Carbon and Platinum Nanostructured Electrodes on Miniaturized Devices for Biomedical Diagnostics

Nowadays, medical devices face several limitations concerning rapid, reliable and simultaneous quantification of a set of ions and metabolites from a micro-nanoliter volume of undiluted samples. The development of minimally-sized devices is, therefore, of key importance. In such a context, electrochemical sensors are particularly advantageous because of the simple, low cost and reproducible fabrication procedures and the rapid analytical measurements. Moreover, they provide easy possibilities for continuous monitoring. However, sensitive and selective detection of molecules in the physio-pathological concentration range is very challenging when conventional electrochemical devices are employed, especially for long-term use. Nanostructured electrodes are considered as one of the most promising strategies to overcome issues of sensitivity because of their large surface area and their excellent electrocatalytic properties. They could also address in part the problem of selectivity due to shifts in potential of the measured Faradic currents. In addition, nanomaterials could provide stable and reproducible potential responses when used as solid-contact materials of ion-selective electrodes. Inappropriate nanointegration methods could decrease the sensor performance so that the development of tailored nanostructuration protocols is extremely important to boost the sensor sensitivity, selectivity and stability over time. Objective of this thesis was to design and electrochemical characterise novel carbon and metal nanostructures for medical sensors. First of all, the integration of carbon nanomaterials on specific sensing sites of a microfabricated sensor was considered. Time-consuming, expensive and hardly-reproducible nanostrucuturation approaches contemplate the co-immobilization of carbon nanomaterials and additives whose presence inevitably masks the nanomaterial promising properties and compromises the time-stability in aqueous environments. The selective CVD growth of carbon nanomaterials was considered as a promising method to enable the coupling nanomaterial-electrode. Deposition parameters were optimised to make the process compatible with CMOS temperatures. Then, new protocols based on rapid electrodeposition methods were developed to integrate differently shaped and sized Pt and Pt-Au nanostructures on electrochemical platforms. Template-free electrodeposition was selected because of the durably-anchored and the contaminant-free coatings resulting after the process. Both nanostructuration approaches generated highly-sensitive electrodes to detect human metabolites as compared with the bare counterparts. Unprecedented sensing performance were obtained by both direct and enzyme-mediated detection mechanisms. Selective sensing was achieved thanks to the capability of the proposed nanostructured electrodes to discriminate the detection potentials of biomarkers from those of interfering species. The developed nanostructures were also excellent solid contacts between an electrode and an ion-selective membrane resulting in stable and reliable solid-contact ion-selective electrodes. To prove their stability and reproducibility for long operating lifetimes, these ion-selective electrodes have been successfully used as standard for continuous acute cell death monitoring

Multiwalled Carbon Nanotubes for Amperometric Array-Based Biosensors

For diagnostic and therapeutic purposes an accurate determination of multiple metabolites is often required. Amperometric devices are attractive tools to quantify biological compounds due to the direct conversion of a biochemical event to a current. This review addresses recent developments in the use of multiwalled carbon nanotubes to enhance detection ca- pability of amperometric array-based biosensors. More specifically, the principal techniques for multiwalled carbon nanotube incorporation onto microelectrode arrays are described. In these types of devices, each electrode is responsible for sensing one metabolite. The specificity is often given by an enzyme since most bio- molecules are not electroactive compounds. Common strategies for the protein immobilization onto multi- walled carbon nanotubes are also presented. After the discussion of nanotube/biomolecule integration onto electrode surfaces, three results are shown. The first one regards the influence on the biodetection signal of differently oriented multiwalled carbon nanotubes. Secondly, a demonstration of enhanced biodetection parameters by using multiwalled carbon nanotubes is given. Finally, a comparative study of three enzymes used to detect the same metabolite and adsorbed onto multiwalled carbon nanotubes is also reported

A novel electrochemical sensor for non-invasive monitoring of lithium levels in mood disorders

Lithium is the main drug for the treatment of mood disorders. Due to its narrow therapeutic window, Therapeutic Drug Monitoring (TDM) is a norm during therapy in order to avoid adverse effects. Consequently, patients are obliged to frequent check-ups in hospitals to determine their serum concentration and optimize accordingly the drug dose. Wearable sensors have attracted a growing interest in the research community in recent years owing to their promising impact in personalized healthcare. In particular, sweat diagnosis has seen an enormous expansion and is currently entering the market thanks to the large availability and simple collection of this fluid. In this paper a novel approach for non-invasive decentralized monitoring of lithium drug concentration through sweat analysis is proposed for the first time. An all-solid-state Ion- Selective Electrode (ISE) with a nanostructured Solid-Contact (SC) is used to detect lithium ions in sweat. The sensor offers near-Nernstian behaviour (57.6±2.1 mV/decade) in the concentration range of interest. In addition, it shows fast response (15-30 s), good reversibility and small potential drift over time. A wide pH stability window (pH 4-12) is also proved

Fully Integrated Biochip Platforms for Advanced Healthcare

Recent advances in microelectronics and biosensors are enabling developments of innovative biochips for advanced healthcare by providing fully integrated platforms for continuous monitoring of a large set of human disease biomarkers. Continuous monitoring of several human metabolites can be addressed by using fully integrated and minimally invasive devices located in the sub-cutis, typically in the peritoneal region. This extends the techniques of continuous monitoring of glucose currently being pursued with diabetic patients. However, several issues have to be considered in order to succeed in developing fully integrated and minimally invasive implantable devices. These innovative devices require a high-degree of integration, minimal invasive surgery, long-term biocompatibility, security and privacy in data transmission, high reliability, high reproducibility, high specificity, low detection limit and high sensitivity. Recent advances in the field have already proposed possible solutions for several of these issues. The aim of the present paper is to present a broad spectrum of recent results and to propose future directions of development in order to obtain fully implantable systems for the continuous monitoring of the human metabolism in advanced healthcare applications

Superior sensing performance of multi-walled carbon nanotube-based electrodes to detect unconjugated bilirubin

The direct electrochemical behaviour of bilirubin in the physio-pathological concentration range and at physio- logical pH was investigated by cyclic voltammetry. Nanostructured electrodes with a thin film of multi-walled carbon nanotubes exhibited a higher sensing performance than bare electrodes. The detection limit obtained with nanostructured electrodes (4.2 ± 0.1 μM) allows the detection of both normal and pathological levels of bilirubin. Due to its sparse solubility in aqueous solvents, in human fluids bilirubin is found in the form of soluble complex with albumin. Therefore, the nanostructured-sensor response was studied in presence of different con- centrations of this protein. A signal weakening was observed with increasing concentrations of albumin due to the decrease of free bilirubin. Finally, bilirubin detection was tested at concentrations typical of newborn jaundice (200–500 μM) and in the presence of normal albumin levels. A detection limit of 9.4 ± 0.3 μM was identified. Since this value is below the minimum critical bilirubin concentration for newborns, our sensor, modified with a thin film of carbon nanotubes, could potentially be used for bilirubin detection in cases of newborn jaundice

A Current-Mode Potentiostat for Multi-Target Detection Tested with Different Lactate Biosensors

Real-time and multi-target detection by wireless implantable devices is of increasing interest for chronic patients. In this work, electrode sharing is proposed to minimize the size of the implantable device when several three-electrode-based sensing sites are needed. An integrated potentiostat and readout circuit for a multi-target biosensor is presented. To realize this, the circuit reads out the sensor current through each working electrode in a current-mode scheme. The maximum detectable current is 8 μA and the simulated input referred current noise of the circuit is 125 pA/pHz at 1 Hz. The circuit was fabricated in 0.18 μm technology and tested for two lactate biosensors fabricated with a commercial lactate oxidase and an engineered one. Chronoamperometry experiments performed with the circuit agree well with a commercial equipment for lactate detection up to 1 mM

Comparison of Two Different Carbon Nanotube-based Surfaces with Respect to Potassium Ferricyanide Electrochemistry

This paper describes the electrochemical investigation of two multi-walled carbon nanotube-based electrodes using potassium ferricyanide as a benchmark redox system. Carbon nanotubes were fabricated by chemical vapor deposition on silicon wafer with camphor and ferrocene as precursors. Vertically-aligned as well as islands of horizontally-randomly-oriented carbon nanotubes were obtained by varying the growth parameters. Cyclic voltammetry was the employed method for this electrochemical study. Vertical nanotubes showed a slightly higher kinetic. Regarding the sensing parameterswe found a sensitivity for vertical nanotubes almost equal to the sensitivity obtainedwith horizontally/randomly oriented nanotubes (71.5±0.3 μA/(mM cm2) and 62.8±0.3 μA/(mMcm2), respectively). In addition, values of detection limit are of the same order of magnitude. Although tip contribution to electron emission has been shown to be greatly larger than the lateral contribution on single carbon nanotubes per unit area, the new findings reported in this paper demonstrate that the global effects of nanotube surface on potassium ferricyanide electrochemistry are comparable for these two types of nanostructured surfaces

Direct and selective synthesis of a wide range of carbon nanomaterials by CVD at CMOS compatible temperatures

Biosensors benefit from specific nano- to monitor human metabolites [1], [2]. Moreover, small structuration of the active bio-interface layer. In this electrodes allow us to design many sensing sites, each perspective, a wide range of carbon nanomaterials including multi-walled carbon nanotubes (MWCNTs), nanographite and carbon nanowalls (CNWs) have been directly synthesised by chemical vapor deposition (CVD) on Pt microelectrodes for the first time down to CMOS-compatible temperatures. This integration process, extremely useful to develop nanostructured multi-sensing site biodevices, has been validated by testing sensors for glucose with enhanced and competitive performance. Moreover it paves the way to the full integration of CMOS circuits, nanostructures and bioprobes

Comparing Sensitivities of Differently Oriented Multi-walled Carbon Nanotubes Integrated on Silicon Wafer for Electrochemical Biosensors

In this study, we report on multi-walled carbon nanotubes fabricated on silicon substrate with four different orientations via chemical vapor deposition. It is well-known that chemical treatments improve the nanotube electrochemical reactivity by creating edge-like defects on their exposed sidewalls. Before use, we performed an acid treatment on carbon nanotubes. To prove the effect of the treatment on these nanostructured electrodes, contact angles were measured. Then, sensitivities and detection limits were evaluated performing cyclic voltammetry. Two target molecules were used: potassium ferricyanide, an inorganic electroactive molecule, and hydrogen peroxide that is a product of reactions catalyzed by many enzymes, such as oxidases and peroxidases. Carbon nanotubes with tilted tips become hydrophilic after the treatment showing a contact angle of 22◦ ± 2◦. This kind of electrode has shown also the best electrochemical performance. Sensitivity and detection limit values are 110.0 ± 0.5 uA/(mM cm2) and 8 uM for potassium ferricyanide solutions and 16.4 ± 0.1 uA/(mM cm2) and 24 uM using hydrogen peroxide as target compound. Considering the results of wettability and voltammetric measurements, nanotubes with tilted tips-based electrodes are found to be the most promising for future biosensing application

Comparing the enhanced sensing interfaces of differently oriented carbon nanotubes onto silicon for bio-chip applications

Carbon nanotubes improve the sensitivity of electrochemical biochips. An optimized integration of the nano/bio/CMOS interface is required to realize accurate and low-cost sensors for applications in personalized medicine. The nanotubes orientation on the chip is a key parameter. The role of the sidewalls and the tips surface on the electro-activity of the nanosensor is still subject of debate in the literature. In this paper, a comparison between vertical densely-packed and randomlyoriented carbon nanotubes directly growth on silicon wafer is proposed in order to identify the best nano-system for biosensing purposes. The comparison is done by using contact-angle measurements, energy dispersive X-ray analysis and electrochemical voltammetry