An ISFET biosensor for the monitoring of maltose-induced conformational changes in MBP

Affinity sensor for haemoglobin A1c based on single-walled carbon nanotube field-effect transistor and fructosyl amino acid binding protein

Haemoglobin A1c (HbA1c) is a significant glycaemic marker for diabetes mellitus. The level of HbA1c reflects the mean blood glucose level over the prior 2-3 months and it is useful for the assessment of therapeutic effectiveness and for diagnosis. In this study, we report the label-free affinity sensor for HbA1c based on the chemiresistor-type field-effect transistor, which has a simple sensor configuration. Single-walled carbon nanotubes (SWNTs) were used as the transducing element. The fructosyl amino acid binding protein from Rhizobium radiobacter (SocA), which binds to α-fructosyl amino acid specifically, was used as the biorecognition element for fructosyl valine (FV), the product of the proteolytic hydrolysis of HbA1c. The developed sensor shows the ability to measure as low as 1.2 nM FV, which is 14-fold more sensitive compared to the previously reported fluorescence-based sensor using SocA. This sensor also exhibits high specificity where no significant response is observed from either fructosyl lysine (FK) or glucose, which are potential interferents. FK is the ε-fructosyl amino acid from glycated albumin, another glycated protein, whereas glucose is naturally present at very high concentration in the blood. We propose that the modulation of the surface charges on the SWNTs caused by the conformational change in SocA upon ligand binding leads to the proportionate changes in the number of carriers in the SWNT channel.

An embedded microretroreflector-based microfluidic immunoassay platform

We present a microfluidic immunoassay platform based on the use of linear microretroreflectors embedded in a transparent polymer layer as an optical sensing surface, and micron-sized magnetic particles as light-blocking labels. Retroreflectors return light directly to its source and are highly detectable using inexpensive optics. The analyte is immuno-magnetically pre-concentrated from a sample and then captured on an antibody-modified microfluidic substrate comprised of embedded microretroreflectors, thereby blocking reflected light. Fluidic force discrimination is used to increase specificity of the assay, following which a difference imaging algorithm that can see single 3 μm magnetic particles without optical calibration is used to detect and quantify signal intensity from each sub-array of retroreflectors. We demonstrate the utility of embedded microretroreflectors as a new sensing modality through a proof-of-concept immunoassay for a small, obligate intracellular bacterial pathogen, Rickettsia conorii, the causative agent of Mediterranean Spotted Fever. The combination of large sensing area, optimized surface chemistry and microfluidic protocols, automated image capture and analysis, and high sensitivity of the difference imaging results in a sensitive immunoassay with a limit of detection of roughly 4000 R. conorii per mL.

Current and emerging challenges of field effect transistor based bio-sensing

Field-effect-transistor (FET) based electrical signal transduction is an increasingly prevalent strategy for bio-sensing. This technique, often termed "Bio-FETs", provides an essentially label-free and real-time based bio-sensing platform effective for a variety of targets. This review highlights recent progress and challenges in the field. A special focus is on the comprehension of emerging nanotechnology-based approaches to facilitate signal-transduction and amplification. Some new targets of Bio-FETs and the future perspectives are also discussed.

Microfluidic chips for point-of-care immunodiagnostics

We might be at the turning point where research in microfluidics undertaken in academia and industrial research laboratories, and substantially sponsored by public grants, may provide a range of portable and networked diagnostic devices. In this Progress Report, an overview on microfluidic devices that may become the next generation of point-of-care (POC) diagnostics is provided. First, we describe gaps and opportunities in medical diagnostics and how microfluidics can address these gaps using the example of immunodiagnostics. Next, we conceptualize how different technologies are converging into working microfluidic POC diagnostics devices. Technologies are explained from the perspective of sample interaction with components of a device. Specifically, we detail materials, surface treatment, sample processing, microfluidic elements (such as valves, pumps, and mixers), receptors, and analytes in the light of various biosensing concepts. Finally, we discuss the integration of components into accurate and reliable devices.

Handheld impedance biosensor system using engineered proteinaceous receptors

We put forward an impedometric protein-based biosensor platform suitable for point-of-care diagnostics. A hand-held scale impedance reader system is described for the detection of corresponding physiochemical changes as the immobilized proteins bind to the analyte molecules in the proximity of the microfabricated electrodes. Specifically, we study the viability of this approach for glucose biosensing purposes using genetically engineered glucokinase as receptor proteins. The proposed reagent-less biosensor offers a high sensitivity of 0.5 mM glucose concentration level in the physiologically relevant range of 0.5 mM to 7.5 mM with less than 10 s response time.

Folding-based electrochemical biosensors: the case for responsive nucleic acid architectures

Biomolecular recognition is versatile, specific, and high affinity, qualities that have motivated decades of research aimed at adapting biomolecules into a general platform for molecular sensing. Despite significant effort, however, so-called "biosensors" have almost entirely failed to achieve their potential as reagentless, real-time analytical devices; the only quantitative, reagentless biosensor to achieve commercial success so far is the home glucose monitor, employed by millions of diabetics. The fundamental stumbling block that has precluded more widespread success of biosensors is the failure of most biomolecules to produce an easily measured signal upon target binding. Antibodies, for example, do not change their shape or dynamics when they bind their recognition partners, nor do they emit light or electrons upon binding. It has thus proven difficult to transduce biomolecular binding events into a measurable output signal, particularly one that is not readily spoofed by the binding of any of the many potentially interfering species in typical biological samples. Analytical approaches based on biomolecular recognition are therefore mostly cumbersome, multistep processes relying on analyte separation and isolation (such as Western blots, ELISA, and other immunochemical methods); these techniques have proven enormously useful, but are limited almost exclusively to laboratory settings. In this Account, we describe how we have refined a potentially general solution to the problem of signal detection in biosensors, one that is based on the binding-induced "folding" of electrode-bound DNA probes. That is, we have developed a broad new class of biosensors that employ electrochemistry to monitor binding-induced changes in the rigidity of a redox-tagged probe DNA that has been site-specifically attached to an interrogating electrode. These folding-based sensors, which have been generalized to a wide range of specific protein, nucleic acid, and small-molecule targets, are rapid (responding in seconds to minutes), sensitive (detecting sub-picomolar to micromolar concentrations), and reagentless. They are also greater than 99% reusable, are supported on micrometer-scale electrodes, and are readily fabricated into densely packed sensor arrays. Finally, and critically, their signaling is linked to a binding-specific change in the physics of the probe DNA, and not simply to adsorption of the target onto the sensor head. Accordingly, they are selective enough to be employed directly in blood, crude soil extracts, cell lysates, and other grossly contaminated clinical and environmental samples. Indeed, we have recently demonstrated the ability to quantitatively monitor a specific small molecule in real-time directly in microliters of flowing, unmodified blood serum. Because of their sensitivity, substantial background suppression, and operational convenience, these folding-based biosensors appear potentially well suited for electronic, on-chip applications in pathogen detection, proteomics, metabolomics, and drug discovery.

Ion-sensitive field-effect transistor for biological sensing

In recent years there has been great progress in applying FET-type biosensors for highly sensitive biological detection. Among them, the ISFET (ion-sensitive field-effect transistor) is one of the most intriguing approaches in electrical biosensing technology. Here, we review some of the main advances in this field over the past few years, explore its application prospects, and discuss the main issues, approaches, and challenges, with the aim of stimulating a broader interest in developing ISFET-based biosensors and extending their applications for reliable and sensitive analysis of various biomolecules such as DNA, proteins, enzymes, and cells.