High-resolution imaging of living cell at the micro-/nano-scale is important for life science research. It may help to observe biological activities of cells, and to detect cell responses to external stimuli and even movements of some protein molecules in cell membranes. However, effective methods to realize such goals do not exist.

Scanning ion conductance microscopy (SICM), first introduced by Hansma group and further developed by korchev group, has become a promising tool for high-resolution imaging on biological samples. By measuring the ion conductance change through an electrolyte-filled fine tapered glass nanopipette, the tip-sample distance can be controlled and maintained thus to obtain the topographical images of samples.  Unlike other scanning probe microscopy (SPM) techniques that require tip-sample contact, SICM is naturally a non-contact approach. Therefore, it can be used to probe live cells under physiological conditions, which is difficult by other SPMs that can easily modify and damage the soft living cells. There are at least two imaging modes of SICM: continuous mode and hopping mode. In both imaging modes, the low imaging speed is the common disadvantage that hinders the efficiency of SICM based observation.

To solve this problem, researchers from Shenyang Institute of Automation (SIA), Chinese Academy of Sciences developed a prior knowledge based fast imaging method (as shown in Fig. 1) based on home-built scanning ion conductance microscopy recently. The key idea is to achieve a high resolution image on targeted features by scanning the sample twice, the first scan to obtain prior knowledge on targeted features with low resolution and the second scan to obtain images on targeted features with high resolution. In the first scan, the scanning speed can be very fast because the prior knowledge can be obtained from a low resolution image that requires few scanning points. In the second scan, only the interested areas (targeted features) are scanned. Because the scanning area on targeted features is much smaller than the whole scanning region, it takes much less time to obtain high resolution images on targeted features, As a result, the total time  to  acquire  a  high  resolution  is  much  less  than  the conventional one scan method .

 Fig. 1 Flowchart of fast imaging method.(Image provided by LI Peng et,al.)

To verify the effectiveness of this method, the researchers firstly constructed a home-made SICM according to the setups shown in Fig. 2. Sample is moving up and down driven by the nano stage that is extending and retracting. The  ion  current,  varying  with  respect to  the distan- ce between the nanopipette tip and the sample surface, provides an  input  signal for  feedback  control  system to  adjust the vertical axis of nano stage and ensures that the sample and nanopipette tip are out of contact.

Fig. 2  Photograph of SICM (Image provided by LI Peng et.al,)