Micro- and nanopipettes

On the other hand, in the case of assisted-ion-transfer reactions in which the ligand is inside the pipette, the mass transport regime is completely analogous to a microdisc electrode. Since these early days, many groups have developed the pipette technique to a high level. In particular, Shao and Mirkin have made 

In 1998, Shao et al. developed a dual micropipette to be used in a generator-collector mode, thereby circumventing the restriction due to the potential window 

FIG U RE 1.29 (a) Video micrographs of a 15.5-pm-radius micropipette filled with an aqueous KCl solution and immersed in a DCE solution of DB18C6. No external pressure was applied to the pipette, and the micro-ITIES is flat. The insets show corresponding steady-state voltammograms of facilitated transfer of potassium (From Shao, Y. H. and M. V. Mirkin, 1998, Anal. Chem., Vol. 70, p. 3155. Used with permission), (b) Photomicrograph of a theta-pipette filled with an aqueous solution. A thin ( l-pm-thick) glass wall separates two barrels, one of which is blocked by an air bubble. Both orifices are about 4.5 pm radius. (Liu, B., Y. H. Shao, and M. V. Mirkin, 2000, Anal Chem, Vol. 72, p. 510. Used with permission.) 

A very recent development of ITIES in micropipettes is the electrochemical attosyringe by Laforge et al. [243] who demonstrated electrochemical control of the fluid motion inside the pipette to sample and dispense attoliter-to-picoliter volumes, for example, in an immobilized biological cell. The movement of the interface supported at the tip of a micropipette was recently observed by Dale and Unwin using confocal fluorescence microscopy [244], This motion was found to be reversible upon cycling. 

In sununary, micropipettes have been useful not only for kinetic measuranents of ion-transfer or assisted-ion-transfer reactions but the asymmetry of the diffusion fields can be used to determine which charge-transfer reactions take place. 

Putting a liquid-filled capillary in contact with a surface produces a liquid meniscus, which then spreads onto the substrate, provided the latter is not too hydrophobic. Pipettes have therefore been used for the local deposition of liquids by direct contact on substrates. The downscaling of pipetting techniques by profiling capillaries with micron-scale apertures benefitted from the association with electrochemistry in the conhned droplet volume. Scanning ionic conductance microscopy which has recently become 

---

Prof. Yuan-Huan Shao s group at Peking University has focused on miniaturization of liquid/liquid interfaces and developed novel techniques for characterization of such soft interfaces based on pulled glass micro- and nanopipettes in the past few years.They have developed new programs for pulling high quality micro- and nanopipettes which can be used to measure very fast ion transfer kinetics, to probe the thickness of interfacial structure and ion distribution near the interface combined with scanning ion conductance microscopy, and to evaluate the electrochemical behavior of facilitated anion transfers kinetically. Recently, they also developed a new way to modify the inner walls of micro- and nanopipettes, and to study ionic current rectification. ... 

Since the mass-transfer coefficient at a micropipette is inversely proportional to its radius, the smaller the pipette the faster heterogeneous rate constants can be measured. Micrometer-sized pipettes are too large to probe rapid CT reactions at the ITIES. Such measurements require smaller (nm-sized) pipettes. Nanopipettes are also potentially useful as SECM tips (see Section IV.D) because they can greatly improve spatial resolution of that technique. The fabrication of nanopipettes was made possible by the use of a micro-processor-controlled laser pipette puller capable of puling quartz capillaries [26]. Using this technique, Wei et al. produced nanopipettes as small as 20 nm tip radius and employed them in amperometric experiments [9]. 

In 1982, Samec et al. studied the kinetics of assisted alkali and alkali-earth metal cation-transfer reactions by neutral carrier and conclnded that the kinetics of transfer of the monovalent ions were too fast to be measured [186]. In 1986, Kakutani et al. published a study of the kinetics of sodium transfer facilitated by di-benzo-18-crown-6 using ac-polarography [187]. They concluded that the transfer mechanism was a TIC process and that the rate constant was also high. Since then, kinetic studies of assisted-ion-transfer reactions have been mainly carried out at micro-lTlES. In 1995, Beattie et al. showed by impedance measurements that facilitated ion-transfer (FIT) reactions are somehow faster than the nonassisted ones [188,189]. In 1997, Shao and Mirkin used nanopipette voltammetry to measure the rate constant of the transfer of K+ assisted by the presence of di-benzo-18-crown-6, and standard rate constant values of the order of 1 cm-S were obtained [190]. A more systematic study was then published that showed the following sequence,, which is not in accordance with... 

For measurement of the ion current run through bacterium membrane, the well-known patch-clamp method is useful. Inside the nanopipette there are electrolyte and an electrode. Before working with the bacillus we have designed a model from a membrane with micro-channels of about 10 pm diameter. This membrane is dipped into electrolyte. The nanopipette has outer diameter about 500 nm, and inner diameter 250 nm. During scanning... 

---