Fine chemical manipulations of microscopic liquid samples. 2. Consuming and nonconsuming schemes.
Microscopic liquid particles can be manipulated chemically using a suitable diffusional microburet (DMB), whose tiny tip plugged with a diffusion membrane acts as a well-defined diffusional transfer channel. In part 1 of this work (Gratzl et al. Anal. Chem. 1999, 71, 2751-2756), we discussed the simplest DMB-based operation: addition, i.e., loading a droplet with a chemical that accumulates there without any chemical reaction occurring. Since in this process no consumption of the delivered molecules in the target droplet takes place, addition is a nonconsuming scheme. In this work, another type of nonconsuming scheme is explored, which is the subtraction of a substance from droplets via a DMB. This process has no analogy among macroscopic chemical operations. Both addition and subtraction occur according to an exponential asymptotic process when diffusion is at quasisteady state inside the DMB tip. These nonconsuming operations were characterized using the transport of microscopic quantities of Lucifer Yellow CH, a fluorescent dye, under a fluorescent microscope. The third basic type of chemical manipulation is when the substance delivered by a DMB is consumed in the target droplet instantaneously by a fast chemical reaction. This consuming scheme was studied by delivering EDTA into droplets containing Pb2+ ions and a color indicator. These microscopic titrations were monitored using gray scale transmittance images of the droplets as recorded versus time. A unified theory of the three basic DMB operations is also presented.