Liquid Scanning with Atomic Force Microscopy
Atomic Force Microscopes (AFM) are the only instruments that can measure nanometer-scale-resolution images of samples submerged in a liquid. This unique capability facilitates applications in physical as well as life sciences. Imaging with an AFM in liquid requires a liquid cell to hold the sample and probe while they are submerged in a liquid. There are two types of liquid cells available for AFM's, open cells and closed cells.
There are several reasons to scan in a liquid cell. In a liquid environment, surface contamination is eliminated and does not hinder AFM imaging. Also, in situ imaging on biological samples is possible with a liquid cell. As an example, Figure 1 is an image of a cheek cell imaged in an aqueous environment using AFMWorkshop's TT-2 AFM.
Open cells are often called a microcell. The design for the open liquid cell is illustrated in Figure 2. The sample is held in a small vessel with a small amount of liquid. The probe is on a special holder with a glass window directly above the probe. In operation, the probe is submerged in the liquid while scanning.
Figure 2: Open Liquid Cell
Closed cells are often called environmental cells. In the environmental cell, the sample and probe are held in a small enclosed area. Figure 3 illustrates the design for a closed cell. The cell is formed by a plate of glass at the top, and a rubber diaphragm at the bottom. To operate this type of cell, the probe and sample are mounted at the top and bottom of the cell, respectively. The top and bottom of the cell are sealed, and the liquid is introduced to the cell.
Figure 3: Closed Liquid Cell
In practice an open cell is typically much easier to use than the closed cell. It takes only a few hours to learn to use an open cell, while it can take several days to acquire images with a closed cell. The advantage of using the closed cell is that you can flow liquids through the cell while scanning, and you can also use the closed cell for scanning in an inert gaseous environment.