Electro-polymerization of Nafion perfluorinated resin and 3,4-ethylenedioxythiophene (EDOT) with two different surfactants yielded nanoscale improved surface coatings on carbon fiber microelectrodes. Increased sensitivity and good selectivity for sensing subsecond release of dopamine, serotonin and adenosine in the presence of large concentrations of ascorbic acid and DOPAC was demonstrated. Two different surfactants were used during electro-polymerization: sodium dodecyl sulfate or sodium dodecyl benzene sulfonate. With the improved nanoscale coatings, the dopamine signal is increased by 4X-9X, while the serotonin signal is increased by 4X, and the adenosine signal is increased by 3X compared to bare carbon. We measured the highest sensitivity of ∼ 34 nA/μM (5X increase from the bare carbon fiber) with the EDOT:Nafion and sodium dodecyl benzene sulfonate. Dopamine selectivity can be achieved using the full voltammetry curves with respect to ascorbic acid, serotonin and adenosine using distinguishing features of the voltammograms, such as differences in reduction or oxidation potentials and with respect to DOPAC using lower signals. Serotonin can be distinguished from dopamine by the difference in reduction potentials. Furthermore, a shift toward less positive potentials for the adenosine oxidation was observed with the coated carbon fibers. The novel surface coated electrodes can potentially improve in vivo measurements. Neurotransmitters are molecules that allow neurons to send signals to target cells to communicate, influencing behaviors like reward and motivational habits.1-3 Dopamine (DA) is a neurotransmitter of the central nervous system with a key role in Parkinson desease. 4 Since dopamine is electro-active, it can be easily oxidized at an electrode surface with electrochemical methods, for example using Fast Scan Cyclic Voltammetry (FSCV). 5,6 Other molecules such as serotonin (5-HT), norepinephrine and adenosine are electro-active neurotransmitters that can be measured electrochemically. 6,7 The problem is that these molecules, except for adenosine that is oxidized at higher potentials, 8,9 have all very similar oxidation and reduction potentials and it is difficult to distinguish them in vivo. 1In fact, ascorbic acid (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC) may completely mask dopamine detection in the brain. 10DOPAC is a metabolite of dopamine and ascorbic acid is an antioxidant, both present at high concentration in the brain and may interfere with dopamine.13 Therefore, it is very important to develop electrode materials with sufficient sensitivity and selectivity for in vivo detection of dopamine, serotonin and adenosine.Carbon has been shown to be one of the best material to perform such measurements in vivo. 3,11,12 Previous studies have been done to increase the sensitivity and selectivity of bare carbon fiber electrodes.15 Conductive polymers increase the surface area of the electrode and enhance the sensitivity. Among the various conductive polymers, poly(3,4-ethylenedioxythiophene) (...
Dopamine is a neurotransmitter that modulates arousal and motivation in humans and animals. It plays a central role in the brain "reward" system. Its dysregulation is involved in several debilitating disorders such as addiction, depression, Parkinson's disease, and schizophrenia. Dopamine neurotransmission and its reuptake in extracellular space takes place with millisecond temporal and nanometer spatial resolution. Novel nanoscale electrodes are needed with superior sensitivity and improved spatial resolution to gain an improved understanding of dopamine dysregulation. We report on a scalable fabrication of dopamine neurochemical probes of a nanostructured glassy carbon that is smaller than any existing dopamine sensor and arrays of more than 6000 nanorod probes. We also report on the electrochemical dopamine sensing of the glassy carbon nanorod electrode. Compared with a carbon fiber, the nanostructured glassy carbon nanorods provide about 2× higher sensitivity per unit area for dopamine sensing and more than 5× higher signal per unit area at low concentration of dopamine, with comparable LOD and time response. These glassy carbon nanorods were fabricated by pyrolysis of a lithographically defined polymeric nanostructure with an industry standard semiconductor fabrication infrastructure. The scalable fabrication strategy offers the potential to integrate these nanoscale carbon rods with an integrated circuit control system and with other complementary metal oxide semiconductor (CMOS) compatible sensors.
working conditions. [12-15] More precisely, in human sweat, the concentration of Na + changes between 10 × 10 −3-100 × 10 −3 m, it is sweat rate-dependent and associated with dehydration. [3,14,16] K + concentration level ranges between 1 × 10 −3-18.5 × 10 −3 m with a sweat rate-independent partitioning. [3,16] Also, pH can strongly vary between 3-8 units, [3] with changes associated with dehydration and muscle fatigue. [14,15] For the next generation of wearable ion sensors, key requirements include mechanical flexibility, simple array patterning for multi-parametric analysis, and microfluidics integration for continuous sampling. [17-23] Conventionally, selective ion measurements are performed using potentiometric two-electrode systems, in which the potential drop between an Ion Selective Membrane (ISM) and a reference electrode is measured. [21,24,25] However, standard potentiometric sensors are difficult to integrate into an array configuration in a microfluidic platform, due to their high output impedance, [18] and the difficult miniaturization of a stable reference electrode. [26-28] Organic electrochemical transistors (OECTs) are an interesting alternative to conventional potentiometric sensors, overcoming some of these limitations. [29] The OECTs are three-terminal devices (drain, source, and gate), with the source and the drain electrodes connected by a conducting polymeric channel. The organic channel is based on conjugated polymer-polyelectrolyte blends, such as the mainly used poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS). [22,30-32] This active material enables mixed ionic and electronic charges interaction, with ionic conduction provided by the PSS polyelectrolyte chains and electronic conduction by nanometric-sized PEDOT crystallites. [33-35] In the presence of an electrolyte and once a positive gate voltage is applied, the dissolved cations are injected into the PEDOT:PSS channel. The cations compensate electrostatically the sulfonate anions of the PSS phase, subsequently lowering the drain current (hole de-doping) in the bulk of the layer. [2,29] This technology, without the need for a reference electrode, allows a facile miniaturization. [28,36] Moreover, the mechanical flexibility of the PEDOT:PSS channel, [22,37] the compatibility with digital manufacturing such as inkjet printing, [38-41] and the very low output impedance, [18] make OECTs promising candidates for Organic electrochemical transistors (OECTs) show remarkable promise as biosensors, thanks to their high signal amplification, simple architecture, and the intrinsic flexibility of the organic material. Despite these properties, their use for real-time sensing in complex biological fluids, such as human sweat, is strongly limited due to the lack of cross-sensitivity and selectivity studies and the use of rigid and bulky device configurations. Here, the development of a novel flexible microfluidics-integrated platform with an array of printed ion-selective OECTs enables multi-ion detection in a wearable fashion. T...
We demonstrate fully inkjet-printed graphene-gated Organic Electrochemical Transistors (OECTs) on polymeric foil for the enzymatic-based biosensing of glucose. The graphene-gated transistors exhibit better linearity, repeatability, and sensitivity than printed silvergated devices studied in this work and other types of printed devices previously reported in the literature. Their limit of detection is 100 nM with a normalized sensitivity of 20 %/dec in the linear range of 30 to 5000 M glucose concentrations, hence comparable with state-of-the-art OECT 2 devices made by lithography processes on rigid substrates and with complex multi-layer gates.Electrochemical impedance spectroscopy analysis shows that the improved sensitivity of the graphene-gated devices is related to a significant decrease of the charge-transfer resistance at the graphene electrode-electrolyte interface in the presence of glucose. The optimized sensing method and device configuration are also extended to the detection of the metabolite lactate. This study enables the development of fully-printed high-performance enzymatic OECTs with graphene sensing-gates for multi-metabolites sensing. similar to the one in blood. 17 Instead, the concentration of lactate is similar to the concentration observed in blood for all these biofluids and equal to 1's-10's mM. 7,17,21 Highly selective enzymes such as glucose oxidase (GOx) and lactate oxidase (LOx) are conventionally used for the electrochemical detection of glucose and lactate, respectively, 5,22 with the possible use of artificial mediators for improving the electron transfer rate. 8,23,24 The enzymes are often immobilized on the working electrode of an amperometric three-electrode system. 5,25,26 However, the linear detection range of standard amperometric devices is generally between hundreds of M-1 mM to several mM, [25][26][27] making glucose detection in some biofluids, such as sweat and saliva, difficult to be achieved. Moreover, the compact integration and fully-printing of standard amperometric cells on flexible substrates is not straightforward. 14,28,29 Organic electrochemical transistors (OECTs) [30][31][32][33] are an interesting alternative to conventional amperometric sensors, overcoming some of their limitations. OECTs are three-terminal devices, with the source and drain electrodes connected by a conjugated polymer-polyelectrolyte channel such as the commonly used poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). Thanks to the mixed ionic-electronic properties and the mechanical flexibility of PEDOT:PSS, OECTs allow high chemical signal amplification and better mechanical matching of the device with soft interfaces, such as the human skin, for non-invasive analysis. [34][35][36][37] Also, because digital manufacturing techniques, such as inkjet printing, can be applied for their fabrication, OECTs are promising candidates for the development of low-cost and multi-sensing biochemical platforms on flexible substrates, such as polymeric foil. 38,39 OECT-based device...
In recent years, wearable epidermal sweat sensors have received extensive attention owing to their great potential to provide personalized information on the health status of individuals at the molecular level. For on‐demand medical analysis of sweat in sedentary conditions, a cost‐effective wearable integrated platform combining sweat stimulation, sampling, transport, and analysis is highly desirable. In this work, a printed iontophoretic system integrated into a microfluidic sensing platform, which combines sweat induction, collection, and real‐time analysis of sweat‐ions into a single patch for on‐demand sweat monitoring on human subjects in stationary conditions is reported. The incorporation of microfluidics features facilitates sweat sampling, collection, and guiding through capillary effect. The multisensing sensor array exhibits sensitivity close to Nernstian behavior for sodium, potassium, and pH. The correlation between the concentrations of ions measured with the sweat patch and with ion chromatography analysis demonstrates the applicability of the system for real‐time point‐of‐care monitoring of the health status of individuals. Furthermore, the sweat patch electronic interface with wireless transmission enables real‐time data monitoring and storage over a cloud platform. This printed iontophoretic‐integrated fluidic sweat patch provides a cost‐effective solution for the on‐demand analysis of sweat components for healthcare applications.
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