Several strategies have been employed to immobilise biological compounds onto surfaces for the development of biosensing interfaces. The Self-Assembly method for the anchoring of enzymes or other bio-compounds displays several advantages relative to other approaches, namely, high reproducibility, molecular level control, vicinity from the surface allowing direct electron transfer [1]. In more recent studies [2,3], it has been shown that a variety of secondary amines condense with carbon disulfide (CS₂) onto gold surfaces at room temperature, forming dithiocarbamate monolayers that have been regarded as a versatile chemical alternative for thiols in self-assembled systems [4]. The major structural aspect distinguishing dithiocarbamates from thiolates is the resonance structure among the sulphur atoms, carbon and nitrogen, providing distinct binding properties, namely higher sulphur densities with possible overlapping of molecular and metal states.

The purpose of this work is to use the reaction between CS₂ and amines for direct biomolecule immobilisation onto gold surfaces. To achieve this goal the chemical coupling between carbon disulphide and distinct compounds containing primary (e.g. dopamine) or secondary amines (e.g. tryptophan, hexylmethylamine, epinephrine) have been primarily investigated. Three different approaches were used for the formation of dithiocarbamates on gold (111) electrodes: (i) one pot chemical reaction: gold in contact with a mixture of CS₂ and amines; (ii) self-assembling of a CS₂ monolayer and post-reaction with the amines, and (iii) dithiocarbamate formation through reaction of CS₂ with amines following exposure to the gold surface. The amount of sulphur attached to gold as well as the stability and packing of the monolayers were evaluated by electrochemical reductive desorption studies. In addition, the use of electrochemically active amine compounds (dopamine and epinephrine) enabled to infer on the amount of amines present on the electrode surface, as a result of the successful reaction. Evidence for dithiocarbamate formation has been obtained using the three methodologies, and the modified electrodes were characterised by scanning tunnelling microscopy. Biofunctionalisation of gold surfaces has been also attempted through the reaction of CS₂ with amines present in Glucose Oxidase. Enzyme immobilisation was confirmed through its electrocatlytic response towards glucose.

Acknowledgements

The authors acknowledge Fundação para a Ciência e a Tecnologia for financial support, project (PTDC/QUI/66612/2006).

References

[1] J. F. Cabrita, L. M. Abrantes, A. S. Viana, Electrochimica Acta, 50 (2005) 2117.

[2] Y. Zhao, P.-S. Waleska, Q. Shi, A. Wei, J. Am. Chem. Soc., 127 (2005) 7328.

[3] H. Zhu, D. M. Coleman, C. J. Dehen I. M. Geisler, D. Zemlyanov, J. Chmielewski, G. J. Simpson, Alexander Wei, Langmuir, 24 (2008) 8660.

[4] P. Morf, F. Raimondi, H.-G. Nothofer, B. Schnyder, A. Yasuda, J. M. wessels, T. A. Jung, Langmuir 22 (2006) 658.

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1. Nanostructured gold surfaces for biosensing applications