K.
Bley
a,
N.
Sinatra
a,
N.
Vogel
ab,
K.
Landfester
a and
C. K.
Weiss
*ac
aMax-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
bSchool of Engineering and Applied Sciences, Harvard University, McKay 426, 9 Oxford Street, Cambridge, MA 02138, USA
cUniversity of Applied Sciences Bingen, Berlinstrasse 109, 55411 Bingen, Germany. E-mail: c.weiss@fh-bingen.de
First published on 29th October 2013
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 perylene-based 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 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-assembled colloidal monolayers. The arrangement of broadly distributed functional colloids into ordered monolayers with high addressability was obtained by the formation of binary colloidal monolayers.
The defined 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 defined 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 defined amounts35 and multiple compounds in defined ratios.36 This has not only been shown for metal complexes, but also for the co-encapsulation of a photochromic dye and a fluorophore. Furukawa37 has shown that the simultaneous encapsulation of fluorescent and photochromic dyes following the miniemulsion approach can lead to inter-component energy transfer between the dye molecules to create particles with light-switchable fluorescence. The authors used a BODIPY dye (4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene) as the fluorophore and cis-1,2-dicyano-1,2-bis-(2,4,5-trimethyl-3-thienyl)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 fluorophore, quenching the emission.
In this contribution, we study in detail the optical switching properties of a fluorophore/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 fluorophore 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 influence 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 close-packed monolayer of larger colloids. This strategy is also suitable for PBA colloids, after they have been subjected to a seeded emulsion polymerization process with styrene as a monomer in order to prevent film 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 light-switchable emission in the colloidal monolayers.
To produce functional photoswitchable colloidal monolayers a dye system based on a bis-thienyl photochrome (cis-1,2-dicyano-1,2-bis-(2,4,5-trimethyl-3-thienyl)ethene, CMTE) and a perylene based fluorophore (N-(2,6-diisopropylphenyl)-perylene-3,4-dicarboximide, PMI) was used (Fig. 2A).
The dye system allows switching on and off the fluorescence emission of the PMI. By irradiating the photochrome CMTE with UV and VIS-light the CMTE molecule undergoes a ring-closing or ring-opening cyclization (Fig. 2A and B), respectively. In the ring-closed excited state the CMTE is able to absorb the emission of the PMI between 500 nm < λ < 620 nm (Fig. 2B). Therefore, the emission of PMI is switched off. After VIS-light irradiation the emission can be switched on again.
The excited, ring-closed state thermally relaxes into the ring-open state due to the thermodynamically unfavorable strained ring configuration and the loss of aromaticity of the thiophene rings (Fig. 2A).39 In solutions and even in polymer films the molecules can easily diffuse, thus compromising the addressability of a defined spatial storage “pixel” of information or readout. However, when incorporated into spatially confined colloidal particles, which are subsequently assembled into a two-dimensional array, diffusion of the dye is effectively suppressed and individual pixels with resolution defined 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 defined system of high addressability and easy readout. Therefore, small amounts of the dyes in defined 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 influence of the polymeric environment was investigated using poly(butyl acrylate) as a soft 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 film. To prevent film formation and allow the application of soft 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 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.
The thermodynamically stable state of CMTE at room temperature is the ring-open form (Fig. 2A). By absorption of UV-light 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 λ = 520 nm (light grey spectrum, Fig. 2B). Although the isomerization from the ring-closed 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 = 5.2 min in solutions of styrene and t1/2 = 6.8 min in butyl acrylate, respectively. The rate constants were calculated from the linear decay fit (eqn (1) and (2)) and summarized in Fig. 3.
y = mx + n | (1) |
k = −m | (2) |
Polymer | PMI:CMTE | d/nm | σ/nm | Distribution/% | T g/°C |
---|---|---|---|---|---|
PS | 1:18 | 84 | 14 | 17 | 71 |
PS | 1:37 | 94 | 16 | 17 | 72 |
PBA | 1:18 | 159 | 20 | 13 | −56 |
PBA | 1:37 | 155 | 30 | 19 | −49 |
PS–PS | 1:18 | 191 | 17 | 9 | 88 |
PBA–PS | 1:18 | 260 | 31 | 12 | 93 |
The efficiency of the energy transfer decreases with increasing molecule distance. Thus, we investigated the influence of varying amounts of photochromic dye on the energy transfer or on the switching process, respectively, in different polymeric environments. The development of the fluorescence intensity of the PMI during the irradiation can be followed easily by fluorescence spectroscopy as the emission maximum at a wavelength of λ = 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 fluorescence intensity of PMI. Emission spectra were recorded after 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 fluorescent 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). After about 3–4 min of UV-irradiation no further decay of fluorescence is visible, indicating that the entire CMTE has isomerized into the ring-closed form. From the data, the rate constants for the decrease of the fluorescence intensity at the maximum at λ = 561 nm were calculated with an exponential fit (eqn (3) and (4)).
y = Aexp(−x/t) + y0 | (3) |
k = t−1 | (4) |
Moreover, the effect of CMTE concentration on the switching efficiency was investigated. The temporal evolution of the fluorescence 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 fluorescence stays at a constant level after 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 significantly 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 fluorescent 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).
The initial intensity of PMI emission in polystyrene recovered after 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.
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 UV-light first to ensure all CMTE is present in the excited state, whereas the particles' emission was switched off. Afterwards, the ring-opening reaction was initiated by VIS-light irradiation isomerizing CMTE to the ground state, thus, switching the particles on. After 30 min of irradiation the emission of the sample was investigated by fluorescence spectroscopy. Afterwards, 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 fluorescence emission on/off in colloids for a well-repeatable isomerization, whereas the emission intensities can be restored completely for every switching cycle, indicating good photoswitchability (Fig. 5).
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 ring-closed 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 confirm that the photoswitching with UV-light proceeds faster in a soft polymeric matrix such as poly(butyl acrylate) (Tg = −50 °C) than in a rigid matrix such as polystyrene (Tg = 71 °C). The VIS-light induced ring opening reaction has a rate constant which is about 10 times smaller than the rate constant for UV-light induced ring closing cyclization. The fluorescence emission can be reversibly recovered by alternating irradiation with visible light within the wavelength range of 515 nm < λ < 690 nm and UV-light for several switching cycles without photo-bleaching effects. The states of the photochromic system shows excellent thermal stability when being embedded in polymeric colloids.
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 λ = 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 fluorescence recovery induced by irradiation with visible light with a wavelength range of 515 nm < λ < 690 nm is shown in Fig. 6B. After 15 min the fluorescence intensity reaches the initial value. The hybrid particles with an additional shell of polystyrene show similar switching behavior to the seed 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 fluorescence intensity of PMI at λ = 561 nm was plotted vs. time and the rate constants were calculated from the linear fit (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.
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 = 260 nm) than the PS–PS particles (d = 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 configuration can be adjusted reliably, even with broadly distributed colloids.
The binary colloidal monolayer of non-fluorescent 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 filled with the smaller fluorescent seeded particles. Therefore, it would be possible to address one single interstitial site in a very efficient and defined way because of the high crystallinity of the functional colloidal monolayer.
Irradiation with visible light can be used to reversibly erase the stored information: after illumination for 300 ms, a complete recovery of the fluorescence intensity in the monolayer is observed (ESI 4†).
For irradiation a mercury short arc lamp (HBO 100 W/2, 100 W, Osram) was used. The wavelength was adjusted with different optical band filters. 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 filter (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 UV-light were covered with aluminum foil and stored in the dark at room temperature recording emission spectra after given times.
For the photoswitching of the functional colloidal monolayers an optical wide field fluorescence microscope (Olympus IX81, inverted fluorescence microscope, Hamburg, Germany) with a 2.5× objective (UIS2), a 100 W halogen lamp and different filters, such as a DAPI filter (UMNU 2, λex = 360 nm and λem = 420 nm) for the UV-light irradiation and an eGFP filter (U-MF2, λex = 472 nm and λem = 520 nm) for the VIS-light irradiation were used. The sample was alternately irradiated for 5 min with either UV or VIS-light and images were taken using the eGFP filter for the excitation of PMI.
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. After seeded emulsion polymerization the hybrid core–shell PS–PS particles had a size of d = 191 nm with a size distribution of 9%, whereas the PBA–PS particles were larger with d = 260 nm and a size distribution of 12%. The resulting Tg was similar after 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†).
We investigate light-induced switching of the fluorescence intensity of the fluorophore via selectively enabling and disabling energy transfer to the photochrome molecule in the confined 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 without risking of degradation by uncontrolled back reactions. Moreover, the precise incorporation of defined amounts of the dye molecules by miniemulsion polymerization can be used to optimize the quenching of the fluorophore emission. The light-induced switching of fluorescence 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 μm2 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 self-assembly 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.
Footnote |
† Electronic supplementary information (ESI) available: S1: scanning electron micrographs of the particles prepared by miniemulsion polymerization and seeded emulsion polymerization, S2: fluorescence 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 fluorescence micrographs for the photoswitching of a functional colloidal monolayer. See DOI: 10.1039/c3nr04897g |
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