Modulating the Profile of NanoscopicWater Films with Low Level Laser LightAndrei P. Sommer* and Ralf-Peter Franke

Department of Biomaterials/ENSOMA-Laboratory, Central Institute of BiomedicalEngineering, UniVersity of Ulm, 89081 Ulm, Germany

Received October 9, 2002; Revised Manuscript Received November 5, 2002

ABSTRACTWater films are omnipresent in nature. They control unspecific interactions at biochip surfaces, functional coatings, and charge transferprocesses in thunderclouds. Here we demonstrate how the profile of nanoscopic water films could be modulated by laser light and simultaneouslyvisualized by near-field optical analysis. Most importantly, the intensity and the energy density of the modulating laser light were at low levelsshown to increase the survival rate of stressed cells, in vitro and in vivo, and to accelerate the healing of wounds in low intensity laseractivated biostimulation.

Depending on material properties (polarity) and environ-mental conditions (relative humidity and temperature), thethickness of water films could vary from one to several tenthsof nanometers.1 Temperature-dependent variations of thethickness of liquid layers on ice have been estimated bycontact mode atomic force microscopy (AFM).2 A prominenteffect accompanying water films is a viscosity gradient withan elevated viscosity at interfacial phase transitions. Thepresence of viscosity gradients has been practically exploitedin near-field optical analysis (NOA) employing near-fieldscanning optical microscopes (NSOM),3 facilitating nanos-cale imaging of rough biological sample surfaces at air,4

noninvasive localization of soft and shape-variant cellmembranes, and imaging of cell organelles in living cells.5

NOA has produced the highest optical resolution that hasever been achieved- a method exploring optical surfaceproperties by laser-induced energy transfer6 from an opticalsensor (tip diameterg20 nm) oscillating within the near-field (∼10 nm) of the surface to be analyzed. Scanning inthe near-field of a sample could be performed in two virtuallyequivalent modes: with sensor oscillations normal or parallelto the substrate, respectively. In particular in biosystems,where scanning in liquid is prevalent, sensors oscillatingparallel to sample surfaces could have specific advantageswhen compared to normal mode operations: As the sensorsapproached the sample to distances of the order of the near-field, an organized layer of water molecules7 masking thesample surface dampened the sensor oscillations, allowingin principle to scan noninvasively. By monitoring amplitudevariations as a function of the height of the sensor abovethe sample, topography and optical contrast could be mappedsimultaneously. The performance of the method at air

depended on the relative humidity, and thus on the depth ofthe water layer attached to samples.5

The substrates utilized to investigate the water layersconsisted of a solid translucent hydrophobic polymer film.The film was prepared by photopolymerization of a hydro-philic light-curing fluid homogeneously spread on the surfaceof a 16 mm diameter titanium disk. The solid polymer filmhad a uniform thickness of 90µm. Because of its dualpolarity, the biocompatible polymer (Prime & Bond NT,DENTSPLY DeTrey, Germany) was also used to fabricatebiosensors for NOA.4,5 The mechanochemically polishedtitanium disk had a total surface roughnesse4 nm,5

determined by AFM (DualScope, DME, Denmark). NOAand topographic examination of the water mask on thepolymer were performed via NSOM (DualScope, DME,Denmark). The NSOM was equipped with quartz sensorsoscillating parallel to the sample surface (tip diameter∼30nm) pulled from cleaved optical fibers (diameter) 125µm)by a laser puller (P-2000, Sutter Instruments, Novato, CA).Linearly polarized 488 nm laser light was coupled into thesensor scanning in the near-field of the sample. Reflectedsignals stemming from interaction of the near-field irradiationwith the sample were collected by the sensor; passing apolarizer tuned to block the original polarization of the laser,the signals could be subsequently detected by a photo-multiplier for NOA. The height of the water film attachedto the polymer film was modulated by a 670 nm laser(intensity∼1000 W/m2) mounted in a collinear arrangementto the optical axis of the sensor (Figure 1).

We investigated light-induced topography variations on atransparent polymer film with a NSOM. The film, im-mobilized at the center of the microscope platform wasscanned simultaneously for topography and optical contrast.The 670 nm laser, used to modulate the topography of thewater layer on the polymer was integrated in the microscope* Corresponding author.

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platform. The red beam could be switched on to irradiatethe sample collinearly to the sensor’s optical axis. While thetopographic image (a) in Figure 2, corresponding to an AFMscan, revealed the veritable situation at the film surface, theNOA image (b), with optical contrast on the nanoscale,validated the synchronization between heights variationsdetected by the sensor and their periodic modulation by thered laser, showing an increased light intensity duringactivated red light phases. The low surface roughness of thepolymer film was essential for discrimination of the minimalheight variation visible in the AFM scan. Calibrated nano-particles dispersed on the surface of solid surfaces (includingice at various temperatures and relative humidity) could be

used to directly determine the actual thickness of the waterlayers on surfaces. Such particles have been used to estimatethe thickness of the hydrophobic polymer film coating thebiosensors applied in NOA.8 Both applications nanoscalecharacterization of liquid layers on ice and noninvasiveimaging of living cells require a minimum perturbation ofthe system, including effects triggered by the process ofobservation. The biological impact of NOA on biosystems,and of additional light administrations during NOA has beenrecently estimated.9 By adjusting the primary parameters,laser power, and scanning velocity, local light doses couldbe adjusted to energy density levels known to have a positivebiostimulatory effect on biosystems.10 Laser irradiation ofsuitable wavelength, intensity, and energy density has beenobserved to have beneficial effects on a representative bodyof cells, in particular under stressed conditions.11

Our observations indicate that nanoscale imaging ofwetting dynamics could be realized by NOA and interpretedin synergistic complementarity with the associated AFMimage. Imaging aqueous films represents a major challengeto both technologies, AFM and NOA. The results motivateexperiments with standard nanoparticles to elucidate theimpact of the near-field irradiation itself on the water filmsdeposited on various surfaces. The interplay between non-invasive imaging and photomodulated nanopatterning couldbe instrumental in the design of nanostructured patterns andnondestructive high throughput pattern transfer,12 and incontrolling unspecific interactions of key biomoleculescontacting biochip surfaces.13-15 By realizing that nanoscopicwater films are prerequisite for atmospheric electrificationprocesses,16 and that the height of water films could besensitively modulated by light intensities equivalent to thesolar constant, the results may offer a better understandingof diurnal lightning variations.17

References(1) Freund, J.; Halbritter, J.; Ho¨rber, J. K. H.Micros. Res. Tech.1999,

44, 327-338.(2) Doppenschmidt, A.; Butt, H. J.Langmuir2000, 16, 6709-6714.(3) Betzig, E.; Trautman, J. K.; Harris, T. D.; Weiner, J. S.; Kostelak,

R. L. Science1991, 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.Science2001, 292,

908-912.(8) Sommer, A. P.Langmuir2002, 18, 5040-5042.(9) Sommer, A. P. Proceedings of the 2nd International Conference on

Near-field Optical Analysis: Photodynamic Therapy & PhotobiologyEffects. 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. ProteomeRes. 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.Nature1999, 398, 593-597.

(14) Ariga, K.; Kunitake, T.Acc. Chem. Res.1997, 31, 3371-378.(15) Radler, U.; Heiz, C.; Luisi, P. L.; Tampe´ R. Langmuir 1998, 14,

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5221-5234.

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Figure 1. Optical axis of the near-field light (green) in collinearand antiparallel alignment with the modulating laser (red), bothintegrated into the microscope (small photo). The orientation ofthe light beams is normal to the plane of the sample, allowing tosimultaneously modulate and measure height variations of waterfilms on top of translucent surfaces.

Figure 2. Topography (a) and associated NOA image (b) on atranslucent hydrophobic polymer film scanned at air via quartzsensor. Horizontal scan: 1µm × 1 µm, vertical scan-range: 10.48nm. NOA permits colocalization of light-induced height variationsof 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 profilesreveals the potential of the method to modulate and to imagenanostructured liquid layers. The intensity of the near-field irradia-tion stemming from the 488 nm laser coupled into the sensor wasstable during scanning at constant velocity and could not affectthe pattern observed.

20 Nano Lett., Vol. 3, No. 1, 2003