A fully electronic sensor for the measurement of cDNA hybridization kinetics

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Abstract

Ion sensitive field effect transistors (ISFET) are candidates for a new generation of fully electrical DNA sensors. To this purpose, we have modified ISFET sensors by adsorbing on their Si3N4 surface poly-l-lysine and single (as well as double) stranded DNA. Once coupled to an accurate model of the oppositely charged layers adsorbed on the surface, the proposed sensor allows quantitatively evaluating the adsorbed molecules densities, as well as estimating DNA hybridization kinetics.

Introduction

Surface-based methods are promising alternatives for the detection of biological molecules such as DNA and proteins (Schena et al., 1995, Han et al., 2006). In particular, microelectronic devices can play a major role in this arena, thanks to extremely high repeatability and process control available by using even relatively old technologies. One of the most interesting properties of this class of sensor is the ability to detect unlabelled sample molecules, thus avoiding bias introduced by enzymatic labelling used, for example, in microarray technology. During the last years, several fully electronic and label-free DNA sensors have been developed. Some sensors are based on the modification of redox reaction during hybridization (Ye and Ju, 2003), while others are based on variation of double layer capacitance after hybridization, on variation of surface potential (such as ISFET based sensors, e.g. Pouthas et al., 2004, Kim et al., 2004), and/or on the detection of the intrinsic charge of molecules adsorbed onto the sensors surface (Fritz et al., 2004, Uslu et al., 2004, Sakata et al., 2005, Purushothaman et al., 2006). In literature, ISFET structures have been widely reported for several types of sensors, ranging form the more conventional for these devices pH sensors (Bergveld et al., 1998, Chin et al., 2001, Grattarolla and Massobrio, 2002, Martinoia and Massobrio, 2000, Martinoia et al., 2001), or biochemical sensors (Lauwers et al., 2001), to more exotic ones, such as sensors for detection of micro-organisms in water (Cambiaso et al., 1996), for cell population measurements (Martinoia et al., 2001), and so on. The ISFET sensitive layer is here made of silicon nitride (Si3N4) (Siu and Cobbold, 1979). The majority of studies in literature have considered SiO2 (Pouthas et al., 2004) or Au (Kim et al., 2004) surfaces as interfaces between silicon-based devices and DNA. Si3N4/electrolyte interface features not only silanol groups (Yates et al., 1974, Siu and Cobbold, 1979) (typical of SiO2 surface) but also amino groups (Grattarolla and Massobrio, 2002, Martinoia and Massobrio, 2000) that play an important role in the control of the charge at sensor/electrolyte interface. Another important advantage of ISFET is the possibility to evaluate the DNA charge by measuring a simple parameter such as the variation in the transistor threshold voltage Vth. In this work we are experimentally studying the adsorption of poly-l-lysine (PLL), double stranded DNA (dsDNA), single stranded DNA (ssDNA) onto Si3N4 sensitive area of ISFETs and the hybridization kinetic of a DNA target to a probe linked to the sensitive surface. The electrical charge associated with these molecules can be evaluated by using an accurate model based on the site binding theory of Si3N4 (Cambiaso et al., 1996, Grattarolla and Massobrio, 2002, Martinoia and Massobrio, 2000, Martinoia et al., 2001). Differently from other proposed models (e.g. Landheer et al., 2005, Erickson et al., 2003) this one can be easily included in spice simulator. We therefore propose the surface-modified ISFET as the base for a fully electronic DNA hybridization sensor.

Section snippets

Experimental devices

All the ISFET sensors used throughout this work are manufactured with a well-established technology (Cambiaso et al., 1996, Martinoia and Massobrio, 2000, Martinoia et al., 2001) by ITC-IRST (Trento, Italy). The ion sensitive field effect transistors/complementary metal nitride oxide semiconductor (ISFET/CMNOS) technology consists in a modified 4 μm CMOS process with Al gate, realized on p-well substrate. In particular, a SiO2 layer is at first grown on silicon surface, over which a

A model for the ISFET interface

A schematic representation of the ISFET system is illustrated in Fig. 1a. The ISFET operation is very similar to that of conventional MOSFET, except that a reference electrode/electrolyte solution gate is used instead of metal or polysilicon gate. The electrical equivalent circuit and the relationships between voltages and currents are rather similar for the two device types. In detail, when a voltage is applied by the Ag/AgCl reference electrode through the electrolyte solution the following

Conclusions

In this work silicon nitride ISFETs have been used for bimolecular detection. A detailed interface model that describes the ISFET/PLL system has been developed. The model can accurately predict the charge of adsorbed PLL layer, taking into account not only the potential drop due to PLL layer, but also the pH variation induced on the ISFET surface by the PLL layer itself. Our model demonstrates that the positively charged PLL layer modifies not only the Helmholtz and diffused layers, but also

Acknowledgement

We thank Prof. A. Paccagnella from the Department of Electronic Information for his useful and interesting comments and for his contribution to the paper review.

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