The D - isomer of CFKK - TMR was synthesized inhouse according to standard solid phase peptide synthesis protocols , using D - analogs of the amino acids ( Anaspec , USA ) and Tetramethylrhodamine NHS ester ( Sigma - Aldrich ) . The peptide was conjugated to the nanoneedles according to the same protocol employed for the L - isomer described above . Both L - and D - isomer sensors were interfaced with OE33 and Het-1A cells for 15 minutes according to the interfacing protocol described . The sensors were removed , fixed in 4 % w / v paraformaldehyde in PBS for 15 minutes , and counterstained with DAPI for 5 minutes . The samples were mounted on coverslip with prolong gold ( Life by confocal microscopy . Five separate z - stacks including the nanoneedles and the entirety of the cells were imaged for each sample . The DAPI and TMR channels were acquired sequentially to avoid cross - talk . All stacks were acquired with constant magnification , laser intensities , detector gain and offset on all channels . The data was quantified for all cells in all images from a single Z plane above the needles as described in the confocal microscopy of cells section .