Electrical performance of silicon-on-insulator field-effect transistors with multiple top-gate organic layers in electrolyte solution

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.

Proton-ELISA: Electrochemical immunoassay on a dual-gated ISFET array

Here we report an electrochemical immunoassay platform called Proton-ELISA (H-ELISA) for the detection of bioanalytes. H-ELISA uniquely utilizes protons as an immunoassay detection medium, generated by the enzyme glucose oxidase (GOx) coupled with Fenton's reagent in a proton amplification reaction cascade that results in a highly amplified signal. A proton-sensitive dual-gated ion-sensitive field effect transistor (DG-ISFET) sensor was also developed for sensitive and accurate detection of the proton signal in H-ELISA. The DG-ISFET sensor comprises of a 128 × 128 array of 16,384 sensing transistors each with an individually addressable back gate to allow for a very high signal throughput and improved accuracy. We then demonstrated that the platform could detect C-reactive protein and immunoglobulin E down to concentrations of 12.5 and 125 pg/mL, respectively. We further showed that the platform is compatible with complex biological sample conditions such as human serum, suggesting that the platform is sufficiently robust for potential diagnostic applications.

Label-free SnO2 nanowire FET biosensor for protein detection

Novel tin oxide field-effect-transistors (SnO₂ NW-FET) for pH and protein detection applicable in the healthcare sector are reported. With a SnO₂ NW-FET the proof-of-concept of a bio-sensing device is demonstrated using the carrier transport control of the FET channel by a (bio-) liquid modulated gate. Ultra-thin Al₂O₃ fabricated by a low temperature atomic layer deposition (ALD) process represents a sensitive layer to H⁺ ions safeguarding the nanowire at the same time. Successful pH sensitivity is demonstrated for pH ranging from 3 to 10. For protein detection, the SnO₂ NW-FET is functionalized with a receptor molecule which specifically interacts with the protein of interest to be detected. The feasibility of this approach is demonstrated via the detection of a biotinylated protein using a NW-FET functionalized with streptavidin. An immediate label-free electronic read-out of the signal is shown. The well-established Enzyme-Linked Immunosorbent Assay (ELISA) method is used to determine the optimal experimental procedure which would enable molecular binding events to occur while being compatible with a final label-free electronic read-out on a NW-FET. Integration of the bottom-up fabricated SnO₂ NW-FET pH- and biosensor into a microfluidic system (lab-on-a-chip) allows the automated analysis of small volumes in the 400 μl range as would be desired in portable on-site point-of-care (POC) devices for medical diagnosis.

Biotin- and glycoprotein-coated microspheres: potential surrogates for studying filtration of cryptosporidium parvum in porous media

Cryptosporidium parvum is a waterborne pathogen, yet no suitable surrogate has been established for quantifying its filtration removal in porous media. Carboxyl polystyrene microspheres with size, density, and shape similar to C. parvum were coated with biotin (free and containing amine, NH(2)) and glycoprotein. These biomolecules have isoelectric points similar to C. parvum (pH ≈ 2), and glycoprotein is a major type of surface protein that oocysts possess. Zeta potential (ζ) and filtration removal of particles in sand of two different grain sizes were examined. Compared to unmodified microspheres, modified microspheres achieved a superior match to the oocysts in ζ, concentration, mass recovery, and collision coefficient. They showed the same log reduction in concentration as oocysts, whereas results from unmodified microspheres deviated by 1 order of magnitude. Of the three types of modified microspheres, glycoprotein-coated microspheres best resembled oocyst concentration, despite having ζ similar to NH(2)-biotin-coated microspheres, suggesting that surface protein also played an important role in particle attachment on solid surfaces. With further validation in environmental conditions, the surrogates developed here could be a cost-effective new tool for assessing oocyst filtration in porous media, for example, to evaluate the performance of sand filters in water and wastewater treatment, water recycling through riverbank filtration, and aquifer recharge.

Theory of signal and noise in double-gated nanoscale electronic pH sensors

The maximum sensitivity of classical nanowire (NW)-based pH sensors is defined by the Nernst limit of 59 mV/pH. For typical noise levels in ultra-small single-gated nanowire sensors, the signal-to-noise ratio is often not sufficient to resolve pH changes necessary for a broad range of applications. Recently, a new class of double-gated devices was demonstrated to offer apparent "super-Nernstian" response (>59 mV/pH) by amplifying the original pH signal through innovative biasing schemes. However, the pH-sensitivity of these nanoscale devices as a function of biasing configurations, number of electrodes, and signal-to-noise ratio (SNR) remains poorly understood. Even the basic question such as "Do double-gated sensors actually resolve smaller changes in pH compared to conventional single-gated sensors in the presence of various sources of noise?" remains unanswered. In this article, we provide a comprehensive numerical and analytical theory of signal and noise of double-gated pH sensors to conclude that, while the theoretical lower limit of pH-resolution does not improve for double-gated sensors, this new class of sensors does improve the (instrument-limited) pH resolution.

Functional polymers in protein detection platforms: optical, electrochemical, electrical, mass-sensitive, and magnetic biosensors

The rapidly growing field of proteomics and related applied sectors in the life sciences demands convenient methodologies for detecting and measuring the levels of specific proteins as well as for screening and analyzing for interacting protein systems. Materials utilized for such protein detection and measurement platforms should meet particular specifications which include ease-of-mass manufacture, biological stability, chemical functionality, cost effectiveness, and portability. Polymers can satisfy many of these requirements and are often considered as choice materials in various biological detection platforms. Therefore, tremendous research efforts have been made for developing new polymers both in macroscopic and nanoscopic length scales as well as applying existing polymeric materials for protein measurements. In this review article, both conventional and alternative techniques for protein detection are overviewed while focusing on the use of various polymeric materials in different protein sensing technologies. Among many available detection mechanisms, most common approaches such as optical, electrochemical, electrical, mass-sensitive, and magnetic methods are comprehensively discussed in this article. Desired properties of polymers exploited for each type of protein detection approach are summarized. Current challenges associated with the application of polymeric materials are examined in each protein detection category. Difficulties facing both quantitative and qualitative protein measurements are also identified. The latest efforts on the development and evaluation of nanoscale polymeric systems for improved protein detection are also discussed from the standpoint of quantitative and qualitative measurements. Finally, future research directions towards further advancements in the field are considered.