Organic biosensors: when electronics meets biology to diagnose rare diseases
“I will start with a premise: I am not a doctor, I am a chemist — a physical chemist, to be more precise,” Bortolotti began frankly. This perspective “external” to the medical world brought a valuable contribution to the conference, demonstrating how Precision Medicine requires the integration of very different competencies: from clinical medicine to genetics, from bioinformatics to physical chemistry.
The stated objective of the presentation was “to arouse curiosity about this technology, show its potential and also its limitations, in order to stimulate interest in a possible application in the field of rare diseases as well.” An objective fully achieved through the presentation of three concrete applications of the technology.
What is a biosensor?
Before entering into the technical details, Bortolotti clarified the fundamental concept: a biosensor is a device that detects — and ideally quantifies — the presence of one or more analytes, such as biomarkers of physiopathological conditions, present in solution.
The functioning is based on two main components. First there is the biorecognition element: an antibody, an aptamer, an enzyme or a nucleic acid strand that selectively and specifically recognises its target within the sample. Part of Bortolotti’s laboratory research is dedicated precisely to the selection of this recognition element, which is crucial for the specificity of the sensor.
Then there is the transduction phase: the transformation of the biochemical event (the binding between the recognition element and its target) into a measurable signal. In the case of the biosensors developed in Modena, this signal is electronic — although it can also be optical or magnetic in other technologies.
To be clinically reliable, a biosensor must guarantee three fundamental characteristics: a low detection limit — the ability to recognise even very few target molecules; high sensitivity — small concentration variations must produce large signal variations; and reproducibility — results must be consistent between different measurements.
Bioelectronics: a bridge between two worlds
The biosensors developed by Bortolotti’s group belong to the field of bioelectronics — “a discipline that connects two apparently distant worlds: that of traditional electronics and that of communication in biological systems.”
In biological systems, communication occurs primarily via ions (sodium, potassium, calcium, chloride) that move through channels and generate electrical potentials. In traditional electronics, on the other hand, communication occurs via electron currents moving in semiconductors or metallic conductors.
Bioelectronic devices serve as a bridge between these two worlds. They can be used in two directions: as actuators to stimulate a biological system (for example, cardiac pacemakers or cochlear implants), or as sensors to detect biological signals such as the concentration of biomarkers. The work of Bortolotti’s group focuses on this latter application.
Organic transistors that function in liquid
The bioelectronic biosensors developed in Modena are three-electrode transistors that function immersed in a liquid environment, using an electrolyte that allows the passage of current. The distinguishing characteristic is the use of organic materials instead of silicon: molecules, polymers, carbon nanotubes or graphene.
“The structure can vary: in some devices, the gate electrode is above the other two (source and drain), in others they are all coplanar,” explained Bortolotti. The liquid — which can be a biological sample such as blood, saliva or urine — connects the electrodes and allows the device to function.
When a biomarker binds to the recognition element immobilised on the gate of the transistor, it changes the gate’s capacity to control the current flowing between source and drain. This change in current is the signal that is measured and that indicates the presence and concentration of the biomarker.
Versatility: from small molecules to large aggregates
After ten years of work on these devices, the laboratory has demonstrated impressive versatility. “We can detect both small molecules such as cortisol, and large aggregates such as extracellular vesicles,” stated Bortolotti. The group has developed biosensors for cytokines (IL-4, IL-6, TNF-α), neurofilaments, viruses and small molecules such as urea.
This versatility is particularly relevant for rare diseases, where biomarkers can be very diverse: small metabolic molecules, altered proteins, specific antibodies, nucleic acid fragments. Having a technological platform that can be adapted to such different targets represents a significant advantage.
First example: cortisol
The first applicative case presented concerns the detection of cortisol — a steroid hormone produced by the adrenal glands in response to stress. Cortisol levels are important indicators in several pathological conditions, from Cushing’s syndrome to adrenal insufficiency.
The biosensor developed uses cortisol-specific antibodies immobilised on the gate of the transistor. When cortisol present in the sample binds to the antibodies, it causes a variation in the electrical current of the device. Laboratory experiments produced dose-response curves with good sensitivity, clearly distinguishing specific signals from non-specific ones through controls with non-corresponding antibodies.
The ability to distinguish the specific signal from background noise is crucial for clinical reliability. Negative controls (devices with antibodies non-specific for cortisol) show no significant current variations, confirming that the signal observed with specific antibodies is genuinely due to cortisol binding.
Second example: anti-drug antibodies (ADA)
The second applicative example concerns the detection of anti-drug antibodies (ADA) — in particular against nivolumab, a monoclonal antibody used in oncological immunotherapy. Some patients treated with biological drugs develop an immune response against the drug itself, producing antibodies that can neutralise its efficacy or cause adverse effects.
In this case, the recognition element is the drug itself: nivolumab was immobilised on the electrode via protein G, so as to correctly orient the molecule and favour binding with any anti-nivolumab antibodies present in the sample.
“When the solution containing ADA comes into contact with the device, if there is interaction, this modifies the electrical current between source and drain, in a manner dependent on the concentration of the antibody,” explained Bortolotti. The result is dose-response curves that allow estimation of important parameters such as the affinity constant and the detection limit.
The detection limit achieved is impressive: 100 femtomolar (fM) — that is, 100 × 10⁻¹⁵ moles per litre. This extremely high sensitivity means that the device could detect undesirable immune responses in patients treated with biological drugs at an early stage, allowing timely therapeutic adjustments.
Third example: extracellular vesicles
The final example concerns extracellular vesicles — small particles released by cells that contain proteins, nucleic acids and lipids. Extracellular vesicles are emerging as important biomarkers in oncology and other conditions, as they reflect the state of the cell of origin.
This work was developed within the HEAL ITALIA project, in collaboration with the group of Prof. Massimo Dominici. Extracellular vesicles represent a more complex target compared to the previous ones: they are nanometre-scale structures (50–200 nm) with a lipid membrane.
The first challenge was to prevent the vesicles from fusing with the electrode surface, which would have caused non-specific signals. “We selected chemically inert surfaces (such as decanethiol) to inhibit this undesired interaction,” explained Bortolotti.
Subsequently, antibodies specific for tetraspanins (CD9, CD63, CD81) — membrane proteins characteristic of extracellular vesicles — were immobilised. Albumin was added to saturate non-functionalised sites and reduce background signals.
The addition of extracellular vesicles containing the target proteins led to a clear variation in electrical current, indicative of specific binding. Negative controls with non-specific antibodies produced no signal variation, confirming the specificity of the detection.
Flexibility and the need for customisation
A key message of the presentation is that “the platform is extremely flexible: it can be adapted to detect very different biomarkers, both in terms of size and concentration.” This flexibility is essential for applications in rare diseases, where relevant biomarkers can vary enormously from one condition to another.
However, Bortolotti also emphasised that “every target requires targeted work: in the selection of the recognition element, in the surface chemistry, and in the device architecture.” There is no universal biosensor: every application requires specific optimisation.
This balance between platform flexibility and the need for customisation is typical of technologies for Precision Medicine. The challenge is to develop platforms sufficiently versatile to be adapted to many different targets, while then investing the time necessary to optimise each specific application.
Potential application to rare diseases
Although biosensors specifically for rare diseases have not yet been developed, Bortolotti highlighted why this technology would be particularly promising in this field. The elevated sensitivity of the platform is crucial, as many rare diseases are characterised by biomarkers present at very low concentrations, and the ability to detect concentrations in the femtomolar range is essential. The dimensional versatility of the platform is also significant, since rare diseases may have as biomarkers small metabolic molecules, proteins, protein complexes or vesicles, and the ability to adapt the platform to targets of very different sizes is a notable advantage. The specificity of the biorecognition elements — antibodies or aptamers — can be selected to discriminate even subtle molecular differences, which is essential in rare diseases where biomarkers may resemble normally present molecules but with small distinctions. The rapidity of the measurement is relevant as well, since unlike some laboratory techniques that require hours or days, electronic biosensors can provide results in much shorter times — potentially important for urgent clinical decisions. Finally, the miniaturisation potential of organic bioelectronic devices opens the way to point-of-care or even wearable devices for the continuous monitoring of certain parameters.
Applications beyond diagnosis
Beyond direct diagnostic application, Bortolotti mentioned other possible uses for these biosensors. They can be used for the study of binding affinity — measuring how strongly a drug binds to its target, or how strongly an antibody recognises its antigen. This information is valuable in the development of new drugs. They can also be applied to drug screening — rapidly testing many compounds to identify those that bind to a therapeutic target of interest. Finally, therapeutic monitoring is another application: following over time the concentrations of drugs or biomarkers of therapeutic response in treated patients, in order to optimise dosages or identify problems at an early stage.
All of these applications are relevant in the context of rare diseases, where the development of new drugs is particularly difficult due to the small number of patients, and where personalised therapeutic monitoring is essential given the heterogeneity of clinical manifestations.
Collaborations and the role of HEAL ITALIA
The presentation concluded with thanks to collaborators — in particular to Dr. Marcello Berto, who carried out much of the experimental work, and to Prof. Dominici’s team for the collaboration on extracellular vesicles.
Specific acknowledgement was given “to the HEAL ITALIA project, which gave us the opportunity to explore applications also in the field of rare diseases.” This highlights how HEAL ITALIA is not only an infrastructure for already-planned research, but also a catalyst for new collaborations and for the exploration of innovative applications of existing technologies.
The HEAL ITALIA project on extracellular vesicles allowed Bortolotti’s group to apply their biosensors to a complex and clinically relevant biological target, potentially opening new research directions. It is a concrete example of how the interdisciplinary collaborations facilitated by major programmes such as HEAL ITALIA can generate innovation.
Towards biosensors for rare diseases
Prof. Bortolotti’s presentation showed how technologies developed initially for other purposes can have valuable applications in the field of rare diseases. Organic bioelectronic biosensors — with their combination of sensitivity, specificity, versatility and miniaturisation potential — represent a promising tool for addressing some of the diagnostic challenges posed by these conditions.
Early diagnosis is crucial in rare diseases, where diagnostic delays often lead to irreversible damage. Having devices capable of detecting specific biomarkers at very low concentrations — potentially in point-of-care settings — could significantly accelerate diagnostic pathways.
Therapeutic monitoring is equally important: many treatments for rare diseases require personalised dosage adjustments, and the ability to rapidly and frequently measure the levels of drugs or biomarkers of therapeutic response would be of great clinical utility.
As the example of anti-drug antibodies demonstrated, biosensors could also identify problems such as the development of immunogenicity against biological drugs at an early stage, allowing timely interventions.
The integration of these technologies in the HEAL ITALIA Precision Medicine Centers — with their biobanks, specialised clinical expertise and research infrastructures — could significantly accelerate the development and validation of specific biosensors for different rare diseases, translating technological potential into concrete benefits for patients.
