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Switching light with light – advanced functional colloidal monolayers† Cite this: DOI: 10.1039/c3nr04897g

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K. Bley,a N. Sinatra,a N. Vogel,ab K. Landfestera and C. K. Weiss*ac Colloidal monolayers comprising of highly ordered two dimensional crystals are of high interest to generate surface patterns for a variety of different applications. Mostly, unfunctionalized polymer or silica colloids are assembled into monolayers. However, the incorporation of functional molecules into such colloids offers a convenient possibility of implementing additional properties to the two-dimensional crystal. Here, we present the formation of novel functional colloidal monolayers with photoswitchable fluorescence. The miniemulsion polymerization technique was used to incorporate an appropriate dye system of a perylenebased fluorophore and a bis-arylethene as a photochrome in polymeric colloids in defined ratios. Upon irradiation with UV or visible light the photochrome reversibly isomerizes from the ring-closed form, which is able to absorb light of the emission wavelength of the fluorescent dye and the ring-open form, which is not. The fluorescence emission of the dye can thus be reversibly switched on and off with light even when embedded in colloids. The colloids were self-assembled at the air–water interface to produce hexagonally ordered functional monolayers and more complex binary crystals. We investigate in detail the influence of Received 13th September 2013 Accepted 27th October 2013

the polymeric matrix on the switching properties of the fluorophore/photochrome system and find that the rate constants for the photoswitching, which all lie in the same range, are less influenced by the polymeric environment than expected. We demonstrate the reversible switching of the fluorescence emission in self-

DOI: 10.1039/c3nr04897g

assembled colloidal monolayers. The arrangement of broadly distributed functional colloids into ordered

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monolayers with high addressability was obtained by the formation of binary colloidal monolayers.

Introduction Highly ordered two-dimensional colloidal crystals nd many applications in science and technology.1,2 In addition to the investigation of their optical properties,3–5 colloidal monolayers are mainly used in lithographic processes, using colloids as templates for metal evaporation,6–10 as a powerful platform for surface patterning,11 for example to mimic vivid structural color occurring in nature or to control wetting properties and for the creation of plasmonic structures.7–9,12–16 Typically, hexagonally close-packed monolayers are used, which can be prepared in a

Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

b

School of Engineering and Applied Sciences, Harvard University, McKay 426, 9 Oxford Street, Cambridge, MA 02138, USA

c University of Applied Sciences Bingen, Berlinstrasse 109, 55411 Bingen, Germany. E-mail: c.weiss@-bingen.de

† Electronic supplementary information (ESI) available: S1: scanning electron micrographs of the particles prepared by miniemulsion polymerization and seeded emulsion polymerization, S2: uorescence emission spectra of the primary colloidal dispersions and the development of PMI's emission during UV-light and VIS-light irradiation, S3: calculations for the size of the interstitial sites of the template colloids and detailed geometric evaluation of the possible assignment of seeded particles, and S4: optical uorescence micrographs for the photoswitching of a functional colloidal monolayer. See DOI: 10.1039/c3nr04897g

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high quality using a variety of methods.17,18 The complexity of the structural design can be increased by the creation of binary monolayers. Here, the two-dimensional (2D) crystal is generated from colloids of two distinct sizes. The small colloids can be located in varied, yet dened amounts in the interstitial sites of a hexagonally close-packed monolayer of large colloids,19–21 or the large colloids are separated by the small colloids randomly22 or in high order.21,23 Kingshott et al.24–26 created binary monolayers using colloids of different materials, i.e. polymer and silica, and different sizes, where one colloid population could be removed selectively by solvents, offering an easy access to sophisticated surface functionalization. Another strategy to implement the functionality to colloidal monolayers and to extend lithographic possibilities is the use of hybrid or functional colloids. This strategy proved very viable, particularly for the preparation of regular arrays of metal nanoparticles. As an extension of micellar lithography,27,28 and an alternative to using hybrid hydrogels,29,30 metal complex loaded colloids were used to prepare hexagonal 2D arrays of metal nanoparticles.31,32 These were subjected to plasma or thermal treatment to eventually generate regular arrays of metal nanoparticles. The metal is determined by the choice of the encapsulated complex,29,33 the size of the metal nanoparticles by the amount of complex,31 and the lattice spacing by the size of the initial colloids.34

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Nanoscale The dened and controlled encapsulation of metal complexes or other materials in polymeric colloids requires special formulation techniques. Although it is possible to use emulsion polymerization for the encapsulation of metal complexes,33,34 the amount of encapsulated material is hard to control, as the process is controlled by the diffusion of the reactants to the locus of polymerization.35 The encapsulation of multiple compounds in dened ratios, as e.g. required for the formation of alloy particles,29 is virtually impossible. To overcome this issue, the miniemulsion polymerization technique can be used. This heterophase polymerization technique allows using various monomers, and encapsulating compounds in dened amounts35 and multiple compounds in dened ratios.36 This has not only been shown for metal complexes, but also for the co-encapsulation of a photochromic dye and a uorophore. Furukawa37 has shown that the simultaneous encapsulation of uorescent and photochromic dyes following the miniemulsion approach can lead to inter-component energy transfer between the dye molecules to create particles with light-switchable uorescence. The authors used a BODIPY dye (4,4-diuoro1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene) as the uorophore and cis-1,2-dicyano-1,2-bis-(2,4,5-trimethyl-3thienyl)ethene (CMTE) as the photochromic compound.37 The use of photochromic dyes enables light-switchable emission, as photochromic dyes like CMTE reversibly isomerize upon irradiation with light.38,39 The isomers have different optical absorption properties and, thus, are used in self darkening glasses or color changing commodities. Here, however, one isomer of CMTE can absorb at the emission wavelength of the uorophore, quenching the emission. In this contribution, we study in detail the optical switching properties of a uorophore/photochrome system in polymeric colloids of different materials and assemble such colloids into two-dimensional structures. Thus, the individual colloids become addressable and may be used for optical barcoding, data storage or switching. First, the optical properties of the system CMTE as a photochrome and N-(2,6-diisopropylphenyl)perylene-3,4-dicarboximide (PMI) acting as a uorophore are investigated in solutions of the monomers used for colloid preparation. Two polymers are used for the preparation of the colloids: polystyrene (PS), with a glass transition temperature well above room temperature, and poly(butyl acrylate) (PBA) with a glass transition temperature below room temperature. As the photoisomerization of CMTE is accompanied by geometrical rearrangement the rigidity (depending on the Tg) of the polymeric matrix is expected to inuence the velocity of the isomerization and the kinetic stability of the thermodynamically unfavored isomer.40 Thus, the optical properties of the photosystem incorporated into colloids are investigated. Although it is possible to create monolayers of PS colloids with some ordering, PBA is not suitable for the formation of colloidal monolayers. It is possible to transform the low-ordered monolayer formed by PS colloids into higher order structures by assembling the colloids into a binary monolayer with larger, unfunctionalized PS colloids. Here, the functional colloids regularly assemble in the interstitial sites of a hexagonally closepacked monolayer of larger colloids. This strategy is also

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Paper suitable for PBA colloids, aer they have been subjected to a seeded emulsion polymerization process with styrene as a monomer in order to prevent lm formation of the PBA polymer with low glass transition temperature (Tg). Thus, rigid colloids are generated, which can be deposited on solid substrates. Finally, we provide direct experimental evidence of lightswitchable emission in the colloidal monolayers.

Results and discussion Concept for the preparation of functional photoswitchable colloidal monolayers The concept for the creation of photoswitchable polymeric colloidal monolayers as a model for data storage application or light erasable barcoding is shown in Fig. 1. Here, a special dye system consisting of a uorophore and a photochrome is encapsulated in a polymeric environment in dened ratios for the production of photoswitchable colloids. Thus, miniemulsion polymerization is the appropriate technique for encapsulation of various materials such as dyes in a controllable way. First, we perform a detailed investigation of the optical properties of the photoswitchable dye system containing PMI as the uorophore and CMTE as the photochrome in solution and in polymeric colloids. The different isomerization states of CMTE were obtained by irradiation with UV and visible light, respectively. The resulting colloids were covered with an additional rigid shell of polystyrene by seeded emulsion polymerization as protection for low glass temperature (Tg) polymers such as PBA and to narrow the size distribution of the colloids.41,42 The seeded colloids can be selfassembled for the creation of functional photoswitchable and highly ordered colloidal monolayers. To produce functional photoswitchable colloidal monolayers a dye system based on a bis-thienyl photochrome (cis-1,2dicyano-1,2-bis-(2,4,5-trimethyl-3-thienyl)ethene, CMTE) and a perylene based uorophore (N-(2,6-diisopropylphenyl)-perylene-3,4-dicarboximide, PMI) was used (Fig. 2A). The dye system allows switching on and off the uorescence emission of the PMI. By irradiating the photochrome CMTE with UV and VIS-light the CMTE molecule undergoes a ringclosing or ring-opening cyclization (Fig. 2A and B), respectively.

Fig. 1

Concept to produce functional photoswitchable colloidal monolayers.

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Fig. 2 (A) Dye system consisting of the photochrome CMTE in the ring-opened ground state (yellow, left) and in the ring-closed excited state (red, right) obtained by irradiation with light of different wavelengths or temperature induced isomerization in relation to the fluorescent dye PMI (green, lower part), (B) absorption and emission spectra showing the spectral overlap of the emission of the fluorophore (PMI, dark grey compact line) and the photochrome (CMTE, ground state black dashed line, excited state light grey dashed line) in styrene. The black shaded area above 620 nm shows weak residual emission of the PMI.

In the ring-closed excited state the CMTE is able to absorb the emission of the PMI between 500 nm < l < 620 nm (Fig. 2B). Therefore, the emission of PMI is switched off. Aer VIS-light irradiation the emission can be switched on again. The excited, ring-closed state thermally relaxes into the ringopen state due to the thermodynamically unfavorable strained ring conguration and the loss of aromaticity of the thiophene rings (Fig. 2A).39 In solutions and even in polymer lms the molecules can easily diffuse, thus compromising the addressability of a dened spatial storage “pixel” of information or readout. However, when incorporated into spatially conned colloidal particles, which are subsequently assembled into a two-dimensional array, diffusion of the dye is effectively suppressed and individual pixels with resolution dened by the size of the colloids are obtained. Increasing the stability of the dye system to prevent the thermally induced ring-opening cyclization that will compromise the storage of information over longer periods of time is obtained by embedding the dyes into a polymeric matrix. Moreover, addressable units of this dye system are necessary for a dened system of high addressability and easy readout. Therefore, small amounts of the dyes in dened ratios were encapsulated in polymeric colloids using the miniemulsion technique. As the polymeric matrix can affect the switching process of the dye because of geometric restrictions the inuence of the polymeric environment was investigated using poly(butyl acrylate) as a so polymer with low Tg (Tg
room temperature [RT]) and polystyrene as a rigid polymer (Tg [ RT). Polymers with low Tg such as poly(butyl acrylate) are not suited for a subsequent assembly of the formed colloids into a colloidal monolayer because they immediately fuse together and form a lm. To prevent lm formation and allow the application of so PBA colloids in self-assembly, a seeded emulsion polymerization approach was used to generate a rigid shell around the PBA colloids for protection.42,43 Another advantage of the seeded emulsion polymerization is the adjustment of size and reduction of size distribution,43 which promotes the formation of monolayers of higher order during

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the self-assembly process. The size distribution plays an important role in the creation of densely packed colloidal monolayers. The highest overall ordering quality can be obtained using particles of uniform size, whereas colloids of broader size distribution reduce the ordering quality at the air– water interface. We assemble the functional colloids into ordered monolayers and more complex binary monolayers and demonstrate optical switching with high resolution. With simple geometric models the particle size range for the small particles which can be co-crystallized can be determined (see ESI 3†).19 The binary monolayers prepared by the co-crystallization method are of high crystallinity and ordering degree because the interstices tolerate a broad range of sizes for the smaller colloids without disturbance of the self-assembly of the larger particles.

Investigation of optical properties of the dyes in solution The optical properties of the photochromic dye (CMTE) and the uorophore (PMI) were investigated in the monomers used for the preparation of colloids. Thus, solutions of CMTE and solutions of PMI in styrene and butyl acrylate were used for absorption (CTME) and uorescence emission (PMI) spectroscopy, respectively. The thermodynamically stable state of CMTE at room temperature is the ring-open form (Fig. 2A). By absorption of UVlight the molecule undergoes a cyclization reaction to form the ring-closed form (Fig. 2A). In contrast to the open form (black spectrum, Fig. 2B), the ring-closed isomer shows a broad absorption band with a maximum of l ¼ 520 nm (light grey spectrum, Fig. 2B). Although the isomerization from the ringclosed into the ring-open state is thermodynamically favoured, excellent thermal stability has been reported giving the photochrome a kind of “memory” ability.40 Investigation of the thermal stability of CMTE in the monomers styrene and butyl acrylate showed that the half-life of the ring-closed CMTE is about t1/2 ¼ 5.2 min in solutions of styrene and t1/2 ¼ 6.8 min in butyl acrylate,

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respectively. The rate constants were calculated from the linear decay t (eqn (1) and (2)) and summarized in Fig. 3. y ¼ mx + n

(1)

k ¼ m

(2)

y is the measured emission, x is the time, n is the intercept on the y-axis, and m is the slope from which the rate constant k is calculated (eqn (2)). As expected, the rate constants show that the thermally activated ring opening reaction of CMTE proceeds with almost similar reaction rates in both monomers (Fig. 3). The solvents do not seem to restrict conformational changes during isomerization. This should differ when CMTE is embedded in a polymeric matrix. In this conguration, the stiffness or exibility of the polymer, expressed by the glass transition temperature (Tg), is expected to have a bigger inuence on the geometry and conformational changes during cyclization and therefore on the switching process. Colloids with photoswitchable emission The miniemulsion polymerization is the appropriate technique to encapsulate multiple compounds in dened ratios in a variety of different polymeric materials. Therefore, a quantitative encapsulation of the dye system for switching uorescence emission with adjustable ratios in the used polymers, polystyrene and poly(butyl acrylate), is guaranteed. To investigate whether the polymeric matrix restricts the switching process we used styrene and butyl acrylate as monomers for the miniemulsion approach. The glass transition temperature of polystyrene is lowered from that of pure PS (100  C)44 because of the encapsulation of hexadecane and the dye molecules (Table 1). The rigidity of PS may lead to conformational restrictions for the geometric changes of the CMTE molecules during the irradiation process. Poly(butyl acrylate) on the other hand is a “so” polymer with Tg z 50  C (Table 1). We expect the photoswitching to occur faster in a soer and more exible polymeric environment, which means that the emission intensity of the uorescent dye should be reduced with higher rate constants in poly(butyl acrylate) than in polystyrene. The

Fig. 3 Rate constants in logarithmic scale for the isomerization induced by irradiation with UV (dark grey) and VIS-light (light grey) as well as thermally induced isomerization (black) for the colloids with varying amounts of CMTE.

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Table 1

Physicochemical characterization of the nanoparticles

Polymer

PMI : CMTE

d/nm

s/nm

Distribution/%

Tg/ C

PS PS PBA PBA PS–PS PBA–PS

1 : 18 1 : 37 1 : 18 1 : 37 1 : 18 1 : 18

84 94 159 155 191 260

14 16 20 30 17 31

17 17 13 19 9 12

71 72 56 49 88 93

irradiation experiments with UV and visible light of the different polymeric colloids with varying amounts of photochromic dye are described in the following.

Photocyclization by UV-light irradiation The absorption of PMI's uorescence emission can be described as intercomponent energy transfer between the uorophore and the photochrome. Thus, the distance between the CMTE and the PMI molecules, which is determined by the concentration, affects the efficiency of the absorption of PMI's emission. The efficiency of the energy transfer decreases with increasing molecule distance. Thus, we investigated the inuence of varying amounts of photochromic dye on the energy transfer or on the switching process, respectively, in different polymeric environments. The development of the uorescence intensity of the PMI during the irradiation can be followed easily by uorescence spectroscopy as the emission maximum at a wavelength of l ¼ 561 nm changes with time due to the formation of the ring-closed or ring-open state of CMTE and, therefore, varying concentrations of molecules absorbing the uorescence intensity of PMI. Emission spectra were recorded aer given intervals of UV-light irradiation until no further increase of the emission signal was visible (ESI 2†). Fig. 4A shows the time dependent decrease of the emission of the uorescent dye PMI in a colloidal system of polystyrene and poly(butyl acrylate) in the presence of the photochromic dye CMTE (ratios of PMI to CMTE 1 : 18 and 1 : 37). Aer about 3–4 min of UV-irradiation no further decay of uorescence is visible, indicating that the entire CMTE has isomerized into the ringclosed form. From the data, the rate constants for the decrease of the uorescence intensity at the maximum at l ¼ 561 nm were calculated with an exponential t (eqn (3) and (4)). y ¼ A exp(x/t) + y0

(3)

k ¼ t1

(4)

y is the measured emission, x is the time, A is the amplitude of the exponential t, y0 is the offset, and t is the time constant from which the rate constant k is calculated. The rate constants (Fig. 3) show that the uorescence emission intensity of the PMI in PS and in PBA can be successfully reduced by the ring-closed form of the CMTE. As expected, the switching process in poly(butyl acrylate) is about 60% faster than the photoswitching in polystyrene. The stiffness of the polystyrene matrix could

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Fig. 4 Time dependence of the fluorescence intensity (l ¼ 561 nm) of the colloidal dispersions of PS and PBA particles during (A) UV-light irradiation, (B) VIS-light irradiation and (C) storage of the dispersions in the dark at room temperature for PMI/CMTE ratios of 1 : 18 (black dots and red triangles pointing up) and 1 : 37 (blue triangles pointing down and green rhombi). Only one representative red dashed fitting curve per diagram is shown for clarity.

decelerate the isomerization process because of geometric or steric restrictions. Moreover, the effect of CMTE concentration on the switching efficiency was investigated. The temporal evolution of the uorescence emission of the dispersions upon irradiation with UV-light containing a higher amount of CMTE (PMI : CMTE as 1 : 37, Fig. 4A, blue triangles pointing down and green rhombi as data points) shows that the uorescence stays at a constant level aer 3 min for PS and PBA, respectively. The emission of the system containing the dyes with ratio 1 : 37 in PS as well as in PBA is signicantly lower than that with a ratio of PMI : CMTE of 1 : 18 (Fig. 4A, black and red dots). As expected, the switching process with higher concentration of CMTE was faster with increasing amount of activated quenching molecules next to the uorescent dye (Fig. 3). Compared to the rate constants of the particles with a ratio of PMI : CMTE of 1 : 18, the rate constants of the intercomponent energy transfer process between the ring-closed form of CMTE and the PMI can be increased by 37% for the polystyrene particles and 21% for the poly(butyl acrylate) nanoparticles when using a higher amount of CMTE (ratio PMI : CMTE 1 : 37).

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VIS-light induced reverse cyclization isomerization For the reverse isomerization process the samples were irradiated with visible light of wavelength range 515 nm < l < 690 nm. The corresponding emission spectra (Fig. 4B) show a time dependent increase of the uorescence maximum at l ¼ 561 nm, which correlates with the increasing amount of CMTE molecules isomerized into the ring-open state. These are not able to quench the emission because of the absence of spectral overlap of CMTE absorption and PMI emission. The initial intensity of PMI emission in polystyrene recovered aer 20 min of irradiation. The switching rate constants were similar in the different polymeric environments of PS and PBA colloids and generally about an order of magnitude lower than that of the UV-induced isomerization (Fig. 3). For the polystyrene particles with a dye ratio of PMI : CMTE of 1 : 37 a rate constant 20 times slower and for the dye ratio 1 : 18 a rate constant 10 times slower compared to the isomerization in solution were calculated for the VIS-light induced ring-opening reaction. For the PBA particles we observed similar behavior but the isomerization is about 20 times slower for a ratio of PMI/ CMTE of 1 : 18 and 30 times slower for a ratio of 1 : 37 as the isomerization in solution.

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Reversibility of the light induced cyclization process We have successfully shown that the uorescence emission of the colloids with the chosen dye system can be switched by optical stimulation at different wavelengths between UV and VIS-light. For any application a high reversibility and high number of cyclization cycles of the emission are necessary. To perform several switching cycles we used alternating irradiation with visible and UV-light of PS colloids with the ratio of PMI/CMTE of 1 : 18. The dispersion was irradiated with UVlight rst to ensure all CMTE is present in the excited state, whereas the particles' emission was switched off. Aerwards, the ring-opening reaction was initiated by VIS-light irradiation isomerizing CMTE to the ground state, thus, switching the particles on. Aer 30 min of irradiation the emission of the sample was investigated by uorescence spectroscopy. Aerwards, the dispersion was again irradiated with UV-light to initiate the ring-closing reaction. This procedure was repeated several times to obtain a higher number of switching cycles. The resulting emission intensities show the presence of the two states for uorescence emission on/off in colloids for a wellrepeatable isomerization, whereas the emission intensities can be restored completely for every switching cycle, indicating good photoswitchability (Fig. 5).

Thermal stability of colloids prepared by miniemulsion polymerization The stability of the individual on/off states in the absence of a stimulus is of great importance. Storage of information can only be realized with stable and dened states. Although the ringclosed form of CMTE is the thermodynamically unstable state, the thermally activated ring opening isomerization can be decelerated by embedding the dye molecules in a polymeric environment of a colloid. Fig. 4C shows the time evolution of the uorescence intensity at l ¼ 561 nm of UV-light irradiated dispersions (room temperature). In both polymeric particles, the uorescence recovery is very slow. Complete recovery of the emission was not observed even aer more than 10 days. The data acquired from the system with a ratio of PMI : CMTE of

Fig. 5 Fluorescence emission at l ¼ 561 nm of the PS dispersion (PMI/CMTE 1 : 18) for reversible switching between the on and the off state induced by alternating irradiation with UV and VIS-light over five switching cycles.

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Paper 1 : 37 are also shown in Fig. 4C. Compared to the nanoparticles with a lower ratio of PMI : CMTE (1 : 18) the uorescence intensity decreases with a similar rate constant. Comparing the rate constants for the thermally induced restoration it is obvious that the reverse cyclization reaction into the ring-open form proceeds with a similar velocity not depending on the nature of the surrounding polymeric matrix. The results underline the excellent thermal stability of the ringclosed state of CMTE and the steric hindrance of the isomerization. The dye system is at least 20 times more stable when being embedded in a polymeric matrix than in solution and light-induced information can easily be stored for more than 10 days. To summarize, the optical properties and the rate constants conrm that the photoswitching with UV-light proceeds faster in a so polymeric matrix such as poly(butyl acrylate) (Tg ¼ 50  C) than in a rigid matrix such as polystyrene (Tg ¼ 71  C). The VIS-light induced ring opening reaction has a rate constant which is about 10 times smaller than the rate constant for UVlight induced ring closing cyclization. The uorescence emission can be reversibly recovered by alternating irradiation with visible light within the wavelength range of 515 nm < l < 690 nm and UV-light for several switching cycles without photobleaching effects. The states of the photochromic system shows excellent thermal stability when being embedded in polymeric colloids.

Optical properties of colloids prepared by seeded emulsion polymerization As mentioned earlier, the formation of a PS shell around the functional colloids serves two purposes. First, PBA is covered by a rigid shell so that it can be used for the self-assembly and deposition process, and second, the size distribution of the colloids can be narrowed by the process.44 The optical properties of these colloids were also investigated and they largely resemble that of the primary colloids. The data are summarized in Fig. 6. As the seed particle ideally does not change, the resulting optical properties are expected to be similar to that of the seed particle, with the exception of a lower emission intensity. Although the emission intensity is less intense than that from the seed particles the decrease in the emission maximum at l ¼ 561 nm with proceeding isomerization is still visible and follows an exponential decay (Fig. 6A). The corresponding rate constants of the seeded particles (Fig. 3) show that the switching process during irradiation with UV-light was decelerated compared to the original seed particles from miniemulsion. The differences may arise from scattering effects due to the enlargement of the particles and the additional shell of polystyrene whereby less light reaches the inner functional core of the hybrid seeded particles. The uorescence recovery induced by irradiation with visible light with a wavelength range of 515 nm < l < 690 nm is shown in Fig. 6B. Aer 15 min the uorescence intensity reaches the initial value. The hybrid particles with an additional shell of polystyrene show similar switching behavior to the seed

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Fig. 6 Time dependent development of the fluorescence intensity at l ¼ 561 nm of the colloidal dispersions of seeded PS–PS and PBA–PS particles during (A) UV-light irradiation, (B) VIS-light irradiation and (C) storage of the dispersions in the dark at room temperature for the ratio of PMI/CMTE of 1 : 18 (black dots and red triangles).

particles. The switching process seems to be slightly decelerated (Fig. 3). This effect might be attributed to scattering effects of the larger particles whereas a low light intensity reaches the dye system. Fig. 6C shows the time dependent development of the thermally induced ring opening reaction of CMTE. The increase of the uorescence intensity of PMI at l ¼ 561 nm was plotted vs. time and the rate constants were calculated from the linear t (eqn (1) and (2)) (Fig. 3). An undesired back-switching to the initial state was not observed for PS–PS colloids in the timeframe of the experiment (several days), thus underlining the excellent stability of the photo-states when incorporated in polymer particles. Moreover, the rate constants for the hybrid particles lie in the same range as the seed colloids without an additional PS shell (Fig. 3). In addition, the excellent thermal stability of the system was also shown in hybrid particle systems.

Direct self-assembly of seeded colloids at the air–water interface We investigated the optical properties of photoswitchable colloids and showed an excellent thermal stability for the on/off states in all cases. For the creation of a model system for data

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storage or light erasable barcoding the colloids need to be precisely located in dened positions. This can be realized using the self-assembly approach of colloids at the air–water interface on a Langmuir trough into two dimensional highly ordered colloidal monolayers. The primary colloids with a size distribution of s > 10% were not suitable for the production of high quality colloidal monolayers. Therefore, we produced larger particles (Table 1) by seeded emulsion polymerization with a narrower size distribution of s ¼ 9%. If the size distribution is too broad, the particles cannot form a hexagonal lattice and the arrangement at the interface is disturbed. Another possibility to create highly ordered monolayers is the formation of binary colloidal monolayers. Thus, two differently sized particle systems are co-assembled at the interface. We used carboxylated polystyrene particles of about 1150 nm as the template for the particles with photoswitchable emission. The smaller seeded colloids are located in the interstices of the resulting hexagonal lattice of the large particles. Therefore, it was possible to create binary colloidal crystals of high ordering quality, which can be seen in the resulting scanning electron micrographs (Fig. 7A and D). These two dimensional crystals, having domain sizes of several hundreds of mm2, were evaluated graphically by assessing the possible arrangements of the

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Fig. 7 (A) SEM image of an ordered binary colloidal monolayer with d ¼ 1150 nm template colloids and photoswitchable PS–PS secondary seeded particles with a number ratio of Nlarge/Nsmall of 1 : 6 and (B) the corresponding statistical analysis for the possible arrangements of the smaller photoswitchable colloids located in the interstitial positions with an arrangement of max. 6 colloids, (C) confocal laser scanning micrograph of a binary monolayer from seeded PS–PS particles shown in (A), (D) SEM image of a binary monolayer with d ¼ 1150 nm template colloids and small photoswitchable PBA–PS seeded particles with a number ratio of Nlarge/Nsmall of 1 : 6 and (E) the statistical analysis for the monolayer shown in (D) for PBA–PS colloids with an arrangement of max. 3 colloids in the interstices.

particles at the interstitial sites. Therefore, the size of the interstitial gaps between the particles was calculated (see ESI 3† for a detailed evaluation) and the seeded PS–PS and PBA–PS particles correlated with the size of the gaps. The resulting geometries were distinguished and identied in the scanning electron micrographs (Fig. 7A and D). The number ratio of larger to small particles Nlarge/Nsmall was adjusted to 1 : 6 at the interface. The statistical evaluation of the binary monolayers shows a predominance of 3 colloids (76%) located in the interstitial positions for the PS–PS particles (Fig. 7B), rather than the maximum arrangement of 6 colloids (up to 6%), which is also in agreement with the adjusted number ratio of Nlarge/ Nsmall of 1 : 6 for template to photoswitchable particles. Fig. 7E shows the statistical evaluation of the binary monolayer of template particles with photoswitchable PBA–PS colloids. As the PBA–PS particles are larger (d ¼ 260 nm) than the PS–PS particles (d ¼ 191 nm), the maximum number of particles for the co-localization at the interstitial site is a geometrical arrangement of 3 colloids (see ESI 3†). The histogram shows again a predominance of 3 colloids (63%) located in the interstitial positions (Fig. 7E). As can be seen in the scanning electron microscopy (SEM) images and from the statistical evaluation (Fig. 7A, B, D and E) the binary colloidal monolayers tolerate a relatively broad size distribution of the smaller colloids located at the interstitial sites of the carboxylated PS template particles, without disturbing the high order of the template colloids. With the number ratio of large particles to small particles (Nlarge/Nsmall) the conguration can be adjusted reliably, even with broadly distributed colloids.

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The binary colloidal monolayer of non-uorescent template PS particles and photoswitchable PS–PS particles was also investigated by confocal laser scanning microscopy. The corresponding CLSM image (Fig. 7C) shows a very high degree of ordering, where the larger template colloids arranged in a hexagonal lattice (dark spots) and the interstitial sites are lled with the smaller uorescent seeded particles. Therefore, it would be possible to address one single interstitial site in a very efficient and dened way because of the high crystallinity of the functional colloidal monolayer.

Photoswitching in colloidal monolayers Finally, we demonstrate the successful use of a colloidal monolayer to reversibly store (and erase) information with light. An optical uorescence microscope was used for the investigation of the photoswitching of the colloidal monolayers. The substrate was rst irradiated with visible light to obtain the ring-open state of the CMTE and the highest possible uorescence intensity of the colloidal particles (Fig. 8A). Aerwards, part of the substrate was exposed to UV-light to induce the photocyclization reaction of the CMTE into the ring-closed state, whereas the uorescence emission of the uorophore is absorbed and the uorescence is switched off in the exposed areas (Fig. 8B). Irradiation with visible light can be used to reversibly erase the stored information: aer illumination for 300 ms, a complete recovery of the uorescence intensity in the monolayer is observed (ESI 4†).

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Fig. 8 Optical fluorescence micrographs of photoswitchable PBA colloidal monolayer (A) with maximum emission of the fluorophore PMI after 5 min irradiation with visible light and (B) partly switched off PBA monolayer (left circular part of the sample) after being exposed to UV-light for 5 min.

Nanoscale through the dispersion for 30 min. 500 mg of the initiator ammonium peroxodisulfate (APS) were dissolved in 3 mL water and added using a syringe to the stirred dispersion. 2 g of styrene and 150 mg of acrylic acid were mixed and added with a syringe pump at a ow rate of 1 mL h1 to the stirred dispersion. Once the addition of monomer was nished, the reaction was le for polymerization at 80  C for 24 h under an argon atmosphere. The dispersion was ltered and dialyzed (membranes with MWCO 14 000 g mol1, Carl Roth, Karlsruhe, Germany) for three days in demineralized water changing the water twice a day.

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Self-assembly of colloidal monolayers on a Langmuir trough

Sodium dodecyl sulfate (SDS, Sigma Aldrich), 2,20 -azobis(2methylbutyronitrile) (V59, WAKO-Chemicals), ammonium peroxodisulfate (APS, Sigma Aldrich), hexadecane (99%, Sigma Aldrich), N-(2,6-diisopropylphenyl)-perylene-3,4-dicarboximide (PMI, a donation of BASF), cis-1,2-dicyano-1,2-bis-(2,4,5-trimethyl-3-thienyl)ethene (CMTE, TCI Europe) and ethanol (VWR) were used as received. The monomers styrene (S, Sigma Aldrich) and butyl acrylate (BA, Sigma Aldrich) were puried using ash column chromatography with aluminum oxide to remove any containing inhibitor. Water of Milli-Q quality was used throughout the experiments.

Prior use, the trough (242 cm2) was thoroughly cleaned with ethanol and several times with Milli-Q water. For spreading the colloidal dispersions (1.5 wt%) were puried with an ethanol– water mixture (1 : 1 v/v) by centrifugation and redispersion. Subsequently, the dispersions were diluted with ethanol to a nal dispersion of 1.5 wt% containing 50 vol% ethanol. The dispersion was added to the interface via a tilted glass slide reaching into the subphase.45,46 Aer 15 min, the monolayer was compressed at a speed of 20 mm min1 and deposited onto a silicon substrate by surface lowering transfer.19 The monolayers were analyzed using scanning electron microscopy (SEM, LEO (Zeiss) 1530 Gemini, Oberkochen, Germany), uorescence and confocal laser scanning microscopy (CLSM, LSM SP5 STED Leica, Germany).

Miniemulsion polymerization

UV-VIS and uorescence spectroscopy

Experimental section Materials

100 mg of sodium dodecyl sulfate (SDS) were dissolved in 24 g of water and stirred at 1000 min1. Another solution was prepared separately from the monomer (styrene or butyl acrylate), 250 mg of the osmotic pressure agent hexadecane, 100 mg of the initiator 2,20 -azobis(2-methylbutyronitrile) (V59) and the desired amounts of uorescence dye PMI and photochromic dye CMTE. The monomer solution was poured into the aqueous solution and stirred for 1 h. Subsequently, the emulsion was homogenized by ultrasonication with a Branson digital sonier 450-D (Dietzenbach, Germany) under ice-cooling using 90% amplitude, ½00 tip for 120 s with a 10 s pulse–10 s pause program. The miniemulsion was transferred into a round bottom ask and polymerized under continuous stirring for 12 h at 72  C. The resulting dispersion was ltered and dialyzed (membranes with MWCO 14 000 g mol1, Carl Roth, Karlsruhe, Germany) for three days in demineralized water, changing the water twice a day. Particle sizes were determined by photon cross-correlation spectroscopy (Nanophox, Sympatec GmbH, Clausthal-Zellerfeld, Germany). The solid content was determined gravimetrically aer freeze-drying. Seeded emulsion polymerization A dispersion of 100 mL Milli-Q water containing 0.1 wt% of seed particles and 0.01 wt% of SDS was heated to 75  C under continuous stirring in a three necked round bottom ask equipped with a condenser and septa. Argon gas was bubbled

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The absorption spectra of the photochromic dye in solution were obtained using a Perkin Elmer Lambda 25 UV-VIS spectrometer. 3 mL of a solution with a concentration of c ¼ 1  106 mol L1 of the photochrome in the corresponding monomer (S or BA) was analyzed. Fluorescence emission spectra were recorded on a TIDAS 3D uorescence spectrometer with an excitation wavelength of lex ¼ 490 nm. For the measurements in monomer 3 mL of a solution of PMI with a concentration of c ¼ 5.2  106 mol L1 were used. The investigations of the colloidal dispersions were made using 3 mL with a solid content of 1.5 wt%. For irradiation a mercury short arc lamp (HBO 100 W/2, 100 W, Osram) was used. The wavelength was adjusted with different optical band lters. For UV-irradiation a dark violet band pass (UG 1, Schott, 270–430 nm) and for VIS-light a combination of a light blue broad band (BG39, Schott) and a yellow optical lter (OG515, Schott) were used transmitting light within a wavelength range of 515 to 690 nm. To investigate the thermal stability of the system the samples irradiated with UVlight were covered with aluminum foil and stored in the dark at room temperature recording emission spectra aer given times. For the photoswitching of the functional colloidal monolayers an optical wide eld uorescence microscope (Olympus IX81, inverted uorescence microscope, Hamburg, Germany) with a 2.5 objective (UIS2), a 100 W halogen lamp and different lters, such as a DAPI lter (UMNU 2, lex ¼ 360 nm and lem ¼ 420 nm) for the UV-light irradiation and an eGFP

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Physicochemical characterization of nanoparticles Each dispersion was characterized using photon cross-correlation spectroscopy with auto-NNLS analysis for the determination of the particle size and size distribution with s as standard deviation. Differential scanning calorimetry was used for the analysis of glass transition temperatures. The corresponding results are summarized in Table 1. Polystyrene colloids produced by miniemulsion polymerization had an average diameter of around 90 nm with a size distribution of 17%, whereas the PBA colloids were about 155 to 160 nm in size with a distribution of 13–19%. The corresponding glass transition temperatures were measured by DSC and result around 71  C for polystyrene and 56 and 49  C for the poly(butyl acrylate) particles. Aer seeded emulsion polymerization the hybrid core–shell PS–PS particles had a size of d ¼ 191 nm with a size distribution of 9%, whereas the PBA–PS particles were larger with d ¼ 260 nm and a size distribution of 12%. The resulting Tg was similar aer the seeded emulsion polymerization step with around 90  C for both batches (PS–PS, PBA–PS). Compared to the colloids from miniemulsion the glass transition temperature is about 20  C higher. For the miniemulsion technique hexadecane is used as an osmotic agent against Ostwald's ripening, lowering the glass transition temperature of the polystyrene colloids. With an additional rigid shell of polystyrene containing less or even no hexadecane, a higher glass transition temperature is reached. The amount of PBA was too low (23%) to see both Tg steps in the thermogram. SEM images of all particles were obtained (see ESI 1†).

Paper without risking of degradation by uncontrolled back reactions. Moreover, the precise incorporation of dened amounts of the dye molecules by miniemulsion polymerization can be used to optimize the quenching of the uorophore emission. The lightinduced switching of uorescence is completely reversible and cyclable without observation of intensity loss. Functional colloidal monolayers were prepared using the co-assembly method of colloids of two distinct sizes at the air–water interface. The obtained binary monolayers of functional seeded particles and plain larger particles showed high crystallinity of several hundreds of mm2 resulting in addressability of the functional colloids. The possible arrangements for the seeded particles located at the interstices of the hexagonally ordered template particles were assessed from the scanning electron micrographs, resulting in a preferred number of three particles at the interstices for the seeded polystyrene as well as for the seeded poly(butyl acrylate) particles. The preparation method of functional binary monolayers offers the possibility of selfassembly of colloids with broad size distribution into highly ordered hexagonal lattices. Therefore, even single particles or smaller particle arrangements can be addressed. We demonstrate successful reversible optically induced storage and elimination of information in such monolayer structures. We envision this technology to be of broad interest in information technology as it provides a cheap, simple and efficient way to generate precise substrates for optical manipulation at micro- and nanoscale.

Acknowledgements Gabriele Sch¨ afer is thanked for providing the 1150 nm carboxylated polystyrene colloids, Petra R¨ ader for DSC measurements, and Dr Sandra Ritz for her help in confocal laser scanning microscopy. NV acknowledges funding from the Leopoldina Fellowship Program.

Conclusion In summary, we present an easy and fast method for the production of functional colloidal monolayers with reversibly photoswitchable colloids. The photoswitchable functional polystyrene and poly(butyl acrylate) colloids were synthesized using the miniemulsion polymerization process to incorporate the appropriate dye system based on a perylene as a uorophore (N-(2,6-diisopropylphenyl)-perylene-3,4-dicarboximide) and a bis-arylethene as a photochrome (cis-1,2-dicyano-1,2-bis-(2,4,5trimethyl-3-thienyl)ethene). Thus, addressable photoswitchable entities are generated. We investigate light-induced switching of the uorescence intensity of the uorophore via selectively enabling and disabling energy transfer to the photochrome molecule in the conned environment of a colloidal particle. By embedding the dyes in a polymeric matrix of colloids the thermal stability of the photochrome in the excited ring-closed state can be increased by a factor of more than 20 compared to the stability in solution. Information stored in the colloids by the photoswitching process can thus be retained over at least several days

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