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Reversible Pattern Formation through Photolysis Timothy R. Kline and Ayusman Sen* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed April 28, 2006. In Final Form: July 7, 2006 We report a photolytic method to induce spatial and temporal patterning/deposition of particles at the micron scale on a time scale of seconds. Reversible pattern formation by negatively charged particles occur around micron-sized silver features on different substrates when exposed to UV light in the presence of aqueous hydrogen peroxide. Diffusiophoretic motion due to a spatially defined ion gradient accounts for our observations. Atomic force and optical microscopy, as well as conductivity measurements, support this hypothesis.
Directing fluid flow and pattern formation at the micro/ nanoscale remains an important challenge in nanotechnology. Flow driven by externally applied pressure, electric field, and thermal and concentration gradients have been reported.1-5 Photoactive organic molecules and inorganic surfaces have also been employed in generating patterns.6-9 A potential alternative is photoinduced generation of ion gradient that results in mechanical motion. We and others have recently described catalytically induced motion at the nano/microscale, some which involves ion gradients, in nonbiological systems.10-15 However, although spatial control has been achieved over fluid movement and pattern formation through such a process, the lack of an “on/off” switch prevented reversible temporal control. Here we report the achievement of both spatial and temporal control over movement and patterning at the micron scale using photolytically generated ion gradients. The patterning appears to arise from the formation of silver (Ag+) and hydroperoxide (OOH-) ions by photolytic decomposition of hydrogen peroxide (H2O2) at a silver feature. The difference in diffusion coefficients between these two ions establishes a diffusion-induced electric field that is responsible for the observed duffusiophoretic motion. This mechanism differs from the previously reported electrokonetically driven flow that requires the presence of two different metals in contact and that occurs in the absence of light.15 * To whom correspondence should be addressed. E-mail: asen@ chem.psu.edu. (1) Van Lintel, H. T. G.; Van De Pol, F. C. M.; Bouwstra, S. A. Sens. Actuators 1988, 15, 153-67. (2) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abott, N. L. Science 1999, 283, 57-60. (3) Dawn, E.; Troian, K.; Troian, S. Nature 1999, 402, 794-7. (4) Anderson, J. L. Ann. ReV. Fluid Mech. 1989, 21, 61-99. (5) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Nature 2003, 299, 371-4. (6) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46-49. (7) Zhao, Y.; Tong, X. AdV. Mater. 2003, 15, 1431-5. (8) Tanifuji, N.; Irie, M.; Matsuda, K. J. Am. Chem. Soc. 2005, 127, 1334453. (9) de Jong, J. J. D.; Hania, P. R.; Pugzlys, A.; Lucas, L. N.; de Loos, M.; Kellogg, R. M.; Feringa, B. L.; Duppen, K.; van Esch, J. H. Angew. Chem. Int. Ed. 2005, 44, 2373-6. (10) Ismagilov, R. F.; Schwartz, A.; Bowden, N.; Whitesides, G. M. Angew. Chem. Int. Ed. 1999, 41, 652-4. (11) Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St. Angelo, S. K.; Cao, Y.; Mallouk, T. E.; Lammert, P. E.; Crespi, V. H. J. Am. Chem. Soc. 2004, 126, 13242-31. (12) Kline, T. R.; Paxton, W. F.; Mallouk, T. E.; Sen, A. Angew. Chem. Int. Ed. 2005, 44, 744-6. (13) Fournier-Bidoz, S.; Arsenault, A. C.; Manners, I.; Ozin, G. A. Chem. Comm. 2004, 441-3. (14) Vicario, J.; Eelkema, R.; Browne, W. R.; Meetsma, A.; La Crois, R. M.; Feringa, B. L. Chem. Comm. 2005, 3936-8. (15) Kline, T. R.; Paxton, W.; Wang, Y.; Velegol, D.; Mallouk, T. E. and Sen, A. J. Am. Chem. Soc 2005, 127, 17150-1
We employed conventional microfabrication using a bi-layer resist (lift-off resist, LOR5A, from Microchem and a positive resist, 3012, from Shipley) to design arrays of silver disks (2080 µm in diameter) with a chromium adhesion layer deposited via thermal deposition onto a thermally grown silicon dioxide on a silicon wafer. Upon UV radiation (mercury lamp), the formation of dynamic patterns around the silver disks on a time scale of seconds was observed (video I in the Supporting Information) in an assembled fluid cell consisting of negatively charged tracers (either silica or negatively charged polystyrene) in dilute aqueous solutions of H2O2 (0.5% w/w). On the other hand, positively charged tracers (e.g. amidine functionalized polystyrene tracers) moved toward the silver disk (8 µm/s) and deposit under the same conditions (video II in the Supporting Information). Previous reports of electrophoretic patterning from externally applied fields were on the time scale of hours.16 As soon as the UV light is turned off by closing the aperture, the pattern relaxes or particles stop moving toward the silver disk. In Figure 1a, we show silica tracers (2.3 µm in diameter) located in the vicinity of the silver disk before opening the aperture and then another snapshot after opening the aperture showing that the colloid particles move approximately 10 microns away from the silver disk (Figure 1b). Furthermore, as shown in Figure 1, panels c and d, patterning is quenched by placing a yellow filter absorbing light below 500 nm in the light path. A more complete representation of the pattern formation with and without a filter in place is obtained by tracking the trajectories of the tracers in terms of their distance from the edge of the silver disk (Figure 1). At the bottom of Figure 1, we show a single particle trajectory as a function of time compiled from the average of eight particles surrounding the silver disk with and without a UV absorbing filter in place. In the presence of UV radiation, the particles move away from the silver disk at a speed of 0.6 µm/s and relax back at 0.4 µm/s when the aperture is closed. In the presence of a yellow filter, nothing happens regardless of the state of the aperture. Curiously, the observed pattern formation is associated with a significant isotope effect (kH/kD): the use of D2O2 + D2O in place of H2O2 + H2O effectively quenches pattern formation (video III in the Supporting Information). The use of a filter does not rule out thermal effects as all filters absorb some infrared irradiation based on their design. We used a thermopile to measure the intensity of light being delivered to the surface from different light sources and filters in place. Unfiltered light with an open aperture was 30 times more intense than the same light with a closed aperture. The yellow filter with an open aperture was 1 order of magnitude lower than unfiltered (16) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56-9.
10.1021/la061165+ CCC: $33.50 © 2006 American Chemical Society Published on Web 07/20/2006
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Figure 1. (top) UV light-induced reversible pattern formation in the presence of a spatially defined silver catalyst (Ag) on silicon dioxide (a) before and (b) after opening the aperture. (c) The absence of patterning when a UV absorbing filter is placed in the light path before and (d) after opening the aperture. Scale bar represents 40 µm. (bottom) Trajectory of tracers around silver disk without (solid line) and with (dashed line) a UV absorbing yellow filter in place. No filter results in dynamic patterning in the presence of an open aperture (UV on) which relaxes upon closing the aperture (UV off), whereas in the presence of a filter the tracers are indifferent to UV irradiation.
light also with an open aperture. Unfiltered light from a mercury lamp did not induce pattern formation in the absence of H2O2, ruling out thermal effects as responsible for pattern formation and positive particle deposition. In the presence of H2O2, a halogen lamp was ineffective for pattern formation whereas a mercury lamp with a 360 nm long-pass filter and with similar intensity (∼6 W/cm2) induced pattern formation. On the other hand, patterning was not observed when the 360 nm long-pass filter was replaced by a 420 nm long-pass filter. Hydrogen peroxide has been reported to decompose in ultraviolet light, a process that does not occur for light of wavelength longer than 365 nm.17 We have confirmed that aqueous solutions of H2O2 do not absorb at wavelengths longer than 360 nm. Pattern formation requires H2O2, UV light, and silver. Silver is a known catalyst for the decomposition of H2O2 to water and oxygen (O2) in the absence of light;18 however, this reaction does not appear to be responsible for pattern formation. Instead, the UV light initiates an independent reaction cascade that eventually leads to an ion gradient and accompanying pattern formation. The photochemical decomposition of H2O2 by UV radiation is well documented.17,19 The steps postulated are shown in eqs 1 and 2. The hydroperoxy radical is a strong oxidizing agent and can readily oxidize metallic silver in accordance with (17) Baxendale, J. H.; Wilson, J. A. Trans. Faraday Soc. 1957, 53, 344-56. (18) Maggs, F. T.; Sutton, D. Trans. Faraday Soc. 1959, 55, 974-80. (19) Smith, P. J. Phys. Chem. 1958, 62, 120-2.
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Figure 2. (top) Conductivity measurements involving 30 mL aqueous hydrogen peroxide (0.5% w/w), and Ag under different conditions including irradiation by a halogen lamp (halogen) rather than UV lamp, without Ag, and UV irradiation of water only. (bottom) AFM images (before UV/H2O2) of silver on SiO2 (∼200 nm thick) appearing smooth and (after 25 min of UV/H2O2) showing same silver feature now completely removed with only the underlying chromium adhesion layer (20 nm thick) remaining.
eq 3. As discussed later, the proposed formation of Ag+ and HOO- ions can account quantitatively for the pattern formation. The observed isotope effect arise from the hydrogen-abstraction step (eq 2). This step is also involved in the UV-initiated decomposition of H2O2 to oxygen. We have measured the rate of oxygen evolution in the systems H2O2 + H2O + UV and D2O2 + D2O + UV and observed an isotope effect (kH/kD) of 7 (hydrogen peroxide concentration: 0.147 M, respective oxygen formation rates, 2.5 × 10-8 and 3.5 × 10-9 mol/s). Furthermore, patterns did not form in the presence of a noncoordinating radical scavenger, sodium nitrate (NaNO3).20 In general, we found that the rate of pattern formation or particle deposition increased linearly with the intensity of light. UV
H2O2 98 2 HO•
(1)
HO• + H2O2 f H2O + HOO•
(2)
HOO• + Ag f HOO- + Ag+
(3)
To confirm the formation of ions and dissolution of Ag, we measured conductivity and performed atomic force microscopy (AFM) in contact mode. We measured the conductivity of samples in a number of scenarios to demonstrate that, in the presence of H2O2, UV, and silver, more ions were generated than with hydrogen peroxide/UV alone or H2O2/silver alone (Figure 2). (20) Nikitenko, S. I.; Venault, L. Moisy, Ph. Ultrasonics Sonochem. 2004, 11, 139-42.
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Further suggesting that some of the ions being produced are silver ions were a series of AFM images showing the disappearance of the silver feature (Figure 2). Our AFM studies before and after 30 min of UV/H2O2 treatment revealed that the silver feature is almost completely removed revealing the Cr adhesion layer (confirmed by energy dispersive spectroscopy). Unlike electrophoretic/electroosmotic patterning reported previously, pattern formation in the present case occurs without the necessity of a second metal in electrical contact.15 Movement due to an interfacial tension gradient can also be ruled out based on the direction of the observed motion (away from the silver surface to the more hydrophilic silica surface).11 Diffusiophoresis in an electrolyte gradient is one possible mechanism described theoretically and confirmed experimentally by Anderson and co-workers,21,22 where the phoretic movement of particles in an electrolyte gradient is predicted by eq 4.
U)
ζp d ln C 2 kT 2 ln(1 - ξ2) E+ η η Ze dx
( )
(4)
In eq 4, is the dielectric constant (7.1 × 10-10 F/m); ζp, the zeta potential of the particle (-80 mV for silica, -60 mV for sulfated polystyrene, and +40 mV for amidine functionalized polystyrene measured in deionized water via electrophoretic lightscattering method on ZetaPALS instrument); η, the viscosity of the medium (1 × 10-3 kg/m‚s); E, electric field induced by difference in ion diffusion coefficient (anion and cation); Z, the charge of the ion creating the gradient; e, elementary charge (1.60 × 10-19 C); and ξ, the double layer potential decay described by Gouy-Chapman theory. The diffusion induced electric field is described by eq 5
E)
kT d ln C β Ze dx
(5)
where β is the ratio of the difference in diffusion coefficients for the anion and cation to the sum of their diffusion coefficients. The decay of the zeta potential in the double layer (the double layer of the surface over which the flow is occurring, thermally grown silicon dioxide) is described by eq 6, where ζw, is the zeta potential of the surface.
ξ ) tanh
( ) Zeζw 4kT
(6)
In eq 4, the first term is the electrophoretic component predicting particle motion dependent on the zeta potential of the moving particle. The second component is the chemiphoretic component and causes particle motion toward lower electrolyte concentrations attributed to a pressure gradient akin to the pressure gradient described by nonelectrolyte diffusiophoresis.23 For diffusiophoresis from an electrolyte gradient to be a plausible mechanism we require two ions: one a cation and the other an anion diffusing at different rates away from the silver surface thereby establishing a diffusion induced electric field described previously.21,22 The kinetic isotope studies suggest that O-H bond breaking is the rate determining step in the decomposition of hydrogen peroxide and the hydroperoxide ion (HOO-) may be an intermediate (see eqs 1-3).19 The diffusion coefficient of hydroperoxide ion has been reported to be 0.3 × (21) Prieve, D. C.; Anderson, J. L.; Ebel, J. P.; Lowell, M. E. J. Fluid Mech. 1984, 148, 247-69. (22) Ebel, J. P.; Anderson, J. L.; Prieve, D. C. Langmuir 1988, 4, 396-406. (23) Anderson, J. L.; Lowell, M. E.; Priever, D. C. J. Fluid Mech. 1982, 117, 107-21.
Table 1. Comparison of Experimental Particle Velocities to Those Predicted by Diffusiophoresis in Electrolyte Gradienta theory velocity variation (µm/s) Ag+ (µm/s) (µm/s) UV and OOH-
tracer
-0.4 0.6 0 -8 0.7 18 -8
silica on SiO2 silica on SiO2 amidine polystyrene on SiO2 amidine polystyrene on SiO2 sulfated polystyrene on SiO2 silica on parylene amidine polystyrene on parylene a
0.4 0.2 0 1 0.4 4 3
off on off on on on on
0 15 0 -21 9 24 -12
Velocity: positive, away from silver; negative, toward silver.
10-9 m2/s (in 8 M KOH).24,25 As discussed above, the cation is Ag+ with a diffusion coefficient of 1.6 × 10-9 m2/s. Now that we knew the source of our ions we substituted the diffusion coefficients for the silver ion (1.6 × 10-9 m2/s) and the hydroperoxy ion (0.3 × 10-9 m2/s) into the expression for β (0.69). To obtain the ion gradient we consider the silver disk as a point source and solve for the concentration gradient from the experimental bulk conductance data (L) for silver + H2O2 + UV light from Figure 2. Photolysis results in the increase in bulk conductance from 2.2 to 3.3 µS. This translates into to an increase in the ion concentration of 1.78 × 10-6 mol/m3, half of which is Ag+ and the other half HOO- calculated from (eq 7)
Ci )
Ll AFzui
(7)
where l is the distance over which conductance is measured (5 × 10-3 m), A is the cross-sectional area of conductivity meter probe (5 × 10-5 m2), F is Faraday’s constant (96 485 C/mol), z is the ion charge, and u is the ion mobility (6.425 × 10-4 m2S/C). Assuming that these ions are formed at the silver disk and diffuse away into the “bulk” (approximated as 20 µm from the silver point source, calculated from diffusion-induced root-meansquare displacement (2DT)1/2) over the time-span of 7 min (Figure 2), we calculate a concentration gradient of 2.23 × 104 mol/m4 from eq 8
d ln C/dx )
ln(Cf - Ci) xf - xi
(8)
where Cf and Ci are the final (5.33 × 10-6 mol/m3) and initial concentrations (3.55 × 10-6 mol/m3) and the difference of xf and xi is the distance over which the concentration gradient is established (20 µm). We then substituted β and the concentration gradient into eq 5 to obtain an electric field of 4.2 V/cm. The zeta potential of the surface over which the pattern was occurring is clearly significant. For example, a parylene, poly(chloro-pxylylene), coated silicon dioxide surface (zeta potential ∼ -1 mV)26 exhibited enhanced patterning (see video IV in the Supporting Information) where particles moved at ∼18 µm/s as compared to 0.6 µm/s for the silver on uncoated silicon dioxide (zeta potential ∼ -70 mV) for particles of identical zeta potential.27,28 Upon substituting ξ obtained from eq 6 into eq 4, (24) Arai, H. Muller, S.; Haas, O. J. Electrochem. Soc. 2000, 147, 3584-91. (25) Katayama, Y.; Sekiguchi, K.; Yamagata, M.; Miura, T. J. Electrochem. Soc. 2005, 152, E247-50. (26) Wang, J.; Yang, L. Tamkang J. Sci. Eng. 2005, 8, 231-6. (27) Ermakova, L. E.; Bogdanova, N. F.; Sidorova, M. P. Colloid J. 2003, 65, 428-33. (28) Kirby, B. J.; Hasselbrink, E. F. Electrophoresis 2004, 25, 187-202.
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Figure 3. (a) The focusing iris is mostly over the silver with the aperture fully open and patterning is observed with the dotted line showing the pattern and the solid line showing the edge of the silver disk. (b) As the focusing aperture is moved away from the silver disk tracers move back toward the silver disk indicated by tracers entering region between dashed and solid lines. (c) The focusing iris is moved back over the silver disk to restore the original pattern. Images d-f also illustrates the reversibility of pattern formation with a fully open aperture as the focusing iris is moved off of the silver disk with subsequent relaxation of the pattern (d, f), and onto the silver to restore the pattern (e). Scale bar represents 40 µm.
we can solve for the theoretical velocity of the particle where velocity is both a function of the zeta potential of the particle (electrophoretic component) and the pressure gradient created by the chemical reaction (chemiphoretic component). Despite the approximations discussed above, our predicted velocities (Table 1) have the correct direction and are generally within an order of magnitude of our experimental results for a series of negative particles with different zeta potentials and positively charged amidine particles and for two different surfaces (silver/ silicon dioxide and silver/parylene). Note that if HO- rather than HOO- is the diffusing anion, particle motion in a direction opposite to that observed would be expected since HO- diffuses faster than Ag+. We have also illustrated the fully reversible spatial and temporal control over pattern formation, by using a focusing iris to regulate patterning with the aperture fully open (Figure 3). The novelty here is the possible application of a maskless method to scan the surface and select one catalyst feature (or at most a few) for selective pattern formation because patterns only form with UV light present (field of view is 0.44 mm at 50× when aperture is fully open). In Figure 3, we show pattern formation with the focusing aperture on the silver surface (panels a, c, and e) and the disappearance of the pattern when the aperture is almost entirely off the silver surface (panels b, d, and f). The reversible formation and relaxation of patterns occur on a time scale of seconds. We have demonstrated that photolysis can induce reversible patterning for negative particles and particle deposition for
positively charged particles due to a diffusion-induced electric field. Our diffusion-induced electric field was formed by the slow dissolution of silver feature forming silver and hydroperoxy ions during photochemical decomposition of hydrogen peroxide over silver. Conductivity measurements, optical microscopy, and AFM support this hypothesis. The patterning or particle deposition that we have observed should be independent of Ag disk thickness. On the other hand, a thicker disk should increase the lifetime of the ion source, and different geometries may alter the profile of the gradient. Acknowledgment. We thank Prof. Thomas Mallouk for laboratory use and helpful suggestions and Prof. Darrell Velegol for theoretical discussions. This work is supported by a NSFNIRT grant and was performed in part at the Penn State Nanofabrication Facility, a member of the NSF National Nanofabrication Users Network. Supporting Information Available: Real-time videos showing the formation of patterns with negative particles on silver patterned SiO2 surface (video I), movement and then deposition of positive particles onto the silver disk of a silver patterned SiO2 surface (video II), quenching pattern formation in D2O2/D2O for silver patterned on SiO2 (video III), and formation of larger patterns on silver patterned on parylene (video IV). All experiments done in 0.5% w/w hydrogen peroxide. This material is available free of charge via the Internet at http://pubs.acs.org. LA061165+
