Metal-Organic Frameworks (MOF) formed by self-assembly of organic ligands and metal ions have emerged as an important class of porous crystalline materials.[1] Arising from their high variability in the choice of organic ligands and metal centers, suitable for the MOF synthesis, a large number of crystal structures have been developed making them attractive candidates for various applications such as gas storage, [2] separation, [3] [4] catalysis, [5] sensing, [6] electronics [7] and drug delivery. [8] Next to the design of new crystal structures and incorporation of novel functional groups recently larger attention has also been drawn to the structuring of MOFs at the mesoscopic/macroscopic scale [2] as well as their positioning.[9] The possibility of incorporating MOF crystals inside superstructures combined with precise positioning will play an important role towards improving their performance and incorporating them into functional devices.

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Unmodified and ferrocene-functionalized mesoporous silica layers exhibiting vertical nanochannels have been grown through electrochemically assisted self-assembly onto indiumtin oxide (ITO), and the resulting film electrodes have been applied for the electrochemical detection of cysteine. When using the unmodified film electrode and ferrocenedimethanol (Fc(MeOH) 2 ) as a mediator in solution, both cysteine and glutathione can be oxidized by the Fc(MeOH) 2 + generated at the electrode surface, but the electron transfer rates were three times faster with cysteine than with glutathione. This can be exploited for the reagent-free selective detection of cysteine over glutathione by using a ferrocene-functionalized mesoporous silica film on an ITO electrode, based on a combination of charge transfer kinetic and mass transport limitations through the oriented nanochannels. This has been demonstrated here by using cyclic voltammetry and amperometry in flow-injection analysis mode. A sensitivity of 1.28 mA M À 1 was measured for the cysteine concentration range of 3-20 μM with a repeatability of the signal between 3 and 12 %. The lowest concentration of cysteine experimentally detected was 3 μM.

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