Mixed Self-Assembled Monolayers of Mercaptoundecanoic Acid and Thiolactic Acid for the Construction of an Enzymatic Biosensor for Hydroquinone Determination

Abstract

A horseradish peroxidase (HRP) biosensor was constructed using binary self-assembled monolayers (SAM) of 11-mercaptoundecanoic acid (MUA) and thiolactic acid (TLA) on gold surface. The advantages of using mixed SAM for the enzyme immobilization is that the long carbon chain molecules act as a support for the enzyme while the short chain molecules favor the electron transfer process. In order to obtain this modified surface, the gold electrode was incubated in a solution containing different proportions of MUA and TLA and the best concentration ratio of these molecules was 0.5 and 1.0 mmol L −1 , respectively. The preparation steps and the biosensor response were monitored by electrochemical techniques. The biosensor proposed was applied to determine hydroquinone in a 0.10 mol L −1 phosphate buffer solution containing H 2 O 2 0.3 mmol L −1 . The Au-SAM mix -HRP electrode, in the presence of hydrogen peroxide, catalyzes the oxidation of hydroquinone to the corresponding quinone, which is electrochemically reduced back to hydroquinone at −0.08 V vs Ag/AgCl. The analytical curve was linear for hydroquinone concentrations from 5.0 to 30 μmol L −1 and the detection limit was 1.26 μmol L −1 . The lifetime of this biosensor was 15 days. The modified electrode displayed good reproducibility, sensitivity and stability for the determination of hydroquinone.The modification of electrode surfaces using the self-assembled monolayers (SAM) technique is more convenient, because they form spontaneously, easy to handle mechanically and relatively stable in electrolyte solutions. 1 The main interest in developing mixed selfassembled monolayers (SAM mix ) is related to the development of attractive methods to promote different arrangements on the electrode surface, which enables the control of specific reaction sites. In this way, SAM mix can be configured as an appropriate platform for the immobilization of biomolecules 2 which have been frequently applied in electroanalysis for the development of biosensors, 3,4 since well-organized and compact monolayers present advantages like selectivity, sensitivity and reduced overpotentials in electrocatalytic reactions. The SAM mix can be obtained by combining the properties of alkanethiols with different carbon chain lengths or different functional group. 5 Several methods for the preparation of mixed SAMs are reported in the literature, being the most commonly used the coadsorption of different thiols, 6,7 and also by electrochemical substitution modification. 8,9 The SAM mix obtained through co-adsorption of different functional groups was described by Ngunjiri et al, 10 formed by molecules octadecanethiol and 11-mercaptoundecanoic acid for immobilization of proteins. Another example, Ji et al. 11 developed the SAM mix formed by thioctic acid (T-COOH) and thioctic acid amide (T-NH 2 ) which was used to immobilize tyrosinase to construct an electrochemical biosensor for phenolic compounds. Studies with this biosensor showed that mixed SAMs improved protein adsorption and they were more co...

Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu

“…Since ADT shows higher stability than C3, the film thickness apparently outweighs the destabilizing influence of weaker lateral interactions and defects. 50,51 The greater oxidative stability of ADT can be explained in the same way.…”

“…15 Therefore, the geometric area of the Cu electrode is the normalization factor for the current reported on Cu in CVs and LSVs, as well as C, throughout the paper and SI. 16 After electrochemically pretreating each electrode, C was measured in each case by fitting the double layer current measured at different scan rates (50,100,150,200,250, 300 mV/s) from cyclic voltammograms confined to the double layer potential window. Thiol SAMs were prepared by soaking each electrode in an ethanol-thiol solution (1-5 mM) for 10 minutes.…”

Probing a Reversible Cationic Switch on a Mixed Self-Assembled Monolayer Using Scanning Electrochemical Microscopy

“…The result can be a powerful evidence for the ideal compact monolayer. Theoretically, the SAM was constructed by the long carbon chain molecules, which has stronger intermolecular interactions to reduce defects. , The long carbon chain molecules also represent a large thickness of the layer and effectively prevent the electron tunneling.…”

Label-Free Detection of Chikungunya Non-Structural Protein 3 Using Electrochemical Impedance Spectroscopy

“…The surface functionalization by the mixed thiol was based on the strong Au-thiolate bond, with the carboxylic acid end-groups exposed at the monolayer-liquid interface for further immoblization. 29 Here, 6-MH was used as a spacer molecule to reduce the steric hindrance of the surface carboxylic end group. 30 After SAM formation, the electrodes were subsequently washed with ethanol and Milli-Q water.…”