Surface nanobubbles studied by time resolved fluorescence microscopy methods combined with AFM: The impact of surface treatment on nanobubble nucleation
Nicole Hain, Daniel Wesner, Sergey I. Druzhinin, and Holger Schönherr*
Physical Chemistry I, Department of Chemistry and Biology & Research Center of Micro and Nanochemistry and Engineering (Cμ), University of Siegen, Adolf-Reichwein-Strasse 2, 57076 Siegen, Germany
*Corresponding Author Tel.: +49 271 740 2806. Fax: +49 271 740 2805. E-mail:
[email protected].
Supporting Information
1
Degassing of the water for the FLIM and the combined AFM - FLIM experiment.
For the degassing experiments on the Microtime 200, a 250 mL aqueous Rh6G solution was degassed for 3 hours at 20°C by using a diaphragm vacuum pump (Type MZ 2C, Vacuubrand, Germany). The pressure was set to 10 mbar, while the solution was stirred continuously. For microscopy experiments, the Asylum MFP-3D-Bio atomic force microscope fluid-cell was used. The piranha-cleaned cover slide was placed in the AFM cell and the top part of the cell was closed with a thick glass also pre-cleaned with piranha solution. The fluid-cell was pre-filled with aqueous Rh6G solution using a cleaned glass syringe. The fluid cell was then mounted on the confocal setup and tubes (Tygon 2001 SC0818 Ismatec, IDEX Health & Science GmbH, Wertheim, Germany) for pumping the liquid were connected to the fluid cell. Nanobubbles were nucleated with the ethanol - water exchange by using the peristaltic pump and Tygon tubes. The tubes were pre-cleaned before starting the experiment by pumping ethanol through for 10 minutes with a flow rate of 1.5 mL/min. For the ethanol-water exchange the flow rate was set to 800 µL/min for 10 minutes. Then the degassed aqueous Rh6G solution (850 nM) was pumped through. While pumping the degassed solution, the pressure in the degassing reservoir was kept at 60 mbar. The pump was set to the desired flow rate. For obtaining a FLIM image, the liquid flow was interrupted and the pressure for the degassed water reservoir was set to 10 mbar. The system was then equilibrated for 30 minutes to minimize thermal drift (i.e. to avoid the shift of the focus).
2
Figure S-1. Statistical analysis of the surface nanobubbles displayed in the intermittent contact mode AFM height image shown in Figure 4: a) Histogram of the nanobubble diameter. b) Histogram of the height. c) Histogram of the calculated contact angle. d) Histogram of the calculated radius of curvature of the nanobubbles.
From the width and the height of the nanobubbles the contact angle (θnano) and the radius of curvature (Rc) were calculated. It is assumed that the bubbles possess a spherical cap shape with Rc, a base width W, and a height H. The contact angle θnano of the nanobubbles is defined as the contact angle through the liquid. θnano was estimated according to (S1).
2
(S1)
/
(S2)
The average contact angle for the nanobubbles used in the Figure S-1 is 145° ± 14°. 3
Figure S-2. (a) Intermittent contact mode AFM height image and (b) corresponding fluorescence intensity image of oxygen plasma cleaned glass after ethanol-water exchange in aqueous Rh6G solution (850 nM). (c) The decay curve displays a double exponential decay with lifetimes of 3960 and 1530 ps and practically no short-lived component < 1 ns.
4
Figure S-3. XPS survey spectra of differently cleaned glass.
5
Figure S-4. Binary images of the intensity images of surface nanobubbles on a) Piranhacleaned glass, b) APMES c) OTS, and d) ODT. e) Histogram displaying the high intensity pixel coverage.
6
Figure S-5: Intermittent contact mode AFM height images of a) piranha cleaned glass, b) APMES on a silicon surface, c) OTS on a silicon surface, and d) ODT on a thin layer of gold on a silicon surface.
7
Table S-1
Roughness (Rq) measured from 5m x 5 m TM height images (512 x 512 pixels2) of the different substrates.
Substrates
Roughness Rq [nm]
Glass
0.47 ± 0.01
APMES
1.99 ± 0.97
OTS
0.70 ± 0.40
ODT
0.48 ± 0.52
8