Dye, peptide and protein retention in paper-based microfluidic devices
Dr. rer. nat. Sonja Wendenburg (2013 – 2018)
Paper-based microfluidic devices provide interesting applications, including clinical “point-of-care” diagnostics, food quality control and environmental monitoring. Paper as a matrix for microfluidic devices offers many advantages compared to other materials, particularly due to its capability to transport liquid by capillary forces.Current research on paper-based microfluidic devices focuses only on commercial available filter paper. Thus, a huge engineering potential for paper-intrinsic parameters is lost. In our group in general, and in particular in this project, the influence of paper sheet parameters on lateral flow in paper-based micro-engineering devices as well as the retention of dyes, peptides and proteins is under investigation with the aim to improve “lab-on-a-chip” devices. Firstly, we are investigating the influence of paper substrate (different grammages, fibre types, …) and solution parameters (type of retaining molecules, pH-value…) on retention. In a second step, we plan to control the retention. This will be realized by modification of paper substrates with functional, photo-reactive polymers. “0-retention” is interesting for “lab-on-chip” devices to reduce sample volume and increase sensitivity, while site-specifically huge retention might be interesting for analyte filtration.
In our group a novel lithographic process for site-specific modification of cellulose fibers in paper-sheets using photo-reactive, functional polymers was developed. In our approach, paper is site-specifically and covalently microstructured in Y-shaped channels by UV-irradiation using copolymers based on methyl methacrylate or styrene and 4-methacryloyloxy benzophenone (MABP), as a photo-crosslinker. The polymer can be transferred onto the fibers by various means, including dip-coating or doctor-blading. For covalent attachment, polymers are bound to the fibers by illumination of the sheets with UV-light, and non-bound polymers (i.e. polymers within shaded areas) are subsequently removed by simple solvent extraction. So prepared, chemically structured papers consist of hydrophilic channels that are defined between hydrophobic coats. To quantify the retention by the retention factor Rf, a (dye) solution (here: 0,8 mg/ml tatrazine in H2O, pH1) is applied to a micro-structured paper substrate (here: Linters, 45 g/m²) and the migration distance of the dye is divided by the migration distance of the water front.
 D.D. Liana, B. Raguse, J.J. Gooding, E. Chow, Sensors, 2012, 12, 11505.
 A. Böhm, F. Carstens, C. Trieb, S. Schabel, M. Biesalski, Microfluid Nanofluid, 2014, 16, 5, 789-799.
 A. Böhm, M.Gattermayer. F. Carstens, S. Schabel, M. Biesalski, Advances in Pulp and Paper Research, Trans XVth Fund.Res.Symp. Cambridge 2013, (S.J.I’Anson, ed.) 599-618.
Polymer-modified paper substrates that reversibly respond to light
M.Sc. Wei Li (2013 – 2017)
Light responsive polymers that change their properties when irradiated with light of the appropriate wavelength open access to promising applications, such as for example safety-papers and bank-notes, as well as low-cost UV sensors. Once they are confined on the solid substrates, surface properties (e.g. wettability, permeability and adhesion) can be dynamically changed leading to the formation of smart surfaces. Besides, the use of paper as substrate is highly attractive owing to its unique features, such as low-cost, nontoxic, flexible, and light-weight. Based on the above information, we designed spiropyran containing polymer modified paper substrates. Such functional paper can change chromatic properties according to the switching behaviors between spiropyran (pale yellow) and merocyanine (purple) by UV and visible light (or heat).
 H. Schenderlein, A. Voss, R. W. Stark, M. Biesalski, Langmuir 2013, 29, 4525.
Photo-reactive polyvinyl acetate copolymers
M.Sc. Michael Graf (2012 – 2016)
as a functional coating can be applied onto various substrates from solution by dip-coating or doctor blading. These copolymers are synthesized using free radical copolymerization and offer uncomplicated photo-chemically induced surface-attachment and network generation properties by direct C-C bond formation via UV excitation of benzophenone moieties, which have been incorporated into the copolymer in desired ratios. Additionally, the hydrophobic polyvinyl acetate can be converted into hydrophilic polyvinyl alcohol by simple and fast ester hydrolysis in alkaline methanolic sodium hydroxide solution. This change in surface chemistry allows direct micro structuring of a substrate surface into non-wetting and wetting domains and ultimately the fabrication of microfluidic structures or channels on a substrate by spatially resolved application of methanolic sodium hydroxide solution.
Tailor-made adhesive agents for surface modifications and specialty papers
Dr. Florian Loyal (2010 – 2015)
Catechol-functionalized polymers show impressive adhesive properties towards various metal and organic surfaces. Among other materials the interaction between catechol groups and TiO2 surfaces point out to be one of the strongest. Due to the coordinative interactions which work both in dry and wet environment, various new applications areas arise from these properties especially for fixation and modification processes in aqueous environments. In the field of paper making TiO2 particles are mostly used due to their extraordinary optical properties as pigments in specialty papers. Additionally over the last decade an increasing interest in using its semiconductor properties for photocatalytic application like waste water treatment, or the degradation of pollutants from air is found.There has been a high interest in so called mussel-inspired polymers during the last decade. These new polymers carry catechol groups which can mimic the ultra strong adhesion properties of proteins secreted by marine mussels to different oxidic surfaces, including TiO2-surfaces even under harsh marine environments (slightly alkaline, shear forces from tides, etc.). TiO2 is a broadly used, but high cost pigment in the paper manufacture process, in order to adjust important optical properties of specialty papers (e.g. opacity). Because of its characteristic agglomeration behavior, to date a number of different additives are required to get an acceptable cost-quality output. In our research we describe a completely novel attempt to use mussel-inspired, charged and non charged, copolymers to effect different parameters of the paper making process, especially the retention, adhesion and distribution of TiO2 and other particles in the paper.
Under irradiation with light of energy higher than the band gap of the material, the photo-catalyst is able to fully mineralize organic media completely to water, CO2 and nontoxic anions, including harmful bacteria and their endo-toxines. Due to the fact that only the catalyst material and in the best way solar light is required for the mineralization, this process have huge potential as a very sustainable process for air and wastewater cleaning. Nowadays TiO2 nano particles are already used in this field, however separation of the small catalyst particles are challenging and very cost effective. Here the combination with paper based materials as a sustainable matrix material with attractive mechanical properties lead to a much easier handling. Furthermore it opens the door for a much broader product area, due to the compatibility with standard paper product, like wallpapers etc. Nevertheless paper, mostly seen as a cheap writing material, is very complex in his structure and therefore several challenges have to overcome to set up a working catalytic composite material.
In our research we are interested in synthesizing new multi-functional polymers with adhesive and photo-reactive properties and further functional side groups for the modification of filter papers as well as other organic and inorganic surfaces. Through our tailor-made polymers we are able to homogeneously attach (photo) catalytic particles, if needed in a local resolved manner. Due to our combination of functional side groups our polymers are able to attach the particles quite strong and are stable even under rigid conditions in aqueous environments. Therefore those systems are promising for the production of photocatalytic papers used for wastewater cleaning or catalytic processes.
 Loyal, F.; Biesalski, M., Molekulare und mesoskopische Effekte der Faser-Faser-/Faser-Füllstoff-Retentionsmittel-Wechselwirkungen, Abschlussbericht INFOR Projekt 142R, 2012, CPF-Ressourcenschwerpunkt, VDP.
 Loyal, F., Catechol-funktionale Copolymere als neuartige Prozess- und Funktionsadditive zur Herstellung funktionaler Papiere, Makromolekulare Chemie und Papierchemie Bd. 4, 2014, ISBN: 978-3-86247-486-8
Development of novel wet-strength additives
M.Eng. Michael Jocher (2011 – 2014)
Wet strength resins are widely used in manufacturing paper and paper products, which need to preserve mechanical strength even if they are completely saturated with water. There are many paper grades which need to be wet strengthened for example: wall paper, security papers, tissue grades, laminating papers, label papers, filter papers, tea bags, abrasive papers, some liner and more. The most important wet strength additives are polyamidoamine epichlorohydrin resins, melamin- and urea formaldehyde resins, which imply permanent wet strength and glyoxal resins, which serve as temporary wet strength agents.
There are big disadvantages with respect to common additives used in paper making:
- ➢ Low curing off machine
- ➢ Problematic low molecular chlorine compounds (by-products)
- ➢ Hazardous to health and detrimental to AOX load
- ➢ Application just in acidic paper making conditions
- ➢ Limited use in wet end application
- ➢ Formaldehyde emission (carcinogeniticity)
We are currently working on alternative approaches in using polymers, which shall not contain problematic substances and shall chemically attach to cellulose fibers, both through heat and/or light exposure.
- ➢ Achieving a well defined wet strength with less further post-process curing events
- ➢ No bleeding-off organic substances from paper
- ➢ Curing during converting step on demand, thus easier broke handling
- ➢ Adjusting different rates of wet strength even in cross-process direction