Inclusion of phthalate esters by a self-assembled monolayer of thiolated cyclodextrin on a gold electrode

Substrate Monolayers as Electrochemical Sensing Elements for α-Chymotrypsin

A disulfide, which carried two L-phenylalanyl p-nitroanilide (Phe-pNA) moieties at both ends, was prepared by the coupling of 11,11'-dithiodiundecanoic acid (DTUA) with Phe-pNA. The compound obtained (DTUA-Phe-pNA) formed a self-assembled monolayer (SAM) on a gold electrode and vacuum-evaporated gold thin film as proven by cyclic voltammetry, and reflection absorption spectroscopy, respectively. Incubation of alpha-chymotrypsin with the SAM-modified electrode induced both a decrease in anodic and cathodic peak currents (-DeltaIa and -DeltaIc) and an increase in potential difference (DeltaEp) in the cyclic voltammogram of potassium ferricyanide as a probe, which suggested the attack of the enzyme at the amide group between Phe and pNA groups of the SAM, resulting in the formation of an intermediate. The linear relationship between the initial rate of increase in the amount of enzyme bound to the SAM and in both DeltaIa (absolute value of the decrease in anodic peak current) and DeltaEp values was confirmed by the quartz crystal microbalance method. The binding rate of the enzyme to the Phe-pNA SAM was dependent on the surface density of the Phe-pNA group in the SAM. The alpha-chymotrypsin-induced increases in the DeltaIa and DeltaEp values were inhibited by the addition of N-acetyl-D-phenylalanine methyl ester (N-Ac-D-Phe-OMe). In contrast with alpha-chymotrypsin, trypsin did not show a significant increase in the DeltaIa and DeltaEp values upon incubation with Phe-pNA-carrying SAM. These results could be attributed to the specific attack of alpha-chymotrypsin to the amide group in the SAM. The inhibition constant for N-Ac-D-Phe-OMe in the SAM system was quite similar to that in the free substrate system, showing that the enzymatic reaction above the SAM proceeds in a similar way to that in the homogeneous solution system.

Electrochemical sensor for electrochemically inactive beta-D(+)-glucose using alpha-cyclodextrin template molecules

We report an electrochemical sensor for an electrochemically inactive organic compound using a self-assembled monolayer (SAM) formed on the gold surface from a solution of thiolated alpha-cyclodextrin (alpha-CD). The SAM makes up an array of ultramicroelectrodes, which capture electroactive molecules such as those of ferrocene. When this SAM-modified electrode is exposed to a solution containing an electrochemically inactive compound, e.g., glucose, the captured ferrocene molecules are replaced by the electroinactive molecules via an equilibrium established between the two compounds, lowering the current for ferrocene oxidation. The decrease in current is directly proportional to the amount of glucose added and the glucose can be analyzed up to approximately 0.80 mM, which is approximately 1/10 of a typical physiological concentration in blood serum. Formation constants of the surface-bound alpha-CD molecules with ferrocene and glucose are also reported.

Influencing the binding selectivity of self-assembled cyclodextrin monolayers on gold through their architecture

Cyclodextrin derivatives modified with seven thioether moieties (1) or with one thiol moiety (2) bind to gold. Monolayers on gold of 1 or mixed monolayers of 2 and mercaptoundecanol were characterized by electrochemistry, wettability, and atomic force microscopy (AFM). Monolayers of 1 are well-ordered, but the order in the mixed monolayers depends on the ratio of 2 to mercaptoundecanol. With sufficient alkyl chains to fill the space under the cyclodextrin moiety of 2, the monolayers are densely packed. Guest recognition at these monolayers in water was studied by surface plasmon resonance (SPR) spectroscopy. For simple organic guests, monolayers of 1 showed the same selectivity and binding strength as beta-cyclodextrin in solution: however, the selectivity towards steroidal bile salts differs from solution. The mixed monolayers of 2, in which the cyclodextrin is less substituted and has more flexibility, bind steroidal guests (6a-6e) with the same selectivity as beta-cyclodextrin in solution.