Controllable fabrication of pico/femtoliter pipette sampling probes and visual sample volume determination

We propose a hydraulically assisted eddy-current etching method for the controllable fabrication of pico/femtoliter sampling probes with equal inner diameters along the length of the probe. The relative standard deviations of the outer and inner diameters (O.D. and I.D., respectively) of several 1.07-μm-I.D. sampling probe tips fabricated in a single batch using this method were 2.2% and 2.8%, respectively, and the average O.D./I.D. ratio of these probes was 1.17. The probe fabrication method has high reproducibility, and sharp tips are produced, which is advantageous for the transfer of ultra-small sample volumes. Further, the narrow, equal diameter, cylindrical inner bore allows the online, visual determination of the pipetted sample volume by utilizing a microscopic imaging system to measure the liquid length. This value is converted linearly to the pipetted volume, whose measurement error is on a sub-pixel level. Image-based sampling using the fabricated probes was achieved by connecting the probe to an electroosmotic pump, which allowed the controlled pipetting of pico/femtoliter samples. Because the pipette allows samples of the same volume to be measured accurately, the pipette was applied for the semiquantitative mass spectrometry analysis of the metabolites in individual epidermal cells sampled from different parts of Portulaca oleracea plants. The results show that different types of cells have distinct metabolite profiles. Further, the experiments showed that, in dopamine-rich leaves, the content of dopamine in the petioles is higher than that in the foliage. The probe fabrication strategy opens new avenues for the controllable pipetting of ultra-small volume samples and the realization of the visual pipetting of pico/femtoliter samples.