A fully electronic sensor for the measurement of cDNA hybridization kinetics

Surface regeneration and reusability of label-free DNA biosensors based on weak polyelectrolyte-modified capacitive field-effect structures

The reusability of capacitive field-effect electrolyte-insulator-semiconductor (EIS) sensors modified with a cationic weak polyelectrolyte (poly(allylamine hydrochloride) (PAH)) for the label-free electrical detection of single-stranded DNA (ssDNA), in-solution- and on-chip-hybridized double-stranded DNA (dsDNA) has been studied. It has been demonstrated that via simply regeneration of the gate surface of the EIS sensor by means of an electrostatic adsorption of a new PAH layer, the same biosensor can be reused for at least five DNA-detection measurements. Because of the reversal of the charge sign of the outermost layer after each surface modification with the cationic PAH or negatively charged DNA molecules, the EIS-biosensor signal exhibits a zigzag-like behavior. The amplitude of the signal changes has a tendency to decrease with increasing number of macromolecular layers. The direction of the EIS-signal shifts can serve as an indicator for a successful DNA-immobilization or -hybridization process. In addition, we observed that the EIS-signal changes induced by each surface-modification step (PAH adsorption, immobilization of ssDNA or dsDNA molecules and on-chip hybridization of complementary target cDNA) is decreased with increasing the ionic strength of the measurement solution, due to the more efficient macromolecular charge-screening by counter ions. The results of field-effect experiments were supported by fluorescence-intensity measurements of the PAH- or DNA-modified EIS surface using various fluorescence dyes.

Electrical characterization of DNA supported on nitrocellulose membranes

Integrated DNA-based nanoscale electronic devices will enable the continued realization of Moore’s Law at the level of functional devices and systems. In this work, the electrical characterization of single and complementary base paired DNA has been directly measured and investigated via the use of nitrocellulose membranes. A radio frequency DAKS-3.5 was used to measure the reflection coefficients of different DNA solutions dotted onto nitrocellulose membranes. Each DNA solution was exposed to a radio frequency signal with a power of 10 dBm and with a sweep from 200 MHz up to 13.6 GHz. The conducted measurements show some distinctions between the homomeric and complementary bases due to their different electrical polarization. As revealed from the measurements conducted, with the addition of DNA oligonucleotides, the measured capacitance increased when compared with buffer medium alone. The DNA molecules could be modeled as dielectric material that can hold electrical charges. Furthermore, the complementary paired DNA molecule-based inks solutions had a higher capacitance value compared with single DNA molecules (A, C, G and T) solutions.

Field effect sensors for nucleic Acid detection: recent advances and future perspectives

In the last decade the use of field-effect-based devices has become a basic structural element in a new generation of biosensors that allow label-free DNA analysis. In particular, ion sensitive field effect transistors (FET) are the basis for the development of radical new approaches for the specific detection and characterization of DNA due to FETs’ greater signal-to-noise ratio, fast measurement capabilities, and possibility to be included in portable instrumentation. Reliable molecular characterization of DNA and/or RNA is vital for disease diagnostics and to follow up alterations in gene expression profiles. FET biosensors may become a relevant tool for molecular diagnostics and at point-of-care. The development of these devices and strategies should be carefully designed, as biomolecular recognition and detection events must occur within the Debye length. This limitation is sometimes considered to be fundamental for FET devices and considerable efforts have been made to develop better architectures. Herein we review the use of field effect sensors for nucleic acid detection strategies—from production and functionalization to integration in molecular diagnostics platforms, with special focus on those that have made their way into the diagnostics lab.

Wireless integrated biosensors for point-of-care diagnostic applications

Recent advances in integrated biosensors, wireless communication and power harvesting techniques are enticing researchers into spawning a new breed of point-of-care (POC) diagnostic devices that have attracted significant interest from industry. Among these, it is the ones equipped with wireless capabilities that drew our attention in this review paper. Indeed, wireless POC devices offer a great advantage, that of the possibility of exerting continuous monitoring of biologically relevant parameters, metabolites and other bio-molecules, relevant to the management of various morbid diseases such as diabetes, brain cancer, ischemia, and Alzheimer's. In this review paper, we examine three major categories of miniaturized integrated devices, namely; the implantable Wireless Bio-Sensors (WBSs), the wearable WBSs and the handheld WBSs. In practice, despite the aforesaid progress made in developing wireless platforms, early detection of health imbalances remains a grand challenge from both the technological and the medical points of view. This paper addresses such challenges and reports the state-of-the-art in this interdisciplinary field.

Programmable ion-sensitive transistor interfaces. III. Design considerations, signal generation, and sensitivity enhancement

We report on factors that affect DNA hybridization detection using ion-sensitive field-effect transistors (ISFETs). Signal generation at the interface between the transistor and immobilized biomolecules is widely ascribed to unscreened molecular charges causing a shift in surface potential and hence the transistor output current. Traditionally, the interaction between DNA and the dielectric or metal sensing interface is modeled by treating the molecular layer as a sheet charge and the ionic profile with a Poisson-Boltzmann distribution. The surface potential under this scenario is described by the Graham equation. This approximation, however, often fails to explain large hybridization signals on the order of tens of mV. More realistic descriptions of the DNA-transistor interface which include factors such as ion permeation, exclusion, and packing constraints have been proposed with little or no corroboration against experimental findings. In this study, we examine such physical models by their assumptions, range of validity, and limitations. We compare simulations against experiments performed on electrolyte-oxide-semiconductor capacitors and foundry-ready floating-gate ISFETs. We find that with weakly charged interfaces (i.e., low intrinsic interface charge), pertinent to the surfaces used in this study, the best agreement between theory and experiment exists when ions are completely excluded from the DNA layer. The influence of various factors such as bulk pH, background salinity, chemical reactivity of surface groups, target molecule concentration, and surface coatings on signal generation is studied. Furthermore, in order to overcome Debye screening limited detection, we suggest two signal enhancement strategies. We first describe frequency domain biosensing, highlighting the ability to sort short DNA strands based on molecular length, and then describe DNA biosensing in multielectrolytes comprising trace amounts of higher-valency salt in a background of monovalent saline. Our study provides guidelines for optimized interface design, signal enhancement, and the interpretation of FET-based biosensor signals.

Programmable ion-sensitive transistor interfaces. II. Biomolecular sensing and manipulation

The chemoreceptive neuron metal-oxide-semiconductor transistor described in the preceding paper is further used to monitor the adsorption and interaction of DNA molecules and subsequently manipulate the adsorbed biomolecules with injected static charge. Adsorption of DNA molecules onto poly-L-lysine-coated sensing gates (SGs) modulates the floating gate (FG) potential ψ(O), which is reflected as a threshold voltage shift measured from the control gate (CG) V(th_CG). The asymmetric capacitive coupling between the CG and SG to the FG results in V(th_CG) amplification. The electric field in the SG oxide E(SG_ox) is fundamentally different when we drive the current readout with V(CG) and V(ref) (i.e., the potential applied to the CG and reference electrode, respectively). The V(CG)-driven readout induces a larger E(SG_ox), leading to a larger V(th_CG) shift when DNA is present. Simulation studies indicate that the counterion screening within the DNA membrane is responsible for this effect. The DNA manipulation mechanism is enabled by tunneling electrons (program) or holes (erase) onto FGs to produce repulsive or attractive forces. Programming leads to repulsion and eventual desorption of DNA, while erasing reestablishes adsorption. We further show that injected holes or electrons prior to DNA addition either aids or disrupts the immobilization process, which can be used for addressable sensor interfaces. To further substantiate DNA manipulation, we used impedance spectroscopy with a split ac-dc technique to reveal the net interface impedance before and after charge injection.

Real-time monitoring of PCR amplification of proto-oncogene c-MYC using a Ta₂O₅ electrolyte-insulator-semiconductor sensor

We present a new approach for real-time monitoring of PCR amplification of a specific sequence from the human c-MYC proto-oncogene using a Ta(2)O(5) electrolyte-insulator-semiconductor (EIS) sensor. The response of the fabricated EIS sensor to cycle DNA amplification was evaluated and compared to standard SYBR-green fluorescence incorporation, showing it was possible to detect DNA concentration variations with 30 mV/μM sensitivity. The sensor's response was then optimized to follow in real-time the PCR amplification of c-MYC sequence from a genomic DNA sample attaining an amplification profile comparable to that of a standard real-time PCR. Owing to the small size, ease of fabrication and low-cost, the developed Ta(2)O(5) sensor may be incorporated onto a microfluidic device and then used for real-time PCR. Our approach may circumvent the practical and economical obstacles posed by current platforms that require an external fluorescence detector difficult to miniaturize and incorporate into a lab-on-chip system.

Label-free detection of DNA using novel organic-based electrolyte-insulator-semiconductor

In this study, we have constructed the first organic field effect sensor based on an electrolyte-insulator-semiconductor structure (OEIS) and applied this novel device to pH and DNA sensing. Variations in the insulator-electrolyte surface potential, which originate from either the change of the ionization states of the insulator surface groups or the binding of charged molecules to the insulator surface, modify the flat band voltage (V(FB)) of the OEIS sensor. The pH sensing experiments of OEIS sensor showed that the output signal linearly depended on pH solution in the range from pH 2 to pH 12, and an average sensitivity of 44.1 mV/pH was obtained. In the biosensing experiments, the absorption of positively charged poly-L-lysine on the insulator surface resulted in the reduction of the V(FB) value, whereas the subsequent binding of negatively charged single-stranded DNA probe (ssDNA) via electrostatic interaction increased the V(FB) value. Furthermore, the ssDNA-immobilized OEIS device was successfully used for the detection of DNA hybridization. The detection limit of complementary DNA was as low as 1 microM, and the output signal of OEIS biosensor linearly increased with the logarithm of complementary DNA concentration in the range from 5x10(-5) to 10(-7) M. The easy and inexpensive fabrication of the OEIS device allows to be served as a potentially disposable and sensitive biosensor.

Overview of electrochemical DNA biosensors: new approaches to detect the expression of life

DNA microarrays are an important tool with a variety of applications in gene expression studies, genotyping, pharmacogenomics, pathogen classification, drug discovery, sequencing and molecular diagnostics. They are having a strong impact in medical diagnostics for cancer, toxicology and infectious disease applications. A series of papers have been published describing DNA biochips as alternative to conventional microarray platforms to facilitate and ameliorate the signal readout. In this review, we will consider the different methods proposed for biochip construction, focusing on electrochemical detection of DNA. We also introduce a novel single-stranded DNA platform performing high-throughput SNP detection and gene expression profiling.

Electron transfer mediating properties of hydrocarbons as a function of chain length: a differential scanning conductive tip atomic force microscopy investigation

The development of novel molecular and biomolecular devices relies on the understanding of charge transport across molecule-substrate interfaces. However, different strategies adopted so far for fabricating and studying transport through metal-molecule-metal junctions yield values for the transport coefficients that differ by up to orders of magnitude even for the same junction. Conductive tip atomic force microscopy (CT-AFM) allows for the simultaneous measurement of transport and morphological properties of molecular assemblies, but absolute transport measurements depend on the nature of the AFM tip-molecule contact. In this work we present a differential approach to the study of metal-molecule-metal junctions based on the combination of AFM-driven nanolithography and CT-AFM. We nanograft patches of alkanethiol molecules in a self-assembled monolayer of alkanethiol molecules of different chain length and measure by CT-AFM the morphology and the transport properties of the nanopatches and of the reference layer. The method allows for the determination of the differential resistance between the two molecular layers and is thus independent of environmental factors. The validity of this approach is demonstrated by measuring the tunneling decay constant of alkanethiols as a function of their length.