Characterization of aptamer cy-apt 20 in vitro. The specific binding capacity of aptamer cy-apt 20 to gastric carcinoma cells was also assessed with flow cytometry by varying the concentration of FITC-cy-apt 20 (from 0 nM to 500 nM) for varied time length of incubation (from 0 min to 50 min). Equivalent FITC-Lib ssDNA were used as controls and results were presented as mean ± standard error. (A-B) increased binding rates were seen with increasing the concentrations of FITC-cy-apt 20 after 40 min of incubation and peaked at 400 nM (arrow notified). (C-D) increased binding rates were also seen by increasing the incubation time length of AGS cells with 400 nM of FITC-cy-apt 20 (arrow notified) and peaked at 40 min. (E) The specificity of cy-apt 20 in recognizing AGS cells was further visualized by fluorescence imaging. All the three kinds of tumor cells were separately incubated with 400 nM of FITC-labled cy-apt 20 for 40 min, and then observed with an invert fluorescence microscope. (Ea) most of AGS cells were stained by FITC-labled cy-apt 20, whereas, few living HepG2 (Eb) and SW620 (Ec) cells exhibited detectable fluorescence. (F) The fluorescence signals of tumor cells relative to background were quantified using NIH Image J software and results were presented as fold changes vs. Background ± standard error. MFI: mean fluorescence intensity. OM: optical microscope; FM: fluorescence microscope.