotny, “Tip-enhanced optical spectroscopy for surface analysis in biosciences,” Surf. Interface Anal. 381472–1480 (2006). [CrossRef]

5.

D. Courjon and C. Bainier, “Near field microscopy and near field optics,” Rep. Prog. Phys. 57, 989–1028 (1994). [CrossRef]

6.

B. Dragnea and S. R. Leone, “Advances in submicron infrared vibrational band chemical imaging,” Int. Rev. Phys. Chem. 20, 59–92 (2001). [CrossRef]

7.

F. Keilmann and R. Hillenbrand, “Near-field microscopy by elastic light scattering from a tip,” Phil. Trans. R. Soc. Lond. A 362, 787–805 (2004). [CrossRef]

8.

T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13, 8893–8899 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-22-8893. [CrossRef] [PubMed]

9.

N. Anderson, P. Anger, A. Hartschuh, and L. Novotny, “Subsurface Raman Imaging with Nanoscale Resolution,” Nano Lett. 6, 744–749 (2006). [CrossRef] [PubMed]

10.

J. S. Samson, G. Wollny, E. Bründermann, A. Bergner, A. Hecker, G. Schwaab, A. D. Wieck, and M. Havenith, “Setup of a scanning near field infrared microscope (SNIM): Imaging of sub-surface nano-structures in gallium doped silicon,” Phys. Chem. Chem. Phys. 8, 753–758 (2006). [CrossRef] [PubMed]

11.

A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy,” Opt. Express 15, 8550–8565 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-14-8550. [CrossRef] [PubMed]

12.

J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16, 1529–1545 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-3-1529. [CrossRef] [PubMed]

13.

U. Merker, P. Engels, F. Madeja, M. Havenith, and W. Urban, “High-resolution CO-laser sideband spectrometer for molecular-beam optothermal spectroscopy in the 5-6.6 µm wavelength region,” Rev. Sci. Instrum. 70, 1933–1938 (1999). [CrossRef]

14.

I. Kopf, J. S. Samson, G. Wollny, Ch. Grunwald, E. Bründermann, and M. Havenith, “Chemical imaging of microstructured self-assembled monolayers with nanometer resolution,” J. Phys. Chem. C 111, 8166–8171 (2007). [CrossRef]

15.

Z. Arsov and L. Quaroni, “Direct interaction between cholesterol and phosphatidylcholines in hydrated membranes revealed by ATR-FTIR spectroscopy,” Chem. Phys. Lipids 150, 35–48 (2007). [CrossRef] [PubMed]

16.

T. Buffeteau, B. Desbat, and D. Eyquem, “Attenuated total reflection Fourier transform infrared microspectroscopy: Theory and application to polymer samples,” Vib. Spectrosc. 11, 29–36 (1996). [CrossRef]

17.

F. Picard, T. Buffeteau, B. Desbat, M. Auger, and M. Pézolet, “Quantitative orientation measurements in thin lipid films by attenuated total reflection infrared spectroscopy,” Biophysical J. 76, 539–551 (1999). [CrossRef]

18.

N. Ocelic, A. J. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89, 101124-1–101124-3 (2006). [CrossRef]

19.

I. Horcas, R. Fernandez, J. M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero, and A. M. Baro, “WSxM: A software for scanning probe microscopy and a tool for nanotechnology,” Rev. Sci. Instrum. 78, 013705-1–013705-8 (2007). [CrossRef] [PubMed]

20.

B. Knoll and F. Keilmann, “Near-field probing of vibrational absorption for chemical microscopy,” Nature 399, 134–137 (1999). [CrossRef]

21.

A. J. Guarino, T. N. Tulenko, and S. P. Wrenn, “Cholesterol crystal nucleation from enzymatically modified low-density lipoproteins: Combined effect of sphingomyelinase and cholesterol esterase,” Biochemistry 43, 1685–1693 (2004). [CrossRef] [PubMed]

OCIS Codes
(110.3080) Imaging systems : Infrared imaging
(180.5810) Microscopy : Scanning microscopy
(300.6340) Spectroscopy : Spectroscopy, infrared

ToC Category:
Microscopy

History
Original Manuscript: February 27, 2008
Revised Manuscript: April 13, 2008
Manuscript Accepted: April 30, 2008
Published: May 8, 2008

Virtual Issues
Vol. 3, Iss. 6 Virtual Journal for Biomedical Optics

Citation
Götz Wollny, Erik Bründermann, Zoran Arsov, Luca Quaroni, and Martina Havenith, "Nanoscale depth resolution in scanning near-field infrared microscopy," Opt. Express 16, 7453-7459 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-10-7453


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