NANO LETTERS
Modulating the Profile of Nanoscopic Water Films with Low Level Laser Light
2003 Vol. 3, No. 1 19-20
Andrei P. Sommer* and Ralf-Peter Franke Department of Biomaterials/ENSOMA-Laboratory, Central Institute of Biomedical Engineering, UniVersity of Ulm, 89081 Ulm, Germany Received October 9, 2002; Revised Manuscript Received November 5, 2002
ABSTRACT Water films are omnipresent in nature. They control unspecific interactions at biochip surfaces, functional coatings, and charge transfer processes in thunderclouds. Here we demonstrate how the profile of nanoscopic water films could be modulated by laser light and simultaneously visualized by near-field optical analysis. Most importantly, the intensity and the energy density of the modulating laser light were at low levels shown to increase the survival rate of stressed cells, in vitro and in vivo, and to accelerate the healing of wounds in low intensity laser activated biostimulation.
Depending on material properties (polarity) and environmental conditions (relative humidity and temperature), the thickness of water films could vary from one to several tenths of nanometers.1 Temperature-dependent variations of the thickness of liquid layers on ice have been estimated by contact mode atomic force microscopy (AFM).2 A prominent effect accompanying water films is a viscosity gradient with an elevated viscosity at interfacial phase transitions. The presence of viscosity gradients has been practically exploited in near-field optical analysis (NOA) employing near-field scanning optical microscopes (NSOM),3 facilitating nanoscale imaging of rough biological sample surfaces at air,4 noninvasive localization of soft and shape-variant cell membranes, and imaging of cell organelles in living cells.5 NOA has produced the highest optical resolution that has ever been achieved - a method exploring optical surface properties by laser-induced energy transfer6 from an optical sensor (tip diameter g20 nm) oscillating within the nearfield (∼10 nm) of the surface to be analyzed. Scanning in the near-field of a sample could be performed in two virtually equivalent modes: with sensor oscillations normal or parallel to the substrate, respectively. In particular in biosystems, where scanning in liquid is prevalent, sensors oscillating parallel to sample surfaces could have specific advantages when compared to normal mode operations: As the sensors approached the sample to distances of the order of the nearfield, an organized layer of water molecules7 masking the sample surface dampened the sensor oscillations, allowing in principle to scan noninvasively. By monitoring amplitude variations as a function of the height of the sensor above the sample, topography and optical contrast could be mapped simultaneously. The performance of the method at air * Corresponding author. 10.1021/nl025839m CCC: $25.00 Published on Web 12/03/2002
© 2003 American Chemical Society
depended on the relative humidity, and thus on the depth of the water layer attached to samples.5 The substrates utilized to investigate the water layers consisted of a solid translucent hydrophobic polymer film. The film was prepared by photopolymerization of a hydrophilic light-curing fluid homogeneously spread on the surface of a 16 mm diameter titanium disk. The solid polymer film had a uniform thickness of 90 µm. Because of its dual polarity, the biocompatible polymer (Prime & Bond NT, DENTSPLY DeTrey, Germany) was also used to fabricate biosensors for NOA.4,5 The mechanochemically polished titanium disk had a total surface roughness e4 nm,5 determined by AFM (DualScope, DME, Denmark). NOA and topographic examination of the water mask on the polymer were performed via NSOM (DualScope, DME, Denmark). The NSOM was equipped with quartz sensors oscillating parallel to the sample surface (tip diameter ∼30 nm) pulled from cleaved optical fibers (diameter ) 125 µm) by a laser puller (P-2000, Sutter Instruments, Novato, CA). Linearly polarized 488 nm laser light was coupled into the sensor scanning in the near-field of the sample. Reflected signals stemming from interaction of the near-field irradiation with the sample were collected by the sensor; passing a polarizer tuned to block the original polarization of the laser, the signals could be subsequently detected by a photomultiplier for NOA. The height of the water film attached to the polymer film was modulated by a 670 nm laser (intensity ∼1000 W/m2) mounted in a collinear arrangement to the optical axis of the sensor (Figure 1). We investigated light-induced topography variations on a transparent polymer film with a NSOM. The film, immobilized at the center of the microscope platform was scanned simultaneously for topography and optical contrast. The 670 nm laser, used to modulate the topography of the water layer on the polymer was integrated in the microscope
Figure 1. Optical axis of the near-field light (green) in collinear and antiparallel alignment with the modulating laser (red), both integrated into the microscope (small photo). The orientation of the light beams is normal to the plane of the sample, allowing to simultaneously modulate and measure height variations of water films on top of translucent surfaces.
Figure 2. Topography (a) and associated NOA image (b) on a translucent hydrophobic polymer film scanned at air via quartz sensor. Horizontal scan: 1 µm × 1 µm, vertical scan-range: 10.48 nm. NOA permits colocalization of light-induced height variations of the nanoscopic water film on top of the polymer. The darker, high light-intensity intervals in the picture on the right (NOA), correspond to the thinner water layers in the scan on the left (AFM). A detailed inspection of the complementary AFM/NOA profiles reveals the potential of the method to modulate and to image nanostructured liquid layers. The intensity of the near-field irradiation stemming from the 488 nm laser coupled into the sensor was stable during scanning at constant velocity and could not affect the pattern observed.
platform. The red beam could be switched on to irradiate the sample collinearly to the sensor’s optical axis. While the topographic image (a) in Figure 2, corresponding to an AFM scan, revealed the veritable situation at the film surface, the NOA image (b), with optical contrast on the nanoscale, validated the synchronization between heights variations detected by the sensor and their periodic modulation by the red laser, showing an increased light intensity during activated red light phases. The low surface roughness of the polymer film was essential for discrimination of the minimal height variation visible in the AFM scan. Calibrated nanoparticles dispersed on the surface of solid surfaces (including ice at various temperatures and relative humidity) could be 20
used to directly determine the actual thickness of the water layers on surfaces. Such particles have been used to estimate the thickness of the hydrophobic polymer film coating the biosensors applied in NOA.8 Both applications nanoscale characterization of liquid layers on ice and noninvasive imaging of living cells require a minimum perturbation of the system, including effects triggered by the process of observation. The biological impact of NOA on biosystems, and of additional light administrations during NOA has been recently estimated.9 By adjusting the primary parameters, laser power, and scanning velocity, local light doses could be adjusted to energy density levels known to have a positive biostimulatory effect on biosystems.10 Laser irradiation of suitable wavelength, intensity, and energy density has been observed to have beneficial effects on a representative body of cells, in particular under stressed conditions.11 Our observations indicate that nanoscale imaging of wetting dynamics could be realized by NOA and interpreted in synergistic complementarity with the associated AFM image. Imaging aqueous films represents a major challenge to both technologies, AFM and NOA. The results motivate experiments with standard nanoparticles to elucidate the impact of the near-field irradiation itself on the water films deposited on various surfaces. The interplay between noninvasive imaging and photomodulated nanopatterning could be instrumental in the design of nanostructured patterns and nondestructive high throughput pattern transfer,12 and in controlling unspecific interactions of key biomolecules contacting biochip surfaces.13-15 By realizing that nanoscopic water films are prerequisite for atmospheric electrification processes,16 and that the height of water films could be sensitively modulated by light intensities equivalent to the solar constant, the results may offer a better understanding of diurnal lightning variations.17 References (1) Freund, J.; Halbritter, J.; Ho¨rber, J. K. H. Micros. Res. Tech. 1999, 44, 327-338. (2) Do¨ppenschmidt, A.; Butt, H. J. Langmuir 2000, 16, 6709-6714. (3) Betzig, E.; Trautman, J. K.; Harris, T. D.; Weiner, J. S.; Kostelak, R. L. Science 1991, 251, 1468-1470. (4) Sommer, A. P.; Franke, R. P. Micron 2002, 33, 227-231. (5) Sommer, A. P.; Franke, R. P. J. Proteome Res. 2002, 1, 111-114. (6) Kuhn, H. J. Chem. Phys. 1970, 53, 101-108. (7) Scatena, L. F.; Brown, M. G.; Richmond G. L. Science 2001, 292, 908-912. (8) Sommer, A. P. Langmuir 2002, 18, 5040-5042. (9) Sommer, A. P. Proceedings of the 2nd International Conference on Near-field Optical Analysis: Photodynamic Therapy & Photobiology Effects. Johnson Space Flight Center, May 2001, Houston, TX, NASA Conference Publication (in press). (10) Sommer, A. P.; Pinheiro, A. L. B.; Mester, A. R.; Franke, R. P.; Whelan, H. T. J. Clin. Laser Med. Surg. 2001, 19, 29-33. (11) Sommer, A. P.; Oron, U.; Kajander, E. O.; Mester, A. R. J. Proteome Res. 2002, 1, 475. (12) Liu J. F.; Cruchon-Dupeyrat, S.; Garno, J. C.; Frommer, J.; Liu, G. Y. Nano Lett. 2002, 2, 937-940. (13) Shi, H.; Tsai, W. B.; Garrison, M. D.; Ferrari, S.; Ratner, B. D. Nature 1999, 398, 593-597. (14) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1997, 31, 3371-378. (15) Ra¨dler, U.; Heiz, C.; Luisi, P. L.; Tampe´ R. Langmuir 1998, 14, 6620-6624. (16) Sommer, A. P.; Levin, Z. Atmos. Res. 2001, 58, 129-139. (17) Williams, E. R.; Heckman, S. J. J. J. Geophys. Res. 1993, 98, 5221-5234.
NL025839M Nano Lett., Vol. 3, No. 1, 2003
