Controlling Arrangement of 60 nm Nanospheres in Evaporating Sessile Drops with Low Level Laser Light Andrei P. Sommer*
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 559-563
Central Institute of Biomedical Engineering, University of Ulm, 89081 Ulm, Germany Received June 16, 2004
ABSTRACT: The interplay between liquid flow and surface interaction can arrange colloidal particles in drops evaporating on substrates into crystalline rings. Recently, it was reported that magnetic fields can affect the crystallinity in rings formed by suspensions containing paramagnetic particles. Here, it is demonstrated that laser light, applied at intensities as low as 1000 W m-2, has a manifest impact on the attachment and self-organization of 60 nm particles on wet substrates. Results indicate that the physical state of water layers attached to substrates can be modulated by low level laser light even in an aqueous milieu, and imply the subaquatic persistence of an ordered interlayer formed by molecules with less degrees of freedom, as compared to those in the bulk liquid. Low level laser light has already been shown to change the height of nanoscopic water layers on substrates in air. Nanoscopic water layers are not only important in determining the arrangement of nanoparticles into two-dimensional crystals, they also control the lifetime of scanning near-field optical microscopy sensors, adhesion processes on biochips, nonspecific interactions at biomaterial/cell interfaces, charge transfer between ice crystals and graupel pellets in thunderclouds, and presumably the attachment of nanobacteria and proteins to tissues in blood. Modalities by which laser light and nanoscopic water layers cooperatively control the attachment of nanoparticles to substrates are illustrated in representative scenarios. Results could be converted into various practical applications, especially in nanotechnology and biomedicine, and unlock a novel and promising field of research. Introduction We owe much of the present knowledge on the nature of nanoscopic water layers to sensitive imaging techniques, in particular atomic force microscopy (AFM) and near-field optical analysis (NOA) performed by nearfield scanning optical microscopy (NSOM). Freund et al.1 have shown that nanoscopic water layers coating surfaces play a critical role in imaging techniques. In our earlier work, we have shown how NOA, carried out in air by a NSOM equipped with hydrophobic biosensors2 operated in the shear force mode, exposed the presence of nanoscopic water layers on both hydrophilic and hydrophobic substrates. In AFM (and NOA), the performance of shear force mode systems depends essentially on sensing density variations and viscosity gradients at the air-water interface: As we have reported in previous work, increasing the relative humidity in the lab regularly resulted in a longer lifetime of the fragile biosensors,3 presumably due to an increase of the height of the water layer attached to the substrates. The thicker this water layer, the larger the distance between the scanning sensor and the substrate. However, distances exceeding about 10 nm are often sufficient to completely impede NSOM. The NSOM principle, whose viability was inspired by Kuhn,4 exploits laser-induced energy transfer from an optical sensor (tip diameter g 20 nm), oscillating parallel to the sample plane, within the near-field (∼10 nm) of the surface to be analyzed. Thus, reducing the distance between the sensor tip and the sample can facilitate NSOM. One way to minimize the separation between * To whom correspondence should be addressed. E-mail: samoan@ gmx.net.
the sensor and the sample is to modulate the height of the water layer above the sample. Modulation can be done by the use of laser light. Importantly, low level laser irradiation induced a transient depletion of nanoscopic water layers masking substrates. The effect has been first visualized in our lab by irradiating a translucent polymer film with a laser beam (670 nm, 1000 W m-2).5 The film was 90 µm thick and had a total surface roughness of 4 nm, facilitating discrimination of minimal height variations. Both the film topography and the intensity of the laser light passing it were simultaneously imaged by AFM and NSOM, respectively. The NSOM, originally designed for imaging optical contrast on the nanoscale, served here as an instrument allowing us to validate the synchronization between the variations in the AFM scan and those in the complementary NOA image. Light modulation showed a correlation between the depletion of the water layer and an increase in light intensity.5 Previously, we have determined that laser light of an intensity around 1000 W m-2 can have beneficial effects on a representative body of biosystems, in particular in stressed conditions.6,7 The prototype employed in our lab (Figure 1) permits us to alleviate stress in living cells in extended NSOM examinations with intensities and doses of light shown to elevate cell vitality levels and to reduce the height of nanoscopic water layers attached to transparent substrates in air.8 In previous experiments, we have recognized structural differences observed in macroscopic rings formed by drops of nanosuspensions evaporating on different substrate materials as preliminary indicators for the persistence of nanoscopic water layers in a subaquatic environment and their action influencing the assembly
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Figure 1. Left: Heart of conventional NSOM (DualScope, DME/Denmark) with mounted 670 nm medical laser (Lasotronic/ Switzerland). The setup permits us to elevate the survival level of living cells in NOA, and to modulate and simultaneously measure height variations of water films on surfaces in air. Translucent samples can be irradiated from below via a 670 nm laser (0.8 mW), implemented in the instrument platform. Right: Brilliant light spot at the sensor tip, reflected by a mirror-polished titanium substrate used in NOA to image living cells. The use of the titanium substrates excluded irradiation from below.5,8
of 60 nm nanospheres on substrates.9 If indeed nanoscopic water layers affected self-organization patterns produced by nanoparticles deposited from drops onto substrates, and low level laser light modulated nanoscopic water layers on substrates, then it appears now plausible that low level laser light will also affect the attachment of nanoparticles to substrates in a wet milieu. Materials and Methods To demonstrate the effect of low level laser light on the attachment of nanoparticles to substrates, six drops of an aqueous suspension containing 60 nm polystyrene nanospheres (Duke Scientific, Palo Alto, CA) of a volume of 15 µL ((1 µL) each have been placed successively on a sterile polystyrene Petri dish (side not specified for cell culture). The dish was positioned on the NSOM platform shown in Figure 1, which was equipped with a 670 nm laser (beam Ø ∼ 1 mm, intensity ∼ 1000 W m-2) and closed to slow the process of evaporation. The orientation of the laser beam was normal to the plane of the dish. The setup allowed us to irradiate exclusively the center of one drop from below, until its complete evaporation (Figure 2a,b), a process taking at room temperature up to 16 h. In previous work, we have illustrated that by slowing down the process of evaporation, i.e., by using closed and sealed Petri dishes, symmetrical rings of an extreme reproducibility could be realized. Such rings were superior in both circularity and crystallinity to the patterns that we have obtained in open Petri dishes.10 In the present experiment, slow evaporation was performed in closed Petri dishes with a diameter of 35 mm and a height of 9 mm, giving a volume of 9 mL. The total volume of the six nanosuspension drops was 0.09 mL. Exemplarily for the situation presented in Figure 2a, a quick calculation shows that at standard conditions (room temperature, pressure ) 105 Pa) the volume of the Petri dish was saturated with water vapor. Treating water vapor as an ideal gas, we can use the ideal gas equation to estimate the volume which water vapor would take in the case that the total mass of the six drops would exist as vapor. We obtain 120 mLsa value exceeding the volume of the Petri dish by far. It now becomes evident that the rate of evaporation must depend on the leakage from the Petri dish. The result is of paramount relevance for the use of drops of nanosuspensions to design rings in general and microrings in particular (which form
Figure 2. (a) Photography of drops of an aqueous suspension containing 60 nm nanospheres and of rings formed by them. The drops were placed successively on the hydrophobic side of a (sterile) Petri dish and allowed to evaporate together in the closed dish. Only the drop at the center was irradiated with low level laser light (from below) and formed, as can be clearly seen in the image on the right, the largest ring. (b) Light microscopy images of rings shown in panel a. The crystalline structure can be verified via crossed polarizers11 or by AFM.12 Left: Representative segment of the ring formed from the laser-exposed drop. Right: Representative structure of one collateral ring, formed from a nonirradiated drop. The bond between the larger ring and the substrate was more stable than between the smaller rings and the substrate. The images were taken at same the magnification. almost instantaneously in air) and helps us to understand the reason for variations in deposition patterns formed from drops of aqueous nanosuspensions evaporating in Petri dishes with different leakage rates. A better approximation of the vapor volume implies solving the Clausius Clapeyron equation, with integration of an extra term, accounting for the vapor pressure due to the curvature of the drops. As can be clearly seen in the representative image in Figure 2a, the largest (and most circular) ring formed concentric to the laser-irradiated site. Remarkably, the laser light did not accelerate the process of
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Figure 3. (a) Drops of the nanosuspension evaporating simultaneously on Thermanox. Evaporation time, ∼ 90 min. The drop at the center was continuously irradiated. Evaporation occurred under partial coverage by a 35 mm Petri dish, placed on the sample to protect the area with the drops. Laser light had no effect on the ring geometry. (b) Drops of pure water evaporating together in air on Thermanox. The drop at the center was continuously irradiated. (c) Drops of pure water evaporating together in air on a Petri dish. The drop at the center was continuously irradiated. evaporation but retarded it, as can be verified in detail in Figure 3a, showing four drops of the nanosuspension of a volume of 15 µL ((1 µL) each, evaporating on a translucent Thermanox cover slip (Nunc, Naperville, IL), under a 35 mm Petri dish. In the experiments reported here, the drop selected for irradiation was always placed first onto the substrate. The retard effect was reproduced with a number of equal-sized drops of an aqueous suspension containing hydroxyapatite nanoparticles of the same mean size, drops of a physiological salt solution, and drops of pure water evaporating under identical conditions on both substrates, Thermanox (Figure 3b) and the Petri dish (Figure 3c).
Results Obviously, irradiation with low level laser light can alter the geometry of rings formed by aqueous nanosuspensions. A closer inspection of the proportions shown in Figure 2a reveals that in all cases the ring diameters are significantly below the initial drop diameters. Thus, the peripheral anchoring of suspended nanospheres to the substrate was less pronounced here than on substrate materials with a high surface energy and a thick water layer, e.g., on titanium, however, still better than on Teflon or polyethylene. The prevalence of a substantial water layer on titanium was described by Freund et al.1 In numerous cases, the final ring diameter corresponded exactly to the initial drop size. This special case is documented in Figure 3a for Thermanox and more precisely in Figure 4 for mirrorpolished titanium. Comparison of the situation presented in Figures 2a and 3a provides evidence that
Figure 4. Left: Drops of an aqueous suspension containing 60 nm polystyrene nanospheres on a mirror-polished titanium disk (Ø ) 16 mm).3 Right: Associated ring patterns formed via slow evaporation in a closed, 35 mm Petri dish. The quartz fiber (Ø ) 125 µm) in the middle (arrow) helps to visualize modifications. Photographs confirm that the initial drop and final ring diameter are practically equal.
although the laser affected the evaporation time of drops on both substrate materials Thermanox and polystyrene (Petri dish), it had no noticeable effect on the arrangement of the suspended nanospheres on Thermanox. Apparently, the laser light did not modify the ring architecture on Thermanox simply because here the final ring diameter and the initial drop diameter coincided continuously. Discussion In view of the luminosity of the irradiated drop presented in Figure 2a, the aforementioned findings
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could indicate that laser light, scattered by the particles suspended in the drop, has depleted the water film separating the nanospheres aligned at the drop margin from the substrate beneath. Thinner peripheral water films could result in a more intimate contact between nanospheres and substrate and encourage attraction between them (pinning). This picture seems to justify the relatively large ring diameter for the irradiated drop displayed in Figure 2a. However, the component of the reflected/scattered laser light appears to be definitely too small to cause such a massive effect, because of the small laser power of ∼ 0.8 mW. Based on our study,5 the most plausible assumption is that the applied laser irradiation has modified the viscosity profile of the nanoscopic water layer consisting here of a number of molecules loosely attached to the substrate. Fluctuations induced by the resonant laser energy in one fraction of the ordered water molecules, partly immobilized in the near-field of the substrate, have probably changed the order profile and thus the rheological properties of this layer. The assumed energy transfer seems reasonable because the subaquatic water layers cannot simply escape into the air. Depending on the materials employed and possible spatial constraints, a correlated reduction in viscosity, in the film masking the substrate, could have a dramatic impact on both the transport and the mobility of the nanoparticles parallel to the surface of the substrate. Consequently, nanoparticles landing on the laser irradiated site of the substrate would experience a difference in damping, while migrating with the evaporative flow sideward, compared to those facing an uninterrupted nanoscopic water layer. The possibility that the laser light interacts with the interfacial water mask at the solid-liquid interface could be motivated by an orientational effect in water at hydrophobic surfaces, found by Scatena et al.,13 implying the coexistence of ordered (interfacial) and unordered (bulk) water. The effect of the laser irradiation has been indirectly verified by us by blocking the light path between the platform holding the laser and substrates. In all of these cases, the prominent difference in size between the drop at the center and the collateral drops, observed in the final phase of the evaporation, disappeared. It is instructive to review the principal mechanism by which drops of evaporating suspensions form rings on substrates in general. A few groups, principally Deegan et al.,14,15 have investigated the dynamics of ring formation in evaporating sessile drops. When rings are formed, the sedimenting particles are first immobilized along a stationary (or temporarily stationary) airliquid-solid contact line. As we have illustrated in a simplified extension of existing models, drops of nanosuspensions tend to produce rings when the action of the evaporative flux, carrying suspended material to the line coinciding with the edge of the drop, exceeds the (attractive) interaction between the particles and the substrate.16 In recent work, we have indicated that thinner and potentially less viscous nanoscopic water masks could shift the balance between the major competing factors determining pattern formation in drops of evaporating nanosuspensions. Depending on the nature of the interaction between the nanoparticles and the substrate,9 the shift can be favorable or unfa-
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vorable for the transport of the suspended nanoparticles by the convective flux. This concept now permits us to explain the retard effect documented in Figure 3a-c. The delayed evaporation in the laser-irradiated drops seems to be an indication of the absence of an interlayer of organized water molecules, exposing its apparent normal function to reduce dissipative effects in the liquid flow parallel to the surface of the substrate. Controlling the self-organization of nanoparticles nondestructively on wet substrates via moderate laser light intensities (by targeting the interlayer) seems promising for the design of 2D crystals. Helseth and Fisher17 demonstrated that by the use of colloidal particles susceptible to magnetic fields, it was possible to address the particles directly, a possibility to change the fine and coarse structure of 2D crystals formed by them on substrates. Because evaporation is slower in drops irradiated with low level laser light, nanoparticles could have more time to arrange into rings, eventually facilitating the production of textures with a higher degree of crystallinity, without a necessity to reduce the temperature of the carrier liquid. That slow evaporation encourages the formation of perfect ring patterns with highly crystalline structures has already been demonstrated by us for binary nanosuspensions.12 Shmuylovich et al.18 have created imperfect ring patterns from diluted microparticle suspensions and provided valuable data of the first formation phase of the 2D crystals. The picture of the interplay between light and water layers, stimulated by the present data, could be employed to interpret electrification phenomena in clouds. The hydrometeors known to exchange charge in clouds are ascending ice crystals and falling graupel pellets. Nanoscale water layers attached to ice crystals represent the primary ionic charge reservoirs, essential for atmospheric electrification processes. Nanoscopic water layers on ice were measured quantitatively by Do¨ppenschmidt and Butt via AFM.19 Charge is preferably transferred in the lower parts of the clouds, in a main charging zone at -15 °C, when a high curvature surface (ice crystal) is in the near-field of a low curvature surface (graupel pellet). The curvature asymmetry paradigm was proposed in our earlier work.20 According to Nelson and Baker,21 it also plays a key role in collisional charge exchange. Notably, the intensity of the laser light, found by us to reversibly modulate the height of the water layers deposited from air onto translucent polymer films, was virtually equivalent to the integral value of the solar constant.5 Assuming that to some extent the nonresonant sun light will deplete nanoscopic water layers coating ice crystal dendrites, it appears now reasonable to discriminate between a short and an extended exposure of the ascending ice crystals to solar light. While short exposures will deplete the ionic charge reservoirs, melting, induced by extended exposures, will increase them, however, at the expense of the dendrite curvature. Both effects may contribute to less thunderstorm charging, i.e., in the sun-exposed side of the clouds, depending on cloud temperature and altitudes a novel detail, possibly enriching the traditional charge transfer paradigm based on parameters including number and geometry of ice crystals and graupel, collision
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probability, cloud liquid water content, and number of solid aerosols. Conclusions The impact of low level laser light on nanoscale water layers seems clear. The presented effects could be useful for advancing several practical fields, including nanopatterning, bioimaging, and prediction of atmospheric charge transfer phenomena. The selection of suitable nanoparticle-substrate combinations and the synergistic use of both methods, laser irradiation and magnetic fields, may help to assemble new colloidal crystals with unexpected properties. Biomedical applications may imply the use of laser light to locally control the attachment of biomolecules to substrates, and as suggested by the models established in our earlier study, for noninvasive inactivation of living nanovesicles in the body by both their possible transient desiccation22 and temporal destabilization of the adhesive bond stemming from interfacial water layers promoting their attachment to tissues. References (1) Freund, J.; Halbritter, J.; Ho¨rber, J. K. H. Microsc. Res. Tech. 1999, 44, 327. (2) Sommer, A. P.; Franke, R. P. Micron 2002, 33, 227. (3) Sommer, A. P.; Franke, R. P. J. Proteome Res. 2002, 1, 111. (4) Kuhn, H. J. Chem. Phys. 1970, 53, 101.
Crystal Growth & Design, Vol. 5, No. 2, 2005 563 (5) Sommer, A. P.; Franke, R. P. Nano Lett. 2003, 3, 19. (6) Sommer, A. P.; Oron, U.; Kajander, E. O.; Mester, A. R. J. Proteome Res. 2002, 1, 475. (7) Sommer, A. P.; Pinheiro, A. L. B.; Mester, A. R.; Franke, R. P.; Whelan, H. T. J. Clin. Laser Med. Surg. 2001, 19, 29. (8) Sommer, A. P. Proceedings of the 2nd International Conference on Near-Field Optical Analysis: Photodynamic Therapy & Photobiology Effects, May 2001, Houston, TX; NASA Publication CP-2002-210786; Johnson Space Center, Houston, TX, 2002; pp 78-83. (9) Sommer, A. P. J. Phys. Chem. B 2004, 108, 8096. (10) Sommer, A. P.; Franke, R. P. Nano Lett. 2003, 3, 573. (11) Sommer, A. P.; Franke, R. P. Nano Lett. 2003, 3, 321. (12) Sommer, A. P.; Ben-Moshe, M.; Magdassi, S. J. Phys. Chem. B 2004, 108, 8. (13) Scatena, L. F.; Brown, M. G.; Richmond, G. L. Science 2001, 292, 908. (14) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (15) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62, 756. (16) Sommer, A. P. J. Proteome Res. In press. (17) Helseth, L. E.; Fischer, T. M. Phys. Rev. E 2003, 68, 042601 (18) Shmuylovich, L.; Shen, A. Q.; Stone, H. A. Langmuir 2002, 18, 3441. (19) Do¨ppenschmidt, A.; Butt, H. J. Langmuir 2000, 16, 6709. (20) Sommer, A. P.; Levin, Z. Atmos. Res. 2001, 58, 129. (21) Nelson, J.; Baker, M. Atmos. Chem. Phys. 2003, 3, 1237. (22) Sommer, A. P. J. Proteome Res. 2004, 3, 670.
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