Label-free electronic detection of biomolecules using a-Si:H field-effect devices D. Gonc ¸alves a,b, * , D.M.F. Prazeres b,c , V. Chu a , J.P. Conde a,c a INESC Microsistemas e Nanotecnologias (INESC-MN), Rua Alves Redol, 9, 1000-029 Lisbon, Portugal b Center of Biological and Chemical Engineering, Instituto Superior Te ´cnico, Lisbon, Portugal c Department of Chemical Engineering, Instituto Superior Te ´ cnico, Lisbon, Portugal Available online 4 April 2006 Abstract Electrolyte-gate amorphous silicon thin-film transistors (EG-TFT) are used for pH and label-free sensing of biological molecules. In top-gate EG-TFTs, the metal gate is replaced by an electrolyte solution, in which a biasing Pt electrode is immersed. It is possible to switch the TFT on by increasing the voltage of the Pt electrode above a threshold value (V T ). EG-TFTs show a decrease in V T when the pH of the electrolyte solution is increased. EG-TFTs also show a shift in V T when exposed to solutions containing molecules of either DNA (V T decreases) or the protein horseradish peroxidase (HRP) (V T increases). Exposure to these solutions results in the adsorption of biomolecules at the dielectric-electrolyte interface which changes the TFT transfer curve and consequently allows electronic label-free detection of the biomolecules. Ó 2006 Elsevier B.V. All rights reserved. PACS: 07.07.Df; 85.30.Tv; 81.05.Gc Keywords: Thin film transistors; Biosensors; Sensors; Adsorption 1. Introduction Hydrogenated amorphous silicon (a-Si:H) thin films allow the fabrication of arrays of electronic devices such as thin film transistors (TFT) or diodes on a variety of inex- pensive, large area, and flexible substrates [1]. The low-tem- perature processing, optoelectronic characteristics, and ability to be deposited and processed on a wide variety of substrates (e.g., glass and polymers) makes a-Si:H an attrac- tive material for lab-on-a-chip applications as the active sensing component for the detection of biomolecules. The development of lab-on-a-chip systems for the rapid detection of small quantities of biological molecules is a topic of great current interest [2–4]. Most of the sensors proposed require the biomolecule to be labelled for detec- tion (e.g., with a fluorescent marker) [2,3]. Label-free elec- tronic detection of biomolecules would considerably simplify the biological assay [3,4]. In 1988 Gotoh et al. reported for the first time an a-Si:H ion sensitive field effect transistor (ISFET) [5]. The sensitivity of ISFETs to pH is usually described by the ion-site binding model and the Nernst law [6], for which a theoretical limit of 59.2 mV/ pH at 298 K is predicted. Some reports show nearly Nerns- tian responses: 54 mV/pH with polycrystalline silicon ISFETs [7] and 57.3 mV/pH with GaN ISFETs [8]. Supra-Nernstian responses were also reported – 72 mV/ pH for hydrogenated diamond ISFET devices [9]. Non- Nernstian responses can be explained, according to Berg- veld [10,11], by a workfunction change due to the forma- tion of surface dipole layers, in addition to changes in the interfacial potential, as in the case of a conventional ISFET. Workfunction modulation is also described by de 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.058 * Corresponding author. Address: INESC Microsistemas e Nanotecnol- ogias (INESC-MN), Rua Alves Redol, 9, 1000-029 Lisbon, Portugal. Tel.: +351 213100237; fax: +351 213145843. E-mail address: dgoncalves@inesc-mn.pt (D. Gonc ¸alves). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 2007–2010