Aída Martín

Controlled chemistry of tailored graphene nanoribbons for electrochemistry: a rational approach to optimizing molecule detection

This work describes a rationalization of the interactions between two fully characterized graphene nanoribbons (GNRs) and a set of significant target molecules. The GNRs were carefully synthesized by unzipping multi-walled carbon nanotubes (MWCNTs) to yield graphene oxide nanoribbons (GNRox) containing 44 wt% oxygen. The GNRox were reduced to yield reduced graphene oxide nanoribbons (GNRred) containing 14 wt%. Each material was characterized by atomic force microscopy, transmission electronic microscopy, Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and voltammetry techniques. Differential pulse voltammetry was used to assess the detection of two strategically selected groups of molecules, including benzenediols, hydroquinone, catechol, and resorcinol, as well as, L-dopa, ascorbic acid, uric acid, and L-tyrosine. The results showed that GNRs provided significantly better electrochemical responses compared to MWCNTs and the non-modified glassy carbon electrode. The chemistry of the few layers of graphene strongly influenced the electrochemical properties of the material. GNRox may be the material of choice for sensing molecules having high oxidation potentials. GNRred, on the other hand, yielded an excellent sensitivity for aromatic molecules in which π–π interactions were dominant or the number of conjugated 1,2-diols present was high. GNRred combines the advantages of the high proportion of sp2-carbon atoms with the presence of a few oxygen moieties remaining in the lattice after the reduction step. The primary interactions responsible for the shift in oxidation potentials were elucidated. This work presents new opportunities for tailoring graphene to a particular sensing application based on the specific chemistry of the molecule.

Food analysis on microchip electrophoresis: An updated review

From 2008 to date, basically, single‐cross microchip electrophoresis (ME) design has been used for food analysis with electrochemicaland laser‐induced fluorescencedetection being the most commonprinciples coupled. In the last 4 years, the main outlines were: (i) the exploration of new analytes such as heavy metals, nitrite, micotoxins, microorganisms, and allergens; (ii) the development of electrokinetic microfluidic (bio‐) sensors into microchip format for the detection of toxins; and interestingly (iii) although sample preparation is still performed off‐chip, an important increase in works dealing with complicated food samples has been clearly noticed. Although microchip technology based on electrokinetics is emerging from important fields such as authentication of foods, detection of frauds, toxics, and allergens; the marriage between micro‐ and nanotechnologies and total integration approaches has not reached the expected impact in the field but it is still a great promise for the development of ME of new generations for food analysis.

Template Electrosynthesis of High-Performance Graphene Microengines.

Template-prepared graphene/Pt and graphene/Au tubular microengines, with extremely high electrocatalytic activity and propulsion efficiency, are described. The new bubble-propelled graphene/metal micromotors are synthesized rapidly and inexpensively by the direct electrodeposition of graphene oxide (GO) within the conical pores of a polycarbonate template membrane followed by deposition of the inner metal layer. The presence of high number of edges and defects in the graphene layer results in highly reactive microporous Pt or Au catalytic structures. The high catalytic activity leads to an ultrafast bubble propulsion (as high as 170 body lengths/sec) and operation at extremely low levels (0.1%) of the peroxide fuel. The effect of such dramatically enhanced catalytic surface area on the bubble growth and motor speed has been theoretically modeled. The template-prepared graphene-based microengines display distinct moving trajectories, along with long microbubble tails. The fast catalytic locomotion and attractive performance of the new graphene/Pt micromotors hold considerable promise for diverse applications.

Enzyme-based microfluidic chip coupled to graphene electrodes for the detection of D-amino acid enantiomer-biomarkers.

An electrochemical microfluidic strategy for the separation and enantiomeric detection of D-methionine (D-Met) and D-leucine (D-Leu) is presented. These D-amino acids (D-AAs) act as biomarkers involved in relevant diseases caused by Vibrio cholerae. On a single layout microfluidic chip (MC), highly compatible with extremely low biological sample consumption, the strategy allowed the controlled microfluidic D-AA separation and the specific reaction between D-amino acid oxidase (DAAO) and each D-AA biomarker avoiding the use of additives (i.e., cyclodextrins) for enantiomeric separation as well as any covalent immobilization of the enzyme into the wall channels or on the electrode surface such as in the biosensor-based approaches. Hybrid polymer/graphene-based electrodes were end-channel coupled to the microfluidic system to improve the analytical performance. D-Met and D-Leu were successfully detected becoming this proof-of-the-concept a promising principle for the development of point-of-care (POC) devices for in situ screening of V. cholerae related diseases.

Multidimensional carbon allotropes as electrochemical detectors in capillary and microchip electrophoresis.

The main multidimensional carbon allotropes could be classified into carbon nanotubes as 1D material, graphene as 2D material, as well as graphite and diamond as 3D carbon materials. Along with this review, a discussion using these four structures as electrochemical detectors in CE and ME will permit us to explore the recent advances in this field.

Graphene oxide nanoribbon-based sensors for the simultaneous bio-electrochemical enantiomeric resolution and analysis of amino acid biomarkers.

In this work, a straightforward in-situ measurement of L and D-amino acids (AAs) has been developed using disposable graphene oxide nanoribbon (GON) screen printed electrodes. For that, we took advantage of the electroactivity of certain clinically relevant AAs, such as tyrosine (Tyr) and methionine (Met), which are involved in important bacterial diseases (Bacillus subtilis and Vibrio cholera, respectively). The strategy is based on a dual electrochemical and enzymatic approach. The D-AA with the class enzyme D amino acid oxidase (DAAO) generates H2O2. This H2O2 is simultaneously detected with the L-AA, electroactive molecule by differential pulse voltammetry (DPV). These GON disposable platforms use just 50 μL of sample and a total analysis time of 360 s. Both L and D enantiomers calibration and quantitative analysis were explored and were simultaneously detected with accuracy and precision in urine samples. Any interference of uric acid and other electroactive AAs was noticed. This proposed electrochemical GON-based enantiomeric bio-sensor becomes a highly promising tool as future point of care for fast and reliable early diagnosis of diseases related to the presence of D-AAs.

Epidermal Microfluidic Electrochemical Detection System: Enhanced Sweat Sampling and Metabolite Detection

Despite tremendous recent efforts, noninvasive sweat monitoring is still far from delivering its early analytical promise. Here, we describe a flexible epidermal microfluidic detection platform fabricated through hybridization of lithographic and screen-printed technologies, for efficient and fast sweat sampling and continuous, real-time electrochemical monitoring of glucose and lactate levels. This soft, skin-mounted device judiciously merges lab-on-a-chip and electrochemical detection technologies, integrated with a miniaturized flexible electronic board for real-time wireless data transmission to a mobile device. Modeling of the device design and sweat flow conditions allowed optimization of the sampling process and the microchannel layout for achieving attractive fluid dynamics and rapid filling of the detection reservoir (within 8 min from starting exercise). The wearable microdevice thus enabled efficient natural sweat pumping to the electrochemical detection chamber containing the enzyme-modified electrode transducers. The fabricated device can be easily mounted on the epidermis without hindrance to the wearer and displays resiliency against continuous mechanical deformation expected from such epidermal wear. Amperometric biosensing of lactate and glucose from the rapidly generated sweat, using the corresponding immobilized oxidase enzymes, was wirelessly monitored during cycling activity of different healthy subjects. This ability to monitor sweat glucose levels introduces new possibilities for effective diabetes management, while similar lactate monitoring paves the way for new wearable fitness applications. The new epidermal microfluidic electrochemical detection strategy represents an attractive alternative to recently reported colorimetric sweat-monitoring methods, and hence holds considerable promise for practical fitness or health monitoring applications.

Microchip electrophoresis-single wall carbon nanotube press-transferred electrodes for fast and reliable electrochemical sensing of melatonin and its precursors.

In the current work, single‐wall carbon nanotube press‐transferred electrodes (SW‐PTEs) were used for detection of melatonin (MT) and its precursors tryptophan (Trp) and serotonin (5‐HT) on microchip electrophoresis (ME). SW‐PTEs were simply fabricated by press transferring a filtered dispersion of single‐wall carbon nanotubes on a nonconductive PMMA substrate, where single‐wall carbon nanotubes act as exclusive transducers. The coupling of ME–SW‐PTEs allowed the fast detection of MT, Trp, and 5‐HT in less than 150 s with excellent analytical features. It exhibited an impressive antifouling performance with RSD values of ≤2 and ≤4% for migration times and peak heights, respectively (n = 12). In addition, sample analysis was also investigated by analysis of 5‐HT, MT, and Trp in commercial samples obtaining excellent quantitative and reproducible recoveries with values of 96.2 ± 1.8%, 101.3 ± 0.2%, and 95.6 ± 1.2% for 5‐HT, MT, and Trp, respectively. The current novel application reveals the analytical power of the press‐transfer technology where the fast and reliable determination of MT and its precursors were performed directly on the nanoscale carbon nanotube detectors without the help of any other electrochemical transducer.

Carbon nanomaterial scaffold films with conductivity at micro and sub-micron levels

This work presents novel porous carbon nanomaterial scaffold films (CNSFs) with conductivity at micro and sub-micron levels. The carbon nanomaterial scaffold films (CNSFs) were carefully evaluated by current sensing atomic force microscopy (CS-AFM) to assess their roughness, level of porosity and spatial homogeneity of the electric resistance at the micro and sub-micron levels. CS-AFM showed the arrangement of nanomaterials on the filter and the local surface conductivity, which was highly dependent on the nanomaterial explored and demonstrated that the nanomaterial was, indeed, the only transducer of the electrical signal. The electrical continuity of the films exhibited ohmic and hopping conductivity for all carbon nanomaterial films, being remarked for multi-walled carbon nanotubes revealing, consequently, its potential for molecule detection at the microscale. The antifouling of these CNSFs (RSDs < 7%, n = 10) was demonstrated towards the electrochemical transduction at the microscale of neurotransmitters and phenolic markers as the target molecules in complex media. These CNSFs provide a simple and affordable alternative for expanding new frontiers for (bio-)molecule detection and other chemistry applications at micro and sub-micron levels.

Electrochemical detectors based on carbon and metallic nanostructures in capillary and microchip electrophoresis

Carbon and metallic‐based nanostructures have been progressively implemented as innovative electrochemical detectors in CE and microchip electrophoresis (ME). For both type of nanomaterials and toward selected examples, this review details the impact of these nanomaterials for enhanced detection performance in CE, ME, and paper‐based microfluidic devices. The analytical performance and the analytical potential in real world applications is also presented and discussed.