Imaging probes double tipping

Several of the procedures described in the previous sections can be advantageously carried out with double barrel tips. Such a probe consists of two capillaries (see Sec. V.B), one of which acts as the potentiometric sensor, while the other is used to determine the tip-substrate distance. For example (79), a gallium microdisk was combined with an ion-selective (K+) potentiometric probe to image K+ activity near the aperture of a capillary (see Fig. 7). Similarly (77), a double barrel tip with one channel as an open Ag/ AgCl micropipette for solution resistance measurement and the other channel as an ion-selective neutral carrier-based microelectrode for potentiometric measurements was successfully used to image concentration distributions for NH4 (Fig. 8) and Zn2+ (Fig. 9). While dual-channel tips facilitate the approach of the substrate and permit a direct determination of the absolute tip-substrate distance, their difficult fabrication severely limits their use. Reference 80 compares the above methods. 

SECCM probes are fabricated from either borosilicate or quartz double-barrel pipettes, pulled to a sharp point in a laser puller (typically a P-2000 from Sutter Instruments). The size of the probe can be controlled by adjusting the pulling parameters, with probes between 100 nm and tens of microns across at the tip end fabricated easily and quickly. A field emission scanning electron microscopy (FE-SEM) image of a typical (relatively large) SECCM probe is shown in Figure 19.2a. The pulling parameters differ between laser pullers, but for a typical 500 nm borosilicate probe on a P-2000 laser puller, the parameters are as follows line 1 heat=600, filament=4, velocity=30, delay= 150. pull=20 line 2 heat=500, filament=4, velocity=30, delay=150. pull=60 line 3 heat=500, filament=3, velocity=30, delay = 135. pull=60. 

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