pH-dependent binding interaction between functionalized thiol monolayers and electroactive ions

Mediator free cholesterol biosensor based on self-assembled monolayer platform

Self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) has been investigated for immobilization of bi-enzymes (ChOx and ChEt) towards development of enzyme biosensors for detection of free and total cholesterol. This enzyme immobilized SAM surface has been characterized by scanning electron microscopy and electrochemical measurements. The results of electrochemical response studies reveal fast enzymatic reaction in phosphate buffer saline solution without using any artificial mediator. This may be attributed to the molecular wire type behavior of short 4-ATP molecule that promotes electron transfer between enzyme and the electrode surface due to its conjugated structure. Interference free estimation of free and total cholesterol has been realized at low operating potential of 0.33 V with range of detection as 25 to 400 mg dl(-1), sensitivity of 542.3 nA mM(-1) (for ChOx/4-ATP/Au) and 886.6 nA mM(-1) (for ChEt-ChOx/4-ATP/Au) with a response time of 20 s at pH 7.4.

Interactions of adsorbed albumin with underpotentially deposited copper on gold piezoelectrodes

The adsorption of a model protein, bovine serum albumin (BSA), on Au electrodes was investigated using the Cu adatom probe method and Electrochemical Quartz Crystal Nanobalance (EQCN) technique. The adsorption of BSA was confirmed by AFM imaging and has been found to be controlled by kinetics. Using the Cu adatom probe method, we were able to reconstruct the entire BSA adsorption transient Theta(BSA) vs. t. The adsorption rate constant k(1), determined from this transient is k(1)=2.45x10(5) L mol(-1) s(-1). We have found that the bulk Cu(0) deposition process is blocked by BSA adsorption and it decays exponentially with time during BSA adsorption. It ceases completely when a full monolayer of BSA is formed. In contrast to that, the mass associated with Cu-u.p.d. decreases only to ca. 50% of that in the absence of BSA, indicating that Cu adatoms can penetrate (wedge) into the space between the surface Au atoms and the adsorbed BSA molecules. In addition to that, we have found that the degree of penetration of Cu adatoms can be controlled by the applied deposition potential. By selecting a sufficiently cathodic potential, we were able to deposit a full Cu-u.p.d. monolayer, independent of the BSA surface coverage extending from Theta(BSA)=0 to Theta(BSA) approximately 1. The positive shift of Cu(ad) desorption peak potential E(p), observed in the presence of adsorbed BSA, has been interpreted in terms of Frumkin exchange interaction forces between Cu(ad) and BSA(ad), on the basis of our earlier theoretical model, expanded here to include adsorbed species in two monolayers. This expansion is possible owing to the fast rate of Cu adatom penetration in the interfacial region. From the plots of E(p) vs. Theta(BSA), the presence of strong attractive interactions between Cu(ad) and BSA(ad) was deduced. These interactions result in a super-shift of the Cu-u.p.d. desorption peak potential, corresponding to the exchange interaction coefficient g(M,X)<-4, indicating on a possibility of the formation of a stable interface complex.