The effect of PeakForce tapping mode AFM imaging on the apparent shape of surface nanobubbles

Abstract

Until now, TM AFM (tapping mode or intermittent contact mode atomic force microscopy) has been the most often applied direct imaging technique to analyze surface nanobubbles at the solid-aqueous interface. While the presence and number density of nanobubbles can be unequivocally detected and estimated, it remains unclear how much the a priori invasive nature of AFM affects the apparent shapes and dimensions of the nanobubbles. To be able to successfully address the unsolved questions in this field, the accurate knowledge of the nanobubbles' dimensions, radii of curvature etc is necessary. In this contribution we present a comparative study of surface nanobubbles on HOPG (highly oriented pyrolytic graphite) in water acquired with (i) TM AFM and (ii) the recently introduced PFT (PeakForce tapping) mode, in which the force exerted on the nanobubbles rather than the amplitude of the resonating cantilever is used as the AFM feedback parameter during imaging. In particular, we analyzed how the apparent size and shape of nanobubbles depend on the maximum applied force in PFT AFM. Even for forces as small as 73 pN, the nanobubbles appeared smaller than their true size, which was estimated from an extrapolation of the bubble height to zero applied force. In addition, the size underestimation was found to be more pronounced for larger bubbles. The extrapolated true nanoscopic contact angles for nanobubbles on HOPG, measured in PFT AFM, ranged from 145° to 175° and were only slightly underestimated by scanning with non-zero forces. This result was comparable to the nanoscopic contact angles of 160°-175° measured using TM AFM in the same set of experiments. Both values disagree, in accordance with the literature, with the macroscopic contact angle of water on HOPG, measured here to be 63° ± 2°.

Exposing nanobubble-like objects to a degassed environment

“…In addition, these features are soft, could be swept away using an AFM tip and disappeared after drying the surface 35 . This finding, in retrospect, also may or may not affect some of our and others previous work in which plastic syringes in combination with disposable needles were used 7,10,18,34,35,56,58,59 . Whether the features observed in these experiments are actual nanobubbles with a gas-impermeable shell induced by the PDMS or are in fact PDMS droplets is something to be investigated and lies outside the scope of this work.…”

“…The literature on nanobubbles states that their apparent shape is very much dependent on the set-point used for scanning the surface in the AFM [7][8][9][10]56 . The set-point dependence of PDMS droplets was therefore compared to that of nanobubble-like objects created using plastic syringes in combination with disposable needles, see Figure 6.…”

“…Real-space images of nanobubbles had to wait until advancements in atomic force microscopy (AFM) immersed in liquids resulted in the observation of soft spherical cap shaped features by Lou et al 5 and Ishida et al 6 in 2000. Unfortunately, the AFM tip disturbs these soft features and properly imaging nanobubbles is not a trivial task [7][8][9][10] . Nanobubbles have been observed on a wide variety of surfaces [11][12][13][14][15][16] and found to be stable in a broad range of conditions like elevated temperature 14,17,18 , low pH 19 and salt solutions 19,20 .…”

Automatic morphological characterization of nanobubbles with a novel image segmentation method and its application in the study of nanobubble coalescence

“…With the selected cross sections, the NB diameter and height can be directly measured. By fitting the cross sections as arcs, the NB contact angle and radius of curvature can be obtained [36,37,38,39].…”

Pharmacokinetics of quercetin-loaded nanodroplets with ultrasound activation and their use for bioimaging

“…Atomic force microscopy was also used to assess the appearance of NQ, but the apparent dimensions of the NQ were altered during preparation of the sample for processing by atomic force microscopy. 49 Therefore, we used TEM to confirm the morphology of NQ. We optimized the method used for TEM sample preparation, including the dye concentration, formulation concentration, and dye settling time, and obtained a clean TEM image (Figure 2).…”