Nanospheres Promote the Storage ofPerfluorocarbons in Water: CouldNanoscale Aerosols Reduce OzoneKiller Concentrations in StratosphericClouds?Andrei P. Sommer* and Ralf-Peter Franke

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

Received December 12, 2002

ABSTRACT

In previous experiments designed to mimic cell-receptor structures on biomaterial surfaces, we demonstrated that the evaporation of water-based suspensions containing polystyrene nanospheres created translucent nanostructured films enclosed by highly regular ring-shapedpatterns. Here we show that similar nanospheres promote partial miscibility between liquids regarded as completely immiscibleswater andperfluorocarbons. The result could indicate that solid nanoscopic aerosols from urban and industrial sources may ultimately be instrumentalin reducing the concentration of ozone killers in the stratosphere.

In previous experiments,1 we analyzed deposition patternsformed by the evaporation of 10-µL droplets of a water-based nanosphere suspension on different materials: tita-nium, stainless steel, silicone (wafer quality), glass, andpolystyrene (Petri dishes). The polystyrene nanospheres(Duke Scientific) had diameters of 60( 2.5 nm. Patternformation depended mainly on the dynamical interaction ofthree basic parameters: interfacial tension between thedroplet and the substrate, speed of evaporation, and surfaceroughness of the substrate. In principle, the patterns allowus to mimic both micro- and nanostructures of cells onbiomaterials. Extremely regular ring patterns formed onmechanochemically polished titanium disks placed in closedPetri dishes to reduce the speed of evaporation.1 The physicalmodel applied to interpret the observed patterns indicatedthat a high interfacial tension between the substrate and theliquid had a favorable effect on the ring symmetry.1 For fast-evaporating nanosphere suspensions with low interfacialtension relative to the substrates, the model predicted aconventional sedimentation of the nanospheres (no ringformation). To test this prediction, we explored ways toproduce nanosphere suspensions based on perfluorocarbons(PFCs). The principal reason motivating the choice of thesevolatile liquids was, however, their extreme chemical inert-ness: PFCs are practically immiscible with water andevaporate from the majority of solid surfaces without leaving

an observable trace behind. The speed of evaporationdepended in particular on the chemical nature of thesubstrate: the effect could be illustrated in preliminaryexperiments by measuring the evaporation time of 10-µLdroplets of perfluorodecalin (density≈ 1.9 kgl-1) on variousmaterials at room temperaturestitanium, 30 min; stainlesssteel, 18 min; polycarbonate and polystyrene, 17 min; andcybernox, 13 min. Cybernox (Sitram/France) is a quasi-crystalline material used for coating frying pans. Titaniumhad the highest interfacial tension relative to perfluorodecalin(and relative to water1).

The interplay between (1) the difference in density, (2)the complete immiscibility of the liquids, and (3) the inertnessof perfluorodecalin, resulting in the stability of the interfacialbarrier between the liquid phases, has been exploited inlaboratory experiments to demonstrate self-assembly phe-nomena at perfluorodecalin/water interfaces using millimeter-scale objects.2-4 Extended interfacial contacts (24 h) betweenthe opaque water-based nanosphere suspension and the clearperfluorodecalin did not create any increase in turbidity inthe transparent phase. Similarly, mixing the phases by handin a closed syringe did not change the translucency of thelower, high-density phase. Figure 1 shows the turbid nano-suspension column on top of the clear perfluorodecalin phase.

According to its noble nature, perfluorodecalin does notvoluntarily incorporate the nanospheres. Thus, producingnanosphere suspensions based on perfluorodecalin in equiva-* Corresponding author. E-mail: samoan@gmx.net.

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lent concentration to that of the aqueous suspension byinterfacial crossing between the water-based nanospheresuspension and perfluorodecalin is in no way trivial. Visibleopacity could be realized in the clear phase by placing theliquids coexisting in a hermetically closed syringe for 20min into an ultrasound bath operating at 20°C. The liquidphases were allowed to rest in interfacial contact understationary conditions for 16 h in the closed syringe arrangedvertically and were then consecutively pushed out, separated,and extracted via a pipet. Since the opacity of the previouslyclear phase remained stable, it was reasonable to assume amoderate accumulation of the nanospheres in it. This couldbe virtually verified, for example, by investigating possiblesedimentations from the evaporation of droplets, extractedfrom this phase, on 16-mm titanium disks positioned insealed Petri dishes (Ø 35 mm). The mechanochemicallypolished titanium disks designed and employed for opticalcontrast enhancement in near-field optical analysis (NOA)5

had a total surface roughness of<4 nm.The evaporation of 10-µL droplets of the opaque per-

fluorodecalin phase on the titanium disks resulted in ahomogeneous sedimentation of nanospheres, nanosphereaggregates, and presumably water droplets encapsulated bynanospheres6 (Figure 2). The evaporation of 10-µL dropletsextracted from the original suspension, from which nano-spheres have probably passed into the perfluorodecalin,produced on the titanium disks circular nanosphere clayarchitectures analogous to the reported rings,1 however, withmassive microstructured patterning in the field enclosed bythe ring (Figure 3a). Examinations of the nanostructured filmin the center of the rings formed by the evaporation of purewater-based nanosphere suspensions,1 reproduced for clarityin Figure 3b, and of the structures shown in Figure 3a revealdifferences that could be traced back to significant quantitiesof perfluorodecalin stored in the water-based suspension!

Most likely, the storage is realized in the form of ananoemulsion in the aqueous suspension,6 stabilized as aminority phase of perfluorodecalin droplets encapsulated byequiradial nanospheres self-assembled in the transient inter-face zones.2-4 Figure 4 (crossed polarizers view) showingone optical microscopy photograph of the objects presentedin Figure 3 exposed the double refractive organization inthe larger and the dried amorphous nanosphere clay in thesmaller ring, as could be expected for the assembly ofidentical and unlike components, respectively.

The physicochemical mechanism forming the ring patternfrom intrinsic water-based nanosphere suspensions and thetranslucent nanostructured film enclosed by the ring, whichwas revealed by atomic force microscopy (AFM), have beenanalyzed in theory and from the experimental side in detail.1

In general, the circularity of the rings formed by theevaporation of water-based nanosphere suspension dropletsincreased with decreasing drop size. Droplets evaporatingon cybernox had a tendency to form oval-shaped depositionpatterns of nanospheres, as shown exemplarily for a puresuspension in Figure 5. Most importantly, the area wettedby droplets of the nanosphere suspensions did not contractduring the evaporation, as known for simple liquids. Thishas a practical consequence: the final ring diameter couldbe predetermined by the initial droplet size. In addition,nanosphere suspension droplets evaporated significantlyfaster than water droplets of the same volume: 10-µLdroplets placed on a polished titanium disk positioned onthe bottom of a closed and sealed Petri dish (Ø 35 mm)needed 385 min for total evaporation at 20°C for thesuspension and 660 min for water. Since pattern formationneeds time, the different time scales might explain the lackingring structure in the case of the evaporation of the per-fluorodecalin suspension.

Encouraged by the permanence of the structures betweenthe aqueous, nanosphere-rich phase and perfluorodecalinachieved via ultrasound-induced interfacial crossing stimu-lated us simply to shake the incompatible phases coexistingin a syringe by hand and to repeat the routine describedabove. Indeed, manual shaking induced manifest structure

Figure 1. Milky nanosphere suspension in the bottle on the leftand clear perfluorodecalin in the bottle on the right. The syringecontains approximately equal volumes of both liquids.

Figure 2. Uniform sedimentation of nanospheres, nanosphereaggregates, and presumably water droplets encapsulated by self-assembled nanospheres on a titanium disk. Light microscopypicture. Bar) 0.2 mm.

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formations on both sides: Figure 6, showing a representativedeposition pattern generated by the evaporation of a dropletof the nanosphere-rich suspension on a titanium disk,revealed a pattern with evident structural modificationsresembling the ultrasound-treated sample shown in Figure3a. The chemical nature of the components and the structuralchanges induced in the liquids by manual shaking, indirectlyproved by comparison of the microstructures shown in Figure6 with the structures stemming from the evaporation of thepure suspension (Figure 3b), provide a model for the micro-physical processes related to the depletion of the earth’sozone layer.

Evaporation-controlled pattern transfer has been alreadydiscussed as a possible mechanism by which nanoscopicaerosols are probably collected and deposited onto the surfaceof ice particles in clouds.1 Nanostructures on ice crystalsurfaces are expected to have a considerable impact onatmospheric charge-transfer processes occurring during col-lisions of ice particles in thunderclouds, influencing both themagnitude and the sign of the electrical charge transferred.7,8

Solid aerosols from urban and industrial sources (e.g., petroland diesel particles) have diameters on the order of that ofthe nanospheres. Representing cloud condensation nuclei(CCN), such solid nanoscopic aerosols could reach the

Figure 3. Patterns formed by the evaporation of nanosphere suspensions on titanium, visualized by optical microscopy: (a) Massivesedimentation in the zone enclosed by the ring, stemming from the ultrasound-facilitated transient incorporation of perfluorodecalin in theaqueous suspension and the formation of a stable nanoemulsion. Ring≈ 5.1 mm. (b) Representative deposition pattern with nanostructuredfilm enclosed by the ring, formed by the evaporation of a pure nanosphere suspension on titanium. Ring≈ 5.5 mm.

Figure 4. Light microscopy via crossed polarizers exposing double-refractive structures revealed partial crystallinity in the larger andamorphous material in the smaller ring presented in Figure 3.

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stratospheric cloud zones within supercooled water dropletsor in the water films coating the ice crystals.9 This is theaction zone of the chlorofluorocarbons (CFCs)sinert liquidswith physicochemical properties similar to the PFCs andthought to be primarily responsible for the destruction ofthe earth’s protective ozone layer.10-13 Our observationsindicate that by analogy to the way perfluorodecalin istransferred and stored in water, inert ozone killers may becollected in a turbulent stratospheric environment by aerosolcarrying supercooled cloud drops or ice crystals. Intensethunderstorm activity could stimulate the incorporation ofCFCs into hydrometeors, which may ultimately reduce theconcentration of the ozone killers causing the Antarctic ozonehole by returning their load to earth as nondestructiveprecipitation. The large reduction in surface solar radiation14

and rain suppression15 are the known environmental factorsrelated to aerosols. Here, we enrich the physical picture,introducing a novel paradigm.

In regard to the symmetry of the representative patternon titanium (Figure 3), the adjustability of the ring size tothe dimension of individual cells and of the nanostructureof the film enclosed by the ring to cell-integrative structures,it is expected that the finding described here could stimulatethe design of practical production strategies for scaffolds withboth micro- and nanostructure.1 Topographies with suchsubstructures are known to elevate the biocompatibility ofbiomaterials. The controlled evaporation of PFC-based

nanosphere suspensions, for example, carried out underwater, could open promising high throughput nanopatterningand biofunctionalization techniques and various technologi-cally utilizable possibilities.

References

(1) Sommer, A. P.; Franke, R. P.Nano Lett., in press.(2) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M.Science

(Washington, D.C.)1997, 276, 233-235.(3) Whitesiders, G. M.; Grzybowski, B.Science (Washington, D.C.)2002,

295, 2418-2421.(4) Breen, T. L.; Tien, J.; Oliver, S. R. J.; Hadzic, T.; Whitesides, G.

M. Science (Washington, D.C.)1999, 284, 948-951.(5) Sommer, A. P.; Franke, R. P.J. Proteome Res. 2002, 1, 111-114.(6) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.;

Bausch, A. R.; Weitz, D. A.Science (Washington, D.C.)2002, 298,1006-1009.

(7) Sommer, A. P.; Levin, Z.Atmos. Res. 2001, 58, 129-139.(8) Sommer, A. P.Langmuir2002, 18, 5040-5042.(9) Sommer, A. P.; Franke, R. P.Nano Lett.2003, 3, 19-20.

(10) Wuebbles, D. J.; Calm, J. M.Science (Washington, D.C.)1997, 278,1090-1091.

(11) Reilly, J. et al.Nature (London)1999, 401, 549-555.(12) Kerr, R. A.Science (Washington, D.C.)2002, 297, 1623-1625.(13) Crutzen, P. J.Nature (London)2002, 415, 23.(14) Lelieveld, J. et al.Science (Washington, D.C.)2002, 298, 794-799.(15) Rosenfeld, D.Science (Washington, D.C.)2000, 287, 1793-1796.

NL025940R

Figure 5. Representative deposition pattern with an enclosednanostructured film (rainbow field in oval ring) formed by theevaporation of a pure nanosphere suspension on cybernox. MeanØ ≈ 6.2 mm.

Figure 6. Pattern formed on a titanium disk by the evaporation ofa nanosphere suspension contaminated with droplets of perfluoro-decalin encapsulated by nanospheres, visualized by optical micros-copy. In contrast to the smaller ring shown in Figure 3a, theintegration of perfluorodecalin was achieved via manual shaking.Ring ≈ 5.5 mm.

324 Nano Lett., Vol. 3, No. 3, 2003