Temperature dependence of 1∕f noise mechanisms in silicon nanowire biochemical field effect transistors

General spectral characteristics of human activity and its inherent scale-free fluctuations

The scale-free nature of daily human activity has been observed in different aspects; however, the description of its spectral characteristics is incomplete. General findings are complicated by the fact that-although actigraphy is commonly used in many research areas-the activity calculation methods are not standardized; therefore, activity signals can be different. The presence of 1/f noise in activity or acceleration signals was mostly analysed for short time windows, and the complete spectral characteristic has only been examined in the case of certain types of them. To explore the general spectral nature of human activity in greater detail, we have performed Power Spectral Density (PSD) based examination and Detrended Fluctuation Analysis (DFA) on several-day-long, triaxial actigraphic acceleration signals of 42 healthy, free-living individuals. We generated different types of activity signals from these, using different acceleration preprocessing techniques and activity metrics. We revealed that the spectra of different types of activity signals generally follow a universal characteristic including 1/f noise over frequencies above the circadian rhythmicity. Moreover, we discovered that the PSD of the raw acceleration signal has the same characteristic. Our findings prove that the spectral scale-free nature is generally inherent to the motor activity of healthy, free-living humans, and is not limited to any particular activity calculation method.

Rapid and facile method to prepare oxide precursor solution by using sonochemistry technology for WZTO thin film transistors

In this paper, a rapid and facile method of preparing metal-oxide semiconductor precursor solution using sonochemistry technology is proposed. Compared with the traditional method (water bath above 60 °C for several hours), the efficiency of preparing solution is improved, because sonochemical reaction is found to accelerate the dissolution of solutes and the agitation of solution. The color comparison and thermal gravimetric and differential scanning calorimetry of solution confirme the formation of W-doped zinc tin oxide (WZTO) precursor solution with good performance. The effects of sonochemical reactions on the film structure, surface morphology, optical properties and chemical composition of WZTO thin films are analyzed by atomic force microscopy, X-ray diffraction, UV visible spectrum and X-ray photoelectron spectroscopy. The results show that the film has a smooth surface, an amorphous structure, a high transmittance and more M-O bonding. Hence, a rapid process of preparing WZTO solution (sonochemical treatment for 10 min) and fabricate TFT with high electron mobility (2.7 cm² V^-1 s^-1) is established, while the corresponding mobility of the traditional method is 1.2 cm² V^-1 s^-1. The results show that the sonochemical reaction can improve the efficiency of preparing solution by 1800% and it is a fast and efficient method for preparing precursor solutions.

Specific and label-free immunosensing of protein-protein interactions with silicon-based immunoFETs

The importance of specific and label-free detection of proteins via antigen-antibody interactions for the development of point-of-care testing devices has greatly influenced the search for a more accessible, sensitive, low cost and robust sensors. The vision of silicon field-effect transistor (FET)-based sensors has been an attractive venue for addressing the challenge as it potentially offers a natural path to incorporate sensors with the existing mature Complementary Metal Oxide Semiconductor (CMOS) industry; this provides a stable and reliable technology, low cost for potential disposable devices, the potential for extreme minituarization, low electronic noise levels, etc. In the current review we focus on silicon-based immunological FET (ImmunoFET) for specific and label-free sensing of proteins through antigen-antibody interactions that can potentially be incorporated into the CMOS industry; hence, immunoFETs based on nano devices (nanowire, nanobelts, carbon nanotube, etc.) are not treated here. The first part of the review provides an overview of immunoFET principles of operation and challenges involved with the realization of such devices (i.e. e.g. Debye length, surface functionalization, noise, etc.). In the second part we provide an overview of the state-of-the-art silicon-based immunoFET structures and novelty, principles of operation and sensing performance reported to date.

Field-effect sensors - from pH sensing to biosensing: sensitivity enhancement using streptavidin-biotin as a model system

Field-Effect Transistor sensors (FET-sensors) have been receiving increasing attention for biomolecular sensing over the last two decades due to their potential for ultra-high sensitivity sensing, label-free operation, cost reduction and miniaturisation. Whilst the commercial application of FET-sensors in pH sensing has been realised, their commercial application in biomolecular sensing (termed BioFETs) is hindered by poor understanding of how to optimise device design for highly reproducible operation and high sensitivity. In part, these problems stem from the highly interdisciplinary nature of the problems encountered in this field, in which knowledge of biomolecular-binding kinetics, surface chemistry, electrical double layer physics and electrical engineering is required. In this work, a quantitative analysis and critical review has been performed comparing literature FET-sensor data for pH-sensing with data for sensing of biomolecular streptavidin binding to surface-bound biotin systems. The aim is to provide the first systematic, quantitative comparison of BioFET results for a single biomolecular analyte, specifically streptavidin, which is the most commonly used model protein in biosensing experiments, and often used as an initial proof-of-concept for new biosensor designs. This novel quantitative and comparative analysis of the surface potential behaviour of a range of devices demonstrated a strong contrast between the trends observed in pH-sensing and those in biomolecule-sensing. Potential explanations are discussed in detail and surface-chemistry optimisation is shown to be a vital component in sensitivity-enhancement. Factors which can influence the response, yet which have not always been fully appreciated, are explored and practical suggestions are provided on how to improve experimental design.

On the capacity of diffusion-based molecular communications with SiNW FET-based receiver

Molecular communication (MC) is a bio-inspired communication method based on the exchange of molecules for information transfer among nanoscale devices. Although MC has been extensively studied from various aspects, limitations imposed by the physical design of transceiving units have been largely neglected in the literature. Recently, we have proposed a nanobioelectronic MC receiver architecture based on the nanoscale field effect transistor-based biosensor (bioFET) technology, providing noninvasive and sensitive molecular detection at nanoscale while producing electrical signals at the output. In this paper, we derive analytical closed-form expressions for the capacity and capacity-achieving input distribution for a memoryless MC channel with a silicon nanowire (SiNW) FET-based MC receiver. The resulting expressions could be used to optimize the information flow in MC systems equipped with nanobioelectronic receivers.

Ultrasensitive Detection of Dual Cancer Biomarkers with Integrated CMOS-Compatible Nanowire Arrays

A direct, rapid, highly sensitive and specific biosensor for detection of cancer biomarkers is desirable in early diagnosis and prognosis of cancer. However, the existing methods of detecting cancer biomarkers suffer from poor sensitivity as well as the requirement of enzymatic labeling or nanoparticle conjugations. Here, we proposed a two-channel PDMS microfluidic integrated CMOS-compatible silicon nanowire (SiNW) field-effect transistor arrays with potentially single use for label-free and ultrasensitive electrical detection of cancer biomarkers. The integrated nanowire arrays showed not only ultrahigh sensitivity of cytokeratin 19 fragment (CYFRA21-1) and prostate specific antigen (PSA) with detection to at least 1 fg/mL in buffer solution but also highly selectivity of discrimination from other similar cancer biomarkers. In addition, this method was used to detect both CYFRA21-1 and PSA real samples as low as 10 fg/mL in undiluted human serums. With its excellent properties and miniaturization, the integrated SiNW-FET device opens up great opportunities for a point-of-care test (POCT) for quick screening and early diagnosis of cancer and other complex diseases.

Performance limitations for nanowire/nanoribbon biosensors

Field-effect transistor-based biosensors (bioFETs) have shown great promise in the field of fast, ultra-sensitive, label-free detection of biomolecules. Reliability and accuracy, when trying to measure small concentrations, is of paramount importance for the translation of these research devices into the clinical setting. Our knowledge and experience with these sensors has reached a stage where we are able to identify three main aspects of bioFET sensing that currently limit their applications. By considering the intrinsic device noise as a limitation to the smallest measurable signal, we show how various parameters, processing steps and surface modifications, affect the limit of detection. We also introduce the signal-to-noise ratio of bioFETs as a universal performance metric, which allows us to gain better insight into the design of more sensitive devices. Another aspect that places a limit on the performance of bioFETs is screening by the electrolyte environment, which reduces the signal that could be potentially measured. Alternative functionalization and detection schemes that could enable the use of these charge-based sensors in physiological conditions are highlighted. Finally, the binding kinetics of the receptor-analyte system are considered, both in the context of extracting information about molecular interactions using the bioFET sensor platform and as a fundamental limitation to the number of molecules that bind to the sensor surface at steady-state conditions and to the signal that is generated. Some strategies to overcome these limitations are also proposed. Taken together, these performance-limiting issues, if solved, would bring bioFET sensors closer to clinical applications.

Low-frequency flicker noise in a MSM device made with single Si nanowire (diameter ≈ 50 nm)

: Low-frequency flicker noise has been measured in a metal-semiconductor-metal (MSM) device made from a single strand of a single crystalline Si nanowire (diameter approximately 50 nm). Measurement was done with an alternating current (ac) excitation for the noise measurement superimposed with direct current (dc) bias that can be controlled independently. The observed noise has a spectral power density ∝1/fα. Application of the superimposed dc bias (retaining the ac bias unchanged) with a value more than the Schottky barrier height at the junction leads to a large suppression of the noise amplitude along with a change of α from 2 to ≈ 1. The dc bias-dependent part of the noise has been interpreted as arising from the interface region. The residual dc bias-independent flicker noise is suggested to arise from the single strand of Si nanowire, which has the conventional 1/f spectral power density.

Silicon nanowires with high-k hafnium oxide dielectrics for sensitive detection of small nucleic acid oligomers

Nanobiosensors based on silicon nanowire field effect transistors offer advantages of low cost, label-free detection, and potential for massive parallelization. As a result, these sensors have often been suggested as an attractive option for applications in point-of-care (POC) medical diagnostics. Unfortunately, a number of performance issues, such as gate leakage and current instability due to fluid contact, have prevented widespread adoption of the technology for routine use. High-k dielectrics, such as hafnium oxide (HfO(2)), have the known ability to address these challenges by passivating the exposed surfaces against destabilizing concerns of ion transport. With these fundamental stability issues addressed, a promising target for POC diagnostics and SiNWFETs has been small oligonucleotides, more specifically, microRNA (miRNA). MicroRNAs are small RNA oligonucleotides which bind to mRNAs, causing translational repression of proteins, gene silencing, and expressions are typically altered in several forms of cancer. In this paper, we describe a process for fabricating stable HfO(2) dielectric-based silicon nanowires for biosensing applications. Here we demonstrate sensing of single-stranded DNA analogues to their microRNA cousins using miR-10b and miR-21 as templates, both known to be upregulated in breast cancer. We characterize the effect of surface functionalization on device performance using the miR-10b DNA analogue as the target sequence and different molecular weight poly-l-lysine as the functionalization layer. By optimizing the surface functionalization and fabrication protocol, we were able to achieve <100 fM detection levels of the miR-10b DNA analogue, with a theoretical limit of detection of 1 fM. Moreover, the noncomplementary DNA target strand, based on miR-21, showed very little response, indicating a highly sensitive and highly selective biosensing platform.

Optimal signal-to-noise ratio for silicon nanowire biochemical sensors

The signal-to-noise ratio (SNR) for silicon nanowire field-effect transistors operated in an electrolyte environment is an essential figure-of-merit to characterize and compare the detection limit of such devices when used in an exposed channel configuration as biochemical sensors. We employ low frequency noise measurements to determine the regime for optimal SNR. We find that SNR is not significantly affected by the electrolyte concentration, composition, or pH, leading us to conclude that the major contributions to the SNR come from the intrinsic device quality. The results presented here show that SNR is maximized at the peak transconductance.