Article pubs.acs.org/JPCC
Thin Polyelectrolyte Multilayers Made by Inkjet Printing and Their Characterization by Nanomechanical Cantilever Sensors Masaya Toda,*,†,‡ Yun Chen,† Sebastian K. Nett,† Akiko N. Itakura,⊥ Jochen Gutmann,†,§ and Rüdiger Berger*,† †
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Graduate School of Engineering, Tohoku University, 6-6-01 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan § Institute of Physical Chemistry and CENIDE, University Duisburg-Essen, Universitätsstrasse 5, 45117 Essen, Germany ⊥ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ‡
S Supporting Information *
ABSTRACT: Measurements with nanomechanical cantilever (NMC) sensors often reveal only qualitative results. Here we overcome this issue by inkjet printing well-defined polyelectrolyte multilayers (PEMs). We present a method that allows fabricating a 40 bilayer (BL) thick and 5 mm long line made of poly(allylamine hydrochloride) (PAH) and polystyrene sulfonate (PSS). NMC sensors were used to quantify the uptake of water in thin PEMs. We measured and analyzed the mass loading and the swelling response of the PEMs upon exposure to relative humidity between 5% and 80%. For a film made of 5 BLs we determined a Young’s module of ∼390 MPa for low humidity (95%, Merck, Darmstadt, Germany) is added. The salt concentration of NaCl in the polyelectrolyte water solutions was 0.1−0.5 mol/L and that for CaCl2 is 0.5 mol/L. In total, four kinds of solutions were prepared in stock: PAH/NaCl, PSS/NaCl, PAH/CaCl2, and PSS/CaCl2. All the chemicals were used without further purification. Preparation of Substrates. Silicon wafers were cleaned in ultrasonication bath of acetone, isopropyl alcohol, distilled water, freshly prepared piranha solution (H2O2:H2SO4 = 1:3), and distilled water in sequence and finally dried in filtered N2 flow. To increase the contact angle of the plotted PE solutions, the silicon wafer was immersed in a PAH/CaCl2 solution for 5−10 min and then rinsed with a large amount of distilled water, followed by immersing in PSS/CaCl2 solution for the same duration, rinsing again with water, and finally drying in N2 flow. These standard conditions resulted in a first bilayer of PAH/PSS having a thickness of 2−3 nm on flat Si surface.28,29 Nano-Plotter. A Nano-Plotter 2 from GeSim GmbH (Grosserkmannsdorf, Germany) was used to drop PE solutions at local regions on the surface. Two ejection nozzles (nozzle 1 and 2) were used for PEM deposition. The nozzles were piezoelectrically actuated to create small droplets from the tip with a high reproducible volume from drop to drop. The ejecting frequency was set to 100 Hz (100 drops/s). The height of the nozzle above the sample surface was set to 0.5 mm. The volume of 1 drop was 0.4 nL. In 1000 droplets, the deviation in volume was evaluated to be about 2% by measuring the size using video images. The position of the nozzle was controlled by a motorized xyz-stage with an accuracy of 1 μm, respectively. Each drop could be ejected on the same position, thus allowing a defined volume to be deposited at the local region on the sample surface. The nozzle tips were moved with a velocity of 5 mm/s for drawing a line. 8072
dx.doi.org/10.1021/jp501464j | J. Phys. Chem. C 2014, 118, 8071−8078
The Journal of Physical Chemistry C
Article
Figure 2. (a) Sketch of plotting lines. (b) Optical images taken with a bright field light microscopy of plotted PEM directly on the surface of Si substrate freshly cleaned by the Piranha solution. (c) The same plot on a PE pretreated substrate with a bilayer of PAH and PSS. Using PE solutions of 3 mM concentration containing 0.1 mol/L NaCl, 10 BL lines were plotted. (d) Sketch of plotting lines orthogonal to each other. (e) At the crossovers the thickness of the deposited layer was doubled. (f) Here different numbers of lines were plotted, respectively.
in which 0.1 mol/L of NaCl was used.30 By using this pretreated surface, we found a smaller diameter of 180−200 μm for a line consisting of 20 BLs. Furthermore the corrugation decreased to a value
