Melanin as an active layer in biosensors

Marina Piacenti Da Silva,Mirko Congiu,Carlos Frederico De Oliveira Graeff,Marcelo Mulato,Natália Biziak De Figueiredo,Jéssica Colnaghi Fernandes

doi:10.1063/1.4869638

Abstract

The development of pH sensors is of great interest due to its extensive application in several areas such as industrial processes, biochemistry and particularly medical diagnostics. In this study, the pH sensing properties of an extended gate field effect transistor (EGFET) based on melanin thin films as active layer are investigated and the physical mechanisms related to the device operation are discussed. Thin films were produced from different melanin precursors on indium tin oxide (ITO) and gold substrates and were investigated by Atomic Force Microscopy and Electrochemical Impedance Spectroscopy. Experiments were performed in the pH range from 2 to 12. EGFETs with melanin deposited on ITO and on gold substrates showed sensitivities ranging from 31.3 mV/pH to 48.9 mV/pH, depending on the melanin precursor and the substrate used. The pH detection is associated with specific binding sites in its structure, hydroxyl groups and quinone imine.

Highlights

Thin films were produced from different melanin precursors on indium tin oxide (ITO) and gold substrates and were investigated by Atomic Force Microscopy and Electrochemical Impedance Spectroscopy
Extended gate field effect transistors (EGFET) have been employed instead of traditional ionsensitive field-effect transistor (ISFET) in pH and ion concentrations measurement in the last few years, due its flexible structure, in which a sensitive layer is deposited on an extended gate that isolates the FET from the chemical environment, promoting extended gate field effect transistor (EGFET) a better long-term stability.[1,2,3,4,5,6] pH sensitive biosensors are extensively studied, since it has many applications, especially in the monitoring of biological systems such as blood, and for chemical analysis and environment monitoring.[7,8,9,10,11]
The analysis of melanin films morphology by Atomic Force Microscopy (AFM) is presented on Figure 3, where it is possible to observe the different roughness in the thin films A, B and C

Summary

Introduction

There are many options of materials that can be used as ion sensitive layers. There is a continuous search for layers with higher sensitivities, specially using materials like biomolecules that could present sensitivities comparable to inorganic sensors.[1,6,9] Recent studies shows that these sensors can have high sensitivities depending on the material used, like zinc oxide based sensors have sensitivity of 38 mV/pH,[1] tin oxide 56–58 mV/pH,[12] and indium tin oxide 55 mV/pH.[3] In the case of organic semiconductors, which are in general less used, nanostructured polyaniline and poly(vinylsulfonic acid) have a sensitivity of 58 mV/pH,[13] carbon nanotubes with 50.9 mV/pH,[5] and hibrid materials like vanadium oxide/hexadecylamine, 38.1 mV/pH.[14]. Specific binding-sites, can donate or accept protons from the solution, making the material surface

Methods

Results

Conclusion