By definition, a biosensor is a bioanalytical device that incorporates a molecular recognition element (bioreceptor) associated to, or integrated with, a physicochemical transducer. According to this, a biosensor is formed by three components: a bioreceptor able to capture specifically the target of interest, a signal transducer able to convert target binding into a measurable electrical signal, and a signal processor that quantifies, analyzes and displays the results. In this way, analyte biocapture can be directly translated into a measurable signal. To date, numerous biosensors have been developed using a wide variety of biorecognition elements (ranging from “classical” bioreceptors such as antibodies, nucleic acid probes, antigens or enzymes, to novel alternatives like aptamers, biomimetic polymers or phage displayed peptides) and types of transducers (principally clustered into optical, electrochemical and micromechanical).

Electrochemical biosensors are among the most commonly used nowadays thanks to their portability, cost effectiveness, small size, rapidity, and robustness. Additionally, electrochemical transducers and equipment are relatively easy to miniaturize into multiplexed platforms, which coupled to integrated microfluidics is highly compatible with multi-analyte testing. This favours the development of point-of-care (POC) devices, to be used directly by the patient or at the surgery. The two clearest examples are probably the glucose electrochemical biosensor, which has facilitated life to diabetic patients to a great extend, and the hand held i.STAT clinical analyser (Abbot), which combines several electrochemical biosensors on a single chip and can be used to monitor simultaneously multiple electrolytes and metabolites in clinical samples.

It is well know that, in spite of the successful examples having been reported, classical immunosensors display a number of limitations too. For example, electrode modification (e.g.: for Ab incorporation, as well as surface appropriate blocking for prevention of non-specific adsorption of non-target components) correlates also its partial blocking. This has a negative effect on electron transfer across the transducer compared to the use of bare electrodes. Besides, immunosensor reutilization is constrained by the limited success of the Ab regeneration protocols, which means that immunosensors have to be cheap enough to be disposable, as well as sufficiently robust, sensitive and reproducible for their analytical application.

In this context, the utilization of magnetic particles (MP) for the development of magneto-immunosensors provide a number of advantages. On the one hand, MP can be actively incubated with the samples (for instance, by rotation or shacking). This reportedly provides faster assay kinetics, higher maximal signals and lower limits of detection than immunocapture on two-dimensional sensing surfaces, and allows also the fast and simple separation and concentration of target molecules from other sample components. In consequence, immunocapture can be performed in smaller sample volumes and shorter assay times, which entails a significant resource saving.

On the other hand, MP-bound target analytes can be confined with the aid of a magnet onto the surface of a transducer for immunoassay detection, and be then released by removing the magnet for sensor regeneration, which is specially interesting if expensive and sophisticated devices are being used. In the case of electrochemical detection, this guarantees that the incubation with potentially complex samples and reagents is carried out far away from the electrode where the electrochemical detection takes place. Accordingly, unmodified electrodes can be used for detection (in opposition to bioengineered devices in classical biosensing) and the surface of the working electrode is easily accessible for improved performance.

Nanotechnology is a rapidly emerging field that is having an enormous impact on assay and biosensor development and, by extension, in their application to diagnostics. Nanomaterials can display almost unlimited combinations of composition, size, dimensions and shape, which can be tailored and coupled to bioreceptors in order to produce nanoprobes with desired properties. In the specific field of Biosensor Development, nanomaterials are regularly exploited with three different purposes.

First, and thanks to the fact that nanostructures are characterized by high surface to volume ratios, they have been extensively used as multi-label carriers for signal amplification. In opposition to detection using bioreceptors conjugated to a single label unit, multi-label nanoparticles have been claimed to provide higher and faster responses, contributing to improved bioassay/biosensor detection limits by up to three orders of magnitude.

Alternatively, a variety of nanomaterials, such as fullerene derivatives, gold nanoparticles, rare earth nanoparticles and ferromagnetic nanoparticles, have been explored as artificial enzymes or enzyme mimics (nanozymes) thanks to their intrinsic non-enzymatic catalytic activity for a variety of molecular substrates. Used as labels in bioassay/biosensor development, nanozymes appear to be more stable and cheaper alternatives than natural enzymes.

Finally, the incorporation of nanomaterials to a transducer’s surface contributes to increase its active area and may also improve its performance in an important way. A very good example are carbon nanotubes (CNT). Among others, CNT incorporation onto an electrode allows taking advantage of the high mechanical resistance, chemical stability and electronic conductivity of this nanocomponent. For instance, CNT-modified electrodes exhibit active surfaces of increased roughness and area, electrocatalytic activity towards a wide variety of molecules, and faster electron transfer than unmodified electrodes.

We recently demonstrated that CNT dispersed in aqueous media have a strong tendency to adsorb non-specifically onto the surface of magnetic particles (MP). The resulting MP/CNT composite can then be arranged onto an electrode using a magnet, which is extremely fast, simple and reversible, allowing easy electrode regeneration/reutilisation. CNT magnetic co-entrapment can then serve to produce nanostructured electrodes of enhanced performance, but has been also applied to the electrochemical monitoring of the MP surface itself. In this context, CNT serve as wires that connect MP between them and with the electrode and allow the electrochemical detection of any electroactive targets and labels that have been previously bound to the MP surface. For instance, we have shown that dopamine, an electroactive neurotransmitter, could be first immunocaptured and concentrated using specific MP, which eliminates interference by non-targeted sample components, followed by CNT magnetic co-entrapment onto an electrode, which promotes surface electrical wiring and allows straightforward electrochemical sensing of the captured molecule.