Crystalline Water at Room Temperature - Under Water and in Air Andrei P. Sommer,*,† Dan Zhu,† Horst-Dieter Fo¨rsterling,‡ Tim Scharnweber,§ and Alexander Welle§ Nanobionic Laboratory, Institute of Micro and Nanomaterials, UniVersity of Ulm, 89081 Ulm, Germany, Department of Chemistry, Philipps UniVersity of Marburg, 35032 Marburg, Germany, and Institute for Biological Interfaces, Research Center Karlsruhe, 76021 Karlsruhe, Germany
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2620–2622
ReceiVed April 14, 2008; ReVised Manuscript ReceiVed May 25, 2008
ABSTRACT: In a visionary work published in 1971, Albert Szent-Gyo¨rgyi predicted that crystalline interfacial water layers would play a fundamental role in biological processes and evolution. However, interfacial water layers are so sensitive to observation that they have never been imaged on relevant surfaces, so far. Here we show that crystalline interfacial water layers prevail at room temperature on both hydrophilic and hydrophobic surfaces - in ambient air and subaquatically. We probe the interfacial water layers by monitoring the resonance frequency responses of quartz crystal microbalance sensors to their irradiation with 633 and 670 nm lasers. Our results are consistent with the fractional picture of confined water, previously explored by atomic force microscopy, near-field scanning optical microscopy, and atomic force acoustic microscopy. Since we provide both structural information and quantitative data on the thickness of interfacial water layers, our approach promises progress in biomedicine and life sciences. Introduction.Interfacial water layers (IWLs) exhibiting a molecular order different from that of bulk water have been identified at room temperature for hydrophobic and hydrophilic surfaces in ambient air, and for hydrophobic surfaces subaquatically. For hydrophobic surfaces in air, the different order was exposed by their spontaneous depletion following their exposure to 670 nm laser light.1 Recently, we used atomic force acoustic microscopy (atomic force microscope operating on an ultrasonic transducer) to demonstrate that in ambient air IWLs are tunable with 670 nm laser light on both hydrophobic and hydrophilic surfaces.2 For hydrophilic surfaces in air, a difference in molecular ordering manifested itself in abnormally high viscosity levels, as measured by atomic force microscopy.3–5 Subaquatically, the difference in ordering has been exposed so far indirectly, that is, by monitoring the effect of 670 nm laser light on the time of evaporation of sessile water drops.6 Experimental Section. Here we use a quartz crystal microbalance (Q-Sense D300, Sweden) to investigate the effect of low intensity laser light on IWLs - in ambient air (relative humidity 32.2%, temperature 23.8 °C) and subaquatically (under a 3 mm column of ultra pure water). The heart of the microbalance is a quartz crystal disk carrying gold electrodes on top (Ø 11 mm) and bottom and performing shear oscillations parallel to its plane. The measuring principle exploits the proportionality between the oscillating mass and the resonance frequency of this sensor system. The sensor was mounted in a window chamber that permitted irradiation of the gold electrode at an angle of 90° from top, and 45° from the side (Figure 1). We used two different 5 mW lasers, both linearly polarized, operating at a wavelength of 633 and 670 nm, respectively, with a spot diameter of about 4 mm. Light intensities for beam incidences normal to the sensor surface were 400 W m-2. The hydrophilic target surfaces were gold electrodes (water contact angle 42°), cleaned by immersion in a solution of boiling aqueous ammonia (30%), H2O2 (30%) and water (ratio 1:1:5), followed by rinsing in ultra pure water and drying under a nitrogen stream. The hydrophobic target surfaces were gold electrodes spin-coated with polystyrene7 (water contact angle 94°). Figure 2 is representative for the increase (decrease) in sensor resonance frequency when the lasers (laser position 1 in Figure 1) are switched on (off). To discriminate between possible effects of irradiative heating on the one hand, and interaction of the photons with interfacial water * Corresponding author. E-mail:
[email protected]. † University of Ulm. ‡ Philipps University of Marburg. § Institute for Biological Interfaces, Research Center Karlsruhe.
Figure 1. Experimental setup to probe the organization of interfacial water layers by a quartz crystal microbalance. The temperature of the sensor is kept constant by a sensitive thermostat.
Figure 2. Resonance frequency change ∆f of the polystyrene coated quartz crystal in response to its irradiation with 633 nm laser light. Upon irradiation, ∆f increases, indicating a decrease of the mass of the surface layer. Interestingly, the responses in resonance frequencies increased with increasing light intensity, signifying intensity levels below saturation.
molecules on the other hand, we irradiated the sensors at an angle of 45° using the 633 and 670 nm lasers (Figure 1), whereby we rotated the laser housing around the beam path (parallel beam), using a beam diameter of about 1 mm. Results and Discussion. Earlier findings indicated that the molecules constituting IWLs are more densely packed than those in bulk water.2 An increase in density follows from considering the intuitive picture of less fluctuating water molecules at liquid-solid interfaces, conforming to their restriction in mobility by unilateral spatial confinement. Recent study of the water structure on hydrogen-terminated nanocrystalline diamond provided evidence for the coexistence of bulk water and a compact crystalline water layer in contact with the diamond.8,9 The present results confirm
10.1021/cg800382x CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
Communications
Crystal Growth & Design, Vol. 8, No. 8, 2008 2621
Figure 3. (a) Changes in sensor resonant frequencies ∆f in air and subaquatically in response to laser irradiation normal to the surface plane on gold (hydrophilic) and polystyrene (hydrophobic). (b) Analogous changes for lasers applied at 45°. Smaller (larger) frequency values stem from laser polarization parallel (normal) to the sensor plane.
the picture of compact, densely packed (rigid) IWLs on the tested substrates - thereby following the shear oscillations of the sensor crystal. Figure 3a is a representative synopsis of the increase in target resonance frequency (as a consequence of the depleted mass) in response to its irradiation. As a general tendency, increases were more pronounced on the hydrophobic surfaces than on the hydrophilic ones, both in air and subaquatically, thus in accordance with the picture of a more solid-like and presumably less bound layer of H2O molecules on hydrophobic surfaces, compared to glue-like water structures, reported to prevail on hydrophilic surfaces.4 The quartz crystal microbalance measuring principle is based on the approximation that for thin and rigid films attached to the sensor surface the frequency change ∆f is proportional to the change in the mass of the film ∆m. In order to calculate ∆m and the thickness d of the depleted IWLs, we use the Sauerbrey equation10
∆m ) - C
VFQ ∆f A, with C ) 2 n 2f
(1)
where V ) 3340 m s-1 is the speed of sound in the quartz crystal, FQ ) 2.651 g cm-3 is the density of the quartz crystal, and f ) 5 MHz is its resonance frequency. Thus C ) 17.7 ng cm-2 s. n ) 7 is the actual overtone number of the oscillator circuit (that is, the actual frequency of the oscillator is 35 MHz). A is the surface area. In the derivation of the Sauerbrey equation it is assumed that the mass change ∆m is uniformly distributed over the total surface of the quartz crystal, Aquartz. In this case the area A in eq 1 equals Aquartz. In our case, however, only a small fraction of the area Aquartz is illuminated by the laser beam. Therefore, we identify A with the cross section area of the laser beam. We start from the simplest possible picture to estimate the thickness x of the layer of H2O molecules, which is depleted by the laser light. xA ) -∆m/F is the volume of the depleted water layer, where F is its density and A is the cross section area of the laser beam. For simplicity we assume a normal water density of F ) 1 g cm-3, using n ) 7 and ∆f ) 22 Hz, from eq 1 we obtain
x)-
17.7 ng · cm-2 · s ∆m C 22 s-1 ) 0.55 nm ) ∆ f) -3 FA Fn 1 g · cm · 7
(2)
It is instructive to divide this value by 0.28 nm (the diameter of one H2O molecule) to find that this corresponds to a thickness of about two monolayers. Regarding the relatively low light intensities it is tempting to ascribe the observed frequency changes to a collective interaction of photons with a layer of interfacial water molecules, that is, to a polarization of the electron clouds of the water molecules close to the sensor. Irradiation at angles of 45° (Figure 1), whereby the plane
of the laser polarization was parallel or normal to the sensor surface, exposed the intrinsic interaction of the polarized photons with polarized water dipoles. Importantly, when the laser polarization was parallel to the sensor plane, there was a significant drop in the frequency shift, compared to the laser polarization perpendicular to the sensor surface. Results for the laser polarization normal and parallel to the sensor surface are displayed in Figure 3b. For both polarizations, irradiation at 45° (laser position 2 in Figure 1) occurred at the same sensor coordinates; light intensities and beam profiles were practically equal, depending on the specific medium (air or water). Thus, here we can safely conclude that the observed differences in frequency shift were not due to irradiative heating but solely due to the interaction of photons with the water molecules prevailing at the liquid-solid interface. Clearly, for water molecules with randomly distributed dipole moments, one would expect no difference in frequency change in response to the change in laser polarization. Our approach offers unprecedented insight into the organization of interfacial water layers and receives importance from the anticipated biological relevance of subaquatic crystalline water layers.11,12 It is instructive to point out that this insight is obtained with nondestructive laser light intensities (400 W m-2) as opposed to the 10 · 1012 W m-2 applied in sum-frequency vibrational spectroscopy to probe the water at solid-water interfaces.13 There are uncertainties in our apparently straightforward experiments, which should be clearly mentioned. Presently, we can not answer the question of why the 633 nm laser is more effective in depleting interfacial water layers. Clarification of this critical point requires further study. Conclusions. Our experiments performed on substrates as different in polarity as gold and polystyrene suggest that the virtually spontaneous change in the resonance frequency of the sensor crystal, in response to the applied laser irradiation, is at least partially due to the depletion of the IWLs by the electric field of the laser. It is worth noting that both laser wavelengths 633 and 670 nm have profound biological effects.14 It is thus expected that our results will provide a new understanding of how to precisely control interfacial processes in the nanoworld in general and interfacial water in biology in particular.
Acknowledgment. We are grateful to the Landesstiftung BadenWu¨rttemberg Bionics Network for supporting this research.
References (1) Sommer, A. P.; Franke, R. P. Nano Lett. 2003, 3, 19–20. (2) Sommer, A. P.; Caron, A.; Fecht, H. J. Langmuir 2008, 24, 635–636. (3) Goertz, M. P.; Houston, J. E.; Zhu, X. Y. Langmuir 2007, 23, 5491– 5497.
2622 Crystal Growth & Design, Vol. 8, No. 8, 2008 (4) Jinesh, K. B.; Frenken, J. W. M. Phys. ReV. Lett. 2006, 96, 166103. (5) Li, T. D.; Gao, J.; Szoszkiewicz, R.; Landman, U.; Riedo, E. Phys. ReV. B 2007, 75, 115415. (6) Sommer, A. P.; Pavla´th, A. E. Cryst. Growth Des. 2007, 7, 18–24. (7) Welle, A.; Kro¨ger, M.; Do¨ring, M.; Niederer, K.; Pindel, E.; Chronakis, I. S. Biomaterials 2007, 28, 2211–2219. (8) Sommer, A. P.; Zhu, D.; Bru¨hne, K. Cryst. Growth Des. 2007, 7, 2298– 2301. (9) Sommer, A. P.; Zhu, D.; Fo¨rsterling, H.-D. Science Online 2008, Feb. 28.
Communications (10) Sauerbrey, G. Z. Phys. 1959, 155, 206–222. (11) Szent-Gyo¨rgyi, A. Perspect. Biol. Med. 1971, 14, 239–249. (12) Sommer, A. P.; Zhu, D.; Wiora, M.; Fecht, H. J. J. Bionic Eng., in press. (13) Ostroverkhov, V.; Waychunas, G. A.; Shen, Y. R. Chem. Phys. Lett. 2004, 386, 144–148. (14) Sommer, A. P.; Pinheiro, A. L.; Mester, A. R.; Franke, R. P.; Whelan, H. T. J. Clin. Laser Med. Surg. 2001, 19, 29–33.
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