@article{UmlandtFeldmannSchnecketal.2020,
  author    = {Umlandt, Maren and Feldmann, David and Schneck, Emanuel and Santer, Svetlana A. and Bekir, Marek},
  title     = {Adsorption of photoresponsive surfactants at solid-liquid interfaces},
  series = {Langmuir},
  volume    = {36},
  journal   = {Langmuir},
  number    = {46},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0743-7463},
  doi       = {10.1021/acs.langmuir.0c02545},
  pages     = {14009 -- 14018},
  year      = {2020},
  abstract  = {We report on the adsorption kinetics of azoben-zene-containing surfactants on solid surfaces of different hydrophobicity. The understanding of this processes is of great importance for many interfacial phenomena that can be actuated and triggered by light, since the surfactant molecules contain a photoresponsive azobenzene group in their hydrophobic tail. Three surfactant types are studied, differing in the spacer connecting the headgroup and the azobenzene unit by between 6 and 10 CH2 groups. Under irradiation with light of a suitable wavelength, the azobenzene undergoes reversible photoisomerization between two states, a nonpolar trans-state and a highly polar cis-state. Consequently, the surfactant molecule changes its hydrophobicity and thus affinity to a surface depending on the photoisomerization state of the azobenzene. The adsorption behavior on hydrophilic (glass) and hydrophobic (TeflonAF) surfaces is analyzed using quartz crystal microbalance with dissipation (QCM-D) and zeta-potential measurements. At equilibrium, the adsorbed surfactant amount is almost twice as large on glass compared to TeflonAF for both isomers. The adsorption rate for the trans-isomers on both surfaces is similar, but the desorption rate of the trans-isomers is faster at the glass-water interface than at the Teflon-water interface. This result demonstrates that the trans-isomers have higher affinity for the glass surface, so the trans-to-cis ratios on glass and TeflonAF are 80/1 and 2/1, respectively, with similar trends for all three surfactant types.},
  language  = {en}
}
@article{AryaJelkenLomadzeetal.2020,
  author    = {Arya, Pooja and Jelken, Joachim and Lomadze, Nino and Santer, Svetlana A. and Bekir, Marek},
  title     = {Kinetics of photo-isomerization of azobenzene containing surfactants},
  series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistry},
  volume    = {152},
  journal   = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistry},
  number    = {2},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-9606},
  doi       = {10.1063/1.5135913},
  pages     = {10},
  year      = {2020},
  abstract  = {We report on photoisomerization kinetics of azobenzene containing surfactants in aqueous solution. The surfactant molecule consists of a positively charged trimethylammonium bromide head group, a hydrophobic spacer connecting via 6 to 10 CH2 groups to the azobenzene unit, and the hydrophobic tail of 1 and 3CH(2) groups. Under exposure to light, the azobenzene photoisomerizes from more stable trans- to metastable cis-state, which can be switched back either thermally in dark or by illumination with light of a longer wavelength. The surfactant isomerization is described by a kinetic model of a pseudo first order reaction approaching equilibrium, where the intensity controls the rate of isomerization until the equilibrated state. The rate constants of the trans-cis and cis-trans photoisomerization are calculated as a function of several parameters such as wavelength and intensity of light, the surfactant concentration, and the length of the hydrophobic tail. The thermal relaxation rate from cis- to trans-state is studied as well. The surfactant isomerization shows a different kinetic below and above the critical micellar concentration of the trans isomer due to steric hindrance within the densely packed micelle but does not depend on the spacer length.},
  language  = {en}
}
@article{MuravevaBekirLomadzeetal.2022,
  author    = {Muraveva, Valeriia and Bekir, Marek and Lomadze, Nino and Großmann, Robert and Beta, Carsten and Santer, Svetlana A.},
  title     = {Interplay of diffusio- and thermo-osmotic flows generated by single light stimulus},
  series = {Applied physics letters},
  volume    = {120},
  journal   = {Applied physics letters},
  number    = {23},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/5.0090229},
  pages     = {5},
  year      = {2022},
  abstract  = {Flow control is a highly relevant topic for micromanipulation of colloidal particles in microfluidic applications. Here, we report on a system that combines two-surface bound flows emanating from thermo-osmotic and diffusio-osmotic mechanisms. These opposing flows are generated at a gold surface immersed into an aqueous solution containing a photo-sensitive surfactant, which is irradiated by a focused UV laser beam. At low power of incoming light, diffusio-osmotic flow due to local photo-isomerization of the surfactant dominates, resulting in a flow pattern oriented away from the irradiated area. In contrast, thermo-osmotic flow takes over due to local heating of the gold surface at larger power, consequently inducing a flow pointing toward the hotspot. In this way, this system allows one to reversibly switch from outward to inward liquid flow with an intermittent range of zero flow at which tracer particles undergo thermal motion by just tuning the laser intensity only. Our work, thus, demonstrates an optofluidic system for flow generation with a high degree of controllability that is necessary to transport particles precisely to desired locations, thereby opening innovative possibilities to generate advanced microfluidic applications.},
  language  = {en}
}
@article{BapolisiKielbBekiretal.2022,
  author    = {Bapolisi, Alain Murhimalika and Kielb, Patrycja and Bekir, Marek and Lehnen, Anne-Catherine and Radon, Christin and Laroque, Sophie and Wendler, Petra and M{\"u}ller-Werkmeister, Henrike and Hartlieb, Matthias},
  title     = {Antimicrobial polymers of linear and bottlebrush architecture},
  series = {Macromolecular rapid communications : publishing the newsletters of the European Polymer Federation},
  volume    = {43},
  journal   = {Macromolecular rapid communications : publishing the newsletters of the European Polymer Federation},
  number    = {19},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1521-3927},
  doi       = {10.1002/marc.202200288},
  pages     = {14},
  year      = {2022},
  abstract  = {Polymeric antimicrobial peptide mimics are a promising alternative for the future management of the daunting problems associated with antimicrobial resistance. However, the development of successful antimicrobial polymers (APs) requires careful control of factors such as amphiphilic balance, molecular weight, dispersity, sequence, and architecture. While most of the earlier developed APs focus on random linear copolymers, the development of APs with advanced architectures proves to be more potent. It is recently developed multivalent bottlebrush APs with improved antibacterial and hemocompatibility profiles, outperforming their linear counterparts. Understanding the rationale behind the outstanding biological activity of these newly developed antimicrobials is vital to further improving their performance. This work investigates the physicochemical properties governing the differences in activity between linear and bottlebrush architectures using various spectroscopic and microscopic techniques. Linear copolymers are more solvated, thermo-responsive, and possess facial amphiphilicity resulting in random aggregations when interacting with liposomes mimicking Escheria coli membranes. The bottlebrush copolymers adopt a more stable secondary conformation in aqueous solution in comparison to linear copolymers, conferring rapid and more specific binding mechanism to membranes. The advantageous physicochemical properties of the bottlebrush topology seem to be a determinant factor in the activity of these promising APs.},
  language  = {en}
}
@article{ReifarthBekirBapolisietal.2022,
  author    = {Reifarth, Martin and Bekir, Marek and Bapolisi, Alain M. and Titov, Evgenii and Nusshardt, Fabian and Nowaczyk, Julius and Grigoriev, Dmitry and Sharma, Anjali and Saalfrank, Peter and Santer, Svetlana A. and Hartlieb, Matthias and B{\"o}ker, Alexander},
  title     = {A dual pH- and light-responsive spiropyrane-based surfactant},
  series = {Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition},
  volume    = {61},
  journal   = {Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition},
  number    = {21},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1433-7851},
  doi       = {10.1002/anie.202114687},
  pages     = {10},
  year      = {2022},
  abstract  = {A cationic surfactant containing a spiropyrane unit is prepared exhibiting a dual-responsive adjustability of its surface-active characteristics. The switching mechanism of the system relies on the reversible conversion of the non-ionic spiropyrane (SP) to a zwitterionic merocyanine (MC) and can be controlled by adjusting the pH value and via light, resulting in a pH-dependent photoactivity: While the compound possesses a pronounced difference in surface activity between both forms under acidic conditions, this behavior is suppressed at a neutral pH level. The underlying switching processes are investigated in detail, and a thermodynamic explanation based on a combination of theoretical and experimental results is provided. This complex stimuli-responsive behavior enables remote-control of colloidal systems. To demonstrate its applicability, the surfactant is utilized for the pH-dependent manipulation of oil-in-water emulsions.},
  language  = {en}
}
@misc{SharmaBekirLomadzeetal.2020,
  author    = {Sharma, Anjali and Bekir, Marek and Lomadze, Nino and Santer, Svetlana A.},
  title     = {Photo-Isomerization Kinetics of Azobenzene Containing Surfactant Conjugated with Polyelectrolyte},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1101},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-48942},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-489427},
  pages     = {14},
  year      = {2020},
  abstract  = {Ionic complexation of azobenzene-containing surfactants with any type of oppositely charged soft objects allows for making them photo-responsive in terms of their size, shape and surface energy. Investigation of the photo-isomerization kinetic and isomer composition at a photo-stationary state of the photo-sensitive surfactant conjugated with charged objects is a necessary prerequisite for understanding the structural response of photo-sensitive complexes. Here, we report on photo-isomerization kinetics of a photo-sensitive surfactant in the presence of poly(acrylic acid, sodium salt). We show that the photo-isomerization of the azobenzene-containing cationic surfactant is slower in a polymer complex compared to being purely dissolved in aqueous solution. In a photo-stationary state, the ratio between the trans and cis isomers is shifted to a higher trans-isomer concentration for all irradiation wavelengths. This is explained by the formation of surfactant aggregates near the polyelectrolyte chains at concentrations much lower than the bulk critical micelle concentration and inhibition of the photo-isomerization kinetics due to steric hindrance within the densely packed aggregates.},
  language  = {en}
}
@article{TitovSharmaLomadzeetal.2021,
  author    = {Titov, Evgenii and Sharma, Anjali and Lomadze, Nino and Saalfrank, Peter and Santer, Svetlana A. and Bekir, Marek},
  title     = {Photoisomerization of an azobenzene-containing surfactant within a micelle},
  series = {ChemPhotoChem},
  volume    = {5},
  journal   = {ChemPhotoChem},
  number    = {10},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2367-0932},
  doi       = {10.1002/cptc.202100103},
  pages     = {926 -- 932},
  year      = {2021},
  abstract  = {Photosensitive azobenzene-containing surfactants have attracted great attention in past years because they offer a means to control soft-matter transformations with light. At concentrations higher than the critical micelle concentration (CMC), the surfactant molecules aggregate and form micelles, which leads to a slowdown of the photoinduced trans -> cis azobenzene isomerization. Here, we combine nonadiabatic dynamics simulations for the surfactant molecules embedded in the micelles with absorption spectroscopy measurements of micellar solutions to uncover the reasons responsible for the reaction slowdown. Our simulations reveal a decrease of isomerization quantum yields for molecules inside the micelles. We also observe a reduction of extinction coefficients upon micellization. These findings explain the deceleration of the trans -> cis switching in micelles of the azobenzene-containing surfactants.},
  language  = {en}
}
@article{SharmaBekirLomadzeetal.2020,
  author    = {Sharma, Anjali and Bekir, Marek and Lomadze, Nino and Santer, Svetlana A.},
  title     = {Photo-Isomerization Kinetics of Azobenzene Containing Surfactant Conjugated with Polyelectrolyte},
  series = {Molecules},
  volume    = {29},
  journal   = {Molecules},
  number    = {1},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {1420-3049},
  doi       = {10.3390/molecules26010019},
  pages     = {12},
  year      = {2020},
  abstract  = {Ionic complexation of azobenzene-containing surfactants with any type of oppositely charged soft objects allows for making them photo-responsive in terms of their size, shape and surface energy. Investigation of the photo-isomerization kinetic and isomer composition at a photo-stationary state of the photo-sensitive surfactant conjugated with charged objects is a necessary prerequisite for understanding the structural response of photo-sensitive complexes. Here, we report on photo-isomerization kinetics of a photo-sensitive surfactant in the presence of poly(acrylic acid, sodium salt). We show that the photo-isomerization of the azobenzene-containing cationic surfactant is slower in a polymer complex compared to being purely dissolved in aqueous solution. In a photo-stationary state, the ratio between the trans and cis isomers is shifted to a higher trans-isomer concentration for all irradiation wavelengths. This is explained by the formation of surfactant aggregates near the polyelectrolyte chains at concentrations much lower than the bulk critical micelle concentration and inhibition of the photo-isomerization kinetics due to steric hindrance within the densely packed aggregates.},
  language  = {en}
}
@article{BekirSharmaUmlandtetal.2022,
  author    = {Bekir, Marek and Sharma, Anjali and Umlandt, Maren and Lomadze, Nino and Santer, Svetlana A.},
  title     = {How to make a surface act as a micropump},
  series = {Advanced materials interfaces},
  volume    = {9},
  journal   = {Advanced materials interfaces},
  number    = {12},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2196-7350},
  doi       = {10.1002/admi.202102395},
  pages     = {11},
  year      = {2022},
  abstract  = {In this paper, the phenomenon of light-driven diffusioosmotic (DO) long-range attractive and repulsive interactions between micro-sized objects trapped near a solid wall is investigated. The range of the DO flow extends several times the size of microparticles and can be adjusted to point towards or away from the particle by varying irradiation parameters such as intensity or wavelength of light. The "fuel" of the light-driven DO flow is a photosensitive surfactant which can be photo-isomerized between trans and cis-states. The trans-isomer tends to accumulate at the interface, while the cis-isomer prefers to stay in solution. In combination with a dissimilar photo-isomerization rate at the interface and in bulk, this yields a concentration gradient of the isomers around single particles resulting in local light-driven diffusioosmotic (l-LDDO) flow. Here, the extended analysis of the l-LDDO flow as a function of irradiation parameters by introducing time-dependent development of the concentration excess of isomers near the particle surface is presented. It is also demonstrated that the l-LDDO can be generated at any solid/liquid interface being more pronounced in the case of strongly absorbing material. This phenomenon has plenty of potential applications since it makes any type of surface act as a micropump.},
  language  = {en}
}
@article{BekirJelkenJungetal.2021,
  author    = {Bekir, Marek and Jelken, Joachim and Jung, Se-Hyeong and Pich, Andrij and Pacholski, Claudia and Kopyshev, Alexey and Santer, Svetlana},
  title     = {Dual responsiveness of microgels induced by single light stimulus},
  series = {Applied physics letters},
  volume    = {118},
  journal   = {Applied physics letters},
  number    = {9},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/5.0036376},
  pages     = {6},
  year      = {2021},
  abstract  = {We report on the multiple response of microgels triggered by a single optical stimulus. Under irradiation, the volume of the microgels is reversibly switched by more than 20 times. The irradiation initiates two different processes: photo-isomerization of the photo-sensitive surfactant, which forms a complex with the anionic microgel, rendering it photo-responsive; and local heating due to a thermo-plasmonic effect within the structured gold layer on which the microgel is deposited. The photo-responsivity is related to the reversible accommodation/release of the photo-sensitive surfactant depending on its photo-isomerization state, while the thermo-sensitivity is intrinsically built in. We show that under exposure to green light, the thermo-plasmonic effect generates a local hot spot in the gold layer, resulting in the shrinkage of the microgel. This process competes with the simultaneous photo-induced swelling. Depending on the position of the laser spot, the spatiotemporal control of reversible particle shrinking/swelling with a predefined extent on a per-second base can be implemented.},
  language  = {en}
}