Digital approaches in Electronic biochips

Download Citation Email Print Request Permissions Save to Project Electronic biochips are defined by the merge of integrated electronics, containing diverse sensors, with reaction solution and sample hold in reaction chamber(s). Based on their overall structure, electronic biochips can work in either well-confined or in area-confined configurations. Extreme parceling of the sample (digital approach), coupled with electronic biochips, can radically enhance the throughput performance of the assay, especially in an area-defined configuration. This takes advantage of the increased sensitivity that follows device miniaturization, as illustrated in our experiments on Silicon Nano Ribbons

Multi-Wire Tri-Gate Silicon Nanowires Reaching Milli-pH Unit Resolution in One Micron Square Footprint

The signal-to-noise ratio of planar ISFET pH sensors deteriorates when reducing the area occupied by the device, thus hampering the scalability of on-chip analytical systems which detect the DNA polymerase through pH measurements. Top-down nano-sized tri-gate transistors, such as silicon nanowires, are designed for high performance solid-state circuits thanks to their superior properties of voltage-to-current transduction, which can be advantageously exploited for pH sensing. A systematic study is carried out on rectangular-shaped nanowires developed in a complementary metal-oxide-semiconductor (CMOS)-compatible technology, showing that reducing the width of the devices below a few hundreds of nanometers leads to higher charge sensitivity. Moreover, devices composed of several wires in parallel further increase the exposed surface per unit footprint area, thus maximizing the signal-to-noise ratio. This technology allows a sub milli-pH unit resolution with a sensor footprint of about 1 \ub5m2, exceeding the performance of previously reported studies on silicon nanowires by two orders of magnitude

Comparison between front- and back-gating of Silicon Nanoribbons in real-time sensing experiments

Field-effect transistors (FETs) with open gate structures such as Silicon Nanoribbons (SiNRs) are promising candidates to become general platforms for ultrasensitive, label-free and real-time detection of biochemical interactions on surface. This work proposes and demonstrates the viability of a solution for integrating Ag/AgCl reference electrodes with the microfluidics. A comparison between different polarization schemes is carried out with an analysis of the respective advantages and disadvantages

Detecting particles flowing through interdigitated 3D microelectrodes

Counting cells in a large microchannel remains challenging and is particularly critical for in vitro assays, such as cell adhesion assays. This paper addresses this issue, by presenting the development of interdigitated three-dimensional electrodes, which are fabricated around passivated pillarshaped silicon microstructures, to detect particles in a flow. The arrays of micropillars occupy the entire channel height and detect the passage of the particle through their gaps by monitoring changes in the electrical resistance. Impedance measurements were employed in order to characterize the electrical equivalent model of the system and to detect the passage of particles in real-time. Three different geometrical micropillar configurations were evaluated and numerical simulations that supported the experimental activity were used to characterize the sensitive volume in the channel. Moreover, the signal-to-noise-ratio related to the passage of a single particle through an array was plotted as a function of the dimension and num ber of micropillars

Tri-gate silicon nanowire transistors for ultra-low pH resolution and improved scalability

The scalability of on-chip analytical systems based on field-effect transistors as pH sensors is limited by the degradation of the signal to noise ratio when decreasing the device size. Nano-sized tri-gate transistors such as silicon nanowires and nanoribbons exhibit improved coupling with the electrolyte environment thanks to their tri dimensional structure. Even though in the framework of solid-state tri-dimensional transistors an enhancement of the channel con-ductivity is achieved by reducing the device width, so far the sensitivity performance of liquid-gate devices has been mostly considered in terms of threshold voltage readout. In pH sensing applications, however, the threshold voltage change is independent on the device size, thus overlooking the potential advantage derived from the improved electrical properties of nano sized devices. In this thesis, sensitivity characterization coupled with a comprehensive noise analysis has been performed on devices ranging from 50 nm to 70 µm in width and from 350 nm to 4250 nm in length. Such comprehensive characterization is made possible thanks to the employement indus-trial top down CMOS compatible technology, which ensures high control over process variation and enables the fabrication of nanowires with a wide range of widths and lengths. The results show that reducing the width below few hundreds of nanometers results in an increase of the conductivity properties of silicon nanoribbons, which ultimately improves the sensitivity with respect to surface charge, hence to pH. This enhanced sensitivity of nanoscaled devices can be further exploited within a multi wire configuration, in which the exposed surface is increased and the noise reduced. We provide experimental evidence that multi wire devices maximize the signal to noise ratio achieving, in the presented technology, a resolution of 0.0028 pH·µm2. This result could have a great impact on the improvement of the scalability of on-chip analytical systems requiring high pH resolution, such as DNA sequencing and quantitative PCR

Robust Microelectrodes Developed for Improved Stability in Electrochemical Characterization of Biomolecular Layers

This paper presents a robust electrochemical detection system composed of microfabricated electrodes and a potentiostat circuit developed for quantitative detection of biomolecules by cyclic voltammetry (CV) measurements. Compared to electrochemical cells employing external counter electrodes (CE) and polymer passivation layers, this system offers more reliable and stable operation owing to on-chip Pt CE and very stable oxide passivation layer, withstanding aggressive cleaning techniques and chemicals involved. The adhesion of oxide to Au and Pt is significantly enhanced by adding slots to the metals and optimizing the metal lift-off process. Different electrode configurations and sizes are tested by CV of redox species before and after self-assembled organic molecular layer formation, and it is concluded that the proposed system offers a low-cost and reliable microelectrode array solution for real time and high sensitive biomolecular detection

Analysis of Dielectric Microbead Detection by Impedance Spectroscopy with Nanoribbons

We present a quantitative numerical analysis of dielectric microbead detection experiments with nanoribbons operated in the AC small signal regime. To this purpose, a comprehensive model of nanoribbon operation in electrolyte environment and with DC/AC signals is extended to include the effect of site-binding charges in DC, transient and AC conditions. The model is calibrated against DC measurements and pH-transients data from the literature. The impact of the microfluidic chamber size, the interconnects and the site-binding charge is investigated. The calibrated model suggests that the impedance response to micron-sized beads cannot be explained without also taking into account the bead surface charge

Peak Shift Measurement of Localized Surface Plasmon Resonance by a Portable Electronic System

In recent years, the characterization of surface molecular layers by localized surface plasmon resonance (LSPR) has attracted a lot of interest thanks to its ability to provide a higher spatial resolution with respect to standard SPR. LSPR can be observed as a peak in the extinction spectrum of metal nanoparticles such as gold non-connected surface patterns. A plasmon peak red shift is caused both by the presence of molecular layers on the gold surface and by molecular binding events. The current study presents a portable transmission system to observe the LSPR phenomenon that extracts the peak location employing a discrete number of light sources. The peak location extraction is performed by an algorithm that takes into account the spectral characteristics of all the components. The performance of our LSPR measurement system has been characterized on a set of Fluorinated Tin Oxide-coated slides covered with nanoislands with a diameter of approximately 30 nm. The samples have been modified with a single-stranded DNA layer and the plasmonic peak location has been determined before and after surface treatment. The samples have been characterized in parallel with a high-end spectrophotometer. The results presented demonstrated the performance of our measurement system in determining the peak location with 1 nm precision. (C) 2012 Elsevier B.V. All rights reserved