Spatial and Temporal Control of Information Storage in Cellulose by

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Spatial and Temporal Control of Information Storage in Cellulose by Chemically Activated Oscillations Irene Vassalini and Ivano Alessandri* INSTM and Chemistry for Technologies Laboratory, Mechanical and Industrial Engineering Department, University of Brescia, via Branze 38, 25123 Brescia, Italy S Supporting Information *

ABSTRACT: Chemical oscillations are exploited to achieve self-expiring graphical information on paper-based supports with precise temporal and spatial control. Writing and self-erasing processes are chemically activated by exciting nonoscillating Belousov−Zhabotinsky (BZ) solutions infiltrated in cellulose paper filters. Exhausted supports can be reactivated many times by adding new BZ medium. Different parameters can be independently controlled to program mono- or multipaced information storage.

KEYWORDS: smart paper, Belousov−Zhabotinsky reaction, chemical writing/self-erasing, stimuli-responsive materials, information storage

1. INTRODUCTION Research on paper-based devices is currently blooming in a large variety of applications, in particular advanced diagnostics and flexible electronics.1−7 The capability to store information with controlled lifetime is expected to have major impact on cellulose-based materials, with interesting applications in security, anticounterfeiting, device self-diagnostics, labeling, packaging, quality control of food and other perishables.8−10 A few examples of self-erasing smart interfaces were achieved through the use of either photochromic organic molecules,11 phosphorescent transition-metal complexes12 or metal nanoparticles functionalized with photoactive ligands.13 In both cases, writing and self-erasing are based on cis−trans isomerization of azobenzene moieties, which is selectively switched on/off by light irradiation at two different wavelengths. Plasmonic heating-mediated self-healing and self-erasing processes were also demonstrated as an efficient alternative to produce light-writable supports.14−17 However, most of those works were addressed to generate self-expiring data on organogel films, synthetic polymer blends or semiconductors,18 which differ from cellulose-based paper in terms of physicochemical properties and applications. Moreover, in all of these cases, external sources of light (high power lamps or lasers) are needed to activate the temporal control of data storage. Here we report a new approach to store self-expiring information, which is particularly suitable for cellulose-based supports. This goal was pursued through the chemical excitation of a Belousov−Zhabotinsky-like (BZ) oscillating reaction, consisting in the oxidation of an organic substrate (citric or malonic acid) by bromate ions, under acidic conditions.19 The writing/erasing process was accomplished © XXXX American Chemical Society

by means of a catalyst (e.g., ferroin), which oscillates between two redox states, characterized by different colors, as the reaction proceeds. The BZ reaction is an excellent artificial model for the reproduction and understanding of biological oscillating processes.20,21 Depending on the ratio between the concentration of the reagents, in BZ-like reactions the oscillations can occur spontaneously or by introducing an external perturbation. The latter condition, usually referred as an excitable nonoscillating state, is particularly suitable for fabricating stimuli-responsive materials. Several efforts have been addressed to the conversion of chemical into mechanical oscillations.22,23 In this regard, chemical waves were exploited for microfreight delivering in liquid media,24 as well as to produce self-oscillating polymeric gels, with potential applications in touch sensors,25 soft-robots,26 ciliary-like motion actuators27 and actuators for autonomous mass transport.28 BZ was investigated for information storage purposes in few pioneering studies, yet limited to bulk solutions.29 In this case, the writing process was based on the projection of an image on the surface of a liquid medium where the BZ reaction takes place, with the assistance of a light-sensitive ruthenium catalyst. Image storage and processing (contrast modification, “skeletonizing”, or smoothing) was achieved by exploiting the phase shift in the oscillating medium between irradiated and dark regions. This principle led the research toward reactiondiffusion-based data storage in liquids.30,31 In the present work, we extend the use of BZ reaction as a pacemaker to store information with spatial and temporal Received: December 6, 2015 Accepted: December 10, 2015

A

DOI: 10.1021/acsami.5b11857 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Br2 + OA → Br − + BrOA + H+

control in paper-based devices. This approach differs from those utilized so far for information storage in liquid or gel phases, because the writing/self-erasing process takes advantage of oscillations activated through an external stimulus, which is triggered by an iron tip working as a “chemical pen”. Moreover, tip-activated oscillations can be synchronized with oscillations originated from the absorption of the nonoscillating BZ medium on the same cellulose support and several parameters (temperature, chemical composition, spatial extension of the graphical tracks) can be easily tuned to drive selferasing and controlled data storage.

OA is an organic acid like citric or malonic acid. The concentration of Br− ions is progressively lowered, until process I is replaced by process II, characterized by the following reactions:

2. EXPERIMENTAL SECTION The Belousov−Zhabotinsky reaction medium in an excitable nonoscillating state was prepared by mixing 7.5 mL of a solution of 0.45 M KBrO3 in 0.9 M H2SO4 and 7.5 mL of a solution of 0.09 M KBr and 0.3 M citric acid. A dark-yellow solution was obtained, where the coloring is indicating of the production of Br2. 7.5 mL of a solution of 0.0052 M Fe(phen)3SO4 was added. After the mixing, the final concentrations were: 0.15 M KBrO3, 0.3 M H2SO4, 0.03 M KBr, 0.1 M citric acid and 0.001 74 M Fe(phen)3SO4. A bright blue solution was obtained. The color was maintained steady and no chemical waves were spontaneously generated. The writing/self-erasing experiments were carried out by dropping different amounts (from 10 to 500 μL) of BZ solution on cellulose filters. Data discussed in the present paper refer to Whatman n.1 filters (thickness, 180 μm; diameter, 7−9 cm; pore size, 11 μm). Analogous results, but with different kinetics were achieved using other types of filters (e.g., Whatman n.5, thickness, 200 μm; diameter, 7 cm; pore size, 2.5 μm). Writing/self-erasing was activated by means of iron tips or bullets. Analogous tests were carried out using small pieces of zinc, aluminum, magnesium, glass and polyethylene terephtalate (PET). Further experiments of chemical activation were carried out by dropping 10 μL of 0.1 M aqueous solutions of different salts (FeCl2, FeCl3, KBr, KNO3, NaCl, KCl and AgNO3) onto the paper supports. The optical evaluation of the self-erasing rate was performed using an open source software (ImageJ) on the basis of the images acquired with a commercial photocamera. Temperature-dependence of the self-erasing process was evaluated by carrying out the self-erasing experiments under temperaturecontrolled heating of the paper-filters, set using a Linkam HFS91 temperature controlled stage, combined with a Linkam TP93 controller system. Heating-induced writing experiments were carried out either by means of a temperature-controlled Bosch heat gun, modified by means of a homemade nozzle.

(I.1)

HBrO2 + Br − + H+ → 2HOBr

(I.2)

HOBr + Br − + H+ → Br2 + H 2O

(I.3)

HBrO2 + BrO3− + H+ → 2BrO2 · + H 2O

(II.1)

BrO2 · + Fe2 + + H+ → HBrO2 + Fe3 +

(II.2)

2HBrO2 → BrO3− + HOBr + H+

(II.3)

HOBr + OA → BrOA + H 2O

(II.4)

Process II yields HOBr2 through an autocatalytic reaction and oxidizes the redox catalyst (in the present case ferroin, [Fe(phen)32+], which is red, is converted into blue ferriin [Fe(phen)33+]). As the latter process is completed, the autocatalytic reaction is terminated and reaction III takes place: 4Fe3 + + BrOA + 2H 2O → 4Fe 2 + + OA + 2CO2 + 5H+ + Br −

(III)

−

The freshly regenerated Br resets the cycle to process I. In the present case, upon mixing, the solution rapidly turned from red into a blue color, indicating the oxidation of the ferroin catalyst (eq II.2). The blue color is maintained and neither chromatic nor spatial oscillations were observed over a significantly long (several hours) time scale (Figure 1a). 100 μL of the nonoscillating BZ medium were dropped onto a commercial cellulose filter (Whatman n.1), giving rise to a 10 cm2 track. Upon absorption, the chemical oscillations were excited, resulting in self-sustained cycles alternating between red and blue color. Figure 1b shows the progress of the oscillations as a function of time. In contact with the cellulose substrate, ferriin (blue) was immediately reduced to ferroin (red). The original blue color was progressively restored, starting from the center of the droplet track and proceeding radially toward the borders. The catalyst oxidation was completed within a few seconds over a circular region, which roughly corresponds to more than 70% of the whole area of the droplet track (see Figure 2a). At 18 °C, the first red-to-blue transition was completed in 30 min. The overall red/blue/red cycle took 1 h. Afterward, the oscillations proceeded, carrying out a second red/blue/red cycle. Drying is completed in 2 h, resulting in a red-colored stationary state, which indicates that the redox catalyst is in reduced form (ferroin). No further oscillations can be activated from dried supports. Both environmental temperature and volume of the droplet can significantly affect the oscillation rate, as summarized in Figure 2b,c. The porosity of cellulose is another factor that influence the oscillation rate. For example, 100 μL of BZ solution dropped onto a Whatman n.5 filter (pore size: 2.5 μm) undergo a complete red-to-blue conversion in 30 min at 25 °C, whereas the same process takes 20 min using Whatman n.1 filters (pore size: 11 μm). These results indicate that the simple absorption/drying of BZ-medium droplets onto a cellulose substrate introduces local gradients of concentration, allowing periodic chemical oscillations to be excited. This approach can be also extended to other hydrophilic, porous supports, provided that they are stable in contact with the BZ solution. For example, polycarbonate filters are

3. RESULTS AND DISCUSSION In a typical proof-of-concept experiment, potassium bromate, sulfuric acid, citric acid, potassium bromide and ferroin were mixed in proper amounts to obtain an excitable nonoscillating BZ reaction medium (see the Experimental Section). The kinetics of the oscillatory BZ reaction is very complex, involving a number of intermediate steps. According to the mechanism proposed by Field, Körös and Noyes (FKN), the overall reaction consists of three main processes (I, II, III), which are summarized below.32 In the first process, BrO3− species react with Br− with the following reactions: BrO3− + 2Br − → HBrO2 + HOBr

(I.4)

B

DOI: 10.1021/acsami.5b11857 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Evolution of the absorption-driven oscillation during its first stages of the example reported in Figure 1 and in the green portion of the inset. The blue color propagates radially from the center toward the borders (see the main text for details); red-to-blue complete conversion time for different droplets at a fixed temperature (25 °C) (b) and at three different values of room temperature (18, 25, and 32 °C). (c) Each experiment was replicated three times. The error bars are included in the point size.

Figure 1. Absorption-driven activation of nonoscillating reactions at 18 °C. (a) The nonoscillating BZ solution utilized in the writing/selferasing experiments. The stability of the stationary blue state was monitored for 3 h in open beakers at room temperature. No changes of color were observed; (b) the deposition of the BZ medium on cellulose-filters activates color oscillations. An example of absorptiondriven oscillations for 100 μL of BZ solution deposited onto a Whatman n.1 filter is shown on the right. The red-to-blue oscillations were monitored as a function of time. The oscillations terminate when the filter is fully dried. Scale bars: 1 cm.

proof of that is given by the red contour of the droplet halo, which is still observed after 16 min from droplet deposition and disappears when the first half-cycle is completed. As a result, both absorption- and tip-induced oscillations can coexist on the same cellulose support at the same time, each one characterized by a well-defined lifetime (Figure 3c). Unlike the absorption-driven oscillations, which can be extended over many cycles, the writing/self-erasing process activated by an iron tip is carried out over only one single blue/ red/blue cycle: the first half-cycle (from blue-to-red) corresponds to writing and the second one (from red-toblue) to self-erasing. This binary sequence ensures that the information is permanently erased and cannot be spontaneously self-restored, which is a key aspect for secure communication. Moreover, because the system can be fully reactivated with the same excitation source, writing can be operated many times on the same blue region of the same substrate. This recyclability was tested in more than 10 writing/erasing cycles operated by writing 0.5 cm2-sized dots in the same region of each sample. As observed before, excitability vanished when the paper filter was completely dried. This means that temporary information can be stored on each filter for 1−2 h. However, each support can be easily regenerated by adding further BZ solution. Figure 4 shows an example of an exhausted (i.e., completely dried) paper filter support, which was reloaded with 500 μL of BZ solution. Eight loading/writing/self-erasing/ drying cycles were carried out with the same procedure in either 1 day- or 1 week-long experimental sessions. No differences in writing/self-erasing performances were found from one cycle to another, demonstrating full recyclability of the same support. Moreover, different dried supports aged for

significantly damaged and cannot be used for information storage purposes (see the Supporting Information). As the reaction is running, its progress can be further perturbed on a local scale by other external inputs. For example, small pieces of iron, like bullets, tips or wires, can serve as a pen for writing graphical tracks on the paper substrate, as shown in Figure 3a. The writing process induces a local, spatially controlled color change from blue to red. In response to this stimulus, the system resets the blue color within a certain period of time (in the order of minutes), which can be precisely controlled by the size of the graphical tracks, as well as other different external parameters, like temperature (vide infra). As a result, the original information is completely erased (see also the video uploaded in the Supporting Information, as well as in snapshots shown in Figure 3a). Figure 3b shows the example of a red footprint, about 1 cm2 in size, obtained by pressing an iron tip with the corresponding size onto the paper substrate. At 18 °C, the track undergoes progressive self-erasing, which is completed in 16 min. In parallel, the oscillations resulting from spatially nonuniform absorption/drying of the BZ-medium droplet, proceed independently with their characteristic oscillation period, resulting unaffected by the local, tip-induced perturbation. A C

DOI: 10.1021/acsami.5b11857 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Tip-induced activation of chemical oscillations. (a) Schematic representation of the writing and self-erasing process. On the right side, examples of graphical tracks that can be created. Scale bars: 1 cm. Bottom: Sequence of snapshots taken from the video uploaded in the Supporting Information. The written track is generated with a paper clip on a Whatman n.1 filter (diameter: 7 cm) previously soaked in the BZ solution. (b) Illustration of the writing/self-erasing process activated by an iron tip (1 cm2 in size) on a cellulose filter functionalized with 100 μL of BZ medium. Scale bars: 3 cm. (c) Color change of a cellulose filter infiltrated with BZ medium as a function of time. Tip-induced activation of writing/self-erasing process was triggered at precise intervals of time, indicated by a red cross.

Figure 4. Reactivation of a dried support. (a) Schematic representation of the writing and self-erasing process and the reactivation of the fully dried, exhausted filters by BZ solution reloading. (b) Pictures of the same cellulose filter functionalized with 500 μL of BZ solution. Every line corresponds to a cycle of writing, self-erasing and drying that takes about 1 h. After each cycle, writing/ self-erasing was reactivated through the deposition of 500 μL of BZ solution. Scale bars: 1 cm.

out with passivated zinc, aluminum and magnesium were unsuccessful (Figure 5a). On the other hand, when the natural

several months were fully reactivated by BZ solution reloading, showing a long shelf life that can be attractive for many applications. A closer inspection revealed interesting findings on the nature of the excitation exploited to obtain the writing/selferasing process. The mechanical activation of a nonoscillating BZ medium was modeled by the Balazs group33,34 and demonstrated in several examples including Nafion membranes35 and gels.25 In those reports, the onset of oscillation is ascribed to the local modification in concentration of the reactants, which is induced by mechanical compression. The mechanical reactivation of chemical oscillations in gels is driving research in BZ-based pressure sensors.25 At first sight, such a purely mechanical activation could be taken to explain the machinery that drives the writing/selferasing process described above. The pressure applied by the pen was in the 1−10 kPa range for all the experiments, the same utilized for activating bulk BZ-gel disks.25 However, in the present case, there is a number of experimental observations suggesting that the chemical nature of the “pen” is actually the crucial factor for exciting writing/self-erasing. First of all, no activation and, as a consequence, no writing/self-erasing, was observed using plastic (e.g., polyethylene terephtalate, PET) or glass tips pressed on the paper substrate. The same tests carried

Figure 5. Chemical pen. (a) Dependence on the chemical nature of the tip surface: schematic summary showing the results of tipactivation tests using different materials as a tip. The writing process was performed in 1−10 kPa pressure range; (b) writing tests with different salt solutions (paper filter: Whatman n.1, diameter: 7 cm).

surface passivation layer was removed, all of these reducing metals were able to produce graphical tracks, although with different kinetics. Iron tips gave the best results in terms of immediate, precise writing. These observations suggest that the direct contact with reducing species drives a localized transformation of Fe3+(blue) into Fe2+ (red). This local change of concentration is promptly restored by reoxidation occurring D

DOI: 10.1021/acsami.5b11857 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces through the BZ process. However, we note that ferroin by itself, in the absence of BZ medium, is not able to exploit the Fe2+/ Fe3+ redox switching to produce a writing/self-erasing process suitable for information storage purposes. In fact, when an iron tip is put in contact with a cellulose filter previously soaked with a ferroin solution, mixed with sulfuric acid at the same concentration as that utilized in the BZ medium (0.3 M), no color oscillation are activated. On the other hand, changes in concentration of BZ ions are essential for activating writing/ self-erasing. We observed other examples of chemical activation by dissolving salts containing species involved in the BZ or analogous reactions, like bromide (KBr) or iron(II) (FeCl2) ions, in BZ-loaded filters. KNO3, which does not release any BZ ions, was not active. The same unsuccessful results were achieved using sodium and potassium chloride. FeCl3, releasing Fe3+ ions, yields very weak red footprints that are hardly detectable by naked eye (Figure 5b). Interestingly, silver nitrate (AgNO3) can be used as a source of Ag+ ions to achieve writing from the red-colored stationary state, inducing the oxidation of ferroin to ferriin. However, the precipitation of Ag particles yields permanent greyish tracks, which can be still distinguished from the blue color. Although further work needs to be done for a more quantitative evaluation of the activation mechanism, these experimental observations demonstrated that a purely mechanical activation, which is able to excite BZ-gel disks, is not sufficient in the case of paper-based supports. On the other hand, local changes in concentration of the BZ species (in particular a local increase of Fe2+ ions) are sufficient to trigger repeatable color oscillations. As already observed in the case of oscillations activated by the deposition of BZ droplets onto the cellulose substrates, volume of the BZ medium and temperature are additional parameters that can be externally modified to control the writing/self-erasing process. The erasing rate can be slightly, yet significantly enhanced by soaking the cellulose filter with a larger volume of the BZ medium (it increased by 0.02 mm2/s on passing from 100 to 500 μL) (Figure 6a). The presence of a higher amount of liquid absorbed on the filter enables a faster diffusion of the reactants, which results in a faster resetting of the steady state. On the other hand, the erasing rate can be increased by external heating, showing an exponential trend in the range 25−150 °C (Figure 6b). Heating increases the drying rate of the BZ medium and diffusion of the reactants. When heating is prolonged up to the complete drying of the support, selferasing is inactivated. This effect can be harnessed to store information on the same support with two different erasing rates. Figure 6c shows the images of a paper filter with two differently written tracks. One was obtained by iron-tipactivation (chemical pen), the other one by blowing hot air through a nozzle. Self-erasing was faster in the case of tipinduced activation, whereas the local heating left tracks characterized by longer lifetime. This “multipaced” process allows for synchronized storage of different self-expiring information, opening the way to several possible combinations.

Figure 6. Influence of external parameters on self-erasing. (a) Erasing rate as a function of: (a) BZ medium volume, (b) external temperature; (c) comparison of the rate of self-erasing process activated by means of either an iron tip or heating-nozzle, on a cellulose filter functionalized with 200 μL of the BZ medium.

the local modification in concentration of the chemical species involved in the oscillatory reaction. Unlike mechanical activation, the use of a chemical pen does not significantly alter the mechanical properties of cellulose supports, avoiding hysteresis and surface damages, which prevent their recyclability. The chemical activation of clock-reactions from nonoscillating media is harnessed for writing graphical tracks that self-expire according to the reaction kinetics. Exhausted supports can be reactivated by addition of new reactive medium. Other external parameters, like the volume of the reaction medium, spatial extension of the graphical tracks or temperature, can be independently controlled to program mono- or multipaced information storage. These results offer a number of possible applications for cellulose-based materials, with particular regard to smart labeling. Further advances might be also reached by using nanostructured tips as activators, which could allow for obtaining temporally controlled information storage at the nanoscale. This concept can be extended to other types of oscillating reactions and supports, opening a rich palette of applications for sensing, diagnostics, energy conversion and paper-based electronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11857. Real-time video showing an example of writing/selferasing on cellulose supports (AVI). Image of polycarbonate filters treated with BZ medium (PDF).

4. CONCLUSION In summary, this work demonstrates that oscillating reactions, like BZ, can be exploited for storing information on cellulosebased supports, with precise, reproducible temporal and spatial control of both writing and self-erasing processes. Different graphical inputs can be introduced by means of chemical pens, inducing a change of the electrochemical potential, as well as

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AUTHOR INFORMATION

Corresponding Author

*I. Alessandri. E-mail: [email protected]. E

DOI: 10.1021/acsami.5b11857 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Author Contributions

(19) Zhabotinsky, A. M. A History of Chemical Oscillations and Waves. Chaos 1991, 1, 379−386. (20) Epstein, I. R.; Showalter, K. Nonlinear Chemical Dynamics: Oscillations, Patterns and Chaos. J. Phys. Chem. 1996, 100, 13132− 13147. (21) Epstein, I. R. Coupled Chemical Oscillators and Emergent System Properties. Chem. Commun. 2014, 50, 10758−10767. (22) Yoshida, R. Self-Oscillating Gels Driven by the BelousovZhabotinsky Reaction as Novel Smart Materials. Adv. Mater. 2010, 22, 3463−3483. (23) Smith, M. L.; Slone, C.; Heitfeld, K.; Vaia, R. A. Designed Autonomic Motion in Heterogeneous Belousov-Zhabotinsky (BZ)Gelatin Composites by Synchronicity. Adv. Funct. Mater. 2013, 23, 2835−2842. (24) Ichino, T.; Asahi, T.; Kitahata, H.; Magome, N.; Agladze, K.; Yoshikawa, K. Microfreight Delivered by Chemical Waves. J. Phys. Chem. C 2008, 112, 3032−3035. (25) Chen, I. C.; Kuksenok, O.; Yashin, V. V.; Balazs, A. C.; Van Vliet, K. J. Mechanical Resuscitation of Chemical Oscillations in Belousov-Zhabotinsky Gels. Adv. Funct. Mater. 2012, 22, 2535−2541. (26) Maeda, S.; Hara, Y.; Sakai, T.; Yoshida, R.; Hashimoto, S. SelfWalking Gel. Adv. Mater. 2007, 19, 3480−3484. (27) Tabata, O.; Hirasawa, H.; Aoki, S.; Yoshida, R.; Kokufuta, E. Ciliary Motion Actuator Using Self-Oscillating Gel. Sens. Actuators, A 2002, 95, 234−238. (28) Yoshida, R.; Murase, Y. Self-Oscillating Surface Gel for Autonomous Mass Transport. Colloids Surf., B 2012, 99, 60−66. (29) Kuhnert, L. A New Photochemical Memory Device in a LightSensitive Chemical Active Medium. Nature 1986, 319, 393−394. (30) Kaminaga, A.; Vanag, V. K.; Epstein, I. R. A Reaction-Diffusion Memory Device. Angew. Chem., Int. Ed. 2006, 45, 3087−3089. (31) Rambidi, N. G.; Shamayaev, K. E.; Peshkov, G. Y. Image Processing Using Light-Sensitive Chemical Waves. Phys. Lett. A 2002, 298, 375−382. (32) Field, R. J.; Körös, E.; Noyes, R. M. Oscillations in Chemical Systems. II. Thorough Analysis of Temporal Oscillation in the Bromate-Cerium-Malonic Acid System. J. Am. Chem. Soc. 1972, 94, 8649−8664. (33) Kuksenok, O.; Yashin, V. V.; Balazs, A. C. Mechanically Induced Chemical Oscillations and Motion in Responsive Gels. Soft Matter 2007, 3, 1138−1144. (34) Yashin, V. V.; Kuksenok, O.; Balazs, A. C. Modeling Autonomously Oscillating Chemo-Responsive Gels. Prog. Polym. Sci. 2010, 35, 155−173. (35) Suzuki, K.; Yoshinobu, T.; Iwasaki, H. Induction of Chemical Waves by Mechanical Stimulation in Elastic Belousov-Zhabotinsky Media. Chem. Phys. Lett. 2001, 349, 437−441.

I.V. performed all the experiments and data analysis. I.A. supervised the experiments. I.V. and I.A. discussed the results and wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank N. Bontempi for valuable discussion. REFERENCES

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DOI: 10.1021/acsami.5b11857 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX