The detection of hydrogen in the Earth's deep underground poses a major challenge due to the lack of oxygen and continuous changes in environmental conditions. An innovative class of hydrogen gas sensors based on AlGaN/GaN High Electron Mobility Transistors (HEMTs) with Platinum (Pt) gates as a functionalization layer, has been developed and optimized for geo-sensing. The study investigated the sensor characteristics using two carrier gases to simulate underground conditions, namely air (with 20 % O2) and N2 (with 0 % O2), across a range of temperatures from 50 degrees C to 300 degrees C and with hydrogen concentrations varying from 25 ppm to 400 ppm. The detection limit was found to be approximately 1 ppm of hydrogen in the atmospheric air. The gas sensor transduction is based on the modification of the conductivity of a 2-Dimensional Electron Gas (2DEG). In this study, the principle was investigated using two gases, O2 and H2, with different electronegativities relative to platinum. The adsorption competition between H2 and O2 on platinum was evaluated, and this allowed the calculation of the ratio of thermodynamic adsorption constants between these two gases.

This study presents the use of new kind of carbon electrode materials as ultramicroelectrodes (UMEs) in the field of electrochemical DNA biosensors which has already been proven to be effective in protocols to DNA sequences hybridization. In contrast to other carbon materials such as diamond like carbon, that are difficult to integrate in microfluidic devices due to their high temperature deposition, amorphous carbon nitride (a-CNx) is easily synthesized at room temperature on various materials using sputtering techniques. Here, we report a-CNx use as microband electrodes in Glass/PDMS microfluidic devices. a-CNx electrodes were activated and then biofuntionalized by covalent grafting of a DNA probe as self-assembled monolayer (SAM) with a view to future development of a detection platform targeting circulating DNA or RNA sequences in microfluidic channels.

Gas sensors based on AlGaN/GaN high-electron-mobility transistors (HEMTs) with indium tin oxide (ITO) gates as functional layers were fabricated to detect hydrogen gas. Several sensing metrics such as the changes in current, sensitivity, and response time confirmed the dependence of the sensing response on the gas concentration.

This review is an update of two previous ones focusing on the limit of detection of electrochemical nucleic acid biosensors allowing direct detection of nucleic acid target (miRNA, mRNA, DNA) after hybridization event. A classification founded on the nature of the electrochemical transduction pathway is established. It provides an overall picture of the detection limit evolution of the various sensor architectures developed during the last three decades and a critical report of recent strategies.

Willard Gibbs formulated in 1873 a mathematical function, the Gibbs energy, whose variation governs the perfect gas evolution. In 1895, Ludwig Boltzmann presented a microscopic description of the perfect gas in concordance with the phenomenological approach of Gibbs. The chemical thermodynamics has been historically developed using the model of the perfect gas from the existence of the chemical potential postulated in 1876 by Gibbs. The chemical potential, a quantity ascribed to every chemical species, represents the driving force of a chemical reaction. Chemists and physicists use since the beginning of the 20th century Gibbs and Boltzmann model to express the evolution laws of chemical reactions and phase transitions. This model, which presents a weakness to describe certain systems so-called as not ideal systems, was gradually declined in numerous successive empirical models of complexities and increasing predictive performances but restricted to particular systems. A unified model of microscopic evolutions based on kinetics is presented in this manuscript which is a clearly-cut breakthrough with thermodynamic chemistry: the existence of the chemical potential is not recognized and the calorimetric entropy from the second law of thermodynamics, which is consistent for heat transfer prediction, is not consistent to predict microscopic evolutions. Kinetic entropy rising from this approach replaces calorimetric entropy in the function that governs chemical reactions and phases equilibria evolutions.

Willard Gibbs formulated in 1873 a mathematical function, the Gibbs energy, whose variation governs the perfect gas evolution. In 1895, Ludwig Boltzmann presented a microscopic description of the perfect gas in concordance with the phenomenological approach of Gibbs. The chemical thermodynamics has been historically developed using the model of the perfect gas from the existence of the chemical potential postulated in 1876 by Gibbs. The chemical potential, a quantity ascribed to every chemical species, represents the driving force of a chemical reaction. Chemists and physicists use since the beginning of the 20th century Gibbs and Boltzmann model to express the evolution laws of chemical reactions and phase transitions. This model, which presents a weakness to describe certain systems so-called as not ideal systems, was gradually declined in numerous successive empirical models of complexities and increasing predictive performances but restricted to particular systems. A unified model of microscopic evolutions based on kinetics is presented in this manuscript which is a clearly-cut breakthrough with thermodynamic chemistry: the existence of the chemical potential is not recognized and the calorimetric entropy from the second law of thermodynamics, which is consistent for heat transfer prediction, is not consistent to predict microscopic evolutions. Kinetic entropy rising from this approach replaces calorimetric entropy in the function that governs chemical reactions and phases equilibria evolutions.

a-CNx films are well-known polarizable interface materials with a large potential window (3 V-4 V [1]) interesting electrochemical reactivity [2] and tunable chemical surfacic composition [3]. Indeed, the surface structure, surface properties, number of grafting sites available are tunable according to the atomic percentage of nitrogen in the a-CNx [4]. High surface reactivity are highly dependent on the surface state and can be affected by synthesis (by varying the Csp3/sp2 ratio), and by pre-treatment (electrochemical or plasma). In this work, we elaborated a microfluidic chip integrating a-CNx as working microelectrodes. In brief, a titanium/ platinum (50/500 Å) underlayer was first deposited on the glass wafer and patterned as working microelectrodes for the future thin film a-CNx adhesion and conductivity. On the top of the working electrode, a 220 nm a-CNx (x=0.12), layer was deposited with DC magnetron sputtering (time =20 min, power=200 W) with a graphite target under a flow of nitrogen (PN2/Ptot =3%, Ptot =0.4 Pa). Then, a classical PDMS fluidic circuit was microfabricated and pre-treated with nitrogen plasma to favor its adhesion onto the glass slide containing the Ti/Pt/ a-CNx microelectrodes. The a-CNx microelectrodes were also electrochemically pre-treated in galvanostatic mode using 0.1 M KOH solution (anodic pre-treatment). The hydrodynamic properties of the fluidic channel a-CNx microelectrodes (as-grown and pre-treated) were screened with the [Fe(III)(CN)6]3−/[Fe(II)(CN)6]4− redox couple by cyclic voltammetry and impedance spectroscopy to provide a basis for further investigations such as the influence of a-CNx electrode pre-treatment on the DNA probe density issues in order to enhance the hybridization reaction efficiency with its specific DNA target. [1]. A. Lagrini, C. Deslouis, H. Cachet, M. Benlahsen, and S. Charvet. Elaboration and electrochemical characterization of nitrogenated amorphou scarbon films, Electrochem. commun., 6(3) (2004) 245–248. [2]. G Adamopoulos, C Godet, C Deslouis, H Cachet, A Lagrini, and B Saidani. The electrochemical reactivity of amorphous hydrogenated carbon nitrides for varying nitrogen contents: the role of the substrate. Diam. Relat. Mater.,12 (2003) 613– 617. [3]. R A Medeiros, A Benchick, R C Rocha-Filho, O Fatibello-Filho, B Saidani, C Debiemme-Chouvy, and C Deslouis. Simultaneous detection of ascorbic acid and dopamine with electrochemically pretreated carbon nitride electrodes: Comparison with boron-doped diamond electrodes. Electrochem. commun., 24 (2012) 61–64. [4]. M Faure, F Billon, A M Haghiri-Gosnet, B Tribollet, C Deslouis, A Pailleret, and J. Gamby. Influence of the atomic nitrogen content in amorphous carbon nitride thin films on the modulation of their polarizable interfaces properties. Electrochim. Acta, 280 (2018) 238-247.

A new electrochemical hybridization trans-duction pathway, obtained by coupling electrochemical adsorption and long-range electron transfer through double-stranded DNA, was investigated using ultrami-croelectrode (UME). The results show that long-range electron transfer does not occurs exclusively throws well-packed and organized self-assembled DNA monolayers. This approach is used to investigate long-range electron transfer properties of both single-and double-stranded short synthetic DNA and DNA plasmids. Single mismatch electrochemical detection protocol of non-labelled short synthetic DNA, without heating or probe labelling, in a 10 minutes protocol, was in fine performed.

In this paper we critically review detection limits of electrochemical DNA biosensors enabling DNA detection without target labelling. The review includes transduction principles and latest breakthroughs. To compare the efficiency of each type of electrochemical DNA biosensor, a simple DNA biosensors classification is established on the basis of the nature of the bio-electrochemical transduction.

Here, we describe the transposition of an ultramicroelectrode (UME) setup into a microfluidic chip configuration for DNA biosensors. The hydrodynamic properties of the fluidic channel microelectrode were screened with an [FeIJIII)IJCN)6] /[Fe(II)(CN)6] 4− redox couple by cyclic voltammetry to provide a basis for further biological processes. A 23-base DNA probe was self-assembled into a monolayer on gold microelectrodes both in classical configuration and integrated in a microfluidic setup. Special interest was focused on the DNA target mimicking the liver-specific micro-ribonucleic acid 122 (miRNA122). Long-range electron transfer was chosen for transducing the hybridization. This direct transduction was indeed significantly enhanced after hybridization due to DNA-duplex π-stacking and the use of redox methylene blue as a DNA intercalator. Quantification of the target was deduced from the resulting electrical signal characterized by cyclic voltammetry. The limit of detection for DNA hybridization was 0.1 fM in stopped flow experiments, where it can reach 1 aM over a 0.5 μL s−1 flow rate, a value 10-fold lower than the one measured with a conventional UME dipped into an electrolyte droplet under the same analytical conditions. An explanation was that forced convection drives more biomolecules to the area of detection even if a balance between the speed of collection and the number of biomolecules collected has been found. The latter point is discussed here along with an attempt to explain why the sensor has reached such an unexpected value for the limit of detection.

A 23-base DNA probe monolayer was self-assembled on a 25-μm gold microelectrode via thiol adsorption. Long-range electron transfer and the use of the redox methylene blue as DNA intercalator were chosen for the monitoring of the hybridization step. The electrochemical properties of the sensor were screened with the [Fe(III)(CN)6]3-/[Fe(II)(CN)6]4- redox couple in cyclic voltammetry in a two-electrode configuration well adapted in the case of microliter biological samples. A study of the stability of the self-assembled monolayer is included in this work. The femtomolar limit of detection for DNA target quantification was deduced from the current density measured and blank measurements depicting the desorption rate of the thiolated DNA probes.

We report on the design of electrochemical DNA-biosensors made from 15, 25 and 50μm diameter gold ultramicroelectrodes (UMEs) in a two-electrode setup. The biosensors were prepared by chemical adsorption of 22-base thiol-labeled DNA-probes in a self-assembled monolayer configuration onto the surface of the gold UME. Hybridization events were transduced by FeIII/FeII redox process electro-mediated by DNA-redox intercalated mediators. The optimization of the DNA-biosensors was achieved by using different FeIII/FeII ligands (cyanide, cefadroxil, deferoxamine) and intercalators (methylene blue, proflavine). Optimized biosensors allowed direct and selective amperometric detection in the femtomolar range and of single mismatch within a 35minutes protocol.

Here, we describe the transposition of an ultramicroelectrode (UME) setup into a microfluidic chip configuration for DNA biosensors. The hydrodynamic properties of the fluidic channel microelectrode were screened with an [Fe(iii)(CN)6]3-/[Fe(ii)(CN)6]4- redox couple by cyclic voltammetry to provide a basis for further biological processes. A 23-base DNA probe was self-assembled into a monolayer on gold microelectrodes both in classical configuration and integrated in a microfluidic setup. Special interest was focused on the DNA target mimicking the liver-specific micro-ribonucleic acid 122 (miRNA122). Long-range electron transfer was chosen for transducing the hybridization. This direct transduction was indeed significantly enhanced after hybridization due to DNA-duplex π-stacking and the use of redox methylene blue as a DNA intercalator. Quantification of the target was deduced from the resulting electrical signal characterized by cyclic voltammetry. The limit of detection for DNA hybridization was 0.1 fM in stopped flow experiments, where it can reach 1 aM over a 0.5 μL s-1 flow rate, a value 104-fold lower than the one measured with a conventional UME dipped into an electrolyte droplet under the same analytical conditions. An explanation was that forced convection drives more biomolecules to the area of detection even if a balance between the speed of collection and the number of biomolecules collected has been found. The latter point is discussed here along with an attempt to explain why the sensor has reached such an unexpected value for the limit of detection.

Biosensors are all about the limit of detection 1 imposed by the molecular level changes in expression of biomolecules to hope to perform reliable diagnostics before symptoms of a disease appears. A special interest was driven to the DNA target, mimicking the liver-specific micro-Ribonucleic Acid 122 2 (miRNA122). DNA hybridization is the most prized method compared to direct sequencing all the more now that long range electron transfer through the DNA duplex π-staking has been demonstrated 3 . The electronic coupling within its inner core of stacked array of heterocyclic aromatic base pairs is very sensitive to local disruptions such as mismatches making a DNA biosensor particularly sequence specific. In a DNA biosensor, the target sequence is recognized by a complementary DNA probe and hybridized. The basis pairing according to Watson and Crick’s rules 4 is converted into an electrical signal. A 23-base DNA probe was self-assembled on the gold microelectrode via thiol adsorption. Long-range electron transfer was chosen for the monitoring of the hybridization step. Indeed, this direct transduction was significantly enhanced due to DNA-duplex π-stacking and the use of the redox methylene blue as DNA intercalator 5 . The electrochemical properties of the sensor were screened with the [Fe(III)(CN) 6 ] 3- /[Fe(II)(CN) 6 ] 4- redox couple in droplets by using cyclic voltammetry (CV) in a two-electrode configuration which is more adapted in the case of microliter biological samples. Voltammograms centered at potential zero are observed as expected for a working UME and a counter electrode made from the same metal and immersed in the same electrolyte. The 2 mm-gold counter electrode (very high area compared to the 25 μm gold working electrode) can be considered as a pseudo-reference electrode allowing a feeble potential drift during measurements. When working with thiolated SAM absorbed on gold substrates, the matter of desorption is not trivial all the more in chloride electrolytes. The reliability of 10 -14 M limit of detection determined for DNA target quantification was deduced from differential current density measured compared with the blank measurements. The data analysis highlights the possible role of the desorption phenomenon of the thiolated DNA probes from the sensor surface that could explained the residual current densities measured for concentrations lower than 10 -14 M. 1 M. Lazerges and F. Bedioui, Anal. Bioanal. Chem. , 2013, 405(11), 3705-3714. 2 J. a. Wilson and S. M. Sagan, Curr. Opin. Virol. , 2014, 7, 11–18. 3 S. O. Kelley, E. M. Boon, J. K. Barton, N. M. Jackson and M. G. Hill, 1999, 27, 4830–4837. 4 J. D. Watson and F. H. C. Crick, Nature , 1953, 171, 737–738. 5 M. Lazerges, V. T. Tal, P. Bigey, D. Scherman and F. Bedioui, Sensors Actuators B Chem. , 2013, 182, 510–513. Figure 1

Linear nanostructures resulting form self-association of a nonapeptide (left) yield progressively to ring-shaped nanostructures (right).

The proof of concept of a DNA-biosensor based on a two-electrode electrochemical setup and using a microelectrode as working electrode, well adapted for detection in microliter samples and miniaturization, is presented herein. A 23-base DNA-probe self-assembled monolayer was first formed onto a 50 μm-diameter gold surface. The microelectrode extremity was then immersed in a 50 μL DNA-target solution drop itself deposited onto a 2 mm-diameter gold counter electrode. Transduction occurs via long-range electron transfer, which is enhanced subsequently to hybridization, due to DNA-base π-stacking. Single mismatch detection of this first prototype was matched at room temperature in the nanomolar range without any optimization.

A heterogeneous system between vitamin C and stearic acid was characterized by thermal and crystallographic analyses. The results showed that such a system prevents vitamin C from thermal decomposition. The evidence was provided by implementing a new protocol associating chromatic assays and thermal analyses in order to quantify the percentage of non-degraded vitamin C. The results collected with the vitamin C-stearic acid mixtures allowed deducing coherent interpretation of the results obtained with pure vitamin C at different heating scan rates. Vitamin C mainly degrades upon melting but also in the solid state for temperature close to the melting point when the heating rates are very low. Under these conditions, the temperature determined at the onset of the DSC graphs cannot be associated with the melting temperature but with a fusion-degradation phenomenon. At higher scan rates, the onset as well as the endothermic value of the signal increase to reach plateau values. These values have been identified as the temperature and enthalpy values of melting of vitamin C according to the results obtained from the heterogeneous system.